Experimental study of electrical switching characteristics of vanadium oxide thin films on bipolar plates for improving thaw-at-start

Abstract

The ultimate goal of cold start of hydrogen-powered polymer electrolyte fuel cell vehicles is to minimize the significant system thaw energy requirement and to achieve the short time period desired for freeze start (e.g. less than 30 seconds) in a subfreezing environment. As part of an effort to improve cold start capability for fuel cell vehicles, this work presents a new thaw-at-start strategy using electrical characteristics of vanadium oxide thin films as self-heating source at sub-zero temperature. Vanadium-based thin film coated on the surface of flat bipolar plates (e.g. carbon-based graphite and metallic bipolar plates) have been synthesized by a dip-coating method via aqueous sol-gel chemistry. Subsequently, the detailed in-/ex-situ analyses of the thin films have been carried out using diverse diagnostic techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) to investigate the chemical composition, crystallinity, and microstructure. In addition, electrical switching characteristics of the thin films on bipolar plates was cautiously observed over a temperature range from -20°C to 80°C by means of 4-point probes installed in a thermo -hygrostat. By doing so, it has been possible to correctly infer the relationship between a tendency of the thermally-induced electrical switching hysteresis and bipolar plate materials. Also, comprehensive theoretical study on the basis of the experimental results have been performed to estimate the heat dissipation rate by Joule heating from the solid thin films on bipolar plates for the rapid cold-start operation of fuel cell vehicles.