Pattern formation in electrochemical systems depends sensitively on the relative arrangement of the electrodes.We present simulations of spatiotemporal patterns during the electrodissolution of a metal disk electrode inwhich the reference electrode is located on the axis of the disk and close to the metal working electrode. Thisgeometry introduces a strong but asymmetric negative feedback which tends to destabilize the homogeneouspotential distribution. We demonstrate that the feedback gives rise to a variety of complex antiphase-typepatterns. Some of these patterns have been observed in experiments; others present novel manifestations ofdynamic instabilities in electrochemical systems.