Improving the electrocatalytic properties of Pd-based catalyst for direct alcohol fuel cells: effect of solid solution

Synthesis and Characterization of Supported Pd Catalysts for Potential Application in Glycerol Electro-Oxidation

Ceria-supported Pd catalysts encompassing oxides of Cu, Co, and Fe were synthesized and characterized using XRD, TEM, SEM-EDX, TPR, BET, and Raman. After the incorporation of the metal oxides, the surface area and pore volume of the ceria support decreased. XRD showed the presence of the metal oxide phases as well as the support, CeO2. TPR showed that the bimetallic catalyst had improved reducibility compared to the monometallic Pd/CeO2. TEM images showed irregular-shaped particles with an average size distribution of 2–10 nm. SEM-EDX showed that the metal oxides were evenly distributed over the surface of the support. The electro-oxidation of glycerol in an alkaline environment was evaluated using cyclic voltammetry, and the products formed were identified and quantified using GC-MS. Glyceric acid was the dominant product over Pd-CuO/CeO2, while glyceraldehyde and dihydroxyacetone were dominant over Pd-Co3O4/CeO2 and Pd-Fe2O3/CeO2, respectively.

PdAg/C Electrocatalysts Synthesized by Thermal Decomposition of Polymeric Precursors Improve Catalytic Activity for Ethanol Oxidation Reaction

An efficient ethanol oxidation reaction (EOR) is required to enhance energy production in alcohol-based fuel cells. The use of bimetallic catalysts promises decreasing reliance on platinum group metal (PGM) electrocatalysts by minimizing the use of these expensive materials in the overall electrocatalyst composition. In this article, an alternative method of bimetallic electrocatalyst synthesis based on the use of polymeric precursors is explored. PdAg/C electrocatalysts were synthesized by thermal decomposition of polymeric precursors and used as the anode electrocatalyst for EOR. Different compositions, including pristine Pd/C and Ag/C, as well as bimetallic Pd80Ag20/C, and Pd60Ag40/C electrocatalysts, were evaluated. Synthesized catalysts were characterized, and electrochemical activity evaluated. X-ray diffraction showed a notable change at diffraction peak values for Pd80Ag20/C and Pd60Ag40/C electrocatalysts, suggesting alloying (solid solution) and smaller crystallite sizes for Pd60Ag40/C. In a thermogravimetric analysis, the electrocatalyst Pd60Ag40/C presented changes in the profile of the curves compared to the other electrocatalysts. In the cyclic voltammetry results for EOR in alkaline medium, Pd60Ag40/C presented a more negative onset potential, a higher current density at the oxidation peak, and a larger electrically active area. Chronoamperometry tests indicated a lower poisoning rate for Pd60Ag40/C, a fact also observed in the CO-stripping voltammetry analysis due to its low onset potential. As the best performing electrocatalyst, Pd60Ag40/C has a lower mass of Pd (a noble and expensive metal) in its composition. It can be inferred that this bimetallic composition can contribute to decreasing the amount of Pd required while increasing the fuel cell performance and expected life. PdAg-type electrocatalysts can provide an economically feasible alternative to pure PGM-electrocatalysts for use as the anode in EOR in fuel cells.

Preparation of Carbon-Supported Ternary Nanocatalysts Palladium-Vanadium-Cobalt for Alcohol Electrooxidation

Carbon-supported nanocatalysts palladium-vanadium-cobalt (PdVCo) were synthesized via ethylene glycol (EG) reduction reaction and NaBH4-assisted reduction. The electrocatalytic performance for alcohol oxidation in alkaline solutions was investigated. The XRD and EDX results confirmed the incorporation of V and Co with Pd lattice to form the ternary nanocatalysts PdVCo in a single phase. The NaBH4-assisted EG reduction process exhibited highly dispersed nanoparticles with a uniform size, and the electrochemical surface area (ECSA) determined by cyclic voltammetry in 1 M KOH was also superior. In electrocatalysis performance, the cyclic voltammetry (CV) and chronoamperometry (CA) results presented an excellent electrocatalytic activity and stability of the PdVCo-20EG-20NaBH4 sample in the alcohol electrooxidation as compared to other synthesized samples with the steady current of 52 mA/cm2 and 21.9 mA/cm2 in methanol and ethanol, respectively.

Nanocatalysts for Electrocatalytic Oxidation of Ethanol

The use of ethanol as a fuel in direct alcohol fuel cells depends not only on its ease of production from renewable sources, but also on overcoming the challenges of storage and transportation. In an ethanol-based fuel cell, highly active electrocatalysts are required to break the C-C bond in ethanol for its complete oxidation at lower overpotentials, with the aim of increasing the cell performance, ethanol conversion rates, and fuel efficiency. In recent decades, the development of wet-chemistry methods has stimulated research into catalyst design, reactivity tailoring, and mechanistic investigations, and thus, created great opportunities to achieve efficient oxidation of ethanol. In this Minireview, the nanomaterials tested as electrocatalysts for the ethanol oxidation reaction in acid or alkaline environments are summarized. The focus is mainly on nanomaterials synthesized by using wet-chemistry methods, with particular attention on the relationship between the chemical and physical characteristics of the catalysts, for example, catalyst composition, morphology, structure, degree of alloying, presence of oxides or supports, and their activity for ethanol electro-oxidation. As potential alternatives to noble metals, non-noble-metal catalysts for ethanol oxidation are also briefly reviewed. Insights into further enhancing the catalytic performance through the design of efficient electrocatalysts are also provided.

Growth Kinetics of Individual Co Particles Ex-solved on SrTi0.75Co0.25O3-δ Polycrystalline Perovskite Thin Films

A precise control of the size, density, and distribution of metal nanoparticles dispersed on functional oxide supports is critical for promoting catalytic activity and stability in renewable energy and catalysis devices. Here, we measure the growth kinetics of individual Co particles ex-solved on SrTi_0.75Co_0.25O_3-δ polycrystalline thin films under a high vacuum, and at various temperatures and grain sizes using in situ transmission electron microscopy. The ex-solution preferentially occurs at grain boundaries and corners which appear essential for controlling particle density and distribution, and enabling low temperature ex-solution. The particle reaches a saturated size after a few minutes, and the size depends on temperature. Quantitative measurements with a kinetic model determine the rate limiting step, vacancy formation enthalpy, ex-solution enthalpy, and activation energy for particle growth. The ex-solved particles are tightly socketed, preventing interactions among them over 800 °C. Furthermore, we obtain the first direct clarification of the active reaction site for CO oxidation-the Co-oxide interface, agreeing well with density functional theory calculations.

Anodic Oxidation of Butan-1-ol on Reduced Graphene Oxide-Supported Pd-Ag Nanoalloy for Fuel Cell Application

Reduced graphene oxide (RGO)-supported bimetallic Pd _x Ag _y alloy nanoparticles of various compositions were synthesized by one-pot coreduction of respective precursors with hydrazine for use in the anode catalysis of oxidation of butan-1-ol in alkali. The as-synthesized catalyst materials were characterized by microscopic, spectroscopic, and diffraction techniques. Cyclic voltammetry (CV), chronoamperometry, and polarization studies infer that a few Pd _x Ag _y materials exhibit an enhanced and synergistic catalytic activity in reference to Pd and Ag nanomaterials. Among the various RGO composites of Pd _x Ag _y alloy on graphite support, the one containing the Pd₇₀Ag₃₀@RGO composite is the best in catalytic activity. The cycle life of the catalyst is found to be very high, and PdO and Ag₂O are found to be generated in the catalyst material with little change in the catalytic capability during the 100th cycle of CV operation. The addition of Ag upto 30 atom % in the Pd _x Ag _y alloy causes greater formation of butyraldehyde and butyl butanoate among the various products. Larger atom % of Pd helps to form sodium butyrate and sodium carbonate, as evident from the ex situ Fourier transform infrared and high-performance liquid chromatography study of the product mixtures and the separate CV studies of the intermediate products. A suitable mechanism is also proposed to fit the findings.

Electro-oxidation competency of palladium nanocatalysts over ceria–carbon composite supports during alkaline ethylene glycol oxidation

The effect of varying Pd–CeO₂ ratios on the oxidation efficiency of ethylene glycol and low molecular weight alcohols in alkaline medium.

Harvesting resonantly-trapped light for small molecule oxidation reactions at the Au/α-Fe2O3 interface

Plasmonic metal nanoparticles (NPs) extend the overall light absorption of semiconductor materials. However, it is not well understood how coupling metal NPs to semiconductors alters the photo-electrochemical activity of small molecule oxidation (SMO) reactions. Different photo-anode electrodes comprised of Au NPs and α-Fe2O3 are designed to elucidate how the coupling plays not only a role in the water oxidation reaction (WO) but also performs for different SMO reactions. In this regard, Au NPs are inserted at specific regions within and/or on α-Fe2O3 layers created with a sequential electron beam evaporation method and multiple annealing treatments. The SMO and WO reactions are probed with broad-spectrum irradiation experiments with an emphasis on light-driven enhancements above and below the α-Fe2O3 band gap. Thin films of α-Fe2O3 supported on a gold back reflective layer resonantly-traps incident light leading to enhanced SMO/WO conversion efficiencies at high overpotential (η) for above band-gap excitations with no SMO activity observed at low η. In contrast, a substantial increase in the light-driven SMO activity is observed at low η, as well as for below band-gap excitations when sufficiently thin α-Fe2O3 films are decorated with Au NPs at the solution-electrode interface. The enhanced photo-catalytic activity is correlated with increased surface oxygen content (hydroxyl groups) at the Au/α-Fe2O3 interface, as well as simulated volume-integrated near-field enhancements over select regions of the Au/α-Fe2O3 interface providing an important platform for future SMO/WO photo-electrocatalyst development.

Highly Active PdNi/RGO/Polyoxometalate Nanocomposite Electrocatalyst for Alcohol Oxidation

A PdNi/RGO/polyoxometalate nanocomposite has been successfully synthesized by a simple wet-chemical method. Characterizations such as transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction analysis, and X-ray photoelectron spectroscopy are employed to verify the morphology, structure, and elemental composition of the as-prepared nanocomposite. Inspired by the fast-developing fuel cells, the electrochemical catalytic performance of the nanocomposite toward methanol and ethanol oxidation in alkaline media is further tested. Notably, the nanocomposite exhibits excellent catalytic activity and long-term stability toward alcohol electrooxidation compared with the PdNi/RGO and commercial Pd/C catalyst. Furthermore, the electrochemical results reveal that the prepared nanocomposite is attractive as a promising electrocatalyst for direct alcohol fuel cells, in which the phosphotungstic acid plays a crucial role in enhancing the electrocatalytic activities of the catalyst.