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This discussion relates to Technology Review's article Solar-Power Breakthrough.

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  danimalia
  

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  07/31/2008 08:12 PM

  This is not artificial photosynthesis

  The article is misleading and implies that this discovery is artificial photosynthesis. But its really just an improved electrolysis electrode. Its cool, but its a far cry from solar power.
  
  i am often irritated by articles in tech review that hype discoveries so much. This article is a good example.

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  • 
    aquariumsW
    

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    08/01/2008 02:10 AM

    Re: This is not artificial photosynthesis

    No, not yet.  Coincidently, I was pondering this in the car today!  What catalyst (or any substance) when added to water exposed to sunlite, will produce cheaper hydrogen gas than possible using other electrolysis.  Thats all.

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    • 
      maclod
      

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      08/01/2008 10:39 AM

      Re: This is not artificial photosynthesis

      Indeed, this is nowhere near artificial photosynthesis. The headlines seems to suggest that the catalyst, when added to water, will allow the set-up to absorb sunlight and produce O2 directly. This discovery, while a great one, is an improvement towards electrolysis rather than solar power! Very misleading article.

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        SteveL
        

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        08/06/2008 01:53 PM

        Re: This is not artificial photosynthesis

        Kevin,
        
        Perhaps your readers would be interested in watching a 10-minute video about the Nocera-Kanan discovery. It's the pilot for a project called Chemical Explorers, a series of Internet videos about interesting developments in modern chemistry. Because it's intended for a general audience, it doesn't go into the kind of technical detail that some of the earlier posts do. But it does allow viewers to hear directly from the two chemists behind this discovery, it shows the cobalt catalyst in action, and it tells the interesting story of how the discovery came about. The video can be watched at the following site:
        
        http://chemicalexplorers.blip.tv/#1150780
        
        SteveL

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  • 
    Kevin Bullis
    

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    08/01/2008 09:11 AM

    Re: This is not artificial photosynthesis

    Please read the second page, including the paragraph that lays out clearly how this advance contributes to the goal of achieving artificial photosynthesis.

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  • 
    cjameshuff2
    

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    08/01/2008 10:01 AM

    Re: This is not artificial photosynthesis

    Indeed. The article appears to be either the work of a crank or of the information being filtered through someone who lacked any understanding of what was being done. Presumably the latter, but the subject's full of the former, and the author does Nocera no favors by giving such a confused and vague description.
    
    Electrolysis is not a difficult way to store energy, but it is inefficient largely due to more energy being imparted at the electrode surfaces to neutralize H+ and O-- ions and recombine them into  H2 and O2 than is stored in the products, due to an energy threshold that must be reached to make the reaction possible. The oxygen has the higher losses due to the nature of its chemical bonds...in water, it is bonded to two hydrogen atoms, while each hydrogen is bonded only to a single oxygen atom, and in O2, it has a double bond compared to the single bond in H2. It sounds as though they have created a catalyst that reduces the minimum energy required to do this, or perhaps to use the energy being released from recombining O2 molecules to push others over the threshold. This catalyst is unlikely to be at all useful in developing one that does the same for hydrogen, but reducing losses at the oxygen-producing electrode (the anode) will still make the cell as a whole more efficient at making hydrogen, and there is less to gain by doing the same on the hydrogen side anyway.
    
    But anyway...this isn't at all revolutionary for power leveling, only for electrolytically producing hydrogen. There are already numerous far more efficient and higher-capacity methods for storing up energy for later use, including things like flywheel and pumped water energy storage. The bigger issues with solar are the inefficiency and the resulting monetary *and environmental* costs of the giant solar panel farms.

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    • 
      vv111y
      

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      08/01/2008 01:47 PM

      Re: This is not artificial photosynthesis

      AFAIK, I agree that there are many candidates for storing energy, and as with the generation, there is a horse race going on as to which is the clear winner. (Correct me if I am wrong, and the optimal storage solution has been found).
      What is the efficiency of this conversion process? How does it do better than any of the other promising finds?
      The previous article using dyed plates as waveguides/concentrators still sounds much more promising...IF the efficiency is good.
      I should add - I have read that the efficiency for hydro storage is poor, but it can certainly store a large amount.

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      vv111y
      

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      08/01/2008 01:50 PM

      Re: This is not artificial photosynthesis

      Also,
      What environmental costs are there for very large solar farms?
      Is it anywhere close to the costs for any of the other solutions?

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  • 
    Emosson
    

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    08/01/2008 10:45 AM

    Re: This is not artificial photosynthesis

    Cobalt based catalysts have been used for decades in the electrolysis of water. Their oxygen evolution activity is thousand times higher in alkaline solution than what Kanan and Nocera report for neutral water (1 mA/cm2 at an overvoltage of 0.41 V). Still, the efficiency of splitting water into hydrogen and oxygen by electrolysis is only around 70%. The efficiency of storing hydrogen at high pressure is about 90% (only 70% if hydrogen is liquefied). Fuels cells, which convert hydrogen back into electricity, are about 40% efficient under practical conditions. Hence, the overall efficiency of storing (solar) electricity as hydrogen is 0.7 * 0.9 * 0.4 = 25%. By comparison, lithium batteries are more than 80% efficient in storing electricity.
    
    Kanan and Nocera propose to store solar energy directly by splitting water with an artificial photosynthetic system (photoelectrochemical cell) instead of having separate solar cells for electricity generation and water electrolysers for hydrogen production. The idea is certainly attractive, but it faces serious difficulties in practice.
    
    First of all the current densities and hence gas evolution rates of photoelectrodes in sunlight are hundred times lower than in an electrolyser. The highly diffusive and volatile hydrogen has to be separated from oxygen (by a membrane that conducts protons) and collected over the whole surface of the solar array. The water consumed by splitting has to be replenished and mixed with electrolyte salts for electrical conductivity.
    
    Water splitting requires theoretically a voltage of 1.23 V at 25 C, practically about 1.8 V due to overvoltage losses. To produce this voltage with visible light at least two photosystems have to be connected in series, as plants have been doing for billion years with photosystems I and II.
    
    To summarize: you need at least two illuminated electrodes connected to catalysts for hydrogen and oxygen evolution, separated by a membrane, bathing in an aqueous electrolyte that is continuously renewed and a system to collect the hydrogen without mixing it with oxygen. The distance between the electrodes has to be kept very short (a few millimetres) in order to limit ohmic resistance losses in the electrolyte. And the whole has to be stable under sunlight and heat for many years…
    
    Isn’t it much easier and more practical to connect a field of photovoltaic panels, which are optimized for solar energy conversion, to a compact electrolyser that is optimized for hydrogen generation? But, as mentioned above, electricity is by far more efficiently stored in batteries. And if you just want to store solar energy during the night the storage of heat in solar thermal power plants is the cheapest and most efficient solution.

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    • 
      pascal1960
      

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      08/01/2008 05:13 PM

      Re: This is not artificial photosynthesis

      OK, so the comparison of efficiency is 25% to 80%.  So this system becomes economically viable if the cost to build, maintain, etc. is 25%/80% = 31.25% over a period of time.  So what are the comparative costs?
      
      An argument about effiency has to include a cost-benefit analysis to carry the argument to its logical conclusion for the purpose of identifying if this has economic legs.

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        jjuzer
        

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        08/02/2008 05:30 PM

        Re: This is not artificial photosynthesis

        Add to this lack of efficiency the fact that a mole of hydrogen (2 grams) occupies 22.4 Liters of space (assuming process works at atmospheric pressure). Compressing hydrogen to any usable pressure for transport or storage for later use requires further energy, which is very large since compressing the light gas is not easy. Further, fuel cells are only 52% efficient, at best, so off the top, electricity generated at such a heavy (PV Capital)cost is discounted by a huge amount for storage. It may be a technical chemical feat towards the mimicking of photosynthesis, but from the data provided, certainly not a useful and economically viable one for power storage. maybe good to make sugar and carbohydrates from CO2 and sunlight as a next step....Aah! that will be the next news release. Scientists sequester CO2 to make sugar......Many of these impractical lab efforts get more hype and attention than practical, economically viable smaller steps towards solving the world's energy problems. The media, unfortunately gobbles up and is only looking for (hype about) earth shattering discoveries, and it is a shame most of the prestigious institutions fall prey to such tactics, perhaps to impress alumni, donors, research grant decision makers, etc.

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    • 
      stas peterson
      

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      08/01/2008 05:49 PM

      Re: This is not artificial photosynthesis

      Oh My God. 
      
      An analogue to Ice9. Put some put some magic pixie catalyst dust, in the Ocean and watch it all dissociate and just ...DISAPPEAR!

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  • 
    dpmartin
    

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    08/03/2008 08:58 PM

    Re: This is not artificial photosynthesis

    They leave out the efficiency of converting electricity to hydrogen. If the percentage is small, the whole discussion could be meaningless.

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