The Role of Surface Hydroxylation, Lattice Vacancies and Bond Covalency in the Electrochemical Oxidation of Water (OER) on Ni-Depleted Iridium Oxide Catalysts

Abstract

The usage of iridium as an oxygen-evolution-reaction (OER) electrocatalyst requires very high atom efficiencies paired with high activity and stability. Our efforts during the past 6 years in the Priority Program 1613 funded by the Deutsche Forschungsgemeinschaft (DFG) were focused to mitigate the molecular origin of kinetic overpotentials of Ir-based OER catalysts and to design new materials to achieve that Ir-based catalysts are more atom and energy efficient, as well as stable. Approaches involved are: (1) use of bimetallic mixed metal oxide materials where Ir is combined with cheaper transition metals as starting materials, (2) use of dealloying concepts of nanometer sized core-shell particle with a thin noble metal oxide shell combined with a hollow or cheap transition metal-rich alloy core, and (3) use of corrosion-resistant high-surface-area oxide support materials. In this mini review, we have highlighted selected advances in our understanding of Ir–Ni bimetallic oxide electrocatalysts for the OER in acidic environments.

Vacancy Promotion in Layered Double Hydroxide Electrocatalysts for Improved Oxygen Evolution Reaction Performance

“…The intense shoulder peak in the 10% Ce-doped sample indicates more unoccupied states at O (2p) mixed with Ce (4f). , It is to be noted that the shoulder peak at ∼529 eV could also be an indication of the O ligand redox activity. These electrophilic O ligands were found to be the active sites for OER on the surface of various oxides such as Ir, Ru, and NiFe LDHs as reported elsewhere. ,, The intensity of peak (a) is related to the unoccupied electronic states in O (2p). The unoccupied states in O (2p) hybridized with Co/Ni (3d) increase on replacing Co with Ce.…”

“…These electrophilic O ligands were found to be the active sites for OER on the surface of various oxides such as Ir, Ru, and NiFe LDHs as reported elsewhere. 30,50,51 The intensity of peak (a) is related to the unoccupied electronic states in O (2p). The unoccupied states in O (2p) hybridized with Co/Ni (3d) increase on replacing Co with Ce.…”

Cation-Exchange Method Enables Uniform Iridium Oxide Nanospheres for Oxygen Evolution Reaction

“…We evaluated the initial stability of both catalysts by applying a constant potential of 1.6 V (vs RHE) for 5 h, following the protocols established by different research groups. ,, The OER polarization curves after the 5 h test are shown as dashed lines in Figure A, and the overpotentials to obtain 10 mA·cm –2 before and after the durability test are compared in Figure C. The overpotential of the IrO 2 nanospheres was 352 mV and increased by 5.7% after the durability test.…”

Electrocatalysis for the Oxygen Evolution Reaction in Acidic Media: Progress and Challenges

“…The first one is that the AEM requires a higher reaction energy barrier (>320 mV) than the LOM theoretically [56]. In terms of active sites, the catalytic process predominantly involves a cationic redox (i.e., transition metal ions) in the AEM mechanism and an anion redox (i.e., lattice oxygen) in the LOM [57]. However, it varies according to actual conditions.…”

“…For metal alloy/oxide-based electrocatalysts, the surface oxidation of metal atoms accompanied by de-alloying (surface dissolution of unstable metals) under acid OER conditions is considered to be a "surface engineering" strategy to design stable and efficient OER electrocatalysts. For example, Travis Jones and Peter Strasser found that electrochemical de-alloying and surface oxidation treatment of IrNi 3.2 nanoparticle precursors can lead to the dissolution of Ni, and the formed Ni nanoparticles showed better catalytic performance than core-shell-structured IrNi@IrO x [57]. In order to investigate the relationship between the reconstructed structure and the enhanced OER performance, an operando X-ray absorption spectroscopy (XAS) analysis was performed to characterize the local electronic properties under the OER process.…”