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     

CO tolerance of PdPt/C and PdPtRu/C anodes for PEMFC

The performance of H₂/O₂ proton exchange membrane fuel cells (PEMFCs) fed with CO-contaminated hydrogen was investigated for anodes with PdPt/C and PdPtRu/C electrocatalysts. The physicochemical properties of the catalysts were characterized by energy dispersive X-ray (EDX) analyses, X-ray diffraction (XRD) and “in situ” X-ray absorption near edge structure (XANES). Experiments were conducted in electrochemical half and single cells by cyclic voltammetry (CV) and I-V polarization measurements, while DEMS was employed to verify the formation of CO₂ at the PEMFC anode outlet. A quite high performance was achieved for the PEMFC fed with H₂ + 100 ppm CO with the PdPt/C and PdPtRu/C anodes containing 0.4 mg metal cm⁻², with the cell presenting potential losses below 200 mV at 1 A cm⁻², with respect to the system fed with pure H₂. For the PdPt/C catalysts no CO₂ formation was seen at the PEMFC anode outlet, indicating that the CO tolerance is improved due to the existence of more free surface sites for H₂ electrooxidation, probably due to a lower Pd-CO interaction compared to pure Pd or Pt. For PdPtRu/C the CO tolerance may also have a contribution from the bifunctional mechanism, as shown by the presence of CO₂ in the PEMFC anode outlet.     

CO tolerance effects of tungsten-based PEMFC anodes

The performance of proton exchange membrane fuel cells (PEMFC) fed with CO-contaminated hydrogen was investigated for anodes with PtWO_x/C and phosphotungstic acid (PTA) impregnated Pt/C electrocatalysts. A quite high performance was achieved for the PEMFC fed with H₂ + 100 ppm CO with anodes containing 0.4 mg PtWO_x cm⁻² and also for those with 0.4 mg Pt cm⁻² impregnated with ca. 1 mg PTA cm⁻². A decay of the single cell performance with time is observed, and this was attributed to an increase of the membrane resistance due to the polymer degradation promoted by the crossover of the tungsten species throughout the membrane.     

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.     

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.     

Novel layer wise anode structure with improved CO-tolerance capability for PEM fuel cell

This study proposes a novel layer wise anode structure to improve the CO-tolerance ability and utilization efficiency of catalyst. The layer wise structure consists of an outer and an inner catalyst layer. The outer catalyst layer acting as a CO barrier is composed of two nano-Ru layers (0.06 mg cm⁻²) by magnetron sputtering deposition method and a Pt₅₀-Ru₅₀ layer (0.10 mg cm⁻²) by screen-printing method on the GDL. The inner catalyst layer providing the hydrogen oxidation reaction is a pure Pt layer (0.07 mg cm⁻²) prepared by direct-printing method on PEM. The roles of the outer and inner catalyst layer relating to the improvement of CO-tolerance ability and utilization efficiency of catalyst for the proposed catalyst layer structure are investigated in this paper. SEM, X-ray, EDS and EPMA analysis were used to characterize microstructures, phases, chemical composition and distributions for the obtained electrocatalyst layers. The hydrogen fuel containing 50 ppm CO/hydrogen fuel containing 50 ppm CO + 2% O₂ is continuously fed to the anode side to investigate the dependence of CO-tolerance ability over time for the MEAs, respectively. The results demonstrate that this proposed anode catalyst layer structure presents a superior CO-tolerance ability and performance to those of conventional and Huag's structures in both oxygen free and oxygen present CO containing hydrogen fuels as well as pure hydrogen fuel. The filtering effect of the outer catalyst layer causes the improved CO-tolerance capability.     

The inclusion of Mo, Nb and Ta in Pt and PtRu carbon supported electrocatalysts in the quest for improved CO tolerant PEMFC anodes

The effect of the inclusion of Mo, Nb and Ta in Pt and PtRu carbon supported anode electrocatalysts on CO tolerance in proton exchange membrane fuel cells (PEMFC) has been investigated by cyclic voltammetry and fuel cell tests. CO stripping voltammetry on binary Pt_xM/C (M: Mo, Nb, Ta) reveals partial oxidation of the CO adlayer at low potential, with PtMo (4:1)/C exhibiting the lowest value. At 80 °C, the operating temperature of the fuel cell, CO oxidation was observed at potentials close to 0 V versus the reversible hydrogen electrode (RHE). No significant difference for CO electro-oxidation at the lower potential limit, compared to PtRu/C, was observed for PtRuM_y/C (M: Mo, Nb). Fuel cell tests demonstrated that while all the prepared catalysts exhibited enhanced performance compared to Pt/C, only the addition of a relatively small amount of Mo to PtRu results in an electrocatalyst with a higher activity, in the presence of carbon monoxide, to PtRu/C, the current catalyst of choice for PEM fuel cell applications.     

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.     

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.     

Pt-Ru catalysts prepared by high energy ball-milling for PEMFC and DMFC: Influence of the synthesis conditions

High energy ball-milling was used to prepare several unsupported Pt-Ru anode catalysts for PEM- and direct methanol fuel cells. Pt and Ru with a 50:50 nominal Pt/Ru ratio were ball-milled at various ball-to-powder weight ratios (from 4/1 to 12/1) and with various Pt:Ru:MgH₂ proportions (from 1:1:2 to 1:1:10), where MgH₂ is a leacheable dispersive agent. The presence of MgH₂ is necessary to obtain unsupported catalysts with a specific surface area of between 50 and 75 m² g⁻¹. The ball-milling parameters greatly affected the relative proportions of the three phases constituting the catalysts. These phases are: Pt(Ru) alloy nanocrystallites, unalloyed Ru crystallites and nanocrystallites. The best CO tolerant catalyst is obtained by using a 12/1 ball-to-powder ratio and a 1:1:8 Pt:Ru:MgH₂ proportion of dispersive agent. It is made of 57 at.% of a nanocrystalline (3 nm) Pt₈₀Ru₂₀ alloy, 42 at.% of a nanocrystalline (3 nm) Ru phase and 1 at.% of a crystalline (∼40 nm) Ru phase. This catalyst has the lowest Pt/Ru surface ratio (0.9), the highest content in nanocrystalline Ru, and the highest ratio of oxidized/metallic Ru (3.3). Both Pt-Ru alloy and nanocrystalline Ru participate to the CO tolerance. The best CO tolerant catalyst is, however, not the best catalyst in DMFC. The latter is obtained by using a 4/1 ball-to-powder ratio and a 1:1:6 Pt:Ru:MgH₂ proportion. Within the starting 50:50 Pt-Ru nominal atomic ratio, no specific correlation was found between catalyst performance in DMFC and atomic surface Pt/Ru ratio, nor nanocrystalline Ru content, nor oxidized/metallic Ru ratio. Performances of the best ball-milled catalysts are compared to those of commercial unsupported catalysts in PEMFC and DMFC.     

CO tolerance and CO oxidation at Pt and Pt-Ru anode catalysts in fuel cell with polybenzimidazole-H3PO4 membrane

CO tolerance of H₂-air single cell with phosphoric acid doped polybenzidazole (PA-PBI) membrane was studied in the temperature range 140-180 °C using either dry or humidified fuel. Fuel composition was varied from neat hydrogen to 67% (vol.) H₂-33% CO mixtures. It was found that poisoning by CO of Pt/C and Pt-Ru/C hydrogen oxidation catalysts is mitigated by fuel humidification. Electrochemical hydrogen oxidation at Pt/C and Pt-Ru/C catalysts in the presence of up to 50% CO in dry or humidified H₂-CO mixtures was studied in a cell driven mode at 180 °C. High CO tolerance of Pt/C and Pt-Ru/C catalysts in FC with PA-PBI membrane at 180 °C can be ascribed to combined action of two factors—reduced energy of CO adsorption at high temperature and removal of adsorbed CO from the catalyst surface by oxidation. Rate of electrochemical CO oxidation at Pt/C and Pt-Ru/C catalysts was measured in a cell driven mode in the temperature range 120-180 °C. Electrochemical CO oxidation might proceed via one of the reaction paths—direct electrochemical CO oxidation and water-gas shift reaction at the catalyst surface followed by electrochemical hydrogen oxidation stage. Steady state CO oxidation at Pt-Ru/C catalyst was demonstrated using CO-air single cell with Pt-Ru/C anode. At 180 °C maximum CO-air single cell power density was 17 mW cm⁻² at cell voltage U = 0.18 V.     

Synthesis, characterization, and hydrotreating activity of carbon-supported transition metal phosphides

A series of nickel, molybdenum, and tungsten metal phosphides deposited on a carbon black support (Ni₂P/C, MoP/C, and WP/C) were synthesized by means of temperature-programmed reduction. The samples were characterized by BET surface area, CO uptake, X-ray diffraction (XRD), elemental analysis, and extended X-ray absorption fine structure (EXAFS) measurements. The activity of these catalysts was measured at 613 K and 3.1 MPa in a three-phase, packed-bed reactor for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) with a model liquid feed containing 500 ppm sulfur as 4,6-dimethyldibenzothiophene (4,6-DMDBT), 3000 ppm sulfur as dimethyl disulfide, and 200 ppm nitrogen as quinoline. The Ni₂P/C catalyst was found to exhibit the best hydroprocessing performance based on equal CO chemisorption sites (70 μmol) loaded in the reactor. An optimum Ni loading for HDS and HDN activity was found as 1.656 mmol g⁻¹ (11.0 wt.% Ni₂P) which gave an HDS conversion of 99% and an HDN conversion of 100% at a molar space velocity of 0.88 h⁻¹. These were much higher than those of a commercial Ni-Mo-S/γ-Al₂O₃ catalyst which gave an HDS conversion of 68% and an HDN conversion of 94%, and a previously reported best Ni₂P/SiO₂ catalyst which gave an HDS conversion of 76% and an HDN conversion of 92%. The use of carbon instead of silica as a support gave rise to other differences, which included smaller particle size, higher CO uptake, lessened retention of P on the support, and reduced sulfur deposition. The stability of the 11.0 wt.% Ni₂P/C catalyst was also excellent with no deactivation observed over 110 h of time on stream. The activity and stability of the Ni₂P/C catalyst were affected by the phosphorous content, both reaching a maximum with an initial Ni/P ratio of 1/2. EXAFS and elemental analysis of the spent samples indicated the formation of a surface phosphosulfide phase on the Ni₂P, which was beneficial for hydrotreating activity, while the bulk structure of the phosphides was maintained during the course of reaction as revealed from the XRD patterns.     

Insights on the SO2 poisoning of Pt3Co/VC and Pt/VC fuel cell catalysts

SO₂ poisoning of carbon-supported Pt₃Co (Pt₃Co/VC) catalyst is performed at the cathode of proton exchange membrane fuel cells (PEMFCs) in order to link previously reported results at the electrode/solution interface to the FC environment.First, the surface area of Pt₃Co/VC catalyst is rigorously characterized by hydrogen adsorption, CO stripping voltammetry and underpotential deposition (upd) of copper adatoms. Then the performance of PEMFC cathodes employing 30 wt.% Pt₃Co/VC and 50 wt.% Pt/VC catalysts is compared after exposure to 1 ppm SO₂ in air for 3 h at constant cell voltage of 0.6 V. In agreement with results reported for the electrode/solution interface, the Pt₃Co/VC is more susceptive to SO₂ poisoning than Pt/VC at a given platinum loading.Both catalysts can be recovered from adsorbed sulfur species by running successive polarization curves in air or cyclic voltammetry (CV) in inert atmosphere. However, the activity of Pt₃Co/VC having ∼3 times higher sulfur coverage is recovered more easily than Pt/VC. To understand the difference between the two catalysts in terms of activity recovery, platinum-sulfur interaction is probed by thermal programmed desorption at the catalyst/inert gas interface and CV at the electrode/solution interface and in the FC environment.     

CoPdx oxygen reduction electrocatalysts for polymer electrolyte membrane and direct methanol fuel cells

The electrochemical activity of carbon-supported cobalt-palladium alloy electrocatalysts of various compositions have been investigated for the oxygen reduction reaction in a 5 cm² single cell polymer electrolyte membrane fuel cell. The polarization experiments have been conducted at various temperatures between 30 and 60 °C and the reduction performance compared with data from a commercial Pt catalyst under identical conditions. Investigation of the catalytic activity of the CoPd_x PEMFC system with varying composition reveals that a nominal cobalt-palladium atomic ratio of 1:3, CoPd₃, exhibits the best performance of all studied catalysts, exhibiting a catalytic activity comparable to the commercial Pt catalyst. The ORR on CoPd₃ has a low activation energy, 52 kJ/mol, and a Tafel slope of approximately 60 mV/decade, indicating that the rate-determining step is a chemical step following the first electron transfer step and may involve the breaking of the oxygen bond. The CoPd₃ catalyst also exhibits excellent chemical stability, with the open circuit cell voltage decreasing by only 3% and the observed current decreasing by only 10% at 0.8 V over 25 h. The CoPd₃ catalyst also exhibits superior tolerance to methanol crossover poisoning than Pt.     

Performance of CO preferential oxidation reactor with noble-metal catalyst coated on ceramic monolith for on-board fuel processing applications

On-board fuel processors are being developed to provide hydrogen-rich gas to the polymer electrolyte fuel cell automotive propulsion systems. Whereas the anode catalyst in the fuel cell has low tolerance for carbon monoxide, 10–100 ppm, reforming of gasoline and other hydrocarbon fuels generally produces 1–2% of CO. Of the many methods of removing CO from the reformer gas, preferential oxidation (PrOx) of CO over noble-metal catalysts is practiced most frequently. In this paper, we present experimental data for CO conversion on a Pt-based catalyst that is active at room temperature and was coated on a ceramic monolith. The data is used to develop an empirical correlation for selectivity for CO oxidation as a function of CO concentration and oxygen stoichiometry at 30,000–80,000/h space velocity. The selectivity correlation is used in a model to analyze the performance of multi-stage, adiabatic PrOx reactors with heat exchange between the stages to cool the reformate to 100 °C. An optimization algorithm is used to determine the operating conditions that can reduce CO concentration to 10 ppm while minimizing parasitic loss of H₂ in the reformate stream. It is found that the 10 ppm constraint limits the maximum inlet CO concentration to 1.05% in a single-stage reactor and to 3.1% in a two-stage reactor. The results clearly show the incremental reduction in parasitic H₂ loss by addition of second and third stages.     

Cyclic operation of porous combustor-heater (PCH)

A porous combustor-heater (PCH) with cyclic flow reversal combustion (CFRC) was investigated on the basis of simulation for practical applications. Emphasis was placed on determination of the optimal switching period under which the system will offer a high thermal efficiency with low emissions. It was shown that by using the CFRC thermal performance, emissions are strongly affected by the switching period through the interaction of the hot zone movement within the reactor and by heat transfer to the tube bank. An optimal switching period of about 120-180 s was found with maximum thermal efficiency to be as high as 85% with low emissions of CO and NO_x of 200 and 20 ppm, respectively. The CFRC system yields higher thermal efficiency with higher combustion efficiencies than the unidirectional steady state regime at the same experimental conditions.     

Integration of magnetically stabilized bed and amorphous nickel alloy catalyst for CO methanation

A novel process for the methanation of CO, involving an amorphous nickel alloy (ANA) catalyst incorporated in a magnetically stabilized bed (MSB), has been developed. The effects of varying the operating conditions on the reaction yield and catalyst lifetime have been investigated. The new process was shown to be markedly superior to existing processes for CO methanation in that the concentration of CO in an H₂-rich gas mixture could be reduced from 2000 ppm to less than 1 ppm at a temperature of 433 K, a pressure of 3.0 MPa, a gas hourly space velocity of , and a magnetic field intensity of 40 kA/m. Compared with conventional fixed bed processes, the operating temperature of the ANA/MSB process can be decreased by about 100 K and the gas velocity can be increased by a factor of 5-10. In durability tests, the ANA catalyst does not undergo any significant structural change and retains its initial high activity. These preliminary results indicate that the novel process has good prospects for application in the petrochemical industry.     

CO tolerance on PtMo/C electrocatalysts prepared by the formic acid method

In this work the activity of PtMo/C based materials prepared by the formic acid method was evaluated as electrocatalysts for the hydrogen oxidation reaction in the presence of CO, in polymer electrolyte fuel cells. A very high electrocatalytic activity was observed for an anode formed by PtMo/C (60:40) and supplied with H₂ containing 100 ppm of CO, which presented an overpotential loss of 100 mV at 1 A cm⁻², compared with pure hydrogen. Several electrode configurations based on Mo/C and PtMo/C CO filtering layers, having a Pt-based catalyst layer, were evaluated. In all cases an enhancement of electrocatalytic performance was observed, as compared with the standard Pt/C electrode. It is concluded that the CO tolerance is achieved through an electrochemical surface reaction of adsorbed CO with surface oxides, as proposed by the bifunctional mechanism, acting together with a heterogeneous chemical reaction of CO with water molecules catalyzed by Mo species and resulting in a lowering of the CO concentration in the gas channels of the electrode.     

Nanostructured Pt-M/C (M = Fe and Co) catalysts prepared by a microemulsion method for oxygen reduction in proton exchange membrane fuel cells

Nanostructured Pt-M/C (M = Fe and Co) catalysts have been synthesized by a microemulsion method and a high-temperature route. They have been characterized by cyclic voltammetry in 1 M H₂SO₄ and for oxygen reduction in proton exchange membrane fuel cells (PEMFC). The Pt-M alloy catalysts synthesized by the microemulsion method show higher electrochemical active surface area than those prepared by the high-temperature route, and some of them exhibit improved catalytic activity towards oxygen reduction compared to pure Pt. Among the various alloy catalysts investigated, the Pt-Co/C catalyst prepared by the microemulsion method shows the best performance with the maximum catalytic activity and minimum polarization loss. Mild heat treatment of the catalysts prepared by the microemulsion method at moderate temperatures (200 °C) in reducing atmosphere is found to improve the catalytic activity due to a cleaning of the surface and an increase in the electrochemical surface area.     

A novel nickel/carbon catalyst for CH4 and H2 production from organic compounds dissolved in wastewater by catalytic hydrothermal gasification

A novel Ni/carbon catalyst recently developed by the authors was used to gasify organic compounds dissolved in the wastewater with TOC concentration from 0.2 to 2%. The process removes the organic compounds by gasifying them into high calorific gases like methane and hydrogen. The investigations were focused on the efficiency of the Ni/carbon catalyst in terms of carbon conversion, conversion of big organic molecules, and catalyst deactivation due to sintering. The preliminary results showed that up to 99% carbon conversion can be achieved at 360 °C, and 20 MPa. A conversion mechanism was suggested which consists of: first, decomposition of big molecules to small molecules on the metal surface, steam gasification of small molecules to produce CO and H₂ followed by CO methanation and CO shift reaction to produce CH₄ and CO₂. The catalyst was found to be highly active and stable and no sintering was observed even after 100 h of reaction time.