| Vehicle applications require that proton exchange membrane (PEM) fuel cells, which electrochemically produce water, must survive and reliably start at sub-freezing temperatures. This thesis investigates the performance and behavior of PEM fuel cells operating at such temperatures. Since they constitute the majority of voltage loss at non-frozen conditions, this work begins with fundamental studies of the oxygen reduction reaction (ORR) kinetics and proton conductivity. In carefully designed experiments, ORR kinetics are measured in-situ from -40°C to 55°C. Measured kinetic parameters are consistent with values reported at non-frozen conditions, and activation energies are found constant across the measured temperature range, indicating ORR mechanism (within electrode ionomer) is not influenced by ice, beyond the additional reactant transport resistance it introduces. Membrane conductivity, measured with volume-less 4-point DC conductivity cell from 30°C to -50°C, is found to decrease with temperature, by nearly an order of magnitude by -20°C at practically relevant water contents, and exhibits a change in activation energy near 0°C in well hydrated membranes. The state-of-water in Nafion is investigated using differential scanning calorimetry (DSC) to provide mechanistic understanding of conductivity behavior. Below 0°C, some fraction of water absorbed in the membrane exists as liquid while the remaining fraction as ice. At progressively lower sub-freezing temperatures, a smaller non-frozen water fraction, responsible for proton conduction, exists in the membrane. Successful cold starting of PEM fuel cells requires self-heating due to voltage losses to raise the temperature near 0°C before electrode ice formation blocks reactant access. The maximum operating time before 0°C is achieved is proportional to the amount of product water uptake in the membrane and ice formation capacity in the electrode, which were measured by isothermal, galvanostatic experiments. Verified by cryo-SEM imaging and predicted by a numerical model, at low currents the membrane and electrode completely fill and ice formation occurs uniformly across electrode thickness, while at higher currents, electrode voids only filled to 40% with ice formation within electrode beginning near the membrane, proceeding outwards. Finally, incorporating the findings of this work, a transient model is developed to predict voltage and temperature during PEM fuel cell start-up from sub-freezing temperatures. |
