Hierarchical metal-organic framework derived nitrogen-doped porous carbon by controllable synthesis for high performance supercapacitors

Highlights

• The N-doped porous carbon has metal-organic frameworks.

• The N-doped porous carbon is synthesized by controllable condition and appropriate temperature.

• The N-doped porous carbon shows excellent performances.

Abstract

The N-doped porous carbon (HPNCs) are prepared via facile carbonization of zeolitic imidazolate frameworks (ZIF-8) by controllable synthesis and appropriate temperature. In this work, we have discussed different preparation methods and conditions about HPNCs. The material which is carbonated at 800 °C and used zinc acetate during preparation process exhibits excellent electrochemical performance with a high specific capacitance of 245 F g^− 1 in 6 M KOH electrolyte at a current density of 1 A g^− 1 and low resistance. Moreover, the symmetric supercapacitor of HPNCs-C8//HPNCs-C8 exhibits a special energy density about 24.8 Wh kg¹ at the power density of 0.79 kW kg^− 1 in the wide voltage region of 0–1.8 V. Such outstanding electrochemical behavior shows that the HPNCs-C8//HPNCs-C8 symmetric supercapacitor is a promising practical energy-storage facility.

Introduction

From an economic perspective, energy storage devices have increased in demand. As a unique electrical energy storage facility, supercapacitor is widely used because of the high-energy density, fast charge and discharge, well electric conductivity and long-cycling stability. According to the mechanism of charge storage, supercapacitors are divided into two types, pseudocapacitor based on the Faraday reaction at the interfaces of electrolyte and electrode materials to obtain high specific capacitance [1], [2] such as conducting polymer electrodes or metal oxides and electrical double-layer capacitors (EDLCs) which store energy based on charge transfer between electrolyte and electrode materials including carbon material electrodes [3], [4], [5], [6]. Carbon materials have increasingly become popular because of good chemical stability, superior conductivity and highpower density [7]. However, the large aperture greatly impedes the specific capacitance of carbon materials [8]. Thus, ideal aperture with bigger size than the de-solvated ions is important, which is associated with proper selection of electrolyte and rational pore size distribution of material rather than merely high surface areas [6].

Above the previous report, it is well known that carbon materials can be successfully prepared by varieties of porous carbon precursors, such as organic molecules [9], [10], fossil materials [11], [12], polymers [13], [14], [15], [16] and biomass materials [17], [18], [19], [20], [21], [22]. Among these, organic molecules, having stable structures and commercial utilities, can become one of the candidates in aspects of the preparation of porous activated carbon materials with specific capacitance enhancement. Particularly, N-doped porous activated carbon can be synthesized by a single suitable precursor, and N-doped surfaces endow activated carbon with the characteristic of improvement of wettability between electrolyte and electrode materials, which plays an important role in enhancing the specific capacitance performance through increasing the active sites for pseudocapacitance [23], [24], [25]. The strategies for synthesizing N-doped porous activated carbon materials involves in post-processing method which achieved by adding into the high N-doped carbon (like urea, amine) [13], [27], [28] and firsthand carbonization method by direct pyrolysis at a definite temperature [26]. Although the former method can retain most of its properties, it takes several problems such as long- time consumption and uneven nitrogen doping. However, the first technique can be controlled by chemical methods to make N-doped more uniform, while the complicated technological conditions remain a challenge. In this aspect, the morphology of high N-doped carbon precursors is allowed for regulating the electrochemical properties, and the direct carbonization synthesis is closer to commercialization. Nitrogen atoms favorably adjust the electronic bulk and electrochemical properties, which reduce charge conversion resistance and improve wettability for further enhancing specific capacitance.

Recently, metal organic frameworks (MOFs) which have an inorganic-organic hybrid structure is a novel multifunctional crystal style and have wide applications in many areas such as catalysis [29], selective adsorption and separation [30], drug delivery [31] and gas storage [32], and for constructing various nanostructured electrodes; they prove to be promising precursors [33], [34], [35], [36], [37], [38]. A zeolitic imidazolate frameworks (ZIF-8) as a N-doped subclass of MOFs possesses crystalline three-dimensional networks that metal ions or clusters were crossed tetrahedrally by imidazolate-mold linkers [33], [39], [41]. However, the method is easy to accumulate during synthesis, thus presenting a challenge.

In this work, ZIF-8 with imidazole groups as precursors [42] compounded through the controllable synthesis, and followed by direct carbonizations to achieve N-doped porous carbon materials (HPNCs). HPNCs also inherit the metal-organic framework morphology with large specific surface areas, as well as favor the transmission of electrons to enhance the conductivity. We also study the electrochemical behaviors of HPNCs on two preparation methods and different carbonization temperatures, and assemble a better symmetric supercapacitor to investigate its electrochemical properties; we demonstrate a worthwhile practical energy-store facility.