Diffusion Layer Thickness Research Articles

Effect of Thickness and Outlet Area Fraction of Macroporous Gas Diffusion Layers on Oxygen Transport Resistance in Water Injection Simulations

Enhanced water removal through the gas diffusion layer (GDL) is important for the design of high-performance proton exchange fuel cells. In this work, the effects of GDL thickness and open area fraction at the GDL/flow field interface are examined under water invasion for a carbon-paper GDL (similar to Toray TGP-H series). Both uncompressed and inhomogeneously compressed samples are considered. Transport in heterogeneous, macroporous GDLs is modeled by means of a hybrid 3D discrete/continuum formulation based on a subdivision of the porous medium into control volumes due to the lack of a well-defined separation between pore and layer scales. Capillary-dominated transport of liquid water is simulated with an invasion percolation algorithm, while oxygen diffusion is simulated with a continuum formulation. Model predictions are validated with previous numerical and experimental data. It is shown that the combination of thin GDLs (mathrm {thickness} sim 100; mu mathrm {m}) and high GDL/flow field open area fractions can facilitate water removal/oxygen supply from/to the catalyst layer and can provide a more uniform oxygen distribution over large cell active areas. In agreement with previous work, porous flow fields with pore sizes comparable to the GDL thickness are good candidates to meet the above requirements, while improving water removal from the flow field (higher gas-phase velocity than conventional millimeter-sized channels) and ensuring a more uniform assembly compression.

Mass Transfer from Ion-Sensing Component-Loaded Nanoemulsions into Ion-Selective Membranes: An Electrochemical Quartz Crystal Microbalance and Thin-Film Coulometry Study.

Recent work has shown that ion-selective components may be transferred from nanoemulsions (NEs) to endow polymeric membranes with ion-selective sensing properties. This approach has also been used for nanopipette electrodes to achieve single-entity electrochemistry, thereby sensing the ion-selective response of single adhered nanospheres. To this date, however, the mechanism and rate of component transfer remain unclear. We study here the transfer of lipophilic ionic compounds from nanoemulsions into thin plasticized poly(vinyl chloride) (PVC-DOS) films by chronoamperometry and quartz crystal microbalance. Thin-film cyclic coulovoltammetry measurements serve to quantify the uptake of lipophilic species into blank PVC-DOS membranes. Electrochemical quartz crystal microbalance data indicate that the transfer of the emulsion components is insignificant, ruling out simple coalescence with the membrane film. Ionophores and ion-exchangers are shown to transfer into the membrane at rates that correlate with their lipophilicity if mass transport is not rate-limiting, which is the case with more lipophilic compounds (calcium and sodium ionophores). On the other hand, with less lipophilic compounds (valinomycin and cation-exchanger salts), transfer rates are limited by mass transport. This is confirmed with rotating disk electrode experiments in which a linear relationship between the diffusion layer thickness and current is observed. The data suggests that once the nanoemulsion container approaches the membrane surface, transfer of components occur by a three-phase partition mechanism where the aqueous phase serves as a kinetic barrier. The results help better understand and quantify the interaction between nanoemulsions and ion-selective membranes and predict membrane doping rates for a range of components.

In English

The goal of this study is unraveling the specific features of non-stationary surface concentration distribution of electroactive and inactive species in a model electrochemical process with a preceding homogeneous first-order chemical reaction (CE mechanism). For this purpose, the exact analytical expressions for the non-stationary concentration distributions of electroactive and inactive species in the thin layer attached to a planar electrode are analyzed. The both cases of equal and unequal diffusion coefficients of species taking part in the preceding chemical reaction are considered. In the former case, the exact analytical expressions for the concentration distributions of electroactive and inactive species on a planar electrode are obtained. The peculiarities of the limiting cases of zero and infinite frequency of an applied alternating current for the both cases of equal and unequal diffusion coefficients of species are discussed. It is shown that there is a phase shift between AC and the surface concentration of species that changes under the action of this current. At low frequencies, the phase angle tends to p/2, whereas at high frequencies it decreases to p/4. The phase angle is the function of the two important measures, namely, the ratio of the Nernst diffusion layer thickness to the oscillation diffusion layer thickness, and the ratio of the Nernst diffusion layer thickness to the reaction layer one. It is shown that the phase angle depends on the diffusion coefficient of species in different manner for low and high values of the rate constants of the chemical reaction. At low values of these parameters, the phase angle shifts slightly to the range of high frequencies with an increase of diffusion coefficient. At the high rate constants, the phase angle decreases with frequency more slowly, and its dependence on diffusion coefficient is observed only at middle frequencies. The surface concentration of electroactive and inactive species decreases with an increase of frequency, but for the inactive species this process is faster than that for the electroactive species. The influence of the inactive species on the surface concentration of electroactive species decreases at high frequencies and at low rate constants of the preceding chemical reaction. The results obtained shed the light on complex dynamics at an electrode/electrolyte interface under non-stationary conditions.

Sono-sorption versus adsorption for the removal of congo red from aqueous solution using NiFeLDH/Au nanocomposite: Kinetics, thermodynamics, isotherm studies, and optimization of process parameters

As anionic alloy or layered compounds, layered double hydroxides (LDH) have recently attracted much attention due to their excellent adsorption and catalytic properties. In this work, NiFe layered double hydroxide (NiFeLDH) was utilized as a precursor to synthesize NiFeLDH/Au nanocomposites via a simple urea-hydrolyzed hydrothermal process under UV–vis light irradiation. The as-prepared nanomaterials were characterized using XRD, FT-IR, XPS, BET, UV–vis, TEM, and FESEM analysis. The performance of NiFeLDH and NiFeLDH/Au nanocomposites was evaluated in the adsorptive removal of congo red (CR) dye from aqueous media using two methods of adsorption, i.e. liquid phase sorption (MS) and sono-sorption (US). The sono-sorption technique exhibited higher removal efficiency due to the generation of small bubbles, strong shock waves, high-velocity microjets, and diminished thickness of diffusion layers. Furthermore, the NiFeLDH/Au nanocomposite offered much higher performance in CR removal, exhibiting a dye removal efficiency of 95% in 7 minutes of sono-sorption time. The superior performance of the NiFeLDH/Au composite can be assigned to the high specific surface area of the sorbent, better dispersion of Au nanoparticles onto the surface of NiFeLDH materials, and synergistic effects between Au and NiFeLDH. Also, the isothermal evaluation showed that the data of CR adsorption onto fabricated nanohybrids could be well-matched with the Langmuir model indicating uniform surface energies. Moreover, kinetics studies revealed that the best data fitting for CR adsorption can be provided by Ho’s pseudo-second-order kinetics. The thermodynamic study revealed that the adsorption of CR dye onto NiFeLDH/Au was an endothermic and spontaneous process at the investigated temperatures and CR concentrations. Finally, this research suggests a simple method for the synthesis of an LDH-based nano-sorbent capable of almost completely removing organic dyes from aqueous media.

Effect of fluid velocity and particle size on the hydrodynamic diffusion layer thickness

The aim of this study was to determine the thickness of the hydrodynamic diffusion layer (hHDL) of three poor water-soluble compounds under laminar fluid flow using a single particle dissolution technique. The single particle dissolution experiments were performed in a flowing aqueous medium using four different fluid velocities (v), ranging from 46 to 103 mm/s. The particles used had an initial radius (r) of 18.8 to 52.3 μm. The determined hHDL values were calculated from both dissolution experiments and computational fluid dynamics (CFD) simulation. In this study, single particle dissolution experiments gave, with one exception, hHDL values in the range of 2.09 to 8.85 µm and corresponding simulations gave hHDL values in the range of 2.53 to 4.38 µm. Hence, we found a semi-quantitative concordance between experimental and simulated determined hHDL values. Also, a theoretical relation between the dependence of hHDL on particle radius and flow velocity of the medium was established by a series of CFD simulations in a fluid velocity range of 10–100 mm/s and particle size (radius) range of 5–40 µm. The outcome suggests a power law relation of the form hHDL∝r3/5v-2/5. In addition, the hHDL seems to be independent of the solubility, while it has a diffusion coefficient dependence. In conclusion, the hHDL values were determined under well-defined conditions; hence, this approach can be used to estimate the hHDL under different conditions to increase the understanding of the mass transfer mechanisms during the dissolution process.

Effect of Post-Heat Treatment Temperature on Interfacial Mechanical Properties of Cold-Rolled Ti/Al Clad Material.

Titanium and titanium alloys possess low density, high specific strength, and excellent corrosion resistance, but are expensive and have low formability at room temperature. Therefore, to reduce cost and achieve excellent properties, titanium and titanium alloys are jointed with aluminum and its alloys, which are inexpensive and have low density and excellent room temperature formability. Cladding is a widely used solid-state bonding technique, and the post-heat treatment of titanium/aluminum clad materials is required to improve their interfacial properties, which is important to ensure the reliability of Ti/Al-clad materials. The interfacial properties of Ti/Al-clad materials are significantly affected by changes in the microstructure and mechanical properties after the post-heat treatment. Thus, in this study, the relationship between the microstructure and mechanical properties at the interface of Ti/Al-clad materials was analyzed after the post-heat treatment at several different temperatures. The thick diffusion and intermetallic compound layer was formed with post-heat treatment owing to the active diffusion of Al atoms. As a result, their uniaxial and nanomechanical properties were varied with the interfacial characteristics of the Ti/Al-clad material.

Analysis of evolution law and mechanism of stacking fault density of M7C3 carbides under the action of the electric current pulse

Stacking faults are often found in M7C3 carbides, and stacking fault density has an important impact on their properties. This study explores the evolution law and mechanism of the stacking fault density of M7C3 carbides under the action of an electric current pulse (ECP). ECP at different current densities was passed into the alloy melt during solidification. The results show that the Cr/Fe ratios in the carbides first decreased and then increased as the current density increased, and the stacking fault density first increased and then decreased. The high-density stacking faults cause the carbide microhardness to be approximately 200Hv0.3 higher than those with low stacking faults when the Cr/Fe ratio is the same. First-principles calculations show that the M7C3 stacking fault energy decreased with increasing Fe content. The increase in iron content was the thermodynamic factor leading to the evolution of stacking fault density. During alloy solidification, ECP led to rapid carbide growth and a decrease in the thickness of the solute diffusion layer at the solid-liquid interface, which induced a change in the Cr/Fe ratio of the carbides. This is the dynamic factor for the evolution of stacking fault density.

Optimization of electrochemical hydrogen compression through computational modeling

One of the main challenges in developing the hydrogen infrastructure is the distribution and storage of hydrogen. A common method to store hydrogen is as a compressed gas. Electrochemical compression (ECC) is a promising technology that can overcome some of the disadvantages of conventional mechanical compressors. ECC employs an externally powered electrochemical cell containing a polymer electrolyte membrane to compress the gas. This work presents a comprehensive 3D ECC model developed for a single cell using COMSOL Multiphysics 5.6 that incorporates all relevant physical and electrochemical processes, and examines the effect of key parameters on ECC performance. It also considers the important phenomenon of back diffusion resulting from the high-pressure differential between the cathode and anode during compression. Results from the current simulations were validated against experimental results obtained previously in our lab. Simulations were first conducted for the unpressurized cathode to understand the effect of membrane thickness, relative humidity of the anode hydrogen supply, temperature, and gas diffusion layer thickness on ECC performance. Next, simulations were conducted for the pressurized cathode, with and without considering back diffusion. In the absence of back diffusion, the pressure ratio reaches the value predicted by the Nernst equation. However, the presence of back diffusion greatly reduces the pressure ratio as was also observed in experiments. The study reveals that three parameters in particular viz. Membrane thickness, operating temperature, and voltage must be carefully selected to optimize ECC operation. These results also suggest that ECC is a viable alternative to conventional technologies for hydrogen compression. This work also provides a foundation for the modeling and analysis of full-scale ECC systems.

Influence factor of Pr(III) recovery kinetics from rare-earth simulant wastewater by PAN microtubule hyperfiltration reactor

Abstract PAN microtubule hyperfiltration reactor (PMHR) with organic phosphate system containing simulant wastewater section with buffer solution acetic acid and enrichment section with nitric acid solution and organic phosphate C16H35O4P (Di (6-methylheptyl) phosphate) dissolved in benzin, has been studied for the praseodymium (III)[expressed as Pr(III) or Pr3+] recovery in the rare-earth simulant wastewater. Many factors of Pr(III) recovery using PMHR need to explore, including hydrogen ion concentration (or pH), cinit of Pr(III) and different ionic strength of rare-earth simulant mine wastewater, volume ratio of C16H35O4P with benzin and nitric acid solution (O/W), nitric acid concentration, C16H35O4P concentration, different acid solution of enrichment section on Pr(III) recovery with PMHR were investigated. The experimental results show that the best recovery conditions of Pr(III) were obtained as that nitric acid concentration was 4.00 mol/L, C16H35O4P concentration was 0.200 mol/L, and O/W was 0.6 in the enrichment section, and pH value was 4.90 in the wastewater section. Ionic strength of rare-earth simulant mine wastewater had no obvious effect on Pr(III) recovery. When cinit of Pr(III) concentration was 1.67 × 10−4 mol/L, the recovery percentage of Pr(III) was up to 94.7% in 160 min. The kinetic equations were developed, and diffusion coefficient in the microtubule reactor and thickness of diffusion layer between the wastewater section and the microtubule pipe wall were obtained by linear slope method. They were 4.16 × 10−6 m2/s and 4.65 μm, respectively. PMHR, owing to a large number of C16H35O4P was used in the enrichment section, can renewal the losing carrier of microtubule reactor. As a result, the recovery percentage of Pr(III) was increased, the stability of PMHR system was enhanced, and the life span of the PMHR system was extended.

Engineering the NiNC Catalyst Microenvironment Enabling CO2 Electroreduction with Nearly 100% CO Selectivity in Acid.

CO2 electrolysis in acid has emerged as a promising route to achieve high CO2 utilization due to the inhibition of undesired carbonate formation that generally occurs in alkaline or neutral conditions. However, the efficiency and stability of this system need to be further improved through tailoring of the electrocatalyst and its working environment. Here, a working microenvironment of structurally engineered NiNC catalyst for acidic CO2 electrolysis is probed and optimized by adding hydrophobic poly(tetrafluoroethylene) (PTFE) nanoparticles in the catalytic layer of gas-diffusion electrodes. The PTFE-modified electrode delivers nearly 100% CO Faradaic efficiency at an industry-relevant current density of 250mA cm-2 , and a high single-pass CO2 utilization of 75.7% at a current density of 200mA cm-2 under 20 sccm CO2 gas flow rate. Moreover, compared to a conventional electrode without added PTFE, the PTFE-modified electrode exhibits a substantially enhanced water-flooding-resistant ability. Mechanistic investigations reveal that a moderate PTFE modification can optimize the local CO2 /H2 O ratio in the catalytic layer, favoring the reduction of the diffusion layer thickness and the formation of a highly active and stable solid-liquid-gas interfacial microenvironment.

Numerical Analysis on Impact of Thickness of PEM and GDL with and without MPL on Coupling Phenomena in PEFC Operated at Higher Temperature Such as 363 K and 373 K

The aim of this study is to clarify the impact of the thickness of a gas diffusion layer (GDL) and a micro porous layer (MPL) on the distributions of gas, H2O, and current density in a polymer electrolyte fuel cell (PEFC) which is operated at 363 K and 373 K and with various thicknesses of polymer electrolyte membrane (PEM) as well as a relative humidity (RH) of supply gas. These investigations are carried out by numerical simulation using the 3D model with COMSOL Multiphysics. In the case of Nafion 115, which is the thicker PEM, the change in the molar concentration of H2O from the inlet to the outlet with MPL is larger than that without MPL irrespective of the thickness of GDL, Tini and RH condition. In the case of Nafion NRE-212, which is the thinner PEM, the change in the molar concentration of H2O from the inlet to the outlet is larger with MPL than that without MPL in the case of TGP-H-060 (the thicker commercial GDL), while that is smaller with MPL than that without MPL in the case of TGP-H-030 (the thinner commercial GDL). These results exhibit the same tendency as the results of the numerical simulation on the current density.

Improvement of diffusion layer model for steam condensation in the presence of non-condensable gas under turbulent free convection

Steam condensation plays a crucial role in predicting the pressure behavior and hydrogen distribution in nuclear power plant containment during a postulated loss-of-coolant accident or a severe accident. The stagnant film model and diffusion layer model are two commonly used heat and transfer analogy models for predicting film-wise condensation in the presence of non-condensable gas on a vertical wall in the nuclear engineering field. This study proposed a general formula for the equivalent condensation thermal conductivity, which can be used to convert between the traditional diffusion layer model and the stagnant film model. An improved diffusion layer model is established, which can be used to analyze steam condensation in the presence of non-condensable gas on a vertical flat surface or outer surface of condenser tubes under turbulent free convection. The density difference of water vapor at the bulk and at the gas–liquid interface is the driving force for steam condensation in the improved model, the traditional diffusion layer model’s assumption, e.g., diffusion layer thickness, constant gas mixture density, and the application of modified Clausius-Clapeyron equation, are no longer used. The newly established model also divides the condensation process into liquid film heat transfer, latent heat transfer, and sensible heat transfer. The three parts of heat transfer processes are linked together by the equivalent heat flux on both sides of the gas–liquid interface. The effects of gas compressibility, suction, fog formation, and liquid film waviness have been considered in this study. The influences of gas compressibility, the thermal resistance of condensate film, and sensible heat transfer on modeling steam condensation were discussed. Compared with various sources of experimental data (619 data points), more than 97% of the model predicted results are within the ±25.0% error band, and the mean absolute relative error is 10.2%, which indicates that the newly proposed model has high accuracy and widely applicable ranges.

Strengthening nanocrystalline immiscible bimetallic composite by high-entropy effect

Immiscible bimetallic composites are a kind of transpiration cooling material with potential in high-temperature service. Aiming at boosting their load-bearing capacity, the refractory phase was replaced with the multi-principal refractory high-entropy phase in the present work. Bi-phase metallic nanocrystalline NbMoTaW–Cu composites were fabricated successfully by the powder metallurgy method. The average grain size of the NbMoTaW phase in the sintered composite was kept to be 15 nm. Two interfacial configurations of BCC/FCC and BCC/amorphous/FCC were found in the composite. Using atom probe tomography, notable compositional inter-diffusion between the immiscible metals was disclosed, and the thickness of the mutual diffusion layer in the NbMoTaW–Cu composite was 2.2 times that in the W–Cu composite. The mechanisms of entropy effect on the formation of amorphous configuration and interfacial mutual diffusion were explained based on thermodynamic calculations. The yield strength and Vickers hardness of the nanocrystalline NbMoTaW–Cu composite are 52% and 27% higher than those of the W–Cu counterpart, respectively. In addition, the NbMoTaW–Cu composite processes excellent resistance to high-temperature softening even at 900 °C. The improved mechanical properties were associated with solid solution strengthening of the refractory metal phase, as well as the constraint effect and strengthening of interface between the refractory metal and Cu phases. This work provides novel guidance for designing advanced immiscible metallic composites with excellent mechanical performance.

(Digital Presentation) Effect of the Length of Carbon Nanofibers on Selective Dopamine Sensing

The engineering of carbon nanofibers (CNFs) has recently received extensive attention in the field of biosensors due to their high surface area, rich nanoscale geometries, ability to selectively detect the analyte of interest (e.g., dopamine) in the presence of interfering molecules that exist in biological environment and inherent resistance to biofouling1 - 3. CNFs with the individual fiber length/size adjusted to the diffusion layer thickness can confine the analyte molecules within the nanostructures and result in the formation of a thin liquid layer, increasing their sensitivity 4. This points towards the potential of altering the aspect ratios and growth densities of CNFs to tune their biosensing properties. Thus, here we aim to evaluate the effect of the length, distribution, and microstructure of fibers on their electrochemistry and how this affects the sensitivity and selectivity for dopamine (DA) detection.Herein, CNFs of varying lengths have been grown on a silicon substrate using the thin layers of Ni and Cr as catalysts. Samples with different length and distribution of CNFs have been synthesized by controlling the duration of the growth phase. The length and the detailed structure of the CNFs have been investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The lengths of CNFs were found to be in the range of 900 nm-1µM, 500-600 nM, 200-300 nM, and <150 nM for 30 mins, 10 mins, 5 mins and 1 min growth times, respectively. Cyclic voltammetry measurements showed that the double-layer capacitance (Cdl) and oxidation current (Ipa) of dopamine increase continuously with the increase in the length of fibers indicating the increase in the surface area (Figure 1A). DA reaction kinetics tend to be reversible for the longest fibers and becomes slightly more sluggish with the decrease in the fiber length. However, it should be noted that carbon nanofiber length (hundreds of nM to 1µm) may match the diffusion layer thickness at certain scan rates, causing the thin liquid layer electrochemical behavior to contribute to the total kinetics. This implies that the variations in the peak separation (ΔEp) can be due to the combined changes in reaction kinetics and geometry. CNFs grown for 30 mins, 10 mins, 5 mins, and 1 min exhibited log Ipa vs v slope of 0.72, 0.63, 0.62 and 0.51, indicating that contribution of adsorption/thin layer formation is coming into effect with the increase in fiber length (Table 1). Washout experiments will be performed to distinguish further between thin layer and adsorption behavior affecting the electrochemistry of DA. It appears that DA reaction kinetics is fully under semi-infinite linear diffusion-control only at 1 min grown CNFs due to the relatively smooth surface of the electrode material. The selectivity of dopamine in the presence of physiological concentrations of ascorbic acid (AA) and uric acid (UA) greatly improves with the increase in the length of the fibers (Figure 1B). Electrodes with 1 min grown CNFs possessing metal particles and exhibiting little CNF growth did not show selectivity towards DA. Interestingly, with the increase in the growth time and consequently the length of the fiber, well-defined oxidation peaks of AA and DA and UA are observed. Moreover, the position of the oxidation potential appears to be affected both by the fiber length and presence/absence of other molecules, and often in the opposite directions (Table 1, Figure 1). The results discussed above point towards the significance of matching the CNF length with the scan rate used for in vivo detection of DA for optimized sensitivity and selectivity. References 1 S. Sainio, T. Palomäki, N. Tujunen, V. Protopopova, J. Koehne, K. Kordas, J. Koskinen, M. Meyyappan and T. Laurila, Mol. Neurobiol., 2015, 52, 859–866.2 S. Sainio, E. Leppänen, E. Mynttinen, T. Palomäki, N. Wester, J. Etula, N. Isoaho, E. Peltola, J. Koehne, M. Meyyappan and others, Mol. Neurobiol., 2020, 57, 179–190.3 A. Kousar, E. Peltola and T. Laurila, ACS omega, 2021, 6, 26391–26403.4 Q. Cao, Z. Shao, D. K. Hensley, N. V Lavrik and B. J. Venton, Langmuir, 2021, 37, 2667–2676. Figure 1

Dynamic Ionic Transport Actuated by Nanospinbar‐Dispersed Colloidal Electrolytes Toward Dendrite‐Free Electrodeposition

AbstractInhibiting uneven dendritic Li electroplating is crucial for the safe and stable cycling of Li metal batteries (LMBs). Homogeneous and fast Li+ transport towards the Li surface is required for uniform and dendrite‐free deposition. However, the traditional ionic transport of static liquid electrolytes involving electromigration and molecular diffusion can trigger a greater disparity in the Li concentration over the Li surface, leading to irregular dendrite growth. Here, a convective Li+ transfer for suppressing dendrite growth through magnetic nanospinbar (NSB)‐dispersed colloidal electrolytes is presented. An ultrahigh‐aspect‐ratio NSB consisting of a paramagnetic Fe3O4 nanoparticle array and silica outer coating is synthesized. Manipulating the external electromagnetic force can remotely control the rotation of individual NSBs without dispersion failure, thereby generating mesoscale turbulence inside the cells. Regardless of the electrolyte composition, rotating the NSB can reduce the Li+ diffusion layer thickness from the bulk and evenly redistribute the Li+ flux over the Li surface, thereby suppressing Li dendrite growth. The NSB‐dispersed electrolyte with advanced salt/solvent compositions demonstrates stable cycling of LMBs over 600 cycles with 70% capacity retention, thereby outperforming the NSB‐free cell.

Effects of operating conditions, various properties of the gas diffusion layer, and shape of endplate on the open-cathode proton exchange membrane fuel cell performance

Research on increasing the power density per unit volume by removing and miniaturizing the balance of plants to operate proton exchange membrane fuel cells (PEMFCs) is essential. For drone applications, it is critical to reduce the weight of the PEMFC system. Therefore, the cathode compressor and humidifier are typically removed by directly using atmospheric air, and this operating method is called an open-cathode (OC). However, in the case of the OC-PEMFC, research has not been actively conducted compared to the PEMFC operating using the conventional method. In this study, the effects of various experimental variables on the OC-PEMFC performance are investigated. First, the effects of the flow rate of fans, whether the fan is used or not, operating temperature, humidification of hydrogen, and direction of the cathode opening area on the OC-PEMFC performance are systematically investigated. Second, the effects of substrate thickness of the gas diffusion layer, adding hydrophilic carbon black to the microporous layer, and perforations in the gas diffusion layer, one of the main components in the PEMFC, on the OC-PEMFC performance are identified. Finally, the shapes of endplates that prevent the accumulation of generated water in the cathode opening area are proposed.

Optimization of gas diffusion layer thickness for proton exchange membrane fuel cells under steady-state and load-varying conditions

This work proposes a design principle of the optimal thickness for cathode gas diffusion layers (GDLs) of proton exchange membrane fuel cells (PEMFCs) based on a balance of cell performances under both steady-state and load-varying conditions. Using a three-dimensional and two-phase model, the effects of cathode GDL thickness (50, 100, 200, and 400 μm) on steady-state performances and transient response characteristics are studied. The results show that under steady-state conditions, an extremely thin cathode GDL would lead to non-uniform oxygen and liquid water distributions, whereas an especially thick cathode GDL would increase oxygen transport resistance. Thus, there is an optimal cathode GDL thickness, yielding the best steady-state performance. A current overshoot phenomenon, arising from high local oxygen concentrations at cathode catalyst layers (CLs), is observed when the load voltage decreases abruptly. Thinner cathode GDLs would weaken the overshoot and shorten the recovery time of current density. Conversely, low local oxygen concentrations at cathode CLs caused by high liquid water saturation levels lead to a current undershoot phenomenon when the load voltage increases abruptly. Thinner cathode GDLs would strengthen the undershoot but shorten the recovery time of current density. The optimal choice of cathode GDL thickness should weigh up the steady-state and the transient performance. Based on this principle, the optimal cathode GDL thickness is selected as 100 µm.

Engineering a concordant microenvironment with air-liquid-solid interface to promote electrochemical H2O2 generation and wastewater purification

Electroreduction of O2 to H2O2 provides an appealing route for both renewable energy storage and wastewater purification. Although grand endeavors are devoted into synthesizing high-performance electrocatalysts, the understanding and engineering of local microenvironment around reactive sites are also critical. Herein, a concordant gas/liquid microenvironment with plentiful air–liquid-solid interfaces is constructed and evaluated, which not only mediates the harmonious transport of O2, H+ and electron (especially reducing the O2 diffusion layer thickness and raising O2 local concentration to accelerate O2 transport), but also induces massive highly active triphase boundary sites for high-rate H2O2 electro-generation. Consequently, even under natural air diffusion, the cathode with the polytetrafluoroethylene proportion of 20% achieves a preeminent H2O2 productivity (40.7 mg h−1 cm−2 at 100 mA cm−2, corresponding to the maximum H2O2 concentration of 813.8 mg L-1) with superior current efficiency (64.2%). Long-term electrolysis stability and reusability indicate the practicability of the air-breathing cathode without the energy-intensive external aeration. Efficient purification of dye wastewater by the modified cathode meets the expectation, showing its huge potential in environmental protection. This work deeply elucidates the effect mechanism of a concordant gas/liquid microenvironment with abundant air–liquid-solid interfaces, which has considerable guiding significance for similar gas-involving electrocatalysis.

Влияние борирования и алитирования на структуру и микротвердость низкоуглеродистых сталей

Introduction. Boriding and aluminizing are among the effective methods for improving the performance properties (corrosion resistance, heat resistance and wear resistance) of machine parts and tools. Solid-phase methods of carrying out techniques of thermochemical treatment (TCT) require long-term exposure at elevated temperatures, which negatively affects the structure and properties of the base material. From these positions, the selection of reasonable temperature-time parameters of solid-phase boriding and aluminizing processes is an urgent task. The purpose of this work is to assess the effect of low-temperature boriding and aluminizing processes on the structure and microhardness of diffusion layers on the surface of low-carbon steels. The paper considers two grades of steels with a carbon content of up to 0.4%: low-carbon steel St3 and alloy steel 3Cr2W8V. The use of the second steel is due to the need to identify the effect of alloying elements in steel on the thickness of diffusion layers and its composition. Powder mixtures based on boron carbide and aluminum carbide are selected as sources of boron and aluminum. Results and discussions. It is found at a process temperature of 900 °C and holding for 2 hours after boriding, iron borides are formed on the surface of both steels. At the same time, two borides FeB and Fe2B are detected on St3 steel by X-ray phase analysis (XRD), and only the Fe2B phase is detected on 3Cr2W8V steel. After aluminizing, aluminum-containing phases such as Al5Fe2, Na3AlF6 and Al2O3 are formed in both steels. The thickness of the resulting diffusion layer on St3 after boriding is 35 μm, after aluminizing – 65 μm. The thickness of the diffusion layer on 3Cr2W8V steel is equal to 15 μm after boriding and 50 μm after aluminizing, which is significantly less than on carbon steel and is obviously due to the effect of alloying elements. It is established that TCT leads to a significant increase in the microhardness of the samples surface. Thus, the maximum microhardness of St3 steel increased to 2,000 HV, and the maximum microhardness of 3Cr2W8V steel increased to 1,700 HV after boriding. The microhardness after aluminizing is comparable for both steels and is equal to 1,000-1,100 HV. Elemental analysis of the upper sections of the diffusion layers shows that the content of boron (7-9%) and aluminum (50-53%) corresponds to the detected XRD iron borides and aluminides. In all cases, there is a gradual decrease in the diffusing elements in the direction from the surface to the base.

The Influence of Grade P6M5 Tool Steel Induction Nitriding Process Parameters on the Diffusion Layer Microhardness and Thickness

The effect the inductor current and the duration of thermal exposure in a nitrogen medium have on the microhardness and depth of the diffusion layers formed in grade P6M5 steel is studied. The investigation was carried out on 5-mm-thick samples 10 mm in diameter that had not been subjected to preliminary heat treatment. The samples were nitrided in a specially designed chamber in the form of a sealed quartz vessel placed in an inductor. The steel samples were placed in the chamber at the inductor location level. After filling the chamber with nitrogen gas to a pressure of 0.2±0.05 MPa, the samples were subjected to high-frequency induction heating. The temperature of the samples was adjusted by changing the inductor current from 4.8 to 5.8 kA. It was found that at an inductor current of 4.8 kA, the sample was heated to a temperature above 800°C; with a further increase in the current, the sample temperature increased above 1000°C. As a result of nitriding with a duration of 300–1200 s at a temperature of 800–1000°C, a surface nitride layer with a thickness of up to 20&nbsp;μm and a diffusion sublayer with a thickness of up to 200 μm were produced. The surface layers were characterized by a nitrogen content of up to 9 at % and microhardness of 1930–1950 HV (200 gf). The diffusion layers were characterized by a lower nitrogen content of up to 6 at %. and microhardness of 1450–1500&nbsp;HV (100 gf). Based on the obtained experimental data, empirical models describing the effect of the inductor current and the duration of thermal exposure on the diffusion layer thickness and microhardness have been developed.