연료전지의 냉시동성 향상을 위한 복합 바나듐 산화물 막의 전기적 발열 특성

Abstract

The applicability of negative temperature coefficient materials to the cold-start enhancement of fuel cells was investigated by both theoretical and experimental method. To estimate the capability of the proposed technique, we first theoretically estimated the minimum electrical resistance required for cold starting at -20℃ ambient temperature with an electrical current density of 0.1 A/cm2. On the basis of the thermal energy balance inside fuel cells, the minimum surface-specific electrical requirement of thin solid negative temperature coefficient film was computed as 0.035 Ω-cm for a 2cm x 2cm sample. Subsequently, we selected vanadium oxide compounds as the most suitable materials of all candidate negative temperature coefficient materials based on their theoretically predicted temperature coefficient of resistance values. To fabricate the thin films, a well mixed precursor solution of vanadium alkoxide and organic co-solvents was prepared by the hydrolytic sol-gel route and then coated on the precleaned flat surface of conductive substrates (e.g. graphite plates with a gas-impermeable resin and stainless steel plates with natural passive oxide layers) by dip-coating method. Subsequently, the variation of the nonlinear electrical resistance of the thin films was measured simultaneously over a wide temperature range of -20 to 80℃, allowing direct detection of the surface temperature of the thin films. In addition, we performed a series of investigations to evaluate the mechanical, chemical, structural, and physical properties such as the adhesion, surface energy, internal contact resistance, microstructure, compositions of the vanadium oxide thin films using the ASTM D3359, ASTM D5946, XRD, XPS, EDS, FE-SEM, and 4-point probe methods. XRD, EDS and XPS revealed that the thin solid film generated was mainly composed of V2O3, which acted as an negative temperature coefficient material from -20 to 80℃. It was also found that a linked structure with cleavages was dispersed homogeneously in the films, and the thin films demonstrated excellent adhesion to the metallic separators. Furthermore, we observed that the vanadium oxides synthesized in this study have approximately 1.85-folded greater surface-specific electrical resistance than the minimum requirement estimated theoretically for the cold staring of fuel cell engines. A remarkable result from this study was that a temperature increase of 41.65℃ was induced by significant Joule heating of the vanadium oxide films on metallic plates, i.e. 0.24 W, at a current density of 0.1 A/cm2 at -20℃. In addition, it should be noted that 4.0 wt.% Mo-doped vanadium oxide thin film shows approximately 4-folded greater specific resistance than the pure vanadium oxide thin films at -20℃ since Mo added to vanadium oxides as a dopant reduces the band-energy gap between the valance band and the conduction band and as a result it facilitates electron transfer. Therefore, we can expect that the phase transition temperature, maximum electrical resistivity, and the spectrum of the resistivity variation can be adjusted to applicable values by conditioning the fabrication processes with mixed-valence vanadium oxides and dopants. In summary, the experimental studies demonstrated that thermal dissipation from resistive vanadium oxide films with a negative temperature coefficient can be effectively used as a self-heating source to melt frozen water at subzero ambient temperatures, particularly for fuel cell transportation applications.