Reactive depth and performance of an electrochemical carbon nanotube network as a function of mass transport

Three-dimensional porous electrodes often suffer from diffusional mass-transfer limitations that may be overcome by having the target solution flow through the electrode. Here, we examine the reactive depth and performance of an electrochemical carbon nanotube (CNT) network toward phenol removal and oxidation in the batch and flow configurations where mass transport into the CNT network is predominantly via diffusion and convection, respectively. Scanning electron microscopy depth profile imaging of phenol electropolymerization is used as a direct probe of the reactive depth. In the batch case, electropolymerization is observed to be greatest at the network surface nearest the cathode and decreases linearly to near zero at a depth of 25 μm. In stark contrast, electropolymerization is observed to be independent of the depth in the flow configuration. In agreement with the depth profile results, phenol removal is increased up to 10-fold, the current efficiency is increased by at least 2-fold, and susceptibility toward passivation is reduced in the flow versus batch configuration. Thus, the enhanced electrochemical performance in the flow configuration is partially due to the convective "activation" of the internal CNT network electron-transfer sites that are diffusion-inaccessible.