Positive Effects of H2O on the Hydrogen Oxidation Reaction on Sr2Fe1.5Mo0.5O6−δ-Based Perovskite Anodes for Solid Oxide Fuel Cells

The steam is commonly supplied with H2 to the anodes of Ni-based solid oxide fuel cells (SOFCs). Humidified H2 is also widely used for emerging perovskite anodes of SOFCs, although the influences of H2O on the hydrogen oxidation reaction (HOR) on their surfaces have not been well understood yet. In this work, the effects of H2O on the HOR on Sr2Fe1.5Mo0.5O6−δ (SF1.5M), La0.5Sr1.5Fe1.5Mo0.5O6−δ (LSFM), and Pr0.5Sr1.5Fe1.5Mo0.5O6−δ (PSFM) were systematically investigated using the electrochemical impedance spectroscopy (EIS) and electrical conductivity relaxation (ECR) methods. The EIS spectra suggested the possible promotional effect of H2O on decreasing the polarization resistance of SF1.5M. The ECR results confirmed that the presence of H2O in H2 can significantly increase the oxygen exchange coefficients of SF1.5M, LSFM, and PSFM. To gain a mechanistic understanding of the role of H2O, the density functional theory-based calculations and thermodynamic modeling were performed to unravel the effect of H2O on the HOR on the SF1.5M (001) surfaces. Benefiting from the increment of the oxygen chemical potential and decrement of the free electron concentration upon increasing humidity, the plateau intermediate state in the HOR energy landscape, the step of H2O plus surface oxygen vacancy formation, is reduced on the SF1.5M (001) BO2 (B = Fe and Mo)-terminated surfaces, when decreasing the slab oxygen nonstoichiometry. Furthermore, by comparing the scenarios of the HOR on the dry and hydrated surfaces, the H2O plus surface oxygen vacancy formation energies are lower in the latter case. These two proposed factors, i.e., (i) change of electron chemical potential upon the change of near-surface δ and (ii) enhanced interaction of surface H species with the hydrated perovskite surfaces, contribute to the promoted HOR in the presence of H2O. This work provides important insights into the effects of H2O on the HOR for SOFCs.