High capacitive amorphous barium nickel phosphate nano ﬁ bers for electrochemical energy storage †

Ultra ﬁ ne amorphous Ba x Ni 3 (cid:1) x (PO 4 ) 2 (0 < x < 3) nano ﬁ bers are synthesized for the ﬁ rst time through a facile cation exchange reaction method at room temperature. Both the phase transformation and growing process of the nano ﬁ bers are systematically investigated. A dramatic morphology transformation from Ba 3 (PO 4 ) 2 ﬂ akes to Ba x Ni 3 (cid:1) x (PO 4 ) 2 nano ﬁ bers with the addition of Ni 2+ ions is observed. The as-prepared nano ﬁ ber material shows a diameter less than 10 nanometers and length of several micrometers. The material possesses a BET surface area of 64.8 m 2 g (cid:1) 1 . When it is used as a supercapacitor electrode material, speci ﬁ c capacitances as high as 1058 F g (cid:1) 1 at 0.5 A g (cid:1) 1 and 713 F g (cid:1) 1 at 5 A g (cid:1) 1 are achieved, indicating the promising energy storage property of this material.


Introduction
Materials based on metal phosphates have attracted attention in the scientic community due to their promising performance when used in varying applications including sensors, gas adsorption/desorption, supercapacitors, optoelectronics, lithium-ion batteries etc. [1][2][3][4][5][6][7][8] Atteld et al. rstly synthesized a series of metal phosphates with a molecular formula of M 11 (HPO 3 ) 8 (OH) 6 (M ¼ Zn, Co, or Ni) through so hydrothermal treatments. 9 Inspired by this work, various phosphates containing different cations or cation combinations like nickel, cobalt, ammonium and sodium, have been synthesized by hydrothermal method. [10][11][12][13][14][15] Most metal phosphates have a layered structure. This unique structure characteristic is desired for applications such as energy storage devices which require fast transport of ions.
Supercapacitors, also called electrochemical capacitors, are one of the most efficient energy storage devices that have been used in practice due to their advantages of high power density and ultra-long cycle life. [16][17][18][19][20] However, current commercial supercapacitors suffer from a limited energy density (less than 10 W h kg À1 ), which prevents their broader application in practice. 21 One of the methods to improve the energy density of supercapacitors is by employing nanostructured pseudocapacitive electrode materials which possess larger specic surface areas, more active sites and shorter transport/diffusion path for electrolytic ions and electrons compared to bulk materials. 22 Besides metal oxides which are widely studied as pseudocapacitive materials, [23][24][25][26][27] limited research has shown that onedimensional (1D) phosphate materials can also be effective supercapacitive materials. Pang 30 which were made by hydrothermal method. Beneting from the high pressure and heating of hydrothermal reaction condition, the material showed good crystallinity and exhibited higher energy density compared to conventional carbon based materials.
However, more recent studies have shown that amorphous state of nanomaterials may be more benecial in supercapacitor devices compared to their crystallinity counterpart owing to the higher structural disorder, larger density of active site of the former, leading to higher energy density. Yang et al. reported amorphous Ni(OH) 2 nanospheres whose capacitive performance was commensurate with the crystalline materials. 31 Moreover, Cao et al. reported the supercapacitive performance of ultrathin amorphous Co 3 (PO 4 ) 2 nanowires which exhibited higher capacitance than the crystalline counterpart material. 32 Herein, we report the synthesis of a new phosphate material based on amorphous Ba x Ni 3Àx (PO 4 ) 2 nanobers with diameter less than 10 nanometres through a facile cation exchange reaction method in aqueous solution at room temperature. By monitoring the phase transformation and growth process of Ba x Ni 3Àx (PO 4 ) 2 material by SEM and XRD, we have observed a clear morphology evolution from Ba 3 (PO 4 ) 2 microplates to microowers and nally to Ba x Ni 3Àx (PO 4 ) 2 nanobers with the increased concentration of Ni(NO 3 ) 2 in the precursor solution. A growth mechanism which involves self-assembly of Ba 3 (PO 4 ) 2 microplates that gradually transformed into Ba x Ni 3Àx (PO 4 ) 2 nanobers is proposed. The study of argon adsorptiondesorption isotherm shows the as-prepared nanobers has a large BET surface area of 64.8 m 2 g À1 , which provides sufficient active sites for the faradaic charge transfer process when used as electrode materials of SCs. The maximum specic capacitance of 1058 F g À1 (at 0.5 A g À1 ) and 713 F g À1 (at 5 A g À1 ) were obtained, which is much higher than the corresponding crystalline material made by annealing the material at 800 C. The remarkable capacitance and good electrochemical properties of Ba x Ni 3Àx (PO 4 ) 2 material are attributed to the unique amorphous nanostructure and the efficient faradaic charge transfer process involving the reversible redox reaction of nickel cations.

Materials
All the chemicals of analytical grade were purchased from Alfa Aesar and used as received without any further purication, unless otherwise stated. Deionized water with a resistivity of 18.2 MU cm was used in all reactions. Nickel foams were provided by Sigma-Aldrich.
Preparation of Ba x Ni 3Àx (PO 4 ) 2 nanobers Ba x Ni 3Àx (PO 4 ) 2 nanobers were prepared through a facile solution method based on cation exchange reaction. In a standard synthesis procedure, 20 mM Na 3 PO 4 $12H 2 O aqueous solution was dropwise added into Ba(NO 3 ) 2 aqueous solution (30 mM) under continuous magnetic stirring. A white precipitate was formed gradually during this process. Aer 2 min reaction, 30 mM Ni(NO 3 ) 2 $6H 2 O aqueous solution was further dropwise added into above reaction solution. Aer 48 h reaction under continuous vigorous stirring, all white precipitates were found to be transformed into light green colour, which was then collected through centrifuge. The material was rinsed at least three times with large amount of deionized water before being dried at 80 C in an electrical oven.

Material characterizations
The morphology and elemental distributions of the products were characterized by eld emission scanning electron microscope (FESEM, JSM-7001F, JEOL) employing an accelerating voltage of 5.00 kV with an energy dispersive spectrometer (EDS). Transmission electron microscope was used to measure the crystallinity and morphology of as-prepared materials (TEM, JEOL 2100). X-ray diffraction analysis (XRD, PANaytical MPD Cu Powder XRD) using Cu Ka radiation at 40 keV and 40 mA was employed to determine the crystalline structure and composition of the as-prepared products. X-ray photoelectron spectroscopy (XPS) was used to obtain the elemental composition and valence state, which was performed by a photoelectron spectrometer using a non-monochromatic Mg Ka (1253.6 eV) Xray source (DAR 400, Omicron Nanotechnology) with incident angle at 65 to the sample surface. The signal was collected by a 125 mm hemispherical electron energy analyser (Sphera II, 7 channels detector, Omicron Nano-technology). Micromeritics Tristar II 3020 Surface Area Analyser was used to test the BET surface area of the sample.

Electrochemical measurements
The electrochemical properties of Ba x Ni 3Àx (PO 4 ) 2 nanobers were comprehensively investigated using a three-electrode conguration cell with as-synthesized material as working electrode, Pt wire as counter electrode and calomel electrode as reference in 2.0 M KOH aqueous electrolyte. The working electrodes were fabricated by following a similar procedure reported by other groups previously. 33 Firstly, a mixture of Ba x Ni 3Àx (PO 4 ) 2 nanobers, carbon black, and poly(vinylidene uoride) (PVDF) with mass ratio of 80 : 10 : 10 was dissolved in N-methylpyrrolidinone (NMP) to make a slurry under continuous stirring for 24 h, which was then pasted on a nickel foam (1 Â 1 cm 2 ). Aer drying in a vacuum oven at 110 C for 10 h, the coated nickel foam was pressed into a thin lm under 10 MPa pressure. Each working electrode contained 1-2 mg active material. All the electrochemical properties were performed with an electrochemical workstation (SP-150, BioLogic Science Instruments) at room temperature (22 C). The electrochemical impedance spectral (EIS) measurements were carried out in the frequency range of 100 kHz to 10 mHz with an AC amplitude of 5 mV.
The specic capacitance (C T ) of the electrode is calculated based on the measured galvanostatic charging/discharging (GCD) curves, according to eqn (1): 34-36 where I (in A) is the discharge current, m (in g) is the total mass of active materials, t (in second) is the discharge time, V (in V) is the potential during the discharge process and dV/dt is the slope of the discharge curve.

Results and discussion
As stated in experimental section, a white precipitate was rstly observed by addition of Na 3 PO 4 to Ba(NO 3 ) 2 , which then transformed to light green color by further addition of Ni(NO 3 ) 2 $6H 2 O. In order to understand this process, the XRD and SEM of both the white precipitate and the nal product of light green precipitate were recorded. The results show that the white precipitate is barium phosphate (Ba 3 (PO 4 ) 2 , Fig. S1(a) †), which adopts rhombohedral crystal phase (PDF#80-1615) and shows good crystallinity. SEM image shows the barium phosphate has a regular shape of hexagon plate with size of several micro-meters ( Fig. S1( The SEM images of the light green precipitate shown in Fig. 1(a and b) conrm that the nal products consist of nanobers which are micrometer in length and nanometer in width. The TEM images (shown in Fig. 1(c and d)) further reveal that the nanobers are bundled together and each ber is less than ten nanometers in diameter. The as-prepared nanobers show an amorphous nature because no lattice fringe could be detected by the HRTEM image as shown in Fig. 2. The corresponding SAED pattern further conrms their amorphous structure (inset in Fig. 2). In order to conrm the elemental composition of as-prepared nanobers, EDS was carried out and the result is shown in Fig. S2 (ESI †). Elements including Ba, Ni, P, and O are clearly detected in the EDS prole with the ratio of Ba/Ni ¼ 4 : 7, conrming the chemical composition of the obtained material. Above result conrms the partial replacement of Ba 2+ in Ba 3 (PO 4 ) 2 by Ni 2+ during the transformation from white precipitate to light green product.
The XRD patterns of as-prepared Ba x Ni 3Àx (PO 4 ) 2 are shown in Fig. 3. Apparently, only a broad and weak diffraction pattern is observed with the fresh product (named as BaNiP), which is in agreement with the HRTEM result shown in Fig. 2, con-rming the amorphous nature of the as-synthesized material. This is probably related with the low synthesis temperature process used. To conrm this, a high temperature calcination was employed to investigate the crystallization process of assynthesized materials. The XRD of the material annealed at 350 C for 2 h in air (referred as BaNiP-350, green line) shows that the broad diffraction peak in BaNiP becomes narrower. Meanwhile, several additional broad peaks are observed in the XRD pattern of BaNiP-350, which suggests the crystallinity of the material is improved with the annealing at 350 C but it is still poor. By further increasing the annealing temperature to 800 C, the XRD peaks of the obtained product (named as BaNiP-800) are strong and sharp. All the peaks of the XRD pattern can be well indexed to the crystal structure of rhombohedral BaNi 2 (PO 4 ) 2 (ICSD-01-089-6720). The absence of other peaks detected in the XRD pattern of BaNi-800 demonstrates the high purity of the material.
Furthermore, XPS was employed to analyse the surface composition and oxidation state of Ba x Ni 3Àx (PO 4 ) 2 nanobres, in which C 1s (284.8 eV) was used to calibrate the binding energies. As shown in Fig. 4(a), the full range XPS spectrum clearly shows the characteristic peaks of Ni, Ba, P, O and C elements. It is worth noting that the C element comes from the residual carbon-based contaminants instead of the as-prepared Ba x Ni 3Àx (PO 4 ) 2 nanobres. The high resolution XPS (HRXPS) spectrum of Ni 2p (Fig. 4(b)) shows the binding energy peaks at 872.7 eV and 855.0 eV, which are corresponding to Ni 2p 1/2 and Ni 2p 3/2 , respectively, indicating that nickel cations are Ni 2+ binding state. 37 It is worth to note that the intensive shoulder peaks (860.1 and 879.1 eV, marked as Sat 1 and Sat 2) along with the main peaks of Ni 2p 3/2 and Ni 2p 1/2 are due to two shakeuptype peaks of nickel. 38 Fig. 4(c) presents the HRXPS spectrum of Ba 3d 1/2 and Ba 3d 5/2 with the characteristic peaks detected at 795.0 eV and 779.6 eV, respectively. The peak positions and binding energy gap between the two signals clearly demonstrate the existence of Ba 2+ . [39][40][41] Moreover, the HRXPS spectra in     42 During the process of fabrication of Ba x Ni 3Àx (PO 4 ) 2 nano-ber, we noticed that the content of precursor Ni(NO 3 ) 2 plays a key role in the formation of nanobers. Fig. 5 and S3 † show the SEM images and XRD patterns of Ba x Ni 3Àx (PO 4 ) 2 material synthesized with different concentration of Ni(NO 3 ) 2 . When the content of Ni(NO 3 ) 2 is only 5 mM, the product consists of material with microower structure (Fig. 5(a)), which is believed due to the self-assembly of partially reacted Ba 3 (PO 4 ) 2 akes. By increasing the concentration of Ni(NO 3 ) 2 solution to 10 mM, the mircroowers are disassembled and lots of akes are found in the material (Fig. 5(b)). Further increase the concentration of Ni(NO 3 ) 2 to 20 mM in the precursor solution, nanobers start to form among the main morphology of akes which are interweaved together as indicated in Fig. 5(c). The akes are completely transformed into nanobers when 30 mM Ni(NO 3 ) 2 is used in the reaction solution (Fig. 5(d)). Beyond this, the morphology of the material does not change. XRD results (Fig. S3 †) conrm strong diffraction peaks of Ba 3 (PO 4 ) 2 akes when 10 mM Ni(NO 3 ) 2 is used in the reaction. A complete transformation from Ba 3 (PO 4 ) 2 into Ba x Ni 3Àx (PO 4 ) 2 nanobers occurs when the concentration of Ni(NO 3 ) 2 solution is over 30 mM along with the disappearance of the characteristic XRD peaks of Ba 3 (PO 4 ) 2 .
In order to further understand the formation mechanism of the nanobers, the SEM images of Ba x Ni 3Àx (PO 4 ) 2 with different reaction time were recorded and are shown in Fig. 6. It can be seen that the smooth surface of pristine Ba(PO 4 ) 2 material becomes rough within only 5 min reaction (Fig. 6(a)). Further elongation of the reaction duration, Ba 3 (PO 4 ) 2 microplates turn into uffy and some of them merge together to form thin lms gradually ( Fig. 6(b)). Nanobers start to form on the amorphous lms aer 2 h reaction (Fig. 6(c) and inset). Further extending the reaction to 48 h leads to the full transformation of all the material into nanobers (Fig. 6(d)).
Based on the above results, the formation mechanism of the nanobers is proposed as shown in Scheme 1. It is known that Ba 3 (PO 4 ) 2 adopts a layer-structure with a characteristic of rod sequence of polyhedral in the form of PO 4 -Ba(2)O 10 -Ba(1)O 12 -Ba(2)O 10 -PO 4 along the c axis. [43][44][45] The weak bonding strength between Ba 2+ and oxygen makes Ba 2+ easy to be substituted by other divalent cations. This explains the partial substitution of Ba 2+ by Ni 2+ in the cations exchange process. In this process, the added Ni 2+ rstly intercalates into the interlayer of the microplated structure of Ba 3 (PO 4 ) 2 . The subsequent cation exchange reaction results in the formation of a rough surface of the original Ba 3 (PO 4 ) 2 akes in the rst few minutes of reaction. As more Ni 2+ ions intercalate into the host, the layered structure is expanded and nally set apart into thin layered lms (Fig. 6(b)). Sufficient Ni 2+ ions are critical in this process to ensure fully penetration and separation of all the pristine Ba 3 (PO 4 ) 2 layers. It is expected that the replacement of Ba 2+ should occur along the c axis direction by considering the molecule structure of Ba 3 (PO 4 ) 2 , which is responsible for the formation of nanober   structure (Fig. 6(c)). The long intensively magnetic stirring further facilitates split of the weakly bonded lms into nanobers.
The energy storage properties of as-synthesized Ba x Ni 3Àx (PO 4 ) 2 nanobers were investigated in a typical three-electrode electrochemical system. The cyclic voltammetry (CV) and GCD plots of Ba x Ni 3Àx (PO 4 ) 2 nanober electrode are shown in Fig. 7(a and b), respectively. It can be seen from Fig. 7(a) that a distinct pair of oxidation and reduction peaks can be clearly observed in the CV plots with different scan rates, demonstrating a strong pseudocapacitive behaviour. 46 The pseudocapacitance should be originated from the redox reaction of Ni 2+ / Ni 3+ during the faradaic charge transfer process other than Ba 2+ considering the sole valence state of the cations of barium. 47,48 Moreover, the high current response in the CV curves indicates that Ba x Ni 3Àx (PO 4 ) 2 nanobers possess considerable charge storage capability.
The GCD plots under different charging current densities ( Fig. 7(b)) further conrm the pseudo-capacitive behaviours of the electrode material with two distinct plateaus during the charging/discharging process. The specic capacitance of Ba x -Ni 3Àx (PO 4 ) 2 nanober electrode under various discharging current densities was calculated according to the GCD curves. As shown in Fig. 7(c), the specic capacitance of the electrode is 1058, 968, 870, 713, 575 and 400 F g À1 at the discharging current density of 0.5, 1, 2, 5, 10 and 20 A g À1 , respectively. Clearly, the electrode retains over 50% of the initial capacitance even when the discharging current density is increased by 20 times from 0.5 A g À1 to 10 A g À1 , indicating a good rate capability. This is ascribed to the high conductivity of the material as conrmed by electrochemical impedance spectrum (EIS) of the electrode in Fig. 7(d). The EIS data was tted using an equivalent circuit by Zview soware. As shown in the inset of Fig. 7(d), the equivalent circuit consists of a series resistance R s , a chargetransfer resistance R ct , and constant phase elements CPE and CPE1 corresponding to a pseudocapacitance of the electrode and double-layer capacitance at the electrode material/ electrolyte interface, respectively. 49 The EIS tting results show that the synthesized Ba x Ni 3Àx (PO 4 ) 2 nanober electrode has a low series resistance (R s ¼ 0.80 U) and charge transfer resistance (R ct ¼ 0.56 U), which is believed to be one of the reasons for the excellent supercapacitive performance of the material.
The cycling performance of the as-prepared electrode is also studied. As shown in Fig. 8, around 80% of the initial capacitance of the electrode is retained aer 500 cycles at a discharging current of 3 A g À1 , indicating a relatively good cycling stability. However compared to other reported crystalline Scheme 1 Schematic illustration of the formation of Ba x Ni 3Àx (PO 4 ) 2 nanofiber. Fig. 7 (a and b) The cyclic voltammetry and galvanostatic charging/ discharging curves of Ba x Ni 3Àx (PO 4 ) 2 nanofiber electrode in 2 M KOH electrolyte, respectively; (c) calculated specific capacitance of Ba x -Ni 3Àx (PO 4 ) 2 ; (d) Nyquist plot of the impedance of Ba x Ni 3Àx (PO 4 ) 2 electrode from 0.01 Hz to 100 kHz. phosphates materials, [27][28][29] further improvement of the cycling stability of the material is needed. In order to understand the reason for the unsatisfactory stability of the material, the SEM image of the working material aer 500 cycles was recorded (Fig. S4 †). It is found that the nanober structure of the synthesized Ba x Ni 3Àx (PO 4 ) 2 is destroyed aer the stability test. This suggests a large volume change occurring within the material during the charging/discharging process, which results in the broken of nanober structure into pieces. Previous work by other researchers have shown that incorporation of more conductive and stable materials such as carbon materials or metal oxides can normally improve the stability and electrochemical performance of non-carbon materials. 50,51 This will be the subject of our future research.
The remarkable supercapacitive performance of the synthesized Ba x Ni 3Àx (PO 4 ) 2 nanobers is attributed to the unique amorphous nanostructure and large surface area. The argon adsorption-desorption isotherm plot reveals that Ba x Ni 3Àx (-PO 4 ) 2 nanobers have a high BET surface area of 64.8 m 2 g À1 (Fig. 9). As shown in Fig. S5, † the amorphous sample owns a 3 times higher current peak than the crystalline material (800 C annealed sample) in the cyclic voltammetry measurement, conrming the superior energy storage property of the amorphous structure. Similar phenomenon has been reported by Li et al. who found superior supercapacitive performance with a series of amorphous Ni-Co-Fe hydroxides. 52 It is ascribed to the unique nature of amorphous materials which is disorder in long-range but is structurally order in short-range. Moreover amorphous material possesses much more unsaturated ligand atoms and efficient electrochemically active sites compared to crystalline counterpart. 52 These features favour the charge transport process in the material. This is conrmed by the low interface charge transfer resistance and good rate ability of the Ba x Ni 3Àx (PO 4 ) 2 material shown above. Moreover, the relatively large surface area provides sufficient pathway for the transport of charged ions, which further boosts the supercapacitive performance of the material.

Conclusions
In summary, nanober-structured amorphous Ba x Ni 3Àx (PO 4 ) 2 material has been synthesized for the rst time through a facile cation-exchange reaction method at room temperature. The asprepared materials exhibited typical pseudocapacitive behaviour with high electrical storage capacity. The specic capacitance of 1058 F g À1 at a discharging current density of 0.5 A g À1 was achieved. The good electrochemical properties of Ba x -Ni 3Àx (PO 4 ) 2 materials are attributed to the ultrane amorphous nanober structure and high surface area of 64.8 m 2 g À1 . Just as other pseudocapacitive materials, the cycling stability of individual Ba x Ni 3Àx (PO 4 ) 2 material also needs to be improved for further application in supercapacitors. The investigation of the formation mechanism shows that the concentration of precursor Ni(NO 3 ) 2 solution plays a critical role in the formation of the nanober structure. The method of synthesizing nanostructured phosphate materials using cation exchange reaction paves a facile way for synthesis of materials with controlled morphology and good electrochemical properties to meet the high-energy density requirement of practical applications in energy storage systems, such as high-performance supercapacitors.