The global environmental crisis necessitates viable alternatives to fossil fuels, with hydrogen energy emerging as a promising zero-emission option. Significant effort has been devoted to developing carbon-free hydrogen production methods, with economical and scalable production using surplus renewable energy gaining prominence. Polymer electrolyte membrane water electrolysis (PEMWE) is a leading candidate due to its high energy efficiency, pure hydrogen output, and capacity for high current densities. However, widespread PEMWE adoption is hampered by high overpotentials and the substantial use of expensive noble metals like iridium (Ir), primarily due to the sluggish kinetics of the oxygen evolution reaction (OER) in acidic conditions. Industrial applications demand high Ir loadings (≥0.5 mg cm⁻²), significantly increasing costs. Therefore, research focuses on enhancing Ir-based catalyst activity and stability while minimizing Ir usage. IrOₓ, formed during catalyst activation, exhibits higher OER activity than metallic Ir or IrO₂, attributed to its multiple valence states. Specifically, a strong correlation exists between IrOₓ activity and the fraction of Ir(III) species. However, Ir(III) readily oxidizes to Ir(IV) or Ir(V) during OER, forming soluble intermediates that lead to Ir dissolution and performance degradation. Previous research has primarily focused on activity enhancement without addressing catalyst deterioration, highlighting the need for strategies to sustainably maintain active Ir(III) species to overcome the activity-durability trade-off. Recent efforts explored metal oxide supports to enhance charge transfer from the support to the Ir catalyst, thereby replenishing charge and stabilizing Ir(III). However, common metal oxides have inherently limited charge transfer capabilities. While defect engineering, like creating oxygen vacancies, can improve charge transfer, it suffers from cation leaching and structural collapse. This study proposes a novel approach using an excess electron reservoir (EER) on a metal oxide support to sustain Ir(III) through charge replenishment.

Extensive research has been conducted to improve the activity and durability of Ir-based catalysts for the oxygen evolution reaction (OER) in acidic conditions. Studies have focused on various approaches such as optimizing IrOₓ structure, exploring different supports, and modifying the electronic properties of the catalyst. The importance of maintaining a high fraction of Ir(III) species for optimal catalytic activity has been highlighted, along with the challenges associated with Ir dissolution and degradation during OER. Researchers have investigated different metal oxide supports to enhance charge transfer to the Ir catalyst, leading to improved activity and durability. However, many common metal oxide supports have limitations in their charge transfer capabilities, necessitating the exploration of alternative strategies. Studies on defect engineering in metal oxide supports have also been reported, but these approaches often face challenges related to cation leaching and structural instability. This research builds upon the existing literature by introducing a novel approach to overcome the limitations of traditional methods and improve both the activity and durability of Ir-based OER catalysts.

This study introduces a unique support modification strategy using an excess electron reservoir (EER) to enhance the OER performance of Ir-based catalysts. The EER, an electron-donating layer, is incorporated between the support and the IrOₓ catalyst to maintain Ir(III) states via charge replenishment. Density functional theory (DFT) calculations were used to screen potential EER materials, considering criteria such as processability, adsorption energy, and charge transfer capability. Charged oxygen species (O⁻ and O₂⁻) were identified as promising EER candidates. Antimony-doped tin oxide (ATO) was chosen as the base support material. ATO supports with varying EER densities (sparse, moderate, and dense) were fabricated using e-beam deposition, with the EER content controlled by adjusting the deposition parameters. Ir catalysts were deposited onto these modified ATO supports using solvent-assisted nanotransfer printing (S-nTP). The resulting Ir/ATO catalysts were characterized using techniques such as X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), and inductively coupled plasma mass spectrometry (ICP-MS). Electrochemical half-cell measurements were conducted in 0.05 M H₂SO₄ electrolyte using a rotating disk electrode (RDE) setup to evaluate OER activity and stability. Linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and chronopotentiometry (CP) were employed to assess catalytic performance. DFT calculations were also used to further investigate the charge transfer mechanism between the EER and the Ir catalyst. Finally, single-cell PEMWE tests were performed to evaluate the performance of the Ir/ATO catalysts in a practical electrochemical cell, comparing it to commercial Ir catalysts. The MEA was prepared by spraying catalyst ink onto a Nafion membrane, followed by electrochemical characterizations.

The study's key findings demonstrate the significant enhancement in OER performance achieved through the incorporation of an excess electron reservoir (EER). DFT calculations successfully identified charged oxygen species (O⁻ and O₂⁻) as suitable EER materials. The fabrication of ATO supports with varying EER densities was achieved through precise control of e-beam deposition parameters. The resulting Ir catalysts supported on ATO with dense EER (Ir/D-ATO) exhibited dramatically enhanced performance compared to commercial Ir catalysts (Ir/C, Ir black, and Ir/TiO₂) and other Ir-based catalysts reported in the literature. Specifically, Ir/D-ATO displayed a remarkable 75-fold increase in mass activity at 1.6V and 55-fold increase at 1.8V compared to Ir black. The improved mass activity was further supported by the superior specific activity, highlighting the intrinsic activity enhancement provided by the EER. Electrochemical Impedance Spectroscopy (EIS) analysis revealed significantly reduced charge transfer resistance for Ir/D-ATO, demonstrating the EER's ability to accelerate the charge transfer during the OER process. Long-term stability tests showed exceptional durability for Ir/D-ATO, maintaining activity for over 250 h at 1.0 A cm⁻² with a minimal degradation rate (0.624 mV/h). Even under a high current density of 10 mA cm⁻², Ir/D-ATO exhibited remarkable stability compared to other catalysts, maintaining activity for about 18 h. ICP-MS analysis confirmed low Ir mass loss during stability tests, further indicating the EER's role in preventing Ir dissolution. XPS analysis revealed a positive correlation between EER density and the fraction of Ir(III) species, which is directly linked to catalytic activity, providing experimental support for the charge transfer mechanism facilitated by the EER. The results clearly demonstrate that enhanced charge transfer plays a critical role in improving OER performance, and the EER effectively controls the Ir oxidation states. Single-cell PEMWE measurements confirmed the superior performance of Ir/D-ATO, achieving the highest mass activity and Ir-specific power (74.8 kW g⁻¹) reported to date for Ir-based catalysts in PEMWE. This high Ir-specific power holds significant promise for achieving gigawatt-scale H₂ production. The DFT modelling confirmed that higher O2 anion concentrations in the EER enhance electron transfer from the ATO support to the Ir catalyst, which leads to lower charge states of Ir, and a higher concentration of Ir(III) that is responsible for the enhanced activity.

This study successfully demonstrates a novel strategy for significantly enhancing the activity and durability of Ir-based oxygen evolution reaction (OER) catalysts in PEM water electrolyzers. The use of an excess electron reservoir (EER) on metal oxide supports provides a practical solution to overcome the limitations of conventional approaches. The findings address the longstanding challenge of balancing activity and stability in OER electrocatalysis by providing a method to effectively control the Ir oxidation state and prevent Ir dissolution. The substantial improvement in mass activity (75 times higher than commercial catalysts) and Ir-specific power (74.8 kW g⁻¹) surpasses previous results, positioning this strategy as a significant advance in the field. The ability to achieve this high performance with a remarkably low Ir loading (7.2 µgIr cm⁻²) further enhances the economic viability and sustainability of hydrogen production. The results underscore the importance of metal-support interactions in electrocatalysis, offering a new design principle for developing high-performance OER catalysts. This approach has significant implications for various energy-related technologies and could pave the way for large-scale, cost-effective hydrogen production from renewable energy sources. Future research could focus on exploring other EER materials and support structures to further optimize the OER performance and expand the applicability of this strategy to other electrocatalytic reactions.

This research successfully demonstrates a new strategy for enhancing the activity and durability of Ir-based OER catalysts using an excess electron reservoir (EER) on metal oxide supports. The Ir/D-ATO catalyst showed a remarkable 75-fold increase in mass activity compared to commercial catalysts and exceptional long-term stability. The achieved high Ir-specific power (74.8 kW g⁻¹) holds significant promise for gigawatt-scale hydrogen production. This work provides valuable insights into the role of charge transfer in electrocatalysis and presents a new design principle for developing high-performance OER catalysts with broader applications in various energy technologies. Future work could explore other EER materials and support structures to further optimize performance and investigate scalability for industrial applications.

While this study demonstrates remarkable improvements in OER performance, several limitations should be considered. The synthesis method, while achieving high performance, might not be easily scalable to large-scale industrial production. The long-term stability tests, though extensive (250 h), should be further extended to ensure long-term durability under various operating conditions. The study focused on a specific catalyst (Ir) and support material (ATO); further investigation is necessary to evaluate the generality of the EER approach for other catalyst-support combinations. Finally, detailed cost analysis considering the overall production process is required for a full economic assessment of the proposed technology.