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A review of nitrogen-doped graphene catalysts for proton exchange membrane fuel cells-synthesis, characterization, and improvement

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Abstract

Platinum group metals (PGM), such as platinum (Pt) or ruthenium (Ru), are the most common catalyst materials for the oxygen reduction reaction (ORR) because of their excellent catalytic performance. However, the high raw material cost of PGM catalysts has become a significant issue. Currently, the nitrogen-doped graphene (N-G) catalyst emerges as one of the promising non-PGM catalysts with the advantages of low cost and high ORR catalytic performance to replace expensive PGM catalysts in electrochemical systems. This paper reviews the investigation of N-G catalysts through the synthesis, characterization, and improvement methodologies. And comparisons between various chemical and mechanochemical synthesis methods and the properties of final N-G catalysts are discussed as well. The paper also reviewed a nanoscale high energy wet ball milling (NHEW) method which was investigated recently for the synthesis of N-G catalysts. Recent research results show that the performance of the N-G catalyst is already comparable to the commercialized Pt/C catalyst. It is also possible to enhance the electrochemical performance of N-G catalysts by the modification of metal organic framework (MOF) materials. The new MOF-modified N-G catalyst shows higher current density than Pt/C catalyst.

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... Chemical Vapor Deposition (CVD) is the most common method to synthesize N-graphene 10 . However, it has the disadvantages of elaborate experimental arrangement 11 , possible contamination of the electrocatalyst with metal impurities 10 , explosion hazard due to H 2 and CH 4 gases used in the process 12 , use of toxic NH 3 as nitrogen source 10 , time-consuming transfer process and high cost 12 . ...
... Thermal treatment, on the other hand, is a simple and straightforward doping method that does not require toxic precursors. It can be easily scalable and provides flexible control of experimental parameters 13,14 , consisting in the high temperature heating of a solid mixture of carbon and nitrogen precursors under inert atmosphere 10 . ...
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Fuel cell is considered an energetic solution of low environmental impact which large-scale commercialization is hindered by the expensive Pt-based cathodic electrocatalyst. Therefore, its replacement by an active less-expensive metal-free electrocatalyst such as nitrogen-doped graphene (N-graphene) is desirable. To investigate nitrogen-doping influence on graphene’s activity in alkaline oxygen reduction reaction (ORR), we prepared N-graphene through thermal treatment of graphene and urea at 800 °C for 30 min. N-graphene exhibited higher onset potential, half-wave potential, diffusion-limited current density and kinetic current density in ORR (0.78 V vs RHE, 0.63 V vs RHE, -3.36 mA cm-2, -3.00 mA cm-2, respectively) than undoped graphene (0.73 V vs RHE, 0.62 V vs RHE, -2.80 mA cm-2, -2.23 mA cm-2, respectively), which may be attributed to nitrogen-doping, edges and defect sites in N-graphene, as revealed by elemental analysis, scanning electron microscopy and Raman spectroscopy results. This activity improvement also affected ORR’s selectivity and rate-determining step.
... The catalytic activity of N-G catalysts is directly related to the nitrogen doping rate and the structure of the active site. The nitrogen doping ratio of N-G synthesized by conventional chemical approaches is around 4e16% [3,11,12]. Our group has successfully synthesized high-performance N-G catalysts with controlled nitrogen content ranging from 10 to 32%, by a facile synthesis approach named nanoscale high-energy wet (NHEW) ball milling [13e15]. ...
... It is believed that this C-O group is from the BM-N-G-16-500 component in the precursor, due to the 0 at% of oxygen in ZIF-8. Also, it has been characterized that the nitrogen-doped graphene structure always contains Sp 2 C-C, Sp 3 C-C, and Sp 2 C-N groups with BEs similar to that of ZIF-8 [2,3,11]. Hence, there are no significant shifts of these three peaks from ZIF-8 state to N-G þ ZIF precursor state. ...
Article
Here we report an advanced nitrogen-doped graphene-based catalyst with metal-reduced organic framework structure (N-G/MOF) prepared by functionalizing ZIF-8 and nitrogen-functionalized graphene oxide using the nanoscale high energy wet ball milling method. The chemical structure control of N-G/MOF was studied by characterizing the variation of the chemical structure of synthesized samples throughout targeted grinding speeds. The results proved that the chemical interaction between ZIF-8 and N-G caused the reduction of nitrogen, oxygen and zinc atoms, and the variation of chemical bonding composition in N-G/MOF. The reduction rate of zinc was gradually increased with the increasing grinding speed and reached 82% of zinc loss at 650 RPM. The characterization of carbon and nitrogen bonding composition confirmed that the reduction of nitrogen, oxygen and zinc atoms was caused by the decomposition of C-N-Zn heteroatom contents in ZIF-8 and the O-containing functional groups in N-G which were influenced by the grinding speed. The decomposition of ZIF-8 not only affected the framework and the pore structure but also modified the chemical structure and the surface distribution of C-N-containing functional groups-constituted active sites. The variation of physical and chemical properties enhanced the electrochemical performance of N-G/MOF and made it comparable to the 10 wt% Pt/C catalyst. The full paper is available here: https://authors.elsevier.com/a/1XWVk1zUAE4Tq
... Carbonaceous materials have become indispensible and highly promising in the fields of electronics [1][2][3], catalysis [4][5][6][7][8][9][10][11], organic synthesis [12][13][14], sensors [15], energy harvesting [16][17][18][19] and storage [1,20,21], biomedicine [1,22,23], ferrofluids [24,25], hyperthermia materials [26], MRI contrast agents, magnetic data storage and metamaterials [27,28] etc. Among the carbon based materials, carbon nanotubes (CNT) and graphene have been in the limelight since their discovery [1,29]. ...
... Owing to their large surface area these carbonaceous materials can also be used in adsorption of various chemical species [30]. Nitrogen-doped graphene and CNT have appeared to be promising replacement for the expensive platinum group catalysts (Pt, Pd, Ru) in fuel cells for oxygen reduction reaction [17]. Recent reports also mention promising supercapacitive behaviour of graphene [20], multi-walled CNT and their polymer composites [21]. ...
Article
The effect of reaction temperature, heating rate of the precursors and length of the reaction zone of the furnace on the morphology of carbon nanostructures synthesized via a single-step pyrolysis route is studied. When the furnace was heated at a slow heating rate the synthesized products have highly amorphous globular morphology regardless of the length of the heating zone and the reaction temperature. The amorphous globules contain a dispersion of Fe-based particles. Higher graphitization around the Fe-based particles is observed upon increasing the synthesis temperature. The heating rate has the highest influence on the morphology of the products. When the furnaces were heated (naturally) at a low rate of ∼3 °C/min up to the reaction temperature amorphous carbon globules formed, while carbon nanotubes formed when the heating rate was increased to 20 °C/min in a controlled manner. When the synthesized samples were calcined in air at 400 °C for 1 h, the amorphous carbon graphitizes and the graphitic phase forms agglomerated nanotube-like structure.
... In recent years, research aimed at the development of efficient and inexpensive oxygen reduction reaction (ORR) catalysts has involved the use of the N-doped graphene materials, coupled or not with some transitional metals, such as iron or cobalt [10][11][12][13]. Due to the electronic interaction between the lone pair electrons of nitrogen and the pielectronic system of the graphitic structure, these materials exhibited electrocatalytic performance towards ORR that was comparable with the Pt/C catalyst [14]. Thus, N-doped graphene has received a lot of attention and has opened new insights in the fields of the energy application technologies [15]. ...
... These images indicate the nitrogen doping of the graphene and the presence of oxygen species on the graphene surface. Moreover, the graphene obtained by ball milling procedure appears to have a fluffy structure, very different from that synthetised by CVD or chemical methods [14,19,20] or the graphite structure (Figures 1 and S1, Supplementary Materials). It is clear from these pictures that multilayer and crumpled graphene are synthetized by this method, suggesting the idea of multiple defective sites and edges on the graphene and explaining the high density of heteroatoms (nitrogen and oxygen) observed in the EDX elemental mapping of these hybrid nanomaterials (Figure 1). ...
Article
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Four N-doped graphene materials with a nitrogen content ranging from 8.34 to 13.1 wt.% are prepared by the ball milling method. This method represents an eco-friendly mechanochemical process that can be easily adapted for industrial-scale productivity and allows both the exfoliation of graphite and the synthesis of large quantities of functionalized graphene. These materials are characterized by transmission and scanning electron microscopy, thermogravimetry measurements, X-ray powder diffraction, X-ray photoelectron and Raman spectroscopy, and then, are tested towards the oxygen reduction reaction by cyclic voltammetry and rotating disk electrode methods. Their responses towards ORR are analysed in correlation with their properties and use for the best ORR catalyst identification. However, even though the mechanochemical procedure and the characterization techniques are clean and green methods (i.e., water is the only solvent used for these syntheses and investigations), they are time consuming and, generally, a low number of materials can be prepared, characterized and tested. In order to eliminate some of these limitations, the use of regression learner and reverse engineering methods are proposed for facilitating the optimization of the synthesis conditions and the materials’ design. Thus, the machine learning algorithms are applied to data containing the synthesis parameters, the results obtained from different characterization techniques and the materials response towards ORR to quickly provide predictions that allow the best synthesis conditions or the best electrocatalysts’ identification.
... To overcome this issue, much effort on preparation of a proper catalyst has been made. Most of commercial ORR catalysts use noble metals on carbon support (Pt/C, Ru/C, Ir/C) [6][7][8][9][10][11] or their alloys (Pt/Ru) [12,13]. Unfortunately, noble metal catalysts are quickly poisoned by various impurities, which are always present in practical systems [14]. ...
Article
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The oxygen reduction reaction (ORR) is a key process for the operation of fuel cells. To accelerate the sluggish kinetics of ORR, a wide range of catalysts have been proposed and tested. In this work, a nano-dispersed copper-impregnated platinum catalyst prepared by electrodeposition of platinum on a poly[Cu(Salen)] template followed by polymer destruction is described. In addition to the high activity of the thus prepared catalyst in the oxygen reduction reaction surpassing that of both polycrystalline platinum catalyst and the commercial carbon-platinum catalyst (“E-TEK”), it showed remarkable tolerance to the presence of methanol in solution.
... Among PAHs, nitrogen (N)-doped PAHs are one of the most widely studied materials. N-doped PAHs are regarded as small molecular models of N-doped graphene, which is a promising nonprecious metal-based heterogeneous catalyst for the oxygen reduction reaction in fuel cell technology [7,8]. ...
Article
The synthesis of a novel conjugated polymer containing fused pyridinium units in its main chain is reported. A precursor polymer possessing pyridine and tetrafluorophenylene moieties was reacted in the presence of Lewis acid additives to promote the intramolecular nucleophilic aromatic substitution (SNAr) reaction. The polymer reaction with BE3-OEt2 gave a product polymer soluble in polar organic solvents, and the successful formation of fused pyridinium ring structures was spectroscopically confirmed. The electrochemical and optical properties of the synthesized polymers were also investigated, suggesting that the polymer product has a narrower band gap than the precursor polymer. The synthesis of a novel conjugated polymer containing fused pyridinium units in its main chain is reported. A precursor polymer possessing pyridine and tetrafluorophenylene moieties was reacted in the presence of Lewis acid additives to promote the intramolecular nucleophilic aromatic substitution (SNAr) reaction. The polymer reaction with BE3-OEt2 gave a product polymer soluble in polar organic solvents, and the successful formation of fused pyridinium ring structures was spectroscopically confirmed. The electrochemical and optical properties of the synthesized polymers were also investigated.
... It is also worth mentioning that nitrogen-doped carbon materials are high-performance catalysts for electrochemical processes, such as the oxygen reduc-tion reaction [13,27,108]. In such reactions, carbon bonded to nitrogen atoms is an active site for oxygen reduction [109]. ...
Article
In recent years, the topic of catalysis by carbon materials (carbocatalysis) has experienced rapid growth. Every year the scope of application of carbocatalysts is expanding swiftly, spreading to areas in which metal catalysts seemed to have no alternative until recently. Due to their structural diversity and functional-ization possibilities, carbon materials offer almost limitless possibilities for creating various catalytically active sites. It is of crucial importance to understand the mechanisms of action of chemically active sites of carbocatalysts for the development of this field. The latest discoveries have unveiled the complexity of this topic and the need to take into account the effects associated with the simultaneous presence of various active sites on the surface of carbon materials. Despite the significant advances in modern research in the field of carbocatalysis, it is important to remember the origins of the use of carbon materials in catalysis. New methods of studying old and almost forgotten reactions often lead to very interesting discoveries. This review focuses on the historical development of ideas about the catalytic activity of carbon materials, modern concepts of the mechanisms of carbocatalytic reactions, and recent studies of the role of carbene centers of graphene materials in catalytic processes.
... Conclusions and recommendations were included at the end of the manuscript. The current work is not limited to one derivative of the graphene [76] or one type of the fuel cells, but it covers the available works done on graphene and all graphene derivatives in four different types of fuel cells. At the same time, this review focuses on the role of graphene in the enhancement of the membrane specifically, not other parts such as the electrodes [77,78]. ...
Article
Nafion membrane is a commercial type of electrolyte membrane that is widely used in low-temperature fuel cells. Nafion membrane is expensive and allows fuel to crossover from anode to cathode. Therefore, it decreases fuel utilization and results in mixed potential at the cathode. Modifying the Nafion membrane and/or replacing it with non-Nafion-based materials has shown promising results in solving these problems. Graphene is a two-dimensional material with exceptional properties that positively affect the performance of several energy conversion/storage devices. With its large surface functional groups, graphene oxide exhibits considerable ionic conductivity with low fuel crossover. This paper reviews and discusses the recent progress in utilizing graphene or its derivatives as a blend with Nafion and non-Nafion based membranes as well as a standalone membrane in low-temperature fuel cells. Graphene or its derivatives possess a high potential as a standalone membrane or a composite membrane in the various low temperature fuel cells applications in terms of high ionic conductivity at low humidity and relevant high temperature, high thermal, mechanical, and chemical stabilities. This work summarizes the recent progress in utilizing graphene and its derivatives (graphene oxide and other doped graphene) in different types of fuel cells, whether as a standalone or as a modifier of the Nafion and non-Nafion based membranes. The review covers the usage of graphene and its derivatives in PEMFCs, DMFCs, anion exchange DMFCs, and MFCs. Furthermore, it elaborates on the science and the engineering aspects of utilizing graphene and its derivatives in different fuel cells. This work demonstrates the importance of the graphene and its derivatives in improving the performance of low temperature fuel cells.
... CVD is attractive as a synthesis approach because it is well established within industry and is suitable for large scale production. Moreover, it is also easily available in laboratories as a research tool and so has gained enormously in popularity both commercially and academically [35]. In our CVD approach using ethanol as the feedstock, it allows a very easy experimental set up and so is both facile and cost effective. ...
Article
There is a growing demand for nanoparticles coated with graphene due their promise in various applications such as batteries and supercapacitors, sorbents, and biomedical applications. A good example is Si where its combination with carbon is considered important for electronics and energy applications, such as lithium ion batteries, polymer based composites and even bio-medical applications. In this study, we aim to develop a very simple chemical vapor deposition approach to form graphitized Si nanoparticles in which ethanol serves as the C feedstock. This differs from other CVD approaches which tend to use a gaseous hydrocarbon (e.g. CH4) as the carbon feedstock. The use of ethanol in which Ar is simply bubbled through liquid ethanol leads to a simpler and cheaper approach. Moreover, in conventional CVD, often an oxidant (e.g. CO2) is added to aid graphitization and minimize the formation of SiC at the Si surface. Ethanol provides a source of O which serves as an inbuilt oxidizer to aid graphitization and limit the formation of SiC. The simple synthesis approach allows one to tailor the number of graphene layers coating the Si nanoparticles through the three main synthesis parameters of ethanol supply, temperature and reaction time. Moreover, using ethanol as the precursor, lower graphitization temperatures than for methane can be used.
... Graphene, a single sheet of carbon atoms bound in a honeycomb network, has demonstrated several potential applications in the energy field area [17][18][19] . It shows unique thermal conductivity, high optical transparency, good thermochemical stability, large surface area along with superior electron conductivity [ 20 , 21 ] . ...
Article
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Development of low-cost electrocatalyst for oxygen reduction reaction (ORR) is the main requirement for advanced chlor-alkali (ACA) cell. Herein, we report a simple and economical one-pot microwave assisted synthesis of nitrogen and phosphorus co-doped graphene (NPG) from graphene oxide in presence of ammonium dihydrogen phosphate. The optimized NPG showed well exfoliated graphene oxide with few layers of graphene and the XPS assured that about 6% of nitrogen and 4% of phosphorus co-doping into the graphene lattice through the various active forms of nitrogen and phosphorus. The large N and P content in the catalyst with all the active forms could facilitate faster ORR kinetics as well as it stability. The oxygen depolarized cathode containing electrocatalyst was fabricated and utilized in ACA cell. According to the result, the minimum cell voltage (2.01 V) was found under conditions: brine concentration 300 g/L, temperature 80°C, pH 2 and current density 1.0 kA/m².
... N-graphene can be prepared through chemical vapor deposition, segregation growth, nitrogen plasma processes and arc-discharge approaches. However, these methods have high production costs [23,24]. Heat-treatment of graphene oxide (GO) with different amines can not only remove oxygen-containing functional groups from GO sheets, but also introduces nitrogen atoms into the carbon backbone [25]. ...
Article
Carbon dioxide (CO2) and hydrogen (H2) adsorptions were investigated on 3D nitrogen-doped porous graphene (GO-PAA) produced by chemical activation of graphene oxide impregnated with polyallylamine (PAA). GO-PAA characterizations by Raman and X-ray photoelectron spectroscopies, thermogravimetric analysis, N2 adsorption-desorption isotherms, scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed that GO-PAA shows excellent thermal stability, decomposing at temperatures higher than 450 °C, specific surface area of 1155 m² g⁻¹, with pyrrolic, pyridinic and graphitic nitrogen atoms homogeneously dispersed throughout its 3D porous structure. The mesoporous nature of GO-PAA and the nitrogen doping level of 7.5 wt% resulted in remarkable H2 (1.3 wt%) and CO2 (20 mmol g⁻¹) adsorption capacities at room temperature (RT, 25 °C) under high-pressure regime (40 bar). This H2 adsorption capacity at RT is among the highest values reported in the open literature. The role of the nitrogen atoms on the gas adsorption properties was investigated by combining experimental data, such as isosteric heat and Diffuse Reflectance Infrared Fourier Transform spectroscopy, with theoretical calculations using Density-Functional Theory. Inclusion of nitrogen atoms in the graphene structure reduces the energies of the Frontier Molecular Orbitals, facilitating polarization and increasing the interaction energy.
... On the contrary to graphene oxidation, which is relatively easy to perform and has been known for a long time by standard wet chemistry oxidants like KMnO 3 [23], the nitrogenation of graphene requires a more complex procedure and even sophisticated instruments. Several methods for the nitrogenation of graphite/graphene are commonly exploited [24][25][26] (Table 1). ...
Article
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The design and fabrication of a new effective manufacturing method of heteroatom-doped carbon materials is still ongoing. In this paper, we present alternative and facile methods to obtain N-rich graphene with the use of low energy gamma radiation. This method was used as a pure and facile method for altering the physical and chemical properties of graphene. The obtained materials have an exceptionally high N content—up to 4 wt %. (dry method) and up to 2 wt %. (wet method). High-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectra and X-ray photoelectron spectroscopy (XPS) studies allowed us to evaluate the quality of the obtained materials. The presented results will provide new insights in designing and optimizing N-doped carbon materials potentially for the development of anode or cathode materials for electrochemical device applications, especially supercapacitors, metal–air batteries and fuel cells. Nitrogen atoms are exclusively bonded as quaternary groups. The method is expandable to the chemical insertion of other heteroatoms to graphene, especially such as sulfur, boron or phosphorus.
... However, the commercialization of PEMFCs is extremely difficult due to some technical issues, typically the insufficient performance of membrane electrode assembly (MEA), which is responsible for efficiently transporting vapors, liquid water, fuels, oxidants, protons, and electrons. The MEA components usually include a polymer electrolyte membrane also known as PEM, anode/cathode catalyst layers, and two gas diffusion layers [6]. In MEA components, however, the PEM is the primarily core component that determine the performance of a fuel cell system due to its multiple roles as an electric i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( x x x x ) x x x insulator, proton-conducting medium, and fuel barrier. ...
Article
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As the critical component of proton exchange membrane fuel cell (PEMFC), proton exchange membrane (PEM) determine its overall performance. Current PEMs hardly meet the operating requirements of fuel cells, limiting their commercial applications. With the development of nanocomposite technology, nanofibers introduced into PEMs to prepare nanofiber composite proton exchange membranes (NCPEMs) have been widely studied. In an NCPEM, nanofibers can form long-range channels for proton transport, and reinforced skeleton to reach the target performance of PEMFCs. Focusing on NCPEM, this paper reviews on recent progresses in nanofiber preparation and NCPEM preparation techniques. Furthermore, different types of nanofibers incorporated into NCPEMs are reviewed in terms of fiber composition. The challenges and future perspectives regarding NCPEMs are also discussed.
... POC devices take advantage of LOC technology and have a more comfortable, cheap, lab-free and convenient diagnosis process instead of the traditional laboratory process [1][2][3][4][5][6][7]. In the development of LOC technology, sample preparation is always an obstacle, especially in microfluidic platforms [8][9][10][11][12][13][14][15][16][17]. Blood is the most commonly used biofluid sample in LOC devices. ...
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Blood plasma is the most commonly used biofluid in disease diagnostic and biomedical analysis due to it contains various biomarkers. The majority of the blood plasma separation is still handled with centrifugation, which is off-chip and time-consuming. Therefore, in the Lab-on-a-chip (LOC) field, an effective microfluidic blood plasma separation platform attracts researchers’ attention globally. Blood plasma self-separation technologies are usually divided into two categories: active self-separation and passive self-separation. Passive self-separation technologies, in contrast with active self-separation, only rely on microchannel geometry, microfluidic phenomena and hydrodynamic forces. Passive self-separation devices are driven by the capillary flow, which is generated due to the characteristics of the surface of the channel and its interaction with the fluid. Comparing to the active plasma separation techniques, passive plasma separation methods are more considered in the microfluidic platform, owing to their ease of fabrication, portable, user-friendly features. We propose an extensive review of mechanisms of passive self-separation technologies and enumerate some experimental details and devices to exploit these effects. The performances, limitations and challenges of these technologies and devices are also compared and discussed.
... Sensors are made from chemical or biological receptors and transducers. The receptor detects and interacts with the analyte and the transducer converts it into a recognizable signal [104][105][106][107]. Biosensors use enzymes, antibodies, or nucleic acids in tandem with a transducer and detector to provide output. ...
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COVID-19, also known as SARS-CoV-2 is a novel, respiratory virus currently plaguing humanity. Genetically, at its core, it is a single-strand positive-sense RNA virus. It is a beta-type Coronavirus and is distinct in its structure and binding mechanism compared to other types of coronaviruses. Testing for the virus remains a challenge due to the small market available for at-home detection. Currently, there are three main types of tests for biomarker detection: viral, antigen and antibody. Reverse Transcription-Polymerase Chain Reaction (RT-PCR) remains the gold standard for viral testing. However, the lack of quantitative detection and turnaround time for results are drawbacks. This manuscript focuses on recent advances in COVID-19 detection that have lower limits of detection and faster response times than RT-PCR testing. The advancements in sensing platforms have amplified the detection levels and provided real-time results for SARS-CoV-2 spike protein detection with limits as low as 1 fg/mL in the Graphene Field Effect Transistor (FET) sensor. Additionally, using multiple biomarkers, detection levels can achieve a specificity and sensitivity level comparable to that of PCR testing. Proper biomarker selection coupled with nano sensing detection platforms are key in the widespread use of Point of Care (POC) diagnosis in COVID-19 detection.
... The preparation of porous material using the CVD method involves chemically etching the metal substrate on which graphene structure growth occurred. While even long-lasting etching does not allow for the complete removal of metals, it can still positively influence the electrochemical properties of the obtained materials [104]. When the CVD method is used to obtain graphene, the product is a material of high quality and homogeneity, which can be beneficial for the next step of doping with graphitic C3N4 [14]. ...
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The exploration and development of different carbon nanomaterials happening over the past years have established carbon electrodes as an important electrocatalyst for oxygen reduction reaction. Metal-free catalysts are especially promising potential alternatives for replacing Pt-based catalysts. This article describes recent advances and challenges in the three main synthesis manners (i.e., pyrolysis, hydrothermal method, and chemical vapor deposition) as effective methods for the production of metal-free carbon-based catalysts. To improve the catalytic activity, heteroatom doping the structure of graphene, carbon nanotubes, porous carbons, and carbon nanofibers is important and makes them a prospective candidate for commercial applications. Special attention is paid to providing an overview on the recent major works about nitrogen-doped carbon electrodes with various concentrations and chemical environments of the heteroatom active sites. A detailed discussion and summary of catalytic properties in aqueous electrolytes is given for graphene and porous carbon-based catalysts in particular, including recent studies performed in the authors’ research group. Finally, we discuss pathways and development opportunities approaching the practical use of mainly graphene-based catalysts for metal–air batteries and fuel cells.
... Carbon materials with nitrogen (N)-containing functional groups such as pyridinic N, primary amine (primary N), secondary amine (secondary N), tertiary amine (tertiary N) which means N in the basal plane, and quaternary N (QN) (Fig. 1a) have been intensively studied for decades [1][2][3] because of various attractive applications such as catalysts [4][5][6][7], electrodes for electrical double layer capacitor [8][9][10], fuel cells [11][12][13][14], metal-air batteries [15][16][17], sensors [18][19][20][21], and water purification [22][23][24] owing to the high electrical conductivity [25][26][27], high specific surface area [28][29][30], and high chemical inertness [31][32][33]. It is well known that the N doping can dramatically change their electrical properties and enhance their catalytic performance by facilitating electron transfer in reaction processes [34][35][36]. ...
Article
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Nitrogen-doped carbon materials, especially pyridinic nitrogen, have attracted attention because of the high performance for various applications such as electrodes for fuel cells and other catalytic reactions. However, the selective introduction of pyridinic nitrogen without using catalysts and the mass production of such carbon materials are challenging. In this work, carbon materials with selectively introduced pyridinic nitrogen were synthesized by just simple heat treatment of aromatic compounds containing pyridinic nitrogen in the absence of catalysts. Dibenzacridine isomers, C21H13N, with different molecular frameworks such as either zigzag or armchair edges were selected as precursors to investigate the influence of edges on the percentage of pyridinic nitrogen. The percentage of pyridinic nitrogen of precursors heated at 973 K was in the order of dibenz[c,h]acridine (DCHA) (80%) > dibenz[a,h]acridine (60%) > dibenz[a,j]acridine (39%) > dibenz[a,i]acridine (33%). The percentage of pyridinic nitrogen of DCHA was further enhanced to 94% at 923 K. Experimental X-ray photoelectron, infrared, and Raman spectra combined with calculation of spectra and formation energy and calculation of carbonization reaction pathways by molecular dynamics simulation with reactive force field (ReaxFF) precisely revealed the reasons why the high percentage of pyridinic nitrogen was attained. The steric hindrance generated by the bay structure of DCHA provided the high formation energy of C–N bond (tertiary nitrogen) and N–H bonds (secondary nitrogen), avoiding the side reactions such as the formation of tertiary nitrogen and secondary nitrogen. Graphical abstract
... Consequently, N1s (Fig. S10f) is constituted with pyridinic-N and pyrrolic/pyridonic-N in G-CDs. Although, B-CDs is constituted by pyridinic-N, pyrrolic/pyridonic-N, and quaternary-N (Fig. S10i) [46]. The G-CDs have additional peak at 1024 and 1047 for core state of Zn2p 3/2 and Zn2p 1/2 [47]. ...
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Carbon dots (CDs) have recently emerged as a very encouraging family of materials with augmented performance in WLEDs. Facile synthesis of CDs with high optical tuning using sustainable green precursors is a great challenge in the applications of WLEDs. In this work, we prepare red, green, and blue-color emissive CDs (RGB CDs) using an extract of Star Jasmine leaves (Trachelospermum jasminoides). The red emission originates due to the presence of an extensive conjugated carbonic system with a few esters (-COOR) and amide (-CONR) moieties on their surface. And the shorter wavelength emission appears due to the presence of carboxylic (-COO) moieties. Specifically, in green emissive CDs, the formation of the O-Zn-O bond causes the fluorescence shifting (>100 nm) and a higher lifetime as it restricts the making of the conjugated -COOR/-CONR groups. The detailed study revealed that an average lifetime of CDs has a linear relationship with the core state lifetime. These highly stable and effective naturally produced RGB CDs have been used as fabrication of WLEDs, which exhibited CCT and CIE coordinates of 4283 K and (0.36,0.32), respectively. Additionally, the red CDs detected water and chloroform visually and fluorometrically in a broad range. This work will open up a new route for developing high-performance WLEDs using natural precursors.
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Most of the transition metal based heteroatom doped carbon electrocatalysts, utilizes the fossil fuel derived commercially available precursors as source of nitrogen and carbon which may question our environmental generosity. Herein, we have developed Ni-based efficient bifunctional electrocatalysts using apple seeds (that contains cyanogenic glycosides) as the precursor for nitrogen and carbon. With tuning the temperature, we were able to optimize the nitrogen doping up to ∼3 at.%. The optimized electrocatalyst catalyses the oxygen reduction reaction (ORR) process with muted peroxide generation (for 0.750–0.1 V the % HO2⁻ generation ∼3 - 2%), preferential 4e⁻ reduction pathways (n ∼ 3.93 to 3.98 in 0.75–0.1 V range) and electron transfer via inner-sphere electron transfer mechanism which ensures the maximum utilization of instituted active centres owing to the direct interaction of reactant species. Alike to ORR, the superior oxygen evolution reaction (OER) performance with smaller Eonset, EJ=10, Tafel slope and enduring accelerated stability test advocates its potential as a bifunctional oxygen electrocatalyst. Moreover, smaller potential gap ΔE (EJ10_OER - E1/2_ORR) of 0.845 V further warrants the energy efficient OER/ORR process. A porotype of Al-air battery system using our catalysts as oxygen electrode and chocolate wafer as anode material is well capable of powering the light emitting diodes. This study hopefully opens a new avenue to explore cyanogenic glycosides plants product to develop multifunctional electrocatalysts.
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Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play curial roles in electrochemical energy conversion and storage, including fuel cells and metal-air batteries. Having rich multidimensional nanoarchitectures [for example, zero-dimensional (0D) fullerenes, 1D carbon nanotubes, 2D graphene, and 3D graphite] with tunable electronic and surface characteristics, various carbon nanomaterials have been demonstrated to act as efficient metal-free electrocatalysts for ORR and OER in fuel cells and batteries. We present a critical review on the recent advances in carbon-based metal-free catalysts for fuel cells and metal-air batteries, and discuss the perspectives and challenges in this rapidly developing field of practical significance.
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Today, most metal and nitrogen doped carbon catalysts for ORR reveal a rather heterogeneous composition. This can be reasoned by a non-optimized precursor composition and different steps in the preparation process to get the required active material. The significant presence of inorganic metal species interfere the assignment of descriptors related to the ORR activity and stability. In this work we present a simple and feasible way to reduce the contribution of inorganic metal species in some cases even down to zero. Such catalysts reveal the desired homogeneous composition of MeN4 sites in the carbon that is accompanied by a significant enhancement in ORR activity. Among the work of other international groups, our iron-based catalyst comprises the highest density of FeN4 sites ever reported without interference of inorganic metal sites.
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Efficient preparation of composite materials consisting of ZIF-8 nanocrystals embedded inside the channels of macroporous anodic aluminum oxide membranes is reported. 1-D self-supported ZIF-8 superstructures are recovered through matrix dissolution. (Figure Presented).
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The preparation of graphitic oxide by methods described in the literature is time consuming and hazardous. A rapid, relatively safe method has been developed for preparing graphitic oxide from graphite in what is essentially an anhydrous mixture of sulfuric acid, sodium nitrate and potassium permanganate.
Article
Platinum-free oxygen reduction reaction (ORR) catalysts could help reduce the cost of future generations of polymer electrolyte membrane fuel cells (PEFCs). One class of non-precious catalyst for PEFCs are nanostructured Fe/C/N-based materials. In these, the nature of the active site is still hotly contested. Resolving this issue could lead to the development of better catalysts. One approach to achieve this is to study nitrogen-doped carbons, without any Fe content. Such materials have been studied, but largely in alkaline media where high activity is routinely obtained. Studies of metal-free catalysts in acid are rare, and Fe-contamination is often an issue. To truly shed light on the ORR mechanism of Fe/C/N-based catalysts, measurements on metal-free catalysts in acid media are required to simulate proton-based PEFC systems. Here we present synthesis of a metal-free defective nitrogen:doped graphene powder with remarkable surface area. We apply this as an ORR catalyst in acid medium and comment on the reaction mechanism. (C) 2014 The Electrochemical Society.
Article
The present work has conducted a comprehensive life-cycle analysis of energy consumption and greenhouse gas (GHG) emission for various fuel/vehicles systems. Focus is placed on the hydrogen-based fuel cell vehicle (FCV) technology, while the gasoline vehicle (GV) equipped with an internal combustion engine (ICE) serves as a reference technology. A fuel-cycle model developed at Argonne National Laboratory, the GREET model, is employed to evaluate the well-to-wheels (WTW) energy and emissions impacts caused by various fuel/vehicle systems. Six potential hydrogen pathways using renewable and non-renewable energy sources are simulated, namely, steam reforming of natural gas and corn ethanol, water electrolysis using grid generation and solar electricity, and coal gasification with and without carbon sequestration. Results showed that the FCVs fuelled with solar electrolysis hydrogen have the greatest benefits in energy conservation and GHG emission reduction. However, by incorporating with the economic consideration, hydrogen from the natural gas reforming is likely to be the primary mode of production for the initial introduction of FCVs.
Article
A low platinum loading model, considering both the platinum loading and platinum particle distribution on carbon support, is developed. This model takes into account the interfacial transport resistances at ionomer, water film and Pt particle surfaces in order to capture the effects of Pt loading and electrode composition on fuel cell performance. After coupling this electrode model into a comprehensive PEM fuel cell model, i.e. M2 model, experimental validation is performed for a wide range of Pt loading from 0.2 to 0.025 mg/cm2 for two electrode compositions with and without carbon dilution. Good agreement between the predicted and measured polarization curves is achieved under wide-ranging operating conditions. The agglomerate size effect is also examined and it is shown that the agglomerates have virtually no effect on cell performance for agglomerate radius smaller than 150 nm. Since in realistic fuel cell catalyst layers, agglomerates may not exist, or may only exist with sizes no larger than 150 nm based on SEM observations, the present work suggests that the standard homogeneous electrode model is suitable and sufficient for analyses of transport losses in PEM fuel cell electrodes where interfacial transport resistances exist.
Article
A two-step method consisting of solid-state microwave irradiation and heat treatment under NH3 gas was used to prepare nitrogen-doped reduced graphene oxide (N-RGO) with a high specific surface area (1007 m(2) g(-1) ), high electrical conductivity (1532 S m(-1) ), and low oxygen content (1.5 wt %) for electrical double-layer capacitor applications. The specific capacitance of N-RGO was 291 F g(-1) at a current density of 1 A g(-1) , and a capacitance of 261 F g(-1) was retained at 50 A g(-1) , which indicated a very good rate capability. N-RGO also showed excellent cycling stability and preserved 96 % of the initial specific capacitance after 100 000 cycles. Near-edge X-ray absorption fine-structure spectroscopy results provided evidenced for the recovery of π conjugation in the carbon networks with the removal of oxygenated groups and revealed chemical bonding of the nitrogen atoms in N-RGO. The good electrochemical performance of N-RGO is attributed to its high surface area, high electrical conductivity, and low oxygen content. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Article
The scientific community is focused on the development of inexpensive and high-performing membrane materials for proton exchange membrane (PEM) fuel cells (FCs). The major approach to reducing the cost of FCs, which is crucial for the widespread acceptance of FCs as energy sources for various practical applications, is reducing the cost of the membrane. Efforts are being made in the development of advanced polymeric materials, which will satisfy the technical and economic demands of the consumers. Because most alternative membranes are outperformed by Nafion membranes over an entire set of important properties, it may be worthwhile to compromise on certain parameters to develop alternative specialized membranes. This review presents the properties (mainly conductivity and chemical and mechanical stability) of modern solid polymer electrolytes (SPEs) for PEM FCs.
Article
Developing highly efficient catalysts for the oxygen reduction reaction (ORR) is key to the fabrication of commercially viable fuel cell devices and metal-air batteries for future energy applications. Herein, we review the most recent advances in the development of Pt-based and Pt-free materials in the field of fuel cell ORR catalysis. This review covers catalyst material selection, design, synthesis, and characterization, as well as the theoretical understanding of the catalysis process and mechanisms. The integration of these catalysts into fuel cell operations and the resulting performance/durability are also discussed. Finally, we provide insights into the remaining challenges and directions for future perspectives and research.
Article
Carbon supported platinum metal alloy catalysts (Pt–M/C) are widely used in low temperature fuel cells. Pt alloyed with first-row transition elements is used as improved cathode material for low temperature fuel cells. A major challenge for the application of Pt–transition metal alloys in phosphoric acid (PAFC) and polymer electrolyte membrane (PEMFC) fuel cells is to improve the stability of these binary catalysts. Dissolution of the non-precious metal in the acid environment can give rise to a decrease of the activity of the catalysts and to a worsening of cell performance. The purpose of this paper is to provide a better insight into the stability of these Pt–M alloy catalysts in the PAFC and PEMFC environments and the effect of the dissolution of the non-precious metal on the electrocatalytic activity of these materials, in the light of the latest advances on this field. Additionally, the durability of a PtCo/C cathode catalyst was evaluated by a short test in a single PEM fuel cell.
Article
Challenging precious Pt-based electrocatalysts for dye-sensitized solar cells (DSSCs), graphene nanoplatelets that are N-doped at the edges (NGnPs) are prepared via simply ball-milling graphite in the presence of nitrogen gas. DSSCs based on specific nanoplatelets designated "NGnP5" display superior photovoltaic performance (power conversion efficiency, 10.27%) compared to that of conventional Pt-based devices (9.96%). More importantly, the NGnP counter electrode exhibits outstanding electrochemical stability and electrocatalytic activity with a cobalt-complex redox couple.
Article
In the present work, direct hydroxylation of benzene to phenol by using in situ generated H2O2 with electricity cogeneration was carried out in a proton exchange membrane fuel cell (PEMFC) reactor. Phenol was produced only when there was current through the external circuit. No other organic products were detected during this electrochemical process. A rotating ring-disc electrode (RRDE) technique was used to quantitatively detect the intermediate H2O2 in an acid electrolyte solution at different potentials and temperatures. The RRDE studies showed that the in situ generated H2O2 may play a crucial role during the formation of phenol. The formation rate of phenol could be controlled by adjusting the current or cell potential.
Article
The mechanism of the solvent-free solid-state dibenzophenazine synthesis by dry co-grinding in a vibratory ball-mill has been investigated. The kinetics of the transformation was followed by HPLC and granulometry evolutions were quantified after co-grinding. The mechanism assumed involves a quinone imine intermediate formed during the first step of the reaction (addition of an amino group to a carbonyle) which is favored by the orbital overlaps between reagents. A water molecule formation occurs during the following step and hydrogen bonds are formed: the water molecule forms a bridge between the reactive centers of the quinone imine, and acts as a catalyst for the completion of the reaction. A push-pull mechanism involving the water bridge is proposed: the energy barrier is reduced by this way. Finally, two thermodynamic drivers favor the dibenzophenazine formation: the increased aromacity number in the product and the stabilization, thanks to water molecules.
Article
We present two different ways to fabricate nitrogen-doped graphene (N-graphene) and demonstrate its use as a metal-free catalyst to study the catalytic active center for the oxygen reduction reaction (ORR). N-graphene was produced by annealing of graphene oxide (G-O) under ammonia or by annealing of a N-containing polymer/reduced graphene oxide (RG-O) composite (polyaniline/RG-O or polypyrrole/RG-O). The effects of the N precursors and annealing temperature on the performance of the catalyst were investigated. The bonding state of the N atom was found to have a significant effect on the selectivity and catalytic activity for ORR. Annealing of G-O with ammonia preferentially formed graphitic N and pyridinic N centers, while annealing of polyaniline/RG-O and polypyrrole/RG-O tended to generate pyridinic and pyrrolic N moieties, respectively. Most importantly, the electrocatalytic activity of the catalyst was found to be dependent on the graphitic N content which determined the limiting current density, while the pyridinic N content improved the onset potential for ORR. However, the total N content in the graphene-based non-precious metal catalyst does not play an important role in the ORR process.
Article
Metal–organic frameworks (MOFs) have been shown to be excellent materials for storage of carbon dioxide, implying that they could be useful for removal of carbon dioxide from flue gas stacks, however their performance in industrially relevant swing adsorption processes for carbon capture has not been studied. Here we show that the efficacy of MOFs for carbon capture depends dramatically on the process and that some MOFs can provide significant carbon capture under typical pressure and vacuum swing processes. In particular, MOFs that possess coordinatively unsaturated metal centers offer as much as 9 mmol g−1 swing capacity under certain conditions. The results herein clearly show that there is no single ideal compound for carbon capture applications and that different materials can perform better or worse depending on the specific process conditions. In addition to their capture performances, we have also investigated their selectivity to carbon dioxide over that of nitrogen and methane. The analysis provided clearly demonstrates that the performance of a given MOF cannot be determined without also considering the detailed industrial process in which the MOF is to be applied.
Article
A significant advance toward achieving practical applications of graphene as a two-dimensional material in nanoelectronics would be provided by successful synthesis of both n-type and p-type doped graphene. However reliable doping and a thorough understanding of carrier transport in the presence of charged impurities governed by ionized donors or acceptors in the graphene lattice are still lacking. Here we report experimental realization of few-layer nitrogen-doped (N-doped) graphene sheets by chemical vapor deposition of organic molecule 1, 3, 5-triazine on Cu metal catalyst. By reducing the growth temperature, the atomic percentage of nitrogen doping is raised from 2.1 % to 5.6 %. With increasing doping concentration, N-doped graphene sheet exhibits a crossover from p-type to n-type behavior accompanied by a strong enhancement of electron-hole transport asymmetry, manifesting the influence of incorporated nitrogen impurities. In addition, by analyzing the data of X-ray photoelectron spectroscopy, Raman spectroscopy, and electrical measurements, we show that pyridinic and pyrrolicN impurities play an important role in determining the transport behavior of carriers in N-doped graphene sheets.
Article
Insufficient catalytic activity and durability are key barriers to the commercial deployment of low temperature polymer electrolyte membrane (PEM) and direct-methanolfuelcells (DMFCs). Recent observations suggest that carbon-based catalyst support materials can be systematically doped with nitrogen to create strong, beneficial catalyst-support interactions which substantially enhance catalyst activity and stability. Data suggest that nitrogen functional groups introduced into a carbon support appear to influence at least three aspects of the catalyst/support system: 1) modified nucleation and growth kinetics during catalystnanoparticle deposition, which results in smaller catalyst particle size and increased catalyst particle dispersion, 2) increased support/catalyst chemical binding (or “tethering”), which results in enhanced durability, and 3) catalystnanoparticle electronic structure modification, which enhances intrinsic catalytic activity. This review highlights recent studies that provide broad-based evidence for these nitrogen-modification effects as well as insights into the underlying fundamental mechanisms.
Article
The temperature rise of powder accompanying with heavy plastic deformation during ball milling is expected to play an important role in determining the kinetics of synthesising and the properties of the final products. In this paper, temperature rise of powder during milling was estimated by using a microforging model. Relationships between the temperature rise and the deformation work of powder and between the temperature rise of mill container and the consumed power during ball milling have been found out. The results show that the temperature rise of mill container increased rapidly with the increased milling speed, and the temperature rise of powder increased with increasing milling speed and the weight ratio of ball to powder. The temperature of mill container calculated was in accordance with tested temperature of the mill container. The maximum temperature rise of powder ΑDTmax is not above 125 K under the maximum milling intensity.
Article
As fuel cell technology matures and time scale to commercialization decreases, the need for a more comprehensive knowledge of materials’ aging mechanisms is essential to attain specified lifetime requirements for applications. In this work, the membrane electrode assembly (MEA) degradation of an eight-cell PEM low power stack was evaluated, during and after fuel cell aging in specified testing conditions of load-cycling that may compromise the durability of the catalyst. The stack degradation analysis comprised observation of catalytic layers, morphology and composition. Examination of the MEAs cross sections, in a joint SEM and TEM study, revealed thickness variation of catalytic layer (up to 47% for the cathode layers), and cracking, delamination, and catalyst migration were observed even though catalyst sintering and consequent loss of electrochemical active area seem to be predominant together with F loss from the ionomer used as binder in the catalytic layers.
Article
A new kind of nitrogen-doped graphene/carbon nanotube nanocomposite can be synthesized by a facile hydrothermal process under mild conditions, which exhibits synergistically enhanced electrochemical activity for the oxygen reduction reaction. This research provides a new route to access a metal-free electrocatalyst with a high activity under mild conditions.
Article
Edge-selectively functionalized graphene nanoplatelets (EFGnPs) with different functional groups were effi-ciently prepared simply by dry ball-milling graphite in the presence of hydrogen, carbon dioxide, sulfur trioxide, or carbon dioxide/sulfur trioxide mixture. Upon exposure to air moisture, the resultant hydrogen- (HGnP), carboxylic acid- (CGnP), sulfonic acid- (SGnP), and carboxylic acid/sulfonic acid- (CSGnP) functionalized GnPs readily dispersed into various polar solvents, including neutral water. The resultant EFGnPs were then used as electrocatalysts for oxygen reduction reaction (ORR) in an alkaline electrolyte. It was found that the edge polar nature of the newly-prepared EFGnPs without heteroatom doping into their basal plane played an important role in regulating the ORR efficiency with the electrocatalytic activity in the order of SGnP > CSGnP > CGnP > HGnP > pristine graphite. More importantly, the sulfur-containing SGnP and CSGnP were found to have a superior ORR performance to commercially available platinum-based electrocatalyst (Pt/C).
Article
The high cost of platinum-based electrocatalysts for the oxygen reduction reaction (ORR) has hindered the practical application of fuel cells. Thanks to its unique chemical and structural properties, nitrogen-doped graphene (NG) is among the most promising metal-free catalysts for replacing platinum. In this work, we have developed a cost-effective synthesis of NG by using cyanamide as a nitrogen source and graphene oxide as a precursor, which led to high and controllable nitrogen contents (4.0% to 12.0%) after pyrolysis. NG thermally treated at 900 °C shows a stable methanol crossover effect, high current density (6.67 mA cm(-2)), and durability (∼87% after 10 000 cycles) when catalyzing ORR in alkaline solution. Further, iron (Fe) nanoparticles could be incorporated into NG with the aid of Fe(III) chloride in the synthetic process. This allows one to examine the influence of non-noble metals on the electrocatalytic performance. Remarkably, we found that NG supported with 5 wt % Fe nanoparticles displayed an excellent methanol crossover effect and high current density (8.20 mA cm(-2)) in an alkaline solution. Moreover, Fe-incorporated NG showed almost four-electron transfer processes and superior stability in both alkaline (∼94%) and acidic (∼85%) solutions, which outperformed the platinum and NG-based catalysts.
Article
Graphene and its derivatives are attractive for electrocatalytical application in fuel cells because of their unique structures and electronic properties. The electrocatalytical mechanism of nitrogen doped graphene in acidic environment was studied by using density functional theory (DFT). The simulations demonstrate that the oxygen reduction reaction (ORR) on N-doped graphene is a direct four-electron pathway, which is consistent with the experimental observations. The energy calculated for each ORR step shows that the ORR can spontaneously occur on the N-graphene. The active catalytical sites on single nitrogen doped graphene are identified, which have either high positive spin density or high positive atomic charge density. The nitrogen doping introduces asymmetry spin density and atomic charge density, making it possible for N-graphene to show high electroncatalytic activities for the ORR.
Article
In this work, CNTs have been bombarded with low energy N ions. X-ray photoelectron spectroscopy have been used to determine the binding configuration of the N-doped CNTs. AFM was also used to determine their morphology and mechanical properties. The same morphology is maintained after the N2+ bombardment. XPS analysis shows that the N 1s core level spectra for N-doped CNTs can be deconvoluted in terms of two peaks related to sp2 (C–N=C) and sp3 hybridization (C–N configuration). This interpretation is in concordance with the increase of the hardness observed by AFM nanoindentations when the sp3 contributions increase. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Article
Hollow carbon nanowires (HCNWs) were prepared through pyrolyzation of a hollow polyaniline nanowire precursor. The HCNWs used as anode material for Na-ion batteries deliver a high reversible capacity of 251 mAh g(-1) and 82.2% capacity retention over 400 charge-discharge cycles between 1.2 and 0.01 V (vs Na(+)/Na) at a constant current of 50 mA g(-1) (0.2 C). Excellent cycling stability is also observed at an even higher charge-discharge rate. A high reversible capacity of 149 mAh g(-1) also can be obtained at a current rate of 500 mA g(-1) (2C). The good Na-ion insertion property is attributed to the short diffusion distance in the HCNWs and the large interlayer distance (0.37 nm) between the graphitic sheets, which agrees with the interlayered distance predicted by theoretical calculations to enable Na-ion insertion in carbon materials.
Article
Different C–N bonding configurations in nitrogen (N) doped carbon materials have different electronic structures. Carbon materials doped with only one kind of C–N bonding configuration are an excellent platform for studying doping effects on the electronic structure and physical/chemical properties. Here we report synthesis of single layer graphene doped with pure pyridinic N by thermal chemical vapour deposition of hydrogen and ethylene on Cu foils in the presence of ammonia. By adjusting the flow rate of ammonia, the atomic ratio of N and C can be modulated from 0 to 16%. The domain like distribution of N incorporated in graphene was revealed by the imaging of Raman spectroscopy and time-of-flight secondary ion mass spectrometry. The ultraviolet photoemission spectroscopy investigation demonstrated that the pyridinic N efficiently changed the valence band structure of graphene, including the raising of density of p states near the Fermi level and the reduction of work function. Such pyridinic N doping in carbon materials was generally considered to be responsible for their oxygen reduction reaction (ORR) activity. The 2e reduction mechanism of ORR on our CN x graphene revealed by rotating disk electrode voltammetry indicated that the pyridinic N may not be an effective promoter for ORR activity of carbon materials as previously expected.