Some additional “TIPS” about electrochemical fuel cells are provided that we trust will be helpful and answer some frequently asked questions.
Platinum electrochemical fuel cells designed for breath alcohol analysis are physically small in size, light in weight and have no power requirements of their own (apart from the electronic signal measuring circuitry). The alcohol fuel cell consists of a porous, chemically inert layer (wafer) coated on one or both sides with finely divided platinum (called platinum black). The porous layer is impregnated with an acidic electrolyte solution and platinum wire electrical connections are applied to the platinum black surfaces.
The entire assembly is mounted in a plastic case (see pictures under fuel cells in this web site) which is provided with a gas inlet that allows a breath sample to be introduced. The various manufacturers employ numerous proprietary nuances in their construction but the basic configuration is as described above.
When alcohol is introduced, the alcohol is electrochemically oxidized at a selective, catalytic platinum electrode to produce a flow of electrons (electric current) from the alcohol molecule itself. The stronger the alcohol content of the sample (breath), the greater the current that flows.
At first glance, this would seem to be straight forward and simple, but many things combine to make the actual output current from the call a rather complex function of time and temperature which is not totally explainable on the basis of just the simplistic explanation of the chemistry of the cell given above. Each phase of the conversion described has a different rate of completion: i.e.,
- All of the molecules of alcohol in the gaseous sample must reach the electrolyte-wetted platinum surface and be captured by that surface.
- The captured alcohol molecules must reach selected, non-occupied points on the platinum surface called “active site”, where the catalytic reaction can take place.
- The rate of completion of the reaction is slowed by the accumulation of negative charge on the active surface.
- The drift of H+ ions through the electrolyte is a relatively slow process.
This is not an exhaustive list, but rather to indicate why a fuel cell does not illustrate the ideal behavior one might expect, and why regardless of strict “quality control” in the manufacturing process, each fuel cell is quite individualistic in its exact behavior, consequently deserving of more expertise in order to design, develop and perfect a breath alcohol analyzer. At PAS Systems International, we pride ourselves in providing that “expertise”.
Some Helpful Notes:
Above we explained “How it Works”, “What it’s Made of”, and “Why”.
When a problem develops, don’t be so quick to “blame the fuel cell”.
For example, have you considered?
- The Impact of Resistance Value on Cell Output Response
- The Linearity of Response to Alcohol Concentration
- Fuel Cell Recovery or Cleanup After a Positive Alcohol Test
- Response to Repeated testing in Close Succession
- Temperature Coefficient of Fuel Cell Output
- Response Versus Temperature of the Fuel Cell
- Effects of Age on Fuel Cell
Advanced literature on these topics is available and can be obtained by contacting PAS Systems International, Inc. (800) 660-7643.
Some fuel cell manufacturers make an issue of fuel cell construction using a wafer coated on both sides with platinum black or a wafer coated on a single side and assembled as a “wafer sandwich” in the case. After years of testing both types and approaches, we have found little or no difference in fuel cell performance. In fact, we often see higher and more consistent outputs in the “wafer sandwich” design. Perhaps it’s simply due to the rigorous attention given to the quality and type of the platinum black and platinum wire leads used.
We find neither difference in internal resistance nor any problem with achieving the suitable electrical connections. It’s simply a matter of knowing what you are doing in the case preparation, design and assembly process. Some tests suggest the two wafer approach is better in high temperature conditions although we make no such claims because so many other factors, as mentioned above, must be managed for optimal performance in wide ranging operating conditions.
If a manufacturer elects to use the double sided wafer, they must use an electrolyte wetted blank along with the platinum electrode wafer, which if not carefully controlled, can lead to excess drying as the blank absorbs electrolyte from the platinum coated wafer, or may even lead to the opposite, excess electrolyte and flooding.
Our advice; don’t be misled by any of these claims. Accept as “proof”, the actual cell performance in your particular application. Any quality fuel cell manufacturer will seek to provide you with the cell configuration that in their judgment should work best for you in your particular application.
In the final analysis, the results of your instrument design will depend on several factors: accurate and reliable sampling system (we have them), proper software design, selecting quality electronic components, accurate calibration, and finally a high quality electrochemical fuel cell backed by an experienced manufacturer who cares about your success.