Fuel quality and operational issues for polymer electrolyte membrane (PEM) fuel cells

Polymer electrolyte membrane (PEM) fuel cells are susceptible to degradation due to the catalyst poisoning caused by CO present in the fuel above certain limits. Although the amount of CO in the fuel may be within the permissible limit, the fuel composition (% CO₂, CH₄, CO and H₂O) and the operating conditions of the cell (level of gas humidification, cell temperature and pressure) can be such that the equilibrium CO content inside the cell may exceed the permissible limit leading to a degradation of the fuel cell performance. In this study, 50 cm² active area PEM fuel cells were operated at 55–60 °C for periods up to 250 hours to study the effect of methane, carbon dioxide and water in the hydrogen fuel mix on the cell performance (stability of voltage and power output). Furthermore, the stability of fuel cells was also studied during operation of cells in a cyclic dead end / flow through configuration, both with and without the presence of carbon dioxide in the hydrogen stream. The presence of methane up to 10% in the hydrogen stream showed a negligible degradation in the cell performance. The presence of carbon dioxide in the hydrogen stream even at 1–2% level was found to degrade the cell performance. However, this degradation was found to disappear by bleeding only about 0.2% oxygen into the fuel stream.     

流道结构和几何尺寸对燃料电池性能影响的实验研究

流场板是质子交换膜燃料电池重要部件之一。本文对以氢气和氧气作为反应气体的质子交换膜电池的极化曲线进行了实验测定,研究了不同流场板结构、流场板深度和宽度对电池性能的影响．研究发现采用组合流道的电池性能最佳．     

Influence of fuel and media on membraneless sodium percarbonate fuel cell

This paper reports the media flexibility of membraneless sodium percarbonate fuel cell (MLSPCFC) using acid/alkaline bipolar electrolyte in which the anode is in acidic media while the cathode is in alkaline media, or vice versa. Investigation of the cell operation is conducted by using formic acid as a fuel and sodium percarbonate as an oxidant for the first time under ‘acid–alkaline media’ configurations. The MLSPCFC architecture enables interchangeable operation with different media combinations. The experimental results indicate that operating under acid–alkaline media conditions significantly improves the fuel cell performance compared with all-acidic and all-alkaline conditions. The effects of flow rates and the concentrations of various species at both the anode and cathode on the cell performance are also investigated. It has been demonstrated that the laminar flow-based microfluidic membraneless fuel cell can reach a maximum power density of 25.62 mW cm^?2 with a fuel mixture flow rate of 0.3 mL min^?1 at room temperature.     

Effect of nitrogen and carbon dioxide as fuel impurities on PEM fuel cell performances

Polymer electrolyte membrane (PEM) fuel cells are considered to have the highest power density of all the fuel cells. They operate on hydrogen fuel, which is generally produced by reforming of hydrocarbons, and may contain large amounts of impurities such as carbon dioxide, nitrogen, and trace amounts of carbon monoxide. We studied the effect of dilution of hydrogen gas with carbon dioxide on PEM fuel cells by polarization studies. The polarization curves were different when hydrogen gas was diluted with same quantities of carbon dioxide and with nitrogen. It may be due to carbon monoxide formation by reverse shift reaction and poisoning of anode platinum catalyst. Use of Pt–Ru alloy catalyst was found to suppress the poisoning. The effects of hydrogen gas composition, temperature, current density, and anode catalyst on fuel cell performances were examined in this study.     

Phosphorus-doped carbon nanoparticles supported palladium electrocatalyst for the hydrogen evolution reaction (HER) in PEM water electrolysis

Hydrogen production by PEM water electrolysis is one of the most efficient methods, due to the produced high purity of gases, high efficiency, and devoid of harmful emissions. In this study, phosphorus-doped carbon nanoparticles (P-CNPs) were synthesized by spray pyrolysis method in chemical vapor deposition (CVD). The synthesized P-CNPs were used as electron carrier support materials for the preparation of P-CNPs-supported palladium (Pd/P-CNPs) electrocatalyst and also used as the hydrogen evolution reaction (HER) electrode in PEM water electrolysis. These synthesized Pd/P-CNPs were characterized by field emission scanning electron microscope, energy-dispersive X-ray spectroscopy, X-ray diffraction, and cyclic voltammetry methods. The membrane electrode assemblies (MEAs) were fabricated using Pd/P-CNPs as a cathode catalyst for the HER and RuO₂ as the anode for oxygen evolution reaction (OER). The fabricated MEA electrochemical performances along with their corresponding yields of hydrogen production were evaluated in PEM water electrolyzer single cell assemblies at various experimental conditions. The obtained results showed that the synthesized Pd/P-CNPs observed a current density of 1 A cm^?2 at 2 V at 80 °C. Further, long-term stability tested for up to 500 h continuously and showed the reasonable stability with similar electrochemical activity compared to commercial Pt/CB. Hence, the synthesized Pd/P-CNPs could be used as the alternative to Pt-based catalysts for HER.     

Influence of methanol impurity in hydrogen on PEMFC performance

Proton exchange membrane fuel cells [PEMFC] have become highly attractive for stationary as well as mobile energy applications due to their good efficiency compact cell design and zero emissions. PEM fuel cells mainly consist of anode and cathode containing platinum/platinum alloy electrocatalysts and Nafion membrane as the electrolyte. They operate on hydrogen fuel, which is generally produced by reforming of hydrocarbons, alcohols such as methanol and may contain large amounts of impurities such as methanol, carbon dioxide, trace amounts of carbon monoxide, etc. The studies on the effect of methanol impurity in hydrogen on fuel cell performance and methods of mitigation of poisoning are very important for the commercialization of fuel cells and are described in a limited number of papers only. In this paper, we present the studies on the influence of methanol impurity in hydrogen for the PEM fuel cells. The effect of various parameters such as methanol concentration, cell voltage, current density, exposure time, reversibility, operating temperature, etc. on the cell performances was investigated using pure hydrogen. Various methods of methanol poisoning mitigation were also investigated.     

质子交换膜燃料电池电流分布的动态响应

动态特性是理解质子交换膜燃料电池性能的重要参数之一.运用燃料电池测试系统、恒电流/恒电压多通道测试仪和燃料电池电流密度分布测试装置,试验测量了质子交换膜燃料电池在不同加湿温度、电池温度和压力下的电流分布动态响应和动态特性.研究发现:不同区域的局部电流达到新的平衡所需的时间不同;加湿温度变化时,不同区域的局部电流的变化趋...     

Laser-based dynamic evaporation and surface shaping of fused silica with assist gases: a path to rimless laser machining

Evaporation and ablation are fundamental processes which drive laser-material processing performance. In applications where surface shape is important, control of the temperature field and the resulting spatially varying material response must be considered. For that purpose, assist gases are useful in, first, lowering treatment temperatures and, second, in changing interfacial and bulk chemistry to limit capillary-driven flow. Additionally, laser-matter coupling is influenced by pulse length as it determines the heat affected zone. Using infrared imaging of CO₂ laser-heated fused silica and surface profile measurements, we derive temperature and time dependent pitting rates along with shapes for a range of gases that include hydrogen, nitrogen, air, and helium. In the range of 1,500–4,500 K, evaporation, flow, and densification are shown to contribute to the pit shape. Analysis reveals a strong and complex dependence of rim formation on heating time and gas chemistry, mostly by lowering treatment temperature. Under dynamic heating, chemicapillarity appears to help in lowering rim height, in spite of the reactants mass transport limitations. Results on this gas-assisted approach suggest the possibility for sub-nanometer “rimless” laser-based machining.     

Kinetic analysis of n-decane–hydrogen blend combustion in premixed and non-premixed supersonic flows

Numerical analysis of ignition and combustion of an n-decane–hydrogen fuel blend in a premixed supersonic flow and in a model scramjet duct is performed using a reduced reaction mechanism built especially to describe the oxidation of blended n-C₁₀H₂₂–H₂ fuel in air at the temperature T₀ > 900–1000 K in the pressure range P₀ = 0.1–13 atm. The developed kinetic mechanism involves the principal reactions responsible for chain mechanism development both for n-decane and for hydrogen oxidation. It has been shown that using blended n-C₁₀H₂₂–H₂ fuel makes it possible to enhance the ignition and combustion both in premixed and in non-premixed supersonic fuel–air flows compared to burning pure hydrogen–air and n-decane–air mixtures. This allows high combustion completeness in the scramjet duct at the distance of ～1 m even at extremely low air temperature T₀ = 1000 K and pressure P₀ = 0.3 atm. This is due to the interaction of kinetics of the formation of highly reactive atoms and radicals, carriers of chain mechanism, in H₂–air and n-C₁₀H₂₂–air mixtures.     

Design of 1 kW high temperature PEM fuel cell system and performance analysis under different operating conditions

This paper demonstrates the mathematical and MATLAB simulation model of 1 KW (28.8V_dc) PEM fuel cell system with boost convertor and RL load to analyze the yield behavior in accordance to control the hydrogen fuel utilization. Two cases have been designed to evaluate the performance of this model. In the first case, fuel cell parameters are examined with and without a fuel regulator that controls the hydrogen fuel rate while in the second case, the operating temperature of a fuel cell stack is varied to observe the impact on the system. PEM fuel cell based power systems can become an alternate choice in the transportation sector to overcome contamination concerns, especially in South Asia where the environmental issues are at peak. The purpose of this work is to introduce such environmentally friendly system of transportation in South Asia, especially in Pakistan and this stack model can be used as a prototype for developing FC based motorbike as currently no practical models have been tested in this region. Therefore, this model has unique advantages over the existing in the literature.     

操作参数对PEM燃料电池中水迁移的影响

质子膜内水分和阴极多孔电极中液态水含量是PEM燃料电池正常运行的控制因素。本文给出了一个用于研究PEM燃料电池内水迁移的稳态、等温、两相流模型。模型耦合了连续方程、动量守恒方程和物质守恒方程,以及水在质子膜中传递方程。运用试验结果验证了模型的有效性。分析模拟结果表明,增大系统操作压力、升高电池操作温度和降低加湿温度将会使质子膜中水的净迁移通量增大;增大操作压力、降低操作温度和升高加湿温度会增加阴极CTL与GDL界面上液态水含量。     

Protic ionic liquid-containing silica-based ionogels for nonhumidified PEMFC applications

This paper provides details on the design of protic ionic liquids (PILs) containing silica-based ionogel for nonhumidified proton exchange membrane (PEM) applications. Specifically, we described herein a simple method of encapsulating individually four different PILs within the silica network via the sol-gel technique. The choice of the PIL, with primary consideration on its chemical structure and the appropriate ratio of tetraethylorthosilicate (TEOS)/PIL to bring about effective encapsulation inside the silica matrix, would be integral to the design of an ionogel PEM with good ionic conductivity and high thermal and stable mechanical properties. Conductivity data suggests that both vehicle and Grotthus mechanisms could play a role in the proton conduction in the PIL-based ionogels. Further, results showed that the ionogel prepared from triethylammonium methanesulfonate PIL (ES) yielded the highest ion conductivity of 3.85 × 10^?2 S/cm measured at 120 °C under nonhumidified condition. Moreover, the same ionogel when used in real proton exchange membrane fuel cell (PEMFC) systems under nonhumidified condition possessed a maximum power density of 43.3 mW/cm² at a current density of 92 mA/cm² at 30 °C, thus confirming, for the first time, the potential of PIL-containing silica-based ionogels fabricated via the facile sol-gel technique for PEMFC applications. Results presented herein merit further investigation and optimization on the use of ionogels as PEM for nonhumidified and moderately high-temperature PEMFC applications.     

A novel design and micro-fabrication for copper (Cu) electroforming bipolar plates

A novel design and micro-fabrication were developed for micro-proton exchange membrane (PEM) fuel cell stack bipolar plates with cross section of 5 cm² and thickness of about 650 μm. Copper metals were used to make bipolar plates (BPs) by using a LIGA-like micro-fabrication process of deep UV lithography in order to obtain SU-8 resist patterns/and SU-8 mould. Through two-sided exposure and development, copper metal bipolar plates with serpentine (meandering) flow field configurations for a micro PEM fuel cell stack, were fabricated and performance tests through polarization characteristics were conducted; this made it possible for the first time to consider copper sheets as suitable for BPs in micro PEM fuel cell stacks.     

Fabrication and device characterization of potassium fluoride solution treated CZTSSe solar cell

Post deposition treatment (PDT) for Cu₂ZnSn(S,Se)₄ (CZTSSe) was carried out by simply dipping the absorber into the KF solution at 80 °C. The dipping time of absorber in KF solution was found to be crucial to device parameters of CZTSSe solar cell. The K-doping improved the solar cell efficiency from 4.4% to 7.6% by 1 min dipping whereas the longer than 5 min dipping solar cells showed distorted kink J-V curves. The activation energy of CZTSSe solar cell was increased upto 1 min KF treatment from 0.83 eV to 0.92 eV which indicates interface recombination is reduced significantly. However, the activation energies of 5 min and 10 min dipping solar cells were found to be 0.81 eV and 0.63 eV where dominant recombination was interface recombination. Furthermore, trap energies of 49 meV and 298 meV of pristine CZTSSe solar cell were modified to 33 meV and 117 meV for 1 min treated CZTSSe solar cell. Trap energies of 5 min were calculated to be 112 meV and 147 meV. The proper KF doping passivated the shallow as well as deep defects of CZTSSe solar cell which is reflected in photovoltaic performances directly.     

Electron temperature and ion density measurements in a glow discharge of an Ar–N2 mixture

A DC glow discharge produced in N₂ gas can generate several species that are important in different applications, such as the modification of surface properties of materials. A low-pressure glow discharge apparatus was used for the the analysis of the Ar–N₂ mixture at a total pressure of 2.0 Torr, a power of 20 W and 40 l/min flow rate of gases. The emission bands were measured in the wavelength range of 200–1100 nm. The principal elements are N₂, N ₂⁺ and Ar I. The electron temperature was found in the range of 1.72–2.08 eV, and the ion density was in the order of 10¹⁰ cm^?3.     

Flexible paper-based borohydride-vanadium fuel cell for powering micro-nanosystems

This work deals about the development of paper-based fuel cells with high open-circuit voltages for application in powering the micro-nanosystems such as bio-sensors. The developed fuel cell employs Whatman paper as the ion conductor placed between the anode and the cathode to replace expensive ion-conducting membranes such as Nafion. A maximum open-circuit voltage of ～2.1 V per single cell and a power output of ～4.5 mW cm^?2 at ～6 mA cm^?2 are obtained when employing 1 M NaBH₄ in 20 wt% NaOH solution and 1 M VO₂ ⁺ in 2.5 M H₂SO₄ solution. Chronoamperometric measurement performed at 1 V indicates that the output power density varies from 6 to 1 mW cm^?2 in about 100 min. The power delivered at the end of even 100 min is comparable to that of the peak power delivered by many of the micro-fuel cell systems reported in the literature.     

SOFC characteristics along the flow path

Solid oxide fuel cells (SOFCs) in metallic housings were integrally and locally characterised. The tests were performed in counter flow operation for hydrogen concentrations from 2% to 100%, to identify concentration limitations and to optimise fuel utilisation. Cell characterisations were performed by spatially resolved electrochemical impedance spectroscopy (EIS), current density/voltage (i–V) and temperature measurements as well as gas chromatography measurements at 16 distinct points across the cell. The results show a substantial variation of current density and voltage distribution along the flow path with varying hydrogen content and fuel utilisation. The fuel utilisation was calculated from the local current densities and compared to the values measured by gas chromatography. Both sets of results showed good agreement. At low hydrogen inlet concentrations the voltage at the fuel outlet drops to values that might be harmful for the stability of the anode since reoxidation of nickel can occur. The impedances obtained by local EIS did not show an overall coherent dependency on the hydrogen concentration. EIS under load revealed two distinct domains: in the range of hydrogen concentrations of 2–10% H₂ the impedance decreased significantly with increasing hydrogen content whereas at higher hydrogen contents the impedance was hardly affected. This indicates significant concentration and diffusion overpotential at low hydrogen concentrations. The local data showed differing behaviour in the middle of the cell compared to the fuel outlet. Leakage at the sealing could be identified as a possible reason. As an additional method of investigation, the voltage drop over the contact resistance of the cathode side was measured. Temperature measurements show that local temperatures differ significantly depending on the load applied to the cell. This observation emphasizes the importance of a thermal management adapted to the characteristics on operation conditions of the cells, particularly when the stack itself has only a low mass.     

Strain rate and fuel composition dependence of chemiluminescent species profiles in non-premixed counterflow flames: comparison with model results

A detailed comparison has been conducted between chemiluminescence (CL) species profiles of OH^?, CH^?, and C₂ ^?, obtained experimentally and from detailed flame kinetics modeling, respectively, of atmospheric pressure non-premixed flames formed in the forward stagnation region of a fuel flow ejected from a porous cylinder and an air counterflow. Both pure methane and mixtures of methane with hydrogen (between 10 and 30 % by volume) were used as fuels. By varying the air-flow velocities methane flames were operated at strain rates between 100 and 350 s^?1, while for methane/hydrogen flames the strain rate was fixed at 200 s^?1. Spatial profiles perpendicular to the flame front were extracted from spectrograms recorded with a spectrometer/CCD camera system and evaluating each spectral band individually. Flame kinetics modeling was accomplished with an in-house chemical mechanism including C₁–C₄ chemistry, as well as elementary steps for the formation, removal, and electronic quenching of all measured active species. In the CH₄/air flames, experiments and model results agree with respect to trends in profile peak intensity and position. For the CH₄/H₂/air flames, with increasing H₂ content in the fuel the experimental CL peak intensities decrease slightly and their peak positions shift towards the fuel side, while for the model the drop in mole fraction is much stronger and the peak positions move closer to the fuel side. For both fuel compositions the modeled profiles peak closer to the fuel side than in the experiments. The discrepancies can only partly be attributed to the limited attainable spatial resolution but may also necessitate revised reaction mechanisms for predicting CL species in this type of flame.     

Preparation of graphene/polyaniline-modified carbon nanotubes and their electrochemical properties in microbial fuel cell

Using chemical vapor deposition methods to prepare carbon nanotubes growing in situ on a carbon felt, graphene and polyaniline were applied to the carbon felt for modifying carbon nanotubes. Microbial fuel cell was constructed with graphene/polyaniline-modified carbon nanotubes as anode, graphite as cathode, and glucose solution as substrate. The effects of electrodes, substrate concentration, and temperature on the properties of microbial fuel cell have been studied. At 38 °C using glucose solution of 1450 mg L^?1 and external resistance of 2500 Ω, the optimum output voltage of 687 mV and removal rate of 83% for chemical oxygen demand were obtained in the microbial fuel cell. The prepared nanomaterials are stable and reusable.     

Numerical modelling of methanol liquid pool fires

The focus of this paper is on numerical modelling of methanol liquid pool fires. A mathematical model is first developed to describe the evaporation and burning of a two-dimensional or axisymmetric pool containing pure liquid methanol. Then, the complete set of unsteady, compressible Navier-Stokes equations for reactive flows are solved in the gas phase to describe the convection of the fuel gases away from the pool surface, diffusion of the gases into the surrounding air and the oxidation of the fuel into product species. Heat transfer into the liquid pool and the metal container through conduction, convection and radiation are modelled by solving a modified form of the energy equation. Clausius–Clapeyron relationships are invoked to model the evaporation rate of a two-dimensional pool of pure liquid methanol. The governing equations along with appropriate boundary and interface conditions are solved using the flux-corrected transport algorithm. Numerical results exhibit a flame structure that compares well with experimental observations. Temperature profiles and burning rates were found to compare favourably with experimental data from single- and three-compartment laboratory burners. The model predicts a puffing frequency of approximately 12 Hz for a 1 cm diameter methanol pool in the absence of any air co-flow. It is also observed that increasing the air co-flow velocity helps in stabilizing the diffusion flame, by pushing the vortical structures away from the flame region.