Carbon-based Metal-free Electrocatalysts Research Articles

Recent Advances on Carbon-Based Metal-Free Electrocatalysts for Energy and Chemical Conversions.

Over the last decade, carbon-based metal-free electrocatalysts (C-MFECs) have become important in electrocatalysis. This field is started thanks to the initial discovery that nitrogen atom doped carbon can function as a metal-free electrode in alkaline fuel cells. A wide variety of metal-free carbon nanomaterials, including 0D carbon dots, 1D carbon nanotubes, 2D graphene, and 3D porous carbons, has demonstrated high electrocatalytic performance across a variety of applications. These include clean energy generation and storage, green chemistry, and environmental remediation. The wide applicability of C-MFECs is facilitated by effective synthetic approaches, e.g., heteroatom doping, and physical/chemical modification. These methods enable the creation of catalysts with electrocatalytic properties useful for sustainable energy transformation and storage (e.g., fuel cells, Zn-air batteries, Li-O2 batteries, dye-sensitized solar cells), green chemical production (e.g., H2O2, NH3, and urea), and environmental remediation (e.g., wastewater treatment, and CO2 conversion). Furthermore, significant advances in the theoretical study of C-MFECs via advanced computational modeling and machine learning techniques have been achieved, revealing the charge transfer mechanism for rational design and development of highly efficient catalysts. This review offers a timely overview of recent progress in the development of C-MFECs, addressing material syntheses, theoretical advances, potential applications, challenges and future directions.

Inclusion Strategy for Constructing S/N Orbital Hybridization-Regulated sp3/sp2 Carbon toward Boost Electrocatalysis.

Metal-free carbon-based electrocatalysts have gained significant attention in the field of zinc-air batteries (ZABs) due to their affordability, good conductivity and chemical stability. However, unmodified carbon materials typically fall short in adsorbing and activating the substrates and intermediates involved in oxygen reduction reactions (ORR). Here, a metal-free carbon-based electrocatalyst with S atom p orbital hybrid modified N-sp3/sp2 carbon structure (C/NS) were prepared by cyclodextrins inclusion. The catalyst demonstrates impressive ORR activity (E1/2=0.885 V vs. RHE) and robust ZABs performance with a power density of 171.3 mW cm-2 and a specific capacity of 781.2 mAh g-1. Density functional theory (DFT) calculation reveals that S atom effectively regulates the charge distribution and p-band center of active site carbon atom in the N-sp3/sp2 carbon structure. This modification prompts the adsorption and dissociation of O2 and intermediates, resulting in higher reactive activity. This work provides a valuable and practical strategy for preparing cost-effective metal-free carbon-based electrocatalysts for ORR with high performance.

N-P covalent bond regulation of mesoporous carbon-based catalyst for lowered oxygen reduction overpotential and enhanced zinc-air battery performance

Developing sustainable metal-free carbon-based electrocatalysts is essential for the deployment of metal-air batteries such as zinc-air batteries (ZABs), among which doping of heteroatoms has attracted tremendous interest over the past decade. However, the effect of the heteroatom covalent bonds in carbon matrix on catalysis was neglected in most studies. Here, an efficient metal-free oxygen reduction reaction (ORR) catalyst is demonstrated by the N-P bonds anchored carbon (termed N,P-C-1000). The N,P-C-1000 catalyst exhibits superior specific surface area of 1362 m2 g−1 and ORR activity with a half-wave potential of 0.83 V, close to that of 20 wt% Pt/C. Theoretical computations reveal that the p-band center for C-2p orbit in N,P-C-1000 has higher interaction strength with the intermediates, thus reducing the overall reaction energy barrier. The N,P-C-1000 assembled primary ZAB can attain a large peak power density of 121.9 mW cm−2 and a steady discharge platform of ∼1.20 V throughout 120 h. Besides, when served as the cathodic catalyst in a solid-state ZAB, the battery shows flexibility, conspicuous open circuit potential (1.423 V), and high peak power density (85.8 mW cm−2). Our findings offer a strategy to tune the intrinsic structure of carbon-based catalysts for improved electrocatalytic performance and shed light on future catalysts design for energy storage technologies beyond batteries.

Metal-free N, B, F ternary-doped carbon electrocatalyst for boosting the oxygen reduction reaction and high-performance zinc–air battery

Developing efficient metal-free carbon-based electrocatalysts for the oxygen reduction reaction (ORR) is significant and challenging to reduce the cost of fuel cells and metal–air batteries for large-scale applications. Herein, a novel metal-free N, B, F ternary-doped carbon electrocatalyst (FBNCΔ(1:6)-950) exhibited an excellent ORR performance with half-wave potential of 0.858 V vs. reversible hydrogen electrode (RHE) via an almost exclusively four-electron transfer pathway in 0.1 M KOH, even better than the commercial Pt/C (20 %). Moreover, the FBNCΔ(1:6)-950-based zinc–air battery (ZAB) shown an excellent power density of 147 mW·cm−2 and long-term stability of 100 h. The FBNCΔ(1:6)-950 was constructed via pyrolyzing zeolite imidazole framework (ZIF-8) precursor containing BF4– at 950 °C under N2 to volatilize the Zn. According to the structural characterization and theoretical calculations, B and F from BF4– not only increased the number of active sites of pyridine-N by etching intact carbon matrix, but also co-modified part of pyridine-N to further optimize the electronic structure and reduce the adsorption energy of *OH, both of which together result in the excellent performance of FBNCΔ(1:6)-950. This work provides a fresh perspective on the advancement of highly active metal-free carbon-based electrocatalysts for the ORR and ZAB.

Tailoring the Atomic-Local Environment of Carbon Nanotube Tips for Selective H2 O2 Electrosynthesis at High Current Densities.

The atomic-local environment of catalytically active sites plays an important role in tuning the activity of carbon-based metal-free electrocatalysts (C-MFECs). However, the rational regulation of the environment is always impeded by synthetic limitations and insufficient understanding of the formation mechanism of the catalytic sites. Herein, the possible cleavage mechanism of carbon nanotubes (CNTs) through the crossing points during ball-milling is proposed, resulting in abundant CNT tips that are more susceptible to be modified by heteroatoms, achieving precise modulation of the atomic environment at the tips. The obtained CNTs with N,S-rich tips (N,S-TCNTs) exhibit a wide potential window of 0.59V along with H2 O2 selectivity for over 90.0%. Even using air as the O2 source, the flow cell system with N,S-TCNTs catalyst attains high H2 O2 productivity up to 30.37molgcat. -1 h-1 @350mAcm-2 , superior to most reported C-MFECs. From a practical point of view, a solid electrolyzer based on N,S-TCNTs is further employed to realize the in-situ continuous generation of pure H2 O2 solution with high productivity (up to 4.35mmolcm-2 h-1 @300mAcm-2 ; over 300h). The CNTs with functionalized tips hold great promise for practical applications, even beyond H2 O2 generation.

Quasi-Three-Dimensional Cyclotriphosphazene-Based Covalent Organic Framework Nanosheet for Efficient Oxygen Reduction

HighlightsJUC-610-nanosheet exhibits highly efficient oxygen reduction reaction (ORR) catalytic activity in alkaline electrolyte with half-wave potential of 0.72 V versus reversible hydrogen electrode, which is one of the best covalent organic frameworks (COF)-based ORR electrocatalysts reported so far.It has been confirmed by experiments and density functional theory calculations that the abundant electrophilic structure in Q3CTP-COFs induces a highly density of carbon active sites, and the unique bilayer stacking facilitates the exposure of active carbon sites and accelerates the mass diffusion during ORR.JUC-610-nanosheet can also serve as a promising cathode for Zn-air batteries (power density of 156 mW cm–2 at 300 mA cm–2), which promotes the development of metal-free carbon-based electrocatalysts.

Monomicelle-Directed Engineering of Strained Carbon Nanoribbons as Oxygen Reduction Catalyst.

To date, precisely tailoring local active sites of well-defined earth-abundant metal-free carbon-based electrocatalysts for attractive electrocatalytic oxygen reduction reaction (ORR), remains challenging. Herein, the authorssuccessfully introduce a strain effect on active C-C bonds adjacent to edged graphitic nitrogen (N), which raises appropriate spin-polarization and charge density of carbon active sites and kinetically favor the facilitation of O2 adsorption and the activation of O-containing intermediates. Thus, the constructed metal-free carbon nanoribbons (CNRs-C) with high-curved edges exhibit outstanding ORR activity with half-wave potentials of 0.78 and 0.9V in 0.5m H2 SO4 and 0.1m KOH, respectively, overwhelming the planar one (0.52 and 0.81V) and the N-doped carbon sheet (0.41 and 0.71V). Especially in acidic media, the kinetic current density (Jk ) is 18 times higher than that of the planar one and the N-doped carbon sheet. Notably, thesefindings show the spin polarization of the asymmetric structure by introducing a strain effect on the C-C bonds for boosting ORR.

Recent progress in carbon-based electrochemical catalysts: From structure design to potential applications

Advances in research and development of carbon-based metal-free electrocatalysts (C-MFECs) have provided potential alternatives to precious metal catalysts for various reactions important to renewable energy and environmental remediation. This timely but critical review provides an overview of recent breakthroughs (within the past 5 years or so) on C-MFECs in all aspects, including the design and regulation of intrinsic catalytic active sites, design and synthesis of carbon composite and hybrid carbon catalysts, mechanism understanding, and potential applications in clean energy storage and energy/chemical conversion. Current challenges and future opportunities in the field of metal-free carbon electrocatalysis are also discussed to provide forward-looking opportunities for their potential applications in various catalytic processes of practical significance.

Pre-Oxidation-Tuned Oxygen and Nitrogen Species of Porous Carbon as an Advanced Electrocatalyst of the Oxygen Reduction Reaction for Flow Zn–Air Batteries

The co-engineering of defects and N doping is highly important for the development of advanced metal-free carbon-based electrocatalysts for the oxygen reduction reaction (ORR). It remains a great challenge to efficiently tune the doped nitrogen species, and the role of oxygen species is often overlooked. Here, a facile strategy is developed for fabricating three-dimensional oxygen and nitrogen codoped porous carbon (ONPC) via air pre-oxidation, followed by carbonization. The optimal catalyst, i.e., ONPC-300, mainly contains pyridinic N and graphitic N species with oxygen-rich functional (ketone C═O) groups and has an enhanced specific surface area, superhydrophilic surface structure, and abundant defect sites for ORR, which endows it with excellent ORR performance, as evidenced by a half-wave potential of 0.862 V and high long-term stability over 100 h in an alkaline medium. Density functional theory (DFT) calculations reveal that ketone C═O groups can optimize the adsorption energy of intermediates and decrease the energy barrier for the rate-determining steps, thus promoting the ORR activity. A flow Zn–air battery using ONPC-300 as the cathode shows outstanding performance, including a remarkable peak power density of 258.5 mW cm–2, high discharge stability over 180 h, and an ultralong cycle life exceeding 800 h.

Unifying the origin of catalytic activities for carbon-based metal-free electrocatalysts

Carbon-based metal-free electrocatalysts (C-MFECs) have received great attention owing to their high efficiency, earth-abundance and tolerance to crossover effect. Recent research and development have demonstrated that various strategies, including heteroatom doping, defect doping, and molecular modification (doping), can induce catalytic active sites into carbon materials to create efficient and multifunctional electrocatalysts. Theoretical research has revealed that the catalytic activity in C-MFECs origins from the charge/spin redistributions induced by the change in electronic structures of doped carbon materials. In this review article, recent advances in the development of C‐MFECs are discussed and the origin of catalytic activities for all C-MFECs is unified to the heteroatom/defect/molecular doping-induced charge transfer/spin redistribution. The progress in theoretical studies of the unified mechanistic understanding is also overviewed for the rational design of C-MFECs. Finally, current challenges and future perspectives in this exciting field are presented.

Facile synthesis of hollow carbon spheres by gas-steamed bifunctional NH4F for efficient cathodes in microbial fuel cells

A facile gas-steamed strategy is reported for preparing heteroatom dual-doped hierarchical porous hollow carbon catalysts via carbonization of a mixture of carbon sphere precursor and ammonium fluoride (NH4F). Notably, NH4F can be decomposed into NH3 and HF by pyrolysis, in which HF gas can etch SiO2 pellets to form hollow structure while the N and F atoms can be introduced at the same time. The FCS-900 exhibits admirable electrocatalytic properties with the highest onset potential and limiting current density in neutral electrolytes (0.944 V vs. RHE and 6.44 mA cm−2). In comparison to MFC-Pt, much higher output voltage and power density (0.617 V and 1093.6 ± 6.26 mW m−2) are obtained by MFC-900. Such results can be attributed to the largest specific surface area of FCS-900 to supply exposed active sites and fast transportation channels. Based on X-ray photoelectron spectroscopy, the FCS-900 catalyst possesses the active substances of pyridinic/graphitic N and C-F bonds. The synergism of N and F can effectively facilitate the adsorption of O2 during ORR, as further supported by density functional theory calculation. The facile and green synthesis strategy can be extended to design metal-free carbon-based electrocatalysts with superior electrocatalytic performance.

NiCo2O4 spinel supported N-dopped porous Hollow carbon derived MOF functionalized SiO2 for efficient ORR electrocatalysis

The improvement of carbon-based metal-free electrocatalysts for the cathodic ORR that are environmentally benign, cost-effective, and highly durable is required for large-scale commercial applications of fuel cells. Herein, we developed the TiO2 nanoparticles supported by a nitrogen-doped carbon matrix with high surface defects derived by the pyrolysis of NH2-MIL-125 (MOF). The construction of controllable morphology, porosity, and particle size through the pyrolysis process can be obtained by introducing the SiO2 template during the NH2-MIL-125 synthesis. Moreover, the synergic covalent coupling between metal oxide spinel (AB2O4, especially NiCo2O4) and TiO2/N- doped nanocarbon effectively enhanced the ORR catalytic activity, particularly with high content of nickel atoms in the spinel due to the various valences of metals in spinel which accelerated the electron transfer during ORR. As a highly effective ORR electrocatalyst with an onset potential of −0.14 V vs Ag/AgCl, the as-prepared N3CO/Es-TiO2/NC exhibited not only high activity but also good stability in an alkaline environment compared to the Pt/C catalyst. The less current change after continuous 1000 cycles of cyclic voltammetry or the methanol addition may be due to the protective effect of the N-doped carbon carrier and the corrosion resistance of TiO2.

Heterocyclic Modulated Electronic States of Alkynyl-Containing Conjugated Microporous Polymers for Efficient Oxygen Reduction.

The oxygen reduction reaction (ORR) is a key process in green energy conversion technology. Heteroatom doping has been proven to be a prospective strategy to prepare metal-free carbon-based electrocatalysts, but such methods often suffer from uncontrollable catalyst frameworks and imprecise active sites. Herein, an organic heterocyclic strategy is adopted to modulate the charge redistribution of alkynyl-containing conjugated microporous polymers (CMPs) by introducing varied five-membered heterocyclic structures. Among these CMPs, the S, 2N-containing thiadiazole heterocyclic molecule (CMP-Tdz) with carbonized alginate materials (CCA ) displays a remarkable quasi-four-electron-transfer ORR pathway, exhibiting an excellent half-wave potential (E1/2 ) of 0.77 V, coupled with superior methanol tolerance and electrochemical stability, which are among the highest performance in the metal-free organic catalytic material systems. Density functional theory calculations prove that the high catalytic performance of these catalysts originates from the sp-hybridized C atom (site-2) which is activated by their adjacent heterocyclic structures. Importantly, the five-membered heterocyclic structures can also modulate the local charge distribution, and increase dipole moment, with significantly improved catalytic kinetics. This incorporation of chemically designed heterocyclic-containing alkynyl-CMPs provides a new approach to developing efficient metal-free carbon-based electrocatalysts for fuel cells.

Fullerene-Derived Porous and Defective N-Doped Carbon Nanosheets as Advanced Trifunctional Metal-Free Electrocatalysts.

Dopants and defects are crucial for multifunctional carbon-based metal-free electrocatalysts, but the rational design and facile production remain as a big challenge. Herein, we report a novel strategy using salt-assisted pyrolysis of derivatized fullerenes to fabricate defect-rich and N-doped carbon nanosheets. Molecular level modification of C60 via amination and hydroxylation gives rise to well-defined fullerol molecules bearing N-containing groups (FNG). Subsequent calcination of FNG and NaCl at 750 °C generates porous carbon nanosheets (FNCNs-750) and turns the N-containing groups into high-level N dopants (12.43 at.%). Further pyrolysis of FNCNs-750 at 900 °C (FNCNs-900) leads to a reduced N content populated by graphitic-N. Meanwhile, abundant intrinsic defects (e. g., topological defects and edges) are created due to the breakdown of fullerene cages and partial removal of edged N atoms. These structural features endow FNCNs-900 with outstanding trifunctional catalytic performance, better than or close to the noble metal-based benchmark catalysts.

Ternary (N, B, F)-Doped Biocarbon Derived from Bean Residues as Efficient Bifunctional Electrocatalysts for Oxygen Reduction and Evolution Reactions

Development of efficient metal-free carbon-based electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are of great significance for Zn-air batteries. Herein, a porous biocarbon bifunctional catalyst (C-NBF-G) was directly synthesized via simple alkali activation and carbonization from bean residues. C-NBF-G exhibited hierarchical porous structures, ternary heteroatom (N, B, and F) doping, a large specific surface area, and a relatively high degree of graphitization. The synergistic action of these characteristics contributed to the outstanding catalytic properties of C-NBF-G for ORR and OER. The catalyst demonstrated an onset potential of 0.94 V, half-wave potential of 0.824 V, and a limiting current density of 5.92 mA cm−2, comparable to those of the commercial 20 wt% Pt/C catalysts. C-NBF-G also exhibited an OER overpotential of 333 mV at 10 mA cm−2 and a Tafel slope of 114 mV dec−1, lower than those of the commercial Pt/C and RuO2 catalysts. These results proved the promising performance of C-NBF-G as a bifunctional catalyst for the ORR and OER.

Enhanced oxygen reduction of porous N-doped carbon nanosheets with graphitic N and defects obtained from coal-based graphene quantum dots

Metal-free carbon-based electrocatalysts for oxygen reduction reaction (ORR) have received considerable attention owing to their abundance and low cost. However, designing carbon-based electrocatalysts with excellent performance in a facile manner remains a challenge. In this work, porous nitrogen-doped carbon nanosheet (PNCNs) catalysts with high contents of graphitic N and defects were readily prepared via a simple self-doping method, in which coal-based graphene quantum dots (GQDs) were used as the carbon and nitrogen resource. Based on the results, PNCNs-900 (obtained at 900 °C) displayed good ORR performance, with a half-wave potential (E1/2) of 0.80 V. It also exhibited high stability and methanol tolerance compared with commercial Pt/C. Experimental investigations and theoretical calculations prove that the increased ORR electrocatalytic activity of PNCNs-900 is due to the synergistic effect of a large number of defects and graphitic N. Furthermore, its higher electrocatalytic activity for ORR also benefits from the porous lamellar structure, which facilitates the mass transfer process. This study offers a novel approach for developing effective ORR catalysts and for the high value-added utilization of coal resources.

Poly(ionic liquids) derived N, S co-doped carbon nanorod from in situ and template-free method as an efficient metal-free bifunctional electrocatalysts for direct methanol fuel cells

Developing highly efficient strategy for fabrication of bifunctional electrocatalysts for oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) is a challenging topic. In this work, we reported the in situ synthesis of N, S co-doped carbon nanorod material (N/S-C) by the facile template-free, one-step pyrolysis approach using N and S co-containing poly(ionic liquid) (PIL) of p[BZ][HSO 4 ] 2 as precursor derived from the simple chemically oxidative polymerization of ionic liquid benzidine sulfate ([BZ][HSO 4 ] 2 ) in the presence of ammonium persulfate as oxidant. The N/S-C catalysts have nanorod morphology with hierarchical porous architectures of abundant micropores and well-defined mesopores featuring high specific surface areas. The ORR performance of N/S-C-800 is extremely satisfactory in alkaline media with the onset and half-potentials of 0.985 V and 0.857 V (vs. RHE), respectively, which are comparable to that of commercial Pt/C (0.990 V and 0.852 V vs. RHE). It also delivers the favorable long-term stability and methanol tolerance, significantly outperforming commercial Pt/C. Moreover, the N/S-C-800 was served as a support for Pt nanoparticles, showing the higher methanol electrooxidation activity (806.74 mA mg Pt -1 ) compared with commercial Pt/C (356.32 mA mg Pt -1 ). These results show an effective route to design the attractive metal-free carbon-based electrocatalysts for direct methanol fuel cells. • The N/S co-doped carbon nanorod material was synthesized in situ via facile one-step pyrolysis approach using the poly(ionic liquids) (PILs) as precursor. • The N/S-C-800 catalyst exhibited the superior ORR activity and stability in alkaline media. • The Pt nanoparticles supported on the N/S-C-800 catalyst showed superior MOR activity.

Carbon-based metal-free oxygen reduction reaction electrocatalysts: Past, present and future

Carbon-based metal-free oxygen reduction reaction electrocatalysts: Past, present and future

Nanosheets with High-Performance Electrochemical Oxygen Reduction Reaction Revived from Green Walnut Peel.

The synthesis of metal-free carbon-based electrocatalysts for oxygen reduction reactions (ORR) to replace conventional Pt-based catalysts has become a hot spot in current research. This work proposes an activation-assisted carbonization strategy, to manufacture N-doped ultra-thin carbon nanosheets (GWS180M800) with high catalytic activity, namely, melamine is used as an accelerator/nitrogen source, and walnut green peels biological waste as a carbon source. The melamine acts as a nitrogen donor in the hydrothermal process, effectively enhancing the nitrogen doping rate. The content of pyridine nitrogen groups accounts for up to 48.5% of the total nitrogen content. Electrochemical tests show that the GWS180M800 has excellent ORR electrocatalytic activity and stability, and makes a quasi-four-electron ORR pathway clear in the alkaline electrolyte. The initial potential and half slope potential are as high as 1.01 and 0.82 V vs. RHE, respectively. The GWS180M800 catalyst has a better ability to avoid methanol cross poisoning than Pt/C has. Compared with 20 wt% Pt/C, GWS180M800 has improved methanol tolerance and stability. It is a metal-free biochar ORR catalyst with great development potential and application prospects. This result provides a new space for the preparation of valuable porous nano-carbon materials based on carbonaceous solid waste and provides new ideas for catalyzing a wide range of electrochemical reactions in the future.

Charge transfer of carbon nanomaterials for efficient metal‐free electrocatalysis

AbstractRecently, carbon‐based metal‐free electrocatalysts (C‐MFECs) have drawn considerable research attention because of their attractive physicochemical characteristics, cost‐effectiveness, and ability to convert and store energy efficiently. Efficient intramolecular charge transfer among different parts of the carbon electrocatalyst and/or intermolecular charge transfer between electrocatalyst and electrolyte dictate the ultimate energy conversion performance. Experimental results and theoretical analyses have demonstrated that rational design of metal‐free carbon nanomaterials, coupled with proper intramolecular charge transfer through heteroatom doping, incorporation of Stone–Wales defects, and/or intermolecular charge transfer through adsorption of appropriate molecules/moieties, can promote efficient electrocatalysis. In this article, we will first provide the related theoretical principles and then present an overview on the rational design and development of C‐MFECs for efficient charge transfer, followed by elucidating charge‐transfer processes for different electrocatalytic reactions related to renewable energy conversion and environmental remediation technologies. Finally, the current challenges and future perspectives in this exciting field will be discussed.