Microwave activation of electrochemical processes: Convection, thermal gradients and hot spot formation at the electrode | solution interface

Microwave activation of electrochemical processes is possible by self-focussing of intense microwave radiation at the electrode I solution (electrolyte) interface of an electrode immersed in a solution and placed in a microwave cavity. Considerable changes in voltammetric current responses are observed experimentally for the one-electron reduction of Ru(NH₃)₆³⁺ in aqueous 0.1 M KCl and for the stepwise two-electron reduction of the methylviologen dication (MV²⁺) in aqueous 0.1 M NaCl. The formation and interconversion of two distinct forms of solid deposits, MV(am)/⁰ and MV(cryst)/⁰, on a mercury electrode surface is investigated, both in the presence of microwave activation and with conventional heating. It is shown that microwave activation achieves (i) high temperatures in the vicinity of the electrode, (ii) thermal desorption of deposits from the electrode surface and (iii) limiting currents an order of magnitude higher compared to those induced by conventional isothermal heating to the same electrode temperature. A simple physical model based on Joule heating of the aqueous solution phase is employed in a finite element simulation (FIDAP(TM)) procedure to explain the differences observed experimentally between conventional heating and microwave activation. Based on the comparison of simulation and experimental data, a considerable thermal gradient and 'hot spot' region in the diffusion layer of the electrode, together with convective mass transport are proposed.