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

Abstract

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 , La2 O2 CO3 , and La2 O3 , have been synthesized through an in situ urea hydrolysis method, followed by annealing at different temperatures. Among these lanthanum compounds, the layer-structured La2 O2 CO3 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, La2 O2 CO3 exhibits the best electrocatalytic activity with the lowest H2 O2 production and highest durability for ORR, which proves the close correlation between electronic energy level and electrocatalytic ORR activity. During the ORR process over La2 O2 CO3 , 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, La2 O2 CO3 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.