A method of manufacturing a coated nickel substrate

Abstract

The present invention relates to a method of manufacturing a coated nickel substrate for use as an alkaline oxygen evolution anode, the coated nickel substrate obtainable by the method and the use of the coated nickel substrate as an alkaline oxygen evolution anode in an alkaline water electrolyser.

Description

[0001] A METHOD OF MANUFACTURING A COATED NICKEL SUBSTRATE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a method of manufacturing a coated nickel substrate for use as an alkaline oxygen evolution anode, the coated nickel substrate obtainable by the method and the use of the coated nickel substrate as an alkaline oxygen evolution anode in an alkaline water electrolyser.

[0004] BACKGROUND

[0005] Nickel is a metal widely used in alkaline water electrolysis due to its high corrosion resistance in alkaline environments (high pH) at a wide range of electrochemical potentials. This can be understood from the Pourbaix diagram, which illustrates that at relevant pH, which here is pH > 13, nickel is either stabilized as metallic nickel at cathodic potentials (below 0.2 V measured against the reversible hydrogen electrode, RHE) or will form a protective oxide / hydroxide layer at anodic potentials (above 0.2 V measured against the reversible hydrogen electrode, RHE). This protective oxide / hydroxide layer makes nickel one of the only pure metals that withstand the chemically harsh environment that the anode endures in an alkaline water electrolyser.

[0006] The oxide / hydroxide layer that forms on nickel should not be understood as an exotic type of material. If nickel is submerged in a strong alkaline electrolyte (could be 0.1 - 10 M xOH, where X = Li, Na, K or Cs), an oxide / hydroxide layer would naturally form as either oxygen gas, 02, is dissolved in the electrolyte or the nickel is subjected to electrochemical potentials above 0.2 V vs RHE. The grown oxide / hydroxide would initially only become 1-2 atomic layers thick. Still, if the electrochemical potential is further increased in the range of 0.2-1.3 V vs RHE, the oxide / hydroxide layer would continue to grow at a decreasingly slower rate. It should be noted that a few atomic layers of nickel oxide / hydroxide suffice in stabilising nickel, and a thicker layer would not create a more stable anode.

[0007] The oxide and hydroxide layers are, in fact, two distinct materials. The oxide layer consists of a three-dimensional unit cell with one nickel atom for each oxygen atom and is described with the chemical formula NiO. The hydroxide layer is a two-dimensional structure consisting of one nickel atom for every two oxygen atoms. The arrangement of the atoms is formally described as a brucite structure, where the nickel and oxygen atoms are octahedrally coordinated. Furthermore, a hydrogen atom is loosely bound to every oxygen atom. The chemical formula for this structure is Ni(OH)2. In both the oxide and the hydroxide layer, the neutral nickel atom is oxidised to Ni2+; however, where nickel oxide is a dense material, like metallic nickel, nickel hydroxide is a more open structure where water and dissolved ions can diffuse in between the hydroxide layer. When the nickel metal is subjected to dry oxygen gas, O2, a nickel oxide layer mainly forms, yet when nickel is oxidised in an alkaline electrolyte, OH- will react with nickel and form a hydroxide layer.

[0008] There exist various types of nickel hydroxide. They all share the fundamental Ni(OH)2chemical formula, but the alpha-Ni(OH)2is characterised by a more disordered structure (many defects, the layers do not stack perfectly), whereas the beta-Ni(OH)2is more dense and ordered. At electrochemical potentials where oxygen evolution takes place, alpha- and beta-Ni(OH)2are further oxidised (Ni2+oxidises to Ni3+) to gamma- and beta-NiOOH, known as an oxyhydroxide. How these structures form and transform into each other are summarised in the Bode scheme, and the properties of the various forms of Ni(OH)2and NiOOH have been characterised to a notable extent due to these materials' scientific and industrial relevance.

[0009] Prior art methods for generation of nickel hydroxide layers on a nickel substrate include electrodeposition, hydrothermal process, co-precipitation and topochemical synthesis.

[0010] Electrodeposition is a method that uses a technique similar to regular electroplating of metals. Usually, metal ions dissolved in an electrolyte are reduced on the cathode and plated in a metallic form. Here, nickel ions are still present in the electrolyte, but the cathode facilitates the reduction of water (production of H2). This causes a local increase of pH close to the surface. The nickel ions close to the surface react with the hydroxide from the increased pH and precipitate onto the cathode as hydroxide. Iron and various additives can be included to control the composition and structure of the deposited hydroxide.

[0011] In hydrothermal process a nickel-containing salt (e.g. Ni(NO3)2or NiCI2) is added to an autoclave together with a nickel substrate. The autoclave is heated to 100-150 °C and cooled down, and hydroxide is grown on the substrate. The temporal change in temperatures and the increased pressure facilitate the growth. Iron and various additives can again be included to control the composition and structure of the grown hydroxide.

[0012] Co-precipitation is a method in which a solution with a nickel-containing salt is added to an alkaline solution. The increased pH causes the nickel to precipitate as nickel hydroxide. It will not precipitate into any substrate (the electrode), so this would need to be done in a separate step. Iron and various additives can again be included to control the composition and structure of the grown hydroxide.

[0013] In topochemical synthesis an oxide, MOF (metal-organic framework), or some other sacrificial precursor is transformed into a hydroxide through various chemical treatments. This method is easily distinguished from the others as it relies on a solid pre-structure that is not already a hydroxide and is then transformed into the catalytically active hydroxide.

[0014] Under the conditions of alkaline electrolysis, nickel anodes (e.g., expanded sheet, foams) are activated during operation (i.e., in situ) due to (1) the formation of a nickel oxyhydroxide (NiOOH) layer on the surface, and (2) the absorption of iron impurities from the electrolyte (naturally present due to the use of steel as a construction material) into the NiOOH host. This process results in the formation of nickel-iron oxyhydroxide (Nii.xFexOOH) phases that provide the actual active sites for the oxygen evolution reaction (OER) at decreased overpotentials. It has been observed that the effectiveness and durability of this activation depends on the initial condition of the anode surface, and that it is typically beneficial to deposit a Ni(OH)2layer on the anode prior to operation, as this results in lower cell voltages compared to pristine nickel electrodes. In most approaches, Ni(OH)2powders are first prepared through electrochemical or hydrothermal methods (often in the presence of Fe3+ions) and then deposited onto the anode surface. However, these preparation methods require a high energy input and involve complex multi-step procedures, making them unsuitable for upscaling. Moreover, the resulting Ni(OH)2coatings often show low conductivity with the underlying substrate as well as poor mechanical stability under realistic operation conditions.

[0015] SUMMARY OF THE INVENTION

[0016] The inventors of the present invention have surprisingly found a new single-step synthesis, which is able to grow a robust nickel hydroxide layer directly on a nickel anode, conferring sufficient conductivity and mechanical stability, with a suitable structure for subsequent in situ activation and iron absorption from the lye. Compared to traditional approaches, the novel method according to the present invention is more amenable to upscaling, while iron incorporation from the electrolyte during operation (iron is not part of the electrode synthesis process) obviates the need for lye preelectrolysis and possibly mitigates the negative effect of iron impurities on the activity of the cathode and on the lifetime of the diaphragm. In a first aspect, the present invention is directed to a method of manufacturing a coated nickel substrate for use as an alkaline oxygen evolution anode, said method comprising the steps of: a. providing a nickel substrate, b. providing an aqueous alkaline solution comprising an oxidant, such as H2O2or (NH4)2S2O2, a nickel-containing salt, such as in form of Ni(NO3)2and a metal hydroxide, such as KOH, c. heating the solution obtained in step b to a temperature in the range of 20 °C to 80 °C, such as about 50°C, d. immersing the nickel substrate provided in step a in the heated solution obtained in step c, e. leaving the nickel substrate in the heated solution for a period of 2 - 36 hours or longer, such as about 18 hours, thereby obtaining a coated nickel substrate, f. removing the coated nickel substrate from the heated solution, g. cleaning the surface of the coated nickel substrate by washing with a cleaning agent, such as water, and h. drying the coated nickel substrate at ambient temperature.

[0017] In a second aspect, the present invention is directed to the coated nickel substrate obtainable by the method.

[0018] In a third aspect, the present invention is directed to the use of the coated nickel substrate as an alkaline oxygen evolution anode in an alkaline water electrolyser, where said electrolyser further comprises an electrolyte comprising iron ions as impurity or added in an amount of 0.5 mg / L - 20 mg / L.

[0019] BRIEF DESCRIPTION OF THE FIGURES

[0020] Fig. 1 shows the coating in a scan with the insert of a similar scan of a HCL washed nickel reference,

[0021] Fig. 2 time versus voltages obtained during electrolysis using the shown scheme of electric current feed to experimental cell,

[0022] Fig. 3 are IV curves obtained at 100 hours for the two different electrodes and without, as well as with, iron ions in the electrolyte,

[0023] Fig. 4 are IV curves obtained at 250 hours for the two different electrodes and without, as well as with, iron ions in the electrolyte. DETAILED DESCRIPTION OF THE INVENTION

[0024] In the first aspect, the present invention provides a novel method of manufacturing a coated nickel substrate for use as an alkaline oxygen evolution anode.

[0025] In step a, a nickel substrate is provided. The nickel substrate is preferably a pure nickel substrate which is elementally clean. Nickel 200 or any metallurgical grade of higher purity is preferred. Alternatively, the nickel substrate is made of transition metal alloys with a high nickel content. Suitable examples of transition metals include iron, chromium, cobalt, copper and molybdenum. The nickel substrate may be formed as a solid plate, a mesh, expanded sheet or sheets, a foam or woven threads or combinations of the mentioned. The nickel substrate may be cleaned prior to use in any strong acid, which would remove the natural oxide layer present on the nickel substrate. The cleaned nickel substrate can be stored in air, in water or in ethanol for any amount of time.

[0026] In step b, an aqueous alkaline solution comprising an oxidant, a nickel-containing salt, and a metal hydroxide is provided. Suitable examples of the oxidant include H2O2and (NH4)2S2O2, where (NH4)2S2O2, is preferred. In cases where (NH4)2S2O2is used as oxidant the concentration of (NH4)2S2O2is typically in the range 0.1 M to 3 M, and preferably about 0.33 M.

[0027] The nickel-containing salt can be any salt containing nickel, provided that the salt only contains a very low amount of other transition metals than nickel. Suitable examples include NiSO4, NiCI2and Ni(NO3)2. In a preferred embodiment, the nickel-containing salt is Ni(NO3)2and the concentration of Ni(NO3)2in the solution obtained in step b is about 1 mM.

[0028] The metal hydroxide can be any alkali hydroxide, such as LiOH, NaOH, KOH, RbOH and CsOH. In a preferred embodiment the metal hydroxide is KOH and the concentration of KOH in the solution is in the range 0.1 M to 3 M, preferably about 2 M KOH.

[0029] In a preferred embodiment, the aqueous alkaline solution of step b is formed by providing an oxidant-containing solution, an aqueous metal hydroxide-containing solution and a nickel-containing solution, and then mixing the oxidant-containing solution and the aqueous metal hydroxide-containing solution to form an alkaline oxidant-containing solution. Thereafter the nickel-containing solution is added to the alkaline oxidant-containing solution, whereby the aqueous alkaline solution of step b is obtained. Preferably, the oxidant-containing solution comprises (NH4)2S2O2and the concentration of (NH4)2S2O2is typically in the range 0.5 to 2 M, preferably about 1 M.

[0030] The aqueous metal hydroxide-containing solution preferably comprises KOH and the concentration of KOH is typically in the range of 1 to 6 M and preferably about 3 M.

[0031] The nickel-containing solution preferably comprises Ni(NO3)2and the concentration of Ni(NO3)2is typically in the range 0.01 and 0.5 M, preferably about 0.1M.

[0032] Any of the prepared solutions should preferably not contain more than 1 ppm of any transition metal impurity other than nickel.

[0033] In step c, the solution obtained in step b is adjusted to a temperature in the range of 20°C to 100°C. Preferably the solution is heated to a temperature about 50°C.

[0034] In step d, the nickel substrate is immersed in the heated solution. The solution can be stirred to create some form of flow and uniform temperature over the nickel substrate. In some embodiments, the solution with the nickel substrate is further pressurized to increase the growth rate, optionally by increasing the temperature above 100 °C.

[0035] In step e, the nickel substrate is left in the heated solution for a period of time, whereby a coated nickel substrate is obtained. If the nickel substrate is only left in the solution for a short period of time, then only a very thin layer of coating is formed on the surface, and it is therefore preferred to leave the nickel subject in the solution for a period of 2 - 36 hours, preferably about 18 hours.

[0036] In step f, the coated nickel substrate is removed from the heated solution and thereafter it is cleaned in step g by washing with a cleaning agent, such as water. Finally, the coated nickel substrate is dried at ambient temperature in step h.

[0037] In the second aspect, the present invention relates to a coated nickel substrate obtainable by the method described above.

[0038] A major advantage of the method according to the invention is that basically it is a one- step simple dipping method, once the growth solution is obtained, and electrode base metal has been cleaned as prescribed. The mixture forming the solution may be stored and can thus be made in advance. Precipitates formed during the mixing process are preferably left within the solution and stirring prior to pumping the mixture in and out of dipping vessels in an industrial scale setting is preferred. The once mixed solution may be used for dipping and growth of coating may be used a multitude of times adding to the ease of industrialisation of the method.

[0039] The coated nickel electrode substrate obtained by the process may be stored any length of time at ambient temperatures and remain stable also during humidity and temperature swings over seasons. The coating is mechanically robust and can withstand handling by operators and machinery during manufacturing processes relevant to electrolyser stack production.

[0040] In the third aspect, the present invention relates to the use of the coated nickel substrate of claim 3 as an alkaline oxygen evolution anode in an alkaline water electrolyser, where said electrolyser further comprises an electrolyte comprising iron ions in an amount of 0.5 mg / L to 20 mg / L of electrolyte

[0041] When an anode electrode with the prescribed surface is used in alkaline water electrolysis with no presence of iron ions in the electrolyte, the results are as expected meagre, however as soon as iron ions are present good results appear. In some industrial electrolyser plants, iron ions are present in the electrolytes, more as an unavoidable impurity than as an addition. In such cases, this particular coating of the anode will not only reduce adverse effects of the iron ions usually observed but will actually increase production efficiency.

[0042] EXAMPLES

[0043] The method of preparing the coated nickel substrate belongs to the order of corrosion engineering: A metallic substrate is left in an ionic solution with a corrosion promotor that accelerates the growth of hydroxide on the substrate. Typical corrosion promoters are Cl- ions or Na2S2O3, but this process would still require that the corrosion happens spontaneously, which would be the case if metallic iron was submerged in a solution containing Ni2+ions. Here, the energetic difference between Ni2+and Fe2+ / 3+would drive the reaction. Alternatively, the corrosion would be driven by a strong oxidant like H2O2or (NH4)S2O8.

[0044] This synthesis procedure can be described as a corrosion engineering approach since the obtained nickel hydroxide is obtained by a spontaneous redox reaction of the nickel surface.

[0045] A typical method of preparing the coated nickel substrate is as described below. A pure nickel substrate is ultrasonicated in 1 M HCI for ten minutes, followed by a thorough washing in Mill! Q water, followed by 5 minutes ultrasonication in Mill! Q water. The cleaned nickel substrate is left to dry for 0-2 hours. A solid plate of nickel 200 was used.

[0046] In an inert container (such as a PP or PTFE container, and used below when nothing else is noted), a 1 M (NH4)2S2O2 solution is prepared. In a separate inert container, a 3 M KOH is prepared. In yet a separate inert container, a 0.1 M solution of Ni(NO3)2is prepared. The prepared solutions do not contain more than 1 ppm of any transition metal impurity.

[0047] 1 part of the (NH4)2S2O2solution is added to 2 parts of the KOH solution. The Initial concentration of the solution is thus 0.33 M (NH4)2S2O2and 2 M KOH. The solution needs to be contained in an inert container. White crystals will start precipitating when the two solutions are mixed. The mixing procedure is not essential to the outcome.

[0048] The 0.1 M Ni(NO3)2solution is added to the (NH4)2S2O2and KOH solution such that the final concentration of Ni(NO3)2is 1 mM. The solution will turn black when Ni(NO3)2is added. This is because the majority of the nickel ions are oxidised in the (NH4)2S2O2 / KOH solution and precipitate as NiOOH. With time, all the precipitates will fall to the bottom, leaving a clear solution. In the following the precipitate was left in the reaction container during treatment of the samples.

[0049] The solution in the container is heated to 50 degrees C. Once the solution has reached the desired temperature, the cleaned nickel substrate is fully immersed in the solution. Having the nickel stand freely in the solution is the best practice. For instance, a nickel plate is not supposed to lie flat on the bottom of the container. This is to avoid any mass transport limitation during the synthesis. No effect from nickel substrate being partially submerged in the precipitates from the solution are observable.

[0050] The nickel substrate is left in the (NH4)2S2O2 / KOH / Ni(NO3)2solution for 18 h, at 50 deg.

[0051] When the 18 h has passed, the now-coated nickel substrate is removed from the warm solution and washed gently with Mill! Q water.

[0052] The now-coated nickel substrate is left to dry (at ambient temp, in air), and this completes the synthesis method. The structure, composition and appearance of the now-coated nickel substrate is described in the last section. In Fig. 1, a scan of an electrode coated with a hydroxide layer as proposed above is shown, with an insert of a scan prior to the actual coating procedure, but after HCL cleaning.

[0053] Fig. 2 displays tests with 3 distinct test cell voltage schemes. Throughout the duration of the 3 voltage schemes, the performance of the treated anode and a clean nickel anode (reference) were compared.

[0054] The following test conditions were applied :

[0055] Temp 80°C Electrolyte 8 M KOH Current density 600 mA / cm2 Cathode Clean nickel

[0056] Diaphragm Zirfon 500 UTP

[0057] The test was performed on the potentiostat, allowing simultaneous recording of the cell and anode potentials.

[0058] The upper curves marked V vs. RHE records the IR compensated cell potentials with hours on the horizontal axis. The lover set of curves, marked Analog V vs. RHE records the Anode potential, however not IR compensated.

[0059] From 0 to 120 hours, marked "phase 1) Constant current" the cell was driven with a constant current. From 120 to 220 hours marked "phase 2) On / off cycling", the cell was driven through an on / off cycling scheme and from 220 to 310 hours, marked "Phase 3) Constant current), a constant current scheme was employed again.

[0060] During all 3 phases, IV curves 20 were recorded as indicated. The IV curve results after 100 hours and after 250 hours test are displayed in Fig. 3 and Fig. 4.

[0061] In each of Figs. 3 and 4, an upper row 21 and a lower row 22 of diagrams are shown. Upper row diagrams 21 show cell voltage IR-compensated, where lower row diagrams 22 shows Anode vs. RHE: Not IR-compensated. Upper row diagrams 21 are aligned vertically and cell voltage at 600 mA / cm2 for the reference electrode in column 23 is indicated across all diagrams in the upper rows 21. Likewise, the lower row diagrams 22 are aligned vertically and anode versus RHE voltage for the reference electrode in column 23 is traced through all of the aligned diagrams in row 22. Moving from left to right, there are 4 columns 23, 24, 25, 26 and each column show the results from the following electrodes and electrolyte combinations:

[0062] 23: Clean Ni - No Fe

[0063] 24: Clean Ni - 1 mg Fe / L

[0064] 25: Ni-Ni(OH)2- No Fe 26: Ni-Ni(OH)2- 1 mg Fe / I

[0065] All of the diagrams in Figs. 3 and 4 have current density (A / cm2) along the horizontal axis.

[0066] In the diagrams in column 26 in Fig. 3 arrows 27, 28 mark the difference in voltage at 600 A / m2 between the clean Ni - No Fe electrode electrolyte combination in column 23 and the electrode with the Ni-Ni(OH)2surface and an amount of iron in the electrolyte in column 26.

[0067] Arrow 27 indicates an improvement of 150 mV decrease on the cell level, and arrow 28 indicates an improvement of 180 mV decrease on the anode voltage. The cell level improvement is smaller than the anode level improvement because probably there was a bit of cathode degradation. This is nevertheless a considerable improvement after 100 hours.

[0068] In the diagrams in column 26 in Fig. 3 arrows 29, 30 likewise are indicators of improvements but here after 250 hours of test, between the clean Ni - No Fe electrode electrolyte combination in column 23 and the electrode with the Ni-Ni(OH)2surface and an amount of iron in the electrolyte in column 26.

[0069] Again, even after 250 hours of test and the stress of the on-off cycling, arrow 29 indicates 70 mV decrease in cell voltage and arrow 30 indicates 100 mV decrease on the anode voltage.

[0070] This also shows how the anode with the synthesized layer of Ni-Ni(OH)2according to the invention will work even in high concentration levels of iron ions in the electrolyte.

Claims

CLAIMS1. A method of manufacturing a coated nickel substrate for use as an alkaline oxygen evolution anode, said method comprising the steps of: a. providing a nickel substrate, b. providing an aqueous alkaline solution comprising an oxidant, such as H2O2or (NH4)2S2O2, a nickel-containing salt, such as Ni(NO3)2and a metal hydroxide, such as KOH, c. heating the solution obtained in step b to a temperature in the range of 20 °C to 80 °C, such as about 50 °C, d. immersing the nickel substrate provided in step a in the heated solution obtained in step c, e. leaving the nickel substrate in the heated solution for a period of 2 hours to 36 hours, such as about 18 hours, thereby obtaining a coated nickel substrate, f. removing the coated nickel substrate from the heated solution, g. cleaning the surface of the coated nickel substrate by washing with a cleaning agent, such as water, and h. drying the coated nickel substrate at ambient temperature.

2. The method according to claim 1, wherein step b comprises the steps of: bl. providing an oxidant-containing solution comprising an oxidant, such as H2O2or (NH4)2S2O2, b2. providing an aqueous metal hydroxide-containing solution comprising a metal hydroxide, such as KOH, b3. providing an aqueous nickel-containing solution comprising a nickel-containing salt, such as Ni(NO3)2, and b4. mixing the oxidant-containing solution of step bl and the aqueous metal hydroxide-containing solution of step b2 to obtain an alkaline oxidant-containing solution, and b5. adding the aqueous nickel-containing solution of step b3 to the alkaline oxidantcontaining solution of step b4 to provide the aqueous alkaline solution of step b.

3. A coated nickel substrate obtainable by any of the claims 1 and 2.

4. Use of the coated nickel substrate of claim 3 as an alkaline oxygen evolution anode in an alkaline water electrolyser, where said electrolyser further comprises an electrolyte comprising iron ions in an amount of 0.5 mg / L to 20 mg / L.

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