High-Performance Flexible Solid-State Asymmetric Supercapacitors

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High Performance Flexible Solid-State Asymmetric Supercapacitors Based on Bi-Metallic Transition Metal Phosphide Nanocrystals Nan Zhang, Yifan Li, Junyuan Xu, Junjie Li, Bin Wei, Yu Ding, Isilda Amorim, Rajesh Thomas, Sitaramanjaneva Mouli Thalluri, Yuanyue Liu, Guihua Yu, and Lifeng Liu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b04810 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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High Performance Flexible Solid-State Asymmetric Supercapacitors Based on Bi-Metallic Transition Metal Phosphide Nanocrystals Nan Zhang†, Yifan Li‡, Junyuan Xu†, Junjie Li†,, Bin Wei†, Yu Ding‡, Isilda Amorim†, Rajesh Thomas†, Sitaramanjaneva Mouli Thalluri†, Yuanyue Liu‡,*, Guihua Yu‡,* and Lifeng Liu†,*

†

International Iberian Nanotechnology Laboratory, Av. Mestre Jose Veiga, 4715-330

Braga, Portugal

‡

Materials Science and Engineering Program and Department of Mechanical

Engineering, University of Texas at Austin, Austin, Texas 78712, United States

||

Key Laboratory of Functional Materials and Devices for Special Environments, Xinjiang

Technical Institute of Physics & Chemistry, CAS; Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, China.

ACS Paragon Plus Environment

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ABSTRACT

Transition metal phosphides (TMPs) have recently emerged as an important type of electrode materials for use in supercapacitors thanks to their intrinsically outstanding specific capacity and high electrical conductivity. Herein, we report the synthesis of bimetallic CoxNi1-xP ultrafine nanocrystals supported on carbon nanofibers (CoxNi1-xP/CNF) and explore their use as positive electrode materials of asymmetric supercapacitors. We find that the Co:Ni ratio has a significant impact on the specific capacitance/capacity of CoxNi1-xP/CNF, and CoxNi1-xP/CNF with an optimal Co:Ni ratio exhibits an extraordinary specific capacitance/capacity of 3514 F g-1 / 1405.6 C g-1 at a charge/discharge current density of 5 A g-1, which is the highest value for TMP based electrode materials reported by far. Our density functional theory (DFT) calculations demonstrate that the significant capacitance/capacity enhancement in CoxNi1-xP/CNF, compared to the mono-metallic NiP/CNF and CoP/CNF, originates from the enriched density of states near the Fermi level. We further fabricate a flexible solid-state asymmetric supercapacitor using CoxNi1xP/CNF

as positive electrode materials, activated carbon as negative electrode materials


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and a polymer gel as the electrolyte. The supercapacitor shows a specific capacitance/capacity of 118.7 F g-1 / 166.2 C g-1 at 20 mV s-1, delivers an energy density of 32.2 Wh kg-1 at 3.5 kW kg-1, and demonstrates good capacity retention after 10000 charge/discharge cycles, holding substantial promise for applications in flexible electronic devices.

KEYWORDS: asymmetrical supercapacitor, transition metal phosphide, nickel phosphide, cobalt phosphide, high specific capacitance


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Flexible and epidermal electronics, such as stretchable displays, foldable smart phones and skinmounted health monitors, have recently attracted considerable interest and are expected to grow into a huge market. Flexible supercapacitors (SCs) represent one of the most important components that can be integrated into flexible electronic devices, storing energy and providing power to active components (e.g. light-emitting diodes, sensors) upon demand. In the last few years, considerable effort has been dedicated to the research of novel materials and design concepts that may offer improved storage performance and better compatibility to flexible SCs.1-8 From electrode materials standpoint, battery-type positive electrode materials, particularly transition metal oxides and hydroxides, have been demonstrated to be able to increase specific capacitance and energy density values in comparison to conventional carbonaceous materials.9-19 However, metal oxides and hydroxides usually have poor electrical conductivity, and therefore they are kinetically unfavorable for fast charge transport to deliver satisfying rate performance. Recently, transition metal phosphides (TMPs), as an emerging type of important functional materials, have been extensively investigated for many applications such as electrocatalytic water splitting,20-22 photocatalytic reactions,23-25 rechargeable batteries,26-28 and supercapacitors,29-42 owing to their excellent electrochemical performance and outstanding electrical conductivity. Moreover, TMPs show prominent thermal stability and good resistance to the ambient environment, allowing them to be used as stable electrode materials. Hence, various TMPs such as nickel phosphide,29-36 cobalt phosphide,37-40 germanium phosphide,41 and copper phosphide,42 have been explored as novel positive electrode materials in asymmetric supercapacitors and showed superior specific capacitance and rate performance. For example, Jin et al. fabricated a flexible supercapacitor based on Ge5P nano-flakes obtained via liquid phase exfoliation, which revealed a high volumetric energy density and good charge/discharge rate capability.41 In


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particular, nickel phosphides and cobalt phosphides have been demonstrated to show prominent advantage over conventional transition metal oxides and hydroxides in energy storage. Zhou et al. reported the synthesis of Ni2P and Co2P nanosheet arrays on nickel foam current collectors (Ni2P NS/NF and Co2P NS/NF), and found that Ni2P NS/NF exhibited a specific capacitance of 2141 F g-1 at 50 mV s-1, much higher than that of Ni(OH)2 NS/NF having similar morphology and microstructure.43 Moreover, they assembled an asymmetric supercapacitor using Ni2P NS/NF as a positive electrode and activated carbon as a negative electrode, which achieved a specific capacitance of 96 F g-1 at 5 mV s-1 and delivered an energy density of 26 W h kg-1. It is known that introducing a secondary metal into mono-metallic transition metal compounds may markedly improve the number of redox centers and thereby their pesudocapacitive performance, as demonstrated previously for NiCo2O4 electrode materials.44 Recently, bi-metallic TMPs have drawn considerable attention because of the enhanced storage capacity they demonstrated for use as electrode materials in supercapacitors.45-54 For instance, Liang and coworkers compared the supercapacitive performance of NiCoP to that of Ni2P and CoP, and found that both the storage capacity (194 mAh g-1 / 698.4 C g-1 at 1 A g-1) and cycling stability of NiCoP were higher than those of mono-metallic phosphides49. Notwithstanding some progress, most bimetallic TMP electrode materials reported so far only have one specific stoichiometry. Fine-tuning the composition of bi-metallic TMPs may further improve the charge storage capacity of the supercapacitor electrodes, but how the ratio of two metal components affects the capacitive properties and where the synergy originates from have been rarely explored both experimentally and theoretically. In this work, we have developed ultrafine and highly active bi-metallic CoxNi1-xP nanoparticle (NPs) loaded on carbon nanofibers (CNF) as the positive materials (CoxNi1-xP/CNF) and


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investigated the influence of Co:Ni ratios on the pesudocapacitive properties. We found that introducing a certain amount of Co into NiP may markedly enhance the specific capacitance/capacity, but if the Co content surpasses that of Ni, the specific capacitance/capacity starts to decrease dramatically. Our density functional theory (DFT) calculations disclose that the composition-dependent capacitance/capacity variation is correlated nicely with the density of states of CoxNi1-xP near the Fermi level, and the synergy between Co and Ni can significantly improve the capacitive properties. Thanks to the high electrochemical activity of the ultrafine NPs and the synergy between Co and Ni, the CoxNi1-xP with an optimized Co:Ni ratio shows an extraordinary specific capacitance/capacity of 3514 F g-1 / 1405.6 C g-1 at a charge/discharge current density of 5 A g-1, which is the highest value reported so far for TMP-based electrode materials. To demonstrate the application potential, we further fabricated an asymmetric supercapacitor using CoxNi1-xP/CNF and commercially available activated carbon (AC), which reveals good performance in terms of both energy/power densities and cycling stability.

RESULTS AND DISCUSSIONS

The preparation of CoxNi1-xP/CNF electrode materials is schematically illustrated in Figure 1a, and the fabrication procedures are detailed in Supporting Information. In brief, CoxNi1-x alloy clusters were firstly loaded on acid-treated CNFs by solution-based chemical reduction of metal cations in ethylene glycol (EG) solution using sodium borohydride (NaBH4) as the reductant. To study the influence of Co:Ni ratio on the pseudocapacitance, precursor solutions containing different Co:Ni ratios were used to prepare CoxNi1-x/CNF. The CoxNi1-xP/CNF was obtained by post-phosphorization treatment of CoxNi1-x/CNF at 300 °C using sodium hypophosphite (NaH2PO2) as the source of phosphor and high purity nitrogen (N2, 99.999%) as the carrier gas.


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Inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses revealed that nearly all TMP/CNF samples have a similar metal loading of about ca. 17 wt.%, and the measured Co:Ni ratios in CoxNi1-xP/CNF are close to those expected (Figure S1, Supporting Information).

Figure 1. (a) Schematic illustration of the preparation of CoxNi1-xP/CNF active materials. (b, c) SEM images showing the morphology of representative Co0.1Ni0.9P/CNF. (d) TEM and (e) HRTEM images of Co0.1Ni0.9P/CNF. Insets: NP size distribution, zoomed view of a single Co0.1Ni0.9P NP and the corresponding FFT-ED pattern. (f) STEM-HAADF image and elemental maps of mixed elements, C, Ni, Co and P.

The morphology of the as-synthesized CoxNi1-xP/CNF was examined by scanning electron microscopy (SEM). The CNFs were found to interconnect with each other forming a reticulate structure (Figures 1b and 1c). This may potentially offer numerous electron conducting channels


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and ion diffusion paths, and help the infiltration of electrolyte. To further investigate the microstructure of CoxNi1-xP/CNF, transmission electron microscopy (TEM) investigations were carried out. Figure 1d shows a representative TEM image of Co0.1Ni0.9P/CNF, where fine Co0.1Ni0.9P NPs with diameters ranging from 2 to 10 nm (mean diameter: 4 nm, Figure 1e inset) are observed to distribute on both the inner and outer walls of the hollow tubular CNFs with a high density. High-resolution TEM (HRTEM) imaging confirmed that these Co0.1Ni0.9P NPs are highly crystalline and may adopt an orthorhombic crystal structure with lattice constants of a = 6.031 Å, b = 4.707 Å, and c = 6.815 Å (Figure 1e, inset), matching well with that of NiP (ICDD No. 00018-0882, a = 6.044 Å, b = 4.878 Å, and c = 6.887 Å). The elemental mapping was performed in the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mode, which reveals that Co, Ni, and P elements uniformly cover the CNF surface (Figure 1f).


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Figure 2. (a) XPS survey spectrum of Co0.1Ni0.9P/CNF. High resolution XPS spectra of (b) Ni 2p, (c) Co 2p and (d) P 2p of the Co0.1Ni0.9P/CNF sample. The elemental composition and valence states of the CoxNi1-xP/CNF were investigated by Xray photoelectron spectroscopy (XPS). Figure 2a shows the representative XPS survey spectrum of Co0.1Ni0.9P/CNF, which verifies the presence of Ni, Co, P, O, and C elements in the sample. A weak peak from Na KL1 is also visible, which might result from the remnant of the NaBH4 reductant used during the wet-chemical reduction process. Figure 2b is the high-resolution spectrum of Ni 2p3/2 for both Co0.1Ni0.9P/CNF and NiP/CNF, where it is observed that the binding energy (BE) peaks of Co0.1Ni0.9P/CNF and NiP/CNF basically appear at the same positions, indicating that doping of 10% Co into NiP does not give rise to a significant change in the surface electronic structure. As far as the Co 2p3/2 spectrum is concerned, the BE peaks of Co0.1Ni0.9P/CNF are found to shift toward higher BE values relative to those of CoP/CNF, implying more electrons are transferred from transition metal species to phosphor atoms in the bi-metallic Co0.1Ni0.9P/CNF (Figure 2c). The peaks appearing at 856.6 and 782.6 eV for CoxNi1-xP/CNF are likely ascribed to phosphate salts of Ni and Co species, because of the superficial oxidation of phosphide particles in air.43, 55 Moreover, the peak centred at 130.2 eV may be assigned to Pδ− in TM – P bonding of metal phosphide (Figure 2d), and the peak at 134.8 eV is typical of phosphate species, which have been reported previously in cobalt and nickel phosphides.56, 57 The crystallinity of CoxNi1-xP/CNF, CoP/CNF and NiP/CNF controls was further examined by X-ray diffractometry (XRD, Figure S2, Supporting Information). However, due to the small crystallite size, the XRD patterns of all samples are featureless without any resolvable diffraction peaks, similar to the result that we reported before.21


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We first tested the pesudocapacitive performance of the CoxNi1-xP/CNF in a three-electrode configuration in 2 M KOH solution. As shown in the cyclic voltammograms (CVs) (Figure S3, Supporting Information), CoP/CNF doesn’t show marked redox peaks in the potential window of 0 – 0.4 V vs. saturated calomel electrode (SCE), while NiP/CNF displays an oxidation peak at about 0.34 V vs. SCE (VSCE) and a reduction peak at about 0.2 VSCE, respectively; moreover, NiP/CNF exhibits a substantially higher redox current density compared to CoP/CNF. When introducing Co into NiP/CNF, the redox peaks firstly shift negatively as the Co content increases, and then positively when the Co:Ni is larger than 1:1. Furthermore, the peak current density of CoxNi1-xP/CNF changes with the varying Co:Ni ratio, and Co0.1Ni0.9P/CNF shows the highest peak current density among all the samples. The CV behaviors of CoxNi1-xP/CNF suggest that there is a strong electronic interaction between Co and Ni species which makes the oxidation of Ni easier, and that tuning Co:Ni ratio can change the Faradaic charge transfer and thereby the pesudocapacitive properties of CoxNi1-xP/CNF. Our observation agrees with recent reports on the CV studies of different Co-Ni-P nanostructures.45, 52


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Figure 3. (a) Specific capacitances/capacities of CoxNi1-xP/CNF with different Co:Ni ratios, calculated based on CV curves recorded at 50 mV s-1. b) CV curves of the best-performing Co0.1Ni0.9P/CNF electrode recorded at different scan rates. Inset: plot of the anodic peak current density vs. the square root of the scan rate (ν). (c) Specific capacitances/capacities of Co0.1Ni0.9P/CNF, CoP/CNF and NiP/CNF calculated at different scan rates. (d) Galvanostatic charge and discharge curves of the Co0.1Ni0.9P/CNF electrode recorded at different current densities. The specific capacitance values of CoxNi1-xP/CNF as well as NiP/CNF and CoP/CNF control samples are calculated based on the CV curves. Given the battery-like feature of CoxNi1-xP, the specific capacity values of all samples are also given along with the specific capacitance. As shown in Figure 3a, the specific capacitance/capacity of NiP/CNF (i.e., Co:Ni = 0:1) is 1987 F g-1 / 794.8


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C g-1, which is markedly higher than that of Ni2P NPs (745 C g-1),58 Ni2P/rGO/NiO composites (742 C g-1),59 and even the bi-metallic Ni-Co-P/POx (647 C g-1).45 After introducing Co into NiP, the specific capacitance/capacity of the resulting CoxNi1-xP/CNF is firstly enhanced, reaching a maxima as high as 3093 F g-1 / 1237.2 C g-1 for Co0.1Ni0.9P/CNF, which is one of the highest capacitance/capacity values reported by far for TMP-based pesudocapacitive materials. Further increase in Co content leads to a gradual decrease in the specific capacitance/capacity of CoxNi1xP/CNF,

for instance, down to 2296 F g-1 / 918.4 C g-1 for Co0.5Ni0.5P/CNF and 1463 F g-1 / 585.2

C g-1 for Co0.66Ni0.33P/CNF. All Ni-rich CoxNi1-xP/CNF (i.e. Co:Ni  1:1) samples exhibit a specific capacitance/capacity value higher than that of NiP/CNF (1987 F g-1 / 794.8 C g-1), showing preferable synergistic promotion in the pesudocapacitive properties. However, when Co becomes predominant in CoxNi1-xP/CNF, the specific capacitance/capacity drops dramatically in comparison to NiP/CNF. It is worth mentioning that the pseudocapacity of CoxNi1-xP/CNF originates from the CoxNi1-xP NPs, instead of the CNF support, as evidenced by the CV curves of the Co0.1Ni0.9P/CNF and the CNFs phosphorized under conditions the same as those used to prepare CoxNi1-xP/CNF (Figure S4, Supporting Information), where it’s seen that the capacitive contribution from CNFs is negligible. Therefore, the CNF just serves as a conductive support of TMP NPs herein. This is further corroborated by the surface area measurement. According to the N2 adsorption/desorption porosimetry (Figure S5, Supporting Information), the surface area of Co0.1Ni0.9P/CNF is 73.5 m2 g-1, smaller than that of the bare acid-treated CNFs (101.4 m2 g-1), indicating that loading of Co0.1Ni0.9P reduces the accessible surface area of CNFs and that the significantly enhanced capacitance/capacity truly results from the ultrafine Co0.1Ni0.9P NPs, instead of the physical surface area increase of the Co0.1Ni0.9P/CNF composite. It is also noted that compared to previous literature reports,34, 37, 55 even the mono-metallic CoP/CNF and NiP/CNF


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show specific capacitance values substantially higher than those of some other TMP-based pesudocapacitive electrode materials, highlighting the prominent advantage of TMP/CNF active materials developed. The high specific capacitance/capacity likely originates from the ultrafine crystal size of CoxNi1-xP and the high electrical conductivity of CNF, which can offer considerable redox-active sites and 3D connected conductive networks, respectively. Figure 3b presents the CV curves of the best-performing Co0.1Ni0.9P/CNF measured at different scan rates. The integral area of each CV curve increases as the scan rate goes up, and redox peaks can be observed even at high scan rates, indicating good capacitive behaviors. As the scan rate increases, the oxidation peak shifts positively and the reduction peak moves negatively, which is presumably caused by the increased internal diffusion resistance at high scan rates. The anodic peak current density shows a linear relationship with the square root of the scan rate (Figure 3b, inset), suggesting that a diffusion-controlled process involve during the charge storage.46 The diffusion coefficient is calculated to be 2.64×10-10 cm2 S-1 (see details in Supporting Information), greater than 1.6×10-16 cm2 S-1 for Ni2P and 4.9×10-18 cm2 S-1 for CoNiP reported previously in the literature,47 indicating that the ultrafine TMP nanoparticles supported on porous interconnected CNF networks are conducive to hydroxyl ion diffusion. We further investigated the variation of the specific capacitance/capacity of Co0.1Ni0.9P/CNF as a function of the scan rate, and compared it to that of NiP/CNF and CoP/CNF control samples (Figure 3c). The specific capacitance/capacity value of Co0.1Ni0.9P/CNF is significantly higher than that of NiP/CNF and CoP/CNF at each scan rate, and it retains 83% of its initial value even at 100 mV s-1, showing very good capacitance/capacity retention. The galvanostatic charge/discharge characteristics of the Co0.1Ni0.9P/CNF were further studied in a three-electrode configuration (Figure 3d and S6). At the current density of 5 A g-1,


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the specific capacitance/capacity of Co0.1Ni0.9P/CNF is as high as 3514 F g-1 / 1405.6 C g-1, which is the highest value reported by far for TMP-based pesudocapacitive materials, to the best of our knowledge, and also higher than some transition metal oxide/hydroxide electrodes reported very recently.60-62 Moreover, even at a fairly high charge/discharge current density of 500 A g-1, Co0.1Ni0.9P/CNF still possesses a high specific capacitance/capacity of 2996 F g-1 / 1198.4 C g-1 (Figure S6, Supporting Information), exhibiting exceptional capacitive retention. Within the potential range under investigation, the Co0.1Ni0.9P/CNF shows excellent pesudocapacitive performance according to both CV measurements and galvanostatic charge/discharge tests, outperforming most TMP-based electrodes reported recently in the literature (Table S1, Supporting Information). Furthermore, the Co0.1Ni0.9P/CNF exhibits good long-term stability. Upon continuous charge/discharge tests at 10 A g-1, the specific capacitance first dropped to about 80% of the initial value after 1000 cycles and then got stabilized up to 10000 cycles without significant variation, showing good capacitance retention (Figure S7a, Supporting Information). The coulombic efficiency had been above 96% during the stability test, exhibiting outstanding electrochemical reversibility. The structure and morphology of Co0.1Ni0.9P/CNF virtually remained unchanged after 10000 charge/discharge cycles, as evidenced by XRD and SEM examinations (Figure S7b-S7d, Supporting Information). It is hypothesized that the variation in the specific capacitance/capacity likely results from the synergistic effect of the transition metal species in bi-metallic CoxNi1-xP, given that the morphology and microstructure of CoxNi1-xP/CNF, CoP/CNF and NiP/CNF are similar (Figure S8, Supporting Information). To verify this hypothesis, we performed DFT calculations for a supercell of CoxNi1-xP with 16 atoms, as shown in Figure 4a. We started from a NiP structure and substituted some of the Ni by Co, thereby generating three representative hybrid systems: Ni8P8,


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Co1Ni7P8, and Co2Ni6P8. The Co:Ni ratios in these systems (Table S2, Supporting Information) are close to those of experimentally measured NiP, Co0.1Ni0.9P, and Co0.2Ni0.8P (Figure S1, Supporting Information). For each ratio, we considered various doping sites for Co, and selected the lowest energy one. Since the capacitance/capacity is closely related with the number of the electronic states available near the Fermi level, we integrated the density of states (DOS), and used this quantity to qua (Figure 4b). A similar voltage range has also been used for modeling graphene-based supercapacitors.63 We find that the trend of the integrated DOS of CoxNi1-xP matches well with the trend of specific capacitance across different ratios (Figure 4c), suggesting that the difference in capacitance is due to the difference in integrated DOS, i.e. the number of electronic states near the Fermi level. Mixing Co and Ni with a proper ratio can increase these states, and thus enhance the capacitance.

Figure 4. (a) The optimized structure of Co0.1Ni0.9P. (b) DOS of Co0.1Ni0.9P; inset zooms in the DOS within 0.5 eV near the Fermi level. (c) The calculated electronic occupation (orange) and experimentally measured specific capacitance (black) of NiP, Co0.1Ni0.9P, and Co0.2Ni0.8P. To demonstrate practical applications of Co0.1Ni0.9P/CNF active materials, we fabricated a solid-state asymmetric supercapacitor comprising Co0.1Ni0.9P/CNF dispersed on a flexible carbon cloth (CC) substrate (i.e. Co0.1Ni0.9P/CNF/CC) and commercially available activated carbon (AC) powders dispersed on CC (i.e. AC/CC), sandwiched by a cellulose paper and a solid-state gel


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electrolyte (KOH/PVA) that is penetrated into both Co0.1Ni0.9P/CNF/CC positive electrode and AC/CC negative electrode, as schematically illustrated in Figure S9 (Supporting Information). The details are described in Methods. SEM observation revealed that thanks to the knitted architecture of carbon cloth, the Co0.1Ni0.9P/CNF could readily integrate into the CC fabric and uniformly graft on the surface of individual carbon fibers, enabling the formation of a hierarchical structure (Figure S10, Supporting Information). The interwoven microfibers and interconnected CNFs can offer a highly conductive network and good diffusion pathways for electrolyte, favourable for electrochemical performance. Based on the charge balance theory (q+ = q−) and the specific capacitance/capacity values of Co0.1Ni0.9P/CNF/CC and AC/CC electrodes obtained from the CV measurements (Figure S11, Supporting Information), the mass ratio of Co0.1Ni0.9P and AC was set to 1:8 (see details in Methods). It is worth noting that the carbon cloth itself has negligible contribution to the capacitance of positive and negative, according to our CV studies (Figure S12, Supporting Information), so it only acts as a flexible conducting substrate.


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Figure 5. (a) CV curves of the Co0.1Ni0.9P/CNF/CC||AC/CC supercapacitor recorded within different working potential windows. Scan rate: 50 mV s-1. (b) CV curves of the Co0.1Ni0.9P/CNF/CC||AC/CC supercapacitor measured at different scan rates. (c) Galvanostatic

charge/discharge

curves

of

the

Co0.1Ni0.9P/CNF/CC||AC/CC

supercapacitor recorded at different current densities. (d) Nyquist plots of the Co0.1Ni0.9P/CNF/CC||AC/CC supercapacitor recorded at the open circuit voltage in the frequency range of 100 mHz to 100 kHz. (e) Ragone plots of some TMP-based asymmetric supercapacitors. (f) Specific capacitance and coulombic efficiency as a function of charging/discharging cycle number. The current density is 10 A g-1.

Figures 5a and S13 (Supporting Information) show CV and chronopotentiometry (CP) profiles of the Co0.1Ni0.9P/CNF/CCAC/CC asymmetric supercapacitor measured in different potential windows. Owing to the asymmetric configuration, the working potential window can be markedly extended to 0.8~1.6 V. With the increase in potential window, the specific capacitance/capacity of the device raises from 76 F g-1 / 60.8 C g-1 (0 ~ 0.8 V) to 144 F g-1 / 230.4 C g-1 (0 ~ 1.6 V). Furthermore, the electrochemical performance of the supercapacitor was evaluated at different scan rates while working within a potential window of 1.4 V (Figure 5b), and the specific capacitance/capacity of the device is calculated accordingly based on the total mass of active materials in both positive and negative electrodes. At 20 mV s-1, the specific


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capacitance/capacity of the asymmetric SC is 118.7 F g-1 / 166.2 C g-1, which is higher than that of TMP-based supercapacitors reported before such as Ni2P nanosheets/Ni foam  AC (96 F g-1 at 5 mV s-1).43 When the scan rate is increased to 200 mV s-1, the specific capacitance/capacity of the device drops to 85.1 F g-1 / 119.1 C g-1 (Figure S14a, Supporting Information), showing good capacitance/capacity retention. It is worth mentioning that the specific capacitance/capacity of the integrated device is mainly restricted by the poor performance of the negative AC electrode, whose maximal achievable specific capacitance is only about 170 F g-1 based on our CV test at 10 mV s1. The

rate performance of the Co0.1Ni0.9P/CNF/CCAC/CC SC was investigated by galvanostatic

charge/discharge tests (Figures 5c and S14b, Supporting Information). The device shows a reasonably high specific capacitance/capacity value of 79.5 F g-1 / 111.3 C g-1 even at a high charge/discharge

current

density

of

40

A

g-1,

implying

that

the

asymmetric

Co0.1Ni0.9P/CNF/CCAC/CC SC has the potential to be utilized for fast energy storage. Moreover, preliminary tests demonstrated that mechanically bending the SC does not markedly change supercapacitive performance (Figure S15, Supporting Information), proving its potential for use in flexible electronic devices. The Nyquist plot of the SC derived from the electrochemical impedance spectroscopy (EIS) measurement is displayed in Figure 5d. A nearly vertical line is observed which indicates a quasi-ideal capacitive behavior of the device. Moreover, the SC shows a small equivalent series resistance (ESR) of 7.9 , which may benefit from the shortened pathways for ion diffusion and electron transport of the hierarchical electrode as well as the ultrasmall size of Co0.1Ni0.9P NPs. We further calculated the energy and power densities of the asymmetric SC. At the power density of 3.5 kW kg-1, the energy density of the SC can reach 32.2 Wh kg-1, and still retains 27.0 Wh kg−1 at a high power density 14.0 kW kg−1, outperforming that of most TMP based asymmetric SCs reported recently in the literatures (Figure 5e). Furthermore,


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the solid-state Co0.1Ni0.9P/CNF/CCAC/CC asymmetric SC exhibits good cycling stability. The capacitance/capacity of the SC decreases gradually upon charging/discharging at a current density of 10 A g-1, and retains 72% of its initial value after 10000 cycles. The degradation likely results from the slow, but inevitable, oxidation of Co0.1Ni0.9P during the repetitive redox reactions. Further effort to improving the cycling stability is under way.

CONCLUSIONS

In summary, we report the synthesis of carbon nanofiber supported ultrafine CoxNi1xP

nanocrystals and systematically investigate how the Co:Ni ratio affects the

pesudocapacitive properties of the bi-metallic phosphide. By fine-tuning the Co:Ni ratio, an extraordinary specific capacitance/capacity of 3514 F g-1 / 1405.6 C g-1 can be achieved for Co0.1Ni0.9P/CNF at 5 A g-1, which is the highest value reported by far for transition metal phosphide based pesudocapacitive materials. DFT calculations demonstrate that introducing a small amount of Co into NiP can substantially increase the number of electronic states near the Fermi level and thereby the material’s capacitance/capacity, and the variation in density of states of CoxNi1-xP matches very well with the trend in specific capacitance/capacity of CoxNi1-xP/CNF measured experimentally.


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Using the best-performing Co0.1Ni0.9P/CNF as positive electrode materials and commercial activated carbon as negative electrode materials, a flexible solid-state asymmetric supercapacitor is fabricated, which can deliver an energy density of 32.2 Wh kg-1 at a power density of 3.5 kW kg-1, and demonstrates good cycling stability upon 10000 charge/discharge cycles. Our work highlights the importance of composition engineering of metal phosphides in achieving high electrochemical performance for pesudocapacitive charge storage. Cobalt is a critical raw material and much more expensive than nickel. Delivering high storage performance with little cobalt element in electrode materials is preferable and shows great promise in lowering the production costs of supercapacitors. Moreover, the synthetic method of CoxNi1-xP/CNF reported herein is simple and inexpensive, and can be readily scaled up and extended to synthesize other multi-metallic TMP electrode materials. It is believed that the Co0.1Ni0.9P/CNF has substantial potential for use in highly performant and low-cost flexible supercapacitors and likely in other energy storage devices.

METHODS


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Assembly of supercapacitors: Before assembling the asymmetric supercapacitor, we first calculated the mass ratio of active materials in positive and negative electrodes. To do so, we measured the specific capacitance of Co0.1Ni0.9P/CNF (CS+) and AC (CS-), respectively, by CV measurements (Figure S11, Supporting Information). Based on the charge balance theory (q+ = q−), the mass ratio of electrode materials was then estimated according to the following equation:47, 52

𝑚+ 𝑚―

𝐶s ― × ∆𝑈 ―

(1)

= 𝐶s + × ∆𝑈 +

where m+ and m− represent the mass of positive electrode and negative electrode, respectively, ΔU+ and ΔU− stand for the potential window within which the CV of positive or negative electrode materials was measured. According to Figure S11 (Supporting Information), CS+ = 3093 F g-1, CS− = 155 F g-1, ΔU+ = 0.4 V, and ΔU− = 1 V, and therefore the mass ratio of Co0.1Ni0.9P/CNF and AC was calculated to be 1:8. The assembling procedures of asymmetric supercapacitors are schematically illustrated in Figure S6 (Supporting Information). Firstly, 5 mg CoxNi1-xP/CNF or 5 mg AC powders were ultrasonically dispersed in 1 mL ethanol solution with 50 L 5 wt. % Nafion®. The ink was then drop-cast into a carbon cloth substrate, and the loading of electrode materials could be controlled by the volume of the drop-cast solution. The typical loadings of CoxNi1-xP and AC on CC are 0.5 and 4 mg cm-2, respectively. Subsequently, the electrode was dried in a vacuum oven at 60 °C for 1 h. To assemble the device, polymer-based gel was used as the solid electrolyte. 2 g of polyvinyl alcohol (PVA) and 3 g of potassium hydroxide (KOH) were added into 20 mL DI water, then the solution was heated to 85 °C and stirred vigorously until it became clear. The hot electrolyte (KOH-PVA aqueous solution, 85 °C) was then dipped onto the CoxNi1-xP/CNF/CC and AC/CC electrodes, respectively. After that, the KOH-PVA coated CoxNi1-xP/CNF/CC and AC/CC were


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pressed against each other with a cellulose paper in between as a separator. Thus-obtained asymmetric supercapacitors were dried for 24 h in the fume hood before testing. DFT calculations. Spin-polarized density functional theory calculations were performed to calculate the geometric and electrical structure of CoxNi1-xP using Vienna Ab-initio Simulation Package (VASP),64-66 with projector augmented wave (PAW) pseudopotential67, 68 and Perdew– Burke–Ernzerhof (PBE) exchange-correlation functional.69, 70 A kinetic energy cut-off of 400 eV was adopted for the plane-wave expansion. For structural optimization, all atomic positions were fully relaxed until the final energy and force on each atom were less than 10-5 eV and 0.05 eV/Å. For density of states calculation, 15 × 15 × 15 k-point was carried out to ensure the accuracy of calculation.

ASSOCIATED CONTENT

Supporting Information. Actual metal loadings in each sample; XRD patterns of the electrode materials; CVs of CoxNi1-xP/CNF electrodes with different Co:Ni ratios; CVs of CNF and Co0.1Ni0.9P/CNF; Isotherms of bare acid-treated CNFs and Co0.1Ni0.9P/CNF; Specific capacitances/capacities of Co0.1Ni0.9P/CNF; SEM images and XRD patterns of the Co0.1Ni0.9P/CNF/CC electrode after the cycling stability test; SEM images of CoP/CNF and NiP/CNF; Schematic showing the fabrication of electrodes and asymmetric


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Page 24 of 34

supercapacitor; SEM images of Co0.1Ni0.9P/CNF/CC; CVs of activated carbon and Co0.1Ni0.9P/CNF; CVs of CC and Co0.1Ni0.9P/CNF/CC; Chronopotentiometric curves of the Co0.1Ni0.9P/CNF/CC||AC/CC

asymmetric

supercapacitor;

Specific

capacitances/capacities of Co0.1Ni0.9P/CNF/CC||AC/CC asymmetric supercapacitor; CVs of the Co0.1Ni0.9P/CNF||AC/CC supercapacitor recorded at the pristine and bending states; SEM images of pristine and acid-treated CNF; Capacitive performance comparison of TMP-based electrodes; Co:Ni ratio and actual loading used for DFT calculations. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *Email:

[email protected] (Y. L.).

*Email:

[email protected] (G. Y.).


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*Email:

[email protected] (L.L.).

ACKNOWLEDGMENT This work was financially supported by the Portuguese Foundation of Science & Technology (FCT) through an Exploratory Grant under FCT-UT Austin Collaborative programme (grant no. UTAP-EXPL/CTE/0008/2017). L. Liu also acknowledges the support

of

FCT

Investigator

Grant

(IF/01595/2014)

and

Exploratory

Grant

(IF/01595/2014/CP1247/CT0001). The DFT calculations are supported by Welch Foundation (Grant No. F-1959-20180324) and the startup grant from The University of Texas at Austin (UT Austin), and used computational resources located at the National Renewable Energy Laboratory (NREL) sponsored by the DOE’s Office of Energy Efficiency and Renewable Energy (EERE), and used the Texas Advanced Computing Center (TACC) at UT Austin.


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SYNOPSIS

Table of Contents graphic


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High-Performance Flexible Solid-State Asymmetric Supercapacitors

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