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.     

Maximizing the efficiency of a HT-PEMFC system integrated with glycerol reformer

The efficiency and output power density of an integrated high temperature polymer electrolyte fuel cell system and glycerol reformer are studied. The effects of reformer temperature, steam to carbon ratio (S/C), fuel cell temperature, and anode stoichiometric ratio are examined. An increase in anode stoichiometric ratio will reduce CO poisoning effect at cell’s anode but cause lower fuel utilization towards energy generation. High S/C operation requires large amount of the energy available, however, it will increase anode tolerance to CO poisoning and therefore will lead to enhanced cell performance. Consequently, the optimum gas composition and flow rate is very dependent on cell operating current density and temperature. For example, at low current densities, similar efficiencies were obtained for all the S/C ratio studied range at cell temperature of 423.15 K, however, at cell temperature of 448.15 K, low S/C ratio provided higher efficiency in comparison to high S/C ratio. High S/C is essential when operating the cells at high current densities where CO has considerable impact on cell performance. Optimal conditions that provide the maximum power density at a given efficiency are reported.     

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.     

Transient evolution of carbon monoxide poisoning effect of PBI membrane fuel cells

High temperature polybenzimidazole membrane fuel cells are the focus of attention due to high CO tolerance and overcoming water managements. This paper develops a transient, one-dimensional mathematical model to predict CO tolerance, and validates it with experiments. Experimental results are measured at different temperatures. Fuel cell performance degradations with time are also measured under various fuel compositions. Transient evolutions of current density, H₂ coverage, CO coverage, and ionic potential are shown during the CO poisoning process. The theoretical results show that hydrogen coverage decreases with time, reducing hydrogen oxidation reactions and dropping ionic potential loss. The effects of temperature, CO contents, and H₂ dilutions on fuel cell performance and the time to reach steady t_ss are all investigated. Predictions of fuel cell current density degradation also show good agreement with experimental results.     

The effect of anode flow characteristics and temperature on the performance of a direct methanol fuel cell

An experimental direct methanol fuel cell (DMFC), designed and manufactured in-house, was used in this study. The cell is of standard filter-press configuration with parallel rectangular single-pass anode channels. The membrane electrode assembly (MEA), with a suitable Pt–Ru anode electrocatalyst, was purchased from E-TEK Inc. A 1.0 M methanol in water solution was used as the fuel and pure oxygen was used as the oxidant in all experiments. Three graphite anode plates were machined with the same flow channel configuration but each with different depth of channels. The cathode was kept the same for all experiments. Polarisation curves and ac impedance spectra were obtained for varying temperatures and channel depths. To separate the contribution of the oxygen reduction reaction to the overvoltage from the anode and membrane contributions, reference hydrogen electrode (RHE) measurements were taken. By comparing the RHE polarisation with the methanol–oxygen polarisation experiments, it was found that polarisation losses at the oxygen cathode accounted for a 40–50% of the overpotential.The variation in the performance of the cell with flow of methanol/water mix, with temperature and with current density was studied. Polarisation measurements indicate that the medium channel depth flow channels performed better than either the shallow depth or deep depth flow channels indicating that there is a complex relationship between the effect of flow velocity and the influence of the rate of production of product CO₂. AC impedance spectroscopy measurements confirmed the observed polarisation results. This method proved to be able to provide a reliable indication of the performance of the cell even when the cell had not yet achieved steady-state. In the case of the shallow channel depth anode, ac impedance revealed that it required considerably longer to achieve steady-state than the time required for the medium and deep channel depths.     

Two-dimensional simulation of cold start processes for proton exchange membrane fuel cell with different hydrogen flow arrangements

Proton exchange membrane (PEM) fuel cells with an off-gas recirculation anode (ORA) or dead-ended anode (DEA) are widely adopted in engineering. However, those two hydrogen flow arrangements may cause anodic water and nitrogen accumulation in comparison with the flow-through anode (FTA) mode, which causes significant performance degradation. In this paper, a two-dimensional cold-start model is developed with detailed consideration of water phase changes and the nitrogen crossover phenomenon. A simplified electrochemical module is built to calculate the current density distribution in the model. The simulation results are consistent with the experimental data at both subzero temperatures and normal operating temperatures. The effects of hydrogen flow arrangements, flow configurations, and startup strategies are investigated during startup process from subzero to normal operating temperatures. Much less ice is generated in counter-flow cases than in co-flow cases during constant current operation. A relatively lower startup voltage can effectively shorten the cold-start process and enhance the cold-start capacity for the PEM fuel cell. The ORA mode has the best hydrogen flow arrangement due to its general abilities, including higher hydrogen utilization efficiency, higher anodic nitrogen tolerance, better output performance and better startup capability.     

Performance analysis of solid oxide fuel cell using reformed fuel

Fuelling SOFC with reformed fuel can be beneficial due to it being cheaper compared to pure hydrogen. A biomass fuel can be easily modeled as a reformed fuel, as it can be converted into H₂ and CO using gasification or biodegradation, the main composition of product from a reformer. Hence in this study it is assumed that feed to the fuel cell contains only H₂ and CO. A closed parametric model is formulated. Performance is analyzed with changes in temperature, pressure and fuel ratio; considering the possible voltage losses, like ohmic, activation, mass transfer and fuel crossover. Performance curves consisting of operating voltage, fuel utilization, efficiency, power density and current density are developed for both pure hydrogen and mixture of CO and H₂. Variations of open circuit voltage with temperature, power density with current density, operating voltage with current density and maximum power density with fuel utilization are also evaluated.     

Performance of carbon-supported PtPd as catalyst for hydrogen oxidation in the anodes of proton exchange membrane fuel cells

In this work, the replacement of platinum by palladium in carbon-supported catalysts as anodes for hydrogen oxidation reaction (HOR), in proton exchange membrane fuel cells (PEMFCs), has been studied. Anodes with carbon-supported Pt, Pd, and equiatomic Pt:Pd, with various Nafion^® contents, were prepared and tested in H₂|O₂ (air) PEMFCs fed with pure or CO-contaminated hydrogen. An electrochemical study of the prepared anodes has been carried out in situ, in membrane electrode assemblies, by cyclic voltammetry and CO electrooxidation voltammetry. The analyses of the corresponding voltammograms indicate that the anode composition influences the cell performance. Single cell experiments have shown that platinum could be replaced, at least partially, saving cost with still good performance, by palladium in the hydrogen diffusion anodes of PEMFCs. The performance of the PtPd catalyst fed with CO-contaminated H₂ used in this work is comparable to Pt, thus justifying further work varying the CO concentration in the H₂ fuel to assert its CO tolerance and to study the effect of the Pt:Pd atomic ratio.     

In-situ investigation on the CO tolerance of carbon supported Pd–Pt electrocatalysts with low Pt content by electrochemical impedance spectroscopy

Carbon supported Pd–Pt electrocatalysts (Pd–Pt/C) with low Pt content were investigated in proton exchange membrane fuel cells (PEMFCs) with pure H₂ and CO/H₂ as the feeding fuels, respectively. The Pd–Pt/C catalysts showed high activity for hydrogen oxidation reaction (HOR) and improved CO tolerance. Electrochemical impedance spectroscopy (EIS) was employed to probe the in-situ information of the improved CO tolerance. The dependence of Nyquist plots and Bode plots on current density and feeding gas was investigated in low polarization region. The results of EIS analysis indicated that the improved CO tolerance of Pd–Pt/C catalysts can be attributed to the lower coverage of CO on the Pd–Pt bimetal than that on the pure Pt.     

Simultaneous measurement of current and temperature distributions in a proton exchange membrane fuel cell

Using a specially designed current distribution measurement gasket in anode and thin thermocouples between the catalyst layer and gas diffusion layer (GDL) in cathode, in-plane current and temperature distributions in a proton exchange membrane fuel cell (PEMFC) have been simultaneously measured. Such simultaneous measurements are realized in a commercially available experimental PEMFC. Experiments have been conducted under different air flow rates, different hydrogen flow rates and different operating voltages, and measurement results show that there is a very good correlation between local temperature rise and local current density. Such correlations can be explained and agree well with basic thermodynamic analysis. Measurement results also show that significant difference exists between the temperatures at cathode catalyst layer/GDL interface and that in the center of cathode endplate, which is often taken as the cell operating temperature. Compared with separate measurement of local current density or temperature, simultaneous measurements of both can reveal additional information on reaction irreversibility and various transport phenomena in fuel cells.     

Electro-catalytic activity of enhanced CO tolerant cerium-promoted Pt/C catalyst for PEM fuel cell anode

Cerium-promoted Pt/C catalysts were prepared by one-pot synthesis process and applied as an anode material for CO tolerance in PEM fuel cell. Its physical properties were characterized by XRD and TEM techniques, which indicated that Pt nano-particles are highly dispersed on the carbon supports. The investigation focused on examining the CO tolerance in sulfur acid solution of Pt–CeO₂/C compared to Pt/C (JM). The hydrogen oxidation activity was strongly depended on the content of the cerium in the Pt catalyst which was detected by CV, LSV, CO-stripping and EIS techniques. Effect of the anode catalyst poisoning on hydrogen oxidation in the presence of CO was studied in single cells. Pt–CeO₂/C catalyst at the appropriate content of 20% Ce presented a very higher CO tolerant activity. A tentative mechanism is proposed for a possible role of a bi-functional synergistic effect between Pt and CeO₂ for the enhanced electro-oxidation of CO. CeO₂-promoted Pt/C catalyst may be one of the attractive candidates as CO tolerance anode material in PEMFC.     

Evaluating the impact of enhanced anode CO tolerance on performance of proton-exchange-membrane fuel cell systems fueled by liquid hydrocarbons

Recent advances in anode electrocatalysts for low-temperature PEM fuel cells are increasing tolerance for CO in the H₂-rich anode stream. This study explores the impact of potential improvements in CO-tolerant electrocatalysts on the system efficiency of low-temperature Nafion-based PEM fuel cell systems operating in conjunction with a hydrocarbon autothermal reformer and a preferential CO oxidation (PROx) reactor for CO clean-up. The incomplete H₂ clean-up by PROx reactors with partial CO removal can present conditions where CO-tolerant anode electrocatalysts significantly improve overall system efficiency. Empirical fuel cell performance models were based upon voltage-current characteristics from single-cell MEA tests at varying CO concentrations with new Pt-Mo alloy reformate-tolerant electrocatalysts tested in conjunction with this study. A system-level model for a liquid-fueled PEM fuel cell system with a 5 kW full power output is used to study the trade-offs between the improved performance with decreased CO concentration and the increased penalties from the air supply to the PROx reactor and associated reduction in H₂ partial pressures to the anode. As CO tolerance is increased over current state-of-the-art Pt alloy catalysts, system efficiencies improve due primarily to higher fuel cell voltages and to a lesser extent to reductions in parasitic loads. Furthermore, increasing CO tolerance of anode electrocatalysts allows for the potential for reduced system costs with minimal efficiency penalty by reducing PROx reactor size through reduced CO conversion requirements.     

质子交换膜燃料电池双极板的设计与模拟分析

质子交换膜燃料电池是直接将化学能转换为电能的装置,双极板上的流道结构对燃料电池的工作性能具有较大的影响。根据应用要求设计了具有平行流道、蛇形流道及希尔伯特分形流道的双极板结构,模拟计算了氢气在不同类型的流道和气体扩散层中的分布状态,分析了燃料电池的输出电流密度和功率密度随电极间电压的变化特点,比较了不同的流道结构对燃料电池输出电流密度的影响,以及不同的工作温度及气体压强的情况下,燃料电池输出电流密度随温度及压强的变化规律。     

Operational characteristics of the direct methanol fuel cell stack on fuel and energy efficiency with performance and stability

This paper is presented to investigate operational characteristics of a direct methanol fuel cell (DMFC) stack with regard to fuel and energy efficiency, including its performance and stability under various operating conditions. Fuel efficiency of the DMFC stack is strongly dependent on fuel concentration, working temperature, current density, and anode channel configuration in the bipolar plates and noticeably increases due to the reduced methanol crossover through the membrane, as the current density increases and the methanol concentration, anode channel depth, and temperature decreases. It is, however, revealed that the energy efficiency of the DMFC stack is not always improved with increased fuel efficiency, since the reduced methanol crossover does not always indicate an increase in the power of the DMFC stack. Further, a lower methanol concentration and temperature sacrifice the power and operational stability of the stack with the large difference of cell voltages, even though the stack shows more than 90% of fuel efficiency in this operating condition. The energy efficiency is therefore a more important characteristic to find optimal operating conditions in the DMFC stack than fuel efficiency based on the methanol utilization and crossover, since it considers both fuel efficiency and cell electrical power. These efforts may contribute to commercialization of the highly efficient DMFC system, through reduction of the loss of energy and fuel.     

On the performance of membraneless laminar flow-based fuel cells

This paper reports on the characterization and optimization of laminar flow-based fuel cells (LFFCs) for both performance and fuel utilization. The impact of different operating conditions (volumetric flow rate, fuel-to-electrolyte flow rate ratio, and oxygen concentration) and of different cell dimensions (electrode-to-electrode distances, and channel length) on the performance (both power density and fuel utilization) of individual LFFCs is investigated. A finite-element-method simulation, which accounts for all relevant transport processes and electrode reactions, was developed to explain the experimental results here. This model can be used to guide further LFFC optimizations with respect to cell design and operation conditions. Using formic acid as the fuel, we measured a peak power density of 55 mW cm⁻². By hydrodynamically focusing the fuel to a thin stream on the anode we were able to reduce the fraction of fuel that passes through the channel without reacting, thereby increasing the fuel utilization per pass to a maximum of 38%. This paper concludes with a discussion on the various trade-offs between maximizing power density and optimizing fuel utilization per pass for individual LFFCs, in light of scaling out to a multichannel LFFC-based power source system.     

Transient analysis of carbon monoxide poisoning and oxygen bleeding in a PEM fuel cell anode catalyst layer

The presence of carbon monoxide in the fuel stream hinders the performance of a polymer electrolyte membrane (PEM) fuel cell, known as carbon monoxide (CO) poisoning. Introducing oxygen in the fuel stream lessens CO poisoning. Since CO poisoning is a phenomenon that occurs over a substantial period of time, a transient model has been developed in this study, taking into account the effect of CO concentration, operating pressure and temperature, as well as oxygen bleeding on the performance of the cell. It is found that at a lower CO concentration the poisoning effect takes a much longer time to reach the steady state, even though for a better steady state anode performance. A higher operating temperature results in a better steady state performance, but the performance drops faster toward the steady state value at higher temperature. A higher operating pressure leads to an enhanced performance over the entire transient history, although the benefit diminishes as pressure is increased. Even with a small amount of oxygen (0.5%) introduced into the fuel stream, the anode performance can be improved significantly. Finally, it is observed that the use of pure hydrogen interspersed in carbon monoxide containing fuel improves the anode performance. However, performance recovery when operating on pure hydrogen is much slower than the performance degradation due to the CO poisoning.     

Effects of operating conditions on the performance of a micro-tubular solid oxide fuel cell (SOFC)

A parametric analysis is carried out to study the effects of the operating conditions on the performance and operation of a micro-tubular solid oxide fuel cell. The computational fluid dynamics model incorporates mass, momentum, species and energy balances along with ionic and electronic charge transfers. Effects of temperature, fuel flow rate, fuel composition, anode pressure and cathode pressure on fuel cell performance are investigated. Polarization curves are compared to allow an understanding of the effects of different operating conditions on the performance of the fuel cell. Effects of anode flow rate on fuel cell efficiency and fuel utilization are also investigated. Moreover, influence of operating temperature on the internal electronic current leaks is outlined. Temperature distributions, current density profiles and hydrogen mole fraction profiles are also utilized to have a better understanding of the spatial effects of operating parameters. It is predicted that at 550 °C, for an output current demand of 0.53 A cm⁻², fuel cell needs to generate 0.65 A cm⁻² ionic current density where the difference in these values is attributed to internal current leaks. On the other hand for temperatures lower than 500 °C, the effect of electronic leakage currents are not significant.     

Studies on the initial behaviours of the molten carbonate fuel cell

Mathematical modelling of the unsteady-state of a unit molten carbonate fuel cell (MCFC) has been made. The behaviour of the fuel cell at the beginning of the operation is observed. The effects of the molar flow rates of gases and the utilization of fuel gas are studied. The current density decreases with time and reaches a steady-state value of 0.14 A cm⁻² at 0.58 s for the chosen reference conditions. As the inlet gas-flow rates or the hydrogen utilization are increased, the time required to reach a steady-state decreases. With increased flow rates of the anode and cathode gases, the average current density is high and the total concentration is low. The current density increases with increasing utilization of hydrogen.     

Systematic approach for modeling methanol mass transport on the anode side of direct methanol fuel cells

A systematic method for modeling direct methanol fuel cells, with a focus on the anode side of the system, is advanced for the purpose of quantifying the methanol crossover phenomenon and predicting the concentration of methanol in the anode catalyst layer of a direct methanol fuel cell. The model accounts for fundamental mass transfer phenomena at steady state, including convective transport in the anode flow channel, as well as diffusion and electro-osmotic drag transport across the polymer electrolyte membrane. Experimental measurements of methanol crossover current density are used to identify five modeling parameters according to a systematic parameter estimation methodology. A validation study shows that the model matches the experimental data well, and the usefulness of the model is illustrated through the analysis of effects such as the choice fuel flow rate in the anode flow channel and the presence of carbon-dioxide bubbles.     

Effect of operating parameters and anode diffusion layer on the direct ethanol fuel cell performance

A parametric study was conducted on the performance of direct ethanol fuel cells. The membrane electrode assemblies employed were composed of a Nafion^® 117 membrane, a Pt/C cathode and a PtRu/C anode. The effect of cathode backpressure, cell temperature, ethanol solution flow rate, ethanol concentration, and oxygen flow rate were evaluated by measuring the cell voltage as a function of current density for each set of conditions. The effect of the anode diffusion media was also studied. It was found that the cell performance was enhanced by increasing the cell temperature and the cathode backpressure. On the contrary, the cell performance was virtually independent of oxygen and fuel solution flow rates. Performance variations were encountered only at very low flow rates. The effect of the ethanol concentration on the performance was as expected, mass transport loses observed at low concentrations and kinetic loses at high ethanol concentration due to fuel crossover. The open circuit voltage appeared to be independent of most operating parameters and was only significantly affected by the ethanol concentration. It was also established that the anode diffusion media had an important effect on the cell performance.