Degradation of iridium oxides via oxygen evolution from the lattice: correlating atomic scale structure with reaction mechanisms

Abstract

Combination of atom probe tomography, isotope-labelling and online electrochemical mass spectrometry provides direct correlation of atomic scale structure of Ir oxide catalysts with the mechanism of oxygen formation from the lattice atoms.

Nanotubular Titanium Oxynitride with an Ultra-Low Iridium Loading as a High-Performance Oxygen-Evolution-Reaction Thin-Film Electrode

“…51 Additionally, we assume that at least to some extent the high performance of the present Ir-TiON system can be ascribed to the different composition of the Ir valence state in comparison to the Ir Black analogue, i.e., the presence of a larger number of Ir species with a mixed valence (Ir 3+/5+ ), which have been shown to be the OER-active surface states of iridium electrodes. 52,53,54,55,41,56 Although such species could neither be confirmed nor refuted by the present XPS analysis (see Table 1 and the associated text), their presence was indirectly demonstrated in our previous study, which showed that the TiON support significantly affected the Ir redox chemistry and the related transient behavior of the oxide layer by preserving the lower Ir oxidation states. 46 To induce electrode degradation, a potentiostatic treatment at 1.8 V vs RHE was employed (we remark again that a degradation time of 1 h for the TiON-Ir and 30 min for the Ir-Black was used).…”

Ir/TiONx/C high-performance oxygen evolution reaction nanocomposite electrocatalysts in acidic media: synthesis, characterization and electrochemical benchmarking protocol

“…This strongly indicates that in the case of Ir/TiON x /C the active Ir sites are regenerated via electrochemical reduction (presumably Ir(III) [55][56][57][58][59][60]), whereas in the case of Ir Black the oxide layer grows too thick (is irreversibly oxidized) to be electrochemically reduced during the activation protocol. These results indicate that the SMSI between the TiON x support and Ir nanoparticles most likely inhibits the growth of oxide, which can be electrochemically reduced to more OER-active [55][56][57][58][59][60], lower-valency Ir species. This inhibition of Ir oxide formation was shown in our previous study where Ir on TiON x exhibited the HUPD peaks, specific of metallic iridium, even after cycling until oxidative potential [17].…”

On the role of microkinetic network structure in the interplay between oxygen evolution reaction and catalyst dissolution

“…In the latter mechanism, the involvement of lattice-oxygen atoms results in weakening of the metal oxide structure, causing catalyst dissolution during OER. The participation of lattice oxygen during OER with Iridium-based catalysts has been confirmed using isotope labelling methods 6 , 14 , 15 . Basic thermodynamic reasoning for catalyst dissolution on metal oxides was given by Binninger et al who showed that, to some extent, dissolution must inevitably occur under OER-conditions due to the thermodynamic instability of the oxygen anion in the metal oxide lattice 8 .…”

“…In terms of both activity and stability in acidic media, a qualitatively different behaviour is observed with a reactively sputtered (RS) Iridium oxide 4 , 58 , 59 compared to thermal Iridium oxide 4 , 6 , 15 . Analysis of the dissolution data of the RS-surface (inset Fig.…”

“…Based on this hypothesis, in the following a quasi-steady state model for a surface of reactively sputtered is derived, considering the superposition of two pathways via high- and low oxidation state intermediate. The model is compared with experimental data measured in acidic media 4 , 15 . For the high oxidation state path (‘HOSP’), the same mechanism shall be assumed as in section “ Thermal Iridium oxide ” for the TO-model.…”