Water vapor adsorption kinetics on small and full scale zeolite coated adsorbers; A comparison

A possibility to enhance both heat and mass transfer characteristics of an adsorber heat exchanger is to apply the adsorbent directly to its surface in form of a consolidated layer. As the majority of the available publications on the effect of both coating technology and adsorbent layer thickness on the adsorption kinetics deals with small scale adsorbent samples, the results obtained can only represent the best case design of an adsorber heat exchanger. This article presents, therefore, a comparison between the adsorption kinetics of water vapour on small as well as two different full scale coated adsorber heat exchanger types with AQSOA-Z02 layers of Mitsubishi Plastics Incorporation under quasi isobaric conditions of adsorption heat pumps. The small scale coated samples have a zeolite dry mass of 200 mg and layer thicknesses of 200, 300 and 500 μm while the full scale adsorbers have a coated zeolite mass between 1.5 and 2.5 kg and layer thicknesses of 150, 200, 300, 400 and 500 μm. In the investigated adsorption heat pump module, up to 52.7 and 57.3% of the equilibrium differential water loading measured with the small scale coated substrates have been obtained after an adsorption-evaporation times of 300 and 600 s, respectively.     

Adjustable TiN coatings deposited with HiPIMS on titanium bipolar plates for PEMFC

TiN coatings were deposited by HiPIMS at different N₂ flow rate to improve the corrosion resistance and conductivity of metallic bipolar plates. The results show that the surface microstructure of TiN coating depends strongly on the N₂ flow rate, all the samples (N₂ flow rate: 4 to10 sccm) meet the DOE 2025 standard and exhibit good hydrophobicity, which has great potential for industrial application. Among them, at the N₂ flow rate of 8 sccm, the TiN coating shows high compactness and the optimum surface microstructure with the lowest surface roughness of 1.083 nm and the highest hardness of 31.172 GPa. The optimized TiN coating exhibits excellent corrosion resistance with corrosion current density of 0.278 μA cm^?2 and a low interfacial contact resistance value of 3.51 mΩ cm². This work has opened a new way for the large-scale preparation of high-performance metal bipolar plate coatings.     

Fast synthesis of thin high silica SSZ-13 zeolite membrane using oil-bath heating

Traditionally, zeolite membranes were prepared using oven heating which usually resulted in lengthy synthesis time and thick membrane. Previous result indicated that oil-bath heating can significantly reduce the synthesis time and membrane thickness of SAPO-34 membrane due to the elimination of thermal-lag. SSZ-13 membrane was prepared using oil-bath heating method. Significant reduction on synthesis time (from 2 to 6 d to 2 h) and membrane thickness (from 5 to 10 μm–1.5 μm) were realized, which is the result of enhanced nucleation and crystallization from ultrafast heat transfer of oil-bath heating. Template removal was realized with an optimized rapid thermal processing method (O-RTP). Outstanding CO₂-CH₄ separation performance was obtained. The high permeance mainly comes from the thinner membrane. The high selectivity can be attributed to O-RTP, which strengthened the bonding between zeolite crystals and minimized thermal exposure time. The combination of oil-bath heating and O-RTP led to high performance SSZ-13 membrane.     

Study on the hydrogen barrier performance of the SiOC coating

SiOC coatings were prepared on X70 pipeline steel substrate by a simple dipping method at low temperatures, and their performance of hindering hydrogen penetration was studied through electrochemical hydrogen permeation experiment. The sample thermal-treated at 120 °C achieved a low diffusion coefficient of hydrogen of 8.20 × 10^?9 cm² s^?1, which was nearly three orders of magnitude lower than 3.58 × 10^?6 cm² s^?1 for the X70 steel. This was due to that the amorphous coating did not provide a stable hydrogen diffusion channel, thus limiting hydrogen diffusion. Density functional theory (DFT) calculation further proved that hydrogen moleculars were difficult to be adsorbed at different sites on the surface of the coating.     

Correlation of critical heat flux and two-phase friction factor for subcooled convective boiling in structured surface microchannels

Critical heat flux (CHF) and pressure drop of subcooled flow boiling are measured for a microchannel heat sink containing 75 parallel 100 μm × 200 μm structured surface channels. The heated surface is made of a Cu metal sheet with/without 2 μm thickness diamond film. Tests and measurements are conducted with de-ionized water, de-ionized water +1 vol.% MCNT additive solution, and FC-72 fluids over a mass velocity range of 820–1600 kg/m² s, with inlet temperatures of 15(8.6)°C, 25(13.6)°C, 44(24.6)°C, and 64(36.6)°C for DI water (FC-72), and heat fluxes up to 600 W/cm². The CHF of subcooled flow boiling of the test fluids in the microchannels is measured parametrically. The two-phase pressure drop is also measured. Both CHF and the two-phase friction factor correlation for one-side heating with two other side-structured surface microchannels are proposed and developed in terms of the relevant parameters.     

Preparation,electrical, mechanical and thermal properties of composite bipolar plate for a fuel cell

A novel composite bipolar plate for a polymer electrolyte fuel cell has been prepared by a bulk-moulding compound (BMC) process. The electrical resistance of the composite material decreases from 20 000 to 5.8 mΩ as the graphite content is increased from 60 to 80 wt.%. Meanwhile, the electrical resistance of composite increases from 6.5 to 25.2 mΩ as the graphite size is decreased from 1000 to 177 μm to less than 53 μm. The thermal decomposition of 5% weight loss of composite bipolar plate is higher than 250 °C. The oxygen permeability of the composite bipolar plate is 5.82×10⁻⁸ (cm³/cm² s) when the graphite content is 75 wt.%, and increases from 6.76×10⁻⁸ to 3.28×10⁻⁵ (cm³/cm² s) as the graphite size is longer or smaller than 75 wt.%. The flexibility of the plate decreases with increasing graphite content. The flexural strength of the plate decreases with decrease in graphite size from 31.25 MPa (1000–177 μm) to 15.96 MPa (53 μm). The flexural modulus decreases with decrease of graphite size from 6923 MPa (1000–177 μm) to 4585 MPa (53 μm). The corrosion currents for plates containing different graphite contents and graphite sizes are all less than 10⁻⁷ A cm⁻². The composite bipolar plates with different graphite contents and graphite sizes meet UL-94V-0 tests, and the limiting oxygen contents are higher than 50. Testing show that composite bipolar plates with optimum composition are very similar to that of the graphite bipolar plate.     

Cu doped Ni–Co spinel protective coatings for solid oxide fuel cell interconnects application

Four different amount of Cu doped Ni–Co alloy coatings were fabricated on SUS430 substrate by electroplating for solid oxide fuel cells (SOFCs) interconnects application. After oxidation at 800 °C, the microstructure and oxide phase of samples were tested by scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Our experimental results indicated that the Cu addition improved the electrical behavior of Ni–Co alloy coating. Cu doping reduced the activation energy (Ea) of electrons hopping and inhibited the growth of Cr₂O₃ oxide layer. Furthermore, the oxidation kinetics and electrical properties of the alloy coatings were obtained. These results showed that the 9% Cu doped Ni–Co coated steels achieved the minimum parabolic rate constant (2.05 × 10⁻¹⁴ g²cm⁻⁴s⁻¹) and area specific resistance (14.11 mΩ cm²) after the thermostatic oxidation process.     

Rapid coating preparation strategy for chromium nitride coated titanium bipolar plates of proton exchange membrane fuel cells

It is critical to develop a coating with sufficient comprehensive performances and efficient preparation strategy for the commercial application of metallic bipolar plate in proton exchange membrane full cell (PEMFC). In this work, chromium nitride coatings prepared by a rapid multi-arc ion plating (MIP) process with various nano thicknesses are investigated in the simulated PEMFC cathodic environments. Both the corrosion resistance and conductivity of the coatings increase with the growth of the coating thickness, which can be attributed to the increasing equivalent diameter, density, and area fraction of the droplets formed on the coating surfaces. The chromium nitride coating with a thickness of approximately 1.0 μm has the lowest I_0.6 V (0.594 μA cm^?2) and interfacial contact resistance (ICR, 6.54 mΩ cm²@1.4 MPa after corrosion test), achieving the 2025 technical targets proposed by the US Department of Energy for bipolar plates. This work shows that rapid preparation by MIP within 12 min is a potential strategy for chromium nitride coated titanium bipolar plates of PEMFCs at industrial scale.     

Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells

Superior corrosion resistance and high electrical conductivity are crucial to the metallic bipolar plates towards a wider application in proton exchange membrane fuel cells. In this work, molybdenum carbide coatings are deposited in different thicknesses onto the surface of 316 L stainless steel by magnetron sputtering, and their feasibility as bipolar plates is investigated. The microstructure characterization confirms a homogenous, compact and defectless surface for the coatings. The anti-corrosion performance improves with the increase of the coating thickness by careful analysis of the potentiodynamic and potentiostatic data. With the adoption of a thin chromium transition layer and coating of a ∼1052 nm thick molybdenum carbide, an excellent corrosion current density of 0.23 μA cm⁻² is achieved, being approximately 3 orders of magnitude lower than that of the bare stainless steel. The coated samples also show a low interfacial contact resistance down to 6.5 mΩ cm² in contrast to 60 mΩ cm² for the uncoated ones. Additionally, the hydrophobic property of the coatings’ surface is beneficial for the removal of liquid water during fuel cell operation. The results suggest that the molybdenum carbide coated stainless steel is a promising candidate for the bipolar plates.     

Manufacturing of zeolite based catalyst from zeolite tuft for biodiesel production from waste sunflower oil

In the present work, zeolite based catalyst was prepared from zeolite tuft by impregnation methods. The zeolite tuft was initially treated with hydrochloric acid (16%) and then several KOH/zeolite catalysts were prepared by impregnation in KOH solutions. Various solutions of KOH with different molarities (1–6 M) were used. Further modification for the catalyst was performed by a 2nd step impregnation treatment by heating and stirring the KOH/zeolite to 80 °C for 4 h. The zeolite tuft and the prepared catalysts were characterized by several analytical techniques in order to explore their physicochemical properties. These tests include: X-Ray Fluorescence (XRF), Scanning Electron Microscopy (SEM), Zero point of Charge (PHzpc), Fourier Transform Infrared (FT-IR), Energy-dispersive X-Ray analysis (EDX) and X-Ray Diffraction (XRD). The catalysts were then used for transesterification of waste sunflower vegetable oil in order to produce biodiesel. Among the different catalysts prepared, the 1–4M KOH/TZT catalyst provided the maximum biodiesel yield of 96.7% at 50 °C reaction temperature, methanol to oil molar ratio of 11.5:1, agitation speed of 800 rpm, 335 μm catalyst particle size and 2 h reaction time. The physicochemical properties of the produced biodiesel comply with the EN and ASTM standard specifications.     

Dynamic ammonia adsorption by FAU zeolites to below 0.1 ppm for hydrogen energy applications

Hydrogen can be released from ammonia (NH₃) by cracking, but the residual ammonia is harmful to polymer exchange membrane fuel cells and should be less than 0.1 ppm (μmol/mol). In this paper, the adsorption of NH₃ by commercial faujasite (FAU) zeolites to below 0.1 ppm have been investigated. The results show that the Si/Al ratio of zeolite is inversely proportional to the adsorption capacity, and the strength of ammonia adsorption by cation Li⁺ is more than that of Na⁺, thus the ammonia adsorption capacity of LiLSX zeolite is greater than that of 13X–HP zeolite. However, the small granule size of crystalline microspheres and the rough surface of 13X–HP zeolite were the factors that lead to the dynamic NH₃ adsorption capacity of 13X–HP zeolite close to LiLSX zeolite. In the dynamic 1700 ppm NH₃ adsorption, with a breakthrough point of 0.1 ppm, the adsorption capacity is 9.27 wt% for LiLSX zeolite and 8.73 wt% for 13X–HP zeolite.     

Thin chromium nitride PVD coatings on stainless steel for conductive component as bipolar plates of PEM fuel cells: Ex-situ and in-situ performances evaluation

In this paper, two types of chromium PVD coatings (100 nm) have been elaborated on 316L stainless steel (SS) by adjusting the nitrogen flow rate. The first coating is a mixture of Cr₂N and Cr, the second one is a single phase CrN. It is shown that the performances of the material are strongly dependant of the nature of the passive film formed on the chromium nitride layers due to the galvanic coupling between the coating and the substrate. The CrN coated SS shows very good corrosion resistance in simulated PEMFC media. The surface conductivity of the SS is also greatly improved and the CrN coated SS shows an interfacial contact resistance of 10 mΩ cm² at 140 N cm⁻². Five single cells of stainless steel bipolar plates coated with the CrN film were assembled for performance test. This 5 cell stack does not show any mean voltage degradation over 200 h dynamic cycling. Moreover, the performances of the CrN coated SS bipolar plates are very close to the Au-coated SS bipolar plates.     

Evaluation of the corrosion resistance of Ni(P)Cr coatings for bipolar plates by electrochemical impedance spectroscopy

In the present research, the corrosion resistance of Ni–P and Ni–P–Cr coatings on AA7075-T6 aluminum plates under simulated anodic and cathodic conditions of polymer electrolyte membrane fuel cells (PEMFC) has been studied by electrochemical impedance spectroscopy (EIS). Three Ni–P coatings 20 μm, 30 μm, and 40 μm thick applied by electroless deposition were tested. Besides, a two-layer Ni–P–Cr coating 30 μm thick was also analyzed. It was formed by an inner Ni–P layer, and an outer 10 μm thick chromium one added by electroplating. Corrosion tests were combined with interfacial contact resistance (ICR), roughness, contact angle, and SEM-EDX measurements. The best results were obtained for the 20 μm Ni–P and the two-layer Ni–P–Cr coatings, although the latter showed a high ICR value due to the high electrical resistivity of the chromium oxide surface formed. It was verified that coating degradation occurs when the electrolyte penetrates the micro-cracks and the nodular surface interfaces, reaching the base metal and causing the coating delamination. This behavior is associated with a sharp decrease in the polarization resistance (R_p) of the equivalent circuit model fitted to the EIS results.     

Tuning of electrocatalytic activity of mixed metal dichalcogenides supported NiMoP coatings for alkaline hydrogen evolution reaction

The mixed metal dichalcogenides combination of WS₂–MoS₂ was coated onto Cu substrate by electroless NiMoP plating technique and the electrocatalytic hydrogen evolution reaction (HER) performance was investigated. The enhanced structural, morphological parameters and boosted electrocatalytic performance of the various metal-metal molar ratio of WS₂–MoS₂ onto NiMoP plate were identified under variable operating conditions and it was successfully evaluated by various characterization techniques. The well-defined crystalline nature, phase, particle size, structure, elemental analysis and surface morphology of prepared coatings were analyzed by FESEM, XRD, AFM and EDS mapping. The electrochemical analysis was performed using open circuit potential (OCP) analysis, chronoamperometry (CA), electrochemical impedance spectroscopy (EIS), Tafel curves, linear sweep voltammetry (LSV), cyclic voltammetry (CV) and polarization studies to find the activity of prepared electrocatalyst towards electrochemical hydrogen evolution reactions. The performance of bare NiMoP and WS₂–MoS₂/NiMoP plates were compared and found that the HER activity of NiMoP can be reinforced by composite incorporation through the synergic effect arises with in the catalytic system, which improves surface roughness and enhances the magnitude of electrocatalyst toward HER. The achievement of enhanced catalytic performance of coatings was authenticate by the kinetic parameters such as decreases in Tafel slope (98 mV dec^?1), enhanced exchange current densities (9.32 × 10^?4 A cm^?2), and a lower overpotential. The consistent performance and durability of the catalyst were also investigated. The enhanced electrocatalytic activity of WS₂–MoS₂/NiMoP coatings increased with respect to the surface-active sites associated with combination of mixed dichalcogenides and the synergic effect arises in between different components present in the coating system. This work envisages the progressive strategies for the economical exploration of a novel WS₂–MoS₂/NiMoP water splitting catalyst used for large scale H₂ generation. The prepared WS₂–MoS₂/NiMoP embedded Cu substrate possess high catalytic activity due to its least overpotential of 101 mV at a benchmark current density of 10 mA cm^?2, which demonstrated the sustainable, efficient and promising electrocatalytic property of prepared catalyst towards HER under alkaline conditions.     

Preparation and characterization of C/Ni-PTFE electrode using Ni-PTFE composite plating for alkaline fuel cells

Graphite particles (80 μm) and PTFE particles (40 μm) were coated with Ni (18-50 wt.%) and PTFE fine particles (0.3 μm; 8 wt.%) via electroless Ni-PTFE composite plating. The conductivity of Ni-PTFE plated graphite (C/Ni-PTFE) and PTFE (PTFE/Ni-PTFE) particles increased with the Ni content. At 35 wt.% Ni content, the conductivity (300 Sm⁻¹) of C/Ni-PTFE particles was about 2 times higher than that of PTFE/Ni-PTFE particles. The particles were pressed into plates under a pressure of 10-500 kg cm⁻² and the plates were then subjected to heat treatment at 350 °C. The surface of C/Ni-PTFE plates contained infinitely many gaps of 0.01-20 μm; these gaps are useful as a pathway for reacting gases. The conductivities in a direction perpendicular and parallel to the C/Ni-PTFE plates were respectively about 3.5 times (510 Sm⁻¹) and 16 times (48 × 10³ Sm⁻¹) higher than those of the PTFE/Ni-PTFE plates. Furthermore, the total pore volume (0.145 cm³ g⁻¹) of C/Ni-PTFE plates was higher than that of PTFE/Ni-PTFE plates, which improved the gas permeability of the former. The current density (84 mA cm⁻² at 0.3 V) of C/Ni-PTFE electrode was about 2 times higher than that of PTFE/Ni-PTFE electrode. This increase in the current density might be attributed to the improvement in the total conductivity and gas permeability of C/Ni-PTFE electrode.     

Inverse analyses of diffusion processes in type 13X zeolite particles

To determine effective surface self-diffusion coefficients of CO₂ within type 13X zeolite particles, at various temperatures (25–70 °C), inverse analyses of observed CO₂-uptake curves were successfully performed. The obtained effective surface self-diffusion coefficients increase with both the amount adsorbed and the temperature and ranged from 7.8 × 10⁻¹⁰ to 1.95 × 10⁻⁹ m²/s under the present experimental conditions.     

Formable chromium nitride coatings for proton exchange membrane fuel cell stainless steel bipolar plates

Among the different coatings developed for proton exchange membrane fuel cell steel bipolar plate, nitride-based coatings present several advantages compared to gold or polymeric coating: high chemical stability, low interfacial contact resistance and reasonable cost. In this work, 50 nm thick chromium nitride coatings are deposited by reactive magnetron sputtering on 316L stainless steel foil. They are optimized to fulfill the Department of Energy targets in terms of interfacial contact resistance (ICR) and corrosion resistance, with values of 8.4 mΩ cm⁻² (at 100 N cm⁻²) and 0.10 μA cm⁻² (in 0.6 M H₂SO₄ solution at 0.48 V_vs. SCE potential) respectively. Moreover, they retain their excellent properties after high deformation (biaxial deformation of 20% in x-axis and 5% in y-axis), giving the possibility to achieve, in line, the stamping of a bipolar plate from a coated foil. The etching of the substrate, prior to the coating deposition, appears to be determinant to obtain low and stable corrosion current and ICR. The removing of interfacial oxyde leads to better coating adhesion and improves the corrosion resistance and electrical conductivity. The enhancement of the properties (low ICR and high corrosion resistance) is durable, with no signicant change of the ICR value up to 200 days after deposition.     

Niobium doped amorphous carbon film on metallic bipolar plates for PEMFCs: First principle calculation,microstructure and performance

Metallic bipolar plates (BPPs) are a promising candidate to replace conventional graphite BPPs due to higher power density and lower cost in proton exchange membrane fuel cells (PEMFCs). Surface coating is essential to enhance interfacial conductivity and corrosion resistance. Amorphous carbon (a-C) films have attracted broad attention from both academia and industry. This study incorporated Niobium (Nb) into a-C to further enhance its performance. First principle calculation was introduced to investigate the evolution mechanism of bond and phase by Nb doping and instruct the coating design. Then, a series of Nb-doped a-C samples were deposited by closed field unbalanced magnetron sputtering ion plating (CFUBMSIP) method. The microstructure was systematically characterized by SEM, XRD, Raman spectrum, and XPS. Interfacial contact resistance (ICR) and electrochemical corrosion were also tested to evaluate the effect of Nb doping. A higher C-sp²/C-sp³ ratio was observed in a-C film with moderate Nb doping. As a result, the ICR decreased to 1.22 mΩ•cm² from initial value of 4.41 mΩ•cm². Besides, the doped Nb also refined grain size and increased the film compactness, which was beneficial for corrosion resistance. Rct, which reflects the relative anti-corrosive property, was increased from 1.8 × 10⁶ Ω•cm² to 3.29 × 10⁶ Ω•cm². Moreover, the current density in 0.84 V (vs. SHE) potentiodynamic polarization is 3.59 × 10⁻⁷ A/cm², lower than 4.59 × 10⁻⁷ A/cm² of a-C films. Besides, the current density in 0.84 V (vs. SHE) potentiostatic polarization shows the same tendency. The enhanced performance of Nb-doped a-C coatings would advance the commercialization of metallic BPPs for PEMFCs.     

Performance predictions for a new zeolite 13X/CaCl2 composite adsorbent for adsorption cooling systems

Composite adsorbents synthesized from zeolite 13X and CaCl₂ were investigated for applications in solar adsorption cooling systems. The effects of Ca ion exchange on the adsorption properties of zeolite 13X were studied. Ca ion exchange was found to decrease the specific surface area of the zeolite while increasing the total pore volume. Soaking zeolite 13X in 46 wt.% CaCl₂ solution for 24 h gave the optimum Ca ion exchange. The increase in the total pore volume facilitated further impregnating the zeolite with CaCl₂. In all, 41.5 mol% of CaCl₂ was impregnated in the Ca-ion-exchanged zeolite from a 40 wt.% CaCl₂ solution to form the zeolite 13X/CaCl₂ composite adsorbent. A 0.4 g/g difference in equilibrium water uptake between 25 and 75 °C at 870 Pa was recorded for the composite adsorbent. This was 420% of that of zeolite 13X under the same conditions. Numerical simulation predicts that an adsorption cooling system using the composite adsorbent could be powered by a low grade thermal energy source using the temperature range 75–100 °C. Greatly improved efficiency is predicted compared to a system using pure zeolite 13X or impregnated silica gel.