WO2024003930A1

WO2024003930A1 - An electrolyzer system with nonprecious electrocatalysts for green h2 production by electrolysis of water

Info

Publication number: WO2024003930A1
Application number: PCT/IN2023/050392
Authority: WO
Inventors: S Ramaprabhu; Anamika GHOSH; Dipsikha GANGULY
Original assignee: Indian Institute Of Technology Madras
Priority date: 2022-06-30
Filing date: 2023-04-21
Publication date: 2024-01-04

Abstract

The present invention discloses an electrolyzer system comprising an anode catalyst layer; a cathode catalyst layer; and a catalytic support for the anode catalyst layer and cathode catalyst layer, characterized in that the anode catalyst layer comprises Prussian blue analogue (PBA) and nitrogen doped carbon nanotube (NCNT or CNT) composite, the cathode catalyst layer comprises Metallic nickel particles encapsulated inside nitrogen-doped carbon tubules (Ni/NCT). The present invention also discloses a method for developing electrolyzer system.

Description

AN ELECTROLYZER SYSTEM WITH NONPRECIOUS ELECTROCATALYSTS FOR GREEN H₂ PRODUCTION BY ELECTROLYSIS OF WATER

FIELD OF INVENTION

The present invention relates to the development of an electrolyzer system comprising an anode catalyst layer, a cathode catalyst layer and a support for the anode catalyst layer and cathode catalyst layer for seawater electrolysis.

The present invention also relates to the development of the non-precious, cost-effective, and straightforward synthesis of cathode and anode catalysts for seawater electrolysis.

BACKGROUND OF THE INVENTION

Hydrogen (H2) is a green, renewable, and storable fuel and storing hydrogen can help combat climate change and reach zero-emission. Besides, hydrogen can be easily distributed in households, transport and industry, and the hydrogen economy is expected to grow by 33% to US$155 billion in 2022.

Currently, much research is focused on the cost-effective production of H2. Water splitting driven by electricity or solar power is one of the most promising approaches for clean H2 production. Further, most state-of-the-art electrolyzers uses ultrapure water in acidic polymer electrolyte membrane (PEM) electrolyzer or alkaline electrolyzers.

At the same time, water distribution issues may arise if pure or fresh waters are used for large scale electrolyzers. Being an abundant source, seawater which comprises 96.5% of the water resources of the world, can solve the issues related to large scale production.

Noble catalysts such as platinum (Pt), ruthenium oxide (RuCh), iridium oxide (IrCh) are currently the state of art catalysts. However, the high-cost factor and abundance still limit their application.

Therefore, research is focused on designing robust and non-precious catalysts for water splitting. Although many studies are done on freshwater oxidation, a few reports show overall water splitting from low-grade or seawater.

Few reports show H2 production from the seawater electrolysis but most of them are based on nitrides, phosphides, sulfides, or selenides. Most of the non-precious catalysts which exhibits good catalytic activity and stability in treated or untreated seawater are grown on nickel foam and involves multiple steps of synthesis. Nickel foam itself has catalytic activity towards Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER).

Therefore, it is necessary to use some cost-effective substrate as catalyst support to evaluate the actual catalytic activity of the catalyst. Additionally, a scalable and simple synthesis approach is highly desirable.

Yu et al (L. Yu, Q. Zhu, S. Song, B. McElhenny, D. Wang, C. Wu, Z. Qin, J. Bao, Y. Yu, S. Chen, Z. Ren, Non-noble metal-nitride based electrocatalysts for high-performance alkaline seawater electrolysis, Nat. Commun. 10 (2019) 1-10. https://doi.org/10.1038/s41467-019-13092-7) designed 3D core-shell type NiFeN/NiMoN /nickel foam anode and NiMON/nickel foam cathode. Their cathode and anode assembly show overall seawater (IM KOH + seawater) splitting potential of 1.625V with a current density of lOOmA/cm² The electrolyzer shows the stability of 100 h. The same group coupled NiCoN/NixP/NiCoN anode with S-(Ni, Fe) OOH cathode and observed overall natural seawater splitting voltage of 1.81V with a current density of lOmA/cm²

Kuang et al. (Y. Kuang, M. J. Kenney, Y. Meng, W.H. Hung, Y. Liu, J.E. Huang, R. Prasanna, P. Li, Y. Li, L. Wang, M.C. Lin, M.D. McGehee, X. Sun, H. Dai, Solar-driven, highly sustained splitting of seawater into hydrogen and oxygen fuels, Proc. Natl. Acad. Sci. U. S. A. 116 (2019) 6624-6629. https://doi.org/10.1073/pnas.1900556116) reported overall seawater electrolysis (IM KOH + natural seawater) potential of 2.12V at a current density 400mA/cm² using NiFe/NiS_x/nickel foam as anode and Ni-NiO-CnOi as a cathode. Their developed catalyst shows the stability of 1000 h. Their cell runs for 24h with a 50mV deviation in the applied potential. Apart from the nitrides, phosphides, sulfides, and selenides, Prussian blue analogues (PBA) are emerging materials for electrolysis. PBAs are the subcategory of Metal Organic Framework (MOFs) due to their metallic centers and extended 3D network.

Two significant advantages of the PBA are a) simple and scalable synthesis and b) the presence of the earth-abundant transition metals gives comparable electrocatalytic activity to the current state of art catalyst.

Li Quan and the group (L. Quan, S. Li, Z. Zhao, J. Liu, Y. Ran, J. Cui, W. Lin, X. Yu, L. Wang, Y. Zhang, J. Ye, Hierarchically Assembling CoFe Prussian Blue Analogue Nanocubes on CoP Nanosheets as Highly Efficient Electrocatalysts for Overall Water Splitting, Small Methods. 5 (2021) 1-10. https://doi.org/10.1002/smtd.202100125.) demonstrated overall water spilling (1MK0H) at 1.570V using CoFe/CoP/nickel foam as anode and cathode.

Ma F. et al. (F. Ma, Q. Wu, M. Liu, L. Zheng, F. Tong, Z. Wang, P. Wang, Y. Liu, H. Cheng, Y. Dai, Z. Zheng, Y. Fan, B. Huang, Surface Fluorination Engineering of NiFe Prussian Blue Analogue Derivatives for Highly Efficient Oxygen Evolution Reaction, ACS Appl. Mater. Interfaces. 13 (2021) 5142-5152. https://doi.org/10.1021/acsami.0c20886) showed overall water splitting electrolyzer (IM KOH) with NiFe-F/nickel foam and Mo2N/CeO2 as anode and cathode with a water splitting potential of 1.49V for a current density of lOmA/cm² PBA derived bimetallic (Ni, Co) Se2 nanocages on graphene aerogel decorated nickel foam as both anode and cathode shows water splitting potential of 1.60V at lOmA/cm² in IM KOH solution.

Bimetallic NiCo core with an amorphous NiCoPBA shell/nickel foam showed 13.4mA/cm² at a cell voltage 1.6V in IM KOH solution.

Franziska and the group (N. Lopez, J.R. Galan-Mascaros, F.S. Hegner, F.A. Garces-Pineda, J. Gonzalez-Cobos, B. Rodriguez-Garcia, M. Torrens, E. Palomares, Understanding the catalytic selectivity of cobalt hexacyanoferrate toward oxygen evolution in seawater electrolysis, ACS Catal. 11 (2021) 13140-13148. https://doi.org/10.1021/acscatal.lc03502.) reported H2 and O2 production from untreated seawater using cobalt hexacyanoferrate (CoFe/PB) as anode and carbon felt cube as the cathode. The cell maintained a current density of 0.7mA/cm² for 24h at 2.7V.

One of the major issues associated with real seawater electrolysis is that seawater contains several salts that form precipitate on the cathode surface. The precipitate formation makes Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) very sluggish and achieved current density doesn’t reach the benchmark current density (lOmA/cm² according to the solar fuel conversion). Further, in KOH treated seawater, Mg (OH)2, Ca (OH)2 precipitate can form on the electrode surface during the electrolysis. The precipitates can block the electrochemically active surface sites and further cause degradation of the cell performance. To overcome the challenge, the development of cost-effective catalysts with enriched electrocatalytic active sites are needed, which should be highly electroactive and stable for the long duration of the cell operation.

OBJECTIVES OF THE INVENTION An objective of the present invention is to provide an electrolyzer system for green H2 production that shows enhanced stability and reduced Chlorine or Hypochlorite formation for both KOH treated seawater and untreated seawater.

Another objective of the present invention is to provide a method for developing electrolyzer system that shows enhanced stability and reduced Chlorine or Hypochlorite formation for both KOH treated seawater and untreated seawater.

SUMMARY OF THE INVENTION

In an aspect of the present invention, an electrolyzer system comprising an anode catalyst layer, a cathode catalyst layer, and a catalytic support for the anode catalyst layer and cathode catalyst layer, wherein the anode catalyst layer comprises Prussian blue analogue (PBA) and nitrogen doped carbon nanotube (NCNT or CNT) composite, the cathode catalyst layer comprises Metallic nickel particles encapsulated inside nitrogen-doped carbon tubules (Ni/NCT).

In another aspect of the present invention, catalytic support for the anode catalyst layer and cathode catalyst layer is selected from a group comprising of carbon material and stainless-steel mesh (SS mesh).

In yet another aspect of the present invention, in anode catalyst layer Prussian blue analogue (PBA) is selected from a group comprising of Cobalt hexacyanoferrate or Co-FePBA, Ni-FePBA, Co-Co PBA, Fe- Fe PBA, Ni-Ni PBA or among single or bimetallic transition metal-based PBA.

In still another aspect of the present invention, nickel particles are encapsulated inside the nitrogen- doped carbon tubules.

In another aspect of the present invention, carbon material is selected from carbon paper, carbon cloth, and porous conducting carbon support.

In yet another aspect of the present invention, electrolyzer system is Co-FePBA/NCNT/carbon paper||Ni/NCT/carbon paper.

In still another aspect of the present invention, the electrolyzer system act as an electrolyser to electrolyse the natural water resources. In yet another aspect of the present invention, wherein the natural water resources comprise of sea water, ground water, salty ground water, or from other natural water resources, preferably sea water.

In still another aspect of the present invention, sea water is selected from a group comprising of untreated sea water or sea water treated with KOH.

In yet another aspect of the present invention, untreated seawater exhibits splitting voltage of 2.68V at a current density of 10 mA/cm²

In still another aspect of the present invention, KOH treated seawater exhibits splitting voltage of 1.88V for a current density of 10 mA/cm².

In yet another aspect of the present invention, KOH treated seawater and untreated seawater exhibits stability from 24 h.

In still another aspect of the present invention, wherein selectivity of the system towards oxygen evolution reaction (OER) over chlorine evolution reaction (CER) is higher as chlorine evolution and hypochlorite formation is suppressed after long duration of electrolysis.

In yet another aspect of the present invention, invention provides the scalable and simple synthesis of cathode and anode for a full cell electrolyzer for sea water, ground water, salty ground water, or other natural water resources.

In another aspect of the present invention, the anode is based on the CoFe based PBA, i.e., cobalt hexacyanoferrate. Co-FePBA is mixed with nitrogen doped carbon nanotube (NCNT) and heated at different temperatures (300-400 °C) to obtain Co-FePBA/NCNT composite. Co-FePBA/NCNT anode is coupled with Ni/NCT cathode.

In yet another aspect of the present invention, Ni/NCT represents metallic nickel encapsulated inside a nitrogen doped carbon tubules. Ni/NCT was synthesized by heating mixture of melamine and nickel chloride (NiCh) in argon (Ar) atmosphere. The best anode cathode assembly (Co- FePBA/NCNT500||Ni/NCT) exhibits overall seawater splitting voltage of 1.88V for a current density of lOmA/cm² (solar fuel conversion). The said electrolyzer system shows overall seawater splitting voltage of 2.68V at a current density of lOmA/cm² in untreated seawater. In still another aspect of the present invention, carbon material was used as catalyst support instead of nickel foam to reduce cost factor and evaluate the better catalytic activity of both anode and cathode catalyst. The desirable performance of the catalysts indicates that the designed cathode and anode can be used in an alkaline electrolyzer using abundant natural water resources.

In yet another aspect of the present invention, besides, large scale H2 production is also possible by increasing the electrode and electrolyzer dimension.

In another aspect of the present invention, a method for developing electrolyzer system comprising the following steps is provided: a) Preparing the slurry for the anode, wherein the optimum amount of CoFe-PBA/NCNT is dispersed in organic solvent for suitable time period; b) Preparing the slurry for the cathode, wherein the optimum amount of Ni/NCT is dispersed in organic solvent for suitable time period; c) Coating the slurries of steps a) and b) on the carbon papers to form Co-FePBA/NCNT /carbon paper and Ni/NCT/carbon paper; and d) Placing the Co-FePBA/NCNT /carbon paper and Ni/NCT/carbon paper inside the glass cell with electrolyte and applying the positive potential to Co-FePBA (300-500/NCNT/carbon paper, and negative potential to Ni/NCT/carbon paper to form electrolyzer system.

In another aspect of the present invention, anode is prepared by steps comprising:

I. Mixing KiFe (CN)e, cobalt acetate and 2 methyl imidazole in organic solvent to produce PBA;

II. Synthesizing CNT by the Chemical Vapour Deposition (CVD) method; III. Heating the CNT of step II) with melamine to synthesize NCNT;

IV. Mechanically grounding PBA of step I) and NCNT of step III) or CNT of step II); and

V. Heating of mechanically grounded PBA of step IV) and nitrogen doped carbon nanotube (NCNT or CNT) of steps III) or II) to obtain anode Co-FePBA/NCNT or Co-FePBA/CNT.

In yet another aspect of the present invention, cathode is prepared by grounding the NiCh and melamine followed by heating for different times in Argon (Ar) atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 : represents X-ray crystallography (XRD) of Co-FePBA and Co-FePBA/NCNT (anode catalyst layer) (a), Ni/NCT (Cathode catalyst layer) (b).

Fig 2: represents Scanning Electron Microscope (SEM) images of Co-FePBA(a), NCNT(b), Co- FePBA/NCNT300(c), Co-FePBA/NCNT400(d), Co-FePBA/NCNT500(e), Ni/NCT(f).

Fig 3: represents the Linear Sweep Voltammetry (LSV) of three different electrolyzers (seawater+lM KOH). The dotted line shows the potential corresponds to the benchmark current density of lOmA/cm². Carbon paper is used as catalyst support.

Fig 4: represents 24 h chronopotentiometry study of three different electrolyzers (seawater + IM KOH). Carbon paper is used as catalyst support.

Fig 5: represents a Digital image for the chlorine test. The chlorine detection kit shows no colour change is visible after adding the reagent. It confirms the selectivity of the OER of the developed catalyst.

Fig 6: represents LSV (a) and chronopotentiometry (b) in untreated seawater.

Fig 7: represents electrolysis in untreated seawater using Co-FePBA/NCNT500||Ni/NCT. Salt precipitates at the cathode surface. Carbon paper is used as catalyst support. Fig 8: a) represents a digital photograph of lab-scale electrolysis in KOH treated seawater (40mL) using Co-FePBA/NCNT500||Ni/NCT, b) magnified image for OER and HER. Carbon paper is used as the catalyst support.

Fig 9: a) represents digital photograph of demonstration of the lab-made simple electrolyzer in KOH treated seawater (250 mL) using Co-FePBA/NCNT500||Ni/NCT, b) Produced O2 and H2 are collected in an inverted glass tube. Carbon paper/SS mesh (2x0.5cm²) was used as catalyst support.

Fig 10: represents LSV (a) and chronopotentiometry (b) for salted ground water.

Fig 11: represents LSV (a) and chronopotentiometry (b) for treated ground water.

Fig 12: represents digital photograph of demonstration of the lab-made simple electrolyzer for salty groundwater.

DETAIL DESCRIPTION OF THE INVENTION

The present invention shows an approach to design a full cell electrolyzer to split natural water resources with a simple and cost-effective cathode and anode assembly.

It is to be understood that the present disclosure is not limited in its application to the details of method set forth in the following description. The present disclosure is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

All publications and patents cited in this specification are herein incorporated as if each individual publication or patent were specifically and individually indicated to be incorporated and are incorporated herein to disclose and describe the methods and/or materials in connection with which the publications are cited. The citation of any publication is for its disclosure prior to the filing date and should not be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed. Each embodiment is provided by way of explanation of the invention and not by way of limitation of the invention. In fact, it will be apparent to person skilled in the art that various modifications and variations can be made to the methods described herein without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be applied to another embodiment to yield a still further embodiment. Therefore, it is intended that the present invention includes such modifications and variations and their equivalents. Other objects, features, and aspects of the present invention are disclosed in or are obvious from, the following detailed description. It is to be understood by person skilled in the art that the present discussion is a description of exemplary embodiments only and is not to be construed as limiting the broader aspects of the present invention.

An embodiment of the present invention discloses an electrolyzer system comprising an anode catalyst layer, a cathode catalyst layer, and a catalytic support for the anode catalyst layer and cathode catalyst layer, wherein the anode catalyst layer comprises Prussian blue analogue (PBA) and nitrogen doped carbon nanotube (NCNT or CNT) composite, the cathode catalyst layer comprises Metallic nickel particles encapsulated inside nitrogen-doped carbon tubules (Ni/NCT).

In another embodiment, the present disclosure provides Prussian blue analogue (PBA) i.e., Cobalt hexacyanoferrate or Co-FePBA in composite with NCNT(Co-FePBAZNCNT) is utilized as anode and Metallic nickel particles encapsulated inside nitrogen-doped carbon tubules (Ni/NCT) is used as cathode. The use of the costly nickel foam is avoided, which itself has catalytic activity for hydrogen evolution (HER) and oxygen evolution reaction (OER).

In still another embodiment of the present invention, carbon material was used as catalyst support instead of nickel foam for better estimation of catalytic activity and to reduce the cost factor.

In yet another embodiment of the present invention, best anode-cathode assembly of the present invention shows overall water splitting voltage of 1.88V in IM KOH treated seawater with a benchmark current density of lOmA/cm² required for a 12.8% efficiency solar cell driven electrolyzer.

In still another embodiment of the present invention, besides, the chlorine test reveals that no chlorine produces during the electrolysis, which depicts the selectivity of OER over chlorine evolution reaction (CER). In another embodiment, the present disclosure provides, a potential of 2.6V is required to achieve a current density of lOmA/cm² in untreated seawater. In present invention, anode-cathode assembly shows stability for 24h for both KOH treated seawater and untreated seawater. Apart from seawater, the cathode and anode assembly are also tested in KOH treated groundwater and salty groundwater (15% salt). A desirable performance is observed in both cases, indicating that the designed cathode and anode can be used in an alkaline electrolyzer using abundant natural water resources. This further opens the opportunity for the large-scale production of green H2 by electrolysis of any natural water resources by increasing the dimension of the electrode and electrolyzer.

In still another embodiment, carbon paper as catalyst support is used and developed Co- FePBA/NCNT as anode and Ni/NCT as cathode for electrolysis of abundant water resources such as seawater, ground water or salty ground water (15% salt).

In yet another embodiment, XRD, SEM are done to characterize the structural and morphological properties of the synthesized catalysts. Cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronopotentiometry (CP) were performed to evaluate the catalytic activity. Different anode and cathode assemblies were tested for seawater electrolysis. Among all, an electrolyzer system comprises of Co- FePBA/NCNT500 as anode and Ni/NCT as cathode (Co-FePBA/NCNT500||Ni/NCT) exhibits best performance with overall seawater (KOH treated seawater) splitting voltage of 1.88V at a benchmark current density 10 mA/cm² (current density achieved by 12.8% efficient solar cell). The electrolyzer system exhibits 24 h stability without formation of any chlorine species after long hours of operation. The electrolyzer system is also stable in untreated seawater and shows required 2.6V to achieve a current density of lOmA/cm². The higher potential in untreated seawater is attributed to precipitation of the salts around cathode and anode surface which blocks the electroactive area and makes the HER and OER more sluggish. However, long term stability (24h) in the harsh conditions indicates that presently developed catalyst can be used in treated or untreated seawater for green H2 production from the abundant water sources.

In another embodiment of the present invention, catalytic support for the anode catalyst layer and cathode catalyst layer is selected from a group comprising of carbon material and stainless-steel mesh (SS mesh). In yet another embodiment of the present invention, anode catalyst layer Prussian blue analogue (PBA) is selected from a group comprising of Cobalt hexacyanoferrate or Co-FePBA, Ni-FePBA, Co-Co PBA, Fe-Fe PBA, Ni-Ni PBA or among single or bimetallic transition metal-based PBA.

In still another embodiment of the present invention, carbon material is selected from carbon paper, carbon cloth, and porous conducting carbon support.

In yet another embodiment of the present invention, electrolyzer system is Co- FePBA/NCNT/carbon paper||Ni/NCT/carbon paper.

In still another embodiment of the present invention, the electrolyzer system act as an electrolyzer to electrolyse the natural water resources.

In yet another embodiment of the present invention, wherein the natural water resources comprise of sea water, ground water, salty ground water, or from other natural water resources, preferably sea water.

In still another embodiment of the present invention, sea water is selected from a group comprising of untreated sea water or sea water treated with KOH.

In yet another embodiment of the present invention, untreated seawater exhibits splitting voltage of 2.68V at a current density of 10 mA/cm²

In still another embodiment of the present invention, KOH treated seawater exhibits splitting voltage of 1.88V for a current density of 10 mA/cm².

In yet another embodiment of the present invention, KOH treated seawater and untreated seawater exhibits stability from 24 h.

In still another embodiment of the present invention, selectivity of the system towards oxygen evolution reaction (OER) over chlorine evolution reaction (CER) is higher as chlorine evolution and hypochlorite formation is suppressed after long duration of electrolysis.

In yet another embodiment of the present invention, invention provides the scalable and simple synthesis of cathode and anode for a full cell electrolyzer for sea water, ground water, salty ground water, or other natural water resources. In another embodiment of the present invention, the anode is based on the CoFe based PBA, i.e., cobalt hexacyanoferrate. Co-FePBA is mixed with nitrogen doped carbon nanotube (NCNT) and heated at different temperatures (300-400 °C) to obtain Co-FePBA/NCNT composite. Co-FePBA/NCNT anode is coupled with Ni/NCT cathode.

In yet another embodiment of the present invention, Ni/NCT represents metallic nickel encapsulated inside a nitrogen doped carbon tubules. Ni/NCT was synthesized by heating mixture of melamine and nickel chloride (NiCb) in argon (Ar) atmosphere. The best anode cathode assembly (Co- FePBA/NCNT500||Ni/NCT) exhibits overall seawater splitting voltage of 1.88V for a current density of lOmA/cm² (solar fuel conversion). The said electrolyzer system shows overall seawater splitting voltage of 2.68V at a current density of lOmA/cm² in untreated seawater.

In still another embodiment of the present invention, carbon material was used as catalyst support instead of nickel foam to reduce cost factor and evaluate the better catalytic activity of both anode and cathode catalyst. The desirable performance of the catalysts indicates that the designed cathode and anode can be used in an alkaline electrolyzer using abundant natural water resources.

In yet another embodiment of the present invention, besides, large scale H2 production is also possible by increasing the electrode and electrolyzer dimension.

In another embodiment of the present invention, a method for developing electrolyzer system comprising the following steps is provided: a) Preparing the slurry for the anode, wherein the optimum amount of CoFe-PBA/NCNT is dispersed in organic solvent for suitable time period; b) Preparing the slurry for the cathode, wherein the optimum amount of Ni/NCT is dispersed in organic solvent for suitable time period; c) Coating the slurries of steps a) and b) on the carbon papers to form Co-FePBA/NCNT /carbon paper and Ni/NCT/carbon paper; and d) Placing the Co-FePBA/NCNT /carbon paper and Ni/NCT/carbon paper inside the glass cell with electrolyte and applying the positive potential to Co-FePBA (300-500/NCNT/carbon paper, and negative potential to Ni/NCT/carbon paper to form electrolyzer system. In another embodiment of the present invention, anode is prepared by steps comprising:

I. Mixing IV Fe (CN)e, cobalt acetate and 2 methyl imidazole in organic solvent to produce PBA;

II. Synthesizing CNT by the Chemical Vapour Deposition (CVD) method;

III. Heating the CNT of step II) with melamine to synthesize NCNT;

In another embodiment of the present invention, the cathode is prepared by grounding the NiCh and melamine followed by heating for different times in Argon (Ar) atmosphere.

In still another embodiment of the present invention, the electrolyte in step d) is selected from KOH treated or untreated natural water resources.

In yet another embodiment of the present invention, organic solvent in steps a) or b) is ethanol.

In still another embodiment of the present invention, the dispersion time in steps a) or b) is 15 minutes.

In yet another embodiment of the present invention, in step c) prepared slurries were coated on the carbon papers in the area of 1 * 1 cm².

In still another embodiment of the present invention, organic solvent in step I) is methanol.

In yet another embodiment of the present invention, step I) is carried out for 12 to 36 hour, preferably for 24 hour.

In still another embodiment of the present invention, wherein step I) is carried out at 30 °C.

In yet another embodiment of the present invention, the temperature in step III) is in the range of 700 to 900 °C, preferably 800 °C.

In still another embodiment of the present invention, step IV) is carried out for 5-10 minutes. In yet another embodiment of the present invention, step V) is carried out at a temperature in the range of 100 to 700 °C, preferably from 200 °C to 600 °C, more preferably from 300 °C to 500 °C.

In still another embodiment of the present invention, step V) is carried out for 1 to 3 hours, preferably for 2 hours.

In yet another embodiment of the present invention, in cathode preparation heating is carried out at a temprature from 600 to 900 °C, preferably at 800 °C.

In still another embodiment of the present invention, in cathode preparation heating is carried out for time ranging from 1 hour to 3 hours, preferably from 1 :30 hour to 2:30 hour.

EXPERIMENTAL SECTION:

Anode:

The PBA is prepared by mixing KiFe (CN)e, cobalt acetate and 2-methyl-imidazole in methanol for 24 h at 30 °C. After 24 h, the solution was filtered with Deionized (DI) water three times and dried in an ambient atmosphere to get Metal Organic Framework (MOF) derived CoFe-PBA. Additionally, PBA can also be synthesized by co-precipitation method.

CNT was synthesized in the Chemical Vapor Deposition (CVD) method. NCNT was synthesized by heating the mixture of CNT and melamine at 800 °C in a tubular furnace.

Further, PBA and NCNT or CNT were mechanically grounded in a mortar pestle for 5-10 mins and loaded in an alumina boat placed inside a quartz tube in a tubular furnace. The mixture was heated in an inert atmosphere at different temperatures (from 300 °C to 500 °C) for two hours to obtain anode i.e., Co-FePBA/NCNT or Co-FePBA/CNT (refer to Figure 1 a). The PBA structure remains stable up to 400 °C. At 500 °C, the partial breaking of the cyanide ring (C=N) exposes some open metal sites. This helps in improving catalytic activity and lowering the water splitting potential.

Cathode: To synthesize the cathode layer NiCh and melamine were properly grounded in a mortar pestle for 5 min and further loaded in a quartz boat inside the tubular furnace. The sample mixture was heated at 800 °C at different times (from 1 :30 h -2:30 h) in Argon (Ar) atmosphere to obtain Ni/NCT (Figure 1 b).

All the samples were characterized using X-Ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD confirms the CoFe-PBA phase, while at 500 °C, some different XRD pattern is observed. This could be due to the partial breaking of the C=N bond. The results are described in Fig 1 that represents XRD of Co-FePBA and Co-FePBA/NCNT (anode) (a), Ni/NCT (Cathode) (b). The SEM morphology reveals that CNT or NCNTs are distributed all over the sample, wrapping the CoFePBA NPs. The results are described in Fig 2 that represents SEM images of Co-FePBA(a), NCNT(b), Co- FePBA/NCNT300(c), Co-FePBA/NCNT400(d), Co-FePBA/NCNT500(e), Ni/NCT(f).

XRD of Ni/NCT reveals metallic Ni particles. The morphology indicates that the Ni particles are encapsulated inside nitrogen-doped carbon tubules uniformly over the sample.

Electrolyzer System:

For the full cell electrolyzer, CoFe-PBA/NCNT is used as anode and Ni/NCT as a cathode. To prepare the slurry for the anode, an optimum amount of CoFe-PBA/NCNT is dispersed in ethanol for 15 mins. Further, 5pL Nafion was used as a binder. For the cathode side, a similar slurry preparation procedure was followed. Before coating, carbon papers were cleaned with DI water. The prepared slurries were then coated on the 1 x1 cm² area of the carbon papers. Then Co-FePBA/NCNT /carbon paper (anode catalyst layer) and Ni/NCT/carbon paper (cathode catalyst layer) were then placed inside the glass cell (comprising the electrolyte) and positive potential was applied to Co-FePBA(300-500)/NCNT/carbon paper, and negative potential was applied to Ni/NCT/carbon paper. KOH treated (refer to Fig 3) or untreated seawater was used as an electrolyte. Seawater was collected from the Besant Nagar beach, Chennai.

The stability test (chlorine test) was carried out at a constant current density, i.e., benchmark current density (10 mA/cm²) (The dotted line in Figure 3 shows the potential corresponds to the benchmark current density of lOmA/cm²) for 24 h or more than that. Further, 24 h chronopotentiometry study of three different electrolyzers (seawater+lM KOH) is indicated in Fig 4. The cell shows stability over a long period. Moreover, the electrolysis was also carried out in groundwater or salty groundwater. CV and LSV were run to check the catalytic activity and optimize the potential for seawater splitting (Please refer to Figures 10 to 12).

A chlorine test was also carried out to confirm if any chorine gas evolution or hypochlorite has formed during the electrolysis using aquatic remedies wild chlorine test (detection) kit. According to the procedure, as soon as the reagent is added to the chlorine-containing solution, the color of the solution should change from pale yellow to dark yellow depending on the chlorine concentration. Depending on the intensity of the color, chlorine levels can be identified using the colorimetric test chart. Fig 5 indicates the digital image for the chlorine test for electrolyzers (seawater+lM KOH). The chlorine detection kit shows no colour change is visible after adding the reagent. It confirms the selectivity of the OER of the developed catalyst. This further proves that presently synthesized catalysts are efficient enough to produce green H2 from seawater or groundwater.

Furthermore, untreated seawater was also employed for the seawater electrolysis (refer Figure 7). A current density of 1 OmA/cm² is achieved at 2.68V, which is better than the other reported similar system. The high potential and limited current density are mainly attributed to the solid precipitation around the cathode and numerous salt adsorption on cathode and anode surfaces, making the reaction very sluggish.

Considering these difficulties, present electrolyzer still exhibits good stability for 24 h at 30 mA/cm² for 24 h in untreated seawater. LSV and chronopotentiometry in untreated seawater are indicated in Fig 6. Further, a lab-scale setup is developed to show the accumulation of H2 and O2 in the cathodic and anodic compartments, respectively.

Example 1:

CoFe-PBA/NCNT is used as anode and Ni/NCT as a cathode. Before coating, carbon papers (Catalyst support) were cleaned with DI water. The prepared slurries were then coated on the 1 x1 cm² area of the carbon papers. Then Co-FePBA/NCNT /carbon paper (anode catalyst layer) and Ni/NCT/carbon paper (cathode catalyst layer) were then placed inside the glass cell and positive potential was applied to Co- FePBA(300-500)/NCNT/carbon paper, and negative potential was applied to Ni/NCT/carbon paper. KOH treated seawater (40 mL) was used as an electrolyte. Seawater was collected from the Besant Nagar beach, Chennai. A current density of 10 mA/cm² is achieved at 1.88V.

The stability test was carried out at a constant current density, i.e., benchmark current density (10 mA/cm²) for 24 h or more than that. The cell shows stability over a long period. No chlorine or hypochlorite formation is observed after long time electrolysis in seawater (Refer to Figure 3-5, 7, 8).

Example 2:

CoFe-PBA/NCNT is used as anode and Ni/NCT as a cathode. Before coating, SS mesh (Catalyst support) were cleaned with DI water. The prepared slurries were then coated on the 1 x1 cm² area of the SS mesh. Then Co-FePBA/NCNT / SS mesh (anode catalyst layer) and Ni/NCT/ SS mesh (cathode catalyst layer) were then placed inside the glass cell and positive potential was applied to Co-FePBA(300- 500)/NCNT/ SS mesh, and negative potential was applied to Ni/NCT/ SS mesh. KOH treated seawater (40 mL) was used as an electrolyte. Seawater was collected from the Besant Nagar beach, Chennai. A current density of 10 mA/cm² is achieved at 1.88V.

The stability test was carried out at a constant current density, i.e., benchmark current density (10 mA/cm²) for 24 h or more than that. The cell shows stability over a long period. No chlorine or hypochlorite formation is observed after long time electrolysis in seawater (Refer to Figure 9).

Example 3:

CoFe-PBA/NCNT is used as anode and Ni/NCT as a cathode. Carbon paper was taken as catalyst support. The prepared slurries were then coated on the 1 x1 cm² area of the carbon paper. Then Co- FePBA/NCNT /carbon paper (anode catalyst layer) and Ni/NCT/ carbon paper (cathode catalyst layer) were then placed inside the glass cell and positive potential was applied to Co-FePBA(300-500)/NCNT/ carbon paper, and negative potential was applied to Ni/NCT/ carbon paper. 40 mL seawater (without KOH) was used as an electrolyte. Seawater was collected from the Besant Nagar beach, Chennai. A current density of 10 mA/cm² is achieved at 2.68V.

The stability test was carried out at a constant current density, i.e., 30 mA/cm² for 24 h or more than that. Slight hypochlorite/chlorine formation was observed (Refer to Figure 6).

Example 4: CoFe-PBA/NCNT is used as anode and Ni/NCT as a cathode. Carbon paper was taken as catalyst support. The prepared slurries were then coated on the 1 x1 cm² area of the carbon paper. Then Co- FePBA/NCNT / carbon paper (anode catalyst layer) and Ni/NCT/ carbon paper (cathode catalyst layer) were then placed inside the glass cell and positive potential was applied to Co-FePBA(300-500)/NCNT/ carbon paper, and negative potential was applied to Ni/NCT/ carbon paper. 250 mL seawater (Treated with 1 M KOH) was used as an electrolyte. Seawater was collected from the Besant Nagar beach, Chennai. A current density of 10 mA/cm² is achieved at 2.01.

The stability test was carried out at a constant current density, i.e., 10 mA/cm² for 6 h.

Inverted glass tube cell assembly was used to collect accumulated gas by a water displacement method for 6 h (Refer to Fig 9).

Example 5:

CoFe-PBA/NCNT is used as anode and Ni/NCT as a cathode. SS mesh was taken as catalyst support. The prepared slurries were then coated on the 1 x1 cm² area of the SS mesh. Then Co-FePBA/NCNT / SS mesh (anode catalyst layer) and Ni/NCT/ SS mesh (cathode catalyst layer) were then placed inside the glass cell and positive potential was applied to Co-FePBA(300-500)/NCNT/ SS mesh, and negative potential was applied to Ni/NCT/ SS mesh. 250 mL seawater (Treated with 1 M KOH) was used as an electrolyte. Seawater was collected from the Besant Nagar beach, Chennai. A current density of 10 mA/cm² is achieved at 2.01.

Example 6:

CoFe-PBA/NCNT is used as anode and Ni/NCT as a cathode. Carbon paper was taken as catalyst support. The prepared slurries were then coated on the 1 x1 cm² area of the carbon paper. Then Co- FePBA/NCNT / carbon paper (anode catalyst layer) and Ni/NCT/ carbon paper (cathode catalyst layer) were then placed inside the glass cell and positive potential was applied to Co-FePBA(300-500)/NCNT/ carbon paper, and negative potential was applied to Ni/NCT/ carbon paper. 250 mL ground water (treated with IM KOH) was used as an electrolyte. Ground water was collected from the Adyar, Chennai. A current density of 10 mA/cm² is achieved at 2.03 V (refer to fig 11).

Inverted glass tube cell assembly was used to collect accumulated gas by a water displacement method for 6 h (refer to Figure 12).

Example 7:

CoFe-PBA/NCNT is used as anode and Ni/NCT as a cathode. SS mesh was taken as catalyst support. The prepared slurries were then coated on the 1 x1 cm² area of the SS mesh. Then Co- FePBA/NCNT / SS mesh (anode catalyst layer) and Ni/NCT/ SS mesh (cathode catalyst layer) were then placed inside the glass cell and positive potential was applied to Co-FePBA(300-500)/NCNT/ SS mesh, and negative potential was applied to Ni/NCT/ SS mesh. 250 mL ground water (treated with IM KOH) was used as an electrolyte. Ground water was collected from the Adyar, Chennai. A current density of 10 mA/cm² is achieved at 2.03 V (refer to fig 11).

Example 8:

CoFe-PBA/NCNT is used as anode and Ni/NCT as a cathode. Carbon paper was taken as catalyst support. The prepared slurries were then coated on the 1 x1 cm² area of the carbon paper. Then Co- FePBA/NCNT /carbon paper (anode catalyst layer) and Ni/NCT/ carbon paper (cathode catalyst layer) were then placed inside the glass cell and positive potential was applied to Co-FePBA(300-500)/NCNT/ carbon paper, and negative potential was applied to Ni/NCT/ carbon paper. 250 mL ground water (15% salt) (treated with IM KOH) was used as an electrolyte. Ground water was collected from the Adyar, Chennai. A current density of 10 mA/cm² is achieved at 2.1 V.

The stability test was carried out at a constant current density, i.e., 10 mA/cm² for 6 h (refer to fig 10). Inverted glass tube cell assembly was used to collect accumulated gas by a water displacement method for 6 h (refer to Figure 12).

Example 9:

CoFe-PBA/NCNT is used as anode and Ni/NCT as a cathode. SS mesh was taken as catalyst support. The prepared slurries were then coated on the 1 x1 cm² area of the SS mesh. Then Co- FePBA/NCNT / SS mesh (anode catalyst layer) and Ni/NCT/ SS mesh (cathode catalyst layer) were then placed inside the glass cell and positive potential was applied to Co-FePBA(300-500)/NCNT/ SS mesh, and negative potential was applied to Ni/NCT/ SS mesh. 250 mL ground water (15% salt) (treated with IM KOH) was used as an electrolyte. Ground water was collected from the Adyar, Chennai. A current density of 10 mA/cm² is achieved at 2.1 V.

The stability test was carried out at a constant current density, i.e., 10 mA/cm² for 6 h (refer to fig 10).

Table 1 : Experiments 1 to 9 are summarized in the tabular form.

Claims

We Claim

1. An electrolyzer system comprising: an anode catalyst layer; a cathode catalyst layer; and a catalytic support for the anode catalyst layer and cathode catalyst layer, characterized in that the anode catalyst layer comprises Prussian blue analogue (PBA) and nitrogen doped carbon nanotube (NCNT or CNT) composite, the cathode catalyst layer comprises Metallic nickel particles encapsulated inside nitrogen-doped carbon tubules (Ni/NCT).

2. The electrolyzer system as claimed in claim 1, wherein catalytic support for the anode catalyst layer and cathode catalyst layer is selected from a group comprising of carbon material and stainless-steel mesh (SS mesh).

3. The electrolyzer system as claimed in claim 1, wherein the Prussian blue analogue (PBA) is selected from a group comprising of Cobalt hexacyanoferrate or Co-FePBA, Ni-FePBA, Co-Co PBA, Fe-Fe PBA, Ni-Ni PBA or among single or bimetallic transition metal-based PBA.

4. The electrolyzer system as claimed in claim 2, wherein carbon material is selected from carbon paper, carbon cloth, and porous conducting carbon support.

5. The electrolyzer system as claimed in claim 1, which is Co-FePBA/NCNT/carbon paper||Ni/NCT/carbon paper. The electrolyzer system as claimed in claim 1, which electrolyse the natural water resources. The electrolyzer system as claimed in claim 6, wherein the natural water resources comprise of sea water, ground water, salty ground water, or from other natural water resources. The electrolyzer system as claimed in claim 7, wherein natural water resource is sea water. The electrolyzer system as claimed in claim 8, wherein sea water is selected from a group comprising of untreated sea water or sea water treated with KOH. The electrolyzer system as claimed in claim 9, wherein untreated seawater exhibits splitting voltage of 2.68V at a current density of 10 mA/cm². The electrolyzer system as claimed in claim 9, wherein KOH treated seawater exhibits splitting voltage of 1.88V for a current density of 10 mA/cm². The electrolyzer system as claimed in claims 1 to 11, wherein KOH treated seawater and untreated seawater exhibits stability from 24 h. The electrolyzer system as claimed in claim 1, wherein selectivity of the system towards oxygen evolution reaction (OER) over chlorine evolution reaction (CER) is higher as chlorine evolution and hypochlorite formation is suppressed after long duration of electrolysis. A method for developing electrolyzer system as claimed in claim 1, comprising the following steps: a) Preparing the slurry for the anode, wherein the optimum amount of CoFe- PBA/NCNT is dispersed in organic solvent for suitable time period; b) Preparing the slurry for the cathode, wherein the optimum amount of Ni/NCT is dispersed in organic solvent for suitable time period; c) Coating the slurries of steps a) and b) on the carbon papers to form Co- FePBA/NCNT /carbon paper and Ni/NCT/carbon paper; and d) Placing the Co-FePBA/NCNT /carbon paper and Ni/NCT/carbon paper inside the glass cell with electrolyte and applying the positive potential to Co-FePBA (300-500)/NCNT/carbon paper, and negative potential to Ni/NCT/carbon paper to form electrolyzer system. The method as claimed in claim 14, wherein anode is prepared by steps comprising:

I. Mixing KaFe (CN)e, cobalt acetate and 2 methyl imidazole in organic solvent to produce PB A;

II. Synthesizing CNT by the Chemical Vapour Deposition (CVD) method;

III. Heating the CNT of step II) with melamine to synthesize NCNT;

IV. Mechanically grounding PB A of step I) and NCNT of step III) or CNT of step II); and

V. Heating of mechanically grounded PBA of step IV) and nitrogen doped carbon nanotube (NCNT or CNT) of steps III) or II) in an inert atmosphere at a temperature of 300-500 °C to obtain anode Co-FePBA/NCNT or Co- FePBA/CNT. The method as claimed in claim 14, wherein cathode is prepared by grounding theNiCh and melamine followed by heating for different times in Argon (Ar) atmosphere. The method as claimed in claim 14, wherein the electrolyte in step d) is selected from KOH treated or untreated natural water resources. The method as claimed in claim 17, wherein the natural water resources comprise of sea water, ground water, salty ground water, or from other natural water resources. The method as claimed in claim 18, wherein the natural water resource is sea water. The method as claimed in claim 14, wherein organic solvent in steps a) or b) is ethanol. The method as claimed in claim 14, wherein dispersion time in steps a) or b) is 15 minutes. The method as claimed in claim 14, wherein in step c) prepared slurries were coated on the carbon papers in the area of 1 x 1 cm². The method as claimed in claim 15, wherein organic solvent in step I) is methanol. The method as claimed in claim 15, wherein step I) is carried out for 12 to 36 hour. The method as claimed in claim 24, wherein step I) is carried out at 24 hour. The method as claimed in claim 15, wherein step I) is carried out at 30 °C. The method as claimed in claim 15, wherein the temperature in step III) is in the range of 700 to 900 °C. The method as claimed in claim 27, wherein the temperature in step III) is 800 °C. The method as claimed in claim 15, wherein step IV) is carried out for 5-10 minutes. The method as claimed in claim 15, wherein step V) is carried out at a temperature in the range of 100 to 700 °C. The method as claimed in claim 30, wherein in step V) is carried out at a temperature in the range of 200 °C to 600 °C. The method as claimed in claim 31, wherein in step V) is carried out at a temperature in the range of 300 to 500 °C. The method as claimed in claim 15, wherein step V) is carried out for 1 to 3 hours. The method as claimed in claim 33, wherein step V) is carried out for 2 hours. The method as claimed in claim 16, wherein heating is carried out at a temprature from

600 to 900 °C. The method as claimed in claim 35, wherein heating is carried out at temperature of 800 °C.

37. The method as claimed in claim 16, wherein heating is carried out for time ranging from 1 hour to 3 hour, preferably from 1:30 hour to 2:30 hour.