The investigation of highly effective, durable, and cost-effective electrocatalysts for the hydrogen evolution reaction (HER) is a prerequisite for the upcoming hydrogen energy society. To establish a new hydrogen energy system and gradually replace the traditional fossil-based energy, electrochemical water-splitting is considered the most promising, environmentally friendly, and efficient way to produce pure hydrogen. Compared with the commonly used platinum (Pt)-based catalysts, ruthenium (Ru) is expected to be a good alternative because of its similar hydrogen bonding energy, lower water decomposition barrier, and considerably lower price. Analyzing and revealing the HER mechanisms, as well as identifying a rational design of Ru-based HER catalysts with desirable activity and stability is indispensable. In this review, the research progress on HER electrocatalysts and the relevant describing parameters for HER performance are briefly introduced. Moreover, four major strategies to improve the performance of Ru-based electrocatalysts, including electronic effect modulation, support engineering, structure design, and maximum utilization (single atom) are discussed. Finally, the challenges, solutions and prospects are highlighted to prompt the practical applications of Ru-based electrocatalysts for HER.

Single-atom catalysts (SACs) are one of the most promising candidates for catalyzing various electrochemical reactions, due to their maximum atom utilizations and unique electronic structures. However, the general methods for preparing the SACs usually require complex processes, delicate regulation, and unique properties for the anchoring metals and supports. Herein, for the first time, we developed a facile, highly efficient, and universal plasma-etching strategy to fabricate various single-metal and dual-metal SACs. As an example, prepared Cu-SAC/NC exhibited superb oxygen reduction reaction (ORR) activity and a long period of circulation stability in an alkaline medium. More importantly, Cu-SAC/NC also exhibited an excellent battery performance when implemented as the air-cathode catalyst for assembled zinc-air batteries. Furthermore, density functional theory calculations revealed that pyrrolic-type three-coordinated Cu-po-N₃ sites exhibited considerable stability and a theoretical overpotential of only 0.21 V for the ORR, and were the main active sites for the high performance of the catalyst. This work proposes a facile, general, highly efficient, and controllable strategy for preparing SACs, and it is expected to open new approaches for the practical applications of SACs.

The electrode, as one of the most critical components in a proton exchange membrane water electrolysis (PEMWE) cell for hydrogen production, has a significant impact on cell performance. Electrodes that are fabricated via various techniques may exhibit different morphologies or properties, which might change the kinetics and resistances of the PEMWE. In this study, we have successfully fabricated several electrodes by different techniques, and the effects of electrode coating methods (ultrasonic spray, blade coating, and rod coating), hot press, and decal transfer processes are comprehensively investigated. The performance differences between various electrodes are due to kinetic or high frequency resistance changes, while the influences are not significant, with the biggest deviation of about 26 mV at 2.0 A cm⁻². In addition, the effects of catalyst ink compositions, including ionomer to catalyst ratio (0.1 to 0.3), water to alcohol ratio (1:1 to 3:1), and catalyst weight percentage (10% to 30%), are also studied, and the electrodes’ performance variations are less than 10 mV at 2.0 A cm⁻². The results show that the PEMWE electrode has superior compatibility and redundancy, which demonstrates the high flexibility of the electrode and its applicability for large-scale manufacturing.

Zinc-iodine batteries (ZIBs) are considered a promising energy storage system, but are still plagued by low energy density and rampant side reactions originating from active H2O molecules in the liquid electrolyte. Realizing the coupled redox reactions within I-/I0/I+ species, i.e., four-electron transfer reactions, is deemed an effective strategy for boosting the energy density of ZIBs, which is mainly blocked by the rapid hydrolysis of nucleophilic I+ ions. To address these issues, urea with diamine ligand sites (-NH2) was introduced into the liquid electrolyte [urea electrolyte (UE)] to achieve durable four-electron ZIBs. As demonstrated by the spectroscopic characterization results, -NH2 groups can bundle the active H2O molecules by reconfiguring the hydrogen bonds, and provide additional electrophilic ligand sites for I+ ions. Based on these advantages, both the side reactions on the Zn anode and the I+ hydrolysis reaction on the I2@AC cathode are remarkably mitigated, and four-electron transfer is realized at low zinc salt concentrations. As a result, the optimized UE electrolyte effectively stabilizes the zinc metal anode, and endows the I2@AC cathode with a high reversible capacity of 187.2 mAh g-1 after 250 cycles at 1 A g-1. The disclosed intermolecular force modulation strategy in this work will offer a comprehensive perspective for the future design of liquid electrolytes for high-energy-density ZIBs.