Charge Modulation in Graphitic Carbon Nitride as a Switchable Approach to High-Capacity Hydrogen Storage

Tunable electronic and optical properties in buckling a non-lamellar B3S monolayer

We propose a novel polymorph of a hexagonal B₃S monolayer by combing structure swarm intelligence and first-principles calculations. Phonon spectrum analysis and ab initio molecular dynamics simulation indicate that the new structure is dynamically and thermally stable. Furthermore, the structure is mechanically stable and has a satisfactory elastic modulus. Our results show that the B₃S monolayer is a semiconductor with strong visible-light optical absorption. More importantly, the electronic properties of the structure are tunable via surface functionalization. For example, hydrogenation or fluorination could transform the monolayer from the semiconducting to metallic state. On the other hand, surface oxidation could significantly enhance both carrier mobility and near-infrared optical absorption. Furthermore, we also discovered that the monolayer possesses satisfactory storage capacity for H₂.

Regulating Pd/Al2O3 catalyst by g-C3N4 toward the enhanced selectivity of isoprene hydrogenation

The catalytic performance of Pd NPs in the selective hydrogenation of isoprene is modulated by g-C₃N₄ deposits on commercial alumina supports.

Potential hydrogen storage materials from metal decorated 2D-C2N: an ab initio study

Two dimensional nitrogenated holey graphene (2D-C₂N) is often considered as an ideal material for hydrogen storage applications owing to its lower mass density and high surface-to-volume ratio. As the interaction between H₂ and pristine 2D-C₂N is very weak with an adsorption energy of only 0.10 eV per H₂, it is important to improve it through appropriate materials design. Using density functional theory calculations, we investigated the hydrogen storage properties of metal (M = Mg, Ca, Ti, V, Mn, Fe, Co, Ni, Cu, and Zn) decorated 2D-C₂N. From this study, we found that M binding energy on 2D-C₂N is greater than the cohesive energy of the respective bulk metals, indicating that the metal is strongly bonded with the 2D-C₂N, which rules out the metal clustering issue. In particular, 2D-C₂N with Mg decoration leads to 6.79 wt% hydrogen storage capacity with a desirable adsorption energy which is above the Department of Energy's target. The electronic structure analyses show that the Mg decoration leads to a semiconductor-to-metallic transition in 2D-C₂N. Our chemical bonding analyses through partial density of states, charge density, electron localization function, charge transfer, and Bader effective charge confirm the presence of an iono-covalent character for Mg decorated 2D-C₂N. This indicates that the H₂ molecules are adsorbed by a polarization mechanism. Overall, our results suggest that Mg decorated 2D-C₂N is a promising candidate for potential hydrogen storage applications.

Fermi Level Determination for Charged Systems via Recursive Density of States Integration

Determining the Fermi level position for a given material is important to understand many of its electronic and chemical properties. Ab initio methods are effective in computing Fermi levels when using charge-neutral supercells. However, in the case where charges are explicitly included, the compensating homogeneous background charge, which is necessary to maintain charge neutrality in periodic models, causes the vacuum potential to be ill-defined - which would otherwise have been a reliable reference potential. Here, we develop a method based on recursively integrating the density of states to determine shifts in the Fermi level upon charging. By introducing incremental charges, one can compute the density of states profile and determine the shift in the Fermi level that corresponds to adding or removing a given increment of charge δq, which allows the evaluation of the Fermi level for any arbitrary charge q. We test this method for a range of materials (graphene, h-BN, C₃N₄, Cu, and MoS₂) and demonstrate that this method can produce a reasonable agreement with models that rely on localized compensating background charges.

Modelling molecular adsorption on charged or polarized surfaces: a critical flaw in common approaches

A number of recent computational material design studies based on density functional theory (DFT) calculations have put forward a new class of materials with electrically switchable chemical characteristics that can be exploited in the development of tunable gas storage and electrocatalytic applications. We find systematic flaws in almost every computational study of gas adsorption on polarized or charged surfaces, stemming from an improper and unreproducible treatment of periodicity, leading to very large errors of up to 3 eV in some cases. Two simple corrective procedures that lead to consistent results are proposed, constituting a crucial course correction to the research in the field.

Conductive Boron-Doped Graphene as an Ideal Material for Electrocatalytically Switchable and High-Capacity Hydrogen Storage

Electrocatalytic, switchable hydrogen storage promises both tunable kinetics and facile reversibility without the need for specific catalysts. The feasibility of this approach relies on having materials that are easy to synthesize, possessing good electrical conductivities. Graphitic carbon nitride (g-C₄N₃) has been predicted to display charge-responsive binding with molecular hydrogen-the only such conductive sorbent material that has been discovered to date. As yet, however, this conductive variant of graphitic carbon nitride is not readily synthesized by scalable methods. Here, we examine the possibility of conductive and easily synthesized boron-doped graphene nanosheets (B-doped graphene) as sorbent materials for practical applications of electrocatalytically switchable hydrogen storage. Using first-principle calculations, we find that the adsorption energy of H₂ molecules on B-doped graphene can be dramatically enhanced by removing electrons from and thereby positively charging the adsorbent. Thus, by controlling charge injected or depleted from the adsorbent, one can effectively tune the storage/release processes which occur spontaneously without any energy barriers. At full hydrogen coverage, the positively charged BC₅ achieves high storage capacities up to 5.3 wt %. Importantly, B-doped graphene, such as BC₄₉, BC₇, and BC₅, have good electrical conductivity and can be easily synthesized by scalable methods, which positions this class of material as a very good candidate for charge injection/release. These predictions pave the route for practical implementation of electrocatalytic systems with switchable storage/release capacities that offer high capacity for hydrogen storage.

First-Principle Framework for Total Charging Energies in Electrocatalytic Materials and Charge-Responsive Molecular Binding at Gas-Surface Interfaces

Heterogeneous charge-responsive molecular binding to electrocatalytic materials has been predicted in several recent works. This phenomenon offers the possibility of using voltage to manipulate the strength of the binding interaction with the target gas molecule and thereby circumvent thermochemistry constraints, which inhibit achieving both efficient binding and facile release of important targets such as CO2 and H2. Stability analysis of such charge-induced molecular adsorption has been beyond the reach of existing first-principle approaches. Here, we draw on concepts from semiconductor physics and density functional theory to develop a first principle theoretical approach that allows calculation of the change in total energy of the supercell due to charging. Coupled with the calculated adsorption energy of gas molecules at any given charge, this allows a complete description of the energetics of the charge-induced molecular adsorption process. Using CO2 molecular adsorption onto negatively charged h-BN (wide-gap semiconductor) and g-C4N3 (half metal) as example cases, our analysis reveals that - while adsorption is exothermic after charge is introduced - the overall adsorption processes are not intrinsically spontaneous due to the energetic cost of charging the materials. The energies needed to overcome the barriers of these processes are 2.10 and 0.43 eV for h-BN and g-C4N3, respectively. This first principle approach opens up new pathways for a more complete description of charge-induced and electrocatalytic processes.

A new strategy for hydrogen storage using BNNS: simultaneous effects of doping and charge modulation

The adsorption behavior of hydrogen molecule (H₂) on neutral and charged states of C-, Si- and P-doped boron nitride nanosheets (BNNSs), is investigated using density functional theory (DFT) method.