Highly Durable Barium Iridates for Oxygen Evolution Electrocatalysis Via Atomic Ordering of Nb5+/Ta5+(d 0) and Suppression of Lattice Oxygen Participation

Proton exchange membrane water electrolyzers (PEMWEs) play a pivotal role in the journey towards a sustainable hydrogen society [1]. Despite offering remarkable current density and exceptional hydrogen purity (99.99%), the highly acidic nature of sulfonated fluoropolymer membranes hinders material selection for anode catalysts to facilitate the oxygen evolution reaction (OER) [2]. Therefore, iridium-based electrocatalysts are regarded as indispensable choices owing to their strong resistance to corrosion under acidic environments based on Pourbaix diagram [3]. However, despite their remarkable stability and performance in acid, two main unsolved issues remained in iridium-based electrocatalysts. The first is the scarcity of iridium, which hampers the widespread adoption of PEMWEs. Iridium accounts for 25 % of the total cost of membrane electrode assemblies (MEA), making the reduction of iridium content an urgent necessity. The other unsolved issue is the suppression lattice oxygen evolution reaction (LOER) [4]. While LOER offers an opportunity to overcome the theoretical OER overpotential, such lattice oxygen participation also induces oxygen gas evolution from [IrO 6 ] crystals and consequently lattice collapse. Hence, there is a pressing need for alternative catalysts with lower iridium content but limited LOER to achieve comparable activity and stability to commercial IrO 2 . In this study, using hexagonal 9R-perovskite BaIrO 3 as starting materials, we systematically demonstrate the d orbital contribution to structural durability according to lattice oxygen participation, especially d 0 states of Nb 5+ and Ta 5+ . The choice of BaIrO 3 as a reference catalyst was governed by its structural robustness from face-sharing connectivity between [IrO 6 ] clusters, which is more difficult to disassemble compared to IrO 2 with edge-, and corner-sharing configurations [5]. Two notable features were observed when transition-metals (Mn, Co, Ni, In, Nb, and Ta) were doped into BaIrO 3 ; i) phase transformation occurs from 9R- to 6H- or 12R-type polymorphs and 2) dopants occupy specific sites in a very ordered manner ( Figure 1a ). We directly observed such ordering natures by atomic-column resolved scanning transmission electron microscopy (STEM). When Nb 5+ and Ta 5+ added, 12R-Ba 4 NbIr 3 O 12 and Ba 4 TaIr 3 O 12 were formed with Nb and Ta cations located in bridging sites between face-sharing [Ir 3 O 12 ] trimers. Density functional theory (DFT) calculations were conducted to study the electronic structure of Ba 4 NbIr 3 O 12 ( Figure 1b ). The electron configuration of Nb 5+ consists of fully filled 4 p 6 and empty 4 d 0 states, thereby Nb 4 d orbitals contribute to the upper electronic band above +2 eV. Consequently, in contrast to the large overlap between O 2p states (black arrow) with Ir 5d states in [IrO 6 ], O 2p states (red arrow) of [NbO 6 ] beneath the Fermi level are significantly suppressed. These findings indicate that [NbO 6 ] serves as an OER inactive site and prevents structural disassembly by limiting lattice oxygen participation. We also carried out time-of-flight SIMS to quantitatively compare lattice oxygen contribution between pristine BaIrO 3 and Ba 4 NbIr 3 O 12 during OER using anodic cycling samples in 1 M HClO 18 4 . As depicted in Figure 1b, substantially lower amount of O 18 was detected in Ba 4 NbIr 3 O 12 than in BaIrO 3 , which directly demonstrated the significant suppression of LOER. The catalytic longevity of IrO 2 , BaIrO 3 , and Ba 4 NbIr 3 O 12 was evaluated by cyclic voltammetry (CV) and chronoamperometry (CA) tests (see Figure 1c ). The relative catalytic behavior was compared in terms of preserving OER activities by normalizing with initial current densities. Both CV and CA results provide straightforward evidence, validating that Ba 4 NbIr 3 O 12 is prominent among the three catalysts. As a result, we have developed promising electrocatalysts with both low iridium content and higher durability, which stem from the synergism of chemical ordering of d 0 cations to suppress lattice oxygen participation and robust face-sharing trimers in the structures. Our research proposes the proper management of chemical ordering in oxides offers a straightforward yet effective approach for developing electrocatalysts with significantly enhanced longevity. References [1] Chu et al., Nature , 2012, 488(7411), 294-303. [2] Liu et al., Chem. Soc. Rev ., 2023, 52(16), 5652-5683. [3] Wang et al., NPJ Comput. Mater ., 2020, 6(1), 160. [4] Geiger et al., Nat. Catal ., 2018, 1(7), 508-515. [5] Song et al. Energy Environ. Sci ., 2020, 13(11), 4178-4188. Figure 1