shows a stepwise decay, while both 3D-structured devices exhibit gradual decay (red and black lines, Figure ). We attribute this stepwise decay to initial growth of the IDZ in the z-direction from the planar electrode until the IDZ contacts the channel 'ceiling', which leads to a sudden increase in resistance (drop in current at approx. t = 5 s). An important point is that contact of the IDZ to the 'ceiling' can be disrupted at higher flow rates. Figure compares these results to current transients obtained for the PSC/Au control, for which stepwise decay occurs at t = 3 s. This result agrees with the observed onset of IDZ propagation (Figure ). A key point is that the current and rate of IDZ growth is lower for the PSC/Au control than for either of the other two cases due to the high electrical resistance and electrochemically inert nature of the PSC beads.