WO2020010410A1

WO2020010410A1 - Synthesis of manganese oxide and zinc oxide nanoparticles simultaneously from spent zinc-carbon batteries using a thermal nanosizing process

Info

Publication number: WO2020010410A1
Application number: PCT/AU2019/050738
Authority: WO
Inventors: Veena Sahajwalla; Rifat Farzana; Ravindra Rajarao
Original assignee: Newsouth Innovations Pty Limited
Priority date: 2018-07-12
Filing date: 2019-07-12
Publication date: 2020-01-16
Also published as: AU2019302568A1

Abstract

The present invention relates to a process for the simultaneous synthesis of manganese oxide and zinc oxide nanoparticles from spent zinc-carbon batteries, and to nanoparticles obtained by the process. The nanoparticles may have several applications, such as in the production of supercapacitors and sensors.

Description

SYNTHESIS OF MANGANESE OXIDE AND ZINC OXIDE NANOPARTICLES SIMULTANEOUSLY FROM SPENT ZINC-CARBON BATTERIES USING A THERMAL

NANOSIZING PROCESS

Field of the invention

[0001] The present invention relates to a process for the simultaneous synthesis of manganese oxide and zinc oxide nanoparticles from spent zinc-carbon batteries, and to nanoparticles obtained by the process.

Background of the invention

[0002] Any discussion of the prior art throughout this specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0003] In Australia, about 183 million batteries (equivalent to 8,000 tonnes) are disposed of in landfill each year. Disposed batteries are a source of heavy metals which represent a significant environmental and human health concern. Moreover, landfill costs are increasing and it is becoming more and more difficult to obtain permission from local authorities in Australia and abroad to dispose of batteries in landfill. In this regard, Directive 2006/66 of the European Commission has prohibited landfilling spent batteries before they have been treated to recover the valuable materials contained therein.

[0004] Zinc-carbon (Zn-C) batteries are a valuable source of metals including Zn and Mn. In recent years, preliminary mechanical methods of separation followed by pyrometallurgical, hydrometallurgical or bio-hydrometallurgical treatments have been employed to recover valuable metal content from spent Zn-C batteries. There are a number of disadvantages associated with these methods. For example, hydrometallurgical processes typically employ leaching of battery waste with strong acids, such as sulfuric acid. Hydrometallurgical processes also require complex operational infrastructure which leads to increased costs. Pyrometallurgical processes are also costly due to their substantial energy requirements.

[0005] The present inventors have developed a thermal process which allows the convenient simultaneous synthesis of manganese oxide and zinc oxide nanoparticles from spent Zn-C batteries. The nanoparticles may have several applications, such as in the production of supercapacitors and sensors.

Summary of the invention

[0006] In a first aspect the present invention provides a process for the simultaneous preparation of zinc oxide nanopartic!es and manganese oxide nanoparticles, the process comprising:

(i) providing cathodic material obtained from a spent zinc-carbon battery;

(ii) heating the cathodic material so as to produce manganese oxide nanoparticles and zinc oxide vapour; and

(iii) condensation of the zinc oxide vapour at a location remote from the cathodic material so as to provide zinc oxide nanoparticles.

[0007] The cathodic material may comprise Zn, ZhMh₂q₄, Zn₅(0H)₈CI₂H₂0, ZnO, Mh₃q₄, Mn₂0₃ and MnOOH, or any combination thereof. In some embodiments, the cathodic material may comprise ZnMn₂0₄ and Zn₅(0H)₈CI₂H₂0.

[0008] The cathodic material may be heated at a temperature of at least 800 °C. The cathodic material may be heated at a temperature between about 800 °C and about 1200 °C. In another embodiment, the cathodic material may be heated at a temperature between about 850 °C and about 950 °C. In a further embodiment, the cathodic material may be heated at a temperature of about 900 °C.

[0009] Step (ii) may be carried out for a period of time between about 10 minutes and about 8 hours. In another embodiment, step (ii) may be carried out for a period of time between about 10 minutes and about 2 hours. In a further embodiment, step (ii) may be carried out for about 1 hour.

[0010] Condensation of the zinc oxide vapour may occur at a temperature below about 300 °C. In another embodiment, condensation of the zinc oxide vapour may occur at a temperature between about 200 °C and about 300 °C. [0011] The zinc oxide vapour may be condensed on a substrate, such as for example, a porous substrate. The porous substrate may be porous silicon.

[0012] The process may further comprise drying the cathodic material prior to performing step (ii).

[0013] The process may further comprise: converting Zn(OH)CI which co-condensed with the zinc oxide in step (iii), to zinc oxide.

[0014] The Zn(OH)CI may be converted to zinc oxide by heating, for example, at a temperature of at least 150 °C. In another embodiment, the Zn(OH)CI may be converted to zinc oxide by heating at a temperature between about 150 °C and about 500 °C.

[0015] Steps (ii) and (iii) may be carried out in an inert atmosphere, for example under an atmosphere of argon or other inert gas, such as nitrogen.

[0016] The process may further comprise heating the manganese oxide so as to convert MnO to Mn₃0₄. Heating may be performed at a temperature of at least about 100 °C. In another embodiment, heating may be performed at a temperature between about 100 °C and about 800 °C.

[0017] The cathodic material may be in the form of a powder.

[0018] In a second aspect, the present invention provides zinc oxide and manganese oxide nanoparticles, whenever obtained by the process of the first aspect.

Brief Description of Drawings

[0019] Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 (a) is a schematic representation of a typical Zn-C battery and (b) a dismantled spent Zn-C battery.

Figure 2 (a) shows the steps and apparatus to synthesise ZnO and MnO nanoparticles in accordance with one embodiment of the invention, and (b) shows the steps of in-situ fabrication of a ZnO nanoparticle thin film on a substrate (porous silicon/P-Si) in accordance with one embodiment of the invention.

Figure 3 shows an XRD pattern and EDS spectra of spent Zn-C battery powder.

Figure 4 shows low and high magnification Field Emission Scanning Electron Microscope (FESEM) images of spent Zn-C battery powder.

Figure 5 shows a thermogravimetric analysis of spent Zn-C battery powder.

Figure 6 (a) shows an XRD pattern of MnO and ZnO nanoparticles synthesised in accordance with an embodiment of the invention (dotted lines shows reference peak position), and (b) shows the presence of Zn(OH)CI impurities in the ZnO nanoparticles prior to oxidation, and (c) shows an XRD pattern of a ZnO nanoparticle film on a P-Si substrate, and (d) shows an XRD pattern of synthesised Mn₃04 nanoparticles.

Figure 7 shows EDS spectra of (a) MnO and ZnO nanoparticles, (b) ZnO film, and (c) EDS mapping of Mn₃0₄ synthesised in accordance with an embodiment of the invention.

Figure 8 shows XPS spectra of (a) MnO nanoparticles synthesised in accordance with an embodiment of the invention (a1 ) survey scan (a2) Mn2p (a3) 01s, and (b) ZnO nanoparticles (b1 ) survey scan (b2) Zn2p (b3) 01 s, (c) ZnO film (d ) survey spectra, (c2) Zn 2p, and (c3) Ol s for ZnO NPs/P-Si, and (d) Mn₃0₄ nanoparticles (d1 ) Mn 2p and (d2) 0 1 s region.

Figure 9 shows Raman spectra of (a) ZnO nanoparticles (Note that MnO was not Raman active and therefore no Raman spectra are provided), and (b) synthesised Mn₃0₄ nanoparticles.

Figure 10 shows FESEM images of synthesised (a) MnO nanoparticles, (b) ZnO nanoparticles, and TEM images of synthesised (c) MnO nanoparticles, (d) ZnO nanoparticles.

Figure 1 1 shows a high-resolution lattice image of MnO and ZnO nanoparticles and a SAED pattern of MnO and ZnO nanoparticles. Figure 12 shows (a1 ) N₂ adsorption/desorption isotherm (a2) BJH pore size distribution of MnO nanoparticles and (b1 ) N₂ adsorption/desorption isotherm (b2) BJH pore size distribution of ZnO nanoparticles.

Figure 13 shows (a) cross-sectional dark field TEM image and (b-d) elemental mapping of ZnO ultra-thin film on a P-Si substrate.

Figure 14 shows (a) low (b,c) high magnification FESEM image, (d) TEM image, (e) Lattice fringes and (f) SAED image of synthesised Mn30₄.

Figure 15 shows electrochemical performance of the ZnO nanoparticles/P-Si in 0.6 M KOH aqueous electrolyte at room temperature: a CV at a scan rate of 100 mV s-1 (inset shows the charge transfer across interface through surface redox reaction), (b) CV curves at different scan rates (from 100 to 5 mV s-1 ), (c) specific capacitance as a function of different scan rates, and (d) Nyquist plots of the ZnO NPs/P-Si electrode.

Figure 16 shows electrochemical performance of Mn₃0₄ electrode in 0.6 M KOH aqueous electrolyte at room temperature (a1 ) CV curves at different scan rates from 5 to 150 mV/s in the potential range of 0 to +0.6 V (b1 ) specific capacitance as function of different scan rate and (a2) Galvanostatic charge-discharge curve of Mh3q₄ nanoparticles in aqueous solution of 0.6 M KOH at different current densities and (b2) specific capacitance as a function of different current densities.

Detailed description

[0020] In one aspect the present invention provides a process for the simultaneous preparation of zinc oxide nanoparticles and manganese oxide nanoparticles. The process comprises the following steps:

(i) providing cathodic material obtained from a spent zinc-carbon battery;

(iii) condensation of the zinc oxide vapour at a location remote from the cathodic material so as to provide zinc oxide nanoparticles. [0021] In some embodiments, the zinc oxide vapour is condensed onto a porous substrate so as to form a thin film. The porous substrate may be porous silicon.

[0022] The powdered cathodic material may be obtained from spent zinc-carbon batteries by dismantling the batteries as shown in Figure 1. Cylindrical Zn-C batteries comprise a negative anode (which is a zinc casing) and a positive cathode which is a powdered material that typically contains Mn0₂ wetted with ZnCI₂. A carbon rod is located in the centre of the battery which acts as a current collector. Once the battery is spent, it has been found by the inventors that the cathodic material typically contains the following compounds: ZnMn₂0₄, Zns(0H)₈Cl₂FI₂0, MnOOH and ZnCI₂4Zn0.5H₂0 (which is equivalent to Zn5(0H)8CI₂H₂0).

[0023] In step (ii) the powdered cathodic material is heated, which results in the simultaneous generation of MnO nanoparticles and zinc oxide vapour. Without wishing to be bound by any particular theory, it is believed that ZnO and MnO are produced from the cathodic material by the reactions depicted in Figure 2a (which are discussed in more detail below in the Examples section). Figure 2b depicts ZnO thin film formation.

[0024] In some embodiments the cathodic material may be heated at a temperature of at least 800 °C. The cathodic material may be heated at a temperature between about 800 °C and about 1200 °C, or heated at a temperature between about 800 °C and about 1 150 °C, or heated at a temperature between about 850 °C and about 1100 °C, or heated at a temperature between about 850 °C and about 1000 °C, or heated at a temperature between about 850 °C and about 950 °C. In one embodiment, the cathodic material may be heated at a temperature of about 900 °C.

[0025] In some embodiments, step (ii) may be carried out for a period of time between about 10 minutes and about 8 hours, or between about 10 minutes and about 6 hours, or between about 10 minutes and about 4 hours, or between about 10 minutes and about 2 hours, or between about 10 minutes and about 1 hour. In one embodiment, step (ii) may be carried out for about 1 hour.

[0026] In some embodiments, the process may further comprise heating the manganese oxide so as to convert MnO to Mn30₄. For example, the process may further comprise heating the manganese oxide at a temperature of at least about 100 °C, such as between about 100 °C and 800 °C so as to convert MnO to Mn30₄. Heating may be carried out in an atmosphere of air. The heating may be performed after step (ii) or after step (iii).

[0027] In step (iii) the zinc oxide vapour is condensed at a temperature below that employed in step (ii), and at a location remote from the cathodic material. Condensation at a remote location permits separation of the MnO and ZnO nanoparticles.

[0028] In one embodiment, the process may be carried out in an apparatus such as that depicted in Figure 2a. The apparatus is a horizontal tube furnace comprising a quartz tube, gas supply system, graphite rod and a crucible. The quartz tube comprises two temperature zones denoted Ti and T₂ and a gas inlet and outlet at opposing ends of the tube. In this embodiment, the crucible is charged with the cathodic material, placed on the graphite rod and then inserted into the tube and placed in the Ti temperature zone. In this temperature zone the cathodic material is heated at a temperature of about 900 °C resulting in the production of MnO nanoparticles and ZnO vapour. The ZnO vapour then travels from the crucible to the cooler T₂ temperature zone (in which the temperature is less than 300 °C, for example, about 250 °C to 300 °C) where it condenses on the tube as nanoparticles. The MnO nanoparticles remain in the crucible. It is therefore apparent that the process of the invention not only permits simultaneous preparation of MnO and ZnO nanoparticles, but also convenient separation of the nanoparticles once formed.

[0029] As described herein, due to the optimal temperature and flow of the carrier gas, formation of ZnO pulp was higher, which predominantly increased the density of the ZnO pulp inside the crucible. The lighter weight of the ZnO pulp inside the crucible chamber facilitated good contact with the targeted substrate. A P-Si substrate may provide a larger internal surface, high resistivity and strong and high surface absorbability. The wettability and surface energies of the P-Si substrate may also play an important role in forming uniform ZnO nanoparticles ultra-thin film without agglomeration. As described herein, due to the hydrophobic nature and lower surface energy of the P-Si substrate, when the pulp of ZnO came into the contact with the substrate, the grain size of ZnO became smaller, along with reduced inter-grain distances, and created a discrete ultra- thin film. [0030] The process may further comprise drying the cathodic material prior to performing step (ii). For example, the cathodic material may be heated for about 2 hours at a temperature of about 90 °C prior to performing step (ii).

[0031] In some embodiments, Zn(OH)CI may be formed in step (ii) and may co condense with ZnO. Again, without wishing to be bound by an particular theory, it is thought that Zn(OH)CI is formed by decomposition of ZnsiOFOsCh following prolonged heating (see reaction 6 below). Conveniently, Zn(OH)CI may be converted to ZnO. Accordingly, the process of the invention may further comprise converting Zn(OH)CI which co-condensed with zinc oxide to zinc oxide. The conversion may be achieved via oxidation, for example by heating the Zn(OH)CI in air to a temperature of at least about 150 °C. MnO may be converted to Mn30₄ by heating, for example, at a temperature of at least 100 °C.

[0032] Relatively pure ZnO and MnO nanoparticles may be recovered from the process. Depending on the end use, additional purification of the nanoparticles may be performed. For example, impurities may be removed by selective dissolution using acids.

[0033] The present invention provides a convenient process for the simultaneous synthesis of MnO and ZnO nanoparticles directly from spent Zn-C batteries. In that context, the present invention may provide a ZnO thin film, and Mn₃0₄ nanoparticles directly from spent Zn-C batteries. Being a thermal process, implementation does not require a sophisticated apparatus, thereby minimising set up and running costs.

[0034] In another aspect, the present invention provides zinc oxide nanoparticles and manganese oxide nanoparticles when produced by a process of the present invention. The zinc oxide nanoparticles and manganese oxide nanoparticles may be used in the production of supercapacitors or sensors.

[0035] MnO has many potential applications, including catalysis, energy storage and generation devices. In particular, MnO particles can be used as anode materials for lithium ion batteries as an attractive alternative to graphite (capacity 372 mAhg-1 ) due to the particle's high theoretical capacity of -756 mAhg ¹, high weight density, low electromotive force and moderate discharge potential. [0036] Likewise ZnO is used in many applications including electronics, optics, optoelectronics, biomedical, sensors, lasers, UV detectors, solar cells, photo catalysts and pharmaceuticals.

[0037] The preparation of ZnO, MnO, ZnO thin film and Mh3q₄ for use in industry from spent batteries will not only reduce reliance on mining to obtain these compounds, but also reduce heavy metal pollution in the environment.

Examples

Preparation of nanoparticles

[0038] Powdered cathodic material from spent Zn-C batteries was dried in an oven at 90 °C for 2 hours to remove moisture. Preparation of the nanoparticles was carried out in a horizontal tube furnace at 900°C under an argon atmosphere at atmospheric pressure. A schematic representation of the furnace and a flow diagram of the steps involved are shown in Figure 2. The furnace comprises a quartz tube, gas supply system with gas flow meter and a graphite rod. The quartz tube employed was 150 mm long and arranged in such a way that the centre of the tube was set at a temperature of about 900°C (Ti - high temperature zone), and the end of the tube adjacent to the gas outlet was set at a lower temperature of about 280 ± 20°C (T₂ - low temperature zone). Condensation of ZnO occurred in the low temperature zone. The carrier gas, (in this case argon) flowed at a rate of 1 L/min via the gas inlet and was controlled by a gas flowmeter throughout the preparation method. The dried powdered cathodic material (~4 g) was loaded onto the ceramic crucible and placed on the graphite rod. The graphite rod was then pushed into the high temperature zone and left there for 1 hour. During this time period a white material condensed on the quartz tube in the lower temperature zone. This material was collected and found to contain about 10 wt.% of ZnO. MnO (~48 wt.%) produced in the crucible was also collected. In order to remove Zn(OH)CI from the ZnO-containing material, the material was heated at 500 °C in air prior to analysis.

[0039] To fabricate a ZnO nanostructured thin film, the powdered cathodic material of spent Zn-C battery samples and the porous Si substrate were kept in a covered crucible, maintaining an optimum height, and placed into the front path of the furnace (cold zone; ~ 180 °C) using a graphite rod. Samples were left in that position for 10 min. The graphite rod was then gradually moved toward the hot area of the furnace where a high temperature (900 °C) was set and left for 60 min. To fabricate Mn30₄, a ceramic crucible loaded with dried battery powder was placed on the graphite rod. The rod was pushed into the furnace and left for 1 hour under argon atmosphere. The greenish solid residue was pulled out and cooled after 1 hour and again left in the furnace under air atmosphere for another 1 hour at 800 °C. The sample was used for analysis and as an active material for electrode fabrication.

Characterisation methods

[0040] Thermogravimetric (TGA) analysis of the powdered cathodic material obtained from spent Zn-C batteries was conducted using simultaneous thermal analyser STA 8000 (PerkinElmer). The chemical composition was analysed by X-ray fluorescence spectroscopy (XRF) of Axios Advanced WDXRF with Rh end-window tube "Superq Software". The crystal structure was identified by PANalytical X’Pert Pro multipurpose X-ray diffraction (XRD) using CuKa radiation (l=1.54 A). Phase identification was conducted by Xpert high score plus software. Elemental compositions were determined by Energy-dispersive X-ray Spectroscopy (EDS) of Bruker X flash 5010. The morphology and microstructure were observed by JEOL 7001 F, field emission scanning electron microscope, transmission electron microscope (TEM) of JEOL-1400 along with selected area electron diffraction (SAED). All powders were dispersed on carbon tape and then coated with platinum for sample preparation for EDS and microscopic analysis. X-Ray Photoelectron Spectroscopy (XPS) analysis was conducted with a Thermo Scientific ESCALAB250Xi using mono-chromated Al K alpha X-ray source with a spot size of 500 micrometres. A Renishaw inVia Raman spectrometer coupled with a microscope was used to characterize the nanoparticles using 514 nm argon ion lasers. Surface area and pore size distribution of the nanoparticles was determined using N₂ physisorption on a Micromeritics Tristar II Plus absorption analyser using Brunauer- Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models from relative pressure (p/pO) 0 to 1. Prior to analysis, the samples (~ 0.5 g) were dried at 1 10^°C in oven and then degassed for at least 3 h at 150^°C under vacuum. Characterisation of spent powdered cathodic material

[0041] Determination of the chemical composition of powdered cathodic material obtained from spent Zn-C batteries was conducted by XRF analysis and is shown in Table 1 below.

Table 1 : Chemical composition of powdered cathodic material obtained from spent Zn-C batteries.

[0042] The major elements present include Mn, Zn and Cl, with minor amounts of Fe, Co, Si etc., and trace elements including Hf, W, Ba and V. A high amount of moisture was present in the mixture which can be attributed to the high loss of ignition (LOI) value. The results of XRD and EDS analyses of the powdered cathodic material are shown in Figure 3. [0043] XRD diffraction peaks showed the presence of mainly ZhMh₂0₄ (hetaerolite) and Zn₅(0H)sCl₂H₂0 (simonkolleite) phases. The presence of these compounds confirms that the major elements present in the powdered cathodic material are Zn, Mn, Cl and O, which is in agreement with XRF analysis data.

[0044] Zn, Mn, Cl and O peaks dominated the EDS analysis. The presence of a C peak can be attributed to carbon present in the batteries and/or from the carbon rod during dismantling.

[0045] Low and high magnification FESEM images of the powdered cathodic material are shown in Figure 4. The morphology of the powder showed no particular orientation or shape. Both coarse and fine particles are present, however particle size could not be clearly distinguished. Fine particles of a spherical shape in an aggregated form were observed throughout the mixture.

[0046] TGA analysis of the powdered cathodic material at a rate of 20^°C min^-1 from room temperature to 1200^°C under a N₂ atmosphere is shown in Figure 5. A total weight loss of ~52 wt.% was observed, which is in agreement with weight loss observed from experimental data in which the powder was subjected to thermal nanosizing. Initial weight loss below 350^°C was attributed to moisture loss. Decomposition of the material that occurred at 450^°C to 800^°C may be attributed to loss of oxygen (see reaction 8 below). Major weight loss observed from 800^°C may be attributed to evaporation of Zn/ZnO.

[0047] A battery cell produces electricity through a number of electromagnetic reactions between the anode, cathode and electrolyte. The oxidation reaction in the anode creates electrons, and the reduction reactions in the cathode absorb electrons and combine to form compounds. The main electrochemical reactions occurring during the discharge process of a Zn-C battery are shown below (reactions 1 , 2 and 3). The electrolyte concentration, temperature, rate and depth of discharge may influence the reactions.

Anode: Zn -® Zn²⁺ + 2e reaction 1

Cathode: Mn0₂ + H₂0 + e- ® MnOOH + OH- reaction 2 Overall: 4Zn + 8Mn0₂ + ZnCl₂ + 9H₂0 8MnOOH + ZnCI₂4Zn0.5H₂0 reaction 3

[0048] It is evident that zinc chloride and manganese oxide react in a battery cell to produce manganese-bearing compounds like MnOOH (manganese oxy-hydroxide), which may decompose to form Mn0₂ over time. ZnCI₂4Zn0.5H₂0 or Zn₅(0H)₈CI₂H₂0 (zinc chloride hydroxide monohydrate or simonkolleite) are also formed. As part of the present analysis, an additional ZnMn₂0₄ (hetaerolite) phase was detected in the powder, along with Zns(0H)₈CI₂H₂0 phases (see the XRD pattern in Figure 3). It is therefore assumed that formation of ZnMn₂0₄ occurs via reactions between Mn0₂ and Zn according to reactions 4 and 5 below. The presence of ZnMn₂0₄ and Zn₅(0H)₈CI₂H₂0 phases in the XRD pattern is also in agreement with high Zn and Mn concentration in chemical analysis performed by XRF.

2Mn0₂ + Zn ZnO + Mn₂0₃ reaction 4

ZnO + Mn₂03 ZnMn₂0₄ reaction 5

[0049] In the present invention, a thermal nanosizing technique is employed in which ZnO is vaporised at 900°C and then condensed, leaving behind residual MnO. Upon heating, simonkolleite starts to lose a single mole of water (reaction 6). Prolonged heating results in decomposition to ZnO and Zn(OFI)CI (zinc chloride hydroxide) (reaction 7). ZnO is reduced to Zn by carbothermal reduction (C being present in battery mixture) and produces Zn vapour. The Zn vapour is converted to ZnO via in-situ oxidation (oxygen coming from reaction 8 and/or residual oxygen in the system) followed by reactions 9 and 10. XRD of the condensate material prior to oxidation at 500°C showed ZnO and Zn(OH)CI phases (Figure 6b). Oxidation of the condensate in air converted the Zn(OH)CI present to ZnO. The ZnO thin film on substrate as shown in Figure 2(b) may be used as a supercapacitor electrode. During oxidation at 800 °C in an air atmosphere, residual manganese oxide transforms to Mh₈0₄ as per reaction 1 1. The Mn30₄ powder was used for analysis and electrode fabrication.

Zh₅(0H)₈a₂H₂0 Zn₅(OH)₈CI₂ + H₂0 reaction 6

Zn₅(OH)₈CI₂ 2Zn(OH)CI + 3ZnO + 3H₂0 reaction 7 [0050] ZnMn₂0₄ started to transform to MnO at 500^°C, and to ZnO at 600^°C (see reaction 8). As such, at 900^°C evaporation of ZnO occurred through the initial carbothermal reduction (carbon present in battery mixture) of ZnO into Zn vapor (reaction 9) and further in situ oxidation (0₂ from reaction 8) of Zn vapor into ZnO (reaction 10).

ZnMn₂0₄ ZnO + MnO + 1/20₂ reaction 8

ZnO + C Zn + CO reaction 9

Zn + 0₂ 2ZnO reaction 10

6MnO + 0₂ 2Mn₃0₄ reaction 1 1

Characterisation of synthesized nanoparticles

[0051] XRD patterns of the MnO and ZnO nanoparticles that were synthesised are shown in Figure 6. Powder samples obtained were scanned in 2Q range from 20° to 80° diffraction angle, with step size of 0.026, 1 ° slit and 10 mm mask. The XRD pattern of the MnO nanoparticle with cubic crystal structure (a, b, c = 0.44 nm) showed a well- defined diffraction peak of (1 1 1 ), (200), (220), (31 1 ) and (222), which matched the standard data of the crystal plane of MnO (ICDD 04-006-5363). ZnO nanoparticles with hexagonal crystal structure (a, b = 0.32 nm, c = 0.52 nm) showed a sharp diffraction peak of (100), (002), (101 ), (102), (1 10), (103), (200), (1 12), (201 ), (004) and (202) which correlated with the crystalline structure of ZnO (ICDD 03-065-341 1 ). The absence of additional peaks in the XRD analysis confirmed the purity of the MnO and ZnO nanoparticles obtained.

[0052] Figure 6c shows the XRD patterns of the synthesized pure ZnO nanoparticles on the porous Si substrates. The presence of peaks that are characteristic of pure ZnO (corresponding to (002), (101 ), (102), and (103) planes at 2Q = 33.8°, 35.9°, 46.8°, and 63.5°, respectively), accord with standard XRD peaks of crystalline ZnO with a hexagonal wurtzite structure. No characteristic peaks from the intermediates - for instance, Zn(OH)₂ - was observed in the samples. This result shows the formation of high purity ZnO nanoparticles. The estimated crystalline size of the ZnO nanoparticles was measured from the dominant peak (002) using the formula of Scherrer and was found to be about 47 nm. The XRD pattern of the as-synthesised powder (Figure 6d) was assigned to a tetragonal Mh3q₄ (Hausmannite) phase (ICDD Code: 04-006-8183) with space group 141/amd (141)36. The diffraction peak positions at 2Q values ~ 18, 29, 31 , 32, 36, 38, 44, 51 , 59, 60 and 65 were assigned to corresponding crystal planes of (101 ), (1 12), (200), (103), (21 1 ), (004), (220), (105), (321 ), (224) and (400), respectively.

[0053] EDS analysis of the nanoparticles is shown in Figure 7a. The EDS spectra of the MnO nanoparticles showed Mn and O peaks, and the EDS spectra of the ZnO nanoparticles showed only Zn, along with an expected O peak. A low intensity carbon peak can be attributed to carbon tape and/or carbon from the battery carbon rod. Whilst XRD analysis of the MnO nanoparticles did not show the presence of other phases except MnO, EDS analysis detected a low concentration of Zn from residual ZnO, which may be removed with a longer time interval. High purity MnO may also be obtained by removing impurities via selective dissolution using acids.

[0054] The elemental composition of the as-synthesized ZnO film on the P-Si substrate was characterized by EDS spectra (Figure 7b). The existence of several peaks of Zn, O, and Si and the lack of any peaks associated with contaminants, settled the formation of a pure nanostructured material. EDS mapping (Figure 7c) of M 0₄ nanoparticles revealed that Mn and O were uniformly distributed which also established the formation of n₃04. A minor distribution of Zn peaks could be attributed to residual ZnO which could be removed with longer times or higher temperatures. The XRD, FTIR, Raman and EDS results demonstrated that the synthesized powder was crystalline Mns0₄with a hausmannite structure having little or no impurities.

[0055] The XPS spectra shown in Figure 8a also confirmed the chemical composition of the metal oxidation states of the synthesised MnO and ZnO nanoparticles. The MnO nanoparticles showed two distinct peaks of Mn2pi_/2 and Mn2p_3/2 at ~ 653 eV and 641.3 eV with a spin energy of separation of 1 1.7 eV. These are in good agreement with spin orbital lines in the literature. The highest 01 s A peak at 529.7 eV may be assigned to the Mn-O, and the 01 s B peak at 531.3 eV can be assigned to loosely bonded Mn-OH, either due to adventitious contamination, oxidation or water. The small 01 s C peak at 532.6 eV may be attributed to the residual O² species bonded to carbon. The ZnO nanoparticles showed a sharp peak at 1021.87 eV corresponding to Zn2p3 in the oxide state. The highest 01 s A spectrum demonstrating a peak at 530.36 eV was assigned to oxidized metal ions, specifically O-Zn in the ZnO lattice. The low intensity peak at 531.6 eV is attributed to loosely bound oxygen (O² ions) on the surface, or an oxygen deficient region within the ZnO matrix. The peak at 532.7 eV may be assigned to OH species in H₂0 molecules absorbed on the surface of the ZnO nanoparticles. The C1 s signal at around -285 eV of the survey scan may be attributed to adventitious carbon/residue from the carbon rod of the battery.

[0056] Figure 8b shows the XPS with widespread survey spectra of the ZnO nanoparticles. As shown in Figure 8b, Zn, O, and C peaks were spotted in survey spectra with minor impurity. The spotted carbon is associated with the adsorbed carbon on the surface at the time of exposing the sample to the ambient atmosphere. Total binding energies were corrected for the charge shift by the C1 s peak of graphitic carbon (BE = 284.6 eV). It was observed that the Zn 2p core-level of ZnO nanoparticles has two fitting peaks located at about 1044.6 and 1021.5 eV, attributable to Zn 2p1/2 and Zn 2p3/2, respectively. These results indicate that the chemical valence of Zn at the surface of the nanoparticles is in an oxidation state. For the ZnO nanoparticles, the binding energy difference between the Zn 2p1/2 and Zn 2p3/2 was 23.1 eV. Figure 9c shows XPS spectra of 01s region of ZnO nanoparticles. The 01s core-level spectrum of ZnO nanoparticles showed three different forms of oxygen. Three fitting Gaussian peaks marked (1 ), (2) and (3) were used to fit the experimental data. Peaks (1 ) and (2), positioned at the lower binding energy of 530.26 eV and 531.47 eV was assigned to 02- ions in the Zn-0 bonding of the wurtzite structure of ZnO. Peak (3), located at 532.41 eV, is related to Zn-O/Si.

[0057] XPS analysis of the as-received powder was conducted using the C1 s binding energy (284.8 eV) as an internal standard. Mn2p and 01 s peaks were observed along with low atomic concentrations of K, Zn, Ca, Cl as impurities. Figure 8c shows the high- resolution spectra of Mn2p and 01 s to confirm the composition and oxidation state of synthesised Mp3q₄ nanoparticles. It was observed that the Mn 2p peak includes two main spin-orbital lines with binding energies at -641.65 and 653.4 eV which are attributable to Mn 2p3/2 and Mn 2p1/2, respectively. The difference in binding energy of -1 1.75 eV between the spin-orbit splitting of Mn 2p3/2 and Mn 2p1/2 levels and their position are in agreement with the reported value for Mn₃0₄. The highest 01s A peak was observed at 529.7 eV and was attributable to the Mn-O and 01s B peak observed at 531.08 eV. This can be attributed to loosely bonded Mn-OH either due to oxidation or water or adventitious contamination. Low intensity peak of 01 s C and D at 532.18 eV, 533.4 eV could be assigned to the residual 02- species bonded with carbon.

[0058] Wurtzite-type ZnO belongs to the C⁴e_v (P63mc) space group. Optical phonon modes, at the point near Brillouin zone, will be represented by Topt = A1+2B1+E1+2E2. As ZnO is ionic, polar A1 and E1 modes are split into transverse optical (TO) and longitudinal optical (LO) components and doubly degenerate B1 modes are silent. The non-polar E2 mode consists of two modes of low (E2 low) and high (E2 high) frequency phonons which are connected with the vibration of the heavy Zn sub-lattice and oxygen atoms, respectively. The peak at -437 cm^-1 is high and fairly narrow and arises from first-order Raman scattering by the E2 phonons of ZnO. The E2 (low) mode was observed at -1 10 cm^-1 , and the peak around 570 cm^'1 has been attributed to the E1 (LO) mode of ZnO. This provides confirmation that the nanoparticles possess a wurtzite hexagonal phase. The peak at 392 cm^'1 is attributed to A1 (TO) mode, and the peak at 574 cm^'1 corresponds to A1 (LO) phonon. The acoustic phonon overtone and optical phonon overtone with A1 symmetry located at -203 and 331 cm ¹ , respectively and the peak at 260 cm^"1 may be attributed to the laser plasma lines. The Raman spectra of the ZnO nanoparticles is shown in Figure 9a.

[0059] Raman scattering (Figure 9b) showed one sharp band at -663 cm-1 along with two weak Raman bands at -370, 317 cm-1. Raman bands at -663, 370, 317 cm-1 were assigned to the characteristic A1g, T2g and Eg active modes which usually originates from stretching of modes of tetragonal hausmannite with spinal structure.

[0060] FESEM images of MnO and ZnO nanoparticles are shown in Figure 10 (a, b). It is apparent from the images that both MnO and ZnO are present as spherical shaped particles in the nano range. It was also observed that the particles are almost uniformly dispersed. A transmission electron micrograph (TEM) of the MnO and ZnO nanoparticles is shown in Figure 10 (c, d). TEM confirmed that the MnO and ZnO nanoparticles are spherical-shaped and similar in morphology and size. The TEM images also showed that both nanoparticles were not uniformly scattered but poly- dispersedly distributed. The average diameter of both the MnO and ZnO nanoparticles is below 50 nm and typically 10-30 nm. [0061] The diameters of the ZnO and MnO nanoparticles were also compared with the XRD data using the Schemer formula as follows: d=(k l)/(b cosO) where k = 0.9 (the shape factor), l is the X-ray wavelength of CuKa radiation (0.154 nm), Q is the Bragg diffraction angle and b is the full width at half maximum of the diffraction peak. The mean particle diameters calculated using this formula are d(101 ) = 11 nm for ZnO and d(200) = 22 nm for MnO, which are in agreement with the particle diameters determined from TEM.

[0062] The SAED patterns of MnO and ZnO are shown in Figure 1 1. Small spots making up a ring were evident in the patterns for both the MnO and ZnO nanoparticles, which represent the Bragg reflection from an individual crystal. As such, the SAED patterns confirm the polycrystalline nature and symmetrical orientation of the MnO and ZnO nanoparticles. The measured inter planar distance values are 0.25 and 0.22 nm, corresponding to the (101 ) plane of ZnO and the (200) plane of MnO, respectively.

[0063] An N₂ adsorption/desorption isotherm of the MnO and ZnO nanoparticles (see Figure 12 a1 and b1 ) showed a similar trend with a poor hysteresis loop and formation of multilayers. Based on the observed isotherms, a BET surface area of 9.63 m²g^~1 and 9.26 m²g^~1 was measured for the MnO and ZnO nanoparticles respectively. The moderate BET surface area of MnO could be attributed to the presence of carbon and is in agreement with a reported value (~1 1 m²g^_1). The BET surface area of the ZnO nanoparticles is also comparable with available literature data (2-15 m²g ¹). As displayed in Figure 12 a2 and b2, the BJH pore size distribution showed dominant pore diameter of around 5 nm.

[0064] The thickness of the ZnO ultra-thin film was measured by taking a cross-sectional TEM image from the ZnO/P-Si substrate (Figure 13a). As shown in Figure 13a, the ZnO ultra-thin film comprises several ZnO film layers stacked periodically to make one thin layer. Uniform pores formed between every two layers, making the ultra-thin film discrete. The thickness of the ultra-thin film was approximately 390 nm, and the single layer was approximately 45 nm. The number of layers in the ultra-thin film could be controlled by changing the time of deposition. Elemental mapping was performed to confirm the distribution of ZnO on the surface of the lattice (Figure 13b-d). The results shown in Figure 13b-d indicate that Zn and O are spread homogenously over the entire P-Si substrate, and further confirms the formation of a uniform ZnO film.

[0065] The morphology of the as-prepared Mn30₄ particles produced from waste batteries was analysed using FESEM and TEM. Referring to Figure 14a, the low magnification micrograph showed mainly spherical particles. Referring to Figure 14b, the high magnification micrograph showed that the particles were generally either spherical or cubic and some were uneven. The cubic particles are highlighted in the figure. The size of the particles was largely in the nanometer range, but some larger particles within about 1 iti were also observed (Figure 14c). In the HRTEM micrographs (Figure 14e), the lattice fringes were observed and matched with d-spacing of 0.25 nm and 0.31 nm, which correspond to d-spacing values of (21 1 ) and (112) planes of crystalline Mn30₄ nanoparticles. The SAED pattern (Figure 14f) showed small spots forming rings coming from the Bragg reflection of each crystallite. This confirmed the polycrystalline nature of Mn₃0₄. The diffraction rings were well matched with inter planner distance corresponding to (103), (21 1 ), (105) and (224) planes of Mn₃0₄. The nanoparticle diameter of Mh3q₄ was measured with XRD data using Scherrer equation. The mean particle diameter calculated using this formula for (21 1 ) plane at 2Q = 36° was approximately 15 nm, which is within the particle diameter range observed using TEM.

[0066] The overall performance of a ZnO nanoparticle-based electrode was analysed using electrochemical impedance spectroscopy (EIS) as well as cyclic voltammetry (CV). Figure 15a shows the CV curve over a voltage range from - 0.5 to 0.8 V for the ZnO/P-Si-based electrodes measured at a scan rate of 100 mV s-1 in 0.6 M KOH electrolyte. As shown in Figure 15a, two peaks at - 0.23 V and 0.6 V were observed, caused by redox reaction between the ZnO and electrolyte. The redox process is primarily governed by intercalation as well as deintercalation of K+ from electrolyte into ZnO: ZnO + K+ + e- ZnOK . These outcomes suggest that the high specific capacitance related to the ZnO electrode originated from pseudocapacitance of the electrochemically active ZnO nanoparticles. Figure 15b shows the CV curves for the as- fabricated electrode at a scan rate of 5 mV s-1 to 100 mV s-1. The specific capacitance (Csp) has extensively been used to calculate the overall performance of electrochemical supercapacitors, and it can be measured by dividing the capacitance by total weight of the deposited ZnO, that is:

where i refers to the current density, t refers to the time, x and y refer to the time at lowest and highest voltage range (V), AV refers to the potential window width (in V) and w refers to the sample weight. Figure 15c shows the differences in specific capacitance of ZnO nanoparticles as a function of scan rates. From the figure it is visible that, with the rise of scan rate from 5 to 100 mV s— 1 , the specific capacitance decreased. At the high scan rates, the movement of the electrolyte ions are bounded by diffusion because of the time restrictions, and simply the outer active surface is employed for storage of charge. Thus, at a high scan rate, the specific capacitance value was low. At lower scan rates, all of the active surface areas of ZnO thin film can be used for storage of charge. The highest specific capacitance of 547 F g-1 was achieved at a 5 mV s-1 scan rate for the ZnO/Psi electrode. The discrete structure of ZnO nanoparticles inhibited the stacking of the porous silicon substrate as a film and improved the electrochemical uses of ZnO nanoparticles.

[0067] The electrochemical properties of Mh3q4 nanoparticles were studied using cyclic voltammetry and galvanostatic charge-discharge and cyclic stability measurements. Figure 16a1 shows the cyclic voltammograms of the Mn30₄ nanoparticles in aqueous 0.6 M KOH as electrolyte at scan rates of 5-150 mVs-1 in the potential range 0 to +0.6 V vs. HgaC . Referring to Figure 16a1 , the electrode showed a pseudo-rectangular-like shape which increased with increasing scan rate. This confirmed that the voltametric current was directly proportional to the scan rates of CV, indicating an ideally capacitive behaviour. All of the curves were near rectangular shape and showed mirror image characteristics even at higher scan rates. This may be attributable to the reversible Faradaic redox reactions and electrochemical stability along with high rate performance. The specific capacitance of the electrodes was calculated from the respective CV curve using the following equation:

where I (A) is the current, AV (V) is the potential window, v (mVs-1 ) is scan rate and m (g) is the mass of active material (Mn30₄) of electrode. Figure 5b shows the specific capacitance with different scan rates. The specific capacitance increased with a decrease in scan rate. The highest specific capacitance of 125 Fg-1 was calculated at lowest scan rate of 5 mVs-1. At scan rate of 150 mVs-1 , lowest capacitance 36 Fg-1 was measured. The higher specific capacitance at lower scan rate is due to sufficient time being provided for the electrolyte to diffuse on the electrode surface interface.

[0068] Figure 16a2 illustrates the galvanostatic charge-discharge (GCD) curve of the Mh3q₄ nanoparticles at different current densities (0.8-3.6 Ag-1 ). Almost all of the charge-discharge curves shown in Figure 16a2 are symmetric in charging counterpart and their corresponding discharge counterparts like triangular charging-discharging characteristics. This may be due to the fast charge propagation with an ohmic drop (IR drop) in the conductive ink. The specific capacitance value from the galvanostatic charge-discharge measurement was calculated using the following equation:

where, I is the discharge current (A), At is the discharge time (s), AV is the potential window (V) and m is the mass (g) of the active material. Figure 16b2 shows the effect of applied current on specific capacitance of the Mh3q₄ electrode in 0.6 M KOH aqueous electrolyte at room temperature. The decrease in the specific capacitance with increasing current density may be attributable to the diffusion limited process. The highest specific capacitance of 1 17.56 Fg-1 was calculated at lowest current density of 0.8 A/g. At current density of 3.6 A/g lowest capacitance 32.06 Fg-1 was measured. At higher current density, the electrolyte ions do not get adequate time for the diffusion into the inner pores and therefore provides lower capacitance.

[0069] Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. Thus, in the context of this specification, the term "comprising" means "including principally, but not necessarily solely". [0070] The term "about" is understood to refer to a range of numbers that a person of skill in the art would consider equivalent to the recited value in the context of achieving the same function or result.

[0071] Although the invention has been described with reference to specific embodiments, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims

1. A process for the simultaneous preparation of zinc oxide nanoparticles and manganese oxide nanoparticles, the process comprising:

(i) providing cathodic material obtained from a spent zinc-carbon battery;

2. The process of claim 1 , wherein the cathodic material comprises ZnMn₂C>4 and Zn₅(0H)₈Cl₂H₂0.

3. The process of claim 1 or claim 2, wherein the cathodic material is heated at a temperature of at least 800 °C.

4. The process of claim 3, wherein the cathodic material is heated at a temperature between about 800 °C and about 1200 °C.

5. The process of claim 4, wherein the cathodic material is heated at a temperature between about 850 °C and about 950 °C.

6. The process of claim 5, wherein the cathodic material is heated at a temperature of about 900 °C.

7. The process of any one of claims 1 to 6, wherein step (ii) is carried out for a period of time between about 10 minutes and about 8 hours.

8. The process of claim 7, wherein step (ii) is carried out for a period of time between about 10 minutes and about 2 hours.

9. The process of any one of claims 1 to 8, wherein condensation of the zinc oxide vapour occurs at a temperature below about 300 °C.

10. The process of claim 9, wherein condensation of the zinc oxide vapour occurs at a temperature between about 250 °C and about 300 °C.

1 1. The process of any one of claims 1 to 10, wherein the zinc oxide vapour is condensed on a substrate.

12. The process of any one of claims 1 to 11 , further comprising drying the cathodic material prior to performing step (ii).

13. The process of any one of claims 1 to 12, further comprising:

converting Zn(OH)CI which co-condensed with the zinc oxide in step (iii), to zinc oxide.

14. The process of claim 13, wherein the Zn(OH)CI is converted to zinc oxide by heating.

15. The process of claim 14, wherein the Zn(OH)CI is converted to zinc oxide by heating at a temperature between about 150 °C and about 500 °C.

16. The process of any one of claims 1 to 15, wherein steps (ii) and (iii) are carried out in an inert atmosphere.

17. The process of any one of claims 1 to 16, further comprising heating the manganese oxide so as to convert MnO to Mh3q4.

18. The process of any one of claims 1 to 17, wherein the cathodic material is in the form of a powder.

19. Zinc oxide and manganese oxide nanoparticles, whenever obtained by the process of any one of claims 1 to 18.