Electrochemical Transport Phenomena in Hybrid Pseudocapacitors under Galvanostatic Cycling

Abstract

This study aims to provide insights into the electrochemical transport and interfacial phenomena in hybrid pseudocapacitors under galvanostatic cycling. Pseudocapacitors are promising electrical energy storage devices for applications requiring large power density. They also involve complex, coupled, and multiscale physical phenomena that are difficult to probe experimentally. The present study performed detailed numerical simulations for a hybrid pseudocapacitor with planar electrodes and binary, asymmetric electrolyte under various cycling conditions, based on a first-principles continuum model accounting simultaneously for charge storage by electric double layer (EDL) formation and by faradaic reactions with intercalation. Two asymptotic regimes were identified corresponding to (i) dominant faradaic charge storage at low current and low frequency or (ii) dominant EDL charge storage at high current and high frequency. Analytical expressions for the intercalated ion concentration and surface overpotential were derived for both asymptotic regimes. Features of typical experimentally measured cell potential were physically interpreted. These insights could guide the optimization of hybrid pseudocapacitors. Electrochemical capacitors (ECs) are promising electrical energy storage devices for applications requiring large power density, rapid response, and/or long cycle life.1-3 They can be divided into two categories, namely electric double layer capacitors (EDLCs) and pseudocapacitors. Both types of devices consist of two porous electrodes on either side of a separator impregnated with electrolyte. EDLCs store electric charge within the electric double layer (EDL) consisting of a layer of electronic charge at the surface of the electrode and an oppositely charged layer of ions in the electrolyte.1,4 Pseudocapacitors combine the energy storage mechanisms used in batteries with those used in EDLCs by storing electric charge chemically via redox reactions and electrostatically within the EDL.1,4,5 The electrical performance of pseudocapacitors closely resembles that of EDLCs rather than that of batteries despite their use of faradaic charge storage. 1,5,6 In fact, an ideal battery operates at a constant cell potential independent of its state of charge (SOC), whereas the cell potential of an EDLC or a pseudocapacitor varies continuously with its SOC.3 Finally, hybrid or asymmetric pseudocapacitors can be designed by pairing a redoxactive or pseudocapacitive electrode (e.g., TiO 2 , MnO 2 , Nb 2 O 5 ) with an EDLC-type electrode made of carbon.3 In general, electrochemical capacitors exhibit electrical performance between that of batteries and that of dielectric capacitors.1-3 They typically have larger power densities, cycle life, and cycle efficiencies than batteries as well as much larger energy densities than dielectric capacitors.2 Among electrochemical capacitors, pseudocapacitors yield larger capacitances and energy densities than EDLCs because they combine faradaic and EDL charge storage and can accommodate more...

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