Rational Design of Fe/N/S-Doped Nanoporous Carbon Catalysts from Covalent Triazine Frameworks for Efficient Oxygen Reduction

Bimetallic-Coordinated Covalent Triazine Framework-Derived FeNi Alloy Nanoparticle-Decorated Coral-Like Nanocarbons for Oxygen Electrocatalysis

It is highly desirable to fabricate transition bimetallic alloy-embedded porous nanocarbons with a unique nanoarchitecture for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in rechargeable zinc-air batteries. In this work, we introduce a template-assisted in situ alloying synthesis of FeNi alloy nanoparticle-decorated coral-like nanocarbons (FeNi-CNCs) as efficient OER/ORR dual-functional electrocatalysts. The present materials are produced through polycondensation of a covalent triazine framework (CTF), the coordination of Ni and Fe ions, and sequential pyrolytic treatment. Through the pyrolysis process, the nanolamellar FeNi-CTF precursors can be facilely converted into FeNi alloy nanoparticle-decorated nanocarbons. These nanocarbons possess a distinctive three-dimensional (3D) coral-like nanostructure, which is favorable for the transport of oxygen and the diffusion of electrolyte. As a result, FeNi-CNC-800 with the highest efficiency exhibited remarkable electrocatalytic performance and great durability. Additionally, it also can be assembled into rechargeable zinc-air batteries that can be assembled in both liquid and solid forms, offering a superior peak power density, large specific capacity, and outstanding reusability during charging/discharging cycles (e.g., 5160 charging-and-discharging cycles at 10 mA cm-2 for the liquid forms). These traits make it a highly promising option in the burgeoning field of wearable energy conversion.

Fabrication of hybrid covalent triazine framework-zinc ferrite spinel to uplift visible light-driven photocatalytic organic pollutant degradation

The tunability of porous covalent triazine frameworks (CTFs) can mitigate poor photostability and rapid hole-electron recombination. Herein, an excellent improvement of visible light-driven photocatalytic pollutant degradation was achieved using a hybrid semiconductor of covalent triazine framework-zinc ferrite spinel catalysts (CTF-ZnFe₂O₄). The as-prepared CTF-ZnFe₂O₄ composites were fabricated using a facile one-pot ionothermal method. The hybrid photocatalysts were identified using X-ray diffraction (XRD), scanning electron microscopy/energy-dispersive X-ray (SEM-EDX), X-ray photoelectron spectrometer (XPS), Brunauer-Emmett-Teller (BET), Fourier transform infrared (FTIR), and UV-visible diffuse reflection spectroscopy (UV-vis DRS) characterizations. The analysis reveals that hybridization successfully ensued and altered the crystallinity structure, morphology, surface area, and bandgap energy of hybrid material. It was found that CTF-ZnFe₂O₄ 90:10 is very effective for the degradation of MB in a UV-vis light photocatalytic process with the efficiency of 95.4% and k_obs of 0.421 min^-1 for degradation of 50 mg/L MB with 0.5 g/L dosages for 120 min. Additionally, the scavenger study, effect of additional oxidants, and stability were performed for the practical application of a hybrid photocatalyst. CTF-ZnFe₂O₄ 90:10 shows outstanding pollutant degradation in sunlight irradiation and high stability with only a 5.2% reduction after a five-times sequential recycling process. Moreover, the photocatalytic mechanism of as-prepared CTF-ZnFe₂O₄ was mainly influenced by [Formula: see text] radical compared to [Formula: see text] and [Formula: see text] radicals. Overall, The as-prepared CTF-ZnFe₂O₄ shows significant potential to be utilized for photocatalytic wastewater treatment.

Electrocatalytic Oxygen Reduction Reaction by the Pd/Fe-N-C Catalyst and Application in a Zn–Air Battery

Developing a non-platinum catalyst that effectively catalyzes the oxygen reduction reaction (ORR) is highly significant for metal–air batteries. Metal and nitrogen co-doped carbons (M-N-Cs) have emerged as alternative candidates to platinum. In this work, dual-metal Pd/Fe-N-C electrocatalysts were synthesized by the one-step pyrolysis of phytic acid, melamine, and Pd/Fe-based salts. The Pd/Fe-N-C catalyst exhibited a good catalytic ability during the ORR process and outperformed the commercial Pt/C catalyst as regards mass activity, catalytic stability, and methanol tolerance. It was found that Pd-Nx is the active center, and the synergistic effect from the Fe component introduction endowed the Pd/Fe-N-C with an excellent catalytic performance towards the ORR. When assembled into a Zn–air battery, its specific capacity was ~775 mAh gZn−1. Meanwhile, the peak power density could reach 3.85 W mgPd−1, i.e., 3.4 times that of the commercial Pt/C catalyst (1.13 W mgPt−1). This implies that the Pd/Fe-N-C catalyst has potential applications in metal–air batteries.

Construction of three-dimensional cobalt sulfide/multi-heteroatom co-doped porous carbon as an efficient trifunctional electrocatalyst

Exploring cost-effective non-precious metal electrocatalysts is vital for the large-scale application of clean energy conversion devices (i.e., fuel cells, metal-air batteries and water electrolysers). Herein, we present the construction of a three-dimensional cobalt sulfide/multi-heteroatom co-doped carbon composite as a trifunctional electrocatalyst for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) through one-step sulfidation of zeolitic-imidazolate frameworks (ZIFs) using sulfur powder as a sulfur source. By virtue of the distinct periodic metal-nitrogen coordination structure and the abundant micropores within the ZIF precursor, sub-10 nm Co₉S₈ nanoparticles (NPs) are homogenously anchored on a Co, S and N multi-heteroatom co-doped carbon framework with a large specific surface area that exposes sufficient reactive sites for these electrocatalytic reactions. The optimized Co₉S₈/CoNSC exhibits outstanding ORR, OER and HER performance, comparable or even superior to those of commercial Pt/C and RuO₂. The small Co₉S₈ NPs and Co-N_x species embedded in the carbon matrix cooperatively catalyze the OER and ORR, while the HER catalysis is mainly contributed by Co₉S₈ NPs. Furthermore, the Co₉S₈/CoNSC shows outstanding anti-poisoning capability towards sulfur species during ORR catalysis with no obvious activity degradation observed in 0.1 M KOH containing 50 μM SO₃^2- species, significantly outperforming commercial Pt/C. The assembled rechargeable Zn-air battery using the Co₉S₈/CoNSC as a cathode shows a high power density (150 mW cm^-2) and the assembled water electrolyzer only requires 1.585 V at a current density of 10 mA cm^-2 when using this material as an anode and a cathode. This work provides an effective strategy to design and synthesize efficient, durable and anti-poisoning cobalt chalcogenide-based trifunctional electrocatalysts for the large-scale application of clean energy conversion devices.

Graphitic Aza-Fused π-Conjugated Networks: Construction, Engineering, and Task-Specific Applications

2D π-conjugated networks linked by aza-fused units represent a pivotal category of graphitic materials with stacked nanosheet architectures. Extensive efforts have been directed at their fabrication and application since the discovery of covalent triazine frameworks (CTFs). Besides the triazine cores, tricycloquinazoline and hexaazatriphenylene linkages are further introduced to tailor the structures and properties. Diverse related materials have been developed rapidly, and a thorough outlook is necessitated to unveil the structure-property-application relationships across multiple subcategories, which is pivotal to guide the design and fabrication toward enhanced task-specific performance. Herein, the structure types and development of related materials including CTFs, covalent quinazoline networks, and hexaazatriphenylene networks, are introduced. Advanced synthetic strategies coupled with characterization techniques provide powerful tools to engineer the properties and tune the associated behaviors in corresponding applications. Case studies in the areas of gas adsorption, membrane-based separation, thermo-/electro-/photocatalysis, and energy storage are then addressed, focusing on the correlation between structure/property engineering and optimization of the corresponding performance, particularly the preferred features and strategies in each specific field. In the last section, the underlying challenges and opportunities in construction and application of this emerging and promising material category are discussed.

Triazine 2D Nanosheets as a New Class of Nanomaterials: Crystallinity, Properties and Applications

Based on the recent (2015–2021) literature data, the authors analyze the mutual dependence of crystallinity/amorphism and specific surface area and porosity in covalent triazine frameworks (CTFs), taking into account thermodynamic and kinetic control in the synthesis of these 2D nanosheets. CTFs have now become a promising new class of high-performance porous organic materials. They can be recycled and reused easily, and thus have great potential as sustainable materials. For 2D CTFs, numerous examples are given to support the known rule that the structure and properties of any material with a given composition depend on the conditions of its synthesis. The review may be useful for elder students, postgraduate students, engineers and research fellows dealing with chemical synthesis and modern nanotechnologies based on 2D covalent triazine frameworks.

Ultrasound-Triggered Assembly of Covalent Triazine Framework for Synthesizing Heteroatom-Doped Carbon Nanoflowers Boosting Metal-Free Bifunctional Electrocatalysis

The construction of multiple heteroatom-doped porous carbon with unique nanoarchitectures and abundant heteroatom active sites is promising for reversible oxygen-involving electrocatalysis. However, most of the synthetic methods required the use of templates to construct precisely designed nanostructured carbon. Herein, we introduced an ultrasound-triggered route for the synthesis of a piperazine-containing covalent triazine framework (P-CTF). The ultrasonic energy triggered both the polycondensation of monomers and the assembly into a nanoflower-shaped morphology without utilizing any templates. Subsequent carbonization of P-CTF led to the formation of nitrogen, phosphorus, and fluorine tri-doped porous carbon (NPF@CNFs) with a well-maintained nanoflower morphology. The resultant NPF@CNFs showed high electrocatalytic activity and stability toward bifunctional electrolysis, which was better than the commercial Pt/C and IrO₂ electrocatalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), respectively. As a further demonstration, employing NPF@CNFs as air electrode materials resulted in an excellent performance of liquid-state and solid-state Zn-air batteries, showing great potentials of the obtained multiple heteroatom-doped porous carbon electrocatalysts for wearable electronics.

Rational design of a Fe/S/N/C catalyst from ZIF-8 for efficient oxygen reduction reaction

Fe/N/C catalysts have been regarded as prospective electrocatalysts for oxygen reduction reactions (ORR). As reported, doping S into the Fe/N/C catalyst is an effective strategy to further enhance its ORR performance. Herein, a rational design is demonstrated to synthesize Fe/S/N/C catalysts with a flexible ratio of the doped Fe and S. Through atomic substitution and molecular confinement methods, Fe and S were incorporated into the ZIF-8 precursor, respectively. After further pyrolysis, the Fe/S/N/C catalyst was obtained with uniformly dispersed Fe-N_x, C-S-C active sites and high specific surface area. The Fe/S/N/C catalyst shows a high half-wave potential in alkaline medium, nearly 32 mV higher than the commercial Pt/C, owing to the strong synergistic effect from Fe-N_x and C-S-C active sites. Additionally, the Fe/S/N/C catalyst exhibits good long-term electrocatalytic durability and high endurance to methanol crossover, implying it is a suitable candidate to take the place of conventional Pt or Pt-based catalysts in electrochemical devices.

Mass Transfer in a Co/N/C Catalyst Layer for the Anion Exchange Membrane Fuel Cell

Developing highly efficient non-noble metal catalysts for the cathode of fuel cells is an urgent requirement for reducing the cost. Although the intrinsic activity of non-noble metal materials has been greatly improved, the fuel cell performance is also determined by the mass transfer within the catalyst layer (CL), particularly at high current density. Electrochemical impedance spectroscopy (EIS) combined with rotating disk electrode (RDE) analysis is a powerful tool to quantitatively analyze the influence of the structural properties on CL performance. Here, Co/N/C CLs with gradient pore structures are constructed based on the controllable synthesis of zeolitic imidazolate framework (ZIF)-derived catalyst. The influences of the carbon support, active site, and catalyst loading are comprehensively studied by EIS in different regions (kinetic and mixed-diffusion). The results indicate that a high micro-/mesopore ratio is beneficial to increasing the density of active sites while reducing the mass-transfer efficiency. Inversely, abundant mesopores promote mass transfer, but they result in low active site density. By carefully adjusting the pore structure and chemical composition of the ZIF-derived catalyst, the Co/N/C CL shows a low mass-transfer resistance (95.5 Ω at 0.75 V vs RHE). This work demonstrates the importance of mass transfer within the fuel cell CL, beyond seeking only high activity.

Synergistic effect of cobalt boride nanoparticles on MoS2 nanoflowers for a highly efficient hydrogen evolution reaction in alkaline media

The development of efficient, stable and low-cost electrocatalysts for the alkaline hydrogen evolution reaction (HER) is critical for large-scale, economically viable water splitting. In this work, we successfully prepared non-precious metal CoB@MoS2 hybrid electrocatalysts for the HER in alkaline media by the reductive growth of cobalt boride nanoparticles (CoB NPs) on the surface of MoS2 nanoflowers (MoS2 NFs). The CoB@MoS2-0.5-300 hybrid showed an HER overpotential of only 146 mV at a current density of 10 mA cm-2 and a Tafel slope of 80.9 mV dec-1 in 1.0 M KOH solution. The significantly enhanced HER activity of the hybrid is primarily attributable to the ability of CoB to drive the OER in alkaline solution and improved electrical conductivity of the hybrid electrocatalyst relative to the pristine MoS2. Furthermore, the synthetic strategy used to prepare the CoB@MoS2 electrocatalyst was successfully applied to prepare NiB@MoS2 and FeB@MoS2 hybrid electrocatalysts, which similarly showed very good HER activity in 1.0 M KOH solution. Thus, this work conclusively demonstrates that the introduction of transition metal borides is an effective approach for enhancing the HER performance of MoS2 in alkaline media.

Iron Nanoparticles Encapsulated in S,N-Codoped Carbon: Sulfur Doping Enriches Surface Electron Density and Enhances Electrocatalytic Activity toward Oxygen Reduction

Development of highly efficient nonprecious metal (NPM) catalysts for oxygen reduction reaction (ORR) in acidic media is challenging but of great significance. Herein, an effective ORR catalyst based on Fe nanoparticles encapsulated by S,N-codoped few-layer defective carbon (Fe@S,N-DC) was synthesized via a microwave-assisted strategy. The obtained Fe@S,N-DC nanocomposite showed a remarkable electrocatalytic activity toward ORR in acidic media, with a half-wave potential (E_1/2) of +0.785 V versus reversible hydrogen electrode, which was 80 mV more positive than that of the sulfur-free counterpart (Fe@N-DC). Furthermore, due to the protection by the S,N-codoped carbon shell, the Fe@S,N-DC nanocomposite displayed apparent stability with only a 13 mV negative shift of E_1/2 after 10,000 cycles and excellent tolerance to methanol. X-ray absorption near-edge spectroscopy measurements confirmed the formation of multiple defective sites on the S,N-codoped carbon surface and strong interfacial electron transfer from the Fe core to the outer carbon surface, as compared to the sulfur-free counterpart. The enriched electron density on the defective carbon surface of Fe@S,N-DC, induced by the interfacial electron transfer, facilitated the reduction of O₂ to OOH*, leading to enhanced ORR performance. These results shed light on the significance of S doping in Fe-N-C catalysts in the design of high-performance NPM catalysts for ORR in acidic media.

Atomic site electrocatalysts for water splitting, oxygen reduction and selective oxidation

This review summarized the fabrication routes and characterization methods of atomic site electrocatalysts (ASCs) followed by their applications for water splitting, oxygen reduction and selective oxidation.

New and Advanced Porous Carbon Materials in Fine Chemical Synthesis. Emerging Precursors of Porous Carbons

The efficiency of porous carbons in fine chemical synthesis, among other application fields, has been demonstrated since both the porous structure and chemical surface provide the appropriated chemical environment favoring a great variety of relevant chemical transformations. In recent years, metal organic frameworks (MOFs) and covalent organic frameworks (COFs) have emerged as interesting opportunities in the preparation of porous carbons with improved physico-chemical properties. Direct calcination of MOFs or COFs, in the presence or not of others carbon or heteroatom sources, could be considered an easy and practical approach for the synthesis of highly dispersed heteroatom-doped porous carbons but also new porous carbons in which single atoms of metallic species are present, showing a great development of the porosity; both characteristics of supreme importance for catalytic applications. The goal of this review is to provide an overview of the traditional methodologies for the synthesis of new porous carbon structures together with emerging ones that use MOFs or COFs as carbon precursors. As mentioned below, the catalytic application in fine chemical synthesis of these kinds of materials is at present barely explored, but probably will expand in the near future.

An N-doped porous carbon network with a multidirectional structure as a highly efficient metal-free catalyst for the oxygen reduction reaction

Metal-free catalysts have gained substantial attention as a promising candidate to replace the expensive platinum (Pt) catalysts for the oxygen reduction reaction (ORR) which is a key process in low temperature fuel cells. Development of highly efficient and mass-producible N-doped carbon catalysts, however, remains to be a great challenge. In this study, N-doped porous carbon (NPC) materials were synthesized via a simple, cost-effective and scalable method for mass production by using the d-gluconic acid sodium salt, pyrrole, Triton X-100 and KOH. The resulting NPC possessed a multidirectional porous carbon network (SBET: 1026.6 m2 g-1, Vt: 1.046 cm3 g-1) with hierarchical porosity and plenty of graphitic N species (49.1%). Electrochemical tests showed that the NPC itself was highly active for the ORR under alkaline and acidic conditions via a four electron pathway for the complete reduction of O2 in water. More importantly, this NPC catalyst demonstrated better performance than commercial Pt/C catalysts in terms of long-term durability and methanol tolerance under both conditions.

A ZIF-derived hierarchically porous Fe–Zn–N–C catalyst synthesized via a two-stage pyrolysis for the highly efficient oxygen reduction reaction in both acidic and alkaline media

A ZIF-derived Fe–Zn–N–C catalyst synthesized by a two-stage pyrolysis process exhibits superior activity for the ORR.

N-doped hierarchical porous metal-free catalysts derived from covalent triazine frameworks for the efficient oxygen reduction reaction

The hierarchical structure enhances oxygen diffusion, improves electron transfer, and exposes more catalytic active sites for the ORR.