Efficient and cost-effective hydrogen production is crucial for a sustainable energy future. Electrocatalytic water splitting, specifically the hydrogen evolution reaction (HER), is a promising method, but requires highly active and durable catalysts to overcome significant activation barriers. While considerable progress has been made in developing high-performance catalysts, a major challenge remains: creating catalysts that function efficiently and stably under high current densities (>1000 mA cm⁻²) needed for industrial-scale hydrogen production. The mechanisms governing HER catalysis differ significantly at low and high current densities. At high current densities, factors beyond the Sabatier principle, such as interfacial charge transfer resistance, reaction intermediate coverage, catalyst mechanical stability, and hydrogen bubble release kinetics, become critical. Slow proton diffusion and hydrogen adsorption, along with large hydrogen bubbles covering active sites, can significantly hinder performance. The conductivity of the catalyst is another key factor, as low conductivity leads to excessive Ohmic drop and inaccurate measurements. Layered transition metal oxides have emerged as promising candidates due to their tunable electronic structures and surface properties. While their thermodynamically stable surfaces are often inert, modifications or reconstructions can significantly improve their catalytic efficiency. Methods such as in situ exsolution have shown promise in creating well-dispersed, stable metal catalysts within oxide supports. Sr2RuO4 (SRO), a layered oxide perovskite, is of particular interest due to its unique electronic properties, including superconductivity and surface magnetism. While some studies have explored SRO's catalytic properties using polycrystalline samples and low current densities, a comprehensive investigation using single crystals and high current densities is lacking. This research aims to address this gap by examining the catalytic properties of bulk SRO single crystals under high current densities, investigating the underlying mechanisms, and assessing their long-term stability for potential industrial applications.

The literature extensively documents the search for efficient HER catalysts, with many studies reporting catalysts surpassing the performance of traditional noble metals like platinum. However, a persistent challenge involves achieving high activity and stability at industrially relevant current densities. Several studies have highlighted the importance of considering factors beyond the Sabatier principle for high-current-density HER. These include interfacial charge transfer resistance, the coverage of reaction intermediates, mechanical stability, and hydrogen bubble release kinetics. For example, the Tafel slope, which reflects the reaction kinetics, is often reported at low current densities but can increase significantly at higher current densities, even for state-of-the-art catalysts. Hydrogen bubble dynamics is another crucial aspect, as large bubbles can block active sites and induce stress, affecting both activity and stability. The conductivity of the catalyst is essential, as low conductivity can lead to significant Ohmic drop, distorting the measured overpotential and leading to misleading conclusions. Several papers discuss the use of layered transition metal oxides as HER catalysts, highlighting the potential to enhance activity through surface modification and reconstruction. In situ exsolution, a technique where nanoparticles are generated and anchored within the bulk oxide, offers a promising approach to improve the stability and dispersion of active metal catalysts. While several studies have investigated SRO's properties, most have focused on polycrystalline samples and low current densities. This work expands upon the existing knowledge by evaluating the catalytic performance of SRO single crystals under high current densities, examining structural changes, and proposing a mechanistic understanding.

SRO single crystals were grown using the floating-zone method. The crystal structure and surface electronic properties were characterized using techniques such as X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The HER catalytic activity was evaluated using electrochemical methods, including linear sweep voltammetry (LSV) and cyclic voltammetry (CV), in both acidic (0.5 M H2SO4) and alkaline (1 M KOH) electrolytes. The stability of the catalyst was assessed using long-term chronoamperometric measurements under various current densities and temperatures. Electrochemical impedance spectroscopy (EIS) was employed to analyze the charge transfer kinetics. Digital image correlation (DIC) was used to study hydrogen bubble dynamics and wettability. The turnover frequency (TOF) was calculated considering the electrochemically active surface area (ECSA). The Faradaic efficiency was verified by collecting and quantifying the produced hydrogen gas. Density functional theory (DFT) calculations were performed to investigate the hydrogen adsorption behavior and the effect of surface reconstruction. The changes in crystal structure during the catalytic reaction were examined to understand the underlying mechanisms. The mass activities of the SRO/Ru catalyst were also calculated by assuming the catalytic activity stems solely from the Ru clusters on the crystal surface.

The SRO single crystals exhibited exceptional HER activity under high current densities, achieving 1000 mA cm⁻² with overpotentials of 182 mV (H2SO4) and 278 mV (KOH). These results are comparable to or even superior to state-of-the-art noble-metal catalysts under similar conditions. The catalyst exhibited remarkable stability, maintaining high performance for extended periods (up to 56 days) at high current densities and temperatures up to 70°C. In-situ analysis revealed the formation of ferromagnetic Ru clusters on the SRO surface after activation. This reconstruction played a crucial role in enhancing catalytic performance. DFT calculations revealed that the Ru clusters act as the primary active sites, showing optimized hydrogen adsorption behavior due to electron redistribution at the Ru-SRO interface. The high conductivity of the SRO bulk contributed to low charge transfer resistance. The improved wettability of the modified surface facilitated rapid hydrogen bubble release, further contributing to the high performance and stability of the catalyst. The TOF value at 100 mV overpotential was calculated to be 121 s⁻¹, surpassing many previously reported catalysts. The catalyst demonstrated a Faradaic efficiency close to 100%. Mass activities of 16.6 A mg⁻¹ at 50 mV overpotential were observed, making it one of the best HER catalysts among noble metal-based compounds. The Tafel slopes, reflecting the reaction kinetics, remained low, even at high current densities, indicating fast HER kinetics. The high stability was confirmed by LSV curves before and after long-term testing, showing negligible activity loss.

The findings of this study successfully address the crucial need for highly efficient and durable HER catalysts capable of operating under industrial-scale current densities. The exceptional performance of the SRO-based catalyst can be attributed to the synergistic combination of several factors. First, the in-situ formation of ferromagnetic Ru clusters on the surface provides highly active catalytic sites with optimized hydrogen adsorption. Second, the high conductivity of the SRO bulk minimizes charge transfer resistance. Third, the improved surface wettability significantly enhances gas bubble removal kinetics. The observed stability demonstrates the robustness of the catalyst against degradation under harsh conditions. These results offer a new strategy for designing high-performance HER catalysts by controlling the interface between the active phase and the support material. This work moves beyond the typical focus on nanostructured catalysts and demonstrates the potential of using bulk single crystals for high-current density electrocatalysis.

This research demonstrates a highly efficient and robust HER catalyst based on the in-situ activation of SRO single crystals, forming ferromagnetic Ru clusters. This approach successfully addresses the challenge of achieving high performance at industrially relevant current densities. The excellent activity and long-term stability, coupled with mechanistic insights from DFT calculations, highlight the potential of this catalyst for practical hydrogen production. Future research could explore further modifications of the SRO surface to enhance performance and explore the application of this strategy to other catalytic reactions. Investigating the scalability of the crystal growth and activation processes is also a crucial next step towards industrial implementation.

While the study demonstrates excellent performance under specific conditions (0.5 M H2SO4 and 1 M KOH), further investigations are needed to assess the catalyst's performance in other electrolytes. The activation process is relatively time-consuming; further optimization could reduce the activation time. The long-term stability tests were conducted at fixed overpotentials; more comprehensive studies under various operating conditions are required. The DFT calculations focused on specific Ru cluster geometries; exploring other possible structures could provide a more complete picture of the catalytic mechanism. Finally, while the Faradaic efficiency was near 100%, scaling up the system for industrial applications requires further investigation of potential mass-transport limitations.