Discussion of 'high-temperature thermodynamic properties of the vanadium carbides V2C and VC0.73 determined using a galvanic cell technique'

Coltters and Nava report the results of their galvanic cell studies of the ΔG0 for the formation of V2C and VC0.73. These authors have used BaF2-BaC2 solid solution as the solid electrolyte. This material, which is a C2-2 ion conductor, has earlier been suggested as a solid electrolyte useful for measuring carbon potentials. The primary purpose of the present discussion is to raise some questions regarding the suitability of BaF2-BaC2 as a solid electrolyte in such studies. Some apparent inconsistencies with regard to the measurements involving VC0.73/V2C electrodes are also raised in this discussion. An important criterion regarding the choice of an electrolyte is that it should be chemically stable with respect to the electrode components. Any reaction between the electrode and the electrolyte would not only affect the chemical potential of the electrode species at the interface but also the nature of the electrolyte as an ionic conductor. The possible interactions between the electrodes and BaF2-BaC2 electrolyte are (a) formation of complex carbides, (b) dissolution of fluoride in the electrode, and (c) displacement reaction. Of these, the last mentioned may be very important, leading to serious complications of the experimental measurements