high temperature operation of a solid polymer electrolyte fuel cell stack based on a new ionomer...

High Temperature Operation of a SolidPolymer Electrolyte Fuel Cell Stack Basedon a New Ionomer MembraneA. S. Aricò1*, A. Di Blasi1, G. Brunaccini1, F. Sergi1, G. Dispenza1, L. Andaloro1,M. Ferraro1, V. Antonucci1, P. Asher2, S. Buche2, D. Fongalland2, G. A. Hards2,J. D. B. Sharman2, A. Bayer3, G. Heinz3, N. Zandonà3, R. Zuber4, M. Gebert5,M. Corasaniti5, A. Ghielmi5, D. J. Jones6

1 CNR-ITAE, Via Salita Santa Lucia Sopra Contesse 5, I-98125 Messina, Italy2 Johnson Matthey Fuel Cells Ltd, Blounts Court, Sonning Common, Reading, Berks, RG4 9NH, UK3 SolviCore GmbH&Co KG, Rodenbacher Chaussee 4,63457 Hanau, Germany4 Umicore AG&Co KG, Dept. RD-EP, Rodenbacher Chaussee 4, 63457 Hanau, Germany5 Solvay Solexis, viale Lombardia, 20 20021 – Bollate (MI) Italy6 Institut Charles Gerhardt, Agrégats, Interfaces et Matériaux pour l’Energie, UMR 5253 CNRS Université Montpellier II, Place EugèneBataillon, 34095 Montpellier, France

Received February 10, 2010; accepted August 30, 2010

1 Introduction

Polymer electrolyte membrane fuel cell (PEMFC) technol-ogy for automotive applications requires operation at highworking temperatures to improve efficiency, tolerance of con-taminants and to enable simplification of the thermal andwater management sub-systems [1–5]. The operating condi-tions for the fuel cell stack targeted by the automakers are

low pressure (P ≤ 1.5 bar abs.) and low relative humidity (RH≤ 25%) [1]. A high stack performance with suitable electricalefficiency (cell voltage ≥ 0.65 V) is desired over a wide tem-perature range from ambient temperature to about 110–120 °C [1–2]. Moreover, an easy and rapid start-up, as well as

–[*] Corresponding author, [email protected]

AbstractPolymer electrolyte fuel cell stacks assembled with JohnsonMatthey Fuel Cells and SolviCore MEAs based on the Aqui-vion™ E79-03S short-side chain (SSC), chemically stabilisedperfluorosulphonic acid membrane developed by SolvaySolexis were investigated at CNR-ITAE in the EU Sixth Fra-mework ‘Autobrane’ project. Electrochemical experimentsin fuel cell short stacks were performed under practicalautomotive operating conditions at pressures of 1–1.5 barabs. over a wide temperature range, up to 130 °C, with vary-ing levels of humidity (down to 18% R. H.). The stacks usinglarge area (360 cm2) MEAs showed elevated performance inthe temperature range from ambient to 100 °C (cell powerdensity in the range of 600–700 mWcm–2) with a moderatedecrease above 100 °C. The performances and electrical effi-

ciencies achieved at 110 °C (cell power density of about400 mWcm–2 at an average cell voltage of about 0.5–0.6 V)are promising for automotive applications. Duty-cycle andsteady-state galvanostatic experiments showed excellentstack stability for operation at high temperature. A perfor-mance comparison of AquivionTM and NafionTM-basedMEAs under practical operating conditions showed a signif-icantly better capability for the Solvay Solexis membrane tosustain high temperature operation.

Keywords: Automotive, High temperature, Polymer electro-lyte membrane fuel cells; Proton exchange membrane, Per-fluorinated membranes, Perfluorosulphonic acid ionomers,Stack, Short-side chain ionomers

FUEL CELLS 10, 2010, No. 6, 1013–1023 © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1013

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DOI: 10.1002/fuce.201000031

Aricò et al.: High Temperature Operation of a Solid Polymer Electrolyte Fuel Cell Stack

the capability to sustain specific duty cycles are pre-requisites for automotive applications [1].

In the last decade, considerable progress has beenmade on new proton conducting membrane materialsfor high temperature PEMFCs [6–20]. Several of thesematerials have shown interesting properties, but fewefforts have been made to assess the new materials at astack level [5, 14]. An investigation of performance andstability in a practical system, over a wide temperaturerange from ambient temperature to 130 °C, is necessaryfor a full assessment of the PEMFC components and toclearly identify progress beyond the state-of-art.

The ‘Autobrane’ project was an interdisciplinary EUSixth Framework programme with a clear focus onmaterial research and development, driven by automo-tive application needs, to overcome existing technologicalbarriers for the introduction of PEMFCs to the market. Theobjective of this work was the demonstration of the mem-brane-electrode assembly technology developed in the Auto-brane project in a state of the art stack adapted to higher tem-perature demands. In particular, the demonstration of a proofof concept of the new membrane and MEA technology wastargeted to a short-stack with realistic active area and a poweroutput of about 1 kW.

As is well known, perfluorosulphonic acid ionomer mem-branes such as Nafion® are the most widely used membranesin PEMFCs [21]. This choice is based on the high conductivityas well as the excellent mechanical, and electrochemical sta-bility characteristics that satisfy the main requirements of fuelcell devices [22]. However, perfluorosulphonic acid basedmembranes that require water molecules for the proton trans-port mechanism (proton mobility) [8, 13] need high levels ofreactant gas humidification, especially at high temperature,to maintain suitable conductivity characteristics. The amountof humidification required varies depending on the operatingtemperature and membrane properties and it influences thesize and complexity of the PEMFC system. Furthermore, theconventional Nafion® type membranes easily dehydrate attemperatures higher than 95 °C with a significant increase incell resistance [7]. A low cost and high temperature mem-brane, with suitable ionic conductivity and stability fromambient temperature up to 120–130 °C, would provide animportant solution to the main drawback presently affectingthe rate of commercialisation of the PEMFC for automotiveapplications.

Nafion® is known as a long-side-chain (LSC) ionomer. Inrecent years, alternative polymers with a structure similar toNafion® but with a shorter pendant side-chain (SSC) carryingthe sulphonic group, e.g Dow, 3M, Hyflon® Ion (see Figure 1)have been developed and investigated for fuel cell operation[23–30]. However, only in a few cases, has a wide commercia-lisation of these membranes been pursued. Probably, this wasdue to the complex process of the polymer synthesis. This isnot the situation with the SSC Hyflon® Ion material devel-oped by Solvay Solexis (now known under the tradename

Aquivion™) that is prepared by using a simple and low costsynthesis route for the base monomer [26, 28]. SSC ionomersare characterised by both larger crystallinity and higher glasstransition temperature than LSC polymers at a given equiva-lent weight (EW) [31, 32]. However, there is a lower limit ofabout 600 g eq–1 for the disappearance of crystallinity in theSSC ionomers [27]. The standard Aquivion™ membrane hasan EW of 850 g eq.–1 compared to 1,100 g eq–1 of Nafion witha glass transition temperature of 127 vs. 67 °C of Nafion®.Both characteristics are promising in terms of mechanical sta-bility and conductivity for operation in the high temperaturerange [29]. In the Autobrane project, the Aquivion™ mem-brane was modified further to obtain an ionomer with an EWof 790 g eq–1 (Aquivion™ E79-03S) whilst retaining good me-chanical properties and characterised by improved conduc-tivity at high temperature.

A key aspect of the Autobrane project was the assemblyand testing of short stacks (about 1–1.5 kW power output)based on large area (360 cm2 active area) Johnson MattheyFuel Cells (JMFC) and Solvicore membrane electrode assem-blies (MEAs) developed in the Autobrane project. Both typesof MEA comprised proprietary catalysts developed by JMFCand Umicore and the Aquivion™ short-side chain (SSC),chemically stabilised perfluorosulphonic acid membranedeveloped by Solvay Solexis in the Autobrane project. Freu-denberg FCCT developed gas-diffusion layers (GDL) materi-als [33] suitable for the high temperature range addressed inAutobrane. To adapt the GDL material to the specific Auto-brane operation conditions, a new ex situ test was set up tomeasure water transport through the GDL [33].

This paper reports on the electrochemical investigation ofAutobrane short-stacks under variable operating conditionsof temperature, pressure and relative humidity with the aimof assessing the capability of the new materials to operateunder real practical automotive conditions.

A comparison of the electrochemical behaviour of Aqui-vion™ and Nafion 111™-based MEAs, under practical opera-tion conditions, in a proper temperature (80–110 °C) isreported to assess the progress beyond state of the art.

---CF(CF2 )k---

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Fig. 1 Polymer structure for long-side chain and SSC perfluorosulphonic ionomermembranes.

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2 Experimental

Differential scanning calorimetry (DSC) experiments wereperformed at Solvay Solexis (Italy) on Aquivion™ precursor(SO2F-form) samples of different EW by using a Perkin-ElmerDSC7 calorimeter equipped with a refrigeration unit for con-trolled cooling to –20 °C. Scans were made on samplesweighing around 10 mg in the temperature range from 10 to350 °C at a heating rate of 10 °C min–1. Before measurement,all samples were pretreated at 350 °C for 15 min. The heat offusion was calculated from DSC according to the methodreported in [27]. Data for Nafion were obtained from the lit-erature [27].

Ionic conductivity measurements were carried out at Sol-vay Solexis on the Aquivion™ E79-03S membrane (thickness30 lm) at different temperatures and RH values by using aset-up for conductivity tests similar to that reported in the lit-erature [34].

JMFC and Solvicore membrane-electrode assemblies(MEAs) were prepared using the Aquivion™ E79-03S shortside chain perfluorosulphonic membrane developed by Sol-vay Solexis. In some experiments, Nafion 111™ (thickness25 lm) was used for comparison. The JMFC (UK) and Solvi-core (Germany) MEAs also consisted of catalytic layers basedon proprietary Pt-based catalysts from JMFC and Umicore[35–37], and the Solvicore MEAs employed GDLs providedby Freudenberg [33]. The total Pt loading of the MEAs wasbetween 0.6 and 0.8 mg cm–2. The micro-diffusion layer inthese MEAs was optimised for high temperature operation.The stack hardware, including end-plates, bipolar plates,macro-diffusion layers and gaskets, was supplied by Nuvera(Italy). The geometric characteristics of the MEAs wereselected on the basis of the Nuvera stack design. The activearea of each MEA was 360 cm2. Each active cell was insertedin between two cooling cells. MEAs were assembled intoshort stacks of 5–6 cells connected in series and their electro-chemical behaviour was investigated under different operat-ing conditions as required for automotive applications.

The electrochemical tests were carried out at the CNR-ITAE (Italy) by using an in-house developed fuel cell stacktest station. This included a hydraulic circuit mainly consist-ing of mass flow controllers, gas humidifiers, pre-heaters,water condensers and back-pressure valves. To control thestack temperature, a thermo-cryostat apparatus was used. Athermostatic (cooling) fluid was passed through the coolingcells to maintain an almost constant temperature through thestack during polarisation experiments. The actual stack tem-perature was measured by a thermocouple located inside thestack close to the outlet of the cooling fluid. The temperatureof the thermostatic fluid at the stack inlet was also monitoredand the thermo-cryostat regulated to maintain the thermalgradient between the stack inlet and outlet to less than 2 °C.Electrochemical polarisations were carried out by using anH&H 5600 electronic load. The overall stack voltage and thevoltages of the various cells were measured by AdvancedMeasurements high common mode rejection ratio digital

voltmeters. All the instruments of the test station were con-trolled by a Labview® software and PXI National Instrumentsinterface boards. The test station also included separateinstrumentation for the electrochemical diagnostics i.e., aGamry EIS300 electronic board for ac-impedance spectrosco-py, an Agilent digital memory oscilloscope for the currentinterrupt method, a Powerten power supply for hydrogenpumping measurements. The electrochemical measurementswere carried out according to a characterisation protocoldefined in the Autobrane project. This protocol included areference polarisation at a conventional temperature (80 °C)in a current range from 0 to 360 A, polarisation experimentsat different temperatures as well as relative humidity, shortendurance tests at high temperature and variable pressureand duty cycles. Hydrogen and air stoichiometry values were1.5 and 2, respectively, for currents higher than 72 A (a con-stant flow rate was used at lower currents). The temperaturewas varied over a wide range relevant to automotive applica-tions and mainly ambient pressure (1 bar abs.) or 1.5 bar abs.were used. The stack behaviour at higher pressures wasinvestigated in one case by polarisation and galvanostaticsteady-state experiments. The relative humidity was con-trolled by varying the temperature of the humidifiers withrespect to the operating temperature of the stack. The gasinlets were maintained at the same temperature of the humi-difiers by heating tapes.

3 Results and Discussion

3.1 Membrane Properties

The new SSC perfluorosulphonic membrane Aquivion™E79-03S was characterised by an EW of 790 g eq–1. Larger val-ues for the heat of fusion at the same EW were determined byDSC for the family of short side chain Aquivion™ as com-pared to conventional long-side chain Nafion-type mem-branes (Figure 2). This provides clear evidence of a highercrystallinity in the Aquivion™ with a corresponding increaseof about 60 °C in the glass transition temperature (Tg) com-pared to Nafion®, as indicated in the Introduction [26]. Theproton conductivity of the Aquivion™ membrane was mea-

Fig. 2 Heat of fusion for SSC versus long-side chain perfluorosulphonicionomer membrane precursor polymers.

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sured at different RH values, over a wide temperature rangefrom ambient to 120 °C (Figure 3). At high RH values therewas minimal change in the conductivity as a function of tem-perature, whereas the most significant decrease in protonconductivity occurred when the RH decreased.

From the practical operation standpoint, a limited amountof electrical energy is dissipated on the auxiliaries (e.g., humi-difiers, pre-heaters) when the stack is operated at low temper-ature, even in the presence of a high relative humidity; yet, acumbersome heat exchanger is necessary (to maintain theseconditions), which may not be compatible with the automo-tive requirements. On the other hand, maintaining a high lev-el of RH at high temperature would require a large amount ofwater and energy loss on the auxiliaries as well as the opera-tion at high pressure to avoid water boiling. This implies theuse of an air compressor instead of a blower. The amount ofwater present inside the membrane and at the electrode–elec-trolyte interface depends on both external humidification andoperating conditions. The internal humidification representsan important contribution to the membrane and electrodeshydration. Since the anode dehydrates faster due to the elec-tro-osmotic drag, a thin membrane is required to favourback-diffusion of the water produced at the cathode. For thisreason and to reduce the cell resistance, a 30 lm thicknessAquivion™ membrane was selected for the manufacturing ofthe large area MEAs. The good crystallinity and appropriatemechanical properties of Aquivion™ were essential to main-tain good structural integrity for the thin membrane underthe various operating conditions.

The polarisation curves for the six-cell short stack underconventional temperatures (100% RH) and ambient pressureare shown in Figure 4. The stack showed appropriate perfor-mance at both low temperature and 80 °C. Smaller activationlosses at low current densities and moderate ohmic losses atintermediate current densities were recorded with respect toconventional MEAs [37]. The combination of highly activeelectrocatalysts [35–37] with a thin (30 lm) short side chainperfluorosulphonic membrane with super-acid properties(EW of 790 g eq–1) allowed an enhancement of the reactionrate at low temperatures.

These properties are useful for a rapid start-up of thedevice. Preliminary experiments on cold start-up from ambi-ent conditions (with thermostatic apparatus off) have showna suitable capability for the stack to reach high current densi-ties necessary for a very rapid self-heating of the device.

A slight increase of performance was registered at 80 °Cwhen the pressure was increased from 1 to 1.5 bar abs. at100% RH (Figure 5). An increase of pressure was especiallyuseful at temperatures above 100 °C to maintain a smallamount of liquid water inside the membrane. However, inpractical applications, the pressure cannot exceed a certainlimit, which is determined by the technology of the airblowers for automotive applications presently available onthe market.

The slight increase of performance with the pressureunder fully humidified conditions at a conventional operat-ing temperature (Figure 5) was mainly due to the positivereaction order (∼1) with respect to the oxygen partial pressurefor the oxygen reduction process in acidic electrolytes [1]. Afurther increase in performance at high current densities was

Fig. 3 Variation of the ionic conductivity for a bare Aquivion™ membraneas a function of temperature at different levels of relative humidity (RH).

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achieved at 80 °C and 1.5 bar abs. by decreasing the tempera-ture of the cathode humidifier from 80 to 64 °C. The corre-sponding variation of relative humidity at the cathode wasfrom 100 to 50% RH (Figure 6).

The slight increase in stack voltage at high currentsappears to be related to a lower flooding effect of the cathodeunder 50% RH at high current densities. By testing severalshort stacks, the recorded power densities under these condi-tions were in the range of 600–700 mW cm–2 at an average cellvoltage of 0.6–0.65 V. These results appear appropriate bothin terms of performance and electrical efficiency for automo-tive applications. The cell voltage distribution for the variouscells in the stack was sufficiently homogeneous indicatingsuitable mass transport properties for the MEAs and stackhardware design (Figure 7). However, it should be noted thatsome increase of voltage fluctuation at high current densitywas recorded (Figure 7) probably due to the large amount ofelectrochemically-produced water that may affect the behav-iour of neighbouring cells.

Polarisation and power curves as a function of tempera-ture, from 70 to 110 °C, are reported in Figure 8 a and b

respectively, for the JMFC MEAs-based stack. Above 80 °C,the performance decreased as the temperature increased dueto an increase of resistance caused by dehydration of themembrane (Figure 8). The trend of the stack performancewith temperature (Figure 8) is closely related to the behav-

iour of the average cell resistance. Thelatter showed an increase with the tem-perature, above 80 °C, related to dehy-dration and a decrease with the operat-ing current due to the occurrence ofinternal humidification (Figure 9).

After testing several short stacksassembled under different conditions, itwas determined that the initial value ofstack compression had a significant effecton the stack performance at both lowand high temperature operating condi-tions. This aspect is clearly observed inFigure 10 for conventional operatingconditions. A high compression allowsfor lower contact resistance but, above acertain limit, a large pressure drop wasobserved which caused an increase ofmass transport constraints. The optimum

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value of compression varied indeed for the differentsets of MEAs; it is thus related to the MEA charac-teristics as well as to the stack macroporous GDL/current collectors.

Figure 11 shows polarisation curves as a functionof the temperature in a six-cells short stackassembled with Solvicore MEAs (active area360 cm2). The stack compression was 13 kg cm–2.The decrease of stack performance above 100 °Cwas due to an increase of the ohmic resistance withthe temperature. This aspect was already discussedfor the JMFC MEAs-based stack and attributed tothe dehydration of the membrane above 100 °C.

Figure 12 shows histograms related to the peakpower in the high temperature range for two stacksassembled with the same type of MEAs but underdifferent compressions. The effect of compressionon the stack performance was also observed at the

various temperatures; however, it was less relevantat 110 °C. As reported above, the best performanceof about 700 mW cm–2 was achieved at 80 °C.The short stacks showed a suitable power outputup to 100 °C with power densities exceeding600 mW cm–2 and a moderate decrease at slightlyhigher temperatures (110 °C). However, the perfor-mance decreased in the range 120–130 °C possiblydue to strong dehydration effects as a consequenceof the decrease of relative humidity as indicated inFigure 12.

Typical cell resistances of 0.05–0.08 Ohm cm2

were measured at 80 °C under optimal compressionfor the Solvicore stack whereas these reached thevalues of 0.15–0.18 Ohm cm2 under poor compres-sion. These results were confirmed by cross-com-parison of the data obtained from the current inter-rupt, ac-impedance spectroscopy and hydrogenpumping methods. The cell resistance increasedfrom 0.05 to about 0.2 Ohm cm2 on increasing tem-perature from 80 to 110 °C, due to the membranedehydration similar to that shown in Figure 9 forthe JMFC MEAs based stack. This indicated a pre-dominant effect of the membrane (the same in thetwo stacks) with respect to the other MEAs compo-nents in determining the ohmic resistance.

Figure 13 shows a comparison in single cell, car-ried out at Solvicore, of the polarisation curves forNafion 111™ and Aquivion™-based MEAs withsame electrodes, under practical operating pressure(1.5 bar abs.), in a temperature range from 80 to110 °C. During single cell tests a lower relativehumidity was used for the cathode with respect tothe anode to get insights into the capability of MEAto utilise water produced at the cathode. A tempera-ture limit of 110 °C was necessary because this wasthe maximum operating temperature at which we

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were able to achieve reasonable polarisation curves withNafion™ at the operating pressure of 1.5 bar abs. At tempera-tures higher than 110 °C, due to the strong de-hydration ofNafion™ with consequent decrease of conductivity [38], theinternal resistance increased significantly under practical con-ditions i.e., 1.5 bar abs. and low R. H., causing strong fluctua-tion of the data and poor performance (including low OCV).It is clearly observed from Figure 13 that the Solexis mem-brane is performing better under both conventional and high-er temperature operating conditions. The power density gapwas larger at the maximum operating temperature (110 °C)of Nafion at 1.5 bar.

Although, the AquivionTM membrane shows significantlybetter water retention properties at high temperature than theconventional perfluorosulphonic membrane, the most effec-tive proton conduction mechanism still relies on the vehiclemechanism where proton transport is assisted by water mole-cules [8, 13]. At high temperature, a Grotthus mechanismmay also contribute to the overall conductivity [8, 13] and thethin (30 lm) Aquivion membranes also favour the back-diffu-sion to the anode of the water produced at the cathode. Stackoperation at high current densities is thus essential to pro-mote the internal self-humidification of the MEAs. Typicalpolarisation and power density curves for short stack cells at110 °C, 1.5 bar abs., 33% R. H. (cathode and anode), at differ-ent compressions, are shown in Figure 14. A power densityof about 370–400 mW cm–2 was recorded at a cell voltage of0.5–0.6 V. These results are promising for automotive appli-cations. The results showing normalised performance (powerdensity) for several short stacks equipped with JMFC and Sol-vicore MEAs is reported in Table 1. This data-set is based onfive stacks assembled under different conditions.

It should be pointed out that polarisation experimentsshown above were performed under well-controlled temper-ature conditions i.e., the stack reached the thermal equilib-rium at the operating current density. Under practical opera-tion conditions under automotive applications, the stack mayexperience rapid changes of current at both conventional andhigh working temperatures. Simplified duty cycles of currentand temperature were thus applied to the stack and the corre-sponding variation of the stack voltage and actual tempera-ture was monitored. The initial and final of several consecu-tive duty cycles are shown in Figure 15. The initial stackcompression was 10 kg cm–2 in this case and a stackequipped with five-cells was used in this experiment. Al-though some voltage/temperature fluctuation was observed,no significant decay of performance was recorded after thesecycles; the temperature deviation with respect to the set-pointof the cooling device was slightly larger than that observed insteady-state polarisation experiments; however, the recordedstack voltage was similar to that observed in the polarisationcurves for similar operating conditions and stack compres-sion.

An analysis of the behavior of several stacks showed thatat high temperature the effect of membrane resistance wasdominant compared to the effect of contact resistances

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Fig. 13 a–c Comparison of the polarisation curves for Nafion 111™ andAquivion™-based Solvicore MEAs, under practical operating pressure(1.5 bar abs.), in the temperature range from 80 to 110 °C. Relativehumidity at the cathode (RHC) and anode (RHA) for each operating tem-perature is indicated in the legend.

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Fig. 14 Polarisation and power density curves at 110 °C and 33% RH of single cells in short stacks (active area 360 cm2). (a) Solvicore MEA, compres-sion 15 kg cm–2; (b) JMFC MEA compression 13 kg cm–2.

Table 1 Power density characteristics for JMFC (a) and Solvicore (b)MEAs-based stacks assembled under different conditions.

(a)Temperature°C

RH% Pressure barabs.

Peak Powerdensity mW cm–2

Power density at0.65 V mW cm–2

30 100 1.5 577 51640 100 1.5 582 52250 100 1.5 601 57660 100 1.5 623 58370 100 1.5 630 59280 100 1.5 675 67590 70 1.5 648 648100 50 1.5 604 525110 33 1.5 370 325

(b)Temperature°C

RH% Pressure barabs.

Peak Powerdensity mW cm–2

Power density at0.65 V mW cm–2

30 100 1.5 542–472a) 503–465a)

50 100 1.5 575–580a) 520–580a)

60 100 1.5 593–677a) 522–680a)

70 100 1.5 608–684a) 528–684a)

80 100 1.5 668–730a) 668–730a),b)

90 70 1.5 531–697a) 447–697a),b)

100 50 1.5 485–634a) 390–634a)

110 33 1.5 370–400a) 375–408a)

120 25 1.5 261 261130 18 1.5 235 202

a) After optimisation of compression; b) Measured at 0.7 V Stoich. H2 1.5, air 2.

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between bipolar plates-flow fields and electrodes.The increase of stack compression produced a smalldecrease of cell resistance at high temperature butthere was an associated risk of damaging the MEAs.For a short endurance test, a low stack compressionwas thus selected i.e., 10 kg cm–2 (Figure 16). Thestack showed a stable performance under continualoperation at 110 °C and 33% relative humidity overseveral hundred hours. The experiment had to bestopped after that a minor problem occurred to aback-pressure component of the test station; how-ever, the stack was performing well during theentire test. Stable performance was also attained inhigh temperature (110 °C) endurance tests (500 h)carried out separately by JMFC and Solvicore in sin-gle cell on similar MEAs (see below). The effect ofpressure was investigated on the stack under opera-tion at 110 °C (Figure 17). A clear increase of perfor-

mance with the operating pressure was recorded inboth steady-state galvanostatic experiments (Fig-ure 16) and polarisation curves (Figure 17). At hightemperature, the pressure plays a significant role inenhancing the performance. This was due to the factthat a high pressure enables a small fraction of liq-uid water to be maintained at 110 °C that is essen-tial for the proton conduction according to thevehicle mechanism [8, 13, 39]. This is a further con-firmation of the ionic conduction mechanism insidethis system. It is derived that an increase of waterretention characteristics for the membrane wouldbe appropriate at 110 °C to get advantage of theMEA self-humidification by the electrochemicallyproduced water. It should be mentioned that if thepressure is increased up to 3 bar abs. and full humi-dification (100% RH) is used, suitable polarisationcurves can be obtained (and also with Nafion 111™[2, 40, 41]). But these conditions are of little practicalinterest for automotive applications [1]. Suitableperformance at high temperature and relatively lowpressures have been reported in the literature foralternative membranes such as phosphoric aciddoped polybenzimidazole [14] and phosphotungs-tic acid doped membranes [42]. However, these sys-tems like phosphoric acid fuel cells appear moreappropriate for stationary power generation [43].

The progress beyond the state of the art for thepresent MEAs is represented by the fact that accep-table performances are now possible in a wide tem-perature range from ambient to 110 °C with moder-ate pressures (up to 1.5 bar abs.) and relativehumidity down to 33%.

One of the above mentioned 500 h endurancetests at 110 °C, 500 mA cm–2, 1.5 bar abs. with RHC18% and RHA 33%, carried out on a single cell atSolvicore laboratories is shown in Figure 18. Polari-sation tests were performed at specific time inter-

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Fig. 15 Duty cycles (a) 1st cycle; (b) 5th cycle; (c) Nominal current and temperatureset point of the cooling apparatus. 5-cell short stack assembled at 10 kg cm–2 withJMFC MEAs (active area 360 cm2).

Fig. 16 Steady-state galvanostatic (144 A) stack operation at different pressures withfixed temperature (110 °C), and RH 33%. JMFC MEAs-based stack.

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vals as indicated by the spikes in the chrono-potentiometricanalysis. These conditions are even more critical than thoseused in the stack testing where the RH values for both anodeand cathode were fixed to 33% at 110 °C. However, it is ob-served that, even under such harsh conditions, the MEAshows appropriate stability with a small decay of 6 × 10–

2 mV h–1. The latter may probably be associated with a smallrelease of cell compression whilst operating at high tempera-ture than to an effective degradation of the MEAs characteris-tics.

4 Conclusion

PEMFC short stacks based on new high temperatureMEAs developed in Autobrane, an EU Framework 6 project,

were investigated under various operatingconditions and operated at temperatures upto 130 °C. The stacks showed appropriateperformance in a wide temperature rangefrom ambient to 100 °C. Only a moderatedecrease in performance was observed at110 °C, 1.5 bar abs. with 33% RH MEAhydration in the stack at high temperaturewas mainly assured by the internal humidifi-cation and the back-diffusion of the waterfrom the cathode to the anode through thethin (30 lm) low EW (790 g eq–1) perfluoro-sulphonic acid membrane. The new Aqui-vion™ E79-03S membrane showed highconductivity, good water retention and me-chanical properties above 100 °C as com-pared to the conventional perfluorosulpho-nic acid membranes and appropriatecharacteristics for a rapid start-up in a coldenvironment as well as suitable operation induty-cycles. These promising characteristicswere supported by improved JMFC and Sol-vicore MEA structures, with especially opti-mised catalyst layers including proprietaryelectro-catalysts developed by JMFC andUmicore for high performance at Autobraneconditions. Significant efforts were alsoaddressed to develop GDL allowing suitablemass transport properties in a wide tempera-ture range. Optimised GDL materials asused in MEAs tested exhibited a reducedwater transport capability. Duty cycles stud-ies at high temperature showed appropriateelectrochemical behaviour under variousoperating conditions. The MEAs showedexcellent performance stability in a stackconfiguration during operation at 110 °C.Moreover the enhanced characteristics ofAquivion™ versus Nafion™ were confirmedby single cell tests over the temperature

range (80°–110 °C). These aspects clearly identify a progressbeyond the state-of-art achieved in the Autobrane project inthe field of PEMFCs. To the best of our knowledge, no effortshave been made up to now to demonstrate the endurance ofperfluorinated membranes at high temperature and in a prac-tical stack. These MEA characteristics appear promising forfuture automotive application.

Acknowledgement

The authors acknowledge the financial support of the EUthrough the Sixth Framework Autobrane (SES6-CT-2005-020074) project. The authors are also grateful to Nuvera forthe supply of stack hardware and to Dr. K. Franchi andMr. G. Fleba for the collaboration. The authors are indebted

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Fig. 17 Polarisation and power curves at different pressures and 110 °C, 33% RH after about100 h operation; JMFC MEAs-based stack compressed at 10 kg cm–2.

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Fig. 18 Single cell endurance test at 110 °C, 500 mA cm–2, 1.5 bar abs. with RHC 18% andRHA 33%.

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with the Autobrane project coordinator Dr. Goesta Pfundtner(Daimler) for all efforts made on this programme.

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high temperature operation of a solid polymer electrolyte fuel cell stack based on a new ionomer...

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