NHC-CDI Ligands Boost Multicarbon Production in Electrocatalytic CO2 Reduction by Increasing Accumulated Charged Intermediates and Promoting *CO Dimerization on Cu.

Copper-based materials exhibit significant potential as catalysts for electrochemical CO2 reduction, owing to their capacity to generate multicarbon hydrocarbons. The molecular functionalization of Cu electrodes represents a simple yet powerful strategy for improving the intrinsic activity of these materials by favoring specific reaction pathways through the creation of tailored microenvironments around the surface active sites. However, despite its success, comprehensive mechanistic insights derived from experimental techniques are often limited, leaving the active role of surface modifiers inconclusive. In this work, we show that N-heterocyclic carbene-carbodiimide-functionalized Cu catalysts display a remarkable activity for multicarbon product formation, surpassing bare Cu electrodes by more than an order of magnitude. These hybrid catalysts operate efficiently using an electrolyzer equipped with a gas diffusion electrode, achieving a multicarbon product selectivity of 58% with a partial current density of ca. -80 mA cm-2. We found that the activity for multicarbon product formation is closely linked to the surface charge that accumulates during electrocatalysis, stemming from surface intermediate buildup. Through X-ray photoelectron spectroscopy, we elucidated the role of the molecular additives in altering the electronic structure of the Cu electrodes, promoting the stabilization of surface CO. Additionally, in situ Raman measurements established the identity of the reaction intermediates that accumulate during electrocatalysis, indicating preferential CO binding on Cu step sites, known for facilitating C-C coupling. This study underscores the significant potential of molecular surface modifications in developing efficient electrocatalysts for CO2 reduction, highlighting surface charge as a pivotal descriptor of multicarbon product activity.

A Monolayer Carbon Nitride on Au(111) with a High Density of Single Co Sites.

Carbon nitrides that expose atomically dispersed single-atom metals in the form of M-N-C (M = metal) sites are attractive earth-abundant catalyst materials that have been demonstrated in electrocatalytic conversion reactions. The catalytic performance is determined by the abundance of N-doped sites and the type of metal coordination to N, but challenges remain to synthesize pristine carbon nitrides with a high concentration of the most active sites and prepare homogeneously doped materials that allow for in-depth characterization of the M-N-C sites and quantitative evaluation of their catalytic performance. Herein, we have synthesized and characterized a well-defined monolayer carbon nitride phase on a Au(111) surface that exposes an exceedingly high concentration of Co-N4 sites. The crystalline monolayer carbon nitride, whose formation is controlled by an on-surface reaction between Co atoms and melamine on Au(111), is characterized by a dense array of 4- and 6-fold N-terminated pockets, whereof only the 4-fold pocket is found to be holding Co atoms. Through detailed characterization using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory modeling, we determine the atomic structure and chemical state of the carbon nitride network. Furthermore, we show that the monolayer carbon nitride structure is stable and reactive toward the electrocatalytic oxygen reduction reaction in alkaline electrolyte, with a quantitative performance metric that significantly exceeds comparable M-N-C-based catalyst types. The work demonstrates that high-density active catalytic sites can be created using common precursor materials, and the formed networks themselves offer an excellent platform for onward studies addressing the characteristics of M-N-C sites.

Exploring the Parameters Controlling Product Selectivity in Electrochemical CO2 Reduction in Competition with Hydrogen Evolution Employing Manganese Bipyridine Complexes.

Selective reduction of CO2 is an efficient solution for producing nonfossil-based chemical feedstocks and simultaneously alleviating the increasing atmospheric concentration of this greenhouse gas. With this aim, molecular electrocatalysts are being extensively studied, although selectivity remains an issue. In this work, a combined experimental-computational study explores how the molecular structure of Mn-based complexes determines the dominant product in the reduction of CO2 to HCOOH, CO, and H2. In contrast to previous Mn(bpy-R)(CO)3Br catalysts containing alkyl amines in the vicinity of the Br ligand, here, we report that bpy-based macrocycles locking these amines at the side opposite to the Br ligand change the product selectivity from HCOOH to H2. Ab initio molecular dynamics simulations of the active species showed that free rotation of the Mn(CO)3 moiety allows for the approach of the protonated amine to the reactive center yielding a Mn-hydride intermediate, which is the key in the formation of H2 and HCOOH. Additional studies with DFT methods showed that the macrocyclic moiety hinders the insertion of CO2 to the metal hydride favoring the formation of H2 over HCOOH. Further, our results suggest that the minor CO product observed experimentally is formed when CO2 adds to Mn on the side opposite to the amine ligand before protonation. These results show how product selectivity can be modulated by ligand design in Mn-based catalysts, providing atomistic details that can be leveraged in the development of a fully selective system.

Steering carbon dioxide reduction toward C-C coupling using copper electrodes modified with porous molecular films.

Copper offers unique capability as catalyst for multicarbon compounds production in the electrochemical carbon dioxide reduction reaction. In lieu of conventional catalysis alloying with other elements, copper can be modified with organic molecules to regulate product distribution. Here, we systematically study to which extent the carbon dioxide reduction is affected by film thickness and porosity. On a polycrystalline copper electrode, immobilization of porous bipyridine-based films of varying thicknesses is shown to result in almost an order of magnitude enhancement of the intrinsic current density pertaining to ethylene formation while multicarbon products selectivity increases from 9.7 to 61.9%. In contrast, the total current density remains mostly unaffected by the modification once it is normalized with respect to the electrochemical active surface area. Supported by a microkinetic model, we propose that porous and thick films increase both local carbon monoxide partial pressure and the carbon monoxide surface coverage by retaining in situ generated carbon monoxide. This reroutes the reaction pathway toward multicarbon products by enhancing carbon-carbon coupling. Our study highlights the significance of customizing the molecular film structure to improve the selectivity of copper catalysts for carbon dioxide reduction reaction.

Interfacial Engineering of PVDF-TrFE toward Higher Piezoelectric, Ferroelectric, and Dielectric Performance for Sensing and Energy Harvesting Applications.

The electrical properties of pristine fluoropolymers are inferior due to their low polar crystalline phase content and rigid dipoles that tend to retain their fixed moment and orientation. Several strategies, such as electrospinning, electrohydrodynamic pulling, and template-assisted growing, have been proven to enhance the electrical properties of fluoropolymers; however, these techniques are mostly very hard to scale-up and expensive. Here, a facile interfacial engineering approach based on amine-functionalized graphene oxide (AGO) is proposed to manipulate the intermolecular interactions in poly(vinylidenefluoride-trifluoroethylene) (PVDF-TrFE) to induce ß-phase formation, enlarge the lamellae dimensions, and align the micro-dipoles. The coexistence of primary amine and hydroxyl groups on AGO nanosheets offers strong hydrogen bonding with fluorine atoms, which facilitates domain alignment, resulting in an exceptional remnant polarization of 11.3 µC cm-2 . PVDF-TrFE films with 0.1 wt.% AGO demonstrate voltage coefficient, energy density, and energy-harvesting figure of merit values of 0.30 Vm N-1 , 4.75 J cm-3 , and 14 pm3  J-1 , respectively, making it outstanding compared with state-of-the-art ceramic-free ferroelectric films. It is believed that this work can open-up new insights toward structural and morphological tailoring of fluoropolymers to enhance their electrical and electromechanical performance and pave the way for their industrial deployment in next-generation wearables and human-machine interfaces.

Electrocatalytic Depolymerization of Self-Immolative Poly(Dithiothreitol) Derivatives.

We report the use of electrogenerated anthraquinone radical anion (AQ•-) to trigger fast catalytic depolymerization of polymers derived from poly(dithiothreitol) (pDTT)-a self-immolative polymer (SIP) with a backbone of dithiothreitols connected with disulfide bonds and end-capped via disulfide bonds to pyridyl groups. The pDTT derivatives studied include polymers with simple thiohexyl end-caps or modified with AQ or methyl groups by Steglich esterification. All polymers were shown to be depolymerized using catalytic amounts of electrons delivered by AQ•-. For pDTT, as little as 0.2 electrons per polymer chain was needed to achieve complete depolymerization. We hypothesize that the reaction proceeds with AQ•- as an electron carrier (either molecularly or as a pendant group), which transfers an electron to a disulfide bond in the polymer in a dissociative manner, generating a thiyl radical and a thiolate. The rapid and catalytic depolymerization is driven by thiyl radicals attacking other disulfide bonds internally or between pDTT chains in a chain reaction. Electrochemical triggering works as a general method for initiating depolymerization of pDTT derivatives and may likely also be used for depolymerization of other disulfide polymers.

Challenges and Prospects in the Catalytic Conversion of Carbon Dioxide to Formaldehyde.

Formaldehyde (HCHO) is a crucial C1 building block for daily-life commodities in a wide range of industrial processes. Industrial production of HCHO today is based on energy- and cost-intensive gas-phase catalytic oxidation of methanol, which calls for exploring other and more sustainable ways of carrying out this process. Utilization of carbon dioxide (CO2 ) as precursor presents a promising strategy to simultaneously mitigate the carbon footprint and alleviate environmental issues. This Minireview summarizes recent progress in CO2 -to-HCHO conversion using hydrogenation, hydroboration/hydrosilylation as well as photochemical, electrochemical, photoelectrochemical, and enzymatic approaches. The active species, reaction intermediates, and mechanistic pathways are discussed to deepen the understanding of HCHO selectivity issues. Finally, shortcomings and prospects of the various strategies for sustainable reduction of CO2 to HCHO are discussed.

Promoting Selective Generation of Formic Acid from CO2 Using Mn(bpy)(CO)3Br as Electrocatalyst and Triethylamine/Isopropanol as Additives.

Urgent solutions are needed to efficiently convert the greenhouse gas CO2 into higher-value products. In this work, fac-Mn(bpy)(CO)3Br (bpy = 2,2'-bipyridine) is employed as electrocatalyst in reductive CO2 conversion. It is shown that product selectivity can be shifted from CO toward HCOOH using appropriate additives, i.e., Et3N along with iPrOH. A crucial aspect of the strategy is to outrun the dimer-generating parent-child reaction involving fac-Mn(bpy)(CO)3Br and [Mn(bpy)(CO)3]- and instead produce the Mn hydride intermediate. Preferentially, this is done at the first reduction wave to enable formation of HCOOH at an overpotential as low as 260 mV and with faradaic efficiency of 59 ± 1%. The latter may be increased to 71 ± 3% at an overpotential of 560 mV, using 2 M concentrations of both Et3N and iPrOH. The nature of the amine additive is crucial for product selectivity, as the faradaic efficiency for HCOOH formation decreases to 13 ± 4% if Et3N is replaced with Et2NH. The origin of this difference lies in the ability of Et3N/iPrOH to establish an equilibrium solution of isopropyl carbonate and CO2, while with Et2NH/iPrOH, formation of the diethylcarbamic acid is favored. According to density-functional theory calculations, CO2 in the former case can take part favorably in the catalytic cycle, while this is less opportune in the latter case because of the CO2-to-carbamic acid conversion. This work presents a straightforward procedure for electrochemical reduction of CO2 to HCOOH by combining an easily synthesized manganese catalyst with commercially available additives.

Low-Valence Znδ+ (0<δ<2) Single-Atom Material as Highly Efficient Electrocatalyst for CO2 Reduction.

A nitrogen-stabilized single-atom catalyst containing low-valence zinc atoms (Znδ+ -NC) is reported. It contains saturated four-coordinate (Zn-N4 ) and unsaturated three-coordinate (Zn-N3 ) sites. The latter makes Zn a low-valence state, as deduced from X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, electron paramagnetic resonance, and density functional theory. Znδ+ -NC catalyzes electrochemical reduction of CO2 to CO with near-unity selectivity in water at an overpotential as low as 310 mV. A current density up to 1 A cm-2 can be achieved together with high CO selectivity of >95 % using Znδ+ -NC in a flow cell. Calculations suggest that the unsaturated Zn-N3 could dramatically reduce the energy barrier by stabilizing the COOH* intermediate owing to the electron-rich environment of Zn. This work sheds light on the relationship among coordination number, valence state, and catalytic performance and achieves high current densities relevant for industrial applications.

Dual-Responsive Material Based on Catechol-Modified Self-Immolative Poly(Disulfide) Backbones.

Functional materials engineered to degrade upon triggering are in high demand due their potentially lower impact on the environment as well as their use in sensing and in medical applications. Here, stimuli-responsive polymers are prepared by decorating a self-immolative poly(dithiothreitol) backbone with pendant catechol units. The highly functional polymer is fashioned into stimuli-responsive gels, formed through pH-dependent catecholato-metal ion cross-links. The gels degrade in response to specific environmental changes, either by addressing the pH responsive, non-covalent, catecholato-metal complexes, or by addition of a thiol. The latter stimulus triggers end-to-end depolymerization of the entire self-immolative backbone through end-cap replacement via thiol-disufide exchanges. Gel degradation is visualized by release of a dye from the supramolecular gel as it itself is converted into smaller molecules.

Spectroscopic Evidence for a Covalent Sigma Au-C Bond on Au Surfaces Using 13C Isotope Labeling.

The Au-C linkage has been demonstrated as a robust interface for coupling thin organic films on Au surfaces. However, the nature of the Au-C interaction remains elusive up to now. Surface-enhanced Raman spectroscopy was previously used to assign a band at 412 cm-1 as a covalent sigma Au-C bond for films generated by spontaneous reduction of the 4-nitrobenzenediazonium salt on Au nanoparticles. However, this assignment is disputed based on our isotopic shift study. We now provide direct evidence for covalent Au-C bonds on the surface of Au nanoparticles using 13C cross-polarization/magic angle spinning solid-state NMR spectroscopy combined with isotope substitution. A 13C NMR shift at 165 ppm was identified as an aromatic carbon linked to the gold surface, while the shift at 148 ppm was attributed to C-C junctions in the arylated organic film. This demonstration of the covalent sigma Au-C bond fills the gap in metal-C bonds for organic films on surfaces, and it has great practical and theoretical significance in understanding and designing a molecular junction based on the Au-C bond.

Are Amines the Holy Grail for Facilitating CO2 Reduction?

The selective and efficient reduction of carbon dioxide represents a key solution to producing non-fossil-fuel-based feedstocks for the chemical industry, while alleviating the increasing atmospheric concentration of this greenhouse gas. A variety of catalytic methods for the CO2 reduction reaction (CO2 RR) have been developed, including hydrogenations and electrochemical or photochemical reductions. For many of the most significant breakthroughs reported in the last decade, we realized that amines or closely related functional groups play a critical role for such transformations, and in several cases, are directly associated with the catalyst as a pendant group. Amines play multiple roles, such as CO2 trapping agents, proton shuttles, electron donors, or facilitators of CO2 reductions through formamide derivatives. In this Viewpoint, we compile some of these recent findings, and discuss their significance in a broader context in an attempt to provide guidelines for the design of new catalysts with enhanced activity and selectivity.

Achieving Near-Unity CO Selectivity for CO2 Electroreduction on an Iron-Decorated Carbon Material.

A straightforward procedure has been developed to prepare a porous carbon material decorated with iron by direct pyrolysis of a mixture of a porous polymer and iron chloride. Characterization of the material with X-ray diffraction, X-ray absorption spectroscopy, and electron microscopy indicates the presence of iron carbide nanoparticles encapsulated inside the carbon matrix, and elemental mapping and cyanide poisoning experiments demonstrate the presence of atomic Fe centers, albeit in trace amounts, which are active sites for electrochemical CO2 reduction. The encapsulated iron carbide nanoparticles are found to boost the catalytic activity of atomic Fe sites in the outer carbon layers, rendering the material highly active and selective for CO2 reduction, although these atomic Fe sites are only present in trace amounts. The target material exhibits near-unity selectivity (98 %) for CO2 -to-CO conversion at a small overpotential (410 mV) in water. Furthermore, the material holds potential for practical application, as a current density over 30 mA cm-2 and a selectivity of 93 % can be achieved in a flow cell.

Polymer Brush Coating and Adhesion Technology at Scale.

Creating strong joints between dissimilar materials for high-performance hybrid products places high demands on modern adhesives. Traditionally, adhesion relies on the compatibility between surfaces, often requiring the use of primers and thick bonding layers to achieve stable joints. The coatings of polymer brushes enable the compatibilization of material surfaces through precise control over surface chemistry, facilitating strong adhesion through a nanometer-thin layer. Here, we give a detailed account of our research on adhesion promoted by polymer brushes along with examples from industrial applications. We discuss two fundamentally different adhesive mechanisms of polymer brushes, namely (1) physical bonding via entanglement and (2) chemical bonding. The former mechanism is demonstrated by e.g., the strong bonding between poly(methyl methacrylate) (PMMA) brush coated stainless steel and bulk PMMA, while the latter is shown by e.g., the improved adhesion between silicone and titanium substrates, functionalized by a hydrosilane-modified poly(hydroxyethyl methacrylate) (PHEMA) brush. This review establishes that the clever design of polymer brushes can facilitate strong bonding between metals and various polymer materials or compatibilize fillers or nanoparticles with otherwise incompatible polymeric matrices. To realize the full potential of polymer brush functionalized materials, we discuss the progress in the synthesis of polymer brushes under ambient and scalable industrial conditions, and present recent developments in atom transfer radical polymerization for the large-scale production of brush-modified materials.

Ligand-Controlled Product Selectivity in Electrochemical Carbon Dioxide Reduction Using Manganese Bipyridine Catalysts.

Electrocatalysis is a promising tool for utilizing carbon dioxide as a feedstock in the chemical industry. However, controlling the selectivity for different CO2 reduction products remains a major challenge. We report a series of manganese carbonyl complexes with elaborated bipyridine or phenanthroline ligands that can reduce CO2 to either formic acid, if the ligand structure contains strategically positioned tertiary amines, or CO, if the amine groups are absent in the ligand or are placed far from the metal center. The amine-modified complexes are benchmarked to be among the most active catalysts for reducing CO2 to formic acid, with a maximum turnover frequency of up to 5500 s-1 at an overpotential of 630 mV. The conversion even works at overpotentials as low as 300 mV, although through an alternative mechanism. Mechanistically, the formation of a Mn-hydride species aided by in situ protonated amine groups was determined to be a key intermediate by cyclic voltammetry, 1H NMR, DFT calculations, and infrared spectroelectrochemistry.

Hydrosilane-Modified Poly(2-Hydroxyethyl Methacrylate) Brush as a Nanoadhesive for Efficient Silicone Bonding.

Leaching of chemicals from adhesion promoters is, in particular, problematic for the food, water, pharmaceutical, and MedTech industries where any chemical contamination is unacceptable. A solution to this issue is to employ covalently attached nanoscale polymer brushes as adhesive layers for plastics. One of the industrially most relevant adhesion targets in that respect is poly(dimethylsiloxane) (PDMS), being used for many high-end applications such as catheters and breast implants. In this work, we have synthesized a novel surface-immobilized poly(2-hydroxyethyl methacrylate)-based brush adhesive containing reactive hydrosilane groups that can bond directly to PDMS. Two different medical grades of addition-cured PDMS were molded on top of titanium substrates already coated with the polymer brush. Titanium plates were used for the chemical analysis, and titanium rods were used for adhesion testing. Adhesion testing revealed a high adhesive force, in which cohesive failure was observed in the bulk PDMS. The necessity of the hydrosilane group in the polymer brush adhesive layer was demonstrated in comparative studies using similar brushes lacking this functionality.

Correction to "Hydrosilane Modified Poly(2-Hydroxyethyl Methacrylate) Brush as a Nanoadhesive for Efficient Silicone Bonding".

[This corrects the article DOI: 10.1021/acsomega.9b01282.].

Carbon Isotope Labeling Strategy for ß-Amino Acid Derivatives via Carbonylation of Azanickellacycles.

A series of 4-membered azametallacycles have been prepared by the oxidative addition of Ni(0) with aziridines. Stoichiometric 13C-labeled carbon monoxide could be efficiently incorporated via Ni-C bond insertion to generate air stable and isolable cyclic Ni-acyl complexes. Upon subjection to a range of C-, N-, O-, and S-nucleophiles, 13C-labeled ß-amino acids and derivatives thereof, as well as ß-aminoketones, could be rapidly accessed. The methodology proved highly adaptable for the synthesis of the antidiabetic drug, sitagliptin, with a single carbon isotope label.

Access to ß-Ketonitriles through Nickel-Catalyzed Carbonylative Coupling of α-Bromonitriles with Alkylzinc Reagents.

Herein, we report a nickel-catalyzed carbonylative coupling of α-bromonitriles and alkylzinc reagents with near stoichiometric carbon monoxide to give ß-ketonitriles in good yields. The reaction is catalyzed by a readily available and stable nickel(II) pincer complex. The developed protocol tolerates substrates bearing a variety of functional groups, which would be problematic or incompatible with previous synthetic methods. Additionally, we demonstrate the suitability of the method for carbon isotope labeling by the synthesis of 13 C-labeled ß-ketonitriles and their transformation into isotopically labeled heterocycles.