A Tandem Solar Biofuel Cell: Harnessing Energy from Light and Biofuels

Abstract We report on a photobioelectrochemical fuel cell consisting of a glucose‐oxidase‐modified BiFeO3 photobiocathode and a quantum‐dot‐sensitized inverse opal TiO2 photobioanode linked to FAD glucose dehydrogenase via a redox polymer. Both photobioelectrodes are driven by enzymatic glucose conversion. Whereas the photobioanode can collect electrons from sugar oxidation at rather low potential, the photobiocathode shows reduction currents at rather high potential. The electrodes can be arranged in a sandwich‐like manner due to the semi‐transparent nature of BiFeO3, which also guarantees a simultaneous excitation of the photobioanode when illuminated via the cathode side. This tandem cell can generate electricity under illumination and in the presence of glucose and provides an exceptionally high OCV of about 1 V. The developed semi‐artificial system has significant implications for the integration of biocatalysts in photoactive entities for bioenergetic purposes, and it opens up a new path toward generation of electricity from sunlight and (bio)fuels.

Construction of IO-TiO2|PbS|POs|FAD-GDH Electrodes: FTO-coated glass slides were cleaned as described above. IO-TiO2 structures were constructed by a protocol established by us previously. [3] Briefly, a mixture of 100 mg ml -1 TTIP and 100 mg ml -1 latex beads with a diameter of 0.8 µm was prepared in isopropanol and deposited by a spin coating approach onto the FTO slides. Eight layers of the TTIP/LB mixture were spin coated at 80 rps in order to obtain an IO-TiO2 structure with a thickness of about 10 µm. Subsequently, the electrodes were sintered at 450 °C under air for 2 h. After preparation of the IO-TiO2 architecture PbS QDs were directly grown on the electrodes by a successive ionic layer adsorption and reaction (SILAR) approach following a previously reported protocol. [3] Therefore, the electrodes were alternately immersed four times in aqueous 20 mM Pb(NO3)2 and aqueous 20 mM (NH4)2S solution for 1 min, respectively. In order to remove unbound precursors the electrodes were carefully rinsed with deionized water and ethanol between the deposition steps. The redox polymer and FAD-GDH were integrated into the final IO-TiO2|PbS electrodes by a coassembly approach, Therefore, 4 µl of a mixture containing 5 mg ml -1 POs and 5 mg ml -1 FAD-GDH (in 5 mM MES buffer pH 7) was deposited onto the electrodes in the dark. After 15 min the electrodes were carefully rinsed with buffer (50 mM potassium phosphate buffer pH7) to remove unbound material.

Construction of the Photobioelectrochemical Tandem Cell:
For the construction of the PBTC the BiFeO3|GOx photobiocathode and the IO-TiO2|PbS|POs|FAD-GDH photobioanode were arranged opposite each other. The cell was filled with 50 mM potassium phosphate buffer pH7 with or without 100 mM glucose and illumination was performed through the cathode side.

Photoelectrochemical and Electrochemical Experiments:
Photoelectrochemical and electrochemical measurements were performed with an integrated photoelectrochemical workstation from Zahner, containing a potentiostat for the light control and a second potentiostat (Zennium) for the electrochemical control. In the case of an illumination, a white light source (410−800 nm) was used unless stated otherwise. Wavelength-resolved measurements were performed with a monochromator (Polychrome V, Till Photonics) with a bandwidth of 15 nm. The (photo)electrochemical characterization of the photobioanode and the photobiocathode was performed in a homemade electrochemical cell with a three-electrode arrangement, consisting of a working electrode (photobioanode or photobiocathode), a platinum wire as counter electrode, and an Ag/AgCl, 1 M KCl, reference electrode. For the cell experiments, linear sweep polarization curves were recorded from the open circuit voltage (OCV) to 0 V with the photobioanode connected as working electrode and the photobiocathode as combined reference/counter electrode. All measurements were performed in 50 mM potassium phosphate buffer pH7.

Figure S2
Tauc plot of a BiFeO3 electrode obtained from UV/Vis analysis. [4] From this, a direct optical band gap of about 2.7 eV has been obtained.

Figure S8
For the determination of the enzymatic activity of GOx bound to the BiFeO3 electrode, an activity assay has been performed using 2,6dichlorophenolindophenol (DCPIP) as electron acceptor. Therefore, a calibration curve using different GOx activities in solution has been recorded by correlating the DCPIP conversion rate to known GOx activities. The same assay has been carried out on the BiFeO3|GOx electrode, allowing an estimation of the bound GOx activity using the calibration curve. Since the GOx is bound to the electrode and the calibration curve has been recorded with GOx in solution, this only gives an approximation of the immobilized GOx activity.