An electrolyzer (16) having opposed electrodes (44, 46) to produce hydrogen gas operates at a pressure greater than 5.5 MPa, where SO2 is liquefied, and a reaction phase (48) of liquid SO2 and H2O which is passed to the electrolyzer where liquid SO3 is formed as a bottom phase (54), and where the amount of water present is less than two moles per mole of liquid SO2.
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