Fabrication of g-C3N4 Nanomesh-Anchored Amorphous NiCoP2O7: Tuned Cycling Life and the Dynamic Behavior of a Hybrid Capacitor.

Developing a novel electrode material with better electrochemical behavior and extended cyclability is a major issue in the field of hybrid capacitors. In this work, we propose a novel strategy for the facile synthesis of nickel-cobalt pyrophosphate nanoparticles anchored on graphitic carbon nitride (NiCoP2O7/g-C3N4) via the simple solvothermal method. Field emission scanning electron microscopy and transmission electron microscopy analysis revealed the uniform anchoring of NiCoP2O7 nanocomposite on g-C3N4 nanosheets. Benefitting from the effect of amorphous nature and a conductive matrix of the NiCoP2O7/g-C3N4 (NP3) composite, the material achieves a specific capacitance of 342 F g-1 at a scan rate of 5 mV s-1. Impressively, the electrode shows long-term cycling stability with 100% capacitance retention over 5000 cycles. Employing activated carbon as an anode and as-prepared NP3 as a cathode, the assembled asymmetric hybrid cell exhibits high-energy density and exceptional cyclability (specific capacitance retention over 10 000 cycles). The outstanding electrochemical and cyclic stability is attributed to the shortest electron-ion pathway with effective interfacial interaction. The low electronic resistance of the NiCoP2O7/g-C3N4 nanocomposite is revealed by varying the bias voltage variation in the electrochemical impedance spectroscopy. Our results promise better utilization of the bimetallic pyrophosphate-anchored g-C3N4 matrix as a potential electrode for high-performance energy storage devices.

Introduction

Over the recent past, electrochemists pan class="Chemical">across the world pan class="Chemical">concern more about the affordable efficient energy storage systems in order to meet their demand from simpler commercial electronics to high power electric vehicles. In this aspect, hybrid capacitors have been spotlighted because of their functional capability to merge the double-layer capacitor and pseudocapacitor with remarkably improved energy and power density.[1−3] In general, a hybrid capacitor comprises two distinguishable electrodes which have redox- and electrostatic-type charge storage mechanisms. In comparison with electrostatic-type electrodes, redox-type electrodes possess reversible, rapid faradic redox reaction by which it offers very high-energy densities. Furthermore, redox-type electrodes were again classified into surface redox pseudocapacitance and intercalation pseudocapacitance electrodes. The basic difference among the intercalation pseudocapacitance and surface redox pseudocapacitance electrodes is a redox reaction at precise potential and continuous surface redox reaction over an extended potential range, respectively.[4−7]

Among various types of electrodes, the intercalation pseudocappan class="Chemical">acitance electrode exhibits better electrochemical performance with impn>roved specific capacitance and enriched energy density. Nevertheless, in practical applications, they suffer from cycling and structural stability which constrain their extended application in various fields. To improve the electrochemical performance of the intercalation pseudocapacitance electrode, serious efforts have been undertaken on electrode materials such as metal carbonates, metal hydroxide, metal phosphates, and so on.[8] Recently, transition-metal pyrophosphates, a less explored polyanionic-type material, have been widely studied as a novel electrode material for the upcoming next-generation hybrid capacitors because of their robust three-dimensional (P2O7)4− matrix with several sites for metal ions. The high binding energy of oxygen with polyanionic group renders excellent chemical stability and multidimensional pathway for ionic conduction in these materials.[9]

Huang pang et al. synthepan class="Chemical">sized hierarchal pan class="Chemical">cobalt pyrophosphate and explored its electrochemical performance exhibiting a maximum specific capacitance of 367 F g–1 at a current density of 0.625 A/g.[10] Similarly, amorphous nickel pyrophosphate was prepared which is found to have a high specific capacitance of 1050 F g–1.[11] Chen et al. prepared amorphous nickel cobalt microplates and explored their application as a cathode for a supercapacitor, which gave a highest specific capacitance of 1259 F g–1 at a current density of 1.5 A/g. The stronger faradic redox reaction was rendered by nickel and cobalt ions in ternary nickel cobalt pyrophosphate (NiCoP2O7) which exhibits enhanced electrochemical performance compared to that of monometallic pyrophosphate such as cobalt pyrophosphate.[12] Further, the non-noble metal composite, NiCoP2O7, has better electrical conductivity and improved reversibility with higher theoretical capacity than that of the metal oxides and metal sulfide as a result of the lower electronegativity nature of phosphates.[13,14] However, NiCoP2O7 still suffers from high capacity fade because of the degradation of NiCoP2O7 into nickel and cobalt ions during charging and discharging, by which approximately 20% loss is observed after 2000 cycles[12] along with worse cycling stability. In addition, the self-agglomeration of NiCoP2O7 rationally degrades the electrochemical performance of the corresponding devices, when attempted for commercialization. An effective strategy to address the above issue is to design and fabricate NiCoP2O7 functional electrode within the conductive network, rendering abundant active sites with reduced self-agglomeration and tailored electron pathway, leading to improved faradic performances.

Out of various pan class="Chemical">carbon-based materials, pan class="Chemical">graphitic carbon nitride (g-C3N4) has attracted a great attention as a versatile matrix. Because of the presence of pyrrolic N “hole” surface defect at lattice and short distance ordering of covalently bonded nitrogen at the edge, the material possesses a high rate capability.[15−18] The porosity nature of heptazine and sp2 hybridized nitrogen provides NiCoP2O7 coordination site, and the metal ions bind with three adjacent N atoms. By preparing such interesting architecture of a bimetallic transition-metal pyrophosphate impregnated on the g-C3N4 mesh with improved redox sites, it is possible to improve the electrical conductivity, wettability in the electrolyte, and the surface polarity which could result in excellent cyclability with high rate capability in a hybrid capacitor.[19−25]

Herein, for the first time, we report a pan class="Chemical">simpclass="Chemical">n>le strategy to develop a novel amorpn>hous bimetallic NiCoP2O7/g-C3N4 composite supported on a nickel plate. In this nanocomposite, NiCoP2O7 nanoparticles are homogeneously dispersed and trapped within the g-C3N4 matrix, which can enhance the passage of electrons in the framework. The inhibition of self-agglomeration and inert nature of g-C3N4 improves the stability of the active material. The favorable ion kinetics of the g-C3N4 network benefits easy access of electrolyte ions to the active material by shortening the diffusion pathway and thus promotes the electron transport. The NiCoP2O7/g-C3N4 composites, as a cathode, give excellent specific capacity retention capability of 100% after 5000 cycling at a current density of 15 mA/g. In order to investigate the device performance, we have fabricated hybrid supercapacitor with synergistically modified NiCoP2O7/g-C3N4 and activated carbon (AC) as a cathode and anode, respectively. The fabricated hybrid supercapacitor exhibits exceptional energy densities (65.8 W h kg–1) among ternary metal composite-based supercapacitors with a high rate of electron transfer and outstanding cycling retention.

Formation Mechanism of the NiCoP2O7/g-C3N4 Hybrid

A fpan class="Chemical">acile solvothermal technique was adopted to prepn>are pan class="Chemical">NiCoP2O7/g-C3N4 composite as a high-energy supercapacitor electrode and is schematically demonstrated in Scheme . Initially, g-C3N4 was dissolved in ethanol to form a stable solution followed by the addition of nickel and cobalt precursors along with ammonium dihydrogen phosphate (ADP) and was mixed homogeneously at room temperature. Ni2+ and Co2+ combine with ADP to form a complex ion of CoNiNH4PO4·H2O, and the as-formed complex ions are readily adsorbed on the surface of the carbon mesh because of the negatively charged surface of g-C3N4. Then, the reaction system is maintained at 140 °C for 12 h. Under this temperature, ADP was thermally decomposed to form NiCoP2O7 with the evolution of ammonia gas. The anchored seeds of NiCoP2O7 on the carbon foam proceed for the formation of a NiCoP2O7/g-C3N4 hybrid composite with controlled crystallization and agglomeration rendered by the solvothermal condition. The reason behind adopting a facile solvothermal method is that usually solvent influences the nucleation process because ethanol possesses slightly higher saturated vapor pressure and viscosity than water and it has slower diffusion rate of ions in the saturated state, by which it promotes individual growth of nanoparticles, lowering growth of large crystals.[31] The resultant structure of the NiCoP2O7/g-C3N4 hybrid possesses numerous intermediate mechanisms such as Ostwald ripening, coalescence, thermal decomposition, and self-assembling. From the literature survey, the possible mechanism for the evolution of hybrid composite is suggested in Scheme .

Scheme 1

Schematic Representation of NiCoP2O7/g-C3N4 Preparation

Results and Discussion

Characterization of Pristine g-C3N4 Nanosheets

Figure S1a discloses the powder X-ray diffrpan class="Chemical">action (PXRD) pattern of pure pan class="Chemical">g-C3N4 which can be well indexed with standard JCPDS card no: 87-1526. The two sharp diffraction peaks at 13.1° and 27.7° are attributed to in-plane structural packing (i.e., consecutive hole to hole distance of the tris-s triazine chain) and long-range interplanar stacking of the aromatic system, respectively. To investigate the chemical structure of g-C3N4, Fourier transform infrared (FTIR) analysis was carried out and the resultant spectra are presented in Figure S1b. The observed three characteristic absorption bands at 3167, 1645–1236, and 810 cm–1 are assigned to the stretching mode of N–H bond, stretching vibration of C–N heterocycle, and breathing of s-triazine molecule, respectively.[26−28] UV-diffuse reflectance spectroscopy was performed to determine the band gap energy level of the two-dimensional g-C3N4. From the optical absorption profile (Figure S1c), it is inferred that the absorption band centered at 465 nm corresponds to the π–π*/n−π* transitions. The energy difference between valence and conduction bands that are evaluated from the intercept of tangents drawn from (αhν2) versus photon energy (hν) is 2.75 eV. The field emission scanning electron microscopy (FESEM) analysis was carried out in order to study the morphological features of the as-prepared g-C3N4. As shown in Figure S1d, the pristine g-C3N4 discloses the structure of wrinkled lamellar nanomesh with a high porosity, which is the featured structural property of g-C3N4 developed via the direct calcination method.[29,30] The as-prepared g-C3N4 was used to prepare NiP2O7/g-C3N4 and NiCoP2O7/g-C3N4 composites, and the as-prepared composites were subjected to various characterization processes.

Structural and Morphological Analysis of the Hybrid Composites

The crystallinity and phase purity of the as-synthesized NiP2O7 (pan class="Gene">NP1), NiP2O7/g-CN (NP2), and NiCoP2O7/g-CN (NP3) nanocomposites were investigated by PXRD and are shown in Figure a. The characteristic XRD pattern of NP1 displays narrow peaks at d = 5.88, 4.36, 3.72, 3.47, 3.00, and 2.53 Å, which are assigned to the (11̅1), (002), (3̅11), (121), (212), and (230) diffraction planes of the nanostructure corresponding to the monoclinic nickel pyrophosphate (JCPDS card no-74-1604) with the calculated crystalline size of ∼13.7 nm using Debye Scherer equation, respectively. Notably, the absence of additional peaks indicates the phase purity of the product.

Figure 1

(a) Comparative XRD profile of Hybrid composites; (b) XPS survey spectrum of the NiCoP2O7/g-C3N4 nanocomposite.

(a) pan class="Chemical">Comparative XRD profile of Hybrid composites; (b) XPS survey spectrum of the NiCoP2O7/g-C3N4 nanocomposite.

The NiP2O7/pan class="Chemical">g-C3N4 and pan class="Chemical">NiCoP2O7/g-C3N4 nanocomposites exhibited an amorphous-like XRD pattern, as presented in Figure a. It is noteworthy that the amorphous nature of NiCoP2O7/g-C3N4 comprises several beneficiary properties because of their defect-rich disordered crystallinity and physical isotropy. This isotropic amorphous nature was an appealing feature of the NP3 composite to display enhanced electrochemical performance. It is because it would permit fast and deep diffusivity of OH– ions into the electrode thus enabling improved electrochemical performance than similar crystalline samples. Also, the poor crystallinity of NP3 endows best cycling stability as stress and strain due to intercalation and deintercalation of OH– ions are constant during charging and discharging.[31]

The X-ray photoelectron spectrospan class="Chemical">copy (XPS) was empclass="Chemical">n>loyed to explore the detailed chemical structure of the pan class="Chemical">NiCoP2O7/g-C3N4 composites, which comprises several elements, namely, Ni 2p, Co 2p, P 2p, O 1s, C 1s, and N 1s, that are shown in Figure b. The high-resolution peak fitting of Ni 2p in Figure S2b illustrates two deconvoluted peaks at binding energies of 875.4 and 857.5 eV for Ni 2p1/2 for Ni 2p3/2, respectively. In addition, the satellite peaks at 880.5 and 862.1 eV are attributed to Ni2+. As displayed in Figure S2c, the Co 2p spectrum endows two doublets from splitted spin orbits of Co 2p1/2 and Co 2p3/2 at 799.7 and 782.8 eV, respectively, along with the satellite peak at 802.6 eV that corresponds to Co2+. The difference in the binding energies of Co species is 16.3, which confirms the 2+ oxidation state of Co in the NP3 nanocomposite.[32] The obtained mixed valence states at the NP3 nanocomposite efficiently enhance electronic conductivity by the possible hopping and defect effect processes. The peak at 131.8 eV is attributed to P 2p, which represents +5 oxidation states of P in the material. The wider O 1s spectrum (Figure S2e) was deconvoluted into two peaks corresponding to two species, viz., chemisorbed hydroxyl (OH) at 532.4 eV and lattice oxygen at 531.8 eV. The N 1s high-resolution spectrum consists of three deconvoluted peaks, as shown in Figure S2g. The peaks at 399.4, 400.9, and 402.1 eV are attributed to pyridinic N (N-6), pyrrolic N (N-5), and quaternary N (N–Q) species, respectively.[33] It is well known that the presence of pyrrolic N and pyridinic N would build several additional defects, thus render further active sites and diffusion channels. The high-resolution C 1s spectrum was deconvoluted into three peaks at 285.3, 287.1, and 288.5 eV. The peak centered at 285.3 eV corresponds to the graphitic carbon, and the two peaks at 287.1 and 288.5 eV ascribed to the C–N bond inside the aromatic structure and sp3 carbon atoms, respectively. From the XPS results, it is suggested that NP3 possesses the coupling of bimetallic cations (Ni2+ and Co2+), rendering rich redox sites and higher conductivity than monometallic materials which could result in enhanced faradic processes. On the other hand, the presence of N-containing species (N-6 and N–Q) in NP3 efficiently improves the electrical conductivity of carbonaceous materials.

The morphology and microstructure of the prepared nanocompn>osite NiCoP2O7/g-C3N4 were characterized by FESEM and transmission electron microscopy (TEM) analysis. Figure a–c presents the sheet-like morphology with a uniform size distribution of ∼10 nm smaller-sized NiCoP2O7 nanoparticles over the g-C3N4 mesh, without any significant agglomeration. The hybrid structure could be able to accommodate large volume changes through which it can actively deteriorate the degradation rate of the active material during the cycling process. This, in turn, results in the improved durability of the supercapacitor. Benefiting from the synergistic effect between NiCoP2O7 and g-C3N4, NP3 provides a delicate architecture with continuous ion and electron diffusive pathways at the interface. A closer inspection with high-resolution TEM (HRTEM) analysis confirms the uniform anchoring of NiCoP2O7 nanoparticles on the g-C3N4 matrix, as presented in Figure d–f. The elemental analysis was carried out using energy-dispersive system and elemental mapping techniques, and the results are presented in Figure S3b–g. The experimental results indicate the presence of nickel, cobalt, phosphate, oxygen, carbon, and nitrogen elements and their even distribution across the sample. The structural and morphological analyses confirm the formation of NiCoP2O7/g-C3N4 composite structure with a uniform distribution of NiCoP2O7 nanocomposites over the g-C3N4 sheets.

Figure 2

FESEM (a) and TEM images (b–f) for the NiCoP2O7/g-C3N4 hybrid composite.

Electrochemical Study

To examine the suitability of structurally pan class="Chemical">confined amorphous pan class="Chemical">NiCoP2O7 anchored on two-dimensional (2D) graphitic carbon nitride as an optimal electrode of a high-performance supercapacitor, it has been thoroughly evaluated by cyclic voltammogram (CV), galvanostatic charge discharge (GCD), and cycling and electrochemical impedance spectroscopy (EIS) measurements. Figure a shows the CV curve of the as-synthesized NP1, NP2, and NP3 hybrid materials at a scan rate of 5 mV s–1. It is observed from the CV curve that the peak current of the prepared samples (NP1, NP2, and NP3) exhibits its dependency on the square root of the scan rate, indicating its better reversibility that the reaction process is rapid enough to maintain the stable concentrations of anions and cations at the electrode surface. The obtained CV curves possess two different regions, namely, potential-dependant area (intercalation pseudocapacitance type mechanism) and potential independent current area (surface redox pseudocapacitance mechanism).

Figure 3

(a) CV curves of NP1, NP2, and NP3 electrodes obtained at a scan rate of 5 mV s–1; (b) CV plot of NP3 electrodes obtained at different scan rates; (c) galvanostatic charge–discharge curve of NP1, NP2, and NP3 electrodes obtained at a current density of 5 mA g–1; and (d) NP3 composite at various current densities.

(a) CV curves of pan class="Gene">NP1, pan class="Gene">NP2, and NP3 electrodes obtained at a scan rate of 5 mV s–1; (b) CV plot of NP3 electrodes obtained at different scan rates; (c) galvanostatic charge–discharge curve of NP1, NP2, and NP3 electrodes obtained at a current density of 5 mA g–1; and (d) NP3 composite at various current densities.

The current region below the intercalation-type mechanism was predominant than that of the surfpan class="Chemical">ace redox mechanism. Hence, the pan class="Chemical">contribution of intercalation-type mechanism in the charge storage process is prevalent. The anodic peak emerges because of oxidation of Ni and Co ions accompanied by surface adsorption of an equal number of OH– ions. The cathodic peaks are due to the reduction of Ni and Co ions accompanied by surface desorption of OH– ions. The possible mechanism of the faradic process in the CV study is ascribed to the redox reaction of Ni2+/Ni3+ and Co2+/Co3+ which can be presented as follows

The CV pattern of pan class="Gene">NP1 and pan class="Gene">NP2 composites as a working electrode in a 2 M aqueous KOH electrolyte at different scan rates ranging from 5 to 200 mV s–1 is presented in Figure S4a,b. It is worth noting that the shape of the obtained CV curve is nonrectangular, demonstrating the quasi-reversible reaction while cycling at the electrode–electrolyte interface, and the small redox peaks at 0.41/0.27 and 0.42/0.26 V were observed even at high scan rates. Because of the synergistic effect of better electrochemical utilization and high charge storage capability rendered by NiCoP2O7 and g-C3N4, the CV current area of the NP3 composite is higher than that of the NP1 and NP2 samples [Figure b] and the spread over and broadened redox peaks of NP3 at low scan rate show a sharp increment of anodic current at higher scan rates. As a result, NP3 reveals the possibility of interaction between oxygen evolution and the surface adsorption and desorption of ions at high scan rates. During the CV process of NP3 electrode at higher scan rates, the reduction peaks were not observed clearly which may be due to the surface modification of active material by g-C3N4 and it could promote the significant broadening of fewer faradic peaks. Subsequently, it is also observed that peak current values show linear variation with increasing scan rates in the range of 5–200 mV s−1 which in turn indicates that rapid ionic and electronic mobilities are possible at the applied potentials. The calculated specific capacitance values indicate that NP3 shows a high specific capacitance at a low scan rate and vice versa, which means that with the decrement of scan rate, the capacitance value increases notably because of the effective interaction between ions with the active surface and its bulk counterparts.[34] Hence, the observed CV study suggests that compared to the bare NiP2O7, addition of Co and g-C3N4 content enhances the electrochemical performance by increasing a redox site and charge transport path for intercalation, respectively. The CV analysis reveals the fact that the rational design of carbonaceous material with composites could tailor the electrochemical performance of the supercapacitor.

Galvanostatic charge–discharge measurements were carried out in order to evaluate the electrochemical behavior of the as-prepared samples, within the potential window of 0 to +0.45 V, and the results are presented in Figure c. On compn>aring with pan class="Gene">NP1 and NP2, NP3 contains large plateau region and it is inferred that NP3 possesses significantly improved redox performance than that of other active electrode materials and it happened because of the intercalation–deintercalation of OH–. Consequently, for NP3 samples, the obtained potential plateau was sloping and longer than that of NP1 and NP2 denoting its lower overpotential. Then, the specific capacitances of NP1, NP2, and NP3 were calculated, and this capacitance of the working electrodes decreases with a rise in applied current density because of diminishing mobility of ions in the electrolyte and sluggish charge migration in the course of the redox mechanism.[35]Figure d shows the variation of charge–discharge characteristics of the NP3 sample at various current densities. It shows that the NP3 nanocomposite can deliver a high specific capacitance of 235.5 F g–1 at a current density of 1 mA g–1 and a capacitance of 145 F g–1 is retained at a high current density of 6 mA g–1. The variation of specific capacitance values of the same material from CV to GCD analysis is due to the possibility of utilizing the entire surface in the case of CV, whereas in the charge–discharge profile, the higher current density hinders the surface accession of OH– ions and includes inferior faradic reaction resulting in reduced specific capacitance. The improved performance of the NP3 electrode is ascribed to the following reasons: amorphous multivalent NiCoP2O7 composites anchored on g-C3N4 not only give back superior catalytic performance and efficient electron conductive channels but also effectively increase the penetration of ions along the direction of the matrix. As a suitable conductive matrix, g-C3N4 provides increased electrode–electrolyte interface for electrical double-layer capacitor (EDLC)-type charge storage and also the pyrrolic N, a better electron donor, promotes large electron transport reaction through unimpeding channels.[36] As discussed earlier, the presence of nitrogen in the carbon matrix (N-6 and N–Q) gives extra capacitance with high wettability and ion diffusion thus promoting the rate capability, as verified by the following EIS measurements.

Cappan class="Chemical">acitance durability is a decipan class="Chemical">sive parameter for an active electrode to be efficiently used in practical applications. Therefore, galvanostatic charge–discharge measurements for 5000 cycles were further carried out at a current density of 5 mA g–1 within a potential range of 0–0.45 V, as shown in Figures b and S5a,b. In cycling analysis (Figure a), NP1 and NP2 show clear enhancement in the capacitance value throughout the cycling process because the activation process and active materials may not be fully exposed to electrolytes initially. However, the specific capacitance value of the NP3 composite maintained nearly constant value with a capacity retention of 100%, resembling effective interface between the gC3N4 and NiCoP2O7 with the formation of the stable state of mixed bi-metals and metal pyrophosphate. The cycling performance is better with recent reports, and some of them are tabulated in Table S1.[37,38] For closer analysis, TEM image of NP3 after 5000 successive cycles given in Figure S7 expresses the structural variation. Although swelling due to intercalation and deintercalation of ions was carried out over the entire cycles, significant variation in the morphology was not observed. However, little change in structure may be correlated with the activation process of the NP3 composite. The improved stability with an elongated discharge time of electrode material after 5000 cycles is due to the combined advantage of the isotropic amorphous phase of Ni–Co pyrophosphate and the effective interfacial area between the electrolytes. In addition, the active material never stimulates structural change or any phase variation.

Figure 4

(a) Cycling stability of NP3 electrode at a current density of 15 mA g–1 over 5000 cycles (b) GCD before and after cycling of the NP3 electrode (c,d) comparative Nyquist plot of NP1, NP2, and NP3.

(a) Cycling stability of NP3 electrode at a current denpan class="Chemical">sity of 15 mA g–1 over 5000 cycles (b) GCD before and after cycling of the NP3 electrode (c,d) pan class="Chemical">comparative Nyquist plot of NP1, NP2, and NP3.

Despite being a highly durable material, it should possess greater electronic kinetics. Therefore, to further study the transfer kinetics of pan class="Gene">NP1, pan class="Gene">NP2, and an NP3 active electrode in the frequency range of 100 mHz to 100 kHz, a Nyquist curve is obtained from EIS spectra and is shown in Figure c,d. The Nyquist plot of nanocomposite comprises three different frequency portions: first, high-frequency element (semicircle) and a low-frequency element (straight sloping line vs imaginary line) which includes a transition area between the above said two elements known as a knee frequency. Notably, the knee frequency where the slope is observed would be misguidedly added to Warburg diffusion. The semicircle and linear slope line are attributed to the electron transfer process at the electrode–electrolyte interface and charge transfer within the electrode, respectively.[39−41]

On analyzing the Nyquist plot of pan class="Gene">NP1, pan class="Gene">NP2, and NP3 samples before and after long-term cycles (Figure c,d), the charge-transfer resistance decreases in the order of NP1 > NP2 > NP3. In the case of NP3, after 5000 charge–discharge cycles, the solution resistance (Rs) increases from 0.463 to 0.477 Ω along with the charge-transfer resistance (Rct) from 6.52 to 13.54 Ω. The Nyquist plots revealed superior electrochemical kinetics of NP3, in the sense. For NP1 and NP2, it is exhibiting increment in the electrode–electrolyte interface access resistance and charge-transfer resistance after cycling. At the same time, NP3 is showing negligible changes in the high- and mid-frequency regions. The lowering of Rct value suggests that a synergistic effect of amorphous bimetallic ions anchored on the carbon matrix possesses short electron diffusive pathway than the other two materials (NP1 and NP2) favoring a long-term stability of the cathode. This facilitates NP3 material to operate as an ideal electrode under such low frequencies.

Hybrid Capacitor

From the CV, GCD, and EIS analypan class="Chemical">sis, NP3 was chosen to be a suitable cathode for pan class="Chemical">constructing a hybrid supercapacitor with AC as an anode and 2 M KOH as the electrolyte. This hybrid supercapacitor delivers high-energy density with excellent cyclability. Figure a shows CV curve of the anode measured in the range of −1.0 to 0 V exhibiting a typical EDLC behavior, whereas the CV pattern of cathode measured in the range of 0–0.5 V shows excellent redox performance. The optimum operating potential of the hybrid supercapacitor was chosen as 0–1.4 V. As shown in Figure b, the CV analysis of the as-prepared full cell was carried out in the potential range of 0–1.5 V. The CV curve shows the mixed energy storage mechanism (capacitive and battery type) for all the scan rates increasing in the order of 5, 7, 10, 20, 30, 40, 50, and 100 mV s–1. The curves expand but no obvious change was obtained in the shape of the CV curve. The experimental observation reveals the rapid kinetic behavior and high Columbic efficiency of the hybrid supercapacitor.

Figure 5

(a) CV curves of AC and NP3 electrodes obtained at the same scan rate; (b) CV plot of the device obtained at various scan rates ranging from 5 to 100 mV s–1; (c) galvanostatic charge–discharge curve of hybrid device at a current density of 1–10 mA g–1; and (d) cycling performance at a current density of 15 mA g–1 over 10 000 cycles.

(a) CV curves of pan class="Chemical">AC and NP3 electrodes obtained at the same scan rate; (b) CV plot of the device obtained at various scan rates ranging from 5 to 100 mV s–1; (c) galvanostatic charge–discharge curve of hybrid device at a current density of 1–10 mA g–1; and (d) cycling performance at a current density of 15 mA g–1 over 10 000 cycles.

Figure c illustrates the charge–discharge plot at a set of current denpan class="Chemical">sities ranging from 1 to 10 mA g–1. The GCD at different current denpan class="Chemical">sities with symmetrical shapes suggests faradic capacitive behavior, and the absence of potential drop indicates negligible loss of energy in the fabricated device. As shown in Figure S6, specific capacitance was calculated as 67.14 F g–1 at a current density of 1 mA g–1; even at high current density, the device maintains reasonable capacitance indicating excellent rate capability. Such enhanced performance of the device is assigned to electric double-layer capacitance of AC and outstanding properties of the NP3 composite.

For prpan class="Chemical">actical applications of hybrid supn>erpan class="Chemical">capacitor, energy density alone is not enough but also it has to maintain specific capacitance for a long time. In Figure d, it has been proved by running 10 000 times charge and discharge cycles. Up to 6000 cycles, the specific capacitance of the hybrid supercapacitor increased and decreased because of the activation process of the hybrid composite electrode. It is because the amorphous NiCoP2O7 is attached with g-C3N4 sheets (see Figure ) so it has to be activated for a long time to achieve the charge flow through the pores with the help of g-C3N4. After 6000 cycles, the device has retained 100% of its performance for the rest of 4000 cycles without any further degradation. The hybrid supercapacitor possesses long-termed cyclic retention, which is proven by the stability analysis. The hybrid supercapacitor was further examined by both CV and GCD test after 10 000 cycles and is shown in Figure a,b; both CV and GCD curve analyses revealed no considerable changes in the electrochemical performance.

Figure 6

(a,b) Before and after cycling CV and GCD curves and (c,d) Nyquist plot at various bias voltages of device.

EIS is a powerful technique to investigate the electrochemical process in depth, where variations in bias voltage and frequency strongly influence the electrochemical performance and their outputs reveal the electrochemical nature of the electrode material. Nowadays, researchers try to understand the intercalation mechanism upan class="Chemical">sing EIS, and there are very few repn>orts for investigating the hybrid pan class="Chemical">capacitor with EIS.[42,43] Here, we have analyzed the NP3 composite-based hybrid capacitor by changing the bias voltage. EIS measurements at different bias potentials ranging from 0.1 to 1.4 V were carried out for the device before and after the stability test and are shown in Figure c,d. The Nyquist plot can be divided into, viz., three major parts high, mid, and low frequencies which indicate the electrode–electrolyte interface, charge-transfer resistance, and adsorptive nature of ions with the surface of electrode, respectively. The straight line at low-frequency range displays a linear variation, and at the lower potential nearby, vertical line along the imaginary axis is obtained. In particular, when the dc bias voltage ranges from 0.1 to 0.6 V, the impedance plot initially exhibits nearly vertical line together with the imaginary plane and later tends to exhibit a significant variation (i.e., a deviation from 90°) along the imaginary axis denoting more facile OH– ionic intercalation in the metallic pyrophosphates. Later, on increasing the bias potential from 0.7 to 1.4 V, the charge-transfer resistance before cycling and after cycling shows slight increment in the Rct value by 6.4 Ω and the line at low frequency shows a tendency of getting closer to 90° along an imaginary axis representing the ideal capacitive behavior. Also, the length of the slope line decreases indicating higher diffusion resistance of OH– ions at higher bias voltages. In addition, when the bias voltage is increased from 0.1 to 1.4 V initially, the deviation from the nearly right angle takes place linearly and gets back when it approaches 1.4 V. This deviation is assigned to reversible intercalation of the OH– type mechanism.[44] Second, the diameter variation of the Warburg-type element corresponding to the diffusion of OH– ions in the mid to low and high-frequency region is significant. On increasing the bias potential, the diameter of the curve decreases with the appearance of a diffusion resistance region (also called Warburg impedance) corresponding to the diffusion of OH– ions through the electrode. On further increase from 0.1 V, the Warburg region tends to decrease and at 1.4 V, the presence of merely absent Warburg region may be attributed to the low resistance offered by the surface layer and short diffusion path for the mobility of OH– ions. Although the biased voltage varies, the intercept at x-axis z′(real part) of higher frequency range coincides with each other, suggesting that (i) untainted charge-transfer resistance, (ii) contact as well as diffusive resistance at the interface between working electrode–current collector, and (iii) electronic and ionic charge-transfer resistance. This variation of impedance spectroscopy with the bias voltage shows that the diffusion-type battery mechanism involved in the process. The optimum bias voltage in which the working electrode can exhibit better capacitance is 0.1–0.5 V and is in good agreement with the obtained CV analysis. At these potentials, z″ imaginary part of impedance shows a steep increment representing a vertical line characteristic of the ideally polarizable active electrode. This nature recommends specifically pseudocapacitance contribution in the overall energy storage. The variation in both terms (such as mechanism and resistance) can be assigned to the pore activation process of the electrode.

Finally, the Ragone plot (Figure ) was used to investigate the electrochemical performance of the hybrid pan class="Chemical">capacitor. The Ragone plot of pan class="Gene">ASC remarkably reached the energy density of ∼65.8 W h kg–1 at a power density of 0.2 W kg–1, displaying an improved energy capability. In addition, it exhibits relatively higher energy density than recent reports and few of those are tabulated in Table S1.[45−48] The overall experimental results indicate a fact that the NiCoP2O7/g-C3N4 exhibits excellent electrochemical performance and a hybrid capacitor constructed with NiCoP2O7/g-C3N4 displays efficient charge storage capability with long-term cycle stability.

Figure 7

Ragone plot (energy density vs power density) of the hybrid capacitor.

Ragone plot (energy denpan class="Chemical">sity vs power denpan class="Chemical">sity) of the hybrid pan class="Chemical">capacitor.

Conclusions

pan class="Chemical">Concisely, in the present work, the pan class="Chemical">NiCoP2O7/g-C3N4 hybrid nanocomposite has been successfully developed by a facile solvothermal technique. The as-synthesized NP3 hybrid nanocomposite showed an improved specific capacitance of 341.58 F g–1 at a scan rate of 5 mV s–1, which was higher compared with NP1 and NP2 composites. The g-C3N4 matrix forbids the degradation of NiCoP2O7 and disintegration of active material during long-term cycling, resulting in excellent cycling retention of 100% after 5000 cycles at a current density of 15 mA s–1. The novel and durable hybrid capacitor assembled using NP3 and AC as a cathode and anode, respectively, delivered a high-energy density of 65.8 W h kg–1 at a power density of 0.233 W kg–1 with enhanced cyclability over 10 000 cycles. Hence, this study sheds light on the development and optimization of various multimetallic pyrophosphate architectures on carbon nitride.

Experimental Section

Synthesis of NiCoP2O7/g-C3N4 Nanocomposite

Analytical grade C3pan class="Species">H6N6, Ni (NO3)2·6pan class="Chemical">H2O, Co (NO3)2·6H2O, and C2·H6O were used as received without any further purification. In a typical procedure, bulk g-C3N4 was synthesized by calcination of melamine at a semi-closed crucible to skip out overreaction with oxygen at 500 °C for 4 h and 520 °C for 2 h. As-prepared bulk g-C3N4 was subjected to liquid exfoliation to obtain 2D g-C3N4 nanosheet morphology. In a typical reaction, 0.1 g of as-prepared bulk carbon nitride was dispersed in double distilled water and ultrasonicated for 30 min. After that, pale yellow color graphitic carbon nitride (g-C3N4) nanosheets were collected. To prepare NiCoP2O7/g-C3N4 nanocomposite, 70 mg of g-C3N4 was thoroughly dispersed in 70 mL of ethanol and then an appropriate amount of nickel nitrate, cobalt nitrate, and (ADP) were added. The g-C3N4 content in the NiCoP2O7/g-C3N4 composite was in the ratio of 0.5:1. The mixture was stirred overnight at room temperature, to attain a homogeneous solution and then transferred into a Teflon-lined stainless autoclave maintained at 140 °C for 12 h. After cooling down to room temperature naturally, the products were collected and centrifuged and dried at 60 °C for 10 h. Then, the sample was annealed at 400 °C for 2 h at a rate of 1 °C min–1 in a flowing atmosphere. Pristine NiP2O7 and NiP2O7/g-C3N4 nanocomposite were also prepared by similar procedure without the addition of Co/g-C3N4 and Co precursors, respectively.

Characterization Techniques

Crystallographic information of the samples was examined upan class="Chemical">sing an X-ray diffractometer (Rigaku mini flux II) equipped with high intense Cu Kα radiation (λ = 1.5418 Å). The elemental composition and valence state of NiCoP2O7/g-C3N4 hybrid were recorded by using XPS (Thermo Scientific MultiLab 2000). Using (FESEM JEOL 7600F), the morphological features and its microstructures were investigated. HRTEM with mapping images and high resolution was recorded from JEOL, 2100F instrument, HORIBA EMAXX-ACT that was connected with TEM. The CHI627 workstation with conventional three electrodes electrochemical cell setup was used for analyzing the electrochemical properties of the prepared samples and fabricated device. Finally, the EIS was performed by using a ZAHNER impedance analyzer.