Research progress of Pt and Pt-based cathode electrocatalysts for proton-exchange membrane fuel cells

Proton-exchange membrane fuel cells (PEMFCs) have been widely used commercially to solve the energy crisis and environmental pollution. The oxygen reduction reaction (ORR) at the cathode is the rate-determining step in PEMFCs. Platinum (Pt) catalysts are used to accelerate the ORR kinetics. Pt's scarcity, high cost, and instability in an acidic environment at high potentials seriously hinder the commercialization of PEMFCs. Therefore, studies should explore electrocatalysts with high catalytic activity, enhanced stability, and low-Pt loading. This review briefly introduces the research progress on Pt and Pt-based ORR electrocatalysts for PEMFCs, including anticorrosion catalyst supports, Pt, and Pt-based alloy electrocatalysts. Advanced preparation technology and material characterization of Pt-based ORR electrocatalysts are necessary to improve the performance and corresponding reaction mechanisms.     

Co3O4 Hollow Polyhedrons as Bifunctional Electrocatalysts for Reduction and Evolution Reactions of Oxygen

It is very important to exploit low‐cost and efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts for the development of renewable‐energy conversion and storage techniques. Although much attention has been made to develop efficient catalysts for ORR and OER, it is still highly desired to create new bifunctional catalysts. In this study, Co₃O₄ hollow polyhedrons are synthesized as efficient bifunctional electrocatalysts for ORR and OER by simple one‐step annealing Co‐centered metal–organic frameworks (ZIF‐67). Due to the large specific surface areas and high porosity, the as‐prepared Co₃O₄ hollow polyhedrons exhibit excellent electrocatalytic activities for ORR and OER in alkaline media. Co₃O₄ hollow polyhedrons show higher peak current density (0.61 mA cm^?2) with four‐electron pathway than Co₃O₄ particles (0.39 mA cm^?2), better methanol tolerance and superior durability (82.6%) than commercial Pt/C electrocatalyst (58.6%) for ORR after 25 000 s. In addition, Co₃O₄ hollow polyhedrons also display excellent OER performances with smaller overpotential (536 mV) for 10 mA cm^?2 than Co₃O₄ particles (593 mV) and superior stability (86.5%) after 25 000 s. This facile one‐step strategy based on metal–organic frameworks self‐sacrificed templates can be used to develop the promising well‐defined porous hollow metal oxides electrode materials for energy conversion and storage technologies.     

Ultrasonic-assisted hydrothermal synthesis of cobalt oxide/nitrogen-doped graphene oxide hybrid as oxygen reduction reaction catalyst for Al-air battery

A persistent ultrasound-assisted hydrothermal method has been developed to prepare cobalt oxide incorporated nitrogen-doped graphene (Co₃O₄/N-GO) hybrids. The electrochemical behaviors and catalytic activity of the prepared hybrids have been systematically investigated as cathode materials for Al-air battery. The results show that ultrasonication can promote the yield ratio of Co₃O₄ from 63.1% to 70.6%. The prepared Co₃O₄/N-GO hybrid with ultrasonication exhibits better ORR activity over that without ultrasonication. The assembled Al-air battery using the ultrasonicated Co₃O₄/N-GO hybrid exhibited an average working voltage of 1.02 V in 4 M KOH electrolyte at 60 mA∙cm⁻², approximately 60 mV higher than that using hybrid without ultrasonication. This should be attributed to large number density of fine Co₃O₄ particles growing on the dispersed GO sheets under the persistent ultrasonication. The related ultrasonic mechanism has been discussed in details.     

Nitrogen doped carbon nano-onions as efficient and robust electrocatalysts for oxygen reduction reactions

We investigated synthesis and electrocatalytic performance of metal-free, nitrogen-doped carbon nano-onions (N-CNOs) for oxygen reduction reactions in alkaline electrolyte. N-CNOs were prepared by chemical oxidation of nanodiamond-derived carbon nano-onions (ox-CNOs), followed by thermal annealing with urea under the flow of argon gas. The chemical oxidation step was critical to successfully internalize nitrogen atoms into carbon network. Morphology, microstructure, and chemical states of carbon nano-onions (CNOs), ox-CNOs, and N-CNOs were characterized by transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Electrocatalytic activity of pristine and modified CNOs was characterized by a series of electrochemical measurements. Electrochemical characterizations were done with thin film electrodes of CNOs mounted on a glassy carbon disk. Compared to CNOs and ox-CNOs, N-CNOs showed remarkably enhanced electron-transfer kinetics with the 4-electron transfer as a dominant reaction pathway. Overall, N-CNOs exhibited electrochemical characteristics comparable to commercial Pt/C catalysts.     

FeNi Layered Double‐Hydroxide Nanosheets on a 3D Carbon Network as an Efficient Electrocatalyst for the Oxygen Evolution Reaction

An efficient electrocatalyst for oxygen evolution has been prepared via the deposition of iron–nickel layered double‐hydroxide (FeNi‐LDH) nanosheets on 3D carbon network as the building scaffold in a one‐step hydrothermal process. It is found that upon the assembling of FeNi‐LDH nanosheets with graphene into the 3D cross‐linked hybrid, the FeNi‐LDH/graphene hybrid features a well‐improved catalytic activity towards the oxygen evolution reaction (OER) with a good stability during the long‐term cycling experiment. Moreover, the hybrid catalyst is also active in the oxygen reduction reaction (ORR), qualifying it as a new type of bifunctional catalyst that can work in metal–air batteries.     

NiWO3 Nanoparticles Grown on Graphitic Carbon Nitride (g‐C3N4) Supported Toray Carbon as an Efficient Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions

Catalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are at the heart of water oxidation reactions. Despite continuous efforts, the development of OER/HER electrocatalysts with high activity at low cost remains a big challenge. Herein, a composite material consisting of TC@WO₃@g‐C₃N₄@Ni‐NiO complex matrix as a bifunctional electrocatalyst for the OER and HER is described. Though the catalyst has modest activity for HER, it exhibits high OER activity thereby making it a better nonprecious electrocatalyst for both OER and HER and is further improved by g‐C₃N₄. The catalytic activity arises from the synergetic effects between WO₃, Ni‐NiO, and g‐C₃N₄. A Ni‐NiO alloy and WO₃ nanoparticles decorated on the g‐C₃N₄ surface supported toray carbon (TC) matrix (TC@WO₃@g‐C₃N₄@Ni‐NiO) by a facile route that show an excellent and durable bifunctional catalytic activity for OER and HER in the alkaline medium are developed. This carbon nitride with binary metal/metal‐oxide matrix supported with TC exhibit an overpotential of 0.385 and 0.535 V versus RHE at a current density of 10 mA cm^?2 (Tafel slopes of 0.057 and 0.246 V dec^?1 for OER and HER, respectively), in 0.1 m NaOH . The catalyst is tested in water electrolysis for 17 h.     

Thermal and chemical approaches for oxygen catalytic recombination evaluation on ceramic materials at high temperature

During the atmospheric entry phase, the physico-chemical phenomena taking place on space shuttle walls can lead to an important excess of heating and damage of the protective materials. The aim of this work is the study of the catalytic recombination of atomic oxygen under plasma conditions chosen to simulate the atmospheric reentry. To do that, we have developed an experimental set-up MESOX (Moyen d'Essai Solaire d'OXydation), which associates a solar radiation concentrator and a microwave generator to reach high temperature, low enthalpy flow and low pressure plasma with an air gas flow. The study of atomic oxygen recombination on silicon- or aluminum-based ceramic materials, at high temperature (1000–1800 K) has been done for different pressures (200–2000 Pa) by a thermal and a chemical understanding. The results give a catalycity scale of materials (thermal recombination flux, q_rec, and coefficient of atomic oxygen recombination, γ). The catalycity activity is weak for the sintered SiC target with atomic oxygen recombination flux reaching 35 kW/m², however, for a target of sintered Al₂O₃, catalytic effect is obtained with energy fluxes between 90 to 180 kW/m². The recombination coefficient γ confirms the catalycity scale of these ceramic materials.     

Nitrogen‐Doped Carbon with Mesopore Confinement Efficiently Enhances the Tolerance,Sensitivity, and Stability of a Pt Catalyst for the Oxygen Reduction Reaction

Electrocatalysts for the oxygen reduction reaction (ORR) present some of the most challenging vulnerability issues reducing ORR performance and shortening their practical lifetime. Fuel crossover resistance, selective activity, and catalytic stability of ORR catalysts are still to be addressed. Here, a facile and in situ template‐free synthesis of Pt‐containing mesoporous nitrogen‐doped carbon composites (Pt‐m‐N‐C) is designed and specifically developed to overcome its drawback as an electrocatalyst for ORR, while its high activity is sustained. The as‐prepared Pt‐m‐N‐C catalyst exhibits high electrocatalytic activity, dominant four‐electron oxygen reduction pathway, superior stability, fuel crossover resistance, and selective activity to a commercial Pt/C catalyst in 0.1 m KOH aqueous solution. Such excellent performance benefits from in situ covalent incorporation of Pt nanoparticles with optimal size into N‐doped carbon support, dense active catalytic sites on surface, excellent electrical contacts between the catalytic sites and the electron‐conducting host, and a favorable mesoporous structure for the stabilization of the Pt nanoparticles by pore confinement and diffusion of oxygen molecules.     

Analytic equation of state for solids with multi-exponential potential based on analytic mean field potential approach and applied to the epsilon phase of solid oxygen

The thermodynamic properties of the ε phase of solid oxygen are studied by using the analytic mean field approach （AMFP）. Analytic expressions for the Helmholtz free energy, internal energy and equation of state of solid oxygen have been derived based on the multi-exponential potential. The formulism for the case of double-exponential （DE） model is applied to the ε phase of solid oxygen. Its four potential parameters are determined through fitting the experimental compression data of the ε phase of solid oxygen. Numerical results of the pressure dependence of the volume calculated by using the AMFP are in good agreement with the original experimental data. This suggests that the AMFP is a useful approach to study the thermodynamic properties of the ε phase of solid oxygen. Furthermore, we predict the variation of the volume, lattice parameters and intermolecular distances with pressure, and some thermodynamic quantities versus volume, at several higher temperatures.     

MFe2O4 and MFe@Oxide Core–Shell Nanoparticles Anchored on N‐Doped Graphene Sheets for Synergistically Enhancing Lithium Storage Performance and Electrocatalytic Activity for Oxygen Reduction Reactions

The intrinsically low electric conductivity and self‐aggregation of MFe₂O₄ during charge/discharge affect their lithium storage performance and electrocatalytic activity. To mitigate these problems, it is shown that N‐doped graphene sheets (NGS), as a highly conductive platform, finely disperse the MFe₂O₄ nanoparticles and rapidly shuttle electrons to and from the MFe₂O₄ nanoparticles. Moreover, by forming a metal@oxide core–shell nanostructure, fast electron transfer from the exterior oxide layer to NGS is achieved. Introducing NGS into MFe₂O₄ allows the composites to exhibit the comparable specific capacity (based on the total mass) to MFe₂O₄, although over 10 wt% of NGS contributes a low specific capacity of around 320–400 mAh g^?1. More importantly, introducing NGS significantly increases the cycling stability performance: 97.5% (CoFe₂O₄/NGS) and ≈100% (NiFe₂O₄/NGS) of the specific capacities have been retained after 80 cycles, far higher than the capacity retentions of CoFe₂O₄ (35.3%) and NiFe₂O₄ (43.7%) tested under otherwise identical conditions. Also demonstrated are the excellent rate capabilities of the composites. For catalyzing the oxygen reduction reaction, the activity is significantly improved when the MFe₂O₄ nanoparticles are transformed into metal@oxide core–shell nanostructure, mainly because the core–shell nanostructure exhibits lower charge transfer resistance.     

Diammonium hydrogen phosphate as a versatile and efficient catalyst for the one-pot synthesis of pyrano[2,3-<Emphasis Type="Italic">d</Emphasis>]pyrimidinone derivatives in aqueous media

Diammonium hydrogen phosphate, (NH(4))(2)HPO(4), was used as a catalyst for one-pot, three-component condensation reactions consisting of aromatic aldehydes, malononitrile and barbituric/thiobarbituric acid in aqueous ethanol at room temperature. This method has the advantages of a simple operation, mild reaction conditions, high yields, by using a less toxic and low cost chemical as a catalyst.