A recently developed class of electrochemical cell based on co-laminar flow of reactants through porous electrodes is investigated. New architectures are designed and assessed for fuel recirculation and rechargeable battery operation. Extensive characterization of cells is performed to determine most sources of voltage loss during operation. To this end, a specialized flow cell technique is developed to mitigate mass transport limitations and measure kinetic rates of reaction on flow-through porous electrodes. This technique is used in in conjunction with cyclic voltammetry and electrochemical impedance spectroscopy to evaluate different treatments for enhancing the rates of vanadium redox reactions on carbon paper electrodes. It is determined that surface area enhancements are the most effective way for increasing redox reaction rates and thus a novel in situ flowing deposition method is conceived to achieve this objective at minimal cost. It is demonstrated that flowing deposition of carbon nanotubes can increase the electrochemical surface area of carbon paper by over an order of magnitude. It is also demonstrated that flowing deposition can be achieved dynamically during cell operation, leading to considerably improved kinetics and mass transport properties. To take full advantage of this deposition method, the total ohmic resistance of the cell is considerably reduced through design optimization with reduced channel width, integration of current collectors and reduction of reactant concentration. With electrodes enhanced by dynamic flowing deposition the cell presented in this study demonstrates nearly a fourfold improvement in power density over the baseline design. Producing more than twice the power density of the leading co-laminar flow cell without the use of catalysts or elevated temperatures and pressures, this cell provides a low-cost standard for further research into system scale-up and implementation of co-laminar flow cell technology. More generally, the experimental technique and deposition method developed in this work are expected to find broader use in other fields of electrochemical energy conversion.
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Thesis advisor: Kjeang, Erik
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