Evaluating Operation Procedures of the Sulfur Depolarized Electrolyser for Hydrogen Production from Water

Abstract

Hydrogen has been identified as an attractive energy carrier for the future in terms of on-site energy generation using for example fuel cells. The production of hydrogen and oxygen (in this case the by-product) from water electrolysis has been commercialised, especially for on-site H2 generation. For high volume H2 production using electrolysis, a more intricate system has been proposed in the form of the sulfur dioxide depolarised electrolyser. The SO2 electrolyser, consisting of a platinum anode and cathode separated by a proton exchange membrane, produces both H2 and H2SO4 from H2O and SO2according to equation 1 (anode reaction) and equation 2 (cathode reaction). SO2 (g)+ 2H2O (l) = 2H+ (aq.) + 2e- + H2SO4 (aq.) Eo = 0.158 VSHE(1) 2H+ (aq.) + 2e- = H2 (g) Eo = 0 VSHE(2) The anode is first depolarised by SO2 to facilitate the electrochemical decomposition of water to protons and electrons. Recombination of the protons (permeating through the PEM) and electrons produces hydrogen at the cathode while H2SO4 is formed at the anode. The primary advantage of the SO2 electrolyser over conventional water electrolysis is based on the theoretical potential needed to drive the reaction. By polarising the anode catalyst with SO2, the voltage is reduced to E0 = 0.158 VSHE, significantly less than the 1.23 VSHErequired for normal water electrolysis. This paper will review the current literature on the operating methods for the SO2 electrolyser while including an in-depth study performed at HySA Infrastructure CoC, such as MEA manufacturing parameters, effect of H2S on cell performance. Additionally, the use of PBI based membranes [1] (manufactured by University of Stuttgart through a joint collaboration) in the electrolyser is evaluated by focussing on membrane stability, MEA acid doping and cell performance. Initially the SO2 assisted water electrolyser was developed as part of the Hybrid Sulfur (HyS) cycle in the 1970’s by the Westinghouse Corporation [2]. The SO2 electrolyser was operated, for the HyS cycle, using SO2saturated sulfuric acid as anode and clean sulfuric acid as cathode. The produced sulfuric acid is then returned to the decomposition step. Sivasumbramanian et al. showed that the electrolyser can also be operated using a gaseous SO2 anode and a liquid cathode with a proton exchange membrane (PEM) [3] as separator. By this configuration the cell performance could be increased significantly. Staser et al. [4] further investigated this operating method in-depth with regards to the influence of water transport across the PEM on cell performance and acid concentration produced. This operating method was also shown for the application of SO2 usage in the flue gas compositions where H2S gas is present. Although a reduced performance was observed it was significantly less than expected when compared to fuel cells for example. The development of high temperature membranes, such as PBI based polymers[1,5], for operating the electrolyser in the 120 – 160 ᵒC range has been shown to perform either better (at 80 ᵒC) or comparable to that of commercial PFSA membranes in the 80 - 90ᵒC range [5]. However, at this stage, only one article is available REF where humidified SO2was used as the anode feed using a sulfonated PBI membrane although low temperature (80 and 90 ᵒC) was used [6].