Hydrogen Purge Research Articles

A dynamic voltage model of a fuel cell stack considering the effects of hydrogen purge operation

Abstract During the operation of proton exchange membrane (PEM) fuel cell, water is easy to accumulate in the anode side, particularly if the fuel cell system is being controlled in ‘dead ended’ mode. The hydrogen purge operation prevents the anode being jammed by excessive water. It is observed that the voltage experiences a recovery and linear degradation after the hydrogen purge operation. In this paper an empirical dynamic voltage model for PEM fuel cell stack is introduced based on the result of experimental investigation. Compared with previous model, the suggested model indicates a better agreement between test and simulation, especially when the effect of hydrogen purge operation is obvious.

Hydrogen and proton exchange membrane fuel cells for clean road transportation

In this paper the main issues related to the working of hydrogen fuel cells and their utilization in the transportation field are presented and discussed, starting from the experimental analysis of dynamic performance of a laboratory fuel cell system based on a 20 kWe H 2/air proton exchange membrane stack. The experiments were carried out on successive load steps characterized by low reactant relative pressures (15–50 kPa) and stack temperature of 310–350 K. The effect of air flow rate, hydrogen purge and membrane humidification strategy on stack output voltage was evaluated evidencing the role of different sub-systems in stack management (for reactant feeding, stack cooling, membrane humidification) and their effect on efficiency losses of the overall system. The results presented in this paper provide experimental indications about potentialities of hydrogen fuel cell systems as clean electric energy generators for automotive applications.

Tritium diffusivity in crystal grain of Li 2TiO 3 and tritium release behavior under several purge gas conditions

It has been reported by the present authors that behavior of tritium release from solid breeder grain is consisted of diffusion in grain, tritium transfer at surface layer and surface reactions on grain surface such as adsorption or isotope exchange reactions. Tritium release curves estimated using the tritium release model gave good agreement with observed tritium release curves from Li 4SiO 4, Li 2ZrO 3 or LiAlO 2. Tritium release behavior from Li 2TiO 3 under humid purge gas, dry purge gas and dry purge gas with hydrogen conditions is discussed in this study, tritium release curves using the release model that we proposed previously give a good agreements with experimental tritium release curves. Tritium effective diffusivity in the crystal grain of Li 2TiO 3 is also estimated in this study using a curve-fitting method applied to the release curves obtained under the humid purge gas condition. It is discussed that change of color of Li 2TiO 3 surface under hydrogen purge gas condition is observed and this phenomenon might affect tritium release behavior from Li 2TiO 3.

Electronic Device for the Control and Energy Management of a PEM Fuel Cell Stack/Supercapacitors Hybrid System

This work shows the development of an electronic system for the control and the management of a hybrid configuration of generation of energy, based on the use of a PEMFC fuel cell stack (50W) and a supercapacitors module. In order to achieve this aim, a Buck DC-DC converter and an electronic control unit (ECU) based on a microcontroller have been set in the electronic system. The ECU controls the DC-DC converter by adapting the voltage given by the fuel cell to the voltage required by the electronic speed regulator of the truck. Moreover, it limits the extraction of energy from the fuel cell according to its electrochemical behaviour. Two digital PID regulators in closed loop are used for the control. They keep the power extracted from the fuel cell constant and limited by regulating the converter, either at constant voltage (CV) or at constant current (CC). Finally, the ECU regulates the fan speed according to the current which is extracted from the fuel cell, in order to provide enough amount of oxygen inside the cell. A supercapacitors module (7.5V, 66.6F) has been installed between the DC-DC converter and the electronic speed regulator of the truck so as to provide the necessary energy during the load peaks required in the start-up or in the acceleration. The supercapacitors will be recharged by the fuel cell stack as soon as the demand of the vehicle decreases and there is an excess of energy. In order to identify faults in the system, as well as to refine and optimize the control algorithm, the ECU keeps a record of all the variables of the system, such as the temperature of the fuel cell stack and the room temperature, the voltage of all the cells, the voltage and current extracted form the stack, the voltage and the current given by the DC-DC converter, the speed of the fan which provides the stack with air, and the activation of the hydrogen purge valve with respect to time.

An Investigation of Transient Carbon Monoxide Poisoning Effects in Polymer Electrolyte Fuel Cells

We describe studies of performance decay and recovery in polymer electrolyte fuel cells in response to step changes in the level of CO in the anode feed stream. Following a step from 0 to 100 ppm CO in the hydrogen anode feed, completion of current decay at constant cell voltage of 0.6V in a 50 cm 2 cell operating at 80°C with anode loading of 0.2 mg Pt cm -2 took 300 seconds. The observed current decay is controlled by the rate of surface dosing, determined primarily, if not exclusively, by inlet flow of CO, and the non-linear dependence of anodic HOR rate on (1-θ CO ). Performance recovery in response to a pure hydrogen purge at 80°C was found to be sluggish, with full recovery taking as long as 1000 seconds. The rate of recovery upon exposure to a hydrogen purge is a function of temperature and is apparently controlled by the rate of thermal desorption of CO. Cell performance decay was found much faster (1-10 sec) for a step from 10 ppm to 200 ppm inlet CO. The recovery time was very slow (> 1500 sec) on return from 200 ppm to 10 ppm inlet. Air bleed into the anode significantly slows down decay times and shorten recovery times in operation with transient levels of CO.

Effect of methyl surface saturation during growth interruption sequences of metalorganic vapor-phase epitaxy of In0.53Ga0.47As using trimethylarsenic

Abstract We have investigated the use of trimethylarsenic (TMAs) as an alternative to arsine in the metal organic vapor-phase epitaxy (MOVPE) of heterostructures containing arsenic-based compounds on InP. We particularly focused this study on the growth interruption sequences (GIS) between two arsenic-based materials using TMAs. We show that a too long stabilization time under TMAs is not desirable. It produces a saturation of the surface with methyl radicals that hampers the growth of the second material (after the GIS). This results in a very poor surface morphology of the sample. An important improvement of the surface morphology is achieved using a long hydrogen purge after the stabilization time under TMAs. This allows the desorption of most of the methyl radicals on the surface before the growth of the second layer. However, even with this hydrogen purge, the surface morphology is not perfect and presents some defects.

Metalorganic vapor phase epitaxial grown heterointerfaces to GaInP with group-III and group-V exchange

Metalorganic vapor phase epitaxial grown heterointerfaces to GaxIn1 − xP (x ≈ 0.5) are studied. Abruptness optimization is carried out by means of a gas switching procedure using a hydrogen purge between AsH3 and PH3 stabilization. X-ray analysis and scanning transmission electron microscopy are used to provide detailed information about interlayers and Ga-segregation from a fitting of simulated to experimental rocking curves in comparison to atomic-number contrast images. InGaAsInAlAs heterostructure field-effect transistors on an InP-substrate are prepared incorporating a highly strained Ga0.5In0.5P spacer layer. Excellent transport data (μH,300 K > 11000 cm2/V·s) are proving that despite the strain and segregation effect in the strained ternary the lower interface at the channel/spacer transition is highly abrupt.

Surface processes of selective growth by atomic layer epitaxy

Characteristics and mechanisms, of the crystallographic selective growth by atomic layer epitaxy (ALE) and the localized ALE growth over the nanometer scale area, are discussed focusing on the surface process. The experimental results indicate that higher growth temperature, and longer hydrogen purge time after AsH 3 supply, promote the crystallographic selectivity of GaAs ALE growth. The characteristics of the ALE selective growth include high crystallographic selectivity, and high uniformity in the nanometer scale. The mechanisms of ALE selective growth seem to be caused by the separate enhancement of As desorption process and the self-limiting mechanism of Ga precursor adsorption process on the GaAs surface.

Electrothermal Atomic Absorption Spectrometric Determination of Potassium in Biological Materials with a Molybdenum Tube Atomizer

Electrothermal atomic absorption spectrometry (ETA-AAS) of potassium in biological materials with a molybdenum tube atomizer has been investigated. The atomization characteristics of potassium on the atomizer are demonstrated. The optimal gas flow rate was 400 ml min−1 Ar and 100 ml min−1 H2. The characteristic mass (the mass of element giving 0.0044 abs.) of potassium by the atomizer was 75 fg, which is better than that obtained with a graphite atomizer. The detection limit (s/n = 3) was 72 fg (corresponding to 7.1 pg ml−1 with 10 μl given). The value was also better than those from flame AA, graphite furnace AA, and flame atomic emission. The relative standard deviation by the use of the molybdenum atomizer was 2.6%. Interferences caused by large amounts of interferents were evaluated. Thiourea as the matrix modifier eliminated the interferences. The determined potassium in biological materials was in good agreement with the certified values. For a graphite furnace AAS, the recommended method will serve as a highly sensitive determination of potassium in biological materials, when a molybdenum or tungsten metal foil lined into the graphite furnace is used in argon + hydrogen purge gas.

The liquid-phase hydrogenation of methylacetoacetate using nickel-exchanged Y zeolite catalysts: Part I. Structure sensitivity

The liquid-phase hydrogenation of methylacetoacetate (MAA) over NiNaY and NiKY zeolites to a racemic methyl 3-hydroxybutyrate (MHB) product has been studied in a stirred reactor under a constant hydrogen purge. The variation of MHB formation with time was monitored and reaction rates accurate to ±2% are reported. The effects of variations in the supported nickel metal particle sizes (in the range 16–75 nm) on the hydrogenation activity were examined by altering either the degree of nickel exchange or the temperature of reduction. It is shown that the hydrogenation of MAA at 343 K is structure-sensitive and a correlation between reaction sensitivity and particle size is presented. Data on the reaction over Ni/Si0 2 catalysts are also provided for comparison purposes. Both the zeolite and silica-based catalysts were reusable and could be stored in the reaction solvent for extended periods of time without an appreciable loss of activity.