Design of anode functional layers for protonic solid oxide electrolysis cells

Abstract

Protonic solid oxide electrolysis cells (P-SOECs) are one of the most efficient devices for hydrogen production from renewable electricity, but the lack of suitable anodes has led to serious concerns in terms of high anodic overpotentials and low conversion efficiencies. Herein, we demonstrated that both the steam electrolysis performances and efficiency of P-SOECs based on a BaZr_0.6Ce_0.2Y_0.1Yb_0.1O_3−δ electrolyte can be significantly improved by employing a Ba_0.95La_0.05Fe_0.8Zn_0.2O_3−δ (BLFZ) H⁺/O²⁻/e⁻ triple-conductor thin film at the anode/electrolyte interface as an anode functional layer (AFL). A broad survey of electrolysis performances was conducted for cells with various AFLs, including H⁺/O²⁻/e⁻ triple conductors and O²⁻/e⁻ double conductors. This investigation clarified that BLFZ can significantly decrease the ohmic and polarization resistances, and thus, greatly increase the electrolysis current. Because of the high proton conductivity and excellent electrochemical kinetics, the BLFZ AFL can allow the anodic reactions to occur over the surfaces of the BLFZ AFL without the long-range diffusion of oxygen species over the anode. Moreover, the BLFZ AFL can depress the hole injection in the electrolyte because the water partial pressure remained relatively high in comparison to the oxygen partial pressure at the AFL/electrolyte interface due to the excellent proton conductivity. Hence, the BLFZ cell offered a high electrolysis current of 570 mA cm⁻² at 1.3 V, and an increased efficiency of 75% from 46% for a cell without the BLFZ AFL at 500 °C. These results reveal that an effective AFL can boost the anodic reaction and optimize the efficiency of obtaining P-SOECs with excellent performances and outstanding ability for hydrogen production. Transition metal oxides combining high proton and low oxide ion conductivities could be promising AFLs for highly efficient P-SOECs.