This post was chosen as an Editor's Selection for ResearchBlogging.org

Update: This post was an Editor’s Selection at Research Blogging! Thanks guys!

In my previous post about water splitting, I talked about how to split water into hydrogen and how it can be useful. The drawback of course was that to make hydrogen from water, you need to input energy. If you’re just using electricity, and that electricity was produced from burning coal or natural gas, then the hydrogen you produce isn’t really any “greener”. However, if one could use sunlight directly to produce hydrogen, then you have a fuel that can be stored and used later, potentially mitigating the problems with solar energy. Namely – it’s not always sunny when you want to use electricity!

Recently Sun Catalytix – a company spun out of Dan Nocera’s lab at MIT – made headlines in a few places with their “artificial leaf” system. You can see it below – light shines on the device and bubbles (tiny bubbles) of hydrogen and oxygen evolve from the surface! No external power, no batteries!

Artificial leaf prototype. Bubbles, the Bubbles! (Source: CNET)

Read on to learn why splitting water using sunlight isn’t as easy as it seems, and how this new artifical leaf you’ve been hearing about is different.

Why So Difficult?

Here’s the reaction required to split water:

Energy + 2H2O –> 2H2 + O2

Suffice to say that the amount of energy required to do this reaction can be found in green light.

“HOLD THE PHONE” you say! If there’s enough energy in green light to split water, why if I set a glass of water out in the sun doesn’t it split itself away? The answer is simply that water doesn’t absorb green light. Water is clear, so in order to make use of the energy in the light, we need some other material present that can absorb the light and make the energy available to the water.

Scientists have known about the ability of certain materials (like titanium dioxide) to accomplish this for some time – the first publication to demonstrate it occurred in 1972.  Since then many research groups have attempted to improve upon this with little success. I spent a few years on this problem myself when I interned at NREL for a few summers under John Turner.  In 1998 he created headlines with a device that could split water from sunlight at about 12% efficiency! However, that material only lasted a couple of hours before completely disintegrating…

The problem seemed to be that the stability and the efficiency of materials are inversely related. The more stable a material, the less efficient it is at water-splitting. An efficient water-splitter typically is very unstable. So unfortunately this simple reaction isn’t as simple as we would like it to be…

Why So Special?

Nocera’s new device however appears to get around some of the problems mentioned above. In essence it is a silicon based solar cell, with an oxygen evolving catalyst on one side and a hydrogen evolving catalyst on the other. The hydrogen catalyst is unfortunately not yet published as I assume they are still attempting to secure the patent on it. All I know is that it is nickel based. The oxygen catalyst however is published and is something rather remarkable. It avoids this stability problem, and a few other problems in an interesting way. Here are details:

  • It’s not platinum! Typical hydrogen and oxygen catalysts involve expensive and/or rare metals, like platinum. Cobalt is relatively cheap and easy to get in comparison. The other catalyst is nickel based – just as abundant as cobalt. This will hopefully keep the cost of these devices reasonable.

    Relative abundance of elements in the Earth's crust. Note that the scale is logarithmic - Cobalt and nickel (blue) are nearly 4 orders of magnitude (10,000x) more abundant than platinum and gold (in red). Iridium (Ir) is the planet's least abundant element because it can only be found where meteorites have landed!

  • It’s self-healing! Typical molecular-based catalysts have a tendency to fall apart over time, get used up by some side chemical reaction, and need to be replaced. This is problem for something that you’d want sitting out in the sun for years. You don’t want to be out there every other week replacing expensive catalysts. The self-healing ability arises from the proposed mechanism wherein the catalyst molecule is continually dissolved and re-formed on the surface of the electrode as part of the catalytic cycle. This is how this catalyst is able to side-step the degradation problem found in other water-splitting materials.
  • It is selective for oxygen evolution. This means that the catalyst will create oxygen over other products. For instance, if we take a look at the table of standard reduction potentials  – which is just a measure of how much energy it takes to do the given reaction – we find:

    O2 + 4H+ + 4 e  is in equilibrium with 2 H2O = 1.23 V
    Cl2 + 2 e  is in equilibrium with 2 Cl = 1.36 V

    The first one is the reaction that we would like to have happen (going from right to left), we use up water and make oxygen gas. However, the second reaction uses up chlorine ions to make chlorine gas. The energy it takes to make chlorine gas is very similar to the energy it takes to make oxygen, only differing by 0.13 Volts. This means that if there is chlorine present in your water when you are trying to make oxygen, you’ll probably waste some energy making chlorine instead of oxygen. One way around this is to deionize your water, but this is expensive and impractical if one wants to scale up to large volumes of water-splitting. The nice thing about this cobalt catalyst is that it creates oxygen selectively over creating chlorine! The company claims that they can use water pulled out of the Charles River with this catalyst to create oxygen without creating chlorine – which is frankly pretty ridiculous, but amazing if true! No one wants to inhale chlorine gas when they are expecting oxygen!

    You can see how well this catalyst works by looking at the figure below. The red line shows how much oxygen was actually produced by the catalyst when a current was applied, and the blue line shows the theoretical maximum amount of oxygen one could get out of that applied current. The lines lie pretty close together indicating that the catalyst performs remarkably well!

This graph shows the efficiency of the cobalt oxygen catalyst. The Y-axis is the amount of oxygen produced, and the X-axis is time in hours. The red line is the actual data collected while the blue line indicates the theoretical maximum amount of oxygen produced. Note that this was accomplished by applying a current to the catalyst, not by using sunlight. Regardless the catalyst works at near 100% efficiency! (Source: Ref #3)

So Energy Crisis Solved Then?

Not so fast. The device is still in a prototyping stage. While all of the items mentioned above show promise from a scientific and technical standpoint, that doesn’t mean that this is a sure thing. For instance, any reasonable solar technology will need to have a lifetime of 15-25 years or so for any investment to make sense. Because this is a new technology, it just hasn’t been around long enough to do those kinds of long-term testing to see if it holds up in real world situations. Secondly, our economy is based on electricity and gasoline – not on hydrogen. While this could be a great device to deploy to developing countries coupled with an easy to use hydrogen burning generator – it would probably only see limited use in the developed world. Also not mentioned in the press release is the full efficiency of the device – i.e. how many of these things or how large of an area needs to be occupied by them in order to provide enough power for a small home or a water pump? If you need 25 square meters of these things just to keep the lights on it’s probably not going to be worth it.

BUT – it still looks promising! Just don’t cancel your utilities quite yet…


References

FUJISHIMA, A., & HONDA, K. (1972). Electrochemical Photolysis of Water at a Semiconductor Electrode Nature, 238 (5358), 37-38 DOI: 10.1038/238037a0

Khaselev O, & Turner JA (1998). A monolithic photovoltaic-photoelectrochemical device for hydrogen production via water splitting Science (New York, N.Y.), 280 (5362), 425-7 PMID: 9545218

Kanan, M., & Nocera, D. (2008). In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+ Science, 321 (5892), 1072-1075 DOI: 10.1126/science.1162018

Lutterman, D., Surendranath, Y., & Nocera, D. (2009). A Self-Healing Oxygen-Evolving Catalyst Journal of the American Chemical Society, 131 (11), 3838-3839 DOI: 10.1021/ja900023k

7 responses »

  1. Pearl says:

    I’m surprised they haven’t published efficiencies…I thought that was one of the fundamental dimensions that devices were compared across.

    • Paul Vallett says:

      I agree. Their press release was light on details and seemed more like a way to build up hype for funding. They claimed it was “10x as efficient” as photosynthesis in a plant – but those are ~0.1% efficient at best, so at best his device is ~1% efficient… (except for sugarcane – 8% efficient!)

  2. Ian Stewart says:

    I think this is the right direction to go in, only, why hydrogen? It is difficult to store. Catalysis to create some low-weight hydrocarbon would be much better, both because the end product is a much more portable and energy-dense fuel, but also because generation of a hydrocarbon sucks CO2 out of the atmosphere. Ethylene is my top candidate because it is a feedstock for many industrial chemicals. I know your thrust is towards inorganic rather than organic catalysis but it is worth remarking that many animals and plants produce ethylene – so there must already be natural enzyme pathways to create it.

    • Paul Vallett says:

      I’ll be the first to admit that hydrogen has problems – storage like you mention is difficult. If one stores it at room temp and atmospheric pressure, it’s going to take up a huge amount of space. If one is going to pressurize it, that’s going to cost energy.

      I think people like the hydrogen route because the feedstock (water) is so ubiquitous, the promise of highly efficient fuel cells is encouraging, and the chemistry seems (at the surface at least) to be simple. I am sure there are lots of other options, like ethylene, that are small molecules with simple chemistries as well that could be good candidates. Maybe the funding-powers-that-be have just decided to focus on hydrogen? That’s a valid thing to point out though, hydrogen isn’t our only option as a small energy storage molecule.

      Just to point out though – there are natural enzymes that split water as well, such as the oxygen evolving complex but as far as I know researchers haven’t had much success using it in devices. I guess my point is that just because nature can accomplish it, that doesn’t make it any easier for us!

      Thanks for your comments!

  3. […] Lectures ← Split That Water – Part II […]

  4. Matt says:

    I like the way you explain things. Can I ask you some questions? I’m an 8th grader doing a science project on catalysts to make oxidation of water more efficient. Can you explain what this equation means? O2 + 4H+ + 4 e− 2 H2O = 1.23 V
    And what does this mean? 2  H2O + 2 e− H2(g) + 2 OH− -0.8277 V
    I mean, I know what the formulas mean, and the e- are electrons and the g means gas. But what does the volts mean? thanks!

    • Paul Vallett says:

      The Volts correspond to how much energy it takes to do the reaction in water – relative to a reference reaction. In this case the reference reaction is that of the Standard Hydrogen Electrode: (http://en.wikipedia.org/wiki/Standard_hydrogen_electrode)

      The SHE is: 2H+ + 2e- —> 2H2 which we say has a potential of 0 volts.

      The other two reactions tell you how much energy is needed to do those two reactions relative to the SHE standard reaction.

      So for instance you’ll need to provide a potential of 1.23 volts versus the SHE at minimum in order to split water to make oxygen gas and four H+.

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