The relationship between the high-frequency performance of supercapacitors and the type of doped nitrogen in the carbon electrode

Abstract

Nitrogen doping has been widely used to improve the performance of carbon electrodes in supercapacitors, particularly in terms of their high-frequency response. However, the charge storage and electrolyte ion response mechanisms of different nitrogen dopants at high frequencies are still unclear. In this study, melamine foam carbons with different configurations of surface-doped N were formed by gradient carbonization, and the effects of the configurations on the high-frequency response behavior of the supercapacitors were analyzed. Using a combination of experiments and first-principle calculations, we found that pyrrolic N, characterized by a higher adsorption energy, increases the charge storage capacity of the electrode at high frequencies. On the other hand, graphitic N, with a lower adsorption energy, increases the speed of ion response. We propose the use of adsorption energy as a practical descriptor for electrode/electrolyte design in high-frequency applications, offering a more universal approach for improving the performance of N-doped carbon materials in supercapacitors.

1. Introduction

High-frequency supercapacitors (SCs), aiming at substitution for aluminum dielectric capacitors (AECs), show great potential in the development trend of lightweight and miniaturization of electronic products[1–4]. To build better high-frequency supercapacitors, the central role is developing electrode materials that can rapidly establish the electrical double layer at high frequencies. Carbon materials including aligned arrays[5–8], macroporous films[9–11], organic frameworks[12,13] and their composites[14–16] have been studied to balance ion transport and charge storage at high frequencies, due to their excellent physical and chemical properties such as good electrical conductivity, high chemical stability and pore tenability. Unlike traditional porous electrodes, the open surface structure characterized by high accessibility of electrolyte and a rapidly established electrical double layer is the key feature to realize the high-frequency performance of carbon electrodes[17–19]. Therefore, the progressive electrode-electrolyte interface is crucial to further improve the high-frequency response ability of SCs.

Doping heteroatoms such as nitrogen, oxygen, sulfur, and phosphorus into a carbon matrix is an effective method to regulate the interface properties between carbon electrodes and electrolytes[20,21]. Specifically, N-doping in carbon electrodes is being explored for high-frequency SCs due to its unique electronic and geometrical structures in the carbon lattice[22–25]. Shi and co-workers applied N-doped carbon electrodes in high-frequency SCs with an enhanced area-specific capacitance of 478 μF cm−2, while maintaining an excellent phase angle of −81.2° and τRC of 203 μs at 120 Hz[26]. The N doping endows the nanocarbon electrodes with additional capacitive active sites and improved electrode conductivity, thereby promoting electronic migration and ionic diffusion[27]. Interestingly, the N-doped nanocarbon electrodes prepared by Grobert and co-workers show an enhanced capacitance of 545 μF cm−2 compared to the pristine electrode but with a cut-off frequency drop of more than 500 Hz, which arises from slow charge transfer at the N-doped sites[28]. Hence, it is unreasonable to attribute the attenuation of high-frequency performance to N content alone in the carbon electrode, ignoring the control of N species.