This study describes a novel flowing deposition method which can be used in situ on a fully assembled electrochemical cell to enhance the electrochemical active surface area of flow-through porous electrodes. As a test case, vanadium redox reaction rates are enhanced by flowing deposition of carbon nanotubes on carbon paper electrodes. The deposition is characterized using a recently developed analytical flow cell technique and shown to increase the electrochemical active surface area and therefore the exchange current for the limiting V2+/V3+ reaction by over an order of magnitude. When applied to the anode of an assembled vanadium redox flow cell, the method nearly eliminates the overpotential associated with this reaction, thereby significantly improving the power output of the cell by up to 70% in the high voltage efficiency regime. As a surface area treatment, this flowing deposition method could be used to similarly enhance the energy conversion efficiency of many other electrochemical flow cells such as fuel cells, flow batteries and capacitive deionization cells.