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 easier urea oxidation reaction (UOR), developing a HER/UOR coupling electrolysis system for hydrogen production can significantly save energy and costs. 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. reversible hydrogen electrode) to drive 10 mA cm−2 for HER and UOR, respectively, in alkaline condition. 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 60% electricity energy comparing to that of tranditional 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 main intermediates in 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.