Enhanced oxygen bulk diffusion of La_0.6Sr_0.4FeO_3-δ fuel electrode by high valence transition metal doping for direct CO₂ electrolysis in solid oxide electrolysis cells

Solid oxide electrolysis cell (SOEC) is a promising energy conversion device for the efficient conversion of CO₂ into valuable chemicals by using renewable energy sources. Fe-based perovskite oxides are highly active for CO₂ electrochemical reduction as fuel electrodes in SOECs. Herein, high valence cations doped La_0.6Sr_0.4FeO_3-δ oxides (La_0.6Sr_0.4FeO_3-δ, La_0.6Sr_0.4Fe_0.9Sc_0.1O_3-δ, La_0.6Sr_0.4Fe_0.9Ta_0.1O_3-δ, La_0.6Sr_0.4Fe_0.9W_0.1O_3-δ) are used as fuel electrodes in SOECs for CO₂ electrolysis, and the crystal structure, electrical conductivity, chemical surface exchange coefficient (K_chem), chemical bulk diffusion coefficient (D_chem) and electrolysis performance are investigated for CO₂ electrolysis. In addition, we determine the electrochemical processes on half cells and single cells with La_0.6Sr_0.4FeO_3-δ series fuel electrodes by distribution of relaxation times (DRT) analysis, respectively. The results show that W-doping introduces more highly oxidative oxygen species O₂²⁻/O⁻ and decreases the activation energy of lattice oxygen diffusion, thus accelerating the oxygen ion diffusion kinetics. The value of D_chem for La_0.6Sr_0.4Fe_0.9W_0.1O_3-δ reaches to 4.671 × 10⁻⁴cm² S⁻¹ at 800 °C, and this value is 3.8 times than that of La_0.6Sr_0.4FeO_3-δ (1.216 × 10⁻⁴cm² S⁻¹). DRT analysis indicates the different rate-limiting steps for CO₂ reduction reaction (CO₂RR) on half cells and single cells, which are the dissociation of carbonate intermediate process on half cells, and the exchange and diffusion of oxygen ions process on single cells. W-doping remarkably improves the oxygen ion transport property, thus enhancing the single cell performance for CO₂ electrolysis. The single cell with La_0.6Sr_0.4Fe_0.9W_0.1O_3-δ fuel electrode demonstrates a high current density of 1.48 A cm⁻² at 800 °C and 1.5 V, with the polarization resistance (Rp) of 0.527 Ω cm² at open circuit voltage (OCV). Meanwhile, the single cell exhibits good stability for 50 h at an applied voltage of 1.2 V. This work reveals the difference in electrochemical processes on half cells and single cells, and provides a strategy to accelerate the CO₂RR kinetics of La_0.6Sr_0.4FeO_3-δ-based materials.