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Although the principle of the fuel cell was discovered over a century ago ( Friedrich Schonbein, 1838) their commercialization began in earnest when power supplies were required for space craft. Pratt and Whitney, General Electric and several others manufactured these supplies in the 1960's and UTC Power are now the sole supplier of fuel cells to NASA for their space program. One is well aware from previous pages on this site that all nations are consuming prodigeous amounts of oil for transportation and that this is
(a) a non-secure fuel
(b) an increasingly expensive fuel
and (c) an environmentally unfriendly fuel
The idea that a fuel cell could be linked to a "hydrogen economy" and provide an alternative fuel for transportation has given a boost to R&D on fuel cells throughout the whole world and an additional benefit from this technology is that it would reduce carbon emmisions by many factors of ten.
A typical diagram of a fuel cell is given below:
The fuel cell converts chemical energy into electricity directly and therefore it is not limited by the second law of dynamics as was the case of the heat engine. It consists of two electrodes separated by an electrolyte which transmits ions but not electrons. Hydrogen is supplied to the porous membrane negative electrode and a catalyst here causes the hydrogen molecules to dissociate into hydrogen ions and electrons. The electrons are forced to travel in an external circuit as the membrane is an insulator but the hydrogen ions diffuse through and migrate across the electrolyte. At the lower electrode the electrons (re-entering the cell from the external circuit) are returned to the hydrogen ions and gaseous hydrogen and oxygen combine chemically to form water. Animations are given on inventor.about.com/od/fstartinventions/a/Fuel_Cells.htm .
The chemical equation, in its basic form is, therefore:
2H2 + O2 = 2H2O
and there are no pollutants to affect the environment. The gases hydrogen and oxygen would be produced by electrolysing water with a photoelectric generator.
Such a cell produces a voltage from 0.6 to 0.7 V and therefore needs to be series connected with other cells to give a workable power source.
A small experimental cell (Part Number 667 401), purchased from Leybold Didactic, www.ld-didactic.co.uk , is illustrated below :
There are a variety of sources of the Hydrogen and Oxygen gases:
1 A Proton Exchange Membrane, PEM, electrolysis unit is fed with a DC voltage and produces both H2 and O2 gases ( Part Number 666 484).
2 An electrolysis cell which is fed with a DC voltage and produces both H2 and O2 gases ( Part Number 664 432).
3 Minican gas cylindars - Part Number 660 997 for H2 , Part Number 660 998 for O2.
The SAFETY NOTES included with the apparatus are listed below:
Hydrogen is extremely flammable and can form a highly explosive mixture with oxygen. This mixture can accumulate in the reservoir when either of the electrolysis units are operated for long periods of time, typically, over one hour. Thus it is advisable to conduct the experiments for periods of less than 15 minutes.
Hydrogen and oxygen are fed into the cell along the central axial ports and water is collected from the bottom ports on the circumference.
A resistive load was connected to the cell as illustrated below
The fuel cell comprises of a voltage source VFC in series with a resistance RFC . Now, if RL is the variable load resistance we can deduce the current I through the circuit by the equation:
VFC = I ( RFC + RL ) but we can only measure the voltage V (where V = I RL ) at the terminals so we can revise the previous equation to give
V = - I RFC + VFC thus, by plotting measured voltage against measured current we can characterize the fuel cell for both the voltage, VFC , and series resistance, RFC .
The hydrogen and oxygen were obtained by method 1 above. The PEM electrolysis unit , operating at 2.8 V and 2.0 A, and the supplies of hydrogen and oxygen fed into the fuel cell.
The output from the fuel cell is given below:
The data shows a reasonably linear voltage output with current and cell voltage on open circuit is 0.64 V; the series resistance is 17 ohms.
Thus, this basic experiment has shown how to characterize a fuel cell and there are several other suggested experiments in the instruction sheets from Leybold Didactic.
Many different configurations of fuel cells are listed in Wikipedia and the choice for, say, motoring will be a closely guarded secret. However, the range of products on the market is vast as HydroPack is unveiled at the Consumer Electronics show in Las Vegas , H-Racer from Horizon gives us its toy car , and the Honda FCX comes to fruition during 2008. Fuel cell costs which have prohibited their use for so long are now beginning to see a noteable reduction.
A recent report on Fuel Cells has been given in the Engineering and Technology magazine, page 50 20 June- 10th July 2009 www.theiet.org/magazine . The good news is that the European Commission is providing funding of 470 million euro's for the next four years for research and development in this area of work.
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