Electrolysis

To induce current to flow through an electrolytic cell and bring about a nonspontaneous cell reaction, the applied potential difference must exceed the zero-current potential by at least the cell overpotential. The cell overpotential is the sum of the overpotentials at the two electrodes and the ohmic drop (IRs, where Rs is the internal resistance of the cell) due to the current through the electrolyte. The additional potential needed to achieve a detectable rate of reaction may need to be large when the exchange current density at the electrodes is small. For similar reasons, a working galvanic cell generates a smaller potential than under zero-current conditions. In this section we see how to cope with both aspects of the overpotential. The relative rates of gas evolution or metal deposition during electrolysis can be estimated from the Butler–Volmer equation and tables of exchange current densities. From eqn 25.46 and assuming equal transfer coefficients, we write the ratio of the cathodic currents as

wherej′ is the current density for electrodeposition and j is that for gas evolution, and j′ 0 and j0 are the corresponding exchange current densities. This equation shows that metal deposition is favoured by a large exchange current density and relatively high gas evolution overpotential (so η −η′ is positive and large). Note that η 0. The exchange current density depends strongly on the nature of the electrode sur face, and changes in the course of the electrodeposition of one metal on another. A very crude criterion is that significant evolution or deposition occurs only if the over potential exceeds about 0.6 V.

A glance at Table 25.6 shows the wide range of exchange current densities for a metal/hydrogen electrode. The most sluggish exchange currents occur for lead and mercury, and the value of 1 pA cm⁻² corresponds to a monolayer of atoms being replaced in about 5 years. For such systems, a high overpotential is needed to induce significant hydrogen evolution. In contrast, the value for platinum (1 mA cm⁻²) corresponds to a monolayer being replaced in 0.1 s, so gas evolution occurs for a much lower overpotential. The exchange current density also depends on the crystal face exposed. For the deposition of copper on copper, the (100) face has j0 = 1 mA cm−2, so for the same overpotential the (100) face grows at 2.5 times the rate of the (111) face, for which j0 = 0.4 mA cm⁻².

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مواضيع ذات صلة

Units

Names of quantities

Working galvanic cells

Experimental techniques

The rate laws

The electric potential at the interface

The structure of the interface

Catalytic activity at surfaces

Mechanisms of heterogeneous catalysis

Mobility on surfaces

The rate of desorption

The rate of adsorption