Iridium nanoparticles for the oxygen evolution reaction: Correlation of structure and activity of benchmark catalyst systems

Strontium Doped IrOx Triggers Direct O-O Coupling to Boost Acid Water Oxidation Electrocatalysis

The discovery of efficient and stable electrocatalysts for the oxygen evolution reaction (OER) in acidic conditions is crucial for the commercialization of proton-exchange membrane water electrolyzers. In this work, we propose a Sr(OH)2-assisted method to fabricate a (200) facet highly exposed strontium-doped IrOx catalyst to provide available adjacent iridium sites with lower Ir-O covalency. This design facilitates direct O-O coupling during the acidic water oxidation process, thereby circumventing the high energy barrier associated with the generation of *OOH intermediates. Benefiting from this advantage, the resulting Sr-IrOx catalyst exhibits an impressive overpotential of 207 mV at a current density of 10 mA cm-2 in 0.5 M H2SO4. Furthermore, a PEMWE device utilizing Sr-IrOx as the anodic catalyst demonstrates a cell voltage of 1.72 V at 1 A cm-2 and maintains excellent stability for over 500 hours. Our work not only provides guidance for the design of improved acidic OER catalysts but also encourages the development of iridium-based electrocatalysts with novel mechanisms for other electrocatalytic reactions.

Innovative Method for Reliable Measurement of PEM Water Electrolyzer Component Resistances

Understanding the sheet resistance of porous electrodes is essential for improving the performance of polymer electrolyte membrane (PEM) water electrolyzers and related technologies. Despite its importance, existing methods often fail to provide reliable and comprehensive data, especially for porous materials with complex morphologies and non-uniform thicknesses. This study introduces a robust and straightforward method for determining the sheet resistance of porous electrodes using a novel probe concept based on industrial printed circuit board (PCB) technology. This probe measures resistance across ten distances, ranging from 250 µm to 2500 µm, enabling local mapping of resistance. The study focuses on the sheet resistance of key components in PEM water electrolyzers, including the gas diffusion layer (GDL), porous transport layer (PTL), and catalyst layers deposited on a membrane. Additionally, an image-processing-based method is presented to obtain the thickness distribution of the studied catalyst layers, facilitating a detailed analysis of the electrical in-plane resistivity with thickness variations. Overall, this methodology has the potential to expedite material integration and bridge the gap between electrode engineering and single-cell testing, thereby advancing the development of PEM water electrolyzers.

Unraveling the Most Relevant Features for the Design of Iridium Mixed Oxides with High Activity and Durability for the Oxygen Evolution Reaction in Acidic Media

Proton exchange membrane water electrolysis (PEMWE) is the technology of choice for the large-scale production of green hydrogen from renewable energy. Current PEMWEs utilize large amounts of critical raw materials such as iridium and platinum in the anode and cathode electrodes, respectively. In addition to its high cost, the use of Ir-based catalysts may represent a critical bottleneck for the large-scale production of PEM electrolyzers since iridium is a very expensive, scarce, and ill-distributed element. Replacing iridium from PEM anodes is a challenging matter since Ir-oxides are the only materials with sufficient stability under the highly oxidant environment of the anode reaction. One of the current strategies aiming to reduce Ir content is the design of advanced Ir-mixed oxides, in which the introduction of cations in different crystallographic sites can help to engineer the Ir active sites with certain characteristics, that is, environment, coordination, distances, oxidation state, etc. This strategy comes with its own problems, since most mixed oxides lack stability during the OER in acidic electrolyte, suffering severe structural reconstruction, which may lead to surfaces with catalytic activity and durability different from that of the original mixed oxide. Only after understanding such a reconstruction process would it be possible to design durable and stable Ir-based catalysts for the OER. In this Perspective, we highlight the most successful strategies to design Ir mixed oxides for the OER in acidic electrolyte and discuss the most promising lines of evolution in the field.

Progress of Heterogeneous Iridium-Based Water Oxidation Catalysts

The water oxidation reaction (or oxygen evolution reaction, OER) plays a critical role in green hydrogen production via water splitting, electrochemical CO₂ reduction, and nitrogen fixation. The four-electron and four-proton transfer OER process involves multiple reaction intermediates and elementary steps that lead to sluggish kinetics; therefore, a high overpotential is necessary to drive the reaction. Among the different water-splitting electrolyzers, the proton exchange membrane type electrolyzer has greater advantages, but its anode catalysts are limited to iridium-based materials. The iridium catalyst has been extensively studied in recent years due to its balanced activity and stability for acidic OER, and many exciting signs of progress have been made. In this review, the surface and bulk Pourbaix diagrams of iridium species in an aqueous solution are introduced. The iridium-based catalysts, including metallic or oxides, amorphous or crystalline, single crystals, atomically dispersed or nanostructured, and iridium compounds for OER, are then elaborated. The latest progress of active sites, reaction intermediates, reaction kinetics, and elementary steps is summarized. Finally, future research directions regarding iridium catalysts for acidic OER are discussed.

Nickel Structures as a Template Strategy to Create Shaped Iridium Electrocatalysts for Electrochemical Water Splitting

Low-cost, highly active, and highly stable catalysts are desired for the generation of hydrogen and oxygen using water electrolyzers. To enhance the kinetics of the oxygen evolution reaction in an acidic medium, it is of paramount importance to redesign iridium electrocatalysts into novel structures with organized morphology and high surface area. Here, we report on the designing of a well-defined and highly active hollow nanoframe based on iridium. The synthesis strategy was to control the shape of nickel nanostructures on which iridium nanoparticles will grow. After the growth of iridium on the surface, the next step was to etch the nickel core to form the NiIr hollow nanoframe. The etching procedure was found to be significant in controlling the hydroxide species on the iridium surface and by that affecting the performance. The catalytic performance of the NiIr hollow nanoframe was studied for oxygen evolution reaction and shows 29 times increased iridium mass activity compared to commercially available iridium-based catalysts. Our study provides novel insights to control the fabrication of iridium-shaped catalysts using 3d transition metal as a template and via a facile etching step to steer the formation of hydroxide species on the surface. These findings shall aid the community to finally create stable iridium alloys for polymer electrolyte membrane water electrolyzers, and the strategy is also useful for many other electrochemical devices such as batteries, fuel cells, sensors, and solar organic cells.

Nanostructured IrOx Coatings for Efficient Oxygen Evolution Reactions in PV-EC Setup

New heteroleptic iridium compounds exhibiting high volatility and defined thermal decomposition behavior were developed and tested in plasma-enhanced chemical vapor deposition (PECVD). The iridium precursor [(COD)Ir(TFB-TFEA)] (COD = 1,5-cyclooctadiene; TFB-TFEA = N-(4,4,4-Trifluorobut-1-en-3-on)-6,6,6-trifluoroethylamin) unifies both reactivity and sufficient stability through its heteroleptic constitution to offer a step-by-step elimination of ligands to provide high compositional purity in CVD deposits. The substitution of neutral COD ligands against CO groups further increased the volatility of the precursor. PECVD experiments with unambiguously characterized Ir compounds (single crystal X-ray diffraction analysis) demonstrated their suitability for an atom-efficient (high molecule-to-precursor yield) gas phase deposition of amorphous iridium oxide (IrOx) phases. Thin films of IrOx were well suited as electrocatalyst in oxygen evolution reaction so that an efficient coupled system in combination with solar cells is viable to perform water-splitting reaction without external bias.

Breaking Long-Range Order in Iridium Oxide by Alkali Ion for Efficient Water Oxidation

Oxygen electrochemistry plays a critical role in clean energy technologies such as fuel cells and electrolyzers, but the oxygen evolution reaction (OER) severely restricts the efficiency of these devices due to its slow kinetics. Here, we show that via incorporation of lithium ion into iridium oxide, the thus obtained amorphous iridium oxide (Li-IrO _x) demonstrates outstanding water oxidation activity with an OER current density of 10 mA/cm² at 270 mV overpotential for 10 h of continuous operation in acidic electrolyte. DFT calculations show that lithium incorporation into iridium oxide is able to lower the activation barrier for OER. X-ray absorption characterizations indicate that both amorphous Li-IrO _x and rutile IrO₂ own similar [IrO₆] octahedron units but have different [IrO₆] octahedron connection modes. Oxidation of iridium to higher oxidation states along with shrinkage in the Ir-O bond was observed by in situ X-ray absorption spectroscopy on amorphous Li-IrO _x, but not on rutile IrO₂ under OER operando conditions. The much more "flexible" disordered [IrO₆] octahedrons with higher oxidation states in amorphous Li-IrO _x as compared to the periodically interconnected "rigid" [IrO₆] octahedrons in crystalline IrO₂ are able to act as more electrophilic centers and thus effectively promote the fast turnover of water oxidation.