NH₄CoPO₄·H₂O microbundles consisting of one-dimensional layered microrods for high performance supercapacitors

Abstract

Unique layered NH₄CoPO₄·H₂O microbundles consisting of one-dimensional layered microrods are firstly synthesized by a facile hydrothermal method. As a result of novel layered structures, there are many nanolayered channels for the diffusion of ions and electrolytes. The preliminary supercapacitor investigation indicates that the specific capacitance of layered NH₄CoPO₄·H₂O microbundle electrodes reaches up to 662 F g⁻¹ at a current density of 1.5 A g⁻¹ and remains at 520 F g⁻¹ even at 15.0 A g⁻¹. The cycle test shows excellent cycling performance of layered NH₄CoPO₄·H₂O microbundle electrodes (the retention 92.7% of initial specific capacitance after 3000 cycles).

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1. Introduction

Due to novel chemical–physical properties, layered micro/nanomaterials with unique architectures have been raised increasing attention for both scientific and technological interests in recent years.^1–4 Among these layered nanomaterials, lots of artificial layered inorganic–organic components structures have been successfully synthesized, such as, ultrathin CoSe₂ amine nanobelts,⁵ layered polydiacetylene/silica mesostructures,⁶ and tungstate-based inorganic–organic nanobelts.⁷ And layered double hydroxides (LDHs) have attracted much attention due to their layered feature and highly tunable interlayer composition.^8–12 Besides, intense research efforts have been made on explorations of layered transition metal oxides.^13–19 For example, layered Li_xMn_yO₂ (ref. 17) and cobalt-substituted manganese oxide has been synthesized with improved capacity and cycling performance as lithium battery electrodes.¹⁸

Metal phosphates, especially for layered metal phosphates and phosphonates have been studied in the years 1987–1990.^20,21 Carling and Yuan et al. obtained NH₄M^IIPO₄·H₂O (M^II = Mn, Fe, Co, and Ni) by precipitations from aqueous solution.^22,23 Zhang et al. have successfully reported a general method to prepare metal ammonium phosphate microspheres.²⁴ However, there are nearly no reports on layered metal ammonium phosphate micro/nanomaterials.

Up until now, layered NH₄CoPO₄·H₂O micro/nanostructures have not been synthesized through a facile method. In our previous work, we firstly found NH₄CoPO₄·H₂O micro/nanostructures can be act as the promising electrodes for supercapacitors.²⁵ Herein, we develop a facile hydrothermal method for synthesis of unique layered NH₄CoPO₄·H₂O microbundles consisting of one-dimensional layered microrods. Moreover, we present the preliminary investigation on electrochemical performance of obtained layered NH₄CoPO₄·H₂O microbundles for supercapacitors, indicating potential applications in high-performance supercapacitors.

2. Results and discussion

Fig. 1 shows XRD pattern of as-prepared samples. All peaks of the pattern are indexed to be in agreement with NH₄CoPO₄·H₂O (JCPDS no. 21-0793). No peaks of other phosphites or phosphates were detected from these patterns. The peaks are strong and narrow, which indicate the good crystallinity of as-prepared samples. Good crystallinity of NH₄CoPO₄·H₂O might be good to improve cycle life of electrodes for its stable structure, and it is not easy to be destroyed during the electrochemical process. In inset of Fig. 1, the schematic crystal structures of NH₄CoPO₄·H₂O super cell (2 × 2 × 2 slabs) projected based on data of Inorganic Crystal Structure Data (ICSD)-15727. NH₄CoPO₄·H₂O has the layered crystal structure and such layered crystal structure might improve the diffusion of ions and electrons.

Fig. 1 XRD patterns of as-prepared samples and in inset of it, crystal structures of NH₄CoPO₄·H₂O super cell (2 × 2 × 2 slabs) projected based on data of ICSD-15727.

The size and shape of as-prepared NH₄CoPO₄·H₂O were examined by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Typical low-magnification SEM image in Fig. 2 shows that the morphology of samples is microbundles composed of many microrods. The size of a single microbundle is ∼50 μm.

Fig. 2 SEM images of as-prepared samples.

A detailed morphology of the single microrod is measured by TEM and HRTEM images in Fig. 3. The microrod has not a uniform diameter, while it has a small end and a large end shown in Fig. 3a. The layered nanoscale properties were further revealed in Fig. 3b. It confirms the nanolayered structures and the size of a layer is about 17.54 Å, which is two times of a layer distance of crystal cell (8.77 Å) in inset of Fig. 1. Fig. 3c show the selected area electron diffraction (SAED) pattern and the high-resolution (HR) TEM images taken from the small end of a microrod shown in Fig. 3a, which demonstrates the highly crystalline nature of the product and may be described as a single crystal with [010] growth direction. The measured distances of the neighboring lattice fringes in Fig. 3c are 2.40 Å, corresponding well to the (020) lattice spacing of NH₄CoPO₄·H₂O.

Fig. 3 (a) Corresponding TEM image, and (b and c) HRTEM images, and SAED patterns.

We also explored the possible formation process of NH₄CoPO₄·H₂O microbundles, and found the reaction time affected the morphology of the product. The product was synthesized under different hydrothermal conditions (seen in ESI Fig. S1†). From ESI Fig. S1a,† microplate clusters were obtained under hydrothermal condition for 8 hours. After hydrothermal condition for 16 hours, many microrods were obtained, and some microbundles assembled short nanorods were observed among these microrods in ESI Fig. S1b.† And after 32 hours of hydrothermal condition, short NH₄CoPO₄·H₂O microbundles assembled short microrods were largely synthesized in ESI Fig. S1c.† When further maintained for 48 hours, long NH₄CoPO₄·H₂O microbundles assembled short microrods have been successfully synthesized, which have formed from short microbundles in Fig. 2.

To gain further insight into the specific surface area of layered NH₄CoPO₄·H₂O microbundles, Brunauer–Emmett–Teller (BET) measurements were performed. And the N₂ adsorption–desorption isotherms of layered NH₄CoPO₄·H₂O microbundles were shown in Fig. 4. The BET surface area of layered NH₄CoPO₄·H₂O microbundles is 89.5 m² g⁻¹. The pore-size distribution (in inset of Fig. 4) was determined by using the Barrett–Joyner–Halenda (BJH) method from the desorption branch of the isotherm. The average pore diameter of the sample is 1.5–6 nm, which is attributed to the layered structures assembled. As widely reported, a high surface area and layered structures usually gives rise to many channels for contacting the electrolyte with surface-interfaces of novel micro/nanostructures.

Fig. 4 Typical N₂ adsorption–desorption isotherms and pore-size distribution curve.

In Scheme 1, it clearly exhibits a layered NH₄CoPO₄·H₂O microbundles model image. NH₄CoPO₄·H₂O microbundles are made up of many layered microrods, which means there are thousands of nanochannels in the microbundle. What is more, thousands of nanochannels might largely improve diffusion of ions and electrolytes, and the microbundle might offer a stable skeleton for ions intercalation–extraction.

Scheme 1 A layered NH₄CoPO₄·H₂O microbundles model image and possible charge–discharge process.

Cyclic voltammogram (CV) studies were employed to characterize the capacitive performance of layered NH₄CoPO₄·H₂O microbundles. Fig. 5a shows CVs of the layered NH₄CoPO₄·H₂O microbundle electrodes (a mass loading of 5 mg) in 3.0 M KOH electrolyte at different scan rates in the range 5–100 mV s⁻¹. As seen in the Fig. 4a, the shapes are different from that of electric double-layer capacitance, suggesting that the capacity mainly results from pseudocapacitive capacitance, and it indicates that the Faradaic pseudocapacitive property of layered NH₄CoPO₄·H₂O microbundles is based on the surface redox mechanism of Co²⁺ to Co³⁺ at the surface.

Fig. 5 (a) Cyclic voltammetry experiments within a 0.0–0.50 V range at a scan rate 5–100 mV s⁻¹ were performed on the layered NH₄CoPO₄·H₂O microbundle electrodes in 3.0 M KOH electrolytes at room temperature, (b) the galvanostatic charge–discharge curves of layered NH₄CoPO₄·H₂O microbundle electrodes during current densities were 1.5–15.0 A g⁻¹ in 3.0 M KOH electrolytes, (c) specific capacitances of layered NH₄CoPO₄·H₂O microbundle electrodes derived from the discharging curves at the current density of 1.5–15.0 A g⁻¹ in 3.0 M KOH electrolytes, (d) charge–discharge cycling test at the current density of 1.5 A g⁻¹ in 3.0 M KOH electrolytes.

Chronopotentiometry (CP) curves at different current densities are shown in Fig. 5b. The symmetrical characteristic of charging–discharging curves is good, which means that the layered NH₄CoPO₄·H₂O microbundle electrodes with excellent electrochemical capability and redox process are reversible. The relationship between the specific capacitances calculated by CP curves and current densities are given in Fig. 5c. Based on the CP curves, layered NH₄CoPO₄·H₂O microbundle electrodes have the large specific capacitance and reach up to 662 F g⁻¹ at a current density of 1.5 A g⁻¹ and remain at 520 F g⁻¹ even at 15.0 A g⁻¹. The specific capacitance of layered NH₄CoPO₄·H₂O microbundles is significantly better than some cobalt based phosphates micro/nanomaterials, such as our previous results – NH₄CoPO₄·H₂O microflowers (<340 F g⁻¹ at 1.5 A g⁻¹),²⁵ cobalt phosphite (Co₁₁(HPO₃)₈(OH)₆) microarchitectures (<312 F g⁻¹ at 1.5 A g⁻¹),²⁶ and cobalt pyrophosphate (Co₂P₂O₇) nano/microstructures (367 F g⁻¹ at 0.625 A g⁻¹),²⁷ but lower than other supercapacitor materials.^28–31

It is very important for electrode materials to have good specific capacitance retention. Supercapacitors should work steadily and safely, which requires the specific capacitance of electrode materials to change as little as possible. Relationships of the specific capacitance against cycling number of layered NH₄CoPO₄·H₂O microbundle electrodes are shown in Fig. 5d. It shows its excellent specific capacitance retention under at 1.5 A g⁻¹. After 300 continuous charge–discharge cycles, layered NH₄CoPO₄·H₂O microbundle electrodes almost retain the same specific capacitance as its initial value. More importantly, layered NH₄CoPO₄·H₂O microbundle electrodes still retain more than 92.7% of its specific capacitance after 3000 continuous charge–discharge cycles.

To identify the exact electrical conductivity of electrodes, we have measured EIS spectrum of layered NH₄CoPO₄·H₂O microbundle electrodes (0 cycle) at room temperature in the frequency range 0.01–10⁵ Hz under open-circuit conditions. Fig. 6 shows the EIS of layered NH₄CoPO₄·H₂O microbundle electrodes at room temperature and its calculated curve by ZSimpWin software. An equivalent circuit used to fit the impedance curve is given in inset of Fig. 6a, which is similar to the circuit employed for the working electrode of supercapacitor. The EIS data can be fitted by a bulk solution resistance R_s, a charge-transfer R_ct and a pseudocapacitive element C_p from redox process of electrode materials, and a CPE to account for the double-layer capacitance. The charge-transfer resistance R_ct of all the sample was calculated by ZSimpWin software, and from the calculated results, we found that R_ct of layered NH₄CoPO₄·H₂O microbundle electrodes is 4.7 Ω. This clearly demonstrates the reduced charge-transfer resistance of the layered NH₄CoPO₄·H₂O microbundle electrodes. In addition, the charge-transfer resistance R_ct, also called Faraday resistance, is a limiting factor for the specific power of the supercapacitor. It is the low Faraday resistance that results in the high specific power of layered NH₄CoPO₄·H₂O microbundle electrodes. This layered structure surface-interface character might also decrease the polarization of the electrode, and thus increase the capacity. The phase angles for impedance plots of layered NH₄CoPO₄·H₂O microbundle electrodes and its calculated curve by ZSimpWin software were observed in Fig. 5b. These phase angles are nearly to 55° in the low frequencies clearly, which means that the layered NH₄CoPO₄·H₂O microbundles allow ions or electrolyte transfer to occur quickly.

Fig. 6 (a) The electrochemical impedance spectra (EIS) of the electrodes at room temperature and its corresponding calculated curve, in inset of (a) the equivalent circuit for the electrochemical impedance spectrum, and (b) the phase angles for impedance plots and its corresponding calculated curve.

The NH₄CoPO₄·H₂O samples obtained before (0 cycle) and after different charging–discharging cycles (1 cycle, 500 cycle, 1000 cycle, 3000 cycle) are further explored by IR and HRTEM measurements (seen in ESI Fig. S2 and S3†). From IR tests in ESI Fig. S2,† five samples almost show the same spectrum, it indirectly deduces that NH₄CoPO₄·H₂O is stable even after 3000 cycle. Detailed information of nanostructures is revealed by HRTEM. It is seen that the nanolayer structure is stable after 500 cycle in ESI Fig. S3.† However, due to ions intercalation-extraction, the nanolayer structure is partly destroyed after 1000 cycle. The phenomenon is enhanced by further prolonging the charging–discharging to 3000 cycle.

The change of nanolayer structures is also proved by XRD patterns of layered NH₄CoPO₄·H₂O microbundles before (0 cycle) and after different charging–discharging cycles (1 cycle, 500 cycle, 1000 cycle, 3000 cycle, seen in Fig. 7). It is found that the intensity of crystal plane (001) has become weak with cycle numbers increasing. More importantly, the (001) peak of samples after 3000 cycles has shifted a little into the small angle of diffraction, which might be caused by ions intercalation into nanolayer structures in Fig. 7b.

Fig. 7 XRD patterns of layered NH₄CoPO₄·H₂O microbundles before (0 cycle) and after different charging–discharging cycles (1 cycle, 500 cycle, 1000 cycle, 3000 cycle). (a) From 5° to 70°; (b) from 5° to 15°.

In order to ascertain the oxidation state of cobalt during charging–discharging proceeding, XPS spectra of layered NH₄CoPO₄·H₂O microbundles before (0 cycle) and after different charging–discharging cycles (1 cycle, 500 cycle, 1000 cycle, 3000 cycle) have been shown in Fig. 8. For the materials (0 cycle and 1 cycle), the Co 2p3/2 main spectral peaks are at about 781 eV with their satellites at the range of 785–786 eV. These results are close to those reported for Co²⁺ in Co(OH)₂ and in Co-contained LDHs.^32–40 The binding energy differences between the Co 2p3/2 and Co 2p1/2 main peaks are around 16.0 eV which is also near to 15.7 eV – a literature value for a cobalt hydrotalcite-like compound.⁴¹ However, for the sample (1000 cycle and 3000 cycle), the Co 2p3/2 main peak is located at 780.4 ± 0.2 eV, and the Co 2p1/2 main peak is 795.3 ± 0.2 eV, respectively. These characteristics are similar to those of CoOOH in a report.³² Moreover, from the O1s spectra (Fig. 8b, for samples 1000 cycle and 3000 cycle), a spectral peak at around 532 eV becomes a little wider, indicating that some Co²⁺ ions on the surfaces are partly oxidized.⁴¹ It means that the oxidization of NH₄CoPO₄·H₂O microbundles' surface can be intensified with cycle numbers increasing. Like CoAl-layered double hydroxide composites, we have proposed the possible mechanism – eqn (1) to explain the intercalation/deintercalation of ions in NH₄CoPO₄·H₂O microbundles:

NH₄CoPO₄·H₂O + OH⁻ = NH₄[Co(OH)PO₄]·H₂O + e⁻ (1)

Fig. 8 X-ray photoelectron spectra (XPS) of layered NH₄CoPO₄·H₂O microbundles before (0 cycle) and after different charging–discharging cycles (1 cycle, 500 cycle, 1000 cycle, and 3000 cycle), (a) Co 2p peak; (b) O 1s peak; (c) N 1s peak, (d) P 2p peak.

Typical electrochemical impedance spectroscopy of the layered NH₄CoPO₄·H₂O microbundle electrodes before (0 cycle) and after different charging–discharging cycles (1 cycle, 500 cycle, 1000 cycle, 3000 cycle) have been shown in Fig. 9. The higher equivalent series resistance (ESR) value, (the equivalent circuit is shown in inset of Fig. 6a), indicates the lower electrical conductivity of the sample and vice versa.⁴² It can be seen that the ESR values in the Nyquist plot of the layered NH₄CoPO₄·H₂O microbundles electrode are very small and increase a little after long charge–discharge cycling (Fig. 9). Moreover, these phase angles have changed a little except that of the electrode after 3000 cycle, which means ions or electrolyte transfer to occur quickly even after long charge–discharge cycling.

Fig. 9 Typical electrochemical impedance spectroscopy measurements of layered NH₄CoPO₄·H₂O microbundle electrodes before-0 cycle, and after different charging–discharging cycles-1 cycle, 500 cycle, 1000 cycle and 3000 cycle.

While a three-electrode cell is valuable for determining electrochemical-specific material characteristics, a two-electrode test cell mimics the physical configuration, internal voltages and charge transfer that occurs in a packaged ultracapacitor and thus providing the best indication of an electrode material's performance.^43,44 Charge–discharge curves of layered NH₄CoPO₄·H₂O microbundles–graphene nanosheets asymmetric supercapacitors measured at different current densities in 3.0 M KOH solutions are shown in Fig. 10a by ref. 45's method. Typical charge–discharge curves of cells from 0 to 1.1 V at different current densities are respectively shown in Fig. 10a. On the curve in 3.0 M KOH, the cell voltage is sharply increased from 0 to ca. 1.1 V when the charging current is applied, attributable to the open-circuit potential differences between graphene and layered NH₄CoPO₄·H₂O microbundles. Then, the cell voltage is gradually increased, and a quasi-linear voltage-time response is visible between 0 and 1.1 V, which is the typical capacitor behavior. The Ragone plots of layered NH₄CoPO₄·H₂O microbundles–graphene nanosheets asymmetric supercapacitors in 3.0 M KOH solutions obtained from the discharge curves are presented in Fig. 10b. The specific energy of cell in 3.0 KOH solution even reaches ca. 26.6 W h kg⁻¹ under a relatively low power operation (e.g., 852 W kg⁻¹). Accordingly, NH₄CoPO₄·H₂O microbundles–graphene nanosheets asymmetric supercapacitors are proposed to be an energy oriented asymmetric supercapacitors in 3.0 KOH solution electrolyte. However, the specific energy of cell in 3.0 KOH solution is not larger than ref. 45–48.

Fig. 10 (a) Charge–discharge curves of layered NH₄CoPO₄·H₂O microbundles–graphene nanosheets asymmetric supercapacitors measured at different current densities in 3.0 M KOH solutions; (b) the Ragone plot of layered NH₄CoPO₄·H₂O microbundles–graphene nanosheets asymmetric supercapacitors in 3.0 M KOH solutions.

3. Conclusions

In this work, unique layered NH₄CoPO₄·H₂O microbundles have been firstly synthesized through a hydrothermal method. More importantly, layered NH₄CoPO₄·H₂O microbundles show novel layered surface-interface condition, which plays key roles to ions intercalating/extracting into/out and electrolyte accesses. The measurement of electrochemical properties of layered NH₄CoPO₄·H₂O microbundles is an important work, which successfully illustrates layered NH₄CoPO₄·H₂O microbundles materials can be applied as an electroactive material for supercapacitor. Despite specific capacitance of layered NH₄CoPO₄·H₂O microbundles materials is not much larger than some previous materials,^45–48 it is a good example to prove that physical and chemical properties of nano/microstructured materials are related with their structures, and the precise control of morphology of nanomaterials will serve for controlling the performance. Exploring the electrochemical characteristics of novel nano/micromaterials might direct a new generation of supercapacitor materials.

4. Experimental section

Synthesis of unique layered NH₄CoPO₄·H₂O microbundles

In a typical synthesis, 0.10 g of cobalt nitrate and 0.15 g ammonium tertiary phosphate were added into 20.0 mL of distilled water under stirring until salts were completely dissolved. The solution mixture was put in a Teflon-lined stainless steel autoclave and heated at 220.0 °C for 48.0 h. After reaction, purple precipitates were obtained. The precipitates were thoroughly washed with distilled water and ethanol to remove ions possibly remaining in the final products, and dried at room temperature in air.

Characterizations

The morphology of as-prepared samples was observed by a JEOL JSM-6701F field-emission scanning electron microscope (FE-SEM) at an acceleration voltage of 5.0 kV. The phase analyses of the samples were performed by X-ray diffraction (XRD) on a Rigaku-Ultima III with Cu K_α radiation (λ = 1.5418 Å). Nitrogen adsorption–desorption measurements were performed on a Gemini VII 2390 Analyzer at 77 K using the volumetric method. The specific surface area was obtained from the N₂ adsorption–desorption isotherms and was calculated by the Brunauer–Emmett–Teller (BET) method. Transmission electron microscopy (TEM) images and HRTEM images were captured on the JEM-2100 instrument microscopy at an acceleration voltage of 200 kV. Fourier transform infrared spectra (FT-IR) were collected in the region from 400 to 4000 cm⁻¹ at room temperature using a Nicolet 670 spectroscopy on KBr pellets. The XP spectra were obtained using a PHI5000 Versa Probe.

Electrochemical study on layered NH₄CoPO₄·H₂O microbundle electrodes was carried out on a CHI 660D electrochemical working station (Shanghai Chenhua Instrument, Inc.). All electrochemical performances were carried out in a conventional three-electrode system equipped with platinum electrode and a saturated calomel electrode (SCE) as counter and reference electrodes, respectively. Before electrochemical measurement, we have purged out O₂ from the solution by the inert gas – Ar. The working electrode was made from mixing of active materials (layered NH₄CoPO₄·H₂O microbundles), acetylene black, and PTFE (polytetrafluoroethylene) with a weight ratio of 80 : 15 : 5, coating on a piece of foamed nickel foam of about 1 cm², and pressing it to be a thin foil at a pressure of 5.0 MPa. The typical mass load of electrode material is 5.0 mg. The electrolyte was 3.0 M KOH solution. Cyclic voltammetry and galvanostatic charge–discharge methods were used to investigate capacitive properties of layered NH₄CoPO₄·H₂O microbundles packages electrode. And electrochemical impedance spectroscopy measurements of all the samples were conducted at open circuit voltage in the frequency range of 100 kHz to 0.01 Hz with AC voltage amplitude of 5 mV by using PARSTAT2273.

Acknowledgements

This work is supported by the Program for New Century Excellent Talents (NCET-13-0645) and National Natural Science Foundation of China (NSFC-21201010, 21071006, U1304504 and 21105002), the Science & Technology Foundation of Henan Province (122102210253, 13A150019), and the China Postdoctoral Science Foundation (2012M521115).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45977b

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