Study on Morphology Control and N-Doping of Porous Carbon Cathode for Zinc-Ion Hybrid Supercapacitor

Abstract

The growing use of renewable energy accelerates research on energy storage systems (ESSs) to accommodate the distributed nature of power sources and fluctuation in supply and demand. The commercially adopted system for ESSs is lithium-ion batteries (LIBs), which are actively studied for recent years. However, the use of rare resources such as Li, Ni and Co, as well as volatile and flammable organic electrolytes, cause environmental problems and fire hazards. Therefore, new type of environmentally friendly and safe ESSs are needed to meet the future demand.In this regard, the aqueous zinc-ion hybrid supercapacitors (ZHSCs) are assumed as the remarkable candidate for ESSs. ZHSCs consist of battery-type Zn metal anode and capacitor-type carbon cathode with different ion storage mechanisms. Because they have unique design of electrodes, they can combine the advantages of existing batteries and supercapacitors such as high energy/power density and long cycle lifespans. In addition, the use of Zn metal has lots of benefits compare to Li-based electrodes, for example, Zn has low standard potential (-0.76 V vs. standard hydrogen electrode), high theoretical gravimetric/volumetric capacity (820 mAh g-1 and 5845 Ah/L) and relatively low cost.In ZHSCs, they store energy through an electric double-layer capacitive mechanism in which Zn2+ ions or anions are adsorbed on the surface of carbon cathode while Zn2+ ions are inserted/extracted in Zn metal anode, or a pseudo-capacitive mechanism generated by redox reaction with functional groups on the carbon surface. To achieve higher electrochemical performance of ZHSCs, previously reported research focused on modifying carbon cathode with highly porous structure or introducing heteroatoms. The porous carbon with abundant micropores (<2nm) and mesopores (2-50nm) can be used to provide high surface area for ions to adsorb/desorb and serve as channels for electrolytes and ions to diffuse rapidly. Besides, modifying surface of carbon cathode by doping heteroatoms (N, O, S, B, P) can improve wettability and conductivity of electrode and produce additional capacitance from redox reactions.However, although many researchers have studied to reform carbon cathode, they rarely study the effect of carbon’s morphology on its electrochemical performance. In the electrode level, irregularly shaped carbon could result in poor volumetric capacitance and high resistance properties caused by low mass loading and poor particle contact. The proposed morphology to solve these issues is spherical shape. As the spherical particles have no edges, they hardly damaged and have good powder fluidity between particles, leading to higher tap density or electrode density. They can also provide short diffusion path for ions to move compared to irregularly shaped particles, since the thin electrode can be made using the same amount of active material. This can enhance overall electrochemical performance of ZHSCs by lowering internal resistance and improving high rate capability of electrodes.In this respect, we suggest “spherical and uniform shape” carbon cathode along with the pore modification and heteroatom doping to compare the factors affecting electrochemical performance in ZHSCs. Thus, we prepared three different types of mesoporous carbons, which are spherical mesoporous carbon foam (S-MCF), N-doped spherical mesocellular carbon foam (S-MCF-N) and irregular shaped mesocellular carbon foam (MSUF-C). The improved electrode density of ZHSCs are achieved in spherically-controlled carbons (S-MCF and S-MCF-N) with 2.9 times higher tap density than irregularly shaped carbon (MSUF-C). Besides, the introduction of nitrogen to carbon (S-MCF-N) affords abundant active sites on the surface of carbon, increasing electrical conductivity and inducing additional pseudo-capacitance. Figure 1