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.     

High surface area graphite as alternative support for proton exchange membrane fuel cell catalysts

The suitability of a high surface area graphite (HSAG) as proton exchange membrane fuel cell (PEMFC) catalyst support has been evaluated and compared with that of the most popular carbon black: the Vulcan XC72. It has been observed that Pt is arranged on the graphite surface resulting in different structures which depend on the catalysts synthesis conditions. The influence that the metal particle size and the metal-support interaction exert on the catalysts degradation rate is analyzed. Temperature programmed oxidation (TPO) under oxygen containing streams has been shown to be a useful method to assess the resistance of PEMFC catalysts to carbon corrosion. The synthesized Pt/HSAG catalysts have been evaluated in single cell tests in the cathode catalytic layer. The obtained results show that HSAG can be a promising alternative to the traditionally used Vulcan XC72 carbon black when suitable catalysts synthesis conditions are used.     

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.     

Physical and electrochemical evaluation of commercial carbon black as electrocatalysts supports for DMFC applications

This work presents results with noble metal catalysts, Pt and PtRu supported on Black Pearl with a higher surface area in comparison with carbon black Vulcan XC-72R and Vulcan XC72. The nanoparticles were synthesized following the alcohol reduction method. Brunauer–Emmet–Teller (BET) surface area analysis, X-ray diffraction (XRD), energy dispersive analysis by X-rays (EDAX), and high resolution transmission electronic microscopy (TEM) experiments were carried out to characterize the materials obtained. Cyclic voltammograms (CV) of catalysts using the porous thin layer electrode technique were obtained for the catalysts surface evaluation and for methanol oxidation to check the electrocatalytic behavior of these nanocatalyst systems.     

Single walled carbon nanotubes as supports for metal hydride electrodes

The electrochemical generation and the storage of hydrogen employing metal hydrides has become a good alternative attending the requirement to search for new sources of clean energy. This work is devoted to study the hydrogen storage as hydrides in an AB₅-type metal alloy (MmNi_4.1Co_0.4Mn_0.4Al_0.5). The behaviour of the alloy containing electrode was evaluated employing several electrodes containing the alloy and diverse carbons. Carbons were prepared by using Single Walled Carbon Nanotubes (SWCNT) with different PTFE percentages (15%, 25% y 33%) and Carbon Vulcan XC72® with 33% of PTFE (VT). Several electrochemical techniques as Cyclic Voltammetry (CV), Charge–Discharge cycles and Electrochemical Impedance Spectroscopy (EIS) were used. Results demonstrate that at low discharge currents, electrodes containing SWCNT exhibit better hydrogen storage than Vulcan XC72® containing electrodes. Studies made with carbon supports only show that this little but not disregarded differences are related to different hydrogen sorption behaviour of SWCNT and Vulcan XC72®. From the kinetic point of view, Vulcan XC72® containing electrodes have a better behaviour than those prepared with SWCNT. On the other hand, the optimal percentage of PTFE for SWCNT was determined to be 25%.     

Influence of activated carbon porous texture on catalyst activity for ethanol electro-oxidation

Direct ethanol fuel cells are based in ethanol electro-oxidation at low temperature and they constitute an alternative energy conversion system. However, they need catalysts in order to improve their efficiency, given that ethanol electro-oxidation is a slow-kinetic reaction. Among those catalysts platinum and platinum alloys play an important role in the increase of the ethanol cleavage kinetics for fuel cell application. However, to maximize the catalyst performance, support materials are needed in order to reduce the catalyst load considering its high cost. One of the more versatile catalyst supports is activated carbon. Recently, attention has focused on wood as carbon material precursor, because of its sustainability and also because the obtained carbons have excellent final properties. In the present work, activated carbon obtained by physical activation of Eucalyptus grandis wood (biocarbon) was tested as Pt and PtSn catalyst support, for ethanol electro-oxidation reaction. For comparison purposes, commercial activated carbon Vulcan XC72 was also tested. The catalyst supports were characterized by textural analysis, elemental analysis and infrared spectroscopy. The obtained catalysts were characterized regarding structure by XRD and their electrochemical behavior was evaluated by cyclic voltammetry. Biocarbon-supported PtSn electrocatalysts showed better electrochemical performance related to the commercial activated carbon (Vulcan XC72)-supported ones, since its developed current density and potential were the highest and its onset potential was the lowest. However, pure platinum showed better values for current density, potential and onset potential in Vulcan XC72-based activated carbons, being the untreated one the best support in this case.     

Preparation and characterization of nano-sized Pt–Pd/C catalysts and comparison of their electro-activity toward methanol and ethanol oxidation

In the present work nano-sized Pt–Pd alloys have been prepared by polyol process on Vulcan XC72. The information on structural characteristics and surface chemistry of the nano-material was obtained using TEM, XRD and XPS.     

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.     

Quantifying potential losses of non-Pt MEAs for gas-phase HBr PEM electrolyzer to produce hydrogen

Gas-phase HBr can be converted to hydrogen and bromine in a proton exchange membrane (PEM) electrolyzer. However, due to high cost and the poisoning of bromine and bromide ions on Pt electrodes, non-Pt MEAs (membrane electrode assembly) need to be developed and evaluated. In this paper, RuO₂, carbon (Vulcan XC 72R) and TiO₂–Nb (10% wt.) are prepared as anodes, and IrO₂/C and MoS₂ are prepared as cathodes for incorporation into MEAs. The individual electrodes in these MEAs are then evaluated by de-convoluting the individual voltage losses in-situ from the total electrolyzer voltage. On the anode, Pt, Vulcan XC 72R, TiO₂–Nb (10% wt.) and RuO₂ are all found to have comparable activity toward bromine evolution. On the cathode, Pt was more active toward the hydrogen evolution reaction (HER) compared to IrO₂/C, and both were far superior to MoS₂.     

Electro-oxidation of ethanol using PtRu/C electrocatalysts prepared by alcohol-reduction process

PtRu/C electrocatalysts were prepared by an alcohol-reduction process using ethylene glycol as solvent and reducing agent and Vulcan Carbon XC72R as support. Two different forms of electrocatalyst preparation were used, one in which the metals were reduced together in a single step (co-reduction of mixed ions) and the other with the metals being reduced separately in two successive steps (successive reduction of metal ions). The electrocatatalysts were characterized by EDAX, XRD, TEM and cyclic voltammetry and the electro-oxidation of ethanol was studied by cyclic voltammetry and chronoamperometry. The electrocatalysts prepared by co-reduction of mixed ions were the most active for ethanol oxidation and the performances increased with the increase of ruthenium content.     

Electrochemical performance of carbon-supported Pt(Cu) electrocatalysts for low-temperature fuel cells

Pt(Cu) nanoparticles supported on carbon nanofibers (CNFs), multi-walled carbon nanotubes (MWCNTs) and Vulcan carbon XC72, have been synthesized by electroless deposition and galvanic exchange. The structural analyses show contracted Pt fcc lattices due to the formation of a PtCu alloy core covered by a Pt-rich shell, mean crystallite sizes of about 3 nm, as well as good dispersion and carbon attachment. The electrochemical surface areas (ECSAs) of Pt(Cu)/CNF and Pt(Cu)/XC72 are comparable to those of commercial Pt/C and PtCu/C. The Pt(Cu) electrocatalysts show more negative onset potentials for CO oxidation than Pt/C and PtCu/C, thus indicating their greater CO tolerance. Pt(Cu)/CNF and Pt(Cu)/MWCNT present the highest mass activity and specific activity for the O₂ reduction, respectively, both with better relative stability than Pt(Cu)/XC72. Pt(Cu)/CNF and Pt(Cu)/MWCNT are then considered good cathode catalysts, yielding estimated savings of about 50 wt% Pt, when applied to low-temperature fuel cells.     

Effect of glidants on LaNi5 powder flowability

Tension accumulation in container walls is a matter of concern in hydride based hydrogen storage systems. As the hydrogen absorbing material swells during hydrogen absorption it will need to flow and accommodate within its container. Failure to do so will result in the build-up of tensions and, eventually, in the failure of the container after a number of absorption-desorption cycles. There have been several ways of avoiding the build-up of mechanical stresses: having a container geometry that allows the swelling of the hydride, combining the hydride forming alloys with other materials that can handle the volume increase or the stresses, and adding solid lubricants to improve the ability of the hydride to accommodate within the container. In the present study we explore the application of nanoscaled powders normally used in the industry as glidant agents for bulk powders. In particular, we address the influence of three different types of glidants in the flowability of LaNi₅ powder: Aerosil R 805, molybdenum disulfide (MoS₂) and Vulcan XC72 carbon black. For this purpose, we have used a pressurized rotating drum device that allows hydrogen pressure or vacuum to test LaNi₅ in hydrogen absorbed or desorbed states. The angle of repose results indicate an improvement on powder flowability when using Aerosil in concentrations of approximately 0.05 wt% or MoS₂ at concentrations of approximately 0.1 wt%. These results are in agreement with models that explain the reduction of interparticle forces when using small quantities of nanoscaled particles. Vulcan XC72 showed no effectiveness as glidant. This unexpected behavior is most likely related to its tendency to become trapped in the cracks of the larger LaNi₅ particles and to form relatively large aggregates.     

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.     

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.     

Novel mesoporous carbon ceramics composites as electrodes for direct methanol fuel cell

In this work, a new family of materials for electrodes of direct methanol fuel cell (DMFC) is presented. Mesoporous carbon ceramics (MCCs) are obtained by the addition of commercial graphite into the synthesis gel of SBA-15 mesoporous silica with SiO₂/C weight ratios of 1/1 and 1/3. X-ray diffraction confirms both the formation of organized silica and the presence of graphite, and nitrogen physisorption measurements show that the presence of a graphitic phase does not interfere in the silica pore diameter although it diminishes the surface area. The MCCs modified with Pt or PtRu are tested as DMFC electrodes and compared with the commercial support Vulcan XC-72R. When used as cathode, the system using MCC-SBA-15 with SiO₂/C weight ratios of 1/1 presents a negligible performance, while the MCC-SBA-15 with SiO₂/C weight ratios of 1/3 is 2.9 times less active than the commercial support. On the other side, when used as anode, the MCC-SBA-15 with SiO₂/C weight ratios of 1/3 displays performances comparable to Vulcan XC-72R.     

Key parameters of active layers affecting proton exchange membrane (PEM) fuel cell performance

The 2ⁿ full factorial design was applied to identify the key parameters of the active layer affecting the performance of a proton exchange membrane (PEM) fuel cell. Three main selected parameters were considered: carbon-type (Vulcan XC 72R and Black Pearls 2000 conducting furnace blacks, Cabot Corporation Boston, MA), Pt loading (0.1 and 0.5 mg/cm²), and Nafion™ sulfonic acid fluoropolymer (Du Pont de Nemours, Wilmington, DE) ionomer content (10% and 60%) for variables A, B, and C, respectively. The results from full factorial analysis indicated that the key factors affecting the exchange current density or activation loss were Pt loading whereas the key factors controlling the resistance due to ohmic loss were Nafion content and carbon type. In addition, there are the interactions between these parameters controlling the thin-film active layer performance, especially the interaction of carbon type and Nafion content. From cyclic voltammograms and cell performance testing, a Nafion content of 30% in a catalyst layer consisting of 0.5 mg/cm² Pt on Vulcan XC 72R is optimal.     

Use of cellulose-based carbon aerogels as catalyst support for PEM fuel cell electrodes: Electrochemical characterization

New nanostructured carbons have been developed through pyrolysis of organic aerogels, based on supercritical drying of cellulose acetate gels. These cellulose acetate-based carbon aerogels (CA) are activated by CO₂ at 800 °C and impregnated by PtCl₆²⁻; the platinum salt is then chemically or electrochemically reduced. The resulting platinized carbon aerogels (Pt/CA) are characterized with transmission electron microscopy (TEM) and electrochemistry. The active area of platinum is estimated from hydrogen adsorption/desorption or CO-stripping voltammetry: it is possible to deposit platinum nanoparticles onto the cellulose acetate-based carbon aerogel surface in significant proportions. The oxygen reduction reaction (ORR) kinetic parameters of the Pt/CA materials, determined from quasi-steady-state voltammetry, are comparable with that of Pt/Vulcan XC72R. These cellulose acetate-based carbon aerogels are thus promising electrocatalyst support for PEM application.     

Electrochemical and capacitive properties of thin-layer carbon black electrodes

Electrochemical properties and porous-structure-dependent capacitive ability of commercial carbon blacks, Black Pearls 2000^® (BP) and Vulcan^® XC 72R (XC), were investigated in H₂SO₄ solution by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The capacitance in-depth profile is correlated to microscopic appearance of carbon blacks in the form of a thin layer applied over Au substrate from water suspensions of BP and XC. The capacitance calculated from voltammetric charge was found to depend on the sweep rate, due to porosity of investigated materials. Impedance (EIS) characteristics upon frequency-dependent charge/discharge process indicate transmission line electric behavior of BP and XC. Capacitance and resistance values obtained by simulations of EIS data, enabled estimation of capacitance and resistance profile throughout carbon black porous electrodes. Capacitance of BP carbon layer increases going from the outer surface towards the bulk of the layer. External capacitance originates from capacitive characteristics of the macroscopic surface consisting of relatively large agglomerates, while internal capacitance originates from “inner” surface of micro-porous agglomerates. Contrary to BP, opposite distribution of the total capacitance to external and internal part was found for XC, caused by its loose structure and considerably lower real surface area in comparison to BP. The XC morphology makes additionally the pseudocapacitive contribution of surface functionalities more pronounced, which indirectly shifts also the “internal” double-layer capacitive response to higher frequencies through the effect of increased wettability of the layer. Thus, the capacitance of XC surface directly exposed to the electrolyte is larger than that of the inner one, which makes it a “fully-utilized” capacitor, while increased capacitive performance of BP emerges only at very low frequencies of charging/discharging process.     

One-step preparation and characterization of PtRu (1:1)/C electrocatalysts by polyol method for polymer electrolyte fuel cells

A one-step process is designed for the preparation of PtRu (1:1) electrocatalysts by the polyol method. Following investigations with an UV–vis spectrophotometer and an inductive-coupled plasma atomic emission spectrophotometer, it is found that an equimolar solution of Pt and Ru salts dissolved in ethylene glycol at temperatures above 190 °C is essential for the formation of the PtRu (1:1) solid solution. X-ray diffraction analysis is used to characterize the composition and size of the prepared electrocatalysts. The lattice parameter is 3.8663 Å, which corresponds to the value for a PtRu solid solution of equiatomic composition and this atomic ratio is confirmed by energy dispersive X-ray spectroscopy. Based on the Scherrer formula, the average particle size of the electrocatalysts is estimated to be 2.6 nm. It is confirmed by transmission electron microscopy that the nanoparticles are distributed uniformly on carbon, Vulcan XC 72. According to the results of unit cell test and CO-stripping voltammetry, the performance of the prepared electrocatalyst is comparable with that of a commercial one of the same composition.     

Electrochemical degradation of Nafion ionomer to functionalize carbon support for methanol electro-oxidation

An effective electrochemical route to produce functional groups on carbon surface is demonstrated. Cyclic voltammetric (CV) sweeps are performed in 0.5 M H₂SO₄ electrolyte on electrodes containing carbon cloth, Vulcan XC72R, and Nafion ionomer. With supply of ambient oxygen, the generation of hydroxyl radicals from the oxygen reduction reaction during CV cycles initiates the decomposition of Nafion ionomer that leads to formation of oxygenated functional groups on the carbon surface. Ion chromatography confirms the dissolution of sulfate anions upon CV scans. Raman analysis suggests a minor alteration for the carbon structure. However, X-ray photoelectron spectroscopy indicates a significant increase of oxygenated functional groups in conjunction with notable reduction in the fluorine content. The amount of the oxygenated functional groups is determined by curve fitting of C 1s spectra with known constituents. These functional groups can also be found by immersing the as-prepared electrode in a solution containing concentrated residues from Nafion ionomer decomposition. The functionalized electrode allows a 170% increment of Pt ion adsorption as compared to the reference sample. After electrochemical reductions, the functionalized electrode reveals significant improvements in electrocatalytic abilities for methanol oxidation, which is attributed to the oxygenated functional groups that facilitates the oxidation of CO on Pt.