Multiphysics software enables accurate membrane modeling in studying physical and electrical behavior of fuel cells.
Scientists have deemed the fuel cell as one of the most promising power sources that can replace the use of fossil fuels as well as satisfy global expectations. The polymer electrolyte membrane fuel cell shows great potential to power homes and vehicles. A special class of fuel cell — Proton Exchange Membrane Fuel Cell (PEMFC) — was investigated during this study. The membrane of the PEMFC is one of the most fundamental parts of the cell, as its properties have great impact on the cell’s output capability.
Most PEMFCs employ the use of hydrated membranes. This poses a problem where the membrane only works properly at temperatures below 80 °C, because above this, the water in the membrane would start to evaporate, leading to membrane degradation and lowering the cell’s output voltage. The Poly(1-vinylimidazole) (PVIm) polymer is a material that does not involve the presence of water for operation as a membrane.
This polymer, when combined with triflic acid, forms an acid-base ternary composite- blend-based membrane. The acidbase interaction within the membrane, as well as the addition of nanocomposite materials, leads to increased proton conductivity and morphology that results in larger output voltages. In this particular membrane, nanotubular titania are included in the base polymer matrix. The membrane also has high chemical, thermal, and mechanical stability making it suitable for use in a PEMFC for temperatures much greater than 80 °C.
In this project, a model of the PVIm polymer membrane was made. This model was applied to different compositions of the acid-base blended membrane, with special attention to the composition of PVIm-Triflic acid-PVDF-HFP in the ratio of 5-2-3. The modeling process was done using COMSOL Multiphysics 4.2a.
A computational domain was created as well as a computational mesh. There are two separate gas channels at the anode and cathode that allow for hydrogen and oxygen gases to enter the fuel cell. The channels are bounded by graphite-sheet bipolar plates that are highly conductive, both thermally and electrically, and are also stable at high temperatures. Between the two channels is the membrane electrode assembly (MEA) that comprises two carbon fiber gas diffusion layers (GDLs), which allow the incoming gases to be dispersed throughout the entire reaction surface layers. Hence, there are also two reaction layers made of platinum where the gases undergo the following chemical reactions:
2H2 => 4H+ + 4e-
O2 + 4H+ + 4e- => 2H2O
The last component of the MEA is the PVIm membrane. The membrane allows protons to flow within the cell, from the anode to the cathode. The membrane thickness is ±542·10-6 [m].
In order to develop the model, several assumptions were made. The fuel cell operates under time-dependent conditions. No water passes through the membrane. Flow of gases is laminar in all channels. The gas inlet conditions are fully developed. All gases are ideal. Gases do not cross over into other channels. Since the fuel cell operates at temperatures higher than 80 °C, there is single-phase water flow. All electrochemical reactions occur in the gaseous phase. The parameters of the materials used in the model are homogenous and isotropic.
Acid-base membranes have increased proton transport ability, thereby increasing the proton conductivity and the output voltage. A stable hydrophobic backbone was created for the membrane with the inclusion of PVDF-HFP (polyvinylidene fluoride-co-hexafluoropropene). Conductivity measurements were obtained for different compositions of PVDF-HFP-triflic acid-PVIm for the membrane. The 5-2-3 membrane with 2% wt. titania showed the highest conductivity. The thermal conductivity and heat capacity of the PVIm-based membrane were calculated from DSC and thermogravimetry studies performed on the membrane.
Thermal insulation boundaries were used for internal boundary conditions. For the gas channels, temperature, pressure, flow rate, and composition were applied. The voltage and temperature of the cell based on the membrane operation were applied at the bipolar plates.
The free and porous media flow within the model was examined, as shown in Figure 1. The secondary current distribution was then obtained across the membrane, as shown in Figures 2 and 3. The voltage is significantly increased from the anodic side of the cell model to the cathodic side as protons flow through the membrane to give a maximum output of 0.95V. The cold gas entering the cell may cause cooling of the membrane. This can lead to a lowering of proton conductivity, causing higher ohmic losses within the cell.
This model shows fairly good agreement with experimental values.
This work was done by S. Beharry of the University of the West Indies using software from COMSOL, Inc., Burlington, MA. For more information, visit http://info.hotims.com/45604-122.