Photoelectrolysis of water is the answer.
Photoelectrolysis of water is the answer.
Using solar energy for electrolysis
Electrolysis of water for biosphere
Photoelectrolysis of water is the answer to fuel and pollution problems. Or rather, it will be, eventually.
I rather belatedly learned of the discovery of ‘photoelectrolysis’ by A. Fujishimi and K. Honda, Nature, vol. 238, pp. 37-38 (1971). Water was directly dissociated, into O2 at a TiO2 electrode and H2 at a Pt electrode, by shining a light of energy greater than the TiO2 bandgap (3.1 eV) onto the TiO2 electrode.
“The photoelectric process involves the photogeneration of charge carriers in the semiconducting oxide electrode and the transfer of those carriers across the electrode/electrolyte interface into solution.” – R. Memming, pp. 79-112 in Electrochemistry II, E. Steckham (ed.) (1988).
The minimum photo energy for the process to occur is 1.23 eV, but in practise a small overvoltage is needed. [as reported Feb. 13 in Thread404-43898]
The problem is that the TiO2 bandwidth is too large to make efficient use of the solar spectrum, which spurred a search for other materials meeting the bandwidth requirements. This was due to the “potential of developing passive catalytic generators to produce H2 as a fuel. One vision was of solar photoelectric panels on the rooftops of homes to generate for use in heating and cooling.” SrTiO3 was found to be more efficient, but the process is not yet cost-competitive [as of 1994]. – The Surface Science of Metal Oxides, V. E. Heinrich and P. A. Cox, pp. 286-288, Cambridge University Press (1994, reprinted with corrections 1996).
By 1998, efficiency had been improved to 12% (conversion of sunlight energy) by the U.S. Department of Energy (http://www.nrel.gov/hot-stuff/press/1998/14scienc.html).
“The overall goal of the Department of Energy's Hydrogen Program is to replace 2 to 4 quads of conventional energy with hydrogen by 2010, and replace 10 quads a year by 2030. A quad is the amount of energy consumed by 1 million households in the U.S.
A further improvement to 18.3% was reported in 2000 using a more complex electrode: “A solar-electric cell that stands above an acid bath on electrode legs has converted light to hydrogen fuel with unprecedented efficiency.” -- Licht, S., et al. 2000.’ Efficient solar water splitting, exemplified by RuO2-catalyzed AlGaAs/Si photoelectrolysis.’ Journal of Physical Chemistry B. 104(Sept 28):8920 http://www.sciencenews.org/20000916/fob6.asp
The U.S. Department of Energy homepage for Hydrogen Resources, last updated Feb. 2003, http://www.eere.energy.gov/hydrogenandfuelcells/hydrogen/production.html summarizes the current status (note that it is possible to use low cost, amorphous Si electrodes):
“Multijunction cell technology developed by the PV [photovoltaic] industry is being used for photoelectrochemical (PEC) light harvesting systems that generate sufficient voltage to split water and are stable in a water/electrolyte environment. Theoretical efficiency for tandem junction systems is 42%; practical systems could achieve 18%-24% efficiency; low-cost multi-junction amorphous silicon (a-Si) systems could achieve 7%-12% efficiency. This is one of the advantages of a direct conversion hydrogen generation system. Not only does it eliminate most of the costs of the electrolyzer, but it also has the possibility of increasing the overall efficiency of the process. Research results for the development of PEC water splitting systems have shown a solar-to-hydrogen efficiency of 12.4% lower heating value (LHV) using concentrated light. Low-cost a-Si tandem designs with appropriate stability and performance are also being developed. An outdoor test of the a-Si cells resulted in a solar-to-hydrogen efficiency of 7.8% LHV under natural sunlight.”
A Photobiological process for H2 is also being researched, but seems to be far behind in terms of development (or even actually operating).
And of course, people and companies are trying to become rich via the patent route:
‘US4011149: Photoelectrolysis of water by solar radiation’
In conclusion, the direct photoelectrolysis of water looks to be simpler and less expensive than earlier, more complex designs involving separate units for photoelectric generation of electricity and electrolysis of water. As the efficiency is improved and/or less expensive semiconductor material is used, photoelectrolysis looks to be something that we will all be familiar with in the future. People might have units to create and store H2 for electricity (home and automobile fuel cells) or for heating.