Molecular NiII Complexes as Bifunctional Electrocatalysts for O2 Evolution and Urea Electro-Oxidation Reaction.

Abstract

Developing resilient and robust electrocatalysts devoid of noble metals is vital for facilitating the generation of O₂/H₂ from water electrolysis, particularly in catalyzing oxygen evolution reaction (OER) and urea oxidation reaction (UOR), respectively. Nickel-based catalysts have attracted as part of global efforts to produce hydrogen from urea-rich wastewater due to their high reaction rates and favorable long-term stability. Two new mononuclear Ni^II-embedded complexes, namely, [C₂₆H₃₈NiN₄O₄] (Complex 1) and [C₂₄H₃₂NiF₂N₄O₂] (Complex 2), are explored as a bifunctional catalyst for the OER and UOR herein. Complexes 1 and 2 crystallize in triclinic and monoclinic with space group $\overline{P}$ 1 (2) & P 21/c (14), respectively. The OER outcomes of Complex 1 exhibit excellent performance, featuring a lower overpotential of 220 mV and reduced Tafel slope value of 82 mV dec^-1 at a benchmarking current density of 10 mA cm^-2 in 1 M KOH compared to Complex 2. Additionally, the results of UOR indicate that Complex 1 only requires 1.30 V potential to achieve the same current density which is significantly lower than other counterparts and most reported materials. Chronopotentiometry analysis reveals that Complex 1 is stable up to a longer period of 100 and 20 h in 1 M KOH and alkaline urea electrolyte, respectively.