Layer-by-Layer (LbL) deposition of multi-walled carbon nanotubes (MWCNT) was used to modify porous electrodes for vanadium redox flow batteries. Preliminary studies performed over glassy carbon electrodes (GCEs) showed that assemblies obtained with MWCNT displayed better mechanical stability than those prepared with other carbon nanoforms. The LbL process involved successive deposition of a positively-charged polymer (in our case polyethylenimine, PEI) and carbon nanotubes modified in such a way to get their surface negatively-charged, and it was found that the use of MWCNT dispersed with poly(acrylic acid) (PAA) allowed a faster and more homogenous film growth than-COOH functionalized SWCNT. The thickness of the MWCNT assembly deposited onto a GCE strongly influenced the electrochemistry of VO 2+ /VO2 + species. The thicker the film, the better the electrochemical reversibility resulted. An electron transfer rate constant k0 of 5x10-4 cm s-1 was determined for the electrode prepared with one hundred bi-layers and treated at 650 °C in inert atmosphere. After optimizing the process on GCEs, the LbL technique was applied to the modification of graphite felt electrodes (GFEs). GF electrodes modified with ten bilayers of PEI-PAA(MWCNT) were tested in a vanadium redox flow battery, showing a net decrease in the charge and discharge overpotentials with respect to the unmodified graphite felt. Lower overpotentials for charge-discharge resulted in higher energy and voltage efficiencies for the battery with modified GF electrodes. Furthermore, cyclability of the system was tested and no fading was observed in the battery performance after one hundred cycles.