Leaf it to Me: Biomimicry and the Artificial Leaf

The concept of environmentalism is almost inevitably coupled to sacrifice and responsibility. In order to reduce our disruption of nature, we are provided with lists upon lists of things we should not do. We should not take long showers. We should not forget to turn the lights off before we leave a room. It’s as if we are at war with nature, constantly striking deals, promising to give up our carelessness and guilty pleasures in return for a longer shot at existence.

While accountability is a crucial part of environmentalism, this negative focus tends to turn people away and does not produce results that are at a large enough scale to create significant changes. Instead, environmentalism has the potential to be, and in many cases, already is, positive, ambitious, and incredibly creative. We should not pursue environmental goals as a desperate last defense. They should represent a step forward, towards attaining greater efficiency and utility in everything we do. Environmentalism should aid us, not stand in our way, of becoming greater than we have ever been before.

By mimicking nature’s design principles, engineers and scientists are creating products that are perfectly adapted to solve problems we face today. Over billions of years, evolution has driven forms to fit their functions. The silk that spiders secrete to spin their webs is the perfect example. It is capable of withstanding the force of wind, the forces exerted by insects trapped in the web, and more1, thereby facilitating the spider’s unique existence. Many of the needs served by nature’s elegant solutions are shared by us, in the human world. The science of biomimicry aims to imitate nature and yield materials and products that are both more green and better performers than their conventional alternatives. In the future, artificial spider silk could be a component of cars, causing dents created by accidents to spontaneously disappear.2 The possibilities are limitless.

Of all the biomimicry projects undertaken, the creation of the artificial leaf is arguably the most imagined and the most anticipated. The concept behind it is simple. An artificial leaf, like its natural brethren, uses sunlight to decompose water into hydrogen and oxygen.3 Either the energy produced by the process or the resultant hydrogen gas can be used to generate fuel. While burning fuels such as coal and gasoline produces carbon dioxide, the consumption of hydrogen fuel only produces water vapor, a cleaner alternative. Companies such as Toyota have already designed viable hydrogen-fueled cars.4 Until now, however, obtaining inexpensive hydrogen gas has been problematic. With the development of the artificial leaf, the production of hydrogen could become very practical, leading to a larger market for hydrogen-powered vehicles.

At the annual meeting of the American Chemical Society (ACS) in 2010, Dr. Tongxiang Fan and his colleagues at Shanghai Jiotang University introduced a prototype for an Artificial Inorganic Leaf. The design mimics the structures of the natural leaf that are involved in focusing solar energy and guiding it through the parts of the leaf that harvest the energy.4 Titanium dioxide, a hydrogen photocatalyst, or compound capable of stimulating the synthesis of hydrogen gas in the presence of sunlight,5 was introduced into the plant Anemone vitefolia in place of its natural photosynthetic pigments. This compound was able to replicate the structures responsible for light-harvesting in the leaf, and is estimated to be able to produce three times as much hydrogen gas as commercial photo-catalysts available today.  In addition to titanium dioxide, nanoparticles of platinum were embedded into the leaf’s surface, to increase its activity. By “biotemplating” titanium dioxide and platinum within a natural leaf, the scientists aim to use “human ingenuity to modify the principles of natural systems for enhanced utility.”4

In a paper published on September 30, 2011, Dr. Daniel Nocera of MIT revealed that he had taken the idea even further by creating the first practical artificial leaf. While the leaf designed by Dr. Fan’s team modified an existing organic leaf, this artificial leaf is composed entirely of three inorganic components. The base of the “leaf” is a thin silicon wafer, which converts sunlight into an electric current, much like a solar cell.6 This electric current also results in the production of “holes” or electron vacancies that travel through the system. These holes use the second component of the artificial leaf, a cobalt-based catalyst, to strip electrons off water molecules, decomposing water into hydrogen and oxygen.7 Oxygen is released out of the side of the wafer on which the catalyst is bound. The other side of the wafer contains a nickel-molybdenum-zinc alloy, which releases hydrogen gas through the other side. These reactions can occur when the artificial leaf is placed in a glass of ordinary water.6 In order to harness the energy produced, a barrier can be constructed separating the two sides of the cell, so that hydrogen ions can stream into one side and oxygen ions the other. Therefore, hydrogen and oxygen can be stored separately, and recombined to generate electricity in a fuel cell.7 While the splitting of a molecule of water is an endothermic reaction that requires energy, the synthesis of water is an exothermic reaction, which releases energy for us to use.

In nature, sunlight absorbed by a leaf results in the photoexcitation of electrons, which leave behind regions of electron vacancies, the holes. These holes are then captured by the oxygen evolving complex to oxidize water and reduce nicotinamide adenine dinucleotide (NAD+) into nicotinamide adenine dinucleotide hydride (NADH). Nocera’s artificial leaf uses the cobalt-oxygen evolving catalyst as a functional model of a leaf’s photosystem II10 , thereby mimicking photosynthesis. Similarly, the recombination of hydrogen and oxygen in a fuel cell can be compared to respiration, where the flow of hydrogen ions down their electrochemical gradient results in both the release of energy and the formation of water.

The most amazing aspect of Nocera’s artificial leaf is how practical it is. The leaf is only made of non-corrosive, earth-abundant materials, such as silicon and cobalt. Furthermore, it is about the size of a poker card,9 lightweight, and requires no wires or external circuits. In the laboratory, the artificial leaf has been able to generate power continuously for 45 hours without a drop in performance, partially due to the self-healing nature of the cobalt catalyst, which reforms its cobalt oxide clusters whenever they are degraded by the reactions. Not only could the leaf one day combat the threat of global warming; it could also make energy both affordable and accessible. If the technology works the way it is expected to, rural villages in remote areas of developing countries will be able to power their homes with ease, just by placing the artificial leaf in a gallon of water in bright sunlight and connecting the system to a fuel cell. This could result in dramatic transformations. Easy access to electricity gives rise to an increased standards of living, as well as access to information through communication devices like cell phones and computers.

Nocera and the Tata group, an Indian multinational conglomerate, have signed an agreement aimed at commercialization.10 Nevertheless, the question of whether the artificial leaf can be useful still remains. An analysis commissioned by the US Department of Energy has determined that the leaf is capable of producing solar hydrogen at a lower cost than an array of photovoltaic panels connected to catalyst-coated electrodes.11,12 Therefore, the main challenge facing the artificial leaf is not the cost or feasibility of production, but instead, how efficiently the system can use solar energy, while keeping costs low. The artificial leaf is more efficient than a natural leaf, which only converts 1% of the sunlight it receives into energy, but not by much. Nocera reports an efficiency of 2.5% without wires and 4.7% with wires.12 Commercial solar panels, on the other hand, display efficiencies upwards of 10%.5 The semiconducting solar cell, not the catalysts, are responsible for most of the energy loss, and in order for Nocera’s catalysts to have any impact in terms of photoelectrical hydrogen production, better semi-conductors must be used. However, better semi-conductors are expensive and will significantly drive up the cost of producing the artificial leaf. Currently, Nocera’s team is testing a higher quality crystalline silicon for the semiconductor. They also plan on improving efficiency by increasing the conductance of the surrounding solution and punching small holes in the semi-conductor to facilitate proton flow.12 Without a great increase in efficiency that brings the number reported into the teens, the artificial leaf will never be able to meet the needs of an American home, though it might be able to power energy-light homes in third-world countries.

Systems to collect, store, and use both the oxygen and the hydrogen gas produced by the artificial remain to be developed,5 indicating that the technology is only in its infancy. However, both Nocera and Fan’s artificial leaves are tangible steps in the right direction, and could lead us into a bold new age. One where there are no soaring gas prices. One where we can save nature by paying her the greatest complement: imitation.

References:

1. M. Baels, L. Gross, and S. Harrell, “SPIDER SILK: STRESS-STRAIN CURVES AND YOUNG’S MODULUS,” Tiem, last modified 1999, http://www.tiem.utk.edu/~gross/bioed/bealsmodules/spider.html.
2. Brian Handwerk, “Artificial Spider Silk Could Be Used for Armor, More,” National Geographic, January 14, 2005, http://news.nationalgeographic.com/news/2005/01/0114_050114_tv_spider.html
3. American Chemical Society, “Blueprint for ‘artificial leaf’ mimics Mother Nature,” ScienceDaily, March 25, 2010, http://www.sciencedaily.com/releases/2010/03/100325131549.htm
4. Darren Quick, “Drawing Inspiration from Mother Nature in Designing an ‘Artifical Leaf,” Gizmag, March 26, 2010, http://www.gizmag.com/artificial-leaf-blueprint/14630/
5. David L. Chandler, “‘Artificial leaf’ makes fuel from sunlight,” MIT news, September 30, 2011, http://web.mit.edu/newsoffice/2011/artificial-leaf-0930.html
6. ScienceNow, “Artificial Leaf Moves Two Steps Closer to Reality,” Wired, September 30, 2011, http://www.wired.com/wiredscience/2011/09/artificial-leaf-solar-fuel/
7. Dinca, Mircea, Yogesh Surendranath, and Daniel G. Nocera. “Nickel-borate oxygen-evolving catalyst that functions under benign conditions.” PNAS 107, no. 23 (June 2010): 10337.
8. Daniel G. Nocera and Matthew W. Kanan, “In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+.” Science , August 22, 2008.
9. Ben Coxworth, “Scientists unveil ‘world’s first practical artificial leaf,” gizmag, March 28, 2011, http://www.gizmag.com/worlds-first-practical-artificial-leaf/18247/
10. Admin. “MIT’s Daniel Nocera Announces Artificial Leaf With Goal To Make Every Home a Power Station, Signs with Tata.” Free Energy Times, March 28, 2011.
11. James, Brian D., George N. Baum, Julie Perez, and Kevin N. Baum. Technoeconomic Analysis of Photoelectrochemical (PEC) Hydrogen Production. http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/pec_technoeconomic_analysis.pdf
12. Noorden, Richard Van. “Secrets of artificial leaf revealed.” Nature News, September 29, 2011. http://www.nature.com/news/2011/110929/full/news.2011.564.html
13. Image: EMSL. “Molecules Frozen Stiff.” Flickr, December 3, 2010. http://www.flickr.com/photos/emsl/5416820160/
14. Image: Yikrazuul. “Manganese cluster.” Wikimedia Commons, May 15, 2009. http://commons.wikimedia.org/wiki/File:Manganese_cluster.svg

Prathima Radhakrishnan is a second year student from the University of Chicago majoring in the biological sciences and biochemistry and minoring in creative writing. Follow The Triple Helix Online on Twitter and join us on Facebook.