Nanoconfinement effects on the reversibility of hydrogen storage in ammonia borane: A first-principles study
Kiok Chang, Eunja Kim, Philippe F. Weck, and David Tománek
Citation: The Journal of Chemical Physics 134, 214501 (2011); doi: 10.1063/1.3594115
View online: dx.doi/10.1063/1.3594115
View Table of Contents: scitation.aip/content/aip/journal/jcp/134/21?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Mechanism of ammonia decomposition and oxidation on Ir(110): A first-principles study
J. Chem. Phys. 138, 144703 (2013); 10.1063/1.4798970
First-principles study of hydrogenated carbon nanotubes: A promising route for bilayer graphene nanoribbons Appl. Phys. Lett. 101, 033105 (2012); 10.1063/1.4737427
First-principles study of hydrogen adsorption in metal-doped COF-10
J. Chem. Phys. 133, 154706 (2010); 10.1063/1.3503654
Strain effects on hydrogen storage capability of metal-decorated graphene: A first-principles study
Appl. Phys. Lett. 97, 103109 (2010); 10.1063/1.3486682
Experimental and computational studies on collective hydrogen dynamics in ammonia borane: Incoherent inelastic neutron scattering
J. Chem. Phys. 130, 024507 (2009); 10.1063/1.3042270
THE JOURNAL OF CHEMICAL PHYSICS134,214501(2011) Nanoconfinement effects on the reversibility of hydrogen storage in ammonia borane:Afirst-principles study
Kiok Chang,1Eunja Kim,2Philippe F.Weck,3and David T ománek1,a)
1Physics and Astronomy Department,Michigan State University,East Lansing,Michigan48824-2320,USA
2Department of Physics and Astronomy,University of Nevada Las Vegas,4505Maryland Parkway,Las Vegas, Nevada89154,USA
3Department of Chemistry,University of Nevada Las Vegas,4505Maryland Parkway,Las Vegas,
Nevada89154,USA
(Received9March2011;accepted5May2011;published online1June2011)
We investigate atomistic mechanisms governing hydrogen relea and uptake process in ammonia borane(AB)within the framework of the density functional theory.In order to determine the most favorable pathways for the thermal inter-conversion between AB and polyaminoborane plus H2,
we calculate potential energy surfaces for the corresponding reactions.We explore the possibility of enclosing AB in narrow carbon nanotubes to limit the formation of undesirable side-products such as the cyclic compound borazine,which hinder subquent rehydrogenation of the system.We also explore the effects of nanoconfinement on the possible rehydrogenation pathways of AB and suggest the u of photoexcitation as a means to achieve dehydrogenation of AB at low temperatures.
©2011American Institute of Physics.[doi:10.1063/1.3594115]
I.INTRODUCTION
Hydrogen is widely regarded as a cost-effective,re-newable,and clean energy alternative to fossil fuels for transportation applications.1From the rearch effort con-ducted in solid-state materials capable of storing hydrogen, the NH3BH3compound called ammonia borane(AB),with an ideal storage capacity of19.5wt.%H2and a reported relea,2,3of up to≈13wt.%H2below200◦C,has emerged as one of the most promising candidate materials to meet the2015volumetric( 82g H2l−1)and gravimetric( 90 g H2kg−1)density targets specified by the U.S.Department of Energy for on-board hydrogen storage.4
At room temperature,AB crystallizes in a stable plastic pha with the tetragonal I4mm structure.5Upon thermally induced decomposition,AB releas H2in a two-step exothermic process.2,3,6–8The initial dehydropolymerization step occurs between343K and385K,yielding≈1mol H2and polyaminoborane(PAB)products,[BH2NH2]n ( H=−1.57kcal mol−1).9PAB further decompos in the temperature range of383−473K,releasing hy-drogen and polyiminoborane(PIB)products,[BHNH]n ( H≈−9.5kcal mol−1).9The ultimate decomposition step leading to the formation of planar BN at≈1500K is not con-sidered practical for storage purpos.Several species,such as borazine(c-B3N3H6),cycloborazanes,or diammoniate of diborane(DADB,NH3BH2NH3+BH4),have been obrved concurrently to the formation of PAB and PIB,depending on the decomposition conditions of AB.3,6–8,10,11
Some mechanistic and thermodynamical aspects of the decomposition process have been investigated in recent computational studies.9,12–15Still,the microscopic pathway of the AB dehydrogenation process has not been understood a)Electronic mail:tomanek@pa.msu.edu.in full detail yet.Also,optimum conditions have yet to be found for AB rehydrogenation and suppression of volatile by-products such as borazine,which can poison the catalyst material of proton exchange membrane fuel cells.Recent ap-proaches to remedy the problems have focud on tuning thermo
dynamic properties and controlling reaction pathways using catalysts,15–17modified AB materials,18,19ionic liquid solvents,10or encapsulating AB in mesoporous materials.20–23 Even though significant experimental advances have been achieved,fundamental understanding of the process is still missing.
In this manuscript,we study the governing mecha-nisms associated with hydrogenation and dehydrogenation process of ammonia borane.Our objective is to better utilize this unique material by examining the energy profiles associated with the conversion of the AB molecular solid to polymeric molecules and hydrogen.Wefirst investigate the two-step dehydrogenation process and then explore encapsu-lation of AB molecules inside nanopores and nanotubes as a potentially viable pathway for AB rehydrogenation.
Details of our computational approach are given in Sec.II,followed by a discussion of our results in Sec.III.
A summary of ourfindings and conclusions is given in Sec.IV.
IIPUTATIONAL METHOD
First-principles total-energy calculations bad on density functional theory(DFT)were carried out using the SIESTA code24to determine the optimum geometry of AB in the solid pha and to examine energetically preferred pathways for the dehydrogenation and rehy-drogenation process.We ud the standard Kohn-Sham lf-consistent method within the local density approxi-mation with the Perdew-Zunger25parametrization of the
0021-9606/2011/134(21)/214501/7/$30.00©2011American Institute of Physics
134,214501-1
214501-2Chang et al.J.Chem.Phys.134,214501(2011) exchange-correlation functional in the uniform electron gas.
Since the weak inter-molecular interactions in our system are dominated by electrostatic and weak covalent interactions causing band dispersion,this energy functional should pro-vide an adequate description of the equilibrium structure and elastic properties.26We furthermore ud a general andflex-ible linear combination of numerical atomic orbital basis and norm-conrving Troullier-Martins pudopotentials27in the nonlocal Kleinman-Bylander form.28Our basis consisted of pudo-atomic-orbitals(PAOs)generated by the split-valence scheme for a double-ζpolarization basis
雅思刘薇t.All calculations were performed using periodic boundary conditions in supercell geometry.Depending on the context of a particular calculation,we ud supercells containing two or more AB molecules.We sampled the reciprocal space by k−point grids with comparable densities in all our calculations,with the minimum number of6k points in the smallest Brillouin zone. We limited the energy shift due to the spatial confinement of the PAO basis functions29,30to less than30meV.The charge density and pudopotentials have been determined on a real space grid with a very high mesh cutoff energy of 200Ry,which is sufficient to converge the total energy to within1meV/atom.We ud the conjugate gradient method for geometry optimization.A structure was considered opti-mized when none of the residual forces exceeded0.01eV/Å. Complementary microcanonical molecular dynamics(MD) simulations were performed at the same level of theory to investigate the thermodynamical properties and the mi-croscopic pathways for the dehydrogenation process.A time-step of1fs was ud in all simulations with a maximum simulation time of2ps.The temperature range of our MD simulations extended up to1500K.
III.RESULTS AND DISCUSSION
my life would suck without you
We have studied the structure and energetics of AB and related compounds in order tofind the most efficient microscopic reaction pathways for the dehydrogenation and rehydrogenation process and
investigated new possible methods for efficient rehydrogenation of AB under mild conditions.We have focud our studies on identifying the different crystal structures of AB,the equilibrium structure of DADB and its formation energetics,and the microscopic pathway of AB dehydrogenation process.We have then ex-plored possible ways to improve the de-and rehydrogenation process of AB by using photoexcitations and by confining BN-bad molecules in narrow carbon nanotubes(CNTs). A.Crystal structures of AB and the role of the dihydrogen bond
Boraneamines in the condend pha show a propensity to form N−Hδ+···δ−H−B clo contacts as a result of the inter-molecular interaction between the NH proton and the adjacent HB bond,as en in Fig.1(a).For this peculiar type of hydrogen bond,commonly referred to as the dihy-drogen bond,the H···H distance is typically in the range of 1.7−2.2Å,thus significantly shorter than the sum of the van der Waals radii of two hydrogen atoms,2.4Å.The hydrogen
(a)
(b)epos
(e)
z
y
(c)(d)
z
y
(f)
z
2015六级真题
y
H
B
N
charge
FIG.1.Optimized geometry of orthorhombic and tetragonal crystals of AB and DADB-AB.(a)Local geometry of AB molecules,withθdenoting the an-gle between neighboring molecules in the orthorhombic crystal.(b)Charge difference plot depicting two AB molecules.Larger lobes,highlighted in blue,indicate the region with excess positive charge and smaller lobes,high-lighted in red,the region with excess negative charge,establishing a per-manent dipole.The dipole-dipole interaction stabilizes the dihydrogen-bond network moment in the crystal.(c)Optimized structure of the orthorhombic crystal withθ=22◦(θexpt=20.4◦).(d)Optimized structure of the tetrag-onal crystal with all AB molecules aligned along the same direction.Opti-mized structure of two AB molecules(e)and DA
DB(f)in the AB tetragonal crystal.The region,where the2AB→DADB transformation takes place,is shaded in(e)and(f).The backbones of AB and DADB involved in this reac-tion are highlighted by the white dotted lines in(e)and(f).B and N atoms in DADB are distinguished by shading in(f).
atom connected to nitrogen carries a partial positive charge (Hδ+)and the hydrogen atom connected to boron a partial negative charge(Hδ−),as en in Fig.1(b).Along with the covalent N−H···σbonds,the weaker N−Hδ+···δ−H−B dihydrogen interactions with a bond strength of≈0.3eV, stabilized by the Coulomb attraction between hydrogen atoms carrying opposite charges,largely contribute to stabilizing the molecular crystal at room temperature.At this point,it is uful to remember that ethane,the homonuclear equivalent to AB with no dihydrogen bonds,is not a solid,but rather forms a gas at room temperature.
Our calculations show that the lowest-energy crystal structures of AB are orthorhombic and tetragonal molecu-lar crystals,depicted in Figs.1(c)and1(d).According to experiment,the low-temperature orthorhombic structure of AB undergoes a pha transition to the high-temperature tetragonal pha31–33at225K.The optimized structure of the Pnm21orthorhombic lattice is characterized by the lat-tice constants a=5.142Å,b=4.588Å,c=4.772Å,and the angleθ=20◦between neighboring AB molecules.This structure,shown in Fig.1(a),agrees rather well with exper-imental dat
a.32,34The small difference between thefinite-temperature experimental valueθexpt=20.4◦and the theo-retical valueθtheo=22◦obtained at T=0may be attributed to anharmonicities in the interactions that are explored in the molecular crystal at nonzero temperatures.
214501-3Nanoconfinement effects on the reversibility J.Chem.Phys.134,214501(2011)
At T=0,wefind the orthorhombic Pnm21crystal to be energetically more stable than the tetragonal I4mm crystal by 49meV per AB molecule.At higher temperatures,changes in the vibrational entropy difference between the phas may overcome this small energy difference,causing a pha tran-sition.Wefind support for this generalfinding in our MD calculations,which indicate a stability reversal between the tetragonal and orthorhombic pha in terms of free energy differences between200and300K,which is very clo to the experimental value of225K.
B.Energetics and structural changes during the DADB formation
Formation of diammoniate of diborane within the AB crystal is governed by changes in the dihydrogen bond net-work,which we study in a suitable supercell geometry.Ac-cording to our calculations,the DADB molecule posss a large dipole moment comparable to that of the AB molecule. Also,the potential energy of DADB in the AB crystal is com-parable to that of AB in crystalli
ne pha.To study the coex-istence of DADB with AB,we substituted DADB for two AB molecules in the tetragonal molecular crystal,as shown in the brown shaded region of Fig.1(e)(optimized AB crystal ge-ometry)and Fig.1(f)(optimized DADB geometry in the AB crystal).The large dipole moment of DADB further stabilizes the dihydrogen bond network,since the potential energy of the DADB tetragonal crystal is lower than that of the perfect AB tetragonal crystal by34meV per AB molecule.
kekeIn terms of potential energy at T=0,the perfect defect-free orthorhombic AB crystal is the most stable structure, followed by the DADB tetragonal crystal,andfinally the perfect defect-free AB tetragonal crystal as the least stable of the three.Considering the fact that the tetragonal and orthorhombic AB crystal structures coexist at room tem-perature,formation of DADB in AB crystals is to be ex-pected on energy grounds,with supporting experimental evidence.33The geometry of NH3BH2NH3in DADB is simi-lar to that of polyaminoborane,the product of thefirst stage of dehydrogenation.
C.Microscopic pathway of the dehydrogenation process
Inspection of our microcanonical molecular dynamics simulations at an average effective temperature of≈1500K reveals that hydrogen atoms attached to nitrogen and boron in ammonia borane are relead and associate to a hydrogen molecule for a20fs time period.This process can b
e under-stood by studying reactions involving chains of AB molecules with one AB molecule per unit cell,as en in the int of Fig.2(a).In this particular study,we artificially incread the inter-chain paration to suppress the influence of neighbor-ing AB chains.To get detailed understanding of the optimum transition path independent of temperature and particular tra-jectories ud in MD runs,we explore the potential energy surface of the system by performing constrained geometry op-timizations and show the results in Fig.2(a).In thefirst dehy-drogenation scenario NH3BH3→NH2BH2+H2,we consider pairs of distances(d N−H,d B−H)within the unit cell as indi-cated in the int of Fig.2(a).Since the dehydrogenation pro-cess occurring in nature can be characterized by a quence of (d N−H,d B−H)distance pairs,we u it as a prejudice-free reac-tion coordinate to characterize the reaction pathway.The po-tential energy surface E(d N−H,d B−H),prented in Fig.2(a), is the result of few hundred structure optimization studies, each of which considered specific values for the d N−H,d B−H distances that were keptfixed along with the unit cell size.The optimum trajectory from the initial geometry M1,reprent-ing the equilibrium structure of AB in the crystal,over the bar-rier B to thefinal state M2containing an H2molecule per unit cell,is shown by the dashed line in Fig.2(a).The correspond-ing energy profile and structural snap shots along this trajec-tory,which corresponds to the reaction coordinate,is depicted in Fig.2(c).The reaction NH3BH3→NH2BH2+H2requires crossing the activation barrier of1.14eV per AB molecule. T
he net process is endothermic,requiring a0.75eV energy investment to occur,which is the reason for the short20fs time period during which a hydrogen molecule was formed as a result of temperaturefluctuations in our microcanonical MD simulation.snake是什么意思
Since dehydrogenation occurs as an activated exothermic process in nature,thefinal product should be more stable than isolated NH2BH2molecules.A possiblefinal product of the dehydrogenation of AB,which satisfies this condition, is polyaminoborane[BH2NH2]n.The cond scenario of the dehydrogenation reaction that involves AB polymerization, n NH3BH3→[NH2BH2]n+n H2,is described in Figs.2(b) and2(d)and is indeed mildly exothermic.In analogy to thefirst dehydrogenation scenario,we considered pairs of distances(d x,A z),shown in the int of Fig.2(b),uful to identify a prejudice-free reaction coordinate with focus on the polymerization.Also in this ca,we artificially incread the size of the unit cell normal to the z axis to suppress the in-fluence of AB molecules away from the z-axis.The potential energy surface E(d x,A z),prented in Fig.2(b),is the result of few hundred structure optimization studies,each of which considered specific values for d x and A z.In our calculation, we consider two AB molecules per unit cell and keep their axes along the BN bond parallel to each other parated by d x.We expect the AB polymerization process to be initiated by reducing d x,accompanied by changes in the unit cell size A z in the axial direction.The most effici
ent pathway for this process,depicted by the white dashed line in Fig.2(b),indeed follows our expectations.Separation of the H2molecule from [NH2BH2]n is a necessary side-effect in thefinal state of the polymerization.Even though the net process is exothermic,it involves rather high activation barriers,as en in Fig.2(d). The activation barrier values of4.29eV and3.35eV in the cond scenario are much larger than the value of2×1.14eV in thefirst scenario with two AB molecules per unit cell.The origin of the high activation energy values is the strong inter-molecular repulsion and the reduced stability of the NH2BH3+H and NH3BH2+H complexes associated with the transition states.Even though the high activation energy values are expected to decrea,when artificial constraints such as relative axis orientation are relaxed,this reaction is unlikely to occur under experimental conditions.
214501-4Chang et al.J.Chem.Phys.134,214501(2011)
(a)d N -H (Å)
d B-H (Å)
0.0
新东方社区0.5
1.0
M 1
M 2
d B-H
港澳留学d N-H
B (c)
(d)E (e V )
B A C
0.0
0.51.0 1.14eV
M 1M 2
Reaction coordinate
E (e V )
0.0
1.02.03.04.05.0
roshan
A z (Å)
1.01.5
try名扬四海歌词2.0
2.5E(eV)
M 1
A
B M 2
C M 3
0.0
D
E
d (Å)
0.5
d x
A z
E(eV)(b)
d x
d N-H d B-H
H
B N
FIG.2.Microscopic pathways of the dehydrogenation process.(a)Contour plot of the potential energy
surface as a function of d B −H and d N −H in the first dehydrogenation scenario,NH 3BH 3→NH 2BH 2+H 2,with d B −H and d N −H defined in the int.The optimum dehydrogenation pathway is indicated by the dashed line.(b)Contour plot of the potential energy surface as a function of d x and A z in the cond dehydrogenation scenario,n NH 3BH 3→[NH 2BH 2]n +n H 2.(c)Potential energy profile along the reaction coordinate for the first scenario.The lected structures (M 1,B ,M 2),are the snapshots in the process.M 1and M 2are the globally relaxed geometries of A and C ,respectively.B is the saddle point geometry on the potential energy surface.The dashed rectangle is the unit cell ud in the calculation.(d)Potential energy profile along the reaction coordinate for the cond scenario.Local minimum (M 1,M 2,M 3)and intermediate structures (B ,D )are shown below.
Another possible scenario of dehydrogenation involves intermediate structures such as NH 3BH 2NH 3+BH 4(DADB)in the process.The formation of DADB in the dihydrogen bond network is energetically favorable due to its high polar-ity.Moreover,from a structural viewpoint,DADB is reminis-cent of a polymer.To check on the viability of this process,we calculate the potential energy surface of the DADB for-mation.In our model calculation we consider the formation of DADB from two isolated AB molecules.We find that a likely reaction may start with an initial dissociation of one of the AB molecules into NH 3and BH 3.Association of the ammonia and AB molecules would lead to th
e formation of NH 3BH 2NH 3,accompanied by the relea of hydrogen from the BH 2site.Even though this reaction is weakly exothermic,in agreement with experimental obrvations,2the activation barrier of 3.59eV for this process appears too high,possibly due to the low stability of intermediate structures in our iso-lated system.Thus,we conclude that this reaction is unlikely to take place.
In view of the fact that the obrved thermolysis of AB is weakly exothermic and occurs under mild conditions,2we must conclude that this process is likely much more complex than described here.We can only speculate about more fa-vorable ways to form DADB in the AB molecular crystal,
including autocatalytic reactions assisted by diffusing hydro-gen atoms in the matrix,since prence of hydrogen may reduce the activation barrier for the formation of DADB.Once NH 3BH 2NH 3forms in the crystal,its prence can pro-mote exothermic polymerization of AB by dehydrogenation.It is fair to assume that at any given point,we may find PAB gments of different length and possibly even branched polymers among the dehydrogenation products.We obrve nonvanishing net charges only at the extremities of the products.Since the dihydrogen bonds,which are responsible for the formation of the molecular crystal,are stabilized by Coulomb attraction between charged extremities,the mixture of different dehydrogenated polymers will likely form a dis-ordered dihydroge
n-bonded network that bears little rem-blance with the ordered AB crystal.
D.Photo-assisted dehydrogenation
Achieving dehydrogenation under mild conditions is an-other challenge if AB is to become a practical hydrogen storage medium.Most rearch effort in this area has fo-cud on the u of chemical catalysts to reduce the activa-tion barrier for dehydrogenation in order to lower the reaction
