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Other than electrolysis - can H2O be cracked into hydrogen and oxygen at what temperature and how are they kept separated?
Graphi
I recently took a corrosion and fuel cell course where we discussed this topic. It is actually a very relevant question and, if this could be done on the cheap, it would be a reliable way of producing hydrogen as a fuel. Using solar concentrators, it is possible to heat water to approximately 1500 to 2500 K. At these temperatures the thermolysis of water is possible and is about 2 % efficient. According to Etievant et al.1 the costs and longevity of the equipment needed for this solar conversion makes its efficacy not "promising."
The problem once creating the hydrogen is how to store it. Some places just use the steam (to drive pistons) or the hydrogen (for combustion to drive pistons) for engines on location to create electricity. This is great! But, if one wants to create hydrogen for sale as an automotive fuel, the hydrogen will have to be efficiently stored for transportation to fuel stations.
Although the hydrogen would be produced at an elevated pressure, it would still need to be compressed for storage. Also, first one would need to remove large amounts of oxygen to produce a useable purity of hydrogen gas. One of the first methods used for the separation of hydrogen from the mixture was porous ceramic membranes composed of zirconia2. These types of membranes allow hydrogen to permeate, while not allowing oxygen to pass. Other methods are available for separating the two gasses. The current technologies for compressing gasses into stainless steel tanks requires a large energy input and creates a relatively low density fuel.
The solution to the storage of hydrogen is to store hydrogen gas in a solid. Some solids, such as the metal palladium, can store more hydrogen per unit volume than current methods of compressing gasses. Other efforts have focused on using nanotube based solids to store hydrogen as opposed to palladium. The issue with this is the difficulty in producing a commercially available solid that can absorb the gas reliably and quickly, then dispense it on demand--as combustion engines and fuel cells will have different fuel/energy requirements depending on driving conditions. As you can see, these ideas are great, but lots of this work is experimental--There are some promising results.
Others have suggested using the sun to power photovoltaic cells used to electrolyze (use electricity to split) water to hydrogen and oxygen. This is a good idea as well! Usually the electricity would come from carbon based sources, not the sun. After water is electrolyzed free of charge by the photovoltaic cells powered by the sun, then the processing of the gasses and same storage problems apply. It's just a carbon free way to electrolyze water. [1] Etievant, C. Solar Energy Muter. 1991, 24, 413. [2] Abraham Kogan, International Journal of Hydrogen Energy, 2000, 25, 1043.
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Water has a very stable molecule, and I don't think there is any dissociation due to temperature. Water is routinely taken into its supercritical region in power plants without dissociation. As you suggest it would be difficult to separate the hydrogen and oxygen in any case.
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aquifier
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