Salmanion Mahya, Nandy Subhajit, Chae Keun Hwa, Najafpour Mohammad Mahdi
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran.
Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea.
Inorg Chem. 2022 Mar 28;61(12):5112-5123. doi: 10.1021/acs.inorgchem.2c00241. Epub 2022 Mar 17.
Metal-organic frameworks (MOFs) are extensively investigated as catalysts in the oxygen-evolution reaction (OER). A Ni-Fe MOF with 2,5-dihydroxy terephthalate as a linker has been claimed to be among the most efficient catalysts for the oxygen-evolution reaction (OER) under alkaline conditions. Herein, the MOF stability under the OER was reinvestigated by electrochemical methods, X-ray diffraction, X-ray absorption spectroscopy, energy-dispersive spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy, nuclear magnetic resonance, operando visible spectroscopy, electrospray ionization mass spectroscopy, and Raman spectroscopy. The peaks corresponding to the carboxylate group are observed at 1420 and 1520 cm using Raman spectroscopy. The peaks disappear after the reaction, suggesting the removal of the carboxylate group. A drop in carbon content but growth in oxygen content after the OER was detected by energy-dispersive spectra. This shows that after the OER, the surface of MOF is oxidized. SEM images also show deep restructures in the surface morphology of this Ni-Fe MOF after the OER. Nuclear magnetic resonance and electrospray ionization mass spectrometry show the decomposition of the linker in alkaline conditions and even in the absence of potential. These experimental data indicate that during the OER, the synthesized MOF transforms to a Fe-Ni-layered double hydroxide, and the formed metal oxide is a candidate for the OER catalysis. Generalization is not true; however, taken together, these findings suggest that the stability of Ni-Fe MOFs under harsh oxidation conditions should be reconsidered.
金属有机框架材料（MOFs）作为析氧反应（OER）的催化剂受到广泛研究。一种以2,5 - 二羟基对苯二甲酸为连接体的镍铁MOF被认为是碱性条件下析氧反应（OER）最有效的催化剂之一。在此，通过电化学方法、X射线衍射、X射线吸收光谱、能量色散光谱、扫描电子显微镜（SEM）、透射电子显微镜、核磁共振、原位可见光谱、电喷雾电离质谱和拉曼光谱等手段，对该MOF在OER条件下的稳定性进行了重新研究。使用拉曼光谱在1420和1520 cm处观察到对应羧基的峰。反应后这些峰消失，表明羧基被去除。能量色散光谱检测到OER后碳含量下降但氧含量增加。这表明OER后MOF的表面被氧化。SEM图像也显示该镍铁MOF在OER后表面形态发生了深度重构。核磁共振和电喷雾电离质谱表明连接体在碱性条件下甚至在无电位情况下都会分解。这些实验数据表明，在OER过程中合成的MOF转变为铁镍层状双氢氧化物，并且形成的金属氧化物是OER催化的候选物。然而，不能一概而论；不过，综合来看，这些发现表明在苛刻氧化条件下镍铁MOFs的稳定性应重新考虑。
