All About the Electrochemical Cell and its Different Types

Rabab salah Ibrahim 5\7\2020 Research gate Cairo University All about the Electrochemical cell and its different types Abstract In brief we are going to discuss electrochemical cells, which have the ability to produce electrical energy from chemical reactions, and also use electrical energy to create chemical reactions, that are divided into two types of cells: galvanic cells which known also as voltaic cells, and electrolytic cells. Galvanic cells are the types that produce electricity such as fuel cells (hydrogen-oxygen fuel cell), and Deniall cell, whilst the electrolytic cells used an external supply of electricity to create the redox reactions. This review also involves Weston standard cell that is being developed to overcome the deficiencies of Clark cell, as well as the role of liquid junction potential in measuring the potential difference of the double layer that made by the diffusion of electron on the both side of the cell, finally the concentration cells that produce voltage when they approaches the equilibrium. Keywords: electrochemical cells, galvanic cells, electrolytic cell, concentration cell, potential, and fuel cell. Introduction In electrical chemistry, the word "cell" is utilized in a wide range of appliances of multiple purposes, forms, and measurements where the reactions of electrochemical typically occur and electrochemical cells generally involve at least two electrodes in association with an electrolyte [1]. An electricity generator from a spontaneous redox reaction or, in return, that uses electricity to conduct a non-spontaneous redox reaction is an electrochemical cell[2], it composed of twoelectron conductors (electrodes) parted by an ionic conductor (electrolyte) and often linked by an electron conductor which is generally called a metal wire conductor [3]. Electrochemical cells that have the ability to generate electrical currents are labeled as voltaic cells or galvanic cells [4]: for example batteries or fuel cells [5]. The chemical reactions or transformations which coupled with appropriate half-cell reactions, can lead to a free energy shift in the cell cycle as a consequence of the electric power generation through electricity [5]. An external electric current is used in other electrochemical cells to drive a chemical reaction that does not happen spontaneously; such cells are termed as Electrolysis cells [4]. The fuel cells are electrochemical cell that transforms a fuel's chemical energy (often a hydrogen) and an oxidizing agent (often oxygen [6,7]) by means of a couple of redox reactions into electricity[8,7]. The chemical energy of the battery obviously comes from supplies of metals and their ions or oxides [9,7]which are already present mainly in the battery, except in flow batteries, and because of this, the fuel cells are different from the most typical battery, as they requiring a continuous sources for fuel and oxygen (usually air) to maintain the chemical reactions[7]. So long as fuel and oxygen are provided, the fuel cells can constantly create electricity [7]. This cell form serves as a renewable source of energy in many isolated regions [10]. In this short review, all the types of electrochemical cells would be discussed in detail, as well as liquid junction potential due to the diffusion of electrons, and other types of the cells. Page | 1 Electrochemical cells, Galvanic cell, and fuel cells. Types of electrochemical cells Electrochemical cells are generally categorized into galvanic cells (or voltaic cells) and electrolytic cells [11,1].  Galvanic cell Galvanic cells, which also termed as voltaic cells [12], are electrochemical cells, in which electrode reactions spontaneously occur, generate an electric current that can be used in batteries or fuel cells to supply electric energy [13], and they can convert chemical reactions to electrical energy directly [14]. A galvanic made up of 2 metals (electrodes), which are linked throughout an electrical conducting solution (an electrolyte) and linked externally to complete the circuit [15]. In such a case, one of the metallic materials (more reactive) begins to solubilize in an electrolyte, whereas the other tends to contain new metal deposited on it [15]. As the metal is dissolved, the more reactive electrons that are used in the metal deposition on the other electrode flow through the external contact (as an electrical current) [15]. The dissolution and deposition reactions, which made by the anode and the cathode reactions in the order, are called half-cell reactions [15], and without the separation of these two half-reactions spatially, this energy would be liberated as heat [12].  Mechanism of galvanic cells A basic example of a galvanic cell containing 2 cell diagram metal / metal ion systems [12]; Zn(s)+Cu2+(aq)→Zn2+(aq)+Cu(s)(1) [16], The galvanic cell shown in figure 1 can be used to perform this reaction; a copper strip is inserted into a beaker that contains a 1 M solution of Cu 2+ ions, and a zinc strip is inserted into a different beaker that contains a 1 M solution of Zn2+ ions[16]. The two metal bands, which function as electrodes, are attached by a wire, and the compartments are linked with a salt bridge, a U-shaped tube that is inserted into the two solutions that involve a concentrated liquid or gelled electrolyte [16]. The ions in the salt bridge are chosen to prevent Page | 2 Electrochemical cells, Galvanic cell, and fuel cells. interfering with the electrochemical reaction through oxidation , reduction or precipitation or by forming a complex; commonly used cations and anions are Na+ or K+ and NO3− or SO42− , (The ions in the salt bridge do not have to be the same as those in the redox couple in either compartment)[16] .When the circuit is closed, a spontaneous reaction occurs: zinc metal is oxidized to Zn2+ ions at the zinc electrode (the anode), and Cu2+ ions are reduced to Cu metal at the copper electrode (the cathode)[16]. As the reaction progresses, the zinc strip dissolves, and the concentration of Zn2+ ions in the Zn2+ solution increases; simultaneously, the copper strip gains mass, and the concentration of Cu2+ ions in the Cu2+ solution decreases (Figure 1 b ). The electrons that are released at the anode flow through the wire, producing an electric current [16]. Galvanic cells therefore transform chemical energy into electrical energy that can then be used to do work[16]. Figure 1 The Reaction of Metallic Zinc with Aqueous Copper (II) Ions in a Galvanic Cell. (a) A galvanic cell can be constructed by inserting a copper strip into a beaker that contains an aqueous 1 M solution of Cu2+ ions and a zinc strip into a different beaker that contains an aqueous 1 M solution of Zn 2+ ions. The two metal strips are connected by a wire that allows electricity to flow, and the beakers are connected by a salt bridge. When the switch is closed to complete the circuit, the zinc electrode (the anode) is spontaneously oxidized to Zn 2+ ions in the left compartment, while Cu2+ ions are simultaneously reduced to copper metal at the copper electrode (the cathode). (b) As the reaction progresses, the Zn anode loses mass as it dissolves to give Zn2+(aq) ions, while the Cu cathode gains mass as Cu2+(aq) ions are reduced to copper metal that is deposited on the cathode.[16] Page | 3 Electrochemical cells, Galvanic cell, and fuel cells. name Role in galvanic cell salt bridge The ions must be permitted to move between the two half-cells to provide electrical contact among them and to close the cell circuit; but in the other hand, it should not be mix between the two solutions as much as possible [17,1]. The reaction would quickly cease in the absence of a salt bridge or some other analogous linkage, because it could not preserve electrical neutrality [16]. electrolyte in the salt By holding electric charge, it completes the circuit and preserves electrical neutrality in bridge both solutions by permitting the ions to pass across them [16]. voltmeter It can be used to measure the difference in electrical potential between the two compartments; as the potential (E°cell) (E°cell = E°cathode – E°anode[18]) of the cell, measured in volts ,which is the difference in electrical potential between the two half-reactions and is related to the energy needed to move a charged particle in an electric field[16].the voltmeter indicates a potential of 1.10 V (Figure 1)[16].  Cell description conventions A special symbolic notation has been adopted to make it simpler to explain the given electrochemical cell. In this notation the cell we described above would be Zn(s) | Zn2+ (aq) || Cu2+(aq) | Cu(s)[19]. There are several other conventions relating to cell notation and nomenclature: o Within actual cell, the identification of the electrodes refers to the direction that the net cells react. The anode is the place of oxidation process and the cathode is where reduction is done. [19] o The potential of the right electrode is higher than the left one, and the cell potential is positive, if electrons move from the left electrode to the right electrode (as shown above in this cell notation) when the cell works in its spontaneous direction [19]. o "Conventional current flow" goes from positive to negative, and that's contrary to the electron flow direction. This indicates that if the electrons move from the left electrode to the right, a galvanometer positioned in the outer circuit would signify a current flow from right to left [19]. Page | 4 Electrochemical cells, Galvanic cell, and fuel cells. A more convenient notation was developed. In this line notation, called a cell diagram, the identification of the electrodes and the chemical components of the containers are demonstrated by their chemical formulae, with an anode written on left and a cathode on right. Phase boundary is displayed by a single vertical line and a salt bridge, which has two-phase boundary lines, by a double vertical line. [19]Thus the cell diagram for the Zn/Cu cell shown in part (a) in Figure(2) Figure 2 cell diagram of Zn/Cu cell [20] Galvanic cells can have other structures, In addition to these examples that we have seen, including the potential cell junction, where the voltage generated by the redox reaction may be calculated more efficiently utilizing two electrodes immersed in a single beaker involving an electrolyte that completes the circuit, thereby reducing the errors caused by the resistance to the charge flow at a boundary {e.g. Pt (s) |H2(g)|HCl (aq)|AgCl (s)Ag (s)} [19]. As galvanic cells can be self-contained and mobile, they could be used as batteries and fuel cells [21]. A battery (storage cell) is a galvanic (or a series of galvanic cells) that has all of the electricity-generating reactants.; such as: Leclanché Dry Cell, Button Batteries, Lithium–Iodine Battery, Nickel–Cadmium (NiCad) Battery, and Lead–Acid(Lead Storage) Battery[21]. A fuel cell, in contrast, is a galvanic cell that necessitates a constant outside source from one or even more reactants to provide electricity, and does not stock chemical or electrical power[21];such as: Hydrogen-Oxygen, Redox, Phosphoric Acid (PAFC), Direct Methanol(DMFC),Polymer Electrolyte Membrane (PEMFC), Solid-Oxide (SOFC, HTSO), and Molten Carbonate (MCFC) Fuel Cells[1]. Page | 5 Electrochemical cells, Galvanic cell, and fuel cells.  Electrolytic cell It is likely to create a cell that functions on a chemical system by pushing an electrical current via the system; these cells are termed as electrolytic cells and run by electrolysis [22,24], which is utilized to induce an oxidation-reduction reaction in a direction where it does not happen spontaneously by running an electric current throughout the system whilst performing work on the chemical system itself, and hence is non-spontaneous [23,24]. As galvanic cells, electrolytic cells made up of two half-cells; one is a half-cell reduction, and the other is a half-cell oxidation [24]. The direction of electron flow in electrolyte cells, though, be reversed from the direction of spontaneous electron flow in galvanic cells, but the concept of both cathode and anode is still the same, where the reduction occurs at the cathode and oxidation happens at the anode [24]. The sign, but not the quantity, of the cell potential, has been reversed, because the directions of both half-reactions have been inverted [24]. As popular examples, the passing of an electron from a cathode to a water molecule induces the creation of an OH-ion and a hydrogen atom; two of these hydrogen atoms are then joined to make a hydrogen molecule, and at the anode, the reverse happens [25]. For a common case, the chloride ion passes its extra electron to the anode and transforms it to a chlorine atom [25]. It should be remembered that the anode and the cathode represent the direction of the current flow, and not the polarity [25]. In a cell that absorbs electrical energy, such as the NaCl electrolysis, the anode is positive, whereas in a cell that produces electrical energy, such as a fuel cell, the anode is negative [25]. The electrodes exchange in a rechargeable battery between charge and discharge [25]. Electrolysis of NaCl: Sodium ions move to the cathode throughout the electrolysis, in which electrons reach the melt and are reduced to sodium metal: Na+ + e-Na[26]. Chloride ions transfer in the opposite direction toward the anode, where they give up their electrons, and then oxidize to chlorine gas: Cl-1/2Cl2+ e-(2) fig3 Page | 6 Electrochemical cells, Galvanic cell, and fuel cells. The overall reaction is the breakdown of sodium chloride into its elements: 2 NaCl 2 Na(s)+ Cl2(g)(3) [26].fig3 Liquid sodium floats to the top of the melt above the cathode and is drained off to a store tank, and the Chlorine gas bubbles out of the melt above the anode [26]. Similarly, other molten ionic compounds which are electrolyzed will lead the metal to be generated at the cathode and the nonmetal at the anode through a redox reaction [26]. Figure 3 electrolysis of molten NaCl cell [27] Electrolysis is the process by which electricity drives the non- spontaneous reaction. In order to cause the redox reaction, the electrolytic cells utilize the flow of electrons (in the power source). The E0cell for electrolytic cells is negative, because the electrons have to flow in the opposite direction. In addition, the electromotive force connecting two electrodes-the battery must be bigger than the E0cell magnitude to force electrons to move in the opposite direction and this extra demand of voltage is called over-potential. [28]  Usage of electrolytic cells: Industrially, in the electrical refining and electrical winning of various non-ferrous metals, electrolytic cells are being used, as well as all high-purity aluminum, copper, zinc, and lead can be produced. [29] Page | 7 Electrochemical cells, Galvanic cell, and fuel cells. Differences between voltaic cell and electrolytic cell electrolytic cell Definition voltaic cell There are electrochemical reactions There are electrochemical reactions that do not occur spontaneously but that occur spontaneously at the are driven by an externally applied electrodes when they are externally voltage greater than the open-circuit connected by a conductor [1]. voltage of the cell [1]. definition It's a device to convert electrical It's a device to convert chemical energy into Design chemical reactions [30]. reactions into electrical energy [30]. It consists of an electrolytic container It consists of two different electrodes in which two electrodes are connected immersed in solutions of their ions that to a DC source [31]. The electrolyte are separated by a semipermeable may be a melt or an aqueous solution membrane or a salt bridge [31]. of some salt, acid or alkali [31]. No salt bridge is used [30]. Electrode polarity The cathode is the negative and the The anode is the negative and the anode is the positive electrode [31]. Chemical reaction cathode is the positive electrode [31]. An outside source is used to trigger a The oxidation reaction takes place at reaction. At the negative electrode, the anode (negative electrode) where the electrons are pushed out of it; so there is a surplus of negative charge. the reduction phase will happen on At the cathode, the reduction reaction the negative electrode. On the positive happens, inducing a positive buildup of electrode the oxidation phase takes charge.[31] place – and this is the anode.[31] Application Some of them are being making Used as a source of electrical current, hydrogen and commercial applications, oxygen gas and industrial batteries or accumulators [31]. electroplating, for and are more commonly referred to as and extracting pure metals from alloys and so on [31]. Page | 8 Electrochemical cells, Galvanic cell, and fuel cells. Liquid junction potential (LJP) Two solutions with different compositions and concentrations can be involved by two half cells in a galvanic cell, and the two solutions must be joined to allow the ionic transmission in order to make this cell function [32-34]. The two solutions are generally separated with fine pores by porous barriers (junction) which permit the diffusion and migration of the ions without combining the two solutions [32-34]. The movement of ions with various speeds through the cross-junction, on one side, produces an excess of positive ions, and on the other is the same excess of negative ions that result in the creation of double-layer separate positive and negative ions system, the device that measures the theoretical potential difference of this double layer is known as liquid junction potential [32-34]. Often the "diffusion potential" Ej in aqueous solution refers to the Liquid junction potential [32-34]; the phrase “diffusion potential” outcomes from the fact that the spatial separation of charge is originally caused by diffusion [35]. The LJP between various solvents is defined by both the solvents and the electrolyte(s) existent at the junction; Hence, the agents forming the LJP were studied along with its appreciation studies; So that the LJP composes of three ingredients: component (a) associated to electrolyte concentrations and ionic mobility, component (b) related to ion solvation (and ionic mobility), and component (c) linked to solvent-solvent interaction [36]. Because of the ionic diffusion of the component(a) which caused by the gradients in ionic concentrations, and the ionic diffusion of component (b)that created by the gradients in ionic standard chemical potentials or ionic solvation energies, Components (a) and (b) are diffusion potentials [36-41]. Component (c) is a dipole potential, and not a diffusion potential [42,36]. Element (c) changes mostly linearly or almost linearly with the volumetric fraction of the combined solvent(s), at a junction with a mixed solvent on one side or with a mixed solvent on the two sides [36,43-45]. If we assume that the two solvents at the junction interact with each other as Lewis acid and Lewis base and some of the solvent molecules are perpendicular to the interphase as shown in fig4c, the solvent side as a Lewis acid is Page | 9 Electrochemical cells, Galvanic cell, and fuel cells. more negative than that as a Lewis base and the value rises with the increase in the strength of the solvent-solvent interaction; so component (c) increases with the increase of the strength of interaction, with the side of the Lewis acid solvent being more negative[36,46,47] Figure4 Characteristics of the three components of the LJP between different solvents [35]. The liquid junction potential intervenes with the measure of the electromotive force of a chemical cell, so its influence should be reduced for precise measurement by locating a salt bridge that made up of a saturated solution of potassium chloride (KCl) and ammonium nitrate (NH4NO3) with lithium acetate (CH3COOLi) between the two solutions forming the junction, when such a bridge is applied, the ions in the bridge are existing in overflow at the junction and they almost carry the entirety of the current across the boundary [48]. Liquid junction potential measurements cannot be carried out with rigorous thermodynamic precision because the individual activity of the electrolyte ions making up the junction cannot be experimentally assessed [49]. Nevertheless, throughout the case of uni-univalent electrolytes with the same solute at varying concentrations, the electromotive force calculations of concentration cells can provide appropriate and adequate values for junction potential [49]. The EMF of a concentration cell without transport is: Where a1 and a2 are activities of HCl in the two solutions, R is the universal gas constant, T is the temperature and F is the Faraday constant [48]. Page | 10 Electrochemical cells, Galvanic cell, and fuel cells. The EMF of a concentration cell with transport (including the ion transport number) is: Where a1 and a2 are activities of HCl solutions of right and left hand electrodes, respectively, and tm is the transport number of Cl−[48]. The difference between the two EMFs of the two concentration cells, with and without ionic transport is the liquid junction potential [48]: Concentration cells A concentration cell is an electrochemical cell composing of two half-cells with the same electrodes and varying concentrations. A concentration cell dilutes the higher concentrated solution and concentrates the diluted solution and generates a voltage when the cell approaches equilibrium, and this happens when the electrons are transferred from the cell with the lower concentration to the cell with the highest concentration [50]. A concentration cell generates a small voltage, produces at room temperature less than 60 millivolts, so the cells are usually not for use in energy storage [51]. The potential difference created by such a cell can be determined using the Nernst equation: . Figure 5 diagram of a concentration cell and its discharge process [52]. Page | 11 Electrochemical cells, Galvanic cell, and fuel cells.  Types of concentration cells There are two general types of concentration cells without transference [52]: o Electrode Concentration Cells Such cells are composed of the same solutions that are used in each half cell as electrolytes. However, the half-cells vary in electrode concentration (electrodes are made of the same substance) [52]. In such types of concentration cells there are no possible liquid junctions; therefore, no error exists; so that they are of benefit in determining electrolyte activity, such as: gas and amalgam concentration cells [32-34]. o Electrolyte Concentration Cells Such cells are made of the same electrodes that are immersed in varying concentrations in the solutions of the same electrolytes. In such cells, electrolytes begin to migrate towards solutions of lower concentrations and away from higher concentration solutions [52]. An example for this type of cell is a cell where the anode composes of Zn/Zn2+ (0.1M), whilst the cathode consists of Zn2+ (0.01M)/Zn. In this cell, due to the reduction of Zn2+ ions at the cathode into metallic zinc, the electrons flow from the anode to the cathode [52]. o Concentration Cells with transference Concentration cells with ion transfer are made through the immersion of two equivalent electrodes (e.g. silver electrodes) in a container separated by a semi-permeable barrier and comprising identical electrolytic solutions (e.g. silver nitrate) on both surfaces of the partition, with different concentrations. In those concentration cells, the electromotive force (emf) increases as a result of the direct transmission of the electrolyte from the extra concentrated solution to the few concentrated solution. The concentration of the electrolyte in the concentration cells of the second type is equalized as outcome of the chemical processes place on the two dissimilar electrodes. [53] Page | 12 Electrochemical cells, Galvanic cell, and fuel cells. Weston standard cell In a patent application in 1891, Edward Weston (1850-1936) illustrated the Weston Cell. He developed a cell that effectively solved the Clark Cell's shortcomings by replacing cadmium with zinc and cadmium sulfate with the zinc sulfate electrolyte. It had a low temperature coefficient and did not crash unless it fell. It was highly reproducible [54]. Cells of Weston are standard cells and are also known as cadmium cells because they use cadmium. The EMF of them will stay unchanged for a prolonged period offered no significant current from the cells is drawn. Weston Cell is very seldom used as an energy source, if any! , but it Figure 6 Weston cell comprises a glass H shaped vessel with a diameter of each leg of can be used 25 mm containing Conc. Cadmium sulfate solution in the upper part and a mercury for sulphate paste underneath; above cadmium sulphate and pure mercury crystals in one electrical leg where the anode set-up is attached, cadmium amalgam crystals and cadmium sulfate in another leg where cathode of the set-up is connected. The two electrodes of the cell analysis and are linked with platinum cables to the outside circuit and air tightly is made of the Hshaped vessel [55]. calculation voltage as a reference of standard EMF [55]. The cadmium sulfate works solution as the electrolyte in the Weston Cell, the mercury behaves as a positive electrode, the cadmium amalgam as a negative electrode, and paste of mercurous cadmium Sulfate acts as a de-polarizer [55]. There are 2 kinds of Weston cells or cadmium cells relying on electrolyte saturation: Saturated and Unsaturated Cadmium / Weston Cell [55]. Page | 13 Electrochemical cells, Galvanic cell, and fuel cells. o Saturated Cadmium/Weston Cell The crystals of Cadmium Sulphate are placed in the cell of Saturated Cadmium / Weston Cell to saturate the electrolyte at a normal temperature. This cell form is often used as a basic or main voltage or EMF standard. This cell is manufactured by using high purity materials. The voltage generated by the individual concentrated Cadmium / Weston cell is very homogeneous and only a small micro voltage difference is noticed in the standard cells. These cells are often used in temperature-controlled environments, as they have elevated temperatures coefficient of-40 microvolts for every degree Celsius increase. The lifetime of these cells is usually 10 to 20 years with only a drift of about 1 micro voltage annually. [55] o Unsaturated Cadmium/Weston Cell The unsaturated cadmium cell or the Weston cell is nearly identical to the saturated cadmium / Weston, except that extra cadmium sulfate crystals are not applied to these cells, which leads the Electrolyte of these cells to be unsaturated under the temperature of the room [55]; so these cell forms are much more popular [54]. The material of electrode and mercurous sulphate are attached to porous plugs to prevent damaging the cell when inverted [54]. The lower temperature coefficient of these cells is around -10 micro voltage every degree Celsius increase [55]. Such cells have lifetimes that are less long than the Saturated Cadmium / Weston cell, but the cell's lifespan is still about ten to twenty years, under the good care, with drift voltage of about 30 micro voltages a year [55]. o Precaution in the use of Standard Cell: During the use of a Standardized Cell, various precautions should be taken to prevent cell damage as well as the reliability and constantness of the EMF of the cell listed below [55]: 1) If a significant current flows from a standard cell the cell is harmed, a quite-minute current of not over than 10 micro amperes should be driven from the standard cells for a quite short Page | 14 Electrochemical cells, Galvanic cell, and fuel cells. period of time. Just a potentiometer or a few open circuit devices should be used for the measurement. High-resistance resistor should also be used as a cell protection. [55] 2) Voltmeter should not be used in measuring the cell's EMF because voltmeter drains too much current and also doesn't accurately read because of the Standard cell's strong internal resistance [55]. 3) The standard cell is not to be transported from one location to another often and it is important not to squeeze the cell, otherwise the Cell's EMF can change [55]. 4) The moisture in the cell must be prevented from condensation [55]. 5) A standard cell should be monitored regularly with other standard cells in order to verify its reliability and EMF [55]. 6) With a constant temperature around 15-20 degrees Celsius, the cell should be kept in a dry area [55]. Daniell's cell The Daniell cell is composed of a copper pot filled with copper (II) sulfate solution, which is immersed in an unglazed vessel full of sulfuric acid and zinc electrode, and is a form of electrochemical cell introduced 1836, by John Frederic Daniel, a British chemist and meteorologist. [56]. Despite its current historical importance, Daniell cell is the most widely example that is being related to a galvanic cell [57,58,1]. Daniell cell was a significant improvement over available technology that is utilized in the early days of battery production [56]. The gravity cell or crowfoot cell was later called Daniell, and it became a popular alternative for electric telegraph systems [56]. Figure 7 Schematic of the Daniell cell [1] Page | 15 Electrochemical cells, Galvanic cell, and fuel cells. In the anodic half-cell a zinc electrode is immersed into a ZnSO4 solution, as shown in fig.7, The metallic zinc is oxidized to zinc ions, which go into the solution [57,58,1]: Zn(s)Zn+2(l)+ 2e- (4) The standard potential of this anodic half reaction is _0.76 V versus SHE In the cathode half-cell a copper metal electrode is immersed into CuSO4 solution, as shown in fig.7. Cu2+ ions from the solution are reduced to metallic copper, which is deposited on the Cu electrode [57,58,1]: Cu2+ (l)+ 2e- Cu(s) (5) The standard potential of this cathodic half reaction is +0.34 V versus SHE. Thus, the overall redox reaction of the Daniell cell is; Cu2+ (l)+ Zn(s) Zn+2(l)+ Cu(s) (6) [57,58,1] These processes lead to solid copper accumulation at the cathode and zinc electrode corrosion as zinc cations in the solution. Around 213 kJ per mole of zinc is produced by the Daniell cell. The weakest 207 kJ / mol bondage (smaller cohesive energy magnitude) in zinc relative to copper metal can be directly linked to this energy, which is described by an absence of bonding through d-orbitals partly filled in zinc. [59,56] Therefore, if the current drawn out of the Daniell cell (or fed to) is small, it is reversible. This cell is being used to produce electricity, by absorbing an electrode or storing electricity. [60,56] In a sequence of same cells, the Daniell cell was developed to make a relativity longer lasting battery, thus improving it over the voltaic pile that was created three decades earlier by the first world batter [61], and also transforms the released chemical energy to the electrical energy during the redox reaction and, if the Zn2 + and Cu2 + ions are unity, has a 1.1 V electrical potentiality [62]. Lead-acid cell Lead peroxide reacts with sulphuric acid in a lead-acid cell that is a secondary electrochemical cell or accumulator to generate a positive charge on the anode. Metallic lead reacts to acid on the cathode, creating a negative charge. The lead in the two electrodes integrates with the sulfate ions to generate the sulfate of the lead that is slightly soluble.[63] This action is described below: Page | 16 Electrochemical cells, Galvanic cell, and fuel cells. The cell representation [32-34,64]: Pb /PbSO4 / H2SO4/ PbSO4/PbO2 / Pb The cathodic process [32-34,64]: PbO2+4H++ SO42- +2e PbSO4 +2H2O The anodic process [32-34,64]: Pb+ SO42- The net process [32-34,64]: PbO2+ Pb+ 2SO42-+4H+ 2PbSO4+2H2O (8)  PbSO4+2e- (6) (7) o Lead acid as a battery The most popular type of battery being used photovoltaic systems is Lead-acid batteries [65]. The energy density of lead-acid batteries is low, thus it is not appropriate for mobile devices [66], they have a longer lifetime and a relatively low cost compared to other types of batteries, but only fairly effective and high maintenance requirements [65]. One of the particular advantages of leadacid batteries is it is being the most common type of battery for the majority of rechargeable battery applications (for example, in start-up motors), and hence have a well-established and fully developed technology foundation [64]. Batteries with lead-acid cells store energy through a chemical reversible reaction [66], therefore the former conditions could be returned via a "charging current" [63]. As shown below: Figure 8 the overall chemical reaction of the Lead acid batteries [63] H2O2 cell H2O2 is a fuel cell type, which is a galvanic cell that instantly transforms the energy from a chemical reaction into electricity. Fuel and oxidation factors are endlessly provided to the fuel cell electrodes in contrast to primary and secondary cells. As long as the outer supply of fuel is kept up, energy can be gained from a fuel cell. [1,67-76] Hydrogen-Oxygen fuel cell generates heat from the reaction of the hydrogen with oxygen, not as a heat, but as usable electricity, as normal combustion in air [77]. Page | 17 Electrochemical cells, Galvanic cell, and fuel cells. The electrodes are composed of porous carbon and a resin with a concentrated aqueous solution of sodium hydroxide in the electrolyte [20,1] as shown in fig 9. The anode is fed hydrogen gas to be oxidized and the cathode is fed oxygen where it is reduced relative to the following chemical reaction [20,1]: Anode: 2H2 (g) + 4 OH- 4 H2O (l)+ 4 e- (9) Cathode: O2 (g) + 2H2O (l) 4 OH-(l) (10) Overall: 2H2 (g) + O2 (g) 2H2O(l) (11) The cell overall reaction generates water, therefore fuel cells are environmentally friendly [20,1]. Figure 9 Schematic of a hydrogen oxygen fuel cell according to [20,1]. Hydrogen-oxygen fuel cells are much more effective than traditional energy sources or batteries (such as zinc-carbon) as they produce instantly the electrical energy by the reaction of the oxygen with the fuel, without complications with boilers, turbines, and dynamos [77]. 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