Self-supported Ni(OH)2/MnO2 on CFP as a flexible anode towards electrocatalytic urea conversion: The role of composition on activity, redox states and reaction dynamics
Nickel-based catalysts accomplish the direct conversion of urea to pure hydrogen via electrochemical oxidation; yet mechanistic understanding is lacking. Synthesizing a series of carbon fiber paper (CFP) supported Ni(OH)2/MnO2 catalysts, we explored relevant redox transitions and catalysis of both UOR (urea oxidation reaction, in KOH-with-urea) and OER (oxygen evolution reaction, in KOH). Cyclic Voltammetry (CV) in KOH-only solution demonstrated a more cathodic transformation from Ni(III/IV) to Ni(II) compared with that in KOH-with-urea solution. The water oxidation overpotential was shifted to higher values (from 0.48 to 0.53 VRHE) as the Mn:Ni atom ratio increases in CFP-NiMn films. In contrast, a higher Mn content results in higher UOR activity and lower onset potential in KOH solution containing urea (1.395-1.375 VRHE). Quasi in-situ, freeze-quench X-ray absorption spectroscopy (XAS) at the Ni and Mn K-edges was employed to uncover oxidation state changes as well as structural transformations at the atomic level showing that CFP-Ni(OH)2 underwent oxidation state changes by about 1.15 e− and 0.21 e− per Ni ion during OER and UOR processes, respectively, versus only 0.71 e− and 0.07 e− per Ni ion in CFP-NiMn2.4. Mn incorporation can stabilize the Ni in lower valent states in a mixed NiMn catalyst without significant changes in oxidation state and structure. The here investigated, readily synthesized CFP-NiMn films exhibit opposite activity trends in KOH and KOH-with-urea electrolytes: Mn incorporation depresses water oxidation, but it promotes the urea oxidation process. We propose that the water oxidation rate (OER) is positively correlated with the capacity for accumulation of Ni and Mn oxidation equivalents, while the urea oxidation (UOR) rate is negatively correlated with this capacity. Our work offers a mechanistic guideline for designing and synthesizing nonprecious metal-coupled Ni-based catalysts with appropriate redox-properties for urea-oxidation applications.