Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2022 / Paper 1 / Section A(iii) / 6

Take unit activity of each dissolved metal ion, unit activity of each metal, $p_{{\mathrm{H}}_2}=1\mathrm{bar}$ and $T=298\mathrm{K}$. The hydrogen-electrode Nernst equation givesThus $E_H=-0.05912\mathrm{V}$ at pH 1 and $-0.82764\mathrm{V}$ at pH 14. For corrosion coupled to hydrogen evolution,For alkaline solution the same electron balance can be written $\mathrm{M}+z{\mathrm{H}}_{2}O\to {\mathrm{M}}^{z+}+z\mathrm{O}{\mathrm{H}}^{-}+(z/2){\mathrm{H}}_2$, before allowing for metal-ion hydrolysis or complexation. Using $z=2$ for zinc and lead, and $z=1$ for lithium, the results per mole of metal areLithium is strongly unstable against hydrogen evolution in both media in this model. The positive alkaline values for zinc and lead apply to the stipulated unit-activity free-ion products; they are not complete predictions of alkaline corrosion once hydroxide precipitation and complexation change the product chemical potentials.

A useful aqueous anode must deliver the desired oxidation current while limiting parasitic corrosion and hydrogen evolution. Thermodynamics sets the driving force, while activation overpotential, surface films and mass transport control the rate.

For zinc, the acid calculation favours dissolution with hydrogen evolution, but a substantial kinetic barrier to hydrogen evolution on a clean zinc surface can slow this loss. Impurities and catalytic inclusions can increase it. In alkaline electrolyte, zinc can be useful in batteries, but the unit-activity $\mathrm{Z}{\mathrm{n}}^{2+}$ calculation is incomplete: soluble zincate and zinc oxide or hydroxide alter the relevant equilibrium. A surface film can protect against corrosion or obstruct useful discharge, depending on its conductivity and transport properties.

For lead, acid corrosion has a much smaller driving force in the stipulated model, and slow hydrogen kinetics can permit practical use. The electrolyte anion matters: a lead sulfate layer in a lead–acid system has a different role from arbitrary soluble lead salts. In alkaline media, hydroxide formation, soluble complexes and passivation again need consideration before drawing a practical conclusion from the free-ion potential.

Metallic lithium is unsuitable as an exposed anode in an ordinary aqueous electrolyte. Its strongly favourable reaction with water overwhelms the argument for simply relying on a hydrogen overpotential. A lithium-ion host electrode is a different material and must be judged by its own operating potential and interfacial chemistry.