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Home , Auxiliary electrode, Chemical species, Electrochemistry

Electrochemical Systems The simplest and traditional electrochemical process occurring at the boundary between an electronically conducting phase (the ) and an ionically conducting phase (the ), is the heterogeneous electro-transfer step between the electrode and the electroactive species of interest present in the solution. An example is the plating of nickel. Ni2+ + 2e- → Ni Physical and Analytical The interface is where the action occurs but connected to that central event are various processes that can : occur in parallel or series. Figure 2 demonstrates a more complex interface; it represents a molecular scale snapshot The Fundamental Core of an interface that exists in a with a solid polymer (capable of conducting H+) as an electrolyte. of Electrochemistry However, there is more to an electrochemical system than a single by Tom Zawodzinski, Shelley Minteer, interface. An entire circuit must be made and Gessie Brisard for measurable current to flow. This circuit consists of the plus external wiring and circuitry The common event for all electrochemical processes is that (power sources, measuring devices, of transfer between ; or between an etc). The cell consists of (at least) electrode and a chemical species situated in the vicinity of the two separated by (at least) one electrolyte solution. Figure 3 is electrode, usually a pure or an . The location where a schematic of a simple circuit. Each the reactions take place is of fundamental electrode has an interface with a importance in electrochemistry because it regulates the solution. flow in the external behavior of most electrochemical systems. It is at this interface circuit but the solution is essentially an electrical . For current to flow, a of the electrode and solution that the electric fi eld present has a chemical to electrical transduction event determinant role in the charge distribution within reactants as must occur at the interface. Electron well as in the position and orientation of the reactant to form transfer between the electrode and solution species, called a faradaic process, the desired products. Figure 1 schematizes a heterogeneous is the means by which this transduction interface. Physical Electrochemistry deals with the occurs. Because the circuit is completed thermodynamics and kinetics (rates and mechanisms) of those via the faradaic process, a current can flow through the cell. To support such processes. Many research topics are concerned with the details a current, the solution must be, to some of reactant and interfacial structure, and how they affect the extent, an ionic conductor. kinetic parameters for electron transfer reactions. Analytical Primary variables associated with the Electrochemistry is the application of electrochemical electrochemical process are the potential of the electrode, which is a measure processes to measure the quantity of a species of interest. In this of the electron energy at the (metal) article, we describe the types of processes and their applications electrode surface, and the current, which in analytical measurements and in electrochemical systems is a measure of the rate of the process, sometimes given as a current density, and devices. In analytical measurements, we try to control or current per unit electrode area. Together, isolate the contributing processes to facilitate the determination these variables define the flux (material of the system’s composition; while for systems and devices per unit time per unit area) as a function of the electron energy. Note that we are the objective is the optimization of the processes to enhance, strict throughout this article in referring for example, the electrical effi ciency or temporal response of to the potential of a single electrode the system. Some examples of electrochemical systems and and the (or potential difference) between two electrodes. Even that devices that play an important role in many areas of sciences nuance has nuances! and technology include electro-organic synthesis, batteries We mentioned that at least two and fuel cells, photoelectrochemical cells, cells, electrodes are used. In most systems electrodialysis, systems, sensors, and measuring for device applications, two electrodes devices in many areas of medicine and biology. are the norm but in analytical systems, however, three electrodes are used. The reason for the various electrodes

62 The Electrochemical Society Interface • Spring 2006 is easily pictured if we consider voltage drops across the electrochemical cell. While current is flowing between electrodes, there is a voltage drop at each electrode/solution interface and across the electrolyte (typically called solution IR loss due to its Ohmic properties). Because the goal of an analytical system is to measure the amount of some species or, perhaps, the rate of a process at a known potential, we must focus on one electrode in the cell, called the , at which this species undergoes a reaction to give us our analytical change in current or voltage. In careful analysis, it is generally important to also know the potential of the working electrode. This can be Fig. 1. Schematic of the structure of a heterogeneous known only relative to that of another electrochemical interface. electrode, which is referred to as a reference electrode. If a two-electrode cell is used, the second electrode is called an . The current flows between the two electrodes and the auxiliary electrode functions both to carry the current (via a redox process) and as a reference electrode. Thus the potential difference between the two electrodes includes both the difference in the redox potentials at the respective interfaces and the IR loss. The latter is not necessarily well known, because the solution resistance can be a complicated, potential and frequency- dependent quantity. To get around this, the two functions are often separated. A third electrode, a reference electrode with a well-defined redox potential, is positioned close to the working electrode FIG. 2. An electrochemical interface of a fuel cell electrode. What is shown is (to minimize IR loss) and electrode two a molecular level picture of a Pt/Ru alloy nanoparticle on a carbon support, the is now referred to as a counter electrode. electrode, surrounded by and polymer electrolyte fragments - (labeled SO3 /polymer backbone) with adsorbed CO and OH species. (Figure Processes courtesy of Professor Matt Neurock, University of Virginia.) Now let us consider what is happening in the cell. It is crucial to keep in mind that all these processes occur simultaneously. Fortunately, we can often think about them one at a time, known as the superposition principle. We can summarize what must happen from the viewpoint of the redox active species. In simplest terms, it must make it to the electrode, react somehow, and then leave the electrode. We are usually concerned with the time scale or rate of these processes, the energetic driving forces for reactions to occur, and the detailed mechanisms of how they occur. Making it to and from the Electrode: Transport. To get to the vicinity of the electrode, there are three transport mechanisms to consider: 1. Diffusion is transport due to random thermal motions. This is a random process on the molecular length scale that acquires directionality at FIG. 3. A complete circuit made of a two-electrode macroscopic length scales because of electrochemical cell representing a in which differences (gradients) the electrolyte is an conductor. The reactions are the oxidation of at the to form H+ and reduction of (continued on next page) dioxygen at the .

The Electrochemical Society Interface • Spring 2006 63 Physical and Analytical... O (continued from previous page) O O O O O in the cell. There is a nuance here: Pt the actual gradient driving diffusion Pt Pt Pt in the macroscopic sense is that of Griffith Pauling Bridge the or activity. Model Model Model For dilute , concentration Pathways is adequate substitute and keeps the O - OH - math simple. We can use Fick’s laws 2e 2e Pt + Pt Pt + 2H2O I 2H + of diffusion to analyze the process. O OH 2H For concentrated solutions, more sophisticated treatments are needed. H+ Pt + H O IIA e- 2 2 2. Convection is transport due to the Pt O Pt O Pt + O2 + concerted bulk flow of the solution. O H OH - In this process, the redox species + 2e 3H IIB of interest is carried along with the Pt + 2H2O flow of solution. In many large- Pt Pt OH scale electrochemical processes, O 2e- 2e- III 2Pt + 2H2O the solution can also be pumped O 2H+ 2H+ through the cell. A mathematical Pt Pt OH description of this process via fluid FIG. 4. Postulated O reduction pathways. mechanics can be complex, but 2 sometimes the geometry is rigged to simplify this description. 3. Migration is transport in response interest, the fraction of charge carried by One final note: all electrocatalytic to a and is strictly that species. processes are strongly influenced by relevant to charged species. the nature of the catalytic surface itself. Applications: Electrocatalysis This nature includes both the nominal Regarding transport, the fastest mode Considering some complex redox composition (e.g., Pt, Au, or Ni) and wins, and will dominate the observed processes may serve to solidify the the actual composition (oxide layers behavior. Convection and migration discussion above. In electrocatalysis, and so on). As indicated above, the are generally faster modes of transport the electrode functions as a catalyst, details of mechanisms involve various than diffusion, in part due to their an unchanged reagent that facilitates a interactions between molecules and intrinsically directional nature. When we . We consider briefly surfaces and the strength or weakness want to be as quick as possible, such as two processes involved in fuel cell of these interactions depends on the producing a product in bulk reactions, the hydrogen oxidation between the true surface or trying to maximize the performance reaction (HOR) and the oxygen reduction and the redox . The spacing of of a battery or fuel cell, this is fine. If we reaction (ORR), both occurring on on the electrode surface, which is are trying to analyze a process, we want Pt electrodes and separated by an different for different , influences control over the mass transport, often electrolyte. The Pt on the oxidation side the adsorption mode of the reactive focusing on diffusion, the slowest mode. is the anode and the Pt on the reduction molecule. For example, O can adsorb in To minimize convection, we work in 2 side is the cathode. either an end-on or a bridging mode. an unstirred system, minimize pressure differentials, and cut vibration. If we In the HOR, a hydrogen molecule Applications: Electroanalysis want to minimize migration, we use an H2 is oxidized in a series of steps. A In our illustration of the various uncharged species (we may not have any dissociative chemisorption step involves processes discussed above, we consider choice in this!) or we can swamp out the the simultaneous and some of the special requirements effect for the ion of interest by putting adsorption of the H2 to form two Pt-H and approaches of electroanalytical in many other to carry the current. moieties. The H then is oxidized to form measurements. Generally, in The latter approach is called using a H+ and is released from the electrode into electroanalysis we are trying to achieve supporting electrolyte. the electrolyte. This is a rapid reaction on Pt. The H-H bond strength is not a well-defined situation enabling How do we characterize the relative particularly large thus Pt-H formation a quantitative relationship to be rates of these processes? Typically, we is facile. Each hydrogen releases a established between a measurable consider the resistance to the process single electron. electrical quantity (current, charge, or its inverse. For diffusion, the rate is or potential) and the concentration or determined by the size of the diffusing The ORR is a much more complicated amount of material (analyte) present species, the solution viscosity, and so reaction sequence (Fig. 4). First the in a sample. Thus, we often control on. That information is summarized in a reaction involves transfer of four current or potential on the analytically diffusion coefficient, D, which has units electrons and four protons to give active (working) electrode. This control of (length)2/time typically on the order water. Again, there is an initial is usually achieved with an instrument of 10-5 cm2/s for a species in aqueous adsorption step followed by a sequence called a . In a typical case, - + solution. For convection, the flow rate of e and H (present in the electrolyte) we scan or step the control variable and and concentration determine the net rate transfers. During this sequence, various measure the response of the dependent of arrival at the electrode. For migration, intermediates are formed. The O-O bond variable. As noted above, we also must we consider two things analogous to is itself strong. At the cathode, lots of control the mass transport behavior in flow rate and concentration, namely, competing side pathways are in progress. our cell and ensure that our measured the electrolyte conductivity, the inverse The ORR is an enormously important or controlled potential is determined of the resistance to migration, and the reaction from a practical viewpoint accurately, entailing a three-electrode transport number of the species of because it is a good oxidant that is cell. essentially free.

64 The Electrochemical Society Interface • Spring 2006 Some typical methods entailed important activity for members of this of research problems include (i) include: Division. The researchers are interested fundamental studies of electrocatalysis Cyclic : the working in the electrochemical properties, reaction pathways for oxidation of is scanned linearly preparation, characterization, responses, fuels using electrochemistry combined with time and cyclicly throughout and applications of novel materials. with reflectance IR a potential window of interest; Some examples are nanoparticles or second harmonic generation the current response is measured, and nanostructured materials, spectroscopy to detect the presence and typically plotted vs. potential. (metallic, nonmetallic, nature of adsorbed intermediates, (ii) or modified materials), magnets, ion electrochemical NMR for the study of the Potential step methods: the working and proton conductors, and glasses nature of adsorbates on electrodes; (iii) electrode potential is stepped from a and amorphous media. Materials electrochemical and ellipsometry studies region in which nothing happens to a for analytical and electronic devices of the formation of oxide layers on metal potential at which the redox reaction such as ceramic, silicon continue to electrodes; and (iv) determination of occurs; the current is measured vs. be improved. In the past decade, the ion and solvent transport rates using time (). This field of energy and the environment conductivity measurements along with approach also forms the basis for such as fuel cells, batteries and energy diffusion measurements employing the family of analytical methods generation and storage has concentrated NMR spectroscopy or specially designed called pulse voltammetry, in which much of the efforts of physical and electrochemical cells.  a combination of multiple steps of analytical electrochemists. A symposium varying magnitude is applied and the on “Three-Dimensional Micro- and Suggested Reading current sampled in time to construct a Nanoscale Battery Architectures” 1. A. J. Bard and L. R. Faulkner, Electrochemical current-voltage response. focuses on a defining characteristic Methods: Fundamentals and Applications, 2nd ed., Wiley & Sons, New York (2001). Rotating disk method: a slow cyclic of 3D batteries: transport between 2. P. T. Kissinger and W. R. Heineman, voltammetric scan is imposed on electrodes remains one-dimensional Laboratory Techniques in Electroanalytical an electrode that is rotated to create (or nearly so) at the microscopic level, Chemistry, 2nd ed., Marcel Dekker, New York a well-defined convective mass while the electrodes are configured in (1996). transport pattern, resulting in a complex geometries (i.e., nonplanar) to 3. L. R. Faulkner, J. Chem. Educ., 60, 262 steady-state voltammetric response increase the of the cell (1983). 4. J. Newman and K. E. Thomas-Ayea, particularly amenable to analysis. within the footprint area. A 3D matrix Electrochemical Systems, 3rd ed., Wiley, New of electrodes (in a periodic array or a Potentiometry: all the above controlled- York (2004). periodic ensemble) is thus required to potential methods have analogues 5. C. H. Hamann, A. Hamnett, and W. meet both the requirements of short in which current is controlled and Vielstich, Electrochemistry, Wiley, New York transport lengths and large energy (1998). potential is measured. Also, the capacity. Improvements in energy 6. C. M. A. Brett and A. M. O. Brett, thermodynamically determined per unit area and high-rate discharge Electroanalysis, Oxford University Press, potential can be measured at open Oxford (1998). capabilities are two of the benefits that circuit (i.e., with no current flow) to 7. I. Rubinstein, Physical Electrochemistry: may be realized for these 3D cells. In yield a measure of concentration. Principles, Methods, and Applications, Marcel the symposium on “Environmental Dekker, New York (1995). These brief descriptions are but a tiny Electrochemistry,” electrochemistry is fraction of the many methods available used both to detect and quantify the About the Authors in the electrochemist’s toolbox. We have pollutants and to directly remediate SHELLEY D. MINTEER is an associate professor of included at the end of this article a list polluted environments. In these areas chemistry at Saint Louis University. Her research of classical readings, excellent available electrochemistry offers the advantages interests include bioelectrocatalysis, biofuel texts, and specialist monographs on of automation, detectability, selectivity, cells, and magnetically modified electrochemical electrochemistry, that cover in more inherent cleanliness, and in many cases, systems. She may be reached at [email protected]. detail the theory and applications of cost effectiveness. GESSIE BRISARD is a professor in the Interfacial physical and analytical electrochemistry. and Applied Electrochemistry Laboratory, Centre Analytical electrochemistry also de Recherche en Énergie, Plasma, et Électrochimie Examples of Research Problems remains a fundamental part of the (CREPE), at the Université de Sherbrooke in of Interest to this Community research such as “Electrochemical Canada. She has developed research programs in Detection of Biomolecules” and the fields of electrocatalysis, lithium batteries, To provide a flavor of the sort of and electroanalytical chemistry. Recently she has “ and Microfluidics.” research problems of current interest become involved in the development and synthesis This year a special symposium is being to this Division, the following of materials for solid oxide fuel cells (SOFCs), organized to honor the memory of including reformer supported medium- topics were presented as symposia Professor Robert A. Osteryoung. Professor SOFC systems based on nanostructured at recent ECS meetings. To this day, Osteryoung’s passing in 2004 was a components using plasma fabrication. Brisard is the topic of “Molecular Structure presently serving as chair of the ECS Physical and tremendous loss to the field of physical Effects in Heterogeneous Electron Analytical Electrochemistry Division. She may be electrochemistry. For over half a century Transfer Kinetics” continues to be a reached at [email protected]. Professor Robert A. Osteryoung sought to fundamental issue; it involves the THOMAS A. ZAWODZINSKI is the F. Alex Nason extend the frontiers of electrochemistry interfacial structure and aspects of Professor of Engineering in the Chemical with contributions spanning the interest include the distribution of Engineering Department at Case Western Reserve gamut from technique development to University. He came to Case as the Ohio Eminent charge in the double layer, the effects of instrument design and automation. Scholar in Fuel Cells in 2002. In addition to specifically adsorbed ions and molecules, fuel cell research, he has led research programs and self-assembled monolayers. The Today’s state of the art is to on a range of fundamental and applied aspects “Electrochemistry of Novel Materials” is focus on the interface between the of electrochemical devices, including lithium another popular symposium organized fields of chemistry and physics. A batteries, sensors, and artificial muscles. Presently, good example is the symposium on his group carries out research on a broad range by the PAED. Materials are the core of of fuel cell topics. He an ARO MURI on new electrochemical systems and the “Electrochemistry and Spectroscopy.” fundamentals of electrocatalysis. Dr. Zawodzinski search for more catalytically active (Many spectroscopists have become is presently a Member-at-Large in the ECS PAE or sensitive electrodes constitutes an addicted to electrochemistry!) Examples Division. He may be reached at [email protected].

The Electrochemical Society Interface • Spring 2006 65