Non-Faradaic reaction

Non-Faradaic reactions or Capacitive type charge injection in which no electrons are transferred between the electrode surface and the electrolyte medium. The reaction includes the redistribution of charged chemical species in the conductive electrolyte medium, for e.g. such reactions are observed in Titanium Nitride (TIN) material. 

Fluang et al. carried out in situ solution electrochemical C NMR spectroscopy to study the reactions on commercial fuel cell electrocatalysts (Pt and PtRu blacks) used for ethanol oxidation reaction. It was concluded that the complete oxidation of ethanol to CO2 only took place dominandy at the very beginning of a potentiostatic chronoamperometric measurement and the PtRu had a much higher activity in catalyzing oxygen insertion reaction that leads to acetic acid [202]. Han et al. investigated the electrochemical oxidation of methanol on Pt and PtRu anode catalysts using in situ NMR spectroscopy and revealed the role of Ru in both Faradaic and non-Faradaic reactions and the spatial distributions of chemicals [203]. 

There is a major difference between faradaic and non-faradaic reactions. Non-faradaic reactions involve no chemical reaction on the electrodes, but faradaic reactions involve chemical reactions including phase transition of active materials. Therefore, the cycle life of battery reactions is limited to several thousand cycles or less, due to irreversible chemical reactions and irreversible phase changes of active materials. On the other hand, the cycle life of capacitors is over 10 10 cycles. 

Wave V (on the reverse scan) could be due to a combination of processes such as surface rearrangement, decrease of the rate of formic acid direct oxidation and increase of the heterogeneous non-faradaic reaction rate (4.23) [143]. 

There are two primary mechanisms of charge transfer at the electrode-electrolyte interface, illustrated in Fig. 1. One is a non-Faradaic reaction, where no electrons are... 

The seminal part of this contribution is that there is a non-Faradaic catalysis, that the catalytic reaction of ethylene with oxygen occurs as well and that it depends on the potential difference across the electrode ... 

Wagner was first to propose the use of solid electrolytes to measure in situ the thermodynamic activity of oxygen on metal catalysts.17 This led to the technique of solid electrolyte potentiometry.18 Huggins, Mason and Giir were the first to use solid electrolyte cells to carry out electrocatalytic reactions such as NO decomposition.19,20 The use of solid electrolyte cells for chemical cogeneration , that is, for the simultaneous production of electrical power and industrial chemicals, was first demonstrated in 1980.21 The first non-Faradaic enhancement in heterogeneous catalysis was reported in 1981 for the case of ethylene epoxidation on Ag electrodes,2 3 but it was only... 

The CO oxidation on Pt was the second reaction, after C2H4 oxidation on Ag, for which a Non-Faradaic rate enhancement was observed.33 Typical measured A values were of the order 102-103 while p was typically below... 

The remarkable NEMCA behavior of the isomerization reaction is shown in Fig. 9.31. At potentials negative with respect to the open circuit potential ( 0.38V) the rates of cis- and tram-2-butene formation start to increase dramatically. At a cell voltage of 0.16 to 0.10V the observed maximum p values are 38 and 46, respectively. The absolute A values are approximately equal to 28, as computed from the ratio Ar/(I/F) (with I/F presenting the rate of proton supply to the catalyst). The system thus exhibits a strong non-faradaic electrophilic behavior. 

The rate of ammonia production was enhanced by more than 1100% in the nitrogen rich regime (Figs 9.33 and 9.34), upon potential application of -IV between the working electrode and the Ag reference electrode. The extent of the NEMCA effect depends strongly on the kinetic regime of the reaction. Very pronounced non-faradaic behavior is observed in the regime 0.33

[Pg.470]

It must be emphasized, however, that since the Faradaic efficiency A is on the order of 2Fr0/I0, one anticipates to observe NEMCA behaviour only for those systems where there is a measurable open-circuit catalytic activity r0. Consequently the low operating temperatures of aqueous electrochemistry may severely limit the number of reactions where Non-Faradaic A values can be obtained. 

Transient measnrements (relaxation measurements) are made before transitory processes have ended, hence the current in the system consists of faradaic and non-faradaic components. Such measurements are made to determine the kinetic parameters of fast electrochemical reactions (by measuring the kinetic currents under conditions when the contribution of concentration polarization still is small) and also to determine the properties of electrode surfaces, in particular the EDL capacitance (by measuring the nonfaradaic current). In 1940, A. N. Frumkin, B. V. Ershler, and P. I. Dolin were the first to use a relaxation method for the study of fast kinetics when they used impedance measurements to study the kinetics of the hydrogen discharge on a platinum electrode. 

In case (a), the galvanic cell under non-faradaic conditions, one obtains an emf of 0.34 - (-0.76) = 1.10 V across the Cu electrode ( + pole) and the Zn electrode (- pole). In case (b), the galvanic cell with internal electrolysis, the electrical current flows in the same direction as in case (a) and the electrical energy thus delivered results from the chemical conversion represented by the following half-reactions and total reaction, repsectively ... 

The electric current connected with chemical conversion is termed the faradaic current in contrast to the non-faradaic current required to charge the electrical double layer. The equation for the electrode reaction is formulated similarly to that for the half-cell reaction (Section 3.1.4) for the cathodic reaction, the electrons are placed on the left-hand side of the equation and, for the anodic reaction, on the right-hand side. 

The first class includes non-redox reactions like isomerisation, dimerisation or oligomerisation of unsaturated compounds, in which the role of the catalyst lies in governing the kinetic and the selectivity of thermodynamically feasible processes. Electrochemistry associated to transition metal catalysis has been first used for that purpose, as a convenient alternative to the usual methods to generate in situ low-valent species which are not easily prepared and/or handled [3]. These reactions are not, however, typical electrochemical syntheses since they are not faradaic they will not be discussed in this review. 

To appreciate that a majority of non-faradaic currents are caused by the effects of adsorption, capacitance and the electrode double-layer, or by competing side reactions such as solvent splitting. 

We have talked about charge being passed. In reality, any current will comprise two components, i.e. faradaic and non-faradaic. Faradaic charge is that component of the overall charge which can be said to follow Faraday s laws, i.e. is linked directly with the sum of the electron-transfer reactions effected. The remainder of the current does not follow Faraday s laws, and hence it is said to be oM-faradaic . To summarize, we could say that ... 

Solvent splitting and electrolytic side reactions are an extremely common contribution to /non-faradaic- We will look in this present section at additional components of the observed non-faradaic charge. 

In the previous section, we introduced the way that coulometry can be employed as an analytical tool, looking speciflcally at some simple forms of the technique. We saw that the charge passed was a simple function of the amount of material that had been electromodified, and then looked at ways in which the coulometric experiment was prone to errors, such as non-faradaic currents borne of electrolytic side reactions or from charging of the double-layer. 

The most common type of errors found during coulometry is the incorporation of non-faradaic charge within the overall charge measured, e.g. as caused by double-layer charging or electrolytic side reactions. These aspects of coulometry have been discussed above. 

A second type of current arises due to the presence of the electrochemical double layer (Sect. 1.2). Additionally, a current may flow due to the adsorption or desorption (Sect. 1.2) or species O and R as well as electroinactive species. In these instances, no chemical reaction occurs and consequently electrons are not transferred across the electrode-solution interface. However, a current may flow elsewhere and this current is called a non-faradaic current. 

Both faradaic and non-faradaic currents may flow when an electrode reaction occurs. Thus, the total current which flows is often the sum of the faradaic and non-faradaic contributions to the current. Most often, it is the faradaic current that is of interest. Many electrochemical techniques have been developed which minimize or eliminate this non-faradaic contribution to the current, but discussion of these is beyond the scope of the present chapter. 

For the evaluation of the non-faradaic component of the response in a more realistic way, different proposals have been made. A useful idea is that corresponding to the interfacial potential distribution proposed in [59] which assumes that the redox center of the molecules can be considered as being distributed homogeneously in a plane, referred to as the plane of electron transfer (PET), located at a finite distance d from the electrode surface. The diffuse capacitance of the solution is modeled by the Gouy-Chapman theory and the dielectric permittivity of the adsorbed layer is considered as constant. Under these conditions, the CV current corresponding to reversible electron transfer reactions can be written as... 

A controlled modification of the rate and selectivity of surface reactions on heterogeneous metal or metal oxide catalysts is a well-studied topic. Dopants and metal-support interactions have frequently been applied to improve catalytic performance. Studies on the electric control of catalytic activity, in which reactants were fed over a catalyst interfaced with O2--, Na+-, or H+-conducting solid electrolytes like yttrium-stabilized zirconia (or electronic-ionic conducting supports like Ti02 and Ce02), have led to the discovery of non-Faradaic electrochemical modification of catalytic activity (NEMCA, Stoukides and Vayenas, 1981), in which catalytic activity and selectivity were both found to depend strongly on the electric potential of the catalyst potential, with an increase in catalytic rate exceeding the rate expected on the basis of Faradaic ion flux by up to five orders of... 

This type of electrode is a source or sink of electrons, permitting electron transfer without itself entering into the reaction, as is the case for the first or second type of electrodes. For this reason they are called redox or inert electrodes. In reality the concept of an inert electrode is idealistic, given that the surface of an electrode has to exert an influence on the electrode reaction (perhaps small) and can form bonds with species in solution (formation of oxides, adsorption, etc.). Such processes give rise to non-faradaic currents (faradaic currents are due to interfacial electron transfer). This topic will be developed further in subsequent chapters. 

NEMCA effect — The term NEMCA is the acronym of Non-faradaic Electrochemical Modification of Catalytic Activity. The NEMCA effect is also known as electrochemical promotion (EP) or electropromotion. It is the effect observed on the rates and selectivities of catalytic reactions taking place on electronically conductive catalysts deposited on ionic (or mixed ionic-electronic) supports upon application of electric current or potential (typically 2 V) between the catalyst and a second (counter or auxiliary) electrode also deposited on the same support. The catalytic reactants are usually in the gas phase. 

The observed change in catalytic rate is typically 5 to 105 times larger than the electrochemical reaction rate (i.e., the rate of ionic transport in the support, or the rate of ion supply to or ion removal from the catalyst) thus the effect is strongly non-faradaic. The electropromoted catalytic reaction rate is typically 2-500 times larger than the open-circuit (i.e., unpromoted) catalytic rate. 

---