B, N Co‐Doped Three‐Dimensional Carbon Aerogels with Excellent Electrochemical Performance for the Oxygen Reduction Reaction

Enhancing the supercapacitive performance of a carbon-based electrode through a balanced strategy for porous structure, graphitization degree and N,B co-doping

Regarding carbon-based electrodes, simultaneously establishing a well-defined meso-porous architecture, introducing abundant hetero-atoms and improving the graphitization degree can effectively enhance their capacitive performance. However, it remains a significant challenge to achieve a good balance between defects and graphitization degree. In this study, the porous structure and composition of carbon materials are co-optimised through a 'dual-function' strategy. Briefly, K3Fe(C2O4)3 and H3BO3 were hybridised with a gelatin aqueous solution to form a homogeneous composite hydrogel, followed by lyophilisation and carbonisation. Owing to the dual functionality of raw materials, the graphitization, activation and hetero-atom doping processes can occur simultaneously during a one-step high-temperature treatment. The resultant carbon material exhibits a high graphitization degree (ID/IG = 0.9 ± 0.1), high hetero-atom content (N: 9.0 ± 0.3 at.%, B: 6.9 ± 0.5 at.%) and a large specific area (1754 ± 58 m2/g). The as-prepared electrode demonstrates a superior capacitance of 383 ± 1F g-1 at 1 A/g. Interestingly, the cyclic voltammetry (CV) curves exhibit a distinctive pair of broad redox peaks, which is uncommon in KOH electrolyte. Experiment data and density functional theory (DFT) simulation verify that N-5, B co-doping enhances the activity of the faradic reaction of carbon electrodes in KOH electrolyte. Furthermore, the fabricated Zn-ion hybrid supercapacitor (ZHSC) based on this carbon electrode delivers a high-energy density of 140.7 W h kg-1 at a power density of 840 W kg-1.

Biomass-Derived Carbon Aerogels for ORR/OER Bifunctional Oxygen Electrodes

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial electrochemical reactions that play vital roles in energy conversion and storage technologies, such as fuel cells and metal-air batteries. Typically, noble-metal-based catalysts are required to enhance the sluggish kinetics of the ORR and OER, but their high costs restrict their practical commercial applications. Thus, highly active and strong non-noble metal catalysts are essential to address the cost and durability challenge. Based on previous research, carbon-based catalysts may present the best alternatives to these precious metals in the future owing to their affordability, very large surface areas, and superior mechanical and electrical qualities. In particular, carbon aerogels prepared using biomass as the precursors are referred to as biomass-derived carbon aerogels. They have sparked broad attention and demonstrated remarkable performance in the energy conversion and storage sectors as they are ecologically beneficial, affordable, and have an abundance of precursors. Therefore, this review focuses on various nanostructured materials based on biomass-derived carbon aerogels as ORR/OER catalysts, including metal atoms, metal compounds, and alloys.

Bimetallic ZIF-derived conductive network of Co–Zn@NPC@MWCNT nanocomposites for efficient electromagnetic wave absorption in the whole X-band

Bimetallic ZIFs-derived Co-Zn@NPC@MWCNTs nanocomposites are successfully fabricated, which possess double absorption peaks of −76.18 dB and −33.09 dB with a thickness of 3.187 mm. The composites exhibit a bandwidth of 6.56 GHz with 3.0 mm thickness.

Identifying active sites of boron, nitrogen co-doped carbon materials for the oxygen reduction reaction to hydrogen peroxide

The electrochemical synthesis of hydrogen peroxide (H₂O₂) from two-electron oxygen reduction reaction (2e^- ORR) is a promising alternative for producing chemicals on demand, but its widespread application is still hampered by the low efficiency. Here, we successfully prepared a boron and nitrogen co-doped porous carbon (B/NC) aerogel with a tunable B, N co-doped configuration by the gelation of PVA-graphene, borax and PANI, followed by pyrolysis. Due to a hierarchical porous structure and optimized B, N co-doping, B/NC aerogel showed an excellent electrocatalytic performance for H₂O₂ production in alkaline solution with a high H₂O₂ selectivity (94.16%) at positive applied potential (0.6 V vs. RHE), superior than most of the other reported electrocatalysts. Density functional theory (DFT) calculations reveal that the hexagonal boron nitride (hBN) coupled with neighboring pyridinic-N species act as the active sites to lower free energy barrier for formation of HOO* intermediate, thus facilitating H₂O₂ production. Practically, B 2p electron plays an important role for the adsorption of HOO* intermediates. B and Nco-doping into carbon materials provides an effective and facile method to reasonably construct carbon-based catalysts for electroreduction of O₂ to H₂O₂.

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.

Metal-Free B, N co-Doped Hierarchical Porous Carbon Electrocatalyst with an Excellent O2 Reduction Performance

Fuel cells have attracted increasing attention due to their low cost, high energy density, low environmental pollution, and abundant raw materials. Oxygen reduction reaction (ORR) is a core technology of fuel cells, and the development of new electrocatalysts with high ORR performance is highly desirable. Herein, we synthesize a series of B, N co-doped hierarchical porous carbons using a soft template method with the integration of self-assembly, calcination and etching. The obtained materials exhibit hierarchical porous structures, controllable pore distribution, partial graphite structures, and B, N co-doping. They can function as the cost-effective and metal-free electrocatalysts, facilitating the diffusion of electrolyte ions and the improvement of ORR performance. Especially, the B, N co-doped porous carbon with the B-to-N molar ratio of 5 (BNC-5) displays a high ORR activity with a half-wave potential (E1/2 ) of 0.73 V, an onset potential (Eonset ) of 0.94 V, and a high limiting current density (JL ) of 5.98 mA cm-2 , superior to the N-doped C (NC) and BNC-1 (the B-to-N molar ratio=1), BNC-3 (the B-to-N molar ratio=3) and BNC-7 (the B-to-N molar ratio=7) under the identical conditions. Moreover, the BNC-5 exhibits good cycling stability after 5000 cyclic voltammetry (CV) cycles and excellent tolerance toward even 3 M methanol. This research provides a new approach for the facile synthesis of dual element-doped carbon electrocatalysts with high ORR performance.

Cu/Cux S-Embedded N,S-Doped Porous Carbon Derived in Situ from a MOF Designed for Efficient Catalysis

The reasonable design of the precursor of a carbon-based nanocatalyst is an important pathway to improve catalytic performance. In this study, a simple solvothermal method was used to synthesize [Cu(TPT)(2,5-tdc)] ⋅ 2H₂ O (Cu-MOF), which contains N and S atoms, in one step. Further in-situ carbonization of the Cu-MOF as the precursor was used to synthesize Cu/Cu_x S-embedded N,S-doped porous carbon (Cu/Cu_x S/NSC) composites. The catalytic activities of the prepared Cu/Cu_x S/NSC were investigated through catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The results show that the designed Cu/Cu_x S/NSC has exceptional catalytic activity and recycling stability, with a reaction rate constant of 0.0256 s^-1 , and the conversion rate still exceeds 90 % after 15 cycles. Meanwhile, the efficient catalytic reduction of dyes (CR, MO, MB and RhB) confirmed its versatility. Finally, the active sites of the Cu/Cu_x S/NSC catalysts were analyzed, and a possible multicomponent synergistic catalytic mechanism was proposed.