Computational chemistry for graphene-based energy applications: progress and challenges

Exploring the potential of MB2 MBene family as promising anodes for Li-ion batteries

In recent years, finding high-performance energy storage materials has become a major challenge for Li-ion batteries. B-based two-dimensional materials have become the focus of attention because of their abundant reserves and non-toxic characteristics. A series of two-dimensional transition metal borides (MBenes) are reported and their electrochemical properties as anode materials for Li-ion batteries are investigated by density functional theory (DFT) calculations. The surface of MB2 possesses medium adsorption strength and diffusion energy barrier for Li atoms, which are conducive to the insertion and extraction of Li-ions during the charge/discharge process of Li-ion batteries. Herein, we explore the potential of MB2 (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu and Zn) as the anode material for LIBs. Excitingly, the Li atom can be stably adsorbed on the surface of MB2 (M = Sc, Ti, V, Nb, Mo, W) monolayers, and the theoretical capacity of the MB2 monolayer is high (521.77-1610.20 mA h g-1). The average open circuit voltage range is within 0.10-1.00 V (vs. Li/Li+). The relationship between the p-band center of the B atom and the adsorption energy of Li on the surface of MB2 is also investigated. Furthermore, it is found that the charge transfer of Li atom and metallic center in the most stable position is strongly related to the corresponding value of diffusion energy barrier. These results confirm that MB2 monolayers are promising 2D anode materials for Li-ion batteries, demonstrating the application prospects of B-based 2D materials.

Temperature-dependent differential capacitance of an ionic liquid-graphene-based supercapacitor

One of the critical factors affecting the performance of supercapacitors is thermal management. The design of supercapacitors that operate across a broad temperature range and at high charge/discharge rates necessitates understanding the correlation of the molecular characteristics of the device (such as interfacial structure and inter-ionic and ion-electrode interactions) with its macroscopic properties. In this study, we use molecular dynamics (MD) simulations to investigate the influence of Joule heating on the structure and dynamics of the ionic liquid (IL)/graphite-based supercapacitors. The temperature-dependent electrical double layer (EDL) and differential capacitance-potential (CD-V) curves of two different ([Bmim][BF4] and [Bmim][PF6]) IL-graphene pairs were studied under various thermal gradients. For the [Bmim][BF4] system, the differential capacitance curves transition from 'U' to bell shape under an applied thermal gradient (∇T) in the range from 3.3 K nm-1 to 16.7 K nm-1. Whereas in [Bmim][PF6], we find a positive dependence of differential capacitance with ∇T with a U-shaped CD-V curve. We examine changes in the EDL structure and screening potential (ϕ(z)) as a function of ∇T and correlate them with the trends observed in the CD-V curve. The identified correlation between the interfacial charge density and differential capacitance with thermal gradient would be helpful for the molecular design of the IL-electrode interface in supercapacitors or other chemical engineering applications.

Two-dimensional Dirac TiB2C2 as a potential anode material for Li-ion batteries: a first-principles study

The development and design of anode materials with good stability, high capacity, low diffusion barrier and excellent cyclability is an important challenge for further improvement of the battery industry. In this context, a promising 2D anode material TiB2C2 with Dirac cone states is investigated through the first-principles prediction. We found this material to be thermodynamically, dynamically, and thermally stable, suggesting the possibility of its experimental synthesis. Considering its lightweight, planar structure and Dirac cone features, we systematically investigated the feasibility of the TiB2C2 monolayer as an anode material for Li-ion batteries (LIBs). Based on the adsorption energy of lithium on the monolayer surfaces, we determined the sites that can hold lithium ions with high adsorption energy. Moreover, TiB2C2 exhibits good ionic and electronic conductivity, a suitable voltage profile, and high structural stability upon the Li-loading process; it also shows 1.12% change in cell parameters. Importantly, a high storage capacity of up to 1075 mA h g-1 was found. All these criteria conclude the appealing electrochemical performance of the TiB2C2 monolayer as a promising anode material for LIBs.

Nanoantibiotic effect of carbon-based nanocomposites: epicentric on graphene, carbon nanotubes and fullerene composites: a review

Carbon in many different forms especially, Graphene, Carbon nanotubes (CNTs), and Fullerene is emerging as an important material in the areas of the biomedical field for various applications. This review comprehensively describes the nano antibiotic effect of carbon-based nanocomposites: epicenter on graphene, carbon nanotubes, and fullerene Composites. It summarises the studies conducted to evaluate their antimicrobial applications as they can disrupt the cell membrane of bacteria resulting in cell death. The initial section gives a glimpse of both "Gram"-positive and negative bacteria, which have been affected by Graphene, CNTs, and Fullerene-based nanocomposites. These bacteria include Staphylococcus Aureus, Bacillus Thuringiensis, Enterococcus faecalis, Enterococcus faecium, Bacillus subtilis, Escherichia coli, Klebseilla pneumoniae, Pseudomonas aeroginosa, Pseudomonas syringae , Shigella flexneri,Candida Albicans, Mucor. Another section is dedicated to the insight of Graphene, and its types such as Graphene Oxide (GO), Reduced graphene oxide (rGO), Graphene Nanoplatelets (GNPs), Graphene Nanoribbons (GNRs), and Graphene Quantum Dots (GQDs). Insight into CNT, including both the types SWCNT and MWCNT, studied, followed by understanding fullerene is also reported. Another section is dedicated to the antibacterial mechanism of Graphene, CNT, and Fullerene-based nanocomposites. Further, an additional section is dedicated to a comprehensive review of the antibacterial characteristics of Graphene, CNT, and nanocomposites based on fullerene. Future perspectives and recommendations have also been highlighted in the last section.

Development of accurate potentials for the physisorption of water on graphene

From coupled-cluster singles and doubles model including connected triples corrections [CCSD(T)] calculations on the water dimer and B97D/CC on the water-circumcoronene complex at a large number of randomly generated conformations, interaction potentials for the physisorption of water on graphene are built, accomplishing almost sub-chemical accuracy. The force fields were constructed by decomposing the interaction into electrostatic and van der Waals contributions, the latter represented through improved Lennard-Jones potentials. Besides, a Chemistry at Harvard Macromolecular Mechanics (CHARMM)-like term was included in the water-water potential to improve the description of hydrogen bonds, and an induction term was added to model the polarization effects in the interaction between water and polyaromatic hydrocarbons (PAHs) or graphene. Two schemes with three and six point charges were considered for the interactions water-water and water-PAH, as Coulomb contributions are zero in the water-graphene system. The proposed fitted potentials reproduce the ab initio data used to build them in the whole range of distances and conformations and provide results for selected points very close to CCSD(T) benchmarks. When applied to the water-graphene system, the obtained results are in excellent agreement with p-CCSD(T), revised symmetry-adapted perturbation theory based on density functional theory monomer properties (DFT-SAPT), and diffusion Monte Carlo reference values. Furthermore, the stability of the various conformers water-PAH and water-graphene, as well as the different trends observed between these systems are rationalized in terms of the modifications of the electrostatic contribution.

Palladium-Functionalized Graphene for Hydrogen Sensing Performance: Theoretical Studies

The adsorption characteristics of H2 molecules on the surface of Pd-doped and Pd-decorated graphene (G) have been investigated using density functional theory (DFT) calculations to explore the sensing capabilities of Pd-doped/decorated graphene. In this analysis, electrostatic potential, atomic charge distribution, 2D and 3D electron density contouring, and electron localization function projection, were investigated. Studies have demonstrated the sensing potential of both Pd-doped and Pd-decorated graphene to H2 molecules and have found that the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), i.e., the HOMO-LUMO gap (HLG), decreases to 0.488 eV and 0.477eV for Pd-doped and Pd-decorated graphene, respectively. When H2 is adsorbed on these structures, electrical conductivity increases for both conditions. Furthermore, chemical activity and electrical conductivity are higher for Pd-decorated G than Pd-doped G, whereas the charge transfer of Pd-doped graphene is far better than that of Pd-decorated graphene. Also, studies have shown that the adsorption energy of Pd-doped graphene (−4.3 eV) is lower than that of Pd-decorated graphene (−0.44 eV); a finding attributable to the fact that the recovery time for Pd-decorated graphene is lower compared to Pd-doped graphene. Therefore, the present analysis confirms that Pd-decorated graphene has a better H2 gas sensing platform than Pd-doped graphene and, as such, may assist the development of nanosensors in the future.

Determination of Quantum Capacitance of Niobium Nitrides Nb2N and Nb4N3 for Supercapacitor Applications

The density of states and quantum capacitance of pure and doped Nb2N and Nb4N3 single-layer and multi-layer bulk structures are investigated using density functional theory calculations. The calculated value of quantum capacitance is quite high for pristine Nb2N and decent for Nb4N3 structures. However for cobalt-doped unpolarized structures, significant increase in quantum capacitance at Fermi level is observed in the case of Nb4N3 as compared to minor increase in case of Nb2N. These results show that pristine and doped Nb2N and Nb4N3 can be preferred over graphene as the electrode material for supercapacitors. The spin and temperature dependences of quantum capacitance for these structures are also investigated.

Advancing Rational Control of Peptide-Surface Complexes

Understanding peptide-surface interactions is crucial for programming self-assembly of peptides at surfaces and in realizing their applications, such as biosensors and biomimetic materials. In this study, we developed insights into the dependence of a residue's interaction with a surface on its neighboring residue in a tripeptide using molecular dynamics simulations. This knowledge is integral for designing rational mutations to control peptide-surface complexes. Using graphene as our model surface, we estimated the free energy of adsorption (ΔA_ads) and extracted predominant conformations of 26 tripeptides with the motif LNR-CR-Gly, where LNR and CR are variable left-neighboring and central residues, respectively. We considered a combination of strongly adsorbing (Phe, Trp, and Arg) and weakly adsorbing (Ala, Val, Leu, Ser, and Thr) amino acids on graphene identified in a prior study to form the tripeptides. Our results indicate that ΔA_ads of a tripeptide cannot be estimated as the sum of ΔA_ads of each residue indicating that the residues in a tripeptide do not behave as independent entities. We observed that the contributions from the strongly adsorbing amino acids were dominant, which suggests that such residues could be used for strengthening peptide-graphene interactions irrespective of their neighboring residues. In contrast, the adsorption of weakly adsorbing central residues is dependent on their neighboring residues. Our structural analysis revealed that the dihedral angles of LNR are more correlated with that of CR in the adsorbed state than in bulk state. Together with ΔA_ads trends, this implies that different backbone structures of a given CR can be accessed for a similar ΔA_ads by varying the LNR. Therefore, incorporation of context effects in designing mutations can lead to desired peptide structure at surfaces. Our results also emphasize that these cooperative effects in ΔA_ads and structure are not easily predicted a priori. The collective results have applications in guiding rational mutagenesis techniques to control orientation of peptides at surfaces and in developing peptide structure prediction algorithms in adsorbed state from its sequence.

Computational methods towards increased efficiency design of graphene membranes for gas separation and water desalination

The potential impact of climate change is widely known as having serious consequences. The themes of water desalination and gas separation are closely related to the environment and energy industry. Graphene-based membranes are promising filtration devices for the two tasks. This review aims to supply a comprehensive overview of the recent computational studies investigating the performance of graphene-based membranes used in water desalination or gas separation. With the use of computational methods, the literature covered finds evidence for key factors, such as pore shape and density, affecting the performance of the investigated membranes. The reviewed studies are expected to act as an impulse towards more computational studies and eventually actual design of graphene-based membranes for water desalination and gas separation.

Adsorption of amino acids on graphene: assessment of current force fields

We compare the free energies of adsorption (ΔAads) and the structural preferences of amino acids on graphene obtained using the non-polarizable force fields-Amberff99SB-ILDN/TIP3P, CHARMM36/modified-TIP3P, OPLS-AA/M/TIP3P, and Amber03w/TIP4P/2005. The amino acid-graphene interactions are favorable irrespective of the force field. While the magnitudes of ΔAads differ between the force fields, the relative free energy of adsorption across amino acids is similar for the studied force fields. ΔAads positively correlates with amino acid-graphene and negatively correlates with graphene-water interaction energies. Using a combination of principal component analysis and density-based clustering technique, we grouped the structures observed in the graphene adsorbed state. The resulting population of clusters, and the conformation in each cluster indicate that the structures of the amino acid in the graphene adsorbed state vary across force fields. The differences in the conformations of amino acids are more severe in the graphene adsorbed state compared to the bulk state for all the force fields. Our findings suggest that the force fields studied will give qualitatively consistent relative strength of adsorption across proteins but different structural preferences in the graphene adsorbed state.

Graphene membranes with nanoslits for seawater desalination via forward osmosis

Stacked graphene (GE) membranes with cascading nanoslits can be synthesized economically compared to monolayer nanoporous GE membranes, and have potential for molecular separation. This study focuses on investigating the seawater desalination performance of these stacked GE layers as forward osmosis (FO) membranes by using molecular dynamics simulations. The FO performance is evaluated in terms of water flux and salt rejection and is explained by analysing the water density distribution and radial distribution function. The water flow displays an Arrhenius type relation with temperature and the activation energy for the stacked GE membrane is estimated to be 8.02 kJ mol^-1, a value much lower than that of commercially available FO membranes. The study reveals that the membrane characteristics including the pore width, offset, interlayer separation distance and number of layers have significant effects on the desalination performance. Unlike monolayer nanoporous GE membranes, at an optimum layer separation distance, the stacked GE membranes with large pore widths and completely misaligned pore configuration can retain complete ion rejection and maintain a high water flux. Findings from the present study are helpful in developing GE-based membranes for seawater desalination via FO.

Self-assembled nitrogen-doped fullerenes and their catalysis for fuel cell and rechargeable metal-air battery applications

In this study, we report self-assembled nitrogen-doped fullerenes (N-fullerene) as non-precious catalysts, which are active for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), and thus applicable for energy conversion and storage devices such as fuel cells and metal-air battery systems. We screen the best N-fullerene catalyst at the nitrogen doping level of 10 at%, not at the previously known doping level of 5 or 20 at% for graphene. We identify that the compressive surface strain induced by doped nitrogen plays a key role in the fine-tuning of catalytic activity.

Graphene nanoribbons anchored to SiC substrates

Graphene nanoribbons are quasi-one-dimensional planar graphene allotropes with diverse properties dependent on their width and types of edges. Graphene nanoribbons anchored to substrates is a hybrid system, which offers novel opportunities for property modifications as well as experimental control. Here we present electronic structure calculations of zigzag graphene nanoribbons chemically attached via the edges to the Si or C terminated surfaces of a SiC substrate. The results show that the edge characteristics are rather robust and the properties are essentially determined by the individual nanoribbon. While the localized spin polarization of the graphene nanoribbon edge atoms is not significantly affected by the substrate, secondary energy gaps in the highest conduction and lowest valence region may emerge in the anchored structures. The van der Waals interaction together with the electrostatic interactions due to the polarity of the surface bonds are found to be important for the structure parameters and energy stability.

Non-covalent modification of reduced graphene oxide by a chiral liquid crystalline surfactant

In order to effectively disperse reduced graphene oxide (RGO) in functional materials and take full advantage of its exceptional physical and chemical properties, a novel and effective approach for non-covalent modification of RGO by a chiral liquid crystalline surfactant (CLCS) consisting of chiral mesogenic units, nematic mesogenic units with carboxyl groups and non-mesogenic units with a polycyclic conjugated structure is firstly established. The polycyclic conjugated structure can anchor onto the RGO surface via π-π interactions, the chiral mesogenic units possess affinity for chiral materials by joining the helical matrix of chiral material and the carboxyl groups in nematic mesogenic units are supposed to form coordination bonds with nano zinc oxide (ZnO) to fabricate functional nano hybrids. The transmittances of CLCS-RGO hybrids exhibit S-shaped nonlinear increase with the increase of wavelength, but the total transmittances from 220 nm to 800 nm show a linear decreasing trend with the increase of RGO content in the CLCS-RGO hybrid. Due to the superior thermal properties of RGO and the interactions between RGO and CLCS, the dispersed RGO can improve the glass transition and increase the thermal stability and decomposition activation energy of CLCS. The intercalation of RGO can decrease the thermochromism temperature and improve the pitch uniformity of CLCS. Furthermore, CLCS can promote the dispersion of RGO in chiral nematic liquid crystals (CNLCs), and the CNLC-RGO-CLCS hybrids present decreased driving voltage and accelerated electro-optical response. The CLCS non-covalently modified RGO can strengthen the photocatalytic degradation of ZnO by suppressing the aggregation of ZnO and RGO.

Modelling of graphene functionalization

This perspective describes the available theoretical methods and models for simulating graphene functionalization based on quantum and classical mechanics.

Molecular origin of high free energy barriers for alkali metal ion transfer through ionic liquid–graphene electrode interfaces

We study mechanisms of solvent-mediated ion interactions with charged surfaces in ionic liquids by molecular dynamics simulations, in an attempt to reveal the main trends that determine ion–electrode interactions in ionic liquids.

Capacitive Energy Storage: Current and Future Challenges

Capacitive energy storage devices are receiving increasing experimental and theoretical attention due to their enormous potential for energy applications. Current research in this field is focused on the improvement of both the energy and the power density of supercapacitors by optimizing the nanostructure of porous electrodes and the chemical structure/composition of the electrolytes. However, the understanding of the underlying correlations and the mechanisms of electric double layer formation near charged surfaces and inside nanoporous electrodes is complicated by the complex interplay of several molecular scale phenomena. This Perspective presents several aspects regarding the experimental and theoretical research in the field, discusses the current atomistic and molecular scale understanding of the mechanisms of energy and charge storage, and provides a brief outlook to the future developments and applications of these devices.

Effect of graphene with nanopores on metal clusters

Graphene with nanopores can enhance the stability of metal clusters and decrease the CO adsorption. Pd supported on graphene with nanopores will act as a superior CO tolerance catalyst.