Where's the oxygen?

[liberty]

In 8th grade science class our teacher demonstrated the combustion of soap bubbles filled with hydrogen and then bubbles filled with a mixture of hydrogen and oxygen. To our young eyes and ears, it appeared that mixing the reactants yielded at least an order of magnitude more power than just relying on ambient oxygen to react with the bubbles filled only with H2. The space shuttle does not launch on H2 alone - they need to bring a molar-compatible quantity of O2 along. Personally I am an internal combustion engine fan, but I would like to pose this question to those of you who are knowledgeable about the electrochemical workings inside a FC.

If you mixed "pure" O2 gas with the fuel inside a H2 FC...

- Would power output efficiency improve and offset costs of bringing the O2 along?
- Regarding impurities in the fuel or different fuels (i.e. methane), would the design be more or less forgiving?
- Would the life of the FC be impacted?
- Are there safety reasons that you don't carry pure O2 around in a chemically-powered vehicle?

Thanks for your thoughts!
Mark

 

[HydroScope]

not sure if I can answer it all but i will try, assume the same fuel cell at the same power output, and only change is air to pure oxygen, and the reaction is with hydrogen then . . . . .

Fc stack efficiency losses are from activation, concentration and ohmic polarization.

Activation polarization remain the same, as the same reactant being used is the same.

concentration polarization would also remain the same their is only so much oxygen the electrode can react with in one go.

Ohmic polarization- due to electron resistance in electrodes etc. and resistance of ions through electrolyte. Thus would remain the same.

Thus fuel cell stack efficiency remains the same.

Stack power density remains the same if all the above remains the same.

Fuel cells do not operate without auxillary equipment, find out what I mean by reading this.

http://www.eere.energy.gov/hydrogenandfuelcells/tech_validation/pdfs/fcm05r0.pdf

If the oxygen was stored at high pressure then that would result in higher efficiency due no need to run a compressor for air intake. but the power density would decrease due to added weight of oxygen tank.

Stored as a liquid. lower power density for the above reason with no increase in efficiency as once oxygen is vapourised it will need to be compressed. similar to Fc on air.

electrodes of platinium don't corrode due to Oxygen instead posioned by CO, thus useing pure Oxygen would likely increase life of electrodes, but their may be some other parts of the Fc that may corrode over time I'm not aware of???

High temp Fc can run on methane or other fuels, I beleive a similar story would be true for them too.

there are always safety concerns when you want a cryogenic stored liquid or high pressure container on-board a moving vehicle that need to be designed with the possible impact in mind. main reason for not storing oxygen is because it is easy to get from air, adding tanks, adds weight, bulk complexity and cost.

 

[moltenmetal]

There's no energy efficiency benefit to carrying oxygen around with you in a tank for a vehicle application, even if you got the oxygen "for free" as a by-product of electrolysis to produce hydrogen as fuel. You'll need to waste some energy compressing the oxygen to store it, you won't get virtually any of that back when you expand the oxygen to use it, and there'll be considerable parasitic mass to carry around in the form of pressurized oxygen cylinders. Some energy is lost in compressing atmospheric air to feed the fuelcell, but that's small in comparison. Different, of course, if you're operating outside the atmosphere...

As far as stationary electrolysis/fuelcell energy storage systems are concerned, if you go to the bother of capturing and compressing the product oxygen, you'd probably want to sell the electrolysis oxygen for other uses (i.e. uses which benefit from or require pure oxygen) rather than burning it in your fuelcell.