Water Oxidation and Surface Protonation of Small IrOx Clusters

Electrochemical water oxidation on the anode is a common bottleneck of electrocatalytic processes. For their efficiency and relative stability, IrO x -type anodes are among the best ones in acidic environment. In spite of all the efforts, however, it is still a complicated system from theoretical point of view. The small band gap and complicated band structure around Fermi level, strong electron correlation makes Ir(IV) oxides challenging itself, and the amorphous structure or nanoparticle structure of the electrodes just adds to the complications. We focused on these later aspects and studied the behavior of small cluster models of IrOx (Ir 3 and up), instead of regular slab models. Crystalline IrO 2 has a rutile lattice where Ir atoms are forming Ir-O-Ir chains along the x/y and Ir-O 2 -Ir chains z direction. In amorphous oxides and nanoclusters those chains are terminated by –OH or –OH 2 groups, to compensate the missing positive charges. In our cluster models we mostly kept the local environment around most Ir similar to bulk, but studied the effect of terminating –OH and –OH 2 groups, i.e. the H isomerism on the cluster surface. For the calculation we used DFT (DMol3) with the PBE (GGG) functional and DNP (4.4) basis set. The convergence criteria of for geometry optimization was 1.0 -5 Ha and 1.0 -6 for the SCF. Due to the large number of states around the HOMO-LUMO gap, we applied thermal smearing. The accuracy of the calculations was assessed compared to calculations with B3LYP hybrid functional, however the geometry and charges remained essentially the same. We found that surface protonation can significantly stabilize the lower valence states of Ir, therefore mobile H on the surface can reduce the energy required for each step of the water oxidation cycle.