Vapor Flow Research Articles

The Effect of Differential Pressure Driven Tensile Stress on Chemical Degradation of Perfluorosulfonic Acid Membranes Using a Hydrogen Peroxide Vapor Flow Cell Test

Abstract A modified ex-situ H2O2 vapor flow cell test has been developed to study the effect of tensile stress on the chemical degradation of perfluorosulfonic acid membranes. The test cells compose of two flowfield plates with a membrane sandwiched in between. A N2 gas stream with higher pressure is directed through one side while a lower pressure 30% RH N2 stream with 175 ppm of H2O2 vapor flows on the other. The test offers advantages which include the ability to test as-received production membranes utilizing the residual trace iron in the membrane as catalyst to generate hydroxyl radicals, good control of H2O2 concentration, and versatile control of test conditions. While applying various tensile stresses from zero to the near creep-rupture strength of Nafion® N211, we measure the fluoride emission rates (FER) at 90°C/30%RH of both N211 and a chemically stabilized and mechanically reinforced state-of-the-art (SOA) 12 µm membrane. The FER of N211 is 10-15 times higher than that of the SOA membrane, showing the effectiveness of using cerium for chemical mitigation. The result also shows that the tensile stress expected during fuel cell operation has insignificant effect on the membrane chemical degradation rates compared with the effect of RH.

Enhancing Mass Transfer Coefficient Prediction from Field Emission Scanning Electron Microscope Images Through Convolutional Neural Networks and Data Augmentation Techniques

With the growing demand for drug products requiring lyophilization, it is essential to either expand aseptic drying capacity or improve the efficiency of existing capacity through process intensification, ensuring that resources are utilized to their full potential. In this regard, mathematical models are highly recommended to assist professionals in process optimization. To effectively utilise these models, it is also essential to develop robust techniques for determining key parameters, including the product resistance to vapour flow. Traditional experimental methods for evaluating this coefficient are time-intensive and/or require the insertion of probes into the product, which is not feasible at a manufacturing scale. This study addresses these challenges by introducing a novel deep learning framework designed to predict the mass transfer coefficient directly from Field Emission Scanning Electron Microscope images. This approach significantly streamlines the evaluation process, leveraging the high-resolution capabilities of Field Emission Scanning Electron Microscope for detailed analysis. In this work, we focus on advanced Field Emission Scanning Electron Microscope image processing, choice of strategic convolutional neural network configuration, and thorough model performance evaluation to predict the mass transfer coefficient. Given the frequent scarcity of datasets in this field, we have employed data augmentation techniques to enhance the robustness of our model. The results demonstrate good predictive accuracy (error on the interpolation test data lower than 5%), highlighting the potential of this framework to facilitate the assessment of mass transfer coefficients in freeze-dried products.

Oscillatory nature in melt-gas-powder interactions during laser powder bed fusion process revealed by CFD-DEM coupled modelling

ABSTRACT The laser powder bed fusion (LPBF) process encompasses interactions between different material states, including melt, gas, and powder. While observational techniques can capture specific states, they cannot be combined to produce a comprehensive monitoring how they are mutually interacted. Thus, an integrated modelling approach is a crucial solution for revealing their interactions. This study investigates melt-gas-powder dynamics under varying laser scan speeds through a coupled CFD-DEM multiphase model. Findings reveal that metal vapour consistently transitions from an initialisation phase to a processing phase, exhibiting consistent behaviour in the former but varied responses in the latter. A relationship is identified between scan speed and vapour flow angle (VFA), with higher speeds broadening the VFA. Significantly, three oscillatory behaviours are observed: first, the oscillation of locally intensified pressure (LIP) sites on keyhole walls, where vapour ejection modes shift and may become periodic at intermediate scan speeds; second, the oscillation of two induced argon vortex flows with opposite swirling directions; and third, a simultaneously induced varying drag force on powder spatter. These oscillations clarify various spatter mechanisms, offer insights for process optimisation, and suggest implications for process stability. The study also proposes future research directions to further elucidate these mechanisms.

Origin of the formation of isostructural bcc-Fe+bcc-Cu nanocomposites in Fe–Cu alloy via vacuum co-deposition

This study examined the influence of substrate temperature and the ratio of iron and copper vapor flow on the structural state of Fe–Cu vacuum condensates. XRD patterns of the condensates deposited under specific electron beam physical vapor deposition process parameters revealed peaks corresponding to either bcc or bcc+fcc phases. Analysis of the lattice parameter of the single bcc structure indicates that only a portion of the copper atoms dissolve in the bcc-Fe lattice, while undissolved copper atoms form bcc-Cu inclusions that are coherent with the bcc-Fe lattice. A model for the formation of the bcc-Cu structure as an adaptive phase is proposed. The formation of the adaptive phase in vacuum condensates is driven by the bcc-Cu epitaxy on the surface of bcc-Fe crystallites and excess vacancies in the condensate structure. As the copper concentration in the isostructural Fe–Cu composite increases, the level of microstrain in its bcc crystal lattice also increases. The transformation of bcc-Cu particles into fcc-Cu via a shear mechanism occurs when heated above 400 °C. It was found that increasing the deposition temperature reduces the concentration range for the formation of the isostructural composite. It is suggested that higher deposition temperatures and increased copper concentration lead to larger copper particle sizes and reduced excess vacancy concentration, which disrupts their coherence with the bcc-Fe matrix. As a result, a composite with a eutectic-like microstructure consisting of bcc-Fe and fcc-Cu phases forms directly during vapor phase condensation.

Revealing the Hidden Role of Capacitance in the Water Flow Through Plants to the Atmosphere

Summary statementThe overlooked role of capacitance is revealed theoretically for vapor flow and by dimensional analogy for liquid flow. It combines with conductance to attribute distinctly the physiological and physical contributions to the quantitative formulation of plant transpiration and sap flow.

A framework for validating efficiency models of thermal separation columns with tray internals

Abstract Tray columns are globally used for distillation processes, and their performance must be optimized to reduce their cost and energy expenditures as well as carbon emissions. A prerequisite to achieve these targets demands a realistic account of the tray performance based on tray efficiency model application. The proof of concept of a Refined Residence Time Distribution (RRTD) model, recently proposed by the authors, is presented. The multifaceted challenges in acquiring hydrodynamic and performance data suitable for model validation are discussed in this work. In that regard, an unprecedented experimental campaign was performed in a representative large-scale air-water tray column simulator based on the application of multiplex flow profiler, new chemical system, and novel data processing schemes. Using these data and additional case studies, the capabilities of the RRTD model were demonstrated. This model successfully accounted for the impacts of varying local liquid flow characteristics and vapor flow maldistributions on the tray efficiency thereby advancing the state of the art of the tray efficiency models. This work particularly aimed at proposing a constructive framework to evaluate tray efficiency models in the future campaigns, and those campaigns will benefit from the learnings of the present work.

Boiling performance enhancement and self-recovery of nucleate boiling regime on micro- and nanostructured porous surfaces

In this paper, we report the pool boiling experimental results for significant enhancements in the boiling heat transfer coefficient (BHTC) and CHF on various microporous surfaces. The microporous surfaces were fabricated by metal (Copper) powder sintering process, and nanostructured porous surfaces were additionally fabricated through the thermal oxidation of the metal porous surfaces. Among the examined substrates, a boiling surface with a thin metal porous layer and millimeter-sized porous pillar structures exhibited optimal boiling performance; the BHTC and CHF of the microporous surface were measured to be approximately 500 % and 270 % higher than those of a smooth reference surface, respectively. We conjecture that the results come from the combined effect of active nucleation site density enhancement, capillary wicking promotion, and separation of liquid-vapor flow paths on the microporous surface with porous pillars. On the other hands, the porous pillar surface with needle-like nanostructures showed the interesting phenomena that can be regarded to the recovery of the boiling regime from transition boiling to nucleate boiling. At the CHF, the temperature increase on the surface was slower than that on the other surfaces. Additionally, at high heat fluxes below the CHF, the overheated surface self-recovered to the nucleate boiling state, indicating the rewetting of local dry spots. We consider that the capillary wicking capability strongly improved by the nanostructures on the surface was presumably responsible for inhibiting the irreversible expansion of local dry spots. From the experimental observations throughout this study, we propose the following key requirements to design an ideal boiling surface: increasing the nucleation site density, ensuring a liquid supply path by capillary wicking, separating the liquid and vapor flow paths, and improving the liquid wettability of the solid surface by forming nanostructures.

A hybrid approach portraying the dynamics of free convection condensation around a circular cylinder

We introduce a hybrid framework to model the fluid dynamics of free convection condensation of saturated steam outside a circular cylinder. The liquid film is modeled analytically, and the adjacent vapor flow field is resolved numerically. The model incorporates temperature-dependent thermophysical properties of the condensate. We explore the subtle role of Jakob number (Ja) and Weber number (We) in the two-phase flow and establish the criteria for dynamic similarity of the flow field with two new dimensionless numbers, viz., free-fall Reynolds number and similarity number. For fixed base Prandtl number of the condensate, the increases in the subcooling rate with Ja and the cylinder's surface area to volume ratio with We cause film thickening. We develop a theoretical correlation to predict the average film thickness (δm). We identify entrainment and bypass zones in the vapor flow field demarcated by a separating streamline. The entrainment zone's streamlines converge to the interface, yielding a net condensate drainage. The bypass streamlines never reach the interface and reduce the condensation efficiency. The results show that the tangential velocity is dominant within the liquid film, the radial velocity is dominant within the vapor flow field, and they are of the same order at the interface. We locate the point of flow separation influenced by a surface tension-induced adverse pressure gradient. The location of the flow separation point shifts upstream, and the separating streamlines become steeper with the increase in We. Our investigation reveals the zone of tangential flow reversal near the interface, promoted with increasing We.

Experimental study on heat transfer performance of mesh-type ultra-thin vapor chamber for large-area pouch battery

Ultra-thin vapor chambers (UTVCs) with high heat transfer characteristics in tight spaces are ideal for the heat dissipation needs of compact, high-energy-density battery systems for electric vehicles (EVs). This has also led to the expansion of the UTVC area to accommodate the battery size. In this study, a large-area (197 mm × 215 mm) UTVC with cost-effective flat meshes as the internal structure was designed for LiNi1-x-yCoxMnyO2 (NCM) pouch battery. Different from traditional mesh-type UTVCs, the vapor core is designed with multiple partially-through vapor channels to reduce vapor flow resistance. Experimental methods were used to investigate the impact of the UTVC placement and the position of the vapor channel in the vapor core on the heat transfer performance, as well as to examine the actual heat dissipation effect on the battery. The results show that gravity affects the performance of UTVC through placement. The UTVC under gravity-assisted placement has an advantage in both heat transfer performance and maximum heat dissipation power, reaching 0.142 °C/W and 144 W, respectively. And it is beneficial to reduce the thermal resistance of the UTVC by locating the vapor channel deeper into the evaporation section. The heat dissipation of the large-area battery by UTVC reduces the maximum temperature of the battery at the rate of 2.4C charging and 3C discharging by 22.3 and 23.8 °C, and the temperature difference is reduced by 62.31 % and 67 %, respectively.

A Diffuse Interface Model for Cavitation, Taking Into Account Capillary Forces

ABSTRACTWe consider the moving least squares method to solve compressible two‐phase water‐water vapor flow with surface tension. A diffuse interface model based on the Navier–Stokes and Korteweg equations is coupled with a suitable system of state equations that allows for a more realistic estimation of the pressure jump across the liquid–vapor interface as a function of temperature. We propose a simple formulation for computing the capillarity coefficient based on the surface tension and the thickness of the diffuse interface. A convergence analysis using pressure jump in the test case of static bubble is conducted to verify our solver. We present several numerical test cases that illustrate the ability of our model to reproduce qualitatively and quantitatively the effects of surface tension on cavitation bubbles in general situations.

Vapor-mediated impact of droplet on superheated powder bed

Droplet impacting on a sufficiently heated powder bed resembles those on a superheated solid surface, as the surface deformation is mediated by the spontaneously generated vapor flow from the bottom surface of the droplet. This emerged impacting behavior is denoted as vapor-mediated impact to differentiate from the wetting impact, which involves the wetting and absorption of the particles due to capillarity. We systematically vary the impacting velocity and the temperature of the powder bed to characterize the impacting dynamics for these two behaviors. For the vapor-mediated impact, the contact time and the maximum spreading diameter are found to have the same scaling laws derived for impact on the superheated surface. We construct a phase diagram of the impacting behaviors based on experimental observation, and propose a simplified model to predict the transition between these two behaviors. The predicted values match well with the experimental results, suggesting the proposed model captures the physical mechanism of the vapor-mediated impact.

Exploration of microwave absorptivity and catalysis of CMF@Co-MoS2 for microwave-initiated depolymerization of lignin

Obtaining key platform chemicals from lignin is a sustainable refining concept of the non-petroleum route. Rapid pyrolysis is a potential refining technology, and microwave-assisted catalytic depolymerization with low energy consumption is preferred. Herein, we reported the strategy of microwave-initiated depolymerization using a self-designed dynamic vapor flow reaction system with a facilely prepared CMF@Co-MoS2-0.18 catalyst, achieving efficient conversion of lignin (Dealkalize) to monophenol. Under constant microwave irradiation (2.45 GHz), the CMF@Co-MoS2-0.18 catalyst had excellent dielectric properties (Dielectric loss factor: 0.56) and microwave heating properties (∼140 s rise to ∼ 600℃). Therefore, microwave-initiated depolymerization of lignin showed that the CMF@Co-MoS2-0.18 catalyst can improve the monophenol content (54.72 %) and phenol selectivity (35.84 %) under the synergistic effect of microwave absorptivity and catalysis. The reaction of the lignin model and kinetics analysis show that the CMF@Co-MoS2-0.18 catalyst increases the yield of monophenol by reducing depolymerization activation energy and selectively breaking the β-O-4 bond. The finite element analysis pointed out that under microwave irradiation, CMF@Co-MoS2-0.18 continuously conducted heat to lignin and the lignin vapor would have sufficient energy to collide with the catalyst active site at a high frequency, which led to an increase in the pre-exponential factor and initiated catalytic depolymerization lignin. In summary, we provide a potential pathway for the rapid conversion of lignin into key platform chemicals.

Porous mesh manifold for enhanced boiling performance

High-performance electronics are continuously demanding cooling of higher heat fluxes. Phase-change cooling, including pool boiling, is a useful approach to address this challenge; however, competition between liquid and vapor flows generally limit the heat fluxes that can be dissipated. A range of strategies to control these flows have been investigated previously, including capillary guides. Here a manifold structure formed from a metallic mesh is investigated to control the disposition of liquid and vapor phases above a pool fed boiling surface enhanced with porous structures. Copper mesh forms defined liquid flow paths, using capillary action to guide and distribute liquid evenly over the heated surface, along with open channels to facilitate vapor escape. The mesh provides a novel structure for liquid guidance that imposes low resistance to liquid flow while occluding a minimal area of heated surface underneath. The manifold performance is characterized in boiling fed by a pool of water above a laser-textured aluminum nitride heat dissipation surface with pin–fin structures having heights of 110 µm and spacing of 30 µm with a heated area of 5mm x 5mm. A maximum heat flux of 490 W/cm2 is reached with the manifold in the pool fed configuration, representing an increase of more than 65% over the porous pin fin surface alone. The maximum stable superheat observed for the manifold of 36K is 14K higher than that for the porous surface without the manifold. The factors limiting performance of the manifold are analyzed. High superheat is attributed to partial flooding of the boiling surface as suggested by the reduction in superheat using external suction. Similar systems and structures for enhanced two-phase cooling are compared.

Numerical and analytical investigation of vapor flows in a flat plate heat pipe: Effects of length ratio and Reynolds number

Heat pipes are crucial in a wide range of applications, ranging from space satellites and industrial systems to electronic cooling and X-ray tube thermal management. This study introduces a method investigation into vapor flows within a flat plate heat pipe, utilizing the collocation method (CM) and the fourth-order Runge-Kutta-Fehlberg (RKF45) method. Building on previous efforts, this work explores the effects of the evaporator-to-condenser length ratio and Reynolds number on velocity and pressure distributions along the entire heat pipe. The significance of this research lies in its ability to elucidate critical parameters that directly influence heat pipe performance, offering deeper insights that are vital for optimizing design and efficiency. The primary motivation of this study is to fill existing gaps in the literature by developing a comprehensive analytical model that accurately characterizes vapor and liquid flow in asymmetrical flat plate heat pipes. The model's validity is confirmed through a satisfactory agreement with numerical results, underscoring the reliability of the methods used. Notably, the findings reveal that higher Reynolds numbers reduce pressure drop and shift the maximum velocity toward the bottom wick in the evaporation section, providing valuable guidance for future design improvements. Additionally, this research presents a powerful method for solving non-linear ordinary differential equations, offering significant time savings and enabling predictive functions. These contributions are poised to enhance the performance of thermal management systems across various engineering disciplines.

Variance investigation on the microstructural characteristics of vessels among three lignocellulosic biomasses

Especially, the processing and utilization of biomass-based material is closely related to the vessel, e.g. the flow of vapour and additive. It is conventional that vessels in most plants can influence on water and nutrients transport between adjacent cells, which could just infer to be important in the wood-based panel industries. In this work, a complete characterization of vessels and pits is presented for three conventional biomasses in wood-based panel: poplar (Populus deltoides) (P), moso bamboo (Phyllostachys edulis) (B), and the fruit shell of oil camellia (Camellia Oleifera) (FS_OC). Every material is analyzed by combining several techniques including: light microscopy, scanning electron microscopy and surveying calculations from resin casting. The results show that among the three biomass materials, B has a significantly larger vessel width (164.8 ± 6.0 μm for B, 2.2 ± 6.2 μm for P, 10.0 ± 0.8 μm for FS_OC) and smaller inclination angle of the perforation plates (6.8° for B, 44.7° for P), which is more conductive to improving moisture transfer between the vessels. The vessel length of P varies widely from 676.8 μm to 1025.2 μm, which is related to its seasonal growth. By resin casting analysis, more differences in the morphology and distribution of pits in the vessel walls were observed between the three species. Such as, For B, there are numerous pits between vessel cells, while very few to none between vessel and parenchyma cells or fiber. In addition to pits, B and FS_OC also have spiral thickening structures on their vessel walls. The pit membrane is an elliptical shape in P, while slit-like shape in FS_OC and a combination of both elliptical and slit-like shape in bamboo. The unique microstructural characteristics of vessels is related to the individual plant growth traits, which is the basis for biomass-based material processing and utilization.

Integrated substrate and adsorbent layer additive manufacturing for asymmetrically operating minichannel adsorbent bed

Adsorption heat pumps (AHP) have conventionally used packed bed design, making their operation sluggish leading to difficulty in using heat for the adsorbent regeneration. While thin adsorbent coatings improve the performance of AHPs when implemented in microscale geometries like minichannels, integrating the adsorbent coating procedure into the scaled-up adsorbent bed manufacturing has been impractical because of the finite time required for the coating process. Meanwhile, separately coated plates manifest sealing issues. This research details an innovative avenue to fabricate a unified adsorbent bed integrated with thin adsorbent coatings. Here, a compact adsorbent bed was fabricated through the integration of additive manufacturing using a fused deposition modeling (FDM) 3D printer for the substrate and resin curing for the MIL-101 (Cr) adsorbent layer. The multi-layered bed was designed to exchange heat between heat transfer fluid (HTF) and the passages through which the refrigerant vapor flows. The vapor channels were coated with a MIL-101 (Cr) adsorbent. The research incorporated both experiments and computational models to evaluate the operation of this adsorbent bed through the measurement of pressure during the adsorption and desorption stages for this component. Various HTF temperatures (80 °C − 90 °C) and flow rates (0.125 kg min−1 – 0.25 kg min -1) were tested. The findings revealed the much-desired asymmetrical operation with long adsorption (75–80 % of the cycle time) and short desorption stages (20 %-25 % of the cycle time), opening the possibility of employing a single bed in adsorption cooling systems to produce a near continuous cooling effect leading to more compact AHPs. The computational model developed for this test setup using MATLAB strongly aligned with experimental findings, with an error margin of 4–12 %.

Influence of large-scale free-stream turbulence on bypass transition in air and organic vapour flows

The free-stream turbulence (FST) induced transition in perfect and non-ideal gas zero-pressure-gradient flat-plate boundary layers is investigated by means of large-eddy simulations. The study focuses on the influence of large incoming disturbances over the laminar-to-turbulent transition, by comparing two different integral length scales $L_f$ , which differ by a factor of seven, at different FST intensities $T_u$ . High-subsonic dense-gas boundary layers of the organic vapour Novec649, representative of organic Rankine cycle applications, are compared with air flows at Mach numbers 0.1 and 0.9. Compressibility and non-ideal gas effects are shown to be of minor importance in comparison to the influence of the FST integral length scale $L_f$ . An increase of the inlet turbulent intensity always promotes transition, whereas an increase of $L_f$ has a double effect on the transition onset. At $T_u=2.5\,\%$ , increasing $L_f$ promotes the transition, while it tends to delay transition for an FST intensity of 4 %. Larger FST integral scales tend to increase the spanwise distance between laminar streaks generated in the boundary layer. Two competing transition scenarios are observed. When the incoming turbulence intensity and length scale are moderate, the classical bypass route consists in the linear non-modal growth of streaks, which then experience secondary instabilities (sinuous or varicose) and lead to the generation of turbulent spots. The second scenario is characterized by the appearance of $\Lambda$ -shaped structures near the inlet, which are further stretched to hairpin vortices before breaking down to turbulence. Spot inceptions can therefore occur at earlier locations than the streak growth. We are then faced with a competition between the classical bypass transition and nonlinear response mechanisms that ‘bypass’ this route. The present case at high $L_f$ and low $T_u$ is an example of a competing scenario, but even for the higher $T_u$ and $L_f$ conditions, only approximately one-third of turbulent spots are due to the $\Lambda$ -shaped events. The nonlinear alternative route has strong similarities with scenarios described previously in the literature in the presence of leading edge effects or due to passing wakes. Such a path is governed by the turbulence intensity, but also by the integral length scale, with both parameters playing a critical role in the generation of the $\Lambda$ -shaped structures near the inlet. This alternative mechanism is found to be robust under varying flow and thermodynamic conditions.

Effect of pin fins on heat transfer during condensation in minichannel heat exchanger

In this study, condensation heat transfer of water vapor in a minichannel heat exchanger on smooth and pin fins surfaces was investigated experimentally. The experimental study was carried out to evaluate heat transfer coefficient, overall heat transfer coefficient, and Nusselt number over different ranges of vapor mass flux from 0.0064 to 0.0368 kg m−2 s−1 and cold-water flow rate range between 0.0013 kg s−1 to 0.0057 kg s−1. The minichannel has a rectangular shape with a hydraulic diameter of 1.3 mm. The experimental testing is carried out on aluminum surface with a channel that has a length of 270, width of 30 mm, and height of 1.3 mm. The pin fins surface is on the bottom of the condensing channel and the fins are circular and have diameter, height, and spacing of 1 mm, and are in-inline arrangement. It is found that condensation heat transfer coefficient on pin fins surface is about 15 % higher than that on smooth surface. It is found that the condensation heat transfer coefficient increases significantly with increasing the vapor mass flux. In addition, the effect of increasing the cold fluid flow rate is lower than that of the hot vapor flow rate, but it becomes more significant for higher vapor flux.

Stacking Fault Nucleation in Films of Vertically Oriented Multiwall Carbon Nanotubes by Pyrolysis of Ferrocene and Dimethyl Ferrocene at a Low Vapor Flow Rate

Recent observations of superconductivity in low-dimensional systems composed of twisted, untwisted, or rhombohedral graphene have attracted significant attention. One-dimensional moiré superlattices and flat bands have interestingly been identified in collapsed chiral carbon nanotubes (CNTs), opening up new avenues for the tunability of the electronic properties in these systems. The nucleation of hexagonal moiré superlattices and other types of stacking faults has also been demonstrated in partially collapsed and uncollapsed carbon nano-onions (CNOs). Here, we report a novel investigation on the dynamics of stacking fault nucleation within the multilayered lattices of micrometer-scale vertically oriented films of multiwall CNTs (MWCNTs), resulting from the pyrolysis of molecular precursors consisting of ferrocene or dimethyl ferrocene, at low vapor flow rates of ~5–20 mL/min. Interestingly, local nucleation of moiré-like superlattices (as stacking faults) was found when employing dimethyl ferrocene as the pyrolysis precursor. The morphological and structural properties of these systems were investigated with the aid of scanning and transmission electron microscopies, namely SEM, TEM, and HRTEM, as well as X-ray diffraction (XRD) and Raman point/mapping spectroscopy. Deconvolution analyses of the Raman spectra also demonstrated a local surface oxidation, possibly occurring on defect-rich interfaces, frequently identified within or in proximity of bamboo-like graphitic caps. By employing high-temperature Raman spectroscopy, we demonstrate a post-growth re-graphitization, which may also be visualized as an alternative way of depleting the oxygen content within the MWCNTs’ interfaces through recrystallization.

Vapor-feed direct methanol fuel cells using pure methanol

This work optimizes the performance of the direct methanol fuel cell (DMFC) to increase its efficiency and strengthen its validity in portable power generation. Specifically, this work focuses on optimizing vapor-feed supply techniques and incorporating water management layers (WMLs) to analyze their effect on methanol crossover. The significance of the vapor-feed supply technique is to enhance the reaction kinetics of the methanol oxidation reaction (MOR) and enable the use of pure methanol (MeOH) as a fuel. Pure methanol is the ideal fuel for the DMFC as it has the highest possible energy density compared to dilute concentrations. However, use of pure methanol is hindered by methanol crossover, which is regarded as the largest technical barrier to commercializing DMFCs. This study measured methanol crossover through a CO2 sensor attached to the cathode outlet and added hydrophobic WMLs to the cathode to alleviate the methanol crossover. The hydrophobic WMLs increased the mass transfer resistance to generate a pressure gradient that encourages water backflow for use in both the proton exchange membrane (PEM) and anode reactions. The influence of vapor flow rate and fuel concentration will also be explored to show their impact on performance and methanol crossover. Likewise, long-term consumption and durability tests were conducted with and without a WML to dictate the WML’s superior fuel efficiency, total efficiency, energy density, and reduced methanol crossover using pure methanol. The addition of the WML increased the energy density of the vapor feed DMFC, using pure methanol, from 705.9 Wh kgMeOH-1 to 867.7 Wh kgMeOH-1 and lowered the crossover current density by 14.8 % when discharged at a constant 200 mA cm−2.