19 research outputs found

    In Situ Infrared Spectroscopic Investigations of Pyridine-Mediated CO<sub>2</sub> Reduction on Pt Electrocatalysts

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    Pyridine-mediated electrochemical reduction of CO<sub>2</sub> has attracted much attention owing to the promise of producing valuable oxygenates with high yields. However, no detectable level of methanol was observed in the pyridine-mediated CO<sub>2</sub> electrolysis on Pt over the entire potential range investigated (−0.2 to −0.8 V vs RHE) in this study. Formate was observed at a potential below −0.6 V vs RHE in the absence and presence of pyridine, but the presence of pyridine does accelerate the rate of formate production. Numerous reaction mechanisms have been proposed on the basis of reactivity measurements, cyclic voltammetry, or computational methods; however, a direct experimental mechanistic investigation has been lacking. By employing surface-enhanced infrared absorption spectroscopy, we identified an adsorbed unidentate COOH<sub>L</sub> intermediate on Pt regardless of the presence of pyridine. Surface coverage of the COOH<sub>L</sub> intermediate relative to that of adsorbed CO appears to increase with the concentration of pyridine in the electrolyte, which is consistent with the observed production rates for formate and CO. We propose that adsorbed COOH<sub>L</sub> is a common intermediate in the formation of both formate and CO, and the presence of pyridinium promotes the formate pathway

    Platinum-Coated Copper Nanowires with High Activity for Hydrogen Oxidation Reaction in Base

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    Platinum (Pt)-coated copper (Cu) nanowires (Pt/CuNWs) are synthesized by the partial galvanic displacement of CuNWs and have a 100 nm diameter and are 25–40 μm length. Pt/CuNWs are studied as a hydrogen oxidation reaction (HOR) catalyst in base along with Cu templated Pt nanotubes (PtNT (Cu)), a 5% Cu monolayer on a bulk polycrystalline Pt electrode (5% ML Cu/BPPt), BPPt, and carbon supported Pt (Pt/C). Comparison of these catalysts demonstrates that the inclusion of Cu benefited the HOR activity of Pt/CuNWs likely by providing compressive strain on Pt; surface Cu further aids in hydroxyl adsorption, thereby improving the HOR activity of Pt/CuNWs. Pt/CuNWs exceed the area and mass exchange current densities of carbon supported Pt by 3.5 times and 1.9 times

    Correlating Hydrogen Oxidation/Evolution Reaction Activity with the Minority Weak Hydrogen-Binding Sites on Ir/C Catalysts

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    Fundamental understanding of the active sites mediating hydrogen oxidation/evolution reaction (HOR/HER) is critical to the design of an efficient HOR/HER electrocatalyst for affordable hydrogen exchange membrane fuel cells and electrolyzers. Here we report the existence of the most active sites on carbon supported iridium nanoparticles (Ir/C) for HOR/HER in alkaline electrolyte by investigating activities of Ir/C with varying particle sizes in the range of 3–12 nm. The distribution of surface sites is quantified by deconvoluting the H<sub>upd</sub> desorption peak in cyclic voltammograms. The portion of the sites with the lowest hydrogen binding energy (HBE) increases with the increase of the particle size or the decrease of the total electrochemical active surface area (t-ECSA). The HOR/HER activity normalized to t-ECSA decreases as t-ECSA increases while it remains constant when normalized to the surface area of the sites with an average HBE of −0.33 eV, accounting for only about 15−30% of the total sites, shoulder the great majority of the HOR/HER activity

    Preparation of Zeolite NaA Membranes on Macroporous Alumina Supports by Secondary Growth of Gel Layers

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    Zeolite NaA membranes are synthesized on tubular α-Al<sub>2</sub>O<sub>3</sub> supports by the secondary growth of gel layers. The gel layer is prepared by wetting–rubbing hydrogel with a composition similar to the secondary synthesis solution <i>x</i>Na<sub>2</sub>O:2SiO<sub>2</sub>:Al<sub>2</sub>O<sub>3</sub>:150H<sub>2</sub>O. The hydrogel’s good uniformity makes it easier to gain a uniform gel layer. Zeolite NaA membranes are characterized by SEM and pervaporation separation of a 90 wt % ethanol/water mixture at 75 °C. The effects of the pretreatment time, pretreatment temperature, gel loading, and Na<sub>2</sub>O/Al<sub>2</sub>O<sub>3</sub> ratio of the hydrogel on properties of zeolite NaA membranes are investigated. It is found that zeolite NaA membranes with high separation factors up to 10 000 can be obtained by secondary growth of gel layers. The optimal pretreatment conditions of the hydrogel are as follows: pretreatment temperature, 50 °C; pretreatment time, ≥18 h; and gel loading, 0.6–0.9 mg/cm<sup>2</sup>. The optimal composition of the hydrogel is 2.2Na<sub>2</sub>O:2SiO<sub>2</sub>:Al<sub>2</sub>O<sub>3</sub>:150H<sub>2</sub>O, which is the same as the synthesis solution. Like crystal seeds, the gel layer improves the zeolite membrane formation on the support surface, and the role of the gel layer becomes more significant with greater amounts of crystals

    Platinum-Coated Palladium Nanotubes as Oxygen Reduction Reaction Electrocatalysts

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    Platinum (Pt) coated palladium (Pd) nanotubes (Pt/PdNTs) with a wall thickness of 6 nm, outer diameter of 60 nm, and length of 5–20 μm are synthesized via the partial galvanic displacement of Pd nanotubes. Pt coatings are controlled to a loading of 9 (PtPd 9), 14 (PtPd 14), and 18 (PtPd 18) wt % and estimated to have a thickness of 1.1, 1.7, and 2.2 Pt atoms, respectively, if a uniform and continuous coating is assumed. Oxygen reduction experiments have been used to evaluate Pt/PdNTs, Pt nanotubes, Pd nanotubes, and supported Pt nanoparticle activity for proton exchange membrane fuel cell cathodes. The dollar and area (specific surface area) normalized ORR activities of Pt/PdNTs exceed the United States Department of Energy (DOE) targets. PtPd 9, PtPd 14, and PtPd 18 produce dollar activities of 10.4, 9.4, and 8.7 A<sup>1</sup>,respectively;PtPd9exceedstheDOEdollaractivitytarget(9.7A<sup>–1</sup>, respectively; PtPd 9 exceeds the DOE dollar activity target (9.7 A<sup>–1</sup>) by 7%. Pt/PdNTs further exceed the DOE area activity target by 40–43%

    Enhanced Electrocatalytic Activity and Durability of Pt Particles Supported on Ordered Mesoporous Carbon Spheres

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    Three-dimensionally ordered mesoporous carbon sphere array (OMCS)-supported Pt nanoparticles (Pt/OMCS) were synthesized and studied as electrocatalysts for the methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). In the Pt/OMCS, the Pt particles with a mean size of ∼1.6 nm are homogeneously dispersed on the mesopore walls of the carbon spheres. The Pt/OMCS catalyst exhibits smaller Pt particle size, greater Pt dispersion, larger specific electrochemically active surface area (ECSA), higher activity for MOR and ORR, and better electrocatalytic stability than the carbon black (Vulcan XC-72R)-supported Pt and commercial Pt/C catalysts

    Platinum Coated Copper Nanowires and Platinum Nanotubes as Oxygen Reduction Electrocatalysts

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    Platinum (Pt) coated copper (Cu) nanowires (Pt/CuNWs) are synthesized by the partial galvanic displacement of Cu nanowires (CuNWs) with a Pt loading of 18 wt %. Pt/CuNWs have an outer diameter of 100 nm, a length of 25–40 μm, and a theoretical Pt layer thickness of 2 nm. Cu templated Pt nanotubes (PtNTs (Cu)) with a wall thickness of 11 nm, an outer diameter of 100 nm, and a length of 5–20 μm are synthesized by the complete galvanic displacement of CuNWs. CuNWs are synthesized by the hydrazine reduction of Cu nitrate in sodium hydroxide. Oxygen reduction reaction (ORR) and durability experiments are conducted on Pt/CuNWs, PtNTs (Cu), silver templated PtNTs (Ag), and carbon supported Pt nanoparticles (Pt/C) to evaluate catalyst activity for use as proton exchange membrane fuel cell cathodes. The ORR area activities of Pt/CuNWs and PtNTs (Cu) are 1.501 and 1.506 mA cm<sub>Pt</sub><sup>–2</sup>, respectively. Pt/CuNWs produce a dollar activity of 9.8 A $<sup>–1</sup> (dollar activity calculated from the DOE mass activity target for 2017–2020 of 0.44 A mg<sub>PGM</sub><sup>–1</sup>). Durability testing of each catalyst shows improved retention of surface area and ORR activity in comparison to Pt/C

    Ordered Mesoporous Nickel Sphere Arrays for Highly Efficient Electrocatalytic Water Oxidation

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    We report the preparation and use of the three-dimensionally ordered mesoporous Ni sphere arrays (3D-OMNiSA) as a highly effective OER catalyst in alkaline electrolyte. The 3D-OMNiSA is fabricated through lyotropic liquid crystal templating within a polymer inverse opal. The prepared 3D-OMNiSA catalyst exhibits a low overpotential of 254 mV at 10 mA cm<sup>–2</sup> and a small Tafel slope of 39 mV decade<sup>–1</sup>, better than the commercial precious RuO<sub>2</sub> catalyst. The mass activity (166.5 A g<sup>–1</sup>) and turnover frequency (0.0281 s<sup>–1</sup>) of 3D-OMNiSA are about 4.3 and 2.2 times that of RuO<sub>2</sub>, respectively. Additionally, this 3D-OMNiSA catalyst shows a high durability under harsh water oxidation cycling test. The outstanding OER performance of the 3D-OMNiSA could be attributed to the large surface area, efficient mass and charge transport, and high structural stability arising from the unique 3D hierarchical porous structure of the 3D-OMNiSA consisting of ordered close-packed mesoporous spheres

    Examination of Near-Electrode Concentration Gradients and Kinetic Impacts on the Electrochemical Reduction of CO<sub>2</sub> using Surface-Enhanced Infrared Spectroscopy

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    Localized concentration gradients within the electrochemical double layer during various electrochemical processes can have wide-ranging impacts; however, experimental investigation to quantitatively correlate the rate of surface-mediated electrochemical reaction with the interfacial species concentrations has historically been lacking. In this work, we demonstrate a spectroscopic method for the in situ determination of the surface pH using the CO<sub>2</sub> reduction reaction as a model system. Attenuated total reflectance surface-enhanced infrared absorption spectroscopy is employed to monitor the ratio of vibrational bands of carbonate and bicarbonate as a function of electrode potential. Integrated areas of vibrational bands are then compared with those obtained from calibration spectra collected in electrolytes with known pH values to determine near-electrode proton concentrations. Experimentally determined interfacial proton concentrations are then related to the resultant concentration overpotentials to examine their impact on electrokinetics. We show that, in CO<sub>2</sub>-saturated sodium bicarbonate solutions, a concentration overpotential of over 150 mV can be induced during electrolysis at −1.0 V vs RHE, leading to substantial losses in energy efficiency. We also show that increases in both convection and buffering capacity of the electrolyte can mitigate interfacial concentration gradients. On the basis of these results, we further discuss how increases in concentration overpotential affect the mechanistic interpretations of the CO<sub>2</sub> reduction electrocatalysis, particularly in terms of Tafel slopes and reaction orders

    Efficient Water Oxidation Using Nanostructured α‑Nickel-Hydroxide as an Electrocatalyst

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    Electrochemical water splitting is a clean technology that can store the intermittent renewable wind and solar energy in H<sub>2</sub> fuels. However, large-scale H<sub>2</sub> production is greatly hindered by the sluggish oxygen evolution reaction (OER) kinetics at the anode of a water electrolyzer. Although many OER electrocatalysts have been developed to negotiate this difficult reaction, substantial progresses in the design of cheap, robust, and efficient catalysts are still required and have been considered a huge challenge. Herein, we report the simple synthesis and use of α-Ni­(OH)<sub>2</sub> nanocrystals as a remarkably active and stable OER catalyst in alkaline media. We found the highly nanostructured α-Ni­(OH)<sub>2</sub> catalyst afforded a current density of 10 mA cm<sup>–2</sup> at a small overpotential of a mere 0.331 V and a small Tafel slope of ∼42 mV/decade, comparing favorably with the state-of-the-art RuO<sub>2</sub> catalyst. This α-Ni­(OH)<sub>2</sub> catalyst also presents outstanding durability under harsh OER cycling conditions, and its stability is much better than that of RuO<sub>2</sub>. Additionally, by comparing the performance of α-Ni­(OH)<sub>2</sub> with two kinds of β-Ni­(OH)<sub>2</sub>, all synthesized in the same system, we experimentally demonstrate that α-Ni­(OH)<sub>2</sub> effects more efficient OER catalysis. These results suggest the possibility for the development of effective and robust OER electrocatalysts by using cheap and easily prepared α-Ni­(OH)<sub>2</sub> to replace the expensive commercial catalysts such as RuO<sub>2</sub> or IrO<sub>2</sub>
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