PEMFC and Thermal Stress Challenge
An electrochemical reaction in the fuel cell between the oxidizing agents (hydrogen, methane) with oxygen converts the chemically stored energy into electrical power. Fuel cells can be broadly categorized based on the working principle and fuel type utilized, but the common components among the different fuel cells are a cathode, an anode and an electrolyte which supports positively charged hydrogen protons (ions) to migrate between the two sides of the fuel cell. Oxidation reactions at the anode cause hydrogen to generate positively charged hydrogen ions (protons) and electrons. After the reaction, the proton circulates from an anode to cathode through the medium electrolyte. In the meanwhile from an external circuit electrons are collected from an anode to the cathode generating direct current electricity. In the cathode area, another set of catalysts causes hydrogen ions, electrons, and oxygen to react to form water.
Fuel cells are mainly classified based on working principle, nature of fuel and electrolyte employed. The categorization describes the electro-chemical reaction, type of catalysis to be selected and temperature limits during operation.
· Polymer electrolyte membrane fuel cells (PEMFCs)
· Direct Methanol Fuel Cells (DMFCs)
· Alkaline Fuel Cells (AFCs)
· Phosphoric Acid Fuel cells (PAFCs)
· Molten Carbonate Fuel Cells (MCFCs)
· Solid Oxide Fuel Cells (SOFCs)
· Reversible Fuel Cells
In the above mentioned fuel cells, PEMFC has advantages such low weight/volume ratio, compact structure, low start up time of below 3 seconds, can work with high power density, easy to cool after the operation, the maximum temperature of operation for low temperature PEMFC is 80–100˚C and for high temperature PEMFC is 120–140˚C which utilizes Nafion membrane or water based acidic polymer electrolyte membrane as electrolyte or in some other cases with null emissions in both cases. By considering the advantages of PEMFC, it has been widely used in transportation purposes, generation of electricity (DC or AC, depends on requirement).
At operating conditions, different types of stresses developed in the fuel cell; among them in this article concentration is focused on thermal stress. Thermal stress is one of the types of mechanical stresses developed in the system when different parts in the fuel cell are not free to expand or contract in response to the change in temperature. It is a function of applied material coefficient of thermal expansion which has immediate impact on the outcome and efficiency of a system and it is mainly dependent on thermal mismatch between the layers of any fuel cell.
Some of the main reasons for the development of thermal stresses are oxygen activity gradient, external mechanical loading, residual stress generated during preparation/geometry/design of the fuel cell, operating conditions, nature of fluid flow (Co-flow or counter flow). Therefore management of thermal stress plays a major role in PEMFC designing, manufacturing, working and improvement.
Nafion/ Polytetrafluoroethylene (PTFE) is a composite membrane which is hydrophobic in nature than Nafion 115 membrane because of the higher percentage of PTFE content, but it is much thinner than Nafion 115 membrane.
Thermal stress distribution for Nafion/PTFE membrane and Nafion 115, its effect on Proton and water transport for PEMFC was simulated using Matlab. Results displayed positive change in the working temperature during fuel cell operation increases thermal stress exponentially. In spite of higher thermal stresses in Nafion 115 compared to PTFE, Nafion 115 allows net flow rate of water. Exceeding this value causes flooding in the fuel cell. Hence Nafion 115 is best suited for PEMFC. But a wide research gap is there to replace the Nafion 115 with appropriate membrane with better heat insulating property.