Carbon-supported Pd-Pt cathode electrocatalysts for proton exchange membrane fuel cells

A series of carbon-supported Pd-Pt alloy (Pd-Pt/C) catalysts for oxygen reduction reaction (ORR) with low-platinum content are synthesized via a modified sodium borohydride reduction method. The structure of as-prepared catalysts is characterized by powder X-ray diffraction (XRD) and transmission electron microscope (TEM) measurements. The prepared Pd-Pt/C catalysts with alloy form show face-centered-cubic (FCC) structure. The metal particles of Pd-Pt/C catalysts with mean size of around 4-5 nm are uniformly dispersed on the carbon support. The electrocatalytic activities for ORR of these catalysts are investigated by rotating disk electrode (RDE), cyclic voltammetry (CV), single cell measurements and electrochemical impedance spectra (EIS) measurements. The results suggest that the electrocatalytic activities of Pd-Pt/C catalysts with low platinum are comparable to that of the commercial Pt/C with the same metal loading. The maximum power density of MEA with a Pd-Pt/C catalyst, the Pd/Pt mass ratio of which is 7:3, is about 1040 mW cm⁻².     

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.     

Surfactant-assisted synthesis of platinum nanoparticle catalysts for proton exchange membrane fuel cells

High-performance platinum nanoparticle catalysts (Pt–NPCs) remain the most widespread applied electrocatalysts for oxygen reduction reaction (ORR). Here, cetyltrimethylammonium bromide (CTAB), a surface-controlling agent, is introduced to modulate the microstructure and size of Pt nanoparticles (NPs) via a microwave-assisted heating process. The Pt-NPC assisted by 5 wt% CTAB exhibits the highest mass activity (MA) of 0.072 A mg_Pt^?1 and specific activity (SA) of 0.077 mA cm^?2, higher than those of commercial Pt/C (0.023 A mg_Pt^?1 and 0.035 mA cm^?2). Transmission electron microscopy (TEM) results indicate that Pt NPs are uniformly dispersed onto carbon supports with an average size of 2.39 nm. When applied in membrane electrode assembly (MEA), it exhibits the highest power density of 1.142 W cm^?2, which is about 1.24 times larger than that of commercial Pt/C.     

A mini-type hydrogen generator from aluminum for proton exchange membrane fuel cells

A safe and simple hydrogen generator, which produced hydrogen by chemical reaction of aluminum and sodium hydroxide solution, was proposed for proton exchange membrane fuel cells. The effects of concentration, dropping rate and initial temperature of sodium hydroxide solution on hydrogen generation rate were investigated. The results showed that about 38 ml min⁻¹ of hydrogen generation rate was obtained with 25 wt.% concentration and 0.01 ml s⁻¹ dropping rate of sodium hydroxide solution. The cell fueled by hydrogen from the generator exhibited performance improvement at low current densities, which was mainly due to the humidified hydrogen reduced the protonic resistivity of the proton exchange membrane. The hydrogen generator could stably operate a single cell under 500 mA for nearly 5 h with about 77% hydrogen utilization ratio.     

A novel self-humidifying membrane electrode assembly with water transfer region for proton exchange membrane fuel cells

A novel self-humidifying membrane electrode assembly (MEA) with the active electrode region surrounded by a unactive “water transfer region (WTR)” was proposed to achieve effective water management and high performance for proton exchange membrane fuel cells (PEMFCs). By this configuration, excess water in the cathode was transferred to anode through Nafion membrane to humidify hydrogen. Polarization curves and power curves of conventional and the self-humidifying MEAs were compared. The self-humidifying MEA showed power density of 85 mW cm⁻² at 0.5 V, which is two times higher than that of a conventional MEA with cathode open. The effects of anode hydrogen flow rates on the performance of the self-humidifying MEA were investigated and its best performance was obtained at a flow rate of 40 ml min⁻¹. Its performance was the best when the environmental temperature was 40 °C. The performance of the self-humidifying MEA was slightly affected by environmental humidity. The area of WTR was optimized, and feasible area ratio of the self-humidifying MEA was 28%.     

Investigation of membrane electrode assembly (MEA) hot-pressing parameters for proton exchange membrane fuel cell

The hot-pressing conditions for fabricating the membrane electrode assembly (MEA) of a proton exchange membrane fuel cell (PEMFC) was investigated by using a 2ⁿ full factorial design. Time, temperature and pressure were key parameters that were varied from 500 to 1500 psi, 1 to 5 min and 100 to 160 °C, respectively. The results from the full factorial analysis indicated that the order of significance of the main MEA fabricating effects was temperature, pressure, time–temperature interaction and pressure–time–temperature interaction. By examining the cell performance curves, the lower fabrication conditions of temperature and pressure were suitable for MEA preparation. The conductive layer between the membrane and the catalyst layer became thin at high pressure and high temperature, as seen from scanning electron microscopy (SEM) images. In the ranges of condition studied, the most suitable hot-pressing condition for MEA fabrication was at 100 °C, 1000 psi and 2 min. This condition provided the highest maximum power density from the MEA and the best contact at the interfaces between the gas diffusion layer, the active layer and the electrolyte membrane. The experimental results were verified by testing with a commercial MEA in the same operating condition and with the same equipment. The performance of the fabricated MEA was better than that of the commercial one.     

A facile synthesis of Pd/C cathode electrocatalyst for proton exchange membrane fuel cells

A carbon supported palladium (Pd/C-NaBH₄-NH₃) catalyst was synthesized via modified sodium borohydride reduction method using ammonia as the complexing reagent. The Pd/C catalysts were characterized by means of powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). Rotating disk electrode (RDE), cyclic voltammetry (CV), electrochemical impedance spectra (EIS) and single cell measurements were employed to evaluate the activities of the catalysts. The as-prepared catalysts with face-centered cubic (fcc) structure are uniformly dispersed on the carbon supports. Twinned and polycrystalline structures are observed in the HRTEM image of Pd/C-NaBH₄-NH₃. The results indicate that the Pd/C-NaBH₄-NH₃ catalyst shows high activity for the oxygen reduction reaction. Single cell with Pd/C-NaBH₄-NH₃ as the cathode displays a maximum power density of 508 mW cm⁻². The favorable performance of the Pd/C-NaBH₄-NH₃ catalyst may be attributed to the uniformly dispersed nanoparticles and more crystalline lattice defects.     

Development of shut-down process for a proton exchange membrane fuel cell

Several different shut-down procedures were carried out to reduce the degradation of membrane electrode assembly (MEA) in a proton exchange membrane fuel cell (PEMFC). The effects of close/open state of outlets of a single cell and application of a dummy load during the shut-down on the degradation of the MEA were investigated. Also, we elucidated the relationship between the thickness of the electrolyte membrane and the degradation of the MEA for different shut-down procedures. When a thin electrolyte membrane was used, the closer of outlets mitigated the degradation during on/off operation. For the thicker electrolyte membrane, the dummy load which eliminates residual hydrogen and oxygen in the electrodes should be applied to lower the degradation.     

The effect of materials on proton exchange membrane fuel cell electrode performance

This paper describes the optimisation in the fabrication materials and techniques used in proton exchange membrane fuel cell (PEMFC) electrodes. The effect on the performance of membrane electrode assemblies (MEAs) from the solvents used in producing catalyst inks is reported. Comparison in MEA performances between various gas diffusion layers (GDLs) and the importance of microporous layers (MPLs) in gas diffusion electrodes (GDEs) are also shown. It was found that the best performances were achieved for GDEs using tetrahydrofuran (THF) as the solvent in the catalyst ink formulation and Sigracet 10BC as the GDL. The results also showed that our in-house painted GDEs were comparable to commercial ones (using Johnson Matthey HiSpec™ and E-TEK catalysts).     

Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells

The electrode ionomer is a key factor that significantly affects the catalyst layer morphology and fuel cell performance. Herein, sulfonated poly(arylene ether sulfone)-based electrode ionomers with polymers of various molecular weights and alcohol/water mixtures were prepared, and those comprising the alcohol/water mixture showed a higher performance than the ones prepared using higher boiling solvents, such as dimethylacetamide; this is owing to the formation of the uniformly dispersed ionomer catalyst layer. The relation between ionomer molecular weight for the same polymer structure and the sulfonation degree was investigated. Because the chain length of polymer varies with molecular weight and chain entanglement degree, its molecular weight affects the electrode morphology. As the ionomer covered the catalyst, the agglomerates formed were of different morphologies according to their molecular weight, which could be deduced indirectly through dynamic light scattering and scanning electron microscopy. Additionally, the fuel cell performance was confirmed in the current-voltage curve.     

Influence of ink preparation with the untreated and the burned Pt/C catalysts for proton exchange membrane fuel cells

This research discovers the burning reaction of the Pt/C catalyst on ink preparation and the effect of the untreated and burned Pt/C catalysts for the proton exchange membrane fuel cells (PEMFCs). The platinum nanoparticles on the carbon support aggregate to form bigger cluster sizes due to the burning reaction of the untreated Pt/C catalyst reacting with the Nafion solution or isopropyl alcohol. After several times of “purposely” burning reaction, the specific surface area of the fully burned Pt/C reduces from 150.9 to 46.6 m² g⁻¹ which is 3 times smaller than the untreated Pt/C catalyst. The crystallite size of platinum catalyst changes from 8.4 to 46.2 nm via the calculation of Debye–Scherrer equation from X-ray diffraction (XRD) and the electrochemical surface area (ECSA) obviously decreases from 85.6 to 14.8 m² g⁻¹. The variation of the ratio of Pt/C to Nafion influences the consequent electrochemical performances. Three catalyst coated membranes (CCMs) coated with untreated, fully burned, and partial burned Pt/C catalysts are analyzed and compared in this study. The CCM coated with the untreated Pt/C catalyst shows the best polarization curve which presents the peak power density, 897 mW cm⁻². Moreover, it presents the slowest degradation rate (0.1 mA min⁻¹) at a constant voltage of 0.4 V for 4000 min, even though the result of Nyquist plots is slightly worse than others The work confirms that the burning reaction of Pt/C catalyst influences the electrochemical performance and structural balance of the catalyst layer.     

Development of efficient membrane electrode assembly for low cost hydrogen production by anion exchange membrane electrolysis

Electrochemical production of hydrogen from water using anion exchange membranes (AEMs) can be achieved with non-noble catalysts, other than traditional proton exchange membranes that use platinum group metals. Using non-noble metals in the catalyst layer will reduce the capital costs associated with water electrolysis systems. The objectives of this study were to develop an effective membrane electrode assembly (MEA) for AEM electrolysis and to determine the effects of various operating parameters on AEM electrolysis. Here, the MEA consisted of the commercially available A-201 AEM and non-noble transition metal oxides as catalysts. The best electrolysis performance recorded was 500 mA cm^?2 for 1.95 V at 60 °C with 1% K₂CO₃ electrolyte. For the purpose of comparison, we also considered commercially available AEMs for AEM electrolysis: Fumapem^® FAA-3 and Fumapem^® FAA-3-PP-75. The performances achieved with these AEMs were comparable with the performance recorded for the conventional AEM A-201. Overall, our results indicated that AEM electrolysis clearly manifests the feasibility of commercial viability.     

Dynamic characteristics and mitigations of hydrogen starvations in proton exchange membrane fuel cells during start-ups

Hydrogen starvation during a start-up process in proton exchange membrane (PEM) fuel cells could result in drastic local current density variations, reverse cell voltage and irreversible cell damages. In this work, variations of local current densities and temperatures are measured in situ under both potentiostatic and galvanostatic modes. Experimental results show that when the cell starts up under potentiostatic mode with hydrogen starvation, current density undershoots occur in the downstream; while under the galvanostatic mode, local current density in the downstream almost drops to zero, but the current density near the outlet remains almost constant. The phenomenon of near constant current density near the outlet leads to a novel approach to alleviate hydrogen starvations - a hydrogen reservoir is added at the anode outlet. Experimental results show that the exit hydrogen reservoir can significantly reduce the zero current region and alleviate hydrogen starvations. A non-dimensional current-density variation coefficient is proposed to measure the magnitude of local current density changes during starvations. Experimental results show that the exit hydrogen reservoir can significantly reduce the current-density variations coefficient over the entire flow channel, indicating that adding an exit reservoir is an effective approach in mitigating hydrogen starvations.     

Maximization of high-temperature proton exchange membrane fuel cell performance with the optimum distribution of phosphoric acid

A proton exchange membrane fuel cell (PEMFC) using a controlled amount of phosphoric acid (PA) in a membrane-electrode assembly (MEA) is operated at 150 °C without humidification of the cells. The effects on MEA performance of Pt loading and the amount of PA in the cathode are investigated. The catalyst utilization is maximized by optimizing the PA content in the cathodes and results in lowering of the Pt loading in the MEA. In-situ cyclic voltammetry is used to confirm that the highest value of the active electrochemical area is achieved with the optimum amount of PA in the cathode. The transient response of cell voltage during current density–voltage experiments (I–V curve) is also found to be affected by the amount of PA in the electrodes.     

An air-cooled proton exchange membrane fuel cell with combined oxidant and coolant flow

Combining the oxidant and coolant flow in an air-cooled proton exchange membrane fuel cell can significantly simplify the fuel cell design. In this paper, an air-cooled PEM fuel cell stack with an open cathode flow field, which supplied the oxidant and removed the heat produced in the fuel cell, was fabricated and tested. The influence of different operating parameters on cell voltage performance and the overall cell ohmic resistance, such as cell temperature and airflow rate, was investigated. The cell temperature and the temperature difference between the cell and the hydrogen humidifier were shown to serve important roles in reducing the fuel cell ohmic resistance. The test results also showed a noteworthy temperature gradient between each cell of a 5-cell stack. A hydrophilic treatment of the cathode flow field channels was demonstrated to be an effective way to mitigate water management issues caused at elevated operating temperatures.     

Synthesis methods of low-Pt-loading electrocatalysts for proton exchange membrane fuel cell systems

While the use of a high level of platinum (Pt) loading in proton exchange membrane fuel cells (PEMFCs) can amplify the trade off towards higher performance and longer lifespan for these PEMFCs, the development of PEMFC electrocatalysts with low-Pt-loadings and high-Pt-utilization is critical and the limited supply and high cost of the Pt used in PEMFC electrocatalysts necessitate a reduction in the Pt level. In order to make such electrocatalysts commercially feasible, cost-effective and innovative, catalyst synthesis methods are needed for Pt loading reduction and performance optimization. Since a Pt-deposited carbon nanotube (CNT) shows higher performance than a commercial Pt-deposited carbon black (CB) with reducing 60% Pt load per electrode area in PEMFCs, use of CNTs in preparing electrocatalysts becomes considerable. This paper reviews the literature on the synthesis methods of carbon-supported Pt electrocatalysts for PEMFC catalyst loading reduction through the improvement of catalyst utilization and activity. The features of electroless deposition (ED) method, deposition on sonochemically treated CNTs, polyol process, electrodeposition method, sputter-deposition technique, γ-irradiation method, microemulsion method, aerosol assisted deposition (AAD) method, Pechini method, supercritical deposition technique, hydrothermal method and colloid method are discussed and characteristics of each one are considered.     

Reduction of hydrogen peroxide production at anode of proton exchange membrane fuel cell under open-circuit conditions using ruthenium–carbon catalyst

This study examines the effect of hydrogen peroxide (H₂O₂) on the open-circuit voltage (OCV) of a proton exchange membrane fuel cell (PEMFC) and the reduction of H₂O₂ in the membrane using a ruthenium/carbon catalyst (Ru/C) at the anode. Each cathode and anode potential of the PEMFC in the presence of H₂O₂ is examined by constructing a half-cell using 1.0 M H₂SO₄ solution as an electrolyte and Ag/AgCl as the reference electrode. H₂O₂ is added to the H₂SO₄ solution and the half-cell potential is measured at each H₂O₂ concentration. The cathode potential is affected by the H₂O₂ concentration while the anode potential remains stable. A Ru catalyst is used to reduce the level of H₂O₂ formation through O₂ cross-over at the interface of a membrane and the anode. The Ru catalyst is known to produce less H₂O₂ through oxygen reduction at the anode of PEMFC than a Pt catalyst. A Ru/C layer is placed between the Nafion^® 112 membrane and anode catalyst layer and the cell voltage under open-circuit condition is measured. A single cell is constructed to compare the OCV of the Pt/C only anode with that of the Ru/C-layered anode. The level of hydrogen cross-over and the OCV are determined after operation at a current density of 1 A cm⁻² for 10 h and stabilization at open-circuit for 1 h to obtain an equilibrium state in the cell. Although there is an increase in the OCV of the cell with the Ru/C layer at the anode, excessive addition of Ru/C has an adverse effect on cell performance.     

Experimental study of proton exchange membrane fuel cells using Nafion 212 and Nafion 211 for portable application at ambient pressure and temperature conditions

Low humidification, large air stoichiometry, dry hydrogen and low operational temperature makes open-cathode proton exchange membrane fuel cells (PEMFCs) with forced-air convection, which is designed for portable applications, quite different from that used in automobile vehicles. In this paper, PEMFCs humidified at 30 °C using Nafion 212 and Nafion 211 as electrolytes were systematically investigated under simulating conditions. These conditions included air stoichiometry from 3 to 100 and cell temperature from 30 °C to 60 °C. The results indicate that the thinner membrane (Nafion 211) had better performance and more stable voltage output under air dual-function configuration than Nafion 212. Furthermore, the dynamic response of the voltage with cell temperature was also studied during rising and cooling procedure between 30 °C and 60 °C.     

Effect of catalyst layer with zeolite on the performance of a proton exchange membrane fuel cell operated under low-humidity conditions

Proton exchange membrane fuel cells (PEMFCs) employ a proton conductive membrane as the separator to transport a hydrogen proton from the anode to the cathode. The membrane's proton conductivity depends on the water content in the membrane, which is affected by the operating conditions. A membrane electrode assembly (MEA) that can self-sustain water is the key component for developing a light-weight and compact PEMFC system without humidifiers. Hence, zeolite is employed to the anode catalyst layer in this study. The effect of the gas diffusion layer (GDL) materials, catalyst loading, binder loading, and zeolite loading on the MEA performance is investigated. The MEA durability is also investigated through the electrochemical impedance spectroscopy (EIS) method. The results suggest that the MEA with the SGL28BCE carbon paper, Pt loadings of 0.1 and 0.7 mg cm^?2 in the anode and cathode, respectively, Nafion-to-carbon weight ratio of 0.5, and zeolite-to-carbon weight ratio of 0.3 showed the best performance when the cell temperature is 60 °C and supplies with dry hydrogen and air from the environment. According to the impedance variation measured by EIS, the MEA with zeolite in the anode catalyst layer shows higher and more stable performance than those without zeolite.     

Carbon-supported Ir–V nanoparticle as novel platinum-free anodic catalysts in proton exchange membrane fuel cell

The active, carbon-supported Ir–V nanoparticle catalysts were successfully synthesized using IrCl₃ and NH₄VO₃ as the Ir and V precursors in ethylene glycol refluxing at 120 °C with varying pH values, then further reduction under hydrogen atmosphere at 200 °C. The nanostructured catalysts were characterized by X-ray diffraction (XRD) and high resolution transmission electron microscopy (TEM). These carbon-supported catalysts give a good dispersion of Ir–V/C electrocatalysts with mean particle size of 2–3 nm, thus leading to a marked promotion of hydrogen oxidation reaction. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry techniques (CV) were used to characterize on-line the performance of the proton exchange membrane fuel cell (PEMFC) using several anode catalysts at different pH values. It was found that the pH value for the synthesis of catalysts affects the performance of electrocatalysts significantly, based on the discharge characteristics of the fuel cell. High cell performance on the anode was achieved with a loading of 0.4 mg cm⁻² 40%Ir–10%V/C catalyst synthesized at pH 12, which results in a maximum a power density of 1008 mW cm⁻² at 0.6 V and 70 °C. This is 50% higher performance than that for commercial available Pt/C catalyst. Fuel cell life test at a constant current density of 1000 mA cm⁻² demonstrated an initial stability up to 100 h generating a cell voltage of 0.6 V, which strongly suggests that the novel Ir–V/C nanoparticle catalysts proposed in this work could be promising for PEMFC.