The Polymer Electrolyte Membrane Fuel Cell (PEMFC) is attracting much interest as a novel energy conversion device because it offers a higher coefficient of energy conversion than that of a conventional internal combustion engine, as well as having advantages with respect to environmental problems. The bipolar plate is a key component in a PEMFC. Many companies and institutes are using graphite plates as bipolar plates in research on fuel cells. The high cost of machining these graphite plates, however, is a practical obstacle to their widespread use.
Alternatively, metals seem to be a good candidate for bipolar plates, because they can be easily deformed and have a better electrical conductivity than that of graphite. Recently, there has been much research on the use of austenitic and ferritic stainless steels in the development of metal-bipolar plates, particularly in view of the good corrosion resistance of those steels. Stainless steels are an excellent candidate material for bipolar plates because they exhibit good corrosion behavior in PEMFC environments. However, the passive films of these steels may increase contact resistance and reduce fuel cell performance. Accordingly, researchers have been actively seeking solutions to this problem
Tarutani reported that stainless steel in which Cr-base carbides was precipitated can continuously maintain low contact resistance. However, the precipitation of Cr-carbides such as $Cr_{23}C_6$ causes the formation of a Cr-depleted zone adjacent to grain boundaries, resulting in an acceleration of corrosion in stainless steel. Thus, the inclusion of $Cr_{23}C_6$ is considered undesirable.
In this research, we will try to improve the interfacial contact conductance of metal-bipolar plates using austenite stainless steels by employing electrically conductive precipitates in 316 stainless steels (SS). We have added the elements of Nb and Ti to produce the conductive precipitates in the 316SS and estimated the interfacial contact conductance and the corrosion resistance. Nb and Ti were added to 316 stainless steels, and then heat-treatments and surface treatments were performed on the 316 stainless steel and the Nb- and Ti-added alloys. All samples indicated enhanced electrical conductivity after surface treatments, whereas they showed low electrical conductivity before surface treatments due to the existence of non-conductive passive film on the alloy surfaces. The Nb- and Ti-added alloys in particular showed remarkable enhancement of electrical conductivity and cell performance compared to the original alloy, 316 stainless steels. Surface characterization revealed that small carbide particles formed on the alloy surface after surface treatments, while the alloys indicated flat surface structure before surface treatments. $Cr_{23}C_6$ formed on the 316 stainless steel, and NbC and TiC mainly formed on the Nb- and Ti-added alloys, respectively. We attribute the enhanced electrical conductivity after surface treatments to the formation of these carbide particles, which possibly act as an electro-conductive channel through the passive film. Furthermore, NbC and TiC are supposed to be more effective carbides than $Cr_{23}C_6$ as electro-conductive channels for stainless steel.
Surface modification by thermal nitridation is one of the solutions. Brady et al. reported that the electrical properties of stainless steel are improved when high-temperature thermal nitridation ($\gt1000\degC$) is used to form transition metal nitrides, such as $Cr_2N$ and CrN, on the surface of the stainless steel. However, Cr-nitride is not continuous and protective. The formation of discrete external Cr-nitrides leads to exposure of the stainless steel surface, including Cr-depleted regions. Thus, high-temperature nitridation produces higher rates of corrosion than a base material. The reduced corrosion resistance of stainless steels after nitridation is associated chiefly with the fact that the formation of Cr-nitrides leads to a loss of Cr from the matrix .
In this research, we use stainless steels in an effort to improve the interfacial contact conductance of metal-bipolar plates; in particular, we focus on the electrically conductive and protective Cr-nitride in 446M stainless steels. We have examined thermal nitridation at low temperature ($600\degC$ and $700\degC$) in 446M stainless steel and estimated the interfacial contact conductance. Using 446M stainless steel, we perform the nitridation in pure nitrogen gas. We then evaluate the interfacial contact resistance and corrosion resistance of nitrided stainless steels. Low-temperature nitrided stainless steels show superior electrical conductivity and corrosion resistance than bare 446M stainless steel.
The interfacial contact resistances for 446M stainless steel nitrided at $600\degC and 700\degC$, which are 3 $m\Omega-cm^2$ and 6 $m\Omega-cm^2$, respectively, are much lower than the corresponding values of bare 446M stainless steel (about $77 m\Omega-cm^2$) at a compaction force of $140 N/cm^2$. The corrosion resistance of low-temperature nitrided 446M stainless steel is examined in potentiodynamic and potentiostatic tests under simulated polymer electrolyte membrane fuel cell (PEMFC) conditions with pH 3 $H_2SO_4$ at $80\degC$. In both a simulated anode condition and a simulated cathode condition, low-temperature nitrided stainless steels show superior corrosion resistance than bare 446M stainless steel.
The carburization of 446M stainless steel in a reducing atmosphere was investigated. The 446M stainless steel was using pack cementation process. The mixture consisted of 20 wt.% $BaCO_3$ powder as an activator and 80 wt.% carbon black as a filler. The ground samples and pack mixtures were placed in an alumina crucible. The crucible was placed into a tube furnace and held at $925\degC$ for 100 min in $5%H_2/Ar$ mixture gas. We then evaluate the interfacial contact resistance and corrosion resistance of carburized 446M stainless steels. The carburized stainless steels show superior electrical conductivity and corrosion resistance than bare 446M stainless steel. Therefore, this process consequently facilitates the application of carburized steel as bipolar plate material for PEMFC.
본 연구에서는 고분자전해질 금속분리판용 446M 스테인리스을 질화처리시 발생하는 내식성 저하의 문제를 해결하기 위해 저온 질화처리를 수행하였다. 이로 인해 표면에 Cr-질화물에 의해 형성되는 Cr-depleted zone 형성을 최소화 하고자 하였고, 이에 부식저항성을 개선고자 하였다. $600~700\degC$에서 질화처리 수행한 후, 미세구조 및 상분석을 통해 Cr-질화물의 형성을 확인하였고, 내식성 및 접촉저항 측정을 통해 금속분리판재로서의 적용가능성에 대해 평가하였다. 446M 스테인리스강을 $600\degC$ 및 $700\degC$에서 저온 질화처리를 수행한 시, 강의전기전도도가 개선되는 것을 확인하였다. 이러한 경향은 $600\degC$에서 저온 질화처리를 수행한 경우 더욱 현저히 나타났다. 또한 부식저항성이 고온 질화처리한 경우에 비하여 크게 감소되지 않았으며, 현저한 개선효과가 나타났다. $700\degC$에서 저온 질화처리한 경우, $600\degC$에서 저온 질화처리한 것 보다 부식저항성의 개선효과가 더욱 크게 나타났다. TEM 및 AES 분석을 통하여 표면분석을 수행한 결과, 저온 질화처리 후 강 표면에 각각의 경우 40 nm 두께의 $Cr_2N/Cr_2O_3$ 과 120 nm 두께의 $CrN/Cr_2O_3$ 층이 비교적 균일하게 형성된 것을 확인할 수 있었다. 따라서, 강의 최 표면에 형성된 전도성 Cr-질화물의 형성으로 인해 접촉저항이 개선되었으며, 균일한 두께의 질화물/산화물 층에 의해 부식저항성의 개선효과가 나타난 것을 확인 할 수 있었다.
