A doping element improving the properties of catalysis: in situ Raman spectroscopy insights into Mn-doped NiMn layered double hydroxide for the urea oxidation reaction

Abstract

Elemental doping has been proved to be an efficient means to adjust the performance of Ni-based hydroxides for the electrochemical urea oxidation reaction. However, the mechanism by which elemental doping affects the catalytic activity of such bimetallic catalysts remains unclear and there is a lack of evidence. Herein, we dope Ni(OH)₂ with Mn, a typical dopant, and show that Mn doping reduces the onset potential of Ni hydroxide oxidation, which favors the generation of electrochemically active Ni^IIIOOH, and increases the electrochemical reversibility of the Ni redox pair in NiMn layered double hydroxides (LDHs). In situ Raman spectroscopy analysis provides direct spectroscopic evidence that Mn promotes the production of high-valence Ni^IIIOOH, specifically from ca. 0.4 V vs. Hg/HgO for Ni(OH)₂ to ca. 0.3 V vs. Hg/HgO for Ni_0.2Mn_0.8 LDHs, i.e., a negative shift of 100 mV. The Raman spectroscopy results also reveal that Mn doping elongates the Ni–O bond in NiOOH and leads to increased phase structure disorder. Consequently, the UOR performance of Ni_1−xMn_x LDH surpasses that of Ni(OH)₂. This study provides important information and a clear mechanistic explanation for the crucial roles of dopants in bimetallic electrocatalysts.