Electrochemical and capacitive properties of thin-layer carbon black electrodes

Abstract

Electrochemical properties and porous-structure-dependent capacitive ability of commercial carbon blacks, Black Pearls 2000® (BP) and Vulcan® XC 72R (XC), were investigated in H2SO4 solution by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The capacitance in-depth profile is correlated to microscopic appearance of carbon blacks in the form of a thin layer applied over Au substrate from water suspensions of BP and XC. The capacitance calculated from voltammetric charge was found to depend on the sweep rate, due to porosity of investigated materials. Impedance (EIS) characteristics upon frequency-dependent charge/discharge process indicate transmission line electric behavior of BP and XC. Capacitance and resistance values obtained by simulations of EIS data, enabled estimation of capacitance and resistance profile throughout carbon black porous electrodes. Capacitance of BP carbon layer increases going from the outer surface towards the bulk of the layer. External capacitance originates from capacitive characteristics of the macroscopic surface consisting of relatively large agglomerates, while internal capacitance originates from "inner" surface of micro-porous agglomerates. Contrary to BP, opposite distribution of the total capacitance to external and internal part was found for XC, caused by its loose structure and considerably lower real surface area in comparison to BP. The XC morphology makes additionally the pseudocapacitive contribution of surface functionalities more pronounced, which indirectly shifts also the "internal" double-layer capacitive response to higher frequencies through the effect of increased wettability of the layer. Thus, the capacitance of XC surface directly exposed to the electrolyte is larger than that of the inner one, which makes it a "fully-utilized" capacitor, while increased capacitive performance of BP emerges only at very low frequencies of charging/discharging process.