Influence of S Contamination on CO2 Reduction at Ag Electrodes

The influence of S poisoning on Ag electrodes for the production of synthesis gas (syn-gas) was evaluated at 20 and 70°C using a flow-cell electrolysis system with a gas diffusion electrode (GDE). Before poisoning, a mixture of CO and H 2 (syn-gas) was produced. After exposure to Na 2 S at open circuit potential (OCP) the overpotential for H 2 evolution decreased resulting in a significant decrease in the Faradaic efficiency for CO. It was found that poisoning was mostly reversed by performing electrolysis at 20°C in S free electrolyte but not at 70°C. Measurements on planar disk electrodes showed distinct stripping waves for S adsorbed at OCP. The stripping waves were influenced by both temperature and the presence of CO 2 . These measurements show the potential dependant nature of electrode recovery where at elevated temperature adequate polarization to strip S was prevented by H 2 evolution. Poisoning during flow-cell operation led to a temporary decrease in CO produced which can be mostly recovered by exchanging the catholyte. SEM analysis of S-exposed GDEs demonstrated a blocking of the pore-structure at the GDE surface. Preliminary experiments with economical grades of CO 2 showed only minor poisoning due to S-containing species leading to only marginal changes in syn-gas composition.