Evaluation of proton-conducting membranes for use in a sulfur dioxide depolarized electrolyzer

Abstract

The chemical stability, sulfur dioxide transport, ionic conductivity, and electrolyzer performance have been measured for several commercially available and experimental proton exchange membranes (PEMs) for use in a sulfur dioxide depolarized electrolyzer (SDE). The SDEs function is to produce hydrogen by using the Hybrid Sulfur (HyS) Process, a sulfur-based electrochemical/thermochemical hybrid cycle. Membrane stability was evaluated using a screening process where each candidate PEM was heated at 80 °C in 60 wt% H 2SO 4 for 24 h. Following acid exposure, chemical stability for each membrane was evaluated by FTIR using the ATR sampling technique. Membrane SO 2 transport was evaluated using a two-chamber permeation cell. SO 2 was introduced into one chamber whereupon SO 2 transported across the membrane into the other chamber and oxidized to H 2SO 4 at an anode positioned immediately adjacent to the membrane. The resulting current was used to determine the SO 2 flux and SO 2 transport. Additionally, membrane electrode assemblies (MEAs) were prepared from candidate membranes to evaluate ionic conductivity and selectivity (ionic conductivity vs. SO 2 transport) which can serve as a tool for selecting membranes. MEAs were also performance tested in a HyS electrolyzer measuring current density vs. a constant cell voltage (1 V, 80 °C in SO 2 saturated 30 wt% H 2SO 4). Finally, candidate membranes were evaluated considering all measured parameters including SO 2 flux, SO 2 transport, ionic conductivity, HyS electrolyzer performance, and membrane stability. Candidate membranes included both PFSA and non-PFSA polymers and polymer blends of which the non-PFSA polymers, BPVE-6F and PBI, showed the best selectivity.