Electrochemical CO2 Reduction to Multicarbon Products on Non-Copper Based Catalysts.

Electrochemical CO2 reduction reaction (eCO2RR) to value-added multicarbon (C2+) products offers a promising approach for achieving carbon neutrality and storing intermittent renewable energy. Copper (Cu)-based electrocatalysts generally play the predominant role in this process. Yet recently, more and more non-Cu materials have demonstrated the capability to convert CO2 into C2+, which provides impressive production efficiency even exceeding those on Cu, and a wider variety of C2+ compounds not achievable with Cu counterparts. This motivates us to organize the present review to make a timely and tutorial summary of recent progresses on developing non-Cu based catalysts for CO2-to-C2+. We begin by elucidating the reaction pathways for C2+ formation, with an emphasis on the unique C-C coupling mechanisms in non-Cu electrocatalysts. Subsequently, we summarize the typical C2+-involved non-Cu catalysts, including ds-, d- and p-block metals, as well as metal-free materials, presenting the state-of-the-art design strategies to enhance C2+ efficiency. The system upgrading to promote C2+ productivity on non-Cu electrodes covering microbial electrosynthesis, electrolyte engineering, regulation of operational conditions, and synergistic co-electrolysis, is highlighted as well. Our review concludes with an exploration of the challenges and future opportunities in this rapidly evolving field.

Identifying the distinct roles of dual dopants in stabilizing the platinum-nickel nanowire catalyst for durable fuel cell.

Stabilizing active PtNi alloy catalyst toward oxygen reduction reaction is essential for fuel cell. Doping of specific metals is an empirical strategy, however, the atomistic insight into how dopant boosts the stability of PtNi catalyst still remains elusive. Here, with typical examples of Mo and Au dopants, we identify the distinct roles of Mo and Au in stabilizing PtNi nanowires catalysts. Specifically, due to the stronger interaction between atomic orbital for Ni-Mo and Pt-Au, the Mo dopant mainly suppresses the outward diffusion of Ni atoms while the Au dopant contributes to the stabilization of surface Pt overlayer. Inspired by this atomistic understanding, we rationally construct the PtNiMoAu nanowires by integrating the different functions of Mo and Au into one entity. Such catalyst assembled in fuel cell cathode thus presents both remarkable activity and durability, even surpassing the United States Department of Energy technical targets for 2025.

Stability Issues in Electrochemical CO2 Reduction: Recent Advances in Fundamental Understanding and Design Strategies.

Electrochemical CO2 reduction reaction (CO2 RR) offers a promising approach to close the anthropogenic carbon cycle and store intermittent renewable energy in fuels or chemicals. On the path to commercializing this technology, achieving the long-term operation stability is a central requirement but still confronts challenges. This motivates to organize the present review to systematically discuss the stability issue of CO2 RR. This review starts from the fundamental understanding on the destabilization mechanisms of CO2 RR, with focus on the degradation of electrocatalyst and change of reaction microenvironment during continuous electrolysis. Subsequently, recent efforts on catalyst design to stabilize the active sites are summarized, where increasing atomic binding strength to resist surface reconstruction is highlighted. Next, the optimization of electrolysis system to enhance the operation stability by maintaining reaction microenvironment especially mitigating flooding and carbonate problems is demonstrated. The manipulation on operation conditions also enables to prolong CO2 RR lifespan through recovering catalytically active sites and mass transport process. This review finally ends up by indicating the challenges and future opportunities.

Steering CO2 electroreduction pathway toward ethanol via surface-bounded hydroxyl species-induced noncovalent interaction.

Selective electroreduction of carbon dioxide (CO2RR) into ethanol at an industrially relevant current density is highly desired. However, it is challenging because the competing ethylene production pathway is generally more thermodynamically favored. Herein, we achieve a selective and productive ethanol production over a porous CuO catalyst that presents a high ethanol Faradaic efficiency (FE) of 44.1 ± 1.0% and an ethanol-to-ethylene ratio of 1.2 at a large ethanol partial current density of 501.0 ± 15.0 mA cm-2, in addition to an extraordinary FE of 90.6 ± 3.4% for multicarbon products. Intriguingly, we found a volcano-shaped relationship between ethanol selectivity and nanocavity size of porous CuO catalyst in the range of 0 to 20 nm. Mechanistic studies indicate that the increased coverage of surface-bounded hydroxyl species (*OH) associated with the nanocavity size-dependent confinement effect contributes to the remarkable ethanol selectivity, which preferentially favors the *CHCOH hydrogenation to *CHCHOH (ethanol pathway) via yielding the noncovalent interaction. Our findings provide insights in favoring the ethanol formation pathway, which paves the path toward rational design of ethanol-oriented catalysts.

A universal synthesis of ultrathin Pd-based nanorings for efficient ethanol electrooxidation.

Metallic nanorings (NRs) with open hollow structures are of particular interest in energy-related catalysis due to their unique features, which include the high utilization of active sites and facile accessibility for reactants. However, there is still a lack of general methods for synthesizing Pd-based multimetallic NRs with a high catalytic performance. Herein, we develop a template-directed strategy for the synthesis of ultrathin PdM (M = Bi, Sb, Pb, BiPb) NRs with a tunable size. Specifically, ultrathin Pd nanosheets (NSs) are used as a template to steer the deposition of M atoms and the interatomic diffusion between Pd and M, subsequently resulting in the hollow structured NRs. Taking the ethanol oxidation reaction (EOR) as a proof-of-concept application, the PdBi NRs deliver a substantially improved activity relative to the Pd NSs and commercial Pd/C catalysts, simultaneously showing outstanding stability and CO tolerance. Mechanistically, density functional theory (DFT) calculations disclose that the incorporation of Bi reduces the energy barrier of the rate-determining step in the EOR C2-path, which, together with the high ratio of exposed active sites, endows the PdBi NRs with an excellent EOR activity. We believe that our work can illuminate the general synthesis of multimetallic NRs and the rational design of advanced electrocatalysts.

CO2 electroreduction to multicarbon products in strongly acidic electrolyte via synergistically modulating the local microenvironment.

Electrochemical CO2 reduction to multicarbon products faces challenges of unsatisfactory selectivity, productivity, and long-term stability. Herein, we demonstrate CO2 electroreduction in strongly acidic electrolyte (pH ≤ 1) on electrochemically reduced porous Cu nanosheets by combining the confinement effect and cation effect to synergistically modulate the local microenvironment. A Faradaic efficiency of 83.7 ± 1.4% and partial current density of 0.56 ± 0.02 A cm-2, single-pass carbon efficiency of 54.4%, and stable electrolysis of 30 h in a flow cell are demonstrated for multicarbon products in a strongly acidic aqueous electrolyte consisting of sulfuric acid and KCl with pH ≤ 1. Mechanistically, the accumulated species (e.g., K+ and OH-) on the Helmholtz plane account for the selectivity and activity toward multicarbon products by kinetically reducing the proton coverage and thermodynamically favoring the CO2 conversion. We find that the K+ cations facilitate C-C coupling through local interaction between K+ and the key intermediate *OCCO.

Intermetallic Compounds: Liquid-Phase Synthesis and Electrocatalytic Applications.

Characterized by long-range atomic ordering, well-defined stoichiometry, and controlled crystal structure, intermetallics have attracted increasing attention in the area of chemical synthesis and catalytic applications. Liquid-phase synthesis of intermetallics has arisen as the promising methodology due to its precise control over size, shape, and resistance toward sintering compared with the traditional metallurgy. This short review tends to provide perspectives on the liquid-phase synthesis of intermetallics in terms of both thermodynamics and methodology, as well as its applications in various catalytic reactions. Specifically, basic thermodynamics and kinetics in the synthesis of intermetallics will be first discussed, followed by discussing the main factors that will affect the formation of intermetallics during synthesis. The application of intermetallics in electrocatalysis will be demonstrated case by case at last. We conclude the review with perspectives on the future developments with respect to both synthesis and catalytic applications.

The adsorption of aromatic macromolecules on graphene with entropy-tailored behavior and its utilization in exfoliating graphite.

Aromatic macromolecules tend to form a compact conformation after physically adsorbed on graphene and it brings about great entropy loss for physisorption, due to the strong interaction between aromatic macromolecules and graphene. However, previous researches have validated the availability of aromatic macromolecules to stabilize graphene based on physisorption. In order to clarify the underlying mechanism of this physisorption process on graphene, a series of aromatic polyamide copolymers are used as models in this research. Apart from their adsorbed conformations on graphene, the conformations of these copolymers as the free states in diluted solutions are taken into consideration. Although these copolymers present the fully extended conformation on graphene, their conformations in diluted solutions vary largely with the copolymer composition. It is verified that the copolymer with smaller conformational change could have the better stabilization effectiveness for graphene, rather than the one having stronger interaction with graphene. Therefore, the entropy-tailored behavior for the adsorption of aromatic macromolecules on graphene is put forward. Based on this mechanism, the chemical structure of aromatic polyamide is optimized and furthermore it is utilized to directly exfoliate natural graphite flakes. Eventually, high-quality graphene nanosheets with a large dimension and low defects are obtained. Moreover, its exfoliating effectiveness is superior to those of the commonly used exfoliating agents nowadays.

Suzuki-Miyaura reaction of C-F bonds in fluorographene.

We report the first successful covalent modification of fluorographene (FG) based on Suzuki-Miyaura reaction of the C-F bond. The origin of the reaction efficiency of the C-F bond can be linked to the two-dimensional structure of FG and the synergistic effect of a phosphine ligand. This extends the application of the Suzuki reaction of the C-F bond into two-dimensional chemistry.

Correction: Dependence of the fluorination intercalation of graphene toward high-quality fluorinated graphene formation.

[This corrects the article DOI: 10.1039/C9SC00975B.].

Dependence of the fluorination intercalation of graphene toward high-quality fluorinated graphene formation.

A direct gas-solid reaction between fluorine gas (F2) and graphene is expected to become an inexpensive, continuous and scalable production method to prepare fluorinated graphene. However, the dependence of the fluorination intercalation of graphene is still poorly understood, which prevents the formation of high-quality fluorinated graphene. Herein, we demonstrate that chemical defects (oxygen group defects) on graphene sheets play a leading role in promoting fluorination intercalation, whereas physical defects (point defects), widely considered to be an advantage due to more diffusion channels for F2, were not influential. Tracing the origins, compared with the point defects, the unstable hydroxyl and epoxy groups produced active radicals and the relatively stable carbonyl and carboxyl groups activated the surrounding aromatic regions, thereby both facilitating fluorination intercalation, and the former was a preferential and easier route. Based on the above investigations, we successfully prepared fluorinated graphene with an ultrahigh interlayer distance (9.7 Å), the largest value reported for fluorinated graphene, by customizing graphene with more hydroxyl and epoxy groups. It presented excellent self-lubricating ability, with an ultralow interlayer interaction of 0.056 mJ m-2, thus possessing a far lower friction coefficient compared with graphene, when acting as a lubricant. Moreover, it was also easy to exfoliate by shearing, due to the diminutive interlayer friction and eliminated commensurate stacking. The exfoliated number of layers of less than three exceeded 80% (monolayer rate ≈ 40%), and no surfactant was applied to prevent further stacking.

In Situ Radical Polymerization and Grafting Reaction Simultaneously Initiated by Fluorinated Graphene.

Fluorinated graphene (FG) showed interesting electrochemical, electronic, and mechanical properties, as well as chemical reactivity for multifarious functionalization of graphene material. This work reported a free radical polymerization and grafting from polymerization of a styrene monomer directly initiated by FG, which simultaneously provided free polymers and functionalized graphene with polymer chains grafted. The FG exhibited an almost comparative initiation efficiency to equivalent commercial initiator azodiisobutyronitrile under similar conditions, resulting in a high yield of free polystyrene (40.9%) with a high molecular weight ( Mn = 114.7 kg/mol). It was demonstrated that FG-triggered polymerization presented some special characteristics, such as a long lifetime of chain radical centers even when the reaction was stopped and insensitivity to oxygen molecules. The mechanistic study indicated that the polymerization was initiated by single-electron transfer reaction between FG and a monomer leading to formation of primary radicals; in addition, FG also played an important role in chain transfer and termination reactions during the polymerization process.

Toward Excellent Tribological Performance as Oil-Based Lubricant Additive: Particular Tribological Behavior of Fluorinated Graphene.

The poor dispersibility, strong interlayer interaction, and inferior crack resistance ability restrict the employment of graphene as a lubricant additive. Herein, we prepared fluorinated graphene with different F/C ratios by direct fluorination of multilayer graphene utilizing F2. Among them, highly fluorinated graphene (HFG) with an F/C ratio of about 1.0 presented prominent thermal stability and excellent tribological performance as an oil-based lubricant additive, whose friction coefficient and wear rate had a 51.4 and 90.9% decrease compared to that of pristine graphene, respectively. It was confirmed that C-F bonds perpendicular to the graphene plane contributed to increasing the interlayer distance and tribological performance of fluorinated graphene, while the randomly oriented CF2 and CF3 groups did not count as influential, as demonstrated via X-ray diffraction, X-ray photoelectron spectroscopy, and polarized attenuated total reflection-Fourier transform infrared spectroscopy. Meanwhile, Raman measurements traced the formation process of integrated and stable HFG tribofilm during friction process, and the corresponding stability was attributed to the physical and chemical interactions between HFG and friction pairs. More interestingly, the outstanding crack resistance ability of HFG preserved the sheet structure from destruction due to decreased in-plane stiffness and out-plane stress, thus constructing the tough tribofilm. The simple and feasible preparation makes HFG a promising candidate as advanced lubricant in industrial fabrication.

The Friedel-Crafts reaction of fluorinated graphene for high-yield arylation of graphene.

Herein, we report the Friedel-Crafts reaction of fluorinated graphene with aryl molecules including methylbenzene, chlorobenzene and polystyrene. The reaction achieved the high-yield arylation functionalization of graphene under mild reaction conditions and extends the range of the Friedel-Crafts reaction to the field of two-dimensional materials.

Towards enhanced tribological performance as water-based lubricant additive: Selective fluorination of graphene oxide at mild temperature.

HYPOTHESIS: It is still difficult to prepare hydrophilic fluorinated graphene with abundant oxygen functional groups and high F/C ratio. EXPERIMENTS: Herein, selective fluorination for graphene oxide (GO) was realized to prepare highly oxygen/fluorine dual functionalized graphene (OFG) under a mild temperature without damaging the sheet structure. Its O/C and F/C ratios came up to 0.58 and 0.19 respectively, which contained the intactly reserved oxygen functional groups from GO during fluorination and C-F bonds from addition reaction between aromatic region and F2, as demonstrated via X-ray photoelectron spectroscopy (XPS), thermo gravimetric analysis (TGA), Raman measurements and so on. FINDINGS: Experiments and density functional theory (DFT) calculations revealed that oxygen functional groups in-situ promoted addition reaction between aromatic region and F2, and the critical point was confirmed that oxygen functional groups began to react with F2 by substitution reaction. As-prepared OFG presented a 47% and 31% decrease of wear rate compared with that of pure water and GO, respectively. During friction process, the abundant oxygen functional groups of OFG contributed to hydrophilic property and introduced fluorine played an important role in enhancing the tribological performance by self-lubricating behavior. Such mild and simple method is achieved to expand the application of fluorinated graphene in aqueous environment.

Low temperature preparation of highly fluorinated multiwalled carbon nanotubes activated by Fe3O4 to enhance microwave absorbing property.

Conventional approaches to preparing highly fluorinated multiwalled carbon nanotubes (MWCNTs) always require a high temperature. This paper presents a catalytic approach to realizing the effective fluorination of MWNCTs at room temperature (RT). Fe3O4/MWCNTs composites with Fe3O4 loaded on MWCNTs were first prepared using the solvothermal method, followed by fluorination treatment at RT. The attachment of Fe3O4 changes the charge distribution and dramatically improves the fluorination activity of MWCNTs. Consequently, the fluorine content of fluorinated Fe3O4/MWCNTs (F-Fe3O4/MWCNTs) can reach up to 17.13 at% (almost six times that of the unloaded sample) only after fluorination at room temperature, which leads to an obvious decrease in permittivity. Besides, the partial fluorination of Fe3O4 brings about abnormally enhanced permeability due to strengthened exchange resonance. Benefiting from the lower permittivity and higher permeability, F-Fe3O4/CNTs composite exhibits increased impedance matching and thus an enhanced microwave absorption property with a minimal reflection loss of -45 dB at 2.61 mm when the filler content is 13 wt%. The efficient absorption bandwidth (<-10 dB) reaches 4.1 GHz when the thickness is 2.5 mm. This work illustrates a novel catalytic approach to preparing highly fluorinated MWCNTs as promising microwave absorbers, and the design concept can also be extended to the fluorination of other carbon materials.

Crystallization of silica promoted by residual hydrogen bonding interactions at high temperature.

A novel approach to prepare crystalline silica through calcination of the composite of silica and highly fluorinated graphene at a relatively low temperature is demonstrated. Silica and its composites with graphene and its derivatives (graphene, graphene oxide and graphene with various degrees of fluorination) were synthesized and then calcined at 900 °C in an air atmosphere. The results of X-ray-diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy reveal that cristobalite was produced through calcining composites of silica and highly fluorinated graphene under ambient air at a relatively low temperature (900 °C), while for the composites of silica and graphene and its derivatives, the calcined products are all amorphous. Thermal gravimetric analysis results indicate that the maximum decomposition temperature of functional groups in highly fluorinated graphene at air temperature is 457 °C, which is higher than that in medium fluorinated graphene, lower fluorinated graphene and graphene oxide (411.3 °C, 313.4 °C and 238.9 °C). A high degradation temperature of highly fluorinated graphene contributes to strong residual hydrogen bonding interactions at high temperature. FTIR results further illustrate that many residual hydrogen bonding interactions in composites of silica and highly fluorinated graphene at higher temperature result in enough linear structures. As a consequence, stronger residual hydrogen bonding interactions at high temperature in composites of silica and highly fluorinated graphene restrain the self-condensation of Si-OH groups and promote the formation of crystalline structures.

Radical mechanism of a nucleophilic reaction depending on a two-dimensional structure.

The mechanism of nucleophilic substitution deserves more investigation to include more reaction systems such as two-dimensional (2D) materials. In this study, we used fluorinated graphene (FG) as a representative 2D material to reveal the in-depth mechanism of its defluorination and nucleophilic substitution reaction under attack of common nucleophiles to explore the chemistry of 2D materials and enrich the research on the nucleophilic substitution reaction. DFT calculations and electron paramagnetic resonance spectroscopy (EPR) demonstrated that defluorination of FG occurred via a radical mechanism after a single electron transfer (SET) reaction between the nucleophile and C-F bond, and a spin center was generated on the nanosheet and fluorine anion. Moreover, neither the SN1 nor SN2 mechanism was suggested to be appropriate for the substitution reaction of FG with a 2D structure due to the corresponding kinetics or thermodynamics disadvantage; hence, its nucleophilic substitution was proved to occur via a radical mechanism initiated by the defluorination step. The proposed substitution mechanism of FG demonstrates that nucleophilic substitution via a radical mechanism can also be applied to the attacking process of common nucleophiles without any particular conditions. Furthermore, it has been discovered that triethylamine without active hydrogen can be covalently attached to graphene nanosheets via a nucleophilic substitution reaction with FG; this further indicates a radical process for the nucleophilic substitution of FG rather than an SN1 or SN2 mechanism. The detailed process of the nucleophilic substitution reaction of FG was revealed to occur via a radical mechanism depending on the 2D structure of FG, which could also represent the typical characteristic of 2D chemistry.

Investigation of the dispersion behavior of fluorinated MWCNTs in various solvents.

The investigation of the dispersion behavior of fluorinated MWCNTs (F-MWCNTs) is very important to understand their structure and take full advantage of their good properties. In this present paper, the dispersion behavior of F-MWCNTs with a low content and a high content of fluorine (denoted as lF-MWCNTs and hF-MWCNTs) was explored in 18 kinds of common solvents. The surface of hF-MWCNTs is considered to be a heterostructure consisting of fluorinated regions and aromatic regions, while lF-MWCNTs are inclined to be a homogeneous structure on the basis of their dispersion behavior. According to dispersion theory based on surface energy and Hansen solubility parameters (HSPs), it was indicated that the corresponding preferable solvents are different for different regions. As a result, good solvents of hF-MWCNTs are distributed in a quite wide scope while lF-MWCNTs can be dispersed only in a significantly narrow range of solvents. The HSPs of lF-MWCNTs and hF-MWCNTs are determined to be δD = 17.6 MPa1/2, δP = 11.8 MPa1/2, δH = 8.8 MPa1/2 and δD = 16.9 MPa1/2, δP = 9.3 MPa1/2, δH = 13.5 MPa1/2, respectively. As a result, mixed solvents of acetone and water were carefully tuned to be compatible with hF-MWCNTs. The dispersion behaviors of lF-MWCNTs and hF-MWCNTs in epoxy were also predicted according to HSPs. It was found that hF-MWCNTs maintain a stable dispersion in epoxy due to their heterogeneous structure at elevated temperatures.