Nontemplating Porous Carbon Material from Polyphosphamide Resin for Supercapacitors

摘要

class="head no_bottom_margin" id="sec1title">IntroductionWith the depletion of conventional energy resources, green and sustainable energy conversion and storage technologies are attracting more and more attention. Among various energy storage devices, supercapacitors have their advantages of high charge-discharge rate, long cycle life, high energy conversion efficiency, etc. (). According to the energy storage mechanisms, supercapacitors can be categorized into electrical double-layer capacitors (EDLCs) and pseudocapacitors (). EDLCs store energy through ion adsorption-desorption at the electrode-electrolyte interfaces. The typical materials for EDLCs always have high specific surface area (SSA), such as porous carbon (), carbon nanotube (), and graphene (). Pseudocapacitors store energy by reversible faradaic reactions of the electrode materials, such as Ni(OH)2 (), MnO2 (), and V2O5 (). Up to now, EDLCs still hold the dominant market position owing to their low cost and high reliability.Porous carbon materials (PCMs) are widely used as electrode materials in supercapacitors, especially EDLCs, owing to their stable physical and chemical properties, large SSA, controllable pore structure, high electronic conductivity, and low cost (, , , ). The capacitance of PCM-based supercapacitors is mainly determined by the SSA and pore structure of PCMs, providing ion storage interface and facilitating the ion transportation, respectively (, ). Therefore much research has been devoted to optimize the pore structure by preparing ordered and hierarchical (micropores and mesopores) PCMs on the premise of remaining large SSA to enhance the EDLC capacity (, href="#bib15" rid="bib15" class=" bibr popnode">Largeot et al., 2008, href="#bib24" rid="bib24" class=" bibr popnode">Qie et al., 2013, href="#bib34" rid="bib34" class=" bibr popnode">Tran and Kalra, 2013). Although metal-organic frameworks (href="#bib10" rid="bib10" class=" bibr popnode">Hu et al., 2010) and metal carbides (href="#bib3" rid="bib3" class=" bibr popnode">Chmiola et al., 2006b) have been used to prepare pore-controllable PCMs, organic polymers are promising precursors because they can be handily designed and synthesized with specific structures and composition; these features are important to obtain PCMs with the desired pore structure (href="#bib5" rid="bib5" class=" bibr popnode">Dutta et al., 2014, href="#bib41" rid="bib41" class=" bibr popnode">Wei et al., 2013, href="#bib43" rid="bib43" class=" bibr popnode">Xu et al., 2013, href="#bib54" rid="bib54" class=" bibr popnode">Zhong et al., 2012).Polymerization of monomers for preparing organic polymers provides the possibility of tuning the final structures, during which monomers may be restricted to the specific space for in situ polymerization or self-assembly. For instance, Böttger-Hiller et al. used spherical SiO2 particles as hard templates to allow in situ monomer polymerization and prepared hollow carbon spheres with porous shell by carbonization and washing of the templates (href="#bib1" rid="bib1" class=" bibr popnode">Böttger-Hiller et al., 2013). Using surfactants or block copolymers as soft templating can direct the polymerization of monomers, and after drying and carbonization, PCMs can be obtained (href="#bib4" rid="bib4" class=" bibr popnode">Chuenchom et al., 2012). For instance, block copolymers, such as Pluronic F-127 (EO106PO70EO106), are commonly used as structure-directing agents for the self-assembly of monomers (href="#bib8" rid="bib8" class=" bibr popnode">Hasegawa et al., 2016, href="#bib38" rid="bib38" class=" bibr popnode">Wang et al., 2018b, href="#bib42" rid="bib42" class=" bibr popnode">Xiong et al., 2017). Liang et al. reported that, by changing the mixture of F127, phloroglucinol, and formaldehyde and the processing conditions, different forms of fibers, sheets, films, and monoliths can be readily synthesized (href="#bib19" rid="bib19" class=" bibr popnode">Liang and Dai, 2006). Estevez et al. reported a dual-templating and post-activation strategy to prepare hierarchical porous carbon (href="#bib6" rid="bib6" class=" bibr popnode">Estevez et al., 2013). Combined ice-template and colloidal silica followed by physical activation was applied to generate interconnected macro-, meso-, and microporosity. However, most of the templates are rather expensive and nonrenewable, which limits their application.In recent years, several studies focused on developing new methods from direct carbonization of special polymers without using any template (href="#bib11" rid="bib11" class=" bibr popnode">Hu et al., 2012, href="#bib51" rid="bib51" class=" bibr popnode">Zhang et al., 2013, href="#bib56" rid="bib56" class=" bibr popnode">Zhu et al., 2015). Porous structure can be formed by regulating the cross-linking style of the polymer or inserting specific elements into the framework. For instance, Puthusseri et al. reported a nontemplating method to synthesize interconnected microporous carbon material by direct pyrolysis of poly (acrylamide-co-acrylic acid) potassium salt without any additional activation. During the pyrolysis, the potassium in the polymer reacted with the carbon to form K2CO3, which creates pores in the carbon framework (href="#bib23" rid="bib23" class=" bibr popnode">Puthusseri et al., 2014). Sevilla et al. reported similar results by using potassium citrate as the precursor (href="#bib26" rid="bib26" class=" bibr popnode">Sevilla and Fuertes, 2014). Apart from K2CO3, chemical compounds, such as KOH, NH3, and NH4H2PO4, were also helpful in the formation of porous carbon (href="#bib14" rid="bib14" class=" bibr popnode">Krüner et al., 2018, href="#bib16" rid="bib16" class=" bibr popnode">Li et al., 2010, href="#bib40" rid="bib40" class=" bibr popnode">Wang and Kaskel, 2012, href="#bib55" rid="bib55" class=" bibr popnode">Zhou et al., 2015).In this study, we proposed a nontemplating method to prepare PCM with high capacitance. We first designed and synthesized a hyper-cross-linked N, P-rich polymer containing the P–N groups, denoted as polyphosphamide resin (PAR). Hyper-cross-linked N, P-rich polymer with hierarchical framework can not only facilitate the formation of well-defined pore structure but also increase the SSA by activation of the formed N, P species (such as NH₃ and NH₄H₂PO₄). Afterward, the PAR was pyrolyzed and post-activated to prepare PCMs. The properties of PAR and the as-prepared PCM were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman spectrum, scanning electron microscopy (SEM), etc. The electrochemical capacitance performance of the as-obtained PCMs was evaluated both in the three-electrode and two-electrode systems. The role of N, P in improving the PCM performance was also investigated.