Researchers Developing Nanotube Arrays to Produce Hydrogen From Visible Light
15 August 2007
 |
| An FESEM image of a Ti-Fe-O nanotube array. Click to enlarge. |
A research group headed by Professor of Electrical Engineering Craig Grimes at Penn State University is developing an inexpensive and easily scalable technique for water photoelectrolysis—the splitting of water into hydrogen and oxygen using light energy.
In a paper published online in Nano Letters, lead author Gopal K. Mor, along with Haripriya E. Prakasam, Oomman K. Varghese, Kathik Shankar, and Grimes, describe the fabrication of thin films made of self-aligned, vertically oriented titanium iron oxide (Ti-Fe-O) nanotube arrays that demonstrate the ability to split water under natural sunlight.
Previously, the Penn State scientists had reported the development of titania nanotube arrays with a photoconversion efficiency of 16.5% under ultraviolet light. Titanium oxide (TiO2), which is commonly used in white paints and sunscreens, has excellent charge-transfer properties and corrosion stability, making it a likely candidate for cheap and long lasting solar cells. However, as ultraviolet light contains only about 5% of the solar spectrum energy, the researchers needed to finds a means to move the materials band gap into the visible spectrum.
They speculated that by doping the TiO2 film with a form of iron called hematite, a low band gap semiconductor material, they could capture a much larger portion of the solar spectrum. The researchers created Ti-Fe metal films by sputtered titanium and iron targets on fluorine-doped tin oxide coated glass substrates. The films were anodized in an ethylene glycol solution and then crystallized by oxygen annealing for 2 hours. They studied a variety of films of differing thicknesses and varying iron content. In this paper they report a photocurrent of 2 mA/cm2 with a sustained, with a time-energy normalized hydrogen evolution rate of 7.1 mL/W·hr and a photoconversion rate of 1.5%, the second-highest rate achieved with an iron oxide related material.
The team is now looking into optimizing the nanotube architecture to overcome the low electron-hole mobility of iron. By reducing the wall thickness of the Ti-Fe-O nanotubes to correspond to the hole diffusion length of iron which is around 4nm, the researchers hope to reach an efficiency closer to the 12.9% theoretical maximum for materials with the band gap of hematite.
As I see it, we are a couple of problems away from having something that will revolutionize the field of hydrogen generation by use of solar energy.
—Craig Grimes
Resources:
Mor, G. K.; Prakasam, H. E.; Varghese, O. K.; Shankar, K.; Grimes, C. A.; “Vertically Oriented Ti-Fe-O Nanotube Array Films: Toward a Useful Material Architecture for Solar Spectrum Water Photoelectrolysis”; Nano Lett.; 2007; 7(8); 2356-2364. DOI: 10.1021/nl0710046
Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A.; “Enhanced Photocleavage of Water Using Titania Nanotube Arrays”; Nano Lett.; 2005; 5(1); 191-195. DOI: 10.1021/nl048301k
Can You all tell me what that means in American?
Posted by: Hillbilly | 15 August 2007 at 12:47 PM
It means give me another $10 million so I can mess around for another 10 years and still not come close to anything that cracks open the laws of physics and makes it sensible to make hydrogen. The H2 gravy train is spluttering a bit, who hears of the hydrogen highway these days....hence the continuing need to manufacture spurious breakthroughts.....
In a world short of energy, with high energy prices, we cannot afford the energy idiocy of making hydrogen. Solar and wind are great, but use the energy to displace fossil fuels such as coal and gas. QED.
Posted by: John Baldwin | 15 August 2007 at 01:01 PM
This is all interesting science, but keep in mind that the competition is PV panels feeding electrolyzers, a combination now at some 15-30% efficiency and climbing with huge advantages of scale and wide applicability. Efficiency is even higher if you feed PHEVs instead of hydrogen vehicles. Plus you don't need to mess with clear fluid-circulating panels bubbling explosive gases, preventing deposits to maintain optical clarity etc. Let's not let this embryonic research contribute to our inattention to (disinvestment in) this here-and-now solution, or others.
Posted by: P Schager | 15 August 2007 at 01:26 PM
On the plus side: Hydrogen is a very clean way to store energy for later use...say at night or during the winter, if your enclosed photoreactors are near a gas turbine (or eventually fuel cell) based power plant. A stationary plant could probably overcome most of the H2 containment issues with very large, low pressure storage tanks.
On the negative side: does the water have to be dimineralized to avoid silt and scale clogging the nano tubes? If so, there goes some of the energy. Then you calculate the cost of moving the water, purifying the water, the inherent energy cost of the photoreactors, the inherent energy cost of the tanks, the energy cost of pressurizing/storing the Hydrogen, and the cost of the High voltage wires to get from the the cheap sunny land to the expensive urban places the power is needed.
Some of these problems are shared by photovoltaics, and come on, solar is the coolest energy answer...but the conversion efficiency probably has to get quite a bit higher and the production cost pretty reasonable before this scheme starts to make sense at scale.
Posted by: HealthyBreeze | 15 August 2007 at 01:33 PM
John:
It appears that you, like myself are getting a bit tired of reading about all the future wonders. While this is really interesting stuff, I would like to see some of this Pie Sky stuff put into production. And, we spend too much on research with few returns on the investment. I would like to see grant money incentives if the research can be brought to produce a useful product. The research on LiIon batteries is a good example. Lots of research for the last ten years; however, so far all I see is batteries for power equipment and laptops. Even Tesla uses laptop cells. Surely someone has solved the safety issues by now and is ready to take a chance on large format PHEVs and BEVs. Times a' wasting and there's money just waiting for the risk-taker. Worse case bankruptcy, best case...billions.
Posted by: Lad | 15 August 2007 at 01:41 PM
Even the maximum theoretical efficiency at 12.5% would be too low in comparison to typical PV at 15-20% efficiency, or concentrated PV at 30-40% efficiency.
High-temp electrolysis is still the key to efficient H2 synthesis.
Using ammonia as a H2-carrier seems to be a promising route that promises to solve the difficulty in H2 transportation and storage. Liquid ammonia can be stored at room temperature and at only 8 atm of pressure, at 1/2 the energy density of gasoline. If an H2-vehicle has twice the mpg as a gasoline car, then the fuel tank can be of the same size to get the same range. (See previous GCC articles)
Ultra-lean burning H2-ICE seems to be a promising FC substitute, with thermal efficiency estimated to be near 50% (see previous GCC articles). Using Ammonia as a fuel source, you can use the ammonia to reduce the NOx in a SCR without having to worry about an urea storage tank on board.
Don't count out the Hydrogen economy as yet. It may come a lot sooner than you think, Big Oil permitting, that is!
Posted by: Roger Pham | 15 August 2007 at 01:52 PM
The only way a hydrogen economy will fly is if Big Oil invests a bazillion dollars in it. They might just do that to keep their monopoly on transportation fuels. The path of least resistance is HEV->PHEV->(BEV &/| FCEV &/| ????).
Posted by: Neil | 15 August 2007 at 02:17 PM
I agree very much with those who think H2 is wasted on cars, but it's going to be a mighty long time before anyone figures out how to get 747s or container ships to run on batteries for any useful distance (as in across an ocean). It's also far from clear that batteries will ever be viable for utility-scale energy storage from intermittent sources such as wind or PV. H2, despite its well-known conversion inefficiencies, may well be one of the primary solutions to both of these problems, and any research devoted to figuring out how to make it from a renewable source is well worth the effort. The stated 12.5% theoretical limit on this particular process may turn out to be only a milestone on the way to better results further down the road with different materials as more knowledge is gained from these efforts.
Some posts on this thread have claimed that Grimes' procedure is less efficient than existing silicon PV/electrolyzer technology. A more detailed explanation would be welcome. It's not clear to me that Grimes' photolytic conversion percentages mean the same as PV conversion percentages, and I would also imagine that there is a significant conversion inefficiency in the electrolysis. Besides which, don't some/all electrolysis processes require additional ingredients (either as reagent or equipment) that may or may not be rare and not easily scalable?
Posted by: Roger Davis | 15 August 2007 at 02:30 PM
Everyone seems focused on the conversion rate rather than the cost. Low PV conversions could be cost effective if the production costs are low enough. It seems the same here. I also agree that we should be looking at hydrogen, not as a fuel for vehicles, but a storage medium for intermittent renewables like solar and wind.
Posted by: JMartin | 15 August 2007 at 03:15 PM
JMartin: You have a good point. People have become obsessed with well-to-wheels efficiency because they have operated for so long in the oil-to-ice paradigm where the object was to get the most from a high energy but limited resource. It's even built in to the description of the measuring stick.
A re-adjustment in thinking is necessary when dealing with an unlimited resource that is difficult to harvest. The new measure for solar and other renewable sources needs to incorporate overall cost, availability of required materials and environmental impact.
Posted by: HenryP | 15 August 2007 at 04:25 PM
Storing intermittent alternative energy sources like solar wind and water, could be done with Sodium-sulfur and/or Vanadium redox batteries, with greater efficiencies and economics then hydrogenm, Not to mention grid energy storage pilot plants in the megawatts using those type of batteries have been built (and not one using hydrogen) Also I should mention smart recharging of EVs.
Posted by: | 15 August 2007 at 10:31 PM
Roger Davis,
Storing intermittent power from wind and solar in BEV/PHEV is utility scale!
The installed power capacity in American cars is some 20 times larger than power capacity in power plants and the energy amount sitting in gas tanks overnight is on the order of 10-30 days worth of power production.
Of course, future electric vehicles would have to use electricity much more efficiently and batteries are not likely to capacities exceeding 20-30 kWh anytime soon. Still that's about one days worth of electricity consumption for a family.
Plus there are many other ways of tackling "intermittency", such as long-distance power lines, flexible consumption, hotwater storage for driving your dish washer/washing machine/dryer, etc.
Charging PHEV's on the company parking lot could certainly make a huge dent in the "intermittency problem". No need for seperate utility-scale battery solutions.
Ships could go back to using coal for propulsion and airplanes can use biofuel. Hydrogen, however, is an extremely poor fuel for airplanes. A jumbo jet would need a liquid hydrogen tank as large as the plane itself!
The only places where producing hydrogen by electrolysis (or TiFeO nanotubes) is in processes where hydrogen is required, such as ammonia production or crude oil upgrading, where huge amounts of natural gas is used today for conversion to hydrogen.
Posted by: Thomas | 16 August 2007 at 03:57 AM
A lot could be done with better interconnected power grids in Canada-USA and in Russia-Western Europe.
Our local hydro system can produce about 40,000 megawatts for extended periods. Most of the capacity is required on cold winter days (heating) but the surplus is as much as 25,000 megawatts during the other three seasons. High consumption periods in southern USA States are opposite to ours. Better power sharing would help to even the load on both sides of the border.
Another 40,000 megawatts of hydro power could be installed (about 10,000 are currently being planned or installed) and another 75,000 in wind power could be co-located or installed near the power lines.
Hydro plants and Wind mills make excellent partners because energy can easily be stored in the huge hydro water reservoirs during high wind production periods. Hydro plants can easily be over-equipped to produce much more to meet higher demands during peak load periods. In other words, huge water reservoirs are the best batteries we have. Why not use them more.
Posted by: | 16 August 2007 at 08:43 AM
All this research of nanotechnology for electricial generation is great for battery development.
Posted by: Henry | 16 August 2007 at 11:46 AM
Utility-scale energy storage in batteries of any kind exists only in the vision of posters to sites such as these. I share that vision to a certain degree but the fact is that no one has yet produced any of the next generation BEV/PHEV batteries we are reading about in any kind of volume at an affordable price with demonstrated lifetime and reliability, let alone tested V2G with anything more than a half-dozen cars. To my knowledge flow batteries have only been used to date at relatively modest scale for such things as load levelling and backup for fairly small wind installations. It's a long way from there to providing full power for a major city for any length of time. It's also far from clear that there is enough lithium/vanadium/whateverium in the world to fully implement any battery-based strategy on a meaningful scale. Pumped hydro backup requires particular terrain and/or geological formations which are not always available (probably not where I live on O'ahu where the
rock is crappy nor in ideal wind locations such as offshore or the US midwest plains).
It's certainly true that no one has ever implemented H2 backup on a large scale either, but it's a possible solution that would be foolish to ignore. Even the most diehard doomers are not yet warning about peak seawater, and H2 generation would be an excellent match with offshore wind and OTEC where other backup solutions are particularly ill-suited. It's unfortunate that all of the misinformed hype about H2 in cars has given it such a bad name in these discussion groups that many here just flame on as soon as they hear H2 mentioned in any context whatsoever, even those where it might make sense.
Posted by: Roger Davis | 16 August 2007 at 12:54 PM
I certainly second the opening comment that this looks like a tease, sort of like NASA finding life on Mars every year or so.
And I also want to point out that strengthing our power transmission system would allow us to build fewer power plants, and if they were nukes, they could be built in more remote locations where evacuation is actually feasible. It would also allow us to curtail the coal burners somewhat because we could utilize the greenest power from a larger area.
Posted by: Van | 16 August 2007 at 01:41 PM
If there's a showstopper for hydrogen-fueled aircraft, it's more likely the cost and safety issues of liquid hydrogen handling at airports.
Posted by: Roger Arnold | 16 August 2007 at 10:36 PM
If there's a showstopper for hydrogen-fueled aircraft, it's more likely the cost and safety issues of liquid hydrogen handling at airports.
Posted by: Roger Arnold | 16 August 2007 at 10:37 PM
Roger Davies,
You don't have to pump water back into the reservoirs. Use wind power directly (as main power source). During high wind production, reduce or turn down the water turbines output and let the water reservoirs fill up. Fuller reservoirs represent higher power production potential and can be used more efficiently during peak power demand periods.
Since water turbines can easily be tuned (adjusted) to variable loads or demands they can be used to compensate for wind mills variability and for load (demand) variations.
Wind mills do not have to be colocated with hydro plants. They can be connected just about anywhere on the power grid. Of course it is more economical to locate your wind mills close to existing high voltage power lines to avoid the installation of new costly power lines.
The best winds are in the northern part of Canada along the Labrador and Hudson Bay coasts where more hydro plants are being installed. Hydro and Wind could share the same power lines. Maximum quality winds are during the cold winter months when water reservoirs are lower and more electricity is required for heating.
A very good combination.
The wind power potential in that area is evaluated at 75000 to 95000 magawatt. Since the quality of the wind is very high, the average wind power production could be as high as 50% (and even more) with large (5 to 10 megawatt each) low speed units installed on higher towers (100+ meters)
There is a golden opportunity to produce enough clean power to operate 50+ million PHEVs or BEVs for generations to come.
Posted by: | 17 August 2007 at 10:56 AM