High Electrochemical Surface Area Research Articles

Highly Efficient Graphene Supports for Fuel Cell Electrocatalysts

Graphene nanomaterials are increasingly important with their distinct properties such as high electrical conductivity, high contact surface area and enormous stability. Therefore, graphene has been used as promising catalyst support in energy conversion and storage systems. In order to achieve high catalytic activity a specifically guided growth of catalyst on graphene support surface with delicate controllability is highly preferred. Therefore, we modify graphene surface in two different ways such as graphene functionalization with various active functional groups and fabrication of nanocomposites with intrinsically conducting polymers e.g. polypyrrole (PPy). We have successfully modified graphene oxide by the functionalization with free amine groups (GO-NH2), RGD peptide (GO-RGD) and Nitrogen doping (N-GO). Platinum (Pt) catalyst nanoparticles have been deposited on these functionalized GO (f-GO) using ethylene glycol modified method. The dispersion of Pt deposited F-GO has been enhanced and stable optimized dispersions in organic solvents were obtained. The cyclic voltametry (CV) results showed a high electrochemical surface area (ECSA) of 147 m2/g for GO-RGD compared to Pt/carbon black (Pt/C, 80 m2/gPt) and Pt/GO (99 m2/gPt). On the other hand, we also fabricated GO/PPy/CB (carbon black) hybrid nanocomposites as catalyst support and deposited Pt catalyst nanoparticles. The CV results showed a high ECSA of 153 m2/gPt. These modified graphene nanomaterials including Pt/f-GO and hybrid composites have been successfully used in proton exchange membrane fuel cell (PEMFC) electrodes. The Pt nanoparticles distribution on various modified graphene surfaces, which may significantly influence their properties such as electrical conductivity, electrocatalytic activity when they used as catalyst support and fuel cell performance will be discussed more in details.

The reduction of benzylbromide at Ag-Ni deposits prepared by galvanic replacement

A two-step procedure was applied to prepare bimetallic Ag-Ni glassy carbon supported catalysts (Ag-Ni/GC). First Ni layers were prepared by means of electrodeposition in an aqueous deaerated nickel chloride+nickel sulfamate+boric acid solution. Second, the partial replacement of Ni layers by Ag was achieved upon immersion of the latter in solutions containing silver nitrate. Three different pretreatment protocols were used after preparation of the Ag/Ni deposits; as prepared, cathodised in alkali and scanned in acid. After the pretreatment the surface was characterised by means of spectroscopy techniques (scanning electron microscopy and energy dispersive x-ray) and electrochemically in an alkali NaOH solution through cyclic voltammetry (CV). Afterwards the modified electrodes were tested for the reduction of benzylbromide in acetonitrile solutions by using CV and were found to show improved activity compared to bulk Ag electrode. The highest activity towards benzylbromide reduction was observed for pre-cathodised Ag-Ni electrodes. A final stage of the research focuses on the development of a practical Ag/Ni foam catalyst for the reduction of benzylbromide. Due to the high electrochemical active surface area of Ag/Ni foam, a higher conversion of benzyl bromide was obtained in comparison with bulk Ag.

Investigation of PtNi/C as methanol tolerant electrocatalyst for the oxygen reduction reaction

Platinum–nickel electrocatalysts prepared by a simple and cost-effective procedure have been investigated for the oxygen reduction reaction (ORR) in direct methanol fuel cells (DMFCs). The formic acid method was used to deposit well-dispersed PtxNiy nanoparticles (x:y=3:2) on the surface of a carbon black support (Ketjenblack). Carbothermal reduction of the as-prepared catalyst in an inert gas (Ar) at 900°C produced an alloy and a partial modification of the crystalline structure from the face centered cubic, typical of Pt, to a tetragonal structure of PtNi alloy. An acid leaching treatment was carried out to partially remove unalloyed surface nickel atoms in the outermost layers, leading to an enrichment of Pt concentration (x:y=2:1). Cyclic voltammetry analysis in acidic medium indicated a relatively high electrochemical active surface area (>70m2g−1) for both untreated and thermally treated PtxNiy catalysts. The activity towards the ORR was evaluated in half-cell configuration (0.5M H2SO4) using the rotating disk electrode technique. Similar behavior towards the ORR was found for the untreated and thermally treated PtNi/C catalysts, with an increase in activity compared to a commercial Pt/C catalyst. A remarkably better tolerance to methanol was observed for both PtNi/C cathode catalysts compared to the commercial Pt/C catalyst.

Hydrogen production in single chamber microbial electrolysis cells with stainless steel fiber felt cathodes

Microbial electrolysis cell (MEC) is a promising technology for sustainable production of hydrogen from biodegradable carbon sources. Employing a low-cost and high efficient cathode to replace platinum catalyzed cathode (Pt/C) for hydrogen generation is a challenge for commercialization of MEC. Here we show that a 3D macroporous stainless steel fiber felt (SSFF) with high electrochemical active surface area has an excellent catalytic activity for hydrogen generation, which is comparable to Pt/C cathode and superior to stainless steel mesh (SSM) cathode in the single-chamber MEC. The SSFF cathode (mean filter rating 100 μm) produces hydrogen at a rate of 3.66 ± 0.43 m3 H2 m−3d−1 (current density of 17.29 ± 1.68 A m−2), with a hydrogen recovery of 76.37 ± 15.04% and overall energy efficiency of 79.61 ± 13.07% at an applied voltage of 0.9 V. The performance of SSFF cathode improves over time due to a decrease in overpotential which caused by corrosion. These results demonstrate that SSFF can be a promising alternative for Pt catalytic cathode in MEC for hydrogen production.

Pt-decorated TiO2 Electrocatalysts for PEFCs

For polymer electrolyte fuel cell (PEFC) electrocatalysts, carbon corrosion is one of the important technological issues to be solved. TiO2 is a possible alternative support material. In this study, vapor-grown carbon fiber (VGCF-H) conductive fillers are applied as the built-in electron-conductive backbone, on which Nb-doped TiO2 is coated and Pt nanoparticles are then impregnated. FESEM images of Pt/Ti0.95Nb0.05O2/VGCF-H reveal selective impregnation of Pt nanoparticles on the TiO2 grains rather than the VGCF-H conductive fillers. This conductive-filler-integrated electrocatalyst exhibits relatively higher electrochemical surface area (ECSA) and oxygen reduction reaction (ORR) activity, compared to the electrocatalysts without conductive fillers.

Deoxyribonucleic acid directed metallization of platinum nanoparticles on graphite nanofibers as a durable oxygen reduction catalyst for polymer electrolyte fuel cells

Effective surface functionalization to the hydrophobic graphite nanofibers (GNF) is performed with the biomolecule, namely deoxy-ribo-nucleic-acid (DNA) via π–π interactions. Pt nanoparticles are impregnated on GNF–DNA composite by ethylene glycol reduction method (Pt/GNF–DNA) and its effect on electro catalytic activity for oxygen reduction reaction (ORR) is systemically studied. Excellent dispersion of Pt nanoparticles over GNF–DNA surfaces with no evidence on particle aggregation is a remarkable achievement in this study. This result in higher electro chemical surface area of the catalyst, enhanced ORR behavior with significant enhancement in mass activity. The catalyst is validated in H2–O2 polymer electrolyte fuel cell (PEFC) and a peak power density of 675 mW cm−2 is achieved at a load current density of 1320 mA cm−2 with a minimal catalyst loading of 0.1 mg cm−2 at a cell temperature of 70 °C and 2 bar absolute pressure. Repeated potential cycling up to 10000 cycles in acidic media is also performed for this catalyst and found excellent stability with only 60 mV drop in the ORR half wave potential. The superior behavior of Pt/GNF–DNA catalyst is credited to the robust fibrous structure of GNF and its effective surface functionalization process via π–π interaction.

Preparation and characterization of Pt/TiO2 nanofibers catalysts for methanol electro-oxidation

Pt/TiO2 nanofibers (Pt/T-NFs) composite catalysts are prepared through the combination of an electrospinning technique with a reductive impregnation method. Obtained samples are analyzed by various techniques, such as transmission electron microscopy and X-ray diffraction, which reveal that 21wt.% Pt/T-NFs sample has a homogeneous distribution of smaller Pt nanoparticles of about 4.5nm in size on the TiO2 nanofibers surface without large aggregations. Electrochemical studies are carried out using cyclic voltammetry, chronoamperometry and CO stripping voltammetry, which indicate that 21wt.% Pt/T-NFs catalyst has higher electrochemical surface area and much better activity for methanol electro-oxidation. All Pt/T-NFs catalysts have enhanced stability and CO tolerance for methanol electro-oxidation than commercial Pt/C. The results suggest that Pt/TiO2 nanofibers catalysts exhibit the better activity for methanol oxidation and are favorable for improving the tolerance to poisoning species.

Synthesis and Characterisation of Pd-Ni-Sn Electrocatalyst for Use in Direct Ethanol Fuel Cells

One critical challenge to commercialise direct ethanol fuel cells (DEFCs) is catalyst poisoningassociated with the strongly adsorbed CO intermediate on catalyst’s surface at low temperature. The present worktherefore aimed to develop a highly electrochemically active catalyst which is durable to the presence of COcontaminant. Different Pd-Ni-Sn compositions impregnated on carbon black were synthesised by sodium borohydridereduction method which can be classified as mono-, binary- and ternary-catalyst systems at which 20%w Pd/C wasutilised as a base catalyst. The addition of 5-20%w Ni and/or 5-20%w Sn metals was found to increase catalyticactivity and catalyst stability for ethanol oxidation reaction (EOR). Several diagnostic techniques were employed toassess their suitability for use in DEFCs including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS),transmission electron microscopy (TEM), scanning electron microscopy-energy dispersive spectrometry (SEM-EDX)and electrochemical techniques. The XRD and XPS spectra verified that the existence of Pd, Ni and Sn in the aspreparedcatalysts was in the oxidation state of Pd0 and Pd2+, Ni2+ (in form of Ni(OH)2) and Sn4+ (in form of SnO2),respectively, which were known to promote the EOR and CO oxidation. The average particle size was in the range of5.46-10.56 nm for all samples. By using SEM-EDX, it was found that the actual ratios of metal loadings were relativelysimilar to the desired compositions and their surface morphology was uniformly distributed. Moreover, electrocatalyticactivity towards EOR was evaluated using cyclic voltammetry in 1M ethanol (in 1M KOH). Among various catalyticcompositions, 20%w Pd/C containing 10%w Ni and 10%w Sn exhibited excellent catalytic activity and CO tolerance asthe maximum current density of 146 mA cm-2 for EOR and the highest electrochemical surface area were obtainedwhile lowering the CO-stripping peak potential to -0.525 V (vs Ag/AgCl).

ORR Activity Enhancement of PtML/Pd/C Core-Shell Catalysts By Potential Cycling

It is now widely recognized that the amount of expensive Pt catalyst in PEFCs should be decreased for cost reduction. Pd core/Pt shell structured catalyst supported on carbon (Pt/Pd/C) is a strong candidate for improving the utilization of Pt. It has been also reported that the activity oxygen reduction reaction (ORR) activity of the Pt shell is enhanced by the strain effect of the Pd core [1,2]. Recently, we developed a new synthetic method for carbon supported Pd core/PtMLshell structured catalyst (Pt/Pd/C) using a modified Cu-UPD process [3] and found that ORR activity of the Pt/Pd/C catalyst was drastically enhanced after a potential cycling accelerated durability test (ADT) [4]. In this study, we investigated microstructural changes of the Pt/Pd/C catalyst after potential cycling and discussed the mechanism for the enhanced ORR activity. In addition, we optimized the protocol for potential cycling to further enhance the ORR activity. Pt/Pd/C catalyst was obtained from Pd/C core (4.5 nm, 30 wt.%, Ishifuku Metal Industry) using the modified Cu-UPD method [3]. The Pd/C core and Pt/Pd/C catalyst were characterized by TG, XRD, XRF, TEM, TEM-EDX and CV. The ORR activity of the catalyst was measured by the RDE technique at 1,600 rpm in O2 saturated 0.1 M HClO4 at 298 K. The ADT was performed using rectangular-wave potential cycling (FCCJ, 0.6 V (3 s)/1.0 V (3 s) vs. RHE for 10,000 cycles) in Ar saturated 0.1 M HClO4at 353 K. The ORR activity of the Pt/Pd/C catalyst was enhanced after ADT, showing by 2-fold higher ORR mass activity (512 A/g-Pt) than that of a commercially available Pt/C catalyst (2.8 nm, 46 wt.%, TEC10E50E, TKK). Morphological changes of the Pt/Pd/C catalyst after ADT is shown in Figure 1. The average size of the catalyst decreased from 5.6 to 4.9 nm and the catalyst particles changed into spherical particles. TEM-EDX analysis revealed that the Pd core dissolved out by 73 % and the Pt shell was thickened (to approximately 2.3 ML) after ADT. The microstructural change of the Pt/Pd/C catalyst after ADT is schematically illustrated in Figure 2. The Pd core dissolved out through defects on the Pt shell during ADT. As a result, the Pt atoms at the shell were rearranged to give a defect-free surface to the Pt shell, by which the number of lower coordinated Pt atoms decreased. EXAFS analysis revealed that the Pt-Pt bond length decreased after ADT. It was concluded that the enhanced ORR activity after ADT was caused by a moderately induced compressive strain at the Pt shell, in addition to a decreased number of the lower coordinated Pt atoms. Though the ADT protocol gave an enhanced mass activity (~ x2) to the Pt/Pd/C catalyst, there remains one problem that the electrochemical surface area (ECSA) decreased from 140 initially to 40-50 m2/g-Pt after ADT. Hence we optimized the potential cycling protocol to minimize the decrease in ECSA during ADT. The optimized protocol (0.4 V (300 s)/1.0 V (300 s) for 60 cycles) gave a high ECSA (> 80 m2/g) and a very high ORR mass activity (1002 A/g-Pt, x4 of Pt/C) after potential cycling. Acknowledgment This study was supported by NEDO, Japan.

Nanostructured Transition Metal Nitride (MN) As a Potential Support for Pt(Ru) Anode Electro-Catalyst for Direct Methanol Fuel Cells (DMFCs)

The rapid depletion of fossil fuels and increased environmental pollution due to vast fossil-fuel consumption is a thrust for efficient use of energy and exploration of renewable and clean energy sources.1 Generation of electricity from renewable energy sources, such as solar, wind without producing carbon dioxide-an undesired green house pollutant, offers enormous potential for meeting future energy demands. In this direction, fuel cell technology has gained special attention as it provides promising and sustainable approach for the production of continuous power with reduced greenhouse gas emissions and higher efficiencies compared to combustion based technologies. In particular, direct methanol fuel cells (DMFCs) have got attention as power sources due to advantages like use of methanol as fuel which has higher energy density (3800 kcal/l) than hydrogen (658 kcal/l, used as a fuel in proton exchange membrane fuel cells), safety and ease of handling the fuel (than H2) and low operating temperature. This offers quick start-up and extended durability of system components. The simple system design would be reflected as an ease in operation, reduced cost and high reliability. Hence, they are considered to be an ideal energy source for applications like automobile, portable devices, stationary systems, etc.2One of the major constraints in the development of high performance DMFCs is the low power density of the system due to poor catalytic performance at the electrodes and high capital cost due to use of expensive noble metal electro-catalysts. Hence, it is important to explore new electro-catalyst with reduced noble metal content at both anode and cathode to enhance the efficiency and power density of DMFC. Currently, Pt(Ru) alloy has been identified as a most active anode electro-catalyst for methanol electro-oxidation in DMFC. The synthesis of high surface area Pt(Ru) electro-catalyst is important to improve the reaction kinetics. In addition, efficient utilization of electro-catalyst by achieving high electrochemical active surface area is important, which can be accomplished by dispersing Pt(Ru) electro-catalyst on support with high surface area and superior electrical conductivity. The present study explores highly conductive nanostructured transition metal nitride (MN) as a potential support for Pt(Ru) electro-catalyst for methanol eletcro-oxidation. High surface area Pt(Ru)/MN (~125 m2/g) has been synthesized by wet chemical approach employing non-halide precursors of Pt and Ru. The electrochemical characterization has been carried out in 1 M methanol and 0.5 M sulfuric acid, used as fuel and electrolyte, using Pt wire as counter electrode and Hg/Hg2SO4 as reference electrode (+0.65 V with respect to NHE) and Pt loading of 0.3 mg/1cm2 at 400C. Pt(Ru)/MN shows promising performance for methanol oxidation, showing ~52% improved catalytic activity at ~0.65 V (vs NHE) in half-cell configuration and ~56% higher maximum power density in single DMFC study (Fig. 1) than that of commercial JM-Pt(Ru). This is attributed to well-dispersion of Pt(Ru) on MN (support of surface area of ~125 m2/g) offering high utilization of noble metals Pt and Ru, along with superior electrochemical active surface area and low charge transfer resistance for Pt(Ru)/MN than that of JM-Pt(Ru). Pt(Ru)/MN also shows superior electrochemical stability in half-cell configuration and single DMFC study than JM-Pt(Ru) electro-catalyst. Thus, the present study demonstrates the potential of MN as a support in improving the electrochemical activity of Pt(Ru) electro-catalyst, in the aim of achieving superior electrochemical performance and stability with reduced noble metal content for the electro-catalyst of DMFC. This portends significant reduction in the overall capital cost of DMFC system. Results of the synthesis, structural characterization and electrochemical activity of these electro-catalysts will be presented and discussed.

Assessment of Nanofiber Electrode MEA Durability By Analytical Electron Microscopy

Carbons typically used as the support for Platinum catalysts in proton-exchange membrane fuel cells (PEMFCs) are vulnerable to electrochemical corrosion, especially during potential excursions generated by startup/shutdown cycling and local anode starvation [1]. Pintauro and coworkers have shown that an electrospun nanofiber cathode, composed of Pt/C particles and a binder of NafionÒ+ poly(acrylic acid), performs remarkably well in a hydrogen/air PEMFC, with high power output at low Pt loading and excellent durability, as determined from end-of-life polarization curves after an accelerated start-stop voltage cycling (carbon corrosion) test. For example, after 1,000 simulated start-stop cycles (voltage cycling between 1.0 and 1.5 V), a nanofiber membrane electrode assembly (MEA) with a Johnson Matthey Pt/C nanofiber catalyst maintained 53% of its initial power at 0.65 V and 85% of its maximum power, as compared to a 28% power retention at 0.65 V and 58% maximum power for a sprayed electrode MEA [2]. In another study [3], constant power output, or even and increase in power, was observed after a carbon corrosion voltage cycling test with a nanofiber electrode MEA, where the cathode Pt/C catalyst binder was a blend of Nafion + poly(vinylidene fluoride) (PVDF). For a Nafion-poly(acrylic acid) binder, the superior end-of-life performance of the nanofiber MEA after a carbon corrosion test was tentatively attributed to the combined effects of a high initial electrochemical cathode surface area, the retention of the nanofiber structure after testing (no collapse of the cathode), and the rapid/effective expulsion of product water from the cathode, which minimizes/eliminates flooding. Nafion-PVDF binders appear to suppress corrosion by increasing the nanofiber hydrophobicity, thus decreasing the concentration of water at the catalyst surface. The unusual but desirable behavior of nanofiber cathodes during a carbon corrosion voltage cycling test warranted further studies, in particular imaging and chemical analysis using scanning transmission electron microscopy (STEM), of cathodes at beginning-of-life (BOL) vs. end-of-life (EOL) (after 1,000 voltage cycles, triangular wave at 1.0 V to 1.5 V at a scan rate of 500 mV/s). Characterization experiments were performed at Oak Ridge National Laboratory using nanofiber MEAs prepared and tested for power output at Vanderbilt University. All MEAs were made with Johnson Matthey HiSpec® 4000 (40% Pt on Vulcan carbon) catalyst at an electrode loading (anode and cathode) of 0.1 mgPt/cm2. The anode binder was always Nafion + poly(acrylic acid) whereas the cathode nanofiber binder was either Nafion + poly(acrylic acid) or Nafion + poly(vinylidene fluoride) (PVDF), where the Nafion/PVDF weight ratio was varied from 80/20 to 20/80. We were particularly interested in understanding how the nanofiber electrode morphology and/or binder composition (e.g., PVDF content and binder hydrophobicity) affected cathode carbon corrosion and power output. In this presentation, results of STEM imaging and compositional analysis of nanofiber MEAs at BOL and EOL will be presented, along with fuel cell polarization curves for the same MEAs. References C. A. Reiser, et al. Electrochemical and Solid State Letters, 8(6): A273-A276 (2005)M. Brodt, T. Han, N. Dale, E. Niangar, R. Wycisk, and P. Pintauro, , J. Electrochem. Soc., 162, F84-F91 (2015).M. Brodt, R. Wycisk, J. Slack, and P. N. Pintauro, “New Developments in Electrospun Nanofiber Electrodes for Hydrogen/Air Fuel Cells” paper #1250, Electrochemical Society Meeting, Cancun, Mexico (2014).

Electrospun Nanofiber Fuel Cell Membrane-Electrode-Assemblies with Pt-Alloy Cathode Catalyst

The hydrogen/air proton-exchange membrane (PEM) fuel cell is a promising candidate for automotive power plants but Pt/C catalyst electrode cost and durability are still issues that require further study. In a series of previous papers and presentations [1-4], Pintauro and coworkers have shown that an electrospun nanofiber cathode, composed of Pt/C particles and a binder of Nafion + poly(acrylic acid), performs remarkably well in a hydrogen/air proton exchange membrane fuel cell. For example, a nanofiber electrode MEA with a 0.055 mgPt/cm2 cathode and 0.059 mgPt/cm2 anode produced more than 900 mW/cm2 at maximum power in a H2/air fuel cell at 80°C, 100% RH and high feed gas flow rates at 2 atm backpressure. In another study, electrospun nanofiber cathodes exhibited excellent durability, as determined from end-of-life polarization curves after an accelerated start-stop voltage cycling (carbon corrosion) test. For example, after 1,000 simulated start-stop cycles, a nanofiber MEA with Johnson Matthey Pt/C catalyst maintained 53% of its initial power at 0.65 V and 85% of its maximum power, as compared to a 28% power retention at 0.65 V and 58% maximum power for a sprayed electrode MEA. The excellent initial performance of nanofiber fuel cell electrodes was attributed to the unique nanofiber electrode morphology, with inter-fiber and intra-fiber porosity which results in better accessibility of oxygen to Pt catalyst sites and efficient removal of product water. The superior end-of-life performance of the nanofiber MEA after a carbon corrosion test was attributed to the combined effects of a high initial electrochemical cathode surface area, the retention of the nanofiber structure after testing (no collapse of the cathode, as confirmed by SEM imaging), and the rapid/effective expulsion of product water from the cathode which minimizes/eliminates flooding. In this presentation, the performance of Pt-alloy catalysts in electrospun nanofiber fuel cell cathodes will be discussed. The method for creating nanofiber mat cathodes with PtCo and PtNi powders, using a mixture of Nafion and poly(acrylic acid) as the cathode binder, will be described. These cathodes, with a Pt loading of 0.10 mg/cm2 were incorporated into a fuel cell membrane-electrode-assembly (MEA) with a Nafion 211 membrane and a Pt/C nanofiber anode (0.1 mg/cm2). Short-term fuel cell performance was assessed at 80oC with 100% RH using hydrogen and air at flow rates of 500 and 2,000 sccm, respectively. Power output was compared with an MEA containing conventional painted electrode gas diffusion electrodes, with neat Nafion binder and with Nafion + poly(acrylic acid) binder (same as the nanofibers). The performance of nanofiber MEAs was also examined and contrasted with that of a conventional electrode MEA after an accelerated carbon corrosion voltage cycling experiment, with 1,000 voltage cycles between 1.0 and 1.5V. Representative performance results for a nanofiber MEA with a PtCo cathode catalyst at a loading of 0.10 mg/cm2 are shown in Table 1. Under ambient pressure, the nanofiber morphology consistently provides a 25% improvement in power density at both 0.65V and at maximum power. This effect is even greater under backpressure where the improvement is consistently around a 35% increase in power density at both 0.65 and maximum power. In this case at 0.65V a power density above 1W/cm2 has been achieved. Additional results and analyses will be discussed in the presentation. Acknowledgments This work was funded in part by the National Science Foundation (NSF EPS-1004083) through the TN-SCORE program under Thrust 2. Figure 1

Three-dimensional crumpled graphene-based platinum–gold alloy nanoparticle composites as superior electrocatalysts for direct methanol fuel cells

High performance of electrocatalysts for direct methanol fuel cells was demonstrated by three-dimensional (3D) graphene (GR) decorated with platinum (Pt)–gold (Au) alloy nanoparticles (3D-GR/PtAu). The 3D-GR/PtAu composite with a morphology like a crumpled paper ball was synthesized from a colloidal mixture of GR and Pt–Au alloy nanoparticles with aerosol spray drying. The 3D-GR/PtAu had a high specific surface area and electrochemical surface area of up to 238 and 325m2/g(Pt), respectively, and the electrocatalytic applications of the 3D-GR/PtAu were examined through methanol oxidation reactions. The 3D-GR/PtAu had the highest electrocatalytic activity for methanol oxidation reactions compared with commercial Pt–carbon black and Pt-GR. The 3D-GR/PtAu was also highly sensitive electrocatalytic activity in the methanol oxidation reaction compared with the 2D-GR/Pt–Au. Furthermore, the electrocatalytic activity of the 3D-GR/PtAu had the highest performance among the catalysts containing Pt, Au, and GR for the methanol oxidation reactions. The increased electrocatalytic activity is attributed to the high specific surface area of the 3D formation and the effective surface structure of the Pt–Au alloy nanoparticles.

Performance of a High Temperature Polymer Electrolyte Membrane Fuel Cell with Low Catalyst Loading Produced by Reactive Spray Deposition Technology

Traditional polymer electrolyte membrane fuel cells (PEMFCs) typically operate at temperatures around 80 °C. As a result of these low operating temperatures PEMFCs suffer from several performance issues such as problems with water and thermal management, poor electrochemical kinetics, and intolerance to poisons such as carbon monoxide. If the temperature can be increased to upwards of 200 °C the kinetics for the sluggish oxygen reduction reaction can be greatly improved and upwards of 1% carbon monoxide in the feed gas stream will not deteriorate cell performance. Additionally, water in the cell will exist primarily in the vapor phase helping mitigate the flooding of the electrodes. Finally, the greater discrepancy between the cell temperature and the surrounding environment enhances heat rejection and simplifies thermal management [1]. In the current work, gas diffusion electrodes (GDE) for a high temperature PEMFC are fabricated via Reactive Spray Deposition Technology (RSDT) [2,3]. This flame based deposition technique produces Pt nanoparticles through combustion of a precursor solution consisting of a metal organic compound (platinum(II) acetylacetonate) dissolved in a high enthalpy solvent (a mixture of xylene, acetone, and propane). The Pt catalyst is attached to a carbon support, Vulcan XC-72R in this case. This is accomplished by spraying a carbon based slurry, containing a PTFE binder, through secondary nozzles into the path of the Pt nanoparticles in the post luminous zone of the flame. In order to produce the GDE (5×5cm), the resulting spray from the RSDT process is directed at a gas diffusion layer substrate (SIGRACET® GDL25BC, 5×5cm). The resulting GDEs were then hot pressed to an ADVENT® membrane doped with 85wt% phosphoric acid to produce the 5×5cm single cell. A study of the effect of the binder to carbon ratio (PTFE/Vulcan XC-72R) on cell performance and structure was performed through the use of polarization scans, cyclic voltammetry, mercury porosimetry, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). PTFE/Vulcan XC-72R ratios ranging from 0.1 to 1 were studied. It was determined experimentally that the best performance resulted from a binder to carbon ratio of 0.9. This is believed to be due to the well-developed Pt nanoparticle distribution, small pores and uniform pore size distributions, and high electrochemical surface area. These experimental results will be coupled with a semi empirical model to mimic cell performance based on the catalyst agglomerate concept [4]. An attempt will be made to link the physical characteristics of the RSDT produced GDE to the appropriate parameters of the model to develop a firm understanding of the influence of the RSDT process parameters on catalyst structure and performance.

Electrocatalytic Oxygen Evolution over Nano Scaled Amorphous Ni−Fe Particles in Alkaline Electrolyte

Electrocatalytic water oxidation plays the key role as efficient and stable platform in some of the promising renewable energy conversion and storage devices, such as water electrolyzers, solar water-splitting cells, and lithium-air batteries. The electrocatalytic oxygen evolution reaction (OER) at the anode often couples with electron or photoelectron CO2 reductionand, H2 evolution reactions at the cathode for the above devices but limits the performance of the entire system because of the sluggish OER kinetics, insufficient reaction sites, or low stability of precious metal catalysts. Series of different transition metallic catalysts were investigated in order to significantly increase the activity of OER catalysts and efficiently improve the performance of energy conversion and storage systems. Ni-Fe system is found to be the most promising bimetallic catalytic system with robust stability, much lower cost, and considerably higher activity than the IrO2, the well-known best OER catalyst. Nano scaled catalysts with high electrochemical surface area (ECSA) not only exhibits a promising feasibility to effectively increase the performance of OER catalysts, but also to be simply utilized in renewable energy conversion and storage devices Herein, we reported 4 nm Ni−Fe nanoparticles (NiyFe1−yOx/C) featuring amorphous structures prepared via a solution-phase nanocapsule method for active and durable OER electrocatalysts in alkaline electrolyte. The Ni−Fe nanoparticle catalyst containing 31% Fe (Ni0.69Fe0.31Ox/C) shows the highest activity, exhibiting a 280 mV overpotential at 10 mA cm−2 (equivalent to 10% efficiency of solar-to-fuel conversion) and a Tafel slope of 30 mV dec−1 in 1.0 M KOH solution. The achieved OER activity outperforms NiOx/C and commercial Ir/ C catalysts and is close to the highest performance of crystalline Ni−Fe thin films reported in the literature. In addition, a Faradaic efficiency of 97% measured on Ni0.69Fe0.31Ox/C suggests that carbon support corrosion and further oxidation of nanoparticle catalysts are negligible during the electrocatalytic OER tests. Ni0.69Fe0.31Ox/C further demonstrates high stability as there is no apparent OER activity loss (based on a chronoamperometry test) or particle aggregation (based on TEM image observation) after a 6 h anodization test. The high efficiency and durability make these supported amorphous Ni−Fe nanoparticles potentially applicable in the (photo)electrochemical cells for water splitting to make H2 fuel or CO2 reduction to produce usable fuels and chemicals.

Gold film supported nanostructured Cu/Pt catalyst with high electrochemical surface area and enhanced electrocatalytic activity for methanol oxidation

The rough Cu film was prepared by electrochemical reduction of copper ions at the surface of gold film electrode (GFE). Nanostructured Cu/Pt catalyst was prepared by partially galvanic replacement of Cu with platinum by simply immersion of Cu-coated gold film electrode in a K2PtCl6 aqueous solution. The structure and nature of the fabricated Cu and Cu/Pt species were characterized by scanning electron microscopy and energy dispersive X-ray spectrometry. Cu-/Pt-coated gold film electrode exhibits really high electrochemical active surface area and remarkable electrocatalytic activity toward methanol oxidation. The effect of electrodeposition potential on the stability of the catalyst during the formation of Cu layer was comparatively investigated by performing methanol oxidation using voltammetry methods.

Preparation of new titanium nitride-carbon nanocomposites in supercritical benzene and their oxygen reduction activity in alkaline medium

Titanium nitride-carbon nanocomposites are synthesized by the reaction of TiCl4 and NaN3 in supercritical benzene medium that also serves as a carbon source. The as-prepared precursors (SI, SII) are subjected to several heat treatments (SIII–SV). The synthesized nanoparticles are characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). The samples are tested as electrocatalyst for oxygen reduction reaction in an alkaline electrolyte. It is shown that the electrocatalytic properties of the synthesized nanoparticles are highly dependent on the heat treatment atmosphere and duration. The heat treatment under ammonia atmosphere at 1000°C for 10h (SIV) increased the ORR mass activity from −0.64 (SI) to −1.28mAmg−1 (SIV) at −0.6V vs. SCE. Moreover, the heat treated sample (SIV) shows almost twice ORR mass activity as commercial TiN. Rotating disk electrode (RDE) measurements exhibit ORR on commercial TiN proceeds via two parallel pathways including two and four electrons, resulting in almost 60% H2O2 production, while SIV sample improves ORR by reducing H2O2 formation to almost 16%. The high ORR activity and stability of the sample SIV are mainly due to (i) random layer structure of carbon that combines through a hybrid state with TiN nanoparticles, (ii) unstoichiometric nitrogen and oxygen doped into TiN lattice, and (iii) higher electrochemical surface area. Moreover, the possible pathways of carbon phase formation in vicinity of supercritical reaction are discussed.

Decoration of a bio-inspired carbon nanosphere with Pt nanoparticles via a polymer-assisted strategy for enhanced electrocatalytic activity

Despite the significant advances in nanostructured supporting materials used for metal-based catalysts, intractable problems including poor dispersion and the agglomeration of metal nanoparticles (NPs) within their complicated structure limit the practical applications. We report herein on a facile polyethyleneimine (PEI)-assisted strategy for the high dispersion and controllable coverage of Pt catalysts onto a 3-dimensional mesoporous carbon material, referred to as a “bio-inspired carbon nanosphere (BCN)”. The PEI adsorbed on the carbon surface serves as anchoring sites for metal ions and as templates for metal nanoparticle growth. The resulting spherical Pt NPs are highly dispersed on the surface to the center of the BCN without any agglomeration even after a thermal annealing. The amount of loaded Pt can be adjusted from 7.3 to 27.7wt% independently, which can be easily controlled by adjusting the precursor concentrations. Given their open pore structures and large surface area, Pt/BCN composites exhibit promising properties for use as electrocatalysts with high electrochemical active surface areas (84.9m2g−1) and oxygen reduction reaction activity. The results suggest that this strategy can be widely applied to the deposition of metal NPs on highly controlled 3-D nanostructures.

Electrochemical performance and biosensor application of TiO2 nanotube arrays with mesoporous structures constructed by chemical etching.

Novel mesoporous TiO2 nanotube arrays (TiO2 NTAs) were synthesized by an anodization method combined with chemical etching in HF solution, and the electrochemical performance was studied. Glucose oxidase (GOx) was immobilized on the mesoporous TiO2 NTAs to achieve an efficient biosensor for amperometric detection of glucose. The morphology, structure, component and electrochemical performance of mesoporous TiO2 NTAs were characterized by scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffractometry, X-ray photoelectron spectrometry and an electrochemical workstation, respectively. The influence of the mesoporous structure on the electrochemical performance is discussed in detail by comparing the cyclic voltammograms and electrochemical impedance spectrum of TiO2 and mesoporous TiO2 NTAs in different conditions. High electrochemical active surface area and electron transfer rate play key roles in enhancing the electrochemical performance of mesoporous TiO2 NTAs. When used as the basis of a biosensor, the amperometric response of glucose on a GOx/TiO2-0.5 NTAs electrode is linearly proportion to the glucose concentration in the range from 0.1 to 6 mM with a sensitivity of 0.954 μA mM(-1) cm(-2), which is 14.3 times that of un-etched GOx/TiO2 NTAs.

Morphological Influence of Polypyrrole Nanoparticles on the Performance of Dye–Sensitized Solar Cells

Polypyrrole (PPy) with various morphologies were synthesized by chemical oxidative polymerization and further used as the counter electrode (CE) in dye–sensitized solar cells (DSSCs). The conventional anionic surfactant, docusate sodium salt (AOT), cationic surfactant, cetylmethyl ammonium bromide (CTAB), and the newly developed polymeric dispersant, poly(oxyethylene)–imide (POEM), were employed in the PPy synthesis. Scanning electron microscopy images (SEM) revealed diversified morphologies of the synthesized PPy nanoparticles with irregular sheet (IS), hierarchical nanosphere (HNS), and nanosphere (NS), corresponded to the choice of the commercial surfactants AOT and CTAB, as well as the home–made POEM, respectively. Fourier transform spectroscopy (FT–IR) and X–ray diffraction (XRD) analysis were used to confirm the PPy structures and crystalline properties. When fabricated into films and used as CE in DSSCs, the PPy–HNS demonstrated the superior cell efficiency of 6.71±0.16% to those of PPy–IS (5.46±0.31) and PPy–NS (6.31±0.24), respectively. The excellent electrocatalytic ability of PPy–HNS was essentially attributed to its high electrochemical surface area (Ae), which was quantitatively calculated through a rotating disk electrode system by using the Koutecky–Levich equation. Brunauer–Emmett–Teller (BET) surface area measurement, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) were also used to substantiate the explanation for the DSSC performances.