THIS INVENTION relates to nanostructures. It relates also to a method of making nanostructures, and to a pseudocapacitor.

Considerable research attention is being devoted to renewable and clean energy technologies globally. The research on improving the electrical energy storage is crucial for increasing the supply of more energy from renewable sources to curb the present energy challenges. There are two main classes of electrochemical capacitors (ECs): all-carbon based electrical double layer capacitors (EDLCs) that store energy by charge-separation at the solid-electrolyte interface, and pseudocapacitors that store energy by redox or Faradaic processes. EDLCs (also known as supercapacitors or ultracapacitors) are important energy storage devices with adequate energy and high power densities compared to conventional electrochemical energy storage systems such as batteries and fuel cells. An extraordinary storing of electrical energy with exceptional power has been projected to increase the awareness and development of important technologies such as hybrid electric vehicles, portable electronics and power-saving units. It is well- established that the performance of supercapacitor-driven technologies is dependent on the physicochemical properties of their electrode materials. Flexible and wearable electronics have begun to attract intense research interests due to their reliability, ease of handling, and great promises for use as flexible energy storage devices. Pseudocapacitors have emerged as the most successful and substantial electrical energy storage devices for flexible and wearable electronics. However, a major short-coming of pseudocapacitors over their supercapacitor counterparts to date is their poor rate-capability compared to supercapacitors. Therefore, there is an urgent need for the development of high-performance pseudopercapacitor materials. Phosphate-rich materials (PRMs), such as mesoporous NH ₄ NiPO ₄ .H ₂ O nanoalmonds, one-dimensional (1 -D) layered NH ₄ CoPO ₄ .H ₂ O microrods and two- dimensional (2-D) VOPO ₄ nanosheets have been reported as high-performance pseudocapacitors. For example, the energy and power densities of these PRMs range between 30 - 140 Wh/kg and 1 - 27 kW/kg for 3-electrode configurations. However, one of the major challenges with the PRMs is their low electrical conductivity, which explains why, for example, the VOPO ₄ nanosheets have to be integrated with high-electrical-conducting graphene sheets. There is a need therefore to make PRMs that allow for compatible conduction pathways in their structures for improved redox-activity and pseudocapacitive behaviour. An important strategy for achieving this is the preparation of hierarchical 1 -D PRM with morphologies of nanorods, nanowires, or the like. The inventors have, hitherto, not been aware of the existence of PRMs of these types of morphologies. Thus, according to a first aspect of the invention, there is provided a nanostructure having a one-dimensional body comprising ammonium metal phosphate.

The one-dimensional (1 -D) body may be in the form of a rod, i.e. may be a nanorod; a wire, i.e. may be a nanowire; a tube, i.e. may be a nanotube; or the like; however, it is preferably a nanorod. In the morphology of nanostructures, a nanostructure is deemed to be 1 -D when one of its dimensions substantially exceeds its other dimensions. Thus, for example, a nanorod is considered to be 1 -D since its length far exceeds its other dimensions. The metal of the body may be nickel, cobalt or manganese; however, the metal may, in particular, be nickel.

According to a second aspect of the invention, there is provided a method of making ammonium metal phosphate nanostructures which includes reacting a metal based acetate and ammonium phosphate, in the presence of a carrier liquid comprising water and a polyol, to produce nanostructures having one-dimensional bodies comprising ammonium metal phosphate.

As set out hereinbefore, the metal may be nickel. The polyol of the carrier liquid may be a dialcohol. The dialcohol may be ethylene glycol (EG) so that the carrier liquid comprises a mixture of EG and water. EG is wholly miscible with, i.e. soluble in, water. Hence, the carrier liquid comprising the EG and water mixture constitutes a solvent for the metal based acetate and the ammonium phosphate. The metal based acetate and ammonium phosphate are hence preferably soluble in the carrier liquid, i.e. the carrier liquid preferably constitutes a solvent for the metal based acetate and the ammonium phosphate. The solvent or mixture may comprise EG and water in a volumetric ratio of about 1 :1 .

The reaction may be effected at elevated temperature. In other words, the reaction may be a hydro/solvo thermal reaction. The reaction may be carried out at a reaction temperature which is above the boiling point of the polyol. Thus, when the solvent is a mixture of EG and water, the reaction temperature may be about 200°C.

The reaction may be carried out for a period in excess of 36 hours. Typically, the reaction may be carried out for a period of about 48 hours.

According to a third aspect of the invention, there is provided a pseudocapacitor comprising nanostructures according to the first aspect of the invention, or nanostructures made by the method of the second aspect of the invention.

The invention will now be described in more detail with reference to the following non-limiting Example, and the accompanying drawings.

In the drawings,

FIGURE 1 shows, for the Example, a schematic representation of the synthetic strategy adopted for the formation of three different nanostructures of NH ₄ NiPO ₄ .H ₂ O;

FIGURE 2 shows, for the Example, SEM micrograph of as-synthesized various morphologies of (a) ANPw (nanoplatelets), (b) ANPeg (nanodendrites) and (c) ANPweg (nanorods), respectively; as well as TEM and HR-TEM images (d) ANPw with diameter of 428nm, (e) ANPeg nanodendrites with > 100nm diameter and (f) single nanorod of ANPweg with 35.8 nm diameter, the inset of 2f showing lattice fringes corresponding to the (121 ) plane of nanorods and their SAED pattern ;

FIGURE 3 shows, for the Example, XRD pattern of ANP samples of ANPw,

ANPeg and ANPweg; the inset is an expanded view of (121 ) and (200) peaks;

FIGURE 4 shows, for the Example, electrochemical performance of ANP electrodes in 3-electrode system (a) CV curves at a scan rate of 10 mVs ^"1 in 3M KOH aqueous electrolyte, insert shows the charge-discharge curves at a current density of 5 A g ^"1 ; (b) Galvanostatic charge - discharge curves of ANPweg at different current densities from 3 to 50 A g ^"1 ; (c) Rate capability of ANP electrodes with current densities; (d) durability test at a current density of 10 A g ^"1 over 5000 continues charge-discharge cycles; (e) Regone plot of energy and power density of ANP electrodes compared with the other materials reported in literature; (f) Nyquist plot of ANP electrodes in 3 M KOH solution (inset; magnified view);

FIGURE 5 shows, for the Example, electrochemical performances of symmetric pseudocapacitors of ANPweg (a) specific capacitance calculated from CV curve against voltage at a scan rate of 10 mVs-1 in 3M KOH aqueous electrolyte (b) galvanostatic CD profiles of ANPweg at the current density of 10mA cm-2 within a cell voltage range of 0-0.8V; (c) Areal capacitance calculated from CD curves as a function of current density; (d) Cycle stability test at a current density of 10 mA cm-2 over 5000 continues charge-discharge cycles; (e) Regone plot of ANPweg symmetric supercapacitor compared with other symmetric supercapacitor values reported in literature; (f) Nyquist plot of ANPweg symmetric supercapacitor (inset; magnified view);

FIGURE 6 shows, for the Example, electrochemical performances of asymmetric pseudocapacitors of ANP-weg//AC (a) specific capacitance calculated from CV curve against voltage at a scan rate of 25 mVs ^"1 in 1 M Na ₂ SO ₄ neutral electrolyte; (b) galvanostatic CD profiles of ANPweg at the current density of 10mA cm ^"2 within a cell voltage range of 0 - 1 .4 V; (c) Areal capacitance calculated from CD curves as a function of current density; (d) voltage- holding (floating) curves of ANPweg asymmetric capacitors for 50 h at the voltage of 1 .4V at a current density of 10 mA cm ^"2 inset infer the CD curves after 50 h holding; (e) Regone plot of ANPweg asymmetric supercapacitor compared with other symmetric supercapacitor values reported in literature; (f) Nyquist plot of ANPweg asymmetric supercapacitor (inset; magnified view);

FIGURE 7 shows, for the Example, electrochemical performances of all solid- state flexible symmetric pseudocapacitors (a) galvanostatic CD profiles of ANP electrodes with a current density of 0.2 imA cm ^"2 within a cell voltage range of 0-0.8V; (b) Areal capacitance of ANPweg calculated from CD curves as a function of current density; (c) durability test of ANPweg electrode measured at a current density of 0.6 imA cm ^"2 ; and (d) Regone plot of all solid-state flexible symmetric pseudocapacitors of ANP electrodes and compared the values with the other all-solid-state supercapacitors reported in literature; and

FIGURE 8 shows, for the Example, (a) photograph of as-prepared flexible all- solid-state symmetric pseudocapacitor (ASSSP); (b) 3 ASSSPs connected in series and lighting up the 1 .67 V LED; (c) charge - discharge profiles of 2 and 3 ASSSPs connected in series giving 1 .5 and 2 V, respectively, and (d) cycle stability measured nearly at 120° bending angle and their coulombic efficiency, inset shows picture of an ASSSP bent at 120°.

EXAMPLE

Experimental

Synthesis and characterisation of NH ₄ NiP0 ₄ .H ₂ 0 nanorods

Analytical grade chemicals of Nickel (II) acetate tetrahydrate, NH ₄ H ₂ PO ₄ and ethylene glycol (EG) were procured from Aldrich, and used as received. In a typical synthesis, 0.5 g of both Ni (CH ₃ COO) ₂ .4H ₂ O and NH ₄ H ₂ PO ₄ were dissolved thoroughly into 40 ml deionized water. Subsequently, an equal amount of EG was added into the above solution (water and EG volume ratio is 1 :1 ). After vigorous stirring for 1 h, the mixture was then transferred into autoclave and heated at 200°C for 48 h. The ANPweg products obtained by heating at 200°C for 24 and 36 h duration were also tested for comparison. The resulted greenish yellow precipitates were thoroughly washed with deionised water and ethanol to remove the unreacted ions. Finally, the powder was dried at room temperature in air (hereinafter referred to as ANPweg). The controlled experiments were also done with ethylene glycol and water separately at 200 °C for 48 h, and the products were designated as ANPeg and ANPw, respectively. The formation of NH ₄ NiPO ₄ .H ₂ O ("ANP") was investigated by PANalytical X'Pert PRO diffractometer equipped with Ni-filtered Cu K-alpha radiation (λ = 1 .541841 A). The morphology of the as-synthesized powders was analysed using JEOL- JSM 7500F scanning electron microscope operated at 2.0 kV. TEM and HRTEM images were obtained from JEOL-Jem 2100 microscope operated at an acceleration voltage of 200 kV. BET measurements were performed to measure the specific surface area and pore size based on the N ₂ adsorption - desorption method by using Micromeritics TriStar II instrument.

Material preparation and pseudocapacitor fabrication

Typically, area of 1 cm ² conducting carbon cloths (B-1 /C, E-TEK) were used both as substrate and current collector for all solid-state flexible pseudocapacitors and 1 .6 cm ² for symmetric and asymmetric pseudocapacitors and nickel foam was used for three electrode system. The composite electrode was prepared by spreading a slurry mixture of NH ₄ NiPO ₄ .H ₂ O nanorods, carbon black and polyvinylidene fluoride (PVDF) (80:15:5 weight ratio) on a piece of carbon cloth and nickel foam and dried in a vacuum oven at 80°C for overnight. The mass of active material on nickel foam was 0.32 mg for ANPweg, 0.35 mg for ANPeg and 0.34mg for ANPw. For symmetric and asymmetric pseudocapacitors, activated carbon (AC) (Norit ^® supra 30) was used as the conducting carbon material and ANPweg electrodes were made by coating slurry mixture of ANPweg, AC and PVDF (50:40:10 weight ratio) on carbon cloth. Polyvinyl alcohol (PVA), potassium hydroxide (KOH) and sodium sulphate (Na ₂ SO ₄ ) were procured from Sigma-Aldrich and used as received. In a typical polymer electrolyte preparation, 8 g of PVA and 4 g of KOH were dissolved in deionised water (40 ml) and the mixture was stirred at 90°C for 1 h. Two configurations of pseudocapacitors were made for solid-state device; active material coated on 1 cm ² carbon cloths were coated individually with prepared gel electrolyte (PVA-KOH) and then allowed to dry before being mounted as thin flexible device. Symmetric pseudocapacitors of ANPweg // ANPweg construct in 3M KOH and asymmetric pseudocapacitor of ANPweg // AC on 1 .6 cm ² carbon cloths in 1 M Na ₂ SO ₄ . Electrochemical measurements

All electrochemical tests were carried out on a Bio-Logic VMP3 at room temperature. In a typical three-electrode system, ANP materials coated on nickel foam was used as the working electrode, platinum mesh and Ag/AgCI (KCI) were used as a counter and reference electrodes, respectively, in 3 M KOH aqueous solution. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency ranging from 10 kHz to 10 imHz at the open circuit voltage with AC voltage amplitude of 1 .5 mV. The specific capacitance (C _sp ), maximum specific power density (P _ma x) and specific energy density (E _sp ) were calculated from the slope of the charge-discharge curves using the following established equations.

(3)

V ²

mR

R _s =dV(ir)/2i (4) where I (A) is the applied current, At (s) the discharge time and m (g) the total mass of electrode, Csp (F/g) the calculated specific capacitance, V (V) is the maximum voltage obtained during charge, and R _s is the equivalent series resistance (ESR). The area was used instead of mass to calculate the areal capacitance, energy and power densities in all solid state, symmetric and asymmetric pseudocapacitors.

Results and discussion

A schematic representation of the synthetic strategy adopted for the formation of three different nanostructures of NH ₄ NiPO ₄ .H ₂ O is summarised in Figure 1 . The nanodendrites of NH ₄ NiPO ₄ .H ₂ O (ANP _eg ), nanoplatelets (ANP _W ) and nanorods (AN Pweg) were synthesized in EG, water, and EG/water mixture via facile solvo/hydrothermal processes, respectively. The morphology and size of as- synthesized NH ₄ NiPO ₄ .H ₂ O were confirmed with the SEM and TEM analyse. Fig. 2 (a-e) shows the SEM and TEM images of ANP _eg , ANP _W and ANP _weg showing the unique formation of different morphologies of nanoplatelets, nanodendrites and nanorods. It is interesting to note from Fig. 2 that the obtained nanoplatelets (Fig. 2a) have average dimensions of 400-600 nm in length and nearly 428 nm in diameter (Fig. 2d), with few of them as micro-platelets. Fig. 2b and e illustrate the SEM and TEM images of ANPeg nanodendrites with average dimension of 100-300 nm in length. As shown in Fig. 2c, the obtained nanorods have average dimensions of 200-300 nm in length and 35-40 nm in diameter, with few of them as microrods. As illustrated by the HRTEM image, (Fig. 2f and insets), the observed nanorods are polycrystalline in nature with clear lattice fringes. The d-spacings of the lattice fringes is found to be 0.29 nm, corresponding to the (121 ) plane of ANPweg along with SAED pattern corresponding to (121 ) plane. A single nanorod has a length and a width of 105 and 35.8 nm, respectively.

From the results, it is evident that spatial localization of water molecules in solvent mixture is critical in mediating the shape growth; the highly viscous solvent EG ^=21 mPa s, 20°C) restrains the mobility of reactants compared to water ( η = 1 .0087x10 ^"3 mPa s, 20°C). The solubility and mobility of reactants considerably favours the homogeneous nucleation process when an appropriate amount of EG/H ₂ O is used. In general, the formation mechanism of the nanostructures seems to occur via the hydrolysis of acetate to acetic acid and hydroxide ion in aqueous solution, followed by the reaction between the PO ₄ ^3" , NH ₄ ⁺ and Ni ²⁺ ions. The diffusion of the active sites of PO ₄ ^3" , NH ₄ ⁺ and Ni ²⁺ ions increases with a rise in temperature thus enhancing the nucleation process. The viscosity of EG decreased with increase in temperature thereby facilitating fast nucleation by reducing the interlayer spacing thus enhance the anisotropic growth of nanorods. The NH ₄ NiPO ₄ .H ₂ O layers are formed by sharing the highly distorted NiO ₆ octahedra corners with cross-linked distorted PO ₄ ^3" tetrahedra and NH ₄ ⁺ ions inserted between the inorganic layers via hydrogen bonding. There are other 1 -D nanomaterials, such as LnPO ₄ and CePO ₄ , whose formation is driven by diffusion-controlled growth mechanism, i.e., attachment of infinite linear chains along the axis of its crystalline phase. The structural arrangement of NH ₄ NiPO ₄ .H ₂ O also contains open channels of octahedra along the parallel and perpendicular axis in (010) and (001 ) planes which may tend to form linear chain extending of octahedral along the axis. Additionally, a drastic change in morphology has also been reported when using different reactants. For instance, it has been reported that nickel nitrate gives almond-like NH ₄ NiPO ₄ .H ₂ O whereas nickel acetate gives nanodendrite-like NH ₄ NiPO ₄ .H ₂ O morphology. The nitrate anion, which is a weak base and a good- leaving group, can easily substitute with one another and proceeds to 3-D growth process, whereas strong base of acetate anions surrounding the cation impede the 3-D growth. However, acetate ions and equal mixture of water and EG play a crucial role in the formation of unique morphologies of 1 -D NH ₄ NiPO ₄ .H ₂ O nanomaterials, a facile and scalable approach which is more suitable to be extended to the preparation of various 1 -D ammonium metal phosphate (NH ₄ MPO ₄ .H ₂ O, where M = Ni, Co, Mn, etc) nanomaterials. Fig. 3 shows the XRD pattern of NH ₄ NiPO ₄ .H ₂ O samples of ANPw, ANPeg and ANPweg. The XRD patterns indicate the characteristic peaks of a pure phase of NH ₄ NiPO ₄ .H ₂ O and all the observed peaks can be readily indexed to a pure orthorhombic phase (space group: Pmn2) with the cell parameters of a = 5.425 A, b = 8.77 A and c = 4.31 A in accordance with the JCPDS card no 86-0585. Interestingly, an increase in intensity of the (121 ) peak is observed compared to the (200) peak infers the preferential orientation growth along the c-axis (insert Fig. 3). The broad peak of the ANPweg is a clear indication that its particle size is smaller than those of the ANPw and ANPeg. An increase in intensities of particular diffraction peak was also observed with increase in synthesis time periods from 24 to 48 h thus confirming the anisotropic growth processes along the c-axis .

Electrochemical characterization

Three-electrode system

Fig. 4 compares the electrochemical performance of the three-electrode configurations of the three nanostructures. The cyclic voltammetric evolutions (Fig. 4a) depict a redox couple arising from the redox-active nickel (Ni ² 7Ni ³⁺ ), confirming the pseudocapacitive behaviour of the NH ₄ NiPO ₄ .H ₂ O. The emergence of the redox couple is represented as follows:

NH ₄ NiPO ₄ .H ₂ O + OH ^" < > NH ₄ [Ni(OH)PO ₄ ].H ₂ O + e ^" The nanorods showed well-defined electrochemistry with large current density and narrower peak-to-peak potential (ΔΕ _Ρ « 150 mV at 20mVs ^"1 ) compared to the >200 mV shown by the nanodendrites and nanoplatelets, meaning that ANPweg exhibits better electrochemical reversibility and faster electron transfer kinetics. Also, unlike the others, the nanorods showed no additional oxidation peaks, indicating that the only oxidation process is that of Ni ² 7Ni ³⁺ without any other phase changes.

Fig. 4b shows typical charge/discharge curves of the ANPweg at different current densities (1 to 50 A g ^"1 ). At all the current densities investigated (Fig. 4c), the ANPweg electrode showed the best performance compared to the AN Peg and ANPw, achieving remarkable a maximum reversible specific capacity of -1400 F g ^"1 . Interestingly, the ANPweg showed an extra-ordinary high rate capability proven by the high capacitance of 545 F g ^"1 at a very high current density of 50 A g ^"1 which is extremely high in comparison with the capacitance value reported in the literature to date. The high value reflects the effective ion migration even at a higher speed which is influenced by high surface area of nanorods with minimum diffusion length of ion accessibility. Upon continuous cycling, the electrode experienced capacitance loss at the initial cycles, stabilized at about 200 cycles and then retained nearly ~ 80% of its original specific capacitance after 5000 cycles (Fig. 4d). Fig. 4e highlights the Ragone plot of observed power (1 .61 kW kg ^"1 ) and energy density (347 kWh kg ^"1 . As far as the inventors are aware, this is the highest energy value observed compared with the values of recently reported supercapacitor nanomaterials based on 3-electrode configurations. Fig. 4f compares the electrochemical impedance behaviour of the electrodes at open-circuit voltage at room temperature. Small semicircle followed by an inclined line at the higher frequency domain is due to the low charge transfer resistance (Ret), meaning that the ANPweg showed the least resistance to charge transport compared to the others. Obviously, the very low charge transfer resistance facilitate the fast ion moment which is an important factor for improving the specific power densities. In addition, the ANPweg showed near-vertical line as expected of a high- performing pseudocapacitance compared to others. The "knee" or "onset" frequency ( ₀ ), which is a measure of the power capability of a supercapacitor, decreases as ANPweg (5 kHz) > ANPeg (1 .7 kHz) > ANPw (1 .18 kHz), confirming the higher energy-storage capability of the ANPweg over other electrodes. The experimentally observed impedance curve was best fitted with the equivalent circuit and the calculated value of ESR of ANPweg, ANPeg and ANPw were found to be 0.79, 1 .72 and 1 .48 Ω. Further, the impedance curves were measured after 5000 consecutive charge-discharge cycles.

Symmetric pseudocapacitors in alkaline electrolyte

Considering the high-performance of the ANPweg electrode at half-cell configuration, subsequent studies on full-cell pseudocapacitor devices were devoted to the ANPweg. Fig. 5 summarises the performance of the ANPweg as a symmetric pseudocapacitor in an aqueous alkaline electrolyte (3M KOH), showing typical CV curve at a scan rate of 10 mVs ^"1 (Fig. 5a), charge-discharge curves at current density 10 imA cm ^"2 (Fig. 5b), areal capacitance (Fig. 5c), cycle stability (Fig. 5d), Ragone plots (Fig. 5e) and Nyquist plot (Fig. 5f) of ANPweg symmetric pseudocapacitors. Interestingly, the electrode gave high areal capacitance of 138 imF cm ^"2 at 20 imA cm ^"2 , 92% of which (126 imF cm ^"2 ) was retained even at a current density of 50 imA cm ^"2 . The high rate capability can be attributed to the nanorods maintaining their excellent structural stability and charge propagation even at higher current densities. It was found that the ANPweg has the highest areal capacitance compared with recently reported symmetric supercapacitor materials. More importantly, the areal capacitance retain more than 97% of its initial values after 5000 continuous charge-discharge cycles with 100% columbic efficiency (Fig. 5d). As shown in Fig. 5e, the ANPweg delivered the highest energy and power densities of 69 mWh cm ^"2 and 145 mW cm ^"2 at a current density of 20 imA cm ^"2 and was found that the ANPweg has energy values compared to that of literature values. As shown in the EIS spectra (Fig. 5f), the observed high frequency intercept show that ANPweg (0.13Ω) have much smaller ESR with inclined vertical line after the semicircle with the response time of 8 ms was lower than the values reported in liquid electrolyte used supercapacitors, onion- like carbon (26 ms) ²⁵ and biscrolled yarn (17 ms) ²⁶ . Asymmetric pseudocapacitors in neutral aqueous electrolyte

The inventors also prepared asymmetric pseudocapacitors in order to further increase the energy density of the device. Typically, ANPweg coated carbon cloth electrodes were used as positive and activated carbon (Norit ^® supra) coated carbon cloth electrode as negative in 1 M Na ₂ SO ₄ neutral aqueous electrolyte. The cyclic voltammograms of ANPweg (Fig. 6a) obtained at a scan rate of 25 mVs ^"1 shows rectangular shapes. Fig. 6b show the charge-discharge at a current density of 10 imA cm ^"2 , the cell gave high areal capacitance of 221 imF cm ^"2 at 20 imA cm ^"2 , 90% of (201 imF cm ^"2 ) which was retained even at a current density of 50 imA cm ^"2 . It was found that this asymmetric capacitor has best areal capacitances compared to many nanostructured electrodes reported earlier in asymmetric capacitors, such as H-TiO ₂ @ MnO ₂ (0.9 F cm ^"3 ), TiO ₂ /NiO nanotube array (2.9 imF cm ^-2 at 0.4 imA cm ^-2 ) and Fe ₃ O ₄ -SnO ₂ core-shell nanorod film (7 imF cm ^-2 ) As shown in Fig. 6d, 50 h voltage-floating tests show excellent capacity retention for the ANPweg cell with areal capacitance of 135 imF cm ^"2 , and long cycle stability with an almost 100% columbic efficiency. Ragone plot of ANPweg asymmetric capacitors (Fig. 6e) exhibits extraordinary energy (134.6 mWh cm ^"2 ) and power (325.6 mW cm ^"2 ) densities at a current density of 20 imA cm ^"2 . These values are much higher when compared with other asymmetric pseudocapacitors. Fig. 6f shows the Nyquist plot of ANPweg // AC asymmetric pseudocapacitor in 1 M Na ₂ SO ₄ neutral aqueous electrolyte showed very small ESR (0.55 Ω) with inclined vertical line with the response time of 18 s. All-solid-state flexible symmetric pseudocapacitors

Finally, the inventors explored the performance of the as-prepared ANP materials (ANPw, AN Peg and ANPweg) as all-solid-state flexible symmetric pseudocapacitors using PVA-KOH polymer electrolyte (Fig. 7). From every analysis, the ANPweg showed better electrochemical performance than the other two ANP materials. Fig. 7a exemplifies typical galvanostatic charge-discharge experiments, while Fig. 7b compares the specific capacitance values obtained at various current densities (0.1 - 0.8 imAcm ^"2 ). The ANPweg gave an excellent specific areal capacitance of 66 imF cm ^"2 at 0.1 imA cm ^"2 (Fig. 7b), and even at a higher current density of 0.8 imA cm ^"2 the capacitance remained as high as 3 imF cm ^"2 . This value is much higher compared to the values reported for other all-solid-state symmetric and asymmetric supercapacitors. For example, areal capacitance achieved with ANPweg was better than the electrochemical double layer microcapacitors which delivered 0.4 - 2 imF cm ^"2 at scan rates of 1 -100 mVs ^"1 , and graphene or carbon nanotube based flexible supercapacitors that showed 3 - 50 imF cm ^"2 . From the prolonged cycle stability performed at the scan rate of 0.6 imA cm ^"2 , the ANPweg retained -97% of its initial capacitance even after 5000 consecutive cycles (Fig. 7c). The Ragone plot (Fig. 7d) showed significantly higher energy (21 .2 mWh cm ^"2 ) and power (12.7mW cm ^"2 ) densities compared to the values reported in the literature to date for all-solid-state SCs (Fig. 7d). From the Nyquist plots of ANP-weg the equivalent series resistance (ESR) values obtained before and after 5000 cycles were 4.5 and 23 Ω, respectively. The response time (60 ms) was lower than the reported values for solid electrolytes (80 ms), and activated carbon (700 ms) ²⁵ . To further understand the reason for the high-performance of the ANPweg, we examined its specific surface area and porosity by performing the Brunauer- Emmett-Teller (BET) measurements. The observed BET surface area was 214 m ² g ^"1 with an average pore size distribution of 2-20 nm with a pore volume of 0.7 m ³ g ^"1 (BJH desorption), inferring the co-existence of mesoporous and microporous nanorods.

As a proof of concept, Fig. 8 describes the bendability of the ANPweg-based all- solid-state symmetric pseudocapacitor (ASSSP) and its ability to light up a 1 .67 V LED when connected in series. Interestingly, when the ASSSP was bent to nearly 120° and subjected to 1000 charge-discharge cycles, it was able to maintain its performance with ca. 100% coulombic efficiency.

Summary

1 -D nanomaterials maximize supercapacitive properties due to their unique ability to permit ion propagation. Also, 1 -D nanomaterials address the space-confined transport phenomena thereby improving the charge accumulation and Faradaic redox reactions. Motivated by the properties of 1 -D nanomaterials in pseudocapacitor applications, and the current challenges identified in the literature, the inventors accordingly investigated synthesis of 1 -D NH ₄ MPO ₄ nanorods with a view to increasing the energy and power densities. The inventors introduced a facile strategy to synthesize various morphologies of NH ₄ NiPO ₄ .H ₂ O by hydro/solvothermal route in ethylene glycol (EG), water and mixed solvents of EG/H ₂ O using nickel-based acetate and ammonium phosphate, particularly without the use of a template and additives. It was found that, when subjected to various experimental conditions from half-cell and symmetric to asymmetric and flexible all- solid-state configurations, NH ₄ NiPO ₄ nanorods in accordance with the invention maintained excellent performance.

Conclusions

The invention provides a novel ammonium metal (especially nickel) phosphate hydrate (NH ₄ MPO ₄ .H ₂ O) with 1 -D morphology, especially nanorod morphology. The nanorod morphology gave an extraordinarily high specific capacitance, power and energy densities in half-cell and full-cell configurations (i.e., symmetric and asymmetric cells, including all-solid-state flexible pseudocapacitors) in different electrolytes. The well-aligned nanorods (ca. 35.8 nm diameter) with meso- and microporous surface enhance ion propagation and interfacial interactions compared to the long range plates (428 nm in diameter) with larger thickness or the nanodendrites with smaller branched structure (ca. 100 nm) with uneven surface. The all-solid-state symmetric pseudocapacitor fabricated from the ANP nanorods proved it can generate power even when bent to 120° and can drive an LED when connected in series. The nanorod structures of the invention provide a significant and promising direction for novel 1 -D materials to obtain high-performance pseudocapacitors, especially for flexible and wearable electronics.

High-performance electrochemical capacitors will drive the next generation portable, flexible and wearable electronics. Unlike the conventional all-carbon supercapacitors (electric double layer capacitors, EDLC) with high power but poor energy density, pseudocapacitors capitalize the high energy density inherent to reversible redox reactions and provide a facile means to enhancing the energy ratings of supercapacitors. The invention hence provides 1 -D nanostructures, especially nanorod structures, of ammonium metal phosphate hydrate, especially ammonium nickel phosphate hydrate, NH ₄ NiPO ₄ .H ₂ O, (abbreviated herein as AN Pweg) as a pseudocapacitor with high energy rating and power handling. In every configuration investigated (symmetric, asymmetric, and flexible all-solid-state) the pseudocapacitor gave excellent values compared to existing literature. The inventors found that a flexible all-solid-state ANPweg-based pseudocapacitor achieved high areal capacitance of 66 imFcm ^"2 with extra-ordinary energy (21 .2 mWh cm ^"2 ) and power (12.7 mW cm ^"2 ) densities. The all-solid-state symmetric pseudocapacitor proved it can generate power even when bent to 120° and can drive an LED when connected in series. It was thus found that ammonium metal phosphate (NH ₄ MPO ₄ , where M = Ni, Co, Mn, etc) nanorods have the potential to be used as high-performance pseudocapacitors, especially for flexible and wearable electronics.