Enhancing bifunctional electrocatalytic performance of electrodeposited medium entropy alloy films by configurational entropy engineering

Abstract

A series of single-phase ternary and quaternary Co-Fe-Sn-(Ni) medium entropy alloy (MEA) films with configurational entropy (Sconf) varying from 1.0R to 1.4R (where R is the universal gas constant) were synthesized by electrodeposition followed by heat treatment. Phase purity and ferromagnetic nature of the heat-treated films were confirmed at room temperature by powder X-ray diffraction and magnetometry, respectively. The hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) exhibited a clear dependence on configurational entropy of the MEA films. The film with Sconf = 1.39R (designated as CFNS-4) delivered markedly reduced overpotentials of 100.3 mV for HER and 319.7 mV for OER at 10 mA/cm2 , accompanied by low Tafel slopes of 46.7 and 55.5 mV/dec, respectively, and stable operation for over 10 h. A monotonic decrease in charge-transfer resistance was observed with increasing Sconf, yielding minimum values of 3.0 Ω for HER and 6.7 Ω for OER for the CFNS-4 film. Correspondingly, the overpotential required to reach 10 mA/cm² also decreased progressively with increasing Sconf, indicating a strong correlation between Sconf and electrochemical activity. When employed as both anode and cathode, CFNS-4 film demonstrated overall water splitting at a cell voltage of 1.67 V, which is significantly lower than that of the ternary film (1.83 V) with S_conf = 1.1R. This work demonstrates the effectiveness of entropy engineering in enhancing bifunctional electrocatalytic performance as well as establishing the entropy-activity correlation in MEA films.