 Welcome back to the lecture series in NPTEL on bioelectricity. So, we are on the thirty fourth lecture. So, last lecture we talked about photosynthesis. So, now we will be talking about we finish the photosynthesis. Now we will be talking about the inspiration we have drawn from photosynthesis and we will be talking about this topic at two level. One the inspiration which has led to the development of a grad cell cells or disensitized solar cells. And the second one which we will be talking about is the inspiration drawn from the manganese cluster. Just a brief recap for those of you who are joining in this lecture. So, talking about photosynthesis in the last lecture or last two lectures we talked about. So, there are two major things what we observed. One thing we observed that chlorophyll dye has the ability to absorb light and ejects an element. So, essentially this chlorophyll dye is a natural photovoltaic material. So, this is one thing what has inspired generations. Hence, we know this thing. The second important thing about chlorophyll in terms of the inspiration is the ability to split water which is essentially if you remember my last lecture I told you the most currently in this current earth ecosystem most abundant source of electrons. And this water splitting cluster which was which we talked about is present on just underneath PS 2 or for system 2 or P 6 80. So, we want to know recollected is a manganese cluster ascertation surrounded by a series of proteins which is called the oxygen evolving complex OEC. So, these are the two zones. So, if you look at the slide these are the two zones where we have drawn inspiration. Synthetic chlorophyll as solar cells it is this part and the second one mimicking water splitting cluster for generation of hydrogen. There is significant amount of hydrogen which is generated out here and this is what we call as hydrogen as where you see all these hybrids and hybrid cars and everything. These are the two things. So, talking about the chlorophyll dye there are several other dye and we will have exclusive one lecture on this one which will fall under dye sensitized solar cells. We will talk about this in detail which is also commonly called as after the name of its innovator Gretzel called Gretzel cells. So, we have a totally to least separate one lecture on Gretzel cell or dye sensitized solar cell what are the different dyes and in that you will come across several dyes you know ruthenium dyes and you will talk about natural flower dyes likewise. Today we will be talking about hydrogen as a fuel by mimicking the manganese cluster water or if not mimicking by deriving inspiration from the manganese cluster which is in which is present in the photosystem 2 of the thylakoid membrane. So, let us move on what we meant by the artificial photosynthesis for solar water splitting. So, essentially hydrogens generated from the solar driven water splitting has a potential to be a clean sustainable and abundant energy source inspired by natural photosynthesis. Artificial solar splitting complex or devices are now being designed and tested recent development based on molecular nanostructure designs have led to advancement in our understanding of light induced charge separation and subsequent catalytic water oxidation and reduction reaction. There is a very nice review which I have cited here in the nature photonics came in the year 2012. Please go through this review and most of the presentation which is which I will be doing today is from that specific paper and there are several good references and you will see the reference of Gretzel wonderful paper there is a paper by Gretzel in published nature. The title of the paper is photo electrochemical cell please go through this paper there is a cross reference of this paper in this particular paper what I am referring now go through this paper this will help you to you know appreciate this whole thing far better than there is a wonderful review wonderful the good paper and that by Helmut Rebocch you will again see the reference in this nature photonics paper goes through the work of Helmut Rebocch. So, these all will help you to get a very global idea how the world is now thinking where it is all moving. So, coming to the solar to chemical energy conversion solar to chemical energy conversion is the ultimate goal for the scientist in the field of energy generation. This process does not emit any greenhouse gases and the chemical energy can be stored and used when required which is in the form of hydrogen. Plants form the conversion through natural photosynthesis which navarivated as NPS in which oxygen and carbohydrates are produced from water and carbon dioxide using sunlight that we have already discovered. The energy conversion efficiency of NPS can reach around 7 percent under optimum conditions although an efficiency of less than 1 percent is usually expected for agricultural crop over their entire life cycle. So, in the previous lecture while I was talking to you about the efficiency of natural photosystem. So, if you remember I was telling you that you are compromising on compromising efficiency of natural photosynthetic systems which is approximately 7 percent. The reason possibly with what I cited is in those one thylakoid complexes there are lot of when the chlorophylls are getting are exciting getting excited and they are getting oxidized and they are ejecting an electron. There are lot of free radicals which are formed. Now, in order to prevent free radical damage this process is slowed down and that is what you see the 7 percent efficiency is in order to ensure that the free radicals which are formed there does not damage the system. So, that we fail to you know recuperate the system and it loses it ages very faster. Possibly that could be one of the possible reasons. Now, moving on to the next slide see the artificial photosynthesis what we really meant by that. So, you have two words which are coming for you now. One is natural photosynthetic system and the other one is APS which is A stands for artificial and it stands for the natural. So, coming back to the slide a potentially more controlled technology for the solar to energy conversion chemical energy conversion processes artificial photosynthesis which aims to emulate natural photosynthesis using manmade materials. APS has been a fascinating scientist in field ranging from material science to physical to inorganic chemistry. However, it remains a significant challenge to construct an efficient APS device capable of producing molecular fuels such as hydrogen at a scale and cost that can compete with fossil fuels significant advances in efficiency are required before such devices will be able to compete with conventional energy sources. So, this is what is competing with the other fuels like coal, petrol, diesel and of course, you have the nuclear. So, this process has to be cheap green and fast and fast scalable without this this cannot really be achieved the way we are dreaming of a world of clean and green energy and nature has already done it nature is already doing electron donation. And if you remember in the last thing I told you that if I have to put three words for the whole evolution to electron donor who is the electron and acceptor third one who is the abundant very abundant source of electron that is it this is all about this how you really can manipulate this to the next level coming back to the slide. So, what are the basis of the basic of the natural photosystems photosynthetic reactions are determined by three reactions we look at there is a light harvesting process if you remember it there is a chlorophyll P 680 P 700 which is essentially P 4 system 2 and 4 system 1 respectively which is supported by a series of molecules which is funneling the light and ejecting the electron this is the first foremost thing. So, you need light harvesting because light energy is if you come here who is the donor of electron. So, we are talking about what is the energy which will help you eject the electron if you if you know the donor then that donor needs an energy which could. So, this energy is provided by the sun and this energy is funneled through the anti-nepigment to the light harvesting process. Second thing if you go through this slide charge generation and separation process. So, what is the charge generation we talked about talk about whenever there is a chlorophyll molecule sitting here it ejects an electron and it becomes oxidized this is the whole charge generation process and the combination electron whole pairing and the separation. So, you have to have a way by which you can separate these processes otherwise what will happen they will recombine back. So, the chlorophyll which is formed the electron is close to it. So, this is the problem. So, this chlorophyll which is there one second which is devoid of an electron plus the electron which is ejected it will recombine again then and there there will be a recombining combination process and it will bring it back to its ground state it will reduce it. So, there has to be a way by which you can separate you can separate the charges you have to separate them otherwise they will recombine then comes the catalytic process which essentially is the splitting of the water. Now, overall efficiency is determined by the balance of the thermodynamics and the kinetics of all these processes in recent decades intensive studies have been focused on further investigating the mechanism involved in NPS. In particular researcher recently revealed the structure of oxygen evolving complex site in photo system 2. Thus providing new inspiration for design of APS structures. So, I was talking to you about oxygen evolving complex OEC underneath P 680 or you know photo system 2. So, recently couple at least 3 years back this structure has been revealed. So, now since this structure is known there is a lot of inspiration to design better oxygen evolving complex or better water splitting complexes. Now, moving on to the next slide NPS to APS what are the recent development overall the past decade fundamental progress has been made in developing novel material structures for water splitting reactions. Particularly in those that target an efficient oxygen evolving catalyst for using APS devices nanomaterial composition structures including inorganic molecular and hybrid organic inorganic material have been explored to meet the specific requirements such as light absorbing wavelength modification photo induced charge separation and faster water splitting reaction. Now, let us have a glimpse of diagrammatically what is happening what are the difference between NPS and APS. The upper picture you see is the NPS where you see the P 680 which is just in top of the water splitting cluster which is the manganese cluster on the right hand side it is shown. Now, P 680 electron gets excited that electron through pheophyte in Q A to Q B to plasticuino to iron sulphur to cytochrome B F to plaster cyanine moves on it is hopping down down down if you look at it. In the Z scheme of things it moves to P 700 where already there is another chlorophyll molecule which has oxidized so that chlorophyll molecule which has oxidized has to be brought back to its ground state. So, at P 700 it is this electron which brings back that P 700 excited molecule back to its ground state and the electron which is ejected by P 700 then moves to AO to A 1 complex to F x and FA Fe big complex and eventually it will works with NADPH reductase to make NADPH molecules and in the meantime on the extreme left if you see the manganese cluster is splitting the water into returns and oxygen is a byproduct and it is supplying the electron the rich source of electron what I was kind of highlighting on the board. Now, if you look at the B and C B and C are essentially these molecules they are the dyes which have been mimicked the chromophore or the dye or semiconductor which upon absorbing light gets excited and donates an electron. This electron is being accepted by protons so you have to supply proton if you see the B you have to supply protons those protons form ox forms hydrogen gas whereas, you need a electron donor. So, in the both cases electron donor is 2 H 2 O. So, electron donor is the water, water is splitting and forming protons these protons are filtered. So, basically there are two reactions which are happening here. So, you have this water splitting cluster which is generating protons and oxygen these protons are filtered through a proton membrane and added with the electron to form hydrogen gas and on the other hand this oxygen is a byproduct of this whole process when this light reaction is taking place. Whereas, you can do the same thing by having by mimicking or you know reducing the efficiency of the system by having two different dyes the chromophore 1 and chromophore 2 and exactly follow the mechanism what the z scheme of things which is being followed by the natural photo system. So, this is the overall A and if you kind of go through the caption of this figure which has been derived from that nature photonics paper. So, this is exactly what it will tell you that and a two step z scheme of reactions of the B is a chromophore it is the last two lines of a single step reaction P excited state. So, that is that is pretty much you can go through this and that will give you an idea that how this photo systems are working. Now, if you have to emulate these kind of photo systems now next I am coming to the structural designs of APS reaction processes. So, what you need essentially is if you see the first picture which is the A and the B. So, we will talk about the structure of the carotenoid porphyrin and fullerene molecular diet system which is basically this one carotenoid porphyrin and fullerene. So, you have the carotenoid you could see long stretch molecule long tail molecule at A when you see the porphyrin which is sitting out there which is in molecule which is the same molecule as your chlorophyll and you have the fullerene ball on the other side. So, there is a single step semiconductor particle attached hydrogen oxygen evolving co catalyst. So, this is the oxygen evolving evolution catalyst and hydrogen evolution catalyst. So, these are the kind of geometries what are being followed if you go to the two excitation process where you have two different molecule. So, which is follow the C you see the two step make system mixture of semiconductor particle with attached hydrogen oxygen evolving co catalyst and redox electrolyte couple. So, this is very interesting out here where you have two system which is basically trying to emulate this kind of system where you see that one chromophore and the second chromophore mimicking the z scheme of things and then you have the electrode system. So, this whole area has evolved at two different level. So, you need to develop high end electrodes for this and you need to develop molecules which will split the water as well as you need molecules which will in the by the action will absorb light and do this job. So, where this all started you have to go back in time. So, this is the major breakthrough in 1972 by Honda and Fujishima who actually during his this was his PhD problem in Tokyo university while he was doing it what he was doing he was trying he was developing titanium dioxide for a totally different purpose and what he found out is that pretty much lead the foundation stone of self cleaning glasses. He was using. So, now, if you kind of go through the slide who used a titanium dioxide film as a photo electrode, but no co catalyst on the titanium oxide surface and this titanium oxide surface the photo generated electron was separated from the whole at titanium dioxide and electrolyte interface. So, this was the first evidence where titanium dioxide could split the water the oxygen and hydrogen generated at a titanium dioxide electrode and a platinum counter electrode respectively in separate compartments inside the photo electrochemical cell. So, this was the first you can say that this was the first breakthrough which lead the foundation stone for photo electrochemical cell and what a photo electrochemical cell is I will give you the reference, but just for your understanding here just for its simple understanding. So, basically the principle of operation of based on the n type semiconductors and I will give you the exact reference you please go through what is PECs basically PECs are a hybrid device of semiconductor and electrolytes these are not the pure dry kind of situation. So, there is an electrolyte and there is semiconductor and I will give you a very nice reference which I have already highlighted here by Michael Gretzel on the photo electrochemical cell that is the title of the paper please go through that paper. I really wish you people to go through that I will be supplying the paper anyway go through that paper because that paper will give you an idea about you know how this whole concept of photo electrochemical cell because that will be needed in order to understand the Gretzel cells of the dye sensitized solar cells. So, once again before I coming back to where I was. So, yeah. So, there are two level where these kind of things have to be developed the other level is the. So, let us divide the problem. So, you need to develop you need the water splitting chemicals which will split the water you need an electrode system which will capture the electrons and then you have to put this whole thing in one assembly. So, the last two DEF what you see is essentially your the electrode system what has been developed single excitation step of water splitting cell containing a semiconductor electrode with water oxidation co-catalyst and a counter electrode to reduce water. So, you have both the things on one electrode you are oxidizing the water there is another electrode where reducing the water then you have dye sensitized transparent metal oxide water splitting cell and the two step tandem water splitting cell. So, these are some of the examples of it now coming back to what are the new concept of nanomaterial development. So, if you follow this part one which is zinc oxide nanoware array electrode on the flow on the fluorine doptane oxide transparent conducting substrate. So, you need different kind of transparent conducting substrate because this process is has to be light dependent light is the source of energy in order to you know carry out this process. So, there are different kind of you know fluorine doptane oxide which are being developed over a period of time which acts as a transparent conducting substrate. So, there is a lot of work which is kind of currently going on on zinc oxide surfaces and you know developing different kind of material which are transparent enough to allow this process to take place. Please go through the paper reference what I have given you because that will give you a better idea about you know what all development has happened in this area. And this is what will give you an idea of the photo induced charge separation and recombination of a semiconductor photo anode. This is basically the basic basic very fundamental principle of the photo electrochemical cell where you have a interface of a semiconductor and a electrolyte and the where the water splitting is taking place. So, this is basically what is happening in a photo electrochemical water splitting cell employing a photo active anodic electrode that induces a space charge region at the semiconductor solution interface. The photo generated electron is separated from the interface and then consumed to generate hydrogen at the counter electrode. A hole is generated at the interface and consumed for water oxidation reaction. So, this is pretty much what is happening. So, you are splitting the water you are getting the protons through a membrane on one side where you are again reducing it by adding electron to make hydrogen. So, this is the overall kind of scheme of things and if someday if this whole thing really works out that we are able to you know mimic the manganese cluster in near future on far future these are some of the. So, if you look at this. So, this is kind of a dream where we will have hydrogen power plants which will be present which will be generating hydrogen and hydrogen will be supplied as the major energy source for you know pretty much everything what you see all over the apart from the other sources which are like photovoltaic power underground electron and the wind power and likewise and so on and so forth. So, what I wanted to highlight here is that you have to realize the biology is no more just community we are studying all these molecules, but biology is an inspiration now biomimicry biomimetics. It is an inspiration for generations now of physicists chemist electronics who are trying to emulate these wonderful structures what nature has already developed because even if you could understand as I always tell that you know 10 percent of it we can really not only we can dream we can really achieve clean and green energy all over the earth. So, in that whole process. So, as I told you in the beginning of the lecture that will be we will be talking about the two set of development one set of development is in terms of the hydrogen field the other set of development will be where the chlorophyll dye has been mimicked or there are synthetic chlorophylls or the inorganic molecules like ruthenium and all which is the ability to behave like chlorophyll, but those things always needed whenever we talk about natural whenever we talk about those kind of dyes they always needed a system again that what I was trying to tell you this one you always needed a source of electron which will ensure that once they get oxidized by donating an electron they should be brought back to their ground state. So, this is very very essential for us to understand that this whole process if you follow these three lines very carefully who is the donor of electron who is the acceptor of electron and who is the perennial source of electron it is all about these things where this whole photosynthesis water splitting for system two for system one hydrogen generation and pretty much pretty much all the power generation processes happening in biological systems are kind of governed. So, this is the overall scheme of things and in next lecture we will be talking about the dye sensitize solar cells followed by one of the lectures probably the thirty eight lecture we will be talking about iron disulfide solar cells and iron disulfide. So, these are the different inspiration what has been which has been drawn over the period of time in order to you know emulate some of the feed which nature has already achieved we are just trying to mimic nature. So, with this I will close in and please go through all the references what I am giving because they are really very good references please because within limits of a class you cannot you know explain everything. So, you have to really go through some of these references wherever it is needed I tried to you know go as much as I can, but you know it is a generic audience. So, I will have to be careful that, but please read through those references because they will help you to clear some of your basics especially please read through this nature photo electrochemical cell references this will be very helpful to understand grad cell cell as well as the hydrogen generation thanks a lot.