Lattice contraction has been regarded as an important factor influencing oxygen reduction reaction (ORR) activity of Pt-based alloys. However, the relationship between quantitative lattice contraction and ORR activity has rarely been reported. Herein, using Pt–Cu alloy nanoparticles (NPs) with similar particle sizes but different compositions as examples, we investigated the relationship between quantitative lattice contraction and ORR activity by defining the shrinking percentage of Pt–Pt bond distance as lattice contraction percentage. The results show that the ORR activities of Pt–Cu alloy NPs exhibit a well-defined volcano-type dependent relationship toward the lattice contraction percentage. The dependent correlation can be explained by the Sabatier principle. This study not only proposes a valid descriptor to bridge the activity and atomic composition but also provides a reference for understanding the composition–structure–activity relationship of Pt-based alloys.Keywords: dependent relationship; lattice contraction percentage; oxygen reduction reaction; Pt-based alloy; X-ray absorption spectroscopy;

Understanding exact electronic configurations of carbon atoms bonded by nitrogen (N) functionalities at atomic-level may literally open a door to advance metal-free carbon materials as efficient catalysts for the oxygen reduction reaction (ORR). In this paper, a set of well-defined carbon nanotubes with controlled doping of various N species, such as pyrrolic, pyridinic and graphitic N, have been achieved by in-situ pyrolysis of polyaniline (PANI) nanotubes at different temperatures. Among these synthesized samples, the carbon nanotubes fabricated at 700 °C exhibit the highest electro-catalytic ORR activity, long-standing stability and good tolerance against methanol in alkaline medium. The improved activity is mainly attributed to the high nitrogen level of the active pyridinic and graphitic N. But, the pyridinic N possesses higher activity than the graphitic N because of their different sp2 electronic structures. Pyridinic N, after bonding with two carbon(C) atoms, has two distorting N−C orbitals and one dangling orbital occupied by a lone electron pairs which are exposed, as the N sits at the edge of the carbon planes. Such unique electronic configuration makes the nitrogen and surrounding carbon atoms, bonded in the CN bonds, can serve host of active sites or work as active sites for the ORR.

•The modification approach for graphene is easier than typical oxidation methods.•The prepared Pt-Co/CA-G catalyst is proton conductive.•The Pt-Co/CA-G catalyst exhibits superior ORR activity and stability.•The enhanced proton transfer is responsible for excellent ORR performance.Designing highly efficient electro-catalysts for the oxygen reduction reaction (ORR) has been regarded as a demanding task in the development of renewable energy sources. However, little attention has been paid on improving Pt-based catalysts by promoting proton transfer from the electrolyte solutions to the catalyst layer at the cathode. Herein, we design proton conductive Pt-Co alloy nanoparticles anchoring on citric acid functionalized graphene (Pt-Co/CA-G) catalysts for efficient ORR. The facile modification approach for graphene can introduce oxygenated functional groups on the graphene surface to promote proton transfer as well as keeping the high electron conductivity without destroying the graphene original structure. The electrochemical results show that the Pt-Co/CA-G catalyst exhibits more excellent ORR activity and stability than the commercial Pt/C catalyst, which can be attributed to its improved proton transfer ability. The fast proton transfer comes from the hydrogen-bonding networks formed by the interaction between the oxygenated functional groups and water molecules. This work provides not only a novel and simple approach to modify graphene but also an effective strategy to improve Pt-based catalysts for the ORR.Download high-res image (396KB)Download full-size image

AbstractThe achievement of crystal-lattice tuning along low Miller index planes to decrease the bandgap of spinel transition-metal oxides may be an effective way to enhance their electrocatalytic activity for the oxygen reduction reaction (ORR). Herein, we have prepared spherical Co3O4 nanoparticles with a preferred orientation along the (111) plane by direct nucleation and growth of the oxide on graphitized carbon black (GCB). The formation of the preferred (111) oxide is attributed to a unique chemical interaction at the interface between Co3O4 and carbon, which results in covalent C-O-Co bonds in the hybrid. Electrocatalysis experiments in an alkaline environment revealed that the electrocatalytic activity for ORR on the preferred (111) oxide increased as a function of the degree of crystal-lattice orientation, which implies a closely intrinsic correlation between the predominant (111) plane and the catalytic activity. Because Co2+ cations are enriched in this plane, they possess a narrow bandgap and unfilled conduction bands at low energy with respect to Co3+ ions in the preferred (111) Co3O4, which can contribute to the absorption and activation of active oxygen and lead to improved ORR activity

Here we propose amorphous CuPt alloy hollow nanotubes as efficient catalysts for the methanol oxidation reaction (MOR) prepared by Na2S2O3-assisted galvanic replacement reaction. The formation mechanism can be explained by the nanoscale Kirkendall-effect-induced hollowing process of the galvanic replacement reaction. The electrochemical tests suggest that the amorphous CuPt alloy exhibits better MOR activity and stability than the crystalline CuPt and commercial Pt/C catalysts, which can be ascribed to the enhanced CO tolerance ability of amorphous alloy. XPS measurements demonstrate that the enhanced anti-CO poison characteristic of amorphous CuPt alloy originates from the strong interaction between Pt and Cu atoms as a result of a unique crystallization state. This research not only provides a facile approach to synthesize amorphous alloy but also opens up an interesting way for amorphous Pt-based alloy to apply to the MOR.Keywords: amorphous alloy; bifunctional mechanism; CO tolerance; galvanic replacement reaction; methanol oxidation reaction

Through in-situ nucleation and growth of perovskite-type LaMnO3 nanocrystals on a modified carbon black, a set of LaMnO3/C hybrids with different LaMnO3 loadings (40 wt%, 80 wt%, 100 wt% and 140 wt%, relative to carbon) were facilely fabricated via hydrolysis and precipitation in an ethanol solution, followed by calcination at 700 °C. The obtained LaMnO3 nanoparticles feature perovskite-type structure and are dispersed uniformly on the carbon surface. The electrochemical experiments in 1 mol L−1 NaOH solution illustrate that the hybrid catalyst (100 wt% of LaMnO3) exhibits the highest ORR electrocatalytic activity and the lowest hydrogen peroxide yield. X-ray photoelectron spectroscopy (XPS) reveals that the catalyst with the best catalytic activity has the highest content of covalent COMn bonds formed at the interface between the LaMnO3 and carbon, which implies a close correlation between the loading and the covalent bond content within the hybrids. The synergy between the oxide and carbon, raised by the formed COMn bonds in their hybrid, is responsible for the remarkably improved ORR activity. This information is important to achieve synergy between non-noble metal oxides and carbon in their composites as efficient and economical ORR catalysts.

Investigating the synergy in hybrids between rare earth (La, Ce, Y) oxides and carbon may be an effective way to develop new efficient and cheap catalysts for catalyzing the oxygen reduction reaction (ORR). In this work, a set of La2O3/C hybrids with different La2O3 loadings were prepared by chemical precipitation in alkaline solution, followed by calcination treatment. The prepared La2O3 nanoparticles with hexagonal structure covered uniformly on the carbon surface. X-ray photoelectron spectroscopy (XPS) indicated different contents of covalent C–O–La bonds at the interface between the La2O3 and carbon in these hybrids. The electrochemical experiments in alkaline solution show that the catalyst with 80 wt% of La2O3 exhibits the highest electrochemical activity in catalyzing ORR and the lowest production of hydrogen peroxide among the synthesized hybrids. The remarkably enhanced ORR activity is attributed to the maximum content of the C–O–La bonds formed in the hybrid. Interestingly, the above C–O–La covalent bonds can promote the electron transfer from the supported carbon (π electron) to the La2O3 phase, and the transferred electron can fill the unoccupied eg orbital splitted by the La 5d orbital of La2O3, which should be responsible for the improved ORR performance.

The ability to precisely control the nanoscale phase structure of bimetallic catalysts is required to achieve a synergistic effect between two metals for the oxygen reduction reaction (ORR). In this work, we synthesized Pt–Ag bimetallic nanoparticles (NPs) with Ag@Pt core–shell, highly alloyed solid and hollow nanostructures respectively, via a galvanic replacement reaction by modifying H2PtCl6 concentration in an aqueous solution containing homemade Ag NPs as the sacrificial templates. The nanophase and corresponding electronic structures of the synthesized Pt–Ag NPs were characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The formation of these Pt–Ag NPs with different nanophase structures is closely ascribed to a defect-induced Kirkendall effect that involves the accelerated interdiffusion of Ag and Pt atoms, triggered by the high density of defects along the Ag NP surface generated by the galvanic replacement reaction. The nanophase structure-dependent electrocatalytic activity of three Pt–Ag bimetallic NPs was determined in 0.5 M H2SO4 solution by using a rotating disk electrode (RDE). The results showed that the core–shell and hollow alloy NPs exhibit excellent ORR activity in acidic solution, which is remarkably higher than that of the commercial Pt/C (E-TEK). The physical origin of the enhancement in the ORR activity can be explained by a mutual ligand effect, raised by the substantial electronic transfer between Pt and Ag at the atomic level, which results from the downshift of the d-band center for Pt and the increased number of the unpaired electrons for Ag in these bimetallic catalysts. Thus two factors achieve a synergistic effect that dominates the remarkably improved electrocatalytic activity for the ORR.

•A composite with a 3D stacked-up nanostructure composed by Mn3O4, Co3O4 and graphene.•The novel composite exhibits superior ORR activity to Co3O4/GO and Mn3O4/GO alone.•The enhanced synergy in this composite is responsible for the ORR activity.•There exists an interphase ligand effect between two spinel oxides and graphene.Constructing composite materials with a smart nanostructure, by using various transition metal oxides and carbon carriers as building blocks, is of great importance to develop highly active, economical noble metal-free catalysts for oxygen reduction reaction (ORR). We have synthesized a novel ternary composite with a special 3D stacked-up nanostructure, composed of Co3O4, Mn3O4 and graphene oxide (GO), via a facile two-step aqueous synthesis without adding any structure directing agent. The composite was characterized by X-ray diffraction, scanning transmission electron microscope, Raman spectroscopy, and X-ray photoelectron spectroscopy. The results revealed that Mn3O4 nanocrystals had been successfully epitaxially deposited onto the surface of Co3O4 nanoparticles to form Mn3O4-on-Co3O4 nanostructures on surface of the graphene. In an alkaline environment, the Co3O4-Mn3O4/GO composite exhibits much better electrocatalytic activity and durability towards ORR than individual Mn3O4/GO and Co3O4/GO catalysts. The recorded kinetic current density (JK) of O2 reduction for the composite is 2.078 mA/cm2, which is comparable to that of a commercial Pt/C (20%) but far exceeding the sum of that obtained from the Co3O4/GO and Mn3O4/GO. The remarkably improved ORR activity is closely attributed to the enhanced synergy between these two oxides and the graphene, raised by the 3D stacked-up structure in this composite. The oxide-on-oxide heterostructure comprising Co3O4 and Mn3O4 can promote covalent electron transfer from carbon support to the oxides as a result of the interphase ligand effect between them, which facilitate the ORR kinetics. Moreover, Mn3O4 phase acting as a co-catalyst, located at the top of Co3O4 phase, also favor the chemical disproportionation of H2O2 intermediates generated by the composite during the ORR.

A novel class of Pt–Co secondary solid solution catalysts with long-range ordered intermetallic CoPt3 as the solvent and Co as the solute for the oxygen reduction reaction (ORR) is proposed. The catalysts exhibit a volcano-type dependence on Co solid solubility in ORR activity and the optimum catalyst with 10% Co solid solubility shows remarkably enhanced catalytic activity and durability, which can be ascribed to the unique electronic structure of secondary solid solution catalysts.

Constructing nanoscale hybrid materials with unique interfacial structures by using various metal oxides and carbon supports as building blocks are of great importance to develop highly active, economical hybrid catalysts for oxygen reduction reaction (ORR). In this work, La2O2CO3 encapsulated La2O3 nanoparticles on a carbon black (La2O2CO3@La2O3/C) were fabricated via chemical precipitation in an aqueous solution containing different concentrations of cetyltrimethyl ammonium bromide (CTAB), followed by calcination at 750 °C. At a given CTAB concentration 24.8 mmol/L, the obtained lanthanum compound nanoparticles reach the smallest particle size (7.1 nm) and are well-dispersed on the carbon surface. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results demonstrate the formation of La2O2CO3 located on the surface of La2O3 nanoparticles in the hybrid. The synthesized La2O2CO3@La2O3/C hybrid exhibits a significantly enhanced electrocatalytic activity in electrocatalysis experiments relative to pure La2O3, La2O2CO3, and carbon in an alkaline environment, by using the R(R)DE technique. Moreover, its long-term stability also outperforms that obtained by commercial Pt/C catalysts (E-TEK). The exact origin of the fast ORR kinetics is mainly ascribed to the La2O2CO3 layer sandwiched at the interface of carbon and La2O3, which contributes favorable surface-adsorbed hydroxide (—OH–ad) substitution and promotes active oxygen adsorption at the interfaces. The unique covalent —C—O—C(═O)—O—La—O— bonds, formed at the interfaces between La2O2CO3 and carbon, can act as active sites for the improved ORR kinetics over this hybrid catalyst. Therefore, the fabrication of lanthanum compound-based hybrid material with an unique interfacial structure maybe open a new way to develop carbon-supported metal oxides as next-generation of ORR catalysts.Keywords: carbon black; chemical bonds; interfacial structure; La2O2CO3 encapsulated La2O3; oxygen reduction reaction

The ability to precisely tune the chemical compositions and electronic structures of nanoalloy catalysts is essential to achieve the goals of high activity and selectivity for the oxygen reduction reaction (ORR) on the catalysts by design. In this work, we synthesized carbon-supported Pt–Co alloy nanoparticles with controlled bimetallic compositions (Pt/Co atomic ratio = 81:19, 76:24, 59:41, 48:52, 40:60 and 26:74) by regulating solution pH and the amount of Pt and Co precursor salts to elucidate the effect of catalyst composition on ORR activity. The obtained Pt–Co alloy nanoparticles have face-centred cubic (fcc) structures and are well-dispersed on the surface of the carbon support with a narrow particle size distribution (2–4 nm diameters). The electrocatalysis experiments in alkaline solution reveal a strong correlation between ORR activity and the alloy composition of the catalysts. Interestingly, the mass-specific activities of the catalysts manifest a typical double-volcano plot as a function of alloy composition. In this Pt–Co alloy series, the catalyst with a Pt:Co atomic ratio of 76:24 exhibits the best ORR performance, which is remarkably higher than that of the commercial Pt/C (E-TEK). X-ray photoelectron spectroscopy (XPS) measurements demonstrate that the electronic structures of these catalysts can be tuned by controlling their alloy compositions, which are highly correlated with the trends in ORR activity. The origin of the enhancement in ORR activity may be strongly related to the unique chemical surface structures of the catalysts.

The ability to precisely tune the chemical compositions and electronic structures of nanoalloy catalysts is essential to achieve the goals of high activity and selectivity for the oxygen reduction reaction (ORR) on the catalysts by design. In this work, we synthesized carbon-supported Pt–Co alloy nanoparticles with controlled bimetallic compositions (Pt/Co atomic ratio = 81:19, 76:24, 59:41, 48:52, 40:60 and 26:74) by regulating solution pH and the amount of Pt and Co precursor salts to elucidate the effect of catalyst composition on ORR activity. The obtained Pt–Co alloy nanoparticles have face-centred cubic (fcc) structures and are well-dispersed on the surface of the carbon support with a narrow particle size distribution (2–4 nm diameters). The electrocatalysis experiments in alkaline solution reveal a strong correlation between ORR activity and the alloy composition of the catalysts. Interestingly, the mass-specific activities of the catalysts manifest a typical double-volcano plot as a function of alloy composition. In this Pt–Co alloy series, the catalyst with a Pt:Co atomic ratio of 76:24 exhibits the best ORR performance, which is remarkably higher than that of the commercial Pt/C (E-TEK). X-ray photoelectron spectroscopy (XPS) measurements demonstrate that the electronic structures of these catalysts can be tuned by controlling their alloy compositions, which are highly correlated with the trends in ORR activity. The origin of the enhancement in ORR activity may be strongly related to the unique chemical surface structures of the catalysts.

A novel class of Pt–Co secondary solid solution catalysts with long-range ordered intermetallic CoPt3 as the solvent and Co as the solute for the oxygen reduction reaction (ORR) is proposed. The catalysts exhibit a volcano-type dependence on Co solid solubility in ORR activity and the optimum catalyst with 10% Co solid solubility shows remarkably enhanced catalytic activity and durability, which can be ascribed to the unique electronic structure of secondary solid solution catalysts.