Electrochemistry Communications 9 (2007) 2045–2050 www.elsevier.com/locate/elecom
Acid–gel-immobilized, nanoporous composite, protonic membranes as low cost system for Direct Methanol Fuel Cells J. Hassoun a, F. Croce b
b,*
, C. Tizzani a, B. Scrosati
a
a Dipartimento di Chimica, Universita` ‘La Sapienza’, P.le A. Moro, 5, 00185 Roma, Italy Dipartimento di Scienze del Farmaco, Universita` D’Annunzio’, Via dei Vestini 31, 66013 Chieti, Italy
Received 14 May 2007; received in revised form 22 May 2007; accepted 23 May 2007 Available online 9 June 2007
Abstract An alternative, low cost, proton conducting electrolyte, designed for low and intermediate temperature DMCF and formed by new type of nanoporous, composite membrane in which sulphuric acid is immobilized by gelification, is here reported. The membrane is based on a polyvinylidene fluoride polymer matrix containing dispersed SiO2 ceramic powder at nanoparticles size. The stability of the immobilized sulphuric acid gel combined with the favorable swelling effect of the extended membrane porosity give to the membrane a high proton conductivity, a low methanol crossover and a satisfactory thermal stability. Good performances both at room and intermediate temperatures are showed by Direct Methanol Fuel Cells which use this system as separator. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Acid; Gel; Membrane; Composite; Proton; Fuel cells
1. Introduction A today epochal challenge for humankind is to deal with the consequences of CO2 production. Vehicle transportation contribute greatly to the production of this dangerous green-house gas with the complication of being a delocalized source. Fuel cells, both Direct methanol [1], DMFC, and Solid Oxides [2], SOFC, seem potentially attractive devices for power generation in very low-emission electrical vehicles. These systems are capable to utilize liquid fuel for power generation with an intrinsic efficiency higher than that of internal combustion engines and with almost negligible polluting emissions [3]. Respect to SOFCs , DMFCs join to the attractiveness of using a liquid fuel, which could be obtained from natural products, the possibility to operate at relatively low temperature by using proton-conducting plastic membranes. *
Corresponding author. E-mail address:
[email protected] (F. Croce).
1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.05.032
Such membranes, in order to guarantee to the fuel cell a high power output and a high energy transformation efficiency, should associate high protonic conductivity with low methanol crossover. Moreover, an eventual mass production of fuel cells will necessarily require low cost membranes. The presently ‘state of art’ membranes, such as perflorosulphonate polymers, whose prototype is NafionÒ, suffer by low thermal stability and high methanol permeability. Besides, their cost is comparable, if not higher, than that of the noble metals used as catalysts in both anode and cathode electrodes to boost the sluggish electrodic reactions [4]. In a previous paper [5] we have demonstrated that micro-porous membranes prepared out of PVdF–CTFE copolymer (Solef 32008Ò) could be usefully utilized in DMFC. To confer proton conductivity, concentrated sulphuric acid aqueous solution was entrapped into the micro-cavities of the membranes. In such a case, the acid solution is held inside the micro-pores of the membrane by the capillary forces exerted between the ceramic filler surfaces, the pore walls and the ionic and molecular
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solution components. This procedure does not guarantee a complete confinement of the acid inside the membrane and could represent a challenge for fuel cell stack engineering. To try to tackle with this problem we have prepared new membranes following the same procedure previously described [5] with the difference that the concentrated sulphuric acid solution has been transformed in a gel inside the micro-pores of the membrane. Here we show that these new ‘‘gel’’ membranes, while still maintaining an elevated ionic conductivity, are characterized by both improved thermal stability and methanol crossover. Furthermore, they show a promising response when tested in laboratory scale prototype DMFCs. 2. Experimental The nanoporous membranes utilized to obtain the proton conducting separators are constituted by an inert matrix of PVdF–CTFE (Solvay-Solexis-SolefÒ 32008) copolymer with dispersed SiO2 ceramics. The PVdF–CTFE is a copolymer formed by the monomers VF2 (vinylidene fluoride; –CH2@CF2–) and CTFE (chlorotrifluoethylene; –CFCl@CF2–) in the molar ratio 80:20. This matrix has the structural role of assuring mechanical support to both the gel electrolyte and to the electrodes in the final Membrane Electrodes Assembly (MEA). The procedure of synthesis is essentially the same as that illustrated in our previous work [5] with the addition of a final step for the formation of the gel inside the pores of the polymeric matrix. First, the PVdF–CTFE copolymer was intimately mixed in a ball miller with the required amount of the ceramic powder (SiO2 fumed silica, 99.8% Aldrich, particle size 14 nm, surface area 200 ± 25 m2/g, Cat. N. S5505). Separately, dibutylphthalate (DBP, Aldrich), which acts as plasticizer and pore precursor component, was dissolved in acetone. The resulting solution was then added to the PVdF–CTFE– SiO2 mixed powders and magnetically stirred overnight at room temperature. As a result, the polymer and DBP completely dissolve in the acetone and a homogeneous slurry with dispersed SiO2 is obtained. The slurry was at this point poured on a glass sheet and cast into a 100 lm thick film by Doctor Blade. After drying the films were repeatedly washed with diethyl ether to extract DBP to finally obtain flexible and highly porous membranes. Several samples, with different SiO2 content, were prepared. Table 1 lists the samples studied in this work and their related composition.
Table 1 PVdF–CTFE composite membranes examined in this work Sample
SiO2 content (wt%)
PVdF-0 PVdF-5 PVdF-10 PVdF-20 PVdF-30 PVdF-40
0 5 10 20 30 40
At this point, the dry porous films were impregnated, at room temperature and under vacuum, with LudoxÒ HG40 (40% colloidal silica, Aldrich) and, subsequently, soaked for 3–4 h in a 6 M H2SO4 solution to promote the formation of a protonic conducting gel entrapped inside the open porosity of the membranes. The through-plane conductivity of the swelled membranes was obtained by impedance spectroscopy run on symmetric Pt/membrane sample/Pt cells in a 1 Hz–1 MHz frequency range using a computer controlled Solarton 1260 FRA. The differential scanning calorimetry, DSC, was performed using a Mettler Toledo DSC821e with a scan rate of 10 °C min 1, starting from the temperature of 25 °C and heating to the temperature of 160°, then cooling to the temperature of 40 °C and, finally, heating to a temperature of 25 °C. The thermal gravimetric analysis, TGA, was performed using a Perkin Elmer instrument at a scan rate of 5 °C min 1 in the 25–220 °C temperature range. Two different kinds of electrodic membranes were used to realize the MEA for the fuel cell tests. Namely, for room temperature tests, a single-layer electrodic membrane (SLEM) was utilized, while, for intermediate temperature (50 °C) tests, a three-layer electrodic membrane (TL-EM) was chosen. SL-EMs were fabricated by a procedure quite similar to that adopted for the preparation of the electrolyte membrane samples. Accordingly, a single-layer was obtained by first intimately mixing a blend of Super P carbon and Pt black (6:4 weight ratio) with PVdF powder (6020 Solvay-Solef Binder) in a 20% total weight ratio. The blend was dispersed in acetone and added with a Teflon emulsion in a 1:1 weight ratio. The final resulting suspension was mixed with DBF in a 1:2 weight ratio. The slurry was dried for 15 min at 70 °C. This procedure gave a highly viscous paste which was pressed at 70 °C and 1 ton/cm2 to obtain a homogeneous, thin, membrane. This membrane was finally washed with diethyl ether to extract DBF and thus, promote porosity. The Pt loading in this porous, electrodic membrane was 4 mg/cm2. Two of these SL-EMs, one at the anode side and the other at the cathode side, were combined with the selected electrolyte membrane and pressed together to obtain the final MEA. TL-EMs, carbon based, were fabricated according to Refs. [6,7]. The TL-EMs are constituted by: (a) A commercial porous Carbon-Paper support (ElectrochemÒ), which could be untreated or treated with Teflon (PTFE), having 100 lm thickness, and employed as the backing layer for both anode and cathode. (b) A diffusive layer, whose role is that of homogenize the flow of reactants directed toward the catalytic layer, realized by Doctor-Blade spreading on the Carbon-Paper support (point a) a homogeneous water/isopropanol suspension prepared by mixing
J. Hassoun et al. / Electrochemistry Communications 9 (2007) 2045–2050
an appropriate amount of a PTFE emulsion with nanometric size carbon powders (Super P). The manufacture was subsequently thermally treated at 120 °C for 1 h, at 280 °C for 30 min and, finally, at 350 °C for 30 min. (c) A catalyst layer prepared out of a homogeneous suspension, formed from the desired amount of Pt/C catalyst (20 wt% Pt, ElectrochemÒ) dispersed into a NafionÒ solution (5% Nafion with ethanol as solvent, Aldrich), spray deposited onto the diffusion layer (point b) and dried at 70 °C for 30 min. Two of these three-layer electrodic membranes, one at the anode side and the other at the cathode side, were sandwiched with the selected electrolyte membrane and pressed together to obtain the final MEA. The current–voltage curves of the cells were obtained by a PAR 273A potentiostat under air flux at the cathode side and a 2 M methanol aqueous solution flux, acidified by H2SO4, at the anode side. The methanol crossover was determined using a U-shaped cell having two compartments separated by the given membrane sample. One compartment was filled by water and the other by a methanol aqueous solution. At fixed time intervals, samples at the water side were analyzed by gas chromatography to monitor the methanol crossing through the membrane. The chromato-
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graphic stationary phase was polyethylene glycol (Carbowax 66). 3. Results and discussion Fig. 1 shows the scanning electron microscopy, SEM, images of PVdF-0 (A), of PVdF-10 (B) samples and the energy dispersive X-ray spectrometry (EDS) of PVdF-10 (C) sample. The high and uniformly distributed porosity is clearly visible in both samples. The presence of this extended porosity, involving a sequence of empty cavities of nanometric size (198 nm), is essential for favoring the entrapment of immobilized acid gel [5]. Fig. 1C shows also a uniform distribution of ceramic filler which induces an uniform absorption of LudoxÒ colloidal silica and consequently, an uniform distribution of the immobilized acid gel. These properties suggest that a good behavior can be expected for this membranes when used as electrolyte in a Fuel Cell. Fig. 2 reports the differential scanning calorimetry, DSC (A) and the thermal gravimetric analysis (B) of the gel formed by acidification of Ludox HS40 colloidal silica with 6 M H2SO4. Observing the DSC profile, Fig. 2A, it is possible to see a large endothermic peak, starting from a temperature of 115 °C. This peak can be ascribed to the thermal decomposition of the gel in a crystalline silica phase and in an aqueous acid phase. The absence of the re-crystallization peak during the cooling step confirms
Fig. 1. SEM pictures of PVdF-0 (A), of PVdF-10 (B) and EDS analysis of PVdF-10 (C) membrane samples. For samples identification see Table 1.
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Fig. 2. Differential scanning calorimetry, DSC, (A) and the thermal gravimetric analysis (B) of the immobilized acid gel based on Ludox HS40.
the absence of the amorphous–crystalline transition and, accordingly the hypothesis of the decomposition. The thermal gravimetric analysis, TGA, of the Ludox HS40 colloidal silica gel, reported in Fig. 2B, shows a loss of weight associated with a peak centered at the temperature of 100 °C in the derivate curve, ascribed to the water evaporation process. The room temperature conductivity of the membranes remains unchanged for various days, as shown by Fig. 3A which reports the time evolution of the conductivity of various membrane samples at 25 °C. The related Arrhenius plots, reported in Fig. 3B, clearly demonstrate that the conductivity remains constant passing from room temperature to 80 °C. This demonstrates that the membranes are stable and no appreciable release of the gel phase does occur at intermediate temperatures. The best membranes have a conductivity of the order of 10 2 S cm 1 with a high ceramic content, and, thus, they are the most appropriate for application in fuel cells designed for operation in the 25–80 °C temperature range. Accordingly, laboratory prototype fuel cells have been assembled by using the best PVdF-based composite ‘acid– gel-immobilized’ membrane as the electrolyte separator. A single-layer electrodic membrane, SL-EM, based MEA has been used for tests at the room temperature, and a three-layer electrodic membrane, TL-EM, based MEA has been used for tests at the intermediate temperature of 50 °C. Fig. 4A compares the current–voltage and the current–power curves of laboratory type DMFCs measured at room temperature and using the sample PVdF30 ‘acid–gel-immobilized’ membrane and the commercial NafionÒ 117, respectively, as the electrolyte separator. The cell based on PVdF-30 ‘acid–gel-immobilized’ membrane shows a much better response (i.e. a power density of about 2 mW cm 2 and a current of the order of 33 mA cm 2) in comparison with the cell based on the commercial NafionÒ 117 (i.e. a power density of about 1.1 mW cm 2 and a current of the order of 18 mA cm 2).
Fig. 3. Time evolution of the conductivity of various membrane samples at 25 °C (A) and the related Arrhenius plots (B). For samples identification see Table 1.
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Fig. 5. Comparison between the methanol crossover level of various membrane samples investigated in this work and the methanol crossover level of a commercial NafionÒ 117 membrane. For samples identification see Table 1.
Fig. 4. (A) Comparison between the current–voltage, current–power curves, obtained at room temperature, of laboratory type DMFCs using, respectively, the sample PVdF-30 ‘acid–gel-immobilized’ membrane and the commercial NafionÒ 117 as electrolyte separators. (B) Comparison between the current–voltage, current–power curve of laboratory type DMFCs obtained using the sample PVdF-30 ‘acid–gel-immobilized’ membrane at room temperature and at the temperature of 50 °C. For sample identification see Table 1.
The performances of the cell based on PVdF-30 ‘acid–gelimmobilized’ membrane increase substantially at the intermediate temperature of 50 °C, reaching a power density of about 6.1 mW cm 2 and a current density of the order of 71 mA cm 2, see Fig. 4B. This increase can be ascribed to the higher activity of the catalyst at this temperature, as well as to the positive effect of the three-layer electrodic membrane, TL-EM, based MEA, in terms of flow homogeneity and optimal reactant diffusion. The optimized performances of laboratory prototype fuel cells using the PVdF-based composite,‘acid–gel-immobilized’ membrane as the electrolyte separator suggest that this membrane is quite promising as a new type of membrane suitable for application in low-rate, room and intermediate temperature DMFCs. The value of the methanol crossover plays a fundamental role for membranes proposed for DMFC application.
Fig. 6. The time evolution of the open circuit voltage, at room temperature, of the laboratory prototype fuel cell using the PVdF-10 ‘acid–gel-immobilized’ membrane sample as the electrolyte separator. For sample identification see Table 1.
In fact, the methanol crossover influences various cell parameters, including energy efficiency [8]. The comparison between the methanol crossover level of the various membrane samples investigated in this work and that of a commercial NafionÒ 117 membrane is shown in Fig. 5. As expected [5] the methanol crossover increases for the membranes having a progressively higher ceramic content [5]. In addition, we can observe, for the membranes with a filler content up to 10% a methanol crossover much lower than that experienced by the commercial NafionÒ 117 membrane. An interesting approach for the evaluation of the average crossover consists on the observation of the time evolution of the open circuit voltage (OCV) of a given DMFC [9,10]. Fig. 6 shows the time evolution of the open circuit voltage at the room temperature of the laboratory prototype fuel cell using the PVdF-10 ‘acid– gel-immobilized’ membrane sample as the electrolyte separator [5]. The occurrence of methanol crossover is revealed by the decay of the OCV. However, the loss is
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limited to 3.9%, this confirming the good selectivity of the membrane.
brane over that based on NafionÒ is undoubtedly its low cost and its simple fabrication procedure.
4. Conclusions
Acknowledgements
In this paper, we prepared novel ‘acid–gel-immobilized’ membranes based on a PVdF polymeric micro-porous matrix in which a concentrated H2SO4 solution was immobilized by gelification. The membranes were studied in terms of their morphology, thermal properties, ionic conductivity and methanol crossover. The membranes were used as separators to realize MEAs for DMFC by utilizing, respectively, two different kind of electrodic configurations: SL-EM and TL-EM. The electrochemical performances of such MEAs were characterized at room and at intermediate temperatures (50 °C) and compared with the performances of similar MEAs prepared by commercial NafionÒ 117 membranes. The single cell performances of the PVdF-‘acid–gel-immobilized’ electrolyte membranes overcome those of conventional NafionÒ membranes tested in the same operating conditions either in terms of current density and power density. Moreover, these new, innovative, ‘acid–gel-immobilized’ membranes show a methanol crossover significantly lower than that of the NafionÒ 117 membrane, when measured in the same operating conditions. It is worth underlining that one of the major potential advantage of the MEA based on this ‘acid–gel-immobilized’ electrolyte mem-
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