KAUST张华彬课题组Angew. Chem.: 金属（羟基）氢氧化物结构无序与电催化析氧活性的关联

一、 【导读】

开发高效的析氧反应（OER）电催化剂并了解其结构-性能关系对能源转换和储存至关重要。然而，多相电催化剂的结构复杂性使得阐明其动态结构演变和析氧反应机制产生了巨大的挑战。基于此，阿卜杜拉国王科技大学张华彬教授和韩宇教授利用过渡金属氧化物，硫化物和硒化物作为模型预催化剂，通过OER过程中的结构自重构过程，利用反应前硫化物/硒化物预催化剂与反应后的（羟基）氢氧化物之间的金属-非金属键长差异，衍生出不同程度的结构无序的（羟基）氢氧化物。结果表明，具用更高结构无序度的催化剂显示出更优异的析氧反应电催化活性。原位X射线吸收精细结构（XAFS）实验揭示了模型催化剂在OER过程中的结构无序度的演变过程。密度泛函理论（DFT）计算表明，结构无序和局域结构畸变能够调控催化剂电子结构和调控催化剂的反应热动力学，增强其OER性能。本工作证实了OER电催化中催化剂结构无序与其催化活性之间的相关性，有望为相关催化剂的制备以及揭示其结构-性能关系提供新的思路。

 三、【核心创新点】

1.本工作提出利用XAFS来研究催化剂在OER反应过程中结构无序性的变化。并用Debye-Waller因子对结构的无序度进行定量分析。

2.将结构无序与其催化本征活性之间进行关联，揭示了结构无序与催化剂本征活性之间的关系，有望为相关催化剂的制备提供指导意义。

 四、【数据概览】

 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.

研究团队对模型预催化剂氧化物、硫化物、硒化物进行结构表征，XAFS表征结果表明衍生的（羟基）氢氧化物D(S)-Fe-Ni-OOH, D(Se)-Fe-Ni-OOH中Fe/Ni-O的键长与其对应的硫化物和硒化物预催化剂中的Fe/Ni-S, Fe/Ni-Se的键长具有明显的差异。

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.

小波变换分析确认了催化剂在结构演变之后衍生得到的结构为（羟基）氢氧化物。研究团队对衍生得到的（羟基）氢氧化物进行EXFAS拟合，得出了详细的结构信息。

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.

电化学性能测试表明，相比较于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）。 在电流密度为50 mA cm^-2时，催化剂对析氧反应的过电位为303 mV。EXAFS分析表明D(Se)-Fe-Ni-OOH具有更高的结构无序度（Fe-O path σ²=0.009 Ų; Ni-O path σ²=0.008 Ų）。

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.

以硫化物为例，双金属D(S)-Fe-Ni-OOH催化剂与单金属D(S)-Fe-OOH和D(S)-Ni-OOH催化剂的XAFS分析表明双金属D(S)-Fe-Ni-OOH催化剂的结构具有更高的无序度。对分布函数分析和高分辨透射电镜显示从硫化物预催化剂衍生出的D(S)-Fe-Ni-OOH比从直接制备得到的Fe-Ni-OOH衍生出的D-Fe-Ni-OOH催化剂表现出更高的结构无序度。

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技术研究了硫化物在OER过程中的价态以及结构无序度的演变过程。在OER过程中，催化剂的价态在逐渐增高并且催化剂由硫化物向（羟基）氢氧化物转变。EXAFS拟合结果表明在结构演变的过程中其无序度（Debye-Waller factor）逐渐增大。小波分析也证实了上述结构演变的过程。

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计算结果表明结构无序和局域结构畸变能够调控催化剂电子结构和调控催化剂的反应热动力学，增强OER性能。

五、【成果启示】

综上所述，本工作选取了过渡金属氧化物，硫化物和硒化物等做为模型预催化剂，研究了在OER过程中其结构无序的演变过程。XAFS分析表明预催化剂与原位衍生催化剂之间的键长差异会产生更加无序的结构，显示出更强的析氧反应电催化活性。本工作从结构无序的角度，揭示析氧电催化结构演化过程中结构无序与活性的相关性，有望为相关催化剂的制备以及揭示其结构-性能关系提供新的思路。

原文详情：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