Science // Energy
Fuel cell gets energy from water
08 April 2015 , 16:37Neftegaz.RU1115
Concern about the impacts of petroleum as the world's major energy source has driven research into alternative energy sources, such as solar radiation, wind and biomass.
One of the tools for converting these sustainable energy sources into usable forms of electricity is the fuel cell. Fuel cells are devices that use chemical reactions to change energy from one form to another.
Because of their efficiency and their ability to harvest energy from diverse sources, fuel cells show promise as one of the keys to sustainable power generation. Fuel cell technology has already matured to the point where it can be used in power plants to supply electricity to buildings, and portable fuel cells have been demonstrated in prototype cars as well as military vehicles. Smaller fuel cells can be used in place of batteries for electronic devices like radios or laptop computers.
A fuel cell has an anode and a cathode, just like the positive and negative terminals of a battery. Chemical reactions occur at the two electrodes.
Generally, conventional fuel cells get power from a carrier molecule - usually hydrogen - that is added to the device. The fuel cell harvests electrons from the carrier molecule, producing electrical current. When hydrogen is used in fuel cells, it combines with oxygen in air to make water as a byproduct, also releasing heat. Fuel cells generally do not require wind or sun and operate quietly.
Tens of thousands of scientific studies have focused on fuel cell technology in attempts to find the chemical fuels and components that produce the greatest energy efficiency. While many varieties of fuel cells have been demonstrated, one thing they have in common is that they depend on energy that was previously generated - in effect they are just tools for converting one form of energy into another. A fuel cell that consumes hydrogen generated from natural gas, for example, still depletes fossil resources. In other types of fuel cells, the carrier molecules are toxic, or the byproducts produced are harmful. Ideally a fuel cell would derive its energy from a renewable source and avoid unsafe chemicals in the process.
To move toward this goal, there is another way to drive the reactions inside a fuel cell. This not-so-common approach is to use entropy. Entropy is a measure of disorder, and nature favors increasing disorder: it is one of the fundamental laws of thermodynamics.
What did they do?
The authors purchased a commercially available fuel cell, the kind that would ordinarily be used to harvest power from methanol, an energy carrier chemical commonly used in fuel cells. They poured water into the anode side of the cell. The cathode side was exposed to a temperature-controlled stream of air.
In the study, the pH of the water was changed by adding sulfuric acid or sodium hydroxide, and the electrical current and voltage were measured with a multimeter. The pH was adjusted because it affects the rate at which electrons can be harvested by the cell.
What did they find?
The fuel cell was driven by evaporation of water from one end of the cell, "pulling" the chemical reactions forward. This form of entropy powered the cell.
Considering the chemical reactions involved, at one end of the cell, water breaks down to make oxygen, hydrogen ions, and electrons. At the other end of the cell, the oxygen, hydrogen ions and electrons recombine to make water. In other words, the chemical input (water) is the same as the output. The net enthalpy, or heat balance, is zero, because any heat released at one side of the cell would be balanced by an equal absorption of heat at the other side, so release of heat cannot be the driving force for the fuel cell.
However, this fuel cell used two different forms of water: liquid and vapor. Liquid water is fed into the cell and vapor escapes. Vapor is more disordered than liquid, so the entropy increases and the chemical reaction moves forward. The electrons are forced through a circuit in the process, to make electricity.
The researchers found that temperature and pH affected performance. The optimum temperature was 70 degrees celsius (about 160 degree farenheit) and the voltage was highest at pH 11 (about the same pH as household ammonia).
What did they mean?
This paper reports a unique approach in developing fuel cells. It takes advantage of the fact that a chemical reaction can be "downhill" or spontaneous even if no heat is released, as long as there is an increase in entropy.
The fuel cell design reported in this study uses this technique to drive the overall chemical reaction.
Remarkably, the fuel cell setup can be used to generate electricity from water and air, producing just water and oxygen as byproducts. This is extremely attractive compared to hydrogen, methanol or other common fuel cell chemicals that only carry energy that was generated elsewhere. For example, it is possible to derive hydrogen from solar or wind power, but in practice most hydrogen comes from natural gas or oil, and a fuel cell would only be capturing the energy from those non-renewable sources.
In addition, the use of water as the energy source avoids problems of flammability and toxicity. Many existing fuel cell technologies based on liquids have the drawbacks that the fuels, byproducts or sometimes both are very hazardous. For example, methanol fuel cells can produce formaldehyde, a cancer-causing chemical. The authors noted that high pH gave better results; in other words the use of a pH-raising additive like sodium hydroxide would be necessary to give optimum performance. This could be considered a drawback but it might be possible through further research to improve the reaction rate by other means.
Because evaporation of water is critical to the process, the researchers suggested that the technology would be most effective in dry, windy areas.
The main limitation of the water fuel cell is that it needs more space, and materials, to generate the same amount of power as a denser fuel cell. The researchers suggest that the water fuel cell is perhaps 100 times less dense than would be practical for most applications. The performance, though, is similar to fuel cells that produce electricity from microbial activity. The researchers suggest the cell would be adequate to power sensors or wireless transmitters.
Further research on these fuel cells might compare the amount of energy used to construct the fuel cell and its component parts, including membranes and metal catalysts, to the amount of energy that could be generated in the fuel cell's expected lifetime.