Quantitative Characterization of Pseudocapacitance on Carbon-Based Active Electrode Materials

Abstract

A pseudocapacitive reaction (PCR) is a surface and/or near surface charge storage behavior accompanying a charge transfer between a guest ion and host active material. The surface induced charge transfer reaction is very fast and stable, because it occurs without solid-state-diffusion of the guest ion and volume changes in the active materials. In addition, a large amount of charge surpassing that gathered from electrochemical double layer (EDL) formation can be stored by PCRs. The more compelling characteristic of the PCR is its collaborative charge storage phenomenon coinciding with EDLs. When the PCR occurs on nanoporous carbon (NPC)-based electrode materials, a drastic improvement in capacity can be achieved with preservation of their intrinsic characteristics that arise from the storage nature of the surface-driven charge. Several reports have demonstrated the superiority of NPCs with PCRs, detailing their significantly high electrochemical performances as active electrode materials for Li and Na ion batteries as well as supercapacitors. Nevertheless, there is little information for the chemisorption-based alkali cation storage behaviors on NPCs. Moreover, there is no method to characterize the level of contribution of the PCR and EDL capacitance, limiting deeper understanding of the PCR in NPCs. Herein, we focused on that the PCRs on NPCs occur through various reaction pathways which have intrinsic activation energy barriers. Therefore, voltage hysteresis of their electrochemical charge/discharge profiles is inevitable in a given voltage window. Owing to the voltage hysteresis, the charge storage capacitance is dependent on the operating voltage window, which could results in variation in the capacitance value according to the characterized voltage range. The voltage-dependent pseudocapacitive behaviors (VD-PCBs) may be used to differentiate the respective contributions of the EDL capacitance and PCR to the overall capacity of NPCs. In this study, heteroatom-enriched NPCs (H-NPCs) were prepared from a nitrogen-containing conjugated polymer precursor by pyrolysis with potassium hydroxide. The voltage-dependent charge storage behaviors of H-NPCs were then characterized by cyclic voltammetry (CV) in different electrolyte systems. From the results, it was confirmed that the pseudocapacitance value can be quantitatively measured through a comparison of CV obtained from different charge carriers. The VD-PCBs on H-NPCs were kinetically fast and highly stable, leading to a high electrochemical performance. In addition, H-NPCs-based lithium ion hybrid capacitors achieved a high specific energy of ~220Wh kg-1 and a specific power of ~5400Wkg-1 with a long-term cycle life of over 500 cycles, demonstrating the usefulness of the VD-PCBs-based H-NPCs.