Questions about hydrogen fuel cell.....

[albertlee]

In a hydrogen fuel cell,

 

1. At anode, why does Hydrogen gas have to release electrons????? what makes that do this??

 

2. If the electrons are released, why they travel along the metal wire, instead of joining the positive ions in the electrolyte??

 

3. What do the characteristics of the hydrogen fuel cell's electrolyte suppose to be?? (Give some examples of electrolyte)....

 

 

Apreciate for responds

 

Albert

[albertlee]

Can any one explain me the above????

[albertlee]

Can any body please help me???

 

By the way, I found a good site, but I am not able to run the java applet animation,

 

can you guys' computers run the applet??

 

Albert

[albertlee]

oh...

 

the site is: http://www.micro.magnet.fsu.edu/primer/java/fuelcell/index.html

 

any one please help........

 

thx in advance

[albertlee]

Ok, do you guys understand my questions????

[Xavier]

I'll have a go at explaining hydrogen fuel cells.

The molecular hydrogen and oxygen molecules are separated a non conducting membrane that is permeable to protons. A platinum catalyst encourages the molecular hydrogen to separate into H- ions and H+ (protons). A few of the protons slip through the membrane as a rare diffusive event before reintegrating with their negatively charged pair. Now it gets interesting.

On the other side of the membrane, the protons will speedily latch onto an oxygen molecule (I admit I am hazy on the exact chemistry which, I gather, involves some weird and wonderful transitional states as this is a highly exothermic reaction that drives all the rest) but the end products are water molecules and O2+ (that is doubly positively charged oxygen ions) which aggressively try to get electrons. The source of additional electrons is the other side of the non-conducting membrane but a wire conducting them from the platinum catalyst to the O2+(aq) will draw a current. The energy of the electron acquisition by O2+ aids the platinum in producing more protons in a positive feedback loop til the membrane transport process is saturated or the fuels run out.

An electrolyte solution is not strictly necessary at all in the fuel cell. On the hydrogen side the gas will be dissociated by platinum on the membrane surface, reduced to a proton and zip through the membrane. On the oxygen side the O2+ ions would find their way to a large eletrode nearby by diffusion, though removing the waste steam and resupplying the O2 makes this a little tricky. This is the design of most interest in making microscale - and in future possibly nanoscale fuel cells to power nanobots.

[Xavier]

I have had a look at that website you mentioned. The bit that they call an Electrolyte clearly cannot be. If it were conductive the electron would cross directly from electrode to electrode, bypassing the wire.

The applet shows protons crossing the 'electrolyte' (though in the text these are mislabelled as electrons at one point) carrying their positive charge from one side of the cell to the other along a concentration gradient that is actively maintained by the reaction that generates the water scouring the protons from the oxygen side of the cell whilst the O2+ ions actively 'pulling' electron from the hydrogen side through the wire increases the rate of creation of protons there.

 

The key element to a hydrogen fuel cell is the barrier between the two gases that conducts protons but not electrons.

 

Actually, a quick primer through the internet reminds me that there are other designs than the proton exchange membrane fuel cells I have been describing (which are the only choice for miniaturised systems). The main rivals are solid oxide fuel cells (SOFC) and molten carbonate fuel cells (MCFC). Their main differences are in what is used in place of the membrane and their advantages are in their higher operating temperatures giving better efficiency on the large scale.

For further research, may I recommend Google Scholar (http://scholar.google.com/) as it focuses on research papers and (claims to) sort by research impact.

[albertlee]

How can the permeable membrane stop electrons form getting through??

[albertlee]

The electrolyte plays a key role. It must permit only the appropriate ions to pass between the anode and cathode. If free electrons or other substances could travel through the electrolyte, they would disrupt the chemical reaction.

 

from: http://fuelcells.si.edu/basics.htm

 

what kind of electrolyte permits only the ions (in this case, the oxygen), but does not allow the elctrons to pass through???

 

Secondly, do the oxygen molecules become 2 O^2- ions??? when they receive electrons at cathode???

 

Thirdly, does the typical hydrogen fuel cell using electrolyte instead of proton exchange membrane call the alkali fuel cell??

 

Albert

[Xavier]

I realise belatedly that the diagrams in the various 'fuel cell basics' websites were trying to describe the original Francis Bacon type fuel cells of which alkaline fuel cells are the most prominent type. I apologise for some misleading here as I had essentially forgotten about their existence in following up my own interest in PEMFCs.

 

The AFCs do indeed have an alkaline electrolyte between the two gases in their solutions. The electrodes do not have to be in contact with the electrolyte as ions made can meander across from the catalyst/electrolyte region through the gas solution which must be pure and hence a good insulator. I imagine that designing the components such that there is not a substantial leak of electrons between the halves of the cell by way of the electrolyte is the great challenge in making the things.

 

The different types of fuel cell all have particular sequences of ion formations.

From your first question, I believe the solid oxide fuel cells allow ions of oxygen to pass across the cell but in the PEMFCs (which I was previously, possibly misleadingly, considering as the 'standard' fuel cell type) it is the proton that crosses whilst in the AFC it is hydroxide ions. The constituents of the gap between the sides of the fuel cell are specifically designed to conduct a particular ion. Proton exchange membranes are often made of very clever polymers that are electrical insulators but which allow protons to cascade along their backbone forming temporary bonds with each monomer unit in turn and tending towards travel right across the membrane due to the concentration gradient (ie. more protons pushing them from behind than from in front).

As an aside there is a suggestion that if a phospholipid bilayer - like a biological cell membrane - were put in this place and studded with as many hydrogen ion channels as are found in certain cells in the kidney, a tiny but hugely efficient fuel cell could be created (if you could only keep the temperature low enough).

 

Regarding the second question, in the PEMFC case the O2 molecule is first attacked by newly arrived protons to make an H2O in a sufficiently energetically favourable reaction that the exotic O2+ ion is formed - carrying in itself most of the energy made by the that reaction. Thus when the O2+ ion reaches an electrode it pulls eletrons from the electrode 'with force', converting the chemical energy to electrical potential. In the other cases the oxygen molecules are prepared by catalytic action (which requires some energy so only occurs in a small fraction of the oxygen atoms but the energy budget is swiftly balanced). In AFCs the oxygen molecules oxidise water to OH- and OOH- ions fairly readily in the strongly alkaline conditions, which contributes to their efficiency.

 

There is a good paper from ChemPhysChem 1(4) pp162-93 Dec 2000 (http://www3.interscience.wiley.com/cgi-bin/abstract/76504449/ABSTRACT) though you'll need an Athens Login, a friendly librarian or $25

[albertlee]

Regarding the second question, in the PEMFC case the O2 molecule is first attacked by newly arrived protons to make an H2O in a sufficiently energetically favourable reaction that the exotic O2+ ion is formed - carrying in itself most of the energy made...

 

I am asking the Alkali fuel cell, not the PEMFC one....

[albertlee]

Any body can help??