The influence of CO on the current density distribution of high temperature polymer electrolyte membrane fuel cells

In this work the poisoning effect of carbon monoxide (CO) on the performance of high temperature polymer electrolyte membrane (PEM) fuel cell is reported. The poisoning of the anode is assessed at 160 °C and 180 °C based on the transient behavior of the fuel cell potential and current density distribution. The current density distribution at similar cell potential and global current density is also critically compared for CO-free hydrogen feed and for CO-contaminated hydrogen feed. Furthermore, the current–cell potential (I–V) and power density curves and impedance spectra are obtained.The presence of CO causes a performance loss which is aggravated for higher CO concentrations and higher current densities and for lower temperatures. The transient behavior of the fuel cell potential and current density distribution show that the poisoning effect of carbon monoxide at the anode is very fast.The use of CO contaminated hydrogen at the anode yields an anisotropic distribution of carbon monoxide, which is accentuated for higher carbon monoxide concentrations and current densities.     

Mathematical Model of Proton Exchange Membrane Fuel Cell with Consideration of Water Management

One‐dimensional model on the membrane electrode assembly (MEA) of proton exchange membrane fuel cell is proposed, where the membrane hydration/dehydration and the possible water flooding of the respective cathode and anode gas diffusion layers are considered. A novel approach of phase‐equilibrium approximation is proposed to trace the water front and the detailed saturation profile once water emerges in either anode or cathode gas diffusion layer. The approach is validated by a semi‐analytical method published earlier. The novel approach is applicable to the polarization regime from open circuit voltage to the limiting current density under practical operation conditions. Oxygen diffusion is limited by water accumulation in the cathode gas diffusion layer as current increases, caused by excessive water generation at the cathode catalyst layer and the electro‐osmotic drag across the membrane. The existence of liquid water in the anode gas diffusion layer is predicted at low current densities if high degrees of humidification in both anode and cathode feeds are employed. The influences of inlet relative humidity, imposed pressure drop, and cell temperature are correlated well with the cell performance. In addition, the overpotentials attributed from individual components of the MEA are delineated against the cell current densities.     

Water transport through a PEM fuel cell: A one-dimensional model with heat transfer effects

Models play an important role in fuel cell design/development. The most critical problems to overcome in the proton exchange membrane (PEM) fuel cell technology are the water and thermal management. In this work, a steady-state, one-dimensional model accounting for coupled heat and mass transfer in a single PEM fuel cell is presented. Special attention is devoted to the water transport through the membrane which is assumed to be a combined effect of diffusion and electro-osmotic drag. The transport of heat through the gas diffusion layers is assumed to be a conduction-predominated process and heat generation or consumption is considered in the catalyst layers. The analytical solutions for concentration and net water transport coefficient are compared with recent published experimental data. The operating conditions considered are various cathode and anode relative humidity (RH) values at and 2 atm. The studied conditions correspond to relatively low values of RH, conditions of special interest, namely, in the automotive applications. Model predictions were successfully compared to experimental and theoretical I-V polarization curves presented by Hung et al. [2007. Operation-relevant modelling of an experimental proton exchange membrane fuel cell. Journal of Power Sources 171, 728-737] and Ju et al. [2005a. A single-phase, non-isothermal model for PEM fuel cells. International Journal of Heat and Mass Transfer 48, 1303-1315]. The developed easy to implement model using low CPU consumption predicts reasonably well the influence of current density and RH on the net water transport coefficient as well as the oxygen, hydrogen and water vapour concentrations at the anode and cathode. The model can provide suitable operating ranges adequate to different applications (namely low humidity operation) for variable MEA structures.     

质子交换膜燃料电池温度和电流分布同步测定

发展了一种新的质子交换膜燃料电池（PEMFC）局部特性测试方法,该方法实现了在不改变PEMFC膜电极和电池结构的情况下对PEMFC电流密度和局部温度分布的同步测量。对一种PEMFC在自然冷却状态下,阳极流场板背面温度分布以及阴极电流密度分布进行了实验测定。实验结果表明,阳极流场板背面最大温差小于1℃;电流密度分布主要受电极内液态水分布影响;同时,水蒸气冷凝放热导致电池局部温度升高是造成电池温度分布不均的主要因素。     

PEM fuel cell Pt anode inhibition by carbon monoxide: Non-uniform behaviour of the cell caused by the finite hydrogen excess

Inhibition of platinum surfaces by carbon monoxide, in particular in polymer membrane electrolyte fuel cells (PEMFC) has been observed for decades by electrochemists. Significant effects have been observed in the hydrogen stream fed to the anode of the fuel cell with concentrations ranging from 1 to 100 ppm depending on the operating conditions e.g. temperature, pressure and excess in reacting gases. As a matter of fact, the gas composition and the surface coverage by CO and H₂ vary in the cell, because of the hydrogen consumption at the anode: this is to result to non-uniform distributions of electrode poisoning, current density, and overvoltage, from the inlet to the outlet of the cell. A simple 1D-model has been developed for prediction of the profiles of the above variables in the fuel cells, with the support of experimental data obtained with a 25 cm² PEMFC: interpretation of polarization curves and impedance spectra yielded the kinetic laws of the two electrode reactions, with both neat hydrogen and CO-containing hydrogen at ppm levels. Simulations show that for low excess in hydrogen – as for practical use of fuel cells – the coverage fractions of the various species can greatly vary in the cell, resulting in non-uniform distributions of current density in the cell and enhanced electrode poisoning near the cell outlet. In contrast working with very high hydrogen excess, as can be done at bench scale, leads to uniform behaviour of the cell, and far less visibility of the anode poisoning by carbon monoxide.     

Simplified Model for Oxygen Transport With Reaction in a Polymer‐Electrolyte Fuel Cell

A simplified mathematical model for an ion‐exchange membrane attached to a gas‐fed porous electrode is developed to simulate the oxygen electrode of a solid‐polymer‐electrolyte fuel cell. In particular, the present model is derived from an earlier rigorous one of Bernardi and Verbrugge(1991) by neglecting the Peclet number for the transport of dissolved oxygen within the catalyst. The advantage of this simpler model is that it can be solved analytically, eliminating the need for numerical simulation. Longitudinal profiles for the dissolved oxygen concentration and catalyst current density calculated from the present model are in good agreement with results from the earlier rigorous model.     

Mathematical modeling of solid oxide fuel cells at high fuel utilization based on diffusion equivalent circuit model

Mass transfer and electrochemical phenomena in the membrane electrode assembly (MEA) are the core components for modeling of solid‐oxide fuel cell (SOFC). The general MEA model is simply governed with the Stefan‐Maxwell equation for multicomponent gas diffusion, Ohm's law for the charge transfer and the current‐overpotential equation for the polarization calculation. However, it has obvious discrepancy at high‐fuel utilization or high‐current density. An advanced MEA model is introduced based on the diffusion equivalent circuit model. The main purpose is to correct the real‐gas concentrations at the triple‐phase boundary by assuming that the resistance of surface diffusion is in series with that of the gaseous bulk diffusion. Thus, it can obtain good prediction of cell performance in a wide range by avoiding the decrement of effective gas diffusivity via unreasonable increment of the electrode tortuosity in the general MEA model. The mathematical model has been validated in the cases of H₂? H₂O, CO? CO₂ and H₂? CO fuel system. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Transient carbon monoxide poisoning of a polymer electrolyte fuel cell operating on diluted hydrogen feed

The transient behavior of a 50 cm² PEM fuel cell fed on simulated reformate containing diluted hydrogen and trace quantities of carbon monoxide (CO) was experimentally investigated. It was found that the overall cell performance throughout the CO poisoning process can be described with a lumped model of hydrogen and CO adsorption, desorption, and electro-oxidation coupled with a current-voltage relationship for fuel cell performance. It was shown that while hydrogen dilution alone does not have an appreciable effect on cell polarization, in the presence of trace amounts of CO, hydrogen dilution amplifies the problem of CO poisoning. This is a result of the diluent reducing the partial pressure of reactants in the anode fed stream, thus retarding the already CO-impaired hydrogen adsorption onto the catalyst surface. In a diluted hydrogen stream, even low CO concentrations (i.e. 10 ppm), which are traditionally considered safe for PEM fuel cell operation, were found to be harmful to cell performance.     

Components for PEM fuel cell systems using hydrogen and CO containing fuels

Proton exchange membrane fuel cells (PEMFC) show a significant performance drop in CO containing hydrogen as fuel gas in comparison to pure hydrogen. The lower performance is due to CO adsorption at the anode thus poisoning the hydrogen oxidation reaction. Two approaches to improve the cell performance are discussed. First, the use of improved electrocatalysts for the anode, such as PtRu alloys, can significantly enhance the CO tolerance. On the other hand, CO poisoning of the anode could be avoided by the use of non-electrochemical methods. For example, the addition of liquid hydrogen peroxide to the humidification water of the cell leads to the formation of active oxygen by decomposition of H₂O₂ and the oxidation of CO. In such a way a complete recovery of the CO free cell performance is achieved for H₂/100 ppm CO.     

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).     

Polarization effects in electrolyte/electrode-supported solid oxide fuel cells

The effects of activation, ohmic and concentration polarization on the overall polarization in solid oxide fuel cells are presented. A complete analysis was conducted based on thermodynamic principles for the calculation of cell voltage. Treating the fuel cell as a control volume, the irreversibility term in a steady flow thermodynamic system was related to the overall polarization. The entropy production was calculated and related to the lost work of the fuel cell, while the heat loss from the cell was determined from the entropy balance. To generalize the cell voltage–current density expression, the Butler–Volmer model was used in the calculation of activation polarization and both ordinary and Knudsen diffusions were considered in the calculation of concentration polarization. The overall cell resistance was deduced from the generalized cell voltage–current density expression. The concentration resistance at the anode can be minimized by humidifying the hydrogen with an appropriate amount of water, depending on the thickness of the anode used. Comparison of polarization effects on the cell performance between the electrolyte-supported and anode-supported cells showed that the latter would give a better cell performance.     

Influence of temperature and relative humidity on performance and CO tolerance of PEM fuel cells with Nafion-Teflon-Zr(HPO4)2 higher temperature composite membranes

The carbon monoxide (CO) poisoning effect on carbon supported catalysts (Pt-Ru/C and Pt/C) in polymer electrolyte membrane (PEM) fuel cells has been investigated at higher temperatures (T > 100 °C) under different relative humidity (RH) conditions. To reduce the IR losses in higher temperature/lower relative humidity, Nafion^®-Teflon^®-Zr(HPO₄)₂ composite membranes were applied as the cell electrolytes. Fuel cell polarization investigation as well as CO stripping voltammetry measurements was carried out at three cell temperatures (80, 105 and 120 °C), with various inlet anode relative humidity (35%, 58% and 100%). CO concentrations in hydrogen varied from 10 ppm to 2%. The fuel cell performance loss due to CO poisoning was significantly alleviated at higher temperature/lower RH due to the lower CO adsorption coverage on the catalytic sites, in spite that the anode catalyst utilization was lower at such conditions due to higher ionic resistance in the electrode. Increasing the anode inlet relative humidity at the higher temperature also alleviated the fuel cell performance losses, which could be attributed to the combination effects of suppressing CO adsorption, increasing anode catalyst utilization and favoring OH_ads group generation for easier CO oxidation.     

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.     

Membrane electrode assembly with Pt/SiO2/C anode catalyst for proton exchange membrane fuel cell operation under low humidity conditions

In this work, a novel self-humidifying membrane electrode assembly (MEA) with Pt/SiO₂/C as anode catalyst was developed to improve the performance of proton exchange membrane fuel cell (PEMFC) operating at low humidity conditions. The characteristics of the composite catalysts were investigated by XRD, TEM and water uptake measurement. The optimal performance of the MEA was obtained with the 10 wt.% of silica in the composite catalyst by single cell tests under both high and low humidity conditions. The low humidity performance of the novel self-humidifying MEA was evaluated in a H₂/air PEMFC at ambient pressure under different relative humidity (RH) and cell temperature conditions. The results show that the MEA performance was hardly changed even if the RHs of both the anode and cathode decreased from 100% to 28%. However, the low humidity performance of the MEA was quite susceptible to the cell temperature, which decreased steeply as the cell temperature increased. At a cell temperature of 50 °C, the MEA shows good stability for low humidity operating: the current density remained at 0.65 A cm⁻² at a usual work voltage of 0.6 V without any degradation after 120 h operation under 28% RH for both the anode and cathode.     

Water and methanol crossover in direct methanol fuel cells—Effect of anode diffusion media

Various anode diffusion media have been experimentally studied to reduce water crossover in a direct methanol fuel cell (DMFC). A two-phase water transport model was also employed to theoretically study their effects on water transport and saturation level in a DMFC anode. It is found that wettability of the anode microporous layer (MPL) has a dramatic effect on water crossover or the water transport coefficient (α) through the membrane. Under different current densities, the MEA with a hydrophobic anode MPL has consistently low α, several times smaller than those with a hydrophilic MPL or without an anode MPL. Methanol transport in the anode is found to be not influenced by a hydrophobic anode MPL but inhibited by a hydrophilic one. Constant-current discharge shows that the MEA with hydrophobic anode MPL displays much smaller voltage fluctuation than that with the hydrophilic one. A modeling study of anode water transport reveals that the liquid saturation in the anode is lowered significantly with the increase of anode MPL contact angle, which is thus identified as a key parameter to minimize water crossover in a DMFC.     

Methods for the Quantitative Characterization of the CO Tolerance in a One‐Dimensional Polymer Electrolyte Fuel Cell

The use of reformed fuels in polymer electrolyte fuel cells (PEFC) exhibits distinctive advantages over pure hydrogen in terms of fuel supply infrastructure and on‐board fuel storage in a vehicle. Yet small residual CO in the reformate in the order of 10–100 ppm cause severe degradation of fuel cell performance. The CO tolerance of the fuel cell is therefore an important characteristic calling for adequate quantitative evaluation. In this study, we assessed a range of methods to quantify the CO tolerance based on cell polarization data. Generally, no method has the claim to be ideal, but the appropriate choice of CO tolerance quantity rather depends on the design target of the fuel cell system. Analysis of literature data revealed, however, that comparison of fuel cell performance using pure and CO‐contaminated hydrogen, respectively, at constant current density seems to be a popular method, because it yields immediate evidence of anode performance. It was also recognized that CO tolerance manifests on different levels: on the electrocatalyst, the electrode, and the fuel cell level. In order to investigate the CO tolerance on the electrode level, a fuel cell of one‐dimensional construction, like the one used in our experiments, needs to be employed.     

Role of hydrophobic anode MPL in controlling water crossover in DMFC

The importance of reducing water crossover from anode to cathode in a direct methanol fuel cell (DMFC) has been well documented, especially if highly concentrated methanol fuel is to be used. A low-α membrane electrode assembly (MEA) with thin membrane is key to achieving this goal. The low water crossover from anode to cathode for these types of MEAs has traditionally been attributed to the use of a hydrophobic cathode micro-porous layer (MPL). However, it has recently been discovered that a hydrophobic anode MPL also reduces the water crossover, possibly even more significantly than a hydrophobic cathode MPL. In this work, we develop and use a 1D, two-phase transport model that accounts for capillary-induced liquid flow in porous media to explain how a hydrophobic anode MPL controls the water crossover from anode to cathode. We further show that a lower water crossover can lead to a lower methanol crossover via dilution of methanol in the anode catalyst layer. Finally, we perform a parametric study and show that a thicker anode MPL with greater hydrophobicity and lower permeability is more effective in reducing the water crossover.     

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 H₂ or 200 ppm CO-containing H₂ 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 H₂ 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.     

Influence of preparation process of MEA with mesocarbon microbeads supported Pt–Ru catalysts on methanol electrooxidation

The effects of mesocarbon microbeads support for platinum–ruthenium (Pt–Ru) catalysts on anode performance of the direct methanol fuel cell (DMFC) were investigated. Polarization characteristics of the anode electrode were low due to the fast rate of mass transport in the electrode. The effects of the Nafion^® content in the catalyst, the MEA hot press condition, the cell operation temperature and methanol concentration on the polarization curves of the anode were also investigated.     

Investigation of phosphoric acid and water transport in the high temperature proton exchange membrane fuel cells using a multiphase model

A three-dimensional, nonisothermal, and multiphase model of high temperature proton exchange membrane fuel cells is built to investigate water and phosphoric acid transportation, in which a spherical agglomerate model considering catalyst layer structure and liquid saturation is applied to determine the electrochemical kinetics in the cathode catalyst layer. Experimental polarization curve, water proportion in the anode outlet gas, and phosphoric acid distribution are selected for validation. It is found that the simulated results can represent the experimental data with reasonable accuracy. Based on the model, the effects of current density and stoichiometry on the variable distributions are analyzed. The results show that water in anode is mainly from cathode by concentration diffusion of liquid water, and the proportion of anode outlet water to the total produced water decreases slightly with the increase of current density. A higher current density leads to a greater electromigration of phosphoric acid from cathode to anode and a higher liquid phase fraction in anode, while a lower phosphoric acid concentration in the fuel cells.