Membrane and nickel electrode screens for the PEM electrolyzer.
Assembly & Test
Careful assembly of a cell stack is critical to good operation. With concerns for gas flows, conductivity, and proper contact pressures, assembly becomes very labor intensive. Even with the best commercial designs, the labor cost component is high. Proper design and automation can address most of this at the commercial level.
We did many calculations of performance based on variables such as gas flow, temperature, and pressures. It was the real world testing that proved to be the most rewarding as a learning experience. We used a fairly sophisticated test setup. It included a digital mass flow meter calibrated for H2, a velocity meter and manometer for air flows, a pressure gauge that accurately reads to 1/10 psi, and several very precise pressure regulators and needle valves. Most of this equipment was purchased as surplus from a local supplier. We acquired a fairly sophisticated lab on a bargain basement budget.
Although a fuel cell is similar to a battery in that they both produce DC power, the similarity ends there. In the fuel cell, gas flow and pressures, temperature, and loads all have to be adjusted and accurately balanced. When we did an open circuit test of a single cell, we were very pleased with the performance. We were not ready for some of the later surprises when testing the assembled stacks.
A fuel cell uses hydrogen as a fuel and the oxygen in the air as an oxidizer. This is much the same as an internal combustion engine, except for the fact that the fuel is not burned. The process is more like rusting than
like fire or combustion. Just as an engine can be starved for air when choked out with a mixture too rich with gasoline, a fuel cell must also have a balanced fuel-to-air (or stoichiometric) ratio, so all hydrogen gets turned into power and H2O.
The air in our fuel cell is forced into the cell stack and across the membranes using a small, 1 watt, brushless DC fan. To simplify the design, we decided to suck the air in, instead of blowing it in under pressure. We assumed it would make little difference. After failing to get the cell stack to deliver anywhere near full output, we realized that we needed to change the air flow direction. Careful observation of current and voltage levels relative to air and fuel flow lead us to this conclusion. Because air is composed of only 20 percent oxygen, any increase in pressure also increases oxygen density for a given volume flow rate. After a change of fans and manifold, our power output doubled.
The next problem was internal resistance causing excessive voltage drops and excessive heat under load. The problem proved to be a lack of contact pressure between the membrane and the electrodes. After carefully tightening the compression bolts, we increased our voltage by 2 volts at the same load
current. We originally designed the stack to operate with H2 under a fixed pressure. After careful testing, we found that a small weep hole was needed to let a little of the hydrogen flow over the diffuser layer and exit at the top of the cell stack.
Our remote control car has high and variable surge loads. Our fuel cell stack lacks a fuel-control management circuit, and has rather poor surge capacity. So a capacitor was placed in parallel with the load to aid in starting the motor. We managed to obtain capacitors with extremely high storage per unit weight. Maxwell Technologies manufactures a complete line of these ultra-large capacity, PowerCache capacitors rated at up to 2,500 farads.
Large size capacitors such as these are being added to hybrid and fuel cell autos for some of the same reasons. These capacitors are excellent choices for delivering large bursts of power, while batteries are better at delivering a lot of energy over time. We purchased ten PC10s. These capacitors, at ten farads, weigh just over 6 grams (0.2 oz.), and are smaller than a postage stamp. Yet they deliver more than 2.5 amps at 2.5 volts. They have proven to be indispensable in leveling out the surge loads placed on the fuel cell, and have increased overall system efficiency.
From H2O to H2 & Back Again
The device that made this project especially interesting was the PEM electrolyzer. It converts water into hydrogen and stores it at pressure. Later, we run the fuel cell from the same hydrogen, converting it back to water. This is nothing short of magic, with a technological twist. The sidebar on page 55 shows a schematic of the reactions taking place in the electrolyzer.
Unlike many designs, the PEM uses no acids or bases as electrolyte. The PEM membrane acts as the electrolyte and also serves to keep the oxygen and hydrogen separated. The beauty of this design is that the hydrogen creates its own pressure. It has no known upper limit except for the mechanical strength built into the cell design. It also produces 99.9 percent pure gas.
Using Delrin and nickel mesh and plate, an eight-cell stack was fabricated. The diagram on page 55 shows its assembly. The testing is where some interesting problems cropped up.
With raw Nafion in a single, clear Plexiglas cell, we could generate gases at a pretty good rate. Unfortunately, when we assembled our stack using this raw Nafion, it refused to electrolyze for long. It would gas for several minutes as the voltage dropped lower and lower. Finally it would just stop conducting. Disconnecting it for several hours seemed to restore conduction, but then it stopped again.
We needed a platinum layer on the anode side to help keep the process going. It was also discovered that the voltage across the PEM must be carefully regulated to maintain conduction. This conclusion was derived with a lot of blind testing. So far, we don't understand the relationships between voltage and generated pressures and what reactions are taking place within the membrane. We hope we can discover it on our own, so
Oregon Institute of Technology students Dan Hill and Jacob Pelzer.
Oregon Institute of Technology students Dan Hill and Jacob Pelzer.
Raw Nafion 117, 0.41 x 0.41 m2
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