Transport in fuel cells: Electrochemical impedance spectroscopy and neutron imaging studies

This dissertation focuses on two powerful methods of performing in-situ studies of transport limitations in fuel cells. The first is electrochemical impedance spectroscopy (EIS) while the second is neutron imaging. Three fuel cell systems are studied in this work: polymer electrolyte membrane fuel cells (PEMFCs), microbial fuel cells (MFCs) and enzyme fuel cells (EFCs).;The first experimental section of this dissertation focuses on application of EIS and neutron imaging to an operating PEMFC. The effects of cathode-side humidity and flow rate, as well as cell temperature and a transient response to cathode-side humidity, were studied for a PEMFC via EIS. It was found that increased air humidity in the cathode resulted in greatly reduced cathode resistance as well as a significant reduction in membrane resistance. The anode resistance was only slightly reduced in this case. Increased air flow rate was observed to have little effect on any resistance in the PEMFC, though slight reductions in both the anode and the cathode were observed. Increased cell temperature resulted in decreased cathode and anode resistances. Finally, the transient response to increased humidity exhibited unstable behavior for both the anode and the cathode resistances and the PEMFC power output. Neutron imaging allowed the calculation of water content throughout the PEMFC, showing a maximum in water content at the cathode gas diffusion layer - membrane interface.;The second experimental section of this dissertation delves into the world of microbial fuel cells. Multiple long-term observations of changes in internal resistances were performed and illustrated the reduction in anode resistance as the bacterial community was established. Over this same time period, the cathode resistance was observed to have increased; these two phenomena suggest that the anode improved over time while the cathode suffered from degradation. Increased anode fluid ionic strength and flow rate both led to significant reductions in cathode resistance, while the anode resistance was relatively unchanged. Improvement of the cathode after changes effected in the anode led to the conclusion that proton transport to and in the cathode limited the MFC, not the bioanode. The experiments performed over time and on the MFC anode demonstrate how the limiting resistance in an MFC can change and that some alterations to the MFC can reduce those limitations. EIS was also used to observe the response of the MFC to such material changes as replacing the membrane and cathode. It was found that increased concentration of carbon source to the bioanode had little effect on the MFC resistances. This suggested that the bioanode had a maximum loading of carbon that it could process and further carbon source was excessive.;The third experimental section involves enzyme fuel cells. EIS was utilized to study the response of the EFC to such operating changes as increased cathode humidity, enzyme loading, air flow rate, and temperature. It was expected that the enzymatic cathode of the EFC was limiting the EFC power output. It was found that the resistance in the cathode dominated the EFC resistances but it decreased markedly as the enzyme loading increased. The effect of cathode flow rate was relatively small on all internal resistances as long as the flow rate was maintained above a minimum level needed for operation. In addition, changes in cathode humidification temperature resulted in a small reduction in resistances throughout the EFC. This suggested that water content in the EFC did not greatly affect internal resistances or power output on a new EFC. Time had a very strong influence on internal resistance and power density; however, power output decreased by 95% after ∼20 hours of operation while internal resistance went up by a factor of 10 over the same period. This drastic increase in resistance, with an attendant loss of power output, was attributed both to enzyme/mediator degradation and water loss. To explore water loss from the EFC cathode, neutron imaging was performed on a working EFC. These studies allowed quantification of the water loss rate from the enzymatic cathode. It was found that increasing the air-side humidity and cathode solution salt content contributed to water retention in the cathode, which resulted in more stable power output over time. However, even saturated air in the cathode resulted in water loss. It was hypothesized that heating due to the cathode reaction in the EFC warmed the cathode and imposed an unsaturated state. Mass and energy balances were performed that support this hypothesis. (Abstract shortened by UMI.)...