Oxygen Evolution Reaction Activity and Stability Benchmarks for Supported and Unsupported IrO x Electrocatalysts

Advanced materials are needed to meet the requirements of devices designed for harvesting and storing renewable electricity. In particular, polymer electrolyte membrane water electrolyzers (PEMWEs) could benefit from a reduction in the size of the iridium oxide (IrO x ) particles used to electrocatalyze the sluggish oxygen evolution reaction (OER). To verify the validity of this approach, we built a library of 18 supported and unsupported IrO x catalysts and established their stability number ( S -number) values using inductively-coupled plasma mass spectrometry and electrochemistry. Our results provide quantitative evidence that (i) supported IrO x nanocatalysts are more active towards the OER but less stable than unsupported micrometer-sized catalysts, e.g. commercial IrO 2 or porous IrO x microparticles; (ii) tantalum-doped tin oxides (TaTO) used as supports for IrO x nanoparticles are more stable than antimony-doped tin oxides (ATO) and carbon black (Vulcan XC72); (iii) thermal annealing under air atmosphere yields depreciated OER activity but enhanced stability; (iv) the beneficial effect of thermal annealing holds both for micro-and nano- IrO x particles, and leads to one order of magnitude lower Ir atom dissolution rate with respect to non-annealed catalysts; (v) the best compromise between OER activity and stability was obtained for unsupported porous IrO x microparticles after thermal annealing under air at 450°C. These insights provide guidance on which material classes and strategies are the most likely to increase sustainably the OER efficiency while contributing to diminish the cost of PEMWE devices.