Efficient Metal-Free Electrocatalysts for Oxygen Reduction: Polyaniline-Derived N- and O-Doped Mesoporous Carbons

Role of Pyridinic-N for Nitrogen-doped graphene quantum dots in oxygen reaction reduction

Nitrogen-doped graphene quantum dots (N-GQDs) exhibit exciting properties in the oxygen reduction reaction (ORR) for ample electrocatalytic edging and N-doped active sites. However, low yield and high dispersity of N-GQDs prohibit their direct application as the electrocatalyst. In this paper, two facile hydrothermal progress were developed to synthesize the high-yield N-GQDs with the diameter of ca. 2-6nm and the hybrid of N-GQDs/Reduced Graphene Oxide (N-GQDs/r-GO). The results demonstrated that the N-GQDs/r-GO display remarkable electrocatalytic activity. Moreover, it can be found that the pyridinic-N plays a major role in ORR. Both the average electron transfer number and the onset potential depend on the content of pyridinic-N. The proposed synthesis strategy is facile and low-cost, serving as a feasible method for the development of highly efficient electrocatalysts.

Double Soft-Template Synthesis of Nitrogen/Sulfur-Codoped Hierarchically Porous Carbon Materials Derived from Protic Ionic Liquid for Supercapacitor

Heteroatom-doped hierarchical porous carbon materials derived from the potential precursors and prepared by a facile, effective, and low-pollution strategy have recently been particularly concerned in different research fields. In this study, the interconnected nitrogen/sulfur-codoped hierarchically porous carbon materials have been successfully obtained via one-step carbonization of the self-assembly of [Phne][HSO₄] (a protic ionic liquid originated from dilute sulfuric acid and phenothiazine by a straightforward acid-base neutralization) and the double soft-template of OP-10 and F-127. During carbonization process, OP-10 as macroporous template and F-127 as mesoporous template were removed, while [Phne][HSO₄] not only could be used as carbon, nitrogen, and sulfur source, but also as a pore forming agent to create micropores. The acquired carbon materials for supercapacitor not only hold a large specific capacitance of 302 F g^-1 even at 1.0 A g^-1, but also fine rate property with 169 F g^-1 at 10 A g^-1 and excellent capacitance retention of nearly 100% over 5000 circulations in 6 M KOH electrolyte. Furthermore, carbon materials also present eximious rate performance with 70% in 1 M Na₂SO₄ electrolyte.

Heteroatom-Doped Carbon Materials for Electrocatalysis

Fuel cells, water electrolyzers, and metal-air batteries are important energy systems that have started to play some roles in our renewable energy landscapes. However, despite much research works carried out on them, they have not yet found large-scale applications, mainly due to the unavailability of sustainable catalysts that can catalyze the reactions employed in them. Currently, noble metal-based materials are the ones that are commonly used as catalysts in most commercial fuel cells, electrolyzers, and metal-air batteries. Hence, there has been considerable research efforts worldwide to find alternative noble metal-free and metal-free catalysts composed of inexpensive, earth-abundant elements for use in the catalytic reactions employed in these energy systems. In this concept paper, a brief introduction on catalysis in renewable energy systems, followed by the recent efforts to develop sustainable, heteroatom-doped carbon and non-noble metal-based electrocatalysts, the challenges to unravel their structure-catalytic activity relationships, and the authors' perspectives on these topics and materials, are discussed.

Three-Dimensional Multi-Doped Porous Carbon/Graphene Derived from Sewage Sludge with Template-Assisted Fe-pillared Montmorillonite for Enhanced Oxygen Reduction Reaction

Three-dimensional multi-doped porous carbon/graphene (Fe-Mt-SS-C) was prepared by carbonization of sewage sludge with template-assisted Fe-pillared montmorillonite. The material consisted of nanosheet- and particle- carbon had a high specific surface area (784.46 m²·g⁻¹) and hierarchical porous structure of micro-, meso- and macropores. The prepared Fe-Mt-SS-C had a high degree of graphitization and large amount of defect atoms. The pyrolysis process made full use of the C, N, Fe, and S by turning them into the carbon framework of the as-obtained material in situ. Template-assisted Fe-pillared montmorillonite contributed to more characteristics of morphology and composition on Fe-Mt-SS-C than other three materials (SS-C, Mt-SS-C and Fe-SS-C), and enhanced the electrocatalytic ORR activity by providing more adsorption sites and the electronic structure, resulting in the increase of conductivity and electrochemical activity. The ORR activity performance of Fe-Mt-SS-C, including the value of onset potential (0.03 V) and E_1/2 (−0.09 V), was better than that of commercial 20 wt% Pt/C (−0.02 V and −0.18 V, respectively). Moreover, the Fe-Mt-SS-C possessed excellent durability and outstanding immunity toward methanol crossover effects. Therefore, the resultant Fe-Mt-SS-C has great potential to applied as a high-efficiency ORR electrocatalyst, more importantly, it realizes the utilization of the sludge at the same time.

Nitrogen-Doped Graphitic Porous Carbon Nanosheets Derived from In Situ Formed g-C3 N4 Templates for the Oxygen Reduction Reaction

Heteroatom-doped carbon materials have been considered as potential substitutes for Pt-based electrocatalysts for the oxygen reduction reaction (ORR) in alkaline fuel cells. Here we report the synthesis of oxygen-containing nitrogen-doped carbon (ONC) nanosheets through the carbonization of a mixture that contained glucose and dicyandiamide (DCDA). In situ formed graphitic carbon nitride (g-C₃ N₄ ) derived from DCDA provided a nitrogen-rich template, thereby facilitating the formation of ONC nanosheets. The resultant ONC materials with high nitrogen content, high specific surface areas, and highly mesoporous total volume displayed excellent electrochemical performance, including a similar ORR onset potential, half-potential, a higher diffusion-limited current, and excellent tolerance to methanol than that of the commercial Pt/C catalyst, respectively. Moreover, the ONC-850 nanosheet displayed high long-term durability even after 1000 cycles as well as a high electron transfer number of 3.92 (4.0 for Pt/C). Additionally, this work provides deeper insight into these materials and a versatile strategy for the synthesis of cost-effective 2D N-doped carbon electrocatalysts.

Surface-rough Fe-N/C composite wrapped on carbon nanotubes as efficient electrocatalyst for oxygen reduction reaction

Fe-N/C composites are considered one of the most promising non-precious-metal electrocatalysts for oxygen reduction reaction (ORR). In this paper, we fabricate a novel and efficient carbon nanotube (CNT)-supported Fe-N/C composite catalyst, via the surface-self-polymerization of polydopamine and then the incorporation with Fe species on CNTs, followed by the pyrolysis process. The obtained catalyst demonstrates excellent electrocatalytic performance towards ORR in alkaline media. The modification of Fe-incorporated nitrogen-rich-carbons (Fe-CN_x) on CNTs lowers the ORR half-wave-potential by ∼190 mV, giving this catalyst with an onset ORR potential of 0.95 V (versus reversible hydrogen electrode (RHE)), a half-wave potential of 0.82 V (versus RHE), and the limiting current density of 5.39 mA cm^-2 in 0.1 M KOH. The performance of the as-prepared catalyst is comparatively better than the commercially available Pt/C in terms of positive half-wave potential and larger limiting current, superior durability, and higher tolerance to the methanol.

An Efficient Bifunctional Electrocatalyst for a Zinc-Air Battery Derived from Fe/N/C and Bimetallic Metal-Organic Framework Composites

Efficient bifunctional electrocatalysts with desirable oxygen activities are closely related to practical applications of renewable energy systems including metal-air batteries, fuel cells, and water splitting. Here a composite material derived from a combination of bimetallic zeolitic imidazolate frameworks (denoted as BMZIFs) and Fe/N/C framework was reported as an efficient bifunctional catalyst. Although BMZIF or Fe/N/C alone exhibits undesirable oxygen reaction activity, a combination of these materials shows unprecedented ORR (half-wave potential of 0.85 V as well as comparatively superior OER activities (potential@10 mA cm^-2 of 1.64 V), outperforming not only a commercial Pt/C electrocatalyst but also most reported bifunctional electrocatalysts. We then tested its practical application in Zn-air batteries. The primary batteries exhibit a high peak power density of 235 mW cm^-2, and the batteries are able to be operated smoothly for 100 cycles at a curent density of 10 mA cm^-2. The unprecedented catalytic activity can be attritued to chemical coupling effects between Fe/N/C and BMZIF and will aid the development of highly active electrocatalysts and applications for electrochemical energy devices.

Iron–nitrogen co-doped hierarchically mesoporous carbon spheres as highly efficient electrocatalysts for the oxygen reduction reaction

Iron and nitrogen co-doped hierarchically mesoporous carbon spheres (Fe–N–CS) were successfully prepared by using hierarchically mesoporous silica spheres as hard template.

Protic salt-based nitrogen-doped mesoporous carbon for simultaneous electrochemical detection of Cd(ii) and Pb(ii)

Nitrogen-doped mesoporous carbon (NMC) derived from a single small-molecule protic salt (p-phenylenediamine bisulfate) is used for sensing toxic heavy metal ions.

Nitrogen doped nanoporous graphene: an efficient metal-free electrocatalyst for the oxygen reduction reaction

Nitrogen doped nanoporous graphene electrocatalysts have been successfully prepared and are shown to be useful electrocatalysts towards the ORR.

Mesoporous C, N-codoped TiO2 hybrid shells with enhanced visible light photocatalytic performance

Mesoporous C, N-codoped TiO₂ (C/N-TiO₂) hybrid shells incorporated with graphite carbon were synthesized. Catalytic results indicated that C and N codoping could remarkably improve the photocatalytic performance of TiO₂.

Pyridinic and pyrrolic nitrogen-rich ordered mesoporous carbon for efficient oxygen reduction in microbial fuel cells

Pyridinic and pyrrolic nitrogen-rich, ordered mesoporous carbon was used as the cathode catalyst in MFCs and showed excellent power generation capabilities.

Capacitive performance of ruthenium-coordinated polypyrrole

Ruthenium coordinated polypyrrole is developed as an active electrode material for supercapacitors.

Advances, challenges and promises of carbon dots

Carbon-dots with unique physical and chemical properties have versatile applications in environmental and energy fields.

The mechanism change by switching the reactants from water to hydroxyl ions for electrocatalytic water oxidation: a case study of copper oxide microspheres

The as-obtained CuO microspheres can serve as an active and stable water oxidation catalyst under electrochemical reaction conditions and operate at modest overpotential providing an alternative to the Co-WOC catalyst for applications in solar energy storage.

Hybrid of g-C3N4 Assisted Metal-Organic Frameworks and Their Derived High-Efficiency Oxygen Reduction Electrocatalyst in the Whole pH Range

A highly active electrocatalyst in the whole pH range for oxygen reduction reaction (ORR) is produced by employing the g-C₃N₄ assisted metal-organic frameworks (MOF) of C₃N₄@NH₂-MIL-101 as the precursor. By pyrolyzing the hybrid at 700 °C, the C₃N₄@NH₂-MIL-101 could be easily transformed into an abundant iron and nitrogen codoped porous carbon skeleton. The selective use of g-C₃N₄ as a support template plays a critical role in facilitating the formation of the architecture with high surface area and rich N content. The obtained catalyst of C₃N₄@NH₂-MIL-101-700 manifested remarkable oxygen reduction activity over the pH 0-14. Noteworthy, the catalyst displayed outstanding ORR activity with more positive half-wave potential than that of the commercial Pt/C catalyst in both alkaline and neutral conditions. Additionally, the optimal C₃N₄@NH₂-MIL-101-700 also exhibited prominent ORR activity which is almost equal to that of commercial Pt/C in acidic electrolyte with high selectivity and very low H₂O₂ yield. Most importantly, the better methanol tolerance and much higher stability than the commercial Pt/C of C₃N₄@NH₂-MIL-101-700 no matter under alkaline, neutral, or acid conditions further demonstrate the catalyst to be a promising candidate for practical electrocatalytic applications.

In-Situ-Activated N-Doped Mesoporous Carbon from a Protic Salt and Its Performance in Supercapacitors

Protic salts have been recently recognized to be an excellent carbon source to obtain highly ordered N-doped carbon without the need of tedious and time-consuming preparation steps that are usually involved in traditional polymer-based precursors. Herein, we report a direct co-pyrolysis of an easily synthesized protic salt (benzimidazolium triflate) with calcium and sodium citrate at 850 °C to obtain N-doped mesoporous carbons from a single calcination procedure. It was found that sodium citrate plays a role in the final carbon porosity and acts as an in situ activator. This results in a large surface area as high as 1738 m²/g with a homogeneous pore size distribution and a moderate nitrogen doping level of 3.1%. X-ray photoelectron spectroscopy (XPS) measurements revealed that graphitic and pyridinic groups are the main nitrogen species present in the material, and their content depends on the amount of sodium citrate used during pyrolysis. Transmission electron microscopy (TEM) investigation showed that sodium citrate assists the formation of graphitic domains and many carbon nanosheets were observed. When applied as supercapacitor electrodes, a specific capacitance of 111 F/g in organic electrolyte was obtained and an excellent capacitance retention of 85.9% was observed at a current density of 10 A/g. At an operating voltage of 3.0 V, the device provided a maximum energy density of 35 W h/kg and a maximum power density of 12 kW/kg.

Ordered mesoporous carbons codoped with nitrogen and iron as effective catalysts for oxygen reduction reaction

Doping with foreign atoms is an effective approach to significantly enhance the catalytic performance of carbon materials for oxygen reduction reaction (ORR). In this paper, a colloidal silica template method was employed to synthesize nitrogen and iron codoped ordered mesoporous carbon for ORR electrocatalysis. The carbon materials were thoroughly characterized by transmission electron microscopy, Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy measurements. The porosity was quantified by nitrogen adsorption/desorption measurements that showed the formation of ordered mesoporous structures with a BET specific surface area up to 953.8 m² g^-1 and the mesopores mostly centered at ca. 25 nm, close to the size of the colloidal silica. The resulting mesoporous carbon exhibited apparent ORR activity in alkaline media, which was highly comparable to that of commercial Pt/C (20 wt%), with the onset potential at +0.99 V vs. RHE. This was ascribed largely to nitrogen dopants, with additional contributions from the trace amounts of iron dopants, and the reactions appeared to be facilitated by the formation of a mesoporous structure. Moreover, the mesoporous carbon showed better stability, resistance against fuel crossover, and selective activity than Pt/C. This work demonstrates a new paradigm for the preparation of heteroatom-doped carbon materials that are promising alternatives to Pt-based catalysts for fuel cells.

Surface-nitrogen-rich ordered mesoporous carbon as an efficient metal-free electrocatalyst for oxygen reduction reaction

Exploring efficient metal-free electrocatalysts for oxygen reduction reactions (ORR) will have a great impact on the field of fuel cells and metal-air batteries. In this paper, we report a simple and efficient routine to coat ordered mesoporous carbon (CMK-3) with nitrogen-doped carbon via pyrolysis of the surface-self-polymerized polydopamine. The optimized CMK-3 catalyst with a coating of nitrogen-doped carbon demonstrates excellent electrocatalytic activity towards ORR in alkaline media. The coating procedure under optimized conditions lowers the ORR half-wave-potential by 80 mV, giving a genuine metal-free catalyst with an onset ORR potential of 0.96 V (vs reversible hydrogen electrode (RHE)) and half-wave potential of 0.83 V (vs RHE) in 0.1 M KOH, which is much better than other carbon material-based catalysts (such as carbon nanotubes and their composites). The performance of this surface-nitrogen-rich CMK-3 catalyst is also superior to that of N-doped ordered mesoporous carbon synthesized by means of the 'nanocasting' technique. Furthermore, the as-prepared catalyst performs comparably in terms of activity, superior durability, and higher tolerance to methanol compared with commercially available Pt/C.

Graphene Oxide Sheathed ZIF-8 Microcrystals: Engineered Precursors of Nitrogen-Doped Porous Carbon for Efficient Oxygen Reduction Reaction (ORR) Electrocatalysis

Nitrogen containing mesoporous carbon obtained by the pyrolysis of graphene oxide (GO) wrapped ZIF-8 (Zeolitic Imidazolate Frameworks-8) micro crystals is demonstrated to be an efficient catalyst for the oxygen reduction reaction (ORR). ZIF-8 synthesis in the presence of GO sheets helped to realize layers of graphene oxide over ZIF-8 microcrystals and the sphere-like structures thus obtained, on heat treatment, transformed to highly porous carbon with a nitrogen content of about 6.12% and surface area of 502 m²/g. These catalysts with a typical micromeso porous architecture exhibited an onset potential of 0.88Vvs RHE in a four electron pathway and also demonstrated superior durability in alkaline medium compared to that of the commercial Pt/C catalyst. The N-doped porous carbon derived from GO sheathed ZIF-8 core-shell structures could therefore be employed as an efficient electrocatalyst for fuel cell applications.

Layered Transition Metal Oxynitride Co3Mo2OxN6-x/C Catalyst for Oxygen Reduction Reaction

Transition metal oxynitrides have now garnered growing interest in our quest for highly efficient alternatives to Pt in direct methanol alkaline fuel cells. Herein, carbon supported Co₃Mo₂O_xN_6-x was synthesized via a simple two-step approach wherein the reactants undergo refluxing and heat treatment in NH₃. For the as-prepared Co₃Mo₂O_xN_6-x catalyst, uniformly dispersed on XC-72, with the particle size averaged at 5 nm, the catalytic activities toward oxygen reduction reaction in alkaline media are related to the commercial Pt/C, such as the comparable onset potential (0.9 V vs RHE), half-wave potential (0.8 V vs RHE), and even higher specific activity (82.7 mA cm^-2 at 0.7 V). Significantly, the Co₃Mo₂O_xN_6-x catalyst was highly stable in terms of 95% current retention after 12 h chronoamperometry measurement, indicative of favorable prospect for the non-noble cathodic catalyst in alkaline fuel cell.

Porous N-Doped Carbon Prepared from Triazine-Based Polypyrrole Network: A Highly Efficient Metal-Free Catalyst for Oxygen Reduction Reaction in Alkaline Electrolytes

Metal-free N-doped carbon (NC) materials have been regarded as one of the most promising catalysts for the oxygen reduction reaction (ORR) in alkaline media because of their outstanding ORR catalytic activity, high stability, and good methanol tolerance. Up to now, only a small minority of such catalysts have been synthesized from triazine-based polymeric networks. Herein, we report the synthesis of such NC catalyst by directly pyrolyzing a nitrogen-rich, triazine-based polypyrrole network (TPN). The TPN is fabricated by oxidative polymerization of 2,4,6-tripyrrol-1,3,5-triazine monomer using TfOH as the protonating agent and benzoyl peroxide as the oxidizing agent. The obtained NC-900 (pyrolyzed at 900 °C) catalyst exhibits excellent ORR activity in alkaline media with a high ORR onset potential (0.972 V vs RHE), a large kinetic-limiting current density (15.66 mA cm^-2 at 0.60 V), and good MeOH tolerance and durability. The as-synthesized NC-900 material is a potential candidate as a highly active, stable, and low-cost ORR catalyst for alkaline fuel cells.

Mesoporous NiCo2O4 Nanoplates on Three-Dimensional Graphene Foam as an Efficient Electrocatalyst for the Oxygen Reduction Reaction

Catalysts for the oxygen reduction reaction (ORR) are highly important in fuel cells and metal-air batteries. Cheap ORR catalysts with ultrahigh electrochemical activity, selectivity, and stability are extremely desirable but still remain challenging. Herein, mesoporous NiCo₂O₄ nanoplate (NP) arrays on three-dimensional (3D) graphene foam are shown to be a highly economical ORR catalyst. This mesoporous mixed-valence oxide can provide more electrocatalytic active sites with increased accessible surface area. In addition, graphene-foam-supported NiCo₂O₄ NP arrays have a 3D hierarchical porous structure, which is of great benefit to ion diffusion and electron transfer. As a result, the mesoporous NiCo₂O₄ NP arrays/graphene foam catalyst exhibits outstanding ORR performance with the four-electron reduction of O₂ to H₂O in alkaline media. Furthermore, the mesoporous catalyst shows enhanced electrocatalytic activity with a half-wave potential of 0.86 V vs RHE and better stability compared with a commercial Pt/C catalyst.

Heteroatom-Doped Carbon Nanostructures Derived from Conjugated Polymers for Energy Applications

Heteroatom-doped carbon materials have been one of the most remarkable families of materials with promising applications in fuel cells, supercapacitors, and batteries. Among them, conjugated polymer (CP)-derived heteroatom-doped carbon materials exhibit remarkable electrochemical performances because the heteroatoms can be preserved at a relatively high content and keep stable under harsh working conditions. In this review, we summarized recent advances in the rational design and various applications of CP-derived heteroatom-doped carbon materials, including polyaniline (PANI), polypyrrole (PPy), and their ramification-derived carbons, as well as transition metal-carbon nanocomposites. The key point of considering CP-derived heteroatom-doped carbon materials as important candidates of electrode materials is that CPs contain only nonmetallic elements and some key heteroatoms in their backbones which provide great chances for the synthesis of metal-free heteroatom-doped carbon nanostructures. The presented examples in this review will provide new insights in designing and optimizing heteroatom-doped carbon materials for the development of anode and cathode materials for electrochemical device applications.

Tuning the Catalytic Activity of a Metal-Organic Framework Derived Copper and Nitrogen Co-Doped Carbon Composite for Oxygen Reduction Reaction

An efficient non-noble metal catalyst for the oxygen reduction reaction (ORR) is of great importance for the fabrication of cost-effective fuel cells. Nitrogen-doped carbons with various transition metal co-dopants have emerged as attractive candidates to replace the expensive platinum catalysts. Here we report the preparation of various copper- and nitrogen-doped carbon materials as highly efficient ORR catalysts by pyrolyzing porphyrin based metal organic frameworks and investigate the effects of air impurities during the thermal carbonization process. Our results indicate that the introduction of air impurities can significantly improve ORR activity in nitrogen-doped carbon and the addition of copper co-dopant further enhances the ORR activity to exceed that of platinum. Systematic structural characterization and electrochemical studies demonstrate that the air-impurity-treated samples show considerably higher surface area and electron transfer numbers, suggesting that the partial etching of the carbon by air leads to increased porosity and accessibility to highly active ORR sites. Our study represents the first example of using air or oxygen impurities to tailor the ORR activity of metal and nitrogen co-doped carbon materials and open up a new avenue to engineer the catalytic activity of these materials.

Growth of MoSe2 nanosheets with small size and expanded spaces of (002) plane on the surfaces of porous N-doped carbon nanotubes for hydrogen production

MoSe₂ nanosheets with small size and expanded spaces between the (002) planes were grown on the surfaces of porous N-doped carbon nanotubes (NCNTs) with much higher HER activity than carbon nanotubes without N dopants. Owing to the synergistic effects between the MoSe₂ nanosheets and the porous NCNT substrate, MoSe₂/NCNTs exhibit superior HER activity to layered metal chalcogenides reported previously.

Metal-Free and Noble Metal-Free Heteroatom-Doped Nanostructured Carbons as Prospective Sustainable Electrocatalysts

The large-scale deployment of many types of fuel cells and electrolyzers is currently constrained by the lack of sustainable and efficient catalysts that can replace the less earth-abundant, noble metal-based catalysts, which are commonly used in these renewable energy systems. This burgeoning issue has led to explosive research efforts worldwide to find alternative, metal-free and noble metal-free catalysts that are composed of inexpensive and earth-abundant elements. Hence, the recent discoveries that doping carbon nanomaterials with heteroatoms (such as N, S, B, etc.) can give sustainable materials with good electrocatalytic activity for reactions carried out in fuel cells and electrolyzers have been not only quite exciting but also very promising to address these challenging issues. Interestingly, even though they contain no metals or involve only the inexpensive, more earth-abundant ones, the catalytic activity of some of these materials fares well with those of the commercially used noble metal-based electrocatalysts, such as Pt/C. However, research efforts to improve the catalytic activity, selectivity, and stability of some of these materials for various reactions are still necessary and thus continuing. While some of these efforts have focused on finding synthetic methods that can tune the structures and compositions of already known materials and thereby improve their catalytic properties (activity, selectivity, stability, etc.), others have focused on developing entirely new materials that can exhibit better or superior catalytic properties. In these efforts, additional considerations are also being paid to find facile synthetic routes or renewable and inexpensive precursors that can lead to such types of catalysts in order to make the entire process highly sustainable and widely applicable. In this Account, notable heteroatom-doped carbon catalysts that have been developed for reactions in fuel cells and water electrolyzers, the various synthetic procedures employed to make them, and the challenges involved in their synthesis as well as their characterizations are discussed. The methods used to systematically vary the structures and compositions of the precursors of these materials, as well as the materials themselves, and the different experimental, imaging, and spectroscopic methods used to investigate the properties and structure-property relationships of the materials for various energy related reactions are also included. The discussions focus mainly around the recent notable results reported on these materials by the author's and other research groups worldwide, albeit not exhaustively. Finally, the author's perspective about the challenges remaining in the field that need to be addressed, the many existing unanswered questions that beg for more research, and the future prospects for research related to the above topics are also mentioned.

Cobalt oxide nanosheets wrapped onto nickel foam for non-enzymatic detection of glucose

Ultra-sensitive and highly selective detection of glucose is essential for the clinical diagnosis of diabetes. In this paper, an ultra-sensitive glucose sensor was successfully fabricated based on cobalt oxide (Co3O4) nanosheets directly grown on nickel foam through a simple hydrothermal method. Characterizations indicated that the Co3O4 nanosheets are completely and uniformly wrapped onto the surface of nickel foam to form a three-dimensional heterostructure. The resulting self-standing electrochemical electrode presents a high performance for the non-enzymatic detection of glucose, including short response time (<10 s), ultra-sensitivity (12.97 mA mM(-1) cm(-2)), excellent selectivity and low detection limit (0.058 μM, S/N = 3). These results indicate that Co3O4 nanosheets wrapped onto nickel foam are a low-cost, practical, and high performance electrochemical electrode for bio sensing.

Synergistically Enhanced Electrocatalytic Activity of Sandwich-like N-Doped Graphene/Carbon Nanosheets Decorated by Fe and S for Oxygen Reduction Reaction

Although N-doped graphene-based electrocatalysts have shown good performance for oxygen reduction reaction (ORR), they still suffer from the single-type active site in the as-prepared catalyst, limited accessible active surface area because of easy aggregation of graphene, and harsh condition for preparation process of graphene. Therefore, further developing a novel type of graphene-based electrocatalyst by a facile and environmentally benign method is highly anticipated. Herein, we first fabricate a sandwich-like graphene/carbon hybrid using graphene oxide (GO) and nontoxic starch. Then the graphene/carbon hybrid undergoes postprocessing with iron(III) chloride (FeCl3) and potassium sulfocyanide (KSCN) to acquire N-doped graphene/carbon nanosheets decorated by Fe and S. The resultant displays the features of interpenetrated three-dimensional hierarchical architecture composed of abundant sandwich-like graphene/carbon nanosheets and low graphene content in as-prepared sample. Remarkably, the obtained catalyst possesses favorable kinetic activity due to the unique structure and synergistic effect of N, S, and Fe on ORR, showing high onset potential, low Tafel slope, and nearly four-electron pathway. Meanwhile, the catalyst exhibits strong methanol tolerance and excellent long-term durability. In view of the multiple active sites, unique hierarchical structure, low graphene content, and outstanding electrochemical activity of the as-prepared sample, this work could broaden the thinking to develop more highly efficient graphene/carbon electrocatalysts for ORR in fuel cells.

Recent Strategies for Improving the Catalytic Activity of 2D TMD Nanosheets Toward the Hydrogen Evolution Reaction

Two-dimensional (2D) transition-metal dichalcogenide (TMD) nanosheets have emerged as a fascinating new class of materials for catalysis. These nanosheets are active for several important catalysis reactions including hydrogen evolution from water. The rich chemistry of TMDs combined with numerous strategies that allow tuning of their electronic properties make these materials very attractive for understanding the fundamental principles of electro- and photocatalysis, as well as for developing highly efficient, renewable, and affordable catalysts for large-scale production of hydrogen. Recent developments are highlighted and important challenges in using TMDs as catalysts are also discussed.

Monodisperse Mesoporous Carbon Nanoparticles from Polymer/Silica Self-Aggregates and Their Electrocatalytic Activities

In our quest to make various chemical processes sustainable, the development of facile synthetic routes and inexpensive catalysts can play a central role. Herein we report the synthesis of monodisperse, polyaniline (PANI)-derived mesoporous carbon nanoparticles (PAMCs) that can serve as efficient metal-free electrocatalysts for the hydrogen peroxide reduction reaction (HPRR) as well as the oxygen reduction reaction (ORR) in fuel cells. The materials are synthesized by polymerization of aniline with the aid of (NH4)2S2O8 as oxidant and colloidal silica nanoparticles as templates, then carbonization of the resulting PANI/silica composite material at different high temperatures, and finally removal of the silica templates from the carbonized products. The PAMC materials that are synthesized under optimized synthetic conditions possess monodisperse mesoporous carbon nanoparticles with an average size of 128 ± 12 nm and an average pore size of ca. 12 nm. Compared with Co3O4, a commonly used electrocatalyst for HPRR, these materials show much better catalytic activity for this reaction. In addition, unlike Co3O4, the PAMCs remain relatively stable during the reaction, under both basic and acidic conditions. The nanoparticles also show good electrocatalytic activity toward ORR. Based on the experimental results, PAMCs' excellent electrocatalytic activity is attributed partly to their heteroatom dopants and/or intrinsic defect sites created by vacancies in their structures and partly to their high porosity and surface area. The reported synthetic method is equally applicable to other polymeric precursors (e.g., polypyrrole (PPY)), which also produces monodisperse, mesoporous carbon nanoparticles in the same way. The resulting materials are potentially useful not only for electrocatalysis of HPRR and ORR in fuel cells but also for other applications where high surface area, small sized, nanostructured carbon materials are generally useful for (e.g., adsorption, supercapacitors, etc.).

Hollow CoP nanopaticle/N-doped graphene hybrids as highly active and stable bifunctional catalysts for full water splitting

An alkaline electrolyzer fabricated by employing hollow CoP nanoparticles/N-doped graphene as bifunctional catalysts exhibits remarkable activity with a current density of 10 mA cm(-2) at a cell voltage of 1.58 V and considerable stability over 65 h of continuous electrolysis operation, favorably comparable to the integrated performance of commercial Pt/C and IrO2.

A Highly Efficient Metal-Free Oxygen Reduction Electrocatalyst Assembled from Carbon Nanotubes and Graphene

A novel carbon-nanotube-graphene hybrid nanostructure is developed using an aerosol-assisted assembly approach. After doping with nitrogen and phosphorus, the prepared hybrid nanomaterials exhibit excellent electrocatalytic performance for oxygen reduction in both alkaline and acidic media. This research presents a continuous and low-cost route to prepare high-performance metal-free electrocatalysts while replacing Pt-based materials.