Pt/WC as an anode catalyst for PEMFC: Activity and CO tolerance

A mesoporous tungsten carbide of WC-phase was synthesized by using ammonium meta tungstate as tungsten precursor and resorcinol–formaldehyde polymer as carbon source in the presence of a surfactant. The platinum supported on this material with a low loading (7.5 wt%) served as an effective CO tolerant electro anode catalyst. The Pt/WC catalyst showed two times higher activity per mass of Pt for hydrogen electro-oxidation compared to a commercial Pt/C catalyst (E-Teck). In addition, it exhibited much improved resistance to CO poisoning relative to the Pt/C catalyst. Since the catalyst is also stable in electrochemical environment, it could become an alternative anode catalyst for PEMFC.     

Characterization of Membrane Electrode Assemblies for High‐Temperature PEM Fuel Cells

This paper will present the characterization of two types of membrane‐electrode‐assemblies (MEAs) for high‐temperature polymer electrolyte membrane fuel cells (HT‐PEMFC) working under reformate stream. The important aspects to be considered in the characterization of these MEAs are: (i) presence of contaminants, and (ii) composition of the anode. Start/stop cycling test were performed for two different Dapozol® MEAs using different GDL materials, using first hydrogen and then synthetic reformate as a fuel gas, both with a dew point of 80 °C. With these results the influence of contaminants present in the reformate was compared for the two types of MEAs, showing the superior performance of the Dapozol® 101 MEA under these conditions. The possibility to further enhance the MEAs' resilience against the operation of reformates by changing the anode catalyst composition was evaluated in a half MEA configuration, considering that the impact of the H₂S present in the fuel presents a major issue. For this reason the hydrogen oxidation reaction (HOR) was evaluated for two types of Pt‐based electrocatalysts in an anodic half MEA configuration using different hydrogen‐rich fuel mixtures. These results provide valuable information for the optimization of the MEA and the anode catalyst for HT‐PEMFC.     

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).     

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.     

Effect of Carbon Dioxide and Ammonia on Polymer Electrolyte Membrane Fuel Cell Stack Performance

Proton exchange membrane fuel cell (PEMFC) performance degrades when impurities are present in the anode fuel gas, referred to as catalyst poisoning. This paper investigates the effect of carbon dioxide and ammonia as impurities in the anode gas of the PEMFC, and found that the presence of CO₂ decreases the performance of the fuel cell by up to 10%. The performance loss depends on the CO₂ concentration and the exposure time. The voltage loss is recoverable on passing pure hydrogen gas, indicating that a permanent poisoning of the catalyst layer has not taken place. Exposure of the fuel cell to ammonia beyond 20 ppm, even for a short duration, causes permanent PEMFC failure, probably due to the deterioration of the membrane.     

Development of prox (preferential oxidation of CO) system for 1 kWe PEMFC

PROX is known as one of the most promising technologies which prevent the anode of PEMFC from being poisoned by carbon monoxide. Hence, commercialization of the PEMFC system is highly dependent on the development of the corresponding PROX system. This study is focused on the development of the PROX system for 1 kW_e PEMFC, and the results can be used to predict the performance of a higher-scaled system. Pt-Ru/Al₂O₃ catalyst made by incipient wetness method has been used for the reaction, since this catalyst shows high activity and selectivity for CO oxidation over a wide range of temperature. With the catalyst, a 1 kW_e proto-type PROX system was set up and its performance was evaluated for the steady state as well as the transient conditions. The outlet CO con-centration of the system was below 10 ppm at its steady state. Also, even at transient conditions, in which sudden flow rate change occurred, the resulting CO concentration still remained under 10 ppm. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University     

Investigation of a Novel Catalyst Coated Membrane Method to Prepare Low‐Platinum‐Loading Membrane Electrode Assemblies for PEMFCs

In this work, a novel catalyst coated membrane (CCM) approach–a catalyst‐sprayed membrane under irradiation (CSMUI)–was developed to prepare MEAs for proton exchange membrane fuel cell (PEMFC) application. Catalyst ink was sprayed directly onto the membrane and an infrared light was used simultaneously to evaporate the solvents. The resultant MEAs prepared by this method yielded very high performance. Based on this approach, the preparation of low‐platinum‐content MEAs was investigated. It was found that for the anode, even if the platinum loading was decreased from 0.2 to 0.03 mg cm^–2, only a very small performance decrease was observed; for the cathode, when the platinum loading was decreased from 0.3 to 0.15 mg cm^–2, just a 5% decrease was detected at 0.7 V, but a 35% decrease was observed when the loading was decreased from 0.15 to 0.06 mg cm^–2. These results indicate that this approach is much better than the catalyst coated gas diffusion layer (GDL) method, especially for the preparation of low‐platinum‐content MEAs. SEM and EIS measurements indicated ample interfacial contact between the catalyst layer and the membrane.     

Electrochemical promotion of CO conversion to CO2 in PEM fuel cell PROX reactor

The possibility of electrochemically promoting the water–gas-shift reaction and the CO oxidation reaction in a PEM fuel cell reactor supplied with a methanol reformate mixture was investigated in PEM fuel cells with Pt or Au state-of-the-art E-TEK anodes, in order to explore the use of PEMFC units as preferential oxidation of CO (PROX) reactors. The electropromotion of CO removal was investigated both with air or H₂ fed to the cathode side and also by O₂ bleeding to the anode during normal PEMFC operation. It was found that the catalytic activity of the anode for CO conversion to CO₂ can be modified significantly by varying the catalyst potential. The magnitude of the electrochemical promotion depends strongly on the anodic electrocatalyst (Pt or Au), on the CO concentration of the fuel mixture, on the operating temperature and on the presence of oxygen. The electropromotion effect and the Faradaic efficiency were found to be much higher in CO-rich anode environments.     

Computational fluid dynamics simulation of polymer electrolyte membrane fuel cells operating on reformate

Carbon monoxide (CO) can extremely diminish the polymer electrolyte membrane fuel cell (PEMFC) performance since it is preferentially absorbed on the platinum catalyst layer blocking and reducing the number of catalyst sites available for the hydrogen oxidation reaction. To gain a good insight of CO poisoning characteristics so as to provide a remedial solution for CO-poisoned PEMFCs, a two-dimensional, isothermal, and single phase CO poisoning numerical model taking into account the transport phenomena, electrochemical reactions and multi-component gas mixture transport is developed for such purpose. Linear and bridged-bonded adsorbed CO modes were considered to occur in parallel on the highly dispersed nano-crystalline Pt/C and PtRu/C catalysts. By performing computational fluid dynamics numerical simulations, this study clearly demonstrates the CO poisoning mechanisms and characteristics of PEMFCs. The numerical results obtained are in reasonably good agreement with the experimental data showing the predictive capability of the model.     

Recent progress in selective CO removal in a H2-rich stream

In this review, recent works related to the selective CO removal in a H₂-rich stream for the application of the low-temperature fuel cell are discussed. The membrane separation, the selective CO hydrogenation, and the preferential CO oxidation (PROX) have been generally studied to meet the requirement for the polymer electrolyte membrane fuel cell (PEMFC) where the CO concentration should be controlled to be less than 10 ppm not to degrade the electrochemical performance of Pt anode. For the membrane separation, the thin layer of Pd-based alloy metal on the porous ceramic material coupled with the catalytic purification is the most advanced method at present. For PROX catalysts, supported Ru catalysts and Pt-based alloy catalysts have been successfully developed so far. The combination of highly selective PROX catalysts and the CO methanation catalyst can provide the extended temperature range to achieve the acceptable CO removal. Because each method has presently its own weak points, the further advance is still in need. The non-noble metal-based membrane requiring smaller pressure differentials is highly plausible in the membrane separation. The highly selective catalyst for CO methanation in the presence of excess CO₂ and H₂O can simplify the CO removal step. The PROX catalyst should be operative over a wide reaction temperature as well as at low temperatures not to cause the reverse water–gas shift reaction. During the development of these catalysts, the progress on the high-temperature PEM fuel cell or the CO-tolerant anode should be carefully evaluated.     

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.     

燃料电池多组分阳极催化剂的最新研究进展

介绍了近年来适用于质子交换膜燃料电池(PEMFC)和直接甲醇燃料电池(DMFC)的阳极催化剂的国内外研究情况,着重介绍了一些近年来发展起来的制备方法和新的多组分催化剂体系,特别是一些表现出了良好的活性和抗CO中毒性能的新催化体系。     

Preparation and characteristics of pretreated Pt/alumina catalysts for the preferential oxidation of carbon monoxide

The polymer electrolyte membrane fuel cell (PEMFC) needs purified hydrogen fuel from hydrocarbon reforming and water-gas shift (WGS) reaction. Concentration of CO should be 10 ppm level to avoid poisoning of the platinum anode electrode. For this, preferential oxidation of carbon monoxide (PROX) reaction is essential. In this study, a novel pretreatment technique was applied to a conventional Pt/γ-Al₂O₃ catalyst. Oxygen-treated, water-treated, and conventional Pt/γ-Al₂O₃ catalyst were prepared and their performances in the PROX reaction were investigated in a simulated hydrogen-rich reaction conditions. Our results showed that catalytic activity of the oxygen-treated 5% Pt/γ-Al₂O₃ catalyst for the CO conversion increased dramatically especially at the low temperature below 100 °C. The enhancement is attributed to the formation of well-dispersed small Pt particles.     

Heterogeneous Catalytic Reactor Design with Non‐Uniform Catalyst Considering Shell‐Progressive Poisoning Behavior

A novel methodology has been developed to design an optimum heterogeneous catalytic reactor, by considering non‐uniform catalyst pellet under shell‐progressive catalyst deactivation. Various types of non‐uniform catalyst pellets are modelled in combination with reactor design. For example, typical non‐uniform catalyst pellets such as egg‐yolk, egg‐shell and middle‐peak distribution are developed as well as step‐type distribution. A progressive poisoning behavior is included to the model to produce correct effectiveness factor from non‐uniform catalyst pellet. As opposed to numerical experiment with limited type of kinetic application to the model in the past, this paper shows a new methodology to include any types of kinetic reactions for the modeling of the reactor with non‐uniform catalyst pellet and shell‐progressive poisoning. For an optimum reactor design, reactor and catalyst variables are considered at the same time. For example, active layer thickness and location inside pellet are optimised together with reactor temperature for the maximisation of the reactor performance. Furthermore, the temperature control strategy over the reactor operation period is added to the optimization, which extends the model to three dimensions. A computational burden has been a major concern for the optimization, and innovative methodology is adopted. Application of profile based synthesis with the combination of SA (Simulated Annealing) and SQP (Successive Quadratic Programming) allows more efficient computation not only at steady state but also in dynamic status over the catalyst lifetime. A Benzene hydrogenation reaction in an industry scale fixed‐bed reactor is used as a case study for illustration.     

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.     

PBI‐Based Polymer Membranes for High Temperature Fuel Cells – Preparation,Characterization and Fuel Cell Demonstration

Proton exchange membrane fuel cell (PEMFC) technology based on perfluorosulfonic acid (PFSA) polymer membranes is briefly reviewed. The newest development in alternative polymer electrolytes for operation above 100 °C is summarized and discussed. As one of the successful approaches to high operational temperatures, the development and evaluation of acid doped polybenzimidazole (PBI) membranes are reviewed, covering polymer synthesis, membrane casting, acid doping, physicochemical characterization and fuel cell testing. A high temperature PEMFC system, operational at up to 200 °C based on phosphoric acid‐doped PBI membranes, is demonstrated. It requires little or no gas humidification and has a CO tolerance of up to several percent. The direct use of reformed hydrogen from a simple methanol reformer, without the need for any further CO removal, has been demonstrated. A lifetime of continuous operation, for over 5000 h at 150 °C, and shutdown‐restart thermal cycle testing for 47 cycles has been achieved. Other issues such as cooling, heat recovery, possible integration with fuel processing units, associated problems and further development are discussed.     

SAXS Analysis of Catalyst Degradation in High Temperature PEM Fuel Cells Subjected to Accelerated Ageing Tests

The loss in performance during fuel cell operation is one of the critical factors that hamper fuel cells commercialization. This paper presents a research activity related to high temperature polymer electrode membrane fuel cell (HT‐PEMFC) degradation. The aim of the study is to investigate catalyst degradation of membrane electrode assemblies (MEAs) subjected to load cycles. Two HT‐PEM MEAs have been subjected to accelerated ageing tests based on load cycling. The cycles profile has been chosen in order to enhance catalyst degradation. Both the tests show a fuel cell performance loss lower than 30 mV after 100,000 cycles at 600 mA cm⁻². In order to analyze the catalyst evolution, synchrotron small angle X‐ray scattering (SAXS) has been employed. The catalyst degradation of the two conditioned samples has been compared with the data obtained from a new MEA that has been used as reference sample. The SAXS results showed a mean size increase of the platinum nanoparticles up to the 100%.     

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.     

质子交换膜燃料电池冷启动的数值模拟

冷启动是质子交换膜燃料电池（PEMFC）商业化所面临的挑战之一，在PEMFC冷启动实验中，通过中子成像技术已经观测到电池内部存在过冷水，因此本文模型重点考虑过冷水对电池冷启动性能的影响。通过引入结冰概率函数对过冷水结冰过程的随机性进行描述，从而建立了PEMFC冷启动的三维、瞬态和多相流动数学模型。基于该模型，研究电池阴极催化层中离子聚合物的体积分数和质子交换膜的厚度对电池冷启动性能的影响。研究结果表明，增加阴极催化层中离子聚合物的体积分数，可有效促进阴极催化层中的反应生成水向质子交换膜中进行扩散，从而充分利用膜内的储水空间；减少质子交换膜的厚度，能促进质子交换膜中的离聚物水向阳极催化层扩散，在大电流密度工况下可有效缓解阳极的“膜干”现象。     

Benefits of Membrane Electrode Assemblies with Asymmetrical GDL Configurations for PEM Fuel Cells

Membrane electrode assemblies (MEAs), based on commercial catalyst‐coated membranes combined with various gas diffusion layers (GDLs) on anode and cathode, were studied in terms of their specific advantages for different operations regimes of proton exchange membrane fuel cells (PEMFCs.) It is verified that MEAs with optimized gas diffusion layer designs (backing and micro‐porous layers) on anode and cathode are able to provide improved cell performance combined with a largely reduced sensitivity towards changes in the relative humidity as compared to MEAs with symmetrical gas diffusion layer configuration.