http://www.treehugger.com/files/2010/03/breakthrough-producing-hydrogen-water-sunlight.php Breakthrough Producing Hydrogen from Water + Sunlight
by Christine Lepisto, Berlin on 03. 7.10
Science & Technology (alternative energy)
Sunlight + Water = Hydrogen Gas
Scientists at the University of East Anglia, led by Dr. Thomas Nann, report a
http://www3.interscience.wiley.com/journal/123275459/abstract?CRETRY=1&SRETRY=0">breakthrough in the production of hydrogen from water using the energy of sunlight. Amidst all the hype about a potential hydrogen economy, which would rely upon the highly energetic and clean burning hydrogen atom, one of the big questions has been whether sufficient hydrogen can be produced without using yet more energy to create the hydrogen. Typical production methods include stripping hydrogen from other fuels like methane or using electrolysis to split the hydrogen out of water. But with efficiencies between 20 and 40% for producing energy from traditional photovoltaic processes, the hydrogen economy cannot be solar powered. Or can it?
The Hydrogen Tipping Point?
In fact, producing H2 is so energy intensive that some have even referred to hydrogen as more "battery" than fuel. Breakthroughs in the generation of hydrogen from solar power could tip the balance in favor of hydrogen fuel cell technologies. Enter Thomas Nann and colleagues. They report 60% efficiency for a process in which hydrogen is produced from water by the photons in light that strike a specially designed submersed electrode.
The concept of
http://www.treehugger.com/files/2007/08/water_sunlight.php">water + sunlight = hydrogen is not new. But turning 60% of the energy in light into hydrogen power is. The trick lies in the nanophotocathode used by Nann's team. A gold electrode coated with nanoclusters of indium phosphide absorb incoming photons of light (that is the wavy line marked "hv" in the image). The nanoclusters then pass electrons liberated by the sun's energy into an iron-sulfur complex which acts like a match-maker between the negatively charged electron and a hydrogen proton in the surrounding water molecules. Gaseous hydrogen results.
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From the paper: (DOI: 10.1002/anie.200906262)
Water Splitting by Visible Light: A Nanophotocathode for Hydrogen Production
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Herein we show that an inexpensive and environmentally benign inorganic light harvesting nanoarray can be combined with a low-cost electrocatalyst that contains abundant elements. This system provides a stable photoelectrochemical platform for hydrogen production. The device is constructed by first building-up a cross-linked indium phosphide (InP) nanocrystal array layer by layer and then incorporating an iron-sulfur electrocatalyst. Iron-sulfur carbonyl assemblies related to the subsite of -hydrogenase have been shown to electrocatalyze the reduction of protons to dihydrogen under dark conditions at potentials between -0.7 and -1.4 V versus the standard calomel electrode (SCE) in non-aqueous electrolytes.<7>, <8> Of these assemblies, we chose |Fe2S2(CO)6|, which has sulfide bridges that are potentially capable of binding to indium as a catalyst for photoelectrochemical reduction of protons in a solid-state assembly, and a modest reduction potential of -0.90 V versus SCE. With this system, we were able to achieve a photoelectrochemical efficiency of more than 60 %, which is a major breakthrough in this field.
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A photocurrent could be sustained for at least one hour without degradation at a bias potential of -400 mV (in practice, a slight enhancement of I was observed at longer times), thereby demonstrating the robustness of the system. We used a closed cell, as depicted in Figure 6, for the bulk electrolysis, which allowed us to analyze the gas headspace by gas chromatography. The right hand side of Figure 6 shows the energy diagram of the photocathode and the suggested mechanism for the photocatalytic reduction of protons. After the passage of 2.58 C (±10 %) of charge, we detected a substantial quantity of 16.2 nanomoles of H2 in the headspace above the electrolyte by gas-chromatography, which corresponds to a current yield of approximately 60 %. Control experiments did not show any production of hydrogen.
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The mechanism of the H2 photoproduction probably involves the absorption of incident light by the InP nanocrystals and excitation of an electron into the conducting band of the nanoparticles. Subsequently, electrons are transferred from the conduction band, which lies at about -1.0 V versus Ag|AgCl,<14> into the LUMO of the catalytic subsite at circa -0.90 V versus Ag|AgCl, thereby effecting the reduction of protons. The intimate mechanism of the electrocatalytic hydrogen is likely to involve Fe---H and/or S---H intermediates.<15>,<16> The holes generated in the valence band of the nanocrystals are immediately filled by electrons available from the underlying gold substrate held at -400 mV versus Ag|AgCl. The mechanism is depicted on the right hand side of Figure 6.
In conclusion, we have discovered a robust and efficient system for the photoelectrochemical production of hydrogen. The system does not rely on excited states of organic molecules/ligands, unlike most of state-of-the-art approaches. The whole system comprises inexpensive and non-toxic elements, and may be a promising alternative for the inexpensive production of hydrogen.
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