Methods of forming exfoliated graphene-based materials, exfoliated graphene-based materials, and uses thereof

Abstract

Electrochemically exfoliated graphene-based materials, such as aerogels, and methods of making and using the same.

Description

METHODS OF FORMING EXFOLIATED GRAPHENE-BASED MATERIALS, EXFOLIATED GRAPHENE-BASED MATERIALS, AND USES THEREOF FIELD

[0001] The present disclosure relates generally to electrochemically exfoliated graphene-based materials, methods of making, and uses thereof.

[0002] BACKGROUND

[0003] One of the factors contributing to a rise in global average temperatures since the middle of the 20th century is the release of anthropogenic carbon dioxide (CO2), which is accountable for almost 60% of the impacts of global warming.[1,2]However, reducing the concentration of CO2in the atmosphere remains a daunting task both scientifically and economically.[4]Addressing this problem requires the development of cost-effective and environmentally friendly materials for CO2capture.[3,5]SUMMARY

[0004] In one or more embodiments of the present disclosure, there is provided: 1. A method of forming functionalized exfoliated graphene, the method comprising providing an electrochemical cell, the cell comprising an electrode comprising graphite, and an electrolyte comprising a transition metal salt; applying a voltage to the electrochemical cell to exfoliate the graphite of the electrode; and forming a functionalized exfoliated graphene comprising a metal oxide, the metal oxide being formed from the transition metal salt. 2. The method of embodiment 1, further comprising selecting the voltage to be applied to the electrochemical cell based on the redox potential of the transition metal of the transition metal salt. 3. The method of any preceding embodiment, wherein applying a voltage comprises applying a constant voltage. 4. The method of any preceding embodiment, wherein applying a voltage comprises applying a constant voltage until current of the electrochemical cell goes to 0A.5. The method of any preceding embodiment, wherein the metal oxide is present in a wt% of about 5 to about 50 wt%. 6. The method of any preceding embodiment, wherein the transition metal salt comprises early transition metals, late transition metals, or a combination thereof. 7. The method of any preceding embodiment, wherein the transition metal salt comprises Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ru, Rh, Pd, Ag, C, W, Ir, Pt, Au, or a combination thereof. 8. The method of any preceding embodiment, wherein the electrolyte further comprises a salt, and forming the functionalized exfoliated graphene further comprises forming the functionalized exfoliated graphene comprising the metal oxide and a heteroatom dopant derived from the salt. 9. The method of any preceding embodiment, wherein the salt comprises an inorganic salt, an organic salt, or a combination thereof. 10. The method of any preceding embodiment, wherein the salt comprises a polyatomic inorganic salt, a polyatomic organic salt, or a combination thereof. 11. The method of any preceding embodiment, wherein the salt comprises polyatomic ions comprising C2H3O2−,HCO₃⁻ , CO₃²⁻ , NH₄⁺ , HSO₄-, SO₄²-, SO3²-, NO3- , NO2- , H2PO4- , HPO42-, PO43-, PO33−, MnO4−, ClO4−, ClO3−, ClO2−, ClO−, OH- , CN-, or a combination thereof. 12. The method of any preceding embodiment, wherein the heteroatom dopant is present at an atomic percentage of about 0.1 to about 5 atomic% . 13. A functionalized exfoliated graphene formed by the method of any one of embodiments 1 to 12. 14. Use of the functionalized exfoliated graphene of embodiment 13 as an electrode of a supercapacitor.15. A method of forming a hierarchical aerogel, the method comprising providing a dispersion comprising a phosphorous-functionalized exfoliated graphene; providing a nitrogen-functionalized polymer into the dispersion; adjusting the pH of the dispersion to couple the phosphorous-functionalized exfoliated graphene and the nitrogen-functionalized polymer; ageing the dispersion to form a hydrogel comprising the phosphorous- functionalized exfoliated graphene and the nitrogen-functionalized polymer; and freeze drying the hydrogel to form a hierarchical aerogel comprising the phosphorous-functionalized exfoliated graphene and the nitrogen-functionalized polymer. 16. The method of embodiment 15, further comprising thermally treating the aerogel. 17. The method of any preceding embodiment, further comprising providing a crosslinker into the dispersion for crosslinking the phosphorous-functionalized exfoliated graphene and the nitrogen-functionalized polymer. 18. The method of any preceding embodiment, wherein the crosslinker comprises a carbodiimide, an N-hydroxysuccinimide ester, or a combination thereof. 19. The method of any preceding embodiment, wherein the crosslinker comprises (1- ethyl-3-(3-dimethylaminopropyl)carbodiimide), (N-hydroxysuccinimide), or a combination thereof. 20. The method of any preceding embodiment, wherein freeze drying the hydrogel comprises drying the dispersion at a temperature of about -90 °C to about -105 °C and a pressure of about 0.003 mbar to about 0.0133 mbar. 21. The method of any preceding embodiment, wherein thermally treating the aerogel comprises heating the aerogel under vacuum. 22. The method of any preceding embodiment, wherein thermally treating the aerogel comprises heating the aerogel under vacuum at a temperature of about 100°C to about 150 °C.23. The method of any preceding embodiment, wherein the phosphorous- functionalized exfoliated graphene comprises electrochemically exfoliated graphene doped with polyatomic ions comprising phosphorous. 24. The method of any preceding embodiment, wherein the polyatomic ions comprising phosphorous comprise H2PO4-, HPO42-, PO43-, PO33−, or a combination thereof. 25. The method of any preceding embodiment, wherein the phosphorous- functionalized exfoliated graphene further comprises a metal oxide. 26. The method of any preceding embodiment, wherein the phosphorous- functionalized exfoliated graphene further comprising a metal oxide is made by the method of any one of embodiments 1 to 12. 27. The method of any preceding embodiment, wherein the nitrogen-functionalized polymer comprises an amino-functionalized polymer. 28. The method of any preceding embodiment, wherein the nitrogen-functionalized polymer comprises a polysaccharide. 29. The method of any preceding embodiment, wherein the nitrogen-functionalized polymer comprises chitosan, functionalized chitosan, or a combination thereof. 30. The method of any preceding embodiment, wherein the nitrogen-functionalized polymer comprises glycated chitosan. 31. The method of any preceding embodiment, wherein the coupling of the phosphorous-functionalized exfoliated graphene and the nitrogen-functionalized polymer comprises intermolecular forces. 32. The method of any preceding embodiment, wherein the coupling of the phosphorous-functionalized exfoliated graphene and the nitrogen-functionalized polymer comprises van der Waals forces, electrostatic forces, dipole-dipole forces, ion–dipole forces, ion–induced dipole forces, hydrogen bonding, or a combination thereof.33. The method of any preceding embodiment, wherein the hierarchical aerogel has a mesoporosity of about 2 nm to about 50 nm. 34. The method of any preceding embodiment, wherein the hierarchical aerogel has a microporosity of <2 nm. 35. The method of any preceding embodiment, wherein the hierarchical aerogel has a macroporosity of about >50 nm to about 0.5 µm. 36. The method of any preceding embodiment, wherein the hierarchical aerogel has a surface area of about 80 m2 / g to about 360 m2 / g. 37. The method of any preceding embodiment, further comprising using the hierarchical aerogel in electrochemical separations, metal extractions, gas adsorptions, gas separations, or a combination thereof. 38. The method of any preceding embodiment, wherein the gas to be absorbed or separated comprises CO2or N2. 39. The method of any preceding embodiment, further comprising regeneration the hierarchical aerogel after use via direct electrical heating. 40. A hierarchical aerogel formed by the method of any one of embodiments 1 to 39. 41. Use of the hierarchical aerogel of embodiment 40 as an adsorbent. 42. Use of the hierarchical aerogel of embodiment 40 as an electrode. 43. Use of the hierarchical aerogel of embodiment 40 as a catalyst. 44. Use of the hierarchical aerogel of embodiment 40 as an electrode of a supercapacitor. 45. Use of the hierarchical aerogel of embodiment 40 in electrochemical separations. 46. Use of the hierarchical aerogel of embodiment 40 in metal extraction.47. Use of the hierarchical aerogel of embodiment 40 as an ion sieve. 48. Use of the hierarchical aerogel of embodiment 40 in gas adsorption, gas separation, or a combination thereof. 49. A hierarchical aerogel comprising a phosphorous-functionalized exfoliated graphene coupled with a nitrogen-functionalized polymer. 50. The aerogel of embodiment 49, wherein the phosphorous-functionalized exfoliated graphene comprises electrochemically exfoliated graphene doped with polyatomic ions comprising phosphorous. 51. The aerogel of any preceding embodiment, wherein the polyatomic ions comprising phosphorous comprise H2PO4-, HPO42-, PO43-, PO33−, or a combination thereof. 52. The aerogel of any preceding embodiment, wherein the phosphorous- functionalized exfoliated graphene further comprises a metal oxide. 53. The aerogel of any preceding embodiment, wherein the phosphorous- functionalized exfoliated graphene further comprising a metal oxide is made by the method of any one of embodiments 1 to 12. 54. The aerogel of any preceding embodiment, wherein the nitrogen-functionalized polymer comprises an amino-functionalized polymer. 55. The aerogel of any preceding embodiment, wherein the nitrogen-functionalized polymer comprises a polysaccharide. 56. The aerogel of any preceding embodiment, wherein the nitrogen-functionalized polymer comprises chitosan, functionalized chitosan, or a combination thereof. 57. The aerogel of any preceding embodiment, wherein the nitrogen-functionalized polymer comprises glycated chitosan.58. The aerogel of any preceding embodiment, wherein the coupling of the phosphorous-functionalized exfoliated graphene and the nitrogen-functionalized polymer comprises intermolecular forces. 59. The aerogel of any preceding embodiment, wherein the coupling comprises van der Waals forces, electrostatic forces, dipole-dipole forces, ion–dipole forces, ion–induced dipole forces, hydrogen bonding, or a combination thereof. 60. The aerogel of any preceding embodiment, wherein the hierarchical aerogel comprises a 3-D structure. 61. The aerogel of any preceding embodiment, wherein the hierarchical aerogel comprises a 3-D structure defining an open, interconnected pore morphology. 62. The aerogel of any preceding embodiment, wherein the hierarchical aerogel comprises a 3-D structure defining mesopores, micropores, macropores, or a combination thereof. 63. The aerogel of any preceding embodiment, wherein the hierarchical aerogel has mesopores between about 2 nm to about 50 nm in size. 64. The aerogel of any preceding embodiment, wherein the hierarchical aerogel has micropores <2 nm in size. 65. The aerogel of any preceding embodiment, wherein the hierarchical aerogel has macropores between about >50 nm to about 0.5 µm in size. 66. The aerogel of any preceding embodiment, wherein the hierarchical aerogel has a surface area of about 80 m2 / g to about 360 m2 / g. BRIEF DESCRIPTION OF THE FIGURES

[0005] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

[0006] Embodiments of the present disclosure will now be described, by way of example only, with reference to the attached Figures.

[0007] FIG.1 depicts a schematic illustration of the synthesis of flexible graphene films.

[0008] FIG. 2 depicts (a) and (b) SEM images of EEG / δ-MnO2under different magnifications. (c) TGA analysis. (d) XRD patterns and crystallographic structure of δ- MnO2and α-MnO2. (e) The Raman Spectra of EEG and EEG / δ-MnO2.

[0009] FIG.3 depicts (a), (b), and (c) TEM images of EEG / δ-MnO2under different magnifications. d) STEM image and corresponding EDS elemental mapping images of C, Mn, O, S, N. (e) The XPS survey spectrum of EEG / δ-MnO2. (f) High-resolution spectra of C 1s, g) Mn 2p and h) N 1s.

[0010] FIG. 4 depicts (a) top-view SEM images and (b) cross-sectional SEM images of EEG / δ-MnO2film. (c) Volumetric, areal and gravimetric capacitance versus scan rate for EEG / δ-MnO2. (d) CV curves of EEG / δ-MnO2film at different scan rates. (e) Logarithmic plot of charge and discharge currents as a function of scan rates for EEG / δ- MnO2. (f) Percentage capacitive contributions obtained at different scan rates for EEG / δ- MnO2.

[0011] FIG. 5 depicts (a) Top-view SEM images and (b) cross-sectional SEM images of NP-EEG film. (c) Volumetric, areal and gravimetric capacitance versus scan rate for NP-EEG. (d) CV curves of NP-EEG film at different scan rates. (e) Logarithmic plot of charge and discharge currents as a function of scan rates for NP-EEG. (f) Percentage capacitive contributions obtained at different scan rates for NP-EEG.

[0012] FIG.6 depicts (a) illustration of components of the solid-state device (ACS). (b) CV curves obtained at different scan rates between 0 and 1.6 V. (c) charge / discharge curves at various areal current densities. (d) CV curves of the flexible ASC at different bending angles using a scan rate of 10 mV s-1. (e) Capacitance retention of ASC after 10000 cycles (the inset shows the first and last cycle). (f) Ragone plots of ASC in comparison to other MnO2and carbon-based solid-state supercapacitors.

[0013] FIG. 7 depicts (a), (b) SEM images of EEG / α-MnO2 under different magnifications.

[0014] FIG. 8 depicts (a), (b), (c) TEM images of EEG / α-MnO2 under different magnifications. d) STEM image and corresponding elemental mapping images of C, Mn, O, S, N.

[0015] FIG.9 depicts (a) The full-range XPS survey spectrum of EEG / α-MnO2. (b) High-resolution spectrum of C 1s, (c) Mn 2p and d) N 1s.

[0016] FIG. 10 depicts (a) The full-range XPS survey spectrum of NP-EEG. (b) High-resolution spectrum of C 1s, ( c) P 2p and (d) N 1s.

[0017] FIG.11 depicts CV curves of EEG / δ-MnO2 films with different mass loading of MnO2at the scan rate of 10 mV sି^.

[0018] FIG. 12 depicts CV curves of graphene films synthesised by different electrolytes as the positive electrode at the scan rate of 10 mV sି^.

[0019] FIG. 13 depicts CV curves of graphene films synthesised by different electrolytes as the negative electrode at the scan rate of 10 mV sି^.

[0020] FIG. 14 depicts a Nyquist plot of NP-EEG and EEG / δ-MnO2films (Inset shows the electrical equivalent circuit model for the Nyquist plot).

[0021] FIG.15 depicts (a) FT-IR spectra for CS and GC6, GC12, GC18, and GC24 after 6, 12, 18 and 24 hours of glycation, respectively. The main characteristics bands are shown in the figure. (b) XRD pattern of EEG-GC18, GC18, CS and EEG.

[0022] FIG. 16 depicts (a) LSCM images of 1 wt.% 1: 2 EEG-GC6, EEG-GC12, EEG-GC18 and EEG-GC24 precursor gels as a function of the reaction time of glycation of chitosan. (b) SEM images of aerogels from 1 wt.% 1:2 EEG-GC6, EEG-GC12, EEG- GC18 and EEG-GC24 precursors. The top row has a scale bar of 15 μm, the middle row has a scale bar of 1 μm and the bottom row has a scale bar of 200 nm. (c) The mean domain size in EEG-GC 1 wt.%, 1:2 EEG: GC wt. ratio precursor gels with varying extent of glycation calculated by image processing software Avizo 3D. (d) CO2uptake as a time sweep for EEG-GC aerogels derived from 1 wt.% 1:2 EEG: GC wt. ratio precursor gels, with varying reaction times of glycation of chitosan, at 298 K and 1 bar.

[0023] FIG.17 depicts SEM images of 1 wt.%, 1:2 aerogels- (a) GO-GC18 aerogels (b) EEG(S)-GC18 aerogels. (c) CSP-GC18 aerogels (d) SMP -GC18 aerogels. The top row for each of the cases has a scale bar of 15 μm while the bottom row has a scale bar of 1 μm.

[0024] FIG.18 depicts the mechanism of amine-phosphate interaction.

[0025] FIG. 19 depicts (a) Mean domain size in EEG-GC18 precursor gels as a function of EEG loading (%) in 1 wt.% and 2 wt.% precursor gels calculated by image processing software Avizo 3D. (b) Rheological characterizations of EEG-GC18 mixed gels of different total solids content in the precursor gels as a function of EEG loading (%) at a frequency range of 0.1-10 Hz and shear strain of 1%. (c) BET surface area (m2g−1) and Porosity (%) of EEG-GC18 aerogels as a function of composition. (d) CO2uptake as a time sweep for EEG-GC181 wt.% and 2 wt.% total solids content in precursor gels with varying EEG: GC18 wt. ratio at 298 K and 1 bar.

[0026] FIG.20 depicts (a) N2adsorption-desorption isotherm at 77 K of EEG-GC18 1 wt.%, 1:2 EEG: GC18 wt. ratio aerogels. (b) Pore size distribution profile of EEG-GC181 wt.%, 1:2 EEG: GC wt. ratio aerogels. (c) CO2 and N2 isotherms at 273 K, 288 K, 298 K and 308 K of EEG-GC181wt.%, 1:2 EEG: GC wt. ratio aerogels. (d) CO2 / N2selectivity of EEG-GC18 1 wt.%, 1:2 EEG: GC wt. ratio aerogels at 298 K, calculated from pure adsorption isotherms using the IAST model, as a function of pressure. (e) Regeneration efficiency over adsorption and regeneration via. electrical heating cycles for 20 cycles for EEG-GC181 wt.%, 1:2 EEG: GC18 wt. ratio aerogels. (f) Adsorption-Regeneration cycles of EEG-GC18, 1 wt.%, 1:2 sample, for 5 cycles as a pressure sweep from 0 to 1 bar, at 298 K.

[0027] FIG. 21 depicts a comparison of selectivity of CO2 / N2of 3D-structured adsorbents at 298 K and 1 bar, at 25% CO2and 75% N2flue gas conditions as a function of CO2uptake at 298K and 1 bar, in mmol g-1, with this work. [14,37,88,103,115,116,148- 157]

[0028] FIG.22 depicts (a) Thermogravimetric analysis of the four MEEG samples under air. (b) FTIR spectra for EEG, MEEG-12, MEEG-24, MEEG-32, and MEEG-42. The main characteristics bands are shown in the figure. (c) SEM images of 1 wt.% 1:1 MEEG- 32-CS aerogels. The top row has a scale bar of 20 μm while the bottom row has a scale bar of 1 μm. (d) BET surface area and Porosity of MEEG-CS aerogels as a function of MnO2loading% in MEEG. (e) CO2breakthrough curve for MEEG-CS aerogels derived from 1 wt.% precursor with 1:1 MEEG: CS ratio, with varying loading of MnO2on MEEG, at 298 K and 1 bar.

[0029] FIG.23 depicts (a) FTIR spectra for MEEG, CS and MEEG-CS (1 wt.%, 1:1 MEEG: CS ratio) aerogel. The main characteristics bands are shown in the figure. (b) XRD pattern of MEEG, CS and MEEG-CS 1 wt.%, 1:1 aerogel.

[0030] FIG. 24 depicts (a) SEM micrographs of 1 wt.% 1:1 aerogel- EEG-CS without crosslinking; MEEG-CS without crosslinking; EEG-CS with crosslinking and MEEG- CS with crosslinking. The top row has a scale bar of 20 μm while the bottom row has a scale bar of 1 μm. (b) BET surface area and Porosity (%) of the derived aerogels. (c) BET surface area and Porosity (%) of the derived aerogels. d) CO2breakthrough curves for the derived aerogels at 298 K and 1 bar. (d) Overall reaction and crosslinking mechanism between MnO2functionalized electrochemically exfoliated graphene (MEEG) and chitosan (CS).

[0031] FIG. 25 depicts (a) Correlation between the BET surface area and CO2uptake for the derived aerogels. (b) Schematic demonstrating the hybrid CO2adsorption mechanism of the derived aerogels.

[0032] FIG.26 depicts (a) CO2and N2isotherms at 273 K, 288 K, 298 K and 308 K on MEEG-CS 1 wt.%, 1:1 MEEG: CS wt. ratio aerogel. (b) CO2 / N2selectivity of MEEG-CS 1 wt.%, 1:1 MEEG: CS wt. ratio aerogel calculated from pure adsorption isotherms using the IAST model, as a function of pressure. (c) Regeneration efficiency over adsorption and regeneration via. electrical heating cycles for 20 cycles for MEEG-CS 1 wt.%, 1:1 MEEG: CS wt. ratio aerogel. (d) SEM images of the MEEG-CS 1 wt.%, 1:1 MEEG: CS wt. ratio aerogel took before and after 20 adsorption- regeneration via. electrical heating cycles. The top row for each of the cases has a scale bar of 20 μm while the bottom row has a scale bar of 1 μm.

[0033] FIG. 27 depicts a comparison of selectivity of CO2 / N2of different 3D- structured adsorbents at 298 K and 1 bar, at 25% CO2and 75% N2flue gas conditions as a function of CO2uptake at 298K and 1 bar, in mmol g-1, with the present disclosure [11,24- 26,89-100].

[0034] FIG.28 depicts a reaction for the synthesis of glycated chitosan.

[0035] FIG.29 depicts the particle size distribution of (a) CS (b) GC6 (c) GC12 (d) GC18 (e) GC24 (f) Average particle size of GC as a function of the extent of glycation in terms of reaction time, obtained from the Dynamic light scattering (DLS) method.

[0036] FIG. 30 depicts SEM images of 1 wt.%, aerogels from CS and GC18, respectively. The top row for each of the cases has a scale bar of 15 μm while the bottom row has a scale bar of 1 μm.

[0037] FIG. 31 depicts (a) Full-range XPS spectrum of EEG.High-resolution spectra of (b) C 1s (c) P 2p (d) N 1s.

[0038] FIG. 32 depicts SEM images of pristine CS, and the synthesized GC6, GC12, GC18 and GC24 polymer particles. Each image has a scale bar of 400 nm.

[0039] FIG. 33 depicts rheological characterizations (Amplitude sweep) of EEG- GC18 mixed gels at different EEG: GC18 wt. ratios; percentage shows the amount of total solids content in the precursor gels, performed at a Frequency of 10 Hz (a) 1 wt.% EEG- GC18 and (b) 2 wt.% EEG-GC18. Rheological characterizations (Frequency sweep) of EEG-GC18 mixed gels at different EEG: GC18 wt. ratios; percentage shows the amount of total solids content in the precursor gels, performed at a Shear Strain of 1%. (c) 1 wt.% EEG-GC18 and (d) 2 wt.% EEG-GC18.

[0040] FIG.34 depicts LSCM images of 1 wt.% and 2 wt.% EEG-GC18 precursor gels as a function of EEG: GC18 wt. ratio. and total solids content.

[0041] FIG.35 depicts SEM images of 1 wt.% and 2 wt.% EEG: GC18 aerogels as a function of EEG: GC18 wt. ratio. The top row for each of the cases of 1 wt.% and 2 wt.% aerogels have a scale bar of 15 μm while the bottom row has a scale bar of 1 μm.

[0042] FIG. 36 depicts high magnification SEM images of 1 wt.% EEG: GC18 aerogels as a function of EEG: GC18 wt. ratio. Each image has a scale bar of 200 nm.

[0043] FIG.37 depicts CO2and N2isotherms at 298 K on EEG-GC181 wt.%, 1:2 EEG: GC18 wt. ratio aerogels fitted to (a) Freundlich Model, (b) Toth Model.

[0044] FIG. 38 depicts a comparison of H2O isotherms of different adsorbents at 298 K and 1 bar with EEG-GC181 wt.%, 1:2 EEG: GC18 wt. ratio aerogels as a function of relative humidity (%).[95-98]

[0045] FIG.39 depicts SEM images of EEG-GC181 wt.%, 1:2 aerogels before and after exposure to 100% RH. The top row for each of the cases has a scale bar of 15 μm while the bottom row has a scale bar of 1 μm.

[0046] FIG. 40 depicts SEM images of the EEG-GC181 wt.%, 1:2 EEG: GC wt. ratio aerogels took before and after 20 adsorption- regeneration via. electrical heating. The top row for each of the cases has a scale bar of 15 μm while the bottom row has a scale bar of 1 μm.

[0047] FIG. 41 depicts a thermogravimetric analysis of EEG, GC18 and EEG- GC18, 1 wt.%, 1:2 aerogel under N2gas.

[0048] FIG. 42 depicts (a) LSCM images of 1 wt.%, 1:1 wt. ratio MEEG-CS precursor dispersions as a function of reaction time for crosslinking. (b) SEM images of 1 wt.%, 1:1 wt. ratio MEEG: CS aerogels as a function of reaction time for crosslinking. The top row has a scale bar of 20 μm while the bottom row has a scale bar of 1 μm. (c) BET surface area and Porosity of MEEG-CS aerogels (1 wt.%, 1:1 MEEG: CS ratio) as a function of reaction time for cross-linking. (d) CO2uptake as time sweeps for MEEG-CS aerogels (1 wt.%, 1:1 MEEG: CS ratio), with varying reaction times for crosslinking, at 298 K and 1 bar.

[0049] FIG.43 depicts a thermogravimetric analysis of MEEG, CS and MEEG-CS, 1 wt.%, 1:1 aerogel.

[0050] FIG.44 depicts CO2and N2isotherms at 298 K on MEEG-CS 1 wt.%, 1:1 MEEG: CS wt. ratio aerogel fitted to (a) Freundlich Model and (b) Toth Model.

[0051] FIG.45 depicts a) N2adsorption-desorption isotherm of MEEG-CS 1 wt.%, 1:1 MEEG: CS wt. ratio aerogels at 77 K. (b) Pore size distribution profile of MEEG-CS 1 wt.%, 1:1 MEEG: CS wt. ratio aerogels.

[0052] FIG.46 depicts rheological characterizations (Amplitude sweep) of MEEG- CS mixed gels at different MEEG: CS wt. ratios; percentage shows the amount of total solids content in the precursor gels, performed at a Frequency of 10 Hz (a) 1 wt.% MEEG- CS and (b) 2 wt.% MEEG-CS. Rheological characterizations (Frequency sweep) of MEEG- CS mixed gels at different MEEG: CS wt. ratios; percentage shows the amount of total solids content in the precursor gels, performed at a Shear Strain of 1%. (c) 1 wt.% MEEG- CS and (d) 2 wt.% MEEG-CS.

[0053] FIG.47 depicts (a) SEM images of 1 wt.% and 2 wt.% MEEG: CS aerogels as a function of MEEG: CS wt. ratio. The top row has a scale bar of 20 μm while the bottom row has a scale bar of 1 μm. (b) BET surface area and Porosity of 1 wt.% and 2 wt.% MEEG-CS aerogels as a function of MEEG: CS wt. ratio. (c) CO2uptake as time sweeps for 1 wt.% and 2 wt.% MEEG-CS aerogels, with varying MEEG: CS wt. ratio, at 298 K and 1 bar.

[0054] FIG.48 depicts SEM images of MEEG-CS 1 wt.%, 3:2 aerogel, where the flexibility of the aerogels was lost. The top row for each of the cases has a scale bar of 20 μm while the bottom row has a scale bar of 2 μm.

[0055] FIG.49 depicts adsorption-Regeneration cycles of MEEG-CS, 1 wt.%, 1:1 sample, for 5 cycles as a pressure sweep from 0 to 1 bar, at 298 K.

[0056] FIG.50 depicts Comparison of H2O isotherm of different adsorbents at 298 K and 1 bar with MEEG-CS 1 wt.%, 1:1 MEEG: CS wt. ratio aerogels as a function of relative humidity (%).

[0057] FIG.51 depicts SEM images of MEEG-CS 1 wt.%, 1:1 aerogel before and after exposure to 100% RH. The top row for each of the cases has a scale bar of 20 μm while the bottom row has a scale bar of 2 μm.

[0058] FIG.52 depicts LSCM images of 1 wt.%, 1:1 wt. ratio MEEG-CS precursor dispersions as a function of MnO2% loading in MEEG.

[0059] FIG.53 depicts LSCM images of 1 wt.% and 2 wt.% MEEG-CS precursor dispersions as a function of MEEG: CS wt. ratio. and total solids content.

[0060] FIG. 54 depicts (a) Full-range XPS spectrum of MEEG. High-resolution spectra of (b) C 1s (c) Mn 2p (d) N 1s.DETAILED DESCRIPTION

[0061] Unless defined otherwise, all technical and scientific terms used herein have the meaning as commonly understood in the art.

[0062] As used in the specification and claims, the singular forms "a", "an" and "the" include plural references unless the context dictates otherwise.

[0063] The growing field of compact wearable electronics and portable devices is in demand of high-performance electrochemical energy storage devices such as batteries and supercapacitors.[1-5]Supercapacitors are known as clean energy storage devices with high power densities, fast charge / discharge rates and long-term stability that can meet the needs of miniature devices.[6-8]In recent years, various electrode materials have been explored to improve the characteristics of the supercapacitors including carbon-based materials,[9-10]transition metal oxides,[11-12]conductive polymers,

[0013] MXenes

[0014] and MOFs.

[0015] However, to meet commercial needs, it is important to keep the material synthesis process simple, scalable and environmentally friendly.[16-18]In addition, due to the limited available space inside the miniature devices, and the small mass loading of the electrodes, volumetric energy density plays a more important role compared to gravimetric energy density.[19-20]Therefore, it is helpful to choose electrode materials that not only show high volumetric performance and retain it for thousands of cycles, but also simultaneously benefit from a facile, green and scalable synthesis method.

[0064] MnO2as a pseudocapacitive transition metal oxide has shown notable capacitive behaviour due to its high theoretical capacitance (~1370 F g-1) stemming from the fast and reversible redox reactions at the electrode surface. Furthermore, its availability, low cost and environmentally friendly nature have made it more attractive for commercial applications.[21-25]The capacitance of the MnO2intensely depends on its crystal structure and varies in the following order: α≈δ>γ>λ>β due to the different cation insertion gaps.

[0026] However, the low electrical conductivity of MnO2(10-3to 10-4S m-1) and slow ion diffusion can hinder its high mass loading and electrochemical performance.[27-28]To overcome this problem, MnO2has been composited with highly conductive nanostructures including carbon-based materials (e.g., graphene,[29-31]carbon nanotubes,

[0032] mesoporous carbon),

[0033] conductive polymers

[0034] and MXenes.

[0035] Till now, most of the synthesis methods are based on the separate synthesis of the mentioned electrically conductive structures and the ideal MnO2phase and then their hybridization.

[0065] Graphene is one of the more promising 2D carbon-based supports for MnO2due to its high conductivity (106 S m-1), stability, large surface area (2630 m2g-1) and wide stable working potential window.

[0036] To hybridize the graphene with MnO2various methodssuch as electrochemical deposition,[37-39]hydrothermal,

[0040] microwave irradiation,

[0041] electrostatic self-assembly

[0042] and solution plasma process

[0043] have been reported. For example, Hamade et al. thermally treated expandable graphite flakes using a microwave followed by a modified hummer’s method to prepare the graphene oxide, GO. Then, GO was functionalized with α-MnO2using a double-solvent reacting system at 80 ℃. The results showed a high gravimetric capacitance of 225 F g-1and capacitance retention of 97.8% after 1000 cycles. In another study, in a modified hummer’s method, GO was decorated with α-MnO2by in-situ anodic pulse electrodeposition. The maximum specific capacitance of the composite was 693.4 F g-1while maintaining 79.2% of its capacitance after 5000 cycles. Pimklang et al.

[0043] developed a simple method based on the solution plasma process to hybridize the graphene with MnO2at ambient pressure and temperature. The prepared electrode reached the capacitance of 218 F g-1and 85% stability after 1000 cycles. Zhu et al.

[0039] proposed a method to simultaneously reduce the GO synthesized using hummer’s method and electrodeposit MnO2nanowall shape. However, it must be pointed out that the preparation of GO and MnO2are two separate steps in all of the mentioned methods.

[0066] Described herein is a one-step, tunable and environmentally friendly method to synthesize functionalized exfoliated graphene, such as graphene / MnO2composites doped with nitrogen and sulfur, using electrochemical exfoliation of graphite. In one or more examples, the crystal structure of the MnO2can be modified by changing the electrolyte. Two electrolyte mixtures of (NH4)2SO4and (NH4)2SO4+ NH4Ac in combination with MnSO4as the source of manganese were utilized to prepare δ-MnO2and α-MnO2, respectively, which are the phases with highest reported capacitances. In addition, another aqueous solution of inorganic salts such as (NH4)2HPO4, (NH4)2SO4and (NH4)2SO4+ NH4Ac without adding MnSO4was also used as the electrolyte to synthesize nitrogen, sulphur and phosphor doped graphene. The flexible, freestanding graphene films were prepared by vacuum-assisted filtration of the graphene dispersions. The EEG / δ-MnO2and the nitrogen and phosphor co-doped graphene films exhibited notable electrochemical performance as the positive and negative electrodes respectively. Consequently, an asymmetric solid-state supercapacitor (ASC) based on the films with an extended operating potential window of 1.6 V was fabricated. The as-prepared ASC exhibited a volumetric energy and power density of 17.4 mWh cm-3and 4.8 W cm-3with capacitance retention of 98.4% after 10,000 cycles.

[0067] Adsorption processes are considered a promising substitute to conventional liquid amine absorption for separating CO2from mixtures, such as flue gases. Flue gases may include CH4, SO2, NOx - generally as minor components - as well as CO2 and N2 are more major components. Several solid materials, including metal-organic frameworks (MOFs), alumina, porous carbons, silica gels and zeolites, have been evaluated for CO2physisorption.[12,99-105] Many factors, including temperature, pressure and moisture content can affect the CO2adsorption capacity of such physical sorbents.[15,106-110] High temperatures and humidity can decrease the CO2adsorption capacity of physical sorbents, while the CO2 / N2selectivity limits their effectiveness for CO2adsorption from flue gases.[89,111,112] MOFs can exhibit notable performance, but they are challenging to structure.[6] Silica is an extensively studied physical sorbent due to its ease of processing and high surface area. However, the large pressure drop across the nanoporous material can increase the operational costs in packed adsorption columns.[6]

[0068] Chemisorbents, in comparison to sorbents based on physisorption, can have higher CO2adsorption capacity and selectivity towards CO2, due to the strong chemical interaction of CO2with the sorbent.[6] A drawback for chemisorbents can be the requirement for high regeneration energies of 80-250 kJ mol−1. Metal oxides like CaO and MgO are often only suitable for high-temperature operations.

[0113] Amine-impregnated and amine-functionalized sorbents have demonstrated potential in achieving the requisite CO2adsorption capacity of 2–4 mmol g−1under actual flue gas conditions and can be durable over several cycles.[13,14,114-116] The theoretical regeneration energies of a few of the tested sorbents are considerably lower than the industry standard.[6,117,118] Since amines' basic character facilitates interaction with the acidic CO2gas, materials based on amines and polyamines can adsorb CO2from even ultra-diluted sources, including flue gas or air.[5,6,16,78]

[0069] Hierarchically structured 3D materials can offer the advantages of high surface area and tunable pore sizes ranging from several nanometers to hundreds of micrometres.[119-121] The use of soft material templating methods has gained attention in the structuring of porous materials due to their versatility, controllability, affordability, and straightforward synthesis process.[122,123] Among these methods, the freeze-drying approach has been implemented for the development of porous materials with tunable porosity and microscopic morphology owing to the use of water as a safe solvent.[36,124,125] The properties of derived aerogels can be influenced by several factors including polymer and particle loading, particle size distribution, dispersibility of precursors, ionic strength, pH, mixing and ageing times, drying methods, and freezingconditions.[26,30-37,39,40] Aerogels can be suited for adsorption applications due to the combination of customizable mechanical properties and porosity, as well as the straightforward and reproducible synthesis approach.[14,126] Compared to conventional packed columns that use packed particle adsorbents, aerogels with their three-dimensional structure and interconnected pores can offer accessible surface areas with lower pressure drop during operation.[127,128] Several aerogels have recently been studied for CO2adsorption because of their ultra-low density, easy tunability and high surface area.[5,14,18]

[0070] In the past decade, graphene, a two-dimensional material, has gained attention in the field of electrochemistry owing to its properties such as high surface area, superior conductivity and catalytic activity.

[0041] The electrochemical exfoliation of graphite has gained interest due to its safety and scalability for producing high-quality graphene.

[0046] Electrochemically exfoliated graphene (EEG) can be produced using various electrolytes, including acidic media, ionic liquids, and inorganic salts.[47-52] Inorganic salts have been discovered to produce graphene with fewer oxygen functional groups and a greater lateral dimension than other electrolyte types.

[0053] The presence of an optimal number of oxygenated groups on the basal planes as well as edges of EEG can provide an opportunity to produce desirable materials for CO2-capturing applications.

[0054]

[0071] Chitosan, a naturally occurring polysaccharide that ranks second in abundance after cellulose, contains several basic amine groups (-NH2). These amine groups can play a role in enabling the adsorption of acidic CO2molecules onto the surface of chitosan. Chitosan can have several advantages, including low cost compared to other amine-based sorbents, environmental friendliness, and excellent mechanical properties. However, chitosan in its flake and powdered states tends to have a minimal surface area and limited porosity, which can render these forms unsuitable as standalone adsorbents.[14,116] Incorporating nanofillers such as carbon nanotubes, clay, carbon black and graphene oxide, for the development of nanocomposite materials has been found to enhance the performance of chitosan for gas separation processes.[14,129,130] Nonetheless, chitosan’s limited dispersibility can hinder its application and impede control of the mesoporosity in chitosan-nanofiller systems.[64,65]

[0072] Described herein is the fabrication of a composite aerogel, which may be used for the adsorption of carbon dioxide from flue gas, using environmentally friendly glycated chitosan (GC) and electrochemically exfoliated graphene (EEG), and employing the coupling between amine and phosphate functional groups. The results demonstrate that the glycation of chitosan can increase precursor gel dispersion and improve themorphology of aerogels. The coupling between the amine functional group on GC and the phosphate functional group on EEG can increase the mesoporosity and derive aerogels with an order of magnitude higher adsorption capacity (2.88 mmol g−1) in comparison to the building blocks (EEG- 0.16 mmol g−1, GC18- 0.81 mmol g−1), at 298 K and 1 bar. By incorporating EEG, the derived aerogels display increased electrical conductivity (20.3 ± 0.2 S m−1), allowing for the sorbent's direct electrical heating for thermal regeneration.[7- 9,94,95] In contrast to external heating, which can sometimes require several minutes to reach the regeneration temperatures, Joule heating can reach the regeneration temperature on a time scale of seconds, which can reduce the operational costs associated with the adsorbent regeneration process and downtime during operation.[7,9,94] The mass ratio of EEG and GC, as well as the aerogel density / solids content, were investigated to achieve a balance between adsorption capacity and structural integrity, resulting in adsorbent stability.

[0073] The rise in atmospheric carbon dioxide (CO2) levels is a major contributing factor to global warming and climate change. To address this issue and achieve climate targets, CO2capture solutions are required in power plants, industrial processes, and transportation emissions.[1,2] The deployment of carbon capture can substantially reduce greenhouse gas emissions, mitigate climate change consequences, decrease air pollution, and create new economic opportunities.[4] Carbon capture has become an important tool in the fight against climate change and provides numerous benefits for the environment and economy.[3,5]

[0074] Adsorption-based carbon capture processes use adsorbent materials. Physical adsorption-based sorbents for CO2include porous carbonaceous substances, zeolites, alumina, silica gels, and metal-organic frameworks (MOFs).[12,99-105] The surface chemistry and structure of physisorbent materials can be optimized for improved CO2adsorption capacity and selectivity.

[0161] However, physisorbent materials can suffer from drawbacks such as limited selectivity and low uptake in the presence of moisture.[15,89,108,111,112] On the other hand, the chemical adsorption of CO2can use the nucleophilic properties exhibited by functional groups present on the surface of the sorbent material for chemical interactions with CO2molecules, which can overcome some of the drawbacks of physisorbents.[6] Metal oxides like CaO and MgO, amine-grafted and amine-impregnated sorbents, are examples of some chemisorbent materials that have been investigated for CO2capture.[13,14,114-116,162] These materials can offer advantages such as high selectivity, capacity, and good performance in the presence of moisture.[117,118] Chemisorbents can complement other carbon capture technologies buttypically require high regeneration energies of approximately 80-250 kJ mol-1, compared to 10-50 kJ mol-1in case of physisorption.[5,16,78] To compete with existing MEA scrubbing systems, it is desirable for solid sorbents to have a CO2 working capacity in the range of 2- 4 mmol g-1of sorbent.[6]

[0075] Hierarchically structured materials, which may be used for gas adsorption processes such as CO2capture, have also attracted interest in the past decade. They can offer an increased surface area and improved pore accessibility, which can result in a higher adsorption capacity for gas molecules, such as CO2molecules.

[0163] The multi-scale organization of these materials can enable the simultaneous utilization of micropores, mesopores, and macropores, which can facilitate the diffusion and adsorption of gas molecules, such as CO2molecules. This hierarchical arrangement maximizes the contact area between the material and CO2, leading to higher adsorption capacities and improved capture efficiency.

[0164] The surface chemistry of hierarchically structured materials can be tailored to increase their selectivity and affinity for gas capture, such as CO2capture. For example, by functionalizing the surface with specific chemical groups such as -NH2, the materials can exhibit chemisorption behaviour with preferential adsorption of CO2over other gases.[117,118] This selective adsorption behaviour can facilitate a high purity in the captured CO2stream and an increase in the adsorption efficiency.[117,118] Their three- dimensional structure with the open interconnected pore morphology offers accessible pores with reduced pressure drop compared to traditional packed bed adsorption columns.[5,165] In this regard, recent years have witnessed an interest in the freeze-drying method for synthesizing porous materials with tailored porosity and morphology.[166-168] For example graphene aerogels are a studied hierarchically structured material for CO2capture.

[0028] Several studies have modified graphene aerogels' structure and surface chemistry to enhance their CO2adsorption capacity and selectivity by adding metal nanoparticles or metal-organic frameworks.[169-173]

[0076] A challenge for the development of graphene aerogels can be in the development of methods to produce aerogels with controlled mesoporosity, in order to increase their specific surface area. This challenge arises from graphene’s hydrophobic nature and its tendency to aggregate through ^^ െ ^^ interactions.[121,174] The introduction of additives to prevent the ^^ െ ^^ interactions may increase the surface area of the aerogels.

[0174] Manganese dioxide (MnO2) has been found to be an effective additive for inducing mesopores in graphene aerogels.[175,176] MnO2interacts with graphene sheets, disrupting aggregation by acting as a steric barrier to the ^^ െ ^^ stacking of graphene layers, thus promoting mesopore formation in the derived aerogels.[177,178] The resultingmesoporous graphene aerogels can be used to create lightweight, high-performance materials for various applications, including catalysis, sensing, and energy storage.[179,180]

[0077] Hybridized adsorbents, that incorporate both physisorption and chemisorption properties, can offer a synergistic approach, combining the benefits of each mechanism to for gas adsorption, such as to optimize CO2uptake.

[0017] By prioritizing surface area enhancement, even if it involves sacrificing a fraction of active sites, the overall adsorption capacity can be increased. The increased surface area promotes greater accessibility, compensating for the reduced active sites and enabling increased adsorption efficiency. For example, the glycation of chitosan can result in increased dispersion in a phosphate EEG-glycated chitosan (GC) system, as well as increased surface area and CO2uptake. However, aggregation due to ^^ െ ^^ stacking may still be an issue for a phosphated EEG-GC system at higher EEG loadings. This may be managed by the introduction of MnO2.

[0078] As described herein, in an embodiment, a hierarchical aerogel was developed by synthesizing MnO2functionalized electrochemically exfoliated graphene (MEEG) and crosslinking it with chitosan (CS), facilitated through amide covalent bonding employing EDC-NHS (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide) as a crosslinker. The crosslinking facilities the derivation of aerogels with an increased surface area due to both MnO2acting as a steric barrier between graphene sheets and the chemical crosslinking limiting the preferred direction of ice crystal growth during the freeze-drying step. Utilization of the herein described hierarchical aerogel in CO2adsorption was studied.

[0126] The aerogel's electrical conductivity (18.4 ± 0.2 S m-1) allowed for direct electrical heating of the sorbent, leading to shorter regeneration times and lower energy costs. The effect of MnO2loading of the MEEG, the weight ratio of MEEG and CS, as well as the aerogel density / solids content, were studied in order to achieve a balance between adsorption capacity and structural integrity. The aerogels with 32.1 wt.% MnO2loading in MEEG, 1:1 MEEG: CS ratio and 1 wt.% of total solids loading exhibited the highest BET surface area of 374.2 m2g-1, and a corresponding CO2adsorption capacity of 3.94 mmol g-1and a selectivity of 65.2 at 298 K and 1 bar. This aerogel withstood 20 cycles of regeneration with a 1.1% of reduction in its adsorption capacity.

[0079] In one or more examples, the present disclosure generally provides: 1. A method of forming functionalized exfoliated graphene, the method comprising providing an electrochemical cell, the cell comprisingan electrode comprising graphite, and an electrolyte comprising a transition metal salt; applying a voltage to the electrochemical cell to exfoliate the graphite of the electrode; and forming a functionalized exfoliated graphene comprising a metal oxide, the metal oxide being formed from the transition metal salt. 2. The method of example 1, further comprising selecting the voltage to be applied to the electrochemical cell based on the redox potential of the transition metal of the transition metal salt. 3. The method of any preceding example, wherein applying a voltage comprises applying a constant voltage. 4. The method of any preceding example, wherein applying a voltage comprises applying a constant voltage until current of the electrochemical cell goes to 0A. 5. The method of any preceding example, wherein the metal oxide is present in a wt% of about 5 to about 50 wt%. 6. The method of any preceding example, wherein the transition metal salt comprises early transition metals, late transition metals, or a combination thereof. 7. The method of any preceding example, wherein the transition metal salt comprises Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ru, Rh, Pd, Ag, C, W, Ir, Pt, Au, or a combination thereof. 8. The method of any preceding example, wherein the electrolyte further comprises a salt, and forming the functionalized exfoliated graphene further comprises forming the functionalized exfoliated graphene comprising the metal oxide and a heteroatom dopant derived from the salt. 9. The method of any preceding example, wherein the salt comprises an inorganic salt, an organic salt, or a combination thereof.10. The method of any preceding example, wherein the salt comprises a polyatomic inorganic salt, a polyatomic organic salt, or a combination thereof. 11. The method of any preceding example, wherein the salt comprises polyatomic ions comprising C2H3O2−,HCO₃⁻ , CO₃²⁻ , NH₄⁺ , HSO₄-, SO₄²-, SO3²-, NO3- , NO2- , H2PO4-, HPO42-, PO43-, PO33−, MnO4−, ClO4−, ClO3−, ClO2−, ClO−, OH- , CN-, or a combination thereof. 12. The method of any preceding example, wherein the heteroatom dopant is present at an atomic percentage of about 0.1 to about 5 atomic% . 13. A functionalized exfoliated graphene formed by the method of any one of examples 1 to 12. 14. Use of the functionalized exfoliated graphene of example 13 as an electrode of a supercapacitor. 15. A method of forming a hierarchical aerogel, the method comprising providing a dispersion comprising a phosphorous-functionalized exfoliated graphene; providing a nitrogen-functionalized polymer into the dispersion; adjusting the pH of the dispersion to couple the phosphorous-functionalized exfoliated graphene and the nitrogen-functionalized polymer; ageing the dispersion to form a hydrogel comprising the phosphorous- functionalized exfoliated graphene and the nitrogen-functionalized polymer; and freeze drying the hydrogel to form a hierarchical aerogel comprising the phosphorous-functionalized exfoliated graphene and the nitrogen-functionalized polymer. 16. The method of example 15, further comprising thermally treating the aerogel. 17. The method of any preceding example, further comprising providing a crosslinker into the dispersion for crosslinking the phosphorous-functionalized exfoliated graphene and the nitrogen-functionalized polymer.18. The method of any preceding example, wherein the crosslinker comprises a carbodiimide, an N-hydroxysuccinimide ester, or a combination thereof. 19. The method of any preceding example, wherein the crosslinker comprises (1-ethyl- 3-(3-dimethylaminopropyl)carbodiimide), (N-hydroxysuccinimide), or a combination thereof. 20. The method of any preceding example, wherein freeze drying the hydrogel comprises drying the dispersion at a temperature of about -90 °C to about -105 °C and a pressure of about 0.003 mbar to about 0.0133 mbar. 21. The method of any preceding example, wherein thermally treating the aerogel comprises heating the aerogel under vacuum. 22. The method of any preceding example, wherein thermally treating the aerogel comprises heating the aerogel under vacuum at a temperature of about 100°C to about 150 °C. 23. The method of any preceding example, wherein the phosphorous-functionalized exfoliated graphene comprises electrochemically exfoliated graphene doped with polyatomic ions comprising phosphorous. 24. The method of any preceding example, wherein the polyatomic ions comprising phosphorous comprise H2PO4-, HPO42-, PO43-, PO33−, or a combination thereof. 25. The method of any preceding example, wherein the phosphorous-functionalized exfoliated graphene further comprises a metal oxide. 26. The method of any preceding example, wherein the phosphorous-functionalized exfoliated graphene further comprising a metal oxide is made by the method of any one of examples 1 to 12. 27. The method of any preceding example, wherein the nitrogen-functionalized polymer comprises an amino-functionalized polymer.28. The method of any preceding example, wherein the nitrogen-functionalized polymer comprises a polysaccharide. 29. The method of any preceding example, wherein the nitrogen-functionalized polymer comprises chitosan, functionalized chitosan, or a combination thereof. 30. The method of any preceding example, wherein the nitrogen-functionalized polymer comprises glycated chitosan. 31. The method of any preceding example, wherein the coupling of the phosphorous- functionalized exfoliated graphene and the nitrogen-functionalized polymer comprises intermolecular forces. 32. The method of any preceding example, wherein the coupling of the phosphorous- functionalized exfoliated graphene and the nitrogen-functionalized polymer comprises van der Waals forces, electrostatic forces, dipole-dipole forces, ion–dipole forces, ion–induced dipole forces, hydrogen bonding, or a combination thereof. 33. The method of any preceding example, wherein the hierarchical aerogel has a mesoporosity of about 2 nm to about 50 nm. 34. The method of any preceding example, wherein the hierarchical aerogel has a microporosity of <2 nm. 35. The method of any preceding example, wherein the hierarchical aerogel has a macroporosity of about >50 nm to about 0.5 µm. 36. The method of any preceding example, wherein the hierarchical aerogel has a surface area of about 80 m2 / g to about 360 m2 / g. 37. The method of any preceding example, further comprising using the hierarchical aerogel in electrochemical separations, metal extractions, gas adsorptions, gas separations, or a combination thereof. 38. The method of any preceding example, wherein the gas to be absorbed or separated comprises CO2or N2.39. The method of any preceding example, further comprising regeneration the hierarchical aerogel after use via direct electrical heating. 40. A hierarchical aerogel formed by the method of any one of examples 1 to 39. 41. Use of the hierarchical aerogel of example 40 as an adsorbent. 42. Use of the hierarchical aerogel of example 40 as an electrode. 43. Use of the hierarchical aerogel of example 40 as a catalyst. 44. Use of the hierarchical aerogel of example 40 as an electrode of a supercapacitor. 45. Use of the hierarchical aerogel of example 40 in electrochemical separations. 46. Use of the hierarchical aerogel of example 40 in metal extraction. 47. Use of the hierarchical aerogel of example 40 as an ion sieve. 48. Use of the hierarchical aerogel of example 40 in gas adsorption, gas separation, or a combination thereof. 49. A hierarchical aerogel comprising a phosphorous-functionalized exfoliated graphene coupled with a nitrogen-functionalized polymer. 50. The aerogel of example 49, wherein the phosphorous-functionalized exfoliated graphene comprises electrochemically exfoliated graphene doped with polyatomic ions comprising phosphorous. 51. The aerogel of any preceding example, wherein the polyatomic ions comprising phosphorous comprise H2PO4-, HPO42-, PO43-, PO33−, or a combination thereof. 52. The aerogel of any preceding example, wherein the phosphorous-functionalized exfoliated graphene further comprises a metal oxide.53. The aerogel of any preceding example, wherein the phosphorous-functionalized exfoliated graphene further comprising a metal oxide is made by the method of any one of examples 1 to 12. 54. The aerogel of any preceding example, wherein the nitrogen-functionalized polymer comprises an amino-functionalized polymer. 55. The aerogel of any preceding example, wherein the nitrogen-functionalized polymer comprises a polysaccharide. 56. The aerogel of any preceding example, wherein the nitrogen-functionalized polymer comprises chitosan, functionalized chitosan, or a combination thereof. 57. The aerogel of any preceding example, wherein the nitrogen-functionalized polymer comprises glycated chitosan. 58. The aerogel of any preceding example, wherein the coupling of the phosphorous- functionalized exfoliated graphene and the nitrogen-functionalized polymer comprises intermolecular forces. 59. The aerogel of any preceding example, wherein the coupling comprises van der Waals forces, electrostatic forces, dipole-dipole forces, ion–dipole forces, ion–induced dipole forces, hydrogen bonding, or a combination thereof. 60. The aerogel of any preceding example, wherein the hierarchical aerogel comprises a 3-D structure. 61. The aerogel of any preceding example, wherein the hierarchical aerogel comprises a 3-D structure defining an open, interconnected pore morphology. 62. The aerogel of any preceding example, wherein the hierarchical aerogel comprises a 3-D structure defining mesopores, micropores, macropores, or a combination thereof. 63. The aerogel of any preceding example, wherein the hierarchical aerogel has mesopores between about 2 nm to about 50 nm in size.64. The aerogel of any preceding example, wherein the hierarchical aerogel has micropores <2 nm in size. 65. The aerogel of any preceding example, wherein the hierarchical aerogel has macropores between about >50 nm to about 0.5 µm in size. 66. The aerogel of any preceding example, wherein the hierarchical aerogel has a surface area of about 80 m2 / g to about 360 m2 / g.

[0080] To gain a better understanding of the invention described herein, the following examples are set forth. It should be understood that these examples are for illustrative purposes only. Therefore, they should not limit the scope of this invention in anyway.

[0081] EXAMPLES

[0082] Example 1 - Electrochemically Exfoliated Graphene (EEG)

[0083] Materials

[0084] Graphite foil (2 ൈ 1 cm2, 0.5 mm thick, ^99% pure), ammonium sulphate (NH4)2SO4, ammonium phosphate (NH4)2HPO4, chitosan chloride (98% deacetylated, from shrimp, Mw<100 kDa, used without further purification), manganese sulphate MnSO4, ammonium sulphate (NH4)2SO4, EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and NHS (N-hydroxysuccinimide), chitosan (98% deacetylated, from shrimp, Mw<100 kDa, used without further purification), D-(+)-Galactose, toluene, graphene oxide, sodium mono-dodecyl phosphate and sodium borohydride were purchased from Millipore Sigma. CalPhos soft rock phosphate was purchased from Custom Hydro-nutrients. Glacial acetic acid (CH3COOH) was purchased from Fisher Scientific.

[0085] Synthesis of Electrochemically Exfoliated Graphene (EEG)

[0086] Electrochemical exfoliation of graphite was performed in a two-electrode cell using a DC power supply as shown in Fig 1. A graphite foil was used as the anode and a stainless-steel foil as the cathode. Mixtures of inorganic salts including (NH4)2SO4, (NH4)2HPO4, and the mixture of (vol. %): [(NH4)2SO490% + NH4OAc 10%] were dissolved in deionized (DI) water with a total salt concentration of 0.1 M as the electrolyte. The distance between the anode and cathode was adjusted to 2 cm and a constant voltage of 10 V was applied for 1-2 hours until the current dropped to 0 A and the graphite inside the electrolyte is fully exfoliated, and to form MnO2nanoparticles based on Pourbaix diagrams.The exfoliated products were washed multiple times with DI water using a vacuum filtration setup and a cellulose membrane (0.45 μm) and redispersed in water by bath sonication for one hour. The dispersion was left overnight to allow any large unexfoliated graphite particles to settle, then the supernatant was separated and vacuum filtered to prepare the graphene films. The electrochemically exfoliated graphene (EEG) films which were synthesized using electrolytes containing (NH4)2SO4, (NH4)2HPO4or a mixture of [(NH4)2SO490% + NH4OAc 10%], and these are named NS-EEG, NP-EEG and Ac-EEG, respectively.

[0087] Synthesis of MnO2-EEG (MEEG)

[0088] The EEG-MnO2composites were synthesized using the same setup, as described above for the synthesis of EEG, while different concentrations of MnSO4were added to the electrolyte. A range of concentrations of MnSO4(5, 10 and 15 mM) was added to the 0.1 M (NH4)2SO4and the mixed (NH4)2SO4 / NH4OAc electrolytes. The graphite foil was exfoliated at 10 V and the EEG-MnO2graphene films were prepared by the same method described above.

[0089] Also see Figures 9 and 10.

[0090] Method and Characterization

[0091] Electrochemical exfoliation of graphite was performed in a two-electrode cell using a DC power supply as shown in Fig.1. A graphite foil was used as the anode and a stainless-steel foil as the cathode. Various types of inorganic salts and their mixture including (NH4)2SO4, (NH4)2HPO4, (NH4)2SO4+ NH4Ac were dissolved inside the water with a concentration of 0.1 M as the electrolyte. The distance between the anode and cathode was adjusted to 2 cm, and a constant voltage of 10 V was applied. The exfoliated products were washed multiple times with deionized (DI) water using a vacuum filtration setup and a cellulose membrane (0.45 μm) and dispersed inside water by one-hour bath sonication. The dispersion was left overnight to let the larger unexfoliated graphite particles settle down, and then the supernatants were separated and vacuum filtered to prepare the graphene films. The graphene films which are synthesized using (NH4)2SO4, (NH4)2HPO4, (NH4)2SO4+ NH4Ac are named NS-EEG, NP-EEG and Ac-EEG, respectively.

[0092] The EEG-MnO2composites were synthesized using the same setup described above, while different concentrations of MnSO4was also added to the electrolyte. 5, 10 and 15 mM MnSO4(33, 66 and 100 mg MnSO4inside 40 ml of DI water) was dissolved inside two different electrolytes of 0.1 M (NH4)2SO4and (NH4)2SO4+ NH4Ac solutions to prepare the exfoliation electrolyte. The graphite foil was exfoliated by applying a constantof 10 V and the EEG-MnO2graphene films were prepared by the same steps as described above.

[0093] The synthesized EEG and EEG-MnO2 samples were characterized using SEM, TEM, XRD, XPS, Raman and TGA. Scanning electron microscopy (SEM, Quanta FEG-250, Thermo Fisher Scientific, USA) was performed to characterize the morphological features of the samples. TEM analysis was conducted on a JEOL JEM-ARM200F analytical (JEOL USA Inc, atomic resolution) microscope at 200 kV accelerating voltage, operated with a Gatan Imaging filter. Raman spectra were obtained using a WITec confocal Raman microscope (Alpha 300R WITec GmbH, Germany) coupled to a 532-nm laser operating at a 5 mW power. Thermogravimetric analysis was performed by a TGA Q500 analyzer under nitrogen and air atmospheres from 25 to 900 ℃ with 10 ℃ min-1heating rate. The crystal structure of the samples was detected using an X-ray diffraction analyzer (Nonius Kappa APEX2 CCD diffractometer) by applying a step size of 0.02° and scan speed of 2 s step-1. X-ray photoelectron spectroscopy (XPS) spectra were obtained on an ESCALAB 250Xi (Thermo Scientific) to indicate the elemental composition in the samples.

[0094] The EEG and EEG-MnO2films were cut into 1 × 2 cm2shapes and used directly as a working electrode in 6 M KOH aqueous electrolyte while 1 cm2of the film was inside the electrolyte. A platinum foil and Ag / AgCl saturated in KCl were used as the counter electrode and the reference electrode respectively. The cyclic voltammetry (CV) measurements were carried out between 0 and ±0.8 V for different films with scan rates in the range of 2-100 mV s-1. The areal, volumetric and gravimetric capacitance of the films were calculated based on the CV curves using the following three equations, respectively: ^^ ൌொ^^∆^(1)

[0095] charge, ∆ ^^ isthe potential are area, mass of the graphene film respectively. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 0.01 Hz to 100 kHz with 10 mV amplitude.

[0096] An asymmetric solid-state supercapacitor (ASC) was fabricated through the assembly of two graphene films pressed on the nickel foam and a PVA / KOH gel electrolyte. First, graphene films were cut into 1 × 1 cm2shapes and pressed on the same-sized nickel foam using a hot press at 10 MPa. To prepare the gel electrolyte, 10 ml of 1 M KOH solution was prepared and 1 g of Polyvinyl alcohol (PVA, Mw: 89,000-98,000 g mol-1) was added tothe solution. The whole mixture was heated to 85 ℃ with vigorously stirring until the solution became clear. The hot gel electrolyte was drop cast on the graphene films and left under the ambient conditions for 2 hours to let the extra water be vaporized. Then, the coated graphene films with the gel were packed by Kapton tape and pressed together under 1 MPa pressure.

[0097] To analyze the electrochemical performance of the manufactured device, CV tests were performed between the potential limits of ±0.8 with the scan rates of 2-100 mV s-1. The galvanostatic charge-discharge (CDC) curves were obtained by applying a constant current density between 0.5 mA cm-2to 15 mA cm-2. The volumetric capacitance of the device was obtained based on the CDC curves using the following equation: ^^ ൌூ∆௧௩^∆^(4)

[0098] Where ^^ is the discharge current, ∆ ^^ is the discharge time and ^^௧is the total volume of two graphene films. The volumetric energy ( ^^) and power ( ^^) density of the ASC were calculated using the following equations: మ ^^ ൌ^∆^ଶൈଷ.^(5)

[0099]

[0100] The was analyzed by SEM as shown in Fig 2a and b and Figure 7. The SEM images show that graphene sheets are uniformly coated with MnO2nanoparticles. The MnO2nanoparticles for both phases of δ and α are in the nanoflower shape with a diameter of 300-500 nm. Three EEG / MnO2samples with different MnO2mass loadings were synthesized by varying the concentration of MnSO4(5, 10 and 15 mM) during the electrochemical exfoliation. As shown in Fig 2c, thermogravimetric analysis (TGA) was performed in the air to evaluate the MnO2mass loading of the composites. With a gradual increase in temperature, graphene is oxidized to carbon dioxide, and MnO2is converted to Mn2O3.

[0134] The residual Mn2O3weight percentages were 11 wt.%, 22 wt.% and 31 wt.% corresponding to MnO2mass loadings of 12 wt.%, 24 wt.% and 34 wt.% for the EEG / MnO2composites, respectively.

[0101] To further explore the crystal structure of the samples, X-ray diffraction analysis was conducted, and the results are shown in Fig 2d. The XRD patterns of electrochemically synthesized MnO2nanoparticles show two different phases of MnO2were formed depending on the electrolyte composition. The MnO2synthesized using an (NH4)2SO4electrolyte shows the predominant peaks ascribed to the δ-MnO2(JCPDS 18- 0802). The addition of NH4OAc to the electrolyte changed the crystal structure, with peakscorresponding to the α-MnO2phase (JCPDS 44-0141). The XRD patterns of EEG / MnO2composites show the (002) plane of the aromatic carbon layers of graphene, while the peaks ascribed to the respective phases of MnO2 are detected, which confirms the crystal structure of the MnO2is maintained during the electrochemical exfoliation of graphene.

[0102] Raman spectroscopy was performed to further investigate the chemical structure and vibrational properties of as-prepared samples. Typically, two feature peaks of graphene are the D band (located at 1350 cmି^), corresponding to defects and disorders in the graphene layers, and the G band (located at 1580 cmି^), resulting from sp2- hybridized CെC bonds in a 2D lattice. As shown in Fig 2e, the D band and G bands for EEG / δ-MnO2are located at 1351.1 cmି^and 1594.7 cmି^, respectively, while these peaks for EEG occur at 1361.0 cmି^and 1602.1 cmି^, respectively. The negative shift observed in the G band of EEG / δ-MnO2can be attributed to charge transfer between EEG and MnO2nanoparticles, indicative of the formation of strong covalent bonds between graphene and MnO2.

[0133] In addition, as a higher level of the disorder can lead to a D band with higher intensity, the higher intensity ratio between the D band and G band (ID / IG) can be a sign of strong interaction between the EEG and MnO2. In the case of the EEG / δ-MnO2composite and EEG, the ID / IGratios are 1.44 and 1.23, respectively, reflecting the greater disorder arising from the functionalization of graphene with MnO2.

[0133]

[0103] The morphology and crystal structure of the composites were further investigated by TEM imaging. Fig 3abc and Fig 8abc show that the graphene nanosheet is uniformly functionalized with MnO2. The scanning TEM (STEM) and the EDS elemental mapping (Fig 3d and Fig 8d) further confirm the uniform distribution of MnO2and the homogenously co-doping of the graphene with nitrogen and sulphur.

[0104] X-ray photoelectron spectroscopy (XPS) was conducted to analyze the chemical composition of the prepared EEG / MnO2composites. The survey spectrum of the EEG / δ-MnO2(Fig 3e) shows peaks for C, O, S, N and Mn and atomic percentage of each element (Table 1 and 2). In order to further understand the chemical characteristics of the sample, the high resolution C 1s, N 1s, Mn 2p spectra are presented and deconvoluted as shows in Fig 3f, g, and h. Deconvolution of C 1s shows five peaks at binding energies of 284.8 eV, 285.6 eV, 286.1 eV, 287.2 eV and 289 eV corresponding to CെC graphitic carbon, CെO / ൌN / െS, CെO / െN, CൌO, and OെCൌO, respectively.[7-8][135-136] In addition, the Mn 2p XPS spectra of the composite shows two main peaks at 642.6 eV and 654.3 eV, corresponding to Mn 2p3 / 2 and Mn 2p1 / 2. The energy gap of 1.7 eV is characteristic of the octahedral MnO6structure and is consistent with the previously reported XPS spectra for MnO2.[137-138] The N 1s peaks of the EEG / δ-MnO2aredeconvoluted to three peaks located at binding energies of 399.4 eV, 400.2 eV and 401.9 eV corresponding to pyridinic, pyrrolic and graphitic nitrogen, respectively.[139-140] During the electrochemical exfoliation, water molecules get oxidized and generate hydroxyl (OH•) and (O•) radicals at high potentials (i.e., 10 V in this case), which can lead to the formation of defects on the edges and grain boundaries of graphene.[7] Subsequently, the graphitelayers get expanded and the intercalation of electrolyte ions (e.g., OHି, NHାସ, Hା, SOଶିସ andPOଷିସ ) happens. Meanwhile, the heteroatom dopants can chemically interact with the defectsites leading to the formation of doped graphene with different heteroatoms.

[0105] The electrochemical properties of the as-synthesized films were tested in a three-electrode system to analyze their performance as both positive and negative supercapacitor electrodes. The as-synthesized films were tested for supercapacitance in view of the higher electrical conductivity generally exhibited by graphene and the relatively higher theoretical capacitance ascribed to MnO2. Cyclic voltammetry (CV) was performed to compare the electrochemical performance of samples with various mass loadings of MnO2. Fig 11 shows that increasing the MnO2mass loading from 12 wt.% to 24 wt.% increased the capacitance from 86.9 F cmିଷ(141 mF cmିଶand 56.6 F gି^) to 180 F cmିଷ(274 mF cmିଶand 110 F gି^). The CVs for EEG / MnO2with loadings of up to 24 wt.% have shown a characteristic rectangular shape. Further increase of the MnO2mass loading to 34 wt.% changed the CV to an oval shape, due to the higher ohmic resistance of the electrode. This phenomenon can be attributed to the weak electrical conductivity of the semiconductive MnO2and the increasing spacing between the graphene platelets due to the MnO2particles. Based on the CV results, EEG / MnO2composites with an MnO2mass loading of 24 wt.% was selected for further analysis.

[0106] Graphene (EEG) films doped with nitrogen, sulphur and phosphorus, and functionalized with δ-MnO2and α-MnO2were tested as the positive electrode for supercapacitors. Fig 12 shows the CV curves of the as-synthesized films with a potential window of 0 to 0.8 V versus Ag / AgCl at a scan rate of 10 mV sି^. The NS-EEG film shows a capacitance of 67.2 F cmିଷoriginating from both electrochemical double-layer and the following surface redox faradaic reactions:[141-142]

[0107] െC െ OH ↔ െC ൌ O ^ Hା^ ^^ି(3.10)

[0108] െCOOH ↔ െCOO ^ Hା^ eି(3.11)

[0109] െC ൌ O ^ eି↔ C െ Oି(3.12)

[0110] N െ H ↔ N ^ Hା^ eି(3.13)

[0111] C െ SOଶെ C ^ 2eି^ HଶO ↔ C െ SO െ C ^ 2OHି(3.14)

[0112] C െ SO െ C ^ 2eି↔ C െ SOH െ C ^ OHି(3.15)

[0113] The presence of sulphur, nitrogen and oxygen functional groups increased the wettability (e.g., the hydrophilicity) of the graphene films in aqueous electrolytes, leading to a better interaction between the electrolyte and electrode surface. To further investigate the effect of graphene chemical structure on its electrochemical performance, (NH4)2HPO4and a mixture of (NH4)2SO4and NH4OAc were used as the electrolyte to prepare NP-EEG and Ac-EEG respectively. The NP-EEG and Ac-EEG films show a higher capacitance of 83.6 and 74.8 F cmିଷcompared to NS-EEG, and a rectangular CV curve was obtained in each test at the scan rate of 10 mV sି^(Fig 12) The phosphorus functional groups boosted the capacitance through a higher wettability and potentially due to the redox reaction:

[0143]

[0114] P ൌ O ^ eି↔ P െ Oି(3.16)

[0115] However, the highest capacitances were achieved with the MnO2functionalized graphene films, where EEG / δ-MnO2outperforms the EEG / α-MnO2with a capacitance of 180 F cmିଷ(compared to 91.3 F cmିଷfor EEG / α-MnO2) at a scan rate of 10 mV sି^. Based on the raman analysis, the intensity of defects is higher in the EEG / MnO2samples. These defects can also facilitate the penetration of ions into the graphene film, leading to a higher capacitance. The MnO2nanoparticles enhanced the capacitance due to the fast and reversible redox reactions:

[0144]

[0116] ^MnOଶ^^^୰^ୟୡ^^ Kା^ eି↔ ^MnOOK^^^୰^ୟୡ^(3.17)

[0117] ^MnOଶ^ୠ^୪୩^ Kା^ eି↔ ^MnOOK^ୠ^୪୩(3.18)

[0118] Based on the CV results, EEG / δ-MnO2was selected for further electrochemical characterization. Surface and cross-section SEM images of the EEG / δ- MnO2film are shown in Fig 4a and b. The thickness of the prepared film was 15.2 μm. The MnO2nanoparticles can not only increase the capacitance through the redox reactions (equations 3.17 and 3.18) but can also prevent the restacking of the graphene sheets, providing spacing between the platelets. The SEM images show a dense micro-wrinkled structure due to the introduction of MnO2nanoparticles to the structure. The areal, volumetric and gravimetric capacitance of the film, as well as the CVs of the film obtained at scan rates from 2 to 100 mV sି^are shown in Fig 4c and d. A quasi-rectangular CV shape corresponding to the capacitive behaviour of the EEG / δ-MnO2film was observed at all scan rates. In addition, the film has areal, volumetric and gravimetric capacitances of 328 mF cmିଶ, 216 F cmିଷ, and 131 F gି^, respectively, at a scan rate of 2 mV sି^, and 41% of this capacitance was retained when the scan rate was increased to 100 mV sି^(Fig 4c).

[0119] To further investigate the charge storage and ion transport mechanism of the EEG / δ-MnO2film as the positive electrode, a power law relationship between the maximum current ( ^^) from the CV and scan rate ( ^^) was fitted to the data by the linear regression:

[0145]

[0120] ^^ ൌ ^^ ^^^(3.19)

[0121] Where ^^ and ^^ are fitted parameters. The value of ^^ is close to 0.5 for an ideal semi-infinite diffusion-controlled process and is close to 1.0 for a surface-controlled capacitive process.

[0145] As shown in Fig 4e, the values of ^^ for the charge and discharge curves were 0.81 and 0.78, respectively, indicating that the charge storage mechanism of EEG / δ-MnO2electrode is intermediate between diffusive and capacitive control. To further explore the relative contributions of capacitive and diffusive processes, the followingequation was used at each scan rate:

[0145] capacitive contribution^%^ ൌ ^భ௩^భ௩ା^మ௩బ.ఱ ൈ 100^%^ (3.20)diffusitive contribution^%^ൌ ^భ௩ା^మ௩బ.ఱ ൈ 100^%^ൌ 100% െ capacitive contribution^%^(3.21)

[0122] Where ^^^^^ is the capacitive contributed, and ^^ଶ^^^.ହis associated with the diffusive processes. As the scan rate was increased from 2 mV sି^to 100 mV sି^, it was found that the surface contribution increases from 32% to 77% (Fig 4f), suggesting a high capacitive contribution resulting from fast electron transfer of graphene, and a short ion diffusion length in the EEG / δ-MnO2composite.

[0123] In order to determine the optimal graphene film for the negative electrode of the supercapacitor, CV was performed with a potential window from െ0.8 to 0 V vs. Ag / AgCl. The performance of the NS-EEG, NP-EEG and Ac-EEG was assessed. As shown in Fig 13, the NP-EEG film had the highest capacitance of 160.7 F cmିଷand a more rectangular shape in a negative potential. Therefore, the NP-EEG has been selected as the negative electrode for more characterization.

[0124] The surface and cross-sectional SEM images of the NP-EEG film are shown in Fig 5a and b. The film has a thickness of 10.5 μm. The surface images of the film show fewer wrinkles and a lower thickness compared to EEG / δ-MnO2film, due to the restacking of the graphene sheets in the absence of the MnO2nanoparticles. CVs were carried out for the NP-EEG at a range of scan rates ranging from 2 mV sି^to 100 mV sି^, using a potential window of െ0.8 to 0 V vs. Ag / AgCl. The CV curves and the calculated capacitances are presented in Fig 5c and d. The areal, volumetric and gravimetric capacitances of the NP-EEG film were found to be 192.2 mF cmିଶ, 183.1 F cmିଷ, and 76.9 F gି^at a scan rate of 2 mV sି^(Fig 5c).

[0125] The CV current and scan rate data was fitted using a power-law relationship (Eq. 3.19) and the values of ^^ obtained were of 0.9 and 0.87, for charge and discharge, respectively (Fig 5e). This indicates that the diffusive contribution was much lower than the capacitive contribution. With increasing scan rate (Fig 5f), the capacitive contribution increased from 67% at a scan rate of 2 mV sି^to 93% at 100 mV sି^. ‘Diffusive’ generally refers to the capacitance that is achieved through the diffusion of electrolyte ions into a graphene film. ‘Capacitive’ refers to the capacitance as a result of the formation of a electrochemical double layer and redox reactions on the surface of the graphene film.

[0126] The electrochemical behaviour of the films was evaluated using electrochemical impedance spectroscopy (EIS), as shown in the Nyquist plots in Fig 14. The NP-EEG film demonstrated capacitive behavior that was almost parallel to the vertical (imaginary impedance) axis in the low-frequency range. In contrast, the EEG / δ-MnO2electrode exhibited a curve with a lower slope than that for the NP-EEG film, indicating a lower fraction capacitive behaviour. This observation is consistent with the findings from the CVs performed at different scan rates. The high frequency intercept with the horizontal axis indicates the internal ohmic resistance of the electrode. The EEG / δ-MnO2electrode was found to have a lower internal bulk resistance of 24.7 Ω, compared to 36.5 Ω for the NP-EEG. This may be due to the higher hydrophilicity associated with the MnO2nanoparticles in the graphene structure, leading to more infiltration of the electrolyte into the electrode pores.

[0147]

[0127] A flexible asymmetric solid-state supercapacitor (ASC) was fabricated using the EEG / δ-MnO2film, NP-EEG film and PVA / KOH, as the positive electrode, negative electrode, and gel electrolyte, respectively (Fig 6a). As shown in Fig 6b, the supercapacitor works in the operating potential window of 1.6 V and a stable, rectangular shaped CV was observed at the scan rates ranging from 2 to 100 mV sି^. As shown in Fig 6c, charge-discharge measurements were conducted at a range of current densities from 0.5 to 15 mAcmିଶ. The charge discharge curves of the ASC at current densities of up to 15 mA cmିଶhave a symmetric triangular shape, with only slight distortion. The low distortion and symmetric shapes indicate that the ASC has excellent reversibility and pseudocapacitive behaviour.

[0128] To meet the practical demand for flexible devices, it is desirable for the supercapacitor to retain its performance at extreme bending. The CV was performed at different bending curvatures to analyze the performance of the ASC. As shown in Fig 6d,the CV curves were almost coincident for the as-fabricated ASC with the length of 1 cm, even at an extreme bending degree of 180° showing the flexibility of the fabricated device. The cyclic stability of the ASC was also tested using CV. As shown in Fig 6e, the device retained 98.4% of its initial capacitance after 10,000 cycles, demonstrating long-term electrochemical stability compared to other MnO2-carbon based supercapacitors.

[0129] Benefiting from the operating potential window of 1.6 V and compact structure, the ASC exhibits a maximum volumetric energy density of up to 17.4 mWh cmିଷand a power density of 0.17 W cmିଷ(Fig 6f). The achieved energy and power density outperformed commercial Li thin-film batteries with reported energy and power densities ranging from 0.3 to 10 mWh cmିଷand 0.02 to 0.06 W cmିଷ.

[0148] The EEG based ASC also showed a higher energy density compared to those of some current commercial supercapacitors and capacitors, reported to be 0.86 mWh cmିଷand 0.002 mWh cmିଷ, respectively.

[0149] The EEG-based ASC outperformed recently reported graphene and MnO2-based solid-state supercapacitors including devices using: a 3D MnO2 / holey graphene film (0.9 V; 2.8 mWh cmିଷ),

[0150] carbon nanotubes / graphene hybrid fibers (1 V;8.63 mWh cmିଷ),

[0151] high mass loading MnO2 on carbon cloth (2 V; 8.25 mWhcmିଷ),

[0152] graphene-wrapped polyaniline (0.8 V; 0.22 mWh cmିଷ),

[0153] a graphene / CNTfilm (1 V; 14.8 mWh cmିଷ),

[0154] a MnO2 / carbon nanotube composite (0.8 V; 4.2 mWhcmିଷ),

[0155] Fe-doped MnO2 (1 V; 1.13 mWh cmିଷ),

[0156] and 3D ZIF (Zeolitic ImidazolateFramework)-8 / ZnO / graphene / polyaniline (1.2 V, 0.24 mWh cmିଷ),

[0157] and others (see Table 3).

[0130] The energy storage characteristics, combined with the one-step synthesis method of the electrode materials, suggests that electrochemically exfoliated and functionalized graphene composites may be a good candidate for the electrode material in high energy density, miniaturized supercapacitors.

[0131] Table 1: Atomic percentage of the elements obtained by the XPS analysis. Atomic %C1s O1s N1s S2p P2p Mn2pEEG / ^^-MnO2 75.2 21.7 0.9 - 1.6EEG / ^^- MnO272.8 24.1 1.1 0.7 - 1.3NP-EEG 75 21.6 1.6 - 1.8 -

[0132] Table 2: The fitting result for the high resolution XPS of nitrogen (N1s): Pyridinic, Pyrrolic and Graphitic nitrogen. Pyridinic N (%) Pyrrolic N (%) Graphitic N (%) EEG / ^^-MnO220.4 32.2 47.4 EEG / ^^-MnO217.5 44.9 37.6 NP-EEG 3.4 28 68.6

[0133] Table 3. Comparison of the electrochemical performance of MnO2-based solid-state supercapacitors. Electrode ElectrolyteSynthesisVoltage Energy and method(V)power densityStability Reference17.4 mWh cm-3MnO2 / Electro- 4.8 W cm-398.4% emical25.7 Wh kg (1000 Present GrapheneKOH / PVAch1.6 -10 cycle work 7.1 kW kg-1s) Hydro- 64.6-1MnO2 / rGONa2SO4 / Wh kg 88% (3500 PVA thermal315 kW kg-1cycles)[1]BC / 2al 1.811. -185.5% MnO / rGO PAAS-Chemic7 Wh kg0.441 kW-1(5000 [2] Na2SO kg 4 cycles) E-390.1% MnO2Na2SO4 / lectro- 8 mWh cm PVA deposition20.98 W cm-3(8000 [4] cycles) 2CNT LiCl / PVA4.2 mW-3MnO / Electro-h cm 80% (1000 deposition20.121 W cm-3 [5]-3cycles)Ni-doped Na2SO4 / Hydro- MnO2 PVAthermal2.20.69 mWh cm 85% (1000 0.031 W cm-3cycles)[6]Fe-doped 1.13 mWh-32LiCl / PVA Chemicm 79% (5200 MnOcal 10.11 W cm-3cycles)[7]MXene / H2SO4 / PV Chemical 1.6.4 Wh kg-185% (2000 MnO2 A61.1 kW kg-1cycles)[8]CNT / NaSO Hydro--391.8% MnO2 / grap 2 4 10.2 mWh cm / PVAthermal2.20-3(10000 [9] hene .153 W cm cycles) MnO / NaSO / Electro- 1.16-381.5% x 2 4 graphene PVAdeposition1.8mWh cm 0.021 W cm-3(10000

[0010] cycles) N-doped NaSO / Hydro- 3.5 mWh c-390.5% graphene / 2 4 O PVA 2therm1.8m -3(1500

[0011] Mnal0.019 W cm cycles) nO2Electro-398.6%Ppy / MH3PO4 / PV- 8.67 mWh cm Adeposition1.80.012 W cm-3(1000

[0012] cycles) MnO 92% 2 / Electro- 1.06 mWh cm-3grapheneLiCl / PVAdeposition20.371 W cm-3(10000

[0013] cycles) nO2 LiCl / PVA Chemical 29.4 -396% MmWh cm50.4 W cm-3(20000

[0014] cycles)MnO2 / Water bath 4.1 mWh cm-3Fe2O3LiClO4method2.10.022 W cm-3 -

[0015] Ppy / MnO2 / Na2SO / Hydro- 3.85 mWh cm384.6% 4 CNT PVAthermal1.80.63 W cm-3(20000

[0016] cycles) rGO / CNT / 5.9 mWh cm-386.7% MnO2LiCl / PVA Chemical 11.1 W cm-3(5000

[0017] cycles) MnO2 / FAF Hydro- 32.4 Wh kg-169% (1800 SCKOH / PVAthermal1.62.5 kW kg-1cycles)

[0018] MnO2 / H3PO4 / PV Hydro- 29.5 Wh kg-184.1% MoS Atherm1 -1(5000

[0019] 2al1.3 W kg cycles) MnO2 / 88.4 Wh kg-197.7% PANI / KOH / PVA Chemical 1.60-1(5000

[0020] silica .8 kW kg cycles) -393%MnO2 / CNTNa2SO4 / Electro- 0.52 mWh cm PVA deposition10.017 W cm-3(10000

[0021] cycles) MnO / N, S PVDF-co- Hy-175% 2 doped rGO HFP / ([DEM dro- 110 Wh kg ther4.5 -1(10000

[0022] E][TFSA])mal0.55 kW kg cycles)MnO2 / CNFNa2SO4 / Hydro- 59.5 Wh kg-179% ( PVA thermal0.85000 1.58 kW kg-1cycles)

[0023] MnO2 / HSO / PV-381.8% graphene / 2 4 5.2 mWh cm ADipping 1.20.008 W c-3(2000

[0024] PANI m cycles) In-situ NaSO / oxidation25.38 Wh -177.6% MnO22 4PVAby Mn-2kg(20000

[0025] ased 2-1b 0 kW kg cycles) MOF PVDF- Microwave MnO2- HFP / EMIM assisted1 -188% (1100 CNT / MoS2BF4 / EMIM hydro-424 Wh kg916 kW kg-1cycles)

[0026] TFSI thermal Hydro- NaSO / Cthermal1.93 mWh c -392.6% MnO2 / Ppy2 4MCelectro-2m0.01 W cm-3(10000

[0027] deposition cycles) MnO Chemical 2 / -193% Ni2P2O7KOH / PVAbath1.666 kWh kgdeposition 0.64 kW kg-1(10000

[0028] cycles) MnO2 / Hydro- 0.59 Wh cm380% (500 24KOH / PVAthermal1.0 NiGaO648 W cm-3cycles)

[0029] MnO2 / 4 NaSO / Hydro- 38.8-193.4% FeMoO / 2 41.8Wh kg (5000

[0030] O PVAtherm-1rGal1.34 kW kg cycles)

[0134] Example 2 - Aerogels

[0135] Materials

[0136] Graphite foil (2 ൈ 1 cm2, 0.5 mm thick, ^99% pure), ammonium sulphate (NH4)2SO4, ammonium phosphate (NH4)2HPO4, chitosan chloride (98% deacetylated, from shrimp, Mw<100 kDa, used without further purification), manganese sulphate (Mn)2SO4,ammonium sulphate (NH4)2SO4, EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and NHS (N-hydroxysuccinimide), chitosan (98% deacetylated, from shrimp, Mw <100 kDa, used without further purification), D-(+)-Galactose, toluene, graphene oxide, sodium mono-dodecyl phosphate and sodium borohydride were purchased from Millipore Sigma. CalPhos soft rock phosphate was purchased from Custom Hydro-nutrients. Glacial acetic acid (CH3COOH) was purchased from Fisher Scientific.

[0137] Synthesis of Electrochemically Exfoliated Graphene (EEG)

[0138] Electrochemical exfoliation of graphite was performed in a two-electrode cell using a DC power supply as shown in Fig 1. A graphite foil was used as the anode and a stainless-steel foil as the cathode. Mixtures of inorganic salts including (NH4)2SO4, (NH4)2HPO4, and the mixture of (vol. %): [(NH4)2SO490% + NH4OAc 10%] were dissolved in deionized (DI) water with a total salt concentration of 0.1 M as the electrolyte. The distance between the anode and cathode was adjusted to 2 cm and a constant voltage of 10 V was applied for 1-2 hours until the current dropped to 0 A and the graphite inside the electrolyte is fully exfoliated. The exfoliated products were washed multiple times with DI water using a vacuum filtration setup and a cellulose membrane (0.45 μm) and redispersed in water by bath sonication for one hour. The dispersion was left overnight to allow any large unexfoliated graphite particles to settle, then the supernatant was separated and vacuum filtered to prepare the graphene films. The electrochemically exfoliated graphene (EEG) films which were synthesized using electrolytes containing (NH4)2SO4, (NH4)2HPO4or a mixture of [(NH4)2SO490% + NH4OAc 10%], and these are named NS-EEG, NP-EEG and Ac-EEG, respectively.

[0139] Synthesis of MnO2-EEG (MEEG)

[0140] The EEG-MnO2composites were synthesized using the same setup, as described above for the synthesis of EEG, while different concentrations of MnSO4were added to the electrolyte. A range of concentrations of MnSO4(5, 10 and 15 mM) was added to the 0.1 M (NH4)2SO4and the mixed (NH4)2SO4 / NH4OAc electrolytes. The graphite foil was exfoliated at 10 V and the EEG-MnO2graphene films were prepared by the same method described above.

[0141] Synthesis of GC:

[0142] To create a homogenous suspension, 1 g of chitosan chloride was dissolved in 100 mL of deionized water in a beaker using a magnetic stirrer. As the chitosan solution was being stirred, 0.3 mL of toluene was added as a preservative and 3 g of galactose was progressively added to add hydrophilicity.

[0066] Aluminum foil was used to cover the beaker and the solution was stirred at 200 rpm for 6, 12, 18, and 24 hours, respectively to produce4 different batches of glycated chitosan (GC6, GC12, GC18, GC24, respectively). To prevent excessive foaming, 6 mL of sodium borohydride solution (1.6 g of sodium borohydride in 6 mL of 0.1 N NaOH) was added. The mixture was bath sonicated for 30 minutes.[64,65]Glacial acetic acid was added to prevent excessive foaming when the borohydride had decomposed. After being autoclaved for five minutes at 130 °C, the GC solution was cooled to ambient temperature. The solution pH was adjusted to 6.4–6.6 before freeze-drying (LABCONCO Benchtop Freeze Dryer, operating conditions: 0.003 mbar and −105 °C). The resulting powder was rinsed with deionized water repeatedly until the pH of the filtrate approached 6.5.

[0143] Synthesis of EEG-GC Aerogels

[0144] EEG dispersions (40 mL) containing 1% and 2% NP-EEG in deionized water were prepared. The dispersions were probe sonicated (Fisherbrand model 705 sonic dismembrator) at a frequency of 50 Hz for 2 hours. Hybrid aerogels were made from aqueous gels with different EEG to glycated chitosan (GC) ratios of 1:9, 1:4, 1:2, and 1:1, and total solid contents of 1 and 2 wt. %. For example, a 1 wt.% sample of EEG-GC in a 1:2 ratio was created by first dispersing 33 mg of EEG in 10 mL of deionized water using ultrasonication for an hour, leading to EEG dispersion in water. Then, 67 mg of glycated chitosan was introduced to the mixture, followed by stirring at 500 rpm for 1 hour and subsequent bath sonication for 40 minutes. Acetic acid was added to shift the pH to 5 to facilitate the specific interaction between the phosphate groups on the EEG and the amine groups on GC. The homogeneous colloidal solution was then aged in a glass vial for two days to form a hydrogel. The gel was then washed several times and frozen using liquid nitrogen at -196 °C and ambient pressure. The EEG-GC aerogel was obtained by freeze drying, the frozen gel was freeze-dried for 48±1 hours, at 0.003 mbar and −105 °C. The adsorbents were then thermally treated in a vacuum oven at 160 °C for 10±1 hours. Thermal treatment can act as an activation step, that facilitates increasing the aerogel’s adsorptive capacity.

[0145] EEG dispersions (40 mL) containing 1% and 2% EEG in deionized water were prepared.[54,159]The dispersions were probe sonicated (Fisherbrand model 705 sonic dismembrator) at a frequency of 50 Hz for 2 hours. The hybrid aerogels were made from aqueous gels with different EEG to GC ratios of 1:9, 1:4, 1:2, and 1:1, and total solid contents of 1 and 2 wt. %. For example, a 1 wt.% sample of EEG-GC in a 1:2 ratio was created by first dispersing 33 mg of EEG in 10 mL of deionized water using ultrasonication for an hour, leading to EEG dispersion in water. Then, 67 mg of glycated chitosan was introduced to the mixture, followed by stirring at 500 rpm for 1 hour and subsequent bathsonication for 40 minutes. Acetic acid was added to shift the pH to 5 to facilitate the specific interaction between the phosphate groups on the EEG and the amine groups on GC.

[0160] The homogeneous colloidal solution was then aged for two days, in a glass vial to form a hydrogel. The gel was then washed several times and frozen using liquid nitrogen at -196 °C and ambient pressure. The EEG-GC aerogel was obtained by freeze drying, the frozen gel was freeze-dried for 48±1 hours, at 0.003 mbar and −105 °C. The adsorbents were then thermally treated in a vacuum oven at 160 °C for 10±1 hours.[14,115,116,126]

[0146] Synthesis of MEEG-CS Aerogels

[0147] MEEG dispersions (50 mL of 1% and 2%) were prepared in deionized water (>18 M cm, Milli-Q) and probe-sonicated at a frequency of 50 Hz for a duration of 30 minutes. Different ratios of MnO2functionalized electrochemically exfoliated graphene (MEEG) to chitosan (MEEG-CS) were used to generate hybrid aerogels with solid contents of 1 and 2 wt. %. For instance, MEEG-CS 1 wt.%, 1:1 sample was produced by dispersing 50 mg of MEEG in 5 ml of deionized water, and then adding 50 mg of chitosan dispersion in 5 ml of 1% acetic acid solution to it.5 mg of EDC-NHS in a 1:3 ratio was gradually added to the resultant solution at 60 °C and was magnetically stirred for 24 hours before being poured into a glass vial and aged for two days. The gel was then frozen using liquid nitrogen and placed in a freeze dryer (LABCONCO Benchtop Freeze Dryer, operating conditions: 0.005 mbar and −104 °C) for 48±1 hours, resulting in the aerogel. Finally, the adsorbents were thermally treated by keeping them in a vacuum oven at 190 °C for 24±1 hours.

[0148] MEEG dispersions (50 mL of 1% and 2%) were prepared in deionized water (>18 M cm, Milli-Q) and probe-sonicated at a frequency of 50 Hz for a duration of 30 minutes. Different ratios of MEEG to chitosan (MEEG-CS) were used to generate hybrid aerogels with solid contents of 1 and 2 wt. %. For instance, MEEG-CS 1 wt.%, 1:1 sample was produced by dispersing 50 mg of MEEG in 5 ml of deionized water, and then adding 50 mg of chitosan dispersion in 5 ml of 1% acetic acid solution to it.5 mg of EDC-NHS in a 1:3 ratio was gradually added to the resultant solution at 60 °C and was magnetically stirred for 24 hours before being poured into a glass vial and aged for two days (Detailed optimization of crosslinking time is shown in Figure 42). The resultant gel was then frozen using liquid nitrogen and placed in a freeze dryer (LABCONCO Benchtop Freeze Dryer, operating conditions: 0.005 mbar and −104 °C) for 48±1 hours, resulting in the aerogel. Finally, the adsorbents were thermally treated by keeping them in a vacuum oven at 190 °C for 24±1 hours.[14,115,116,126]

[0149] Optimization of crosslinking reaction time

[0150] The extent of crosslinking between MEEG and CS was controlled by manipulating the crosslinking reaction times to 0, 12, 24 and 36 hours. LSCM images and SEM micrographs of MEEG-CS (1 wt.%, 1:1 MEEG: CS ratio) aerogels as a function of cross-linking reaction time, are depicted in Figure 42a and b. Figure 42c depicts that on increasing the crosslinking reaction time, the surface area and porosity of the aerogels also increase steadily. However, beyond 24 hours there was observed a negligible increase in surface area and porosity. This could be attributed to reaching a point where all carboxylic groups of MEEG-CS are covalently bonded to the primary amines of the chitosan. The CO2uptake study presented in Figure 42d demonstrates that prolonging the reaction time by up to 24 hours enhances the number of mesopores, which in turn increases the surface area of the sorbent, facilitating faster CO2internal diffusion and providing greater accessibility to the active NH2adsorption sites present in the developed structured adsorbent. However, beyond 24 hours the enhancement in CO2uptake was not considered insignificant. Thus, 24 hours was considered the crosslinking reaction time that provides a balance between enhancing the surface area and minimizing energy consumption.

[0151] Characterization

[0152] A Leica SP8 TSC inverted microscope with an 8 kHz resonant scanner and a white laser excitation source was utilized to examine the precursor gels. In the case of MEEG, which exhibits inherent fluorescence in the 500-700 nm range, a 480 nm laser was used to generate an emission spectrum ranging from 520-700 nm. CS, on the other hand, does not exhibit fluorescence and was imaged using reflection mode with a 450 nm laser. 2D images of the MEEG-CS aerogels were captured with a 63x magnification objective, 1024×1024 image format, 180.38 nm × 180.38 nm pixel size, 0.8 AU pinhole, 184.52 mm × 184.52 mm image size, and 400 Hz scanning speed. Avizo software was used to apply an image processing technique to determine the mean particle diameter. To analyze the pore morphology of the MEEG-CS crosslinked aerogels, the cross-sections were examined by a ZEISS Supra 55-VP field emission scanning electron microscope (SEM) was used with Pt pre-deposition. The images were captured using a 10 kV acceleration voltage with 1024 × 1024-pixel resolution, and the samples were mounted on conductive carbon glue tabs on an aluminum stub.

[0153] The Perkin Elmer Frontier FT-IR spectrometer was used to collect Fourier- transform infrared spectroscopy (FT-IR) spectra, with a resolution of 4 cm-1and 32 scans. X-ray diffraction (XRD) analysis was carried out on the aerogels using a Rigaku tabletop diffractometer, which utilized Cu Karadiation at 40 kV and 25 mA to determine their phase structure and composition. To determine the elemental composition of the synthesizedMEEG, X-ray photon spectroscopy (XPS) was performed using Thermo-Scientific's ESCALAB 250Xi. To determine the moisture uptake, the Cincinnati sub-zero humidifier chamber was employed. The Malvern Instruments’ Zetasizer was used to conduct Zeta potential and Dynamic Light Scattering (DLS) tests.

[0154] To examine the viscoelastic properties of the MEEG-CS crosslinked gels, oscillatory frequency sweep measurements were carried out using an Anton Paar Modular Compact Rheometer (MCR 702). The rheological measurements were performed with a 50 mm cone plate geometry having a 0.286 mm gap. The oscillatory frequency was ramped from 0.1 to 100 Hz, while the amplitude was logarithmically ramped between 0.01-100 % shear strain and the measurements were repeated thrice to verify reproducibility of the results.

[0155] To examine the nitrogen adsorption isotherms of the aerogels, a BELSORP- mini was employed at a temperature of 196 °C (MicrotracBEL Corp.). The specific surface area (SBET) was determined using the Brunauer-Emmett-Teller (BET) method in the pressure range of 0.05-0.35. The total pore volume (Vt) was calculated from the quantity of N2adsorption at P / P0= 0.99. The Barrett-Joyner-Halenda (BJH) approach was used to evaluate the pore size distribution by analyzing the N2adsorption-desorption isotherm at 77 K.

[0156] The apparent bulk density (rb) of the aerogel was obtained from the aerogel mass (m) and its volume (V) as described in (eq.4.1): ρ^ൌ^^ (eq.4.1)

[0157] The values of skeletal density (rs) were obtained by using helium in a gas- displacement pycnometer. The porosity of the aerogels was calculated according to the following relation, as described in (eq.4.2): ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^, % ൌ^1 െ ^್^ೞ^*100 (eq.4.2)to the presence ofspace, or pores may to the measurement techniques. Dead space can lead to underestimation or overestimation of porosity values, impacting reliability of results. Therefore, conducting repeated measurements at various sample orientations and positions facilitates obtaining a comprehensive understanding of porosity distribution and to mitigate the impact of dead space, ensuring more accurate and representative data.

[0159] Through thermogravimetric analysis (TGA) on a TA Instruments High Resolution 2950, the thermal stability of the organic aerogels was assessed. The TGAthermograms were produced using air / nitrogen and a ramp rate of 10 °C min-1at temperatures between 20 and 900 °C.

[0160] CO2 adsorption tests were conducted on the MEEG-CS crosslinked aerogels using TGA, with a sample weighing around 5 mg subjected to 100 ml min-1of pure Ar gas for degassing. After retaining the sample under the Ar gas flow, the temperature was lowered to 25 °C and the feed gas was changed to CO2for a 30-minute adsorption procedure. The experimental error in the sample's capacity to trap CO2was estimated within 5% using three repeated adsorption tests using the MEEG-CS crosslinked aerogels.

[0161] Adsorption Isotherms: The adsorption isotherms for CO2and N2were measured using the Accelerated Surface Area & Porosimetry System (ASAP) 2020 by Micromeritics Instruments Inc. The glass analytical tube was filled with a dry sample of 50- 100 mg. Prior to analysis, the samples were activated for 6 hours at 120°C in an argon gas environment, as determined by a thermogravimetric activation screening method (Figure 42d). The samples were then subjected to two stages of activation under vacuum (10-6mbar) - first to 60°C for two hours, followed by heating to 150°C for 15 hours, while maintaining an outgassing rate of less than 10 µbar per minute. Subsequently, the sample was backfilled with N2and transferred to the analysis port, where it was evacuated for at least an additional 120 minutes prior to commencing the pressure swing analysis, where the pressure is varied from 0 to 1 bar and each data point is taken after the samples have reached equilibrium. The Freundlich isotherm model and the Toth isotherm model (Figure 44) were studied to characterize the adsorption of CO2and N2on MEEG-CS (1 wt.% 1:1 MEEG: CS) aerogel.[81,83]

[0162] Ideal Adsorption Solution Theory (IAST) has been employed to assess multi-component (CO2-N2) adsorption systems using solely pure component experimental data.

[0079] The post-combustion process of power plants typically results in flue gas consisting of approximately 75-85% N2and 15-25% CO2. For this study, the partial pressures of CO2and N2were taken as 0.15 and 0.85, respectively, to determine the selectivity of CO2over N2and investigate whether the adsorption performance of MEEG- CS aligns with that of flue gas in humid power plants. The general procedure of calculating selectivity by the IAST model and the expression is given as: ^^ ൌ^^ / ^ೕ^^ / ^ೕ(eq.4.3)

[0163] where S is the selectivity coefficient; qiand qjare amounts adsorbed of components i and j, respectively; piand pjare partial pressure of components i and j, herein are 0.15 and 0.85, respectively.

[0086] Thermal Regeneration via Electrical Heating: Joule heating was employed to thermally regenerate aerogels via electrical heating, post CO2adsorption of the electrically conducting aerogels. The setup used was an Electro- Rheological Device (ERD) in the MCR 702 rheometer to regenerate the MEEG-CS aerogels, which allowed applying a variable electric current of up to 20 A. To control the regeneration temperature, combined was a 25 mm parallel plate measuring system with a Peltier device. Adsorption was performed at 25°C for 30 minutes, and desorption was achieved by passing a DC current of 1A through the aerogel, raising its temperature to 150°C in around 20 seconds, and holding it at that temperature for 50 minutes. Performed were 20 adsorption-regeneration cycles using the electrical heating regeneration method and measured the change in mass by weighing the aerogels before and after each cycle. For instance, when testing a 50 mg sample of MEEG-CS, 1 wt.%, 1:1 MEEG: CS, under pure CO2at 25°C and 1 bar pressure, the CO2adsorption results in a change in mass of 8.668 mg to be observed via a weighing balance with a resolution of 0.01 mg.

[0164] 53. XPS Characterization of MEEG

[0165] X-ray photoelectron spectroscopy (XPS) was conducted to analyze the chemical composition of MEEG (Figure 54a). Survey spectrum of the MEEG showed the peaks belonging to C, O, S, N and Mn elements which indicated the successful doping of the graphene with nitrogen and sulphur and its functionalization with MnO2. In order to further understand the chemical characteristics of the sample, high resolution of the C 1s, N 1s, Mn 2p were presented and deconvoluted as shown in Figure 54 b,c,d. Deconvolution of C 1s shows five peaks at binding energies of 284.8 eV, 285.6 eV, 286.1 eV, 287.2 eV and 289 eV responding to CെC graphitic carbon, CെO / ൌN / െS, CെO / െN, CൌO, and OെCൌO. In addition, the Mn 2p XPS spectra of the composite shows two main peaks at 642.6 eV and 654.3 eV, corresponding to Mn 2p3 / 2and Mn 2p1 / 2with the energy gap of 1.7 eV which is characteristic of MnO6octahedral structure. The N 1s peaks of the MEEG are deconvoluted to three peaks locating at binding energies of 399.4 eV, 400.2 eV and 401.9 eV corresponding to pyridinic, pyrrolic and graphitic nitrogen respectively.

[0166] See Figures.28 to 41; and Figures 42 to 54. Also see Tables 4-7; and Tables 8-10.

[0167] CO2Capturing EEG-GC18 Aerogels

[0168] Effect of Varying Extent of Glycation of Chitosan

[0169] Glycated chitosan was synthesized from chitosan (CS) by binding galactose molecules to introduce hydrophilicity. A Schiff base that is in equilibrium with the Amadori Product is produced when the primary amine group on chitosan reacts with the aldehyde group of galactose.[70,71] After further treatment with sodium borohydride, the double bonds are reduced and stable glycated chitosan is produced.

[0077] Glycation of chitosanimproves its dispersion in aqueous media but at the expense of converting primary amines to secondary amines, which have considerably lower CO2adsorption capacity. Thus, finding a balance between achieving good dispersion and preserving primary amines groups is needed.[33,78] To vary the extent of glycation of chitosan, glycation reactions were conducted for 6, 12, 18 and 24 hours. These samples are referred to as GC6, GC12, GC18 and GC24, respectively.

[0170] Figure 15a shows the Fourier-transform infrared (FT-IR) spectra of CS (pH = 4.0 ± 0.2) and GC samples (pH = 6.5 ± 0.2) as generated after 6, 12, 18, and 24 hours of glycation. The OH stretching and NH stretching overlapped at 3,400 to 3,200 cm-1. The absorption peaks at 3,000 to 2,900 cm-1, 1650 to 1645 cm-1, 1635 cm-1, 1,580 to 1,550 cm-1, and 1,460 to 1,390 cm-1corresponds to CH stretching, primary amine and amide II, C = O stretching amide I, NH bending and CH bending, respectively in the FTIR spectrum of CS.

[0079] The vibrational bands observed at 1,160 and 995 cm-1can be associated with C- O-C saccharide bridge vibrations. Additionally, the absorptions in the ranges of 1,010 to 1,060 cm-1and 1,080 to 1,120 cm-1are due to the vibrations resulting from the stretching of the C-O bonds in the polysaccharide. These vibrations indicate the presence of the primary alcohol (C6-OH) and secondary alcohol (C3-OH), respectively.

[0080] As the degree of glycation increases from GC6 to GC24, the galactose moiety's introduction of chemical groups to the polymer chain (GC) such as the principal amine band at 1649 cm−1begins to diminish and the secondary amine band at 764 cm-1starts to appear.

[0081] With an increase in glycation extent from 6 to 24 hours of reaction time, the band at 1070 cm-1also becomes sharper and more pronounced, which is another trait of secondary amine groups.

[0082] These modifications are brought on by the interactions of the OH groups on the galactose with the NH2groups of CS via hydrogen bonding.

[0082]

[0171] Figure 15b illustrates the X-ray diffraction (XRD) patterns of EEG-GC18, GC18, CS, and EEG. In the XRD pattern, a prominent and well-defined peak for EEG is observed at 2θ = 26.5°, indicating the presence of a crystal plane (002) of graphene. The XRD pattern of CS exhibits two broad diffraction peaks approximately at 2θ = 10° and 20°, which can be attributed to the reflection planes of (020) and (110) respectively.

[0027] These broad peaks suggest a lack of well-defined crystallinity in the biopolymer. However, the XRD pattern of GC18 depicts two sharp peaks at 2θ = 19.2° and 23.3°, demonstrating a higher crystallinity of 75.7% as compared to pristine chitosan (CS), which is 41.2% crystalline. This suggests that the proportion of amorphous regions was reduced during the glycation process (Figure 29). The XRD of EEG-GC18, 1 wt.% 1:2 aerogel includes peaksat 2θ = 19.2° and 23.3° from the GC18 and 2θ = 26.4◦ indicating the presence of the EEG in the composite.

[0172] To understand the impact of the extent of glycation on the EEG-GC precursor gels and the corresponding aerogel microstructure, 1:2 EEG-GC gels (1 wt.% solids content) were visualized using laser scanning confocal microscopy (LSCM) (Figure 16a) and the corresponding aerogels were imaged using scanning electron microscopy (SEM) (Figure 16b). As EEG and GC are oppositely charged (zeta potential of -38.9 ± 0.6 mV and 33.2 ± 0.5 mV, respectively), strong interparticle interaction was expected. LSCM images confirmed this strong interaction with EEG and GC occupying the same regions in the precursor gels. The EEG-GC domain size, estimated from the acquired LSCM images, was observed to decrease with increasing extent of glycation in EEG-GC 1 wt.% 1:2 gels, as depicted in Figure 16c. Improved dispersion quality was observed with increasing glycation extent, with considerable improvement obtained after 12 hours of glycation. Fewer features detected in the LSCM images indicate better dispersion, as the resolution limit in the reflection mode is limited to the 180 nm × 180 nm pixel size. Increasing the reaction time for glycation from 0 to 24 hrs led to a decrease in the average particle size (Figure 29). The microstructure of the freeze-dried aerogels, as seen in SEM micrographs (Figure 16b), was consistent with the microstructure of the corresponding gels observed via LSCM. The pore size of the 1 wt.% 1:2 EEG-GC aerogels decreased and the number of mesopores increased with increasing glycation. The SEM micrographs demonstrate that aggregates are present on the pore wall in the EEG-GC6 aerogel, while EEG-GC18 and EEG-GC24 display a hierarchical porosity with large pores being on the order of several microns, and the mesopores of nanometers dimension.

[0173] The CO2uptake profile (Figure 16d) demonstrates that all tested aerogels substantially outperformed their building blocks (EEG and CS). This was considered significant because typically the structuring process results in a reduction of the adsorption capacity. For example, in a study by Alhwaige et al., an aerogel showed improved adsorption capacity over chitosan, by grafting chitosan onto the carbon support, with calcination to enhance its surface area and porosity.[27,83] The increase in the adsorption capacity of EEG-GC aerogels on increasing glycation reaction time from 6 to 24 hours can be attributed to the mesopores observed in the aerogel structure. The presence of mesopores enhances the surface area of the sorbent, improves the CO2diffusion rate into the material and increases the accessibility to the active sites for adsorption in the structured adsorbent. The adsorption curves in Figure 16d also confirm the fast adsorption kinetics of CO2for the structured adsorbent. The porous aerogel structure enables low CO2gas phase mass transfer resistance within the sorbent aerogels. At 298 K, EEG-GC18 exhibited the highest CO2adsorption capacity of 2.88 mmol g−1whereas increasing the glycation time to 24 hrs (EEG-GC24) led to an aerogel with a lower adsorption capacity of 2.41 mmol g−1, despite the higher surface area and porosity. This can be attributed to the presence of more primary amine groups in the EEG-GC18 (as shown in Table 4). Thus, GC18 was identified as the GC sample that may have a suitable amount of glycation in terms of CO2adsorption capacity in EEG-GC aerogels.

[0174] Amine-Phosphate Link Enables Mesoporosity

[0175] To investigate the underlying cause of mesopore formation in otherwise non-porous GC18 upon introduction of EEG, experiments were conducted using different building blocks. To examine whether the opposite charge of the building blocks may be necessary for nanochannel formation, aerogels with GC18 combined with graphene oxide (GO) (zeta potential of -32.5 ± 1.2 mV) and an EEG(S) with no phosphate groups (EEG prepared using 1M (NH4)2SO4electrolyte) (zeta potential of -31.7 ± 0.9 mV) were prepared. These aerogels did not result in hierarchical porosity, which ruled out the opposite charge effect (Figure 17 a-b). On the other hand, to explore the impact of the interaction between the amine groups in chitosan and the phosphate groups in EEG, GC18 composites were prepared using two other inorganic materials containing phosphate groups, namely CalPhos soft rock phosphate (CSP) and sodium mono-dodecyl phosphate (SMP). These aerogels were found to have similar mesoporosity in the pore walls (Figure 17 c-d), highlighting the phosphate-amine interaction in obtaining hierarchically structured porous materials.

[0176] The mechanism depicted in Figure 18 illustrates the steps involved in amine- phosphate interaction leading to the formation of mesopores. The first step is hydration, in which the reactants are dispersed in water and the phosphate groups in EEG are hydrated. This process can be facilitated by the addition of acetic acid, which shifts the pH to 4 and helps to stabilize the phosphate groups.[83,84] In the second step of the process, the amine group in GC18 is protonated which is promoted by the hydrated phosphate anions. This further leads to the formation of electrostatic interactions between the hydrogen of the protonated amine and the deprotonated oxygen atom of the phosphate. This process is driven by dipole-dipole interactions, which result in the formation of coordinate bonds between the two groups.

[0085] The formation of these coordinate bonds between the amine and phosphate groups results in a network of interconnected particles, which is important for the subsequent steps. In the third step, the phosphate group is deprotonated, which leads to the elimination of a hydrogen ion from the system.

[0086] When this proton iseliminated in the presence of water in the system, the resulting hydronium ions are solvated by six adjacent water molecules through hydrogen bonding. Freeze-drying of these hydronium ion-water clusters results in the formation of the ordered mesoporous structures as seen in EEG-GC18 aerogels.

[0087]

[0177] The particle size distribution of GC also contributes to the mesoporosity in EEG-GC18 aerogels. The glycation process has a significant impact on the particle size distribution, causing a shift from unimodal with an average particle size of around 1 micron, to a bimodal distribution with the first peak having an average size ranging between 20 and 50 nm, depending on the extent of glycation. Micrographs of the pristine CS and GC precursors are in good agreement with this observation. It is posited that during glycation, amorphous parts of the biopolymer are largely eliminated, leading to smaller and more crystalline particles remaining.

[0088] This is consistent with the observed increase in the crystallinity of GC18 in comparison to CS (see Figure 15b). In EEG-GC18 hydrogels, these smaller nanoparticles of GC18 allow better dispersion of the polymer within the EEG network. The presence of small particles (20-50 nm in size) of GC18 also implies enhanced interaction with EEG through favourable amine phosphate links, leading to an overall increase in the formation of mesopores. Thus, the proposition is that this amine-phosphate interaction coupled with the bimodal particle size distribution of GC leads to the formation of mesopores generated by the liquid-to-solid phase change of the trapped hydronium ion- water clusters during the freezing step before the freeze-drying process.

[0178] Optimization of EEG: GC18 wt. ratio

[0179] To optimize the composition of the EEG-GC18 aerogels, for each of 1 wt.% and 2 wt.% total solids content, the EEG: GC18 weight ratio of 1:9, 1:4, 1:2 and 1:1 was studied. The mean domain size in the precursor gels estimated from LSCM images (Figure 19a) shows that increasing the loading of EEG led to an increase in domain size in the EEG-GC18 network. The larger domain size observed can be attributed to a higher probability of interaction between EEG and GC18 as the EEG content was increased. EEG can form hydrogen bonds with GC18 as well as weak CH- ^^ and heteroatom- ^^ interaction between EEG and GC18.[89,90]At higher solids loadings, the interparticle distance is significantly smaller, giving rise to a higher probability of links between EEG and GC18, as well as EEG-EEG interaction through ^^ - ^^ stacking. The enhanced aggregation results in a more solid-like behaviour of the 2 wt.% gels with bigger mean domain sizes, which is also evident from their rheological signature. In particular, the storage modulus G' is larger than the loss modulus G", and both moduli are frequency independent, confirming gel-like characteristics. Increasing the EEG loading led to an increase in both G' and G" at both 1wt.% and 2 wt.% solids contents (Figure 19b). However, G' and G" were almost an order of magnitude higher with 2 wt.% EEG-GC18 gels in comparison to the 1 wt.% EEG-GC18 gels.

[0180] BET surface area and porosity as a function of EEG loading and total solids content are shown in Figure 19c. These results demonstrate that EEG-GC181 wt.%, 1:2 EEG: GC wt. ratio aerogels resulted in the aerogel with the highest surface area (136.3 m2g−1) and porosity (97.8%). The BET surface area for the 1 wt.% EEG-GC18 aerogels increased from 91.6 m2g−1to 136.3 m2g−1as the EEG: GC18 wt. ratio was increased from 1:9 to 1:2, but a decrease in the surface area to 127.6 m2g−1was observed when it was further increased to 1:1 in EEG-GC18 aerogels. This reduction in BET surface area can be attributed to the aggregation of EEG sheets and is consistent with the pore wall structure observed in the SEM micrographs (see Fig. 30). The BET surface area of the 2 wt.% aerogels was considerably lower than the 1 wt.% aerogels due to the presence of thicker walls.

[0181] SEM micrographs of the freeze-dried aerogels were consistent with the microstructure of the corresponding gels observed via LSCM. The pore size of the 1 wt.% EEG-GC18 decreased with increasing EEG loading and the structures also became more porous. The 2 wt.% EEG-GC18 aerogels were found to have thicker walls and hence less accessibility to the active sites and lower surface area. The SEM micrographs of 2 wt.% aerogels also indicate the presence of aggregates of EEG platelets, which can be held together by ^^− ^^ interactions resulting in aggregation, and hence fewer mesopores. Higher magnification images show the presence of several mesopores in the network formed by shorter amorphous chains of GC18, which are connecting the EEG sheets.

[0182] Based on the CO2breakthrough analysis (Figure 19d), the EEG-GC181 wt.% 1:2 aerogel had the highest CO2adsorption capacity of the materials studied at 2.88 mmol g-1. Overall, the 1 wt.% EEG-GC18 aerogels can outperform the 2 wt.% aerogels, which had thicker walls and suffered from aggregation, resulting in lower surface area. Upon changing the EEG: GC18 wt. ratio from 1:9 to 1:2, the adsorption capacity increased from 2.56 to 2.88 mmol g−1due to greater accessibility to the active sites, even though the overall number of primary amine groups is lower. However, there was a drop in adsorption capacity to 2.66 mmol g−1as the EEG loading was increased to 50% in 1:1 EEG: GC18 due to the aggregation of EEG sheets. For the EEG-GC18 aerogels, a direct correlation was observed between the BET surface area and CO2adsorption capacity.

[0183] Figure 19d shows a rapid increase in CO2uptake followed by plateauing, which is typical of fast kinetics when effluent CO2concentration is monitored as a functionof time.

[0027] The adsorption kinetics regulate the total cycle time of a fixed-bed adsorption system and can impact how much sorbent would be required for a particular adsorption cycle. Faster kinetics leads to less adsorbent requirement, which can make EEG-GC18 more cost-effective. The kinetics of CO2adsorption on the EEG-GC18 aerogels are improved by the inherent acid-base reaction kinetics of CO2with the amine functional groups and the low gas phase mass transfer resistance in the porous sorbent.[4,23]

[0184] Adsorption Isotherms and Regeneration

[0185] At 77 K, N2adsorption-desorption isotherms with relative pressures (P / P0) ranging from 0.001 to 0.995 were studied for the selected aerogels, EEG-GC18, 1 wt.% 1:2 (Figure 20a). Due to the filling of mesopores within the sample, the N2adsorption capacity increases in the relative pressure range of 0.2 to 0.9. Interparticle voids account for the abrupt rise in N2adsorption that occurs after P / P0= 0.9. When reversed, the isotherms also show a minor hysteresis loop. The presence of mesopores and macropores in the sample explains the observed increase in uptake at high relative pressures. This further suggests that an open hierarchical structure with more mesopores (2-50 nm) than micropores (< 2 nm) was present in the aerogel. According to IUPAC, the final sample showed characteristics of the type IV isotherm, as depicted in Figure 20a. Based upon the pore size distribution profile (Figure 20b), the EEG-GC18 1 wt.% 1:2 aerogels have pores with diameters ranging from 20 to 500 Å. The BET surface area of the selected (EEG-GC181 wt.% 1:2) aerogels was found to be 136.29 m2g−1and its total pore volume was 0.166 cm3g−1at P / P0= 0.98.

[0186] Adsorption temperature can be a significant factor in the CO2adsorption process, especially for porous materials.

[0083] For the EEG-GC181 wt.% 1:2 aerogel, the CO2adsorption process was studied at four different temperatures, 273, 288, 298 and 308 K, for a pressure range of 0-1200 mbar. The maximum uptake capacity of 3.88 mmol g-1was achieved at 273 K, and the minimum adsorption capacity of 2.4 mmol g−1was obtained at 308 K, which reveals the exothermic nature of the adsorption. EEG-GC181 wt.% 1:2 aerogel demonstrated characteristics of both physisorption as well as chemisorption (Figure 20c). The significant drop in the adsorption capacity with increasing temperature is a typical characteristic of physisorption.

[0091] Figure 20c shows that the CO2uptake of the EEG-GC181 wt.% 1:2 aerogel increases with pressure at low-pressure ranges (<50 mbar), illustrating the typical chemisorption characteristics due to the strong acid-base interaction between the amine moieties of the adsorbent and CO2dipole molecules to form carbamate species.

[0092]

[0187] At pressures of >400 mbar, the isotherm curve for EEG-GC181 wt.% 1:2 aerogel flattens out, corresponding to weak chemisorption. The steric hindrance that results from the interaction of the primary amines in chitosan with galactose molecules prevents active amine groups from initiating the acid-base reaction with CO2molecules, reducing the strength of the chemisorption. The isotherm curve of EEG-GC181 wt.% 1:2 aerogel flattens further in the high-pressure region, suggesting a shift towards physisorption, where the mesopore surfaces participate in CO2adsorption. This adsorbent's relatively high CO2uptake capacity is associated with nitrogen-rich amine functional groups on the surface of aerogel and its mesoporous structure and specific surface area. EEG-GC181 wt.% 1:2 aerogel has an amine loading of 7.62 mmol g-1. In dry conditions, the maximum theoretical amine efficiency is typically 0.5.

[0023] The experimentally achieved amine efficiency of 0.38 indicates excellent accessibility and effective utilization of the amine functional groups present. Contrary to the adsorption isotherm curve of CO2, the N2adsorption capacity was lower, which is due to a limited interaction of the molecules of N2with the basic amine functional groups present in the framework of EEG-GC181 wt.% 1:2 aerogel. Overall, the high accessibility of the specific surface area and porous framework can result in an increase in N2adsorption with pressure.

[0188] The fitting parameters and Pearson coefficient (R2) of Freundlich and Toth adsorption isotherm models were studied. The value of R2was higher for CO2adsorption when fitted with the Toth model, as compared to the Freundlich model. The correlation coefficients of the Toth model indicate that the adsorption takes place on a heterogeneous surface rather than a homogeneous surface.

[0093] Moreover, the parameters of the Freundlich model infer that the adsorption strength of CO2on the surface of EEG-GC181 wt.% 1:2 aerogel was not uniform. The value of n determined from the Freundlich model greatly deviates from 1, indicating that the surface of EEG-GC181 wt.% 1:2 aerogel is non- uniform with multi-layer adsorption.

[0094] The heat of adsorption calculated from the adsorption isotherms for the EEG-GC18, 1 wt.%, 1:2 aerogels was -57.8 kJ mol−1, which lies between the physisorption range of -10 to -50 kJ mol−1and chemisorption range of -80 to -250 kJ mol−1. The Toth isotherm fitted to the data was utilized in the IAST equation to determine the selectivity for the EEG-GC18 1 wt.% 1:2 aerogel. The selectivity of the aerogel adsorbent is shown in Figure 20d, which reveals that the selectivity decreased with pressure. The maximum selectivity obtained was 117 at 0 bar and reduced to 43.8 at 1 bar, which is in a typical range of many porous adsorbents.

[0189] The H2O sorption profile obtained from EEG-GC18 was generally considered unusual in that, for a solid with hydrophilic building blocks such as GC18, itexhibited a lower water uptake of 7.8 ± 0.4 mmol g−1, at 100% relative humidity, which indicated partial hydrophobization of the final composite during the freeze-drying and thermal treatment. For example, a lower water uptake suggests a higher hydrophobicity, which may improve the aerogel’s selectivity for more hydrophobic gases, such as CO2. The H2O uptake of the EEG-GC181 wt.%, 1:2 is lower than Zeolite 13X, and three other water- resistant CO2adsorbents, namely, CAU-10, CALF-20 and Al fumarate.[95-98] The microstructure of the aerogel was not disrupted on exposure to 100% relative humidity, showing potential for humid flue gas applications.

[0190] To evaluate the regeneration by Joule heating exploiting the conductive nature of the EEG, the EEG-GC181 wt.%, 1:2 aerogel was cycled over 20 adsorption and electrical heating regeneration steps (Figure 20e). The EEG-GC18, 1 wt.% 1:2 aerogel was successfully regenerated after each of the twenty cycles. The local nature of the resistive heating method, which enables direct heating of the adsorbent, eliminates inefficiencies and slow heating rate associated with heat transfer through the adsorbent with external heating. Regeneration temperatures may be attained in a matter of seconds, lowering the losses during the sorbent regeneration procedure, and allowing rapid switching of beds between adsorption and regeneration, reducing the amount of adsorbent required. The performance observed in the 5 cyclic pressure swing adsorption cycles (from 0 to 1 bar), depicted in Figure 20f, appears to be in good agreement with the performance observed when regeneration is performed via electrical heating. The SEM images of adsorbents after the adsorption-regeneration cycles confirmed minimal changes in the adsorbent structure due to the electrical heating. The surface morphology of aerogel remained intact so that they retained 98.5% of the initial adsorption capacity after 20 cycles of adsorption and electrical heating regeneration. No further decrease in the adsorption capacity was observed after the 15thcycle.

[0191] A comparison of the CO2uptake to comparable structured adsorbents reported in recent research articles, is shown in Figure 21.[11,27-29,52,99-109] Compared to previously reported hierarchically structured CO2adsorbents, the EEG-GC181 wt.% 1:2 aerogel shows a balance between selectivity and adsorption capacity at 298 K and 1 bar. Though several non-structured CO2adsorbents (e.g., Zeolite-13X, CALF-20, etc.) have been reported to depict better adsorption capacity, hierarchically structured adsorbents can stand out due to the very low-pressure drop across the adsorption columns, which substantially reduce the operating costs in practical applications.[95,97]

[0192] In summary, EEG-GC18 has exhibited improved performance - for example, at 298 K and 1 bar operating conditions - as compared to other 3D-structured adsorbentswhile using a simple preparation process and low-cost biodegradable materials. For example, Cellulose-silica composites, calcined at over 1000 ºC, have been reported to have lower selectivity. In the process of structuring potential adsorbents, the adsorption capacity is often compromised, however, it was herein observed that the EEG-GC18 aerogels can perform better than its building blocks, EEG and GC18. An additional advantage of these aerogels is that they can be regenerated by a simple Joule heating process because of the electrical conductivity imparted by the EEG. A comparison study of adsorbents regenerated using Joule heating was conducted. Compared to these previously reported sorbents, the EEG-GC18 aerogel sorbent exhibited promising results in terms of capacity, selectivity, pressure drop, regeneration stability, conductivity, and speed of heating, with additional characteristics of simple synthesis and the use of more sustainable raw materials (EEG and GC).

[0193] CO2Capturing MEEG-CS Crosslinked Aerogels

[0194] Mechanistic Design of MEEG-CS Aerogels

[0195] The surface area enhancement of MEEG-CS systems can be influenced by several parameters, including the loading of MnO2on MEEG, the conditions of the crosslinking reaction, and the weight ratio of MEEG to CS. These factors play an important role in increasing the surface area of the resulting aerogels, thereby enhancing the accessibility to active sites, and facilitating improved CO2capture performance. To investigate the MnO2loading, four MEEG samples were prepared by varying the MnSO4concentration (7.5, 10, 20 and 30 mM) during the electrochemical exfoliation. As shown in Figure 22a, TGA was performed under air to determine the mass loading of MnO2, for each of the four MEEG samples prepared. By increasing the temperature from 25 ℃ to 900 ℃, the graphene was combusted, resulting in MnO2conversion into Mn2O3

[0181] . The residual mass percentages are 10.9 wt.%, 22.1 wt.%, 29.2 wt.% and 37.9 wt.% which corresponds to the remaining Mn2O3 mass and is equivalent to MnO2 mass loading of 12 wt.%, 24.3 wt.%, 32.1 wt.% and 41.7 wt.%. These MEEG samples are denoted as MEEG-12, MEEG- 24, MEEG-32, and MEEG-42 from here onwards.

[0196] Figure 22b depicts the FTIR spectra of the control EEG (electrochemically exfoliated graphene, without MnO2functionalization) and MEEG samples. The spectrum of EEG exhibited an absorption peak at 1565-1580 cm-1, corresponding to the C=C bonds present in the graphene sheets. Additionally, a peak at 1220-1230 cm-1was observed, which was attributed to the presence of carboxylic groups

[0061] . Following the intercalation of MnO2between the MEEG sheets, a new prominent peak emerged at 550-560 cm-1, indicating the presence of Mn-O bonds in MnO2

[0061] , which indicated the functionalizationof MEEG with MnO2nanoparticles. On increasing MnO2content from 12 wt.% in MEEG-12 to 41.7% in MEEG-42, 1220-1230 cm-1peak corresponding to the carboxylic groups becomes weaker and almost disappears for MEEG-42. However, simultaneously the MnO2 band becomes more and more prominent, thus suggesting that MnO2was introduced at the expense of -COOH groups, which are primarily responsible for crosslinking with CS.

[0197] SEM micrographs (Figure 22c) show the hierarchical porosity of the freeze- dried MEEG-CS aerogels, exhibiting the coexistence of macropores and nanoscopic mesopores embedded within the pore wall (bottom row of Figure 22c). The hierarchical structure of the aerogels improves adsorption kinetics by reducing mass-transfer resistance and increasing accessibility to active adsorption sites. Among the derived aerogels, MEEG- 32-CS exhibited a maximum surface area of 374.2 m2g-1(Figure 22d) and a CO2adsorption capacity of 3.94 mmol g-1(Figure 22e). N2adsorption-desorption isotherms of MEEG-32- CS, 1 wt.% 1:1 sample at 77 K exhibited a small reversible hysteresis loop. Thus, the sample depicted a typical IUPAC type IV isotherm characteristic, indicating an open hierarchical structure with more mesopores (2-50 nm) than micropores (< 2 nm). The pore size distribution profile also showed that the MEEG-CS 1 wt.% 1:1 aerogel has pores with diameters ranging from 20 to 500 Å. The total pore volume at P / P0= 0.99 was determined to be 0.34 cc g-1. The BET analysis data for all other derived aerogels have been tabulated. The reduced number of carboxylic groups present in MEEG-42 leads to an insufficient extent of crosslinking with CS, resulting in lower surface area and, consequently, decreased CO2adsorption capacity. To achieve a balance between the introduction of MnO2, which acts as a steric barrier preventing π-π stacking of MEEG sheets, and the preservation of carboxylic groups responsible for amide crosslinks between MEEG and CS, the MnO2loading was set at 32.1%. Henceforth, herein, MEEG refers to MEEG-32, unless otherwise specified.

[0198] The FTIR spectra of MEEG-CS aerogels (Figure 23a) show the overlapping peaks of OH and NH stretching at 3,400-3,200 cm−1, CH stretching at 3,000-2,800 cm−1, primary amine, and amide II at 1650-1645 cm−1, C=O stretching amide I at 1640 cm−1, NH bending at 1,590-1,560 cm−1, and CH bending at 1,450-1,380 cm−1

[0062] . The bands observed at 1,160 and 996 cm−1were assigned to the vibrations of the C-O-C saccharide bridge, while those at 1,020-1,050 cm−1and 1,070-1,110 cm−1corresponded to the C-O stretching vibrations in chitosan, attributed to the C6-OH primary alcohol and C3-OH secondary alcohol, respectively

[0063] . With crosslinking, a noticeable decrease in the intensity of the primary amine band at 1649 cm−1for MEEG-CS was observed as compared to CS, while a new band appeared at 1552 cm−1(Amide I), which confirms successfulcrosslinking between MEEG and CS. Furthermore, the appearance of the secondary amine band at 764 cm−1was also detected. Another trait of the secondary amine groups was a sharp and pronounced band at 1070 cm−1, which became more distinct with crosslinking

[0064] .

[0199] Figure 23b demonstrates the XRD patterns of MEEG, CS, and MEEG-CS, 1 wt.%, 1:1 MEEG: CS. The diffraction peak at 2θ = 26.4° corresponds to a crystal plane (002) of graphene and indicates the regular spatial structure of MEEG. Additionally, a diffraction peak at 36.1° corresponding to MnO2was observed, which corresponded to the (006) plane. MEEG shows the predominant peaks ascribed to the δ-MnO2. The main peaks ascribed to the mentioned phases of MnO2were still detected in MEEG which indicated that the crystal structure of the MnO2was maintained during the electrochemical exfoliation of graphene. Introducing MnO2onto EEG leads to an increase in d-spacing from 0.36 nm to 0.74 nm, as calculated from Bragg’s law. This is primarily due to the intercalation of MnO2layers between the graphene sheets. The presence of MnO2expands the interlayer distance, resulting in an increased d-spacing compared to pristine graphene. CS displays two broad diffraction peaks at approximately 2θ = 10.01° and 20°, which correspond to the reflection planes (020) and (110), respectively, indicating a poorly crystalline nature of the biopolymer (42.1% crystalline)

[0024] . The XRD spectra of MEEG-CS aerogel demonstrate that the phase transformation of EEG, from crystalline (83.4%) to semi-crystalline (59.1%) took place during MEEG-CS aerogel synthesis. The presence of peaks at 2θ = 20°, 26.4° and 36.1° indicated that the characteristic bands of MEEG and CS were still intact during the preparation of aerogel and resulted in a new composite.

[0200] Increasing the MEEG content resulted in increased values of both the storage modulus (G') and loss modulus (G") for MEEG-CS precursor gels, due to an increased likelihood of MEEG-CS links. However, increasing the total solids loading to 2 wt.% resulted in an order of magnitude higher G' and G". G’ and G” obtained for the MEEG- CS hydrogels are independent of frequency, confirming gel-like behaviour. SEM micrographs demonstrate that the pore size of the MEEG-CS aerogels decreased as the MEEG: CS ratio increased from 1:9 to 1:1. BET surface area and CO2uptake of 1 wt.% MEEG-CS aerogels increased with increasing MEEG: CS wt. ratio from 1:9 to 1:1. However, the BET surface area and adsorption capacity for 2 wt.% aerogels were slightly lower than that of the 1 wt.% aerogels, where was considered due to comparatively more aggregated structures obtained due to the reduced inter-particle distance on increasing total solids loading resulting in reduced accessibility to the active amine sites. The results reveal that aerogels prepared with 1 wt.% of total solids and 1:1 MEEG: CS wt. ratio exhibitsthe maximum surface area and CO2adsorption capacity. It was observed that the structural integrity of the aerogel appeared to be lost when the MEEG: CS wt. ratio is further increased to 1.5:1.

[0201] Increasing the surface area in graphene-based systems can often be hindered by the formation of aggregates at higher particle loadings caused by π-π stacking between consecutive graphene sheets. However, in the case of MEEG-CS systems, a two- fold increase in surface area was observed compared to the EEG-CS system without MnO2functionalization or crosslinking (Figures 24a and 24b). The presence of MnO2impedes the proximity and subsequent ^^ െ ^^ stacking of graphene sheets

[0061] . The zeta potential decreases from -33.2 ± 0.8 mV in EEG to-36.7 ± 1.3 mV in MEEG MnO2functionalization onto the graphene surface results in enhanced interparticle repulsion by the presence of negatively charged oxygen atoms in MnO2layer [65-67]. Consequently, an increase in surface area from 52.8 m2g-1for EEG-CS aerogel (without crosslinking) to 103.6 m2g-1in MEEG-CS aerogel (without crosslinking) and the corresponding increase in CO2adsorption capacity from 1.13 mmol g-1to 1.74 mmol g-1(Figure 24c) was observed.

[0202] Secondly, the crosslinking process involving the formation of strong amide bonds between MEEG and CS inhibited the growth of ice crystals during aerogel synthesis, thus resulting in mesoporous composites, after freeze-drying

[0053] . The crosslinking reaction, facilitated by EDC-NHS (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide), involved the activation of carboxyl groups (- COOH) on the MEEG surface by EDC. This reaction lead to the formation of an unstable O-acylisourea intermediate, which reacted with the NHS (N-hydroxysuccinimide) to form a semi-stable NHS ester intermediate [68-70]. The intermediate NHS ester undergoes a reaction with the amine groups present on CS, leading to the formation of covalent amide bonds, as illustrated in Figure 24d. The resulting composite, crosslinked MEEG-CS, is observed to be stable and well-dispersed with covalent amide bonds connecting the carboxyl groups on the MEEG surface and the amine groups on CS[45,71,72]. Additionally, MEEG could also establish hydrogen bonds with CS and engage in weak CH-π

[0073] and heteroatom-π

[0074] interactions with it. Relative to other crosslinkers, EDC-NHS crosslinking method enables controlled and reproducible crosslinking of carboxyl groups in MEEG and amino groups in CS, reducing side reactions and the need for harsh conditions or toxic chemicals, making it more environmentally friendly and more easily scalable [75, 76]. In EDC-NHS media, the zeta potential of MEEG was further enhanced to -40.3 ± 1.2 mV. Upon crosslinking, the surface area increases to 220.4 m2g-1for the EEG-CS system and to 374.23 m2g-1for the MEEG-CS system (Figures 24a and 24b). The adsorption capacitywas also correspondingly increased to 3.52 mmol g-1for the EEG-CS system and to 3.94 mmol g-1for the MEEG-CS system, respectively (Figure 24c). Despite the loss of some primary amine sites during the crosslinking process, an objective of increasing the surface area and creating a hybrid adsorbent that combines physisorption and chemisorption was effectively accomplished.

[0203] This was further supported by the direct proportionality of CO2adsorption capacity to their surface area (Figure 25a). The increased surface area provided more sites for CO2interaction, enhancing physisorption, while primary amine sites on the surface facilitate chemisorption. The synergy between these mechanisms was observed to increase CO2uptake (Figure 25b). The MEEG-CS adsorbent encountered constraints arising from either insufficient carboxyl groups for crosslinking or inadequate MnO2content to prevent aggregation. Although the introduction of additional carboxylic group defects showed the possibility of further enhancing the surface area, MnO2may be incorporated as a repulsive barrier, hence compromising further enhancement of crosslinking, thus emphasizing the synergistic dual effect described herein.

[0204] Adsorption Isotherms and Thermal Regeneration via Electrical Heating

[0205] Pure CO2and N2isotherm analysis was conducted on the MEEG-CS 1 wt.% 1:1 aerogel at four different temperatures (273, 288, 298, and 308 K), within a pressure range of 0-1200 mbar (Figure 26a). The MEEG-CS aerogel showed characteristics of both physisorption and chemisorption. The decreasing nature of CO2uptake capacity from 5.43 mmol g-1at 273 K to 3.87 mmol g-1at 308 K, suggested that the adsorption is thermodynamically controlled, which is a defining characteristic of physisorption

[0077] . However, Figure 26a also shows that despite having a mesoporous structure, the CO2uptake of MEEG-CS 1 wt.% 1:1 aerogel is notably higher at even lower-pressure regions, which may indicate the adsorption is governed by chemisorption. The high CO2adsorption capacities observed in the low-pressure range, below 200 mbar, may be attributed to the abundant presence of basic amine groups, which serve as abundant and active sites for chemical adsorption, resulting in carbamate formation

[0078] . MEEG-CS, 1 wt.%, 1:1 aerogel has an amine loading of 9.62 mmol g-1. Under dry conditions, the maximum amine efficiency achievable is known to be 0.5

[0020] . Notably, MEEG-CS, 1wt.% 1:1 aerogel has yielded an efficiency of 0.41, indicative of accessibility and utilization of the amine functional groups within the sorbent. The presence of amine functional groups on the surface of the aerogel contributes to the enhancement of CO2uptake capacity, especially at lower pressures where CO2interaction with basic NH2groups is more significant than the volume of pores and specific surface area. With increasing pressure, the mesopores aid in accessibility tothe active adsorption sites, thus increasing the overall CO2uptake. The isotherms indicate a synergistic combination of physisorption and chemisorption mechanisms. In contrast, the N2 uptake was observed to be low due to no possible electrostatic interactions between the N2molecules, and the basic amine functional groups present on the MEEG-CS 1 wt.% 1:1 aerogel. However, due to the presence of mesopores the N2adsorption is increased with an increase in pressure. The CO2heat of adsorption value calculated using the Clausius- Clapeyron equation from the four adsorption isotherms of MEEG-CS, 1 wt.%, 1:1 aerogel was -49.2 kJ mol-1. The experimental data obtained from CO2and N2adsorption was fitted to Freundlich and Toth isotherm models. The correlation coefficients of the Toth isotherm model suggest that the adsorption takes place on a heterogeneous surface

[0057] . The parameters obtained from the Freundlich isotherm model implied that the adsorption strength of CO2on the surface of 1 wt.% 1:1 MEEG-CS aerogel was non-uniform, with a deviated n value from 1, also indicating multi-layer adsorption on a non-uniform surface

[0056] . The graph of selectivity shows a decreasing trend with an increase in pressure (Figure 26b). The maximum selectivity is 174.9 at 4 mbar, decreasing to 65.2 at 1 bar, which is consistent with the characteristics of porous materials.

[0206] The resistive heating method used herein enabled the direct heating of MEEG nanoparticles on their support structure, made possible due to the electrical conductivity incorporated due to the presence of graphene in these structures. This can overcome the need for other and often inefficient external heating procedures, resulting in a decrease in energy usage as the MEEG-CS 1 wt.% 1:1 aerogel was heated in seconds instead of several minutes in external heating systems, which was an added advantage. The surface morphology of the aerogels remained intact after 20 adsorption and electrical heating regeneration cycles, retaining 98.9% of its initial adsorption capacity (Figure 26c). The SEM images of adsorbents showed minimal changes in the surface morphology due to electrical heating regeneration (Figure 26d). Cyclic pressure swing adsorption (PSA) studies also demonstrate the long-term stability of the sorbent (Figure 49).

[0207] The water sorption profile for MEEG-CS 1 wt.%, 1:1 aerogel is depicted in Figure 50. At a relative humidity of 100%, the water uptake was only 6.2 ± 0.5 mmol g-1, which indicated that the final composite underwent partial hydrophobization during crosslinking with hydrophobic CS at 60 °C and freeze-drying. The water uptake of MEEG- CS 1 wt.%, 1:1 was lower than Zeolite 13X

[0079] , as well as three other water-resistant adsorbents, namely CAU-10

[0080] , CALF-20

[0081] , and Al fumarate

[0082] . Additionally, exposure to 100% relative humidity did not disrupt the microstructure of the aerogel (Figure 51), which may make it a promising candidate for humid flue gas applications..

[0208] In Figure 27, a comparison of CO2uptake capacities of comparable hierarchically structured adsorbents reported in recent research articles is presented[11,24- 26,89-100]. The MEEG-CS 1 wt.%, 1:1 aerogel shows a greater balance between selectivity and adsorption capacity at 298 K and 1 bar, compared to other hierarchically structured CO2adsorbents. For example, hierarchically structured adsorbents such as MEEG-CS aerogels may exhibit a reduced pressure drop across adsorption columns during operation. This results in reduced operational costs, making them an attractive solution for large-scale systems. Hybrid silica aerogels have shown a CO2adsorption capacity of 4.9 mmol g-1at 273 K and 1 bar, coupled with a surface area of 756 m2g-1

[0017] . MEEG-CS exhibits a greater adsorption capacity (5.43 mmol g-1at 273 K and 1 bar), despite possessing a surface area of about half that of the silica aerogel, 374.2 m2g-1. This may be attributed to its hierarchical structuring, which results in an increased number of accessible amine sites, as evidenced by its high amine efficiency. Further, the derived aerogels may be regenerated through a Joule heating process facilitated by the conductivity imparted by MEEG. MEEG-CS aerogels described herein show improved results in terms of CO2adsorption capacity due to hybrid adsorption mechanism, CO2 / N2selectivity, operational costs, adsorption kinetics, and regeneration stability, with additional characteristics of simple synthesis and a sustainable approach. Table 4. Characteristic results for different compositions of EEG-GC aerogels BET Po Sample Bulk re Average Por CO2Uptake Densit osity surface volume, Pore-1Composition y, ^^^^(%) area (mmol g ) (g cm-3) Vp Diameter, (m2g-1) (cm3g-1) Dp(nm) (298 K; 1 bar) EEG-GC18, 1wt.% 1:90.058 94.5 91.6 0.27 7.50 2.56 ± 0.022EEG-GC18, 1wt.% 1:40.046 95.9 113.2 0.34 6.26 2.77 ± 0.026EEG-GC6, 1wt.% 1:20.093 85.1 53.6 0.16 12.89 1.68 ± 0.015EEG-GC12, 1wt.% 1:20.064 92.9 92.1 0.28 7.42 2.53 ± 0.03EEG-GC18, 1wt.% 1:20.039 97.8 136.3 0.41 5.11 2.88 ± 0.027EEG-GC24, 1wt.% 1:20.026 98.3 142.9 0.43 4.81 2.41 ± 0.02EEG-GC18, 1wt.% 1:10.027 97.2 127.6 0.38 5.47 2.66 ± 0.028EEG-GC18, 0.07293.2 2wt.% 1:973.8 0.22 9.64 1.21 ± 0.028EEG-GC18, 2wt.% 1:40.065 94.2 82.4 0.25 8.49 1.32 ± 0.03EEG-GC18, 2wt.% 1:20.048 96.1 90.5 0.27 9.17 1.58 ± 0.032EEG-GC18, 2wt.% 1:10.051 96.3 78.2 0.23 10.70 1.14 ± 0.033Table 5. BET characterization results for SMP-GC18 and CSP-GC18 aerogels Bulk Density, BET Pore Average Pore Sample ^^ Porosity surface volume, V Diam Composition^^ peter, Dp(g cm-3) (%) area3 -1(m2g-1) (cm g ) (nm) SMP-GC18, 1wt.% 1:20.047 96.8 112.3 0.31 6.45CSP-GC18, 1wt.% 1:20.051 95.7 98.5 0.29 7.23Table 6. Fitting results for Freundlich and Toth models for CO2and N2adsorption data at 298 K temperature Correlation Models coefficientsCorrel2 98R2 ation coefficientsfor N2for CO at 22 at 298 K RK FreundlichkF0.620.985kF 0.016n 4.56 n 2.260.991qs0.99 qs0.01 Toth kT3.05 0.996 kT0.48 0.989 m 0.92 m 0.76 Table 7. Comparison study of CO2adsorbents regenerated using electrical heating (Adsorption conditions: Pure CO2, 298 K and 1 bar) BET urface CO2 / N2CO Cycles s 2 Conducti nts area Selectivity Uptake vity studied; Refer- Adsorbe (S m-1) Regeneration en g ) (mm-1ces (m2 -1ol g ) efficiency CNT-zeolite _ 33.6 4.2 58.63 cycles;99% [8]MgAl-MMO / rGO ls963 cycles; aeroge.8 _ 0.64 1599.5%[7]Activated carbon-_ 29.74 cycles: Zeolite 13X2.3 12.1998%[9]3D printed Zeolite AC1985 34 cycles; 13X / 0.7 3.1 23.590.4%

[0095] CNT films 562 _ 3.96 48.18 cycles;93.8%

[0093] PANI aerogels 94.2 _ 2.14 285 cycles;96%

[0158] EEG-GC18 20 cycles; Present aerogels136.3 43.8 2.88 20.3 ± 0.297% workTable 8. Characteristic results for different compositions of MEEG-CS aerogels Bulk BET Aver Sample Density, Porosity surface Pore age CO2 Uptake volume, V Pore (-1Composition ^^ p mmol g ) ^^(%) area3 -Diameter, (g cm-3) (m2g-1) (cm g1) D (298 K; 1 bar) p (nm) EEG-CS, 1 wt.% 1:1; No 0.086 85.7 52.8 0.05 33.39 1.13 ± 0.03 crosslinking EEG-CS, 1 wt.% 1:1; 24 hrs 0.031 97.1 220.4 0.21 7.95 3.52 ± 0.04 crosslinking MEEG-12-CS, 1wt.% 1:10.021 97.8 281.97 0.27 6.18 3.54 ± 0.07MEEG-24-CS, 1wt.% 1:10.023 98.3 315.74 0.33 4.97 3.75 ± 0.06MEEG-32-CS, 1wt.% 1:10.025 98.8 374.23 0.34 4.83 3.94 ± 0.02MEEG-42-CS, 1wt.% 1:10.018 96.7 130.62 0.19 8.75 2.96 ± 0.01MEEG-CS, 1 wt.% 1:1; No 0.016 94.2 103.61 0.17 9.94 1.74 ± 0.03 crosslinking MEEG-CS, 1 wt.% 1:1; 12 hrs 0.021 96.7 258.54 0.33 5.02 3.15 ± 0.08 crosslinking MEEG-CS, 1 wt.% 1:1; 24 hrs 0.025 98.8 374.23 0.34 4.83 3.94 ± 0.09 crosslinking MEEG-CS, 1 wt.% 1:1; 36 hrs 0.027 98.9 374.36 0.34 4.77 4.01 ± 0.04 crosslinking MEEG-CS, 1 wt.% 1:90.032 96.9 210.62 0.24 6.88 3.47 ± 0.09MEEG-CS, 1 wt.% 1:40.029 97.5 295.89 0.26 6.23 3.65 ± 0.10MEEG-CS, 1 wt.% 1:20.027 98.2 352.15 0.32 5.07 3.72 ± 0.01MEEG-CS, 1 0.025 98.6 374.23.94 ± 0.06 wt.% 1:13 0.34 4.83MEEG-CS, 1 wt.% 3:20.034 91.6 137.8 0.20 8.94 2.12 ± 0.06MEEG-CS, 2 wt.% 1:90.037 96.8 154.27 0.22 7.61 3.16 ± 0.07MEEG-CS, 2 wt.% 1:40.033 97.3 208.35 0.23 7.15 3.30 ± 0.03MEEG-CS, 2 wt.% 1:20.028 98.1 215.65 0.25 6.47 3.56 ± 0.09MEEG-CS, 2 wt.% 1:10.026 98.8 230.39 0.32 5.12 3.89 ± 0.07MEEG-CS, 2 wt.% 3:20.042 90.5 119.4 0.18 9.31 2.07 ± 0.06Table 9. Fitting results for Freundlich and Toth model for CO2and N2adsorption data at 298 K Correlation Correlation Models coefficients for CO2 R2coefficients R2at 298 K for N2 at 298 K FreundlichkF 0.97 kF 0.017n 4.560.966n 2.240.992qs1.54 qs0.02 Toth kT3.07 0.993 kT0.12 0.978 m 0.93 m 0.77 Table 10. Comparison study of CO2adsorbents regenerated using different methods BET Cycles nts surface CO / N CO Up Conduct- studied; Regener- Adsorbe2 2 2take Refer- area Selectivity (mmol g-1) ivity Regener- ation (S m-1) a ences (m2g-1) tion Method efficiency rGO-CS 2.72 (298 K5 cycles;Pressure aerogels415 39,_

[0014] 1 bar)99% swingHybrid Silica 5.21 10 756.4(298 K, _ cycles; Pressure aerogels_

[0017] 1 bar) 84.4%swing3.96 CNT films 562 _ (298 K,48.18 cycles;Joule 93.8% H

[0093] 1 bar)eatingPANI 2.14 (298 K,25 cycles;Joule aerogels94.2 _8

[0158] 1 bar)96% Heating4.2 CNT-zeolite _ 33.6 (298 K,58.63 cycles;Joule 99% Heat[8]1 bar)ingMgAl- 0.64 MMO / rGO 96.8 _ (298 K,153 cycles;Joule ogels 1 bar)99.5%[7]aerHeatingActivated 2.3 carbon- _ 29.7 (298 K,12.194 cycles:Joule e 13X 1 bar)98% Hea[9]Zeolitting3D printed 3.1 Zeolite 1985 30.7 (298 K,23.54 cycles;Joule X / AC 1 bar)90.4

[0095] 13% HeatingN-doped 2.3 10 carbon 1631 37 (298 K, _ cycles; Pressure[8posite 1 bar) 99.4%swi8]comngAPTES- 2.51 10 TEOS 22 29 (298 K, _ cycles; Pressure swin

[0152] aerogels 1 bar) 65%gAmine 2.72 100 grafted silica 767 19 (298 K, _ cycles; External hea

[0153] aerogels 1 bar) 90%tingClay- 2.21 Chitosan 710 27 (298 K,_6 cycles;External 92% heat

[0154] composite 1 bar)ingCellulose 1.96 10 132(298 K, _ cycles; Pressure aerogels10

[0156] 1 bar) 98.7%swingEEG-GC18 2.88 20.3 ± 20 Joule Pre rogels(298 K, cycl sent ae136.3 43.8es; 1 bar) 0.2 98.5% Heating work 3.94 (298 K, MEEG-CS 1 bar); 18 20 374.2 65.4 ± cycles; Joule Present aerogels.25.43 0.2 Heating work (273 K, 99% 1 bar)

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[0210] The embodiments described herein are intended to be examples only. Alterations, modifications, and / or variations can be affected to the particular embodiments by those of skill in the art. The scope of the claims should not be limited by the particular embodiments set forth herein but should be construed in a manner consistent with the specification as a whole.

[0211] The aspects, embodiments, and / or examples of the present disclosure being thus described, it should be recognized that said aspects, embodiments, and / or examples may be varied in ways that do not depart from the spirit and scope of the present disclosure, and that said variations are intended to be included within the scope of the following claims.

[0212] All publications, patents and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication patent, or patent application was specifically and individually indicated to be incorporated by reference.

Claims

WHAT IS CLAIMED IS:

1. A method of forming functionalized exfoliated graphene, the method comprising providing an electrochemical cell, the cell comprising an electrode comprising graphite, and an electrolyte comprising a transition metal salt; applying a voltage to the electrochemical cell to exfoliate the graphite of the electrode; and forming a functionalized exfoliated graphene comprising a metal oxide, the metal oxide being formed from the transition metal salt.

2. The method of claim 1, further comprising selecting the voltage to be applied to the electrochemical cell based on the redox potential of the transition metal of the transition metal salt.

3. The method of any preceding claim, wherein applying a voltage comprises applying a constant voltage.

4. The method of any preceding claim, wherein applying a voltage comprises applying a constant voltage until current of the electrochemical cell goes to 0A.

5. The method of any preceding claim, wherein the metal oxide is present in a wt% of about 5 to about 50 wt%.

6. The method of any preceding claim, wherein the transition metal salt comprises early transition metals, late transition metals, or a combination thereof.

7. The method of any preceding claim, wherein the transition metal salt comprises Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ru, Rh, Pd, Ag, C, W, Ir, Pt, Au, or a combination thereof.

8. The method of any preceding claim, wherein the electrolyte further comprises a salt, and forming the functionalized exfoliated graphene further comprises forming the functionalized exfoliated graphene comprising the metal oxide and a heteroatom dopant derived from the salt.

9. The method of any preceding claim, wherein the salt comprises an inorganic salt, an organic salt, or a combination thereof.

10. The method of any preceding claim, wherein the salt comprises a polyatomic inorganic salt, a polyatomic organic salt, or a combination thereof.

11. The method of any preceding claim, wherein the salt comprises polyatomic ions comprising C2H3O2−,HCO₃⁻ , CO₃²⁻ , NH₄⁺ , HSO₄-, SO₄²-, SO3²-, NO3- , NO2- , H2PO4-, HPO42-, PO43-, PO33−, MnO4−, ClO4−, ClO3−, ClO2−, ClO−, OH- , CN-, or a combination thereof.

12. The method of any preceding claim, wherein the heteroatom dopant is present at an atomic percentage of about 0.1 to about 5 atomic% .

13. A functionalized exfoliated graphene formed by the method of any one of claims 1 to 12.

14. Use of the functionalized exfoliated graphene of claim 13 as an electrode of a supercapacitor.

15. A method of forming a hierarchical aerogel, the method comprising providing a dispersion comprising a phosphorous-functionalized exfoliated graphene; providing a nitrogen-functionalized polymer into the dispersion; adjusting the pH of the dispersion to couple the phosphorous-functionalized exfoliated graphene and the nitrogen-functionalized polymer; ageing the dispersion to form a hydrogel comprising the phosphorous- functionalized exfoliated graphene and the nitrogen-functionalized polymer; and freeze drying the hydrogel to form a hierarchical aerogel comprising the phosphorous-functionalized exfoliated graphene and the nitrogen-functionalized polymer.

16. The method of claim 15, further comprising thermally treating the aerogel.

17. The method of any preceding claim, further comprising providing a crosslinker into the dispersion for crosslinking the phosphorous-functionalized exfoliated graphene and the nitrogen-functionalized polymer.

18. The method of any preceding claim, wherein the crosslinker comprises a carbodiimide, an N-hydroxysuccinimide ester, or a combination thereof.

19. The method of any preceding claim, wherein the crosslinker comprises (1-ethyl-3- (3-dimethylaminopropyl)carbodiimide), (N-hydroxysuccinimide), or a combination thereof.

20. The method of any preceding claim, wherein freeze drying the hydrogel comprises drying the dispersion at a temperature of about -90 °C to about -105 °C and a pressure of about 0.003 mbar to about 0.0133 mbar.

21. The method of any preceding claim, wherein thermally treating the aerogel comprises heating the aerogel under vacuum.

22. The method of any preceding claim, wherein thermally treating the aerogel comprises heating the aerogel under vacuum at a temperature of about 100°C to about 150 °C.

23. The method of any preceding claim, wherein the phosphorous-functionalized exfoliated graphene comprises electrochemically exfoliated graphene doped with polyatomic ions comprising phosphorous.

24. The method of any preceding claim, wherein the polyatomic ions comprising phosphorous comprise H2PO4-, HPO42-, PO43-, PO33−, or a combination thereof.

25. The method of any preceding claim, wherein the phosphorous-functionalized exfoliated graphene further comprises a metal oxide.

26. The method of any preceding claim, wherein the phosphorous-functionalized exfoliated graphene further comprising a metal oxide is made by the method of any one of claims 1 to 12.

27. The method of any preceding claim, wherein the nitrogen-functionalized polymer comprises an amino-functionalized polymer.

28. The method of any preceding claim, wherein the nitrogen-functionalized polymer comprises a polysaccharide.

29. The method of any preceding claim, wherein the nitrogen-functionalized polymer comprises chitosan, functionalized chitosan, or a combination thereof.

30. The method of any preceding claim, wherein the nitrogen-functionalized polymer comprises glycated chitosan.

31. The method of any preceding claim, wherein the coupling of the phosphorous- functionalized exfoliated graphene and the nitrogen-functionalized polymer comprises intermolecular forces.

32. The method of any preceding claim, wherein the coupling of the phosphorous- functionalized exfoliated graphene and the nitrogen-functionalized polymer comprises van der Waals forces, electrostatic forces, dipole-dipole forces, ion–dipole forces, ion–induced dipole forces, hydrogen bonding, or a combination thereof.

33. The method of any preceding claim, wherein the hierarchical aerogel has a mesoporosity of about 2 nm to about 50 nm.

34. The method of any preceding claim, wherein the hierarchical aerogel has a microporosity of <2 nm.

35. The method of any preceding claim, wherein the hierarchical aerogel has a macroporosity of about >50 nm to about 0.5 µm.

36. The method of any preceding claim, wherein the hierarchical aerogel has a surface area of about 80 m2 / g to about 360 m2 / g.

37. The method of any preceding claim, further comprising using the hierarchical aerogel in electrochemical separations, metal extractions, gas adsorptions, gas separations, or a combination thereof.

38. The method of any preceding claim, wherein the gas to be absorbed or separated comprises CO2or N2.

39. The method of any preceding claim, further comprising regeneration the hierarchical aerogel after use via direct electrical heating.

40. A hierarchical aerogel formed by the method of any one of claims 1 to 39.

41. Use of the hierarchical aerogel of claim 40 as an adsorbent.

42. Use of the hierarchical aerogel of claim 40 as an electrode.

43. Use of the hierarchical aerogel of claim 40 as a catalyst.

44. Use of the hierarchical aerogel of claim 40 as an electrode of a supercapacitor.

45. Use of the hierarchical aerogel of claim 40 in electrochemical separations.

46. Use of the hierarchical aerogel of claim 40 in metal extraction.

47. Use of the hierarchical aerogel of claim 40 as an ion sieve.

48. Use of the hierarchical aerogel of claim 40 in gas adsorption, gas separation, or a combination thereof.

49. A hierarchical aerogel comprising a phosphorous-functionalized exfoliated graphene coupled with a nitrogen-functionalized polymer.

50. The aerogel of claim 49, wherein the phosphorous-functionalized exfoliated graphene comprises electrochemically exfoliated graphene doped with polyatomic ions comprising phosphorous.

51. The aerogel of any preceding claim, wherein the polyatomic ions comprising phosphorous comprise H2PO4-, HPO42-, PO43-, PO33−, or a combination thereof.

52. The aerogel of any preceding claim, wherein the phosphorous-functionalized exfoliated graphene further comprises a metal oxide.

53. The aerogel of any preceding claim, wherein the phosphorous-functionalized exfoliated graphene further comprising a metal oxide is made by the method of any one of claims 1 to 12.

54. The aerogel of any preceding claim, wherein the nitrogen-functionalized polymer comprises an amino-functionalized polymer.

55. The aerogel of any preceding claim, wherein the nitrogen-functionalized polymer comprises a polysaccharide.

56. The aerogel of any preceding claim, wherein the nitrogen-functionalized polymer comprises chitosan, functionalized chitosan, or a combination thereof.

57. The aerogel of any preceding claim, wherein the nitrogen-functionalized polymer comprises glycated chitosan.

58. The aerogel of any preceding claim, wherein the coupling of the phosphorous- functionalized exfoliated graphene and the nitrogen-functionalized polymer comprises intermolecular forces.

59. The aerogel of any preceding claim, wherein the coupling comprises van der Waals forces, electrostatic forces, dipole-dipole forces, ion–dipole forces, ion–induced dipole forces, hydrogen bonding, or a combination thereof.

60. The aerogel of any preceding claim, wherein the hierarchical aerogel comprises a 3-D structure.

61. The aerogel of any preceding claim, wherein the hierarchical aerogel comprises a 3-D structure defining an open, interconnected pore morphology.

62. The aerogel of any preceding claim, wherein the hierarchical aerogel comprises a 3-D structure defining mesopores, micropores, macropores, or a combination thereof.

63. The aerogel of any preceding claim, wherein the hierarchical aerogel has mesopores between about 2 nm to about 50 nm in size.

64. The aerogel of any preceding claim, wherein the hierarchical aerogel has micropores <2 nm in size.

65. The aerogel of any preceding claim, wherein the hierarchical aerogel has macropores between about >50 nm to about 0.5 µm in size.

66. The aerogel of any preceding claim, wherein the hierarchical aerogel has a surface area of about 80 m2 / g to about 360 m2 / g.

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