Unfolding the significance of regenerative active species in nickel hydroxide-based systems for sustained urea electro-oxidation

Abstract

Electrochemical urea oxidation (UOR) has gained attention as an alternative to the oxygen evolution reaction (OER) because of its low thermodynamic energy barrier for hydrogen generation. The activity enhancement of the conventional Ni-based catalysts can be addressed by enhancing NiOOH active species, but ensuring stability for a prolonged duration can be a challenging feat. The catalyst degradation is usually attributed to the lack of tolerance for CO_x released during UOR; however, in this study, we show that the catalyst activity and durability are also governed by the dynamics of the generated NiOOH species in Ni-hydroxides. We have studied the UOR activity and stability of Ni-based hydroxide systems, Ni(OH)₂ and Ni₃O₂(OH)₄, having Ni in different oxidation states. Ni₃O₂(OH)₄ shows better UOR activity in 0.1 M KOH as compared to Ni(OH)₂, but the activity trend is reversed in 1 M KOH. The time-dependent UOR indicates drastic degradation of activity for Ni₃O₂(OH)₄ in both electrolytes compared to Ni(OH)₂. The in situ X-ray absorption study reveals the regeneration of NiOOH active species in Ni(OH)₂ during the urea electrooxidation, whereas in Ni₃O₂(OH)₄, the active species gets reduced to Ni(OH)₂ rather than getting regenerated. The Ni₃O₂(OH)₄ catalyst seems to undergo fast degradation accompanied by an enhancement in the Ni–O and Ni–Ni coordination number as a function of KOH concentration derived from extended X-ray absorption fine structure analysis. The active species dynamics are further supported through in situ FT-infrared spectral studies. The variation in the contributions of direct and indirect mechanisms to UOR and their correlation with the regeneration of active species are analyzed through an electrochemical impedance study. The present study reveals that the regenerative active species of UOR catalysts have a profound effect on catalyst durability.