The influence of CO2, CO and air bleed on the current distribution of a polymer electrolyte fuel cell

The influence of CO₂, CO and air bleed on current distribution was studied during transient operation, and the dynamic response of the fuel cell was evaluated. CO causes significant changes in the current distribution in a polymer electrolyte fuel cell. The current distribution reaches steady state after approximately 60 min following addition of 10 ppm CO to the anode fuel stream. Air bleed may recover the uneven current distribution caused by CO and also the drop in cell voltage due to CO and CO₂ poisoning. The recovery of cell performance during air bleed occurs evenly over the electrode surface even when the O₂ partial pressure is far too low to fully recover the CO poisoning. The O₂ supplied to the anode reacts on the anode catalyst and no O₂ was measured at the cell outlet for air bleed levels up to 2.5%.     

Potential oscillations in a proton exchange membrane fuel cell with a Pd-Pt/C anode

We report in this paper the occurrence of potential oscillations in a proton exchange membrane fuel cell (PEMFC) with a Pd-Pt/C anode, fed with H₂/100 ppm CO, and operated at 30 °C. We demonstrate that the use of Pd-Pt/C anode enables the emergence of dynamic instabilities in a PEMFC. Oscillations are characterized by the presence of very high oscillation amplitude, ca. 0.8 V, which is almost twice that observed in a PEMFC with a Pt-Ru/C anode under similar conditions. The effects of the H₂/CO flow rate and cell current density on the oscillatory dynamics were investigated and the mechanism rationalized in terms of the CO oxidation and adsorption processes. We also discuss the fundamental aspects concerning the operation of a PEMFC under oscillatory regime in terms of the benefit resulting from the higher average power output.     

Carbon monoxide poisoning of proton exchange membrane fuel cells

Proton exchange membrane fuel cell (PEMFC) performance degrades when carbon monoxide (CO) is present in the fuel gas; this is referred to as CO poisoning. This paper investigates CO poisoning of PEMFCs by reviewing work on the electrochemistry of CO and hydrogen, the experimental performance of PEMFCs exhibiting CO poisoning, methods to mitigate CO poisoning and theoretical models of CO poisoning. It is found that CO poisons the anode reaction through preferentially adsorbing to the platinum surface and blocking active sites, and that the CO poisoning effect is slow and reversible. There exist three methods to mitigate the effect of CO poisoning: (i) the use of a platinum alloy catalyst, (ii) higher cell operating temperature and (iii) introduction of oxygen into the fuel gas flow. Of these three methods, the third is the most practical. There are several models available in the literature for the effect of CO poisoning on a PEMFC and from the modeling efforts, it is clear that small CO oxidation rates can result in much increased performance of the anode. However, none of the existing models have considered the effect of transport phenomena in a cell, nor the effect of oxygen crossover from the cathode, which may be a significant contributor to CO tolerance in a PEMFC. In addition, there is a lack of data for CO oxidation and adsorption at low temperatures, which is needed for detailed modeling of CO poisoning in PEMFCs. Copyright © 2001 John Wiley & Sons, Ltd.     

Effects of anode air bleeding on the performance of CO-poisoned proton-exchange membrane fuel cells

In the present work, the dynamic behavior of a PEM fuel cell under CO-poisoning and the effects of air bleeding on the recovery ratio are reported. Pt-Ru catalyst is used as the anode in a single cell and the hydrogen is pre-mixed with 53 ppm of CO as the fuel. The result indicates that even using a CO-tolerant catalyst, CO-poisoning cannot be avoided with the operating conditions in our study. About 80% of the output current is lost within 20 min. Upon anode air bleeding with 5% air, 90% of the current is recovered within 1 min. Higher air bleeding ratio only results in minor improvement of the cell performance. We have developed a transient model to estimate the current reduction due to CO-poisoning and to evaluate the amount of air bleeding needed for a given recovery ratio. A long-term durability test has also been conducted using simulated reformatted gas, in which 1% O₂ is injected into the fuel stream. After more than 3000 h, the cell voltage degradation is less than 3%.     

Electrochemical analysis of hydrogen membrane fuel cells

An electrochemical analysis was conducted with respect to a hydrogen membrane fuel cell with SrZr_0.8In_0.2O_3−δ electrolyte, which is a new type of fuel cell featuring an ultra-thin proton conductor supported on a dense metal anode. Most of the voltage loss derives from the cathode and the electrolyte, and a small amount of anode polarization was observed only in regions with high current density. The cathode polarization was approximately an order of magnitude lower than that of SOFCs. Furthermore, the conductivity of the film electrolyte was almost identical to that of the sinter at 600 °C; however, it was several times as large at 400 °C. A TEM micrograph revealed that the film electrolyte consists mainly of long columnar crystals, and this crystal structure can be related to the conductivity enhancement below 600 °C.     

Composite anode for CO tolerance proton exchange membrane fuel cells

Fuel of proton exchange membrane fuel cells (PEMFC) mostly comes from reformate containing CO, which will poison the fuel cell electrocatalyst. The effect of CO on the performance of PEMFC is studied in this paper. Several electrode structures are investigated for CO containing fuel. The experimental results show that thin-film catalyst electrode has higher specific catalyst activity and traditional electrode structure can stand for CO poisoning to some extent. A composite electrode structure is proposed for improving CO tolerance of PEMFCs. With the same catalyst loading, the new composite electrode has improved cell performance than traditional electrode with PtRu/C electrocatalyst for both pure hydrogen and CO/H₂. The EDX test of composite anode is also performed in this paper, the effective catalyst distribution is found in the composite anode.     

The effect of H2S 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 (H₂S). Trace amount of CO and H₂S can cause considerable cell performance losses. A comparison between the effect of CO and that of H₂S on PEMFC performance was made in this paper. Under the same conditions, the H₂S 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 H₂S), the fuel cell performance deteriorates more quickly than 50 ppm CO but slowly than 50 ppm H₂S and can be only partially recovered by reintroducing neat H₂. 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.     

Carbon monoxide poisoning and mitigation strategies for polymer electrolyte membrane fuel cells – A review

Polymer electrolyte membrane fuel cells (PEMFC) have received much attention due to their high power density, good start-stop capabilities and high gravimetric and volumetric power density compared with other fuel cells. However, certain technological challenges persist, which include the fact that conventional anode electrocatalysts are poisoned by low levels (few ppm) of carbon monoxide (CO). This review considers the mechanism of CO poisoning and the effects that it has on the PEMFC performance. The key parameters affecting CO poisoning are identified and methods used to mitigate the effects are discussed. These mitigation strategies are divided into three groups according to the means by which the technologies are applied: pre-treatment of reformate, on board removal of CO and in operando mitigation strategies.     

Effect of anode iridium oxide content on the electrochemical performance and resistance to cell reversal potential of polymer electrolyte membrane fuel cells

An ideal polymer electrolyte membrane fuel cell (PEMFC) is one that continuously generates electricity as long as hydrogen and oxygen (or air) are supplied to its anode and cathode, respectively. However, internal and/or external conditions could bring about the degradation of its electrodes, which are composed of nanoparticle catalysts. Particularly, when the hydrogen supply to the anode is disrupted, a reverse voltage is generated. This phenomenon, which seriously degrades the anode catalyst, is referred to as cell reversal. To prevent its occurrence, iridium oxide (IrO₂) particles were added to the anode in the membrane-electrode assembly of the PEMFC single-cells. After 100 cell reversal cycles, the single-cell voltage profiles of the anode with Pt/C only and the anodes with Pt/C and various IrO₂ contents were obtained. Additionally, the cell reversal-induced degradation phenomenon was also confirmed electrochemically and physically, and the use of anodes with various IrO₂ contents was also discussed.     

Investigation of the temperature‐related performance of proton exchange membrane fuel cell stacks in the presence of CO

H₂ is generally used as the fuel in proton exchange membrane fuel cells (PEMFCs). However, H₂ produced from reformate gas usually contains a trace of CO, which may severely affect the fuel cell performance. With the adoption of domestic short side chain, low equivalent weight perfluorosulphonic acid (PFSA) membrane, a 100 W stack is built and evaluated at elevated temperature of 95 °C for the purpose of improving its CO tolerance. The stack is operated with 5 ppm, 10 ppm and 20 ppm CO/H₂, respectively; better performance is obtained as expected. Furthermore, a 5 kW PEMFC stack is prepared with home‐made Ir–V/C and Pt/C as anode catalysts for the membrane electrode assemblies to compare their CO tolerance. Physical and electrochemical characterizations, such as transmission electron microscope and linear scan voltammogram are employed for catalyst investigation. The results demonstrate that the employment of domestic PFSA membrane enables the stack to be operated at 95 °C, which can improve the CO tolerance of all the anode catalysts. In addition, the effect of CO on cell polarization is insignificant at lower current densities. Under the same operating conditions, cells with Ir–V/C catalyst show better CO tolerance. Copyright © 2013 John Wiley & Sons, Ltd.     

Development of an air bleeding technique and specific duration to improve the CO tolerance of proton-exchange membrane fuel cells

This study investigated transient CO poisoning of a proton-exchange membrane fuel cell under either a fixed cell voltage or fixed current density. During CO poisoning tests, the cell performance decreases over time. Experiments were performed to identify which method yields better performance in CO poisoning tests. The results revealed that a change in cell voltage did not affect the stable polarization behavior after CO poisoning of the cell. On the other hand, a higher fixed current density yielded better tolerance of 52.7 ppm CO. The air bleeding technique was then applied using different timings for air introduction during CO poisoning tests. Air bleeding significantly improved the CO tolerance of the cell and recovered the performance after poisoning, regardless of the timing of air introduction. The effects of different anode catalyst materials on cell performance were also investigated during poisoning tests. Without air bleeding, a Pt–Ru alloy catalyst exhibited better CO tolerance than a pure Pt catalyst. However, the air bleeding technique can effectively increase the CO tolerance of cells regardless of the type of catalyst used.     

The effects of hydrogen sulfide on the polymer electrolyte membrane fuel cell anode catalyst: H2S-Pt/C interaction products

The performance of a polymer electrolyte membrane fuel cell (PEMFC) operating on a simulated hydrocarbon reformate is described. The anode feed stream consisted of 80% H₂, ∼20% N₂, and 8 ppm hydrogen sulfide (H₂S). Cell performance losses are calculated by evaluating cell potential reduction due to H₂S contamination through lifetime tests. It is found that potential, or power, loss under this condition is a result of platinum surface contamination with elemental sulfur. Electrochemical mass spectroscopy (EMS) and electrochemical techniques are employed, in order to show that elemental sulfur is adsorbed onto platinum, and that sulfur dioxide is one of the oxidation products. Moreover, it is demonstrated that a possible approach for mitigating H₂S poisoning on the PEMFC anode catalyst is to inject low levels of air into the H₂S-contaminated anode feeding stream.     

Development of a methodology to optimize the air bleed in PEMFC systems operating with low quality hydrogen

The use of hydrogen with lower quality than that specified in current regulation is an attractive option for stationary PEMFC power production. In this paper, the effect of CO is mitigated using air bleed levels up to 2% in an H₂ PEMFC fed with CO concentrations below 20 ppm. A methodology to optimize the air bleed levels is developed using a novel arrangement of cells coupled to a gas chromatograph. The methodology relies on evaluating the distributed performance of the cell and on determining the CO and CO₂ molar flow rates at the anode outlet. Furthermore, the amount of CO adsorbed onto the catalyst and the fraction of catalytic sites covered by CO are estimated. The results show that different parameters, such as the H₂ volumetric flow rate, CO concentration and air bleed level, influence both the steady state and dynamics of PEMFCs operated with low quality hydrogen.     

The study of Au/MoS2 anode catalyst for solid oxide fuel cell (SOFC) using H2S-containing syngas fuel

Au/MoS₂ is a promising anode catalyst for conversion of all components of H₂S-containing syngas in solid oxide fuel cell (SOFC). MoS₂-supported nano-Au particles have catalytic activity for conversion of CO when syngas is used as fuel in SOFC systems, thus preventing poisoning of MoS₂ active sites by CO. In contrast to use of MoS₂ as anode catalyst, performance of Au/MoS₂ anode catalyst improves when CO is present in the feed. Current density over 600 mA cm⁻² and maximum power density over 70 mW cm⁻² were obtained at 900 °C, showing that Au/MoS₂ could be potentially used as sulfur-tolerant catalyst in fuel cell applications.     

Effect of CO and CO2 impurities on performance of direct hydrogen polymer-electrolyte fuel cells

Mechanisms by which trace amounts of CO and CO₂ impurities in fuel may affect the performance of direct hydrogen polymer-electrolyte fuel cell stacks have been investigated. It is found that the available data on CO-related polarization losses for Pt electrodes could be explained on the basis of CO adsorption on bridge sites, if the CO concentration is less than about 100 ppm, together with electrochemical oxidation of adsorbed CO at high overpotentials. The literature data on voltage degradation due to CO₂ is consistent with CO production by the reverse water–gas shift reaction between the gas phase CO₂ and the H₂ adsorbed on active Pt sites. The effect of oxygen crossover and air bleed in “cleaning” of poisoned sites could be modeled by considering competitive oxidation of adsorbed CO and H by gas phase O₂. A model has been developed to determine the buildup of CO and CO₂ impurities due to anode gas recycle. It indicates that depending on H₂ utilization, oxygen crossover and current density, anode gas recycle can enrich the recirculating gas with CO impurity but recycle always leads to buildup of CO₂ in the anode channels. The buildup of CO and CO₂ impurities can be controlled by purging a fraction of the spent anode gas. There is an optimum purge fraction at which the degradation in the stack efficiency is the smallest. At a purge rate higher than the optimum, the stack efficiency is reduced due to excessive loss of H₂ in purge gas. At a purge rate lower than the optimum, the stack efficiency is reduced due to the decrease in cell voltage caused by the excessive buildup of CO and CO₂. It is shown that the poisoning model can be used to determine the limits of CO and CO₂ impurities in fuel H₂ for a specified maximum acceptable degradation in cell voltage and stack efficiency. The impurity limits are functions of operating conditions, such as pressure and temperature, and stack design parameters, such as catalyst loading and membrane thickness.     

Effect of CO in the anode fuel on the performance of PEM fuel cell cathode

Even trace amounts of CO in the fuel for a proton-exchange membrane fuel cell (PEMFC) could poison not only the anode, which is directly exposed to the fuel, but also the cathode, which is separated from the fuel by a proton-exchange membrane; and the performance decline of the cathode is sometimes more than that of the anode. Adsorption of CO on the cathode catalyst has been detected electrochemically, and this indicates that CO can pass through the membrane to reach the cathode. To reduce such a poisoning effect, fuel cell operation conditions (e.g. level of membrane humidification, gas pressure difference between cathode and anode), membrane and catalyst layer structures, and CO-tolerant cathode catalysts should be further explored.     

Ethanol internal steam reforming in intermediate temperature solid oxide fuel cell

This study investigates the performance of a standard Ni-YSZ anode supported cell under ethanol steam reforming operating conditions. Therefore, the fuel cell was directly operated with a steam/ethanol mixture (3 to 1 molar). Other gas mixtures were also used for comparison to check the conversion of ethanol and of reformate gases (H₂, CO) in the fuel cell. The electrochemical properties of the fuel cell fed with four different fuel compositions were characterized between 710 and 860 °C by I-V and EIS measurements at OCV and under polarization. In order to elucidate the limiting processes, impedance spectra obtained with different gas compositions were compared using the derivative of the real part of the impedance with respect of the natural logarithm of the frequency.Results show that internal steam reforming of ethanol takes place significantly on Ni-YSZ anode only above 760 °C. Comparisons of results obtained with reformate gas showed that the electrochemical cell performance is dominated by the conversion of hydrogen. The conversion of CO also occurs either directly or indirectly through the water-gas shift reaction but has a significant impact on the electrochemical performance only above 760 °C.     

Effects of reformate on performance of PBI/H3PO4 proton exchange membrane fuel cell stack

The present study aims to examine the effect of nitrogen and carbon monoxide concentrations as well as the working temperature and the stoichiometry number on the performance of a self-made five-cell high-temperature Proton-exchange membrane fuel cell stack (PEMFC). The concentration of hydrogen in a reformed gas can be varied, and it may contain poisonous substances such as carbon monoxide. Hence, the composition of the fuel gas could affect the performance of the PEMFC. The polarization curve and the electrochemical impedance spectrogram are utilized to examine the behaviors of PEMFC. The cell temperature of 160 °C is found as an optimal working temperature in this study for high-temperature PEMFC. Measured results show that the stoichiometry of the anode gas has a minimal effect on the PEMFC performance. A high percentage of nitrogen makes hydrogen dilute and leads to poor cell performance. When carbon dioxide exceeds 3%, the pt-catalyst was covered with the CO and the cell performance significantly decreased. Finally, a raise of the PEMFC temperature boosted the catalyst energy and improved the detachment of the carbon monoxide and eventually enhanced carbon monoxide tolerance.     

Polarization measurements on single-step co-fired solid oxide fuel cells (SOFCs)

Anode-supported planar solid oxide fuel cells (SOFC) were successfully fabricated by a single step co-firing process. The cells comprised of a Ni + yttria-stabilized zirconia (YSZ) anode, a YSZ or scandia-stabilized zirconia (ScSZ) electrolyte, a (La_0.85Ca_0.15)_0.97MnO₃ (LCM) + YSZ cathode active layer, and an LCM cathode current collector layer. The fabrication process involved tape casting of the anode, screen printing of the electrolyte and the cathode, and single step co-firing of the green-state cells in the temperature range of 1300–1330 °C for 2 h. Cells were tested in the temperature range of 700–800 °C with humidified hydrogen as fuel and air as oxidant. Cell test results and polarization modeling showed that the polarization losses were dominated by the ohmic loss associated with the electrodes and the activation polarization of the cathode, and that the ohmic loss due to the ionic resistance of the electrolyte and the activation polarization of the anode were relatively insignificant. Ohmic resistance associated with the electrode was lowered by improving the electrical contact between the electrode and the current collector. Activation polarization of the cathode was reduced by the improvement of the microstructure of the cathode active layer and lowering the cell sintering temperature. The cell performance was further improved by increasing the porosity in the anode. As a result, the maximum power density of 1.5 W cm⁻² was achieved at 800 °C with humidified hydrogen and air.     

A novel proton exchange membrane fuel cell anode for enhancing CO tolerance

A novel proton exchange membrane fuel cell (PEMFC) anode which can facilitate the CO oxidation by air bleeding and reduce the direct combustion of hydrogen with oxygen within the electrode is described. This novel anode consists of placing Pt or Au particles in the diffusion layer which is called Pt- or Au-refined diffusion layer. Thus, the chemical oxidation of CO occurs at Pt or Au particles before it reaches the electrochemical catalyst layer when trace amount of oxygen is injected into the anode. All membrane electrode assemblies (MEAs) composed of Pt- or Au-refined diffusion layer do perform better than the traditionary MEA when 100 ppm CO/H₂ and 2% air are fed and have the performance as excellent as the traditionary MEA with neat hydrogen. Furthermore, CO tolerance of the MEAs composed of Au-refined diffusion layer was also assessed without oxygen injection. When 100 ppm CO/H₂ is fed, MEAs composed of Au-refined diffusion layer have the slightly better performance than traditionary MEA do because Au particles in the diffusion layer have activity in the water gas shift (WGS) reaction at low temperature.