Uniform nanoporous graphene sponge from natural polysaccharides as a metal-free electrocatalyst for hydrogen generation

Waste-Derived Catalysts for Water Electrolysis: Circular Economy-Driven Sustainable Green Hydrogen Energy

The sustainable production of green hydrogen via water electrolysis necessitates cost-effective electrocatalysts. By following the circular economy principle, the utilization of waste-derived catalysts significantly promotes the sustainable development of green hydrogen energy. Currently, diverse waste-derived catalysts have exhibited excellent catalytic performance toward hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water electrolysis (OWE). Herein, we systematically examine recent achievements in waste-derived electrocatalysts for water electrolysis. The general principles of water electrolysis and design principles of efficient electrocatalysts are discussed, followed by the illustration of current strategies for transforming wastes into electrocatalysts. Then, applications of waste-derived catalysts (i.e., carbon-based catalysts, transitional metal-based catalysts, and carbon-based heterostructure catalysts) in HER, OER, and OWE are reviewed successively. An emphasis is put on correlating the catalysts' structure-performance relationship. Also, challenges and research directions in this booming field are finally highlighted. This review would provide useful insights into the design, synthesis, and applications of waste-derived electrocatalysts, and thus accelerate the development of the circular economy-driven green hydrogen energy scheme.

Baldovi H, Dhakshinamoorthy A, Primo A, Garcia H. Tridimensional N, P-Codoped Carbon Sponges as Highly Selective Catalysts for Aerobic Oxidative Coupling of Benzylamine

Two tridimensional N-doped porous carbon sponges (3DC-X) have been prepared by using cetyltrimethylammonium chloride (CTAC) and cetyltrimethylammonium bromide (CTAB) as soft templates and alginate to replicate the liquid crystals formed by CTA+ in water. Alginate is a filmogenic polysaccharide of natural origin having the ability to form nanometric defectless films around objects. Subsequent pyrolysis at 900 °C under an Ar flow of the resulting CTA+-polysaccharide assemblies result in 3DC-1 and 3DC-2, with the N percentages of 0.48 and 0.36 wt % for the materials resulting from CTAC and CTAB, respectively. Another four 3DC materials were obtained via pyrolysis of the adduct of phytic acid and chitosan, rendering an amorphous, N and P-codoped carbon sample (3DC-3 to 3DC-6). The six 3DC samples exhibit a large area (>650 m2 × g-1) and porosity, as determined by Ar adsorption. The catalytic activity of these materials in promoting the aerobic oxidation of benzylamine increases with the specific surface area and doping, being the largest for 3DC-4, which is able to achieve 73% benzylamine conversion to N-benzylidene benzylamine in solventless conditions at 70 °C in 5 h. Quenching studies and hot filtration tests indicate that 3DC-4 acts as a heterogeneous catalyst rather than an initiator, triggering the formation of hydroperoxyl and hydroxyl radicals as the main reactive oxygen species involved in the aerobic oxidation.

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.

Porous Graphitic Carbons Containing Nitrogen by Structuration of Chitosan with Pluronic P123

Using Pluronic P123 as a structure-directing agent and chitosan as a carbon precursor, different porous carbons with remarkable morphologies such as orthohedra or spheres with diametrically opposite holes are obtained. These particles of micrometric size are constituted by the stacking of thin sheets (60 nm) that become increasingly bent in the opposite sense, concave in the upper and convex in the bottom hemispheres, as the chitosan proportion increases. TEM images, after dispersion of the particles by sonication, show that besides micrometric graphene sheets, the material is constituted by nanometric onion-like carbons. The morphology and structure of these porous carbons can be explained based on the ability of Pluronic P123 to undergo self-assembly in aqueous solution due to its amphoteric nature and the filmogenic properties of chitosan to coat Pluronic P123 nanoobjects undergoing structuration and becoming transformed into nitrogen-doped graphitic carbons. XPS analysis reveals the presence of nitrogen in their composition. These porous carbons exhibit a significant CO2 adsorption capacity of above 3 mmol g-1 under 100 kPa at 273 K attributable to their large specific surface area, ultraporosity, and the presence of basic N sites. In addition, the presence of dopant elements in the graphitic carbons opening the gap is responsible for the photocatalytic activity for H2 generation in the presence of sacrificial electron donors, reaching a H2 production of 63 μmol g-1 in 24 h.

Design, Fabrication, and Mechanism of Nitrogen-Doped Graphene-Based Photocatalyst

Solving energy and environmental problems through solar-driven photocatalysis is an attractive and challenging topic. Hence, various types of photocatalysts have been developed successively to address the demands of photocatalysis. Graphene-based materials have elicited considerable attention since the discovery of graphene. As a derivative of graphene, nitrogen-doped graphene (NG) particularly stands out. Nitrogen atoms can break the undifferentiated structure of graphene and open the bandgap while endowing graphene with an uneven electron density distribution. Therefore, NG retains nearly all the advantages of original graphene and is equipped with several novel properties, ensuring infinite possibilities for NG-based photocatalysis. This review introduces the atomic and band structures of NG, summarizes in situ and ex situ synthesis methods, highlights the mechanism and advantages of NG in photocatalysis, and outlines its applications in different photocatalysis directions (primarily hydrogen production, CO₂ reduction, pollutant degradation, and as photoactive ingredient). Lastly, the central challenges and possible improvements of NG-based photocatalysis in the future are presented. This study is expected to learn from the past and achieve progress toward the future for NG-based photocatalysis.

3D defective graphenes with subnanometric porosity obtained by soft-templating following zeolite procedures

By applying the well-known templating mechanism employed for the synthesis of mesoporous silicas to the structuration of sodium alginate, a novel defective 3D tubular graphene material (graphenolite) with hierarchical macro/meso/micro-porous structure, very high powder specific surface area (1820 m2 g-1) and regular micropore size (0.6 nm) has been obtained. The key feature of the process is the filmogenic property of alginate that is able to replicate the liquid crystal rods formed by the CTAC template in the aqueous phase. The 3D graphene exhibits 2.5 times higher capacitance using Li+ electrolyte compared to K+, indicating that Li+ can ingress to the ultramicropores which, in contrast, are not accessible to K+. Electrochemical impedance measurements also indicate much lower resistance for Li+ in comparison to K+ electrolyte, confirming the benefits of controlled microporosity of 3D graphene granting selective access to Li+, but not to K+. The present report opens the door for the synthesis of a wide range of 3D graphene materials that could be prepared following similar strategies to those employed for the preparation of zeolites and periodic mesoporous aluminosilicates.

Recent Advances of Porous Graphene: Synthesis, Functionalization, and Electrochemical Applications

Graphene is a 2D sheet of sp² bonded carbon atoms and tends to aggregate together, due to the strong π-π stacking and van der Waals attraction between different layers. Its unique properties such as a high specific surface area and a fast mass transport rate are severely blocked. To address these issues, various kinds of 2D holey graphene and 3D porous graphene are either self-assembled from graphene layers or fabricated using graphene related materials such as graphene oxide and reduced graphene oxide. Porous graphene not only possesses unique pore structures, but also introduces abundant exposed edges and accelerates mass transfer. The properties and applications of these porous graphenes and their composites/hybrids have been extensively studied in recent years. Herein, recent progress and achievements in synthesis and functionalization of various 2D holey graphene and 3D porous graphene are reviewed. Of special interest, electrochemical applications of porous graphene and its hybrids in the fields of electrochemical sensing, electrocatalysis, and electrochemical energy storage, are highlighted. As the closing remarks, the challenges and opportunities for the future research of porous graphene and its composites are discussed and outlined.