Fundamentals of SO2 depolarized water electrolysis and challenges of materials used

Anu Lokkiluoto

    Research output: ThesisDoctoral ThesisMonograph


    Sulfur dioxide depolarized water electrolysis (SDE) produces sulfuric acid and hydrogen. Due to its lower cell voltage, the process requires far less electricity than traditional water electrolysis. When SO2 is obtained from flash smelting, sulfides roasting, sulfur combustion, or any other similar operation, SDE is a part of the OutotecR open cycle process. In the present work, materials to be used in SDE were studied together with the fundamentals of the process. Data on the co-production of concentrated acid and hydrogen are scarce in the earlier literature. Theoretical modeling work covering the entire concentration range was deemed necessary in addition to experimental testing to settle this issue. Based on the literature concerning PEM Fuel Cells, it was assumed that the water transport properties of the membrane used as a separator in the electrolysis have a significant effect on the water balance. The activities of the electrolyte components were calculated for the whole concentration range of the ternary H2O-H2SO4-SO2 system relevant for the SDE process using a mathematical model elaborated during the present work. The calculated activities were combined with the model for water transport through a Nafion membrane, and an overall model for SDE was built. The reversible cell potential was calculated for the entire concentration range. The model was used to predict the overall water balance of the electrolysis process with changing electrolyte concentrations. The stability of thin gold coatings and their activity towards electrochemical oxidation of SO2 were studied first in autoclave tests and with linear sweep voltammetry, and the performance of the coatings was compared to that of gold. Next, the performance of the gold-coated electrodes was tested in bench-scale SDE. Even though the oxidation of SO2 has previously been studied on gold, such experiments have seldom been carried out in as high concentrations of sulfuric acid as in this work, and the results obtained with gold coatings are believed to be original. The modeling results were compared with the actual performance of the electrolyzer. Based on the combined results of the experimental and theoretical work, it was possible to explain and predict the remarkable changes in the SDE process that take place with changing electrolyte concentrations. A significant amount of water is transported from the anolyte to the catholyte through the membrane due to the electro-osmotic water drag effect. This phenomenon is beneficial for the production or concentration of sulfuric acid by SO2 depolarized electrolysis. Thin gold coatings on stainless steel surface can be used to catalyze the anodic oxidation of SO2, and similar electrodes can be used as cathodes.
    Translated title of the contributionSO2-depolarisoitu elektrolyysi vedyn valmistamiseksi
    Original languageEnglish
    QualificationDoctor's degree
    Awarding Institution
    • Aalto University
    • Gasik, Michael, Supervising Professor
    • Taskinen, Pekka, Thesis Advisor
    Print ISBNs978-952-60-5077-5
    Electronic ISBNs978-952-60-5078-2
    Publication statusPublished - 2013
    MoE publication typeG4 Doctoral dissertation (monograph)


    • SO2 depolarized electrolysis
    • sulfuric acid
    • hydrogen
    • Nafion membrane
    • water transport
    • gold


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