Electrocatalytic coating

Abstract

Electrocatalytic Coating The invention discloses a process for coating electrolyser plate comprising the steps of pre- treating the plate and coating the plate with nickel and cobalt alloy by electrodeposition. The nickel and cobalt alloy is electrodeposited from a deep eutectic solvent to form an 5 electrocatalytic coating with corrosion resistance that is nanocrystalline.

Description

[0001] Electrocatalytic Coating

[0002] Field of Invention

[0003] The invention relates to electrocatalytic coating for electrolyser plate.

[0004] Background

[0005] Hydrogen is an ideal clean energy source when compared with other energy sources, as it has high calorific value, high energy density, and multiple storage methods. There are several hydrogen production methods. Hydrogen production by water electrolysis has a position in the market due to its high purity, simple operation and zero pollution, and it is also the most commonly used method for converting renewable energy into hydrogen storage. However, today less than 4% of hydrogen production is based on electrolysis processes, of which the main part is hydrogen as a by-product of chlorine production. Hence, the major share of the needed hydrogen depends on the fossil path through the steam reforming of natural gas. This due to the higher production costs of electrolysis processes compared to conventional fossil sources, due to high electricity costs and interfering laws. To reduce CO2emissions and to become independent of fossil energy carriers, the share of hydrogen produced using renewable power sources needs to be increased significantly in the next few decades. Therefore, water electrolysis is a key technology for splitting water into hydrogen and oxygen by using renewable energy. After drying and removing oxygen impurities, the hydrogen purity is higher than 99.9%, and the hydrogen can be directly utilised.

[0006] For water electrolysis, there are three common technologies available: Alkaline water electrolysis (AWE), proton exchange membrane (or polymer electrolyte membrane) electrolysis (PEMEL), and solid oxide electrolysis (SOEL). While the low-temperature technologies, AWE and PEMEL, both provide high technology readiness levels, the high- temperature SOEL technology is still in the development stage. Hence, the investment costs and the lifetime determine whether AWE or PEMEL is the most favorable system design for a large-scale application. Today, the investment costs for AWE is lower than that of PEMEL. Furthermore, the lifetime of alkaline water electrolysers is higher and the annual maintenance costs are lower compared to a PEMEL system.

[0007] The shortcomings of AWE are gradually being overcome by further development. The AWE system has low capital expenditure due to non-requirement of precious metal, but faces corrosion challenges affecting durability and efficiency.

[0008] An aim of the invention therefore is to enhance the efficiency of alkaline water electrolysis (AWE) by providing an electrocatalytic coating for electrolyser plate. Summary of Invention

[0009] In an aspect of the invention there is provided a process for coating electrolyser plate comprising the steps of: pre-treating the plate; and coating the plate with nickel and cobalt alloy by electrodeposition; characterized in that the nickel and cobalt alloy is electrodeposited from a deep eutectic solvent to form an electrocatalytic coating with corrosion resistance that is nanocrystalline.

[0010] Advantageously pre-treatment ensures the plate is free of any impurities or foreign materials which then enables the coating to strongly adhere to the surface of the substrate.

[0011] Advantageously electrodeposition using deep eutectic solvent (DES); a non-aqueous electrolyte is selected as gas evolution at the anode / solution interface is negligible, high current efficiencies are obtained and when compared to aqueous acid-based solutions, deep eutectic solvent is benign and non-corrosive.

[0012] Preferably the plate is stainless steel 316 (SS316).

[0013] In one embodiment pre-treating the plate can be achieved by grinding, acid pickling, chrome stripping, Wood’s nickel strike or a combination thereof.

[0014] In one embodiment the molar ratio of nickel to cobalt is preferably within the range of 0.1 :0.1 to 2:2.

[0015] Preferably the molar ratio of nickel to cobalt is 2:1 .

[0016] In one embodiment the deep eutectic solvent is a mixture of choline chloride, ethylene glycol, nickel chloride and cobalt chloride.

[0017] In one embodiment chronopotentiometry electrodeposition with an applied current within the range of 1 mA to 3 mA for a duration of up to 20 minutes is performed to coat the plate with nickel and cobalt alloy.

[0018] In one embodiment the average grain size of the coating is less than 500 nm. Advantageously the Ni-Co alloy coated SS316 showed better durability when utilised for alkaline water electrolysis compared to bare SS316 (without Ni-Co alloy coating) without any apparent catalyst layer peel-off from the coated SS316 surface. Advantageously the process presents a viable approach to deposit a smooth nanocrystalline Ni-Co alloy coating on SS316 by chronopotentiometry electrodeposition at ambient conditions using deep eutectic solvent.

[0019] In an aspect of the invention, there is provided a plate comprising; a coating of nickel and cobalt alloy; characterized in that the nickel and cobalt alloy is electrodeposited from a deep eutectic solvent to form an electrocatalytic coating with corrosion resistance that is nanocrystalline. In an aspect of the invention, there is provided a use of a plate in alkaline water electrolysis and anion exchange membrane electrolysis.

[0020] Advantageously the plate enhances the efficiency of alkaline water electrolysis thus potentially reducing the cost of hydrogen.

[0021] Brief Description of Drawings

[0022] It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.

[0023] Figure 1a illustrates a chronopotentiometric deposition curve at a constant current of 1 mA / cm2.

[0024] Figure 1 b illustrates stainless steel plates coated with Ni-Co alloy achieved by chronopotentiometry electrodeposition.

[0025] Figure 1c illustrates scanning electron microscope (SEM) images of stainless steel plates coated with Ni-Co alloy achieved by chronopotentiometry electrodeposition.

[0026] Figure 2 illustrates pretreated stainless steel plates coated with Ni-Co alloy achieved by chronopotentiometry electrodeposition.

[0027] Figure 3 illustrates Ni-Co alloy nanocrystallites deposited on pretreated stainless steel plate.

[0028] Figures 4a and 4b illustrate scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX) of pretreated stainless steel plates coated with Ni-Co alloy according to varying chronopotentiometry electrodeposition duration and applied current respectively.

[0029] Figure 5 illustrates corrosion analysis by anodic polarization curves of pre-treated bare and Ni-Co alloy coated stainless steel plates before and after alkaline water electrolysis.

[0030] Figure 6 illustrates hydrogen evolution reaction (HER) potentials on a cyclic voltammogram using pre-treated bare and Ni-Co alloy coated stainless steel plates.

[0031] Detailed Description

[0032] The present invention discloses a process to coat electrolyser plates with an electrocatalytic coating that is able to sustain in harsh alkaline media.

[0033] The use of platinum known as a catalyst for water electrolysis is hampered by its cost and availability. Non platinum group metals such as nickel and nickel cobalt alloy are able to serve as a catalytic coating for alkaline water electrolysis.

[0034] The present invention discloses the process of electrodeposition of nickel (Ni) and cobalt (Co) alloy using deep eutectic solvent (DES) on electrolysers plate to establish a smooth electrocatalytic coating for alkaline water electrolysis (AWE).

[0035] The process for coating the plate comprises the steps of pre-treating the plate and coating the plate with nickel and cobalt alloy by electrodeposition. The nickel and cobalt alloy is electrodeposited from a deep eutectic solvent to form an electrocatalytic coating that is nanocrystalline with an average grain size of less than 500 nm. Preferably the plate is stainless steel 316 (SS316) and the electrolyte utilised for electrodeposition comprises choline chloride and ethylene glycol deep eutectic solvent dissolved by nickel chloride and cobalt chloride. The molar ratio of Ni:Co is within the range of 0.1 :0.1 to 2:2. Preferably the molar ratio of Ni:Co is 2:1.

[0036] Pre-treatment requires the surface of the stainless steel plate to be cleaned to ensure the plate is free of any impurities or foreign materials which then enables the coating to strongly adhere to the surface of the substrate. Examples of pre-treatment includes polishing, grinding, acid treatment, chrome-stripping, Wood’s nickel strike or a combination thereof. Preferably Wood’s nickel strike is the selected pre-treatment as the coating of Ni-Co alloy does not peel off when the plate is utilised for alkaline water electrolysis.

[0037] Electrodeposition of electrocatalytic coating using deep eutectic solvent (DES); a nonaqueous electrolyte is selected as gas evolution at the anode / solution interface is negligible, high current efficiencies are obtained and when compared to aqueous acid-based solutions, deep eutectic solvent is benign and non-corrosive. Also, by utilising DES, the use of corrosive acidic electrolytic baths and volatile solvents that are harmful to the environment can be avoided. Non-aqueous volatile solvents on the other hand limit the solubility of metal precursors and also electrochemical stability. Further, hydrogen embrittlement is noticed when metals are deposited from conventional aqueous electrolytes. Chronopotentiometry electrodeposition with an applied current within the range of 1 mA / cm2to 3 mA / cm2for a duration of up to 20 minutes is performed to coat the plate with nickel and cobalt alloy. Chronopotentiometry technique was selected based on physical observation of the coating deposited as well as scanning electron microscope (SEM) analysis of the morphology of the coating on the plate.

[0038] A Ni-Co alloy coating on the plate in a 2:1 molar composition from a of deep eutectic solvent with an applied current density of 2 mA / cm2and electrodeposition duration of 10 minutes demonstrated lower hydrogen evolution reaction (HER) overpotential of -0.20 V when compared to a reversible hydrogen electrode (RHE) amongst different variables tested and improves efficiency in terms of overpotential reduction by almost 30% when compared to a bare plate.

[0039] Figure 1a illustrates a chronopotentiometric deposition curve at a constant current of 1 mA / cm-2demonstrating a smooth coating on the plate. The figure depicts a typical voltage-time curve for instantaneous nucleation and diffusion-limited island growth. Initially the double layer charging the potentials were more negative and increased overpotential; when reached sufficient potential favouring ion-discharge, instantaneous nucleation representing electrodeposition occurred. Following on, decrease of the potential also demonstrates the increase in the electroactive area on the working electrode which is the stainless steel plate.

[0040] Figure 1 b illustrates stainless steel plates coated with Ni-Co alloy achieved by chronopotentiometry electrodeposition. Stainless steel plates labelled Ni-Co 1 , Ni-Co 2 and Ni-Co 3 were subjected to chronopotentiometry electrodeposition of Ni-Co alloy in a 2:1 molar composition at a current of 1 mA / cm2, 2 mA / cm2and 3 mA / cm2respectively for 5 minutes. With a higher current density, a thicker coating was observed for the same duration of deposition.

[0041] Figure 1c illustrates scanning electron microscope (SEM) images of stainless steel plates coated with Ni-Co alloy achieved by chronopotentiometry electrodeposition. The SEM image on the left is that of Ni-Co alloy in a ratio of 2:1 (Ni: 20.945 wt.%; Co: 9.823 wt. %) electrodeposited at a current of 1 mA / cm2for 5 minutes while the image on the right is that of Ni-Co alloy in a ratio of 1 :1 (Ni: 14.70 wt.%; Co: 22.71 wt.%) electrodeposited at a current of 1 mA / cm2for 5 minutes. From the images obtained it is observed that no cracks are observed in the Ni-Co alloy coating deposited on both the stainless steel plates. Smooth coatings without cracks relates to better adhesion to the substrate (SS316 plate) and thereby increasing the area of electrocatalytic activity while simultaneously reducing possibilities of exposing the stainless steel plate surface to alkaline media which causes corrosion to happen at a faster rate. Varying the Ni:Co ratio in the electrolytic DES bath resulted in variable Ni-Co alloy content on the electrodeposits. Ni rich coatings provided better performance in terms of hydrogen evolution reaction (HER).

[0042] Figure 2 illustrates pretreated stainless steel plates coated with Ni-Co alloy achieved by chronopotentiometry electrodeposition. The stainless steel plates are pretreated by means of polishing prior to being deposited with the Ni-Co alloy coating according to varying electrodeposition parameters (Ni:Co alloy ratio, electrodeposition duration). In comparison to the stainless steel plates illustrated in Figure 1 , a smoother coating of the Ni-Co alloy is deposited on the pretreated stainless steel plates of Figure 2.

[0043] Figure 3 illustrates Ni-Co alloy nanocrystallites deposited on pretreated stainless steel plate. A smooth and packed coating of Ni-Co alloy (molar ratio of 2:1) nanocrystallites with an average grain size of less than 500 nm electrodeposited from the choline chloride and ethylene glycol deep eutectic solvent (DES) on stainless steel by chronopotentiometry is achieved. Smooth coatings under sub-micron levels relate to better adhesion to the substrate (SS316 plate) and thereby increasing the area of electrocatalytic activity while simultaneously reducing possibilities of exposing the substrate surface to alkaline media which causes corrosion to happen at a faster rate.

[0044] Figures 4a and 4b illustrate pretreated stainless steel plates coated with Ni-Co alloy according to varying chronopotentiometry electrodeposition duration and applied current respectively. In Figure 4a it is observed that a thicker Ni-Co alloy coating of 598.4 nm is achieved when the electrodeposition duration is 10 minutes when compared to 5 and 15 minutes. Although the thickness of the coating is directly proportionate to the electrodeposition duration, thicker electrocoatings arise from 3D nucleation of electrodeposits leading to cracks and the coating peels off eventually due to poor adherence to the stainless steel plate. In Figure 4b it is observed that a thicker Ni-Co alloy coating of 598.4 nm is achieved when the electrodeposition applied current is 2 mA / cm2which has been determined to be the optimum current density when compared to 1 mA / cm2and 3mA / cm2.

[0045] To test the durability of the Ni-Co alloy coating deposited on the pre-treated stainless steel (SS316) substrate, alkaline water electrolysis was performed continuously using 1.0 M KOH as the electrolyte for more than 40 minutes. The Ni-Co alloy coated substrate showed better durability when compared to bare substrate (without Ni-Co alloy coating) without any apparent catalyst layer peel-off from the coated substrate surface. Table 1 : Alkaline water electrolysis utilising Ni-Co alloy coated substrate (coating area 0.5 cm2).

[0046] Table 1

[0047] The Ni-Co alloy is able to serve as an electrocatalytic coating for alkaline water electrolysis as well as sustains the durability of the alkaline water electrolysis system when compared to bare stainless steel under harsh conditions such as alkaline medium. Figure 5 illustrates corrosion analysis of bare and coated stainless steel substrates before and after alkaline water electrolysis. Polarization curves show similar trend for both bare and Ni-Co alloy coated substrates, where the Ecorr values (Table 2) shifted positively after water electrolysis. After electrolysis, the Ecorr shifted positively, indicating an increase in the passivating layer and corrosion resistance. Both Ecorr of bare (after electrolysis) and Ni-Co alloy coated (after electrolysis) substrates shifted positivity to -0.26877 V and -0.23963 V respectively. Comparing the corrosion rate values, both Ni-Co alloy coated (before and after electrolysis) and bare (after electrolysis) substrates exhibited higher values than the bare substrate, indicating an improvement in the corrosion resistance of the stainless steel substrate. However, in the case of the bare substrate, higher passivation may lead to higher overpotentials during alkaline water electrolysis leading to a more energy intense process. There is no significant change in the current for Ni-Co alloy coated substrates before and after water electrolysis in the passivating region (Region 3), indicating a stable coating layer. In Region 4, the slope in current for the bare substrate is higher than Ni-Co alloy coated substrates. This shows that the Ni-Co alloy exhibits slower transpassivation, meaning that the coating has successfully slowed down corrosion. Even after electrolysis, the slope for Ni-Co alloy coated substrates remains constant, indicating a stable coating that can improve corrosion resistance.

[0048] Table 2: ICORR and ECORR values for bare and Ni-Co alloy coated substrates.

[0049] Table 2

[0050] Figure 6 illustrates hydrogen evolution reaction (HER) potentials on a cyclic voltammogram using pre-treated bare substrates and Ni-Co alloy coated substrates. HER overpotentials for coated substrates are reduced up to 30% when compared to the bare substrate indicating electrocatalytic activity.

[0051] Table 3: Electrochemical overpotentials of HER compared between bare and Ni-Co alloy coated substrates.

[0052] Table 3

[0053] This present invention demonstrates a viable approach to deposit a smooth nanocrystalline Ni-Co alloy coating on SS316 by chronopotentiometry electrodeposition at ambient conditions using deep eutectic solvent whereby the coated SS316 can be applied in alkaline water electrolysis and anion exchange membrane electrolysis.

Claims

CLAIMS1 . A process for coating electrolyser plate comprising the steps of: pre-treating the plate; and coating the plate with nickel and cobalt alloy by electrodeposition; characterized in that the nickel and cobalt alloy is electrodeposited from a deep eutectic solvent to form an electrocatalytic coating with corrosion resistance that is nanocrystalline.

2. The process according to claim 1 , wherein the plate is preferably stainless steel 316 (SS316).

3. The process according to claim 1 , wherein pre-treating the plate can be achieved by grinding, acid pickling, chrome stripping, Wood’s nickel strike or a combination thereof.

4. The process according to claim 1 , wherein the molar ratio of nickel to cobalt is within the range of 0.1 :0.1 to 2:2.

5. The process according to claim 1 , wherein the molar ratio of nickel to cobalt is preferably 2:1.

6. The process according to claim 1 , wherein the deep eutectic solvent is a mixture of choline chloride, ethylene glycol, nickel chloride and cobalt chloride.

7. The process according to claim 1 , wherein chronopotentiometry electrodeposition with an applied current within the range of 1 mA / cm2to 3 mA / cm2for a duration of up to 20 minutes is performed to coat the plate with nickel and cobalt alloy.

8. The process according to claim 1 , wherein the average grain size of the coating is less than 500 nm.

9. A plate according to the process of claim 1 comprising; a coating of nickel and cobalt alloy; characterized in that the nickel and cobalt alloy is electrodeposited from a deep eutectic solvent to form an electrocatalytic coating with corrosion resistance that is nanocrystalline.

10. Use of a plate according to claim 9 in alkaline water electrolysis and anion exchange membrane electrolysis.

Images