Exploring the Impact of Blend and Graft of Quinoline Derivative in Low-Temperature Curable Polyimides.

The utilization of accelerators has been a common approach to prepare low-temperature curable polyimide (PI). However, the accelerators have gradually fallen out of favor because of their excessive dosages and negative effect on the properties of PI. In this work, a new strategy of introducing accelerators by grafting to eliminate these disadvantages is presented. A novel quinoline derivative named 6-([1,1'-biphenyl]-4-yl)-4-chloroquinoline (NQL) is designed for this purpose, and an ultralow dosage of only 2.5 mol% is sufficient to prepare low-temperature curable PI. The favorable low-temperature curing effect of NQL is attributed to its strong alkalinity (pKa = 18.47) and electron-donating ability. At a curing temperature of 200 °C, the PI with 2.5 mol% NQL showed outstanding properties (Young's modulus of 5.73 GPa, elongation of 37.3%, tensile strength of 237 MPa, and coefficient of thermal expansion of 16 ppm K-1 ). In particular, NQL can even lower the curing temperature to 180 °C and the ultralow temperature curable PI film still retains excellent properties. These results demonstrate that introducing low-temperature curable accelerators by partial grafting instead of blending is a promising way to furnish low-temperature curable PI, and provide insights into the preparation of polyimide with high performance in advanced packaging.

Insight into the Reactivity of Carbon Structures for Nitrogen Reduction Reaction.

Graphene-based structures have been widely reported as promising metal-free catalysts for nitrogen reduction reaction. To explain the reactivity origin, various structures have been proposed and debated, including defects, functional groups, and doped heteroatoms. This computational work demonstrates that these structures may evolve from one to another under electrochemical conditions, generating weakly coordinated carbons, which have been identified as the active sites for N2 adsorption and activation.

Electrocatalytic dinitrogen reduction reaction on silicon carbide: a density functional theory study.

It is challenging to identify effective electrocatalysts for nitrogen reduction in order to advance electrochemical nitrogen fixation under ambient conditions using methods that are powered by renewable energy. Silicon carbide was investigated computationally as a metal-free, surface-derived catalyst for the electrocatalytic nitrogen reduction reaction. As demonstrated by first-principle calculations, Si-terminated and C-terminated surfaces, with the Si and C as active sites, are all reactive for dinitrogen capture and activation, resembling the catalytic behavior of popular B-based electrocatalysts, but the latter (C-terminated) offers an ultralow over-potential of 0.39 V, which is lower than most metals and alloys, while retarding hydrogen evolution. This research enriches the design of catalysts for dinitrogen fixation under ambient conditions, and also highlights a new direction for Si-based materials for nitrogen reduction.

Hydrogen Evolution in [NiFe] Hydrogenases: A Case of Heterolytic Approach between Proton and Hydride.

The mechanism for Hydrogen Evolution Reaction (HER) in [NiFe] hydrogenase enzymes distinguishes them from inorganic catalysts. The first H+/e- pair injected to the active site of the hydrogenases transforms into hydride, while the second H+/e- pair injection leads to the formation of the H-/H+ pair both binding to the active site. The two opposite charged hydrogens heterolytically approach each other in order to form dihydrogen (H2), which is enhanced by the Coulomb force. Two previously proposed reaction routes for this process have been examined by Conceptual Density Functional Theory (DFT) in this work. One presents better agreement with experimental spectra, while the other is thermodynamically more favorable. Both paths suggest that the approach and the charge transfer between the proton and hydride are motivated by the stabilization of the electronic activity and the electrophilicity of Ni. After the heterolytic approach of the proton and hydride moieties, the two hydrogen atoms attach to the Ni ion and combine homolytically.

Impact of H-termination on the nitrogen reduction reaction of molybdenum carbide as an electrochemical catalyst.

Transition metal molybdenum (Mo) exhibits a strong capacity to adsorb nitrogen (N2), but the Mo-N2 interaction is too strong and thus it is difficult for ammonia (NH3) to be released from the catalyst surface. Bonding with nonmetals with strong electronegativity is helpful to weaken the Mo-N2 interaction, while the effect of hydrogen termination on catalyst surfaces needs to be evaluated given that the hydrogen evolution reaction (HER) is a key side reaction. This computational work aims to explore α-molybdenum carbide (Mo2C, orthorhombic phase) as an electrochemical catalyst for the full nitrogen reduction reaction (NRR). Our density functional theory (DFT) calculations focus on a (100) surface and demonstrate that (i) surface molybdenum and carbon can be terminated by hydrogen via the Volmer step and (ii) the NRR can occur on H-terminated Mo2C(100) with an energy requirement of 1.0-1.4 eV, depending on H-coverage. Although C-Mo bonding can remarkably reduce difficulty in NH3 release from a Mo-site, H-terminals result in performance deterioration. These results provide new insights into the development of NRR catalysts.

The oxygen reduction reaction on [NiFe] hydrogenases.

Oxygen tolerance capacity is critical for hydrogen oxidation/evolution catalysts. In nature, [NiFe] hydrogenases show excellent O2-tolerance and can rapidly reactivate the active site. This work aims to understand the reduction of O2 on the active site of [NiFe] hydrogenases. From the density functional theory (DFT) calculations, the free energy diagram for the oxygen reduction reaction (ORR) has been derived and the rate-determining step is found to be the Ni-B to Ni-SIb' step. Our calculation explains the slow reactivation for the Ni-A state compared to the Ni-B state, which is due to the particularly stable structure of the Ni-A state.

Hydrogen bonding effect between active site and protein environment on catalysis performance in H2-producing [NiFe] hydrogenases.

The interaction between the active site and the surrounding protein environment plays a fundamental role in the hydrogen evolution reaction (HER) in [NiFe] hydrogenases. Our density functional theory (DFT) findings demonstrate that the reaction Gibbs free energy required for the rate determining step decreases by 7.1 kcal mol-1 when the surrounding protein environment is taken into account, which is chiefly due to free energy decreases for the two H+/e- addition steps (the so-called Ni-SIa to I1, and Ni-C to Ni-R), being the largest thermodynamic impediments of the whole reaction. The variety of hydrogen bonds (H-bonds) between the amino acids and the active site is hypothesised to be the main reason for such stability: H-bonds not only work as electrostatic attractive forces that influence the charge redistribution, but more importantly, they act as an electron 'pull' taking electrons from the active site towards the amino acids. Moreover, the electron 'pull' effect through H-bonds via the S- in cysteine residues shows a larger influence on the energy profile than that via the CN- ligands on Fe.

An Aluminum-Sulfur Battery with a Fast Kinetic Response.

The electrochemical performance of the aluminum-sulfur (Al-S) battery has very poor reversibility and a low charge/discharge current density owing to slow kinetic processes determined by an inevitable dissociation reaction from Al2 Cl7- to free Al3+ . Al2 Cl6 Br- was used instead of Al2 Cl7- as the dissociation reaction reagent. A 15-fold faster reaction rate of Al2 Cl6 Br- dissociation than that of Al2 Cl7- was confirmed by density function theory calculations and the Arrhenius equation. This accelerated dissociation reaction was experimentally verified by the increase of exchange current density during Al electro-deposition. Using Al2 Cl6 Br- instead of Al2 Cl7- , a kinetically accelerated Al-S battery has a sulfur utilization of more than 80 %, with at least four times the sulfur content and five times the current density than that of previous work.

Why is a proton transformed into a hydride by [NiFe] hydrogenases? An intrinsic reactivity analysis based on conceptual DFT.

The hydrogen evolution reaction (HER) catalysed by [NiFe] hydrogenases entails a series of chemical events involving great mechanistic interest. In an attempt to understand and delve into the question about 'Why does nature work in that way?', an in-depth intrinsic reactivity analysis based on conceptual DFT has been carried out focusing on the so-called to step, i.e. our work tries to answer how and why the proton attached to the reactive sulphur atom from one of the exo-cyclic cysteine residues is transformed into a bridging hydride to be shared between the Ni/Fe metals in the active site of [NiFe] hydrogenases, which involves not only H migration, but also a change of the charge state on Ni from Ni(i) to Ni(iii). Our DFT results suggest that the transformation is motivated by spontaneous rearrangements of the electron density, and stabilisation comes from the decrease of both electronic activity and electrophilicity index from Ni.

Role of water environment in chemical degradation of a covalent organic framework tethered with quaternary ammonium for anion exchange membranes.

The anion exchange membrane (AEM) is a main component for AEM fuel cells. Recently, a series of electrolytes based on covalent organic frameworks (COFs) functionalized with quaternary ammonium (QA) of showed extraordinary ionic conductivities thanks to the intrinsic porosity of the COF structures, which also provide a robust backbone for good mechanical strength. However, the chemical stability of the COF-based AEMs in alkaline conditions is yet to be understood. Here we systematically investigate the chemical degradation of the COF-based structures tethered with alkyl spacers by combining molecular dynamics (MD) simulations and density functional theory (DFT) calculations. We find that the water environment protects the cationic groups from chemical degradation in terms of both physical and chemical effects, which play a synergistic role. Moreover, we introduce the effective density of water as an order parameter to quantitatively characterize the level of degradation of the COF-based systems with similar design of architecture. The results provide guidance for estimation of the chemical stability of COF-based AEMs.

Defective 2D silicon phosphide monolayers for the nitrogen reduction reaction: a DFT study.

Electroreduction of N2 is a highly promising route for NH3 production. The lack of efficient catalysts that can activate and then reduce N2 into NH3 limits this as a pragmatic application. In this work, a 2D layered group IV-V material, silicon phosphide (SiP), is evaluated as a suitable substrate for the electrochemical nitrogen reduction reaction (ENRR). To capture N2, one phosphorus (P) defect was introduced on the plane of SiP. DFT calculations found that the defective SiP monolayer (D1-SiP, which is defined by the P-defect on SiP) exhibits enormous prospects towards the ENRR because of enhanced electron conductivity, good activation on N2, lower limiting potential (UL = -0.87 V) through the enzymatic pathway, smooth charge transfer between the catalyst and the reaction species, and robust thermal stability. Importantly, D1-SiP demonstrates the suppressed activities on producing of H2 and N2H4 side-products. This research demonstrates the potential of 2D metal-free Si-based catalysts for nitrogen fixation and further enriches the study of group IV-V materials for the ENRR.

p-Block element-doped silicon nanowires for nitrogen reduction reaction: a DFT study.

Photocatalytic nitrogen reduction reaction (NRR) is a promising, green route to chemically reducing N2 into NH3 under ambient conditions, correlating to the N2 fixation process of nitrogenase enzymes. To achieve high-yield NRR with sunlight as the driving force, high-performance photocatalysts are essential. One-dimensional silicon nanowires (1D SiNWs) are a great photoelectric candidate, but inactive for NRR due to their inability to capture N2. In this study, we proposed SiNWs doped by p-block elements (B, C, P) to tune the affinity to N2 and demonstrated that two-coordinated boron (B2C) offers an ultra-low overpotential (η) of 0.34 V to catalyze full NRR, which is even much lower than that of flat benchmark Ru(0001) catalysts (η = 0.92 V). Moreover, aspects including suppressed hydrogen evolution reaction (HER), high-spin ground state of the B2C site, and decreased band gap after B-doping ensure the high selectivity and photocatalytic activity. Finally, this work not only shows the potential use of metal-free p-block element-based catalysts, but also would facilitate the development of 1D nanomaterials towards efficient reduction of N2 into NH3.

A Ta-TaS2 monolith catalyst with robust and metallic interface for superior hydrogen evolution.

The use of highly-active and robust catalysts is crucial for producing green hydrogen by water electrolysis as we strive to achieve global carbon neutrality. Noble metals like platinum are currently used catalysts in industry for the hydrogen evolution, but suffer from scarcity, high price and unsatisfied performance and stability at large current density, restrict their large-scale implementations. Here we report the synthesis of a type of monolith catalyst consisting of a metal disulfide (e.g., tantalum sulfides) vertically bonded to a conductive substrate of the same metal tantalum by strong covalent bonds. These features give the monolith catalyst a mechanically-robust and electrically near-zero-resistance interface, leading to an excellent hydrogen evolution performance including rapid charge transfer and excellent durability, together with a low overpotential of 398 mV to achieve a current density of 2,000 mA cm-2 as required by industry. The monolith catalyst has a negligible performance decay after 200 h operation at large current densities. In light of its robust and metallic interface and the various choices of metals giving the same structure, such monolith materials would have broad uses besides catalysis.

Coenzyme Coupling Boosts Charge Transport through Single Bioactive Enzyme Junctions.

Oxidation of formate to CO2 is catalyzed via the donation of electrons from formate dehydrogenase (FDH) to nicotinamide adenine dinucleotide (NAD+), and thus the charge transport characteristics of FDH become essential but remain unexplored. Here, we investigated the charge transport through single-enzyme junctions of FDH using the scanning tunneling microscope break junction technique (STM-BJ). We found that the coupling of NAD+ with FDH boosts the charge transport by ∼2,100%, and the single-enzyme conductance highly correlates with the enzyme activity. The combined flicker noise analysis demonstrated the switching of the coenzyme-mediated charge transport pathway and supported by the significantly reduced HOMO-LUMO gap from calculations. Site-specific mutagenesis analysis demonstrated that FDH-NAD+ stably combined own higher bioactivity and boosts charge transport, and the coupling has been optimized via the natural selection. Our work provides evidence of hydrogen bond coupling in bioactivity but also bridges the charge transport through single-enzyme junctions and enzyme activities.

Confined Fe-Cu Clusters as Sub-Nanometer Reactors for Efficiently Regulating the Electrochemical Nitrogen Reduction Reaction.

Electrochemical nitrogen reduction reaction (NRR) over nonprecious-metal and single-atom catalysts has received increasing attention as a sustainable strategy to synthesize ammonia. However, the atomic-scale regulation of such active sites for NRR catalysis remains challenging because of the large distance between them, which significantly weakens their cooperation. Herein, the utilization of regular surface cavities with unique microenvironment on graphitic carbon nitride as "subnano reactors" to precisely confine multiple Fe and Cu atoms for NRR electrocatalysis is reported. The synergy of Fe and Cu atoms in such confined subnano space provides significantly enhanced NRR performance, with nearly doubles ammonia yield and 54%-increased Faradic efficiency up to 34%, comparing with the single-metal counterparts. First principle simulation reveals this synergistic effect originates from the unique Fe-Cu coordination, which effectively modifies the N2 absorption, improves electron transfer, and offers extra redox couples for NRR. This work thus provides new strategies of manipulating catalysts active centers at the sub-nanometer scale.

Tuning the Hydrogen Evolution Performance of Metallic 2D Tantalum Disulfide by Interfacial Engineering.

Metallic transition metal dichalcogenides, such as tantalum disulfide (TaS2), have recently emerged as promising electrocatalysts for the hydrogen evolution reaction. This work reports an effective strategy to further tune their performance through interfacial engineering, including lattice mismatch and electron injection between electrocatalysts and the underlying substrates. A unique two-zone chemical vapor deposition technique has been developed, and 2D TaS2 has been successfully grown on four different substrates, including glassy carbon, carbon fibers, Mo foil, and Au foil, providing excellent platforms to study catalyst-substrate interactions. Among them, TaS2 on Au foil offers the best performance with lowest overpotential and smallest charge transfer resistance, due to a suitable lattice mismatch and charge injection between TaS2 and Au, as revealed by theoretical calculations and experimental measurements. This work highlights the key roles the substrate plays in the catalysis and demonstrates the validity of interfacial engineering in catalyst design.