Water electrolysis, a process involving the electrochemical splitting of water into hydrogen and oxygen, offers a promising pathway towards sustainable hydrogen production. This method, while environmentally benign, currently accounts for only 4% of global hydrogen production, primarily due to the high cost and sluggish kinetics of the oxygen evolution reaction (OER), particularly in acidic environments. The OER, a complex four-electron/four-proton process, necessitates catalysts capable of operating at low overpotentials and exhibiting exceptional durability. While the hydrogen evolution reaction (HER) proceeds efficiently in acidic media with the aid of Pt catalysts, the OER remains a critical bottleneck. Most metals readily dissolve in the potential region required for OER in acid, according to Pourbaix diagrams. Ir oxides show some stability, but still require high overpotentials. Ruthenium, while highly active, suffers from significant degradation. Existing strategies to enhance Ru stability, such as heavy Ir doping or thermal calcination, often compromise its activity. This research addresses the critical need for a non-degradable, highly active OER catalyst suitable for acidic conditions, aiming to overcome this long-standing challenge in electrochemistry.

Numerous studies have explored OER catalysts, focusing on materials design principles for alkaline media. However, relatively few reports address the specific challenges of acidic OER. Iridium oxides have emerged as moderately stable OER catalysts in acidic environments; however, they still demand high overpotentials, typically exceeding 300 mV. Ruthenium, while exhibiting significantly higher activity than iridium, faces severe degradation issues. Previous attempts to improve Ru stability have involved strategies such as heavy Ir doping (at least 30 at.%), thermal calcination, and strong support-metal interactions. These methods, while achieving some success in improving stability, generally lead to diminished catalytic activity. The lack of highly efficient and durable OER catalysts capable of operating in acid at low overpotentials presents a significant barrier to the widespread adoption of acidic water electrolysis.

The study synthesized a novel Ru-Ir catalyst, RuIr-NC, via a hot-injection method involving the addition of RuCl3·nH2O and H2IrCl6 aqueous solutions to a triethylene glycol (TEG) solution containing polyvinylpyrrolidone (PVP) at 230 °C. Advanced characterization techniques, including transmission electron microscopy (TEM), aberration-corrected scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray spectroscopy (EDS), and 3D tomography, revealed the unique coral-like structure of RuIr-NC, composed of 3 nm-thick sheets with extended (0001) facets and a homogeneous distribution of Ru and Ir atoms at a ratio of 94:6. The catalysts' OER performance was evaluated using a rotating disk electrode (RDE) in 0.05 M H2SO4 via linear sweep voltammetry (LSV), measuring geometric, mass, and specific activities. Long-term stability was assessed through chronopotentiometry (CP) at a constant current density. The HER activity was evaluated similarly. Operando X-ray absorption near-edge spectroscopy (XANES) and ex situ hard X-ray photoelectron spectroscopy (HAXPES) were employed to monitor structural and electronic changes during the OER process. Inductively coupled plasma mass spectrometry (ICP-MS) quantified metal dissolution. A home-made two-electrode water-splitting cell utilizing RuIr-NC as both anode and cathode was constructed and tested for overall water-splitting performance and long-term stability. Comparative studies were conducted with Ru NPs, Ir NPs, and RuIr nanospherical particles (RuIr-NS) synthesized using a heat-up method.

RuIr-NC demonstrated exceptional OER activity in acidic media, requiring only 165 mV overpotential to achieve 10 mA cm⁻²geo, significantly outperforming Ir NPs (371 mV), Ru NPs (550 mV), and other state-of-the-art catalysts. Mass and specific activities were 1-2 orders of magnitude higher than those of benchmark catalysts. Remarkably, RuIr-NC exhibited exceptional stability, showing no noticeable degradation over 122 h at 1 mA cm⁻²geo and maintaining performance for 40 h even at 10 mA cm⁻²geo. In contrast, RuIr-NS and other reference catalysts showed significant degradation within 1-12 h. Operando XANES revealed that RuIr-NC’s superior stability resulted from the resistance of its extended {0001} facets to oxidation and dissolution. HAADF-STEM images confirmed that the coral-like structure and (0001) facets remained intact even at high potentials, while RuIr-NS particles underwent significant amorphization and dissolution. Furthermore, RuIr-NC exhibited comparable HER activity to commercial Pt/C, with a Tafel slope of 32.0 mV/dec. The home-made overall water-splitting cell using RuIr-NC as both electrodes achieved 10 mA cm⁻²geo at a low cell voltage of 1.485 V and maintained this performance for over 120 h, significantly exceeding the performance of cells using commercial IrO2 and Pt/C catalysts.

The findings demonstrate that the unique coral-like structure of RuIr-NC, characterized by its anisotropic 3 nm-thick nanosheets with extended {0001} facets, is the key to its superior OER activity and stability. The preferentially exposed {0001} facets effectively resist the formation of dissolvable metal oxides, preventing catalyst degradation. This contrasts sharply with the behavior of spherical Ru-Ir catalysts, which readily dissolve under OER conditions. The exceptional performance of RuIr-NC addresses a critical bottleneck in acidic water electrolysis, showcasing the potential of structural engineering for achieving highly active and stable electrocatalysts. The combined high activity for both OER and HER makes RuIr-NC an ideal bifunctional catalyst for efficient overall water splitting.

This study successfully synthesized and characterized a highly active and stable Ru-Ir nanocoral catalyst (RuIr-NC) for overall water splitting in acidic media. The unique anisotropic nanosheet structure with exposed {0001} facets is crucial for its superior performance. RuIr-NC significantly outperforms existing catalysts in both activity and stability, enabling a low-cost, highly efficient water electrolyzer. Future research could explore the synthesis of similar anisotropic nanostructures with other metal combinations to further optimize performance and explore different synthesis strategies to achieve even higher activity and broader applications.

While RuIr-NC shows exceptional performance, further research is needed to assess its long-term stability under industrial-scale operating conditions. The synthesis method might require optimization for large-scale production. The study primarily focuses on H2SO4 electrolyte; additional investigations are warranted to explore the catalyst's performance in other acidic electrolytes.