Storing the Sun in a tank

How electrolysis could produce hydrogen as a way to store renewable energy: During the day, solar panels supply surplus electricity for electrolysis, producing hydrogen from water, which is then stored in a tank. At night, hydrogen would be combined with oxygen from the air in a fuel cell to generate electricity. Credit: Jakob Kibsgaard

Among the several hurdles in making renewable energy a cost effective, reliable and diffuse reality is the storage of energy between the time it is captured and the one it is used. 

This goes for tides, for wind and for solar. They are all important sources of energy that can be harvested only in certain window and most of the times these windows don't match with the usage needs.

There are several ways of storing energy but again the problem is finding something that is both cheap (affordable) and easy to use. Techniques include heating various chemicals, like sodium, pressurising air, spinning big wheels, moving water to high reservoirs...

Now researchers at Stanford are proposing to use energy harvested from the Sun (turned into electricity) to separate hydrogen in the H2O molecule (water) and storing it into a tank. The advantage of this method lies in the very high energy density of hydrogen. In a relatively small tank you can store plenty of energy. Once you need it you just add air (and in particular oxygen) and you get water back and along with it plenty of energy ready to use.

The problem with this approach is that the electrolytical process (electrolyses) used to split the water molecule in hydrogen and oxygen results in the production of protons, that is hydrogen atoms that miss the electron. In order to have the proton capturing an electron and therefore becoming a stable hydrogen atom you need to keep it "quiet" for a little while and that requires a catalyst. The ideal one is platinum, but platinum is not cheap! What researchers at Stanford have been able to do is to use a much cheaper element, molybdenum, to do the same thing.

They got the inspiration from looking at petroleum refineries where the molybdenum sulfide is being used. It does not work well for what they needed but a variation of it does, almost as well as platinum: molybdenum phosphosulfide.

So now we have a way to place Sun energy in a bottle, literally!

Author - Roberto Saracco

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