Proton Exchange or Polymer Electrolyte Membrane Fuel Cells - Coursework Example

Proton Exchange or Polymer Electrolyte Membrane Fuel Cells (PEMFCs) Name Course Name and Code Instructor’s Name Date Introduction Fuel cells refer to electrochemical energy conversion mechanism that has higher rates of efficiency compared to internal combustion engines in conversion of fuel into power as described by Smitha et al. (2005). Mench (2011) indicates that they do so by the direct and incessant conversion of externally supplied fuel and oxidant. There are varied types of fuel cells, which are categorized based on electrolyte they utilize. Among the types of fuel cells are the polymer electrolyte membrane fuel cells also known as the proton exchange membrane fuel cells. Primarily, these fuel cells are described as solid and organic polymer, which uses oxygen and hydrogen fuel to produce power (Buchi, et al., 2009). They are advantageous due to their low weight and volume and reduced warm up time hence starting quickly (Haile, 2003). The report seeks to discuss two aspects of polymer electrolyte membrane fuel cells namely the solid electrolyte and catalysis. Solid electrolyte Haile (2003) notes that the efficiency and effectiveness of PEMFCs is greatly influenced by the membranes and the operating conditions such as pressure, the operating temperature and relative humidity. PMFCs utilize a solid polymer membrane as the primary electrolyte and the electrodes are porous carbon coated with platinum catalyst (Smitha, et al., 2005). The main function of an electrolyte which is a solid polymer membrane in PEMFCs is to separate the cathode and the anode in order to ensure the supplied fuel and oxidant does not mix (Mench, 2011). In addition, supplying an ionically-conductive route for proton Figure 1 illustrates a schematic of a PEMFCs Figure1 For enhanced performance of PEMFCs, the solid electrolytes used should have certain attributes which includes exceptionally minimal fuel bypass to enhance coulombic competence, cost of production that is attuned to purposed application and increased proton conductivity to facilitate high currents with low resistive losses and no electronic conductivity (Haile, 2003). In addition, the solid electrolytes need to have sufficient mechanical power and stability with resistance to swelling, chemical and electrochemical constancy under operating parameters and moisture control. Haile (2003) highlights that there are varied factors that influence the efficiency of the membranes in PEMFCs which includes hydration where increased rates of hydration generate high levels of conductivity. However, a wet electrolyte enhances the risk of flooding the cathode thus the oxidation reaction is slowed down. The other factor is thickness where reduced thickness in membranes minimizes water drag hence increasing performance of fuel cell while lowering membrane resistance. Be it as it may, the thickness should be ideal to prevent problems caused by fuel bypass and ensure durability (Mench, 2011). Membrane materials used in PEMFCs are categorized in varied membrane systems namely non-fluorinated hydrocarbons, perfluorinated ionomers, non-fluorinated membranes with aromatic backbone, partially fluorinated polymers and acid–base blends all of which have varying structures, physical characteristics and situ performance (Smitha, et al., 2005). Presently, the perfluorinated ionomers are the most preferred membrane system used in PEMFCs. According to Smitha et al. (2005), the commonly used perfluorinated polymer made presently is branded Nafion, which has high levels of proton conductivity, sufficient chemical constancy and mechanical power. The perfluorinated membranes are stable and strong in oxidative and reductive conditions and their situ performance is relatively good with membrane durability of up to more than sixty thousand hours, 0.2S/cm proton conductivity in humid membranes under operating temperatures, sufficient cell resistance and low voltage loss. Despite the advantages generated by perfluorinated membranes such as Nafion, their main limitations include the high costs of the membranes, the low safety while producing and using the membranes, the need for supporting materials and temperature induced difficulties (Smitha, et al., 2005). Figure 2 is the chemical structure of Nafion figure 2: chemical structure of Nafion For the solid polymer electrolyte membrane to work, it should not only be resistant to the decreasing conditions at the cathode and the oxidative conditions at the anode but also, conduct the protons which are the hydrogen ions and not the electrons in order to ensure the PEMFC is not short-circuited (Zaidi, 2009). In addition, it must not permit the gas to move to the other side of the cell. In PEM FCs where the fuel is hydrogen, a proton conductors generates the advantage of producing water at the cathose as the by product and hence, the hydrogen fuel is not diluted as a function of usage and as a result, so long as oxygen is adequate, the cell voltages remain relatively high (Buchi, et al., 2009). Catalysis In PEMFCs, the fuel used is hydrogen and the hydrogen ions are the charge carriers. At the anode side, the hydrogen molecule breaks into protons and electrons where the former filter through to the cathode while the latter flows through an external circuit and generate electric energy (Haile, 2003). On the other hand, oxygen gas is supplied at the cathode side and it mixes with the hydrogen electrons to generate water as the by-product (Mench, 2011). The reaction at the cell occurs in two half-cell reactions namely the hydrogen oxidation reaction (HOR) at the Anode where H2 = 2H+ + 2e- with subsequent anode thermodynamic potential of Eoa = 0.00 And an oxygen reduction reaction (ORR) at the cathode where ½O2 + 2H+ + 2e- = H2O with subsequent cathode potential of Eoc = 1.229, both reactions at optimal conditions. The overall reaction is H2 + ½O2 = H2O + energy, where standard equilibrium electromotive power is 1.229V Figure 3 illustrates the ORR and the HOR Figure 3 While breaking the hydrogen molecule into protons and electrons as earlier mentioned, a catalyst is used which in PEMFCs, is usually made from platinum. Typically, PEMFCs function at 800C which are very low thus slowing down the reactions. This necessitates the use of platinum catalyst at the anode and cathode sides to speed up reactions. The performance and the effectiveness of the platinum catalyst can be improved by optimizing the size and shape of the particles of the platinum (Buchi, et al., 2009). Reducing the size of the particles enhances the total surface area of the catalyst involved per amount of platinum utilized The oxygen molecule is relatively strong compared to the hydrogen molecule owing to the pi-bond and therefore they are hard to split as noted by Mench ( 2011). Four hydrogen electrons are needed to break one oxygen molecule which influence the speed of the reaction since the higher the number of electrons in an electrochemical process, the slower the reactions becomes (Haile, 2003). In order to finish an oxygen reduction reaction (ORR), the four hydrogen electrons have to occur in single transfer, one electron after the other, which therefore causes the reaction to slow down necessitating an electro catalyst. On the other hand, the hydrogen oxidation reaction (HOR) is relatively simple while using a platinum catalyst since it has a much weaker sigma bond compared to the oxygen pi-bond and only two electrons are involved in the process. Once the electrons in the hydrogen are removed at the anode, the hydrogen cations are adsorbed on the platinum. Exchange current density is integral in reducing activation overvoltage during oxygen reduction reaction and hydrogen oxidation reaction (Smitha, et al., 2005). Buchi et al. (2009) indicates that to enhance the exchange current density therefore, one can increase the operating temperature, utilize an effective catalyst, enhance the roughness of the electrode which translates to an increase in the electro active surface area and augment the reactant concentration. Moreover, increase the operating pressure as suggested by Haile (2003). According to Zaidi (2009), there are tendencies for ohmic losses and internal resistance of the cell during reactions in the PEFMCs which can adequately be minimized by using electrodes with high conductivity, reducing the thickness of the electrolyte without compromising on integrity of the structure of the cell and using effective materials and design for the interconnects of the PMFCs. On the other hand, mass transport losses can be reduced effectively by increasing the concentration of the reaction, enhancing the flow rate of the reactant, increasing pressure and doing improvements to the gas diffusion layer as echoed by Smitha et al. (2005). Conclusion PEMFCs are the most preferred and advantageous fuel cells owing to their high rates of power density and the minimal effects it has on the environment when used in transport vehicles. Their ability to start quickly reduces wear and tear on systems components which translates to enhanced durability. The report has analyzed in detail the solid electrolyte and catalysis in PEMFCs. References Buchi, F.N., Inaba, M., & Schmidt, T.J. 2009. Polymer Electrolyte Fuel Cell Durability. Sidney: Springer. Haile, S.M. 2003. Fuel cell materials and components. Acta Materialia, vol. 51, pp. 5981–6000 Mench, M.M., Kumbur, E.C., & Veziroglu, T.N. 2011. Polymer Electrolyte Fuel Cell Degradation. London: Academic Press. Smitha, B., Sridhar, S., & Khan, A.A. 2005. Solid polymer electrolyte membranes for fuel cell applications—a review. Journal of Membrane Science, vol. 259, pp. 10–26 Zaidi, S.M.J. 2009. Polymer membranes for fuel cells. Sidney: Springer. Read More