Stationary Pattern Formation in Iron Electrodissolution with Two Cells: Effects of Serial or Parallel Coupling

Abstract

We investigate the stationary pattern formation for two coupled cells at constant circuit potential. Two configurations are explored: two cells in serial and parallel. In each cell the anode is iron electrodissolution and the cathode is hydrogen ion reductions on a large surface area Pt electrode with sulfuric acid electrolyte. Iron electrodissolution in sulfuric acid exhibits negative differential resistance due to electrode passivation by the surface oxide where the stability of the surface oxide is dependent on electrode potential and pH at the electrode interface. In the presence of ohmic potential drops (e.g., due to solution or external resistance), bistability can occur where the active and passive states co-exist at a given circuit potential. First, we investigated the patterns that emerge for two serially coupled iron electrodissolution in sulfuric acid. Investigation of the forward scan potentiodynamic I-V curves reveals sequential passivation of electrodes in no particular order and a potential region is identified where one electrode is passive and the other is inactive, and thus a stationary pattern is formed. Numerical simulations and equivalent circuit network modeling of the experimental set-up reveals a bidirectional negative coupling topology as a consequence of serial configuration, providing insight into the pattern formation. The patterns are interpreted with a kinetic model which shows that the coupled NDR system can exhibit symmetry breaking transitions in the presence of negative coupling. Next, we studied stationary pattern formation in two parallel cells. In contrast to the serial placement, the potentiodynamic I-V curves reveal simultaneous passivation of the two electrodes. The electrode potential versus circuit potential plot displays a uniform state of electrodes throughout the scan. The corresponding modeling showed that the lack of sequential passivation and the presence of uniform states were due to the inherent bidirectional positive coupling in the system. The experiments demonstrate the rich dynamical response of a coupled negative differential resistance systems to generate stationary patterns similar to Turing patterns that are used to interpret morphogenesis in biological systems. The theoretical understanding and insight gained from modeling and simulating observed patterns of relatively smaller electrochemical cells could be crucial in the accelerated understanding, design, and control of battery packs that utilize serial and parallel coupled electrodes.