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Abstract The effect of the presence of a microporous layer on the propensity for corrosion of metallic bipolar plates in an operating polymer electrolyte membrane fuel cell is investigated using an in situ reference electrode array. The local potential at the surface of the cathode bipolar plate is significantly more negative in the presence of the microporous layer, which is attributed to the higher ionic resistance of the aqueous phase in the reactant transport layer associated with more effective removal of water from the catalyst layer/reactant transport layer interface. As a result the bipolar plate is effectively shielded from elevated potentials that may be present at the cathode electrode, even during start-up and shutdown of the cell. Revision of ex situ test protocols for candidate bipolar plate materials, surface treatments and coatings is recommended to reduce unnecessary conservatism in testing. Highlights • In situ mapping of local potential at a fuel cell metallic bipolar plate. • Demonstrated beneficial effect of MPL on corrosion of metallic bipolar plates. • More effective water removal increases ionic resistance in reactant transport layer. [ABSTRACT FROM AUTHOR]

Abstract This paper presents a novel method of structural optimization by using graphite particles ranging in size from 35 to 500 μm to fabricate conductive fluorinated ethylene–propylene composites for high-temperature bipolar plates. To investigate the effects of dispersion and packing density, the large graphite particles were decorated with fluorinated ethylene–propylene powders by ball milling, and the master batch of well-dispersed small graphite particles and polymer master batch was mixed with large graphite particles. The resulting fluorinated ethylene–propylene/graphite composite bipolar plates, which contained 65 wt% graphite, exhibited high electrical conductivity of 550 S cm−1. In particular, by modulating the electrical transportation paths between the large graphite particles with the well-dispersed fluorinated ethylene–propylene/graphite master batch, the orientation and dispersion of the graphite particles in the matrix resulted in enhanced electrical conductivity and mechanical properties. The preparation of structurally optimized fluorinated ethylene–propylene/graphite composite bipolar plates with well-dispersed graphite particles of different sizes provides a robust and scalable strategy for realizing high-performance and large-area bipolar plates. Graphical abstract Image 1 Highlights • FEP/graphite composites were prepared by melt-processing from a master batch. • Graphite particles of different sizes were dispersed in the master batch. • FEP/graphite composite bipolar plates (BPs) were prepared by compression molding. • Structurally optimized BPs had high electrical conductivity and mechanical properties. • The composites represent a robust and scalable strategy for large-area BPs. [ABSTRACT FROM AUTHOR]

The performance of polymer electrolyte membrane fuel cell (PEMFC) greatly depends on the properties of the components. Bipolar plate is one of the key components of PEMFC and the properties of the plates on fuel cell performance are critical. Electrical conductivity and surface roughness of the bipolar plate appear very important properties to minimize ohmic resistance and optimize water management, respectively. The composite bipolar plates having conductive fillers and thermosetting resin binder are produced by compression molding. Response Surface Methodology (RSM) has been applied to optimize the production conditions of bipolar plate. Electrical conductivity, physical appearance and roughness are chosen as response parameters and molding temperature, pressure and time are chosen as independent parameters. The main challenges of this study are observation of the individual and combined effects of these parameters on bipolar plate properties by RSM. The optimization of the production conditions of polymer composite plate is carried out by aiming maximum electrical conductivity and minimum time. The maximum electrical conductivity of 107.4 Scm −1 is obtained at temperature of 187 °C, pressure of 119 bar and time of 5 min. It is found that the polymer composite plates produced by the compression molding process at minimum time satisfied the electrical conductivity target of Department of Energy (DoE). [ABSTRACT FROM AUTHOR]

The vanadium redox flow battery (VRFB) is a promising electrochemical storage system for stationary megawatt-class applications. The currently limited cell area determined by the bipolar plate (BPP) could be enlarged significantly with a novel extruded large-area plate. For the first time a techno-economic assessment of VRFB in a power range of 1 MW–20 MW and energy capacities of up to 160 MWh is presented on the basis of the production cost model of large-area BPP. The economic model is based on the configuration of a 250 kW stack and the overall system including stacks, power electronics, electrolyte and auxiliaries. Final results include a simple function for the calculation of system costs within the above described scope. In addition, the impact of cost reduction potentials for key components (membrane, electrode, BPP, vanadium electrolyte) on stack and system costs is quantified and validated. [ABSTRACT FROM AUTHOR]

Bipolar plates are one of the most important parts of proton exchange membrane fuel cells (PEMFCs) which their performance has appreciable effect on cell operation. In this study, Ni–Mo and Ni–Mo–P alloy coatings were electrodeposited on AISI 304 stainless steel specimens as metallic bipolar plates in order to improve corrosion resistance, hydrophobicity and electrical conductivity of the specimens. Scanning electron microscopy (SEM) micrographs showed that the prepared coatings were uniform and compact. Moreover, X-ray diffraction (XRD) analysis indicated that the coatings were composed of a solid solution where Ni–Mo and Ni–Mo–P coatings had nanocrystalline and amorphous structures, respectively. The potentiodynamic and potentiostatic polarization tests in a simulated PEMFC cathode environment showed that corrosion resistance of the coated specimens was considerably improved compared to that of the bare one. Furthermore, interfacial contact resistance (ICR) before and after potentiostatic test was significantly decreased by applying Ni–Mo and Ni–Mo–P coatings, where ICR values of the as-deposited bipolar plates were reduced by approximately eight times in comparison to that of the bare 304 stainless steel at typical compaction force of 220 N/cm 2 . Therefore, the investigated Ni–Mo and Ni–Mo–P coatings would be potentially good candidates as metallic bipolar plates in PEMFC. Moreover, Ni–Mo and Ni–Mo–P alloy coatings showed higher surface hydrophobicity than the bare 304 stainless steel, which is advantageous for water management and also for corrosion resistance improvement. [ABSTRACT FROM AUTHOR]

In this study, the effect of a high temperature nitriding treatment on the characteristics of the Ni-free stainless steel (SUS445) used as the metallic bipolar plate of a polymer electrolyte fuel cell (PEFC) was investigated. The phase structure, composition, electrical conductivity and electrochemical corrosion resistance of the stainless steel after the nitriding treatment (SUS445-N) were analyzed. The supersaturated nitrogen phase γ N , nitrides CrN and Cr 2 N in the nitride layer with a good electrical conductivity were observed and confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The corrosion resistance of the nitrided SUS445-N stainless steel was significantly improved due to the formation of the chromium-containing nitrides. The conducted fuel cell durability test revealed that when using the nitrided SUS445-N stainless steel as the bipolar plate, the fuel cell voltage performance was almost the same as that using the graphite carbon as the bipolar plate for 2500 h. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced polymer composites are ideal substitutes for brittle graphite for electrically conductive bipolar plates because of their high mechanical property and productivity. Early composite bipolar plates suffered from high electrical contact resistance due to the resin-rich area formed on the surface. To decrease the electrical contact resistance, the “soft layer method” was developed, which exposed bare carbon fibers on the composite surface. This method significantly improved the electrical as well as mechanical properties of composite bipolar plates. The remaining problem for wider commercialization of composite bipolar plates is their high fabrication pressure. In this work, a non-woven carbon felt composite bipolar plate was developed to obtain improved electrical properties with low fabrication pressure. The soft layer method was also used to expose bare carbon fibers. Compared to conventional high fiber volume fraction composite bipolar plates, the developed bipolar plate exhibited superior electrical properties. [ABSTRACT FROM AUTHOR]

Stainless steels, as low cost materials, are attractive for proton exchange membrane fuel cells (PEMFC) bipolar plates. However, these metallic alloys require surface coatings or treatments to enhance its corrosion resistance and surface conductivity in PEMFC environments. In this study, response surface methodology based on Box–Behnken design was employed to investigate the influence of time, activator content and temperature on corrosion current density of pack chromised 304 stainless steel in simulated PEMFC environment of aerated 0.5 M H 2 SO 4 + 2 ppm HF at 70 °C. These process parameters were optimised and the performance of the optimised coatings in simulated and real PEMFC environments was investigated. The results indicated that temperature had the most significant influence on the performance of chromised coatings in the selected PEMFC environment. The optimised coating produced at 1040 °C for 3 hours with powder containing 6.84 wt% activator content exhibited better corrosion resistance than the substrate in typical PEMFC cathode and anode environments respectively as well as about six fold decrease in the contact resistance of the substrate at 150 N/cm 2 . Hence, the single fuel cell assembled with the bipolar plate coated with the optimised process parameters exhibited a two-fold increase in the maximum power density of the cell with the uncoated bipolar plates. [ABSTRACT FROM AUTHOR]

The corrosion behavior and interfacial contact resistance (ICR) of Mg–Nd–Zn–Zr magnesium alloy in simulated PEMFC environments are investigated systematically. The corrosion rate decreases logarithmically with the immersion time due to accumulation of corrosion products. At first, Mg–Nd–Zn–Zr magnesium alloy exhibits lower corrosion rate in anodic environment because the reactions involving formation of hydrogen gas and dissolution of magnesium are inhibited. After long time immersion, it presents higher corrosion rate because of pitting corrosion and cracked corrosion products layer. In the cathodic environment, a compact corrosion products layer composed of amorphous MgCO 3 and Mg(OH) 2 is formed as a result of precipitation of carbonate-containing corrosion products. The ICR of Mg–Nd–Zn–Zr magnesium alloy immersed in cathodic environment is higher than that in anodic one due to the accumulation of amorphous Mg(OH) 2 and MgCO 3 corrosion products layer. [ABSTRACT FROM AUTHOR]

Insufficient corrosion resistance and surface conductivity are two main issues that plague large-scale application of stainless steel (SS) bipolar plates in proton exchange membrane fuel cells (PEMFCs). This study explores the use of nanocrystalline Ta/TaN multilayer coatings to improve the electrical and electrochemical performance of polished 316L SS bipolar plates. The multilayer coatings have been deposited by (reactive) magnetron sputtering and characterized by X-ray diffraction, field-emission scanning electron microscopy and transmission electron microscopy. The electrochemical behavior of bare and coated substrates has been evaluated in simulated PEMFC working environments by potentiodynamic and potentiostatic polarization tests at ambient temperature and 80 °C. The results show that the Ta/TaN multilayer coating increases the polarization resistance of 316L SS by about 30 and 10 4 times at ambient and elevated temperatures, respectively. The interfacial contact resistance (ICR) shows a low value of 12 mΩ × cm 2 before the potentiostatic test. This ICR is significantly lower than for the bare substrate and remains mostly unchanged after potentiostatic polarization for 14 h. In addition, the high contact angle (92°) with water for coated substrates indicates a hydrophobic character, which can improve the water management within the cell in PEMFC stacks. [ABSTRACT FROM AUTHOR]

Two layers of AlN–TiN coating were deposited onto SS316L as PEM bipolar plate using plasma focus device for 5, 10 and 20 shots number. The XRD patterns of all coated samples reveal formation of TiN, Ti 2 N and AlTi 3 N crystallite phases on the surface. Using the Scherrer formula the crystallite size of the coating layer is estimated about 20–40 nm. Thickness of the two layers coating obtained from cross sectional SEM of the samples is in the range of 5–15 μm. Through the potentiostatic test considerable decrease of the current density of the 20 shots coated samples is achieved. The ICR of the samples reduced as follows: 20 shots < 10 shots < 5 shots < bare sample which confirms that the electrical conductivity of the bipolar plate is increased due to the formation of AlN and TiN coating. SEM morphology of all samples before and after potentiostatic tests shows that the coated samples are exposed to less corrosion compare with the bare sample. These results are in good agreement with electrochemical tests and the 20 shots sample has the lowest corrosion. [ABSTRACT FROM AUTHOR]

The carbon/epoxy composite bipolar plate is an ideal substitute for the brittle graphite bipolar plate for proton exchange membrane fuel cells (PEMFCs) and vanadium redox flow batteries (VRFBs) because of its high mechanical properties and easy manufacturing. However, due to the resin-rich area that forms on its surface, the carbon/epoxy composite bipolar plate requires an expanded graphite coating to decrease the areal specific resistance (ASR). The expanded graphite coating not only increases the manufacturing costs but also has very low mechanical properties. In this work, an innovative manufacturing method that exposes carbon fibers on the surface of the carbon/epoxy composite bipolar plate was developed. Soft release films were inserted between the mold and the composite to prevent the formation of a resin-rich area and to expose carbon fibers on the surface of the bipolar plate. The developed method considerably decreased the ASR of the carbon composite bipolar plate without an expanded graphite coating, which satisfied the target established by the Department of Energy (DOE) of United States. The effects of the soft layer on the mechanical and electrical properties of the carbon composite bipolar plate were investigated. [ABSTRACT FROM AUTHOR]

In order to improve the interfacial contact resistance (ICR) and corrosion resistance of STS316 bipolar plates of PEMFCs, after surface pretreatment at mild plasma condition, the fluorine-doped tin oxides (FTO) thin films were deposited by electron cyclotron resonance-metal organic chemical vapor deposition. The mild plasma pre-treatment of the bipolar plate substrate before its coating with the FTO film has rendered a higher efficiency to the pertinent fuel cell, which can be attributed to a smooth charge transfer at the interface of the plate and the film due the removal of fine impurities on the substrate that could not be done just by washing. The deposition of FTO as a protect layer of fuel cell electrode after mild plasma pre-treatment improved the electrochemical stability. In our experimental range, with the increases in microwave power up to 1000 W, both of the corrosion resistance and ICR significantly decreased, however, those values sharply increased above 1100 W due to the crack and unstable film formation with the void space. Selecting the optimized value of the microwave power appeared to be the critical process step on the formation of the corrosion-protective layer for bipolar plates. [ABSTRACT FROM AUTHOR]

In this paper, corrosion behavior of an AISI 304 stainless steel modified by niobium or niobium nitride (denoted as niobized 304 SS and - 304 SS, respectively) is investigated in simulated solid polymer fuel cell (SPFC) operating conditions. Potentiodynamic polarizations show that the corrosion potentials of surface modified 304 SS shift to positive direction while the corrosion current densities decrease greatly comparing with the bare 304 SS in simulated anodic SPFC environments. The order of corrosive resistance in corrosive potential, corrosive current density and pitting potential is: - 304 SS > niobized 304 SS > bare 304 SS. In the methanol-fueled SPFC operating conditions, the results show that the corrosion resistance of bare and niobized 304 SS increases with the methanol concentration increasing in the test solutions. [ABSTRACT FROM AUTHOR]

The high temperature proton exchange membrane fuel cell (HT-PEMFC) operating from 120 °C to 220 °C is able to overcome many technical issues related to the low-temperature PEMFC (LT-PEMFC), such as their complex water/thermal management to prevent flooding/drying out of the membrane and their low tolerance to carbon monoxide. In a PEMFC stack, the bipolar plates are a major component, contributing 50% of the total cost and 80% of the total weight. In this study, a composite bipolar plate was developed using resole type phenolic and continuous carbon fiber to improve the high-temperature performance (∼220 °C) for use in HT-PEMFC applications. To improve the mechanical and electrical properties, the nano-size carbon black was mixed with phenolic resin. A pre-cure process using hot rolling was developed to achieve partial wetting of the carbon fiber felt on the carbon/phenol composite bipolar plate surface. The randomly oriented carbon fiber felt was co-cure bonded with the pre-cure process to modify the surface of the composite bipolar plates by exposing the bare carbon fibers on the outer surface, which reduced the ASR (areal specific resistance). The developed composite bipolar plate satisfied the DOE technical targets. [ABSTRACT FROM AUTHOR]

The crossover effect of the land/channel width ratio of bipolar plates in polymer electrolyte membrane fuel cells is experimentally investigated in this study. To isolate the effect of the land/channel width ratio, three different types of bipolar plates of a fixed sum and channel width are specially prepared. With three different bipolar plates, measurements are taken of electrochemical performance, inlet pressure, and hydrogen crossover rate. When the stoichiometric ratio of hydrogen is 1.5, the standard type of bipolar plate, BP2 (land width = 0.75 mm, channel width = 1.05 mm) show the best performance. However, according to increasing stoichiometric ratio of hydrogen, BP3 (land width = 1.12 mm, channel width = 0.68 mm) has the best performance, especially at the medium and high current range. For the crossover rate, the biggest amount of hydrogen gas crossover to the cathode in BP3. This is because of the anode inlet pressure caused by the largest land/channel ratio of BP3. [ABSTRACT FROM AUTHOR]

Electrochemical behaviors of bulk metallic glass Zr55Cu30Al10Ni5 are investigated in the solution of 0.05M H2 SO4 + 2 ppm F-, and the effect of potentiostatic polarization on interfacial contact resistance (ICR) between Zr55Cu30Al10Ni5 and carbon paper is evaluated. Zr55Cu30Al10Ni5 undergoes passivation, and passive film is very stable under anode or cathode operation condition. Only general corrosion takes place on the surface of Zr55Cu30Al10Ni5 during potentiostatic polarization at applied potential. The ICR of Zr55Cu30Al10Ni5 is around 8 mΩ cm² at compaction force of 150 N cm-2, and the increased ICR results from the formation of surface passive film. The bulk metallic glass Zr55Cu30Al10Ni5 is a potential material using as bipolar plate for proton exchange membrane fuel cell (PEMFC) stacks. [ABSTRACT FROM AUTHOR]

Abstract: In this study, synthetic graphite and carbon nanotube (CNT) filled polyphenylene sulfide (PPS) based bipolar plates are produced by using co-rotating twin-screw extruder and injection molding. Graphite is the main conductive filler and CNTs are used as bridging filler between graphite particles. To improve the dispersion of the fillers and the flow behavior of the composite, titanate coupling agent (KR-TTS) is used. The concentration effect of CNTs and coupling agent on the properties of bipolar plates are examined. At 72.5 wt.% total conductive filler concentration, by addition of 2.5 wt.% CNT and 3 wt.% KR-TTS; through-plane and in-plane electrical conductivities increase from 1.42 S cm−1 to 20 S cm−1 and 6.4 S cm−1 to 57.3 S cm−1 respectively compared to sample without CNTs and additive. Extruder torque value and apparent viscosity of samples decrease significantly with coupling agent and as a result; the flow behavior is positively affected. Flexural strength is improved 15% by addition of 1.25 wt.% CNT. Differential scanning calorimeter (DSC) analysis shows nucleating effect of conductive fillers on PPS matrix. Corrosion measurements, cyclic voltammetry and galvanostatic charge–discharge tests are performed to examine the electrochemical stability and the performance of produced bipolar plates in all-vanadium redox flow battery. [Copyright &y& Elsevier]

Abstract: Bipolar plates are one of the most important components in PEMFC stack and have multiple functions, such as separators and current collectors, distributing reactions uniformly, and etc. Stainless steel is ideal candidate for bipolar plates owing to good thermal and electrical conductivity, good mechanical properties etc. However, stainless steel plate still cannot resist the corrosion of working condition. In this work, ternary Ti–Mo–N film was fabricated on 316L stainless steel (SS316L) as a surface modification layer to enhance the corrosion resistance. Effects of Mo content on the microstructure and corrosion resistance of Ti–Mo–N films are systematically investigated by altering sputtering current of the Mo target. XRD results reveal that the preferred orientation changes from [111] to [220] direction as Mo content in the film increases. The synthesized Ti–Mo–N films form a substitutional solid solution of (Ti, Mo)N where larger Mo atoms replace Ti in TiN crystal lattice. The TiN-coated SS316L sample shows the best corrosion resistance. While Mo content in the Ti–Mo–N films increases, the corrosion resistance gradually degrades. Compared with the uncoated samples, all the Ti–Mo–N film coated samples show enhanced corrosion resistance in simulated PEMFC working condition. [Copyright &y& Elsevier]

Abstract: The corrosion behaviour of an AISI 304 stainless steel (304 SS) is investigated in aqueous acid methanol solutions (0.5 M H2SO4 + 2 ppm HF + x M CH3OH, x = 0, 1, 5, 10 and 20) at 50 °C to simulate the varied anodic operating conditions of direct methanol fuel cells. Electrochemical measurements including potentiodynamic polarisation, potentiostatic polarisation and electrochemical impedance spectroscopy tests, are employed to analyse the corrosion behaviour. The results reveal that the corrosion resistance of 304 SS is enhanced in solutions with higher methanol content. Scanning electron microscopy and inductively coupled plasma atomic emission spectrometry data indicate that the surface corrosion on 304 SS is alleviated when the methanol concentration is increased. According to the X-ray photoelectron spectroscopy and Mott–Schottky analyses, the passive films formed on the 304 SS after potentiostatic tests in all the test solutions are composed of a duplex electronic structure with an external n-type semiconductor layer and an internal p-type semiconductor layer. Further analyses of the surface conductivity conducted by measuring the interfacial contact resistance between the 304 SS and carbon paper reveal that the passive film formed in the solution with higher methanol content exhibits lower conductivity. [Copyright &y& Elsevier]

Abstract: A niobium carbide diffusion layer with a cubic NbC phase surface layer (∼6 μm) and a Nb and C diffusion subsurface layer (∼1 μm) is fabricated on the surface of AISI 304 stainless steel (304 SS) bipolar plate in a proton exchange membrane fuel cell (PEMFC) using plasma surface diffusion alloying. The electrochemical behaviour of the niobium carbide diffusion-modified 304 SS (Nb–C 304 SS) is investigated in simulated PEMFC environments (0.5 M H2SO4 and 2 ppm HF solution at 80 °C). Potentiodynamic, potentiostatic polarisation and electrochemical impedance spectroscopy measurements reveal that the niobium carbide diffusion layer considerably improves the corrosion resistance of 304 SS compared with untreated samples. The corrosion current density of Nb–C 304 SS is maintained at 0.058 μA cm−2 and 0.051 μA cm−2 under simulated anodic and cathodic conditions, respectively. The interfacial contact resistance of Nb–C 304 SS is 8.47 mΩ cm2 at a compaction force of 140 N cm−2, which is significantly lower than that of the untreated sample (100.98 mΩ cm2). Moreover, only a minor increase in the ICR of Nb–C 304 SS occurs after 10 h potentiostatic tests in both cathodic and anodic environments. [Copyright &y& Elsevier]

Abstract: Metallic bipolar plate (BPP) with high corrosion and low contact resistance, durability, strength, low cost, volume, and weight requirements is one of the critical parts of the PEMFC. This study is dedicated to understand the effect of the process sequence (manufacturing then coating vs. coating then manufacturing) on the corrosion resistance of coated metallic bipolar plates. To this goal, three different PVD coatings (titanium nitride (TiN), chromium nitride (CrN), zirconium nitride (ZrN)), with three thicknesses, (0.1, 0.5, 1 μm) were applied on BPPs made of 316L stainless steel alloy before and after two types of manufacturing (i.e., stamping or hydroforming). Corrosion test results indicated that ZrN coating exhibited the best corrosion protection while the performance of TiN coating was the lowest among the tested coatings and thicknesses. For most of the cases tested, in which coating was applied before manufacturing, occurrence of corrosion was found to be more profound than the case where coating was applied after manufacturing. Increasing the coating thickness was found to improve the corrosion resistance. It was also revealed that hydroformed BPPs performed slightly better than stamped BPPs in terms of the corrosion behavior. [Copyright &y& Elsevier]

SUMMARY In this research, a novel woven nanocomposite was manufactured as bipolar plate of proton exchange membrane fuel cell. The developed composite includes 40 wt.% phenolic resin as binder and 45 wt.% graphite, 10 wt.% nanosheet expanded graphite, and 5 wt.% carbon fiber as fillers, as well as a woven carbon fiber cloth in the middle. A single-cell assembly was manufactured by using the developed composite bipolar plate. Subsequently, the performance and power density curves were obtained from the developed cell. The results showed that the cell performs the maximum power density as high as 812 mW/cm2 and current density 900 mA/cm2 at 0.6 V that are better than the cells prepared by the metallic and some commercial bipolar plates. In this research, in order to achieve the best cell performance, some of the operation factors containing the cell body temperature, oxygen inlet temperature, and hydrogen inlet temperature, as well as back pressure were optimized, and the results were properly discussed. Last, in order to select a suitable material for bipolar plate, using simple additive weighting method approach, a material selection was performed upon several alternatives including composite, graphite, SS316L, SS304, Hastelloy, Al, and Ti, which are currently being used for bipolar plate. The result of material selection specified that Al/CrN is the most suitable candidate to be used for bipolar plates. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: The bipolar plate is an important component of the PEMFC (polymer electrolyte membrane fuel cell) because it supplies the pathway of electron flow between each unit cell. Fe–Ni–Cr alloy is considered as a good candidate material for bipolar plate, but it is limited to use as a bipolar plate due to its high ICR (interfacial contact resistance) and corrosion problem. In order to explore a cost-effective method on surface modification, various chemical and electrochemical treatments are performed on Fe–Ni–Cr alloy to acquire the effect of the surface modification on the ICR and corrosion behavior. The ICR and corrosion resistance of Fe–Ni–Cr alloy can be effectively controlled by the chemical treatment of immersion in the mixed acid solution with 10 vol% HNO3, 2 vol% HCl and 1 vol% HF for 10 min at 65 °C and then was placed in 30 vol% HNO3 solution for 5 min. The chemical treatment is more effective on reducing ICR and improving corrosion resistance than that of electrochemical methods (be carried out in the 2 mol/L H2SO4 solution with the electrical potential from −0.4 V to 0.6 V) for Fe–Ni–Cr alloy as a bipolar plate for polymer electrolyte membrane fuel cells. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to prepare a polymer-based carbon nanocomposite reinforced by carbon fiber cloth (CF) to be utilized as bipolar plate of proton exchange membrane (PEM) fuel cell. For this purpose, some single, double, and triple-filler composites were manufactured by using phenolic resin as polymer (P) and graphite (G), carbon fiber (CF) and expanded graphite (EG) as fillers. The production method was compression-molding technique. The electrical conductivity, flexural strength, toughness, hardness, porosity, and hydrogen permeability tests were then measured to determine the mechanical and physical properties. A triple-filler composite containing 45 wt.% G, 10 wt.% CF, 5 wt.% EG, reinforced by a layer of CF cloth, was selected as composite bipolar plate. The electrical conductivity, thermal conductivity, and flexural strength of this composite were 74 S/cm, 9.6W/mK, and 74MPa, respectively, which are higher than the specified value by department of energy in USA (DOE). The composite bipolar plate used in the single fuel cell assembly showed a maximum power density 810mW/cm2. In this paper, a material selection was performed on the different materials of bipolar plates. It can be concluded that the composite bipolar plates are more suitable for high life time stationary applications. [Copyright &y& Elsevier]

Abstract: Aluminum has many advantages for commercial bipolar plate of PEM fuel cell such as light weight, low cost and easy manufacturing. However, it has a low corrosion resistance under a PEM fuel cell operation condition that is a special issue of all metal bipolar plates. In this study, polypropylene composite coated with aluminum bipolar plates were fabricated to improve the corrosion resistance. However, contact resistance of polymer composite coated aluminum bipolar plate is highly increased due to high contact resistance between aluminum substrate and composite layer. Two different types of inter layers were added to improve the contact resistance. Carbon paper attached and carbon black added samples were fabricated between aluminum substrate and composite. Polyamide-imide/carbon black composite adhesive was used for carbon paper attached on the aluminum plate. The contact resistance of carbon paper attached sample was lower than that of carbon black added sample. And, corrosion resistance was tested by potentiodynamic and potentiostatic methods. The composite coated aluminum attached to carbon paper exhibited properties suitable for PEM fuel cells. [Copyright &y& Elsevier]

Abstract: A manufacturing and single-cell fuel cell performance study of stamped, laser welded, and gas nitrided ferritic stainless steel foils in an advanced automotive bipolar plate assembly design was performed. Two developmental foil compositions were studied: Fe–20Cr–4V and Fe–23Cr–4V wt.%. Foils 0.1mm thick were stamped and then laser welded together to create single bipolar plate assemblies with cooling channels. The plates were then surface treated by pre-oxidation and nitridation in N2–4H2 based gas mixtures using either a conventional furnace or a short-cycle quartz lamp infrared heating system. Single-cell fuel cell testing was performed at 80°C for 500h at 0.3A/cm2 using 100% humidification and a 100%/40% humidification cycle that stresses the membrane and enhances release of the fluoride ion and promotes a more corrosive environment for the bipolar plates. Periodic high frequency resistance potential-current scans during the 500h fuel cell test and post-test analysis of the membrane indicated no resistance increase of the plates and only trace levels of metal ion contamination. [Copyright &y& Elsevier]

Abstract: Proton exchange membrane fuel cells (PEMFCs) is a promising candidate as energy systems. However, the stability and lifetime of cells are still important issues. The effect of surface roughness on metallic bipolar plate is discussed in this paper. Various roughness on the bulk surface are obtained by the sandblasting method. The grain sizes of sand are selected as 50, 100 and 200μm. The Ac impedance experiment results show that the bipolar plate roughness and carbon paper porosity are well matched when the surface roughness is within 1–2μm. Superior condition decreases the contact resistance loss in the fuel cell. The high frequency resistance of the coarse surface was larger than that of the substrate by around 5mΩ. Furthermore, a new arc was formed at the low frequency region. Hence, the unmatch roughness condition of the bipolar plate significantly increases the contact resistance and mass transfer resistance. This paper develops a sequential approach to study an optimum surface roughness by combining the whole performance (I–V) curve and AC impedance result. It benefits us to quantify the contact and mass transfer resistance exists in the PEMFC. The proposed surface treatment improves the surface effect and promotes the implement of potential metallic bipolar plate in near future. [Copyright &y& Elsevier]

Abstract: The bipolar plates have several functions in Proton Exchange Membrane Fuel Cell (PEMFC); separating flow channels between gases, connecting electrical paths between cells in series and providing structural supports for the stack. Although the carbon fiber epoxy composite bipolar plates recently developed, have high specific stiffness and strength, they have relatively high electrical resistance compared to graphite bipolar plates due to their high surface electrical contact resistance, which requires additional surface treatment such as graphite layer coating. In this research, a new manufacturing method for the carbon fabric/graphite hybrid bipolar plate which consists of two carbon fiber fabrics and an expanded graphite foil liner between them, has been developed in order to increase the productivity of PEMFC and reduce its manufacturing cost by exposing bare carbon fabrics to decrease the surface electrical contact resistance of bipolar plate. Phenolic resin was chosen rather than epoxy resin for the matrix material to decrease both material cost and manufacturing time, and the optimum areal density of phenolic resin was determined experimentally. Also the corrosion resistance, electrical contact resistance and mechanical properties of the hybrid bipolar plates were measured. [Copyright &y& Elsevier]

SUMMARY Bipolar plates are responsible for functions of vital importance to the long-term performance of fuel cells. They play crucial roles in water and gas management, mechanical strength and electrical conductivity. It also significantly contributes to the volume, weight and cost of fuel cell stacks. The properties of bipolar plates are affected by the materials and processes used in the manufacturing of the plates. The objective of this article is to review the use of thermoplastic materials as polymer matrices in bipolar plate applications. Conductive composites consisting of different types and blends of thermoplastic polymers are detailed discussed. The effects of filler types and processing conditions are given. Several thermoplastic blends consisting of carbon black, carbon nanotube and graphite are evaluated. The dispersion of conductive fillers, in particular, polymer composites and polymer blend composites is also given. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Bipolar plates are one of the crucial components of proton exchange membrane fuel cells. Because of the expensive production costs of traditional graphite bipolar plates, which require a few millimeters thickness over the space, the resulting metal bipolar plate reduces the cost, and the thickness can be reduced to the micron range. This study explored the application of micro-stamping technology to produce thin metal bipolar plates with the relevant process parameters. In this study, the channel design was 0.8 ×0.75 mm for the use of a rigid punch on a 50-μm-thick stainless steel sheet (SUS 304) for micro-channel stamping processes. The finite element method and the experimental results were used to analyze the main parameters of the micro-stamping process. The traditional material model and the scale factor modified material model were used for simulation. The experimental results verified that the modified material model is more realistic for products and has superior similarity because the punch load is relatively small. This study used updated Lagrangian formulation concept to establish an elastic-plastic deformation finite element analysis model and scale factor to modify the calculation to effectively simulate the micro-stamping process for metal bipolar plates. [ABSTRACT FROM AUTHOR]

Abstract: Polymer electrolyte membrane fuel cell or proton exchange membrane fuel cell (PEMFC) is composed of bipolar plates, end plates, membrane electrode assemblies (MEAs) and gas diffusion layers (GDLs). Among the constituents of PEMFCs, the bipolar plates are major components, which make up a large portion of the stack volume and cost. In this study, the bipolar plate has been fabricated using carbon fabric/phenolic composite for mass production because the phenolic resin cures much faster than epoxy resin. Because the phenolic resin becomes brittle after being fully cured, it is mixed with carbon black to increase the ductility. The surface of the carbon fabric/phenolic composite bipolar plates has been treated with flame to reduce the interfacial contact resistance. The experimental results show that the contact resistance of the carbon fabric/phenolic composites bipolar plate decreases as the flame-treatment temperature and time increase due to the degree of carbonization of the phenolic resin, while the flexural strength decreases because the carbonization of phenolic resin increases its brittleness. Therefore, the optimum flame surface treatment temperature and processing time for the carbon fabric/phenolic composite bipolar plates are investigated to optimize the electrical conductivities and mechanical properties. [Copyright &y& Elsevier]

A bipolar plate is one of the crucial components that strongly influence the power density of the proton exchange membrane fuel cell stack. Especially, an aluminium bipolar plate not only has better mechanical properties such as density, electrical resistivity and thermal conductivity but also has more corrosion resistance than bipolar plates made of stainless steel and graphite composites. Furthermore, the use of aluminium reduces the cost and weight of the plates, and the plates are easily machinable. This study presents the results of experiments conducted to investigate the feasibility of developing bipolar plates made of aluminium that consist of microchannels. This paper explains experimental investigations performed to investigate the formability of an aluminium sheet. The effects of forming velocity and die temperature on the microchannels in thin sheets of Al5052 having a thickness of 0.3 mm are investigated. The roundness of the core-die channels ranged from 0.3 to 0.1 mm, and the height is 0.6 mm. Formability tests are conducted for forming velocities of 0.1 and 1.0 mm/s and die temperatures in the range from room temperature to 300°C. The experimental results indicate the feasibility of the proposed technique for manufacturing the bipolar plates of aluminium. [ABSTRACT FROM AUTHOR]

Abstract: Metallic PEM fuel cell bipolar plate (BPP) materials such as stainless steel typically suffer from insufficient corrosion resistance and/or interfacial contact resistance (ICR). Therefore, coatings with both high corrosion resistance and low ICR are required. Closed field unbalanced magnetron sputter ion plating (CFUBMSIP) was used to produce dense and well adhered TiN, TiN+C and Graphit-iC™ coatings on an AISI316L foil substrate. The ICR of the coatings was measured before and after a potentiostatic measurement at a cathodic standby PEMFC potential of 1V/SHE. Following the corrosion measurement only the TiN+C, Graphit-iC™ and 10nm Au coatings met the ICR and corrosion US DoE targets of <10mΩcm2 and <1μAcm−2. Potentiodynamic corrosion measurements of the applied coatings were carried out under the same conditions. The multilayer coating TiN+C showed improved corrosion resistance over single layer coatings of TiN and Graphit-iC™. Multilayer coatings and 10nm Au coatings were identified as the best candidates for future research. [Copyright &y& Elsevier]

Abstract: The bipolar plates of proton exchange membrane (PEM) fuel cell systems serve many functions, such as electrically connecting cells in series, separating gases in adjacent cells and providing structural support for the stack and flow channels of water and gases. Although carbon fiber composite bipolar plates have high strength and stiffness as well as corrosion resistance in acidic environments, they do not yield a sufficiently high electrical conductance, which compromises the efficiency of PEM fuel cells. To increase the electrical conductance of the bipolar plate, a composite hybrid bipolar plate with bypass-connected gas diffusion layers (GDLs) has been developed. The graphite foil coating method, which was developed in preceded study, is applied to decrease the contact resistance between the bipolar plates and the GDLs. Moreover, to minimize the contact resistance between the hybrid bipolar plates and gas diffusion layers by allowing continuous electron conduction though the unit cell, a bypass connection is made between the bipolar plates and the GDLs. The experimental results show that the developed hybrid bipolar plate has a 98% lower electrical resistance than the conventional composite bipolar plates. [Copyright &y& Elsevier]

Abstract: This study investigated the electrical properties of the gas diffusion layer (GDL) and the bipolar plates that are key components of a polymer electrolyte fuel cell (PEFC). Observations of the contact condition between the GDL and a bipolar plate under compression showed no noticeable increase in contact area with increasing compression pressure above a level of 0.8 MPa. Contact resistance between the GDL and an Au-coated bipolar plate decreased with increasing Au coverage. However, the decrease in contact resistance relative to Au coverage was small, less than 1 mΩ cm2, at coverage of approximately 0.6 or higher. That is presumably attributable to the flow of electrons to the Au contact spot with little voltage loss in the in-plane direction of the GDL. One reason for that is the fact that sufficient contact with the GDL was secured, as indicated by the observation results. Additionally, the in-plane resistance of the GDL was markedly smaller than the contact resistance. The change in contact resistance relative to Au coverage was calculated for GDLs having different in-plane resistivities. The results revealed that the increase in contact resistance relative to Au coverage became smaller as the resistivity of the GDL decreased. [Copyright &y& Elsevier]

Abstract: An empirical corrosion model for SS316L in simulated proton exchange membrane fuel cell (PEMFC) environments is developed based on systematic experimental data on the effects of various factors, such as acidity, fluoride ion concentration, temperature and polarization potential. Correlation parameters under different conditions are provided in tabulated forms and comparisons of the empirical model with experimental results are shown in graphical forms. The results show that the empirical model agrees very well with the experimental data except at the short initial polarization time and the model is applicable up to a polarization potential of 0.7 V. The results also show that polarization potential is the most sensitive parameter among all the parameters studied. [Copyright &y& Elsevier]

Abstract: To examine durability of metallic bipolar plates (BPs) under reverse current conditions, the degradation of PEMFC employing graphite, bare 316L, and CrN-coated 316L BPs is investigated via a 1.4V pulse cycling test. After 20 cycles, the average voltage decay rate at 160mAcm−2 is 6.8, 16.8, and 12.0mVcycle−1 for the single cell using graphite, bare 316L, and CrN-coated 316L BPs, respectively. SEM, EPMA, and TEM analyses of the cathodes that experienced an extraordinary high voltage of 1.4V show that carbon corrosion and Pt migration/agglomeration occur similarly for the single cells, irrespective of the bipolar plate material. In contrast, in the membrane tested with bare 316L and CrN-coated 316L, Fe and Cr are detected; the amounts of Fe and Cr in the membrane are higher for bare 316L than for CrN-coated 316L. The membrane contamination results in a decrease in the ionic conductivity of the membranes, which mainly contributes to the faster performance decay of the single cells employing bare 316L and CrN-coated 316L bipolar plates. Thus, if automotive PEMFCs using metallic BPs are exposed to reverse current conditions upon start/stop cycles, metal contamination of the membrane could accelerate the performance decay in addition to the cathode degradation, such as carbon corrosion and Pt migration/agglomeration. [Copyright &y& Elsevier]

Abstract: Electrically conductive and hydrophilic coatings for proton exchange membrane fuel cell (PEMFC) stainless steel bipolar plates have been developed in order to minimize voltage losses at the plate and gas diffusion layer (GDL) interface and facilitate liquid water transport in plate channels for efficient stack operation. The coatings are based on a multifunctional silane, 1,2-bis(triethoxysilyl)ethane (BTSE), mixed with conductive, hydrophilic carbon black. Vulcan® XC72 carbon black was modified with either polar phenylsulfonic acid (PSA) or carboxylic acid (COOH) groups to increase hydrophilic character and wetting behavior. Wetting and electrical contact resistance performance was compared with coatings based on nano-particle titania and silica. These conductive silane and carbon composite coating precursors are conveniently formulated in alcohol solution for scalable application via spray coating. Cured films exhibit negligible contact resistance increase (<2mΩcm2) at 1.4MPa when deposited on both physical vapor deposited (PVD) carbon and electroplated gold coated stainless steel. The coatings were tested for hydrophilicity retention under wet and dry fuel cell conditions where the BTSE-COOH coating remained hydrophilic on stamped stainless steel bipolar plate prototypes after greater than 1200h of simulated fuel cell testing with only moderate loss of hydrophilicity. [Copyright &y& Elsevier]

Abstract: The purpose of this report is to directly observe the passive layer and concentration behavior of nitrogen interstitially incorporated in Ni-saving high nitrogen stainless steel (HNS) using an aberration corrected scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) or – electron energy loss spectroscopy (EELS). The thickness of the passive layer barely changed after 1000h single cell operation, compared with the as-polished state. The observed passive layer was thin (3nm) and mainly composed of chromium oxide, as confirmed by STEM-EDS. It was confirmed that nitrogen was not present in the passive layer, but was concentrated at the interface between the passive layer and the metal bulk. The concentrated area ranged approximately 2nm to steel bulk from the interface. With help of the STEM-EDS and EELS, we were able to understand the nature of the passive layer for Ni-saving HNS, which caused remarkable improvement of the cell performance due to superior corrosion resistance. [Copyright &y& Elsevier]

The rubber pad forming process has been used to fabricate metallic bipolar plates from aluminum 1050 alloy for use in a proton-exchange membrane fuel cell. The plates were fabricated using a 200 ton hydraulic press and the effects of the process parameters of the rubber pad forming process (plate thickness, punch speed, press pressure, rubber thickness and rubber hardness) on the forming depth and thinning were evaluated. [ABSTRACT FROM AUTHOR]

Abstract: A new design of metallic bipolar plates with pure graphite coating on metal substrates has been developed. The top layer was a continuous graphite sheet made by the expanded graphite flakes. The pure and layered graphite surface provides high electrical conduction and high chemical inertness in PEMFC environments. Between the graphite sheet and the metal substrate was a binder layer, which mainly consisted of ester resin and conductive fillers. It functions as a binder for the graphite layer and as the second protection layer from acid solution as well. The results from the potentiodynamics tests showed rather low corrosion current densities for our proposed materials, even after soaking tests for over 2000h. The graphite-coated 316L revealed negative (cathodic) current densities either in the PEMFC cathodic or in the anodic environment for the whole polarization period in the potentiostatic experiments. Thus, the graphite-coated SS316L could potentially be used as a bipolar plate material in the PEM fuel cell applications. [Copyright &y& Elsevier]

Abstract: Corrosion performance of 316L stainless steel as a bipolar plate material in proton exchange membrane fuel cell (PEMFC) is studied under different simulated PEMFC anode conditions. Solutions of 1 × 10−5 M H2SO4 with a wide range of different F− concentrations at 70 °C bubbled with hydrogen gas are used to simulate the PEMFC anode environments. Electrochemical methods, both potentiodynamic and potentiostatic, are employed to study the corrosion behavior. Scanning electron microscope (SEM) and atomic force microscope (AFM) are used to examine the surface morphology of the specimen after it is potentiostatic polarized in simulated PEMFC anode environments. X-ray photoelectron spectroscopy (XPS) analysis is used to identify the compositions and the depth profile of the passive film formed on the 316L stainless steel surface after it is polarized in simulated PEMFC anode environments. Mott–Schottky measurements are used to characterize the semiconductor passive films. The results of potentiostatic analyses show that corrosion currents increase with F− concentrations. SEM examinations show that no localized corrosion occurs on the surface of 316L stainless steel and AFM measurement results indicate that the surface topography of 316L stainless steel becomes slightly rougher after polarized in solutions with higher concentration of F−. From the results of XPS analysis and Mott–Schottky measurements, it is determined that the passive film formed on 316L stainless steel is a single layer n-type semiconductor. [Copyright &y& Elsevier]

Abstract: The effect of RuO2 electrodeposition on ferritic stainless steel as a bipolar plate is evaluated in terms of the surface morphology, interfacial contact resistance (ICR), potentiostatic polarization, contact angle, and X-ray photoelectron spectroscopy. The surface morphology of deposited RuO2 is greatly stabilized by addition of HNO3 in 10 mM RuCl3·xH2O solution. The RuO2-deposition on stainless steel shows a high contact angle indicating the high surface energy and hydrophobic characteristics. The ICR measurement indicates that the deposition of conductive RuO2 on stainless steel is very effective in decreasing ICR value. Moreover, after potentiostatic polarization, the ICR value shows only 2.4 and 2.2 mΩ cm2 at 150 N/cm2 under air and H2 purged environments, respectively. In electrochemical test, even though the current density of RuO2-deposited stainless steel is slightly higher than that of bare stainless steel, it is acceptable value for the relevant DOE 2015 target for metallic bipolar plates (less than 1 μA/cm2). Because the RuO2-deposition on stainless steel shows a low ICR value and good corrosion resistance and high contact angle, the RuO2-deposition is a sufficiently feasible method for the bipolar plate material of PEMFC. [Copyright &y& Elsevier]

Abstract: Honeycomb-like nanocomposite Ti-Ag-N films are prepared by pulsed bias arc ion plating, and for the first time served as surface modification layer on TA1 titanium as bipolar plates for unitized regenerative fuel cells (URFCs). Phase structure and surface morphology of the coated samples are investigated by XRD and SEM. Corrosion resistance of bare Ti plates and Ti-Ag-N coated samples is evaluated in simulated URFC conditions, and the interface contact resistance (ICR) between bipolar plates and carbon papers is measured before and after potentiostatic polarization tests. Ti2N phase with a preferred orientation of (110) plane is observed in the Ti-Ag-N film. SEM observation indicates that the film appears honeycomb-like and forms a nanocomposite microstructure with Ag nanoparticles embedded in Ti2N matrix. Compared to bare titanium plates, the coated sample shows improved corrosion resistance and ultra-low ICR (2mΩcm2 under a compaction pressure of 1.4MPa). The superior conductivity of the coated sample is attributed to nanocomposite microstructure containing highly conductive Ag nanoparticles, and honeycomb-like rough surface obtained in this study. Nanocomposite Ti-Ag-N coated titanium bipolar plate exhibits prominent interfacial conductivity and excellent corrosion resistance at high potential, being a promising candidate as bipolar plate for applications in URFCs. [Copyright &y& Elsevier]

Abstract: Tribological variations, surface conditions (roughness, hardness, coating) and surface interactions between micro-stamping dies and bipolar plate blanks play a critical role in determining the surface quality, channel formation and precision of bipolar plates. This study is aimed to understand the cause, mechanism and consequences of interactions between micro-stamping process conditions and bipolar plate quality. A total of 2000 repeated micro-stamping of 51 μm-thick uncoated and 1 μm-thick ZrN coated SS316L sheet blanks into an array of 750 μm micro-channels were performed using 175–220 kN force levels with constant stamping speed of 1 mm/s. Microscopic examinations were conducted periodically on both die and coated & uncoated plate surfaces to observe topographic variations. In addition, corrosion and contact resistance tests were carried out in the same intervals. Analysis of variance (ANOVA) technique was used to determine the significance of the process parameters on channel height, roughness, corrosion and contact resistance differences. The results revealed similar roughness trends for die and plate surfaces during 2000 micro-stampings. ZrN coating with 1 μm thickness dramatically improved corrosion and contact resistance behavior of plates. [Copyright &y& Elsevier]

Abstract: Carbon composite bipolar plates were developed by compression molding of novolac type phenol formaldehyde resin with natural graphite, carbon black, and carbon fiber. The General Effective Media equation was adapted to model the electrical conductivity of the bipolar plate. The experimental values of the electrical conductivity of the composites with different reinforcements were well predicted by the model. For resin–graphite system (2-component), the most effective in-plane and through-plane electrical conductivities for 70% graphite content were found to be 201.26 and 40.91 S cm−1, respectively. Similarly, for optimum resin–graphite–carbon black system (3-component), these values were found as 269.55 and 82.77 S cm−1, respectively. The most effective in-plane and through-plane electrical conductivities were found to be 285.54 and 91.79 S cm−1, respectively, for the composite with resin–graphite–carbon black–carbon fiber system (4-component). The predicted electrical conductivities for all the three systems were found to be in well agreement with the experimental values. [Copyright &y& Elsevier]

Abstract: 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−, SO4 2− and Cl− anions) and accelerated conditions (0.5 M and 1 M H2SO4 + 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. [Copyright &y& Elsevier]

Abstract: The surface of 446M ferritic stainless steel (FSS) is modified by immersing in NaOH solution to understand its effect on the interfacial contact resistance (ICR) and corrosion. Immersion in NaOH solution under optimum condition can lead to decrease the ICR value of 446M FSS with no decrease in the corrosion resistance. Immersion of 446M FSS in NaOH solution increases the ratio of Cr oxy-hydroxide/oxide, which contributes to decrease the ICR value. This means that the bound water present in the form of the OH− group in the passive film acts as a donor-type impurity and provides the active sites for electrical conduction in the oxide. This imparts positive effect on the electrical conduction and leads to decrease in the ICR value even after the long-term immersion in the simulated PEMFC environment. [Copyright &y& Elsevier]

Abstract: The electrochemical corrosion cells will be generated from the possible pinholes of the promising CrN and TiN coatings in a PEMFC environment. To prevent the elution of possible pinholes, CrN/TiN multi-coatings on SS have been considered. This study examined the electrochemical behavior of three CrN/TiN coatings on 316L stainless steel deposited at different CrN/TiN thickness ratios by rf-magnetron sputtering as potential bipolar plate materials. Potentiodynamic tests of CrN/TiN-coated 316L stainless steel carried out in a 1M H2SO4 +2ppm HF solution at 70°C revealed a significantly lower corrosion current density than that of uncoated 316L SS, as well as a decrease in the corrosion current density with decreasing inner-layer CrN thickness. Electrochemical impedance spectroscopy also showed that the CrN/TiN-coated 316L SS sample had higher charge transfer resistance than the uncoated 316L SS sample, which increased with decreasing inner-layer CrN thickness. This was attributed to the crystalline-refined CrN/TiN(200). [Copyright &y& Elsevier]