Become a fan of Slashdot on Facebook

 



Forgot your password?
typodupeerror
Check out the new SourceForge HTML5 internet speed test! No Flash necessary and runs on all devices. ×
Science Technology

Scientists Achieve Perfect Efficiency For Water-Splitting Half-Reaction (phys.org) 130

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.
This discussion has been archived. No new comments can be posted.

Scientists Achieve Perfect Efficiency For Water-Splitting Half-Reaction

Comments Filter:
  • Oh well. (Score:5, Funny)

    by sims 2 ( 994794 ) on Monday February 29, 2016 @11:55PM (#51612637)

    Too bad there aren't any uses for oxygen.

    • 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.

      • Nowhere does it state Oxygen is produced in the process.

      • by michelcolman ( 1208008 ) on Tuesday March 01, 2016 @05:01AM (#51613449)

        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.

        • by tburkhol ( 121842 ) on Tuesday March 01, 2016 @10:05AM (#51614413)

          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.

        • They're also not splitting water because in order to prevent the hydroxyl radicals (formed from hydroxide ions after taking one electron out) from chewing up everything or taking their electron back (thereby lowering the photonic efficiency), they quench them with isopropanol. So, they have made an isopropanol-fueled photoelectrochemical cell that uses water as an intermediate electron donor. Next.
    • Just set the date to Jan 1 1970
    • The energy stored in the hydrogen is released by recombining it with the oxygen to create water which can then be re-split using sunlight. It's a closed loop, that is driven by sunlight.
    • Wait, wait! Mix it with carbon to create a marvellous earth greening gas called CO2! It's hugely beneficial for life on earth!
  • by Anonymous Coward

    I make that one ten millionth of a mole of H2 per second. Not exactly going to be making industrial quantities via this method, are we?

    • by synaptic ( 4599 )

      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."

      • Yes, but a mole of hydrogen weighs less than a penny, while a mole of nanorods weighs about as much as you or maybe your car or house.

      • by orpheus ( 14534 )

        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."

        Yes, but the specific numbers given would indicate 360,000 MOLECULES/hour, which makes it seem much more likely that the article itself misspoke by saying "moles" where it should have said "molecules"

        Sanity check: 360,000 moles/hr per mole of catalyst = 100 moles/sec per mole of catalyst = 6x10^25 reactions/sec per molecule of catalyst.
        You can't get a reaction time of 1/(6x10^25) sec = 1.7x10^-26 sec for chemical reactions in our current universe (maybe in a Big Bang)
        1.7x10^-26 sec is FAR less time

  • OK, so all the atoms make it into the product, but what's the energetic cost, or yield? Isn't that what really matters? Yes, I could read the article, but that's what a good summary is for -- spelling out the result in a sentence or two.
    • 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.

    • Activated by photons, you know like a solar cell?

      • With a 100% photon-to-electron conversion efficiency? If that is is the case, why are they concerned with the water-splitting?
        • Got to capture those electrons in useful form - no good liberating an electron and having it resorbed a few nanoseconds later.

  • Corroding nanorods (Score:3, Insightful)

    by lannocc ( 568669 ) <shawn@lannocc.com> on Tuesday March 01, 2016 @12:05AM (#51612669) Homepage
    And the nanorods still corrode, something they say needs to be addressed. I think this takes the practical efficiency below 100%.
    • When I see platinum nanorods in water, I worry that anything more than absolutely pure water will leave its "more" deposited on the nanorods, rendering them useless in a big big hurry.

  • and a typical sample contains about 600 trillion nanorods.

    That sounds like a lot, but Avogadro's number is still way, way bigger than that. How many decades before this produces 1 mole (2g) of hydrogen gas?

    • by suutar ( 1860506 )

      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.

      • by PPH ( 736903 )

        Or one could use multiples of "a typical sample".

        • by Dunbal ( 464142 ) *
          In which case I will put it to you differently: Hydrogen gas is currently priced around $1/kg = $2/1000 mole = $0.002/mole. Excluding the cost of collecting, compressing and bottling your gas (let's say you can pay for all that with the oxygen you sell - since you also get oxygen) - it will take you 158 years to pay back $1 per "typical sample". I am sure these cost more that $1 to make. Therefore while academically interesting, these are not and will probably never be industrially/economically viable. I d
      • You forgot to factor in that the sun is only up for half the day (on average). So only 12 hours a day will have usable light. My math rounds out at 231 days per mole. Factor in real world limitations like the effect of weather and I would estimate somewhere in the neighborhood of a year per mole when it's all said and done. I think they're going to need a whole lot of these things to be actually useful.

        Still in all, it's a good advancement in the right direction.

        • You forgot to factor in that the sun is only up for half the day (on average). So only 12 hours a day will have usable light.

          But then there's cosine error from the light not hitting the surface straight on. (At a solar-farm level it applies even to tracking systems, since you're really interested in the amount of ground surface area involved.)

          Then there's things like non-noon light taking a longer path through the admosphere (especially near sunrise and sunset) latitude, weather, season, altitude, near-hor

      • 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.

        The paper is restricted (paywalled?), but assuming 600 trillion nanostructures, sqrt(600 trillion) is a square about 24 million on a side. The Platinum atomic radius is about 0.13 nanometers, so rods 1 nanometer wide mean that the "typical sample" is 24 million nanometers on a side, or 24 mm on a side.

        Assume a typical sample is 1cm (10 mm) on a side, a square meter would be 100cm x 100cm, or 10,000 times more.

        Using your figures, 1e7 becomes 1e3 seconds, or about 15 minutes, unless I dropped a decimal or my

  • This process requires day light to function, therefore it's potential output is limited by the amount of time the sun shines. Not only is there night but also clouds.

    I hate solar power, not just because it is so limited but because so many tree hugger types flock to it. The chemistry of this process is very interesting but if used to convert sunlight to hydrogen then I believe this is a waste of time. Solar power is a distraction from energy production schemes that actually work.

    Let's take a look at the

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

    • by dbIII ( 701233 )

      I hate solar power, not just because it is so limited but because so many tree hugger types flock to it.

      Just ignore that and think manly thoughts about the Apollo program like the rest of us. It's an engineering thing and not a "tree hugger" thing.

      it requires carbon nano-tubes laced with platinum. Can we think of a material that is even more expensive than that?

      Since it's on a tiny scale with potentially a vanishingly thin coating I'd say just about everything else, especially since it's just about as corr

    • by wbr1 ( 2538558 ) on Tuesday March 01, 2016 @04:49AM (#51613419)
      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.
      • I think your best bet at the moment is good old alkaline electrolysis and demand response, preferably with the electrolyzers colocated at large utility-scale PV plants and using direct DC wiring.
    • by omnichad ( 1198475 ) on Tuesday March 01, 2016 @11:22AM (#51614963) Homepage

      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.

      • by wwalker ( 159341 )

        You are forgetting geothermal, which might be partially nuclear in nature, if I'm not mistaken. Come to think of it, solar power is also nuclear at its root (except fusion, not fission). So it's all nuclear all around.

        • OK. Geothermal, yes. But the cause of the energy in solar power isn't really relevant since that in itself is outside the Earth's energy system.

  • How many nimrods does it take to make hydrogen?
  • by ls671 ( 1122017 ) on Tuesday March 01, 2016 @12:28AM (#51612789) Homepage

    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/... [salon.com]

    Truly yours,

    Rex Wayne Tillerson

    • by Anonymous Coward

      There is definitely more energy (hydrogen) in the oceans than there is in those silly oil patches.

      No there isn't. There is energy in light and it can be captured by breaking the bond in water.
      And it's this energy that you get back by burning hydrogen to water.

  • Oh my God... I can hear the "Run your car on water" scammers firing up their computers right now. There's bound to be another wave of crappy scams for sale online which will tout this as the kind of breakthrough that has finally made it practical to boost your mileage by 500% and reduce gas costs by 600%. Just send $49.95 now for the secrets of how this nano-technology can let you split water into hydrogen and run your car for free!

    Groan!

    Even PT Barnum would roll in his grave!

  • 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.
    • by Anonymous Coward

      Well, with global warming and all, we can start to reverse the ocean's rise with this wonderful new technology.

    • Re:Peak Water (Score:4, Interesting)

      by bluefoxlucid ( 723572 ) on Tuesday March 01, 2016 @10:56AM (#51614759) Journal

      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.

  • 100 H2/s/rod * 600 Trillion rods / sample = 6 * 10^16 H2/s/sample

    High number, but small compared to Avogadro's number: Na = 6*10^23
    I.e. it takes approximately 10^7 s (~117 days) to produce one mole (~1g) of hydrogen gas (per sample).

    If you would construct a factory which produces a ton (a bit modest, but still) on H2 every day, you'd need 10^6 grams per day. That leads to 10^6*117 ~ 10^8 samples.

    I wonder that a sample costs... and what the price of a ton H2 is currently on the market. Let me make a wild gue

  • by jcr ( 53032 ) <jcr@mac.cOPENBSDom minus bsd> on Tuesday March 01, 2016 @06:38AM (#51613661) Journal

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

    -jcr

  • Entropy (Score:4, Insightful)

    by wkwilley2 ( 4278669 ) on Tuesday March 01, 2016 @08:18AM (#51613859)

    Entropy just isn't what it used to be.

  • Elon Musk says it best. "Hydrogen is an incredibly dumb” alternative fuel

    http://thinkprogress.org/climate/2015/02/12/3621136/tesla-elon-musk-hydrogen-dumb/

  • I would take Hydrogen Combustion engine car over a Electric car any day. Electric cars are dumb, batteries are expensive to produce and generate a lot of waste. Hydrogen combustion engines produce water vapor.
  • Yeah... I guess that's pretty good... but I hope this success doesn't keep scientists from striving for even higher efficiency numbers.
  • is there any researches going on quantum mechanical level about this splitting reaction
  • And then pump it into the ground. Use nuclear energy to power the process and we could have net negative carbon emissions!

"I have just one word for you, my boy...plastics." - from "The Graduate"

Working...