电极反应动力学 燃料电池.ppt

Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,Towards a Sustainable Energy Future,2011 SpringFuel Cell and Electrochemistry,Lecture 9,Kinetics of electrode reactions,Dynamic Equilibrium,Dynamic Equilibrium,The kinetic theory therefore predicts a constant concentration ratio at equilibrium,just as thermodynamics does;,Kinetics describe the evolution of mass flow throughout the system,including both the,approach,to equilibrium and the,dynamic maintenance,of that state.,Thermodynamics describe only equilibrium.,Exchange velocity:,equilibrium features nonzero rates of conversion of A to (and vice versa),but those rates are equal.v,0,The,Arrhenius,Equation and Potential Energy Surfaces,A,is the,pre-exponential factor,k,is the,reaction rate coefficient,E,A,activation energy,Transition State Theory,we focus on the special condition in which the entire systemA,B,and all other configurationsis at thermal equilibrium,Essentials of electrode reactions,we always saw that current is often limited wholly or partially by the rate at which the,electroreactants,are transported to the electrode surface.,This kind of limitation does not concern a theory of interfacial kinetics.More to the point is the case of low current and efficient stirring,in which,mass transport,is not a factor determining the current.Instead,it is controlled by,interfacial dynamics.,Tafel,equation,Butler-,volmer,model of electrode kinetics,Suppose the electrode potential is equal to,E,.,The,cathodic,and anodic activation energies are,Where,the,transfer coefficient,Standard Rate Constant,It simply is a measure of the kinetic facility of a,redox,couple.,A system with a large,k,0,will achieve equilibrium on a short time scale,but a system with small,k,0,will be sluggish.,The largest measured standard rate constants are in the range of 1 to 10 cm/s,and are associated with particularly simple electron-transfer processes.,Molecular rearrangement upon electron transfer or,multistep,process can be very sluggish,The Transfer Coefficient,In most systems,a,turns out to lie between 0.3 and 0.7,and it can usually be approximated by 0.5 in the absence of actual measurements.,The transfer coefficient,a,should generally be a potential-dependent factor;,In a typical chemical system,the free energies of activation are in the range of a few electron volts,but the full range of measurable kinetics usually corresponds to a,change,in activation energy of only 50-200,meV,or a few percent of the total.,Exchange current,At zero current,At equilibrium,the bulk concentrations of and R are found also at the surface,Exchange current,Exchange current density,The current-,overpotential,Equation,Current-,overpotential,Equation,The current-,overpotential,Equation,The solid curve shows the actual total current,which is the sum of the components,ic,and,ia,shown as dashed traces.,For large negative,overpotentials,the anodic component is negligible;hence the total current curve merges with that for,ic,.At large positive,overpotentials,the,cathodic,component is negligible,and the total current is essentially the same as,ia,.,In going either direction from,Eeq,the magnitude of the current rises rapidly,because the exponential factors dominate behavior,At extreme,overpotential,the current levels off.the current is limited by mass transfer rather than heterogeneous kinetics.,Approximate Forms of the i-,Equation,(1)No Mass-Transfer Effects,The exchange current can be viewed as a kind of idle current for charge exchange across the interface.If we want to draw a net current that is only a small fraction of this bidirectional idle current,then only a tiny,overpotential,will be required to extract it.,Even at equilibrium,the system is delivering charge across the interface at rates much greater than we require.The role of the slight,overpotential,is to unbalance the rates in the two directions to a small degree so that one of them predominates.,On the other hand,if we ask for a net current that exceeds the exchange current,the job is much harder.We have to drive the system to deliver charge at the required rate,and we can only do that by applying a significant,overpotential,.,From this perspective,we see that the exchange current is a measure of any systems ability to deliver a net current without a significant energy loss due to activation.,Approximate Forms of the i-,Equation,(2),Linear Characteristic at Small,For small values of,x,the exponential,e,x,can be approximated as 1+,x,Approximate Forms of the i-,Equation,(3),Tafel,Behavior at Large,at large negative,overpotentials,exp(-,f,),exp(l,-,)f,Approximate Forms of the i-,Equation,Tafel,Plots,Exchange Current Plots,Very Facile Kinetics and Reversible Behavior,When i,0,becomes very large compared to any current of interest,The ratio i/i,0,then approaches zero,Effect of Mass Transfer,