Energy News Monitoring

Artificial photosynthesis is a technique that shows promising results in the search for viable alternatives to fossil fuels, as it is a way to harvest solar energy to create hydrogen from water. For this process, so-called water splitting devices are used, which have yet to live up to their potential because their design still does not have the right properties to function efficiently. Now, researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the Joint Center for Artificial Photosynthesis (JCAP) have developed a new device that turns sunlight and water into two different types of energy: hydrogen fuel and electricity.

In a recent study (2018) the researchers came up with a patent-pending design that comprises a single hybrid photoelectrochemical and voltaic (HPEV) cell. The HPEV utilizes the formerly unused potential of the photon-excited electrons and thus increases its total efficiency, much like cogeneration power plants which produce both heat and power from natural gas or coal.

The HPEV device has dual back contacts, which are already a widely applied technology applied in high-performance PV panels. To eliminate the drawbacks of the conventional systems the authors of the study added an additional contact to the rear of the silicon component, thus creating a device with two contacts at the back instead of one, as in conventional systems. By means of this additional contact, the current can be split in two: one part can be used to create solar fuels and the other can be used for extraction of electrical power. Thus the problem of energy losses in unused excited electrons could be avoided.

The possibility of producing hydrogen and electricity by means of artificial photosynthesis has been widely researched during the past decade. In 2013, a team of scientists used TiO2 electrodes which were sensitized to incoming light by means of a light-absorbing dye composed of ruthenium tris (bipyridine). The ruthenium dye improved the antenna effect and oxidized water with the help of an iridium oxide catalyst. The scientists also added an artificial electron transfer mediator which increased the efficiency of the dye considerably. Activated by light in the blue range, the mediator-aided dye-sensitized system split water molecules with a quantum efficiency of about 2.3%.

In 2016, scientists developed a solar fuel system that used sunlight to split water into hydrogen for storage as fuel. It consisted of a base unit that was made up of three thin-film silicon solar cells connected in series. On the back of each cell, the researchers installed two electrolyser electrodes made from nickel foam. This device, however, only showed an efficiency of 3.9%.

There are several apparent advantages to this new artificial photosynthesis system. Hydrogen is an important energy source because it is easy to transport and clean. It can be harvested in a twofold manner, to store and to produce electricity. The most recent artificial photosynthesis system discussed in this report has an extra outlet on the back which allows for the joint production of solar fuel as well as electrical power and overcomes the problem of the performance of mismatched components of previous designs. Also, the new device is able to use leftover electrons that do not participate in the fuel generation process to produce electricity, which increases the overall efficiency of the system. The new system has a total efficiency of 20.2%, which is three times better than traditional solar hydrogen cells. Also, conventional artificial photosynthesis systems do not perform as efficiently as the new system presented in this report as silicon inhibits the current flowing freely. Therefore, they generate relatively less current than they normally would and produce less solar fuel.

The researchers are confident that their continued collaboration will help to further develop the HPEV concept so that one day it can also be applied in other fields, such as CO₂ emission reduction. “This was truly a group effort where people with a lot of experience were able to contribute,” said Sergev. “After a year and a half of working together on a pretty tedious process it was great to see our experiments finally come together.”