Impedance Spectroscopy and Performance of Cross-Linked New Naphthalenic Polyimide Acid Membranes Sen Yuan, † Carmen del Rio, ‡ Mar Lo ´pez-Gonza ´lez, ‡ Xiaoxia Guo, † Jianhua Fang,* ,† and Evaristo Riande* ,‡ School of Chemistry and Chemical Technology, Shanghai Jiao Tong UniVersity, Shanghai 200240, China, and Instituto de Ciencia y Tecnologı ´a de Polı ´meros (CSIC), 28006 Madrid, Spain ReceiVed: October 17, 2010; ReVised Manuscript ReceiVed: NoVember 17, 2010 This paper reports the synthesis, characterization, and thermal stability of cation-exchange membranes based on naphthalenic copolyimides. To improve the mechanical stability, the membranes were cross-linked through the reaction of sulfonic acid groups with hydrogen atoms of activated phenyl groups located in neighboring chains using appropriate catalysts. The ion-exchange capacity of the membranes is higher than 2 meq/(g of dry membrane), and the number of molecules of water per ion-exchange group lies in the vicinity of 20. The response of the membrane electrode assembly (MEA) to electrical perturbation fields in a wide range of frequencies was measured, and the equivalent electrical circuit governing the response is discussed. The conductivity of the MEAs at 75 °C is of the order of 10 -2 S/cm. Hydrogen crossover in the membranes was determined by electrochemical measurements and the permeability coefficient of the gas in the membranes determined. Finally, the single cell exhibits a fair performance, the power of the cell at 75 °C being only 225 mW/cm 2 or lower, at 0.5 V. Introduction Perfluorinated membranes exhibit rather good properties that make them appropriate to be used as polyelectrolytes in many applications, for example, automotive, portable and/or stationary power, etc. However, these membranes present some shortcom- ings related to environmental issues, high dependence on humidification that limits the working temperature to the vicinity of 80 °C, high methanol crossover, and, above all, a nearly prohibitive cost. As a result, the search for alternatives to perfluorinated acid type membranes has remained in the last years a large branch of fuel cell science. 1,2 The basic require- ments for good performance acidic membranes are high protonic conductivity, low permeability to fuel and oxidant, chemical stability, and good mechanical properties. High counterion permselectivity is an additional characteristic for membranes used in electrodialysis. 1 Carriers of current inside fuel cells are protons; electronic conductivity is of little concern because the polyelectrolytes usually employed are inherently isolating materials. However, perfluorinated composite membranes con- taining electronically conducting intercalations, such as poly- aniline 3,4 and poly(1-methylpyrrole), 5 have been developed in order to decrease methanol crossover. In these cases, the formation of electronically conducting paths should be avoided. Some of the characteristics required for good performance membranes are opposite properties. For example, high ion- exchange capacity (IEC) promotes proton diffusion, thus positively affecting the protonic conductivity of acidic mem- branes. However, high IEC increases membrane swelling, thus inducing disentanglement of the polyelectrolyte chains, which adversely affects the mechanical properties of membranes. Therefore, the optimization of the performance of acidic membranes is often the result of a compromise between oppo- site properties. 1 Although efforts have been made to simulate the conductivity of membranes from their chemical structure, 6,7 the screening of a large number of polyelectrolytes having different chemical structures is still the method preferred to increase the chances of finding membranes with high proton conductivity, chemical stability, and good mechanical properties. In this context, acidic ion-exchange membranes based on polyimides, polyetherke- tones, polysulfones, polyethersulfones, polyphosphazenes, etc. may meet the requirements necessary for their use as solid polyelectrolytes in fuel cells. 2,8 To reduce to a minimum hydrolytic degradation, naphthalenic, rather than phtahlic, polyimides have been used as a material base for the preparation of polyimide acidic membranes. 2 Moreover, the improve- ment of mechanical stability requires not only the use of a mixture of sulfonated and unsulfonated diamines in the synthesis of the membranes but also further cross-linking of the poly- electrolyte chains. Cross-linking reduces chain mobility and, therefore, hinders the formation of percolation paths or channels resulting from micro- or nanophase separation of hydrophilic proton-exchange sites from hydrophobic moieties. 9,10 It should be noted that hydrophobic and hydrophilic domain segregation is easier in flexible membranes, such as Nafion, containing the acidic moieties in flexible side groups rather than in membranes with the ionic groups attached to a rigid backbone. The aim of this work was to investigate the characteristics of acidic membranes prepared from new naphthalenic polyim- ides in which cross-linking of the molecular chains was performed using either phosphorus pentoxide or polyphosphoric acid. The conductivity of the membrane electrode assemblies (MEAs) was measured by impedance spectroscopy, and the performance of the membranes in single cells was explored. The chemical structures of the polyelectrolytes used to prepare the membranes and their compositions and acronyms are shown in Figure 1 and Table 1. Special attention is paid to the complex * To whom correspondence should be addressed. E-mail: riande@ ictp.csic.es (E.R.), [email protected] (J.F.). † Shanghai Jiao Tong University. ‡ Instituto de Ciencia y Tecnologı ´a de Polı ´meros (CSIC). J. Phys. Chem. C 2010, 114, 22773–22782 22773 10.1021/jp109941t 2010 American Chemical Society Published on Web 12/06/2010
Transcript
Impedance Spectroscopy and Performance of Cross-Linked New Naphthalenic PolyimideAcid Membranes
Sen Yuan,† Carmen del Rio,‡ Mar Lopez-Gonzalez,‡ Xiaoxia Guo,† Jianhua Fang,*,† andEvaristo Riande*,‡
School of Chemistry and Chemical Technology, Shanghai Jiao Tong UniVersity, Shanghai 200240, China, andInstituto de Ciencia y Tecnologıa de Polımeros (CSIC), 28006 Madrid, Spain
ReceiVed: October 17, 2010; ReVised Manuscript ReceiVed: NoVember 17, 2010
This paper reports the synthesis, characterization, and thermal stability of cation-exchange membranes basedon naphthalenic copolyimides. To improve the mechanical stability, the membranes were cross-linked throughthe reaction of sulfonic acid groups with hydrogen atoms of activated phenyl groups located in neighboringchains using appropriate catalysts. The ion-exchange capacity of the membranes is higher than 2 meq/(g ofdry membrane), and the number of molecules of water per ion-exchange group lies in the vicinity of 20. Theresponse of the membrane electrode assembly (MEA) to electrical perturbation fields in a wide range offrequencies was measured, and the equivalent electrical circuit governing the response is discussed. Theconductivity of the MEAs at 75 °C is of the order of 10-2 S/cm. Hydrogen crossover in the membranes wasdetermined by electrochemical measurements and the permeability coefficient of the gas in the membranesdetermined. Finally, the single cell exhibits a fair performance, the power of the cell at 75 °C being only 225mW/cm2 or lower, at 0.5 V.
Introduction
Perfluorinated membranes exhibit rather good properties thatmake them appropriate to be used as polyelectrolytes in manyapplications, for example, automotive, portable and/or stationarypower, etc. However, these membranes present some shortcom-ings related to environmental issues, high dependence onhumidification that limits the working temperature to the vicinityof 80 °C, high methanol crossover, and, above all, a nearlyprohibitive cost. As a result, the search for alternatives toperfluorinated acid type membranes has remained in the lastyears a large branch of fuel cell science.1,2 The basic require-ments for good performance acidic membranes are high protonicconductivity, low permeability to fuel and oxidant, chemicalstability, and good mechanical properties. High counterionpermselectivity is an additional characteristic for membranesused in electrodialysis.1 Carriers of current inside fuel cells areprotons; electronic conductivity is of little concern because thepolyelectrolytes usually employed are inherently isolatingmaterials. However, perfluorinated composite membranes con-taining electronically conducting intercalations, such as poly-aniline3,4 and poly(1-methylpyrrole),5 have been developed inorder to decrease methanol crossover. In these cases, theformation of electronically conducting paths should be avoided.Some of the characteristics required for good performancemembranes are opposite properties. For example, high ion-exchange capacity (IEC) promotes proton diffusion, thuspositively affecting the protonic conductivity of acidic mem-branes. However, high IEC increases membrane swelling, thusinducing disentanglement of the polyelectrolyte chains, whichadversely affects the mechanical properties of membranes.Therefore, the optimization of the performance of acidic
membranes is often the result of a compromise between oppo-site properties.1
Although efforts have been made to simulate the conductivityof membranes from their chemical structure,6,7 the screeningof a large number of polyelectrolytes having different chemicalstructures is still the method preferred to increase the chancesof finding membranes with high proton conductivity, chemicalstability, and good mechanical properties. In this context, acidicion-exchange membranes based on polyimides, polyetherke-tones, polysulfones, polyethersulfones, polyphosphazenes, etc.may meet the requirements necessary for their use as solidpolyelectrolytes in fuel cells.2,8 To reduce to a minimumhydrolytic degradation, naphthalenic, rather than phtahlic,polyimides have been used as a material base for the preparationof polyimide acidic membranes.2 Moreover, the improve-ment of mechanical stability requires not only the use of amixture of sulfonated and unsulfonated diamines in the synthesisof the membranes but also further cross-linking of the poly-electrolyte chains. Cross-linking reduces chain mobility and,therefore, hinders the formation of percolation paths or channelsresulting from micro- or nanophase separation of hydrophilicproton-exchange sites from hydrophobic moieties.9,10 It shouldbe noted that hydrophobic and hydrophilic domain segregationis easier in flexible membranes, such as Nafion, containing theacidic moieties in flexible side groups rather than in membraneswith the ionic groups attached to a rigid backbone.
The aim of this work was to investigate the characteristicsof acidic membranes prepared from new naphthalenic polyim-ides in which cross-linking of the molecular chains wasperformed using either phosphorus pentoxide or polyphosphoricacid. The conductivity of the membrane electrode assemblies(MEAs) was measured by impedance spectroscopy, and theperformance of the membranes in single cells was explored.The chemical structures of the polyelectrolytes used to preparethe membranes and their compositions and acronyms are shownin Figure 1 and Table 1. Special attention is paid to the complex
† Shanghai Jiao Tong University.‡ Instituto de Ciencia y Tecnologıa de Polımeros (CSIC).
J. Phys. Chem. C 2010, 114, 22773–22782 22773
10.1021/jp109941t 2010 American Chemical SocietyPublished on Web 12/06/2010
impedance associated with the response of the MEAs to electricperturbation fields.
Experimental Section
Materials. 1,4,5,8-Naphthalenetetracarboxylic dianhydride(NTDA) was purchased from Beijing Multi Technology Co.,Ltd. 9,9-Bis (4-aminophenyl)fluorene (BAPF) was purchasedfrom Aldrich. Triethylamine (Et3N), m-cresol, dimethyl sulfox-ide (DMSO), benzoic acid, polyphosphoric acid (PPA), phos-phorus pentoxide, and methanesulfonic acid (MSA) werepurchased from Sinopharm Chemical Reagent Co., Ltd. (SCRC).NTDA was dried at 160 °C in vacuo for 20 h before use. DMSOand Et3N were distilled under reduced (for DMSO) or normal(for Et3N) pressure and dried with a 4A molecular sieve priorto use. Other materials were used as received. Eaton’s reagent(PPMA) was prepared by slightly heating the mixture of MSAand phosphorus pentoxide (10:1, by weight) until a clear solutionwas formed. 4,4′-Diaminodiphenyl ether-2,2′-disulfonic acid(ODADS), 4,4′-bis(4-aminophenoxy)biphenyl-3,3′-disulfonic
acid (BAPBDS), and 3,3-bis(4-sulfophenoxy)benzidine (BSPOB)weresynthesizedaccordingtoourpreviouslyreportedmethods.11-13
Polymerization. The sulfonated polyimides (SPIs) weresynthesized by random copolymerization of NTDA, sulfonateddiamines (ODADS, BAPBDS, or BSPOB), and commonnonsulfonated diamines. A typical procedure using the SPIB51,NTDA-BSPOB/BAPF (5/1, molar ratio of the diamines), isdescribed as an example.
To a 100 mL completely dried 3-neck flask were added 1.056g (2.0 mmol) of BSPOB, 10 mL of m-cresol, and 0.64 mL ofEt3N under a nitrogen flow with magnetic stirring. After BSPOBwas completely dissolved, 0.1392 g (0.4 mmol) of BAPF, 0.6432g (2.4 mmol) of NTDA, and 0.5856 g (4.8 mmol) of benzoicacid were added. The mixture was stirred at room temperaturefor 30 min and then heated at 80 °C for 4 h and 180 °C for20 h. After cooling to room temperature, the highly viscoussolution mixture was diluted with an additional 10 mL ofm-cresol and then poured into 200 mL of acetone with stirring.
Figure 1. Schemes of the chemical compositions of the membranes.
TABLE 1: Acronyms, Membrane Composition, Cross-Linking Agent, Temperature, and Duration of the Coss-Linking Reaction
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The fiber-like precipitate was filtered off, thoroughly washedwith acetone, and dried in vacuo at 80 °C for 10 h.
Membrane Formation and Proton Exchange. The SPIs intheir triethylammonium salt form were dissolved in DMSO (forODADS and BSPOB-based SPIs) or m-cresol (for BAPBDS-based SPI) to form clear ∼5 wt % solutions. The polymersolutions were then cast onto glass plates and heated at 80 °C(for DMSO solutions) or 120 °C (for m-cresol solution) for 10 h.The as-cast membranes were soaked in methanol at 60 °C for10 h to remove the residual solvents, followed by immersingin 1.0 N hydrochloric acid at room temperature for 2 days forthe proton exchange. The resulting membranes were thoroughlywashed with deionized water until the rinsed water becameneutral and finally dried in vacuum at 120 °C for 20 h.
Cross-Linking Treatment. The dry SPI membranes in theirproton form were immersed in Eaton’s reagent in a glass vesselat 80 °C for a period of time (NTDA-BSPOB/BAPF (5/1), 0.5 h;NTDA-BAPBDS/BAPF (4/1), 24 h; NTDA-ODADS/BAPF (3/1), 48 h) under a nitrogen atmosphere. The membranes weretaken out, thoroughly rinsed with deionized water until the rinsedwater became neutral, and dried in a vacuum at 120 °C for 20 h.For NTDA-ODADS/ODA (3/1), the cross-linking treatment wasperformed by a similar procedure except that PPA instead ofEaton’s reagent was used as the cross-linking medium and thecross-linking temperature and time were 180 °C and 7.5 h,respectively.
Thermogravimetric Experiments Coupled with MassSpectrometry (TGA-MS). The thermal stability of the mem-branes was examined under a nitrogen atmosphere using a TQ100 apparatus from TA Instruments coupled with a massspectrometer ThermoStar from Pfeiffer Vacuum provided witha Channeltron detector with a tungsten filament. Thermogravi-metric experiments were carried out at the heating rate of 10K/min to a final temperature of 900 °C. MS peaks of differentfragments from m/z 10 to 70 were detected as a function oftemperature.
Water Uptake and Ion-Exchange Capacity. The weighed,dried cation-exchange membrane of interest in the acid formwas immersed in distilled water. After a certain time, themembrane was removed from the water, gently blotted withfilter paper, and weighed. This operation was repeated severaltimes until the weight of the wet membrane remained constant.The water uptake, wu, in grams of water/(g of dry membrane)was estimated as wu ) (mw/md) - 1, where mw and md are,respectively, the masses of the wet-dry membrane.
The ion-exchange capacity (IEC) was measured by immersingthe membrane of interest in the acid form in 1 N sodium chloridesolution. Protons released in the exchange reaction R-H + Na+
f R-Na + H+ were titrated with a low normality NaOHsolution. The IEC of the membrane was estimated as IEC ) VN/md, where V is the volume of the NaOH solution of normalityN spent in the titration and md is the mass of the dry membrane.
Membrane Electrode Assembly (MEA) Preparation. Bothcathode and anode catalyst layers consisted of Pt/C catalyst (40wt % Pt, Vulcan XC-72, E-TEK) (0.78 ( 0.07 mg Pt/cm2) and30 wt % Nafion ionomer (5 wt % solution, EW 1100, Fluka).Catalyst ink was prepared using isopropanol/H2O (2:1, v/v) asa dispersion media and ultrasonicating for 30 min. Catalyst-coated gas diffusion layers (CCGDL) having an electrodegeometrical surface area of 5 cm2 were fabricated by man-ual spray-coating on Toray carbon paper TGP-H-090 with 40wt % wet proofing.14 Polyimide membrane electrode assemblies(MEAs) were fabricated by hot pressing at 130 °C, 75 bar during150 s.
MEA Performance Tests. The experimental single cell fromElectroChem with an active area of 5 cm2 consists of twographite separator plates with a serpentine flow pattern, silicongaskets with a high-precision thickness, and heaters. Preliminarytests on the MEAs’ polarization curve performance were carriedout at 75 °C and different temperatures of hydrogen humidifi-cation. According to the results obtained in these tests, the MEAsbehave better when the temperature of the water humidifyingthe fuel is about 5 °C above the cell working temperature. Theuse of a gas diffusion layer with a 40% Teflon treatment (wetproofing) facilitates the evacuation of the possible formationof condensation water. Finally, a constant gas flow of 40 mL/min was chosen due to technical limitations of the fuel cell teststation used for the experiments, which does not allow astoichiometric reactant flow rate control.
MEA Conductivity. Electrochemical impedance spectros-copy (EIS) experiments were conducted in the single cell insitu, at 75 °C, using a potentiostat Autolab PGStat30 equippedwith a FRA module. The cell was continuously supplied (50mL/min) with humidified H2 (SHE, anode) and N2 (workingelectrode, cathode). The amplitude of the sinusoidal signal was10 mV and the frequency range 10 kHz to 1 Hz. The spectrawere recorded under a dc bias potential of 0.45 V.
H2 Crossover through the Membranes. The H2 crossoverwas measured at 75 °C by potential step voltametry (potentiostatAutolab PGStat 30) supplying humidified H2 and N2 gases tothe cell (50 mL/min). The anode served as the reference standardhydrogen electrode and the cathode as the working electrode.The potential was stepped from 0.2 to 0.5 V in 0.1 V incrementsof 180 s of duration each.
Results
To increase their mechanical stability, the acidic membranesSPIB31, SPIB41, and SPIB51 were cross-linked using Eaton’sreagent (methanesulfonic acid/P2O5 ) 10/1), whereas the cross-linking of the membrane SPIO31 was accomplished withpolyphosphoric acid. An inspection of the cross-linking reactiontime collected in Table 1 and the water uptake of the membranesshown in Table 2 suggest that both parameters are uncorrelated.
TABLE 2: Water Uptake and Ion-Exchange Capacity (IEC) Obtained by Different Methods and Moles of Water PerEquivalent of the Fixed -SO3
For example, the membrane SPIB31 cross-linked for 48 h hasa water uptake (wu) of 1.26 g of water/(g of dry membrane),significantly higher than that of the membrane SPIB51 cross-linked for 0.5 h for which wu ) 0.69 in the same units. Thenumber of moles of water per anionic fixed group, λ, is 25.45for the SPIB31 membrane and decreases to 16.45 for the SPIB51membrane.
The thermograms of Figure 2 show that loss of mass of themembranes decreases rather sharply as the temperature increasesfrom room temperature up to 100 °C, reaching a plateau between120 and ca. 300 °C. The percentage of loss of mass in this firststep ranges from ca. 14% for the SPIB41 and SPIO31membranes to 11% for the SPIB51 membrane. A second lossof weight step, ranging between 14.6% for the SPIB41 andSPIO31 membranes and 11.5% for the SPIB51 membrane,occurs in the approximate range of temperature of 300-400°C. Finally, a third loss of weight step of the order of 23.1,33.8, and 31% takes place between 425 and 900 °C for theSPIB41, SPIB51, and SPIO31 membranes, respectively. Ac-cording to the mass spectroscopic results for the SPIO31membrane, shown in Figure 3, the mass loss in the first step isdue to the evaporation of absorbed moisture in the samples (m/z) 18) and OH+ (m/z ) 17). The mass loss in the second stagecontains SO+ (m/z ) 48) and SO2
+ (m/z ) 64) ionic species,and therefore, it is caused by the decomposition of sulfonic acidgroups. This behavior has been observed with many othersulfonated polymers.11,12 Finally, the appearance of CO2
+ (m/z) 45), C3H7
+ (m/z ) 43), C2H6+ (m/z ) 30), and C3H8
+ (m/z) 44) in the third step suggests the decomposition of thebackbone of the copolyimides at temperatures above 425 °C.
The equivalent circuit of the MEAs in conductivity measure-ments carried out in the single cell is a resistance RM represent-ing the Ohmic resistance that the membranes oppose to transportprotons in series with a parallel RC circuit that accounts for therelaxation of polarization processes. Notice that RC has thedimensions of time, and a circuit of this type represents a Debyepolarization process. Aside from the acidic membrane, otherenvironments involving capacity layers between the catalyst andthe Nafion and between the Nafion and the carbon paper of theelectrodes intervene in the measurements of the resistance ofthe MEAs to proton transport carried out in situ. Moreover,relaxation processes with a single relaxation time are unlikelyin condensed matter, and the parallel RC circuit accounting fordipolar relaxation processes should be replaced by a polarizationresistance Rp in parallel with a constant phase element ofadmittance, Y*(ω) ) Y0(jωτ1)n, where ω and τ1 are, respectively,the angular frequency of the perturbation field and the meanrelaxation time of the polar relaxation processes.15 The exponentn lies in the range of 0 < n e 1. Notice that the lower the n, the
wider the distribution of relaxation times of the relaxationprocesses. The impedance of the equivalent circuit can be writtenas
where Y1 ) RpY0 is a dimensionless quantity. The real Z′(ω)and loss Z′′(ω) components of the complex permittivity are
and
According to these expressions, the complex Z′′ versus Z′ planeplot, called a Nyquist diagram, is a curve intersecting theabscissa axis at extreme frequencies, that is, limωf0Z′(ω) )RM + Rp, limωf0Z′′(ω) ) 0, limωf∞Z′(ω) ) RM, andlimωf∞Z′′(ω) ) 0. The intersection of the curve with the abscissaaxis in the high-frequency region then gives the Ohmicresistance, RM, of the MEAs directly measured in the singlefuel cell. Nyquist plots for the complex impedances obtainedfor the MEAs are shown in Figure 4. Values of the area specific
Figure 2. Thermograms for SPIB41 (continuous line), SPIB51 (dashedline), and SPIO31 (dots) acidic membranes.
Figure 3. Mass spectroscopy results for the SPIO31 membrane.
Z*(ω) ) RM +Rp
1 + Y1(jωτ1)n
(1)
Z′(ω) ) RM +Rp[1 + Y1(ωτ1)
n cosnπ2 ]
1 + Y12(ωτ1)
2n + 2Y1(ωτ1)n cos
nπ2
(2)
Z″(ω) ) -Rp[Y1(ωτ1)
n sinnπ2 ]
1 + Y12(ωτ1)
2n + 2Y1(ωτ1)n cos
nπ2
(3)
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resistances of the membranes, r ) RMS, where S is the area ofthe membrane, are collected in the fifth column of Table 3.
Membrane hydrogen crossover, an important measurementof membrane quality and suitable preparation of the MEAs, wasperformed by potential step voltammetry using different workingelectrode potentials, V. Illustrative plots of the limiting hydrogenoxidation current, iL, versus V, shown in Figure 5, are straightlines, and the results for iL at V ) 0 are collected in the secondcolumn of Table 4. The fourth and third columns of the tableshow the values of the fuel crossover and the electronicresistance, the latter being obtained from the reciprocal of theslope of the straight lines of Figure 5. The electronic resistanceis related to the electrons crossing the membrane from the anodeto the cathode.
Polarization curves for the MEAs SPIB41, SPIB51, andSPIO31 at 75 °C and 2 bar are shown in Figure 6. The MEA
SPIB31 showed rather poor mechanical properties at 75 °C, andits performance was not studied. The open-circuit voltage VOC
sharply decreases from the thermodynamic electromotive forceof the cell (1.23 V) to 0.95-1 V. The open voltages of thecells at 75 °C are shown in Table 3. The voltage of the fuelcells drops rather sharply at low current densities as the resultof the electrode reaction. The voltage continues decreasing withincreasing current density mainly due to the Ohmic resistanceof the membrane, and the maximum power of the fuel cellsoccurs in the vicinity of 0.4-0.5 V with i ) 350-400 mA/
Figure 4. Nyquist plots for the MEAs obtained from in situ complex impedance measurements in water-saturated H2/N2, at 75 °C. Arrows in theSPIB51 MEA indicate an increase in frequency.
TABLE 3: Open-Circuit Voltage, OCV, Maximum PowerDensity of the Single Cell, Wmax, Membrane Thickness, t,Area Specific Resistance, r, and Conductivity, σ, of theMEAs, at 75° Ca
a The measurements of the resistance of the membranes wereperformed under water-saturated H2/N2, at 1 bar.
Figure 5. Limiting current density as a function of the workingelectrode potential for SPIB41 (circles), SPIB51(squares), and SPIO31(triangles) MEAs, at 75 °C.
cm2. The SPIB51 MEA exhibits the best performance with apower density of ca. 210 mW/cm2 (current density, 420 mA/cm2, and 0.5 V). In general, the cells’ performance is inconsonance with the MEAs’ conductivity decreasing in the orderof SPIB51 > SPIB41 > SPIO31.
Discussion
By changing the molar ratio of sulfonated to unsulfonateddiamines, polyelectrolytes with an ion-exchange capacity vary-ing in a wide range are obtained. Polyelectrolytes with IEC >2.0 meq/(g of dry membrane) can be used to prepare membranesthat, conveniently humidified, exhibit rather high proton con-ductivity. However, the high swelling degree of the membranesdecreases their mechanical stability. As a result, it is necessaryto design procedures to enhance membrane durability withoutaffecting too negatively the conductivity. The decrease ofmembrane swelling can be accomplished by cross-linking.Several procedures for this purpose have been reported in theliterature. For example, a series of cross-linked SPI membraneswere prepared by self-cross-linking of maleimido-end-cappedSPI oligomers and further reaction of the oligomers with adiacrylate compound. However, the membranes exhibited poorwater stability in fuel cell operating conditions, presumably dueto low hydrolytic stability of the maleimido rings.16 Morehydrolytically stable SPI membranes have been developedthrough reaction of anhydride-end-capped SPI oligomers and atriamine monomer.17 Also, cross-linking of SPI membranes wasaccomplished by chemical reaction between the carboxyl groupsfrom a nonsulfonated diamine moiety and the hydroxyl groupsof N,N-bis(2-hydroxyethyl)-2-aminothanesulfonicacid (cross-linker), though stability tests concerning stability under severeconditions are not available.18 In our laboratories, a cross-linkingmethod has been developed for polyimides that can be extendedto polysulfones, polyetherketones, etc.13,19-22 The method is
based on the condensation reaction between sulfonic acid groupsof a sulfonated diamine moiety and the activated hydrogen atomof a nonsulfonated diamine moiety using a condensation agent(PPMA or PPA). A scheme of the cross-linking reaction isshown in Figure 7. The cross-linking mechanism has beenconfirmed in our previous paper by FT-IR spectra13,20,21 andmodel compound synthesis.13 As has been reported earlier,13
the cross-linking conditions (cross-linking medium, temperature,and time) are closely related to polymer structure. Cross-linkingof different polymer membranes may require that it be carriedout under different conditions because of different cross-linkingrates that depend on the reactivity of sulfonic acid groups andthe benzene rings that provide activated hydrogen atoms forthe cross-linking reaction. The sulfonic acid group with nosubstituent(s) in its ortho position(s), apart from the polymermain chain and attaching to activated (high electron density)benzene rings, is highly reactive. In this study, the reactivity ofsulfonic acid groups varies in the following order: SPIB51 .SPIB41 > SPIB31 ) SPIO31 (Figure 8). The sulfonic acidgroups of the SPIB51 show the highest reactivity because theyare pendant groups apart from the polymer backbone and thereare no substituents in the ortho positions of the sulfonic acidgroups, leading to less steric effects in the cross-linking reaction.The sulfonic acid groups of the SPIB41 have a much lowerreactivity than that of the SPIB51 due to the steric effect (thepresence of ortho substituents). The sulfonic acid groups of theSPIB3 have the lowest reactivity because of not only the stericeffect but also the relatively low electron density of the benzenerings to which the sulfonic acid groups are attached. On theother hand, the reactivity of the “activated hydrogen atoms” isrelated to the electron density of the benzene rings providingthe “activated hydrogen atoms”. The 2,7-positions of fluoreneunits of SPIB31, SPIB41, and SPIB51 are much more reactivethan the diphenyl ether unit of SPIO31 (Figure 9). As a result,the SPIB51 membrane shows the fastest cross-linking rate andcan be readily cross-linked in PPMA at 80 °C for 0.5 h. TheSPIB41 membrane shows a lower reaction rate than the SPIB51membrane, and a longer reaction time (24 h) is needed in PPMAat 80 °C. The SPIB31 has an even lower cross-linking rate thanthat of SPIB41, and very long reaction time (48 h) is needed inPPMA at 80 °C. It is observed that the SPIO31 mem-brane cannot be cross-linked in PPMA, and a more powerfulcross-linking agent, polyphosphoric acid (PPA), and a highercross-linking temperature (180 °C) are required to achieve cross-linking. The sulfonyl linkages are very stable, and the cross-linked SPI membranes of high conductivity are more stable thannon-cross-linked ones of similar conductivity. The analysis ofthe mechanical properties of the membranes shows that themaximum stress, MS, and elongation at break, EB, are,respectively, higher and longer for the wet cross-linked SPImembranes than for the wet un-cross-linked ones, indicatingbetter mechanical properties resulting from cross-linking. How-ever, if the comparison is made for the dry membranes, theopposite occurs. For example, the values of MS and EB for thecross-linked wet SPIB41 membrane, at room temperature, are39 MP and 42%, but the values of these quantities for the un-cross-linked membrane are 28 MP and 14%, respectively. Thevalues of MS for the un-cross-linked and cross-linked drymembranes are 67 and 54 MPa, respectively, whereas the resultsfor EB amount to 34 and 22%, respectively. Cross-linking resultsin the restriction of polymer chain displacement and, therefore,lower EB and even lower MS values were obtained with thecross-linked dry membranes. For the cross-linked wet mem-branes, however, the absorbed water in the membranes acted
TABLE 4: Values of the Intercepts of the Straight Lines ofFigure 4 with the Ordinate Axis, iL0, Fuel Crossover, J(H2),and Electronic Resistance, Re, and the PermeabilityCoefficient of Hydrogen at 75° C
Figure 6. Polarization curves for single cells with SPIB41 (filledcircles), SPIB51 (open circles), and SPIO31 (filled squares) MEAs, at75 °C and 2 bar.
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as a plasticizer, which improved the flexibility of the membranes,and thus, longer EB values were obtained. For the un-cross-linked wet membranes, the high IECs caused excess swellingand, therefore, rather poor mechanical properties were obtained.
Although a severe loss of weight is observed in the TGAcurves at low temperatures, the mass spectrometry techniqueshows that such a loss of mass comes from the evaporation ofwater occluded in the membranes. In this regard, it has beenreported23 that 0.05-0.10 g of water residual/(g of dry mem-brane) is not easily removed in naphthalenic copolyimides. 1HNMR spectra of hydrated naphtahalenic polyimide membranesshow chemical peaks at chemical shifts spanning from 1 to 14
ppm.7 Peaks appearing at high fields correspond to waterprotons, difficult to remove under heating experimental condi-tions, that do not affect the chemical stability of the membranes.For example, the peaks at 0.7-1.1 ppm still appear after heatingthe membranes at 130 °C for 2 days in vacuum.7 These peakswere attributed to water associated with the polymer in cavitiesprimarily formed by segments with highly delocalized electrondensity. As Figure 3 shows, degradation of the sulfonic acidgroups starts at temperatures slightly higher than 200 °C,reaching a maximum in the vicinity of 375 °C. Traces ofdegraded products containing sulfur atoms are still detected inthe vicinity of 500 °C.
Let us now examine the Nyquist plots for the MEAs. Theloss impedance at high frequencies is positive, and both Z′′ andZ′ decrease with decreasing frequency until Z′′ ) 0. Bydecreasing the frequency even further, Z′′ becomes negative andboth the absolute values of Z′′ and Z′ increase, the plot departingfrom the arc that accounts for the dipolar relaxations of themolecular chains. The positive loss impedance at high frequen-cies is presumably produced by parasitic inductances. Theequivalent electric circuit of the MEAs in the single cell thatmodels the response must contain an inductance, ωL, where Lis the inductance coefficient, in series with RM and the resistance-
Figure 7. Mechanisms of the cross-linking reaction by Eaton’s reagent and polyphosphoric acid (PPA).
Figure 8. Scheme of the reactivity of sulfonic acid groups in the cross-linking reactions.
Figure 9. Scheme of the reactivity of Car-H bonds in the cross-linkingreactions.
constant phase element parallel circuit (see Figure 10). Thecomplex impedance of this configuration is given by
Equation 4 suggests that the presence of parasitic inductancesdoes not affect the values of the components of the impedanceat ω f 0, but it does for ω . 0. For example, limωf∞Z′(ω) )RM and limωf0Z′(ω) ) RM + Rp, while limωf∞Z′′(ω)f ∞ andlimωf0Z′′(ω) ) 0. That is, the imaginary component of thecomplex impedance is governed by the inductance at highfrequencies. Therefore, a pure inductance does not model theinductance curve of Figure 4 because, experimentally, both Z′′and Z′ for the inductance contribution increase with increasingfrequency. Taking into account that, at extremely low andextremely high frequencies, an inductance acts, respectively,as a short and an open circuit (the opposite occurs with thecapacitance), the pure inductance in eq 4 should be replacedby a resistance R in parallel with an inductance ωL. In this case,the components of the complex impedance are given by
For ω > ωc, where ωc is the frequency at which Z′′ ) 0 inFigure 4, the components of the impedance are
and
The extreme limits of eqs 6 and 7 arelimωLf0Z′(ω) ) RM, limωLf∞Z′(ω) ) RM + R, limωLf0Z′′(ω) ) 0,and limωLf∞Z′′(ω) ) 0. The plot of the Z′′ versus Z′ for theinductance curve shows that the experimental results in the low-frequency side fit reasonably well to a semicircumference,suggesting that the impedance coefficient is lower than 10-5
henry. At high frequencies, the values of Z′′ undergo ananomalous increase, presumably due to nonlinear contributionsto the response.
Water plays a determinant role in proton transport because itintervenes in the dissociation of the sulfonic acid groups, transferof the protons to the aqueous medium, and screening of theprotons from the sulfonated anions anchored to the molecularchains and because it is the medium through which protonsdiffuse in the membrane. According to ab initio simulations,
protons in bulk water and in water clusters fluctuate betweenmore localized hydronium ion-like states or Eigen ions and moredelocalized H5O2
+ or Zundel ions.24,25 Proton transport may bethe result of forming and breaking hydrogen bonds in theneighborhood of proton locations. Whatever the transportmechanism is, low proton conductivity requires the existenceof percolation paths in the membranes through which protonsdiffuse. Percolation paths are formed by segregation of hydro-philic groups from hydrophobic ones, and their formation iseasier the higher the flexibility of the polyelectrolyte chains is.Formation of percolation paths in the naphthalenic polyimidesused in this work is hindered by cross-linking that decreasesthe mobility of the already rigid molecular chains. As aconsequence, and taking into account their high water uptkakeand high IEC, the polyimide membranes in the MEAs onlyexhibit a fair conductivity if the comparison is made with theconductivity of perfluorinated membranes.
The fuel crossover can be obtained from the data of iL0 givenin Table 4 by means of the following expression
where J(H2) is the flux of hydrogen in cm3 (STP)/(cm2 s) andF is Faraday’s constant. The results for J(H2) presented in Table4 show that the fuel crossover is negligible in comparison withthe flux of hydrogen flowing in the anode. The permeabilitycoefficient of hydrogen in the wet membranes can be determinedbearing in mind that the difference of the pressure of H2 betweenthe anode and the cathode is 76 cm Hg. The results obtainedfor the permeability coefficient P of the SPIB41 and SPIO31membranes are of the same order as that reported in the literaturefor the permeability coefficient of the gas in dry membranesused for gas separation.
As indicated above, the polarization curves show, as usual,a sharp decrease of the electromotive force from 1.24 V to avalue in the vicinity of 0.9-1 V caused by internal currentsand fuel crossover. Moreover, owing to the overvoltage arisingfrom the redox reactions at the electrodes, the Ohmic resistanceof the membrane to proton transport, and the concentrationlosses of reactants at the electrodes, the voltage of the singlefuel cell can be expressed in terms of the current density by26
where A ) RT/2RF corresponds to the slope of the Taffelequation; R and F being, respectively, the charge transfercoefficient that lies in the range of 0.25-0.50 and the Faraday’sconstant; i0 is the exchange current density; r is the area specificresistance in Ωcm2; and m and n are empirical parameters relatedto the concentration of gas at the electrodes. An inspection ofthe polarization curves of Figure 6 reveal that, between themembranes used in this work, the SPIB51 membrane exhibitsthe best performance, arising, in part, from its conductivity,higher than the conductivities of the other two membranes, andalso from its smaller thickness. Notice that r ) RA ) l/σ, whereR, A, and l are, respectively, the Ohmic resistance of themembrane, the active area of the MEA, and the thickness ofthe membrane, and therefore, the decrease in voltage by effectof the specific Ohmic resistance of a membrane of conductivityσ is lower the lower its thickness is. By using the experimentalvalues of r, the fitting of eq 9 to the polarization curves is rather
Figure 10. Equivalent electric circuit governing the response of theMEAs to electric perturbation fields.
Z*(ω) ) RM +Rp
1 + Y1(jωτ1)n+ jωL (4)
Z*(ω) ) RM +Rp
1 + Y1(jωτ1)n+ jRωL
R + jωL(5)
Z′(ω) ) RM + Rω2L2
R2 + ω2L2(6)
Z″(ω) ) R2ωL
R2 + ω2L2(7)
J(H2) ) 22 400iL0
2F(8)
V ) VOC - A ln( ii0
) - ri - m ln(ni) (9)
22780 J. Phys. Chem. C, Vol. 114, No. 51, 2010 Yuan et al.
poor for values of i > 200 mA/cm2. This behavior is reflectedin Figure 11, where an example of the fit of eq 9 to thepolarization curve of the SPIB41 MEA is shown. It can be seenthat eq 9 describes the polarization curves of the single cell,assuming that the specific Ohmic resistance of the membraneis roughly twice the value of r experimentally measured forthe membrane. To test the reliability of the values of r directlymeasured in the cell unit, additional experiments were carriedout by measuring the resistance of the water-saturated SPIO31membrane with a Novocontrol dielectric apparatus operatingin the frequency range of 0.01-106 Hz. Nyquist diagrams forthis membrane at some temperatures are shown in Figure 12.At 75 °C, the value of r is 0.29 Ωcm2, in rather good agreementwith that obtained in the cell unit, 0.32 Ωcm2. Moreover, theresults of the inset of Figure 12 show that r obeys Arrheniusbehavior with activation energy of ca. 7.2 kcal/mol. In view ofthe similarity of the values of r obtained by the two techniques,it suggests that the relatively high values found for this parameterin the analysis of the polarization curves may be due, in part,to the Ohmic resistance arising from diffusion of the ionicspecies produced in the catalyst sites across the interfaces ofimide-Nafion polyelectrolytes surrounding them.
The performance of the MEAs is usually expressed as themaximum of the power density in the curves shown in Figure13. It can be seen that the value of this quantity lies in the range
of 150-225 mW/cm2 at 0.5 V. For comparative purposes, thepower density curve for a Nafion 112 MEA, measured in thesame conditions as those of the naphthalenic copolyimidemembranes, is shown in Figure 13. According to these results,the performance of the Nafion and the naphtahlenic copolyimideMEAs is rather similar. The exchange current lies in the vicinityof 10-5 A/cm2 in the naphthalenic copolyimide MEAs, reflectingsluggish reaction kinetics at the electrodes that results in a severeperformance penalty.
Conclusions
At high frequencies, parasitic inductances govern the responseof the MEAs in the single cells to perturbation fields, whereasat moderate frequencies, a polarization resistance in parallel witha constant phase element governs the response. The realcomponent of the complex impedance at which Z′′ in theNyquist diagrams is 0 is taken as the Ohmic resistance of theMEAs.
Despite the relatively high ion-exchange capacity of themembranes, the MEAs only exhibit a conductivity of the orderof 10-2 S/cm, at 70 °C, as a consequence of the rigidity of thechains, on the one hand, and chains cross-linking, on the otherhand, that hinder the segregation of hydrophilic groups fromhydrophobic ones to form percolation paths through whichprotons formed in the anode travel to the cathode. Cross-linkingreduces the membranes’ conductivity but greatly increases themechanical stability of the membranes.
The polarization curves obtained for the naphthalenic co-polyimide MEAs are similar to those of Nafion 112 MEAs,measured in the same conditions. The good chemical andmechanical stabilities of the cross-linked naphthalenic copoly-imide membranes make them candidates as solid electroytesfor fuel cells.
Acknowledgment. This work was supported by the CICYTthrough the project MAT2008-06725-C03-01 and ShanghaiMunicipal Natural Science Foundation (Grant No. 08ZR1410300).
References and Notes
(1) Sata, T. Ion Exchange Membranes RSC; Royal Society of Chem-istry: Cambridge, U.K., 2004.
(2) Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath,J. E. Chem. ReV. 2004, 104, 4587.
Figure 11. Fitting of eq 9 (continuous and dashed lines) to theexperimental results (circles) (see text for details).
Figure 12. Nyquist diagrams for the membrane SPIO31 equilibratedwith distilled water and sandwiched between gold electrodes at severaltemperatures: 303 K (filled squares), 323 K (open circles), 333 K (filledcircles), and 348 K (open squares). Inset: temperature dependence ofr for the SPIO31 membrane.
Figure 13. Power density curves for single cells with SPIB41 (filledcircles), SPIB51 (open circles), and SPIO31 (filled squares) MEAs, at75 °C and 2 bar. For comparative purposes, the curve for a Nafion 112MEA (open squares), measured in the same conditions as those of thenaphthalenic copolyimide acidic membranes, is also shown.
T. J. Membr. Sci. 2004, 230, 111.(13) Fang, J.; Zhai, F.; Guo, X.; Xu, H.; Okamoto, K.-i. J. Mater. Chem.
2007, 17, 1102.(14) Jurado, J. R.; Chinarro, E.; Colomer, M. T. ES Patent 2209657, 2005.(15) Barsoukov, E., Macdonald, J. R., Eds. Impedance Spectroscopy:
