Coupling Biocatalysis with Photocatalysis for Construction of a Photobioelectrochemical Cell
Authors: Riedel Marc , Lisdat Fred 
Published: July 2021

The integration of photoactive proteins such as photosystem I and II in electrode architectures has gained much attention during the last years for light-to-current and light-to-chemical conversion [1,2]. Here, we have combined the photophysical properties of the biological photosystem II (PSII) with a second artificial, light-sensitive entity (PbS quantum dots). This biohybrid approach mimics the natural photosynthesis since also here two photoactive entities are combined for the whole process of water splitting and usage of electrons for energetic purpose.
For the electrode construction quantum dots (QDs) and PSII (from T. elongatus) have been integrated in a 3D TiO2 electrode architecture in order to ensure a large interface for the loading of high amounts of both photoactive entities. The electrode structure is prepared by a template-based approach employing a simple spin coating procedure to adjust the thickness of the 3D structure [3]. TiO2 is chosen as a basic electrode material since it provides advantageous properties for charge carrier separation of photo-excited electron-hole pairs from QDs. The PbS QDs are directly synthetized via a successive ionic layer adsorption and reaction approach (SILAR) on the TiO2 surface and allow the excitation with visible light [3]. In order to connect the PSII efficiently with the TiO2/PbS electrode an osmium-based redox polymer (poly(1-vinylimidazole- co-allylamine) - Os(bipy)2Cl) has been used, which mediates the electron transfer from PSII towards the QDs under illumination [4].
The electrode architectures have been characterized by scanning electron microscopy, wavelength dependent measurements, photochronoamperometric and photovoltammetric experiments. It is shown that the photocurrent magnitude can be well correlated to the amount of PSII integrated within the 3D electrode architecture. Interestingly the electrons from the water oxidation can be collected at very low potential starting from -0.55 V vs Ag/AgCl. This is about 0.2 V below the redox potential of the acceptor site in PSII and illustrates one beneficial feature of the new developed system.
This photobioanode has been combined with a biocathode hosting bilirubin oxidase (BOD). Here, a transparent electrode has been developed which also allows the illumination of the whole cell through the cathode. For this purpose 3D antimony tin oxide (ATO) electrodes have been prepared with the same template procedure. A high transparency can be ensured even when 8 layers of inverse-opal ATO are deposited on FTO electrodes (Fluor-doped tin oxide). The ATO surface has to be modified with pyrenecarboxylic acid in order to provide a suitable surface for the direct electron transfer of BOD. Here, current densities of about 135 µA/cm2 have been obtained in air-saturated solution for the oxygen reduction process.
By combing both electrodes, a photobioelectrochemical cell (PBC) can be fabricated, which does not need any fuel to be supplied, but allows the light-driven generation of electricity by regeneration of O2 and H2O at the photobioanode and biocathode, respectively. The PBC gives an extraordinary high open cell voltage of about 1V under illumination and a maximum power density of about 50µW/cm2.
[1] V. M. Friebe, R. N. Frese, Curr. Opin. Electrochem. 2017, 5, 126.
[2] D. Ciornii, M. Riedel, K. R. Stieger, S. C. Feifel, M. Hejazi, H. Lokstein, A. Zouni, F. Lisdat, J. Am. Chem. Soc. 2017, 139, 16478.
[3] M. Riedel, W. J. Parak, A. Ruff, W. Schuhmann, F. Lisdat, ACS Catal. 2018, 8, 5212.
[4] M. Riedel, J. Wersig, A. Ruff, W. Schuhmann, A. Zouni, F. Lisdat, Angew. Chem. Int. Ed. 2019, 58, 801.