From thin air: Making fuel like a tree

If we were to pick one thing to mimic in nature that would bring the most good to our planet, what would it be? Would it be based on greatest need, steepest challenge, widest application or biggest impact?

Maybe it would be a point on a graph where feasibility met added value. Or perhaps it would be something else, something un-graphable that would inspire millions of hearts.

One such holy grail quest of bio-inspired design is the development of artificial photosynthesis (AP). It is hard to propose a rival with as many possible consequences. Our world, both natural and technological, runs on the sun, and the basis for most of our wealth can be directly traced to the complex and everyday miracle going on, right now, as every leaf in almost every plant on the globe converts that sunlight into sugar. We already have photovoltaic panels and solar thermal arrays, but these apparatuses do not do what nature does: precisely and quickly turn light energy into electrical energy into chemical energy, and produce something that can be stored easily and used at will.

AP has the potential to produce more than one type of fuel. The photosynthetic process could be adjusted so that the reactions between light, carbon dioxide and water ultimately produce liquid hydrogen. Liquid hydrogen could fuel hydrogen-powered engines or be funneled into a fuel-cell setup, which would effectively reverse the photosynthesis process, creating electricity by combining hydrogen and oxygen into water.

Methanol is another possible output. Instead of emitting pure hydrogen in the photosynthesis process, the photoelectrochemical cell could generate methanol fuel (CH3OH).

Game changer

What would our world look like if we were able to make sugars from only sunlight, carbon dioxide and water? An AP system or a photoelectrochemical cell that mimics what happens in plants potentially could create an endless, relatively inexpensive supply of all the clean liquid fuel and electricity we needed in a storable form.

The ability to produce a clean fuel without generating any harmful byproducts -- such as greenhouse gasses -- makes AP an ideal energy source for the environment. It wouldn't require mining, growing or drilling. Both water and carbon dioxide are currently in great supply, so it could be a virtually limitless source, perhaps less expensive than other energy forms in the long run. This type of photoelectrochemical reaction even could be used to remove harmful CO2 from the air in the process of producing fuel.

Getting there

So, if AP is the greatest thing since sliced bread, why don't we employ it everywhere on earth? Good question. The answer appears to be: complexity and cost of the sheer mass of materials needs for light-harvesting infrastructure.

Because of the costs, the public sector has borne much of the research and development. The three main publicly funded research centers include the Joint Center for Artificial Photosynthesis (JCAP), one of three Energy Innovation Hubs in the United States; the Towards Bisolar Cells consortium in the Netherlands; and the Korean Center for Artificial Photosynthesis in South Korea.

Within the private sector, there is also a serious effort at cracking this challenge, including Sun Catalytix and HyperSolar Inc. Both U.S. companies illustrate the ebb and flow of market-based biotechnology development.

Sun Catalytix and HyperSolar lead AP field

The AP goal for these companies is to mimic only the first part of photosynthesis -- the splitting of water into hydrogen and oxygen. Hydrogen is then used in a fuel cell to produce electricity. Water is split traditionally through an electrocatalytic process. Expensive platinum is used as the catalyst and the electricity needed is produced by the burning of fossil fuels. When using solar power to provide the electricity, the solar panel and the so-called electrolyzer typically have been built as separate units. Much of the research involves eliminating rare materials and fossil fuels from the process, lowering the energy input and combining the solar collection with the water splitting. Nanotechnology offers significant prospects for this.

Sun Catalytix of Cambridge, Mass., has been a frontrunner in the AP field, but seems to have paused in its quest for a commercial AP device. It has worked since 2009 to develop an artificial leaf based on the original research of Daniel G. Nocera, the Patterson Rockwood professor of energy at the Massachusetts Institute of Technology, and his team. Their most recent prototype had advanced the field by combining light harvesting and catalyzing functions into one device, and by using cheap catalysts made without extremely toxic solutions. The artificial leaf is a catalyst-coated silicon wafer.

At the American Chemical Society's 2013 national meeting in April, Nocera touted his artificial leaf as a possible answer to local energy production for the developing world, claiming that one could put the leaf into a quart of dirty water and generate 100 watts of power, 24 hours a day. The leaf also was self-healing, and they designed the catalyst to break up into a bumpy nanosurface to discourage the forming of biofilms. It was described as "personalized energy for the third world."

Despite this groundbreaking work, the company announced in May 2012 that it could not make an economically competitive product and would investigate a yet-to-be-developed semiconductor material before field testing another prototype. The company remains committed to commercializing a wireless solar-hydrogen device with earth-abundant materials. Much of the Sun Catalytix work had been funded by Polaris Venture Partners, India-based Tata Group and a $4 million ARPA + E grant from the U.S. Department of Energy.

According to the announcement in Nature News, it costs $7 per kilogram to make hydrogen from a solar panel and electrolysis unit, while the artificial leaf would come in at $6.50. In contrast, it costs $1 to $2 to make a kilogram of hydrogen from fossil fuels. With the falling price of solar cells, the company will consider "cheaper designs -- but these require yet-to-be-invented semiconductor materials."

This may be moot for the short term as the company seems to have shifted to promoting a flow battery. This is typically used as a backup battery for a power grid and comprises an aqueous electrolyte pumped across a membrane to produce an electrochemical reaction harvested for its power. Although competition is strong in this sector, the technology is more familiar and the market is known. The company just received about $4 million in an equity offering to five new, unnamed investors and hopes to raise $8 million more.

HyperSolar's high hopes

In contrast, HyperSolar Inc., a publicly traded company based in Santa Barbara, Calif., remains optimistic about its prospects and announced recently that it had reached an important milestone in the quest for making hydrogen fuel in a zero-carbon process for a competitive price.

The company has developed a fuel cell that generates one volt of power, generally seen as an important milestone. It takes 1.5 volts to split water into hydrogen and oxygen, so this is still shy of the mark but a significant improvement over the 0.8 volt production the company started with. To date, the cheapest way to split water is using a silicon solar cell technology, but these are typically low power producers at 0.7 volts per cell. The trick is to make more power at lower cost, with the key being how you manufacture the semi-conductor material.

HyperSolar has demonstrated a small prototype panel of their H2Generator cell that can be immersed in dirty water and directly produce hydrogen. The company intends to scale this up to a commercial size, but it remains to be seen what shape that will take. A major consideration in the design process is protecting the semiconductor material from degradation by the sun.

"A big hurdle in using a solar-to-fuel conversion process is the stabilization of the semiconductor material against photocorrosion," explained CEO Tim Young. "Our development of an efficient and low-cost protective polymer coating that also allows good electrical conductivity is a significant achievement in our development of a cost-effective means for using the power of the sun to extract renewable hydrogen from water."

HyperSolar claims that it will develop this material by making a nanoparticle from cheap materials using a batch chemical process, not using traditional and more expensive semiconductor processes and facilities.

Each nanoparticle "is a complete hydrogen generator that contains a novel high voltage solar cell bonded to chemical catalysts by a proprietary encapsulation coating," the company said in a recent press release.

The world still waits for a ubiquitous device that can turn thin air, water and sunlight into carbon-based fuel, but the intense chase goes on, powered by the immense potential and a fascination with this miracle of nature.

Green leaf image by Chyrko Olena via Shutterstock.