The surface evolution of La_0.4Sr_0.6TiO_3+δ anode in solid oxide fuel cells: Understanding the sulfur-promotion effect

The ideal solid oxide fuel cells (SOFCs) can be powered by readily available hydrocarbon fuels containing impurities. While this is commonly recognized as a key advantage of SOFC, it also, together with the elevated operating temperature, becomes the main barrier impeding the in-situ or operando investigations of the anode surface chemistry. Here, using a well-designed quenching experiment, we managed to characterize the near-surface structure of La_0.4Sr_0.6TiO_3+δ (LST) anode in SOFCs fuelled by H₂S-containing methane. This new method enabled us to clearly observe the surface amorphization and sulfidation of LST under simulated SOFC operating conditions. The ∼1 nm-thick two dimensional sulfur-adsorbed layer was on top of the disordered LST, containing –S, –SH and elemental sulfur species. In SOFC test, such “poisoned” anode showed increased performances: a ten-fold enhanced power density enhancement (up to 30 mW cm⁻²) and an improved open circuit voltage (from 0.69 V to 1.17 V). Moreover, its anodic polarization resistance in methane decreased to 21.53 Ω cm², a difference of 95% compared with the sulfur-free anode. Control experiments confirmed that once the adsorbed sulfur species were removed electrochemically, methane conversion slowed down simultaneously till full stop.