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 H2 (syn-gas) was produced. After exposure to Na2S at open circuit potential (OCP) the overpotential for H2 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 CO2. These measurements show the potential dependant nature of electrode recovery where at elevated temperature adequate polarization to strip S was prevented by H2 evolution. Poisoning during operation leads 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 CO2 showed only minor poisoning due to S-containing species leading to only marginal changes in syn-gas composition.