WO2021199057A1

WO2021199057A1 - Hydrogen generation from waste water using self- healing electrodes

Info

Publication number: WO2021199057A1
Application number: PCT/IN2021/050027
Authority: WO
Inventors: Snehangshu PATRA
Original assignee: Patra Snehangshu
Priority date: 2020-04-03
Filing date: 2021-01-11
Publication date: 2021-10-07

Abstract

Hydrogen generation from waste water using self-healing electrodes The invention discloses a process by which H2, O2 and/or mixture of H2+O2 can efficiently be extracted from domestic waste water by a smart waste water (ww) electrolyzer. The present invention discloses the synthesis of electrocatalytic ww- electrodes by utilizing the biochemical reactions of sulfate reducing bacteria (SRBs) present in waste water. The heterogenous waste water splitted by a ww-electrolyzer in to H2/O2/mixture of H2+O2 which is possible energy application in cooking/soldering/home heating etc. The ww-electrodes when packed in to stack of multiple electrodes produces 625-650 litre of H2 in the expense of 1 unit of electricity (1 kWh) from waste water. The ww-electrolyzer ran continuously for 5 days without decreasing the efficiency of gas production.

Description

Hydrogen generation from waste water using self-healing electrodes

FIELD OF INVENTION

The present invention relates to the development a process for production of ¾, O2 and/or mixture of Fb and O2 efficiently by electrolyzing waste water using a smart waste water (ww) electrolyzer. More specifically this invention discloses the catalytic waste water (ww)-electrodes which split the domestic waste water in to pure Fb and O2 with higher efficiency than that of distilled water. The ww-electrodes are stable for a stretch of minimum five days on continuous operation.

BACKGROUND OF INVENTION

Waste water utilization is an extremely challenging because of its heterogeneity which includes heavy metals, bacteria, metal sulfate/phosphate and organic dirt. The present invention relates to the development of a ww-electrolyzer that extract Fb and O2 and/or in the form of fire/energy from waste water irrespective of its heterogeneity.

The enhancement of CO2 induced atmospheric temperature, i.e global warming as well as pollution led us in the brink of massive destruction of ecosystem, rise in sea level, increase in forest fire, epidemic of respiratory related diseases etc. In the context, clean energy technologies especially Fb economy is being proposed. The basics of Fb economy is the utilization of Fb as fuel which can provide high energy (higher energy density than that of gasoline) when combined with O2 producing water as a byproduct. The Fb is abundant in earth in the form of water (2/3 portion). At present most of the Fb is being produced by cost effective steam reformation of methane (SMR). However, to produce 1 kg of ¾, SMR process ejects 8 kg of CO2 into the atmosphere. Thus, the importance of alternative methods which can produce ¾ without emission of CO2 or other green-house gasses cannot be undermined. The direct electrolysis of H2O is one of the sought out technologies as it can be completely “green” and cost effective especially when it utilizes solar/wind energy as the source of electricity. Consequently research and development were dedicated in developing efficient electrocatalyst/electrodes for high productivity. The state-of-the- art electrocatalysts are based on the noble metals such as Platinum (Pt), Ruthernium Ruthenium (Ru), Gold (Au), Iridium (Ir). As these are expensive and rare elements, researcher dedicated to search electrocatalyst based on earth abundant material such as Fe, Ni, Cu, Co etc. Over the last decade pure and mixed Ni, Fe and Co based phosphides, oxides, oxyhydroxides, sulphides and selenides electrocatalysts have been intensely investigated in form of different nanostructures. Apart from electrodes, researchers also developed Fh generator which can efficiently split water especially distilled water into Fh and O2 separately or mixture of Fh and O2 and is further commercialized.

US 9034167B2 discloses hydrogen/oxygen generation system which includes an electrolyzer cell, a servo integrated controller which was used to efficiently and effectively produce hydrogen and oxygen gases, a power control module, a voltage/current feedback device and a temperature feedback device. US 2016/0285118A1 relates a hydrogen gas generator, and more particularly relates to a hydrogen generator for providing hydrogen gas to a fuel cell System. US2015/0107990A1 describe a hydrogen/oxygen (HHO) system consist of a HHO generator and a Zeer pot. Zeer pot is designed to dissipate the heat inside the HHO generator, thereby reducing the temperature of electrolyte and restricting the electrolyte to evaporate. The evaporation of electrolyte is one of the bottle-necks of the HHO system. US patent no: 4455152 demonstrate hydrogen generator which includes an induction coil for heating up to temperature where water decomposes. Oxygen and hydrogen produced by decomposition of water when passed through the ferrous oxide pellets which was positioned in a fire-resistant crucible. US 2010/0320083 A1 proposed a mechanism by that produces 4 litres per minute HHO gas in stoichiometric ratio in expense of 360 W at atmospheric pressure and increased from 50% to 100% output by increase in vacuum. US 8864974B2 related to a hydrogen generator and particularly related to a hydrogen generator comprising a plurality of cells. US patent number: 3892653 exhibited a hydrogen generator constitute by a voltaic cell having reactive Mg which decomposed to produce ¾ and non-reactive electrode immersed in a salt-water. These innovations essentially stress on producing the energy/th and O2 by electrolysis of distilled water. However for wide spread technological advances, availability of distilled water would need a separate industrial set-up thus possesses a bottle-neck. Therefore, exploring other sources of water especially waste water would be interesting choices. At the same time, human activities have led to astronomical amount of waste water. For example city of Kolkata ejects 750 million litres of waste water per day (MLD). Waste water flow through the open canals and mixes with main water bodies of earth without any purification. Reutilization of waste water is difficult task as it possesses no or low quantity O2 which does not let it to be utilized in pisciculture, agriculture or other human activities. This also is den of various bacteria such as sulphate reducing bacteria (SRBs) whose biochemical reaction produces foul smell around the canal. CN 107188326A discloses a kind of method that hydrogen peroxide oxidation adsorbs combined treatment chemical nickle-plating wastewater with ion exchange. The study of CN107188326A describe a method for reclaiming metallic Ni from the waste water that is exhausted from Nickel-plating industries. This innovation is completely different from the proposed patent application on “Hydrogen production from waste water by self-healing electrodes”. The main aim of the present innovation is to draw ¾ efficiently by electrolyzing waste water. The main innovative part is the development of a stack of self-healing electrodes which shows exceptional activity for the production of H2/O2 and/or mixture of H2+O2 which is 3 times higher than that of stainless steel based distilled water electrolyzer. I also found that the ¾ production from waste water is higher than that of electrolysis of distilled water. There by justifying the importance of self-healing electrodes.

WO2016079746A1 describes a methods and system for hydrogen production by water electrolysis which discloses effect of utilization of redox-active electrodes for generating hydrogen/oxygen by applying potential between a hydrogen evolution and oxygen evolution electrode from aqueous water solution. However, the present invention is related to production of ¾ by utilizing waste-water elctrolyzer which comprises of a stack of self-healing electrodes that are synthesized in waste-water. Therefore, to the best of our knowledge, none of the above mentioned prior art attempts, individually or collectively, proposed the system and embodiments indicated and disclosed by the present invention and which even outnumber the efficiency of state-of-the-art Pt electrodes or stainless steel electrodes OBJECTIVE OF INVENTION

The objective of the present innovation is to obtain value added chemicals/products (H2/O2/H2+O2) from waste water.

Further objective is to develop energy independent, cheap, scalable catalytic ww- electrode by waste water fabrication process which follows thorough characterization by various physiochemical and electrochemical techniques (FESEM-EDAX, XRD, Raman, AFM)

Another objective is to fabricate the flow cell stack with thin film catalytic ww- electrodes, for example ww-Ni3S2/Ni electrode,

Further objective of the invention is to translate the present stack fabrication technology to sulphides of pure or mixed reactive metals such as Fe, Zn, Co, Cu, W etc as well as on nanoparticles substrate.

Further objective of the invention is to develop a large scale ww -electrolyzer along with electrode stack, bubbler, electrolyte reservoir, voltammeter, ammeter etc. for electrolysis of waste water for the production of ¾ in expense of low amount of energy. Further objective is to develop ww-electrolyzer for various energy applications such as fire for gas welding, for cooking, for heating home, producing electricity and mechanical work if connected to steam generator etc.

Further objective is to purify the waste water or other non-clean water in to pure drinking water

SUMMARY OF INVENTION

The invention discloses a process by which domestic waste water can be efficiently electrolyzed to Fh, O2 and/or mixture of H2+O2 by a smart waste water (ww) electrolyzer. The present invention discloses utilization of waste water in two ways. In an aspect of the invention, electrocatalytic ww-electrodes is synthesized by utilizing sulfate reducing bacteria (SRBs) present in waste water which acts as an external energy. In another aspect of the invention, the waste water is split by a ww- electrolyzer in to Fh/C /mixture of H2+O2 which follows their possible energy application in cooking/soldering/home heating/purification of waste water into drinking water etc.

The ww-electrolyzer consists of mainly a stack of catalytic ww-electrodes (as they are synthesized by dipping them in waste water), a water reservoir, a voltammeter for supplying DC voltage, an ammeter for measuring the current passed, a bubbler, and flashback arrestor and a burner/torch. The main innovative component of this innovation is the stack of the catalytic ww-electrodes which possess the potential of splitting the heterogenous waste water with high efficiency.

In another aspect of the invention, the single electrode possess potential to split the waste water in to pure H2 and O2. The ww-electrodes are stable for at least 5 days on continuous operation which even outnumber the efficiency of state-of-the-art Pt electrodes or stainless steel electrodes. The ww-electrodes when packed in to stack of multiple (24 electrodes or more as per requirement ) of dimension of 5 inchx6 inchxl.5 mm produces 625-650 litre (55.8 to 58.0 gm) of H2 at the expense of 1 unit of electricity (1 kWh) from waste water which exhibits more efficient system than commercial distilled water based electrolyzer (here after named as dw-electrolyzer) (only produces 200 litre (17.8 gm) in expense of 1 unit of electricity). The ww- electrolyzer runs continuously for 5 days without decreasing the efficiency of gas production. The device can run with similar efficiency from any sources of domestic water (The waste water was tested from the four different metro cities of India, Delhi, Chennai, Mumbai and Kolkata and various waste water sources from Kolkata).

Brief Description of Drawing:

Fig 1 illustrates the schematic diagram of the transformation of bare Ni electrode to Ni3S2/Ni electrode. The sulphate reducing bacteria, predominant bioagent in urban wastewater, converts sulfate ions SO4² into HS during sulfurogenesis. The nascent HS ions are the main constituent of surface transformation into N13S2 thin film.

Fig. 2. illustrates the physiochemical characterization of thin film metal sulfide. The morphological studies of (A) Bare Ni and (B) ww-Ni3S2/Ni foil electrode showed the formation of globular agglomerates of size less than 100 nm. (C) The Cross sectional view from focused ion beam (FIB) study confirm ww-Ni3S2/Ni electrode composed of a porous thin film of about 80 nm (D). The 2D atomic force microscopy study reiterated that the thin film is composed of globular nanoparticles (E) X-ray Photoelectron Spectroscopy analysis of ww-Ni3S2/Ni in comparison with bare Ni is presented to check the presence of Ni and S species and (F)The Raman spectroscopy confirms the existence of 6 bands corresponding to vibration modes of rhombohedral N13S2.

Fig. 3. (A) illustrates the deconvoluted 4^th cycle and (B) Multicycling of CV (Cyclic Voltameter) of ww-Ni3S2/Ni Foil in 0.1M phosphate buffer (pH 7) at 25 mV.s ¹ in comparison to bare Ni electrode in order to infer the redox activity and long term stability. The CV for comparative study of ww-Ni3S2/Ni Foil with respect to (C) asrb- Ni3S2/Ni Foil (fabricated in artificial SRB culture medium) and (D) e-Ni3S2/Ni Foil (electrodeposited N1₃S₂) and h-Ni₃S₂/Ni Foil (hydrothermal synthesis) was performed under same experimental conditions

Fig. 4. illustrates the development of N13S2 from the urban wastewater collected from various parts of India (New Delhi, Mumbai, Chennai and Kolkata) and N13S2 thin film was developed in offsite condition. These ww- N13S2 /Ni Foil electrodes were studied with the help of cyclic voltammetry (CV) in 0.1 M phosphate buffer (pH 7) at 25 mV.s ¹.

Fig. 5 illustrates the iR corrected linear sweep voltammetry study for ww- Ni3S2/Ni Foam electrode in alkaline distilled water and in wastewater @ 10 mV.s ¹ for (A) Hydrogen evolution reaction (HER) and (B) Oxygen evolution reaction (OER). The Tafel analysis was performed under stirring condition at 1 mV.s ¹ to study (C) HER and (D) OER.

Fig. 6. (A) illustrates the long term stability experiment was performed in two electrode configuration based on bifunctional ww-Ni3S2/Ni Foam electrodes under high current density of 200 mA.cm ² for wastewater splitting, in comparison to bare Ni Foam and Pt coated Ni Foam (B). The detection of gaseous products via gas chromatography after electrolysis of distilled and wastewater by bifunctional ww- Ni3S2/Ni Foam electrodes. (D) The cyclic voltammogram of post OER and HER ww- Ni3S2/Ni Foil electrode at sweep rate of 25 mV.s ¹ in 0.1M phosphate buffer pH 7 illustrates complete or partial loss of redox activity of N1₃S₂, NiS or Ni. The OER and HER was continuously run for 6 h at a current density of 200 mA. cm². The post OER N1₃S₂ electrode was regenerated just by subjecting to wastewater treatment as is clear from cyclic voltammogram marked in blue color.

Fig. 7. illustrates the set-up for production of ¾ and its application in energy applications such as fueling torch for soldering Fig. 8 illustrates the Cu and Cu_xS_y electrode displayed the versatility of the waste water synthesis technique. The linear sweep voltammetry of Cu_xS_y/Cu in phosphate buffer execute a successful formation of cupper sulfide, through exact composition is yet to be determined. The electrode also showed extremely high efficacy towards water splitting and can as effective as of Ni3S2/Ni catalyst.

Fig. 9 illustrates the linear sweep voltammetry of nickel foam (NF), ww-(Ni-Cu)S/NF and ww-etch-(Ni-Cu)S/NF electrode for (A) HER and (B) OER in 1 M KOH waste water at sweep rate of 10 mV s ¹.

Detailed Description of the invention: The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and together with the description, serve to explain the principles of the inventions.

The embodiments are in such detail as to clearly communicate the disclosure. The amount of detail offered has the intention to cover all modifications, equivalents, and alternatives falling within the scope of the present disclosure as defined by the appended claims.

The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the present disclosure. Accordingly, it should be apparent to those skilled in the art that the following description of various embodiments of the present disclosure is provided for illustration purpose only and not for the purpose of limiting the present disclosure as defined by the appended claims and their equivalents.

It is to be understood that the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a component surface" includes reference to one or more of such surfaces. The terminology used herein is for the purpose of describing particular various embodiments only and is not intended to be limiting of various embodiments. As used herein, the singular forms "a," "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising" used herein specify the presence of stated features, integers, steps, operations, members, components, and/or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, members, components, and/or groups thereof. Also, expressions such as "at least one of," when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list.

The exemplary embodiments shown in the accompanying drawings will now be described more fully hereinafter. The exemplary embodiments are provided only for illustrative purposes and this disclosure will be thorough and complete and will fully convey the scope of the invention to those of ordinary skill in the art.

The ww electrolyzer consist of mainly a stack of catalytic ww-electrodes (as they are synthesized by dipping them in waste water), a water reservoir, a voltammeter for supplying DC voltage, an ammeter for measuring the current passed, a bubbler, and flashback arrestor and a burner/torch. The main innovative component of this innovation is the stack of the catalytic ww-electrodes which possess the potential of splitting the heterogenous waste water with high efficiency. In a stack the number of electrodes can be varied from 23 to 440 electrodes depending on the amount of hydrogen required. The size of the electrodes can be varied from 4-8 inch (l)x4-8 inch (w) xl mm (t), preferably, the size of the electrodes may be (5inch (l)x6 inch (w) xl mm (t)), (4 inchx4 inchxl mm) or (8 inchx8 inchxl mm) for optimum production of H2. Different sizes of the electrodes are required for varying production. For making amaking a stack, the gap between electrodes is one of the important factors to obtain H2 efficiently. The gap between two electrodes may vary from 3- 8 mm, preferably 3, 5, 6, 8 mm. A gap lower than 3 mm can enhance the temperature of the cell which is unwanted and a gap higher than 8 mm may decrease the current efficiency of the cell. Thus to control the temperature of the cell and high current efficiency, optimum gap is required. In accordance with a preferred embodiment of the inventionthe invention, a 6 mm gap is employed. The electrodes are arranged vertically to obtain high efficiency of hydrogen production.

Synthesis of Electrocatalyst

In the preferred embodiment, the present invention discloses synthesis technique to form thin film metal sulfides in the first step on a Ni electrode (as a test substrate) in SRB (Sulphur Reducing Bacteria) medium either artificially grown or in urban wastewater. No additional energy or chemical reagents was required for the formation of Ni based metal sulfide such as N13S2. The bare Ni metal electrodes of varying dimension of either 1 cmxl cm or 5 inchx6 inch were chosen herein and subjected to the synthesis technique as illustrated in Fig 100 in artificially developed SRB environment in laboratory condition or in waste water bodies for a duration of 1 to 7 days before initiation of biocorrosion for development of catalytic layer. A change in color from shining white to black was observed which ensures the chemical modification of Ni surface after removal of all physically attached dirt by profuse washing with distilled water .

The surface modification of these test substrates provides the characteristic of N13S2 or other nickel sulfide color. This was observed both for artificially grown SRB as well as across different types of waste water environment (biogas plant, stagnant and under flow wastewater locations) and across the waste water bodies of four different metro cities such as Kolkata, Delhi, Mumbai and Chennai. N13S2 on Ni foil was subjected to various physiochemical and electrochemical characteristics for better efficiency of the process. Additionally, the N13S2 modified Ni foam was further developed to test bi-functional hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysis in comparison to electrodes developed through various other synthesis techniques. The N13S2 modified Ni foil and Ni foam electrode developed in SRB enriched environment either developed in laboratory designated as a SRB-Ni3S2/Ni foil or in wastewater are hereafter designated as ww- Ni3S2/Ni foil and ww-Ni3S2/Ni foam, respectively.

Table 1 discloses herein the dependence of synthesis time of metal sulphide catalyst on the concentration of SRB

Table 1. The dependence of synthesis time of metal sulphide catalyst on the concentration of SRB

In the preferred embodiment, physicochemical characterization of ww-electrocatalyst for effective hydrogen generation illustrated in Fig. 2-Fig. 9 ensures uniformity in surface modification and other related characterization for effective hydrogen generation. Fig.2 illustrates morphology, exact compositions, nature of the surface, etc. The FESEM images illustrated the formation of globular agglomerates of size less than 100 nm. The consistency of such structure is observed at all scales, depicting uniformity in surface modification. Furthermore, the EDS analysis depicts the uniform distribution of S element across the Ni foil as well as foam surface that was evident from area mapping. No other impurities other than C, O and traces of Si and Al, which were originally present in same concentration in case of bare Ni electrode. Such similar morphology and sulfur concentration was seen across the Ni foil treated in different sources of wastewater, which is a favorable outcome for reproducibility of this methodology. Furthermore, it is interesting to note that in spite of the electrode fabrication in such heterogeneous conditions, absence of any other elements leads to the conclusion that biogenic sulphurization is the dominant reaction in these environments. The focused ion beam (FIB) study as illustrated in the Fig. 2C was performed in order to understand the cross-section characteristic of the thin film. It reveals two distinct layers, the highly corrugated and porous top layer, and a denser bottom layer, with the thickness of 70-90 nm. The atomic force microscopy (AFM) study (Fig. 2D) reveals the globular type of morphology coherent to FESEM study. The broad X-Ray Photoelectron spectroscopy scan of ww-Ni3S2/Ni foil electrode shows the presence of Sulphur 2s and 2p states along with nickel 2s, 2p, 3s and 3p states (Fig. 2E). An increase in O Is in WW-N13S2 layer w.r.t. bare Ni is an indication of its surface oxidation on exposure to air which might cause two distinct layers as observed with FIB. The conclusive chemical state of nickel sulfide thin film could be inferred from the Raman spectrum analysis(Fig. 2F), which showed a total of six bands at 188, 200, 222, 304, 324, and 350 cm ¹, which corresponds to a heazlewoodite N13S2 phase for both Ni foils.

Electrochemical characterization in phosphate buffer

In order to evaluate the redox behavior, the ww-Ni3S2/Ni foil electrode was investigated using CV in 0.1 M phosphate buffer at pH=7 as illustrated in Fig. 3 A, devoid of any redox species in the electrolyte. The choice of the neutral pH was in order to prevent the anodic dissociation of Ni²⁺ in the acidic region or formation of Ni(OH)_x for basic medium. The deconvolution of peaks suggests that the contribution of Ni to Ni²⁺ in the film of N13S2 layer was observed to be negligible ( 1.72%), thereby depicting the exclusive electrochemical activity of N13S2 thin film. Furthermore in absence of any redox species in the electrolyte, the ww-Ni3S2/Ni foil electrode appeared to be highly electroactive and exhibited as much as 7xl0³ times higher faradaic charge compared to bare Ni foil electrode. A quasi-reversible redox pair centered at a potential of -0.45 V was observed, which corresponds to Ni3S2/Ni3-_xS2 redox couple, with x being the upper limit of metastability of heazlewoodite.

The conversion of N13S2 into a lower nickel content meta stable intermediate phase N13-_XS2 (may be N17S6) was observed to be highly facile and stable in nature as observed over 25 cycles redox study in Fig. 3B due to eutectoid nature of these phases. On further oxidation, a large irreversible peak was observed at -0.08 V, indicating the formation of more stable sulfur-rich NiS state.

N13S2 to NiS conversion decreases over consecutive cycles as the increase in redox activity of Ni3S2-Ni3-_xS2 dominates over NiS formation. It was observed a small reduction peak at -0.1V from 10^th cycle onwards. This may be due to the increase in cone of Ni²⁺ ions in diffusion layer on multiple cycling resulting in reduction of NiS into nickel enriched states such as Ni3-_xS2 or N13S2. On sweeping beyond 0.2 V, a trail of oxidation current was observed may be due to the formation of the higher oxidation state of Nickel (N13S4, N1S2) and the further increase would lead to NiOx. The formation of NiO_x or NiOOH would be a dominating reaction with increase in pH(alkaline), instead of Ni²⁺ dissociation in neutral/acidic medium. Interestingly, this formation serve as a “pre-catalysts” for oxygen evolution reaction in alkaline medium.

The redox activity of ww-Ni3S2/Ni Foil electrode was found to be stable over consecutive 25 cycles as illustrated in Fig. 3B. Heterogeneity of such natural synthesis medium, the comparative study with artificial synthesis medium is an important prerequisite. As a control experiment, the present invention SRB medium was cultured in the laboratory and after which the bare Ni foil (dimension 1 cmx 1 cm or 5 inchx6 inch) was kept in it for 1 to 7 days. The developed electrode was termed as asrb-Ni₃S₂/Ni Foil and was further studied in phosphate buffer, as illustrated in Fig. 3C. It was observed that asrb-Ni₃S₂/Ni Foil exhibited exactly similar redox activity with comparable current density to that of ww-Ni₃S₂/Ni Foil. Thus, the artificial synthesis medium is redundant on the additional time period for SRB culture and cost of chemicals, and moreover same output can be achieved with natural wastewater synthesis. It is also crucial to study this synthesis from different sources to understand the role of these medium and further the reproducibility of such self- sustaining synthesis chamber. In order to perform comparative study, ww-Ni₃S₂/Ni foil was furthered developed through other conventional methodologies such as electrodeposition and hydrothermal which are termed as e-Ni₃S₂/Ni foil and h- Ni₃S₂/Ni foil, respectively as specified in Fig. 3D A much inferior redox activity in terms of current density as well as the stability of these electrodes was observed. The Ni S₂-Ni _-xS₂ redox couple was observed to be at 0.5 V for the electrode developed through both the electrodeposition and hydrothermal technique, exactly same as wastewater fabrication, but with at least 10 times lower in current density. Also, interestingly entire redox activity was lost within 5 consecutive cycles. This could be the dissolution of N1₃S₂ film due to poor substrate adhesivity, which was prominently observed in dissimilar electrode substrate (FTO, ITO, stainless steel electrode). Thus, the properties of biosynthesized N1₃S₂ thin film outperforms other conventional synthesis methodologies, and will be further evaluated for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) applications.

The redox activities were found reproducible in as much as 11 wastewater sources across India, as illustrated in the maps of Fig. 4. These ww-Ni₃S₂/Ni foils were studied electrochemically and the redox properties was found reproducible, as given in the inset of Fig.4, and depicting a deviation in current density of about 0.8 mA.cm ². It was thus inferred that (i) the growth of N1₃S₂ can be possible even in extremely heterogeneous condition irrespective of composition (many known and unknown organic, inorganic, biological components) of wastewater, (ii) only biochemical reaction of SRB is responsible for such metal sulfide formation and (iii) the synthesis in natural wastewater bodies is a local phenomenon, nullify other competing biochemical reactions. SRB produced HS ions by a biochemical reaction between organic matters and SO4² ions in the anoxic environment, which is as shown in the following simple reaction.

SEBs

SO +8 [H] +H⁺ — ÷ HS ( Eqn 1)

As expected, a high concentration of SRBs population (10⁵-10³ units ml ¹) was detected in wastewater bodies with the help of the commercial SRB kit. Moreover, the elemental analysis of wastewater by inductively coupled plasma atomic emission spectroscopy (ICP-AES) revealed the concentration of Sulphur to be 10.8 ppm at urban wastewater in Kolkata, India (22°32'07.8"N 88°23'40.1"E). Thus the two important components, Sulphur sources and biocatalyst SRBs, for the biochemical synthesis of metal sulfides were found in these wastewater bodies. The absence of any other impurities on ww-Ni3S2/Ni Foil (from EDAX), in turn, and following generic reaction (Eqn. 2) is specifically taking place.

2HS +3M ²⁺ 2e M₃S₂ | +2H⁺ (Eqn. 2)

Domestic Waste water electrolysis

In a preferred embodiment, the commercially available 3D nickel foam is used for various electrochemical applications, mainly due to its low cost, large surface area, high conductivity, porosity and excellent mass transportation under flow dynamics. The present invention depicts the use of Ni foam as a base electrode for loading catalysts as a current collector and support matrix for growth of nanostructure electrocatalysts. Thus, the WW-N13S2/ Ni foam electrode was developed by exactly similar process as was discussed earlier section which follows their characterization for HER and OER activity as specified in Fig. 5 for splitting of wastewater. However, in order to study such applications, one should include the concept of iR corrections for these voltammograms as the ionic conductivity of such medium would differ. In an aspect of the of invention at iR compensated linear sweep voltammetry, it was observed a huge shift of onset potential by 150-200 mV for ww-Ni3S2/Ni Foam in comparison to bare Ni Foam for HER in both the sources of water. In case of HER, 115 and 117 mV over potential was required to achieve 10 mA.cm ² (Fig. 5 A) and OER current shoot attaining a current density of 200 mA.cm ² at 310 mV for distilled water, 184 mV over potential lower in comparison to wastewater. Furthermore, similar electrocatalytic activity at the low over potential range was observed mostly in domestic waste water whereas in high over potential range it deviates because of high solution resistance (3W). The collected domestic waste water was filtered to remove the suspended particles before performing the electrochemical characterizations. Besides, LSV was performed for both HER and OER at a scan rate of 1 mV.sec ¹ under stirring conditions of 1000 rpm in order to extract Tafel slope, as depicted in Fig. 5C and D. The Tafel slope for wastewater splitting was found to be 81 and 67 mV per decade for HER and OER, respectively which are comparable to the Tafel slope in distilled water. Additionally, the reproducibility studies of ww- Ni3S2/Ni Foam in depicted a standard deviation (SD) of 1.5 %. In the preferred embodiment, the electrocatalyst was tested for its long-term stability in order to infer the possibility of poisoning or degradation leading to an over potential shift for unexplored wastewater splitting. It is a thumb rule that the electrocatalysts exhibiting steady potential usually at 10 mA.cm ² over 10 h is viable to scale up for water electrolyzers. Chronoamperometry was performed in two identical electrodes configurations at harsher conditions, i.e. the current density of 200 mA.cm ² over 116 h. It was found that ww-Ni3S2/Ni Foam electrodes showed profound stability at 2.5 V, as shown in Fig. 6A. An occasional shift of 100 mV was seen due to decrease in solution volume, which was rectified on adjusting the level. Furthermore, for comparative purpose, same experiment was conducted with Pt coated Ni Foam as a state-of-the-art catalyst, termed as Pt/Ni Foam. It is observed Pt/Ni Foam was stable up to 46 h of continuous electrolysis. Thereafter a rapid degradation of catalytic activities occurred, which was may be due to Pt poisoning or dissolution of the film from the surface. Finally, it steadily approaches the potential of bare Ni foam, which was IV more polarized in comparison to ww-Ni3S2/Ni Foam.

Furthermore, the gas chromatography was performed to identify the gaseous evolution for both the distilled and the wastewater electrolysis with two identical ww- Ni3S2/Ni Foam electrodes (Fig. 6B) at various over potential ranging from 0.77 to 2.3 V. it was found ¾ and O2 as the sole gaseous products for all of the above conditions. In spite of such heterogeneous conditions of wastewater, gaseous constituents through electrolysis are free from any impurities.

The N1₃S₂ under different electrochemical condition has the tendency to change states (NiS, Ni(OH)₂, NiOOH, NiO). For example, the post OER FESEM showed amorphous morphology of NiOOH upon conversion from N1₃S₂, along with EDAX data depicting higher concentration of O. NiOOH layer is of 140-170 nm thickness along with the underline very thin N1₃S₂ layer at Ni interface. Such surface modifications can further be studied with the Raman spectroscopy, as shown in Fig. 6C. The typical bands of N1₃S₂ was found to be non-existent for post OER. However two bands were observed at 318 and 452 cm ¹, which can be attributed to E type lattice vibration of Ni-OH and Ai type vibration of the Ni-0 bonds respectively. Similarly, a change in morphology on performing HER was observed. The Raman spectra of post HER ww-Ni₃S₂/Ni Foam showed the presence of all distinct bands depicting nickel sulfide but with lower intensity. Moreover, the post HER redox study in phosphate buffer pH 7 in Fig. 6D also exhibit similar redox nature of WW-N1₃S₂ even after 5 cycles (red line). Whenever a complete loss of redox activity of ww- Ni₃S₂/Ni Foam was observed for the ww-Ni₃S₂/Ni electrode undergo OER for 6 h continuously as depicted in Fig. 6D (black line), it regain the lost electrochemical activities of the catalyst by using the synthesis process in SRB medium as illustrated in Fig 1 . Complete regeneration of redox activity with similar current density was observed through a CV as depicted in Fig. 6D (blue line). It is noteworthy that it would serve to reuse or regenerate any damaged or modified electrode for any other applications.

APPFICATION

In electrochemical systems, the ww-catalytic electrodes are primarily being investigated as a promising electrocatalysts for water electrolyzer, batteries, supercapacitor and photoelectrochemical systems. Thus, the catalytic electrodes developed from wastewater were evaluated for water splitting electrocatalysis as described in Fig. 5 and 6, especially from waste water.

The ww-Ni3S2/Ni catalytic electrode exhibited bi-functional electrocatalytic activity towards HER and OER from waste water, as illustrated in Fig. 5. The values of Tafel slope for waste water electrolysis is similar to that of the distilled water suggesting high implication of catalytic ww-electrodes in waste water electrolysis. The catalytic ww-electrodes showed excellent stability for the production of ¾ and O2 during at least for 5 continuous days (Fig. 6). The only product for waste water electrolysis is ¾ and O2 as clear from gas chromatography analysis. Example:

The following example is given by way of illustration and therefore should not be construed to limit the scope of invention

Example 1 : Development of stack and waste water electrolyzer The waste water synthesis technique was applied to large size of the ww-electrode, 5 inchx6 inch which indicate the scalability of the synthesis technique. Domestic waste water was used in this application. The multi stack generator was built by assembling multiple (24) catalytic ww-electrode plates of dimension of 5 inchx6 inch or as per requirement, as shown in Fig. 7. The catalytic electrodes were stacked in parallel with gasket to prevent leakage of current and electrolyte as common for a complete dry cell. Flow field in the catalytic ww-electrode was introduced so that waste water easily pass through the ww-electrode while carrying FL and O2 or mixture of H2+O2 and transferred back to the water tank made of PVC. The H2 and O2 is then pass through the bubbler and flash back arrestor which finally be collected through a pipe or optionally attached to a torch or oven. The torch/oven then can be ignited by spark to obtain fire for various high temperature applications such as brazing, cooking, producing electricity and mechanical work if connected to steam generator etc. This ww-electrolyzer with the catalytic ww-Ni3S2/Ni electrode produced 625-650 litre of H2 (55.8 to 58.0 gm) in expense of 1 kWh electricity. The production of H2 was compared with the conventional stainless steel (SS) based dw-electrolyzer which produced 200 Litre of H2 (17.8 gm) from distilled water in expense of same amount of electricity. The conventional dw-electrolyzer initially produced H2 with 80% efficiency in waste water but soon the production decreases to 20% because of the fouling of the electrodes.

Example 2: Development of Waste water Electrolyser for other reactive metal catalytic electrode

The present invention was easily translated to make other interesting electrocatalyst such as Cu_xS_y/Cu electrode, as illustrated in Fig. 8. A piece of Cu metal foil was cut in to size of 1 cmxl cm dimension and was dipped in SRB containing waste water medium for 3 to 7 days. A change in color from orange -red to black was observed. The electrochemical redox behavior of Cu_xS_y/Cu electrode in a phosphate buffer electrolyte was tested by performing cyclic voltammetry (CV). The CV clearly demonstrated the enhancement of redox behavior in comparison to bare Cu substrate. The idea is not to restrict by just making electrodes, but to check their usefulness for splitting waste water. The Fig. 9 shows the efficacy of ww-Cu_xS_y-Ni_xS_y/NF and ww- etchCu_xS_y-Ni_xS_y /NF for splitting of water especially in waste water. At first Ni-Cu was electrochemically deposited on a Ni foam from a solution of 0.1 to 1 M N1SO4 along with 0.01-0.05 M CuS0₄ in 0.1-0.5 M boric acid, is further termed as Ni- Cu/NF electrode. The electrochemical etching of Cu from Ni-Cu/NF electrode was performed by applying a high anodic potential of 0.5 -1.045 V, now termed as etch- Ni-Cu/Ni electrode. The etching solution was 0.1 to 1 M N1SO4 along with 0.01-0.05 M CuSC>4 in 0.1-0.5 M boric acid under stirring. For sulfidation, both the Ni-Cu/NF and etch(Ni-Cu/NF) electrode was further subjected to treatment with waste water for the duration of 3-7 days, as depicted in Fig. 1. After sulfidation, the electrodes were termed as ww-Cu_xS_y-Ni_xS_y/NF and ww-etch-Cu_xS_y-Ni_xS_y /NF. Following the synthesis, the samples were washed thoroughly and electrochemical HER and OER studies were performed. In the first glance at non-iR compensated linear sweep voltammetry, a huge shift of onset potential by 300 mV in comparison to bare Ni Foam was observed in HER region. Furthermore, Fig. 9 depicted current density of 200 mA.cm ² for both ww-Cu_xS_y-Ni_xS_y/NF and for ww-etch-Cu_xS_y-Ni_xS_y /NF electrode at an over potential of -0.5 V and 0.46 V, respectively. These values are 0.24 to 0.28 V lower than that of the bare Ni foam electrode. The conventional benchmark parameter for HER is to measure potential at 10 mA cm ². However, redox behaviour at 10 mA cm ² at an overpotential of 0.009 was observed which is due to conversion of Cu²⁺ to Cu°. It was presumed that this redox conversion enhances the hydrogen evolution rate as Cu is good catalyst for HER. In case of OER, a conventional electrochemical behavior with a peak at 1.5 V followed a sudden rise in current was noted. The peak was attributed to the oxidation of 3d transitional metal sulfides, oxides, hydroxide into oxyhydroxide (Ni3S2 NiOOH) in alkaline medium. This Ni3S2 NiOOH conversion, the sole participant for OER, was found to be non-existent in case of bare Ni foam (Fig. 9), and thereby consequently poor OER kinetics. On further sweeping beyond 1.5 V, an OER current shoot was exhibited attaining a current density of 200 mA.cm ² at a potential of 1.75 V and 1.88 for ww-etch-Cu_xS_y-Ni_xS_y/NF and ww-Cu_xS_y-Ni_xS_y/NF electrode, respectively. This numbers are far improved than the NF electrode yet another observation signifies the importance of surface modification.

Following large scale sulfide based electrode (or photoelectrode) material can be synthesized by waste water synthesis followed by their exploitation in waste water electrolysis: Mn, Fe, Co, Zn, Ga, Ge, Mo, Ag, Cd, In, Sn, Zr, Mo, Ru, Pd, W, Sb, Pb, Bi. These electrodes can further be tested for developing large scale electrolyzer as shown in Fig. 7.

ADVANTAGES OF THE PRESENT INVENTION: The efficacy of the present innovative ww-electrolyzer is at least 2 times better than that of dw-electrolyzer by utilizing same stack of the ww-electrodes because of self- healing properties in waste water as illustrated in Fig. 7. The ww-electrolyzer is also 3 times more efficient than that of conventional stainless steel based dw-electrolyzer, clearly indicating the advantage of such electrolyzer. Domestic waste water is properly utilized in this invention. Catalytic electrodes are developed by incubating them in waste water. These catalytic electrodes are extremely efficient in splitting the same waste water as explained in the invention. The invention disclosed may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Various modifications will be readily apparent to persons skilled in the art. The general principles defined herein may be applied to other embodiments and applications without departing from the scope of the invention. Different embodiments of the invention are possible to achieve the best method of performance and to obtain the effective composition as describes. It will be understood that the invention may be carried out into practice by skilled persons with many modifications, variations and adaptations without departing from its spirit or exceeding the scope of the claims in describing the invention for the purpose of illustration. It is also to be noted that the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope of the present invention, which is further set forth under the following claims:-

Claims

I claim,

1. Method of generation of self-healing thin film metal sulfides electrocatalyst (100) comprising providing a SRB medium (Sulphur Reducing Bacteria) present in domestic waste water or artificially grown environment and a Ni based electrode as a test substrate, immersing the Ni electrode in SRB environment for 1-7 days till initiation of biocorrosion for development of catalytic layer of N13S2 on the substrate, followed by characterisation of the N13S2 for hydrogen generation. removing all physically attached dirt to generate the N13S2 modified Ni electrode, for bifunctional hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).

Regenerating the activity of electrode with redox activity by reversing the polarity of the electrodes or by immersing it in domestic waste water or artificially grown SRB medium after HER and OER,

2. The method as claimed in claim 1, wherein said electrode is a Ni sheet, Ni foil, Ni foam electrode or a sheet or foil of any metal coated comprised-with Ni particles of various sizes.

3. The method as claimed in claim 1 wherein bare Ni metal electrodes of varying dimensions is used for development of large / small scale electrolyser and the thin film (100) is produced in artificially developed SRB environment in laboratory condition or in waste water bodies.

4. The method as claimed in claims 1& 2, wherein the redox activity of ww-

Ni3S2/Ni Foil electrode is stable over consecutive 25-50 cycles.

5. The method as claimed in claim 1 wherein the ww-Ni₃S₂/Ni electrode shows strong electroactivity over either conventional electrochemical or hydrothermally synthesized Ni₃S₂/Ni electrode (300D).

6. The method as claimed in claim 1 wherein there is a huge lowering of onset potential by 150-200 mV for WW-N1₃S₂/N1 Foam for HER.

7. Method of hydrogen generation from domestic waste water using N1₃S₂ modified Ni electrode, the method comprising the steps of providing a SRB medium (Sulphur Reducing Bacteria) present in waste water or artificially grown environment and a Ni based electrode as a test substrate, immersing the Ni electrode in SRB environment for 1-7 days till initiation of biocorrosion for development of catalytic layer of N1₃S₂ on the substrate, followed by characterisation of the N1₃S₂ for hydrogen generation, - removing all physically attached dirt to generate the N₁₃S₂ modified Ni electrode, for bifunctional hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), filtering the domestic waste water to remove suspended particles, suspending the electrode/generator in the filtered waste water , - initiation of flow field in the ww-electrode for carrying ¾ and O₂ or mixture of H₂+O₂ back to the water tank, passing the ¾ and O₂ through a bubbler and flash back arrestor and finally collecting the same for further use, regenerating said electrode with redox activity after HER and OER, by reversing the polarity of the electrodes or by immersing it in domestic waste water or artificially grown SRB medium.

8. The method as claimed in claim 7, wherein said generator comprises a plurality of large size catalytic ww-electrode plates (700) stacked for hydrogen generation.

9. The method as claimed in claim 7, wherein ¾ and O2 are produced as the sole gaseous products in waste water electrolysis.

10. The method as claimed in claim 7, wherein ww-electrolyzer with the catalytic ww- Ni3S2/Ni electrode produces 625-650 litre of H2 (55.8-58.0 gm) at an expense of

1 kWh electricity .

11. The method as claimed in claim 7, wherein sulfides of Ni, Cu, Mn, Fe, Co, Zn, Ga, Ge, Mo, Ag, Cd, In, Sn, Zr, Ru, Pd, W, Sb, Pb, Bi or sulfides of mixed metal such as NiCu, CuZn, NiCo, NiMo, NiZn, NiFe, FeCo, CuFe, NiFeCo, NiCuFe, NiCuZn, NiFeZn produces ¾_, O2 and combination thereof without any impurities.

12. The method as claimed in claim 7, wherein effective hydrogen generation time varies as inversely proportional to the concentration of SRB in the growth medium, wherein when the concentration of SRB varies in the range of 10³-10⁵ bacteria/ml, the time for the synthesis of electrodes varies between 1-7 days; when concentration is less than 10³ bacteria/ml, the time required is greater than 7 days and when the concentration is greater than 10⁵ bacteria/ml, the time required is less than 1 day.

13. A ww-electrolyzer for generation and for the application thereafter of hydrogen from domestic waste water comprising a stack of catalytic ww-electrodes, a water reservoir, a voltammeter for supplying DC voltage, an ammeter for measuring the current passed, a bubbler, and flashback arrestor and a burner/torch, wherein, in the stack, the number of electrodes varies in the range of from 23 to 440 electrodes, the size of the electrodes varies in the range of from 4-8 inchx4-8 inchxl m and the gap between electrodes varies in the range of from 3- 8 mm.

14. The ww- electrolyzer as claimed in claim 13, wherein the gap between the electrodes is preferably 3, 5, 6, 8 mm, most preferably 6 mm.

15. The ww-electrolyzer as claimed in claim 13, wherein the electrodes are arranged vertically to obtain high efficiency of hydrogen production.