Sulfur-containing activated carbons with greatly reduced content of bottle neck pores for double-layer capacitors: a case study for pseudocapacitance detection

Abstract

Synthesis of S-doped activated carbons (ACs) by carbonization and simultaneous activation of S-based polymers was found to be an efficient route to produce porous carbons for double layer capacitors (EDLCs) with high specific energy and power densities combined with low self-discharge. Here we investigate for the first time the processing-structure–property relationships related to the formation of polythiophene-derived ACs for EDLC applications. Sulfide bridges present in the polymer precursor were found to depress the shrinkage of the smallest micropores during the carbonization process and allow for the enhanced ion transport within the produced AC electrodes. The cyclic voltammetry (CV) measurements on S-doped ACs produced at 800 and 850 °C showed high specific capacitance (up to ∼200 F g⁻¹) and no significant self-discharge in neutral aqueous electrolytes. More importantly, these capacitance values remained virtually identical for a sweep rate increasing from 1 to 50 mV s⁻¹. The observed capacitance retention is quite remarkable for thick electrodes of ∼200 μm and a large AC particle size of 10–100 μm. It indicates great potential of the proposed synthesis technology for EDLCs operating at high frequencies and high currents. In the course of our systematic studies of AC performance in different electrolytes we found a strong correlation between the large pseudocapacitance and the significant self-discharge in ACs. We harness the difference between the characteristic times required to establish a double layer and that of the pseudocapacitive redox reactions and propose a simple method to estimate the fraction of pseudocapacitance. The proposed method is particularly valuable in cases when CV measurements do not show clear characteristic reduction–oxidation peaks.