Noah+Johnson+-+Final+Project

Chemistry Information Retrieval 2011 Research Project Noah Johnson


 * The relationship between structure and conductivity in proton conductive materials**


 * Abstract**

Hydrogen fuel cells are extremely important for the purposes of lowering human greenhouse gas emissions, but they have been limited by lack of cheap, highly proton conductive materials. The creation of proton conductive materials relies on a fundamental understanding of proton transport, which requires a combination of experimental and theoretical research. Thus far, research has shown that the most common material used (Nafion), is composed of ionic domains connected by water filled channels, where conductivity increases as water content becomes less restricted by sulfonate groups, allowing for more transport via low energy mechanisms. Therefore, the ideal proton conductive material will involve well-connected water, where conductivity can occur via low-energy hopping modes between adjacent proton carriers.


 * Introduction**

In recent years, the subject of climate change has become a great concern in the scientific community, with the majority of research pointing to anthropogenic sources of greenhouse gases as a major contributor(1). One potential solution to this is the hydrogen fuel cell, which would drastically lower those emissions caused by fossil fuel combustion, particularly in transportation(2). They vary between each either by fuel type (gas, liquid), composition (hydrogen, methanol, phosphoric acid), and material (polymer, solid acid, etc). The most common type of fuel cell is the proton exchange membrane fuel cell (PEMFC). This is composed of a proton conductive membrane with a high surface area, impregnated on either side with a noble metal catalyst. The anode splits hydrogen into electrons (sent through an external circuit to do work), transfers these protons through the membrane to the cathode, where they are recombined with oxygen gas to form water. This proton conductive membrane (Nafion) is generally highly expensive(3) and while it has a high proton conductivity for equivalent membranes, measured around 0.01 S/cm(4), it is still low in comparison to electron conductive materials. This is problematic, because the lower the conductivity, the greater the effective resistance of the cell, and therefore the greater the drop in voltage and conductivity. For this reason, much effort has gone into trying to find an inexpensive alternative to this membrane (trademarked Nafion), with a higher conductivity. In order to do this, we must first understand how proton conductivity works in these systems.


 * Mechanisms**

In general, proton conductivity can occur in a combination of three different transport phenomena. The simplest of these is vehicular transport(5).

Vehicular transport is the simple movement of ions as a water-solvated species, where that species for a proton can either be an Eigen (H3O+), Zundel (H5O2+), or strongly solvated Eigen (H9O4+) cation(6). This method of transport predominates in simple water solutions, and so would be expected to predominate under conditions similar to bulk water(6).

The second mode of transportation is the Grotthuss mechanism7. This is a transport mechanism that occurs when protons are transported between forming hydrogen bonds with different water species(7). As such, this has a low activation energy, equivalent to the breaking of a weak hydrogen bond(7).

The third mode of transportation is surface transport, where protons can be transferred between acidic surface groups such as -SO3-(8). This has the highest associated activation energy, as electrostatic interaction must be overcome to separate the positively charged proton from the negatively charged acid group8. As such, it will only become an important factor when water concentration becomes very low. Conductivity

The Nernst-Einstein is a simple relationship of the conductivity of an ionic system, where the conductivity of the ion is directly proportional to the activity and mobility of that same ion(9).



However, as these are extremely difficult quantities to measure, the equation can be simplified to the conductivity of the system being proportional to an effective proton mobility multiplied by the sulfonic acid concentration(10).

This equation can account for a multitude of immeasurable factors, and it makes the important prediction that conductivity will increase with increasing acid concentration. This acid concentration is often termed the ion-exchange capacity (IEC)(10). The hidden assumption here is that increasing ion concentration will cause an influx of water by osmotic pressure, therefore causing a greater dissociation of sulfonic acid groups, and allowing for more free proton conduction (i.e. higher mobility).

If this were the only consideration then creating a highly proton conductive material would be a simple task. A simple increase in acid concentration would linearly increase the conductivity by increasing water content. However, this is very obviously not the case.

9

As this graph shows, the relationship between water and conductivity is not linear. It increases linearly up to a certain point, where it then begins to deviate from ideal behavior. If these systems behaved ideally, they would quickly reach the conductivity of a proton in a pure water solution, but the majority of systems are much lower,(10) likely due to the presence of sulfonic acid groups on the surface. Therefore we must now consider not only the mechanism and condition of proton conductivity, but the morphology as well.


 * Morphology by measurement**

The simple prediction from our earlier discussion was that when surface transport dominates, in a low water system, then conductivity would drop with increased activation energy. However, this does not take into account the shape of the system; morphology could very well limit the distance of protons from acid groups. This has been measured using small angle X-ray scattering, and the results show that in the relatively high conductivity Nafion polymer, proton conductive channels are large, with a limited amount of tortuosity, where equivalent low conductivity polymers (in this case, sulfonated poly ether ether ketone, or SPEEK) have very narrow channels, with a large quantity of “dead ends.”(11). This has been confirmed by small-angle neutron scattering (SANS), which also concludes that Nafion transitions from separated spherical ionic domains, to spherical domains connected by channels, to ionic rods(11).




 * Morphology by simulation**

Experimental measurements are not the only way to discuss the relationship between morphology of a system and its conductivity. Molecular simulations are often more useful, as they allow complete control over the environment, allowing a researcher to see the effect of minute changes in a system. In application to proton conductivity in membranes, these simulations are often run in two forms: atomistic and mesoscale.

Atomistic simulations have the advantage of being extremely simple, relating a sulfonic acid anion, water, proton, and hydronium ion, yet even these can obtain accurate data for proton conductive membranes(12). With a slight increase in complexity, a prediction that the morphology of a Nafion membrane is simply that of water filled channels connecting small clusters of ionic groups(13). This same simulation predicts that there are two types of water in the system: water that is bound, or constrained, by the surface sulfonic acid groups, and water that is free in the center of the pore(13). This matches our assumption that smaller pore size will decrease conductivity; it will decrease the relative amount of free water, thus dropping the conductivity. This has been very clearly shown in other atomistic simulations, where at low water content, sulfonate groups were shown to cluster around water molecules, preventing their motion(14). This has further side effects, as the clustering of the sulfonate groups has an effect on the morphology of the system. Namely, this clustering introduces “kinks” into the backbone, thus increasing the tortuosity of the membrane(15). Atomistic simulations have an inherent weakness, in that they cannot account for conductivity over long periods of time, as well as over a great distance(16). Because of this, atomistic simulations cannot include all of the interactions that they normally would and still run in a reasonable amount of time, and so more coarse-grain simulations are necessary. These mesoscale simulations can give more information about the overall structure of the Nafion membrane. For example, they have predicted that that the channels can be separated into three layers: two layers of side groups with a strong association to water, and an inner layer of only water(17). These simulations also show that the membrane morphology transitions from the isolated clusters to randomly connected water channels, as was measured before(18). This type of prediction is impossible with an atomistic simulation, using current technology.


 * Nanostructuring**

By looking at this data, it can be easily seen that conductivity could be greatly improved if the limitations of the electrolyte membrane could be removed. First, the tortuosity inherent in the polymeric membranes limits conductivity by forcing protons through a greater path length, and by limiting the motion of water in the kinked portions of the backbone. This can be easily observed in systems where conductivity has been limited to one-dimensional pathways. The easiest comparison is by aligning Nafion polymer through the technique of electrospinning. This is the technique of using a charge to draw out thin fibers of material. In Nafion, this serves to align the ionic domains, thus creating a more ordered pathway for protons to travel(19). This serves to increase conductivity over that of simple Nafion by more than an order of magnitude(19), more than illustrating the point. This effect can also be seen in one-dimensional iron coordination polymers, where conductivities as high as in bulk Nafion can be reached without the benefit of additional acid groups(20).



The second change is that the mechanism of surface transport, which in every model is a limiting mechanism, must be reduced or eliminated, allowing bulk transport, or even Grotthuss hopping, to dominate. The difficulty with this limitation is that the path must still be constrained in order to prevent free contact between the anode and the cathode. This will allow crossover, which will greatly reduce, or even eliminate, the voltage difference between the two electrodes. This would lower efficiency to almost nothing. One example was seen above, in the example of the iron CP. Without any acid groups, there could be no limiting surface transport, and proton conductivity must be entirely by a hopping mechanism. Another example can be seen in imidazole, where conductivity is greatly improved by confining motion to one dimension, but only in the absence of constraining side groups(21).



One material that combines these properties is a hydrogen bonding complex of trimesic acid and melamine(22). While the materials themselves provide little contribution to the proton conductivity (besides the addition of sulfonic acid groups), the crystal structure of the compound includes water bound into a pair of ordered helices in every pore.

So, similar to the iron coordination polymer, water is bound into an ordered structure which serves to greatly enhance conductivity. However, this compound has the benefit of including acid groups, which serves to make its proton conductivity the highest yet recorded22, further supporting the conclusions reached earlier about the relationship between morphology and conductivity.


 * Conclusion**

In order to design a highly proton conductive material, the mechanisms by which said proton conductivity occurs must first be understood. To this end, experimental measurements along with molecular simulations of the current standard for proton conductivity (Nafion) have been conducted. These drew a picture of Nafion which highlighted the nature of transport within the membrane in relation to a number of conditions, as well as the weaknesses which, when corrected, could create a superior material. When these types of materials are able to be incorporated into current fuel cells, it could drive the cost down significantly, allowing for more widespread deployment. This would then drive further research, allowing for a not-too-distant future where transportation is nearly emissions free.


 * Bibliography**

(1) Oreskes, N. Beyond the ivory tower. The scientific consensus on climate change. Science (New York, N.Y.) 2004, 306, 1686. [|link]

(2) Keith, D. W.; Farrell, A. E. Environmental science. Rethinking hydrogen cars. Science (New York, N.Y.) 2003, 301, 315-6. [|Link]

(3) Gebert, M. Höhlein, B.; Stolten, D. Benchmark Cost Analysis of Main PEFC-Ionomer Membrane Solutions. Journal of Fuel Cell Science and Technology 2004, 1, 56. [|Link]

(4) Tan, S.; Bélanger, D. Characterization and transport properties of Nafion/polyaniline composite membranes. The journal of physical chemistry. B 2005, 109, 23480-90. [|Link]

(5) Venkatnathan, A. Devanathan, R.; Dupuis, M. Atomistic simulations of hydrated nafion and temperature effects on hydronium ion mobility. The journal of physical chemistry. B 2007, 111, 7234-44. [|Link]

(6) Kirchner, B. Eigen or Zundel ion: news from calculated and experimental photoelectron spectroscopy. Chemphyschem : a European journal of chemical physics and physical chemistry 2007, 8, 41-3. [|Link]

(7) Agmon, N. The Grotthuss mechanism. Chemical Physics Letters 1995, 50, 456. [|Link]

(8) Eikerling, M.; Kornyshev, a a Proton transfer in a single pore of a polymer electrolyte membrane. Journal of Electroanalytical Chemistry 2001, 502, 1-14. [|Link]

(9) Peckham, T. J. Schmeisser, J. Rodgers, M.; Holdcroft, S. Main-chain, statistically sulfonated proton exchange membranes: the relationships of acid concentration and proton mobility to water content and their effect upon proton conductivity. Journal of Materials Chemistry 2007, 17, 3255. [|Link]

(10) Peckham, T. J. Schmeisser, J.; Holdcroft, S. Relationships of Acid and water content to proton transport in statistically sulfonated proton exchange membranes: variation of water content via control of relative humidity. The journal of physical chemistry. B 2008, 112, 2848-58. [|Link]

(11) Kreuer, K. D. On the development of proton conducting polymer membranes for hydrogen and methanol fuel cells. Journal of Membrane Science 2001, 185, 29-39. [|Link]

(12) Ennari, J. Elomaa, M.; Sundholm, F. Modelling a polyelectrolyte system in water to estimate the ion-conductivity. Polymer 1999, 40, 5035-5041. [|Link]

(13) Elliott, J. a; Hanna, S. Elliott, A. M. S.; Cooley, G. E. Atomistic simulation and molecular dynamics of model systems for perfluorinated ionomer membranes. Physical Chemistry Chemical Physics 1999, 1, 4855-4863. [|Link]

(14) Urata, S. Irisawa, J. Takada, A. Shinoda, W. Tsuzuki, S.; Mikami, M. Molecular dynamics simulation of swollen membrane of perfluorinated ionomer. The journal of physical chemistry. B 2005, 109, 4269-78. [|Link]

(15) Elliott, J. a; Paddison, S. J. Modelling of morphology and proton transport in PFSA membranes. Physical chemistry chemical physics : PCCP 2007, 9, 2602-18. [|Link]

(16) Galperin, D. Y.; Khokhlov, A. R. Mesoscopic Morphology of Proton-Conducting Polyelectrolyte Membranes of Nafion® Type: A Self-Consistent Mean Field Simulation. Macromolecular Theory and Simulations 2006, 15, 137-146. [|Link]

(17) Khalatur, P. G. Talitskikh, S. K.; Khokhlov, A. R. Structural Organization of Water-Containing Nafion: The Integral Equation Theory. Macromolecular Theory and Simulations 2002, 11, 566. [|Link]

(18) Malek, K. Eikerling, M. Wang, Q. Liu, Z. Otsuka, S. Akizuki, K.; Abe, M. Nanophase segregation and water dynamics in hydrated Nafion: molecular modeling and experimental validation. The Journal of chemical physics 2008, 129, 204702. [|Link]

(19) Dong, B. Gwee, L. Salas-de la Cruz, D. Winey, K. I.; Elabd, Y. a Super proton conductive high-purity nafion nanofibers. Nano letters 2010, 10, 3785-90. [|Link]

(20) Yamada, T. Sadakiyo, M.; Kitagawa, H. High proton conductivity of one-dimensional ferrous oxalate dihydrate. Journal of the American Chemical Society 2009, 131, 3144-5. [|Link]

(21) Bureekaew, S. Horike, S. Higuchi, M. Mizuno, M. Kawamura, T. Tanaka, D. Yanai, N.; Kitagawa, S. One-dimensional imidazole aggregate in aluminium porous coordination polymers with high proton conductivity. Nature materials 2009, 8, 831-6. [|Link]

(22) Wang, H. Xu, X. Johnson, N. M. Dandala, N. K. R.; Ji, H.-F. High Proton Conductivity of Water Channels in a Highly Ordered Nanowire. Angewandte Chemie (International ed. in English) 2011. [|Link]