Lanthanum-Based Compounds: Electronic Band-Gap-Dependent Electrocatalytic Materials for Oxygen Reduction Reaction

5 years ago

Lanthanum-Based Compounds: Electronic Band-Gap-Dependent Electrocatalytic Materials for Oxygen Reduction Reaction

Jingjun Liu, Weiwei Gu, Feng Wang, Ye Song

The electronic energy level of lanthanum compounds plays an important role in the oxygen reduction reaction (ORR) electrocatalytic process. In this work, three lanthanum compounds, LaOHCO3, La2O2CO3, and La2O3, have been synthesized through an in situ urea hydrolysis method, followed by annealing at different temperatures. Among these lanthanum compounds, the layer-structured La2O2CO3 has the smallest band gap and moderate values of the conduction band (CB) and valence band (VB). Electrochemical measurements in 0.1 m KOH solution have shown that, compared with the other catalysts, La2O2CO3 exhibits the best electrocatalytic activity with the lowest H2O2 production and highest durability for ORR, which proves the close correlation between electronic energy level and electrocatalytic ORR activity. During the ORR process over La2O2CO3, some covalent electrons from the VB are first excited to the CB and then transfer to the unoccupied π*2p orbitals of an active oxygen molecule, leading to strengthened oxygen adsorption and promotion of the reduction of oxygen. Moreover, La2O2CO3 has an ability to chemically disproportionate hydrogen peroxide (to give HO2−), and the produced HO2− at the energy level of O2/HO2− can undergo prompt chemical disproportionation into O2 and OH−. The O2 generated at this stage is adsorbed on the catalyst surface, which can be utilized for further oxygen reduction. La La La land: Three lanthanum compounds, LaOHCO3, La2O2CO3, and La2O3, have been synthesized on a carbon support as potential catalysts for oxygen reduction. The energy level diagram of La2O2CO3 (see graphic) confirms its eminent suitability for electrocatalytic oxygen reduction with the generation of HO2−.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201701136