Graphene-based transition metal oxide nanocomposites for the oxygen reduction reaction

Advanced Functionalized Nanoclusters (Cu, Ag, and Au) as Effective Catalyst for Organic Transformation Reactions

A considerable amount of research has been carried out in recent years on synthesizing metal nanoclusters (NCs), which have wide applications in the field of optical materials with non-linear properties, bio-sensing, and catalysis. Aside from being structurally accurate, the atomically precise NCs possess well-defined compositions due to significant tailoring, both at the surface and the core, for certain functionalities. To illustrate the importance of atomically precise metal NCs for catalytic processes, this review emphasizes 1) the recent work on Cu, Ag, and Au NCs with their synthesis, 2) the parameters affecting the activity and selectivity of NCs catalysis, and 3) the discussion on the catalytic potential of these metal NCs. Additionally, metal NCs will facilitate the design of extremely active and selective catalysts for significant reactions by elucidating catalytic mechanisms at the atomic and molecular levels. Future advancements in the science of catalysis are expected to come from the potential to design NCs catalysts at the atomic level.

Microwave assisted synthesis of Mn3O4 nanograins intercalated into reduced graphene oxide layers as cathode material for alternative clean power generation energy device

Mn₃O₄ nanograins incorporated into reduced graphene oxide as a nanocomposite electrocatalyst have been synthesized via one-step, facile, and single-pot microwave-assisted hydrothermal technique. The nanocomposites were employed as cathode material of fuel cells for oxygen reduction reaction (ORR). The synthesized product was thoroughly studied by using important characterization, such as XRD for the structure analysis and FESEM and TEM analyses to assess the morphological structures of the material. Raman spectra were employed to study the GO, rGO bands and formation of Mn₃O₄@rGO nanocomposite. FTIR and UV–Vis spectroscopic analysis were used to verify the effective synthesis of the desired electrocatalyst. The Mn₃O₄@rGO-10% nanocomposite with 10 wt% of graphene oxide was used to alter the shiny surface of the working electrode and applied for ORR in O₂ purged 0.5 M KOH electrolyte solution. The Mn₃O₄@rGO-10% nanocomposite electrocatalyst exhibited outstanding performance with an improved current of − 0.738 mA/cm² and shifted overpotential values of − 0.345 V when compared to other controlled electrodes, including the conventionally used Pt/C catalyst generally used for ORR activity. The tolerance of Mn₃O₄@rGO-10% nanocomposite was tested by injecting a higher concentration of methanol, i.e., 0.5 M, and found unsusceptible by methanol crossover. The stability test of the synthesized electrocatalyst after 3000 s was also considered, and it demonstrated excellent current retention of 98% compared to commercially available Pt/C electrocatalyst. The synthesized nanocomposite material could be regarded as an effective and Pt-free electrocatalyst for practical ORR that meets the requirement of low cost, facile fabrication, and adequate stability.

Ceria Boosting on In Situ Nitrogen-Doped Graphene Oxide for Efficient Bifunctional ORR/OER Activity

In the present work, a highly efficient and excellent electrocatalyst material for bifunctional oxygen reduction/evolution reaction (ORR/OER) was synthesized using the microwave-assisted hydrothermal method. In brief, ultrafine hexagonal cerium oxide (CeO₂) nanoparticles were tailored on the layered surface of in situ nitrogen-doped graphene oxide (NGO) sheets. The nanocomposites exhibited a high anodic onset potential of 0.925 V vs. RHE for ORR activity and 1.2 V for OER activity with a very high current density in 0.5 M KOH. The influence of oxygen cluster on Ce³⁺/Ce⁴⁺ ion decoration on outward/inward in situ nitrogen-coupled GO enhanced the physicochemical properties of composites and in turn increased electron transferability. The microwave-assisted hydrothermal coupling technique provides a higher density, active sites on CeO₂@NGO composites, and oxygen deficiency structures in ultrafine Ce-O particles and boosts higher charge transferability in the composites. It is believed that the physical states of Ce-N- C, Ce-C=O, and a higher amount of oxygen participation with ceria increase the density of composites that in turn increases the efficiency. N-doped graphene oxide promotes high current conduction and rapid electron transferability while reducing the external transport resistance in oxygen electrocatalysis by sufficient mass transfer through in-built channels. This study may provide insights into the knowledge of Ce-enabled bifunctional activity to guide the design of a robust catalyst for electrochemical performance.

Ultrafine Mo2C nanoparticles embedded in an MOF derived N and P co-doped carbon matrix for an efficient electrocatalytic oxygen reduction reaction in zinc-air batteries

Exploring high-activity electrocatalysts for an oxygen reduction reaction (ORR) is of great significance for a variety of renewable energy conversion and storage technologies. Here, ultrafine Mo₂C nanoparticles assembled in N and P-co-doped carbon (Mo₂C@NPC) was developed from ZIF-8 encapsulated molybdenum-based polyoxometalates (PMo₁₂) as a highly efficient ORR electrocatalyst and shows excellent performance for zinc-air batteries. The well distribution of the PMo₁₂ in ZIF-8 results in the formation of ultrafine Mo₂C nanocrystallites encapsulated in a porous carbon matrix after pyrolysis. Significantly, from experimental and theoretical investigations, the highly porous structure, highly dispersed ultrafine Mo₂C and the N and P co-doping in the Mo₂C@NPC lead to the remarkable ORR activity with an onset potential of ∼1.01 V, a half-wave potential of ∼0.90 V and a Tafel slope of 51.7 mV dec^-1 at 1600 rpm in 0.1 M KOH. In addition, the Mo₂C@NPC as an ORR catalyst in zinc-air batteries achieved a high power density of 266 mW cm^-2 and a high specific capacity of 780.9 mA h g^-1, exceeding that driven by commercial Pt/C. Our results revealed that the porous architecture and ultrafine Mo₂C nanocrystallites of the electrocatalysts could facilitate mass transport and increase the accessibility of active sites, thus optimizing their performances in an ORR. The present study provides some guidelines for the design and synthesis of efficient nanostructured electrocatalysts.

Enhanced Electrocatalytic Activity of Cobalt-Doped Ceria Embedded on Nitrogen, Sulfur-Doped Reduced Graphene Oxide as an Electrocatalyst for Oxygen Reduction Reaction

N, S-doped rGO was successfully synthesized and embedded Co-doped CeO2 via hydrothermal synthesis. The crystal structure, surface morphology and elemental composition of the prepared catalyst were studied by XRD, Raman spectra, SEM, TEM and XPS analyses. The synthesized electrocatalyst exhibits high onset and halfwave potential during the ORR. This result shows that a combination of N- and S-doped rGO and Co-doped CeO2 leads to a synergistic effect in catalyzing the ORR in alkaline media. Co–CeO2/N, S–rGO displays enhanced ORR performance compared to bare CeO2. The superior stability of the prepared catalyst implies its potential applications beyond fuel cells and metal–air batteries.

Density functional theory study of active sites on nitrogen-doped graphene for oxygen reduction reaction

Oxygen reduction reaction (ORR) remains challenging due to its complexity and slow kinetics. In particular, Pt-based catalysts which possess outstanding ORR activity are limited in application with high cost and ease of poisoning. In recent years, nitrogen-doped graphene has been widely studied as a potential ORR catalyst for replacing Pt. However, the vague understanding of the reaction mechanism and active sites limits the potential ORR activity of nitrogen-doped graphene materials. Herein, density functional theory is used to study the reaction mechanism and active sites of nitrogen-doped graphene for ORR at the atomic level, focusing on explaining the important role of nitrogen species on ORR. The results reveal that graphitic N (GrN) doping is beneficial to improve the ORR performance of graphene, and dual-GrN-doped graphene can demonstrate the highest catalytic properties with the lowest barriers of ORR. These results provide a theoretical guide for designing catalysts with ideal ORR property, which puts forward a new approach to conceive brilliant catalysts related to energy conversion and environmental catalysis.

Theoretical Study on a Potential Oxygen Reduction Reaction Electrocatalyst: Single Fe Atoms Supported on Graphite Carbonitride

In recent years, one of the research directions of proton-exchange membrane fuel cells (PEMFCs) was to exploit efficient electrocatalysts for oxygen reduction reaction (ORR) instead of precious metals. In this study, on the basis of the density-functional theory (DFT) calculations, we designed a new type of single-atom ORR electrocatalyst by doping single iron atoms into the N-coordination cavity of the substrate graphite carbonitride (Fe/g-C₃N₄). The adsorption site and the adsorption energy of all the intermediates, the reaction energy barriers, potential energy surface, and Mulliken charges have been analyzed. The feasible ORR reaction paths and the most favorable ORR reaction mechanism were performed. Our calculation results prove that Fe/g-C₃N₄ is a potential electrocatalyst toward ORR. This work proposes a novel notion for the development of cathode materials in PEMFCs.

Electrocatalysis for CO2 conversion: from fundamentals to value-added products

This timely and comprehensive review mainly summarizes advances in heterogeneous electroreduction of CO₂: from fundamentals to value-added products.

Fabrication of polyoxometalate-modified palladium–nickel/reduced graphene oxide alloy catalysts for enhanced oxygen reduction reaction activity

A novel nanocatalyst, polyoxometalate-modified palladium–nickel/reduced graphene oxide (Pd8Ni2/rGO-POM), is prepared and served as an effective ORR nanomaterial in alkaline media.

Carbon Xerogel Nanostructures with Integrated Bi and Fe Components for Hydrogen Peroxide and Heavy Metal Detection

Multifunctional Bi- and Fe-modified carbon xerogel composites (CXBiFe), with different Fe concentrations, were obtained by a resorcinol-formaldehyde sol-gel method, followed by drying in ambient conditions and pyrolysis treatment. The morphological and structural characterization performed by X-ray diffraction (XRD), Raman spectroscopy, N2 adsorption/desorption porosimetry, scanning electron microscopy (SEM) and scanning/transmission electron microscopy (STEM) analyses, indicates the formation of carbon-based nanocomposites with integrated Bi and Fe oxide nanoparticles. At higher Fe concentrations, Bi-Fe-O interactions lead to the formation of hybrid nanostructures and off-stoichiometric Bi2Fe4O9 mullite-like structures together with an excess of iron oxide nanoparticles. To examine the effect of the Fe content on the electrochemical performance of the CXBiFe composites, the obtained powders were initially dispersed in a chitosan solution and applied on the surface of glassy carbon electrodes. Then, the multifunctional character of the CXBiFe systems is assessed by involving the obtained modified electrodes for the detection of different analytes, such as biomarkers (hydrogen peroxide) and heavy metal ions (i.e., Pb2+). The achieved results indicate a drop in the detection limit for H2O2 as Fe content increases. Even though the current results suggest that the surface modifications of the Bi phase with Fe and O impurities lower Pb2+ detection efficiencies, Pb2+ sensing well below the admitted concentrations for drinkable water is also noticed.

Phase Segregated Pt-SnO2 /C Nanohybrids for Highly Efficient Oxygen Reduction Electrocatalysis

Strengthening the interfacial interaction in heterogeneous catalysts can lead to a dramatic improvement in their performance and allow the use of smaller amounts of active noble metal, thus decreasing the cost without compromising their activity. In this work, a facile phase-segregation method is demonstrated for synthesizing platinum-tin oxide hybrids supported on carbon black (PtSnO₂ /C) in situ by air annealing PtSn alloy nanoparticles on carbon black. Compared with a control sample formed by preloading SnO₂ on carbon support followed by deposition of Pt nanoparticles, the phase-segregation-derived PtSnO₂ /C exhibits a more strongly coupled PtSnO₂ interface with lattice overlap of Pt (111) and SnO₂ (200), along with enhanced electron transfer from SnO₂ to Pt. Furthermore, the PtSnO₂ active sites show a strong ability to degrade reactive oxygen species. As a result, the PtSnO₂ /C nanohybrids exhibit both excellent activity and stability as a catalyst for the oxygen reduction reaction, with an overall performance which is superior to both the control sample and commercial Pt/C catalyst. This phase-segregation method can be expected to be applicable in the preparation of other strongly coupled nanohybrids and offers a new route to high-performance heterogeneous catalysts for low-cost energy conversion devices.

Photocatalytic Degradation of Phenol Using Chemical Vapor Desposition Graphene Column

In the field of wastewater treatment, the advanced oxidation process (AOP) is a widely employed method. It uses reactive oxygen species (ROS) to degrade harmful organic and inorganic chemicals. Metal catalysts are the conventional standard when using these methods. However, they have drawbacks such as harsh activation conditions and poor recyclability. We previously suggested chemical vapor deposition (CVD) graphene film as an alternative metal-free catalyst. In this study, we enhanced the catalytic activity of the CVD graphene film by synergistically adding UV light irradiation. The result was complete degradation of phenol on a wafer-scale in a reduced timeframe. To further enhance the degradation process, we devised a graphene-based column for continuous in situ chemical oxidation and analyzed the intermediates over time, proving the potential of graphene-assisted AOP in industrial wastewater applications.

An Efficient Bifunctional Electrocatalyst of Phosphorous Carbon Co-doped MOFs

It is eager to develop high-performance and cheap bifunctional electrochemical catalysts for both of the oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) for the energy crisis and environmental problems. Herein, we report a series of ZIF-derived Co-P-C co-doped polyhedral materials with a well-defined morphology. The optimized catalyst Co/P/MOFs-CNTs-700 exhibited favorable electrochemical activities with the lowest overpotential of 420 mV to achieve the current density of 10 mA cm^-2 for OER and the half potential of 0.8 V for ORR in 0.1 M NaOH. The performance can be well improved by doping phosphorous resource which greatly changed its morphology. Meanwhile, the doped carbon resources also improve the conductivity, which makes it a promising bifunctional electrochemical catalyst and can be comparable with the commercial electrocatalysts.

Remarkably efficient charge transfer through a double heterojunction mechanism by a CdS-SnS-SnS2/rGO composite with excellent photocatalytic performance under visible light

In this work, a novel CdS-SnS-SnS₂/rGO photocatalyst with two tin valence states (Ⅱ and IV) was successfully synthesized by a one-pot solvothermal method. For comparison, CdS-SnS₂/rGO (GCS2) with tin in only the IV valence state was made by the same method. Based on a series of characterizations, CdS, SnS and SnS₂ were shown to be successfully loaded onto the rGO surface. The introduction of rGO may increase charge carrier separation. The degradation efficiency increased gradually with increasing rGO loading content, and the optimum photocatalytic activity was observed at 6.0 wt% rGO loading content (GCS1), which achieved the efficient removal (84.46%) of ibuprofen over 60 min. Compared with GCS2, the CdS-SnS-SnS₂/rGO composite exhibited significantly improved photocatalytic performance, which can be ascribed to the formation of a double heterostructure. rGO worked as a transfer mediator to transfer electrons from the conduction band (CB) of SnS to the CB of SnS₂ at the heterointerface, which then flowed to the CB of CdS because of another heterojunction, further enhancing the separation efficiency of photogenerated carriers. Therefore, this study highlights a novel double heterojunction system with a facial preparation method, visible light response and good recyclability, which is beneficial for environmental remediation.

Platinum Group Metal-Free Catalysts for Oxygen Reduction Reaction: Applications in Microbial Fuel Cells

Scientific and technological innovation is increasingly playing a role for promoting the transition towards a circular economy and sustainable development. Thanks to its dual function of harvesting energy from waste and cleaning up waste from organic pollutants, microbial fuel cells (MFCs) provide a revolutionary answer to the global environmental challenges. Yet, one key factor that limits the implementation of larger scale MFCs is the high cost and low durability of current electrode materials, owing to the use of platinum at the cathode side. To address this issue, the scientific community has devoted its research efforts for identifying innovative and low cost materials and components to assemble lab-scale MFC prototypes, fed with wastewaters of different nature. This review work summarizes the state-of the-art of developing platinum group metal-free (PGM-free) catalysts for applications at the cathode side of MFCs. We address how different catalyst families boost oxygen reduction reaction (ORR) in neutral pH, as result of an interplay between surface chemistry and morphology on the efficiency of ORR active sites. We particularly review the properties, performance, and applicability of metal-free carbon-based materials, molecular catalysts based on metal macrocycles supported on carbon nanostructures, M-N-C catalysts activated via pyrolysis, metal oxide-based catalysts, and enzyme catalysts. We finally discuss recent progress on MFC cathode design, providing a guidance for improving cathode activity and stability under MFC operating conditions.

A bimetallic MOF@graphene oxide composite as an efficient bifunctional oxygen electrocatalyst for rechargeable Zn-air batteries

The development of low-cost bifunctional catalysts for replacing precious metal Pt-group electrocatalysts is highly desirable but remains challenging in the energy conversion and storage field. Metal-organic frameworks (MOFs) with a large specific surface area and a controllable pore structure provide a great opportunity to prepare bifunctional catalysts, but their low activity in oxygen electrocatalysis hinders their applications. Herein, we rationally design and fabricate a bimetallic ZnCo-zeolite imidazole framework@graphene oxide (ZnCo-ZIF@GO) hybrid electrocatalyst through a simple in situ growth of a bimetallic ZIF on graphene oxide nanosheets. The obtained ZnCo-ZIF@GO hybrid displays superior electrocatalytic activities toward both the ORR and OER in alkaline solution compared to the pure ZnCo-ZIF. The outstanding bifunctional electrocatalytic performance is attributed to the synergy and strong interaction between the ZnCo-ZIF and GO, enhanced ionic conductivity, and hierarchical porosity. A rechargeable Zn-air battery (ZAB) assembled using ZnCo-ZIF@GO as the air cathode displays excellent charge and discharge performance, high energy density, and cycling stability, demonstrating its great potential as an advanced bifunctional electrocatalyst in the field of energy conversion and storage.

Highly Dispersed Nonprecious Metal Catalyst for Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells

This study reports a high-performing nonprecious metal catalyst for the oxygen reduction reaction that is composed of highly dispersed Fe centered active sites on bamboolike carbon nanotubes. NH₂-MIL-88B is used as the iron source and ZIF-8 as the carbon source. The precursors are uniformly mixed by ball milling, which destroys their crystal structures. A bamboolike carbon nanotube network results from the pyrolysis of the mixed precursors. The morphology is controlled by the proportion of the precursors and the pyrolysis temperature. The catalyst shows excellent oxygen reduction activity in both half-cell and full-cell tests. The onset potential and half-wave potential are 0.96 and 0.78 V vs RHE, respectively. In the fuel cell test, the current density reaches 0.85 A cm^-2 at 0.7 V and 1.24 A cm^-2 at 0.6 V (iR-corrected). The novel synthesis approach of the highly dispersed catalyst provides new strategy in the design of high effective nonprecious metal catalysts for fuel cell.

Metal Oxide (Co3O4 and Mn3O4) Impregnation into S, N-doped Graphene for Oxygen Reduction Reaction (ORR)

To address aggravating environmental and energy problems, active, efficient, low-cost, and robust electrocatalysts (ECs) are actively pursued as substitutes for the current noble metal ECs. Therefore, in this study, we report the preparation of graphene flakes (GF) doped with S and N using 2-5-dimercapto-1,3,4-thiadiazole (S₃N₂) as precursor followed by the immobilization of cobalt spinel oxide (Co₃O₄) or manganese spinel oxide (Mn₃O₄) nanoparticles through a one-step co-precipitation procedure (Co/S₃N₂–GF and Mn/S₃N₂–GF). Characterization by different physicochemical techniques (Fourier Transform Infrared (FTIR), Raman spectroscopy, Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD)) of both composites shows the preservation of the metal oxide spinel structure and further confirms the successful preparation of the envisaged electrocatalysts. Co/S₃N₂–GF composite exhibits the best ORR performance with an onset potential of 0.91 V vs. RHE, a diffusion-limiting current density of −4.50 mA cm⁻² and selectivity for the direct four-electron pathway, matching the results obtained for commercial Pt/C. Moreover, both Co/S₃N₂–GF and Mn/S₃N₂–GF showed excellent tolerance to methanol poisoning and good stability.

Electrochemistry in Carbon-based Quantum Dots

Electrochemistry belongs to an important branch of chemistry that deals with the chemical changes produced by electricity and the production of electricity by chemical changes. Therefore, it can not only act a powerful tool for materials synthesis, but also offer an effective platform for sensing and catalysis. As extraordinary zero-dimensional materials, carbon-based quantum dots (CQDs) have been attracting tremendous attention due to their excellent properties such as good chemical stability, environmental friendliness, nontoxicity and abundant resources. Compared with the traditional methods for the preparation of CQDs, electrochemical (EC) methods offer advantages of simple instrumentation, mild reaction conditions, low cost and mass production. In return, CQDs could provide cost-effective, environmentally friendly, biocompatible, stable and easily-functionalizable probes, modifiers and catalysts for EC sensing. However, no specific review has been presented to systematically summarize both aspects until now. In this review, the EC preparation methods of CQDs are critically discussed focusing on CQDs. We further emphasize the applications of CQDs in EC sensors, electrocatalysis, biofuel cells and EC flexible devices. This review will further the experimental and theoretical understanding of the challenges and future prospective in this field, open new directions on exploring new advanced CQDs in EC to meet the high demands in diverse applications.

Mixed Copper/Copper-Oxide Anchored Mesoporous Fullerene Nanohybrids as Superior Electrocatalysts toward Oxygen Reduction Reaction

Developing a highly active, stable, and efficient non-noble metal-free functional electrocatalyst to supplant the benchmark Pt/C-based catalysts in practical fuel cell applications remains a stupendous challenge. A rational strategy is developed to directly anchor highly active and dispersed copper (Cu) nanospecies on mesoporous fullerenes (referred to as Cu-MFC₆₀ ) toward enhancing oxygen reduction reaction (ORR) electrocatalysis. The preparation of Cu-MFC₆₀ involves i) the synthesis of ordered MFC₆₀ via the prevalent nanohard templating technique and ii) the postfunctionalization of MFC₆₀ with finely distributed Cu nanospecies through incipient wet impregnation. The concurrence of Cu and cuprous oxide nanoparticles in the as-developed Cu-MFC₆₀ samples through relevant material characterizations is affirmed. The optimized ORR catalyst, Cu(15%)-MFC₆₀ , exhibits superior electrocatalytic ORR characteristics with an onset potential of 0.860 vs reversible hydrogen electrode, diffusion-limiting current density (-5.183 mA cm^-2 ), improved stability, and tolerance to methanol crossover along with a high selectivity (four-electron transfer). This enhanced ORR performance can be attributed to the rapid mass transfer and abundant active sites owing to the synergistic coupling effects arising from the mixed copper nanospecies and the fullerene framework.

Synthesis of a manganese dioxide nanorod-anchored graphene oxide composite for highly sensitive electrochemical sensing of dopamine

A novel manganese dioxide nanorod-anchored graphene oxide (MnO₂ NRs/GO) composite was synthesized by a simple hydrothermal method for the development of a highly sensitive electrochemical sensor for dopamine.

Application of Graphene-Based Materials for Detection of Nitrate and Nitrite in Water-A Review

Nitrite and nitrate are widely found in various water environments but the potential toxicity of nitrite and nitrate poses a great threat to human health. Recently, many methods have been developed to detect nitrate and nitrite in water. One of them is to use graphene-based materials. Graphene is a two-dimensional carbon nano-material with sp² hybrid orbital, which has a large surface area and excellent conductivity and electron transfer ability. It is widely used for modifying electrodes for electrochemical sensors. Graphene based electrochemical sensors have the advantages of being low cost, effective and efficient for nitrite and nitrate detection. This paper reviews the application of graphene-based nanomaterials for electrochemical detection of nitrate and nitrite in water. The properties and advantages of the electrodes were modified by graphene, graphene oxide and reduced graphene oxide nanocomposite in the development of nitrite sensors are discussed in detail. Based on the review, the paper summarizes the working conditions and performance of different sensors, including working potential, pH, detection range, detection limit, sensitivity, reproducibility, repeatability and long-term stability. Furthermore, the challenges and suggestions for future research on the application of graphene-based nanocomposite electrochemical sensors for nitrite detection are also highlighted.

Leaching- and sintering-resistant hollow or structurally ordered intermetallic PtFe alloy catalysts for oxygen reduction reactions

Carbon supported Pt-based alloy materials that have been developed for proton exchange membrane fuel cells (PEMFCs) are vulnerable to deactivation due to the loss of non-noble metal components (leaching) or detachment, migration and aggregation of active nanoparticles (sintering). Until now some methods have been developed to inhibit leaching or sintering individually. However, a route able to avoid leaching and sintering simultaneously is still lacking. Herein, we develop a thermally driven interfacial diffusion alloying route that allows for the direct evolution of solid Pt nanoparticles (NPs) supported on carbon (Pt/C) into a Pt-skin-like hollow PtFe alloy or a structurally ordered intermetallic PtFe alloy, together with in situ encapsulation of PtFe alloy NPs with a thin layer porous nitrogen-doped carbon (NC) shell. The robust NC shells not only effectively prevent Pt-based NPs from detachment, migration, and aggregation during accelerated durability tests but also allow smoother access of electrolyte to the Pt surface, thus allowing the catalysts to well preserve their high catalytic activity. The well-defined shape and atomic arrangement of PtFe alloy NPs exhibit over 600% increase in mass activity and specific activity when compared with that of the pristine Pt/C catalyst. Stability tests confirm that the ordered PtFe alloy is more electrochemically stable than the disordered hollow PtFe alloy and Pt/C catalysts due to its ordered atomic arrangement and the robust NC shell.

Interwoven Molecular Chains Obtained by Ionic Self-Assembly of Two Iron(III) Porphyrins with Opposite and Mismatched Charges

We report ionic self-assembly of positively charged Fe^III meso-tetra(N-methyl-4-pyridyl) porphyrin (Fe^IIINMePyP) with negatively charged Fe^III meso-tetra(4-sulfonatophenyl) porphyrin (Fe^IIITPPS₄), leading to the formation of flower-like nanostructures composed of unprecedented three-dimensional (3D) entangled chains of porphyrin dimers. Molecular dynamics (MD) simulations show that the 3D entanglement of porphyrin chains closely correlates to mismatched charges present in porphyrin dimers like [Fe^III(H₂O)₂NMePyP]⁵⁺/[Fe^III(H₂O)₂TPPS₄]^3- that requires extra interactions or entanglement with neighboring ones to achieve electric neutrality. Interestingly, the interwoven chains bring in excellent thermal stability as evidenced by well maintenance of the flower-like morphology after pyrolysis at 775 °C in argon, which is in good agreement of high-temperature MD simulations. Meanwhile, heat treatment of the flower-like porphyrin nanostructure leads to the formation of a non-noble metal electrocatalyst (NNME) with largely inherited morphology. This exemplifies a new approach by combining ionic self-assembly with subsequent pyrolysis for the synthesis of NNMEs with desired control over the morphology of template-free NNMEs that has rarely been achieved prior to this study. Furthermore, our electrocatalyst exhibits excellent activity and durability toward oxygen reduction reaction as well as much better methanol tolerance compared with commercial Pt/C in alkaline solutions.

Functional molecule guided evolution of MnO x nanostructure patterns on N-graphene and their oxygen reduction activity

In this work, we systematically followed the growth of MnO_x nanostructures on trimesic acid (TMA)/benzoic acid (BA) functionalised nitrogen doped graphene (NG) and studied their electrocatalytic activity towards oxygen reduction reaction (ORR). In these hybrid materials the MnO_x phase, their morphology and Mn surface valency were guided by the functional molecules, their concentration and the duration of reaction, which in turn significantly affected the ORR activity. During the growth in the presence of TMA, agglomerated nanostructures were formed at 2 h reaction, which transformed to well dispersed 4–7 nm particles at 6 h over a large area of NG. However, in the presence of BA, MnOOH nano-flecks were formed at 2 h and transformed to MnOOH nanowires and oval shaped Mn₃O₄ particles at 8 h of reaction. The valency of surface Mn on the different MnO_x nanostructures was ascertained by X-ray photoelectron spectroscopy (XPS). The ORR activity of samples were studied by cyclic voltammetry (CV) and rotating disc electrode (RDE) in alkaline medium. Among all the studied samples, the highest ORR activity with most efficient 4e⁻ transfer process is observed for TMA modified NG-MnO_X obtained at 6 h of reaction, which is due to its well dispersed nanostructure morphology.

In this work, we systematically followed the growth of MnO_x nanostructures on trimesic acid (TMA)/benzoic acid (BA) functionalised nitrogen doped graphene (NG) and studied their electrocatalytic activity towards oxygen reduction reaction (ORR).

Graphene wrapped Fe7C3 nanoparticles supported on N-doped graphene nanosheets for efficient and highly methanol-tolerant oxygen reduction reaction

Green and efficient non-precious metal electrocatalysts for oxygen reduction reaction (ORR) are prepared to meet the increasing demand for clean, secure and sustainable energy. Herein, we report a novel and environmentally friendly strategy for synthesis of graphene-wrapped iron carbide (Fe₇C₃) nanoparticles supported on hierarchical fibrous N-doped graphene with open-mesoporous structures (Fe₇C₃/NG) by simply annealing the mixture of melamine, iron (II) phthalocyanine (FePc) and Fe₂O₃. The effects of the pyrolysis temperature and the molar ratio of FePc to melamine were critically examined in the controls. Remarkably, the Fe₇C₃/NG obtained at 800 °C (i.e. Fe₇C₃/NG-800) manifested the forward shifts in the onset potential (0.98 V) and half-wave potential (0.85 V) with respective to commercial Pt/C (50 wt%) in 0.1 M KOH, coupled with the great enhancement in the durability (still remained 92.11% of its initial current density even after 40,000 s) and strong methanol tolerance. This research presents a promising strategy for developing Pt-free non-precious efficient ORR electrocatalysts in fuel cells.

Next-Generation Multifunctional Carbon-Metal Nanohybrids for Energy and Environmental Applications

Nanotechnology has unprecedentedly revolutionized human societies over the past decades and will continue to advance our broad societal goals in the coming decades. The research, development, and particularly the application of engineered nanomaterials have shifted the focus from "less efficient" single-component nanomaterials toward "superior-performance", next-generation multifunctional nanohybrids. Carbon nanomaterials (e.g., carbon nanotubes, graphene family nanomaterials, carbon dots, and graphitic carbon nitride) and metal/metal oxide nanoparticles (e.g., Ag, Au, CdS, Cu₂O, MoS₂, TiO₂, and ZnO) combinations are the most commonly pursued nanohybrids (carbon-metal nanohybrids; CMNHs), which exhibit appealing properties and promising multifunctionalities for addressing multiple complex challenges faced by humanity at the critical energy-water-environment (EWE) nexus. In this frontier review, we first highlight the altered and newly emerging properties (e.g., electronic and optical attributes, particle size, shape, morphology, crystallinity, dimensionality, carbon/metal ratio, and hybridization mode) of CMNHs that are distinct from those of their parent component materials. We then illustrate how these important newly emerging properties and functions of CMNHs direct their performances at the EWE nexus including energy harvesting (e.g., H₂O splitting and CO₂ conversion), water treatment (e.g., contaminant removal and membrane technology), and environmental sensing and in situ nanoremediation. This review concludes with identifications of critical knowledge gaps and future research directions for maximizing the benefits of next-generation multifunctional CMNHs at the EWE nexus and beyond.

A room-temperature interfacial approach towards iron/nitrogen co-doped fibrous porous carbons as electrocatalysts for the oxygen reduction reaction and Zn-Air batteries

The development of nonprecious and efficient catalysts to boost the oxygen reduction reaction (ORR) is imperative. However, the majority of previously reported approaches suffered from a complicated fabrication procedure, both time consuming and difficult to scale up. Herein, large-scale iron ion embedded polyaniline fibers were successfully fabricated as precursors for preparing iron/nitrogen co-doped fibrous porous carbons (Fe/NPCFs) through an interfacial engineering strategy at room temperature. As ORR electrocatalysts in an alkaline medium (0.1 M KOH), Fe/NPCFs display a positive half-wave potential of 0.827 V (vs. RHE), and high limited current density (up to 5.76 mA cm-2), which are better than those of commercial Pt/C (E1/2 = 0.815 V, JL = 5.47 mA cm-2). Also, Fe/NPCFs exhibit a high ORR catalysis activity (E1/2 = 0.632 V, JL = 5.07 mA cm-2) in acidic medium (0.5 M H2SO4). When used as an air cathode in a primary Zn-air battery, high power density (158.5 mW cm-2) and specific capacity (717.8 mA h g-1) can be easily achieved, outperforming the commercial Pt/C.