The search for artificial photosynthesis: everything Nature can do we can do...not quite as well?

"Natural selection is not a master engineer, but a tinkerer. It doesn't produce the absolute perfection achievable by a designer starting from scratch, but merely the best it can do with what it has to work with."

- Jerry Coyne

The human brain is a brilliant organ, capable of goal-oriented behavior and creativity that has birthed the Pyramids, the iPhone, Romeo and Juliet, and the New York City skyline. This purposeful action leads to amazing technological advancements at breakneck speeds, mirroring our species' dramatic population growth and rapacious resource use.

But we are not the only forces of creation. Evolution also has its canvas, but uses a different brush. Over millions of years, without the intent or purpose of human activity, natural selection has woven a world of complex species, each with its own genetic fingerprint, that rival any human creation in design efficiency and resilience (the "absurd" perfection of the human eye mystified even Charles Darwin).

So what can we learn from Nature's expertise in refinement? What if we developed technologies that mimicked the best machinery developed from eons of evolution? Photosynthesis, the ubiquitous process that converts solar energy to usable chemical fuel that serves as the foundation for all life on Earth, is a prime example. As fossil fuels dwindle, scientists hope to replicate plant photosynthesis using human-made materials to store light as chemical energy in hydrogen, which has three times the energy density of gasoline. Finding a cheap method to do this could provide clean hydrogen fuel as the basis for a future, fossil-fuel-free economy. So far, attempts have succeeded in reproducing the basic process but have had difficulties keeping devices stable for long periods of time. Now, a recent study¹ may have found a solution, bringing artificial photosynthesis closer to a practical reality.

I'm here to help you react

Splitting water is the chemical process at the heart of photosynthesis. Plants absorb carbon dioxide (CO₂) from the atmosphere and suck up water (H₂O) through their roots so that the two molecules meet in chloroplasts deep inside the leaves. Here, each pair of water molecules splits into two hydrogen molecules (H₂) and one oxygen molecule (O₂). This reaction is endothermic - the molecules only react with an input of energy - so surrounding chlorophylls have to absorb sunlight to provide the necessary boost. The hydrogen product reacts with CO₂ to form sugars ((CH₂O)_n) that serve as the basic energy unit for plants (the oxygen eventually finds its way to our lungs). In this way, plants cleanly (no fossil fuels!) convert solar to chemical energy to repair and build their structure.

There's just one problem: even with solar energy to initiate the reaction, the rate will still be too slow to be of any use to the plant. But a catalyst comes to the rescue! A catalyst is not consumed by a chemical reaction, but its mere presence provides a chemical environment that speeds up the reaction rate. It's easy to see catalysis as a bit of magic - we still don't understand how most types work at the atomic level - but it's definitely not anything supernatural! The catalytic material just alters chemical reaction pathways in a complicated fashion that can involve many intermediate states that help speed up the process.

In plants, chlorophylls with magnesium clusters and various enzymes serve as photocatalysts to speed up the water reaction. Evolution has refined an extremely complex pathway to split water and make sugar, but we know the basics we would need to replicate: use solar energy to split water in the presence of a catalyst, then collect the hydrogen product as fuel, either in pure form or as part of an organic molecule (like the sugar in the plant). This would be a fossil-fuel free way to power fuel cells and the so-called hydrogen economy. So how have we done so far to reproduce this natural feat?

A bit of nickel does the trick

The core of human-made water-splitting devices is a semiconductor. These materials - usually silicon, gallium arsenide, or cadmium telluride² - absorb mostly visible light, exciting electrons that hang around long enough to provide the energy to initiate the water-splitting reaction. The semiconductor and a catalyst are immersed in an an electrolyte, usually an alkaline or acidic solution with water. Excited electrons move through the solution, find water near the presence of a catalyst, and split it, using the collected product as fuel.

Sounds like a pretty simple construction, right? Actually, it's a bit more complicated. Most of the semiconductors used to absorb light corrode away when exposed to the electrolyte, reducing the device lifetime and making it impractical for commercial use. Therefore, these semiconductors, which are relatively cheap with refined manufacturing methods from their use in the photovoltaic industry, haven't been of use for water splitting devices.

But researchers may have found a way around this previously intractable problem. The idea is to use another material that is stable in the electrolyte as a protective layer to prevent corrosion. This has been done before, but designs become much more cumbersome because a separate catalytic material must also be present. Finding an optimal geometry that allows the excited electron, water, and catalyst all to arrive at the same place in the presence of a protective barrier can be difficult. To solve this, Sun et al found a way to reduce the number of materials required. They placed nickel oxide (NiO_x) on top of a silicon semiconductor to act as the protective layer. When submersed in the electrolyte, they found that the nickel oxide served a dual role as catalyst for the reaction! Water splitting took place for over one hundred hours of operation without the semiconductor corroding, a good sign that the device geometry could be viable for long-term use. The nickel oxide film is also transparent and anti-reflective, so visible light will pass through it to hit the semiconductor protected below and excite electrons. The researchers appear to have discovered a material that fulfills the many, stringent requirements for efficient water splitting - transparent, photocatalytic, and chemically stable.

These types of protective layers have been used before to prevent corrosion³, but this is the first demonstration of such a technique with conventional semiconductors (silicon) that are cheaper and offer potentially higher efficiency (more output energy is provided per input light energy). More work will be done on testing various ways that the nickel oxide is deposited on the semiconductor to optimize photocatalytic activity, but the early signs suggest this is a very promising route to artificial photosynthesis. And it isn't the only way! Over at Bio 2.0, another great blog within Nature's Scitable network, Dan Kramer discusses new research about a hybrid, nanowire-bacteria system used to reduce carbon dioxide from solar energy. Many creative approaches are appearing in this field, a great sign of significant progress in the near future.

Artificial water-splitting devices, built with inorganic crystals and acids, electrical wires winding away like tentacles, may offer no resemblance to the flexible green organics of plant photosynthesis. But the chemistry at the core is the same, and we are just now beginning to match the elegance of Nature's million-year-old machinery.

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