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Self-powered hydrogen generation from bacteria

catsmate

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catsmate
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According to this interesting article in the US Proceedings of the National Academy of Sciences US researchers have managed to generate hydrogen by bacterial action without needing an external power source. If correct this is a major breakthrough in practical hydrogen fuel production, though currently cost is too great for commercial use.
There's a good article on the paper on the BBC website.

Abstract said:
There is a tremendous source of entropic energy available from the salinity difference between river water and seawater, but this energy has yet to be efficiently captured and stored. Here we demonstrate that H2 can be produced in a single process by capturing the salinity driven energy along with organic matter degradation using exoelectrogenic bacteria. Only five pairs of seawater and river water cells were sandwiched between an anode, containing exoelectrogenic bacteria, and a cathode, forming a microbial reverse-electrodialysis electrolysis cell. Exoelectrogens added an electrical potential from acetate oxidation and reduced the anode overpotential, while the reverse electrodialysis stack contributed 0.5–0.6 V at a salinity ratio (seawater:river water) of 50. The H2 production rate increased from 0.8 to 1.6 m3-H2/m3-anolyte/day for seawater and river water flow rates ranging from 0.1 to 0.8 mL/ min. H2 recovery, the ratio of electrons used for H2 evolution to electrons released by substrate oxidation, ranged from 72% to 86%. Energy efficiencies, calculated from changes in salinities and the loss of organic matter, were 58% to 64%. By using a relatively small reverse electrodialysis stack (11 membranes), only ∼1% of the produced energy was needed for pumping water. Although Pt was used on the cathode in these tests, additional tests with a nonprecious metal catalyst (MoS2) demonstrated H2 production at a rate of 0.8 m3/m3/d and an energy efficiency of 51%. These results show that pure H2 gas can efficiently be produced from virtually limitless supplies of seawater and river water, and biodegradable organic matter.
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By not requiring an external power source they are meaning that they don't need to apply a current across the cells to generate the hydrogen. There will still be a requirement to run pumps, cultivate, and maintain the organisms, etc.
 
I didn't quite understand the sciencey bit. Where does the energy come from?
 
Iiiiiiinteresting.

I'm a bit confused as well, but the BBC link says:

The MECs use something called "reverse electrodialysis" (RED), which refers to the energy gathered from the difference in salinity, or salt content, between saltwater and freshwater.
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ETA: wiki link for MEC (Microbial Electrolysis Cell)

ETA2: A good diagram for the process is here on wiki too
 
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phildonnia said:
I didn't quite understand the sciencey bit. Where does the energy come from?
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Yeah - come on big brains explain to us where the magic comes from! Also as others have mentioned there would be be energy inputs into such a system such as the pumps and so on - is it likely that the net output would be positive when those energy costs are taken into account or is this one that would still rely on being manufactured using non-renewable energy sources?
 
phildonnia said:
I didn't quite understand the sciencey bit. Where does the energy come from?
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From the quote in the OP:
Here we demonstrate that H2 can be produced in a single process by capturing the salinity driven energy along with organic matter degradation using exoelectrogenic bacteria.
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Salinity differences and stuff rotting.
 
Darat said:
Yeah - come on big brains explain to us where the magic comes from! Also as others have mentioned there would be be energy inputs into such a system such as the pumps and so on - is it likely that the net output would be positive when those energy costs are taken into account or is this one that would still rely on being manufactured using non-renewable energy sources?
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Yes there will be extraction/containment/cultivation energy costs. The process is analogous to fermentation of sugar to alcohols, provide feedstock and the micro-organisms will produce the hydrogen; the output gas will need to be pumped and compressed. With development the process should be cheaper than current methods for hydrogen production, steam reforming requires fossil fuels and heat and electrolysis is very energy intensive.
It's a start, not a solution.
 
Darat said:
Yeah - come on big brains explain to us where the magic comes from! Also as others have mentioned there would be be energy inputs into such a system such as the pumps and so on - is it likely that the net output would be positive when those energy costs are taken into account or is this one that would still rely on being manufactured using non-renewable energy sources?
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Part of it could I suppose be easily linked to a large freshwater pump system containing degredable organic matter present in all large cities (ie, the sewer system)
The sewage would provide the energy for the bacteria to grow and produce H2.
The salt water is harder. For seaside cities this could quite conceivably be a process with a net energy gain, but I suspect that pumping seawater any real distance negates such a gain.
Of course if treating sewage in seaside cities becomes an energy producer rather than consumer this would in the long run benefit any city hooked up to the same grid.
 
Wouldn't it be quite easy to make salt water in places that have none, by just adding salt to the fresh water? A lot easier than moving salt water around.
 
nvidiot said:
Wouldn't it be quite easy to make salt water in places that have none, by just adding salt to the fresh water? A lot easier than moving salt water around.
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Salt may be cheap, but I doubt it's so cheap that would make sense.
 
Bacteria don't just live on nothing. They need an energy source. For most bacteria this is chemical energy derived from their environment. For photosynthetic or radiosynthetic bacteria, an external energy source is captured and converted into chemical energy for the cell to use.

Nothing here has shown what they eat.
 
Bacteria don't just live on nothing. They need an energy source. For most bacteria this is chemical energy derived from their environment. For photosynthetic or radiosynthetic bacteria, an external energy source is captured and converted into chemical energy for the cell to use.

Nothing here has shown what they eat.
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Of course they don't live on nothing Ben, the paper apparently suggests wastewater as the organic material source. The BBC link in the OP has a quote from the authors.
 
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phildonnia said:
I didn't quite understand the sciencey bit. Where does the energy come from?
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http://en.wikipedia.org/wiki/Reverse_electrodialysis

Basically, when you dissolve something in water, energy is either used or released. So when you put two volumes of water with different concentration next to each other you get an energy gradient, and like any gradient it can be exploited to do work, at least in theory. It's essentially the same principle as two different metals producing a voltage when put together.

From the Wiki article, it appears this principle is already being looked into as a means of producing electricity. This new research is instead looking at using it to produce hydrogen directly, without the inefficiencies that would be present from first making electricity and then using that for electrolysis. The main question is likely to be how efficient this method actually is, and whether it would actually be better than just producing electricity without the bacteria getting involved.
 
nvidiot said:
Of course they don't live on nothing Ben, the paper apparently suggests wastewater as the organic material source. The BBC link in the OP has a quote from the authors.
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Ah, OK. That is a very limited resource, believe it or not. You might think that well, there is a lot of crap in the world, but when you look at the energy content of a city's sewage, it does not amount to much in terms of the energy the city uses.
 
Darat said:
Yeah - come on big brains explain to us where the magic comes from! Also as others have mentioned there would be be energy inputs into such a system such as the pumps and so on - is it likely that the net output would be positive when those energy costs are taken into account or is this one that would still rely on being manufactured using non-renewable energy sources?
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There's a partial answer in the quoted abstract:

By using a relatively small reverse electrodialysis stack (11 membranes), only ∼1% of the produced energy was needed for pumping water.
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phildonnia said:
Ok, so basically, fresh water has higher potential than salt water?
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Pretty much. It's the same basic principle as putting salt on a slug to kill it, or why salt and sugar work as preservatives - water will tend to flow through a membrane from high concentration to low concentration, even when there are other pressures trying to stop it. The slug keeps its water inside just fine, but put salt on it and the osmotic pressure is enough to suck it out. The same sucking can be harnessed to produce electricity or, in this case, to directly produce hydrogen, or in other ways such as simply acting as a water pump.
 
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