Proton Exchange Membrane Fuel Cell Environment Research Articles

Synthesis and Characterization of Platinum on Carbon Nanoparticles Selectively Coated with Titanium Nitride (TiN)

The serious impact that high emissions of pollutants, derived from fossil fuel-based energy power generation, have on climate change resulted in the growing demand for cleaner and more efficient power sources. From the currently available green technologies, Proton Exchange Membrane Fuel Cell (PEMFC) offers the prospect of zero-emissions power production by using the electrochemical reactions of hydrogen and oxygen to generate electrical energy. Platinum (Pt)/Platinum alloy nanoparticles distributed on a carbon (C) catalyst support are the most efficient catalyst materials due to their high activity, even though the scarcity of Pt makes it expensive for widespread fuel cell application and commercialization. Furthermore, the carbon support is prone to corrosion and subsequent degradation when operating in typical PEMFC environment. This phenomenon can induce the loss of Pt nanoparticles (NPs) active surface area due to its detaching from the support, Pt NPs growth and Pt NPs agglomeration, which leads to premature performance losses in a PEMFC. Here we propose a novel way to prevent the carbon corrosion by depositing a corrosion-resistant layer of titanium nitride (TiN) on platinum on carbon (Pt/C) catalyst (in powder state). The goal of fabricating such designed catalysts is to selectively coat the carbon support with TiN, while the Pt catalyst centers are left uncoated and accessible for reagents reactions (oxygen O and H+ protons). Another attractive property of this TiN coating is its electronic conductivity, which is enough to enable the free movement of electrons from the current collector toward the catalytic centers. The deposition of the approximately 1.5 nm thick layer of TiN was carried out employing hollow-cathode plasma-assisted atomic layer deposition (HCPA-ALD) while using tetrakis(dimethylamino)titanium (IV) (TDMAT) and Ar/N2 plasma as the metal precursor and nitrogen co-reactant, respectively, at 150°C. To maintain the Pt NPs protected from TiN deposition on its surfaces during HCPA-ALD, its surface was selectively coated with a thin film of oleylamine, a polymer that would only absorb onto Pt NPs, which is later removed from the catalyst and TiN system (after the ALD process is over), by heat treatment. Here we will mainly focus on the TiN deposition process and the subsequent Transmission Electron Microscopy (TEM) characterization performed on these nanoparticles, which allows us to prove the successful formation of homogenous coatings and architecture characteristics of these nanoparticles after oleylamine application and TiN deposition. Our preliminary results have shown that highly conformal TiN can be successfully grown onto Pt/C powder nanoparticles by using HCPA-ALD with a custom-made agitator mechanism. The second phase of this research will be focused on the electrochemical responses of the TiN coated catalysts and the effect TiN coating has on its catalytic response, as well as inspecting the degradation mechanisms this novel catalyst system experiences after potential cycling.

Investigation of Corrosion Performance of Pure Aluminum as Bipolar Plate in PEMFC Environment

The global climate changes occurred because of the emissions of greenhouse gases such as CO2, NO x and SO x that are ongoing throughout the world pose a gradually higher demand for replacing today's fossil fuel based energy system by less pollutant technologies.[1-2] Among the alternative energies available, proton exchange membrane fuel cells (PEFCs) have been considered to power transportation vehicles such as automobiles and buses due to their high power density, relatively quick start-up and low operating temperatures.[3] The bipolar plate is one of the most important parts that impacts the manufacturing cost of the PEFC.[4] The high electrical conductivity, high gas impermeability, good mechanical performance and low cost are some of the desired properties of bipolar plate materials. Also, mainly two types of materials are used as bipolar plates in PEFC as carbon and metals. Carbon materials show a very good corrosion resistance but lower performance in separating reaction gases compare with metals. Among the many candidate materials, aluminum (Al) is considered as better bipolar plate applications in PEFC due to its low cost and low density.[5] Moreover, bipolar plate is required to have sufficient corrosion resistance even with a sulfuric acid solution having a pH of around 3.0, but it is considered that Al cannot tolerate this condition without its well surface modification which is proposed by Yashiro et al.[6] Sulfuric acid solutions are widely used for aluminum anodizing operations. That’s why it was our interest to study the corrosion of Al material for using as a gas separating plate in PEFC environment. Also, the influence of sulfate and fluoride ions on the stability of films formed on Al surfaces in aqueous solutions of sulfuric acid and sodium fluoride particularly at low concentration levels through finding the amount of corrosion was examined. Scanning electron microscopic (SEM), energy dispersive x-ray (EDX) and cross-sectional analyses were also done to observe the surface morphology and elemental composition.In the present investigation, pure Al having 6 cm2 of area were used as the test pieces for immersion in 2.5×10-6M H2SO4 (pH 5.41), 5×10-6M H2SO4 (pH 5.14), 2.5×10-6M H2SO4 + 2ppm F- (pH 5.57) and 5×10-6M H2SO4 + 2ppm F- (pH 5.45) solutions using reagent grade of H2SO4 and NaF. Test pieces were polished by emery paper 4/0, 6/0, 1200 and 2000, rinsed and immersed in around 100 cm3 aqueous solutions for a period of 24h, 48h, 72h, 96h and 120h at 80°C. A potentiodynamic polarization test was performed to check the corrosion resistance of Al in the simulated PEFC environment using 0.05M SO4 2- and 2ppm F- solutions (pH 3.3 and 5.8) and high current density of Al was found. Moreover, a 500h single cell operation was also performed to investigate the corrosion of Al as the gas separating plates. A glassy carbon was used as the channel former and aluminum foil was used as the gas separating plate. The single cell results showed that the corrosion of the gas separation plate was much milder than the corrosion expected in polarization test which showed the possibility of using Al as a gas separating plate in PEFC.Additionally, the oxide film was formed in the Al surfaces because of the oxidized aluminum at the time of immersion. As a result, the corrosion products were formed and graphed as amount of corrosion versus immersion time. Firstly, the amount of corrosion was increased with increasing the concentration of sulfuric acid and increased more after the addition of 2ppmF- ions in the acid solutions. From the results found in this study, it is concluded that pure as-polished Al undergoes severe corrosion in sodium fluoride whereas sulfate ions form much lower corrosion as well as act as inhibitor when Al is used as bipolar plate in the PEFC environment. Acknowledgements This work was supported by the Ministry of Education, Sports, Science and Technology (MEXT), Japan. References Carrette, K. A. Friedrich, U. Stimming, Fuel Cells, 1 (1), 5-39 (2001).O. Collantes, Technol Forecasting Soc. Change, 74(3), 267-280 (2007).Schäfer, J. B. Heywood, M. A. Weiss, Energy, 31 (12), 2064-2087 (2006).Cunningham, D. Baird, J. Mater. Chem, 16, 4385-4388 (2006).A. A. El-Enim, O. E. Abdel-Salam, H. El-Abd, A. M. Amin, J. Power Sources, 177 (1), 131-136 (2008).Yashiro, T. Ichikawa, S. -T. Myung, M. Kumagai, S. Kozutsumi, Zairyo-to-Kankyo, 60 (10), 432-434 (2011).

Enhanced mechanical properties and corrosion behavior of polypropylene/multi-walled carbon nanotubes/carbon nanofibers nanocomposites for application in bipolar plates of proton exchange membrane fuel cells

Polypropylene (PP) nanocomposites with multi-walled carbon nanotubes (MWCNT) alone or combined with carbon nanofibers (CNF) at different loadings have been fabricated by melt mixing with a focus on their mechanical properties and corrosion resistance for bipolar plates applications in proton exchange membrane fuel cells (PEMFCs). The incorporation of up to 20 wt% MWCNT in the PP matrix produces enhancements of 71, 47, 56, and 30% in microhardness, elastic modulus, and tensile and flexural strength, respectively. Combined additions of MWCNT and CNF allow producing hybrid nanocomposites with increased strength than when neat MWCNT as reinforcement, preserving their processability even at a total filler content of up to 30 wt%. The measured values of icorr in both PP/MWCNT and PP/MWCNT/CNF suggest a slow degradation rate in the PEMFC environment. Based on the US Department of Energy (DOE) targets, PP/20MWCNT, PP/21.5MWCNT, and PP/15MWCNT/15CNF nanocomposites are good candidates to produce bipolar plates for PEMFCs.

Evaluation of vacuum heat-treated α-C films for surface protection of metal bipolar plates used in polymer electrolyte membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs) assembled with metal bipolar plates (BPPs) is a promising power source for new energy vehicles. However, metal BPPs have serious corrosion issues and the surface electrical performance degrades in the harsh PEMFC environment. Amorphous carbon (α-C) films exhibit improved properties with both high corrosion resistance and electrical conductivity. Subsequent vacuum heat treatment of the as-coated α-C films can change their phase composition and film structure, modifying their performance. This study prepared α-C films with a titanium interlayer on a SS316L substrate by DC balance magnetron sputtering and then subjected them to vacuum heat treatment at different temperatures (400–700 °C). These treated α-C films are systemically analyzed in terms of surface and cross-sectional morphologies, sp bond hybridization, interfacial electrical conductivity, and corrosion resistance. The results indicate that the conversion of sp3 to sp2 and the compact density of α-C films are greatly enhanced with an increase in temperature, greatly improving the corrosion resistance and surface conductivity of the films. These promising results lead to a potential direction for the post-coating treatment of α-C films on metal BPPs in PEMFCs.

Accelerated Test of Silicone Rubbers Exposing to PEMFC environment

The durability of sealing material is one of the major challenges to the overall lifetime of proton exchange membrane fuel cell (PEMFC). In order to study the failure mode and aging mechanism of fuel cell seals, this paper focuses on the most widely used commercial silicone rubber, and an ex-situ accelerated aging method was employed by simulating the circumstance of PEMFC, including higher temperature and clamping force, lower pH value and F− with higher concentration. The main factors that affect the performance of the seal were obtained through the test of their morphologies and mechanical properties. The failure mode of the aging mechanism of the rubbers were analyzed through scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), attenuated-total-reflection infrared spectroscopy (ATR-IR), mass loss, compression set (CS), etc. It was found that temperature played an important role in the deterioration of seal performance, and the change of the parameters of the compression set with time conformed the dynamic empirical formula, which provides the experimental basis for the lifetime prediction of the seals.

Study on conductivity and corrosion resistance of N-doped and Cr/N co-doped DLC films on bipolar plates for PEMFC

In order to obtain films with good conductivity and high corrosion resistance for proton exchange membrane fuel cell (PEMFC) bipolar plate applications, N-doped and Cr/N co-doped diamond-like carbon (DLC) were deposited on stainless steel 316L and silicon wafer with constant nitrogen flow rate (4 sccm) and different Cr target power (0, 60, 90, 120 W). The electrical properties and corrosion resistance of the uncoated stainless steel and coated samples were tested under the simulated working environment of PEMFC. Scanning electron microscope (SEM) results show that the surface of Cr/N co-doped DLC film is smoother. Through the analysis of X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) results, it is found that Cr/N co-doing DLC film is a combination of amorphous C, CrC, CN, Cr2N and nanocrystalline CrN. Contact resistance test indicates that the Cr/N co-doped DLC film (with Cr target power of 90 W) has the best electrical conductivity (9.20 mΩ cm2 at 150 N/cm2 pressure). Potentiodynamic polarization test shows that both N-doped DLC and Cr/N co-doped DLC films had better corrosion resistance (the corrosion current density could reach E−6 ~ E−7 A/cm2) than uncoated SS316L. To sum up, Cr/N co-doped DLC film (with Cr target power of 90 W) shows outstanding performance in both electrical conductivity and corrosion resistance, which is suitable for the surface treatment of stainless steel bipolar plates.

ZrC Coating Modified Ti Bipolar Plate for Proton Exchange Membrane Fuel Cell

AbstractThe electrical conductivity, corrosion resistance and hydrophobic property of bipolar plate are important factors to affect the overall performance of proton exchange membrane fuel cell (PEMFC). In order to improve the performance of bipolar plate, ZrC coating has been prepared on bare Ti plate by plasma surface modification technology. From the microscopic morphology characterization, it can be found that the surface of ZrC modified plate is smooth without obvious defect. The potentiodynamic polarization and potentiostatic polarization tests are measured in cathodic and anodic environment of PEMFC. The result shows that the corrosion current density of ZrC coated Ti plate decreases by 1–2 orders of magnitude compared with bare Ti plate. Moreover, the interfacial contact resistance (ICR) decreases from 110 mΩ cm2 to 7.70 mΩ cm2 under the compaction force of 140 N cm−2. In addition, the ICR of ZrC coated Ti plate slightly increases after potentiostatic polarization, indicating that a good electrical conductivity and durability is obtained by ZrC coated Ti plate. Furthermore, the ZrC coated Ti plate has a lower water contact angle (105°) than bare Ti plate (76°), indicating a better hydrophobic performance for water management.

Influence of heat treatment on corrosion behaviors of 316L stainless steel in simulated cathodic environment of proton exchange membrane fuel cell (PEMFC)

The influences of two types of heat-treatments on the corrosion behavior of 316L SS in the simulated cathodic environment of PEMFC, are investigated using potentiodynamic curve, electrochemical impedance spectroscopy (EIS), Mott-Schottky plot and auger electron spectroscopy (AES), respectively. The results show that 316L SS is in the passive state within the potential region from −0.1VSCE to 0.8VSCE in the simulated cathodic environment of PEMFC, and a passive film can be formed on 316L SS. The passivity of 316L SS in the simulated cathodic environment of PEMFC firstly increases and then degrades with the increased solid solution temperature or time, and the best passivity corresponds to the solid solution temperature at 1050 °C for 40 min among other solid solution treatments. While 316L SS heat-treated with the solid solution treatment at 1050 °C for 40 min plus aging treatment at 900 °C for 4 h, also has the best passivity in the same solution among other solid solution plus aging treatments. The best corrosion protection, lowest donor density and the highest thickness of the passive film corresponding to the solid solution temperature at 1050 °C for 40 min among other solid solution or solid solution plus aging heat treatments, and this treatment is mostly suitable for improving the anti-corrosion property of 316L SS in the simulated cathodic environment of proton exchange membrane fuel cell (PEMFC).

Influence of Time on Low Temperature Salt Bath Nitriding and its Corrosion Behavior of 316L ASS in PEMFC Environment

Austenitic stainless steels (ASSs) offer many advantages over carbon and/or graphite composites for bipolar plates of proton exchange membrane fuel cells (PEMFCs). However, its corrosion resistance in PEMFCs operating conditions is lower. A low temperature (450°C) salt bath nitriding technique for different durations (3, 6 and 12 h) was employed to modify the surface of 316L ASS to enhance its corrosion resistance. The scanning electron microscopy of the transverse sections of nitrided sample reveals a white layer and its thickness increases on increasing nitriding durations. The X-ray photon spectroscopy results show that on nitriding, nitrogen (N) is present in the free form (on 3 h duration) and nitrides (CrN and FexN) precipitates (on 6 and 12 h durations). The corrosion resistance increased on nitridation. The best corrosion resistance was obtained for 3 h nitriding. The corrosion resistance decreases on increasing durations which can be attributed to the formation of nitrides.

Electrochemical behavior and surface conductivity of NbC modified Ti bipolar plate for proton exchange membrane fuel cell

The drawback of corrosion resistance and surface conductivity is the main deficiency to restrict the widespread application of Ti bipolar plate in proton exchange membrane fuel cell (PEMFC). A NbC modified layer has been prepared on the surface of Ti plate by plasma surface modification technology to improve the performance of bipolar plate. The NbC modified layer possesses dense, continuous microstructure and exhibits well interface bonding with substrate. The electrochemical characteristic of NbC-Ti bipolar plate is investigated in simulated PEMFC environment (0.5 M H2SO4 + 3 ppm HF solution at 75 °C). The NbC modified bipolar plate exhibits more positive corrosion potential and lower current density at anode and cathode potential. The interfacial contact resistance (ICR) of NbC-Ti bipolar plate shows a low value of 16.6 mΩ cm2 at compaction force of 140 N cm−2, lower than that of bare Ti plate (91.9 mΩ cm2). In addition, the ICR of NbC modified bipolar plate only exhibits slight increase after potentiostatic test of 4 h in both anode and cathode environment. Thus, the NbC modified Ti bipolar plate can act as candidate supplies for PEMFC.

Interface-induced degradation of amorphous carbon films/stainless steel bipolar plates in proton exchange membrane fuel cells

In the field of proton exchange membrane fuel cells (PEMFCs), amorphous carbon (a-C) films have attracted considerable attention as a surface functional coating for metallic bipolar plates (BPPs), given that they can endow metallic BPPs with both high corrosion resistance and electrical conductivity under harsh PEMFC environments. In this study, to determine the role of a-C/metallic substrate interfaces on the performance of PEMFCs, and clarify the associated degradation mechanism, a series of a-C films are deposited on 316L stainless steel (316Lss) samples at different sputtering powers. The composition, microstructure, interfacial contact resistance (ICR), and corrosion resistance of the a-C films are then systemically investigated, before and after electrochemical corrosion tests. The results obtained reveal that all the a-C films can greatly improve the performance of 316Lss under simulated PEMFC operational conditions, and the a-C film deposited at 0.9 kW exhibits the lowest corrosion current density (~7.52 × 10−3 μA cm−2) and minimum ICR values (2.91 and 4.00 mΩ cm2, before and after the long-time potentiostatic polarization tests, respectively). Furthermore, residual chromium oxides, which possibly result in an increased in ICR values, are observed at the a-C/316Lss interface after corrosion test; thus, an interface-induced a-C film degradation mechanism is proposed.

Material behavior of rubber sealing for proton exchange membrane fuel cells

Reliable sealing is necessary for the stable operation of proton exchange membrane fuel cell (PEMFC). In practical application, various materials have been tried in PEMFC sealing. However, the mechanical properties of these sealing materials, which play a key role in the sealing stability, have not been fully understood in PEMFC environment, especially after long-term operation. In this paper, according to the operating environment of PEMFC, sealing material experiments are carried out to explore the differences in mechanical behaviors of sealing materials, including silicone rubber (SR), fluororubber (FR), nitrile rubber (NBR) and ethylene-propylene-diene-terpolymer rubber (EPDM) and the variation of mechanical properties of these sealing materials is predicted as time goes on. The results indicate that compression rate has a great influence on sealing contact stress. SR and EPDM, with the variation of 0.15 MPa and 0.45 MPa in stress, show the best and worst mechanical stability at different compression rates, respectively. In terms of temperature, it is found that SR can adapt to different operating temperature of PEMFC and only 18% variation is found from 20 °C to 100 °C. Finally, based on Time-Temperature Superposition (TTS), high temperature experiments are conducted to predict long-term relaxation stress under PEMFC working condition. The analysis results are beneficial for choosing suitable sealing material, and it can also be applied to predict sealing ability in PEMFC.

Accelerated Stress Tests on Fuel Cell Cathode Catalysts: Thin Film (Ex-situ) Vs. Catalyst Layer (In-situ)

For proton exchange membrane fuel cells (PEMFC), cathode catalyst durability is presently a key limiting factor to commercialization for automotive applications. The oxygen reduction reaction takes place on the cathode – transforming O2 + 4H+ + 4e- to 2H2O. Degradation occurs during normal operational load profiles for idle-to-peak power transients (rapid stop-go drive cycles), as the cathode voltage quickly switches from 0.9-1.2V (idle) to ~0.6V (peak). Cathode catalyst durability is dependent upon multiple operational factors, including; potential limits, temperature, and pH. As a result of the transient potentials, the electrochemical surface area (ECSA) of the carbon supported platinum catalyst (Pt/C) is reduced, due to a combination of Pt dissolution, Pt particle growth (migration and/or redeposition), and carbon corrosion (Pt detachment). Typically, accelerated stress tests (AST) are performed on thin films to evaluate catalyst durability and decouple contributions from ancillary fuel cell materials and structures. The thin films consists of catalyst deposited on a conductive rotating disk electrode and analyzed in liquid electrolyte (HClO4 or H2SO4) vs. a separate counter and reference electrode – ex-situ to the actual PEMFC environment. This removes the additional complexity of the catalyst layers 3-deminsional structure (containing both Pt/C and proton conducting ionomer), diffusion media and membrane – in-situ. The AST used in this study includes 25,000 potential cycles from idle to peak potential limits using either square or triangular waveforms (SW or TW, respectively) at 10 sec intervals, for both Pt/C thin films and catalyst layers. ECSA was measured periodically during the cycles, either by integration of the hydrogen desorption area or by CO stripping analysis. The following operational limits were used with a lower potential limit of 0.6V – several upper potential limits (UPL) from 0.9 to 1.2V, multiple temperature ranges between 25°C and 70°C, and different electrolyte pH ranges 0 to 1. Ex-situ AST results in 0.1 M HClO4 at 70°C show that the loss in ECSA was affected by both the UPL and the potential wave profile. For the 0.9V UPL dominated by Pt dissolution, the SW was more aggressive than the TW, resulting in a 45.6% and 36.8% ECSA loss, respectively. While the higher UPL of 1.2V had nearly identical ECSA losses for both potential wave profiles of 89%, due to the dominance of carbon corrosion. Figure 1A shows the effects of pH on AST induced ECSA losses at room temperature for the UPL of 1.2V using a TW. The pH values correspond to different concentrations of HClO4 (0.1 M HClO4 = pH of 1). As the pH is lowered there is an increase in ECSA loss. In Figure 1B the TW was found to cause more catalyst degradation at lower pH values. This potentially could be related to in-situ relative humidity dependent degradation – to be further studied in-situ on a catalyst layer. Figure 1

Influence of oxygen adsorption on the chemical stability and conductivity of transition metal ceramic coatings: First-principle calculations

Considering their excellent chemical stability and conductivity, transition metal carbide and transition metal nitride coatings are adopted to protect metallic bipolar plates in this study. However, the inevitable adsorption of oxygen on these coatings in the cathodic environment of proton exchange membrane fuel cell (PEMFC) influences their chemical stabilities and conductivities. To asses this influence, first-principle calculations are conducted using CrN, TiC and TiN as examples. We show that in the case of CrN, CrOx ionic bonds are formed on CrN (111), and TiOx formed on TiC (111) and TiN (111) for the case of TiC and TiN, respectively. In addition, the overlapping in the electron density difference distribution of CrO is more significant than that of TiO (TiN) and TiO (TiC). This suggests that CrN has a better chemical stability than TiN or TiC, and therefore, is more suitable for the protective coating of metallic bipolar plates, especially in the cathodic environment of PEMFC.

Durable degradation resistance of graphene coated nickel and Monel-400 as bi-polar plates for proton exchange membrane fuel cell

Degradation/corrosion of metallic bi-polar plates in proton exchange membrane fuel cell (PEMFC) environment is a vexing problem in durable performance of PEMFCs. Graphene coatings have been demonstrated to provide robust and durable corrosion resistance in PEMFC environment, without compromising the essential criterion, i.e., high electric conductivity. Multilayer graphene coatings were deposited on nickel and a commercial Ni–Cu alloy by chemical vapour deposition technique. Time dependent electrochemical tests carried out in the simulated PEMFC environment (0.5 M H2SO4 solution) showed up to two orders of magnitude improvement in corrosion resistance of the metal substrate, and this resistance sustained for the entire test duration of 750 h. The magnitude of corrosion resistance of graphene-coated Ni was considerably superior than that for graphene-coated Monel 400 (investigated in this study) or graphene-coated Cu (reported in an earlier study). As suggested by the post-corrosion scanning electron microscopy, the graphene coatings on Ni and Monel 400 remained largely intact after long exposures to the aggressive H2SO4 solution. The durable corrosion resistance of nickel and a nickel-copper alloy due to graphene coating is attributed to the ability of nickel to develop multilayer graphene (however, the nickel-copper alloy is less efficient in developing a robust multilayer graphene).

Poly(m‐phenylenediamine)‐coated 316L SS: A promising material for bipolar plates in PEMFC environment

AbstractThe applicability of conducting polymer coatings to enhance corrosion resistance of bipolar plates in proton exchange membrane fuel cells (PEMFC) is gaining greater significance as electrical conductivity is as important as corrosion resistance. Metaphenylenediamine (mPD) monomer was electropolymerized to poly(m‐phenylenediamine) (PmPD) conducting polymer over 316L SS and characterized by attenuated total reflectance infrared spectroscopy to confirm the formation of P mPD polymer. Scanning electron microscopy was used to study the surface morphology of the polymer. Open‐circuit potential, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization studies were conducted to assess the corrosion protection performance of the PmPD polymer coating in PEMFC environment. The charge‐transfer resistance measured from EIS for the coated substrates was higher than the uncoated substrate. Potentiodynamic polarization studies showed lower corrosion current density for the PmPD‐coated substrates. All the results proved that the PmPD‐coated substrates could exhibit enhanced corrosion resistance in PEMFC environment.

Study on the Properties of Vertical Carbon Nanotube Films Grown on Stainless Steel Bipolar Plates.

Research on the conductivity and corrosion resistance of stainless steel bipolar plates in a proton exchange membrane fuel cell (PEMFC) is commonly performed in a normal-temperature environment (about 20 °C). However, these fuel cells must function in low-temperature environments (lower than 0 °C) in some conditions, such as in vehicle fuel cells and in portable power supplies that operate during the winter in northern China. Stainless steel bipolar plates have higher requirements in terms of their hydrophobic and anti-icing properties, in addition to needing high conductivity and corrosion resistance. In this study, carbon nanotubes (CNTs) are grown on the surface of 304 stainless steel (304 SS) without a catalyst coating by plasma-enhanced chemical vapor deposition (PECVD), which is a simple and cheap method that allows stainless steel to be used as bipolar plates in low-temperature environments. The Raman spectroscopy and scanning electron microscopy (SEM) results show that the CNTs grown on the surface of 304 SS have different morphologies. The stainless steel samples with different CNT morphologies are tested by hydrophobicity and in situ icing experiments to prove that vertical CNTs can achieve a superhydrophobic state and have good anti-icing properties. The interfacial contact resistance (ICR) of the bare 304 SS and the 304 SS with vertical CNTs is compared by voltammetry, and then the corrosion resistances of both types is compared in a simulated PEMFC environment via a three-electrode system. Consequently, the ICR of the 304 SS with vertical CNTs was lower than the bare 304 SS. The corrosion potential was positive, and the corrosion current density was greatly reduced for the stainless steel with vertical CNTs grown directly on its surface when compared with the bare 304 SS. The experimental results show that vertical CNTs have good application prospects as bipolar plates for PEMFCs in low-temperature environments.

In-situ electrodeposition of conductive polypyrrole-graphene oxide composite coating for corrosion protection of 304SS bipolar plates

The enhancement in long-term corrosion resistance of conductive polymer coating is essential for the practical application, particularly to protect metallic bipolar plates in the aggressive environment of proton exchange membrane fuel cell (PEMFC). In this study, graphene oxide (GO) was introduced to polypyrrole (PPY) matrix, furthermore, PPY-GO composite coatings with different GO content were in-situ electrodeposited on 304 stainless steel (SS) bipolar plates to protect them from corrosion in the aggressive working environment. The electrochemical results imply that the conductive PPY-GO composite coatings significantly decrease the polarization current density of 304SS during potentiostatic polarization in the simulated PEMFC environment. The existence of GO in PPY matrix enhances the coatings adhesion strength and makes the diffusion pathway of corrosive species longer and more circuitous, further restricts their inward penetration. The PPY-GO coating with the optimized GO content of 1 mg mL−1 in the electrodeposition electrolyte shows the best corrosion resistance among the prepared coatings. This enhanced long-term corrosion protection of the PPY-GO composite coating in comparison with the pristine PPY coating is resulting from the enhanced physical barrier and anodic protection.

Degradation of Silicone Rubbers as Sealing Materials for Proton Exchange Membrane Fuel Cells under Temperature Cycling.

Gaskets are compressed in proton exchange membrane fuel cells (PEMFCs) to keep fuel, oxidant and coolant within their respective regions and are very important for sealing and maintaining electrochemical performance of fuel cells during their long-term operation. It has been proved that the gas leakage caused by the failure of the gaskets following long-term operation is one of the main reasons for PEMFC performance degradation. In this work, degradation of silicone rubbers, the potential gasket materials for PEMFCs, were investigated in the simulated PEMFC environment solution, weak acid solution, de-ionized water and air, respectively, under alternating temperature cycling from −20 °C to 90 °C. The changes in hardness, weight, chemical properties, mechanical behavior and surface morphology of the samples of silicone rubbers were studied after a certain number of temperature cycles. The results show that with the increase in temperature cycles, the hardness of the samples increases and the weight of the samples decreases gradually. Scanning electron microscopy reveals that cracks and caves constantly appear on the surface of the samples. Attenuated total reflection Fourier transform infrared spectra (ATR-FTIR) results demonstrate that the surface chemistry changes via de-crosslinking and chain scission in the backbone due to the exposure of samples to the environments over time under alternating temperature cycles.

Fabrication of microporous layer - free hierarchical gas diffusion electrode as a low Pt-loading PEMFC cathode by direct growth of helical carbon nanofibers.

Improving interfacial contact between each component in the proton exchange membrane fuel cell (PEMFC) can lead to a significant increase in power density and Pt utilization. In this work, the junction between the catalyst layer and gas diffusion layer (GDL) is greatly enhanced through direct attachment of helical carbon nanofibers, giving rise to a hierarchical structure within the electrical interconnections. The alternative novel GDL is produced by spraying a thin layer of Pd2C60 precursor on commercial carbon paper, followed by chemical vapor deposition growth resulting in a surface morphology of well-attached nanofibers surrounding the microfibers present in the commercial carbon paper. Subsequent solvothermal deposition of platinum nanoparticles allowed evaluation of its suitability as gas diffusion electrode in cathodic H2/O2 PEMFC environment. A combination of lowered charge transfer resistance and enhanced Pt-utilization is attributed to its unique wire-like appearance and its robust properties. The fabricated microporous layer – free GDL is suitable for relatively aggressive membrane electrode assembly fabrication procedures and is produced by industrially favorable techniques, rendering it capable of efficiently supporting small amounts of precious metal catalyst nanoparticles in various PEM applications.