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WHY PEC RESEARCH?
Hydrogen, H2, has the potential to meet the requirements of a sustainable and carbon-neutral fuel in the future, if it can be produced from our Sun, the world's most abundant energy source, and stored and transported safely.
At present, there is still a large gap between our present global energy consumption (around 13 TW), our use of solar energy to supply the world's energy demand (less than 2 %), and the enormous untapped potential of the sun (120'000 TW).
The development of photoelectrochemical cells (PEC) is promoted by increasing public awareness that the Earth's oil reserves could run out during this century. Public concern has been heightened as well by the environmental pollution and the climatic consequences of the greenhouse effect caused by fossil fuel combustion.
Photoelectrochemical cells (PEC) have been shown to directly split water into H2 and O2 (photoelectrolysis of water) thereby providing a basis for a renewable, clean production of hydrogen from sunlight. They rely on a photoactive material (a semiconductor) capable of harvesting and converting solar energy into stored chemical fuel, i.e. hydrogen.
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The challenge arises from being the semiconductors robust in water splitting not responsive to a wide portion of the solar radiation spectrum, yielding relatively low efficiencies of conversion solar-to-hydrogen.
Closing this gap will lead to the development of cheap and efficient systems for hydrogen production directly from sunlight and therefore directly address, not only the conversion, but also the storage issues.
HYDROGEN ECONOMY
Hydrogen is not a primary energy source - such as coal or gas - but is an energy carrier. It could provide a pathway for a gradual transition from a fossil fuel economy to a virtually carbon-free energy future using the key emerging technology of fuel cells for interconversion between hydrogen and electricity. Hydrogen can also be burnt in internal combustion engines.
Very little hydrogen gas is present in Earth's atmosphere. Hydrogen is locked up in enormous quantities in water (H2O), hydrocarbons (such as methane, CH4), and other organic matter. Efficiently producing hydrogen from these compounds is one of the challenges of using hydrogen as a fuel. Currently, steam reforming of methane accounts for about 95 % of the hydrogen produced in the United States. The first widespread utilization of hydrogen in our economy [3] is foreseen in the transport sector, where gasoline is to be replaced with the sustainable fuel. However, this utilization of hydrogen is strongly dependent on the development of mobile storage and conversion technologies. The current storage technology does not satisfy the standards that are set by the US department of energy (DOE) in order to compete with gasoline.
HYDROGEN STORAGE
The energy in 1 kg of hydrogen gas is about the same as the energy in 1 gallon (3.78 kg) of gasoline. A light-duty fuel cell vehicle must store 5-13 kg of hydrogen to enable an adequate driving range of about 500 km. Because hydrogen has a low volumetric energy density (a small amount of energy by volume compared with fuels such as gasoline), storing this much hydrogen on a vehicle using currently available technology would require a very large tank-larger than the trunk of a typical car. Advanced technologies are needed to reduce the required storage space and weight.
Storage technologies under development include high-pressure tanks with gaseous hydrogen compressed at up to 10,000 pounds per square inch, cryogenic liquid hydrogen cooled to -253 °C in insulated tanks and chemical bonding of hydrogen with another material (such as metal hydrides).
COST CHALLENGE FOR HYDROGEN PRODUCTION
The primary challenge for hydrogen production is reducing the cost of production technologies to make the resulting hydrogen cost competitive with conventional transportation fuels. If a 15 % efficient solar cell provides the required potential difference to 70 % efficient electrolyzer, a total conversion efficiency of 10 % can be obtained under optimal illumination conditions. The US DOE has projected that an increase in efficiency of silicon solar cells to 20 % would result in a hydrogen price of 8 $/kg. In order to achieve cost reduction a low cost technology is proposed by Grätzel et al. that is based on the photoelectrolysis of water in a tandem-cell without the use of silicon. In this tandem cell configuration the required potential difference for the water splitting cell to work is provided by a Dye Sensitized Solar Cell (DSC). A conversion efficiency of 10 % is set as a threshold for industrial application of this technology.
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EPFL - ENERGY CENTER
BAC 104 (Château En Bassenges)
Station 5
CH - 1015 Lausanne
Tél. : +41 21 693 02 33
Fax : +41 21 693 00 00
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