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 towards the OER but less stable than unsupported micrometer-sized catalysts, e.g. 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 microand nano-IrOx 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 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.

TiO2 photocatalyst is of interest for antimicrobial coatings on hospital touch-surfaces. Recent research has focused on visible spectrum enhancement of photocatalytic activity. Here, we report TiO2 with a high degree of nanostructure, deposited on stainless steel as a solid layer more than 10 μm thick by pulsed-pressure-MOCVD. The TiO2 coating exhibits a rarely-reported microstructure comprising anatase and rutile in a composite with amorphous carbon. Columnar anatase single crystals are segmented into 15–20 nm thick plates, resulting in a mille-feuilles nanostructure. Polycrystalline rutile columns exhibit dendrite generation resembling pine tree strobili. We propose that high growth rate and co-deposition of carbon contribute to formation of the unique nanostructures. High vapor flux produces step-edge instabilities in the TiO2, and solid carbon preferentially co-deposits on certain high energy facets. The equivalent effective surface area of the nanostructured coating is estimated to be 100 times higher than standard TiO2 coatings and powders. The coatings prepared on stainless steel showed greater than 3-log reduction in viable E coli after 4 hours visible light exposure. The pp-MOCVD approach could represent an up-scalable manufacturing route for supported catalysts of functional nanostructured materials without having to make nanoparticles.