COMPONENT DEVELOPMENT NEEDS FOR THE HYBRID SULFUR ELECTROLYZER

Abstract

Fiscal year 2008 studies in electrolyzer component development have focused on the characterization of membrane electrode assemblies (MEA) after performance tests in the single cell electrolyzer, evaluation of electrocatalysts and membranes using a small scale electrolyzer and evaluating the contribution of individual cell components to the overall electrochemical performance. Scanning electron microscopic (SEM) studies of samples taken from MEAs testing in the SRNL single cell electrolyzer test station indicates a sulfur-rich layer forms between the cathode catalyst layer and the membrane. Based on a review of operating conditions for each of the MEAs evaluated, we conclude that the formation of the layer results from the reduction of sulfur dioxide as it passes through the MEA and reaches the catalyst layer at the cathode-membrane interface. Formation of the sulfur rich layer results in partial delamination of the cathode catalyst layer leading to diminished performance. Furthermore we believe that operating the electrolyzer at elevated pressure significantly increases the rate of formation due to increased adsorption of hydrogen on the internal catalyst surface. Thus, identification of a membrane that exhibits much lower transport of sulfur dioxide is needed to reduce the quantity of sulfur dioxide that reaches the cathode catalyst and is reduced to produce the sulfur-rich layer. Three candidate membranes are currently being evaluated that have shown promise from preliminary studies, (1) modified Nafion{reg_sign}, (2) polybenzimidazole (PBI), and (3) sulfonated Diels Alder polyphenylene (SDAPP). Testing examined the activity for the sulfur dioxide oxidation of platinum (Pt) and platinum-alloy catalysts in 30 wt% sulfuric acid solution. Linear sweep voltammetry showed an increase in activity when catalysts in which Pt is alloyed with non-noble transition metals such as cobalt and chromium. However when Pt is alloyed with noble metals, such as iridium or ruthenium, the kinetic activity decreases. We recommend further testing to determine if these binary alloys will provide the increased reaction kinetic needed to meet the targets. We also plan to test the performance of these catalyst materials for both proton and sulfur dioxide reduction. The latter may provide another parameter by which we can control the reduction of sulfur dioxide upon transport to the cathode catalyst surface. A small scale electrolyzer (2 cm{sup 2}) has been fabricated and successfully installed as an additional tool to evaluate the effect of different operating conditions on electrolyzer and MEA performance. Currently this electrolyzer is limited to testing at temperatures up to 80 C and at atmospheric pressure. Selected electrochemical performance data from the single cell sulfur dioxide depolarized electrolyzer were analyzed with the aid of an empirical equation which takes into account the overpotential of each of the components. By using the empirical equation, the performance data was broken down into its components and a comparison of the potential losses was made. The results indicated that for the testing conditions of 80 C and 30 wt% sulfuric acid, the major overpotential contribution ({approx}70 % of all losses) arise from the slow reaction rate of oxidation of sulfur dioxide. The results indicate that in order to meet the target of hydrogen production at 0.5 A/cm{sup 2} at 0.6 V and 50 wt% sulfuric acid, identification of a better catalyst for sulfur dioxide oxidation will provide the largest gain in electrolyzer performance.