Electrochemical capacitive behaviors of ordered mesoporous carbons with controllable pore sizes

Abstract

Ordered mesoporous carbons (OMCs) with controllable pore sizes in the range of 4–10 nm are prepared by a template procedure using 2D hexagonal MSU-H and 3D cubic KIT-6 as hard templates and boric acid as the pore expanding agent. The electrochemical performances of the as-synthesized OMCs as electrode materials for electrochemical double layer capacitors (EDLCs) are characterized by cyclic voltammetry (CV), galvanostatic charge/discharge (GC) and electrochemical impedance spectroscopy (EIS) experiments in 30 wt% KOH electrolyte. The influence of the pore size distributions of OMCs on the electrochemical capacitive performances is discussed. The prepared OMCs exhibit good capacitive behaviors with the specific capacitance values ranging from 143 to 205.3 F g⁻¹ at a voltage scan rate of 5 mV s⁻¹ and 81 to 86% retained at a high scan rate of 100 mV s⁻¹. OMC-M-2 shows the highest specific surface capacitance value of 27.5 μF cm⁻² at 5 mV s⁻¹ with a peak pore size of 7.8 nm and a Brunauer–Emmet–Teller (BET) surface area of 729.3 m² g⁻¹. The analysis of two kinds of pore symmetries of OMCs with the same pore size of about 6.5 nm shows that the 3D cubic OMC exhibited superior capacitive performance than the 2D hexagonal OMC.

Introduction

Over the last decade, OMCs have been studied widely in many fields such as adsorbents, catalysts, gas storage, and biosensors due to their narrow pore size distributions, high surface areas and high pore volumes [1], [2], [3], [4], [5]. Recently, many researches have demonstrated that OMCs have promising electrochemical capacitive properties in the application of EDLCs [6], [7], [8], [9]. EDLCs utilize the electric double layer formed at the electrode/electrolyte interface where charges are accumulated on the electrode surfaces and ions of opposite charge compensate them. On the basis of this mechanism, the electrode material should have a high surface area for charge accumulation. Activated carbons with high surface area are currently used as the electrode materials for EDLCs due to their low cost and a wide variety of carbon precursors. However, activated carbons that only contain micropores (<2 nm) have no interconnected porous network with sufficient open pore windows for electrolyte wetting and rapid ionic transport. Moreover, even if the micropores are wetted by the electrolyte, the ionic motion is still restricted in such small pores, inducing the high rate capability unrealised. Considering the above limitations, ordered mesoporous carbons with regularly interconnected mesopores (2–50 nm) are desired for EDLCs electrodes owning to that the ordered mesoporous channels and a noticeable volume of micropores can provide a large surface area for dispersion of the active sites [10], [11], [12], [13], [14]. Thus, OMCs should have better electrochemical capacitive properties than conventional activated carbons at high current densities. Xing et al. [15] prepared a series of highly OMCs with 3D cubic and 2D hexagonal spacer group, the results indicated that there are different electrochemical capacitive properties between the two kinds of materials with different pore structures, and OMC-3D demonstrated excellent high-frequency performances due to its higher surface area in pores larger than 3 nm. Unfortunately, the capacitive performance of conventional OMCs is still not satisfying owning to the low surface capacitance. Generally speaking, the surface capacitance of OMC can achieve 20 μF cm⁻² [8], but it is only about 8–10 μF cm⁻² according to the reported literatures [16], [17], [18]. Therefore, systemic research into the effect of pore structure of OMCs on the electrochemical capacitive performance is required to optimize the capability of the OMCs. Herein, OMCs with controllable pore size distribution and different pore symmetries were synthesized by the template method and the influence of pore structure on the capacitive behaviors was discussed in detail.

The template carbonization method is very attractive to synthesize the OMC materials for the reason that the carbon structure in terms of various aspects, such as pore structure and microscopic morphology, is easily controllable. Since the first synthesis of ordered mesoporous carbon, denoted as CMK-1, was reported by Ryoo et al. using the MCM-48 silica with cubic Ia3d symmetry as the template [19], many kinds of OMC materials with different structures have been prepared using various types of silica templates [20], [21], [22], [23], [24]. Recently, Lee et al. reported the synthesis of OMC materials with tunable mesopore sizes in the range of 3–10 nm employing boric acid as the pore expanding agent [25]. According to the pore expansion mechanism, the boric acid was mixed with the carbon precursor firstly, and then the boron species spontaneously separated from the mixture of boron and carbon precursors to the silica surface of mesoporous silica template during the carbonization process. Subsequently, the boron oxide and borosilicate nanolayers were formed between the silica and carbon frameworks. By removal of the silica and the boron layer using HF solution, the OMC materials with an increased pore size was obtained finally. A 2D hexagonal mesoporous silica, MSU-H, was used as the silica template in their previous report [26].

In the present work, we synthesized two kinds of OMC materials with controllable pore sizes using the aforementioned 2D hexagonal MSU-H and cubic KIT-6 (Ia3d), which contains 3D bicontinuous channel networks, as hard templates. The sucrose was used as the carbon precursor and the boric acid as an agent aiming to expand the pore sizes gradually by controlling the amount of the boric acid during the process of carbonization. The prepared OMC materials were studied as electrode materials for supercapacitor applications and the relationship between the pore structures and electrochemical capacitive properties of the carbon materials was investigated.