The design of a microfabricated air electrode for liquid electrolyte fuel cells

Abstract

In this dissertation, the microfabricated electrode (MFE) concept was applied to the design of an air electrode for liquid electrolyte fuel cells. The catalyst layer of the electrode is envisioned to be fabricated by using a microfabricated die to apply a three-dimensionally patterned macro-texture upon a microporous carbon matrix. The resulting dual porosity structure consists of an array of cylindrical holes that are formed from the die and micropores present in the carbon matrix. The holes are used for gas transport while the micropores are saturated with a liquid electrolyte for ion transport. The catalyst is loaded into the microfabricated structure by electrodepositing thin catalyst films within the cylindrical holes. In this dissertation, three issues concerning the design of the MFE were investigated: 1) identification of the best material to use for the microporous carbon matrix, 2) the study of electrokinetic parameters of electrodeposited Pt films, and 3) the study of oxygen transport behavior within a Pt film supported on the surface of a microporous carbon matrix. Two types of polymer-bonded carbon materials have been identified as suitable materials for the carbon matrix. They are carbon black particles bonded into a microporous matrix either by polytetrafluoroethylene (PTFE) fibrils or by polyethersulfone (PES), which is a soluble polymer in common solvents. Experiments and modeling have indicated that these materials will allow the microfabricated catalyst layer to have an effective ionic conductivity that is 4 to 5 times greater than the conventional catalyst layer. Rotating disk electrode experiments on electrodeposited Pt films in 0.5 M sulfuric acid show that these films have an oxygen reduction reaction mass activity that is 2.5 times greater than that of Pt particles supported on carbon black.

(cont.) Furthermore, oxygen gain experiments on electrodeposited Pt films supported on a microporous membrane indicate that these films experienced no oxygen transport losses in air, up to a current density of 130 mA/cm2. These results strongly support the use of thin catalyst film technology in catalyst layers of fuel cells. The experimental results presented this dissertation were used to develop a half-cell model of the MFE in concentrated phosphoric acid. The results of the model suggest that the MFE is capable of producing a current density 3.5 times greater than that of the conventional electrode. It is believed that such potential improvements in the performance of the air electrode support continued efforts to fabricate and test the MFE design concept presented in this dissertation.