Anode and Cathode of Polymer Electrolyte Membrane Fuel Cells - Coursework Example

Anode and Cathode of Polymer electrolyte membrane fuel cells Name: Course: Lecturer: Institution: City & State: Date: Anode and Cathode of Polymer electrolyte membrane fuel cells Introduction Polymer electrolyte fuel cells are also known as proton exchange fuel cells. Generally, they have high power delivery as well as having an advantage of low volume and weight as compared to other types of cells. PEM cells utilize electrolyte, which is a solid polymer, while the electrodes are porous carbon that contain platinum catalyst. Their operation does not need use of corrosive electrolytes such as sulphuric acid, but use hydrogen or oxygen present in the air and water. Usually, these cells receive hydrogen from storage tanks. PEM fuel cells operate at comparatively low temperatures ranging about  as this allows the PEM fuel cells to start quickly (Vielstich, et al. 2009). This protects the cells from fast wear thus making PEM fuel cells more durable and operates for a longer period. Nevertheless, for a detarchment of hydrogen protons and electrons a metal catalyst is necessary thereby increasing the cost of the system. Addition of the platinum catalyst in the system calls for addition of an extra reactor that would reduce CO poisoning. This is because platinum catalyst is highly sensitive to CO poisoning. Therefore, this has called for researchers to explore on the ruthenium/platinum catalyst that is resistant to Poisoning caused by CO. Since PEM fuels have a fast start up time, low power to weight ratio, and low orientation sensitivity, they are usable in transportation applications as well as stationary applications. Advantages of PEM fuel cells They quick start up at low temperatures They require low temperature for start-up They use solid electrolyte thus they have easy electrolyte management Challenges of PEM fuel cells Platinum catalyst is highly sensitive to CO poisoning The types of the catalyst required in the reaction are expensive Principle of operation of PEM fuel cells Management of water in PEM is the most critical issue in the operation of the cell. The membrane absorbs water to ionize the acid groups while the excess water flows to the cathode to reduce the power output. The PEM hydrogen fuel cell is shown below: Figure 1: PEM fuel cell made of hydrogen-oxygen At the anode, the hydrogen molecules adsorb dissociatively as they oxidize the protons. As a result, the electrons flow through the external load while the protons diffuse creating electrochemical gradient in the PEM to the cathode. On the other hand, at the cathode, the Oxygen molecules adsorbs thus they are reduced and react producing water molecules. The water formed evaporates at the anode and cathode or gets absorbed in the PEM (Gregor, 2003). Materials and key cell components of PEM fuel cell Microscopically, the membranes form a single phase from a solid polymer electrolyte. This membrane bond to each other and has hydrophobic or hydrophilic moieties. PEM fuel cells use sulfonic acid groups since they are hydrophilic and this helps in transfer of protons from anode to cathode. They also help in the separation of reactants between the anode and the cathode thus acting as electronic current insulators as they provide ionic current conductivity. An example of the polymer used as in PEM is the Nafion that has a structure that resemble that of Teflen (Frano, 2005). The structure of Nafion as shown below: Figure 2: structure of Nafion Catalyst layer formation Electrode used in PEM fuel cells are made out of two techniques the first method is gas diffusion layer where, the diffusion layer is coated using the catalyst ink. The second method is a membrane electrode assembly where a catalyst ink is coats the membrane thus forming catalyst-coated membrane. The figures below show gas diffusion layer method and membrane electrode assembly methods respectively. Figure 3: gas diffusion layer Figure 4: Membrane electrode assembly Electrochemical reactions (thermodynamics) of PEM fuel cell PEM fuel cells made of hydrogen have two half reactions, that is, at the anode there is hydrogen oxidation (HOR) while at the cathode there is oxygen reduction reaction (ORR). The cathodic and anode reactions are as indicated in the equations below: At the anode: …………………………………………………………………..…… (i) Which give potential of  under standard conditions which after oxidation forms water as shown in equation (ii) for the cathode reaction …………………………………………………….…….….. (ii) At the cathode and anode, the following general reaction takes place where heat is generated because of the reduction reaction. ………………………………………………………….………. (iii) Thus, this generates a cathodic potential of  In essence, oxygen-oxygen bond are immensely strong, therefore, they are hard to break. Therefore, for the purpose of reduction, one oxygen molecule requires four electrons. It is necessary to understand that when the number of atoms is higher in an electrochemical reaction, the harder and slower the reaction. It is necessary to consider that each half reaction as is a series of sub-reactions that compose the whole reaction mechanism, it is always only a single electron transfer can take place in any half reaction for a given reaction mechanism. For a complete ORR, there must be a reduction of four electrons from the mechanism to allow single electron transfer. Most of the electron transfer reactions in the fuel cells like the ORR are slow and complex in nature; thereby need electro-catalysts that help the reactions to happen a noticeable rate within room temperature and pressure (rt p). The oxygen reduction reaction has the following reaction pathways: The possible electrocatalysts that may be used in such reaction as indicated in the table below: Table 1: oxygen reduction catalysts Noble metals Non-noble metal electrodes Organometric complexes Bulk noble metals Nanoparticulate Platinum Copper ruthenium Gold ruthenium Titanium dioxide Palladium nanoparticles on gold Silver nanoparticles on gold Nickel Copper Titanium dioxide Lanthanum magnate Transition metal complexes with non-porphyrin ligards Transition metal complexes with porphyrin ligards Voltage loss and Polarization of the PEM fuel cells Due to the voltage in the cell, the cell current (I), a current density () results due to the relationship between the electric cell and the area of the curve, when the open circuit is closed, the open circuit voltage decreases as a result of electrode polarization (Larminie, & Dicks, 2003). This occurs due to the shift of the cathode and the anode potentials of the cell from their equilibrium. This is as shown in the graph below: Conclusion In spite of the challenges that face PEM fuel cells, the PEM fuel cells have a higher possibility of becoming better option. This is in regard to other models of cells which include alkaline cells, phosphoric cells and other cells. This is as shown indicated below: Cell Electrolyte used Stack size Efficiency Applications Advantages challenges PEM Perfluoro sulfonic acid Read More