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Hydrogen from sunlight, but as a dark reaction; time-delayed photocatalytic H2 production

9 December 2016

A team at the Max Planck Institute for Solid State Research, Germany, and collaborators at ETH Zurich and the University of Cambridge, have developed a system that enables time-delayed photocatalytic hydrogen generation—essentially, an artificial photosynthesis system that can operate in the dark. A paper on their work is published in the journal Angewandte Chemie International Edition.

The system uses a carbon nitride-based material that can harvest and store sunlight as long-lived trapped electrons for redox chemistry in the dark. More specifically, the system comprises a partially anionic, cyanamide-functionalized heptazine polymer, which, in the presence of an appropriate electron donor, forms a radical species under irradiation that has a lifetime of more than 10 hours. This ultra-long-lived radical can reductively produce hydrogen in the presence of a hydrogen evolution catalyst in the dark on demand.

The as-modified graphitic nitride is a yellow solid, which turns blue upon exposure to light. This “blue radical” state contains trapped electrons. The scientists found out that when the light was switched off and a hydrogen-evolution co-catalyst was added, the polymer turned yellow again while producing hydrogen by releasing the stored electrons.

Thus it is possible to decouple the generation of photoinduced electrons from their use, for example, in fuel production, within one single, inexpensive material. This could be a significant advance for the production of storable solar fuels independent of the intermittency of solar irradiation.

The storage of trapped electrons within a carbon nitride backbone may open the prospect for overcoming limitations of the diurnal availability of sunlight for solar fuel production, provided that a scalable photo-oxidation process can be identified. This finding not only reveals a hitherto undescribed property of heptazine-based materials exploitable for specialized applications,but may also inspire rational design of photo-catalytic materials with long-lived, photo-induced states, a challenge that currently limits their applicability. Finally, the long-lived radical can also be prepared electrochemically by applying a cathodic voltage. The continuous charging of the carbon nitride with electrons reveals a capacitor-like function, which suggests that energy storage by this carbon nitride photocatalyst in the form of a “solar battery” may ultimately become possible.

—Lau et al.

Resources

  • Lau, V. W.-h., Klose, D., Kasap, H., Podjaski, F., Pignié, M.-C., Reisner, E., Jeschke, G. and Lotsch, B. V. (2016) “Dark Photocatalysis: Storage of Solar Energy in Carbon Nitride for Time-Delayed Hydrogen Generation” Angew. Chem. Int. Ed.. http://dx.doi.org/10.1002/anie.201608553

December 9, 2016 in Hydrogen, Hydrogen Production, Solar, Solar fuels | Permalink | Comments (13)

Comments

Interesting but no information on efficiency either theoretical or actual.

Produce synthetic low cost gasoline and put that for sale near where i live without tax.

Ok. So you have a Photo-Sensitive polymer that stores photo-ionized electrons with a Photo-Remission Half-Life of 10 hrs.

So, I have to carry this material around and find a suitable catalyst reductant, perhaps H2O, so that I can get some useful Hydrogen from it after reduction.

Isn't this just an extra step?

The kids Play with a Similar Material on Halloween. You either shake-it around, heat it or place it in sunlight to Ionize some unstable electrons. Photo-Remission half life is probably in the minutes and not 10 hours though.

I think they are trying to sell the 10 hour half life and some ability to "Absorb" and react with a Reductant like H2O.

I don't quite get this.
You generate H2 so you can store it, because it is a chemical fuel and you can store lots of it, not like electricity.
So why would you want to delay generating and storing hydrogen ? Why not just make as much as you can, whenever, and use it later when you want it?

Ok, geniuses, this is how one can take advantage of this wonderful discovery:
Expose the chemical to sunlight together with electron donor source, pump away those exposed chemical, then separate out the radicalized molecules in the dark and generate hydrogen and then pump out those de-radicalized molecules again to the light.

What the above has accomplished is the elimination of solar PV AND of electrolyzer which are both costly, and replace with just translucent tubing running on top of any roof to circulate the chemical while being exposed to sunlight.

A potential to eliminate H2 storage tank and the H2 compressor, if the sun-exposed radicalized molecule collected during the day can be turned directly into H2 on-demand later for used in home-based fuel cells that can generate both power and heat. Power to run the house lights and appliances, while the waste heat from the FC unit can make hot water for bathing, laundry and dishwashing.
Since it is easier to store liquids at ambient pressure than to store H2 at high pressures, this can replace the Tesla Powerwall for daily storage of solar energy to be used later in the evening.

I doubt that a significant amount of H2 can be produced from a large amount of (toxic and expensive) radicalized chemicals.

Every chemical advancement has its intrinsic value, but I am quite confident that cheap photovoltaics combined with a cheap (soon without noble metals) electrolyzer will be much more efficient and convenient. Transporting electrons from the photovoltaics to the electrolyzer will always be easier than pumping radicalized chemicals.

In addition, the efficience of the electrolyzer depends largely on the needed overpotential for the redox reactions (that's why Pt is so popular).

Voltage convertors can easily optimise the current/voltage for maximal efficiency. Trying something equal with radicalised chemicals will be quite a challenge.

Also the flexibility of a photovoltaics/electrolyzer combination is key: use the electricity when you need electricity, or when there are batteries to charge. Use the excess electricity to produce H2 when no electricity is needed.

The electrolyzer can also use excess wind or nuclear electricity.

"Ok, geniuses,"
Roger, don't insult.
BTW you used one too many commas.

The only relevant questions should be energy density expected and toxicity.
SJ - I'd be insulted if he did't notice!

Nothing in the article about the cost or useful lifespan of this material (does it photodegrade?).  Is it cheaper to store energy as the excited state of this stuff than as hydrogen?  If so it might solve the evening-peaking demand problem, but you still need the HFC.

Another problem it might solve is production rate of a hydrogen-driven chemical fuels plant.  You wouldn't have to design for peak rate, you could just put the immediate excess of excited material into a tank and keep running the H2 production and reaction process into the evening.

Cost is everything, though.  Yellow material suggests that it absorbs red, blue and green photons.  The cost per watt of output, the useful life and the thermal stability (how much cooling do you need?) are going to be the big factors.

Good post EP. They suggest 10 Hr half life in excited state. How stable is it sitting idle excited or unexcited? How does it mix with various reductants?

Dunno, but I must correct myself:  a yellow material absorbs mostly blue light.

This suggests that you could have a dual-mode panel, with the blue light going to deferred hydrogen production and the rest passing through to conventional PV cells for immediate electric generation.

"limitations of the diurnal availability of sunlight"
wtf? they store electrons during the day and make h2 at night! what's the value of that?

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