Plasma nitrided titanium as a bipolar plate for proton exchange membrane fuel cell

Plasma nitriding was applied to improve the surface performance of titanium bipolar plate. XRD and SEM results showed a titanium nitride layer was formed after nitridation. In comparison with pure titanium, the interfacial contact resistance of plasma nitrided titanium was reduced to some extent by the nitridation treatment. However, high corrosion current was observed under electrochemical tests in 0.5 M H₂SO₄ + 5 ppm HF. Both the electrical conductivity and corrosion resistance of the surface of plasma nitriding titanium did not reach the level of graphite. Some more improvements are expected in the plasma nitriding process or another surface modification on pure titanium.     

Improvement of corrosion and electrical conductivity of 316L stainless steel as bipolar plate by TiN nanoparticle implantation using plasma focus

The present work reports the results of TiN-ions implantation into the SS316L samples as bipolar plates by a 4 kJ Mather type Plasma Focus (PF) device operated with nitrogen gas for 10, 20, and 30 shots in order to improve the corrosion resistance and electrical conductivity of samples. The PF can generate short lived (10–100 ns) but high temperature (0.1–2.0 keV) and high density (10¹⁸–10²⁰ cm⁻³) plasma, and the whole process of PF lasts just a few microseconds. X-ray diffraction (XRD) results reveal the formation of a nanocrystalline titanium nitride coating on the surface of substrate. The interfacial contact resistance (ICR) of samples is measured, and the results show that the conductivity of samples increase after coating because of high electrical conductivity of TiN coating. The electrochemical results show that the corrosion resistances are significantly improved when TiN films are deposited into SS316L substrate. The corrosion potential of the TiN coated samples increases compared with that of the bare SSI316L and corrosion currents decrease in TiN implanted samples. Scanning Electron Microscopy (SEM) indicates changes in surface morphology before and after potentiostatic test. The thickness of coated layer which is obtained by cross sectional SEM is about 19 μm.     

Optical properties of tungsten and titanium oxide thin films prepared by plasma sputter deposition

Tungsten oxide and titanium oxide thin films were prepared by RF reactive magnetron sputter deposition. The stationary and rotating substrate holders were applied to analyze the rotating effect. The optical properties and thicknesses of oxide films were determined by a proposed optical model and the measured transmittance spectra. The dispersed refractive indices of thin films have a wide range distribution in different sputtering conditions. In the situation of rotating substrate holder, the refractive index was lower than that of the stationary substrate holder. Also, amorphous TiO₂ structure can be prepared by using rotating substrate holder. The transmittance spectrum of crystalline TiO₂ reveals that the textured structure on the film surface affects the transmittance characteristic.     

Effect of pH value on corrosion resistance and surface conductivity of plasma‐nitrided 304L bipolar plate for PEMFC

Low temperature plasma nitriding is developed to meet the requirements for corrosion resistance and interfacial contact resistance (ICR) of stainless steel 304L as the bipolar plate for PEMFC. A dense and supersaturated‐nitrogen nitrided layer has formed on the surface of the stainless steel 304L. Electrochemical behavior for the untreated and plasma‐nitrided 304L was measured in H₂SO₄ (pH=1–5)+2 ppm F^? simulating PEMFC environment, and the ICR was evaluated before and after corrosion tests. The experimental results have shown that the ICR for the plasma nitrided 304L is lower than the requirement of U.S. DOE (<10 mΩ cm² to 2010). Corrosion resistance and the ICR at the compaction force of 150–200 N cm^?2 increase with increasing pH value for the untreated and plasma‐nitrided 304L. The passive current densities for the untreated and plasma‐nitrided 304L are all lower than 16 µA cm^?2. The ICR between passive film and carbon paper are increased markedly because of passive film formed on the surface of both studied 304L. However, the passive current density and the ICR are lower for the plasma nitrided 304L than those for the untreated one at the given pH value, which results from the different composition of the stable passive film formed on the surface. The low temperature plasma nitriding provides a promising method for 304L using as bipolar plate for PEMFC. Further research is needed to evaluate the long‐term stability of passive film and the performance of single fuel cell. Copyright © 2010 John Wiley & Sons, Ltd.     

Compositional dependence of electrical conductivity of Ce1−xTbxO2−δ (0x1)

Well dispersed Ce_1−xTb_xO_2−δ nano-powders were synthesized by using the carbonate coprecipitation method in an entire compositional range of 0x1, which allowed the preparation of highly dense pellets by sintering at 1673 K and the systematical study of the electrical conductivity in such a wide compositional range. It was found that the conductivity increased with increasing Tb concentration except that of x=0.80. Secondary phase were observed by using X-ray diffraction in the samples with x0.80, which might have negative impact on the conductivity of the samples.     

Nitrogen plasma-implanted titanium as bipolar plates in polymer electrolyte membrane fuel cells

Nitrogen plasma immersion ion implantation (PIII), a non-line-of-sight surface treatment technique suitable for bipolar plates in polymer electrolyte membrane fuel cells, is conducted at low and high temperature to improve the corrosion resistance and conductivity of titanium sheets. X-ray photoelectron spectroscopy (XPS) shows that high-temperature (HT) nitrogen PIII produces a thick oxy-nitride layer on the titanium surface. This layer which provides good corrosion resistance and high electrical conductivity as verified by electrochemical tests, inductively coupled plasma optical emission spectroscopy, and interfacial contact resistance (ICR) measurements renders the materials suitable for polymer electrolyte membrane fuel cells. In comparison, the low-temperature (LT) PIII titanium sample exhibits poorer corrosion resistance and electrical conductivity than the untreated titanium control.     

Corrosion resistance of functionally graded TiN/Ti coatings for proton exchange membrane fuel cells

Bipolar Plates (BPP) are important components of proton exchange membrane fuel cell (PEMFC) stacks. In the development of innovative fuel cell designs, it is advantageous to use aluminum for these applications, however, this material lacks the necessary corrosion resistance. Since the performance of PEMFC stacks depends on BPP properties, in particular, corrosion resistance, depositing titanium nitride (TiN) thin films onto aluminum substrates may improve their efficiency and durability. The present work focuses on improving corrosion resistance and hydrophobicity of TiN/Ti by using N graded films deposited onto aluminum substrates (AA-1100) by grid-assisted magnetron sputtering (GAMS). Electrochemical impedance spectroscopy (EIS) and potentiodynamic and potentiostatic polarization are used to investigate the performance of the substrate/film system at room temperature and 70 °C, thus simulating a prototypic PEMFC electrolyte environment. Electrochemical test results showed that graded TiN films improved corrosion resistance when compared with both the homogeneous films and the AA1100 uncoated substrate. Furthermore, contact angle results reveal improved hydrophobicity for both homogeneous and graded TiN coatings when compared with the AA1100 substrate.     

Improvement of corrosion resistance and electrical conductivity of 304 stainless steel using close field unbalanced magnetron sputtered carbon film

Carbon film has been deposited on 304 stainless steel (SS304) using close field unbalanced magnetron sputter ion plating (CFUBMSIP) to improve the corrosion resistance and electrical conductivity of SS304 acting as bipolar plates for proton exchange membrane fuel cells (PEMFCs). The corrosion resistance, interfacial contact resistance (ICR), surface morphology and contact angle with water of the bare and carbon-coated SS304 are investigated. The carbon-coated SS304 shows good corrosion resistance in the simulated cathode and anode PEMFC environment. The ICR between the carbon-coated SS304 and the carbon paper is 8.28-2.59 mΩ cm² under compaction forces between 75 and 360 N cm⁻². The contact angle of the carbon-coated SS304 with water is 88.6°, which is beneficial to water management in the fuel cell stack. These results indicate that the carbon-coated SS304 exhibits high corrosion resistance, low ICR and hydrophobicity and is a promising candidate for bipolar plates.     

Distinguishing between apparent and authentic conductivity of protonic ceramic electrolytes using differential resistance analysis applied to conductivity measurements

Differential resistance analysis (DRA) is used for determination of authentic bulk conductivity of yttrium-doped barium cerate proton conductors (BCY) using electrochemical impedance spectroscopy (EIS), by eliminating all electrode and fixture resistance as well as interfacial or surface phenomena. Bulk conductivity determined using DRA in the range 500–800 °C was higher by a factor of 1.5–3 than values obtained for conductivity of the same electrolyte using EIS alone. Application of DRA with EIS provides geometrically independent conductivity values.     

Characterization and corrosion behaviour of grade 2 titanium used in electrolyzers for hydrogen production

The decentralized production of hydrogen as a renewable energy vector targets large markets and potential applications: mobility, electricity and fixed heat generation, electricity storage in the form of gas. Hydrogen is today in France and in the industrialized countries an emerging industrial sector, leading to an increasing development of electrolyzers. The study of the behaviour of grade 2 titanium used in these electrolyzers for hydrogen production, using a characterization of parts by Scanning Electron Microscopy (SEM), surface analysis by Glow Discharge Optical Emission Spectrometry (GDOES) demonstrating the hydrogen diffusion in titanium, and electrochemical measurements, will make it possible to understand and interpret the mechanisms that caused degradation after 5000 h of operation and to validate the choice of grade 2 titanium.     

Hydrogen induced microstructure evolution of titanium matrix composites

The effect of melt hydrogenation on microstructure evolution of Ti-6Al-4V matrix composites was investigated in this study. Molten alloy was hydrogenated with a mixture of hydrogen and argon, and reinforced at 5% total volume fraction with a 1:1 mol ratio mixture of TiB and TiC particles. Microstructure of as cast composites showed hydrogen induced more TiB whiskers with higher length-diameter ratio (LDR), because hydrogen accelerated atomic diffusion and then increased growth rate of TiB whiskers. Hot compression results indicated hydrogen reduced peak flowing stress. Microstructure of as compressed composites indicated hydrogen encouraged decomposition of residual lamellas. Hydrogen eliminated most cracks and holes along the interface between ceramic particles and matrix. Compared with unhydrogenated composites, the original ceramic particles in hydrogenated composites were fragmented into smaller pieces after compression. Electron back-scattered diffraction and transmission electron microscopy results indicated hydrogen increased volume fraction of dynamic recrystallization (DRX). And hydrogen decreased the density of dislocations nearby the interface.     

Effects of niobium and titanium addition and surface treatment on electrical conductivity of 316 stainless steel as bipolar plates for proton-exchange membrane fuel cells

Niobium and titanium are added to 316 stainless steel, and then heat treatment and surface treatment are performed on the 316 stainless steel and the Nb- and Ti-added alloys. All samples exhibit enhanced electrical conductivity after surface treatment but have low electrical conductivity before surface treatment due to the existence of non-conductive passive films on the alloy surfaces. In particular, the Nb- and Ti-added alloys experience a remarkable enhancement of electrical conductivity and cell performance compared with the original 316 stainless steel. Surface characterization reveals the presence of small carbide particles on the alloy surface after treatment, whereas the untreated alloys have a flat surface structure. Cr₂₃C₆ forms on the 316 stainless steel, and NbC and TiC forms on the Nb- and Ti-added alloys, respectively. The enhanced electrical conductivity after surface treatment is attributed to the formation of these carbide particles, which possibly act as electro-conductive channels through the passive film. Furthermore, NbC and TiC are considered to be more effective carbides than Cr₂₃C₆ as electro-conductive channels for stainless steel.     

Enhancement of Bi-based niobate pyrochlores conductivity with Ru-doping. Structural,optical, and electrical properties

New Ru-doped bismuth-based pyrochlores were synthesized. The effects of Ru-doping in Bi_1.5Mg_0.375Cu_0.375Nb_1.5-xRu_xO_7-δ on the formation of the crystal structure, stability, optical, and electrical properties of the compositions were studied. For compositions up to x = 0.1, only a phase of the pyrochlore type was detected by X-ray diffraction method (XRD) and energy dispersive X-ray analysis (EDX). Small impurities of Bi₂Ru₂O₇, RuO₂ and CuO were found for samples with x ≥ 0.25. The lattice parameters of pyrochlores decrease upon doping with Ru. X-ray diffraction modeling revealed the structural stability of the doped compositions and the distribution of Ru⁴⁺ cations in the B sites of the A₂B₂O₆O' pyrochlore structure. A decrease in the band gap energy E_g was observed upon doping with Ru: 2.40 (x = 0) – 2.10 eV (x = 0.5) for the direct transition and 1.52 (x = 0) – 0.16 eV (x = 0.5) for indirect transition as a result of optical spectra analysis and DFT-HSE03 calculation. The chemical compatibility of the obtained pyrochlores with the manganite perovskite La_0.7Sr_0.3MnO₃ was studied. The pyrochlores can be considered as a component of cathode composite material for using in intermediate temperature solid oxide fuel cells (IT-SOFC).     

Thermally and air–plasma-oxidized titanium and stainless steel plates as solar selective absorbers

Solar selective surfaces can be constructed in many ways. In particular, low emissivity (ε) is currently sought for by starting with surfaces bearing high infrared reflectance. It is well known that metallic surfaces behave in this way. Then, high solar absorptance (α) can be achieved by adding an appropriate thin layer through a wide variety of possibilities. In this work, a simple direct method to produce such type of coatings on titanium and 304 stainless steel plates is assessed: thermal oxidation. Good selectivities S(=α/ε) 10 or higher in some cases were obtained mainly on steel substrates. An alternative is to expose these metallic surfaces to ionized oxygen species, rather than to neutral oxygen molecules. This was accomplished by oxidizing some plates in a typical glow discharge capacitive system. In this case, acceptable but not so high selectivities, as in the first case, were obtained. Some comments on the metallic surface morphology and the stability of the oxidized surfaces are also presented.     

The optical micrographs and texture of the hydrogenated Ti60 titanium alloy by the equal-channel angular pressing

The optical micrographs and texture after one pass and two passes of equal-channel angular pressing for the hydrogenated Ti60 titanium alloy had been investigated. The slight grain refinement after two passes of equal-channel angular pressing for the hydrogenated Ti60 titanium alloy was obtained. The pass number of equal-channel angular pressing affected the texture of the hydrogenated Ti60 titanium alloy.     

Pre-oxidized and nitrided stainless steel alloy foil for proton exchange membrane fuel cell bipolar plates: Part 1. Corrosion, interfacial contact resistance, and surface structure

Thermal (gas) nitridation of stainless steel alloys can yield low interfacial contact resistance (ICR), electrically conductive and corrosion-resistant nitride containing surface layers (Cr₂N, CrN, TiN, V₂N, VN, etc.) of interest for fuel cells, batteries, and sensors. This paper presents results of scale-up studies to determine the feasibility of extending the nitridation approach to thin 0.1 mm stainless steel alloy foils for proton exchange membrane fuel cell (PEMFC) bipolar plates. Developmental Fe-20Cr-4V alloy and type 2205 stainless steel foils were treated by pre-oxidation and nitridation to form low-ICR, corrosion-resistant surfaces. As-treated Fe-20Cr-4V foil exhibited target (low) ICR values, whereas 2205 foil suffered from run-to-run variation in ICR values, ranging up to 2× the target value. Pre-oxidized and nitrided surface structure examination revealed surface-through-layer-thickness V-nitride particles for the treated Fe-20Cr-4V, but near continuous chromia for treated 2205 stainless steel, which was linked to the variation in ICR values. Promising corrosion resistance was observed under simulated aggressive PEMFC anode- and cathode-side bipolar plate conditions for both materials, although ICR values were observed to increase. The implications of these findings for stamped bipolar plate foils are discussed.     

Plasma-nitrided austenitic stainless steel 316L as bipolar plate for PEMFC

Stainless steel bipolar plates for the polymer electrolyte membrane (PEM) fuel cell offer many advantages over conventional machined graphite. Austenitic stainless steel 316L is a traditional candidate for metal bipolar plates. However, the interfacial ohmic loss across the metallic bipolar plate and membrane electrode assembly due to corrosion increases the overall power output of PEMFC. Plasma nitriding was applied to improve the surface performance of 316L bipolar plates. A dense _γN${\gamma }_{\mathrm{N}}$ phase layer was formed on the surface. Polarization curves in the solution simulating PEMFC environment and interfacial contact resistance were measured. The results show that the corrosion resistance is improved and the interfacial contact resistance (ICR) is decreased after plasma nitriding. In comparison with the untreated 316L, the ICR between the carbon paper and passive film for the plasma-nitrided 316L decreases at the same condition and lowers with increasing pH value.     

Photo-electrochemistry of metallic titanium/mixed phase titanium oxide

In this study, the effect of potassium hydroxide concentration in anodization bath, anodization time, and calcination temperature on the photo-electrochemical behavior of metallic titanium/mixed phase titanium oxide is investigated. Further, the phase structure of a titanium oxide photocatalyst prepared on a titanium electrode through a high-voltage anodization method is examined. The study exploits photo-electrochemical, Fourier transform infrared spectroscopy attenuated total reflectance (FTIR–ATR), X-ray diffraction, and Raman spectroscopic methods to obtain better insights into the mechanism of mixed-phase titanium oxide formation. In this regard, the photo-electrochemical properties of the photocatalysts prepared in single excitation energy, violet light (410 nm), were investigated. The anodization time and the potassium hydroxide concentration in the anodization bath have significant effects on the photo-electrochemical properties of the photocatalysts. The experiments show that the effect of potassium hydroxide concentration is a function of the anodization potential applied, demonstrating different patterns as the anodization potential changes. Furthermore, FTIR-ATR, X-ray diffraction, and Raman spectroscopic studies reveal that the extended anodization times decrease the population of OH-containing groups, leading to lower photo-electrochemical performance. On the other hand, the formation of anatase phases becomes more favorable only in the extended anodization times before application of the calcination process. Additionally, the calcination temperature has a significant impact on the anatase to rutile ratio. Finally, increasing potassium hydroxide concentration leads to the formation of an amorphous titanium oxide layer. It can be concluded that the obtained information might have a significant impact on the preparation of titanium oxide and other metal oxide photocatalysts through the high voltage anodization process.     

Investigation on electrochemical behavior and surface conductivity of titanium carbide modified Ti bipolar plate of PEMFC

The titanium carbide (TiC) modified layer is prepared by plasma surface modification technology on the surface of Ti plate (TA1) to meet the performance requirements of bipolar plate of PEMFC. The microstructure characterization confirms that a compact and defectless TiC modified layer is formed on the surface of Ti bipolar plate. The corrosion current density of TiC modified plate in simulated PEMFC environment is reduced by approximately an order of magnitude and the self-corrosion potential is significantly improved compared with bare Ti plate. The interfacial contact resistance (ICR) of TiC modified plate (7.5 mΩ cm², under loading pressure of 140 N) is evidently lower than bare Ti plate (98.1 mΩ cm²). Even after potentiostatic polarization, the ICR of TiC modified plate still remains at satisfactory values. Furthermore, the contact angle of TiC modified plate reaches a higher value of 112°, which is beneficial to the water discharge of PEMFC.     

The impact of etching during microfabrication on the microstructure and the electrical conductivity of gadolinia-doped ceria thin films

Gadolinia-doped ceria, Ce_0.8Gd_0.2O_1.9−x (CGO), thin films deposited by spray pyrolysis and annealed to different degree of crystallinity between 0% and 95% are exposed to different etchants and etching methods. The attack of the etchants on the CGO thin films is analyzed with respect to changes in microstructure and in-plane electrical conductivity. It is found that amorphous CGO films are dissolved in hydrochloric acid after elongated etching times. Hydrofluoric acid severely attacks CGO thin films after already short times of exposure (1 min), more intense the less crystalline the thin film is. Ar ion etching smoothens the surface of the CGO thin films without considerable removal of material. No microstructural attack of NaOH, CHF₃/O₂ and SF₆/Ar is found. The electrical conductivity is in general only affected when microstructural changes are severe. Therefore, it is concluded that CGO thin films can be well used as functional layers in micro-fabricated devices and that micro-fabrication is, with the exception of hydrofluoric, not harmful for the electrical properties of crystalline CGO thin films.