Decorating carbon nanotubes with Ni particles using an electroless deposition technique for hydrogen storage applications

A facile and low-cost electroless deposition technique is utilized to decorate multi-walled carbon nanotubes (CNTs) with Ni. The obtained composites are attempted to use as hydrogen storage materials, whose performance is evaluated with a high-pressure microbalance. Effects of the concentration of plating solution, deposition time, and reaction temperature on the loading amount, particle size, morphology, and distribution density of Ni are studied using a transmission electron microscope. With proper deposition parameters, highly dispersed Ni nanoparticles with a uniform diameter can be fabricated on CNTs, causing a notable hydrogen spillover reaction on the composite. The optimum hydrogen storage capacity of the prepared Ni-decorated CNTs with a average diameter of 5 nm, measured at 6.89 MPa and 25 °C, is 1.02 wt%, which is almost three times higher than that (0.35 wt%) of pristine CNTs.     

Coated stainless steels evaluation for bipolar plates in PEM water electrolysis conditions

Proton exchange membrane water electrolysis (PEMWE) is a promising technology to be incorporated in the production of green hydrogen, but one of its limitation to market penetration is the cost of bipolar plates (BPP).Aiming to reduce the cost of PEMWE stack, different surface engineered coating systems based on CrN/TiN, Ti/TiN, Ti and TiN deposited by physical vapor deposition on SS 316L, SS 904L and SS 321 were tested, as potential cost effective solutions to be implemented on bipolar plates. A corrosion evaluation has been carried out in anodic PEMWE conditions in order to determine the best substrate/coating combination for bipolar plates. Ti/TiN multi-layered coating on SS 321 shown the best performance with ?0.02% weight loss, current at 2 V_SHE to 436 μA cm^?2 and ICR after corrosion test to 9.9 mΩ cm².     

Single-particle investigation on the activation process of a hydrogen storage alloy

Using an improved apparatus to investigate the activation process of single particles of hydrogen storage alloy LaNi_3.55Co_0.75Mn_0.4Al_0.3, we directly showed the particle's morphology changes during the activation process. Electrochemical properties were systematically monitored during the activation process of the single particles. We finally proposed a new parameter – normalized output rate (NOR) – to evaluate the output performance of the electrode material. In addition to revealing what occurred during the activation process, we provided a program for the quick testing of electrode materials.     

Electroless deposition of Co(Mn)/Pd-decorator into Y2O3-stabilized ZrO2 scaffold as cathodes for solid oxide fuel cells

In this work, we show that higher Co or Mn dopant content of up to 25 mol% on Pd can be obtained without modifying the original crystal structure of Pd via co-reduction synthesis route using hydrazine (N₂H₄), which is not achievable via conventional sol-gel route. The synthesized Co/Mn-doped Pd alloys can inhibit Pd agglomeration that hinders Pd use as an oxygen reduction reaction (ORR) promoter on solid oxide fuel cell (SOFC) cathode. The presence of Co or Mn on Pd lattice of Co/Mn-doped Pd alloy-decorated YSZ (Y₂O₃ stabilized ZrO₂) cathode provides dual advantages of improving the ORR performance of the cathode and enhancing the ORR performance stability during long term operation. Between 600 °C and 800 °C, Pd_0.90Co_0.10O-impregnated YSZ cathode displayed the highest ORR performance among PdO-, Pd_0.90Co_0.10O-, Pd_0.75Co_0.25O-, Pd_0.90Mn_0.10O-, and Pd_0.75Mn_0.25O-impregnated cathodes as indicated by its relatively low area specific resistance of 0.14 Ω cm² at 700 °C. Our thermal gravimetric analyses and scanning electron microscopy images revealed that the high electrochemical performance stability of Co/Mn-doped Pd alloys during 30 hour-cathodic current test correlates with their higher metal ? metal oxide conversion reversibility and microstructure stability under thermal cycling between room temperature and 900 °C (relative to Pd).     

Study on dimensional and corrosion properties of thixoformed A356 and AA7075 aluminum bipolar plates for proton exchange membrane fuel cells

Metallic bipolar plates for polymer exchange membrane (PEM) fuel cells are currently manufactured by stamping of thin sheets. However, there are dimensional and shape errors of microchannels because of forming limitation such as spring back of thin sheets after stamping. On the other hand, stamping process is limited to commercially available sheet alloys, which restricts the development of a high corrosion resistant substrate aluminum alloy. Here, thixoforming (a commercial semisolid route) that is applicable to a wide range of aluminum alloys is proposed for net-shape micromanufacturing of aluminum bipolar plates with high dimensional stability. High corrosion resistance cast A356 (Cu-free) and wrought AA7075 (∼2% Cu) aluminum billets are used for this study. Initial billets are heated at different semisolid temperatures. Subsequently, the semisolid slurries are injected into the die cavity. A356 and AA7075 aluminum bipolar plates are successfully fabricated by thixoforming with very small deviation of 0.7% and 1.5% from the nominal value of 0.300 mm in the microchannel depth, respectively. A multilayer coating of TiN/CrN is deposited on the surface of thixoformed bipolar plates through a commercially available magnetron sputtering technique. Electrochemical corrosion tests show that coated-thixoformed A356 (Cu-free) bipolar plates have significantly lower corrosion current densities than coated-thixoformed AA7075 (∼2% Cu) bipolar plates. This seems to be due to the deleterious effect of Cu alloying element on the corrosion resistance of aluminum alloys that clearly confirms the importance of substrate material development for corrosive PEM fuel cell environment. It is suggested that specific high corrosion resistance aluminum alloy for PEM fuel cell application can be simply designed and then thixoforming can be efficiently and cost effectively employed to fabricate net-shape aluminum bipolar plates.     

Fabrication of nickel boride-coated carbon nanotube films by electrophoresis and electroless deposition for electrochemical hydrogen storage

Network-like carbon nanotube (CNT) films free of polymer binder were deposited directly onto a stainless steel substrate by electrophoresis. Results of experiments indicated that a CNT film with duplex surface treatment (nitric acid etching and nickel boride coating) provides satisfactory electrochemical hydrogen storage. Nitric acid treatment increased the hydrogen storage capacity of the CNT film due to the opening of CNTs, the formation of micropores in the CNT surface, and the improvement of surface wettability. Coating the CNT film with nickel boride (Ni₂B) greatly improved the electrochemical activity of the film and consequently increased the hydrogen storage capacity. The optimal amount of nickel boride coating on the CNT film was found to be about 45 wt%. Smaller amounts of nickel boride cannot provide enough electrochemical activity. Excessive nickel boride, on the other hand, leads to a decrease in the number of active sites on CNTs that are available for hydrogen storage.     

A review of modified metal bipolar plates for proton exchange membrane fuel cells

In recent years, the proton exchange membrane fuel cell (PEMFC) has been widely studied due to its high energy efficiency and non-polluting products. As a key component of PEMFC, bipolar plates (BPPs) play an important role in isolating reaction gas, distributing flow field, collecting electrons and conducting heat. Metal BPPs have excellent manufacturing performance, low cost, and mechanical strength. Therefore, it is considered to be a powerful substitute for traditional graphite BPP. The surface modification of metal BPP is essential for its application in PEMFC. In this review, the latest developments in popular coatings were reviewed from the perspective of corrosion resistance, conductivity and contact angle of metal BPPs in PEMFC environments. The strengths and weaknesses of different surface modification methods were analyzed. Meanwhile,the development trend of future commercialization was also considered.     

Hydrogen generation from the hydrolysis of ammonia borane using cobalt-nickel-phosphorus (Co-Ni-P) catalyst supported on Pd-activated TiO2 by electroless deposition

Catalytically active, low-cost, and reusable transition metal catalysts are desired to develop on-demand hydrogen generation system for practical onboard applications. By using electroless deposition method, we have prepared the Pd-activated TiO₂-supported Co-Ni-P ternary alloy catalyst (Co-Ni-P/Pd-TiO₂) that can effectively promote the hydrogen release from ammonia-borane aqueous solution. Co-Ni-P/Pd-TiO₂ catalysts are stable enough to be isolated as solid materials and characterized by XRD, SEM, and EDX. They are isolable, redispersible and reusable as an active catalyst in the hydrolysis of AB. The reported work also includes the full experimental details for the collection of a wealth of kinetic data to determine the activation energy (E_a = 54.9 kJ mol⁻¹) and effects of the amount of catalyst, amount of substrate, and temperature on the rate for the catalytic hydrolysis of AB. Maximum H₂ generation rate of ∼60 mL H₂ min⁻¹ (g catalyst)⁻¹ and ∼400 mL H₂ min⁻¹ (g catalyst)⁻¹ was measured by the hydrolysis of AB at 25 °C and 55 °C, respectively.     

Investigation of single-layer and multilayer coatings for aluminum bipolar plate in polymer electrolyte membrane fuel cell

Aluminum bipolar plates offer good mechanical performance and availability for mass production while allow up to 65% lighter than stainless steel. To improve the corrosion resistance and surface electrical conductivity of aluminum bipolar plates, several coatings, including TiN, CrN, C, C/TiN and C/CrN, are deposited on aluminum alloy 5052 (AA-5052) by close field unbalanced magnetron sputter ion plating. Scanning electron microscope (SEM) results show that the coatings containing carbon layer are denser than TiN and CrN. Although the potentiodynamic test results show improved corrosion resistance by all the coatings, the potentiostatic test results reveal different stability of these coatings in PEMFC environments. Comparing the SEM images of these coatings after potentiostatic test, C/CrN multilayer coating exhibits the best stability. C/CrN multilayer coated AA-5052 has the lowest metal ion concentration after potentiostatic test, being 11.12 ppm and 1.29 ppm in PEMFC cathodic and anodic environments, respectively. Furthermore, the interfacial contact resistance (ICR) of the bare AA-5052 is decreased from 61.58 mΩ-cm² to 4.08 mΩ-cm² by C/CrN multilayer coating at the compaction force of 150 N-cm⁻². Therefore, C/CrN multilayer coating is a good choice for surface modification of aluminum bipolar plate.     

Electrophoretic deposition of polypyrrole/Vulcan XC-72 corrosion protection coatings on SS-304 bipolar plates by asymmetric alternating current for PEM fuel cells

In this study, corrosion protection composite films composed of polypyrrole (formed from the electropolymerization of pyrrol monomer) and Vulcan XC-72, labeled as PPy-C, were deposited on SS-304 substrates by EPD using an asymmetric AC signal (AAC) in acetone and acetone:methanol solutions. Prior to deposition, Vulcan powders were functionalized in order to create functional groups that develop a negative surface charge. Under the applied electric field, both pyrrole monomer and carbon colloidal particles migrated simultaneously toward the SS anode surface to create the corrosion protection PPy-C coatings. The effect of applied voltages (50, 70, 90 and 120 V) and deposition time (2, 6, 10 and 15 min) on the film characteristics was evaluated. The coatings were analyzed by SEM and the results showed improved characteristics of the films deposited by AAC, in terms of homogeneity and porosity, compared to samples prepared by a continuous DC process. Polarization curves were conducted in order to evaluate the corrosion characteristics of the bipolar plates in 0.1 M H₂SO₄ at room temperature. The experimental results indicated that the PPy-C composites significantly enhanced the corrosion resistance of the SS substrates. For example, the T1 sample increased the corrosion potential by about 541 mV and decreased the corrosion current density by two orders of magnitude, compared to uncoated SS. FTIR studies confirmed the formation of PPy after electropolymerization of the pyrrole monomer under the EPD process. Even though this characterization has been carried out in a half cell, it has been proposed that the PPy-C-coated SS bipolar plates may find application in the acidic atmosphere of PEMFCs.     

Surface modifications of gasketless carbon composite bipolar plates for gas tightness of PEM fuel cells

A carbon fiber composite gasketless bipolar plate for proton exchange membrane (PEM) fuel cells was developed using the groove of a trapezoidal shape, which develops large contact stresses due to a wedge mechanism. The surface of the composite groove was modified to increase gas tightness without using rubber gaskets. Graphite, polyimide (PI) and polyethylene terephthalate (PET) film were co-cure bonded on the surface of the bipolar plate during the compression molding process to change its surface hardness, which could affect the gas tightness. Additionally, plasma surface treatment and mechanical abrasion were used to modify the surface morphologies. Corrosion and gas tightness tests on the surface modified bipolar plates were performed to verify the reliability of the gasketless sealing method.     

Some fatty acids used for latent heat storage: thermal stability and corrosion of metals with respect to thermal cycling

The present study includes thermal stability of some fatty acids as phase change materials (PCMs). The selected fatty acids were stearic, palmitic, myristic and lauric acid with melting temperatures between 40–63°C and industrial-grade with 90–95 % purity. Latent heat storage capacity and phase transition temperature of the PCMs were determined by Differential Scanning Calorimetry (DSC) technique as a function of after repeated thermal cycles such as 40, 410, 700 and 910. The present work also comprises the investigation of corrosion resistance of some construction materials to the fatty acids over a long period. The containment materials tested were stainless steel (SS 304 L), carbon steel (steel C20), aluminium (Al) and copper (Cu). Gravimetric analysis as mass loss (mg/cm²), corrosion rate (mg/day) and a microscopic or matellographic investigation were performed for corrosion tests after 910 thermal cycles. DSC measurements showed that all fatty acids investigated as PCMs have a good thermal stability as a function of latent heat and phase transition temperature range for an actual middle-term thermal energy storage utility. However, in long-term solar thermal applications, the palmitic acid and myristic acid may be considered more suitable PCMs than the others. From the gravimetric and metallographic results, it can be concluded that stainless steel (SS 304L) with chromium oxide (Cr₂O₃) surface layer and Al with aluminium oxide (Al₂O₃) surface layer are essentially compatible with the investigated fatty acids. Carbon steel (Steel C20) and Cupper (Cu) are only preferantially compatible with PCMs.     

Protective coatings for metal bipolar plates of fuel cells: A review

Proton exchange membrane fuel cell has attracted much attention in recent years due to their advantages of environmental protection and high resource utilization, which is important for improving the global environment. Bipolar plates are the important components of fuel cell, which accounts for most of the weight and high cost. Compared with graphite bipolar plates, metal bipolar plates are easier to machining and have lower cost because of its good mechanical properties. However, in the acidic environment of proton exchange membrane fuel cell operation, metal bipolar plates are prone to corrosion, which leads to lower output efficiency of fuel cell and seriously affected the application. Applying a protective coating to the metal bipolar plates is an effective way to improve its corrosion resistance. This paper mainly introduces the research progress of several anti-corrosion coatings for metal bipolar plates in recent years, and summarizes the challenges and future requirements of metal bipolar plates.     

Pre-oxidized and nitrided stainless steel alloy foil for proton exchange membrane fuel cell bipolar plates. Part 2: Single-cell fuel cell evaluation of stamped plates

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 proton exchange membrane (PEM) single-cell fuel cell studies of stamped and pre-oxidized/nitrided developmental Fe-20Cr-4V weight percent (wt.%) and commercial type 2205 stainless steel alloy foils. The single-cell fuel cell behavior of the stamped and pre-oxidized/nitrided material was compared to as-stamped (no surface treatment) 904L, 2205, and Fe-20Cr-4V stainless steel alloy foils and machined graphite of similar flow field design. The best fuel cell behavior among the alloys was exhibited by the pre-oxidized/nitrided Fe-20Cr-4V, which exhibited ∼5-20% better peak power output than untreated Fe-20Cr-4V, 2205, and 904L metal stampings. Durability was assessed for pre-oxidized/nitrided Fe-20Cr-4V, 904L metal, and graphite plates by 1000+ h of cyclic single-cell fuel cell testing. All three materials showed good durability with no significant degradation in cell power output. Post-test analysis indicated no metal ion contamination of the membrane electrode assemblies (MEAs) occurred with the pre-oxidized and nitrided Fe-20Cr-4V or graphite plates, and only a minor amount of contamination with the 904L plates.     

The effect of pH and halides on the corrosion process of stainless steel bipolar plates for proton exchange membrane fuel cells

Stainless steel is attractive as material for bipolar plates in proton exchange membrane fuel cells, due to its high electrical conductivity, high mechanical strength and relatively low material and processing cost. Potentiostatic and potentiodynamic tests were performed in H₂SO₄ solutions on AISI 316L stainless steel bipolar plates with etched flow fields. The effect of pH and presence of small amounts of fluoride and chloride on the corrosion rate and interfacial contact resistance of the stainless steel bipolar plate were investigated. The tests performed in electrolytes with various pH values revealed that the oxide layer was thinner and more prone to corrosion at pH values significantly lower than the pH one expects the bipolar plate to experience in an operating proton exchange membrane fuel cells. The use of solutions with very low pH in such measurements is thus probably not the best way of accelerating the corrosion rate of stainless steel bipolar plates. By use of strongly acidic solutions the composition and thickness of the oxide layer on the stainless steel is probably altered in a way that might never have happened in an operating proton exchange membrane fuel cell. Additions of fluoride and chloride in the amounts expected in an operating fuel cell (2 ppm F⁻ and 10 ppm Cl⁻) did not cause significant changes for neither the polarization- nor the contact resistance measurements. However, by increasing the amount of Cl⁻ to 100 ppm, pitting was initiated on the stainless steel surface.     

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.     

Development and evaluation of nitride coated titanium bipolar plates for PEM fuel cells

In order to improve the conductivity of titanium bipolar plate under the premise of ensuring its corrosion resistance for the proton exchange membrane fuel cell (PEMFC), the nitride coatings are deposited on the surface of titanium bipolar plate via a powder immersion reaction assisted coating (PIRAC) method. Both the scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) show that the dense titanium nitride coatings with the thickness around 1.5–2.5 μm are successfully prepared. Furthermore, the X-ray photoelectron spectroscopy (XPS) results confirm the presence of TiN, TiN_xO_y and TiO₂ phases on the surface of nitride coatings, and the content of these phases is tunable by adjusting the prepared temperatures. Both the microstructure, the thickness and the composition of the nitride coatings could be associated with the corrosion resistance and the interfacial contact resistance of the nitrided samples. We find that the nitrided samples prepared at 1000 °C could be the ideal mixed coating materials with the proper combination of the corrosion resistance and the interfacial contact resistance, which also show the best long-term stability in simulated PEMFC cathode environment.     

Hydrogen storage properties and thermal stability of V35Ti20Cr45 alloy by heat treatment

The crystal structure, microstructure, hydrogen storage properties and thermal stability of the as-cast and annealed V₃₅Ti₂₀Cr₄₅ alloys prepared by arc-melting were studied in this work. It was confirmed that the as-cast alloy is a body-centered cubic (bcc) single phase, while it consists of bcc main phase and C14-typed Laves secondary phase after annealed at 973 K for 72 h. As a result of the microstructure change, the activation performance and kinetic properties of the annealed alloy are improved greatly due to the catalysis of C14-typed Laves secondary phase in the annealed alloy. The kinetic mechanism of hydrogen absorption/desorption processes in the as-cast and annealed alloys was discussed using the Johnson-Mehl-Avrami (JMA) equation. Based on the plateau pressure data from pressure-composition-temperature (PCT) measurements with the Van't Hoff equation, the calculated formation enthalpies of the hydride formed in the as-cast and annealed alloys indicate that heat treatment results in lower thermal stability of the hydride in the as-cast alloy. Furthermore, using the Kissinger method with the peak temperatures from differential scanning calorimeter (DSC) measurements at different heating rates, the calculated activation energies of the dehydrogenation in the as-cast and annealed alloys suggest that heat treatment is very beneficial to improve hydrogen absorption/desorption capacities in the alloy.     

Surface microstructure and performance of TiN monolayer film on titanium bipolar plate for PEMFC

The influence of bias voltage on surface microstructure of TiN films deposited on Ti substrate by multi-arc ion plating was systematically investigated. The TiN films were characterized using X-ray diffraction, scanning electron microscopy and atomic force microscopy. The corrosion resistance was tested by potentiodynamic polarization and electrochemical impedance spectroscopy at 70–80 °C in the simulated PEMFC cathode environment. The results show that the surface microstructure of TiN film depends strongly on the bias voltages. At the bias voltage of −100 V, TiN film shows the optimum surface microstructure with the lowest surface roughness R_z of 0.039 μm tested by AFM and relatively high compactness. The optimized TiN film exhibits excellent corrosion resistance with corrosion current density of 0.87 μA/cm² in a 0.5 M H₂SO₄ + 2 ppm HF solution at 80 °C with air and a low interfacial contact resistance (ICR) value of 3.0 mΩ cm² at a compaction force of 140 N/cm². These results support TiN as a promising coating material for Ti bipolar plates.     

An investigation of Zr-based bulk metallic glasses as bipolar plates for proton exchange membrane fuel cells

The electrochemical properties and interfacial contact resistance (ICR) of four Zr-based bulk metallic glasses with different compositions are evaluated for PEMFC applications. Based on the results and market demands, the corrosion behavior of the Zr_41·2Ti_13·8Cu_12·5Ni₁₀Be_22.5 (numbers indicate at.%) BMG and 304 stainless steel (SS304) in accelerated simulated anode and cathode environments, such as 0.5 M H₂SO₄ and 2 ppm HF solutions bubbled with pure hydrogen and air at 80 °C, respectively, is further investigated through potentiodynamic polarization, potentiostatic polarization, and electrochemical impedance spectroscopy. The performance tests of the single cell with the Zr-based BMG as BPPs are conducted and the maximum power density of the single cell has exceeded 470 mW/cm². The combination of these results and other properties demonstrate that the Zr-based BMG can be used as the anode or cathode material for metallic bipolar plates.