Performance limitations and improvements of small-scale free-breathing polymer electrolyte membrane fuel cells

Fuel cell is an electrochemical device that converts the chemical energy of fuel directly into electricity and heat without combustion with flame. The range of potential applications is from small-scale portable electronics to transportation and large-scale power production. An interesting approach for small-scale applications are free-breathing fuel cells, i.e. cells that take the oxygen needed in the reactions passively from ambient air. This thesis concentrates on performance limitations caused by the use of passive air supply in small-scale polymer electrolyte membrane fuel cells (PEMFCs) and aims to find solutions for these limitations. A conventional free-breathing PEMFC with vertical cathode channels was studied with a current distribution measurement system at different cell temperatures and ambient conditions. In addition, different overpotential distributions were determined with a flow pulse method, and a mass transport model for the cathode side of the cell was developed. The results showed that with this kind of a cell design, the main limiting factor is the mass transfer in the cathode channels. In order to enhance the air flow and thus to improve mass transfer, there should be a high temperature difference between the cell and ambient air. High cell temperature would also decrease liquid water saturation, which would be beneficial since it causes additional mass transfer limitations. The mass transfer limitations can also be reduced with wider channels. However, wide channels require a rigid gas diffusion backing in order to offer adequate support for the membrane. A potential mechanically rigid material, titanium sinter, was introduced in the thesis. Another possible solution for reduction of mass transfer limitations is a so-called planar free-breathing cell design in which there is more effective area for cathode side mass transfer than in traditional design. This cell design was introduced and its performance with different cathode structures was studied in the thesis. The results with certain cathode structure showed very high power densities for a free-breathing fuel cell. According to the results achieved in this thesis, it seems that free-breathing PEMFCs are a very potential power source for small-scale applications such as consumer electronics. As an outcome of this thesis the performance limitations of free-breathing PEMFCs are better known, and possible solutions for them are introduced.reviewe