电极过程动力学 (3).pdf

1-1 CHAPTER 1:INTRODUCTION AND OVERVIEW 1.What is electrochemistry?2.Modes of Cell Operation 3.Cell Components 4.Fundamental concepts 5.Energy requirements 6.Chemical vs.Electrochemical conversion 7.Unique characteristics of electrochemical processes.Advantages,disadvantages and barriers.8.Scope of applications,9.Electrochemical engineering 1-2 CHAPTER 1:INTRODUCTION AND OVERVIEW 1.What is electrochemistry?All chemical interactions are electrical at the atomic level so that in a sense all chemistry is electrochemistry.W.J.Moore,Physical Chemistry,Prentice-Hall,1962.Electrochemistry covers a broad range of research and applications including diversified subjects ranging from batteries and electroplating to manufacturing of chlorine,corrosion and electrochemical solar cells.An adequately broad definition will probably state that electrochemistry is the science studying the interaction of charged ionic species under an applied electric field.Most often electrochemistry is concerned with chemical processes occurring at the interface of an ionic solution(=electrolyte)and an electronically conducting material(=electrode)with the simultaneous gain or loss of electrons)i.e.oxidation or reduction at an interface).2.Modes of Operation of Electrochemical Cells Electrochemical cells can generally take place in one of the two following modes.a.Application of external energy(voltage to drive a chemical reaction(e.g.electrolysis or plating).b.Utilization of spontaneous chemical reaction to obtain(electrical)energy.Because of the reversibility of electrochemical processes,typically,the same cell can be operated in either mode,supplying or removing energy as needed to drive the reaction in the desired mode(direction).If the reactions are such that the reactants contain more energy than the products,the reaction will be spontaneous,releasing electrical energy,such as in a battery or a fuel cell.In the opposite case,when the reactants contain less energy than the products,the reaction will not be spontaneous,and in order to drive it,we need to 1-3 supply external energy(such as in e.g.plating or the charging of a rechargeable battery.The quantitative treatment of this subject will be provided in chapter 4 dealing with thermodynamics.The energy release by spontaneous reactions occurring in electrochemical cells is unique,as it involves the direct conversion of chemical into electrical energy,a process that can proceed at high efficiency in batteries or fuel cells.The direct conversion of solar energy into electrical energy taking place in a liquid junction solar cell can also be considered as longing in this class.1-4 Fig.1-1:Modes of operations of electrochemical cells.Electrochemical Cells-Modes of Operation 1.Electrical Energy Chemical Products SO4-M+Zn Cu Zn+Cu+e-+e-Cathodic Reduction:Anodic Oxidation:Zn+2e Zn Cu Cu+2e SO4-M+Zn Cu Zn+Cu+Zn Zn+2e Cu+2e Cu+e-Anodic Oxidation:Cathodic Reduction:2.Chemical Reaction Electrical Energy e-LOAD Electrolysis,Plating or Battery Charging External driving force is applied to drive a chemical rxn against its spontaneous direction Battery or Fuel-Cell A spontaneous chemical reaction is used to produceelectrical energy 1-5 3.The Electrochemical Cell:Any electrochemical reaction takes place in a cell.The electrochemical cell must consist at least of:(i)Minimum of two electrodes(ii)Electrolyte(iii)An external conductor,or power source.The reason for having at least two electrodes,is that we cannot generate charge,only circulate it.Furthermore,we cannot accumulate any substantial amount of charge anywhere within the cell(due to electroneutrality,as discussed later),hence any oxidation reaction on one electrode(anode)must be accompanied by a second reduction reaction on another electrode(cathode).In some case,such as in corrosion,we encounter bi=polar electrodes,i.e.,a single electrode that has two separate zones on it,one supporting a cathodic reaction,the second,serves as an anode(Fig.1-3).The electrolyte is the source sink for products and reactants,and provides ionic conduction.The electrodes provide electronic conduction.The electrode-solution interface is the region where charge passes from electronic medium to ionic medium.Electrochemical reactions always take place at the interface between the electrode which is an electronic conductor(electrons move within the conduction band),and the electrolyte-in which current is carried only through ions.Since all electrochemical reactions involve oxidation or reduction,electrons must be transported across this inter-face,to or from the ions.The important difference in the mechanism of charge transport through any electrochemical system must be emphasized:Ionic charge carriers in the electrolyte(both positive and negative ions move in opposite directions and both contributing to the current),and only electrons,(i.e.,negative charge carriers)in the electrode and wires.(We should recognize that liquid mercury in which electronic conduction takes place is an electrode,whereas solid B alumina in which current is carried by movement of ions is considered an electrolyte).4.The Electrochemical Interface:Right at the electrode and extending into the solution is a narrow region of molecular dimension scale(10-100 A)called the electrical double layer.This layer which consists of 1-6 charged ions absorbed or attracted to the electrode,acts like a capacitor and is unique by virtue of the fact that the electroneutrality condition which must persist in the bulk of the electrolyte,and inside the electrode,does not hold inside this double layer.Although the double layer and the phenomena associated with it are of extreme importance to many electrochemical processes,because of its thinness it usually can be neglected in macroscopic studies of current and voltage balances.e-+Zn Cu Zn+Cu+-e-MAJOR COMPONENTS:1.Electrolyte(ions)2.Cathode(s)3.Anode(s)4.Electron Pathway(Power Supply or Load)5.Separator Conservation of Charge-Charge may only be circulated around,not generated Electroneutrality:Zi Ci=0 Electrode/Electrolyte Intrface-The Double Layer(serves as a capacitor)Energy requirements at the electrode:10-100 KCal 4 eV 0-4 Volts Water decomposition:1.23 V Organics(CHO)1 V Li:-4 V Fig.1-2:Electrochemical Cell Components THE ELECTROCHEMICAL CELL 1-7 Electrochemical process at an open-circuit cell(single electrode)A bi-polar or corroding electrode Fe Fe+2 +2e 2H+2e H2 Fe+2H+H2+Fe+2 Electrode Types Active plating or dissolving Passivated covered with a resistive layer,typically anodic film,often highly polarized Inert(or dimensionally stable)typically gas evolving electrode Single or multiple simultaneous reactions Separator Types Porous diaphragm(non-selective,slows diffusion)Ion-selective membrane(permeable to only single ion or groups of ions)o Ion exchange(Nafion,PBI)Cationic Anionic o Ceramic(beta-alumina,Ceria stabilized zirconia)Fe+2 H2 HCl Fe+cathode anode Overall:H2H+Fig.1-3:Electrode and Separator types 1-8 1-9 1-10 1-11 1-12 5.Role of the electrode:Any electrochemical cell consists of at least two electrodes,a cathode and an anode,immersed inside an electrolyte.The role of the electrode is:1.Source or sink for electrons 2.Site for chemical reactions 3.Source of reactants,sink for products Illustrations:1.redox couple:Fe2+(aq)Fe 3+(aq)+e(oxidation)2.metal.electrodeposition:Nit+(aq)+2e Ni(metal lattice)3.electrochemical generation of H2:2 H2O+2e+2 sites(metal)2 H-sites(metal)+2 OH 2 H-sites(metal)H2+2 sites(metal)4.battery oxide reaction(mercury cell)HgO(solid)+2H 2O+2e Hg(liquid)+2 OH-(solution)Electrodes can be divided into different kinds:Catalytic electrodes(Platinum black,DSA)Consumable electrodes(Chlor-alkali,aluminum)Dimensionally Stable Electrodes(DSA)6.Function of the electrolyte phase:1.Source of reactants,sink for products 2.Provide ionic conduction path between the electrodes.7.Comparison of electrochemical processes to chemical processes in general:Electrochemical reactions are directly driven by applying(or removing)external energy through the voltage applied to the electrodes,causing current to flow.This implies that unlike a regular chemical reaction which can proceed only spontaneously,in the direction of an overall lower Gibbs free energy,an electrochemical reaction can normally be driven 1-13 in any desired direction.Furthermore,since one can control the amount and rate of this external energy application,the rate of electrochemical reactions can be precisely monitored and controlled and their endpoint accurately determined.This makes electrochemical reactions extremely useful as far as their utilization is concerned.They can be used reversibly for energy storage and generation,or production of chemicals under extremely well controlled conditions.A regular chemical reaction usually involves breaking(or making)a number of different chemical bonds,with the constraint that the overall free energy of the system is decreased.Electrochemical reactions usually involve ionized species and always involve oxidation and reduction.Unlike regular oxidation-reduction reactions,these however take place at separate locations which can be far removed from one another.Electrochemical reactions always occur at an electrode-electrolyte interface and therefore always belong to the class of heterogeneous reactions.Chemical processes in general can either be homogenous,i.e.,involving one phase only,or heterogeneous.The surface,or electrode area available for the electrochemical reaction is therefore often a critical issue(porous,fluidized bed,or particulate electrodes are examples of addressing this problem).1-14 CHEMICAL VS.ELECTROCHEMICAL CONVERSION Chemical Conversion:Direct charge transfer Homogenous reaction Output:Heat H(Limited by Carnot efficiency)Indirect conversion to electricity Electrochemical Conversion:Charge transfer at electrodes Heterogenous reaction(surface dependent)2 distinctly separate sites(cathode&anode)Separator often required Energy output:G=H-T S Direct conversion to electricity +_ _+e e e+_ e e _ _+e e H+SO4-2 2H2+O2 2 H2O C+O2 CO2 Zn+Cu+Cu+Zn+e 2H2 4 H+4 e(anode)O2+4e+4H+H2O _ 2H2+O2 2 H2O Zn Zn+2e Cu+2 e Cu _ Zn+Cu+Cu+Zn+Fig.1-4:Chemical vs.electrochemical conversion 1-15 7.Energy Requirements at the Electrode:Since the electrochemical reaction,very much like a regular chemical reaction involves the breaking(or forming)of chemical bonds,we can expect the associated energy to be in the range of 10-100 kcal/bond,or up to 4 ev.This corresponds to a range of 0-4 volts.More specifically:splitting water requires 1.23v(implying that a single cell of an aqueous battery can not produce over 1.23 volts);the discharge of Lithium requires 4 volts,and electro-organic reactions involving forming or breaking of CHO type bonds require about 1 volt.8.Unique characteristics of electrochemical processes:A.Even the most elementary electrochemical cell must consist of at least two electrodes immersed in an electrolyte.Any electrochemical reaction is characterized by two or more products,each produced at one of the electrodes.An oxidation reaction takes place at the anode,reduction at the cathode.ILLUSTRATION:Water Electrolysis B.Separator is often required to separate the two products,prevent or slow down their back mixing and their reacting back to form the reactant thereby reducing the process efficiency.The separator must allow the passage of at least some of the ionic species of the solution thereby providing electric continuity.A good separator will provide good separation,will be chemically inert,and will introduce the least amount of ohmic drop into the cell.Separators can be made of fritted glass(pH electrodes),ceramics,plastic Cathode:4 H+4 e 2H2 (Proton reduction)Anode:H2O O2+4e+4H+(Hydroxyl oxidation)Overall:2 H2O 2H2+O2 1-16 materials(batteries),asbestos(diaphragm in the chlor-alkali cells)or be ion selective such as the NafionR membrane which is used in the chlor alkali process and in ambient temperature fuel-cells.C.The potential applied between the electrodes in an electrochemical cell is directly converted into chemical energy;hence an electrochemical cell is a unique device for direct energy conversion,not restricted by the Carnot efficiency which imposes severe limitations on heat engines.D.An electrochemical process provides essentially limitless oxidation and reduction possibilities.The maximum energy associated with a chemical bond is about.5 eV corresponding to 5 Volts,which can easily be provided by a simple power supply,rectifier,or a battery.(The assembly of the first batteries led Davy to promptly discover and separate sodium.)E.Electrochemical processes are characterized by high-purity products.A 99.999 pure product can be typically produced from a contaminated raw material in one stage electrochemical process(copper refining).Electrochemical processes can also be extremely selective.F.The reaction rate in electrochemical processes can be precisely controlled and monitored.The current is directly proportional to the reaction rate,and the applied voltage is the driving force.This makes the monitoring and control of electrochemical processes,easy,accurate and very reliable.Electrochemical processes are conveniently used as sample reactions for mass transport studies,since the reaction rate can be so easily monitored(the limiting current technique).Electrochemical systems offer the unique advantage in that local fluxes can accurately and conveniently be determined by measuring the current.Furthermore,by simply varying the applied potential,i.e.,the driving force,the electrochemical reaction rate can be conveniently changed and often reversed.G.Electrochemical processes provide good control over the structure and appearance of the product.This is used advantageously in a large number of electrochemical surface finishing processes such as applying protective or decorative coatings by electroplating,in anodic polishing or in electrolytic powder formation.H.Electrochemical processes are driven by the electric field,a vector quantity providing directionality to the electrochemical reaction.This feature is important 1-17 when considering the distribution of reaction rates,i.e.,the variation of current density along the electrodes and is utilized in processes such as electro-machining.I.Detailed understanding of electrochemical systems requires however,additional important information which is often difficult to obtain.This is mainly due to the fact that several charge and mass transport processes occur simultaneously even in the simplest systems.Any useful analysis must be capable of separating the effects due to each of these mechanisms.The analysis is further complicated by the fact that the transport processes are inherently coupled and often depend on local system parameters such as concentration or the electric potential which may vary considerably over very short distances near the electrode,and often cannot be locally probed.The potenti