Operando Identification of Dual Active Sites in Ca₂IrO₄ Nanocrystals with Yttrium Substitutions Boosting Acidic Oxygen Evolution Reaction

Abstract

Non-noble metal substitutions of the iridium sites in iridium-based complex oxides while boosting acidic oxygen evolution reaction (OER) is a promising method for developing efficient electrocatalysts, but this remains a huge challenge. Herein, we report yttrium substitution of iridium in Ca 2 IrO 4 (Ca 2 Y x Ir 1−x O 4 ) nanocrystals for the first time, which is demonstrated by detailed structural characterizations using X-ray absorption spectra, X-ray diffraction patterns, and elemental mapping images. The synthesized Ca 2 Y 0.2 Ir 0.8 O 4 catalyst requires a low overpotential of only 213 mV, achieving an acidic OER current density of 10 mA cm −2 , which represents an approximately 203.7-fold improvement in iridium mass activity and 204.4-fold improvement in turnover frequency in comparison to that of IrO 2 at 1.5 V vs RHE, respectively. Systematic characterizations of electronic structures reveal the synergistic effects between high-valence iridium sites and increased lattice oxygen concentration induced by Y 3+ substitutions, which greatly enhance the intrinsic OER activity of Ca 2 Y 0.2 Ir 0.8 O 4 . Operando X-ray absorption spectra and Raman spectra reveal the iridium and yttrium dual active sites during acidic OER. Our results provide a method for designing dual active sites in iridium-based complex oxides for highly active acidic OER electrocatalysts.

IrO2 Nanoparticle-Decorated Ir-Doped W18O49 Nanowires with High Mass Specific OER Activity for Proton Exchange Membrane Electrolysis

“…All of these demonstrate that the valence state of W increases while that of Ir decreases, revealing the transfer of electrons from W 18 O 49 to IrO 2 . Such observation confirms the existence of strong catalyst-support interactions between W 18 O 49 and IrO 2 , , and the electronic structure of the catalyst is regulated. ,, In addition, the valence state variability of W could generate abundant oxygen vacancies on tungsten oxide, which is conducive to electron transport in the catalyst during the reaction . These would be beneficial for OER performance. ,, From Figure d and Table S2, the O 1s XPS spectra show three peaks with binding energy at 530.08–530.58, 530.88–531.38, and 532.28–532.88 eV, which are assigned to the lattice oxygen, surface absorbed oxygen, and water, respectively .…”

“…Such observation confirms the existence of strong catalyst-support interactions between W 18 O 49 and IrO 2 , 21,27 and the electronic structure of the catalyst is regulated. 28,51,52 In addition, the valence state variability of W could generate abundant oxygen vacancies on tungsten oxide, which is conducive to electron transport in the catalyst during the reaction. 15 These would be beneficial for OER performance.…”

Recent advances in iridium-based catalysts with different dimensions for the acidic oxygen evolution reaction

“…Ca 2 Y x Ir 1− x O 4 ( x = 0.1, 0.2, 0.3) nanocrystals fabricated by replacing some iridium sites with yttrium (Y) in Ca 2 IrO 4 were optimized for the acidic OER. 112 The proportion of lattice oxygen in Ca 2 Y 0.2 Ir 0.8 O 4 nanocrystals was about 21.5%, much higher than that in Ca 2 IrO 4 nanocrystals (14.9%); the abundant O (II− δ )− (electrophilic lattice oxygen) species favored the formation of *O–O during the acidic OER process and significantly lowered the kinetic barriers. The formation of *O–O bonds was generally important to the lattice oxygen mechanism rate-limiting step.…”

Unraveling the Most Relevant Features for the Design of Iridium Mixed Oxides with High Activity and Durability for the Oxygen Evolution Reaction in Acidic Media

“…Ca 2 IrO 4 crystallizes in a hexagonal symmetry with IrO 6 significantly distorted. Ca 2 IrO 4 and Ca 2 Y x Ir 1–x O 4 have been reported to display high OER activity, achieving 1.443 V at 10 mA cm –2 geo or 632.6 A g –1 Ir at 1.5 V for Ca 2 Y 0.2 Ir 0.8 O 4 , which is among the best values reported in literature for any Ir compound . The authors proposed that the OER activity is strongly affected by the distortion of the IrO 6 octahedra.…”

“…This group includes iridates in which IrO 6 octahedra are connected through the edges. The oxides in this group can be arranged into: 71 The authors proposed that the OER activity is strongly affected by the distortion of the IrO 6 octahedra. This is an interesting observation, which also applies to Ir-mixed oxides from other groups.…”