KAUST張華彬課題組Angew. Chem.: 金屬（羥基）氫氧化物結構無序與電催化析氧活性的關聯

一、 【導讀】

開發高效的析氧反應（OER）電催化劑并了解其結構-性能關系對能源轉換和儲存至關重要。然而，多相電催化劑的結構復雜性使得闡明其動態結構演變和析氧反應機制產生了巨大的挑戰。基于此，阿卜杜拉國王科技大學張華彬教授和韓宇教授利用過渡金屬氧化物，硫化物和硒化物作為模型預催化劑，通過OER過程中的結構自重構過程，利用反應前硫化物/硒化物預催化劑與反應后的（羥基）氫氧化物之間的金屬-非金屬鍵長差異，衍生出不同程度的結構無序的（羥基）氫氧化物。結果表明，具用更高結構無序度的催化劑顯示出更優異的析氧反應電催化活性。原位X射線吸收精細結構（XAFS）實驗揭示了模型催化劑在OER過程中的結構無序度的演變過程。密度泛函理論（DFT）計算表明，結構無序和局域結構畸變能夠調控催化劑電子結構和調控催化劑的反應熱動力學，增強其OER性能。本工作證實了OER電催化中催化劑結構無序與其催化活性之間的相關性，有望為相關催化劑的制備以及揭示其結構-性能關系提供新的思路。

?三、【核心創新點】

1.本工作提出利用XAFS來研究催化劑在OER反應過程中結構無序性的變化。并用Debye-Waller因子對結構的無序度進行定量分析。

2.將結構無序與其催化本征活性之間進行關聯，揭示了結構無序與催化劑本征活性之間的關系，有望為相關催化劑的制備提供指導意義。

?四、【數據概覽】

?Figure 1. Characterizations of catalysts. a) HRTEM images of as-prepared Fe-Ni-OOH, Fe-Ni-O, Fe-Ni-S and Fe-Ni-Se. b) Fourier transformed magnitudes of Ni K-edge EXAFS spectra for different pre-catalysts and in situ-derived catalysts. c) Bond length extracted from EXAFS fitting results for catalysts. d) Experimental Ni K-edge XANES spectra for D-Fe-Ni-OOH, D(O)-Fe-Ni-OOH, D(S)-Fe-Ni-OOH and D(Se)-Fe-Ni-OOH. e) A schematic illustration showing the structural evolution from pre-catalysts to in situ-derived catalysts. Atoms with transparent color denote the corresponding dissolved anions during the reaction.

研究團隊對模型預催化劑氧化物、硫化物、硒化物進行結構表征，XAFS表征結果表明衍生的（羥基）氫氧化物D(S)-Fe-Ni-OOH, D(Se)-Fe-Ni-OOH中Fe/Ni-O的鍵長與其對應的硫化物和硒化物預催化劑中的Fe/Ni-S, Fe/Ni-Se的鍵長具有明顯的差異。

Figure 2. k2-weighted Fourier transformed EXAFS at the Ni K-edge of the D-Fe-Ni-OOH a), D(O)-Fe-Ni-OOH b), D(S)-Fe-Ni-OOH c) and D(Se)-Fe-Ni-OOH d) with EXAFS fitting. e) Wavelet transform for k2-weighted EXAFS signals at the Ni K-edge of catalysts.

小波變換分析確認了催化劑在結構演變之后衍生得到的結構為（羥基）氫氧化物。研究團隊對衍生得到的（羥基）氫氧化物進行EXFAS擬合，得出了詳細的結構信息。

Figure 3. Electrocatalytic performance. a) polarization curves of D-Fe-Ni-OOH, D(O)-Fe-Ni-OOH, D(S)-Fe-Ni-OOH and D(Se)-Fe-Ni-OOH in 1M KOH at a scanning rate of 5mV s^-1. b) Comparison of the overpotential at a current density of 50 mA cm^-2 and the current density at an applied potential of 1.55 V vs. RHE for D-Fe-Ni-OOH, D(O)-Fe-Ni-OOH, D(S)-Fe-Ni-OOH and D(Se)-Fe-Ni-OOH. c) Tafel plots derived from the polarization curves in (a); d) EIS Nyquist plots. e) Comparison of the Debye-Waller factor (σ²) for catalysts extracted from EXAFS fitting results and the applied potential at a current density of 0.1 mA cm-2 ECSA.

電化學性能測試表明，相比較于D(O)-Fe-Ni-OOH和D(S)-Fe-Ni-OOH，D(Se)-Fe-Ni-OOH具有更加出色的OER活性和更小Tafel斜率（39.63 mv dec^-1）。 在電流密度為50 mA cm^-2時，催化劑對析氧反應的過電位為303 mV。EXAFS分析表明D(Se)-Fe-Ni-OOH具有更高的結構無序度（Fe-O path σ²=0.009 ?²; Ni-O path σ²=0.008 ?²）。

Figure 4. a) Ni K-edge XANES of D(S)-Ni-OOH and D(S)-Fe-Ni-OOH. b) k2-weighted Fourier transformed EXAFS at the Ni K-edge of the D-Ni-OOH with EXAFS fitting. c) Debye-Waller factor (σ²) for D(S)-Fe-OOH, D(S)-Ni-OOH and D(S)-Fe-Ni-OOH extracted from EXAFS fitting results. d) PDF analysis for D-Fe-Ni-OOH and D(S)-Fe-Ni-OOH. Inset is a magnified view of the red box. e) HRTEM image of D(S)-Fe-Ni-OOH.

以硫化物為例，雙金屬D(S)-Fe-Ni-OOH催化劑與單金屬D(S)-Fe-OOH和D(S)-Ni-OOH催化劑的XAFS分析表明雙金屬D(S)-Fe-Ni-OOH催化劑的結構具有更高的無序度。對分布函數分析和高分辨透射電鏡顯示從硫化物預催化劑衍生出的D(S)-Fe-Ni-OOH比從直接制備得到的Fe-Ni-OOH衍生出的D-Fe-Ni-OOH催化劑表現出更高的結構無序度。

Figure 5. In-situ XAFS characterizations. a) In situ XANES spectra recorded at the Ni K-edge of Fe-Ni-S at different applied potentials from 1.23 to 2.03 V vs. RHE during the OER process. b) Oxidation state evolution of Ni species during the OER process. ΔE denotes the K-edge position difference of samples in comparison with that of Ni foil. c) Wavelet transform of Ni K-edge EXAFS spectra for Fe-Ni-S under various potentials during OER. d) Fourier transformed magnitudes of Ni K-edge EXAFS spectra under various potentials during OER. e) k2-weighted Fourier transformed EXAFS at the Ni K-edge of the Fe-Ni-S at 1.43 V vs. RHE with EXAFS fitting. f) The coordination number (CN) and bond length extracted from EXAFS fitting results. g) The Debye-Waller factor (σ²) for catalysts extracted from EXAFS fitting results.

利用原位XAFS技術研究了硫化物在OER過程中的價態以及結構無序度的演變過程。在OER過程中，催化劑的價態在逐漸增高并且催化劑由硫化物向（羥基）氫氧化物轉變。EXAFS擬合結果表明在結構演變的過程中其無序度（Debye-Waller factor）逐漸增大。小波分析也證實了上述結構演變的過程。

Figure 6. DFT calculations. a) TDOS of D-Fe-Ni-OOH, D(O)-Fe-Ni-OOH, D(S)-Fe-Ni-OOH and D(Se)-Fe-Ni-OOH. b) Free energy diagram for the OER over Ni sites for catalysts. c) Comparison of the theoretical calculated overpotentials and change in the octahedral distortion parameters of different DFT models. d) Linear scaling relationship for catalysts between the adsorption energy of OOH* versus the adsorption energy of OH* and between the adsorption energy of O* versus the adsorption energy of OH*. e) Calculated volcano plot of OER overpotential η with ΔG_OH and ΔG_O-ΔG_OH as the descriptors.

DFT計算結果表明結構無序和局域結構畸變能夠調控催化劑電子結構和調控催化劑的反應熱動力學，增強OER性能。

五、【成果啟示】

綜上所述，本工作選取了過渡金屬氧化物，硫化物和硒化物等做為模型預催化劑，研究了在OER過程中其結構無序的演變過程。XAFS分析表明預催化劑與原位衍生催化劑之間的鍵長差異會產生更加無序的結構，顯示出更強的析氧反應電催化活性。本工作從結構無序的角度，揭示析氧電催化結構演化過程中結構無序與活性的相關性，有望為相關催化劑的制備以及揭示其結構-性能關系提供新的思路。

原文詳情：Shouwei Zuo, Zhipeng Wu, Guikai Zhang, Cailing Chen, Yuanfu Ren, Lirong Zheng, Jing Zhang, Yu Han, Huabin Zhang. Correlating Structural Disorder in Metal (Oxy)hydroxides and Catalytic Activity in Electrocatalytic Oxygen Evolution. Angew. Chem. Int. Ed.2023, e202316762.

https://doi.org/10.1002/anie.202316762

本文由Holmes供稿