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 O₂ 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 H₂ 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.     

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.     

Degradation of proton exchange membrane fuel cells due to CO and CO2 poisoning

The CO and CO₂ poisoning effects on the degradation of cell performance of proton exchange membrane fuel cell (PEMFC) under transient stage were investigated. The mechanism of CO poisoning lies in the preferential adsorbing of CO to the platinum surface and the blocking of active sites of hydrogen. These phenomena were described with adsorption, desorption, and electro-oxidation processes of CO and hydrogen in the present work. In addition, it is well known that the reverse water gas shift reaction (RWGS) is the main effect of the CO₂ poisoning, through which a large part of the catalytic surface area becomes inactive due to the hydrogen dissociation. The predicted results showed that, by contaminating the fuel with 10 ppm CO at the condition of P_H = 0.8 atm and PCO₂=0.2 atm$P_{\text{C}{\text{O}}_2}=0.2\text{atm}$, the current density of the PEM fuel cell was lowered 28% with rate constant of RWGS k_rs from zero to 0.02. With 50 ppm CO, the performance drop was only 18%. For the reformed gas, CO₂ poisoning became much more significantly when the CO content in the reactant gas was small.     

Experimental characterization and mechanistic modeling of carbon monoxide fueled solid oxide fuel cell

The paper presents an elementary reaction based solid oxide fuel cell (SOFC) model coupled with anodic elementary heterogeneous reactions and electrochemical charge transfer reactions for CO/CO₂ fuel based on an anode supported button cell. The model is calibrated and validated using experimental data obtained for various CO/CO₂ fuel compositions at 750, 800 and 850 °C. The comparison shows that the modeling results agree well with the experimental data. The effects of operating conditions on the cell performance and the detailed species concentration distribution are predicted. Then, the carbon deposition on the SOFC anode with CO/CO₂ fuel is experimentally measured and simulated using the elementary reaction model. The results indicate that lower temperature and lower operation voltage are helpful to reduce the possibilities of carbon deposition on Ni particle surfaces.     

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%.     

A direct carbon fuel cell with (molten carbonate)/(doped ceria) composite electrolyte

A composite electrolyte containing a Li/Na carbonate eutectic and a doped ceria phase is employed in a direct carbon fuel cell (DCFC). A four-layer pellet cell, viz. cathode current collector (silver powder), cathode (lithiated NiO/electrolyte), electrolyte and anode current collector layers (silver powder), is fabricated by a co-pressing and sintering technique. Activated carbon powder is mixed with the composite electrolyte and is retained in the anode cavity above the anode current collector. The performance of the single cell with variation of cathode gas and temperature is examined. With a suitable CO₂/O₂ ratio of the cathode gas, an operating temperature of 700 °C, a power output of 100 mW cm⁻² at a current density of 200 mA cm⁻² is obtained. A mechanism of O²⁻ and CO₃²⁻ binary ionic conduction and the anode electrochemical process is discussed.     

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.     

Demonstrating feasibility of a high temperature polymer electrolyte membrane fuel cell operation with natural gas reformate composition

Experimental data on the performance of a single cell PBI-based HT-PEMFC operated with a fuel composition similar to natural gas reformate and oxygen enriched cathode air are presented. A test studying the effect of CO₂, H₂O and CO in the fuel on fuel cell performance revealed that the presence of CO₂ mainly worsens mass transport, H₂O improves proton conduction and CO influences reaction kinetics as well as causing mass transport limitations. A small increase of the O₂ concentration in the oxidant provided a boost on performance. Electrical efficiency of the fuel cell was improved from 36.6% with H₂/air operation up to 38.2% with synthetic reformate gas/30% O₂ enriched air. Three 1000 h long-term tests at constant load conditions were performed. The first test showed a degradation rate of ?21.4 μV/h and was operated with H₂/30% O₂. The second test was performed with the same kind of MEA but different fuel composition (54% H₂, 15% CO₂ and 31% H₂O) and exhibited a reduction of the degradation rate to ?5.5 μV/h. The main reason for this lifetime improvement is H₂O because its transport from anode to cathode may sweep along PA that soaks catalyst active sites and limits HOR. Moreover, water in rich H₂ reformate streams also relieves formation of CO from CO₂ via RWGS. The third test was performed with a different kind of MEA (extra PTFE content in GDE) but the same fuel composition than the second one. A higher degradation rate of ?22.2 μV/h was observed but it was mainly caused by unprotected shut-downs during operation. Two preliminary long-term tests were also performed with a fuel composition similar to natural gas reformate (54% H₂, 14% CO₂, 1% CO and 31% H₂O). These latest tests revealed that the fuel cell should be operated at higher temperatures to diminish CO catalyst coverage, and that anode purge with dry gases avoids water condensation in gas pipes. In addition, CO poisoning on anode catalyst is time dependent and operation at high current densities enhances CO catalyst coverage.     

Efficient electrochemical oxidation of ethanol to carbon dioxide in a fuel cell at ambient temperature

A carbon dioxide monitor has been used to follow the Faradaic yield of CO₂ from the oxidation of ethanol vapour in a direct ethanol PEM fuel cell at ambient temperature. The time resolution of the CO₂ measurements (ca.15 s at half height for a burst of CO₂) was sufficient to observe stripping of adsorbed CO from the anode, and to monitor CO₂ yields as a function of time during linear sweep and pulse experiments. It has been demonstrated that CO₂ yields can be increased dramatically by pulsing the potential or current such that adsorbed CO is stripped from the electrode and then ethanol is allowed to readsorb. Yields of CO₂ as high as 80% have been sustained for as long as 50 s under current pulsing conditions. An average CO₂ yield of 45% was obtained during 600 s of pulsing the current between 0 and 4 mA cm⁻² at 1 Hz.     

Characteristics of a fluidized bed electrode for a direct carbon fuel cell anode

The characteristics of a fluidized bed electrode applied as a direct carbon fuel cell anode, which has an inner diameter of 35 mm and height of 520 mm and employed bamboo-based activated carbon (BB-AC) as a feedstock, are vigorously studied under various experimental conditions. The optimal performance of the fluidized bed electrode direct carbon fuel cell (FEBDCFC) anode with the BB-AC as a fuel is obtained under the following conditions with a limiting current density of 95.9 mA cm⁻²: reaction temperature, 923 K; N₂ flow rate, 385 ml min⁻¹; O₂/CO₂ flow rate, 10/20 ml min⁻¹; nickel particle content, 30 g; and a cylindrically curved nickel plate as a current collector. Under the same optimal conditions, the limiting current density of the FEBDCFC anode with oak wood-based activated carbon and activated carbon fiber as the fuel is determined to be 94.5 and 88.4 mA cm⁻², which is lower than that determined for BB-AC as the fuel. Comparatively, the limiting current density for graphite, which is utilized as the carbon fuel for this fuel cell system, could not be unequivocally determined because no plateau of the limiting current density against the overpotential is observed.     

Relationship between carbon corrosion and positive electrode potential in a proton-exchange membrane fuel cell during start/stop operation

Fuel starvation during start-up and shut-down processes can adversely affect the performance of proton-exchange membrane fuel cells. In this study, fuel starvation is induced intentionally by supplying hydrogen and air to the negative electrode (anode) side alternately, and the individual electrode potential is measured in situ using a dynamic hydrogen electrode. The positive electrode (cathode) potential is increased to 1.4 V when air/hydrogen boundaries developed on the anode side. The development of a high cathode potential causes oxidation of the carbon support with the amount of CO₂ evolution proportional to the cathode potential above 1.0 V. Above ∼1.2 V, CO and SO₂ are generated electrochemically or chemically and the rate of CO production is higher than that of SO₂. Although a higher cathode potential is induced irrespective of the cell temperature, oxidation of the carbon support is retarded significantly at low temperatures.     

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.     

Initialization of a methane-fueled single-chamber solid-oxide fuel cell with NiO + SDC anode and BSCF + SDC cathode

The initialization of an anode-supported single-chamber solid-oxide fuel cell, with NiO + Sm_0.2Ce_0.8O_1.9 anode and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ + Sm_0.2Ce_0.8O_1.9 cathode, was investigated. The initialization process had significant impact on the observed performance of the fuel cell. The in situ reduction of the anode by a methane–air mixture failed. Although pure methane did reduce the nickel oxide, it also resulted in severe carbon coking over the anode and serious distortion of the fuel cell. In situ initialization by hydrogen led to simultaneous reduction of both the anode and cathode; however, the cell still delivered a maximum power density of ∼350 mW cm⁻², attributed to the re-formation of the BSCF phase under the methane–air atmosphere at high temperatures. The ex situ reduction method appeared to be the most promising. The activated fuel cell showed a peak power density of ∼570 mW cm⁻² at a furnace temperature of 600 °C, with the main polarization resistance contributed from the electrolyte.     

Impacts of air bleeding on membrane degradation in polymer electrolyte fuel cells

A long-term accelerated test (4600 h) of a 25 cm² single cell with excess air bleeding (5%) was carried out to investigate the effects of air bleeding on membrane degradation in polymer electrolyte fuel cells. The rate of membrane degradation was negligibly low (fluoride-ion release rate = 1.3 × 10⁻¹⁰ mol cm⁻² h⁻¹ in average) up to 2000 h. However, membrane degradation rate was gradually increased after 2000 h. The CO tolerance of the anode gradually dropped, which indicated that the anode catalyst was deteriorated during the test. The results of the rotating ring–disk electrode measurements revealed that deterioration of Pt–Ru/C catalyst by potential cycling greatly enhances H₂O₂ formation in oxygen reduction reaction in the anode potential range (∼0 V). Furthermore, membrane degradation rate of the MEA increased after the anode catalyst was forced to be deteriorated by potential cycling. It was concluded that excess air bleeding deteriorated the anode catalyst, which greatly enhanced H₂O₂ formation upon air bleeding and resulted in the increased membrane degradation rate after 2000 h.     

Coke formation and performance of an intermediate-temperature solid oxide fuel cell operating on dimethyl ether fuel

Dimethyl ether (DME) as a fuel of SOFCs is investigated with great attention paid to coke formation over the Ni-YSZ anode. DME is easily decomposed to CH₄, CO and H₂ at temperatures above 700 °C, with total conversion occurring at 850 °C over the Ni-YSZ catalyst. These data suggest that the DME electro-oxidation likely proceeds via an indirect pathway. O₂-TPO analysis, laser Raman spectroscopy and SEM-EDX characterizations demonstrate coke formation over Ni-YSZ, which is obvious and become more prevalent at higher temperatures. The introduction of CO₂ in the fuel gas decreases the CH₄ selectivity and effectively suppresses coke formation above 700 °C. The suppression effect is increasingly apparent at higher temperatures. At 850 °C, the anode still maintains geometric integrity after exposure to DME-CO₂ (1:1, volume ratio) under OCV condition. With DME or DME-CO₂, the fuel cell power output is comparable to results obtained by operating with 3% water humidified hydrogen. No obvious cell degradation from the anode is observed when operating with DME-CO₂, while it is obvious with DME. The introduction of CO₂ may be a good choice to suppress the coke formation when operating on DME; however, the proper selection of operation temperature is of significant importance.     

Study on proton-conducting solid oxide fuel cells with a conventional nickel cermet anode operating on dimethyl ether

This study investigates dimethyl ether (DME) as a potential fuel for proton-conducting SOFCs with a conventional nickel cermet anode and a BaZr_0.4Ce_0.4Y_0.2O_3−δ (BZCY4) electrolyte. A catalytic test demonstrates that the sintered Ni + BZCY4 anode has an acceptable catalytic activity for the decomposition and steam reforming of DME with CO, CH₄ and CO₂ as the only gaseous carbon-containing products. An O₂-TPO analysis demonstrates the presence of a large amount of coke formation over the anode catalyst when operating on pure DME, which is effectively suppressed by introducing steam into the fuel gas. The selectivity towards CH₄ is also obviously reduced. Peak power densities of 252, 280 and 374 mW cm⁻² are achieved for the cells operating on pure DME, a DME + H₂O gas mixture (1:3) and hydrogen at 700 °C, respectively. After the test, the cell operating on pure DME is seriously cracked whereas the cell operating on DME + H₂O maintains its original integrity. A lower power output is obtained for the cell operating on DME + H₂O than on H₂ at low temperature, which is mainly due to the increased electrode polarization resistance. The selection of a better proton-conducting phase in the anode is critical to further increase the cell power output.     

Syngas reactivity over (LaAg)(CoFe)O3 and Ag-added (LaSr)(CoFe)O3 anodes of solid oxide fuel cells

The syngas, H₂ + CO gas mixture with various H₂/CO ratios, is used as the anode fuel of solid oxide fuel cell with La_0.7Ag_0.3Co_0.2Fe_0.8O₃ (LACF) and 2 wt% Ag-added La_0.58Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) as the anode, respectively, both being in composite with 50 wt% Ce_0.9Gd_0.1O_1.95 (GDC). Both the current-voltage and the fixed-voltage measurements are performed at 800 °C. The reactivity with H₂ as the fuel is larger than that with CO. The syngas reactivity increases with increasing H₂ content. The results of the current-voltage and the fixed-voltage measurements are in agreement with each other. Ag-added LSCF-GDC has better reactivity with H₂, CO and syngas and better stability in the H₂ atmosphere than LACF-GDC as the anode material.     

Samarium doped ceria-(Li/Na)2CO3 composite electrolyte and its electrochemical properties in low temperature solid oxide fuel cell

A composite of samarium doped ceria (SDC) and a binary carbonate eutectic (52 mol% Li₂CO₃/48 mol% Na₂CO₃) is investigated with respect to its morphology, conductivity and fuel cell performances. The morphology study shows the composition could prevent SDC particles from agglomeration. The conductivity is measured under air, argon and hydrogen, respectively. A sharp increase in conductivity occurs under all the atmospheres, which relates to the superionic phase transition in the interface phases between SDC and carbonates. Single cells with the composite electrolyte are fabricated by a uniaxial die-press method using NiO/electrolyte as anode and lithiated NiO/electrolyte as cathode. The cell shows a maximum power density of 590 mW cm⁻² at 600 °C, using hydrogen as the fuel and air as the oxidant. Unlike that of cells based on pure oxygen ionic conductor or pure protonic conductor, the open circuit voltage of the SDC-carbonate based fuel cell decreases with an increase in water content of either anodic or cathodic inlet gas, indicating the electrolyte is a co-ionic (H⁺/O²⁻) conductor. The results also exhibit that oxygen ionic conductivity contributes to the major part of the whole conductivity under fuel cell circumstances.     

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.     

Measurement of oxygen chemical potential in Gd2O3-doped ceria-Y2O3-stabilized zirconia bi-layer electrolyte, anode-supported solid oxide fuel cells

Solid oxide fuel cells (SOFC) were fabricated with gadolinia-doped ceria (GDC)-yttria stabilized zirconia (YSZ), thin bi-layer electrolytes supported on Ni + YSZ anodes. The GDC and YSZ layer thicknesses were 45 μm, and ∼5 μm, respectively. Two types of cells were made; YSZ layer between anode and GDC (GDC/YSZ) and YSZ layer between cathode and GDC (YSZ/GDC). Two platinum reference electrodes were embedded within the GDC layer. Cells were tested at 650 °C with hydrogen as fuel and air as oxidant. Electric potentials between embedded reference electrodes and anode and between cathode and anode were measured at open circuit, short circuit and under load. The electric potential was nearly constant through GDC in the cathode/YSZ/GDC/anode cells. By contrast, it varied monotonically through GDC in the cathode/GDC/YSZ/anode cells. Estimates of oxygen chemical potential, μO₂, variation through GDC were made. μO₂ within the GDC layer in the cathode/GDC/YSZ/anode cell decreased as the current was increased. By contrast, μO₂ within the GDC layer in the cathode/YSZ/GDC/anode cell increased as the current was increased. The cathode/YSZ/GDC/anode cell exhibited maximum power density of ∼0.52 W cm⁻² at 650 °C while the cathode/GDC/YSZ/anode cell exhibited maximum power density of ∼0.14 W cm⁻² for the same total electrolyte thickness.