WO2024147010A1

WO2024147010A1 - Improved electrodes

Info

Publication number: WO2024147010A1
Application number: PCT/GB2024/050007
Authority: WO
Inventors: Peter James Dobson; Alexander William Faris REIP; Claudio PICCHI; Augusto Bartolome DIAZ DE BUDALLES
Original assignee: Oxford Nanosystems Ltd
Current assignee: Oxford Nanosystems Ltd
Priority date: 2023-01-05
Filing date: 2024-01-04
Publication date: 2024-07-11
Anticipated expiration: 2025-07-05
Also published as: GB2630010A

Abstract

The present invention discloses an electrode suitable for alkaline electrolysis that comprises at least two layers of coating wherein the exterior layer of the cathode is a catalytic alloy having a specific surface area configuration that confers improved efficiency and durability to the electrode in alkaline electrolysis.

Description

IMPROVED ELECTRODES

Introduction

Water electrolysis has been proposed as a desirable and clean way to address many issues with the production and storage of energy. Hydrogen production from water electrolysis is not found in many applications yet because the hydrogen produced by water electrolysis is not yet economically viable.

Given that the supply of water is freely available, the hydrogen and oxygen production in water alkali electrolysers can be, in principle, almost limitless and if the technical issues around electrolysis are solved it can be highly economical. In practice, however, large scale electrochemical production of hydrogen by renewable electricity from water splitting is greatly constrained by the performance and stability of the cathode and anode and by the negative effect of polarisation effects due to shut down events i.e. when there is no power through the system as might occur if renewable energy is used.

US2019/119822, EP3604619 and EP3388553 disclose examples of electrodes designed for water electrolysis which comprise a single layer with modifications made thereon and which, in some cases, involve use of expensive or undesired materials.

The selection criteria of an electrode for water electrolysis include low over-potential, high electrocatalytic activity, high electrical conductivity, electrochemical stability, and reasonable low cost to produce and use. There is thus a need to provide a stable and efficient electrode that fits the above criteria, particularly by promoting hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).

Summary of the Invention

The above selection criteria are met and solved by an electrode comprising a substrate, a first layer present on the exterior surface of the substrate to promote and define the surface area and/or porosity and a second layer present on the exterior surface of the first layer and wherein the second layer is an electrolysis promoting catalyst and the combination has a surface area of 0.1-5 m²/g. It has been found that a catalyst that can operate in an alkaline electrolyser within the above surface area range provides an increased efficiency in oxygen and/or hydrogen evolution reaction while also possessing durability.

Preferred embodiments of the present invention and alternative embodiments of the present invention are further explained in the description here below.

Figures

Figure 1 A and A’ shows images from scanning electron microscopy (SEM) with the results from each step of the addition of the first layer and subsequent addition of the second layer (catalytic alloy) to an electrode substrate in accordance with the present invention. This contrasts with Figure B which shows the results of the single step of the direct addition of the catalytic alloy to the same substrate.

Figure 2 shows polarization curves of electrodes according to the present invention and the electrode created according to direct addition of the catalytic alloy to the substrate electrode

Figure 3 shows the SEM of a different multi-layer electrode where the catalytic alloy has a higher surface area than the range according to the present invention

Figure 4 shows, through polarization curves of electrodes, the difference in electrolysis efficiency between the electrode according to the present invention and the control multi-layer electrode of Example 2

Figure 5A shows how the electrode according to the invention does not suffer any degradation in its electrolysis catalysis after an accelerated lOOh KOH durability test. Figure 5B shows how the surface area morphology (globular morphology) of the second layer (catalytic layer) according to the present invention is not changed after an accelerated lOOh KOH durability test

Figure 6 shows, through polarization curves of electrodes, how the difference in catalysis efficiency and in degradation of a control multi-layer electrode of Example 2 with respect to the multi-layer electrode according to the present invention after a 2h KOH durability test.

Figure 7 shows how the electrode according to the invention in a finite cell system does not suffer any degradation in its electrolysis catalysis after a lOOh durability test as per Example 5 Figure 8 shows how the electrode according to the invention in a zero gap cell system does not suffer any degradation in its electrolysis catalysis after a lOOOh durability test as per Example 6.

Detailed description

One aspect of the present invention is an improved electrode. The electrode comprises a substrate, a first layer present on the exterior surface of the substrate to promote surface area and/or porosity and a second layer present on the exterior surface of the first layer and wherein the second layer is an electrolysis promoting catalyst and has a surface area of 0.1-5m²/g. Preferably the surface area of the second layer according to the present invention has a lower limit of 0.15m²/g, even more preferably of 0.2m²/g. Preferably the surface area of the second layer according to the present invention has a higher limit of 4.5 m²/g, even more preferably 4 m²/g.

The surface area range confers an advantage to the electrode according to the present invention for hydrogen or oxygen evolution. The usual state of the art for catalysts is to create a surface with as high a surface area as possible to create a significantly large number of catalytic sites (ref. A Powder Metallurgy Route to Produce Raney-Nickel Electrodes for Alkaline Water Electrolysis; C I Bernacker et al 2019 J. Electrochem. Soc. 166 F357 DOI 10.1149/2.0851904jes). Some catalysts such as Raney nickel can have surface areas of over 100m²/g. The surprising feature of the present invention is that despite having a lower surface area than the state of the art, it provides an increase in performance due to the multiple layer effect causing an increase in bubble nucleation and release compared to the state of the art. In fact, the surface area of the present invention (0.1-5m²/g) is significantly lower than the state of the art. Another advantage of the electrode according to the present invention is that it demonstrates markedly improved durability compared to materials with similar composition such as the multi-layer electrode control mentioned in the Examples.

The substrate of the electrode according to the present invention can be any electrode material. Thus another advantage of the method of the present invention is that the nature of the substrate electrode is less or not relevant. With the right pretreatment methods known in the art, non- metallic surface (e.g. graphite, silicon etc.) or any metallic surface (e.g. copper, brass, nickel, mild steel, stainless steel, aluminium etc.) can be used as substrate. Within the context of the present invention, “outer” or “exterior” surface means the surface facing towards the electrolyte solution and away from the substrate of the electrode.

The first layer of the electrode coating according to the present invention is present on the outer surface of the substrate and this is added by any suitable method known in the art. The first layer can be any metallic/non-metallic alloy that is able to provide an array that allows the second layer to be added onto it so that it results with a surface area of 0.1-5 m²/g or the preferable embodiments as discussed above. It has been found that the resultant hierarchical structure (i.e. structures formed one upon the other at successively smaller length scales) is present on the substrate so that the second layer according to the present invention added onto its exterior and has the resultant desired surface area. It is also preferable that the first layer array allows the second layer to be present in a hierarchical structure. This is shown in the SEM images shown in Fig. 1 A and A’. The deposition of the catalyst layer directly onto the substrate without the addition of the first layer according to the present invention first gives a different structure as compared to the resultant deposition of the second layer onto the first layer according to the present invention. This is advantageous because the combination of layers provides a synergistic effect by having a second layer that is able to decrease the energy required to split water molecules with the specific fractal or globular morphology to increase the hydrogen and oxygen bubbles release from the surface.

The first layer according to the present invention is preferably a metal alloy comprising of Nickel (Ni), Copper (Cu) and Phosphorus (P). Preferably the first layer is added by deposition, more preferably electroless deposition.

The second layer according to the present invention can be any alloy suitable for hydrogen or oxygen evolution in water alkali electrolysis (catalytic alloy). It is not an improvement on a single layer, but its own layer of catalytic alloy deposited on the first layer. This is advantageous because the second layer according to the invention is durable and relatively inexpensive, as explained below and above, while maintaining high efficiencies required for hydrogen or oxygen electrolysis. It has also been found that the addition of the second layer onto the first layer according to the invention allows one to use a more varied type of catalytic alloy without sacrificing any benefit or efficiency of hydrogen or oxygen evolution. Preferably the alloy in the second layer according to the present invention comprises essentially of 4 metals. The 4 metals are preferably Chromium (Cr), Iron (Fe), Cobalt (Co) and Nickel (Ni). This is advantageous because they do not contain any precious metal thus reducing significantly the costs involved in producing the electrode according to the present invention.

It is even more preferable if the second layer according to the present invention is composed essentially of the following components present in the quantities listed in Table 1.

Table 1 - Preferred composition of the second layer according to the present invention

Within the context of the present invention, surface area can be determined using any method known in the art. Surface area is also known as Specific Surface Area (SSA). Electrochemical experiments (cyclic voltammetry (CV), galvanostatic experiment (GS) and Tafelplot are 3 methods known in the art. Scanning SEM with specific software is also another method known in the art.

BET (Brunauer-Emmett-Teller) analysis of gas adsorption-desorption at 77K is a preferred method of measuring the surface area as it can determine the specific surface area as opposed to other methods that can prove differences in activity without giving specific surface area measurements. The BET model is well-known method that calculates the SSA from the adsorption-desorption at 77 K. The surface area is determined from the adsorbed amount of gas, preferably Krypton gas. The BET analysis relates the quantity of gas adsorbed to the surface area.

Another aspect of the present invention is the use of the electrode according to the present invention in water alkali electrolysis because of its improved performance, improved durability and cheaper costs to produce. Thus an electrolyser comprising one or more electrodes according to the present invention is also contemplated within the scope of the present invention. The above disclosure is concentrated on electrodes for use in water alkali electrolysis, but variations of the above invention could also apply to other types of electrolysis and thus the above present invention could apply to other types of electrolysis with variations that could easily be applied by the person skilled in the art.

The electrode according to the invention may be present or used in a variety of electrolyser stacking systems (cathode and membrane). Two preferred methods known in the art that confer specific and added advantages are the finite cells and the zero gap cells.

Another aspect of the present invention is the method of producing the electrode according to the present invention. Any method suitable for its use in water alkali electrolysis is contemplated.

It is preferred that the method of producing the electrode according to the present invention comprises the two following separate steps: i) adding the first layer upon the substrate and subsequently ii) adding the second layer upon the first layer

This is preferred because it allows the first layer according to the present invention to form an array of surface asperities suitable to produce the desired surface area and morphology for the subsequently added second layer according to the present invention as discussed above.

Any method known in the art for adding the first layer according to the present invention in step (i) is contemplated within the scope of the present invention. Examples include electrolytic, electroless, sintering, ion beam etching, vacuum or plasma deposition methods known in the art. A preferred method of addition is by electroless deposition, even more preferable the nanoFLUX® method (nF method) as described in W02019021016 (the nanoFLUX deposition, also known as the nF method of deposition). Any aspect of the electroless deposition described therein is incorporated by reference to the present invention where allowed by relevant patent Law. These are the preferred embodiments of the method according to the present invention. This is because it is believed they add the porosity and surface area necessary so the addition of the second layer according to the present invention presents the above cited features and advantages over known prior art once added to an electrode according to the present invention.

Addition of the second layer in step (ii) of the method according to the present invention is preferably carried out by deposition. Examples of deposition techniques are vacuum or plasma deposition or electrochemical deposition. If electrochemical deposition is used, the depositing solution preferably contains one or more of the sources of the metals and phosphorus that are present in preferred embodiments of the second layer. The depositing solution of step (ii) preferably comprises one or more of the following components: CrCh*6H20, FeCh, NiCh*6H2O, CoCh*6H2O and NaFbPC^FbO. The sodium hypophosphite is preferably used as source of phosphorus (if phosphorus is desired in the second layer). It is noted that sodium hypophosphite is advantageous also because it prevents the formation of Cr^VI.

Preferably said depositing solution of step (ii) comprises said sources of metal in the solution at a concentration as listed in Table 2.

Table 2 - List of preferred ranges of concentration for components in the depositing solution of step (ii) of the method according to the present invention

Preferably said depositing solution of step (ii) also comprises malic acid and glycine. Malic acid and glycine act as a complexing agent for those metals and it is preferable to have such complexing agents to prevent powdering. The benefit of the electrode prepared according to the method of the present invention is that it is not powdery as, for instance, some versions of Raney’s nickel known in the art (ref. C I Bernacker et al 2019 J. Electrochem. Soc. 166 F357

DOI 10.1149/2.0851904jes article cited above) and which are the standard by which other electrodes are assessed. This is advantageous as it prevents contamination of the systems involved. Preferably malic acid and glycine are present in the depositing solution of step (ii) at concentration in the ranges shown in Table 2. NaCl is also preferably present in the depositing solution of step (ii) as a conducting salt to increase the overall conductivity of the solution. Preferably NaCl is present in the depositing solution of step (ii) at a concentration in the range shown in Table 2.

Boric acid is also preferably present in the depositing solution of step (ii) to function as a pH buffer. Preferably boric acid is present in the depositing solution of step (ii) at a concentration in the range shown in Table 2.

The deposition of the depositing solution of step (ii) according to the method of the present invention is preferably carried out at a pH adjusted to 3.2-4.5. This is even more preferably adjusted to pH 3.8. The pH in the depositing solution of step (ii) is adjusted preferably with NH₄OH.

The current density during deposition of the depositing solution of step (ii) according to the method of the present invention is kept between 5 and 10 A/dm², more preferably 7 A/dm² and during the deposition the solution is kept under vigorous stirring.

In an even more preferred embodiment, the depositing solution of step (ii) of the method according to the present invention comprises the following components at the concentration listed in Table 3.

Table 3 - List of preferred concentrations for components in the depositing solution of step (ii) of the method according to the present invention

The temperature during step (ii) of the method according to the present invention is preferably kept between 35 and 55° C, more preferably at 45°C. The addition in step (ii) of the method according to the present invention preferably takes between 20 and 80 min, more preferably between 55-65min, even more preferably around 60 min.

From the above description, it is clear that to the person skilled in the art that the above disclosure of the electrode according to the present invention benefits from some advantages of being produced according to preferred embodiments of the method of the present invention also disclosed here above. Resultant features of the method according to the present invention are thus incorporated as features that could apply to the electrode according to the present invention and the uses thereof also.

The above cited advantages are exemplified in the following examples.

Examples

SEM-EDS Characterization:

All the images were acquired with a Hitachi 4200TM Plus in BSE acquisition mode with an acceleration voltage of 15KV. The magnification used was 1200x. All the EDX data were acquired with a Bruker ESPRIT Quantax 200 EDS system, the acquisition time was 10 min.

Chemicals:

All the chemicals used were purchased from Sigma Aldrich with an ACS purity grade. nanoFLUX® deposition:

Nickel meshes were used as substrate and degreased with acetone for one minute then pickled in HC1 10% v/v for 2 minutes. After this pre-treatment, the samples were activated in a solution containing 0.01 g/L of PdCh for 1 minute and transferred in nanoFLUX® (nF) kept at 70°C. the deposition time was 45 mins. The samples were rinsed with deionized water in between each step. More details of this method can be found in the description of the method W02019021016 but are known in the art already.

The nanoFLUX® (nF) process deposits a micro dendritic structure on the surface of a Ni-Cu- P alloy. The process is deposited via electroless deposition utilising the reduction of the metals with a reducing agent. The structures can be deposited to create different sized microstructures by varying the deposition time. The nanoFLUX deposition bath contains between 5-15g CuSO4.5H₂O, 0.5-lg N1SO4.6H2O 10- 50g HBO3 10-50g NasCeFFO?, 10-50g NaPCEFE FEO in 1 litre of distilled water. The solution is adjusted to pH 9 with NaOH 6M is heated to 80 degrees and 0.05g PEG added.

A metal substrate is first cleaned and then added to a O. lg/L PdCl₂ solution for 2 minutes followed by washing and added to the nanoFLUX bath. The sample is left in the bath for a specific timeframe determined by the dendrite size before being removed and washed.

Catalytic alloy deposition:

The Nickel meshes coated with nF are degreased with acetone for 3’, rinsed and pickled with HC1 10% v/v for 30”. The are then transferred in the catalyst electroplating bath, which is kept at 40°C. The deposition time is fixed at 40 minutes and the current density used is 7.65A/dm².

The solution was kept under agitation using a stirring bar at 1200rpm.

Example 1 - Characterization of the multilayer electrode according to the present invention with respect to a single layer

Samples of substrate were coated with the catalytic alloy according to the method according to the present invention. In step (i) nanoFLUX® deposition occurred. In step (ii) the catalytic alloy was deposited using the following solution:

Samples were also coated directly with the catalytic alloy. SEM were taken to show the resultant morphology in Fig. 1 A and A’ respectively and it is clear how the deposition of nanoFLUX® in step (i) creates a different morphology on the resultant catalytic alloy where hydrogen evolution occurs.

The multi-layered sample according to the present invention showed improved performance compared to the same alloy coated on a “naked” sample (i.e. without prior nF deposition). The peak current density with a polarization curve was circa 10% larger for the multi-layered sample (Fig. 2).

Example 2 - Characterization of the surface area range of the multilayer electrode according to the present invention

A multi-layer control was created to demonstrate the advantages of the surface area range according to the present invention. Ni-Co alloy nanocone array were electrodeposited onto the Cu microcone arrays. The Cu microcones were obtained by an electroless deposition method known in the art. Subsequently, Ni-Co alloy nanocone arrays were electroplated on the as- prepared Cu microcones arrayed substrate at a constant current of 0.1 A/ dm² and 60°C for 5- 10 min.

The content of NiC12*6H2O in the plating bath was fixed at 1 mol L’¹, while the content of COC12*6H2O was varied in the range of 0-0.05 mol L’¹. H3BO3 (0.5 mol L'¹) and ethylene diamine dihydrochloride (1.0 mol L'¹) were added as buffer agent and crystallization modifier, respectively. The pH value of the plating bath was adjusted to 4.0 using ammonium hydroxide.

A SEM image of the resultant multi-layer deposit is shown in Fig. 3 and it shows a higher surface area than the range claimed in the present invention.

The coating was tested for catalytic activity and compared to the electrode according to the present invention as made in Example 1 (the OnS catalyst) and which showed a remarkably higher activity as shown in Fig. 4.

The onset voltage for hydrogen production was remarkably lower for the OnS catalyst and the current achieved at -2.2 V was 25% higher. Example 3 - Specific surface area tests

A BET (Brunauer-Emmett-Teller) analysis using Krypton gas at 77K was carried out to determine the specific surface area of two samples: one with uncoated nickel mesh (baseline) and the other with coated nickel mesh. Three mesh samples were added to sample cells. SSA were carried out and shown in the table below. Similar tests were carried out on coated mesh samples. The difference between the uncoated and coated samples were then determined by appropriate software and are shown in the following table:

Example 4 - Durability tests The multi-layer electrode according to the present invention produced in Example 1 underwent accelerated durability tests known to those skilled in the art.

These tests involve cells comprising the electrodes being kept under flowing KOH at a flow rate of 50 mL/min. KOH used was in the concentration of 8M and at 90° C. After a period of 36h of a steady current (0.8A/cm²), periods of on and off current 1 hour long each were alternated over the total testing time. The off time is the most corrosive as, when the current is on, cathodic protection is provided to the cathode For the electrode according to the present invention, the testing time was lOOh and, as is shown in Fig. 5 A, the electrode did not show any reduction in efficiency. A SEM image was taken before and after the lOOh durability test and it showed no change in the morphology of the catalytic alloy (Fig. 5B), thus showing its extraordinary durability.

An accelerated durability comparison test was run against the multi-layer control sample produced in Example 2. The electrode according to Example 1 and the control according to Example 2 were tested for activity after being kept in KOH 8M at 130° C for two hours.

The polarization curve were recorded and are shown in Fig. 6. As is seen in Fig. 6, the multilayer control sample produced in Example 2 has degraded and catalyst activity drops by 10% while the multi-layer electrode sample according to the present invention shows no degradation (when compared to polarization curves of Fig. 4).

Example 5 - Durability test in a Finite Gap Cell System

Samples of expanded nickel mesh were coated using the above method of Example 1 with the difference that in step (ii) the catalytic alloy was deposited using the preferred solution of Table 3 here above.

The baseline uncoated samples were added to an electrolyser test cell stack and the coated samples were coated using the above method. The coated samples were then added to the test stack with uncoated mesh being used as anodes. A gap of 2mm was created between the anode and cathode with a Zirfon Perl UTP 500 membrane added between.

The test was run by pumping electrolyte (KOH 20% 60oC) through the cells and keeping the current density at 0.45A/cm2. The voltage was measured between each cell. The cell was cleaned ready for testing by cycling the current density from 0.45A/cm2 to 0 A/cm2 over 1 hour and then back. Once this had been carried out twice the current density was kept constant. After 100 hours of testing a further set of current density cycling was carried out for 16 hours to prove durability.

The results are shown in Figure 7, which shows not only high performance but also demonstrates good durability.

Example 6 - Durability test in a Zero Gap Cell System Samples of expanded nickel mesh were coated using the above method of Example 1 with the difference that in step (ii) the catalytic alloy was deposited using the preferred solution of Table 3 here above.

The baseline uncoated samples were added to an electrolyser test cell stack and the coated samples were coated using the above method. The coated samples were then added to the test stack with uncoated mesh being used as anodes. A Zirfon Perl UTP 500 membrane was added between and the cell pressurised to 5bar.

The test was run at industrially relevant conditions by pumping electrolyte (KOH 30% 90oC) through the cells and keeping the current density at 0.9/cm2. The voltage was measured between each cell. The cell was cleaned ready for testing by cycling the current density from 0.9A/cm2 to 0 A/cm2 over 1 hour and then back. Once this had been carried out twice the current density was kept constant for 36 hours. After this, 950 hours of durability testing was carried out by cycling the current density ever hour.

The results are shown in Figure 8, which shows not only high performance constant at a voltage close to PGM and there was hardly any change in durability over the 1000 hours.

Claims

1. An electrode comprising a substrate, a first layer present on the exterior surface of the substrate to promote surface area and/or porosity and a second layer present on the exterior surface of the first layer and wherein the second layer is an electrolysis promoting catalyst and has a surface area of 0.1-5 m²/g.

2. The electrode according to claim 1 wherein the surface area is 0.15-4.5 m²/g.

3. The electrode according to claims 1-2, wherein the second layer is present in a fractal or globular morphology.

4. The electrode according to any of claims 1-3, wherein the first layer is porous and has a microstructured topography on its exterior surface.

5. The electrode according to any of claims 1-4, wherein the first layer is an alloy.

6. The electrode according to claim 5, wherein the alloy comprises Nickel (Ni), Copper (Cu) and Phosphorus (P)

7. The electrode according to any of claims 1-6, wherein the second layer is an oxygen or hydrogen evolving catalytic alloy.

8. The electrode according to claim 7, wherein the oxygen or hydrogen evolving catalytic alloy comprise essentially of 4 metals.

9. The electrode according to claim 8, wherein the 4 metals are Cr, Fe, Co and Ni.

10. The electrode according to claim 8 or 9, wherein the alloy is comprised within the following range:

11. An electrolyser comprising an electrode according to any one of claims 1-10.

12. A method for producing the electrode according to any of claims 1-10 wherein the method comprises the following step: i) adding the first layer upon the substrate and subsequently ii) adding the second layer upon the first layer

13. The method according to claim 12, wherein nanoFLUX deposition is used to add the first layer upon the substrate in step (i).

14. The method according to claim 12 or 13, wherein the addition of step (ii) is done by deposition with a solution that comprises CrCh*6H20, FeCh, NiC12*6H2O, COC1₃*6H₂O and NaH₂PO2*H₂O.

15. The method according to any of claims 12-14, wherein the addition of step (ii) is done by deposition with a solution that comprises malic acid and/or glycine and/or NaCl and/or boric acid.

16. The method according to any of claims 12-15, wherein the addition of step (ii) is done by deposition with a solution and wherein the pH is adjusted to 3.2-4.5 with NH4OH and/or the temperature is kept between 35 and 55° C and/or deposition time range is 20-80 min and/or the current density is kept between 5 and 10 A/dm² and/or during the deposition the solution is kept under vigorous stirring.