Nontemplating Porous Carbon Material from Polyphosphamide Resin for Supercapacitors.

The nontemplating preparation of porous carbon materials by using specially designed polymer precursors for supercapacitor is attracting considerable research attention because of the more controllable frame structure and easier processes than templating methods. Herein, a deliberately designed cross-linking polyphosphamide resin with defined N and P structure is synthesized and then carbonized to obtain porous carbon material. The as-obtained porous carbon material has a specific surface area of 2,620 m2 g-1, high porosity of 1.49 cm3 g-1, and well-distributed micro/mesoporous carbon structure. Different from activation by post-added NH4H2PO4, the confined N and P in the polymer frame are confirmed to play an important role in pore structure development by forming in situ highly dispersed NH4H2PO4 during carbonization. When evaluated as the electrode material for supercapacitors, the polyphosphamide-resin-based porous carbon material demonstrates excellent capacitance (440 F g-1 under 0.5 A g-1) and high stability (retention of 93% over 10,000 cycles).

Introduction

With 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. (Wang et al., 2012). According to the energy storage mechanisms, supercapacitors can be categorized into electrical double-layer capacitors (EDLCs) and pseudocapacitors (Salanne et al., 2016). 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 class="Chemical">carbon (Yao et al., 2018), class="Chemical">n class="Chemical">carbon nanotube (Yu et al., 2014), and graphene (Strauss et al., 2018). Pseudocapacitors store energy by reversible faradaic reactions of the electrode materials, such as Ni(OH)2 (Su et al., 2014), MnO2 (Wang et al., 2015), and V2O5 (Wang et al., 2018a). Up to now, EDLCs still hold the dominant market position owing to their low cost and high reliability.

Porous class="Chemical">carbon materials (class="Chemical">n class="Chemical">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 (Liu et al., 2017, Simon and Gogotsi, 2008, Zhai et al., 2011, Zhang and Zhao, 2009). 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 (Chmiola et al., 2006a, Pandolfo and Hollenkamp, 2006). 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 (Kondrat et al., 2012, Largeot et al., 2008, Qie et al., 2013, Tran and Kalra, 2013). Although metal-organic frameworks (Hu et al., 2010) and metal carbides (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 (Dutta et al., 2014, Wei et al., 2013, Xu et al., 2013, Zhong et al., 2012).

class="Chemical">Polymerizatioclass="Chemical">n of moclass="Chemical">nomers for preclass="Chemical">n class="Chemical">paring 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 (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 (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 (Hasegawa et al., 2016, Wang et al., 2018b, 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 (Liang and Dai, 2006). Estevez et al. reported a dual-templating and post-activation strategy to prepare hierarchical porous carbon (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 class="Chemical">carbonizatioclass="Chemical">n of special class="Chemical">n class="Chemical">polymers without using any template (Hu et al., 2012, Zhang et al., 2013, 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 (Puthusseri et al., 2014). Sevilla et al. reported similar results by using potassium citrate as the precursor (Sevilla and Fuertes, 2014). Apart from K2CO3, chemical compounds, such as KOH, NH3, and NH4H2PO4, were also helpful in the formation of porous carbon (Krüner et al., 2018, Li et al., 2010, Wang and Kaskel, 2012, Zhou et al., 2015).

In this study, we proposed a nontemplating method to preclass="Chemical">pare PCM with high capacitaclass="Chemical">nce. We first desigclass="Chemical">ned aclass="Chemical">nd syclass="Chemical">nthesized a hyper-cross-liclass="Chemical">nked class="Chemical">n class="Chemical">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.

Results

Preparation of Polyphosphamide Resin

class="Chemical">PAR was preclass="Chemical">n class="Chemical">pared according to our previous work (Zeng et al., 2011). The synthetic route of the PAR is shown in Scheme 1.

Scheme 1

Scheme for the PAR Synthesis

Scheme for the n class="Chemical">PAR Syclass="Chemical">nthesis

In the synthesis of class="Chemical">PAR, N–P boclass="Chemical">nds were formed to liclass="Chemical">nk the class="Chemical">n class="Chemical">phosphoryl trichloride and hexane-1,6-diamine. Phosphoryl trichloride acted as a center to connect three molecules of hexane-1,6-diamine, and the other head of hexane-1,6-diamine was bonded to another phosphoryl trichloride. Thus, a branch-like network grew. Therefore the O=P–(N–C)3 structures were well-dispersed as nodes in the PAR network, as shown in Figure 1.

Figure 1

Simulated Molecular Model of PAR

(A) Formation of O=P–(N–C)3 node with one phosphoryl trichloride molecule and three hexane-1,6-diamine molecules.

(B) Possible structural unit of PAR.

(C) Simulated molecular structure of PAR. Atoms involved in simulation: carbon, gray; nitrogen, blue; oxygen, red; phosphorus, pink; and hydrogen, white.

Simulated Molecular Model of n class="Chemical">PAR

(A) Formation of O=P–(N–C)3 node with one n class="Chemical">phosphoryl trichloride molecule aclass="Chemical">nd three class="Chemical">n class="Chemical">hexane-1,6-diamine molecules.

(B) Possible structural unit of n class="Chemical">PAR.

(C) Simulated molecular structure of class="Chemical">PAR. Atoms iclass="Chemical">nvolved iclass="Chemical">n simulatioclass="Chemical">n: class="Chemical">n class="Chemical">carbon, gray; nitrogen, blue; oxygen, red; phosphorus, pink; and hydrogen, white.

The FTIR spectrum of n class="Chemical">PAR is showclass="Chemical">n iclass="Chemical">n Figure 2A. The broad baclass="Chemical">nd at 3,422 cm−1 iclass="Chemical">ndicated the existeclass="Chemical">nce of N–H. The doublet peaks occurriclass="Chemical">ng at 2,930 aclass="Chemical">nd 2,858 cm−1 were ascribed to the symmetric aclass="Chemical">nd asymmetric stretchiclass="Chemical">ngs of alkyl C–H. The baclass="Chemical">nd at 1,640 cm−1 was the stretchiclass="Chemical">ng of P=O. The peaks at 1,546 aclass="Chemical">nd 1,463 cm−1 were the deformatioclass="Chemical">n vibratioclass="Chemical">n of N–H aclass="Chemical">nd beclass="Chemical">ndiclass="Chemical">ng vibratioclass="Chemical">n of C–H, respectively. The peaks at 1,267 aclass="Chemical">nd 1,096 cm−1 were attributed to the symmetric aclass="Chemical">nd asymmetric stretchiclass="Chemical">ng vibratioclass="Chemical">ns of C–N. Notably, the peak at 982 cm−1 iclass="Chemical">ndicated the preseclass="Chemical">nce of N–P. Furthermore, the XPS spectra showclass="Chemical">n iclass="Chemical">n Figure S2 democlass="Chemical">nstrated the existeclass="Chemical">nce of N–P boclass="Chemical">nds. All these peaks fouclass="Chemical">nd iclass="Chemical">n the FTIR spectrum aclass="Chemical">nd XPS spectra suggested that the class="Chemical">n class="Chemical">phosphoramide structure of the resin was formed, consistent with the initial design.

Figure 2

Materials Characterization of PAR

(A) FTIR spectrum of the PAR.

(B) TG-differential scanning calorimeter (DSC) curves of the PAR.

Materials Characterization of n class="Chemical">PAR

(A) FTIR spectrum of the n class="Chemical">PAR.

(B) TG-differential scanning calorimeter (DSC) curves of the n class="Chemical">PAR.

The thermal degradation behavior of class="Chemical">PAR is showclass="Chemical">n iclass="Chemical">n Figure 2B. Two major degradatioclass="Chemical">n peaks are showclass="Chemical">n iclass="Chemical">n the differeclass="Chemical">ntial thermal aclass="Chemical">nalysis curve (red liclass="Chemical">ne). Oclass="Chemical">ne is from ∼381°C to 495°C, with 18.6% loss of the total weight attributed to the decompositioclass="Chemical">n of the P–N, C–N, aclass="Chemical">nd P=O groups. The other is from ∼495°C to 779°C, with 52.1% loss of the total weight due to class="Chemical">n class="Chemical">polymer carbonization (e.g., the cleavage of C–H and C–C bonds) to form the carbon framework.

Physicochemical Properties of PCMs

Based on the thermogravimetric analysis (TGA), the class="Chemical">PAR was pyrolyzed uclass="Chemical">nder aclass="Chemical">n Ar atmosphere to preclass="Chemical">n class="Chemical">pare PCMPAR. The morphology and structure of PCMPAR were first examined by SEM and transmission electron microscopy (TEM). The SEM image shows that PCMPAR has rough surface with multiple micropores (Figure 3A). The TEM image exhibits large number of micropores in the inner structure of the material (Figures 3B and S3), indicating that it is a porous material. The porous nature of PCMPAR can enhance the performance of supercapacitor (Hou et al., 2015, Huang et al., 2016, Tian et al., 2015). N2 adsorption-desorption method was used to analyze the SSA and pore texture of the PCMs. In Figure 3C, compared with PCMPFR, PCMPAR and PCMPFR+NP exhibit a significant increase in volume when P/P0 increases from 0 to 0.3. This phenomenon suggests that the curves are between types I and IV (IUPAC classification), which is attributed to the presence of certain mesopores in the materials (Tao et al., 2006). The result can also be proved by pore size distributions (Figure 3D), which indicates that all the PCMs possess a certain fraction of mesopores (>2 nm). The figure clearly shows that the micropores of PCMs concentrated at 1–3 nm along with a small proportion at approximately 0.5 nm. Notably, the proportion of mesopores showed an order of PCMPAR>PCMPFR+NP>PCMPFR. The Brunauer–Emmett–Teller (BET) SSAs of PCMPAR, PCMPFR, and PCMPFR+NP were 2,620, 2,233, and 3,062 m2 g−1, respectively. The SSA, pore volume, and average pore size of the PCMs are summarized in Table 1.

Figure 3

Characterization of the PCMs

(A) SEM image of PCMPAR (scale bar, 100 nm).

(B) TEM image of PCMPAR (scale bar, 100 nm).

(C) N2 adsorption and desorption isotherms of PCMPAR, PCMPFR, and PCMPFR+NP.

(D) Pore size distributions of PCMPAR, PCMPFR, and PCMPFR+NP.

Table 1

Textural and Chemical Properties of PCMPAR, PCMPFR, and PCMPFR+NP

Sample Code	SBET (m2 g−1)	VPa (cm3 g−1)	Pore Size (nm)	Elemental Analysis		XPS Analysis
				N (wt %)	P (wt %)	N (at%)	P (at%)
PCMPAR	2,620	1.49	2.3	1.07	0	1.14	0.08
PCMPFR	2,233	1.19	2.1	0.80	0	1.09	0
PCMPFR+NP	3,062	1.68	2.2	0.27	0	0.60	0.16

Pore volume determined at P/P0 = 0.99.

Characterization of the n class="Chemical">PCMs

(A) SEM image of PCMn class="Chemical">PAR (scale bar, 100 class="Chemical">nm).

(B) TEM image of PCMn class="Chemical">PAR (scale bar, 100 class="Chemical">nm).

(C) class="Chemical">N2 adsorptioclass="Chemical">n aclass="Chemical">nd desorptioclass="Chemical">n isotherms of PCMclass="Chemical">n class="Chemical">PAR, PCMPFR, and PCMPFR+NP.

(D) Pore size distributions of PCMclass="Chemical">PAR, class="Chemical">n class="Chemical">PCMPFR, and PCMPFR+NP.

Textural and Chemical Properties of PCMclass="Chemical">PAR, class="Chemical">n class="Chemical">PCMPFR, and PCMPFR+NP

Influence of the Precursor on the Formation of PCMs

To reveal the roles of class="Chemical">nitrogen aclass="Chemical">nd class="Chemical">n class="Chemical">phosphorus in the formation of special pore structure of PCMPAR, PFR was selected as a common precursor to fabricate PCMPFR, which contains no nitrogen and phosphorus. The transformation of N and P during the entire process was investigated to explore their effect on the formation of PCMs. The change of P content during the porous carbon preparation is shown in Figure 4A. Briefly, 13.2 wt % P in original PAR concentrated to 18.8 wt % in PAR-char after pre-carbonization and further decreased to 0 wt % after activation (Table 1). This result can also be proved by the XPS result (Figures 4C and 4D). The high-resolution XPS spectra of N 1s were fitted and presented in Figure 4E. The peak at 401.0 eV of PCMPAR and PCMPFR+NP can be ascribed to the binding energy of graphitic-N, whereas the peak at 399.3, 401.0, and 401.9 eV of PCMPFR can be fitted to pyrrolic-N, graphitic-N (eV), and oxided-N, respectively (Sun et al., 2018). The content of the doped-N of all PCMs are around or below 1 atom % (1.14 atom % for PCMPAR, 1.09 atom % for PCMPFR, and 0.60 atom % for PCMPFR+NP). Therefore we concluded that the low content of doped N has little influence on the capacitance performance, which is similar to the effect of phosphorus. X-ray diffraction (XRD) results showed that after PAR pre-carbonization, the nitrogen and phosphorus were transformed to NH4H2PO4 in PAR-char. NH4H2PO4 may act as an activating agent to create pores.

Figure 4

Analysis of Nitrogen and Phosphorus during Thermal Treatment

(A) Transformation of P content during the material preparation.

(B) XRD patterns of PAR and PAR-char.

(C) XPS survey spectra of PCMPAR, PCMPFR, and PCMPFR+NP.

(D) XPS P 2p spectra of PCMPAR, PCMPFR, and PCMPFR+NP.

(E) XPS N 1s spectra of PCMPAR, PCMPFR, and PCMPFR+NP.

Analysis of n class="Chemical">Nitrogen aclass="Chemical">nd class="Chemical">n class="Chemical">Phosphorus during Thermal Treatment

(A) Transformation of P content during the material pren class="Chemical">paratioclass="Chemical">n.

(B) XRD patterns of n class="Chemical">PAR aclass="Chemical">nd class="Chemical">n class="Chemical">PAR-char.

(C) XPS survey spectra of PCMclass="Chemical">PAR, class="Chemical">n class="Chemical">PCMPFR, and PCMPFR+NP.

(D) XPS P 2p spectra of PCMclass="Chemical">PAR, class="Chemical">n class="Chemical">PCMPFR, and PCMPFR+NP.

(E) XPS N 1s spectra of PCMclass="Chemical">PAR, class="Chemical">n class="Chemical">PCMPFR, and PCMPFR+NP.

PCMclass="Chemical">PAR had ficlass="Chemical">ne textural properties with high class="Chemical">n class="Chemical">nitrogen uptakes and a BET surface area of 2,620 m2 g−1, which was higher than that of PCMPFR (2,233 m2 g−1). Moreover, PCMPAR had more mesopores than PCMPFR, as proven by the pore size distribution and the average pore diameters (2.3 nm of PCMPAR and 2.1 nm of PCMPFR). To verify the importance of chemically bonded N and P in PAR, equal weight of NH4H2PO4 was added in PFR-char for the following activation (PFR-char:NH4H2PO4:KOH = 0.59:0.41:4). The results showed that the introduction of NH4H2PO4 to the PFR can significantly improve the micropore formation. Thus the pore volume (1.68 cm3 g−1) and SSA (3,062 m2 g−1) of PCMPFR+NP increased, which was 1.37 times higher than that of PCMPFR and 1.17 times higher than that of PCMPAR. Therefore both external and in situ NH4H2PO4 acted as an activating agent to create pores (Li et al., 2010, Xu et al., 2017). However, the in situ NH4H2PO4 may be helpful in forming mesopores than externally added NH4H2PO4. It can be clearly found that PCMPAR has a higher proportion of mesopores/micropores than PCMPFR+NP (Figure 4D). This condition may affect the capacitance performance of PCM.

There are no sharp peaks in the XRD pattern of PCMclass="Chemical">PAR, class="Chemical">n class="Chemical">PCMPFR, and PCMPFR+NP, indicating the amorphous state of carbon in PCMs (Figure S4). Raman spectra of the PCMs are shown in Figure S5. Two characteristic peaks at 1,340 and 1,580 cm−1 are assigned to the defect-induced band (D) and graphitic band (G), respectively (Wang et al., 2018c). The disorder degree of the PCMs can be generally described by the intensity ratio between the D and G bands (ID/IG). The ID/IG of PCMPAR, PCMPFR, and PCMPFR+NP were 0.90, 0.98, and 0.92, respectively. The slightly lower ID/IG of PCMPAR indicated that it had lower disorder degree and higher degree of graphitization, which was beneficial to improve the electric conductivity.

Explanation of the Effect of N and P

The role of class="Chemical">N, P iclass="Chemical">n the formatioclass="Chemical">n of porous class="Chemical">n class="Chemical">carbon was proposed (Figure 5). In pre-carbonization, N–P and C–N bonds decomposed, and part of the N, P combined with H to form NH3 and PH3. The others combined with H and O and transformed to NH4H2PO4. The polymer framework was partially carbonized to form a NH4H2PO4-decorated raw char. During the activation process, KOH could firstly react with the NH4H2PO4 as follows: NH4H2PO4 + 3KOH = NH3↑+ K3PO4 + 3H2O↑. To confirm this hypothesis, we conducted Thermogravimetric Analysis and Fourier Transform infrared spectroscopy (TG-FTIR) characterizations of PAR samples. The pre-carbonized PAR (PAR-char) and the KOH-added sample (PAR-char + KOH) were first analyzed with a TG analyzer, and the released gaseous products were online analyzed by an FTIR spectrometer. TG-differential thermal gravity (DTG) results show that PAR-char + KOH has a significant weight loss in the range of 100°C –400°C, which is rather different from PAR-char (>600°C). Given that PAR-char was already pre-carbonized at 500°C and exhibited no apparent weight loss (Figure S6), the weight loss of PAR-char + KOH should be caused by the reactions between NH4H2PO4 in PAR-char and KOH during carbonization and released gaseous products. Furthermore, FTIR results show that H2O, NH3, and CO2 were the main gaseous products of PAR-char + KOH, whereas little H2O and NH3 were released from PAR-char (Figures 5B–5E and S7), demonstrating that the reaction of NH4H2PO4 in PAR-char and KOH occurred. Therefore the NH4H2PO4 in PAR-char is not easy to decompose below 500°C, but can react with KOH during the heating process. In addition, the main gaseous products (H2O, NH3, and CO2) as well as K3PO4 and the excess KOH can all act as activating agents to improve the porosity of carbon substrate. Besides, the pre-carbonized PAR was just physically mixed with KOH powder, thus the NH4H2PO4 embedded in carbon was partially in contact with KOH. Importantly, the special N–P structure of PAR formed highly dispersed NH4H2PO4 and thus resulted in the development of well-distributed micro/mesopores, which is beneficial to improve the capacitance performance (Li et al., 2014, Zheng et al., 2015).

Figure 5

Mechanism of the Formation of Micro/Mesopores

(A) Scheme for PCMPAR synthesis from PAR, and the possible roles played by N and P during the formation of micro/mesopores.

(B) 3D infrared spectrum of gaseous compounds released during pyrolysis of PAR-char + KOH.

(C) 3D infrared spectrum of gaseous compounds released during pyrolysis of PAR-char.

(D) FTIR spectra of pyrolysis products of PAR-char + KOH at selected temperatures.

(E) FTIR spectra of pyrolysis products of PAR-char at selected temperatures.

(A) Scheme for PCMclass="Chemical">PAR syclass="Chemical">nthesis from class="Chemical">n class="Chemical">PAR, and the possible roles played by N and P during the formation of micro/mesopores.

(B) 3D infrared spectrum of gaseous compounds released during pyrolysis of n class="Chemical">PAR-char + class="Chemical">n class="Chemical">KOH.

(C) 3D infrared spectrum of gaseous compounds released during pyrolysis of n class="Chemical">PAR-char.

(D) FTIR spectra of pyrolysis products of n class="Chemical">PAR-char + class="Chemical">n class="Chemical">KOH at selected temperatures.

(E) FTIR spectra of pyrolysis products of n class="Chemical">PAR-char at selected temperatures.

Electrochemical Performance of the PCMs

The electrochemical performance of the class="Chemical">PCMs was iclass="Chemical">nvestigated with CV aclass="Chemical">nd class="Chemical">n class="Chemical">GCD measurements. The cyclic voltammetry (CV) curves and galvanostatical charge–discharge (GCD) profiles of PCMs are shown in Figure S8. The CV curves of PCMPAR under wide scan rates from 5 mV s−1 to 100 mV s−1 showed a similarly rectangular and symmetric shape (Figure S8A). Importantly, the CV curves of PCMPAR still maintained a rectangular shape even at a sweep rate of 100 mV s−1 and an inapparent reduction in the CV area. This phenomenon indicated that PCMPAR has a fine ion transporting and accessible surface area. The PCMPFR and PCMPFR+NP had similar CV shapes with PCMPAR but suffered an area distortion at a high scan rate and a prominent area reduction with the increase in scan rate. This condition indicated a poor capacitance performance of PCMPFR and PCMPFR+NP at high charge-discharge rate. Instead of the typical rectangular shape, a region of reversible pseudofaradaic reaction can be observed at about −0.5 V in the CV curves. In addition, the approximate triangular charge-discharge profiles and their slightly nonlinear characteristic exhibited the primary EDLC performance and additional pseudocapacitance performance (Figure S8B). The pseudocapacitance can be attributed to the high oxygen content (7%–8%) of the PCMs (Table S1) (Guo et al., 2014, Liu et al., 2018, Tang et al., 2017).

CV curves of the class="Chemical">PCMs at scaclass="Chemical">n rate of 50 mV s−1 show that the specific capacitaclass="Chemical">nce has aclass="Chemical">n order of PCMclass="Chemical">n class="Chemical">PAR>PCMPFR+NP>PCMPFR (Figure 6A). The result can also be proved by the GCD profiles under current density of 5 A g−1 (Figure 6B). The GCD measurements were performed under different current densities to assess specific capacitance (Figure 6C). The PCMPAR showed a high specific capacitance of 440 F g−1 under a current density of 0.5 A g−1 and still maintained at 278 F g−1 at a high current density of 20 A g−1. This PCM material shows a competitive performance among carbon-based supercapacitor electrodes (Table S2). As a comparison, the specific capacitances of PCMPFR and PCMPFR+NP are also listed in Figure 6C. PCMPFR had the lowest specific capacitance (348 F g−1 at 0.5 A g−1 and 150 F g−1 at 20 A g−1) attributed to the lowest SSA (2,233 m2 g−1) because the specific capacitance of EDLC mainly depended on the SSA and porosity characteristics of electrode materials. PCMPFR+NP (3,062 m2 g−1) had larger SSA than PCMPAR (2,620 m2 g−1), but the specific capacitance of PCMPFR+NP was not higher than that of PCMPAR under overall current densities. When the current density was below 1 A g−1, PCMPFR+NP exhibited higher specific capacitance than PCMPAR. However, when the current density was above 1 A g−1, PCMPAR showed high specific capacitance. At low charge-discharge rate, ions from the electrolyte had enough time to accumulate on the electrode surface. Therefore, SSA dominated the capacitance performance. Meanwhile, at high charge-discharge rate, ions cannot be delivered to the surface in time owing to the limitation of the pore structure. This phenomenon indicated that SSA was not finely utilized. As a result, the pore characteristics and the accessible surface area determined the specific capacitance at high charge-discharge rate. Researchers found hierarchical porous microstructures, namely, proper macroporous, mesoporous, and microporous distributions, which are favorable for the diffusion of electrolyte ions, thus leading to a high-rate electrochemical capacitance (Li et al., 2013, Zhang et al., 2016, Zheng et al., 2010). The pore size distribution showed that PCMPAR has more mesopores than PCMPFR and PCMPFR+NP (Figure 2D). With the increase of the proportion of mesopores, PCMPAR can provide a sufficient electrode-electrolyte interface for the accumulation of ions. Thus capacitance performance was improved.

Figure 6

Electrochemical Characterization of the PCMs in Three-Electrode System

(A) CV curves of the PCMs at scan rate of 50 mV s−1.

(B) GCD profiles of the PCMs under current density of 5 A g−1.

(C) Specific capacitance of the PCMs calculated by GCD profiles.

(D) EIS plots of the PCMs.

Electrochemical Characterization of the n class="Chemical">PCMs iclass="Chemical">n Three-Electrode System

(A) CV curves of the n class="Chemical">PCMs at scaclass="Chemical">n rate of 50 mV s−1.

(B) n class="Chemical">GCD profiles of the class="Chemical">n class="Chemical">PCMs under current density of 5 A g−1.

(C) Specific capacitance of the n class="Chemical">PCMs calculated by class="Chemical">n class="Chemical">GCD profiles.

(D) EIS plots of the n class="Chemical">PCMs.

Electrochemical impedance spectroscopy (EIS) was performed to further understand the ion or charge transport of the class="Chemical">PCMs. As showclass="Chemical">n iclass="Chemical">n Figure 6D, iclass="Chemical">n the low-frequeclass="Chemical">ncy regioclass="Chemical">n, the class="Chemical">n class="Chemical">PCMs showed almost vertical curves, wherein PCMPAR had the highest slope. This phenomenon indicated the nearly ideal EDLC behavior and the small equivalent diffusion resistance. In the high-frequency region (inset in Figure 6D), the intercept on the real axis presented low values (<0.4 Ω). This finding corresponded to low equivalent series resistances and indicated the excellent conductivity of the PCMs in aqueous electrolyte system. From the high- to low-frequency region, the low interfacial charge transfer resistances calculated from the diameter of the semicircle were 0.09 Ω of PCMPAR, 0.29 Ω of PCMPFR, and 0.10 Ω of PCMPFR+NP. The EIS results showed that PCMPAR had smaller contact resistance among the electrode materials and electrolyte ions, indicating lower charge transfer resistance on the electrode surface. Furthermore, the cycling stability of the PCMPAR was evaluated by long-term CV cycling measurement at a scan rate of 50 mV s−1. The result showed that PCMPAR has fine capacitance retention of 93% over 10,000 cycles (Figure S9). Besides, PCMPAR, PCMPFR, and PCMPFR+NP are all soaked well in water (Figure S10 and Table S3), which has no negative effect when used as electrode materials in aqueous solution. Combining the results of CV, GCD, EIS, and long-term cycling, PCMPAR exhibited a favorable performance as supercapacitor electrode.

Electrochemical Tests in Two-Electrode System

The electrochemical performance of PCMclass="Chemical">PAR iclass="Chemical">n a two-electrode system is showclass="Chemical">n iclass="Chemical">n Figure S8. The PCMclass="Chemical">n class="Chemical">PAR showed similar rectangular CV curves as well as approximate triangular GCD profiles (Figures 7A and 7B), indicative of a primary EDLC performance. The specific capacitances of PCMPAR calculated by GCD profiles were 279, 255, 237, 210, 191, and 174 F g−1 at current densities of 0.5, 1, 2, 5, 10, and 20 A g−1, respectively (Figure 7C). These values were competitive during the carbon materials. Furthermore, the energy density and power density were calculated, and the results are shown in Figure 7D. PCMPAR exhibited an energy density of 9.7 Wh kg−1 or 6.0 Wh kg−1. This value corresponded to a power density of 0.25 kW kg−1 or 10 kW kg−1 in 6 M KOH electrolyte.

Figure 7

Electrochemical Characterization of the PCMPAR in Two-Electrode System

(A) CV curves of PCMPAR at different scan rates in a two-electrode system.

(B) GCD profiles of PCMPAR under different current densities.

(C) Specific capacitance of PCMPAR calculated by GCD profiles.

(D) Ragone plot of PCMPAR.

Electrochemical Characterization of the PCMn class="Chemical">PAR iclass="Chemical">n Two-Electrode System

(A) CV curves of PCMn class="Chemical">PAR at differeclass="Chemical">nt scaclass="Chemical">n rates iclass="Chemical">n a two-electrode system.

(B) n class="Chemical">GCD profiles of PCMclass="Chemical">n class="Chemical">PAR under different current densities.

(C) Specific capacitance of PCMn class="Chemical">PAR calculated by class="Chemical">n class="Chemical">GCD profiles.

(D) Ragone plot of PCMn class="Chemical">PAR.

Discussion

class="Chemical">Phosphoramide resiclass="Chemical">n was syclass="Chemical">nthesized, whereiclass="Chemical">n the special N–P structure was chemically boclass="Chemical">nded as the class="Chemical">nodes for liclass="Chemical">nkiclass="Chemical">ng the moclass="Chemical">nomers iclass="Chemical">nto class="Chemical">n class="Chemical">polymers. The N and P in the PAR framework can be transformed to NH4H2PO4 during carbonization. The well-distributed NH4H2PO4 acts as the in situ activating agent to form micro/mesopores, which has better effect on activation than post-added NH4H2PO4. The as-prepared PCMPAR has a high SSA of 2,620 m2 g−1 and pore volume of 1.49 cm3 g−1 and fine micro/mesoporous structure. Electrochemical tests show that PCMPAR has approximately rectangular CV curves and triangular GCD profiles. This finding indicates that PCMPAR is a typical EDLC electrode. The PCMPAR exhibits a specific capacitance of 440 F g−1 at 0.5 A g−1 and 278 F g−1 at 20 A g−1. The high capacitance retention at high charge-discharge rates signifies that PCMPAR has fine micro/mesoporous structure for the transport of electrolyte ions. The results show that PAR is a good candidate to prepare PCMs for supercapacitor because of its unique form of N and P in the framework.

Limitations of Study

Based on the current preclass="Chemical">paratioclass="Chemical">n method of class="Chemical">n class="Chemical">PCMs from PAR involving drastic activation, it is hard to precisely control the pore size. Therefore we would design and synthesize more N, P resins by changing the monomers in future work and adopt the mild activation process. The effect of monomer structures on the characteristics of the PCMs should be investigated to optimize the design and preparation of new PCMs for energy storage, catalysis, environmental remediation, and so on.

Methods

All methods can be found in the accompanying Transn class="Chemical">pareclass="Chemical">nt Methods supplemeclass="Chemical">ntal file.