Enhancement of the Passive Direct Methanol Fuel Cells Performance by Modification of the Cathode Microporous Layer Using Carbon Nanofibers

Mass transfer is a key parameter affecting the performance of the passive direct methanol fuel cells (DMFCs), which work under natural convection. In this study, effect of carbon nanofibers (CNFs) addition to the cathode microporous layer (MPL) on the performance of the passive DMFCs was investigated. The results indicated that CNFs content has a significant influence on both of the mass transport and the electrochemical surface area (ECSA). Interestingly, addition of the CNFs (20 wt.%) leads to increase the power density of the passive DMFC to 160% compared to pristine carbon black MPL. At low current density, the CNFs content has no influence on the performance, while at high current density the maximum performance can be obtained at 20 wt.% CNFs then the performance decreases with further increase in the CNFs content. Although the highest catalyst utilization is observed at 40 wt.% CNFs, a maximum power density of 36 mW cm^–2 can be obtained at 20 wt.% CNFs and this is related to the significant effect of the mass transfer resistance under the passive operation conditions. Overall, addition of CNFs to the MPL can be considered an effective strategy to modify the passive DMFCs performance.     

Electrochemical Characteristics and Performance of Platinum Nanoparticles Supported by Vulcan/Polyaniline for Oxygen Reduction in PEMFC

We report a Pt/Vulcan carbon–polyaniline (VC–PANI) catalyst for the oxygen reduction reaction (ORR). This electrocatalyst was prepared from Pt nanoparticles supported by a VC–PANI composite substrate. Electrochemical performance was measured using potentiostat/galvanostats technique and a proton exchange membrane fuel cell (PEMFC) test station. The electrochemical properties of the electrodes were characterized using linear sweep voltammetry, AC impedance spectroscopy and chronoamperometry. Electrochemical characterization by hydrogen adsorption/desorption cyclic voltammetry and CO stripping voltammetry indicates that the electrochemical active surface areas of the Pt/VC–PANI are comparable to the commercial catalyst. The performance of the Pt/VC–PANI and Pt/C(E‐TEK) + PANI electrocatalysts were found to be 1.82 and 1.33 times higher than of the Pt/C(E‐TEK) electrode. The surface morphologies of the electrodes were characterized by using scanning electron microscopy (SEM). PANI has a fibrous structure and the improved performance was attributed to the PANI effect and synergistic effects between the carbon Vulcan and the PANI fiber. These results indicate that Pt/VC–PANI is a promising catalyst for the ORR in PEMFCs using an H₂/O₂ feed.     

Magnetic Field,Flow Field and Inclusion Collision Growth in a Continuous Caster with EMBR

A three‐dimension mathematical model has been developed to describe the magnetic field, fluid flow and inclusion transport in a continuous caster with electromagnetic brake (EMBR). According to the model, all the governing equations can be expressed as a general differential equation, so a general numerical method was developed to solve these equations. The numerical results agree well with the experimental result. In the continuous caster, the inclusion distributions have ‘M’ shape under the nozzle and ‘W’ shape at the exit, which come from the centrifugal effect and the collision and aggregation among inclusions. The three‐dimensional static magnetic field can effectively damp local flows and affect the inclusion transport in a continuous caster. If EMBR is installed under the nozzle, it can promote the inclusion removal and the inclusion ‘M’ distribution disappears.     

Comparing Anode Gas Recirculation with Hydrogen Purge and Bleed in a Novel PEMFC Laboratory Test Cell Configuration

In automotive‐type polymer electrolyte membrane fuel cell (PEMFC) systems, impurities and inert gases accumulate in the anode gas recirculation loop. Therefore, the impurity limits, dictated by the current hydrogen fuel specification (ISO 14687‐2:2012), also require quantification with representative fuel cell test systems applying anode gas recirculation, that enables high fuel utilization rates and accumulation of impurities.We report a novel PEMFC laboratory test cell configuration mimicking automotive conditions. This setup enabled comparison of two operation modes, hydrogen bleed and purge, within 84.4%–98.6% fuel utilizations. The results indicate that similar enrichment dynamics apply to both bleed and purge modes.The configuration employed a membrane dryer to circumvent the 60 °C limit of commercially available recirculation pumps. The membrane dryer allows heat and humidity extraction from the anode exit gas stream, enabling the adoption of conventional recirculation pumps, minimizing water condensation, and making sampling with on‐line gas analysis instruments easier. The results show that anode gas recirculation systems with hydrogen bleed can be implemented in conventional test stations by resorting to commercially available recirculation pumps. This enables realistic and cost‐effective determination of impurity effects for fuel cell system development and new hydrogen fuel standards.     

Effect of Serpentine Multi‐pass Flow Field Channel Orientation in the Liquid Water Distributions and Cell Performance

A commercial 50 cm² polymer electrolyte membrane (PEM) fuel cell with serpentine flow fields was operated at 2.0 bar and 60 °C with two orientations of the flow field channels with respect to gravity, i.e. horizontal and vertical channels. A 3 × 3 test matrix of anode and cathode reactants relative humidity was used for the performance assessment of the cell in both orientations. The cell performance and operating data, including cell voltage and resistance, were measured, and neutron radiographs were recorded during the entire operation in order to gain knowledge of the liquid water distributions within the cell for both orientations. A quantitative analysis of the results is presented in this work, comparing the cell operation for both flow field orientations. It is observed that the configuration with horizontal cathode flow field channels presents a better cell performance, and less amount of liquid water blocking the flow field channels. Thus, the results show that the selection of the cell orientation has an influence on the final performance, and it is therefore, a design parameter to be considered for a real application. The differences in the cell water content are quantitatively analyzed and discussed.     

Current Advances in Polymer Electrolyte Fuel Cells Based on the Promotional Role of Under‐rib Convection

Literature data on the promotional role of under‐rib convection for polymer electrolyte fuel cells (PEFCs) fueled by hydrogen and methanol are structured and analyzed, thus providing a guide to improving fuel cell performance through the optimization of flow field interaction. Data are presented for both physical and electrochemical performance showing reactant mass transport, electrochemical reaction, water behavior, and power density enhanced by under‐rib convection. Performance improvement studies ranging from single cell to stack are presented for measuring the performance of real operating conditions and large‐scale setups. The flow field optimization techniques by under‐rib convection are derived from the collected data over a wide range of experiments and modeling studies with a variety of components including both single cell and stack arrangements. Numerical models for PEFCs are presented with an emphasis on mass transfer and electrochemical reaction inside the fuel cell. The models are primarily used here as a tool in the parametric analysis of significant design features and to permit the design of the experiment. Enhanced flow field design that utilizes the promotional role of under‐rib convection can contribute to commercializing PEFCs.     

Dynamic Simulation of Oxygen Transport Rates in Highly Ordered Electrodes for Proton Exchange Membrane Fuel Cells

Highly ordered Pt electrode has been recognized as an important technology for reducing Pt usage in fuel cells due to its improved oxygen transport capability. However, ordered Pt electrode can lead to the decrease in roughness of electrode, which in turn makes it unclear whether the improved oxygen transport can offset the decreased roughness of ordered electrode. Herein, we theoretically investigate the oxygen distribution, generated current, and minimum Pt loading of ordered Pt electrode based on kinetic model of oxygen transport. The results reveal that ordered Pt electrodes do not exhibit concentration polarization with the electrode thickness up to 100 μm. For ordered Pt electrode with diameter of nanorod of 60 nm, the limited current density reaches 110.2 A cm⁻² that is much higher than that for conventional electrode without considering Ohmic loss and mass transport loss outside electrode. To generate a current of 1.5 A cm⁻² at 0.67 V for fuel cell, the minimum Pt loading of cathode in PEMFC reaches 0.029 mg cm⁻² assuming that the electrocatalyst nanorods contain 1 nm Pt layer at the outmost surface.     

Performance Improvement of Wave‐Like PEMFC Stack with Compound Membrane Electrode Assembly

The novel architecture of wave‐like proton exchange membrane fuel cell (PEMFC) stack developed in our previous work achieved peak volumetric power density and specific power of 2,715.9 W L^–1 and 2,157.9 W kg^–1, respectively. However, there still existed perforated bipolar plates and the carbon fiber gas diffusion layer (GDL) was easy to cause damage during the fabrication process of undulate membrane electrode assembles (MEAs). In the present study, sintered stainless steel fiber felt (SSSFF) was employed to work as metallic GDL (MGDL) and bipolar plates simultaneously. Compound membrane electrode assembles (CMEAs) with serpentine and interdigitated flow channels were designed and fabricated using stamping process. A single cell with CMEA was assembled in house and the output performance was evaluated systemically. The results indicated that the peak volumetric power density and specific power of wave‐like PEMFC single with CMEA are 5,764.0 W L^–1 and 4,693.5 W kg^–1 respectively. This study achieved a significant performance improvement due to the concept of CMEA and may propose a possible means to meet the DOE's 2020 technical target that volumetric power density is 2,500 W L^–1 and specific power is 2,000 W kg^–1 for stack.     

Three Dimensional Model of a High Temperature PEMFC. Study of the Flow Field Effect on Performance

A three‐dimensional isothermal model of a high temperature polymer membrane fuel cell equipped with polybenzimidazole membrane is described. All major transport phenomena were taken into account except the species crossover through the membrane. The cathode catalyst layer was treated as spherical catalyst agglomerates with porous inter‐agglomerate spaces. The inter‐agglomerate spaces were filled with a mixture of electrolyte (hot phosphoric acid) and polytetrafluoroethylene (PTFE). This approach proved to be an essential requirement for accurate simulation. In this particular paper, the influence of different flow field designs and dimensions on performance was intensely study. Traditional configurations were tested (straight, serpentine, pin‐in, and interdigitated), and new designs were proposed. With these new designs, we tried to maximize performance by providing homogeneous reactants distribution over the active area keeping low‐pressure drop and relatively high velocity. The dimension and position of the inlet and outlet manifolds were also analyzed. From the obtained results a massive influence of the manifolds position and dimension on performance was observed. This fact can provide important guidelines for future bipolar plates optimization.     

CO2 Absorption Using Nanofluids in a Wetted‐Wall Column with External Magnetic Field

A new method for enhancing the mass transfer coefficient in the gas absorption process is reported. CO₂ absorption experiments were carried out in a wetted‐wall column using different aqueous nanofluids as the solvent. The mass transfer characteristics were found to increase by applying Al₂O₃/water nanofluid. The mass transfer coefficient decreased with TiO₂/water nanofluid. In the case of Fe₃O₄/water nanofluid, the mass transfer rate was enhanced by increasing the nanoparticle volume fraction, but the mass transfer coefficient was lower than that obtained with water for all experimental conditions studied. Finally, applying a downward magnetic field resulted in higher mass flux and mass transfer coefficient in comparison with experiments without a magnetic field.     

High Performance Fuel Cell Operation with a Non‐fluorinated Polyphosphazene Electrode Binder

Non‐fluorinated sulphonated polyphosphazene (SPOP) was synthesised and characterised for utilisation as an electrode binder. Polarisation curves were obtained at 80 °C and 95% RH in a H₂/air fuel cell. SPOP has the proper properties for a cathode binder in a fuel cell, as its polarisation curve traced that of Nafion® binder for the same operating conditions.     

Investigation of mass transport in gas diffusion layer at the air cathode of a PEMFC

In a polymer electrolyte membrane fuel cell (PEMFC), slow diffusion in the gas diffusion electrode may induce oxygen depletion when using air at the cathode. This work focuses on the behavior of a single PEMFC built with a Nafion^® based MEA and an E-TEK gas diffusion layer and fed at the cathode with nitrogen containing 5, 10 and 20% of oxygen and working at different cell temperatures and relative humidities. The purpose is to apply the experimental impedance technique to cells wherein transport limitations at the cathode are significant. In parallel, a model is proposed to interpret the polarization curves and the impedance diagrams of a single PEMFC. The model accounts for mass transport through the gas diffusion electrode. It allows us to qualitatively analyze the experimental polarization curves and the corresponding impedance spectra and highlights the intra-electrode processes and the influence of the gas diffusion layer.     

Electropolishing of Vertical Plates under Intensified Mass Transfer Conditions in a Gas Stirred,Modified Parallel Plate Reactor

The rates of electropolishing of vertical copper plates in H₃PO₄ placed downstream of a H₂ evolving cathode were studied by measuring the limiting current of the diffusion controlled process under different conditions of H₂ discharge rates and H₃PO₄ concentrations. The mass transfer coefficient was found to increase with H₂ discharge rate raised to the exponent of 0.4. Within the studied experimental range of conditions, the rate of polishing increased by a factor ranging from 1.95–5.8, depending on the H₂ discharge rate and H₃PO₄ concentration compared to the natural convection value. The results are explained in terms of surface renewal theory. The proposed electrochemical reactor offers the advantage of increasing the rate of electropolishing without consuming external stirring energy as opposed to traditional electrochemical reactors.     

Use of Grooved Microchannels to Improve the Performance of Membrane‐Less Fuel Cells

In this work, the fluid dynamics within a membrane‐less microchannel fuel cell is analyzed computationally. The membrane‐less design is a result of the laminar nature of the fluid flow at small Reynolds numbers, restricting the fuel and oxidant to the vicinity of the corresponding electrodes, without the need of a proton exchange membrane (PEM). However, the performance of such cells is limited by the slow diffusive mass transport near the electrodes, with a large fraction of the reactants leaving the channel without coming in contact with the catalytic surfaces, and thus not being used. We mitigate this problem through the introduction of channel surface modification consisting of angled grooves designed to create convective flows that direct the reactants toward the active surfaces. The grooved structures are optimized for maximum fuel utilization. Operation of this type of cells at Péclet numbers close to 2,500 leads to a performance doubling compared with unmodified cells. Moreover, this increase in efficiency is accompanied by a more uniform distribution of the current across the electrodes, reducing the possibility of hot spots being developed.     

Comparison of Ozone and Oxygen Mass Transfer in a Laboratory and Pilot Plant Operation

Mass transfer of ozone and oxygen to water was investigated both in pilot plant countercurrent bubble column and in a Rushton type laboratory stirred reactor supplied with a variable speed turbine agitator. A comparison was made for different hydrodynamic conditions with the main task of developing an engineering approach for determination of the physical volumetric mass transfer coefficient (K_L ^oa), specific interfacial area (a), and physical masstransfer coefficient (K_LO). The mass transfer characteristics of ozone and oxygen can be determined quickly in a pilot plant or laboratory apparatus, and employed to optimize the performance ofthe full scale water treatment plant.     

Analysis of Leakages in a Solid Oxide Fuel Cell Stack in a System Environment

A solid oxide fuel cell (SOFC) stack can exhibit both anodic and cathodic leakages, i.e. a fuel leak from the anode side and an air leak from the cathode side of the stack, respectively. This study describes the results of an in‐situ leakage analysis conducted for a planar SOFC stack during 2000 hours of operation in an actual system environment. The leakages are quantified experimentally at nominal system operating conditions by conducting composition analysis and flow metering of gases for both fuel and air subsystems. Based on the calculated atomic hydrogen‐to‐carbon ratio of the fuel and air gases, it is found that the fuel leakages are mostly selective by nature: the leaking fuel gas does not have the same composition as the fuel system gas. A simple diffusive leakage model, based on the leakage being driven by concentration differences weighted by diffusion coefficients, is applied to quantify the amount of leakages. The leakage model provides a good correspondence with the experimental results of the gas analysis.     

松南气田气井合理产量模式建立

松南气田开采层位为火山岩储层,受构造和火山活动的双重控制,非均质性强,连通性差,而且有边底水,现有的经验难以满足开发需要。目前松南气田已投入开发,如何确保火山岩储层气藏高效开发,首先要解决气井合理产量确定问题。针对松南气田火山岩储层的特点,对不同类型的气井开展试井工作,利用试井成果确定气井的产能方程。松南气田火山岩储层气井在生产上表现为产气量差别大、压力下降快慢不同,因此,在单井合理产量模式确定上,主要是以气井产能方程为基础,考虑气井的边底水特征、井筒积液、稳产期等因素,对不同类型的气井分别建立了合理产量模式。     

Microbial Power‐Generating Capabilities on Micro‐/Nano‐Structured Anodes in Micro‐Sized Microbial Fuel Cells

Microbial fuel cells (MFCs) are an alternative electricity generating technology and efficient method for removing organic material from wastewater. Their low power densities, however, hinder practical applications. A primary limitation in these systems is the anode. The chemical makeup and surface area of the anode influences bacterial respiration rates and in turn, electricity generation. Some of the highest power densities have been reported using large surface area anodes, but due to variable chemical/physical factors (e.g., solution chemistry, architecture) among these studies, meaningful comparisons are difficult to make. In this work, we compare under identical conditions six micro/nano‐structured anodes in micro‐sized MFCs (47 μL). The six materials investigated include carbon nanotube (CNT), carbon nanofiber (CNF), gold/poly (ϵ‐caprolactone) microfiber (GPM), gold/poly(ϵ‐caprolactone) nanofiber (GPN), planar gold (PG), and conventional carbon paper (CP). The MFCs using three dimensional anode structures (CNT, CNF, GPM, and GPN) exhibited lower internal resistances than the macroscopic CP and two‐dimensional PG anodes. However, those novel anode materials suffered from major issues such as high activation loss and instability for long‐term operation, causing an enduring problem in creating widespread commercial MFC applications. The reported work provides an in‐depth understanding of the interplay between micro‐/nano‐structured anodes and active microbial biofilm, suggesting future directions of those novel anode materials for MFC technologies.     

A Visual Study of Bubble Formation in Microfluidic Fuel Cells

Two microfluidic fuel cells having microchannel widths of 1.0 mm (cell #1) and 0.5 mm (cell #2) with electrode spacing of 0.4 mm were tested at volumetric flow rates ranging from 0.1 to 1.0 ml min^–1. The concentration of hydrogen peroxide was tested at 0.1, 0.3, and 0.6 M. An additional microfluidic fuel cell (cell #3) having microchannel width of 0.5 mm and electrode spacing of 0.2 mm was also tested. Bubble formation under various tested conditions in different microchannels are presented. The open circuit voltage of the cells increased as reactant volumetric flow rate increased. Effect of electrode spacing on cell performance depends on the reactant concentration and volumetric flow rate. Also reported was the area‐specific internal resistance of the present cells and their fuel utilization corresponding to peak power density at a given flow rate with [H₂O₂] = 0.1 M. For cell #1, cell #2, and cell #3, respectively, the maximum power densities were 9, 40, and 16 mW cm^–2 at 1.0 ml min^–1 and 0.6 M, while the maximum power densities were 5, 11, and 15 mW cm^–2 at 1.0 ml min^–1 and 0.1 M.     

Instability of a Magnetic Fluid Jet in a Transverse Magnetic Field

The instability behavior of moving magnetic fluid jet in the presence of a constant uniform transverse magnetic field is investigated theoretically within the framework of linear stability analysis. The corresponding dispersion relation between the wave growth rate of instability and the magnitude of its wavenumber is obtained. The dependence of the wave growth rate on the polar angle in the presence of magnetic field is found. The influence of the magnetic field on the length of undisturbed jet section and the instability wavelength is studied. The correlation with the experimental results is demonstrated.