Peak sunlight flux is about 1 kW per square metre. 20% conversion efficiency would be good going (200 W). The sun shines half the day, and most of that not vertically downwards (50 W). A playing card is maybe 55 mm by 90 mm, so there are about 200 per square metre (50/200 W).
Average output power looks to be 1/4 W at best.
45 sunlight hours
"efficiently electrolyse the water in the presence of sunlight. In testing, he showed it could operate for a continuous 45 hours without a hiccup"
Wow - where/when did they test it with 45 hours of continuous sunshine?
You could do this above the arctic circle during the summer..... wouldn't be intense sunlight but it would be constant.
Antarctica, for starters.
Antartica is pretty much south of the Antarctic circle. From September to March, sunlight's pretty much continuous down there. The other months, the Arctic Ocean and parts of Alaska and northern Scandinavia and Russia get the distinction.
"...the leaf could be a significant step towards green energy becoming a sustainable reality."
It's already a sustainable reality. Freiburg, Germany proves it.
Still, a very potentially cool discovery. A lot of important details are missing from the article, but there's a lot of promise there.
Well Done! and yes, I'd call it photosynthesis.
Admittedly it was only a matter of time to find the right catalyst, but its great to see it has been achieved.
The key concept here is the same as the key concept of photosynthesis, whilst it does not have the exact same chemicals, the principle of a two stage, two photon absorbtion, to split water is exactly what photosynthesis is.
The energy required to split a water molecule is greater than the energy of one photon (otherwise water would be falling apart everywhere!) so a multiple stage process is required to convert the water in two stages using one photon for each stage, via an intermediate step using a suitable catalyst.
What I like about this is the fact that it can operate as a closed system, the water is split into 2xH2+O2 and a fuel cell will combine the two again returning pure water 2xH2O, there is no need for water refills, and no need for massive amounts of pure water to be available. maybe just some cooling?
I'd be inerested to know if this can work at high pressures, as compresing hydrogen is daft, the only way to effectivly make compressed hydrogen for storage is to pressurise the water before splitting it. So does this mean we need a suitible glass window that will withstand the pressure and allow in enough light... Or perhaps a two stage process, where a low pressure system running dependant on light, powers a second high pressure electroliser for splitting for the purpose of storage.
Catalyst not required...
To split water into Hydrogen and Oxygen does NOT require a catalyst. To do the opposite and combine Hydrogen and Oxygen to make water and electricity does require a catalyst. I don't know where the notion that a catalyst is required to perform electrolysis of water came from, but it just is not required. No platinum, no nickel, no gold. Precious metals need not apply. Just get yourself a cathode, an anode, some water doped with an electrolyte (don't use salt unless you can breathe Chlorine gas) and apply current with sufficient voltage to do the job.
split water into Hydrogen and Oxygen does NOT require a catalyst
Electrolysis it doesn't require a catalyst but this isn't electrolysis.
If we're being pedantic a catalyst does NOT alter an equilibrium only the kinetics.
2H2 + O2 > 2H2O + energy : the equilibrium lies heavily to the right, a catalyst allows the reaction to proceed usefully at low temperatures rather than needing a match - the match will give rather a large increase in rate !!
It most certainly is electrolysis of water...
Using electricity to split water into its two components is indeed electrolysis, and that is what the article claims the artificial leaf is doing.
"The key is Nocera's use of inexpensive catalysts, made from nickel and cobalt, which efficiently electrolyse the water in the presence of sunlight."
A catalyst is not required to electrolyse water. The reaction 2H2 + O2 > 2H2O + Heat is the overall reaction that takes place in a fuel cell, which is exactly the opposite of water electrolysis. Since we're being pedantic, the fuel cell reaction is
Anode Reaction: 2H2 => 4H+ + 4e-
Cathode Reaction: O2 + 4H+ + 4e- => 2H2O
Overall Cell Reaction: 2H2 + O2 => 2H2O (plus heat)
The fuel cell process is exothermic (gives off heat).
Electrolysis of water
Cathode (Reduction): 2H(aq) +2e > H2(g)
Anode (Oxidation): 2H2O(/) > O2(g) +4H(aq) +4E
Overall Reaction: 2H2O(/) > H2(g) + O2(g)
The half reactions are balanced with the electrolyte (usually an acid but it can also be a base).
Electrolysis of water is endothermic (requires heat).
A catalyst may help speed up the process of electrolysis of water, it is however not a requirement.
Typos & Journalistic Errors.
Shall we take that quote from the original source:
"The key to this breakthrough is Nocera's recent discovery of several powerful new, inexpensive catalysts, made of nickel and cobalt, that are capable of efficiently splitting water into its two components, hydrogen and oxygen, under simple conditions. Right now, Nocera's leaf is about 10 times more efficient at carrying out photosynthesis than a natural leaf. However, he is optimistic that he can boost the efficiency of the artificial leaf much higher in the future. "
Note the lack of the word electrolyse, since in this case it is wrong. The word that should have been used is 'Photolsye' since the energy is being provided in the form of photons not electrons. So this is NOT electrolysis, no matter what the article states.
The general reaction of photosynthetic photolysis can be given as:
[H2X] + [2 photons] --> [2e-] + [2H+] + [X]
The chemical nature of "X" depends on the type of reaction. Both H2O and H2S are photolysed in Nature.
Whilst you are correct that a catalyst is not required to electrolyse water, This is not electrolysis.
It is also true that with very high levels of photons you can get Photodissociation of water without a catalyst but you are talking about the levels of light produced by say a Carbon Dioxide Laser. where two photons may simultainiously enter a reaction (see http://en.wikipedia.org/wiki/Photodissociation#Multiple_photon_dissociation)
But where the level of light/photons is such that only one photon will enter the reaction at any time a catalyst MUST be used. And in this case a catalyst was used!
A catalyst is not inert in a reaction, it provides a middle phase of a reaction, in this case that middle phase is after one photon is absorbed, the second photon completes the reaction and returns the catalyst. Water dissociation is not possible with just one photon.
So going back to equations the overall excluding the catalyst should be as follows:
2[H2O] + 4[photons] --> [Photolysis Eq] --> 2[H2] + [O2] --> [Fuel Cell Eq] --> 2[H2O] + 4[e-]
Giving the key result: photons in electrons out.
Nickel and cobalt aren't that cheap
And you can do the same with TiO2 actually.
No need for the silicon either. I know nothing at all about the relative efficiencies mind. But juswt good old TiO2 containing slag (of which there is a monstrous amount lying around) made into roof tiles can do he same.
No way this is photosynthesis!
I bet they just got a plain solar cell and stuck the electrodes in a glass of water and, voila, electrolysis! Now, just pack it all up and lets call it a Leaf!
Couldn't we just stick a hypodermic into a trees vein's (equivalent) and steal it's sugars. Or come to think of it use palm oil to make fuel. Kind of cuts out the middleman ;op
But let's not belittle early stage research - sounds like a promising new technology in its infancy.
If you read the source
You'd see this is TEN times more efficent than a leaf.
And [my derrivation] using a fuel cell at 80% efficency is way better than buring palm oil at 40% efficency.
I think this guy is on to a winner here.
Full roof cover ?
I guess they imply an area comparable to what we now consider a standard photovoltaic installation would be enough to power ... etc...