First Spectroscopic Observation of Peroxocarbonate/ Peroxodicarbonate in Molten Carbonate

Abstract

An active oxygen species in molten carbonate is the key to elucidate the complex mechanism of the cathode reaction in MCFC. However, under acidic condition of MCFC operation (P CO 2 > 1.01 × 10 4 Pa), the exact chemical state of the active oxygen species remains unclear and is a subject of continuous debates. In this work, we present the first experimental observation of peroxocarbonate/peroxodicarbonate species in acidic molten carbonate by using in-situ Raman spectroscopic technique. The results indicate that the predominant oxygen species (O 2 2-) in basic lithium-rich melts became unstable with the increase of acidity level in the melts and reacted with CO 2 to produce peroxocarbonate or peroxodicarbonate anion. In combination with the result of theoretical calculation, it is deduced that the peroxodicarbonate mechanism is the dominant feature in the cathode reaction path in MCFC.

DFT Models of Molecular Species in Carbonate Molten Salts

“…The broad peak at 982 cm −1 is well represented by the strong Raman vibrations 6−9 (905−1037 cm −1 ) for CO 4 2− as given in Table 1. The broad Raman peak at approximately 610 cm −1 is 1.00 dp none also matched by the theoretical O−C−O bend and stretch vibration at 603 cm −1 .…”

“…The peak at 1062 cm −1 remains essentially constant. The authors 4 suggest that the addition of CO 2 leads to the formation of CO 4 2− via eq 2:…”

“…The broad Raman peak at approximately 610 cm −1 is 1.00 dp none also matched by the theoretical O−C−O bend and stretch vibration at 603 cm −1 . A modification of the above prediction (eq 2) is to include the reaction of CO 4 2− with K + to form KCO 4 − via eq 3:…”

“…There is also a weaker vibration at 602 cm . It is likely that the observed spectrum is due to a combination of these vibrations for CO 4 Table 2 …”

A perspective on liquid salts for energy and materials

“…21 In molten hydroxides and carbonates we can anticipate the presence of species such as superoxide, ܱ ଶ ି ; peroxide, ܱ ଶ ଶି ; and percarbonate, ‫ܱܥ‬ ସ ଶି especially on the anodic side. 32 The presence of small concentrations of dissolved alkali metals is also possible in cathodic environments, giving further opportunities for "exotic" chemistry.…”