Geometry optimization and pressure analysis of a proton exchange membrane fuel cell stack

For proton exchange membrane fuel cells (PEMFCs), the distribution of reactant streams in the reactor is critical to their efficiency. This study aims to investigate the optimal design of the inlet/outlet flow channel in the fuel cell stack with different geometric dimensions of the tube and intermediate zones (IZ). The tube-to-IZ length ratio, the IZ width, and the tube diameter are adjusted to optimize the geometric dimensions for the highest pressure uniformity. Four different methods, including the Taguchi method, analysis of variance (ANOVA), neural network (NN), and multiple adaptive regression splines (MARS), are used in the analyses. The results indicate the tube diameter is the most impactive one among the three factors to improve the pressure uniformity. The analysis suggests that the optimal geometric design is the tube-to-IZ length ratio of 9, the IZ width of 14 mm, and the tube diameter of 9 mm with the pressure uniformity of 0.529. The relative errors of the predicted pressure uniformity values by NN and MARS under the optimal design are 1.62% and 3.89%, respectively. This reveals that NN and MARS can accurately predict the pressure uniformity, and are promising tools for the design of PEMFCs.     

A novel computational approach to study proton transfer in perfluorosulfonic acid membranes

Density functional theory (DFT) calculations were done to obtain the energies of two perfluorosulfonic acid membranes at low humidity conditions. For the first time, an artificial neural network (ANN) approach along with statistical methods were employed for modeling, prediction, and analysis of the energies derived by the DFT method. The ANN method substantially does speed up the ab initio electronic structure calculations and has superior accuracy to mimic the results of such calculations. The designed ANNs for modeling the total and binding energies had high performance since the computed R² values were 0.99998 and 0.990, and the calculated root mean squared error (RMSE) values were 0.612173 Ha and 0.084901 Ha in predicting the total and binding energies, respectively. Statistical analysis of binding energies per water molecule using analysis of means (ANOM) and analysis of variance (ANOVA) methods showed that the hydration level has significant influence on the proton transfer in the perfluorosulfonic acid membranes. ANOM and ANOVA methods were also employed to determine the quantitative effect of other parameters (i.e., temperature and total charge of the system) as well as the combined effect of these parameters. The ease of the proton transfer was also assessed with the aid of the obtained potential energy surfaces.     

Optimization of injection parameters for mechanical properties of specimens with weld line of polypropylene using Taguchi method

This study optimized effect of injection parameters and weld line on the mechanical properties of polypropylene (PP) moldings. The mold with an insert was designed to create weld line in the experimental specimen. Melt temperature, packing pressure and injection pressure were investigated to study their effects on the mechanical strength of specimens with/without weld lines. Taguchi's L₉ (3³) orthogonal array design was employed for the experimental plan. Mechanical properties such as maximum tensile load, extension at break and charpy impact strength (notched) of the specimens were measured. Signal to noise ratio for mechanical properties of PP using Taguchi method was calculated and effect of the injection parameters and weld line on mechanical properties was determined using the analysis of variance (ANOVA). Linear models were also created by using regression analysis. The most important parameter affecting the maximum tensile load and the extension at break (for specimen without/with weld line) was injection pressure and melt temperature, and for charpy impact strength (notched) (without/with weld line) was melt temperature and injection pressure, respectively.     

Preparation of palladium membrane by bio-membrane assisted electroless plating for hydrogen separation

Palladium membrane was prepared on the inner surface of alumina tube by bio-membrane assisted electroless plating combined with osmosis method (BELP). In this preparation technique, an egg-shell film not only served as a semipermeable membrane to form osmotic system for preparing palladium membrane, but also acted as a protection layer to prevent the contamination of the palladium membrane from the osmotic solution. Moreover, the plating solution was circulated through the tube side to promote the mass transfer on the solid–liquid interface between the plating surface and the solution. The detailed depositing process of the palladium membrane was studied by scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDXS). Both long term operation and temperature cycling test carried out for hydrogen and nitrogen permeation confirmed that the palladium membrane was stable.     

Preparation, testing and modelling of a hydrogen selective Pd/YSZ/SS composite membrane

A palladium selective tubular membrane has been prepared to separate and purify hydrogen. The membrane consists of a composite material, formed by different layers: a stainless steel support (thickness of 1.9 mm), an yttria-stabilized zirconia interphase (thickness of 50 μm) prepared by Atmospheric Plasma Spraying and a palladium layer (thickness of 27.7 μm) prepared by Electroless Plating. The permeation properties of the membrane have been tested at different operating conditions: retentate pressure (1-5 bar), temperature (350-450 °C) and hydrogen molar fraction of feed gas (0.7-1). At 400 °C, a permeability of 1.1 × 10⁻⁸ mol/(s m Pa^0.5) and a complete selectivity to hydrogen were obtained. The complete retention of nitrogen was maintained for all tested experiment conditions, with both single and mixtures of gases, ensuring 100% purity in the hydrogen permeate flux.A rigorous model considering all the resistances involved in the hydrogen transport has been applied for evaluating the relative importance of the different resistances, concluding that the transport through the palladium layer is the controlling one. In the same way, a model considering the axial variations of hydrogen concentration because of the cylindrical geometry of the experimental device has been applied to the fitting of the experimental data. The best fitting results have been obtained considering Sieverts’-law dependences of the permeation on the hydrogen partial pressure.     

Modeling of transient hydrogen permeation process across a palladium membrane

Transient mass transfer processes of hydrogen permeating through a Pd membrane are modeled to aid in predicting the hydrogen transport behavior. The model is established in terms of the quasi-steady time and the steady permeation rate. Meanwhile, four important parameters are considered; they are the permeation lag time, the initial permeation rate, the concave up period and the concave down period. A unit step function is embedded in the model to account for the effect of the hydrogen permeation lag at a lower pressure difference. Corresponding to the lower, the moderate and the higher pressure differences (i.e. 3, 5 and 8 atm), though the hydrogen permeation undergoes a three-stage, a two-stage and a one-stage processes, respectively, these processes can be predicted well by an arc tangential function. By introducing an adjusting parameter in the arc tangential function, there exists an optimal value of the adjusting parameter when the pressure difference is lower. In regard to the moderate and higher pressure differences, the predictions agree with experiments well if the adjusting parameter is sufficiently large. Physically, the unit step function is used to account for the controlling mechanisms of hydrogen diffusion toward the membrane and the spillover of the hydrogen across the membrane. The initial jump parameter represents the rapid response of the initial hydrogen permeation. The adjusting parameter can be used to describe the relative importance of the concave up and the concave down periods.     

Palladium composite membrane fabricated on rough porous alumina tube without intermediate layer for hydrogen separation

A thin palladium composite membrane without any modified layer was successfully obtained on a rough porous alumina substrate. Prior to the fabrication of palladium membrane, a poly(vinyl) alcohol (PVA) layer was first coated onto the porous substrate by dip-coating technique to improve its surface roughness and pore size. After deposition of palladium membrane on the PVA modified substrate, the polymer layer can be completely removed from the composite membrane by heat treatment. The microstructure of the palladium composite membrane was characterized in detail using SEM, EDXS and XRD analysis. Permeation measurements were carried out using H₂ and N₂ at temperatures of 623 K, 673 K, 723 K and 773 K. The results indicated that the hydrogen permeation flux of 0.238 mol m^?2 s^?1 with H₂ separation factor α(H₂/N₂) of 956 for the as-prepared palladium membrane was obtained at 773 K and 100 kPa. Furthermore, the good membrane stability was proven during the total operation time of 160 h at the temperature range of 623 K–773 K and gas exchange cycles of 30 between hydrogen and nitrogen at 723 K.     

Toward effective membranes for hydrogen separation: Multichannel composite palladium membranes

Composite palladium membranes can be used as a hydrogen separator because of their excellent permeability and permselectivity. The total membrane area in a hydrogen separator must be reasonably large for industrial use, and it is important that each membrane provides a large enough area. Such a demand can be well met by introducing multichannel composite membranes. In this work, a commercially available microporous ceramic filter with 19 channels was used as a membrane substrate, and the diameter of each channel was 4 mm. A uniform thin palladium layer was fabricated inside the narrow channels by using an electroless plating method, and the resulting membranes were highly permeable and selective. This membrane concept provides a high surface-to-volume ratio without causing significant pressure loss, making the hydrogen separator compact and capable. However, special attention should be paid to cleaning the membrane after electroless plating.     

One-step electroplating of palladium–copper alloy layers on a vanadium membrane for hydrogen separation: Quick,easy, and low-cost preparation

Palladium–copper (PdCu) alloy layers with 150 nm thickness were prepared by one-step electroplating on both sides of a vanadium foil. The entire process, including pretreatment, washing, and electroplating, was complete in <5 min. The PdCu layer had a high surface coverage and almost completely covered the vanadium foil. The composition difference between the center and the edges was within 3 at%, and the alloy composition was controlled to nearly the target composition (50:50). Peel tests revealed that the as-electroplated layer had sufficient adhesion for practical applications. XRD analysis confirmed the successful formation of a face-centered cubic (FCC) PdCu alloy. In-situ XRD analysis performed under a hydrogen atmosphere revealed the structural change of PdCu from FCC to body-centered cubic (BCC) immediately after the temperature reached 300 °C. The hydrogen permeability of electroplated PdCu/V/PdCu at 200–300 °C (1–3 × 10⁻⁸ mol m⁻¹ s⁻¹ Pa^−1/2) was considerably higher than that of the 10 μm-thick electroplated BCC PdCu.