Dopants fixation of Ruthenium for boosting acidic oxygen evolution stability and activity

Abstract

Designing highly durable and active electrocatalysts applied in polymer electrolyte membrane (PEM) electrolyzer for the oxygen evolution reaction remains a grand challenge due to the high dissolution of catalysts in acidic electrolyte. Hindering formation of oxygen vacancies by tuning the electronic structure of catalysts to improve the durability and activity in acidic electrolyte was theoretically effective but rarely reported. Herein we demonstrated rationally tuning electronic structure of RuO₂ with introducing W and Er, which significantly increased oxygen vacancy formation energy. The representative W_0.2Er_0.1Ru_0.7O_2-δ required a super-low overpotential of 168 mV (10 mA cm^-2) accompanied with a record stability of 500 h in acidic electrolyte. More remarkably, it could operate steadily for 120 h (100 mA cm^-2) in PEM device. Density functional theory calculations revealed co-doping of W and Er tuned electronic structure of RuO₂ by charge redistribution, which significantly prohibited formation of soluble Ru^x>4 and lowered adsorption energies for oxygen intermediates.

Fig. 1. Schematic illustration of W_0.2Er_0.1Ru_0.7O_2−δ toward acidic OER.

a The illustration of lattice oxygen oxidation way of W_0.2Er_0.1Ru_0.7O_2−δ toward acidic OER. b The illustration of adsorbate evolution way of W_0.2Er_0.1Ru_0.7O_2−δ toward acidic OER. c the reaction paths of W_0.2Er_0.1Ru_0.7O_2−δ for lattice oxygen oxidation. d the reaction paths of W_0.2Er_0.1Ru_0.7O_2−δ for adsorbate evolution oxidation (* represents the surface-bound species on W_0.2Er_0.1Ru_0.7O_2−δ).

Fig. 2. DFT for RuO₂ and W_0.2Er_0.1Ru_0.7O_2−δ toward acidic OER.

a DOS plots of Ru 4d and O 2p states in RuO₂ and W_0.2Er_0.1Ru_0.7O_2−δ−1. The dashed line means the Fermi level energy (E_F). b Schematic diagrams of rigid band models for RuO₂ and W_0.2Er_0.1Ru_0.7O_2−δ−1 toward acidic OER. c The calculated energy for formation of V_O in different positions of RuO₂, W_0.2Ru_0.8O_2−δ−1, Er_0.1Ru_0.9O_2−δ−1, and W_0.2Er_0.1Ru_0.7O_2−δ−1. d The calculated energy barriers diagram for W_0.2Er_0.1Ru_0.7O_2−δ−1. The corresponding models of W_0.2Er_0.1Ru_0.7O_2−δ−1 to oxygen intermediates such as OH*, O* as well as OOH* (The number 1 or other numeral represents different established models for these structures).

Fig. 3. The designing strategy and TEM characterization for the prepared W_0.2Er_0.1Ru_0.7O_2−δ nanosheets.

a Schematic route for synthesis of W_0.2Er_0.1Ru_0.7O_2−δ nanosheets. b TEM image, c HAADF-TEM image, d the corresponding elemental maps, e HR-TEM image (inset: Fourier transform analyses for W_0.2Er_0.1Ru_0.7O_2−δ), and f SAED image for the W_0.2Er_0.1Ru_0.7O_2−δ nanosheets.

Fig. 4. XPS characterization for W_0.2Er_0.1Ru_0.7O_2−δ, Er_0.1Ru_0.9O_2−δ, W_0.2Ru_0.8O_2−δ, and RuO_2−δ nanosheets.

a Ru 3d spectra, b W 4f spectra, c Er 4d spectra, d O 1s spectra for the prepared W_0.2Er_0.1Ru_0.7O_2−δ, Er_0.1Ru_0.9O_2−δ, W_0.2Ru_0.8O_2−δ, and RuO_2−δ nanosheets. In order to precisely analyze the valance state of Ru, X-ray absorption near-edge spectroscopy (XANES) was applied to characterize RuO_2−δ and W_0.2Er_0.1Ru_0.7O_2−δ. Ru foil and C-RuO₂ were employed as reference materials^,. Compared with Ru K-edge position of Ru foil, the C-RuO₂, RuO_2−δ, and W_0.2Er_0.1Ru_0.7O_2−δ all shifted to higher energy, resulting from the Ru–O bonds in these materials (Fig. 5a). Additionally, the Ru K-edge spectra in Fig. 5a showed the adsorption energy of the prepared RuO_2−δ and W_0.2Er_0.1Ru_0.7O_2−δ were different from that for C-RuO₂. This result mainly resulted from the fact that Ru valence state in RuO_2−δ and W_0.2Er_0.1Ru_0.7O_2−δ were mainly dominated Ru⁴⁺ accompanied with Ru^x+ (x < 4). Simultaneously, compared with adsorption energy of RuO_2−δ, the adsorption energy for W_0.2Er_0.1Ru_0.7O_2−δ shifted to lower energy region, indicating that Ru valence state in W_0.2Er_0.1Ru_0.7O_2−δ was a little lower than that in RuO_2−δ due to introduction of W and Er. Additionally, the adsorption energy (E₀) for RuO_2−δ (22,119.99 eV) was also a little higher than that for W_0.2Er_0.1Ru_0.7O_2−δ (22,118.92 eV). These results were consistent with the valence state analysis in XPS. Furthermore, extended X-ray absorption fine structure (EXAFS) with Fourier transform as well as its counterpart (k³-weighted EXAFS) was applied to analyze the structure of RuO_2−δ and W_0.2Er_0.1Ru_0.7O_2−δ (Fig. 5b). Compared with the bond of Ru–Ru in Ru foil (2.68 Å), W_0.2Er_0.1Ru_0.7O_2−δ exhibited a slightly longer interatomic distance (2.71 Å), which could be related with the strained effect in HRTEM (Supplementary Table 7). Additionally, the Ru–Ru and Ru–O bonds in RuO_2−δ and W_0.2Er_0.1Ru_0.7O_2−δ showed different interatomic distances is due to the existence of lower Ru^x<4 valence state, compared with that in C-RuO₂ (3.12 and 3.56 Å). Besides that, the different Ru–Ru, Ru–O bonds between RuO_2−δ and W_0.2Er_0.1Ru_0.7O_2−δ should be related with introducing W and Er into RuO_2−δ. Furthermore, wavelet transform (WT) for Ru K-edge EXAFS in Fig. 5c–f was applied to exhibit the length changes of Ru–Ru and Ru–O bonds in W_0.2Er_0.1Ru_0.7O_2−δ. The intensities at ≈6.5 Å⁻¹ increased gradually, indicating the Ru^x<4 had strong influence on W_0.2Er_0.1Ru_0.7O_2−δ, compared with that for C-RuO₂. Besides that, compared with RuO_2−δ, the intensities changed slightly at ≈13.5 Å⁻¹ in W_0.2Er_0.1Ru_0.7O_2−δ is due to the coordination of Ru–W/Er.

Fig. 5. XANES characterization for W_0.2Er_0.1Ru_0.7O_2−δ and RuO_2−δ nanosheets.

a Ru K-edge spectra for Ru foil, C-RuO₂, W_0.2Er_0.1Ru_0.7O_2−δ, and RuO_2−δ. b FT-EXAFS spectra of Ru K-edge for Ru foil, C-RuO₂, W_0.2Er_0.1Ru_0.7O_2−δ, and RuO_2−δ. c-f WT-EXAFS of Ru foil, C-RuO₂, RuO_2−δ, and W_0.2Er_0.1Ru_0.7O_2−δ, respectively.

Fig. 6. Acidic OER activities for these samples tested in 0.5 M H₂SO₄ electrolyte.

a Polarization curves, b corresponding Tafel slopes calculated from a. c C_dl plots inferred from CV curves. d EIS plots for W_0.2Er_0.1Ru_0.7O_2−δ, Er_0.1Ru_0.9O_2−δ, W_0.2Ru_0.8O_2−δ, and RuO_2−δ nanosheets.

Fig. 7. The long-durable stability investigation for OER and PEM in acidic electrolyte.

a The current-time (500 h) stability of W_0.2Er_0.1Ru_0.7O_2−δ nanosheets in 0.5 M H₂SO₄. b The comparison of stabilities in acidic electrolyte for various electrocatalysts, x-axis stands for the initial ɳ for electrocatalysts reaching 10 mA cm⁻², y-axis represents the final ɳ of various electrocatalysts after stability measurement. c ICP analysis for W_0.2Er_0.1Ru_0.7O_2−δ after 500 h operation in acidic electrolyte. d Photograph of the employed PEM reaction device. e The current–time (120 h) stability of W_0.2Er_0.1Ru_0.7O_2−δ as anodic side in acidic PEM (inset: PEM reaction device, detection of H₂).

Fig. 8. Chemical recognition and characterization for W_0.2Er_0.1Ru_0.7O_2−δ after 500 h stability.

a Ru 3d spectra, b W 4f spectra, c Er 4d spectra, d O 1s spectra for W_0.2Er_0.1Ru_0.7O_2−δ before and after 500 h stability. e ESR spectra for W_0.2Er_0.1Ru_0.7O_2−δ before and after 500 h stability. f HR-TEM image, g Elemental mapping for W_0.2Er_0.1Ru_0.7O_2−δ after acidic OER stability.