Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: scientizscht on 11/07/2018 08:17:30
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Hello
Can you please tell me in short how a fuel cell works?
Also do you have any metric of its efficiency?
Thanks!
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A fuel cell is essentially like a battery. There are two chemical reactions that occur, one releases electrons (oxidation), and one accepts electrons (reduction). The oxidation occurs at one electrode, and reduction occurs at the other electrode. When the circuit is completed between the two electrodes and the reactants for each reactant are provided, then electricity flows. Efficiency can be calculated in a few different ways, but probably the most useful is to look at the energy efficiency at a given current density: we can determine the maximum theoretical amount of energy that the chemical reactions can put out, and measure how much energy is actually usable for doing work. This can vary from nearly 0% efficiency to over 90% (nothing over 100% is possible), but typical fuel cells under typical working conditions are around 80% efficient.
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A fuel cell is essentially like a battery. There are two chemical reactions that occur, one releases electrons (oxidation), and one accepts electrons (reduction). The oxidation occurs at one electrode, and reduction occurs at the other electrode. When the circuit is completed between the two electrodes and the reactants for each reactant are provided, then electricity flows. Efficiency can be calculated in a few different ways, but probably the most useful is to look at the energy efficiency at a given current density: we can determine the maximum theoretical amount of energy that the chemical reactions can put out, and measure how much energy is actually usable for doing work. This can vary from nearly 0% efficiency to over 90% (nothing over 100% is possible), but typical fuel cells under typical working conditions are around 80% efficient.
Thanks but that doesnt explain much. Where is the precious metal catalyst needed?
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A fuel cell is essentially like a battery. There are two chemical reactions that occur, one releases electrons (oxidation), and one accepts electrons (reduction). The oxidation occurs at one electrode, and reduction occurs at the other electrode. When the circuit is completed between the two electrodes and the reactants for each reactant are provided, then electricity flows. Efficiency can be calculated in a few different ways, but probably the most useful is to look at the energy efficiency at a given current density: we can determine the maximum theoretical amount of energy that the chemical reactions can put out, and measure how much energy is actually usable for doing work. This can vary from nearly 0% efficiency to over 90% (nothing over 100% is possible), but typical fuel cells under typical working conditions are around 80% efficient.
Thanks but that doesnt explain much. Where is the precious metal catalyst needed?
You never asked about the catalyst. Since it is not a fundamental requirement for a fuel cell, I did not mention it on the first pass. Perhaps this will be more helpful:
The catalyst is typically on one or both of the electrodes. It doesn't have to be precious metal, although most of the most stable and most efficient catalysts are (or contain) precious metals.
The catalyst is required for high current densities (current divided by surface area of the electrode) because chemical reactions, even when exergonic (releasing energy, ie favorable), can be quite sluggish at reasonable temperatures, when compared to the rates required for supplying high powers. You can think of a fuel cell as "cold" combustion--it is theoretically possible to extract more usable energy per unit of fuel than for combustion because less energy is being converted into heat, but on the flip side, chemical reactions go more slowly at these lower temperatures, so it is hard to release as much energy per unit time. To be competitive with the reaction rates found in combustion, catalysts are needed to reduce the activation barriers for the reactions to proceed as quickly as possible.
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A fuel cell is essentially like a battery. There are two chemical reactions that occur, one releases electrons (oxidation), and one accepts electrons (reduction). The oxidation occurs at one electrode, and reduction occurs at the other electrode. When the circuit is completed between the two electrodes and the reactants for each reactant are provided, then electricity flows. Efficiency can be calculated in a few different ways, but probably the most useful is to look at the energy efficiency at a given current density: we can determine the maximum theoretical amount of energy that the chemical reactions can put out, and measure how much energy is actually usable for doing work. This can vary from nearly 0% efficiency to over 90% (nothing over 100% is possible), but typical fuel cells under typical working conditions are around 80% efficient.
Thanks but that doesnt explain much. Where is the precious metal catalyst needed?
You never asked about the catalyst. Since it is not a fundamental requirement for a fuel cell, I did not mention it on the first pass. Perhaps this will be more helpful:
The catalyst is typically on one or both of the electrodes. It doesn't have to be precious metal, although most of the most stable and most efficient catalysts are (or contain) precious metals.
The catalyst is required for high current densities (current divided by surface area of the electrode) because chemical reactions, even when exergonic (releasing energy, ie favorable), can be quite sluggish at reasonable temperatures, when compared to the rates required for supplying high powers. You can think of a fuel cell as "cold" combustion--it is theoretically possible to extract more usable energy per unit of fuel than for combustion because less energy is being converted into heat, but on the flip side, chemical reactions go more slowly at these lower temperatures, so it is hard to release as much energy per unit time. To be competitive with the reaction rates found in combustion, catalysts are needed to reduce the activation barriers for the reactions to proceed as quickly as possible.
can you describe please fuel cells with oxygen ion conductors?
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can you describe please fuel cells with oxygen ion conductors?
they are a subset of fuel cells, which contain a solid material that conducts oxygen ions...
for more info, see here: http://lmgtfy.com/?q=oxygen+ion+conductors
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How does a fuel cell work and what is its efficiency?
You could start here: https://en.wikipedia.org/wiki/Fuel_cell#Efficiency_of_leading_fuel_cell_types
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"t is also important to take losses due to fuel production, transportation, and storage into account. Fuel cell vehicles running on compressed hydrogen may have a power-plant-to-wheel efficiency of 22% if the hydrogen is stored as high-pressure gas, and 17% if it is stored as liquid hydrogen.[67] Fuel cells cannot store energy like a battery,[68] except as hydrogen, but in some applications, such as stand-alone power plants based on discontinuous sources such as solar or wind power, they are combined with electrolyzers and storage systems to form an energy storage system. Most hydrogen is used for oil refining, chemicals and fertilizer production and therefore produced by steam methane reforming, which emits carbon dioxide.[69] The overall efficiency (electricity to hydrogen and back to electricity) of such plants (known as round-trip efficiency), using pure hydrogen and pure oxygen can be "from 35 up to 50 percent", depending on gas density and other conditions.[70]. The electrolyzer/fuel cell system can store indefinite quantities of hydrogen, and is therefore suited for long-term storage." This statement to me rules hydrogen fuel cells out as totally inefficient.
The best way to store hydrogen is to combine it with carbon as a hydrocarbon liquid. Easy to store at room temperature and economical on use of precious materials.
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I still cannot comprehend how a fuel cell works.
Can you list its components in its simplest form and describe the process?
So far, I understand that there is an electrode from precious metal which oxidises oxygen and converts it to oxygen ion. What happens then to the ion? What happens in the other side? How is energy produced?
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Anyone please?
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We have a platinum plate. On the left side of the plate we dump some hydrogen. That makes the left side of the plate negative. That's because platinum rips an electron off of any hydrogen atom that touches it.
On the right side of the plate we dump some positive water, which we have made by burning the positive hydrogen. By positive hydrogen I mean the hydrogen from which electrons were ripped off.
So we are charging the the plate on the left side and then we are discharging the plate on the right side. So we know there is a current going through the plate.
Maybe the working of the fuel cell has been explained now?
Positive water can rip electrons out of platinum, while positive hydrogen can't.
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Thanks although what you described is slightly different to what I see on diagrams online. Also I don't understand what you mean positive water, there's no such thing.
Would the following description be accurate?
We have two plates of catalyst divided by water. The outer side of one plate is exposed to atmospheric oxygen which converts it to oxygen ion. The outer side of the other plate is exposed to hydrogen that we supply and it converts it to protons. Obviously there's potential difference generated between the two plates.
Protons and oxygen are somehow diffused to the middle common space between the plates and they react to form water.
1) how are protons and oxygen ions diffused?
2) what's the purpose of the middle space?
We have a platinum plate. On the left side of the plate we dump some hydrogen. That makes the left side of the plate negative. That's because platinum rips an electron off of any hydrogen atom that touches it.
On the right side of the plate we dump some positive water, which we have made by burning the positive hydrogen. By positive hydrogen I mean the hydrogen from which electrons were ripped off.
So we are charging the the plate on the left side and then we are discharging the plate on the right side. So we know there is a current going through the plate.
Maybe the working of the fuel cell has been explained now?
Positive water can rip electrons out of platinum, while positive hydrogen can't.
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Would the following description be accurate?
We have two plates of catalyst divided by water. The outer side of one plate is exposed to atmospheric oxygen which converts it to oxygen ion. The outer side of the other plate is exposed to hydrogen that we supply and it converts it to protons. Obviously there's potential difference generated between the two plates.
Protons and oxygen are somehow diffused to the middle common space between the plates and they react to form water.
1) how are protons and oxygen ions diffused?
2) what's the purpose of the middle space?
Yes, that's a reasonable description.
1) If the space between the plates is filled with a liquid like water, then protons and (hydr)oxide ions are dissolved in the liquid as mobile ions, and are free to migrate/diffuse. If the space is filled with a solid, then it needs to be a special type of solid that is permeable to the ions.
2) the gap is needed to prevent short circuit--the electrons coming from the hydrogen need to pass through the external load on their way to the oxygen to do useful work--otherwise you just have a fire.
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Can anyone suggest me a person from Cambridge University to advise me on oxygen and hydrogen consumption rate in fuel cells? I want to know the rate per volume of cell or per effective area.