Study on Electrocatalytic Activity of Metal Oxides

Electrocatalysis represents a critical branch of catalysis research, characterized by its interdisciplinary nature and drawing interest from chemists, physicists, biochemists, surface scientists, materials scientists, and engineers [1,2,3,4]. Over the past few decades, electrocatalytic reactions such as water splitting, oxygen reduction, hydrogen oxidation, carbon dioxide reduction, nitrogen reduction, and alcohol oxidation have garnered attention from academia, industry researchers, and governmental bodies [5,6,7]. Noble-metal-based catalysts have traditionally been regarded as benchmarks for these reactions due to their high levels of activity [8,9]. However, their limited availability and high cost render them economically impractical for large-scale industrial applications [10]. To address this challenge, in recent years, researchers have made significant efforts to explore low-cost metal oxide catalysts that exhibit both satisfactory intrinsic activity and stability.

This Special Issue aims to highlight the most recent advancements in metal oxide catalysts for electrocatalytic applications, showcasing cutting-edge research with potential for practical application and in-depth mechanistic insights.

Water electrolysis is a pivotal technology in the production of hydrogen energy. Given the rising industrial demand for green hydrogen, the electrode size required for traditional alkaline water electrolyzers has been progressively increasing [11]. Extensive research efforts have been dedicated to developing highly active and stable catalysts for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in water electrolysis.

In Contribution 1, commercially available Raney Ni-coated Ni mesh was used as a precursor to synthesize a large-scale water electrolysis anode (NiFe(OH)x@Raney Ni, exceeding 300 mm) through simple immersion in a solution containing Ni(NO₃)₂ and Fe(NO₃)₃ at 60 °C. The prepared electrode exhibited an overpotential of only 240 mV at a current density of 10 mA cm⁻², comparable to that of hydrothermally synthesized NiFe-layered double hydroxides (LDHs). It also demonstrated stable operation for more than 100 h at 500 mA cm⁻². The large-scale electrode showed consistent overpotentials across different regions, and when incorporated into an alkaline water electrolysis device, it achieved an average cell voltage of 1.80 V and direct-current hydrogen production energy consumption as low as 4.3 kWh/Nm³, demonstrating its viability for industrial applications. In Contribution 2, ultrathin high-entropy LDHs containing Ni, Co, Fe, Zn, and Cr (U-NiFeZnCoCr-LDH) were prepared using a modified co-precipitation method. By introducing formamide (FA) during synthesis, the thickness was controlled at approximately 2 nm. Through the rational selection of metal elements, a dual-improvement in activity and stability was achieved in U-NiFeZnCoCr-LDH, which operated stably for 180 h at 800 mA cm⁻². The inductively coupled plasma mass spectroscopy (ICP-MS) results showed that the dissolution of active sites during stability testing was reduced by 42.7% compared to NiFe-LDH, demonstrating the industrialization potential of this material.

Electrolytic hydrogen production from seawater offers an economical, sustainable, and efficient strategy for renewable hydrogen generation. However, the abundant presence of Cl⁻ ions in seawater poses a challenge to anodic catalysts [12]. Contribution 3 reported self-supported NiCoFeS as a carrier and used cyclic voltammetry to load single-atom Au onto NiCoFeS nanosheets as alkaline seawater electrolysis catalysts. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) confirmed the existence of Au in a single-atom form. The Au@NiCoFeS catalyst exhibited excellent electrochemical activity and stability in both alkaline and seawater electrolytes, requiring only 201 mV and 183 mV overpotentials, respectively, to achieve a current density of 10 mA cm⁻². Moreover, it maintained stable operation at a current density of 200 mA cm⁻² for 250 h in both alkaline and seawater electrolytes. The loading of Au single atoms effectively increased the electrochemical specific surface area of the catalyst and reduced the charge transfer resistance. Additionally, its activity in seawater electrolytes even surpassed that in alkaline electrolytes, which may be attributed to the in situ regulation of the Au coordination environment by the abundant Cl⁻ ions in seawater.

In Contribution 4, ultrafine Ru nanoparticles were anchored on a hierarchical Ni₂P promoter to fabricate a Ru/Ni₂P electrode with both excellent HER activity and stability. The Ni₂P promoter received electrons from Ru and weakened the proton adsorption at Ru sites during HER. Moreover, the hierarchical Ni₂P promoter endowed the Ru catalysts with an enlarged surface area and a stable electrode structure. As a result, the as-achieved Ru/Ni₂P electrode displayed low overpotentials of 57 and 164 mV at HER current densities of 10 and 50 mA cm⁻², respectively, comparable to commercial Pt catalysts. Moreover, the Ru/Ni₂P electrode was able to operate steadily at a high current density of 50 mA cm⁻² for over 96 h. After pairing with a commercial RuO₂ anode, the Ru/Ni₂P anode drove water splitting at 1.73 V with a current density of 10 mA cm⁻², which was 160 mV lower than that of its commercial Ni counterpart. These findings indicated the importance of the hierarchical phosphide promoter in enhancing Ru nanocatalysts for water splitting hydrogen production.

Using electrochemical oxidation catalysis, urine-containing waste water can be converted into green hydrogen, eliminating the need for complex and high-carbon-emission waste water purification processes [13]. This approach helps promote waste water management and the development of new energy sources. In Contribution 5, rotten banana juice was utilized as a reducing, capping, and stabilizing agent for the design of new composite systems of CuO/NiO during modified hydrothermal processing. As a result of coating NiO onto proper CuO, the as-prepared catalyst was found to be highly active in terms of the oxidation of urea in alkaline media. The proposed urea sensor exhibited a wide linear range of 0.1 to 17 mM and a low limit of detection of 0.004 mM. It was shown, through structural and electrochemical investigations, that the CuO content, particle size, shape orientation, crystal defects, surface modification, rapid charge transport, and enriched surface active sites are all important factors in driving urea oxidation in alkaline conditions. This paper demonstrates that rotten banana juice could be used as a green tool to synthesize new electrocatalytic materials using metal oxides for a wide range of electrochemical applications.

The scientific contributions presented in this Special Issue shed significant light on the state of the art in emerging catalysts for electrocatalysis reactions that could be incorporated into the industrial sectors.

We would like to express our gratitude to MDPI and the Catalysts journal for the opportunity to serve as Guest Editors and contribute to the current state of the art in metal oxide catalysts for electrochemical catalysis, as well as to the Section Managing Editor, Dr. Maeve Yue, who worked hard to help us publish this Special Issue. In addition, we would like to thank all of the authors who shared their research, and the referees for their invaluable contributions.