Elsevier

Electrochimica Acta

Volume 176, 10 September 2015, Pages 982-988
Electrochimica Acta

Nitrogen, oxygen and phosphorus decorated porous carbons derived from shrimp shells for supercapacitors

https://doi.org/10.1016/j.electacta.2015.07.094Get rights and content

Highlights

  • Nitrogen, oxygen and phosphorus decorated porous carbon is in situ synthesized by H3PO4 activation of shrimp shell.

  • Porous carbons with abundant pores and heteroatom functionalities are good electrode materials for supercapacitors.

  • The highest specific capacitance reaches 206 F g−1 at 0.1 A g−1 in 6 M KOH solution in KOH electrolyte.

  • The excellent electrochemical properties depend on the synergistic effect of N, O, P -doped nature and porous structures.

  • The energy density and power density of obtained carbons can be substantially improved by widening the operation voltage window with the assistance of doped P.

Abstract

Nitrogen (N), oxygen (O) and phosphorus (P) decorated porous carbon (PC) is in situ synthesized for supercapacitors by H3PO4 activation of shrimp shell. The results show that shrimp shell with 5.32 wt% nitrogen, 30.1wt% carbon and 40.2 wt% oxygen is a good carbon precursor, and H3PO4 plays an important role on the formation of porous structure and P doping. The contents of N, O and P functional groups in the resultant carbons along with their porosities are temperature-dependent. PCs with abundant pores and heteroatom functionalities are good electrode materials for supercapacitors in basic medium. Typically, PC with moderate N, O and P contents, large surface area and co-existence of mesopores with micropors exhibits a high specific capacitance of 206 F g−1 at 0.1 A g−1 in 6 M KOH solution. Furthermore, the energy density and power density of obtained PC electrodes can be substantially improved by widening the operation voltage window with the assistance of doped P. Specifically, its energy density can increase greatly from 2.9 Wh kg−1 at 0.9 V to 5.2 Wh kg−1 at 1.1 V. The present study demonstrates a feasible and effective strategy to develop high-performance supercapacitors economically from waste biomass and also helps to understand the roles of heteroatom on the electrochemical performances of PC electrodes.

Introduction

Supercapacitors have attracted great attention owing to their high rate capability, long cycle life, large power density and low maintenance [1], [2]. Based on the different charge-storage mechanisms, supercapacitors can be divided into two categories: one is the electrical double-layer capacitors (EDLCs), as characterized by high power density and long cycle life, in which the capacity arises from the electrosorption of ions on porous carbon (PC) electrodes at the electrode/electrolyte interface [3], [4], [5], [6]. The other is the pseudocapacitors with high energy density but poor power density and cycle stability, its capacity comes from faradaic reactions at the electrode/electrolyte surface [3], [4], [5], [6], [7]. The electrode materials are the key components determining their capacitance performance, and PCs are the most reported electrode materials owing to their large surface area, tunable pore structure and good electrical conductivity [8], [9], [10], [11]. Many strategies have been carried for optimizing the energy density of capacitors while keeping a high power density for supercapacitors [12], [13], and pseudocapacitance materials are important candidates in this respect. Surface-functionalized carbon materials containing heteroatoms such as nitrogen or oxygen exhibit pseudacapacitance due to charge or mass transfer between the electrode and the ions of the electrolyte [14], [15]. It is known that oxygen functionalities are usually present on the surface of PC as a residue from the carbon source or a result of activation process. The pseudocapacitance contribution of nitrogen to the capacitance of PC has also been widely reported [16]. Various techniques have been focus on the introduction of nitrogen species into the carbon framework, which is achieved either by using nitrogen-containing precursors such as melamine and polyvinylpyridine or via treating carbon materials with ammonia gas [17], [18], [19]. Despite the great efforts on synthesis of nitrogen-doped carbon electrodes with high capacitive performance, the exploration of renewable biomass with lots of heteroatom as raw material to produce electrode materials would be more worthwhile considering the potential scale of supercapacitor applications [20].

Shrimp shell, a kind of food waste in high quantity, is a cheap and reliable biomass source without increasing competition for food. Shrimp shell is composed of nitrogen containing polysaccharide named as chitin (poly-b(1/4)-N-acetyl-D-glucosamine), which can be used as the precursor to directly produce nitrogen doped carbons by pyrolysis technique [21]. However, such carbons usually have poor capacitance when used as the supercapacitor electrode because of their relatively low porosity. Usually, physical or chemical activation of carbon precursors can be used to introduce pores into the resultant carbons, allowing good performance as electrodes for supercapacitors due to the synergistic effect of EDLCs and pseudocapacitors [22], [23]. Herein, we report a simple, sustainable preparation of nitrogen/oxygen/phosphorus (N/O/P) heteroatom decorated PCs via in-situ H3PO4 activation of shrimp shell. Following two advantages brought by H3PO4 as the activating agent can be actualized: (1) Porous structures can be introduced into the carbon materials, thus improve the power density of supercapacitors. (2) Phosphorus atom can be in situ introduced into the carbon framework, which can improve its energy density. As expected, the resultant PCs exhibit good electrochemical performances when used as supercapacitor electrodes.

Section snippets

Synthesis of N/O/P decorated PCs from shrimp shell

After being cooked and eaten, Bohai shrimp shell was used as the raw material in this study. The shell was sufficiently dissolved in 10 wt% HCl solution at room temperature until the complete removal of CaCO3, then recovered by filtration and washed abundantly with distilled water and dried at 80 °C for 4 h. Activation process was performed as follows. Firstly, above treated shell and phosphoric acid (H3PO4, 85 wt%) with a weight ratio of 1:3 were thoroughly mixed together and dried at 80 °C

Results and discussion

Fig. 1 illustrates the strategy for the synthesis of N/O/P decorated carbons with porous structures using shrimp shell as the carbon precursor by one-step H3PO4 activation. CaCO3 in shrimp shell was firstly removed by HCl, then in situ activated by H3PO4 at 400∼600 °C in Ar. PCs with 21-24 wt% of yield were obtained and used as electrode materials for supercapacitors.

Shrimp shell undergoes thermal decomposition and activation by H3PO4 during heat treatment, and lots of heteroatoms including N, O

Conclusions

N, O, P heteroatom decorated PCs for supercapacitors have been successfully prepared by one step H3PO4 activation of shrimp shell. The contents of N, O and P functional groups in the resultant PCs along with their porosities are temperature-dependent. PCs with abundant pores and heteroatom functionalities are good electrode materials for supercapacitors. N, O, P decorated PCs with suitable porous structure exhibit good capacitive behaviors, including large capacitance, long cycle life, high

Acknowledgements

This work is supported by the NSFC (Nos. 51372277, 50902066), China Postdoctoral Science Foundation (2013M530922, 2014T70253) and Program for Liaoning Excellent Talents in University (LJQ2014118) and the Fundamental Research Funds for the Central Universities (No.15CX08005A).

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