Highly Durable Barium Iridates for Oxygen Evolution Electrocatalysis Via Atomic Ordering of Nb5+/Ta5+(d
0) and Suppression of Lattice Oxygen Participation
Authors: Park Ki Hyun , Sim Yelyn , Yun Tae Gyu , Kim Dongho , Kim Junseop , Chung Sung-Yoon 
Published: December 2024

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 [IrO6] 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 IrO2.

In this study, using hexagonal 9R-perovskite BaIrO3 as starting materials, we systematically demonstrate the d orbital contribution to structural durability according to lattice oxygen participation, especially d
0 states of Nb5+ and Ta5+. The choice of BaIrO3 as a reference catalyst was governed by its structural robustness from face-sharing connectivity between [IrO6] clusters, which is more difficult to disassemble compared to IrO2 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 BaIrO3; 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 Nb5+ and Ta5+ added, 12R-Ba4NbIr3O12 and Ba4TaIr3O12 were formed with Nb and Ta cations located in bridging sites between face-sharing [Ir3O12] trimers.

Density functional theory (DFT) calculations were conducted to study the electronic structure of Ba4NbIr3O12 (Figure 1b). The electron configuration of Nb5+ consists of fully filled 4p
6 and empty 4d
0 states, thereby Nb 4d 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 [IrO6], O 2p states (red arrow) of [NbO6] beneath the Fermi level are significantly suppressed. These findings indicate that [NbO6] 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 BaIrO3 and Ba4NbIr3O12 during OER using anodic cycling samples in 1 M HClO18
4. As depicted in Figure 1b, substantially lower amount of O18 was detected in Ba4NbIr3O12 than in BaIrO3, which directly demonstrated the significant suppression of LOER.

The catalytic longevity of IrO2, BaIrO3, and Ba4NbIr3O12 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 Ba4NbIr3O12 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 d0 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