Microwave activation of electrochemical processes: convection, thermal gradients and hot spot formation at the electrode vertical bar solution interface

Microwave activation of electrochemical processes is possible by self-focus sing of intense microwave radiation at the electrode\solution (electrolyte) interface of an electrode immersed in a solution and placed in a microwave cavity. Considerable changes in voltammetric current responses are observe d experimentally for the one-electron reduction of Ru(NH3)(6)(3+) in aqueou s 0.1 M KCl and for the stepwise two-electron reduction of the methylviolog en dication (MV2+) in aqueous 0.1 M NaCl. The formation and interconversion of two distinct forms of solid deposits, MVam0 and MVcryst0, on a mercury electrode surface is investigated, both in the presence of microwave activa tion and with conventional heating. It is shown that microwave activation a chieves (i) high temperatures in the vicinity of the electrode, (ii) therma l desorption of deposits from the electrode surface and (iii) limiting curr ents 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 phas e is employed in a finite element simulation (FIDAP(TM)) procedure to expla in the differences observed experimentally between conventional heating and microwave activation. Based on the comparison of simulation and experiment al data, a considerable thermal gradient and 'hot spot' region in the diffu sion layer of the electrode, together with convective mass transport are pr oposed.