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

Abstract

Proton exchange membrane water electrolysis (PEMWE) is the technology of choice for the large-scale production of green hydrogen from renewable energy. Current PEMWEs utilize large amounts of critical raw materials such as iridium and platinum in the anode and cathode electrodes, respectively. In addition to its high cost, the use of Ir-based catalysts may represent a critical bottleneck for the large-scale production of PEM electrolyzers since iridium is a very expensive, scarce, and ill-distributed element. Replacing iridium from PEM anodes is a challenging matter since Ir-oxides are the only materials with sufficient stability under the highly oxidant environment of the anode reaction. One of the current strategies aiming to reduce Ir content is the design of advanced Ir-mixed oxides, in which the introduction of cations in different crystallographic sites can help to engineer the Ir active sites with certain characteristics, that is, environment, coordination, distances, oxidation state, etc. This strategy comes with its own problems, since most mixed oxides lack stability during the OER in acidic electrolyte, suffering severe structural reconstruction, which may lead to surfaces with catalytic activity and durability different from that of the original mixed oxide. Only after understanding such a reconstruction process would it be possible to design durable and stable Ir-based catalysts for the OER. In this Perspective, we highlight the most successful strategies to design Ir mixed oxides for the OER in acidic electrolyte and discuss the most promising lines of evolution in the field.

Figure 1

Schematic representation of (a) AEM and (b) LOM mechanisms for the OER. Adapted with permission from ref (20). Copyright 2021 MDPI.

Figure 2

Crystallographic structures of the most typical Ir-mixed oxides. IrO₆ octahedra are represented in orange, IrO₄ in tetrahedra in yellow, and IrO₄ in square-planar in light orange. The compounds identified in bold have been studied as electrocatalysts for the OER.

Figure 3

(a) Different polytypes of BaIrO₃ depending on the synthesis conditions. (b) Example of the crystal structure of the 5H polytype along [0 1 0], showing the stacking of hexagonal (h) and cubic (c) layers corresponding to face-sharing or corner-sharing IrO₆ octahedra, respectively. (c) Close look at the dimer and corner-shared octahedra. Figure reproduced from ref (65). Copyright 2009 American Chemical Society.

Figure 4

(a) Ir–O bond length distributions of IrO₆ on different oxides and (b) their corresponding k⁰-normalized Ir-L_III edge EXAFS values. (c) iR corrected specific mass activity of the oxides. The scan rate is 10 mV s^–1, the mass loading of all catalysts is 0.2 mg cm^–2, and the electrode area is 0.25 cm^–2. (d) A comparison of TOF values of the different catalysts at η = 0.37 V. (e) Different IrO₆ distortion types. (f) Relationship between the TOF and the distortion of the Oh in terms of a distortion parameter (Γ). Larger Γ values mean larger distortion. Reproduced with permission from ref (34). Copyright 2019 Royal Society of Chemistry.

Figure 5

(a) Crystal structure of a distorted double perovskite in which IrO₆ and MO₆ octahedra share corners. Reproduced with permission from ref (80). Copyright 2022 Springer Nature. (b) Ir L₃-edge XAS spectra Sr₂BIrO₆ with B = Ca, Fe, Co, Zn (top) and Sc, Ca, Ni and Zn (bottom) along with those of elemental Ir and IrO₂. Reproduced with permission from ref (83). Copyright 2015 Wiley-VCH Verlag GmbH & Co. (c) Current densities of Sr₂MIrO₆ (M = Ca, Mg, Zn, Ni, Co, Sc and Fe) measured at 1600 rpm in O₂-saturated 0.1 M HClO₄ using catalyst loading of 0.25 mg_cat cm^–2 and a scan rate of 10 mV s^–1. Adapted with permission from ref (81), Copyright 2021 Royal Society of Chemistry and ref (80), Copyright 2022 Springer Nature. (d) Influence of the Irⁿ⁺ oxidation state of each double perovskites on the OER potentials necessary to reach 10 mA cm^–2. Adapted with permission from ref (81). Copyright 2021 Royal Society of Chemistry. Oxidation state and activity data for Zn, Mg, and Ca adapted with permission from ref (80). Copyright 2022 Springer Nature. (e) Correlation between the distortion of the IrO₆ octahedra and the Irⁿ⁺ oxidation state of different iridates. Reproduced with permission from ref (83). Copyright 2015 Wiley-VCH Verlag GmbH & Co.. (f) Evolution of the cell potential (E_cell) and cell temperature (T_cell) during a PEMWE measurement (1000 h) at constant 2 A cm^–2 using Sr₂CaIrO₆ and Pt/C as anode and cathode respectively. Reproduced with permission from ref (84). Copyright 2023 Wiley-VCH GmbH.

Figure 6

(a) Effect of the size of the A-site cation on the Ir–O–Ir bond angles and lattice parameter a of several pyrochlore iridates (R₂Ir₂O₇, R= Ho, Tb, Gd, Nd, and Pr). (b) OER mass activities recorded with the different iridates in 0.1 M HClO₄. (c) Electronic phase diagram around room temperature and the corresponding schematic band structures of Ir 5d orbitals. Figures a, b, and c reproduced with permission from ref (86). Copyright 2018 Wiley-VCH Verlag GmbH & Co. (d) Electron filling in the e″ orbital of Ir 5d states and its contribution to the improved OER activity of Y₂Ir₂O₇. Reproduced from ref (87). Copyright 2018 American Chemical Society.

Figure 7

(a) Stability-number calculated for different catalysts as powders and films in 0.1 M HClO₄ (slow scan rate of 5 mV s^–1 to 1.55 V vs RHE and 1.65 V vs RHE for films and powders respectively). (b, c) Lifetime estimated for the different powders (b) and films (c) based on the use of the stability number. Figures a, b, and c reproduced with permission from ref (99). Copyright 2018 Springer Nature. (d, e) Pourbaix diagrams for monometallic Pd and Ir with respect to the water system (the color indicates the value of ΔG_pbx for one of the phases with respect to the other phases; bluer color means closer values of ΔG_pbx to zero). (f) Pourbaix diagram for bimetallic Ir–Pd oxides. (g) Convex hull diagram of ΔG_Pourbaix at pH = 0 and E = 1.23 V_RHE. Yellow and blue symbols are data taken from Materials Project and calculated in ref (104). Figures d, e, f, and g reproduced from ref (104). Copyright 2020 American Chemical Society.

Figure 8

Representative TEM micrographs of Ir-mixed oxides after the OER cycles. (a) SrTi_1–xIr_xO₃ without surface restructuration after OER. Reproduced with permission from ref (75). Copyright 2019 Wiley-VCH Verlag GmbH & Co. KGaA. (b) Example of the formation of crystalline IrO₂ nanoparticles La₂LiIrO₆. Reproduced with permission from ref (19). Copyright 2016 Springer Nature Limited. (c) IL-TEM images showing the evolution of the same regions during OER. HRTEM and aberration corrected TEM images showing hollow particles of dimers of edge-shared IrO₆ octahedra generated upon the reconstruction of Sr₂CaIrO₆ after 5000 OER cycles. Reproduced with permission from ref (80). Copyright 2022 Springer Nature. (d) Amorphous layer of IrO_x formed on Sm₃IrO₇. Reproduced with permission from ref (92). Copyright 2023 American Chemical Society.

Figure 9

Proposed protocol for the assessment of the OER activity of mixed oxides in acidic electrolyte, including steps for evaluating electrocatalyst stability in acidic electrolyte (step 1), RDE activity and durability (steps 3–6), and activity in PEMWE (step 7).