journal of PowerSources195(2010)46-53 Contents lists available at ScienceDirect Journal of Power Sources jou rna I homepa ELSEVIER ge: www.elsevier.com/locate/j powsou r Evaluation of silver as a miniature direct methanol full cell electrode Yong Gao, Xiangxing Kong, Norman Munroe. Kinzy jones> Department of Mechanical and Materials Engineering, Florida International ARTICLE INFO Article history; Received29 May2009 Receivedin revisedform 2 july 2009 Accepted3july 2009 Availableonline 5 August2009 Keywords: Poroussilvertape Electrodeevaluation Miniaturedirect methanol fuelcells Lowtemperature cofiredceramics University, 10555 W Flagler Street, Miami, FL 33174, USA ABSTRACT Miniature direct methanol fuel cells (DMFCs) and direct hydrogen fuel cells are promising candidates for future polymer electrolyte membrane (PEM) based micro-power sources. Currently, most miniature DMFCsare developed using a silicon based microelectromechanical system (MEMS) technique, which requires complex and precise processing. Low temperature cofire ceramic (LTee) technology offers an attractive alternative for a ceramics MEMSconstruction, allowing the integration of high density interconnect and embedded electronic components with microchannels and hermetic cavities from the mesoto the microscale. Silver is a major metallization source for LTee. which can be fabricated in a range of configurations, from a solid hermetic layer to a porous open structure with microchannels that can easily be integrated into the structures. Silver based LTee provides an ideal technology for the fabrication of an integrated fuel cell into a high density ceramic-based microelectronic assembly. A silver electrode was evaluated in a simulated DMFCoperating environment and found to exhibit good corrosion resistance and chemical stability, essential properties for electrode systems. Potentiodynamic analysis of a catalyzed silver electrode (prepared by thermal decomposition of a Pt/Ru resinate) revealed excellent corrosion resistance under anodic and cathodic DMFCoperating conditions. The Pt/Ru catalyst on the silver electrode enhanced the methanol oxidation reaction (MOR) as well as oxygen reduction reaction (ORR)as compared with similar reactions on carbon electrodes. The potential at which methanol is oxidjzed was lower than the silver oxidation potential, which served to protect the silver electrode. The aetermination of a contact angle of 30' on the silver electrode indicated wettability, which is deleterious for its application in DMFCs.Nevertheless, the results of good corrosion resistance derived from this investigation as well as the high electrical and thermal conductivities of silver all auger well for it usage as an electrode in DMFC © 2009 Elsevier B.V.All rights reserved. 1. Introduction Direct methanol fuel cells (DMFCs) have been considered to be promising power sources for portable electronic devices such as cellular phones, personal digital assistants (PDAs), and portable computers due to their simple construction, environmentally benign nature, and projected ease of use [1-3). Miniature DMFCs are key components in future applications integrating microsystem technologies, which will enable sensing, computing, actuation, control, and communication [4,5). Optimized miniature DMFCs utilize small volumes and integrated structures. Current miniature fuel cells employ microelectromechanical system (MEMS) technique in their design and fabrication. However, MEMS structures are difficult to process and possess limitations in fabrication and sealing of multilayer structures [6-9). Low temperature cofired ceramics (LTCC) is becoming more integrated as ceramic microsystems. Microsystems allow the integration of numerous * Correspondingauthor. Tel.:+13053482345; fax:+13053481932. E·mail address: jones@fiu.edu(K.jones). 0378-7753/$- see front matter © 2009 ElsevierB.V.Allrights reserved. doi:10.1016/j.jpowsour.2009.07.008 components, including embedded passives, high density interconnect, high performance thermal management systems, sensors and actuators, mechanical, fluidic and optical components. The development of these systems have led to enhanced processing capabilities including enhanced properties by controlled sintering or the development of cavities and micro-electromechanical structures using fugitive inserts which are removed during firing. LTee devices have produced meso- and macroscale channels, large volume cavities, micro-cavities, wick, valves and controlled porosity structures. Recent work on embedded micro-heat pipes in LTCChas lead to the development of large integrated cavities (1 em x 1 em minimum) with cavity thicknesses varying from 25!-Lm to several rnm. Additionally, sub-tOuurn 3-D microchannels and silver structures of controlled porosity have been cofired in LIeC [10-12). LTee has been used by Motorola in the manufacture of a mechanically assembled DMFC [13) with the LICC providing the mechanical structure containing microfluidic channels and cavities for fuel containment while utilizing a regular carbon-based PEM sandwiched between the electrodes. The system was mechanically assembled, loosing the packaging efficiency which a cofired, integrated LTee structure Y. Gao etaLj Journal of Power Sources 195 (2010)46-53 HoleforPEM feeding 100~~--------------------------------, Ag via for electron connection 90 LTCC with cavity for fuel container LTCC with cavity for PEM Porous Ag tape 'with catalyst in LTCC as anode -~~;;m;;~~ Porous Ag tape with catalyst in as anode LTCC LTCC with pores for airflow 47 Ag via for electron connection 80 ~ 70 'if!. ::- 60 s: Cl ~ Fig.1. A schematic illustration of an integrated LTCC-based DMFCsingle cell structure (not to scale). 50 40 30 would provide. To evaluate the possibility of a ceramic-based electrode system, a silver electrode structure was fabricated and used as the anode in a commercial carbon electrode DMFC(H-Tec Methanol Fuel Cell Junior) [14]. This demonstrated that silver metallization with a platinum/ruthenium catalyst could be developed in a cofired LTCCstructure and warranted further investigation as an electrode structure DMFCs. This paper considers the development of porous silver tape electrodes cofired into a hermetic LTCCstructure with integrated cavities for fuel and micro-cavities into which the PEM is added after LTCCfiring. Silver is widely used as conductive metallization in LTCCfabrication [15,16]. LTCC-basedDMFCconsists of the assembly of individual layers of unfired "green" LTCCtape, silver tape and fugitive materials such as carbon tape and thick film inks. The structure is laminated to develop a uniform green density, which requires all cavities/channels/voids spaces to be filled with a fugitive material such as carbon to allow uniform pressure during lamination. The LTCestructure is then fired with a multi-step profile to allow burnout of the-organics. and fugitive graphite until a peak of 850°C to produce q fully sinrered ceramic. Any carbonbased fugitive material burns out during sintering of the ceramic to produce cavities. Fig. 1 illustrates an integrated LTCC-basedsingle cell. The MEAconsists of anode and cathode-electrodes made of porous silver tape with the catalyst added, and a [TCCcavity into which the PEMis introduced. The Silver/LTCCstructure enables the fabrication of hermetic multi-fuel cells, equipped with silver-based bipolar plates. In order to ensure good performance and extended service of the DMFC,the electrode materials must posses good electrical conductivity, chemical stability, corrosion resistance, and be capable of developing a bipolar configuration. In current DMFC, carbon (or porous carbon paper) is employed as the electrode to support the catalyst due to its high corrosion resistance and low specific density. However, the electrical conductivity and thermal conductivity of carbon are much less than those of metallic materials. Furthermore, the process for manufacturing porous carbon is very complex and time-consuming [17]. Additionally, the corrosion of carbon caused by non-uniform distribution offuel on the anode and crossover of oxygen through the membrane separator has recently been recognized [18]. When used as a cathode, carbon is thermodynamically unstable at typical electrode.operating.conditions, The equilibrium potential for carbon oxidation to carbon dioxide is -0.034 VSCEat 25°C [19]. PEM fuel cell cathodes generally operate at potentials in the range of 0.26-0.66VsCE' significantly more anodic than the equilibrium potential for carbon corrosion, which could result in its degradation over time. Although thermodynamically unstable, slow kinetics allows the use of carbon in low temperature fuel-cell applications. As a result, alternative materials are being developed to replace carbon-based materials, such as conducting polymer and Ti mesh [20-22]. Such concerns with carbon electrode degradation over time are what motivated this investigation on the usage of silver as a potential electrode material. 20 10 O+-~--~--r-~--~--r-~--~--r-~~ o 50 100 150 200 250 300 350 400 450 500 550 Temperature (0 C) Fig. 2. The TGAcurve of Pt and Ru resinare. 2. Experimental 2.1. Electrode materials and electrolytes Silver foil (99.998% Ag, Alfa, Ward Hill, MA) was used as the electrode test material and Poco graphite sheet (Poco Graphite Inc., Decatur, Texas) was used as the carbon electrode for comparative analysis. The anode catalyst was prepared by brushing a 1:1 molar mixture of platinum resinate and ruthenium resinate (BASF,East Newark, NJ) onto the electrode (silver foil or carbon) followed by firing. The catalyst firing temperature was determined by performing a thermogravinometric analysis (Hi-Res TGA2950, TA Instrument, New Castle, DE) on the electrode over a temperature range of 30-500°C, with a ramp rate of 5cCmin-1 and a hold time of 30min at 500eC in air. The TGAcurve (Fig. 2) indicated that Pt and Ru precipitated at 350 and 460 =C,respectively. In order to obtain the Pt-Ru alloy, a catalyst firing temperature of 500°C for 4 h was chosen. In the case of the cathode, only platinum resinate was applied to the electrode followed by sintering under the aforementioned firing regime. Three electrolytes, 05 M H2S04, 1 M H2S04 and 05 M H2S04 with 1 M methanol, were used in the test. All electrolytic solutions were made from analytical grade chemicals. 2.2. Electrochemical testing A conventional three-electrode set-up in a glass corrosion cell equipped with a Luggin capillary was used in the electrochemical experiments. A platinum electrode served as the counter electrode and a saturated calomel electrode (SCE,0.241 Vvs. VSHElwas used as the reference electrode. Allelectrochemistry measurements were conducted using a computer controlled Solartron 1287/1260 impedance system. In order to evaluate the electrochemical behavior of silver, potentiodynamic corrosion tests were conducted to simulate the DMFCoperating conditions. The anodic conditions involved the usage of 1 M methanol and 05 M H2S04. The cathodic conditions involved the usage of 1 M H2S04 with air purge. Both tests were conducted at 70 'C with a sweep rate of 15 mV S-l. The potential range for the anodic and cathodic scan was from -05 V(vs. open circuit) to 1.2VSCEand from 1.2V (vs. open circuit) to -1.0 V (vs. open circuit) respectively. The dissolved oxygen and pH were measured in each test using a water quality multi measure sensor (PS-2169, PASCO,Roseville, CA).The solution was purged by N2 for 30 min \.1.. Y. Caoet al. / Journal afPower Sources 195 (2010) 46-53 48 carbon), dispersants, binders, plasticizers and a solvent into a slurry that was deaerated and cast on a mylar film by doctor blading. Cast thickness was adjusted by the spacing of the doctor blade. In order to produce a porous silver tape, silver powder (PM 225, Heraeus, Conshohocken, PAlwas mixed with graphite powder (fugitive pore forming agent) to form the functional powder. Typically, 0.25 mm thick tapes were cast for the porous electrode, which was fired using standard LICCfiring procedures: at a heat rate of 2 "Cmirr ! to 450 CCand hold for 2 h for binder burn-out, heating 2 =Cmirr ! to 600 "C for 2 h for graphite burn-out, then heating at 2 <C min '! to 850 for 15 min followed by a nature cooling ramp, which did not exceed 50 "Cmin-1 until cooled. The microstructure of the porous silver tape (see Fig. 3) was characterized by JEOLJSM-6330F field emission scanning electron microscope (SEM).The porosity (£) was calculated by the following equation: C ( e = 1Fig. 3•.The 5£M photomicrograph of porous silver tape. prior to each test and then either aerated by air or deaerated by N2 during the test. The corrosion resistance of silver was also assessed by potentiostaic polarization at 70 "C, by employing the abovementioned solutions but adding 2 ppm F- [23],in an attempt to simulate the DMFCelectrode environment. TheappH~d voltages were -0.1 VSCE (~0.14 VSHEJfor the anode and 0.6 VSCE (~0.84 VSHE )for the cathode, respectively. The electrode stability w~?<assessed by conducting cyclic voltammetry withvarious com:~.fltrations?f H2S04 and voltage scan ranges. Alicyclic voltamrnetrytests were conducted after the electrolyte was purged with nitrogen for about 30min to remove dissolved oxygen. Firstly,the silver elestrodewas scanned from 0 to 1.2VsCEat 25°C to evaluate the effect of acid concentration. Secondly, cyclic voltarnmetry was conducted frornO toOA VSCEfor 600 cycles at a scan rate of25 mVs-1 at 25 "Cj,nO.5-MH2S04 with 1 M methanol. Additionally, methap0l.0xiciation reaction (MOR) and oxygen reduction reaction (ORR)were evaluated by cyclic voltammetry over a scan range ofO-l.OVscEat20 mV S-l scan rate at60 =C, The electrolyte employed was 0.5 M H2S04 with 1M methanol for the MORand 0.5 M H2S04 for the ORR. 2.3. Tape casting The silver electrode was fabricated by tape casting, which consisted of mixing the desired "functional" powder (ceramic, silver, WA (1) Preald where WA = areal weight (g crrr-'): Preal = solid phase density; d = thickness. The pore size of the silver tape was measured at approximately 2-3 p.m, which is consistent with the particle size of graphite powder. However, the actual porosity was determined to be 70%. 3.. Results and discussion 3.1. Corrosion test The DMFCanodic, cathodic and overall electrode reactions are shown in Eqs. (2)-(4). The potential at which MOR occurs is -0.191 VSCEand ORR0.99 VSCEunder standard conditions. Eo = -0.191 VSCE (2) (3) Eo = 0.99VsCE Overall reaction Ecell= 0.94VsCE (4) The silver electrode was evaluated by potentiodynamic polarization analysis over the entire potential range of DMFCoperating conditions. The operating conditions experienced by an electrode in a DMFCwere simulated as previously mentioned in Section 2.2. Fig.4 illustrates potentiodynamic polarization curves (anodic scan) under anode and cathode operating conditions for the silver foil and the porous silver tape. There was no significant different in 2.0 2.0 1.5 1.5 J f-AgfOil -. - Porous Autaoe 1.0 t.l 1.0 U t.l U o: '"> ;;t.l ;;- ---~ t.l -- 0.0 0.5 ;:;;=::::: --~-0.0 -0.5 -1.0 f--,-~.,....,~nnr~ lE-8 1 '"'"> 0.5 1E-7 lE-6 •••••.. ~.,."","-~ lE-5 lE-4 II Acm· 2 ••• ~""~mnr~cnmI 1E-3 0.01 0.1 '~- -----<, .:;;'-.-~-. ,_._.- -0.5f-~~..--~~..--~~"--~~,,--~~...-~,..,.,.,,,J IE-4 1E-3 1E-5 lE-6 0.01 0.1 I I A em-2 Fig.4. Potentiodynamic polarization curves (anodic scan) of (a) silver foil and porous silver tape in 0.5M H2S04 in 1 M methanol; and (b) 1 M H2S04 bubble with air at 70'(. y. Gao et al./ Journal of Power Sources 195 (2010)46-53 Tablet Corrosion parameters of silver foil subjected toanodic polarization under both anode and cathode operating conditions. Corrosion parameters Anode condition Cathode condition Ba (mV) Be (mV) 59.59 97.04 1.94 . 0.131 2.578 0.605 0.57 70.02 442.73 22.74 0.216 30.166 0.645 0.63 leorr (!LA Eeorr cm-2) (VSCE) Corrosion rate (MPY) Passivation potential (VSCE) Ipas'ive (Acrrr+) Table 2 Corrosion parameters of silver foil in 1 M H2S04 potentiodynarnic polarization (both anodic and cathodic scan) with air purge. Corrosion parameters. Anodic scan Cathodic scan Ba (II)V) Be (mv) 10 (fLAcm-2) 70.2 442.73 22.74 0.221 19.1 133.56 220 0.398 491. .0;704 0.08 Eo (VSCE) Corrosion rate (MPY) Passivation potential (VSCE) lpassive (Acm-2) 30.166 0,6450.63 49 a comparison of corrosion parameters obtained from anodic and cathodic scans of silver foil under air purge. Fig. 5(b) showed the comparison of cathodic scan of silver under aerated and deaerated conditions. An increase in the open circuit potential was observed with the catalyst coated electrode. The polarization curve of deaerated silver foil showed a region that is mass transport controlled. This is to be expected. since the solution was dearated with nitrogen. A decrease in potential at region C revealed no change in the reaction rate and hence the measured current. Since this reaction is limited by how fast oxygen may diffuse (mass transport controlled) there is a limiting current density as compared to a mixed potential as previously discussed for bare silver. The measured dissolved oxygen concentration (D02) and pH are tabulated in Table 3. The catalyst coating on the silver electrode raised the open circuit potential as depicted in Fig. 6. Since DMFCs operate at 0.36VSCEand even lower potentials [24). it can be concluded that minimal oxidation would occur with silver electrode as well as catalyst coated electrodes. This corrosion behavior is partially attributed to the formation of the Pt-Ag alloy on the electrode surface. a phenomenon that was also observed in Pt-Ag alloy in HCI solution [25). \ \ 3.2. Electrode stability corrosion parameters. The corrosion parameters of silver foil are summarized in Table 1. An open circuit or rest potential of between 0.13 and 0.216VsCE was obtained for the anodic scan, At this potential the sum of the anodic and cathodic reaction rate on the electrode surface is zero and the measured current will be close to zero. According to Eq. (2). the equilibrium methanol oxidation.voltage is-::0.196VscE.Therefore. silver is stable at the MORpotential because metal oxidation cannot take place at such low'potential. The cathodic polarization sc~n illuStr~tedinFig. 5indicated an open circuit potential of 0.4 VSCEfor silver foil, This potential, as with the anodic scan represents the potential at which the sum of the anodic and cathodic reaction occurring on silver surface is zero. Depending on the pH and dlssolvedoxygen.concentration in the solution. region B represents a mixed potential of oxygen reduction and H+ reduction (hydrogen evolution or water reduction). This is apparent in the absence of a limiting current density due to limits of mass transport of 02 to the silver surface. The potentiodynarnic analysis of porous silver-tape (Fig. Sea)) resulted in the same Ecorr and [corr but some limitation of mass transport. Table 2 provides 1.5 I-Agfoi' - - - PorousAg tap Electrode stability in DMFCs is of paramount importance because any dissolution of metal ions may result in catalyst poisoning. and fowling of the membrane. The evaluation of electrode corrosion resistance under DMFC operating conditions was conducted by potentiostatic measurement of the current-time relationship as shown in Fig. 7. The transient current decayed rapidly in the beginning and remained constant at -5.0 x 10-4 A due to the formation of a passive film. The negative current indicated that the electrode surface was cathodically protected. which implied no further active dissolution of the electrode under those conditions. From the relatively steady current. it can be concluded that the passive film remained very stable over time. Potentiostatic measurement of the DMFCcathode is shown in Fig.8. where a similar current-time relationship to that of the anode is displayed. The current in this oxidative state exhibited good stability and remained at a very low level at 0.3 J.LA. This current-time curve indicated that the passive film was very stable under cathodic conditions. 2.0 J -..... _._. 1.5 1.0 1.0 f;I;l U 0.5 A - ---~"'\. CI.l '" > :> [oJ \~ -- -0.5 f;I;l U 0.5 '\ '"> :> 1 CI.l ) 0.0 0.0 [oJ -0.5 -1.0 -1.0 -1.5 -1.5 lE-S IE-4 IE-3 Ag foil (aerated) Ag foil (dcaerated) Ag coated witb Pt (aerated) 0.01 I I A cm-2 0.1 IE-S lE-4 IE-3 0.01 0.1 I I A em-2 Fig. 5. Potentiodynamic polarization (cathodic scan) of (a) silver foil and porous silver tape; (b) silver foil (aerated). silver foil (deaerated), and silver foil coated with Pt in 1 M H2S04 at 70·C bubbled with air. 50 Y. Gao etal. / Journal o!Power Sources 195 (2010) 46-53 Table 3 Comparison dissolved oxygen concentration (D02) and pH for cathodic scan under cathode conditions. Samples Conditions Dissolved oxygen concentration (002 )(mg L-I) N2purge 30 min; air purge during test N2 purge 30 rnin: air purge during test N2 purge 30 min; N2 purge during test Porous silver tape Silver foil (aerated) Silver foil (deaerated) Before test After test 6.3 5.2 10.0 20 16.9 10.8 Before test After test -2.59 -0.01 -0.11 -2.33 0.5 0.16 2.0.--------------------, 2.0...---------------------, ---Ag ---Ag ..... AgwithPtIRu Coatin 1.5 pH ------------------ ..... An wit Pt coatin~ 15 t3 1.0 t3 1.0 00. 00. '"... ~ >- 0.5 ~ - >- ~ 05 -- ..~~- •...•:-.=:::.:.~.•.. 0.0 0.0 -0.5 1E-8 1E-7 1E-6 1E-5 lE-4 1E-3 0.01 0.1 -0.5 \---T~'""I"~~....,...~~""~~"O""~~,....~"""~~nni lE-7 1E-4 lE-3 1E-61E-5 II A cm-2 0.010.1 I I A cm· 2 Fig. 6. Potentiodynamic polarization curves (anodic scan) of (a) silver foil coated with catalyst (Pt-Ru for anode and Pt for cathode) in 0.5 M H2S04 with 1 M methanol and (b) 1 M H2S04 bubble with air at 70<C. <;'8 0.003,--------------------, 33. Cyclic volmmmerry 0.002 In addition to contamination of the PEM and catalytic sites [26], any electrode oxidation would result in an increase in ohmic resistance and charge transfer resistance. Cyclic voltammetry was conducted on silver in three H2S04 concentrations (0.1,0.5, and 1 M) at 25°C as shown in Fig. 9. The onset potential of silver oxidation occurred at around 0.48 VSCE with little variation as the concentration of H2S04 was increased. A higher current density (larger current peak value) was obtained at higher acid concentration, which indicated greater degree of silver oxidation. It can be concluded from Fig.8 that a change of acid environment in DMFCshas no significant effect on silver oxidation potential. Cyclicvoltammetry was conducted to test the electrode stability from 0 to 0.4 VSCE at 25 -c for 600 cycles in 0.5 M H2S04. The last cycle is shown in Fig. 10. There was no change in the diameter of 0.001 -e'" o~ ------ -0.001 -0.002 -O.003L--~--L-~~-~-~-~--~-~ 1000 o 2000 3000 4000 Time I see Fig. 7. Potentiostatic analysis for silver foil in 1 M methanol and 0.5 M H2S04 + 2 pprn F- at 70'C at -0.1 VSCE. 0.15 ...,-----------------------; 0.12 3.50E-06 -... -- 0.5M H2S04 1M H2S04 /1 <;'80.06 '" <: 0.1 M H2S04 0.09 3.00E-06 '"'8 -- 2.50E-06 '" 2.00E-06 ::: 0.03 1.50E-06 o 1.00E-06 lrl-- ':t'-~"'7'-'0.2 -0.03 \.;014; 0.6 '-----1 I 0.8 1.2 1.4 1.6 5.00E-07 -0.06 O.OOE+OO 0 500 1000 1500 2000 2500 Time! S 3000 3500 4000 -0.09 -1- -' E/VvsSCE Fig. 8. Potentiostatic testing for silver foil in 1 M H2S04 + 2 ppm F- at 70 C purged with air at 0.6VSCE. G Fig. 9. Cyclic voltammetry of silver foil in different concentrations of H2S04 with scan rate of 20 mv S-I at 25'C. Y. Gao et al.] Journal of Power Sources 195 (2010)46-53 3.5E-03 0.010 3.0E-03 --2nd 2.SE-03 -- cycle e -e'" -,-------------------, -- 0.008 Pt-Ru on Ag 600th cycle _ '"I 51 2.0E-03 0.006 'OJ) '"I 1.SE-03 a a -<'" 1.0E-03 ... 5.0E-04 0.004 0.002 0.000 t~~:i:S2:~~::=~~::... ...• --f...• ---1 O.OE+OO os 0.1 -S.OE-04 -1.0E-03 0.1 -0.002 00 -0.004 -'--------------------' 0.2 E/VvsSCE Fig. 10. Cyclic voltammetry 25 mVs· 1 at 25°C. of silver electrode Fig. 12. Cyclic voltammetry ofPt-Ru catalyst coated on silver in 0.5 M H2S04 with 1 M methanol solution at 60°C with scan rate 20 mV S-l. after 600 cycles at scanning speed the silver wire after 600 cycles under the above condition. However, the silver oxidation peak decreased after 600 cycles indicating the development of a passivating film. Another important concern was whether the silver was oxidized before the oxidation of methanol by the catalyst. If so, it would greatly decrease the DMFCperformance for two reasons: (1) oxidation of the silver electrode decreases its conductivity and (2) methanol oxidation would be limited due to less availability of catalyst. This problem depends on the onset potential of methanol oxidation by the catalyst. Fig. 11 compares the cyclic voltammograms of catalyst on silver foil and carbon. Due to the strong oxidation peak of the silver, the effect of catalyst on the carbon electrode cannot be shown on the same scale (Fig. 11(a». Nevertheless, Fig. 11(b) shows the onset oxidation potential of methanol on a carbon electrode with catalyst at 0.32 VSCE,while the onset oxidation potential of silver was 0.48VSCEas previously described in Section 3.3. It is clear that the oxidation of methanol is catalyzed by Pt-Ru before the onset of silver electrode oxidization. It is therefore apparent in Fig.l1(a) thatthe peak associated with methanol oxidation is obscured by the peak for silver electrode oxidation. Ascan be seen in Fig.11(b), the voltage over which methanol oxidation occurs ranged between 0.32 and 0.82VsCE,which partially overlaps the potential range over which silver is oxidized (Fig. 9). It should be noted that the peak beyond 0.8 VSCEcorresponded to 02 adsorption [27]. solutions respectively. Fig. 12 shows cyclic voltammograms of silver with Pt-Ru catalyst in 0.5 M H2S04 with 1 M methanol solution over a scan range of 0-0.5 VSCE.Comparing with Fig. 11(b), the MaR peak current on silver with catalyst was 3.6 times greater than that obtained on carbon. This enhancement was attributed to the Pt-Ru alloy on silver and will be analyzed in detail in another paper. Methanol oxidation on noble metal alloy has been reported elsewhere [28]. Fig.13 compares the cyclic voltammetry for the electro-catalysis of ORRby Pt/Ag vs. Pt on carbon substrates. The peak corresponding to the mass catalytic activity ofPt/Ag (ORR)at 0.53 VSCE was greater than that ofPt (ORR).There have been reports of electro-catalyzed ORRby Ag and Pt in the literature [29-32] but not as a combined catalyst. Soitis reasonable to conclude that the enhanced ORReffect was primarily due to the catalytic effect of Pt-Ag that was formed on the electrode's surface. The peaks at 0.41 and 0.3 VSCEin anodic and cathodic scan correspond to silver oxidation and reduction in acid solution. 3.5. Other properties Other properties of concern include wettability and thermal conductivity of silver electrode. The presence of water results in catalyst submergence, which results in a loss of power. Electrode wettability affects cell performance, thus water should be removed immediately to avoid flooding. Surface wettability was assessed using a microscope to measure the contact angle of water on the silver electrode. Fig. 14 shows the contact angle (30') for water on the silver surface. This value indicated that the silver electrode possessed a low surface energy, which could result in flooding of the 3.4. Comparison of silver electrode and carbon electrode ORR and MaR on a silver electrode were evaluated by cyclic voltammetry in 0.5 M H2S04 and 0.5 M H2S04 with 1 M methanol (b) (a) 0.04 ~--Carbon .. e s..• I-_carbonl 0.002 OJ) OJ) '"I ..••.. 0.0025 0.06 •• T E/VvsSCE a e -<'" 0.02 '"I -e 0.8 0.001S 0.001 12 O.OOOS -0.04 -02 E/VvsSCE Fig. 11. The comparison of cyclic voltammetry 1. eo E/VvsSCE between (a) catalyst on silver foil and (b) carbon at 60·C in 0.5 M H2S04 with 1 M methanol with the scan rate 20 mVs-1• y. Coo et 01./ Journal of Power Sources 195 (2010) 46-53 52 .4 E/VvsSCE E/VvsSCE Fig. 13. Cyclic voltammetry of Pt on silver in 0.5 M H2S04 with scan rate 20 mV,1 Fig. 14. Wettabiliry of water on silver surface. DMFC.Several approaches can be taken to enhance water removal. Firstly, a hydrophobic agent such as fluoro-octyltrichlorosilane [33], an ethanol solution ofhexadecanethiol [J4] or lH,lH,2H,2Hperfluorodecyltrichlorsilane[35] could be placed on the cathodic silver surface by post-fired processing or evaporation. Secondly, laminated multilayer silver tapes with different pore size can form a wick system to expedite the removal of water. Fuel cells can release large quantities of heat when generating electric energy during operation [36]. Less than about 30% can be expected as electricity, while the remainder is converted into heat [37]. Thermal management in DMFCs is therefore a major challenge because released thermal energy can cause discomfort for the device user. Such issues become more apparent as a growing number of miniature fuel cells become available for portable electronic power sources. The heat produced has to be dissipated, or else the accumulated heat in the system might represent a strong disadvantage for compact portable systems. High thermal conductivity materials facilitate the thermal release and are useful for thermal management in fuel cells. Since silver has the highest thermal conductivity (theoretical value 429Wm-J K-J) among all metals, its usage with thermal conductivity (over 100 W m-J K-J ) is more than adequate for thermal management in DMFCs[38]. 4. Conclusion The silver electrode was evaluated by simulating aggressive environments for both anode and cathode in DMFC.Potentiodynamic test revealed the corrosion profile of silver over the entire potential range of DMFC operation. The results indicated that silver was stable under anodic conditions and was passively protected under cathodic conditions. Potentiodynamic polarization tests (cathodic scan) revealed that the cathodic reactions on the silver electrode coated with Pt was mass transport controlled (due to at 60°C from 0 to 0.45 VSCE• limits of mass transport of 02) as compared with a mixed potential of oxygen reduction and H+ ion reduction on a bare silver electrode. Potentiostatic analyses revealed that the silver electrode has a very low passivating current for both anodic and cathodic reactions. Cyclicvoltammetry experiments demonstrated that the silver electrode was stable (no dissolution) after 600 cycles under a DMFC operation potential of 0.4 VSCE. The catalyst prepared by thermal decomposition of resinate resource on the silver electrode formed Pt-Ru alloy, which increased its corrosion resistance. Experimental results indicated that MORwas more efficient on silver with catalyst under anodic conditions. Similarly, ORRreactions were also more efficient on silver with catalyst as compared with carbon. 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