Jing
Xu
a,
Nianjun
Yang
*a,
Siyu
Yu
ab,
Anna
Schulte
c,
Holger
Schönherr
c and
Xin
Jiang
*a
aInstitute of Materials Engineering, University of Siegen, 57076 Siegen, Germany. E-mail: nianjun.yang@uni-siegen.de; xin.jiang@uni-siegen.de
bSchool of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China
cPhysical Chemistry I & Research Center of Micro and Nanochemistry and Engineering (Cμ), Department of Chemistry and Biology, University of Siegen, 57076 Siegen, Germany
First published on 2nd June 2020
The low energy density of traditional supercapacitors has strongly restricted their applications. The utilization of novel capacitor electrodes to enhance the energy densities of supercapacitors is thus of great significance. Herein, a binder-free Ni12P5/Ni/TiC nanocomposite film is synthesized and further employed as the capacitor electrode. This nanocomposite film is grown by means of a chemical vapor deposition process, where Ni5TiO7 nanowires and a TiO2 layer are in situ converted into hierarchical interconnected three-dimensional (3D) Ni/Ni12P5 nanoparticles and a porous TiC matrix, respectively. Such a nanocomposite film exhibits an extremely high specific surface area and excellent conductivity, leading to its high capacitive performance. Remarkably, the multiple redox states of Ni species, namely two pairs of redox waves are observed in neutral aqueous solutions. At a current density of 10 mA cm−2, its specific capacitance in 1 M Na2SO4 aqueous solution is as high as 160.0 mF cm−2. The maximal energy density of a supercapacitor fabricated with this nanocomposite capacitor electrode is 42.6 W h kg−1 at a power density of 1550 W kg−1. Such an ultra-high energy density is even comparable with that of Li-batteries. The proposed supercapacitor thus has high potential for industrial applications.
To date, various PC capacitor electrodes have been synthesized and applied for the construction of high-performance SCs.1–6 Among them, Ni-based materials are one of the extensively reported PC capacitor electrodes. This is due to their abundance and easy availability. For example, nickel oxide/hydroxide (NiO/Ni(OH)2) PC capacitor electrodes have been frequently utilized. Unfortunately, their low electric conductivities led to relative low power densities of these PCs. Due to the excellent electric conductivities and superior redox activities of nickel phosphides (e.g., NiP2, Ni12P5, Ni2P, etc.), they have been proposed to replace these oxide/hydroxide PC capacitor electrodes.7,8 For example, a specific capacitance of 2141 F g−1 has been obtained for a Ni2P/Ni capacitor electrode, which is about three times higher than that of a Ni(OH)2/Ni capacitor electrode under identified conditions.9 Among nanocapsule Ni12P5, flower-like NiO, and flower-like Ni(OH)2 PC capacitor electrodes, the Ni12P5 nanocapsules exhibit the highest capacitance of 949 F g−1 at a current density of 1 A g−1.10 Although various methods (e.g., such as ball-milling11 and hydrothermal process12) have been utilized to synthesize nickel phosphides, the as-produced nickel phosphides by these methods contain a lot of byproducts. In other words, the purities of nickel phosphides synthesized by these methods are low.13,14 Moreover, they are only in the form of powders. A binder(s) and current collector are thus required for the construction of these nickel phosphide based PC capacitor electrodes. The novel approach to synthesize binder-free nickel phosphide based PC capacitor electrodes is thus highly demanded.
Herein, a binder-free Ni12P5/Ni/TiC nanocomposite film is synthesized by means of in situ carbonization/reduction of the precursors of Ni5TiO7 nanowires in a chemical vapor deposition (CVD) reactor. The P dopant is introduced into a porous TiO2 layer in the course of a plasma electrolytic oxidation (PEO) process.15 On this hierarchical film, Ni/Ni12P5 nanoparticles are coated on the top of a TiC layer. A three-dimensional (3D) network structure of this nanocomposite film leads to an extremely high specific surface, good permeability, and full exposure of active sites. Moreover, the TiC layer is porous and has excellent electric conductivity. In other words, it can serve as the current collector. Consequently, this novel Ni12P5/Ni/TiC/Ti nanocomposite film possesses all advantages of a capacitor electrode that are required for the construction of high-performance PCs. In this contribution, the investigation of the capacitive performance of this nanocomposite film (e.g., its capacitance, capacitance retention) in 1 M Na2SO4 solution and the evaluation on power densities and energy densities of the SCs constructed using this nanocomposite PC capacitor electrode are presented.
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Fig. 1 XRD spectra of a Ni5TiO7/TiO2 nanocomposite film before (a) and after (b) the CVD treatment for 15 min. |
The SEM images and related EDX profiles of the as-prepared nanocomposite films before and after the CVD process are shown in Fig. 2. Straight and dense Ni5TiO7 nanowires are seen on the Ni5TiO7/TiO2(P) nanocomposite film before the CVD process (Fig. 2a). The average diameter of these nanowires is about 190 nm and their surface is smooth (Fig. S1b†). The EDX line profile reveals that the overall surface consists mainly of the elements of Ti, Ni, O, and P (Fig. 2b). The element of P is from the used PEO electrolyte. It is believed that the electrolyte components can be dissociated to active ion- or atom-species due to the high energy discharges during the PEO.17,18 This is confirmed by the EDX analysis of the PEO sample before the growth of the Ni5TiO7 nanowires (Fig. S2a†). Hence, the phosphorus ions are incorporated into the porous TiO2 surface/matrix during the growth of the TiO2 layer. Such phenomena are similar to those reported previously.19,20 After the CVD treatment, the morphology of these Ni5TiO7 nanowires is greatly changed (Fig. 2c). Their surface is not smooth anymore. The surface of these nanowires appears to be coated with different particles, the sizes of which vary from 89 to 221 nm. These fibrous nanoparticles are actually aggregated with each other or further interconnected, leading to the formation of a 3D network structure. The EDX line profile of a local nanoparticle (Fig. 2d) reveals that these nanoparticles are mainly composed of the elements of Ni, P and C as well as a tiny amount of the elements of Ti and O. The existence of C element on the local nanoparticle suggests the formation of amorphous carbon. Further analysis of EDX line profiles on different nanoparticles reveals that the average atomic ratio of Ni to O is approximately 21:
5, higher than the stoichiometry of Ni12P5. This indicates an excess amount of Ni element in this composite film, probably due to the existence of pure Ni atoms, as already observed from the XRD analysis of these nanoparticles. Note here that, it is believed that the variation of the atomic ratios of these elements (e.g., Ni, P, and O) inside these nanoparticles actually affects their properties (e.g., the capacitive performance of the as-fabricated Ni12P5/Ni/TiC composite capacitor electrode). For a more precise calculation of the atomic ratios of these elements (e.g., Ni, P, and O) in different nanoparticles and their structure (e.g., a core–shell structure), the high-resolution transmission electron images of these nanoparticles are required.
To further investigate the structure of this nanocomposite film, it was treated in an ultrasonic bath for a few seconds. In this way, superficial nanoparticles were partially removed. The SEM image of a treated sample and its corresponding EDX mapping are shown in Fig. 2e and f, respectively. Two distinct structures are observed beneath the nanoparticle network: a porous bottom layer and a “melt-like” interlayer. The diameter of the pores in the bottom layer ranges from 0.7 to 2.8 μm. Such a porous layer is derived from the porous coating that is formed after the PEO treatment (Fig. S2b†). The formation of these pores is due to the discharging behavior during the PEO process.21 The bottom of this porous layer is a Ti-rich area (Fig. 2f), indicating that this TiC layer is originally transformed from the as-formed TiO2 layer during the PEO process. Moreover, a “melt-like” layer is covered on the top of pores. Such an interlayer is actually a Ni-rich area. This “melt-like” layer is assumed to be formed via the reduction of NiO, which is derived from the thermal decomposition of Ni(NO3)2 during the annealing process.22
To further analyze the composition and the chemical bonding states of the Ni5TiO7/TiO2 nanocomposite film after the CVD treatment, its XPS spectra were recorded. Its XPS survey spectrum (Fig. 3) confirms that the dominant element of the CVD treated Ni5TiO7/TiO2 nanocomposite film is carbon (79.9%), suggesting the successful carbonization of Ni5TiO7 during the CVD process. Its high-resolution XPS spectra were then utilized to investigate the bonding state of Ti, C, Ni and P elements. For example, in its Ti 2p spectrum (Fig. 3a), two peaks are centered at 460.9 and 454.9 eV, which can be assigned to the Ti 2p1/2 and Ti 2p3/2 of the Ti4+ species, respectively.23 The shoulder between them (at around 457.8 eV) indicates the existence of Ti in a lower valence. In its C 1s spectrum (Fig. 3b), the peak at 281.9 eV is attributed to metal–C binding, confirming again the formation of TiC. The rest of the peaks located at the binding energies of 284.4, 286.2, and 288.6 eV correspond to carbon in C–C (sp2), C–O, and CO, respectively. The sp2 carbon indicates the existence of amorphous carbon in the composite film. In its Ni 2p spectrum (Fig. 3c), there are two dominating peaks centered at binding energies of 852.8 and 870.0 eV, ascribed to Ni 2p3/2 of metallic Ni0.24 Another two broad peaks at 855.1 and 873.5 eV can be assigned to NiII. Two additional peaks at 859.8 and 876.5 eV are assigned to the shake-up satellite structure of Ni 2p1/2. In its P 2p spectrum (Fig. 3d), the binding energy at 129.8 eV can be ascribed to phosphide (Ni–P). The peak appearing at 133.3 eV can be attributed to phosphate species, which may have been formed due to the exposure of the sample to air.10,25 Therefore, in Ni12P5 the Ni species exhibits a slight positive charge (Nix+, 0 < x < 2), while the P species has a small negative charge. Consequently, such a nanocomposite film features pseudocapacitive behavior.7,8,10 Once it is utilized as capacitor electrodes, a high performance SC is expected to be constructed.
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Fig. 3 High-resolution XPS spectra of (a) Ti 2p, (b) C 1s, (c) Ni 2p, and (d) P 2p core levels for the Ni12P5/Ni/TiC nanocomposite film. |
Ni2+ + 2OH− ↔ Ni(OH)2 | (1) |
Ni(OH)2 + OH− ↔ NiOOH + H2O + e− | (2) |
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Fig. 4 (a) CVs and (b) GCD curves of a Ni12P5/Ni/TiC nanocomposite capacitor electrode in 1.0 M Na2SO4 aqueous solution at different scan rates and current densities; (c) capacitance retention as a function of cycling numbers where the applied current density is 20 mA cm−2. The insets in (c) are the SEM images of the Ni12P5/Ni/TiC nanocomposite film after (left) 2000 and (right) 10![]() |
An increase of the scan rates leads to the linear enhancement of peak currents and enlarged peak difference (namely a positive shift of anodic peak potentials and meanwhile a negative shift of cathodic peak potentials). The redox process of this Ni12P5/Ni/TiC nanocomposite capacitor electrode is thus quasi-reversible. Notice that, these CVs are distinct from those for EDLCs, which are close to the ideal rectangular shape. Therefore, the Ni12P5/Ni/TiC nanocomposite capacitor electrode exhibits pseudocapacitive behavior. The calculated pseudocapacitances are 133.9, 92.5, 56.2, and 34.7 mF cm−2 at a scan rate of 10, 20, 50, and 100 mV s−1, respectively.
Fig. 4b presents the galvanostatic charging/discharging (GCD) curves of the Ni12P5/Ni/TiC nanocomposite capacitor electrode at the current densities ranging from 10 to 30 mA cm−2. The curves are non-linear but with equivalent charging and discharging times. Take the current density of 10 mA cm−2 as an example, the discharging time is ca. 21 s and the charging time is 29 s. This confirms the good reversibility of this PC electrode during the charging/discharging processes. Two voltage plateaus (from −0.1 to −0.07 V in the charging region, and from 0.08 to 0.11 V in the discharging domain) indicate that the redox reactions are dominated in the charging/discharging processes, in good agreement with the CV measurements. The estimated capacitances are 160.0, 84.3, 43.0, and 4.9 mF cm−2 at a current density of 10, 15, 20 and 30 mA cm−2, respectively. The ultra-high specific capacitance of this Ni12P5/Ni/TiC nanocomposite capacitor electrode is believed to mainly originate from its unique nano-structural feature or its high surface area. In other words, the extremely high specific surface of the Ni12P5/Ni nanoparticles provides a large and pseudocapacitive surface area. Meanwhile, its hierarchical 3D network benefits accessibility to the electrolytes in solutions.
The long-term stability of the Ni12P5/Ni/TiC nanocomposite capacitor electrode was tested by means of the GCD technique at a current density of 20 mA cm−2. The capacitance retention is shown in Fig. 4c as a function of the cycling number. After the first 2000 cycles, the capacitance is reduced to 65% of its initial capacitance. To figure out possible reasons for this rapid reduction of the as-obtained capacitance, the Ni12P5/Ni/TiC composite capacitor electrode was then examined using SEM after the cycling test. Surprisingly, its 3D network (shown in Fig. 2c) is partially destroyed (the left inset in Fig. 4c). Namely, some nanostructural features disappear, probably originating from surface damage during such a surface-controlled redox process and/or quasi-/ir-reversible surface re-construction in the course of the charging/discharging process. To avoid the rapid reduction of the as-obtained capacitances, the Ni12P5/Ni/TiC nanocomposite capacitor electrode was then coated with a thin Nafion membrane. Under this condition, the capacitance of the Ni12P5/Ni/TiC nanocomposite capacitor electrode is only reduced to 82.1% of its initial value, even after 10000 charging/discharging cycles. As expected, the surface morphology of the Ni12P5/Ni/TiC composite capacitor electrode (the right inset in Fig. 4c) is nearly unchanged after such a long-term stability test.
A symmetrical two-electrode system was then constructed using Ni12P5/Ni/TiC nanocomposite capacitor electrodes. It was further applied to calculate the energy (E) and power (P) densities of this PC device. The mass of active electrode materials was estimated by weighing the active films peeled from the Ti substrate. Fig. 4d shows a comparison of the gravimetric Ragone plot of this PC (namely P vs. E) with that of other capacitors and batteries. For example, the maximal E is 42.6 W h kg−1, which is obtained at a P of 1550 W kg−1. This energy density is even close to that of lithium batteries. Compared to some other reported phosphide based supercapacitors,14,27 our device exhibits not only much higher E, but also higher P. For example, its energy density remains as 3.5 W h kg−1 even at a power density of 15762 W kg−1. Such an excellent SC or PC performance is due to high pseudocapacitive behavior (or redox activity) of the Ni12P5/Ni species in the Ni12P5/Ni/TiC nanocomposite film. In addition, the excellent conductivities of Ni12P5, Ni, and TiC accelerate the exchange of the electrons between the electrode and the electrolytes, eventually bringing in improved pseudocapacitive behavior or SC performance of the Ni12P5/Ni/TiC nanocomposite capacitor electrode.
A supercapacitor demonstrator was further constructed to check the application potential of the proposed Ni12P5/Ni/TiC nanocomposite capacitor electrodes. The voltage-time curve shown in Fig. 5 interprets the real working performance of this SC device. From the recorded voltage-time curve, one can see outstanding reversibility and repeatability of such a demonstrator. Namely, it features good stability. The red LED can be lighted up for about 20 s after a charging time of 10 s with a voltage decrease from 1.6 to 2.4 V. Note that, only one PC device is employed here to light up a red light-emitting diode (LED, Fig. S4†). Hence, such PCs have great potential in practical applications.
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Fig. 6 Schematic illustration of the synthesis of the Ni12P5/Ni/TiC composite film on a Ti substrate. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nr01984d |
This journal is © The Royal Society of Chemistry 2020 |