国外电化学讲义
电化学基本概念ppt课件
i i
两相间建立平衡电势
电极（Electrode)
电极材料/电解质
Zn|Zn2+, SO42Pt|H2,H+ Fe|Fe3O4|Fe2O3|水溶液
电极（Electrode)
电极材料/电解质 •传递电荷
Zn|Zn2+,SO42-,
•氧化或还原反应
Pt|H2,H+
的地点
•“半电池”
Fe|Fe3O4|Fe2O3|水溶液
法拉弟定律的几个要点
1. 电和化学反应相互作用的定量关系 2. 不受电极、外界条件的影响 3. 适用于多个电化学装置的多个反应（串联）
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阴极
阳极
H+
NO3-
银阴极 银阳极
法拉弟定律的几个要点
1. 电和化学反应相互作用的定量关系 2. 不受电极、外界条件的影响 3. 适用于多个电化学装置的多个反应（串联） 4. 适用于单个电化学装置的多个反应（并联）
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铅酸蓄电池 (1860年--)
充电
(吸收电能)
负极 e
e 正极
Pb2+ PbSO4
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Pb2+
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麻省理工电化学教程系列6
I. Equivalent Circuit ModelsLecture 6: Impedance of ElectrodesMIT Student (and MZB)1. Flat ElectrodesIn the previous lecture, we leant about impedance spectroscopy. Electrochemical impedance spectroscopy is the technique where the cell or electrode impedance is platted versus frequency. Thus, the impedance is measured as a function of the frequency of the AC source.Electrochemical impedance spectroscopy is a useful method for investigating porous electrodes, which are extensively used in the field of batteries, fuel cells, and electrochemical capacitors. With the impedance spectroscopy analysis, we can characterize various electrodes in terms of AC frequency and model the equivalent RC circuits. In this lecture, we start with flat electrodes shown to Figure 6.1 to learn how the impedance spectroscopy can be applied to the analysis of electrodes.Figure 6.1 Schematic of a flat electrode cell which is composed of a neutral bulkelectrolyte and thin double layers, where L is the electrode spacing and λD is DebyeScreening Length (double layer thickness)a. Ideally conducting electrodesIn flat electrode cells, if faradaic reactions occur fast and the charge transfer resistance negligible, we can only consider the bulk capacitance and bulk resistance. Under the assumptions, the equivalent RC circuit of the flat electrode cell can be expressed as shown in Figure 6.2.Figure 6.2 Equivalent RC circuit of ideally conducting electrodes or ideally non-polarizable cell, where C b is the bulk or geometrical capacitance per area and R b is thebulk resistance per area.The bulk capacitance per area, C b, can be calculated by ε/L, where ε is the permittivity of electrolyte and L is the electrolyte spacing. The bulk resistance per area, R b, can be expressed by L/σb, where σb is the conductivity of electrolyte. In addition, we can set the bulk time scale for charge relaxation as τb, which can be obtained by,Normally, the time scale is a range of MHz. As we leant in the previous lecture, the impedance of the equivalent RC circuit can be expressed by,As a dimensionless form, the impedance can be alternatively expressed by,Where, the dimensionless impedance and the dimensionless frequency . The corresponding Nyquist plot can be obtained in terms of the dimensionless frequency as Figure 6.3.Figure 6.3 Nyquist plot of the equivalent circuit for ideally conducting electrodeswhich is expressed by,, so is translated by +1 and stretched by 1/2 .b. Ideally blocking electrodesIf flat electrode cells don’t have faradaic reactions, current leads to capacitive charging of the double layers. Thus, we should consider the double layer capacitance when the equivalent circuit is determined. In other words, it can be deserved that the cell has ideally polarizable electrodes. In this case, the equivalent RC circuit can be drawn as Figure 6.4.Figure 6.4 Equivalent RC circuit for the ideally polarizable electrodes, where C D is thedouble layer capacitance per area and can be obtained by C D=ε/λD.The double layer charging time scale can be considered as and we can know that the time scale of the double layer charging is much longer than that of the bulkcharging relaxation due to much thinner double layers than the electrode spacing. As the same way to the ideally conducting electrodes, we can express the impedance and the corresponding dimensionless form as,With the impedance forms, two limits of and can be considered and we can get simplified forms of the dimensionless impedance for the two limits asFrom the limits, we can easily imagine the corresponding Nyquist plot which has two different regimes shown as Figure 6.5.Figure 6.5 Nyquist plot of ideally blocking electrodesc. Partially polarizable electrodesUntil now, we studied about two ideal electrodes of no charge transfer resistance with fast faradaic reactions and no faradaic reactions. Then, in the case of flat electrode cells which have not-fast faradaic reactions, how can we express the RC circuit and Nyquist plot? In this case, we can say the electrodes as partially polarizable electrodes, so the faradaic charge-transferresistance, R F, and double layer capacitance, C D, should be considered, simultaneously. Based on the consideration, the RC circuit can be expressed as shown in Figure 6.6.Figure 6.6 Equivalent RC circuit of partially polarizable electrodesAs the same way to the previous ones, the impedance can be expressed by,Depending on the speed of faradaic reactions, we can imagine different forms of Nyquist plot for the equivalent RC circuit. Figure 6.7(a) shows a typical Nyquist plot of partially polarizable electrodes. When faradaic reaction speeds are fast, the Nyquist plot can be as Figure 6.7(b) and it can be considered as blocking electrodes. With the increase of the faradaic reaction speed, the charge transfer resistance decreases and the Nyquist plot become the same to that of ideally conducting electrodes, shown in Figure 6.3, at the limit of R F →0. In contrast, in the case of slow faradaic reactions, the Nyquist plot is shaped as Figure 6.7(c) and we can expect that conducting electrodes show the shape. The slower faradaic reaction speed makes the higher value of the charge transfer resistance and the Nyquist plot eventually has the same to that of ideally blocking electrodes, shown in Figure 6.5, at the limit of R F →∞.(a)(b)(c)Figure 6.7 Nyquist plots of cells with partially polarizable electrodes . (a) Balanced bulk and Faradaic resistances, (b) more highly conducting electrodes with relatively large bulk resistance, (c) nearly blocking electrodes with relatively small bulk resistance.2. Porous ElectrodesPorous electrodes have been used for various applications since porous electrodes have many advantages especially for electrochemical systems such as capacitors and batteries. First of all, porous electrodes provide large surface areas which become interfaces between electrodes and electrolytes, resulting in high capacitances and compensating slow electrochemical reactions. In this chapter, we will build up equivalent RC circuits for two types of porous electrodes and study about corresponding Nyquist plots, leading to constant-phase elements (CPEs) with special phase angles (π/4 and π/8). We will also discuss an impedance formula that has a constantphase element with arbitrary phase angle by employing a self-affine fractal model for a rough electrode surface. These models show that microstructural complexity in rough or porous electrodes can lead to CPE behavior, and we will close by noting that similar behavior can also result for flat electrodes with adsorption or reaction processes possessing a broad distribution of relaxation times.a. Homogeneous microstructureWhen an electrode is composed of homogeneous micro-porous structures as shown in Figure 6.8(a), we can construct the equivalent RC circuit model with the surface impedance per length, Z s, and the resistance of pore electrolyte per length, R p, induced by ionic conductions in electrolyte. To simplify the problem, we assume that the electronic conduction in metal is sufficiently fast to neglect the resistance and the assumption is reasonable for normal electrode-electrolyte cells. In the previous lecture, the homogeneous microstructured electrode which has thin double layers compared to the pore thickness can be expressed by the transmission line model in single microscale as shown in Figure 6.8(b).(a)(b)Figure 6.8 Schematic of homogeneous pore in single microscale (a), the equivalenttransmission line model of the homogeneous microstructured porous electrodes with thin electric double layersIf we can assume that the transmission line is infinite, the total impedance, Z, can be expressed as Figure 6.9.Figure 6.9 Total impedance of the infinite transmission line model can be considered as the same to the recursive model adding a surface impedance per length and a resistance of pore electrolyte per length to the total impedanceFrom the recursive model, the total impedance of Z can be expressed and two limits can be considered as following equation,Where, and(i) Smooth pore wallsTo determine the surface impedance, we can separately consider the transmission line as two different wall surface conditions. First, let’s consider smooth pore walls which have single length scale pores as shown in Figure 6.8(a). The corresponding transmission line can be composed of capacitances of surfaces per length, C s, and the resistances of pore electrolyte per length, R p, as shown in Figure 6.10.Figure 6.10 Equivalent transmission line of the homogeneous microstructures whichhave smooth pore wallsIn the transmission line, the surface impedance can be expressed as so that we can obtain the total impedance and consider two limits as following forms,The corresponding Naquist plot can be as Figure 6.11 and we can confirm that the impedance curve with respect to frequency is a hyperbola.Figure 6.11 Naquist plot of smooth pore wallsIf electrodes have a finite length, at very low frequencies, , charging propagates across the entire porous electrode of length L, and the electrode behaves like a pure capacitance, C=C s L. The Nyquist plot of the finite length porous electrode can be shown as Figure 6.12.Figure 6.12 Nyquist plot of finite length porous electrodes(ii) Porous pore wallsIn the previous section, we built the equivalent RC circuit model in smooth pore walls, which have single length scale of pores. Then, let’s suppose the electrode has two different length scales of pores such as that a single large length scale pore has a lot of small scale of pores on the surface as shown in Figure 6.13.Figure 6.13 Schematic of a pore which has porous pore wallsIn this case, we can consider a simple model letting the surface impedance, Z s, is close to the surface resistance which is much higher than the pore electrolyte resistance. With the assumption,the dimensionless total impedance, and we can model small pores by an array of transmission lines. Finally, we can obtain the dimensionless total impedance as blow,The corresponding Nyquist plot can be obtained as shown in Figure 6.14.Figure 6.14 Nyquist plot of a pore which has porous pore wallsFrom the study of the porous electrodes, we can figure out the impedances have a form of, where A and β are constant values, at low frequencies. The impedance form is normally called as a constant phase element. According to the values of β, we can classify the impedance form with four cases. First, β=1/2 is called as Warbug impedance for a simple RC transmission line or diffusion limitations, which will be learnt in the future lecture. Second,0<β<1 can be due to complex microstructure of the surface or anomalous diffusion or reaction kinetics. When β=0 and β=1, we can consider the impedance form as resistor and capacitor, respectively.b. Fractal rough surfacesA fractal surface is a kind of self affine surfaces and can be a good model to possibly explain the AC response of an interface between a metal and an electrolyte. A simple model of the fractal rough surfaces is the self-affine Cantor block, shown in Figure 6.15.Figure 6.15 A self-affine Cantor block model for the electrolyte surface at the metal-electrolyte interface which has CPE impedance.In each integration, there are N2=4 branches of width smaller by 1/a and length smaller by 1/a z, so the surface area of sides is reduced by 1/a2. Theodore Kaplan et al. (1987) showed that the Cantor block model also has a constant element form of impedance:c. Distribution of relaxation timesIt is worth to noting that the constant-phase-angle impedance does not require rough or porous surfaces. It can also arise from anomalous kinetics or transport when there is a broad range of time scales (e.g. due to multistep reactions, or heterogeneous catalysis at many different surface sites), even at an atomically flat surface. In both cases, one can think of the surface or porous material as providing many different circuit pathways for charge storage in parallel, each with a different relaxation time τ=RC. We can model this as a continuous integral over a random distribution of relaxation times,Z=R ∞n(τ)dτ∫1+iωτMIT OpenCourseWare10.626 / 10.462 Electrochemical Energy SystemsSpring 2011For information about citing these materials or our Terms of Use, visit: /terms.。
电化学分析法导论PPT课件
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3、辅助电极或对电极
它们是提供电子传导的场所，与工作电极组成电池，形 成通路，但电极上进行的电化学反应并非实验中所需研究或 测试的。当通过的电流很小时，一般直接由工作电极和参比 电极组成电池，但是，当通过的电流较大时，参比电极不能 负荷，其电位不再稳定，或体系的iR降低太大，难以克服， 此时需再采用辅助电极，即构成所谓三电极系统来测量或控 制工作电极的电位。
5
以北京大学的高小霞、中国地质科学院的姚修仁为代表进行的极谱催化波的
研究和应用，取得了突出的成绩，特别是用于稀土元素的测定，处于国际领先水 平。
以南京大学高鸿为代表进行的研究各类电极和电极过程的电流理论与示波极谱 滴定，跻身于世界前列。
复旦大学的邓家祺教授研究活化分析。
中国科学院环境生态研究中心研究库仑分析。
究电分析化学技术发展的前沿领域。
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电分析化学虽然只有一、二百年历史，但作为分析手段，方法是多样
化，应用是广泛化，有经典的成熟方法，也有新创的刚露头的方法，不但 在技术上日新月异，而且在理论上也是不断深入提高和前进的，因此要把 许多电分析化学方法恰当地分类是有一定困难的。
应用电化学课件
Methods for
studying the
...
interface
电化学作为技术的研究范围
energy Batteries
Electrolysis & electrosynthesis
resources
Electrochemical protection of materials
environments
mistry
现代电化学
涉及学科领域：
Chemistry, Physics, Mathematics Spectroscopes (spectroscopic electrochemistry) Biological science （bio-electrochemistry） Environmental science（environmental electrochemistry）
H2 + O2 H2O + G (W) Chemical energy → Work
Electrons act as reactant or product
不自发的电化学反应—电解池(Voltaic cell)
Chemical reaction
Fe2O3 + C + H (Q) Fe + CO2
Electric phenomena /
processes
Electrochemical reactions
Chemical phenomena /
processes
Charge formation & transfer
Electrochemical Science & Technologies
formation & transformation
最新电化学原理要点讲解学习精品课件
• 即对于有一个质子参与的电化学反应，溶 液pH每增加1，其表观(biǎo ɡuān)电极电 位移动的理论数值为-0.058 V
第二十九页，共42页。
pH梯度(tī dù)实验
Q:已知其电位随pH的变 化为-0.032 V/pH, 那么 几个质子参与(cānyù)其中? A:
析…循环伏安/计时安培/交流阻抗 (zǔkàng)…） • 色谱分析（液相/气相）
第十一页，共42页。
二、电化学的基本原理
第十二页，共42页。
原电池与电解池
原电池：能自发地将化学能转化(zhuǎnhuà)为电能 电解池：需要消耗外部电源提供的电能，使电池内部发生化学反
应
第十三页，共42页。
无/有液体接界(jiē jiè)电池
循环(xúnhuán)伏安法的适 用范围
• 研究一个(yī ɡè)新物质的电化学性质时的 首选
• 用于电极反应的性质、机理和电极过程动 力学参数的研究
• 可用于定量分析 • 其他
第三十五页，共42页。
其他(qítā)电化学方法
• 示差脉冲伏安(fú ān)法(DPV) • 方波伏安(fú ān)法(SWV) • 交流阻抗法( EIS ) • 计时安培法 • ……
第三十六页，共42页。
更灵敏(línɡ mǐn)的检测方 法
循环伏安法
检测限10-5 mol/L
改变(gǎibiàn)加载 电位的波形
示差脉冲伏安法(DPV) 方波伏安法(SWV)
检测限10-8 mol/L 扫描速率快
第三十七页，共42页。
示差脉冲(màichōng)伏安法DPV Differential-Pulse Voltammetry
电化学讲义
题型四电化学考情分析五年大数据分析在2021年高考中，对于电化学的考查将继续坚持以新型电池及电解应用装置为背景材料，以题干（装置图）提供电极构成材料、交换膜等基本信息，基于电化学原理广泛设问，综合考查电化学基础知识及其相关领域的基本技能，包括电极与离子移动方向判断、电极反应式书写、溶液的酸碱性和pH变化、有关计算及其与相关学科的综合考查等。
预测以二次电池以及含有离子交换膜的电解池为背景的命题将成为热点题型，因为二次电池不仅实现电极材料循环使用，符合“低耗高效”的时代需求，而且命题角度丰富，便于同时考查原电池和电解池工作原理；含有离子交换膜的电解池设问空间大，便于考查考生的探究能力。
回归教材教材知识体系（对照一轮资料认真梳理）知识迁移能力（回归课本探寻问题本源练就“吓不死”神功）课本原图陌生装置图电池类型燃料电池电解池二次电池金属腐蚀完成习题后自己补充一下如何识破“纸老虎”在教材中找到问题本源并解决解题策略总体思路：什么池→什么极→什么反应→什么现象→电子离子流向→相关计算。
一、电极判断原电池：电解池二、盐桥的组成和作用（1）盐桥中装有饱和的KCl、KNO3等溶液和琼胶制成的胶冻。
（2）盐桥的作用：Ａ．连接内电路，形成闭合回路；Ｂ．平衡电荷，使原电池不断产生电流。
（3）一室电池缺点：由于氧化剂与还原剂直接接触，工作一段时间后，锌片表面观察到有少量红色铜析出，说明锌片表面存在腐蚀电流，该电池的效率不高，电池不工作时，锌片继续被氧化，使得该电池的可贮存时间不长。
（4）双室盐桥电池优点：利用盐桥防止氧化剂与还原剂直接接触，提高电池效率，延长电池的可贮存时间。
三、多室电解池（IE班一轮复习时编写的微专题，很有用很重要）多室电解池是利用离子交换膜将电解池隔成多个极室，借助离子交换膜的选择透过性，自动把产品分离开，得到的目标物质更加纯净，降低分离提纯的成本。
但多室电解池因交换膜多，离子转移复杂，进出物质种类繁多等因素，导致学生常常分析混乱。
麻省理工电化学教程系列2
I. Equivalent Circuit ModelsLecture 2: Electrochemical Energy ConversionMIT StudentGalvanic cells convert different forms of energy (chemical fuel, sunlight, mechanical pressure, etc.) into electrical energy and heat. In this lecture, we are interested in some examples of galvanic cells.Current Chemical fuelSunlightMechanicalpressure……1. Voltage Sources1.1 Polymer Electrolyte Membrane (PEM) Fuel CellPEM fuel cells employ a polymer membrane to move ions from anode to cathode. Figure 2 shows the structure of Hydrogen-oxygen PEM fuel cell. Hydrogen molecules are oxidized to protons at the anode. Electrons travel through an external load resistance. Protons diffuse through the PEM under an electrochemical gradient to the cathode. Oxygen molecules adsorb at the cathode, are reduced and react with the protons to produce water. The product water is absorbed into the PEM, or evaporates into the gas streams at the anode and cathode. The detailed reactions areFigure 1: Energy Conversion of Galvanic CellH2 O2gas gas2OResistorAnode (oxidation reaction, produces electrons):Cathode (reduction reaction, consumes electrons):Net reaction:The equivalent circuit of PEM fuel cell is shown in Figure 3. The resistors and capacitors are related to different effects:R a D: gas diffusion at anodeR c D: gas diffusion at cathodeR a i: interfacial charge transfer at anodeR c i: interfacial charge transfer at cathodeR M: ion transfer in membraneC a i: interfacial charge storage at anodeC c i: interfacial charge storage at cathodeFigure 2: Hydrogen-oxygen PEM fuel cella c0 R ext1.2 Solid Oxide Fuel Cell (SOFC)Solid oxide fuel cells are a class of fuel cell characterized by the use of a solid oxidematerial as the electrolyte. In contrast to PEM fuel cells, which conduct positive hydrogen ions (protons) through a polymer electrolyte from the anode to the cathode, the SOFCs use a solid oxide electrolyte to conduct anions, e.g. negative oxygen ions, from the cathode to the anode, as shown in Figure 2. The electrochemical oxidation of the oxygen ions thus occurs on the anode side. The detailed reactions areAnode (oxidation reaction, produces electrons):Cathode (reduction reaction, consumes electrons):Net reaction:Figure 3: Equivalent circuit of PEM fuel cellH 2O 2gas gasH 2 Resistor2. Current Sources2.1 Silicon p-n Junction Solar CellBefore we learn silicon solar cell, we need to understand some basic concepts of semiconductor physics.A fundamental result of solid-state physics states that in semiconductor electrons with specific energies are allowed to stay in sharply define bands leaving bandgaps of forbidden energies in between. As shown in Figure 5, the two key bands around the fundamental bandgap are given special names. The band immediately below is called the valence band while the band immediately above is called the conduction band. In pure silicon, all states in valence band are occupied by electrons while all states in conduction band are empty at temperature of 0 K. At high temperatures a few states of the conduction band may actually be occupied by electrons. The electrons in the partially filled conduction band can move around in response to an applied voltage. In addition, there are empty states, called holes, in valence band which now allow electrons to move about.The band structures of silicon can be changed by doping. Doping brings foreign atoms which do not change the silicon lattice very much but can donate or accept electrons. The atom from Column V of the periodic table can “donate” one of its electrons to the conduction band and become positively charged, which is called donor. The semiconductor with doped donors is call n-type semiconductor. The atom from Column III of the periodic table can “accept” abonding electron and become negatively charged, which is called acceptor. The semiconductorFigure 4: Solid oxide fuel cellwith doped acceptors is call p-type semiconductor. Electron states associated with donor and acceptor atoms can be represented in the energy band diagram as shown in Figure 6. Donor states are inside the forbidden gap slightly below the conduction band edge at E D . The reason is that it only takes a small amount of energy to release the electron the conduction band. Similarly, acceptor states are represented slightly above the valence band at E A , as a little energy will result in the release of a hole to the valence band.Egg(E)Figure 5: Band structure of Intrinsic Si at 0KFigure 6: Band structure of doped Si. (a) n type. (b) p type.An ideal p-n junction is a semiconductor structure where the doping level abruptly changes from n-type with a uniform donor concentration to p-type with a uniform acceptor concentration,shown in Figure 7. Electrons on the n-side will tend to flow towards the p-side since they are presented with empty states at the same energy. At the same time, holes on the p-side will flow towards the n-side where there are lots of available states at the same energy. Once thermal equilibrium has been established and charge redistribution has stopped, a charge dipole has developed across the junction. This results in net positive charge on the n-side and net negation charge on the p-side. So there is a build-in electric field at the junction.When the p-n junction absorb a photon with hv>E g, a new electron-hole pair is produced. The build-in electric field drives the electron to the n-side and the hole to the p-side. If we connect the p-n junction to the external circuit, we will “harvest” the electron-hole pair current at electrodes.lightR extFigure 7: p-n junction solar cellThe equivalent circuit of p-n junction solar cell is shown in Figure 8(a). A p-n junction diode is in parallel with a current source where I p is the photocurrent. Figure 8(b) shows the I-V curve of the diode. In forward bias, large current is easy to pass. But in reverse bias, only a small saturation current I s is allowed. The I-V relation of diode can be calculate asAssuming R int=0, the I-V relation isp =0 (dark environment), the solar cell performs as a diode. When I p>0 (light environment), the open circuit voltage isI D V I Dsreverse biasforward bias (a) (b) VI p >0V 0I p =0I p +I s0 I s I p IFigure 8: Equivalent circuit of p-n junction solar cellFigure 9: Current-voltage relation of p-n junction solar cell2.2 Graetzel Cell (dye-sensitized solar cell)A dye-sensitized solar cell is based on a semiconductor formed between a photo-sensitized anode and an electrolyte, as shown in Figure 10. The cell has three primary parts. On the top is a transparent anode made of fluorine-doped tin dioxide (S n O2:F) deposited on the back of a (typically glass) plate. On the back of the conductive plate is a thin layer of titanium dioxide(T i O2), which forms into a highly porous structure with an extremely high surface area. T i O2 only absorbs a small fraction of the solar photons. The plate is then immersed in a mixture of a photosensitive ruthenium-polypyridine dye and a solvent. After soaking the film in the dye solution, a thin layer of the dye is left covalently bonded to the surface of the T i O2. A separate backing is made with a thin layer of the iodide electrolyte spread over a conductive sheet, typically platinum metal.Sunlight enters the cell through the transparent S n O2:F top contact, striking the dye on the surface of the T i O2. Photons striking the dye with enough energy to be absorbed will create an excited state of the dye, from which an electron can be "injected" directly into the conduction band of the T i O2, and from there it moves by diffusion (as a result of an electron concentration gradient) to the clear anode on top. Meanwhile, the dye molecule has lost an electron and the molecule will decompose if another electron is not provided. The dye strips one from iodide in electrolyte below the T i O2, oxidizing it into triiodide. This reaction occurs quite quickly compared to the time that it takes for the injected electron to recombine with the oxidized dye molecule, preventing this recombination reaction that would effectively short-circuit the solar cell. The triiodide then recovers its missing electron by mechanically diffusing to the bottom of the cell, where the counter electrode re-introduces the electrons after flowing through the external circuit.R extFigure 10: Dye-sensitized solar cellFigure 11 shows the equivalent circuit of dye-sensitized solar cell. The basic idea is still the same: “harvest” the electron-hole pairs from the absorbed photons.0 c 02.3 Electro Kinetic Energy ConversionA pressure-driven fluid flow through micro channels carries a net electrical charge with it, inducing both a current and a potential when the charge accumulates at the channel ends. These so-called streaming currents and streaming potentials can drive an external load and therefore represent a means of converting hydrostatic energy into electrical power, as shown in Figure 12. The notion of employing such electro kinetic effects in an energy conversion device is called electro kinetic energy conversion. Physical modeling of electro kinetic energy conversion is needed to guide the optimization of these properties. The relations of electro kinetic phenomena can be described by the following equations:where P is pressure, Q is flow rate, V is voltage and I is current. K H is hydrodynamicconductivity of the porous material, K E is electric conductivity and S is electro kinetic coupling.The detailed model of electro kinetic energy conversion will be discussed in Lecture 33.R extFigure 11: Equivalent circuit of dye-sensitized solar cellFigure 12: Electro Kinetic Energy ConversionMIT OpenCourseWare10.626 / 10.462 Electrochemical Energy SystemsSpring 2011For information about citing these materials or our Terms of Use, visit: /terms.。
物理化学课件：电化学
03
脉冲伏安法优缺点
脉冲伏安法具有高灵敏度、高分辨率等优点，但也存在一些缺点，如
仪器复杂、对实验条件要求较高、数据处理繁琐等。
循环伏安法
循环伏安法原理
循环伏安法是一种常用的电化学分析方法，通过在一定时间间隔内反复扫描电压，并测量 电流响应的方法，可以获得电流随电压变化的关系曲线。
循环伏安法应用
该方法广泛应用于电化学反应机理研究、电极材料研究、电池性能测试等领域。通过循环 伏安法可以研究反应机理、电极过程动力学参数等重要信息。
2
燃料电池具有高能量密度、低污染等优点，使 其成为未来能源发展的重要方向之一。
3
当前研究的重点是如何提高燃料电池的效率和 降低成本，以实现商业化应用。
太阳能电池研究
01
太阳能是一种清洁、可再生的能源，而太阳能电池则是将太阳 能转化为电能的关键器件。
02
目前，太阳能电池的研究主要集中在提高光电转换效率、降低
阴阳极反应
在电合成过程中，阳极和阴极上 的反应可以控制，以制备所需的 化学物质。
产品分离
电合成过程中产生的物质可以通 过适当的分离技术进行分离，以 获得纯度较高的产品。
电解和电合成应用
工业生产
01
电解和电合成过程在工业生产中具有广泛的应用，如电解法制
备碱、电解法制备酸、电合成法制备有机化合物等。
环境治理
电池及电极过程
电池的组成和类型
电池的组成
电池通常由正极、负极、电解质、隔膜和外壳等组成。
电池的类型
根据不同的特点和应用场景，电池有多种类型，如锂离子电池、铅酸电池、 镍氢电池等。
电极过程动力学
电极过程
在电池中，电极过程是指发生在电极和电解质之间的电化学反应。
电化学讲义沉积法制备氧化锌
❖ 3) 采用阴极恒电压模式电沉积ZnO 薄膜, 电压控制在- 019 V, 沉积溶液为011 mol/ L Zn(NO3) 2 水溶液( O2 浓度为反应温 度时的饱和浓度) , 恒温水浴控制反应温 度为65 e , 沉积时间为10 min。
❖ 4) 电沉积后用去离子水漂洗制备的ZnO 薄膜,然后自然晾干。
透明性好。未经过这种预活化处理而直接用步骤3 制备的ZnO 薄膜则表面粗糙, 呈乳白色不透明。图
1 给出了经预活化处理后制备的ZnO 薄膜的透射光 谱( 以ITO 导电玻璃作为参比) , 在400~ 2000 nm 的 波长范围内表现出大于80% 的透过率。透射边在 370 nm 左右, 对应ZnO 的光学带隙为3135 eV。由 于平整的薄膜上下表面对光的干涉作用, 导致了透 射谱上出现周期性的起伏, 根据这个特征可以计算 薄膜的厚度[ 5] 。由图1 估算制备的ZnO 薄膜的厚 度为234 nm 左右。图2 表示经预活化处理后制备的 ZnO 薄膜的XRD 谱。
2. 7eV, 拓宽了薄膜的吸光范围, 这对ZnO 薄膜在 光学方面的应用具有重要意义。
关键词 氧化锌薄膜 电沉积 掺杂 带隙能
1引言
❖ 氧化锌( ZnO) 是一种性能很好的材料, 在电子、光 学、声学及化学等领域都有广泛应用。ZnO 为纤锌 矿结构的直接带隙半导体材料, 室温下禁带宽度为 3137 eV, 激子结合能高达60 meV, 因此具备了发射 蓝光或近紫外光的优越条件。而且, ZnO 可实现p型或n-型掺杂, 有很高的导电、导热性能, 化学性质 稳定, 用它来制备发光器件必然具有高的稳定性和 较低的价格。1997 年报道了ZnO 的光抽运近紫外 受激发射现象[ 1] , 由于其发射的波长比GaN 蓝光 更短, 将在提高光记录密度和光信息的存取速度方 面起到非常重要的作用, 引发了ZnO 半导体激光器 件的研究热潮。
电化学课件-纯英文_4Why Electrons Transfer
Luggin Capillary
Reference electrode placed in glass capillary containing test solution
Narrow end placed close to working electrode
Exact position determined experimentally
Let’s consider:
case 1: at equilibrium case 2: at E more negative than Eeqbm case 3: at E more positive than Eeqbm
Case 1: At Equilibrium
E = Eo - (RT/nF)ln(CR*/CO*) E - E0 = - (RT/nF)ln(CR*/CO*) E = Eo so, CR* = Co*
Eredox E
E
F
•Net flow of electrons from solute to M •Ef more positive than Eredox •more anodic •more oxidizing
The Kinetics of Electron Transfer
Consider:
= 0.5 activated complex halfway between reagents/ products on reaction coordinate; typical case for ET at type III M electrode
O R
Reaction Coordinate
Electroanalytical Chemistry
电化学课件
通过测量交流信号作用下的电极响 应，分析电极过程的阻抗特性。
电导测量技术
溶液电导测量
测量溶液的电导率，了解溶液中 离子的迁移性质。
电极电导测量
测量电极材料的电导率，研究电 极的导电性能。
电导滴定法
通过测量滴定过程中溶液电导的 变化，确定滴定终点及待测物质
的浓度。
电化学测量实验方法
循环伏安法
掌握电化学基本原理和基础知识，了解电化学在各个领域的应用，培养分析和 解决电化学问题的能力。
学习内容
包括电解质溶液、原电池与电解池、电极过程动力学、电化学热力学与电化学 动力学、电化学分析方法等。通过实验和案例分析，加深对理论知识的理解和 应用。
02 电化学基础
电解质溶液
01
02
电解质溶液的定义和分类
电化学的历史与发展
18世纪末，意大利物理学家伏特发明了电池，为电化学的研究奠定了基础。
19世纪，英国科学家法拉第发现了电解定律，揭示了电流与化学反应之间的关系。
20世纪以来，随着理论和实验技术的不断发展，电化学在能源转换与存储、环境科 学、生物医学等领域取得了重要突破。
课程目标与学习内容
课程目标
交流阻抗谱
利用交流阻抗技术，研究金属腐蚀过程中的电荷转移和物 质传输过程。
金属腐蚀与防护实验技术
失重法
通过测量金属在腐蚀前后的质量 损失，评估金属的腐蚀速率。
电化学测试
运用电化学工作站进行电位、电 流、阻抗等参数的测量和分析。
表面分析技术
利用扫描电子显微镜（SEM）、 能谱仪（EDS）等手段，观察和 分析金属表面的腐蚀形貌和成分 变化。
离子选择性电极
利用离子选择性电极对特 定离子的响应，测量离子 浓度及电位。
电化学 第1章 绪论
第1章 绪论1.1 电化学的发展与研究对象1.1.1 电化学的产生及其在历史上的作用1、电化学的产生电化学的产生与发展始于18世纪末19世纪初。
1791年意大利生物学家伽伐尼（Galvanic ）从事青蛙的生理功能研究时，用手术刀触及解剖后挂在阳台上的青蛙腿，发现青蛙腿产生剧烈的抽动。
分析原因后认为，由于肌肉内有电解液，这时是偶然地构成了电化学电路。
这件事引起了很大的轰动。
当时成立了伽伐尼动物电学会，但未搞明白。
1799年伏打（Volta ），也是意大利人，他根据伽伐尼实验提出假设：认为蛙腿的抽动是因二金属接触时通过电解质溶液产生的电流造成的。
故将锌片和银片交错迭起，中间用浸有电解液的毛呢隔开，构成电堆。
因电堆两端引线刺激蛙腿，发生了同样的现象。
该电堆被后人称为“伏打电堆”，是公认的世界历史上第一个化学电源。
2、电化学在历史上的作用伏打电堆的出现，使人们较容易地获得了直流电。
科学家们利用这种直流电得以进行大量的研究，大大地扩展了人们对于物质的认识，同时促进了电化学的发展，也极大地促进了化学理论的发展。
1）扩展了对于物质的认识。
最初人们认为自然界中有33种元素，实际上其中有一部分是化合物。
如：KOH 、NaOH 、NaCl 、O H 2等。
1800年尼克松（Nichoson ）、卡利苏（Carlisle ）利用伏打电堆电解水溶液，发现有两种气体析出，得知为2H 和2O 。
此后人们做了大量的工作：如电解4CuSO 得到Cu ，电解3AgNO 得到Ag ，电解熔融KOH 得到K 等等。
10年之内，还得到了Na 、Mg 、Ca 、Sr 、Ba 等，这就是最早的电化学冶金。
10年时间，人们所能得到或认识的元素就已多达55种。
没有这个基础，门捷列夫周期表的产生是不可能的。
2）促进了电学的发展1819年，奥斯特用电堆发现了电流对磁针的影响，即所谓电磁现象。
1826年，发现了欧姆定律。
这都是利用了伏打电堆，对于电流通过导体时发生的现象进行了物理学的研究而发现的。
电化学讲义
一、原电池基础知识：形成条件：②、电解质溶液（一般与活泼性强的电极发生氧化还原反应）； 原 ③、形成闭合回路（或在溶液中接触）电 负极：用还原性较强的物质作负极，负极向外电路提供电子；发生氧化反应。
池 基本概念： 正极：用氧化性较强的物质正极，正极从外电路得到电子，发生还原反应。
原 电极反应方程式：电极反应、总反应。
理氧化反应 还原反应反应原理：Zn-2e -=Zn 2+ 2H ++2e -=2H 2↑电极反应： 负极（锌筒）Zn-2e -=Zn 2+正极（石墨）2NH 4++2e -=2NH 3+H 2↑总反应：Zn+2NH 4+=Zn 2++2NH 3+H 2↑干电池： 电解质溶液：糊状的NH 4Cl特点：电量小，放电过程易发生气涨和溶液②、碱性锌——锰干电池 电极：负极由锌改锌粉（反应面积增大，放电电流增加）；电解液：由中性变为碱性（离子导电性好）。
PbO 2） PbO 2+SO 42-+4H ++2e -=PbSO 4+2H 2O 负极（Pb ） Pb+SO 42--2e -=PbSO 4铅蓄电池：总反应：PbO 2+Pb+2H 2SO 4 4+2H 2O1.25g/cm 3~1.28g/cm 3的H 2SO 4 溶液蓄电池 特点：电压稳定。
Ni ——Cd ）可充电电池； 其它蓄电池 Cd+2NiO(OH)+2H 2O Cd(OH)2+2Ni(OH)2 Ⅱ、银锌蓄电池锂电池不是把还原剂、氧化剂物质全部贮藏在电池内，而是工作时不断从外界输入，同时燃料 电极反应产物不断排出电池。
电池 ②、原料：除氢气和氧气外，也可以是CH 4、煤气、燃料、空气、氯气等氧化剂。
2H 2+2OH --4e -=4H 2O ；正极：O 2+2H 2O+4e -=4OH -失e -,沿导线传递，有电流产生化学电源简介放电充电放电③、氢氧燃料电池：总反应：O2 +2H2 =2H2O特点：转化率高，持续使用，无污染。
电化学课件-纯英文7Ultramicroelectrodes
EQCM - Principles of Operation
• Solution exerts a dampening effect:
– Under potential control – XPS, SEM, etc. not in situ
• Very sensitive
– Good for complex matrices
• Physiological fluids
• Detection limit
– pmole - fmole
ele–ctr2odeelectrodes sandwiching quartz (piezoelectric)
Quartz cry• sotaslcillating E surface of quartz wafer • oscillation at characteristic frequency (2 - 20 MHz)
• Stray capacitance
– Impurities
• Maintenance
– Cost – Fragile construction
UME’s - Applications
• Determine [analyte] • Study reactions
– at low temperatures = frozen glasses
• Biological systems
– Single cells – organelles
• Fast kinetics • Development of analytical tools for
chap7电化学(3)
+ –
稀溶液
浓溶液
扩散方向
减小液体接界电势的办法:在两液体间连接上一个" 减小液体接界电势的办法 在两液体间连接上一个"盐 在两液体间连接上一个 桥".
KCl 饱和溶液是最合适作盐桥,NH4NO3 溶液. 饱和溶液是最合适作盐桥, 溶液.
§ 7.8 电极的种类
1. 第一类电极
将某金属 吸附了某种气体的惰性金属置于 金属或 置于含有该元素 将某金属或吸附了某种气体的惰性金属置于含有该元素 中构成的. 离子的溶液中构成的 包括金属电极,氢电极, 离子的溶液中构成的.包括金属电极,氢电极,氧电极和卤 素电极. 素电极. (1)金属电极和卤素电极:均较简单, 金属电极和卤素电极:均较简单, 金属电极 Zn2++2e-→Zn 如: Zn2+|Zn: Cl– | Cl2|Pt: Cl2(g)+2e– →2Cl– (2)氢电极: 氢电极: 氢电极 结构:将镀有铂黑的铂片浸入含有 结构 将镀有铂黑的铂片浸入含有 H+或OH–的溶液中,并不断通H2(g) 的溶液中,并不断通 构成了酸性或碱性氢电极. 就构成了酸性或碱性氢电极.
1. 电极电势
单个电极的电势的绝对值是无法测得的, 单个电极的电势的绝对值是无法测得的,它其实是一个 相对电势. 相对电势.
电极电势E(电极 是利用下列电池的电动势定义的 电极电势 电极)是利用下列电池的电动势定义的: 电极 是利用下列电池的电动势定义的: Pt | H2(g,100kPa) | H+{ a(H+)=1 } 给定电极 给定电极中各组分均处在各自的标准 标准电极电势 E :给定电极中各组分均处在各自的标准 态时的电极电势. 态时的电极电势. 由此标准,氢电极的标准电极电势为零. 由此标准,氢电极的标准电极电势为零.
电化学理论知识英文版重要
2. Types of EESSs and their working principlesThe working mechanism of any EESS relies on an inherent potential difference between two electrodes known as the operating voltage. The operating voltage of the device is dictated by the differences in redox potential of the positive and negative electrode. The potential difference is used to drive electrochemical reactions on either electrode when they are connected through an external circuit. This creates a flow of electrons from the negative electrode to the positive electrode. The flow of electrons induces oxidation reactions on the negative electrode (anode) and reduction reactions on the positive electrode (cathode) when discharging. The charged electrodes are balanced by a concomitant flow of counter-ions. EESSs are grouped into a number of different categories depending on the composition of the electrodes, the counter-ions, and the nature of the redox reactions (Fig. 1).2.1 Solid electrode batteries2.1.1 Metal-ion battery working principle. Batteries operate with a constant voltage defined, approximately, by the potential difference between the anode and cathode. Because of this, In a galvanostatic charge/discharge experiment the potential of the electrode or device ideally remains constant until the active material has been fully reduced (oxidized). In a cyclic voltammogram experiment, one observes a reversible, sharp redox peak when a redox event occurs (inset Fig. 1a–c).Metal-ion batteries are the most common type of EESSs. They are typically composed of an anode (negative electrode), a cathode (positive electrode), electrolyte (either aqueous, organic, solid-state,18or polymeric10,19), a separator (to prevent short circuiting), current collectors (to collect charge at each electrode), and a cell casing (to keep the components together and prevent exposure to the external environment). Metal-ion batteries are used for a wide variety of both portable and stationary applications for either primary or back-up power. In metal-ion batteries the charged anodes and cathodes are balanced by the metal ion in a ‘rocking-chair’ type mechanism (Fig. 1a). This is a strict requirement imposed by the definition of metal-ion batteries that should be clearly distinct from dual-ion batteries described below. Metal-ion batteries can be constructed with relatively small amounts of electrolyte because the ions balancing the charge at one electrode are constantly being replenished. Additionally, metal-ion batteries are very attractive candidates for use with solid-state electrolytes because the mobility of only one ion needs to be considered.2.1.2 Dual-ion battery working principle.In a dual-ion battery the charged anodes and cathodes are balanced by cations and anions respectively (Fig. 1b). Dual-ion batteries encompass a wide variety of electrolytes and electrodes. The anodes range from negative charge-accepting compounds to reduced metals and inorganic materials. The cathodes can also be a wide variety of materials as long as they are balanced by anions when charged. We will adhere to this definition throughout this review, but we note that others have referred to these systems as organic batteries, metal organic batteries, and radical polymer batteries.14,22Although these terms may be used to describe the electrodes, the convention of naming solid electrode batteries based on the mobile counter-ions is upheld with this nomenclature.2.1.3 Performance metrics of solid electrode batteries.A number of performance metrics need to be considered for the development of electrodematerials for solid electrode batteries. These performance metrics can be used to estimate the overall performance of the device (Tables 1–7). The theoretical capacitance (C theor) of a material is the maximum amount of charge a material can hold with respect to its mass. It is typically reported in m A h gÀ1and is calculated using eqn (1):Here, n is the maximum number of charges the compound can accept (or give up), F is Faraday’s constant, and M is the molecular weight of the compound in g molÀ1. Typically, the C theo r is used to assess how well the material could perform under optimized conditions. If the C theor is reached, then it is expected that the electrode cannot accept any more charge.The specific capacity (C sp) is the measured capacity of the electrode at a specific current density for either charging or discharging. The C sp is reported in m A h gÀ1and by measuring C sp at different rates (usually reported as a C-rate, where 1C is the amount of current it would take to collect the total charge of the C theor in 1 hour) the rate capabilities of the electrode can be determined. The C sp is typically calculated from galvanostatic charge/discharge curves using eqn (2):Here, i is the current in milliamperes, t is the time of discharging (charging) in seconds, and m is the mass of the active material in grams. If the C sp at low and high rates are similar, it can be said that the electrode has high rate capabilities. This typically depends on the electron transfer kinetics of the compound, and the electronic and ionic conductivity of the electrode and electrolyte.The coulombic efficiency (CE) is measured by dividing the C sp for discharging by the C sp for charging. This provides insight into the reversibility of the redox reactions and indicates whether any side reactions occur with the electrode and electrolyte. The CE is a good indicator of whether a stable solid electrolyte interface (SEI)is formed in the charging cycles and if the material itself will be stable upon extended cycling. If the CE is low in the first charging cycles but increases to B100% afterwards, it is typically attributed to SEI formation.The cycling stability is an important parameter that quantifies the retention of capacity upon charging and discharging the electrode multiple times. Usually this measurement is performed under galvanostatic conditions and is reported as a percentage of the initial capacity after a specified number of cycles. The current density (or C-rate) must be specified for these measurements because the rate can h ave a significant effect on the cycling stability. This effect is especially pronounced if capacity fading is due to electrode dissolution, which is a common problem with organic electrode materials.The potential at which the redox process(es) occur(s) is also a very important parameter. Combined with the capacity, the redox potential can be used to predict the energy density of the device when paired with an anode/cathode of known redox potential. To have a high energy density, the potential of cathode material should be as high as possible while that of anode material should be as low as possible within the electro-chemical window of the electrolyte, or within the electrolytes’ ability to form a stable SEI. Although an ideal battery maintains a constantvoltage while it discharges, real batteries tend to have a decreased voltage with decreasing state-of-charge (SOC).This creates a sloping voltage plateau that is especially apparent in polymeric electrodes or in electrodes with multiple redoxevents.23The reduction and oxidation peak splitting is also important to provide insight into electron-transfer kinetics, and to predict the energy efficiency of the device.While energy and power density are important parameters to gauge the performance of energy storage, we chose to exclude them from our evaluation of solid organic electrode materials since they pertain to fully assembled devices and relate to the combined performance of all aspects of the device including both the anode and cathode, the electrolyte, membrane, and resistances associated with various aspects of the device. Additionally, it is important to report the electrode formulations and procedure for electrode manufacturing, electrode morphologies, electrode thicknesses, electrolyte, and the conditions under which the experiments are being performed. All of these factors can have an enormous effect on device performance. For example, in our lab we have observed that changes in the electrolyte solvent can influence the electrochemical properties, such as the capacity, by as much as an order of magnitude. Therefore, we encourage others to report the details of electrode preparation and testing in full.。
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Section 5: Voltammetric MethodsVoltammetric Methods •Historical•Electrolysis at DME -1920’s•Usually 3-electrode cells•Measurement of current that results from the application of potential.•Different voltammetric techniques are distinguished primarily by the potential function that is applied to the working electrode and by the material used as the working electrode.Types of VoltammetryPolarographyLinear sweep and Cyclic Voltammetry Hydrodynamic VoltammetryPulsed methodsAC Voltammetry (not here)ip=2.69 x 105n3/2A D O1/2v1/2C OIt is instructive to start with PolarographyVoltammetry at a dropping mercury electrodePolarography uses mercury droplet electrode that is regularly renewed during analysis.Applications:Metal ions (especiallyheavy metalpollutants) -highsensitivity.Organic species ableto be oxidized orreduced at electrodes:quinones, reducingsugars and derivatives,thiol and disulphidecompounds, oxidationcofactors (coenzymesetc), vitamins,pharmaceuticals.Alternative whenspectroscopic methodsfail.HistoryJaroslav Heyrovskýwas the inventorof the polarographic method, and thefather of electroanalytical chemistry,for which he was the recipient of theNobel Prize. His contribution toelectroanalytical chemistry can notbe overestimated. All modernvoltammetric methods used now inelectroanalytical chemistry originatefrom polarography.On February 10, 1922, the "polarograph" was born as Heyrovskýrecorded the current-voltage curve for a solution of 1 M NaOH.Heyrovskýcorrectly interpreted the current increase between -1.9 and -2.0 V as being due to deposition of Na+ions, forming an amalgam.Typical polarographic curves (dependence of current I on the voltage E applied to the electrodes; the small oscillations indicate the slow dropping of mercury): lower curve -the supporting solution of ammonium chloride and hydroxide containing small amounts of cadmium, zinc and manganese, upper curve -the same after addition of small amount of thallium.Swedish king Gustav Adolf VI awards theNobel Prize to Heyrovskýin Stockholm on 10.12.1959Potential RampsLinear sweep PolarographyIn order to derive the the current responseone must account for the variation of drop area with time:A = 4%(3mt/4%d)2/3= 0.85(mt)2/3Density ofMass flow rate of drop dropWe can substitute this into Cottrell Equation (see Mass Transport Notes)i(t) = nFACD1/2/ %1/2t1/2We also replace D by 7/3D to account for the compression of the diffusion layer by the expanding dropGiving the Ilkovich Equation:i708nD1/2m2/3t1/6Cd=I has units of Amps when D is in cm2s-1,m isin g/s and t is in seconds. C is in mol/cm3This expression gives the current at the end of thedrop life. The average current is obtained byintegrating the current over this time periodi607nD1/2m2/3t1/6Cav=The diffusion current is determined by subtracting away the residual currentFurther improvements can be made by reducing charging currents (see Pulse methods later)E 1/2 = E 0 + RT/nF log (D R /D o )1/2(reversible couple)Usually D’s are similar so half wave potential is similar to formal potential. Also potential is independent of concentration and can therefore be used as a diagnostic of identity of analytes. For exampleClassFunctional Group E 1/2 (v SCE)Azo-N=N--0.4carbonyl>C=O -2.2NitroNO 2-0.9Quinone C=O -0.1•Organic reductions often involve hydrogen ions•R + nH+ + ne RH n•Good buffering reqd in organic polarographyMetal Complexes+ ne + Hg M(Hg) + pL•MLp•Difference between half-wave potential for complexed and uncomplexed metal ion is given by:•E1/2(c) -E1/2(free) =-RT/nF p ln [L] +RT/nF ln (D free/D (c))1/2 RT/nF ln Kd•Stoichiometry can thus be determined by plotting E1/2 versus [L]•Also possible to improve resolution between neighbouring waves by carefully choosing ligand and concentrationHeyrovsky-Ilkovich Equation•Describes wave shape for reversible systems (withfast electron transfer kinetics)•E = E 1/2+ RT/nF ln((i d -i)/i)•Plot E vs log((i d -i)/i) gives straight line of slope0.059/n•Convenient way to get n•Intercept is half wave potential•Irreversible systems•The waves are more “drawn out”than forreversible systems•Limiting currents still show a linear function ofconcentration•Shape of polarogram is given by:•E = E 0 + RT/ nF ln (1.35k f ((i d -i)/i)(t/D)1/2)•transfer coefficient forward rate constantCharging Currents : first look•Drop acts as capacitor•Double layer•Since potential change during drop life is very small we can neglect charging changing with potential•charging thus depends on time and electrode areai c = dq/dt = (E-Epzc)CdldA/dtbut A = 4%(3mt/4%d)2/3= 0.85(mt)2/3i c = 0.00567 (E-Epzc) Cdlm2/3t-1/3so I(total) = id + ic= kt1/6+ k’t-1/3Charging current sets limit of detection Various ways of reducing ite.g., Sample current at end of drop life (TAST polarography)Other methods involve Pulse voltammetry -see laterCyclic Voltammetry•potential is continuously changed as a linear function of time. The rate of change of potential with time is referred to as the scan rate (v).•cyclic voltammetry, in which the direction of the potential is reversed at the end of the first scan. Thus, the waveform is usually of the form of an isosceles triangle.•advantage that the product of the electron transfer reaction that occurred in the forward scan can be probed again in the reverse scan. •powerful tool for the determination of formal redox potentials, detection of chemical reactions that precede or follow the electrochemical reaction and evaluation of electron transfer kinetics.Potential SweepGives:WhyI-E-t surface revisited (see Chronoamp notes)•Polarography -constant time E-IChronamp -i -t•CV -I-E-t (diagonal cut) -notice waveshape•Diffusional “tails”-governed by masstransport•You can simulate this: for exampleusing the ensemble method of finitedifferences. See Bard and Faulkner (oldedition) Appendix B p675 et seq.Sign Conventions:•2 conventions in the literature:•American -plot cathodic current positive, plot negative potentials decreasing to the right.•IUPAC -plot anodic currents positive, plot positive potentials increasing to the right.•The slightly illogical American convention arose for historical reasons.•IUPAC stands for International Union of Pure & Applied Chemistry.anodiccathodicIUPAC StyleThe Randles-Sevcik equationReversible systemsi p= 0.4463 n F A C (n F v D / R T)1/2n: number of electrons, v scan rate (V / sec),F:Faraday’s constant (96485 C /mol), A: electrode area (cm2), R is the universal gas constant (8.314 J /mol K), T is the absolute temperature (K), and D is the analyte’s diffusion coefficient (cm2/sec). Note that if the temperature is assumed to be 25°C (298.15 K), the Randles-Sevcik equation can be written in a more concise form,i p= (2.687x105) n3/2v1/2D1/2A Cwhere the constant is understood to have units (i.e., 2.687x105 C mol–1V–1/2).•i pa = i pc for a reversible couple•Peak ratios are often stronglyaffected by chemical reactionscoupled to the redox process•See later and also OrganicElectrochemistry (Prof. Houmam)•Peak positions are related to formal potential ofredox process•E 0= (E pa + E pc ) /2•Separation of peaks fr a reversible couple is0.059/n V•1-e fast electron transfer thus gives 59mVseparation•Peak potentials are then independent of scan rate(see previous page)•Half-peak potential E p/2 = E 1/2 0.028/n•sign is + for a reduction•shape of voltammogram depends on transfer coefficient ( ) and on a dimensionless parameter• = k[RT/%DF ]1/2• is the scan rate•for >7 votammogram is reversible •when deviates from 0.5 the voltammograms become asymmetric -cathodic peak sharper as expected from Butler Volmer eqn.= 0.5=10,1.0.1,0.01 = 0.25, 0.5 .75Microelectrodes•wave shape is differentFerrocene redox at a 10 m diameter glassy carbon electrode.-why?•This steady-state current is explained by envisioning that the microelectrode is a "dot" with the diffusion layer being hemispherical in shape extending out into the solution. The amount of ferrocene diffusing to the electrode surface is defined by the volume enclosed by an expanding hemisphere, not a plane projecting into the solution as in the case of a planar electrode.•One salient feature of the microelectrode is the small current magnitude, which means that iR loss is negligible even at high scan rates. This allows the determination of kinetic rates of electron transfer that are very fast by going to high scan rates.•Also time constant RC is small (see Lecture One) -fast electron transfer can be studied•The fundamental reason you have met before inthe Mass Transport lectures•recall that diffusional properties depend on thegeometry (and size) of the electrode•we now have a brief look at microelectrodesVoltammetry at microelectrodes•microelectrodes have at least one dimension of the order of microns•In a strict sense, a microelectrode can be defined as an electrode that has a characteristic surface dimension smaller than the thickness of the diffusion layer on the timescale of the electrochemical experiment•small size facilitates their use in very small sample volumes. -opened up the possibility of in vivo electrochemistry. This has been a major driving force in the development of microelectrodes and has received considerable attention..Mass Transport•at small electrodes, growth of diffusion layer is initially similar to that at larger electrodes;•at longer times, it slows significantly. Thus the size of the diffusion layer at t> 0 is smaller at small electrodes, and the concentration gradient and therefore the rate of (diffusional) mass transport are considerably greater at microelectrodes.•At short times size of the diffusion layer is smaller thanthat of the electrode, and planar diffusion dominates--evenat microelectrodes.•at very short time scale experiments (e.g., fast-scan cyclic voltammetry) a microelectrode will exhibit macroelectrode (planar diffusion) behavior.•at longer times, the dimensions of the diffusion layerexceed those of the microelectrode, and the diffusion becomes hemispherical. The molecules diffusing to the electrode surface then come from the hemisphericalvolume (of the reactant-depleted region) that increases with time; this is not the case at macroelectrodes, where planar diffusion dominates•so current at microelectrode is a sum of both planar and spherical diffusion•magnitude of each will depend on time andsize of microelectrode= nFADC[ ( 1/%Dt)1/2 + 1/r]idso:= C(1/ + 1/r)idplanar diffusion sphericaldiffusionThe first term predominates at short times ( <<r ),while the second at a sufficiently long time ( >>r ).is the diffusion layer thickness,.This quantity is defined,for planar semi-infinite diffusion.= (%Dt)1/2Irreversible and Quasi-Reversible Systems: Macroelectrodes FastSlow•For 'slow’reactions (so called quasi-reversible or irreversible electron transfer reactions) the voltage applied will not result in the generation of the concentrations at the electrode surface predicted by the Nernst equation.•kinetics of the reaction are 'slow' and thus the equilibria are not established rapidly (in comparison to the voltage scan rate).•position of the current maximum,E p) shifts depending upon the reduction rate constant (and also the voltage scan rate). This occurs because the current takes more time to respond to the the applied voltage than the reversible case.For irreversible processes (those with sluggish electron exchange), the individual peaks are reduced in size and widely separated. Totally irreversible systems are characterized by a shift of the peak potential with scan rate:E= E°-(RT/ nF)[0.78 −ln(kο/(D)1/2) +lnαnFν/RΤ)1/2]pα is the transfer coefficient and n a is the number of electrons involved in the charge-transfer step.•Thus,E p occurs at potentials higher than E°, with the overpotential related to k°and α.•Independent of the value k°, such peakdisplacement can be compensated by anappropriate change of the scan rate.•peak potential and the half-peak potential (at25°C) will differ by 48/αn mV.•Hence, the voltammogram becomes moredrawn-out as αn decreases.The peak current, given by:•i p= (2.99x105)n(αn a)1/2ACD1/2n1/2•i p is still proportional to the bulk concentration, but will be lower in height (depending upon the value of α).•Assuming α= 0.5, the ratio of the reversible-to-irreversible current peaks is 1.27 (i.e.the peak current for the irreversible process is about 80% of the peak for a reversible one).•For quasi-reversible systems (with 10-1> k°> 10-5cm/s) the current is controlled by both the charge transfer and mass transport.•Shape of the cyclic voltammogram is a function of the ratio k°/(πνnFD/RT)1/2•As ratio increases, the process approaches the reversible case. For small values of it, the system exhibits an irreversible behavior. Overall, the voltammograms of a quasi-reversible system are more drawn out and exhibit a larger separation in peak potentials compared to a reversible system.Reaction mechanisms•Cyclic voltammetry can be used todiagnose presence of reactions that precedeor follow electron transfer•Classified by EC•affects surface concentrations ofelectroactive species•changes in shape of voltammogram•info on intermediates•IntroductionCyclic voltammetry can be used to investigate the chemical reactivity of species. To illustrate this let us consider a few possible reactions.First we consider the EC reaction:•The voltammogram will exhibit a smaller reversepeak because the product (R) is chemicallyremoved from the electrode surface.•The mass transport equations for this reactionwhen diffusional transport is dominant are:•mass transport equation for (O) is identical to the case when no chemical reaction occurs •species (R) however has an additional term to account for the fact that it is destroyed chemically by a first order reaction.It is possible to gain information about the chemical rate constant k EC by studying the reaction via cyclic voltammetry.EC reaction for reversible electron transfer reaction and rate constant is extremely large.kEC•back peak height ⇒k EC.•wave shifts as k EC increases•results from the desire of the electrochemical system to set up an equilibrium controlled by the applied voltage.•for reversible electron transfer reactions the ratio of (O) and (R) at the surface can be predicted by the Nernst equation at any particular value of applied voltage. •chemical reaction removes (R) so when this happens the applied voltage forces more (O) to convert to (R) electrochemically to restablish the required ratio.•As more (O) is converted to (R) this results in the flow of more current and the wave begins to shift anodically(for a reduction).E = E0+RT/nF ln[O]/[R]CE Reaction: Scan rate dependence fastslow•Many other (endless) possibilities -too many to discuss here•e.g., ECEHere S is more difficult to reduce than OECE:product easier to reduce•First cathodic scan is normal -then you see SEC’(catalytic)O is regenerated -re-reduced etcAdsorption in cyclic voltammetryRepetitive voltammograms for micromolarriboflavin at a HMDE•note peak separation is smaller than for solution phase couple•if it is ideal then separation = 0•peak half-width = 90.6/n mV•peak current is directly proportional to surface coverage ( ) and scan rate•i p= (n2F2 A )/4RT•Peak area also gives coverage•Q=nFA can be used to determine area occupied by molecule -can give orientational informationRotating Disk Voltammetry•important advance in voltammetry•rotating disk electrode (RDE)•and later the rotating ring-disk electrode (RRDE) byLevich and co-workers in the former Union of SovietSocialist Republics. Although steady-statevoltammograms had previously been obtained forstirred solutions most of these voltammograms werenot amenable to rigorous mathematical treatments.•RDE shows hydrodynamic behavior that could betreated mathematically•allowed the RDE to be applied to solution andkinetic studies. and rapid homogeneous reactions under steady-state conditionsRDE is constructed from a disk of electrode material (e.g. gold, glassy carbon or platinum) imbedded in a rod of insulating material (e.g. Teflon). The electrode is attached to a motor and rotated at a certain frequency. The movement of rotation leads to a very well defined solution flow pattern. The rotating device acts as a pump, pulling the solution upward and then throwing it outward.The Levich EquationVeniamin Grigorievich(Benjamin)Levichwas a leading scientist in electrochemical hydrodynamics, -invented and developedby him. The famous Levich equationdescribing a current at a rotating diskelectrode is named after him.It is important to note that the layer of solution immediately adjacent to the surface of the electrode behaves as if it were stuck to the electrode. While the bulk of the solution is being stirred vigorously by the rotating electrode, this thin layer of solution manages to cling to the surface of the electrode and appears (from the perspective of the rotating electrode) to be motionless. This layer is called the stagnant layer in order to distinguish it from the remaining bulk of the solution.•Analyte is conveyed to the electrode surface by a combination of two types of transport.•vortex flow in the bulk solution continuously brings fresh analyte to the outer edge of the stagnant layer.•analyte diffuses across stagnant layer. The thinner the stagnant layer, the faster the analyte can diffuse across it and reach the electrode surface.•Faster electrode rotation makes the stagnant layer thinner. faster rotation rates permit the analyte to diffuse to the electrode faster, resulting in a higher current being measured at the electrode.The act of rotation drags material to the electrode surface where it can react. Providing the rotation speed is kept within the limits that laminar flow is maintained then the mass transport equation is given bywhere the x dimension is the distance normal to the electrode surface. It is apparent that the mass transport equation is now dominated by both diffusion and convection and both these terms effect the concentration of reagent close to the electrode surface. Therefore to predict the current for this type of electrode we must solve this subject to the reactions occurring at the electrode.Rotated disk voltammetry is similar to cyclicvoltammetry in that the working electrode potential is (slowly) swept back and forth across the formal potentialof analyte.The Levich equation This equation takes into account both the rate of diffusion across the stagnant layer and the complex solution flow pattern. In particular, the Levich equation gives the height of the sigmoidal wave observed in rotated disk voltammetry. The sigmoid wave height is often called the Levich current,i L, and it is directly proportional to the analyte concentration, C. The Levich equation is written as:i L= (0.620) n F A D2/3w1/2v–1/6Cwhere w is the angular rotation rate of the electrode (radians/sec) and v is the kinematic viscosity of the solution (cm2/sec). The kinematic viscosity is the ratio of the solution's viscosity to its density.•results for a series of rotated disk voltammograms taken at different scan rates.•a Levich study. The limiting current (or Levich current) is measured and plotted against the square root of the rotation rate, producing a Levich plot. Note that the experimental rotation rate, f , is measured in RPM and must be converted to the angular rotation rate, w, so that it has units of radians per secondin experiment shown the electrode area, A, was 0.1963 cm2, the analyte concentration, C, was2.55x10–6mol/cm3, and the solution had a kinematic viscosity, v, equal to 0.00916 cm2/sec. After careful substitution and unit analysis, you can solve for the diffusion coefficient, D, and obtain a value equal to 4.75x10–6 cm2/s.The kinematic viscosity is the ratio of the absolute viscosity of a solution to its density. Absolute viscosity is measured in poises (1 poise = gram cm–1sec–1). Kinematic viscosity is measured in stokes (1 stoke = cm2sec–1). Extensive tables of solution viscosity and more information about viscosity units can be found in the CRC Handbook of Chemistry and Physics.Rotating Ring Disk VoltammetryRotating-ring-disk electrode:A variant of the rotating-disk electrode which includes a second electrode -a concentric ring electrode -that is placed outside the disk and used to analyze the species generated on the disk. The ring is electrically insulated from the disk so that their potentials can be controlled independently. Abbreviated as RRDE•convenient way to measure post-electron transfer reactions of products•relationship between disk current and ring current depends on rate of movement of product from the disk•only a fraction of disc products will reach ring •each ring-disk electrode must be calibrated with a well-behaved reversible couple to determine the collection efficiency (N)•N=iR / iD•couples used -ferri/ferocynadide,quinone/hydroquinone•efficiency depends on electrode geometry (radii of disk and ring)Example: peroxide-study of post electron transfer •rapid disproportionation•electrochem HOOH -e @HOO•followed by HOO + HOO @HOOH + O2•we now look at cyclic voltammetry and RRDE studyCV in MeCNdisk•CV:i pa increases with [HOOH] (A to D) •i pa proportional to square root of scan rate •note plateau in wave after peak -indicative of secondary redox processRing•b: Ring currents •RHS: cathodic scan•A ED is disconnected: B EDis at +2.6V vs SCE•products can be characterized by scanning ring from +1 to -2V•first wave at 0.4V is indicative of HOO。