Changes in the Electrochemical Behavior of Polymeric Carbon Induced by Heat‐Treatment and Doping with Lithium Ions

Abstract

The electrochemical properties of glassy carbon (GC) and GC doped with lithium ions were investigated as a function of heat‐treatment temperature (HTT). A phenolic resin precursor (liquid resol, was heat‐treated to 650, 700, 1000, 2000, or 2500 °C to form GC. was made by dissolving 5% and 10% by weigh 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 −1.2 V 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 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 (∼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 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.