Research progress on carbon-based zinc-ion capacitors

Abstract

Zinc-ion capacitors (ZICs), which consist of a capacitor-type electrode and a battery-type electrode, not only possess the high power density of supercapacitors and the high energy density of batteries, but also have other advantages such as abundant resources, high safety and environmental friendliness. However, they still face problems such as insufficient specific capacitance, a short cycling life, and narrow operating voltage and temperature ranges, which are hindering their practical use. We provide a comprehensive overview of the fundamental theory of carbon-based ZICs and summarize recent research progress from three perspectives: the carbon cathode, electrolyte and zinc anode. The influence of the structure and surface chemical properties of the carbon materials on the capacitive performance of ZICs is considered together with theoretical guidance for advancing their development and practical use.

1. Introduction

To address the challenges of global energy scarcity and environmental pollution, the urgent development and utilization of green renewable energy sources such as solar energy, geothermal energy, and tidal energy are necessary. However, many renewable energy sources are intermittent and time-sensitive, making continuous utilization of these energy challenging. Consequently, efficient and stable energy storage and conversion devices have been developed to harness renewable energy. Among these energy storage devices, batteries[1–3] and supercapacitors[4,5] have emerged as the most promising options. The electrochemical performance of batteries and supercapacitors varies depending on different energy storage mechanisms. Batteries achieve energy storage through the intercalation/de-intercalation of cations on the electrodes, involving redox reactions during charging and discharging[1]. Although rechargeable batteries offer high energy density, high specific capacity, wide operating voltage, and low rate of self-discharge, challenges such as shortages of lithium resource, low power density, short cycle life and safety concerns still exist[6]. Supercapacitors achieve energy storage through the rapid adsorption/desorption of ions, as well as pseudocapacitance reactions on the electrode surface during charging and discharging[5]. Although supercapacitors boast high power density, outstanding rate performance, long cycle life, and wide temperature range, their application is still limited by insufficient energy density[4]. Despite their respective advantages and disadvantages, both batteries and supercapacitors play vital roles in various fields and require further development to address their limitations.

To combine the advantages and overcome the disadvantages of batteries and supercapacitors, hybrid capacitors, also called asymmetric electrochemical capacitors, have been successfully developed. These hybrid capacitors effectively integrate the intercalation/de-intercalation mechanism from electrodes of batteries and the rapid adsorption/desorption mechanism from electrodes of supercapacitors, thereby balancing and optimizing the electrochemical performance simultaneously[7]. Initially, hybrid capacitors with univalent metal ion such as Li+, Na+ and K+ were studied, but these were constrained by resource scarcity and safety concerns. Subsequently, it has been discovered that polyvalent metal ions, such as Zn2+, Ca2+, Mg2+ and Al3+, offer more stable thermodynamics and faster charge transfer kinetics compared to univalent metal ions. This advantage arises because they have the capacity to store multiple electrons[8,9] although the dynamic diameters of multivalent ions are similar to those of univalent ions. Among polyvalent metal ions, zinc stands out due to its plentiful resources, high theoretical specific capacity, and suitable redox potential, making it a promising candidate for hybrid capacitors.