Improving a Sulfur-Tolerant Ruddlesden–Popper Catalyst by Fluorine Doping for CO2 Electrolysis Reaction

We report a highly improved cathode catalyst by doping fluorine anions in oxygen sites of a Ruddlesden–Popper material for CO 2 electrolysis to produce CO in solid oxide electrolysis cells (SOECs). The obtained fluorine-doped catalyst of La 0.9 Sr 0.8 Co 0.4 Mn 0.6 O 3.9−δ F 0.1 (R.P.LSCoMnF) exhibited the higher electrochemical performance of 499 mA/cm 2 at 1.3 V and 850 °C with the smaller polarization resistance of 0.853 Ω·cm 2 than those of the undoped catalyst of La 0.9 Sr 0.8 Co 0.4 Mn 0.6 O 4−δ (R.P.LSCoMn). Moreover, a high faradaic efficiency of 98.6% and a CO production rate of 135 μmol/cm 2 ·min were achieved for CO 2 electrolysis reaction in the single cell with the R.P.LSCoMnF cathode catalyst. These enhanced performances are mainly attributed to the improved properties of surface exchange of oxygen and bulk oxygen diffusion by fluorine doping. More importantly, a negligible sign of performance degradation was observed from the galvanostatic test under CO 2 gas streams containing H 2 S gas, and its structure was maintained even after exposure to 100 ppm H 2 S in an N 2 gas stream at 850 °C for 10 h, suggesting that R.P.LSCoMnF is highly robust against poisonous sulfur species. Therefore, this newly developed fluorine-doped Ruddlesden–Popper catalyst could be a promising cathode for the electrolysis of sulfur-containing CO 2 gas stream, which is emitted from steel-making blast furnaces or power plants.