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Molecular shuttle speeds up hydrogen production by the photocatalytic splitting of water

15 August 2014

In their latest experiments with semiconductor nanocrystals as light absorbers, physicists led by Professor Jochen Feldmann (Ludwig-Maximilians-Universität München, LMU Munich), in collaboration with a team of chemists under the direction of Professor Andrey Rogach (City University of Hong Kong), have succeeded in significantly increasing the yield of hydrogen produced by the photocatalytic splitting of water.

The crucial innovation, reported in the latest issue of the journal Nature Materials, is the use of a so-called molecular shuttle to markedly improve the mobility of charge carriers in their reaction system.

The apparent quantum yield and the formation rate under 447 nm laser illumination exceeded 53% and 63 mmol g−1 h−1, respectively. The fast hole transfer confers long-term photostability on the system and opens new pathways to improve the oxidation side of full ​water splitting.

—Simon et al.

The amount of solar radiation that reaches the Earth in a year exceeds current annual energy needs by more than 10,000-fold; however, it is not yet possible to store sufficiently high amounts of solar energy in an efficient way. One approach is to utilize incoming solar radiation for the photocatalytic generation of molecular hydrogen (H2) from water.

When a quantum of light (a “photon”) with sufficient energy excites a semiconductor nanocrystal, it produces a negative charge (electron) and a positive charge (hole). Photocatalytic synthesis of hydrogen gas from water requires the transfer of electrons to the hydrogen, while the holes interact with the oxygen or are scavenged by other molecules.

However, before any of this can happen, the photogenerated electrons and holes must be quickly separated from each other. If the semiconducting nanocrystals are decorated with nanoparticles of a metal catalyst—such as the precious metal platinum—the electron can rapidly transfer to the metal and hydrogen production ensues.

However, unless the positively charged holes are effectively removed, they will accumulate and eventually bring H2 synthesis to a halt.

One problem for an efficient removal of holes is the need for polar molecules ot be attached to the nanocrystals as surface ligands in order to make the nanocrystals water-soluble. By doing so, however, the resulting “ligand forest” of the attached polar molecules makes it difficult for the holes to interact with water or larger scavenger molecules.

An analogy is the delivery of airline passengers to their final destination. Spatial constraints make it impossible for the aircraft to convey its passengers directly to their hotels in town. Instead, smaller and more maneuverable carriers, such as the shuttle buses, are used for the short last stage of the trip.

In a similar way, the research teams in Munich and Hong Kong hit on the idea of using one of the smallest constituents of their system—the hydroxyl ion formed by the dissociation of water—to penetrate the ligand forest, collect the holes from the surface of the crystals and transport them to a larger acceptor molecule.

The concentration of this molecular shuttle in the system can be easily controlled by altering the pH of the solution. Indeed, raising the pH of the solution drastically increases the rate of hydrogen production.

I was amazed the first time I tried it. As soon as I increased the pH I could see, with the naked eye, bubbles of hydrogen rising to the surface.

—Thomas Simon, a PhD student at Professor Feldmann’s chair

The new system also has other advantages besides the increase in yield. First, its long-term stability could be markedly improved. Furthermore, it turns out that the costly platinum catalyst can be replaced by nickel, a far less expensive metal.

The discovery of this new mechanism could lead to entirely new approaches to the photocatalytic production of hydrogen.

— Dr. Jacek Stolarczyk, head of the Photocatalysis group, chair of Photonics and Optoelectronics (PhOG) at LMU

Resources

  • Thomas Simon, Nicolas Bouchonville, Maximilian J. Berr, Aleksandar Vaneski, Asmir Adrović, David Volbers, Regina Wyrwich, Markus Döblinger, Andrei S. Susha, Andrey L. Rogach, Frank Jäckel, Jacek K. Stolarczyk and Jochen Feldmann (2014) “Redox shuttle mechanism enhances photocatalytic H2 generation on Ni-decorated CdS nanorods,” Nature Materials doi: 10.1038/nmat4049

August 15, 2014 in Catalysts, Hydrogen Production, Solar, Solar fuels | Permalink | Comments (7) | TrackBack (0)

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Comments

Hurry-up hydrogen, im awaiting a breakthrough like this since a long time. Till 1973 we need something else then petrol-gasoline-diesel, hydrogen can be used almost in every vehicles. I hope they will put this method in use for real and bring down the cost of hydrogen at the pump.

Are we one step closer to the magic bullet of energy independence, using sunshine to split H2O into its component elements?

If the Solar Hydrogen Trends announcement is accurate, it will be far and above the production this implies.

Im getting old but im 53 y old. I always taught that some day I will get rid of polluting atmosphere with my car. Im lucky for electricity as here in quebec Canada our electricity is cheap and come almost 100% from hydro-electricity contrary to numerous places where it come from coal, nat gas or nuclear. Im ready today to witness a change from polluting petrol to clean renewable hydrogen for cars and trucks and electricity generation. Please begin right now to produce and sell this clean hydrogen everywhere so I can witness for myself. Petrol should be avoided right now cause it's costly and polluting. Please put this projet on top of priority list and also there is not only fuelcell cars that can use hydrogen but also ice engines that can be fueled by hydrogen and that would be less costly. Cover the desert of these solar panels and start producing clean electricity good for eon of time, it will even not consume water, this will be a marvel. Put documentary on tv showing the implementation of this as I will even enjoy more this invention. Show the demise of petrol and I don't bother that my actual car a gasoline dodge neon 2005 lose it's value because of a technology change, it's only value at 1500$ actually. Anyway the price of gasoline might shrink so I will use my neon for another 4 to 5 years. Im starting counting the weeks till this invention hit the market, there is no reasons that I will not be put in use, it will be money making.

53% efficiency is only at 447nm.

What's the efficiency for the solar spectrum?

I doubt this strategy will ever be more efficient (and certainly will ever be cheaper) than [photovoltaic + electrolysis].

The flexibility of the latter is also a major advantage: electricity when needed, H2 when electricity in excess.

I would not be so sure, PV operates in a narrow band, this is why it is 15-30% efficient over all. Even WITH 30% concentrated PV and positioning, you have 60% electrolysis for 18% efficiency now.

Both, water and sunlight are free and plentiful.

Even at low efficiency, the H2 produced could be stored, distributed and used to supply e-energy for all uses.

CPPs, NGPPs, NPPs etc could be progressively phased out.

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