Active Surface Area and Intrinsic Catalytic Oxygen Evolution Reactivity of NiFe LDH at Reactive Electrode Potentials Using Capacitances

Abstract

Determination of the electrochemically active surface area (ECSA) is essential in electrocatalysis to provide surface normalized intrinsic catalytic activity. Conventionally, ECSAs of metal oxides and hydroxides are estimated using double layer capacitance (Cdl) measured at nonfaradaic potential windows. However, in the case of Ni-based hydroxide catalysts for the oxygen evolution reaction (OER), the nonfaradaic potential region before the Ni(II) oxidation peak is nonconductive, which hinders accurate electrochemical measurements. To overcome this problem, in this work, we have investigated the use of electrochemical impedance spectroscopy (EIS) at reactive OER potentials to extract the capacitance that is hypothesized to arise due to reactive OER intermediates (O*, OH*, OOH*) adsorbed on the catalyst surface. This allowed the estimation of ECSA and intrinsic activity of NiFe layered double hydroxide (NiFe LDH), the most active, state-of-the-art OER electrocatalyst in alkaline media. We analyzed the OER adsorbates capacitance (Ca) on NiFe LDH and Ni(OH)2 at different electrode potentials and identified a suitable potential range for accurate ECSA evaluation. Finally, we validated our method and the choice of potential range through rigorous catalyst loading and support studies.