Article pubs.acs.org/Langmuir
Stereochemistry-Dependent Proton Conduction in Proton Exchange Membrane Fuel Cells Ravikumar Thimmappa, Mruthyunjayachari Chattanahalli Devendrachari, Alagar Raja Kottaichamy, Omshanker Tiwari, Pramod Gaikwad, Bhuneshwar Paswan, and Musthafa Ottakam Thotiyl* Department of Chemistry and Centre for Energy Science, Indian Institute of Science Education and Research (IISER) Pune, Dr. Homi Bhabha Road, Pashan, Pune 411008, India S Supporting Information *
ABSTRACT: Graphene oxide (GO) is impermeable to H2 and O2 fuels while permitting H+ shuttling, making it a potential candidate for proton exchange membrane fuel cells (PEMFC), albeit with a large anisotropy in their proton transport having a dominant in plane (σIP) contribution over the through plane (σTP). If GO-based membranes are ever to succeed in PEMFC, it inevitably should have a dominant through-plane proton shuttling capability (σTP), as it is the direction in which proton gets transported in a real fuel-cell configuration. Here we show that anisotropy in proton conduction in GO-based fuel cell membranes can be brought down by selectively tuning the geometric arrangement of functional groups around the dopant molecules. The results show that cis isomer causes a selective amplification of through-plane proton transport, σTP, pointing to a very strong geometry angle in ionic conduction. Intercalation of cis isomer causes significant expansion of GO (001) planes involved in σTP transport due to their mutual H-bonding interaction and efficient bridging of individual GO planes, bringing down the activation energy required for σTP, suggesting the dominance of a Grotthuss-type mechanism. This isomergoverned amplification of through-plane proton shuttling resulted in the overall boosting of fuel-cell performance, and it underlines that geometrical factors should be given prime consideration while selecting dopant molecules for bringing down the anisotropy in proton conduction and enhancing the fuel-cell performance in GO-based PEMFC.
1. INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) are promising energy conversion devices where chemical energy stored in the chemical bonds of H2 and O2 is converted into electricity with zero emission and higher efficiency.1 It is an integral part of sustainable energy production because, the intermittent nature and geographic variation of renewable energy resources demand their storage either in the chemical bonds of fuel molecules by splitting water in to H2 and O2 or in terms of certain electrochemical reactions in batteries.2−4 The former require the development of efficient fuel-cell technology for the conversion of chemical energy of H2 and O2 into electricity. For a PEMFC there are multitudes of performance-limiting parameters, with the most important one being the protonshuttling capability of the membrane.5 Although the proton conductivity of the state of the art Nafion membranes is superior to other membranes, they are very expensive, hindering their worldwide commercialization.6,7 In this context, enormous efforts are dedicated across the globe to bring in cost-effective and alternate membranes for PEMFC.8−11 The most important alternate membrane that caught the attention of the fuel-cell community is graphene oxide (GO)-based membranes because of their impermeable nature to H2 and O2 gases while exhibiting decent H+ transport, satisfying the important prerequisites for a successful fuel-cell membrane. Their ease of synthesis from widely available raw materials and © XXXX American Chemical Society
processability are added advantages, making them very inexpensive and cost-effective. The reported in-plane proton conductivity (σIP) value for GO-based membranes is higher than that through plane proton conductivity (σTP).8,9,11 Because in a practical fuel cell the pathway for proton shuttling is through the plane, it is imperative to enhance the σTP conductivity selectively12 for achieving the fuel-cell performance required for practical applications. Here we show how geometry factors of the dopant molecules bring down the anisotropy in proton conductivity in GO-based membranes. This investigation revealed that dopant molecules with an all-cis configuration presents GO with higher through plane proton conductivity (σTP) compared with a trans-configuration of the same molecule. By the judicious choice of dopant molecules we have successfully reduced the anisotropy in proton transport, thereby edging a step further in overcoming one of the greatest difficulties faced by GO-based PEMFC.
2. EXPERIMENTAL SECTION Preparation of Graphene Oxide. Graphene oxide was prepared by modified Hummers method.13 In brief, 3.0 g of graphite flakes was Received: October 28, 2015 Revised: December 7, 2015
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Langmuir weighed and mixed with 4:1 ratio of acid mixture of H2SO4/H3PO4, followed by the gradual addition of 18 g of potassium permanganate. The mixture was kept for stirring for ∼3 days. After that, 15 mL of 36% H2O2 was slowly added to the reaction mass. The reaction mass turned to yellow color, and it was allowed to stir for 3 h. The formed graphene oxide was collected by centrifugation and washed with 1 M HCl, followed by distilled water. Preparation of Inositol-Doped Graphene Oxide Membranes. Required amount of graphene oxide and inositol (cis or myo) was dispersed in water and sonicated for 1 h. The homogeneous solution of myo and cis inositol graphene oxide solution was then vacuumfiltered using cellulose acetate membrane; then, the membranes were air-dried and finally peeled off from the filter paper. The membrane thickness was measured by using digital micrometer, and the average thickness was found to be 40−45 μm. Physicochemical Characterization. To see cross-section morphology of the composite membranes, we first cut the membrane samples by nitrogen snapping and mounted on a holder in such a way that the edges of the sample are toward the light source. Morphology was investigated by a JSM-5300LV (Japan) scanning electron microscope (SEM). A Bruker D8 Advance X-ray diffraction machine was used for diffraction pattern analysis. Fourier transform infrared spectroscopy (FTIR) was measured on ATR-FT-IR using Bruker Alpha FTIR spectrometer system in the wavenumber between 4000 and 500 cm−1. UV−vis spectrum of the samples was (PerkinElmer Lambda 950) carried out in the range 200−800 nm. H1 NMR spectra were recorded with Bruker 500 MHz. The thermal stability of the membranes was measured by thermogravimetric analysis using a STA6000 machine over a temperature range of 25−600 °C with a scan rate of 5 °C min−1 under nitrogen flow (30 mL min−1). XPS spectrum of membranes was acquired using AXIS ULTRA, integrating the Kratos patented Magnetic Immersion Lens and Charge Neutralisation System. A Shirley background subtraction was used for all XPS spectra. XPS peak fit 4.1 software was used for all deconvolutions. A Gaussian−Lorentzian production function was used for all deconvolution with a self-consistent fit. Electrochemical Characterization of the Membrane. AC impedance measurements were carried out between frequencies of 100 kHz and 10 mHz to measure the in-plane and through-plane proton transport using a two-probe method with a PARSTAT potentiostat at a relative humidity of 30%. The method involves two equally spaced probes in contact with the measured material, and from the Nyquist plot, the resistance was extracted from the high-frequency intercept on the real axis and then the proton conductivity was calculated using the following eqs 1 and 2. The cell was operated with a variable temperature controller (30−70 °C at 30% RH). σIP = L /R*A
3. RESULTS AND DISCUSSION GO was prepared and characterized as per the procedure reported in the literature.13 The thoroughly cleaned and dried GO was then dispersed in ultrapure water with different geometrical isomers of the dopant molecules (inositol), and the membranes were prepared by vacuum filtration technique (see Experimental Section).11 GO was chosen as the membrane matrix mainly because of its reported impermeability to H2 and O2 fuels while permitting H+ transport. Its inexpensive nature makes GO a truly scalable membrane if opportunities exist to enhance its H+ transport and temperature resistance. Proton conduction in GO-based membranes is reported to be due to the existence of oxygen functionalities like carboxylic, phenyl hydroxide, epoxide, and so on. The negative charge associated with GO’s nanofluidic channels formed by individual GO sheets can effectively shuttle ionic species.11,14 We have chosen inositol as the intercalating molecules because of the abundance of hydroxyl groups around the cyclohexane ring that can participate in H-bonding interaction and proton shuttling as per Grotthuss-type mechanism.15 To investigate the geometrical factors affecting the proton transport, we have selected myoinositol (MI) and cis-inositol (CI) as the dopant molecules. The structure of these two molecules and their −OH group orientation around the cyclohexane ring is shown in Figure S1 (Supporting Information). In MI, two −OH groups (second and sixth) are found to be in trans to the other four − OH groups, whereas in CI all −OH groups are in cis configuration. The digital photograph of the membranes with these two dopant molecules (30% w/w) are shown in Figure S1 (Supporting Information). C (1s) X-ray photoelectron spectra (XPS) of GO (Figure 1) show the binding energies
(1) −1
where σIP is the in-plane proton conductivity (S·cm ), L is the distance between the Pt electrodes (0.5 cm), R is the resistance (ohms), and A is the area of the membrane (cm2).
σTP = L /R*A
(2) −1
where σTP is the through-plane proton conductivity (S·cm ), L is the membrane thickness, R is the bulk resistance of the membrane, and A is the area of the membrane (cm2). Fuel-Cell Characterization. The membrane electrode assembly was prepared by sandwiching the membrane between 40% Pt/C anode (0.2 mg cm−2) and cathode (0.2 mg cm−2) electrodes. The catalyst ink was made by ultrasonicating with 2-propanol and 20 wt % Nafion ionomer (5%), and the ink was sprayed onto gas diffusion electrodes (carbon paper) with a wet proofed microporous layer (H2315 T10AC1), purchased from Freudenberg (FFCCT, Germany). The MEA was placed between two graphite blocks, and the active electrode area formed by the parallel gas flow channels was 1 cm2. Gold-plated steel bolts were screwed into the blocks to allow electrical contact. Humidified (30% RH) H2 (0.1 dm3 min−1) and O2 (0.07 dm3 min−1) were fed to anode and cathode sides, respectively. The cell was conditioned at 0.3 V for 1 h before polarization studies, and fuel-cell performance was carried out at 30 °C at 30% RH.
Figure 1. (a) Deconvoluted C 1s XPS spectra of GO, (b) CIG, and (c) MIG membranes.
corresponding to CC at 284.44 eV, C−OH/C−O−C at 285.88 eV, and CO at 287.63 eV.16 The relative intensity of C−OH/C−O−C increased in CI intercalated GO (CIG) and MI intercalated GO (MIG) (Figure 1b,c, respectively), in comparison with GO (Figure 1a), indicating the successful incorporation of dopants in GO matrix. X-ray diffraction pattern of inositol-doped GO (Figure 2a) suggests that on doping with inositol 2θ values for graphene B
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The marked disparity in the expansion of GO planes despite the identicalness of dopant’s functional groups (−OH) yet with different orientations around the cyclohexane ring suggests that there is a strong geometric angle to it. FTIR is the benchmark technique to probe H-bonding interactions as the spectral features are very sensitive to H bonding with concomitant intensity modulations and spectral shifts.17 The FTIR spectra of 30% MIG and CIG in the range 2000−3500 cm−1 are shown in Figure S3 (Supporting Information) along with their individual counterparts. In MIG the features observed (in the range 3000−3500 cm−1 corresponding to − OH stretching frequency) for pure components are almost reproduced as such in the composite membrane (Table S1, Supporting Information); however, in CIG, the features for the pure components are replaced by broader features in the composite membrane, hinting to a significant interaction between CI and GO. It is known that the extensive H bonding broadens the FTIR bands.18,19 Therefore, H-bonding interaction between the host (GO) and the guest (CI or MI) should be quite extensive in the case of CIG compared with MIG. The FTIR spectra in the range 700−2000 cm−1 (Figure 2b) indicate that the features of pure components were retained with negligible broadening or spectral shifts in MIG composite membrane; however, notable differences were prominent in the CIG composite membrane (Figure 2c). Almost all of the FTIR bands were significantly broadened, some got shifted, and some exhibited intensity modulation in CIG membrane compared with their pure counterparts (Figure 2c and Table S1). The features in the range 1400−1475 cm−1 (in-plane and out-of-plane deformation of methylene hydrogen) in pure CI were replaced by a broader but an amplified peak centered at 1404 cm−1 in CIG, suggesting a modulation in the force constant of the corresponding chemical bonds due to H bonding. The peaks at 1189, 1247, 1270, and 1348 cm−1, observed in pure CI, were replaced by very broad features in the composite membrane. The peaks at 1123 and 1148 cm−1 corresponding to C−O and C−C vibrations in pure CI were down shifted in the composite membrane. One region that deserves special attention is the wavenumber region 850−1220 cm−1 corresponding to epoxide vibrations, as these functional groups are reported to be mainly responsible for H+ transport in GO membranes.20 H bonding with epoxide functionality should aid proton transport as reported by Matsumoto et al.28
Figure 2. (a) X-ray diffraction patterns. (b,c) FTIR spectra of GO, myo-inositol doped GO (MIG). and cis-inositol doped GO (CIG) membranes. (d) TGA curves of GO, MIG and CIG membranes. Insets in panels b and c show the expanded region for the range 850−1500 cm−1.
oxide (001) planes are down-shifted significantly compared with pure GO. These suggest that the GO planes considerably expand for accommodating inositol, indicating the molecules mainly prefer to occupy the interplanar sites in GO. The scanning electron micrographs of doped and undoped GO (Figure S2, Supporting Information) show the stacking of GO sheets, which demonstrate a tendency to expand on doping with inositol, supporting this observation. The interlayer distance calculated using Bragg’s equation is found to be 0.83, 0.98, and 1.069 nm for GO, myo-inositol GO (MIG), and cis-inositol GO (CIG), respectively. The incorporation of CI causes significant expansion of graphene oxide planes (29%) compared with MI (18%), pointing to a marked difference in the way these molecules are interacting with the functional groups in GO planes. GO with a variety of functional groups like −OH, −COOH, epoxy, and so on can form H bonding with the present set of dopant molecules; therefore, the results presented in Figure 2a bring out a marked difference in the way these isomeric molecules engage in H bonding with the host.
Figure 3. Schematics of H-bonding interactions in MIG and CIG composite membranes. C
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concluded that in CIG membranes there exists a substantial host−guest interaction by H bonding, as opposed to a much weaker interaction in MIG, bringing out a clear geometrical influence. NMR spectra of MIG and CIG show that there is a downshift for the protons in CIG composite membrane (Figure S5 Supporting Information), further affirming that H-bonding interaction between CI and GO is extensive. Overall, the characterization techniques adopted here affirm that doping of CI to GO results in extensive H-bonding interaction with the host, thereby expanding the GO planes. It is a well-known fact that the abnormally high proton conductivity in ice and water is due to extensive H bonding.22,23 Because the H-bonding interaction in CIG is quite extensive (from XRD, FTIR, TGA, XPS, and NMR) between the individual GO planes, the strategy is expected to increase the σTP selectively. This is because the interlayer proton transport will be aided by the extensive H bonds between the planes (Figure 3), thereby bringing down the activation energy required for proton movement through the plane. This process should be relatively difficult for MIG as the interaction with GO is minimal, relatively raising the energy barrier required for σTP transport. Because the 001 plane is the plane of σTP proton transport, the significant interaction and the corresponding expansion of GO planes in CIG is expected to increase the proton shuttling capability of such membranes. The facile interlayer proton transport is expected to aid its further transfer through the nanopores of GO to the subsequent planes.24 This sort of bridging of individual GO planes by extensive Hbonding network may favor Grotthuss-type proton transport over vehicular shuttling, which, in turn, is expected to improve the σTP proton transport.8 Furthermore, a significant expansion of GO planes as observed in CIG (Figure 2a) is anticipated to aid selectively the through-plane proton transport by incorporation of more water molecules at the interplanes. The σIP proton conductivity, which is inversely related to electrolyte resistance (extracted from the high-frequency intercept on the real impedance axis), increased significantly with increase in the proportion of inositol (cis or myo), Figure S6a (Supporting Information), confirming the presence of inositol improves proton shuttling. We have chosen 30% of inositol as the maximum dopant concentration, because beyond this loading limit the membrane tends to brittle. Consequently, 30% loading of inositol delivered the maximum proton conductivity and hence it is selected as the optimal membrane for further studies. No significant disparity is observed between CI and MI for the entire composition studied as far as σIP conductivities are concerned, Figure S6a (Supporting Information); however, a disparity in σTP conductivities is evident at higher compositions of inositol with CI delivering higher σTP conductivity over MI, Figure S6b (Supporting Information). It should be noted that loading of MI undoubtedly improves the proton conductivity of GO (compared with pure GO) because there can be some H-bonding interaction between them; however, the selective improvement of σTP in CIG has got a clear geometrical angle to it. Irrespective of the nature of the intercalating agent, σTP is improved over GO, Figure S7 (Supporting Information), suggesting that doping with molecules having proper functionality can amplify ionic transport in the nanochannels in GO. High σTP conductivity is considered to be a desirable parameter for proton exchange membrane fuel cells because it is the direction in which proton get transported when H2 gets oxidized to H+ at the anode.25,26 Therefore, an ideal proton
This vibration is reproduced almost as such in MIG, however, with noticeable broadening and spectral shifts in CIG (Table S1, Supporting Information), indicating significant involvement of epoxide functionality in H-bonding interaction with CI. The only vibration that is noticeably affected in MIG is −CO stretching vibration (Table S1, Supporting Information), which is found to be down-shifted in the composite membrane of MIG compared with pure GO. All of these bring out that there is extensive H-bonding interaction between CI and GO as opposed to marginal interactions in MIG. Therefore, it can be presumed that MI intercalates into GO planes without having much interaction with the host (GO), whereas CI intercalates by participating in extensive H-bonding interaction with the functional groups in the GO plane. Because the only factor that differentiates these molecules is the orientation of −OH groups around the ring, the disparity in their H bonding with GO stems out of their geometric factors. This may be because of the all-cis configuration of −OH groups in CI, thereby facilitating facile host guest interactions (Figure 3) apart from intra- and intermolecular H bonding. Because of this extensive interaction, GO planes may prefer an expanded state not only to accommodate the molecules but also to minimize the energy. As shown in Figure 3, in the most stable chair conformation of MI, the −OH groups are not in a position to significantly interact with the functional groups at GO planes; therefore, the expansion observed is mainly to accommodate the molecule. As of now the interactions shown in Figure 3 are only a possibility, and we can only confirm the significant host−guest interaction CIG compared with a marginal engagement in MIG. FTIR suggests that H bonding is extensive for CIG compared with MIG (Figure 2b,c), yet the XRD reveals an expansion of GO sheets (Figure 2a) when CI is intercalated. An extensive H bonding is anticipated to bring down the individual GO sheets in CIG. The expansion observed (Figure 2a, blue trace) contrary to this expectation could be due to the different orientation of intercalating molecules at GO interplanes. Because the intercalating molecules are isomeric, the extensive H-bonding interaction between CI and GO may demand a particular orientation for maximizing such interactions and adopting an energy-minimized conformation. On the basis of this, it can be presumed that on intercalation CI adopts an orientation that is different from MI and that considerably expands the GO interlayer distance in CIG, and work is underway in this direction to understand it further. Nonetheless, it should be noted that an expansion of GO interlayer distance improves proton conductivity due to the increased presence water in the interlayers through extended hydrogenbonding network, as reported by Medhekar et al.21 Thermogravimetric analysis of GO versus CIG and MIG (Figure 2d) suggests the weight loss around 100 °C is because of evaporation of free and adsorbed water and the mass loss from 100 to 200 °C is due to reactive oxygenated functionalities producing CO, CO2, and steam. The noticeable weight loss above 200 °C is due to the oxidation of carbon skeleton. The loss observed up to 200 °C is slightly lower for CIG compared with MIG and GO, further pointing to the existence of extensive H-bonding interaction in CIG membrane. n-π* transition observed in the UV−vis spectrum of GO at 300 nm is found to be very much diminished in intensity in CIG (Figure S4, Supporting Information), corroborating further the existence of extensive H bonding in CIG. Those almost identically strong n-π* peaks for MIG and GO indicate a weaker interaction between MI and GO. Therefore, it can be D
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Langmuir exchange membrane (PEM) should have higher σTP, a property seldom realized in the state of the art PEMs, including the best performing Nafion.27 The σTP conductivity values for GO was reported to be lower than the σIP conductivity in line with our results.11,28 Arrhenius plot (Figure 4a) demonstrates a decrease
undoped GO, demonstrating that the dopant molecules improve proton transport irrespective of their geometry, however, with a marked disparity in their σTP proton transport. The membrane resistance extracted when it is sandwiched between the anode and the cathode electrocatalysts (MEA) in an actual fuel-cell configuration (with H2 and O2 fuels) should reflect the intrinsic σTP proton transport for such fuel cells. The complex plane impedance plot of the H2−O2 fuel cell constructed out of CIG and MIG (Figure 4c) demonstrates a significantly improved proton conductivity for the former (0.0013 S/cm for CIG vs 0.00096 S/cm for MIG). The σTP values of the corresponding MEAs calculated from the current−voltage curves (in the ohmic region) are slightly higher (0.0048 S/cm for CIG vs 0.0026 for MIG) than those obtained from Nyquist plots and are attributed to the contribution of composite resistance of the MEAs and contact resistance to the impedance extracted from the latter. The lower proton conductivity observed in an actual fuel-cell configuration compared with the isolated membrane resistance (Figure 4a) could be due to the contributions of composite resistance from the MEAs and associated internal resistance in the former. Nevertheless, because the measurement is done in an actual fuel-cell configuration, the results in Figure 4c affirm that an enhanced σTP is the responsible factor for the improved fuel-cell performance in CIG-based fuel cells (Figure 4b). A slight increase in charge-transfer resistance (∼0.2 Ω/cm2) was observed for CIG membranes, and because the catalyst layer compositions are the same in either membranes and the semicircle will have contributions from the cathodic as well as anodic reactions, it could be due to inhomogeneity of the MEA and the contributions arising out of change in surface properties due to the incorporation of inositols to GO. The long-term stability plot (Figure 4d) suggests that CIG-based fuel cells maintain high activity for a longer duration compared with MIG membranes, demonstrating their decent stability. A sudden drop in voltage, followed by a small increase in voltage, was observed for either MEA in the initial stages of stability tests. When the fuel cell is subjected to sudden load of 100 mA/ cm2 the system will have to stabilize itself to sustain the sudden change in the mass transport, prompting the cell voltage to go down to certain values as per the concentration of available reactants. Cathodic reaction generates water, and back diffusion of water due to concentration gradient may hydrate the anodic side of the membrane, slightly activating the anodic reaction. This could be the reason for the small increase in voltage in the initial part of stability tests. Ultimately, an equilibrium will be attained between water production and back diffusion, leading to a stable voltage profile as long as the fuel is supplied. Kinetic parameters are extracted from iR-corrected (resistance values are extracted from the impedance spectra, Figure 4c) polarization curves (Figure 5a). After iR correction, the polarization curves in the kinetic region for CIG MEA occurred around 0.7 V, as opposed to ∼0.6 V in MIG MEA. Because the electron-transfer reactions are identical on either MEAs as the catalysts and half cell reactions are similar, the results in Figure 5 highlight a difference in the way the membranes conduct protons through the plane. The higher operating voltage observed even after iR correction indicate that proton shuttling is inherently faster in CIG compared with MIG, pointing to the fact that geometric factors of dopant molecules modify the proton conduction mechanism in such membranes. The parameters displaying the intrinsic rate of the electrochemical reactions, that is, Tafel slopes, were extracted from the
Figure 4. (a) Arrhenius plots (TP, through-plane; IP, in-plane) in the temperature range of 30−70 °C at 30% relative humidity (RH). At each temperature, an average of three different measurements are plotted. (b) H2−O2 (30 °C, flow rate 100 mL/cm3 at 30% RH) fuelcell performance. (c) Nyquist plots in the frequency range 100 kHz to 10 mHz at OCV. (d) Durability performance tests for MIG and CIG membranes at a galvanostatic current density of 100 mA/cm2.
in the anisotropy between σTP to σIP proton conductivity in CIG membranes. From this plot, the activation energy (Table S2 Supporting Information) of σTP proton transport in CIG (26.89 kJ/mol) is lower than MIG (34.39 kJ/mol), supporting the previous argument that significant host−guest interaction and the corresponding expansion of GO planes in CIG favor σTP proton transport by a Grotthuss-type mechanism. It should be noted that no significant deviation in the activation energy for σIP proton transport is observed (11.87 kJ/mol for CIG vs 11.12 kJ/mol for MIG) between CIG and MIG. As previously explained, it may be that an expansion of GO interlayer distance aids the incorportaion of more water molecules at the interplanes, selectively amplifying the through-plane proton transport. Therefore, our strategy of doping GO with abundant −OH-containing molecules with different geometric arrangement is a strategy that could selectively enhance the σTP conductivity and therefore is a step forward in overcoming one of the greatest difficulties faced by the state of the art graphene-based membranes. Furthermore, this geometry governed proton conductivity is reflected in the actual fuelcell performance in terms of enhanced open-circuit voltage, peak current, and peak power densities, as discussed later in the section for Figure 4b. Orientation-dependent proton shuttling ability of dopants molecules in GO is further compared in a fuel-cell architect, with humidified (30% RH) H2 and O2 fuels (Figure 4b). The results confirm our proton conductivity studies unambiguously with CIG (vs MIG) delivering higher OCV (1 V vs 0.9 V), power density (145 mW/cm2 vs 110 mW/cm2), and peak current density (432 vs 330 mA/cm2), underlining the importance of geometrical factors while selecting dopant molecules. It should be noted that the performance of inositol (CI or MI)-doped GO membranes is significantly higher than E
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with inositol. Such inositol-doped membranes displayed inferior proton conductivities (MI/PVDF; 9.1 × 10−5 S/cm and CI/PVDF;7.4 × 10 −5 S/cm), demonstrating the importance of H bonding by host−guest interaction for proton shuttling. Furthermore, in such membranes, CI dopant displayed inferior fuel-cell performance compared with MI dopant, Figure S8 (Supporting Information), underlining the importance of H bonding by host−guest interaction for selective amplification of σTP transport.
Figure 5. (a) iR free polarization curves of CIG and MIG MEAs; red and green curves represent the polarization curves without iR correction. (b) Tafel plots obtained from the iR free polarization curves of CIG and MIG MEAs.
4. CONCLUSIONS We have demonstrated stereochemistry-dependent proton conductivity in GO-based PEMFC membrane, and the results point to the fact that anisotropy in proton conduction in GObased fuel-cell membranes can be brought down by selectively tuning the geometric arrangement of functional groups around the dopant molecules. Intercalation of GO with an all-cis configuration leads to a selective amplification of through-plane proton transport, σTP, a parameter well necessitated by the state-of-the-art GO-based membranes, indicating a strong geometrical angle in proton shuttling. Substantial expansion (29%) and efficient bridging of GO planes by an all-cis configuration of dopant bring down the activation energy required for the interlayer proton movement, aiding its further transport to subsequent planes through the nanopores of GO. The fuel-cell performance data unambiguously confirm that cisisomer-doped membrane is superior to the corresponding trans configuration in line with the proton conductivity studies. Fuelcell kinetic analysis reveals that geometric arrangement of dopant molecules tunes the proton conduction mechanism in such membranes. This study underlines that functional groups like epoxides on GO and a suitable geometric orientation of functional groups in the dopants are prerequisites for bringing down the anisotropy in proton transport and the overall amplification of fuel-cell performance in GO-based PEMFC.
corresponding polarization curves for CIG and MIG MEAs and are presented in Figure 5b. Tafel slopes are found to be lower on CIG MEAs compared with MIG MEAs (94 vs 120 mV/ dec), indicating an improvement in the intrinsic kinetic of the overall electrochemical reaction when the geometry of the dopants is tuned. The good preservation of dopant molecules in GO matrix after long-term stability tests becomes a critically important parameter for the proposed strategy to be viable. XRD patterns of CIG and MIG membranes before and after long-term stability tests were almost identical with some minor differences (Figure 6a). FTIR spectra (Figure 6b,c) did not reveal
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03984. Schematics of Myo and Cis inositol, SEM images of composite membranes, FTIR spectra, deconvoluted XPS, UV−vis, NMR data, proton conductivity of various loadings of Myo and Cis inositol-doped GO membranes, assignments of FTIR bands, and XPS comparison data. (PDF)
Figure 6. Comparison of CIG and MIG membranes before and after durability test. (a) X-ray diffraction patterns, (b,c) FTIR spectra, and (d) through-plane proton conductivity of CIG and MIG at 30 °C and 30% RH during the long-term stability test.
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noticeable changes after long-term stability tests. σTP (Figure 6d) measured during the long-term stability tests demonstrated almost a stable proton conductivity profiles. Taken together, XRD, FTIR, and conductivity data affirm that the dopants are well-preserved in GO matrix and the strategy proposed here to selectively amplify the through-plane proton transport is feasible. To prove unambiguously that H-bonding interactions between the host and guest is the responsible factor for the selective amplification of σTP transport and the concomitant enhancement in fuel-cell performance, we selected a host lacking functional groups capable of forming H-bonding networks (for example polyvinylidene diflouride (PVDF)),
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +912025908261. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.O.T. acknowledges MHRD and DST of India for financial assistance. M.O.T. thanks Dr. P.A. Joy, National Chemical Laboratory, Pune for his valuable suggestions on XRD and Dr. Nirmalya Ballav, IISER Pune for his inputs on XPS data. F
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DOI: 10.1021/acs.langmuir.5b03984 Langmuir XXXX, XXX, XXX−XXX
