Direct methanol fuel cells with reticulated vitreous carbon,uncompressed graphite felt and Ti mesh anodes

Reticulated vitreous carbon (RVC, 39 pores per cm), uncompressed graphite felt (UGF) and Ti mesh were investigated as 3-D anode catalyst supports for direct liquid methanol fuel cells with the aim of improving the catalyst mass specific activity. Mesoporous Pt–Ru layers composed of nano-particle agglomerates were electrodeposited on the 3-D substrates using a micellar deposition media composed of Triton X-100, isopropanol, and an aqueous phase containing H₂PtCl₆ and (NH₄)₂RuCl₆. The effect of deposition current density, support type, and counter electrode design on the catalyst layer morphology, mass loading and elemental composition is discussed. In direct methanol fuel cell experiments using 1 M CH₃OH—0.5 M H₂SO₄ the 3-D anodes with PtRu load between 2.8 g m⁻² (on Ti mesh) and 12.0 g m⁻² (on RVC) and Pt:Ru atomic ratio of about 4:1 provided peak power outputs based on catalyst mass of 50.4 W g⁻¹ and 40.5 W g⁻¹, respectively, at 333 K. The mass specific activity of the catalyst supported on the 3-D matrix is determined by the synergy between catalyst deposition procedure and support physico-chemical properties.     

High activity PtRu/C catalysts synthesized by a modified impregnation method for methanol electro-oxidation

A modified impregnation method was used to prepare highly dispersive carbon-supported PtRu catalyst (PtRu/C). Two modifications to the conventional impregnation method were performed: one was to precipitate the precursors ((NH₄)₂PtCl₆ and Ru(OH)₃) on the carbon support before metal reduction; the other was to add a buffer into the synthetic solution to stabilize the pH. The prepared catalyst showed a much higher activity for methanol electro-oxidation than a catalyst prepared by the conventional impregnation method, even higher than that of current commercially available, state-of-the-art catalysts. The morphology of the prepared catalyst was characterized using TEM and XRD measurements to determine particle sizes, alloying degree, and lattice parameters. Electrochemical methods were also used to ascertain the electrochemical active surface area and the specific activity of the catalyst. Based on XPS measurements, the high activity of this catalyst was found to originate from both metallic Ru (Ru⁰) and hydrous ruthenium oxides (RuO_xH_y) species on the catalyst surface. However, RuO_xH_y was found to be more active than metallic Ru. In addition, the anhydrous ruthenium oxide (RuO₂) species on the catalyst surface was found to be less active.     

Effect of transition metals (Ni,Sn and Mo) in Pt5Ru4M alloy ternary electrocatalyst on methanol electro-oxidation

Pure Pt, PtRu and Pt₅Ru₄M (M = Ni, Sn and Mo) electrocatalysts were prepared using a NaBH₄ reduction method. The alloy formation and particle size of the electrocatalysts were determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The formation of crystalline Pt was confirmed regardless of the addition of Ru and transition metals. The average particle size was found to be about 2.5–3.5 nm.The electrochemical properties of the electrocatalysts were analyzed by methanol electro-oxidation and CO stripping in the half cell. The mass activity and specific activity were obtained through these experiments. Methanol electro-oxidation and the specific activity of the PtRuNi electrocatalyst were much higher than that of PtRu electrocatalyst. The specific activity of methanol electro-oxidation based on EAS for the PtRuSn and PtRuMo electrocatalysts was higher than that of the PtRu, although their mass activity of methanol electro-oxidation was lower.     

Ternary Pt<Subscript>45</Subscript>Ru<Subscript>45</Subscript>M<Subscript>10</Subscript>/C (M=Mn,Mo and W) catalysts for methanol and ethanol electro-oxidation

Ternary Pt₄₅Ru₄₅Mn₁₀/C, Pt₄₅Ru₄₅Mo₁₀/C and Pt₄₅Ru₄₅W₁₀/C catalysts were synthesized and physical and electrochemical properties were characterized. Particle sizes of the catalysts were determined by X-ray diffraction to be 3.9, 4.8 and 4.6 nm for the Mn, Mo and W incorporated catalysts, respectively. Electrochemically active surface areas were calculated from CO stripping results, which were 17.7, 17.2 and 15.7 m²/g _catal for the Pt₄₅Ru₄₅Mn₁₀/C, Pt₄₅Ru₄₅Mo₁₀/C and Pt₄₅Ru₄₅W₁₀/C catalysts, respectively. In methanol electro-oxidation, the Pt₄₅Ru₄₅W₁₀/C catalyst showed highest mass and specific activities of 2.78 A/g _cat. and 177 mA/m², respectively, which were 22 and 100% higher than those of commercial PtRu/C. In the case of ethanol electro-oxidation, the Pt₄₅Ru₄₅Mo₁₀/C catalyst exhibited highest mass and specific activities of 4.8 A/g _catal and 280 mA/m², respectively. Specific activity of the Pt₄₅Ru₄₅Mo₁₀/C catalyst was 56% higher than that of the commercial PtRu/C.     

Characterization and electrocatalytic properties of PtRu/C catalysts prepared by impregnation-reduction method using Nd2O3 as dispersing reagent

A simple impregnation-reduction method introducing Nd₂O₃ as dispersing reagent has been used to synthesize PtRu/C catalysts with uniform Pt-Ru spherical nanoparticles. X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis have been used to characterize the composition, particle size and crystallinity of the catalysts. Well-dispersed catalysts with average particle size about 2 nm are achieved. The electrochemically active surface area of the different PtRu/C catalysts is determined by the CO_ad-stripping voltammetry experiment. The electrocatalytic activities of these catalysts towards methanol electrooxidation are investigated by cyclic voltammetry measurements and ac impedance spectroscopy. The in-house prepared PtRu/C catalyst (PtRu/C-03) in 0.5 M H₂SO₄ + 1.0 M CH₃OH at 30 °C display a higher catalytic activity and lower charge-transfer resistance (R_t) than that of the standard PtRu/C catalyst (PtRu/C-C). It is mainly due to enhanced electrochemically active specific surface, higher alloying extent of Ru and the abundant Pt⁰ and Ru oxides on the surface of the PtRu/C catalyst.     

Graphitic spherical carbon as a support for a PtRu-alloy catalyst in the methanol electro-oxidation

Spherical carbons were prepared using sucrose as a carbon precursor via hydrothermal method for use as supports for PtRu-alloy catalysts in the methanol electro-oxidation. Spherical carbon particles with an average diameter of 1μm (SC-1) were prepared under static condition (without stirring), while spherical carbon materials with a diameter of 500–600 nm (SC-2) were obtained under dynamic condition (with stirring). A graphitic spherical carbon material (SC-g) was successfully prepared by the addition of Fe salt under dynamic condition. It was revealed that the catalytic action of Fe species during the hydrothermal process was essential for the formation of a graphitic structure of SC-g. The surface areas were found to be 112, 383, and 252 m²/g for SC-1, SC-2, and SC-g, respectively. PtRu nanoparticles were then supported on the spherical carbons by a NaBH₄-reduction method for use in the methanol electro-oxidation. The average metal particle sizes were 3.5, 2.6, and 2.7 nm for PtRu/SC-1, PtRu/SC-2, and PtRu/SC-g, respectively. The PtRu/SC-1 and PtRu/SC-2 showed a lower catalytic performance in the methanol electro-oxidation than the PtRu/Vulcan. However, the PtRu/SC-g exhibited a higher catalytic performance than the PtRu/Vulcan. It is believed that the high graphitic nature of SC-g was responsible for the enhanced catalytic performance of PtRu/SC-g.     

Particle size effects of PtRu nanoparticles embedded in TiO2 on methanol electrooxidation

Size-controlled PtRu nanoparticles embedded in TiO₂ were prepared by simultaneous multi-gun sputtering from pure targets of Pt, Ru, and TiO₂. The mean diameter of the PtRu nanoparticles, as confirmed by their high-resolution transmission electron microscopic images, can be varied from ∼1.8 to ∼3.7 nm by changing the RF power ratio of PtRu and TiO₂. The transmission electron diffraction and grazing incidence wide angle X-ray scattering patterns of the PtRu nanoparticles embedded in TiO₂ confirmed that the PtRu exists as a mixed alloy structure consisting of both fcc and hcp phases, whereas the TiO₂ matrix is present as an amorphous phase. The size-controlled PtRu/TiO₂ electrodes were found to exhibit unique electronic properties depending on their size, which affected the potential of zero total charge and methanol oxidation reaction. The mass activity of PtRu/TiO₂ for methanol oxidation was determined by the interplay of the surface electronic factors at the metal-solution-interface and the value of the oxophilicity of the nanoparticles was increased by decreasing the particle size.     

Effect of heat treatment on PtRu/C catalyst for methanol electro-oxidation

The effect of heat treatment on a commercial PtRu/C catalyst was investigated with a focus on the relationship between electrochemical and surface properties. The heat treated PtRu/C catalysts were prepared by reducing the commercial PtRu/C catalyst at 300, 500, and 600 °C under hydrogen flow. The maximum mass activity for the methanol electro-oxidation reaction (MOR) was observed in the catalyst heat treated at 500 °C, while specific activity for the MOR increased with increasing heat treatment temperature. Cyclic voltammetry (CV) results revealed that the heat treatment caused Pt rich surface formation. The increase in surface Pt was confirmed by X-ray photoelectron spectroscopy; the surface (Pt:Ru) ratio of the fresh catalyst (81:19) changed to (87:13) in the 600 °C heat treated catalyst. Quantitative analysis of the Ru oxidation state showed that the ratio of metallic Ru increased with an increase in heat treatment temperature. On the other hand, RuO_xH_y completely reduced at 500 °C and the ratio of RuO₂ slightly decreased with increasing heat treatment temperature.     

Phosphomolybdic acid modified PtRu nanocatalysts for methanol electro-oxidation

Phosphomolybdic acid (H₃PMo₁₂O₄₀, PMo₁₂) was employed to modify PtRu nanocatalysts for methanol electro-oxidation. The results show that the performance of PtRu catalysts was improved by the formation of the negatively charged self-assembled PMo₁₂ monolayer on the catalyst surface. Electrochemical measurements indicate that the PMo₁₂-modified PtRu catalyst has a higher catalytic activity and a better poison tolerance than the unmodified one. It is also found that the sequence in which PtRu and PMo₁₂ were deposited onto the carbon support during the preparation process has a major influence on the performance of PMo₁₂-modified PtRu nanocatalysts. X-ray photoelectron spectra results show that the self-assembled PMo₁₂ layer inhibits the formation of metal oxides/hydroxides on the surface of PtRu nanoparticles to some extent.     

Improvements of electrocatalytic activity of PtRu nanoparticles on multi-walled carbon nanotubes by a H2 plasma treatment in methanol and formic acid oxidation

A H₂ plasma has been used to treat the PtRu nanoparticles supported on the plasma functionalized multi-walled carbon nanotubes (PtRu/PS-MWCNTs). The plasma treatment does not change the size and crystalline structure of PtRu nanoparticles, but reduces the fraction of the oxidized species at the outermost perimeter of particles. The electrochemical results show that these plasma treated PtRu/PS-MWCNTs exhibit increased electrochemically active surface area, improved electrocatalytic activity and long term stability toward methanol and formic acid oxidation, and enhanced tolerance to carbonaceous species relative to the sample untreated with the H₂ plasma. The electrocatalytic activities of the plasma treated PtRu/PS-MWCNTs are found to be dependent upon the Pt:Ru atomic ratios of PtRu nanoparticles. The catalysts with a Pt:Ru atomic ratio close to 1:1 show superior properties in the electrooxidation of methanol and formic acid at room temperature and better tolerance to carbonaceous species.     

Preparation of supported PtRu/C electrocatalyst for direct methanol fuel cells

In this work, high-surface supported PtRu/C were prepared with Ru(NO)(NO₃)₃ and [Pt(H₂NCH₂CH₂NH₂)₂]Cl₂ as the precursors and hydrogen as a reducing agent. XRD and TEM analyses showed that the PtRu/C catalysts with different loadings possessed small and homogeneous metal particles. Even at high metal loading (40 wt.% Pt, 20 wt.% Ru) the mean metal particle size is less than 4 nm. Meanwhile, the calculated Pt crystalline lattice parameter and Pt (2 2 0) peak position indicated that the geometric structure of Pt was modified by Ru atoms. Among the prepared catalysts, the lattice parameter of 40-20 wt.% PtRu/C contract most. Cyclic voltammetry (CV), chronoamperometry (CA), CO stripping and single direct methanol fuel cell tests jointly suggested that the 40-20 wt.% PtRu/C catalyst has the highest electrochemical activity for methanol oxidation.     

Effect of reduction conditions on electrocatalytic activity of a ternary PtNiCr/C catalyst for methanol electro-oxidation

The effect of reduction conditions on a Pt₂₈Ni₃₆Cr₃₆/C catalyst was investigated by using two different reduction methods: hydrogen reduction and NaBH₄ reduction. In hydrogen reduced catalysts, dissolution of metallic Ni and Cr was observed during cyclic voltammetry (CV) tests, and a larger amount of Ni and Cr was dissolved when reduced at higher temperatures. For methanol electro-oxidation, the highest specific current density of 1.70 A m⁻² at 600 s of the chronoamperometry tests was observed in the catalyst reduced at 300 °C, which was ∼24 times that of a Pt/C catalyst (0.0685 A m⁻²). In NaBH₄ reduced catalysts, formation of an amorphous phase and a more Pt-rich surface was observed in X-ray diffraction and CV results, respectively, with increasing amounts of NaBH₄. When reduced by 50 times of the stoichiometric amount of NaBH₄, the PtNiCr/C catalyst (PtNiCr-50t) showed a current density of 34.1 A g_{noble metal}⁻¹, which was 81% higher than the 18.8 A g_{noble metal}⁻¹ value of a PtRu/C catalyst at 600 s of the chronoamperometry tests. After 13 h of chronoamperometry testing, the activity of the PtNiCr-50t (15.0 A g_{noble metal}⁻¹) was 110% higher than the PtRu/C catalyst (7.15 A g_{noble metal}⁻¹). The PtNiCr/C catalyst shows promise as a Ru-free methanol oxidation catalyst.     

Effect of electrochemical polarization of PtRu/C catalysts on methanol electrooxidation

To determine the influence of electrochemical polarization of PtRu/C catalysts on methanol electrooxidation, this work investigated methanol electrooxidation on as received and different electrochemically polarized PtRu/C catalysts. Thermogravimetric analysis (TGA) and X-ray diffraction (XRD) were used to characterize the redox state of PtRu/C after different electrochemical polarization. The methanol electrooxidation activity was measured by cyclic voltammetry (CV), Tafel steady state plot and electrochemical impedance spectroscopy (EIS). The results indicate that the metallic state Pt⁰Ru⁰ can be formed during cathodic polarization and contribute to electrooxidation of methanol, while the formation of inactive ruthenium oxides during anodic polarization cause the negative effect on methanol electrooxidation. Different Tafel slopes and impedance behaviors in different potential regions also reveal a change of the mechanism and rate-determining step in methanol electrooxidation with increasing potentials. The kinetic analysis from Tafel plots and EIS reveal that at low potentials indicate the splitting of the first CH bond of CH₃OH molecule with the first electron transfer is rate-determining step. However, at higher potentials, the oxidation reaction of adsorbed intermediate CO_ads becomes rate-determining step.     

Investigation of direct methanol fuel cell performance of sulfonated polyimide membrane

Sulfonated polyimide (SPI) membranes have been evaluated as electrolyte membranes in direct methanol fuel cells (DMFCs). The membrane-electrode assembly (MEA) was made by hot-pressing the membrane, an anode and a cathode, catalyzed with PtRu/CB (PtRu dispersed on carbon black) and Pt/CB bound with Nafion^® ionomer, respectively. The performance of the cell based on SPI was compared with that of Nafion^® 112 in various operation conditions such as cell temperature (T_cell), cathode relative humidity (RH), and methanol concentration (C_MeOH). The methanol crossover at the cell based on SPI was a half of Nafion^® 112, resulting in the improved cell efficiency. Advantage of the use of SPI became much distinctive from the conventional Nafion^® 112 when the DMFC was operated at a higher T_cell or a higher C_MeOH.     

Pt-CeO2/C anode catalyst for direct methanol fuel cells

In order to develop a cheaper and durable catalyst for methanol electrooxidation reaction, ceria (CeO₂) as a co-catalytic material with Pt on carbon was investigated with an aim of replacing Ru in PtRu/C which is considered as prominent anode catalyst till date. A series of Pt-CeO₂/C catalysts with various compositions of ceria, viz. 40 wt% Pt-3–12 wt% CeO₂/C and PtRu/C were synthesized by wet impregnation method. Electrocatalytic activities of these catalysts for methanol oxidation were examined by cyclic voltammetry and chronoamperometry techniques and it is found that 40 wt% Pt-9 wt% CeO₂/C catalyst exhibited a better activity and stability than did the unmodified Pt/C catalyst. Hence, we explore the possibility of employing Pt-CeO₂ as an electrocatalyst for methanol oxidation. The physicochemical characterizations of the catalysts were carried out by using Brunauer Emmett Teller (BET) surface area and pore size distribution (PSD) measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques. A tentative mechanism is proposed for a possible role of ceria as a co-catalyst in Pt/C system for methanol electrooxidation.     

Particle growth behavior of carbon-supported Pt,Ru, PtRu catalysts prepared by an impregnation reductive-pyrolysis method for direct methanol fuel cell anodes

Carbon-supported Pt, Ru, and binary PtRu catalysts were prepared by an impregnation-reductive pyrolysis method at various temperatures, with Pt(NH₃)₂(NO₂)₂ and Ru(NO₃)₃ as precursors. The effect of the reductive pyrolysis temperature on the structure of the metal particles and its relationship to the electrocatalytic activity toward methanol and preadsorbed carbon monoxide (CO_ad) oxidation was examined. The decomposition temperature of the Pt₅₀Ru₅₀ mixed precursor shifted to a temperature lower than that of the Ru single-source precursor. High-resolution scanning electron microscopy, X-ray diffraction, and CO_ad stripping voltammetry of Pt/C and Ru/C indicated that Ru nanoparticles tend to grow drastically when the pyrolysis temperature is increased, whereas Pt nanoparticles are more resistant to particle growth. Scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy analysis showed that there is a slight compositional variation between individual nanoparticles, depending on the particle size. The Pt₅₀Ru₅₀/C catalyst prepared at 200 °C exhibited the maximum electrocatalytic activity toward methanol oxidation per mass of PtRu, which is discussed based on the appropriate balance of precursor decomposition and particle growth.     

Ethanol oxidation on novel, carbon supported Pt alloy catalysts—Model studies under defined diffusion conditions

The structure, surface composition and activity/selectivity for ethanol oxidation of carbon supported Pt alloy catalysts with different composition and catalyst loading, which were synthesized via the polyol-route, were investigated and characterized by microscopic/spectroscopic methods (TEM, EDX, XRD) and electrochemical (RDE, on-line DEMS) measurements under well-defined transport and diffusion conditions. The performance of the polyol-type Pt/C (20 wt.%), PtRu/C (20, 40 and 60 wt.%), and Pt₃Sn/C (20 wt.%) catalysts was compared with that of commercial Pt/C, PtRu/C and Pt₃Sn/C (E-Tek) catalysts. The metal particle sizes of the polyol-type catalysts are significantly smaller than those of the corresponding commercial catalysts, nevertheless both the mass specific activities and, more pronounced, the inherent, active surface area specific activities are lower than those of the commercial catalysts, which is related to the lower degree of alloy formation in the polyol-type catalysts. For all catalysts, incomplete ethanol oxidation to C2 products (acetaldehyde and acetic acid) prevails under conditions of this study, CO₂ formation contributes by ≤1% for potentiostatic reaction conditions. The lower activity of the polyol-type catalysts is mainly due to the lower activity for acetaldehyde formation. Implications and further strategies for fuel cell applications are discussed.     

Ultrathin graphitic carbon nitride nanosheets and graphene composite material as high-performance PtRu catalyst support for methanol electro-oxidation

Graphitic carbon nitride (g-C₃N₄) has been demonstrated as an advanced support material for Pt nanoparticles (NPs) due to its excellent stability and abundant Lewis acid for anchoring metal NPs. However, its non-conductive nature and low surface areas still impede its application in electrochemical fields. Herein, a π–π stacking method is presented to prepare graphene/ultrathin g-C₃N₄ nanosheets composite support for PtRu catalyst. The weaknesses of g-C₃N₄ are greatly overcome by establishing a 2D layered structure. The significantly enhanced performance for this novel PtRu catalyst is ascribed to reasons as follows: the homogeneous dispersion of PtRu NPs on g-C₃N₄ nanosheets due to its abundant Lewis acid sites for anchoring PtRu NPs; the excellent mechanical resistance and stability of g-C₃N₄ nanosheets in acidic and oxidative environments; the increased electron conductivity of support by forming a layered structure and the strong metal-support interaction (SMSI) between metal NPs and g-C₃N₄ NS.     

Rapid synthesis of water-soluble carbon nanotubes-supported PtRu nanoparticles for methanol electrooxidation

A one-pot, rapid microwave-assisted method is proposed to directly synthesize water-soluble surface-aminated carbon nanotubes (CNTs-NH₂) from pristine CNTs by covalently grafting ethylenediamine onto the surface of CNTs, requiring no cumbersome chemical oxidation of CNTs. The excellent water-solubility and abundant surface functional groups of the CNTs-NH₂ lead to the production of uniformly dispersed PtRu nanoparticles (NPs) with diameter ranging from 1.5 to 4.5 nm. Cyclic voltammetry and amperometric studies indicate that the resulting PtRu NPs/CNTs-NH₂ nanohybrids have higher electrochemical surface area, better electrocatalytic performance and higher stability towards methanol oxidation compared to PtRu NPs supported on the pristine CNTs. The high electrocatalytic performance is mainly contributed to the small particle size and high dispersion of PtRu NPs and good proton conductivity of CNTs-NH₂. This work may demonstrate a universal approach to fabricate superior metal nanocrystalline on CNTs for broad applications in energy systems and sensing devices.     

Carbon nanofibers supported Pt-Ru electrocatalysts for direct methanol fuel cells

Oxidized and reduced carbon nanofibers (OCNF and RCNF) were used as supports to prepare highly dispersed PtRu catalysts for the direct methanol fuel cells (DMFC). The structural and surface features and electrocatalytic properties of bimetallic PtRu/OCNF and PtRu/RCNF were extensively investigated. FT-IR spectra show that carboxyl groups exist on the surface of the OCNF, which greatly influence the morphology and crystallinity of the electrocatalysts. Transmission electron microscopy and X-ray diffraction consistently show that PtRu/RCNF has a smaller particle size and more uniform distribution than PtRu/OCNF. However, both catalysts have very similar methanol oxidation peak current densities that are significantly lower than commercial catalyst based on current-voltage (CV) results. These two catalysts also give very similar single cell performance except for some difference in the resistance polarization region. The OCNF supported catalysts give better performance than commercial catalysts when current density is higher than 50 mA cm⁻² in spite of low methanol oxidation peak current density. These results can be ascribed to the specific surface and structural properties of carbon nanofibers.