CHANGES IN THE ELECTROCHEMICAL-BEHAVIOR OF POLYMERIC CARBON INDUCED BY HEAT-TREATMENT AND DOPING WITH LITHIUM IONS

The electrochemical properties of glassy carbon (GC) and GC doped with lithium ions (GC:Li+) were investigated as a function of heat-treatme nt temperature (HTT). A phenolic resin precursor (liquid resol, C7H8O2 ) was heat-treated to 650, 700, 1000, 2000, or 2500 degrees C to form GC. GC:Li+ was made by dissolving 5% and 10% by weight LiNO3, in resol (referred to as 5% WLDR and 10% WLDR, respectively) and following the same heat-treatment programs. Cyclic voltammetry was performed at GC electrodes in aqueous solutions of sulfuric acid and of potassium sulf ate together with the electro-oxidation of potassium ferrocyanide. The hydrogen evolution potential became more negative with rising GC HTT, ranging from -1.2 V for HTT 650 degrees C to -2.0 V for HTT 2500 degr ees C in both sulfuric acid and potassium sulfate solutions, while hys teresis in the voltammograms was reduced and the oxidation and reducti on peaks disappeared The standard rate constant of ferrocyanide oxidat ion at a polished electrode surface increased from 6 to 11 x 10(-3) cm s(-1) with increasing HTT. On adding lithium ions to the resol, the o pen-circuit potentials compared to undoped GC became less positive. In sulfuric acid, new cyclic voltammetric peaks emerged, anodic at +0.6 V and cathodic at;+0.3 V vs the standard calomel electrode, indicative of alterations in the bulk structure of the GC matrix. Lithium doping the resin caused GC: upon heating, to lose 9-10% mass, lowered the de gree of graphitization at HTTs below the melting points of LiOH and Li 2CO3 (similar to 720 degrees C), and enhanced graphitization once the lithium compounds melted and diffused away. X-ray photoelectron spectr oscopy results confirmed that no lithium remained in the 5% WLDR sampl es if the HTT was 1000 degrees C or higher. Scanning electron microsco py showed that the pore-diameter distribution of GC:Li+ differed from GC. Inductively coupled plasma atomic emission spectroscopy showed tha t the 10% WLDR samples heat-treated to 650 degrees C had a higher lith ium-ion release rate than those undergoing HTTs to 500 and 575 degrees C, which identified lithium out-diffusion as the underlying mechanism of change in the permeability of GC as a function of HTT.