High-valence-state manganate(v) Ba3Mn2O8 as an efficient anode of a proton-conducting solid oxide steam electrolyzer

Abstract

Herein, high-valence-state Mn(V) oxide, barium manganate(V) (Ba₃(MnO₄)₂), is examined as an anode electrocatalyst of a H⁺-conducting solid oxide steam electrolysis cell (H-SOEC). Ba₃(MnO₄)₂ comprises C_3v-symmetric MnO₄³⁻ oxo-anions with three long Mn–O bonds and one short Mn–O bond at room temperature. Ba₃(MnO₄)₂ caused a conductivity jump by one order of magnitude at approximately 600 °C owing to the antiferromagnetic/paramagnetic phase transition, accompanied by a shape change of the tetrahedral MnO₄³⁻ anions from C_3v to T_d symmetry, as confirmed by the electrical conductivity measurements and the extended X-ray absorption fine structure at an elevated temperature. Hence, the Ba₃(MnO₄)₂ base anode of the H-SOEC exhibited improved performance, with anode polarization resistances being lower than those of Sm_0.5Sr_0.5CoO₃, a well-known H-SOEC anode material. Impedance analysis in terms of oxygen and water partial pressure revealed that the superior performance of the Ba₃(MnO₄)₂ base anode can be attributed to the extended reaction area. Since abundant unoccupied 3d states of the high-valence-state Mn⁵⁺ cations are favorable for charge transfer interactions with water electron donors, thereby facilitating water adsorption, the oxygen evolution reaction could occur directly over the electrode surface, and thus the reaction sites were not limited to the gas–electrode–electrolyte triple phase boundary.