In Situ Quantification of the Active Sites, Turnover Frequency, and Stability of Ni–Fe (Oxy)hydroxides for the Oxygen Evolution Reaction

Quantification of the active site number and the site-specific turnover frequency (TOF) of nanoscale electrocatalysts is the prerequisite for understanding their intrinsic catalytic properties. Although Fe has been suggested as a highly active site in Ni–Fe (oxy)hydroxides, one of the most active noble-metal-free electrocatalysts for the alkaline oxygen evolution reaction (OER), effective approaches for measuring the Fe active site number and the intrinsic turnover frequency per active Fe site (TOFFe) remain missing, which hinders the understanding of their electrocatalytic activity and stability. Herein, we report the direct in situ quantification of the electrochemically active Fe site number and associated TOFFe values of Ni–Fe (oxy)hydroxides by previously unexplored Fe2+/3+ redox electrochemistry in alkaline solutions. This further enables in situ monitoring of the Fe active site number and the TOFFe of Ni–Fe (oxy)hydroxides during OER electrocatalysis, providing important insights into the activity degradation/regeneration caused by Fe dissolution/adsorption as well as a site-dependent activity and stability. By comparing with the Ni–Fe (oxy)hydroxides, we directly evidence that Fe incorporated at the edges of Ni (oxy)hydroxides holds not only a higher TOF value but also improved durability. In addition to the Fe redox reaction, we also reveal a Ni reduction reaction at low potentials, which is associated with the reduction of irreversibly oxidized Ni at OER potentials. The revealed Fe and Ni redox electrochemistry provides important insights into the site-dependent OER activity and the stability of the state-of-the-art (oxy)hydroxide OER electrocatalysts.