Quasireference Electrodes

The variation in catalytic activity for H2 oxidation. Nevertheless the agreement with Eq. (7.11) is noteworthy, as is also the fact that, due to the faster catalytic reaction of H2 on Pt than on Ag and thus due to the lower oxygen chemical potential on Pt than on Ag,35 the work function of the Pt catalyst electrode is lower than that of the Ag catalyst-electrode over the entire UWr range (Fig. 7.8b), although on bare surfaces O0 is much higher for Pt than for Ag (Fig. 7.8b). 

The electrolyte volume of the STM cells is usually very small (ofthe order of a 100 pi in the above-described case) and evaporation of the solution can create problems in long-term experiments. Miniature reference electrodes, mostly saturated calomel electrodes (SCE), have been described in the literature [25], although they are hardly used anymore in our laboratory for practical reasons Cleaning the glassware in caroic acid becomes cumbersome. For most studies, a simple Pt wire, immersed directly into solution, is a convenient, low-noise quasireference electrode. The Pt wire is readily cleaned by holding it into a Bunsen flame, and it provides a fairly constant reference potential of fcj>i — + 0.55 0.05 V versus SCE for 0.1 M sulfuric or perchloric acid solutions (+ 0.67 0.05 V for 0.1 M nitric acid), which has to be checked from time to time and for different solutions. 

Nonaqueous solutions Silver electrode (as quasireference electrode). 

The solution iR drop at the DME will also be time-dependent because rt, the drop radius, is a function of time. For this reason a stationary hanging-mercury-drop electrode (HMDE) is to be preferred or the vertical orifice (Smo-ler) DME can be used (see Figure 5.14). The tip of a platinium-wire quasireference electrode can be placed as close as 0.1 drop diameter (about 0.003 cm) because the drop grows in the downward direction.7 This gives nearly complete compensation in an electrolyte with a specific resistance of 15,000 Q-cm for a cell with total resistance of about 105 12. The effect of the polargrams of placing the quasi-reference electrode at different distances from the electrode surface is shown in Figure 6.3. 

Fig. 11.21 Cyclic voltammograms of original and regenerated ionic liquid [BMP]Tf2N)). The potential was determined vs. Pt as quasireference electrode. Scan rate 5 mVs 1.

Pseudoreference electrode reference electrodes, and -> quasireference electrode... 

Quasireference electrode (QRE) — (-> reference electrode, pseudoreference electrode). An electrode that maintains a given, but generally not well-defined, potential during the course of a series of electrochemical experiments. It has the advantage of not contaminating the test solution by solvent or ions that a conventional reference electrode might contain and transfer. Thus in studies in aprotic solvents, like acetonitrile, a silver wire can behave as a QRE. It must be calibrated with respect to a true reference electrode or reference redox couple that is added at the end of the experiments to obtain meaningful potential values. 

In many cases platinum or silver wires serve as quasireference electrodes, however, they have to be calibrated by a reference redox systems. [The term pseudoreference (literally false reference) electrode is also used in the literature, however, the term quasireference electrode is preferred]. 

Undivided cell — Electrochemical cells where all electrodes (two or three) are placed in the same compartment. Undivided cells are typically used for analytical experiments at small or -> microelectrodes in aqueous solutions when a -> two-electrode system is applied, or in a -> three-electrode measurements in nonaqueous media with a (platinum) -> quasireference electrode. A requirement in the use of an undivided cell is that the reaction products produced at the counter electrode do not reach or perturb the behavior of the working electrode. 

Infrared absorption spectroscopy Isophthalic acid Low energy electron diffraction Lowest imoccupied molecular orbital Mechanically controlled break-junction Mercury-sulfate electrode Potential of zero charge q = 0 Quasireference electrode Real hydrogen electrode Reference electrode Alkanedithiols HS(CH2)nSH Self-assembled monolayer(s)... 

One of the key components of a photoelectrochemical setup is the photoelectrochemical cell (further abbreviated as PEC cell) in which the photoanode or photocathode is mounted. At bare minimum, a PEC cell consists of a reservoir to hold the electrolyte, the sample to be studied (the working electrode), a counter electrode to supply current, an optically transparent window that allows the sample to be illuminated, and facilities to electrically coimect both electrodes to the outside world. Other components that are often present are a reference electrode, a quasireference electrode (see Sects. 3.2.4 and 3.6.6), and one or more inputs and outputs for a gas circulation system and/or gas purging of the electrolyte. In addition, one may encounter a magnetic stir bar, and a membrane that separates the anode and cathode compartments in order to avoid mixing of the evolved oxygen and hydrogen gases. 

Fig. 7.9 Absorption spectra of MEH-PPPV films in the dry state (a) original sample (b) after three voltammetric cycles (c) oxidized at 1 V vs. Ag-quasireference electrode and (d) cycled back to the original form. (Reproduced from [65] with the permission of Elsevier Ltd.)...

The linear actuation of PP was also studied by electrochemical deformation measurements during cyclic voltammetry and potential step experiments [291], It was found that in TBACFsSOsIpropylenecarbonate electrolyte, the shortest length of the PP strip investigated presents itself at 0 V vs. Ag wire quasireference electrode, while 6.6% expansion was achieved at +1V and ca. 4% at — 1V. The potential-dependent shrinkage and expansion phenomena show long-term stability. 

Choice of reference electrodes is one of the most important points in electrochemical measurements in ILs. The reference electrodes are required to show stable electrode potentials, which are usually determined by an equilibrium between reversible redox couples. The redox reaction between silver and silver cation, Ag/Ag(I), is often used as the redox couple for reference electrode in conventional nonaqueous electrolytes. The reference electrode based on Ag/Ag(l) has been also used in various ILs. However, the potentials of Ag/Ag(l) reference electrodes are different in different ILs since the Gibbs energy for formation of Ag(I) depends on the ions composing the ILs. Therefore, it is necessary to calibrate the potentials of reference electrodes against a conunon standard redox potential. A redox couple of ferrocenium (Fc" ) and ferrocene (Fc) is often used for this purpose although its redox potential is considered slightly dependent on BLs. Platinum or silver electrodes immersed in ILs are sometimes used as quasi-reference electrodes. The potentials of these quasi-reference electrodes may seem to be stable in the ILs without any redox species. However, their potentials are unstable and unreliable since they are not determined by any redox equilibrium. Thus, use of quasireference electrodes should be avoided even when the potentials are calibrated by Fc /Fc couple. 

The electrodeposition of AlSb cannot be achieved from aqueous solution because the reduction potential is far beyond the aqueous electrolyte potential limit. Furthermore, the high volatility of Sb complicates the electrodeposition of AlSb from high-temperature molten salt electrolytes. Consequently, the electrodeposition of AlSb is limited to be in the room-temperature molten salts which are also termed ionic liquids. Freyland and coworkers [164,165] first explored the nanoscale electrodeposition of AlSb on Au(l 11) using in situ scanning probe techniques such as STM and STS from AlCls-l-butyl-S-methylimidzolium chloride (1 1) ionic liquid containing SbCs. At a potential positive to 0.0 V (vs. an A1/A1(III) quasireference electrode), only Sb was deposited. The codeposition of AlSb occurred at more negative potentials. The deposition obtained at —0.9 V was Sb-rich whereas that at —1.5 V was Al-rich. Homogeneous distributed stoichiometric AlSb with a band gap of 2.0 0.2 eV was obtained at —1.1 V. 

FIGURE 1.16 Scheme of nanogap-based SECM measurements of a fast electron-transfer reaction at a macroscopic snbstrate in the (a) steady-state feedback mode and in quasi-steady-state (b) feedback and (c) SG/TC modes, (d) Quasi-steady-state i-j- — Eg voltammograms of TCNQ in acetonitrile (solid cnrves). The tip was held at —0.235 or 0 V versus an Ag quasireference electrode for feedback or SG/TC modes, respectively. Snbstrate potential was cycled at 50 mV/s. Closed circles and dotted lines are theoretical cnrves for quasi-reversible (k = 7 cm/s and a = 0.5) and reversible snbstrate reactions, respectively, with EP = -88 mV. The inset shows a reversible voltammogram with a peak separation of 61 mV simultaneously measured at the substrate. (Reprinted with permission from Nioradze, N. et al.. Quasi-steady-state voltammetry of rapid electron transfer reactions at the macroscopic substrate of the scanning electrochemical microscope. Anal. Chem., Vol. 83, 2011 pp. 828-835. Copyright 2011, American Chemical Society.)... 

Quasi-reference electrodes (Chap. 14) are by and large the most common type of reference electrodes reported in ionic Uquid electrochemistry. However, since the potential of this reference electrode is unknown, and the stability of this reference electrode can vary from electrode to electrode and solution to solution, accurate potentials cannot be measured with this type of reference electrode. Quasireference electrodes can also show considerable drift, especially when using silver wire directly immersed into the IL solutirm [15]. If using a quasi-reference electrode in IL electrochemical measurements for accurate determination of potentials, the electrode should be calibrated against an lUPAC recommended redox couple [15]. However, in many IL electrochemical reports, this is not performed and as such any potential data reported cannot be cOTifirmed by other laboratories. 

The standard potential Em is the polarographic half-wave potential of the U(IV) reduction in a melt with equal concentrations of U(1V) and U(11I). If the potential eq at nonpolarizable platinum wire, which serves as the quasireference electrode, is equated to zero, Eq. (5.16) becomes ... 

A silver wire were used in voltammetric studies as a reference and quasireference electrodes, respectively. In all cases a reference compound was added to the solution as a standard for potential values. The criteria for the choice of such a standard were (i) its chemical inertness towards the species under examination, and (ii) the ability to give reversible, well defined oxidation and/or reduction processes not overlapping with those of the sample. The most used standard was ferrocene, which is known to give a reversible monoelectronic oxidation process [12] and to be a relatively innocent species. 

Quite frequently a large noble metal electrode is used as a quasireference electrode (QRE) in electroanalytical work. Such an electrode is equivalent to the mercury pool electrode in conventional polarography. Since the QRE is normally used in a three-electrode system, the current passing through the reference electrode is extremely small (<10 A). Therefore, the potential of the QRE remains quite constant ( 10 mV for prolonged periods of time) provided oxidants or reductants are not added to the melt. In that case the potential shifts in the direction expected from the Nernst equation. 

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