High-valence Ni and Fe sites on sulfated NiFe-LDH nanosheets to enhance O-O coupling for water oxidation

Abstract High-valence Ni and Fe metal sites have demonstrated a crucial role in enhancing the catalytic performances of NiFe-LDH electrocatalysts in oxygen evolution reaction (OER). Although considerable OER catalytic performances achieved under high overpotential, the catalytic talent of NiFe-LDH electrocatalysts at low overpotential is rarely realized due to the absence of high-valence Ni and Fe sites. We herein report a surface engineering route to fabricate sulfated NiFe-LDH nanosheets via ion exchange strategy in sulfate-rich media. XPS results reveal a modified surface electronic structure with high-valence Ni and Fe after ion exchange reaction. Computational PDOS results suggest that computed d-band centers (ed) of Fe and Ni for sulfated NiFe-LDH show a significant downward shift resulting an increased valence of metal cation with orbital volume shrinkage. The high-valence Fe can facilitate a optimized multi-electron process of Ni center from [NiII-OH]−/[NiIII-OH]− to NiIV-OOH rather than NiII/NiIII to NiIV at low overpotential. The high-valence Ni can serve as the highly active center for O-O coupling during OER process. Combined with the synergetic action of high-valence Fe and Ni, the sulfated NiFe-LDH nanosheets exhibit much larger reaction kinetics and outstanding electrocatalytic activity on glassy carbon electrode (η10 = 219 mV, η50 = 288 mV) with a remarkable long-term stability.