Electrochemical cells driving

In this part of Chapter 12, we study electrolysis, the process of driving a reaction in a nonspontaneous direction by using an electric current. First, we see how electrochemical cells are constructed for electrolysis and how to predict the potential needed to bring electrolysis about. Then, we examine the products of electrolysis and see how to predict the amount of products to expect for a given flow ot electric current. 

Figure 6.6 Electrode arrangement for an electrochemical cell in which both electrodes can function in the same solution. Description The electrochemical cell is found to drive electrons from the hydrogen electrode to the silver electrode with an emf of about 0.2 V. This can be reported by writing the cell as Pt H2 (1 atm) HCI (1.0 M) AgCI Ag and assigning the emf as +0.2 V. If the cell to be written as Ag AgCI HCI (1.0 M) H2 (1 atm) Pt, the emf would be assigned as -0.2 V. In either instance, the emf values show that there is a tendency for electrons to be propelled through the external circuit from the hydrogen to the silver electrode.

Electrode reactions are inner-sphere reactions because they involve adsorption on electrode surfaces. The electrode can act as an electron source (cathode) or an electron sink (anode). A complete electrochemical cell consists of two electrode reactions. Reactants are oxidized at the anode and reduced at the cathode. Each individual reaction is called a half cell reaction. The driving force for electron transfer across an electrochemical cell is the Gibbs free energy difference between the two half cell reactions. The Gibbs free energy difference is defined below in terms of electrode potential,... 

Oxidation—reduction reactions, commonly called redox reactions, are an extremely important category of reaction. Redox reactions include combustion, corrosion, respiration, photosynthesis, and the reactions involved in electrochemical cells (batteries). The driving force involved in redox reactions is the exchange of electrons from a more active species to a less active one. You can predict the relative activities from a table of activities or a halfreaction table. Chapter 16 goes into depth about electrochemistry and redox reactions. 

Figure 1,2. Distribution of potential across a working electrochemical cell. The potential drop across the working electrode-solution interface drives the cell reaction.

Electrolysis is the process of driving a reaction in a nonspontaneous direction by using an electric current. An electrolytic cell is an electrochemical cell in which electrolysis takes place. The arrangement of components in electrolytic cells is different from that in galvanic cells. Specifically, the two electrodes usually share the same compartment, there is usually only one electrolyte, and concentrations and pressures are usually far from standard. 

Thus far, we ve been concerned only with galvanic cells—electrochemical cells in which a spontaneous redox reaction produces an electric current. A second important kind of electrochemical cell is the electrolytic cell, in which an electric current is used to drive a nonspontaneous reaction. Thus, the processes occurring in galvanic and electrolytic cells are the reverse of each other A galvanic cell converts... 

The type of work that we will deal with most often in this book is work of expansion and contraction, which we will call PVwork. Usually, the expansion or contraction is against the pressure of the atmosphere. In cases in which other types of work are involved, such as the work required to stretch an object or increase its surface area or the work of electrochemical cells or driving chemical reactions, we will usually designate these as vvoth or 8vvoth. We then have... 

We can think of chemical reactions occurring reversibly in electrochemical cells, where the driving force of the reaction is opposed by an electrical force. The difficulty of constructing electrochemical cells at 0 K is a practical, not a theoretical limitation. 

A battery (or galvanic or voltaic cell) is a device that uses oxidation and reduction reactions to produce an electric current. In an electrolytic cell, an external source of electric current is used to drive a chemical reaction. This process is called electrolysis. When the electric potential applied to an electrochemical cell is just sufficient to balance the potential produced by reactions in the cell, we have an electrochemical cell at equilibrium. This state also occurs if there is no connections between the terminals of the cell (open-circuit condition). Our discussion in this chapter will be limited to electrochemical cells at equilibrium. 

Charge-transfer reactions are the basis of electrochemical substance-producing cells driven by an external current source and of electrochemical energy-producing cells driving an external load (see Chapter 13). Metallic corrosion, it has been stressed,... 

It has been shown (Section 13.3.1), that in an electrochemical energy converter, the maximum cell potential is the value Vg obtainable when the reaction in the cell is electrically balanced out to equilibrium, i.e., when no current is being drawn from the cell. As soon as the cell drives a current through the external circuit, the cellpotential falls from the equilibrium value Ve to V. The value of the actual potential V at which the cell works when delivering a current i is always less than the equilibrium potential Vg. Hence, one has from Eq. (13.8)... 

The thin semiconductor particulate film prepared by immobilizing semiconductor nanoclusters on a conducting glass surface acts as a photosensitive electrode in an electrochemical cell. An externally applied anodic bias not only improves the efficiency of charge separation by driving the photogenerated electrons via the external circuit to the counter electrode compartment but also provides a means to carry out selective oxidation and reduction in two separate compartments. This technique has been shown to be veiy effective for the degradation of 4-chlorophenol [116,117], formic acid [149], and surfactants [150] and textile azo dyes [264,265]. 

In Fig. 15c, the resistor has been replaced by an electrochemical cell. This cell could be a recharging battery or a corrosion cell that is being studied electro-chemically. In either case, it will be a driven system. The driving is being done by the battery just discussed, or a power supply, or a potentiostat (more on this option below). Nonetheless, replacing the resistor with an electrochemical cell does nothing to change the polarity of the driven system. The electrode on the... 

Figure 15 Types of electrical/electrochemical cell with polarity conventions shown (a) power supply driving a resistor, (b) battery driving resistor, (c) power supply recharging a driven battery, (d) corrosion cell as a nearly short-circuited driving system. The resistance represents the electrical resistance in the metal between anode and cathode sites.

Finite electrolyte conductivities and ionic current flow lead to ohmic voltage components in electrochemical cells. It is constructive at this point to review the effects of ohmic voltage contributions to driven and driving cells in the case of uniform current distributions. It will be shown that for each type of cell, the ohmic resistance lowers the true overpotential at the electrode interface for a fixed cell voltage even in the case of a uniform current distribution at all points on the electrode. 

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