New materials carry out artificial photosynthesis

Brazilian university researchers develop artificial chlorophyll molecules capable of using solar energy and water to generate hydrogen and oxygen; the study was presented at an event in England.

By Elton Alisson, in Chicheley, England

Agência FAPESP – Plants’ capacity for photosynthesis has inspired scientists from various fields to attempt to produce artificial materials with similar properties in the laboratory.

A group of researchers at the Institute of Chemistry of the University of Campinas (Unicamp), for example, is developing nanometer-scale (one nanometer is one-billionth of a meter) materials capable of carrying out artificial photosynthesis to produce energy.

Some of the findings of these studies were presented on February 25 at the UK-Brazil-Chile Frontiers of Science conference. Organized by the Royal Society of the United Kingdom, together with FAPESP and the Brazilian and Chilean Academies of Sciences, the event was held until February 26 in Chicheley, southern England, to promote scientific and interdisciplinary collaboration among young Brazilian, Chilean and British researchers in fields on the frontiers of knowledge.

“Based on existing knowledge about the natural system of photosynthesis carried out by plants, we are trying to reproduce the essential points in the photosynthetic process using artificial materials for electrical power or even fuel from solar energy,” said Jackson Dirceu Megiatto Júnior, professor at the Unicamp Institute of Chemistry.

According to the researcher, who completed his doctorate with a fellowship from FAPESP, the idea for carrying out artificial photosynthesis was proposed in the early 20th century.

The project only began to take shape, however, in recent years as a result of important advances in the field, which have facilitated the synthesis of materials capable of using solar energy and water to generate hydrogen and oxygen in the laboratory.

Some of these advances have included the development of catalyst materials (that accelerate a reaction), which, when activated by sunlight, break down water molecules into hydrogen and oxygen.

This step in the process of photosynthesis is considered to be the most complex because the hydrogen and oxygen atoms are “stuck together” in the water molecules. That is why it was difficult to find a material that could selectively separate them yet not become degraded.

More recently, new materials such as silicon solar panels have been developed with the capacity of carrying out the process, known as “light-induced water splitting.” This, Megiatto said, opened the door to connecting these photoactive materials to conventional fuel cells – electrochemical cells that convert chemical energy into electrical energy by combining hydrogen and oxygen to re-form water molecules.

“The challenge now is to connect these materials to a fuel cell. If we can use the hydrogen and oxygen produced by these new materials in a fuel cell, we’ll be able to generate water along with electricity and complete the cycle of artificial photosynthesis,” Megiatto explained.

Natural materials

According to Megiatto, some of the limitations to using silicon solar panels to separate hydrogen and oxygen from the water molecules using sunlight are that the materials are expensive and difficult to process to the level of purity required for this purpose.

To find an alternative, researchers at the Unicamp Institute of Chemistry are looking to nature itself for materials capable of absorbing sunlight and generating energy (photovoltaic) and also serving as catalysts.

The most promising material identified is chlorophyll – the photosynthetic pigment that, in addition to imparting their green color, is used by plants to carry out photosynthesis.

“These molecules are nature’s way of absorbing sunlight. Their process of chemical synthesis, however, is difficult and expensive,” Megiatto said.

To overcome these barriers, the researcher began to synthesize artificial chlorophyll molecules called porphyrins during his post-doctoral studies in the United States.

In addition to being easier to synthesize than natural chlorophyll, the artificial molecules of the pigment are also easier to chemically manipulate, Megiatto explained.

“We have much more flexibility in designing photoactive materials using porphyrins rather than chlorophyll,” the researcher stated. “Using nanoengineering techniques, for example, we can optimize the properties of these molecules to increase their light absorption efficiency,” he said.

Another advantage of the artificial chlorophyll, according to Megiatto, is the greater chemical stability of the porphyrins. When inside the protein medium of natural photosynthesis, natural chlorophyll molecules are stable. By contrast, they undergo physico-chemical reactions and are rapidly degraded when extracted from the protein medium.

However, porphyrin has less of a tendency to present this type of behavior, said the researcher.

“When connected to catalysts, these materials have been shown to be very promising in transforming sunlight into chemical energy through the oxidation of water molecules; but right now, they are only being studied in aqueous solution and not in an actual photosynthetic device,” stated Megiatto.

“What we’re trying to do now is to form a photoactive polymeric film with these molecules to develop a solid material to put on the metal plates and semiconductors [electrodes] needed to operate a solar cell,” he explained.

Increased efficiency of photosynthesis

According to Megiatto, plants waste large amounts of solar energy during the natural photosynthetic process. He goes on to note that because it depends on energy for a series of needs, such as growth and survival, sugarcane, for example, uses only a small percentage of solar energy to convert carbon dioxide into sugars.

“The maximum efficiency of natural photosynthesis is approximately 10%,” said Megiatto. “Terrestrial plants have less than 1% photosynthetic efficiency, although some are capable of carrying out photosynthesis with an efficiency somewhere between 4% and 5%.”

To increase the efficiency of photosynthesis in plants such as rice, the international research consortium “C4 Rice” funded by the Bill & Melinda Gates Foundation, the International Rice Research Institute (IRRI) and research institutions in the United Kingdom is attempting to genetically modify its metabolism.

Rice and other grains, such as soybeans and beans, are known as C3 plants and have a greater capacity for growth and less photosynthetic efficiency than C4 plants, such as corn and sugarcane. By contrast, C4 plants have more efficient photosynthesis but are less able to grow quickly or cover large crop-growing areas than C3 plants.

Through changes in biochemical routes and the anatomy of the plant leaves, the researchers who took part in the consortium are attempting to develop a variety of rice that combines the properties of C3 and C4 plants.

“A rice variety with the properties of both C3 and C4 plants would have 50% more photosynthetic efficiency in the use of water and nitrogen than a variety that is not genetically modified,” said Sarah Covshoff, researcher from the University of Cambridge and participant in “C4 Rice,” during her presentation at Chicheley.

According to Covshoff, developing a variety of rice with the photosynthetic properties of C4 plants will signify progress in synthetic biology techniques. The goal of the international research consortium is to have a prototype of C4 rice by the end of 2016.

“The knowledge acquired through this project can also be applied to agricultural research to increase the yield of plants used for biofuel production,” she said.