Metal phosphonate-derived cobalt/nickel phosphide@N-doped carbon hybrids as efficient bifunctional oxygen electrodes for Zn−air batteries

The exploration of efficient bifunctional electrocatalysts for oxygen reduction reaction and oxygen evolution reaction is pivotal for the development of rechargeable metal–air batteries. Transition metal phosphides are emerging as promising catalyst candidates because of their superb activity and low cost. Herein, a novel metal phosphonate-derived cobalt/nickel phosphide@N-doped carbon hybrid was developed by a carbothermal reduction of cobalt/nickel phosphonate hybrids with different Co/Ni molar ratios. The metal phosphonate derivation method achieved an intimately coupled interaction between metal phosphides and a heteroatom-doped carbon substrate. The resultant Co₂P/Ni₃P@NC-0.2 enables an impressive electrocatalytic oxygen reduction reaction activity, comparable with those of state-of-the-art Pt/C catalysts in terms of onset potential (0.88 V), 4e^‒ selectivity, methanol tolerance, and long-term durability. Moreover, remarkable oxygen evolution reaction activity was also observed in alkaline conditions. The high activity is ascribed to the N-doping, abundant accessible catalytic active sites, and the synergistic effect among the components. This work not only describes a high-efficiency electrocatalyst for both oxygen reduction reaction and oxygen evolution reaction, but also highlights the application of metal phosphonate hybrids in fabricating metal phosphides with tunable structures, which is of great significance in the energy conversion field.     

氮掺杂科琴黑碳材料的制备及 电催化氧还原性能研究

氮掺杂碳材料是一种有应用前景的电催化氧还原催化剂。以尿素和三聚氰胺作为氮源,在氮气气氛下高温焙烧,制得两种氮掺杂科琴黑碳材料并将其用于电催化氧还原反应。使用X射线衍射仪（XRD）、X射线光电子能谱仪（XPS）、场发射扫描电子显微镜（FESEM）、比表面物理吸附分析仪等对氮掺杂前后的科琴黑的结构和形貌进行了分析。结果表明：氮掺杂之后科琴黑仍保持石墨结构,其形貌和比表面积均无明显改变。在XPS谱图上,氮掺杂后科琴黑上存在氮元素,其中以三聚氰胺为氮源比以尿素为氮源更容易得到吡啶氮。通过循环伏安法和线性扫描伏安法研究了3个样品的电催化氧还原性能。结果表明：氮掺杂能明显提高科琴黑的电催化氧还原性能,未掺杂的 科琴黑（AC）的半波电位为0.746 V,而以尿素和三聚氰胺为氮源掺杂后的科琴黑碳材料的半波电位分别提高到了 0.756 V（尿素-N/AC）和0.786 V（三聚氰胺-N/AC）。     

Graphene-reinforced metal-organic frameworks derived cobalt sulfide/carbon nanocomposites as efficient multifunctional electrocatalysts

Developing cost-effective electrocatalysts for oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is vital in energy conversion and storage applications. Herein, we report a simple method for the synthesis of graphene-reinforced CoS/C nanocomposites and the evaluation of their electrocatalytic performance for typical electrocatalytic reactions. Nanocomposites of CoS embedded in N, S co-doped porous carbon and graphene (CoS@C/Graphene) were generated via simultaneous sulfurization and carbonization of one-pot synthesized graphite oxide-ZIF-67 precursors. The obtained CoS@C/Graphene nanocomposites were characterized by X-ray diffraction, Raman spectroscopy, thermogravimetric analysis-mass spectroscopy, scanning electronic microscopy, transmission electronic microscopy, X-ray photoelectron spectroscopy and gas sorption. It is found that CoS nanoparticles homogenously dispersed in the in situ formed N, S co-doped porous carbon/graphene matrix. The CoS@C/10Graphene composite not only shows excellent electrocatalytic activity toward ORR with high onset potential of 0.89 V, four-electron pathway and superior durability of maintaining 98% of current after continuously running for around 5 h, but also exhibits good performance for OER and HER, due to the improved electrical conductivity, increased catalytic active sites and connectivity between the electrocatalytic active CoS and the carbon matrix. This work offers a new approach for the development of novel multifunctional nanocomposites for the next generation of energy conversion and storage applications.     

A dual metal-organic framework strategy for synthesis of FeCo@NC bifunctional oxygen catalysts for clean energy application

Developing high efficient bifunctional oxygen electrocatalysts for clean energy applications like Zin-air battery (ZAB) is highly desired, because it would reduce the cost and speed up the practical application of ZAB. Here we use a dual metal–organic framework (MOF) synthesis strategy to prepare the N-doped carbon supported bimetallic FeCo nanoparticle catalysts (marked as FeCo@NC) by pyrolysis of ZnCo-ZIF/MIL-101(Fe) composite. The FeCo@NC exhibits remarkable electrocatalytic activity for ORR with half-wave potential of 0.89 V vs. the reversible hydrogen electrode (RHE) and robust durability for both ORR and OER (oxygen reduction reaction and oxygen evolution reaction), which is attributed to the generation of Fe_0.26Co_0.74 crystalline phase and mesopores due to the dual-MOF synthesis strategy. The rechargeable ZAB based on FeCo@NC air electrode shows a maximum energy density of 139.6mW·cm^-2 and excellent cyclic stability over 130 h, significantly surpassing the Pt and Ir-based ZAB. The present work provides a useful dual-MOF synthesis strategy for preparing high-performance multifunctional catalysts for ORR, OER and hydrogen evolution reaction (HER).     

Oxygen reduction at Au nanoparticles electrodeposited on different carbon substrates

The electrocatalytic reduction of molecular oxygen (O₂) has been performed in O₂-saturated 0.5 M KOH solution at Au nanoparticles electrodeposited onto two different carbon substrates, namely glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG). Cyclic voltammetry (CV) technique has been used in this investigation. The electrocatalytic activity of the Au nanoparticle-based electrodes is inherently related to its electrodeposition conditions (i.e., the absence or presence of some additives) as well as the nature of the substrate. For instance, Au nanoparticles electrodeposited onto GC (nano-Au/GC) from K[AuBr₄] in the presence of 25 μM cysteine showed a high electrocatalytic activity towards the oxygen reduction reaction (ORR) as demonstrated by the largest positive shift of the cathodic peak potential (at ca. −0.165 V versus Ag/AgCl/KCl (sat)). On the other hand, two well-separated successive reduction peaks corresponding to the 2-step 4-electron reduction of oxygen were observed at the different nano-Au/HOPG electrodes. The relative ratio of the two peak current heights changed significantly depending on the electrodeposition conditions of the Au nanoparticles. The morphology of the different Au nanoparticles electrodeposited onto the different substrates was depicted by scanning electron microscope (SEM) technique.     

Noble-metal-free cobalt hydroxide nanosheets for efficient electrocatalytic oxidation

Cobalt hydroxide has been emerging as a promising catalyst for the electrocatalytic oxidation reactions, including the oxygen evolution reaction (OER) and glucose oxidation reaction (GOR). Herein, we prepared cobalt hydroxide nanoparticles (CoHP) and cobalt hydroxide nanosheets (CoHS) on nickel foam. In the electrocatalytic OER, CoHS shows an overpotential of 306 mV at a current density of 10 mA·cm^–2. This is enhanced as compared with that of CoHP (367 mV at 10 mA·cm^–2). In addition, CoHS also exhibits an improved performance in the electrocatalytic GOR. The improved electrocatalytic performance of CoHS could be due to the higher ability of the two-dimensional nanosheets on CoHS in electron transfer. These results are useful for fabricating efficient catalysts for electrocatalytic oxidation reactions.     

Heterometallic cluster-based organic frameworks as highly active electrocatalysts for oxygen reduction and oxygen evolution reaction: a density functional theory study

Recently, metal–organic frameworks are one of the potential catalytic materials for electrocatalytic applications. The oxygen reduction reaction and oxygen evolution reaction catalytic activities of heterometallic cluster-based organic frameworks are investigated using density functional theory. Firstly, the catalytic activities of heterometallic clusters are investigated. Among all heterometallic clusters, Fe₂Mn–Mn has a minimum overpotential of 0.35 V for oxygen reduction reaction, and Fe₂Co–Co possesses the smallest overpotential of 0.32 V for oxygen evolution reaction, respectively 100 and 50 mV lower than those of Pt(111) and RuO₂(110) catalysts. The analysis of the potential gap of Fe₂M clusters indicates that Fe₂Mn, Fe₂Co, and Fe₂Ni clusters possess good bifunctional catalytic activity. Additionally, the catalytic activity of Fe₂Mn and Fe₂Co connected through 3,3′,5,5′-azobenzenetetracarboxylate linker to form Fe₂M–PCN–Fe₂M is explored. Compared with Fe₂Mn–PCN–Fe₂Mn, Fe₂Co–PCN–Fe₂Co, and isolated Fe₂M clusters, the mixed-metal Fe₂Co–PCN–Fe₂Mn possesses excellent bifunctional catalytic activity, and the values of potential gap on the Mn and Co sites of Fe₂Co–PCN–Fe₂Mn are 0.69 and 0.70 V, respectively. Furthermore, the analysis of the electron structure indicates that constructing a mixed-metal cluster can efficiently enhance the electronic properties of the catalyst. In conclusion, the mixed-metal cluster strategy provides a new approach to further design and synthesize high-efficiency bifunctional electrocatalysts.     

Synthesis of ordered mesoporous carbon/cobalt oxide nanocomposite for determination of glutathione

In the paper, a novel ordered mesoporous carbon/cobalt oxide nanocomposite (OMC–Co) was easily synthesized. After encapsulating cobalt oxide nanoparticles in the wall of the ordered mesoporous carbon (OMC), the mesostructure of the nanocomposite material remained highly ordered and intact. For the first time, OMC–Co material was used to modify the glassy carbon (GC) electrode, and the obtained OMC–Co/GC modified electrode showed strong electrocatalytic properties towards glutathione (GSH). The use of OMC–Co film to mediate the GSH oxidation exhibited remarkably strong and stable electrocatalytic response compared to that seen at OMC/GC electrode. These results showed that the electrocatalytic properties of OMC could be improved when the cobalt oxide nanoparticles were incorporated into this new material. A sensitive GSH sensor was developed based on the OMC–Co/GC electrode, which showed a high sensitivity and a remarkably low detection limit. Moreover, OMC–Co/GC modified electrode can be used for selective amperometric determination of GSH in the presence of glucose, dopamine (DA) and uric acid (UA).     

The role of trace Fe in Fe–N-doped amorphous carbon with excellent electrocatalytic performance for oxygen reduction reaction

Synthesized by dripping iron acetate into the N-doped carbon film enriched with pyridinic N and followed by annealing at 800 °C, Fe–N-doped amorphous carbon (d_Fe–N-C) with an Fe content of 0.2 at.% showed excellent electrocatalytic activity, stability and methanol tolerance via a four-electron pathway for oxygen reduction reaction (ORR), which outperformed commercial Pt/C catalyst. More importantly, by tuning the Fe content and annealing temperature, the trace Fe in d_Fe–N-C was supposed to form high active FeN₄ sites with pyridinic N and played an important role in the excellent electrocatalytic performance for ORR.     

Co-Fe-Pd纳米粒子的可控制备及其氧还原催化性能

以Co(COOH)₂、FeCl₃和PdCl₂为原料,柠檬酸为稳定剂,乙醇为加速剂,采用超声辅助制备Co-Fe-Pd金属纳米粒子,并评估其氧还原反应(ORR)电催化性能。研究结果表明,Co-Fe-Pd金属纳米粒子平均粒径约3～5 nm,由于Co、Fe固溶于Pd晶格,使Co-Pd、Fe-Pd和Co-Fe-Pd纳米粒子仅显示Pd衍射峰,且伴有不同程度的宽化;相比于Co-Fe、Fe-Pd或Co-Pd纳米粒子,三元Co-Fe-Pd晶格压缩更为明显,晶格缺陷诱使的活性位点增加,氧还原催化能力增强;其氧还原起峰电位为1.03 V(vs RHE),Tafel斜率为?87 mV/dec,可与商用Pt/C催化剂相媲美;氧还原过程中电子转移数为3.80±0.04,说明其主导四电子转移路径;此外,RRDE结果显示氧还原过程中的中间产物H₂O₂含量约10%。     

Ultrafine Fe-modulated Ni nanoparticles embedded within nitrogen-doped carbon from Zr-MOFs-confined conversion for efficient oxygen evolution reaction

Improvement of the low-cost transition metal electrocatalyst used in sluggish oxygen evolution reaction is a significant but challenging problem. In this study, ultrafine Fe-modulated Ni nanoparticles embedded in a porous Ni-doped carbon matrix were produced by the pyrolysis of zirconium metal–organic–frameworks, in which 2,2′-bipyridine-5,5′-dicarboxylate operating as a ligand can coordinate with Ni²⁺ and Fe³⁺. This strategy allows formation of Fe-modulated Ni nanoparticles with a uniform dimension of about 2 nm which can be ascribed to the spatial blocking effect of ZrO₂. This unique catalyst displays an efficient oxygen evolution reaction electrocatalytic activity with a low overpotential of 372 mV at 10 mA·cm^–2 and a small Tafel slope of 84.4 mV·dec^–1 in alkaline media. More importantly, it shows superior durability and structural stability after 43 h in a chronoamperometry test. Meanwhile, it shows excellent cycling stability during 4000 cyclic voltammetry cycles. This research offers a new insight into the construction of uniform nanoscale transition metals and their alloys as highly efficient and durable electrocatalysts.     

Multi-walled carbon nanotube-supported tungsten oxide-containing multifunctional hybrid electrocatalytic system for oxygen reduction in acid medium

Combination of multi-walled carbon nanotubes, cobalt porphyrin and tungsten oxide in the film (deposited onto glassy carbon electrode substrate) produces an electrocatalytic system capable of effective reduction of oxygen in such acid medium as 0.5 mol dm⁻³ H₂SO₄.Co-existence of cobalt porphyrin and tungsten oxide, together with dispersed carbon nanotubes, leads to the enhancement effect evident from some positive shift in the oxygen reduction voltammetric potential and the significant increase of voltammetric currents (relative to those characteristic of the system free of carbon nanotubes and WO₃). The multi-component electrocatalytic film has also exhibited relatively higher activity towards reduction of hydrogen peroxide. It is reasonable to expect that the reduction of oxygen is initiated at the cobalt porphyrin redox centers, and the undesirable hydrogen peroxide intermediate is further reduced at the tungsten oxide support. An important function of carbon nanotubes is to improve transport of electrons within the electrocatalytic multi-component film.     

Electrocatalytic Activity of Carbon Nanotube-Supported Pt–Cr–Co Tri-Metallic Nanoparticles for Methanol and Ethanol Oxidations

In this work, platinum (Pt), Pt–Cr, Pt–Co, and Pt–Cr–Co nanoparticles were synthesized on single-walled carbon nanotubes, and their effects on electrocatalytic activity for methanol and ethanol oxidations were investigated using cyclic voltammetry and electrochemical impedance spectroscopy. In comparison to Pt, Pt–Cr, and Pt–Co, Pt–Cr–Co nanoparticles demonstrate better catalytic characteristics for methanol and ethanol oxidations, such as forward peak current density, resistance to carbon monoxide poisoning, peak potential for oxidation, and charge transfer resistance. This study indicates that Pt-trimetallic nanoparticles could be desirable catalyst candidates for both direct methanol and ethanol fuel cells.     

Functionalized-carbon nanotube supported electrocatalysts and buckypaper-based biocathodes for glucose fuel cell applications

The preparation and testing for electrocatalytic activity of functionalized carbon nanotube (f-CNT) supported Pt and Au–Pt nanoparticles (NPs), and bilirubin oxidase (BOD), are reported. These materials were utilized as oxygen reduction reaction (ORR) cathode electrocatalysts in a phosphate buffer solution (0.2 M, pH 7.4) at 25 °C, in the absence and presence of glucose. Carbon monoxide (CO) stripping voltammetry was applied to determine the electrochemically active surface area (ESA). The ORR performance of the Pt/f-CNTs catalyst was high (specific activity of 80.9 μA cm_Pt⁻² at 0.8 V vs. RHE) with an open circuit potential within ca. 10 mV of that delivered by state-of-the-art carbon supported platinum catalyst and exhibited better glucose tolerance. The f-CNT support favors a higher electrocatalytic activity of BOD for the ORR than a commercially available carbon black (Vulcan XC-72R). These results demonstrate that f-CNTs are a promising electrocatalyst supporting substrate for biofuel cell applications.     

Shape- and Composition-Controlled Pt–Fe–Co Nanoparticles for Electrocatalytic Methanol Oxidation

We report the preparation of several shape and composition-controlled platinum-based nanoparticles: Pt–Fe nanocubes, Pt–Fe–Co nanocubes, Pt–Fe–Co branched nanocubes, Pt–Fe–Co nanoparticles with low cobalt content, and Pt–Fe–Co nanoparticles with high cobalt content. Pt–Fe–Co branched nanocubes, in particular, showed the best activity and durability for electrocatalytic methanol oxidation.     

铁/钴质量比对MWCNT-聚苯胺复合物氧还原电活性的影响

中心金属离子在非贵金属C-N复合物对氧还原反应（ORR）的电催化活性方面有重要作用。通过将含有多壁碳纳米管（MWCNTs）、聚苯胺与过渡金属Fe和Co的前驱体在N₂气氛下于900℃下加热,得到了不同金属比例的C-N催化剂。采用SEM、XRD等对催化剂的结构进行了表征。利用电化学伏安技术,研究了催化剂对ORR的电催化活性及其稳定性。结果表明,当Fe与Co质量比为6：1时催化剂的催化活性最好,在酸性溶液中ORR起始电位达到0.52 V（vs SCE）,电流密度为12.5 mA·mg^-1@-0.3 V （vs SCE）;在碱性溶液中ORR 起始电位为-0.09 V（vs SCE）,电流密度为7.8 mA·mg^-1@-0.8 V （vs SCE）。结果表明,催化剂中Fe与Co的质量比对催化剂的活性有很大影响。     

Exfoliated graphene sheets decorated with metal/metal oxide nanoparticles: Simple preparation from cation exchanged graphite oxide

We produced carbon hybrid materials of graphene sheets decorated with metal or metal oxide nanoparticles of gold, silver, copper, cobalt, or nickel from cation exchanged graphite oxide. Measurements using powder X-ray diffraction, transmission electron microscopy, and X-ray absorption spectra revealed that the Au and Ag in the materials (Au–Gr and Ag–Gr) existed on graphene sheets as metal nanoparticles, whereas Cu and Co in the materials (Cu–Gr and Co–Gr) existed as a metal oxide. Most Ni particles in Ni–Gr were metal, but the surfaces of large particles were partly oxidized, producing a core–shell structure. The Ag–Gr sample showed a catalytic activity for the oxygen reduction reaction in 1.0 M KOH aq. under an oxygen atmosphere. Ag–Gr is superior as a cathode in alkaline fuel cells, which should not be disturbed by the methanol cross-over problem from the anode. We established an effective approach to prepare a series of graphene-nanoparticle composite materials using heat treatment.     

Electrocatalytic performance of carbon nanotube-supported palladium particles in the oxidation of formic acid and the reduction of oxygen

We describe a simple approach for the synthesis of nanosized Pd particles supported on multiwall carbon nanotubes (MWCNTs) and their electrocatalytic performance in the oxidation of formic acid and the reduction of oxygen. The metal precursors are pre-organized on poly(diallyldimethylammonium) chloride-wrapped MWCNTs by electrostatic interaction and chemically reduced to obtain Pd nanoparticles. The MWCNT-supported nanoparticles are characterized by UV-visible spectroscopy, X-ray diffraction (XRD), scanning and transmission electron microscopy and electrochemical measurements. MWCNTs are uniformly decorated with closely packed array of nanoparticles. The nanoparticles on the MWCNTs have spherical and rod-like shapes with size ranging from 5 to 10 nm. XRD and selected area electron diffraction measurements of the nanoparticles show (1 1 1), (2 0 0) and (2 2 0) reflections of the Pd lattice. The electrocatalytic activity of the nanoparticles towards oxidation of formic acid and reduction of oxygen is examined in acidic solution. The MWCNT-supported particles exhibit excellent electrocatalytic activity. The electrocatalytic reduction of oxygen follows the peroxide pathway. Surface morphology and coverage of particles on the nanotubes control the electrocatalytic activity. The large surface area and high catalytic activity of the MWCNT-supported nanoparticles facilitate the electrocatalytic reactions at a favorable potential.     

Cathode catalysts for fuel cell development: A theoretical study based on band structure calculations for tungsten nitride and cobalt tungsten nitrides

Band structure calculations were performed for tungsten nitride, cobalt tungsten nitrides, and platinum slabs. The major requirements for the development of a superior cathode catalyst are: (1) that the Fermi level of the cathode catalyst is close to the energy level of the lowest unoccupied molecular orbital of O₂, the lowest unoccupied atomic orbital of an oxygen atom, and the lowest unoccupied atomic orbital of a hydrogen atom so that they can readily interact with one another; and (2) that the cathode catalysts have smaller ΔE value which represent the difference between the Fermi level and the peak position of the density of states of the O_p orbital of O₂ adsorbed on the catalyst. The active site structures of cobalt tungsten nitrides for activation of the oxygen reduction reaction were found to have the surface structure of Co–O–Co, which lowered the unoccupied orbital of the oxygen atom to approximately that of the Fermi level. However, this structure concomitantly lowered the Fermi level, which resulted in an increase in ΔE. Consequently, the optimal cathode catalyst regarding the surface conformation contains a Co–O–Co structure that is dispersed on the surface of the cobalt tungsten nitride. The cobalt tungsten oxynitride exhibited a catalytic activity for the oxygen reduction reaction. A linear dependence is observed between the ΔE and the oxygen reduction reaction offset potentials of the tungsten nitride, cobalt tungsten nitride, cobalt tungsten oxynitride, and platinum.     

Highly efficient supporting material derived from used cigarette filter for oxygen reduction reaction

Bimodal porous nitrogen (N) doped carbon supported Pt composite was prepared as a catalyst for oxygen reduction reaction (ORR). The N-doped carbon (NCF) support was obtained via one-pot pyrolysis of the used cigarette filters. Physical characterizations and electrochemical tests proved that the presence of N dopant on the surface of the NCF not only provided highly dispersive active sites for the growth of the Pt nanoparticles but also the active centers for ORR itself. It was demonstrated that these combinative effects contributed on higher ORR activity and durability than those for the commercial carbon (Vulcan XC) supported Pt composites.