ultrathin nickel–cobalt phosphate 2d nanosheets for...

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electronic/optoelectronic devices,[10,11] bio-medicine,[12] and sensors.[13] For instance, Liang et al. fabricated a flexible nonvola-tile memory device based on 2D ultrathin WO3⋅H2O nanosheets.[14] Tan et al. suc-cessfully prepared 2D semiconductor het-ero-nanostructures utilized as the anode in a lithium ion battery.[15] Exploring new 2D materials with high electrochemical activity is needed to achieve excellent elec-trochemical performance.

With the social development and the improvement in human lifestyle, energy consumption is regarded as the most important problem facing humanity. Suc-cessful exploitation of renewable but inter-mittent energy sources such as wind and solar power requires efficient and reli-able electrical energy storage, so people are studying new electrochemical energy storage devices, which include batteries,

fuel cells, and electrochemical capacitors.[16–18] Recently, electro-chemical capacitors, especially for the flexible electrochemical capacitors, have attracted considerable attention among scien-tists and engineers for various advantages.[19–25] So far, aqueous and solid electrolytes have been applied in electrochemical capacitor areas. The heat-dissipating capacity of aqueous elec-trolyte is relatively strong, which is suitable for high temper-ature areas, and it is convenient to be replaced. On the other hand, solid flexible electrolyte has good cold resistance; thus, its working efficiency is much better than that of aqueous electro-lyte at low temperature. Liquid electrolytes and solid electrolytes are both needed in the areas where the temperature differs a lot in a small time window.

Transition metal phosphates have been studied for decades and widely applied in many industrial areas.[26,27] However, their applications in the energy storage field have been less concerned until recently.[28–30] Yang et al. synthesized highly ordered mesoporous 2D nanosheets made of transition-metal phosphates with a thickness of about 3.7 nm.[31] Wu et al. reported an inorganic graphene analog, α1-vanadyl phosphate ultrathin nanosheets with the thickness less than six atomic layers.[32] Lately, Xi et al. reported an ultrathin 2D nanosheet and a 1D nanowire of amorphous cobalt phosphate.[33] In our previous work, lamellar potassium cobalt phosphate hydrate nanocrystals were synthesized.[34] However, there are

Ultrathin Nickel–Cobalt Phosphate 2D Nanosheets for Electrochemical Energy Storage under Aqueous/Solid-State Electrolyte

Bing Li, Peng Gu, Yongcheng Feng, Guangxun Zhang, Kesheng Huang, Huaiguo Xue, and Huan Pang*

2D materials are ideal for constructing flexible electrochemical energy storage devices due to their great advantages of flexibility, thinness, and transparency. Here, a simple one-step hydrothermal process is proposed for the synthesis of nickel–cobalt phosphate 2D nanosheets, and the structural influence on the pseudocapacitive performance of the obtained nickel–cobalt phosphate is investigated via electrochemical measurement. It is found that the ultrathin nickel–cobalt phosphate 2D nanosheets with an Ni/Co ratio of 4:5 show the best electrochemical performance for energy storage, and the maximum spe-cific capacitance up to 1132.5 F g−1. More importantly, an aqueous and solid-state flexible electrochemical energy storage device has been assembled. The aqueous device shows a high energy density of 32.5 Wh kg−1 at a power den-sity of 0.6 kW kg−1, and the solid-state device shows a high energy density of 35.8 Wh kg−1 at a power density of 0.7 kW kg−1. These excellent performances confirm that the nickel–cobalt phosphate 2D nanosheets are promising mate-rials for applications in electrochemical energy storage devices.

B. Li, Dr. P. Gu, Y. C. Feng, G. X. Zhang, K. S. Huang, Prof. H. G. Xue, Prof. H. PangSchool of Chemistry and Chemical EngineeringYangzhou UniversityYangzhou 225002, Jiangsu, P. R. ChinaE-mail: [email protected], [email protected]

DOI: 10.1002/adfm.201605784

1. Introduction

Ultrathin 2D nanomaterials are attracting remarkable attention after the realization of graphene[1] and have been extensively investigated.[2,3] Generally speaking, 2D nanomaterials hold a couple of unique characteristics compared to other nanomate-rials. First of all, the electrons in ultrathin 2D nanomaterials cannot be confined by interlayer interactions, which enables greatly compelling electrochemical properties.[4] Second, the thickness of a few atoms offers them maximum mechanical flexibility and excellent optical transparency.[5] In addition, the large lateral size and ultrathin thickness endow them with ultrahigh specific surface area.[5] These advantages make atomically thin 2D inorganic materials good candidates for a variety of applications, such as catalysis,[6,7] energy storage,[8,9]

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few reports about 2D transition metal phosphate, especially ultrathin 2D metal phosphate composite nanosheets synthe-sized by a one-step method.[29,35–37]

In this work, we successfully synthesized nickel–cobalt phos-phate 2D nanosheets in a mild and sample hydrothermal con-dition. Furthermore, we also explored the influence factors of Ni/Co ratio, solvent quantity, surface active agent, reaction tem-perature, and reaction time on the growth of as-prepared mate-rials. It is found that the nickel–cobalt phosphate composite, only with an Ni/Co ratio of 4:5, can obtain a uniform and ultrathin 2D nanosheet structure with the thickness of ≈5 nm. Due to the similar atomic structure of Co and Ni subgroup ele-ments, the ultrathin nickel–cobalt phosphate shows the fine crystalline structure with the Ni/Co ratio close to 1.

In addition, we investigated electrochemical performances of as-prepared samples via a traditional three-electrode system, and the ultrathin nickel–cobalt phosphate 2D nanosheets with the Ni/Co ratio of 4:5 showed the best electrochemical per-formance for energy storage. More importantly, an aqueous electrochemical energy storage device and solid-state flexible electrochemical energy storage device were successfully assem-bled. These devices were based on nickel–cobalt phosphate 2D nanosheets as positive electrode and activated carbon as nega-tive electrode. In aqueous condition, the as-assembled device exhibited a maximum specific capacitance of 162.8 F g−1 with a good cycle stability of 80.4% even after 5000 cycles. The few nanometers of nickel–cobalt phosphate 2D nanosheets may offer them maximum mechanical flexibility, and the large lateral size (ultrathin thickness) endows them with suf-ficient electroactive sites. The solid-state flexible device exhib-ited a maximum specific capacitance of 129.6 F g−1, good rate/mechanical stability, and good cycle stability with 90.5% maintained after 5000 cycles. The aqueous device showed a high energy density of 32.5 Wh kg−1 at the power density of 0.6 kW kg−1. The solid-state device showed a high energy den-sity of 35.8 Wh kg−1 at a power density of 0.7 kW kg−1. The previously mentioned excellent performance reveals that the nickel–cobalt phosphate 2D nanosheets are one of the prom-ising candidates for electrochemical energy storage devices with both aqueous electrolyte and solid electrolyte, which may be applied in the areas where the temperature differs a lot in a small time window.

2. Results and Discussion

The characterization of crystallographic structure and phase purity of the samples were carried out by X-ray powder diffrac-tion (XRD) measurement. As shown in Figure S1 (Supporting Information), the diffraction peaks can be mainly assigned to Co3(PO4)2 (JCPDS No. 80-1997). In the XRD pattern, major dif-fraction peaks at 13.3°, 17.9°, 26.7°, 26.1°, 26.9°, 27.4°, 29.9°, 33.8°, 38.6°, and 48.5° can be indexed to the (101), (111), (112), (202), (021), (012), (221), (031), and (402) facet of the Co3(PO4)2 phases. Meanwhile, other diffraction peaks at 17.5°, 23.5°, 26.0°, 37.2°, and 39.3° refer to the corresponding (002), (102), (111), (211), and (021) facet of the Ni3(PO4)2 phases (JCPDS No. 70-1796). The above results prove that the material is the nickel–cobalt phosphate composite.

To further explore the element composition and the oxida-tion state of our prepared materials, an attempt was made to access the binding energy and chemical states of the bonded elements by X-ray photoelectron spectroscopy (XPS) measure-ment. This aforementioned compound contains elements of Ni, Co, P, and O with an atomic ratio of 7.0:8.4:13.4:57.8. In addition, the corresponding high-resolution spectra are shown in Figure 1. Figure 1a shows that the high-resolution Ni 2p XPS spectrum can be deconvoluted into two spin–orbit dou-blets and two shake-up satellites, in which the binding energy for Ni 2p3/2 and Ni 2p1/2 is observed at 856.3 and 874.1 eV, respectively, yielding characteristics of the Ni(II) state. From Figure 1b, the Co 2p spectrum can be best fitted by Co 2p3/2 and Co 2p1/2 peaks located at 781.5 and 797.5 eV accompanied by two shake-up satellite peaks (786.1 and 802.7 eV), which dis-tinctly verify the presence of the Co(II). As shown in Figure 1c, the peaks located at 133.3, 134.2, 135.7, and 136.5 eV corre-spond to the characteristic P 2p3/2 peaks of P(V). Combined with the results from XRD, the material was further proved to be nickel–cobalt phosphate composite.

The morphology of as-prepared sample was examined by scanning electron microscopy (SEM), atomic force micros-copy (AFM), and transmission electron microscopy (TEM). A typical low-magnification SEM image in Figure 2a shows that the morphology of the obtained sample is a uniform 2D nanosheet, and the width of a nanosheet is about 500–800 nm. In addition, the nanosheets are nearly transparent, as shown in Figure 2b, which indicates the ultrathin nanostructure. The thickness of an ultrathin nanosheet is ≈5 nm, further measured by AFM shown in Figure 2c. The ultrathin nanosheet structure can provide sufficient electroactive sites for surface redox reac-tion and exhibit a high pseudocapacitance.[38] The schematic crystal structure of the sample nickel–cobalt phosphate super cells (2 × 2 × 2 slabs) based on data from the inorganic crystal structure database (ICSD)-69851 is shown in Figure 2d. It can be clearly seen that the sample is a layered structure, and the distance of the neighboring layer is 0.77 nm. It denotes that the ultrathin nanosheet is comprised of 6–7 single layers. Figure 2e is a TEM image of nickel–cobalt phosphate nanosheets at low magnifications, which accords with the above SEM results. Figure 2f shows high-resolution TEM (HRTEM) image and selected area electron diffraction (SAED) of the nanosheets. The HRTEM image exhibits an atomic lattice fringe spacing of 0.33 nm corresponding well to the (202) plane of the nickel–cobalt phosphate nanosheets. As indicated in Figure 2g, the mapping of the elements Co, Ni, P, and O from a selected area suggests that those elements are homogeneously distributed in the whole area without apparent element aggregations or separations observed, which provides further evidence that the material is the nickel–cobalt phosphate composite.

By controlling the time of reactions, we try to explore the process of crystal growth. Figure S2a–d (Supporting Informa-tion) shows the SEM images of the sample with reaction time of 2, 4, 6, and 8 h, respectively. Figure S3 (Supporting Infor-mation) shows the corresponding XRD pattern. As shown in Figure S3 (Supporting Information), the reaction time clearly affects the growth of nanocrystals. When the reaction time is 2 h, the reaction product is Co3(PO4)2⋅8H2O (JCPDS No. 79-0731), and there is no special morphology generated. As

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the reaction time is extended to four hours, the sample loses part of the crystal water and shows branch-like structure. The diffraction peaks can be mainly assigned to Co3(PO4)2 (JCPDS No. 80-1997). Continuing to increase the reaction time to eight hours, the composition does not change, but the morphology turns into sheets. After 6 h, the morphology of the sample is leaflike with a size of about 500 nm. After 8 h, the sample pre-sents a homogeneous lamellar structure. Compared with the sample of 2, 4, and 6 h, the XRD peaks of the sample with reac-tion time of 8 h is much stronger, which demonstrates that the crystal keeps growth from the beginning to 8 h.

Serials of reaction conditions were changed to explore the impact of the morphology and composition. Table S1 (Supporting Information) shows the detailed experimental parameters and the corresponding sample names (Sample S4 (Supporting Information) is the aforementioned sample with optimum structure). As shown in Figure S4a–f (Supporting Information), we obtained a serial of Samples S1–S6 (Supporting Information) with different ratios of cobalt and nickel. From these images, it could be seen clearly that the products are composed of nanosheets with different sizes and thicknesses, and we found that Sample S4 (Supporting Infor-mation) with an Ni/Co ratio of 4:5 is the thinnest and most uni-form. This may be caused by the similar atomic structure of Co and Ni elements, as they are next to each otheron the periodic table.

Since the amount of solvent affects the reaction dynamics, here we tuned the amount of solvent to study the influence on the morphology. The obtained samples were denoted as S7–S12 (Supporting Information), and they are under the same condition of Samples S1–S6 (Supporting Information),

except that the amount of solvent decreased to 5 mL. Figure S6a–f (Supporting Information) shows SEM images of Samples S7–S12 (Supporting Information). After compari-sons, it was obvious that Sample S4 (Supporting Information) is thinner than Samples S7–S12 (Supporting Information) because the concentration of the reactant is increased. In the process of preparing nanomaterials, surfactant can play a key role such as template agent, stabilizing agent, and dispersant. Thus, 0.1 g surfactant of polyvinyl pyrrolidone (PVP, rela-tive molecular weight is 30 000) was added to obtain Samples S13–S18 (Supporting Information), where the other conditions were kept the same as Samples S1–S6 (Supporting Informa-tion). Figure S8 (Supporting Information) shows the trend of morphology changes after adding PVP, which is the same as that of the materials without PVP, and Figure S9 (Supporting Information) shows that there is no difference on the phase of as-prepared materials with or without PVP.

Reactions with increasing temperature and prolonging time were further done. Figure S9a–f (Supporting Information) shows SEM images of Samples S19–S24 (Supporting Informa-tion) that were obtained under 200 °C for 24 h. These samples have the same composition in Figure S9 (Supporting Informa-tion), although the amount of nickel source is different. Those peaks match well with Co2P2O7 (JCPDS No. 34-1378), and the generated Co2P2O7 is probably caused by the long time and high temperature of the reactions.

In a conventional three-electrode system, we characterized the electrochemical capacitive properties of electrode assem-bled from nickel–cobalt phosphate 2D ultrathin nanosheets by cyclic voltammogram (CV) and galvanostatic charge–dis-charge (CP) method in 3.0 m KOH aqueous solution. Figure 3a

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Figure 1. a–c) High-resolution XPS spectra of Ni 2p, Co 2p, and P 2p for nickel–cobalt phosphate (Sample S4, Supporting Information).

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shows the CV curves of nickel–cobalt phosphate 2D ultrathin nanosheet electrode (mass loading is about 5 mg) at scan rates of 5, 10, 20, 30, and 50 mV s−1, respectively. The shapes indicate that the capacity is faradaic pseudocapacitive property, which is different from that of electric double-layer capacitance. The faradaic pseudocapacitive property of the nickel–cobalt phos-phate 2D ultrathin nanosheets electrode could be explained by the surface redox mechanism of Co(II) to Co(III) and Ni(II) to Ni(III).

Figure 3b shows the CP curves of nickel–cobalt phosphate 2D ultrathin nanosheets electrode when current densities were at 1, 2, 5, and 10 A g−1. The shape of the CP curves means that the nickel–cobalt phosphate 2D ultrathin nanosheets electrode has good electrochemical capability, but the shape was not very symmetric. Namely, the Coulombic efficiency was not very high, and there were some irreversible electrochemical behav-iors in the charge–discharge process. The slight internal resist-ance drop implied a small intrinsic series resistance of nickel–cobalt phosphate 2D ultrathin nanosheets.[32] In addition, there is a potential platform in every discharge curve. It is the typical pseudocapacitance behavior of transition metal compounds, which is caused by a charge transfer reaction or electrochem-ical absorption/desorption process at the electrode/electrolyte interface.[19,39]

The specific capacitances were calculated based on CP curves and shown in Figure 3c, which were 1132.5, 980, 852.5, and

717.5 F g−1 at a current density of 1, 2, 5, and 10 A g−1, respec-tively. The large specific capacitance shows the great potential in high power applications. This high specific capacitance can be explained by the ion-exchange mechanism. The OH− needs enough time to transfer between the surface of electrode mate-rials and the solution in order to be intercalated/extracted into/out of the sample when charging/discharging. If the current is low, the OH− has enough time to transfer and has more charge transfers than at high current densities, which means more charge can be stored and thus higher specific capacitance results.

For electrode materials, it is also very important to have a good capacitance retention. The work electrode should work steadily and safely, which requires the specific capacitance of electrode materials should change as little as possible. The cycling performances of the electrode at a current density of 2 A g−1 are shown in Figure 3d. The electrode shows an excel-lent cycling life and good cycling stability. It can be seen that the capacity changed from 979.8 to 911.4 F g−1 during 8000 cycles, which maintained up to 93% after 8000 cycles.

The SEM and TEM images of the as-prepared ultrathin 2D nanosheets nickel–cobalt phosphate after 8000 cycles under a traditional three-electrode system are shown in Figure S12 (Supporting Information). It can be seen from SEM image that ultrathin nanosheets have mainly maintained the orig-inal morphology, which was further confirmed by good cycle

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Figure 2. a,b) SEM images, c) AFM images, and e,f) TEM images of nickel–cobalt phosphate (Sample S4, Supporting Information) at different mag-nifications. d) The schematic crystal structure of the sample nickel–cobalt phosphate super cells (2 × 2 × 2 slabs) based on data from ICSD-69851. g) Elemental mapping showing the homogeneous distribution of all elements of Ni, Co, P, and O in the nanosheet.

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ability; however some nanosheets are broken into small pieces after 8000 cycles, which may cause the little decay. What is more, some nanopores can be seen from the high-resolution TEM image, which may be caused by the occurrence of ion intercalation/deintercalation during the charging/discharging. Most of the nanosheets retain the original nanostructure after 8000 cycles, suggesting the excellent chemical stability of the nanostructure.

To understand the conductivity of the as-prepared elec-trodes, electrochemical impedance spectroscopy (EIS) spectra were measured in the frequency from 0.01 to 105 Hz under open-circuit voltages, and the results are shown in Figure S13 (Supporting Information). As shown in Figure S13 (Supporting Information), the impedance curves present two partial semi-circles in the high- and medium-frequency regions and an inclined line in the low-frequency region. The semicircle diam-eter after 8000 cycles is close to that of the original, indicating that the charge-transfer resistance (Rct) changed little. The curves showed that the resistance changed slightly, which fur-ther evidence the stability of the electrochemical performance. The EIS data can be fitted by a bulk solution resistance (Rs), a charge-transfer resistance, and a pseudocapacitive element from the redox process of the nickel–cobalt phosphate, and a constant phase element to account for the double-layer capaci-tance. The Rs was measured to be 1.87 and 2.44 Ω, respectively, while the Rct was calculated (by ZSimpWin software) to be 16.5 and 21.3 Ω, respectively.

A comparison with previously reported nickel/cobalt phos-phates nanomaterials is given in Table 1. The morphology is of vital importance for the electrochemical performance; 2D

nanomaterials hold a couple of unique characteristics compared to counterparts with different dimensionalities. The electrons in ultrathin 2D nanomaterials have not been confined by interlayer interactions. Furthermore, the large lateral size and ultrathin thickness endow them with ultrahigh specific surface area, and the thickness of few atoms offers them maximum mechanical flexibility. In most cases, the performance of nickel–cobalt hybrid phosphate is better than pure nickel phosphate or cobalt phosphate, because the composite materials benefit from the synergistic effect of the two materials, then exhibits enhanced capacitance behavior.[49] We also characterized the electrochem-ical properties of Samples S1–S18 (Supporting Information) by CV and CP method in 3.0 m KOH aqueous solution, and sev-eral of them were shown in Figures S14 and S15 (Supporting Information). As shown in Figures S14 and S15 (Supporting Information), the specific capacitance of Samples S1, S3, and S6 (Supporting Information) can reach 417.5, 652.8, and 1012 F g−1 at 1 A g−1, respectively. It clearly reveals that the elec-trochemical capacitive property of Samples S3, S4, and S6 (Sup-porting Information) is better than that of Sample S1 (Supporting Information), and the result may be due to the addition of nickel. As shown in Figures S16 and Figure S17 (Supporting Information), the specific capacitance of Samples S19, S21, and S24 (Supporting Information) can reach 206.3, 272.5, and 310.0 F g−1 at 1 A g−1, respectively. Those capacities were obvi-ously lower than those of the samples with nanosheets structure, because different structures lead to different electrochemical performance for electrolyte accesses and ions intercalated/extracted into/out. Chemical composition also shows a remark-able effect on the capacitive behavior of the samples. According

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Figure 3. a) Cyclic voltammetry curves of nickel–cobalt phosphate (Sample S4, Supporting Information) electrode at different scan rates. b) The galvanostatic charge–discharge curves of Sample S4 (Supporting Information) electrode at different current densities. c) Specific capacitances of Sample S4 (Supporting Information) electrode at different current densities. d) Charge–discharge cycling test at a current density of 2 A g−1 in 3.0 m KOH electrolyte.

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Table 1. A comparison with previously reported nickel/cobalt phosphates nanomaterials.

Material Morphology SP [F g−1] (low current density) SP [F g−1] (high current density) Capacity retention [%]

Ni2P2O7[40] 557.7 (1.2 A g−1) 360 (1.2 A g−1) 97.3 (1k cycles, 2.4 A g−1)

Ni2P2O7[41] 1050 (0.5 A g−1) 410 (7 A g−1) 90.5 (6k cycles, 1 A g−1)

MOF-NixPyO2[42] 1627 (1 A g−1) 1044 (20 A g−1) 76.82 (2k cycles, 6 A g−1)

Ni3P2O8[43] 1464 (0.5 A g−1) 1190 (8 A g−1) 84 (1k cycles, 0.5 A g−1)

Ni11(HPO3)8(OH)6[44] 558 (0.5 A g−1) 224 (7 A g−1) 97.6 (10k cycles, 0.5 A g−1)

Ni11(HPO3)8(OH)6[45] 295 (0.625 A g−1) 177 (6.25 A g−1) 99.3 (1k cycles, 0.625 A g−1)

NaCoPO4-Co3O4[46] 268 (0.8 A g−1) 180 (6.4 A g−1) 91 (1k cycles, 1.6 A g−1)

Co3(PO4)2⋅8H2O[47] 350 (1 A g−1) 227 (10 A g−1) 102 (0.5k cycles, 1 A g−1)

CoHPO4⋅3H2O[28] 413 (1.5 A g−1) 338 (15 A g−1) 85.1 (3k cycles, 1.5 A g−1)

Co11(HPO3)8(OH)6[48] 1200 (0.5 A g−1) 1078 (6 A g−1) –

NixCo3−x(PO4)2[29] 940 (1 A g−1) 765 (10 A g−1) 84.5 (1k cycles, 4 A g−1)

Co0.86Ni2.14(PO4)2[35] 1050 (0.25 A g−1) 1032 (10 A g−1) –

Ni3P2O8-Co3P2O8[36] 1980 (0.5 A g−1) 1340 (8 A g−1) 90.9 (1k cycles, 1 A g−1)

(Co,Ni)3(PO4)2[37] 1128 (0.5 A g−1) 997 (24 A g−1) 95.6 (5k cycles, 1 A g−1)

This work 1132 (1 A g−1) 717 (10 A g−1) 93 (8k cycles, 2 A g−1)

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to the analysis above, the nickel–cobalt phosphate 2D ultrathin nanosheets (Sample S4, Supporting Information) electrode showed the best electrochemical energy storage performance.

The aqueous and solid-state flexible devices were success-fully fabricated, which contains nickel–cobalt phosphate 2D ultrathin nanosheets (positive material) and activated carbon (negative material). To obtain the optimal performance, the charge balance between the two electrodes should follow the relationship of q+ = q−. According to the equation, q = m × C × ΔV, the storage of charge usually depends on the mass of the electrode (m), the potential range for the specific capacitance (C), and the charge/discharge process (ΔV).[37] In order to sat-isfy the relationship q+ = q−, the mass ratio between the two electrodes should be m+/m− = (C− × ΔV−)/(C+ × ΔV+) = (200 × 1)/ (1132 × 0.4) ≈ 1:2.2. The specific capacitance and voltage of the positive electrode were taken from the results of the above three-electrode test.

The optical photo of the as-assembled aqueous device is shown as the insert of Figure 4d. As shown in Figure 4a, in the case of aqueous electrolyte, the working voltage window was enlarged to 0–1.2 V. Figure 4b showed the CP curves of the device when current densities were 1, 2, 5, and 10 A g−1. The shape of CP curves is very symmetric, which means that Coulombic efficiency is very high. The specific capacitances were calculated according to CP curves at the current density of 1–10 A g−1, which were shown in Figure 4c. The specific capaci-tance of the device at the current density of 1, 2, 5, and 10 A g−1 was calculated to be 162.8, 155.5, 147.3, and 128.9 F g−1, respec-tively, showing the potential for high-power applications. Figure 4d showed the cycling performance at a current density of 2 A g−1. After 5000 cycles, the specific capacitance of the

device is 80.4% of its initial capacitance, which proves the good cycling performance of the device.

The solid-state flexible device was also fabricated by a facile method (details are shown in the Experimental Section). Gel electrolyte (KOH/polyvinyl alcohol (PVA)) was spread on the active materials manually and was covered by two pieces of poly(ethyelene terephthalate) (PET) in order to prevent the loss of water in the gel electrolyte. CV curves of the flexible device with a potential window of 0.0–1.4 V at different scan rates were measured. As shown in Figure 5a, there is a distinct reduction peak under various scan rates from 5 to 30 mV s−1, not an ideal rectangular shape, and the double-layer capacitance charac-teristic of these curves is not pronounced, indicating that the capacitance characteristics are mainly because of pseudocapaci-tive processes,[50] which are attributed to the Faradic pseudoca-pacitance of nickel–cobalt phosphate. It may be mainly because there is slight charge imbalance of two electrodes and the resist-ance of the PVA/KOH gel electrolyte is slightly larger than that of aqueous electrolyte, which does not affect the charge and dis-charge performance of the supercapacitor.[51] Interestingly, the shape of the CV curve can still be kept even at large scan rate of 30 mV s−1, which manifests a good rate capability of the device. CP curves at different current densities are shown in Figure 5b. The specific capacitance reached 129.6 F g−1 at a current density of 1 A g−1. The device had a specific capacitance of 129.6, 114.6, 100.4, and 73.8 F g−1 at 1, 2, 5, and 10 A g−1, and those results were plotted in Figure 5c.

A charge–discharge cycling test was conducted to study the long-term cycle ability of the device. Figure 5d showed the cycling performance at a current density of 2 A g−1. After 5000 cycles, the specific capacitance was maintained at 90.5%

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Figure 4. Electrochemical characterization of the nickel–cobalt phosphate//activated carbon aqueous device: a) Cyclic voltammetry curves at a scan rate of 5–100 mV s−1. b) The galvanostatic charge–discharge curves with different current densities (1–10 A g−1). c) Specific capacitances at the current density of 1–10 A g−1. d) Charge–discharge cycling test at a current density of 2 A g−1 (inset: an optical picture of aqueous device for liquid electrolyte).

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of its initial capacitance, which proves the excellent cycling performance of the device. The small decay could be caused by the consumption of the gel electrolyte in an irreversible reaction of active materials and electrolytes. To certify the practical application of the device, a solid-state flexible device was employed to power a red light-emitting diode (LED) in the inset of Figure 5d. The device can power a red LED for about 3 min after charging 30 s at 1 A g−1. CV measurements were conducted at a scan rate of 20 mV s−1 with different bending modes to test the flexibility of the device; as shown in Figure 6b, the CV curves with different bending modes change lightly. More importantly, we have measured the performance of the device after 300 bending times; as seen in Figure 6c, the as-prepared device shows only 0.44% decay during different bending modes for 300 times.

The energy and power densities are very important param-eters in optimizing materials for practical applications. Tra-ditionally, the practical applications of the solid-state flexible device are enhanced by high power densities but largely lim-ited due to the lower energy densities. Indeed, most of the recently developed supercapacitor systems present high energy densities but result in great compromise over the power den-sities. The Ragone plot of the as-prepared device in Figure 6 shows the relationship between energy and power density. The aqueous device showed a high energy density of 32.5 Wh kg−1 at the power density of 0.6 kW kg−1. What is more, the solid-state device showed a high energy density of 35.8 Wh kg−1 at the power density of 0.7 kW kg−1. The higher energy density values suggest the potential of the materials for their practical use in real energy storage devices.

Since Co and Ni subgroup elements are next to each other on the 4th period of the periodic table, the similar atomic struc-ture may lead to the fine crystal structure and morphology of the ultrathin nickel–cobalt phosphate 2D nanosheets with an Ni/Co ratio close to 1. Thus, the nickel–cobalt phosphate only with an Ni/Co ratio of 4:5 showed the homogeneous ultrathin 2D nanosheets structure. The large lateral size and ultrathin thickness endow the nanosheets with sufficient electroactive sites. The free interlayer electrons of the nanosheets enable the weak van der Waals’ force on the vertical direction, which may lead to the excellent mechanical flexibility. More importantly, the electrons in ultrathin 2D nanomaterials cannot be confined by interlayer interactions, which enable greatly compelling elec-trochemical properties. The electrochemical experiments show that the electrochemical energy storage performance of the ultrathin nickel–cobalt phosphate with an Ni/Co ratio of 4:5 is the best. The ultrathin nickel–cobalt phosphate is a promising material for electrochemical energy storage devices, which can be applied in the areas where the temperature differs greatly in a small time window.

3. Conclusions

In conclusion, uniform and ultrathin 2D nanosheets nickel–cobalt phosphate were successfully prepared through a series of exploration, and the synthesis process of this composite mate-rial is a one-step hydrothermal strategy, which is very simple and requires low energy consumption. We investigated their electrochemical performance via a traditional three-electrode

Adv. Funct. Mater. 2017, 27, 1605784


Figure 5. Electrochemical characterization of the nickel–cobalt phosphate//activated carbon solid-state flexible device: a) Cyclic voltammetry curves at different scan rates. b) The galvanostatic charge–discharge curves at different current densities. c) Specific capacitances at different current densities. d) Charge–discharge cycling test at a current density of 2 A g−1 (inset: a red LED powered by the flexible device).

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system, aqueous device, and solid-state flexible device, and the electrode showed a maximum specific capacitance of 1132.5 F g−1. Aqueous device and solid-state flexible device which were based on nickel–cobalt phosphate 2D nanosheets and activated carbon were successfully assembled. These devices exhibited high specific capacitance, excellent cycle sta-bility, and high energy density. The aforementioned excellent performance reveals that the ultrathin nickel–cobalt phosphate is a promising material for electrochemical energy storage devices with aqueous/solid electrolyte, which can be used in either high or low temperatures.

4. Experimental SectionFabrication of Nickel–Cobalt Phosphate 2D Nanosheets: For preparation

of nickel–cobalt phosphate 2D ultrathin nanosheets, 0.4 g nickel acetate, 0.5 g cobalt acetate, 0.5 g sodium pyrophosphate anhydrous, and 10 mL H2O were mixed together. The above mixture was stirred at room temperature for 30 min, then transferred into 100 mL stainless-steel autoclave lined with polytetrafluoroethylene (PTFE). After that, the autoclave was sealed and maintained at 160 °C for 8 h. The obtained precipitates were washed several times by distilled water and ethanol, and the sample was denoted as S4 (Supporting Information).

Characterizations: The morphology of as-prepared samples was captured by a Supra 55 SEM at an acceleration voltage of 5.0 kV. The phase analysis of the samples was performed by a Bruker AXS D8 advanced XRD with Cu Kα radiation of 40 kV. The surface chemical species of the samples were examined using a Perkin-Elmer PHI-5702

multifunctional XPS with Al Ka radiation of 1486.6 eV as the excitation source. TEM images were observed on the JEM-2100 instrument microscopy at an acceleration voltage of 200 kV. Scanning transmission electron microscopy images, HRTEM images, SAED images, and energy-dispersive X-ray spectroscopy mapping were captured on a Tecnai G2 F30 transmission electron microscopy at an acceleration voltage of 300 kV. All electrochemical performances were carried out by a CHI 660E instrument.

Fabrication of the Electrodes in a Traditional Three-Electrode System: Electrochemical performance was conducted by a CHI 660E instrument in a traditional three-electrode system. A Hg/HgO electrode and platinum electrode were chosen as reference and counter electrode, respectively. The electrolyte was 3.0 m KOH aqueous solution. The working electrode materials were prepared by grinding the mixture of active materials, acetylene black, and PTFE with a weight ratio of 80:15:5, and coating the mixture on a 1 cm × 5 cm nickel foam. The additive was a certain amount of isopropyl alcohol when grinding and the painted size was about 1 cm × 1 cm. In addition, the nickel foam where coated active materials were pressed to a thin foil at a pressure of 10.0 MPa. The typical mass loading of the electrode material was 5 mg.

Fabrication of the Aqueous Electrochemical Energy Storage Device: Aqueous electrochemical energy storage devices were assembled by employing the nickel–cobalt phosphate 2D nanosheets as positive electrode and activated carbon as negative electrode. In addition, the weight of the sample was 2 mg, and the weight of activated carbon was 4.4 mg. The electrochemical performance of the devices was measured at room temperature in a two-electrode electrochemical full cell. The electrolyte was 3.0 m KOH aqueous solution.

Fabrication of the Solid-State Flexible Electrochemical Energy Storage Device: The positive and negative electrodes were prepared the same way as the electrodes of aqueous device. The PVA/KOH gel electrolyte

Figure 6. a) Ragone plot of aqueous device and solid-state device showing the relationship between energy density and power density. b) Cyclic vol-tammetry within a 0–1.4 V range at a scan rate of 20 mV s−1 with three bending modes, and the bending modes are consistent with panel (b). c) The specific capacitance of as-prepared device after 300 bending times with different bending modes.

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was prepared as follows: 1.52 g PVA were added to 15 mL deionized water and the as-obtained solution was heated to 75 °C for 30 min, then 5 mL KOH (8 m) aqueous solution was added dropwise into the gel solution under stirring. The positive and negative electrodes were placed on different sides of the PET substrate, and then coated with the gel solution covering the active materials. After the excess water was vaporized, the positive and negative electrodes including electrolyte sandwiched between two pieces of PET substrates. Then, the all-solid-state flexible device was fabricated.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis work was supported by the Program for New Century Excellent Talents of the University in China (Grant No. NCET-13-0645) and the National Natural Science Foundation of China (Grant Nos. NSFC-21671170 and 21673203), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (Grant No. 164200510018), Plan for Scientific Innovation Talent of Henan Province, Program for Innovative Research Team (In Science and Technology) in University of Henan Province (Grant Nos. 14IRTSTHN004 and 16IRTSTHN003), the Science and Technology Foundation of Henan Province (Grant Nos. 122102210253 and 13A150019), the Science and Technology Foundation of Jiangsu Province (Grant No. BK20150438), the Six Talent Plan (Grant No. 2015-XCL-030), and the China Postdoctoral Science Foundation (Grant No. 2012M521115). Qinglan project. The authors also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions and the technical support received at the Testing Center of Yangzhou University.

Received: November 5, 2016Revised: December 3, 2016

Published online: February 15, 2017

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