The preparation of porous carbon materials derived from bio-protic ionic liquid with application in flexible solid-state supercapacitors

Highlights

• N/P co-doped carbon materials were facilely prepared from Arg[H₂PO₄]₂ for solid-state flexible supercapacitors derived.

• Arg-2–900 possessed micro-meso-macropore hierarchically porous structure.

• The symmetric supercapacitors exhibited wide work voltage in 6 M KOH and 1 M Na₂SO₄, respectively.

• The devices displayed high energy density of 8.8 Wh kg⁻¹and power density of 320 W kg⁻¹ in 6 M KOH.

Abstract

Ionic liquids have attracted much more attentions for its wide application in catalyst, green solvents and carbon precursors. Herein, N/P co-doped porous carbon materials with developed pore structure were facilely prepared from the phosphoric acid protic ionic liquid of arginine (Arg[H₂PO₄]₂) and (NH₄)₂HPO₄. The former acted as the carbon precursor, heteroatom source and mesopore generator, while the latter worked as the activator which had great impact on the pore distribution and microstructure. The porous carbon materials were characterized by SEM, XRD, Raman and N₂ adsorption analysis in system, indicating that Arg-2–900 was promising electrode materials for supercapacitors. It exhibited high specific capacitance retention of 94% after 10000 cycles with stable electric double layer capacitors. The assembled symmetrical supercapacitors exhibited a wide voltage window in alkaline electrolyte and neutral aqueous electrolyte, displaying high energy density and power density, respectively. In addition, the solid-state supercapacitors were prepared and showed good flexibility after bending the flexible supercapacitor cell at different angles. The results demonstrated the successful synthesis of N/P co-doped porous carbon materials form Arg[H₂PO₄]₂ and broad application in wearable storage device.

Introduction

Ionic liquids with high thermal stability, intrinsic heteroatoms and designability played more and more important role in various fields, such as catalysis, organic synthesis, adsorption and supercapacitors (Jens and Arne, 2012). For supercapacitors, it usually worked as electrolyte in which electrode materials could exhibit stable electrochemical behavior at wide work voltage. In recent years, it has been reported that in-situ heteroatom co-doped functional carbon materials could be synthesized from ionic liquids rich in heteroatoms (Tamar and Calum, 2015). Introducing P heteroatom into carbon materials can increase the wettability and decrease the charge transfer resistance of the electrolyte, which are benefit to the capacitive performance (Jurcakova et al., 2009, Panja et al., 2015, Li et al., 2018a, Li et al., 2018b, Park et al., 2018). Meanwhile, nitrogen doping could modify the shift of Fermi level of valence band in the carbon materials, increasing the electrochemical reactivity between electrolyte and electrode (Wang et al., 2016, Li et al., 2019a, Li et al., 2019b, Li et al., 2019c, Xia et al., 2017a, Xia et al., 2017b). The favorable characteristics of multiple heteroatoms doping will generate more active sites as well as facilitate electron ions transfer, contributing the specific capacitance of supercapacitors by faradaic pseudocapacitance together (Zhang et al., 2017, Hu et al., 2016; Wang et al., 2012). It was a facilely way to develop N/P co-doped carbon materials from amino acid protic ionic liquid rich in heteroatoms (Zhu et al., 2019).

In addition, P doped into carbon materials could widen the potential window by restraining the formation of unstable surface O-Ⅲ group which can enhance the oxidation stability at the positive potential and adsorb hydrogen at the negative potential respectively (Khan et al., 2020, Chen et al., 2018, Yan et al., 2018, Liu et al., 2014). Consequently, supercapacitors based on P co-doped carbon materials could operate under a wider voltage in aqueous electrolyte of 1 M H₂SO₄, 1 M Na₂SO₄ and 6 M KOH. According with the equation of E = 1/2 CV², the wider the work voltage is, the higher power density can be achieved (Li et al., 2020, Jyothibasu and Lee, 2020), indicating that it will be an effective approach to improve energy density of carbon materials with P doping for supercapacitors.

As the key component of supercapacitors, the electrode materials determined the performance of the capacitors decisively. In order to satisfy the rapid utilities of portable electronic products, more and more flexible and wearable storage devices based on different kinds of electrode have been constructed, such as wearable nanogenerators, flexible displays and foldable supercapacitors (Gelinck et al., 2004, Xiong et al., 2015, Yu et al., 2020, Zou et al., 2020). However, considering the real application of the wearable devices, the carbon-based flexible supercapacitors with excellent bending and electrochemical properties (Li et al., 2017, Lv et al., 2018, Xu et al., 2019, Xue et al., 2013) were considered as the most promising new generation wearable electronic products due to the fast charge-discharge capability, high stability and good mechanical property (Cheng et al., 2015, Chen et al., 2016, Guo et al., 2020, Wang et al., 2019a, Wang et al., 2019b, Wang et al., 2019c, Wang et al., 2019d, Zhang et al., 2020). Therefore, it shows great potential fabricating wearable supercapacitors from carbon-based electrode materials.

In this work, N/P co-doping hierarchically porous carbon materials were designed and synthesized for high-performance supercapacitors from Arg[H₂PO₄]₂ and (NH₄)₂HPO₄ as illustrated in Fig. 1. Due to the micro-meso-macropores structure and N/P co-doping, the as-prepared Arg-2–900 showed excellent electroconductivity and longtime cycling stability. Notably, the symmetric supercapacitors exhibited a high specific capacitance of 152 F g⁻¹ at 1 A g⁻¹, displaying high energy density and power density under wide work voltage in alkaline and neutral electrolyte. Additionally, the all solid-state symmetric devices assembled by Arg-2–900 displayed good flexibility and foldability, providing a new strategy for developing high performance electrode for wearable devices.