Template synthesis of structure-controlled 3D hollow nickel-cobalt phosphides microcubes for high-performance supercapacitors

Journal of Colloid and Interface Science 561 (2020) 23–31

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Template synthesis of structure-controlled 3D hollow nickel-cobalt phosphides microcubes for high-performance supercapacitors Xiuli Zhang a, Li Zhang a,b,⇑, Guancheng Xu a, Aihua Zhao a, Shuai Zhang a, Ting Zhao a a Key Laboratory of Energy Materials Chemistry, Ministry of Education; Key Laboratory of Advanced Functional Materials, Autonomous Region; Institute of Applied Chemistry, Xinjiang University, Shengli Road No. 666, Urumqi 830046, China b Physics and Chemistry Detecting Center, Xinjiang University, Shengli Road No. 666, Urumqi 830046, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Metal-formate frameworks are

utilized to prepare hollow structures.
Both synergistic effect between Ni

and Co and hollow structures contribute to facilitating the OH adsorption capability for fast faradaic redox reaction.
NiCoP-2 microcubes deliver enhanced specific capacitance and excellent energy density.

a r t i c l e

i n f o

Article history: Received 18 October 2019 Revised 26 November 2019 Accepted 28 November 2019 Available online 29 November 2019 Keywords: Hollow structures Bimetallic phosphides Metal-formate frameworks Supercapacitors

a b s t r a c t Given that combined merits of both compositions and novel structures play important roles in energy storage of electrode materials, we herein use [CH3NH3][NixCo1-x(HCOO)3] as self-sacrificed precursors to synthesize structure-controlled hollow nickel cobalt phosphides microcubes. The as-obtained hollow NiCoP-2 displays the highest specific capacity of 629 C g1 at 1 A g1 and superior cycling stability with 81.3% capacitance retention at 6 A g1 after 8000 cycles. Moreover, the asymmetric supercapacitor composed of NiCoP-2 and commercial active carbon (YP-80F) presents the remarkable battery storage performance in terms of outstanding specific capacitance (121.5 F g1 at 1 A g1), excellent cycling durability (90.1% capacitance retention after 10,000 cycles) and high energy density of 43.2 Wh kg1 at a power density of 801.6 W kg1. The attractive performance can be ascribed to superiority of the transition metal phosphides, hollow structures, as well as synergism between Ni and Co. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction Nowadays, prominent problems of energy consumption and environmental pollution have awoken the human conscience to pursue promising storage technologies [1,2]. As a newable, clean and sustainable energy storage device, supercapacitors (SCs) have

⇑ Corresponding author at: Key Laboratory of Energy Materials Chemistry, Ministry of Education; Key Laboratory of Advanced Functional Materials, Autonomous Region; Institute of Applied Chemistry, Xinjiang University, Shengli Road No. 666, Urumqi 830046, China. E-mail address: [email protected] (L. Zhang). https://doi.org/10.1016/j.jcis.2019.11.112 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

gained widespread explorations due to their excellent advantages including fast charge/discharge process, higher power density and longer service life [3]. However, their practical applications still suffer from the limited energy density [4,5]. Based on the energy storage mechanism of SCs: E = 1/2CV2, expanding cell voltage and increasing capacitance of electrode materials can address this drawback. In terms of the former, assembling asymmetric supercapacitors (ASCs) can enlarge the operation voltage window [6,7]. For the latter, it is critical to design and fabricate novel electrode materials with high-capacity and fast electron/ion transfer characteristics [8–10].

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Transition metal phosphides, as n-type semiconductors, present metal-like properties and ultra-fast electron conductivity, which is superior to transition metal oxides, and hydroxides [11–14]. As a result, they attract extensive attention for high performance supercapacitors [15]. Among the reported metal phosphides, Ni-based and Co-based phosphides are considered as promising electrode materials. Ni-based phosphides exhibit higher theoretical capacitance while fail to ensure outstanding rate performance and cycling stability [16–18]. In contrast to the former, Co-based phosphides display excellent capacitance rate and cycling stability during the charging and discharging process [19–21]. Therefore, constructing cobalt nickel bimetallic phosphides is regarded as a valuable strategy for high performance electrode materials. In addition, the novel architectures of materials are also conducive to improving performance of electrode materials [22-25]. Up to now, the porous and hollow structure emerges great advantages for supercapacitors [26-29]. Not only can this structure help to increase the capacitance and rate performance of electrode materials by facilitating the contact between electrode materials and electrolyte, but contribute to cycling stability by providing a large amount of buffer space for the volume change during cycling processes [27,29]. For instance, hollow NiCo2O4, designed by Shen et al. possesses both OH storage space and channels for rapid diffusion of OH, redounding to the enhanced capacitance retention and long-term cycling stability [28]. Recently, the sacrificial template method has been considered as economic and efficient for preparation hollow materials [30,31]. As a class of promising templates and precursors, Metal organic frameworks (MOFs) present popular position in fabricating hollow or porous materials on account of their nature of porosity and large surface area [32,33]. When thermally annealed MOFs materials in inert gas or reactant gas, the smaller metal ions in MOFs diffuse outward faster than sulfur/selenium-, or phosphorus-containing species diffuse inward, which provides a route for forming hollow porous structures by the Kirkendall effect [34-37]. Over the past years, hollow materials obtained from sacrificial MOFs template methods are mostly spherical in shape [38]. However, the non-spherical hollow structures manifest their new advantages ranging from anisotropic morphologies to highcurvature surfaces compared with spherical structures, which allows their potential impacts on electrochemical activities [39]. Therefore, utilizing the available non-spherical templates is very necessary to generate non-spherical hollow products Inspired by the abovementioned analysis, we herein use metalformate frameworks as self-sacrificed precursors to synthesize cubic hollow cobalt nickel bimetallic phosphides. The structure and composition are controllable by tuning composition of metal ions in MOFs. Interestingly, when molar ratio of Ni/Co was up to 2, the typical resultant of hollow named as NiCoP-2 microcubes exhibited the highest specific capacity of 629 C g1 at current density of 1 A g1 and an exceptional energy density of 43.2 Wh kg1 at a power density of 801.6 W kg1, indicating its potential applications in storage devices.

2. Experiment section 2.1. Materials Nickel(II) nitrate hexahydrate (Ni(NO3)26H2O, 98%), cobalt(II) nitratehexahydrate (Co(NO3)26H2O, 98%), and sodiumhypophosphite (NaH2PO2H2O, 99%) were purchased from Yongsheng Fine Chemical Co., Ltd (Tianjin). Anhydrous ethanol (CH3CH2OH, 99.7 wt%), formic acid (HCOOH, 88 wt%), and methylamine water solution (CH3NH2, 40 wt%) were purchased from Tianjin Zhiyuan Chemical Reagent Plant. Surfactant polyvinylpyrrolidone K30

(PVP K30, viscosity average molecular wt. 40000) was purchased from Tokyo Chemical Industry Co., Ltd. The commercial active carbon (YP-80F) was purchased from KURARAY (Japan). Notably, all the chemicals were of analytical grade and used without further purification. 2.2. Construction of solid NiCo-MOF-2 microcubes. In a typical co-precipitation method, 0.66 mM Ni(NO3)26H2O, 0.33 mM Co(NO3)26H2O and 0.5 g PVP K30 were dissolved in 25 mL absolute ethanol to provide metal ions. Meanwhile, the colorless organic ligand solution was also prepared by dissolving 5 mM HCOOH, 5 mM CH3NH2 and 0.5 g PVP K30 in another 25 mL absolute ethanol. Afterward, the former, pale pink solution, was dropped into the latter under stirring for 4 h and then aged for 24 h at ambient temperature. Lastly, the light pink product was washed with ethanol several times and collected after vacuumdrying at 60 °C for 8 h. Other precursors containing Ni-MOF, NiCo-MOF-2, NiCo-MOF-1, NiCo-MOF-0.5 and Co-MOF were also prepared by tuning molar ratio of Ni(NO3)26H2O and Co(NO3)26H2O to 1:0, 1:1, 1:2 and 0:1 with a total molar amount of 1 mM. 2.3. Synthesis of 3D hollow NiCoP-2 microcubes NiCoP-2 was obtained by low-temperature phosphating method. Concretely, NiCo-MOF-2 (70 mg) and NaH2PO2H2O (700 mg) were firstly put at the opposite positions of a porcelain boat with the latter at the upstream side. Then, the boat was placed in the center of tube furnace. Under the N2 atmosphere, the samples were annealed up to 350 °C for 2 h with a heating rate of 2 °C min1. At last, the grey-black powder was fabricated after cooling to room temperature with the protection of N2. Besides, NiPx, NiCoP-2, Ni-Co-P-1, Ni-Co-P-0.5 and CoP were also synthesized and the procedure was similar to that of preparing NiCoP-2. 3. Results and discussion The synthesis process of hollow NixCo1-xP microcubes is schematically shown in Scheme 1. The solid [CH3NH3] [NixCo1-x(HCOO)3] microcubes were firstly fabricated through a coprecipitation method at room temperature, and then used as templates to react with PH3 released from NaH2PO2 in a flow of N2. The involved reaction mechanism of phosphorization process is similar to the following formula [40,41]. D

2NaH2 PO2  H2 OðsÞ ! PH3 ðgÞ þ Na2 HPO4 ðsÞ þ H2 OðgÞ 2Ni

2þ

D

þ Co2þ þ 2PH3 ! Ni2 P þ CoP þ 6Hþ

The morphology and structure of the NiCoP-2 were firstly studied via FESEM. Fig. 1a shows that hollow interiors and appropriate pores appear in the rounded cubes inheriting from NiCo-MOF-2 precursors (Fig. S1a-b). The single cubic image (Fig. 1b) also shows rough shells constructed by numerous nanoparticles. TEM image (Fig. 1c) presents the abundant gaps resulted from the interconnected nanoparticles. Additionally, the hollow interior also can be observed in Fig. 1d. This structural differences before and after phosphating can be mainly attributable to the release of gas (CO2, H2O) from the decomposition of organic linkers, and then central Ni2+/Co2+ in MOFs rapidly diffusing outward to convert into metal phosphides. In order to analyze the impacts of both the composition and content of metal ions on the products structure, we also characterized other precursors and corresponding phosphides with different molar ratios of Ni2+/Co2+. The overall morphology of precursors is cubic shape shown in Fig. S1c-f. But their size depends on the nickel content. More H+ from the hydrolysis of nickel ions can

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Scheme 1. Schematic illustration for the preparation of hollow NixCo1-xP cubes.

Fig. 1. SEM images (a-b) of NiCoP-2. TEM images of (c-d) NiCoP-2. XRD patterns (e) of NiPx, NiCoP-2, Ni-Co-P-1, Ni-Co-P-0.5 and CoP. HRTEM image (f) and SEM-EDX mapping (g) of NiCoP-2. Inset is the selected area electron diffraction (SAED) of NiCoP-2.

combine with HCOO to from weak HCOOH, which affects the growth of the precursors [29]. After phosphorization, 3D cubic structures of phosphides (Fig. S1g–i) are maintained well except that NiPx (Fig. S1j) shows partial deformation and collapse in structure. Fig. S2 further shows the size distribution histogram about the precursors and phosphorization products. Obviously, metal ion content contributes a lot to average diameters of precursors and the corresponding phosphides. Co-MOFs and CoP show maximum diameter about 5 mm. With the increasing Ni content, the average diameter of precursors and the phosphides becomes smaller. After phosphorization, bimetallic phosphides containing cobalt can almost maintain the size of original precursors, but average diameter of NiPx is only the half of its precursors. The above structural change reveals that elemental cobalt can be served as a stable agent to improve stability of microstructures during the conversion of the samples, while nickel ion plays a pivotal role in size of the precursors [42,43]. The PXRD was then employed to determine phase purity and compositions of the prepared products. As shown in Fig. S1k, all

precursors display typical diffraction peaks and match well with the simulated crystal phase [44], which was converted from single crystal structure data in the literature by using MERCURY software. During phosphorization, the Ni and Co ions incarnate different the coordination modes with P (shown in Fig. 1e). Pure Co-precursors are converted into CoP (PDF#29-0497) [45]. The pure Ni-MOFs are mostly transformed into Ni5P4 (PDF#18-0883) [46] along with redundant peaks of Ni2P (PDF#03-0953) [47]. The XRD patterns of bimetallic Ni–Co phosphides tend to the gradual conversion from CoP phase to Ni2P phase with the increasing amount of Ni ion. When molar ratio of Ni2+/Co2+ increases to 2:1, the Ni2P characteristic peaks are stronger and slightly shift to the peak positions of NiCoP (PDF#71-2336) [48,49], indicating crystal similarity of the both. The HRTEM (Fig. 1f) further confirms above similarity, in which visible lattice fingers with the interplanar spacing of 0.19 nm and 0.22 nm correspond to (2 1 0) and (1 1 1) crystal planes of NiCoP [50], respectively. Selected area electron diffraction (SAED) (inset in Fig. 1f) indicates the monocrystalline structure of hollow NiCoP-2 cubes. The SEM-EDX mapping (Fig. 1g)

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suggests that the elements of Ni, Co and P are coexistent and uniformly distributed over the entire surface of microcubes, and atomic percentages of Ni, Co, are 25.57, 12.90 at% (listed in the inset of Fig. S3). Atomic molar ratio of Ni and Co is about 2:1, approaching the used chemometric of reactants. X-ray photoelectron spectroscopy (XPS) was further measured to investigate the element compositions, chemical status and electron interactions of the as-synthesized samples around the surface. Fig. 2a is the full survey spectrum of CoP, NiCoP-2 and NiPx. Clearly, Co, Ni, and P elements appear at the corresponding products. The existence of C, O elements can be attributed to the exposure of air [51] and no other strange elements are found in XPS spectra, which accords with the test results of PXRD and SEM-EDX mapping. The detailed analysis for Co 2p, Ni 2p and P 2p were carried out and marked, respectively. Fig. 2b shows that in both NiCoP-2 and CoP, each Co2p peak can be decomposed into two pairs of doublets with two satellites (noted as sat.), which indicates the existence of Co-P [20] and Co-O (Co2+) [11]. Notably, the main Co 2p3/2 peaks (778.6 eV) of NiCoP-2 presents a 0.3 eV negative shift compared to that of pure CoP (778.9 eV). The XPS peaks of P 2p from the three products are visible in Fig. 2c. They can be divided into two peaks at about 129 eV and 134 eV, representing the metal phosphides and phosphate species, respectively [52,53]. Compared with the CoP (129.7 eV), the P 2p3/2 in NiCoP-2 (129.3 eV) shows lower binding energy with negative offset of 0.4 eV while it shifts to the higher binding energy relative to the NiPx (128.7 eV). Lastly, similar to the Co 2p, the peaks of Ni 2p for both NiCoP-2 and NiPx also can be observed in Fig. 2d. Two pairs of doublets, along with two satellites, represent Ni-P and oxidized nickels (Ni2+), respectively [12,37]. The main Ni 2p3/2 peaks of NiPx (853.2 eV) shows positive gap of 0.4 eV relative to that of NiCoP-2 (852.8 eV). In the light of above XPS analysis, the strong electron interactions

between monometallic phosphides occur during the synthesis of NiCoP-2, which leads to the increased OH adsorption capability for fast faradaic reactions [11]. Then, electrochemical properties of the materials also were evaluated by utilizing electrochemical measurements visible in supporting information. As shown in Fig. 3a, at the CV curves of 20 mV s1, except that the CoP exhibits two couples of redox peaks, which can be attributed to two-step oxygen reduction process [19], other samples, around a potential range of 0.2 to 0.55 V, display a pair of well-defined redox peaks, indicating faradaic redox electrochemical process from the Cod+ /Nid+ state to the Co3+/Ni3+ state [54]. Besides, it is evident that the bimetallic phosphides (especially NiCoP-2) show larger curve area than the CoP/NiPx at the same scan rate, which implies the growing specific capacitance for bimetallic phosphides. To uncover electrochemical kinetics of prepared electrodes, reaction plots between scan rate (m) and peak current (i) were drawn according to the following equation [55]:

i ¼ av b In which a and b are constants. The parameter b is used to evaluate the storage mechanisms of electrodes based on varying value between 0.5 and 1.0. The closer the value is to 0.5, the more electrode means a diffusion-dominated behavior, while closer b value of 1.0 indicates capacitive behavior. The b-value is the slope for log i vs. log m plots. Seen from the plots in Fig. 3b, b-value is about 0.97 for CoP, 0.83 for Ni-Co-P-1, 0.87 for Ni-Co-P-0.5, and 0.69 for NiPx, respectively. The b-value of NiCoP-2 eletrode is 0.63 closest to 0.5, suggesting that it is like a battery-type electrode and its capacitive contribution is mainly provided by diffusioncontrolled process. Specific capacity can be measured via GCD curves at 1 A g1. As clearly seen in Fig. 3c, among all samples,

Fig. 2. XPS characterization of survey spectra (a), Co 2p (b), P 2p (c) and Ni 2p spectra (d) related to CoP, NiCoP-2 and NiPx.

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Fig. 3. CV curves (a), log i vs. log m plots (b), CP curves at 1 A g1 (c), capacitance rate curves (d) of CoP, Ni-Co-P-0.5, Ni-Co-P-1, NiCoP-2 and NiPx, respectively. CV curves (e), GCD curves (f), cycle performance and coulombic efficiency at 6 A g1 (g) of NiCoP-2. SEM image (h) of NiCoP-2 after 8000 cycles.

the NiCoP-2 shows the longest discharging time and delivers the excellent specific capacity about 629C g1at 1 A g1, higher than Ni-Co-P-0.5 (570 C g1), Ni-Co-P-1 (483 C g1), NiPx (345 C g1) and CoP (only 130 C g1). Considering tiny CV curve area of nickel Foam (Fig. S4a), the contribution to capacity from it can be ignored. Also, capacitance retention of different products was evaluated and shown in Fig. 3d. It is obvious that the NiCoP-2 exhibits the

superior capacitance retention at 20 A g1 (63.59%), even exceeding the pure CoP (61.50%). The other samples with Co also display ideal retention of 58.75% for Ni-Co-P-0.5 and 55.90% for Ni-Co-P-1, respectively. However, the NiPx shows the lowest capacitance retention of only 28.90%. This further means cobalt element can alter the electronic structure of NiPx, improving conductivity and reactivity of Ni-based phosphides [43]. As further confirmed by

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the electrical resistance responses of the Ni-Co phosphides with different Ni to Co ratios, Fig.S4b shows that except for NiPx, Nyquist plots of other samples with Co manifest a smaller semicircle in the high-frequency region and a straight line in the lowfrequency region. The fitted value of charge-transfer impedance (Rct) is 0.573 X for NiPx, 0.295 X for Ni-Co-P-1, 0.352 X for NiCo-P-0.5, 0.203 X for Ni-Co-P-2 and 0.101 X for CoP, which intimates that Co introduction lowers charge-transfer resistance of metallic phosphides [21,42,56]. Fig. 3e-f were next drawn to further point out the outstanding electrochemical properties of hollow NiCoP-2. Fig. 3e shows that although with the increasing scan rate, positive peak moves positively and the cathode peak moves negatively owing to polarization, the shape of redox peaks has no obvious change. This highly

reversible redox reaction mechanism can be described as follows [48,49]:

NiCoP þ 4ðOH Þ þ xðOH Þ $ NiPðOH Þ2 þCoPðOHÞ2 þ xðOH Þ þ 4e

What’s more, the symmetry of GCD curves (Fig. 3f) at different current density also implies the reversibility of NiCoP-2 electrode materials in charge-discharge process. Corresponding the specific capacitances calculated are about 1258, 1240, 1150, 1104, 1056, 1000 and 800 F g1 at 1, 2, 4, 6, 8, 10 and 20 A g1, respectively. The excellent cyclic stability of the NiCoP-2 electrode is also visible in Fig. 3g. At current density of 6 A g1, the capacitance value of NiCoP-2 electrode first increases and then decreases. After 8000 cycles, it still keeps 81.3% of the initial capacitance. The marked

Fig. 4. CV curves at 20 mV s1 of NiCoP-2 and AC (YP-80F) (a). CV curves of NiCoP-2//AC (YP-80F) at changing potential from 1.4 V to 1.8 V (b). CV curves (c), GCD curves (d), capacitance rate (e) and cycling curve at 4 A g1 (f) of NiCoP-2//AC (YP-80F).

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Fig. 5. Ragone plot of ASCs device (a). A wearable electronic LED (3 V) lighted up by two ASCs in series (b).

increase of capacitance in initial stage can be due to self-activation that allows the electrolyte solution contact the porous shell slowly, and then penetrate into the hollow interiors of electrode materials to more effectively activate the pseudo-capacitor reactions [38]. Moreover, from the SEM image after the repeated cycling test (Fig. 3h), we can see that the morphology and size of 3D NiCoP-2 microcubes don’t change substantially, which not only indicates its excellent mechanical properties, but highlights advantages of hollow structure in alleviating volumetric expansion of electrode materials during the cycling process. Notably, many nanoparticles and nanosheets can be found on the initial smooth surface of the microcubes (shown in the Fig.S5), which is attributed to surficial oxidation of NiCoP-2 materials. The main composition of nanosheets can be Ni(OH)2. Co hydroxides could possibly serve as a surface protective layer by preventing Ni from being oxidized to some degree, and thus improve the overall stability of the NiCoP [19]. Summarily, NiCoP-2 presents pivotal value for excellent performance. This can be due to the following advantages: 1) Nickelrich nature facilitates rounded cube with moderate size, which is profitable to provide abundant redox active sites for ultrahigh capacity [29]. (2) Synergy effect between nickel and cobalt element is in favor of increasing the OH adsorption energy [38]. 3) Novel porous hollow structure can promote fast ion and electron transports [32]. Based on the aforementioned discussion, we further estimated practical application of NiCoP-2 by assembling asymmetric supercapacitors (ASC) device in 6 M KOH electrolyte. Because commercial activated carbon (YP-80F) presents outstanding advantages of higher specific surface area, low resistance and high efficiency [57], it was utilized as negative electrode in our ASCs. Fig. S6a shows that the AC (YP-80F) can still keep rectangular-shaped CV curves without redox peaks even at a higher scan rate, showing typical double-layer behavior. The specific capacitance of it was also calculated to be 230 F g1 at 1 A g1 from GCD curves shown in Fig. S6b. According to the principle of charge balance between cathode and anode described in supporting information Eq. (1), the optimized mass loadings of AC (YP-80F) and NiCoP-2 are 6.8 mg and 2.5 mg with mass ratio of 2.72:2. Fig. 4a shows the CV curves of AC (YP-80F) and NiCoP-2 at 20 mV s1. According to CV curves, the potential window of AC (YP-80F) and NiCoP-2 is from 1 to 0 V and 0.2 to 0.55 V, respectively. In order to determine the appropriate working window of the ASC device, we tried the CV technology with the rising voltage window from 1.4 V to 1.8 V. It can be seen in Fig. 4b that the ASC device starts polarization when potential window exceeds 1.6 V, which can be due to the

oxygen evolution reaction caused by the higher voltage. Therefore, 0–1.6 V is considered as the ideal operation voltage. Seen from Fig. 4c, rectangular-like CV curves coupled with redox peaks display mixed features of both Faraday quasi capacitor and EDLC behaviors of ASCs. This also can be proved in the next GCD curves (Fig. 4d), wherein the small plateaus can be ascribed to the faradaic redox reactions of Cod+ /Nid+ [54]. Additionally, based on the GCD curves at the various current densities, specific capacitance of the ASCs is 121.5 F g1 at 1 A g1, which can be due to rapid redox reaction of electrolytic liquid ions (OH) with electrode active substances NiCoP-2 and charge rearrangement. Gratifyingly, this excellent value of capacitance is superior over the recently reported ASCs that utilize AC as negative electrode, such as SNiCoP-2-300//AC (103.1 F g1 at 1 A g1) [48] P-NiCoP/NF//AC (79.4 F g1 at 0.93 A g1) [49], NixCo1-xP//AC (115.8 F g1 at 1 A g1) [50]. Besides, as shown in Fig. 4e, 68.8 F g1 capacitance can be retained when current density is up to 10 A g1, implying the valuable rate capability. The charge-discharge cycling stability of the ASC device was then measured by constant current limiting voltage method at 4 A g1. As shown in Fig. 4f, after 10,000 cycles, only 9.9% specific capacity is lost and the capacitance retention goes up to 90.1%. Attractively, the original capacity progressively rises to 118% and decreases again after the 1000th cycles, which can be caused by the activation process similar with the threeelectrode system. According to the supporting information Eqs. (4) and (5), this ASCs device presents the higher energy density of 43.2 Wh kg1 at corresponding power density of 801.6 W kg1 (Fig. 5a). Moreover, owing to the excellent advantages of both rich mixed valences of bimetals and P and hollow structure, this outstanding battery energy storage capability and discharge capacity are more excellent than other ASCs device employing similar AC as negative electrode like NixCo1-xO//NiyCo2-yP@C//AC (39.4 Wh kg1 at 394 W kg1) [58], NiCo2S4@Ni(OH)2@PPy//AC (34.67 Wh kg1 at 120.13 W kg1) [59], Ni2P/Ni/C//AC (32.02 Wh kg1 at 700 W kg1) [37], NiPNS/NF//AC (26 Wh kg1at 337 W kg1) [60]. Additionally, it also surpasses other cobalt nickel based supercapacitors reported recently like NiCoP-CoP//PNGF (39 Wh kg1 at 1784 W kg1) [54] and NiCoP//Graphene (32.9 Wh kg1 at 1301 W kg1) [56], and phosphide-based hybrid supercapacitors including Co2P//graphene (24 Wh kg1 at 300 W kg1) [21]. The wearable electronic LED (3 V) was employed to prove the practical applications of NiCoP-2//AC (YP-80F) devices. As shown in Fig. 5b, the 3 V blue LEDs containing XJU letters can be lighted by assembling two ACS devices in series and then being charged fully.

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4. Conclusion In summary, metal-formate frameworks (MFFs) have been utilized as sacrificing template to prepare structure-tailorable hollow NixCo1-xP microcubes. The average diameter of hollow NixCo1-xP microcubes decreases with the content of nickel ion. Meanwhile, Co element serves as stable agent to improve the stability of hollow NixCo1-xP microcubes. Based on the combined merits of hollow structure and synergistic effect between Co and Ni ions, the prepared hollow bimetallic Ni–Co phosphides show better electrochemical activity than monometallic phosphides. The NiCoP-2 exhibits the highest specific capacity of 629 C g1 at 1 A g1. The ASCs device based on the NiCoP-2//AC (YP-80F) shows higher energy density of 43.2 Wh kg1. Hence, the hollow NiCoP-2 microcubes demonstrate great potential for application in highperformance supercapacitors. Additionally, we think that the low-cost and convenient-synthesis metal-formate frameworks could be promising templates to construct other hollow metalrich compounds with non-spherical morphology and structure for future energy storage and conversion applications. CRediT authorship contribution statement Xiuli Zhang: Investigation, Methodology, Software, Writing original draft. Li Zhang: Investigation, Methodology, Supervision, Writing - review & editing. Guancheng Xu: Writing - review & editing. Aihua Zhao: Writing - review & editing. Shuai Zhang: Software. Ting Zhao: Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by National Natural Science Foundation of China (No. 21661029, 21771157, 21965035) and Key Laboratory Open Research Foundation of Xinjiang Autonomous Region (2016D03008). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.11.112. References [1] D. Larcher, J.M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (2015) 19–29. [2] A.R. Dehghani-Sanij, E. Tharumalingam, M.B. Dusseault, R. Fraser, Study of energy storage systems and environmental challenges of batteries, Renew Sustain. Energy Rev. 104 (2019) 192–208. [3] M.B. Winter, J. Ralhp, What are batteries, fuel cells, and supercapacitors?, Chem. Rev. 104 (2004) 4245–4269. [4] H. Hu, B.Y. Guan, X.W. Lou, Construction of complex CoS hollow structures with enhanced electrochemical properties for hybrid supercapacitors, Chem 1 (2016) 102–113. [5] L. Yu, B. Guan, W. Xiao, X.W. Lou, Formation of yolk-shelled Ni-Co mixed oxide nanoprisms with enhanced electrochemical performance for hybrid supercapacitors and lithium ion batteries, Adv. Energy Mater. 5 (21) (2015) 1500981. [6] S.N. Yun, Y.W. Zhang, Q. Xu, J.M. Liu, Y. Qin, Recent advance in new-generation integrated devices for energy harvesting and storage, Nano Energy 60 (2019) 600–619. [7] A. Eftekhari, The mechanism of ultrafast supercapacitors, J. Mater. Chem. A 6 (2018) 2866–2876. [8] X. Lu, M. Yu, G. Wang, Y. Tong, Y. Li, Flexible solid-state supercapacitors: design, fabrication and applications, Energy Environ. Sci. 7 (2014) 2160–2181.

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