Chromium nitride/Cr coated 316L stainless steel as bipolar plate for proton exchange membrane fuel cell

Chromium nitride/Cr coating has been deposited on surface of 316L stainless steel to improve conductivity and corrosion resistance by physical vapor deposition (PVD) technology. Electrochemical behaviors of the chromium nitride/Cr coated 316L stainless steel are investigated in 0.05 M H₂SO₄ + 2 ppm F⁻ simulating proton exchange membrane fuel cell (PEMFC) environments, and interfacial contact resistance (ICR) are measured before and after potentiostatic polarization at anodic and cathodic operation potentials for PEMFC. The chromium nitride/Cr coated 316L stainless steel exhibits improved corrosion resistance and better stability of passive film either in the simulated anodic or cathodic environment. In comparison to 316L stainless steel with air-formed oxide film, the ICR between the chromium nitride/Cr coated 316L stainless steel and carbon paper is about 30 mΩ cm² that is about one-third of bare 316L stainless steel at the compaction force of 150 N cm⁻². Even stable passive films are formed in the simulated PEMFC environments after potentiostatic polarization, the ICR of the chromium nitride/Cr coated 316L stainless steel increases slightly in the range of measured compaction force. The excellent performance of the chromium nitride/Cr coated 316L stainless steel is attributed to inherent characters. The chromium nitride/Cr coated 316L stainless steel is a promising material using as bipolar plate for PEMFC.     

An investigation into TiN-coated 316L stainless steel as a bipolar plate material for PEM fuel cells

In order to reduce the cost, weight and volume of the bipolar plates, considerable attention is being paid to developing metallic bipolar plates to replace the non-porous graphite bipolar plates that are in current use. However, metals are prone to corrosion in the proton exchange membrane (PEM) fuel cell environments, which decreases the ionic conductivity of the membrane and lowers the overall performance of the fuel cells. In this study, TiN was coated on SS316L using a physical vapor deposition (PVD) technology (plasma enhanced reactive evaporation) to increase the corrosion resistance of the base SS316L. X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical methods were used to characterize the TiN-coated SS316L. XRD showed that the TiN coating had a face-centered-cubic (fcc) structure. Potentiodynamic tests and electrochemical impedance tests showed that the corrosion resistance of SS316L was significantly increased in 0.5 M H₂SO₄ at 70 °C by coating with TiN. In order to investigate the suitability of these coated materials as cathodes and anodes in a PEMFC, potentiostatic tests were conducted under both simulated cathode and anode conditions. The simulated anode environment was −0.1 V versus SCE purged with H₂ and the simulated cathode environment was 0.6 V versus SCE purged with O₂. In the simulated anode conditions, the corrosion current of TiN-coated SS316L is −4 × 10⁻⁵ A cm⁻², which is lower than that of the uncoated SS316L (about −1 × 10⁻⁶ A cm⁻²). In the simulated cathode conditions, the corrosion current of TiN-coated SS316L is increased to 2.5 × 10⁻⁵ A cm⁻², which is higher than that of the uncoated SS316L (about 5 × 10⁻⁶ A cm⁻²). This is because pitting corrosion had taken place on the TiN-coated specimen.     

Conductive amorphous carbon-coated 316L stainless steel as bipolar plates in polymer electrolyte membrane fuel cells

Amorphous carbon (a-C) film about 3 μm in thickness is coated on 316L stainless steel by close field unbalanced magnetron sputter ion plating (CFUBMSIP). The AFM and Raman results reveal that the a-C coating is dense and compact with a small size of graphitic crystallite and large number of disordered band. Interfacial contact resistance (ICR) results show that the surface conductivity of the bare SS316L is significantly increased by the a-C coating, with values of 8.3–5.2 mΩ cm² under 120–210 N/cm². The corrosion potential (E_corr) shifts from about −0.3 V vs SCE to about 0.2 V vs SCE in both the simulated anode and cathode environments. The passivation current density is reduced from 11.26 to 3.56 μA/cm² with the aid of the a-C coating in the simulated cathode environment. The a-C coated SS316L is cathodically protected in the simulated anode environment thereby exhibiting a stable and lower current density compared to the uncoated one in the simulated anode environment as demonstrated by the potentiostatic results.     

Titanium oxynitride films for a bipolar plate of polymer electrolyte membrane fuel cell prepared by inductively coupled plasma assisted reactive sputtering

Titanium oxynitride (TiN_xO_y) films are investigated for application as a bipolar plate coating material in a polymer electrolyte membrane fuel cell (PEMFC). TiN_xO_y films with various amounts of oxygen are deposited on stainless-steel substrates by inductively coupled plasma (ICP) assisted reactive sputtering by changing the oxygen gas flow rate. The interfacial contact resistance (ICR) and the corrosion resistance of the TiN_xO_y films are measured under PEMFC simulated conditions. When the amount of oxygen in the TiN_xO_y film is approximately <12 at.% (O₂ flow rate ≤0.2 sccm), the corrosion resistance is enhanced considerably, whereas the interfacial contact resistance does not change. The corrosion current density decreases from 8 × 10⁻⁶ A cm⁻² for the TiN-coated sample to 2.7 × 10⁻⁶ A cm⁻² at 0.6 V vs. SCE as a result of oxygen incorporation in the TiN film. The ICR value remains at 2.5 mΩ cm² at 150 N cm⁻². When a small amount of oxygen is added to the TiN film, it is postulated that the oxygen atoms locate at the column and grain boundaries, and thus prevent corrosive media from penetrating into the substrate while not deteriorating the electrical property of the film.     

An investigation into nickel implanted 316L stainless steel as a bipolar plate for PEM fuel cell

A nickel-rich layer about 100 μm in thickness with improved conductivity was formed on the surface of austenitic stainless steel 316L (SS316L) by ion implantation. The effect of ion implantation on the corrosion behavior of SS316L was investigated in 0.5 M H₂SO₄ with 2 ppm HF solution at 80 °C by potentiodynamic test. In order to investigate the chemical stability of the ion implanted SS316L, the potentiostatic test was conducted in an accelerated cathode environment and the solutions after the potentiostatic test were analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES). The results of potentiodynamic test show that the corrosion potential of SS316L is shifted toward the positive direction from −0.3 V versus SCE to −0.05 V versus SCE in anode environment and the passivation current density at 0.6 V is reduced from 11.26 to 7.00 μA cm⁻² in the cathode environment with an ion implantation dose of 3 × 10¹⁷ ions cm⁻². The potentiostatic test results indicate that the nickel implanted SS316L has higher chemical stability in the accelerated cathode environment than the bare SS316L, due to the increased amount of metallic Ni in the passive layer. The ICP results are in agreement with the electrochemical test results that the bare SS316L has the highest dissolution rate in both cathode and anode environments and the Ni implantation markedly reduces the dissolution rate. A significant improvement of interfacial contact resistance (ICR) is achieved for the SS316L implanted with nickel as compared to the bare SS316L, which is attributed to the reduction in passive layer thickness caused by the nickel implantation. The ICR values for implanted specimens increase with increasing dose.     

Effect of plasma nitriding on behavior of austenitic stainless steel 304L bipolar plate in proton exchange membrane fuel cell

A dense and supersaturated nitrogen layer with higher conductivity is obtained on the surface of austenitic stainless steel 304L by the low temperature plasma nitriding. The effect of plasma nitriding on the corrosion behavior and interfacial contact resistance (ICR) for the austenitic stainless steel 304L was investigated in 0.05 M H₂SO₄ + 2 ppm F⁻ simulating proton exchange membrane fuel cell (PEMFC) environment using electrochemical and electric resistance measurements. The experiment results show that the stable passive film is formed after the potentiostatic polarization at the specified anodic or cathodic potentials under PEMFC operation condition, and the plasma nitriding improves slightly the corrosion resistance and decreases markedly the ICR of 304L. The ICR of the plasma nitrided 304L increases after the potentiostatic polarizations for 4 h, and lower than 100 mΩ cm² at the compaction force of 150 N cm⁻².     

Effects of low-temperature nitridation on the electrical conductivity and corrosion resistance of 446M stainless steel as bipolar plates for proton exchange membrane fuel cell

Low-temperature nitridation was used to form a protective and conductive layer on stainless steel. The surface characterization reveals that a continuous and protective Cr-nitride/oxide layer (CrN and Cr₂O₃) forms on the 446M stainless steel surface after low-temperature nitridation. The electrical conductivity of the sample is investigated in terms of the interfacial contact resistance. This value for nitrided 446M at low temperature is 6 mΩ cm², which is much lower than that of the bare 446M stainless steel (about 77 mΩ cm²) at a compaction force of 140 N/cm². The corrosion resistance of low-temperature nitrided 446M stainless steel is examined in potentiodynamic and potentiostatic tests under simulated polymer electrolyte membrane fuel cell (PEMFC) conditions with pH 3 H₂SO₄ at 80 °C. In a simulated anode condition, the current density is −1 × 10⁻⁶ A/cm². In a simulated cathode condition, the current density is 1 × 10⁻⁷ A/cm². Low-temperature nitrided 446M stainless steel shows superior electrical conductivity and corrosion resistance than bare 446M stainless steel.     

Electrochemical durability of gas diffusion layer under simulated proton exchange membrane fuel cell conditions

An effective ex-situ method for characterizing electrochemical durability of a gas diffusion layer (GDL) under simulated polymer electrolyte membrane fuel cell (PEMFC) conditions is reported in this article. Electrochemical oxidation of the GDLs are studied following potentiostatic treatments up to 96 h holding at potentials from 1.0 to 1.4 V (vs.SCE) in 0.5 mol L⁻¹ H₂SO₄. From the analysis of morphology, resistance, gas permeability and contact angle, the characteristics of the fresh GDL and the oxidized GDLs are compared. It is found that the maximum power densities of the fuel cells with the oxidized GDLs hold at 1.2 and 1.4 V (vs.SCE) for 96 h decreased 178 and 486 mW cm⁻², respectively. The electrochemical impedance spectra measured at 1500 mA cm⁻² are also presented and they reveal that the ohmic resistance, charge-transfer and mass-transfer resistances of the fuel cell changed significantly due to corrosion at high potential.     

Poly(3-methylthiophene)/MnO2 composite electrodes as electrochemical capacitors

Composite electrodes prepared by electrodeposition of manganese oxide on titanium substrates modified with poly(3-methylthiophene) (PMeT) were investigated and compared with Ti/MnO₂ electrodes. The polymer films were prepared by galvanostatic deposition at 2 mA cm⁻² with different deposition charges (250 and 1500 mC cm⁻²). The electrodes were characterized by cyclic voltammetry in 1 mol L⁻¹ Na₂SO₄ and by scanning electron microscopy. The results show a very significant improvement in the specific capacitance of the oxide due the presence of the polymer coating. For Ti/MnO₂ the specific capacitance was of 122 F g⁻¹, while Ti/PMeT₂₅₀/MnO₂ and Ti/PMeT₁₅₀₀/MnO₂ displayed values of 218 and 66 F g⁻¹, respectively. If only oxide mass is considered, the capacitances of the composite electrode increases to 381 and 153 F g⁻¹, respectively. The micrographs of samples show that the polymer coating leads to very significant changes in the morphology of the oxide deposit, which in consequence, generate the improvement observed in the charge storage property.     

Corrosion of carbon support for PEM fuel cells by electrochemical quartz crystal microbalance

During the voltammetry of carbon supports for proton exchange membrane fuel cells (PEMFCs), including commercial carbon blacks, graphitized carbon black and multi-wall carbon nanotubes (MWNTs), in deaerated 0.5 M H₂SO₄ solution results in mass changes as observed by using in situ electrochemical quartz crystal microbalance (EQCM). The mass change and corrosion onset potential during electrochemical carbon corrosion indicate that oxides are formed and accumulated on the carbon surface, leading to an increase in mass. A decrease in the mass is associated with carbon loss from the gasification of carbon surface oxides into carbon dioxide. High BET surface area carbon blacks ECP600 and ECP 300 have a carbon loss of 0.0245 ng cm⁻² s⁻¹ and 0.0144 ng cm⁻² s⁻¹ and as compared to 0.0115 ng cm⁻² s⁻¹ for low surface area support XC-72 and so they are less resistant to corrosion. Graphitized XC-72 and MWNTs, with higher graphitization have higher carbon corrosion onset potential at 1.65 V and 1.62 V and appear to be more intrinsically resistant to corrosion.     

Effect of Ti sublayer on the ORR catalytic efficiency of dc magnetron sputtered thin Pt films

A series of thin Pt films were deposited by dc magnetron sputtering directly on a commercial hydrophobic carbon paper substrate having a thin microporous Vulcan-XC72 layer or upon a thin Ti sublayer sputtered on the top of the microporous carbon film. The electrocatalytic properties of the sputtered Pt films toward the oxygen reduction reaction were investigated in 0.5 M H₂SO₄ solution and in a hydrogen PEM fuel cell. The catalyst with ultralow Pt loading of 22 μg cm⁻² deposited on a 33 Å thick Ti sublayer is robust, mechanically stable, possesses highly developed surface area and improved catalytic efficiency. Its performance as a MEA cathode in a single hydrogen PEM fuel cell (577 mA cm⁻² at 0.4 V cell voltages and a maximum power of 0.954 W) proved to be much superior compared to that of MEA with the same cathode Pt loading but without Ti sublayer (173 mA cm⁻² at 0.4 V, 0.231 W, respectively).     

Performance of Ti-Ag-deposited titanium bipolar plates in simulated unitized regenerative fuel cell (URFC) environment

Titanium with excellent corrosion resistance, good mechanical strength and lightweight is an ideal BPP material for unitized regenerative fuel cell (URFC), but the easy-passivation property accordingly results in poor cell performance. Surface modification is needed to improve the interfacial conductivity. In this study, Ti-Ag film is prepared on TA1 titanium as bipolar plates for URFC by pulsed bias arc ion plating (PBAIP). Interfacial conductivity of Ti-Ag/Ti is improved obviously, presenting an interfacial contact resistance of 4.3 mΩ cm² under 1.4 MPa. The results tested by potentiodynamic, potentiostatic and stepwise potentiostatic measures in simulated URFC environments show that Ti-Ag/Ti has good anticorrosion performance, especially at high potential. The corrosion current density of Ti-Ag/Ti is approximately 10^−5.0 A cm⁻², similar to that of uncoated titanium, at 2.00 V (vs. NHE) in a 0.5 M H₂SO₄ + 5 ppm F⁻ solution at 70 °C with pressured air purging. Ti-Ag/Ti sample also has low surface energy. The contact angle of the sample with water is 102.7°, which is beneficial for water management in URFC. The bipolar plate with cost-effective Ti-Ag film combines the prominent interfacial conductivity with the excellent corrosion resistance at high potential, showing great potential of application in URFC.     

Surface modification and performance of inexpensive Fe-based bipolar plates for proton exchange membrane fuel cells

A reforming pack chromization with rolling pretreatment process is utilized to develop inexpensive and high-performance Fe-based metal bipolar plates (SS 420, SS 430, and SS 316 stainless steels) for PEMFC systems. Rolling process is previously performed to reduce the chromizing temperature and generate a coating possessing excellent conductivity and corrosion resistance on the steels during chromization. The power efficiencies of rolled-chromized and simple chromized bipolar plates are compared with graphite bipolar plates employed in PEMFCs. The results show that the rolled-chromized bipolar plates have a corrosion current (I_corr) of 7.87 × 10⁻⁸ A cm⁻² and an interfacial contact resistance of 9.7 mΩ cm². Moreover, the power density of the single cell assembled with rolled-chromized bipolar plates is 0.46 W cm⁻², which is very close to that of graphite (0.50 W cm⁻²), in the tested conditions of this study.     

Corrosion behaviors and contact resistances of the low-carbon steel bipolar plate with a chromized coating containing carbides and nitrides

This work improved the surface performance of low-carbon steel AISI 1020 by a reforming pack chromization process at low temperature (700 °C) and investigated the possibility that the modified steels are used as metal bipolar plates (BPP) of PEMFCs. The steel surface was activated by electrical discharge machining (EDM) with different currents before the chromizing procedure. Experimental results indicate that a dense and homogenous Cr-rich layer is formed on the EDM carbon steels by pack chromization. The chromized coating pretreated with electrical discharge currents of 2 A has the lowest corrosion current density, 5.78 × 10⁻⁸ Acm⁻², evaluated by potentiodynamic polarization in a 0.5 M H₂SO₄ solution and the smallest interfacial contact resistance (ICR), 11.8 mΩ-cm², at 140 N/cm². The carbon steel with a coating containing carbides and nitrides is promising for application as metal BPPs, and this report presents the first research in producing BPPs with carbon steels.     

Microstructure and electrochemical characterization of solid oxide fuel cells fabricated by co-tape casting

A co-tape casting technique was applied to fabricate electrolyte/anode for solid oxide fuel cells. YSZ and NiO-YSZ powders are raw materials for electrolyte and anode, respectively. Through adjusting the Polyvinyl Butyral (PVB) amount in slurry, the co-sintering temperature for electrolyte/anode could be dropped. After being co-sintered at 1400 °C for 5 h, the half-cells with dense electrolytes and large three phase boundaries were obtained. The improved unit cell exhibited a maximum power density of 589 mW cm⁻² at 800 °C. At the voltage of 0.7 V, the current densities of the cell reached 667 mA cm⁻². When the electrolyte and the anode were cast within one step and sintered together at 1250 °C for 5 h and the thickness of electrolyte was controlled exactly at 20 μm, the open-circuit voltage (OCV) of the cell could reach 1.11 V at 800 °C and the maximum power densities were 739, 950 and 1222 mW cm⁻² at 750, 800 and 850 °C, respectively, with H₂ as the fuel under a flow rate of 50 sccm and the cathode exposed to the stationary air. Under the voltage of 0.7 V, the current densities of cell were 875, 1126 and 1501 mA cm⁻², respectively. These are attributed to the large anode three phase boundaries and uniform electrolyte obtained under the lower sintering temperature. The electrochemical characteristics of the cells were investigated and discussed.     

Development of high performance carbon composite catalyst for oxygen reduction reaction in PEM Proton Exchange Membrane fuel cells

Highly active and stable carbon composite catalysts for oxygen reduction in PEM fuel cells were developed through the high-temperature pyrolysis of Co–Fe–N chelate complex, followed by the chemical post-treatment. A metal-free carbon catalyst was used as the support. The carbon composite catalyst showed an onset potential for oxygen reduction as high as 0.87 V (NHE) in H₂SO₄ solution, and generated less than 1% H₂O₂. The PEM fuel cell exhibited a current density as high as 0.27 A cm⁻² at 0.6 V and 2.3 A cm⁻² at 0.2 V for a catalyst loading of 6.0 mg cm⁻². No significant performance degradation was observed over 480 h of continuous fuel cell operation with 2 mg cm⁻² catalyst under a load of 200 mA cm⁻² as evidenced by a resulting cell voltage of 0.32 V with a voltage decay rate of 80 μV h⁻¹. Materials characterization studies indicated that the metal–nitrogen chelate complexes decompose at high pyrolysis temperatures above 800 °C, resulting in the formation of the metallic species. During the pyrolysis, the transition metals facilitate the incorporation of pyridinic and graphitic nitrogen groups into the carbon matrix, and the carbon surface doped with nitrogen groups is catalytically active for oxygen reduction.     

Corrosion protection of 304 stainless steel bipolar plates using TiC films produced by high-energy micro-arc alloying process

Metallic bipolar plates look promising for the replacement of graphite due to higher mechanical strength, better durability to shocks and vibration, no gas permeability, acceptable material cost and superior applicability to mass production. However, the corrosion and passivation of metals in environments of proton exchange membrane fuel cell (PEMFC) cause considerable power degradation. Great attempts were conducted to improve the corrosion resistance of metals while keeping low contact resistance. In this paper, a simple, novel and cost-effective high-energy micro-arc alloying process was employed to prepare compact titanium carbide as coatings for the type 304 stainless steel bipolar plates with a metallurgical bonding between the coating and substrate. It was found that TiC coating increased the corrosion potential of the bare steel in 1 M H₂SO₄ solution at room temperature by more than 200 mV, and decreased significantly its corrosion current density from 8.3 μA cm⁻² for the bare steel to 0.034 μA cm⁻² for the TiC-coated steel. No obvious degradation was observed for the TiC coatings after 30-day exposure in solution.     

Cathode development for alkaline fuel cells based on a porous silver membrane

Porous silver membranes were investigated as potential substrates for alkaline fuel cell cathodes by the means of polarization curves and electrochemical impedance spectroscopy measurements. The silver membranes provide electrocatalytic function, mechanical support and a means of current collection. Improved performance, compared to a previous design, was obtained by increasing gas accessibility (using Teflon AF instead of PTFE suspension) and by adding a catalyst (MnO₂ or Pt) in the membrane structure to increase the cathode activity. This new cathode design performed significantly better (∼55 mA cm⁻² at 0.8 V, ∼295 mA cm⁻² at 0.6 V and ∼630 mA cm⁻² at 0.4 V versus RHE) than the previous design (∼30 mA cm⁻² at 0.8 V, ∼250 mA cm⁻² at 0.6 V and ∼500 mA cm⁻² at 0.4 V) in the presence of 6.9 M KOH and oxygen (1 atm(abs)) at room temperature. The hydrophobisation technique of the porous structure and the addition of an extra catalyst appeared to be critical and necessary to obtain high performance. A passive air-breathing hydrogen-air fuel cell constructed from the membranes achieves a peak power density of 65 mW cm⁻² at 0.40 V cell potential when operating at 25 °C showing a 15 mW cm⁻² improvement compared to the previous design.     

YSZ films fabricated by a spin smoothing technique and its application in solid oxide fuel cell

A dense single-layer YSZ film has been successfully fabricated by a spin smoothing method. Followed by a simplified slurry coating, an additional spin smoothing process was conducted to obtain a thinner and smoother film. By employment of high-viscosity slurry including high YSZ content, the film has a suitable thickness by a single coating cycle. With Sm_0.2Ce_0.8O_1.9 (SDC)-impregnated La_0.7Sr_0.3MnO₃ (LSM) cathode and porous NiO–YSZ anode, single solid oxide fuel cell (SOFC) based on an 8-μm-thick YSZ film was obtained. Open-circuit voltage (OCV) of the cell was 1.04 V at 800 °C, and maximum power densities were 676, 965 and 1420 mW cm⁻² at 700, 750 and 800 °C, respectively, using H₂ at a flow rate of 40 mL min⁻¹ as fuel and ambient air as oxidant. The power density could be increased to 1648 mW cm⁻² at 800 °C when the flow rate of H₂ was enhanced to 200 mL min⁻¹.     

The effect of sorbitol on the morphological characteristics of lead–tin films electrodeposited from an alkaline bath

An alkaline Pb–Sn plating bath containing sorbitol as additive has been developed, which has the advantage of low toxicity and ease of handling relative to fluoborate baths, etc. The Pb–Sn deposition voltammetric curve from this bath revealed two deposition processes, at −0.87 and −1.17 V. The voltammetric studies at various sweep rates indicate that the Pb–Sn deposition process is controlled by mass transport. The joint diffusion coefficient of the Pb(II) and Sn(II) sorbitate complex species is 1.15 × 10⁻⁶ cm² s⁻¹. SEM analysis showed that the films produced at −0.87 and −1.17 V are, respectively, composed of dendritic or hexagonal crystals, showing that co-deposition of tin hindered dendritic growth. EDS of the Pb–Sn films showed that the deposit obtained at −0.87 V is pure lead, while that at −1.17 V, with 5.0 or 10.0 C cm⁻², has 19.10 wt% Sn or 26.35 wt% Sn, respectively. It was observed that the Pb–Sn electrodeposited films were grey at both deposition potentials (−0.87 and −1.17 V) and deposition charges (5.0 and 10.0 C cm⁻²). X-ray spectra showed that at the potential −0.87 V a mixture of Pb, PbPt₄ and Pb₂PtO₄ were deposited, while at −1.17 V, Pb, β-Sn, and PbSnO₃ were deposited.