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Hydrogen Fuel Cells launching the H-society

 

What are the basic concepts of a solid polymer fuel cell / How does it work?:

The core element of a fuel cell is a semipermeable solid polymer membrane. It is a very simple device, separating two bodies of gasses. One body of gas is hydrogen (H2), the other body of gas is air that has oxygen in it (O2). On either side there is a catalyst and two electrodes that are connected together with a wire. In its simplest form, a hydrogen-oxygen fuel cell consists of an electrolyte solution, such as potassium hydroxide solution, and two inert electrodes. Hydrogen and oxygen gases are bubbled through the anode and cathode compartments, where the following reaction takes place:

Anode: 2H2(g) + 4OH-(aq) ® 4H2O(l) + 4e-
Cathode: O2(g) + 2H2O(l) + 4e-
® 4OH-(aq)
Overall: 2H2(g) + O2(g)
® 2H2O(l)
As can be seen in fig.1, the hydrogen fuel cell is used in the space program. The pure water produced by the cell is used as drinking water by the astronauts.


Fig.1: compact module of a H-fuel cell used by NASA (65kB)

The fuel cell is structured like a sandwich (fig. 2). At its core is a thin plastic foil - the proton exchange membrane (PEM), which is coated on both sides with a thin catalyst layer, preferably platinum (Pt), and a gas/permeable electrode out of a graphite paper. In the outer layers, gas channels have been milled into the two so-called bipolar flow field plates (FFP). Hydrogen flows through the channels on one side, while oxygen through those on the other side. Upon contact of the hydrogen with the Pt-catalyst, it causes the H-atom to ionize (to decompose into a proton H+ and an electron, e-). The positively charged protons permeate through the PEM membrane whereas the negative electrons do not. As a result of this diffusion process a voltage difference between the two electrodes out of graphite paper ensues. Attaching electrodes to the oppositely situated PME layers and connecting them via an external electrical load, the electrical gradient causing the electrons to flow through it, drive this load (e.g. DC motor). While the electrons are externally routed to the other side of the PME membrane, they join the oxygen, giving it a negative charge and finally merge with the protons that migrated directly through the foil. As a result of this electrochemical reaction, pure water (H2O) and a small amount of heat are formed. The standard electromotive force of the cell can be calculated according to the electro-chemical voltage series (ECVS-table), which yields an overall cell-voltage of 1.23V (0.83VO2 + 0.4VH2). Thus, the cell reaction is spontaneous under standard-state conditions. The electrodes have two functions. As mentioned already, they serve as electron conductors, but they also provide the necessary surfaces for the initial deposition of the molecules into atomic species (electrocatalysts that reduce activation energy), prior to electron transfer.
Individual fuel cells can be then combined into a fuel cell "stack". The number of fuel cells in the stack determines the total voltage, and the surface area of each cell determines the total current. Multiplying the voltage (V) by the current (I) yields the total electrical power generated (P = I×V).


Fig.2: Schematic working principle of a fuel cell (40kB)
What advantages do fuel cells offer over the eternal combustion engine?

The thing that most people have focused on is really the environmental factor. It is simply cleaner because there is no combustion taking place inside the fuel cell, they are inherently low temperature devices, so one doesn't get the by-products that are inherit in combustion. One does not either sulfur-oxide (SO2), nitrogen-oxides (NOX), carbon-monoxide (CO), which are just natural by-product of combustion. Fuel cells are also quite a lot more efficient, 2-3 times more efficient (h >40%) than internal combustion engines (h 26%) - see fig. 3.


Fig.3: Efficiency of a fuel cell vs. combustion engine (75kB)

When one looks at various applications, fuel cells have a number of other attributes that makes them more suitable or more desirable for many applications. If one thinks of an automobile, fuel cells are solid state devices, they don't have any moving parts in the fuel cell themselves, therefore, they are inherently a low vibration, low noise device. So it quite obvious that for a number of other reasons besides just environmental reasons that fuel cell powered products will not just be cleaner products but it will ultimately be better ones.
Fuel cells are primary energy producers, not just like batteries that are storage devices, that store energy that was produced elsewhere. In other words, fuel cells take up fuel (H) and produce the energy from that fuel.

The key issue is fuel infrastructure: Fuel cells are going to use a different fuel (H-gas) than conventional fossil fuel. So the issue of determining what that fuel is going to be and ensuring that when these technologies are in the market place, is something that is very important. The infrastructure of production and distribution will be provided by the energy companies. As H-fuel is still expensive and difficult to transport and store; therefore, the introduction of the H-society will be still a matter of patience, it maybe not before the year 2005.

In the meantime, it is much more likely to utilize a hydrocarbon like natural gas or methanol. These would then be treated in a process called "steam reforming"; this releases H which is then used in the fuel cell.

What we need is a cheap, clean and sustainable method of separating H from a compound molecule.
A team at the University of NSW may have an answer. They found a way of speeding up the water splitting reaction first describe 30yrs ago that uses the mineral "rutile" to decompose water. Rutile is found in sandy beaches and it is another main component in titanium dioxide (TiO2) or titania.

Fig.4 depicts a schematic diagram in which hydrogen- (H2) and oxygen- (O2) gases are obtained. Such a photo-electrochemical cell are electrolytic cells that use the energy of light to reverse a spontaneous reaction. They differ from photo-voltaic cells, which generate an electrical current from light, in that they produce a chemical fuel rather than a flow of electrons. Semiconductors make good anodes for photo-electrochemical cells, because light easily excites their valence electrons. The electrons can then leave their parent atoms and move through the solid, generating a current and producing oxygen gas from water. Hydrogen is generated at the cathode, usually an inert metal such as platinum.
Much of the research is directed at improving the electrodes and the efficiency of their operation (h photosynthesis 3% vs.
h photo-electrochemistry 10%). One problem with semiconducting electrodes is that they tend to become corroded in water. Titania instead is a noble exception, therefore the brightest hope in photo-electrochemical cell research.



Fig.4: Conceptual diagram of H2 and O2-generation by photovoltaic cells (60kB)

Mr. Chris Sorrow of the department of ceramic engineering has more:
When titania or rutile comes into contact with water, there is a surface reaction on the rutile that decomposes the water into its component gases hydrogen and oxygen. Those gases can be collected and at the same time it generates electrons which provides electricity. And that is done by making a photo-anode of the titania and a cathode completing the circuit. Solar energy can be thus used to accelerate this process. Currently this process takes place quite slowly. To improve the rate of reaction, research is primarily focused on the surface properties, aiming at boosting the production of H, to simply produce more of it. As this electro-chemical circuit produces electrons, the hydrogen is simply a by-product. As one can see, accelerating this process can only be achieved by modifying the surface property of titania because everything is happening there.
Titania usually is a stoichiometric molecule, i.e. one Ti-atom surrounded by two O-atoms. Although nature produced fairly amounts of it, it is the non-stochiometric composite of this molecule that catalyzes the reaction described above. What needs to be done is a variation of both atoms involved. Doping, i.e. adding minute amounts of impurities to change the amount of titania or to change the amount of oxygen to render the molecule non-stoichiometric (to make it a random mixture of both components). That degree of non-stochiometrics has a huge influence on the electronic behaviour of the material, and that's what they are doing at the Uuniversity of NSW.
This all needs to be done to change the sensitivity of the surface material, and that means the surface reactivity. It is no longer stable so to speak and it is very reactive. And that reactivity is reflected in more rapid reactions between the water and titania, that they hope will ultimately be a cheap and clean way of producing hydrogen.

References: Atkins P.; Jones L.; 1997; Chemistry 3rd ed.; Freeman and Company, New York - USA
Chang R., 1994; Chemistry 5th ed.; McGraw-Hill, New York - USA

Sites on the WWW:

Union of Concerned Scientists: www.ucsusa.org
Fuel Cells Technology: www.hydrogenus.com
The World Fuel Cells Council: www.fuelcellworld.org
Bellona: www.bellona.no
Bellona: www.kettering.edu
Fuel Cells 2000: www.fuelcellworld.org
National Hydrogen Association: www.ucsusa.org
United Technologies Company: www.utcfuelcells.com
The US Fuel Cell Council: www.usfcc.com
American Methanol Institute: www.methanol.org
California Air Resources Board: www.arb.ca.gov
Fuel Cell Partnership: www.drivingthefuture.org
California Hydrogen Business Council: www.ch2bc.org
Earthbeat - edition from the 24th of June 2000: www.abc.net.au/ra/
Earthbeat - edition from the 31st of May 2003: www.abc.net.au/rn/
Ballard Power Systems: www.ballard.com/
Department of Ceramic Engineering, University of NSW - Sorrel C., : www.materials.unsw.edu.au/research/energy.html
Daimler-Chrysler: www.daimlerchrysler.com
Science Directory: www.directory.net