Question

Discuss why cathode flow channel design is less important for SOFCs than for PEMFCs. Hint: Consider the typical operating temperature of a SOFC and its effect on jL.

Answer 1

The PEMFC is also referred to as the Solid Polymer Fuel Cell; the name is derived from the special “plastic” membrane used as an electrolyte. The components of a single cell are: an electron conducting anode (a porous gas diffusion layer as an electrode and an anodic catalyst layer), a proton conducting electrolyte (hydrated solid membrane), an electron conducting cathode (a cathodic catalyst layer and a porous gas diffusion layer as an electrode), and current collectors with the reactant (gas) flow fields. Current collectors are bipolar plates in a stack; they contain over 90% of the volume and 80% of the mass of a fuel cell stack. The bipolar plate is the most expensive part of the fuel cell. Platinum or platinum alloys in nanometer size particles are the electrocatalysts used with Nafion membranes. The anode-electrolyte-cathode assembly is referred to as Membrane Electrode Assemblies (MEAs), only a few hundred 3 4 micron thickness; it is the heart of PEMFC. If heat generated in the fuel cell due to exothermic reaction is large, cooling passages are provided by a central channel in each bipolar plate. A stack of cells are connected in series, and cell stacks (modules) connected in series and parallel to obtain the desired current and voltage. Sources of pressurized air and CO free (very essential to have CO less than 10 ppm) hydrogen gas are required for generating the desired electric power. The cell voltage at the design point is around 0.7 V and power densities of up to 1 W/cm2 of electrode area when supplied with hydrogen and air. Oxygen reduction is more complex and results in significant overpotential at the cathode. The PEMFC relies on the presence of liquid water to conduct protons effectively through the membrane, and moisturization of the membrane limits the operating temperature of the PEMFC. Systems for thermal management in the cells and water management in the MEAs are essential for efficient operation of the PEMFC. The solid oxide fuel cells operate at temperatures where certain oxidic electrolytes become highly conducting oxygen ions O2− . The oxides used are mixtures of yttria and zirconia first demonstrated by Nernst in 1899. Thus the charge carrier is an oxygen ion and not a proton. Overall cell reaction is the formation of water and standard reversible potentials are the same as for other hydrogen/oxygen fuel cells. The typical operating temperature of the SOFC is 700- 1000°C. Very unique tubular SOFC design concept has been developed to avoid the sealing problem for preventing anode/cathode gas leaks at the operating temperature of about 950-1000°C (no seals are available at these temperatures). This fuel cell is very expensive and has low power density. It has been developed and proven the durability requirements; however, due to high cost, it has not been marketable. Planar fuel cells in about 5 kW power generation capacities are being developed now. SOFC is of considerable interest since it has considerably high system efficiency compared to other fuel cell systems with cogeneration as a result of high operating temperatures, and negligible deterioration in performance over several years. Increasing of the cross-sectional area lead to the decreasing of a molar concentration of oxygen at the interface of gas diffusion layer and catalyst layer that then decrease cell performance. On the other hand, increasing aspect ratio lead to the increase of the molar concentration of oxygen and cell performance. The viscosity of the gas mixture depends on the temperature and composition. Pressure diff is required togas flow-through channel. The boundary conditions for cathode flow channel are that the inlet mass flow rate is constant, the inlet gas compositions are constant and the flow is fully developed at cathode flow channel outlet. The solid walls are no slip with zero flux boundary conditions. At the interfaces between the gas channel, gas diffusion layer and the catalyst layer, the velocity, mass fractions, momentum fluxes and mass fluxes are assumed to be equal. A decrease of the cross-sectional area does not have a significant effect on the cell performance. However, when the aspect ratio is increased, one can observe the slight effect on the performance. With higher aspect ratio, the smallest cross-sectional area shows better performance than the bigger one at high current density zone. The reason was that the decrease of the cross-sectional area would increase the oxygen concentration at the interface between gas diffusion layer and catalyst layer, leading to the higher rate of reaction. For open cathode PEM fuel cell, cathode flow channels are straight. Pressure distribution along the flow channels is uniform for all channels. If the pressure difference between two adjacent channels does not occur, the under-rib convection will not observe. Decreasing of cross-sectional area decreases rib area which will give higher oxygen concentration at the interface between gas diffusion layer and catalyst layer. increasing of channel aspect ratio slightly improve the cell performance. Similar to the effect of cross-section area, the increasing channel aspect ratio decreased rib area that caused increasing of oxygen concentration at an interface between gas diffusion layer and catalyst layer