Search

Supramolecular chemistry during the synthesis of carbon-nitrogen-based materials has recently experienced a renaissance in the arena of photocatalysis and electrocatalysis. In this review, we start with the discussion of supramolecular assemblies-derived carbon-nitrogen-based materials’ regulation from the aspect of morphology, chemical composition, and micro/nanostructural control. Afterwards the recent advances of these materials in energy and environment related applications, including degradation of pollutants, water splitting, oxygen reduction reactions, CO2 reduction reactions along with organic synthesis are summarized. The correlations between the structural features and physicochemical properties of the carbon-nitrogen-based materials and the specific catalytic activity are discussed in depth. By highlighting the opportunities and challenges of supramolecular assembly strategies, we attempt an outlook on possible future developments for highly efficient carbon-based photo/electrocatalysts.

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. [ABSTRACT FROM AUTHOR]

The continuously increasing CO2 released from human activities poses a great threat to human survival by fluctuating global climate and disturbing carbon balance among the four reservoirs of the biosphere, earth, air, and water. Converting CO2 to value-added feedstocks via electrocatalysis of the CO2 reduction reaction (CO2RR) has been regarded as one of the most attractive routes to re-balance the carbon cycle, thanks to its multiple advantages of mild operating conditions, easy handling, tunable products and the potential of synergy with the rapidly increasing renewable energy (i.e., solar, wind). Instead of focusing on a special topic of electrocatalysts for the CO2RR that have been extensively reviewed elsewhere, we herein present a rather comprehensive review of the recent research progress, in the view of associated value-added products upon selective electrocatalytic CO2 conversion. We initially provide an overview of the history and the fundamental science regarding the electrocatalytic CO2RR, with a special introduction to the design, preparation, and performance evaluation of electrocatalysts, the factors influencing the CO2RR, and the associated theoretical calculations. Emphasis will then be given to the emerging trends of selective electrocatalytic conversion of CO2 into a variety of value-added products. The structure-performance relationship and mechanism will also be discussed and investigated. The outlooks for CO2 electrocatalysis, including the challenges and opportunities in the development of new electrocatalysts, electrolyzers, the recently rising operando fundamental studies, and the feasibility of industrial applications are finally summarized. [ABSTRACT FROM AUTHOR]

Electrocatalytic nitrogen reduction reaction (NRR) plays a vital role for next‐generation electrochemical energy conversion technologies. However, the NRR kinetics is still limited by the sluggish hydrogenation process on noble‐metal‐free electrocatalyst. Herein, we report the rational design and synthesis of a hybrid catalyst with atomic iron sites anchored on a N,O‐doped porous carbon (FeSA‐NO‐C) matrix of an inverse opal structure, leading to a remarkably high NH3 yield rate of 31.9 μgNH3 h−1 mg−1cat. and Faradaic efficiency of 11.8 % at −0.4 V for NRR electrocatalysis, outperformed almost all previously reported atomically dispersed metal‐nitrogen‐carbon catalysts. Theoretical calculations revealed that the observed high NRR catalytic activity for the FeSA‐NO‐C catalyst stemmed mainly from the optimized charge‐transfer between the adjacent O and Fe atoms homogenously distributed on the porous carbon support, which could not only significantly facilitate the transportation of N2 and ions but also effectively decrease the binding energy between the isolated Fe atom and *N2 intermediate and the thermodynamic Gibbs free energy of the rate‐determining step (*N2 → *NNH). [ABSTRACT FROM AUTHOR]

Though the use of conventional self‐assembled architectures in functional applications involving advanced energy chemistries is an important research area, it remains largely unexplored. The self‐assembly of the threefold and sixfold‐symmetric terpyridines (tpy) with Co(II) salts results in a novel morphological and structural characteristics, regardless of the nature of the self‐assembled fragments. Herein, such metallopolymers are achieved by one‐pot synthesis in CH3OH/CHCl3 (v/v = 5:1) mixture ambient. It is found, for the first time, that Co‐containing polymers can be well dispersed in deionized water to form gel‐like self‐assemblies that consist of a highly interconnected 3D network and exhibit enhanced electrical conductivity and thus are attractive as electrocatalysts. As expected, the optimized Co‐based polymeric structures exhibit a low overpotential of 320 mV at 10 mA cm−2 and high stability over 2000 cycles toward oxygen evolution reaction (OER), surpassing commercial RuO2/C, single‐site Co catalysts, polymer, and metal–organic framework‐based OER catalysts reported to date. X‐ray absorption spectroscopy and density functional theory calculations reveal that the tpy‐Co2+ (3N‐Co or tpy‐Co2+) configurations act as highly active sites. Importantly, this work demonstrates the functional application of the self‐assembled metallopolymers as electrocatalysts for energy conversion. [ABSTRACT FROM AUTHOR]

Metal–CO2 batteries have attracted much attention owing to their high energy density and use of greenhouse CO2 waste as the energy source. However, the increasing cost of lithium and the low discharge potential of Na–CO2 batteries create obstacles for practical applications of Li/Na–CO2 batteries. Recently, earth‐abundant potassium ions have attracted considerable interest as fast ionic charge carriers for electrochemical energy storage. Herein, we report the first K–CO2 battery with a carbon‐based metal‐free electrocatalyst. The battery shows a higher theoretical discharge potential (E⊖=2.48 V) than that of Na–CO2 batteries (E⊖=2.35 V) and can operate for more than 250 cycles (1500 h) with a cutoff capacity of 300 mA h g−1. Combined DFT calculations and experimental observations revealed a reaction mechanism involving the reversible formation and decomposition of P121/c1‐type K2CO3 at the efficient carbon‐based catalyst. [ABSTRACT FROM AUTHOR]

A combined surface and dual electronic modulation strategy is used to realize metal‐free all‐pH catalysis towards the hydrogen evolution reaction (HER) by coupling a N‐doped carbon framework (MHCF, electron acceptors) derived from MOFs with higher‐Fermi‐level pure carbon nanotubes (CNTs, electron donors), followed by surface modification with carboxyl‐group‐rich polymers. Although the three constituents are inactive, as‐assembled ternary membranes yield superior HER performance with low overpotentials and high durability (≤5 % activity loss over 100 h) at all pH values. The C adjacent to pyrrolic N in MHCF is the most active site and the induced directional interfacial electron transfer from CNTs to MHCF coupled with N‐driven intramolecular electron transfer in MHCF optimizes Gibbs free energy for hydrogen adsorption (ΔGH*) near zero, while the polymer modulation enables local H+ enrichment in acidic media and enhanced water adsorption and activation in neutral and basic media. [ABSTRACT FROM AUTHOR]

Abstract: Photo/electrochemical CO2 splitting is impeded by the low cost‐effective catalysts for key reactions: CO2 reduction (CDRR) and water oxidation. A porous silicon and nitrogen co‐doped carbon (SiNC) nanomaterial by a facile pyrolyzation was developed as a metal‐free bifunctional electrocatalyst. CO2‐to‐CO and oxygen evolution (OER) partial current density under neutral conditions were enhanced by two orders of magnitude in the Tafel regime on SiNC relative to single‐doped comparisons beyond their specific area gap. The photovoltaic‐driven CO2 splitting device with SiNC electrodes imitating photosynthesis yielded an overall solar‐to‐chemical efficiency of advanced 12.5 % (by multiplying energy efficiency of CO2 splitting cell and photovoltaic device) at only 650 mV overpotential. Mechanism studies suggested the elastic electron structure of −Si(O)−C−N− unit in SiNC as the highly active site for CDRR and OER simultaneously by lowering the free energy of CDRR and OER intermediates adsorption. [ABSTRACT FROM AUTHOR]

Abstract: Single Fe atoms dispersed on hierarchically structured porous carbon (SA‐Fe‐HPC) frameworks are prepared by pyrolysis of unsubstituted phthalocyanine/iron phthalocyanine complexes confined within micropores of the porous carbon support. The single‐atom Fe catalysts have a well‐defined atomic dispersion of Fe atoms coordinated by N ligands on the 3D hierarchically porous carbon support. These SA‐Fe‐HPC catalysts are comparable to the commercial Pt/C electrode even in acidic electrolytes for oxygen reduction reaction (ORR) in terms of the ORR activity (E1/2=0.81 V), but have better long‐term electrochemical stability (7 mV negative shift after 3000 potential cycles) and fuel selectivity. In alkaline media, the SA‐Fe‐HPC catalysts outperform the commercial Pt/C electrode in ORR activity (E1/2=0.89 V), fuel selectivity, and long‐term stability (1 mV negative shift after 3000 potential cycles). Thus, these nSA‐Fe‐HPCs are promising non‐platinum‐group metal ORR catalysts for fuel‐cell technologies. [ABSTRACT FROM AUTHOR]

Black phosphorus (BP) as a new 2D material has attracted extensive attention because of its unique electronic, optical, and structural properties. However, the difficulties associated with BP synthesis severely hinder the further development of BP for any potential applications. On the other hand, searching for other potential applications of BP is also a big challenge. A facile strategy was developed for preparation of BP supported on Ti foil (BP-Ti) in a thin-film form. Surprisingly, the as-prepared BP shows advanced electrocatalytic activity for the oxygen evolution reaction (OER). To improve the OER activity of the electrocatalyst, BP was grown on a carbon nanotube network (BP-CNT), showing even better activity. The results demonstrate that BP can be prepared by a facile method and may be applied as an electrocatalyst. [ABSTRACT FROM AUTHOR]

Besides their use in fuel cells for energy conversion through the oxygen reduction reaction (ORR), carbon-based metal-free catalysts have also been demonstrated to be promising alternatives to noble-metal/metal oxide catalysts for the oxygen evolution reaction (OER) in metal-air batteries for energy storage and for the splitting of water to produce hydrogen fuels through the hydrogen evolution reaction (HER). This Review focuses on recent progress in the development of carbon-based metal-free catalysts for the OER and HER, along with challenges and perspectives in the emerging field of metal-free electrocatalysis. [ABSTRACT FROM AUTHOR]

Co3O4, which is of mixed valences Co2+ and Co3+, has been extensively investigated as an efficient electrocatalyst for the oxygen evolution reaction (OER). The proper control of Co2+/Co3+ ratio in Co3O4 could lead to modifications on its electronic and thus catalytic properties. Herein, we designed an efficient Co3O4-based OER electrocatalyst by a plasma-engraving strategy, which not only produced higher surface area, but also generated oxygen vacancies on Co3O4 surface with more Co2+ formed. The increased surface area ensures the Co3O4 has more sites for OER, and generated oxygen vacancies on Co3O4 surface improve the electronic conductivity and create more active defects for OER. Compared to pristine Co3O4, the engraved Co3O4 exhibits a much higher current density and a lower onset potential. The specific activity of the plasma-engraved Co3O4 nanosheets (0.055 mA cm−2BET at 1.6 V) is 10 times higher than that of pristine Co3O4, which is contributed by the surface oxygen vacancies. [ABSTRACT FROM AUTHOR]

For the first time, we developed edge-rich and dopant-free graphene as a highly efficient ORR electrocatalyst. Electrochemical analysis revealed that the as-obtained edge-rich graphene showed excellent ORR activity through a one-step and four-electron pathway. With a similar strategy, edge-rich carbon nanotubes and graphite can also be obtained with enhanced ORR activity. This work confirms the important role of edge carbon in efficient ORR electrocatalysis without interruption by any other dopants. [ABSTRACT FROM AUTHOR]

Heteroatom-doped carbon materials have been extensively investigated as metal-free electrocatalysts to replace commercial Pt/C catalysts in oxygen reduction reactions in fuel cells and Li-air batteries. However, the synthesis of such materials usually involves high temperature or complicated equipment. Graphene-based sulfur composites have been recently developed to prolong the cycling life of Li-S batteries, one of the most attractive energy-storage devices. Given the high cost of graphene, there is significant demand to recycle and reuse graphene from Li-S batteries. Herein, we report a green and cost-effective method to prepare sulfur-doped graphene, achieved by the continuous charge/discharge cycling of graphene-sulfur composites in Li-S batteries. This material was used as a metal-free electrocatalyst for the oxygen reduction reaction and shows better electrocatalytic activity than pristine graphene and better methanol tolerance durability than Pt/C. [ABSTRACT FROM AUTHOR]

The electrocatalytic activity of metallic nanostructures is commonly tuned by changing their structure, composition, size and morphology to decrease the activation energy of a rate-determining elementary step. Here, we report that the electrocatalytic activity of the excited plasmonic model Pd nanostructured electrocatalyst for oxygen reduction reaction (ORR) can be enhanced significantly by increasing the kinetic pre-exponential factor. The localized surface plasmon resonance (SPR) effect promotes a two-orders of magnitude increase in the apparent pre-exponential factor in the Arrhenius equation. Combined experimental and theoretical analyses of the kinetic H/D isotope effect indicate that the observed increase in the apparent pre-exponential factor is attributable to an increased quantum–mechanical proton tunneling arising from the local electric field enhancement upon SPR excitation. Intriguingly, the ORR activity enhancement can be observed under simulated solar light illumination, implying potential for fuel cell applications. The methodology developed in this study is applicable to other proton-coupled electron-transfer reactions, thus opening a promising route to manipulating electrocatalytic reactivity by tuning the kinetic pre-exponential factor and suggesting potential for fuel cell applications. [ABSTRACT FROM AUTHOR]

Ceria nanomaterials for heterogeneous catalysis have attracted much attention due to their excellent properties and have been extensively applied in recent years. But the poor electron conductivity and the aggregation behavior severely affect their electrocatalytic performances. In this paper, we prepared a novel catalyst based on CeO 2 nanocrystallines (CeO 2 NCs) ensemble-on-nitrogen-doped graphene (CeO 2 –NG) nanocomposites through a one-step heat-treatment without the need of the precursor. The results confirmed that the high dispersion of CeO 2 NCs with the uniform size distribution of about 5 nm on the surface of nitrogen-doped graphene (NG) sheets could be easily obtained via the one-step procedure and the resultant CeO 2 –NG nanocomposites were an excellent electrode material possessing outstanding electrochemical features for electron transfer. Luminol, an important electroactive substance, was further chosen to inspect the electrocatalytic properties of the as-prepared CeO 2 –NG nanocomposites. The studies showed that the presence of the NG in CeO 2 –NG nanocomposites could facilitate the electrochemical redox process of luminol. Compared with pristine CeO 2 NCs, the synthesized CeO 2 –NG nanocomposites can enhance the electrochemiluminescence (ECL) intensity by 3.3-fold and decrease the onset ECL potential for about 72 mV in the neutral condition. Employing above superiority, selecting cholesterol oxidase (ChOx) as the model oxidase, a facile ECL method for cholesterol detection with the CeO 2 –NG nanocomposites as the matrix to immobilize enzyme ChOx was developed. The results demonstrated CeO 2 –NG nanocomposites exhibited excellent performances in terms of sensitivity and catalytic activities, indicating that NG-based nanomaterials have great promise in electrocatalytic and enzymatic biosensing fields. [ABSTRACT FROM AUTHOR]