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Journal of Power Sources 296 (2015) 433e439

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Journal of Power Sources

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Direct power generation from waste coffee groundsin a biomass fuel cell

Hansaem Jang a, Joey D. Ocon a, c, Seunghwa Lee a, Jae Kwang Lee b, Jaeyoung Lee a, b, *

a Electrochemical Reaction and Technology Laboratory (ERTL), School of Environmental Science and Engineering, Gwangju Institute of Science andTechnology (GIST), Gwangju, 500-712, South Koreab Ertl Center for Electrochemistry and Catalysis, Research Institute for Solar and Sustainable Energies, GIST, Gwangju, 500-712, South Koreac Laboratory of Electrochemical Engineering (LEE), Department of Chemical Engineering, College of Engineering, University of the Philippines Diliman, 1101,Philippines

h i g h l i g h t s

* Corresponding author. Electrochemical Reaction(ERTL), School of Environmental Science and EnginScience and Technology (GIST), Gwangju, 500-712, So

E-mail address: jaeyoung@gist.ac.kr (J. Lee).

http://dx.doi.org/10.1016/j.jpowsour.2015.07.0590378-7753/© 2015 Elsevier B.V. All rights reserved.

g r a p h i c a l a b s t r a c t

� Waste biomass is directly employedas a fuel with no any specialtreatment.

� Waste coffee ground is a fuel forSOFC-based carbon fuel celltechnology.

� Carbonization and gasification takeplace under experimentaltemperature.

� Produced in-situ gaseous compoundshighly enhance electrochemicalreaction.

a r t i c l e i n f o

Article history:Received 8 June 2015Received in revised form9 July 2015Accepted 19 July 2015Available online 1 August 2015

Keywords:Direct carbon fuel cellSolid oxide fuel cellCarbon fuel cellBiomassWaste coffee ground

a b s t r a c t

We demonstrate the possibility of direct power generation from waste coffee grounds (WCG) via high-temperature carbon fuel cell technology. At 900 �C, the WCG-powered fuel cell exhibits a maximumpower density that is twice than carbon black. Our results suggest that the heteroatoms and hydrogencontained in WCG are crucial in providing good cell performance due to its in-situ gasification, withoutany need for pre-reforming. As a first report on the use of coffee as a carbon-neutral fuel, this studyshows the potential of waste biomass (e.g. WCG) in sustainable electricity generation in fuel cells.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

The worldwide clamor towards less dependence on fossil fuels,due to the emission of greenhouse gases and energy security issues,

and Technology Laboratoryeering, Gwangju Institute ofuth Korea.

has led to the strong interest in using biomass energy [1]. As analternative, renewable energy source, biomass absorbs the sameamount of carbon dioxide (CO2) during plant growth, contributingless to global warming. The only remaining issue, however, is howto produce energy from biomass without competing with foodsupply over the use of arable lands [1]. As such, utilization of wastebiomass byproducts, especially from the food and beverage in-dustry, is key to solving this problem.

Fig. 1. Current-potential (j-V) and -power (j-P) curves of carbon fuel cells using(a) carbon black and (b) waste coffee grounds at different temperatures: - 750 �C, C800 �C, : 850 �C, and ; 900 �C.

Fig. 2. The measured high frequency resistance of carbon black (black solid line withcircles, C) and waste coffee grounds (red dash line with squares, -) (For interpre-tation of the references to color in this figure legend, the reader is referred to the webversion of this article.).

H. Jang et al. / Journal of Power Sources 296 (2015) 433e439434

Waste coffee grounds (WCG) are an abundant resource forbiomass-to-energy conversion technologies. The worldwide coffeeconsumption has steadily increased over the past decades, reachingan annual consumption of 8.8 million metric tons and leading toenormous amounts of organic wastes [2]. There have been variousattempts in using WCG but most of these methods generate by-products that should be discarded in landfills or eventually incin-erated [3e8]. For example, Sena da Fonseca et al. [3] found thatWCG can be added to clay ceramics to enhance the mechanicalstrength due to increased water absorption and porosity. Severalresearch groups [4e8] investigated the adsorption properties ofpolyhydroxy polyphenol functional groups in WCG and WCG-derived char on heavy metals. In addition, some researchers[9,10] studied WCG reforming with pyrolysis in order to producebio-oil, which can then be processed to synthesize high valuechemicals. Recently, biodiesel was produced from WCG [11,12],with the oil content, saponifiable lipids, and lipid profile varyingaccording to the regional location and brewing technique [12].

On the other hand, using WCG as fuel in a carbon fuel celltechnology with a solid oxide electrolyte renders multiple advan-tages. This electrochemical technology offers higher efficiencybecause it is not subject to Carnot limitations [13]. It mainly pro-duces CO2, which could be captured and reused, and a smallamount of ashes, from which metals and/or other materials couldbe retrieved [14,15]. Additionally, it does not require intermediateconversion steps or pretreatment, improving the overall processefficiency relative to conventional methods [16e19]. Biomass-based carbon fuel cells can be categorized by a fuel treatmentmethod: i. fuel as a carbonized biomass [14,15,20e22], ii. fuel as agasified biomass (similar technique to that of integrated gasifica-tion fuel cell, IGFC) [23e26], and iii. fuel as an untreated biomass[27]. While there have been previous studies using biomass-basedcarbon fuel cells as recapitulated above, this is the first report onthe performance of WCG-powered fuel cells, without the need forpre-treatment or gasification.

In this study, we showed the direct electrochemical oxidationperformance of WCG-powered anodes, in comparison with that ofcarbon black (CB). A detailed analysis on the chemical compositionand nature of WCG and carbonizedWCGwas performed in order toexplain its electrochemical behavior.

2. Experimental

2.1. Preparation of biomass fuel cells

Waste coffee grounds (WCG, Tanay Hills Coffee Beanery,Hazelnut Arabica, Philippines) were dried at room temperature forthree days (<50% relative humidity). Carbon black powder(ENSACO 350G, Timcal) was used as reference fuel due to its highcarbon purity. The commercially available anode-supported buttontype cell was fixed to a Pt (99.9%, 52 mesh, Alfa Aesar) currentcollector using Ag paste (Fujikura Kasei). The cell, which iscomposed of Ni-YSZ as anode, 8YSZ as electrolyte, and LSM ascathode, was air-sealed using sealants (Thermiculite 866, the USA;Aremco ceramabond 668) to prevent combustion of fuels duringthe electrochemical reaction.

2.2. Electrochemical characterization

The fuel cell experiments were performed using an optimallydesigned apparatus consisting of an alumina ceramic reactor, afurnace and an electrochemical workstation (NARA Cell-Tech). Thepower performance tests were carried out at different tempera-tures: 750 �C, 800 �C, 850 �C, and 900 �C. The reactor was heated upat a ramping rate of 5 �C min�1 up to the desired reaction

temperature. When the temperature reaches up to 600 �C fromroom temperature, the anode chamber was purged out by flushingpure Ar gas (99.999%) at a rate of 30 mL min�1 to eliminate internalO2, which could oxidize the fuel. Meanwhile, pure O2 (99.99%) gaswas continuously fed at a rate of 50 mL min�1 to the cathode side.

Table 1The proximate analysis of waste coffee grounds and carbon black as a reference,performed along with the method in ASTM D3172.

Sample Moisture/% as received

Volatile matter/% dry basis

Ash/% dry basis

Fixed carbon/% dry basis

Carbon black 2.02 3.46 0.03 96.50Waste coffee ground 6.09 84.86 1.29 13.85

H. Jang et al. / Journal of Power Sources 296 (2015) 433e439 435

The gases entering the reactor were preheated and kept at 100 �C toprevent a dramatic temperature drop, which could influence thecell resistance [28,29]. During operation, the internal cell resistancewas measured with an AC-impedance meter (3560 ACmUHiTester,Hioki) at 1 kHz.

2.3. Characterization

Proximate analysis (ASTM D3172) of WCG showed a highamount of volatile matter, which could be continuously ventilatedout by Ar gas up to 600 �C in the experiment. Hence, the carbon-izationwas performed at 950 �C under the presence of atmosphericoxygen without any inflow in order to foster a similar atmospherewith that in the reactor for characterizing and understanding thebehavior of the fuels. Characterization with FE-SEM (S-4700, Hita-chi) and XRD (MiniFlex II, Rigaku) exhibited that carbonization

Fig. 3. Scanning electron microscope (SEM) images of (a) carbon black, (b) coffee ground, (c)waste coffee ground and (f) 30-min-carbonized waste coffee ground.

process left a carbon backbone after volatilization of WCG.To better understand the gasification and oxidation reactions,

TGA was carried out in the experimental temperature range at aramp setting of 5 �C min�1 from room temperature to 900 �C. Toreflect the real experimental condition, TGA was done in air (TGA-50H, Shimadzu) and N2 gas (SDT Q600, TA Instruments Ltd.). Theresults of TGA indicated that theWCG experiment displayed uniquetrends, which were not found in the test with pure carbon. Thus,elemental analyzers were employed for estimating organic mattercomposition (CHN-S: Flash 2000, Thermo Fisher; O: EA-1110,Thermo Quest). The result showed the presence of hydrogen andheteroatoms even after carbonization, which implies various re-actions including gasification are available to occur in the range ofexperimental temperature since the carbonization was proceededat 950 �C. These various reactions could happen due to gasification;therefore, gas analysis was performed to figure out the gascomposition of the anode chamber at the experimental conditionand to identify the gasification reactions. WCG was placed in ahorizontal furnace and then heated at the same temperaturesetting with that in the electrochemical tests. The furnace was putin a vacuum condition and then was fed with Ar gas at a rate of50 mL min�1 until the sampling was finished. At the each pause,product gas was sampled and then analyzed with GC (7590A,Agilent Technologies)/MS (5975C, Agilent Technologies). In addi-tion, surface area analysis (Belsorp max, BEL Japan. INC.) was also

waste coffee ground, (d) 7-min-carbonized waste coffee ground, (e) 15-min-carbonized

Fig. 4. Mass loss curves of carbon black (black solid lines), waste coffee grounds (reddotted lines) and carbonized waste coffee grounds (red dashed lines), under (a) air and(b) N2 gas. The TGA measurements were taken at a ramping rate of 5 �C min�1 (Forinterpretation of the references to color in this figure legend, the reader is referred tothe web version of this article.).

H. Jang et al. / Journal of Power Sources 296 (2015) 433e439436

performed.

Fig. 5. The X-ray diffraction analysis of carbon black (green solid line), carbonizedwaste coffee grounds (red solid line) and waste coffee grounds overlapped withrepresentative monomers and bimer of lignin (black bar, 4-Methoxyphenol), cellulose(red bar, a-D-Glucose; green bar, Cellobiose) and hemicellulose (blue bar b-DL-Arabi-nose) (black solid line). (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

3. Results and discussion

In order to demonstrate the possibility of using WCG as fuel inhigh temperature carbon fuel cells, we assembled the cellscomprised of CB andWCG. Expectedly, the overall cell performanceof the carbon fuel cells improved with the increase in the operatingtemperature. WCG generally has higher operating voltages withincreasing current density than the CB, as seen in Fig. 1. Forinstance, WCG-powered fuel cells exhibited a power density of87.2 mW cm�2 at 900 �C, which is almost twice higher than CB-powered anodes (46.3 mW cm�2). In addition, the open circuitpotential (OCP) of WCG is noticeably higher than that of CB, whichcould suggest that not only carbon electrochemical oxidation oc-curs in the WCG-powered cells but other oxidation reactions aswell.

The predominance of anodic half reactions occurring at thetriple phase boundary (TPB), where fuel, electron-conducting ma-terial (current collector) and ion-conducting material (electrolyte)meet, strongly influences the carbon fuel cell performance [17]. Insolid fuels, however, the probability of forming these TPB reaction

sites is low relative to gaseous fuels. For instance, even in gaseousfuels, reactions can only happen in the adjacency of no more than10 mm [30e37]. For this reason, the indirect electrochemical carbonoxidation preceded by the reverse Boudouard reaction might havebeen more dominant than the direct carbon oxidation [17]. Inaddition, the production of gaseous products during fuel celloperation could lower the polarization resistance since the gaseousreactants can easily reach the active sites in contrast to solid carbonfuels.

The cell ohmic resistance was measured in the high frequencyregion (1 kHz) to investigate the possible reactions in the anode. Asshown in Fig. 2, the reverse Boudouard reaction was dominant inCB, evident from the dramatic drop in cell resistance as the reactortemperature was increased beyond 700 �C. The reverse Boudouardreaction spontaneously occurred over 800 �C, as seen from theconverging cell resistances of all samples. At this point, carbonmonoxide (CO) is the thermodynamically dominant species [38].On the other hand, WCG exhibited simultaneous gasification re-actions other than the reverse Boudouard reaction (e.g. pyrolysis) inthe temperature range considered.

In order to investigate the morphology, crystallinity, and gasi-fication tendencies of the samples, different characterizationtechniques (e.g. proximate analysis, scanning electron microscope(SEM), X-ray diffraction (XRD)) were used. The results (Table 1)showed that WCG contains more than 80% volatile matter, most ofwhich could be removed by Ar gas purging up to 600 �C while therest can be utilized as gaseous fuels. Furthermore, WCG has rela-tively low fixed carbon content (~14%) relative to carbon black(~97%), yet the former exhibited better fuel cell performance thanthe latter. To improve our understanding on the reactions in theanode, we also used carbonized WCG (cWCG) for comparison withthe pristine WCG, since the carbonization removed volatile mate-rials. It could be expected that the actualWCG-powered carbon fuelcells have characteristics in between to that of WCG and cWCG. Asshown in the SEM images in Fig. 3, theWCG samples have the samemorphologies, characterized by a honeycomb-shaped structure.Nonetheless, the removal of volatile materials resulted to asmoother surface and thinner walls of the structure in cWCG.

Table 2Elemental analysis of carbon black, waste coffee grounds and carbonized waste coffee grounds.

Sample Elementa

C H N S O

Carbon black 94.996 0.148 0 0 0.662Waste coffee ground 51.896 7.249 1.651 0.083 21.361Carbonized waste coffee ground 7 min 71.467 1.036 2.235 0 6.128

15 min 75.914 1.112 1.730 0 5.19330 min 75.574 0.618 1.719 0 5.176

a All values tabulated here are in weight percentages.

Fig. 7. Thermodynamically calculated mole fraction of CO (red dash line) and CO2

(black solid line) as a function of temperature at 1 atm (For interpretation of the ref-erences to color in this figure legend, the reader is referred to the web version of thisarticle.).

H. Jang et al. / Journal of Power Sources 296 (2015) 433e439 437

Thermal gravimetric analysis (TGA) under air condition wascarried out on CB,WCG, and cWCG. As shown in Fig. 4a, the onset ofCB oxidation into either carbon dioxide (CO2) or carbon monoxide(CO) occurred at around 550 �C. On the other hand, WCG wasreadily oxidized at a considerable lower temperature, with the gapbetween WCG and cWCG at 300e400 �C representing difference indegree of volatilization. While the XRD patterns of cWCG and CBare similar (Fig. 5), the oxidation initiation temperature of cWCG(350 �C) was notably lower than CB (550 �C). The heteroatoms andhydrogen contained in WCG and cWCG, which are low in concen-tration in CB, could lead to gasification reactions that form addi-tional oxidizable fuels. By heteroatoms, we refer to elements andcomponents aside from carbon and ash. As shown in the general-ized pyrolysis reaction below, the heteroatoms are crucial inexplaining the behavior of WCG and cWCG during carbon fuel celloperation [39].

CxHyOzðsÞ¼H2ðgÞþCH4ðgÞþC2H4ðgÞþCOðgÞþCO2ðgÞþCðsÞ (1)

In the same manner, the TGA under inert gas condition supportsthe observations above (Fig. 4b). Without any oxidants, CB wasstable, as shown by the absence of any mass loss up to 800 �C. Thisis consistent with the high stability of the almost heteroatom-freeCB in inert gas. In the region between 400 �C and 700 �C, bothcWCG and CB have parallel mass loss curves, with slightlydecreasing mass as the temperature was increased. Table 2 displaysthe difference in the elemental composition between CB, WCG, andcWCG at various carbonization times. After only 7 min of carbon-ization, the hydrogen content of WCG dropped from 7.25 wt.% to1.04 wt.%, which could lead to formation of water and small organicmolecules. Consequently, water could initiate the gasification (e.g.

Fig. 6. Gas composition of the gasified waste coffee ground as a function of the reactortemperature. The gasification products are CO (solid), CO2 (dash), CH4 (short dot), C2H4

(dash-dot), and C2H6 (dot).

water-gas shift reaction) [39,40] that improves the oxidation ki-netics and lowers the cell polarization. As observed in Table 2,sulfur seemed to be volatilized first, and then followed by the otherelements. Then, Ni-YSZ anode could deteriorate with sulfur, whichcan be utilized as a possible fuel in liquid tin anode solid oxide fuelcell system [41]. Thus, this system could be considered in biomassfuel cells in order to prevent the sulfur poisoning to anode surfaceand to enhance cell performance. In addition, carbon, hydrogen andoxygen are all involved in the gasification reactions, as shown in thechange in their concentrations, consistent with the reaction ineqn. (1).

Unlike CB, both WCG and cWCG were gasified due to the pres-ence of the intrinsic heteroatoms and its high hydrogen content. Tobetter understand the reactions in the fuel cell, WCG was gasifiedunder the same conditions, except that the gasification was carriedout in the absence of O2� ion supply. The presence of O2� ion duringgas production can further oxidize the gaseous products and thushide the effect of the intrinsic heteroatoms and hydrogen. The exit

Table 3Surface area of carbon black, waste coffee grounds and carbonized waste coffeegrounds.

Sample Analytic modela Surface area/m2 g�1

Carbon black BET eq. 782.23Waste coffee ground BET eq. 3.14Carbonized waste

coffee ground7 min Langmuir eq. 288.4715 min Langmuir eq. 511.2730 min Langmuir eq. 807.54

a A proper model is applied in terms of the shape of isotherm.

Fig. 8. The X-ray diffraction (XRD) analysis of ash retrieved fromwaste coffee grounds,including C MgO, : KCaPO4 and A K6Fe2O5.

Scheme 1. Illustration on a biomass fuel cell powered by waste coffee grounds (WCG).The gasification of the WCG components is crucial in the production of easily oxidiz-able gaseous fuels, leading to good electrochemical conversion in the carbon fuel cell.

H. Jang et al. / Journal of Power Sources 296 (2015) 433e439438

gas distribution during gasification depends mainly on the reactortemperature. The gaseous stream is composed of H2, O2, CO, CO2and various C1 to C3 hydrocarbons, as expected from the biomasspyrolysis reaction in eqn. (1). The yields of some of the pyrolysisproducts from WCG are plotted in Fig. 6. Hydrocarbon productswere predominant at 750 �C, proving that the hydrogen contentdifference between WCG and cWCG played a crucial role in thepyrolytic reactions. Upon reaching 800 �C, the sample seemed tohave been oxidized by the elemental oxygen within the sample.Furthermore, at higher temperatures, the CO/CO2 ratio increasedand became well-matched with the thermodynamically computedCOeCO2 speciation (Fig. 7). Regardless of the reactor temperature,formation of oxidizable gases (e.g. CO and hydrocarbons) providedadditional fuel in the electrochemical system. This is advantageousdue to the higher accessibility of the gaseous reactants to the TPBs,leading to sustained electrochemical oxidation in the cell. This isreflected in the decrease in the anodic polarization and lowering ofthe cell resistance in Fig. 2.

The gasification reactions in Fig. 4b occurred vigorously attemperatures higher than 800 �C, resulting to a steeper mass lossslope in cWCG than in WCG. This is due to the difference in thespecific surface area (SSA) of the samples, as shown in Table 3.During carbonization, the SSA of WCG sharply increased with heat-treatment time. For instance, the SSA of WCG (807.54 m2 g�1) ap-proaches that of CB (782.23 m2 g�1) after only 30 min of carbon-ization. Thus, increased SSA from gasification could provide morereaction sites between the solid fuels and gases.

Unlike the ash contents of most carbon materials [17], theamount of ash in WCG is more than 1% (Table 1) and the ash con-sists not of silicate but mostly of MgO, KCaPO4 and K6Fe2O5 (Fig. 8

Table 4Energy-dispersive X-ray spectroscopy of carbon black, coffee grounds, waste coffee grou

Samplea C O S

Carbon black (Timcal 350G) 97.0897.79

2.922.21

Coffee ground 91.1394.43

5.484.27

1.160.45

Waste coffee ground 89.7293.36

6.835.34

0.720.28

Carbonized waste coffee ground 91.6694.42

6.124.73

0.060.02

a The upper values in each space are in wt.%-basis and the lower ones are at.%-basis.

and Table 4). The impact of these minerals in the fuel cell opera-tion should be considered in future studies. Generally, the presenceof ash in solid oxide fuel cells is not as serious as that in moltencarbonate fuel cells (MCFC) due to the easier separation of ash [17].Nevertheless, the accumulation of ash onto anode surface and porecould eventually lead to fuel cell activity degradation by hidingTPBs.

4. Conclusion

In this paper, we first report on the characteristics and perfor-mance of waste coffee grounds (WCG) as fuel in carbon fuel celltechnology. At a cell operating temperature of 900 �C, WCG-powered fuel cells exhibited a maximum power density that isalmost twice than that of carbon black. Using various character-ization techniques (e.g. TGA, XRD, GCeMS), we show that theheteroatoms and hydrogen initially contained in WCG were crucialin the in-situ gasification of WCG to produce electrochemicallyoxidizable products (i.e. H2, CO and hydrocarbons), see Scheme 1.The gaseous products of gasification could then easily access theTPB reaction sites, resulting to decreased cell polarization. While itwould be relevant to study in detail the gasification reactions inWCG-powered fuel cells, the complexity of gasification reactions inthis setting is limiting. Future studies should focus in looking at thepossible effects of the ash composition inWCG in its long-term fuel

nds and carbonized waste coffee grounds (TEM-EDS; Oxford, INCAx, England).

Mg P K Ca Fe

0.640.33

0.350.14

0.750.24

0.450.14

0.030.01

0.620.32

0.390.16

0.750.24

0.950.30

0.010.00

0.560.29

0.430.17

0.940.30

0.230.07

0.010.00

H. Jang et al. / Journal of Power Sources 296 (2015) 433e439 439

cell performance. Nevertheless, this work provides sufficient evi-dence that WCG can indeed be used as fuels in a high temperaturefuel cell technology.

Acknowledgment

This work was supported by the New & Renewable EnergyDevelopment Program of the Korea Institute of Energy TechnologyEvaluation and Planning (KETEP) grant funded by the Korea Gov-ernment Ministry of Knowledge Economy (No. 20113020030010).

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