Components of artificial leaf

The Artificial Leaf Alternative source of energy

1Outline IntroductionWhat is artificial leaf?Components of artificial leafCo-OEC Catalyst Silicon baseNickel-Molybdenum-Zinc Catalyst (NiMoZn)How it works? MechanismApplicationsConclusion

Genius InventorDaniel G.Nocera is the Professor of Energy at the Massachusetts Institute of Technology.Studies energy conversions in biology and chemistry focusing on the generation of solar fuels.Designed an artificial leaf which is able to produce fuel like it occurs as in real leaves via photosynthesis.

The Artificial Leaf

The artficial leaf is a photosynthesis mimicking construct, which could be used to produce a fuel from the solar energy.Photosynthesis in Green Plants

The electrons are transferred via a series of redox-active cofac- tors to photosystem I (PSI). These cofactors include plastoquinol (PQH2) and plastocyanin (PC) and those bound within the cytochrome b6f (cyt b6f) complex finally arriving at ferredoxin (Fd) bound to PSI (see Figure 2). Through the action of ferredoxinNADP reductase (FNR), these energized electrons reduce protons to produce hydrogen in the form of NADPH. In executing this reaction sequence, the plant stores solar energy in a fuel-forming process resulting from rearranging the bonds of water.

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Basic PrincipleEnergy is stored in the bonds of the carbohydrates. Natural photosynthesis process involves Calvin cycle, where hydrogen particle is introduced as a NADPH and forms carbohydrates for plants metabolic activity. These organic compounds mostly stored as an ATP for further usage.6Components of artificial leaf

Structure of the Artificial Leaf

3 parts of the leaf:Cobalt CatalystNickel-Molybdenum-Zinc CatalystSilicon

OEC water splitting compoundOEC- the compound that achieves water splitting in photosynthesis.Naturally, it resides in photosystem II(PSII).Natural components of OEC is manganese(Mn), calcium(Ca) and Oxygen(O).They form a cubane structure to become activated.OEC(oxygen evolving complex)Cubane is self-assembled when Mn2+ is oxidized to Mn3+ .The formation of cubane is carried by the photooxidation reaction.Natural Cubane Structure

The Co-OEC replaces the OEC in artificial leaf.It also has a self-healing property.It self-assembles when Co+2 oxidized to Co+3.The Co-OEC forms dimer by enhancing its function than original natural OEC.

The Co-OEC

The difference in dimer formationWater splitting in Artificial leaf

Water splitting is accomplished by PCET where Pi (or Bi) anion manages the coupling of the proton to the electron released from water oxidation; the anion also provides a mechanism for repair.

14HEC hydrogen evolving complexHEC (ferredoxin) the compound that reduce protons to produce hydrogen in the form of NADPH.Naturally, it resides in photosystem I(PSI).It is an enzyme, ferredoxin NADP+ reductase (FNR), which has in its structure iron ions.

HEC (hydrogen evolving complex) The NiMoZn catalyst replaces the HEC in the artificial leaf.We can use Pt (platinum) as a reducing catalyst.BUT, the Pt is very expensive and it is very rare metal.Therefore, it was constructed the catalyst from cheaper and more accessible metals like nickel, zinc and molybdenum.Nickel Molybdenum-Zinc CatalystSilicon BridgeSilicon acts as a bridge between the Cobalt Catalyst and NiMoZn Catalyst.Its function is the same as the ETC and photosystem membrane in the natural photosynthesis process.It absorbs solar energy and excites the electrons to deliver to other components.

Silicon JunctionBetween the Co-OEC and silicon, there is a layer of ITO (Indium tin oxide) to protect the silicon from oxidation. Stainless steel is also included as a layer for structural support.Additional Bridge Parts20

Artificial Leaf ModelSilicon itself has 3 distinct regions in electron transfering.They are:1) n-Si part2) p-Si part3) p+-Si part Silicon-semiconductorSilicon-semiconductor parts

In the artificial leaf, the Co-OEC, silicon semiconductor, and NiMoZn catalyst work together. Figure 5 is a simplified model of the leaf. The positively charge holes in the p-Si migrate towards the Co-OEC and accept the electrons that were oxidized from water. The electrons in the n-Si migrate toward the NiMoZn alloy and become available for HER. Because the doped silicon is a semiconductor, energy is required for the conduction of electrons and holes. In the case of the artificial leaf, sunlight is the energy source and allows the entire system to operate. The presence of a semiconductor makes a wireless leaf possible.When photons hit n-Si, electrons move into the conduction band and are available for HER. In the p-Si, electrons move from the conduction band to the valence, producing holes that can accept electrons from the Co-OEC. The p+-Si part serves as a tunnel between p part and ITO.

23A model artificial leaf

Protons travel from the more positive end of the artificial leaf to the more negative end, where they are catalysed into molecular hydrogen. It is clear thatwater and sunlight are needed for the entire process. However, the water does not have to be clean the leaf works in most typical environments. The normal level of sunlight is enough to operate the leaf.24Natural vs artificialPhotosynthesis in naturePhotosynthetic membranes catch photons(PSI and PSII).PSII OEC splits water into O2 and 4H+.Ferredoxin of PSI converts protons into NADPH.

Artificial leafSi junction catches photonsCo-OEC splits water into O2 and 4H+NiMoZn converts protons into H2

How can we use it?!Making fuel from sunlightThrough photolysis of water(by splitting it into O2 and H2)

H2 O2Just as in the photosynthesis in plants, in artificial leaf model we hae the same principle of photolysis-splitting of water into hydrogen and oxegen gases. Both of them can be collected and used in different purposes.Reuse of dirty waterEarlier only pure water was usedAbility of catalysts to obtain energy from bacteria contaminated waterLeaf was improved(self healing catalysts)

Advantages of artificial leafIt can operate in environmental conditions (does not require special conditions).It is made of cheap and earth-abundant materials (Si, Ni, Co, Mo).Wireless (easy to transport and distribute).Not harmful (no harmful byproducts).From 1 liter of water 13.2 MJ could be obtained

It can not be applied in water scarce regions (like African countries).It will not work in the absence of sun.There are some difficulties in separation of H2 and O2.

Disadvantages of artificial leafIndependent electricity production at home

Application

3-5 liters of H2O = 1 dayConclusionThe Artificial Leaf components:Co-OECNiMoZnSi junctionCheap, accessible metals.Personalized source of energy.BUT, it needs further improvements (storage, separation, efficiency).Thank you for your attention!

Do you have any questions?Nocera, D. 2012. The Artificial Leaf. Accounts of Chemical Research, v.45, no.5,p.767-776.Eichelman, K., Leedy, A. 2013. Mimicking photosynthesis using a silicon based andcobalt catalysed artificial leaf to produce hydrogen. University of Pittsburg, SwansonSchool of Engineering. http://136.142.82.187/eng12/Chair/data/papers/3152.pdf (July 31, 2013).Victoria Ng. 2013. A Leaf of Another Kind. Cluster 8:The Chemistry of Life. (August 2, 2013).http://web.mit.edu/newsoffice/2011/artificial-leaf-0930.htmlhttp://cleantechnica.com/2013/04/11/artificial-leaf-low-cost-solar-cell-for-hydrogen-fuel-cell/http://www.euronuclear.org/info/encyclopedia/coalequivalent.htm

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