PEM Materials Performance - Literature review Example

PEM MATERIALS PERFORMANCE By Student’s name Course code and name Professor’s name University name City, State Date of submission 1.0 Introduction Recent technological leaps in energy engineering call for advancement in efficiency as a means of saving the badly needed energy. The PEM (proton exchange membrane) fuel cell material is no exception when it comes to pushing for efficiency and as such, developments that have been seen since the inception of this technology cannot be ignored. Raised from the fuel cell concept PEM, has emerged as one of the most successful technology that has been incorporated into the solid oxide fuel cells, phosphoric acid fuel cells among others (Wilkinson, et al., 2010). According to Ubong et al. (2008), the much anticipated migration from high temperature proton exchange membrane to low temperature substitutes is as a result of material development. Figure 1: Typical illustration of fuel cell technology (Battery University, 2011). The proliferation of this technology into the power industry has resulted to dependable power that hence requires to be progressively developed for sustenance and competitiveness when it comes to choosing among the existing technologies. According to statistics availed in the official development site Fuel Cell Today, Matthey (2014) indicates that the dependability of this technology can be equated to the currently 150,000 sets of fuel cells being used in the market with a combined capability of up to 15MW. It is the keenness of the automotive industry to develop this technology that particularly attracts this study with a focus on PEM materials performance. This article focuses on PEM fuel cells and the present performance analyses for cell membranes in order to determine their suitability with regard to efficiency achievement and 2.0 Developments Made in PEM Fuel Cell Materials Basically the development of fuel cells has attracted a huge following in terms of material research. New materials meant to phase out the old ones have been developed, but have been found to be insufficient in terms of the required efficiency levels. The upcoming modifications look at improving the main cell components which include the proton exchange and nafion-electrolyte membranes (Kolodziejczyk, 2011). Other materials of great importance to this study include the largely utilised perfluorinated sulfonic acid membranes, non-fluorinated proton exchange membrane, composite film, high temperature membrane, alkaline membrane and full ceramic proton exchange membrane. This analysis shall therefore delve into the materials that are largely utilised in the PEM fuel cell. 2.1 PEM Fuel Cell Membranes This section highlights on the significant developments that have been made in the major PEM fuel cell membrane materials and their respective properties impacts towards the achievement of goals set by the department of energy (DoE) with regards to alternative energy. 2.1.1 Perfluorinated Sulfonic Acid Membranes This membrane consists of fluorocarbon ethers that are combined with sulfonic acid group in order to make up a complete component with high chemical stability hence its high usability. The proton conductivity of this material generally lies within 80˚C and a wetting index exceeding 0.10 S.cm-1. This membrane is characterised by a remarkable proton conductivity, a wide electrochemical window, perm selectivity and mechanical stability. These properties are imparted as a result of phase separation of membrane nanostructure which is meant to transport the ions and solves through the surrounding hydrated domains. It is therefore important to highlight that while solvent and humidity are considered as the most important determinants of membrane properties. The performance of this membrane is therefore based on the interaction of the electrochemical, nanostructure and mechanical properties (Kusoglu & Weber , 2013). Figure 2: Factors leading to degradation of fuel cell perfluorinated sulfonic acid membrane degradation (Rodgers, et al., 2012). PEM fuel cells have been found to exhibit a steady moderate power loss which is followed by a sudden failure. This degradation is further aggravated by the factors highlighted in figure 2 above. The initial power output is high enough to sustain effective equipment operation due to the strength that is derived from oxidation treatment of PEM fuel cells. The degradation of this strength in most cases occurs when carbon corrosion, platinum sintering and dissolution coupled with other processes such as recrystallization, fluoride loss backbone split and chain losses. Other modes of determining perfluorinated sulfonic acid membranes strength such as low temperature degradation have shown that contamination is the main cause of loss of strength in this material thus management of these occurrences is highly advocated by developers (Wilkinson, et al., 2010). Figure 3: Degradation factors affecting perfluorinated sulfonic acid membranes’ performance (Wilkinson, et al., 2010). Subjecting perfluorinated sulfonic acid membranes to stresses leads to cracks and tears. This calls for proper membrane preparation to avoid small perforations that attract foreign materials. Increased stresses are also likely to occur in membranes that are in possession of reactant inlet or borders/ edges that may stretch due to compressional forces brought about by flow. Stresses resulting from local swells due to manufacturing defects are also faulted for the electrode misalignments which are the definite causes for abrupt cell failures. Cast Nafion is said to be micro-crack resistant although they tear isotropically perpendicularly to machine direction. Extruded Nafion 115 membrane however responds to creep under accelerated situations such as increase in temperature. Mechanical degradation is more associated to effects of humidity as illustrated by Rodgers, et al. (2012). In their experiments, they established that mechanical degradation is associated with nonhumidification and RH cycling which affects the dimensional changes. Figure 3 above sums up the factors affecting the performance of the perfluorinated sulfonic acid membranes. 2.1.2 Non-perfluorinated Proton Exchange Membrane This types of membranes are generally low in mechanical strength and chemical stability and are normally replaced with non-blended resins such as fluorine and other inorganic fluorides. Non-perfluorinated proton exchange membranes do not meet the required threshold performance for the sake of incorporation into fuel cells. This has led to researches on their improvements as their utilization is likely to plunge end users into high maintenance costs. The maximum conductivity of these membranes is estimated at a maximum of 4.2x10(-3) S/cm at 100% relative humidity and a temperature of 80˚C. The effects of temperature on the performance characteristics of Non-perfluorinated proton exchange membrane is shown in figure 4 below. It is noted that with increase in temperature, the performance depreciates in an exponential manner (Acton, 2013). Figure 4: Effects of temperature on the conventional non-perfluorinated proton exchange membrane (Uma, 2012). 2.1.3 Non-Fluorinated Proton Exchange Membrane Aggressive approach towards the development of better non-fluorinated polymer membranes to offer a competitive approach towards achieving the set targets by the department of energy is perturbing. Non-fluorinated polymer membranes have been basically manufactured in order to substitute the existing commercial perflourosulfonic acid membranes that operate within the same temperature ranges. These membranes are sought for they possess prolonged life that emanates from the membrane mechanical strength. This strength is drawn from the sulfonation process together with hydration which increases the functionality level in comparison to the other membranes. The stability levels of these membranes are also high which is positive towards the achievement of efficiency in such environments that are aggressive. The fact that the material manufacturers have also been able to achieve thermohydrolytic and chemical stability aids in advancing the functionality of the fuel cells as clean energy. The progress made in proton –conducting hydrocarbon especially when it comes to heterocyclic based polymers for proton exchange and polymer exchange has revolutionized the fuel cell industry. The contributions made by modification and synthesis of salient properties of the conventional non-fluorinated proton exchange membrane are very important towards the achievement of power requirements by the department of energy. The support that is given towards the developmental projects for this membrane has yielded to a better microstructure when they are viewed under small angle x-ray diffraction measurements (Rozière & Jones, 2003). 2.2 PEM Fuel Cell Electrodes Fuel cell electrodes are very important in the conductance of electrons, gas and protons. In order to carry out the transportation function effectively, the volume of the conducting media should therefore be optimised through design and assembly methods. Membrane electrode assembly (MEA) is a procedure utilised in coming up with the full grown electrodes. This procedure has been improved to come up with a process that basically mirrors the two field plates for production of bipolar plates. This boosts energy produced as connecting such cells in series offers a greater voltage. Further, MEAs have been developed to consist of gas diffusion layers, catalyst layers and proton exchange membranes. Electrolyte membrane designs have taken another angle as the interfacing between the layers has been the critical factor of consideration. The current collector layers have therefore been formed in a manner that forms bipolar plates. The definition of an effective electrode takes the form of a transport enhancer meant for proton transfer across the catalytic membrane, electron transfer from current collector across the gas diffusion layer and also the reactant gases across the gas channel (Litster & McLean, 2004). Electrode designs have proven that the emerging methods need to be reviewed from time to time in order to obtain the best results when it comes to output. The PTFE binding technology used in coming up with thin film electrodes has replaced electro deposition and spattering giving the final product a homogenous finish. This is one of the major differentiation criteria that has been established by the industry practitioners although the accomplishments made in gas diffusion layers do not go mentioned too. The commercial cells employ conventional carbon cloth as the diffusion layer and also for wet proofing due to its porosity. The PTFE technology has further improved on the surface area exposure as to capitalize on tapping more electrons. Together with this is the Nafion layer that acts as a catalyst for proton transport thus the design process affects the final results. Figure 5: Improved proton exchange membrane fuel cell (Litster & McLean, 2004). 2.3 PEM Fuel Cell Catalysts Electrochemical reactions are often catalysed utilising platinum for an intrinsically fast hydrogen oxidation. The exchange current density however needs to be considered while choosing the type of catalyst to be used (He, et al., 2005). The polarisation losses noted at the anode lead to anode degradation a factor that makes platinum the preferred choice. Among other catalysts that are in existence for the purpose of PEM include carbon supports, interfacial degradation layering and Nafion ionomer. The performance degradation differs from one catalyst to the other owing to the material microstructural/ macrostructural changes on the catalyst layers (Zhang, et al., 2009). Figure 6: Performance of metal free catalysts in comparison to platinum based catalysts (The Kucernak Research Group, 2013). The evolution of oxygen reduction catalysts dates back to platinum black to carbon supported platinum whose feasibility is way higher than the former. The low surface area made platinum black unsuitable for usage as a catalyst due to low performance. When higher platinum loadings are used for the purpose of PEM fuel cells catalytic actions, the performance is noted to be high since the surface area is increased. This is usually supplied in the range of 10% to over 50% platinum depending on the manufacturing technology used for the production process. Due to technological revolution, recognition of electrode structures has become an important note to all material scientists who wish to improve on such. Polytetrafluoroethylene (PTFE)-bonded electrodes have also improved to Nafion-bonded electrodes in order respond to the changes in catalyst components. Protonic conductivity has been aggressively improved by the industry players who have further introduced Nafion layer in order to extend the reaction zone with the major step identified as the improvement in electrode materials. 2.4 Bipolar Plates Bipolar plates are meant to be the key components of the PEM fuel cell – they distribute fuel gas and air while conducting current from one cell to the other. These plates are also responsible of heat removal from the active area in order to prevent leakage of coolants and gases. Materials meant for PEM bipolar plates are speedily developing with the major ones being non-porous coated graphite, polymer composites and coated metallic sheets among others (Hermann, et al., 2005). The basic structure of these materials is advocated to have good electrical conductivity, high thermal conductivity and chemical/ corrosion resistance, recyclable, low weight against volume, low cost with mass production capability, low hydrogen permeability and high mechanical stability (Heinzel, et al., 2009). These materials exist in either graphitic form or metallic form as discussed below. 2.4.1 Metallic Bipolar Plates When PEM fuel cells were mainly used for submarines and space crafts, the production costs were extremely high as compared to what it is today. This is because expensive metallic elements that are corrosion resistance were being utilised during this time. Substrates coatings were later investigated and found to be optimal when combined for this purpose. The point was that since corrosion occurred at the anode and growth of metal oxides which would in turn decrease the conductivity of the metallic bipolar plate materials, there was need to introduce a reducing environment at the anode. This would reduce the formation of oxide layer, hydrides and dissolution of metal in humid hydrogen or product water. The contamination that was brought about by the oxidation process had to be eliminated by all means necessary since this would lead to reduced reactivity (Heinzel, et al., 2009). Metallic bipolar plates are basically made of titanium, austenitic stainless steel, aluminium and other metallic alloys that are naturally corrosion resistance. These are further subdivided into carbon-based and metal based. Mostly carbon based plates are made of polymers with conducting abilities such as polyaniline, diamond-like carbon and polypyrrole. Metal based coatings on the other side include metal nitrides and other metal elements in the early transition groups such as molybdenum, niobium and vanadium. Low cost developments from such alloys as stainless steel have received attention from industry practitioners who have gone further to develop this corrosion resistance material. It has also been championed for since it has a low interfacial contact resistance with other PEM materials. Materials such as titanium is considered to be expensive, thus it does not receive much attention. Aluminium has a lot of requirements when it comes to coating or else it shall be pitted by galvanic reactions. Conclusively, coated bipolar materials achieve corrosion resistance through careful coating in order to make it cheap for other applications such as the automotive industry (Heinzel, et al., 2009). 2.4.2 Graphitic Bipolar Plates The development of bipolar plates from graphitic materials has also advanced the PEM industry in that these materials are known to be readily resistance to phosphoric acid. Pure graphite based materials possess a high electrical and thermal conductivity coupled with a low density as compared to their metallic counterparts. The processing of this material for purpose of flow fields formation is however an affair that is costly, time consuming and complicated. Graphite materials have also been found to be brittle and porous thus it is important for them to have a coating that will reduce their permeability levels. Graphite composites are therefore manufactured in a manner that is likely to counter all these shortcomings. The feasibility of these materials is questionable due to their low producibility thus they are meant for special functions such as space flight and satellites. The polymer matrix that results from the composite formations meant for bipolar plates have a stable mechanical state and good dispersion quality but also depends on the methods of production which range from extrusion to compression moulding (Yuan, et al., 2005). The graph shown below gives a comparison between graphic and metal bipolar plate materials and it is evident that the former performs better than the latter. Figure 7: Performance graph showing contrast in performance between metal and graphite materials (Yuan, et al., 2005). 2.5 Gas Diffusion Layers (GDL) Figure 8: Position of GDLs in a PEM fuel cell (Lee, et al., 1999). Gas diffusion layers shown in the figure 8 above are usually critical in ensuring that the gas reactants are transported effectively to the catalytic layers. They therefore have to have a low electronic resistance and be in possession of surfaces that have proper wetting characteristics when it comes to low temperature applications. Sintering GDLs using Acetylene black has proven to reduce the pore volume – a desirable factor when choosing the materials. The permeability determines the reactivity of the cell thus other methods of regulating the pores during the manufacturing stage such as mercury-intrusion porosimetry have been innovated in order to maintain an optimum porosity across the materials. This procedure has been championed by industry practitioners such as Parikh et al. (2012) and Yakisir (2006) who have indicated that the pressure and volume related matters should be dealt with using high technology approaches. Standard porosimetry meant to develop these materials further has been exhibited to be favoured by high injection pressures which compress GDL materials to smaller pores which are undesired. The regulation of material pores is done by adjusting the injection pressures. Contact angles are also important beside the pore size distribution and other statistical parameters. Physical and chemical homogeneity of these materials is a very important factor when it comes to choice. 3.0 Conclusion The revolution in materials meant for PEM fuel cells development has accelerated over the 21st century with a promise to make them effective and affordable for general purposes. While the industry engages several fields of study ranging from chemical to physical in order to come up with a perfect item, it is notable that the goals set by the Department of Energy (DoE) are dynamic and cannot be specifically met through this technology alone. However, improvements made in material fuel cell membranes include increased deployment of perfluorinated sulfonic acid membranes in place of non-fluorinated proton exchange which is now considered as obsolete. Electrode development has also taken a different turn as design methods are innovated with the latest one being PTFE binding technology that has greatly improved the PME fuel cells performance. Catalysts development has also been adhered to ensure compatibility with upcoming materials – the use of carbon coated catalyst is especially advocated due to the achievable efficiency levels. On the other hand, graphite bipolar plates have been found to react better than their metallic counterparts. List of References Acton, Q. A., 2013. Issues in Hydrogen, Fuel Cell, Electrochemical, and Experimental, and Experimental Technologies. Atlanta, Georgia: ScholarlyEditions. Battery University, 2011. BU-210: Fuel Cell Technology. [Online] Available at: http://batteryuniversity.com/learn/article/fuel_cell_technology [Accessed 20 September 2014]. He, C., Desai, S., Brown, G. & Bollepalli, . S., 2005. PEM Fuel Cell Catalysts: Cost, Performance, and Durability. The Electrochemical Society Interface , pp. pp 41-44. Heinzel, A., Mahlendorf, F. & Jansen, C., 2009. Bipolar Plates. Elsevier B.V., pp. pp 810-816. Hermann, A., Chaudhuri, T. & Spagnol, P., 2005. Bipolar plates for PEM fuel cells: A review. International Journal of Hydrogen Energy, p. pp 1297–1302. Kolodziejczyk, B., 2011. Development of Novel Air Electrode Materials for Fuel Cells , Akureyri, Iceland: The School for Renewable Energy Science. Kusoglu , A. & Weber , A., 2013. Structure-Property Relationship of Perfluorinated Sulfonic Acid (PFSA) Membranes. American Physical Society, 58(1), pp. pp1-10. Lee, W.-k., Ho, C.-H., Van Zee, J. W. & Murthy, M., 1999. The effects of compression and gas diffusion layers on the performance of a PEM fuel cell. Journal of Power Sources, Volume 84, p. pp 45–51. Litster, S. & McLean, G., 2004. PEM fuel cell electrodes. Journal of Power Sources, Volume 130, pp. pp 61-76. Parikh, N., Allen, J. S. & Yassar, R. S., 2012. 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P. et al., 2010. Proton Exchange Membrane Fuel Cells: Materials Properties and Performance. New York#: CRC Press. Yakisir, D., 2006. Development of Gas Diffusion Layer for Proton Exchange Membrane Fuel Cell, PEMFC, Quebec: Universite Laval. Yuan, X. Z., Wang, H., Zhang, J. & Wilkinson, D. P., 2005. Bipolar Plates for PEM Fuel Cells - From Materials to Processing. Journal of New Materials for Electrochemical Systems, Volume 8, pp. pp 257-267. Zhang, S. et al., 2009. A review of platinum-based catalyst layer degradation in proton exchange membrane fuel cells. Journal of Power Sources, p. pp. 588–600. Read More