Interface engineering of stainless steel cathodes via porous oxide layers for high-rate urea electrolysis

Hydrogen technologies are increasingly recognized as a cornerstone of future sustainable energy systems. The use of urea as a feedstock, rather than relying solely on alkaline electrolytes, offers a sustainable and less energy-intensive pathway for hydrogen production; however, cathodes commonly used for urea electrolysis often rely on noble-metal catalysts and are therefore costly, motivating the development of new low-cost, active, and stable cathodes to reduce the overall process cost. This work investigates the use of low-cost nanoporous oxide coatings on stainless steel electrodes to enhance hydrogen production in urea electrolyzer cells (UECs). Electrochemical performance was assessed using nickel-based anodes and stainless-steel cathodes modified with different metal oxide coatings (ZrO 2 , TiO 2 , SiO 2 , AlOOH). Structural analysis confirmed the purity, phase evolution, and porosity of the synthesized oxides. Remarkably, all oxide-coated electrodes fabricated at optimized temperatures outperformed bare stainless steel in hydrogen evolution efficiency, demonstrating a simple and scalable route to improve UEC performance. Among the investigated systems, the ZrO 2 -coated stainless-steel electrode calcined at 400°C showed the most favorable cathodic response among the coated electrodes, with higher cathodic current densities than bare stainless steel in the HER-relevant potential region. Cyclic voltammetry and capacitance analysis indicated modified interfacial behavior and increased double-layer capacitance, consistent with improved charge-transfer kinetics and a larger accessible electrode/electrolyte interface. Flow-cell experiments were performed using a ZrO 2 -based cathode in an electrolyte containing 0.33 M urea and 5 M KOH at three different current densities (37.5, 75 and 150 mA cm −2 ). Under these practical operating conditions, the system achieved a specific energy consumption as low as 37.6 kWh kg −1 H 2 , corresponding to an approximately 20 % reduction in energy demand compared with the uncoated stainless-steel cathode. Overall, these findings demonstrate that nanoporous oxide coatings provide a structurally robust and economically viable strategy to enhance cathodic performance in alkaline urea electrolysis. The combination of reduced overpotential, improved charge-transfer characteristics, and lower energy consumption underscores the scalability and practical relevance of oxide-modified stainless-steel electrodes for renewable hydrogen production.