by : Walt Pyle, Alan Spivak, Reynaldo Cortez, and Jim Healy
A gas fed battery that never needs recharging! This article describes a process for building a fuel cell using tools and techniques any skilled hobbyist with a wellequipped shop can duplicate. The fuel cell that we built can produce direct current electricity from stored hydrogen and oxygen. We obtained the hydrogen for this fuel cell commercially but plan to produce hydrogen and oxygen from a renewable energy system based on solar photovoltaics and water electrolyzers. Cookbook Approach to Building a Fuel Cell In this article we reveal the process we used to make a proton exchange membrane (PEM) fuel cell.
First, we describe what the PEM material is, and where to get it. Then we cover the steps necessary for preparing the membrane to use it in a fuel cell.
Next, we describe the catalyst and binders used on both sides of the PEM and the method of “hotpressing” them all together to form the single fuel cell catalyst-PEM-catalyst “sandwich”. Finally, the holder for the catalyzed PEM fuel cell with its gas supply piping, insulators, and wiring studs is shown. Some PEM fuel cell performance data were obtained using an electrical resistor to provide a variable load. Two digital multimeters and a shunt resistor were used to measure the voltage and current, so we could calculate the power produced. Although the fuel cell described produces a relatively low voltage, several fuel cells of this kind can be wired in series to produce higher voltages and do useful work.
The PEM Material
The PEM (proton exchange membrane) material is a perfluorosulfonic acid polymer film. Several manufacturers make PEMs in one form or another. We used one made by du Pont called Nafion 117. Nafion 117 is a transparent polymer film about 175 microns (0.007 inches) thick. Dow Chemical Co., Asahi Chemical Co., and Chloride Engineers Ltd. make something similar. A patent describing how one PEM manufacturer’s film is processed is listed in the references section at the end of this article. The basic structural unit formula for Nafion 117 is shown below:
CF2 = CFOCF2CFOCF2CF2SO3H CF3
Nafion 117 contains fluorine, carbon, oxygen, sulfur, and hydrogen arranged in repeating polymer molecules. The hydrogen atom on the SO3 part of the
molecule can detach from one SO3 site. The free H+ proton can hop from SO3 site to SO3 site through the material, to emerge on the other side of the membrane. This is the reason it is called a proton exchange membrane. It can be thought of as solid sulfuric acid, an electrolyte.
The PEM is relatively expensive at this point in time. We paid about $100 for a 30.5 centimeter by 30.5 centimeter (12 inch by 12 inch) piece of Nafion 117 from a chemical supply house. Some manufacturers want your first born child in exchange for a sample. However, du Pont really is in the PEM business, and they will sell it to you with no strings attached from their pilot plant production. The price comes down to about $65 for the same size piece when you buy four times as much PEM direct from du Pont. The piece we bought was large enough to make about six of our round fuel cells ($10–$16/cell).
Punching the PEM Disk from a Sheet of Nafion 117 We set the sheet of Nafion 117 on a piece of clean acrylic plastic using clean cotton gloves to avoid contaminating the sheet with fingerprints. Then we punched out some round PEM disks using a 4.76 centimeter (17?8 inch) arch punch and a mechanics hammer filled with lead powder. After one or two tries, we found that several strikes with the hammer at different angles was best for cutting the disk free from the sheet. Striking the punch too hard shattered the acrylic sheet.
Handle the PEM with tweezers or forceps to prevent contamination. We used a pair of stainless steel tweezers which were ground flat and polished on the grasping faces to eliminate burrs and prevent puncturing or denting the soft PEM. Grasp the PEM disks only on the outer peripheral edge, never on the inner active area. Preparing the PEM for Catalyst Application We prepared the film for catalyst application by dipping it in six different heated solutions in glass beakers. The solutions were all held at 80°C (176°F) by immersing
the beakers in a heated pan of water on top of two gas stove burners as shown above right. Each beaker held the PEM film for one hour in sequence. Use safety glasses and gloves while working with the solutions. The sequence of beakers used to dip the PEM was set up as follows:
Beaker 1 = 100 milliliters of distilled water [hydrate the membrane and dissolve surface contaminants]. Beaker 2 = 100 milliliters of 3% hydrogen peroxide solution (USP) [remove organic contaminants from
PEM surface]. Beaker 3 = 100 milliliters of sulfuric acid (new
battery electrolyte) [remove metal ion contaminants from PEM surface, and sulfonate the PEM surface].
Beaker 4 = 100 milliliters distilled water [rinse sulfuric acid from surface and hydrate PEM].
Beaker 5 = 100 milliliters distilled water [repeat rinse].
Beaker 6 = 100 milliliters distilled water [repeat rinse].
While the PEM disk is in a beaker, there may be a tendency for the film to curl and lift on the steam bubbles, rising to the surface. It should be kept submerged so the top side doesn’t get exposed to air. Use a clean inert polyethylene plastic or glass probe to keep it down in the dipping solution.
We used a Taylor candy thermometer for controlling the beaker bath temperature, and adjusted the gas stove burner controls as needed. From time to time, more water had to be added to the bath surrounding
the beakers, due to evaporation. After the PEM disk was dipped in each of the six hot solution beakers for an hour, it was then wiped with a piece of lint-free lens cleaning tissue, and air-dried in a clean place.
The Catalyst Layer Material The catalyst layer is the most expensive part of this fuel cell. It is made from a mixture of platinum, carbon
powder, and PEM powder, bonded to a conductive carbon fiber cloth. We obtained ours from E-Tek Inc. The cost for an order of their ELAT catalyst cloth sheet includes a setup charge. So get together with others for a larger order if you want to keep costs down. We paid $360 for a piece of ELAT 15.2 centimeters by 15.2 centimeters [6 inches by 6 inches] including the $150 setup charge. This piece provides enough for about twelve disks. Each fuel cell requires two disks of ELAT and one larger disk of PEM to make the sandwich, so you can make six cells from this size piece of ELAT ($60/cell). The cost may have come down by now due to increased production at E-Tek. In the future it may be possible to reduce the cost by putting the catalyst coating directly on the PEM with a platinum-carbon ink, as practiced by Los Alamos National Laboratory.
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