Bifunctional Al-Doped Cobalt Ferrocyanide Nanocube Array for Energy-Saving Hydrogen Production via Urea Electrolysis

The very slow anodic oxygen evolution reaction (OER) greatly limits the development of large-scale hydrogen production via water electrolysis. By replacing OER with an easier urea oxidation reaction (UOR), developing an HER/UOR coupling electrolysis system for hydrogen production could save a significant amount of energy and money. An Al-doped cobalt ferrocyanide (Al-Co₂Fe(CN)₆) nanocube array was in situ grown on nickel foam (Al-Co₂Fe(CN)₆/NF). Due to the unique nanocube array structure and regulated electronic structure of Al-Co₂Fe(CN)₆, the as-prepared Al-Co₂Fe(CN)₆/NF electrode exhibited outstanding catalytic activities and long-term stability to both UOR and HER. The Al-Co₂Fe(CN)₆/NF electrode needed potentials of 0.169 V and 1.118 V (vs. a reversible hydrogen electrode) to drive 10 mA cm^-2 for HER and UOR, respectively, in alkaline conditions. Applying the Al-Co₂Fe(CN)₆/NF to a whole-urea electrolysis system, 10 mA cm^-2 was achieved at a cell voltage of 1.357 V, which saved 11.2% electricity energy compared to that of traditional water splitting. Density functional theory calculations demonstrated that the boosted UOR activity comes from Co sites with Al-doped electronic environments. This promoted and balanced the adsorption/desorption of the main intermediates in the UOR process. This work indicates that Co-based materials as efficient catalysts have great prospects for application in urea electrolysis systems and are expected to achieve low-cost and energy-saving H₂ production.