Modified anode for zinc batteries and its process of preparation thereof

Abstract

The present disclosure generally relates to zinc batteries. Specifically, the present disclosure relates to an electrode comprising UV modified polydopamine (PDA) coated zinc and a process of preparation thereof. The present disclosure also relates to a zinc battery comprising the electrode as anode or working electrode. The presence of zincophilic groups over the electrode (Zn@PDA-UV) suppressed zincate cross-over, thus enhancing the recyclability of the zinc air batteries (ZABs).

Description

[0001] PT / 2026 / 11456

[0002] MODIFIED ANODE FOR ZINC BATTERIES AND ITS PROCESS OF PREPARATION THEREOF

[0003] TECHNICAL FIELD OF THE INVENTION

[0004] The present disclosure generally relates to zinc-air batteries. Specifically, the present disclosure relates to an electrode comprising UV modified polydopamine (PDA) coated zinc, and a process of preparation thereof. The present disclosure also relates to a zinc-air battery comprising the said electrode as anode.

[0005] BACKGROUND OF THE INVENTION

[0006] Zinc-air batteries (ZABs) have evolved as a promising energy storage device considering the high theoretical energy density of zinc (Zn) metal, higher compatibility of Zn with aqueous electrolytes, and higher safety and lower cost. The research and development of rechargeable ZABs basically focuses on designing an efficient bifunctional electrocatalyst capable of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), thus enabling smooth charging and discharging of the battery (refer, X. Yang, S. Li, D. Ye, J. Kuang, S. Guo, Y. Zou and X. Cai, 2022, 7, 2100673). Moving away from the state-of-the-art noble metal catalysts like Pt / C and RuCh for ORR and OER, respectively, to transition metal -based oxides, sulphides and perovskites-based catalysts having comparable activity has marked down the cost for ZAB device fabrication. This approach of designing a superior bifunctional electrocatalyst has profoundly accelerated the ZAB research. However, the limited reversibility of Zn anode results in low columbic efficiency, hence lowering the batteries' cycling life. The Zn anodes are often associated with dendrite growth, side reactions, passivation, and shape changes, engendering a non-uniform electric field and byproduct aggregation over the anode surface, hampering the ion transport and formation of ‘dead Zn’, consequently lowering the efficiency of ZABs (refer, Q. Zhang, X. Liu, X. Zhu, Y. Wan and C. J. S. M. Zhong, Small Methods, 2023, 7, 2201277). The Zn anode often undergoes self-corrosion in alkaline media, and the ZnO formed as the discharge product accumulates over the anode surface, creating a passivation layer, hindering subsequent ion transport and further Zn utilization, impeding further discharge of the battery (refer, J. Hao, X. Li, X. Zeng, D. Li, J. Mao and Z. Guo, Energy & Environmental Science, 2020, 13, 3917-3949). Also, the non-uniform surface charge over the Zn foil leads to irregular plating-stripping, paving the way to dendrite formation and, eventually, premature cell failure. For the Zn anode in an alkaline electrolyte, a hydrogenPT / 2026 / 11456

[0007] evolution reaction (HER) can also happen as a side reaction, eventually leading to lower Zn utilization. Hence, the shortcomings associated with the Zn anode must be addressed and mitigated to design efficient and durable ZABs.

[0008] There are some solutions known in the art e.g., coating a protective layer over the Zn anode, constructing a 3D structured anode, and introducing an electrode and electrolyte additives, etc., which enhances the plating / stripping cycling of the Zn anode, thereby improving the cycling life of the battery (refer, J. Chen, Y. Wang, Z. Tian, J. Zhao, Y. Ma and H. N. Alshareef, 2024, 6, el2485). Anode coating is a simple and effective strategy, which often does not require tedious preparation methods, and involves interface tuning by coating a thin layer over the anode surface, modulating the Zn plating and stripping behavior, and thereby enhancing their cycling life.

[0009] Recently, MXene / Zn metal anode interfacial structure with single / few-layer Ti3C2TxMXene as a protective layer was reported (refer, D. Yang, J. Li, C. Liu, J. Ge, W. Xing and J. Zhu, ACS Applied Materials & Interfaces, 2023, 15, 10651-10659). This Ti3C2Txisolates the direct contact between Zn metal anodes and electrolytes and inhibits zincate dissolution due to the ion screening function of Ti3C2Tx, potentially addressing the stubborn issues of Zn anodes. The Ti3C2Tx-protected Zn metal anode exhibits cycle stability (stable for more than 400 cycles) to the bare Zn counterpart (20 cycles) at a high current density of 5.0 mA cm-2. Another report reports a chemical buffer layer coated over Zn metal where ZnO nanorods are uniformly distributed over graphene oxide (GO) to enhance the reversibility of Zn <4> ZnO (refer, W. Sun, M. Ma, M. Zhu, K. Xu, T. Xu, Y. Zhu and Y. Qian, Small, 2022, 18, 2106604). Here, ZnO nanorods play the nuclei role, while the adsorption affinity of GO helps confine ZnO and Zn(OH)4 to the buffer layer. These reports have utilized an ex-situ protective coating strategy, and the synthesis of active substrates like MXene and GO which is tedious. Thus, strategically designing a procedure for an effective protective layer in-situ on the Zn foil (anode) would dramatically ease the anode modification thereby tackling the associated issues mentioned above. The coated layer should strongly adhere to the Zn surface, enabling protection of the anode in harsh electrolytes and modulating the Zn2+ion migration through them to maintain Zn flux during the discharge process. Additionally, it should have zincophilic sites which can suppress zincate cross-over.

[0010] Polydopamine (PDA), having the catechol groups, forms coordinate bonds with metal surfaces at a pH of 8.5, thus forming a self-aggr egated coating over metal surfaces. This coating acts asPT / 2026 / 11456

[0011] a protective layer over the metals, thus controlling the self-corrosion of metals in adverse conditions. Further, the affinity of this PDA with zincate groups confines the zincates formed as a discharge product onto the PDA layer, enabling enhanced reversibility of the battery cycling. However, PDA often forms self-aggregates at higher alkaline concentrations and detaches from the substrate. This detachment of PDA in high alkaline concentrations is detrimental for air-battery operating with 6 M KOH as an electrolyte.

[0012] Thus, there is a need in the art to provide a new modified and coated electrode or anode for zinc battery which will address the aforesaid problems. The present invention by providing an electrode comprising a UV modified polydopamine protective layer which facilitates a long cycle life of ZAB.

[0013] OBJECTIVES OF THE INVENTION

[0014] An objective of the present disclosure is to provide an electrode comprising UV modified polydopamine coated zinc.

[0015] Another objective of the present disclosure is to provide a process for preparation of the electrode.

[0016] Yet another objective of the present disclosure is to provide a three electrode system comprising the said electrode as working electrode.

[0017] Yet another objective of the present disclosure is to provide zinc air battery comprising the said electrode as anode.

[0018] SUMMARY OF THE INVENTION

[0019] In one aspect, the present disclosure relates to an electrode comprising ultraviolet (UV) modified polydopamine (PDA) coated zinc.

[0020] In another aspect, the present invention relates to a process for preparation of an electrode of the present invention, the process comprising:

[0021] a) forming a polydopamine (PDA) solution;PT / 2026 / 11456

[0022] b) coating or depositing the polydopamine solution onto a zinc substrate by dipping or immersing the substrate in the polydopamine solution to obtain a polydopamine coated zinc electrode material; and

[0023] c) irradiating the polydopamine coated zinc electrode material with ultraviolet (UV) radiation having a wavelength in the range of 315-400 nm to obtain the electrode.

[0024] In another aspect, the present invention relates to a three-electrode electrochemical system, comprising:

[0025] i) a working electrode as the electrode of the present invention;

[0026] ii) a reference electrode;

[0027] iii) a counter electrode;

[0028] iv) a power supply / Potentiostat; and

[0029] v) an electrolyte solution,

[0030] In another aspect, the present invention relates to a battery comprising:

[0031] i) an anode, wherein the anode is the electrode of the present invention;

[0032] ii) an air cathode; and

[0033] iii) an electrolyte solution,

[0034] BRIEF DESCRIPTION OF THE DRAWING

[0035] Figure 1 depicts FESEM images of a) bare Zn b) Zn@PDA c) Zn@PDA-UV of Example 1, and cross-sectional ESEM image displaying the coating thickness at different resolutions of d) 50 pm and e) 20 pm and f) cross-sectional ESEM images of PDA coating over one half of the Zn foil while keeping the other half masked.

[0036] Figure 2 shows 3D topography image obtained from AFM for a) bare Zn, b) Zn@PDA-UV of Example 1, and contact angle analysis of c) bare Zn, d) Zn@PDA and e) Zn@PDA-UV of Example 1.PT / 2026 / 11456

[0037] Figure 3 shows a) the comparative XRD spectra for bare Zn, Zn@PDA and Zn@PDA-UV of Example 1 anodes with JCPDS of Zn, and XPS spectra displaying b) N Is, c) Zn 2p, and d) O 1 s spectra.

[0038] Figure 4 shows FESEM images of a) bare Zn b) Zn@PDA c) Zn@PDA-UV electrodes of Example 1 after soaking in 6 M KOH for 24 h and comparative XRD image depicting the extent of self-corrosion in zinc anode before and after corrosion test of d) Bare Zn e) Zn@PDA f) Zn@PDA-UV of Example 1.

[0039] Figure 5 shows electrochemical HER analysis, b) LPR studies, and c, d) Chronoamperometric studies conducted for bare Zn, Zn@PDA and Zn@PDA-UV of Example 1 as anodes.

[0040] Figure 6 shows the (a, b, c) reversible electrochemical stability of Zn@PDA-UV-6, Zn@PDA-UV-4 and bare Zn anodes at a current density of 0.5 mA cm'1and a specific capacity of 1 mAh cm'2.); and SEM images of d) Zn@PDA-UV-6, e) Zn@PDA-UV, and f) Bare Zn after plating stripping cycling.

[0041] Figure 7 shows the electrochemical performances of Zn@PDA-UV||air comprising the electrode of Example 1 and Zn||air full cells, a) the current (I)-voltage (V) polarization plots recorded for ZAB with bare Zn and Zn@PDA-UV of Example 1 as anode (b) the Nyquist plots obtained from the EIS analysis of the bare Zn and Zn@PDA-UV of Example 1 cells at the OCV condition c) the galvanostatic discharge plots recorded at 5 mA cm'2, and d) the corresponding galvanostatic discharge capacities of the fabricated ZABs after normalizing with the consumed amount of the Zn during the discharge process at 5 mA cm'2galvanostatic discharge plot.

[0042] Figure 8 shows a) galvanostatic charge-discharge cycling 12 charge-discharge cycles to confirm the effectiveness of anode coating in bare zinc and the Zn@PDA-UV of Example 1, and b) its corresponding XRD peak depicting ZnO formation over the anode surface.

[0043] Figure 9 shows the FESEM images of a) bare Zn anode surface b) Zn@PDA-UV surface of Example 1 after cycling for 12 charge-discharge cycles; and c) Cycle performance curves for Zn@PDA-UV||air as per the present invention and Znllair cells at a current density of 5 mA cm-2with a charge / discharge time of 5 min along with the initial and final voltage efficiencies.PT / 2026 / 11456

[0044] Figure 10 shows voltage-areal capacity profiles for bare Zn||air cells a) before and b) after cycling studies and voltage-areal capacity profiles for Zn@PDA-UV ||air cells c) before and d) after cycling studies comprising the electrode of Example 1.

[0045] Figure 11 shows (a, b) impedance analysis of ZAB with bareZn as anode; and (c, d) Zn@PDA-UV of Example 1 as anode obtained after different cycles.

[0046] Figure 12 illustrates the three-electrode system comprising the electrode of Example 1 as working electrode.

[0047] Figure 13 illustrates the zinc air battery comprising the electrode of Example 1 as anode.

[0048] DETAILED DESCRIPTION OF THE INVENTION

[0049] In an embodiment, the present invention provides an electrode comprising ultraviolet (UV) modified polydopamine (PDA) coated zinc.

[0050] The electrode comprises of PDA in an amount of 3mg / ml to 4 mg / ml (in trisbuffer) and zinc foil of thickness 2 to 5 pm. Bicarbonate buffer can also be used. The zinc coated with PDA may include a zinc material which is in the form of a zinc foil, zinc foam, zinc plate or zinc mesh. The electrode comprises of PDA coating having thickness of at least 1.5 pm. Preferably, the PDA coating has a thickness in a range from around 1.5 pm - 2.0 pm. Bicarbonate buffer can also be used

[0051] The PDA coating on the electrode shows a globular morphology and has a uniform height throughout the electrode material. Conventionally, the coating undergoes degradation and dissolution from substrate in any highly alkaline conditions. The UV modification of the coating converts the co-ordinate bond of polydopamine coated onto the zinc into covalent bond by converting hydroxyl groups present in dopamine into catechol groups of polydopamine along with secondary cross-linking of the amine groups with the carbonyl group. As a result, the UV modified electrode exhibits enhanced crosslinking within the PDA, thereby providing a thicker, and smoother coating with improved rigidity and resistance to alkaline conditions. This modification further improves the mechanical and chemical stability of PDA coating in harsh alkaline environments such as those created by the 6 M KOH when used as the electrolyte in ZABs.PT / 2026 / 11456

[0052] The crosslinking within the PDA also facilitates the intercalation of catechol groups with the zinc metal, thereby providing a uniform deposition of PDA along with oxygen containing functional groups like OH of catechol towards the zincate group, prevents zincate crossover formed during the charge / discharge process of a battery. As a result, the electrode of the present invention comprises an ordered surface coating or compact coating, resulting in improved smoothness. Further, the electrode of the present invention is hydrophobic in nature having a contact angle of at least 106°, which provides advantageous effects such as corrosion protection, suppression of dendrite formation, and improved cycling stability of the electrode when employed in electrochemical cells such as ZABs.

[0053] Additionally, the electrode of the present invention when employed in a battery such as zinc air batteries (ZAB) shows a reduction in current density, indicating suppressed zinc (Zn) electrodeposition kinetics and an almost constant behavior with a slight increase, attributed to the suppressed dendrite formation. Further, the electrode demonstrates reduced hydrogen evolution reaction (HER) and Zn deposition, which further collectively minimizes the dendrite formation (Figure 7c) onto the electrode resulting in a highly stable and long cycle life of the battery.

[0054] In another embodiment, the present invention provides a process for preparation of an electrode of the present invention, the process comprising:

[0055] a) forming a polydopamine (PDA) solution;

[0056] b) coating or depositing the polydopamine solution onto a zinc substrate by dipping or immersing the substrate in the polydopamine solution to obtain a polydopamine coated zinc electrode material; and

[0057] c) irradiating the polydopamine coated zinc electrode material with ultraviolet (UV) radiation having a wavelength in the range of 315-400 nm to obtain the electrode.

[0058] In step a), the polydopamine solution is prepared by adding dopamine hydrochloride in a concentration ranging from 3 mg / ml to 4 mg / ml to a Tris buffer solution having concentration ranging from 0.5 M to 1 M. The Tris buffer solution comprises of a Tris base (Tri s(hydroxymethyl)aminom ethane), acid (concentrated or ~l-6 M for adjusting pH) and distilled water. Preferably, the pH of the solution is maintained in a range from 8 to 9 using a concentrated acid such as hydrochloric acid, sulfuric acid, nitric acid (HNO3), etc.PT / 2026 / 11456

[0059] In step b), the zinc substrate is dipped or immersed in the polydopamine solution having concentration in a range from 3 to 4 mg / L. The dipping or immersing is done vertically or horizontally. The zinc substrate is selected from a zinc material, zinc foil, zinc foam, zinc plate and zinc mesh. The step b) is carried out at a temperature in a range from 25 to 35 °C for a time period in a range from 2 hours to 8 hours.

[0060] In step c), preferably the UV radiation is carried out using a UV chamber of wavelength 315 to 400 nm. Preferably, before the step of UV radiation the polydopamine coated zinc electrode material is rinsed and washed with water and dried at temperature in a range from 50-70°C for time period in a range from 5 hours-7 hours.

[0061] In a preferred embodiment, the present invention provides a process for preparation of the electrode of the present invention, the process comprising the steps of:

[0062] a) forming a polydopamine (PDA) solution;

[0063] b) coating or depositing polydopamine onto a zinc substrate by dipping or immersing the substrate in the polydopamine solution at a temperature in a range from 25-35 °C for time period in a range from 2 to 8 h to obtain crude polydopamine coated electrode material;

[0064] c) rinsing and washing the crude polydopamine coated electrode material of step b) with water followed by drying at temperature in a range from 50-70°C for time period in a range from 5- 7 h to obtain polydopamine coated electrode material; and

[0065] d) irradiating the polydopamine coated electrode material of step c) by UV irradiation having wavelength in a range from 315 to 400 nm to obtain the electrode.

[0066] In another embodiment, the present invention provides a three-electrode system comprising:

[0067] i. a working electrode as the electrode of the present invention,

[0068] ii. a reference electrode,

[0069] iii. a counter electrode,

[0070] iv. a power supply / Potentiostat, and

[0071] v. an electrolyte solution;PT / 2026 / 11456

[0072] The reference electrode is selected from but not limited to silver (Ag) / silver chloride (AgCl) electrode, mercury (Hg) / mercury oxide (HgO), calomel electrode (Hg2C12) and mercury-mercurous sulfate electrode (Hg / HgSO4), and so on. The counter electrode is selected from but not limited to platinum mesh, platinum wire, graphite rod and so on. The electrolyte solution is selected from but not limited to potassium hydroxide solution having concentration in the range of 4-8 M, anion conducting polymer electrolyte membrane, hydrophilic polymer gel selected from polyvinyl alcohol, polyacrylic acid, polyethylene oxide and so on.

[0073] In another embodiment, the present invention provides a battery comprising:

[0074] i. an anode, wherein the anode is the electrode of the present invention;

[0075] ii. an air cathode, and

[0076] iii. an electrolyte solution,

[0077] Preferably, the battery is a zinc battery, more preferably a zinc air battery (ZAB). The air cathode is selected from but not limited to Pt / C-RuCh (platinum over carbon and ruthenium oxide) coated over gas diffusion layer (GDL), or any bifunctional electrocatalyst capable of ORR and OER. The electrolyte solution is selected from but not limited to combination of potassium hydroxide having concentration in the range of 4-8 M and zinc acetate having concentration in the range of 0.1 -0.3 M, or an anion conducting polymer electrolyte membrane.

[0078] During the discharge phase, the anode of the zinc-air battery undergoes oxidation to form a zincate species which subsequently converts to zinc oxide (Zn —> [Zn(OH)4]2' (zincate) —> ZnO), and during charging, the reverse reaction happens. Hence, during the charging process, ZnO which in close vicinity of the Zn anode will be reduced and plated over zinc. Usually, during discharge, the zincate groups formed tend to move away from the anodic chamber to the cathodic chamber, known as zincate crossover. This process not only results in ZnO deposition at the cathode but also reduces the ZnO concentration near the anode, thus reducing the recyclability of the Zn anode. Hence, the PDA coated electrode of the present invention which is UV modified (Zn@PDA-UV) when employed as an anode, helps in adhering the zincate groups to the anodic chamber due to the zincophilic nature of PDA, thereby enhancing recyclability.PT / 2026 / 11456

[0079] The zinc battery comprising the electrode of the present invention exhibits prolonged platingstripping cycles, lasting at least or over 50 h with constant polarization voltage (refer, Figure 7a), compared to Bare Zn, which showed plating-stripping behavior for only 35 h with an increase in polarization voltage before ceasing due to excessive pressure caused by the HER side reaction (refer, Figure 7b). Further, the zinc battery comprising the electrode of the present invention exhibits a superior peak power density even though the initial OCV was lower due to the enhanced kinetics of Zn ion transport, as depicted from the LSV plot. The zinc battery comprising the electrode of the present invention exhibits a lower bulk resistance (5.3 Q) than with bare Zn anode (6.7 Q), and demonstrates a superior specific capacity of at least 56 h and 800 mAh g'1when compared to the bare Zn having only 42 h and 683 mAh g’1, thus confirming that the electrode of the present invention when employed as an anode inhibits the Zn loss by electrolyte corrosion, facilitating an active surface protective layer (Figure 9c, 9d).

[0080] Additionally, the zinc battery comprising the electrode of the present invention exhibits a uniform plating stripping behaviour exhibiting suppressed dendrite formation (Figure 11), and demonstrates cycling stability beyond 75 h with a constant charge-discharge over potential (initial overpotential of 0.61 V and a final of 0.77 V) in comparison to the Bare Zn ZAB, which showed cyclic stability for a shorter duration of 55 h with a gradual increase in overpotential after 50 h (initial of 0.7 V and a final of 1.24 V). The symmetrical Zn@PDA-UV coin cell comprising the electrode of the present invention achieves a superior stability of 50 h at a current density of 1 mA cm'2.

[0081] EXAMPLES:

[0082] Materials Required: Zn foil was procured from global nanotech; dopamine hydrochloride, tris buffer, potassium hydroxide pellets and RuCh from sigma Aldrich; 40% Pt / C was procured from alfa-acer; and gas diffusion layer from SGL.

[0083] Example 1: Preparation of the electrode of the present invention:

[0084] Step A: Preparation of polymer PDA coating onto zinc anode (Zn@PDA):

[0085] Zn@PDA was prepared by simple self-polymerization of dopamine at room temperature. 30 milligrams of dopamine hydrochloride was added to 30 mL Tris buffer solution (pH adjusted to 8 with concentrated hydrochloric acid). Then, zinc foils of 300 pm were placed vertically inPT / 2026 / 11456

[0086] the mixed solution and reacted for 4 hours. The polydopamine (PDA) coated electrodes were rinsed with deionized water and dried at 60 °C. Different samples were prepared by controlling the PDA deposition times (2, 4, 6, and 8 h).

[0087] Step B: Preparation of final modified anode by UV irradiation: Zn@PDA-UV

[0088] The Zn@PDA electrodes were subjected to UV irradiation of 315 to 400 nm wavelength to obtain Zn@PDA-UV (Zn@PDA-UV-4) electrodes. Similarly, Zn@PDA-UV-2 and Zn@PDA-UV-6 control samples were prepared. The polydopamine coating time was 2 h in case of Zn@PDA-UV-2 and 6 h in Zn@PDA-UV-6. The final electrode had a PDA coating time of 4 h Zn@PDA-UV-4.

[0089] Characterization Data:

[0090] Surface modification of the prepared electrodes was investigated using the morphology analysis of bare Zn foil and the electrode of Example 1 using Field Emission Scanning Electron Microscopy (FESEM). The FESEM images of the bare Zn show a smoother Zn surface (Figure la). Figure lb depicts the coating of PDA over zinc which is not UV modified. As shown in the Figure, the surface of Zn@PDA clearly showed a cataphracted coating layer, implying a larger surface area and porous morphology specific to that of the polydopamine structure, confirming the electroless deposition of PDA over the Zn surface This coated layer enables the formation of a uniform Zn2+flux on the Zn surface. However, PDA tends to aggregate and disintegrate from the substrate under harsh conditions like higher alkaline concentrations. Hence, the secondary modification of PDA by ultraviolet (UV) treatment as per the present inveniton enhanced the mechanical and chemical stability of PDA in harsh alkaline environments created by the 6 M KOH when used as the electrolyte in ZABs. The UV treatment of Zn@PDA facilitates crosslinking within the PDA, enhancing its rigidity and resistance to alkaline conditions. Figure 1c shows the FESEM images of the electrode of Example 1 (Zn@PDA-UV), which depicts a thicker and smoother polymer coverage over the Zn foil. This enhancement is attributed to the UV treatment, which promotes a more ordered surface structure or compact coating, resulting in improved smoothness. The cross-sectional electron microscopic analysis of the Zn@PDA-UV electrode of Example 1 confirms a PDA coating thickness of around E5- 2.0 pm, as shown in Figures Id and e. To confirm the electroless coating process, a Zn foil was partially coated with PDA, while the remaining portion was masked during the coating process, followed by UV modification. The cross-sectional ESEMPT / 2026 / 11456

[0091] image showed that the one-half of the foil exhibits a smooth morphology corresponding to bare Zn, whereas the other half showed a globular morphology characteristic of the PDA coating, thereby confirming the successful deposition of the PDA layer (Figure If).

[0092] The topography images obtained through the Atomic Force Microscopy (AFM) of the bare Zn and Zn@PDA-UV were compared. While both pictures show smooth surfaces, the 3D AFM analysis confirmed that the PDA coating maintained a uniform height throughout, in contrast to the sharp crests and troughs observed on the bare Zn. Additionally, it was observed that the electrode of Example 1 comprising the PDA coating and UV modification slightly altered the hydrophilicity of the Zn anode, as shown in Figures 2a and b. The variation in surface property of the electrode of Example 1 after the UV modification is to be adequately examined as their changes often help in fine-tuning the final output in the device-level demonstration. Hydrophilicity or hydrophobicity is one of the critical factors which determines the performance of the anode. Increased hydrophobicity offers advantages such as corrosion protection, suppression of dendrite formation, and improved cycling stability of the electrode. However, it has certain detrimental effects, as hydrophobic electrodes often impact discharge performance, as effective electrolyte penetration and Zn utilization are hampered. Furthermore, excessive hydrophobicity leads to reduced electrochemical reaction rates, leading to deteriorating battery performance. Therefore, an accurate tuning of electrode properties is essential to effectively balance corrosion protection and electrochemical functionality. Hence, a thin hydrophobic layer ensuring ion transport while still providing protection would be an effective solution for battery anodes. Figures 2c and 2d depict the hydrophilicity of the bare zinc electrode (Figure 2c) and the PDA coated electrode (Figure 2d) . As observed from the said figures, the contact angle of the bare Zn anode was 92°, thus demonstrating borderline hydrophilicity, and upon a thin PDA coating, a slight increase occurred in the contact angle (116°), making it a bit more hydrophobic (Figure 2d). Whereas the electrode of Example 1 comprising the UV modified PDA coating, showed a further decrease in the contact angle to 106° (Figure 2e), thus indicating an effectively tuned protective layer. This decrease in contact angle (increase in hydrophilicity) in the electrode of the present invention is due to an increase in surface polarity due to the formation of functional groups like hydroxyl or carbonyl groups.

[0093] Figure 3a depicts the X-ray diffraction (XRD) analysis of the bare zinc, the PDA coated zinc and the electrode of Example 1 which confirms the surface coverage over the Zn foil. The XRD patterns of bare Zn foil exhibit diffraction peaks at 29 values of 36.2°, 38.9°, 43.2°, 54.3°, 70.0°,PT / 2026 / 11456

[0094] 70.6°, and 77.0° corresponding to the (002), (100), (101), (102), (103), (110), and (004) planes of Zn metal as listed in the JCPDS file No.00-0004-0831. After PDA coating, the intensity of the Zn metal peaks diminishes due to the polymer layer covering the Zn metal surface. Whereas the electrode of the present invention further enhances the crosslinking within the PDA layer, resulting even greater reduction in the intensity of the Zn metal peaks compared to the Zn@PDA counterpart.

[0095] X-ray photoelectron spectroscopy was used to confirm the PDA coating over Zn foil. The N Is spectra of Bare Zn, Zn@PDA and Zn@PDA-UV of Example 1 were obtained. The N peak was absent in N Is spectra of Bare Zn, while the peak appeared for Zn@PDA at 399.88 eV and was retained in Zn @PDA-UV (399.48 eV), confirming the coating of Zn foil with PDA (Figure 3b). The Zn 2p spectra obtained for Bare Zn, Zn@PDA and Zn@PDA-UV of Example 1 as shown in Figure 3c depicted a shift towards higher binding energy, the Zn 2p 3 / 2 peak appearing at 1021.9 to 1022.2 eV in Zn@PDA and 1022.4 eV in Zn@PDA-UV, confirmed the coordination of Zn with catechol oxygen of PDA upon coating of the Zn with PDA layer. A resultant shift was also observed in the O IS spectra, where a change to lower binding energy was observed from bare Zn (532.26 eV) to Zn@PDA (532.08 eV) to Zn @PDA-UV (532.0 eV).

[0096] Additionally, the deconvoluted Zn 2p 3 / 2 spectra provided information regarding the relative proportions of Zn-Zn and Zn-0 peaks, which indicated the intercalation of catechol groups with Zn, thereby conforming the deposition of PDA over the Zn foil. Additionally, the secondary modification of PDA via UV treatment is corroborated by the O Is spectra (Figure 3d) as the intensity of the O Is spectra increases after UV treatment. This enhancement in the O ls spectra indicates further oxidation of PDA, which promotes the formation of additional oxygen-containing functional groups. However, the O ls spectra showed a slight shift towards higher binding energy, contrary to the shift expected during the conversion of hydroxyl to quinone. This is because the quinones formed during UV treatment react with other amine and catechol groups, resulting in covalent crosslinking, thus altering the electronic environment of oxygen atoms in the PDA structure, contributing to an overall increase in binding energy for the O Is electrons.

[0097] To confirm the oxidation and cross-linking of polydopamine upon UV treatment, the deconvoluted C Is, N Is and O ls spectra are compared. The C Is of Zn@PDA-UV spectra displayed an increased intensity in the C=O peak and a corresponding decrease in the C-O / C-PT / 2026 / 11456

[0098] N bond compared to the Zn@PDA anode, thus confirming the oxidation of catechol groups in polydopamine after UV irradiation. Similarly, The N Is spectra of Zn@PDA-UV shows a significant decrease in the peak corresponding to the primary amine group and a subsequent decrease in secondary and tertiary amine groups, hence substantiating the cross-linking involving primary amine groups. Moreover, deconvoluted O Is spectra also confirmed the increased intensity of C=O peaks, thereby confirming the oxidation of catechol groups after UV modification.

[0099] The effect of PDA as a protective layer in controlling self-corrosion was further investigated by monitoring the self-corrosion of bare Zn, Zn@PDA and Zn@PDA-UV of Example 1 after dipping in 6 M KOH for 24 h. A comparison of FESEM images obtained before and after selfcorrosion was studied as depicted in Figure 4a to 4c, which clearly showed that the bare Zn after 24 h in 6 M KOH demonstrated corrosion of Zn with passivation layer formation over the Zn foil (Figure 4a). In contrast, the corrosion was suppressed in Zn@PDA. However, as the PDA layer disintegrates and comes out of the Zn foil in an alkaline electrolyte, Zn@PDA still undergoes self-corrosion to an extent, but less than the bare Zn (Figure 4b). On the other hand, the electrode of the present invention comprising of UV modified PDA coating showed the lowest extent of self-corrosion compared to the control samples (Figure 4c) thereby indicating the enhanced chemical stability of PDA in alkaline electrolytes.

[0100] The XRD analysis of the Zn electrodes before and after self-corrosion was further analyzed to support the FESEM data as depicted in Figures 4d to 4f. The intensity of Zn peaks of the Bare Zn and Zn@PDA diminished after undergoing self-corrosion, implying the formation of a passivation layer over the Zn foil (Figure 4 d, e, and f). On the other hand, the lowering of the intensity is less pronounced in Zn@PDA-UV owing to its corrosion and passivation resistance, which is attributed to the stabilized protective PDA layer over the Zn foil attributed to UV modification.

[0101] Further, the corrosion resistance and the hydrogen evolution reaction (HER) suppression ability of the stabilized PDA coating was investigated through linear polarization resistance (LPR) studies and HER LSV plots in a three-electrode setup in 6 M KOH electrolyte using Ag / AgCl as reference electrode and Zn foil, Zn@PDA and the electrode of Example 1 as working electrode. The HER LSV plots showed that the Zn foil, after protection with PDA and secondary stabilization, was less prone to HER side reactions compared to the Zn@PDA and Zn foil (Figure 5a). This is mainly because the stabilized PDA over the Zn foil acts as a stablePT / 2026 / 11456

[0102] physical barrier between Zn and alkaline electrolytes, suppressing the HER activity. The hydrophobicity of the coated electrode rather than the bare Zn foil also aids in suppressing HER. However, for the Zn@PDA sample, the PDA layer undergoes disintegration and agglomeration upon contact with the alkaline electrolyte, leading to its detachment from the Zn surface. Consequently, the layer fails to suppress the HER and protect the Zn foil. The corrosion studies of the bare Zn, PDA-coated Zn (Zn@PDA) and UV stabilized PDA-coated Zn (Zn@PDA-UV) of Example 1 depicted that the Zn@PDA-UV had the highest corrosion potential (-1.5 V vs. Ag / AgCl) compared to Zn@PDA (-1.534 V vs. Ag / AgCl) and bare Zn (-1.537 V vs. Ag / AgCl), inferring the better corrosion resistance of Zn@PDA-UV (Figure 5b).

[0103] The chronoamperogram for the Zn plating -1.8 V for 2000 s was further recorded for the Zn, PDA-coated Zn (Zn@PDA) and the electrode of Example 1 as depicted in Figures 5c and 5d. As observed, the bare zinc and Zn@PDA showed a gradual rise in current density, indicating a rapid increase in surface area with Zn plating, thus pointing towards dendritic Zn growth (Figure 5c and 5d). Whereas Zn@PDA-UV showed suppression in the current density, indicating suppressed kinetics for Zn electrodeposition. Additionally, current density showed an almost constant behaviour with a slight increase, owing to the suppressed dendrite formation.

[0104] Further, the effect of Zn plating and the stripping behaviour of Bare Zn, Zn@PDA-UV-4 and Zn@PDA-UV-6 (In Zn@PDA-UV-4 the polydopamine coating time was 4 h and in case of Zn@PDA-UV-6 polydopamine coating time was 6 h) were investigated using a symmetric cell at a current density of 0.5 mA cm'1and a specific capacity of 1 mAh cm'2as depicted in Figure 6. The symmetric cell with Zn@PDA-UV-4 exhibited a prolonged plating-stripping cycles, lasting over 50 h with constant polarization voltage (Figure 6a), compared to Bare Zn, which showed plating-stripping behaviour for only 35 h with an increase in polarization voltage before ceasing due to excessive pressure caused by the HER side reaction (Figure 6b). The superior performance of Zn@PDA-UV-4 is attributed to suppressed HER and directed Zn deposition, which minimized the dendrite formation (Figure 6c). In contrast, Zn@PDA-UV-6 demonstrated stable plating-stripping for less than 10 h due to significant HER activity during symmetric cell operation. As expected, the protected Zn anode exhibits superior cycle life to the bare Zn anode, indicating that direct contact of bare Zn anodes with strong alkaline electrolytes and the HER side reaction occurring at the Zn surface in alkaline electrolyte are the major contributors to poor reversibility.PT / 2026 / 11456

[0105] To visualize the surface morphology evolution during the plating stripping, ESEM images of the Zn electrodes were obtained after the plating stripping completion. The FESEM images of Zn@PDA-UV-4 showed an almost uniform Zn deposition with an ameliorated ZnO deposition. At the same time, the bare Zn displayed an irregular Zn deposition leading to dendrite formation along with the appearance of nanorods ZnO deposits, thus indicating incomplete Zn <4> ZnO reversibility (Figure 6 d, e, and f). The hydrophobic nature of the coating layer of the Zn@PDA-UV electrode of Example 1 regulates the Zn dissolution during the discharge thus modulating the ZnO formation and controlling the reversibility. However, in bare Zn, gradual Zn dissolution happens during the discharge, leading to increased ZnO formation at the anode, hampering the reversibility and, thereby, the lifetime of the battery, attributed; to the active Zn loss from the anode surface.

[0106] Example 2: Zinc air battery comprising the electrode of Example 1:

[0107] To demonstrate the practical application of the electrode of Example 1 as anode, ZABs were prepared using bare Zn, Zn@PDA and Zn@PDA-UV of Example 1 as an anode, Pt / C-RuO2 coated over gas diffusion layer (GDL) as air cathode and 6 M KOH + 0.2 M Zn(OAc)2 as electrolyte. These devices demonstrated an open circuit voltage (OCV) of 1.37, 1.27 and 1.28 V, respectively. A decrement in the OCV is due to the PDA coverage over the Zn anode during the preparation of the electrode. The polarization plot obtained exhibited a peak power density of 98.4 mW cm'2(bare zinc) and 110 mW cm'2(Zn@PDA-UV of the present invention) (Figure 7a). The electrode of the present invention as anode showed a superior peak power density than bare zinc, even though the initial OCV was lower due to the enhanced kinetics of Zn ion transport, as depicted from the LSV plot (Figure 7 a). The impedance spectrum obtained at the OCV conditions for bare zinc and Zn@PDA-UV electrode of Example 1 were further compared as demonstrated in Figure 7b. Upon comparison, the ZAB with Zn@PDA-UV-4 anode depicted a lower bulk resistance (5.3 Q) than with bare Zn anode (6.7 Q). Reduced bulk resistance facilitates efficient electron transport, thereby minimizing ohmic losses and enhancing power density. However, the Zn@PDA-UV-4 electrode exhibited a higher charge transfer resistance due to a protective PDA layer (Figure 7b). Further, Figures 7c and 7d depicts the galvanostatic discharge plots recorded at 5 mA cm'2and the corresponding galvanostatic discharge capacities of the ZABs comprising the electrode of the present invention and bare zinc after normalizing with the consumed amount of the Zn during the discharge process at 5 mA cm'2galvanostatic discharge plot. The discharge plot obtained at aPT / 2026 / 11456

[0108] current density of 5 mA cm'2depicted an improved discharge time and, hence, a superior specific capacity for Zn@PDA-UV of the present invention (56 h and 800 mAh g'1) as compared to the bare Zn (42 h and 683 mAh g'1) thus confirming that the electrode of the present invention when employed as an anode is successful in inhibiting the Zn loss by electrolyte corrosion, facilitating an active surface protective layer.

[0109] Dendrite suppression enabled by the protective coverage of Zn foil with PDA coating and the UV modification could be visualized by cycling the ZAB for 12 charge-discharge cycles and recording the ESEM images after cycling as depicted in Figure 8a. Zn passivation and dendrite formation were further confirmed from the XRD analysis (Figure 8b), which displayed an enhanced ZnO deposition after 12 cycles over bare Zn anode. In contrast, the Zn@PDA-UV anode displayed a suppressed intensity for ZnO. Figure 9 depicts the FESEM images of a) bare Zn anode surface and b) Zn@PDA-UV surface of the present invention after cycling for 12 charge-discharge cycles. The ESEM images of bare Zn clearly showed spikes of dendrite formation over the Zn anode whereas, the electrode of the present invention, displayed a more uniform plating stripping behaviour with 12 cycles thereby indicating suppressed dendrite formation (Figure 9).

[0110] Long-term charge-discharge cycling of the fabricated batteries was conducted for bare zinc and the Zn@PDA-UV electrode of the present invention as depicted in Figure 9c. The electrode of the present invention showed superior cycling stability beyond 75 h with a constant chargedischarge over potential (initial overpotential of 0.61 V and a final of 0.77 V) while the bare Zn ZAB showed cyclic stability for a shorter duration of 55 h with a gradual increase in overpotential after 50 h (initial of 0.7 V and a final of 1.24 V). The initial and final voltage efficiency for both devices were also calculated and tabulated as presented in Table 1 below.

[0111] Table 1: Comparison table for initial voltage, final voltage and voltage efficiency for ZAB utilizing bare Zn and Zn@PDA-UV of Example 1 as anode.PT / 2026 / 11456

[0112] Parameters Bare Zn Zn@PDA-l'V

[0113] Initial voltage Discharge 1.26 1.28

[0114] (V) Charge 1.96 1.89

[0115] Final voltage Discharge 1.06 1.12

[0116] tv) Charge 2.3 1.89

[0117] Voltage Initial 64 67

[0118] efficiency (%) Filial 46 60

[0119]

[0120] The Zn@PDA-UV device of the present invention depicted an initial voltage of 67 %, which dropped to 60 % at the end of cycling, while the bare Zn had a comparable initial efficiency of 64 %, which gradually declined to 46 % at the end of cycling. The stable charge-discharge feature and superior voltage efficiency retention exhibited by the electrode of the present invention can be directly attributed to the protective effect of the PDA coating and UV modification, thereby suppressing HER, and exhibiting uniform plating stripping behaviour by the controlled Zn deposition.

[0121] To investigate the failure mechanism of ZABs, continuous impedance analysis was conducted where the impedance of the device was recorded at different time intervals of charging and discharging as depicted in Figure 10a to d. During the cycling of the ZAB with bare Zn anode, the bulk resistance value increased gradually after cycling for 100 cycles and shot after 300 cycles, due to the blocked transport of electrons and ions by ZnO accumulation, coinciding with the long cycling results hence clearly depicting the gradually development of ZnO passivation layer over the anode cycle while cycling the bare Zn in the alkaline electrolyte (Figure 10a and 10b). The ZAB with Zn@PDA-UV-4 anode as per the present invention (Figure 10c and lOd) displayed an almost constant impedance plot even after cycling for 200 cycles, and showed only a slight increase in impedance after 450 cycles, i.e., towards the end of the cycling due to partial ZnO production confirming the screening effect of the UV modified PDA layer thereby clearly articulating that ZnO passivation was effectively suppressed by the electrode of the present invention when employed as an anode.

[0122] Figure 11 shows (a, b) impedance analysis of ZAB with bareZn as anode; and (c, d) Zn@PDA-UV anode obtained after different cycles. A comparison of the obtained plots clearly shows that, even though the battery with bare Zn anode depicted an initial lower impedance the increase in resistance with cycling increases gradually as a result of ZnO formation that occursPT / 2026 / 11456

[0123] at the anode during cycling, while in case of Zn@PDA-UV battery of the present invention a slow rise in impedance with cycling was observed owing to the suppressed byproduct formation.

[0124] Figure 12 depicts a three-electrode system containing the electrode of the present invention as working electrode and Figure 13 depicts the zinc air battery containing the electrode of the present invention as an anode.

[0125] ADVANTAGES OF THE INVENTION:

[0126] • The electrode of the present invention when employed as an anode) for zinc air battery reduces self-discharge, improves kinetics, and electron transport, stabilizes zinc electrodeposition, and reduces HER reaction.

[0127] • The electrode of the present invention provides an increased cycle life, increased stability, decreased dendrite formation, decreased corrosion, and decreased passivation, in zinc air batteries.

Claims

PT / 2026 / 11456WE CLAIM1. An electrode comprising UV modified polydopamine (PDA) coated zinc.

2. The electrode as claimed in claim 1, wherein the UV modified PDA coating has a thickness of at least 1.5 pm.

3. A process for preparation of an electrode as claimed in claim 1, the process comprises:a) forming a polydopamine (PDA) solution;b) coating or depositing the polydopamine solution onto a zinc substrate by dipping or immersing the substrate in the polydopamine solution to obtain a polydopamine coated zinc electrode material; andc) irradiating the polydopamine coated zinc electrode material with ultraviolet (UV) radiation having a wavelength in the range of 315-400 nm to obtain the electrode.

4. The process as claimed in claim 4, wherein the polydopamine solution is formed by adding dopamine hydrochloride to a Tris buffer solution.

5. The process as claimed in claim 4, wherein the zinc substrate is dipped or immersed in the polydopamine solution having concentration in a range from 3 to 4 mg / L6. The process as claimed in claim 4, wherein the step b) is carried out at a temperature in a range from 25 to 35 °C.

7. A three-electrode electrochemical system comprising:i) a working electrode as the electrode as claimed in claim 1;ii) a reference electrode;iii) a counter electrode;iv) a power supply / Potentiostat; andv) an electrolyte solution.

8. A battery, comprising:PT / 2026 / 11456i) an anode, wherein the anode is the electrode as claimed in claim 1;ii) an air cathode; andiii) an electrolyte solution,9. The battery as claimed in claim 8, wherein the electrolyte is selected from potassium hydroxide, zinc acetate or mixtures thereof.

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