Slade, RCT

Varcoe, JR

Electrochemistry Communications

University of Surrey

1An electron-beam-grafted ETFE alkaline anion-exchange membrane inmetal-cation-free solid-state alkaline fuel cells.John R. Varcoe* and Robert C. T. SladeChemistry, The University of Surrey, Guildford GU2 7XH, United Kingdom.Tel: +44 (0)1483 682616 Fax: +44 (0)1483 686851*Author for correspondence: E-mail: j.varcoe@surrey.ac.ukA novel, physically strong, quaternary-ammonium-functionalised radiation-graftedETFE alkaline anion-exchange membrane (AAEM) has been developed forapplication in fuel cells without undesirable addition of M+OH- into the anode fuelstream, thereby facilitating stable performances and long term operational lifetimes. Apromising H2/O2 fuel cell peak power performance of 130 mW cm-2 was obtained foran alkaline membrane electrode assembly, while a maximum power density of 8.5mW cm-2 was obtained in a metal-cation-free methanol/O2 fuel cell. Mass transportphenomena in the electrodes and inadequate interface between the membrane andelectrodes were identified as the major limiters of power performance; the need forelectrode architectures to be optimised for alkaline membrane fuel cells wasidentified.Keywords: Radiation-grafting; Alkaline anion-exchange membrane; Fuel cell; ETFE; Methanol;Metal-cation-free alkaline membrane electrode assembly21. IntroductionThere is an increasing interest in the application of alkaline anion-exchangemembranes (AAEMs, membranes that conduct hydroxide ions, the high pHequivalent to proton-exchange membranes such as Nafion® from Du Pont) in metal-cation-free all-solid-state alkaline fuel cells. The advantages, disadvantages andrationale of this have been explained in detail in a recent review [1]. A recentbreakthrough towards this target was the development of an alkaline polymerelectrochemical interface, an OH--conducting analogue of the acidic Nafion-dispersions used in proton-exchange membrane (PEM) membrane electrodeassemblies (MEAs) [2]; this interface enables the preparation of metal-cation-freealkaline MEAs, which improves performances and allows long-term operation of theresulting fuel cells with direct methanol oxidation at the anode (generating CO2) andCO2-containing air supplied to the cathode. Without this type of alkaline bindingpolymer, non-ideal strategies have been employed, such as addition of highconcentrations of NaOH / KOH into the methanol fuel supply [3-5]. The authors’ previous work on the development of thermally and chemically stableAAEMs for fuel cells has centred on the introduction of quaternary ammoniumfunctionality via radiation-grafting (using high radiation doses of γ-rays) ontopoly(tetrafluoroethylene-co-hexafluoropropylene), FEP, preformed membranes [6,7];these membranes exhibited conductivities between 0.010 – 0.035 S cm-1 in thetemperature range 20 - 80°C when fully hydrated (acceptable for application in fuelcells as these conductivities correspond to 20 – 33% of the levels exhibited byNafion®-115 under the same conditions) [8]. The major problem encountered with theFEP-based AAEMs, with respect to application in fuel cells, is the lack of physicalstrength and stability; the conductive AAEMs could not be tested in fuel cells on3safety grounds (especially with H2/O2 gases) as physical failure was unavoidable. Thesource of this physical weakness is the lack of stability to radiation of FEP; to achieveAAEMs of acceptable conductivity required radiation doses that degraded the FEPpolymer chains to an unacceptable level.The advance reported in this communication relates to the development of an AAEMwhere the FEP has been replaced by a poly(ethylene-co-tetrafluoroethylene), ETFE,preformed film. Partially fluorinated films such as ETFE and poly(vinylidenefluoride), PVDF, exhibit superior radiation-resistance over fully fluorinated films [9](radiation-induced radical formation via C-H bond fission dominate in partiallyfluorinated polymers, while undesirable C-C bond fission becomes significant infully-fluorinated polymers); PVDF is, however, not suitable as a base material as itchemically degrades (dehydrofluorination) in alkaline environments [7]. Thiscommunication reports highly promising initial performances when metal-cation-freeMEAs containing a radiation-grafted ETFE AAEM were evaluated in H2/O2 anddirect methanol fuel cells (DMFCs).2. Experimental2.1 Membrane preparationThe ETFE-based AAEM was produced using widely reported methods [6,8] and sothe procedure will only be summarised here (Scheme 1): ETFE (25 µm thick,Nowoflon ET-6235 film, Nowofol, Germany) was irradiated in air with an electron-beam to a total dose of 7 MRad. The irradiated ETFE was then submerged innitrogen-purged vinylbenzyl chloride monomer (VBC, Dow Chemicals, 1:1 meta-/para- mix, used as received without removal of inhibitors) at 60°C for 120 h. Theresulting ETFE-g-poly(vinylbenzyl chloride) copolymer was then immersed in4aqueous trimethylamine (50%wt, Acros Organics) at room temperature for 4 h.Immersion of the resulting anion-exchange membrane in aqueous potassiumhydroxide (1 mol dm-3) for 1 h yielded the target AAEM. The AAEM produced(thickness = 51 ± 2 µm when fully hydrated) had an ion-exchange capacity of 1.42 ±0.09 x 10-3 mol(OH-) g-1(dry polymer), as determined using a standard back titrationmethod [8] (cf. Nafion®-115 has an ion-exchange capacity of 0.92 ± 0.01 x 10-3 mol(H+) g-1(dry polymer) [10], and a fully hydrated thickness of 153 ± 3 µm). Theshort-term thermal stability of the AAEM was determined to be 130 ± 10°C in 60 cm3min-1 of flowing air (TA instruments TGA Q500 thermogravimetric analyser);detailed studies into the long-term thermal stability and conductivity (differenttemperatures and different relative humidities) of this class of AAEM are now beingundertaken.2.2 Preparation of the membrane electrode assembliesFor initial H2/O2 testing, the anode and cathode Pt/C electrodes (E-Tek, 0.5 mg cm-2 Pt/C(20%wt) on carbon cloth with unspecified (proprietary) loadings of PTFE asbinder) were spray-treated with poly(vinylbenzyl chloride) (Aldrich, used as received)to a loading of 0.51 ± 0.01 mg cm-2 using an airbrush and ethyl acetate as solvent;subsequent immersion in undiluted N,N,N’,N’-tetramethylhexane-1,6-diamine (AcrosOrganics, used as received, toxic) for 24h followed by a treatment in aqueouspotassium hydroxide (1 mol dm-3) for 1 h yielded electrodes coated with thehydroxide-exchanging polymer interface (Scheme 2) on the catalyst layer [2]. Formethanol testing, the anode (E-Tek, 4 mg cm-2 PtRu on carbon cloth with unspecifiedloadings of PTFE as binder) and cathode (E-Tek, 4 mg cm-2 HP Pt black on carboncloth with unspecified loadings of PTFE as binder) were similarly treated with the5same intermediate loading of poly(vinylbenzyl chloride). Lamination was attemptedby pressing the treated electrodes to the membrane at 3000 kgf (120 kgf cm-2) at both60 and 100°C; this was, however, unsuccessful and the electrodes and membranewere therefore simply assembled into the fuel cell fixture, non-laminated, at a torqueof 50 N m so as not to further compromise the electrodes’ and membrane’s physicalintegrity with excessive pressing.2.3 Fuel cell testingThe initial fuel cell testing was conducted on an Arbin Fuel Cell Test Station with a25 cm2 test fixture. H2/O2 testing was conducted with H2 and O2 gases (BOC,laboratory grade) flowing at 2 dm3 min-1 flow rates and controlled at 100% relativehumidity with dew point humidifiers. Voltage (V / V) and power density (Pd / mWcm-2) vs current density (i / mA cm-2) steady-state polarization curves weregalvanostatically collected using the integrated electronic load (from open circuit tohigh current densities), with data points being recorded after stabilisation for 10 minat each current density; before the curves were recorded the MEAs were conditionedby running at high current densities (50 mV potentiostatically-controlled discharge)for 30 min, followed by open circuit operation for 15 min. The MEA area resistancewas monitored automatically by the electronic load (current pulse method), whichgives routine order of magnitude values for resistances during fuel cell tests. DMFCtesting was conducted with aqueous methanol (analytical grade, 2 mol dm-3, 10 cm3min-1 pre-heated to the fuel cell temperature) replacing the H2 at the anode. Theconditioning step was as for the H2/O2 tests. Temperatures and pressures are reportedin the relevant figure captions.63. Results and Discussion3.1 H2/O2 fuel cell performancesThe H2/O2 fuel cell performances (0.5 mg cm-2 Pt/C electrodes) are presented inFigure 1. Peak power densities of 90 and 110 mW cm-2 and in situ internal resistancesof 1.0 and 1.1 Ω cm2 were recorded at 50 and 60°C respectively; these peak powerdensities are more than twice the peak power performance obtained by Agel et. alusing epichlorohdrin-based alkaline membranes [11]. Tafel slopes from plots of iR-corrected voltages vs. log10(current densities) were 71 and 60 mV dec-1 at 50 and60°C respectively. It was observed, from the deviation from linearity of the Tafelplots, that mass-transport limitations / interferences started to appear at as low as 30 -40 mA cm-2 for the AAEM-MEA (the iR-corrected cell potentials were still above 0.8V); this strongly suggests that mass-transport phenomena are significantly limitingperformance, as expected with the use of electrodes that have not been optimised foruse in AAEM-MEAs. This is further complicated by an additional water mass-transport consideration i.e. water is now a reactant at the alkaline cathode as well asbeing present for membrane hydration: ½O2(g) + H2O(g/l) → 2OH-(mem).For initial benchmarking purposes and before methanol testing, the H2/O2 fuel cellperformance at 50°C of the 4 mg cm-2 PtRu/Pt black AAEM-MEA was evaluated(Figure 2) and compared to an analogous Nafion®-115 MEA (tested under the sameconditions). The peak power density obtained with the AAEM-MEA was 130 mWcm-2, compared to the 480 mW cm-2 for the Nafion-MEA; this is a significant result asthe peak power performance achieved with this early stage, and non-laminated,AAEM-MEA is > 25% of the performance obtained with proton-exchange materialsthat have been optimised over decades. Nafion-115 was selected as the benchmarkPEM in anticipation of similar absolute resistances (Nafion is 3 times the thickness of7the ETFE-based-AAEM but also of 3 – 5 time higher conductivity) in order tominimise the number of variables affecting fuel cell performance; in situ internalresistances of 0.79 ± 0.14 Ω cm2 were observed for the AAEM-MEA and 0.39 ± 0.02Ω cm2 for the Nafion-MEA. Tafel slopes of 70 mV dec-1 for the AAEM-MEA and 61mV dec-1 for the Nafion-MEA were measured (at iR-corrected potentials above 0.8V). A deviation from linearity of the Tafel plots (mass-transport limitations) started toappear at only 120 mA cm-2 for the alkaline-MEA, compared to 1050 mA cm-2 for theNafion-MEA. This again highlights the need, with alkaline membrane MEAs, foroptimisation of the electrode structures to minimise mass-transport relatedperformance losses; mass-transport being a primary limiter of performance is fullyconsistent with the observed small (44%) increase in power densities observed at50°C (90 → 130 mW cm-2) when increasing the electrode catalyst loadings by 700%(0.5 → 4 mg cm-2).3.2 Methanol/O2 performancesAs alkaline membranes were reported to exhibit only 30% of the methanolpermeability of Nafion® [5], the ETFE-based AAEM was deliberately synthesisedthin (25 µm base ETFE membrane thickness yielding a 51 ± 2 µm thick anion-exchange membrane when fully hydrated) to offset the lower conductivities ofalkaline membranes [8]. The methanol/O2 performances are presented in Figure 3.Peak power densities of 1.5 mW cm-2 at 50°C increasing to 8.5 mW cm-2 at 80°C(with additional reactant pressurisation) were obtained and represent promisingperformances at such an early stage development of AAEM-MEA (the Nafion-115MEA produced peak power densities in the range 30 – 80 mW cm-2 under the sametesting conditions); the fact that a stable performance was obtained at a temperature as8high as 80°C is significant as AAEMs are generally considered to be stable totemperature only up to 60°C [12]. Long term in situ tests are required to investigatethis further.These AAEM-MEA performances compete well with the 10 mW cm-2 peak powerdensity obtained by Scott et al. with an alkaline-MEA containing a commercialquaternary-ammonium anion-exchange membrane (Morgane®-ADP, Solvay S.A.,Belgium), no alkaline interface polymer and 2 mg cm-2 Pt/C loadings, and tested at60°C, with 2 bar cathode pressure, and 80 cm3 min-1 methanol (2 mol dm-3) inaqueous NaOH (1 mol dm-3) [5]. Countanceau et al. report a peak power density of 18mW cm-2 at 20°C with the same membrane and electrode Pt loadings as Scott et al.with methanol (2 mol dm-3), but with even higher concentrations of aqueous NaOH (4mol dm-3); however < 0.5 mW cm-2 was obtained on the removal of NaOH from thefuel stream. Ogumi et al. obtained a peak power density of 6 mW cm-2 with anothertype of commercial anion-exchange membrane (AHA, Tokuyama, Japan) at 50°Cwith 4 mg cm-2 PtRu at the anode supplied with 50 cm3 min-1 methanol (1 mol dm-3)in aqueous KOH (1 mol dm-3), 1 mg cm-2 Pt/C at the cathode, and quaternised poly(4-vinylpyridine) as a binding material. An interesting observation in that study was thatthe replacement of the Pt/C at the cathode with non-noble Ag/C did not lead to asignificant drop in peak power – the use of non-noble metal catalysts being one of thepotential advantages of using alkaline membranes in fuel cells.4. Concluding RemarksThe results in this study, to the best of our knowledge, represent the highest peakpower performances obtained to date with metal cation-free quaternary-ammonium-functionalised alkaline-membrane-MEAs and demonstrate that the presence of9M+OH- in an aqueous methanol fuel is not needed to obtain a functioning alkalinemembrane DMFC. The absence of metal cations is vital for elimination of carbonateprecipitation in the electrode structures (a methanol / air fuel cell test with a lowperforming, non-ETFE, metal-cation-free alkaline-MEA was previously tested for 233h with very low voltage degradation and insignificant membrane carbonation [2]).Further optimisation of the electrode formulation (e.g. applying the mass-transport-optimised catalysed Ti mesh electrodes of Scott et al. [13]), which bears in mind theadded requirement for optimised water transport to the cathode (possibly byminimising loadings of hydrophobic PTFE in the cathode electrode architectures),will allow the realization of methanol fuel cell performances comparable to thosealready achieved with Nafion®, but potentially with non-Pt-containing electrodes.Determination of the conditions required for optimised alkaline membrane/electrodelamination is also considered a key step towards further performance improvement.AcknowledgmentsWe gratefully thank the Engineering and Physical Sciences Research Council (GrantGR/S60709/01) and the World Gold Council for funding, and EBIS Iotron Ltd.(Harwell, U.K.) for access to electron-beam facilities.10References1 J. R. Varcoe, R. C. T. Slade, Fuel Cells 5 (2005) 187.2 J. R. Varcoe, R. C. T. Slade, E. Lam How Yee, Chem. Commun. accepted and in-press.3 C. Coutanceau, L. Demarconnay, C. Lamy, J.-M. Léger, J. Power Sources in-press.4 K. Matsuoka, Y. Iriyama, T. Abe, M. Matsuoka, Z. Ogumi, J. Power Sources 150(2005) 27.5 E. H. Yu, K. Scott, J. Power Sources 137 (2004) 248.6 H. Herman, R. C. T. Slade, J. R. Varcoe, J. Membr. Sci. 218 (2003) 147.7 T. N. Danks, R. C. T. Slade, J. R. Varcoe, J. Mater. Chem. 13 (2003) 712.8 J. R. Varcoe, R. C. T. Slade, Solid State Ionics, 176 (2005) 585.9 J. Chen, M. Asano, Y. Maekawa, M. Yoshida, J. Membr. Sci. in-press.10 R. F. Silva, M. De Francesco, A. Pozio, J. Power Sources 134 (2004) 18.11 E. Agel, J. Bouet, J. F. Fauvarque, J. Power Sources 101 (2001) 267.12 T. Sata, M. Tsujimoto, T. Yamaguchi, K. Matsusaki, J. Membr. Sci. 112 (1996)161.13 E. H. Yu, K. Scott, J. Appl. Electrochem. 35 (2005) 91.11Scheme 1: The synthesis of radiation-grafted ETFE alkaline anion-exchangemembrane (AAEM).Scheme 2: The synthesis of the alkaline polymer electrochemical interface.Figure 1: The H2/O2 fuel cell performances (no back-pressure) of the 0.5 mg cm-2 Pt/C AAEM-MEA at 50°C (•) and 60°C (O).Figure 2: The H2/O2 fuel cell performance at 50°C (no back-pressure) of the 4 mgcm-2 catalysed AAEM-MEA (•) compared to an analogous Nafion115-MEA (O, thesame electrodes but coated with 0.8 mg cm-2 Nafion dispersion and pressed at 120 kgfcm-2 at 135°C for 3 min).Figure 3 The methanol/O2 performances of the 4 mg cm-2 catalysed AAEM-MEAlisted from lowest to highest peak power performances: 50°C (no back-pressure),60°C (no back-pressure), 60°C (2 bar and 2.5 bar back-pressures respectively at theanode and cathode), and 80°C (2 bar and 2.5 bar back-pressures respectively at theanode and cathode).  

An electron-beam-grafted ETFE alkaline anion-exchange membrane in metal-cation-free solid-state alkaline fuel cells

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University of Surrey