Construction of porous N-doped graphene layer for efficient oxygen reduction reaction

Role of N in Transition-Metal-Nitrides for Anchoring Platinum-Group Metal Atoms toward Single-Atom Catalysis

Single-atom catalysts (SACs) with a maximum atom utilization efficiency have received growing attention in heterogeneous catalysis. The supporting substrate that provides atomic-dispersed anchoring sites and the local electronic environment in these catalysts is crucial to their activity and stability. Here, inspired by N-doped graphene substrate, the role of N is explored in transition metal nitrides for anchoring single metal atoms toward single-atom catalysis. A pore-rich metallic vanadium nitride (VN) nanosheet is fabricated as one supporting-substrate example, whose surface features abundant unsaturated N sites with lower binding energy than that of widely used N-doped graphene. Impressively, it is found that this support can anchor nearly all platinum-group single atoms (e.g., platinum, palladium, iridium, and ruthenium), and even be extendable to multiple SACs, i.e., binary (Pt/Pd) and ternary (Pt/Pd/Ir). As a proof-of-concept application for hydrogen production, Pt-based SAC (Pt₁ -VN) performs excellently, exhibiting a mass activity up to 22.55 A mg^-1 _Pt at 0.05 V and a high turnover frequency value close to 0.350 H₂ s^-1 , superior to commercial platinum/carbon catalyst. The catalyst's durability can be further improved by using binary (Pt₁ Pd₁ -VN) SAC. This work provides inexpensive and durable nitride-based support, giving a possible pathway for universally constructing platinum-group SACs.

Successful Manufacturing Protocols of N-Rich Carbon Electrodes Ensuring High ORR Activity: A Review

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.

Preparation of rGO/MnO2 Composites through Simultaneous Graphene Oxide Reduction by Electrophoretic Deposition

We report the preparation of manganese dioxide (MnO₂) nanoparticles and graphene oxide (GO) composites reduced by an electrophoretic deposition (EPD) process. The MnO₂ nanoparticles were prepared by the electrolysis of an acidic KMnO₄ solution using an alternating monopolar arrangement of a multiple-electrode system. The particles produced were γ-MnO₂ with a rod-like morphology and a surface area of approximately 647.2 m²/g. The GO particles were produced by the oxidation of activated coconut shell charcoal using a modified Hummers method. The surface area of the GO produced was very high, with a value of approximately 2525.9 m²/g. Fourier transform infrared spectra indicate that a significant portion of the oxygen-containing functional groups was removed from the GO by electrochemical reduction during the EPD process after sufficient time following deposition of the GO. The composite obtained by the EPD process was composed of reduced graphene oxide (rGO) and γ-MnO₂ and exhibited excellent electrocatalytic activity toward the oxygen reduction reaction following a two-electron transfer mechanism. This approach opens the possibility for assembling rGO composites in an efficient and effective manner for electrocatalysis.

Carbon-based metal-free electrocatalysts: from oxygen reduction to multifunctional electrocatalysis

Since the discovery of N-doped carbon nanotubes as the first carbon-based metal-free electrocatalyst (C-MFEC) for oxygen reduction reaction (ORR) in 2009, C-MFECs have shown multifunctional electrocatalytic activities for many reactions beyond ORR, such as oxygen evolution reaction (OER), hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO₂RR), nitrogen reduction reaction (NRR), and hydrogen peroxide production reaction (H₂O₂PR). Consequently, C-MFECs have attracted a great deal of interest for various applications, including metal-air batteries, water splitting devices, regenerative fuel cells, solar cells, fuel and chemical production, water purification, to mention a few. By altering the electronic configuration and/or modulating their spin angular momentum, both heteroatom(s) doping and structural defects (e.g., atomic vacancy, edge) have been demonstrated to create catalytic active sites in the skeleton of graphitic carbon materials. Although certain C-MFECs have been made to be comparable to or even better than their counterparts based on noble metals, transition metals and/or their hybrids, further research and development are necessary in order to translate C-MFECs for practical applications. In this article, we present a timely and comprehensive, but critical, review on recent advancements in the field of C-MFECs within the past five years or so by discussing various types of electrocatalytic reactions catalyzed by C-MFECs. An emphasis is given to potential applications of C-MFECs for energy conversion and storage. The structure-property relationship for and mechanistic understanding of C-MFECs will also be discussed, along with the current challenges and future perspectives.

Synthesis of Graphene-Based Biopolymer TiO2 Electrodes Using Pyrolytic Direct Deposition Method and its Catalytic Performance

The traditional methods used to synthesize graphene layers over semiconductors are chemical-based methods. In the present investigation, a novel photoelectroactive electrode was synthesized using a chitosan biopolymer without the usage of chemicals. A chitosan-biopolymer layer over the surface of TiO2 was generated by electrodeposition. Furthermore, the pyrolysis method was used for the conversion of a biopolymer into graphene layers. The catalytic activity of the fabricated electrodes was investigated by the photo-electro-Fenton (PEF) process to oxidize chloramphenicol and nadolol pharmaceutical drugs in wastewater, remove metals (scandium, neodymium, and arsenic) and degrade real municipal wastewater. The PEF operational parameters (pH, voltage, reaction time, and Fenton catalytic dose) were optimized for the overall degradation of chloramphenicol and nadolol pharmaceutical drugs in wastewater. It was observed that at the optimum process operational parameters it took 40 min to degrade chloramphenicol and nadolol pharmaceutical drugs in wastewater. It was proved that biopolymer-based photoelectroactive novel electrodes render good catalytic activity. Furthermore, the reusability study of fabricated electrodes showed excellent storage and self-healing properties.

Selective Permeation of Water through Angstrom-Channel Graphene Membranes for Bioethanol Concentration

Graphene-based laminate membranes have been theoretically predicted to selectively transport ethanol from ethanol-water solution while blocking water. Here, robust angstrom-channel graphene membranes (ACGMs) fabricated by intercalating carbon sheets derived from chitosan into thermally reduced graphene oxide (GO) sheets are reported. ACGMs with robust and continuous slit-shaped pores (an average pore size of 3.9 Å) are investigated for the dehydration of ethanol. Surprisingly, only water permeates through ACGMs in the presence of aqueous ethanol solution. For the water-ethanol mixture containing 90 wt% ethanol, water can selectively permeate through ACGMs with a water flux of 63.8 ± 3.2 kg m^-2 h^-1 at 20 °C and 389.1 ± 19.4 kg m^-2 h^-1 at 60 °C, which are over two orders of magnitude higher than those of conventional pervaporation membranes. This means that ACGMs can effectively operate at room temperature. Moreover, the ethanol can be fast concentrated to high purity (up to 99.9 wt%). Therefore, ACGMs are very promising for production of bioethanol with high efficiency, thus improving its process sustainability.

Hybrid Structures Made of Polyurethane/Graphene Nanocomposite Foams Embedded within Aluminum Open-Cell Foam

This paper focuses on the development of hybrid structures containing two different classes of porous materials, nanocomposite foams made of polyurethane combined with graphene-based materials, and aluminum open-cell foams (Al-OC). Prior to the hybrid structures preparation, the nanocomposite foam formulation was optimized. The optimization consisted of studying the effect of the addition of graphene oxide (GO) and graphene nanoplatelets (GNPs) at different loadings (1.0, 2.5 and 5.0 wt%) during the polyurethane foam (PUF) formation, and their effect on the final nanocomposite properties. Globally, the results showed enhanced mechanical, acoustic and fire-retardant properties of the PUF nanocomposites when compared with pristine PUF. In a later step, the hybrid structure was prepared by embedding the Al-OC foam with the optimized nanocomposite formulation (prepared with 2.5 wt% of GNPs (PUF/GNPs2.5)). The process of filling the pores of the Al-OC was successfully achieved, with the resulting hybrid structure retaining low thermal conductivity values, around 0.038 W∙m−1∙K−1, and presenting an improved sound absorption coefficient, especially for mid to high frequencies, with respect to the individual foams. Furthermore, the new hybrid structure also displayed better mechanical properties (the stress corresponding to 10% of deformation was improved in more than 10 and 1.3 times comparatively to PUF/GNPs2.5 and Al-OC, respectively).

A 3D nitrogen-doped graphene aerogel for enhanced visible-light photocatalytic pollutant degradation and hydrogen evolution

The synergistic effect of the 3D structure and N-doping explain the high surface area of 536 m² g⁻¹ and excellent photocatalytic activity.

Bacterial cellulose/graphene oxide aerogels with enhanced dimensional and thermal stability

We present a novel method for processing bacterial cellulose/graphene oxide (BC/GO) aerogels with multifunctional properties. The addition of a small amount of dimethyl sulfoxide (DMSO) to the aqueous dispersion of the nanomaterials during the gelification process affected the water freezing temperature of the system and thereby affecting the porous structure of the aerogel obtained after liophilization. The possibility to obtain small and elongated pore with axial orientation allowed a significant improvement of the structural stability of the aerogels. Moreover, the aerogels reduction by thermal treatment with ammonia gas induced crosslinking between the different nanophases, thus given an incremental factor for the mechanical performance of the aerogels under harsh conditions. The resulting aerogels also showed significant improvements in terms of thermal stability and electrical conductivity. These multifunctional BC/GO aerogels present high potential as sustainable and ecological alternative materials for lightweight packaging, filters for atmosphere and water treatment, or energy applications.

Catalytic trends of nitrogen doped carbon nanotubes for oxygen reduction reaction

Replacing the state-of-the-art fuel cell catalyst platinum for a cheaper and abundant alternative would make the hydrogen economy viable. Both nitrogen-doped graphene and nitrogen-doped carbon nanotubes (N-CNT) have been shown to be capable of acting as a metal-free catalyst for the oxygen reduction reaction (ORR). Until now, most of the research has been focused on the nitrogen doping and less on the structure of the nanotubes. Here, density functional theory calculations are used to calculate trends in ORR catalytic activity of graphitic-N-doped CNTs of different sizes and chirality of selected tubes between (4,0) and (20,10). This includes 13 armchair tubes, 17 zig-zag tubes and 42 chiral tubes, or 72 N-CNTs in total. 22 tubes are predicted to have a lower overpotential than the platinum catalyst and 46 tubes have lower overpotential than nitrogen doped graphene. The most active tubes are (14,7), (12,6), and (8,8), and display an overpotential of around 0.35 V, or 0.1 V lower overpotential than predicted on Pt(111) with the same level of theory.

Overview on the use of surfactants for the preparation of porous carbon materials by the sol-gel method: applications in energy systems

Porous carbon materials attract great interest because of the wide range of applications in electrochemical energy systems, especially in the case of structured and porosity-tuned carbons prepared by template-assisted methods. The use of surfactant prevents the collapse of the porous structure during the air-drying stage in the sol-gel process, which is regarded as a critical stage in this method. This work offers an overview on the use of surfactants as templates for the manufacture of tunable porous carbon materials by the sol-gel method mainly using the polymerization reaction of resorcinol (R) and formaldehyde (F). The use of surfactants avoids the application of other economically disadvantaged drying techniques such as supercritical fluids and freeze-drying. The surfactant-assisted sol-gel methods reported in the literature for the fabrication of porous carbons are widely discussed, as well as the potentiality of the synthesized materials as electrodes in electrochemical systems, which greatly depends on the final porous structure. Besides, this work offers information on hybrid methods in which surfactants are used not only for the fabrication of porous carbon materials with mesoporous/microporous structure but also for the development of advanced structures and composites, including nanomaterials with enhanced properties. Finally, future prospects in the synthesis of carbon materials prepared by surfactant-assisted sol-gel method are presented.