SO2 permeability and proton conductivity of sPEEK membranes for SO2-depolarized electrolyzer

The SO₂ transport properties of Nafion^® and sPEEK membranes were measured using an electrochemical reaction cell to investigate their application in the electrochemical hybrid sulfur process. The permeability of SO₂ in the membranes was determined from a combined theory based on Faraday's law and Fick's law where the electrochemical reaction rate of SO₂ in the downstream membrane is the same as its diffusion flux through the membrane. Both Nafion^® and sPEEK membranes show higher SO₂ diffusion coefficients at higher temperatures. For sPEEK membranes, increasing the degree of sulfonation resulted in increasing permeability, as more water was imbibed in the membranes with higher degrees of sulfonation. Activation energy was extracted from the temperature-dependence of the diffusion coefficients for both membranes. The sPEEK membranes exhibited similar diffusion coefficients to those of Nafion^®, even at high sulfonation degrees of 70%. Besides SO₂ permeability, proton conductivity and mechanical properties were measured for comparison between the 2 polymer membranes. Although the proton conductivity of the sPEEK was slightly lower than the Nafion^® membrane, it was very competitive considering its higher mechanical strength and much lower cost.