Reversible Electrochemical Graphite Salt Formation from Aqueous Salt Electrolytes

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Abstract
Cyclic electrochemical intercalation/deintercalation of anions from aqueous salt solutions in crystalline graphite electrodes has been investigated. The practical electrode was polypropylene bound natural graphite “CPP”. The main candidates for the salts were NaClO4, NaBF4, Na2SO4 and KF in the concentration range of 0.1 – 12 M. – Current efficiencies α with ClO−4 and BF−4 are lower as in aqueous acids due to side reactions, which increase with increasing dilution and temperature. No pH‐effect on α has been found between pH 1 – 12. Under favourable conditions (8 M NaClO4 at 20 °C or 4 M NaClO4 at 0 °C), α's as high as 80% have been observed. A pronounced formation effect is found upon galvanostatic cycling. In Na2SO4, α drops to zero at pH 3 due to the lack of HSO−4‐ions. No intercalation of F− ions is detected from aqueous KF. – Intercalation potential E1 shows Nernst behaviour with regard to anions in solution and intercalated anions. The slope of Nernst curve is 59 mV per decade as expected. E1 is much more positive than in aqueous acid of the same concentration due to the absence of solvate acid depolarization. The anions are desolvated, but not discharged upon intercalation. No indication of an in situ formation of solvate acid has been found. – For comparison, intercalation of anions from perchlorates in CH3OH, CH3COOH, and CH3CN has been investigated. – The potential application in an aqueous Zn/graphite accumulator is discussed. This battery has – with 4 M NaClO4 – an open circuit voltage of 2.58 V.

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