Achieving favourable oxygen electrocatalytic activity with compositionally complex metal molybdates

Abstract

Tuning the surface binding of the active species or intermediates with the catalyst is an important consideration when designing high-performance electrocatalysts. In this respect, high entropy materials have aroused significant research interest owing to the high entropy-driven enhanced electrochemically active surface, modified electronic configuration and the cocktail effect in the compositionally complex material, which could facilitate the mass transport and charge transfer at the solid–liquid–gas interface. However, research on high entropy materials is only in its infancy. Herein, we demonstrate a simple and scalable approach for designing first-row transition metal based on a new high entropy molybdate as a high-performance bifunctional electrocatalyst for both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). The high entropy molybdate (HEMo; (MnNiCuCoZn)MoO₄) was synthesized by applying an oxalate-based coprecipitation approach, followed by calcinating at different temperatures ranging from 800 to 1000 °C. With MnMoO₄ as the starting molybdate, the successive addition of Ni, Cu, Co, and Zn produced different binary, ternary, quaternary and high entropy molybdates, which showed gradually improved OER electrocatalytic activity with an increasing number of shared metal centres in the molybdate framework. Among the different molybdates, HEMo calcined at 900 °C (HEMo@900) showed the best OER electrocatalytic activity with a very low overpotential of 318 mV at 10 mA cm⁻² with a Tafel slope value of only 49.45 mV dec⁻¹ and long-term stability of >100 h in 1 M KOH electrolyte. The HEMo@900 also exhibited excellent ORR electrocatalytic activity with a high onset potential of ∼0.88 V (vs. RHE), half wave potential of 0.74 V (vs. RHE) and an average electron transfer number (n) of ∼3.4, indicating a favourable four-electron transfer process. The OER/ORR bifunctional electrocatalytic activity was comparable to that of the benchmark OER (RuO₂) and ORR (Pt/C) electrocatalysts tested under similar conditions. To the best of our knowledge, this is the first report on exploring the high entropy molybdates towards oxygen electrocatalytic reactions.