电极反应动力学燃料电池ppt课件

1、Towards a Sustainable Energy Future2019 SpringFuel Cell and ElectrochemistryLecture 9Kinetics of electrode reactionsTowards a Sustainable Energy FutureDynamic EquilibriumTowards a Sustainable Energy FutureDynamic EquilibriumThe kinetic theory therefore predicts a constant concentration ratio at equi

2、librium, 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

3、of A to (and vice versa), but those rates are equal. v0Towards a Sustainable Energy FutureThe Arrhenius Equation and Potential Energy SurfacesAisthepre-exponentialfactorkisthereactionratecoefficientEAactivationenergyTowards a Sustainable Energy FutureTowards a Sustainable Energy FutureTransition Sta

4、te Theorywe focus on the special condition in which the entire systemA, B, and all other configurationsis at thermal equilibriumTowards a Sustainable Energy FutureEssentials of electrode reactionswe always saw that current is often limited wholly or partially by the rate at which the electroreactant

5、s 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 dynamic

6、s.Towards a Sustainable Energy FutureTafel equationTowards a Sustainable Energy FutureButler-volmer model of electrode kineticsTowards a Sustainable Energy FutureSuppose the electrode potential is equal to E. The cathodic and anodic activation energies are Towards a Sustainable Energy Future Where ,

7、 the transfer coefficientTowards a Sustainable Energy FutureTowards a Sustainable Energy FutureStandard Rate Constant It simply is a measure of the kinetic facility of a redox couple. A system with a large k0 will achieve equilibrium on a short time scale, but a system with small k0 will be sluggish

8、. 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 sluggishTowards a Sustainable Energy FutureThe Transfer Coefficien

9、tIn 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.Towards a Sustainable Energy FutureThe transfer coefficienta should generally be a potential-dependent factor;In a typical chemical system, the free energies of ac

10、tivation 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.Towards a Sustainable Energy FutureExchange currentAt zero currentAt equilibrium, the bulk concentration

11、s of and R are found also at the surfaceTowards a Sustainable Energy FutureExchange currentExchange current densityTowards a Sustainable Energy FutureThe current-overpotential EquationCurrent-overpotential EquationTowards a Sustainable Energy FutureTowards a Sustainable Energy FutureThe current-over

12、potential EquationThe 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

13、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 behaviorAt extreme overpotential, the current levels off. the current is limited by mass tra

14、nsfer rather than heterogeneous kinetics.Towards a Sustainable Energy FutureApproximate Forms of the i- Equation(1)No Mass-Transfer EffectsTowards a Sustainable Energy FutureTowards a Sustainable Energy FutureTowards a Sustainable Energy FutureThe exchange current can be viewed as a kind of idle cur

15、rent 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 gre

16、ater 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 deli

17、ver 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.Towards a Sustainable Energy Futu

18、reApproximate Forms of the i- Equation (2) Linear Characteristic at Small For small values of x, the exponential ex can be approximated as 1 + xTowards a Sustainable Energy FutureApproximate Forms of the i- Equation(3) Tafel Behavior at Large at large negative overpotentials, exp(-f ) exp(l- )f Towards a Sustainable Energy FutureApproximate Forms of the i- Equation Tafel PlotsTowards a Sustainable Energy FutureTowards a Sustainable Energy FutureExchange Current PlotsTowards a Sus