Operando Raman Spectroscopy of Transition Metal Oxide Catalysts in Regard to the Oxygen Evolution Reaction

Transition metal oxides (NiO x , MnO x) have been synthesized by reactive magnetron sputtering and analysed regarding their chemical composition with XPS to develop active catalysts for the oxygen evolution reaction in alkaline media. NiO x was investigated towards its temperature dependent deficient structure and the enhancement of the catalytic activity through these defects. The catalytic activity was evaluated with the help of operando Raman spectroscopy to monitor the active surface intermediates of NiO x and investigate a temperature dependent Ni III /Ni IV redox transition. The defect structure was concluded to greatly improve catalytic activity, capacitive charge storage, and redox processes. MnO x was deposited in three different oxidation states (Mn II , Mn III , and Mn IV) as concluded with XPS and investigated towards its suitability as OER catalyst. With Raman spectroscopy, all MnO x revealed δ-MnO 2 structure when in contact with the alkaline electrolyte. Their catalytic activity was detected to differ based on their degree of charging with an applied potential, i.e. the coexistence of Mn III and MnIV species, and an increased distance between layers of MnO 6 octahedra. With a combination of reduction and activation processes, the activity of Mn II was altered greatly, to receive a by more than 100 mV reduced overpotential at 5 mA cm −2 , and a low Tafel slope of 44 mV dec −1 .