The production of hydrogen gas using renewable electricity is a crucial step towards reducing reliance on fossil fuels. Proton exchange membrane (PEM) water electrolysis offers several advantages for hydrogen production, including high purity, efficiency, and scalability. However, a major obstacle is the significant dissolution of electrocatalysts at the anode during the oxygen evolution reaction (OER). Current state-of-the-art OER electrocatalysts are based on iridium and ruthenium oxides, but these materials suffer from degradation, especially at moderate current densities. This necessitates a compromise between activity and stability. Various strategies have been explored to improve the lifetime of these electrocatalysts, including the use of mixed oxides, earth-abundant metal oxides, and perovskites. The use of earth-abundant mixed oxides as stabilizing frameworks for RuO₂ stands out as a promising approach to mitigate Ru dissolution. This work focuses on developing a multicomponent mixed-oxide matrix to host and protect OER active metals from dissolution under harsh acidic conditions and high anodic potentials. The chosen quaternary mixed oxide (MO) comprises Sn, Sb, Mo, and W interconnected oxides forming a continuous porous structure, created using the scalable solution precursor plasma spraying (SPPS) method. This approach offers potential advantages of improved stability in acidic media, large-scale production capabilities, low electrode cost, and potential improvements in OER activity. The research investigates the MO's ability to act as a protective scaffold for RuO₂, analyzing its stability and impact on OER performance, ultimately aiming to extend the lifetime of both RuO₂ and the titanium fiber felt support commonly used in PEM water electrolyzers.

Previous research has explored various strategies to improve the durability of RuO₂-based OER catalysts in acidic media. Studies have investigated RuO₂/IrO₂ mixed-oxide phases, RuO₂ mixed with earth-abundant metal oxides, polychloro oxides, and perovskite-based oxides. However, significant challenges remain in achieving both high activity and long-term stability. The use of earth-abundant mixed metal oxides as supports for RuO₂ has shown promise in enhancing stability and, in some cases, activity. Several metal oxides, including CrO₅, Sb-SnO₂, TaOx, TiO₂, WO₃, and pyrochlore complexes, have been studied as potential supports. These studies highlighted the benefits of such strategies, including improved stability in acidic media, cost-effectiveness, and scalability. This current work builds upon these previous efforts by investigating a quaternary mixed oxide system designed to specifically address the challenges of Ru dissolution and degradation in acidic OER conditions.

The study employed solution precursor plasma spraying (SPPS) to deposit Sn-Sb-Mo-W mixed oxide coatings onto fluorine-doped tin oxide (FTO)-coated glass substrates. The molar ratio of Sn:Sb:Mo:W was 5:5:18:18, selected based on previous theoretical and experimental studies. The SPPS process involves introducing a solution precursor into a plasma plume, where solvent evaporation, precursor decomposition, and particle formation occur. The resulting coating is porous and consists of an interconnected network of nanostructured oxides. After spraying, the coatings were oxidized at 500 °C for 2 h to transform them into a mixture of metal oxides. The morphology and chemical composition were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The stability of the MO coatings was evaluated through cyclic voltammetry (CV) tests at different pH levels (0, 1, and 2) and applied potentials (1.2-1.8 V vs RHE). RuO₂ was then incorporated into the MO matrix by adding RuCl₃ to the precursor solution. The influence of annealing time on the stability of the Ru-MO coating was investigated. Finally, titanium fiber felt, a common gas diffusion layer in PEM electrolyzers, was used as a substrate for Ru-MO and RuO₂ (as a reference) to evaluate the overall stability and performance of the material in a more realistic environment. Electrochemical measurements, including polarization curves, CV stability tests, electrochemical impedance spectroscopy (EIS), and chronopotentiometry, were performed to assess the OER activity and stability. The electrochemical surface area (ECSA) and mass activity (MA) were also calculated.

The quaternary Sn-Sb-Mo-W mixed oxide (MO) coating demonstrated excellent stability in acidic media (pH 0-2) under anodic potentials, with only partial dissolution of Mo observed. This resulted in a porous structure that increased the surface area. Incorporating a small amount of RuO₂ (2.64% w/w) into the MO matrix significantly enhanced the stability of Ru during OER. The improved stability was attributed to the MO hindering the formation of highly reactive Ru⁸⁺, maintaining Ru in lower oxidation states (Ru²⁺ and Ru³⁺). Using titanium fiber felt as a substrate further improved the performance of the Ru-MO catalyst. The Ru-MO@Ti catalyst exhibited an ECSA of 131.4 cm² and a mass activity of 4440 A g⁻¹Ru at 1.8 V vs RHE, comparable to state-of-the-art Ru-based OER electrocatalysts. A long-term stability test (2000 CV cycles) showed that Ru-MO@Ti retained 80% of its initial activity, significantly outperforming a RuO₂@Ti reference sample (which lost ~90% of its activity). Chronopotentiometry also confirmed the superior stability of Ru-MO@Ti, displaying only a minor potential increase (10 mV) after 10 h of operation, in contrast to a substantial increase (500 mV) for RuO₂@Ti. The analysis of Tafel slopes and EIS provided further insights into the reaction mechanisms and charge transfer processes.

The results demonstrate the effectiveness of the quaternary Sn-Sb-Mo-W mixed oxide as a protective scaffold for RuO₂ in acidic OER conditions. The superior stability of the Ru-MO@Ti catalyst compared to the RuO₂@Ti reference sample highlights the crucial role of the MO matrix in preventing Ru dissolution and degradation. The porous structure formed by partial Mo dissolution enhances the surface area and electrolyte accessibility, contributing to improved activity. The findings address the critical need for durable and efficient OER catalysts in PEM water electrolyzers. The high mass activity and long-term stability of the Ru-MO@Ti catalyst show significant potential for industrial-scale renewable hydrogen production. The observed electrochemical behavior suggests that the MO scaffold effectively stabilizes Ru in lower oxidation states, suppressing the formation of highly soluble RuO₄. This strategic approach opens up opportunities to further explore inexpensive OER active metals within the MO matrix.

This study successfully demonstrated the fabrication of a stable and highly active OER catalyst by utilizing a quaternary Sn-Sb-Mo-W mixed oxide as a protective scaffold for RuO₂ on a titanium fiber felt support. The superior stability and activity of the Ru-MO@Ti catalyst compared to the reference RuO₂@Ti catalyst highlight the significance of this novel approach in enhancing the durability and performance of OER electrocatalysts for PEM water electrolysis. Future research could explore optimizing the composition and structure of the MO matrix, investigating alternative sacrificial elements for improved porosity, and extending the approach to other OER-active metals. The findings significantly advance the development of durable and efficient electrocatalysts for large-scale, renewable hydrogen production.

The study focused on a specific composition of the quaternary mixed oxide and Ru loading. Further research is needed to explore a broader range of compositions to optimize performance and stability. The relatively low Ru loading (2.64% w/w) might limit the overall activity; increasing Ru content while maintaining stability could be investigated. While the titanium fiber felt substrate provides a more realistic environment, long-term testing under real operating conditions of a PEM electrolyzer is necessary for a more comprehensive evaluation of the catalyst's durability.