2H-AgNiO2電子結構與透過表面修飾調控析氧反應活性

Abstract

在電化學水分解中，析氧反應（oxygen evolution reaction, OER）相比於析氫反應需要更高的過電位，因此開發高效的 OER 電催化劑是很重要的研究課題。過渡金屬氧化物因其在地殼中豐富的含量和在鹼性電解液中的穩定性而成為有前景的OER電催化劑。Fe 摻雜的 NiOOH 和其他 Ni(III)氧化物（如LaNiO3 和 LiNiO2）皆展現了其頂尖的OER 催化活性，並且證明其性能與電子結構密切相關。在本研究中，我們通過水熱法合成純相的2H-AgNiO2 粉末，並探討其電子結構與 OER 催化性能。在O K-edge X-ray吸收光譜中觀察到了負電荷轉移材料中存在於O位點上存在的配體空穴（ligand hole）。Ni L-edge 共振非彈性 X-ray 散射（resonant inelastic X-ray scattering）的實驗結果與多重態晶體場/多重態配位場理論計算比對後發現， Ni位點的電子組態為3d8L最符合2H-AgNiO₂的情況，證實了2H-AgNiO₂具有負電荷轉移能量。2H-AgNiO2 本身並非良好的電催化劑，在 1 M NaOH 溶液中表現出大於 500 mV 的過電位（η10mA）。然而，向電解液中加入Fe(III)後，η10mA與Tafel slope會大幅下降。我們先在1 M NaOH 溶液中進行OER後再轉移到有加入Fe(III)的電解液中進行反應，發現效能會比直接在加入Fe的電解液中進行OER有更高的活性，η10mA最低可以來到237 mV。根據電子顯微鏡影像與X-ray光電子能譜的結果，2H-AgNiO2在1 M NaOH 溶液中進行OER的過程會沿者ab-plane產生侵蝕的痕跡並改變表面的化學組成，而在1 M NaOH 溶液中加入Fe(III)並吸附在2H-AgNiO2表面則會阻止侵蝕的現象發生。在不同的電解液條件下調控2H-AgNiO2表面的侵蝕與Fe(III)吸附行為是OER效率與活性提升的關鍵。

In electrochemical water splitting, the oxygen evolution reaction (OER) requires a higher overpotential than the hydrogen evolution reaction (HER). Therefore, developing efficient OER electrocatalysts is a key research focus. Transition metal oxides (TMOs) are promising candidates for OER electrocatalysts due to their abundance and stability in alkaline electrolytes. Among TMOs, Fe-doped NiOOH and other Ni(III) oxides such as LaNiO3 and LiNiO2 show excellent OER activities, which are strongly linked to their electronic structure. This study explores the electronic structure and OER performance of 2H-AgNiO2, a semi-metallic compound with a hexagonal delafossite structure. Pure 2H-AgNiO2 powder was synthesized via a hydrothermal method. O K-edge X-ray absorption spectroscopy revealed ligand holes at oxygen sites, attributable to a negative charge-transfer character. Ni L-edge resonant inelastic X-ray scattering (RIXS), combined with multiplet theory simulation, showed that the Ni sites adopt a 3d8L configuration, indicative of a negative charge-transfer energy. The pristine 2H-AgNiO2 is a poor OER catalyst with an overpotential η10mA over 500 mV in 1 M NaOH electrolyte. However, adding Fe(III) into the electrolyte significantly lowers η10mA and the Tafel slope. Intentional aging 2H-AgNiO2 in Fe-free 1 M NaOH before switching to Fe-containing electrolyte reduces η10mA to 237 mV. Microscopy and XPS results show that OER in NaOH etches grooves along the ab-plane and alters surface composition. Surface-adsorbed Fe(III) was found to inhibit further etching of the 2H-AgNiO2 surface. Therefore, controlling surface etching and Fe adsorption is crucial for improving the OER activity of 2H-AgNiO2.