Non-catalytic, instant iridium (Ir) leaching: A non-negligible aspect in identifying Ir-based perovskite oxygen-evolving electrocatalysts

doi:10.1016/S1872-2067(21)63983-9

Abstract

Abstract:

The large-scale application of proton exchange membrane water electrolysis technology requires the development of high-performance oxygen evolution electrocatalysts with as little iridium (Ir) as possible. Ir-based double perovskite oxides (A₂B’IrO₆; A = alkaline, alkaline-earth, or rare-earth elements; B’ = transition metal or rare-earth elements) represent a class of oxides with great potential to replace the commercial catalyst IrO₂. However, the structural evolution of Ir-based double perovskite oxides in electrolytes is incompletely understood, and foundational knowledge of the design principle of the “ideal” material is lacking. In this work, we report the unexpected phenomenon of instant Ir leaching from Ir-based double perovskite oxides in acid under non-catalytic conditions and discuss the implications of this phenomenon for mechanism investigation and material identification. Some well-known Ir-based double perovskite oxides, such as Ba₂PrIrO₆ and Sr₂YIrO₆, undergo instantaneous Ir leaching when they come into contact with acidic electrolytes. The Ir-leaching process is found to be non-persistent and non-thermodynamically determined, and its extent is correlated with the leaching of other B’-site elements in the perovskite oxides. Based on this observation, we revisit the Ir dissolution-precipitation process for surface IrO_x formation during the perovskite-electrolyzed oxygen evolution reaction, emphasizing the non-negligible role of Ir species owing to acid corrosion in the electrolyte. Finally, we modify a screening protocol for low-Ir oxygen evolution electrocatalysts and propose an instant acid corrosion test as an indispensable process to evaluate the structural stability of potential catalysts.

Key words: Oxygen evolution reaction, Water splitting, Iridium, Stability, Perovskite

Qi Zhang, Hui Chen, Lan Yang, Xiao Liang, Lei Shi, Qing Feng, Yongcun Zou, Guo-Dong Li, Xiaoxin Zou. Non-catalytic, instant iridium (Ir) leaching: A non-negligible aspect in identifying Ir-based perovskite oxygen-evolving electrocatalysts[J]. Chinese Journal of Catalysis, 2022, 43(3): 885-893.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(21)63983-9

https://www.cjcatal.com/EN/Y2022/V43/I3/885

Figures/Tables 9

Fig. 1. (a) Crystal structure of A2B’IrO6. (b) XRD patterns of A2B′IrO6 double perovskites with the corresponding Joint Committee on Powder Diffraction Standards cards. (c) HRTEM image of Sr2TiIrO6. Inset: Fast Fourier transform image of (c). (d) STEM-EDS mapping image of Sr2TiIrO6.

Fig. 2. (a) Schematic of the instant acid corrosion test. (b) Optical photographs of the solutions after the instant acid corrosion test of A2B’IrO6. (c) UV-Vis absorption spectra of the solutions after the instant acid treatment of A2B’IrO6. (d) Comparison of the atomic ratios of A and B’ leached instantly from A2B’IrO6. Atomic ratio = (leached metal amount/ corresponding metal content in the catalyst) × 100%. (e) Comparison of the atomic ratios of Ir leached instantly from A2B’IrO6. Atomic ratio = (leached Ir amount)/(Ir content in the catalyst) × 100%. The error bars in (d) and (e) were obtained from the standard deviations of three independent measurements.

Fig. 3. (a) Contents of Ba instantly leached from Ba2PrIrO6 in acidic electrolytes of different pH. (b) Contents of Pr instantly leached from Ba2PrIrO6 in acidic electrolytes of different pH. (c) Contents of Ir instantly leached from Ba2PrIrO6 in acidic electrolytes of different pH.

Fig. 4. (a) Comparison of the HRTEM images of Sr2TiIrO6 before and after instant acid treatment. (b) Comparison of the Ti 2p, Sr 3d, and Ir 4f XPS spectra of Sr2TiIrO6 before and after instant acid treatment. (c) Comparison of the HRTEM images of Ba2PrIrO6 before and after instant acid treatment. (d) Comparison of the Pr 3d, Ba 3d, and Ir 4f XPS spectra of Ba2PrIrO6 before and after instant acid treatment.

Fig. 5. (a) Comparison of the atomic ratios of A leached from Sr2TiIrO6 and Ba2PrIrO6 during long-term acid corrosion. (b) Comparison of the atomic ratios of B’ leached from Sr2TiIrO6 and Ba2PrIrO6 during long-term acid corrosion. (c) Comparison of the atomic ratios of Ir leached from Sr2TiIrO6 and Ba2PrIrO6 during long-term acid corrosion. (d) Comparison of the XRD patterns of Sr2TiIrO6 and Ba2PrIrO6 after long-term acid corrosion.

Fig. 6. Pourbaix diagrams of (a) Sr2TiIrO6 and (b) Ba2PrIrO6. The aqueous ion concentration is 1 × 10-6 mol/L at 25 °C. The black dotted line represents the water oxidation line. The thermodynamically stable species of the numerically labeled regions are listed in Table S2.

Fig. 7. (a) Schematic of the structural changes in Sr2TiIrO6 during instant acid corrosion under non-catalytic conditions. The purple circles represent Sr-site vacancies. (b) Schematic of the structural changes in A2B’IrO6 (except Sr2TiIrO6) during instant acid corrosion under non-catalytic conditions.

Fig. 8. (a) Schematic of two experimental routes for Ba2PrIrO6 in the catalytic corrosion test. (b) Comparison of the atomic ratios of Ir leached from Ba2PrIrO6 in Routes 1 and 2 during OER electrocatalysis. Inset: Schematic of the dissolution-precipitation process of Ir species. The blue star represents the atomic ratio of instant Ir leaching under non-catalytic conditions.

Fig. 9. Protocol for evaluating the performance of low-iridium oxygen evolution electrocatalysts in acidic conditions. LSV, linear sweep voltammetry measurement; CP, chronopotentiometry; CA chronoamperometry; MEA membrane electrode assembly.

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