Anode Flow Rate Research Articles

An experimental and modeling based investigation into the high stoichiometric flow rates required in direct methanol fuel cells

Direct methanol fuel cells (DMFCs) characteristically require high stoichiometric flow rates in order to optimize their performance. Experiments and modeling were performed in order to provide an improved understanding of the physical basis of this requirement. Anode side modeling suggests that high stoichiometry is necessary to keep the carbon dioxide (CO 2) gaseous volumetric generation rate to a fraction of the overall anode flow rate such that CO 2 gaseous clogging may not result. This requires anode stoichiometries generally greater than 20, and higher with increasing molarity. Cathode side modeling and measurements show that saturated exit air is directly related to poor cell performance. The modeling suggests that the cathode side mass transfer effect is quite strong despite the laminar flow conditions. Stoichiometries greater than 5 are seen to reduce the cell's exit relative humidity (RH) and improve cell performance.

Experimental studies of a direct methanol fuel cell

Systematic experiments have been conducted to study the effects of various operating parameters on the performances of a direct methanol fuel cell (DMFC). The effects of cell operating temperature, methanol concentration, anode flow rate, air flow rate, and cathode humidification have been studied. The experimental results showed that all the studied operating parameters, except the cathode humidification, have significant effects on the DMFC cell performances, and the cathode humidification has almost negligible effect. The study revealed that the detrimental effect of methanol crossover can be alleviated by increasing cathode air flow rate or oxygen partial pressure. This result showed that the cathode structure and operating condition may play a very important role in DMFC design and operations. The experimental results are presented in both graphical and tabular forms.

A 3D model for PEM fuel cells operated on reformate

A three-dimensional mathematical model for PEM fuel cells operated on reformate is developed based on our previous established fuel cell model [Int. J. Transport Phenomena 3 (2001) 177], by incorporating the adsorption and oxidation kinetics of CO on platinum surface proposed by Springer et al. [Proceedings of the Electrochemical Society, Montreal, Canada, 1997; J. Electrochem. Soc. 148 (2001) A11]. This model is capable of studying the effect of CO poisoning as well as the hydrogen dilution effect by inert gases. The adsorption and oxidation kinetics of CO on a platinum surface are incorporated in the source terms of the species equations; thus, the basic form of the mathematical equations are the same as those used for PEM fuel cells operated on pure hydrogen. With this model, we can obtain detailed information on the CO poisoning and variation of CO and hydrogen concentrations inside the anode. The results from this 3D model reveal many new phenomena that cannot be obtained from previous 1D or 2D models. Results of the effects of various operating and design parameters, such as anode flow rate, gas diffuser porosity, gas diffuser thickness, and the width of the collector plate shoulder, are also presented. The modeling results demonstrate the value of this model as a design and optimization tool for the anode of PEM fuel cells operating on reformate.

A 3D model for PEM fuel cells operated on reformate

A three-dimensional mathematical model for PEM fuel cells operated on reformate is developed based on our previous established fuel cell model [Int. J. Transport Phenomena 3 (2001) 177], by incorporating the adsorption and oxidation kinetics of CO on platinum surface proposed by Springer et al. [Proceedings of the Electrochemical Society, Montreal, Canada, 1997; J. Electrochem. Soc. 148 (2001) A11]. This model is capable of studying the effect of CO poisoning as well as the hydrogen dilution effect by inert gases. The adsorption and oxidation kinetics of CO on a platinum surface are incorporated in the source terms of the species equations; thus, the basic form of the mathematical equations are the same as those used for PEM fuel cells operated on pure hydrogen. With this model, we can obtain detailed information on the CO poisoning and variation of CO and hydrogen concentrations inside the anode. The results from this 3D model reveal many new phenomena that cannot be obtained from previous 1D or 2D models. Results of the effects of various operating and design parameters, such as anode flow rate, gas diffuser porosity, gas diffuser thickness, and the width of the collector plate shoulder, are also presented. The modeling results demonstrate the value of this model as a design and optimization tool for the anode of PEM fuel cells operating on reformate.

Mechanistic and Bifurcation Analysis of Anode Potential Oscillations in PEMFCs with CO in Anode Feed

A detailed mathematical analysis is performed to understand the anode potential oscillations observed experimentally in a proton exchange membrane fuel cell (PEMFC) with feed (Ref. 9). Temperature and anode flow rate are found to be key bifurcation parameters. The time dependence of all the key surface species must be accounted for in order for the model to predict the oscillatory behavior, while the time dependence of CO concentration in the anode chamber need not necessarily be considered. The bifurcation diagram of CO electro-oxidation rate constant agrees very well with the effect of temperature on the oscillation pattern. The oscillator model is classified as a hidden negative differential resistance oscillator based on the dynamical response of the anodic current and surface species to a dynamic potential scan. A linear stability analysis indicates that the bifurcation experienced is a supercritical Hopf bifurcation. © 2004 The Electrochemical Society. All rights reserved.

Influence of Gas Flow Rate on Performance of H 2 S /Air Solid Oxide Fuel Cells with MoS2 ­ NiS ­ Ag Anode

Performance of a solid oxide fuel cell with the configuration of /yttria-stabilized zirconia/Pt, air is dependent on anode and cathode compartment gas flow rates. The cell open-circuit voltage (OCV) was independent of air flow rate but increased with increasing flow rate. A linear relationship existed between OCV and the logarithm of flow rate. Increasing the flow rate increased the exchange rate of reaction products and feed at the anode catalyst, thereby decreasing the local concentration of reaction products and increasing the local concentration of hence, OCV increased from the Nernst effect. The magnitude of the change in OCV with temperature was consistent with calculated values based on reaction equilibria. It was found that increasing either or both air flow rate and flow rate improved current-voltage and power density performance. The results were consistent with improved gas diffusion in the cathode with increasing air flow rate, and with both improved gas diffusion in anode and increased concentration of anodic electroactive species with increasing flow rate. © 2003 The Electrochemical Society. All rights reserved.

Sustained Potential Oscillations in Proton Exchange Membrane Fuel Cells with PtRu as Anode Catalyst

Sustained potential oscillations are experimentally observed in a proton exchange membrane fuel cell with PtRu as anode catalyst and with CO as the anode feed when operating under a constant current density mode. These oscillations appear at fuel-cell temperatures below 70°C. A threshold value exists for both the current density and the anode flow rate at a given fuel-cell temperature for their onset. The temperature dependence of the oscillation period shows an apparent activation energy around 60 kJ/mol. The potential oscillations are believed to be due to the coupling of anode electro-oxidation of and CO on the PtRu catalyst surface, on which is formed more readily, i.e., at lower overpotentials. A simple kinetic model is provided that can reproduce the observed oscillatory phenomenon both qualitatively and quantitatively. © 2002 The Electrochemical Society. All rights reserved.

Influence of Anode Flow Rate and Cathode Oxygen Pressure on CO Poisoning of Proton Exchange Membrane Fuel Cells

The anode flow rate of a proton exchange membrane (PEM) fuel cell involving Pt anode electrocatalyst is found to strongly influence the single cell performance when containing trace amounts of CO is used as the feed. The performance drops dramatically due to CO poisoning as the anode flow rate increases until a large overpotential is reached when it levels off. This effect of the flow rate on the extent of poisoning is found to be reversible and is explained as depending on the actual concentration of CO in the anode chamber which in turn depends on the feed content, the flow rate, and CO oxidation kinetics on Pt. Further, it is found that oxygen permeating across the PEM from the cathode side also appreciably affects the anode overpotential by providing another route for CO oxidation. A CO inventory model is provided that explains the observed phenomena in a PEM fuel cell operating with as anode feed and a cathode feed with different oxygen pressures. © 2002 The Electrochemical Society. All rights reserved.

Spatial Distribution of the CO Transient Response of a PEFC

We report on the investigation of the current distribution of a PEFC under the influence of CO impurities in the hydrogen feed stream by application of the segmented cell system. Stepping the CO partial pressure of the anode feed stream from 0 to 100 ppm CO the current response of the cell was recorded for different stoichiometric flows. A poisoning time delay occurs along the flow path due to the anode flow rate. Also anode and cathode processes, e.g. CO turnover and oxygen mass transport limitation contribute to the time response of the cell.