Anode Feed Research Articles

Ni-Modified Perovskite Materials for Solid Oxide Fuel Cell Anodes Fed With Glycerol

An investigation of the electrochemical oxidation of glycerol as alternative to hydrogen and methane in an intermediate temperature solid oxide fuel cell (IT-SOFC) by using a noble metal-free anode catalyst was carried out. The anode electrocatalyst was based on a Ni-modified La0.6Sr0.4Fe0.8Co0.2O3 (LSFCO) perovskite. After thermal activation, Ni was mainly present as ultrafine La2NiO4 particles homogeneously dispersed on the perovskite surface. The thermal treatment also caused a modification of perovskite into a lanthanum depleted SrFe0.8Co0.2O3 (SFCO) structure. These results were corroborated by x-ray diffraction (XRD) and transmission electron microscopy (TEM). By using this modified perovskite, a low ohmic resistance was recorded and suitable power density achieved for an electrolyte supported IT-SOFC. The effects on the electrochemical performance produced by the variation of the ratio of glycerol / water for the anode feed were investigated.

Impact of carbon monoxide on PEFC catalyst carbon support degradation under current-cycled operating conditions

It is well known that, even at ppm levels, the presence of CO in a PEFC anode feed stream has a significant impact on the MEA performance. Numerous work on short-term CO impact on PEFC performance under steady-state current demands has been carried out. However, to the best of our knowledge, the impact of long-term (i.e., >600 h) CO contamination on intrinsic Pt and C support aging (Pt oxidation/dissolution/ripening, C oxidation, …) under current-cycled operating conditions has never been explored. In this paper, on the basis of a combined theoretical and experimental approach, we investigate the long-term CO effect on PEFC performance and degradation. Firstly, on the basis of our previously published PEFC materials degradation models, we suggest that anodic CO poisoning could be used to mitigate the cathodic carbon catalyst-support corrosion phenomena and thus to enhance the MEA durability. Secondly, endurance experiments are performed on single fuel cells with current-cycled protocols representative of transport applications. The impact of CO on electrochemical transient response shows a reasonable agreement with simulated behaviors, and it is experimentally demonstrated that the impact of CO on the cell potential degradation rate is strongly dependent on the current-cycle mode.

Analysis of the Ripple Current in a 5 kW Polymer Electrolyte Membrane Fuel Cell Stack

AbstractPolymer electrolyte fuel cell systems are increasingly being used in applications requiring an inverter to convert the direct current (DC) output of the stack to an alternating current (AC). These inverters, and other time‐varying inputs to the stack such as the anode feed pressure, cause deviations from the average stack current, or ripple currents, which are undesirable for reasons of performance and durability. A dynamic fuel cell model has been developed and validated against experimental data for a 5 kW fuel cell stack, examining in detail the ripple current behaviour. It was shown that the ripple currents exceed the 2% maximum recommended value, and may lead to long‐term degradation of the fuel cell stack.

The effect of internal air bleed on CO poisoning in a proton exchange membrane fuel cell

It is found that carbon monoxide (CO) poisoning could be mitigated by increasing only cathode backpressure for a proton exchange membrane fuel cell (PEMFC) with ultra-thin membranes (≤25 μm). This mitigation can be explained by a heterogeneous oxidation of CO on a Pt–Ru/C anode by the permeated O 2 which is known as “internal air bleed” in his paper. A steady-state model which accounts for this internal air bleed has been developed to model the Pt–Ru/C anode polarization data when 50 ppm CO in H 2 is used as anode feed gas. The modeling results show that the mitigation of CO poisoning by the internal air bleed even exists at ambient conditions for a PEMFC with an ultra-thin membrane. Therefore, the effect of internal air bleed must be considered for modeling fuel cell performance or anode polarization data if an ultra-thin membrane and a low level of CO concentration are used for a Pt–Ru/C anode. An empirical relationship between the amount of internal air bleed used for the mitigation of CO poisoning and the fraction of free Pt sites is provided to facilitate the inclusion of an internal air bleed term in the modeling of anode polarization and the fuel cell performance.

Water Transport through a Proton-Exchange Membrane (PEM) Fuel Cell Operating near Ambient Conditions: Experimental and Modeling Studies

In the present work, an experimental study on the performance of an “in-house”-developed proton-exchange membrane (PEM) fuel cell with 25 cm2 of active membrane area is described. The membrane/electrodes assembly (MEA), from Paxitech, has seven layers [membrane/catalyst layers/gas diffusion layers (GDLs)/gaskets]. The catalytic layers have a load of 70% Pt/C and 0.5 mg of Pt/cm2 on both sides, and the membrane is made of Nafion 112. A multiserpentine configuration for the anode and cathode flow channels is used. Experiments were carried out under different anode and cathode relative humidities (RHs) and flow rates. Predictions from a previously developed one-dimensional model, coupling mass- and heat-transfer effects, are compared to experimental polarization curves. The influence of the anode and cathode relative humidification level on the cell performance is explained under the light of the predicted water content across the membrane. Under the operating conditions studied, the net water flux of water is toward the anode. Accordingly, the influence of the anode humidification is not significant, and the influence of the cathode humidification has a high impact in fuel cell performance. Results show that fuel cell performance is better for experiments where higher water content values were obtained. In comparison to the anode feed flow rate influence, the influence of the cathode feed flow rate has a major impact in fuel cell performance.

MEA for alkaline direct ethanol fuel cell with alkali doped PBI membrane and non-platinum electrodes

This paper reports on the fabrication of MEA for alkaline direct ethanol fuel cell (ADEFC). The MEA was fabricated using non-platinum electrocatalysts and a membrane of alkali doped polybenzimidazole (PBI). The employed oxygen reduction catalyst was prepared by pyrolysis of 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine cobalt(II) supported on XC72 carbon. This catalyst is tolerant to ethanol. Electrocatalyst at the anode was RuV alloy supported on XC72 carbon. It was synthesized by reduction of respective salts at elevated temperature. Single cell power density of 100 mW cm −2 at U = 0.4 V was achieved at 80 °C using air at ambient pressure and 3 M KOH + 2 M EtOH anode feed. The developed MEA is considered viable for use in emergency power supply units and in power sources for portable electronic equipment.

Dynamic Behavior of a PEM Fuel Cell During Electrochemical CO Oxidation on a PtRu Anode

The dynamic behaviour of a single PEM fuel cell (PEMFC) with a PtRu/C anode catalyst using CO containing H2 as anode feed was investigated at ambient temperature. The autonomous oscillations of the cell potential were observed during the galvanostatic operation with hydrogen anode feed containing CO up to 1000 ppm. The oscillations were ascribed to the coupling of the adsorption of CO (the poisoning step) and the subsequent electrochemical oxidation of CO (the regeneration step) on the anode catalyst. The oscillations were dependent on the CO concentration of the feed gas and the applied current density. Furthermore, it was found that with CO containing feed gas, the time average power output was remarkably higher under potential oscillatory conditions in the galvanostatic mode than during potentiostatic operation. Accompanying these self-sustained potential oscillations, oscillation patterns of the anode outlet CO concentration were also detected at low current density (<100 mA/cm2). The online measurements of the anode outlet CO concentrations revealed that CO in the anode CO/H2 feed was partially electrochemically removed during galvanostatic operation. More than 90% CO conversion was obtained at the current densities above 125 mA/cm2 with low feed flow rates (100–200 mL/min).

The Impacts of Repetitive Carbon Monoxide Poisoning on Performance and Durability of a Proton Exchange Membrane Fuel Cell

In this paper, we report the effects on cell performance and durability of repeatedly poisoning the anode catalyst of a proton exchange membrane fuel cell (PEMFC) to 2 ppm carbon monoxide (CO) in the hydrogen fuel. The experiments were designed to simulate repeated exposure to low level impurities as may be experienced in a dynamically operated PEMFC. Experiments were conducted over a period of 1000 hours and accompanied by weekly diagnostics to characterize detailed changes in performance. Results for the repeatedly poisoned cell were compared to those of a control cell which was operated identically but without CO in the anode feed stream. Two effects were observed: the electro-chemically active area (ECA) of the test cell anode decreased significantly during the initial 300 hours of the experiment and the hydrogen crossover rates increased by a factor of 67 by the end of the experiment. Both effects contributed to performance losses, while the latter also indicated the end of life for the membrane electrode assembly (MEA). The results of the experiments emphasize the impact low level CO contaminant may have on MEA lifetime and cell performance under dynamic conditions.

The Impact of Trace Carbon Monoxide / Toluene Mixtures on PEMFC Performance

The impact of fuel impurities toluene (C7H8) and carbon monoxide (CO) on the performance of polymer electrolyte membrane fuel cells (PEMFCs) was investigated. Single and mixed impurities were injected into the fuel cell via the anode feed stream. Experiments were accompanied by gas chromatography analysis to determine the conversion and reaction processes occurring during contaminant exposure. Impurity injections of 2 and 20 ppm C7H8 caused an insignificant overpotential change of 2 and 8 mV respectively. An almost complete conversion of C7H8 to methylcyclohexane (C7H14) was observed. Impurity injections of 2 ppm CO caused a significant overpotential change of 247 mV. At steady state conditions more than 60% of the injected CO was converted to CO2 in the fuel cell. When the impurities were injected into the anode feed stream as a mixture the overpotential change was 100 mV greater than the sum of the overpotential changes the single impurities caused .Hydrogenation of C7H8 to C7H14 was observed but in contrast to the single C7H8 impurity experiment it stopped when a certain CO coverage was reached. The strong adsorption of CO caused a competition between the hydrogen reduction reaction and the hydrogenation of C7H8 for the available reaction sites.

Improved electrochemical CO removal via potential oscillations in serially connected PEM fuel cells with PtRu anodes

The polarization performance of two PEM fuel cells (with anode PtRu/C catalyst) connected either in parallel or serial, was compared to the performance of a single PEM fuel cell in galvanostatic operation using CO-free H 2 or 200 ppm CO-containing H 2 stream as anode feed at ambient temperature. Spontaneous potential oscillations were observed experimentally for the coupled configuration with two cells connected in serial or parallel using CO-containing H 2 feed at various current densities applied. The potential oscillations are ascribed by the dynamic CO adsorption and subsequent electrochemical CO oxidation on the anode. The measured anode outlet CO concentration was found to decrease with the order: single cell > parallel cells > serial cells at various current densities and anodic flow rates. The low anode outlet CO concentration (<10 ppm) at high current densities applied showed that CO in the anode feed was removed efficiently by the electrochemical CO oxidation occurring on the PtRu anode. The anode outlet CO concentration decreased as follows: a single cell > the parallel cells > the serial cells at broad range of current densities and anodic flow rates. The highest CO conversion and the highest average power output at equal hydrogen recovery degree were obtained with serially coupled fuel cells.

DMFC performance and methanol cross-over: Experimental analysis and model validation

A combined experimental and modelling approach is proposed to analyze methanol cross-over and its effect on DMFC performance. The experimental analysis is performed in order to allow an accurate investigation of methanol cross-over influence on DMFC performance, hence measurements were characterized in terms of uncertainty and reproducibility. The findings suggest that methanol cross-over is mainly determined by diffusion transport and affects cell performance partly via methanol electro-oxidation at the cathode. The modelling analysis is carried out to further investigate methanol cross-over phenomenon. A simple model evaluates the effectiveness of two proposed interpretations regarding methanol cross-over and its effects. The model is validated using the experimental data gathered. Both the experimental analysis and the proposed and validated model allow a substantial step forward in the understanding of the main phenomena associated with methanol cross-over. The findings confirm the possibility to reduce methanol cross-over by optimizing anode feeding.

Non-isothermal dynamic modelling and optimization of a direct methanol fuel cell

A non-isothermal dynamic optimization model of direct methanol fuel cells (DMFCs) is developed to predict their performance with an effective optimum-operating strategy. After investigating the sensitivities of the transient behaviour (the outlet temperature, crossovers of methanol and water, and cell voltage) to operating conditions (the inlet flow rates into anode and cathode compartments, and feed concentration) through dynamic simulations, we find that anode feed concentration has a significantly larger impact on methanol crossover, temperature, and cell voltage than the anode and cathode flow rates. Also, optimum transient conditions to satisfy the desired fuel efficiency are obtained by dynamic optimization. In the developed model, the significant influence of temperature on DMFC behaviour is described in detail with successful estimation of its model parameters.

LaCrO3−VOx−YSZ Anode Catalyst for Solid Oxide Fuel Cell Using Impure Hydrogen

A new, highly active and stable anode catalyst material LaCrO3−VOx−YSZ has been developed for solid oxide fuel cell systems fueled with impure hydrogen. The presence of CO in the anode feed enhanced initial fuel cell performance when compared to the use of CO-free H2 containing 5000 ppmv H2S. Impedance spectra showed that the anode polarization resistance decreased in the presence of CO. Current density of 450 mA/cm2 at 0.6 V and maximum power density of 260 mW/cm2 (at 450 mA/cm2) were obtained at 900 °C. XRD (X-ray diffraction) and XPS (X-ray photoelectron spectroscopy) analyses showed that the new catalyst was chemically stable in H2S-rich syngas or hydrogen. However, when syngas was used, slow carbon deposition compromised the fuel cell performance during long tests at high temperatures.

DMFC anode polarization: Experimental analysis and model validation

Anode two-phase flow has an important influence on DMFC performance and methanol crossover. In order to elucidate two-phase flow influence on anode performance, in this work, anode polarization is investigated combining experimental and modelling approach. A systematic experimental analysis of operating conditions influence on anode polarization is presented. Hysteresis due to operating condition is observed; experimental results suggest that it arises from methanol accumulation and has to be considered in evaluating DMFC performances and measurements reproducibility. A model of DMFC anode polarization is presented and utilised as tool to investigate anode two-phase flow. The proposed analysis permits one to produce a confident interpretation of the main involved phenomena. In particular, it confirms that methanol electro-oxidation kinetics is weakly dependent on methanol concentration and that methanol transport in gas phase produces an important contribution in anode feeding. Moreover, it emphasises the possibility to optimise anode flow rate in order to improve DMFC performance and reduce methanol crossover.

The effect of H 2S and CO mixtures on PEMFC performance

Proton exchange membrane fuel cells (PEMFCs) most likely will use reformed fuel as the primary source for the anode feed which always contains carbon dioxide (CO) and hydrogen sulfide (H 2S). Trace amount of CO and H 2S can cause considerable cell performance losses. A comparison between the effect of CO and that of H 2S on PEMFC performance was made in this paper. Under the same conditions, the H 2S poisoning rate is much higher than CO because of different adsorption intensity. When the fuel stream contains the gas mixture (25 ppm CO and 25 ppm H 2S), the fuel cell performance deteriorates more quickly than 50 ppm CO but slowly than 50 ppm H 2S and can be only partially recovered by reintroducing neat H 2. The resulting effects of the mixtures can be divided into two parts roughly: during the inception phase, the cell voltage drops quickly and the actual values of anode overvoltage are bigger than the corresponding calculated values; then the deterioration rate of the cell performance decreases gradually.

Reaction Dynamics in a Parallel Flow Channel PEM Fuel Cell

The spatiotemporal dynamic response of a segmented anode parallel channel polymer electrolyte membrane (PEM) fuel cell was monitored following changes in flow rate, temperature and load resistance. Autohumidified operation with dry feeds at 1 bar pressure was achieved at temperatures below 70°C, where the convective transport of water vapor was less than the water production by the fuel cell current. The current could be “ignited” by a single injection of water into the anode feed, or by reducing the temperature and external load resistance. Co-current flow of the hydrogen and oxygen resulted in current ignition at the outlets of the flow channels, followed by a wave of high current density propagating toward the inlets. Counter-current flow of the hydrogen and the oxygen resulted in ignition near the center of the flow channels; over time the ignition front fanned out. The spatio-temporal dynamics of the current ignition along the flow channels can be effectively predicted from a model of a set of coupled differential fuel cells in series. Liquid water condensing in the flow channels gives rise to complex spatio-temporal variations in the current density; these variations are strongly dependent on orientation of the fuel cell with respect to gravity.

Effect of Water on the Electrochemical Oxidation of Gas-Phase SO[sub 2] in a PEM Electrolyzer for H[sub 2] Production

Water plays a critical role in producing hydrogen from the electrochemical oxidation of SO 2 in a proton exchange membrane (PEM) electrolyzer. Not only is water needed to keep the membrane hydrated, but it is also a reactant. One way to supply water is to dissolve SO 2 in sulfuric acid and feed that liquid to the anode, but this process results in significant diffusion resistance for the SO 2 . Alternatively, we have developed a process where SO 2 is fed as a gas to the anode compartment and reacts with water crossing the membrane to produce sulfuric acid. There was concern that the diffusion resistance of water through the membrane is as significant as SO 2 diffusion through water, thus limiting the benefit of a gas-phase anode feed. We show here that water diffusion through the membrane is not as limiting as liquid-phase SO 2 diffusion. Therefore, we can control the cell voltage, the limiting current, and the sulfuric acid concentration by varying the diffusion resistance of the membrane via thickness or temperature. Catalyst loading, however, has a negligible effect on cell performance.

CFD analysis of a solid oxide fuel cell with internal reforming: Coupled interactions of transport, heterogeneous catalysis and electrochemical processes

Direct internal reforming in solid oxide fuel cell (SOFC) results in increased overall efficiency of the system. Present study focus on the chemical and electrochemical process in an internally reforming anode supported SOFC button cell running on humidified CH 4 (3% H 2 O). The computational approach employs a detailed multi-step model for heterogeneous chemistry in the anode, modified Butler–Volmer formalism for the electrochemistry and Dusty Gas Model (DGM) for the porous media transport. Two-dimensional elliptic model equations are solved for a button cell configuration. The electrochemical model assumes hydrogen as the only electrochemically active species. The predicted cell performances are compared with experimental reports. The results show that model predictions are in good agreement with experimental observation except the open circuit potentials. Furthermore, the steam content in the anode feed stream is found to have remarkable effect on the resulting overpotential losses and surface coverages of various species at the three-phase boundary.

Analysis and modeling of PEM fuel cell stack performance: Effect of in situ reverse water gas shift reaction and oxygen bleeding

In this study the performance of a polymer electrolyte membrane (PEM) fuel cell stack is analyzed with a mathematical model when the stack operates on hydrocarbon reformate gas as the anode feed stream. It is shown that the effect of carbon dioxide dilution of the hydrogen dominated reformate gas has a minimal impact on the stack performance. However, the CO-poisoning effect due to the in situ reverse water gas shift reaction in the anode feed stream could have a very serious adverse impact on the stack performance, especially at high current densities. Thermodynamic calculations indicate that the equilibrium concentrations of CO could be as high as 100 ppm, generated by the in situ reverse water gas shift reaction, under the typical conditions of PEM fuel cell operation; and are influenced by the stack operating temperature and water content of the reformate anode feed. This CO-poisoning of the stack performance is shown mitigated effectively by introducing about 0.5–1% oxygen to the anode feed.

The potential of model studies for the understanding of catalyst poisoning and temperature effects in polymer electrolyte fuel cell reactions

In this contribution we demonstrate the potential of model studies for the understanding of electrocatalytic reactions in low-temperature polymer electrolyte fuel cells (PEFCs) operated by H 2-rich anode feed gas, in particular of the role of temperature effects and catalyst poisoning. Reviewing previous work from our laboratory and, for better comparison, focussing on carbon-supported Pt catalysts, the important role of using fuel cell relevant reaction and mass transport conditions will be outlined. The latter conditions include continuous reaction, elevated temperatures, realistic supported catalyst materials and controlled mass transport. The data show the importance of combining electrochemical techniques such as rotating disc electrode (RDE), wall-jet and flow cell measurements, and on-line differential electrochemical mass spectrometry (DEMS) under controlled mass transport conditions.