Insight into the Structure of Mn-NiS2 during Urea Oxidation Using Combined In Situ X-ray Absorption Spectroscopy and Attenuated Total Reflectance Surface-Enhanced Infrared Absorption Spectroscopy

The urea electrocatalytic oxidation reaction (UOR) has enormous potential as an ideal alternative anode reaction for water splitting owing to its lower thermodynamic equilibrium potential of 0.37 V versus reversible hydrogen electrode (vs RHE). Nickel-based materials, especially NiOOH, are considered to be one of the most promising non-noble metal catalysts for UOR due to their inexpensive cost and rich abundance. However, NiOOH displays a high overpotential and poor long-term stability. Herein, our density functional theory calculations show that the rate-determining step for UOR is desorption of CO2 on NiOOH, and Mn-doped NiOOH has the lowest energy for CO2 desorption. Hence, we prepared a Mn-NiS2 precatalyst that would transform into the active form of Mn-NiOOH during the electrochemical process. The catalyst exhibits good performance for UOR, achieving 100 mA cm(-2) at 1.426 V (vs RHE, without IR correction) for 200 h with no significant voltage change, which is rarely reported for nonprecious-metal UOR catalysts. X-ray absorption near-edge spectroscopy and X-ray diffraction characterization show the transformation from sulfide to oxyhydroxide when a voltage is applied, while in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) proves that Mn-NiOOH accelerates the desorption of CO2 compared to NiOOH.