Polymer Electrolyte Membrane Fuel Cell Environment Research Articles

Exploring the La Microalloying Effects on the Electrochemical Properties of AlCoCrFeNi2.1 EHEAs in Simulated PEMFC Environments

Eutectic high-entropy alloys (EHEAs), represented by AlCoCrFeNi2.1, have the potential to be used as bipolar plates in fuel cells. However, there has been relatively little attention paid to their corrosion behavior in polymer electrolyte membrane fuel cell (PEMFC) environments. In this paper, the influence of La element addition on the microstructure and electrochemical properties of AlCoCrFeNi2.1 EHEAs in various simulated PEMFC conditions was investigated. The results showed that La addition reduced the corrosion resistance of the alloy, and with the increase of F- concentration and temperature, the effect of La on the corrosion resistance became more significant. The decrease in corrosion resistance was attributed to the formation of a (Ni, La)-rich BCC structure around the (Ni, Al) phase, which acted as the preferred corrosion site. Accordingly, the electrochemical processes of the La-added alloys were significantly accelerated, and the passive film properties deteriorated, leading to the occurrence of pitting corrosion.

Solution acidity and temperature induced anodic dissolution and degradation of through-plane electrical conductivity of Au/TiN coated metal bipolar plates used in PEMFC

A novel composite coating of gold/titanium nitride on stainless steel (Au/TiN/SS) was developed by introducing Au dots onto a TiN coating to improve the corrosion resistance and electrical conductivity of stainless steel bipolar plates for the polymer electrolyte membrane fuel cell (PEMFC). The effects of temperature and pH on conductivity and corrosion resistance of the Au/TiN/SS in a simulated PEMFC environment were investigated. The results showed that the interfacial contact resistance of TiN coating reduced from 5.48 mΩ cm2 to 1.72 mΩ cm2 after introducing the Au dots, indicating that Au dots can enhance the interfacial conductivity of the composite coating on stainless steel. The increases of solution temperature and acidity enhanced the dissolution of TiN as well as the delamination of Au dots from the composite coating, and the corrosion current density (icorr) of Au/TiN coating increased in high temperature and strong acidic environments. The thin layer of TiNxOy and TiO2 around the Au dots was preferentially dissolved to detach the Au from the composite coating, therefore induced the degradation of through-plane electrical conductivity of the Au/TiN coated metal bipolar plates.

Corrosion Behavior of Aluminum in Dilute Fluoride and Sulfate Solutions for Use as Bipolar Plate of PEFC

ABSTRACT In polymer electrolyte membrane fuel cell (PEFC), bipolar plate (BP) is a vital multifunctional component which constitute more than 80% of the total stack weight and about 30% of the materials cost.[1-3] Among many materials act as BP, aluminum (Al) is one of the most alternative for fulfilling the requirements that mentioned above for its special qualities such as mechanical properties, low cost and low density.[4-5] It is also generally required for a BP to be corrosion resistant even in sulfuric acid solution with a pH around 3.0, but it would be difficult for BP like Al to sustain under this condition. To fulfil this condition, a new type of BP is proposed by Yashiro et al. [6] that made of two sections: a flow path that directly contacts with MEA and a gas isolation plate. The gas isolation plate contacts with the generated water which is near to pure water and bare Al can also be used for this section. Therefore, in our previous works, the effects of fluoride and sulfate ions on pure Al in the simulated PEFC environment was studied at 80°C.In this study, the corrosion behavior of Al in dilute sodium fluoride and sodium sulfate solutions with different concentrations at 40°C was evaluated. Open circuit potential (OCP), scanning electron microscopic (SEM) images, energy dispersive x-ray (EDX) spectra and cross-sectional images were also taken to check the corrosion potential, surface morphology, elemental composition and other analyses.Pure Al having a thickness of 1 mm and area of 10 cm2 were used as the test samples for immersion process. 0.1ppm F- (pH 5.70), 0.1ppm SO4 2- (pH 5.64), mixture of 0.1ppm F- and 0.1ppm SO4 2- (pH 5.66), mixture of 0.1 ppm F- and 1ppm SO4 2- (pH 5.57) and distilled water (DSW) were used as the test solutions using NaF and Na2SO4. Firstly, Al was cut into desired sizes and polished by emery paper down to #2000, rinsed and immersed in around 90 cm3 aerated aqueous solutions. The immersion process was carried out at 40°C for a period of 24h, 48h, 72h, 96h and 120h. After the immersion test, weight was increased in all the test samples with increasing time regardless of the solutions. It is considered that the oxide film was formed in Al surfaces because of the oxidized aluminum at the time of immersion process. It is considered that the oxide films formation occurred because of the corrosive properties of the test samples with test solutions.A significant amount of corrosion of pure as-polished Al immersed in different solutions for 24 to 120h at 40°C was found. The results show that corrosion amount is higher in 0.1ppm F- solution and lower in 0.1ppm SO4 2- solution. It is considered that the inhibition of corrosion was occurred by sulfate ions, and the amount of corrosion was decreased. However, the corrosion amount was increased more when a mixture of 0.1ppm F- and 0.1ppm SO4 2- was used, and vice versa in case of DSW. Furthermore, when the concentration of sulfate ions was increased around 10 times using 1ppm SO4 2- with 0.1ppm F-, the lowest amount of corrosion was found among the used solutions.Fig. 1 shows the SEM images of pure as-polished Al of before and after immersion for 24h. Some irregular shaped particles of different sizes are observed in the Al surfaces that indicate the presence of aluminum oxide as corrosion products. In addition, thick oxide layers along with many cracks are found in the case the Al samples immersed in 0.1ppm F- and 0.1ppm F- + 0.1ppm SO4 2- solutions. Besides, very low amount of cracks are found in the case of DSW and 0.1ppm SO4 2- solution. Therefore, it is concluded that the presence of F- ions were confirmed which act as corrosion promoter and SO4 2- ions were act as corrosion inhibitor when Al is used as bipolar plates in the PEFC environment. Acknowledgements This work was supported by JSPS KAKENHI grant number JP19K05091.

Ni/Ni-Cr-P and Ni/Ni-Mo-Cr-P Electrodeposited Coatings As Bipolar Plate in Polymer Electrolyte Membrane Fuel Cell Environment

Polymer electrolyte membrane (PEM) fuel cells are expected to play a major role in transporting humans and goods. However, one of major bottlenecks is the high materials and manufacturing cost. One way to lower the cost to a great extent is by replacing conventional graphitic bipolar plates with metallic bipolar plates (BPPs), as they costs around 30% of the total cost of stack1. That said, metallic BPPs have durability issues. Over time, they corrode and show higher interfacial contact resistance (ICR). In this work we attempt to engineer the surface of metallic BPPs, so that they perform for longer duration without degrading. To keep the process economical and commercially attractive, a bilayer coating of Ni followed by Ni-Cr-P alloy is pulsed electrodeposited on AISI 1020 low carbon steel. The electrodeposition process is optimized varying the process parameters, such as electrolyte composition, temperature and deposition current. Field emission scanning electron microscopy and energy dispersive x-ray spectroscopy are used to study the surface morphology and compositional analysis of coating, respectively. Variation in microstructure and composition the Ni/Ni-Cr-P coating is observed with change in process parameter such as pH and temperature. Corrosion studies were carried out on Ni/Ni-Cr-P coated samples using potentiodynamic technique in accelerated (0.5 M H2SO4 + 2 ppm HF) solution at 80ºC. Ni/Ni-Cr-P coated samples with higher Cr content exhibited better corrosion resistance behaviour in both cathodic (air-purging) as well as anodic (H2-purging) environment compared to the bare substrate. Long term durability of Ni/Ni-Cr-P coated samples and AISI 1020 low carbon steel samples were studied using potentiostatic polarization technique. Both coated and bare samples are placed in the accelerated electrolyte solution. Ni/Ni-Cr-P coated samples showed better durability with less change in current density, while bare samples completely disintegrate and precipitate into the solution within 2–3 hours in potentiostatic mode in both the environments. Ni/Ni-Cr-P coated sample show lower interfacial contact resistance (ICR) value than that of bare AISI 1020 steel samples. For better corrosion resistance and improvement in surface electrical property, Mo was included in the coating composition2. Ni/Ni-Mo-Cr-P coated samples show improved corrosion resistance than Ni/Ni-Cr-P coated samples and AISI 1020 low carbon steel bare samples in both anodic and cathodic PEM fuel cell environment. Reference: R. Taherian, J. Power Sources, 265, 370–390 (2014) http://dx.doi.org/10.1016/j.jpowsour.2014.04.081.H. Rashtchi et al., Fuel Cells, 16, 784–800 (2016) http://dx.doi.org/10.1002/fuce.201600062

Hardware-In-the-Loop Simulation을 이용한 고분자 전해질 연료전지 냉각시스템 최적 제어기법 연구

Abstract - Polymer electrolyte membrane fuel cell(PEMFC) requires cooling system to maintain the proper operating temperature(about 65℃∼75℃) because the efficiency and power are affected by operating temperature. In order to retain the operating temperature of PEMFC, cooling system and coolant control logic are needed. Hardware-in-the-loop simulation(HILS) is one of effective methods to study and evaluate control algorithm. In this paper, the HILS system was designed to study the coolant control algorithm. The models of HILS system consisted of PEMFC, heat exchanger, and external environment associated with temperature. The hardwares in HILS system are 3-way valves, pumps, and a heat exchanger. The priority control and the control target temperature were investigated to improve the control performance using HILS. The 3-way valve in 1 st cooling circuit was selected as priority control target. The under limit value of 2 nd 3-way valve set as a function of PEMFC power and 2 nd circuit coolant temperature to correct temperature control performance. As a result, the temperature of PEMFC is stably controlled.

A nanocrystalline zirconium carbide coating as a functional corrosion-resistant barrier for polymer electrolyte membrane fuel cell application

A ZrC nanocrystalline coating is engineered onto a Ti–6Al–4V substrate using a double cathode glow discharge technique in order to improve the corrosion resistance and long-term stability of this alloy. The new coating exhibits an extremely dense, homogeneous microstructure composed of equiaxed grains with an average grain size of ∼12 nm and is well adhered on the surface of the substrate. The corrosion behaviour of the coating is systematically investigated using various electrochemical methods, including potentiodynamic, potentiostatic polarizations and electrochemical impedance spectroscopy (EIS), in a simulated polymer electrolyte membrane fuel cell (PEMFC) operating circumstances under different temperatures. The results show that with rising temperature, the corrosion potential (Ecorr) decreases and the corrosion current density (icorr) of the ZrC coated specimen increases, indicating that the corrosion resistance decreased with increasing temperature. However, at a given temperature, the ZrC-coated Ti–6Al–4V alloy has a higher Ecorr and lower icorr as compared to the bare substrate. The results of EIS measurements show that the values of the resistance for the ZrC coated Ti–6Al–4V alloy are three orders of magnitude larger than those of Ti–6A1–4V in the simulated PEMFC environment.

Pt Catalyst Degradation in Aqueous and Fuel Cell Environments studied via In-Operando Anomalous Small-Angle X-ray Scattering

The evolution of Pt nanoparticle cathode electrocatalyst size distribution in a polymer electrolyte membrane fuel cell (PEMFC) was followed during accelerated stress tests using in-operando anomalous small-angle X-ray scattering (ASAXS). This evolution was compared to that observed in an aqueous electrolyte environment using stagnant electrolyte, flowing electrolyte, and flowing electrolyte at elevated temperature to reveal the different degradation trends in the PEMFC and aqueous environments and to determine the relevance of aqueous measurements to the stability of Pt nanoparticle catalyst in the fuel cell environment. The observed changes in the particle size distributions (PSDs) were analyzed to elucidate the extent and mechanisms of particle growth and corresponding mass and active surface area losses in the different environments. These losses indicate a Pt nanoparticle surface area loss mechanism controlled by Pt dissolution, the particle size dependence of Pt dissolution, the loss of dissolved Pt into the membrane and electrolyte, and, to a lesser extent, the re-deposition of dissolved Pt onto larger particles. Based on the geometric surface area loss, mass loss, and mean particle size increase trends, the aqueous environment best reflecting the fuel cell environment was found to be one in which the electrolyte is flowing rather than stagnant. Pt nanoparticle surface area loss resulting from potential cycling can be inhibited by reducing the number of particles smaller than a critical particle diameter (CPD), which was found to be ∼3.5 to ∼4nm, with the CPD dependent on both the cycling protocol (square wave vs triangle wave) and the catalyst environment (fuel cell, aqueous stagnant, aqueous flowing electrolyte, or elevated temperature flowing electrolyte)

Corrosion behavior of polyphenylene sulfide–carbon black–graphite composites for bipolar plates of polymer electrolyte membrane fuel cells

The aim of this work was to study the corrosion behavior of polyphenylene sulfide (PPS) – carbon black – graphite composites regarding their application as bipolar plates of polymer electrolyte membrane (PEM) fuel cells. Electrochemical impedance spectroscopy (EIS), potentiostatic and potentiodynamic polarization tests were used to characterize the electrochemical response of the composites in a simulated PEM fuel cell environment. Cross-sectional views of fractured specimens were observed by scanning electron microscopy (SEM). The results showed that the corrosion behavior depends on the carbon black content incorporated into the composite formulation. There was a trend of decreasing the corrosion resistance for higher carbon black contents. This behavior could be explained based on the porosity and electrical conductivity of the composites.

Application of Carbon Coated Stainless Steel for Bipolar Plate in Proton Exchange Membrane Fuel Cell

A polymer electrolyte membrane fuel cell (PEMFC) which uses stainless steel as a bipolar plate has many advantages over conventional machined graphite. Austenitic stainless steel 316L is a traditional candidate for metal bipolar plates. During a long operation time, metal oxide formation leads to contact resistance, and metal dissolution can cause contamination of the membrane electrode assembly (MEA). A pure Carbon was coated on the surface of Stainless steel and provided a good anticorrosion layer. The carbon film has a low area specific resistance compared to the composite coating because it has a high surface conductivity. In addition, The ICR between the carbon-coated SS316 and the carbon paper is 1100–3000 mΩ-cm2 under compaction forces between 20 and 280 N/cm2. The carbon-coated SS316L shows good corrosion resistance in the simulated PEMFC environment. After 4000hrs of testing time, the power density of a single cell was less than 1% decline. Therefore, the carbon-coated SS316L has an excellent durability property.

Modifying a Stainless Steel for PEMFC Bipolar Plates via Electrochemical Nitridation

AbstractAISI446 stainless steel was electrochemically nitrided at room temperature. X‐ray photoelectron spectroscopy (XPS) analysis indicated that the nitrided steel was covered with surface ammonia and a layer of nitrides (mainly of mixed chromium nitrides). The nitride layer for 4 h nitrided steel at –0.9 V was about 2.5 nm thick. Dominating oxides appear on the steel's surface, so nitrogen incorporated oxides is a suitable term to describe the nitrided surface. The nitrided surface showed very low interfacial contact resistance (ICR) and excellent corrosion resistance in simulated polymer electrolyte membrane fuel cell (PEMFC) environments. The excellent stability of the nitride steel was confirmed by XPS depth profiling before and after testing in the PEMFC environments. Electrochemical nitridation provides an economic way for modifying the steel's surface to approach the U.S. Department of Energy 2015 goal for bipolar plates.

Corrosion Resistance Measurements of Amorphous Ni40Ti40Nb20 Bipolar Plate Material for Polymer Electrolyte Membrane Fuel Cells

Metallic bipolar plates have the advantages of better manufacturability, higher strength over graphite bipolar plates. The higher strength and toughness of the metallic materials permits the reduction of the width of the bipolar plate so, the volume and mass of the fuel cell can also be reduced. In this paper we are investigating the use of Ni-based amorphous material as a bipolar plate for polymer electrolyte membrane fuel cell (PEMFC). The major requirements of the metallic bipolar plate material are low weight, high corrosion and low contact resistance. The corrosion property of the present alloy has been investigated under conditions that simulate the fuel cell environment. Hydrogen gas and air were bubbled into a 1 N H2SO4solution at 70 °C, throughout the experiment to simulate the respective anodic and cathodic PEMFC environment. The Ni-base amorphous alloys displayed higher corrosion resistance than stainless steel.

Comparison of accelerated aging of silicone rubber gasket material with aging in a fuel cell environment

AbstractA polymer electrolyte membrane (PEM) fuel cell stack requires gaskets in each cell to keep the reactant gases within their respective regions. Both sealing and electrochemical performance of the fuel cell depend on the long‐term stability of the gasket materials. In this paper, the change in properties and structure of a silicone rubber gasket brought about by use in a fuel cell was studied and compared to the changes in the same silicone rubber gasket material brought about by accelerated aging. The accelerated aging conditions were chosen to relate to the PEM fuel cell environment, but with more extreme conditions of elevated temperature (140°C) and greater acidity. The dilute sulfuric acid accelerated aging solutions used had pH values of 1, 2, and 4. In an additional test, Nafion® membrane suspended in water was used for accelerated aging, to more closely correspond to a PEM fuel cell environment. The analysis showed that acid hydrolysis was the most likely mechanism of degradation and that similar degradation occurred under both real fuel cell and accelerated aging conditions. It was concluded that the accelerated aging test is a good one for rapidly screening materials for resistance to the acidic environment of the fuel cell. ©2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Corrosion behavior of SS316L in simulated and accelerated PEMFC environments

The electrochemical behavior and change in the passive film formation of SS316L are investigated under polymer electrolyte membrane fuel cell (PEMFC) simulated (pH from 3 to 6 containing F −, SO 4 2− and Cl − anions) and accelerated conditions (0.5 M and 1 M H 2SO 4 + 2 ppm HF). Potentiodynamic, potentiostatic, and EIS measurements are performed to investigate the electrochemical behavior of the SS316L specimens in both the anode and cathode PEMFC environments. The chemical composition of the passive film, surface topography of the specimens, and degree of metal ion release is characterized by XPS, SEM, and ICP-OES, respectively. The results reveal that the nature of the passive film depends on the pH value, external medium/environment, as well as applied potential during polarization. The corrosion behavior of SS316L is closely related to the chemical composition and structure of the passive film.

Dynamic mechanical characteristics of five elastomeric gasket materials aged in a simulated and an accelerated PEM fuel cell environment

Elastomeric materials are used as gaskets or seals in polymer electrolyte membrane (PEM) fuel cells and stacks. These gaskets/seals in PEM fuel cells are exposed to acidic, humid air, mechanical compressive pressure and cyclic temperature environment. Both the physical and chemical long-term stabilities of these gaskets/seals are therefore crucial to the overall performance of the fuel cell. Chemical degradation of five elastomeric gasket materials in a simulated and an aggressive accelerated fuel cell solution at 80 °C up to 63 weeks was investigated in this work using dynamic mechanical analysis (DMA) which assesses the change of dynamic mechanical properties of the five materials samples as they aged. The storage, loss modulus, and tan δ of the materials were presented that can reveal the glass transition temperature and other properties. The five materials tested are copolymeric resin (CR), liquid silicone rubber (LSR), fluorosilicone rubber (FSR), ethylene propylene diene monomer rubber (EPDM), and fluoroelastomer copolymer (FKM).

Characterization and performance of UNS S63019 (21-4N) as bipolar plate material in a simulated polymer electrolyte membrane fuel cell environment

The austenitic stainless steel UNS S63019 was evaluated regarding its potential as bipolar plate material in a polymer electrolyte membrane fuel cell (PEMFC) environment. Segregated grains of niobium carbide (NbC x ) were identified in polished cross-sections of the alloy, offering a possible pathway for enhanced electrical conductivity through the passive surface oxide. Additionally, the alloy was tested for corrosion resistance in a simulated PEMFC environment. It was considered that perhaps the elevated nitrogen concentration in the alloy would provide some benefit for corrosion resistance. Results for interfacial contact resistance (ICR) testing of the air-formed surface film on UNS S63019 showed decreased electrical conductivity as compared to UNS S30400. Niobium carbide particles did not improve film conductivity due to a non-conductive niobium oxide layer that formed on the surface. Corrosion resistance of the alloy was also poor as compared with UNS S30400, demonstrating that elevated nitrogen concentration in the alloy was not adequate in itself to enhance corrosion resistance. Poor corrosion resistance was attributed primarily to high carbon content in the alloy which combined with a significant amount of chromium to form carbides.

Chemical degradation of five elastomeric seal materials in a simulated and an accelerated PEM fuel cell environment

Polymer electrolyte membrane (PEM) fuel cell stack requires gaskets and seals in each cell to keep the hydrogen and air/oxygen within their respective regions. The stability of the gaskets/seals is critical to the operating life as well as the electrochemical performance of the fuel cell. Chemical degradation of five elastomeric gasket materials in a simulated and an aggressive accelerated fuel cell solution at PEM operating temperature for up to 63 weeks was investigated in this work. The five materials are copolymeric resin (CR), liquid silicone rubber (LSR), fluorosilicone rubber (FSR), ethylene propylene diene monomer rubber (EPDM), and fluoroelastomer copolymer (FKM). Using optical microscopy, topographical changes on the sample surface due to the acidic environment were revealed. Weight loss of the test samples was monitored. Atomic absorption spectrometer analysis was performed to study the silicon, calcium, and magnesium leachants from the materials into the soaking solution. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy was employed to study the surface chemistry of the materials before and after exposure to the simulated fuel cell environment over time. Among the five materials studied, CR and LSR in the accelerated solution are not as stable as the other three materials. FSR appears to be the most stable.

Corrosion behavior and electrical conductivity of niobium implanted 316L stainless steel used as bipolar plates in polymer electrolyte membrane fuel cells

The corrosion behavior and interfacial contact resistance (ICR) of niobium implanted SS316L used as the bipolar plate in a polymer electrolyte membrane fuel cell (PEMFC) are investigated. The ICR values of the bare and niobium implanted SS316L are measured to evaluate the electrical conductivity. The effects of ion implantation on the corrosion behavior are investigated by potentiodynamic and potentiostatic tests in the simulated PEMFC anode and cathode environments. The solutions after the potentiostatic test are analyzed by inductively-coupled plasma atomic emission spectrometry (ICP-AES). The surface topography of the samples before and after the potentiostatic test is monitored by SEM in order to investigate the mechanism and degree of corrosion. The XPS results indicate that the composition on the surface is altered by ion implantation. The electrochemical results reveal that the passivation current density of the Nb implanted SS316L decreases and has higher chemical stability in the simulated PEMFC environment. However, the ion implantation fluence affects the current density. The ICP results are in agreement with those of the electrochemical test disclosing that the bare SS316L has the highest dissolution rate in both the cathode and anode environments and niobium implantation reduces the dissolution rate significantly. SEM shows that the bare SS316L undergoes serious corrosion whereas after Nb ion implantation, corrosion is greatly retarded. The XPS depth profiles indicate that a passive film with a new composition consisting mainly of niobium oxide is formed after the potentiostatic test. Our results suggest that niobium implantation with proper ion fluences can significantly improve the corrosion resistance and the electric conductivity of SS316L in the simulated PEMFC environments.

Plasma Nitrided Type 349 Stainless Steel for Polymer Electrolyte Membrane Fuel Cell Bipolar Plate—Part I: Nitrided in Nitrogen Plasma

An austenite 349 stainless steel was nitrided via nitrogen plasma. Glancing angle X-ray diffraction patterns suggest that the nitrided layer is amorphous. X-ray photoelectron spectroscopy analysis indicated that the plasma nitridation process produced bulk-type nitrides in the surface layer. In general, the nitrided layer was composed of iron oxide in the outer layer and chromium oxide in the inner layers. Contaminations of vanadium and tin were detected in the as-grown nitrided layer; these dissolved away after polarization. The influence of these contaminants on the corrosion resistance of the nitrided layer in polymer electrolyte membrane fuel cell (PEMFC) environments is not considered significant. The nitrided sample had a much higher contact resistance than the bare one and the contact resistance increased with the nitriding time. The high interfacial contact resistance values can be related to the thicker oxide film after plasma nitridation. The corrosion resistances obtained for the 1 h nitrided and bare stainless steels in simulated PEMFC environments were similar. The outmost nitrided layer dissolved after polarization in the PEMFC environments leaving a passive film (modified with nitrides), similar to that of bare stainless steel under the same conditions. The passive film thickness was 3.7 nm for nitrided steel in PEMFC cathode environment and 4.2 nm for nitrided steel in PEMFC anode environment.

Plasma Nitrided Type 349 Stainless Steel for Polymer Electrolyte Membrane Fuel Cell Bipolar Plate—Part II: Nitrided in Ammonia Plasma

Austenitic 349 stainless steel was nitrided in an NH3 plasma. A low interfacial contact resistance was obtained with the nitrided steel. Glancing angle X-ray diffraction suggests that the nitrided layer is very thin and possibly amorphous. X-ray photoelectron spectroscopy (XPS) studies show that the nitrided layer is composed of mixed oxides and nitrides of Fe3+ and Cr3+. Contaminations of V and Sn were also observed, though their influence on the as-nitrided surface conductivity is not clear. The nitrided samples were investigated in a simulated polymer electrolyte membrane fuel cell (PEMFC) environment, and showed excellent corrosion resistance. The XPS depth profile indicated that the passive film, which formed on the plasma-nitrided steel in the PEMFC anode environment, is composed of mixed oxides and nitrides, in which chromium oxide/nitride dominates the surface chemistry. No V or Sn was detected on the surface after the polarization tests. For the PEMFC bipolar plate application, nitridation in NH3 plasma is a promising surface treatment approach, though more research is needed to investigate the influence of the plasma density and substrate bias on the surface conductivity and performance of the nitrided steel in PEMFC environments.

Microindentation Test for Assessing the Mechanical Properties of Silicone Rubber Exposed to a Simulated Polymer Electrolyte Membrane Fuel Cell Environment

The elastomeric materials used as seals and gaskets in polymer electrolyte membrane (PEM) fuel cells are exposed to acidic environment, humid air, and hydrogen, and subjected to mechanical compressive load. The long-term mechanical and chemical stability of these materials is critical to both sealing and the electrochemical performance of the fuel cell. In this paper, mechanical degradation of two elastomeric materials, Silicone S and Silicone G, which are potential gasket materials for PEM fuel cells, was investigated. Test samples were subjected to various compressive loads to simulate the actual loading in addition to soaking in a simulated PEM fuel cell environment. Two temperatures, 80°C and 60°C, were selected and used in this study. Mechanical properties of the samples before and after exposure to the environment were studied by microindentation. Indentation load, elastic modulus, and hardness were obtained from the loading and unloading curves. Indentation deformation was studied using Hertz contact model. Dynamic mechanical analysis was conducted to verify the elastic modulus obtained by Hertz contact model. It was found that the mechanical properties of the samples changed considerably after exposure to the simulated environment over time. The temperature and the applied compressive load play a significant role in the mechanical degradation. The microindentation method is proved to provide a simple and efficient way to evaluate the mechanical properties of gasket materials.