WO2010020007A1

WO2010020007A1 - Nanoporous carbon electrodes and supercapacitors formed therefrom

Info

Publication number: WO2010020007A1
Application number: PCT/AU2009/001072
Authority: WO
Inventors: Thomas Edward Rufford; Denisa Jurcakova; Zhonghua Zhu; Gao Qing Lu
Original assignee: The University Of Queensland
Priority date: 2008-08-20
Filing date: 2009-08-20
Publication date: 2010-02-25
Also published as: CN102160135A

Abstract

A method of producing activated carbon by reacting waste coffee grounds with an activating agent in an environment including at least one inert gas. Also disclosed is a nanoporous carbon electrode for use in a supercapacitor, and an Electrochemical Double-Layer Capacitor (EDLC). The constructed supercapacitor, in 1 M H₂SO₄ aqueous electrolyte, exhibited energy densities up to 20 Wh/kg, and excellent stability at high charge-discharge rates. In a two electrode cell the electrode was observed to have a specific capacitance as high as 368 F/g, rectangular cyclic voltammetry curves and stable performance over 10,000 cycles. Also disclosed is an EDLC using an organic electrolyte, the influence of activation agents and ratios, and activated carbon prepared from sugar cane bagasse.

Description

_

NANOPOROUS CARBON ELECTRODES AND SUPERCAPACITORS FORMED THEREFROM

Technical Field

[001] In one form the present invention generally relates to Electrochemical Double- Layer Capacitors (EDLCs), also known as supercapacitors and ultracapacitors, and more specifically to EDLCs having a nanoporous carbon electrode formed from activated carbon. In other forms, the present invention also generally relates to a method of forming a nanoporous carbon electrode from a carbon precursor, and/or to a method of activating carbon.

Background

[002] Electrochemical Double-Layer Capacitors (EDLCs), also known as supercapacitors and ultracapacitors, have a variety of applications, particularly in energy-smoothing and momentary-load devices. For example, EDLCs are a promising alternative to batteries for delivering peak power demands in portable electronic applications. Energy storage in

EDLCs relies on the accumulation of charge at electrodes purely by electrostatic forces, and as no chemical reactions are involved, unlike the case with batteries, high rates of energy delivery, and stable, reversible charge cycling can be achieved.

[003] An electrode material used in known commercial EDLCs is high surface area activated carbon, which is a high cost component. An important challenge is to develop low cost activated carbon electrodes with relatively high energy and power densities.

[004] Activated carbon is a form of carbon that has been processed to become extremely porous and thus to have a very large surface area. Activated carbon is known to be produced from carbonaceous source materials like nutshells, wood or coal.

[005] Activated carbon can be prepared through either physical or chemical methods. In the physical methods, a carbon precursor (e.g. nutshell, wood or coal) is first carbonised in an inert gas atmosphere (e.g. nitrogen or argon). Following the carbonisation of the carbon precursor, a dilute stream of an oxidising gas (e.g. oxygen or carbon dioxide) is introduced. The partial oxidation of the carbonised product produces the porous activated carbon. In the case of chemical methods, the carbon precursor (either before or after carbonization) is mixed with an activating agent (e.g. phosphoric acid, potassium hydroxide or zinc chloride) and then heat treated in a nitrogen atmosphere to yield activated carbon. Generally, chemical activation is preferred over physical activation due to the lower temperatures and shorter time needed for activating material.

[006] There is a need for an improved EDLC or supercapacitor, an electrode for use in an EDLC, a method of forming a nanoporous carbon electrode, and/or a method of activating carbon, which address or at least ameliorate one or more problems inherent in the prior art.

[007] The reference in this specification to any prior publication (or information derived from the prior publication), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from the prior publication) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Brief Summary

[008] In one aspect, activated carbon is produced from coffee grounds, preferably waste coffee grounds. An example use of this form of activated carbon is as a supercapacitor electrode. This form of activated carbon is herein referred to as Coffee Ground Carbon ("CGC").

[009] In another aspect, activated carbon is produced from other sources of carbon, for example sugar cane bagasse. In still another aspect, different activating agents are utilised to produce activated carbon, for example ZnCl₂ (zinc chloride), MgCl₂ (magnesium chloride) or FeCl₃ (iron (III) chloride or ferric chloride). In still another aspect, different ratios or concentrations of an activating agent are used to control or produce different amounts or ratios of micropores and mesopores in the activated carbon.

[010] Using a CGC electrode, a constructed supercapacitor, in 1 M H₂SO₄ aqueous electrolyte, exhibited energy densities up to about 20 Wh/kg, and excellent stability at high charge-discharge rates. In a two electrode cell the CGC electrode was observed to have a _

specific capacitance as high as 368 F/g, rectangular cyclic voltammetry curves and stable performance over 10000 cycles.

[Oi l] The good electrochemical performance of the CGC electrode was attributed to a well developed porosity, with a distribution of micropores about < 1 ran, and mesopores about 2 - 4 ran wide. The presence of electrochemically active quinone oxygen groups and nitrogen functional groups can also contribute to good electrical performance. The mesopores up to 4 ran wide are known to facilitate electrolyte transport at fast charge rates.

[012] According to a first aspect, there is provided a method of producing activated carbon by reacting waste coffee grounds with an activating agent. In a particular form, the reaction occurs in an environment including at least one inert gas.

[013] According to a second aspect, there is provided a method of producing an electrode for use in a supercapacitor, the electrode made at least partially from activated carbon by reacting waste coffee grounds with an activating agent. In a particular form, the reaction occurs in an environment including at least one inert gas.

[014] According to a third aspect, there is provided a nanoporous carbon electrode for use in a supercapacitor, the electrode including activated carbon produced by reacting waste coffee grounds with an activating agent in an environment including at least one inert gas.

[015] According to a fourth aspect, there is provided an Electrochemical Double-Layer Capacitor (EDLC), including an electrode at least partially formed from activated carbon produced by reacting waste coffee grounds with an activating agent. In a particular form, the reaction occurs in an environment including at least one inert gas.

[016] In various, but non-limiting forms: the activating agent is zinc chloride; the at least one inert gas is nitrogen; and/or, the reaction occurs at a temperature of greater than 873 K. Preferably, to produce CGC from waste coffee grounds, the temperature is between about 1100 K and about 1200 K. _

[017] In further various, but non-limiting forms: the surface area of the electrode is between about 800 m²/g and about 1200 m²/g; the total pore volume of the electrode is between about 0.40 cm³/g and about 0.50 cm³/g; the total micropore volume of the electrode is between about 0.20 cm³/g and about 0.30 cm³/g; and/or, the specific capacitance per unit surface area of the electrode at a current load of about 5 A/g is between about 25 μF/cm² and about 35 μF/cm².

[018] In still further various, but non-limiting forms: the EDLC includes an aqueous electrolyte; at a cell voltage of about 1.2 V the energy density is greater than 10 Wh/kg; at a cell voltage of about 1.2 V the energy density is about 20 Wh/kg; the specific capacitance at about 0.05 A/g is greater than 300 F/g; the specific capacitance at about 0.05 A/g is between about 350 F/g and about 380 F/g; and/or, the EDLC has an energy density of greater than 10 Wh/kg at power densities up to 6000 W/kg.

[019] Thus, waste biomass, such as waste coffee grounds, was able to be utilised to produce electrode materials for cost-effective energy storage systems, such as supercapacitors, for the development of renewable energy technologies.

Brief Description Of Figures [020] Example embodiments should become apparent from the following description, which is given by way of example only, of at least one preferred but non-limiting embodiment, described in connection with the accompanying figures.

[021] Figure 1 illustrates an example method for producing CGC.

[022] Figure 2 shows an example cumulative pore size distribution of the CGC;

[023] Figure 3(a) shows an example cyclic voltammetry curve for CGC in a two electrode cell with 1 M H₂SO₄;

[024] Figure 3(b) shows an example the electrochemical stability of CGC and Maxsorb over 5000 cycles at a cell potential of 0 - 1 V (closed symbols) followed by 5000 cycles with a cell potential of 0 - 1.2 V (open symbols); [025] Figure 3(c) shows an example Ragone plot illustrating performance of CGC relative to Maxsorb, HPGC, ALG-C and BFC;

[026] Figure 4 illustrates an example EDLC (i.e. supercapacitor) based on a CGC electrode;

[027] Figure 5 shows a logarithmic plot of energy density as a function of power density for a coffee precursor versus other forms of precursor (all using ZnCl₂ activation).

[028] Figure 6 shows an example Ragone plot in 1 M H₂SO₄ for activated carbons prepared from different waste precursors including sugar cane bagasse.

[029] Figure 7 shows example specific capacitance plots of CGCs in 1 M H₂SO₄ prepared from waste coffee grounds using ZnCl₂ (CGC-Zn), FeCl₃ (CGC-Fe) and MgCl₂ (CGC- Mg). CGC-O was prepared by physical activation in N₂ with no chemical activation agent.

[030] Figure 8(a) shows example discharge current density dependence of specific capacitance for CGC prepared at 1173 K with ZnCl₂ ratios of 0.5, 1, 2, 3.5 and 5.

[031 ] Figure 8(b) shows the influence of carbon mesopore volume (t-plot) on specific capacitance retention at 1, 5, and 10 A g^'1 relative to capacitance at 0.05 A g^"1.

[032] Figure 9 shows example electrochemical performance of CGCs in 1 M TEABF₄/AN and: (a) dependence of specific capacitance on current density; and (b) Ragone plots for CGCs, including performance of CGC-1.0 in 1 M H₂SO₄. Energy density and power density in this plot are based on the mass of active electrode material, and excludes the mass of the electrolyte, current collectors, and cell packaging.

Preferred Embodiments

[033] The following modes, given by way of example only, are described in order to provide a more precise understanding of the subject matter of a preferred embodiment or embodiments. Waste Coffee Grounds

[034] Referring to Figure 1, there is illustrated an example method 100 of producing activated carbon. At step 110 coffee grounds, preferably waste coffee grounds, are obtained, for example as a by-product from the normal procedure of making coffee drinks from ground coffee beans. At step 120 the waste coffee grounds are allowed to react with an activating agent in an environment including at least one inert gas, for example a nitrogen reaction atmosphere. As illustrative examples, the activating agent can be zinc chloride, and the reaction occurs at a temperature of greater than 873 K. More preferably, the temperature is between about 1100 K and about 1200 K. Optionally, at step 130, method 100 further includes the subsequent steps of washing the waste coffee grounds with hydrochloric acid and then rinsing with distilled water. This allows a form of activated carbon to be obtained at step 140. At step 150 the activated carbon (i.e. the Coffee Ground Carbon ("CGC")) is formed as an electrode for use as part of an EDLC.

[035] In alkali activation processes, for example potassium hydroxide activation, one can produce microporous carbons with very high specific surface areas from precursor carbons. As the number of charges accumulated at an electrode is dependent on the surface area of the electrode/electrolyte interface the energy storage capacity of an EDLC is strongly influenced by the surface area of the electrode. However, it is also believed that not just the specific surface area but also the pore size is important to good energy storage capacity. For aqueous electrolytes, pores of less than 0.5 run width are considered too narrow for effective double-layer formation. This means that appropriate identification and selection of a suitable form of precursor carbon, and a suitable activation method, are necessary for formation of an improved and/or cost-effective electrode for use in an EDLC.

[036] Spent coffee grounds, from a coffee machine, were activated with zinc chloride at 1 173 K under a flow of nitrogen gas. Residual reagent was removed by washing with hydrochloric acid, then rinsing with distilled water. The highest capacitance results were obtained at 1173 K, but carbons prepared at 873 K also produced a capacitance C > 300 F/g. The yield of Coffee Ground Carbon (CGC) was 36 wt%, which is more than double the typical yields by alkali activation methods. The textural properties of CGC are given in Table 1.

Maxsorb 1840 0.84 0.34 14

Table 1. Surface texture properties and specific capacitance of CGC compared to Maxsorb. " Surface area determined by BET method with N₂ adsorption isotherm at 77 K. ^b Micropore volume calculated with the Dubinin-Radushkevich equation from the CO₂ adsorption isotherm at 273 K. ^c Specific capacitance per unit surface area at a current load of 5 A/g.

[037] Figure 2 illustrates the cumulative pore size distribution of the CGC. The cumulative pore size distribution (using non-local density functional theory) in CGC and Maxsorb was calculated from CO₂ at 273 K (left graph) and N₂ at 77 K adsorption isotherms (right graph). Clearly the total pore volume of CGC is much less than that of the commercially available activated carbon - Maxsorb, used as a reference (note: Maxsorb is a form of high surface area activated carbon manufactured by Kansai Coke and Chemicals, Japan).

[038] However, the CGC has a greater ratio of narrow micropores (< 1 ran) to total pore volume. Both types of activated carbons contain a small number of mesopores of 2 - 4 nm width and while these pores may not provide a significant number of active sites, such mesopores can facilitate electrolyte transport at fast charge rates.

[039] The electrochemical performance of the CGC was evaluated in a two electrode cell and compared with Maxsorb, a commonly used reference material in supercapacitor research. The Cyclic Voltammetry (CV) curves for CGC in 1 M H₂SO₄ electrolyte are shown in Figure 3(a). The CGC exhibited near ideal EDLC behaviour, and good electrical conductivity, with rectangular CV curves at low voltage scan rates, and a curve shape similar to that of the Maxsorb at 100 mV/s. _

[040] The specific capacitance (C, F/g) of a single electrode can be calculated more accurately from galvanostatic charge-discharge (GC) profiles, using C=4it_dΔE_d, where / is the current density (A/g), t_d is the discharge time (s) and AE_d is the cell voltage (V). The charge-discharge profiles of the CGC supercapacitor were symmetrical for current loads from 0.05 A/g to 5 A/g, indicating good reversible EDLC behavior. At 0.05 A/g the specific capacitance was 368 F/g, which is excellent for an activated carbon as typically only 80 F/g to less than 300 F/g is achieved.

[041] As shown in Table 1, the surface area normalized capacitance, C_SA, of the CGC was nearly double that of the Maxsorb. Thus, the CGC pore structure is more effective for double-layer formation than the pores of Maxsorb. The advantages of a lower surface area carbon with high specific capacitance include higher densities and reduced potential for electrolyte decomposition.

[042] Figure 3(b) shows that the electrochemical stability of the CGC was far superior to that of Maxsorb over 10000 charge cycles, particularly the second 5000 cycles at 1.2 V where the CGC capacitance fell by only 5% compared to 14 % for Maxsorb.

[043] To examine whether the high capacitance of CGC involved any pseudo-faradaic reactions CV measurements were performed in a three electrode cell with a Ag/AgCl reference electrode. In the three electrode cell CV small peaks were observed at around 0.4 V. These peaks could be attributed to pseudo-faradaic reactions involving quinone oxygen functional groups. X-ray photoelectron spectroscopy (XPS) identified 6.8 at% oxygen and only 1.5 at% nitrogen on the CGC surface. Deconvolution of the Ols spectra identified that most of the oxygen was distributed in phenol or ether groups (Binding Energy (BE) = 532.8 eV, 58.8 %), and only a relatively small amount was in the electrochemically active quinone form (BE = 531.4 eV, 19.7 %). These quinone groups contribute to the capacitance of CGC in H₂SO₄. The nitrogen was distributed across pyridinic (N6), pyrollic and pyridone (N5), quaternary (NQ), and pyridine-N-oxide (NX) groups. In contrast, the Maxsorb surface contained less than 2 at% oxygen and no nitrogen. Sample Ol s NIs

N5 - NX

Carbonyl- Phenol- Chemisorbed N6 - NQ -

Pyrollic/ Pyridine qumone ether O₂/H₂O Pyridinic Quaternary pyridone - N-oxide

Peak

531 532 535 398 400 401 403 (eV)

CGC 19.7% 58.8% 21.5% 35.9% 20.5% 30.6% 13.0%

Table 2. Relative concentrations of oxygen and nitrogen functionalities of coffee ground carbon (CGC) obtained by deconvolution analyses of high-resolution XPS spectra.

[044] The excellent performance of the CGC based supercapacitor is demonstrated in the Ragone plot shown in Figure 3(c) by comparison with recently reported carbon ELDCs, including:

(i) the hierarchical porous graphitic carbon (HPGC) tested by Wang et al. (Wang, D.-W.; Li, F.; Liu, M.; Lu, G. Q.; Cheng, H.-M. Angew. Chem., Int. Ed. 2008, 47, 373- 376.);

(ii) biomass derived carbons from a seaweed by-product (ALG-C) (from Raymundo-Pinero, E.; Leroux, F.; Beguin, F. Adv. Mater. 2006, 18, 1877-1882.); and,

(iii) banana fibers (BFC) (from Subramanian, V.; Luo, C; Stephan, A. M.; Nahm, K. S.; Thomas, S.; Wei, B. J. Phys. Chem. C 2007, 111, 7527-7531.).

[045] Energy density is proportional to the square of cell voltage, and higher energy densities can be achieved with organic electrolytes operating at up to 3 V. However, aqueous electrolytes would be beneficial for cost, chemical stability and handling reasons. It is believed that most carbon EDLCs are limited to 0.6 - 0.7 V in aqueous electrolytes, falling short of the maximum theoretical potential (1.23 V). However, the CGC performance is observed to be remarkably stable even at 1.2 V, as shown in Figure 3(b).

[046] At a cell voltage of 1.2 V the CGC supercapacitor could achieve an energy density exceeding 20 Wh/kg. This energy density is comparable to that of a lead-acid or nickel- cadmium battery, but significantly the CGC supercapacitor can provide more than 10 Wh/kg at power densities up to 6000 W/kg. [047] Referring to Figure 3, the weight of the cell components are not included in the calculations. Specific capacitances (F/g) in Figures 3(a) and 3(b) are calculated per single electrode. Specific capacitances of two-electrode cells were used in the calculations of energy densities (E=CV²/2).

[048] Referring to Figure 4, there is illustrated an example EDLC 400 having a first metallic connector 410, a second metallic connector 420, the CGC electrodes 430 (i.e. CGC material coated on connectors 410, 420), and a separator 440 (in this case glassy filter paper). CGC electrodes 430 are nanoporous activated carbon electrode made from waste coffee grounds. CGC electrodes 430 are immersed in aqueous electrolyte 450, for example H₂SO₄. This allows double layer formation at the interface between the carbon electrode and electrolyte.

[049] Referring to Figure 5, there is illustrated a logarithmic plot of energy density as a function of power density for example EDLCs formed from waste coffee grounds as a carbon precursor versus Maxsorb and other forms of precursors including popcorn, sucrose and sawdust (all using ZnCl₂ activation).

[050] Thus, a high-performance carbon electrode material was prepared by activation of waste coffee grounds. An aqueous electrolyte EDLC, i.e. a supercapacitor, was made that can achieve an energy density up to about, and exceeding, 20 Wh/kg using activated carbon as an electrode which is derived from relatively cheap waste coffee grounds.

[051] This waste coffee grounds derived carbon exhibited extraordinary electrochemical capacitance, predominantly due to a well developed porosity, complemented by pseudo- faradaic reactions involving oxygen and nitrogen functional groups. A high ratio of narrow micropores of about 0.5 - 1 run provide a highly effective surface area for double-layer formation, while the presence of mesopores up to about 4 nm facilitate electrolyte transport, which is believed to be particularly critical at fast charge-discharge rates. This pore structure and surface chemistry produced a high specific capacitance with only a moderate specific surface area, and resulted in stable charge cycling. An energy density approaching that of an acid battery was obtained. _ _

[052] These unexpected findings highlight the ability to utilize waste coffee grounds to produce low cost electrode materials for high performance EDLCs. This could allow more cost-effective energy storage systems for the development of renewable energy technologies, such as solar and wind energies.

Further Examples

[053] The following further examples provide a discussion of particular alternate embodiments. The examples are intended to be merely illustrative and not limiting to the scope of the present invention.

Sugar Cane Bagasse

[054] Sugar cane bagasse is a by-product from the milling of sugar cane and large quantities of this waste material are produced each milling season in Australia and other sugar cane producing countries. Activated carbons for supercapacitor electrodes were prepared from sugar cane bagasse using chemical activation with ZnCl₂. The ZnCl₂ activation of bagasse was studied using thermogravimetic analysis and the carbon pore structures were characterised using N₂ and CO₂ adsorption.

[055] Sugar cane bagasse was rinsed in hot water at 100 ⁰C for about 8 hours, filtered and then air-dried at 100 ⁰C for about 48 hours. In a typical synthesis experiment 2 g of bagasse was mixed with ZnCl₂ in 20 mL of distilled water. Activated carbons were prepared using ZnCl₂ to bagasse weight ratios of 0, 1, 2, and 3.5. The mixture was stirred at room temperature for about 4 hours and then dried in an oven at 100 ⁰C. Carbonisation was performed under N₂ gas flow in a tube furnace at a heating rate of 5 ⁰C min^'1 to 900 ⁰C. The maximum temperature was held for 1 hour. The carbonized samples were washed in 0.2 M HCl, rinsed in distilled water, filtered and then dried to obtain the activated sugar cane carbons (SCC). These samples are referred to as SCC-O, SCC-I, SCC-2, and SCC-3.5 with reference to the ZnCl₂ to bagasse weight ratio. Activated carbon SCC- 1-750 was prepared by the same method described above with a ZnCl₂ to bagasse ratio of 1 and a maximum activation temperature of 750 ⁰C. - -

[056] Electrodes were prepared by mixing 90 wt.% activated carbon, 5 wt.% carbon black (Mitsubishi #32), and 5 wt.% polyvinylidene-fluoride in N-methyl pyrrolidone to form slurry. The slurry was painted in a 1 cm² area on titanium strips, with typically 3 mg active material applied to each electrode. Sandwich type electrochemical cells were constructed, with two symmetrical carbon electrodes separated by glassy fiber paper, and the electrodes were immersed in 1 M H₂SO₄ electrolyte.

[057] In two-electrode sandwich type supercapacitor cells containing 1 M H₂SO₄, the sugar cane bagasse carbons (SCCs) exhibit energy densities up to 10 Wh kg^"1 and specific capacitance close to 300 Fg^"1. The electrochemical performance of the SCCs is attributed to high specific surface areas and the development of mesopores with ZnCl₂ impregnation ratios of 1 or greater. In contrast, the pyrolysis of bagasse without ZnCl₂ produces a carbon with low specific capacitance. The SCC prepared with a ZnCl₂ ratio of 3.5 shows the most stable electrochemical performance at fast charge-discharge rates.

Sample Weight Carbon IaJ LaJ

SBET Pore ^[bi Narrow ratio of yield m² g^" volume Micropore Mesopore micropore

ZnCl₂ wt. % 1 cm³ g^"1 volume volume volume to Cm³ g^"1 cm³ g^"1 cm³ g^"1 bagasse

SCC-O 0 19.5 < 10 0.18 - - -

SCC-I 1 24.3 1 155 0.64 0.38 0.26 0.28

SCC-2 2 34.8 1373 0.76 0.54 0.23 0.25

SCC-3.5 3.5 34.2 1788 1.74 0.19 1.55 0.19

SCC- 1 24.7 1452 0.81 0.48 0.33 0.27

750-1

Table 3. Surface texture properties of activated carbons from sugar cane bagasse. ^ Micropore and mesopore volumes calculated from t-plot method with carbon black reference. M Narrow micropore volume calculated with Dubinin-Radushkevich method from CO₂ adsorption isotherm at 0 ⁰C. _ _

[058] Analysis showed that most of the additional pore volume developed in the SCCs at higher ZnCl₂ ratios is due to growth of pores wider than 2 nm. The volume of narrow micropores, less than 1 nm, is not influenced strongly by the ZnCl₂ to bagasse ratio. The specific surface area of ZnCl₂ activated bagasse carbons decreases as the activation temperature increases from 750 ⁰C to 900 ⁰C, as shown in Table 3.

[059] Activated carbon SCC-I was examined using TGA in an air atmosphere up to 750 ⁰C. The carbon weight loss from SCC-I in air is 96 wt. %. Sugar cane bagasse has an ash content of several percent weight, with a high proportion of silica in the ash. The grey material (4 wt.% of the carbon sample) that remains from SCC-I after TGA in air is a residue of the silica, and other mineral ashes, present in the raw bagasse. The presence of silica and alumina is confirmed by two low binding energy peaks (at 155.6 eV and 106.4 eV) in a wide survey XPS spectrum, in addition to the peaks for CIs, Ols and NIs. The low binding energy peaks can be attributed to Si and Al impurities. These results suggest that the bagasse demineralisation (in water at 373 K) and carbon rinsing (in 0.2 M HCl at room temperature) procedures do not completely remove silica or alumina from the bagasse and SCC.

[060] At high current loads SCC-3.5 shows the best retention of capacitance, with the general trend for carbons prepared at 900 ⁰C is for capacitance retention above current loads of 2 A g^'1 as follows: SCC-3.5 > SCC-2 > SCC-I. SCC-3.5 has the greatest mesopore volume and shows the most stable double-layer capacitance at increasing current density. Mesopores are believed to act as reservoirs for electrolyte ions and facilitate ion transport through the carbon pore network at fast charge-discharge rates.

[061] Although SCC- 1-750 shows superior capacitance to SCC-I at low current loads, which can be explained by the greater specific surface area of SCC-1-750, the deterioration of specific capacitance for the carbon prepared at 750 ⁰C and 900 ⁰C is similar. This result shows that the development of mesopore volume with increasing ZnCl₂ ratio has a larger effect on the specific capacitance of SCC than activation temperature in the range 750 ⁰C to 900 ⁰C. This result suggests that an activation temperature of 750 ⁰C is sufficient, when ZnCl₂ is used as a porogen, to achieve adequate carbon electrical conductivity for a supercapacitor electrode prepared from bagasse. [062] The suitability of the sugar cane bagasse derived carbons for supercapacitor applications was evaluated. Energy densities up to 10 Wh kg^"1 (on an active carbon mass basis) were achieved by the SCC supercapacitor cells at low current loads, with the highest energy density achieved by SCC- 1-750. At increasing power density the benefit of mesopores in the carbon electrodes are clearly seen by the stable performance of SCC-3.5 which retains an energy density of 5.9 Wh kg^"1 at a power density of 10000 W kg^"1.

[063] Referring to Figure 6, example Ragone plots in 1 M H₂SO₄ for activated carbons prepared from different waste precursors, including sugar cane bagasse, are shown. The energy density of electrodes made using sugar cane bagasse activated carbon is not as high as the CGC. The lower energy density of sugar cane bagasse carbon is partly due to impurities, including silica.

[064] Activated carbon electrodes prepared by ZnCl₂ activation of sugar cane bagasse were produced with surface areas of more than 1000 m² g^"1 and the surface area was found to increase with the ZnCl₂ to bagasse weight ratio. The volume of mesopores was also found to increase with the ZnCl₂ to bagasse weight ratio. Thermal pyrolysis of sugar cane bagasse without ZnCl₂ did not produce a carbon with a well developed pore structure. The ZnCl₂ activated carbons displayed excellent electrochemical properties, with specific capacitances as high as 300 F g^"1 observed in supercapacitor cells containing 1 M H₂SO₄ electrolyte. The carbon SCC- 1-750 prepared at 750 ⁰C with a ZnCl₂ to bagasse ratio of 1 exhibited the highest specific capacitance at low current loads, however, at current densities greater than I A g^"1 the most stable electrochemical performance and highest specific capacitance was observed for the carbon prepared with the largest ZnCl₂ ratio, SCC-3.5. These electrochemical results provide evidence of the benefit of mesopores to double-layer capacitance at fast charge-discharge rates.

Alternate Activation Agents [065] Coffee ground carbons were activated with FeCl₃ (iron (III) chloride or ferric chloride) and MgCl₂ (magnesium chloride). Referring to Figure 7, the capacitance and energy density of activated carbons prepared with FeCl₃ and MgCl₂ activation agents were lower than for ZnCl₂ (zinc chloride) activated coffee ground carbons. The MgCl₂ activated _

carbon had low surface area and poor capacitance. Although the capacitance of FeCl₃ activated coffee grounds was not as large as ZnCl₂ activated coffee grounds, the FeCl₃ activated coffee ground shows good retention of capacitance at high charge rates.

Activation Using Different Zinc Chloride Ratios

[066] Activated carbons with large mesopore volumes were prepared from waste coffee grounds by chemical activation with ZnCl₂. These carbons exhibited excellent electrochemical double-layer capacitance of up to 368 F g^"1 in 1 M H₂SO₄. The effect of carbonisation temperature and ZnCl₂ ratio on carbon pore development and consequently electrochemical double-layer capacitance in 1 M H₂SO₄ was investigated. Cyclic voltammetry, electrochemical impedance spectroscopy, and galvanic charge-discharge cycles were used to study the effects of mesopores on capacitance at fast charge rates.

[067] Activated carbons with greater mesopore content retained higher specific capacitance at fast charge-discharge rates as the mesopores acts as channels or reservoirs for electrolyte transport. Modelling was used to determine the contribution of the mesopores to double-layer capacitance to be 14 μF cm^"2. The contribution of micropores to capacitance decreased at fast discharge rates and was found to be dependent on the number of mesopores, which influence the transport of ions through the carbon pore network.

[068] Waste coffee grounds were obtained from a domestic espresso machine and dried at 373 K for about 24 hours. In a typical synthesis experiment 2 g of the dried waste coffee grounds were mixed in 20 mL of distilled water with the desired mass of ZnCl₂ for porogen ratios of 0.5, 1, 2, 3.5, and 5 (by weight), labelled CGC-0.5, CGC-I, CGC-2, CGC-3.5 and CGC-5 respectively. The mixture was stirred at room temperature for 4 hours and then dried in an oven at 373 K. Carbonisation was performed under N₂ gas flow in a tube furnace at a heating rate of 5 K min^"1 to 1173 K. The maximum temperature was held for 1 hour. The carbonised CGCs were washed in 0.6 M HCl, and then rinsed in distilled water before being filtered and dried. To test the effect of carbonisation temperature on electrochemical properties a sample was prepared, labelled CGC-ILT, at 773 K using a ZnCl₂ impregnation ratio of 1. The surface structure and porosity of the carbon was examined using N₂ adsorption at 77 K and CO₂ adsorption at 273 K. The specific surface area was calculated from the N₂ adsorption isotherm using the BET equation. [069] Electrodes were prepared by mixing 90 wt% active material (CGCs), 5 wt% carbon black (Mitsubishi), and 5 wt% polyvinylidene-fluoride in N-methyl pyrrolidone to form slurry. The slurry was painted in a 1 cm² area on titanium strips, with typically 2 mg carbon applied to each electrode. A sandwich type cell was constructed from two electrodes, with similar weights, facing each other and separated by glassy fiber paper. The 1 M H₂SO₄ electrolyte was added to the cell under vacuum to reduce air contamination and improve wettability of the electrodes.

[070] Table 4 shows that the specific surface area and pore volume falls when the activation temperature is increased to 1173 K. The reduction in pore volume at temperatures greater than 1173 K results from carbon gasification and continued organisation of the graphite structure after evaporation of ZnCl₂ salt particles, which leads to pore shrinkage. In XRD patterns (not shown) broad diffraction peaks are observed around 28° and 52° for both CGC-I and CGC-ILT, which is typical of the poor crystallinity of activated carbons. However, the intensity of the peak at 28 ° (associated with the graphitic (0 0 2) carbon) is greater for CGC-I prepared at 1173 K than the intensity of this peak for CGC-ILT. Although activation at 773 K produces a higher surface area, the increase in graphitic order with activation at 1173 K should produce a carbon electrode with better conductivity. Increasing the ZnCl₂ to coffee grounds ratio had no significant effect on crystalline structure of the CGCs, with broad peaks at 28 ° and 52 ° observed in the XRD patterns for all CGCs prepared at 1173 K. The results in Table 4 show that pyrolysis of waste coffee grounds at 1173 K without ZnCl₂ fails to develop significant porosity. In contrast, activation with ZnCl₂, even at low concentration, results in significant development of carbon pore volume.

Label ZnCl₂ Activation V₁ c IaJ c LaJ

SBET ^micro '-'meso impregnation temperature (cm V) (m ² g ^" (mV) (mV) ratio (K) ')

CGC-O 0 1 173 0.01 10 0 10

CGC-0.5 0.5 1173 0.23 429 355 74

CGC-I 1 1173 0.48 977 891 86

CGC-2 2 1173 0.66 1030 745 285 - -

CGC-3.5 3.5 1173 0.93 940 324 616

CGC-5 5 1 173 1.30 1021 176 845

CGC-ILT 1 773 0.64 1291 1189 102

Table 4. Pore structure parameters of the activated coffee ground carbons. [^a] Micropore and mesopore volumes calculated from t-plot method with carbon black reference.

[071] CGCs with ZnCl₂ ratios of 1, 2, and 3.5, showed capacitive behaviour at low- frequencies, with near vertical impedance responses closer to that of ideal plate capacitors. CGC-2 and CGC-3.5 both display capacitive EDLC behaviour up to frequencies of 50 Hz. The width of the semi-circle impedance loop at medium frequencies reflects resistance to ion diffusion through the mesopore structure of the carbon, and this resistance to electrolyte transport is observed to be smaller for CGC-3.5 than CGC-I or CGC-2. At high-frequency the ESR of CGCs is largely independent of ZnCl₂ ratio, confirming charge- transfer processes at high-frequency are limited by resistances external to the porous structure of the active carbon material.

[072] The effect of ZnCl₂ ratio on CGC specific capacitance at increasing current density is shown in Figure 8(a). At low current density CGC-I has the highest specific capacitance, 358 F g^"1. CGC-I has the highest micropore surface area (S_mi_cro ⁼891 m² g^"1) of the ZnCl₂ activated coffee grounds, providing the largest area for double-layer formation. The high specific capacitance of CGC-I suggests that most of the micropore surface area in this carbon is accessible at low current loads to electrolyte ions and double- layer formation. In contrast, the specific capacitance of CGC-0.5 is only 20 F g^"1 at low current loads. Although CGC-0.5 has a surface area of 429 m² g^"1 there are few pores larger than 1 nm and impedance analysis confirms that electrolyte ion transport in CGC- 0.5 is restricted. The carbons with greater mesopore volume, CGC-3.5 and CGC-5, show the best retention of capacitance at 1 A g^"1. Figure 8(b) shows that the benefit of mesopores is even greater at 5 A g^'1 and 10 A g^"1. - -

[073] Table 5 compares calculations of C_di_;mi_cro and C_di,_meso for the CGCs to other reported carbon electrode results. At low current loads, the specific double-layer capacitance on the mesopore surface area C_di.m_eso for CGCs is consistent with the other results published for H₂SO₄. Published values for Cdi._mi_cro show greater variation, which reflects: (1) different carbon pore size distributions, (2) different methods for calculation of micropore and mesopore areas, (3) accessibility of the micropore sites to the electrolyte ions under different cell conditions, and (4) errors from the data fitting methods. The carbon's micro crystalline structure also influences Cdi.mi_cro because the capacitance of graphite edge planes is higher than that of basal planes, and it is reasonable to expect that the carbons in Table 5 produced by different synthesis techniques would have varying degrees of graphitic order. The contribution of the micropore surface area to capacitance decreases at high current loads. The drop in C_di.micro with current density occurs because electrolyte transport becomes restricted in micropores at fast charge-discharge rates. C_di._meso does not decrease significantly at high current loads.

Carbon Electrolyte Number Current Micropore Q_/>m/cτo C_//,_me∞ electrode of density calculation (μFcm^"2) (μFcm^'2) electrodes (mA g^"1) method in test cell

Microbeads 30 %wt 2 5 N₂ DFT 19.5 74

KOH

Carbon 30 %wt 2 5 N₂ DFT 14.5 7.5 fibres KOH

Coal 6 M KOH 2 100-500 Benzene by 10.1 9.1 activated Kelvin carbon equation

Coal 2 M H₂SO₄ 2 100-500 Benzene by 10.1 9.1 activated Kelvin carbon equation

Carbon 1 M H₂SO₄ 3 1 N₂ ?-plot 4 22 from (mV s^'1) colloidal - -

SiO₂

Activated 1 M H₂SO₄ 100 N₂ /-plot 4.8 20.4 carbon

Activated 1 M H₂SO₄ 50 N₂ t-plot 40.5 18.2 coffee grounds

Table 5. Comparison of contributions of microporous and mesoporous/external surface areas to capacitance of carbon electrodes in aqueous electrolytes

[074] Activated carbons with mesoporous structures can be produced from waste coffee grounds by chemical activation with high ZnCl₂ impregnation ratios. At low current loads the specific capacitance of the coffee ground carbons was as high as 368 F g^"1, for CGC-I . At high current loads, the carbons prepared with high ZnCl₂ ratios and containing the most mesopore volume show the best retention of capacitance. This demonstrates the benefit of mesopores for ion transport at fast charge-discharge rates. A minimum activation temperature is required to ensure adequate conductivity of the carbon electrodes, with carbon prepared at 773 K showing poor electrochemical performance at current loads above 0.1 A g^"1.

[075] The contribution of micropore and mesopore surface areas to specific capacitance of activated carbons (from waste coffee grounds) was investigated. Modelling of capacitance and pore size distribution was used to determine the specific double-layer capacitance on the micropore surfaces and the mesopore surfaces. The specific capacitance on the mesopore surface area was calculated to be 18.2 μF/cm² for the CGCs, a value close to specific capacitance in H₂SO₄ for other carbon materials reported in the literature. The specific capacitance on micropore surfaces was calculated to be 40.5 μF cm^"2 at low current loads (0.05 A g^"1), and this value decreased at higher current loads. The value of 40.5 μF cm^"2 is higher than other reports and the difference could be due to (1) pore size distributions, (2) definition of the micropore size, (3) accessibility of the micropore sites to the electrolyte ions under different cell conditions, and (4) errors from the data fitting methods. _

Organic Electrolyte

[076] Using ZnCl₂ activation a series of carbon electrodes were prepared from waste coffee grounds to study the effect of mesopores on double-layer capacitance in a tetraethyl ammonium tetrafluoroborate/acetonitrile (TE ABF₄/ AN) electrolyte. The activated carbon with the largest mesopore volume achieved an energy density of 34 Wh kg^"1 at low current loads and, significantly, retained an energy density of 16.5 Wh kg^"1 and specific capacitance of more than 100 F g^"1 at fast charge-discharge rates (20 A g^"1).

[077] Dried waste coffee grounds were mixed with ZnCl₂ at weight ratios of 1.0, 3.5, and 5.0. The mixtures were heated under a N₂ gas flow in a tube furnace at 5 K min^"1 to 1 173 K, and held for 1 h. Carbonised samples were washed in 0.6 M HCl, rinsed with distilled water, and then dried at 373 K. The activated coffee ground carbons (CGCs) are labelled CGC-1.0, CGC-3.5, and CGC-5.0 in reference to the ZnCl₂ weight ratio.

[078] Electrodes were prepared by mixing 90 wt% activated carbon, 5 wt% carbon black (Mitsubishi #32), and 5 wt% polyvinylidenefluoride in N-methyl pyrrolidone to form slurry. The slurry was painted in a 1 cm² area on aluminium strips, with typically 3 mg of carbon applied to each electrode. The performance of the CGCs was compared to commercially available activated carbon Maxsorb in 1 M TEABF₄ in acetonitrile (AN). Sandwich type cells, with two symmetrical carbon electrodes separated by glassy fiber paper, were assembled inside a N₂ filled glove box.

[079] EDLC performance of coffee ground carbons in organic electrolyte can be enhanced by control of the carbon pore structure. Referring to Figure 9, activated carbons prepared with high ratios (3.5:1 and 5: 1) of ZnCl₂ to coffee grounds exhibit higher energy density at high power loads than activated carbons prepared with a ZnCl₂ to coffee ground ratio of 1 : 1.

[080] For EDLC operation at fast charge-discharge rates in an organic electrolyte, activated carbons with a greater volume of mesopores performed better than microporous carbons. Chemical activation can be effectively used to control the carbon mesoporosity, with an increased ZnCl₂ ratio producing larger pores. This activation process can be tailored to produce carbon pore size distributions suitable for organic electrolytes. [081] Optional embodiments of the present invention may also be said to broadly consist in the parts, elements and features referred to or indicated herein, individually or collectively, in any or all combinations of two or more of the parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

[082] Although a preferred embodiment has been described in detail, it should be understood that various changes, substitutions, and alterations can be made by one of ordinary skill in the art without departing from the scope of the present invention.

Claims

The claims:

1. A method of producing activated carbon by reacting waste coffee grounds with an activating agent in an environment including at least one inert gas.

2. The method as claimed in claim 1 , wherein the activating agent is zinc chloride.

3. The method as claimed in claim 1, wherein the activating agent is iron (III) chloride.

4. The method as claimed in any one of claims 1 to 3, wherein the at least one inert gas is nitrogen.

5. The method as claimed in any one of claims 1 to 4, wherein the reaction occurs at a temperature of greater than 873 K.

6. The method as claimed in any one of claims 1 to 4, wherein the reaction occurs at a temperature between about 1100 K and about 1200 K.

7. The method as claimed in any one of claims 1 to 6, further including the step of washing with hydrochloric acid.

8. The method as claimed in any one of claims 1 to 7, wherein a weight ratio of the activating agent to the waste coffee grounds is used to control the carbon mesoporosity.

9. The method as claimed in claim 8, wherein the activating agent is zinc chloride and the weight ratio is about 1 : 1.

10. The method as claimed in claim 8, wherein the activating agent is zinc chloride and the zinc chloride to waste coffee ground weight ratio is between about 3.5:1 and about 5:1.

11. A nanoporous carbon electrode for use in a supercapacitor, the electrode including activated carbon produced by reacting waste coffee grounds with an activating agent in an environment including at least one inert gas.

12. The electrode as claimed in claim 1 1, wherein the activated carbon includes a substantially high ratio of narrow micropores, having a width of about 0.5 nm to about 1.0 nm, to mesopores, having a width of about 2 nm to about 4 nm.

13. The electrode as claimed in either claim 11 or 12, wherein the surface area of the electrode is between about 800 m²/g and about 1200 m²/g.

14. The electrode as claimed in any one of claims 11 to 13, wherein a total pore volume of the electrode is between about 0.40 cm³/g and about 0.50 cm³/g.

15. The electrode as claimed in any one of claims 11 to 13, wherein a total micropore volume of the electrode is between about 0.20 cm³/g and about 0.30 cm³/g.

16. The electrode as claimed in any one of claims 11 to 15, wherein the specific capacitance per unit surface area of the electrode at a current load of about 5 A/g is between about 25 μF/cm² and about 35 μF/cm².

17. An Electrochemical Double-Layer Capacitor (EDLC), including an electrode at least partially formed from activated carbon produced by reacting waste coffee grounds with an activating agent in an environment including at least one inert gas.

18. The capacitor as claimed in claim 17, also including an aqueous electrolyte.

19. The capacitor as claimed in claim 18, wherein the aqueous electrolyte includes an organic compound or mixture.

20. The capacitor as claimed in claim 19, wherein the organic compound or mixture is tetraethyl ammonium tetrafluoroborate/acetonitrile (TEABF₄/ AN). _

21. The capacitor as claimed in any one of claims 17 to 20, wherein at a cell voltage of about 1.2 V the energy density is greater than 10 Wh/kg.

22. The capacitor as claimed in any one of claims 17 to 20, wherein at a cell voltage of about 1.2 V the energy density is about 20 Wh/kg.

23. The capacitor as claimed in any one of claims 17 to 22, wherein the specific capacitance at about 0.05 A/g is greater than 300 F/g.

24. The capacitor as claimed in any one of claims 17 to 22, wherein the specific capacitance at about 0.05 A/g is between about 350 F/g and about 380 F/g.

25. A method of producing activated carbon by reacting sugar cane bagasse with an activating agent in an environment including at least one inert gas.

26. The method as claimed in claim 25, wherein the activating agent is zinc chloride.

27. The method as claimed in either of claims 25 or 26, wherein the weight ratio of zinc chloride to sugar cane bagasse is about 3.5:1.