Micro DMFC for Portable Applications

Abstract

Objective and results Methanol fuel cell systems have the potential of being part of a portable energy system with both high energy density and fast refueling. A hearing aid is an example of a device where fast and easy refueling/recharging and high energy density is an important requirement. The direct methanol fuel cell (DMFC) system presented here is considered novel as it presents new solutions to miniaturization and, to our knowledge, has the smallest volume reported in literature for a self-contained system. Furthermore, it has high energy density, and has been thoroughly tested with many cells actively running for more than 2 years. The fuel cell developed is a vapor feed DMFC. It is omnidirectional and fully self-contained, weighing less than 1 g with fuel and packaging. The design has been optimized for maximum energy and power density for the given footprint. At room temperature (25 °C) a gravimetric energy density of 350 Wh/kg has been obtained. Scaling the reservoir can lead to a cell with an energy density of more than 700 Wh/kg. In the specific case of powering hearing aids, the current design of the micro fuel cell is capable of powering a hearing aid for up to 2 days, with a refueling time of 20 seconds. At the DTI testing facilities, up to 500 cells have been evaluated simultaneously and the oldest cells have been running for 7 years. More than 1500 cells have been tested and currently 60 cells have been operating for more than 20.000 hours. The accumulated runtime of the tested cells is above 6 million hours thus providing significant amount of data for use in development of the cells. Excellent degradation levels have been shown. One example is a cell exhibiting a degradation of 1 µV/h during its 25.000 hours lifetime, but with no measurable degradation over the last 20.000 hours. Fuel cell design The presented work will focus on the fuel cell system design and how it enables the described figures of merit. The cell consists of three primary parts. One part termed the powerpack, which contains the functional parts of the fuel cell. These are the pervaporation membrane, current collectors and membrane electrode assembly (MEA). The two other primary parts are a fuel reservoir, containing the methanol solution, and a valve which enables refueling of the system. Figure 1.A shows a drawing of the powerpack and the different elements inside the powerpack, which will be the key topic of the presented work. The powerpack is build by placing layers of the different parts inside a metal cup (the anode cup), which acts as the anode current collector. The design has been developed to minimize packaging, maximize active MEA area and enable easy assembly. Traditional gaskets take up a large part of the potential active MEA area in small scale systems. In the design shown in Figure 1.A the gasket on the anode side has been removed. In the latest design traditional gaskets have been removed and the polymer electrolyte membrane (PEM) acts as both ion conductor, gasket and insulator. This is achieved by forming the PEM into a cup, which continues up the sides of the anode cup, thus separating the anode side from the cathode side. This simplifies assembly and results in a significant increase in voltage output. The CO2 produced at the anode side is vented through a small hole in the sidewall of the anode cup. A pervaporation membrane, placed in the bottom of the anode cup, prevents direct liquid contact to the fuel reservoir, thus enabling vapor feed operation. This design makes the fuel cell omnidirectional. There is no liquid contact to the CO2 hole, regardless of the direction the fuel cell has. Figure 1: A) An illustration of a powerpack and the layers inside it: 1: Isolator, 2: cathode current collector, 3: water management layer (WML), 4: cathode gas diffusion electrode, 5: gasket, 6: polymer electrolyte membrane, 7: anode gas diffusion electrode, 8: (WML), 9: pervaporation limiter (PL), 10: pervaporation membrane, 11: anode cup (and current collector). B) A picture of an actual fuel cell. The size is approximately 9x8x5.5 mm3. Figure 1