Ultrasound-assisted transformation from waste biomass to efficient carbon-based metal-free pH-universal oxygen reduction reaction electrocatalysts

Efficient carbon-based nitrogen-doped electrocatalysts derived from waste biomass are regarded as a promising alternative to noble metal catalysts for oxygen reduction reaction (ORR), which is crucial to fuel cell performance. Here, coconut palm leaves are employed as the carbon source and a series of nitrogen-doped porous carbons were prepared by virtue of a facile and mild ultrasound-assisted method. The obtained carbon material (ANDC-900-10) conveys excellent pH-universal catalytic activity with onset potentials (E_onset) of 1.01, 0.91 and 0.84 V vs. RHE, half-wave potentials (E_1/2) of 0.87, 0.74 and 0.66 V vs. RHE and limiting current densities (J_L) of 5.50, 5.45 and 4.97 mA cm⁻² in alkaline, neutral and acidic electrolytes, respectively, prevailing over the commercial Pt/C catalyst and, what's more, ANDC-900-10 displays preeminent methanol crossover resistance and long-term stability in the broad pH range (0–13), thanks to its abundant hierarchical nanopores as well as effective nitrogen doping with high-density pyridinic-N and graphitic-N. This work provides sonochemical insight for underpinning the eco-friendly approach to rationally designing versatile metal-free carbon-based catalysts toward the ORR at various pH levels.     

B,N-codoped 3D micro-/mesoporous carbon nanofibers web as efficient metal-free catalysts for oxygen reduction

B, N-codoped carbon nanofibers were massively prepared by heat treatment of electrospun carbon nanofibers with the mixture of boric acid/urea in N₂ (BNC_f-N) and subsequently activated in NH₃ (BNC_f-NA). The directly electrospun self-standing 3D non woven structure with void spaces between each fiber facilitates the mass transport of reactant and resulted molecules. Further NH₃ activation gives BNC_f-NA a high surface area of 306.3 m² g⁻¹ with micro/mesoporous structure, providing abundant passageway for proton transfer. Simultaneously, NH₃ activation also realizes the optimization of surface functionalities, such as more B–N–C and pyridinic-N. These intriguing features render BNC_f-NA excellent catalytic behavior with nearly four-electron oxygen reduction reaction (ORR) process in alkaline media, especially much better stability and methanol tolerance than the commercial Pt/C catalyst. Our work provides a large-scale preparation method for efficient metal-free catalysts toward ORR, thus further intensifying the commercial application of fuel cells.     

Direct pyrolysis and ultrasound assisted preparation of N,S co-doped graphene/Fe3C nanocomposite as an efficient electrocatalyst for oxygen reduction and oxygen evolution reactions

Bifunctional electrocatalysts to enable efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential for fabricating high performance metal–air batteries and fuel cells. Here, a defect rich nitrogen and sulfur co-doped graphene/iron carbide (NS-GR/Fe₃C) nanocomposite as an electrocatalyst for ORR and OER is demonstrated. An ink of NS-GR/Fe₃C is developed by homogeneously dispersing the catalyst in a Nafion containing solvent mixture using an ultrasonication bath (Model-DC150H; power − 150 W; frequency − 40 kHz). The ultrasonically prepared ink is used for preparing the electrode for electrochemical studies. In the case of ORR, the positive half-wave potential displayed by NS-GR/Fe₃C is 0.859 V (vs. RHE) and for the OER, onset potential is 1.489 V (vs. RHE) with enhanced current density. The optimized NS–GR/Fe₃C electrode exhibited excellent ORR/OER bifunctional activities, high methanol tolerance and excellent long-term cycling stability in an alkaline medium. The observed onset potential for NS–GR/Fe₃C electrocatalyst is comparable with the commercial noble metal catalyst, thereby revealing one of the best low-cost alternative air–cathode catalysts for the energy conversion and storage application.     

Preparation of carbon supported Pt-P catalysts and its electrocatalytic performance for oxygen reduction

The carbon supported PtP (PtP/C) catalysts were synthesized from Pt(NO₃)₂ and phosphorus yellow at the room temperature. The content of P in the PtP/C catalysts prepared with this method is high and the average size of the PtP particles is decreased with increasing the content of P. The electrocatalytic performances of the PtP/C catalysts prepared with this method for the oxygen reduction reaction (ORR) are better than that of the commercial Pt/C catalyst. The promotion action of P for enhancing the electrocatalytic performance of the PtP/C catalyst for ORR is mainly due to that Pt and P form the alloy and then the electron density of Pt is decreased.     

One-step ultrasonic synthesis of Co/Ni-catecholates for improved performance in oxygen reduction reaction

The inherent periodically arranged M−N_X, M−S_X and M−O_X units (M are usually Fe, Co, Ni, etc.) in metal–organic frameworks (MOFs) can be promising active centers in electrocatalysis. In previous studies, MOFs were usually constructed by energy-consuming hydro- or solvo-thermal reactions. Ultrasonic synthesis is a rapid and environment-friendly technique when envisaging MOFs’ industrial applications. In addition, different synthetic pathways for MOFs may lead to difference in their microstructure, resulting in different electrocatalytic performance. Nevertheless, only a handful of MOFs were successfully prepared by ultrasonic synthesis and few were applied in electrochemical catalysis. Herein, we constructed Ni/Co-catecholates (Ni/Co-CATs) synthesized by one-step ultrasonic method (250 W, 40 KHz, 25 W/L, Ultrasonic clearing machine) and compared their performance in oxygen reduction reaction (ORR) with that of Ni/Co-CATs synthesized by hydrothermal method. Ni-CAT and Co-CAT prepared by ultrasonic showed the half-wave potential of −0.196 V and −0.116 V (vs. Ag/AgCl), respectively. The potentials were more positive than those prepared by hydro-thermal method. And they showed excellent electrochemical stability in neutral solution. The latter was only 32 mV lower than that of commercial Pt/C. The improved performance in ORR was attributed to higher specific surface area and mesopore volume as well as more structural defects generated in the ultrasonic synthesis process, which could facilitate their exposure of electrocatalytic active sites and their mass transport. This work gives some perspective into cost-effective synthetic strategies of efficient MOFs-based electrocatalysts.     

A facile sonochemical route for the synthesis of MoS2/Pd composites for highly efficient oxygen reduction reaction

For the alkaline fuel cell cathode reaction, it is very essential to develop novel catalysts with superior catalytic properties. Here, we report the synthesis of highly active and stable MoS₂/Pd composites for the oxygen reduction reaction (ORR), via a simple, eco-friendly sonochemical method. The bulk MoS₂ was first transformed into single and few layers MoS₂ nanosheets through ultrasonic exfoliation. Then the exfoliated MoS₂ nanosheets served as supporting materials for the nucleation and further in-situ growth of Pd nanoparticles to form MoS₂/Pd composites via ultrasonic irradiation. Cyclic voltammetry and rotating disk voltammetry measurements demonstrate that as-prepared MoS₂/Pd composites which provides a direct four-electron pathway for the ORR, have better electrocatalytic activity, long-term operation stability than commercial Pt/C catalyst. We expect that the present work would provide a promising strategy for the development of efficient oxygen reduction electrocatalyst. In addition, this study can also be extended to the preparation of other hybrid with desirable morphologies and functions.     

Plasma characterization and room temperature growth of carbon nanotubes and nano-onions by excimer laser ablation

The deposition of carbon nanotubes and carbon nano-onions at room temperature using excimer laser radiation to ablate mixed graphite-metal targets is described. Our deposition conditions are in contrast to other investigations on the pulsed laser deposition of carbon nanotubes that have employed high temperatures and high pressures. We find that the formation of these carbon nanostructures is dependent on the ambient gas employed during ablation. In the presence of O₂ gas, carbon nanotubes and nano-onions are produced, while inert atmospheres such as Ar yield amorphous carbon. High-resolution, in situ, time-resolved emission spectroscopy has been used to track the evolution of species (C₂, C₃, Ni/Co) in the ablation plume. Spectral fits on low and high-resolution spectra reveal that the vibrational-rotational temperatures for C₂ produced in O₂ remain at ∼5000 K for nearly 20 μs but drop rapidly in Ar. Details of the formation of carbon nanotubes and nano-onions, and in situ time-resolved optical emission spectroscopy are described.     

Edge-enriched graphene with boron and nitrogen co-doping for enhanced oxygen reduction reaction

Carbon-based electrocatalysts for oxygen reduction reaction (ORR), especially in anion exchange membrane fuel cells (AEMFCs), have received a lot of attention because they exhibit excellent stability and are comparable to commercial Pt/C catalysts. Currently, to maximize the catalytic activity of carbon-based electrocatalysts, there are two major strategies: heteroatom doping or exposing active edge sites. However, the approach of increasing heteroatomic dopants of active edge sites has been rarely addressed. In this study, we present a simple strategy to prepare edge-enriched graphene catalysts with an increased ratio of heteroatomic dopants suitable for ORR of AEMFCs. The catalysts were prepared under harsh oxidation conditions, followed by a simple co-doping process with boron and nitrogen. The ORR activity of the catalysts was observed to be related to an increase of edge sites with heteroatomic dopants. We believe that the edge-enriched structure leads to accelerated electron transfer with enhanced oxygen adsorption.     

Dealloyed PtCu catalyst as an efficient electrocatalyst in oxygen reduction reaction

Pt-transition metal alloy catalysts with an active Pt surface have exceptional properties for use in oxygen electro-reduction reactions in fuel cells. Herein, we report the simple synthesis of dealloyed PtCu catalysts and their catalytic performance in oxygen reduction. The dealloyed PtCu catalysts consisted of a Pt-enriched shell with a Pt–Cu alloy core and were synthesized through a chemical co-reduction process followed by thermal annealing and chemical dealloying. During synthesis, thermal annealing leads to a high degree of formation of PtCu alloy particles (e.g., PtCu or PtCu₃), and chemical dealloying causes selective dissolution of unstable Cu species from the surface layers of the PtCu alloy particles, resulting in a PtCu alloy@Pt-enriched surface core–shell configuration. Our PtCu₃/C catalyst exhibits a great improvement in the oxygen reduction reaction with a mass activity of 0.501 A/mg_Pt, which is 2.24 times greater than that of a commercial Pt catalyst. In this article, the synthesis details, characteristics and performance improvements in ORR of chemically dealloyed PtCu catalysts are systemically explained.     

An oxygen reduction catalytic process through superoxo adsorption states on n-type doped h-BN: A first-principles study

Dioxygen adsorption and activation on metal-ligand systems are the key elements for biological oxidative metabolisms and also catalyst design for the oxygen reduction reaction (ORR). We show, through first-principles calculations, that similar dioxygen adducts can form on metal-free n-type doped hexagonal boron nitride (h-BN) nanostructures. The density of electron donors determines the charge state of dioxygen, either in superoxo and peroxo, which exactly correlates with the ‘end-on’ and ‘side-on’ configurations, respectively. Activated O₂ in the superoxo state shows a better catalytic performance possibly mediating the direct four-electron reduction. The formation of hydrogen peroxide (H₂O₂) is practically eliminated, and thus we suggest that a surface coated with the n-type doped h-BN can be the basis for an ORR catalyst with increased stability.     

氮掺杂碳载钴催化剂制备及其TG-FTIR和XRD光谱分析

以三聚氰胺甲醛树脂预聚体为氮源、碳源,以乙酸钴为金属前驱体,制备氮掺杂碳载钴氧还原电催化剂。利用傅里叶变换红外光谱与热重联用(thermogravimetry-fourier transform infrared spectroscopy,TG-FTIR)、X射线衍射光谱分析(X-ray diffraction spectra,XRD)等研究了催化剂的制备过程和结构,采用旋转圆盘电极测试(rotating disc electrode,RDE)考察了制备过程中不同炭化温度对催化剂氧还原催化活性的影响。结果显示,在惰性气氛中,随炭化温度升高,样品中部分有机基团以CO,CO2,HCHO,NH3,NO2等形态随保护气流失,催化剂结构出现明显变化,形成典型的面心立方结构。旋转圆盘电极测试结果表明,所制备的催化剂都具有较好的电催化活性,氮掺杂碳载钴催化剂的氧还原起始电位在0.5V(vs.SCE)左右,炭化温度为700℃时制备的催化剂具有最高电催化氧还原活性。     

Exploration of bimetallic Pt-Pd/C nanoparticles as an electrocatalyst for oxygen reduction reaction

In this study, carbon supported Pt and Pt-Pd were synthesized as oxygen reduction reaction electrocatalysts for polymer electrolyte membrane fuel cells (PEMFCs). Pt and Pt-Pd nanoparticles have been synthesized by reduction of metal precursors in presence of NaBH₄. Various techniques such as X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX) and scanning electron microscopy (SEM) were utilized to study the prepared samples. Furthermore, electrochemical properties of the prepared samples were evaluated from cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry and electrochemical impedance spectroscopy (EIS). The results showed, the crystallite size of electrocatalysts (Pt and Pt-Pd) is below 10 nm. The higher catalytic activity was detected for Pt-Pd/C electrocatalyst for oxygen reduction reaction (ORR). In addition, it is believed that the better performance of electrocatalyst is related to the synergic effect between Pt and Pd nanoparticles, weakening of the OO bond on Pd-modified Pt nanoparticles in ORR, uniform dispersion of Pd and Pt on the carbon support and higher electrochemical active surface area (EAS) of Pt-Pd/C electrocatalyst.     

Thermal conductivity and thermal rectification in carbon nanotubes with geometric variations of doped nitrogen: Non-equilibrium molecular dynamics simulations

The thermal conductivity of carbon nanotubes with geometric variations of doped nitrogen is investigated. The phenomenon of thermal rectification shows that the heat transport is preferably in one direction. The asymmetric heat transport of the triangular single-nitrogen-doped carbon nanotubes (SNDCNTs) is larger than that of the parallel various-nitrogen-doped carbon nanotubes (VNDCNTs).     

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.     

First-principles molecular dynamics simulation of O2 reduction on nitrogen-doped carbon

Density-functional calculations were performed to examine oxygen reduction reactions (ORRs) on N-doped graphene sheets. We found that O₂ adsorption becomes energetically favorable as the number of N around a CC bond increases. Pathways for both 4e⁻ and 2e⁻ reductions were identified. The possibility of O poisoning was suggested after calculating the reversible potential of each reduction step.     

Catalytic activity of carbon-supported iridium oxide for oxygen reduction reaction as a Pt-free catalyst in polymer electrolyte fuel cell

Iridium oxide supported on Vulcan XC-72 carbon black (IrO₂/C) as a cathode catalyst for polymer electrolyte fuel cell (PEFC) has been characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD) measurement. The IrO₂ particles were 8-160 nm in diameter. The oxygen electroreduction activity was studied by cyclic voltammetry (CV). It was found that IrO₂/C had high oxygen reduction reaction (ORR) activity. The performance of the membrane electrode assemble (MEA) was also tested in a single PEFC and showed that IrO₂/C catalyst would be potential candidates for use as cathode catalyst in PEFC.     

Facile synthesis of Co-doped SnS2 as a pre-catalyst for efficient oxygen evolution reaction

Co-doped flower-like SnS₂ was synthesized using a one-step hydrothermal method. The Co content of Co-doped SnS₂ was facilely tuned by controlling the [Co]/[Sn] molar concentration ratio (SC-x; x = 0.05, 0.5, 1.0 2.0, where x indicates the [Co]/[Sn] ratio). The morphology of the samples did not significantly change despite changes in the Co dopant content. Compared to SC-0 (667 mV), SC-0.05 (400 mV), SC-0.5 (382 mV), and SC-1.0 (374 mV), SC-2.0 showed higher catalytic performance, with an overpotential of 323 mV at a current density of 10 mA/cm² in 1 M KOH solution. Moreover, SC-2.0 exhibited high stability for 12 h during chronopotentiometry. SC-2.0 was unexpectedly transformed to weakly crystallized CoOOH nanoparticles after the stability test. The transformation rate from Co-doped SnS₂ to CoOOH was decreased with an increase in the Co content.     

First-principles study on the Ni@Pt12 Ih core-shell nanoparticles: A good catalyst for oxygen reduction reaction

The adsorption, diffusion and dissociation properties of O₂ on the icosahedron (Ih) Ni@Pt12 core-shell nanoparticle were investigated using the ab initio density functional theory calculations. It is found that, compared with the Pt(111) surface, the Ih Ni@Pt12 core-shell nanoparticle can enhance the adsorption, diffusion and dissociation of O₂, as well as the adsorption and diffusion of the atomic O (the dissociation product of O₂), and therefore serve as a good catalyst for oxygen reduction reaction. Our study gives a reasonable theoretical explanation to the high catalytic activity of the Ni@Pt core-shell nanoparticles for the oxygen reduction reaction.     

基于片层结构钴卟啉金属有机骨架的高效氧还原反应电催化剂

使用片层结构的金属有机骨架作为前驱物,通过高温煅烧制备出氮掺杂的介孔碳材料,同时在片层碳骨架上均匀分布着石墨化碳包裹的钴纳米粒子.电化学测试表明,800℃制备的样品具有与铂炭（Pt/C）催化剂相接近的氧还原反应催化活性,催化稳定性和甲醇耐受性良好,显著超越商品化Pt/C催化剂.     

Structural and electronic properties of lithium endohedral doped carbon nanocapsules

Endohedral lithium and lithium ion doped carbon nanocapsule (CNC) systems have been theoretically investigated by performing semi-empirical molecular orbital method at the level of PM3 (UHF and/or RHF) type quantum chemical treatment in order to explore the energetics and electronic structures. The geometries of the studied systems have been optimized and the molecular properties, energies, some selected molecular orbital eigenvalues and dipole moments of the studied capsules are reported. Molecular dynamics simulations have been also performed to study the structural properties and energetics of endohedral lithium and lithium ion doped CNCs.