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Researchers Achieve Unprecedented Efficiency in Photoelectrochemical Production of Hydrogen from Water: 42%

Gratzl
HR-SEM images of Fe2O3 films on SnO2:F-coated conducting glass. (A) Cross-section of 500 nm thick mesoporous Si-doped Fe2O3 on 400 nm thick compact SnO2:F. (B) Top view (45° tilted) of the Si-doped Fe2O3 film. (C) Top view (45° tilted) of an undoped Fe2O3 film. Click to enlarge.

Michael Grätzel and his colleagues have developed a device that sets a new benchmark for efficiency in splitting water into hydrogen and oxygen using ordinary sunlight. The research will be published in the 13 December issue of the Journal of the American Chemical Society.

Previously, the best water photooxidation technology had an external quantum efficiency of about 37%. The new technology’s efficiency is 42%, which the researchers term “unprecedented.” The efficiency is due to an improved positive electrode and other innovations in the water-splitting device.

Grätzel and collaborators developed the Grätzel Cell, a dye-sensitized photoelectrochemical cell that uses photo-sensitization of wide-band-gap mesoporous oxide semiconductors. The work originally appeared in a paper in Nature in 1991.

As reported in the Nature article, the original overall light-to-electric energy conversion yield of a Grätzel cell was 7.1–7.9% in simulated solar light and 12% in diffuse daylight.

Iron oxide (-Fe2O3, or hematite) is an especially attractive photoanode due to its abundance, stability, and environmental compatibility, as well as suitable band gap and valence band edge position. Unfortunately, the reported efficiencies of water oxidation at illuminated hematite electrodes are notoriously low.

Grätzel and his team tackled that in this most recent work by producing Fe2O3 photoanodes via deposition of silicon-doped nanocrystalline hematite films by APCVD (atmospheric pressure chemical vapor deposition).

The result was a highly developed dendritic nanostructure of 500 nm thickness having a feature size of only 10-20 nm at the surface. The dendritic nanostructure minimizes the distance photogenerated holes have to diffuse to the Fe2O3/electrolyte interface in a film that is thick enough for strong light absorption.

The efficiency is further enhanced by deposition of a thin insulating SiO2 layer below and a cobalt monolayer on top of the Fe2O3 film.

Under illumination in 1 M NaOH, water is oxidized at the Fe2O3 electrode with higher efficiency (IPCE [incident photon to current efficiencies] = 42% at 370 nm and 2.2 mA/cm2 in AM 1.5 G sunlight of 1000 W/m2 at 1.23 VRHE) than at the best reported single crystalline Fe2O3 electrodes.

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Comments

George

Mark asks: How much water does it take to make a pound of Hydrogen? When it burns the 'waste' byproduct is mostly water vapour, assuming it's pure. Is there a net water loss? How much energy does it take?

In laymans terms please.

To create one pound (454 g) of Hydrogen gas you would need about 4 litres of water. You would also create about 9 pounds of oxygen gas in a typical electrolysis scheme. Aside from hydrogen leaks which should be minimal in a well designed system, you should get essentially all of the water back when the hydrogen is oxidized.

Electrolysis of water requires a significant energy input- more than you get back out when you burn it. The resulting hydrogen costs 7-9 dollars per kilogram. (1kg = 2.2 lb) A kilogram of hydrogen has the energy content of about 1 gallon of gasoline. The high energy cost of creating hydrogen is one of the reasons that some people consider the "Hydrogen Economy" to be a fraud. Gratzel's system, if it is cheap and robust enough (big if), might change that part of the equation.

SJC

"Grätzel and collaborators developed the Grätzel Cell" -
I would say these guys know what they are doing. Consider a PV panel with 15% efficiency used to turn water to hydrogen at maybe 50% efficiency. Abundant solar energy to hydrogen could even be stored and used in a PEM fuel cell at night..

JB

Has anyone thought of using the earth to compress hydrogen? More precisely, what stops a company from running two cables down into the ocean 10,000 feet and supplying the cable with wind powered electricity, thereby producing hydrogen from the ocean at a pressure of 4454 psi. The hydrogen would be collected at depth and piped, at a collection pressure of 4454 psi, up to a surface collection station and further into existing high pressure gas distribution infrastructures.

Cheryl Ho

Since DME has an advantage of decomposition at lower temperature than methane and LPG, R&D for hydrogen source for fuel cell has been carried out. DME has a potential of feedstock for chemicals. DME to olefins is under development in Japan.

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Production of DME/ Methanol through biomass
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