Ce(III)/Ce(IV) in methanesulfonic acid as the positive half cell of a redox flow battery

The characteristics of the Ce(III)/Ce(IV) redox couple in methanesulfonic acid were studied at a platinum disk electrode (0.125 cm2) over a wide range of electrolyte compositions and temperatures: cerium (III) methanesulfonate (0.1 to 1.2 mol dm-3), methanesulfonic acid (0.1 to 5.0 mol dm-3) and electrolyte temperatures (295 to 333 K). The cyclic voltammetry experiments indicated that the diffusion coefficient of Ce(III) ions was 0.5 x 10-6 cm2 s-1 and that the electrochemical kinetics for the oxidation of Ce(III) and the reduction of Ce(IV) was slow. The reversibility of the redox reaction depended on the electrolyte composition and improved at higher electrolyte temperatures. At higher methanesulfonic acid concentrations, the degree of oxygen evolution decreased by up to 50 % when the acid concentration increased from 2 to 5 mol dm-3. The oxidation of Ce(III) and reduction of Ce(IV) were also investigated during a constant current batch electrolysis in a parallel plate zinc-cerium flow cell with a 3-dimensional platinised titanium mesh electrode. The current efficiencies over 4.5 hour of the process Ce(III) to Ce(IV) and 3.3 hour electrolysis of the reverse reaction Ce(IV) to Ce(III) were 94.0 and 97.6 %, respectively. With a 2-dimensional, planar platinised titanium electrode (9 cm2 area), the redox reaction of the Ce(III)/Ce(IV) system was under mass-transport control, while the reaction on the 3-dimensional mesh electrode was initially under charge-transfer control but became mass-transport controlled after 2.5 to 3 h of electrolysis. The effect of the side reactions (hydrogen and oxygen evolution) on the current efficiencies and the conversion of Ce(III) and Ce(IV) are discussed