Sun shines on an old idea to make hydrogen.
By Bill Scanlo
Back in 1998, when American motorists were paying just over a dollar for a gallon (about 3,8 litres) of petrol, John Turner of the US Department of Energy’s National Renewable Energy Laboratory (NREL) dropped jaws all over the energy world by demonstrating that he could use sunlight to extract hydrogen from water at a remarkable 12,4 per cent efficiency.
Three hundred articles and news reports followed, there were predictions of the next Big Thing, and then… almost nothing happened. Energy companies went back to their petrol and their diesel, or their attempts to extract fuel from biofuels, or to extract hydrogen via algae or coal-powered electricity. Now, with petrol selling at $3,89 a gallon, and still the dominant transportation fuel, there is renewed interest in Turner’s work.
In October, the Japan Society of Co-ordination Chemistry acknowledged Turner with its first Lectureship Award for his pioneering work in the fields of solar hydrogen and fuel cells, and for being an international spokesman for hydrogen production via photoelectrochemical water splitting. “It was quite an honour to be recognised that way,” he said, adding: “It’s what I’ve been doing for 30 years… pushing this technology into new areas that are more fruitful. “People are looking at climate change, looking at solar fuels. The tsunami has people in Japan re-thinking nuclear, plus, they import 90 per cent of their oil. Countries are looking at technologies that give them greater energy independence.”
As so often happens in science, researchers are returning to a breakthrough that had lost momentum. Japan, South Korea and Singapore are establishing advanced artificial photosynthesis centres, and last month, the US Department of Energy announced the availability of R8 million in grants to evaluate technology pathways for cost-competitive hydrogen fuel.
In 2010, the DOE established the Joint Centre for Artificial Photosynthesis at the California Institute of Technology (Caltech) as one of its Energy Innovation Hubs, with the aim of finding a cost-effective method to produce fuels using only sunlight, water and carbon dioxide as inputs.
It’s photosynthesis without the green stuff. Turner uses multi-junction solar cells made mostly from elements in the third and fifth columns of the periodic table, such as gallium, arsenic and indium. These so-called III-V solar cells still work the best.
NREL also funds research through its Laboratory Directed Research and Development Programme aimed at making alloys that combine the benefits of the III-V elements with the strengths of elements such as cobalt, manganese, phosphorous or titanium. The trick is to combine the materials in such a way that they are not only stable and robust, but also have the right band gaps to generate enough voltage to split water.
Turner, who has been with NREL since 1979, says the cleanest way to produce hydrogen is by using sunlight to directly split water into hydrogen and oxygen. The amount of greenhouse gases such a method can save is almost unfathomable. Consider that in the United States alone, 9 million tons of hydrogen are made each year, much of it in petroleum refineries and primarily via a process called steam reforming of natural gas.
For each kilogram of hydrogen produced that way, about 12 kg of carbon dioxide is produced. A process that could replace all that natural gas reforming with the Sun and water can save 100 000 trillion kilograms of carbon dioxide from reaching the atmosphere each year. Hydrogen produced from water and sunlight can replace natural gas reforming if it is made near the refinery and used in place of the hydrogen typically made from reforming.
It won’t be easy. In fact, Turner says that the reason his initial breakthrough lost momentum is that improving the technique “was hard work, and people don’t like to do hard work”. Better technology, the cost of petrol, and worries about nuclear power in the post-tsunami era have combined to make researchers believe water splitting may be worth the hard work. Turner’s pioneering work – and the renewed enthusiasm for it – is leading researchers in several directions.
New, more robust materials show promise
Turner’s NREL colleagues, Dan Ruddy and Nate Neale, are researching new materials that can duplicate the elegance and efficiency of the 1998 breakthrough, but are robust enough to remain stable hour after hour, month after month. They’re getting help from the Centre for Inverse Design, a DOE Energy Frontier Research Centre that uses theory and supercomputing to first come up with the ideal properties of a new material, then find the mix of chemicals that holds those properties.
The aim is the creation of materials that can regenerate even as they disintegrate. It’s something the human body can already do, from replacing plasma every 48 hours to regenerating a new stomach lining every five days. Some metal oxides look particularly promising, Ruddy and Neale say. They’re stable enough, but typically have large band gaps – meaning the gap between the band that can conduct energy and the band that cannot is too wide to allow for efficient conversion of photons into electrons.
Ruddy and Neale are taking hints from some previous work at NREL that combines several layers of semiconducting material into a solar cell that has multiple junctions and thus has ideal band gaps to capture the Sun’s energy during the morning, midday and afternoon. Instead of a junction where two layers meet, as in the case of a conventional solar cell, the water-splitting cell has a junction where a layer hits an electrolyte. “We’re seeing a lot of promise in some of these new materials,” Neale says.
Researchers at MIT are bonding a solar cell to a newly developed catalyst to try to efficiently use the Sun to split water into hydrogen and oxygen. The catalyst of the so-called artificial leaf is made of earth-abundant, inexpensive materials; mostly silicon, cobalt, zinc, nickel and molybdenum. Like the NREL approach, MIT’s needs no external wires or control circuits to operate.
“John Turner’s work was seminal,” says MIT and Harvard chemist Daniel Nocera. “He set a new path for photoelectrochemical cells. Before Turner, the path of photoelectrochemistry was to have materials that absorbed light and did the catalysis of water splitting. Turner separated the function of light collection and charge separation from catalysis in what is now called the ‘Turner cell’.”
Adds Nocera: “The field was slow to pick up on this, but we followed Turner’s lead, and with the development of new catalysts – and by following Turner’s approach – we were able to develop a buried junction comprising earth-abundant materials.”
Turner envisions a day when the United States and other nations depend by and large on domestically produced hydrogen as an energy carrier for transportation, heat and electricity. No pollution, no dependency on foreign oil, no national security worries.
“In my view, a hydrogen economy is inevitable,” Turner says, “simply because we can’t run on fossil fuels forever, and because hydrogen makes the most sense when it comes to building a sustainable infrastructure.” Turner says the two big visions for the hydrogen economy are its use in fuel cell vehicles and in producing ammonia, which is essential for food production. “I’m talking about a system that will last for millennia.”
Neale and Ruddy take a shorter-term view. They can envision solar-powered water splitting replacing reforming natural gas as the way refineries produce the hydrogen that is used to remove sulphur from petroleum or to convert heavy crude into lighter, easier-to-refine products.
If all the hydrogen were pulled out of a litre of water, that amount of hydrogen would have 40 per cent of the energy equivalence of one litre of petrol.
One challenge is the number of solar cells needed to generate as much hydrogen fuel each day as that produced by a typical large oil refinery. You might need a few square kilometres of solar arrays around the refinery.
Another hurdle is to show that the process will work as well when it is boosted to manufacturing scale. Recent NREL analysis indicates a path to getting a gallium-arsenide-based solar cell down to about R16 per watt. Using that as the base for the Turner water-splitting device would produce hydrogen at about R25 per kilogram; this would make it fairly competitive with petrol. A kilogram of hydrogen can propel a car almost three times further than a gallon (about 3,8 litres) of petrol, but because hydrogen is so light, a conventional fuel tank would provide a greater range than a vehicle’s hydrogen tank.
Solar power for the long run
Millions of high school students have done the basic experiment: use an electric source and clamps to electrolyse water and watch the bubbles of hydrogen climb to the surface. Using the Sun and a solar cell eliminates the need for electrolysers, or anything else that needs a fossil-fuel source.
“This has been languishing in the weeds, and now all of a sudden it’s in the forefront,” Turner says. “Our funding has been zeroed out twice, but in the past few years we’ve seen more interest.” In fact, Turner already has given 23 invited talks on hydrogen in 2012, a personal record for talks in a year.
“I’m optimistic – if people will focus
on the right concepts,” he says. “That’s what’s necessary to make this field viable. It’s going to take a new material, and people haven’t been interested in looking at new materials. Now they are.”
Of course, there are other pathways to producing hydrogen, including using biomass or algae, or extracting hydrogen using extreme heat. Says Turner: “The one that has the lowest cost for the hydrogen is the one that will prevail.”
NREL has a hydrogen filling station at its National Wind Technology Centre, the stored hydrogen coming from solar- and wind-powered water splitting. And NREL houses the data repository for the DOE’s technology validation project on the performance of fuel cell vehicles.
On track to being a scientist early
Turner was sure of his future career when he was 12 years old and first saw the word “scientist” written on a blackboard in his Idaho elementary school. “I knew that was me,” he says. “I came home and told my mom I was going to be a scientist. She said, ‘That’s cute, Johnny. Tomorrow you’ll want to be a fireman.’
“But it never changed. In high school, I took calculus, chemistry, physics and biology, and decided on chemistry.” Fortunately, he was fast enough in the 100- and 200-yard sprints and adept enough in the hurdles and triple jump to make sure his passion saw fruit. Turner won a track scholarship to Idaho State University, got his PhD in chemistry from Colorado State University, and did post-graduate work at Caltech – now the site of the big DOE Energy Innovation Hub on hydrogen.
He was a celebrity for a short while in 1998. His paper on using the Sun for water splitting even inspired a film starring Keanu Reeves titled Chain Reaction, about a hydrogen-fuelled economy being the Earth’s last best hope. “We need a chemical energy carrier like petroleum, methane, that carries energy in chemical bonds,” Turner says. “It’s all stored sunlight. That’s what hydrogen represents.”
Turner has a couple of big names on his side. In Jules Verne’s 1874 novel, Mysterious Island, a protagonist says: “Yes, my friends, I believe that water will some day be employed as fuel, that hydrogen and oxygen which constitute it… will furnish an inexhaustible source of heat and light. Some day, water will be the coal of the future.”
And in 1931, shortly before he died, Thomas Edison told his friends Henry Ford and Harvey Firestone: “I’d put my money on the Sun and solar energy. What a source of power! I hope we don’t have to wait until oil and coal run out before we tackle that.”
Turner puts it this way: “How much coal will we have in 75 years or 200 years? We have enough coal for ourselves and our kids, but what about our grandkids and great-grandkids? The Sun and the wind are intermittent resources. We’ll run out of fossil fuels. But the Sun and water will be around for a long, long time.”
● Source: National Renewable Energy Laboratory
Nanotech solar cell achieves high efficiency
Using nanotechnology techniques, NREL scientists have produced solar cells at a “competitive” efficiency of 18,2 per cent – a breakthrough that could represent a major step towards lowering the cost of solar energy.
The researchers tailored a nanostructured surface while ensuring that the light-generated electricity could still be collected efficiently from the solar cell. They made nano-islands of silver on a silicon wafer and immersed it briefly in liquids to make billions of nano-sized holes in the wafer surface. The holes and silicon walls are smaller than the light wavelengths hitting them, so the light doesn’t recognise any sudden change in density at the surface and therefore doesn’t reflect back into the atmosphere as wasted energy. The researchers controlled the nanoshapes and the chemical composition of the surface to reach record solar cell efficiencies for this “black silicon” material.
Says Howard Branz, the grant’s principal investigator: “This work can have a big impact on both conventional and emerging solar cell based on nanowires and nanospheres. For the first time, it shows that really great solar cells can be made from nanostructured semiconductors.”