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Controllable Fabrication and Tuned Electrochemical Performance of Potassium Co-Ni Phosphate Microplates as Electrodes in Supercapacitors Bo Liang, Yule Chen, JIANGYU HE, Chen Chen, Wenwen Liu, Yuanqing He, Xiaohe Liu, Ning Zhang, and Vellaisamy A. L. Roy ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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ACS Applied Materials & Interfaces

Controllable Fabrication and Tuned Electrochemical Performance of Potassium Co-Ni Phosphate Microplates as Electrodes in Supercapacitors Bo Liang, †,

∥

Yule Chen, †, ‡,

Xiaohe Liu,† Ning Zhang,†,

⊥,

∥

Jiangyu He,† Chen Chen,† Wenwen Liu,‡ Yuanqing He,† ⊥

* Vellaisamy A. L. Roy , *

† School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, China ‡ Powder Metallurgy Institute, Central South University, Changsha, Hunan 410083, China ⊥Department of Materials Science & Engineering and State Key Laboratory of Millimeter Waves, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, China ∥These authors contributed equally to this work

KEYWORDS

Supercapacitor, Potassium nickel phosphate, Electrochemistry, Electrode, Microplate

ABSTRACT

Mostly reported pristine phosphates, such as NH4MPO4·H2O (M = Co, Ni), is not very stable as supercapacitor electrodes because of their chemical properties. In this work, KCoxNi1-xPO4·H2O microplates were fabricated by a facile hydrothermal method at low temperature and used as electrodes in supercapacitors. The Co and Ni content could be 1

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adjusted and optimal electrochemical performance was found in KCo0.33Ni0.67PO4·H2O, which also possessed superior specific capacitance, rate performance, and long-term chemical stability compared with NH4Co0.33Ni0.67PO4·H2O because of its unique chemical composition and microstructure. Asymmetric supercapacitor cells based on KCo0.33Ni0.67PO4·H2O and active carbon were assembled, which produce specific capacitance of 34.7 mAh g-1 (227 F g−1) under current density of 1.5 A g−1 and retain 82 % as initial specific capacitance after charging and discharging approximate 5000 times. The assembled asymmetric supercapacitor cells (ASCs) exhibited much higher power and energy density than most previously reported transition metal phosphate ASCs. The KCoxNi1-xPO4·H2O electrodes fabricated in this work are efficient, inexpensive, and composed of naturally abundant materials, rendering them promising for energy storage device applications. INTRODUCTION

The most attractive characteristics of electrochemical capacitors are their faster charging and discharging rates, superior long-term cycle stability, and higher power and energy densities compared with conventional batteries.1–3 Pseudocapacitors are an important type of supercapacitor (SC) in which capacitance is derived from the faradaic redox reactions at the surface of electrodes, producing tens to hundreds of times specific capacitance as double-layer capacitors (EDLCs) that store energy as the way of accumulated electrostatic charges near the interfaces of electrode and electrolyte in electric double layers.2,4 To design high-efficiency energy storage pseudocapacitive electrodes, the materials must have high electrochemical activity, favorable conductivity, and high chemical stability in faradaic redox reactions.2,5 Multiple electrode materials have been explored that exhibit unique 2


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electrochemical performance: metal oxides,6 hydroxides,7 sulfides,8 phosphides,9 and others.10 Among these materials, transition metal phosphates and pyrophosphates, such as NH4NiPO4, NH4CoPO4, Ni2P2O7, and Co2P2O7, are highly electrochemically active, inexpensive, and naturally abundant, and have been applied as promising electrochemical active materials in SCs.11–18 Unfortunately, although NH4MPO4 (M = Co, Fe, Ni) materials exhibit high electrochemical activity, they are unstable in prolonged charge and discharge reactions because of the existence of NH4+, which easily reacts with electrolytes, especially in highly alkaline solutions, and induces irreversible structural corruption.19 M2P2O7 (M = Co, Mn, Ni) materials have relatively poor conductivity and induce a lower specific capacitance than H4MPO420; moreover, high-temperature calcination is usually required to obtain M2P2O7, which consumes more energy in preparation.12,20,21 Therefore, it remains desirable to identify more phosphate materials with high electrochemical activity and chemical stability that are easy to prepare. KMPO4·H2O (M = Ni, Co, Mn) materials have a struvite-type orthorhombic structure, similar to the widely reported NH4MPO4·H2O electrode materials if the NH4+ is considered to be replaced by K+. KMPO4·H2O exhibits a layered structure with a layer-slab of P-O-M polyhedrons and intercalated K+ between the layers;22,23 such layered channels benefit the diffusion of ions and electrons in faradaic redox reactions. Furthermore, the existence of K+ in KMPO4·H2O induces greater chemical stability than the NH4MPO4·H2O in an alkaline electrolyte such as KOH solution; this enables it to be used as a highly specific and stable electrode material in SCs.19,24 However, the application of KMPO4·H2O materials as SC electrodes has seldom been reported.

3



In this study, KCoxNi1-xPO4·H2O materials were fabricated and applied as an electrode material in SCs. Co and Ni were selected because the cooperation of Co and Ni in hydroxides, phosphides, pyrophosphate, and so on can induce considerably enhanced capacitance; Co2+ can efficiently reduce the resistance and Ni2+ can provide high theoretical capacitance.20,25–28 Such mechanisms may also be suitable in phosphate materials to generate high-performance electrodes. In addition, both Ni and Co are naturally abundant and inexpensive, meeting the need for cheap electrodes in practical applications. The fabricated KCo0.33Ni0.67PO4·H2O materials exhibited improved specific capacitance and much enhanced chemical stability than the corresponding NH4Co0.33Ni0.67PO4·H2O materials in the evaluation of three-electrodes cell. Based on above characterizations, asymmetric supercapacitor cells (ASCs) were assembled by employing optimized KCo0.33Ni0.67PO4·H2O as active material in positive electrode as well as activated carbon (AC) material in counter negative electrode. The favorable capacitance, cycling ability, and energy densities over assembled ASCs indicate a promising application in energy storage devices. EXPERIMENTAL SECTION

Synthesis of Materials: All reagents were purchased commercially with AR grade. The KCoxNi1-xPO4·H2O materials were synthesized by a solvothermal method. Stoichiometric nickel chloride hexahydrate (NiCl2·6H2O), cobalt chloride hexahydrate (CoCl2·6H2O), and potassium hydrogen phosphate (K2HPO4·3H2O) were added to 30 mL deionized water at room temperature in a 40-mL autoclave. After vigorous stirring for 30 min, the autoclave was maintained at 120 °C for 12 h. After the reactions, the precipitates was collected and washed by deionized water at room temperature and then was dried at approximate 70 °C. These 4


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products are called KCoxNi1−x PO4·H2O (where x is from 0 to 1, depending on the ratio of Ni and Co). For comparison, NH4Co0.33Ni0.67PO4·H2O was prepared by a hydrothermal method adapted from a previously reported method under similar conditions.19 The detailed information for constructing and testing electrodes in three-electrodes system and asymmetric supercapacitor cells are given in supporting information. Materials Characterizations: Powder X-ray diffraction (XRD) was carried out on a RIGAKU Rint-2000 X-ray diffractometer with graphite monochromatized Cu Kα radiation (λ = 1.54184 Å). Analysis of X-ray photoelectron spectroscopy (XPS) was conducted on a spectrophotometer of Thermo Fisher ESCALAB 250Xi. Scanning electron microscopy (SEM) and energy-dispersion X-ray spectroscopy (EDS) mapping images were recorded on a FEI Helios Nanolab 600i field emission (FE) scanning electron microscope equipped with Oxford X-Max20 energy spectrum system. The transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) images were produced from a Tecnai G2 F20 FE transmission electron microscope at 200 kV acceleration voltage. Fourier transform infrared (FT−IR) spectra were recorded on a Nicolet Nexus 670 spectrograph. The atomic crystal structure is drawn by software of VESTA.29 RESULTS AND DISCUSSION

Figure 1 displays the typical XRD patterns of the products prepared at different ratios of Co and Ni. The standard powder diffraction file numbers for both KNiPO4·H2O (JCPDS card No. 86-0591) and KCoPO4·H2O (JCPDS card No. 86-0590) are shown at the bottom of the figure. All products exhibit satisfactory crystallinity and diffraction patterns consistent with the standard patterns (also displayed at the bottom), indicating that the pure phases of the solid 5



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states of KCoxNi1-xPO4·H2O were generated. The right side displays a magnified section around (010), indicating the slight shift of the diffraction peaks of KNiPO4·H2O from Figure 1a toward a smaller degree in Figure 1d with the increase of Co to KCoPO4·H2O, which is a result of increased cell lattice volume with the incorporation of Co. Detailed cell lattice parameters of the products are given in Table S1 of Supporting Information.

Figure

1

XRD

patterns

of

(a)

KNiPO4·H2O,

(b)

KCo0.67Ni0.33PO4·H2O, (d) KCoPO4·H2O.

6


KCo0.33Ni0.67PO4·H2O,

(c)

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Figure

2

SEM

images

of

(a)

KNiPO4·H2O,

(b)

KCo0.33Ni0.67PO4·H2O,

(c)

KCo0.67Ni0.33PO4·H2O, and (d) KCoPO4·H2O; (e) EDS spectra and (f) EDS mapping images of KCo0.33Ni0.67PO4·H2O.

The morphology of KCoxNi1-xPO4·H2O was characterized through SEM analysis. As Figure 2a indicates, KNiPO4·H2O exhibited a 10–20 µm plate-like morphology. These plates are rectangular with smooth surfaces, approximately 500 nm thick as shown by the vertically positioned plate. When Co2+ was incorporated, the morphology was maintained for KCo0.33Ni0.67PO4·H2O (Figure 2b), KCo0.67Ni0.33PO4·H2O (Figure 2c), and KCoPO4·H2O (Figure 2d). In addition, the product compositions were analyzed by EDS. Figure 2e indicates that only the elements of Ni, Co, P, K, and O be observed in EDS patterns over as prepared

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KCo0.33Ni0.67PO4·H2O. The ratio of Co: Ni is close to 0.5, which is consistent with the experimental section. The EDS elemental mappings in Figure 2f indicate that the K, Co, Ni, P, and O elements from a selected area are homogeneously distributed without apparent element aggregations or separations. More detailed compositions of the other products are shown in Table S2 of supporting information, exhibiting that the content of Co and Ni is closed to the experimental process.

Figure 3 (a) TEM image of some microplates, (b) SAED pattern under a microplate, (c) HRTEM

image

of

a

microplate

for

KCo0.33Ni0.67PO4·H2O;

(d)

Ft-IR

over

KCo0.33Ni0.67PO4·H2O powder.

The TEM image of the KCo0.33Ni0.67 PO4·H2O microplate is given in Figure 3a, and shows

8


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typical plate-like morphology identical with the SEM result. In addition, these microplates possess uniform contrast, suggesting a homogeneous thickness. The SAED pattern taken from a microplate is exhibited in Figure 3b, with the diffraction spots indicating that the microplate is single-crystal phase. Two spots can be indexed to (200) and (002), corresponding to a [010] zone axis. Figure 2b is a HRTEM image showing a clear interplanar distance measured as 0.42 nm, corresponding to the (011) crystal face. In addition, Ft-IR was used to reveal the existing chemical bonds in as-prepared KCo0.33Ni0.67 PO4·H2O. As shown in Figure 3d, these peaks are dominated by the fundamental vibrations of the PO43− polyanions, which were approximately 1150, 1028, 925, 848, and 565 cm−1.30,31 Moreover, broad bands locate at approximately 3387 and 2358 cm−1 refer to the hydroxyl vibrations, and the peaks at 1677 cm−1 are ascribed to H-O-H bending mode in KCo0.33Ni0.67PO4·H2O.30,32 The chemical states of elements in the as-prepared products were evaluated via XPS. The full survey spectrum (Figure 4a) shows the presence of K 2p, Ni 2p, Co 2p, P 2s, P 2p and O 1s from KCo0.33Ni0.67PO4·H2O with a peak of adventitious C 1s at approximate 284.8 eV. The Co 2p spectrum in Figure 4b can be fitted as two major binding energies of Co 2p1/2 at 797.1 eV and Co 2p3/2 at 781.1 eV along with two satellite peaks centered 802.1 and 785.0 eV, which can be identified as Co2+.33 Ni 2p spectra in Figure 4c shows two main peaks of Ni 2p1/2 at 873.7 eV and Ni 2p3/2 at 855.9 eV accompanied with two satellite peaks lies binding energies at 879.3 and 860.9 eV, respectively, which indicate that the Ni is in the state of Ni2+.20,33 Figure 4d shows a single peak located at 132.8 eV, corresponding to the characteristic P 2p peak of P5+ in the product.34 The O 2p spectrum in Figure 4e can be fitted into double peaks at 530.8 and 532.8 eV, corresponding to oxygen in lattice and surface

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chemisorbed hydroxyl, repectively.12 The K 2p spectrum is shown in Figure 4f, and the peaks in 292.6 and 295.3 eV are identified as K 2p3/2 and K 2p1/2, respectively, which are consistent with the K+ spectrum.35

Figure 4 XPS spectra of the KCo0.33Ni0.67PO4·H2O: (a) full surveyed, (b) Co 2p, (c) Ni 2p, (d) P 1s, (e) O 1s, and (f) K 2p.

Regarding the electrochemical performance of these materials, Figure 5a shows the typical cyclic voltammetry (CV) curves of KCoxNi1-xPO4·H2O microsheets with a 0–0.55 V potential window in 1 M KOH solution at a scan rate of 5 mV s−1 under three-electrodes cell. The shape of the CV curves showed redox peaks as battery type. In electrode material of

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KNiPO4·H2O, two cathodic peaks are visible at approximately 0.35 and 0.44 V, as well a broad anodic peak at 0.53 V ascribe to the Ni2+/Ni3+ redox process: KNiPO4·H2O + OH− ⇋ KNi(OH)PO4·H2O + e− and KNi(OH)PO4·H2O + OH− ⇋ KNi(OH)2PO4·H2O + e−.19 When Co2+ was incorporated, the redox peaks moved to negative positions due to the following reactions:

KCoxNi1-xPO4·H2O

+

OH−

⇋

KCoxNi1-x(OH)PO4·H2O

+

e−

and

KCoxNi1-x(OH)PO4·H2O+ OH− ⇋ KCoxNi1-x (OH)2PO4·H2O + e−. Notably, the CV curves expanded with increasing Ni content, and the CV integral area of KCo0.33Ni0.67PO4·H2O indicates the largest areas in comparison with other products, implying that the KCo0.33Ni0.67PO4·H2O possesses the highest capacitance. To obtain the detailed specific capacitance of these products, GCD measurements were carried out. Figure 5b exhibits the galvanostatic charge-discharge (GCD) profiles at current density of 1.5 A g−1 for KCoxNi1-xPO4·H2O materials, exhibiting a semisymmetric shape for all samples. The specific capacitances were calculated to be 107 mAh g-1 (701 F g−1) for KNiPO4·H2O, 178 mAh g-1 (1166 F g−1) for KCo0.33Ni0.67PO4·H2O, 109 mAh g-1 (713 F g−1) for KCo0.67Ni0.33PO4·H2O, and 35 mAh g-1 (229 F g−1) for KCoPO4·H2O at 1.5 A g−1 (Figure 5c). With the increase of Ni from KCoPO4·H2O to KNiPO4·H2O, the specific capacitance increased and the highest specific capacitance appeared at KCo0.33Ni0.67PO4·H2O, consistent with the CV results. Thus, rationally adjusting the Co and Ni ratios over KCoxNi1-xPO4·H2O provides an efficient method for tuning their specific capacitance. For rate performance over these materials, 43% of the specific capacitance was retained for KNiPO4·H2O, 65.5% for KCo0.33Ni0.67PO4·H2O, 69.3% for KCo0.67Ni0.33PO4·H2O, and 57.2% for KCoPO4·H2O when the current density increased from 1.5 to 30 A g−1.

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Figure 5 (a) CV curves at 5 mV s−1 and (b) GCD curves at 1.5 A g−1 over KCoxNi1-xPO4·H2O; (c) Plots of specific capacitance versus current densities for KCoxNi1-xPO4·H2O and NH4Co0.33Ni0.67PO4·H2O; (d) rate performance at 1.5, 3, 5, and 1.5 A g−1 and (e) long-term charge

/

discharge

performance

at

1.5

A

g−1 for

KCo0.33Ni0.67PO4·H2O

NH4Co0.33Ni0.67PO4·H2O; (f) Nyquist plots of KCoxNi1-xPO4·H2O over a frequency range MHz- 0.01 Hz (inset: enlarged curves in high-frequency region).

12


and 1

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For comparison, the NH4Co0.33Ni0.67PO4·H2O was prepared under similar conditions and the XRD and SEM data in Figure S1-S2 of supporting information suggests that the crystal structure and morphology is close to the as-prepared KCoxNi1-xPO4·H2O. Figure 4c displays the electrochemical performance with NH4Co0.33Ni0.67PO4·H2O, which produces a specific capacitance of approximately 158 mAhg-1 (1037 F g−1) under 1.5 A g−1 and 66.6 % is retained at when current density increases to 30 A g−1. The NH4Co0.33Ni0.67PO4·H2O shows slightly lower specific capacitance than the KCo0.33Ni0.67PO4·H2O. The rate performance was also examined (Figure 5d), and four platforms can clearly be observed when the products run for approximately 100 cycles at 1.5, 3, 5, and 1.5 A g−1. For KNiPO4·H2O, 106%, of the specific capacitance was retained, as well as 99% for KCo0.33Ni0.67PO4·H2O, 86% for KCo0.67Ni0.33PO4·H2O, and 123% for KCoPO4·H2O. This result suggests a favorable rate performance

over

the

as-prepared

KCoxNi1-xPO4·H2O.

By

contrast,

the

NH4Co0.33Ni0.67PO4·H2O only retained 78%, exhibiting poorer rate performance than the corresponding KCo0.33Ni0.67PO4·H2O materials. The long-term charge and discharge performance of the materials was also studied. Figure 5f indicates that the specific capacitance of KCoxNi1-xPO4·H2O slightly increased in the first 100 cycles and remained stable until approximately 700 cycles. Approximately 94% of the initial capacitance was retained after 1000 cycles (retained 74 % after 2000 cycles as displayed in Figure S3, supporting information). However, only approximately 57% of the specific capacitance was retained over NH4Co0.33Ni0.67PO4·H2O. These results suggest that the rate performance and long-term cycling performance of KCoxNi1-xPO4·H2O are superior to those of the widely reported NH4CoxNi1-xPO4·H2O materials. The KMPO4·H2O shows

13



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much greater stability than NH4MPO4·H2O in electrochemical reactions, probably because of their variations in compositions and microstructure. Figure 6a–b displays the crystal structures of KCo0.33Ni0.67PO4·H2O and NH4Co0.33Ni0.67PO4·H2O, showing that the K+ and NH4+ interact in the interlayers of the P-O-Ni polyhedrons, allowing for the possibility of ion exchange with the surrounding cations in the electrolyte because of the chemical equilibrium. In the KOH solution, the NH4CoxNi1-xPO4·H2O is unstable because the NH4+ could react with OH− to generate NH3·H2O, resulting in the structural corruption of the materials. For KCoxNi1-xPO4·H2O, such ion exchanges do not occur and the material structure remains stable

in

electrochemical

reactions.

In

order

to

prove

this

proposal,

the

NH4Co0.33Ni0.67PO4·H2O and KCo0.33Ni0.67PO4·H2O microplates are grown on Ni foam and then used directly as electrode to more clearly observe the changes of phosphates during the electrochemical reactions (fabrication process is shown in experimental section of supporting information). As illustrated in Figure S4a of supporting information, XRD patterns of KCo0.33Ni0.67PO4·H2O/Ni foam has no obvious changes after 1000 cycles of charge-discharge. But for NH4Co0.33Ni0.67PO4·H2O, the diffraction intensities is much decreased after 1000 cycles as illustrated in Figure S4 of supporting information, indicating that the active materials is in some extent collapsed and dissolved. The SEM images illustrate that the KCo0.33Ni0.67PO4·H2O microplates can successfully grow on Ni foam (Figure S5a-b, supporting information). After 1000 cycles at 1 Ag-1, the morphology of microplates is kept perfectly (Figure S5 c-d, supporting information), indicating that the materials is stable during the electrochemical reactions. For NH4Co0.33Ni0.67PO4·H2O, the compact microplates arrays on Ni foam (Figure S6 a-b of supporting information) is partly dissolved after 1000

14


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cycles (Figure S6 c-d of supporting information), indicating that the NH4Co0.33Ni0.67PO4·H2O materials is not as stable as KCo0.33Ni0.67PO4·H2O. The EIS is illustrated in Figure 5f. The slope of impedance plots over the products tends to a vertical asymptote at low-frequencies, which indicates ideal capacitors for these materials. The fitted electric circuits were added Figure 5f, which is consistent of a solution resistance (Rs), a charge-transfer resistance (Rct), a Warburg impedance (W), a pseudocapacitive capacitance (Cps), and a double layer capacitance (Cdl). The parameter for each component are given in Table S3 of supporting information. The Rs is 1.49 Ω for KNiPO4·H2O, 1.43 Ω for KCo0.33Ni0.67PO4·H2O, 1.49 Ω for KCo0.67Ni0.33PO4·H2O, and 1.44 Ω for KCoPO4·H2O. And the Rct is 0.56 Ω, 0.5 Ω, 0.42 Ω, and 0.31 Ω for KNiPO4·H2O, KCo0.33Ni0.67PO4·H2O, KCo0.67Ni0.33PO4·H2O, and KCoPO4·H2O, respectively. The EIS results suggests that the introducing of Co2+ in KNiPO4·H2O could induce a decreased resistance. However, excessive Co2+ does not improve the specific capacitance, perhaps because Co2+ based compounds have lower theoretical capacitance than the Ni2+ based compounds, as mentioned in previous reports.22–25

15



Figure 6 Atomic structures of (a) KCo0.33Ni0.67PO4·H2O and (b) NH4Co0.33Ni0.67PO4·H2O.

In the next stage of the study, we further assembled asymmetric supercapacitor devices by employing the optimized KCo0.33Ni0.67PO4·H2O (positive electrode) and AC (negative electrode). Figure 7a shows the CV curves of the ASC at various voltage windows at a scan rate of 5 mV s−1. All CV curves exhibit a typical pseudocapacitive behavior from KCo0.33Ni0.67PO4·H2O material, and the shape is maintained with a voltage increase from 0.8 to 1.6 V, indicating that the ASC can be operated steadily at 1.6 V. Figure 7b displays the specific asymmetric capacitance with various voltages. The specific capacitance increases considerably from 8.25 to 34.7 mAh g-1 (54 to 227 F g−1) as the operating potentials are increased from 1 to 1.6 V. The curves of CV measured in 0 -1.6 V under various scan rates 16


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are shown in Figure 7c. The shape of CV was kept with scan rates changing from 5 to 100 mV s−1, implying high rate capability and favorable reversibility for the as-fabricated ASC. GCD curves shown in Figure 7d shows that curves exhibited favorable electrochemical capacitive behaviors at different current densities, but the shape is somewhat asymmetric, indicating that irreversible electrochemical reactions occurred during the charge and discharge reactions. Moreover, the GCD curves is non–linear and shows platform, which implies that the as prepared material have a battery–type electrodes for Faradic redox reactions. Figure 7e summarizes the specific capacitances of KCo0.33Ni0.67PO4·H2O//AC, which are calculated according to these GCD curves. Remarkably, the ASC produces specific capacitance of 34.7 mAh g-1 (227 F g−1) at 1.5 A g−1 and it still retained 19.6 mAh g-1 (128 F g−1) at a high current density of 10 A g−1. In addition, a practical application of as fabricated ASCs is demonstrated in inset of Figure 7e, indicating that two serried ASCs can successfully light an LED. Finally, long-term charge-discharge of the ASC at 5 A g−1 is shown in Figure 7f, suggesting that the specific capacitance is retained as 82% as initial values and the coulombic efficiency maintains at approximately 96% after approximately 5000 charge−discharge cycles, exhibiting favorable long-term stability over the fabricated ASC. In considering that the charge and discharge curves in ASC exhibited not very asymmetric shapes, some irreversible reactions may happened during the electrochemical reactions, which would induce the decrease of columbic efficiency. The last 10 charge and discharge curves is presented in inset of Figure 7f; the maintained quasi-symmetric shapes indicates high stability of the as prepared materials after approximate 5000 cycles.

17



Figure 7 (a) CV curves with a scan rate of 5 mV s−1 at different voltage windows. (b) Gravimetric specific capacitance at 1.5 A g−1 as a different voltage window. (c) CV curves at 1.6 V voltage from 5 mV s-1 to 100 mV s-1. (d) GCD curves and (e) Specific capacitance under various current densities (inset is a demonstration for lighting an LED by two serried devices). (f) Long-term charge–discharge performance at current density of 5 A g−1.

Moreover, to evaluate the performance of the fabricated ASCs as energy storage devices, the power densities (P) and energy densities (E) of the as-prepared KCo0.33Ni0.67PO4·H2O//AC

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ASC were calculated. Figure 8 displays Ragone plot diagrams of P vs. E for the two electrode materials, alongside other materials reported in the literature for comparison. A maximum E of 80.64 Wh kg−1 is achieved at a P of 1200 W kg−1 and maximum P of 8 kW kg−1 is produced at E of 45.48 Wh kg−1. The energy and power fields for lithium ion batteries (LIBs) and EDLC are also displayed. The E for the as-fabricated ASCs are close to the medium field energy density in LIBs and superior to the fields of EDLC. The energy density over the as-fabricated ASC exhibits superior performance to other nickel and cobalt phosphate materials (Figure 8 and Table S4 in supporting information), such as Na-doped Ni2P2O7 // graphene (23.4 Wh kg−1 at 1.29 kW kg−1),12 Ni2P2O7 // porous graphitic carbon (65 Wh kg−1 at 800 W kg−1),36 the high-power device of porous AC//Co3(PO4)2·8H2O nanoparticles (29.29 Wh kg−1 at 468.75 W kg−1),37 and Ni–Co phosphate 2D nanosheets (32.56 Wh kg−1 at 600 W kg−1).11 The energy density is also higher than that of the other transition metal compound ASCs in our previous reports, such as 41.6 Wh kg−1 at 375 W kg−1 and for polypyrrole/NH4NiPO4·H2O/ Ni foam // AC

19

and 42.4 Wh kg−1 at 800 KW kg−1 for

amorphous Co-Ni phosphates // AC.20

19



Figure 8

Ragone plots of the KNi0.67Co0.33PO4·H2O//AC ASC alongside those of other

ASCs reported in the literature.

CONCLUSION

KCoxNi1-xPO4·H2O microplates were fabricated by a facile hydrothermal method at low temperature and exhibited efficient electrochemical performance as electrodes in SCs. The optimal electrochemical performance, which was obtained in KCo0.33Ni0.67PO4·H2O by properly adjusting the Co and Ni content, was 178 mAh g-1 (1166 F g−1) at 1.5 A g−1 in specific capacitance; 94% of this performance was retained after 1000 times charge-discharge under 1.5 A g-1. The KCo0.33Ni0.67PO4·H2O possessed superior specific capacitance, rate performance, and long-term chemical stability compared with the corresponding NH4Co0.33Ni0.67PO4·H2O materials due to the unique compositions and microstructures in the KOH solutions. The ASCs were assembled by employing KCo0.33Ni0.67PO4·H2O and AC, and they exhibited specific capacitance of 34.7 mAh g-1 (227 F g−1) and retained 82% specific capacitance as initial values after 5000 times charge–discharges. The assembled ASCs could produce maximum power densities of 8 kW kg-1 and maximum energy density of 80.64 Wh kg−1, both considerably higher than previously reported for transition metal phosphates. The KCoxNi1-xPO4·H2O reported in this work showed promising applications as an efficient, naturally abundant, and inexpensive electrodes in supercapacitors. ASSOCIATED CONTENT Supporting Information

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A table of microstructure data and EIS spectra of KCoxNi1-xPO4·H2O, XRD patterns and an SEM image of the NH4Co0.33Ni0.67PO4·H2O, and a table comparing the electrochemical performance with previously reported transition metal compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding authors * Email: [email protected] (N. Zhang), [email protected] (V. A. L. Roy)

Funding

This work was supported by the National Natural Science Foundation of China (grant no. 51402364), General Financial Grant from the China Postdoctoral Science Foundation (grant no. 2016M592443), and a General Financial Grant from Central South University graduate student innovation project (grant no. 502211708). Notes The authors declare no competing financial interest.

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