The synergy between multi-wall carbon nanotubes and Vulcan XC72R in microporous layers

In this study, the effects of the addition of multi-wall carbon nanotubes (MWCNTs) into a microporous layer (MPL) containing Vulcan XC72R on the oxygen reduction reaction (ORR) were studied. We tested various percentages of MWCNTs and Vulcan XC72R in the MPLs of gas-diffusion electrodes (GDEs) with various Pt loadings in the catalyst layer. The performance of the ORR in the electrodes was studied with linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and chronoamperometry. The structures of the MPLs were investigated by using scanning electron microscopy (SEM), mercury porosimetry (MP), and gas permeability. In addition, the optimum polytetrafluoroethylene (PTFE) content of the MPL was determined. Our results indicate that the performances of the GDEs are optimal under the following conditions: (a) 60 wt% MWCNTs and 40 wt% Vulcan XC72R with a Pt loading of 0.115 mg/cm²; (b) 80 wt% MWCNTs and 20 wt% Vulcan XC72R with a Pt loading of 0.5 mg/cm²; and (c) 40 wt% MWCNTs and 60 wt% Vulcan XC72R with a Pt loading of 1 mg/cm².     

Influence of the relative volumes between catalyst and Nafion ionomer in the catalyst layer efficiency

Over the years, studies have analyzed the composition of the catalyst layer using commercial platinum catalyst, supported on Vulcan XC72 with 20% of metal loading (Pt/C 20%_Mw), and found that values between 20 and 40% of Nafion ionomer related to the mass of the catalyst layer (% NI_W) have resulted in more efficient electrodes for PEMFC. Recent studies with catalysts synthesized on Vulcan XC72 resulted in 59% NI_W as the best formulation. In this work, the commercial and the synthesized Pt/C 20%_Mw catalyst were evaluated by Gas Pycnometry, Gas Adsorption (through BET and BJH), and Mercury Intrusion Porosimetry. The results showed volumetric differences between the Vulcan XC72 used in commercial catalyst and the Vulcan XC72 commercially available for synthesis (as purchased). These differences impair the synthesized catalyst in comparison with the commercial one. Therefore, the relationship between the quantities of catalysts and Nafion ionomer on the catalyst layers must be calculated as a function of the catalysts volumes.     

Study on the membrane electrode assembly fabrication with carbon supported cobalt triethylenetetramine as cathode catalyst for proton exchange membrane fuel cell

An improved fabrication technique for conventional hot-pressed membrane electrode assemblies (MEAs) with carbon supported cobalt triethylenetetramine (CoTETA/C) as the cathode catalyst is investigated. The V-I results of PEM single cell tests show that addition of glycol to the cathode catalyst ink leads to significantly higher electrochemical performance and power density than the single cell prepared by the traditional method. SEM analysis shows that the MEAs prepared by the conventional hot-pressed method have cracks between the cathode catalyst layer and Nafion membrane, and the contact problem between cathode catalyst layer and Nafion membrane is greatly suppressed by addition of glycol to the cathode catalyst ink. Current density-voltage curve and impedance studies illuminate that the MEAs prepared by adding glycol to the cathode catalyst ink have a higher electrochemical surface area, lower cell ohmic resistance, and lower charge transfer resistance. The effects of CoTETA/C loading, Nafion content, and Pt loading are also studied. By optimizing the preparation parameters of the MEA, the as-fabricated cell with a Pt loading of 0.15 mg cm⁻² delivers a maximum power density of 181.1 mW cm⁻², and a power density of 126.2 mW cm⁻² at a voltage of 0.4 V.     

High-performance anode for Polymer Electrolyte Membrane Fuel Cells by multiple-layer Pt sputter deposition

We investigate the sputtering deposition as a tool for preparing Polymer Electrolyte Membrane Fuel Cell (PEMFC) electrodes with improved performance and catalyst utilization. Anodes of PEMFC with ultra-low loading of Pt (0.05 mg cm⁻²) are developed by alternate sputtering of Pt and painting layers of carbon nanotube ink with Nafion directly on the gas diffusion layer. Sputter depositing alternate layers of Pt on carbon-Nafion layer (CNL) has increased the anode activity over single-layer Pt deposited anode due to improved porosity and the presence of Pt nanoparticles in the inner CNL. Also, we investigated the influence of Nafion content in the CNL. The optimal Nafion content giving less resistance and better performance in an anode is 29 wt.%. This is significantly lower than for standard MEA anodes, indicating sufficient interfacial contact between each CNL. We studied the anodes prepared with 50 wt.% Nafion, which revealed larger ohmic resistance and also, blocks the CNL pores reducing gas permeability. Excellent mass transfer and performance is obtained with three-layer Pt sputter deposited anode with CNL containing 29 wt.% of Nafion.     

Optimum compositions of membrane electrode assemblies (MEAs) for direct dimethyl ether fuel cell

We investigated the effects of the compositions of catalyst layers and diffusion layers on performances of the membrane electrode assemblies (MEAs) for direct dimethyl ether fuel cell. The performances of the MEAs with different thicknesses of Nafion membranes were compared in this work. The optimal compositions in the anode are: 20 wt% Nafion content and 3.6 mg cm⁻² Pt loading in the catalyst layer, and 30 wt% PTFE content and 1 mg cm⁻² carbon black loading in the diffusion layer. In the cathode, MEA with 20 wt% Nafion content in the catalyst layer and 30 wt% PTFE content in the diffusion layer presented the optimal performance. The MEA with Nafion 115 membrane displayed the highest maximum power density of 46 mW cm⁻² among the three MEAs with different Nafion membranes. Copyright © 2009 John Wiley & Sons, Ltd.     

The performance and degradation of Pt electrocatalysts on novel carbon carriers for PEMFC applications

Electrocatalyst stability is an important factor influencing the performance of polymer electrolyte membrane (PEM) fuel cells and is essential in maintaining the cell output. The aim of this work was to elucidate factors which influence the stability of platinum supported onto graphitic nanofibres (Pt/GNFs) and to compare the performance of these materials with the commonly used Pt/Vulcan electrocatalyst. Platinum nanoparticles (average diameter of 6.9 nm) were supported on GNFs which were prepared by chemical vapour deposition over an unsupported nickel oxide (NiO) catalyst precursor. The performance of Pt/GNFs based electrodes were studied by cyclic voltammetry and a single-cell fuel cell test and were compared with a commercially available carbon nanostructure, Vulcan XC-72, which was also impregnated with Pt nanoparticles. Characterisation of the pre- and post-operation of the Pt/GNFs by XRD and TEM showed that structural changes of the Pt had occurred during testing. It was found that the average diameter of each grain and the degree of agglomeration among particles was increased, creating elongated clusters of Pt along the carbon fibre. Analysis of electrocatalyst post-operation also identified that the sulphate from the Nafion membrane was reacting with the Pt surface forming platinum sulphide (PtS). These phases were confirmed by the presence of low intensity, but sharp XRD peaks, attributed to a few large diameter particles (49 nm). These two factors resulted in current density dropping from 0.2 A/cm² to 0.1 A/cm² (at 0.70 V) over a 25 h test period.     

Structure of carbon-supported Pt-Ru nanoparticles and their electrocatalytic behavior for hydrogen oxidation reaction

The electrochemical activity towards hydrogen oxidation reaction (HOR) of a high performance carbon-supported Pt-Ru electrocatalyst (HP 20 wt.% 1:1 Pt-Ru alloy on Vulcan XC-72 carbon black) has been studied using the thin-film rotating disk electrode (RDE) technique. The physical properties of the Pt-Ru nanoparticles in the electrocatalyst were previously determined by transmission electron microscopy (TEM), high resolution TEM, fast Fourier transform (FFT), electron diffraction and X-ray diffraction (XRD). The corresponding compositional and size-shape analyses indicated that nanoparticles generally presented a 3D cubo-octahedral morphology with about 26 at.% Ru in the lattice positions of the face-centred cubic structure of Pt. The kinetics for HOR was studied in a hydrogen-saturated 0.5 M H₂SO₄ solution using thin-film electrodes prepared by depositing an ink of the electrocatalyst with different Nafion contents in a one-step process on a glassy carbon electrode. A maximum electrochemically active surface area (ECSA) of 119 m² g Pt⁻¹ was found for an optimum Nafion composition of the film of about 35 wt.%. The kinetic current density in the absence of mass transfer effects was 21 mA cm⁻². A Tafel slope of 26 mV dec⁻¹, independent of the rotation rate and Nafion content, was always obtained, evidencing that HOR behaves reversibly. The exchange current density referred to the ECSA of the Pt-Ru nanoparticles was 0.17 mA cm⁻², a similar value to that previously found for analogous inks containing pure Pt nanoparticles.     

Structure and electrocatalytic performance of carbon-supported platinum nanoparticles

The structure of Pt nanoparticles and the composition of the catalyst-Nafion films strongly determine the performance of proton exchange membrane fuel cells. The effect of Nafion content in the catalyst ink, prepared with a commercially available carbon-supported Pt, in the kinetics of the hydrogen oxidation reaction (HOR), has been studied by the thin layer rotating disk electrode technique. The kinetic parameters have been related to the catalyst nanoparticles structure, characterized by X-ray diffraction and high-resolution transmission electron microscopy. The size-shape analysis is consistent with the presence of 3D cubo-octahedral Pt nanoparticles with average size of 2.5 nm. The electrochemically active surface area, determined by CO stripping, appears to depend on the composition of the deposited Pt/C-Nafion film, with a maximum value of 73 m² g_Pt⁻¹ for 30 wt.% Nafion. The results of CO stripping indicate that the external Pt faces are mainly (1 0 0) and (1 1 1) terraces, thus confirming the cubo-octahedral structure of nanoparticles. Cyclic voltammetry combined with the RDE technique has been applied to study the kinetic parameters of HOR besides the ionomer resistance effect on the anode kinetic current at different ionomer contents. The kinetic parameters show that H₂ oxidation behaves reversibly with an estimated exchange current density of 0.27 mA cm⁻².     

Sputtered Pt loadings of membrane electrode assemblies in proton exchange membrane fuel cells

The fabrication of electrodes use in proton exchange membrane fuel cells (PEMFCs) by Pt sputter deposition has great potential to increase Pt utilization and reduce Pt loading without loss of cell performance. A radio frequency (RF) magnetron sputter deposition process (RF power = 100 W and argon pressure = 10^?3 Torr) was adopted to prepare Pt catalyst layers of PEMFC electrodes. The effects of cathode Pt and Nafion loadings on membrane electrode assembly (MEA)/cell performance were investigated using cell polarization, cyclic voltammetry, AC impedance, and microstructure analysis. Among the tested MEAs with various cathode Pt loadings (0.02–0.4 mg cm^?2), the one with 0.1 mg‐Pt cm^?2 (grain size = 3.90 nm, mainly Pt(111)) exhibited the best cell performance (320 and 285 mW cm^?2 at 0.44 and 0.60 V, respectively), which was similar to or better than those of some commercial nonsputtered/sputtered electrodes with the same or higher Pt loadings. The electrode Pt utilization efficiency increased as the Pt loading decreased. A Pt loading of greater than or lower than 0.1 mg cm^?2 yielded a lower electrode electrochemical active surface (EAS) area but a higher charge transfer and diffusion resistance. Nafion impregnation (0.1 to 0.3 mg cm^?2) into the sputtered Pt layer (Pt = 0.1 mg cm^?2) noticeably increased the EAS area, consistent with the decrease of the capacitance of the electrode double layer, but did not improve MEA/cell performance, mainly because of the increase in the kinetic and mass transfer resistances associated with oxygen reduction on the cathode. Copyright © 2011 John Wiley & Sons, Ltd.     

Composite‐supported Pt catalyst and electrosprayed cathode catalyst layer for polymer electrolyte membrane fuel cell

A major limitation of the conventional polymer electrolyte membrane fuel cell (PEMFC) catalysts is the fast oxidative degradation of their carbon black supports. Complete replacement of carbon black is difficult because of its low‐cost and high electrical conductivity. Reported here are the development and optimization of composite‐supported Pt catalysts and the electrosprayed cathode catalyst layer with these catalysts for PEMFC. These catalysts are supported by a composite of carbon black (Vulcan XC‐72R) and the electrochemically much more stable carbon‐embedded niobium‐doped titanium dioxide nanofibers (C/Nb_0.1Ti_0.9O₂). Four different catalyst supports with 20 wt.% Pt were prepared by air spraying and electrospraying to compare their activity and stability. Vulcan XC‐72R and C/Nb_0.1Ti_0.9O₂ were tested as pristine support materials for comparison as well as 1:3 and 3:1 mixtures by weight of the two pristine support materials (composite supports). The amount of Nafion in the catalyst ink was optimized for each catalyst layer by a volumetric method. An increase in carbon black content of the support layer from 0% to 100% increases the performance of these catalysts in H₂/air PEMFCs but also increases the loss of oxygen reduction reaction mass activity. The best balance between PEMFC performance and durability was obtained for the Pt catalyst with 25% carbon black in the support layer, while the highest initial oxygen reduction reaction mass activity was obtained for the catalyst with 75% carbon black content. Copyright © 2017 John Wiley & Sons, Ltd.     

Influence of anode diffusion layer on the performance of a liquid feed direct methanol fuel cell by AC impedance spectroscopy

Effect of anode diffusion layer over the performance of the liquid feed direct methanol fuel cell has been studied by AC impedance spectroscopy. The anode employed comprises of the catalyst layer and diffusion layer. The latter comprises of backing layer and catalyst‐supporting layer. The supporting layer is present in between the backing layer and the catalyst layer. The composition of the supporting layer is optimized based on the information obtained from polarization and AC impedance measurements. Among the three types of carbons (Black pearl 2000, Vulcan XC‐72, Shawinigan acetylene black), Black pearls 2000 is found to be the ideal type of carbon used in the supporting layer. The optimized loading compositions of carbon, Nafion and PTFE in the supporting layer are reported to be 3 mg cm^?2, 10 wt%, and 0 wt%, respectively. These values are rationalized on the basis of the transport of methanol and carbon dioxide and the crossover of methanol from the anode to the cathode. Copyright © 2006 John Wiley & Sons, Ltd.     

Low Pt loading high performance cathodes for PEM fuel cells

A simple direct mixing of carbon-supported catalysts with Nafion without adding any additional organic solvents was used to make electrodes for oxygen reduction in PEM fuel cells. For E-TEK 20% Pt/C, a Nafion content of 30% in the catalyst layer exhibited the best performance. Electrode dried from 90 to 150 °C showed little difference in performance. Highest power densities increased almost linearly with cell temperature, and values of 0.52, 0.60, 0.63, and 0.72 W/cm² were achieved at 35, 50, 60, and 75 °C, respectively, for a cathode with a Pt loading of 0.12 mg/cm² and operated using air at ambient pressure. A maximum performance was achieved with Pt loadings of 0.20±0.05 and 0.35±0.05 mg/cm² for 20 and 40% Pt/C, respectively, while the maximum performance using 40% Pt/C was only slightly better than that using 20% Pt/C. A Nafion/carbon sublayer with up to 30% Nafion content added between ELAT and the catalyst layer did not show any effect on performance.     

Effects of platinum loading on the performance of proton exchange membrane fuel cells with high ionomer content in catalyst layers

The effect of Pt loading on the performance of proton exchange membrane fuel cells with atmospheric air feed was evaluated at various relative humidities. The membrane electrode assemblies (MEAs) were fabricated by decal methods with high Nafion ionomer content (30 and 40 wt.%). When the Pt loading was decreased, the performance of the MEAs with an ionomer content of 30 wt.% gradually decreased, mainly due to the insufficient active Pt surfaces with low proton conductivity. With a higher ionomer content of 40 wt.%, the activation overpotential was not significantly increased by the decrease in Pt loading, and the concentration overpotential could be largely reduced by decreasing the Pt loading to 0.25 mg/cm². When the Pt loading was further decreased to 0.15 mg/cm², even though the flooding became more severe, the cell performance at 0.6 V and intermediate relative humidity of 55% was about 71.6%, compared to the MEA with a high Pt loading of 0.35 mg/cm² (ionomer content: 30 wt.%). The cell performance could be further enhanced by decreasing the ionomer content in the anode to enhance the water back diffusion.     

Carbon xerogels as Pt catalyst supports for polymer electrolyte membrane fuel-cell applications

Carbon xerogels prepared by the resorcinol-formaldehyde (RF) sol-gel method with ambient-pressure drying were explored as Pt catalyst supports for polymer electrolyte membrane (PEM) fuel cells. Carbon xerogel samples without Pt catalyst (CX) were characterized by the N₂ sorption method (BET, BJH, others), and carbon xerogel samples with supported Pt catalyst (Pt/CX) were characterized by thermogravimetry (TGA), powder X-ray diffraction (XRD), electron microscopy (SEM, TEM) and ex situ cyclic voltammetry for thin-film electrode samples supported on glassy carbon and studied in a sulfuric acid electrolyte. Experiments on Pt/CX were made in comparison with commercially obtained samples of Pt catalyst supported on a Vulcan XC-72R carbon black support (Pt/XC-72R). CX samples had high BET surface area with a relatively narrow pore size distribution with a peak pore size near 14 nm. Pt contents for both Pt/CX and Pt/XC-72R were near 20 wt % as determined by TGA. Pt catalyst particles on Pt/CX had a mean diameter near 3.3 nm, slightly larger than for Pt/XC-72R which was near 2.8 nm. Electrochemically active surface areas (ESA) for Pt as determined by ex situ CV measurements of H adsorption/desorption were similar for Pt/XC-72R and Pt/CX but those from CO stripping were slightly higher for Pt/XC-72R than for Pt/CX. Membrane-electrode assemblies (MEAs) were fabricated from both Pt/CX and Pt/XC-72R on Nafion 117 membranes using the decal transfer method, and MEA characteristics and single-cell performance were evaluated via in situ cyclic voltammetry, polarization curve, and current-interrupt and high-frequency impedance methods. In situ CV yielded ESA values for Pt/XC-72R MEAs that were similar to those obtained by ex situ CV in sulfuric acid, but those for Pt/CX MEAs were smaller (by 13-17%), suggesting that access of Nafion electrolyte to Pt particles in Pt/CX electrodes is diminished relative to that for Pt/XC-72R electrodes. Polarization curve analysis at low current density (0.9 V cell voltage) reveals slightly higher intrinsic catalyst activity for the Pt/CX catalyst which may reflect the fact that Pt particle size in these catalysts is slightly higher. Cell performance at higher current densities is slightly lower for Pt/CX than the Pt/XC-72R sample, however after normalization for Pt loading, performance is slightly higher for Pt/CX, particularly in H₂/O₂ and at lower cell temperatures (50 °C). This latter finding may reflect a possible lower mass-transfer resistance in the Pt/CX sample.     

l-Ascorbic acid as an alternative fuel for direct oxidation fuel cells

l-Ascorbic acid (AA) was directly supplied to polymer electrolyte fuel cells (PEFCs) as an alternative fuel. Only dehydroascorbic acid (DHAA) was detected as a product released by the electrochemical oxidation of AA via a two-electron transfer process regardless of the anode catalyst used. The ionomer in the anode may inhibit the mass transfer of AA to the reaction sites by electrostatic repulsion. In addition, polymer resins without an ionic group such as poly(vinylidene fluoride) and poly(vinyl butyral) were also useful for reducing the contact resistance between Nafion membrane and carbon black used as an anode, although an ionomer like Nafion is needed for typical PEFCs. A reaction mechanism at the two-phase boundaries between AA and carbon black was proposed for the anode structure of DAAFCs, since lack of the proton conductivity was compensated by AA. There was too little crossover of AA through a Nafion membrane to cause a serious technical problem. The best performance (maximum power density of 16 mW cm⁻²) was attained with a Vulcan XC72 anode that included 5 wt.% Nafion at room temperature, which was about one-third of that for a DMFC with a PtRu anode.     

Thin film Pt/TiO2 catalysts for the polymer electrolyte fuel cell

Thin film Pt/TiO₂ catalysts are evaluated in a polymer electrolyte electrochemical cell. Individual thin films of Pt and TiO₂, and bilayers of them, were deposited directly on Nafion membranes by thermal evaporation with varying deposition order and thickness (Pt loadings of 3–6 μg cm⁻²). Structural and chemical characterization was performed by transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Oxygen reduction reaction (ORR) polarization plots show that the presence of a thin TiO₂ layer between the platinum and the Nafion increases the performance compared to a Pt film deposited directly on Nafion. Based on the TEM analysis, we attribute this improvement to a better dispersion of Pt on TiO₂ compared to on Nafion and in addition, substantial proton conduction through the thin TiO₂ layer. It is also shown that deposition order and the film thickness affects the performance.     

Contribution of Nafion loading to the activity of catalysts and the performance of PEMFC

Different amounts of Nafion loadings (in the range of 0–2.0 mg cm⁻²) were added to a catalyst containing 0.5 mg cm⁻² of Pt; these were prepared by spraying a Nafion solution on an electrode surface. The effect of Nafion loading on the activity of the catalyst and the performance of a proton exchange membrane fuel cell (PEMFC) was investigated by using electrochemical methods such as direct current polarization (using an I–V curve), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear scan voltammetry (LSV). The results of the I–V and EIS were compared in order to resolve the ohmic resistance (R_Ω, calculated from the I–V curve) into interfacial and internal resistances (R_if and R_s, simulated from the EIS). The analysis of the electrochemical data revealed that the interfacial resistance (R_if) is closely related to the reactive region of three-phase zones (interfaces among the reactants, electrolyte and catalyst), and it provides a major parameter for diagnosing the activity of the catalysts and the performance of the PEMFC.     

Hollow core mesoporous shell carbon supported Pt electrocatalysts with high Pt loading for PEMFCs

The aim of this study is to synthesize mesoporous carbon supports and prepare their corresponding electrocatalysts with microwave irradiation method and also increasing the Pt loading over the carbon support by using some additional reducing agents. Pt loadings on hollow core mesoporous shell (HCMS) and commercial Vulcan XC72 carbon supports up to 34% and 44%, respectively, were achieved via polyol process with microwave irradiation method. When hydrazine or sodium borohydride was used in addition to ethylene glycol, Pt loading over the HCMS carbon support was increased. Characterization of the prepared electrocatalysts was performed by ex situ (BET, XRD, SEM, TGA and Cyclic Voltammetry) and in situ (PEM fuel cell tests) analysis. PEM fuel cell performance tests showed that 44% Pt/Vulcan XC72 and 28% Pt/HCMS electrocatalysts exhibited improved fuel cell performances. The results revealed that as the Pt loading increased PEM fuel cell performance was also increased.     

Effect of loading and distributions of Nafion ionomer in the catalyst layer for PEMFCs

The electrode with various contents of Nafion ionomer for inside and/or on the surface in the catalyst layer, respectively, was designed for proton exchange membrane fuel cell (PEMFC) electrode to investigate the effect of Nafion ionomer distribution in the catalyst layer on cell performance and improve electrode performance. The effect of Nafion ionomer on the electrode of each design was judged by a cyclic voltammetry measurement and the cell performance obtained through a single cell test using H₂/O₂ gases. Electrodes with different ionomer distributions for inside and on the surface in the catalyst layer, respectively, were examined. It is found that the electrode where the Nafion ionomer is impregnated on the surface of catalyst layer shows better cell performance than that where the Nafion ionomer is incorporated in the inside of catalyst layer. The best cell performance among the catalyst layers tested in this study was obtained for the electrode with 0.5 mg cm⁻² of Nafion ionomer inside the catalyst layer and 1.0 mg cm⁻² of Nafion ionomer on the surface of the catalyst layer together.     

Optimization of Nafion content in Nafion–polyaniline nano-composite modified cathodes for PEMFC application

In this research several Nafion–Polyaniline nano-composite modified cathodes have been fabricated and evaluated in oxygen reduction reaction (ORR) in order to use in proton exchange membrane fuel cell (PEMFC). Modified cathodes made by the wide range of Nafion content (from 0 to 1.6 mg cm⁻²) and investigated in the acidic solution by different electrochemical techniques at 25 °C. The results indicate the activity of the modified electrodes is increased by employing of Nafion–Polyaniline nano-composite in the reaction layer, but there is an optimum value for Nafion content in the catalyst layer. The modified electrode impregnated by 0.4 mg cm⁻² of Nafion shows the highest activity. Analysis of the surface morphology of the Nafion–polyaniline modified electrodes by scanning electron microscopy and electrochemical data reveal that the existence of polyaniline (PANI) nanofibers in the catalyst layer before adding Nafion solution, improves the homogeneity distribution of the ionomer in catalyst layer, change the morphology of electrode and increase the performance of gas diffusion electrodes (GDEs) in oxygen reduction reaction.