Novel Raney-like nanoporous Pd catalyst with superior electrocatalytic activity towards ethanol electro-oxidation

A novel Raney-like nanoporous Pd catalyst has been fabricated through the combination of ball-milling with alkali-dealloying strategy. The microstructure of this catalyst has been characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results show that the as-fabricated Raney Pd powders are several microns in size, and each particle exhibits an open, bicontinuous interpenetrating ligament-channel structure with a length scale of 3–7 nm. Electrochemical measurements demonstrate that this Raney Pd catalyst has a high electrochemical active surface area and shows remarkable electrocatalytic activity and stability towards ethanol oxidation. Due to the advantages of simple preparation and superior performance, this Raney Pd catalyst can find promising application as a candidate for the anode catalyst of direct ethanol fuel cells.     

Nanoporous PtCo and PtNi alloy ribbons for methanol electrooxidation

Nanoporous (NP) PtCo and PtNi alloy ribbons with predetermined bimetallic compositions are easily fabricated by one step of mild dealloying, which are characterized by uniform three-dimensional bicontinuous network architecture with the ligament size as small as 3 nm. Compared with E-TEK Pt/C catalyst, the as-made NP-PtCo(Ni) alloys exhibit superior specific activity with the lower peak potential and enhanced CO-tolerance toward methanol electrooxidation. More importantly, these nanomaterials also show much higher structure stability with little loss of the electrochemical surface area of Pt upon 5000 potential cycles in acid solution. X-ray photoelectron spectroscopy and DFT calculations revealed that alloying with Co or Ni modifies the electronic structure of Pt with the downshift of Pt d-band center, thus resulting in the improved methanol oxidation activity and decreased CO poisoning.     

Electrochemical catalytic activities of nanoporous palladium rods for methanol electro-oxidation

A novel electrocatalyst, nanoporous palladium (npPd) rods can be facilely fabricated by dealloying a binary Al₈₀Pd₂₀ alloy in a 5 wt.% HCl aqueous solution under free corrosion conditions. The microstructure of these nanoporous palladium rods has been characterized using scanning electron microscopy and transmission electron microscopy. The results show that each Pd rod is several microns in length and several hundred nanometers in diameter. Moreover, all the rods exhibit a typical three-dimensional bicontinuous interpenetrating ligament-channel structure with length scale of 15-20 nm. The electrochemical experiments demonstrate that these peculiar nanoporous palladium rods (mixed with Vulcan XC-72 carbon powders to form a npPd/C catalyst) reveal a superior electrocatalytic performance toward methanol oxidation in the alkaline media. In addition, the electrocatalytic activity obviously depends on the metal loading on the electrode and will reach to the highest level (223.52 mA mg⁻¹) when applying 0.4 mg cm⁻² metal loading on the electrode. Moreover, a competing adsorption mechanism should exist when performing methanol oxidation on the surface of npPd rods, and the electro-oxidation reaction is a diffusion-controlled electrochemical process. Due to the advantages of simplicity and high efficiency in the mass production, the npPd rods can act as a promising candidate for the anode catalyst for direct methanol fuel cells (DMFCs).     

Sawtooth-shaped nickel-based submicrowires and their electrocatalytic activity for methanol oxidation in alkaline media

Sawtooth-shaped nickel and nickel-cobalt submicrowires were fabricated by hydrazine reduction of their salt precursors under magnetic field. The sawtooth-shaped submicrowires exhibited better electrochemical property in electrooxidation of methanol in alkaline media than nickel nanoparticles and smooth submicrowires because the sawtooth-shaped structure benefited more from both geometry of nanowires and nano-size effect. Therefore, the sawtooth-shaped nickel-based submicrowires synthesized in this work have the potential for use as a non-precious electrode catalyst for alkaline fuel cells.     

Carbon-nanosphere-supported Pt nanoparticles for methanol and ethanol electro-oxidation in alkaline media

Carbon nanospheres with diameters ∼200 nm have been synthesized from glucose at 200 °C and normal atmosphere by a novel composite-molten-salt (CMS) method. Pt nanoparticles supported on those carbon nanospheres are used for methanol and ethanol electro-oxidation in alkaline media. Experimental results demonstrate that, in comparison with the carbon black or hydrothermally-synthesized carbon nanosphere support, CMS carbon-nanosphere-supported Pt electrocatalyst shows an enhanced efficiency for both methanol and ethanol electro-oxidation in terms of electrode conductivity, electrochemically active surface, oxidation peak current density and onset potential. This enhancement is considered to be not only due to the high carbonization of the CMS synthesized carbon nanospheres, but also due to the formation of a porous structure by the carbon nanospheres which significantly reduces the liquid sealing effect allowing efficient gas diffusion.     

Nickel nanorods over nickel foam as standalone anode for direct alkaline methanol and ethanol fuel cell

A highly electroactive nickel nanorod (NNR)/nickel foam (NF) electrode was fabricated for direct alcohol fuel cells (DAFCs) using a simple and cost-effective hydrothermal process. The Ni/NiO nanorods were successfully grown on the surface of an NF electrode, which strongly enhanced the anode wettability and increased surface area by 18 times (11.9 m² g⁻¹), resulting in interfacial polarization resistance reduction. The NNR/NF electrode shows high electro-catalytic activity and great stability during alcohol oxidation. The current densities obtained for NNR/NF were four (479 mA cm⁻²) and six (543 mA cm⁻²) times higher than that for pristine NF in the cases of methanol and ethanol oxidations, respectively. This high current density can be attributed to the superhydrophilic surface of the Ni/NiO nanorods and corresponding high mass transfer capability between the electrolytes and Ni/NiO nanorods embedded on the surface of the electrodes. This study presents a new approach for using the novel NNR/NF as a cheap and high performance anode in DAFCs.     

Synthesis of PdNi catalysts for the oxidation of ethanol in alkaline direct ethanol fuel cells

Carbon-supported PdNi catalysts for the ethanol oxidation reaction in alkaline direct ethanol fuel cells are successfully synthesized by the simultaneous reduction method using NaBH₄ as reductant. X-ray diffraction characterization confirms the formation of the face-centered cubic crystalline Pd and Ni(OH)₂ on the carbon powder for the PdNi/C catalysts. Transmission electron microscopy images show that the metal particles are well-dispersed on the carbon powder, while energy-dispersive X-ray spectrometer results indicate the uniform distribution of Ni around Pd. X-ray photoelectron spectroscopy analyses reveal the chemical states of Ni, including metallic Ni, NiO, Ni(OH)₂ and NiOOH. Cyclic voltammetry and chronopotentiometry tests demonstrate that the Pd₂Ni₃/C catalyst exhibits higher activity and stability for the ethanol oxidation reaction in an alkaline medium than does the Pd/C catalyst. Fuel cell performance tests show that the application of Pd₂Ni₃/C as the anode catalyst of an alkaline direct ethanol fuel cell with an anion-exchange membrane can yield a maximum power density of 90 mW cm⁻² at 60 °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).     

Roles of Pb and MnOx in PtPb/MnOx-CNTs catalyst for methanol electro-oxidation

Design of novel nano-scale catalysts with high activity and low cost for methanol oxidation reaction is crucial for the development of direct methanol fuel cell. In this study, MnO_x, Pt and Pb were forced to precipitate successively on the surface of carbon nanotubes for fabricating a PtPb/MnO_x-CNTs catalyst. Physical characterizations indicated that there existed a mass of Mn (IV, Ⅴ), Pb (Ⅱ) and Pt (0) species, and partial alloying between Pt and Pb in this catalyst. Methanol oxidation reaction with this novel composite exhibited over 3 times higher specific activity (140.9 mA cm⁻²) and somewhat lower onset potential (−0.1 V vs. Hg/Hg₂SO₄) than the values on Pt/CNTs (44.2 mA cm⁻² and 0 V, respectively). Fundamental understanding in reaction mechanisms enabled us to reveal the distinguishing functions between Pb and MnO_x in methanol oxidation processes. The addition of Pb resulted in the enhanced intrinsic activity towards electro-oxidation of residual intermediate species, while dehydrogenation in methanol oxidation processes was obviously improved by using MnO_x-CNTs as a support.     

Palladium-ruthenium alloy nanoparticles dispersed on CoWO4-doped graphene for enhanced methanol electro-oxidation

Highly dispersed nanoparticles (NPs) of Pd and Pd-Ru alloys on the 10 wt% CoWO₄-doped GNS (graphene nano sheets) support have been obtained by a microwave-assisted polyol reduction and investigated for their application as efficient electrode materials for methanol oxidation reaction (MOR). Structural and electrocatalytic surface characterization of hybrid materials were carried out by XRD, TEM, XPS, cyclic voltammetry and chronoamperometry. Pure CoWO₄ and CoWO₄-doped GNS follow the monoclinic crystal structure and the Pd NPs (6–7 nm) dispersed on CoWO₄-doped GNS follow the face-centered cubic crystal structure. It is observed that with the increase of Pd loading from 5 to 20 mg on the support, the onset potential (E_op) for MOR shifts negatively and the MOR current density increases, the magnitude of shift in E_op and increase in the MOR peak current density being the greatest in the case of 15 mg Pd loading. Introduction of Ru from 0.6 to 2.0 mg into 15 mg Pd on the catalyst support, the apparent activity of the active catalyst, 15Pd/10 wt% CoWO₄-GNS improved further, the magnitude of improvement, however, being the greatest (≈50%) with 1.0 mg Ru. Thus, novel 15Pd-1.0Ru/10 wt%CoWO₄-doped GNS can be a promising electrode material for MOR in alkaline solutions.     

Pt-modified Au/C catalysts for direct glycerol electro-oxidation in an alkaline medium

Au-based catalysts promoted with Pt were prepared by using polyvinyl alcohol protection method. Different amounts of Pt (5, 10 and 15% of total metal) were added in the Au sol formation step to improve the activity of Au/C toward glycerol electro-oxidation in an alkaline medium. The physical and electrochemical properties of the as-prepared catalysts were explored. The average particle sizes of the Au/C and Pt-modified Au/C catalysts measured by transmission electron microscopy (TEM) were the same at around 4 nm. The PtAu/C alloy formation in the PtAu/C catalysts was confirmed by the increase of lattice parameter calculated from the X-ray diffraction (XRD) patterns and by the absence of Pt ring in the electron diffraction pattern. The change of binding energy in X-ray photoelectron spectroscopy (XPS) results indicated the interaction between Pt and Au. For glycerol electro-oxidation in an alkaline medium, the PtAu/C catalysts were more active than the Au/C catalyst as observed from an early onset potential and a shift of potential at maximum current density to a lower potential. Among the Pt-modified Au/C catalysts, the most active catalyst was Pt₁Au₉/C. The synergistic effects between Pt–Au was proven by a better performance of the PtAu/C compared to the physical mixed catalyst of Au/C and Pt/C at the same Pt:Au ratio. The Pt-modified Au/C catalysts were more stable than the Au/C, especially in a high potential region. This enhancement may be caused by the promotion effect of highly active PtO on the surface of the bimetallic catalyst.     

Synthesis and characterization of Pt-Au/C catalyst for glucose electro-oxidation for the application in direct glucose fuel cell

To minimize the poisoning of Pt-catalyst in glucose electro-oxidation for direct glucose fuel cell, carbon supported low metal loaded platinum-gold (Pt-Au/C) catalyst (1:1) was synthesized by immobilizing metal sols on carbon. The physical characterization of Pt-Au/C, Pt/C and Au/C was carried out using transmission electron microscope (TEM), scanning electron microscope (SEM), energy dispersive X-ray (EDX), X-ray diffraction (XRD) and thermo gravimetric analysis (TGA). SEM indicates the uniformity in loading of metals on Vulcan XC-72 carbon support, whereas TEM picture and XRD pattern confirm the formation of Pt-Au nanoparticles of less than 10 nm size. TGA shows the metal present in Pt-Au/C catalyst is 14.5% by wt. Electrochemical analysis such as cyclic voltammetry (CV) and chronoamperometry (CA) on Pt-Au/C and commercial Pt/C and Au/C (40 wt. % of metal) for glucose electro-oxidation in alkaline media shows that Pt-Au/C is capable of electro-oxidation of glucose at low potential as that of Pt/C catalyst and more active than Au/C catalyst. The poisoning rate of prepared Pt-Au/C (0.0046% s⁻¹) is lower than that of Pt-Ru/C (0.0085% s⁻¹) and Pt/C (0.011% s⁻¹) catalysts. A batch cell operated using Pt-Au/C as anode and activated charcoal as cathode delivered 0.9 V OCV and 0.72 mW cm⁻² peak power density at 0.2 M glucose in 1 M KOH solution.     

Facile synthesis of a Bi-modified PtRu catalyst for methanol and ethanol electro-oxidation in alkaline medium

In this work a facile synthesis for a high-performance PtRuBi/C catalyst was presented through a simple mixture of commercial PtRu/C and Bi(NO₃)₃. Most of the Bimodified the PtRu particle surface via irreversible adsorption and deposition processes. X-ray photoelectron spectroscopy analysis indicated that Bi₂O₃ was the main form in the catalyst and that there exists an interaction between Bi₂O₃ and Pt. The current density of PtRuBi/C (1:1:0.2 for Pt:Ru:Bi) in the cyclic voltammograms for methanol or ethanol oxidation is over 2.6 times higher than that of PtRu/C. The anti-poisoning ability of this catalyst was also greatly improved. The Bi-containing catalyst had abundant oxygenated species and facilitated removal of poisonous intermediate species.     

NixCo1−x alloy nanoparticle-doped carbon nanofibers as effective non-precious catalyst for ethanol oxidation

Alloy structure generates special characteristics for the nano-metallic compounds which make this interesting class of materials promising candidates for many application fields. Moreover, the performance of the nanostructural catalysts is strongly influenced by the morphology; nanofibers reveal distinct catalytic activity compared to the nanoparticles. In this study, non-precious electrocatalysts based on alloy structure and nanofibrous morphology are introduced. Briefly, Ni_xCo_1−x (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0) alloy nanoparticles incorporated in carbon nanofibers are investigated as electrocatalysts for ethanol oxidation. Preparation of the introduced nanofibers could be achieved by calcination of electrospun nanofibers composed of nickel acetate tetrahydrate, cobalt acetate tetrahydrate and poly(vinyl alcohol) in argon atmosphere at 800 °C. Polycondensation characteristic of the utilized metals precursors led to produce good morphology electrospun nanofibers as well as preserved the nanofibrous morphology during the calcination process for all formulations. The catalytic activity of cobalt enhanced the carbonization of the utilized polymer which resulted in producing nickel/cobalt alloys nanoparticles embedded in carbon nanofibers. Electrochemical investigation of the introduced nanofibers toward ethanol oxidation indicated that the alloy structure has a strong influence. For instance, the corresponding current densities of Ni- and Ni_0.9Co_0.1-doped carbon nanofibers were 37 and 142 mA/cm², respectively. Moreover, very low onset potential (−50 mV vs. Ag/AgCl) was observed when Ni_0.1Co_0.9-doped carbon nanofibers were utilized. Furthermore, Ni_0.9Co_0.1-doped carbon nanofibers could oxidize ethanol solution up to 5 M due to the observed active layer regeneration. The introduced nanofibers have good stability because of the alloy structure. Overall, this study opens new avenue for the transition metals alloys and the nanofibrous morphology to produce novel and effective non-precious electrocatalysts.     

Carbon nanotube-supported Pt-Alloyed metal anode catalysts for methanol and ethanol oxidation

To improve the electrocatalytic activity of alcohol oxidation, functionalized carbon nanotubes (CNTs) decorated with various compositions of metal alloy catalyst nanoparticles (Pt_xM_y, where M = Au and Pd; x and y = 1–3) have been prepared via reduction. The CNTs were treated with an nitric acid solution to promote the oxygen-containing functional groups and further load the metal nanoparticles. X-ray diffraction (XRD) scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to probe the formation of catalyst microstructure morphologies. A uniform dispersion of the spherical metal particles with diameters of 2–6 nm was acquired. The catalytic properties of the catalyst for oxidation were thoroughly studied by electrochemical methods that involved in the cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). To maximize the electrocatalytic performance and minimize the metal integration of the loaded CNTs, various compositions of active catalysts with large active surface areas are expected to increase the activity of the enhanced catalysts for alcohol oxidation. Most of the prepared bimetallic catalysts have better alcohol oxidation kinetics than commercial PtRu/C. Among the prepared catalysts, the PtAu/CNTs and PtPd/CNTs catalysts with high electrochemically active surface area (ECSA) show excellent activities for alcohol oxidation resulting in their low onset potentials, small charge transfer resistances and high peak current densities and I_f/I_b ratios, stability, and better tolerance to CO for alcohol oxidation. The integration of Pt and different metal species with different stoichiometric ratios in the CNTs support affects the electrochemical active surface area achieved in the catalytic oxidation reactions.     

Pt supported on mesoporous material for methanol and ethanol oxidation in alkaline medium

Pt nanoparticles supported on a mesoporous material of zeolite Faujasite-C composite is a highly active catalyst for methanol and ethanol oxidation in alkaline media. Pt was synthesized by a simple methodology of chemical reduction using ultrasound method. Faujasite-C composite was prepared by sol-gel method using fly ash as economic precursor. Pt/Faujasite-C was characterized by X-ray diffraction (XRD), scanning (SEM) and transmission (TEM) electron microscopy to investigate its structure, morphology, composition and size. The electrochemical activity of catalyst towards methanol and ethanol oxidation reaction in alkaline media was evaluated by cyclic voltammetry and chronoamperometry techniques. The results obtained were compared with Pt/C synthesized and tested at the same conditions. According to TEM results Pt/Faujasite-C electrocatalyst exhibits a higher Pt agglomeration compared to Pt/C. Pt/Faujasite-C is more active for alcohol oxidation reactions compared to Pt/C. Pt electrocatalysts are more active for ethanol oxidation than methanol oxidation. Chronoamperometric results indicated that Pt deactivation by intermediate poisoning is more severe for ethanol than methanol. Pt/Faujasite-C can be used as anodic electrocatalyst in direct liquid fuel cells.     

Investigation of Pt/WC/C catalyst for methanol electro-oxidation and oxygen electro-reduction

A Pt/WC/C catalyst is developed to increase the methanol electro-oxidation (MOR) and oxygen electro-reduction (ORR) activities of the Pt/C catalyst. Cyclic voltammetry and CO stripping results show that spill-over of H⁺ occurs in Pt/WC/C, and this is confirmed by comparing the desorption area values for H⁺ and CO. A significant reduction in the potential of the CO electro-oxidation peak from 0.81 V for Pt/C to 0.68 V for Pt/WC/C is observed in CO stripping test results. This indicates that an increase in the activity for CO electro-oxidation is achieved by replacing the carbon support with WC. Preferential deposition of Pt on WC rather than on the carbon support is investigated by complementary analysis of CO stripping, transmission electron microscopy and concentration mapping by energy dispersive spectroscopy. The Pt/WC/C catalyst exhibits a specific activity of 170 mA m⁻² for MOR. This is 42% higher than that for the Pt/C catalyst, viz., 120 mA m⁻². The Pt/WC/C catalyst also exhibits a much higher current density for ORR, i.e., 0.87 mA cm⁻² compared with 0.36 mA cm⁻² for Pt/C at 0.7 V. In the presence of methanol, the Pt/WC/C catalyst still maintains a higher current density than the Pt/C catalyst.     

Nitrogen-doped carbon xerogel as high active oxygen reduction catalyst for direct methanol alkaline fuel cell

A novel catalyst based on nitrogen-doped carbon xerogel for oxygen reduction reaction (ORR) was prepared via a sol–gel process, following by the subsequent pyrolysis under ammonia atmosphere. The catalytic activity in alkaline media was optimized by tuning the metal (cobalt) ratio to the gel precursor. Sample with the optimum activity was characterized by transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis and electrochemical measurements. Results show that the catalyst possesses an amorphous microstructure with nitrogen doped on the surface. The nitrogen-doped carbon xerogel displays comparable ORR activity and superior methanol tolerance than Pt/C in alkaline medium, demonstrating its promising application in direct methanol alkaline fuel cells as non-precious cathode catalyst.     

MnO2 modified multi-walled carbon nanotubes supported Pd nanoparticles for methanol electro-oxidation in alkaline media

A novel method to prepare MnO₂ modified multi-walled carbon nanotubes (MnO₂/MWCNTs) supported Pd (Pd-MnO₂/MWCNTs) electrocatalyst is reported. The morphology, component and crystallinity of the catalyst were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The activity of Pd-MnO₂/MWCNTs was tested using methanol electro-oxidation in alkaline media. The results showed that the Pd-MnO₂/MWCNTs exhibited higher electrocatalytic activity and stability than Pd/MWCNTs and Pd/Vulcan (Pd/commercial Vulcan XC-72 carbon black).