From Exfoliation of Graphite to Graphene-based Electrodes for a Biophotoelectrochemical Application

Graphene as a two-dimensional atomic crystal, has gained a tremendous research interest due to its unique electrical, optical, and mechanical properties. In order to utilize the full potential of graphene, there is a need to develop a commercially viable and environmentally friendly method for mass production of high-quality graphene. High-shear exfoliation of graphite is a cost-effective and scalable liquid-phase exfoliation (LPE) method for producing dispersions of defect-free and unoxidized few-layer graphene. However, the drawbacks of LPE, such as low graphene concentrations, long processing times and solvent residuals, hamper the scalability of the process and applicability of the dispersions. In this work, natural flake graphite obtained from Haapamäki, Finland was exfoliated in environmentally friendly aqueous media using either sodium cholate (SC) surfactant or cellulose nanocrystals (CNCs) as stabilizers. Both SC and CNC are non-toxic materials derived from renewable sources. Concentrations as high as 3.0 mg ml-1 (3 % yield) and 4.0 mg ml-1 (4 % yield) were produced only after 2 h of shear exfoliation in SC and CNC dispersing media, respectively. The produced material was of high quality and mostly less than 5 graphene layers thick as investigated with atomic force microscopy and Raman spectroscopy. The high concentrations of the few-layer graphene dispersions are attributed to a proper choice of process parameters such as appropriate
temperatures and stabilizer concentrations. From SC- and CNC-stabilized dispersions, electroconductive thin films (hereafter graphene and graphene-CNC films, respectively) with a high electron transfer efficiency were fabricated by spray coating on non-conductive glass substrates.

The potential of using the prepared graphene-based thin films as anode materials in a biophotovoltaic (BPV) application that harnesses photosynthetic microorganisms, such as cyanobacteria, for solar-to-electricity conversion was investigated. The biocompatibility studies with photosynthetic microorganisms (cyanobacterial cells and microalgae) revealed no toxic effects as the cells maintained their photosynthetic performance and growth when placed in direct contact with the materials. Furthermore, the surface characterization revealed hydrophilic films with nanoscale roughness, which are desired properties for anodes interfacing cyanobacterial cells. Cyclic voltammetry experiments demonstrated the electroactivity and stability of the electrodes in aqueous electrolyte solutions. Both graphene and graphene–CNC electrodes were able to harvest photocurrent from the cyanobacterial cells during illumination in threeelectrode photoelectrochemical set-ups. Furthermore, the average photocharges with both electrode types were higher than with indium tin oxide–coated glass electrodes (more commonly used in BPVs) under similar conditions. As indium is a rare and expensive metal, the few-layer graphene and graphene–CNC composites present a more sustainable alternative as anode materials for renewable electricity generation in BPVs due to their abundant source materials and efficient fabrication method.