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1、,Cyclic Voltammetry（循环伏安法）,Figure 1.8,Figure 1.9,Figure 1.10,Reversible system:,The peak current for a reversible couple(at 250C)is given by Randles-Sevcik equation:,Where n is the number of electrons,A is the electrode area(in cm2),C is the concentration(in mol/cm3),D is the diffusion coefficient(i
2、n cm2/s),and is the scan rate(in V/s).,(1.6),(1.7),(1.8),Figure 1.11,Q.L.Echegoyen et al,J.Am.Chem.Soc.1992,114,3978,Irreversible and Quasi-reversible Systems:,(1.9),(1.10),Figure 1.12 不可逆过程电流和电势随扫描速度的变化,Study of Adsorption Processes,(1.11),Figure 1.13,Figure 1.14,(1.12),Study of Reaction Mechanisms
3、,Quantitative Applications,Polarography(极谱)and Voltammetry(伏安法)的区别？,polarographyA classical electroanalytical technique discovered in 1922 by J.Heyrovsky,for which he was awarded the Nobel Prize for Chemistry in 1959.Essentially,it is linear-sweep voltammetry using a dropping-mercury electrode for w
4、orking electrode and a large mercury pool as counter electrode.,voltammetryAn electrochemical measuring technique used for electrochemical analysis or for the determination of the kinetics and mechanism of electrode reactions.Voltammetry is a family of techniques with the common characteristics that
5、 the potential of the working electrode is controlled(typically with a potentiostat)and the current flowing through the electrode is measured.In one of the most common applications of the technique,the potential is scanned linearly in time;this is called the linear-sweep voltammetry,LSV,or LV.Cyclic
6、 voltammetry(CV)is a linear-sweep voltammetry with the scan continued in the reverse direction at the end of the first scan,this cycle can be repeated a number of times.,6.7多电子转移的电极过程对于两电子还原反应，一般的表达式为：A+e BB+e C,Fig.6.7 两电子转移反应示意图,对于此过程，可以分为三个极限情况进行讨论：(1)第二步在更负的电位下反应，我们可以观察到两个分开的波。即：kf,2-kb,2 kb,1B+
7、eC fast(3)第二步是决速步骤，即：kf,2-kb,2 kb,1A+e B pre-equilibriumkf,2 kb,1B+eC rate-determining stepDue to pre-equilibrium,the voltammetric wave is steeper than in case 2.The activated complex is more sensitive than in case 2 to changes in applied potential.,Fig.6.8 Voltammograms for the reduction of species
8、 A followingA+e B+e C according to the relative rates of the two steps.IL=IL(A B).(a)Second step much more difficult than the first;(b)First step rate-determining;second step fast;(c)First step pre-equilibrium;second step rate-determining.,3.4特性吸附即使电场不存在也能发生的吸附-特性吸附特性吸附使零电荷电势(EPZC)发生移动。负离子使之负移，正离子使之
9、正移。为什么？为什么表面活性剂能够使微分电容在EPZC处变化很大？答案：见吴浩青，李永舫电化学动力学p2426.吸附等温线(isotherm)：(1)Langmuir;(2)Temkin;(3)Frumkin.,图3.20,特性吸附对电毛细曲线的影响,-,+,-,阳离子的活性顺序是：N(C3 H7)4+Ti+K+,阴离子的活性顺序是：S2-I-Br-NO3-,图3.21 阴、阳离子吸附对于电毛细曲线的影响,图3.22,吸附等温线(isotherm)：(1)Langmuir;(2)Temkin;(3)Frumkin,图3.23 各种吸附等温线,Langmuir isotherm:Langmuir
10、 假设：(a)吸附在电极表面的分子彼此没有相互作用；(b)表面吸附是均匀的；(c)在高浓度时电极表面达到饱和浓度(单分子层)，用s 代表。=i/s(覆盖度)。i i=/(1-)=i/(s-i)Esin-Markov coefficient,吸附等温线(isotherm)：,Temkin isotherm:主要考虑到吸附能是覆盖度有关。i=RT/(2g)ln(i i)g是一个与吸附物质之间相互作用能有关的参数。,Frumkin isotherm:i=RT/(2g)ln(i i)ln(s-i)/i g是正值时表示吸附物质之间是吸引作用，g是负值时表示吸附物质之间是排斥作用。,常用的研究物质在电极表
11、面吸附的电化学方法有循环伏安法和电势双阶跃法,循环伏安法,图3.24,吸附物质的循环循环伏安图,图3.25,图3.26,R.H.Wopschall and I.Shain,Anal.Chem.,39,1514(1967),Why do we view adsorbed neutral Species as being intimately bound to the electrode surface,rather thanBeing collected in the diffuse layer?,图3.27,固体电极上的研究Well-Defined Single-Crystal Electrode Surface,测量固体/电解液体系的零电荷电势并非简单的事，因为很难得到电极表面的重现性。,图3.28,图3.29,图3.30,
