Electrochemical reactions, multistep

Lefebvre, M. C. Establishing the Link Between Multistep Electrochemical Reaction Mechanisms and Experimental Tafel Slopes 32... 

In Chapter 6 we considered the basic mles obeyed by simple electrode reactions occurring without the formation of intermediates. However, electrochemical reactions in which two or more electrons are transferred more often than not follow a path involving a number of consecutive, simpler steps producing stable or unstable intermediates (i.e., they are multistep reactions). 

Like other heterogeneous chemical reactions, electrochemical reactions are always multistep reactions. Some intermediate steps may involve the adsorption or chemisorption of reactants, intermediates, or products. Adsorption processes as a rule have decisive influence on the rates of electrochemical processes. 

The above discussion emphasizes the limitations imposed by the use of metal particles on porous substrates, and calls for further efforts in designing model systems for better understanding of PSEs in complex multistep electrochemical reactions. 

Also, in complex electrode reactions involving multistep proton and electron transfer steps, the electrochemical reaction order with respect to the H+ or HO may also vary with pH, indicating a change of mechanism with pH. In this respect, the use of schemes of squares outlined in Sect. 2.2 is very useful in the analysis of these complex kinetics [13]. 

The transfer coefficients are the ones determining how the electrode potential influences the electrochemical reaction rate or, in other words, the inclination of the relation between log I and the over-potential, also called the Tafel slope, of a multistep reaction. The coefficients are an important aid when unravelling the electrochemical reaction mechanisms, because the experimentally determined Tafel slope should correspond to the value that is calculated for the postulated sub-step sequence and RDS. 

In Sect. 6.2, multi-electron (multistep) electrochemical reactions are surveyed, especially two-electron reactions. It is shown that, when all the electron transfer reactions behave as reversible and the diffusion coefficients of all species are equal, the CSCV and CV curves of these processes are expressed by explicit analytical equations applicable to any electrode geometry and size. The influence of the difference between the formal potentials of the different electrochemical reactions on these... 

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

Impedance spectroscopy is a helpful means for studying both the bulk transport properties of a material and the electrochemical reactions on its surface. The importance of impedance spectroscopy arises from the efficacy of the methodology in separating individual reaction-migration paces into a multistep process, since each reaction or migration step has, ideally, a single time constant related with it consequently, each step can be separated in the frequency domain ... 

In order to understand the manner in which the interfacial region influences the observed kinetics, especially in terms of the theoretical models discussed below, it is clearly important to gain detailed information on the spatial location of the reaction site as well as a knowledge of the mechanistic pathway. Information on the latter for multistep processes can often be obtained by the use of electrochemical perturbation techniques in order to detect reaction intermediates, especially adsorbed species [13]. Various in-situ spectroscopic techniques, especially those that can detect interfacial species such as infrared and Raman spectroscopies, are beginning to be used for this purpose and will undoubtedly contribute greatly to the elucidation of electrochemical reaction mechanisms in the future. 

It is well known " that when an electrochemical reaction proceeds in a multistep pathway with the steps in series, the Tafel slope observed for the log i vs. rj relation depends on the partial reaction sequence, and the state and potential dependence of adsorption of intermediates [see Eqs. (12) and (13)]. Various values of b for the reaction, step determines the rate of the overall reaction and (2) the conditions of coverage of the electroactive intermediate (H in the h.e.r.) in the reaction and the potential dependence of that coverage. Values > RT/0.5F (j8 = 0.5) sometimes arise with reac-... 

Usually the kinetics of a multistep electrochemical reaction, as also for analogous regular chemical reactions, can be treated in two complementary ways. 

Eq. (22)]. This then gives Eq. (31), the equation for the net overall rate for a multistep electrochemical reaction having a single rds ... 

Tafel slopes that are not infinite but are substantially greater than 118 mV dec- can be explained by (1) an arbitrary and trivial assumption that P < 1/2 (2) the effect (footnote f) of barrier-layer films such as oxide on Zr02 or Ti0242 (but this is usually only in the case of anodic reactions, particularly those involving valve-metal barrier oxide films) and (3) an electrochemical reaction mechanism where the rds is a chemical step and has a stoichiometric number, v, greater than 2 [refer to Eq. (1)]. This latter possibility will be developed in the next section in terms of a general multistep reaction mechanism. 

The theoretical as of B R [Eqs, (la) and (lb)] have been applied in many investigations of multistep electrochemical reactions. This is largely due to the straightforward means by which the particulars of reaction mechanisms (i.e., parameters such as and v) contribute to the... 

However, traditional polymerizations always require a preengineered separator surface with active groups prior to grafting, which may involve complex even sometimes violet, multistep treatments, such as ultraviolet or heat. Inevitably, these treatments may introduce chemical residue (e.g., catalyst and initiator) and cause irreversible shrinkage and aging of the separators, which adversely degrade the electrochemical reaction and decrease the durability of the membrane. 

SERS can detect not only adsorption/de-sorption behavior, but also redox processes of surface species [88] since the method can provide direct identification of adsorbed intermediates and/or products formed in multistep processes. Thus, when combined with conventional electrochemical measurements, Raman spectroscopy can identify the intermediates and evaluate the reaction pathways of electrochemical reactions. 

Meyer M, Melke J, Gerteisen D (2011) Modeling and simulation of a direct ethanol fuel cell considering multistep electrochemical reactions, transport processes and mixed potentials. Electrochim Acta 56 4299 307... 

---