Photoelectrolysis of water is the answer.

[kenvlach]

This is a reporting of developments relevant to 2 earlier threads:
Using solar energy for electrolysis
thread804-46957 and
Electrolysis of water for biosphere
thread404-43898

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’

http://www.delphion.com/details?pn=US04011149__

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.

 

[GregLocock]

8% is a pretty good efficiency. Interesting that they are using TiO2, the old 'organic' pv cells used it as well. Obviously direct conversion to hydrogen is the neatest solution for a fuel cell based power system, so long as you have a way of storing hydrogen, whic seems problematc at the moment.

Cheers

Greg Locock

 

[25362]

This thread is, in fact, addressing two important issues: hydrogen generation, and the exploitation of solar energy. It seems also to me logical to treat them together.

When electricity is not expensive, water electrolytic splitting is an attractive source of hydrogen to synthesize ammonia (Aswan, Egypt) for the manufacture of fertilizers. Other present economical options for hydrogen generation are based on coal and breeder-reactor technologies.

However, solar-energy is non-polluting and free, and its potential exploitation by mankind should be pursued.

One of the attractive by-products of water electrolysis is oxygen. It could be used for partial oxidation of resids or refuse, to recover more hydrogen. Uses for -or disposal of- the CO₂ output could be found, such as with methane reforming. Electrolytic oxygen could also be used for coal gasification as in the reaction:

CH_0.8 + 0.6 H₂O + 0.7 O₂ -> CO₂ + H₂

Other -still competitive- sources of hydrogen are steam + oxygen "oxidation" of hydrocarbons and thermal decomposition of hydrocarbons. But these are not renewable resources and sooner or later will have to be replaced.

Photovoltaic cells with sunlight concentrators (lenses, mirrors) and solar tracking, can directly convert solar-energy into electricity without the intervention of thermal cycles. Its industrial development is promising but still a way to go, especially with regard to harmful emissions and waste products from the electrodes and the like. A possible solution I heard of is their encapsulation.

Hydrogen can be purified from its usual contaminants, CO₂, CO, O₂, N₂, H₂O, CH₄, by shift catalysis, H₂S and CO₂ removal, PSA process, and nitrogen wash.

As for transportation, I read it somewhere that hydrogen transportation for distances beyond ca. 700 km, is even less expensive than electricity's !

Storing hydrogen in underground used nat-gas fields involves using pressures of 100 atm, and then, of course, there is the contamination with nat-gas components. Greg Locock is right in his comments, but is there another practical idea to store large quantities of hydrogen ? Storage of solar energy is another matter. Solar-powered photovoltaic cells can make this possible by the use of dye molecules which are, in fact, redox electrolytes than can drive reactions thermodynamically uphill, thereby storing chemical energy.

Hydrogen has a myriad of uses apart from the production of mechanical clean energy by reacting it with oxygen.
A hydrogen-based economy would lessen pollution, health hazards and costs, and would diminish overall environmental damage.

 

[h2guy]

Hydrogen fuel additive! What is the best way to add pure hydrogen gas to a engine as an additive?

 

[ScotCan]

This is an interesting thread. Over the past 14 months an associate and myself have been experimenting with different electrodes in a water electrolyser in our lab. The commercial electrolysers normally use expensive and elaborate electrodes working at low volts but high amps. Our system is the opposite using fairly high voltage at much lower amperage. We use commercial solar panels to power the electrolyser and in full sunlight clouds of hydrogen gas in fine bubbles collect in our water filled collectors. The start up times are much faster than conventional electrolysers; we use only tap water (we do not need to spike the water with KOH) and the operating temperature is significantly lower than conventional electrolysers. Admittedly the collected hydrogen is "dirty" having impurities which would require to be removed, but, the significance of the electrodes design is that gas generation starts up faster and is about the same efficiency as commercial systems.
This research is based on a stationary project using PV solar panels to power a household and at peak solar input bleed off the surplus power to make hydrogen. This hydrogen would be stored in low pressure (350 psi) tanks for use in a variety of applications - for cooking, barbecueing, heating and as supplementary feed to an IC engined generator
There are storage batteries in the design to act as buffers for the very large variation in solar input from sun up to noon high to sunset, and the generation of hydrogen is essentially to store surplus solar energy in excess of what the batteries need.
We are in the process of scaling this system up in three stages to determine if there are scale effects which may change the characteristics of the system.
Just thought I would let all of you know that outside the mainstream there are experimenters making discoveries which have not (as yet) shown up in the bigger labs.

 

[moltenmetal]

An interesting approach. But I don't understand how your high voltage electrolysis could be as efficient as a low-voltage one, regardless what your electrode looks like.

I'm not an electrochemist, but I understand the reason a person would try to carry out an electrolysis at the lowest voltage possible. Once you've reached the electrode's overvoltage for the generation of hydrogen, the actual amount of hydrogen generated is determined by the current, not the voltage, because current is electrons transferred per second- it's the electrochemical equivalent of flow. Voltage is just driving force (i.e. the electrochemical equivalent of pressure). Just like in a fluidic system, excessive voltage generally ends up wasted as heat.

One way to use the extra voltage is by carrying out the electrolysis at your desired storage pressure (i.e. running the electrolyzer pressurized). This increases the partial pressure (activity) of hydrogen in the cell, which naturally requires additional voltage. Whether or not this approach is more energy efficient than mechanical re-compression of the product hydrogen is something I don't know for sure, though I suspect it to be true. Regardless, running the electrolyzer pressurized will eliminate a piece of mechanical complexity (and the need for future maintenance).

The alternative to stepping down the voltage to give what is required for electrolysis is to put a number of cells in series. This same approach is used in PEM fuelcell stacks to give the voltages required.

As to the direct electrophotochemical generation of hydrogen, it's an area of much interest (not the least of which because it attracts government research grant money). But any of the dye-sensitized systems or ones which rely on organic electron transfer agents are subject to degradation of these materials- and with the cost of these sensitizers and transfer agents to produce and purify, the rate of consumption of these materials in any of the processes I've seen so far exceeds the value of the produced hydrogen by at least an order of magnitude. And unlike a tree, no man-made "plant" to carry out solar photosynthesis of a valuable material like hydrogen will be able to "grow" its own sensitizer to replace what becomes damaged in the process.

 

[Dmitri]

Hi to everyone, pls consider myself as newbie in TiO2 technology.
I’ve been attended to seminar about TiO2 technology, which fascinated me for its easy application and low cost.

My filed is environmental applications, so I was looking for a technology which could replace (if it is possible) the biological treatment for small and high fluctuate flow rates (during summers) with physical-chemical treatment.


I would appreciate if anyone, who is already in this line of business, informs me about applications and problems of TiO2 technology.