Synthesis and evaluation of carbon nanotube-supported RuSe catalyst for direct methanol fuel cell cathode

Ruthenium selenide (RuSe) supported on a carbon nanotube (CNT) material, i.e., RuSe/CNT, with a controlled composition (Ru:Se = 1:0.2) was synthesized using a modified polyol method as a model catalyst for direct methanol fuel cell (DMFC) cathode. The prepared electrocatalyst was physically characterized by means of Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) Spectroscopy and X-ray Photoelectron Spectroscopy (XPS), and its activity for oxygen reduction reaction (ORR) was examined using Linear-Sweep Voltammetry (LSV). In addition, the methanol tolerance was characterized using Electrochemical Impedance Spectroscopy (EIS). It was found that the prepared RuSe/CNT catalyst has good catalyst morphology, uniform and small particle size, and controllable catalyst composition. After subjecting to a proper heat treatment at 400 °C, the electrocatalyst exhibits a good oxygen reduction activity with high methanol tolerance. From both LSV and XPS analyses, it was concluded that a high Se3d_5/2 content plays an important role for oxygen reduction on RuSe/CNT. The EIS characterization also identified the presence of reaction intermediates during the oxygen reduction process. Based on the test results, the mechanisms underlying the dual function of the RuSe/CNT catalyst are proposed. The prepared catalyst was further evaluated for its potential application to DMFC. At 70 °C, the single-cell DMFC integrated with RuSe/CNT exhibited a performance much better than that incorporated with Pt/C counterpart when operated with a high-concentration (i.e., 6 M) methanol fuel. However, substantial improvements are still needed for practical applications.     

Analysis of performance losses of direct methanol fuel cell with methanol tolerant PtCoRu/C cathode electrode

The long-term stability of PtCoRu/C to methanol crossover has been evaluated in a direct methanol fuel cell (DMFC) configuration. The DMFC has been subjected to continuous operation under potential step cycles. The degradation of the DMFC with PtCoRu/C has been followed by comparison of the power density curves recorded after 0, 60 and 312 h of continuous operation, and compared to that recorded for a DMFC with Pt/C. Electrochemical Impedance Spectra (EIS) were recorded directly from the DMFCs and used to identify the main degradation phenomena responsible for the loss of performance of the used fuel cell. AC impedance spectra show that the resistance of the anode reaction increases while resistance associated to the cathode reaction decreases after the long-term stability tests; however, the analysis of the power density curves unequivocally show that the performance of the DMFCs goes down during the stability tests. This apparent contradiction can be explained by taking into account the changes between the fresh and used PtCoRu/C observed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. During the potential step cycles Ru dissolves form PtCoRu/C leading to Pt-enriched catalysts which are more active for the oxygen reduction reaction (lower resistance) but less tolerant to methanol (lower power density).     

Investigation of AuNi/C anode catalyst for direct methanol fuel cells

AuNi nanoparticles supported on the activated carbon (AuNi/C) are synthesized by the impregnation method in the ethyleneglycol system using NH₂NH₂·H₂O as a reducing agent. The alloying of Au and Ni and the removal of unalloyed Ni in the AuNi/C composition are achieved by heat and acid treatments in sequence. Research results reveal that the average size and alloying degree of the AuNi nanoparticles in the AuNi/C catalyst increase with the enhancement of the annealing temperature. However, the Ni content of the AuNi/C catalyst firstly goes up and then down with the rising of heat treatment temperature due to the AuNi system phase-separates. Moreover, the electrocatalytic activity normalized by the electrochemically active surface area of each AuNi/C catalyst is far better than that of the Au/C catalyst, because of the bifunctional mechanism and the electrocatalytic activity of the NiOOH. In particular, the AuNi/C catalyst annealed at 400 °C exhibits the most excellent activity, due to its small AuNi particles and proper alloying degree. Furthermore, its mass-specific electrochemical activity is higher than that of the Au/C catalyst, although the mean diameter of the AuNi nanoparticles in this catalyst is larger than that of the Au nanoparticles.     

Increased interface effects of PtFe alloy/CeO2/C with PtFe selective loading on CeO2 for superior performance in direct methanol fuel cell

Here, a simple two-step solvothermal approach has been employed to synthesize PtFe alloy (or Pt)/CeO₂/C with PtFe (or Pt) selective loading on CeO₂ nanoparticles. In addition, the selective loading of PtFe alloy or Pt nanoparticles on the surface of CeO₂ is achieved under weak alkaline environment, which is mainly attributed to the opposite electrostatic force between H⁺ enriched on the surface of CeO₂ particles and OH⁻ covered with carbon supporters. As-prepared PtFe alloy (or Pt)/CeO₂/C catalysts with two-stage loading structures show more excellent electro-catalytic efficiency for methanol oxidation as well as duration compared with commercial Pt/C and PtCeO₂/C with random loading structure. Further, single-cell assembly based on Pt₃Fe/CeO₂/C as the anode catalyst exhibits a maximum power density of 31.1 mW cm⁻², which is 1.95 times that of an analogous cell based on the commercial Pt/C. These improved performances with considerable low Pt content (<0.3 mg cm⁻²) are mainly ascribed to the abundant three phase interfaces (PtCeO₂ carbon) induced by the selective and efficient dispersion of Pt nanoparticles on ceria.     

Palladium selenides as active methanol tolerant cathode materials for direct methanol fuel cell

Palladium selenides, PdSe, Pd₃Se and PdSe₂ have been prepared by the hydrothermal method and investigated for their structural and electrocatalytic properties toward the oxygen reduction reaction (ORR) using SEM/TEM, XRD, cyclic and linear sweep voltammetries. The crystallites of PdSe and PdSe₂ are found to follow tetragonal and orthorhombic crystal structures, respectively. The PdSe electrode in 0.5 M H₂SO₄ exhibits significantly higher electrocatalytic activity than the Pd₃Se or PdSe₂ electrode under similar experimental conditions. Further, a change in the palladium/selenium ratio from unity in the catalyst results in low ORR activity.     

Synthesis and optimization of PtRu/TiO2-CNF anodic catalyst for direct methanol fuel cell

Direct methanol fuel cell (DMFC) is a promising power source technology, but it has been unable to be successfully commercialized due to its high cost and low kinetic oxidation. Both problems stem from one of its main components, the catalyst. Therefore, this study is focused on determining and optimizing the electrocatalyst parameters of a high-performance DMFC. The electrocatalyst, PtRu/TiO₂-CNF, is produced by the deposition method and is subjected to electrochemical measurement and cyclic voltammetry (CV) to measure half-cell performance in a DMFC. The optimization process involved two main phases, a screening process followed by response surface methodology (RSM). The resulting optimum parameters were then used for the single cell performance testing. The results show that the mathematical model suggested by RSM is adequate for the optimization of the parameter levels. The optimum parameters suggested by RSM are a PtRu composition of 30.25% and a catalyst loading of 0.59 mg/cm², resulting in almost perfect agreement between the measured current density (603.06 mA/mg_PtRu) and the predicted value (600.63 mA/mg_PtRu). The current density obtained in this study is the highest among other researchers in the same field.     

PtCo catalyst with modulated surface characteristics for the cathode of direct methanol fuel cells

A PtCo catalyst with an ordered cubic primitive structure was synthesized and investigated for the application as a cathode in direct methanol fuel cells. The synthesis involved the preparation of an amorphous PtOx/C precursor by the sulfite complex route, an impregnation with Co(NO₃)₂, a high temperature (800 °C) carbothermal reduction and, finally, a leaching procedure. This method led to the occurrence of a Pt₃Co/C catalyst with a primitive cubic ordered (L1₂) phase and a mean crystallite size of 3.3 nm, as well as a suitable degree of alloying. This electrocatalyst was investigated for the oxygen reduction reaction in a direct methanol fuel cell (DMFC) operating in the range 30–90 °C. At 60 °C, under atmospheric pressure, a maximum power density of 40 mW cm⁻² was obtained with the new PtCo catalyst formulation at low noble metal loading on the electrode (1 mg cm⁻²). This performance was 2.3 times higher than a benchmark Pt catalyst used for comparison.     

A novel Pt/Cr/Ru/C cathode catalyst for direct methanol fuel cells (DMFC) with simultaneous methanol tolerance and oxygen promotion

New carbon supported electrocatalysts Pt/Cr/Ru with distinct compositions and preparation methods were studied with the help of different electrochemical and spectroscopic techniques. The purposes of obtaining these catalysts lie on their possibilities towards methanol/oxygen fuel cells. In this sense, the oxygen reduction reaction and methanol oxidation reaction were analyzed using stationary and fluid dynamic methodologies. Pt_7.8/Ru_1.3/Cr_0.5 and Pt_8.0/Ru_2.0/Cr_0.1 were the most interesting prepared substrates, on which the first one shows the best catalytic properties towards methanol oxidation and the second the finest performance towards oxygen reduction reaction. Reaction orders with respect to oxygen for the oxygen reduction reaction were obtained being equal to ½ at potentials lower than 0.80 V for both catalysts. Polarization curves run for this reaction depicted two Tafel slopes, i.e. 0.09 V dec⁻¹ above 0.8 V and 0.20 V dec⁻¹ below 0.8 V for both catalysts. An analysis of the most likely mechanism for the oxygen reduction was proposed on the base of those reaction orders and Tafel slopes.     

Development of an air-breathing direct methanol fuel cell with the cathode shutter current collectors

An air-breathing direct methanol fuel cell with a novel cathode shutter current collector is fabricated to develop the power sources for consumer electronic devices. Compared with the conventional circular cathode current collector, the shutter one improves the oxygen consumption and mass transport. The anode and cathode current collectors are made of stainless steel using thermal stamping die process. Moreover, an encapsulation method using the tailor-made clamps is designed to assemble the current collectors and MEA for distributing the stress of the edges and inside uniformly. It is observed that the maximum power density of the air-breathing DMFC operating with 1 M methanol solution achieves 19.7 mW/cm² at room temperature. Based on the individual DMFCs, the air-breathing stack consisting of 36 DMFC units is achieved and applied to power a notebook computer.     

Mixed conducting catalyst support materials for the direct methanol fuel cell

Finely dispersed Pt- and Pt/Ru-particles have been deposited on high surface-area ruthenium dioxide (RuO₂) using the Petrow and Allen method [U.S. Patent No. 4,044,193 (23 August 1977)]. RuO₂ has been synthesized according to different preparation methods. It turned out that the product showing the highest surface area could be produced by a simple fast precipitation method. The electrocatalytic activities of catalysts on different ruthenium oxide supports have been investigated in half-cell experiments by stationary current voltage measurements. Pt/Ru-catalysts deposited on a Vulcan XC-72 carbon black have been used for comparison.X-ray analysis methods (XRD, EDX) have been used to characterize the composition and crystallinity of the materials and their geometric surface areas have been determined by the BET method.It turned out that the electric conductivity of the RuO₂ materials was comparable to that observed for Vulcan XC-72. Furthermore, RuO₂ materials having a BET surface area above 125 m²/g could be synthesized. (Vulcan XC-71: ∼250 m²/g).Surprisingly, no significant electrochemical activity was found when Pt/Ru was deposited on freshly precipitated hydrous RuO₂. Deposition of noble metals on calcined RuO₂ resulted in electrochemical activities comparable to the ones obtained for the Vulcan XC-72 support. Thus, no extraordinary enhancement of catalytic activity for the methanol has been observed when RuO₂ oxide was used as a mixed conducting catalyst support.     

Effect of carbon black additive in Pt black cathode catalyst layer on direct methanol fuel cell performance

Vulcan XC-72R, Ketjen Black EC 300J and Black Pearls 2000 carbon blacks were used as the additive in Pt black cathode catalyst layer to investigate the effect on direct methanol fuel cell (DMFC) performance. The carbon blacks, Pt black catalyst and catalyst inks were characterized by N₂ adsorption and scanning transmission electron microscopy (STEM) with Energy dispersive X-ray (EDX) spectroscopy. The cathode catalyst layers without and with carbon black additive were characterized by scanning electron microscopy, EDX, cyclic voltammetry and current-voltage curve measurements. Compared with Vulcan XC-72R and Black Pearls 2000, Ketjen Black EC 300J was more beneficial to increase the electrochemical surface area and DMFC performance of the cathode catalyst layer. The cathode catalyst layer with Ketjen Black EC 300J additive was kept intimately binding with the Nafion membrane after 360 h stability test of air-breathing DMFC.     

Application of hyperdispersant to the cathode diffusion layer for direct methanol fuel cell

In this study, Nafion ionomer, as a kind of hyperdispersant, was added to polytetrafluoroethylene (PTFE) water dispersion system to suppress the size of PTFE particles in the ink of microporous layer (MPL). The agglomeration behavior of PTFE in ethanol and MPL were investigated by laser diffraction, dynamic light scattering (DLS) and metallurgical microscopes. The electronic resistance, pore size distribution, gas permeability and surface hydrophobic/hydrophilic properties were also characterized for prepared gas diffusion layers (GDLs). It was shown that PTFE water dispersion system suffered flocculating when dispersed in ethanol and this agglomeration behavior was reduced by employing Nafion ionomer. With the increase in the Nafion ionomer adopted in the MPL, not only the decreased hydrophobic property was shown in the MPL, but the decreased PTFE particle size was also achieved, which results in improved crosslink of carbon and pores themselves as well as the volume loss of pores in micron scale. The increased gas permeability and electronic conductivity of the GDL made the one employing the PTFE dispersion system with 1% Nafion content own its advantages as the cathode diffusion layer for a direct methanol fuel cell (DMFC) under near-ambient conditions.     

Improved mass transfer using a pore former in cathode catalyst layer in the direct methanol fuel cell

The cathode catalyst layer in direct methanol fuel cells (DMFCs) was prepared using polystyrene beads as a pore former. Field emission scanning electron microscopy showed that the catalyst layer with the pore former contained pores with a uniform shape and size. Mercury intrusion porosimetry showed that the pore former increased the volume of secondary pores in the catalyst layer. The electrochemical properties of the membrane electrode assembly (MEA) were evaluated by current–voltage polarization measurements, electrochemical impedance spectroscopy and cyclic voltammetry. These results suggest that the catalyst layer with the pore former reduces the mass transfer resistance and improves the cell performance by approximately 50% through modification of its morphology.     

Iron phthalocyanine as a cathode catalyst for a direct borohydride fuel cell

A direct borohydride fuel cell (DBFC) is constructed using a cathode based on iron phthalocyanine (FePc) catalyst supported on active carbon (AC), and a AB₅-type hydrogen storage alloy (MmNi_3.55Co_0.75Mn_0.4Al_0.3) was used as the anode catalyst. The electrochemical properties are investigated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), etc. methods. The electrochemical experiments show that FePc-catalyzed cathode not only exhibits considerable electrocatalytic activity for oxygen reduction in the BH₄⁻ solutions, but also the existence of BH₄⁻ ions has almost no negative influences on the discharge performances of the air-breathing cathode. At the optimum conditions of 6 M KOH + 0.8 M KBH₄ and room temperature, the maximal power density of 92 mW cm⁻² is obtained for this cell with a discharge current density of 175 mA cm⁻² at a cell voltage of 0.53 V. The new type alkaline fuel cell overcomes the problem of the conventional fuel cell in which both noble metal catalysts and expensive ion exchange membrane were used.     

Anode catalyst layer with novel microstructure for a direct methanol fuel cell

An anode catalyst layer (ACL) with novel microstructure for direct methanol fuel cells (DMFCs) based on the catalyst coated membrane (CCM) technique is prepared by heating-spray method in this paper. The morphology and pore distribution of the novel ACL are characterized by scanning electron microscopy (SEM) and specific surface area (BET) analysis, respectively. A continuous flocculent structure with abundant micropores and a clear hierarchical distribution of pore diameter are obtained in the ACL. The DMFC performance is investigated by polarization curves measurements. In compared with traditional ACL, the novel structure ACL has larger active surfaces and exhibits higher performance with 39% increase in maximum power density during the single cell test at 80 °C. The possible reason for the increased performance has been analyzed and ascribed to the novel structure for the ordered mass transfer and the reduced internal resistance.     

PtRu/C-Au/TiO2 electrocatalyst for a direct methanol fuel cell

Au/TiO₂ is added to a PtRu/C electrode to improve the performance of a direct methanol fuel cell (DMFC). A high-throughput-screening test is performed for the fast screening of the loading of Au/TiO₂ on PtRu/C. The electrochemically-active surface area of PtRu/C-Au/TiO₂ and PtRu/C is determined from cyclic voltammetry. In CO-stripping and methanol oxidation voltammetry, PtRu/C-Au/TiO₂ exhibits better activity for CO and methanol oxidation than PtRu/C. The performance of the DMFC is also improved by addition of Au/TiO₂ to the PtRu/C electrode. The CO adsorbed on Pt may move to the surface of the Au/TiO₂ by the interaction between PtRu/C and Au/TiO₂. The improved performance of the PtRu/C-Au/TiO₂ catalyst is explained in terms of preferential oxidation of CO or CO-like poisoning species that are generated during the oxidation of methanol on PtRu/C.     

Electrochemical preparation of Pt-based ternary alloy catalyst for direct methanol fuel cell anode

The CoPtRu catalyst was prepared with electrochemical methods on carbon paper. The preparation of Co particles on the carbon paper was performed through an electrodeposition process by varying the deposition potential and time. After Co electrodeposition, Pt and Ru galvanic displacements were carried out by controlling displacement time. The bulk and surface composition of the catalysts were analyzed by using inductively coupled plasma (ICP) mass spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. It was proved that the CoPtRu catalyst was successfully synthesized using the electrochemical process. In this study, the electrochemically prepared catalysts showed superior catalytic activity for methanol oxidation and tolerance to CO poisoning compared to a commercial PtRu/C catalyst (E-tek).     

Development of a passive direct methanol fuel cell stack for high methanol concentration

In order to develop a vertically arranged passive DMFC with a porous carbon plate, PCP, the effect of the head height of the methanol solution in contact with the porous carbon plate on the power generation was investigated for a 55 mm height using a single cell. The single cell was operated at several methanol concentrations greater than 70 wt%. By filling the reservoir with 90 and 100 wt% methanol solutions, power densities greater than 30 mW cm⁻² for over 10 h were demonstrated. Based on the result of the single cell study, a passive DMFC stack consisting of 8 unit cells with the PCP was designed and fabricated. The power generation characteristics were then experimentally measured. The maximum power output of 1.8 W, which was almost 10% lower than that expected from the single cell performance, was obtained with 100% methanol. At the same time, a nonuniform cell voltage among the 8 unit cells was found as a reason for the decreasing power output with the increasing current.     

Methanol-tolerant cathode electrode structure composed of heterogeneous composites to overcome methanol crossover effects for direct methanol fuel cell

A methanol-tolerant cathode electrode composed of heterogeneous composites was developed to overcome CO poisoning and large O₂ mass transfer overpotential generated by methanol crossover as well as the limitation of a single alloy catalyst with methanol-tolerance in direct methanol fuel cells (DMFCs). Two additives, PtRu black and PTFE particles, were well distributed in the Pt/C matrix of the cathode electrode, and had significant effects upon open circuit voltage (OCV) and performance. A small amount of PtRu black protected the Pt surface during the oxygen reduction reaction (ORR) by decreasing CO poisoning. In addition, hydrophobic PTFE particles reduced the O₂ mass transfer overpotential induced by water and permeated methanol in the cathode. Despite only 0.5 mg cm⁻² of metal catalysts in the cathode, the membrane electrode assembly (MEA) with 3 M methanol showed high performance (0.117 W cm⁻²), which was larger than that of the traditional MEA (0.067 W cm⁻²).