Budding Opportunities
for Artificial Leaves


Artificial leaf technology is sprouting new growth. For decades, engineers and scientists have been pursuing a practical technology to copy nature’s trick of converting sunlight and water into pure hydrogen. The effort has been slow, but new nanomaterials, chemical processes, and public-private investments are strengthening the artificial leaf’s position as a promising branch of the sustainable energy industry.

Artificial leaves and other species of faux-photosynthesis, if scaled up, could significantly reduce greenhouse emissions, curb fossil fuel demand, and bring low-cost power to resource-starved regions in the developing world. In the U.S., where cars and trucks produce a fifth of the nation’s heat-trapping emissions, electric cars are steadily gaining ground and an infrastructure to keep them juiced up is developing. Although electric vehicles are free of tailpipe emissions, their green advantage is diminished at the charging station, where they tap into conventionally produced power.

New hydrogen-powered cars like the 2016 Toyota Mirai – selected by the Union of Concerned Scientists as one of the five cleanest new cars on the market – uses a hydrogen-powered fuel cell instead of a battery. Hydrogen is a clean efficient fuel that gives the Mirai and its ilk a range of distance between fill-ups on part with a gasoline-powered car. However, because hydrogen doesn’t occur naturally on Earth, it must be manufactured, and the preferred raw material for most hydrogen made today is natural gas or through electrolysis, which converts high-purity water into hydrogen using large quantities of electricity. The artificial leaf would eliminate the carbon footprint altogether by using only sunlight to achieve the same chemical reaction. 

MIT chemist Dan Nocera displays a device he and co-worker Matthew Kanan used to split water. Image: Len Rubenstein / MIT



The artificial leaf’s potential stems from a process called water splitting. It involves placing specially-coated photoabsorbing electrodes into water, where they catalyze a photoelectrochemical process that breaks the molecular bond between the water’s hydrogen and oxygen. Depending on the need, the hydrogen can be used immediately or stored and further processed. The engineering challenge has been to find the right balance of low cost, high efficiency, and robustness. Materials that work well as photocathodes for hydrogen extraction work best in an acidic water; oxygen-extracting photoanodes, however, require a basic environment. Many of the best catalytic coatings have been costly and scarce materials that are economically off-limits to commercial exploitation.

Early artificial leaves proved the concept on a small scale but were too expensive and unstable to be scaled up. In 2011, however, Massachusetts Institute of Technology chemist Daniel Nocera (now at Harvard) revealed a device hailed as the most practical approach yet. His team developed cobalt and nickel catalysts with the ability to split water from any source using light at ambient temperature and pressure. Reporting 10 times more efficient energy conversion than natural photosynthesis over at least 45 hours of continuous operation, the technology fueled the launch of SunCatalytix. The company shifted emphasis to a closer-to-market flow-battery product before its acquisition by Lockheed Martin last year. Although it’s not yet certain if this particular technology will see the light of day, there’s plenty of life left in the artificial leaf sweepstakes.

JCAP graduate students Eric Verlage and Prineha Narang operate a spectroscopic ellipsometer. Image: JCAP

Take the Joint Center for Artificial Photosynthesis, for example. JCAP was launched in 2010 with the ambitious goal of a functioning artificial leaf prototype during its initial five-year, $116 million grant period. The Department of Energy-supported JCAP involves more than 160 engineers and scientists working at lead sites California Institute of Technology and Lawrence Berkeley National Laboratory, as well as at Stanford University at University of California campuses at Berkeley, Irvine, and San Diego. JCAP is testing potential catalyst and photoabsorber materials – based on nickel, iron, cobalt, and cerium oxides – to find ones that work well together in the same chemical environment. They report analyzing up to 1 million sample materials per day using an ink-jet-printer-like spotting technology to deposit arrays of samples on glass plates for high-throughput assay.

In anticipation of future industrial hydrogen production plants based on the technology, JCAP places a significant emphasis on prototyping and scale-up throughout the research process. Large prototypes help engineers gauge the potential effects of operating factors that would impact a facility based on massive cell arrays and related water handling and storage infrastructure.

Other Approaches

In other areas of development, a UC Berkeley group is taking the direct approach to solar water splitting. The team led by Peidong Yang and Bin Liu developed a free-standing paper-like mesh made of porous inorganic nanowires with properties that abet light absorption, charge carrier extraction, and catalysis. By immersing these structures in water, their proof-of-concept study achieved unassisted water-splitting without the use of electron mediators. They envision a scaled-up version of the process using a low-cost vacuum approach akin to the paper industry’s vacuum filtration process. The Berkeley group’s mesh uses an oxide photocathode made of strontium titanate, which is structurally similar to the perovskite materials used in traditional solar cells and, more recently, in artificial photosynthesis.

In fact, Michael Graetzel – the man credited with coining that very phrase – has recently introduced a high-efficiency device incorporating cheap and easy-to-make perovskite, which offer higher voltages for less cost than silicon or other rare-earth materials. His Laboratory of Photonics and Interfaces at Ecole Polytechnique (Lausanne, Switzerland), reported that its persovkite absorbers converted 12.3% of incoming solar energy into hydrogen. In 1991 Graetzel invented dye-sensitized solar cells, which turn visible light into electricity using photosensitive dyes that mimic the role of chlorophyll in nature. G24 Power (Newport, UK) markets a range of indoor and wearable devices based on DSSCs.

Michael MacRae is an independent writer.

Learn more about the latest energy technologies at
ASME’s Power and Energy 2015.

Artificial leaves, if scaled up, could significantly reduce greenhouse emissions, curb fossil fuel demand, and bring low-cost power to resource-starved regions in the developing world.


April 2015

by Michael MacRae, ASME.org