Recipient Organization
VANDERBILT UNIVERSITY, THE
110 21ST AVENUE S STE 937
NASHVILLE,TN 372032416
Performing Department
(N/A)
Non Technical Summary
Photosynthesis, our planet's vast process for solar energy conversion, is powered by two nanoscale proteins--Photosystems I and II.Working together, these proteins fuel what is termed the Z-scheme of photosynthesis with high potential oxidations by Photosystem II and high (negative) potential reductions by Photosystem I.This project aims to extract these two powerful proteins from leafy green agricultural waste and combine them as the active components with other nanoscale materials in biohybrid solar cells to convert solar energy into electrical power.The preparation of solar cells from vastly abundant proteins via straightforward processing provides distinct advantages over most leading strategies that require energy-intensive processing (i.e. silicon) or rare and/or expensive materials. In addition, our approach represents a cleaner, more sustainable, and potentially more efficient route as compared to biofuels where the plant components are combusted. To close the gap on these more mature, yet less sustainable approaches, the project will provide several key innovations.Before these proteins are incorporated into the solar cell, their active sites will be equipped with improved capabilities for electron transfer, such as through conducting polymers and metal nanoparticles.Cathodes and anodes of high surface area will be prepared with aims to increase the loading of these proteins and the power of the cell.These electrodes will be designed to minimize short circuits and the back transfer of electrons that have hampered many biohybrid cells.A conductive gel electrolyte will be developed to provide more efficient current transfer.These Z-scheme, photosynthesis-inspired solar cells are designed to produce much greater electrical potential than those produced by other biohybrid solar cells, based on either Photosystem I or Photosystem II alone.Generally, this project will create high value-added products of nano-biomaterials from agricultural origins for non-food applications. Our approach of extracting these proteins from agricultural waste sources and equipping them for biohybrid solar energy conversion addresses key USDA goals of reduction of food waste/loss, reduction of greenhouse gas emissions, and increased protection of the environment.
Animal Health Component
10%
Research Effort Categories
Basic
80%
Applied
10%
Developmental
10%
Goals / Objectives
In this proposal, we are combining the relative advantages of wired and diffusible systems-- namely, fast electron injection from/to the electrode and a gel-based medium that is compatible with both planar and porous electrodes--to prepare portable solar cells that contain both PSI and PSII. The principal objectives for the proposed project are stated below:Examine the concentrations of active PSI and PSII from spinach that ranges from just harvested to up to a month beyond harvest time. Investigate the effect of these active concentrations on photoelectrochemical performance.Modify Photosystems I and II at their active sites with metal nanoparticles and wired conducting polymers, respectively, through photoelectrochemical processes.Design electrode interfaces to minimize charge recombination and enhance photovoltages toin vivolevels.Incorporate both PSI- and PSII-polymer conjugates in Z-scheme gel-based solar cells to elevate the cell potential and improve overall efficiencies.
Project Methods
To extract PSI and PSII from spinach, we will use a versatile and classical method,which consists of grinding the leaves in a buffer, filtering away larger components, centrifuging twice, and running a chromatographic column to isolate the proteins. This method enables separation of PSII from PSI during the column step, such that both proteins will beisolated from a single column. SDS-PAGE will be used to characterize each protein fraction for purity by breaking the protein complexes into their respective subunits, all separated by molecular weight in the gel. We will compare PSI and PSII active concentrations from spinach at different dates past harvest in two different ways: (1) the Baba assaywith potassium ferricyanide as a reductant of the P700+reaction center of PSI and an oxygen evolution assayfor the P680 reaction center of PSII; (2) the relative photocurrent of a micron-thick film of each protein on a gold electrode in a standard mediator solution.To prepare protein-polymer conjugates, a monomer from Table 1 of the proposalwill be added to a solution of PSII and placed under a solar simulator for times ranging from 0.5 to 5 h. The ratio of the concentrations of the monomer to the protein will be varied to affect the ratio of conducting polymer to protein, and the size of the resulting conjugate. Time will be used to control the extent of the reaction between the monomer and protein. After the desiredreaction time is completed, the solution will be dialyzed through tubing with a 100 kDa cutoff to remove unreacted monomer and any lose oligomers and then maintained for further use or centrifuged to yield the solid protein-polymer conjugates.The photoexcitedelectron at the FBsite of PSI has sufficient energy to reduce metal ions, such as Pt,and is compatible with numerous metal salts as Hill reagents.Here, we will investigate the photoreduction of Au and Ag salts to form nanoparticles at the FBsite to enable direct wiring between the reducing end of PSI and a suitable anode. In a typical experiment, Triton-solubilized PSI will be exposed to the metal salt while irradiated with red light in a solution containing sodium ascorbate and 2,6-dichlorophenolindophenol (DCPIP) as a fast reductant of the P700+site.We will use themodified photosystems to selectively orient the proteins on their respective electrodes, via reductive deposition and self-assembly. We will molecularly insulate the bound photosystems by growing an insulating polymer around them through surface-initiated polymerization strategies previously employed by the Jennings group.We will design and fabricate gel-based solar cells that incorporate the electrode design of Aim 3 for both PSII-coated cathodes and PSI-coated anodes with wired connections of each photosystem and molecular insulation of the uncoated electrode areas (Fig. 10). For the electrodes of high surface area, mesoporous cathodes and anodes will be fabricated from metals and metal oxides, respectively. We and others have demonstrated the ability to prepare mesoporous ITO films by a polymer templating approach (Fig. 6),and we have shown that the additional electrode interfacial area dramatically enhances PSI photoelectrochemical performance. Further, we have assembled PSI proteins into a nanoporous gold electrode, prepared by dealloying a 100 nm sheet of Au/Ag alloy, with photocurrent enhancements that scale with the electrode area.While gold and FTO or ITO will be the initial substrates of choice for the anode and cathode, respectively, we will modify the materials to alter their work functions with the goal to increase the potential difference of the cell. For example, depositing polyethyleneimine onto FTO reduces its work function by 1 eV. Likewise, the work function of gold can be increased by up to 1 eV by depositing an aromatic thiol onto the surface.Both of these strategies will be employed to more closely align the energy levels of the cathode and anode with the PSII and PSI conjugates, respectively, while maximizing the potential difference of the cell.The past decade has seen tremendous advances in both electron-conducting gels with applications in a host of medical and non-medical areas and ion-conducting gels (or ionogels) with uses in batteries and capacitors. The combination of electron- and ion-conducting gels has been demonstrated by Zhou et al. combining PEDOT:PSS with a biopolymer and an ionic liquid.The polymer gel shows high electrical conductivity (11 S/cm) and 400% stretchability. We feel that polymers with this dual conduction capability would be extremely beneficial for biohydrid solar cells if processed into ultra thin films. Such a mediating gel would enable the photosystems to be in a wet environment, similar to that in nature, while offering electron conduction to speed the charge transfer between electrodes.The electron- and ion-conducting gel will be prepared from an electron conductor such as PEDOT:PSS along with an ion-conducting polymer, an ionic liquid or water, and a redox mediator, such as Fe(CN)6_4-/Fe(CN)6_3-, that is compatible with both PSI, PSII, and PEDOT:PSS. The ionic liquid or water and redox mediator will be deposited into the porosity of the electrodes with the purpose to rapidly transfer electrons from PSII at the cathode or to PSI at the anode in the near-electrode space. The dual electron- and ion-conducting gel will then be spin-coated into a thin (< 500 nm) film atop either electrode.