Quantitative Examination of Aqueous Ferrocyanide Oxidation in a Carbon Nanotube Electrochemical Filter: Effects of Flow Rate, Ionic Strength, and Cathode Material

The unique conductivity, small diameter, and high specific surface area of carbon nanotubes (CNTs) make them suitable for a range of nontraditional electrode structures. For example, an electrochemically active CNT microfilter has been shown to be effective for water treatment. The forced convection through the 3D CNT electrode improves mass transfer relative to conventional 2D electrodes, but the effects of several solution and reactor parameters in this system are poorly understood because previous works focused on strongly sorbing organic species capable of undergoing several multielectron transfer reactions. Here, the use of Fe(CN)(6)(4-/3-) as a model soluble and nonadsorbing redox system enabled a near complete accounting of the predominant electron transfer reactions of anodic Fe(CN)(6)(4-) oxidation and cathodic water reduction to H-2. Subsequently, the effects of liquid flow rate, electrolyte concentration, target molecule concentration, anode potential, and cathode material on anodic Fe(CN)(6)(4-) oxidation kinetics and thermodynamics in an electrochemical filter were investigated. Electrochemical kinetics were observed to increase linearly with increasing flow rate, and electrolyte concentration had negligible effects in the flow configuration due to convective replenishment of Fe(CN)(6)(4-) and electrolyte to the CNT surface. In the electron-transfer-limited regime ([Fe(CN)(6)(4-)](0) = 15 mM), a rate constant for the oxidation of Fe(CN)(6)(4-) was calculated to be in the range k(et), = 2332 - 3705 s(-1) or (6-10) x 10(-3) cm s(-1). The use of a CNT network cathode furthermore enabled the cathodic reaction to proceed at a lower cell potential, indicating that the CNT network catalyzed water reduction. The total cell potential required for facile water reduction decreased from 1.6 V with the Ti cathode to 0.8 V with the CNT cathode, resulting in a total cell energy efficiency (>75%) that approached the current efficiency. Overall, the results quantitatively exemplify some of the advantages of using a 3D CNT electrode in the flow-through configuration.