Crystalline Disorder, Surface Chemistry, and Their Effects on the Oxygen Evolution Reaction (OER) Activity of Mass-Produced Nanostructured Iridium Oxides

Abstract

In the present study, three mass-produced commercial IrO x samples from different suppliers were studied to establish correlations between various properties and their OER activities. The structures of the electrocatalysts at different scales were explored through laboratory instrumentation, powder X-ray diffraction, and synchrotron-based X-ray total scattering experiments combined with pair distribution function analysis. X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy using a transmission electron microscope were used to determine respectively the surface and the bulk elemental compositions of the samples. The coherent domain size (CDS) values of IrO x phases within the catalyst particles were estimated to be ∼10, ∼ 19, and ∼ 54 Å for the three IrO x samples. Surprisingly, the sample with a CDS of ∼19 Å turned out as the best OER electrocatalyst among the three in terms of mass-specific activity, I OER(m), followed by the 10 and 54 Å species. The amount of surface native compound oxygen was found to be a key parameter for the interface electrochemical accessibility. The intrinsic OER activity, evaluated using area-specific activity, I OER(a), suggests that the oxide with lattice disorder presenting a mixture of tetragonal and orthorhombic phases (70:20 w/w) is of superior intrinsic OER activity; however, the oxide with the presence of a monoclinic-like phase is of inferior intrinsic OER activity, which may also be due to the surface presence of Ir3+ along with Ir4+. The classic belief that the pure tetragonal phase is the best crystalline structure as the OER catalyst is challenged. Iridium oxides with disordered crystallinities may offer a class of highly active oxygen evolution electrocatalysts. The knowledge thus obtained should have a significant impact on the understanding, selection, and processing of IrO x -based OER electrocatalysts.

Progress of Heterogeneous Iridium-Based Water Oxidation Catalysts

“…Amorphous iridium oxides are regarded as more active but less stable catalysts for the acidic OER compared with crystalline rutile-phase iridium oxides. ,− Several methods have been developed to obtain amorphous iridium catalysts such as surface oxidation of iridium metal, , leaching of ions in iridium compounds such as SrIrO 3 , , incorporation of alkali metal ions into iridium oxide, light-driven decomposition, and anodic deposition from iridium-based organometallic precursors. The origin of the higher electrochemical activity of amorphous iridium oxides is attributed to several possible structural features, including electronic defects (O 2p hole states), − flexible and disordered [IrO 6 ] octahedrons, cross-linked hollandite-like structure, and possible different reaction mechanism .…”

Multiple Bubble Removal Strategies to Promote Oxygen Evolution Reaction: Mechanistic Understandings from Orientation, Rotation, and Sonication Perspectives

“…Compared to the HER process in the water electrolyzers, the pathway for the OER reaction that includes a four-electron process is slower and more complicated. ,, Therefore, high OER activity is a vital process to improve the efficiency of PEMWE. Although OER mechanisms have not been fully understood, it is shown that structural features of catalysts, for instance, morphology and particle size, significantly influence the OER performance. , Furthermore, it is agreed that efficient transport of charge and mass would enormously affect the functionality of water electrolyzers.…”

“…Although OER mechanisms have not been fully understood, it is shown that structural features of catalysts, for instance, morphology and particle size, significantly influence the OER performance. 14,18 efficient transport of charge and mass would enormously affect the functionality of water electrolyzers. Among responsible loss phenomena in the OER mechanism, bubble coverage of the electrode dramatically contributes to the final efficiency.…”

Boosting the OER activity of amorphous IrOx in acidic medium by tuning its electron structure using lanthanum salt nanosheets

“…27 Despite these efforts, it is still necessary to further enhance the catalytic activity of most OER catalysts to reduce their energy consumption for commercial usage. 3,5,9,28–30 Industrial anodes such as Ir oxide 5,31 also have disadvantages of high cost, scarcity and unsatisfactory catalytic activity. 2,3,32,33 Although a-IrO x structured catalysts show relatively high OER activity, much more active catalysts are needed to meet energy requirements, 3,8,12 which is the major endeavour of the present study.…”