Electrochemical and surface X-ray photoelectron spectroscopy study on the rhodium–carbonate electrode in molten nitrates

Abstract

A combined potentiometric and X-ray photoelectron spectroscopic (X.p.s.) study has been performed on the system (Rh)CO₂, O₂/CO^2–₃ in molten nitrates at two significantly different temperatures (525 and 623 K) in order to obtain further information on molten-salt carbonate electrodes involving transition metals. At both temperatures the electrode response showed Nernstian behaviour different from that relevant to the overall reaction CO^2–₃→ CO₂+½O₂+ 2e^– and was strongly dependent on temperature. The parallel X.p.s. investigation showed that rhodium surfaces undergo chemical microcorrosion when in contact with the melt, with the formation of an oxide layer whose thickness is dependent on the temperature and on the time of contact between the metal and the melt; the relevant kinetics of layer formation were studied by following the modification of the spectroscopic data with time. The results may be rationalized by hypothesizing potential-determining steps involving metal oxide species present on the electrode surface. The paper shows the convenience of using X.p.s. as an addition to conventional electrochemical techniques.