The development of high-performance, cost-effective catalysts for the hydrogen evolution reaction (HER) is crucial for efficient water electrolyzers. Single-atom catalysts (SACs) are promising due to their maximized atom-use efficiency and unique coordination environments, but their catalytic activity, especially in multistep reactions like alkaline HER, needs improvement. Constructing synergistic sites to assist single-atom sites is a potential strategy. Molybdenum disulfide (MoS₂), a cost-effective layered transition metal dichalcogenide, has been widely studied for HER. Strategies to activate its inert basal plane include phase engineering, creating sulfur vacancies (SVs), and single-atom doping. While single-atom doping can create SVs, which are considered active sites, a deeper understanding of their synergistic effect, particularly under reaction conditions, is lacking. Introducing strain to catalysts can optimize the electronic structure of active sites, creating a favorable reaction environment. This study aimed to construct a nanoporous MoS₂ (np-MoS₂) to anchor single-atom Ru (Ru/np-MoS₂) and investigate the synergistic effect between Ru sites and SVs in alkaline HER. The curvature-induced strain is tailored by controlling the ligament size of np-MoS₂. The goal was to amplify the synergistic interaction between SVs and single atoms through strain engineering, thereby maximizing the catalytic activity of SACs.

Extensive research has focused on improving the hydrogen evolution reaction (HER) activity of molybdenum disulfide (MoS2), a cost-effective layered transition metal dichalcogenide. The inert basal plane of MoS2 has been targeted for activation using various methods, including phase engineering, creating sulfur vacancies (SVs), and single-atom doping. Single-atom doping, in particular, has shown promise in creating SVs and enhancing catalytic activity. However, a comprehensive understanding of the synergistic effect between SVs and single-atom dopants remains elusive. The introduction of strain has emerged as a powerful tool to modulate the electronic structure of active sites and enhance catalytic performance. This study builds upon this existing research by exploring the combined effects of strain engineering and single-atom doping on the HER activity of MoS2.

The study involved theoretical calculations using density functional theory (DFT) to investigate the effects of strain on water adsorption energies and reaction barriers on different sites (Ru and Mo_SV) in the Ru/MoS₂ system. Nanoporous MoS₂ (np-MoS₂) was synthesized using chemical vapor deposition and chemical etching to create a 3D bicontinuous nanoporous structure. Single-atom Ru was incorporated through a spontaneous reduction method. The resulting Ru/np-MoS₂ catalyst, along with control samples (plane MoS₂, P-MoS₂; and nanoporous MoS₂ with larger ligaments, Lnp-MoS₂), was characterized using various techniques including scanning electron microscopy (SEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and ambient pressure XPS (AP-XPS). Electrochemical HER performance was evaluated using a three-electrode system in 1.0 M KOH electrolyte, measuring overpotential, Tafel slope, and electrochemical surface area (ECSA). Operando XAS measurements were conducted using a home-built cell to monitor changes in the electronic and atomic structure of the catalyst under operating conditions.

DFT calculations predicted that strain enhances water adsorption on Mo_SV sites and lowers the energy barriers for HER on Ru sites, indicating a synergistic effect amplified by strain. The synthesized Ru/np-MoS₂ catalyst exhibited superior HER performance compared to control samples and commercial catalysts (Ru/C and Pt/C), with an overpotential of 30 mV at 10 mA cm⁻², a Tafel slope of 31 mV dec⁻¹, and excellent stability. Operando XAS revealed that the Ru oxidation state changed during HER, indicating its active role in water dissociation. AP-XPS confirmed that Ru/np-MoS₂ adsorbs more water than np-MoS₂. The results highlight that the strain-induced enhancement of water adsorption on SVs facilitates efficient mass transfer to Ru sites, promoting water dissociation and ultimately enhancing HER activity. The study confirms that the magnitude of strain directly impacts the intrinsic HER activity, with higher strain leading to more significant changes in atomic and electronic structures of Ru sites and SVs. Control experiments comparing ECSA-normalized current density of nanoporous MoS2 and Ru-doped MoS2 under the same strain condition clearly show the higher catalytic activity of the most strained Ru/1T-MoS2 active structure in Ru/np-MoS2. The H2 Faraday efficiency of Ru/np-MoS2 was close to 100% under different applied potentials, and long-term stability was demonstrated by chronoamperometric tests, showing no aggregation of Ru atoms.

The findings demonstrate the success of strain engineering in amplifying the synergistic effect between single-atom Ru sites and SVs in MoS₂ for enhanced HER. The combination of theoretical calculations, experimental characterization, and operando techniques provides a comprehensive understanding of the catalyst's behavior under operating conditions. The improved water adsorption on SVs and the resulting facilitated mass transfer to Ru sites are key factors in the enhanced HER activity. This study showcases the potential of precise strain control to optimize the properties of SACs. The approach of using strain engineering could be extended to other SAC systems by modifying assisting sites or adjusting the strain modulation method.

This work successfully demonstrates a strain engineering strategy to boost the hydrogen evolution reaction (HER) activity of single-atom ruthenium catalysts supported on nanoporous molybdenum disulfide. The optimized catalyst exhibits exceptional performance, exceeding state-of-the-art and commercial catalysts. Future work could focus on expanding this strategy to other SAC systems and investigating the effects of different strain types and magnitudes on catalytic activity. The findings contribute significantly to the design of highly efficient and durable HER catalysts.

The study focuses on a specific catalyst system (Ru/np-MoS₂) and electrolyte (1.0 M KOH). The generalizability of the strain engineering strategy to other SACs and electrolytes needs further investigation. While the long-term stability is demonstrated, long-term studies under industrial conditions are needed for comprehensive validation. The precise mechanism of the synergistic effect between SVs and Ru atoms, especially the role of specific intermediates, could benefit from more detailed studies.