WO2021154332A1

WO2021154332A1 - High-performance supercapacitors from biomass-derived carbon

Info

Publication number: WO2021154332A1
Application number: PCT/US2020/043392
Authority: WO
Inventors: Ram Krishna GUPTA; Pawan K. KAHOL; Timothy R DAWSEY
Original assignee: Pittsburg State University
Priority date: 2020-01-28
Filing date: 2020-07-24
Publication date: 2021-08-05

Abstract

Biomass-derived activated carbon for use in fabricating energy storage devices, such as supercapacitors. Processes for preparing activated carbon from biomass using a potassium hydroxide treatment, including optional nitrogen-doping. Activated carbon from spent coffee grounds.

Description

HIGH-PERFORMANCE SUPERCAPACITORS FROM BIOMASS-DERIVED CARBON CROSS-REFERENCE TO RELATED APPLICATIONS The present application claims the priority benefit of U.S. Provisional Patent Application Serial No. 62/966,668, filed January 28, 2020, entitled WASTE COFFEE MANAGEMENT: DERIVING HIGH-PERFORMANCE SUPERCAPACITORS USING NITROGEN-DOPED COFFEE DERIVED CARBON, incorporated by reference in its entirety herein. BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to biomass-derived activated carbon for use in fabricating energy storage devices. Description of Related Art Since 1200 B.C., coffee has acquired an important place in human society, owing to its aroma and taste. More than half of American population drinks coffee every day, predicting an average consumption of about 5 kg per year, close to that of European community. In 2019, the world's coffee production is forecast to reach a peak value of 174.5 million bags of 60 kgs, much higher than previous years, according to the statistics of the U.S. Department of Agriculture. Owing to its considerable demand, coffee grind waste is substantial. Apart from a rich source of caffeine, it includes several different chemicals such as vitamins, carbohydrates, lipids, minerals, alkaloids, phenolic, and nitrogen-rich compounds. Hence, this affluent source of bio-carbon waste could serve as an exciting precursor to economically produce activated carbon for energy applications. Some reports have been observed for utilizing coffee waste to developing activated carbon for supercapacitor application. Previous reports describe nanoporous activated carbon electrodes from waste coffee beans using ZnCl₂ activation. Coffee shells have been used to obtain activated carbon for supercapacitor applications. Phosphorous and nitrogen co-doped activated carbon from spent coffee grounds has also been synthesized using ammonium polyphosphate via microwave- assisted synthesis. In our previous studies, we have reported the production of bio-waste derived activated carbons using KOH activation, for supercapacitor application. Utilizing a KOH activation technique, we have demonstrated varied pore size distribution, control over the surface area of activated carbons, and resulting in promising charge storage performance of the device. Electric double-layer capacitors have attracted attention due to faster charging and discharging capabilities, long cyclic performance, and relatively high power densities. In order to achieve higher performance, conductive specific surface area, and the pore size distribution are important characteristics for the porous carbon material, which further corresponds to the mobility of the charges within the pores. Ideally, carbon has lower conductivity due to the limited number of electrons in the density of states. Furthermore, during the chemical activation process, the conductivity of carbon can be greatly influenced due to induced porosity and may result in inferior charge storage performance. This can be observed by a reduced graphitic phase of carbon with increasing chemical activation, as reported by previous studies. However, the conductivity of carbon-based materials can be improved by doping of elements such as nitrogen and phosphorus, that could significantly improve the density of states for the carbon. Owing to higher nitrogen content, melamine is a widely used precursor for nitrogen doping and providing n-type behavior to the carbon. Thus, with improved conductive nature of activated carbon, higher charge storage performance could be achieved. SUMMARY OF THE INVENTION The present invention is broadly concerned with methods for preparing improved activated carbon from biomass. For example, methods of preparing nitrogen-doped activated carbon from biomass are disclosed. The methods generally comprise treating biomass with a source of nitrogen to yield nitrogen-doped biomass based carbon; and chemically activating the nitrogen-doped biomass based carbon with potassium hydroxide to yield nitrogen-doped activated carbon. Also described herein are electrodes for supercapacitors, generally comprising a porous metal substrate; and a layer of nitrogen-doped activated carbonized biomass preparing according to any of the disclosed methods adjacent said substrate. Also described herein are methods of preparing activated carbon from biomass. The methods comprise pre-carbonizing dried biomass by heating under nitrogen atmosphere to about 300 °C (+/-50 °C) for approximately 1 hr to about 2 hrs to yield pre-carbonized biomass. The methods involve chemically activating and calcining the pre-carbonized biomass by mixing the pre-carbonized biomass with potassium hydroxide and heating under nitrogen or argon atmosphere to an activation temperature ranging from about 600 °C to about 1,200 °C for approximately 1 hr to 2 hrs to yield carbonized biomass. The carbonized biomass is washed with hydrochloric acid and water to yield the activated carbon from biomass. Any of the described methods can be used to prepare activated carbon from biomass, which can be used to fabricate electrodes for energy storage devices. Such devices comprise generally a porous metal substrate and a layer of activated carbon from biomass adjacent said substrate. The described methods can also be used to fabricate energy storage devices, generally comprising a pair of activated carbon electrodes, a non-conductive separator between the pair of activated carbon electrodes, and an electrolytic liquid, all within a housing. Again, the activated carbon is prepared from biomass according to any of the methods described herein. Two or more devices can be coupled in series to further improve the energy properties. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a schematic of (A) synthesizing nitrogen-doped coffee derived activated carbon, and (B) device fabrication using the activated carbon from coffee waste. Fig.2 is a graph of XRD patterns of carbonized coffee powders. Fig.3 shows graphs of the Raman spectra of the carbonized coffee powders (CP-UA, CP- N, and CP-NA). Fig.4 shows SEM images of the carbonized coffee (a) CP-UA, (b) CP-N, and (c) CP-NA at low and high magnifications. Fig. 5 shows (a) nitrogen adsorption-desorption isotherms, and (b) BJH pore size distributions of coffee derived carbons. Fig.6A shows graphs of XPS core level spectra of C 1s for CP-UA, CP-N, and CP-NA. Fig.6B shows graphs of XPS core level spectra of O 1s for CP-UA, CP-N, and CP-NA. Fig.6C shows graphs of XPS core level spectra of N 1s for CP-UA, CP-N, and CP-NA. Fig.7 shows (a) cyclic voltammogram at 10 mV/s, and (b) specific capacitance versus scan rate for all the samples. Fig.8 is a graph of the CV curves of CP-NA at various scan rates. Fig. 9 shows (a) charge-discharge characteristics at 5 A/g, and (b) specific capacitance versus applied current for all the samples. Fig.10 shows (a) charge-discharge characteristics of CP-NA at various current densities, and b) Ragone plot for all the samples. Fig.11 is a graph of the volumetric capacitance versus applied current for all the samples. Fig.12 is a graph of the variation of specific capacitance as a function of mass loading and current density. Fig.13 shows graphs of CV curves of CP-NA at various scan rates based on (a) weight and (b) area, and (c) variation of specific capacitance versus scan rate. Fig.14 shows graphs of galvanostatic charge-discharge curves of CP-NA at various current densities based on (a) weight and (b) area, and (c) variation of specific capacitance versus applied current. Fig. 15 shows graphs of (a) Ragone plot, (b) stability curves for CP-NA symmetrical supercapacitor device, and (c) galvanostatic charge-discharge for single and two cells in series using CP-NA electrodes. Fig.16 is a comparative graph of surface area of the activated carbons derived from various biomass upon chemical activation with a 1:1 ratio of biomass to the activating agent. Fig.17 shows graphs for (a) cyclic voltammograms of the supercapacitor device fabricated using various electrolytes at 50 mV/s of scan rate, and (b) corresponding charge-discharge characteristics of the device at 5 A/g. GF was used as a separator along with CP-N deposited on carbon cloth as the electrode for the device. Fig.18 shows graphs for (a) variation of specific capacitance and (b) energy density as a function of applied current density for the supercapacitor device fabricated using various electrolytes. GF was used as a separator along with CP-N deposited on carbon cloth as the electrode for the device. Fig.19 shows graphs for (a) cyclic voltammograms at 100 mV/s and (b) charge-discharge characteristics at 10 A/g of one device and two devices in series. (Both devices were fabricated using TEATFB as an electrolyte, CP-N as active material on carbon cloth and GF (Device 1) and Teflon (Device 2) as separators. Fig.20 shows graphs for (a) cyclic voltammograms of the device fabricated using activated carbon from sorghum grains, and (b) variation of specific capacitance as a function of scan rate. The device was fabricated using EMITFB as an electrolyte. Fig. 21 shows graphs for (a) charge-discharge characteristics, (b) variation of specific capacitance as a function of current density, and (c) plot of energy-power densities for the device fabricated using carbon derived from orange peel in 6M KOH electrolyte. Fig. 22 shows graphs for (a) charge-discharge characteristics, (b) variation of specific capacitance as a function of current density, and (c) plot of energy-power densities for the device fabricated using carbon derived from corn-based distiller grain in 1M TEATFB electrolyte. Fig. 23 shows graphs for (a) charge-discharge characteristics, (b) variation of specific capacitance as a function of current density, and (c) plot of energy-power densities for the device fabricated using carbon derived from tea leaves in EMITFB electrolyte. Fig.24 shows graphs for (a) variation of specific capacitance, (b) energy density, and (c) power density of energy storage devices fabricated using carbon from various biomass. The devices were fabricated using 6M KOH, 1M TEATFB, and EMITFB as electrolytes. Fig. 25A-B shows graphs for (a) capacitance retention as a function of the number of charge-discharge cycles for sorghum grain-based device in 6M KOH, inset figure shows impedance vs. frequency plot for the device before and after stability test, (b) capacitance retention as a function of the number of charge-discharge cycles for jute fiber-based device in 1M TEATFB, inset figure shows first few charge-discharge cycles of the same device. Fig. 25C shows capacitance retention as a function of the number of charge-discharge cycles for distiller grain-based device in EMITFB electrolyte, the inset figure shows Coulombic efficiency as a function of the number of charge-discharge cycles. DETAILED DESCRIPTION The present invention is concerned with activated carbon for use in fabricating electrodes for various energy storage devices, such as capacitors, supercapacitors, battery cells, as well as cathodic protection coatings. Processes for preparing activated carbon from biomass are also described. Thus, the term “activated carbon” as used herein refers to carbonaceous material with a two/three-dimensional structure composed primarily of carbon atoms and is derived from the conversion of a biomass feedstock material into carbon char, which has been treated physically or chemically to develop a plurality of pores in the carbon structure. The activated carbon has tunable properties for different applications. The activated carbon is particularly suited for use in fabricating supercapacitors. Supercapacitors typically include a non-conductive separator and an electrolytic liquid sandwiched between a pair of electrodes (cathode/anode) comprising the activated carbon. A housing or case may be provided to contain the capacitor components. Depending upon the biomass starting material and the process parameters used to convert the biomass into activated carbon, important characteristics of the carbon-based electrodes can be adjusted to change the properties of the device, such as the energy density, power density, and the like. Embodiments described herein are concerned with converting low-cost biomass waste materials into activated carbon. In one or more embodiments, the process involves the production of high performance, nitrogen-doped activated carbon from biomass. A particularly preferred biomass starting material is spent coffee grounds (i.e., coffee grounds that have already been used to brew coffee). Other biomass includes bamboo fibers, banana peels, coconut husk, distiller grain, jute fibers recycled from a rope, hemp fiber recycled from a rope, orange peels, sorghum grain, and used tea leaves. The term “biomass” as used herein specifically refers to the material of plant origin (as opposed to animal-derived materials), and includes any portion of the raw plant (e.g., stalk, flowers, leaves, seeds, nuts, fruit, etc.), and processed plant materials (e.g., coffee grounds, fibers, boards, etc.). The process generally involves carbonizing the biomass and activating the resulting carbonized product by increasing the porosity and surface area of the carbonized material. In one or more embodiments, carbonization and activation occur during the same step. Activation includes any process that etches the surface of the carbonized material to create pores or micropores. Examples include physical etching with steam, as well as chemical etching, e.g., using KOH or H₃PO₄. As demonstrated in the working examples, a process has been developed herein that avoids the use of harsh chemicals such as H3PO4 but still yields high performance activated carbon from biomass. In one or more embodiments, the biomass starting materials are first dried to evaporate moisture, such as in an oven at less than 110 °C, preferably from about 60 °C to about 100 °C for about 10 hrs to about 24 hrs. Drying may or may not be necessary depending upon the moisture content of the biomass starting material. In one or more embodiments, the dried biomass is then ground, milled, or shredding into particulate form. In one or more embodiments, the dried biomass is ground into a fine powder (e.g., grain distiller, coffee powder, with an average/mean particle size of 1 micron to 1 mm, although in some cases less than 1 micron). In one or more embodiments, fibrous biomass material (e.g., jute or hemp) may be used in fiber form and grinding is not carried out. In one or more embodiments, fibrous biomass may be cut to reduce the length of the fibers. In one or more embodiments, dried biomass material is then subjected to a pre-carbonization step by heating under nitrogen atmosphere to about 300 °C (+/-50 °C) for approximately 1 hr to about 2 hrs. The pre-carbonized biomass material is then mixed with KOH and heated under nitrogen or argon atmosphere from room temperature up to an activation temperature ranging from about 600 °C to about 1,200 °C, preferably about 600 °C to about 900 °C, more preferably about 800 °C (+/-50 °C) for approximately 1 hr to 2 hrs. In one or more embodiments, the mass ratio of biomass to KOH can range from about 1:0.5 to about 1:4, and values therebetween (e.g., 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5). In one or more embodiments, this calcining step takes approximately 1.5 hrs to about 3 hrs to reach the designated temperature. The material is held at the activation temperature for between 1 hr and 4 hrs (preferably 1 hr to 3 hrs) and then allowed to cool to room temperature (~27 °C) over the course of approximately 1.5 hrs to 3.5 hrs. Advantageously, the process does not require pre-soaking the pre-carbonized biomass material in the KOH. That is, once the biomass material and KOH are mixed, the mixture can be immediately (i.e., 10 minutes or less) placed in the oven. Thus, the reaction/etching with the KOH and the calcining occurs in the same step. The resulting carbonized biomass is then washed with HCl and water to yield activated carbon, characterized as a carbonized biomass with a porous surface. The resulting activated carbon is dried to evaporate moisture, such as in an oven at less than 110 °C, preferably from about 60 °C to about 100 °C for about 2 hrs to about 12 hrs. In one or more embodiments, the process involves nitrogen-doping and activation of the biomass-derived carbon. In this embodiment, the biomass material is reacted with a source of nitrogen before the activation step. In more detail, the biomass starting material is dried (if needed) and reduced in size, e.g., to particulate form, as described above. The dried biomass is then mixed with a source of nitrogen and allowed to pre-soak for a time period of from about 30 minutes to about 1.5 hrs, preferably from about 45 minutes to about 60 minutes. Alternatively, the biomass starting material or dried biomass may be simply mixed with the source of nitrogen and reduced in size, for example by grinding. Exemplary sources of nitrogen include melamine, urea, ammonium, nitrogen-containing salts, as well as antioxidants to activate nitrogen naturally present in the starting feedstock. Melamine is a preferred nitrogen source. The dried biomass is mixed with the source of nitrogen in a mass ratio ranging from about 1:0.5 to about 1:4 (e.g., 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5), preferably about 1:1 ratio. The melamine-soaked biomass is then dried to evaporate moisture, such as in an oven at less than 110 °C, preferably from about 60 °C to about 100 °C for about 1 hr to about 5 hrs. The dried melamine infused biomass is then ground into a powder and heated under a nitrogen atmosphere from room temperature up to a calcining temperature ranging from about 600 °C to about 1,200 °C, preferably about 600 °C to about 900 °C, more preferably about 800 °C (+/-50 °C) for approximately 1 hr to about 3 hrs. The resulting nitrogen-doped carbonized biomass is then washed with water and again dried to evaporate moisture as previously described. The resulting biomass-derived nitrogen-doped carbon material can then be subjected to the KOH activation process as described above. Regular or nitrogen-doped activated carbon can be characterized by increased surface area (0.66 m²/g unactivated carbon), with an average pore diameter of less than 2 nm. The activated carbon has a BET surface area that is about 2,000 × of unactivated carbon. In general, the BET surface area of activated carbon will be at least 60 m²/g, preferably ranging from about 60 m²/g to about 2,000 m²/g. Nitrogen-doped activated carbon is further characterized by Raman spectroscopy to estimate the ratio of graphitic (G) and diamond (D) phases of carbon as compared to regular activated carbon. Diamond is bonded in a way that provides high mechanical strength but almost no electrical conductivity, while the graphitic structure allows the transport of electrons to make it more conductive. Thus, it will be appreciated that a high amount of graphitic phase in carbon will be beneficial for batteries/supercapacitors (i.e., a high IG/ID (ratio of graphitic and diamond phase) will have better conductivity). The IG/ID ratio of unactivated carbon, nitrogen- doped, and nitrogen-doped activated carbon was estimated as 0.94, 0.86, and 1.02, respectively. In general, however, the nitrogen-doped materials demonstrate improved energy storage capacity as compared to regular activated carbon and unactivated carbon. The regular or nitrogen-doped activated carbon can be used to prepare electrodes for energy storage devices. For example, the activated carbon can be mixed with other conductive carbon materials (e.g., carbon black), non-carbon materials, and/or conductive polymers, along with binder materials in a suitable solvent system to form a mixture that can be applied to a suitable electrode substrate. Preferably, at least 50 wt% of the mixture comprises the biomass-derived activated carbon, more preferably at least 60 wt%, even more preferably at least 70 wt%, and more preferably from about 75 wt% to about 95 wt%, based upon the total weight of the solids in the mixture taken as 100% by weight (i.e., excluding the solvent system). Exemplary carbon materials that may be mixed with the activated carbon include carbon black, including acetylene black, super P conducting carbon, graphite, carbon nanotubes and the like. Exemplary non-carbon materials that may be mixed with the activated carbon include copper, titanium, silver, gold, palladium, platinum, and metal oxide thereof. The amount of such additional carbon materials and/or non- carbon materials can range from about 5 wt% to about 10 wt%, based upon the total weight of the solids in the mixture taken as 100% by weight (i.e., excluding the solvent system). Exemplary binder materials include Nafion, polyvinylidene difluoride (PVdF), and polytetrafluoroethylene (PTFE), styrene-butadiene rubber, sodium carboxymethyl cellulose, polyacrylonitrile, poly(ethylene oxide) and mixtures thereof. The amount of such binders can range from about 5 wt% to about 10 wt%, based upon the total weight of the solids in the mixture taken as 100% by weight (i.e., excluding the solvent system). Exemplary solvent systems include water as well as organic solvents such as N-methyl-2-pyrrolidone, acetone, ethylene glycol, and the like. The amount of solvent can vary depending upon the desired consistency of the mixture, e.g., ranging from a liquid consistency suitable for dip-coating, puddling or spin coating on the substrate to a paste consistency applied with a blade or spatula. The mixture is applied to the electrode substrate and typically dried under vacuum at a temperature ranging from 50 °C to about 80 °C for a time period of from about 1 hr to about 10 hrs to evaporate solvents. Depending upon the desired weight of the activated carbon, multiple applications can be applied. In general, the resulting electrode materials (activated carbons) will have weight ranging from about 0.2 mg/cm² to about 20 mg/cm². Exemplary substrates include nickel foams, stainless steel, copper, aluminum, carbon paper, carbon cloth, and the like. Energy storage devices, such as supercapacitors, can be fabricated using the activated carbon described herein. For example, a supercapacitor may be fabricated by arranging the components within a housing, such that a pair of the activated carbon electrodes are separated by a non-conductive separator or membrane and immersed in electrolytic liquid. In certain embodiments, one or both of the activated carbon electrodes can advantageously act as both an electrode and current collector in a supercapacitor, capacitor, or battery cell. Suitable separators (or membranes) include one or more layers of cellulose-based filter paper, glass fibers (GF), porous Teflon, tissue papers, polymer separators e.g., from Celgard (polypropylene membrane, polyethylene membranes, etc.), and the like. Exemplary electrolytic liquids will comprise aqueous, organic, or ionic electrolytes, such as KOH, H2SO4, Na2SO4, tetraethylammonium tetrafluoroborate (TEATFB), LiPF6, 1-ethyl-3-methyl imidazolium tetrafluoroborate (EMITFB)], 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-Butyl-1-methylpyrrolidinium bis(trifluoromethyl-sulfonyl)imide, 1-Methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl) imide, quaternary phosphonium salts, sodium perchlorate, lithium perchlorate, lithium hexafluoride arsenate, acetonitrile, propylene carbonate, tetrahydrofuran, diethyl carbonate, γ- butyrolactone, tetraethylammonium tetrafluoroborate, triethyl (metyl) tetrafluoroborate, and mixtures thereof. Other electrolytic liquid components may also be used within the scope of the present invention. When the energy storage device is a traditional capacitor, a dielectric layer may also be present, which may comprise a dielectric material such as glass, ceramic, plastic film, paper, mica, air, and/or oxide layer. The components may be used in combination with one or more spacers and/or springs before being encased in the housing or device cover. The activated carbon is particularly suited for fabricating capacitors, and preferably supercapacitors. The activated carbon may also be used as an electrode material for other energy storage systems, such as alkaline or lithium battery cells. In one or more embodiments, the resulting biomass-derived supercapacitor has a range of operating voltage between about 1.0 Vand about 3.5 V. In one or more embodiments, the resulting biomass-derived supercapacitor has a capacitance of between about 10 F/g and about 120 F/g at room temperature. In one or more embodiments, the resulting biomass-derived supercapacitor has a range of energy density between about 5 Wh/g and about 110 Wh/kg. In one or more embodiments, the resulting biomass-derived supercapacitor has a range of power density between about 0.2 kW/kg and about 28 kW/kg. As described in the working examples, the properties of the supercapacitor can be tuned by adjusting the nitrogen-doping and/or carbon activation parameters during the conversion of the biomass into activated carbon. For example, charge storage capacity at 5 A/g increased from about 11 F/g to about 28 F/g upon nitrogen doping which further improved to over 125 F/g after chemical activation. Additional advantages of the various embodiments of the invention will be apparent to those skilled in the art upon review of the disclosure herein and the working examples below. It will be appreciated that the various embodiments described herein are not necessarily mutually exclusive unless otherwise indicated herein. For example, a feature described or depicted in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, the present invention encompasses a variety of combinations and/or integrations of the specific embodiments described herein. As used herein, the phrase "and/or," when used in a list of two or more items, means that any one of the listed items can be employed by itself or any combination of two or more of the listed items can be employed. For example, if a composition is described as containing or excluding components A, B, and/or C, the composition can contain or exclude A alone; B alone; C alone; A and B in combination; A and C in combination; B and C in combination; or A, B, and C in combination. The present description also uses numerical ranges to quantify certain parameters relating to various embodiments of the invention. It should be understood that when numerical ranges are provided, such ranges are to be construed as providing literal support for claim limitations that only recite the lower value of the range as well as claim limitations that only recite the upper value of the range. For example, a disclosed numerical range of about 10 to about 100 provides literal support for a claim reciting "greater than about 10" (with no upper bounds) and a claim reciting "less than about 100" (with no lower bounds). EXAMPLES The following examples set forth methods in accordance with the invention. It is to be understood, however, that these examples are provided by way of illustration and nothing therein should be taken as a limitation upon the overall scope of the invention. EXAMPLE 1 In this work, nitrogen-doped activated carbon was produced from waste coffee powder using a two-step chemical activation process. Nitrogen doping was achieved by treating the coffee powder with melamine, prior to chemical activation. The produced nitrogen-doped carbon resulted in a very high surface area of 1,824 m²/g, along with improved nitrogen content, and maintained a high graphitic phase as confirmed by Raman spectroscopy. The elemental composition of the obtained coffee derived carbon was analyzed using XPS spectroscopy. Supercapacitor electrodes were fabricated using coffee waste-derived carbon. The electrochemical performance of synthesized electrodes was analyzed using a three-electrode system and two-electrode system. It was observed that nitrogen-doping improves the electrochemical performance of the carbon and therefore the charge storage capacity. The nitrogen-doped coffee carbon showed a high specific capacitance of 148 F/g at a current density of 0.5 A/g. The symmetrical coin cell device was fabricated using coffee derived carbon electrodes to analyze its real-time performance. The device showed the highest specific capacitance of 74 F/g at a current density of 1 A/g. The highest energy and power density for the device was calculated to be 12.8 Wh/kg and 6.64 kW/kg, respectively. The symmetrical coin cell device showed the highest energy and power density of 12.8 Wh/kg and 6.64 kW/kg, respectively. Moreover, the device maintained about 97% of its initial capacitance even after 10,000 cycles, with about 100% of Coulombic efficiency. These results indicate that the synthesized nitrogen-doped coffee carbon electrode could be used as a high-performance supercapacitor electrode for energy storage applications and at the same time managing the waste generated by using coffee. Materials and Methods Synthesis of Activated Carbonized Coffee Powder Three different types of carbon were synthesized using spent coffee grounds. Spent coffee grounds from a commercial source were dried and then calcined at 800 ^oC for 2 hrs with a ramp rate of 5 ^oC/min under nitrogen atmosphere in a tube furnace to produce Unactivated Coffee Powder-based carbon (CP-UA) as a control. Nitrogen-Doped Coffee Powder-based carbon (CP- N) was prepared by mixing spent dry coffee grounds with melamine at a 1:1 weight ratio in de- ionized (DI) water. For this, the mixture was sonicated for 60 minutes and then dried in an oven. The obtained powder was ground and calcined at 800 ^oC for 2 hrs (5 ^oC/min) under a nitrogen atmosphere. Finally, the powder was washed with DI water several times and dried in the oven to yield CP-N powder. Nitrogen-Doped Activated Coffee Powder-based carbon (CP-NA) was synthesized by chemically activating the CP-N prepared above using KOH as a chemical activating agent. For this, CP-N and KOH were mixed in 1:1 ratio and heated at 800 ^oC for 2 hrs (5 ^oC/min) under the nitrogen atmosphere. After cooling to room temperature, the product was washed with 1M HCl solution, followed by DI water. Then, the product was dried at 70 °C for 12 hrs to yield CP-NA. The percentage yield of CP-UA, CP-N, and CP-NA was about 57%, 56%, and 39%, respectively. Structural Characterization The structural characterization of the carbonized coffee was performed using X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM). The XRD spectra of the samples were taken using Shimadzu X-ray diffractometer using the 2θ scan with CuKα1 (λ = 1.5406 Å) radiation. Raman studies were carried out using excitation source of 532 nm wavelength to determine the fingerprint of all the carbons (Horiba, Xplora Plus). Scanning electron microscopy (JEOL, JSM-840A) was used to characterize the porous morphology of carbons. The chemical state and composition of the prepared coffee-based carbons were studied using X-ray photoelectron spectroscopy (Thermo Scientific Kα XPS system). The X-ray power of 75 W at 12 kV was used for the experiment with a spot size of 400 mm². The surface area was determined by the Brunauer–Emmett–Teller (BET) adsorption method (Micrometrics, USA, ASAP 2020 Models). A coffee sample was first degassed for 24 hrs at a holding temperature of 90 °C, following which the analysis for nitrogen adsorption was done at liquid nitrogen temperature (−196 °C). Electrochemical Characterization Electrochemical characterizations of the carbonized coffee and supercapacitor device were performed using Versastat 4-500 electrochemical workstation (Princeton Applied Research, USA). For electrochemical measurements, the three-electrode system consisting of a platinum wire as a counter electrode, saturated calomel or Hg/HgO as a reference electrode, and carbonized coffee on nickel foam as a working electrode, was used. The working electrode was prepared by mixing 80 wt.% of the carbonized coffee, 10 wt.% of acetylene black, and 10 wt.% of polyvinylidene difluoride (PVdF) in the presence of N-methyl pyrrolidinone (NMP). After mixing, the paste was applied onto pre-cleaned nickel foam and dried at 60 °C under vacuum for 10 hrs. The mass was measured by weighing the nickel foam before and after electrode preparation using an analytical balance (Mettler Toledo, MS105DU, 0.01 mg of resolution). The loading of the active materials on the nickel foam was about 1.4 mg/cm². The three-electrode system allows extracting information about the electrode acting as cathode and anode, however, it is also important to study the performance of the materials in the two-electrode configuration as practical devices are made in the two-electrode configuration. For this, we have fabricated symmetrical coin cells using coffee derived carbon as anode and cathode. Results and Discussion The detailed schematic of nitrogen-doped coffee derived activated carbon is shown in Fig. 1. The structural quality of the synthesized carbon samples was analyzed using X-ray diffraction and Raman spectroscopy. XRD patterns of the unactivated coffee carbon, unactivated nitrogen- doped coffee carbon, and KOH activated nitrogen-doped coffee carbon are given in Fig. 2. The presence of the graphitic phase of carbon can be observed by sharp peaks around 24^o and 44^o corresponding to (002) and (100) planes, respectively. This could be beneficial for efficient charge transfer in supercapacitors, due to the conducting nature of graphitic carbon. The presence of the disordered phase in the synthesized samples was estimated using Raman spectroscopy. Fig. 3 shows the Raman spectra of all the synthesized samples. The peaks at 1,355 and 1,592 cm^-1 are represented as the D band and the G band, respectively, which are the characteristic Raman peaks for carbon materials. The presence of disorders in the coffee derived carbons were measured by calculating the I_D/I_G ratio. The I_D/I_G ratio of CP-UA, CP-N, and CP-NA samples was calculated as 1.06, 1.16, and 0.98, respectively. The decrease in the disorders after nitrogen doping and activation (CP-NA) indicates an increase in the graphitic structure after chemical activation of coffee. This could be due to increased nitrogen content within the carbon resulting in improved graphitic structure even after activation. The graphitization ratio of CP-NA was much higher than that of previously reported N, P co-doped activated coffee, and ZnCl₂ activated coffee shell samples. This improvement could be due to efficient nitrogen doping using melamine and activation using KOH. The surface morphology of coffee derived carbons was examined using scanning electron microscopy. As observed in Fig.4, the SEM images of the activated coffee derived carbon show improved porosity in structure resulting in higher surface area as compared to the unactivated sample. The effect of pre-carbonization, nitrogen doping, and activation, on the surface area and porosity of the carbon were analyzed using nitrogen adsorption/desorption isotherm (Fig. 5a). There was no porosity observed in CP-UA carbon, whereas, a slight improvement in porosity was observed in CP-N. Upon activation with KOH, a significant increase in porosity and surface area was observed. As the coffee carbon reacts with KOH, the reaction proceeds as following: 6KOH + 2C → 2K + 3H₂ + 2K₂CO₃ …. (1) The chemically activating agent KOH reacts with carbon to form potassium carbonate which can be easily decomposed and washed away using 1M HCl to obtain a highly porous carbon structure. Such carbon showed type I isotherm where a significant amount of nitrogen adsorption occurred at relatively low pressure followed by almost constant nitrogen adsorption at high pressures. Type I isotherm shows the characteristic behavior of a material with a microporous structure. The surface area of CP-UA, CP-N, CP-NA was observed to be 0.66, 60, and 1,824 m²/g, respectively. The pore size distribution (PSD) of these samples were examined using Barrett–Joyner–Halenda (BJH) plots (Fig.5b). The PSD plot for nitrogen-doped activated coffee derived carbon displayed a bimodal distribution of micropores and mesopores with maximum pores of the diameter of 2 nm. The t-plot microporous volume of CP-UA, CP-N, CP-NA was estimated to be 0.01, 0.025, and 0.529 cm³/g, respectively. XPS was used to further characterize the samples to determine their elemental composition and oxidation states. The C 1s spectra of all the samples are given in Fig.6A. The Gaussian fitted C 1s spectra of the samples show the presence of several peaks. The peak around 284.4 eV relates to the graphitic carbon (sp² carbon). While the other peaks are related to sp³ carbon, C-N bonding, and C-O bonding. The O 1s spectra of all the samples show the presence of peaks around 531.0, 532.5, 533.3 and 534.4 eV which corresponds to O=C, O-C, HO-C, and O-N, respectively (Fig. 6B). The N 1s XPS spectra of all the samples show peaks around 397.8, 399.5, 401.1, and 403.8 eV corresponding to pyridinic, pyrrolic, graphitic, and pyridine N oxide species, respectively (Fig. 6C). The relative atomic percentage of N 1s was observed to be 1.73, 10.18, and 2.80% for CP- UA, CP-N, and CP-NA, respectively. Table 1 shows the atomic content of coffee derived carbons and the percentage of various nitrogen species. Table 1: Atomic content of coffee derived carbons.

The electrochemical performance of all the coffee derived carbons was studied using cyclic voltammetry (CV) and galvanostatic charge-discharge (CD) measurements. Fig. 7a shows CV curves of all the three samples at 10 mV/s in 3M KOH electrolyte. The curves for CP-UA and CP- N present a slightly deviated behavior as compared to the ideal rectangular shape. While, CP-NA shows an almost ideally rectangular shape with significantly increased area under the curve, indicating its improved charge storage performance. This increase can be correlated to the increased surface area of the sample. After performing CV test at decreasing scan rates for CP- NA, it was observed that the curves attend close to a rectangle shape. This suggests that charge particles get sufficient time at a lower scan rate, to intercalated themselves within the pores of carbon (Fig.7b). This can be correlated to a steady increase in capacitance with a decreasing scan rate from Fig. 8. Specific capacitance (C) from the CV data was calculated using the following expression:

where A is the area under the CV curve, is the scan rate, ΔV is potential window and m is the

mass of the coffee derived carbons. The maximum specific capacitance of 150 F/g was achieved for CP-NA at 1 mV/s, with only 61% of reduction in capacity, at the higher scan rate. After activation of the nitrogen-doped coffee electrode, the specific capacitance increased 150%, suggesting that the activation method is valid. The galvanostatic charge-discharge behavior of all the carbons can be observed from Fig. 9. The carbon samples were charged with a potential window of 0.9 V at a sweeping current of 5 A/g. The CD curve of CP-NA carbon shows higher discharge time as compared to CP-UA and CP-N, suggesting higher charge storage capacity. The triangular shape of CD curves for all the samples suggest electric double layer type capacitive behavior (Fig.10). With decreasing current density, the discharge time increases suggesting enough time was available for absorption and desorption process over the electrode’s surface. The discharge time is directly related to the specific capacitance (C) of the electrode through the following equation: where I is the discharge cur

rent (A), Δt is the discharge time (s), ΔV is the potential window (V), and m is the mass (g) of the coffee derived carbons. The highest specific capacitance of 148.5 F/g was calculated for CP-NA sample at 0.5 A/g and given in Fig.9. The volumetric capacitance (in F/cm³) of the electrodes was also calculated (Fig.11). The highest volumetric capacitance of 51.6 F/cm³ was observed for CP-NA sample at a current density of 0.5 A/g. Only ~27% decrease in volumetric capacitance was observed on increasing current density from 0.5 A/g to 20 A/g. The resistivity of the samples was calculated using iR drops from galvanostatic charge-discharge measurements. The resistivity of CP-UA, CP-N, and CP-NA was observed to be 0.073, 0.22, and 1.8 Ohm/cm², respectively. It can be observed that CP-NA maintains 86.6% retention of initial capacitance upon increasing the current density to 10 A/g, suggesting higher rate capability as compared to previous results. For example, About 25% retention was observed for ZnCl₂ activated coffee carbon by Rufford et al, as the current density was increased from 0.5 A/g to 10 A/g. Higher rate capability can be correlated to higher surface area and graphitization of carbon upon nitrogen doping by melamine. Similar behavior was observed in the previous report where nitrogen-doped activated carbon sheets were synthesized using glucose as a carbon source via melamine-assisted chemical blowing and sequent KOH-activation method. The capacitance retention was observed to be 81.1% at a current density of 10 A/g. The higher capacitance value of CP-NA suggests its promising applicability in energy storage device applications. The energy and power density of the synthesized electrodes can be outlined using the Ragone plot (Fig.10). The energy density (E) and power density (P) of the coffee derived carbons were calculated using the expressions given below: (4) (5)

where C is the specific capacitance of the electrode determined using charge-discharge measurements, ΔV is the potential window (V), and t is the discharge time (s). The maximum energy and power density of CP-NA were calculated to be 16.3 Wh/kg and 5,545 W/kg, respectively. The obtained values were comparable to previously reported values. For example, mesoporous carbon using activate peanut shells showed a decrease in energy density from 6.18 Wh/kg to 0.83 Wh/kg as the current density increased from 0.05 A/g to 20 A/g. Liu et al. used one- step calcium chloride activation for sugar cane bagasse carbon along with urea, for the preparation of nitrogen-rich porous carbons. The power density of 4,892.5 W/kg was observed at an energy density of 4.58 Wh/kg. For the electrodes synthesized by chemical activation of coconut shell using ZnCl₂, the respective energy and power density of 7.6 Wh/kg and 4.5 kW/kg were observed. A variation in specific capacitance as a function of coffee derived carbon loading on the electrode was studied (Fig.12). The specific capacitance was observed to be increasing with the increase in the loading mass, reaching a maximum and then started decreasing with further increases in the loading mass. As observed in Fig.12, when the mass loading was increased from 2.9 mg/cm² to 12.37 mg/cm², the specific capacitance increased from 137 F/g to 177 F/g at a current density of 5 A/g. Our results suggest that the optimum loading of the active material on the electrode is desired to achieve higher charge storage capacity. The coin cell was fabricated using CP-NA electrodes as a supercapacitor device. Cyclic voltammetry and galvanostatic charge-discharge test were performed based on the weight of the material and area of the electrode in 6M KOH. The symmetry of CV curves at higher scan rates indicate uniform absorption and desorption even at a higher scan rate (Fig. 13). The specific capacitance of 32.1 F/g was observed at 300 mV/s, which increased to 71.3 F/g at 2 mV/s and, shown in Fig.13. The galvanostatic charge-discharge curves show symmetrical triangular shape suggesting the electric double-layer type of charge storage. The charge storage capacity increases with decreasing current density and increasing discharge time (Fig. 14). The highest specific capacitance of 74 F/g was observed at a current density of 1 A/g and about 72% retention was observed as the current was increased to 10 A/g, suggesting high rate performance of the CP-NA carbon using a two-electrode system (Fig. 14). The obtained capacitance value for the supercapacitor device is comparable to other derived activated carbon-based materials such as coffee, banana, tire, and jute, as given in Table 2. Table 2. Comparison of specific capacitance and capacitance retention of waste coffee derived carbon-based supercapacitor device with other reports.

The improved performance can be correlated to the high surface area and nitrogen doping in the carbon structure, resulting in improved capacitive performance. A small concentration of nitrogen can induce charged state within the carbon structure and improve charge transfer properties, thereby resulting in high capacitance values. In addition, the nitrogen doping created structural defects in the carbon which serves as active sites for storing charges. The energy and power density of the supercapacitor device can be demonstrated in the Ragone plot (Fig.15). The obtained energy and power densities were comparable to a three-electrode study performed in this study. The highest energy and power density for the device was analyzed to be 12.8 Wh/kg and 6,643 W/kg, respectively, which is among the best reports (Table 3). Table 3: Comparison of Energy and power density with previous carbon-based supercapacitor devices.

The stability of CP-NA supercapacitor device was performed using a galvanostatic charge- discharge test for 10,000 cycles (Fig.15). The stability test showed capacitance retention of about 97% and Coulombic efficiency of about 100%, suggesting long term stability of activated nitrogen-doped coffee carbon electrodes to be used for supercapacitor application. The study was further extended by connecting two devices in series and compared with a single device (Fig.15). The operating voltage can be doubled by coupling the two-singular device in series. Similarly, several devices can be coupled in series to form a battery of cells and satisfy a higher voltage need for real-time application. Conclusion: It can be concluded from this work, that nitrogen-doped activated carbon can be synthesized from waste coffee using a two-step chemical activation process. Nitrogen doping can be achieved by pre-treatment of the coffee powder with melamine, before the activation step. The synthesized carbon results in a very high surface area of 1,824 m²/g and show a higher graphitic phase of carbon, as confirmed from Raman spectroscopy. The XPS analysis showed elemental composition and oxidation states of N-doped coffee carbon with higher nitrogen content. The supercapacitor electrodes fabricated using N-doped coffee carbon showed a high specific capacitance of 148 F/g at a current density of 0.5 A/g. The symmetrical coin cells fabricated using these electrodes showed the highest energy and power density of 12.8 Wh/kg and 6.64 kW/kg, respectively. Moreover, the device maintains stable performance with capacitance retention of about 97% after 10,000 cycles and Coulombic efficiency of about 100%. These results suggest promising applicability of synthesized N-doped coffee carbon electrode for high-performance supercapacitors. References: 1. Huang et al. Wide Electrochemical Window of Supercapacitors from Coffee Bean-Derived Phosphorus-Rich Carbons. ChemSusChem 2013, 6, 2330. 2. Rufford et al. Electrochemistry Communications Double-Layer Capacitance of Waste Coffee Ground Activated Carbons in an Organic Electrolyte. Electrochem. commun.2009, 11, 974. 3. Mi et al. Coconut-Shell-Based Porous Carbons with a Tunable Micro/Mesopore Ratio for High- Performance Supercapacitors. Energy and Fuels 2012, 26, 5321. 4. Subramanian et al. Supercapacitors from Activated Carbon Derived from Banana Fibers. J. Phys. Chem. C 2007, 111, 7527. 5. Zhi et al. Effects of Pore Structure on Performance of an Activated-Carbon Supercapacitor Electrode Recycled from Scrap Waste Tires. ACS Sustain. Chem. Eng.2014, 2, 1592. 6. Zequine et al. High-Performance Flexible Supercapacitors Obtained via Recycled Jute: Bio- Waste to Energy Storage Approach. Sci. Rep.2017, 7, 1174. 7. Olivares-Marín et al. Cherry Stones as Precursor of Activated Carbons for Supercapacitors. Mater. Chem. Phys.2009, 114, 323. 8. Li et al. Top-down Approach for Fabricating Free-Standing Bio-Carbon Supercapacitor Electrodes with a Hierarchical Structure. Sci. Rep.2015, 5, 1. 9. Jeon et al. Controlling Porosity in Lignin-Derived Nanoporous Carbon for Supercapacitor Applications. ChemSusChem 2015, 8, 428. 10. Li et al. Template-Synthesized Hierarchical Porous Carbons from Bio-Oil with High Performance for Supercapacitor Electrodes. Fuel Process. Technol.2019, 192, 239. 11. Wahid et al. Enhanced Capacitance Retention in a Supercapacitor Made of Carbon from Sugarcane Bagasse by Hydrothermal Pretreatment. Energy and Fuels 2014, 28, 4233. 12. Hulicova-Jurcakova et al. Highly Stable Performance of Supercapacitors from Phosphorus- Enriched Carbons. J. Am. Chem. Soc.2009, 131, 5026. EXAMPLE 2 In this Example, chemically activated carbons were prepared using protocols described in Example 1 and various types of biomass: Bamboo fibers, banana peels, coconut husk, used coffee grounds, distiller grain, jute fibers recycled from a rope, hemp fiber recycled from a rope, orange peels, sorghum grain, used tea leaves. The material was observed to have similar performance to coffee grounds. Therefore, for the purposes of this study, nitrogen-doping was not carried out (only chemical activation). The materials were first dried in an oven at 100 °C to evaporate moisture. Except for fibrous materials, the dried materials were then ground into a powder. The dried fiber or powder was then subjected to pre-carbonization at 300 °C for 2 hrs. The powder was then chemically activated using KOH as a chemical activating agent. For this, the powder and KOH were mixed in designated ratios. The mixture was then heated from room temperature up to 800 ^oC. The heating process took approximately 2.5 hrs to reach temperature. The material was held at 800 ^oC for between 1 hr and 3 hrs under nitrogen atmosphere. The product was then allowed to cool to room temperature over a period of approximately 2 hrs to 3 hrs. The product was then washed with 1M HCl solution, followed by DI water, and dried to yield the chemically activated carbon from biomass. Various parameters were investigated to understand how changing certain variables in the process resulted in different properties. For example, the surface area of the chemically activated carbons from the biomass depends on the type of biomass used (Fig.16). The surface area of the chemically activated carbons can also be tuned by varying the ratio of biomass and chemical activating agents. In particular, increasing the ratio of the chemical activating agent increases the porosity of the chemically activated carbon. Variations in the length of the activation process and/or the temperature (range 600 °C to 1,000 °C) did not seem to have as much of an impact. As an example, using different KOH ratios, ranging from 1:0.5 biomass to KOH to 1:4 biomass to KOH, the surface area of banana peel derived carbon can be tuned from 0.2 m²/g to over 2,000 m²/g. Pre-carbonized banana peel derived carbon showed a surface area of 0.28 m²/g which increased to 1,878 m²/g upon activation with potassium hydroxide (1:4 biomass:KOH). As another example, the surface area of distiller grain from corn can be increased from 1 m²/g to over 2,600 m²/g. Pre-carbonized distiller grain based on corn showed a surface area of 1.84 m²/g, while after activating with 1:0.5, 1:1, 1:2, and 1:3 (corn grain distiller: KOH) showed a surface area of 1,074, 1,901, 2,593, and 2,603 m²/g, respectively. Supercapacitor devices were fabricated using the above biomass-derived activated carbon materials. In some of the tests, different aqueous, organic, or ionic electrolytes were used: KOH, tetraethylammonium tetrafluoroborate (TEATFB), LiPF6, 1-ethyl-3-methyl imidazolium tetrafluoroborate (EMITFB)], 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1- Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, and mixed electrolytes. The biomass-derived activated carbon was dispersed in the electrolyte(s) and deposited on different substrates. Substrates tested include nickel foam, stainless steel, copper, aluminum, carbon paper, and carbon cloth. A simple dip-coating method and doctor blade methods were applied to fabricate the electrodes. Various types of separators such as cellulose-based filter paper, glass fibers (GF), porous Teflon, and tissue papers were used to fabricate devices. The output potential can be tuned from 1.0 V to 3.5 V depending on the electrolyte used (Fig.17). As an example, a device fabricated using 6M KOH has a potential range of 0 V to 1.2 V. As a second example, a device fabricated using 1M TEATFB and 1M LiPF6 provides an output voltage in the range of 0 V to 2.4 V. As a third example, a device fabricated using a room- temperature ionic liquid, such as EMITFB, provides an output voltage in the range of 0 V to 3.5 V. Energy and power densities of supercapacitors fabricated using activated carbon from various biomass can be significantly enhanced by using different electrolytes (Fig.18). The output voltage of these devices can also be significantly enhanced by combining them in series. As an example, the output voltage of a coffee-based supercapacitor device can be enhanced from 2.4 V to 4.8 V by combining two devices in series (Fig.19). The CV curves of the devices fabricated using various biomass shows non-redox type behavior. The potential range depends on the type of electrolyte used. The rate capability of these devices varies depending on the type of biomass, separator, and electrolyte (Fig.20). The charge-discharge characteristics of the devices fabricated using various biomass show triangular behavior resembling a typical electrochemical double-layer type of behavior. As an example, shown in Fig.21, a device fabricated using activated carbon from orange peel can reach a voltage of 1.2 V in 6M KOH electrolyte, provides specific capacitance in the range of 60 F/g to 100 F/g, energy density in the range of 2 Wh/kg to 20 Wh/kg, and power density of 0.5 kW/kg to 10 kW/kg. As a second example, shown in Fig. 22, a device fabricated using activated carbon from distiller grain can reach a voltage of 2.4 V in 1M TEATFB as an electrolyte, provides specific capacitance in the range of 45 F/g to 75 F/g, energy density in the range of 20 Wh/kg to 60 Wh/kg, and power density of 0.1 kW/kg to 30 kW/kg. As a third example, shown in Fig. 23, a device fabricated using activated carbon from tea leaves can reach a voltage of 3.5 V in EMITFB electrolyte, provides specific capacitance in the range of 10 F/g to 75 F/g, energy density in the range of 5 Wh/kg to 130 Wh/kg, and power density of 0.1 kW/kg to 20 kW/kg. The energy storage devices fabricated using carbon from the different biomass show a range of electrochemical properties depending on applied charge-discharge current densities. As an example, shown in Fig.24(a), a device fabricated using activated carbon from various biomass shows specific capacitance in the range of 60 F/g to 95 F/g in 6M KOH, 30 F/g to 75 F/g in 1M TEATFB, and 9 F/g to 70 F/g in EMITFB at 3 A/g. As another example, shown in Fig.24(b), a device fabricated using activated carbon from various biomass shows energy density in the range of 10 Wh/kg to 20 Wh/kg in 6M KOH, 20 Wh/kg to 50 Wh/kg in 1M TEATFB, and 10 Wh/kg to 100 Wh/kg in EMITFB at 3 A/g. As another example, shown in Fig.24(c), a device fabricated using activated carbon from various biomass shows power density in the range of 1 kW/kg to 2 kW/kg in 6M KOH, 3 kW/kg to 4 kW/kg in 1M TEATFB, and 4 kW/kg to 5 kW/kg in EMITFB at 3 A/g. The energy storage devices fabricated using carbon from various biomass show high electrochemical stability performance studied using cyclic charge-discharge measurements. As an example, shown in Fig. 25(a), a device fabricated using activated carbon from sorghum grain showed almost 100% capacitance retention over 10,000 cycles of charge-discharge studies. The device was fabricated using 6M KOH and operated between 0 V to 1.2 V. Also, the impedance of such a device remains almost identical after the cyclic stability test (inset Fig.25(a)). As a second example, shown in Fig.25(b), a device fabricated using activated carbon from jute fibers retained over 72% of its initial charge storage capacity after 10,000 cycles of charge-discharge studies. The device was fabricated using 1M TEATFB and operated between 0 V to 2.4 V. The inset of Figure 25(b) shows the first few cycles of charge-discharge behavior. As another example, shown in Fig. 25(c), a device fabricated using activated carbon from distiller grain based on corn retained over 60% of its initial charge storage capacity after 10,000 cycles of charge-discharge studies. The device was fabricated using EMITFB and operated between 0 V to 3.5 V. The inset of Fig.25(c) shows the Coulombic efficiency of the devices measured during cyclic charge-discharge studies. The device maintained almost 100% Coulombic efficiency.

Claims

CLAIMS: 1. A method of preparing nitrogen-doped activated carbon from biomass, said method comprising: treating biomass with a source of nitrogen to yield nitrogen-doped biomass based carbon; and chemically activating the nitrogen-doped biomass based carbon with potassium hydroxide to yield nitrogen-doped activated carbon.

2. The method of claim 1, wherein said source of nitrogen is melamine, and said treating comprises soaking said biomass with melamine in water for up to about 1.5 hrs to yield melamine infused biomass.

3. The method of claim 2, wherein said biomass is mixed with said melamine in a mass ratio ranging from about 1:0.5 to about 1:4 biomass:melamine.

4. The method of claim 2, further comprising drying said melamine infused biomass, and calcining said dried melamine infused biomass for a sufficient period of time to yield said nitrogen- doped biomass based carbon.

5. The method of claim 4, further comprising grinding said dried melamine infused biomass into a powder before calcining.

6. The method of claim 1, wherein said chemically activating comprises mixing said nitrogen- doped biomass based carbon with potassium hydroxide in water, drying said mixture, and calcining said mixture for a sufficient period of time to yield said nitrogen-doped activated carbon.

7. The method of claim 6, further comprising washing said mixture with hydrochloric acid before drying.

8. The method of claim 1, wherein said biomass is selected from the group consisting of spent coffee grounds, bamboo fibers, banana peels, coconut husk, distiller grain, jute fibers recycled from a rope, hemp fiber recycled from a rope, orange peels, sorghum grain, used tea leaves, and combinations thereof.

9. An electrode for a supercapacitor, comprising: a porous metal substrate; and a layer of nitrogen-doped activated carbonized biomass preparing according to any one of claims 1-8 adjacent said substrate.

10. The electrode of claim 5, wherein the porous metal substrate is nickel, preferably nickel foam.

11. The electrode of claim 5, wherein the layer of nitrogen-doped activated carbonized coffee further comprises acetylene black, polyvinylidene difluoride, and/or N-methyl pyrrolidinone.

12. A method of preparing activated carbon from biomass, said method comprising: pre-carbonizing dried biomass by heating under nitrogen atmosphere to about 300 °C (+/- 50 °C) for approximately 1 hr to about 2 hrs to yield pre-carbonized biomass; and chemically activating and calcining the pre-carbonized biomass by mixing the pre- carbonized biomass with potassium hydroxide and heating under nitrogen or argon atmosphere to an activation temperature ranging from about 600 °C to about 1,200 °C for approximately 1 hr to 2 hrs to yield carbonized biomass; and washing said carbonized biomass with hydrochloric acid and water to yield said activated carbon from biomass.

13. The method of claim 12, wherein the mass ratio of biomass to potassium hydroxide in the mixture ranges from about 1:0.5 to about 1:4.

14. The method of claim 12, wherein the biomass and potassium hydroxide mixture is held at the activation temperature for between 1 hr and 4 hrs, and then allowed to cool to room temperature (~27 °C) over the course of approximately 1.5 hrs to 3.5 hrs.

15. The method of claim 12, wherein the activated carbon from biomass has an average pore diameter of less than 2 nm.

16. The method of claim 12, wherein the activated carbon from biomass has a surface area ranging from about 60 m²/g to about 2,000 m²/g.

17. The method of claim 12, further comprising treating said pre-carbonized biomass with a source of nitrogen to yield nitrogen-doped biomass based carbon prior to said activating, wherein said activating comprises mixing the nitrogen-doped biomass based carbon with said potassium hydroxide.

18. An electrode for an energy storage device, comprising: a porous metal substrate; and a layer of activated carbon from biomass preparing according to any one of claims 12-17 adjacent said substrate.

19. The electrode of claim 18, wherein said layer comprising said activated carbon from biomass mixed with second conductive carbon and a binder.

20. The electrode of claim 19, wherein said second conductive carbon is selected from the group consisting of carbon black, including acetylene black, and super P conducting carbon.

21. The electrode of claim 19, wherein said binder is selected from the group consisting of nafion, polyvinylidene difluoride (PVdF), polytetrafluoroethylene (PTFE), styrene butadiene rubber, sodium carboxymethyl cellulose, polyacrylonitrile, poly(ethylene oxide) and mixtures thereof.

22. An energy storage device comprising: a housing; a pair of activated carbon electrodes; a non-conductive separator between said pair of activated carbon electrodes; and an electrolytic liquid, wherein said activated carbon electrodes comprise an activated carbon from biomass prepared according to any one of claims 1-8 or 12-17.

23. The energy storage device of claim 22, wherein the non-conductive separator is selected from the group consisting of one or more layers of cellulose-based filter paper, glass fibers (GF), porous Teflon, tissue papers, and polymer membranes.

24. The energy storage device of claim 22, wherein said electrolytic liquid is selected from the group consisting of potassium hydroxide, sulfuric acid, sodium sulfate, tetraethylammonium tetrafluoroborate, lithium hexafluorophosphate, 1-ethyl-3-methyl imidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-Butyl-1-methylpyrrolidinium bis(trifluoromethyl-sulfonyl)imide, 1-Methyl-1-propylpyrrolidinium bis(trifluoromethyl- sulfonyl)imide. and mixtures thereof.

25. The energy storage device of claim 22, comprising two or more of said devices coupled in series within said housing.