Bio-inspired nickel–iron-based organogel: an efficient and stable bifunctional electrocatalyst for overall water splitting at high current density

Abstract

Developing a platinum group metal (PGM) free electrocatalyst remains a prime challenge for cost effective green hydrogen (H₂) production. Herein, mimicking the PS II catalyst, a bimetallic organogel of nickel (Ni²⁺), iron (Fe³⁺) and benzotriazole (NiFe-gel) is developed as an efficient electrocatalyst. The developed synthetic strategy is simple and scalable, and most importantly, no binder is required for gel-loaded electrode preparation. The respective gel-based electrode showed excellent bifunctionality, in both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), water electrolysis activity in low and high current density (η₁₀: 110 mV and η₁₀₀₀: 260 mV for the OER and η₁₀: 88 mV and η₁₀₀₀: 324 mV for the HER), low Tafel slope and outstanding stability for 100 h at a current density of 1 A cm⁻². The two-electrode electrolyser using the developed NiFe-gel in the anode and cathode setups for overall water splitting attained current densities of 10 mA cm⁻² and 1 A cm⁻² at potentials of 1.49 V and 1.89 V, respectively. Most significantly, NiFe-gel loaded anion exchange membrane based 4 cm² alkaline water electrolysis (AEMAWE) attained a current density of 1.08 A cm⁻² at 50 °C and 2 V and showed stability for at least 100 h. Very nominal performance reduction was observed upon scale-up of the electrolyser from 4 cm² to 9 cm², and the performance was better than the targeted AEMAWE performance of ≥1 A cm⁻² at 2 V. This excellent performance is attributed to the synergistic electronic interaction between Fe³⁺ and Ni²⁺, interaction of nitrogen rich triazole moieties attached to the metal site, similar to the PS II system, and porous electrode microstructure. Thus, the NiFe-gel might be a potential PGM-free electrocatalyst for industrial scale hydrogen production through water electrolysis.