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Autori: Maleki, H., Cojocaru, C.D., Brett, C.M.A., Jenkins, G.M., Selman, J.R.
Editorial: Journal of the Electrochemical Society, Volume 145, Issue 3, p.721-730, 1998.
The electrochemical properties of glassy carbon (GC) and GC doped with lithium ions (GC:Li+) were investigated as a function of heat-treatment temperature (HTT). A phenolic resin precursor (liquid resol, C7H8O2) was heat-treated to 650, 700, 1000, 2000, or 2500°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 sulfate together with the electro-oxidation of potassium ferrocyanide. The hydrogen evolution potential became more negative with rising GC HTT, ranging from -1.2V for HTT 650°C to -2.0 V for HTT 2500°C in both sulfuric acid and potassium sulfate solutions while hysteresis in the voltammograms was reduced and the oxidation and reduction peaks disappeared. The standard rate constant of ferrocyanide oxidation at a polished electrode surface increased from 6 to 11 × 10-3 cm s-1 with increasing HTT. On adding lithium ions to the resol, the open-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 degree of graphitization at HTTs below the melting points of LiOH and Li2CO3 (∼720°C), and enhanced graphitization once the lithium compounds melted and diffused away. X-ray photoelectron spectroscopy results confirmed that no lithium remained in the 5%WLDR samples if the HTT was 1000°C or higher. Scanning electron microscopy showed that the pore-diameter distribution of GC:Li+ differed from GC. Inductively coupled plasma atomic emission spectroscopy showed that the 10%WLDR samples heat-treated to 650°C had a higher lithium-ion release rate than those undergoing HTTs to 500 and 575°C, which identified lithium out-diffusion as the underlying mechanism of change in the permeability of GC as a function of HTT.
Cuvinte cheie: glassy carbon