Tungsten based electrocatalyst for fuel cell applications

DFT Study of α-Keggin-type Iso-polyoxotungstate Anions [HnW12O40](8-n)- (n =1-4): Can [H4W12O40]4- Exist?

The Keggin-type iso-polyoxotungstate (iso-POT) anions [H_nW₁₂O₄₀]^(8-n)- 's, in which their central vacancies are occupied by protons, are attractive materials. It is of importance to reveal if the vacancies can be fully occupied by four protons. For further understanding the speciation of these iso-POT anions, relative stabilities and proton transfer reactions between H₁[H_n-1W₁₂O₄₀]^(8-n)- and [H_nW₁₂O₄₀]^(8-n)- were examined in detail by using the first-principles calculations (the nudged elastic band method, the synchronous transit-guided quasi-Newton method, the intrinsic reaction coordinate method, and frequency analysis calculations). Thermochemistry analysis of the proton transfer was also performed. [H_nW₁₂O₄₀]^(8-n)- was energetically more stable than H₁[H_n-1W₁₂O₄₀]^(8-n)-. This held for n = 4. According to the results of thermochemistry analysis, the rate constant and the Wigner correction were respectively 3.1 × 10¹ s^-1 and 2.2 at T = 298.15 K for the proton transfer from H₁[H₃W₁₂O₄₀]^4- to [H₄W₁₂O₄₀]^4-, indicating that [H₄W₁₂O₄₀]^4- can exist when H₁[H₃W₁₂O₄₀]^4- is formed by protonating [H₃W₁₂O₄₀]^5-.

Hydrochromic Visualization of a Keggin-Type Structure Triggered by Metallic Fluids for Liquid Displays, Reversible Writing, and Acidic Environment Detection

Hydrochromic visualization of a liquid interface shows vital potential applications in liquid displays, reversible writing, and acidic environmental detection, which offers a platform for detection and forewarning due to its intuitive and visual characteristics. Herein, we report a hydrochromic display due to the interfacial effect of liquid metal (LM)-triggered ammonium metatungstate (AMT) with instant dual-mode color switching. The double-electron-transfer reaction of the AMT on the surface of gallium-based LM caused the formation of heteropoly blue in the presence of acidic surroundings, resulting in a reversible color switching from being colorless to blue or blue to colorless. This visual interfacial discoloration phenomenon can be applied to the liquid display on diverse patterns of the LM surface. Furthermore, papers with a functional display were prepared, which can be used for writing up to eight times with dual-mode color switching. In addition, the reactive activity of acid triggering make it a potential candidate for use in visualizing an acidic environment with a detection range of pH = 1 to 0 (0.1-1.5 M). Briefly, this interfacial discoloration phenomenon enriches the interfacial engineering of LM and provides a unique prospective and wide-range platform for the application of LM.

POM-FLPs: [MoIV3]n-polyoxometalate bifunctional catalysis by [MoIV3]n–Om Lewis pairs frustrated by triangular MoIV–MoIV bonds

The largest [Mo@Mo₂₀]-1 and the first [Ge@Mo₁₆Zn_0.5Ge_1.5]-2 incorporating group IV metals are added to the [MoIV3]_n-POMs hybrid cluster family, which allows for the first systematic investigation into their reasonable syntheses, diversified molecular and electronic structures, and their influence on the catalytic hydrogenation performance of such POM-FLPs system.

Platinum-Based Catalysts on Various Carbon Supports and Conducting Polymers for Direct Methanol Fuel Cell Applications: a Review

Platinum (Pt)-based nanoparticle metals have received a substantial amount of attention and are the most popular catalysts for direct methanol fuel cell (DMFC). However, the high cost of Pt catalysts, slow kinetic oxidation, and the formation of CO intermediate molecules during the methanol oxidation reaction (MOR) are major challenges associate with single-metal Pt catalysts. Recent studies are focusing on using either Pt alloys, such as Fe, Ni, Co, Rh, Ru, Co, and Sn metals, or carbon support materials to enhance the catalytic performance of Pt. In recent years, Pt and Pt alloy catalysts supported on great potential of carbon materials such as MWCNT, CNF, CNT, CNC, CMS, CNT, CB, and graphene have received remarkable interests due to their significant properties that can contribute to the excellent MOR and DMFC performance. This review paper summaries the development of the above alloys and support materials related to reduce the usage of Pt, improve stability, and better electrocatalytic performance of Pt in DMFC. Finally, discussion of each catalyst and support in terms of morphology, electrocatalytic activity, structural characteristics, and its fuel cell performance are presented.

A Robust Versatile Hybrid Electrocatalyst for the Oxygen Reduction Reaction

Identifying non-precious-metal catalysts with desirable overall performance for oxygen reduction reaction (ORR) in either acidic or basic media is still a bottleneck. Here, a hybrid material is reported, in which tungsten carbide (WC) and ferberite (FeWO₄) are attached to the Fe and N dual-doped ordered mesoporous carbon (WC-FeWO₄@FeN-OMC) as a superior performance catalyst for the ORR in either acidic or basic media. In comparison with the frequently used Pt/C ORR catalyst (20 wt. %), our hybrid materials exhibit comparable electrocatalytic activity mainly via a 4e ORR process, better stability, and total tolerance to methanol in either acidic or basic media. These advantages, especially the outstanding stability in acidic media, render the WC-FeWO₄@FeN-OMC as a promising potential non-precious-metal ORR catalyst in practical fuel cell applications.

Oxidation of CO and Methanol on Pd-Ni Catalysts Supported on Different Chemically-Treated Carbon Nanofibers

In this work, palladium-nickel nanoparticles supported on carbon nanofibers were synthesized, with metal contents close to 25 wt % and Pd:Ni atomic ratios near to 1:2. These catalysts were previously studied in order to determine their activity toward the oxygen reduction reaction. Before the deposition of metals, the carbon nanofibers were chemically treated in order to generate oxygen and nitrogen groups on their surface. Transmission electron microscopy analysis (TEM) images revealed particle diameters between 3 and 4 nm, overcoming the sizes observed for the nanoparticles supported on carbon black (catalyst Pd-Ni CB 1:2). From the CO oxidation at different temperatures, the activation energy E_act for this reaction was determined. These values indicated a high tolerance of the catalysts toward the CO poisoning, especially in the case of the catalysts supported on the non-chemically treated carbon nanofibers. On the other hand, apparent activation energy E_ap for the methanol oxidation was also determined finding—as a rate determining step—the CO_ads diffusion to the OH_ads for the catalysts supported on carbon nanofibers. The results here presented showed that the surface functional groups only play a role in the obtaining of lower particle sizes, which is an important factor in the obtaining of low CO oxidation activation energies.

Combined Photoemission Spectroscopy and Electrochemical Study of a Mixture of (Oxy)carbides as Potential Innovative Supports and Electrocatalysts

Active and stable non-noble metal materials, able to substitute Pt as catalyst or to reduce the Pt amount, are vitally important for the extended commercialization of energy conversion technologies, such as fuel cells and electrolyzers. Here, we report a fundamental study of nonstoichiometric tungsten carbide (WxC) and its interaction with titanium oxycarbide (TiOxCy) under electrochemical working conditions. In particular, the electrochemical activity and stability of the WxC/TiOxCy system toward the ethanol electrooxidation reaction (EOR) and hydrogen evolution reaction (HER) are investigated. The chemical changes caused by the applied potential are established by combining photoemission spectroscopy and electrochemistry. WxC is not active toward the ethanol electrooxidation reaction at room temperature but it is highly stable under these conditions thanks to the formation of a passive thin film on the surface, consisting mainly of WO2 and W2O5, which prevents the full oxidation of WxC. In addition, WxC is able to adsorb ethanol, forming ethoxy groups on the surface, which constitutes the first step for the ethanol oxidation. The interaction between WxC and TiOxCy plays an important role in the electrochemical stability of WxC since specific orientations of the substrate are able to stabilize WxC and prevent its corrosion. The beneficial interaction with the substrate and the specific surface chemistry makes tungsten carbide a good electrocatalyst support or cocatalyst for direct ethanol fuel cells. However, WxC is active toward the HER and chemically stable under hydrogen reduction conditions, since no changes in the chemical composition or dissolution of the film are observed. This makes tungsten carbide a good candidate as electrocatalyst support or cocatalyst for the electrochemical production of hydrogen.

Electrochemical Behavior of TiO(x)C(y) as Catalyst Support for Direct Ethanol Fuel Cells at Intermediate Temperature: From Planar Systems to Powders

To achieve complete oxidation of ethanol (EOR) to CO2, higher operating temperatures (often called intermediate-T, 150-200 °C) and appropriate catalysts are required. We examine here titanium oxycarbide (hereafter TiOxCy) as a possible alternative to standard carbon-based supports to enhance the stability of the catalyst/support assembly at intermediate-T. To test this material as electrocatalyst support, a systematic study of its behavior under electrochemical conditions was carried out. To have a clear description of the chemical changes of TiOxCy induced by electrochemical polarization of the material, a special setup that allows the combination of X-ray photoelectron spectroscopy and electrochemical measurements was used. Subsequently, an electrochemical study was carried out on TiOxCy powders, both at room temperature and at 150 °C. The present study has revealed that TiOxCy is a sufficiently conductive material whose surface is passivated by a TiO2 film under working conditions, which prevents the full oxidation of the TiOxCy and can thus be considered a stable electrode material for EOR working conditions. This result has also been confirmed through density functional theory (DFT) calculations on a simplified model system. Furthermore, it has been experimentally observed that ethanol molecules adsorb on the TiOxCy surface, inhibiting its oxidation. This result has been confirmed by using in situ Fourier transform infrared spectroscopy (FTIRS). The adsorption of ethanol is expected to favor the EOR in the presence of suitable catalyst nanoparticles supported on TiOxCy.

Preparation of tungsten carbide nanosheets with large surface area using an in situ DWCNT template

H₂WO₄ sheets approximately 50 nm long and 20 nm wide were prepared using a DWCNT template, and WC nanosheets with large surface areas were obtained when the H₂WO₄ was carbonized at a low temperature using a liquid carbon source.

Catalytic Aminohalogenation of Alkenes and Alkynes

Catalytic aminohalogenation methods enable the regio- and stereoselective vicinal difunctionalization of alkynes, allenes and alkenes with amine and halogen moieties. A range of protocols and reaction mechanisms including organometallic, Lewis base, Lewis acid and Brønsted acid catalysis have been disclosed, enabling the regio- and stereoselective synthesis of halogen-functionalized acyclic amines and nitrogen heterocycles. Recent advances including aminofluorination and catalytic enantioselective aminohalogenation reactions are summarized in this review.