A pH-dependent partial electrochemical exfoliation of highly oriented pyrolytic graphite for high areal capacitance electric double layer capacitor electrode

Abstract

High electrolyte resistance severely limits the response time and energy efficiency of an electric double layer capacitor electrode. Vertically oriented graphene nanosheets (VOG) having open, straight channel morphology is a new, attractive morphology with low electrolyte resistance. While CVD remains the dominant synthesis method for VOG electrodes, electrochemical exfoliation of graphite, a facile, economical and scalable method for production of loose graphene flakes, can also provide a VOG electrode. Here we show, that the conditions of electrochemical exfoliation for producing VOG morphology is substantially different from using it to produce loose graphene flakes. In particular, a low pH solution leads to a better expansion of the graphite and substantially better VOG density while a high pH solution leads to the formation of loose graphene flakes. Sulfuric acid is better than other sulfate salt-based electrolyte solutions for the synthesis of a VOG electrode. The electrolyte concentration, exfoliation time, and current density also affect the electrode capacitance. The optimal process conditions provide a VOG electrode of areal capacitance 750 mF/cm2. A capacitor consisting of two identical VOG electrodes shows a volume capacitance density of 1.56 F cm−3 higher than most of the literature values for supercapacitors.