Modulating charge storage mechanism of cobalt-tungsten nitride electrodes using in situ formed metal-p-n heterojunction for ultrahigh energy density supercapattery

Abstract

Supercapatteries combine both diffusion-controlled and capacitive charge storage mechanisms to simultaneously deliver exceptional power density and energy density. Though developing materials that combine both charge storage mechanisms for use as supercapattery electrodes is difficult, one solution has been to modify a material that inherently has one of these mechanisms so that it also exhibits the other form of charge storage. In the present study, we modulate the electrochemical reconstruction of a W5N4 electrode to modify its charge storage from pure battery-type to pseudocapacitive, thus enhancing its specific capacity and cyclic stability. This modulation was carried out by embedding Co in W5N4 (Co-W5N4), leading to the formation of W and N-doped Co(OH)2 as an active phase and producing a high specific capacity of 2211 C g−1 at 2 A g−1. The incorporation of a thin layer of TiO2 (Co-W5N4/TiO2) through atomic layer deposition minimized tungsten dissolution during reconstruction, thus generating in-situ a dynamic metal–p-n junction heterostructure (Co-W5N4||Co(OH)2||TiO2) that modulated the charge flow, achieving high-rate capability (retaining 53.1 % at 50 A g−1 compared to 2 A g−1) and cyclic stability (82.8 % after 100,000 cycles at a high current density of 40 A g−1). As illustrated by operando electrochemical impedance spectroscopy and physicochemical analysis, the dynamic response of metal–p-n junction sustains the pseudocapacitive charge storage mechanism at high current rate. This study thus demonstrates the formation of metal-p-n heterojunction and describes its dynamic response under actual charge/discharge operating conditions, providing useful information for the design of energy storage electrodes and energy conversion catalysts.