Photosynthetic Electrochemical Cell Based on RuO2 Nanosheets Modified Bio-Anode
Authors: Hong Hyeonaug , Lee Jang Mee , Yun JaeHyoung , Kim Yong Jae , Kim Seon Il , Shin HyeIn , Ahn Hyun S , Hwang Seong-Ju , Ryu WonHyoung 
Published: May 2020

Thylakoids are the photosynthetic complexes that absorb light energy to produce photosynthetic electrons (PEs) in the chloroplast of plant cells. PE production starts from water splitting at photosystem II (PSII) in the thylakoid membrane and they are excited by absorbing photon energy. Over the last decade, many researchers have attempted to extract PEs from thylakoid by enhancing attachment of thylakoid membranes to an electrode or using artificial mediators. However, these have some limitations including increased ohmic resistance from the length of tethering molecules and energy loss due to artificial mediators. In this study, we propose a photosynthetic electrochemical cell based on RuO2 nanosheet modified electrode for the photosynthetic energy harvesting from thylakoids. RuO2 is known as one of the most conductive metal oxides. Metal oxides have partially positive charges that can improve attachment of negatively charged thylakoids to the electrode. Also, partially positively and negatively charged characteristics of RuO2 surfaces induce ensemble docking between electron carriers and electrode surface, which can attract both negatively charged electron carriers and positively charged electron carriers. Furthermore, RuO2 nanosheets show a unique characteristic of proton adsorption, which can sustain positive electrical potential during PEs extraction. For photosynthetic energy harvesting, the photosynthetic electrochemical cell was constructed with anode based on thylakoid and air-cathode based on Pt-C, which separated by a proton exchange membrane. The light was controlled by a 100 W halogen lamp to operate the photosystem so that excited PEs can be extracted to the electrode. The surface potential of RuO2 nanosheets modified electrodes was 250 mV vs. Ag/AgCl, which shifted positively more than 100 mV than that of the bare Au electrode. From the inspection of RuO2 lattice by x-ray absorption spectroscopy, positively shifted surface potential is the result of proton adsorption. A positively biased electrode surface can induce a large amount of PEs extraction from the electron carriers than the Au electrode. When the light is illuminated on the thylakoids deposited electrodes, 8.9 ± 6.3 mV and 88.7 ± 13.8 mV of the surface potentials were measured from the Au and the RuO2 modified electrode, respectively. These lowered electrode potentials are due to the redox potential of the electron carriers on the electron transport chain of the thylakoid membrane. The open-circuit voltages (OCV, the subtracted potential between the cathode and the anode) were measured 575.1 ± 6.4 mV and 424.0 ± 17.2 mV from the Au and the RuO2 electrode, respectively. Although OCV of the RuO2 electrode is lower than the Au electrode, the power density of the RuO2 electrode shows 4 times higher than the Au electrode, and this is due to the enhanced PEs extraction by both the surface charge and the proton adsorption characteristics of RuO2 nanosheets.