Bionic leaf converts energy from our sun better than nature does

Researchers at Harvard have created a device that mimics the natural process of photosynthesis, taking solar energy and converting it into chemical energy or liquid fuel. 

Researchers at Harvard University have created a system that allows them to store the energy of the sun, converting solar energy into chemical energy using a hybrid mechanism of inorganic chemistry and living organisms.

Comparing their invention with the natural process of photosynthesis, they refer to it as a “bionic leaf” or “artificial leaf,” and they say the level of efficiency they have achieved far exceeds that of other similar systems – including photosynthesis itself.

The paper, published Thursday in the journal Science, describes the work as addressing two fundamental goals: storing the energy of the sun, rather than merely converting it for immediate use, and building something useful from carbon dioxide in the atmosphere, thereby reducing a major greenhouse gas.

“I think this is actually quite exciting research,” Johannes Lischner of Imperial College, London, who was not involved in the study, tells The Christian Science Monitor in a telephone interview. “Converting sunlight into chemical fuels with high efficiency is something of a holy grail for renewable energy.”

The system works like this. A jar is set up containing little more than two electrodes, Ralstonia eutropha bacteria, and water. Electric current is passed through the electrodes, which then break down the water molecules, releasing hydrogen gas.

“You can use hydrogen as a source of energy, burn it,” says co-author Pamela Silver of Harvard University in a phone interview with the Monitor. “Instead, we decided to take advantage of bacteria that take in hydrogen and carbon dioxide and use them to grow.”

As they grow, explains Dr. Silver, these organisms produce certain compounds. The bacteria can be genetically engineered to make useful things like alcohol and plastic precursors.

Scientists have been trying to grow bacteria off water-splitting electrodes for decades, and while they have succeeded, certain constraints have defeated their efforts to create systems that run with any degree of efficiency.

Chief among these challenges were the leaching of heavy metals from the electrodes and the production of reactive oxygen species, none of which leads to happy, healthy bacteria. The critical innovation in this latest research was to use a cobalt-based water-splitting system.

“It is essentially self-healing,” Michael Strano of Massachusetts Institute of Technology, who was not involved in the research, tells the Monitor in a phone interview. “The anode and cathode synergize: As one degrades it feeds the other, and vice versa.”

Dr. Strano, whose work includes incorporating nanomaterials into actual leaves, describes this latest research as “pioneering,” explaining that the major innovation is the way in which the team has “used water-splitting chemistry and rendered it biocompatible.”

When scientists first started these kinds of experiments, as far back as the 1960s, with all of the obstacles they faced, they could only achieve efficiencies of about one percent, in terms of the conversion of solar energy into biomass. That is the figure the authors of this paper also award to photosynthesis, though some experts take exception to this, saying that it oversimplifies the complexities of energy use in plants. 

Nevertheless, this latest research boosts that figure to 10 percent, putting it well above the generally accepted threshold of eight percent that makes it worth considering for real-world applications.

“The fact that this research exceeds that figure is promising,” says Dr. Lischner, who is a Royal Society Fellow and lecturer in Imperial’s department of materials, “but the question is does it work as a real-world device?”

The researchers talk of using the mechanism in developing countries, where access to infrastructure for storing and accessing energy can be limited, and Lischner wonders whether these biological systems can survive in such areas as, say, the Sahara desert. But while the practicalities of application “remain to be proven,” he insists “the promise that this paper shows is quite intriguing.”

That hope is that this technology can be hooked up to photovoltaic cells, so that the energy of the sun is used to drive the water-splitting reaction. The bacteria could then be engineered to convert the hydrogen's energy into a multitude of carbon-based products, including biofuels and plastics, "essentially making products out of thin air," as co-author Brendan Colón describes it in a podcast.

Thus one of the major drawbacks of solar power – its inability to store energy to tide it through the hours of darkness – could be remedied. This research even has the potential to herald a breakthrough in worldwide efforts to reduce carbon dioxide in the atmosphere and turn that carbon into something useful.

Moreover, as Strano of MIT points out, the paper is suggestive of a new dawn in science, an area still in its nascency.

“There is a whole community that works on microbial biosynthesis, and on the other side you have inorganic chemistry” says Strano. “In this work you see a merging of these worlds.”