Design of high-ionic conductivity electrodes for direct methanol fuel cells

Abstract

Carbon-supported porous electrodes are used in low-temperature fuel cells to provide maximum catalyst surface area, while taking up little volume and using minimum catalyst material. In Direct Methanol Fuel Cells (DMFCs), however, much of the catalyst included in the anode is significantly under-utilized, while a small fraction of the catalyst facilitates the bulk of the oxidation reaction. In this thesis, the porous carbon electrode used as the anode in a DMFC is analyzed using Axiomatic Design theory. The imbalance of catalyst utilization in these electrodes is determined to be a result of coupled design, in which large amounts of catalyst can compromise ionic resistance and fuel transport within the electrode. This design flaw is confirmed experimentally using cyclic voltammetry and impedance spectroscopy. Tests of standard electrodes show that they have a maximum Nafion content of about 30% Nafion by weight and that excessive catalyst loading eventually results in less available catalyst, not more. An alternative design is proposed to alleviate the coupling between functions by applying micron-scale structure to the nano-porous electrode. The proposed design introduces ionically conductive channels through the thickness of the porous electrode to greatly reduce ionic resistance to catalyst particles far from the ion exchange membrane without compromising access to catalyst particles near the membrane accessible for fuel delivery and product removal. The influence of the proposed design on ionic conductivity is analyzed using a twodimensional analog of the transmission line model for porous electrodes. The model suggests that ionic resistance can be decreased by up to 87 % with the addition of ionically conductive posts. Structured electrodes with 75 pm diameter posts spaced 175 tm apart are shown in electrochemical impedance spectroscopy experiments to perform notably better than standard cells. The structured cells show a 6 % increase in available catalyst area and a 46 % decrease in ionic resistance. Peak cell power is estimated to increase by 4 % as a result of the best electrode tested while an electrode with ideal geometry could increase peak cell power by 9 %. Even greater benefits could be realized if, as predicted, structured cells can keep ionic resistance constant while catalyst loading is increased.