Conductive Polymer Composites And Coated Metals As Alternative Bipolar Plate Materials For All-vanadium Redox-flow Batteries

¹Fraunhofer Institute for Chemical Technology (ICT) Applied Electrochemistry, Joseph-von-Fraunhofer str.7, 76327 Pfinztal, Germany

²Fraunhofer Institute for Chemical Technology (ICT) Applied Electrochemistry, Robert-Koch-Platz 8a, 38440 Wolfsburg, Germany

Adv. Mater. Lett., 2014, 5 (6), pp 299-308

DOI: 10.5185/amlett.2014.amwc.1023

Publication Date (Web): Mar 23, 2014

Copyright © IAAM-VBRI Press

E-mail: burak.caglar@ict.fraunhofer.de

Abstract

In this study polypropylene (PP) based conductive composites and metal doped diamond like carbon (DLC) coated metallic substrates are studied as alternative bipolar materials for all-vanadium redox flow battery (VRFB). Graphite and carbon nanotube (CNT) filled PP based bipolar plates were produced via twin-screw co-rotating extruder and injection molding. Addition of 3 wt. % CNTs into highly filled graphite-PP matrix increased in-plane and through-plane electrical conductivities from 10 S/cm to 50 S/cm and from 2 S/cm to 10 S/cm respectively. PP composites with 78 wt. % graphite and 2 wt. % CNT filling ratio showed flexural strength value of 48,01 MPa. Produced bipolar plates were examined with galvanostatic charge-discharge test in a single-cell VRFB. Energy efficiency of 85,43 % at 25 mA/cm2 and discharge power density of 78,48 mW/cm² at 75 mA/cm² were achieved and those values were found to be comparable with commercial bipolar plates. Titanium, vanadium, chromoium and tungsten doped diamond-like coating (DLC) films were coated on metallic substrates (e.g. stainless steel 1.4301 and titanium alloy 3.7165) by a physical vapor deposition. The metallic dopant is necessary to achieve high conductivities in the order of ~100 S/cm. The values range from 0.5 to 35 S/cm for in-plane and from 10 to 110 S/cm for through-plane. The hydrogen evolution reaction (HER) and the anodic corrosions stability in 2 molar sulfuric acid constituted the main focus area for our investigations on metallic bipolar plates. An interesting material for coated metallic bipolar plate is the 10 µm Ti-DLC on 1.4301 which exhibits the highest hydrogen evolution overpotential of all investigated materials (710 mV µA/cm²). It also showed improved corrosion stability for anodic potentials.

Keywords

Vanadium redox flow battery, polymer composites, metallic bipolar plates, diamond like carbon coating, corrosion tests.