Factors affecting the dissolution and reduction of oxygen in molten carbonate electrolytes. Part 1: Effect of temperature and alkali carbonate mixture

Abstract

The effect of temperature, electrolyte and gas composition on the dissolution and reduction of oxygen in molten carbonates has been investigated by a variety of electrochemical techniques and by electron spin resonance (ESR) spectroscopy. In mixed melt compositions (e.g. Li K , Li Na ) at low temperatures and moderate carbon dioxide partial pressures, the reduction process was observed to occur via the mixed diffusion of carbon dioxide with peroxide and/or superoxide ions. The diffusion-controlled current was observed to increase in the order Li K (50:50 mol.) > Li K (70:30) > Li Na (52:48) . This was due to an increase in both the diffusion coefficient and the effective oxygen concentration. In basic media e.g. pO 2 = 1 atm, the reduction process was observed to occur almost entirely by the diffusion of peroxide ions. Although superoxide ions were observed by ESR spectroscopy, the concentration was probably too low to affect the electrochemical results. With the exception of lithium carbonate, the reduction process at temperatures greater than approximately 675°C occurred in two stages. The second wave at higher cathodic overpotentials was most probably due to the reduction of peroxide ions and shifted toward negative potentials with increasing oxygen partial pressure and cation radius. In the Na K melt, the peak separation was the largest. In this case, peroxide ions were produced at the electrode surface by the partial reduction of superoxide ions. In the case of lithium carbonate, the reduction of peroxide ions was the most likely mechaism.