Graphite anode reaction in the KF2HF melt

Effects of traces of water on the graphite anode reaction have been investigated in the KF2HF melt at 100 °C. Cyclic voltammetry shows that with increasing water content from 0.01% to 0.05%, anode potentials for the formation of graphite oxide and graphite fluoride films on graphite electrodes are shifted to lower potentials. This may be ascribed to increase in the reaction of discharged oxygen with graphite and the subsequent decomposition of graphite oxide film by attack of discharged fluorine, which gives ultimately graphite fluoride film on a graphite electrode. When the water content is 0.05 %, the anode effect is thus caused in a short time by graphite fluoride film with a low surface energy. However, when it is 0.01-0.02%, a stage 4 intercalation compound of graphite, C_n⁺HF₂^- was prepared. Addition of 3-6 wt% LiF to the melt gave a stage 3 C_XF(HF)_y without occurrence of anode effect.