In situ XAFS of acid-resilient iridate pyrochlore oxygen evolution electrocatalysts under operating conditions

Abstract

Electrocatalytic water splitting using iridate pyrochlores studied using synchrotron spectroscopy reveals a response from all iridium in the conducting oxide.

Characterization of a Dynamic Y2Ir2O7 Catalyst during the Oxygen Evolution Reaction in Acid

“…This result suggests Ir became more oxidized and the d-band hole count increased during OER relative to the as-deposited sample. We note that final oxidation state exhibiting Ir V+ character and the increase in oxidation state are consistent with in situ XAS measurements for similar AIr x O y OER catalysts. ,, We further acknowledge that the magnitude of the increase shown here for Y 2 Ir 2 O 7 is smaller than those reports, but we expect this difference is due to the higher initial Ir oxidation state in Y 2 Ir 2 O 7 .…”

“…This shoulder feature does not change with consecutive individual scans, suggesting that it is not an artifact of beam damage from the high photon flux of the measurement (Figure S4). We note that this shoulder feature can be subtle (and even go unnoticed) when the Ir L III -edge spectra are collected with standard resolution and is not always reported for Y 2 Ir 2 O 7 or other AIr x O y materials. ,,,,, To prove this, we collected Ir L III -edge XAS on the same material using a beamline with standard resolution and did not observe the same clear splitting of the peak (Figure S6), consistent with previous reports showing the advantages of HERFD-XAS at the Ir L III -edge …”

“…We note that this shoulder feature can be subtle (and even go unnoticed) when the Ir L IIIedge spectra are collected with standard resolution and is not always reported for Y 2 Ir 2 O 7 or other AIr x O y materials. 6,8,14,24,28,29 To prove this, we collected Ir L III -edge XAS on the same material using a beamline with standard resolution and did not observe the same clear splitting of the peak (Figure S6), consistent with previous reports showing the advantages of HERFD-XAS at the Ir L III -edge. 30 This distinctive peak shape measured by HERFD-XAS is attributed to the different electronic configuration of Ir in Y 2 Ir 2 O 7 , where electron correlations and spin−orbit coupling are known to split the Ir 5d t 2g orbital into e″ and u′ orbitals, 23,24,31,32 whereas all five d valence electrons in IrO 2 occupy the t 2g orbital that spans the Fermi level, the d electrons in Y 2 Ir 2 O 7 fill the u′ orbital and half-fill the higher energy e″ orbital with localized empty states above the Fermi level.…”

Operando Direct Observation of Stable Water-Oxidation Intermediates on Ca2–xIrO4 Nanocrystals for Efficient Acidic Oxygen Evolution

“…To unravel the valence state (VS) of Ir sites on Ca 2– x IrO 4 nanocrystals under acidic OER conditions, we perform operando XAS measurements at the iridium L 3 edge. The white-line positions of the Ir L 3 edge XANES spectra have been used to probe the VS of Ir atoms in perovskite compounds , and complex oxides. ,,, Figure c presents the operando XAFS spectra for the Ca 2– x IrO 4 catalyst recorded at different applied potentials. To determine the Ir VS precisely, the fitted average VS from the Ir L 3 edge XANES analysis are shown in Figure d.…”

Exploiting the flexibility of the pyrochlore composition for acid-resilient iridium oxide electrocatalysts in proton exchange membranes

“…Powder neutron diffraction was used to determine the composition and crystal structure of the new pyrochlores prepared in this work due to the sensitivity of the method to oxygen positions in the presence of high atomic number metals (such as Ir) and the possibility of detecting the protons of any occluded water molecules, which would be very challenging using X-ray diffraction methods. The parent material (Ca,Na) 2Àx Ir 2 O 6 $nH 2 -O, which we have recently studied using X-ray absorption spectroscopy, 27 is related to the phase (Na 0.27 Ca 0.59 ) 2 -Ir 2 O 6 $0.66H 2 O which we reported earlier, 28 but since we have optimised the synthesis method to prepare better crystalline materials free of IrO 2 impurity (by replacing the Ca(NO 3 ) 2 $4H 2 O earlier used by CaO 2 ) we performed a full analysis of its structure to compare with the earlier phase. A material prepared in the absence of Na is a novel variant of the iridate pyrochlore and was prepared for comparison of its electrochemical properties.…”