Evaluating the influence of physicochemical characteristics of porous carbons on their cathode performance in aqueous zinc-ion hybrid capacitors

Owing to their high surface area, tunable pore size/hydrophilicity and good electrical conductivity, porous carbons are intensively investigated as a cathode material for aqueous zinc-ion hybrid capacitors (AZICs). Still, the correlation between these features and cathode performance has remained relatively underexplored. Here, we investigate a set of porous carbons with different characteristics as AZIC cathodes, and reveal how their properties impact some of their key performance metrics, such as capacity and rate capability. The minimum pore width that took part in AZIC charge storage was estimated to be ∼0.65 nm by a methodology that is generally applicable to other types of capacitors and electrolytes. The effect of pore width on rate capability was also explored. Specifically, supermicropores (0.7–2 nm wide) contributed to rate capability only up to moderately high current densities (<5 A g −1 ), the presence of small mesopores being required to provide significant capacity retention at higher currents. Further, the hydrophilicity of the porous carbons was quantitatively evaluated in the form of an areal density of hydrophilic sites from the analysis of water adsorption isotherms. However, this parameter did not have any noticeable impact on their performance as AZIC cathodes, at least in the areal density range investigated here. • Correlation between porosity and cathode performance of porous carbons probed in AZIC. • Minimum pore width involved in charge storage in AZICs estimated to be 0.65 nm. • This critical width yields best correlation between carbon capacity and surface area. • Pore hydrophilicity of the carbon has no impact on its performance as AZIC cathode. • Rate capability improved by mesoporosity and increased electrical conductivity.