Electrocatalysis electrode-electrolyte interface

The aim of this review is to first provide an introduction of electrocatalysis with the hope that it may introduce the subject to non-electrochemists. The emphasis is therefore on the surface chemistry of electrode reactions and the physics of the electrode electrolyte interface. A brief background of the interface and the thermodynamic basis of electrode potential is presented first in Section 2, followed by an introduction to electrode kinetics in Section 3. This introductory material is by no means comprehensive, but will hopefully provide sufficient background for the rest of the review. For more comprehensive accounts, please see texts listed in the references.1-3... 

Electrocatalysis and the Rate of Electrochemical Reactions For a given electrochemical reaction A + ne B, which involves the transfer of n electrons at the electrode/ electrolyte interface, the equilibrium potential, called the electrode potential, is given by the Nernst law ... 

In electrocatalysis, the activity of different electrocatalysts is usually expressed via the exchange current I0, and the specific activity, via the exchange current density, iQ (A cm-2), still often computed on the basis of the superficial electrode surface area. Only when the current is normalized using the true surface area of the electrode-electrolyte interface, the comparison between different electrocatalysts is truly meaningful. The determination of the true surface area of porous electrodes is discussed in Sect. 2.3.5. 

As already shown in Fig. 1, a general feature of electrocatalysis is that the current passing through an electrode-electrolyte interface depends exponentially on overpotential, as described by the Butler-Volmer equation discussed in Sect. 2.4.1, so that logi versus r] U — C/rev) gives straight lines, termed Tafel plots (Fig. 1). On this basis, one would expect an exponential-type dependence of current on overpotential in Fig. 12 (curve labeled 7ac). Such a curve would correspond to pure activation control, that is, to infinitely fast mass-transport rates of reactants and products to and from the two electrodes. 

One of the main distinguishing features of electrocatalysis from chemical catalysis is the effect of the electric field across the electrode-electrolyte interface on the rates of reaction. A change in the electric field causes a change in the activation energy of the over-all reaction. Thus, if AJ , and are the heats of activation at an overpotential rj and at the reversible potential, they are related by the equation... 

Electrocatalysis refers to the heterogeneous catalysis of a charge transfer reaction across an electrode-electrolyte interface. The type of reactions which are commonly observed in fuel cells are those in which a type of redox reaction... 

The structure and physicochemical properties of the enzymes which have been used to date to promote electrochemical reactions are briefly outlined. Methods of their immobilization are described. The status of research on redox transformations of proteins and enzymes at the electrode-electrolyte interface is discussed. Current concepts on the ways of conjugation of enzymatic and electrochemical reactions are summarized. Examples of bioelectrocatalysis in some electrochemical reactions are described. Electrocatalysis by enzymes under conditions of direct mediatorless transport of electrons between the electrode and the enzyme active center is considered in detail. Lastly, an analysis of the status of work pertaining to the field of sensors with enzymatic electrodes and to biofuel cells is provided. 

It is well known that the maximum efficiency of electrochemical devices depends upon electrochemical thermodynamics, whereas real efficiency depends upon the electrode kinetics. To understand and control electrode reactions and the related parameters at an electrode and solution interface, a systematic study of the kinetics of electrode reactions is required. When ILs are used as solvents and electrolytes, many oftheelectrochemical processes will be differentandsomenewelectrochemical processes may also occur. For example, the properties of the electrode/electrolyte interface often dictate the sensitivity, specificity, stability, and response time, and thus the success or failure of the electrochemical detection technologies. The IL/electrode interface properties will determine many analytical parameters for sensor applications. Thus, the fundamentals of electrochemical processes in ILs need to be studied in order to have sensor developments as well as many other applications such as electrocatalysis, energy storage, and so on. Based on these insights, this chapter has been arranged into three parts (1) Fundamentals of electrode/electrolyte interfacial processes in ILs (2) Experimental techniques for the characterization of dynamic processes at the interface of electrodes and IL electrolytes and (3) Sensors based on these unique electrode/IL interface properties. And in the end, we wiU summarize the future directions in fundamental and applied study of IL-electrode interface properties for sensor applications. 

Noguchi, H Okada, T. and Uosaki, K. (2008) Molecular structure at electrode/ electrolyte solution interfaces related to electrocatalysis. Faraday Discussion, 140, 125-137. 

Catalytic current — is a -> faradaic current that is obtained as a result of a catalytic electrode mechanism (see - electrocatalysis) in which the catalyst (Cat) is either dissolved in the bulk solution or adsorbed or immobilized at the electrode surface, or it is electrochemi-cally generated at the electrode-electrolyte solution interface [i]. The current obtained in the presence of the catalyst and the substrate (S) must exceed the sum of the currents obtained with Cat and S separately, provided the currents are measured under identical experimental conditions. The catalytic current is obtained in either of the two following general situations ... 

Corrosion of the material used is another factor that limits the selection of the electrocatalyst. The electrochemical corrosion of pure noble metals is not as important as in the case of binary or ternary alloys in strong acid or alkaline solutions, since these catalysts are widely used in electrochemical reactors. In the case of anodic bulk electrolysis, noble metal alloys used in electrocatalysis mainly contain noble metal oxides to make the oxidation mechanism more favorable for complete electron transfer. The corrosion problem that occurs from this type of catalyst is the auto-corrosion of the electrode surface instead of the electrode/electrolyte solution interface degradation. The problem of corrosion is considered in detail in Chapter 22. 

In electrocatalysis, the major subject are redox reactions occurring on inert, nonconsumable electrodes and involving substances dissolved in the electrolyte while there is no stoichiometric involvement of the electrode material. Electrocatalytic processes and phenomena are basically studied in aqueous solutions at temperatures not exceeding 120 to 150°C. Yet electrocatalytic problems sometimes emerge as well in high-temperature systems at interfaces with solid or molten electrolytes. 

More than a decade ago, Hamond and Winograd used XPS for the study of UPD Ag and Cu on polycrystalline platinum electrodes [11,12]. This study revealed a clear correlation between the amount of UPD metal on the electrode surface after emersion and in the electrolyte under controlled potential before emersion. Thereby, it was demonstrated that ex situ measurements on electrode surfaces provide relevant information about the electrochemical interface, (see Section 2.7). In view of the importance of UPD for electrocatalysis and metal deposition [132,133], knowledge of the oxidation state of the adatom in terms of chemical shifts, of the influence of the adatom on local work functions and knowledge of the distribution of electronic states in the valence band is highly desirable. The results of XPS and UPS studies on UPD metal layers will be discussed in the following chapter. Finally the poisoning effect of UPD on the H2 evolution reaction will be briefly mentioned. 

Several important energy-related applications, including hydrogen production, fuel cells, and CO2 reduction, have thrust electrocatalysis into the forefront of catalysis research recently. Electrocatalysis involves several physiochemical environmental dfects, which poses substantial challenges for the theoreticians. First, there is the electric potential which can aifect the thermodynamics of the system and the kinetics of the electron transfer reactions. The electrolyte, which is usually aqueous, contains water and ions that can interact directly with a surface and charged/polar adsorbates, and indirectly with the charge in the electrode to form the electrochemical double layer, which sets up an electric field at the interface that further affects interfacial reactivity. 

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