Electrochemical cell side reactions

Chemical yields from an electrochemical reaction are expressed in the usual way based on the starting material consumed. Cunent efficiency is determined from the ratio of Coulombs consumed in forming tlie product to the total number of Coulombs passed through the cell. Side reactions, particularly oxygen or hydrogen evolution, decrease the current efficiency. 

One of the main uses of these wet cells is to investigate surface electrochemistry [94, 95]. In these experiments, a single-crystal surface is prepared by UFIV teclmiqiies and then transferred into an electrochemical cell. An electrochemical reaction is then run and characterized using cyclic voltaimnetry, with the sample itself being one of the electrodes. In order to be sure that the electrochemical measurements all involved the same crystal face, for some experiments a single-crystal cube was actually oriented and polished on all six sides Following surface modification by electrochemistry, the sample is returned to UFIV for... 

Cathodic hydrogen evolution is one of the most common electrochemical reactions. It is the principal reaction in electrolytic hydrogen production, the auxiliary reaction in the production of many substances forming at the anode, such as chlorine, and a side reaction in many cathodic processes, particularly in electrohydrometallurgy. It is of considerable importance in the corrosion of metals. Its special characteristic is the fact that it can proceed in any aqueous solution particular reactants need not be added. The reverse reaction, which is the anodic ionization of molecular hydrogen, is utilized in batteries and fuel cells. 

Suitable solid electrolytes can be employed as the electrolyte in an electrochemical cell. The electrolyte is used in the form of a membrane which is impermeable to gas phase transport. Electroactive materials, or electrodes, are deposited on both sides of the electrolyte to increase the rates of charge transfer across the electrolyte interface and it is important that the active molecules in the gas phase have easy access to the electrode/electrolyte interface where they can participate in the charge-transfer reactions. For this reason it is necessary, in most cases, to ensure that the electrode has a high porosity while, at the same time, remaining electrically continuous. 

STY for the electrochemical HDH of DCP in paraffin oil. In this case, a sulphuric acid aqueous solution was used as the anolyte, which decreased the cell resistance that arose from the application to the non-aqueous catholyte, making the process possible. As shown in Fig. 13.9a (curves c, d and e), moderate current densities, i.e. around 10 mA cm-2, were necessary to achieve better performance high current densities, such as 20mA cm-2, caused severe side reactions, e.g. hydrogen evolution at the cathode (13.13) and oxygen evolution at the anode ... 

A fuel cell is an electrochemical device that converts the chemical energy of a fuel directly into electricity. The cell consists of three main parts the fuel compartment, the oxidant compartment, and an electrolyte membrane separating the fuel and oxidant. At the fuel side, the fuel is oxidized and electrons are released. At the oxidant side, the oxidant is reduced by accepting the electrons released from the fuel side. The electrons flowing through the fuel side to the oxidizer side can be harnessed, producing electric power. For an H2/air fuel cell, the reactions are ... 

Another type of BE is electrosynthesis, a synthetic technique that takes place in an electrochemical cell. The potential is set to drive the intended redox reaction and electrolysis continues until all the analyte has been reduced or oxidized. Electrosynthesis is often usefiil in industrial applications and is desirable, as side reactions are often minimal compared to conventional synthetic techniques. 

The charged reactant for the sink electrochemical reaction is supplied by the solid electrochemical cell of a PEVD system. The solid phase (E) is an exclusive ionic conductor for (A +) or (A ), and serves as the solid electrolyte. (C) and (W) are solid electronic conducting phases, and contact (E) from both sides as counter and working electrodes, respectively. They coimect with the external electric circuit, which consists of a dc source and other possible measurement devices. Because the conductivity changes in nature from ionic to electronic at the electrode/electrolyte interfaces, the solid electrochemical cell in a PEVD system effectively separates the transport paths of ionic and electronic charged carriers... 

The role of the source (O) in a PEVD system is to provide a constant supply of the solid-state transported reactant (A) during a PEVD process. Theoretically, it can be either a solid, liquid or vapor phase, as long as it can supply the ionic reactant (A ) or (A ) to the solid electrolyte (E) and the electronic reactant (e) or (h) to the counter electrode (C) via a source side electrochemical reaction. Therefore, the source must be in intimate contact with both solid electrolyte (E) and counter electrode (C) for mass and charge transfer between the source and solid electrochemical cell at location I of Figure 3. Practically, it is preferable to fix the chemical potential at the source. Any gas or solid mixture which does not react with the cell components and establishes a constant chenfical potential of (A) is a suitable source. For instance, elemental (A) provides (A +) or (A ) according to the following reaction... 

The current, I, in a PEVD process can be recorded simultaneously by an ammeter in the external circuit to reveal the kinetics of the PEVD reactions. As discussed in the last section, solid-state reactant (A) needs to be transported as a combination of ionic and electronic species from the source to the sink side through the solid electrochemical cell to participate in a PEVD reaction with vapor phase reactant (B). The PEVD reaction rate, and subsequent product (D) formation rate, v(t), can be expressed as... 

Although it is not as severe in PEVD systems as in aqueous electrochemical systems in which various kinds of mobile ions are present in the electrolytes, it should be pointed out that, in the presence of reactants at the sink electrode surface, other electrochemical reactions might also take place in parallel with the desired one at the sink side. If side reactions exist, usually such parallel reactions contributions to the measured current are not easy to quantify. If it is desired to use current to monitor the reaction and product formation in PEVD, side reactions should be eliminated or at least controlled. Fortunately, only one ionic species is usually mobile in a solid electrochemical cell because of the nature of the solid electrolyte. As long as the vapor phase is properly controlled, usually one electrode reaction is predominant over a wide range of PEVD applied potentials. Virtually 100% current efficiency for product formation can be expected. 

This anodic reaction provides sodium ions and electrons to the solid electrolyte and the inert Pt counter electrode, respectively, at the source side. Both the sodium ions and electrons will then travel through the solid electrochemical cell along previously-mentioned ionic and electronic paths to sustain the PEVD cathodic reaction for Na COj product formation at the sink side. Eurthermore, based on anodic reaction 60, the chemical potential of sodium is fixed by the vapor phase at the source side. Under open circuit conditions, this type of source can also serve as the reference electrode for a CO potentiometric sensor. 

This overall electrochemical cell reaction is equivalent to transporting the Na COg phase physically from one side of the substrate to the other. 

Under a negative dc applied potential, reaction (60) goes to the right resulting in Na COj decomposition at the source side. Both sodium ions and electrons are given up by the source Na CO disc at the anode of the electrochemical cell. Sodium ions travel through the soUd electrolyte (Na -(i"-alumina) to the sink reaction sites at the cathode. Electrons are conducted through the external electric circuit... 

The PEVD process takes advantage of the solid electrochemical cell of an SOFC. Oxygen is chosen to be the solid state transported reactant. At the source side (the cathode of the SOFC), oxygen in the source gas phase is reduced to oxygen anions (O ) through a cathodic reaction... 

While reaction parameters were not identified by regression to impedance data, the simulation presented by Roy et al. demonstrates that side reactions proposed in the literature can account for low-frequency inductive loops. Indeed, the results presented in Figures 23.4 and 23.5 show that both models can account for low-frequency inductive loops. Other models can also account for low-frequency inductive loops so long as they involve potential-dependent adsorbed intermediates. It is generally understood that equivalent circuit models are not unique and have therefore an ambiguous relationship to physical properties of the electrochemical cell. As shown by Roy et al., even models based on physical and chemical processes are ambiguous. In the present case, the ambiguity arises from uncertainty as to which reactions are responsible for the low-frequency inductive features. 

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