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Two isoenergetic isomers of the Be3B11− cluster, featuring the smallest trihedral spherical geometries, can interconvert via the mechanism of “triangle-pyramid-triangle”, which facilitates the migration of boron atoms in the B11 skeleton.

A plier-shaped charge-transfer [Mg2]2+[Mg2B7]− complex cluster exhibits double 6π/6σ aromaticity, whose hybrid molecular wheel structure is rationalized using the concept of electronic transmutation.

Boron-based Al2B8 cluster assumes a sandwich structure. The Al2 unit floats above and below a boron wheel, featuring intriguing dynamic fluxionality.

The ternary SiB2Be2 cluster is a planar tetracoordinate silicon (ptSi) system and features 2π/2σ double aromaticity. Its 14-electron count deviates from the 18-electron rule.

The boron-rich B12O cluster features a bridging B–O–B edge, rather than a boronyl ligand. It is effectively isoelectronic with bare B12 cluster in terms of delocalized π/σ bonding.

The masterpiece of planar dodeca-coordination has been achieved in the Y©B8C4 cluster, which sets a record coordination number for a planar structure in chemistry.

This work fabricates mechanically exfoliated graphite paper with layered microstructures, which not only ensures the high flexibility of the resulting electrochemical capacitor, but substantially boosts its electrochemical properties.

The fascinating sandwich K6Be2B6H6 cluster with a real borozene ring, being stabilized collectively by three-fold 2σ/6π/2σ aromaticity.

The global minimum of SiSb3M3+ (M = Ca, Sr, Ba) is a D3h symmetric structure containing an elusive planar hexacoordinate silicon (phSi) atom. Most importantly, the phSi core remains intact in ligand protected environment as well.

The aromatic CB5S5+ cluster is the first global energy minimum containing an exotic planar hypercoordinate carbon inside a boron wheel.

The beryllium-doped Be

The B6Ca2 and B8Ca2 clusters adopt interesting inverse sandwich architectures, featuring a prolate B6 (or perfect B8) ring jammed with two capping Ca atoms. Both clusters show the high thermodynamic stability due to the double (σ and π) electronic delocalization. In present paper, we computationally studied the hydrogen storage of them. The results suggest that each Ca site in B6Ca2 and B8Ca2 clusters could store up six H2, yielding a gravimetric density of 14.2 wt% for B6Ca2 and 12.6 wt% for B8Ca2. The average adsorption energy for H2-adsorbed B6Ca2 and B8Ca2 complexes is within the scope of 0.12–0.15 eV per H2 at wB97XD level, hinting that two clusters could reversibly store and release hydrogen, which is positively confirmed by the Born-Oppenheimer molecular dynamics simulations. Both B6Ca2 and B8Ca2 nanoclusters are feasible hydrogen storage media under the ambient condition.

This work investigates the effect of preparation conditions on the supercapacitive performances of the poly(3,4-ethylenedioxythiophene) (PEDOT) electrodes electrodeposited detailedly. These preparation conditions include electrode substrates, electrodeposition modes, parameters, and counter anions. Electrochemical characterizations indicate an evident impact of electrode substrates on the electrochemical behaviors of the PEDOT electrodes prepared. Graphite foils (GF) as the electrode substrate show better electrochemical capacitive properties relative to conductive glasses (CG). This can be ascribed to lower sheet resistance and better adhesion to the PEDOT films for the former. Moreover, SEM and AFM tests indicate that different electrodeposition modes, parameters, and counter anions would result in different morphologies and roughness for the PEDOT electrodes obtained, consequently affecting their supercapacitive performances. Among numerous preparation conditions, the polystyrene sulfonic acid salt-doped PEDOT deposited on GF substrates prepared with potentiostatic mode at 0.9 V shows the best supercapacitive behaviors, delivering the specific capacitance of 108.2 mF cm−2 at 0.2 mA cm−2, also exhibiting good rate capability and superior cycle performance (keeping 93.4% of initial capacitance after 10,000 cycles). This work indicates that the supercapacitive properties of PEDOT electrodes can be substantially enhanced by changing their preparation conditions.

The design of clusters featuring non-classical planar hypercoordinate atoms (phAs) often depends on the delocalized multicenter bonds involving reactive electron-deficient elements, which both destabilize the clusters and lead to difficulty in achieving the phA arrangement for electronegative elements such as nitrogen due to their preference for localized bonds. In this work, we computationally designed a series of aluminum chalcogenide clusters NAl4X4+ (X = S, Se, Te) with a desired planar tetracoordinate nitrogen and meaningfully improved chemical stability, as evidenced by the wide gaps (6.51–7.23 eV) between their highest occupied molecular orbitals and lowest unoccupied molecular orbitals, high molecular rigidity (dynamically stable up to 1500 K), and exclusively low global energy minima nature (their isomers locate at least 51.2 kcal/mol higher). Remarkably, these clusters are stabilized by peripheral chalcogen atoms, which not only sterically protect the NAl4 core moiety but also electronically compensate for the electron-deficient aluminum atoms via X → Al π back bonds, meeting the description of our recently proposed “electron-compensation” strategy.

Boron displays many unusual structural and bonding properties due to its electron deficiency. Here we show that a boron atom in a boron monoxide cluster (B9O−) exhibits transition-metal-like properties. Temperature-dependent photoelectron spectroscopy provided evidence of the existence of two isomers for B9O−: the main isomer has an adiabatic detachment energy (ADE) of 4.19 eV and a higher energy isomer with an ADE of 3.59 eV. The global minimum of B9O− is found surprisingly to be an umbrella-like structure (C6v, 1A1) and its simulated spectrum agrees well with that of the main isomer observed. A low-lying isomer (Cs, 1A′) consisting of a BO unit bonded to a disk-like B8 cluster agrees well with the 3.59 eV ADE species. The unexpected umbrella-like global minimum of B9O− can be viewed as a central boron atom coordinated by a η7-B7 ligand on one side and a BO ligand on the other side, [(η7-B7)-B-BO]−. The central B atom is found to share its valence electrons with the B7 unit to fulfill double aromaticity, similar to that in half-sandwich [(η7-B7)-Zn-CO]− or [(η7-B7)-Fe(CO)3]− transition-metal complexes. The ability of boron to form a half-sandwich complex with an aromatic ligand, a prototypical property of transition metals, brings out new metallomimetic properties of boron.

Cycling stability of electrodes is a key performance index that determines the working life of supercapacitors. However, unlike specific capacitance and coulombic efficiency, nearly all researches ...

Boron forms a rich variety of low-dimensional nanosystems, including the newly discovered helix Be6 B102- (1) and Be6 B11- (2) clusters. We report herein on the elucidation of chemical bonding in clusters 1/2, using the modern quantum chemistry tools of canonical molecular orbital analyses and adaptive natural density partitioning (AdNDP). It is shown that clusters 1/2 contain a chiral helix Be2 B10 Be2 or Be2 B11 Be2 skeleton with a total of 11 and 12 segments, respectively, which effectively curve into "helical pseudo rings" and chemically consist of two "quasicircles" as defined by their anchoring Be centers. The helix skeleton is connected via Lewis-type B-B and Be-B-Be σ bonds, being further stabilized by island π/σ bonds and a loose π bond at the junction. The Be6 component in 1/2 assumes a distorted prism shape only physically, and it is fragmented into four parts: two terminal Be2 dimers and two isolated Be centers. A Be2 dimer at the far end manages to bend over and cap a quasicircle from one side of B plane. Consequently, each quasicircle of a helical pseudo ring is capped from opposite sides by two Be2 /Be units, facilitating intramolecular charge-transfers of 5 electrons from Be to B. Overall, the folding of B helix involves as many as 10 electrons. The enormous electrostatics offers the ultimate driving forces for B helix formation.

Planar tetracoordinate or pentacoordinate carbon (ptC and ppC) systems are dominated by the 18-electron counting, and a 14-electron ptC cluster is generally considered odd if not impossible. Herein we report computational evidence for a series of ternary CBe4Mnn−2 (M = Li, Au; n = 1–3) clusters, which feature 14 valence electrons and a uniform ptC center. These global-minimum clusters are established via computer global searches, followed by electronic structure calculations at the PBE0-D3, B3LYP-D3, and single-point CCSD(T) levels. In all six species, a core ptC CBe4 unit is stabilized by one to three peripheral Li/Au atoms in a bridging fashion. Chemical bonding analyses reveal delocalized 2π/6σ frameworks around the ptC center, thus rendering double π/σ aromaticity according to the (4n + 2) Huckel rule. The delocalized π/σ frameworks collectively conform to the 8-electron counting and are irrelevant to the “18-electron rule”. The 14-electron ptC clusters can potentially be extended to isoelectronic CBe4Mnn−2 (M = Na, K, Cu, Ag; n = 1–3) systems. Cationic ptC CBe4M3+ (M = Li, Na, K, Cu, Ag, Au) clusters have low vertical electron affinities (2.36–4.77 eV) and belong to the class of exotic species called superalkali or pseudoalkali cations.

Cubic octacoordinate carbon (coC) systems are extremely rare in chemistry. We report on a designer 32-electron coC CBe8H12 cluster, which has high Oh symmetry, being robust chemically and electronically, as well as dynamically up to 1200 K. Chemical bonding analysis establishes it as a core–shell cluster, consisting of cubic Be8H12 shell versus coC CBe8 core. The former is bound via Lewis-type three-center two-electron (3c-2e) Be–H–Be σ bonds and the latter features 8σ cubic aromaticity, collectively leading to a two-fold superatomic electron configuration. Cubic 8σ delocalization occurs in a truly three-dimensional fashion within the coC core, rather than on square surfaces only. The C center is in the s1p3 configuration, which uses 2s/2px/2py/2pz atomic orbitals equally in bonding with the Be8 cube, albeit without sp3 hybridization. The cubic cluster concept can be extended to alternative Oh Be8H122+ and NBe8H12+ clusters, the latter being a superalkali cation, which offers opportunities to tune the electronic properties.

Computational evidence is reported on a boron-based ternary Rb6Be2B6 cluster as the “Big Mac” sandwich on a subnanoscale with thickness of 0.58 nm. The core hexagonal B6 ring, occurring in the naked form due to double 6π/6σ aromaticity, is capped by two tetrahedral BeRb3 ligands. Such a B6 motif is scarce in boron clusters. The sandwich cluster has four-fold 2σ/6π/6σ/2σ aromaticity and its tetrahedral BeRb3 ligand is the simplest case of three-dimensional aromaticity (or spherical aromaticity). The sandwich can be formulated as a charge-transfer complex, [Rb3Be]3+[B6]6−[BeRb3]3+, whose components are held together by robust electrostatics, facilitating dual-mode dynamic fluxionality.

Elemental boron clusters do not form linear chain or monocyclic ring structures, which is in contrast to carbon. Based on computer global searches and quantum chemical calculations, we report on the viability of a curved boron single chain in binary Be6B7− cluster. The boron motif assumes a bow shape, being anchored on a Be6 prism. Such a motif, which appears to be highly strained in its free-standing form, is exotic in boron-based clusters and nanostructures. Chemically, the cluster is analogous to a “clam-and-pearl-chain” system at the nanoscale (about 1 nm in size), in which a Be6 clam moderately opens its mouth, except that a B7 pearl chain is too large to be encapsulated inside. The picture differs from a three-layered sandwich. This cluster features a hybrid Be2B7 monocyclic ring, which is octagonal in nature and supports double 10π/6σ aromaticity. The number of π bonds substantially surpasses that in bare boron clusters of similar sizes. Two Be3 rings in the prism are also σ aromatic, albeit with effective 1σ/1σ electron-counting only. The unique multifold 1σ/10π/6σ/1σ aromaticity governs the geometry of the Be6B7− cluster, which can also be rationalized using the concept of dual electronic transmutation.

Boron is an electron-deficient element. The flatland of planar or quasi-planar (2D) boron clusters is believed to possess aromaticity for all members, which remains a fundamental issue in debate in boron chemistry. Using a selected set of D2h B62−, C2h B282−, and C2v B29− clusters as counter examples, we shall present computational evidence for global or island π antiaromaticity in 2D boron clusters. The latter two are flattened for the purpose of clarity, which model their quasi-planar C2 or Cs monoanion clusters observed in prior gas-phase experiments. Chemical bonding in the clusters is elucidated collectively on the basis of canonical molecular orbital (CMO) analysis, adaptive natural density partitioning (AdNDP), electron localization functions (ELFs), and localized molecular orbital (LMO) analysis. These results are complementary to each other and yet highly coherent. As a quantitative indicator, nucleus-independent chemical shifts (NICSs) are calculated at selected specific points in the clusters, which help differentiate between π aromaticity and antiaromaticity. Intriguingly, triangular sites in the same boron cluster can be aromatic, antiaromatic, or nonaromatic, despite the fact that they are physically indistinguishable. The phenomenon is understood in analogy to hydrocarbons and polycyclic aromatic hydrocarbons (PAHs). Even perfect sheet-like boron clusters are convertible to the PAH analogous systems. This work provides compelling examples for global and island π antiaromaticity in the 2D boron clusters.

Boron oxide clusters have structural richness and exotic chemical bonding. We report a quantum chemical study on the binary B

Boron displays many unusual structural and bonding properties due to its electron deficiency. Here we show that a boron atom in a boron monoxide cluster (B9O−) exhibits transition-metal-like properties. Temperature-dependent photoelectron spectroscopy provided evidence of the existence of two isomers for B9O−: the main isomer has an adiabatic detachment energy (ADE) of 4.19 eV and a higher energy isomer with an ADE of 3.59 eV. The global minimum of B9O− is found surprisingly to be an umbrella-like structure (C6v, 1A1) and its simulated spectrum agrees well with that of the main isomer observed. A low-lying isomer (Cs, 1A′) consisting of a BO unit bonded to a disk-like B8 cluster agrees well with the 3.59 eV ADE species. The unexpected umbrella-like global minimum of B9O− can be viewed as a central boron atom coordinated by a η7-B7 ligand on one side and a BO ligand on the other side, [(η7-B7)-B-BO]−. The central B atom is found to share its valence electrons with the B7 unit to fulfill double aromaticity, similar to that in half-sandwich [(η7-B7)-Zn-CO]− or [(η7-B7)-Fe(CO)3]− transition-metal complexes. The ability of boron to form a half-sandwich complex with an aromatic ligand, a prototypical property of transition metals, brings out new metallomimetic properties of boron., The global minimum of the B9O− cluster is found to have an umbrella-like structure, where the central B atom exhibits transition-metal-like bonding properties, coordinated by a η7-B7 ligand on one side and a BO ligand on the other.

Planar

This work reports on the significant improvement of electrochemical capacitive properties of the polyaniline (PANI) electrode via morphology changes, following an efficient and simple route. Specifically, a surfactant-assisted electropolymerization method is put forward to prepare PANI nanorods (PNRs) electrode and the formation mechanism of PNRs is investigated. In comparison to PANI electrode with compact microstructures, PNRs electrode has a dispersed nanorod-shaped morphology, showing substantially improved supercapacitive performances. The resulting PNRs electrode has the high areal capacitance of 345.9 mF cm−2, which represents a boost of as much as 40% comparing with that of PANI electrode. The PNRs electrode also features superior rate performance and excellent cycling stability, exhibiting 93.1% of capacitance retention after 10000 cycles. The present study has demonstrated a new and simple strategy to prepare nanostructured PANI and the prepared PNRs supercapacitor electrode is very promising for practical applications.

Alloying an elongated B10 cluster with Ca is shown to give rise to a dynamically fluxional B10Ca cluster, the latter behaving like a tank tread at the subnanoscale. Computer global search identifies the B10Ca C2 (1A) global-minimum structure, which is chiral in nature and retains the quasi-planar moiety of bare B10 cluster with Ca capped at one side, forming a half-sandwich. The rotation barrier of B10Ca cluster is reduced with respect to B10 by one order of magnitude, down to 1 kcal mol−1 at the PBE0/6-311+G* level, which demonstrates structural fluxionality at 600 K and beyond via molecular dynamics simulations. Structurewise, the Ca alloying in B10Ca cluster generates rhombic defect holes, preactivating the species and making it flexible against deformation. Chemical bonding analyses indicate that the B10Ca cluster is a charge-transfer [B10]2−[Ca]2+ complex, being doubly π/σ aromatic with the 6π and 10σ electron-counting. Such a pattern offers ideal π/σ delocalization and facilitates fluxionality. In contrast, bare B10 cluster has conflicting aromaticity with 6π and 8σ electrons, which is nonfluxional with a barrier of 12 kcal mol−1. Double π/σ aromaticity versus conflicting aromaticity is a key mechanism that distinguishes between fluxional B10Ca and nonfluxional B10 clusters, offering a compelling example that the concept of aromaticity (and double aromaticity) can be exploited to design dynamically fluxional nanosystems.

Molecules with planar tetracoordinate carbons (ptCs) are exotic in chemical bonding, and they are normally designed according to the 18-electron rule. Here we report on the viability of ptC clusters with as few as 12 valence electrons, which represent the lower limit in terms of electron counting. Specifically, we have computationally designed a class of ternary 12-electron ptC clusters, CBe3X3+ (X = H, Li, Na, Cu, Ag), based on a rhombic CBe32− unit. Computer structural searches reveal that the ptC species are global minima, whose C center is coordinated in-plane by three Be atoms and a terminal X atom via robust C–Be/C–X bonding, either covalent or ionic. The other two X atoms are on the periphery and each bridge two Be atoms. Bonding analyses show that the ptC core is governed by delocalized 2π/6σ bonding, that is, double π/σ aromaticity, which collectively conforms to the 8-electron counting. Additional 4 electrons contribute to peripheral Be–X–Be and Be–Be σ bonding. The delocalized 2π/6σ frameworks appear to be universal for all ptC clusters, ranging from 18-electron down to 12-electron systems. In other words, the ptC species are dictated entirely by the 8-electron counting. Predicted vertical electron affinities of these ptC clusters range from 3.13 to 5.48 eV, indicative of superalkali or pseudoalkali cations.

This Perspective article deals with recent computational and experimental findings in boron-based heterocyclic clusters, which focuses on binary B-O and B-S clusters, as well as relevant ternary B-X-H (X = O, S, N) species. Boron is electron-deficient and boron clusters do not form monocyclic rings or linear chains. Boron-based heterocyclic clusters are intuitively even more electron-deficient and feature exotic chemical bonding, which make use of O 2p, S 3p, or N 2p lone-pairs for π delocalization over heterocyclic rings, facilitating new cluster structures and new types of bonding. Rhombic, pentagonal, hexagonal, and polycyclic clusters are discussed herein. Rhombic species are stabilized by four-center four-electron (4c-4e) π bonding, that is, the o-bond. An o-bond cluster differs from a typical 4π antiaromatic system, because it has 4π electrons in an unusual bonding/nonbonding combination, which takes advantage of the empty 2pz atomic orbitals from electron-deficient boron centers. A variety of examples (notably including boronyl boroxine) possess a hexagonal ring, as well as magic 6π electron-counting, making them new members of the inorganic benzene family. Pentagonal clusters bridge rhombic o-bond systems and inorganic benzenes, but they do not necessarily favor 6π electron-counting as in cyclopentadienide anion. In contrast, pentagonal 4π clusters are stable, leading to the concept of pentagonal o-bond. One electron can overturn the potential energy landscape of a system, enabling rhombic-to-hexagonal structural transition, which further reinforces the idea that 4π electron-counting is favorable for rhombic systems and 6π is magic for hexagonal rings. The bonding analogy between heterocyclic clusters and hydrocarbons goes beyond monocyclic species, which allows rational design of boron-based inorganic analogs of polycyclic aromatic hydrocarbons, including s-indacene as a puzzling aromatic/antiaromatic system. Selected linear B-O clusters are also briefly discussed, featuring dual 3c-4e π bonds, that is, ω-hyperbonds. Dual ω-hyperbonds, rhombic or pentagonal o-bond, and inorganic benzenes share a common chemical origin. The field of boron-based heterocyclic clusters is still in its infant stage, and much new chemistry remains to be discovered in forthcoming experimental and theoretical studies.

Chirality plays an important role in nature. Nanoclusters can also exhibit chiral properties. We report herein a joint experimental and theoretical investigation on the geometric and electronic structures of B31− and B32− clusters, using photoelectron spectroscopy in combination with first-principles calculations. Two degenerate quasi-planar chiral C1 enantiomers (I and II, 1A) with a central hexagonal vacancy are identified as the global minima of B31−. For B32−, two degenerate boat-like quasi-planar chiral C2 structures (VI and VII, 2A) with a central hexagonal vacancy are also found as the global minima, with a low-lying chair-like Ci B32− (VIII, 2Au) also present in the experiment as a minor isomer. The chiral conversions in quasi-planar B31− and B32− clusters are investigated and relatively low barriers are found due to the high flexibility of these monolayer clusters, which feature multiple delocalized σ and π bonds over buckled molecular surfaces.

Boron-based clusters possess unusual structural and bonding properties owing to boron's electron-deficiency. We report on the theoretical prediction of two binary B–Na clusters, Na6B7− and Na8B7+, which assume unique sandwich geometries, featuring a perfectly planar B7 wheel and two triangular Na3 or quasi-tetrahedral Na4 ligands. Despite distinct electronegativities of B/Na, the B–Na clusters do not form typical salts. Both sandwich species are dynamically fluxional at 300 K and beyond. Two dynamic modes are observed: an in-plane rotation of the B7 wheel versus twisting of the two Na3/Na4 ligands. Their energy barriers are negligibly small. Natural bond orbital calculations show that the clusters are charge-transfer complexes [Na3]+[B7]3−[Na3]+ and [Na4]2+[B7]3−[Na4]2+, respectively. Chemical bonding analyses indicate that the B7 wheel in the clusters has 6π/6σ double aromaticity and the Na3/Na4 ligands are 2σ aromatic, collectively leading to four-fold π/σ aromaticity. The quasi-tetrahedral Na4 ligand is the simplest example of spherical aromaticity and can also be considered a superatom. Interlayer bonding in the sandwiches is greater than 20 eV, due to electrostatics, which should not be confused with weakly bound species. Four-fold π/σ aromaticity and robust interlayer ionic bonding offer uniform and dilute electron clouds over the sandwiches, facilitating their dual-mode dynamic fluxionality. The Na8B7+ cluster is also a superalkali cation.

Current collector is an important component of supercapacitors to transport electrons from/to the electroactive materials. However, its role has not received the attention it deserves. In this work, we have put forward a facile electrochemical approach that treats graphite (GP) to obtain superficial expansion-treated graphite (Ex-GP), which is used as the current collector/substrate for electrodeposition of poly(3,4-ethylenedioxythiophene) (PEDOT). We have also examined and optimized the electrodeposition mode and parameter of PEDOT. Electrochemical measurements indicate that Ex-GP/PEDOT electrodes exhibit obviously enhanced electrochemical performances comparing to GP/PEDOT electrodes. This is attributed to the reduced constriction and spreading resistance for the former, due to increased area of contact spots at the current collector/electroactive material interface. Herein Ex-GP/PEDOT electrodes achieve a high specific capacitance of 135.4 mF cm−2 at 0.5 mA cm−2. A highly flexible solid-state supercapacitor is further fabricated with Ex-GP/PEDOT electrodes. The supercapacitor device exhibits high rate capability (maintaining 73.8% of capacitance as current density increases for 20 times), superior energy/power characteristics (1.45 mW h cm−3 at power density of 49.6 mW cm−3), and remarkable cycle performance (maintaining 95.8% of initial capacitance after 5000 cycles), which holds great promise for use in flexible electronics. The present study is expected to stimulate further research activities on optimizing current collector/active material interface for boosted supercapacitive properties.

Elemental boron clusters do not form linear chain or monocyclic ring structures, which is in contrast to carbon. Based on computer global searches and quantum chemical calculations, we report on the viability of a curved boron single chain in binary Be6B7- cluster. The boron motif assumes a bow shape, being anchored on a Be6 prism. Such a motif, which appears to be highly strained in its free-standing form, is exotic in boron-based clusters and nanostructures. Chemically, the cluster is analogous to a "clam-and-pearl-chain" system at the nanoscale (about 1 nm in size), in which a Be6 clam moderately opens its mouth, except that a B7 pearl chain is too large to be encapsulated inside. The picture differs from a three-layered sandwich. This cluster features a hybrid Be2B7 monocyclic ring, which is octagonal in nature and supports double 10π/6σ aromaticity. The number of π bonds substantially surpasses that in bare boron clusters of similar sizes. Two Be3 rings in the prism are also σ aromatic, albeit with effective 1σ/1σ electron-counting only. The unique multifold 1σ/10π/6σ/1σ aromaticity governs the geometry of the Be6B7- cluster, which can also be rationalized using the concept of dual electronic transmutation.

Computational evidence is reported on a boron-based ternary Rb6Be2B6 cluster as the "Big Mac" sandwich on a subnanoscale with thickness of 0.58 nm. The core hexagonal B6 ring, occurring in the naked form due to double 6π/6σ aromaticity, is capped by two tetrahedral BeRb3 ligands. Such a B6 motif is scarce in boron clusters. The sandwich cluster has four-fold 2σ/6π/6σ/2σ aromaticity and its tetrahedral BeRb3 ligand is the simplest case of three-dimensional aromaticity (or spherical aromaticity). The sandwich can be formulated as a charge-transfer complex, [Rb3Be]3+[B6]6-[BeRb3]3+, whose components are held together by robust electrostatics, facilitating dual-mode dynamic fluxionality.

A 17-electron CBe

Boroxol (B3O3) rings and relevant hexagonal B3S3 structural blocks are ubiquitous in boron oxide/sulfide glasses, crystals, and high temperature liquids. However, the isolation of an ultimate heterocyclic B3O3 or B3S3 cluster in the free-standing form, with as few as six atoms, has been unsuccessful so far. We report on computational design of the simplest case of such a system: highly symmetric D3h B3S3+ (1A1) cluster. It is the well-defined global minimum on the potential energy surface, following global searches and electronic structure calculations at the B3LYP and single-point CCSD(T) levels. Chemical bonding analysis reveals an ideal system with skeleton Lewis B3S3 single bonds and unique double 6π/2σ aromaticity, which underlies its stability. The cluster turns out to be an inorganic analog of the 3,5-dehydrophenyl cation, a typical double π/σ aromatic system. It offers an example for chemical analogy between boron-based heterocyclic clusters and aromatic hydrocarbons. Double π/σ aromaticity is also a new concept in heterocyclic boron clusters. Prior systems such as borazine, boroxine, and boronyl boroxine only deal with π aromaticity as in benzene.

This paper reports on the preparation of polyaniline-carbon nanotube (PANi-CNT) hybrid electrodes via a facile electro-co-deposition method. In the process, carboxylated CNTs, which serve as counter anions in the electrodes, are incorporated into PANi. Supercapacitive properties of PANi-CNT hybrid electrodes are observed to depend on the degree of carboxylation of CNTs. Electrochemical measurements show two key results. First, the incorporation of CNTs observably improves the supercapacitive properties of PANi electrodes. Second, the CNTs with higher degree of carboxylation result in further improvement in supercapacitive properties of the hybrid electrodes. The former observation is attributed to the high electrical conductivity and large surface area of CNTs, which improve the electrical conductivities of the hybrids and facilitate the dispersion of PANi. The latter is in line with the fact that more CNTs are incorporated into the hybrids due to higher degree of carboxylation. The PANi-CNT electrodes prepared by CNTs with higher degree of carboxylation demonstrate remarkable supercapacitive performances, including high specific capacitance of 578.2 F g−1 and good rate performance. They also have superior cycling stability, showing 88.2% of capacitance retention after 10,000 cycles.

A novel strategy is proposed to obviously enhance the electrochemical capacitive properties of polypyrrole/graphene oxide/carbon nanotubes (PPy/GO/CNT) ternary composite. Here GO is treated by carboxylation reaction to obtain carboxyl graphene (CG). For comparison, PPy/GO/CNT and PPy/CG/CNT composites are synthesized with the same one-pot electro-co-deposition. Fourier transform infrared spectrometry, energy-dispersive X-ray spectroscopy, X-ray diffraction, scanning and transmission electron microscopy are carried out to characterize the composition and morphology of both composites. Unlike only utilizing the edged carboxyl groups on GO to combine with PPy coating, the current PPy/CG/CNT composite makes use of carboxyl groups distributed on basal planes and edges of CG nanosheets to combine with PPy. Accordingly, electrochemical measurements indicate that the PPy/CG/CNT electrodes markedly improve the supercapacitive properties compared to PPy/GO/CNT electrodes. The as-prepared PPy/CG/CNT composite electrodes show a high areal specific capacitance of 196.7 mF cm−2 at the current density of 0.5 mA cm−2 and superior rate capability, as well as achieve 98.1% of capacitance retention after 5000 CV cycles. The PPy/CG/CNT ternary composite we have developed holds promise for high-efficiency supercapacitor applications.

Planar hypercoordinate carbon molecules are exotic species, for which the 18-electron counting has been considered a rule. We report herein computational evidence of perfectly planar C2v CBe4Li4 (1...

We propose a strategy to enhance the electrochemical performances of electrodes by optimizing charge transfer pathways at the current collector/electroactive material interface. A facile method is developed for electrochemical surface treatment of graphite, and the obtained electrochemical surface-treated graphite is utilized as substrate for electropolymerization of polyaniline, which facilitates the construction of high-quality electrical links at the interface of polyaniline and electrochemical surface-treated graphite. Effect of galvanostatic and potentiostatic polymerization on electrochemical properties of polyaniline electrodes is also examined. Electrochemical measurements indicate that the potentiostatically polymerized polyaniline electrode has superior supercapacitive performances. In particular, polyaniline/electrochemical surface-treated graphite electrode exhibits remarkably enhanced electrochemical properties in comparison with polyaniline/graphite electrode, which can be ascribed to the optimized charge transfer pathways at current collector/electroactive material interface for the former. Polyaniline/electrochemical surface-treated graphite electrode achieves a high specific capacitance of 437.9 mF cm−2 at 0.5 mA cm−2. Moreover, the polyaniline/electrochemical surface-treated graphite electrodes are used to assemble a flexible solid-state supercapacitor. The device demonstrates high flexibility, superior supercapacitive performances, and good cycling stability, which holds great promise for use in flexible electronics.

We seek to improve the electrochemical properties of supercapacitor via engineering the interface of electroactive material/current collector. A facile electrochemical method is put forward to fabricate expanded graphite (ExGP), and polyaniline-carbon nanotube (PANI-CNT) composite electrodes using ExGP as substrate are prepared by one-step co-electrodeposition. Compared with PANI-CNT/GP electrodes, PANI-CNT/ExGP electrodes show significantly improved electrochemical capacitive performances due to reduced constriction/spreading resistance. This effect is ascribed to the increased area of contact points at the interface of electroactive material/current collector for the latter. The PANI-CNT/ExGP electrodes deliver a specific capacitance of 826.7 F g−1, higher than previous reports based on PANI-CNT composites. A highly flexible solid-state supercapacitor is assembled using PANI-CNT/ExGP electrodes. The device shows high rate capability, superior energy/power characteristics (7.1 kW kg−1 at an energy density of 12.0 Wh kg−1), and good cycling stability. This study demonstrates that optimizing electroactive material/current collector interface can implement faster electron transport and represents an effective strategy to promote electrochemical capacitive properties of supercapacitors.