Nanoporous multimetallic Ir alloys as efficient and stable electrocatalysts for acidic oxygen evolution reactions

Quantification of electrochemically accessible iridium oxide surface area with mercury underpotential deposition

Research drives development of sustainable electrocatalytic technologies, but efforts are hindered by inconsistent reporting of advances in catalytic performance. Iridium-based oxide catalysts are widely studied for electrocatalytic technologies, particularly for the oxygen evolution reaction (OER) for proton exchange membrane water electrolysis, but insufficient techniques for quantifying electrochemically accessible iridium active sites impede accurate assessment of intrinsic activity improvements. We develop mercury underpotential deposition and stripping as a reversible electrochemical adsorption process to robustly quantify iridium sites and consistently normalize OER performance of benchmark IrOx electrodes to a single intrinsic activity curve, where other commonly used normalization methods cannot. Through rigorous deconvolution of mercury redox and reproportionation reactions, we extract net monolayer deposition and stripping of mercury on iridium sites throughout testing using a rotating ring disk electrode. This technique is a transformative method to standardize OER performance across a wide range of iridium-based materials and quantify electrochemical iridium active sites.

Composite Anode for PEM Water Electrolyzers: Lowering Iridium Loadings and Reducing Material Costs with a Conductive Additive

To enable the greater installed capacity of proton exchange membrane water electrolysis (PEMWE) for clean hydrogen production, associated costs must be lowered while achieving high current density performance and durability. Scarce and expensive iridium (Ir) required for the oxygen evolution reaction (OER) is a large contributor to the overall cost, yet high loadings of Ir (1-2 mgIr cm-2) are currently needed in commercial systems to maintain sufficient activity, conductivity, and durability. To meet the aggressive targets for low Ir loadings, we introduce a composite anode approach using a conductive additive that is less expensive than Ir to facilitate robust, high-performance operation with low Ir loading by retaining electrode thickness and in-plane electrical conductivity. In this demonstration, we use platinum (Pt) black as the conductive additive given its high electrical conductivity, acid stability, and current price one-fifth that of Ir. Using a high-activity commercial Ir oxide (IrO x ) catalyst, we present a 95% Ir loading reduction and 80% cost reduction of the anode catalyst materials while maintaining equal current density performance at a cell voltage of 1.8 V. Furthermore, we show enhanced stability of a composite anode compared to an IrO x anode with loadings of 0.10 mgIr cm-2 via accelerated stress test (AST) and postmortem imaging. With this approach, we show promising results toward lowering Ir loadings and material costs, addressing a significant barrier to the widespread adoption of PEMWE for clean hydrogen production.

Recent advances in iridium-based catalysts with different dimensions for the acidic oxygen evolution reaction

Proton exchange membrane (PEM) water electrolysis is considered a promising technology for green hydrogen production, and iridium (Ir)-based catalysts are the best materials for anodic oxygen evolution reactions (OER) owing to their high stability and anti-corrosion ability in a strong acid electrolyte. The properties of Ir-based nanocatalysts can be tuned by rational dimension engineering, which has received intensive attention recently for catalysis ability boosting. To achieve a comprehensive understanding of the structural and catalysis performance, herein, an overview of the recent progress was provided for Ir-based catalysts with different dimensions for the acidic OER. The promotional effect was first presented in terms of the nano-size effect, synergistic effect, and electronic effect based on the dimensional effect, then the latest progress of Ir-based catalysts classified into zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) catalysts was introduced in detail; and the practical application of some typical examples in the real PEM water electrolyzers (PEMWE) was also presented. Finally, the problems and challenges faced by current dimensionally engineered Ir-based catalysts in acidic electrolytes were discussed. It is concluded that the increased surface area and catalytic active sites can be realized by dimensional engineering strategies, while the controllable synthesis of different dimensional structured catalysts is still a great challenge, and the correlation between structure and performance, especially for the structural evolution during the electrochemical operation process, should be probed in depth. Hopefully, this effort could help understand the progress of dimensional engineering of Ir-based catalysts in OER catalysis and contribute to the design and preparation of novel efficient Ir-based catalysts.

Electrocatalysts for the Oxygen Evolution Reaction in Acidic Media

The well-established proton exchange membrane (PEM)-based water electrolysis, which operates under acidic conditions, possesses many advantages compared to alkaline water electrolysis, such as compact design, higher voltage efficiency, and higher gas purity. However, PEM-based water electrolysis is hampered by the low efficiency, instability, and high cost of anodic electrocatalysts for the oxygen evolution reaction (OER). In this review, the recently reported acidic OER electrocatalysts are comprehensively summarized, classified, and discussed. The related fundamental studies on OER mechanisms and the relationship between activity and stability are particularly highlighted in order to provide an atomistic-level understanding for OER catalysis. A stability test protocol is suggested to evaluate the intrinsic activity degradation. Some current challenges and unresolved questions, such as the usage of carbon-based materials and the differences between the electrocatalyst performances in acidic electrolytes and PEM-based electrolyzers are also discussed. Finally, suggestions for the most promising electrocatalysts and a perspective for future research are outlined. This review presents a fresh impetus and guideline to the rational design and synthesis of high-performance acidic OER electrocatalysts for PEM-based water electrolysis.