Revealing Anomalous High Capacitance of Individual Porous Nanoparticles Employing Nano-Impact Electrochemistry

Abstract

As a class of promising energy storage devices, supercapacitors have attracted great attention, regarding the deployment of high-power output in renewable energy technologies. The electrification of transportation and the dramatic increase of using portable electronic devices, require significant improvement in this field. Electric double-layer capacitors (EDLCs) are an important class of supercapacitors, in which the electrochemical double layer formed at the electrode/electrolyte interface is used to store energy. However, despite their high power density and remarkable cycle life, the prevalent application of EDLCs is limited because of their still insufficient energy density. Therefore, increasing the capacitance of EDLC electrodes is of great interest. One emerging approach for enhancing energy density has been employment of porous structures as novel electrodes for EDLCs [1].From a theoretical point of view, a conventional EDLC electrode can be considered as a parallel-plate capacitor that delivers a capacitance (C) according to C = ε A / d, where ε is the permittivity of the electrolyte, A is the electrode surface area, and d is the thickness of EDL. Consequently, increasing A of the EDLC electrodes using meso- or nanostructured materials is a classic approach toward realizing large specific capacitance and high energy density. Alternatively, it was experimentally proved that confinement effect on d in microporous EDLCs (pore size < 1 nm) leads to an anomalous increase in the specific capacitance as the pore width decreases. These findings suggest non-classical behaviors of EDLs emerged in sub nanometer pores. However, except for a few theoretical simulation results [2], there has been very limited research carried out on the application of the confinement effect in two-dimensional materials as an additional strategy toward high-energy density capacitors [3]. Herein, we propose single particle electrochemistry to study capacitive properties of individual porous nanoparticles, as an ideal candidate for fabrication of EDLCs.Employing “nano-impact” electrochemistry, an enhanced current is measured, which is caused by randomly colliding nanoparticles with a microelectrode [4,5]. In capacitive nano-impacts, the colliding particle causes the perturbation of a portion of the EDL with the subsequent charging-discharging processes which give rise to transient current-time events. This can be related to the electronic properties/capacitance of the particles [6]. Potentially this method will provide a powerful tool to study the properties of individual nanoparticles without any ensemble effects and overlaying effects of neighboring particles. In the present study, capacitance of individual porous platinum nanoparticles is investigated, by using PtNP impacts at platinum microelectrode in aqueous solution. These experiments are exploited to extract physico-chemical information about the structural effects on the energy density of EDLCs. The gained insights will help to improve our fundamental understanding of the EDL formation and will, thus, provide guidance towards the development of new and improved EDLCs.