Fluorophore Metal-Organic Complexes: High-Throughput Optical Screening for Aprotic Electrochemical Systems.

Combinatorial optical screening of aprotic electrocatalysts has not yet been achieved primarily due to H+-associated mechanisms of fluorophore modulation. We have overcome this problem by using fluorophore metal-organic complexes. In particular, eosin Y and quinine can be coordinated with various metallic cations (e.g., Li+, Na+, Mg2+, Zn2+, and Al3+) in aprotic solvents, triggering changes in their fluorescent properties. These interactions have been used in a reliable screening method to determine oxygen reduction/evolution reaction activities of 100 Mn-based binary catalysts for the aprotic Li-air battery.

Highly Efficient, Selective, and Stable CO2 Electroreduction on a Hexagonal Zn Catalyst.

Electrocatalytic CO2 conversion into fuel is a prospective strategy for the sustainable energy production. However, still many parts of the catalyst such as low catalytic activity, selectivity, and stability are challenging. Herein, a hierarchical hexagonal Zn catalyst showed highly efficient and, more importantly, stable performance as an electrocatalyst for selectively producing CO. Moreover, we found that its high selectivity for CO is attributed to morphology. In electrochemical analysis, Zn (101) facet is favorable to CO formation whereas Zn (002) facet favors the H2 evolution during CO2 electrolysis. Indeed, DFT calculations showed that (101) facet lowers a reduction potential for CO2 to CO by more effectively stabilizing a (.) COOH intermediate than (002) facet. This further suggests that tuning the crystal structure to control (101)/(002) facet ratio of Zn can be considered as a key design principle to achieve a desirable product from Zn catalyst.

Hierarchical Cu pillar electrodes for electrochemical CO2 reduction to formic acid with low overpotential.

To achieve high performance of electrochemical CO2 reduction, a series of Cu pillar electrodes (Cu-2.5 h, Cu-5 h) were fabricated by using an electrodeposition method, and then their catalytic activities and reaction mechanisms were investigated. The series of Cu pillar electrodes exhibited improved electrocatalytic activities toward CO2 reduction to formic acid (HCOOH) as Cu pillars on electrodes developed. The Cu-5 h electrode performed well with a 28% Faradaic efficiency for formic acid at -0.5 V (vs. RHE). X-ray diffraction (XRD) analysis indicated that the enhanced catalytic activities were primarily attributable to the increased (111) facet, which is energetically favourable for the production of HCOOH. Also, ultraviolet photoelectron spectroscopy (UPS) and in situ electrochemical impedance spectroscopy (EIS) results suggested that the series of Cu pillar structure electrodes improved the electron transfer to adsorbed CO2 due to the decreased work function of the Cu pillar structure.

Rational Design of a Hierarchical Tin Dendrite Electrode for Efficient Electrochemical Reduction of CO2.

Catalysis is a key technology for the synthesis of renewable fuels through electrochemical reduction of CO2 . However, successful CO2 reduction still suffers from the lack of affordable catalyst design and understanding the factors governing catalysis. Herein, we demonstrate that the CO2 conversion selectivity on Sn (or SnOx /Sn) electrodes is correlated to the native oxygen content at the subsurface. Electrochemical analyses show that the reduced Sn electrode with abundant oxygen species effectively stabilizes a CO2 (.-) intermediate rather than the clean Sn surface, and consequently results in enhanced formate production in the CO2 reduction. Based on this design strategy, a hierarchical Sn dendrite electrode with high oxygen content, consisting of a multi-branched conifer-like structure with an enlarged surface area, was synthesized. The electrode exhibits a superior formate production rate (228.6 µmol h(-1) cm(-2) ) at -1.36 VRHE without any considerable catalytic degradation over 18 h of operation.

Combinatorial high-throughput optical screening of high performance Pd alloy cathode for hybrid Li-air battery.

Combinatorial high-throughput optical screening method was developed to find the optimum composition of highly active Pd-based catalysts at the cathode of the hybrid Li-air battery. Pd alone, which is one-third the cost of Pt, has difficulty in replacing Pt; therefore, the integration of other metals was investigated to improve its performance toward oxygen reduction reaction (ORR). Among the binary Pd-based catalysts, the composition of Pd-Ir derived catalysts had higher performance toward ORR compared to other Pd-based binary combinations. The composition at 88:12 at. % (Pd: Ir) showed the highest activity toward ORR at the cathode of the hybrid Li-air battery. The prepared Pd(88)Ir(12)/C catalyst showed a current density of -2.58 mA cm(-2) at 0.8 V (vs RHE), which was around 30% higher compared to that of Pd/C (-1.97 mA cm(-2)). When the prepared Pd(88)Ir(12)/C catalyst was applied to the hybrid Li-air battery, the polarization of the cell was reduced and the energy efficiency of the cell was about 30% higher than that of the cell with Pd/C.

Novel Mn-Ce-Ti mixed-oxide catalyst for the selective catalytic reduction of NOx with NH3.

Mn-Ce-Ti mixed-oxide catalyst prepared by the hydrothermal method was investigated for the selective catalytic reduction (SCR) of NOx with NH3 in the presence of oxygen. It was found that the environmentally benign Mn-Ce-Ti catalyst exhibited excellent NH3-SCR activity and strong resistance against H2O and SO2 with a broad operation temperature window, which is very competitive for the practical application in controlling the NOx emission from diesel engines. On the basis of the catalyst characterization, the dual redox cycles (Mn(4+) + Ce(3+) ↔ Mn(3+) + Ce(4+), Mn(4+) + Ti(3+) ↔ Mn(3+) + Ti(4+)) and the amorphous structure play key roles for the high catalytic deNOx performance. Diffuse reflectance infrared Fourier transform spectroscopy studies showed that the synergetic effect between Mn and Ce contributes to the formation of reactive intermediate species, thus promoting the NH3-SCR to proceed.

Long-range electron transfer over graphene-based catalyst for high-performing oxygen reduction reactions: importance of size, N-doping, and metallic impurities.

N-doped carbon materials are considered as next-generation oxygen reduction reaction (ORR) catalysts for fuel cells due to their prolonged stability and low cost. However, the underlying mechanism of these catalysts has been only insufficiently identified, preventing the rational design of high-performing catalysts. Here, we show that the first electron is transferred into O2 molecules at the outer Helmholtz plane (ET-OHP) over a long range. This is in sharp contrast to the conventional belief that O2 adsorption must precede the ET step and thus that the active site must possess as good an O2 binding character as that which occurs on metallic catalysts. Based on the ET-OHP mechanism, the location of the electrode potential dominantly characterizes the ORR activity. Accordingly, we demonstrate that the electrode potential can be elevated by reducing the graphene size and/or including metal impurities, thereby enhancing the ORR activity, which can be transferred into single-cell operations with superior stability.

Combinatorial high-throughput screening for highly active Pd-Ir-Ce based ternary catalysts in electrochemical oxygen reduction reaction.

A combinatorial library having 66 different ternary compositions of Pd-Ir-Ce was prepared via the impregnation method to find the optimum ternary composition with the highest performance toward oxygen reduction reaction (ORR) in acid media. Its performance in ORR activity of the combinatorial array was evaluated through two different combinatorial high-throughput screening methods to gain validity: (1) multielectrode half-cell method and (2) optical screening method. From the combinatorial results, the spot at 79:12:9 for Pd-Ir-Ce (at. %) in the array showed the highest ORR activity. The electrochemical characterizations of the single catalyst demonstrates that the optimized Pd79Ir12Ce9/C catalyst shows 1.5 times the ORR activity compared to that of Pd/C catalyst at 0.85 V (vs. RHE). In the Pd-Ir-Ce based catalysts, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results reveal that Ir and Ce are present in the form of IrO2 and CeO2, respectively, and the electron configuration of Pd is effectively modified through the decoration with IrO2 and CeO2. From the results, we suggest that the electro-modification of Pd through strong metal-metal oxide interaction with IrO2-CeO2 was a reason for the enhanced ORR activity.

A superior catalyst with dual redox cycles for the selective reduction of NO(x) by ammonia.

An environmentally benign Cu-Ce-Ti oxide catalyst exhibited excellent NH3-SCR activity, high N2 selectivity and strong resistance against H2O and SO2 with a broad operation temperature window. The dual redox cycles (Cu(2+) + Ce(3+) ↔ Cu(+) + Ce(4+), Cu(2+) + Ti(3+) ↔ Cu(+) + Ti(4+)) play key roles for the superior catalytic deNOx performance.

Designed synthesis of well-defined Pd@Pt core-shell nanoparticles with controlled shell thickness as efficient oxygen reduction electrocatalysts.

Improving the electrocatalytic activity and durability of Pt-based catalysts with low Pt content toward the oxygen reduction reaction (ORR) is one of the main challenges in advancing the performance of polymer electrolyte membrane fuel cells (PEMFCs). Herein, a designed synthesis of well-defined Pd@Pt core-shell nanoparticles (NPs) with a controlled Pt shell thickness of 0.4-1.2 nm by a facile wet chemical method and their electrocatalytic performances for ORR as a function of shell thickness are reported. Pd@Pt NPs with predetermined structural parameters were prepared by in situ heteroepitaxial growth of Pt on as-synthesized 6 nm Pd NPs without any sacrificial layers and intermediate workup processes, and thus the synthetic procedure for the production of Pd@Pt NPs with well-defined sizes and shell thicknesses is greatly simplified. The Pt shell thickness could be precisely controlled by adjusting the molar ratio of Pt to Pd. The ORR performance of the Pd@Pt NPs strongly depended on the thickness of their Pt shells. The Pd@Pt NPs with 0.94 nm Pt shells exhibited enhanced specific activity and higher durability compared to other Pd@Pt NPs and commercial Pt/C catalysts. Testing Pd@Pt NPs with 0.94 nm Pt shells in a membrane electrode assembly revealed a single-cell performance comparable with that of the Pt/C catalyst despite their lower Pt content, that is the present NP catalysts can facilitate low-cost and high-efficient applications of PEMFCs.

Additional doping of phosphorus and/or sulfur into nitrogen-doped carbon for efficient oxygen reduction reaction in acidic media.

Phosphorus and/or sulfur are additionally doped into N-doped carbon (NDC) using phosphoric acid and cysteine. The resulting catalysts demonstrate excellent oxygen reduction activities coupled with high stabilities in acidic media. Specially, additional S-doping in NDC reveals nearly 2.5 times higher activity than that of NDC at 0.75 V (vs. RHE).

Sustainable production of syngas from biomass-derived glycerol by steam reforming over highly stable Ni/SiC.

The production of syngas was investigated by steam reforming glycerol over Ni/Al(2)O(3), Ni/CeO(2), and Ni/SiC (which have acidic, basic, and neutral properties) at temperatures below 773 K. The complete and stable conversion of glycerol with a yield (higher than 90 %) of gaseous products (mainly syngas) was achieved over Ni/SiC during a 60 h reaction, whereas the conversion of glycerol continually decreases over Ni/Al(2)O(3) (by 49.8 %) and Ni/CeO(2) (by 77.1 %). The deactivation of Ni/Al(2)O(3) and Ni/CeO(2) is mainly caused by coke deposition because of the C-C cleavage of the byproducts produced by dehydration over acidic sites and condensation over basic sites. Gaseous products with a 1.0-1.9 syngas ratio (H(2)/CO) are produced over Ni/SiC. This ratio is required for the Fischer-Tropsch synthesis. However, a syngas ratio of more than 3.0 was observed over Ni/Al(2)O(3) and Ni/CeO(2) because of the high activity of the water-gas-shift reaction. Any dissociative or associative adsorption of water on Al(2)O(3) and CeO(2) promotes a water-gas-shift reaction and produces a higher syngas ratio. H(2) and CO were mainly produced by decomposition of glycerol through dehydrogenation and decarbonylation over Ni sites. Thus, SiC promotes an intrinsic contribution of nickel (dehydrogenation, and decarbonylation) without any byproducts from the dehydration and condensation.

Binary and ternary doping of nitrogen, boron, and phosphorus into carbon for enhancing electrochemical oxygen reduction activity.

N-doped carbon, a promising alternative to Pt catalyst for oxygen reduction reactions (ORRs) in acidic media, is modified in order to increase its catalytic activity through the additional doping of B and P at the carbon growth step. This additional doping alters the electrical, physical, and morphological properties of the carbon. The B-doping reinforces the sp(2)-structure of graphite and increases the portion of pyridinic-N sites in the carbon lattice, whereas P-doping enhances the charge delocalization of the carbon atoms and produces carbon structures with many edge sites. These electrical and physical alternations of the N-doped carbon are more favorable for the reduction of the oxygen on the carbon surface. Compared with N-doped carbon, B,N-doped or P,N-doped carbon shows 1.2 or 2.1 times higher ORR activity at 0.6 V (vs RHE) in acidic media. The most active catalyst in the reaction is the ternary-doped carbon (B,P,N-doped carbon), which records -6.0 mA/mg of mass activity at 0.6 V (vs RHE), and it is 2.3 times higher than that of the N-doped carbon. These results imply that the binary or ternary doping of B and P with N into carbon induces remarkable performance enhancements, and the charge delocalization of the carbon atoms or number of edge sites of the carbon is a significant factor in deciding the oxygen reduction activity in carbon-based catalysts.

Oxygen reduction activity of Pd-Mn3O4 nanoparticles and performance enhancement by voltammetrically accelerated degradation.

Electrochemical properties of Pd-Mn3O4 nanoparticles toward oxygen reduction reaction (ORR) in acidic media were investigated. The catalysts were prepared by polyol reduction of Pd(acac)2 and thermal decomposition of Mn2(CO)10. Surface composition and structure of the particles vary depending on the injection temperature of Mn2(CO)10 and are closely related to the electrochemical properties. The presence of Mn3O4 promotes the performance towards ORR by facilitating oxygen transfer to the active sites of Pd. Through an accelerated degradation test (ADT), nanoparticles with a Pd-rich shell are obtained by dissolution of surface exposed Mn3O4 and at 0.57 V (vs. Ag/AgCl) this catalyst shows the highest activity towards ORR, 149% in mass activity and 142% in specific activity compared to that of Pd/C.

Thermoelectric properties of nanocomposite thin films prepared with poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) and graphene.

Carbon nanotubes (CNTs), either single wall carbon nanotubes (SWNTs) or multiwall carbon nanotubes (MWNTs), can improve the thermoelectric properties of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT : PSS), but it requires addition of 30-40 wt% CNTs. We report that the figure of merit (ZT) value of PEDOT : PSS thin film for thermoelectric property is increased about 10 times by incorporating 2 wt% of graphene. PEDOT : PSS thin films containing 1, 2, 3 wt% graphene are prepared by solution spin coating method. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy analyses identified the strong π-π interactions which facilitated the dispersion between graphene and PEDOT : PSS. The uniformly distributed graphene increased the interfacial area by 2-10 times as compared with CNT based on the same weight. The power factor and ZT value of PEDOT : PSS thin film containing 2 wt% graphene was 11.09 µW mK(-2) and 2.1 × 10(-2), respectively. This enhancement arises from the facilitated carrier transfer between PEDOT : PSS and graphene as well as the high electron mobility of graphene (200,000 cm(2) V(-1) s(-1)). Furthermore the porous structure of the thin film decreases the thermal conductivity resulting in a high ZT value, which is higher by 20% than that for a PEDOT : PSS thin film containing 35 wt% SWNTs.

Combinatorial screening of highly active Pd binary catalysts for electrochemical oxygen reduction.

Electro-catalysts omitting platinum are of interest to reduce the cost of fuel cells. The development of non-Pt alloys for this purpose would require a large number of experiments. Palladium-based bimetallic electro-catalysts using eight different metals were computationally evaluated for the oxygen reduction reaction (ORR) and were made and tested in acidic media using combinatorial methods. A Pd-Co alloy showed the closest oxygen adsorption energy to platinum in simple theoretical model calculations, suggesting the highest ORR activity. This prediction was confirmed experimentally, suggesting that the single parameter of oxygen adsorption energy can be a useful guide to developing non-Pt oxygen reduction catalysts in the future.

On the mechanism of enhanced oxygen reduction reaction in nitrogen-doped graphene nanoribbons.

Nitrogen (N)-doped carbon materials were shown in recent studies to have promising catalytic activity for oxygen reduction reaction (ORR) as a metal-free alternative to platinum, but the underlying molecular mechanism or even the active sites for high catalytic efficiency are still missing or controversial both experimentally and theoretically. We report here the results of periodic density functional theory (DFT) calculations about the ORR at the edge of a graphene nanoribbon (GNR). The edge structure and doped-N near the edge are shown to enhance the oxygen adsorption, the first electron transfer, and also the selectivity toward the four-electron, rather than the two-electron, reduction pathway. We find that the outermost graphitic nitrogen site in particular gives the most desirable characteristics for improved ORR activity, and hence the active site. However, the latter graphitic nitrogen becomes pyridinic-like in the next electron and proton transfer reaction via the ring-opening of a cyclic C-N bond. This inter-conversion between the graphitic and pyridinic sites within a catalytic cycle may reconcile the controversy whether the pyridinic, graphitic, or both nitrogens are active sites.

Chemiluminescence analyzer of NO x as a high-throughput screening tool in selective catalytic reduction of NO.

A chemiluminescence-based analyzer of NO x gas species has been applied for high-throughput screening of a library of catalytic materials. The applicability of the commercial NO x analyzer as a rapid screening tool was evaluated using selective catalytic reduction of NO gas. A library of 60 binary alloys composed of Pt and Co, Zr, La, Ce, Fe or W on Al2O3 substrate was tested for the efficiency of NO x removal using a home-built 64-channel parallel and sequential tubular reactor. The NO x concentrations measured by the NO x analyzer agreed well with the results obtained using micro gas chromatography for a reference catalyst consisting of 1 wt% Pt on γ-Al2O3. Most alloys showed high efficiency at 275 °C, which is typical of Pt-based catalysts for selective catalytic reduction of NO. The screening with NO x analyzer allowed to select Pt-Ce(X) (X=1-3) and Pt-Fe(2) as the optimal catalysts for NO x removal: 73% NO x conversion was achieved with the Pt-Fe(2) alloy, which was much better than the results for the reference catalyst and the other library alloys. This study demonstrates a sequential high-throughput method of practical evaluation of catalysts for the selective reduction of NO.

Optimum concentration gradient of the electrocatalyst, Nafion® and poly(tetrafluoroethylene) in a membrane-electrode-assembly for enhanced performance of direct methanol fuel cells.

A combinatorial library of membrane-electrode-assemblies (MEAs) which consisted of 27 different compositions was fabricated to optimize the multilayer structure of direct methanol fuel cells. Each spot consisted of three layers of ink and a gradient was generated by employing different concentrations of the three components (Pt catalyst, Nafion® and polytetrafluoroethylene (PTFE)) of each layer. For quick evaluation of the library, a high-throughput optical screening technique was employed for methanol electro-oxidation reaction (MOR) activity. The screening results revealed that gradient layers could lead to higher MOR activity than uniform layers. It was found that the MOR activity was higher when the concentrations of Pt catalyst and Nafion ionomer decreased downward from the top layer to the bottom layer. On the other hand, higher MOR activity was observed when PTFE concentration increased downward from the top to the bottom layer.

Enhancement in electro-oxidation of methanol over PtRu black catalyst through strong interaction with iron oxide nanocluster.

One of the major issues in direct methanol fuel cell research is to develop a new catalyst for methanol electro-oxidation reaction (MOR) with high activity and low cost. In this study, a new, simple, and economic way was introduced to improve the catalytic activity of commercial PtRu black catalyst for the MOR. A nanocomposite electrode was fabricated by mixing the PtRu catalyst with Fe(2)O(3) nanoclusters. When 10 wt % of the PtRu catalyst was replaced by the Fe(2)O(3) nanoclusters, mass activity (A/g(Pt)) increased by 80% compared to that of the pure PtRu catalyst. Specific activity of the mixed catalyst was 100% higher than that of the pure PtRu catalyst. The nanocomposite catalysts were also applied to single cells. Although the amount of the PtRu catalyst was reduced by 10 wt %, 10% higher potential was observed in the nanocomposite catalysts at a current density of 100 mA/cm(2).