PEMFC contaminant tolerance limit—CO in H2

Equations are derived from a generic, transient PEMFC contamination model to predict the effect of CO contaminant concentration in H₂ on both steady state performance losses and time constants. The resulting predictions allowed determination of the CO tolerance limit for the cases of a Pt and WC catalyst under specific operating conditions. An increase of the International Organization for Standardization CO tolerance limit of 0.2 ppm is possible because the CO concentration leading to a steady state performance loss of less than 1% is estimated at 0.2-0.9 ppm. An increase in CO tolerance limit is expected to reduce the analytical verification cost (quality control). The steady state performance loss is independent of catalyst loading thus avoiding a future standard change resulting from PEMFC cost reduction activities.     

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.     

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.     

Reaction and Surface Characterization Studies of Ru/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Catalysts for CO Preferential Oxidation in Reformed Gas

In order to investigate the reasons the activation of a Ru/Al₂O₃ catalyst by heating in a H₂/N₂ mixed gas improves the CO preferential oxidation (PROX) activity, the oxidation state of the Ru on the catalyst surface was studied by using ESCA. As the ratio of Ru(0) to total Ru on the surface was increased, the temperature window of the Ru catalyst, where CO was reduced to below 10 ppm, was expanded to the lower temperature side. The activity of CO oxidation by O₂ of the Ru catalyst at lower temperatures was improved by increasing the ratio of Ru(0). However, the selectivity for CO oxidation hardly varied with the change in the surface Ru(0) ratio at these low temperatures. It is considered that O₂ activation on Ru(0) plays an essential role in CO PROX activity on the Ru catalyst at low temperatures.     

CO Oxidation Kinetics over a Nanostructured Cu0.1Ce0.9O2?y Catalyst: A CO/O2 Concentration Cycling Study

Feed composition cycling as a transient kinetic technique provides detailed information about elementary steps in CO oxidation over a nanostructured Cu_0.1Ce_0.9O_2–y catalyst. This catalyst has a great potential as a future PROX reactor catalyst since it has great oxygen storage capacity as well as high reoxidation rate under oxygen rich conditions.     

CO oxidation over CuOx-CeO2-ZrO2 catalysts: Transient behaviour and role of copper clusters in contact with ceria

The effects of ZrO₂ content on the CO oxidation activity in a series of CuO_x/Ce_xZr_1−xO₂ (x = 0, 0.15, 0.5, 0.7 and 1) catalysts were investigated, both in the absence and in the presence of H₂, i.e. preferential CO oxidation—PROX. The investigation was performed under light-off conditions to focus the effects of transients and shut-down/start-up cycles on the performance; such phenomena are expected to affect the activity of PROX catalysts in small/delocalised fuel reformers. Evidence has been obtained for a transition from an “oxidized” towards a “reduced” state of the catalyst under the simulated PROX reaction conditions as a function of the reaction temperature, leading to different active species under the reaction conditions. Both CO oxidation activity and PROX selectivity appear to be affected by this process. IR characterisation of the surface copper species suggests an important role of reduced cerium sites in close contact with copper clusters on the CO oxidation activity at low temperatures.     

Artificial neural network aided virtual screening of additives to a Co/SrCO3 catalyst for preferential oxidation of CO in excess hydrogen

Preferential oxidation (PROX) of 0.7–1 vol% CO was investigated using the stoichiometric amount of O₂ in excess hydrogen. Cobalt supported on SrCO₃ showed high selectivity to PROX of CO, and the new additive to the Co/SrCO₃ catalyst was investigated for the high tolerance towards CO₂ and H₂O. Representative 10 elements (B, K, Sc, Mn, Zn, Nb, Ag, Nd, Re, and Tl) were selected to represent the physicochemical properties of all elements suitable for additives of solid catalyst. A supported cobalt catalyst with one kind of the above additive was prepared for CO PROX reaction. The activities at 240 °C and the physicochemical properties of the 10 elements were used as training data of a radial basis function network (RBFN), a kind of artificial neural network. After the training, the RBFN predicted the catalytic performance of the supported catalyst containing various element X as Co–X/SrCO₃. The elements such as Bi, Ga, and In were predicted to be promising additives. Finally, the catalytic performance of these additives was experimentally verified. Sixty four percent of CO conversion and 70% selectivity for PROX at 240 °C was achieved in the presence of excess carbon dioxide and steam by Co 3.2–Bi 0.3 mol%/SrCO₃ pretreated at 345 °C.     

H2O-tolerant monolithic catalysts for preferential oxidation of carbon monoxide in the presence of hydrogen

Pt–Fe/mordenite (4 wt% Pt–0.5 wt% Fe) powder catalysts were wash-coated onto ceramic straight-channel monoliths by using silica- and/or alumina-sol as a binder, and were evaluated for the preferential oxidation of carbon monoxide (PROX) in a hydrogen-rich gas. In a synthetic reformate gas (1% CO, 1% O₂, 5% H₂O, 20% CO₂, and balance H₂), the CO concentration was reduced to less than 20 ppm at temperatures ranging from 100 to 130 °C. After a certain period of the PROX reaction, condensation of H₂O in the pores of the mordenite-support occurred over the monolithic catalyst, which was wash-coated with alumina-sol, in the lower temperature range (100–120 °C), resulting in a rapid increase in CO concentration. The monolithic catalyst wash-coated with silica-sol, however, showed an excellent tolerance against H₂O condensation and offered a stable catalytic performance, maintaining a CO concentration of ca. 20 ppm for 200 h. The H₂O-tolerant characteristic was attributed to the relatively small adsorption amount of H₂O over the silica-modified monolithic catalyst.     

Combinatorial Design of Al2O3 Supported Au Catalysts For Preferential CO Oxidation

Multi-component Au/Al₂O₃ catalysts were designed and tested for PROX reaction using holographic research strategy. On the bases of our previous study Pb has been selected as the main modifier of the Au. In addition to Au and Pb the catalysts library contained V, Ba, Ce, Sm, Ag and Cu resulting in multi component catalysts tailored for PROX reaction. After preparation and testing of 173 catalysts within five generations new catalyst compositions with excellent performance have been obtained. Upon using the best catalyst CO could be removed almost completely and the selectivity of oxygen towards the CO oxidation was around 75%. In the course of catalyst library design it has been revealed that the selection of the objective function (OF) has high impact both on the rate of optimization and the performance of catalysts designed. The complex OF was created from two single desirability functions related to CO conversion and oxygen selectivity towards CO oxidation. In order to maintain high optimization rate there was a need to change the weights of single desirability functions in the course of catalyst library design. The results show that Pb, Sm, Cu and Ag are the key modifiers for PROX reaction under experimental conditions applied.     

Low-level CO in hydrogen-rich gas supplied by a methanol processor for PEMFCs

The present study developed a low-CO methanol processor for the online supply of hydrogen to a proton exchange membrane fuel cell (PEMFC) composed of a steam reformer, a catalytic combustor and a reactor for the removal of CO. Commercial Cu/ZnO/Al₂O₃- and Pt/Al₂O₃-based catalysts were used in the methanol steam reforming and the preferential oxidation (PROX) reactor, respectively. The steam reformer was successfully heated with a catalytic combustor at room temperature without any additional electrical power supply. Hydrogen gas was obtained at a flow rate of 43.0 L h⁻¹ using a feed flow rate of 39.5 ml h⁻¹ (S/C=1.1) and an operation temperature of 250 °C, corresponding to a power output of 59 W_e. The CO concentration could be maintained at 4–5 ppm for stable operation.     

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.     

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.     

The kinetics of methanol oxidation on a supported Pd model catalyst: molecular beam and TR-IRAS experiments

Combining a multi-molecular-beam approach and in situ time-resolved IR reflection absorption spectroscopy (TR-IRAS), we investigate the kinetics of methanol oxidation on a well-defined supported Pd model catalyst. The model catalyst is prepared under ultra-high-vacuum (UHV) conditions by Pd deposition onto a well-ordered Al₂O₃ film grown on NiAl (110). In previous studies, this system has been characterized in detail with respect to its geometric and electronic structure and its adsorption properties. Crossing molecular beams of methanol and oxygen on the sample surface, we systematically probe the rate of total methanol oxidation to CO₂ as a function of surface temperature and reactant fluxes. The results are compared with equivalent experiments for the related CO oxidation reaction. Pronounced differences are observed in the kinetics of the two processes, both under steady state and under transient conditions. The dissimilarities can be related to the dehydrogenation step of methanol, which is found to be strongly inhibited at high oxygen coverage. At low oxygen fluxes, CO is formed as the main product of methanol decomposition. Via a three-beam isotope-exchange experiment combined with TR-IRAS, the kinetics of CO formation is investigated as a function of reactant fluxes and surface temperature. Mean-field simulations of the kinetics are performed in a two-step procedure. First, the kinetics of CO oxidation is described, both under steady state and transient conditions. In a second step the microkinetic model is extended to include the formation of CO formed by methanol dehydrogenation. A comparison with the experimental data indicates that the transient kinetics cannot be fully described by a mean-field approach.     

The development of a fully integrated micro-channel fuel processor using low temperature co-fired ceramic (LTCC)

A fully integrated micro-channel fuel processor system consisting of vaporizer, steam reformer, heat exchanger and preferential CO oxidation (PROX) was developed using low temperature co-fired ceramic (LTCC). To fabricate a compact all-in-one system, each substrate was stacked to build a multilayered type fuel processor. A CuO/ZnO/Al₂O₃ catalyst and Pt-based catalyst prepared by wet impregnation were deposited inside the micro-channel of steam reformer and PROX, respectively. The performance of the fully integrated micro-channel reformer was measured at various conditions such as the ratio of the feed flow rate, the ratio of H₂O/CH₃OH and the operating temperature of the reactor. In parallel with the experiments, 3-D fluid dynamics simulation (Fluent) was conducted to verify the micro-reformer performance. The fully integrated micro-channel reformer has the dimensions of W: 130 mm × D: 50 mm × H: 3 mm. The fuel processor produced the gas composition of 71% H₂ and 25% CO₂, and more than 93% of methanol conversion was achieved at 300 °C and 2 cm³/h of the feed flow rate when CO concentration was maintained below 100 ppm by PROX.     

XAFS Characterization of Pt–Fe/zeolite Catalysts for Preferential Oxidation of CO in Hydrogen Fuel Gases

We have developed a new Pt–Fe/mordenite (Pt–Fe/M) catalyst which shows remarkably high activity and selectivity for the oxidation of CO in H₂-rich gas compared with Pt/M. In the present work, to understand the role and structure of Pt and Fe in the Pt–Fe/M catalyst, the states of metallic components in ion-exchanged, H₂ pre-treated and post-PROX (preferential oxidation of CO) samples have been studied by means of XAFS. It was confirmed that Pt forms the metallic clusters after H₂ pretreatment or the PROX experiment, whereas a large part of Fe exists as oxides even after the H₂ treatment. At post-analysis of the catalysts used for the PROX experiment, an increase in coordination number of Fe–O was observed. Pt clusters in the Pt–Fe(2:1 weight ratio)/M catalyst, which showed the highest PROX performance, were found to have a different electronic structure from the other catalysts. Additionally, preferential CO adsorption onto Pt sites at Pt–Fe/M was clearly demonstrated by infrared spectroscopy analysis in a stream of 1% CO containing H₂. Based on these results, the superior PROX mechanism was discussed.     

Performance of Pt-Fe/mordenite monolithic catalysts for preferential oxidation of carbon monoxide in a reformate gas for PEFCs

Pt-Fe/mordenite catalysts coated on ceramic straight-channel monoliths were evaluated for the preferential oxidation of carbon monoxide (PROX) in hydrogen-rich gas streams. In a feed gas containing 1% CO, 1% O₂, with the balance H₂, CO conversion reached almost 100% at temperatures ranging from 100 to 130 °C, i.e., an outlet CO concentration of less than 10 ppm. Even in a synthetic reformate gas (1% CO, 1% O₂, 15% H₂O, 20% CO₂, balance H₂), the monolithic catalyst exhibited excellent activity, reducing the CO concentration to less than 100 ppm. In particular, under optimized conditions, an outlet CO concentration of less than 10 ppm was realized. This is the first report that has demonstrated that monolithic catalysts could achieve the 10-ppm target level at a low O₂/CO ratio and a high space velocity in a single-stage reactor. Excellent durability of the monolithic catalyst is expected, based on a lack of deterioration in performance during 500 h of operation.     

Production of H2 for fuel cell applications: methanol steam reforming with sufficiently thorough cleaning of H2 from CO impurity

Methanol steam reforming (MSR) and preferential CO oxidation (PROX) were studied with the view of improving the generation of H₂-rich gases. In MSR, conventional catalysts of methanol synthesis were tested, various Cu-based catalysts were prepared and studied. A theoretic kinetic model (based on the reaction mechanism established using independent methods [1]) is developed and checked out. PROX was studied over various Ru/Al₂O₃ catalysts using a flow “quasi-adiabatic” reactor. On-line recording of gas temperature in the catalyst bed and CO residual concentration at varied reaction conditions allowed to observe ignition and extinction of the catalyst surface and the transition states of the process. It is shown that in the ignition mode a sharp decrease in CO residual concentration can be achieved. The combination of proposed catalyst and the control of the macrokinetic regime of PROX allows high degree of CO removal from gaseous mixtures produced by MSR. Residual CO content in a H₂-rich gaseous mixture can be lowered to < 15 ppm at GHSV∼100 m³/(kg cat)/h and O₂/CO ratio of 1. Obtained data show the possibility of designing a high-throughput set-up for generation of H₂-rich gases from methanol with one-step cleaning from the CO impurity.     

Power Generation from Biogas using SOFC – Results for a 1 kWe Demonstration Unit

A stand‐alone system for power generation from biogas‐based on a commercial SOFC module in the 1 kW_e range shall demonstrate its applicability to biogas, quantify the efficiency gain compared to conventional combined heat and power technology and justify further development toward SOFC modules in the hundreds of kilowatt range. The system includes biogas cleaning, combined dry and steam reforming, electrochemical oxidation of synthesis gas, offgas burning, and heat usage for steam generation and support of the endothermic reforming reaction. The system demonstrated a performance of 1 kW_e at 52% gross efficiency for a synthetic biogas containing 55 vol.% CH₄ during 500 h in the lab. In addition, the performance using real biogas derived from the wastewater treatment process of a sugar plant was demonstrated for different operating points. Based on the experimentally validated results, it is possible to predict the benefit of operating larger SOFC biogas systems. Investment costs of 2.5 times compared to the conventional technology of a 75 kW_e biogas unit get paid off due to higher electricity revenues over time.     

Fuel and emissions properties of Stirling engine operated with wood powder

From viewpoints of the environment and fuel cost reduction, small-scale biomass combined heat and power (CHP) plants are in demand, especially wood-waste fueled system, which are simple to operate and maintenance-free with high thermal efficiency similar to oil fired units. These are requested by wood and other industries located in mountainous region. To meet these requirements, a Stirling engine CHP system combined with simplified biomass combustion process with pulverized wood powder was developed.In an R&D project started in 2004 considering wood powder properties as a fuel, combustion performance and emissions in combustion flue gas were tested using combustion test apparatus with commercial size units. The wood powder combustion system was modified and optimized during the combustion test results, and the design of the demonstration plant combined with 55 kW_e Stirling engine power unit was considered. The demonstration plant was finally completed in March of 2006, and test operation has been progressed for the future commercial CHP system.In the wood powder combustion test, wood powder of less than 500 μm is mainly used, and a combustion chamber length of 3 m is applied. In these conditions, the air ratio can be reduced to 1.1 without increasing CO emission of less than 10 ppm and combustion efficiency of 99.9%. In the same conditions, NO_x emission is estimated to be less than 120 ppm (6% O₂ basis). Wood powder was confirmed to have excellent properties as a fuel for Stirling engine CHP system. This paper summarizes the wood powder combustion test, and presents the evaluation of the burner design parameters for the biomass Stirling engine system.     

Compact Reforming Process Using High-Performance and Long-Lived CO Preferential Oxidation over an Activated Ru/Al2O3 Catalyst for PEFC

Compact natural gas reforming process using high-performance and long-lived CO preferential oxidation (PROX) over an activated Ru/Al₂O₃ catalyst has been developed for residential polymer electrolyte fuel cell (PEFC) systems. The long-term durability of the catalyst was demonstrated for more than 40,000 h. After 40,000 h operation, CO was removed from a reformed gas to below 1 ppm on the activated Ru/Al₂O₃ catalyst at [O₂]/[CO] = 1.5. The high activity and selectivity of the catalyst were maintained for more than 40,000 h. Moreover, the start–stop durability for more than 3,000 cycles of the activated Ru/Al₂O₃ catalyst was also demonstrated without N₂ purge.