MnO2-Modified 3D-Printed Lattice Photo-Bioelectrode for Photosynthetic Energy Conversion from Spinach Thylakoids

Abstract

Photosynthetic bio-solar energy conversion has been widely investigated as a promising potential in the field of renewable energy due to its high internal quantum efficiency. Thylakoid membranes (TMs) are organelles in chloroplasts where photosynthesis occurs. Once photosynthesis begins photosynthetic electrons (PEs) were generated, and they were transferred via proteins embedded in TMs by sequential redox reactions. To efficiently collect PEs, several approaches were reported to improve electrical connections such as carbon nanotube modified electrode [1], graphene oxide (GO)/TMs composites [2], or ruthenium oxide (RuO2) modified electrode [3]. As another approach, TM-alginate films were electrosprayed on SU-8 micro-pillar electrode to enlarge the electrochemical surface area between TMs and electrode [4]. However, the previous works were still limited in using two-dimensional electrodes with a marginal increase of electrode surface area.In this study, we aimed to maximize the electrochemically-active surface of TM-decorated bioelectrodes using 3D-printed polymeric lattices. We designed an octet-truss lattice structure with a high specific surface area. For the large surface area of the electrode with a high volume fraction, a strut diameter of the lattice was set to be 0.4 mm in the unit cell of an octet-truss lattice with a total length of 2.5 mm. To maximize light transmission, the octet-truss lattice was 3D-printed with a clear resin using stereolithography (SLA). To grant electrical conductivity on the hydrophobic nature of poly(urethane dimethacrylate) resin, poly(dopamine) (PDA) was polymerized as a binder on SLA-printed lattices followed by immersion in the 1.3 wt% of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution for 30 min. After immersion in PEDOT:PSS solution, PEDOT:PSS coated lattice was gently dried to remove the remaining solution with N2 gun and annealed in a vacuum oven at 120 ℃ for 15 min. On the other hand, the intrinsic electrical conductivity of PEDOT:PSS is lower than 1 S/cm. Therefore, to enhance the conductivity of annealed PEDOT:PSS layer, PEDOT:PSS coated lattice was immersed 5 wt% of ethylene glycol (EG) solution. Then, EG-treated PEDOT:PSS coated lattice was annealed in a vacuum oven as above described. Due to the removal of surplus insulating PSS chains and reorientation of PEDOT chains, the resistance of EG-treated PEDOT:PSS coated lattice was measured thousands of times lower than no treated lattice. A manganese oxide (MnO2) was electrochemically deposited for improved attachment to TMs at 0.5 V vs Ag/AgCl for 2 min. Then, TMs of 1 mg chl/ml were drop-cast on MnO2/PEDOT:PSS/PDA octet-truss lattice electrode. Each intermediate product was carefully analyzed using SEM, optical microscopy, and fluorescence spectroscopy. The PE currents from TM/MnO2/PEDOT:PSS/PDA octet-truss lattice electrode were measured and compared to a flat electrode and with different layers of lattice structures. With 2 layers of TM/MnO2/PEDOT:PSS/PDA octet-truss lattice electrode, the PE currents were increased 4 times compared to PE currents of the flat electrode. Acknowledgement This work was supported by the National Research Foundation of Korea(NRF) Grant funded by the Korean Government(MSIT) (No. 2020R1A2C3013158), and the Human Resources Development program (No.20204030200110) of the Korea Institute of Energy Technology Evaluation and Planning(KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.