From elemental metals to synergistic electrocatalysis: Comparative theoretical and experimental insights into FeNiCoCuMo high-entropy alloy for alkaline oxygen evolution reaction

Abstract

Self-supported, hot-pressed FeNiCoCuMo high-entropy alloy (HEA) electrodes were fabricated and characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), and energy dispersive spectroscopy (EDS), confirming a face-centered cubic (FCC) matrix with minor body-centered cubic (BCC) phase (~1wt%). We map the redox behavior of the individual constituents (Fe, Ni, Co, Cu, and Mo) and compare it with HEA to reveal solid-solution synergy (“cocktail effect”). Electrochemistry (cyclic voltammetry (CV)/linear sweep voltammetry (LSV)/Tafel in 1.0 M KOH) and X-ray photoelectron spectroscopy (XPS) show broadened redox features for HEA and Ni/Co-rich (oxy)hydroxide signatures with MoOx contributions. Triplicate electrodes (M1–M3) deliver an average overpotential of 370 mV at 10 mA·cm−2 and a Tafel slope of 78 mV·dec−1, outperforming monometallic references and remaining competitive with the literature-reported RuO2. Chronopotentiometry 100 h evidence stable operation; post-mortem XRD indicates a thin reconstructed surface while the bulk remains FCC-dominated. Density functional theory (DFT) supports broadened electronic states near the Fermi level and enhanced charge transfer. Overall, structure and computation link compositional disorder, surface reconstruction, and oxygen evolution reaction (OER) kinetics in a robust anode for alkaline oxygen evolution.

1. Introduction

The rising global demand for sustainable energy has intensified interest in hydrogen production via electrochemical water splitting. Among the two half-reactions involved, the oxygen evolution reaction (OER) remains the major bottleneck due to its sluggish 4e− mechanism and high overpotential [1]. In alkaline and neutral media, the reaction is described as [1]: 4OH− → O2 + 2H2O + 4e– (E0 = 1.23 V vs. reversible hydrogen electrode (RHE), where E0 corresponds to the standard potential).

To date, the most efficient OER catalysts have been based on noble metal oxides, such as RuO2 and IrO2, known for their favorable kinetics and stability. However, their scarcity and cost hinder large-scale deployment [2]. As a result, there is growing interest in earth-abundant alternatives that offer comparable performance and long-term durability in alkaline environments [2].

High-entropy alloys (HEAs), composed of five or more principal elements in near-equiatomic ratios, have emerged as promising candidates for electrocatalysis. Their high configurational entropy stabilizes single-phase solid solutions, typically face-centered cubic (FCC) or body-centered cubic (BCC), with abundant catalytically active sites. Moreover, their compositional complexity enables tunable electronic structures and synergistic surface chemistries [3]. This catalytic enhancement is often attributed to the “cocktail effect”, where interactions between different metals give rise to new electronic configurations capable of stabilizing OER intermediates such as *OH, *O, and *OOH. However, excessive stabilization may be detrimental, as the Sabatier principle dictates that intermediate binding should be neither too strong nor too weak. Interestingly, the diversity of active sites in HEAs may break linear scaling relations, offering further performance benefits [4–5]. Additionally, lattice distortions from atomic size mismatch increase defect densities and grain boundaries, boosting mass transport and charge transfer. Engineering the d-band center through compositional design provides another powerful lever for enhancing electrocatalytic activity [6]. Despite these advantages, many HEA studies have focused on structural analysis via X-ray diffraction (XRD), scanning electron microscopy (SEM), or energy-dispersive spectroscopy (EDS), while their electrochemical identity as true solid solutions remains underexplored. There is limited understanding of how HEAs behave under potential cycling in alkaline media compared to their individual metal constituents. The multielement synergy is not just the coexistence of discrete metal phases. For example, while metals like Ni and Co exhibit distinct redox transitions, HEAs often present broadened and overlapping features in cyclic voltammetry (CV), suggesting the emergence of new hybrid states.