Electrode for use in alkaline water electrolysis systems and electrolyzer using such electrode

Abstract

The present invention relates to an electrode for use in alkaline water electrolysis systems, through and through consisting of a Nickel-Iron (NiFe) alloy. The invention further relates to an electrolyzer comprising such electrode as a cathode, and in an embodiment also as an anode. An important aspect of the present invention is that using NiFe alloys as through-andthrough electrodes enables the material to maintain its catalytic activity even as it erodes. This contrasts with electrodes that rely on thin catalyst layers, which degrade more unpredictably. The solid alloy composition ensures that the electrode's performance remains consistent until it is fully dissolved.

Description

The present invention relates to an . STATE OF THEART The combination of renewable energy sources like wind and solar with water electrolysis technology is essential for producing green hydrogen. However, the fluctuating and intermittent nature ofthese energy sources leads to frequent start-stop cycles in electrolyzer operation, which induce reverse currents in the electrolyzer stack, when electrolyzer is operated in a way to match the amount of renewable energy available. An example of such system is described in Dutch patent application NL2038234.These reverse currents, caused by sudden voltage drops when the electrolyzer is powered down, flow in the opposite direction to the normal operational current and can lead to significant electrode and catalyst degradation. This is the case with alkaline water electrolysis (AWE), proton exchange membrane (PEM) electrolysis, and anion exchange membrane (AEM) electrolysis and has a long-term impact on electrode and catalyst stability. ALKALINE ELECTROLYZERS (AWE) In alkaline water electrolysis (AWE), reverse currents can cause severe degradation of nickel-based electrodes and bipolar plates. AWE systems typically use less expensive materials, but during shutdown, reverse currents can initiate redox reactions at the electrode surfaces. Studies show that nickel-based electrodes suffer significant oxidation during reverse current events. This process, combined with the reduction of active nickel oxides like NiOOH, leads to erosion, directly related to the magnitude ofthe reverse current and the duration of off-cycles [2]. Maintaining a small protective current during shutdowns has been proposed as a mitigation measure, though it cannot completely prevent degradation. The use of Raney nickel catalysts in alkaline systems, known for their high surface area, 1 has been shown to be particularly vulnerable under intermittent operation as after several hundred hours of operation with intermittent cycling, nickel dissolution and loss of catalytic surface area occurred due to frequent exposure to reverse currents. The erosion rate could be reduced by lowering the operational temperature during idle periods, yet the overall impact on lifetime remains substantial. PEM ELECTROLYZERS Proton exchange membrane (PEM) electrolyzers are more flexible in dynamic operation compared to AWE, but they are highly susceptible to degradation under reverse currents, especially in the anode catalyst layer. The high overpotentials needed for the oxygen evolution reaction (OER) in acidic environments, combined with the corrosive nature of the environment, result in rapid degradation of iridium-based catalysts. During intermittent operation, reverse currents promote iridium dissolution and redeposition, leading to a loss of active surface area and a corresponding increase in the cell voltage. In PEM systems, degradation ofthe membrane also occurs due to chemical interactions between hydrogen and oxygen crossover at the membrane interface. Under dynamic conditions, hydrogen peroxide formation and subsequent membrane attack by reactive radicals have been observed, causing localized thinning and the development of pinholes. ANION EXCHANGE MEMBRANE ELECTROLYZERS (AEM) AEM electrolyzers offer a balance between the cost advantages ofAWE and the operational flexibility ofPEM systems, but they are also highly vulnerable to reverse currents. The NiFe-Iayered double hydroxide (LDH) catalysts used inAEM anodes degrade significantly when exposed to reverse currents. The cathode, particularly when using Pt- based catalysts, is even more prone to deterioration. NiMo / C and PtRu / C catalysts lose their activity more rapidly than traditional Pt / C catalysts under reverse current conditions, and the degradation follows the trend NiMo / C > PtRu / C > Pt / C. ForAEM systems, the alkaline environment and low ionic conductivity present further challenges, as reverse currents exacerbate ionomerdegradation. Bipolar configurations are particularly susceptible to shunt currents, leading to localized corrosion on the bipolar plates. 2 INFLUENCE OF TEMPERATURE ON EROSION ANDPERFORMANCE Temperature significantly impacts both the degradation and performance of electrolysis stacks. Higher operating temperatures tend to accelerate the dissolution of catalyst materials, especially inAEM electrolyzers, where catalysts like NiMo / C are highly sensitive to reverse current-induced degradation at elevated temperatures. At 60°C, for instance, NiMo / C catalysts demonstrated significant performance losses and structural changes due to the increased oxidative potential in the alkaline environment. Higher temperatures in PEM electrolyzers lead to faster dissolution of iridium catalysts, with degradation rates rising notably during intermittent operation. On the other hand, higher temperatures also enhance electrolyzer performance by reducing ohmic resistance and improving reaction kinetics. As temperature increases, the kinetics of the Oxygen Evolution Reaction (OER) and Hydrogen Evolution Reaction (HER) improve, leading to higher current densities and overall efficiency. This is particularly evident in PEM electrolyzers, where operating at elevated temperatures (e.g., 60-80°C) significantly reduces overpotentials during dynamicoperation, offsetting some of the degradation effects. However, while lowering the operating temperature could slow down the degradation of electrodes and extend their lifespan, it would come at the cost of significantly reduced energy efficiency, as the reaction rates and overall system performance would suffer. QUANTIFYING ELECTRODE EROSION The degradation rates vary by technology and materials used, resulting in measurable degradation in terms of voltage increase and catalyst loss over time. ln PEM electrolyzers, voltage degradation rates of 35.5 uV / h have been recorded, with even higher resistive losses observed during intermittent operation, indicating the ongoing dissolution of iridium catalysts under dynamic conditions. ln alkaline systems, nickel-based catalysts show visible signs of dissolution after 400-500 hours of intermittent cycling. The challenge of reverse current-induced degradation is amplified when electrolysis systems are coupled with renewable energy sources. lntegrating electrolysis with fluctuating renewables requires better control strategies to minimize downtime and limit reverse current exposure. Strategies such as maintaining a small protective current during 3 off-cycles can reduce the impact of reverse currents, but this comes at the cost of additional energy consumption, which diminishes overall system efficiency. There are materials aimed at resisting degradation, such as iridium oxides in PEM systems and nickel-iron alloys in alkaline systems. However, even these materials, while more durable than their predecessors, still experience significant erosion under frequent cycling conditions, particularly in the harsh environments created by intermittent operation. Although progress has been made in reducing the rate of degradation, these materials still face performance losses over time, indicating that while improvements have been achieved, they do not fully solve the issue, and further innovations are needed to balance longevity with cost and efficiency. ln prior artwater electrolysis is predominantly focused on the optimization of thin catalyst layers on cheaper substrates. These catalyst layers, typically 2 to 10 microns thick, are susceptible to rapid degradation under the frequent cycling conditions that arise when electrolyzers are coupled with fluctuating renewable energy sources. This degradation leads to significant losses in performance, necessitating frequent and costly replacement ofthese layers. SUMMARY OF THE INVENTION Frequent start-stop cycles in electrolyzers, especially when integrated with fluctuating renewable energy sources, induce reverse currents that accelerate the degradation of both electrodes and catalyst layers. ln alkaline water electrolysis, nickel-based electrodes are particularly vulnerable to redox cycling, which causes oxidation and erosion, reducing overall efficiency and longevity. Though significant efforts have focused on improving thin catalyst layers, these layersoften only 2 to 10 microns thickcan degrade after relativelyfew cycles, especially under reverse current conditions. To mitigate these challenges, shifting focus from more durable thin catalyst layers to Nickel-Iron alloys and accept that erosion is inevitable presents a practical solution. Alloys as Permalloy and Mu-metal are used in the electronics industry for magnetic shielding, which offer a durable alternative for electrodes in alkaline electrolyzers. lnstead of relying on a thin, high-performance catalyst layer that erodes rapidly, bulk alloys maintain consistent surface composition even as the electrode gradually degrades. This method 4 ensures more reliable long-term performance and reduces the risk of sudden efficiency loss due to the failure of thin catalyst layers. ln summary, the use of bulk alloy electrodes provides a promising solution to the challenges posed by intermittent operation. By accepting gradual degradation ofthe entire electrode material, rather than focusing solely on preserving thin catalyst layers, this approach could significantly extend the lifespan of electrolysis systems and improve their viability for long-term hydrogen production, especially when coupled with renewable energy sources. The present invention offers both technical and economic advantages, leveraging established supply chains for Nickel-Iron alloys to deliver more durable and cost-effective solutions. DETAILED DESCRIPTION OF THE INVENTION According to the present invention, bulk Nickel-Iron alloys, such as those marketed as Permalloy and Mu-metal, are proposed as through-and-through electrodes. Unlike traditional thin catalyst layers, these alloys are available as a material forthe electronics industry where they serve as magnetic shielding. Permalloy and Mu-metal, which are primarily composed of nickel and iron, possess excellent electrochemical properties that make them suitable for use as both the catalyst and the structural material of the electrode. As the bulk electrode gradually erodes over time due to the harsh operational conditions in alkaline electrolyzers, the surface maintains the same catalytic composition, ensuring consistent performance. This approach eliminates the sharp decline in activity that occurs when thin catalyst layers are depleted. By utilizing these bulk Nickel-Iron alloys, the electrode's entire structure acts as the catalyst, allowing for gradual erosion while maintaining functionality. This contrasts sharply with the traditional model of an expensive, high-performance catalyst layer on a cheaper substrate, which can fail suddenly when the thin layer is degraded. The use of bulk alloys could substantially extend the lifespan of alkaline electrolysis systems, reduce operational costs, and improve system resilience, particularly in renewable energy contexts where intermittent operation iscommon. 5 This approach leverages the existing supply chains for bulk Nickel-Iron alloys, making it a cost- effective and practical alternative to thin-layer catalyst systems. The robustness and durability of these bulk alloys in alkaline environments represent a significant advancement in electrode design, allowing to increase the viability and sustainability of hydrogen production on a large scale. THE STRESS TESTMETHODOLOGY A comparison of the performance and durability of various metal alloy candidates to be used as electrodes in alkaline water electrolysis systems was made, to assess the rate of electrode erosion and its impact on performance under cyclic loading conditions, simulating the operational stresses faced by electrodes in renewable-powered electrolyzers. A multi-channel potentiostat, which allowed for precise control and monitoring of the current applied to each electrode was used. The electrodes were exposed to a forward current of400 mA for 6 seconds, followed by a reverse current of40 mA. This cyclic pattern was repeated continuously to simulate severe dynamic operation. After every 3000 cycles, an IV curve was recorded to monitor the electrochemical behavior of the electrodes and detect any performance changes. This was done at a constant temperature of45°C, and a 30% KOH concentration was used as the electrolyte. The alloys included pure nickel (which is commonly used as a cathode in alkaline systems), NiFe alloys with varying nickel compositions (80% nickel, 50% nickel), and a nickel alloy containing cobalt, which has been reported in the literature as an efficient cathode catalyst for alkaline systems. The waveforms ofthe voltage response foreach alloy during the forward and reverse current cycles provide insight into the evolving surface states of the electrodes. As the electrodes undergo repeated cycles of current, the response patterns begin to reveal significant differences in the surface chemistry of the materials. The invention will now be elucidated into more detail with reference to the following figures. Herein: 6 - Figure 1 shows a graph showing voltage over time for different alloys; and - Figure 2 shows a graph showing weight loss and erosion rate. Figure 1 shows a voltage over time for different alloys. The voltage response ofthe different alloys at the end ofa 400mA forward current pulse is plotted against the number of cycles. For each alloy, the voltage remains stable throughout the majority of the test (indicating minimal change in surface activity), with a sharp increase in voltage nearthe end ofthe electrode's lifespan, corresponding to the point of complete dissolution of the cathode. Pure nickel electrodes eroded fully after 80,000 cycles, while the NiFe alloy with 80% nickel lasted slightly longer at 83,000 cycles. The cobalt-containing alloy exhibited better performance, lasting up to 160,000 cycles. The NiFe alloy with 50% nickel, however, outperforms all other materials as cathode material, maintaining a stable voltage for approximately 300,000 cycles before showing a (less) sharp increase due to electrode failure. This behavior of all alloys shows that the solid alloy electrodes maintain their catalytic activity throughout their operational life, even as they gradually erode. That is the new surface being revealed after dissolution ofthe surface layer hydroxide / oxide maintains the same electrochemical performance. Figure 2 shows a weight loss and erosion rate of the electrodes oftwo alloys (NiCoFe and Ni50Fe) in time. ln both cases the weight loss rate is more significant on the cathode than on the anode, showing that the cathode experiences a higher rate of erosion. For all alloys, the primary mechanism of erosion is the dissolution of the surface layer (likely hydroxide or oxide) formed after each forward current pulse. This loss is compared to a hypothetical equivalent number ofatomic layers lost, based on the insight that the surface dissolution corresponds to the erosion of the top atomic layer of the electrode material after each cycle. The comparison shows that there is a strong correlation between the weight loss expressed as a number of atomic layers lost and the number of cycles the electrode had undergone, again showing that the erosion involves the dissolution of the surface layer during the reverse current. 7 This shows that NiFe alloys with 50% nickel provide significantly greater durability and erosion resistance as cathode compared to pure nickel and other alloys, including those containing cobalt. An important aspect of the present invention is that using NiFe alloys as through-and- through electrodes enables the material to maintain its catalytic activity even as it erodes. This contrasts with electrodes that rely on thin catalyst layers, which degrade more unpredictably. The solid alloy composition ensures that the electrode's performance remains consistent until it is fully dissolved. NiFe alloys, particularly those with 50% nickel, significantly outperform pure nickel and other alloys in terms of operational life and stability. The voltage response ofthe alloy electrodes remains stable until they near complete dissolution, indicating that the solid alloy composition allows for continuous catalytic activity even as the electrode gradually erodes. This invention presents a longer-lasting, more reliable electrode for alkaline electrolysis systems, particularly in renewable energy applications where frequent cycling is common. 8 Clauses 1. Electrode for use in alkaline water electrolysis systems, through and through consisting of a Nickel-Iron (NiFe) alloy. 2. Electrode according to claim 1, wherein the % of nickel is between 40% and 60%, and preferably 50%, and as a consequence, the percentage Iron is between 60% and 40%. 3. Electrode according to claim 1 or 2, wherein the electrode material has a thickness between 100 and 1000 um. 4. Electrode according to any of the preceding claims, wherein the electrode material has a surface area between 10 mm2 and 1 m2. 5. Electrolyzer, comprising at least a cathode formed by an electrode according to any of the preceding claims. 6. Electrolyzer according to claim 5, comprising an anode formed by an electrode according to any of claims 1-4. 9

Claims

1. Electrode for use in alkaline water electrolysis systems, consisting entirely of a nickel-iron (NiFe) alloy.

2. Electrode according to claim 1, where the percentage of nickel lies between 40% and 60%, and at preference at 50%, and as a result the percentage of iron lies between 60% and 40%.

3. Electrode according to claim 1 or 2, where the electrode material has a thickness between 100 and 1000 um.

4. Electrode in accordance with one of the preceding claims, where the electrode material is a has a surface area between 10 mm² and 1 m².

5. Electrolyzer, comprising at least one cathode formed by an electrode according to a of the preceding conclusions.

6. Electrolyzer according to claim 5, comprising an anode formed by an electrode according to a of conclusions 1-4. 10 Figure 1 12

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