electrochimica acta 54 (2009) 7444–7451

Electrochimica Acta 54 (2009) 74447451

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Electrochimica Acta

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Prepar gerecharg

MojtabaDepartment of , UK

a r t i c l

Article history:Received 15 AReceived in reAccepted 28 JuAvailable onlin

Keywords:RechargeableLithiumOxygenCarbonActivationPorosityStorage capaci

by pbonized byAr antive mlectrofounhologstorae size

1. Introduction

Increasehave led tosuch as theSince theseas energy deto portablestorage sysperformancrequiremen

Becausethe forefronof energy ptionof the cbattery canference betof the reactby ensuringof the batte

Lithiumtronegativitexisting me

CorresponE-mail add

positive ions [3]. Owing to these properties, lithiumbased batterieshave a high energy density, high specic energy and high operating

0013-4686/$ doi:10.1016/j.in the price of fossil fuels and their carbon foot printsthe utilization of cheap and renewable energy sourceswind and solar power as alternative energy supplies.energy sources are intermittent and on the other handmand for awide range of applications frommicrochipselectronics to hybrid electric vehicles increases, energytems play a more signicant role in our lives and theires have to be further improved to meet the higherts of future systems.of their inherent simplicity in concept batteries are att of the electrical energy storage systems. The amounter mass or volume that a battery can deliver is a func-ells voltage and capacity. The stored energy content of abe maximized by having a large chemical potential dif-ween the two electrodes; making the mass (or volume)ants per exchanged electron as small as possible; andthat the electrolyte is not consumed in the chemistryry [1].is an extremely light metal and has the lowest elec-y of the standard cell potential (3.045V) in thetals [2]. It can donate electrons the most easily to form

ding author. Tel.: +44 141 548 4084; fax: +44 552 2302.ress: [email protected] (P.J. Hall).

voltage, and are expected as the potential power sources for manyapplications [4].

Commercial rechargeable lithium batteries use lithium tran-sition metal oxides, typically LiCoO2, as cathode and graphite asanode [5]. Their specic capacity depends on the capacities of boththe cathode and anodematerials. Graphite has a theoretical capac-ity of 372mAhg1 [6]. Energy storage in these batteries is limitedby the cathode and does not exceed 200mAhg1 [7].

The capacity of a lithium battery system can be enhancedremarkably by using a completely different approach which com-bines Li as anode directly with oxygen as cathode active materialin a Li/oxygen cell [7,8]. Oxygen accessed from the environmentis reduced catalytically on an air electrode surface to form eitheran oxide or peroxide ion. The catalytically formed anions reactwith lithium cations supplied by the anode and delivered bythe electrolyte to form Li2O2 on the air electrode surface duringdischarge process. Oxygen, the unlimited cathode active mate-rial, is not stored in the battery and therefore the theoreticalspecic energy of the Li/O2 cell (11400Whkg1 excluded and5200Whkg1 included oxygen) is the highest among variousmetal/oxygen batteries and is stillmuch larger than that achievableusing a conventional lithium ion battery [8].

The O2 electrode in lithium/oxygen batteries is a carbonelectrode having a porous structure in which several electrochem-ical and transport processes occur simultaneously. The porous

see front matter 2009 Elsevier Ltd. All rights reserved.electacta.2009.07.079ation of controlled porosity carbon aeroeable lithium oxygen batteries

Mirzaeian, Peter J. Hall

Chemical and Process Engineering, University of Strathclyde, Glasgow, G1 1XJ, Scotland

e i n f o

pril 2009vised form 27 July 2009ly 2009e 5 August 2009

battery

ty

a b s t r a c t

Porous carbon aerogels are preparedby sodium carbonate followed by carstructure of carbon aerogels is adjustpreparation and also pyrolysis underThe prepared carbons are used as acelectrochemical performance of the echarge/discharge measurements, it ischarge voltage) depends on the morpand surface area of carbon affects thevolume (2.195 cm3/g) and a wide por/ locate /e lec tac ta

ls for energy storage in

olycondensation of resorcinol and formaldehyde catalyzedation of the resultant aerogels in an inert atmosphere. Porechanging the molar ratio of resorcinol to catalyst during gel

d activation under CO2 atmosphere at different temperatures.aterials in fabrication of composite carbon electrodes. Thedes has been tested in a Li/O2 cell. Through the galvanostaticd that the cell performance (i.e. discharge capacity and dis-y of carbon and a combined effect of pore volume, pore size

ge capacity. A Li/O2 cell using the carbon with the largest pore(14.23nm) showed a specic capacity of 1290mAhg1.

2009 Elsevier Ltd. All rights reserved.

M. Mirzaeian, P.J. Hall / Electrochimica Acta 54 (2009) 74447451 7445

structure acts as gas transport pores for the diffusion of oxygento the carbon-electrolyte interface (reaction zone), formation andstorageof Li2O2 during thedischargeprocess andalso electrochem-ical decomposition of Li2O2 during the charge process. The kineticsof oxygen diffusion through the cathode limits the performance ofthe Li/oxygdischarge oor choked bmedia for thduring charthe porouseter for ene

This shothe morphochallenge ielectrodes tthe capacitthe energyadvances arwhich could

The usecatalyzed eHence, theture cathodbatteries. Mgen electrocarbon, witsurface areformance oby galvanoequipment.

2. Experim

2.1. Synthe

RF aerogcinol [C6H4to the metwere prepaformaldehytilled wateratio of R tthe molarRF solutionevaporation72h at 363gels have stheir structin acetonethe hydrogdays.

2.2. TGA an

Thermogpyrolysis cosystem for atem. Typicaalumina pato the TGA.for sample.for 30min aheated to 1weight loss

2.3. Carbonization of R/F aerogels

To study the effect of pyrolysis conditions on the porous struc-ture of carbon aerogels, pyrolysis of the dried RF aerogels wasperformed

ing ml wasminm. Tat 42ld fore atrformnace

tivat

actitemmic br owa

in1.pt atthenmin

ateri

porterizmsmerior tofacer mealys

e ofutionof th

ll con

hodel sams biFP 811/1[8,10e andstir fthi

he acmaketroche cor, aoakeode eminrts wt is gAl mosphth a pin pen cells [9]. It has been also discussed that the end-of-f the cell is reached when the carbon pores are lledy the deposition of Li2O2 [7,10]. Electrolyte is the onlye transfer of lithium ions between cathode and anodege/discharge and therefore diffusion of electrolyte intostructure of the electrode is another important param-rgy storage in lithium/oxygen batteries.ws that the cell performance strongly depends onlogy and structure of the carbon. Therefore the mainn this issue is the development of new carbon basedo improve the kinetics of the air cathode and enhancey; energy and power densities; and the stability ofdelivered by these systems. Engineering and chemicale also required to prevent the access of CO2 and H2Oreact with either Li2O2 or lithium metal.of nanomaterials makes it possible to design porouslectrodes with improved kinetics and energy efciency.objective of the present work is to prepare nanostruc-e electrodes based on R-F carbon aerogels for Li/O2any parameters inuence the performance of oxy-de. In this work the role of porous structure of theh emphasize on the activation process, porosity anda measurements is explored. The electrochemical per-f the carbon aerogel based electrodes is examinedstatic charge/discharge tests using a Solartron 1470

ental

sis of R/F aerogels

els were synthesized by the polycondensation of resor-(OH)2] (R) with formaldehyde [HCHO] (F) accordinghod proposed by Pekala et al. [11,12]. RF solutionsred by mixing the required amount of resorcinol andde, and sodium carbonate as catalyst (C) and dis-r (W) under vigorous stirring for 45min. The molaro F (R/F) and ratio of R/W (g/ml) were constant andratio of R to C (R/C) was varied. The homogeneouss were poured into sealed glass vials to prevent theof water during gelation and gelled by aging forK into an oven to obtain RF hydrogels. The hydro-tructures lled with water. To remove water fromures, the gels were solvent exchanged by immersingfor three days prior to the vacuum drying. Finally,els were dried in a vacuum oven at 353K for 3

alysis of aerogels

ravimetric analysis (TGA) was used to determine thenditions for the dried aerogels. The thermal analysisnalysis of aerogel samples was a Mettler TGA/DSC sys-lly between 5 and 10mg of the gel was weighed in ann and placed on the horizontal microbalance connectedThis weight corresponded to a weight percent of 100%The sample was purged with Ar owing at 50mlmin1

t 303K to remove oxygen from the system. It was then273K at 10Kmin1 under Ar and the variation of thewith temperature was determined.

followThe ge200mlprogratainedand heperatuwas pethe fur

2.4. Ac

Theferenta cerawith Asample10Kmwas keIt was200ml

2.5. M

Thecharacisother(Micro2h prifor surused fopore anvolumdistribbranch

2.6. Ce

Cataeroge2801 aPVDF/Hratios:whereacetonleft to200muntil tcut to

Elecbox. Tcollectrator sa cathlow alucell pacontacfor theO2 atmder wiof 30mat different temperatures in a tubular furnace using theethod: 3 g of dried gel was used in each experiment.placed in a ceramic boat and purged with Ar owing at

1 for 30min at room temperature prior to the heatinghe furnace was heated to 423K at 5Kmin1 and main-3K for 30min. Then it was heated to 673K at 5Kmin1

r 30min. It was then heated to the nal pyrolysis tem-10Kmin1 and kept for 3h. Thewhole heating programed under Ar owing at 200mlmin1. After pyrolysis,was cooled in owing Ar to room temperature.

ion of carbon aerogels

vated carbon aerogels were prepared under CO2 at dif-peratures. A sample of carbon aerogel was placed inoat at the centre of the tubular furnace and purged

wing at 200mlmin1 for 30min prior to heating. Thes heated to the appropriate activation temperature atAt this stage gas was changed to CO2 and the samplethe activation temperature under CO2 for varying time.cooled down to room temperature under Ar owing at1.

al characterization

osity of RF areogels and all carbon aerogels wased by the analysis of nitrogen adsorptiondesorptionmeasured by an ASAP 2420 adsorption analyzertics) at 77K. The samples were evacuated at 373K forthe adsorption measurements. BET method was usedarea measurements, BJH adsorptionsdesorption wassopore analysis and t-plot method was used for micro-is. Total pore volumewas calculated from the adsorbednitrogen at P/P0 = 0.99 (saturation pressure). Pore sizes were obtained by the BJH method from adsorptione isotherms [13].

struction and electrochemical measurements

electrodes were prepared by mixing porous carbonple, electrolytic manganese dioxide (EMD), Kynar Flexnder (polyvinylidene diuoride-hexauoropropylene;8/12 by weight) and propylene carbonate (PC) (weight9/15/55) according to the procedure described else-]. The materials were placed in a glass bottle to whicha stirrer barwere added and the resultingmixturewas

or 4h to form a paste. Then the paste was spread into ack lm by an applicator. The electrode was untouchedetone evaporates. Disks of 1.3 cm diameter were thendisk-like cathode electrodes.

hemical cell was constructed in an argon-lled gloveell consists of a stainless steel bar as anode currentlithium metal foil as anode, a glass micro-ber sepa-d with electrolyte (1M LiPF6 in propylene carbonate),lectrode, an aluminum mesh and a spring and a hol-um bar on the top of it as cathode current collector. Theere compressed together to ensure that the electrodeood and then the cell was completely sealed exceptesh window that exposes the porous cathode to theere. The cell then was exposed to O2, from O2 cylin-urity of 99.5%, at atmospheric pressure for a minimumrior to the electrochemical tests and all measurements

7446 M. Mirzaeian, P.J. Hall / Electrochimica Acta 54 (2009) 74447451

Table 1Surface characteristics of aerogel samples.

Sample R/F R/C R/W (g/ml) SBET (m2/g) Vtotal (cm3/g) Vmicro (cm3/g) Vmeso (cm3/g) %Vmicro %Vmeso Davg (nm)

RF001 0.50 100 0.1 278 0.1797 0.1189 0.0608 66 34 2.58RF002 0.50 200 0.1 417 0.5301 0.1801 0.350 33 67 5.10RF003 0.50 500 0.1 149 0.632 0.0075 0.6245 2 98 16.96RF004 0.50 600 0.1 182 1.0631 0.0132 1.0499 1.2 98.8 23.34

were carried out under O2 atmosphere at 298K. Galvanostaticcharge/discharge tests were performed by a Solartron 1470 poten-tiostat by applying a constant current in the voltage range between2 and 4.5V.

3. Results and discussion

3.1. BET analysis of RF aerogels

Fig. 1 shows N2 adsorptiondesorption isotherms at 77K for RFaerogelswith different R/C ratios. All isotherms are type IV showingadsorption hysteresis which indicates the presence of mesopores[13]. For RF001 and RF002 samples with R/C=100 and 200 the ini-tial region of the isotherms experiences a sharper rise at low P/P0indicating the presence of micropores in addition to mesopores inthe gel structure.

Hysteres600 in the Pin their strusent the llthe emptyiisotherms aincrease in Rity and formhigher R/C r

The porein Fig. 2. ThPSD with asample witto pores ofR/C=500 anand show taround 22nship betwedata in Tabume and m

to 600. The mean pore size markedly increases with the R/Cratio.

Tamon et al. have shown that during the polycondensation pro-cess, resorcinol and formaldehyde are consumed to form highlycross-linked particles. The aggregated particles become intercon-nected after gelation showing a structure composed of rings ofparticleswith 2050 particles per ring [15]. The particle size rangesfrom 3 to 20nm and increases with R/C ratio [16]. Hence the R/Cratio is the principal parameter that controls the size of inter-connected particles leading the scale and size of pores in the gelstructure.

3.2. Effect of pyrolysis conditions on the porous structure of thegels

Fig. 3 shows adsorptiondesorption isotherms of nitrogen atRF001 aerogel with R/C ratio of 100 and CRF001-x carbon

ls prher

ms screas3K.es wthetionatess duise aative3 als, thesedwl. Thitemls wiis loops for RF003 and RF004 gels with R/C=500 and/P0 range 0.21.0 show that mesoporosity is dominantctures. The lower part of the hysteresis loops repre-ing of the mesopores while the upper parts representng of the mesopores [13,14]. It can be seen that there shifted to the right at higher relative pressures with/C ratio. This indicates the development ofmesoporos-ation of larger mesopores during gelation process atatios.size distribution (PSD) of RF gel samples are showne RF001 sample with R/C=100 shows the narrowestmaxima around pores of 2nm diameter. For RF002

h R/C=200 the PSD is wider and the maxima is shifted7nm diameter. For RF003 and RF004 samples withd 600 the pore size distribution curves are even widerhat the maxima are shifted to the right and locatedm and 40nm, respectively. This shows the relation-en mesoporous structure and R/C ratio clearly. Thele 1 show a signicant increase in total pore vol-esopose volume when R/C ratio increases from 100

77K onaerogetures wisothergels inT107increas

Asbonizaand cremattersharp ris indic

Fig.1073Kdecreaaerogeat highaerogeFig. 1. N2 adsorptiondesorption isotherms at 77K.Fig. 2. Pore sizVarious R/C raepared by pyrolysis of the gel at different tempera-e x is pyrolysis temperature. Adsorptiondesorptionuggest that the amounts of N2 adsorbed by carbon aero-es with increase in the carbonization temperature forThese results mean that pore volume of carbon gelsith increasing the temperature in this range.carbonization temperature increases to 1073K, car-processmodies the porous structure of the original gelmore micropores in the structure by releasing volatilering carbonization. Evidence for this comes from thet the initial region of the isotherms (P/P0 < 0.01) whichof the presence of micropores in carbon aerogels.o shows that for the carbonization temperature aboveamount of nitrogen adsorbed by carbon aerogelshich shows adecrease in the pore volumeof the carbons is probably due to the collapse of the porous structureperatures. Changes in the pore parameters of the carbonth heat treatment temperatures are listed in Table 2.e distribution of RF gels with for RF aerogels with different R/C ratios.tios.


Table 2Surface characteristics of the RF001 aerogel and CRF001carbon aerogels.

Sample Temperature (K) SBET (m2/g) Vtotal (cm3/g) Vmicro (cm3/g) Vmeso (cm3/g) %Vmicro %Vmeso Davg (nm)

RF001 278 0.1797 0.1189 0.0608 66 34 2.58CRF001-873K 873 286 0.1870 0.1484 0.0386 79 21 2.33CRF001-973K 973 524 0.2874 0.2625 0.0249 91 9 2.27CRF001-1073K 1073 669 0.3800 0.3300 0.0500 86 14 2.19CRF001-1273K 1273 520 0.2905 0.2603 0.0302 89 11 2.23

The datatotal pore vtion tempeof carbon atotal pore vmaximumshow thatdevelopme

Fig. 4 shCRF001carbshow thatpores with

Fig. 4. Pore sisamples.

can be seen that the PSD curves are shifted to smaller diametersafter carbonization probably due to the shrinkage of the aerogelstructure during the carbonization process.

Data in Table 2 suggest that decrease in the mesoprosity of thegel is at the expense of an increase in microporosity during car-bonization.

Fig. 5 shows the weight loss versus temperature and deriva-tive plots for a dried resorcinol formaldehyde gel. The TGA plot(Fig. 5a) shows that the sample mass does not change with tem-peratures above 1073K and the derivative plot (Fig. 5b) shows thatthe signicantmass losses occur at about 423, 673 and 873K. Thesepeaks are due to the desorption of water and carbonization reac-tions involving breakage of CO bonds at 673K and CH bonds at873K, respectively [17]. The TGA shows that above 1073K there isno further weight loss.

Since wer porls ca

fect o

adsorepain Fiisotm, inpart oteauF002Fig. 3. N2 adsorptiondesorption isotherms at 77K.

given in Table 2 shows that both total surface area andolume of the carbon aerogels increase with carboniza-rature for temperatures below 1073K. Heat treatmenterogel above 1073K decreases both surface area andolume. Carbon aerogel prepared at 1073K showed theBET surface area and total pore volume. These resultsa temperature of 1073K is the most effective for the

ture foaeroge

3.3. Ef

Thebons pshown

Theisotherinitialthe plaFor CRnt of the pore structure of the gel.ows the pore size distribution for RF001 aerogel andonspreparedatdifferent temperatures. ThePSDcurvescarbonization process decreases volume of pores fordiameters in the range of 3 to 10nm signicantly. It

ze distribution of RF001 and for RF001and CRF001 samples. CRF001

(P/P0 < 0.01ores in theindicative oto the prese

In the caing and em(i.e. 0.71.0

Fig. 5. Thermotion temperathave found that 1073K is the most effective tempera-osity the effect of R/C ratio on pore structure of carbonn be investigated.

f R/C ratio on the porous structure of carbons

rptiondesorption isotherms of nitrogen at 77Kon car-red from aerogels with different R/C ratios at 1073K areg. 6.herms for CRF001 carbon (R/C=100) show a type Idicating that this sample is microporous [13,14]. Thef isotherm showsmicropore lling and the low slope ofis due to multilayer adsorption on the external surface.sample (R/C=200) the initial region of the isotherms

) experience a sharp rise which is indicative of microp-sample. The hysteresis loop in the P/P0 range 0.40.8 isfmesoporosity in the structure of the carbon in additionnce of micropores.se of CRF003 sample (R/C=500) hysteresis loop for ll-ptying of mesopores is shifted to a higher P/P0 range) which is associated with the presence of mesoporesgravimetric analysis of a dried RF aerogel. (a)Weight % vs. carboniza-ure. (b) Derivative thermogravimetry.


Table 3Porous parameters of carbon samples.

Sample R/C SBET (m2/g) Vtotal (cm3/g) Vmicro (cm3/g) Vmeso (cm3/g) %Vmicro %Vmeso Davg (nm)

CRF001 100 669 0.3800 0.3300 0.0500 86 14 2.19CRF002 200 702 0.714 0.183 0.531 26 74 6.59CRF003 500 647 1.2245 0.2101 1.0144 17 83 21.65CRF004 600 576 1.2779 0.2012 1.0767 16 84 25.27

with largerratio of 600of even larg

Fig. 7 ShCRF001 exhand there idiameters a

The relaobserved thlyst (R/C rattotal pore vcarbon aero2.19nm to

Fig. 7. Pore sidifferent R/C r

porous structure and especially the pore volume and the pore sizedistribution of the aerogels and carbon aerogels can be adjusted bychanging the R/C ratio of the gel precursors.

3.4. Effect of activation on the porous structure of carbon aerogels

Adsorptiondesorption isothermsof nitrogen at 77K for CRF002carbon aerogel and ACRF002 activated carbon aerogels obtained byCO2 activation of CRF002 carbon aerogel at different temperaturesfor varying times are shown in Fig. 8. Activated carbons are namedas ACRF002-x where x is activation temperature.

The adsorptiondesorption isotherms show a steep increase inthe amount of gas adsorbed at P/P0 < 0.01. This shows that largequantity of micropores have been introduced into the carbon aero-gel after physical activation [18]. This might be due to the creationof new micropores by oxidation of certain structural componentsand opening of previously inaccessible pores [19].

teresorosent tthe esteteed vcreasorbety inesis l) indpore9 alssameFig. 6. N2 adsorptiondesorption isotherms at 77K.

diameters in the structure. For CRF004 carbon with R/Chysteresis loop is in the P/P0 range 0.821.0 indicativeer mesopores in the carbon structure.ows the PSD curves for the carbon samples. Sampleibits a micropore peak at the pore diameter of 2nms a shift of pore size distribution to pores with largers R/C ratio increases.ted porous parameters are listed in Table 3. It can beat with increase of themolar ratio of resorcinol to cata-

Hysmesoprepresresentthat hyadsorbthat ingas adporosihyster0.40.9size of

Fig.at theio) from 100 to 600 in the initial gel precursors both theolume and the average pore diameter of the resultantgels increase from 0.38 cm3 g1 to 1.2779 cm3 g1 and25.27nm, respectively. These results indicate that the

ze distribution of carbons with for carbons with different R/C ratios.atios.

carbon samilar pore sicarbon samin Table 4. Iof pores foris loops in the P/P0 range 0.40.9 are indicative ofity in carbons. The lower part of the hysteresis loopshe lling of the mesopores while the upper parts rep-mptying of the mesopores [13,14]. It can be observedsis loops for activated carbons shift vertically to higherolumes at higher activation temperatures. This meanssing the degree of burn off increases the amount ofd by sample which is indicative of the development ofthe carbon structure. For all isotherms shown in Fig. 8oops are located almost over the same P/P0 range (i.e.icating that the activation process does not affect thes and pore size distribution signicantly.o show that for all samples, the PSD curves are centeredpore diameter range (i.e. around 10nm) indicating allples aremainlymesoporousmaterials and possess sim-zes and pore size distributions. Porous parameters ofples activated at different degrees of burn off are givent can be observed that although the average diametersall samples are similar, there is a signicant increaseFig. 8. N2 adsorptiondesorption isotherms at 77K.


Table 4Porous parameters of CRF002 carbon samples activated at different degrees of burn off.

Sample Time (min) Burn off % SBET (m2/g) Vtotal (cm3/g) Vmicro (cm3/g) Vmeso (cm3/g) %Vmicro %Vmeso Davg (nm)

CRF002 .183 0.531 26 74 6.59ACRF002- 10 .282 0.573 33 67 6.47ACRF002- 11 .450 0.87 31 69 6.10ACRF002- 11 .431 0.851 34 66 5.85ACRF002- 12 .369 0.746 33 67 6.15ACRF002- 12 .427 1.007 30 70 5.60

Fig. 9. Pore situres. carbons

in pore volof two samin the adsoincreased. Tbe due to th[20,21].

The porat 1273K, ghigher thanthe collapsevolume and

Fig. 9 shothe range oing to thesepore volumdue to the cof more me

In orderand highertemperaturACRF003-xingN2 adsoisotherms sthe amountreatment raverage potakes place20nm).

Data givlarge increavolumes an

Resultscarbon aero

molo cartempmateanc 0 702 0.714 073K 180 15.72 946 0.855 023K 120 43.76 1545 1.320 073K 60 49.82 1531 1.282 023K 30 65.40 1290 1.115 073K 20 67.00 1725 1.434 0

ze distribution of ACRF002 for ACRF002 carbons at various tempera-at various temperatures.

ume of the samples with burn off. Porous parametersples activated at 1173K and 1223K show a decreaserption capacity even though the degree of burn off hashe decrease in pore volume and BET surface area may

ing theand alsferentactiveperforme pore blockage and collapse at this temperature range

ous parameters of ACRF002- 1273K carbon activatediven in Table 4 show that activation at a temperature1223K results in a higher degree of burn off, openingd and blocked porous structure and increasing the poreBET area signicantly.ws that theporosity developmentmainly takesplace inf small mesopores (i.e. pore diameter10nm). Accord-results, increase in the degree of burn off increases thee, without affecting the mean pore diameter probablyreation and widening of micropores and also openingsopores.to prepare carbon samples with larger mesopores

pore volumes, CRF003 carbon is activated at differentes and varying times. Activated carbons are named aswhere x is activation temperature and the correspond-rptiondesorption isothermsare shown in Fig. 10. Thesehow that the higher the degree of burn off, the greatert of adsorption. Fig. 11 shows that such an activationesults in activated carbons with high pore volumes andre diameters of around 15nm. Porosity developmentin the range of large mesopores (i.e. pore diameter

en in Table 5 shows that activation at 1323K produces ase in thepore characteristics suchas total andmesopored BET surface area.given in Section 3 show that the porous structure ofgels can be engineered on a nanometre scale by chang-

3.5. Electro

All the dcharge/discto less than

Fig. 11. Pore satures. carbonFig. 10. N2 adsorptiondesorption isotherms at 77K.

ar ratio of resorcinol to catalyst during gel preparationbonization and activation under CO2 atmosphere at dif-eratures. Some of these carbons have been utilized asrials for electrode fabrication and their electrochemicale is tested in a lithium oxygen cell as follow.chemical measurements

ata reported below are obtained from the galvanostaticharge of the cells described above with reproducibility5% error in the cell capacities.

ize distribution of ACRF003 for ACRF003 carbons at various temper-s at various temperatures.


Table 5Porous parameters of CRF003 carbon samples activated at different degrees of burn off.

Sample Time (min) Burn off % SBET (m2/g) Vtotal (cm3/g) Vmicro (cm3/g) Vmeso (cm3/g) %Vmicro %Vmeso Davg (nm)

CRF003 0 647 1.2245 0.2101 1.0144 17 83 21.65ACRF003- 1073K 150 9.7 732 1.200 0.253 0.947 21 79 16.77ACRF003- 1123K 125 21 891 1.336 0.313 1.023 24 76 15.93ACRF003- 1173K 120 54 1497 1.942 0.448 1.494 23 77 15.43ACRF003- 1223K 60 58 1506 1.965 0.434 1.531 22 78 15.01ACRF003- 1273K 30 65 1585 2.075 0.488 1.587 23 77 14.76ACRF003- 1323K 20 67 1687 2.195 0.463 1.732 21 79 14.23

EMD usSigma andcharging pron the cathEMD is an imay take pto the dischcharge capathat the pronly onto canormalized

3.5.1. EffectVolume

cycles) basesimilar poreumes. Variasamples atat about 2.7of the cells adata for a sinvolving fo

Table 6change in tIt can be sein the level

Table 6Discharge cap

Sample

ACRF00211ACRF00212ACRF00212

Disched at

ampd the highest storage capacity of 740mAhg and sample02- 1223K with the lowest mesopore volume showed the

1Fig. 12. Discharge capacities of ACRF002.

ed in the electrode formulation was purchased fromused as received. It aids the Li2O2 decomposition andocess [8] and also catalyzes the reduction of O2 to O2

ode electrode during discharge process [7]. Althoughntercalation host for Li and some of discharge reaction

Fig. 13.discharg

bons. SshoweACRF0lace on its surface [10], however it does not contributearge capacity signicantly as its contribution to the dis-city is reported to be atmost 16mAhg1 [8]. Assumingoducts of the discharge reaction in Li/O2 cell depositrbon, the capacity of the lithium/oxygen cells has beento the weight of carbon in the cathode electrode.

of poreFig. 12 shows discharge behavior of the Li/O2 cells (rstd on electrodes prepared from ACRF002 carbons withsizes and pore size distributions but different pore vol-tions of cell voltage against specic capacity of carbona discharge rate of 20mA/g show at discharge proles2.9V. These are associated with the storage behaviornd are in good agreement with the preceding reportedimilar cell discharged in O2 at atmospheric pressurermation of Li2O2 as discharge product [7,8].shows how discharge voltage and capacity vary withhe structure of carbons used in electrode formulation.en that the storage capacity increases with increaseof burn off and development of mesoporosity in car-

acities of ACRF002 carbons with various pore volumes.

Cell voltage (V) Discharge capacity (mAh/g)

23K 2.60 63023K 2.68 18073K 2.76 740

lowest storcarbon sam

Fig. 12 son themorpmight be dupore volum

Activatedistributioncharge capashown in Fi

Fig. 13 asesses highvolume produring thereached whsition of Liunderway tcapacity.

Dischargelectrode fo


Sample

ACRF00310ACRF00311ACRF00313arge capacities of ACRF003 carbons discharged at 20mA/g. carbons20mA/g.

le ACRF002- 1273K with the highest mesopore volume1age capacity of 180mAhg between three ACRF002ples.hows that the discharge voltage of a Li/O2 cell dependshology of the carbon used in its cathode electrode. Thise to the change in the conductivity of carbons as theires are different.d carbonsACRF003with similar pore sizes andpore sizes but different pore volumes have also been tested. Dis-cities of these carbons at a discharge rate of 20mA/g areg. 13.lso shows that a carbon with higher pore volume pos-er discharge capacity. It is believed that larger porevidesmore space for the formation and storage of Li2O2discharge process as the end-of-discharge of the cell isen the carbon pores are lled or choked by the depo-

2O2, however as this is not clear yet, further work iso understand the role of pore volume on the storage

e capacities and voltages of ACRF003 carbons used inrmulation are given in Table 7.

acities of ACRF003 carbons with various pore volumes.


73K 2.68 52823K 2.74 88023K 2.83 1290


Fig. 1


Sample

ACRF00310ACRF00311

3.5.2. EffectFig. 14 c

similar porlarger pore

Pores into come inical reductiimportant tity of Li/O2porous struoxygen acce

The storpore size anrial. In ordedensity, theface area sh

Table 8 sperformancinto the buformation opores and lathe electrollithium ionthe storagein the poroucarbon-elec

4. Conclusions

Porous activated carbons were synthesized through poly-condensation of resorcinol with formaldehyde followed bycarbonization at 1073K in an inert atmosphere and activationunder CO2 at different temperatures. Activation at higher tem-peratures increased total and meso pore volumes while thepore size distribution of the samples remained almost con-stant.

Activated carbons were used as active materials in cath-ode electrodes for Li/O2 batteries and their electrochemicalbehavior was characterized by constant current charge/dischargeexperiments.

Associating the discharge capacitywith the textural parametersof the porous carbons showed that the appropriate pore volumeand pore diameter are the key factors contributing to high capac-

as smorbinedaffee larnm)volt

wled

thane Con

nces

ArmanEndo,Gabaninden9.landroazam. Abragasaw(200

ead, Kctrochead, J.. Pek4. Discharge capacities of carbons with different pore size.

acities of carbons with various pore size.


73K 2.68 52823K 2.74 880

of pore sizeompares the discharge capacities of two carbons withe volumes but different pore diameters. Carbon withdiameter exhibits a larger discharge capacity.carbon electrode allow both the oxygen and electrolytecontact with the carbon and catalyst. The electrochem-on of oxygen takes place on the carbon surface. It isominimize diffusion barriers to increase storage capac-cells by tailoring carbon electrodes having appropriatectures and pore sizes that maximize electrolyte andssibility to the porous network.

age capacity is linked to the physical surface area, to thed to the ionic accessibility of the electrode activemate-

ity. It won thea comcarbonwith th(14.23charge

Ackno

WeStorag

Refere

[1] M.[2] M.[3] J.P.[4] D. L

198[5] S. F[6] R. Y[7] K.M[8] T. O

128[9] J. R

Ele[10] J. R[11] R.Wr to obtain a reasonable storage capacity and energy

electrode pore volume, pore size distribution, and sur-ould be optimized.hows how mesopores are important for improving thee of Li/O2 cells. They lead the electrolyte ions transportlk of material quickly and also provide larger space forf discharge products. Therefore a sample with widerrger pore volumemight results in: higher diffusivity ofyte into the carbon structure and better accessibility ofs to the carbon surface (reaction zone), larger space forof discharge products as larger pore volume is availables structure, and nally better diffusion of oxygen to thetrolyte interface.

[12] R.W. Pek[13] F. Rouqu

solids, in:1999.

[14] S.J. GreggNew York

[15] H. Tamon[16] G.C. Rube

4341.[17] C. Lin, J.A[18] M. Kruk,[19] H. Marsh[20] D. Mowla

Carbon, v[21] Y. Yan, J.

(2008) 30hown that the discharge voltage of a Li/O2 cell dependsphology of the carbon used in its oxygen electrode andeffect of pore volume, pore size and surface area of

cts the storage capacity. A Li/O2 cell using the carbongest pore volume (2.195 cm3 g1) and a wide pore sizeshowed a specic capacity of 1290mAhg1 and dis-age of 2.83V.

gments

k the EPSRC for funding as part of the Supergen Energysortium (grant code EP/D031672/1).

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Preparation of controlled porosity carbon aerogels for energy storage in rechargeable lithium oxygen batteriesIntroductionExperimentalSynthesis of R/F aerogelsTGA analysis of aerogelsCarbonization of R/F aerogelsActivation of carbon aerogelsMaterial characterizationCell construction and electrochemical measurements

Results and discussionBET analysis of RF aerogelsEffect of pyrolysis conditions on the porous structure of the gelsEffect of R/C ratio on the porous structure of carbonsEffect of activation on the porous structure of carbon aerogelsElectrochemical measurementsEffect of poreEffect of pore size

ConclusionsAcknowledgmentsReferences