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Scientists Achieve Perfect Efficiency For Water-Splitting Half-Reaction (phys.org)

Posted by timothy on from the fill-'er-half-up dept.

Dthief writes: Splitting water is a two-step process, and in a new study, researchers have performed one of these steps (reduction) with 100% efficiency. The results shatter the previous record of 60% for hydrogen production with visible light, and emphasize that future research should focus on the other step (oxidation) in order to realize practical overall water splitting. The main application of splitting water into its components of oxygen and hydrogen is that the hydrogen can then be used to deliver energy to fuel cells for powering vehicles and electronic devices. The process involves exposing the water to a mass of platinum-tipped nanorods, with visible light driving the reaction. The 100% efficiency refers to the photon-to-hydrogen conversion efficiency, and it means that virtually all of the photons that reach the photocatalyst generate an electron, and every two electrons produce one H2 molecule. At 100% yield, the half-reaction produces about 100 H2 molecules per second (or one every 10 milliseconds) on each nanorod, and a typical sample contains about 600 trillion nanorods.

28 of 130 comments (clear)

  1. Oh well. by sims+2 · · Score: 4, Funny

    Too bad there aren't any uses for oxygen.

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    1. Re:Oh well. by ShanghaiBill · · Score: 5, Interesting

      the cheapest way to store the oxygen so you can use it later is to not store it, releasing it into the atmosphere, and later just use "air" for the reaction. :-)

      Not when you consider the total cost. Air is about 78% Nitrogen, which leads to a lot of inefficiency. Most engines will run much more efficiently with straight O2, or even O2 and some other substance such as injected water. Coal burning power plants that do carbon sequestration use air stripped of N2 so they don't have to separate the CO2 from the N2 in the exhaust. Nitrogen in a combustion chamber also leads to a lot of nasty pollutants that are difficult and expensive to remove. If you are producing pure O2, just venting it into the air is a big waste.

    2. Re:Oh well. by dbIII · · Score: 5, Funny

      Nitrogen in a combustion chamber also leads to a lot of nasty pollutants that are difficult and expensive to remove

      Not that hard just add water and pipe it away as very weak nitric acid which is pretty well how "scrubbers" work.
      At a power station where I did some work the new manager decided he wanted nice white smoke coming out of the stack for PR reasons. So somebody played with the settings on the scrubber injecting a lot of water into the exhaust which came out as nice white steam. That night was very still, the water condensed out of the air and the utility had to pay for repainting thousands of cars in the city that had nitric acid damaging the paintwork.

    3. Re:Oh well. by michelcolman · · Score: 2

      If you lived on Titan, you might see things the other way around.

    4. Re:Oh well. by Jesus_666 · · Score: 3, Informative

      Whether using pure O2 in an engine make sense depends on where the engine is used. For a fixed installation it could make sense to use pure oxygen but for something like a vehicle I'd recommend against it - unless you think that vehicle accidents need to become more spectacular.

      I do agree, though, that it makes more sense to store the O2 than to vent it. There are plenty of uses for bottles of pure O2.

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    5. Re:Oh well. by zAPPzAPP · · Score: 3, Informative

      The engine is the oxidation part. It outputs water, not O2.

      The process described in the arcticle would happen in an industrial complex of some sort.

    6. Re:Oh well. by Megane · · Score: 2

      The question is more whether this O2 is better to keep around than the O2 that is a by-product of making liquid nitrogen. And that O2 starts out as a liquid, so it is much easier to keep pure until it can be put into canisters.

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    7. Re:Oh well. by currently_awake · · Score: 4, Interesting

      Hydrogen is a terrible energy source. The gas has very low energy density (big gas tank), compressing it takes a lot of energy, and it leaks through holding tanks. The money that is being wasted on the hydrogen economy should be directed into broadcast energy or putting power rails into the roads.

    8. Re:Oh well. by Immerman · · Score: 3, Interesting

      How do you figure that's carbon neutral? You're still converting all that geologically sequestered carbon into CO2.

      Just because you're scrubbing it from the exhaust and temporarily storing it somewhere doesn't mean it's not there. So far as I've heard, we've yet to come up with a way to store CO2 in the long term. Pumping it into the ground is only a temporary buffering solution, as it begins to leak out almost immediately. Undersea storage is a bit more effective, but much more expensive, and has a devastating effect on the fragile sea-bottom ecosystem with unknown long-term consequences.

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    9. Re:Oh well. by Immerman · · Score: 2

      Well, if we were producing H2 on a scale to replace gasoline as a portable fuel source, then I'd venture a guess that the amount of O2 produced would be several orders of magnitude greater than produced as a byproduct of liquid nitrogen production.

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    10. Re:Oh well. by thegarbz · · Score: 5, Interesting

      False.

      Pure O2 is the only thing which breaks the fire triangle (fires require oxygen, fuel, and ignition sources)

      It's oxygen, so that's one corner of the triangle.

      It turns almost anything into a fuel and handling it is incredibly difficult because of this. For example pure O2 pipelines have "fuses" put into them which is basically a section of teflon pipe with high pressure water jets directed at it because the metal pipework itself becomes a fuel in the presence of pure O2. Most things which aren't a fuel in normal air burn spectacularly in the presence of concentrated O2. That's the second corner of the triangle.

      Also pure O2 is easily ignited by any particles in the O2 stream. There's a separate specification for construction and cleaning of O2 carrying systems on top of the standard pipe spec because of this, as well as special ordering requirements for any equipment used in O2 services. If that isn't bad enough it can spontaneously combustion due to adiabatic compression by something as simple as a valve moving too quickly. That's the final corner of the triangle.

        In industry O2 is handled like an explosive and highly volatile substance. The exception is when it is in stationary containment (i.e. a gas cylinder). In most plants handling pure O2, the pipework has special identification on it to ensure people stay the heck away from it.

    11. Re:Oh well. by BoogieChile · · Score: 2

      Yes, your nostrils also need a fuel to burn.

      But seriously, if you have pure O2 coming out of your nostrils, then something is seriously wrong with your respiratory processes and you should probably get back to Area 51 at your earliest convenience.

  2. Corroding nanorods by lannocc · · Score: 2, Insightful

    And the nanorods still corrode, something they say needs to be addressed. I think this takes the practical efficiency below 100%.

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  3. Re:What about energetic yield? by jeffb+(2.718) · · Score: 2

    You, um, need to read the second paragraph of the summary. That 100% DOES refer to quantum efficiency -- they're claiming that essentially every photon that reaches the catalyst frees an atom of hydrogen.

  4. what do they mean with two step? by gl4ss · · Score: 2

    the article is confusing. googling for water splitting half reaction results mostly just in this article or copies of it itself.

    they can pluck the h2 from the O but the O doesn't want to O2? and it just reacts back to water? so they can make H2 from water with 100% efficiency except that they can't?

    in their not-just-water(they mention "high ph") solution? I'm not sure this is big enough news to tout all over the world with a trombone as they seem to be doing.

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    1. Re:what do they mean with two step? by michelcolman · · Score: 5, Informative

      They are not splitting water at all. They are just encouraging individual H atoms (H+ ions, or basically protons) to combine into H2 molecules.

      Normally, even pure water has a whole lot of individual H+ ions floating around in it, and the same number of OH- ions. Those H+ don't combine into H2 because that would require extra electrons, which are stuck in the OH- ions. That's why you can create H2 using an electric current which delivers the missing electrons so the H can pair up. (This doesn't work very efficiently in pure water, but a bit of catalyst helps a lot.)

      Acidic solutions have more H+ (along with negative ions from the acid, like for example HSO4-), basic solutions have less H+ (and more OH-)

      Apparently the researchers are using a basic solution (high pH, lots of OH- and less H+), and then using photons to liberate electrons from the OH- to allow H+ to combine into H2.

      This works best in a basic solution because the problem is not so much the number of H+, but rather the number of OH- ions that can be persuaded to give up an electron by nudging them with a photon. In an acidic solution, most of the eligible electrons are stuck in the negative ions from the acid which are much more possessive of their electrons.

      The big novelty is apparently that they can get the OH- to give electrons to the H+ and let them combine into H2, rather than combining back into H2O.

    2. Re:what do they mean with two step? by tburkhol · · Score: 5, Informative

      Apparently the researchers are using a basic solution (high pH, lots of OH- and less H+), and then using photons to liberate electrons from the OH- to allow H+ to combine into H2.

      That would leave you with uncharged hydroxyl radicals with an unpaired electron, so I don't think that's what they're doing. They are only talking about the reduction half reaction (2 H+ + 2e- --> H2). There has to be an oxidation half reaction. You're proposing (OH- --> OH + e-), but they're talking about splitting water, ie : 2 H2O --> O2 + 4 H+ + 4e-

      I suspect they need the high pH to let the H2 diffuse away.

  5. Re:The thing is by suutar · · Score: 2

    about 3.8 months, unless I dropped a decimal. 6e23 atoms wanted / 6e14 rods means we need to generate 1e9 atoms per rod; at 1e-2 seconds per atom per rod, that's 1e7 seconds or 115ish days.

  6. Re:Still has the problem of night by JoeMerchant · · Score: 2

    All it needs are photons, I wonder if Co60 decay photons do anything good for them?

  7. ExxonMobil president here by ls671 · · Score: 2

    Thanks for the tip guys!

    There is definitely more energy (hydrogen) in the oceans than there is in those silly oil patches. Who needs water anyway?

    We might have to make a deal with Nestlé but this should come along well.

    https://www.salon.com/2015/04/...

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    Rex Wayne Tillerson

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  8. Re:Does it scale? by synaptic · · Score: 2

    Your math is right based on the summary but the article says:

    "With a stable system and a turnover frequency of 360,000 moles of hydrogen per hour per mole of catalyst, the potential here is real."

  9. Peak Water by Ailicec · · Score: 2

    It's a little early to get worried about it, but when we finally get a hydrogen economy going, I wonder how much water we'll lose due to leaking hydrogen. Billions of devices leaking a little bit over many years would add up. Maybe technology will move on the next step before it's a serious problem.

    1. Re:Peak Water by bluefoxlucid · · Score: 3, Interesting

      A hydrogen economy would be useless for exactly that reason. To store hydrogen, you need exotic materials and high-energy cooling systems. Hydrogen leaks through sealed steel; you'd need to use a lot of energy to keep a hydrogen storage unit idle. This is okay when you're making, shipping, burning; but it's not okay when you fill up your gas tank once a month.

      If they could combine the hydrogen with CO2 and make O2+C4H, they could use methane to drive fuel cells or internal combustion. You can actually store methane.

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  10. Re:Still has the problem of night by wbr1 · · Score: 4, Informative

    This is not a solar energy 'scheme'. It is a more efficient method to produce hydrogen for fuel cells. I too am a fan of nuclear, properly done, but solar energy and solar processes have their place in a good energy mix.

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  11. Ok, so what does this mean economically? by jcr · · Score: 3, Interesting

    Hydrogen sells for about $1/Kg today. How much cheaper will it get with this process?

    -jcr

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  12. Entropy by wkwilley2 · · Score: 3, Insightful

    Entropy just isn't what it used to be.

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  13. Re: Does it scale? by Bengie · · Score: 2

    A Mole of Moles: What would happen if you were to gather a mole (unit of measurement) of moles (the small furry critter) in one place?
    https://what-if.xkcd.com/4/

  14. Re:Still has the problem of night by omnichad · · Score: 4, Informative

    Solar power is a distraction from energy production schemes that actually work.

    Solar power is the root of all energy production schemes that actually work. With maybe an exception for nuclear. It's the only energy input into our otherwise closed system. Oil and coal are both sequestered solar power, while wind and hydro are both driven by solar power converted to heat.