How to Artificial Photosynthesis Works

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Plants convert sunlight into energy through photosynthesis. Can we do the same? See more green science pictures.

If the smartest energy source is one that's abundant, cheap and clean, then plants are a lot smarter than humans. Over billions of years, they developed perhaps the most efficient power supply in the world:photosynthesis, or the conversion of sunlight, carbon dioxide and water into usable fuel, emitting useful oxygen in the process.

In the case of plants (as well as algae and some bacteria), "usable fuel" is carbohydrates, proteins and fats. Humans, on the other hand, are looking for liquid fuel to power cars and electricity to run refrigerators. But that doesn't mean we can't look to photosynthesis to solve our dirty-, expensive-, dwindling-energy woes. For years, scientists have been trying to come up with a way to use the same energy system that plants do but with an altered end output.

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Using nothing but sunlight as the energy input, plants perform massive energy conversions, turning 1,102 billion tons (1,000 billion metric tons) of CO₂ into organic matter, i.e., energy for animals in the form of food, every year [source: Hunter]. And that's only using 3 percent of the sunlight that reaches Earth [source: Boyd].

The energy available in sunlight is an untapped resource we've only begun to really get a handle on. Current photovoltaic-cell technology, typically a semiconductor-based system, is expensive, not terribly efficient, and only does instant conversions from sunlight to electricity -- the energy output isn't stored for a rainy day (although that could be changing: See "Is there a way to get solar energy at night?"). But an artificial photosynthesis system or a photoelectrochemical cell that mimics what happens in plants could potentially create an endless, relatively inexpensive supply of all the clean "gas" and electricity we need to power our lives -- and in a storable form, too.

In this article, we'll look at artificial photosynthesis and see how far it's come. We'll find out what the system has to be able to do, check out some current methods of achieving artificial photosynthesis and see why it's not as easy to design as some other energy-conversion systems.

So, what does an artificial photosynthesis system have to be able to do?

Artificial Photosynthesis Approaches

To recreate the photosynthesis that plants have perfected, an energy conversion system has to be able to do two crucial things (probably inside of some type of nanotube that acts as the structural "leaf"): harvest sunlight and split water molecules.

Plants accomplish these tasks using chlorophyll, which captures sunlight, and a collection of proteins and enzymes that use that sunlight to break down H₂O molecules into hydrogen, electrons and oxygen (protons). The electrons and hydrogen are then used to turn CO₂ into carbohydrates, and the oxygen is expelled.

For an artificial system to work for human needs, the output has to change. Instead of releasing only oxygen at the end of the reaction, it would have to release liquid hydrogen (or perhaps methanol) as well. That hydrogen could be used directly as liquid fuel or channeled into a fuel cell. Getting the process to produce hydrogen is not a problem, since it's already there in the water molecules. And capturing sunlight is not a problem -- current solar-power systems do that.

The hard part is splitting the water molecules to get the electrons necessary to facilitate the chemical process that produces the hydrogen. Splitting water requires an energy input of about 2.5 volts [source:Hunter]. This means the process requires a catalyst -- something to get the whole thing moving. The catalyst reacts with the sun's photons to initiate a chemical reaction.

There have been important advances in this area in the last five or 10 years. A few of the more successful catalysts include:

• Manganese: Manganese is the catalyst found in the photosynthetic core of plants. A single atom of manganese triggers the natural process that uses sunlight to split water. Using manganese in an artificial system is a biomimetric approach -- it directly mimics the biology found in plants.
• Dye-sensitized titanium dioxide: Titanium dioxide (TiO₂) is a stable metal that can act as an efficient catalyst. It's used in a dye-sensitized solar cell, also known as a Graetzel cell, which has been around since the 1990s. In a Graetzel cell, the TiO₂ is suspended in a layer of dye particles that capture the sunlight and then expose it to the TiO₂ to start the reaction.
• Cobalt oxide: One of the more recently discovered catalysts, clusters of nano-sized cobalt-oxide molecules (CoO) have been found to be stable and highly efficient triggers in an artificial photosynthesis system. Cobalt oxide is also a very abundant molecule -- it's currently a popular industrial catalyst.

Once perfected, these systems could change the way we power our world.

Artificial Photosynthesis Applications

Image courtesy of Warren Gretz, National Renewable Energy Laboratory
NREL scientist John Turner demonstrates the ability of a photoelectrochemical (PEC) cell to produce hydrogen from water using energy from a light source.

Fossil fuels are in short supply, and they're contributing to pollution and global warming. Coal, while abundant, is highly polluting both to human bodies and the environment. Wind turbines are hurting picturesque landscapes, corn requires huge tracts of farmland and current solar-cell technology is expensive and inefficient. Artificial photosynthesis could offer a new, possibly ideal way out of our energy predicament.

For one thing, it has benefits over photovoltaic cells, found in today's solar panels. The direct conversion of sunlight to electricity in photovoltaic cells makes solar power a weather- and time-dependent energy, which decreases its utility and increases its price. Artificial photosynthesis, on the other hand, could produce a storable fuel.

And unlike most methods of generating alternative energy, artificial photosynthesis has the potential to produce more than one type of fuel. The photosynthetic process could be tweaked so the reactions between light, CO₂ and H₂O ultimately produce liquid hydrogen. Liquid hydrogen can be used like gasoline in hydrogen-powered engines. It could also be funneled into a fuel-cell setup, which would effectively reverse the photosynthesis process, creating electricity by combining hydrogen and oxygen into water. Hydrogen fuel cells can generate electricity like the stuff we get from the grid, so we'd use it to run our air conditioning and water heaters.

One current problem with large-scale hydrogen energy is the question of how to efficiently -- and cleanly -- generate liquid hydrogen. Artificial photosynthesis might be a solution.

Methanol is another possible output. Instead of emitting pure hydrogen in the photosynthesis process, the photoelectrochemical cell could generate methanol fuel (CH₃OH). Methanol, or methyl alcohol, is typically derived from the methane in natural gas, and it's often added to commercial gasoline to make it burn more cleanly. Some cars can even run on methanol alone.

The ability to produce a clean fuel without generating any harmful by-products, like greenhouse gasses, makes artificial photosynthesis an ideal energy source for the environment. It wouldn't require mining, growing or drilling. And since neither water nor carbon dioxide is currently in short supply, it could also be a limitless source, potentially less expensive than other energy forms in the long run. In fact, this type of photoelectrochemical reaction could even remove large amounts of harmful CO₂ from the air in the process of producing fuel. It's a win-win situation.

But we're not there just yet. There are several obstacles in the way of using artificial photosynthesis on a mass scale.

Challenges in Creating Artificial Photosynthesis

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Nature has perfected the photosynthesis process over billions of years. It won't be easy to replicate it in a synthetic system.

While artificial photosynthesis works in the lab, it's not ready for mass consumption. Replicating what happens naturally in green plants is not a simple task.

Efficiency is crucial in energy production. Plants took billions of years to develop the photosynthesis process that works efficiently for them; replicating that in a synthetic system takes a lot of trial and error.

The manganese that acts as a catalyst in plants doesn't work as well in a man-made setup, mostly because manganese is somewhat unstable. It doesn't last particularly long, and it won't dissolve in water, making a manganese-based system somewhat inefficient and impractical. The other big obstacle is that the molecular geometry in plants is extraordinarily complex and exact -- most man-made setups can't replicate that level of intricacy.

Stability is an issue in many potential photosynthesis systems. Organic catalysts often degrade, or they trigger additional reactions that can damage the workings of the cell. Inorganic metal-oxide catalysts are a good possibility, but they have to work fast enough to make efficient use of the photons pouring into the system. That type of catalytic speed is hard to come by. And some metal oxides that have the speed are lacking in another area -- abundance.

In the current state-of-the-art dye-sensitized cells, the problem is not the catalyst; instead, it's the electrolyte solution that absorbs the protons from the split water molecules. It's an essential part of the cell, but it's made of volatile solvents that can erode other components in the system.

Advances in the last few years are starting to address these issues. Cobalt oxide is a stable, fast and abundant metal oxide. Researchers in dye-sensitized cells have come up with a non-solvent-based solution to replace the corrosive stuff.

Research in artificial photosynthesis is picking up steam, but it won't be leaving the lab any time soon. It'll be at least 10 years before this type of system is a reality [source: Boyd]. And that's a pretty hopeful estimate. Some people aren't sure it'll ever happen. Still, who can resist hoping for artificial plants that behave like the real thing?