Researchers from the Israel Institute of Technology (Technion) used a succulent plant to create a living “bio-solar cell” that runs on photosynthesis. The demonstration showed how the natural processes that take place in living plants could lead to a new source of green energy. The development yielded the first such cell to run on photosynthesis, according to the researchers. Typically, the researchers said, harvesting an electrical current from biological photosynthetic systems such as live cells or isolated complexes is achieved by immersion of the system into an electrolyte solution. In all living cells, electrons are shuttled around as part of natural, biochemical processes. If electrodes are present, the cells can generate electricity that can be used externally. Previous researchers created fuel cells in this way with bacteria — though the microbes in the system had to be constantly fed. Instead, the Technion team turned to the photosynthesis process, in which light drives a flow of electrons from water in the plant, and the electron flow generates oxygen and sugar. Living photosynthetic cells produce a constant flow of electrons within the plant, which can be pulled away as a photocurrent and used to power an external circuit, like a solar cell. The researchers used the succulent plant Corpuscularia lehmannii, also called the ice plant, to develop a biologically based photoelectrochemical cell. The researchers used the succulent’s store of internal water and nutrients as the electrolyte solution for an electrochemical cell. They then tested whether the plant’s photosynthesis process could be used to create power and transform the plant into a living solar cell. The researchers inserted an iron anode and platinum cathode into one of the plant’s leaves and found that its voltage was 0.28 V. When connected into a circuit, it produced up to 20 µA/cm2 of bias-free photocurrent density when exposed to light — and could continue to produce current for over a day. When 0.5 V bias was added to the iron anode, the current density increased by about tenfold. Researchers in Israel showed that the ice plant succulent shown here can become a living solar cell and power a circuit using photosynthesis. The researchers inserted an iron anode and platinum cathode into one of the plant’s leaves and found that its voltage was 0.28 V. Adapted from ACS Applied Materials & Interfaces, 2022, DOI: 10.1021/acsami.2c15123. Although these values are less than that of a traditional alkaline battery, they are representative of just one leaf. According to the researchers, previous studies on similar organic devices suggest that connecting multiple leaves in series could increase the voltage. Further, the addition of a photosystem II inhibitor hindered the photocurrent, which indicated that water oxidation is the primary source of electrons in the light. Two-dimensional fluorescence measurements showed that the coenzymes nicotinamide adenine dinucleotide + hydrogen (NADH) and nicotinamide adenine dinucleotide phosphate (NADPH) serve as the primary mediating electron transfer molecules, functionally connecting the photosynthesis process to the metal electrodes. The team designed the living solar cell so that protons within the internal leaf solution could be combined to form hydrogen gas at the cathode, for use in other applications. The approach to green energy offers a way to absorb carbon dioxide CO2 while simultaneously producing an electrical current with minimal engineering requirements. The team is hopeful that its method will enable development of future sustainable, multifunctional green energy technologies. The research was published in ACS Applied Materials & Interfaces (www.doi.org/10.1021/acsami.2c15123).
