Unraveling Sulfur Tolerance Mechanisms in Samarium-Doped Ceria-NiRh Catalysts for Solid Oxide Fuel Cells

Sulfur poisoning remains a critical challenge in the development of durable anode catalysts for fuel-flexible solid oxide fuel cells (SOFCs), yet the atomic-scale mechanisms that govern sulfur tolerance have remained elusive. In this work, we uncover new fundamental insights into the behavior of NiRh catalysts supported on samarium-doped ceria (SDC-NiRh), revealing how the in situ exsolution of Ni and Rh leads to the formation of interactive surface species that resist sulfur accumulation. For the first time, we directly observe the dynamic evolution of sulfur-metal interactions under operating conditions, showing that Rh incorporation not only modifies the local chemical environment but also suppresses the formation of persistent Ni-S species, which also facilitates the self-cleaning of sulfur species. At 650 °C, quantitative analysis reveals that the SDC-NiRh system exhibits enhanced water uptake (mass change up to 0.5%) and stronger exothermic behavior (-33.87 J/g) compared to SDC-Ni (-22.76 J/g), indicating improved surface reactivity and higher water uptake capacity, which help the subsequent sulfur self-cleaning reactions. Under sulfur exposure at 650 °C, the SDC-NiRh catalyst retains a cleaner surface and demonstrates sulfur self-cleaning upon steam treatment, in contrast with the more heavily poisoned Ni-only system. Furthermore, the SOFC with SDC-NiRh catalyst maintains more than 3 times higher power output and lower resistance under 100 ppm of H₂S at 600 °C, highlighting its superior sulfur tolerance. These findings help us to understand and design sulfur-tolerant SOFC anode catalysts, offer mechanistic clarity, and guide principles for future fuel-flexible SOFC material development.