Electrochemical reversibility examples

Examples are tending to be more sophisticated and complex in form. For example, a dinuclear complex featuring a bridging phosphinate and phenolate in addition to peroxide (221) has been reported,966 as a model for phosphodiester systems. Apart from dicobalt(III) systems, a mixed-valence CoII,ni di-/i-superoxo complex (222) has been prepared.967 Transition between the three redox states CoII,n, Co11,111, and Co111111 is electrochemically reversible. 

The enhancement of SWV net peak current caused by the reactant adsorption on the working electrode surface was utilized for detection of chloride, bromide and iodide induced adsorption of bismuth(III), cadmium(II) and lead(II) ions on mercury electrodes [236-243]. An example is shown in Fig. 3.13. The SWV net peak currents of lead(II) ions in bromide media are enhanced in the range of bromide concentrations in which the nentral complex PbBr2 is formed in the solntion [239]. If the simple electrode reaction is electrochemically reversible, the net peak cnnent is independent of the composition of supporting electrolyte. So, its enhancement is an indication that one of the complex species is adsorbed at the electrode snrface. 

The example considered is the redox polymer, [Os(bpy)2(PVP)ioCl]Cl, where PVP is poly(4-vinylpyridine) and 10 signifies the ratio of pyridine monomer units to metal centers. Figure 5.66 illustrates the structure of this metallopolymer. As discussed previously in Chapter 4, thin films of this material on electrode surfaces can be prepared by solvent evaporation or spin-coating. The voltammetric properties of the polymer-modified electrodes made by using this material are well-defined and are consistent with electrochemically reversible processes [90,91]. The redox properties of these polymers are based on the presence of the pendent redox-active groups, typically those associated with the Os(n/m) couple, since the polymer backbone is not redox-active. In sensing applications, the redox-active site, the osmium complex in this present example, acts as a mediator between a redox-active substrate in solution and the electrode. In this way, such redox-active layers can be used as electrocatalysts, thus giving them widespread use in biosensors. 

Figures 11.10 (a) and (b) show that the voltammetry of these couples in a range of RTILs is nearly electrochemically reversible. Note however that, unlike the ferrocene- and cobaltocenium-based couples, the reduction potentials are likely to vary significantly from one RTIL to another. In experimental practice it is also important to verify that the calibration molecules do not interfere chemically with the voltammetric process under study. For example, we have investigated the oxidation of molecular hydrogen in the presence of TMPD and observed a reaction of the two species, as noted by the disappearance of the reverse-peak of the first redox couple (see Figure 11.11). This implies that the peak potentials ofTMPD +/TMPD are no longer obvious, and that this redox couple cannot be used as an internal reference in this type of experiment.

Cyclic chronopotentometry — A controlled current technique where the applied - current step is reversed at every transition time between cathodic and anodic to produce a series of steps in the potential vs. time plot - chronopotentiogram. The progression of transition times is characteristic of the mechanism of the electrode reaction. For example, a simple uncomplicated electron transfer reaction with both products soluble and stable shows relative -> transition times in the series 1 0.333 0.588 0.355 0.546 0.366... independent of the electrochemical reversibility of the electrode reaction. 

Table 2. Selected examples of HTMCC with one redox change with chemical and electrochemical reversibility E° in V, relative to the SCE). ...

The electrochemical behavior of a-tocopherylquinone and related model compounds points out a number of interesting facts. For example, in aprotic media in the absence of protons or proton donors essentially all biological quinones, including a-tocopherylquinone, are electrochemically reduced in stepwise e processes giving first an anion radical and then a dianion. Both these processes are electrochemically reversible. However, addition of protons or a proton donor to the a-tocopherylquinone or other bioquinones in an... 

Furthermore, porous CPs (e.g., polypyrrole, polyanUine) films have been used as host matrices for polyelectrolyte capsules developed from composite material, which can combine electric conductivity of the polymer with controlled permeability of polyelectrolyte shell to form controllable micro- and nanocontainers. A recent example was reported by D.G. Schchukin and his co-workers [21]. They introduced a novel application of polyelectrolyte microcapsules as microcontainers with a electrochemically reversible flux of redox-active materials into and out of the capsule volume. Incorporation of the capsules inside a polypyrrole (PPy) film resulted in a new composite electrode. This electrode combined the electrocatalytic and conducting properties of the PPy with the storage and release properties of the capsules, and if loaded with electrochemical fuels, this film possessed electrochemically controlled switching between open and closed states of the capsule shell. This approach could also be of practical interest for chemically rechargeable batteries or fuel cells operating on an absolutely new concept. However, in this case, PPy was just utilized as support for the polyelectrolyte microcapsules. 

It can be seen that cyclic voltammograms at low scan rate have peak-to-peak separations close to the value theoretically expected for a reversible process of A p = 2.218 X 7 r/ = 57 mV at 298 K [47] and the peak current increases with the square root of the scan rate. Under these conditions, the process is diffusion controlled and termed electrochemically reversible or Nernstian within the timescale applicable to the experiment under consideration. Hence, as with all reversible systems operating under thermodynamic rather than kinetic control, no information concerning the rate of electron transfer at the electrode surface or the mechanism of the process can be obtained from data obtained at slow scan rate. The increase of A p at faster scan rate may be indicative of the introduction of kinetic control on the shorter timescale now being applied (hence the rate constant could be calculated) or it may arise because of a small amount of uncompensated resistance. Considerable care is required to distinguish between these two possible origins of enhancement of A p. For example, repetition of the experiments in Table II.l.l at... 

There are numerous examples of electrochemically reversible pH-dependent couples based on metal eomplexes of aromatie iV-heterocycles. " Haga has extensively investigated the redox reactions of [(bpy)2M (bibzimH2)], (M = Ru, Os bibzimH2 is 2,2 -bibenzimidazole) in 1 1... 

Ru(NH3)6] " in aqueous electrolytes of sufficiently high concentrations represents a real-life example of an extremely fast charge transfer process that is not necessarily burdened by the influence of R, . Therefore, the voltammetry of the [Ru(NH3)6] process should ideally fit the theoretical predictions for an electrochemically reversible process. 

For slow scan rates, i.e., a few volts per second or slower, the voltammetric response of a miniaturized electrode is a sigmoid-shaped steady-state voltammogram [8]. Peak-shaped responses are observed (Mily when the scan rate is sufficiently high so that the depletion layer thickness is smaller than the critical dimension of the electrode. For example, a 50-nm-radius nanoelectrode requires the sweep rate to be of the order of 10,000 Vs to show the classical peak-shaped responses typically observed for electrochemically reversible systems at macroelectrodes at mVs scan rates. The theory describing mass transport to micro- and... 

It is quite possible that a series of intermediate phases forms during the electrochemical process, their exact nature being controlled by the differences in the kinetics of the diffusion of the different ions or atoms in the system. Parallel reactions to products with similar thermodynamic stability may lead to a degradation of the reversible properties, if one of the products is not electrochemically reversible. Hence, knowledge of the real reaction partners and of their properties is the key for understanding the electrochemical processes in the system and elucidating the reaction mechanism. This is possible by making ex situ and/or in situ experiments with methods that supply information about the chemical composition, structural, and thermal properties of the compounds in the reaction mixture. A number of examples have been presented in literature based on the various methods as listed in Table 3.5. 

We have studied a range of different metals and metal oxides with widely varying degrees of electrochemical reversibility and electrodeposition kinetics, hi contrast to silver, many important noble metals and metal oxides electrodeposit irreversibly, hi Figure 16.1.6a, for example, irreversible cyclic voltammograms acquired in plating solutions for MoOj (a metallic oxide of molybdenum) and platinum are compared with the reversible CVs seen for silver. 

The electrochemical reversibility of the employed redox material in a pseudocapacitor normally means that the redox process follows Nerstian behavior [2]. These redox materials include (1) electrochemically active materials that can be adsorbed strongly on an electrically conductive substrate surface such as a carbon particle and (2) solid-state redox materials that can combine with or intercalate into an electrode substrate to form a hybrid electrode layer. For example, adsorption on an electrode substrate surface is commonly observed as underpotential deposition of protons on the surface of a crystalline metal electrode (Ft, Rh, Pd, Ir, or Ru). In the case of Ru, the protons can pass through the surface into the metal lattice by an absorption process, similar to the transitional behavior seen in lithium battery intercalation electrodes. 

The extent of the contribution of the Pt i/-orbitals to the frontier orbitals can be directly assessed by electron paramagnetic resonance (EPR) (spectro) electrochemical experiments as demonstrated in Fig. 3.8. These experiments are only possible for those compounds in which the redox process in question is chemically and electrochemically reversible. For example, the EPR studies on the radical anions revealed 10 % contribution of Pt(II) orbitals in the LUMO of Pt(4,4 -X2-2,2 -bipyridine)Cl2 systems [48]. Likewise, 12 % Pt d( ) contribution in the... 

Tetra peripheral 3,5-bis(trifluoromeihyl)phenylethynyl-substituted manganese (III) chloride phthalocyanines [Cl-Mn(III)Pc] can be given as another example [37]. As shown in Fig. 30, [Cl-Mn(III)Pc] gave three reduction and two oxidation reactions in DCM/TBAP electrolyte system. While the first reduction and oxidation reactions were chemically and electrochemically reversible, the second and third... 

Consider the cyclic voltammetry trace of electrically activated iridium oxide (the so called AIROF) which features reversible reactions (Fig. 3.3). The scan rate is very slow, so the dynamic behavior of the Helmholtz capacitance has a negligible effect on the measured trace. The positive peaks A and B correspond to two distinct oxidation reactions at the surface of the electrode, pertaining to different electrode potentials. The negative peaks C and D correspond to reduction reactions. C matches A and D matches B, as they have similar shape. The reduction potential peak (for example at C, Epc) does not happen at a negative electrode-electrolyte voltage drop, but at a positive one even near to the potential where oxidation potential peak (at A, Epa) is located. If the surface redox reactions are fast and the reaction rate is limited by the diffusion of the reactants in the solution, the difference between the oxidation and reduction peaks is only 59 mV/n for a reaction where n electrons are transferred in the stoichiometry of the reaction. This state is called electrochemical reversibility, which means that the thermodynamic equilibrium in the redox reaction at the surface is established fast at every applied electrode potential. Note that this concept is not the same as the chemical reversibility explained before. A system can be electrochemically irreversible but chemically reversible. As seen in Fig. 3.3, iridium oxide is already electrochemically irreversible even at the very slow potential ramp of 50 mV/s, as the , 4 — is already larger than 59 mV. 

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