Catalysts and a method of fabrication thereof

Abstract

A method of fabrication of transitional metal-based layered double hydroxides, comprising preparation of a 3D conductive substrate and electrodeposition of layered double hydroxides, interconnected lamellar nanosheets of the layered double hydroxides bonding firmly to the substrate. A transitional metal-based layered double hydroxides catalyst comprises lamellar nanosheets of transitional metal-based layered double hydroxides bonding firmly to a porous substrate.

Description

CATALYSTS AND A METHOD OF FABRICATION THEREOFFIELD OF THE INVENTION

[0001] The present invention relates catalytic layers. More specifically, the present invention is concerned with transitional metal-based layered double hydroxides and a method of fabrication thereof.BACKGROUND OF THE INVENTION

[0002] The rapidly increasing demand for clean, efficient, and inexpensive green energy sources has led to a strong interest in alternatives to fossil fuels that have a negative impact on the environment. Electrochemical water splitting, when linked to renewables such as hydroelectric, solar and wind, can generate green hydrogen and is of significance for the development of a sustainable clean energy network. Electrochemical water splitting involves both the hydrogen evolution reaction (HER) at the negative electrode and the oxygen evolution reaction (OER) at the positive electrode. Water electrolysis can be performed in both acidic and alkaline conditions. Alkaline water electrolysis differs from acidic water electrolysis in that non-noble metals can be used to catalyze both the HER and the oxygen evolution reaction (OER), with the potential to decrease production costs. Both the HER and the oxygen evolution reaction (OER) need low cost, stable and active electrocatalysts and electrodes to meet the requirements of any industrial and commercial applications. The oxygen evolution reaction (OER) involves the transfer of four electrons and protons to convert H2O to O2. Due to the complexity of the reaction, the oxygen evolution reaction (OER) overpotential accounts for a large fraction of the cell voltage in water electrolysis.

[0003] The origin of the oxygen evolution reaction (OER) overpotential varies with the current density. At the lowest current density, the activation overpotential is the main component limiting the reaction. Transition metal (Co, Ni, Fe, Mn)-based layered double hydroxides (LDH) have emerged, among others, as promising catalysts to decrease the oxygen evolution reaction (OER) activation overpotential in alkaline medium. At the highest current density, where O2 evolution is important, gas bubbles can accumulate at the electrode surface, partly blocking the active sites of the electrode and causing an increase of the ohmic drop due to the formation of a non-conductive gas layer. Moreover, the release of gas bubbles could erode the electrocatalytic coating, resulting in performance degradation during long-term use. Electrodes with appropriate micro / nano- engineered surface structure to minimize the negative effect of gas bubbles have been studied; they have a captive air bubble contact angle exceeding 150° and are referred to as "superaerophobic". Upon oxygen evolution, their superaerophobic properties lead to a decrease of the diameter of O2 bubbles and of their adhesion strength to the electrode surface.SUMMARY OF THE INVENTION

[0004] More specifically, in accordance with the present invention, there is provided a method of fabrication of transitional metal-based layered double hydroxides comprising preparation of a 3D conductive substrate and electrodeposition of layered double hydroxides.

[0005] There is further provided a material comprising transitional metal-based layered double hydroxides and a porous 3D conductive substrate with interconnected lamellar nanosheets of the transitional metal-based layered double hydroxides bonding firmly to the substrate.

[0006] There is further provided a transitional metal-based layered double hydroxides catalyst comprising interconnected lamellar nanosheets of transitional metal-based layered double hydroxides bonding firmly to a porous substrate.

[0007] Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] In the appended drawings:

[0009] FIGs. 1A, 1 B and 1C are SEM micrographs of Ni plate, FIGs. 1 D, 1 E and 1 F are SEM micrographs of 3D Ni, FIG. 1 G is a micrograph of NICo-LDH-3D Ni-3; FIGs. 1 H and 11 show higher magnification of NICo- LDH-3D Ni-3 from a region outside of the larger pores, FIGs. 1 J and 1 K show higher magnification of NICo- LDH-3D Ni-3 from a region inside the larger pores;

[0010] FIGs. 2 show captive air bubble (5 pL) contact angle (CA) measurements on: FIG. 2A : Ni plate, FIG. 2B : porous 3D Ni; FIG. 2C: NICo-LDH-3D Ni -3;

[0011] FIG. 3 shows relative Ni composition of NICo-LDH deposits as a function of the deposition current density;

[0012] FIG. 4 shows chronopotentiometric curves for the deposition of NICo-LDH (2 mA cm-2) on Ni plate and porous 3D Ni electrode;

[0013] FIG. 5 shows XRD patterns of NICo-LDH deposited on Ni plate and 3D Ni substrate; for the Ni plate, the XRD pattern was recorded in a surface sensitive mode with a grazing incidence angle of 2°; for the 3D porous Ni substrate, the XRD profile was recorded in the Bragg Brentano configuration; the LDH deposition current density was 2 mA cm-2for the Ni plate and 3 mA cm-2for the 3D porous Ni substrate;

[0014] FIGs. 6 show cyclic voltammograms of FIG. 6A: Ni plate, FIG. 6B: pure Ni(OH)2-LDH and pure CO(OH)2-LDH on Ni plate, and FIG. 6C: 3D Ni and NICo-LDH deposited 3D Ni substrate; the electrolyte was 1 M KOH and the scan rate was 5 mV S’1; the current density of the Co(OH)2 - LDH electrode in FIG. 6B was multiply by a factor of 40 to fit on the same current density scale as the N i(OH)2 - LDH electrode;

[0015] FIGs. 7 show galvanostatic curves at 10 mA cm-2of a few selected electrodes in : FIG. 7A : in Ni plate and NICo LDH-Ni plate -2, FIG. 7B: 3D Ni and NICo-LDH-3D Ni - 3; the electrolyte was 1 M KOH at room temperature;

[0016] FIGs. 8 show plots of overpotential vs LDH deposition current density on 3D Ni electrode FIG. 8A: at 10 mA cm-2and FIG. 8B: at 250 mA cm-2;

[0017] FIGs. 9 show variation of log (j) and log (1 / Ret) with respect to the IR-corrected potential for FIG. 9A: the porous 3D Ni and FIG. 9B: a best-performing NICo-LDH 3D Ni-3 electrode in disclosed experiments; the measurements were conducted on two different 3D Ni electrodes and two different NiCo LDH-3D Ni-3 electrodes, represented by squares and circles;

[0018] FIG. 10 shows Log (j) versus the electrode potential (Tafel plot), with the curves in FIGs. 9 to emphasize their difference; the way the exchange current density was determined is illustrated;

[0019] FIGs. 1 1 show Nyquist plots of FIGs. 1 1 A and 11 C: porous 3D Ni, FIGs. 11 B and 1 1 D: NiCo-LDH 3D Ni-3; the high-frequency region of both electrodes are shown in FIGs. 1 1 C and 11 D; filled symbols are for the experimental data, while dotted lines are for the fitted data. A 45° and 22.5° straight line are shown in c) and d), respectively;

[0020] FIG. 12 show variation of the double layer capacitance, Cdi, with respect to the current density, two samples being tested in each case; plotting Cd, values as a function of current density rather than electrode potential enables comparison of electrode behavior under conditions where oxygen production is identical;

[0021] FIG. 13 shows chronopotentiometry curve of the NiCo-LDH-3D Ni- 3 electrode at 250 mA cm-2in 1 M KOH;

[0022] FIG. 14 shows open circuit potential of NiCo-LDH-3D Ni-3 electrode before and after 56-hour electrolysis at 250 mA cm-2in 1 M KOH;

[0023] FIGs. 15 show SEM micrographs of the NiCo-LDH 3D-NI-3 electrode prior (FIGs. 15A, 15B, 15C) and after (FIGs. 15D, 15E, 15F) 56 hours of electrolysis at 250 mA cm-2in 1 M KOH at room temperature;

[0024] FIG. 16 shows CVs of NiCo-LDH-3D Ni- 3 recorded before and after a 56 h galvanostatic test at 250 mA cm-2, recorded in 1 M KOH at 5mV S’1;

[0025] FIGs. 17 show XPS analysis of (FIGs. 17A, 17B, 17C) as-deposited and (FIGs. 17D, 17E, 17F) postelectrolysis NiCo LDH-3D Ni-3 electrodes; the Ni 2p (FIGs. 17A and 17D), Co 2p (FIGs. 17B, 17E) and O 1s (FIGs. 17C, 17F) core level peaks are shown; the electrolysis was performed during 56 h at 250 mA cm-2in 1 M KOH;

[0026] FIG. 18 shows XPS analysis of NiCo LDH-3D Ni-3 electrodes before and after 56 h of electrolysis at 250 mA cm-2;

[0027] FIG. 19 shows XRD patterns of NiCo-LDH 3D-NI-3 prior to and after 56 hours of electrolysis at 250 mA cm-2;

[0028] FIG. 20 shows relative Fe composition of NiFe-LDH deposits as a function of the deposition current density; the error bars are the standard deviations of three measurements made on different samples;

[0029] FIG. 21 shows XRD patterns of sample NIFe-9 deposited on 3D Ni substrate electrode, the diffraction peaks of fee Ni being indicated by an asterisk;

[0030] FIG. 22 shows grazing incidence angle XRD pattern of NiFe-9 deposited on a Ni plate, the position of the Ni (asterisk) and Ni Fe LDH ( circles ) diffraction peaks being indicated.

[0031] FIGs. 23 show HR-SEM micrographs of 3D Ni substrate (FIGs. 23A, 23B) and Ni Fe LDH (FIGs. 23C, 23D) deposited on 3D Ni substrate electrode at two magnifications;

[0032] FIGs. 24 show HAADF-STEM images of NiFe-9 electrodes with EDX elemental mapping images of Ni, Fe, and O elements of : as-deposited NiFe-9 (FIG. 24A) and NiFe-9 electrode after 1000 CV cycles (FIG. 24B);

[0033] FIGs. 25 show high resol ution-TEM images of as-deposited NiFe -9 (FIG. 25A); FIG. 25B and FIG. 25C being enlarged views of the selected areas in FIG. 25A;

[0034] FIGs. 26 show captive air bubble (5 pL) contact angle measurements on 3D porous Ni (FIG. 26A) and NiFe -9 (FIG. 26B);

[0035] FIGs. 27 show cyclic voltammograms of NiFe films deposited on 3D Ni; the electrolyte was 1 M KOH,-1 -2 -2 and the scan rate was 5 mV s (FIG. 27A), plots of overpotentials at 10 mA cm and 250 mA cm vs the NiFe deposition current density (FIG. 27B);

[0036] FIGs. 28 show variation of Qc (FIG. 28A) and Cd with respect to the NiFe LDH deposition current density (FIG. 28B), the variation of Cdi with respect to Qc being depicted in FIG. 28C;

[0037] FIG. 29 shows variation of log (j) with respect to the iR-corrected potential for porous 3D Ni, compared with the best-performing NiFe-9 electrode;

[0038] FIGs. 30 show CV measurements at a scan rate of 5 mV s-1on NiFe -9 electrode recorded between 1000 cycles (CVs at a scan rate of 50 mV s-1) and compared with the initial CV recorded at the beginning of electrochemical measurements (FIG. 30 A); enlarged view of FIG. 30 A (FIG. 30 B); galvanostatic curves of NiFe-9 at 250 mA cm-2before and after 1000 CV measurements (FIG. 30 C);

[0039] FIGs. 31 show galvanostatic curves of NiFe-9 after 1000 CV measurements at 10 mA cm2(FIG. 31 A) and 250 mA cm’2(FIG. 31 B); and

[0040] FIGs. 32 showXPS analysis of as-deposited NiFe-9 (FIGs. 32A, 32B, 32C) and after 1000 CV cycles (FIGs. 32D, 32E, 32F), in FIGs. 32A and 32D, FIGs. 32B and 32E and FIGs. 32C and 32F the high-resolution spectra of Ni 2p, O 1 s, and Fe 2p are shown, respectively.DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0041] The present invention is illustrated in further detail by the following non-limiting examples.

[0042] Fabrication of a superaerophobic support structure for NiCo-layered double hydroxide electrocatalysts for the oxygen evolution reaction is now described hereinbelow, in Relation to Figures 1 -19.

[0043] In a nutshell, 3D Ni substrate with superaerophobic properties were catalyzed through the deposition of NICo layered double hydroxide (NICo-LDH). Deposition of NICo-LDH was achieved using a mixed solution of Ni and Co nitrate salts as precursors. Precipitation of NICo-LDH was achieved by the application of a negative current that caused a local raise of the solution pH. NICo-LDHs were deposited in the form of vertically aligned nanosheets that infiltrate throughout the porous structure of the superaerophobic 3D Ni substrate. Variation of the deposition current changed the composition of the NICo-LDH. The performance and the mechanisms responsible for the oxygen evolution reaction (OER) were investigated through a series of electrochemical experiments that include electrochemical impedance spectroscopy. As exemplified by a change of the Tafel slope from about 60 to 40 mV / decade, the oxygen evolution reaction (OER) mechanism is modified by the deposition the NICo-LDH. In 1 M KOH at room temperature, the best electrode has overpotential as small as 269 and 340 mV at 10 and 250 mA cm-2, respectively. Longer-term electrolysis tests during 56 hours at 250 mA cm-2showed a constant overpotential of 340 mV, without any sign of degradation.

[0044] In the present disclosure, 3D Ni substrate electrodes with superaerophobic properties that were modified subsequently with the addition of NICo-LDHs are investigated, using a precipitation method resulting from a change of pH to deposit NICo-LDH materials throughout the porous structure of a preformed 3D Ni substrate electrode. In a solution containing Ni and Co nitrate salts as precursors, the application of a negative current leads to the reduction of nitrate ions and water molecules that results in the generation of hydroxyl ions at the electrode-electrolyte interface, according to Reactions 1.1 and 1.2 as follows:NO3-+ 7H2O + 8e- NH4++ 10OH- Reaction 1.12H2O + 2e- H2+ 2OH- Reaction 1.2

[0045] These hydroxyl ions lead to a steep increase of the pH, causing the chemical precipitation of NICo- LDHs according to the following reaction: xN i2++ yCo2++ 2(x+y)OH- -> NixCoy(OH)2(x+y) Reaction 1 .3

[0046] Accordingly, it was expected that precipitation of NICo-LDHs could occur in all pores of a 3D electrode as long as these pores were in contact with the electrolyte and Reactions 3.1 and 3.2 actually occurred in the pores.

[0047] Superaerophobic 3D Ni substrate electrode prepared by the DHBT method and modified by the addition of NICo-LDHs are investigated for the oxygen evolution reaction (OER). Deposition of NICo-LDHs was achieved under various current densities to identify the optimum morphology and composition necessary to achieve the largest electrocatalytic activity for the oxygen evolution reaction (OER).

[0048] Ni plates, 1 cm x 1 cm x 0.1 cm (Alfa Aesar, 99.5% metal basis) were abraded with 150 grit SIC and used as substrates. The Ni substrates were sealed in bent glass tubes (90° angle) in order to expose oneside of the substrate and maintain the electrode surface in a vertical position. The Ni plates were soaked in 1 M HCI solution for 10 minutes to remove oxides and then in ethanol for another 10 minutes.

[0049] For the synthesis of porous 3D Ni Substrate, the porous 3D Ni coatings were deposited on Ni plate substrates, by galvanostatic electrodeposition at -2 A cm-2in an aqueous solution of 0.1 M NiCh- 6H2O (ACROS Organics, ACS Reagent) and 2 M NH4CI (Fisher Chemical, Trace Metal grade). In these conditions, both electroreduction of Ni+2cations and H2 evolution are occurring concomitantly. After 420 seconds of electrodeposition, a porous 3D Ni substrate was formed on the Ni plate that has a primary honeycomb-like structure and a cauliflower-like secondary structure. These electrodes were rinsed several times with deionized water and then dried with an Argon stream. The resulting electrodes will be referred to as 3D Ni substrates.

[0050] For the NICo-LDHs, NICo-Layered Double Hydroxides (NICo-LDHs) were synthesized on 3D Ni substrate using a solution of 0.2 mmol Co(NO3)2.6H2O (Aldrich Chem Co. 99.999% (metal basis) and 0.2 mmol Ni(NO3)2.6H2O (Alfa Aesar Puratronic 99.998% (metal basis) dissolved in 25 ml of de-ionized water. A cathodic current was applied during 10 min, at current densities ranging from -1 to -4 mA cnr2geo. After deposition, the electrodes were rinsed several times with de-ionized water and ethanol, and then dried overnight at 60 °C. These samples will be referred to as NICo-LDH-3D Ni- X, where X is the applied current density for the preparation of the NICo-LDH layer. For comparison, the synthesis of NICo-LDHs on Ni plates (denoted as NICo-LDHs-NI Plate X) was achieved using the same procedure but with current densities limited to -1 and -2 mA cm-2. In some cases, pure Ni(OH)2-LDH and pure Co(OH)2-LDH were deposited on Ni plates, using a 0.2 mmol solution of either Co(NO3)2.6H2O or Ni(NO3)2.6H2O at a current density of -2 mA cm-2for 10 minutes. Table 1 -1 herein below presents the list of samples prepared, along with their preparation conditions and acronyms.

[0051] All steps involving deposition of coatings and electrochemical measurements were performed at ambient temperature (25°) in a standard three-electrode glass cell, with a Ni substrate as working electrode, a platinum gauze (Alfa Aesar, 99.9%) as counter electrode, and a saturated calomel electrode (SCE) as reference electrode. A Solartron 1480A multi-potentiostat was used for the deposition of coatings. Deionized water (Milli-Q, resistivity > 18 MO cm) was used throughout this experiment.Table 1 -1

[0052] For a physical characterization of the samples, sample surface morphology was characterized by high-resolution scanning electron microscopy (HR-SEM, Tescan Lyra 3). The chemical composition was assessed by energy-dispersive X-ray spectroscopy (EDX) (VEGA3 TESCAN). The crystallographic structure of the samples was examined by X-ray diffraction (XRD, Panalytical X-Pert PRO MRD) using a Cu tube. XRD data were obtained from 28 = 10 to 90 °, with a step size of 0.02° and a counting time of 0.5 s per step.

[0053] Captive air bubble contact angle measurements were performed to assess the hydrophobicity of the electrodes. The volume of the air bubble was 5 pL. Using a Panasonic CCD camera (model GP-MF552), images were captured of captive air bubbles in contact with the electrode surface. Static contact angles were measured using Image J software with the Drop snake plugin. At least three different measurements were conducted on each sample, and the average value of contact angle was reported. Dynamic contact angle measurements were conducted. For this purpose, the sample holder was initially horizontal and then tilted slowly until the air bubble began to move and the advancing (8adv) and receding (8red) contact angles were measured. The radius of the bubble (R), the extent of its contact with the electrode (w), the sliding angle (8Sid), and the advancing (8adv) and receding (8rec) contact angles were determined using Image J software.

[0054] X ray photoelectron spectroscopy (XPS) measurements were carried out using an Aluminum source (Ka = 1486.6 eV) (PHI Quantes equipment). Spectra were fitted by CasaXPS using a Linear Shirley type background to encompass peaks. Fits were performed with a Gauss / Lorentz profile with a ratio of 70% to 30%. Adventitious carbon was used for calibration by setting the C 1s peak to 284.6 eV.

[0055] For electrochemical analysis, all electrochemical measurements were conducted using a Solartron 1287 potentiostat using a one-compartment electrochemical cell. The electrolyte was 1 M KOH (High purity, Fisher Chemical, ACS Reagent grade, 99.99% (metal basis). In all cases, the electrolyte was saturated with Ar prior to use. A SCE reference was used via a Luggin capillary to measure the electrode potential and converted to the corresponding reversible hydrogen electrode (RHE) scale using ERHE = ESCE + 0.242 V + 0.059V x pH, with the pH of 1 M KOH at room temperature being 13.5. The counter electrode was Pt (negative electrode) in all cases.

[0056] Samples were immersed and the open circuit potential was monitored for 10 minutes. Cyclic voltammograms (CV) were recorded between 0.40 and 1.60 V at a scan rate of 50 mV s-1until steady-state potentiodynamic features were obtained, usually after 10 cycles. Upon stabilization, a final CV was recorded at 5 mV S'1in the same potential window. The overpotentials for the oxygen evolution reaction (OER) were determined by chronopotentiometry at 10 mA cm-2, r|io, and then at 250 mA cm-2, 13250, for 1 hour at room temperature in 1 M KOH.

[0057] Tafel plots were obtained by performing stepwise potentiostatic measurements. In each case, the current was monitored for 600 seconds, and the iR-corrected overpotential was calculated by averaging the data recorded in the last 60 seconds. Long-term electrolysis tests were conducted by measuring the overpotential at a fixed current density of 250 mA cm-2for 56 hours in 1 M KOH.

[0058] The ohmic drop was measured by electrochemical impedance spectroscopy (EIS) and an ohmic drop correction was applied to the potential values reported in this study. All current densities were calculated based on the geometric surface area. The ohmic drop was measured by electrochemical impedance spectroscopy (EIS) and an ohmic drop correction was applied to the potential values reported in this study. All current densities were calculated based on the geometric surface area.

[0059] Electrochemical Impedance Spectroscopy (EIS) measurements between 0.01 Hz and 200 kHz were conducted using a FRA Analyzer (Solartron 1255B) after 600 seconds of polarization. These conditions were sufficient for the electrode to reach a steady state current. The electrochemical impedance spectroscopy (EIS) data were fitted using a transmission line model (TLM) and the de Levie Equation as follows: 1.4where Cdi and rct are the double-layer capacitance and charge-transfer resistance on the pore walls, respectively, Ro is the total solution resistance (Q cm2) within the pores, and Ruis the uncompensated resistance. The complex nonlinear least squares (CNLS) fitting program of Matlab® was used.

[0060] The results will now be presented and discussed. In relation to the surface morphology, high- resolution scanning electron microscopy (HR-SEM) micrographs of Ni plate and porous 3D Ni substrate before and after NiCo-LDH deposition are shown in FIGs. 1. The HR-SEM micrographs of Ni plate in FIG.s 1A, 1 B and 1 C show that the substrate surface is dense, with some directional scratches originating from the sample preparation step. In contrast, the deposition at high current density of the 3D Ni substrate results in the formation of dendritic deposits (in FIGs. 1 D-1 F) with a primary honeycomb-like structure and a cauliflowerlike secondary structure. Micrometer-sized pores were observed, with pore diameters varying between 10 to 30 pm. The walls surrounding the pores show a highly porous semi-spherical cauliflower-like structure. The pores extend from the 3D Ni substrate surface all the way to the underlying Ni plate.

[0061] The structure of the NiCo-LDH-3D Ni-3 sample was shown in FIG. 1 G. At this magnification, the morphology of NiCo-LDH-3D Ni-3 is not different from that of the 3D Ni substrate. However, micrographstaken at higher magnification from both outside (FIGs. 1 H and 11) and inside the 3D Ni pores (FIGs. 11 and 1 K) show that the NiCo-LDH deposit is made of lamellar nanosheets. The thickness of the nanosheets is in a range between about 30 nm and 120 bout and the nanosheets are arranged into an interconnected network with pores of the substrate, of a diameters in a range between about 100 and about 200 nm. The HR-SEM micrographs in FIG. 1 show that NiCo-LDH can be deposited on the surface and into the porous structure of 3D Ni electrodes.

[0062] As part of the captive air bubble contact angle measurements, static contact angle measurements using air were performed on various samples. As seen in FIGs. 2, the captive air bubble contact angle increases from 94 ° for the Ni plate to 150 ° for the porous 3D Ni substrate. Upon deposition of NiCo-LDHs on a 3D Ni substrate, the air bubble contact angle is further increased to 166° (FIG. 2C).

[0063] The adhesion force of an air bubble was assessed by recording the contact angle hysteresis formed when moving on a tilted surface. The adhesion force, Fadh , is given as follows:Fadh = k w Y1V(cos 0adv- cos 0rec) 1 .5 where k is the retentive force factor (dimensionless constant), w is the length of the contact line between the air bubble and the surface (m), is the liquid - vapor surface energy (J rm2), and 0adv and 0recare the advancing and receding contact angles, respectively. Experimentally measured values of w, 0adv and 0recof 5 l gas bubbles on inclined surfaces are given in Table 1-2 below, showing experimentally measured values for w, 0advand 0rec of a 5 pl air bubble on an inclined surface.

[0064] No attempt was made to experimentally determine the value of k. In the literature, values of k ranging from 1 to TT are reported, depending on the shape of the air bubble and wettability of the surface. For surface showing large contact angle hysteresis, i.e. a Ni plate, TC / 2 < k < 2, while for surface showing small contact angle hysteresis, k = 4 / TT = 1 .27. So, using k = 1 .54 for Ni plate with large contact angle hysteresis, and k = 1 .27 for 3D Ni and NICo-LDH-3D Ni with small contact angle hysteresis, a relative comparison of Fadh can be performed by computing k w (cos 0adv - cos ©rec), with being common to all electrodes. From Table 1 -2, Fadh varied in the following order: NICo-LDH-3D Ni-3 < 3D NICo < 3D Ni « Ni plate. There is a factor of about 9 difference between the k w (cos eadv - cos erec) value of a Ni plate and the porous 3D Ni electrode, emphasizing the effect of microstructure on the adhesion force of an air bubble. The deposition of NiCo-LDH on 3D Ni brings another factor of about 7 decrease of the adhesion force, demonstrating the impact of nanostructure on the adhesion force.Table 1 -2

[0065] In relation to the composition, the chemical composition of the samples was characterized by EDX. In the variation of the ratio Ni / (Co + Ni) (in at. %) is shown with respect to the NiCo-LDH deposition current density. On both substrates, increasing the LDH deposition current density led to a decrease in the relative amount of Ni, although the Ni content varied with the nature of the substrate. At the lowest current density investigated (1 mA cm-2), the relative concentration of Ni on the 3D Ni substrate was 100 at.%. This is consistent with the lower solubility product of Ni(OH)2 (5.48 x 10'16) compared to Co(OH)2 (5.92 x 10'15). These values corresponded to molar solubility of 5.16 x 10'6and 1.14 x 10'5mole per liter for Ni(OH)2 and Co(OH)2, respectively. The pH of the electrolyte in close proximity to the 3D electrode increased with increasing current density, causing the precipitation of the less soluble Ni(OH)2 and the more soluble Co(OH)2. In FIG. 3, at any apparent current density, there were less Ni atoms in the NiCo-LDH deposited on the Ni plate than on the porous 3D Ni electrode, because the real current density differs between these two substrates due to their difference in effective surface area. This is consistent with the electrode potential at any apparent current density being smaller on porous 3D Ni electrode than on Ni plate (see In FIG. 4). As a result, the real current density is smaller on the 3D Ni electrode, which translates into lower increase of the pH near the electrode surface, which is less alkaline, than on the flat Ni electrode, causing the preferential deposition of the less soluble Ni-based LDH.

[0066] With X ray diffraction, the XRD profiles of NiCo-LDH deposited on a Ni plate and 3D porous Ni substrate were shown in Figure 1 -5. For the Ni plate, the XRD pattern was recorded at fixed grazing incidence angle to increase the surface sensitivity. It displayed a series of peaks assigned to the Ni substrate and two peaks at 20 = 1 1 .25 and 22.81 ° that may be assigned to the (003) and (006) peaks of a-NiCo(OH)2. For the 3D porous Ni substrate, the XRD profile was recorded in 0-20 mode since the inherent roughness of the substrate is such that the angle of incidence between the incident beam and the surface varies from point to point. Accordingly, the peaks attributed to NiCo-LDH are less intense compared to the Ni plate substrate since the incident X-ray beam probed a large thickness at the sample surface. On 3D Ni, the most intense (003) peak of a-NiCo(OH)2 was observed at 20 = 1 1 .75°, instead of 20 = 1 1 .25° for the Ni plate. This variation in the position of peak (003) is indicative of a change in the Ni and Co composition of a-NiCo(OH)2 between the two substrates. In principle, the position of the (003) peak of a- NiCo-LDH can be used to determine the Ni and Co content of the LDH layer. Reliable and consistent data for the position of this peak in pure a-Co LDH and pure a-Ni LDH are still needed. Part of the reason for this could be that the interlayer spacing of hydrotalcite-like materials have rich interlayer chemistry associated with anion-exchange properties and that the crystalline structure of the final products vary with the preparation conditions and the nature of the anion.

[0067] With cyclic voltammetry, the steady-state CVs of the Ni plate, and Ni(OH)2-LDH and Co(OH)2-LDH deposited on a Ni plate are shown in FIG. 6A and FIG. 6B, respectively. The CV of the Ni plate exhibits a pair of oxidation and reduction peaks at 1.37 and 1.32 V, respectively, associated with the p-Ni(OH)2 / p-Ni(OOH) transition. Upon deposition of Ni(OH)2-LDH, an oxidation peak was observed at 1.39 V, with a shoulder at about 1.43V. The corresponding reduction peaks were observed at 1.35 and 1.32 V, respectively. A current density increase by a factor of 6-8 was observed compared to the Ni plate.

[0068] The deposition of Co(OH)2-LDH on Ni plate led to a significantly different CV response (FIG. 6B). For CO(OH)2-LDH, a pair of redox peaks at 1.02 / 1.08 V was associated with Reaction 1.6, while the peaks centered at 1 .28 / 1 .38 were associated with Reaction 1 .7 hereinbelow. There is no sign of the sharp oxidation peak of the Ni substrate at 1 .37 V, which is an indication that the surface of the electrode is completely covered with Co-LDH. The current associated with the deposition of Co(OH)2-LDH was much smaller than for Ni-LDH:CO(OH)2 + OH- ^CoOOH + H2O + e- Reaction 1 .6CoOOH + OH- «->CoO2 + H2O + e- Reaction 1 .7

[0069] The CV of the 3D Ni electrode is shown in Figure FIG. 6C. On the positive sweep, an oxidation peak was observed at 1 .37 V with a shoulder at about. 1 .40 V. In addition, a small oxidation peak was observed at +1 ,56V, before a current increase at 1 .6 V associated with the oxygen evolution reaction (OER). The oxidation peak at 1.37 V corresponds to the a-Ni(OH)2 / y-NIOOH transition, while the shoulder at 1.40 V may be attributed to the p-Ni(OH)2 / p-NIOOH transition. The small oxidation peak at about 1.56 V was attributed to formation of Ni (IV) species, potentially at the edges of y-Ni(OH)2 / y-NiOOH domains. On the negative sweep, a broad reduction peak was observed at 1 .26 V.

[0070] Compared to the Ni plate, the CV of the porous 3D Ni electrode (FIG. 6C) showed a larger current density due to its larger electrochemically active surface area arising from the primary honeycomb-like structure and the secondary cauliflower-like structure of the DHBT deposit. The 3D porous Ni electrode is not very active for the oxygen evolution reaction (OER) despite its huge electrochemical active surface area. This is consistent with the literature showing that the intrinsic activity of Ni for the oxygen evolution reaction (OER) is small unless extraneous elements like Fe are added.

[0071] The CV of NiCo-LDH deposited on 3D Ni did not show the pair of redox peaks at 1.02 / 1.08 V associated with the one-electron oxidation / reduction of Co(OH)2 to CoOOH (Reaction 1.6). Instead, it exhibited a pair of low intensity redox peaks at 1.11 / 1.18 V that were not present in pure Ni-LDH. This is indicative of an interaction between Co and Ni atoms dissolved in the a-NICo(OH)2 structure. At larger potential, there is a pair of broad and larger intensity redox peaks at 1 .24 / 1 .35 V, close to those appearing in Ni-LDH. In addition, there is a hint of a small oxidative peak at 1 .52 V that preceded at sharp current increase at 1 .6 V associated with the oxygen evolution reaction (OER). The onset potential of NiCo-LDH deposited on 3D porous Ni, measured at 0.01 mA cm-2, is at least 50 mV less positive than for the porous 3D Ni electrode. The current density at 1 .6 V of NiCo-LDH-3D Ni is a factor about 6 larger than porous 3D Ni. Both observationsindicate a reduction of the energy barriers of the intermediates involved in the oxygen evolution reaction (OER). These findings are in line with previous studies, which have shown that incorporating cobalt hydroxide into the nickel structure significantly improves the electrochemical properties of Ni hydroxide for the oxygen evolution reaction (OER).

[0072] Concerning oxygen evolution reaction (OER) overpotentials, FIGs. 7A and 7B display the galvanostatic curves of a few representative electrodes at a current density of 10 mA cm-2. The same behavior was observed for all electrodes. For the Ni plate electrode, a stable potential of 1 .59 V was observed over 1 hour of electrolysis. The galvanostatic curves of NICo-LDH-NI plate-2 exhibited a similar behavior, although the potential was lowered at 1.55 V. This indicates that the NICo-LDH deposit has a beneficial effect on the oxygen evolution reaction (OER) performance of the Ni plate.

[0073] At the beginning of the galvanostatic measurements, an initial transient was observed on the 3D porous Ni electrode, where the electrode potential increased rapidly from about 1 .20 to 1 .58 V. This was due to the transformation of Ni(ll) to Ni(lll) that occurred at about 1.35 V (see FIG. 6C). This transformation occurred over the first 40 s. This transformation was barely discernable on the Ni plate because its electrochemically active surface area was smaller compared to the porous 3D Ni electrode. The charge associated with this transformation was about 0.4 C cm-2. Following the transient, the electrode potential stayed constant at 1 .58 V. The galvanostatic curve of NICo-LDH-3D-3 exhibited the same behavior, with the electrode potential increasing from 1.20 to a steady value of 1.50 V. This transition occurred over the first about 160 s, compared to 40 s for the porous 3D Ni electrode. The associated charge was 1 .6 C cm-2, a factor of 4 increase compared to the porous 3D Ni electrode. The beneficial effect of NICo-LDH on the oxygen evolution reaction (OER) activity of the Ni plate was also observed on the porous 3D Ni electrode.

[0074] Galvanostatic tests at 10 and 250 mA cm-2were conducted on the series of electrodes prepared in this study, and the results are summarized** in FIGs. 8. Increasing the LDH deposition current decreased both r|io and r]25o. NICo-LDH-3D Ni-3 was the best performing electrode, with overpotential as small as 269 and 340 mV at 10 and 250 mA cm-2, respectively. These overpotentials are lower than many nickel-based electrocatalysts reported in the literature. Table 1 -3 below shows a comparison of the oxygen evolution reaction (OER) characteristic of various Ni-based electrocatalysts.Table 1 -3

[0075] In relation to the Tafel plots and EIS measurement, Tafel plots recorded on the porous 3D Ni and best-performing NICo-LDH-3D Ni-3 electrodes were shown in FIGs. 9. Two independent samples were prepared and characterized in both cases. For both electrode types, a single Tafel slope was calculated using the log (j) and log (1 / Rct) data sets shown in Figure FIG. 9A for the porous 3D Ni and in Figure in FIG. 9B for NICo-LDH-3D Ni-3. Upon deposition of NICo-LDH on the porous 3D Ni substrate, the Tafel slope changed from 62 to 40 mV / decade. This would indicate a change in the rate-determining step of the oxygen evolution reaction (OER), from:M - OH + OH -^ M - O - + H2O Reaction s toM - 0 - — > M - 0 + e- Reaction 1 .9

[0076] which was termed the "Krasil1shchikov's Path". The exchange current density for the oxygen evolution reaction (OER) was calculated by extrapolating the Tafel slopes of the log (j) vs potential curves tothe equilibrium reversible potential of the reaction (see FIGs. 10). The exchange current density for 3D Ni was about 1 x 10-8A cm-2, a factor of 10 larger compared to NiCo-LDH-3D Ni-3, which was about 1 x 109A cm-2. Mixing Co and Ni atoms in the LDH layer changed the rate-determining step of the OER, which in turn has a profound effect on the exchange current density and the Tafel slope of the reaction.

[0077] Electrochemical impedance spectroscopy (EIS) data were recorded at different potentials in the oxygen evolution reaction (OER) range and the experimental data and fitted curves are displayed in FIGs. 1 1. For clarity, data were shown only for a few selected potentials larger than the onset potential for the oxygen evolution reaction (OER). The electrochemical impedance spectroscopy (EIS) spectra of the porous 3D Ni and NICo-LDH-3D Ni-3 electrodes displayed both a straight line in the high frequency region and one depressed semicircle in the low frequency region. The shape of the high frequency line did not change with potential, although the intercept with the X-axis was increased slightly as the overpotential is increased. This was an indication that the features in the high-frequency response were not related to the oxygen evolution reaction (OER) kinetics but were characteristic of the porosity of the electrodes. The slope of the straight lines in the high-frequency regions of both electrodes differs, being close to 45° for porous 3D Ni and closer to 22.5° for NiCo-LDH-3D Ni-3. This indicates a change of the electrode porosity because of the NiCo-LDH deposit.

[0078] The de Levie Equation (2 1 .4) used to fit the electrochemical impedance spectroscopy (EIS) data is based on a transmission line model in which pores are parallel with a cross-sectional shape independent of depth. The fit of the electrochemical impedance spectroscopy (EIS) data of the porous 3D Ni electrode with the de Levie Reaction is good over both the low and the high-frequency regions. Thus, it can be inferred that the porous 3D Ni electrode can be assimilated to parallel pores with a cross-sectional shape independent of depth. The de Levie model failed to represent adequately the 22.5° straight line of the high-frequency region of the NICo-LDH-3D Ni-3 electrode, suggesting the porous structure of this electrode is more complex than assumed by the model. This is consistent with the HR-SEM micrographs of FIG.1 showing the existence of two types of pores.

[0079] The de Levie Reaction was used to fit the depressed semi-circle observed at low frequencies in the electrochemical impedance spectroscopy (EIS) data, and the following Reactions were used to determine the charge-transfer resistance (Rct) and the double-layer capacitance (Cdi) as follows:Cdl= cdl / RnReaction 1.10Rct= rctRn Reaction 1.11

[0080] The log(1 / Rct) data were plotted against potential in FIG. 9. These data were analyzed in tandem with the log(j) versus potential data, so that both data sets yield the same Tafel slope. This was indeed the case, confirming the model used in the electrochemical impedance spectroscopy (EIS) data analysis.

[0081] The corresponding Cdi values were plotted as a function of the current density in FIG. 12. This way of presenting Cdi data was chosen instead of as a function of potential because of the large difference incurrent density between the two types of electrode. The double layer capacitance of the porous 3D Ni electrode varied between 0.19 and 0.10 F cnr2, with an average value of 0.13 F cm-2. In comparison, larger Cdi values were observed for the NiCo-LDH-3D Ni-3, with an average value of 1.29 F cm-2, a factor of 10 increase compared to porous 3D Ni electrode.

[0082] This comparison between the Cd values of both electrode would suggest that the electrochemically active surface area of NiCo-LDH-3D Ni-3 is a factor of 10 larger than the 3D Ni electrode. This is consistent with the SEM micrographs of FIG. 1 that showed a dramatic change of the surface morphology between these two electrodes. In FIG. 7, only a factor of 4 was found based on the charge involved in a redox transition occurring before the oxygen evolution reaction (OER) onset potential. Evaluation of the electrochemically active surface based on C values was considered more meaningful because the measurements were carried out in a potential region where O2 is released from the electrode surface. As seen previously, the exchange current of the 3D Ni electrode is a factor of 10 larger than the NICo-LDH-3D Ni-3 electrode, notwithstanding the fact the former have a smaller electrochemically active surface area than the latter. Were it not for a change in the oxygen evolution reaction (OER) mechanism that reduces the Tafel slope of the NICo-LDH-3D Ni-3 electrode from 60 to 40 mV / decade, the overpotential of this electrode at practical current density would be larger than that of the 3D Ni electrode.

[0083] Long-term electrolysis tests (56 h) were carried out at 250 mA cm-2and the results are shown in FIG. 13. The electrode potential shows a decrease during the first few hours of electrolysis, and then remained stable, with T]25O = 340 mV at the end of the 56 h electrolysis period. The Open Circuit Potential (OCP) after 56 hours of electrolysis was about 1 .4 V, about 0.5 V more positive that the OCP of a freshly prepared NICo- LDH-3D Ni-3 (see FIG. 14).

[0084] As shown in FIG. 15, there is no morphological change after 56 hours of electrolysis. As shown previously, the contact angle of captive air bubbles on NICo-LDH-3D Ni-3 was 166°, resulting in a decrease in their adhesion strength that could contribute to the mechanical stability of NICo-LDH deposits. In FIG. 16, the NICo LDH-3D Ni-3 CVs after 56 h of electrolysis indicate an approximately 20% increase of the charge under both the cathodic and the anodic peak. This suggests that an increase of the electrochemically active surface area has occurred during electrolysis. This 20% increase of the electrochemically active surface area is consistent with the initial decrease of the overpotential observed in FIG. 13 and the Tafel slope determined in FIG. 9.

[0085] A comparison was made between the XPS spectra of NICo LDH-3D Ni-3 before and after 56 hours of electrolysis at 250 mA cm-2(see FIGs. 17). The surface composition of as-deposited NICo LDH-3D Ni-3 was [Co] = 15 at.%, [Ni] = 17 at.% and [O] = 68 at.%. After electrolysis, the surface composition was [Co] = 20 at.%, [Ni] = 18 at.% and [O] = 62 at.%.

[0086] There was no radical change in surface composition after 56 hours of electrolysis, indicating that the deposit is stable. Furthermore, no peaks were observed in the about 700 eV region (see FIG. 18), where 2p3 / 2and 2pi / 2 iron peaks would have been expected if the electrode surface had been contaminated after prolonged treatment.

[0087] The 2p3 / 2 core level peak of as-deposited NICo-LDH-3D Ni-3 was observed at 855.2 eV, accompanied by a satellite peak located 5.8 eV higher in binding energy (FIG. 17A). According to the literature, this peak primarily arises from Ni2+in NiCo LDH. After 56 hours of electrolysis, the binding energy of the Ni 2p3 / 2 peak remains constant at 855.3 eV. However, the satellite peak is now positioned 6.7 eV higher in binding energy (FIG. 17D). This observation suggests an increase in the average oxidation state of the Ni atoms. This finding aligns with the difference in binding energy between the main and satellite peaks of NiOOH, which can be as large as 8.3 eV.

[0088] The Co 2p3 / 2 core level peak of as-deposited NiCo-LDH-3D Ni-3 was observed at 780.5 eV, accompanied by a satellite peak located 5.5 eV higher in binding energy (FIG. 17B). These observations are consistent with the literature, indicating that Co atoms are in a +2 oxidation state in as-deposited NiCo LDH. Following 56 h of electrolysis, the Co 2p3 / 2 core level peak appeared at 779.3 eV, and the satellite peak is now positioned 10.0 eV higher in binding energy (FIG. 17E). In comparison to the as-deposited electrode, the Co 2p3 / 2 peak has shifted to a lower binding energy, and the binding energy separation between the Co 3ps / 2 peak and the satellite peak has increased. This may indicate that the valence state of Co atoms is close to +3. This is consistent with the fact that the OCP of the electrode, when it was pulled out of the electrolyte, was about 1.4 V (see FIG. 14). This value is well above the oxidation peak characteristic of the conversion between NiCo(OH)2 and NiCoOOH (see FIG. 6c).

[0089] Figure FIG. 17C depicts the O 1 s core level spectrum of the as-deposited NiCo-LDH-3D Ni- 3 electrode, showing a peak at approximately 530.7 eV. This peak may be attributed to the presence of surface hydroxyl groups (M-OH, where M = Ni or Co). In Figure FIG. 17F, following electrolysis, another peak at about 528.6 eV appeared, which may be attributed to M-0 bond. These findings provide additional confirmation of the formation of oxyhydroxides on the catalyst's surface after electrolysis.

[0090] The XRD patterns of NiCo LDH-3D Ni-3 before and after 56 hours of electrolysis are shown in FIG. 19. The (003) peak of a-NiCo(OH)2 observed at 20 = 1 1.71 ° in the as-deposited electrode was moved to 20 = 12.45° after electrolysis. This shift of the main (003) peak is reminiscent of that observed between a-Ni(0H)2 (JCPDS 38-0715) and y-NiOOH (JCPDS 06-0075). Accordingly, and in agreement with the XPS and OCP data shown previously, this shift was attributed to the transformation of a-NiCo LDH to y-NiCoOOH during the OER process. The LDH structure was preserved after 56 h of electrolysis.

[0091] As presented hereinabove, a porous Ni substrate with a 3D fractal structure and with superaerophobic properties was catalyzed for the OER through the deposition of NiCo-LDH. The deposition method, which involves locally increasing the pH at the electrode / electrolyte interface by reducing the water molecules and nitrate ions in solution, results in the co-precipitation of NiCo-LDH in all the pores of the substrate's 3D structure. NiCo-LDH was deposited in the form of nanosheets firmly, and the resulting electroderetains the superaerophobic properties of the substrate. This resulted in a factor of about 60 reduction of the adhesion force of air bubbles, enabling the resulting electrode to display a stable overpotential at high current density over 56 hours.

[0092] Using electrochemical impedance spectroscopy (EIS), the electrochemically active surface area was evaluated at electrode potential more positive than the onset potential for the OER (oxygen evolving potential region). The deposition of NICo-LDH increased the electrochemically active surface by a factor of 10 compared to the 3D Ni substrate. NICo-LDH deposition modifies the oxygen evolution reaction (OER) mechanism by changing the rate-determining step from a chemical step, with a 60 mV / decade Tafel slope (Reactions 1 -8) to an electrochemical step, with a 40 mV / decade Tafel slope (Reactions 1 -9). At the same time, the exchange current density of the NICo-LDH electrode decreased by a factor of 10 compared to the 3D Ni electrode. At sufficiently high current densities, such as those required for practical applications in an electrolyzer, the reduction in the Tafel slope of the NICo-LDH deposit more than offsets the decrease in exchange current density, resulting in the NICo-LDH electrode showing the best performance for the OER, with r|25o = 340 mV at 250 mA cm-2. This is a typical example of electrocatalysis where a lower exchange current density is counterbalanced by a lower Tafel slope, the two factors combining to give a larger apparent current density than the 3D Ni electrode in a potential region where O2 is produced at a rate compatible with practical application.

[0093] The layered double hydroxides may be ones of: NIMn, NiCr, Nilr, NICu, NIMo, NIV, and NiZn.

[0094] There is now presented achieving OER activity in water splitting through tuned NIFe-LDH catalysts

[0095] As described hereinbelow, a 3D Ni substrate with superaerophobic properties was functionalized with NIFe LDH nanosheets to significantly enhance OER performance. The NIFe LDHs were synthesized using a mixed solution of Ni and Fe nitrate salts, with the deposition achieved by applying varying negative currents to locally increase the pH, facilitating the precipitation of the LDH. The deposition parameters were systematically optimized to maximize catalytic efficiency and stability, which were evaluated through extensive electrochemical testing, including cyclic voltammetry and galvanostatic measurements. Results demonstrated that tuning the deposition current had a direct impact on the composition and catalytic activity of NIFe-LDH. The optimized electrode achieved exceptional performance, with overpotentials of 218 mV at 10 mA cm-2and 254 mV at 250 mA cm-2in 1 M KOH at room temperature. To further enhance the stability and activity of the catalyst, an in-situ electrochemical tuning technique involving 1 ,000 CV cycles was applied, to selectively dissolve less-active Fe phases (FeOxHy), as confirmed by STEM-EDX elemental mapping, thereby exposing a greater number of catalytically active NIFe-LDH sites. This was evidenced by improved OER activity over time. Prolonged durability tests performed at 10 and 250 mA cm-2demonstrated excellent stability, with no signs of degradation after 10 hours.

[0096] In a first step the 3D Ni structure was deposited using the dynamic hydrogen bubble template (DHBT) method, resulting in the formation of dendritic structures with a large electrochemically active surface areaand superaerophobic properties, which mitigates the blocking effects of O2bubbles at high current densities. A subsequent step involves investigating 3D Ni substrate electrodes with superaerophobic properties, catalyzed by the deposition of NiFe LDH. The optimal deposition parameters required to achieve the highest electrocatalytic activity for OER, ensuring uniform deposition of NiFe LDH are determined by electrodeposition with varying negative current densities; during the deposition process, the formation of NiFe LDH along with non-uniform patches of FeOOH, or FeOxHy, depending on the stoichiometry, indicating suboptimal deposition conditions, were observed. To address this, in-situ tuning was used to remove the FeOOH patches, improving both the catalyst’s activity and stability. The mechanism behind these improvements is investigated through advanced characterization methods, including STEM-EDX elemental mapping, to identify the structural and compositional changes that occur during the tuning process. This present study effectively addresses the impact of co-deposited, less-active phases and instability in NiFe LDH catalysts.

[0097] In experiments described herein, the substrates were 1 cm x 1 cm x 0.1 cm Ni plates (Alfa Aesar, 99.5% metal basis). Prior to use, they were abraded with 180 grit abrasive paper, rinsed with deionized water. An electrical contact was established to the back of the Ni plate using a Cu wire. Using a glass tube whose end was bent to 90°, the nickel substrates were maintained in a vertical position. The glass tube was sealed so that only one face of the Ni plate (1 cm2) was exposed to the electrolyte. To remove oxides from the surface, the Ni plates were soaked in nitric acid, chloric acid, and water for two minutes each. They were then ultrasonically cleaned with ethanol for 10 minutes, followed by rinsing with deionized water for ten minutes.

[0098] All the synthesis process and the electrochemical characterizations were fabricated at room temperature (25°C) in a standard three-electrode configuration glass cell, with a Ni substrate as working electrode, a platinum mesh (Alfa Aesar, 99.9%) as a counter electrode, and a saturated calomel electrode (SCE) as reference electrode. All experiments were conducted using a Solartron 1480A multi-potentiostat. The distance between the working and the counter electrode was kept constant at 5 mm. Ultrapure deionized (DI) water (Milli-Q, resistivity > 18 MO cm) was used throughout the experiments.

[0099] For the synthesis of the 3D Ni substrate electrode, the 3D Ni substrate electrodes were prepared by the Dynamic Hydrogen Bubble Template (DHBT) method, by electrodeposition at -2 A cm-2in a 60 ml aqueous solution of 0.1 M NiCh' 6H2O (ACROS Organics, ACS Reagent) and 2 M NH4CI (Fisher Chemical, Trace Metal grade)., using Ni plates as Ni substrates. After 420 seconds of electrodeposition, a porous 3D Ni substrate was formed on the Ni plate, comprising a primary honeycomb-like structure and a cauliflower-like secondary structure. Following electrodeposition, these electrodes were rinsed several times with de-ionized water and dried with compressed air.

[0100] For the synthesis of NiFe LDH films, NiFe-Layered Double Hydroxides (NiFe-LDHs) were synthesized on 3D Ni substrate electrodes using a solution of 0.075 mmol Fe(NO3)3.9H2O (Sigma Aldrich 99.95% (metal basis) and 0.075 mmol Ni(NO3)2.6H2O (Alfa Aesar Puratronic 99.998% (metal basis) dissolved in 25 ml of de-ionized water. A negative current was applied during 10 min, at current densities ranging from -1 to -13 mA cnT2geo. After deposition, the electrodes were rinsed several times with de-ionized water andethanol, and then dried overnight at 60 °C. These samples are referred to as NiFe-X, where X is the applied current density. A list of the samples is provided in Table 2-1 below.Table 2-1

[0101] For physical characterization, the surface morphologies of the produced electrodes were characterized by high-resolution scanning electron microscopy (HR-SEM, Tescan Lyra 3). An energy- dispersive X-ray spectroscopy (EDX) was performed to determine their chemical composition (VEGA3 TESCAN). The X-ray diffraction (XRD) patterns were obtained by Panalytical X-Pert PRO MRD using a Cu Ka radiation (A = 1 .54178 A) operated at 45 kV and 40 mA. XRD data were collected from 28 = 10 to 90 °, with a step size of 0.02° and a counting time of 0.5 s per step.

[0102] Superaerophobicity of electrodes was assessed by measuring the contact angle of captive air bubbles with a volume of 5 pL. Images of captive air bubbles in contact with the electrode surface were captured using a Panasonic CCD camera (model GP-MF552). Contact angles were measured using Image J software and the Drop snake plugin. For each sample, at least two measurements were conducted, and the average contact angle was calculated.

[0103] Air bubble adhesion force was evaluated by measuring advancing (8adv) and receding (8rec) contact angles. In the beginning, the sample holder was horizontal, and then the tilt of the holder slowly caused the air bubble to move. A CCD camera video was taken while the air bubble moved. Using Image J software, the radius (R) and the extent of contact with the electrode (w) of the bubble were measured, along with the sliding angle (0sid), and the advancing (8adv) and receding (8rec) contact angles.

[0104] The process of preparing a TEM sample starts by scratching the surface of NiFe- 9 sample using a plastic pipette to create a thin layer of the sample. Once the sample was scratched, it was sonicated for a fewminutes in high-purity ethanol to ensure good dispersion of the sample and remove any remaining contaminants. Next, a suitable TEM grid (carbon) was selected whereby a small droplet of the dispersed solution is deposited on the grid. The grid was subsequently dried and loaded into the TEM instrument for analysis.

[0105] To investigate the morphology and composition of NIFe- 9 before and after electrolysis, high- resolution transmission electron microscopy (HR-TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging, along with energy dispersive X-ray (EDX) mapping were carried out. All TEM and HAADF-STEM imaging was performed using a Thermo Scientific Talos 200X operating at 200kV available at the Canada Center for Electron Microscopy (CCEM) at McMaster University.

[0106] X ray photoelectron spectroscopy (XPS) measurements were carried out on VG Escalab 220I XL, which was performed with a Specs Phoibos 150 hemispherical analyzer using the Twin Mg Ka source (1253.6eV) XR50 (SPECS). A silver specimen was used as a reference sample to calibrate the system. Adventitious carbon was used for calibration by setting the C 1 s peak to 284.6 eV. 284.6 eV.

[0107] For the electrochemical analysis, a Solartron 1287 potentiostat was used to conduct all electrochemical measurements in 1 M KOH (High purity, Fisher Chemical, ACS Reagent grade, 99.99% (metal basis). All electrolytes were saturated with Ar prior to use. A one-compartment electrochemical cell was used to conduct the measurements with a Pt mesh (99.99%, Alpha Aeser) as the counter electrode, prepared electrodes as the working electrode and Hg / HgO as the reference electrode. Near the electrode surface, the electrode potential was measured with a Luggin capillary. To avoid contamination from glass components in alkaline media, a PTFE (PolyTetraFloroEthylene) cell was used. To prepare the cell for each experiment, the cell was cleaned with 6M HNO3 and 6M HCI at 90° for 3 hours each, then thoroughly rinsed with DI water, and then boiled in DI water.

[0108] All the potential values reported herein were corrected against ohmic drop. The ohmic drop was measured by electrochemical impedance spectroscopy (EIS). All current densities were calculated based on the geometric surface area. A Solartron 1255B frequency analyzer was used for the electrochemical impedance spectroscopy (EIS) measurements. All current densities were calculated based on the geometric surface area.

[0109] In all cases, the experimentally measured electrode potentials (vs. Hg / HgO) were converted to the corresponding reversible hydrogen electrode (RHE) scale using ERHE = Eng / Hgo + 0.098 V + 0.059V x pH, with the pH of 1 M KOH at room temperature being 13.5. The ohmic drop-corrected OER overpotential (q) was calculated using q = ERHE - iR - 1.23 V, where I is the current and R is the ohmic drop. Electrochemical Impedance Spectroscopy (EIS) measurements between 0.01 Hz and 200 kHz were conducted after a constant potential was applied to the electrode during 800 second. These conditions were sufficient for the electrode to reach a steady state current. The electrode potential was varied from 1 .45 to 1 .75 V in the regime of the OER.

[0110] The experiment began with a 15-minute period under open circuit potential (OCP). After achieving steady-state potentiodynamic features with cyclic voltammograms (CVs) recorded between 0.40 and 1.70 V vs. RHE at a scan rate of 50 mV s-1(after 25 cycles), the final CV was recorded at 5 mV s-1in the same potential window. CVs reported in this study come from final recordings at 5 mV s-1. Furthermore, the electrochemically active surface (ECSA) is determined by measuring the cathodic charge under Ni(OH)2 / NIOOH redox peak in the final CVs. Along the same line, the electrochemically active surface (ECSA) of the electrodes was also estimated from CVs in the double layer region of 0.94 to 1 .04 V (vs RHE) at varying scan rates of 1 to 10 mV s-1in 1 .0 M KOH. There were no faradaic processes in this region. Averages of the anodic and cathodic currents recorded at 0.99 V vs. RHE were plotted with respect to scan rate. The slope of the curves was used to determine electrode capacitance.

[0111] Steady-steady Tafel plots were obtained by performing stepwise potentiostatic measurements at increasing higher potentials ranging from 1.45 to 1.75 V vs RHE. In each case, the electrode current was monitored for 800 seconds, and the IR-corrected overpotential was calculated by averaging the data recorded in the last 100 seconds.

[0112] Galvanostatic tests were conducted on the electrode series prepared in this study at 10 and 250 mA cm-2to assess their activity. After identifying the electrode with the highest catalytic activity, focus shifted to studying its stability. A total of 1000 CV cycles were performed at a scan rate of 50 mV s-1, with a potential between 0.9 and 1.6 V vs RHE. Additional CV tests were conducted at a scan rate of 5 mV s-1, within the same potential range. These tests were carried out after 150, 650, and 1000 cycles, and the results were compared with the initial CV to identify any changes that occurred during or after the CV studies. Furthermore, a galvanostatic plot after 1000 CV cycles was recorded for one hour at 250 mA cm-2to further probe the performance alteration. To monitor the stability of the catalyst after optimizing the deposition conditions, galvanostatic tests were conducted at 10 and 250 mA cm-2for extended durations.

[0113] The results will now be presented and discussed.

[0114] Concerning structural characterization : NiFe LDH layers were deposited on three-dimensional (3D) Ni substrate electrodes. In summary, a cathodic current of between 1 and 13 mA cm“2geo was applied, resulting in the reduction of nitrate ions and water molecules and the production of hydroxyl ions (Reactions 2.1 and 2.2, respectively). This subsequently elevated the pH at the electrode surface. In these conditions, the metallic ions in proximity to the electrode surface underwent chemical reactions with the hydroxyl ions, resulting in the precipitation and formation of a NiFe Layered Double Hydroxide (LDH) compound, as illustrated by Reactions as follows:NO3- + 7H2O + 8e- NH4++ 10OH- Reaction 2.12H2O + 2e- H2+ 2OH- Reaction 2.2 xNi2++ yFe3++2(x+ 3y / 2)OH- -> NixFey(OH)2(x+3y / 2) Reaction 2.3

[0115] An EDX analysis was performed to determine the chemical composition of the electrodes. Figure 20 shows the variation of Fe content, expressed as [Fe] / [Fe] + [Ni] in at. %, with respect to the applied current density. As the deposition current density increased up to 9 mA cm-2, the Fe content increased. However, for current densities above 9 mA cm-2, a decrease in Fe content was observed.

[0116] The results shown in Figure 20 can be interpreted in two different ways. An increase in the deposition current density results in the production of a greater number of OH- anions (Reactions 2.1 and 2.2), which in turn increases the pH at the electrode surface. It can be hypothesized that this increase in pH results in a greater incorporation of Fe ions than Ni ions into the structure of the LDH deposited material. Accordingly, the results shown in FIG. 20 can be directly attributed to a change in the composition of the Ni Fe LDH deposited material. However, as suggested by Reaction 2.3, the deposited mass should also increase due to the elevated pH achieved at the highest current density. The thickness of the layer probed during EDX analysis is constant at about 25 microns, which is greater than the thickness of the deposited layer. This indicates that the chemical composition determined during EDX analysis is an average between that of the deposited LDH layer and that of the Ni substrate. Therefore, as the mass and, more importantly, the thickness of the deposited layer increase with the deposition current density, the composition of the sampled layer will show a decrease in its Ni content due to the fact that a smaller fraction of the Ni substrate is being sampled. At this point, it is not possible to distinguish between these two effects.

[0117] The observed reduction in Fe content at current densities exceeding 9 mA cm-2is likely due to the inferior mechanical stability of these deposits, as black residues were observed at the base of the electrochemical cell during these experiments. Consequently, in subsequent experiments, the deposition current density was limited to 9 mA cm-2.

[0118] XRD patterns of several NiFe LDH layers deposited on 3D Ni substrate electrodes were recorded, and a typical example is shown in FIG. 21 . In all cases, only the sharp peaks associated with the Ni substrates were observed, and no peak that could be attributed to NiFe LDH was observed. This is most likely due to the limited thickness of the LDH deposits. Therefore, NiFe LDH was deposited on a flat Ni plate to perform XRD measurements in a surface-sensitive mode at a grazing angle of incidence of 2°. The corresponding XRD pattern in Figure 22 showed two additional peaks at about 28 = 12.11 and 21.40°, corresponding to the (003) and (006) reflections of a-NIFe LDH, respectively. These peaks were broad, indicating that the deposits were poorly crystalline and / or consisted of domains with small coherent length.

[0119] Figure 23 shows scanning electron microscope (SEM) micrographs of the 3D Ni substrate electrode before and after NiFe LDH deposition. The 3D Ni substrate electrode has a hemispherical cauliflower-like structure. After NiFe LDH deposition, the structure was similar to that of the 3D Ni substrate at low magnification (Fig. 23C). However, at high magnification, the structure of the NiFe LDH deposit was clearly observed. It consisted of nanosheets with thicknesses in a range between about 30 nm and about 120 nm. A similar morphology was observed for NiCo LDH deposited on the same substrate using a similar procedure.

[0120] Figure 24 shows the HAADF-STEM image and corresponding elemental mapping of the NiFe-9 electrode. Figure 24a shows that Fe and Ni are distributed over the surface of the sample, with some regions showing an increase in Fe and 0 concentrations relative to the neighboring regions. This indicates the presence of iron oxides (FeOOH or FeOxHy) on the sample surface. Note that the areas of higher Fe and 0 concentration do not correspond to areas of lower Ni concentration. This observation will be important later when the CVs are analyzed.

[0121] The HR-TEM images of as-deposited NiFe-9 are shown in FIG. 25. Two domains were discerned; (I) a thin section made of an amorphous domain (red rectangle); and (ii) a section made of a well-crystallized compound in which individual rows of atoms are observed (green rectangle). The spacing between these rows was 0.267 nm, which corresponds to the (101) plane spacing of NiFe LDH phase. This is consistent with the XRD data of Figure 22.

[0122] Captive air bubble contact angles were measured both before and after the deposition of NiFe-9 on 3D Ni. Figure 26 reveals that the contact angle of the captive air bubble increased from 150° for the 3D Ni to 161 ° for the NiFe-9 electrode, indicating an enhancement in the superaerophobic properties of the 3D Ni surface.

[0123] The contact angle hysteresis of a moving air bubble on a tilted surface was used to determine the relative adhesion force between the air bubble and the electrode surface. The adhesion force, Fadh is given by:Fadh=k w ylv(cos 0adv- cos 0rec) Reaction 2.4 in which k is the retentive force factor (dimensionless constant), w is the air bubble's contact line with the surface (m), is the liquid-vapor surface energy (J nr2), and 0adv and 0rec are the advancing and receding contact angles, respectively. Table 2-2 hereinbelow lists the experimentally determined values of w, 0adv and 0rec of 5 l gas bubbles on inclined surfaces. The value of k was not determined through experimentation. Instead, the value of k = 4 / TT = 1 .27 was used for both electrodes. A comparison of Fadh can then be made according to Reaction 2.4, with yivbeing equal for both electrodes. According to Table 2-2, there is a factor of about 3 difference between the Fadh value of porous 3D Ni and NiFe-9 electrodes, emphasizing the effect of nanostructuration on the adhesion of an air bubble at the electrode surface.Table 2-2

[0124] In relation to cyclic voltammetry: in FIG. 27, the CV of the 3D Ni substrate electrode showed an oxidation peak at about 1.37 V with a weak shoulder at about 1.40 V, corresponding to the well-known a- Ni(OH)2 / y-NIOOH transition and p-NI(OH)2 / p-NIOOH transition, respectively. The corresponding reduction peak was observed at about 1 .26 V. Upon deposition of the NiFe LDH film on the 3D Ni substrate electrode, a significant change in the CV of the resulting electrode was observed. As the deposition current density is increased up to 9 mA cm-2, a shift of the oxidation redox peak from about 1 .37 to 1 .42 V is observed, along with an increase in the peak current density from about 0.020 to 0.054 A cm-2. These changes are accompanied by the appearance of a new reduction peak at about 1.32 V, the intensity of which gradually increased to 0.035 A cm-2. These changes were associated with the deposition of NiFe LDH on the surface of the 3D Ni substrate electrode. As explained above, an increase in the deposition current density causes an increase in the pH near the electrode surface, resulting in a corresponding increase in the mass of the deposit. The increase in the reduction peak at 1 .32 V associated with the presence of NiFe LDH is therefore consistent with the increase in the mass of the deposit.

[0125] As shown in FIG. 27A, the CV curves at potentials more positive than about 1 .45 V become steeper and steeper as the deposition current density is increased, reflecting an increase in the electrocatalytic activity for the OER with the mass of the NiFe LDH deposit. This effect will be better evaluated later on by a series of galvanostatic measurements.

[0126] FIGs. 28A and 28B show the cathodic charge (Qc) and the double layer, Cd, estimated from CVs measurements, which serve as representatives of the electrochemically active surface area, ECSA, respectively. Interestingly, FIG. 28C shows that there is a linear variation of Cdi with respect to Qc, indicating that the following analysis does not depend on the metric used to estimate the EASA. As the deposition current density was increased, both Qcand C were increased. This trend is likely related to the mass of the deposits, which increased as the deposition current density increased. In the case of the best performing electrode, NiFe-9, there was an increase in EASA of 3-4 compared to the bare substrate.

[0127] Concerning the electrocatalytic properties of the electrodes for the OER: galvanostatic tests were performed on all electrodes at 10 and 250 mA cm-2and the results are shown in Figure 27b with respect to the NiFe LDH deposition current density. NiFe LDH deposition on 3D porous Ni significantly improved the electrode activity at both current densities. NiFe-9 outperformed the other electrodes, requiring only 218 and 254 mV overpotentials to produce current densities of 10 and 250 mA cm-2, respectively. These overpotentials are significantly lower than other nickel-based electrocatalysts listed in Table 2-3 hereinbelow, showing a comparison of the OER characteristics of some state-of-the-art catalysts, indicating that NiFe-9 has the potential to be used as a highly efficient industrial electrocatalyst for the OER during hydrogen production.Table 2-3

[0128] Tafel plot measurements were performed, and the results are shown in FIG. 29. The Tafel slope of the NiFe-9 electrode is 28 mV / decade, which is lower than that of the bare 3D Ni substrate electrode (53 mV / decade). This change in Tafel slope indicates a shift in the rate-determining step of the OER upon deposition of NiFe LDH. The recorded slope value is lower than that observed for most electrocatalysts in the literature (see Table 2-3hereinabove), including a present best performing NiCo LDH, which was prepared in the same manner as the NiFe LDHs hereinabove.

[0129] The enhanced performance of the NiFe-9 sample can be attributed to a number of factors. Firstly, the NiFe LDH materials themselves are considered to represent the current state of the art in terms of their ability to catalyze the OER. Additionally, the excellent electrical conductivity of the porous three-dimensional nickel support facilitates enhanced electron transfer, thereby enabling a more efficient electrocatalytic process. This results in a low Tafel slope, which favors the decrease of the overpotential for the OER at elevated current density. Furthermore, the superaerophobic properties of the NiFe-9 electrode result in a weaker air bubble adhesion force, which allows a larger electrochemical active surface area to be exposed to the electrolyte at high current density. This is in contrast to less aerophobic electrodes, which are more obstructed by O2 bubbles, thereby limiting the accessibility of the electrolyte to the active sites.

[0130] The stability of the NiFe-9 electrode was evaluated by performing a series of CVs between 0.9 and 1.6 V (FIG. 30A). Significant changes in the shape and intensity of the redox peaks were observed. After 1000 cycles, the intensity of the oxidation peak increased by 37% while its position and shape remained almost the same. This is in contrast to the change observed in the corresponding reduction peak, which showed a gradual change in shape with cycling, mainly due to an increase in the intensity of a reduction peak at about 1 .33 V (FIG. 30B). Previously (FIG. 27A), this peak was associated with the deposition of NiFe LDH, suggesting a rise in the number of such active sites. At the end of 1000 cycles, the charge under the reduction peak was increased by a factor of about 2 compared to the as-deposited NiFe LDH electrode.

[0131] The chronoamperometric curves at 250 mA cm-2of NiFe-9 before and after 1000 CV cycles are compared in FIG. 30C. The electrode potential remained stable throughout the measurements (1 h). After the series of CVs, the overpotential decreased by approximately 10 mV compared to the as-deposited electrode. This decrease was expected from the about 2-fold increase in EASA and Tafel slope of the NiFe-9 electrode. The observed improvement in electrode performance after cycling would result from an increase in EASAwithout any change in the OER mechanism. This is clearly seen from the Tafel slope measurements after 1000 CV cycles, as shown in FIG. 29, where the Tafel slope remained unchanged.

[0132] Interestingly, the HR-TEM images of the electrode after 1000 cycles and the corresponding elemental mapping (FIG. 24) show that Ni, Fe and O are homogeneously distributed throughout the particle, in sharp contrast to what was observed for the as-deposited NIFe-9, where Fe and O tend to aggregate in the same locations. It is as if the iron oxyhydroxide patches initially present on the surface of the electrode were removed during cycling, exposing the NIFe LDH deposit beneath. This is consistent with Figure 30b, where the intensity of the reduction associated with NIFe LDH increased.

[0133] As iron oxide / hydroxide becomes unstable at high anodic potentials in alkaline electrolytes, resulting in its transformation to the soluble ferrate species, FeO42', it is hypothesized that the discrete iron oxide / hydroxide patches observed in Figure 24a, which are masking the underlying NIFe LDH phase, would dissolve over the course of the CV cycles performed in Figure 30. This dissolution process would gradually reveal the underlying layer, resulting in an increase in the reduction peak associated with NIFe LDH (FIG. 30B). Iron oxide / hydroxide is known to be less active for OER than NIFe LDH. Thus, the progressive dissolution of iron oxide / hydroxide patches would result in the removal of a less active material and the exposure of the underlying more active material, therefore increasing the electrocatalytic activity for the OER of the resulting electrode without any change in the rate-determining step.

[0134] The practical application of NIFe LDH catalysts is hindered by significant instability, particularly at elevated current densities, of at least 100 mA cm-2, where OER activity diminishes markedly over time. To evaluate the durability of the NIFe-9 electrode under extended electrolysis, galvanostatic tests were performed at 10 and 250 mA cm-2for 10 hours in each case. It is noteworthy that the NIFe-9 electrode, which has undergone 1000 CVs, demonstrated consistent electrochemical potentials over an extended period, of about 10 h, at both low and high current densities, as illustrated in Figure 31 .

[0135] The exceptional stability observed in the NIFe-9 electrode may be attributed to its nanosheet structure. This design minimizes the formation of localized acidic environments during the OER, which are known to cause the dissolution of active elements such as Fe and Ni. The slower diffusion of proton acceptors (e.g., OH") within bulk NIFe LDH interlayers leads to localized acidity, contributing to Fe and Ni dissolution and a gradual decline in OER activity. In contrast, the nanosheet configuration improves OH" diffusion, significantly reducing dissolution rates of these elements.

[0136] In the present experiments, this enhanced diffusion mechanism may explain the decreased Fe dissolution observed during OER, contributing to the overall stability of the material. Moreover, the micro / nanostructured structure offers a higher surface area, exposing more active sites and evenly distributing the catalytic load, which reduces mechanical and chemical stress on individual sites, further enhancing the performance and durability of the catalyst. Additionally, the nanosheet structure facilitates the efficient release of gas bubbles during electrolysis, which reduces bubble-induced degradation, improving electrode efficiencyand maintaining consistent catalytic activity over extended periods. Such features highlight the potential of the nanostructured NiFe material as a highly stable and efficient catalyst for water splitting, addressing critical challenges in green hydrogen production.

[0137] In relation to surface characterization: the XPS data for as-deposited NiFe-9 electrodes before and after 1000 CV cycles are shown in Figure 32. The 2p3 / 2 core level peaks of the as-deposited NiFe-9 electrode was observed at about 855.0 eV, accompanied by satellite peaks positioned at about 861.0 eV (FIG. 32A). These peak may be attributed to Ni2+in NiFe LDH. After CVs, the 2p3 / 2 core level peak remained similar to that of the as-deposited electrode (FIG. 32D), indicating that no significant change in the oxidation state of Ni occurred after 1000 CVs.

[0138] In FIG. 32B, the O 1 s spectrum of the as-deposited NiFe-9 electrode shows a peak at about 530.6 eV. This peak may be attributed to hydroxyl groups (M-OH, where M = Ni or Fe) in the LDH structure. A shoulder at about 529.0 eV is observed, which may be attributed to lattice oxygen in the LDH structure. In Figure 32d, the O 1 s spectrum of the NiFe-9 electrode after 1000 CVs appears almost identical to that of the as-deposited electrode, except perhaps for a slight shift of the peak position to a lower binding energy.

[0139] The Fe 2p3 / 2 core level peak of the as-deposited NiFe-9 electrode (FIG. 32C) was observed at about 71 1.0 eV, accompanied by a satellite peak at about 719 .0 eV. The position of these peaks is consistent with an iron atom in the 3+ oxidation state, as would be expected for iron in NiFe LDH and Fe oxyhydroxide. In FIG. 32F, the Fe 2p3 / 2 core level peak of the electrode after 1000 CVs is similar to that of the as-deposited electrode.

[0140] In summary, the binding energies and peak shapes of Ni, O and Fe show no major changes after the NiFe-9 electrode has been subjected to 1000 CVs, suggesting that the chemical environments of these elements are not significantly altered. Although there is virtually no change in the shape of the core-level peaks of Ni, O and Fe, the relative intensity of these peaks varies after the electrode has been subjected to 1000 CVs. In fact, as shown in Table 2-4 hereinbelow, the O concentration increases from 33.3 to 56.8% after 1000 CVs. In addition, the relative concentration of Ni (Fe) increases (decreases) after 1000 CVs. These changes are consistent with previous results showing that the Fe-rich patches on the surface of the as- deposited NiFe-9 electrode disappear after 1000 CVs (FIG. 24) and that the characteristic reduction peak of NIFe-LDH increases during the gradual dissolution of iron oxy / hydroxide (FIG. 30).Table 2-4

[0141] In relation to the electrocatalytic performance of NiFe LDH and NICo LDH deposited on 3D Ni electrode: a direct comparison of the OER performance between the two catalysts: NICo LDH and NIFe LDH investigated hereinabove. The two transition metal-based LDH electrocatalysts were prepared using the same electrodeposition technique and on the same 3D Ni substrate. As indicated in Table 2-5 hereinbelow, the most active NiCo-LDH-3D Ni-3 electrode exhibited overpotentials of 269 mV and 340 mV at 10 and 250 mA cm-2in 1 M KOH, with a Tafel slope of 40 mV / decade. In contrast, the NiFe-9 electrode exhibited noteworthy overpotentials of 218 and 254 mV to attain equivalent current densities of 10 and 250 mA cm-2, respectively, thereby surpassing the performance of its counterpart NiCo-LDH. Furthermore, the NiFe-9 electrode exhibited a Tafel slope of 28 mV / decade, which further underscores its superior electrocatalytic performance. A noteworthy aspect of this development is the markedly higher intrinsic activity of Ni Fe LDHs in comparison to NiCo LDHs. Despite the higher cathodic charge of NiCo LDH-3 surface (approximately 1600 mC cm-2), which serves as a relative measure of electrochemically active surface area, compared to NiFe-9 (960 mC cm-2), the latter electrode still exhibits superior performance. This highlights the greater importance of intrinsic activity in performance enhancement compared to increase of the electrochemically active surface area. The observed enhancement in OER activity positions NiFe LDH as a promising candidate for advanced electrocatalytic applications in comparison to NiCo LDH. This finding indicates that when considering fixed geometric electrode area-based OER current densities, Fe-containing LDH catalysts consistently exhibit the highest kinetic OER activities, followed by Fe-free Co-based LDH catalysts.Table 2-5

[0142] In conclusion, there is thus presented a comprehensive investigation of the optimal preparation conditions for maximizing the catalytic activity of the NiFe LDH catalyst deposited on a superaerophobic 3D Ni support. As the less active FeOOH phase deposited alongside the NiFe LDH, achieving a single-phase deposition of NiFe LDH was a challenge. The non-uniform FeOOH phase partially obstructed and masked the NiFe LDH structure, thereby reducing its accessibility to the electrolyte. However, following a series of cyclic voltammograms (CVs), the less active FeOOH phase was progressively dissolved, thereby exposingthe underlying NiFe LDH phase. Referred to hereinabove as the tuning step, it resulted in improved activity, and uniform deposition and optimization of NiFe LDH, which resulted in high-performance water splitting. The results of the STEM-EDX analysis confirmed the removal of the FeOOH patches from the NiFe LDH nanosheets. As a result, the NiFe-9 electrocatalyst demonstrated an outstanding overpotential of 254 mV at 250 mA cm-2, with this performance remaining stable for over 10 hours of electrolysis. In light of the pressing need for a synthesis technique capable of producing a single-phase, active NiFe LDH on a large scale for practical applications, the presently disclosed electrocatalyst preparation and tuning method offers considerable promise for electrochemical energy storage and conversion.

[0143] The substrate may be one of : a Ni, a Ni foam, a Ni foam loaded with Raney Ni, a Ni mesh and a Ni mesh loaded with Raney Ni.

[0144] The LDH may be ones of: NIMn LDH, NICo LDH, NiCr LDH, Nilr LDH, NIV-LDH, NICu-LDH, and NIZn-LDH.

[0145] In summary, the two-step electrodeposition method disclosed herein comprises fabricating a superaerophobic 3D Ni substrate and electrodeposition of catalytic layers on 3D Ni.

[0146] The superaerophobic 3D Ni substrate is fabricated using the dynamic hydrogen bubble templating (DHBT) method, using a current density of about -2 A / cm2, a duration between about 400 and about 450 seconds, and an electrolyte composed of 0.1 M NiCI2'6H2O and 2 M NH4CI solution. The structure is dried with an argon stream. This structure enhances gas bubble release, minimizes bubble-induced blocking effects during electrolysis, and provides a conductive, high-surface-area substrate for catalytic layers.

[0147] The electrodeposition of catalytic layers on 3D Ni includes electrodeposition of NICo LDH and NiFe LDH,. The deposition parameters for the active NICo and NiFe LDH layers are optimized by adjusting the deposition time and the deposition current density range, for achieving target surface morphology and chemical composition, according to their target efficiency for water splitting.

[0148] Before depositing the LDH layers on the 3D Ni structure, the parameters were initially optimized on a Ni plate. Various deposition times were tested to determine the optimum duration of 10 minutes, with current density ranges of 1 to 2 mA / cm2for NICo LDH and -1 to -15 mA / cm2for NiFe LDH. Subsequently, deposition parameters were further refined for each layer on 3D Ni, maintaining the optimal deposition time of 10 minutes but using current density ranges of -1 to -13 mA / cm2for NiFe LDH and -1 to -4 mA / cm2for NICo LDH.

[0149] Additional optimizations included fine-tuning the furnace drying duration. These controlled parameters ensured uniform surface morphologies and compositions, significantly enhancing the catalytic activity and durability of the electrodes for oxygen evolution reactions (OER).

[0150] Thus, the method presented herein comprises a tuning step applied to NiFe LDH on 3D Ni electrodes, selected to achieve uniform, high-performance, and stable electrocatalysts for OER. While the optimal deposition parameters for NiFe LDH maximize OER performance, they may also result in the formation ofunwanted non-uniform patches of FeOOH, or FeOxHy, depending on stoichiometry, as discussed hereinabove for example in relation to Fig. 24). These patches partially obstruct and mask the NiFe LDH structure, reducing its accessibility to the electrolyte. In-situ tuning using cyclic voltammetry (CV) is used to selectively remove the less-active FeOOH phases while preserving the active NiFe LDH layer. In experiments as described hereinabove, over 1000 CV cycles, the FeOOH patches were progressively dissolved, exposing the underlying NiFe LDH and significantly enhancing both catalytic activity and stability. Such tuning step ensures optimized electrocatalyst composition, providing stable performance for OER. Thus integrating an in- situ tuning with controlled electrodeposition, uniform, high-performing, and stable NiFe LDH catalysts are obtained, in a combined approach providing a novel pathway for advancing water-splitting technologies (see hereinabove and FIG. 30 for example).

[0151] Gas bubble adhesion force measurements were used for assessing super aerophobicity as discussed hereinabove in relation to FIGs. 2 and 26 for example.

[0152] Various post-stability tests (XPS, TEM, SEM / EDX, XRD, CVs, and Tafel) were conducted to gain new insights into material properties, including chemical composition and real ECSA, as described hereinabove hereinabove in relation to FIGs. 19, 24, 29, 30C, and 32 for example.

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[0154] The scope of the claims should not be limited by the embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.

Claims

Claims1 . A method of fabrication of transitional metal-based layered double hydroxides, comprising preparation of a 3D conductive substrate and electrodeposition of layered double hydroxides.

2. The method of claim 1 , wherein the 3D conductive substrate is a 3D Ni substrate fabricated by dynamic hydrogen bubble template.

3. The method of claim 1 , wherein the 3D conductive substrate is a 3D Ni substrate fabricated by dynamic hydrogen bubble template, the layered double hydroxides are NIFe layered double hydroxides, and the method further comprises removing FeOOH deposits from electrodeposited NIFe layered double hydroxides.

4. The method of claim 1 , wherein the 3D conductive substrate is a 3D Ni substrate fabricated by dynamic hydrogen bubble template, the layered double hydroxides are NICo layered double hydroxides, and the method further comprises determining durability of deposited NICo layered double hydroxides.

5. The method of claim 1 , wherein the 3D conductive substrate is a 3D Ni substrate fabricated by dynamic hydrogen bubble template, and the layered double hydroxides are ones of: NICo, NIMn, NiCr, Nilr, NICu; NIMo; NIV, and NiZn.

6. The method of claim 1 , wherein the 3D conductive substrate is a 3D Ni substrate fabricated by dynamic hydrogen bubble template, and the layered double hydroxides are ones of: NIFe, NIMn, NICo, NiCr, Nilr, NIV, NICu, and NiZn.

7. The method of claim 1 , wherein the 3D conductive substrate is a 3D Ni substrate fabricated by dynamic hydrogen bubble template with a current density of about -2 A / cm2, a duration between 400 and 450 seconds; with a 0.1 M NICI2'6H2O and 2 M NH4CI solution as an electrolyte; said electrodeposition of the layered double hydroxides comprises electrodeposition of NICo layered double hydroxides with a current density between -1 and -4 mA / cm2, a deposition time of about 10 minutes and drying at 60°C overnight, with a 0.2 mmol Co(NO3)2-6H2O (99.95%) and 0.2 mmol Ni(NO3)2'6H2O (99.998%) solution in 25 mL deionized water as an electrolyte.

8. The method of claim 1 , wherein the 3D conductive substrate is a 3D Ni substrate fabricated by dynamic hydrogen bubble template with a current density of about -2 A / cm2, a duration between400 and 450 seconds, and a 0.1 M NiCI2'6H2O and 2 M NH4CI solution as an electrolyte; the 3D Ni substrate is dried with an Argon stream; said electrodeposition of the layered double hydroxides comprises electrodeposition of NICo layered double hydroxides with a current density between -1 and -4 mA / cm2, a deposition time of about 10 minutes and drying at 60°C overnight, with a 0.2 mmol Co(NO3)2-6H2O (99.95%) and 0.2 mmol Ni(NO3)2'6H2O (99.998%)solution in 25 mL deionized water as an electrolyte.

9. The method of claim 1 , wherein the 3D conductive substrate is a 3D Ni substrate fabricated by dynamic hydrogen bubble template, with a current density of about -2 A / cm2, a duration between 400 and 450 seconds, with a 0.1 M NiCI2-6H2O and 2 M NH4CI solution as an electrolyte; said electrodeposition of the layered double hydroxides comprises electrodeposition of NIFe layered double hydroxides with a current density between -1 and -13 mA / cm2, a deposition time of about 10 minutes and drying at 60°C overnight, with a 0.075 mmol Fe(NO3)3'9H2O (99.95%) and 0.075 mmol electrolyte.

10. The method of claim 1 , wherein the 3D conductive substrate is a 3D Ni substrate fabricated by dynamic hydrogen bubble template, with a current density of about -2 A / cm2, a duration between 400 and 450 seconds, with a 0.1 M NiCI26H2O and 2 M NH4CI solution as an electrolyte, and the 3D Ni substrate is dried with compressed air; said electrodeposition of the layered double hydroxides comprises electrodeposition of NIFe layered double hydroxides with a current density between -1 and -13 mA / cm2, a deposition time of about 10 minutes and drying at about 60°C overnight, with a 0.075 mmol Fe(NO3)3'9H2O (99.95%) and 0.075 mmol electrolyte.1 1 . The method of claim 1 , wherein the 3D conductive substrate is a 3D Ni substrate fabricated by dynamic hydrogen bubble template, with a current density of about -2 A / cm2, a duration between 400 and 450 seconds, a 0.1 M NiCI26H2O and 2 M NH4CI solution as an electrolyte; said electrodeposition of the layered double hydroxides comprises electrodeposition of NIFe layered double hydroxides with a current density between -1 and -13 mA / cm2, a deposition time of about 10 minutes and drying at 60°C overnight, with a 0.075 mmol Fe(NO3)3'9H2O (99.95%) and 0.075 mmol Ni(NO3)2'6H2O (99.998%) solution in 25 mL deionized water as an electrolyte; the method further comprising in-situ tuning using cyclic voltammetry to selectively remove FeOOH phases.

12. The method of claim 1 , wherein the 3D conductive substrate is a 3D Ni substrate fabricated by dynamic hydrogen bubble template, with a current density of about -2 A / cm2, a duration between 400 and 450 seconds, a 0.1 M NICI26H2O and 2 M NH4CI solution as an electrolyte; and the 3D Ni substrate is dried with an Argon stream; said electrodeposition of the layered double hydroxides comprises electrodeposition of NIFe layered double hydroxides with a current density between -1 and -13 mA / cm2, a deposition time of about 10 minutes and drying at about 60°C overnight, with a 0.075 mmol Fe(NO3)3'9H2O(99.95%) and 0.075 mmol Ni(N03)2'6H20 (99.998%) solution in 25 mL deionized water as an electrolyte; the method further comprising in-situ tuning using cyclic voltammetry to selectively remove FeOOH phases.

13. A material comprising transitional metal-based layered double hydroxides and a porous 3D conductive substrate with interconnected lamellar nanosheets of the transitional metal-based layered double hydroxides bonding firmly to the substrate.

14. The material of claim 13, wherein the substrate is one of : a Ni, a Ni foam, a Ni foam loaded with Raney Ni, a Ni mesh and a Ni mesh loaded with Raney Ni substrates.

15. The material of claim 13, wherein the substrate is one of : a Ni, a Ni foam, a Ni foam loaded with Raney Ni, a Ni mesh and a Ni mesh loaded with Raney Ni substrates; the interconnected lamellar nanosheets are NiCo interconnected lamellar nanosheets forming an interconnected network with pores of the substrate of a diameter between about 100 nm and about 200 nm.

16. The material of claim 13, wherein the 3D conductive substrate is one of : a Ni, a Ni foam, a Ni foam loaded with Raney Ni, a Ni mesh and a Ni mesh loaded with Raney Ni substrates; the interconnected lamellar nanosheets are NiFe interconnected lamellar nanosheets of thicknesses in a range between about 30 nm and about 120 nm.

17. A transitional metal-based layered double hydroxides catalyst comprising interconnected lamellar nanosheets of transitional metal-based layered double hydroxides bonding firmly to a porous substrate.

18. The catalyst of claim 17, wherein the substrate is one of: a Ni, a Ni foam, a Ni foam loaded with Raney Ni, a Ni mesh and a Ni mesh loaded with Raney Ni substrates.

19. The catalyst of claim 17, wherein the substrate is one of: a Ni, a Ni foam, a Ni foam loaded with Raney Ni, a Ni mesh and a Ni mesh loaded with Raney Ni substrates; the interconnected lamellar nanosheets are NiCo interconnected lamellar nanosheets of thicknesses in a range between about 30 nm and about 120 nm.

20. The catalyst of claim 17, wherein the substrate is one of:a Ni, a Ni foam, a Ni foam loaded with Raney Ni, a Ni mesh and a Ni mesh loaded with Raney Ni substrates; the interconnected lamellar nanosheets are NiFe interconnected lamellar nanosheets of thicknesses in a range between about 30 nm and about 120 nm.

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