Electrochemical Cells with Transfer

FIGURE 2.9 The Daniell cell in equilibrium mode. One should avoid using cathode and anode in such a mode. 

It is important to emphasize here the difference between cells without transfer and the cell with transfer. A cell with transfer has two additional potential differences between the salt bridge and the electrolytes at each end of the bridge. These potentials can be minimized and almost eliminated in a number of ways. The additional potentials are referred to as the diffusion or liquid junction potentials, which will be discussed in Chapter 3. 

Any study of an electrochemical system should be started with the equilibrium mode. In such a mode, the electrodes should not be called the cathode or the anode, and both half-reactions should be shown as reduction reactions. It is because, by common convention, the reference data on the (standard) electrode potentials are given for the reduction reactions. The convention will be discussed in Chapter 4. However, one electrode of an electrochemical cell should be more positive than another one, and the polarity can be experimentally found using the high-resistance electrometer. Also, the polarity of the electrodes in the equilibrium cell can theoretically be calculated using thermodynamic data. This will also be discussed in Chapter 4. 

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Electrochemical Cell Without Transference Assume that we want to determine the activities of HCl solutions of various concentrations. We assemble a galvanic cell with hydrogen and calomel electrode ... 

The transfer of electrons sets up an electric current—a flow of electric charges. This flow is vital in electrochemical reactions. As the reaction continues, electrons are injected into the process and then drawn off. Chemists carry out electrochemical reactions within a device known as an electrochemical cell, with electrical conductors called electrodes to inject and withdraw electrons. 

Energetics of oxidation-reduction (redox) reactions in solution are conveniently studied by arranging the system in an electrochemical cell. Charge transfer from the excited molecule to a solid is equivalent to an electrode reaction, namely a redox reaction of an excited molecule. Therefore, it should be possible to study them by electrochemical techniques. A redox reaction can proceed either by electron transfer from the excited molecule in solution to the solid, an anodic process, or by electron transfer from the solid to the excited molecule, a cathodic process. Such electrode reactions of the electronically excited system are difficult to observe with metal electrodes for two reasons firstly, energy transfer to metal may act as a quenching mechanism, and secondly, electron transfer in one direction is immediately compensated by a reverse transfer. By usihg semiconductors or insulators as electrodes, both these processes can be avoided. 

Figure 18.13 Vacuum electrochemical cell with an integrated drying tube (o) and water-cooled jackets (fl, fZ) from (A) a front view and (B) a top view. Schematic representation of the drying operation is shown in A, B and C. The cell is filled with aluminum oxide and electrolyte solution in A. The solution is transferred into the cell by a 90° rotation in B. After back-rotation, the solution flows into the electrode compartment, passing through the cooled alumina drying tube in C. [From Ref. 2, with permission.]...

Liquid-solid mass transfer has also been studied, on a limited basis. Application to systems with catalytic surfaces or electrodes would benefit from such studies. The theoretical equations have been proposed based on film-flow theory (32) and surface-renewal theory (39). Using an electrochemical cell with rotating screen disks, liquid-solid mass transfer was shown to increase with rotor speed and increased spacing between disks but to decrease with the addition of more disks (39). Water flow over naphthalene pellets provided 4-6 times higher volumetric mass transfer coefficients compared to gravity flow and similar superficial liquid velocities (17). 

In the previous chapters the condition of electroneutrality was applied to all systems that contained charged species. In this chapter we study the results when this condition is relaxed. This leads to studies of electrochemical systems, especially those involving galvanic cells. Cells without transference are emphasized, although simple cells with transference are discussed. At the end of the chapter the conditions of equilibrium across membranes in electrochemical systems are outlined. 

Electron Transfer in Electrochemistry. In electrochemical cells electron transfer occurs within the electrode-solution interface, with electron removal (oxidation) at the anode, and with electron introduction (reduction) at the cathode. The current through the solution is carried by the ions of the electrolyte, and the voltage limits are those for electron removal from and electron insertion into the solvent-electrolyte [e.g., H20/(H30+)(C10j ) (Na )(-OH) ... 

The greatly reduced double-layer capacitance of microelectrodes, associated with their small area, results in electrochemical cells with small RC time constants. For example, for a microdisk the RC time constant is proportional to the radius of the electrode. The small RC constants allow high-speed voltammetric experiments to be performed at the microsecond timescale (scan rates higher than 106V/s) and hence to probe the kinetics of very fast electron transfer and coupling chemical reactions (114) or the dynamic of processes such as exocytosis (e.g., Fig. 4.25). Such high-speed experiments are discussed further in Section 2.1. 

Example 5.12 Equipartition principle in an electrochemical cell with a specified duty We desire the electrode to transfer a specified amount of electricity Q over a finite time t0... 

Preparing a clean surface is often a prerequisite for surface-science studies. UHV-based methods of sample preparation and characterization are established, and these may be exploited for studies of surfaces immersed in solution by interfacing an electrochemical cell with an UHV chamber. Samples can then be transferred from UHV and immersed into electrolyte solution under a purified-Ar atmosphere. However, even under these clean conditions, some metals oxidize or get contaminated prior to immersion. Other techniques for the preparation of clean surfaces that do not require UHV techniques are available for some metals. For example, flame annealing and quenching have been successfully used, but this procedure is probably limited to Au, Pt, Rh, Pd, Ir, and Ag substrates. In this technique, substrates are annealed in an oxygen flame and quenched in pure water. 

The case of a liquid junction between electrolyte solutions of the same composition was examined earlier for electrochemical cells with transport (section 9.5). This situation is now re-examined using Onsager s method for dealing with mass transfer. The system considered is... 

FIGURE 10.6 UETV, electrochemical device C, electrochemical cell, with sample SC, auxiliary and reference electrodes E, electrolyte AI, argon inlet V, valve for the separation between the electrochemical pre-chamher and the main UHV chamber SM, sample manipulator SP, sorption pump P, the turbomolecular pumps M, mass spectrometer S, sputter gun SC, sample of single crystals R, x-ray emission tube L, low-energy electron diffraction system H, heat lamp X, x-ray photoelectronic spectrometer and T, transfer rod with sample holder. 

It is worth noting that the remarkable effect described for the carbon support porosity on the metal utilization factor and hence on the specific electrocat-alytic activity in methanol electrooxidation was only observed when the catalysts were incorporated in ME As and measured in a single cell. The measurements performed for thin catalytic layers in a conventional electrochemical cell with liquid electrolyte provided similar specific catalytic activities for Pt-Ru/C samples with similar metal dispersions but different BET surface areas of carbon supports [223]. The conclusions drawn from measurements performed in liquid electrolytes are thus not always directly transferable to PEM fuel cells, where catalytic particles are in contact with a solid electrolyte. Discrepancies between the measurements performed with liquid and solid electrolytes may arise from (1) different utilization factors (higher utilization factors are usually expected in the former case), (2) different solubilities and diffusion coefficients, and (3) different electrode structures. Thus, to access the influence of carbon support porosity... 

Figure 12 describes a simple electrochemical cell with a bare metal, in which the corrosion process is controlled by charge transfer. In this circuit, is the ohmic resistance, corresponding to the solution in the cell plus the cables and connections. Ret is the charge transfer resistance and Cji the capacitance of the double layer at the solution-metal interface. The Nyquist and Bode plots for this circuit are also presented. 

In Case study 5.2, we add the complication of a known faradaic reaction to the CV of the blank cell. Ferricyanide is a well-known, relatively stable iron complex with experimentally observable, reversible electrochemical behavior. For simplicity, in this chapter, we use ferricyanide when we refer to potassium ferricyanide. Ferricyanide follows a single, one-electron reduction to ferrocyanide and has been used as an educational tool for electrochemistry. In particular, two articles cover the primary analyses for CV using ferricyanide under reversible conditions [22, 23], Here, we follow the criteria outlined in the study by Kissinger and Heineman and use the data as a tool to understand biofilm CVs. We evaluate the scan rate dependence, electrode material and addition of rotation (to control mass transfer) and estimate some diagnostic parameters listed in Table 5.2. Figure 5.7 shows a picture of the fully assembled electrochemical cell with the yellow-colored solution containing ferricyanide. It was in this cell that all the ferricyanide results were obtained. 

An electrode in contact with an electrolyte is called a half-cell, often also written half cell. Thus, a simple two-electrode electrochemical cell is composed of two half-cells that contain either the same electrolyte but different electrodes or different electrodes and electrolytes. The first type of chemical cell, where there is no phase boundary between different electrolytes, is a cell without transference. The other type, in which a liquid-liquid junction potential or diffusion potential is developed across the boundary between the two solutions, is a cell with transference. Commercially available reference electrodes can be considered half-cells. ... 

UHV Measurements. Cu(hkl) single crystal was prepared by cycles of sputtering (0.5 keV Ar" ) and annealing (550°C in UHV) until AES and LEED indicated a clean and well-ordered surface, respectively. Afterwards, the crystal was transferred into a thin-layer electrochemical cell with a Pd/H reference electrode and immersed under potential control where no Pb adsorption occurs. 0.3 M HF (Baker, Ultrex) was chosen as supporting electrolyte for these measurements as it affords emersion experiments without the interference from non-volatile molecules (e.g., HCIO4 ) electrochemical experiments were done at room temperature. After electrochemical characterization in the presence of lead, the crystal was emersed from the electrolyte and returned to the UHV environment for postelectrochemical analysis. AES spectra were recorded for each emersed surfaces and the LEED pictures were taking at 46 eV for Cu(lOO) and 60 eV for Cu(l 11). 

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