(141h) Understanding the Electrochemical Behavior of a Thin, Flexible Micro Fuel Cell

Portable fuel cell systems in the power range of few milliwatts to a few hundred watts have become the focus of vigorous research and development activities. Fuel cells promise to provide more reliable, longer portable power than batteries. Micro fuel cell (mFC) technology can take advantage of fuels with energy densities of an order of magnitude higher than the energy stored in batteries. The air breathing cathode reduces the size and weight of the cathodic plate, and simplifies system requirements since oxygen inlet is fully passive. On the other hand, working conditions for the air breathing cell are more demanding due to the absence of an air flow behind the cathode gas diffusion layer to assist the removal of water.

The importance of micro-structures on polymers for micro fuel cell fabrication is enormous, particularly when considered as a low-cost alternative to the silicon- or glass-based MEMS technologies, for disposable small electronic gadgets. Many polymer-based microfabrication techniques via microinjection molding, casting, and micro-hot embossing have been developed.

Hot embossing provides advantages such as a relatively low-cost for embossing tools, simple operation and higher accuracy in the replication of small features. The embossing master stamp can be a silicon wafer, glass, electroplated nickel mold or other stamp with microfeatures. The hot embossing process introduces less residual stress in the polymer because the polymer stretches for a very short distance from the substrate into microstructure during hot embossing. In addition, the temperature variation range for the polymer is smaller than that required in injection molding, thus can reduce shrinkage during cooling and the friction forces acting on the microfeatures during de-molding. However, still micro-hot embossing is facing challenge in terms of process feasibility, since it is difficult to make the polymer to fill completely into microfeatured geometry of high aspect ratio and it is also delicate to separate the embossed structures from the mold without breakage.

An air-breathing micro fuel cell with direct hydrogen flow through porous anode electrode is realized and reported in this study. The performance of the microdevice with embedded flow channels and electrodes is characterized in ambient conditions. To achieve this goal, we implement a SU-8 microchannel stamp to transfer a pattern into a Nafion 1110 membrane by hot embossing. Nafion 212 thermally seals the whole device as a blanket top layer. The fabrication process of microchannels and micro fuel cell construction is evaluated with optical and scanning electron microscopy (SEM) step by step. Variations of hydrogen feed rate on performance were investigated. The shared-anode characteristic design of the double-sided cell is further studied by separate performance and electrochemical impedance spectroscopy (EIS) measurements taken from each side and compared with data for the complete cell. The maximum power density per superficial (footprint) area is 68.4 mWcm-2 for the shared anode stack. In the present micro fuel cell architecture, the external package is constructed of Nafion polymer, which offers a thin, light and flexible energy source with low cost of manufacturing. The device performance offers a high volumetric and gravimetric energy density for portable applications.