Removal of excess product water in a PEM fuel cell stack by vibrational and acoustical methods

The research presented here investigates the use of vibro-acoustic methods to improve the performance of a PEM fuel cell by enhancing water removal from the active reaction sites within the fuel cell. Removing the water increases the available reaction sites and thus increases the available power for a given operating condition. To examine the new water removal methods, first, the production of water in fuel cells and current water removal methods are reviewed. Then, the new methods are proposed that are based on structural and acoustical excitation of the stack. Specifically, the use of flexural waves, acoustic waves and surface waves to remove water from a fuel cell stack are examined. Analytical formulations are given in order to calculate the excitation frequency and amplitude required to move a droplet resting on a vibrating bipolar plate. Depending on the droplet radius and other parameters, it is estimated that a water droplet resting on a bipolar plate can be moved by structural displacement levels as low as 1 μm. The different approaches to droplet removal are compared in terms of the minimum vibration energy required per droplet. Water production in a commercial fuel cell stack is then estimated and used as a test case to compare the power required to effect removal of a certain number of droplets with the amount of power produced by the stack. It is shown that a water droplet clogging a plate channel may be moved with parasitic power requirements as low as 21 mW. For each method, the energy required to effect droplet removal is quite small, although among the three, the use of surface acoustic waves may be the best option in terms of minimal vibration energy and implementation feasibility.     

Passive direct methanol fuel cells fed with methanol vapor

A vapor fed passive direct methanol fuel cell (DMFC) is proposed to achieve a high energy density by using pure methanol for mobile applications. Vapor is provided from a methanol reservoir to the membrane electrode assembly (MEA) through a vaporizer, barrier and buffer layer. With a composite membrane of lower methanol cross-over and diffusion layers of hydrophilic nanomaterials, the humidity of the MEA was enhanced by water back diffusion from the cathode to the anode through the membrane in these passive DMFCs. The humidity in the MEA due to water back diffusion results in the supply of water for an anodic electrochemical reaction with a low membrane resistance. The vapor fed passive DMFC with humidified MEA maintained 20–25 mW cm⁻² power density for 360 h and performed with a 70% higher fuel efficiency and 1.5 times higher energy density when compared with a liquid fed passive DMFC.     

Development of micro direct methanol fuel cells for high power applications

A micro direct methanol fuel cell (μDMFC) with active area of 1.625 cm² has been developed for high power portable applications and its electrochemical characterization carried out in this study. The fragility of the silicon wafer makes it difficult to compress the cell for good sealing and hence to reduce contact resistance in the Si-based μDMFC. We have instead used very thin stainless steel plates as bipolar plates with the flow field machined by photochemical etching technology. For both anode and cathode flow fields, widths of both the channel and rib were 750 μm, with a channel depth of 500 μm. A gold layer was deposited on the stainless steel plate to prevent corrosion. This study used an advanced MEA developed in-house featuring a modified anode backing structure with a compact microporous layer. Maximum power density of the micro DMFC reached 62.5 mW cm⁻² at 40 °C, and 100 mW cm⁻² at 60 °C at atmospheric pressure, which almost doubled the performance of our previous Si-based μDMFC.     

Preparation,electrical, mechanical and thermal properties of composite bipolar plate for a fuel cell

A novel composite bipolar plate for a polymer electrolyte fuel cell has been prepared by a bulk-moulding compound (BMC) process. The electrical resistance of the composite material decreases from 20 000 to 5.8 mΩ as the graphite content is increased from 60 to 80 wt.%. Meanwhile, the electrical resistance of composite increases from 6.5 to 25.2 mΩ as the graphite size is decreased from 1000 to 177 μm to less than 53 μm. The thermal decomposition of 5% weight loss of composite bipolar plate is higher than 250 °C. The oxygen permeability of the composite bipolar plate is 5.82×10⁻⁸ (cm³/cm² s) when the graphite content is 75 wt.%, and increases from 6.76×10⁻⁸ to 3.28×10⁻⁵ (cm³/cm² s) as the graphite size is longer or smaller than 75 wt.%. The flexibility of the plate decreases with increasing graphite content. The flexural strength of the plate decreases with decrease in graphite size from 31.25 MPa (1000–177 μm) to 15.96 MPa (53 μm). The flexural modulus decreases with decrease of graphite size from 6923 MPa (1000–177 μm) to 4585 MPa (53 μm). The corrosion currents for plates containing different graphite contents and graphite sizes are all less than 10⁻⁷ A cm⁻². The composite bipolar plates with different graphite contents and graphite sizes meet UL-94V-0 tests, and the limiting oxygen contents are higher than 50. Testing show that composite bipolar plates with optimum composition are very similar to that of the graphite bipolar plate.     

Effects of ambient pressure,gas temperature and combustion reaction on droplet evaporation

The effects of ambient pressure, initial gas temperature and combustion reaction on the evaporation of a single fuel droplet and multiple fuel droplets are investigated by means of three-dimensional numerical simulation. The ambient pressure, initial gas temperature and droplets’ mass loading ratio, ML, are varied in the ranges of 0.1–2.0 MPa, 1000–2000 K and 0.027–0.36, respectively, under the condition with or without combustion reaction. The results show that both for the conditions with and without combustion reaction, droplet lifetime increases with increasing the ambient pressure at low initial gas temperature of 1000 K, but decreases at high initial gas temperatures of 1500 K and 2000 K, although the droplet lifetime becomes shorter due to combustion reaction. The increase of ML and the inhomogeneity of droplet distribution due to turbulence generally make the droplet lifetime longer, since the high droplets’ mass loading ratio at local locations causes the decrease of gas temperature and the increase of the evaporated fuel mass fraction towards the vapor surface mass fraction.     

Dispersion and vaporization of biofuels and conventional fuels in a crossflow pre-mixer

In lean premixed pre-vaporized (LPP) combustion, controlled atomization, dispersion and vaporization of different types of liquid fuel in the premixer are the key factors required to stabilize the combustion process and improve the efficiency. The dispersion and vaporization process for biofuels and conventional fuels sprayed into a crossflow pre-mixer have been simulated and analyzed with respect to vaporization rate, degree of mixedness and homogeneity. Two major biofuels under investigation are Ethanol and Rapeseed Methyl Esters (RME), while conventional fuels are gasoline and jet-A. First, the numerical code is validated by comparing with the experimental data of single n-heptane and decane droplet evaporating under both moderate and high temperature convective air flow. Next, the spray simulations were conducted with monodispersed droplets with an initial diameter of 80 μm injected into a turbulent crossflow of air with a typical velocity of 10 m/s and temperature of around 800 K. Vaporization time scales of different fuels are found to be very different. The droplet diameter reduction and surface temperature rise were found to be strongly dependent on the fuel properties. Gasoline droplet exhibited a much faster vaporization due a combination of higher vapor pressure and smaller latent heat of vaporization compared to other fuels. Mono-dispersed spray was adopted with the expectation of achieving more homogeneous fuel droplet size than poly-dispersed spray. However, the diameter histogram in the zone near the pre-mixer exit shows a large range of droplet diameter distributions for all the fuels. In order to improve the vaporization performance, fuels were pre-heated before injection. Results show that the Sauter mean diameter of ethanol improved from 52.8% of the initial injection size to 48.2%, while jet-A improved from 48.4% to 18.6% and RME improved from 63.5% to 31.3%. The diameter histogram showed improved vaporization performance of jet-A.     

Preparation and properties of high performance nanocomposite bipolar plate for fuel cell

This study aims at developing lightweight and high performance composite bipolar plates for use in polymer electrolyte membrane fuel cells (PEMFCs). The thin polymer composite bipolar plates (the thickness <1.5 mm) containing of vinyl ester resin, graphite powder, organoclay have been fabricated by bulk molding compound (BMC) process. Organoclay was prepared by ionic exchange of montmorillonite (MMT) with three different molecular weight (M_w) of poly(oxypropylene)-backboned diamine intercalating agents. Results indicate that the basal spacing and content of MMT varied with M_w of POP-diamines are critical in determining the resultant mechanical properties for bipolar plates. Flexural strength of MMT composite plates was increased from 30.21 to 45.66 MPa by adding 2 phr of MMT. The flexural strength of the plate was also ca. 38% higher than the pristine graphite plate as the basal spacing of MMT was increased from 1.71 to 5.43 nm. Meanwhile, the unnotched impact strength of the composite plates was increased from 58.11 to 80.21 J m⁻¹. The unnotched impact strength of the plate was ca. 30% higher than that of the original graphite plates as the basal spacing of MMT was increased from 1.71 to 5.43 nm. The limiting oxygen index (LOI) and the UL-94 test revealed that the bipolar plate possesses excellent flame retardant with LOI >50 and UL-94-V0. The thermal decomposition temperature of each MMT composite plate is also higher than 250 °C. In addition, the bulk electrical conductivity of the bipolar plate with different MMT contents and basal spacing of MMT is higher than 100 S cm⁻¹. The corrosion current is less than 10⁻⁷ A cm⁻². Results confirm that the addition of MMT leads to a significant improvement on the performance of the composite bipolar plate.     

A novel integration approach for combining the micro thermal sensor and stainless steel foil as gas diffusion layer in micro fuel cell

Fuel cells will be used extensively in the future as renewable energy sources and they are the subject of substantial research. However, various problems are encountered with their mass production, such as the bipolar plate, the flow channel, the catalyst, the membrane electrode assembly (MEA), and the gas diffusion layer (GDL). Given the present uneven gas reactions and the difficulty of obtaining information on the temperature in a fuel cell, this novel investigation utilized micro-electro-mechanical-systems (MEMS) to integrate a micro thermal sensor and a stainless steel foil as a gas diffusion layer. The reaction inside a micro fuel cell must be controlled and adjusted in real time. The results of the experiment demonstrated that the accuracy and sensitivity of the micro thermal sensor were 0.5 °C and 1.805 × 10^?3/°C, respectively.     

Experimental and numerical investigation of droplet evaporation under diesel engine conditions

Evaporation of mono-disperse fuel droplets under high temperature and high pressure conditions is investigated. The time-dependent growth of the boundary layer of the droplets and the influence of neighboring droplets are examined analytically. A transient Nusselt number is calculated from numerical data and compared to the quasi-steady correlations available in literature. The analogy between heat and mass transfer is tested considering transient and quasi-steady calculations for the gas phase up to the critical point for a single droplet. The droplet evaporation in a droplet chain is examined numerically. Experimental investigations are performed to examine the influence of neighboring droplets on the drag coefficients. The results are compared with drag coefficient models for single droplets in a temperature range from T = 293–550 K and gas pressure p = 0.1–2 MPa. The experimental data provide basis for model validation in computational fluid dynamics.     

Polymeric composite bipolar plates for vehicle applications

At present membrane electrode assembly performance levels and stack operating conditions of PEM fuel cells, a plate area specific resistance of less than approximately 20 mΩ cm² and a plate thickness of less than 2 mm are required to meet the vehicular volumetric power density target (>2 kW l⁻¹). It is, however, difficult to meet these aggressive requirements, and simultaneously obtain good mechanical properties when using polymeric plate materials. Polymers become brittle and break frequently at the high conductive filler loadings (e.g., >50 v/o graphite) required for high conductivity. This study investigates a potential approach for obtaining high plate conductivity at low conductive filler loadings, thus enabling high volumes of thin and ductile plates to be manufactured at low scrap rates.     

Control of miniature proton exchange membrane fuel cells based on fuzzy logic

A control strategy is presented in this paper which is suitable for miniature hydrogen/air proton-exchange membrane (PEM) fuel cells. The control approach is based on process modelling using fuzzy logic and tested using a PEM stack consisting of 15 cells with parallel channels on the cathode side and a meander-shaped flow-field on the anode side. The active area per cell is 8 cm². Commercially available materials are used for the bipolar plates, gas diffusion layers and the membrane-electrode assembly (MEA). It is concluded from a simple water balance model that water management at different temperatures can be achieved by controlling the air stoichiometry. This is achieved by varying the fan voltage for the air supply of the PEM stack. A control strategy of the Takagi Sugeno Kang (TSK) type, based on fuzzy logic, is presented. The TSK-type controller offers the advantage that the system output can be computed in an efficient way: the rule consequents of the controller combine the system variables in linear equations. It is shown experimentally that drying out of the membrane at high temperatures can be monitored by measuring the ac impedance of the fuel cell stack at a frequency of 1 kHz. Flooding of single cells leads to an abrupt drop of the corresponding single-cell voltage. Therefore, the fuzzy rule base consists of the ac impedance at 1 kHz and all single-cell voltages. The parameters of the fuzzy rule base are determined by plotting characteristic diagrams of the fuel cell stack at constant temperatures. The fuel cell stack can be controlled at T=60 °C up to a power level of 7.5 W. The fuel cell stack is controlled successfully even when the external electric load changes. At T=65 °C, a maximum power level of 8 W is found. A decrease of the maximum power level is observed for higher temperatures.     

Performance of an aluminate cement /graphite conductive composite bipolar plate

Aluminate cement/graphite conductive composite bipolar plates were prepared by mould pressing at room temperature. The effects of the graphite content, the mould pressing pressure and mould pressing time on the electrical conductivity and the flexural strength of composite are discussed. The electrical conductivity and the flexural strength of the composite bipolar plates with 60 wt.% graphite content, prepared with a mould pressing pressure of 5 MPa for 10 min, is >100 S cm⁻¹ and 20 MPa, respectively and can be improved by optimizing the mould pressing conditions, especially mould pressing time. The water content of the composite bipolar plate with different graphite contents was also investigated. The water content of the composite bipolar plate is about 6 wt.% with a graphite content of 60 wt.%. This composite bipolar plate contains capillary pores and has hydrophilicity, which is different from other composite bipolar plates. Therefore, it possesses an inner humidifying function and can use the water produced at the cathode for humidifying the proton exchange membrane during the operation of a PEMFC. In addition, the H₂ permeability of the composite bipolar plate is low.     

Modeling of thermo-physical processes in liquid ceramic precursor droplets injected into a plasma jet

Modeling of liquid ceramic precursor droplets axially injected into a plasma is presented. Droplets undergo heating and solvent vaporization leading to high solute concentration near droplet surface. At a critical solute super-saturation concentration, precipitation is postulated to occur forming a precipitate shell around liquid core. Internal pressurization and rupture of shell occur subsequently. Droplet size, shell porosity and thickness effects were studied. Timescales of internal pressurization and precipitate formation are of the order of microsecond and millisecond, respectively. Small droplets (d ⩽ 5 μm) tend to form thick shells and are less likely to undergo shell fracture compared to larger droplets.     

A quasi-discrete model for heating and evaporation of complex multicomponent hydrocarbon fuel droplets

A quasi-discrete model for heating and evaporation of complex multicomponent hydrocarbon fuel droplets is suggested and tested in Diesel engine-like conditions. The model is based on the assumption that properties of components are weak functions of the number of carbon atoms in the components (n). The components with relatively close n are replaced by the quasi-components with properties calculated as average properties of the a priori defined groups of actual components. Thus the analysis of heating and evaporation of droplets consisting of many components is replaced by the analysis of heating and evaporation of droplets consisting of relatively few quasi-components. In contrast to previously suggested approaches to modelling the heating and evaporation of droplets consisting of many components, the effects of temperature gradient and quasi-component diffusion inside droplets are taken into account. The model is applied to Diesel fuel droplets, approximated as a mixture of 21 components C_nH_2n+2 for 5 ? n ? 25, which correspond to a maximum of 20 quasi-components with average properties for n = n_j and n = n_j+1, where j varies from 5 to 24. It is pointed out that droplet surface temperatures and radii, predicted by a rigorous model taking into account the effect of all 20 quasi-components, are very close to those predicted by the model, using just five quasi-components. Errors due to the assumptions that the droplet thermal conductivity and species diffusivities are infinitely large cannot be ignored in the general case.     

Uniformity analysis at MEA and stack Levels for a Nexa PEM fuel cell system

The Nexa™ power module is evaluated at membrane-electrode-assembly (MEA) and stack levels. The I–V Curves of the Nexa™ PEM fuel cell system is measured using periodic current interruption to maintain isothermal stack temperature. The uniformity analysis is mainly performed on the load of 800 W for all MEAs in 10 individual Nexa™ stacks. Statistical data show that the MEA voltage without an external load averages 224 mV higher than that with a load of 800 W. The MEA voltage difference is especially pronounced around the two cells at the air compressor side. The average difference is 8.8% and the highest difference is 13.1% between the minimum MEA voltage in the stack and the mean value. This voltage difference reveals a possibility to increase the product power capability and cut the cost per kilowatts by improving the weak performance electrodes or MEAs in the stack.     

The auto-ignition of single n-heptane/iso-octane droplets

Numerical simulations including detailed chemical and physical models are performed to investigate the influence of different physical parameters on the auto-ignition of n-heptane/iso-octane droplets in air. Simulations are performed for isobaric conditions with an ambient pressure of 8 bar and a droplet radius of 200 μm. The ambient gas temperature ranges from 800 K to 2000 K and the droplet temperature was varied from 300 K to 400 K. Below an ambient temperature of 1000 K the ignition delay time is found to increase with an increasing volume fraction of iso-octane. Above 1000 K the ignition delay time appears to be almost independent of the mixture composition of the droplet. The local ignition conditions are also studied. It turns out that ignition occurs at points, where the mixture is lean. This trend is more significant, if the ambient temperature increases. The influence of physical properties of the mixture components, like diffusion coefficients, heat conductivity, heat of vaporization and vapor pressure, is investigated. Furthermore, the influences of simplifying assumptions such as the distillation and diffusion limit are studied.     

Durability study of proton exchange membrane fuel cells under dynamic testing conditions with cyclic current profile

This work addresses issues of long-term durability of hydrogen–air proton exchange membrane fuel cells (PEMFCs) under cyclic current loading conditions, simulating the real road driving conditions for automotives. The same type of membrane–electrode assembly (MEA) was also aged under constant current mode as a control and the results were compared with those of the cyclically aged MEA. Both MEAs were characterized for cell polarizations, impedance spectra, Tafel plots, hydrogen crossover rates as well as electrochemical active surface areas at intervals of 100 h of aging. It was demonstrated that hydrogen crossover increased dramatically after 500 h of current cycling due to pinhole formation and was the most dominant degradation source. The fuel cell approached the end of its useful lifetime after 1000 h of operation. On the other hand, the hydrogen crossover rate remained approximately constant for the MEA under constant current operation. Mass transport limitations were identified as the major source of decreased performance during the constant current operation. This decrease in performance was partially reversible when cathode flooding was resolved by setting the cell at lower current densities. At the end, a phenomenological durability model was established successfully to describe the aging processes and cell performance at different time nodes.     

Growth of Cr-Nitrides on commercial Ni–Cr and Fe–Cr base alloys to protect PEMFC bipolar plates

Nitridation of Cr-bearing alloys can yield low interfacial contact resistance (ICR), electrically conductive and corrosion-resistant CrN or Cr₂N base surfaces of interest for a range of electrochemical devices, including fuel cells, batteries, and sensors. This paper presents results of exploratory studies of the nitridation of commercially available, high Cr (30–35 wt%) Ni–Cr alloys and a ferritic high Cr (29 wt%) stainless steel for proton exchange membrane fuel cell (PEMFC) bipolar plates. A high degree of corrosion resistance in sulfuric acid solutions designed to simulate bipolar plate conditions and low ICR values were achieved. Oxygen impurities in the nitriding environment were observed to play a significant role in the nitrided surface structures that formed, with detrimental effects for the Ni–Cr base alloys, but beneficial effects for the stainless steel alloy. Positive results from single-cell fuel cell testing are also presented.     

A polymer electrolyte fuel cell life test: 3 years of continuous operation

Implementation of polymer electrolyte fuel cells (PEMFCs) for stationary power applications requires the demonstration of reliable fuel cell stack life. One of the most critical components in the stack and that most likely to ultimately dictate stack life is the membrane electrode assembly (MEA). This publication reports the results of a 26,300 h single cell life test operated with a commercial MEA at conditions relevant to stationary fuel cell applications. In this experiment, the ultimate MEA life was dictated by failure of the membrane. In addition, the performance degradation rate of the cell was determined to be between 4 and 6 μV h⁻¹, at the operating current density of 800 mA cm⁻². AC impedance analysis and DC electrochemical tests (cyclic voltammetry and polarization curves) were performed as diagnostics during and on completion the test, to understand materials changes occurring during the test. Post mortem analyses of the fuel cell components were also performed.     

Performance of DMFC with SS 316 bipolar/end plates

This work mainly emphasizes the development of new materials and design for a bipolar/end plate in a direct methanol fuel cell (DMFC). According to the DOE requirements, preliminary studies show that SS 316 (Stainless Steel 316) is a suitable candidate. Several flow field designs were studied and a modified serpentine design was proposed. SS 316 end plates were fabricated with an intricate modified serpentine flow field design on it. The performance of a single stack DMFC with SS 316 end plates were studied with different operational parameters. A long-term test was carried out for 100 h with recycling the methanol and the contaminants in the MEA were characterized. The stack efficiency is found to be 51% and polarization losses are discussed. SS 316 with low permeability resulted in an increased pressure drop across the flow field, which increased the fuel cell performance. The use of SS 316 as bipolar plate material will reduce the machining cost as well as volume of the fuel cell stack.