The standard sulfur-iodine thermochemical cycle for the production of hydrogen includes a step for the thermal decomposition of SO₃, that requires temperatures of ³850°C.  The other reactions in the thermochemical cycle can be performed at ≤500°C, and it is highly desirable to be able to carry out the entire process with maximum temperatures less than about 500ºC.

We have investigated the feasibility of using an oxide ion-conducting electrolysis cell to electrochemically decompose SO₃ to SO₂ at the desired lower temperatures,

SO₃ + electricity → SO₂ + ½O₂                                         500-600ºC, E°=0.11 V

We have built a single-atmosphere SO₃ electrode test system to evaluate new electrodes for the electrolytic reduction of SO₃.  Initial tests were performed with Pt electrodes, which confirmed the reduction of sulfuric acid in steam through cyclic voltammetry.  Tests with sample solid oxide fuel cell-type electrode materials gave inconclusive results, with voltammograms showing poor cathodic currents and no indication of the formation of SO₂ or O₂ with the application of voltage.

We have obtained some very promising results with a GaVO₄/Au electrode, however.  We selected this material based on calculated stability diagrams and knowing that V₂O₅ is a good oxidation catalyst for sulfur.  After a cyclic voltammetry experiment, which reduced the electrode to –1.3 V, we noticed an interesting change in the voltammogram, in which the cathodic current began to increase at an increasing rate as voltage was applied.  More significantly, we also observed that the concentrations of both SO₂ and O₂ began to increase rapidly beyond what could be explained by just the faradaic oxygen flux through the cell.  In this paper, we will discuss the materials selection criteria we have developed, the results of our most recent experiments on the SO₃ electrode and the development of the electrochemical cell.

The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”).  Argonne, a U. S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357.  The U. S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.