Science Interviews


Wed, 1st Oct 2014

Artificial photosynthesis

Dr Erwin Reisner, Cambridge University

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Many people agree that hydrogen is one of the fuels of the future. It doesn't releaseMaple Leaf pollutants when used, and can be created from water. However, currently, to break water down into hydrogen and oxygen takes a lot of energy. Many scientists are looking into cheaper and more environmentally-friendly ways of producing hydrogen from water. One of these is Dr Erwin Reisner, from Cambridge University, and he explained to Chris Smith how he uses sunlight to drive the process.

Ginny Smith and Dave Ansell also have a go at splitting water into hydrogen and oxygen, with explosive results!

Erwin -  What we try to do is really produce a renewable fuel such as hydrogen as an example.  So, I think it can be perceived as an extension of photovoltaic research where we take a solar panel, instead of producing electricity, we just try to produce hydrogen directly from water as an example.  In a nutshell, you have water, you shine light on it and hydrogen comes out.  Hydrogen is a very interesting chemical because itís energy rich, itís a fuel, and we can store and transport it.

Chris -  How does this work?  How can you actually split the water up like that?

Erwin -  So essentially, we heard the first part was heavily inspired by photovoltaic research.  So, we need a light absorbing material where we essentially harvest the photons, what weíve heard about, and then instead of producing electricity, we just transfer energy directly to a catalyst.  The catalyst is a substance that facilitates a chemical reaction.  This catalyst weíre using is a catalyst that withdraws hydrogen from water as an example.  Essentially, yes we are physics and when we talk about fuels, we need some chemistry.  Thatís my specialty and thatís why we try to make fuel cell.  Hydrogen is only one example.  We can think all about liquid fuels to replace fossil fuels and with liquid form in all kinds of possibilities.

Chris -  So, you would have a material which would have access to some water.  It could split the water into hydrogen and oxygen.  How do you get the energy back?  What would you do with the hydrogen?

Erwin -  The hydrogen needs to be stored.  Essentially at the moment, what you would do is you would just pump hydrogen out and compress it, and then store it, and transport it.  Or you'd convert it directly into a liquid form of fuel.  Thatís another possibility.  So hydrogen can be converted into all kinds of liquids by established industrial processes.  Itís done under mega ton scale at the momentÖ

Chris -  They're not dangerous though.  I mean, Hindenburg didnít go down to Ė well, it did go down, that was the problem, but it wasnít too much of a success story from a chemistry and engineering perspective, was it?  Isnít there a bit of a danger associated with hydrogen?

Erwin -  Yes, hydrogen is explosive thatís for sure, but so is actually natural gas and gasoline is also actually quite dangerous and explosive.  So, I think we definitely have the technology to handle hydrogen as a gas and itís not much more dangerous than other forms of fuels weíre actually handling at the moment.

Chris -  Well, before we hear more about the technology, I think we should hear a bit more about hydrogen.  Ginny and DaveÖ

Ginny -  So, weíre actually going to look at a more conventional way of getting hydrogen out of water.  So, water is H2O which means itís made up of hydrogen and oxygen.  So, if you want to get hydrogen back out, you have to split those molecules up.  So, how do we go about doing that?

Dave -  So, what you want to do for that Ė so first of all, I have a pot of essentially water.  There's a bit of salt in there called magnesium sulphate.  This basically means it conducts electricity a bit better and makes something work a bit better.  What weíve also got is a power supply and this will apply a voltage to these two screwdrivers.  And so, what I'm going to do is I'm going to make one of the screwdrivers positive and one of the screwdrivers negative.  Now, if you think of water, itís H2O.  The H is a slightly positive and the O, the oxygen is slightly negative.  So, if you put a large voltage across water, the oxygen will be attracted to the positive electrode and the hydrogen will be attracted to the negative electrode.  If you put enough voltage on, that will actually split those water molecules apart and you should get gases coming off.

Ginny -  Okay, so weíve got a small beaker full of liquid and you're now putting a contraption made of two screwdrivers taped together into it.  This looks a little bit dodgy.

Dave -  Weíll try.  So, I've now turned it on and something quite interesting is happening.

Ginny -  Yeah, can you see what's happening?

Boy -  I can see bubbles.

Ginny -  You can see bubbles, exactly.  So, those should be bubbles of hydrogen and oxygen.

Dave -  That is the idea, yes.  So, weíll let those build up nicely for a while and there's an easy way to see whether itís likely to be hydrogen and oxygen; weíll be trying setting fire to it because we can basically release the energy we put in by putting electricity through the liquid and splitting hydrogen and oxygen in from water.  if we set fire to it, the hydrogen will burn with the oxygen to create water again and release lots of energy.

Ginny -  So, weíve just talked about how explosive and dangerous hydrogen is.  Now, weíre going to set fire some of it.

Dave -  Sounds like a good plan to me.

Ginny -  Everyone thinks that sounds like a good plan?

Audience -  Yeah.

Ginny -  So, weíve got quite a good layer of froth on top of this little beaker now and I can see Dave is ready with a box of matches.  Do I need to get back?

Dave -  In a minute, yes.  The reason why itís creating a froth is a little bit of washing up liquid in there which catches the bubbles, so they're trapped nicely.

Ginny -  Because otherwise, they just escape and go off into the atmosphere and we will be able to set fire to them which should be no fun at all.

Dave -  Okay, so here we go.  Iíll light a match and Iíll put it down to the froth at which partÖ 

Ginny -  Did anyone notice that that bang sounded a little bit funny?

Boy -  It sounded like a balloon popping.

Ginny -  It did sound a bit like a balloon popping.  It was quite kind of squeaky wasnít it and thatís characteristic of hydrogen.

Dave -  It was a very, very sharp bang, which is because actually it wasnít just hydrogen you're burning.  It was a mixture of hydrogen and oxygen.  If you mix hydrogen and oxygen together, it will burn exceedingly fast.  Actually, if you get the mixture right, they will burn faster than the speed of sound and you get what's called a detonation which is really destructive.  If it is not perfectly mixed then you won't get a detonation.  With air, it doesnít detonate quite so well, but yes, you have a very, very violent bang.

Chris -  Ginny and Dave, thank you very much.  (applause)  So Erwin, presumably your experiments donít quite go like that.

Erwin -  No, itís safer.  Shall we show it?

Ginny -  So Erwin, youíve brought an example of how you create hydrogen.  Itís a little bit different, right?

Erwin -  Yes, so this is a very simple system thatís why I brought it here.  eEsentially, what Iíd like to show is in a test tube essentially, how you can generate some hydrogen with our energy saving light bulbs which are not very energetic.

Ginny -  We need quite a lot of power to put through our mixture to separate the water out.  You're going to do that using less energy.

Erwin -  I will try, yes.  Essentially, what we have here, this is really just water with some buffer at pH 7.  There's nothing unusual about it.  We'll just pipette this out quickly and you can see as normal water, itís just fully transparent.  This means if you want to generate hydrogen with this mixture, itís very bad because no light is being absorbed.  Thatís why itís transparent.

Ginny -  So, we need to absorb the light for the energy to split the water and transparent things donít absorb light very well.

Erwin -  Precisely.  Thatís why I brought this dye here.  You can see that the deep red coloration and essentially, I just take a bit of that.  Itís just an organic light absorber.  It gives some colour.

Ginny -  Okay, so now, itís got a nice bright red colour.

Erwin -  Yeah, exactly.  So now essentially, this dye will absorb light, but we still donít have this catalyst I mentioned before.  So, at the moment, we absorbed the light, but itís not good enough to generate the hydrogen.  So, we really need this catalyst, this substance that helps and facilitates the evolution of hydrogen.

Ginny -  And a catalyst is just something that is used in a reaction that helps something else be produced but doesnít actually get used up itself.

Erwin -  Precisely, yeah.  This catalyst is what we develop in our laboratory.

Ginny -  So, that would mean you could use the same catalyst over and over again, just adding more water.

Erwin -  If we have a very good catalyst, yes, but at the moment, they do not exist, except they're very, very expensive and like platinum.  But this is a very cheap material, a very cheap catalyst.  At the moment, they're not sufficiently efficient to run for very long.  So at the moment, all I do is shake it.

Ginny -  And itís a nice little bright orange colour now.  It almost looks like itís glowing.

Erwin -  Yeah, it is exactly.  What I will do now, I will just put it here in the back and it will probably take a couple of minutes but then we will see the formation of hydrogen.

Ginny -  So, you're going to pop that under a light thatís going to effectively, you'd normally do this with sunlight, but itís evening and weíre inside.  So, weíre going to put it by a lamp instead.  Weíre cheating basically.  And then weíre going to come back to thatÖ

Chris -  How will you know the hydrogen has been made?

Erwin -  Essentially, we can also light it up or we use analytical facilities in a chemistry laboratory.  We know precisely what gas is being formed.

Ginny -  Setting fire to it sounds like more fun.

Erwin -  Precisely, yeah, I agree.

Chris -  Weíll come back to that in a second.  So, tell us a bit more about how you actually are working on this?  The ultimate goal then would be, so that we have ways of converting plentiful sunlight into a supply of hydrogen.

Erwin -  Yeah, so that is the idea.  At the moment, this is a very new line of research compared to wind technology or solar cells, which means we have no commercial applications at the moment.  So, these are really being proposed at the moment, but it will still take a considerable amount of time really to bring this to the market place.

Chris -  Is it just visible light or can it use heat?  So, if we took a waste industrial process that produces loads and loads of heat or a data centre.  I mean, one statistic is that the data centres that run the internet are chucking more heat into the sky than they're actually using to run the data centre in aircon.  And also, they're contributing more CO2 than the airline industry.  So, can we turn that waste heat into something with this technology for example?

Erwin -  Yeah.  Heat is certainly very interesting and I think should be used much more in the future, but we focus only on solar technology.  If you use solar technology, either you run it like our systems just at room temperature or you work on solar thermal approaches where essentially you also work with heat, with solar concentrate, thatís where you work at a thousand and more degrees Celsius also to produce fuels that way.

Chris -  Whoís got some questions for Erwin on how hydrogen works?

Bryan -  Hello.  I'm Bryan from Cambridge.  My question is, we heard early on that the solar panels turned about 20% of the solar energy into electricity.  When you then generate hydrogen, what percentage of the energy is transferred then?

Erwin -  So at the moment, depending on technology, we have two ways.  Either we convert energy directly, which means sunlight goes directly to fuel.  This way, the record efficiency is about 13%.  But these are achieved with very expensive materials and effectively, systems do not last very long.  An indirect approach would be to couple a solar cell plus an electrolyte system as weíve just seen before.  With such technology, we can probably reach 15% to 20% or even 30% very easily on an industrial scale.

James -  Hi.  My name is James.  I'm from the United States.  You said the technology is new.  However, what commercial applications were you looking for this technology and also, what are hurdles that you face going to towards that?

Erwin -  The main hurdle is essentially Ė there are several - but the main problem at the moment is the cheap price of hydrogen produced from fossil fuels.  So at the moment, all the hydrogen we see which might be quite interesting is effectively not renewable hydrogen.  We might see all the green hydrogen buses driving around but this hydrogen is all produced through industrial processes from fossil fuels.  And this hydrogen at the moment is about an order of magnitude cheaper to produce than any renewable forms of hydrogen.  So at the moment, what is really letting us down is the very low cost of hydrogen derived from fossil fuels.

Joe -  Hi.  It is Joe.  My question is, can we use dirty water or sea water to generate hydrogen because pure and clean water, we are so of it in this world anyway?

Erwin -  Yes, itís certainly feasible and people have shown that this is possible.  So essentially, with seawater, we mainly deal with highly saline water, full of sodium chloride.  But in principle, there's nothing that holds you back to use seawater to generate hydrogen renewably.  In fact, it might even help because there's an electrolyte already in the water.

Chris -  What does that mean?

Erwin -  Electrolytes are simply a conductor in the aqueous solution.  So, if you want to electrolytise  water for example you need a conductor in electrolytes.

Ginny -  So, we had added one to our demonstration here in order to make it carry the charge better.  So actually, if you were doing this kind of electrolysis to split it, it would probably be better with seawater.  In fact, it works very, very slowly if at all, with pure water.

Dave -  The only disadvantage with electrolytising seawater is that instead of producing oxygen in the other end, you'll produce chlorine which is a chemical weapon, so you have to be careful with that one.

Ginny -  We decided not to go for that tonight.

Erwin -  Chlorine is also produced by the industry at the moment.  Itís quite available chemical, so we might even be able to use this one and produce renewable chlorine this way.

Chris -  How is your catalytic breakdown of water going?  How are you getting on?

Erwin -  Okay, so at the moment, we have produced the first couple of bubbles of hydrogen which I'm happy to share with Ginny as an example.  Yeah, gently as I said, our light source is very weakÖ

Chris -  Gently, He says gently - is this because it's explosive?

Ginny -  Can you see?

Erwin -  No.

Ginny -  There are some bubbles appearing.  What is that in the bottom of the test tube?

Erwin -  This is hydrogen that comes from water.

Ginny -  There's a little white thing.

Erwin -  This is stir bath.  This is just to stir the solution.

Ginny -  Okay, so there's something in the bottom to stir it and you can see on there, we formed little bubbles of hydrogen.

Erwin -  Yeah, the bubbles like to accumulate at the stir bath.

Ginny -  Okay, so if Dave takes it around, you should be able to see Ė there's not very many.  I think ours was more impressive, but you did do it with light and we had to use a very high voltage to get that much.

Chris -  Are you effectively trying to recreate photosynthesis here?  I mean, plants are very good at gathering energy from the sun and turning it into a chemical form of energy that they can use elsewhere in the plant or store as sugars turned into starch.  So, is that sort of what you're doing?

Erwin -  Precisely.  So actually, we do look at natural photosynthesis, try to learn from it and try to mimic the processes.  The field is in fact called artificial photosynthesis.  So, we do try to adopt with chemistry, materials chemistry to adopt and mimic processes in photosynthesis.


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On the combustibility of hydrogen, a comment was made about the Hindenburg accident. Recent analysis has shown that a lot of what was burning in the footage was not the hydrogen, but the skin of the vehicle, which was essentially painted with thermite. Ed S, Tue, 14th Oct 2014

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