Stabilizing Electrocatalysts in Alkaline Water Electrolyzers during Dynamic On/Off Operation through Double Layer Capacitance Engineering

Green hydrogen production via alkaline water electrolysis (AWE) powered by renewable electricity is a promising pathway for societal decarbonization. Despite its technological maturity, widespread AWE adoption would face challenges due to the intermittent nature of renewable energy, necessitating frequent start-up/shutdown (on/off) cycles. During shutdown, reverse currents generated in industrial AWE systems—where cathodes and anodes are interconnected through bipolar plates and electrolyte in manifolds—trigger unfavorable redox reactions, accelerating catalyst degradation.1 This study investigates the reverse current dynamics and potential shifts of AWE electrodes during on/off cycles by employing an internal reversible hydrogen electrode (RHE) reference. We identify the double-layer capacitance (C dl) of the electrodes as a reasonable descriptor influencing potential changes during shutdown, and demonstrate that balancing the C dl values of the anode and cathode prevents unfavorable redox reactions, thereby enhancing electrode stability. Nickel-supported ruthenium (Ru) and lanthanum nickel oxide (LaNiO₃) were chosen as cathode and anode catalysts, respectively. Ru was deposited onto nickel felt (NF) via cathodic electrodeposition, while LaNiO₃ was synthesized by thermal decomposition. The electrodes were assembled into a single-cell configuration, with an external resistance mimicking the electrolyte manifold connection in industrial AWE systems, enabling reverse current measurement. First, electrode potential changes upon shutdown were recorded for various cathode-anode combinations, in which the cathode was fixed to Ru while the anode was varied among LaNiO3, porous nickel electrodeposited on NF, and the bare NF. Notably, as shown in figure 1a, Ru cathode paired with LaNiO₃ anode exhibited significant potential increases (up to 1.0 V vs. RHE) while Ru cathode’s potential increases less when the anode is replaced to the Porous Ni and the bare NF. The potential changes of anode and cathode depend on the balance of their discharge capacities.2 Thus, we employed the C dl of the electrodes as an indicator, which may be proportional to the total charge accumulation that leads to the reverse current during shutdown condition. It was found that C dl widely varies among the tested anodes (Bare NF: 1.7 mF cm−2, Porous Ni: 41 mF cm−2, LaNiO3: 280 mF cm−2) while the cathode was maintained to Ru (141 mF cm−2). Stability test with dynamic on (CP 1 A cm−2 10 min) and off cycles revealed that the Ru cathode activity deteriorated with LaNiO3 anode (figure 1b) due to Ru dissolution at the resting potentials above 1.0 V vs. RHE as shown in figure 1a. This is consistent with previous study on Ru’s CV which clarified Ru dissolution in highly alkaline solution as a ruthenate RuO4 2− species around 1.0 V vs. RHE.3 However, as the grey plot shows, pairing the Ru cathode with NF substrates (lower C dl) mitigated potential increases upon on/off cycles, preserving catalyst stability. This is because unlike the case of LaNiO3, Ru cathode’s potential did not reach 1.0 V vs. RHE upon shutdown. To address the issue caused by LaNiO3 catalysts, we developed a porous RuNi catalyst with a higher C dl (464.4 mF cm⁻²) via cathodic electrodeposition in a mixture of NiCl2 and RuCl3 solution. The porous RuNi potential during shutdown was successfully maintained below 0.5 V vs. RHE, which is below the dissolution threshold, even when paired with LaNiO3 anode. The porous RuNi catalyst exhibited excellent stability under dynamic conditions (green symbols in figure 1c), validating the proposed strategy. Finally, a linear correlation between the ratio of potential shifts (ΔE a/ΔE c) and C dl ratios (C dlc/ C dla) was observed (figure 1c), confirming that the C dl reasonably represents the charge accumulation. Here, subscripts ‘a’ and ‘c’ indicate the anode and the cathode, respectively. This work highlights the critical role of C dl in determining electrode potential changes during shutdown. By balancing anode and cathode C dl values, we successfully mitigated unfavorable redox reactions and prolonged catalyst lifespan in AWE systems operating under intermittent conditions. These findings provide valuable insights for optimizing electrode design in dynamic water electrolysis processes. Reference: A. Haleem et al., J. Power Sources 2022, 535, 231454. Oda et al., Electrocatalysis 2023, 14, 499–510. Chalupczok et al, Int. J. Electrochem. 2018, 1273768. Figure 1