Hollow core/mesoporous shell carbon capsule as an unique cathode catalyst support in direct methanol fuel cell

Hollow core mesoporous shell (HCMS) carbon has been explored for the first time as a cathode catalyst support in direct methanol fuel cells (DMFCs). The HCMS carbon consisting of discrete spherical particles possesses unique structural characteristics including large specific surface area and mesoporous volume and well-developed interconnected void structure, which are highly desired for a cathode catalyst support in low temperature fuel cells. Significant enhancement in the electrocatalytic activity toward oxygen reduction reaction has been achieved by the HCMS carbon-supported Pt nanoparticles compared with carbon black Vulcan XC-72-supported ones in the DMFC. In addition, much higher power was delivered by the Pt/HCMS catalysts (i.e., corresponding to an enhancement of ca. 91–128% in power density compared with that of Pt/Vulcan), suggesting that HCMS carbon is a unique cathode catalyst support in direct methanol fuel cell.     

The improved methanol tolerance using Pt/C in cathode of direct methanol fuel cell

Membrane electrode assemblies (MEA) were prepared using PtRu black and 60 wt.% carbon-supported platinum (Pt/C) as their anode and cathode catalysts, respectively. The cathode catalyst layers were fabricated using various amounts of Pt (0.5 mg cm⁻², 1.0 mg cm⁻², 2.0 mg cm⁻², and 3.0 mg cm⁻²). To study the effect of carbon support on performance, a MEA in which Pt black was used as the cathode catalyst was fabricated. In addition, the effect of methanol crossover on the Pt/C on the cathode side of a direct methanol fuel cell (DMFC) was investigated. The performance of the single cell that used Pt/C as the cathode catalyst was higher than single cell that used Pt black and this result was pronounced when highly concentrated methanol (above 2.0 M) was used as the fuel.     

Investigation of bimetallic Pt-M/C as DMFC cathode catalysts

A low temperature preparation procedure, based on a combination of colloidal and incipient wetness methods, was developed to modify the Pt catalyst with transition metals (Fe, Cu and Co). A moderate degree of alloying was obtained with Pt-Fe/C and Pt-Co/C cathode catalysts by using the new low temperature preparation route; whereas, a high degree of alloying was obtained for Pt-Cu/C by using the same procedure. Despite of the high metal concentration (60 wt%) on carbon, all catalysts showed small primary metal particle size and a low degree of agglomeration. These catalysts were investigated as cathodes in direct methanol fuel cells (DMFCs) operating at low temperatures (60 °C). It appeared that Pt-Fe/C catalysts were superior than Pt/C, Pt-Co/C and Pt-Cu/C catalysts both in terms of catalytic activity and tolerance to methanol. Adsorbed methanolic residues stripping analysis indicated a better methanol tolerance and an enhanced activity towards oxygen reduction in the case of the Pt-Fe system. An improvement of the DMFC single cell performance was also observed in the presence of Pt-Fe catalysts.     

Alternative cathodes based on iron phthalocyanine catalysts for mini- or micro-DMFC working at room temperature

Iron phthalocyanine based cathodes were prepared either by dispersion of FePc on carbon or by electropolymerization of aniline in presence of FeTsPc. The macrocycles based cathodes were compared to a classical commercial Pt/C cathode in a standard three-electrode electrochemical cell and under DMFC conditions at room temperature. It was shown that the molecular dispersion of FeTsPc into a PAni film greatly enhances the activity of the macrocycle catalyst towards oxygen reduction reaction (ORR). But, in the same time, the stability under DMFC conditions is drastically decreased compared to the stability obtained with a FePc/C electrode. It was suggested that this instability of the catalytic film was rather due to the release of the FeTsPc from the polymer than to the destruction of the macrocycle active centre. Even if iron phthalocyanine catalysts display total tolerance to methanol when the anode is fed with a 5 M methanol solution, the comparison between a PAni-FeTsPc/C cathode and a Pt/C cathode in DMFC working conditions is in favor of the Pt/C cathode, in term of maximum achieved power density. However, the ratio (platinum atoms per cm²/number of FeTsPc molecules per cm²) is close to 100, which allows to be optimistic for further enhancement of activity of polymer-FeTsPc electrodes. It was suggested that researches to develop new electron conductive polymers stable under oxidative environment and with a high doping capacity could be a direction to use platinum alternative cathode catalysts in DMFC technology.     

Characterization and performances of cobalt-tungsten and molybdenum-tungsten carbides as anode catalyst for PEFC

The preparation of carbon-supported cobalt-tungsten and molybdenum-tungsten carbides and their activity as an anode catalyst for a polymer electrolyte fuel cell were investigated. The electrocatalytic activity for the hydrogen oxidation reaction over the catalysts was evaluated using a single-stack fuel cell and a rotating disk electrode. The characterization of the catalysts was performed by XRD, temperature-programmed carburization, temperature-programmed reduction and X-ray photoelectron spectroscopy. The maximum power densities of the 30 wt% 873 K-carburized cobalt-tungsten and molybdenum-tungsten mixed with Ketjen carbon (cobalt-tungsten carbide (CoWC)/Ketjen black (KB) and molybdenum-tungsten carbide (MoWC)/KB) were 15.7 and 12.0 mW cm⁻², respectively, which were 14 and 11%, compared to the in-house membrane electrode assembly (MEA) prepared from a 20 wt% Pt/C catalyst. The CoWC/KB catalyst exhibited the highest maximum power density compared to the MoWC/KB and WC/KB catalysts. The 873 K-carburized CoW/KB catalyst formed the oxycarbided and/or carbided CoW that are responsible for the excellent hydrogen oxygen reaction.     

The Influence of Pt Loading,Support and Nafion Content on the Performance of Direct Methanol Fuel Cells: Examined on the Example of the Cathode

Nano‐sized Pt colloids were prepared using the polyol method and supported on Ketjen black EC 600J (KB), Vulcan XC‐72 (VC) and high surface area graphite 300 (HG). The effects of the Nafion ionomer content, and the Pt loading of the cathode catalyst layer as well as the Pt loading on the support on the performance of direct methanol fuel cells (DMFCs), were studied. The membrane electrode assemblies (MEAs) were analysed using current–voltage curves, cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and adsorbed CO stripping voltammetry. Optimum Nafion to carbon (N/C) ratios (N/C being defined as the weight ratio of the Nafion ionomer to the carbon) were determined. The optimum N/C ratios were found to depend on the support as follows, 1.4, 0.7 and 0.5 for Pt/KB, Pt/VC and Pt/HG, respectively and to be independent of the Pt/C loading range of 20–80 wt% tested in this work. The highest DMFC performances, as well as the highest electrochemical active surface areas, and improved gas diffusivities, were achieved using these ratios. For the catalysts prepared in this work, the average Pt crystallite size was found to decrease with increasing surface area of the support for a particular Pt loading. MEAs made using KB as support and the optimal N/C ratio of 1.4 showed the best performances, i.e. higher than the VC and HG supports for any N/C ratio. The highest DMFC performance was observed using 60 wt% Pt on KB cathode electrodes of 1 mg Pt cm^–2 loading and an N/C value of 1.4. For all three supports studied, the 60 wt% Pt on carbon loading resulted in the best DMFC performance. This may be linked to the Pt particle size and catalyst preparation method used in this work. In comparison to literature results, high DMFC performances were achieved using relatively ‘low' Pt and Ru loadings. For example, a maximum power density of >100 mW cm^–2 at 60 °C was observed using a 1 mg Pt cm^–2 cathode loading and a 2 mg PtRu cm^–2 anode loading.     

Rational determination of exchange current density for hydrogen electrode reactions at carbon-supported Pt catalysts

The rotating disk electrode (RDE) is a useful technique for precise determination of exchange current density (j₀) in electrochemistry. For the study of powder catalysts, a common practice is to apply the powder onto an inert disk substrate (such as glassy carbon). However, this approach in its usual version will lead to wrong results for the exchange current density of hydrogen electrode reactions at carbon-supported Pt nanoparticles (Pt/C) because of the poor utilization of the loaded Pt nanoparticles. Our new approach is to dilute the Pt/C powder with a large amount of pristine carbon support to make the catalyst layer. In this way, all the catalyst particles in the catalyst layer have nearly the same and much enhanced mass transport so that rational exchange current density can be obtained. Using the new approach, the current density for hydrogen electrode reactions at Pt/C in 0.1 M perchloric acid at 25 °C is found to be 27.2 ± 3.5 mA/cm² with an apparent activation energy 43 kJ/mol. These results are in agreement with the j₀ estimation based on real fuel cell experiments.     

Silica-sol-templated mesoporous carbon as catalyst support for polymer electrolyte membrane fuel cell applications

Mesoporous carbon (MC) samples having especially high specific surface area, pore size, and pore volume (e.g. pore volume in excess of 4 cm³ g⁻¹) were prepared and their suitability as Pt catalyst supports in polymer electrolyte membrane fuel cells was examined. Pt particles on the MC support were slightly larger than those on commercial samples of Pt on carbon black, and they showed a greater tendency to agglomerate on the MC support than on carbon black. Ex situ cyclic voltammetry gave values for electrochemically active surface area that were about half that for a commercial Pt-on-carbon black sample. Preliminary attempts to prepare thin-film electrodes from Pt/MC samples with a Nafion binder using conventional ink formulations failed, probably because much of the Nafion electrolyte was taken up inside support pores and was not available to bind the support particles together. An alternate approach involving painting of catalyst inks directly onto gas diffusion layers was used to prepare membrane electrode assemblies (MEAs) from Pt/MC samples, which were tested using single-cell test hardware. Performance of these Pt/MC sample MEAs was compared with that prepared by decal transfer method with commercially obtained Vulcan XC-72R supported Pt catalyst. The reasons for the lower performance of Pt/MC were discussed.     

Highly methanol-tolerant non-precious metal cathode catalysts for direct methanol fuel cell

This work reports on the oxygen reduction activity of several non-precious metal (non-PGM) catalysts for oxygen reduction reaction (ORR) at the fuel cell cathode, including pyrolyzed CoTPP, FeTPP, H₂TMPP, and CoTMPP. Of the studied catalysts, pyrolyzed CoTMPP (Co-tetramethoxyphenylporphyrin) was found to perform significantly better than other materials. The catalyst underwent a thorough testing in both hydrogen-air polymer electrolyte fuel cell (PEFC) and direct methanol fuel cell (DMFC). It was found that CoTMPP cathode can sustain currents that are only 2-3 times lower than those obtained with a conventional Pt-black cathode in an H₂-air PEFC. DMFC experiments, including methanol crossover and methanol tolerance measurements, indicate high ORR selectivity of the CoTMPP catalyst. Based on results obtained to date, the CoTMPP-based catalyst offers promise for the use in conventional and mixed-reactant DMFCs operating with concentrated methanol feeds. However, hydrogen-air fuel cell life data, consisting of over 800 h of continuous cell operation, indicate that improvement to long-term stability of the CoTMPP catalyst will be required to make it practical.     

Surface-modified carbons as platinum catalyst support for PEM fuel cells

Carbon forms, such as activated carbon, carbon black, carbon nanofibers and nanotubes, can be used as support materials for precious metal catalysts used in fuel cell electrodes. This work first compares the ability of functionalized high surface area graphitic (carbon nanofibers) and amorphous (activated carbon) carbons to homogeneously support finely divided platinum catalyst particles, then contrasts the performance of platinum/carbon composite electrodes within a hydrogen fuel cell. Functionalization by concentrated acid treatment results in the creation of various oxygen carrying functionalities on the otherwise inert carbon surfaces. The degree of surface functionalization is found to be a function of the functionalization treatment strength. Chemical reduction of the platinum precursor complex using milder reducing agents in the temperature range of 75-85 °C, and using ethylene glycol at 140 °C yields the smallest platinum particle sizes observed in this study, a result confirmed by X-ray diffraction and transmission electron microscopy measurements. X-ray photoelectron spectroscopy measurements confirm the existence of platinum in primarily its metallic state on the functionalized carbon surfaces.     

Mass transport of direct methanol fuel cell species in sulfonated poly(ether ether ketone) membranes

Homogeneous membranes based on sulfonated poly(ether ether ketone) (sPEEK) with different sulfonation degrees (SD) were prepared and characterized. In order to perform a critical analysis of the SD effect on the polymer barrier and mass transport properties towards direct methanol fuel cell species, proton conductivity, water/methanol pervaporation and nitrogen/oxygen/carbon dioxide pressure rise method experiments are proposed. This procedure allows the evaluation of the individual permeability coefficients in hydrated sPEEK membranes with different sulfonation degrees. Nafion^® 112 was used as reference material. DMFC tests were also performed at 50 °C. It was observed that the proton conductivity and the permeability towards water, methanol, oxygen and carbon dioxide increase with the sPEEK sulfonation degree. In contrast, the SD seems to not affect the nitrogen permeability coefficient. In terms of selectivity, it was observed that the carbon dioxide/oxygen selectivity increases with the sPEEK SD. In contrast, the nitrogen/oxygen selectivity decreases. In terms of barrier properties for preventing the DMFC reactants loss, the polymer electrolyte membrane based on the sulfonated poly(ether ether ketone) with SD lower or equal to 71%, although having slightly lower proton conductivity, presented much better characteristics for fuel cell applications compared with the well known Nafion^® 112. In terms of the DMFC tests of the studied membranes at low temperature, the sPEEK membrane with SD = 71% showed to have similar performance, or even better, as that of Nafion^® 112. However, the highest DMFC overall efficiency was achieved using sPEEK membrane with SD = 52%.     

Microstructure effects on the electrochemical corrosion of carbon materials and carbon-supported Pt catalysts

The electrochemical corrosion behavior of a set of porous carbonaceous materials of interest as catalyst supports for polymer electrolyte membrane fuel cells was examined in 2 M H₂SO₄ at 80 °C at constant electrode potential of 1.2 V vs. RHE. Correlations have been observed between the specific rates of corrosion of carbon materials and carbon-supported Pt catalysts on the one hand and their substructural characteristics derived from X-ray diffraction analysis on the other hand. Carbon supports of the Sibunit family and catalytic filamentous carbons possess lower specific (i.e., surface area normalized) corrosion currents compared to conventional furnace black Vulcan XC-72 and better stabilize Pt nanoparticles.     

Novel approach to membraneless direct methanol fuel cells using advanced 3D anodes

A conventional membrane electrode assembly (MEA) for a direct methanol fuel cell (DMFC) consists of a polymer electrolyte membrane (PEM) compressed between an anode and cathode electrode. Limitations with this conventional design include: cost, fuel crossover, membrane degradation or contamination, ohmic losses and reduced active triple phase boundary (TPB) sites for catalyst located away from the electrode/membrane interface. In this work, ex situ and in situ characterization of a novel electrode assembly based on a membraneless architecture and advanced 3D anodes was investigated. The approach was shown to be fuel independent and scaleable to a conventional bi-polar fuel cell arrangement. The membraneless configuration exhibits comparable performance to a conventional ambient (25 °C, 1 atm) air-breathing DMFC. However, it has the additional advantages of a simplified design, the elimination of the membrane (a significant component expense) and enhanced fuel and catalyst utilization through the extension of the active catalyst zone.     

Characterization of proton exchange membranes based on SPSEBS/SPSU blends

Proton-conducting polymer membranes are used as an electrolytes in proton exchange membrane fuel cells (PEMFCs). The most widely used commercially available membrane electrolytes are perfluorosulfonic acid polymers, an expensive class of ionomers. In this study, the potential of polymer blends derived from sulfonated polystyrene ethylene butylene polystyrene (SPSEBS) and sulfonated polysulfone (SPSU) for use in electrolyte applications was examined. Although SPSEBS by itself exhibits good conductivity, flexibility, and chemical stability, it has poor mechanical stability. So, in an effort to improve the mechanical properties of SPSEBS while maintaining its good conductivity, it was blended with SPSU. SPSEBS/SPSU blends were therefore prepared by a solvent evaporation method, and the resulting blend membranes were characterized in terms of conductivity, ionic exchange capacity, and water uptake. Sulfonation was confirmed and the crystallinity of the blend membranes was studied by FTIR spectroscopy and X-ray diffraction. The morphologies of the membranes were studied by scanning electron microscope (SEM), and their thermal stabilities by TGA and DSC. Finally, the mechanical strength of SPSEBS was studied using a UTM (universal testing machine). This paper presents the results of recent investigations aimed at developing an optimized in-house membrane electrode assembly (MEA) preparation technique combining catalyst ink spraying and assembly hot pressing. Easy steps were chosen for this preparation technique in order to simplify the method, thus minimizing costs. The influence of MEA fabrication parameters like electrode pressing or annealing on the performance of the hydrogen fuel cell was studied by performing single cell measurements during H₂/O₂ operation. Carbon cloth was used as a gas diffusion layer (GDL), and the composition of the electrode ink was optimized to maximize fuel cell performance. A commercial E-TEK catalyst was used for the anode and cathode, with Pt loadings of 0.125 and 0.37 mg/cm², respectively. The MEA with the best performance delivered approximately 0.50 W/cm² at room temperature. The methanol permeability and the selectivity ratio strongly influenced DMFC performance. Both direct methanol fuel cells (DMFCs) and PEMFCs are discussed in this paper.     

Optimization of the sputter-deposited platinum cathode for a direct methanol fuel cell

The electrodes prepared by a sputtering method were evaluated as the cathodes for direct methanol fuel cells (DMFCs). Pt loading below 0.25 mg cm⁻² achieved higher mass activities than that of 0.5 mg cm⁻² prepared by the paste method, which was general conventional method. However, an increase in Pt loading reduced the catalyst activity for the oxygen reduction reaction (ORR). This result may suggest an increase in only electrochemically inactive Pt. Pt utilization efficiency can be found about ten times higher at Pt loading of 0.04 mg cm⁻². Moreover, addition of Nafion to sputter-deposited Pt cathodes is found possible to improve the catalyst activity for the ORR, but the excess Nafion over the optimum condition reduces the active sites.     

Performance comparison of portable direct methanol fuel cell mini-stacks based on a low-cost fluorine-free polymer electrolyte and Nafion membrane

A low-cost fluorine-free proton conducting polymer electrolyte was investigated for application in direct methanol fuel cell (DMFC) mini-stacks. The membrane consisted of a sulfonated polystyrene grafted onto a polyethylene backbone. DMFC operating conditions specifically addressing portable applications, i.e. passive mode, air breathing, high methanol concentration, room temperature, were selected. The device consisted of a passive DMFC monopolar three-cell stack. Two designs for flow-fields/current collectors based on open-flow or grid-like geometry were investigated. An optimization of the mini-stack structure was necessary to improve utilization of the fluorine-free membrane. Titanium-grid current collectors with proper mechanical stiffness allowed a significant increase of the performance by reducing contact resistance even in the case of significant swelling. A single cell maximum power density of about 18 mW cm⁻² was achieved with the fluorine-free membrane at room temperature under passive mode. As a comparison, the performance obtained with Nafion 117 membrane and Ti grids was 31 mW cm⁻². Despite the lower performance, the fluorine-free membrane showed good characteristics for application in portable DMFCs especially with regard to the perspectives of significant cost reduction.     

Gas diffusion electrodes containing sulfonated polyether ionomers for PEFCs

Gas diffusion electrodes (GDEs) containing sulfonated polyether (SPE) ionomers as proton conducting binder have been prepared and evaluated in H₂/O₂ polymer electrolyte fuel cells. An autoclave treatment has been applied for the first time to a hydrocarbon ionomer for the preparation of GDEs. The GDEs worked well as anode without practical overpotential up to 800 mA/cm² of the current density. As cathode, the GDEs showed significant dependence on the SPE content and its ion exchange capacity (IEC). Higher catalyst utilization was achieved for the GDEs with higher SPE content due to enhanced proton conduction. Cyclic voltammetry implied higher catalyst utilization of the SPE-based GDEs than that of the conventional Nafion^®-based GDEs. Scanning transmission electron microscopy (STEM) revealed that the SPE ionomer coated uniformly on the surface of Pt/carbon black catalysts. Humidification conditions affect proton conductivity and swelling of the SPE ionomer and thus were crucial for the cathode performance. SPE ionomer with medium IEC (2.17 meq/g) served best in GDEs in terms of catalyst utilization.     

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

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

Synthesis and electro-catalytic activity of methanol oxidation on nitrogen containing carbon nanotubes supported Pt electrodes

Template synthesis of various nitrogen containing carbon nanotubes using different nitrogen containing polymers and the variation of nitrogen content in carbon nanotube (CNT) on the behaviour of supported Pt electrodes in the anodic oxidation of methanol in direct methanol fuel cells was investigated. Characterizations of the as-prepared catalysts are investigated by electron microscopy and electrochemical analysis. The catalyst with N-containing CNT as a support exhibits a higher catalytic activity than that carbon supported platinum electrode and CNT supported electrodes. The N-containing CNT supported electrodes with 10.5% nitrogen content show a higher catalytic activity compared to other N-CNT supported electrodes. This could be due to the existence of additional active sites on the surface of the N-containing CNT supported electrodes, which favours better dispersion of Pt particles. Also, the strong metal-support interaction plays a major role in enhancing the catalytic activity for methanol oxidation.     

Mechanical milling of catalyst support for enhancing the performance in fuel cells

In this work, we studied the characteristic variations of catalyst supports caused by mechanical milling and their electrochemical application in fuel cells. Two different catalyst supports, carbon black (XC-72R) and K20 (mesoporous carbon), were crushed and dispersed by mechanical milling using a bead mill. The bead mill operated with 0.3 μm zirconia beads at the rate of 3500 rpm for 30 min. The secondary particle size of the crushed catalyst supports ranged from around 0.1 μm to 10 μm. The secondary particle size of the catalyst supports after crushing represents a decrease of approximately 10% compared with that of raw catalyst supports. To confirm the role of the catalyst supports in the direct methanol fuel cell (DMFC), Pt and Ru were loaded onto these catalyst supports using an impregnation method. In the single cell test, Pt-Ru/XC-Bead and PtRu/K20-Bead showed power densities of 135 mW/cm² and 144 mW/cm² under air at 60 °C, respectively. The performance values of these catalysts, which were fabricated using reformed catalyst supports, were 10% to 20% higher than those of raw catalyst supports. As a result, the catalyst supports crushed by the bead mill helped to improve the electrochemical performance of the direct methanol fuel cell.