Opportunities and Knowledge Gaps of SO₂ Electrocatalytic Oxidation for H₂ Electrochemical Generation

Abstract

The current electrochemical method for H2 production is water electrolysis, a process with a high energy demand, which is limited by the oxygen evolution reaction (OER). One way to handle the problems related to the OER is to use other oxidative reactions by so-called coupled water electrolysis. One option is the SO2 oxidation reaction (SO2OR), a process that generates H2SO4, which has industrial use, by the consumption of an abundant pollutant that demands, under standard conditions, 0.70 V less than the OER according to theoretical predictions. On the basis of theoretical calculations the mechanism is expected to be the same for a range of metallic catalysts, with the best ones being Pt and Au, in order. Here, the SO2 electro-oxidation on Pt and Au electrodes was investigated by in situ infrared reflection–absorption (IRRA) spectroelectrochemistry, aiming to elucidate the mechanism. On Pt, species such as dithionate, S2O62–, not commonly cited in the literature, were found as intermediates, and PtOH and PtO were suggested as oxidative species. On Au electrodes, the situation observed was completely different. The electrolyte chaotropicity strongly influenced the adsorption of SO2 on Au, changing from Au–O for highly kosmotropic media to Au–S for more chaotropic systems. When the Au–S bond is formed, dithionate and S2O52– species were simultaneously present along with the Au(SO3) complex in solution. The observation of these two species was accompanied by potential oscillations, and an HN-NDR (hidden N-shaped negative differential resistor) oscillator was observed. Different mechanisms for different electrolytes are proposed for Au electrodes.