Using Instability of a Non-stoichiometric Mixed Oxide Oxygen Evolution Catalyst As a Tool to Improve Its Electrocatalytic Performance

Abstract

Owing to their superior electrocatalytic performance, non-stoichiometric mixed oxides are often considered as promising electrocatalysts for the acidic oxygen evolution reaction (OER). Their activity and stability can be superior to those of the state-of-the-art IrO 2 catalysts, although the exact nature of this phenomenon is not yet understood. In the current work, a Ir 0.7 Sn 0.3 O 2-x thin-film electrode is taken as a representative example for a thorough evaluation of OER activity of the non-stoichiometric oxides. Complementary activity and stability analysis of Ir 0.7 Sn 0.3 O 2-x electrodes is achieved using a setup based on an electrochemical scanning flow cell and ICP-MS. The obtained ICP-MS data presents an unambiguous proof of the preferential dissolution of the less noble Sn from the mixed oxide during OER. While less than a monolayer of Ir is dissolved after a prolonged electrolysis of 1400 min during which its dissolution rate drops to near zero, the amount of Sn lost is ten monolayers. The latter finding is confirmed by XPS analysis, which besides showing Ir surface enrichment also indicates a gradual transformation of Ir 0 to Ir III species. This transition is beneficial for electrode activity, as the overpotential for OER at j = 5 mA cm −2 was decreasing up to 300 mV. The increase in electrode activity is attributed to several mechanisms including generation of Ir III active sites and overall surface area increase. A generalized description of OER catalysis by Ir-based materials is given, including data from the current work as well as from other Ir-based mixed oxides, such as Ir-Ru-O and Ir-Ni-O.

Electrochemical On‐line ICP‐MS in Electrocatalysis Research

“…As a result of a continuous incease in the surface area during the leaching, electrocatalytic activity of Ir−Ni gradually improves. This is applicable to other mixed oxides, in particular to non‐stoichiometric Ir−Sn mixed oxide with 70 at% of Ir prepared by reactive sputtering, where increase in activity was mainly attributed to an increase in the surface area during preferable dissolution of Sn, as shown in Figure b.…”

“…For this, complementary ICP‐MS post analysis, providing additional information on equilibrium concentration of the dissolved elements, can be performed, e. g. using an H‐cell with divided anodic and cathodic compartments. Presented in Figure b a comparison between SFC‐ICP‐MS and ex situ ICP‐MS analysis of Ir and Sn dissolition reveals that for the relatively short time intervals a good agreement between the two sets of data exists …”

“…Data is taken from . (b) Dissolution of Ir 0.7 Sn 0.3 O 2−n mixed oxide during anodic polarization at 1 mA cm −2 measured online by SFC‐ICP‐MS (opened symbols) or using post analysis approach after collecting the samples from an electrochemical H‐cell with divided anodic and cathodic compartments (solid symbols) . (c) Stability‐numbers corresponding to anodic polarization at 1 mA cm −2 calculated for: Ir−Ru material library mixed oxide catalyst in the whole compositional range; Ir 0.7 Sn 0.3 O 2−n mixed oxide at the beginning of polarization and in steady‐state conditions; metallic Ir; Ir hydrous oxide.…”

Alkaline manganese electrochemistry studied byin situandoperandospectroscopic methods – metal dissolution, oxide formation and oxygen evolution

“…However, a gradual decrease in the Mn dissolution rate upon oxidation of a non-stoichiometric phase cannot be ruled out; such dissolution was recently reported for metallic Ir and its alloys. 58,59 During oxidation (0.1-0.4 V) k-spectra show a peak at E2.9 eV, with a shoulder at E2.2 eV. In situ Raman spectra show formation of a-MnO 2 under these conditions.…”

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

“…8 This can be especially crucial in the case of mixed oxide catalysts, where preferential leaching of a less noble element occurs, 9 resulting in surface reconstruction 10 and the formation of amorphous hydrous oxide of Ir. 9,11,12 By combining isotope labelling and differential/ online electrochemical mass spectrometry (DEMS/OLEMS), several studies evidenced lattice oxygen participation in the OER mechanism for non-noble perovskites 13 and sputtered Ru oxide. 14 In contrast, no lattice oxygen participation in the OER was observed for Pt 15 and thermally prepared RuO 2 .…”