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• Ultrathin cobalt carbide (Co 2 C) nanosheets are firstly employed as photothermal catalysts. • Efficient degradation of mustard-gas simulants (CEES) is realized under solar light. • Catalytic performance of Co 2 C is better than nanocrystalline TiO 2 under identical conditions. • Mechanism of photothermal-driven CEES degradation on Co 2 C is clearly revealed. Here, ultrathin cobalt carbide (Co 2 C) nanosheets are firstly illustrated as effective and robust catalysts toward photothermal degradation of sulfur mustard simulants (e.g., 2-chloroethyl ethyl sulfide, CEES) under solar light. Under the optimal conditions, the degradation rate of CEES by Co 2 C nanosheets is up to 98 %, which is much higher than the widely used P25 and anatase TiO 2 nanoparticles. Moreover, the degradation performance is comparable or even better than those typical photothermal catalysts, including MnO 2 , MnO x -TiO 2 and Co 3 O 4 , under identical conditions. Experimental evidences and density functional theory (DFT) calculations reveal that the superior activity is attributed to three main reasons: (i) the high photo-to-heat conversion efficiency of Co 2 C enables an elevated surface temperature for chemical bond breaking, (ii) the feasible binding of CEES on Co 2 C surface via Co S and Co Cl coordination promotes the process of degradation, and (iii) the surface hydroxyl groups (OH) on Co 2 C nanosheets favor the degradation of CEES. Obviously, this work provides new insights into practical and large-scale application of transition metal carbides (TMCs) as novel photothermal catalysts in the decontamination of chemical warfare agents (CWAs) under ambient conditions (i.e., solar light and room temperature). [ABSTRACT FROM AUTHOR]