Preparation of CO-tolerant anode electrocatalysts for polymer electrolyte membrane fuel cells

The preparation and the thorough characterization of 40 wt% Pt electrocatalysts supported on Ti_(1?x)M_xO₂-C (M = W, Mo; x = 0.3–0.4) composite materials with enhanced stability and efficiency is presented.W-containing composite supported catalyst with different structural characteristics were compared in order to explore the influence of the nature of the W species on the electrocatalytic performance. The assessment of the electrochemical properties of the novel catalysts revealed a correlation between the degree of W incorporation, the hydrogen spillover effect and the stability against initial leaching which influences the activity and CO tolerance of the catalysts.A preparation route for Ti_0.7Mo_0.3O₂-C composite with high extent of Mo incorporation was developed. No significant difference was observed in the activity, stability and CO tolerance of the W- or Mo-containing composite supported Pt catalysts with almost complete incorporation of the oxophilic dopant. Better performance of the Pt/Ti_0.7M_0.3O₂-C (M = W, Mo) electrocatalysts in a single cell test device using hydrogen containing 100 ppm CO compared to the reference Pt/C and PtRu/C (Quintech) catalysts was also demonstrated.     

Pt-based electrocatalysts for polymer electrolyte membrane fuel cells prepared by supercritical deposition technique

Pt-based electrocatalysts were prepared on different carbon supports which are multiwall carbon nanotubes (MWCNTs), Vulcan XC 72R (VXR) and black pearl 2000 (BP2000) using a supercritical carbon dioxide (scCO₂) deposition technique. These catalysts were characterized by using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and cyclic voltammetry (CV). XRD and HRTEM results demonstrated that the scCO₂ deposition technique enables a high surface area metal phase to be deposited, with the size of the Pt particles ranging from 1 to 2 nm. The electrochemical surface areas (ESAs) of the prepared electrocatalysts were compared to the surface areas of commercial ETEK Pt/C (10 wt% Pt) and Tanaka Pt/C (46.5 wt% Pt) catalysts. The CV data indicate that the ESAs of the prepared Pt/VXR and Pt/MWCNT catalysts are about three times larger than that of the commercial ETEK catalyst for similar (10 wt% Pt) loadings. Oxygen reduction activity was investigated by hydrodynamic voltammetry. From the slope of Koutecky–Levich plots, the average number of electrons transferred in the oxygen reduction reaction (ORR) was 3.5, 3.6 and 3.7 for Pt/BP2000, Pt/VXR and Pt/MWCNT, correspondingly, which indicated almost complete reduction of oxygen to water.     

A novel proton exchange membrane fuel cell anode for enhancing CO tolerance

A novel proton exchange membrane fuel cell (PEMFC) anode which can facilitate the CO oxidation by air bleeding and reduce the direct combustion of hydrogen with oxygen within the electrode is described. This novel anode consists of placing Pt or Au particles in the diffusion layer which is called Pt- or Au-refined diffusion layer. Thus, the chemical oxidation of CO occurs at Pt or Au particles before it reaches the electrochemical catalyst layer when trace amount of oxygen is injected into the anode. All membrane electrode assemblies (MEAs) composed of Pt- or Au-refined diffusion layer do perform better than the traditionary MEA when 100 ppm CO/H₂ and 2% air are fed and have the performance as excellent as the traditionary MEA with neat hydrogen. Furthermore, CO tolerance of the MEAs composed of Au-refined diffusion layer was also assessed without oxygen injection. When 100 ppm CO/H₂ is fed, MEAs composed of Au-refined diffusion layer have the slightly better performance than traditionary MEA do because Au particles in the diffusion layer have activity in the water gas shift (WGS) reaction at low temperature.     

Low-cost graphite as durable support for Pt-based cathode electrocatalysts for proton exchange membrane based fuel cells

Proton exchange membrane fuel cell (PEMFC) technology has reached pre-commercial viability, but their insufficient durability acts as a major roadblock in its full-fledged utilization. It has been well established that the issue of durability is majorly due to the corrosion of carbon support used for Pt. Therefore, a search for low-cost and robust alternative support is highly desirable. In this paper, different graphite (and graphene) materials as durable support for Pt-based electrocatalyst are investigated. We followed the top-down approach where a fully graphitized support is mildly wet-milled and surface-treated to give a sufficient surface modification for improved Pt deposition on these supports. All the graphite-supported Pt samples showed better durability than that of state-of-the-art commercial electrocatalysts. Considering both activity and durability the best catalyst among the investigated samples showed a comparable mass specific activity (MSA) of 0.186 A/mg and significantly higher durability (70%) after 7500 stress cycles. For HiSpec9100 and BASF commercial electrocatalysts, the normalized ESA retention value after 7500 stress cycles was 40% and 47%, respectively.     

Critical importance of humidification of the anode in miniature air-breathing polymer electrolyte membrane fuel cells

Although water management at the cathode is known to be critical in miniature polymer electrolyte membrane fuel cells (mPEMFCs), this study shows that control of water transport towards the anode is a determining factor to increase air-breathing mPEMFC performances. An analytical 1D model is developed to capture the water transport and water content profile in the membrane. It shows that drying at the anode and flooding at the cathode can happen simultaneously, mainly due to dominant electro-osmotic drag at low cell temperatures. Experimental results demonstrate that injecting water at the anode, at a rate of 3 times the amount produced at the cathode, increases the cell performances at high current densities. By this method, the limiting current and maximum power densities have been raised by 100% and 30% respectively.     

Particle size dependence of CO tolerance of anode PtRu catalysts for polymer electrolyte fuel cells

An anode catalyst for a polymer electrolyte fuel cell must be CO-tolerant, that is, it must have the function of hydrogen oxidation in the presence of CO, because hydrogen fuel gas generated by the steam reforming process of natural gas contains a small amount of CO. In the present study, PtRu/C catalysts were prepared with control of the degree of Pt-Ru alloying and the size of PtRu particles. This control has become possible by a new method of heat treatment at the final step in the preparation of catalysts. The CO tolerances of PtRu/C catalysts with the same degree of Pt-Ru alloying and with different average sizes of PtRu particles were thus compared. Polarization curves were obtained with pure H₂ and CO/H₂ (CO concentrations of 500-2040 ppm). It was found that the CO tolerance of highly dispersed PtRu/C (high dispersion (HD)) with small PtRu particles was much higher than that of poorly dispersed PtRu/C (low dispersion (LD)) with large metal particles. The CO tolerance of PtRu/C (HD) was higher than that of any commercial PtRu/C. The high CO tolerance of PtRu/C (HD) is thought to be due to efficient concerted functions of Pt, Ru, and their alloy.     

Performance of a poly(2,5-benzimidazole)-based polymer electrolyte membrane fuel cell

The performance of an ABPBI-based High Temperature H₂/O₂ PEMFC system was studied under different experimental conditions. Increasing the temperature from 130 to 170 °C improved the cell performance, even though further increase was not beneficial for the system. Humidification of the H₂ stream ameliorated this behaviour, even though operating above 170 °C is not advisable in terms of cell performance. A significant electrolyte dehydration seems to negatively affect the fuel cell performance, especially in the case of the anode. In the presence of 2% vol. CO in the H₂ stream, the temperature exerted a positive effect on the cell performance, reducing the strong adsorption of this poison on the platinum sites. Moreover, humidification of the H₂ + CO stream increased the maximum power densities of the cell, further alleviating the CO poisoning effects. Actual CO–O₂ fuel cell results confirmed the significant beneficial effect of the relative humidity on the kinetics of the CO oxidation process.     

CO tolerance and stability of PtRu and PtRuMo electrocatalysts supported on N-doped graphene nanoplatelets for polymer electrolyte membrane fuel cells

PtRu and PtRuMo electrocatalysts supported on N-doped graphene nanoplatelets (N-GNPs) were synthesized by a polyol method and utilized as anodes in polymer electrolyte membrane fuel cells (PEMFCs) to measure their CO tolerance and stability. A higher structural stability of the N-GNP supported catalysts, presenting a lower metal loss and a lower particle growth than PtRu/C was observed. Tests in PEMFCs indicated both a higher CO tolerance and a higher electrochemical stability of N-GNP supported PtRu and PtRuMo catalysts than a commercial conventional carbon black (CB) supported PtRu.     

Effect of polyoxometalate amount deposited on Pt/C electrocatalysts for CO tolerant electrooxidation of H2 in polymer electrolyte fuel cells

Polyoxometalate-deposited Pt/C electrocatalysts are prepared by impregnation with various amounts of polyoxometalate (POM) anions (from 2 to 16.7 wt.% PMo₁₂O₄₀^3–) on the Pt/C catalyst. The prepared electrocatalysts show a high CO electrooxidation performance over a half-cell system for CO stripping voltammetry, and CO tolerant electrooxidation of H₂ is further demonstrated over a proton exchange membrane fuel cell by using CO-containing H₂ gas feeds (0, 10, 50, and 100 ppm CO in H₂). In the CO stripping voltammograms, the onset and peak potentials for the CO oxidation appear to decrease as the POM deposition is increased, indicating that the electrooxidation of CO undergoes more efficiently on the catalyst surface with the deposited POMs on the Pt/C catalysts. In the single fuel cell tests with the CO-containing H₂ gases, the higher current density is also generated with the larger amounts of deposited POMs on the Pt/C catalysts. Importantly, the charge transfer resistance R_p appears to decrease monotonically with the POM amounts, which was measured by electrochemical impedance spectroscopy. Physico-chemical characterizations with electrocatalytic analyses show that the deposited POMs hardly affect the active phase of Pt catalyst itself but can help the electrooxidation of H₂ by efficiently oxidizing CO to prevent the Pt catalyst from poisoning. Consequently, this POM-deposited Pt/C catalyst can serve as a promising CO tolerant anode catalyst for the polymer electrolyte fuel cells that are operated with hydrocarbons-reformed H₂ fuel gases.     

Electrocatalytic reduction of dioxygen on PdCu for polymer electrolyte membrane fuel cells

The present research is aimed to study the oxygen reduction reaction (ORR) on a PdCu electrocatalyst synthesized through reduction of PdCl₂ and CuCl with NaBH₄ in a THF solution. Characterization of PdCu electrocatalyst was performed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) spectroscopy. Characterization results showed that the synthesis method produced spherical agglomerated nanocrystalline PdCu particles of about 10 nm size. The electrochemical activity was evaluated using cyclic voltammetry (CV), rotating disc electrode (RDE) and electrochemical impedance spectroscopy (EIS) in a 0.5 M H₂SO₄ electrolyte at 25 °C. The onset potential for ORR on PdCu is shifted by ca. 30 mV to more positive values and enhanced catalytic current densities were observed, compared to that of pure Pd catalyst. The synthesized PdCu electrocatalyst dispersed on a carbon black support was tested as cathode electrode in a membrane-electrode assembly (MEA) achieving a power density of 150 mW cm⁻² at 0.38 V and 80 °C.     

Platinum nanoparticles decorated carbon nanofiber hybrids as highly active electrocatalysts for polymer electrolyte membrane fuel cells

Increasing the efficiency of electrocatalyst is the key demand for the polymer electrolyte membrane fuel cells (PEMFC). To address the activity and performance challenges of commercial electrocatalyst, Pt/C, we introduce a new hybrid catalyst support for Pt nanoparticles. In this regard, combining or mixing specific type of carbon-based supports is a feasible strategy to increase catalyst utilization and performance. In the current study, Pt nanoparticles (NPs) were decorated on a new hybrid network, comprising of carbon nanofiber (CNF) and carbon black (CB), by means of a facile and efficient microwave (MW) assisted reduction method. All synthesized electrocatalysts were characterized to elucidate chemical and morphological structures. Then, the hybrid electrocatalysts were utilized as hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) electrocatalysts and their electrocatalytic activities were investigated by using cyclic voltammetry (CV) and linear sweep voltammetry (LSV), respectively. We found that the hybridization of CNF with CB substantially improved not only the electrocatalytic activity but also the fuel cell performance, which can be attributed to a consecutive conductive network, in which CB acts as a spacer, and synergistic effects between the CNF and CB. The hybrid electrocatalyst (Pt/CNF-CB with 50:50 wt%) showed a superior activity toward HOR and ORR while also offering exceptional fuel cell performance. That hybrid possessed the highest electrochemically active surface area (ECSA) compared with Pt/CNF and Pt/CB. In addition, the mass activity (at 0.80 V vs RHE) of the Pt/CNF-CB (50:50 wt%) is about 3.3 and 3.5 times higher than that of Pt/CNF and Pt/CB, respectively. Furthermore, that hybrid electrocatalyst exhibited enhanced fuel cell performance with 907 mW.cm⁻¹ maximum power density. This work demonstrated that the CNF-CB supported Pt nanoparticles as electrocatalysts are extremely promising for fuel cell reactions.     

Development of self-breathing polymer electrolyte membrane fuel cell stack with cylindrical cells

Self-breathing fuel cells use oxygen directly from the atmosphere and this eliminates the need of air fans and humidifiers, and as such enables applications where a reduction in size and volume of the power systems is important. Herein, we report a novel design and fabrication of a 12 celled cylindrical self-breathing stack made from stainless steel. A cylindrical geometry was implemented to permit higher self-breathing function and means to improve the Membrane Electrode Assembly compaction and thus reduce hydrogen leaks. The stack delivered a maximum voltage of 9.4 V and a maximum power of 1 W cm^?2 while operating under ambient condition (25 °C, 20% relative humidity and with dry hydrogen). The stainless steel bipolar plates provided enough compaction to the stack to minimize ohmic resistance and the hydrogen sealing was done with silicon gaskets which facilitated the stacking of the cells. The developed 12 celled stack delivered a volume power density of 29 W per litre which is 80% higher than previously reported self-breathing polymer electrolyte membrane fuel cells.     

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

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

Enhanced diffusion in polymer electrolyte membrane fuel cells using oscillating flow

This study investigates the enhancement of the oxygen diffusion rate at the cathode of a proton exchange membrane fuel cell (PEMFC) due to pure oscillating flow. A unit cell of PEMFC using hydrogen fuel and oscillating air was tested. The experimental results show that the non-dimensional effective diffusivity varies linearly with the square of the Womersley number, when the Womersley number is close to unity. The non-dimensional effective diffusivity varies linearly with the Womersley number itself when the Womersley number is much larger than unity. Similar trend has been confirmed from the theoretical approach. Under the experimental conditions in this study, the reaction rate of oxygen increased linearly with respect to the sweep distance. The experimental results showed that a power density of 115.4 mW/cm² was obtained from the unit cell with oscillating flow, which is comparable to that obtained with forced flow. Therefore, an oscillating flow is found to be able to increase the concentration of the oxygen in the channel of PEMFCs, and consequently enhances mass-transfer, similarly to the use of forced flow using blowers or compressors.     

Study of a porous membrane humidification method in polymer electrolyte fuel cells

A gas humidification sub-system that does not add to the parasitic power loss is advantageous for water management in PEMFC. A membrane humidifier was fabricated with porous membrane and the performance of the single cell using this humidifier has been evaluated. The study shows that the performance of the humidifier is comparable to that of the bubble humidifier. It was further found that the humidifier is suitable for both water and exhaust cathode air as the humidifying medium.     

High performance electrocatalysts supported on graphene based hybrids for polymer electrolyte membrane fuel cells

In this study, new electrocatalysts for PEM fuel cells, based on Pt nanoparticles supported on hybrid carbon support networks comprising reduced graphene oxide (rGO) and carbon black (CB) at varying ratios, were designed and prepared by means of a rapid and efficient microwave-assisted synthesis method. Resultant catalysts were characterized ex-situ for their structure, morphology, electrocatalytic activity. In addition, membrane-electrode assemblies (MEAs) fabricated using resultant electrocatalysts and evaluated in-situ for their fuel cell performance and impedance characteristics. TEM studies showed that Pt nanoparticles were homogeneously decorated on rGO and rGO-CB hybrids while they had bigger size and partially agglomerated distribution on CB. The electrocatalyst, supported on GO-CB hybrid containing 75% GO (HE75), possessed very encouraging results in terms of Pt particle size and dispersion, catalytic activity towards HOR and ORR, and fuel cell performance. The maximum power density of 1090 mW cm^?2 was achieved with MEA (Pt loading of 0.4 mg cm^?2) based on electrocatalyst, HE75. Therefore, the resultant hybrid demonstrated higher Pt utilization with enhanced FC performance output. Our results, revealing excellent attributes of hybrid supported electrocatalysts, can be ascribed to the role of CB preventing rGO sheets from restacking, effectively modifying the array of graphene and providing more available active catalyst sites in the electrocatalyst material.     

A fluorinated polymer/inorganic composite electrolyte membrane for intermediate temperature fuel cells

Composite membranes based on polytetrafluoroethylene (PTFE) and silicon dioxide (PTFE/SiO₂ × HPO₃) are fabricated to act as a fuel cell membrane for operation at temperatures from 120 to 200°C. A porous PTFE membrane is used as the membrane supporting structure and SiO₂ × HPO₃ sol as the proton conductor. SEM and EDX show that the sol clusters are connected together and adhered to the PTFE polymer. This structure completely fills the pores of the PTFE and minimises the gas cross‐over. The PTFE/SiO₂ × HPO₃ membrane has a high proton conductivity, up to 0.14 S cm^?1 at a relative humidity lower than 0.5%. The PTFE/SiO₂ × HPO₃ composite membrane gives the modest performance when it is tested in a hydrogen fuel cell although it is a potential material for the intermediate‐temperature proton‐conducting membrane fuel cell. Copyright © 2011 John Wiley & Sons, Ltd.     

A nonlinear viscoelastic-viscoplastic constitutive model for ionomer membranes in polymer electrolyte membrane fuel cells

This paper describes a phenomenological constitutive model for ionomer membranes in polymer electrolyte membrane fuel cells (PEMFCs). Unlike the existing approaches of elasto-plastic, viscoelastic, and viscoplastic model, the proposed model was inspired by micromechanisms of polymer deformation. The constitutive model is a combination of the nonlinear visco-elastic Bergström-Boyce model and hydration-temperature-dependent empirical equations for elastic modulus of ionomer membranes. Experiment results obtained from an uniaxial tension test for Nafion NR-111 membrane under well controlled environments were compared with simulated results by the finite element method (FEM) and the proposed model showed fairly good predictive capabilities for the large deformation behavior of the Nafion membrane subjected to the uniaxial loading condition in a wide range of relative humidity and temperature levels including liquid water.     

An experimental study on the bubble humidification method of polymer electrolyte membrane fuel cells

A necessary requirement for polymer electrolyte membrane fuel cell (PEMFC) performance is providing sufficient water content in the membrane. The bubble humidifier is the simplest and inexpensive method for PEMFC humidification. In this study, a prototype of bubble humidifier is designed, fabricated, and tested. The effects of water temperature in the reservoir, water level inside the reservoir and inlet air flow on the humidifier performance are investigated. The results show that the outlet air relative humidity decreases (about 6% - 11%) with an increase in the inlet air flow rate from 1 m³ h^?1 to 3 m³ h^?1 at four different water temperatures. The increase in the water temperature and water level inside the reservoir lead to the better humidifier performance. At the water temperature of 20^°C, increasing water level from 5 cm to 7.5 cm has a significant effect on humidifier performance but increasing water level from 7.5 cm to 15 cm does not offer any advantage.     

Sub-nano-Pt cluster supported on graphene nanosheets for CO tolerant catalysts in polymer electrolyte fuel cells

The carbon monoxide (CO) tolerance performance of polymer electrode fuel cells (PEFCs) was studied for a catalyst composed of graphene nanosheets (GNS) with sub-nano-Pt clusters. The Pt catalysts supported on the GNS showed a higher CO tolerance performance in the hydrogen oxidation reaction (HOR), which was significantly different from that of platinum on carbon black (Pt/CB). It is proposed that the presence of the sub-nano-Pt clusters promotes the catalytic activity and that the substrate carbon material alters the catalytic properties of Pt via the interface interactions between the graphene and the Pt.