Oxygen Evolution Reaction Activity and Stability Benchmarks for Supported and Unsupported IrOx 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 (IrOx) 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 IrOx 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 IrOx nanocatalysts are more active toward the OER but less stable than unsupported micrometer-sized catalysts, for example, commercial IrO2 or porous IrOx microparticles; (ii) tantalum-doped tin oxides (TaTO) used as supports for IrOx 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-IrOx particles and leads to 1 order of magnitude lower Ir atom dissolution rate with respect to nonannealed catalysts; (v) the best compromise between OER activity and stability was obtained for unsupported porous IrOx 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.