Modulating Spin‐States of Active Catalysts for Efficient Urea Oxidation in Anion Exchange Membrane Electrolyzer

ABSTRACT The electronic properties of active M(O)OH catalysts can be effectively modified by tailoring the characteristics of their corresponding precatalysts. Herein, we report a coordination site‐modulated precatalyst to access active catalysts for efficient electrocatalytic urea oxidation in an anion exchange membrane (AEM) electrolyzer. A two‐dimensional NiFe‐coordination polymer (NiFe‐2D) was constructed via selective site engineering using hydrazine. Upon electrochemical reconstruction, NiFe‐2D was transformed into an active Fe‐Ni(O)OH catalyst (AC‐2D). X‐ray absorption spectroscopy (XAS) confirmed the changes in the coordination and electronic environment of AC‐2D with a tuned spin state. Remarkably, AC‐2D delivered a current density of 100 mA cm −2 at 1.50 V vs. RHE for the oxygen evolution reaction (OER) and 1.40 V vs. RHE for the urea oxidation reaction (UOR). AC‐2D also delivered outstanding performance in an AEM electrolyzer, achieving a current of 4 A at 2.9 V cell voltage and maintaining operation for over 8 days for UOR‐assisted water splitting. In situ electrochemical impedance spectroscopy (EIS) and kinetic analysis suggested facile charge transfer during OER and UOR in AC‐2D. In situ Raman studies revealed that Ni 4+ species were identified as the main active centers for OER, while Ni 3+ species served as the catalytic sites for UOR.