The mechanism of OER activity and stability enhancement in acid by atomically doped iridium in γ-MnO2

doi:10.1016/S1872-2067(24)60201-9

Abstract

Abstract:

Construction of iridium (Ir) based active sites on certain acid stable supports now is a general strategy for the development of low-Ir OER catalysts. Atomically doped Ir in the lattice of acid stable γ-MnO₂ has been recently achieved, which shows high activity and stability though Ir usage was reduced more than 95% than that in current commercial proton exchange membrane water electrolyzer (PEMWE). However, the activity and stability enhancement by Ir doping in γ-MnO₂ still remains elusive. Herein, high dispersion of iridium (up to 1.37 atom%) doping in the lattice of γ-MnO₂ has been achieved by optimizing the thermal decomposition of the iridium precursors. Benefiting from atomic dispersive doping of Ir, the optimized Ir-MnO₂ catalyst shows high OER activity, as it has turnover frequency of 0.655 s⁻¹ at an overpotential of 300 mV in 0.5 mol L^-1 H₂SO₄. The catalyst also shows high stability, as it can sustainably work at 100 mA cm^-2 for 24 h. Experimental and theoretical studies reveal that Ir is preferentially doped into β phase rather than R phase, and the Ir site is the active site for OER. The OER active site is postulated to be Ir⁵⁺-O(H)-Mn³⁺ unit structure on the surface. Furthermore, Ir doping changes the potential determining step from the formation of O* to the formation of *OOH, emphasizing the promoting effect toward OER derived from Ir sites. This work not only demonstrates the possibility of achieving atomic-level doping of Ir on the surface of a support to dramatically reduce Ir usage, but also, more importantly, reveals the mechanism behind accounting for the stability and activity enhancement by Ir doping. These important findings may serve as valuable guidance for further development of more efficient, stable and cost-effective low Ir-based OER catalysts for PEMWE.

Key words: Acidic oxygen evolution reaction, Low-iridium, Mn-based oxides, Proton exchange membrane water, electrolysis, Green hydrogen

Yimeng Sun, Jun Chen, Lin Liu, Haibo Chi, Hongxian Han. The mechanism of OER activity and stability enhancement in acid by atomically doped iridium in γ-MnO₂[J]. Chinese Journal of Catalysis, 2025, 69: 99-110.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(24)60201-9

https://www.cjcatal.com/EN/Y2025/V69/I2/99

Figures/Tables 7

Table 1 The Ir contents in precursors and the final working electrodes by ICP analysis.

Catalyst	C₁₄H₂₇Ir₃O₁₈ and Mn(NO₃)₂ Ir/Mn molar ratio	Final product
Catalyst	C₁₄H₂₇Ir₃O₁₈ and Mn(NO₃)₂ Ir/Mn molar ratio	Composition formula	Ir/Mn molar ratio	Ir loading mg cm^-2
0.1%Ir-MnO₂	0.1%	Ir_0.001Mn_0.999O₂	0.10%	0.007
0.5%Ir-MnO₂	0.5%	Ir_0.005Mn_0.995O₂	0.50%	0.033
1%Ir-MnO₂	1%	Ir_0.009Mn_0.991O₂	0.89%	0.059
2%Ir-MnO₂	2%	Ir_0.014Mn_0.986O₂	1.37%	0.091
3%Ir-MnO₂	3%	Ir_0.016Mn_0.984O₂	1.59%	0.103
4%Ir-MnO₂	4%	Ir_0.020Mn_0.980O₂	2.05%	0.128

Fig. 1. (a) Schematic crystal structure of γ-MnO2. (b) XRD patterns of γ-MnO2 and Ir-MnO2 with varying iridium contents (the orange squares represent characteristic peaks of Mn2O3).

Fig. 2. (a) TEM image and statistical particle size distribution of as-synthesized 2%Ir-MnO2. (b,c) Structural characteristic of the intergrowth of two phases as shown in the HR-TEM images. (d) Representative high-magnification HAADF-STEM image of 2%Ir-MnO2, with bright spots highlighted by red circles ascribed to Ir atoms. (e) The corresponding element mapping of O, Mn, and Ir in 2%Ir-MnO2. (f) Enlarged XRD patterns of γ-MnO2 and 2%Ir-MnO2.

Fig. 3. High-resolution XPS spectra. (a) Ir 4f of 2% Ir-MnO2 and commercial IrO2. (b) Fitting the valence state of Ir. Mn 2p (c) and O 1s (d) of γ-MnO2 and 2% Ir-MnO2.

Fig. 4. OER performance of catalysts in a three-electrode test with catalysts loaded on GC working electrodes. (a) Representative LSV curves of γ-MnO2, Ir-MnO2 with different Ir doping levels, and commercial IrO2 in 0.5 mol L-1 H2SO4. (b) The overpotential values of the catalysts at 1.70 VRHE. (c) Tafel slope derived from (a). (d) EIS curves and the fitting results of γ-MnO2, Ir-MnO2, and commercial IrO2 at 1.55 VRHE. The insert in (d) shows the equivalent circuit.

Fig. 5. Stability test by chronopotentiometry of γ-MnO2 and 2%Ir-MnO2 grown on FTO at 10 mA cm-2 (a) and 100 mA cm-2 (b). Ir 4f (c) and Mn 2p (d) XPS spectra of the pristine and utilized 2%Ir-MnO2 catalyst (in 0.5 mol L-1 H2SO4 solution for 24 h at 100 mA cm?2).

Fig. 6. DFT calculations. (a) Structures of Ir-MnO2 with Ir atoms doped in different Mn sites (β phase and R phase site) of γ-MnO2. The red, purple, and yellow spheres are O, Mn, and Ir atoms, respectively. The diagram below illustrates normal COHP and ICOHP value of O(2p)-Ir(4d) on different Ir-MnO2 models. (b) Mn 3d and Ir 5d pDOS spectra of 2%Ir-MnO2. The inset is a magnified view at the Fermi level. (c) Gibbs free energy diagrams for OER on Ir site and Mn site on Ir-γ-MnO2 surface and Mn site on γ-MnO2 (100) surface. (d) Structures of OER intermediates on Ir site on the Ir-γ-MnO2 (100) slab.

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The mechanism of OER activity and stability enhancement in acid by atomically doped iridium in γ-MnO₂

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