Carbon materials have been at the forefront of hydrogen storage research. However, without improvements in the hydrogen binding strength, as provided by transition-metal dopants, they will not meet practical targets. We performed ab initio density functional theory simulations on titanium-atom dopants adsorbed on the native defects of an (8,0) nanotube. Adsorption on a vacancy strongly binds titanium, preventing nanoparticle coalescence (a major issue for atomic dopants). The defect-modulated Ti adsorbs five H2 molecules with H2 binding energies in the range from −0.2 to −0.7 eV/H2, desirable for practical applications. Molecular dynamics simulations indicate that this complex is stable at room temperature, and simulation of a C112Ti16H160 unit cell finds that a structure with 7.1 wt % hydrogen storage is stable.

Bifunctional catalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are highly desirable for rechargeable metal–air batteries and regenerative fuel cells. However, the commercial oxygen electrocatalysts (mainly noble metal based) can only exhibit either ORR or OER activity and also suffer from inherent cost and stability issues. It remains challenging to achieve efficient ORR and OER bifunctionality on a single catalyst. Metal-free structures offer relatively large scope for this bifunctionality to be engineered within one catalyst, together with improved cost-effectiveness and durability. Herein, by closely coupled computational design and experimental development, highly effective bifunctionality was achieved in a phosphorus and nitrogen co-doped graphene framework (PNGF) – with both ORR and OER activities reaching the theoretical limits of metal-free catalysts, superior to their noble metal counterparts in both (bi)functionality and durability. In particular, with the identification of active P–N sites for OER and N-doped sites for ORR, we successfully intensified these sites by one-pot synthesis to tailor the PNGF. The resulting catalyst achieved an ORR potential of 0.845 V vs. RHE at 3 mA cm−2 and an OER potential of 1.55 V vs. RHE at 10 mA cm−2. Its combined ORR and OER overpotential of 705 mV is much lower than those previously reported for metal-free bifunctional catalysts.

We demonstrate a simple and fully scalable method for obtaining hierarchical hyperporous graphene networks of ultrahigh total pore volume by thermal-shock exfoliation of graphene-oxide (exfGO) at a relatively mild temperature of 300 °C. Such pore volume per unit mass has not previously been achieved in any type of porous solid. We find that the amount of oxidation of starting graphene-oxide is the key factor that determines the pore volume and surface area of the final material after thermal shock. Specifically, we emphasize that the development of the hyperporosity is directly proportional to the enhanced oxidation of sp2 CC to form CO/COO. Using our method, we reproducibly synthesized remarkable meso-/macro-porous graphene networks with exceptionally high total pore volumes, exceeding 6 cm3 g−1. This is a step change compared to ≤3 cm3 g−1 in conventional GO under similar synthetic conditions. Moreover, a record high amine impregnation of >6 g g−1 is readily attained in exfGO samples (solid-amine@exfGO), where amine loading is directly controlled by the pore-structure and volume of the host materials. Such solid-amine@exfGO samples exhibit an ultrahigh selective flue-gas CO2 capture of 30–40 wt% at 75 °C with a working capacity of ≈25 wt% and a very long cycling stability under simulated flue-gas stream conditions. To the best of our knowledge, this is the first report where a graphene-oxide based hyperporous carbon network is used to host amines for carbon capture application with exceptionally high storage capacity and stability.

We report a purposely designed route for the synthesis of a promising carbon-based electrocatalyst for both ORR (oxygen reduction reaction) and OER (oxygen evolution reaction) from zeolitic imidazolate frameworks (ZIFs). Firstly, precursor ZIFs are rationally designed with a blend of volatile zinc to induce porosity and stable cobalt to induce graphitic domains. Secondly, the self-modulated cobalt–nitrogen–carbon system (SCNCS) is shown to be an effective ORR catalyst after graphitization at mild temperatures. Finally, the best OER catalyst is developed by enhancing graphitization of the SCNCS. For the first time, solely by switching the graphitization conditions of SCNCS, excellent ORR or OER performance is realized. This approach not only opens up a simple protocol for simultaneous optimization of nitrogen doping and graphitization at controlled cobalt concentrations, but also provide a facile method of developing such active catalysts without the use of extensive synthesis procedures.

Metal-free catalysts, such as graphene/carbon nanostructures, are highly cost-effective to replace expensive noble metals for CO2 reduction if fundamental issues, such as active sites and selectivity, are clearly understood. Using both density functional theory (DFT) and ab initio molecular dynamic calculations, we show that the interplay of N-doping and curvature can effectively tune the activity and selectivity of graphene/carbon-nanotube (CNT) catalysts. The CO2 activation barrier can be optimized to 0.58 eV for graphitic-N doped graphene edges, compared with 1.3 eV in the un-doped counterpart. The graphene catalyst without curvature shows strong selectivity for CO/HCOOH production, whereas the (6, 0) CNT with a high degree of curvature is effective for both CH3OH and HCHO production. Curvature is also very influential to tune the overpotential for a given product, e.g. from 1.5 to 0.02 V for CO production and from 1.29 to 0.49 V for CH3OH production. Hence, the graphene/CNT nanostructures offer great scope and flexibility for effective tunning of catalyst efficiency and selectivity, as shown here for CO2 reduction.

Graphitic carbon nitride is an exemplar material for metal-free photocatalytic hydrogen production, essential to drive the change to a greener economy. However, its bandgap is too large, at 2.7 eV, for visible light harvesting, which hinders uptake in applications. From two sets of independent quantum mechanical simulations, we have determined the effect of two representative interstitial (hydrogen and fluorine) dopants on the electronic structure and optical properties of this material. From defect analysis, we have found that for a significant range of chemical potential the anionic fluorine dopant is favored. This dopant has significant effects on the optical absorption with the valence band edge shifted up by 0.55 eV, which extends light absorption into the visible. In contrast, hydrogen prefers to be cationic, with the conduction band edge shifted down by 0.45 eV, which strongly reduces hydrogen production as the thermodynamic driving force for proton reduction is significantly reduced. Fluorine is advantageous for improved H2 production as band gap reduction is driven by raising of the valence band, with minimal effect on the thermodynamic driving force for hydrogen reduction. We propose that a design principle for improving carbon nitrides for hydrogen production is to use strongly electronegative dopants.

The oxygen reduction reaction (ORR) is critical for electrochemical energy storage and conversion: e.g., in fuel cells and metal–air batteries. A major challenge is to develop cost-effective and durable ORR catalysts, to replace the relatively expensive platinum-loaded carbon (PtC) counterparts, particularly for large-scale applications. Despite progress over the past few decades in developing efficient non-precious-metal (NPM) catalysts, such as Fe/N/C-based materials (the best-known alternatives), most of the reported catalytic activities have yet to match that of PtC. Herein we propose a two-step process for the production of highly efficient NPM catalysts that outperform PtC in alkaline media: (1) a hierarchical porosity of a supporting substrate is generated and optimized in advance, especially to achieve a high total pore volume for rapid mass transfer, and (2) an appropriate amount of NPM precursor is added to the optimized substrate to boost the reduction potential while maintaining the hierarchically porous structure. Such a scheme was successfully applied to a case of nanoconfined maghemite (γ-Fe2O3) in a nitrogen-doped graphene framework. The resulting catalyst system surpasses the performance of the equivalent commercial PtC, in terms of a higher reduction potential, a significantly lower peroxide formation ratio, more than tripled kinetic current density, smaller Tafel slope, better durability, etc. The reported catalyst is also among the best of all the existing Fe-based ORR catalysts, indicating the great potential of γ-Fe2O3 for ORR in practical applications.Keywords: alkaline electrolyte; hierarchical graphene framework; maghemite; nitrogen doping; oxygen reduction reaction

A hybrid membrane of superacid sulfated Zr–MOF (SZM) and Nafion shows much superior performance to Nafion, particularly for fuel cell operating under low humidity. The Brønsted acidic sites in SZM networks retain an ample amount of water which facilitated proton conduction under low humidity. The water retention properties of Nafion–SZM hybrid membranes with 1 wt % loading of SZM increased at 35% relative humidity and outperformed commercial unfilled Nafion membrane. The proton conductivity increases by 23% for Nafion–SZM hybrid compared to unfilled Nafion membrane. The Nafion–SZM membrane also shows higher performance stability at 35% relative humidity than Nafion, as confirmed by close monitoring of the change of open circuit voltage for 24 h.Keywords: fuel cell; membrane; metal−organic frameworks; proton conduction; superacidity

A highly effective nanoporous CO2 sorbent with naturally doped nitrogen and calcium elements, derived from pine-cone biomass, shows superior CO2 capture performance. The sorbent was produced from carbonization and KOH activation of dried pine cones, and possess a highly nanoporous structure with a specific surface area and a pore volume up to 2110 m2/g and 0.89 cm3/g, respectively. The highest CO2 uptake of 20.9 wt % (under 1 bar CO2 and 25 °C) was achieved in a porous structure with relatively high levels of nitrogen and calcium dopants inherited from the biomass precursor. Further porosity and elemental analyses show that CO2 sorption is enhanced by both highly developed ultramicroporous structure (<0.7 nm) and well-dispersed nitrogen and metal dopants in carbon sorbents.Keywords: Biomass; Carbon; Carbon capture; Carbon dioxide; Green materials; Pine cone

Graphene-based materials have generated tremendous interest in a wide range of research activities. A wide variety of graphene related materials have been synthesised for potential applications in electronics, energy storage, catalysis, and gas sorption, storage, separation and sensing. Recently, gas sorption, storage and separation in porous nanocarbons and metal–organic frameworks have received increasing attention. In particular, the tuneable porosity, surface area and functionality of the lightweight and stable graphene-based materials open up great scope for those applications. Such structural features can be achieved by the design and control of the synthesis routes. Here, we highlight recent progresses and challenges in the syntheses of graphene-based materials with hierarchical pore structures, tuneable high surface area, chemical doping and surface functionalization for gas (H2, CH4, CO2, N2, NH3, NO2, H2S, SO2, etc.) sorption, storage and separation.

By means of hybrid DFT calculations and the deformation potential approximation, we show that bilayer phosphorene under slight compression perpendicular to its surface exhibits extraordinary room temperature electron mobility of order 7 × 104 cm2 V–1 s–1. This is approximately 2 orders of magnitude higher than is widely reported for ground state phosphorenes and is the result of the emergence of a new conduction band minimum that is decoupled from the in-plane acoustic phonons that dominate carrier scattering.

A heterogeneously porous “green carbon” structure was derived from abundant London plane leaves and shows excellent performance for both CO2 capture and Oxygen Reduction Reaction (ORR). The carbonised and KOH-activated carbon possesses a high level of micropores, a specific surface area exceeding 2000 m2 g−1 and a large pore volume of over 1 cm3 g−1, leading to an excellent CO2 uptake of 19.4 wt% under ambient conditions and fast four-electron transfer in an alkaline medium for ORR. Furthermore, XPS and X-ray analyses reveal well-dispersed metal elements (such as Mg and Ca) in the porous carbon, which are naturally doped and inherited from the leaf structure, and can help to enhance CO2 adsorption. On the other hand, these metal elements do not positively affect catalytic ORR performance. Hence, a purpose-specific cleaning approach after KOH activation, i.e. by water or acid, has been devised to obtain optimal functionalities for CO2 capture or ORR.

Sub-surface alloying (SSA) can be an effective approach to tuning surface functionalities. Focusing on Rh(111) as a typical substrate for graphene nucleation, we show strong modulation by SSA atoms of both the energetics and kinetics of graphene nucleation simulated by first-principles calculations. Counter-intuitively, when the sub-surface atoms are replaced by more active solute metal elements to the left of Rh in the periodic table, such as the early transition metals (TMs), Ru and Tc, the binding between a C atom and the substrate is weakened and two C atoms favor dimerization. Alternatively, when the alloying elements are the late TMs to the right of Rh, such as the relatively inert Pd and Ag, the repulsion between the two C atoms is enhanced. Such distinct results can be well addressed by the delicately modulated activities of the surface host atoms in the framework of the d-band theory. More specifically, we establish a very simple selection rule for optimizing the metal substrate for high quality graphene growth: the introduction of an early (late) solute TM in the SSA lowers (raises) the d-band center and the activity of the top-most host metal atoms, weakening (strengthening) the C–substrate binding, meanwhile both energetically and kinetically facilitating (hindering) the graphene nucleation, and simultaneously promoting (suppressing) the orientation disordering the graphene domains. Importantly, our preliminary theoretical results also show that such a simple rule is also proposed to be operative for graphene growth on the widely invoked Cu(111) catalytic substrate.

Utilising the hugely abundant waste from spent coffee grounds (CGs), KOH activated highly microporous carbons with surface areas of 2785 m2 g−1 and micropore volumes of 0.793 cm3 g−1 were synthesised that are capable of uptake capacities near 3 mmol g−1 at 50 °C and 1 bar. Importantly such uptake capacities are achieved though the material's superior microporous character and without doping within the carbon matrix, thereby ensuring facile regeneration with a binding enthalpy of only 26 kJ mol−1 and therefore being capable of energy unintensive cycleable adsorption processes. Furthermore, excellent tunability of pore-size is demonstrated from narrow micropores through to narrow mesopores, enabling optimised adsorption over a range of pressures.

We report a new, simple and versatile method to obtain highly active MOF structures by carefully controlled post-synthesis thermal annealing. The active ZIF-8 structure shows highly enhanced CO2/N2 selectivity and a stable cyclic CO2 uptake of ≥1.5 mmol g−1 at 1 bar and 25 °C with a heat of adsorption of ≥30 kJ mol−1, which is over 100% greater than ≈0.7 mmol g−1 and ≈17 kJ mol−1, respectively of ZIF-8.

Passivated phosphorene nanoribbons, armchair (a-PNR), diagonal (d-PNR), and zigzag (z-PNR), were investigated using density functional theory. Z-PNRs demonstrate the greatest quantum size effect, tuning the bandgap from 1.4 to 2.6 eV when the width is reduced from 26 to 6 Å. Strain effectively tunes charge carrier transport, leading to a sudden increase in electron effective mass at +8% strain for a-PNRs or hole effective mass at +3% strain for z-PNRs, differentiating the (mh*/me*) ratio by an order of magnitude in each case. Straining of d-PNRs results in a direct to indirect band gap transition at either −7% or +5% strain and therein creates degenerate energy valleys with potential applications for valleytronics and/or photocatalysis.

Structural stability and porosity characteristics of metal–organic frameworks (MOFs) are of great importance for practical applications, such as gas sorption/storage and catalytic support. By means of a simple and effective method of postsynthesis thermal annealing below its framework decomposition temperature, the annealed MOF-5 shows unexpectedly high CO2 uptake up to 2 mmol g–1 at 25 °C and 1 bar, which is more than double the capacity of the untreated counterpart (0.8 mmol g–1). Structural characterizations reveal that the annealed MOFs are very active with local vacancy sites due to partial decomposition of the bridging carboxylates of the framework linker. The annealed MOFs also show high stability for cyclic CO2 uptake and air/moisture. Such an approach may be effectively applied to other MOF structures or MOF based membranes to enhance their gas uptake or separation.

A new hydrogen “storage” strategy is demonstrated for an ammonia borane (AB) and LiH system by means of moderate mechanical milling of the two species under an inert atmosphere. This not only avoids reactive hydrogen release during milling but also leads to formation of an intermediate composite LixNH3−xBH3 (x < 1) to store hydrogen with good stability at ambient temperature. The presence of the intermediate composite and sufficient dispersion of the AB and LiH particles facilitate further reaction of AB and LiH with hydrogen release at a relatively low temperature. Herein, we report a composite system obtained by this simple but effective approach to destabilize AB for enhanced hydrogen release at a desirable temperature. A 10 min milled 3AB/5LiH can release hydrogen of ∼5.3 wt% at 70 °C, and another ∼5.5 wt% at 92 °C; so a total of ∼10.8 wt% hydrogen can be obtained before 92 °C. With an increase of the LiH content, this temperature can be further reduced down to 61 °C, which is a significant improvement that has not been reported before. Moreover, our results show a much lower dehydrogenation temperature, reduced from 92 to 67 °C, and a fast kinetics, e.g., 5 and 9 wt% mass loss at 95 °C within 415 and 1050 s, respectively, which is 6 times faster than those reported in the literature (5 and 9 wt% of mass loss at 100 °C within 2400 and 6900 s, respectively). To the best of our knowledge, our system possesses the highest dehydrogenation capacity (5 wt%) at the low hydrogen release temperature (67 °C), with great improvement on the dehydrogenation kinetics in the solid-state hydride systems. A systemic investigation on the mechanism of the reaction under different milling conditions, reaction temperatures and LiH contents is reported here for the first time.

A facile and efficient “spheridization” method is developed to produce nitrogen-enriched hierarchically porous carbon spheres of millimeters in diameter, with intricate micro-, meso- and macro-structural features. Such spheres not only show exceptional working capacity for CO2 sorption, but also satisfy practical requirements for dynamic flow in post-combustion CO2 capture. Those were achieved using co-polymerized acrylonitrile and acrylamide as the N-enriched carbon precursor, a solvent-exchange process to create hierarchically porous macro-sphere preforms, oxidization to induce cyclization of the polymer chains, and carbonization with concurrent chemical activation by KOH. The resulting carbon spheres show a relatively high CO2 uptake of 16.7 wt% under 1 bar of CO2 and, particularly, an exceptional uptake of 9.3 wt% under a CO2 partial pressure of 0.15 bar at 25 °C. Subsequent structural and chemical analyses suggest that the outstanding properties are due to highly developed microporous structures and the relatively high pyridinic nitrogen content inherited from the co-polymer precursor, incorporated within the hierarchical porous structures.

Two substrates, γ-alumina (γ-Al2O3) and reduced graphene oxide (rGO), have been used to confine the formation of magnesium oxide (MgO) crystals so as to control the crystal growth, reduce the crystal size, and enlarge the surface area and thus increase the CO2 capture capacity at elevated temperatures. Typically, MgO/γ-Al2O3 was synthesized by a facile sol–gel route, and MgO/rGO was obtained by calcining the hydrothermally grown magnesium hydroxide (Mg(OH)2) on rGO sheets. Distinct morphologies of MgO were observed through the above two synthesis routes: spherical particles were formed when using γ-Al2O3 as the substrate while MgO nanowhiskers appeared when the loading ratio of the precursor was high in rGO-supported samples. The effects of the substrate on the morphology of the confined MgO and the corresponding CO2 uptake are discussed in detail for the first time.

•Dehydrogenation mechanism was studied by first-principle method.•Monomer and dimer has different dehydrogenation mechanism.•The charge transfer determines the mechanism.Our first-principles study of Ca(NH2BH3)2 reveals that the gas phase energy barrier for the first H2 release is 1.90 eV via a Ca⋯H transition state and 1.71 eV via an N–H⋯B transition state for the second H2 release. In the dimer, the barrier for H2 release from the bridging [NH2BH3]− species is 1.60 eV via an N–H⋯B transition state, and 0.94 eV via an N–H⋯B transition state for the non-bridging [NH2BH3]− species. Analysis of the atomic charge distribution shows that the mechanism of dehydrogenation is determined by the charge transfer between the transition state and the initial state: the less the charge transfer, the lower the barrier to dehydrogenation.

Mg(BH4)2·2NH3 is a relatively new compound considered for hydrogen storage. The fundamental properties of the compound were comprehensively studied using first-principles calculations, such as crystal structure and electronic structure, reaction Gibbs free energy and possible reaction pathway. The calculated crystal structure is in good agreement with the experimental and other theoretical results. Results from electronic density of states (DOS) and electron localization function (ELF) show the covalent characteristics of the N–H and the B–H bonds, and the weak ionic interactions between the Mg atom and the NH3 ligands or the (BH4)− ligands. The reaction Gibbs free energies of several possible decomposition reactions were calculated between 0 and 700 K. All the reactions are exothermic. The most likely reaction pathway of the dehydrogenation reaction was clarified to show five distinct steps.Highlights► Structural and electronic properties were studied. ► The most possible overall reaction was determined. ► Up to 500 °C, the dehydrogenation reaction could be divided to five steps.

Ca(BH4)2·2NH3 is a relatively new compound with potential application in hydrogen storage. Here the fundamental properties of the compound, such as electronic structure, energetic and thermodynamic properties, were comprehensively studied using first-principles calculations. Results from electronic density of states (DOS) and electron localization function (ELF) indicate the covalent bond nature of the N–H bond and the B–H bond. Charge density analyses show weak ionic interactions between the Ca atom and the NH3 complexes or the (BH4)− complexes. The calculated vibration frequencies of B–H and N–H are in good agreement with other theoretical and experimental results. Furthermore, we calculated the reaction enthalpy and reaction Gibbs free energy at a range of temperature 0–700 K. Our results are in good agreement with experimental results in literature. Possible reaction mechanism of the decomposition reaction is proposed.Graphical AbstractThe crystal structure of this compound and the calculated decomposition reaction free energy for two different reactions:Reac(2):Ca(BH4)2⋅2NH3⟶162°CCa(BH4)2⋅NH3+NH3⟶230°CCa(BH4)2+2NH3Reac(3):Ca(BH4)2⋅2NH3⟶190°C1/4Ca(BH4)2+1/4Ca3(BN2)2+BN+6H2Highlights► Crystal structure of this compound was studied in detail. ► Electronic properties were calculated for the first time. ► Phonon density of states and reaction free energy at different temperatures were first calculated. ► Possible decomposition mechanism was presented.

Porous carbons were processed by the foaming of two-part polymer precursors with pre-loaded carbon powder (graphitic or amorphous), and then resin impregnation and carbonization to control both porosity and mechanical strength of the resulting foam. Electrical conductivity of the foams was improved by nickel-catalyzed graphitization. Different levels of graphitization were obtained for varied concentrations of nickel to the amorphous carbon foams. The presence of graphitic carbon improves the electrical conductivity by a factor of 50, compared to the amorphous counterparts. Electrochemical studies showed that graphitization of the amorphous structures increased the specific electrochemical surface area and electron transfer rate of the carbon electrodes.

Electronic structure calculations have been used to determine and compare the thermodynamics of H2 release from ammonia borane (NH3BH3), lithium amidoborane (LiNH2BH3), and sodium amidoborane (NaNH2BH3). Using two types of exchange correlation functional we show that in the gas-phase the metal amidoboranes have much higher energies of complexation than ammonia borane, meaning that for the former compounds the B–N bond does not break upon dehydrogenation. Thermodynamically however, both the binding energy for H2 release and the activation energy for dehydrogenation are much lower for NH3BH3 than for the metal amidoboranes, in contrast to experimental results. We reconcile this by also investigating the effects of dimer complexation (2×NH3BH3, 2×LiNH2BH3) on the dehydrogenation properties. As previously described in the literature the minimum energy pathway for H2 release from the 2×NH3BH3 complex involves the formation of a diammoniate of diborane complex ([BH4]−[NH3BH2NH3]+). A new mechanism is found for dehydrogenation from the 2×LiNH2BH3 dimer that involves the formation of an analogous dibroane complex ([BH4]−[LiNH2BH2LiNH2]+), intriguingly it is lower in energy than the original dimer (by 0.13 eV at ambient temperatures). Additionally, this pathway allows almost thermoneutral release of H2 from the lithium amidoboranes at room temperature, and has an activation barrier that is lower in energy than for ammonia borane, in contrast to other theoretical research. The transition state for single and dimer lithium amidoborane demonstrates that the light metal atom plays a significant role in acting as a carrier for hydrogen transport during the dehydrogenation process via the formation of a Li–H complex. We posit that it is this mechanism which is responsible, in condensed molecular systems, for the improved dehydrogenation thermodynamics of metal amidoboranes.

We report a first-principles study of a CO2 gas-sorbent material consisting of calcium atoms and carbon-based nanostructures. In the low gas pressure regime, we find that Ca decoration of nanotubes and graphene possess unusually large CO2 uptake capacities (∼0.4–0.6 g CO2/g sorbent) as a result of their topology and a strong interaction between the metal dopants and CO2 molecules. Decomposition of the gas-loaded nanomaterials into CO gas and calcium oxide (CaO) is shown to be thermodynamically favorable; thus performance of the carbon capture process is further enhanced via formation of calcium carbonate (CaCO3). Gas adsorption CO2/N2 selectivity issues have been also addressed with the finding that N2 molecules bind to the metal-doped surfaces more weakly than CO2 molecules. The predicted molecular binding and accompanying gas selectivity features strongly suggest the potential of Ca-doped carbon materials for CO2 capture applications.

Fine Ni particles are effective in catalyzing hydrogen sorption of MgH2, but there is confusion about the extent of this effect in relation to Ni particle size and content. Here, effects of Ni particles of different sizes on hydrogen desorption of MgH2 were comparatively investigated. MgH2 mixed with only 2 at% of fine Ni particles rapidly desorbs hydrogen up to 6.5 wt% around 200–340 °C, but there is no significant difference in the desorption temperature of the mixture when Ni particles vary from 90 to 200 nm. Increasing the content of Ni to 4 at%, or a combined (2 at% Ni + 2 at% Fe), leads to hydrogen desorption starting from 160 °C. Further analyses of the literature suggest that the effectiveness of Ni catalysis largely depends on its site density over MgH2 surface, i.e., an optimal site density of catalytic particles is important in balancing the sorption properties of MgH2. The projected trend suggests that MgH2 can desorb hydrogen from 100 °C, the targeted temperature for fuel cells, if the number of catalyst sites is around 4 × 1014 per m2 of MgH2, or the number ratio of Ni to MgH2 particles is about a million to one.

This critical review covers the mechanisms underlying density functional theory (DFT) simulations and their relevance in evaluating, developing and discovering new materials. It is intended to be of interest for both experimentalists and theorists in the expanding field of hydrogen storage. We focus on the most studied classes of materials, metal-hydride, -amide, and -borohydride mixtures, and bare and transition metal-doped carbon systems and the utility of DFT simulations for the pre-screening of thermally destabilised reaction paths (170 references).

A high Nb containing TiAl alloy from pre-alloyed powder of Ti–45Al–8.5Nb–0.2B–0.2W–0.1Y was processed by spark plasma sintering (SPS). The effects of sintering temperature on the microstructure and mechanical properties were studied. The optimized conditions yield high densities and uniform microstructure. Specimens sintered at 1100 °C are characterized by fine duplex microstructure, leading to superior room temperature mechanical properties with a tensile strength of 1024 MPa and an elongation of 1.16%. Specimens sintered at 1200 °C are of fully lamellar microstructure with a tensile strength of 964 MPa and an elongation of 0.88%. The main fracture mode in the duplex microstructure was transgranular in the equiaxed γ grains and interlamellar in the lamellar colonies. For the fully lamellar structure, the fracture mode was dominated by interlamellar, translamellar and stepwise failure.

A clean powder metallurgy route was developed here to produce Ti foams, using a fugitive polymeric filler, polypropylene carbonate (PPC), to create porosities in a metal-polymer compact at the pre-processing stage. The as-produced foams were studied by scanning electron microscopy (SEM), LECO combustion analyses and X-ray diffraction (XRD). Compression tests were performed to assess their mechanical properties. The results show that titanium foams with open pores can be successfully produced by the method. The compressive strength and modulus of the foams decrease with an increasing level of porosity and can be tailored to those of the human bones. After alkali treatment and soaking in a simulated body fluid (SBF) for 3 days, a thin apatite layer was formed along the Ti foam surfaces, which provides favourable bioactive conditions for bone bonding and growth.

The dehydrogenation mechanism of M(NH2BH3)2 (M = Mg, Sr) was studied by a first-principles method. The results show that the gas-phase energy barrier for the first H2 release is 2.15 eV via a N–H···B transition state and 1.35 eV via a Mg···H transition state for the second H2 release in Mg(NH2BH3)2. The barrier is 1.21 and 2.27 eV via a N–H···B transition state for the first and second H2 release in Sr(NH2BH3)2, respectively. For the dimer, both compounds release the first H2 via oligomerization and the corresponding barriers are 1.01 eV for Mg(NH2BH3)2 and 1.25 eV for Sr(NH2BH3)2. Further analysis of the charges of the transition states and the initial states leads to a general conclusion: for the same final state, the smaller the charge transfer, the lower the barrier. For Mg(NH2BH3)2, the reaction pathway is determined by the HOMO and LUMO orbitals of the initial state.

A new hydrogen “storage” strategy is demonstrated for an ammonia borane (AB) and LiH system by means of moderate mechanical milling of the two species under an inert atmosphere. This not only avoids reactive hydrogen release during milling but also leads to formation of an intermediate composite LixNH3−xBH3 (x < 1) to store hydrogen with good stability at ambient temperature. The presence of the intermediate composite and sufficient dispersion of the AB and LiH particles facilitate further reaction of AB and LiH with hydrogen release at a relatively low temperature. Herein, we report a composite system obtained by this simple but effective approach to destabilize AB for enhanced hydrogen release at a desirable temperature. A 10 min milled 3AB/5LiH can release hydrogen of ∼5.3 wt% at 70 °C, and another ∼5.5 wt% at 92 °C; so a total of ∼10.8 wt% hydrogen can be obtained before 92 °C. With an increase of the LiH content, this temperature can be further reduced down to 61 °C, which is a significant improvement that has not been reported before. Moreover, our results show a much lower dehydrogenation temperature, reduced from 92 to 67 °C, and a fast kinetics, e.g., 5 and 9 wt% mass loss at 95 °C within 415 and 1050 s, respectively, which is 6 times faster than those reported in the literature (5 and 9 wt% of mass loss at 100 °C within 2400 and 6900 s, respectively). To the best of our knowledge, our system possesses the highest dehydrogenation capacity (5 wt%) at the low hydrogen release temperature (67 °C), with great improvement on the dehydrogenation kinetics in the solid-state hydride systems. A systemic investigation on the mechanism of the reaction under different milling conditions, reaction temperatures and LiH contents is reported here for the first time.

Metal-free catalysts, such as graphene/carbon nanostructures, are highly cost-effective to replace expensive noble metals for CO2 reduction if fundamental issues, such as active sites and selectivity, are clearly understood. Using both density functional theory (DFT) and ab initio molecular dynamic calculations, we show that the interplay of N-doping and curvature can effectively tune the activity and selectivity of graphene/carbon-nanotube (CNT) catalysts. The CO2 activation barrier can be optimized to 0.58 eV for graphitic-N doped graphene edges, compared with 1.3 eV in the un-doped counterpart. The graphene catalyst without curvature shows strong selectivity for CO/HCOOH production, whereas the (6, 0) CNT with a high degree of curvature is effective for both CH3OH and HCHO production. Curvature is also very influential to tune the overpotential for a given product, e.g. from 1.5 to 0.02 V for CO production and from 1.29 to 0.49 V for CH3OH production. Hence, the graphene/CNT nanostructures offer great scope and flexibility for effective tunning of catalyst efficiency and selectivity, as shown here for CO2 reduction.

Electronic structure calculations have been used to determine and compare the thermodynamics of H2 release from ammonia borane (NH3BH3), lithium amidoborane (LiNH2BH3), and sodium amidoborane (NaNH2BH3). Using two types of exchange correlation functional we show that in the gas-phase the metal amidoboranes have much higher energies of complexation than ammonia borane, meaning that for the former compounds the B–N bond does not break upon dehydrogenation. Thermodynamically however, both the binding energy for H2 release and the activation energy for dehydrogenation are much lower for NH3BH3 than for the metal amidoboranes, in contrast to experimental results. We reconcile this by also investigating the effects of dimer complexation (2×NH3BH3, 2×LiNH2BH3) on the dehydrogenation properties. As previously described in the literature the minimum energy pathway for H2 release from the 2×NH3BH3 complex involves the formation of a diammoniate of diborane complex ([BH4]−[NH3BH2NH3]+). A new mechanism is found for dehydrogenation from the 2×LiNH2BH3 dimer that involves the formation of an analogous dibroane complex ([BH4]−[LiNH2BH2LiNH2]+), intriguingly it is lower in energy than the original dimer (by 0.13 eV at ambient temperatures). Additionally, this pathway allows almost thermoneutral release of H2 from the lithium amidoboranes at room temperature, and has an activation barrier that is lower in energy than for ammonia borane, in contrast to other theoretical research. The transition state for single and dimer lithium amidoborane demonstrates that the light metal atom plays a significant role in acting as a carrier for hydrogen transport during the dehydrogenation process via the formation of a Li–H complex. We posit that it is this mechanism which is responsible, in condensed molecular systems, for the improved dehydrogenation thermodynamics of metal amidoboranes.

Sub-surface alloying (SSA) can be an effective approach to tuning surface functionalities. Focusing on Rh(111) as a typical substrate for graphene nucleation, we show strong modulation by SSA atoms of both the energetics and kinetics of graphene nucleation simulated by first-principles calculations. Counter-intuitively, when the sub-surface atoms are replaced by more active solute metal elements to the left of Rh in the periodic table, such as the early transition metals (TMs), Ru and Tc, the binding between a C atom and the substrate is weakened and two C atoms favor dimerization. Alternatively, when the alloying elements are the late TMs to the right of Rh, such as the relatively inert Pd and Ag, the repulsion between the two C atoms is enhanced. Such distinct results can be well addressed by the delicately modulated activities of the surface host atoms in the framework of the d-band theory. More specifically, we establish a very simple selection rule for optimizing the metal substrate for high quality graphene growth: the introduction of an early (late) solute TM in the SSA lowers (raises) the d-band center and the activity of the top-most host metal atoms, weakening (strengthening) the C–substrate binding, meanwhile both energetically and kinetically facilitating (hindering) the graphene nucleation, and simultaneously promoting (suppressing) the orientation disordering the graphene domains. Importantly, our preliminary theoretical results also show that such a simple rule is also proposed to be operative for graphene growth on the widely invoked Cu(111) catalytic substrate.

A facile and efficient “spheridization” method is developed to produce nitrogen-enriched hierarchically porous carbon spheres of millimeters in diameter, with intricate micro-, meso- and macro-structural features. Such spheres not only show exceptional working capacity for CO2 sorption, but also satisfy practical requirements for dynamic flow in post-combustion CO2 capture. Those were achieved using co-polymerized acrylonitrile and acrylamide as the N-enriched carbon precursor, a solvent-exchange process to create hierarchically porous macro-sphere preforms, oxidization to induce cyclization of the polymer chains, and carbonization with concurrent chemical activation by KOH. The resulting carbon spheres show a relatively high CO2 uptake of 16.7 wt% under 1 bar of CO2 and, particularly, an exceptional uptake of 9.3 wt% under a CO2 partial pressure of 0.15 bar at 25 °C. Subsequent structural and chemical analyses suggest that the outstanding properties are due to highly developed microporous structures and the relatively high pyridinic nitrogen content inherited from the co-polymer precursor, incorporated within the hierarchical porous structures.

This critical review covers the mechanisms underlying density functional theory (DFT) simulations and their relevance in evaluating, developing and discovering new materials. It is intended to be of interest for both experimentalists and theorists in the expanding field of hydrogen storage. We focus on the most studied classes of materials, metal-hydride, -amide, and -borohydride mixtures, and bare and transition metal-doped carbon systems and the utility of DFT simulations for the pre-screening of thermally destabilised reaction paths (170 references).

A heterogeneously porous “green carbon” structure was derived from abundant London plane leaves and shows excellent performance for both CO2 capture and Oxygen Reduction Reaction (ORR). The carbonised and KOH-activated carbon possesses a high level of micropores, a specific surface area exceeding 2000 m2 g−1 and a large pore volume of over 1 cm3 g−1, leading to an excellent CO2 uptake of 19.4 wt% under ambient conditions and fast four-electron transfer in an alkaline medium for ORR. Furthermore, XPS and X-ray analyses reveal well-dispersed metal elements (such as Mg and Ca) in the porous carbon, which are naturally doped and inherited from the leaf structure, and can help to enhance CO2 adsorption. On the other hand, these metal elements do not positively affect catalytic ORR performance. Hence, a purpose-specific cleaning approach after KOH activation, i.e. by water or acid, has been devised to obtain optimal functionalities for CO2 capture or ORR.