Improvement of a commercial activated carbon for organic electric double-layer capacitors using a consecutive doping method

doi:10.1016/j.carbon.2020.01.024

Carbon

Volume 160, 30 April 2020, Pages 45-53

https://doi.org/10.1016/j.carbon.2020.01.024 Get rights and content

Highlights

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A consecutive doping method was used to improve a commercial activated carbon.
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Using the method, a carbon with large surface area and high conductivity was prepared.
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The prepared carbon showed good electrochemical properties in an organic EDLC system.
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The obtained results were highly improved results compared to the commercial carbon.
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The consecutive doping is an effective method for preparing carbons for organic EDLC.

Abstract

In this study, a consecutive doping method was used to improve the electrochemical properties of a commercial activated carbon (YP50f) as an electrode material for organic electric double-layer capacitors (EDLCs). In the consecutive doping method, nitrogen doping is carried out after an oxygen-doping process. Using the consecutive doping method, we prepared a carbon material with both a large specific surface area and high electrical conductivity. The specific surface area of the carbon material was mainly developed during the oxygen-doping step, whereas electrical conductivity was enhanced in the nitrogen-doping step. On the basis of the properties of the consecutively doped carbon material, we expected the prepared carbon material to exhibit good electrochemical properties when used as electrode materials for organic EDLCs. From the results of electrochemical characterizations, we observed that the consecutively doped carbon material showed a highly improved capacitance compared to the YP50f under all of the applied charge–discharge rates. Consequently, we concluded that the consecutive doping method is an effective method to prepare carbon materials with good electrochemical properties as electrode materials for organic EDLCs as a result of both their enhanced specific surface area and their enhanced electrical conductivity.

Graphical abstract

Introduction

Electric double-layer capacitors (EDLCs) are being constantly studied as promising candidates for next-generation electrical energy storage devices. The operating principle of EDLCs is based on charge separation in the Helmholtz double-layer at the interface between an electrode and an electrolyte. From this working principle, EDLCs have various characteristics such as fast charge–discharge rate, durable life cycle, and high power density [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]]. Therefore, EDLCs have been used in various fields that require high power density and many researchers have recently attempted to replace rechargeable secondary batteries with EDLCs. However, these attempts have encountered problems because the energy density of EDLCs is relatively low compared to that of rechargeable secondary batteries. In this respect, increasing the energy density of EDLCs is a main research theme, and developing electrode carbon materials that can increase the capacitance of EDLCs is considered a key approach to solving the energy density problem of EDLCs [[7], [8], [9]]. A large surface area and high electrical conductivity are well known requirements for carbon materials used as electrode materials for EDLCs with high energy density.

Activated carbon materials are the most commonly used active materials for commercial EDLCs because of their large specific surface areas [[13], [14], [15], [16], [17], [18], [19], [20], [21]]. MSP20 (Kansai Coke & Chemicals Co., Japan), the CEP series (Power Carbon Technology Co., Korea), and the YP series (Kuraray Co., Japan) are representative examples of commercially available high grade activated carbon materials for EDLC electrodes. Some of the properties of the aforementioned commercial activated carbon materials are summarized in Table S1 [[18], [19], [20], [21]]. A large specific surface area of activated carbon materials certainly produces EDLCs with high capacitance properties because it maximizes the amount of electrolyte ions adsorbed onto the electrode surface. However, because of their high porosity, activated carbon materials have a relatively low electrical conductivity compared with other carbon materials [[22], [23], [24]]. The electrical conductivity of electrode carbon materials for EDLCs is a key factor affecting their capacitance properties during high-rate charge–discharge processes. For this reason, the capacitance properties of activated carbon materials as electrode materials for EDLCs rapidly degrade with increasing charge–discharge rate [6,25,26]. This loss of capacitance is the main issue for researchers in the field of activated carbon and in the field of EDLCs. That is, the electrical conductivity of commercially available activated carbon materials should be enhanced to produce EDLCs with high energy and power densities.

Doping is a well-known method for enhancing the electrical conductivity of carbon materials. In particular, doping processes using oxygen and nitrogen moieties, in which atoms have more valence electrons than carbon atoms, are representative methods to improve the electrochemical properties of EDLCs [[27], [28], [29], [30], [31], [32], [33], [34]]. For this reason, many researchers have reported using oxygen- or nitrogen-doped carbon materials in conjunction with various doping methods to prepare EDLCs with not only high power density but also high energy density [[28], [29], [30], [31],33]. According to the paper reported by Ishimoto et al., however, oxygen-doping processes adversely affect EDLCs in organic electrolyte systems because oxygen functional groups from oxygen-doping processes exhibit high reactivity with organic solutions for electrolytes [35]. On the other hand, nitrogen-doping methods have been actively reported as good methods to enhance electrochemical properties of EDLCs even in organic electrolyte systems [30]. Considering that commercial EDLCs operate in organic electrolyte systems, nitrogen-doping processes are commercially more important than oxygen-doping processes. In this respect, researchers have actively attempted to increase the nitrogen content in carbon materials using various methods to enhance the electrochemical properties of organic EDLCs [18].

In this study, we aim to prepare a carbon material with large surface area and with high electrical conductivity for assembly into organic EDLCs with high electrochemical performance. For this research purpose, we attempted to increase the nitrogen content of a commercially available activated carbon. In this regard, we employed various doping methods: an oxygen doping, a nitrogen doping, and a consecutive doping method. In the consecutive doping method, nitrogen doping is carried out after an oxygen-doping process. Through various characterization methods, we confirmed that the oxygen-doping method can be used to prepare carbon materials with a large surface area and that the nitrogen-doping method can be used to improve the electrical conductivity of carbon materials in an organic electrolyte system. As a result, we successfully transformed a commercial activated carbon material into a carbon material with both a large specific surface area and high electrical conductivity using the consecutive doping method. In addition, the carbon material prepared using the consecutive doping method exhibited highly improved capacitance properties compared with the commercial activated carbon when used as an electrode material for organic EDLCs. Consequently, we concluded that the consecutive doping method is effective for preparing carbon materials for organic EDLCs with enhanced electrochemical properties.

Section snippets

Doping processes

In this study, a commercially available activated carbon (YP50f; YP, Kuraray Co., Japan) was doped using various doping agents and methods. For oxygen doping, YP was heat-treated under air flowing at 50 mL min⁻¹. The oxygen doping was conducted at 500 °C for 1 h in a tube furnace; the oxygen-doped YP is hereafter referred to as YPO. For nitrogen doping, ammonia gas mixed with inert gas was used as a doping agent. The composition of the ammonia–inert gas mixture was fixed at an ammonia/inert gas

Elemental composition of the prepared carbon materials

To confirm the elemental compositions of the YP, YPO, YPN, and YPON, we conducted EA (Table 2). The EA results confirm that YP has a substantial oxygen content of 8.71% and a relatively low nitrogen content of 0.19%. After the oxygen-doping method, the oxygen content of YP increased from 8.71% to 12.18%. By contrast, YPN, which is nitrogen-doped YP, showed a higher nitrogen content than YP; in addition, YPON, which was prepared by the consecutive doping method, also showed increased nitrogen

Conclusions

In this study, we prepared a carbon material with not only a large surface area but also high electrical conductivity with the objective of obtaining electrode materials for organic EDLCs with good electrochemical properties. For this purpose, various doping methods—oxygen doping, nitrogen doping, and consecutive doping—were applied to a commercial activated carbon material. The consecutive doping method is a doping method in which nitrogen doping is used after oxygen doping. We confirmed that

CRediT authorship contribution statement

Inchan Yang: Conceptualization, Investigation, Data curation, Writing - original draft. Jihoon Yoo: Investigation, Data curation. Dahye Kwon: Visualization, Investigation. Dalsu Choi: Writing - review & editing. Myung-Soo Kim: Methodology, Writing - review & editing. Ji Chul Jung: Conceptualization, Funding acquisition, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2018R1D1A1B07048128).

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Improvement of a commercial activated carbon for organic electric double-layer capacitors using a consecutive doping method

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Doping processes

Elemental composition of the prepared carbon materials

Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgments

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