Energy storage is a significant challenge due to the continuous growth of energy production from intermittent renewable energy sources. A promising solution is proton exchange membrane water electrolysis (PEMWE), which produces pure hydrogen at high pressures. Moreover, PEM water electrolyzers operate in acidic environment and thus benefit from the fast kinetics of the cathodic reaction and high-voltage efficiencies at high current densities. At the same time, the acidic environment strongly influences the stability of the anodic water oxidizing material, thereby limiting the oxygen evolution reaction (OER) catalysts to mainly noble metal oxides such as RuO₂ and IrO₂. The high cost and low abundance of the latter hinders the wide implementation of the PEMWE technology making the development of the active, stable, and cheap OER catalysts an extremely important issue.

The present work addresses the synthesis and study of high surface area iridium oxide and iridium pyrochlore materials as anodes for PEMWE. The OER catalysts were synthesized by the modified Adams fusion method, which utilizes molten sodium nitrate for oxide nanoparticles formation. Chlorine-free IrO₂ nanoparticles of different size and shape were synthesized from Ir(acac)₃ at 350−600 °C. The applied approach allowed for controlling the effect of the particle size, morphology, and nature of the surface species on the OER activity of IrO₂ catalysts. IrO₂ synthesized at 350 °C consists of 1.7 ± 0.4 nm particles with a specific surface area of 150 m²g⁻¹ and shows the highest OER activity: 1.499 V to reach 10 Ag_ox⁻¹. In addition to X-ray photoelectron spectroscopy, operando X-ray absorption spectroscopy studies of this oxide catalyst indicate the presence of Ir³⁺ surface sites which were correlated with the formation of iridium hydroxo (Ir−OH) surface species.

Iridium pyrochlore materials of the general formula A₂Ir₂O_6.5+x, where A is Bi, Y or Pb, were synthesized at 500−575 °C. Contrary to the conventional high temperature solid-state synthesis of iridium pyrochlores, the modified Adams fusion method allowed synthesis of the materials with surface areas of up to 40 m²g⁻¹. Obtained catalysts show high OER activity and stability; the lowest overpotentials were observed for Y₂Ir₂O₇ and Bi₂Ir₂O₇. Y₂Ir₂O₇ shows an overpotential about 50 mV lower than that of 30 m²g⁻¹ iridium oxide, which corresponds to about a 10-fold increase in the current. This suggests that the pyrochlore OER materials allow improving the specific activity of Ir-based catalysts, offering a possibility to develop cheap, active and stable anodes for PEMWE.