 In this class, we will see the different components involved in electrochemical cell and what are the different barriers to the electrochemical reaction which occurs in the electrolysis process? What are the different components, their requirements and how did they impact? So, if we look at the different components and there is an electrode, electrolyte and diaphragm and they need to satisfy certain requirements in the electrolytic cell. If you look at the barriers to the reaction which occurs in the electrolytic cell, that all depends upon the different components of the cell. Now, these barriers could be, there could be boundary layers being formed at the electrode surface, then the electrode phase, its characteristics, the characteristics of the electrolyte involved, what kind of separator is being used, what are the different electrical resistances involved in the process and all these determine the behaviour of current, IV efficiency and the amount of hydrogen being produced from a particular electrolytic cell. Let us look at one by one each of these. Now, when we look at the electrode and an electrolyte interface, so this is the interface, at this interface there are differences occurring due to the surfaces, one is a solid surface, the another may be a liquid or a solid surface. There are differences in terms of the electrolyte velocity, there are difference because of the concentration of the active, electroactive species and there are differences which are arising because of the electric potential from the surface of electrode to a certain distance from the electrode. Now, each of these differences they give rise to a boundary layer at the interface. For example, let us consider the difference in the electric potential at this interface. Now, if we try to plot this is a model wherein we can see that if this is a negative electrode on the surface of it there are positive ions, they form a sort of compact layer. Let us say if at the surface the potential is phi E, the electrode surface, this is a compact layer which is being formed and the remaining there is a diffuse layer. So, which has ions distributed more than that in the electrolyte close to the electrode. So, because of that there is a variation in the potential, so at the surface if it is phi E and at the electrode surface in the solution if it is phi S. So, in this compact layer it varies linearly and thereafter it varies in the diffuse layer it varies exponentially. Because of the presence of this boundary layer it acts as if there are two capacitances operating one which is because of the compact layer and because of the diffuse layer and similar to this boundary layer which is occurring because of the electric potential dip changes at the interface of electrode and electrolyte. Similarly, there are changes which are because of the electrolyte velocity, concentration boundary layer because of the change in the concentration of electroactive species or it could be because of the change in the velocity of electrolyte in the vicinity of the electrode surface and that could give rise to a hydrodynamic boundary layer. Now, this concentration boundary layer here it is the driving force is the diffusion while in case of electrode electrolyte velocity the charge separation is the charge separation is the driving force for the difference in the electroelectric potential that occurs at the interface. However, when it is electrolyte velocity different giving rise to hydrodynamic boundary layer the difference or the driving force is the convection in the process. Now, when this difference in the potential phi E minus phi S is existing that double layer at interface has several consequences. Now, this potential difference which is arising phi E minus phi S that provides the required driving force for the electron to transfer between the electrodes across the electrodes and that driving force however, gets affected because of the adsorption of different species at the electrode surface. Now, these could be the reactants, these could be products, these could be the different ions, these could be contaminants involved in the process. Like in the earlier figure we have shown that these are the solvent molecules which get adsorbed onto the electrode surface and that changes this potential difference in the process. There could be variations, localized variations which occur because of the potential difference and that in fact influences the rate of the reaction, the efficiency of the process as well as the selectivity towards hydrogen being produced. At the same time this potential difference which is arising across the interface that can be very high. Like for example, if phi E minus phi S is equal to 2 volt over 0.2 nanometer, the potential gradient will be 10 raise to power 10 volt per meter. Such a high end localized driving force enables energetically difficult processes to be carried out electrochemically. The double layer capacitance measurement can provide information on adsorption at electrodes. However, it tends to create problem in kinetic studies, particularly where high surface area electrodes are involved and when there is rapid potential changes with time due to charging currents. Now, let us look at the electrode phase. Now, the electrode phase, the requirement for electrode phase is that it should be mechanically stable. Mechanically stable it should not undergo erosion, it under when it is acted upon by different reactants, by different products in the presence of different ions. So, there should not be any loss of electrode when it is subjected to different species. At the same time it should remain physically and chemically intact. These electrode which is being used, it should be easily integrable. By easily integrable we means in an electrolytic cell, this can be easily connected giving rise to good electrical connections and when required can be taken out for inspection or for maintenance. The shape and surface of the electrode phase that also is very important in the sense that this is the place where the various electrochemical reaction occurs. So, our phase should be such that it should allow the reactants to get reacted and the products formed to be removed in the process. Now, what happens in these electrodes is usually a very thin coating is considered on a bulk material. The electrodes should be such that the desired reaction should be achieved. Desired reactions which leads to the formation of the caches product, however certain unwanted chemical reactions can be avoided. And these desired reactions should occur with a high rate of reaction and as such that will define the efficiency of the process, the current density involved in the process. The electrode should have good electrical conductivity and it should definitely have a low cost and high life. Now, when we look at the processes involved on the electrode surface then there are two types of electrode processes. One is where it acts as electron source or sink and reaction characteristics are sort of independent of electrode material. Second, electrode surface acts as catalyst and the type as well as rate of reaction depends on interactions between the electrode and electrolyte species. Thus a rough surface can provide a high and active electrode area to increase the electro catalytic effect. In water electrolyzers an electro catalytic electrode surface is required to promote a hydrogen and oxygen evolution at high rate and at low over voltage. Similarly, if we consider the electrolyte phase then there are components of it, a solvent which is required which should remain stable under the reaction conditions and electrolyte and the electroactive species and these electroactive species are nothing but the reactants. So, now when we look at the electrolyte the solvent should be such that it should be cost effective, it should be stable, it should be easy to handle that means its disposal as well as handling should be safe and recycling of that should be safe. So, as such if we look at these solvent then water is the most promising solvent and the reason for water being used as a solvent is that it is not only the low cost stable the same requirements it forms an oligomer. So, it has a high dipole movement and this high dipole movement allows the interaction of the electroactive species. At the same time the splitting up of or the ionization of water it is very small into H plus or OH minus. So, it remains stable under the operating conditions. So, as such it is an ideal solvent. So, self ionization of water is relatively very small. If you look at the electrolytes then the high conductivity in the solution phase is the requirement and that electrolyte that has to disintegrate or dissociate to form anions and cations. What is required is that it should have a high conductivity in the solution phase and this high conductivity will lead to definitely the low resistance it will result in anions and cations which will basically carry the current of current it will have a high ionic strength and that results into equal and constant activity coefficient for the reactants and products. So, if we look at the electrolytic conductivity this is represented by either the molar electrolytic conductivity times the electrolyte concentration molar electrolytic conductivity times the concentration and it can also be written as conductance times the cell constant. And usually for typical cases this electrolytic conductivity it lies in the range of 5 to 25 Siemens per meter and this is when the typical concentration is of the order of say 1 mole per decimeter cube. So, this is usually the electrolytic conductivity being observed. Another important component of an electrolytic cell is a separator. Now, separator although it if it is present it has several advantages but the presence of separator also leads to disadvantages that like it will add on to the cost it will add on to the complexity of the system and also it will increase the resistance to the flow of ions. However, separator also plays certain important role like in the presence of separator the two electrodes they are physically separated prevents sorting the gaseous products which are released formed in the electrochemical reactions they are separately produced. So, their intermixing is prevented at the same time we can use independent cathode catholite and anode anolite in the presence of a separator. It can also prevent the redox shuttle which is in fact parasitic in nature. So, parasitic redox shuttle which is when the species can undergo reduction at one electrode and it can undergo re-oxidation at the another electrode and that can be avoided. At the same time the separator can be selective towards one ion and it can selectively allow that particular ion in transport of ions. There are different types of separators which are available these can be either porous separators or these are also called spacers. Now these porous separators they are such that they can sort of provide support to in fact fragile electrodes they can even provide support to membranes being used and they can promote a sort of turbulence at the electrode surface. However, they cannot prevent the movement of solvent they cannot stop the mixing up of anolite and catholite. Now usually the pore size which is of these separators that lie in the range of 0.5 to 12 mm. Another category of separators which can be used are micro porous wherein the pore size is in micron dimensions, micro porous separators or diaphragms and for them the pore size lies in the range of 0.1 to 50 micron and they allow the transport of solvent solute and ions through it by means of hydraulic permeability. However, there are barriers which are because of the presence of these type of separators the convection and diffusion type of barriers exist when these type of supports separators are being used or there can be another separator which are ion exchange type which selectively allow a particular type of ions to flow through them. These are ion exchange membrane type of separators which can be used and these in fact they divide the electrochemical cell into two compartments and in that case we can use separate anode, anolite or separate catholite we can use. However, they permit only the passage of ions allows passage of selective passage of ions in the process. They have a pore size which is small 10 to the power minus 8 to 10 to the power minus 9 meters and they also function in such a way that they have barrier to convection and diffusion. But there are several requirements when it comes to membranes being used in an electrolyzer. These membranes should have very high selectivity and they should be able to permeate only a particular type of ions and not the neutral molecules. They should have high chemical stability under the reaction conditions when it is subjected to different reactants, products and different ions. These should be mechanically stable. So they should be stable at the same time they should provide strength and flexibility. They should have high ionic conductivity lower electronic conductivity. They should be such that they should operate under variable current densities can be able to operate at high current density when it is operating under full operation or it should be also be able to operate under lower current densities and at the same time they should be homogeneous in structure so that they should not be local variations. It should have throughout the same uniform current densities. It should be easily available, convenient to handle and economical. So low cost and high life of these membranes which can be used in an electrolyzer. To summarize this portion we have seen the different components involved in the electrolytic cell. What are the different requirements for electrode, electrolyte and separator and how these components what kind of roles they play in an electrolyzer. So in the next class we will see the kinetics involved in the process and how to find out the efficiency of an electrolyzer. Thank you.