Japan relies on fossil fuel imported from overseas for the majority of its energy, and from the security perspective, the securing of alternative energy sources has become an issue. Energy supply processes that use green hydrogen produced with renewable energy are garnering attention as a next-generation energy system. Hydrogen is an environmentally friendly energy carrier because it does not emit carbon dioxide (CO₂) when used as an energy carrier and contributes to energy security if combined with renewable energy power generation. It therefore has the potential to serve as an important core of energy processing.

A research group led by Professor Kazuhiro Takanabe at the Graduate School of Engineering, The University of Tokyo and Team Director Ryuhei Nakamura at the RIKEN Center for Sustainable Resource Science, RIKEN has been working to solve problems in proton exchange membrane (PEM) water electrolysis (PEM water electrolysis)^*1, which is attracting attention as a production technology for green hydrogen.

The research group identified that specific interaction between manganese (Mn) and iridium produces hexavalent iridium oxide, a novel iridium catalyst with a high oxidation number (+6), as a catalyst in PEM water electrolysis. This novel iridium catalyst reduced iridium usage by 95% or more compared to the conventional 2-4 mg Ir/cm² while retaining high activity and stability by dispersing iridium at the atomic level.

In the research, the formation process of the novel iridium catalyst was monitored at the large synchrotron radiation facility, SPring-8^*2, enabling both basic understanding of electrocatalysts and practical application of hydrogen production technologies.

*1 Proton exchange membrane (PEM) water electrolysis (PEM water electrolysis)
One of the methods of industrial water electrolysis. It is characterized by its method of soaking water in a film called solid polymer and subsequently breaking down the water, rather than electrolyzing water in its liquid state. By applying electrocatalysts on both sides of the film, the electrodes are brought as close to each other as possible in order to not only suppress electrical resistance but also promote the supply of reactants and thereby increase the efficiency in hydrogen production.

*2 Large synchrotron radiation facility, SPring-8
RIKEN's facility for experimental studies located in Harima Science Garden City, Hyogo Prefecture, that produces the world's highest-performing synchrotron radiation.

Water electrolysis (water electrolysis: 2H₂O → 2H₂ + O₂) using renewable energy is garnering attention as a hydrogen production technology that does not emit CO₂ and is environmentally friendly. Within such technology, PEM water electrolysis, which exhibits high load-following capability, is attracting attention as a technology suitable for green hydrogen production using renewable energy with fluctuating power generation volume, such as solar power generation. However, since oxygen-generation anodes^*3 are exposed to high potential and an acidic environment, iridium oxide (IrO₂) is used as a precious metal catalyst^*4 that exhibits both activity and stability.

Currently, approximately 1 g of iridium is required per 1 kW in current PEM water electrolysis. Meanwhile, the world's annual production of iridium is 7-8 tons. To achieve carbon neutrality^*5 by 2050, an electrolyzer installation of approximately 2,000 GW (1 GW: 1 billion watts) in scale is required, which is equivalent to 150 or more years worth of iridium production volume when operating only for PEM water electrolysis. Therefore, a significant reduction in iridium usage was urgently needed to resolve the issue of iridium scarcity.

*3 Anode
An electrode with a positive voltage is called an anode, and an electrode with a negative voltage is called a cathode. In water electrolysis, oxygen is produced with anodes and hydrogen with cathodes.

*4 Precious metal catalysts
Catalysts containing precious metal elements that are scarce on earth, such as platinum and iridium. The use of precious metal catalysts allows efficient water electrolysis; however, due to reserves and price-related issues, more abundant materials need to be used in order to expand the use of water electrolysis throughout society.

*5 Carbon neutrality
Current industrial activities emit greenhouse gases such as carbon dioxide. Carbon neutrality refers to the state in which the emitted volume is suppressed and equivalent to the amount of greenhouse gases absorbed by afforestation and other measures, i.e., the state in which the net amount of greenhouse gases emitted from human activities is substantially zero.

The research group has been developing Mn oxide (MnO₂) as a catalyst material for PEM water electrolysis. In the process, the research group discovered that Mn oxide specifically adsorbs iridium and that the resultant material excels in oxygen evolution catalytic activity (Fig. 1A). Based on this knowledge, the researchers sought to analyze the electronic state and structure of novel catalyst materials.

First, MnO₂ electrodes were prepared by the electrodeposition method^*6, and the resultant electrode (MnO₂/PTL) was immersed in K₂IrCl₆ precursor solution at 95 degrees for 6 hours or longer (iridium adsorption process in Fig. 1B). Subsequently, it was annealed at 450 degrees to synthesize a novel catalyst material (heat treatment process in Fig. 1B). During the synthesis process, iridium was adsorbed on the MnO₂ surface, and, at the same time, a ligand exchange reaction between K₂IrCl₆ and Mn oxide occurred.

Specifically, X-ray absorption spectroscopy (XAS)^*7 was performed at the large synchrotron radiation facility, SPring-8, to measure the X-ray absorption spectrum of iridium. It was found that iridium atoms were oxidized from 4+ to 6+ (Fig. 1C). Additionally, the shortening of the bonding distance to the adjacent ligand (Fig. 1D) indicated that the iridium ligand^*8 was exchanged from chloride ion to oxide ion.

Assessment of the state of the catalyst obtained by the above synthesis method revealed that the synthesized catalyst was an atomically-dispersed hexavalent iridium oxide, and thus the new catalyst was named "Ir^Ⅵ-ado catalyst."

*6 Electrodeposition method
A method for depositing solid materials by applying voltage. In the present research, gamma manganese oxide was deposited by applying voltage to an aqueous solution containing manganese ions.

*7 X-ray absorption spectroscopy (XAS)
A method for identifying the electronic state and local structure of substances. A wide range of substances can be measured, including gases, solids, liquids, and solutions. To increase the accuracy of measurement, it is often performed using synchrotron radiation facilities, which provide powerful X-rays, as the light source.

*8 Ligand
Ions or molecules bound to central atoms are collectively referred to as "ligands."

The novel iridium catalyst with high oxidation number (+6) developed in the present research can reduce iridium usage by 95% or more (from 2-4 mg Ir/cm² to 0.02-0.08 mg Ir/cm²), as well as exert excellent performance in both aspects of activity and stability. This may reduce the usage of precious metals, which has been an issue in the deployment of PEM-type water electrolysis, and significantly contribute to achieving 2050 carbon neutrality, which aims to reduce the overall greenhouse gas emission to substantially zero by 2050.