Fuel Cells Q&A: Explaining Hydrogen, Electrochemical, & Polymer Cells

[Stanley514]

Could somebody expain me the following questions:
1)Why hydrogen fuel cell needs catalyst (such as platinum) to operate,
while elcectrochemical elements (such as zinc-air) do not?
2)Why power density increase in fuel cells leads to efficiency decrease while in electrochemical batteries it does not always seem to be an issue?For example,in flow cell power density exist quite independently
from efficiency and could be great?There exist high power Li-ion batteries.They didn't mention it would lead to decrease in efficiency.
3)Could we use only solid polymer electrolyte in PEM cells or it could be some liquid such as hydrochloric acid or some other strong electrolyte acid?Maybe it could lead to power density increase?

 

[Topher925]

Could somebody expain me the following questions:
1)Why hydrogen fuel cell needs catalyst (such as platinum) to operate,
while elcectrochemical elements (such as zinc-air) do not?
2)Why power density increase in fuel cells leads to efficiency decrease while in electrochemical batteries it does not always seem to be an issue?For example,in flow cell power density exist quite independently
from efficiency and could be great?There exist high power Li-ion batteries.They didn't mention it would lead to decrease in efficiency.
3)Could we use only solid polymer electrolyte in PEM cells or it could be some liquid such as hydrochloric acid or some other strong electrolyte acid?Maybe it could lead to power density increase?

1. Because hydrogen will not oxidize and oxygen will not reduce (not significantly anyway) at typical ambient temperatures.

2. Because efficiency is proportional to voltage. As you increase the current of a galvanic cell the power output increases to a point, and the voltage decreases. Storage efficiency does decrease in batteries with increased power output but not as significantly since batteries have a much lower current density than fuel cells.

3. No. If you take out the membrane then its no longer a "proton exchange membrane" or "polymer electrolyte membrane" fuel cell. Instead you have a alkaline fuel cell (what you are describing) or a phosphoric acid fuel cell.

 

[Stanley514]

If I no make mistake, action of typical electrochemical element is based on solvation of metal in electrolyte.For example zinc will dissolve in electrolyte,such as salt,on positive ions and electrons.After that ions flow through electrolyte to cathode and electrons run in external circuit.This process doesn't require any catalyst.If we take hydrogen and will bubble it through hydrochloric acid,will it not dissolve on protons and electrons?I thought it supposed to,otherwise hydrochloric acid would not be a strong acid.Also it should conduct protons and unite them with oxygen.Why do we need a catalyst, again?I know there exist phosphoric acid fuel cells,but they still need platinum catalyst.Why?

 

[Ygggdrasil]

If we take hydrogen and will bubble it through hydrochloric acid,will it not dissolve on protons and electrons?

No. This is exactly why a catalyst is required.

I thought it supposed to,otherwise hydrochloric acid would not be a strong acid.Also it should conduct protons and unite them with oxygen.Why do we need a catalyst, again?

The H-Cl bond in hydrochloric acid is much weaker than the H-H bond in hydrogen. The H-Cl bond can be easily broken in the presence of water. The much stronger H-H bond requires a catalyst in order to be broken.

Different types of bonds have different types of properties.

 

[Stanley514]

What is known about Tungsten Carbide as Platinum substitute?
Does that thing really work?

http://www.sciencemag.org/cgi/content/abstract/181/4099/547

Is it true that Alkalune fuel cells have power density many times smaller then PEMs?PEMs are able to deliever 2 Amps per square cm,
while alkaline deliever only few milliamps.Are not they suitable for
mobile applications?

http://www.greencarcongress.com/2009/02/nitrogen-doped.html

 

[Topher925]

What is known about Tungsten Carbide as Platinum substitute?
Does that thing really work?

As far as I know, tungsten carbide has an activity that is several orders of magnitude smaller than platinum of a oxygen reduction reaction. Today, metal and nitrogen based complexes (Fe/N/C, Co/N/C) are thought to arguable hold the most promise for replacing platinum in fuel cells and many other electrochemical devices.

Is it true that Alkalune fuel cells have power density many times smaller then PEMs?PEMs are able to deliever 2 Amps per square cm,
while alkaline deliever only few milliamps.Are not they suitable for
mobile applications?

Yes. Alkaline fuel cells suffer from greater concentration and activation losses due to their (generally, but not always) liquid electrolyte and non-noble metal catalysts. However, because they typically operate at lower current densities they are often more efficient than PEMFCs.

They are not suitable for mobile applications because of their very low power density along with having to use a alkaline solution used for the electrolyte.

 

[Stanley514]

What do you think about advantages and disadvantages of membraneless fuel cells?

http://www.physorg.com/news3476.html"

They claim alkaline chemistry with high power densities.I`ve calculated that if those elements will be stocked and worked as promised 10 liters volume of fuel cells could provide
about 70 KW of power.

What do you think about posibility to use Tesla coil and corona discharge to break down hydrocorbons and hydrogen atoms on protons and electrons,and after that feed them to membrana? Could it replace need for catalyst?

You said that voltage is proportional to efficiency.Could we stock more smaller fuel cells together and in such way increase voltage?

 

[Topher925]

What do you think about advantages and disadvantages of membraneless fuel cells?

Membraneless fuel cells as of right now are only useful for stationary power applications. Theres a huge list of reasons why they can't be used for transport applications including reliability, impurity/contamination, and durability. However, for stationary applications its hard to be a well designed SOFC. They are stable, efficient, can run on just about anything that doesn't contain sulfur (including CO), and have excellent durability.

They claim alkaline chemistry with high power densities.I`ve calculated that if those elements will be stocked and worked as promised 10 liters volume of fuel cells could provide about 70 KW of power.

I have a hard time believing that but even if that's correct it doesn't include all the other equipment required like water tanks, humidifiers, heaters, coupling of the micro-FCs together, etc.

What do you think about posibility to use Tesla coil and corona discharge to break down hydrocorbons and hydrogen atoms on protons and electrons,and after that feed them to membrana? Could it replace need for catalyst?

Do you mean using static electricity to ionize the fuel instead of a catalyst? Its a pipe dream. It may be possible, but no way practical. Might make a good thesis topic for a physics student though.

You said that voltage is proportional to efficiency.Could we stock more smaller fuel cells together and in such way increase voltage?

Yes, and this is commonly done. When a bunch of fuel cells are put together in series they form a fuel cell "stack".

 

[Stanley514]

Because efficiency is proportional to voltage. As you increase the current of a galvanic cell the power output increases to a point, and the voltage decreases. Storage efficiency does decrease in batteries with increased power output but not as significantly since batteries have a much lower current density than fuel cells.

How than a car battery could provide 600 of cranking amps without remerkeable drop in voltage? 600 amp X 12 volts = 7200 W.

Maybe this is no very correct example,but solar cells with light concentrators are more efficient regardless they deliever bigger currents.
Could you explain this physics in detail?

 

[Gokul43201]

The H-Cl bond in hydrochloric acid is much weaker than the H-H bond in hydrogen. The H-Cl bond can be easily broken in the presence of water. The much stronger H-H bond requires a catalyst in order to be broken.

(I imagine I'm preaching to the choir here but) The difference is not strictly in the strength of the bonds (in fact, the H-H bond has a nearly identical enthalpy to the H-Cl bond) but in the polar nature of the H-Cl bond compared to the H-H bond.

 

[Stanley514]

So what prevents to make fuel cell from very many small stocks and such increase voltage a lot? When large current will flow drop in voltage will be compensated by large voltage at the beginning.Will it allow to increase power density a lot?

Could we increase power density by just increase surface area?
http://en.wikipedia.org/wiki/Fuel_cell"
Will it cause decrease in efficiency?

When catalys breaks down hydrogen into protons and electrons what inforces electrons to run in a circuit instead of immediately unite again with protons? Is it not thermodynamically easiest way for them?

Could we let positive ions just to arch to the cathode like in electrostatic generator,istead using PEM? If main motor driving force is electrons,is it not the same how positive ions will unite with air?I think arching system will be more compact and allow to use also carbon atoms which otherwise just lost if we use hydrocarbons in a fuel cell?