Heterostructure interface effect on the ORR/OER kinetics of Ag–PrBa0.5Sr0.5Co2O5+δ for high-efficiency Li–O2 battery

Developing high-efficiency and cost-effective bifunctional electrocatalysts toward the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is an urgent issue for the oxygen-based electrochemical devices. Herein, an interface engineering concept has been proposed to achieve high-performance Ag–PBSC (Ag–PrBa_0.5Sr_0.5Co₂O_5+δ) heterostructure nanofibers catalyst. Benefiting from the significant ligand action and interparticle cooperation of exsolved Ag NPs and PBSC double perovskite, ORR/OER catalytic kinetics have been successfully boosted. In details, the PBSC double perovskite possessing abundant oxygen vacancies can provide oxygen channels and facilitate the transfer of electrons and oxygen. The embedded Ag NPs can deliver superior catalytic durability for the heterostructure interface. As expected, the as-synthesized Ag-PBSC heterostructure catalyst performs a favorable electrochemical performance in the oxygen-based applications. In alkaline media, the catalyst exhibits an excellent activity for ORR (E_onset: 0.88 V vs. RHE and E_1/2: 0.72 V vs. RHE) and OER (1.67 V at 10 mA cm^–2). When adopting the Ag–PBSC heterostructure catalyst in LOBs, the corresponding battery provides an outperforming capacity performance (13 000 mAh g^–1), low discharge–charge polarization (1.37 V), and considerable cycling performance (128 cycles at the restricted capacity of 3 000 mAh g^–1 and 400 mA g^–1). Apparently, the work described here confirms that the interface engineering of perovskites can open up opportunities to develop highly active and durable heterostructure electrocatalysts for multitudinous oxygen-based electrochemical applications.     

Ultrathin amorphous MnO2 modified prawn shells-derived porous carbon towards robust oxygen electrocatalyst for rechargeable Zn-air battery

Precious metal-free bifunctional electrocatalysts with efficient performance to ORR (oxygen reduction reaction) and OER (oxygen evolution reaction) are the prerequisite to the commercialization of rechargeable Zn-air battery (RZAB). Here, ultrathin amorphous MnO₂ modified prawn shells-derived porous carbon (U–MnO₂/PSNC) is designed and synthesized via a self-template assisted pyrolysis coupling in-situ redox reaction strategy. In this composite, the ultrathin amorphous MnO₂ provides high surface defects and homogeneous catalytic active sites. The conductive prawn shells-derived porous carbon displays macro-meso-microporous structure, promoting electric conductivity and offering fast pathway for mass diffusion. With optimized composition, the U–MnO₂-0.01/PSNC delivers an excellent bifunctional to ORR/OER with a small ΔE (bifunctional activity parameter) value of 0.776 V. Moreover, the RZAB equipped with U–MnO₂-0.01/PSNC displays a considerable stability with narrow decay of battery efficiency (1.10%) for ～334 h (500 cycles) at 10 mA cm^?2. This work enlightens a new pathway to design cost-effective bifunctional electrocatalysts for metal-air battery.     

Robust assembly of urchin-like NiCo2O4/CNTs architecture as bifunctional electrocatalyst in Zn-Air batteries

Cost-effective bifunctional electrocatalysts with desirable oxygen catalytic activity to both of ORR and OER are essential to the large-scale commercial application of metal-air batteries. In this work, a robust hydrothermal assembly strategy is presented to fabricate the three-dimensional (3D) urchin-like hybrid material of acid-treated multiwall CNTs (MWCNTs) and needle-like NiCo₂O₄ as a cost-effective bifunctional oxygen electrocatalyst. Benefiting from the particular 3D urchin-like mesoporous structure providing rich transport channels for O₂ diffusion and electrolyte infiltration and abundant active sites for charge transfer reaction, and the synergistic effect of high electrocatalytically active NiCo₂O₄ and high conductive MWCNTs network, the optimized NCO@20%MWCNTs composite exhibits a superior OER and ORR catalytic activity. In its practical application, an ultrahigh initial round-trip efficiency of 79.4% and long-term cycle stability (950 cycles) at 10 mA cm⁻² are achieved in Zn-air battery. This simple strategy for construction of the cost-effective unique 3D porous urchin-like structure together with the improved electrochemical catalytic activity provides a great potential for material design in oxygen catalysis and metal-air batteries.     

Noncovalent hybrid of CoMn2O4 spinel nanocrystals and poly (diallyldimethylammonium chloride) functionalized carbon nanotubes as efficient electrocatalysts for oxygen reduction reaction

We have grown CoMn₂O₄ spinel nanocrystals on poly (diallyldimethylammonium chloride) functionalized carbon nanotubes (PDDA-CNTs) by noncovalent functionalization and solvothermal techniques. PDDA plays an important role in homogeneously increasing the surface density of available functional groups, which can provide active sites for decoration of CoMn₂O₄ on CNTs. In addition, PDDA preserves the intrinsic properties of CNTs, increases the active sites of catalysts, and enhances the durability of the catalysts. Here, CoMn₂O₄ nanocrystals were uniformly deposited on PDDA-CNTs with loading amounts from 36% to 83%. The as-prepared CoMn₂O₄/PDDA-CNT catalyst showed high current densities for the oxygen reduction reaction (ORR) in alkaline and neutral conditions, which outperformed the Co₃O₄/PDDA-CNT and Pt/C catalysts at medium overpotential, mainly through a 4e reduction pathway. The obtained CoMn₂O₄/PDDA-CNT hybrid exhibited excellent activity and durability when subjected to an oxygen evolution reaction. These results indicate that the CoMn₂O₄/PDDA-CNT hybrid represents a promising alternative to Pt for ORR electrocatalysis, and this non-precious bifunctional electrocatalyst provides a corrosion resistant and protective cathode layer to fuel cells. The excellent activity and stability of the hybrid materials demonstrate the potential of noncovalent coupling inorganic/carbon composites as novel catalytic systems for lithium–air batteries and chlor-alkali production.     

Hierarchical FeC/MnO2 composite with in-situ grown CNTs as an advanced trifunctional catalyst for water splitting and Metal−Air batteries

In high demand is developing trifunctional electrocatalysts to simultaneously drive hydrogen evolution reaction (HER) and oxygen evolution/reduction reaction (OER/ORR) for metal-air batteries and water splitting. Here we develop the carbon nanotubes (CNTs)-grafted FeC/MnO₂ nanocomposite catalyst by carbonizing FeMn metal-organic frameworks. The synergistic effect between FeC and MnO₂ dominantly contributes the ORR, OER, and HER. The transition metal-mediated growth of CNTs by an in-situ catalysis mechanism enables high electrical conductivity, abundant active sites, as well as efficient reaction pathways. The optimized chemical composite and unique hierarchical structure endow the FeC/MnO₂ with low overpotentials for multiply electrochemical reactions. Consequently, the composite catalyst successfully serves as the bifunctional electrode for water splitting with a voltage of 1.66 V at 10 mA cm^?2 as well as the cathode for all-solid-state metal-air battery with Pt/C-comparable performance. The advanced transition metal composite presented in this work provides the guidance for rationally developing trifunctional electrocatalysts for efficient integrated energy conversion systems.     

Combined in situ EXAFS and electrochemical investigation of the oxygen reduction reaction on unmodified and Se-modified Ru/C

In this work we study the kinetics of the oxygen reduction reaction on carbon-supported Ru nanoparticles modified with various amounts of Se. Rotating disk electrode is used to determine kinetic currents for the ORR in 0.1 M H₂SO₄ at 298 K and O₂ partial pressures from 1 to 0.01 atm. The dependence of the ORR activity on Se/Ru ratio shows volcano-type behavior with ca. 10 fold increase of the mass activity at 0.1 < Se/Ru < 0.3. The reaction order in O₂ is close to 1 in the interval of overpotentials from 0.4 to 0.7 V, and is independent of the presence of Se. Regardless the amount of Se, the Tafel slope demonstrates continuous increase from ca. 70 mV/dec at 0.4 V to ca. 140 mV/dec at 0.6 V overpotential. In situ EXAFS spectra are measured at Ru K-edge (in the transmission mode) and Se K-edge (in the fluorescence mode) in argon and oxygen saturated 0.1 M H₂SO₄ solutions in the interval of electrode potentials from 0.050 to 0.750 V RHE. The data are used to explore the surface state changes of Ru and Ru_xSe_y particles and clarify the promoting role of Se during the ORR.     

Simple fabrication of strongly coupled cobalt ferrite/carbon nanotube composite based on deoxygenation for improving lithium storage

An easy method to synthesize a strongly coupled cobalt ferrite/carbon nanotube (CoFe₂O₄/CNT) composite with oxygen bridges between CoFe₂O₄ and reduced carbon nanotubes (CNTs) by calcining the precursor material was reported. The precursor was prepared by an electrostatic self-assembly of the exfoliated Co(II)Fe(II)Fe(III)-layered double hydroxide (CoFeFe-LDH) nanosheets and acid treated CNTs. The deoxygenation effect of ferrous ion (Fe²⁺) in CoFeFe-LDH nanosheets on the oxygen-containing groups of acid treated CNTs was investigated by X-ray photoelectron spectroscopy (XPS) measurement. After thermal conversion, the obtained CoFe₂O₄ was bonded to the reduced CNTs through Metal–O–C (oxygen bridge), which was characterized by XPS, Fourier transform infrared spectroscopy, and Raman spectroscopy. When applied as an anode for lithium-ion battery, the CoFe₂O₄/CNT composite exhibited a low resistance of charge transfer and Li-ion diffusion, good cycle performance, and high rate capability. At a lower current density of 0.15 A·g⁻¹, a specific discharge capacity of 910 mA·h·g⁻¹ was achieved up to 50 cycles. When current density was increased to 8.8 A·g⁻¹, the CoFe₂O₄/CNT composite still delivered 500 mA·h·g⁻¹.     

Highly durable platelet carbon nanofiber-supported platinum catalysts for the oxygen reduction reaction

We report the preparation and characterization of highly durable platinum catalysts supported on platelet-structure carbon nanofibers (Pt/p-CNFs) for the oxygen reduction reaction. The p-CNFs were prepared by liquid phase carbonization of polyvinyl chloride using a porous anodic alumina template at 600 °C; their degree of graphitization was increased by the subsequent heat treatment at higher temperatures of up to 1400 °C. The platinum nanoparticles with ∼3 nm diameter were deposited more uniformly on the p-CNFs compared with those on the commercial Ketjen black (KB). The catalytic activity and durability of the Pt/p-CNFs for the oxygen reduction reaction (ORR) in H₂SO₄ solution were improved by increasing the heat-treatment temperature of p-CNFs. The durability of the Pt/p-CNFs was much higher than that of Pt/KB; in particular, a loss of less than 10% was observed in the ORR activity of Pt/p-CNF heat-treated at 1400 °C after potential cycling from 0.5 to 1.5 V vs. RHE for 200 cycles in an argon-saturated H₂SO₄ aqueous solution.     

Flexible magnetoelectric PVDF–CoFe2O4 fiber films for self-powered energy harvesters

Integrating the concept of magnetoelectric in the mechanical energy harvesters through the magneto-mechano-electrical (MME) nanogenerators has been explored to realize the self-powered devices. The magnetoelectric interaction enabled the output performance of the MME nanogenerator under magnetic stimulus of the active components of the energy harvesters. In this perspective, we fabricated a flexible biomechanical and MME nanogenerator using PVDF/CoFe₂O₄ fibers composite films. CoFe₂O₄ fibers were synthesized by the electrospinning technique and the process parameters were optimized to achieve uniform and bead-free fibers. The structural and morphological properties were investigated through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The structural and morphology revealed the fibers calcined at 800 °C with a heating rate of 2 °C/min produced bead-free continuous fibers with a fiber diameter of 210 nm with cubic spinel crystalline structure with a crystallite size of 34 nm. These highly crystalline fibers were used to fabricate PVDF/CoFe₂O₄ fibers composite films. The magnetoelectric behaviour of the films verified through polarization vs. electric field (P-E) loops under magnetic field. The leakage current density and mechanism of the composite films were investigated, and it was discovered that the mechanism was due to Schottky emission. Further the energy harvesting performance of the composite films were estimated where the nanogenerator achieved an output voltage of 13 V under biomechanical tapping force while the MME nanogenerator produced 3.5 V under a low frequency stray magnetic field of 6 Oe with a power density of 28 μW/m².     

Investigation into the activity of heteropolyacids towards the oxygen reduction reaction on PEMFC cathodes

A total of 18 heteropolyacids (HPAs) were investigated to determine their activity as non-Pt oxygen reduction reaction (ORR) catalysts in polymer electrolyte membrane fuel cell cathodes (PEMFCs). Polarization curves, cyclic voltammetry and impedance spectroscopy determined that, of the HPAs tested, only molybdenum based HPAs are active for the ORR and that vanadium substitutions improved the activity. The reduction potentials of the HPAs in the fuel cell environment were determined by cyclic voltammetry. This showed that no activity is seen above 0.55 V, as the catalysts must first be reduced in situ by 4e⁻ before the HPA can reduce oxygen. The potential at which the HPA can be reduced has been determined to be the limiting factor when using these catalysts for ORR in PEMFCs. Power densities of 67 mW/cm² at 0.2 V were obtained using H₅PMo₁₀V₂O₄₀. Molybdenum based HPAs were covalently bonded to the carbon achieving mass loadings 3× that obtained through adsorption. Using this approach catalyst, performance was improved to 86 mW/cm² at 0.2 V. The increased loadings did not significantly increase the potentials at which the HPA becomes active for the ORR. We were able to show that MEA degradation, as measured by F⁻ emission rates, using these catalysts are reduced during accelerated testing protocols.     

Facile electrodeposition of MFe2O4 (M=Co,Fe) on carbon cloth as air cathodes for Li-O2 batteries

To develop high energy Li-O₂ batteries (LOBs), it is important to optimize the air cathode structure. Therefore, MFe₂O₄@carbon cloth (M = Co, Fe) has been prepared by electrodeposition to be an effective cathode for LOBs. This cathode can effectively avoid the disturbance of a polymer binder during the discharge-charge process. Electrocatalytic and electrochemical measurements show that MFe₂O₄@carbon cloth can enhance the ORR/OER kinetics. In particular, CoFe₂O₄@carbon cloth exhibits a good initial discharge specific capacity (7259 mA h/g at 170 mA/g) and long cycle life (over 100 cycles at the upper limit specific capacity of 500 mA h/g at 170 mA/g). The favorable electrocatalytic properties of CoFe₂O₄@CC are ascribed to the presence of Co²⁺.     

Impact of rare earth europium (RE-Eu3+) ions substitution on microstructural,optical and magnetic properties of CoFe2−xEuxO4 nanosystems

In this study, pure cobalt ferrite (CoFe₂O₄) nanoparticles and europium doped CoFe₂O₄ (CoFe_2−xEu_xO₄; x = 0.1, 0.2, 0.3) nanoparticles were synthesized by the precipitation and hydrothermal approach. The impact of replacing trivalent iron (Fe³⁺) ions by trivalent rare earth europium (RE-Eu³⁺) ions on the microstructure, optical and magnetic properties of the produced CoFe₂O₄ nanoparticles was studied. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectra exposed the consistency of a single cubic phase with the evidence of Eu₂O₃ phases for x ≥ 0.2. FTIR transmittance spectra showed that, the all investigated samples have three characteristic metal-oxygen bond vibrations corresponding to octahedral B-site (υ₁ and υ₂) and tetrahedral A-site (υ₃) around 415 cm⁻¹, 470 cm⁻¹ and 600 cm⁻¹ respectively. XRD and energy dispersive X-ray spectroscopy studies affirmed the integration of RE-Eu³⁺ ions within CoFe₂O₄ host lattice and decrease of average crystals size from 13.7 nm to 4.7 nm. Transmission electron microscopy (TEM) analysis showed the crucial role played by RE-Eu³⁺ added to CoFe₂O₄ in reducing the particle size below 5 nm in agreement with XRD analysis. High resolution-TEM (HR-TEM) analysis showed that the as-synthesized spinel ferrite, i.e., CoFe_2−xEu_xO₄, nanoparticles are single-crystalline with no visible defects. In addition, the HR-TEM results showed that pure and doped CoFe₂O₄ have well-resolved lattice fringes and their interplanar spacings matches that obtained by XRD analysis. Magnetic properties investigated by the vibrating sample magnetometer technique illustrated transformation of magnetic state from ferromagnetic to superparamagnetic at 300 K resulting in introducing RE-Eu³⁺ in CoFe₂O₄ lattice. At low temperature (~5 K) the magnetic order was ferromagnetic for both pure and doped CoFe₂O₄ samples. Substitution of Fe³⁺ ions in CoFe₂O₄ nanoparticles with RE-Eu³⁺ ions optimizes the sample nanocrystals size, cation distribution and magnetic properties for many applications.     

Bimagnetic hard/soft and soft/hard ferrite nanocomposites: Structural,magnetic and hyperthermia properties

Recent studies show that the chemical composition and shape of magnetic nanoparticles (NPs) play an important role in their properties. In particular, the bimagnetic NPs display useful and in many cases, more interesting properties than single-phase NPs. In this work, we prepared Fe₃O₄ and CoFe₂O₄ cube-like NPs and bimagnetic hard/soft (CoFe₂O₄/Fe₃O₄) and soft/hard (Fe₃O₄/CoFe₂O₄) nanocomposites (core/coating) using a facile and eco-friendly co-precipitation method that allows the synthesis of the cube-like NPs at temperatures near room temperature. The phase purity and the crystallinity of the NPs with a spinel structure were confirmed by the X-ray diffraction and infrared spectra techniques. Transmission electron microscopy (TEM) images revealed that the NPs have a cubic-like shape with an average dimension of 20 nm. Energy dispersive X-ray analysis, Mössbauer spectroscopy and SQUID magnetic measurements indicated the co-existence of Fe₃O₄ and CoFe₂O₄ phases in nanocomposites. In addition, the hysteresis loops exhibited a single-phase behavior in the nanocomposites that indicates there is a good exchange-coupling interaction between the hard and soft magnetic phases. The CoFe₂O₄/Fe₃O₄ nanocomposites presented a larger saturation magnetization than the CoFe₂O₄ NPs that is effective for their use in magnetic hyperthermia. Finally, we studied the hyperthermia properties of samples in an alternating magnetic field with a frequency of 276 kHz and field amplitude of 13.9 kA/m. Our results showed that magnetic hyperthermia efficiency simultaneously depends on the composition of samples along with magnetic anisotropy and saturation magnetization.     

Partially oxidized niobium carbonitride as a non-platinum catalyst for the reduction of oxygen in acidic medium

Partially oxidized NbC_0.5N_0.5 has been evaluated as a non-platinum catalyst for the reduction of oxygen in acidic medium. NbC_0.5N_0.5 powder was partially oxidized in N₂ gas containing O₂ of 10⁻⁴ atm at the temperature range of 700-1000 °C. Partially oxidized NbC_0.5N_0.5 had a definite oxygen reduction reaction (ORR) activity, while as-prepared NbC_0.5N_0.5 and completely oxidized Nb₂O₅ had a poor catalytic activity for ORR. The onset potential of the partially oxidized NbC_0.5N_0.5 for the ORR achieved 0.92 V vs. RHE in 0.1 M H₂SO₄ at 30 °C. The results of X-ray absorption spectroscopy and ionization potential measurements suggested that oxygen-vacancy defects might be responsible for the oxygen reduction capability by creating electronically favorable oxygen adsorption sites.     

Effects of the fuel type and fuel content on the specific surface area and magnetic properties of solution combusted CoFe2O4 nanoparticles

In this work, the different fuels (citric acid, glycine and urea) at the various fuel to oxidant ratios (ϕ=0.5, 0.75, 1 and 1.25) were used for solution combustion synthesis of CoFe₂O₄ nanoparticles. The phase evolution, microstructure, specific surface area and magnetic properties of the solution combusted CoFe₂O₄ nanoparticles were investigated by X-ray diffraction, thermal analysis, electron microscopy, adsorption-desorption isotherms and vibrating sample magnetometry techniques. The specific surface area of the combusted products decreased with the increase of fuel to oxidant ratio (ϕ), irrespective of the fuel type. However, the specific surface area for the glycine fuel was higher than the others, due to the higher combustion rate for releasing gaseous products. Furthermore, the solution combusted CoFe₂O₄ powders by the glycine fuel exhibited the higher saturation magnetization (63.6 emu/g) on account of their higher crystallinity and particle size.     

Effect of lemon juice on microstructure,phase changes,and magnetic performance of CoFe2O4 nanoparticles and their use on release of anti-cancer drugs

Cobalt ferrite magnetic nanoparticles were synthesized and developed by a modified Pechini method using iron nitrate, cobalt nitrate, ethylene glycol (EG), and sucrose with different volumes of lemon juice (10, 20, 30, 40, 50, 60, and 70 ml) as the source of chelating agent as well as nonmagnetic elements such as Ca and Mg ions. The XRD patterns confirmed that all samples synthesized by different contents of extracted lemon juice had a cubic crystal structure with single-phase spinel. Scanning electron microscopy revealed that cobalt ferrite nanoparticles had a semi-spherical morphology. Also, the vibrating sample magnetometer indicated that the saturation magnetization of CoFe₂O₄ nanoparticles prepared with different values of extracted lemon juice increased from 18.6 emu/g for 10 ml extracted lemon juice to 75.7 emu/g for 50 ml extracted lemon juice, after which the saturation magnetization diminished. Afterwards, the CoFe₂O₄ nanoparticles were coated with polyethylene glycol (PEG) and doxorubicin (DOX) drugs, whereby drug delivery was detected at different pH levels. The CoFe₂O₄-PEG-DOX nanocomposite could release doxorubicin by more than 42% at pH = 5.4 in 75 h.     

Influence of sputtering power on oxygen reduction reaction activity of zirconium oxides prepared by radio frequency reactive sputtering

Zirconium oxides (ZrO_2−x) have been investigated as new cathodes for direct methanol fuel cells without platinum. ZrO_2−x films were prepared using a radio frequency (RF) magnetron sputtering at RF powers from 75 to 175 W. The influence of the RF power on the catalytic activity for the oxygen reduction reaction (ORR) and properties of the ZrO_2−x films were examined. The ORR activity of the ZrO_2−x catalyst increased with the RF power in the range we studied. The onset potential for ORR over ZrO_2−x deposited at 175 W was 0.88 V vs RHE. In addition, the relationship between the ORR activity and the composition, crystallinity, electric conductivity, as well as the ionization potential has been investigated. The zirconium oxide with an oxygen defected state and the higher electric conductivity showed the higher ORR activity, and the electrocatalytic activity for ORR increased with the decreasing in the ionization potential of the ZrO_2−x catalyst.     

Preparation of magnetic composite photocatalyst Bi2WO6/CoFe2O4by two-step hydrothermal method and itsphotocatalytic degradation of bisphenol A

Easily separable magnetic photocatalyst Bi₂WO₆/CoFe₂O₄ was synthesized by a two-step hydrothermal method. Pure spinel CoFe₂O₄ in nano-scale was prepared by a hydrothermal method, which was followed by a second hydrothermal process to coat CoFe₂O₄ with Bi₂WO₆. The prepared Bi₂WO₆/CoFe₂O₄ kept the magnetic property of CoFe₂O₄ and high efficient photocatalytic activity of Bi₂WO₆ as well. The photoactivity of Bi₂WO₆/CoFe₂O₄ (mass ratio 10:1) to degrade bisphenol A (BPA) was close to that of pure Bi₂WO₆ after 120 min of simulated solar light irradiation. After reaction, the catalyst particles could be easily harvested from the suspension by applying an external magnetic field.     

Phenol methylation over nanoparticulate CoFe2O4 inverse spinel catalysts: The effect of morphology on catalytic performance

A series of CoFe₂O₄ nanoparticles have been prepared via co-precipitation and controlled thermal sintering, with tunable diameters spanning 7–50 nm. XRD confirms that the inverse spinel structure is adopted by all samples, while XPS shows their surface compositions depend on calcination temperature and associated particle size. Small (<20 nm) particles expose Fe³⁺ enriched surfaces, whereas larger (50 nm) particles formed at higher temperatures possess Co:Fe surface compositions close to the expected 1:2 bulk ratio. A model is proposed in which smaller crystallites expose predominately (1 1 1) facets, preferentially terminated in tetrahedral Fe³⁺ surface sites, while sintering favours (1 1 0) and (1 0 0) facets and Co:Fe surface compositions closer to the bulk inverse spinel phase. All materials were active towards the gas-phase methylation of phenol to o-cresol at temperatures as low as 300 °C. Under these conditions, materials calcined at 450 and 750 °C exhibit o-cresol selectivities of 90% and 80%, respectively. Increasing either particle size or reaction temperature promotes methanol decomposition and the evolution of gaseous reductants (principally CO and H₂), which may play a role in CoFe₂O₄ reduction and the concomitant respective dehydroxylation of phenol to benzene. The degree of methanol decomposition, and consequent H₂ or CO evolution, appears to correlate with surface Co²⁺ content: larger CoFe₂O₄ nanoparticles have more Co rich surfaces and are more active towards methanol decomposition than their smaller counterparts. Reduction of the inverse spinel surface thus switches catalysis from the regio- and chemo-selective methylation of phenol to o-cresol, towards methanol decomposition and phenol dehydroxylation to benzene. At 300 °C sub-20 nm CoFe₂O₄ nanoparticles are less active for methanol decomposition and become less susceptible to reduction than their 50 nm counterparts, favouring a high selectivity towards methylation.     

Electrocatalysis of oxygen reduction on a carbon supported platinum-vanadium alloy in polymer electrolyte fuel cells

The electrocatalysis of the oxygen reduction reaction (ORR) on carbon supported Pt:V 1:1 catalyst in polymer electrolyte fuel cells (PEFC) was investigated. At an oxygen pressure of 1 atm results indicate a lower electrocatalytic activity for the ORR in the presence of vanadium. However, at an O₂ pressure ≥2 atm an enhanced electrocatalytic property of PtV/C compared with Pt/C is revealed. This result indicates the occurrence of a different electrocatalytic mechanism for the ORR on Pt/C and PtV/C. An increase of mass transport overpotentials is observed for the PtV/C catalyst, and this was related to the presence of vanadium oxide. Indeed, XRD analysis revealed that only about 30% of V present in the catalyst is alloyed with Pt, forming a face centred cubic (fcc) Pt₃V solid solution. A thermal treatment at 850 °C under reducing atmosphere leads to the formation of an ordered fcc Pt₂V phase. After this, the ORR activity of PtV/C at O₂ pressure 1 atm is higher than that of Pt/C.