High Entropy Layered Double Hydroxides as Robust Trifunctional Catalysts for Enhanced Water Splitting and Ethanol Electrooxidation

The desire for a sustainable and clean energy future continues to concern the scientific community. Green hydrogen production via water electrolysis offers a sustainable, carbon-neutral pathway to meet future energy demands. However, most of the energy lost in conventional water electrolysis is limited by the sluggish kinetics of the oxygen evolution reaction (OER), leading to high overpotentials and reduced system efficiency. Considering the overall water splitting, the OER process requires a high voltage to drive it; therefore, substituting OER with an electro-oxidation reaction with a lower theoretical potential is an efficient methodology to alleviate this issue. Recently, as an alternative to traditional hydrogen production from electrocatalytic water splitting, many readily oxidizable molecules, including alcohols, have been deployed to replace the OER process and improve energy efficiency. The ethanol oxidation reaction (EOR) usually competes with the OER at working potentials, serving as an alternative anodic process to overcome the kinetic limitations of OER. Developing highly active electrocatalysts through simple and quick methods remains challenging. In response, high-entropy materials have recently gained attention as promising electrocatalysts due to their structural stability, tunable composition, and synergistic interactions among diverse metal constituents. Herein, we introduce the concept of a high-entropy layered double hydroxide (HE-LDH) catalyst composed of Ni, Fe, Cu, Mg, and In, engineered to function as a robust trifunctional electrocatalyst capable of efficiently driving the HER, OER, and EOR. The HE-LDH was synthesized via hydrothermal treatment process at 120 °C for 12 hours, using a defined metal precursor molar ratio (Ni:Fe:Cu:Mg:In = 3:1:0.5:0.5:0.5). To further optimize performance, etching process using a two-step liquid-phase was applied for different etching times (10, 20, 20, and 60 minutes), with the 20-minutes etched sample demonstrating the best electrochemical properties. Different characterization techniques including X-ray diffraction (XRD), Scanning Electron microscopy (SEM), and Energy-Dispersive X-Ray Spectroscopy (EDS) were used to confirm the HE-LDHs. Electrochemical tests in alkaline media (0.5 M KOH) for HER and OER; (0.5 M KOH + 1 M EtOH) for EOR were measured. The HER overpotential reduced from 507 mV (NiFe-LDH) to 321 mV (HE-LDH), and further to 187 mV after etching. For OER, overpotentials reduced from 169 mV (NiFe-LDH) to 150 mV (HE-LDH), and 143 mV after etching, while EOR saw reductions from 148 mV (HE-LDH) to 143 mV after etching. Corresponding Tafel slope improvements were observed: for HER, from 266.61 mV/dec to 136.69 mV/dec; for OER, from 226.64 mV/dec to 87.98 mV/dec; and for EOR, from 100.01 mV/dec to 56.83 mV/dec. Electrochemical impedance spectroscopy (EIS) and electrochemical active surface area (ECSA) analyses further substantiated these findings. Charge transfer resistance for HER/OER decreased from 160 Ω (NiFe-LDH) to 65 Ω (HE-LDH) and 25 Ω after etching, with ECSA increasing from 0.079 mF/cm² to 0.32 mF/cm². For EOR, EIS dropped from 125 Ω to 40 Ω after etching, indicating improved charge mobility and catalytic site exposure. In addition, after 100 hours of continuous operation for both HER and EOR the catalytic performance of et-HELDH (20 min) shows no obvious deterioration. The rich selectivity of the elements and the fine regulation of the nanostructure injects new vitality into the performance improvement of high-entropy catalyst. These findings highlight the transformative potential of the high-entropy engineering and fine nanostructure control open opportunities to solve the problems of low intrinsic activity, very few active sites, instability, and low conductance.