Efficient capture of solar energy will be critical to meeting the energy needs of the future. Semiconductor photocatalysis is expected to make an important contribution in this regard, delivering ...both energy carriers (especially H2) and valuable chemical feedstocks under direct sunlight. Over the past few years, carbon dots (CDs) have emerged as a promising new class of metal‐free photocatalyst, displaying semiconductor‐like photoelectric properties and showing excellent performance in a wide variety of photoelectrochemical and photocatalytic applications owing to their ease of synthesis, unique structure, adjustable composition, ease of surface functionalization, outstanding electron‐transfer efficiency and tunable light‐harvesting range (from deep UV to the near‐infrared). Here, recent advances in the rational design of CDs‐based photocatalysts are highlighted and their applications in photocatalytic environmental remediation, water splitting into hydrogen, CO2 reduction, and organic synthesis are discussed.
Carbon dots (CDs) have emerged as promising materials for various photocatalytic reactions owing to their tunable light‐harvesting range and outstanding electron‐transfer efficiency stemming from their intrinsic nanostructures. Recent advances in the rational design of CD‐based photocatalysts and their applications in photocatalytic environmental remediation, hydrogen evolution by water splitting, CO2 reduction, and organic synthesis are highlighted.
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Fe single‐atom catalysts (Fe SACs) with atomic FeNx active sites are very promising alternatives to platinum‐based catalysts for the oxygen reduction reaction (ORR). The pyrolysis of metal–organic ...frameworks (MOFs) is a common approach for preparing Fe SACs, though most MOF‐derived catalysts reported to date are microporous and thus suffer from poor mass transfer and a high proportion of catalytically inaccessible FeNx active sites. Herein, NH2‐MIL‐101(Al), a MOF possessing a mesoporous cage architecture, is used as the precursor to prepare a series of N‐doped carbon supports (denoted herein as NC‐MIL101‐T) with a well‐defined mesoporous structure at different pyrolysis temperatures. The NC‐MIL101‐T supports are then impregnated with a Fe(II)‐phenanthroline complex, and heated again to yield Fe SAC‐MIL101‐T catalysts rich in accessible FeNx single atom sites. The best performing Fe SAC‐MIL101‐1000 catalyst offers outstanding ORR activity in alkaline media, evidenced by an ORR half‐wave potential of 0.94 V (vs RHE) in 0.1 m KOH, as well as excellent performance in both aqueous primary zinc–air batteries (a near maximum theoretical energy density of 984.2 Wh kgZn−1) and solid‐state zinc–air batteries (a peak power density of 50.6 mW cm−2 and a specific capacity of 724.0 mAh kgZn−1).
Mesoporous Fe single‐atom catalysts (Fe SAC‐MIL101‐T) are successfully synthesized using NH2‐MIL‐101(Al)‐derived N‐doped carbon as supports. The abundant mesopores in the supports promote mass transport during the oxygen reduction reaction (ORR) and ensure a high proportion of FeNx sites are accessible. Fe SAC‐MIL101‐1000 demonstrates outstanding activity for ORR and excellent performance in both aqueous and solid‐state zinc–air batteries.
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Fe–N–C catalysts offer excellent performance for the oxygen reduction reaction (ORR) in alkaline media. With a view toward boosting the intrinsic ORR activity of Fe single‐atom sites in Fe–N–C ...catalysts, fine‐tuning the local coordination of the Fe sites to optimize the binding energies of ORR intermediates is imperative. Herein, a porous FeN4–O–NCR electrocatalyst rich in catalytically accessible FeN4–O sites (wherein the Fe single atoms are coordinated to four in‐plane nitrogen atoms and one subsurface axial oxygen atom) supported on N‐doped carbon nanorods (NCR) is reported. Fe K‐edge X‐ray absorption spectroscopy (XAS) verifies the presence of FeN4–O active sites in FeN4–O–NCR, while density functional theory calculations reveal that the FeN4–O coordination offers a lower energy and more selective 4‐electron/4‐proton ORR pathway compared to traditional FeN4 sites. Electrochemical tests validate the outstanding intrinsic activity of FeN4–O–NCR for alkaline ORR, outperforming Pt/C and almost all other M–N–C catalysts reported to date. A primary zinc–air battery constructed using FeN4–O–NCR delivers a peak power density of 214.2 mW cm−2 at a current density of 334.1 mA cm−2, highlighting the benefits of optimizing the local coordination of iron single atoms.
An Fe single‐atom catalyst (FeN4‐O–NCR) based on O,N‐codoped carbon nanorods delivers remarkable oxygen reduction reaction (ORR) performance in a primary zinc–air battery. The Fe atoms in FeN4–O–NCR are coordinated by four in‐plane N atoms and a subsurface axial oxygen atom that bridge between N‐doped graphene sheets, thus optimizing the Fe d‐band center position and binding energies of ORR intermediates.
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In this work, porous monolayer nickel‐iron layered double hydroxide (PM‐LDH) nanosheets with a lateral size of ≈30 nm and a thickness of ≈0.8 nm are successfully synthesized by a facile one‐step ...strategy. Briefly, an aqueous solution containing Ni2+ and Fe3+ is added dropwise to an aqueous formamide solution at 80 °C and pH 10, with the PM‐LDH product formed within only 10 min. This fast synthetic strategy introduces an abundance of pores in the monolayer NiFe‐LDH nanosheets, resulting in PM‐LDH containing high concentration of oxygen and cation vacancies, as is confirmed by extended X‐ray absorption fine structure and electron spin resonance measurements. The oxygen and cation vacancies in PM‐LDH act synergistically to increase the electropositivity of the LDH nanosheets, while also enhancing H2O adsorption and bonding strength of the OH* intermediate formed during water electrooxidation, endowing PM‐LDH with outstanding performance for the oxygen evolution reaction (OER). PM‐LDH offers a very low overpotential (230 mV) for OER at a current density of 10 mA cm−2, with a Tafel slope of only 47 mV dec−1, representing one of the best OER performance yet reported for a NiFe‐LDH system. The results encourage the wider utilization of porous monolayer LDH nanosheets in electrocatalysis, catalysis, and solar cells.
Porous monolayer nickel‐iron layered double hydroxide (PM‐LDH) nanosheets with a lateral size of ≈30 nm and a thickness of ≈0.8 nm are successfully synthesized by a facile one‐step strategy. Abundant multivacancies increase the electropositivity of the nanosheets, while also enhancing H2O adsorption and bonding strength of the OH* intermediate formed during water electrooxidation, endowing PM‐LDH with outstanding water oxidation performance.
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Abstract
The electrochemical CO
2
reduction reaction (CO
2
RR) represents a very promising future strategy for synthesizing carbon-containing chemicals in a more sustainable way. In spite of great ...progress in electrocatalyst design over the last decade, the critical role of wettability-controlled interfacial structures for CO
2
RR remains largely unexplored. Here, we systematically modify the structure of gas-liquid-solid interfaces over a typical Au/C gas diffusion electrode through wettability modification to reveal its contribution to interfacial CO
2
transportation and electroreduction. Based on confocal laser scanning microscopy measurements, the Cassie-Wenzel coexistence state is demonstrated to be the ideal three phase structure for continuous CO
2
supply from gas phase to Au active sites at high current densities. The pivotal role of interfacial structure for the stabilization of the interfacial CO
2
concentration during CO
2
RR is quantitatively analysed through a newly-developed in-situ fluorescence electrochemical spectroscopic method, pinpointing the necessary CO
2
mass transfer conditions for CO
2
RR operation at high current densities.
Covalent organic frameworks (COFs) represent an emerging class of organic photocatalysts. However, their complicated structures lead to indeterminacy about photocatalytic active sites and reaction ...mechanisms. Herein, we use reticular chemistry to construct a family of isoreticular crystalline hydrazide-based COF photocatalysts, with the optoelectronic properties and local pore characteristics of the COFs modulated using different linkers. The excited state electronic distribution and transport pathways in the COFs are probed using a host of experimental methods and theoretical calculations at a molecular level. One of our developed COFs (denoted as COF-4) exhibits a remarkable excited state electron utilization efficiency and charge transfer properties, achieving a record-high photocatalytic uranium extraction performance of ~6.84 mg/g/day in natural seawater among all techniques reported so far. This study brings a new understanding about the operation of COF-based photocatalysts, guiding the design of improved COF photocatalysts for many applications.
Rechargeable zinc–air batteries (ZABs) are presently attracting a lot of attention for electrical energy storage, owing to their low manufacturing cost and very high theoretical specific energy ...density. Currently, the large‐scale application of ZABs is hampered by the sluggish kinetics of the oxygen‐reduction reaction (ORR) and oxygen evolution reaction (OER), which underpin battery discharging and charging processes, respectively. In recent years, metal single‐atom catalysts (SACs) have emerged as promising candidates for driving oxygen electrocatalysis in ZABs, offering both high electrocatalytic activity and high metal atom utilization through unique metal coordination environments (typically porphyrin‐like MNx species on N‐doped carbon supports). Herein, recent breakthroughs in the design of SACs for ORR and OER electrocatalysis are summarized, with a general view towards improving ZAB performance. This Review begins by introducing the operating principles of ZABs and the reaction mechanisms of the ORR and the OER on the air electrode, after which the various types of SAC‐based materials developed to date for oxygen electrocatalysis and ZABs are discussed. Special emphasis is placed on the relationships between the structure of the SAC active site and electrocatalytic performance. Finally, challenges and opportunities for SACs in practical ZABs are explored.
Recent breakthroughs in the development of metal single‐atom catalysts (SACs) for the oxygen reduction reaction and the oxygen evolution reaction are reviewed, with the goal of improving the future performance of zinc–air batteries. Particular emphasis is placed on the relationships between the structure of the metal SAC sites and electrocatalytic performance in oxygen electrocatalysis.
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We report a new methodology for producing monometallic or bimetallic nanoparticles confined within hollow nitrogen-doped porous carbon capsules. The capsules are derived from metal–organic framework ...(MOF) crystals that are coated with a shell of a secondary material comprising either a metal–tannic acid coordination polymer or a resorcinol–formaldehyde polymer. Platinum nanoparticles are optionally sandwiched between the MOF core and the shell. Pyrolysis of the MOF–shell composites produces hollow capsules of porous nitrogen-doped carbon that bear either monometallic (Pt, Co, and Ni) or alloyed (PtCo and PtNi) metal nanoparticles. The Co and Ni components of the bimetallic nanoparticles are derived from the shell surrounding the MOF crystals. The hollow capsules prevent sintering and detachment of the nanoparticles, and their porous walls allow for efficient mass transport. Alloyed PtCo nanoparticles embedded in the capsule walls are highly active, selective, and recyclable catalysts for the hydrogenation of nitroarenes to anilines.
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A facile synthetic strategy for nitrogen‐deficient graphitic carbon nitride (g‐C3Nx) is established, involving a simple alkali‐assisted thermal polymerization of urea, melamine, or thiourea. In situ ...introduced nitrogen vacancies significantly redshift the absorption edge of g‐C3Nx, with the defect concentration depending on the alkali to nitrogen precursor ratio. The g‐C3Nx products show superior visible‐light photocatalytic performance compared to pristine g‐C3N4.
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Designing bifunctional catalysts capable of driving the electrochemical hydrogen evolution reaction (HER) and also H2 evolution via the hydrolysis of hydrogen storage materials such as ammonia borane ...(AB) is of considerable practical importance for future hydrogen economies. Herein, we systematically examined the effect of tensile lattice strain in CoRu nanoalloys supported on carbon quantum dots (CoRu/CQDs) on hydrogen generation by HER and AB hydrolysis. By varying the Ru content, the lattice parameters and Ru‐induced lattice strain in the CoRu nanoalloys could be tuned. The CoRu0.5/CQDs catalyst with an ultra‐low Ru content (1.33 wt.%) exhibited excellent catalytic activity for HER (η=18 mV at 10 mA cm−2 in 1 M KOH) and extraordinary activity for the hydrolysis of AB with a turnover frequency of 3255.4 mol(H2)
mol−1(Ru) min−1 or 814.7 mol(H2)
mol−1(cat) min−1 at 298 K, respectively, representing one of the best activities yet reported for AB hydrolysis over a ruthenium alloy catalyst. Moreover, the CoRu0.5/CQDs catalyst displayed excellent stability during each reaction, including seven alternating cycles of HER and AB hydrolysis. Theoretical calculations revealed that the remarkable catalytic performance of CoRu0.5/CQDs resulted from the optimal alloy electronic structure realized by incorporating small amounts of Ru, which enabled fast interfacial electron transfer to intermediates, thus benefitting H2 evolution kinetics. Results support the development of new and improved catalysts HER and AB hydrolysis.
Ultra‐low Ru induced CoRu nanoalloy lattice strain for robust hydrogen evolution reaction (HER) and hydrolysis of ammonia borane (AB) bifunctional hydrogen production is introduced. The CoRu0.5/CQDs displayed excellent stability during each reaction, including seven alternating cycles of HER and AB hydrolysis.
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