Spontaneous hydrogen evolution in direct methanol fuel cells

A direct methanol fuel cell (DMFC) is an electrochemical energy conversion device that converts chemical energy of liquid methanol into electrical energy. Because of its simplicity and high energy density, the DMFC has been identified as the most promising power source for portable and mobile applications. In this thesis, for the first time, hydrogen was found to evolve spontaneously on the anode of the DMFC under the open-circuit condition and at low oxygen flow rates. This finding is contrary to the conventional wisdom that electrochemical reactions cease as the external load is removed. It was also found that a transient spontaneous hydrogen evolution took place on the DMFC anode under the open-circuit condition whenever the oxygen supply was started up or turned off. Subsequently, this thesis puts forward a theoretical explanation of the mechanism leading to the peculiar phenomenon of hydrogen evolution. The remainder of this thesis work was then focused on investigating the effect of spontaneous hydrogen evolution on the operation characteristic of the DMFC. For example, it was found that the spontaneous hydrogen evolution induced by oxygen supply interruptions accelerated the membrane electrode assembly (MEA) activation process and created a temporary rise in the DMFC performance. Another finding is that when the oxygen flow rate was reduced to a critical value, the open circuit voltage (OCV) of the DMFC declined abruptly from the normal value when the oxygen flow rate was sufficiently high. It was found that the abrupt decline in the OCV at this critical oxygen flow rate (COFR) coincided with the onset of the spontaneous hydrogen evolution in the DMFC. Theoretically, it is shown that the mass flux of oxygen furnished at the COFR balances the oxidation rate of methanol that permeates from the anode to cathode, which means that the COFR is a measure of the methanol permeation rate through the membrane. The equivalent methanol permeation current density determined from the COFR were found to be in reasonable agreement with that measured from the permeation cell for different methanol concentrations and at different temperatures. Finally, this thesis presents a transient OCV overshoot behavior during the startup period of oxygen supply, which was also due to the hydrogen evolution on the DMFC anode. It was found that the OCV of the DMFC underwent an overshoot before it stabilized during the startup period of oxygen supply. More importantly, it is shown that within the OCV overshoot period the DMFC anode behaved like a hydrogen electrode due to the existence of hydrogen at the catalyst layer, resulting in a transient reference hydrogen electrode, which allows quantifying the respective anode and cathode potentials of the DMFC.