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1、substantial/大量的实质的/ impact/影响/ on carbon emissions, with more than132 million vehicles/工具/ in the United States alone.2Battery and fuel-cell technologies are strong candidates/候选者/ toreplace gasoline/汽油/ and diesel/柴油/ engines. In particular, hydrogenis an attractive energy carrier because it is car

2、bon-free,abundantly/丰富的/ available from water, and has an exceptional massenergy density.3 Unfortunately, hydrogen is an extremelyvolatile/挥发性的/ gas under ambient/周围的/ conditions, resulting in avolumetric energy density that is much too low for practicaapplications. For on-board use, hydrogen must b

3、e compressedto very high pressures or stored cryogenically/低温/, both of whichcost energy and substantial/大量的实质的/ly increase vehicle weight. The goaltherefore is to design low-cost, light-weight materials that canreversibly/可逆的/ and rapidly store hydrogen near ambient/周围的/ conditionsat a density equa

4、l to or greater than liquid hydrogen. The USDepartment of Energy 2010 targets for a hydrogen storagesystem are: a capacity of 45 g H2 per L, a refuelling/补充燃料/ time of10 min or less, a lifetime of 1000 refuelling/补充燃料/ cycles, and an abilityto operate within the temperature range _30 to 50 1C.4,5 It

5、 isimportant to note that these targets are for the entire storagesystem, such that the performance of a storage material mustbe even higher in order to account for the storage containerand, if necessary, temperature regulating/校正/ apparatus/设备/.Hydrogen binds to surfaces by weak dispersive/分散的/ int

6、eractions（physisorption） or through stronger chemical associations（chemisorption）. Physisorption correlates with surface area,with greater gas uptake favored by higher surface area. Thus,materials with large surface areas and low densities, such asmetalorganic frameworks and certain activated carbon

7、s, areattractive for hydrogen storage applications. In the temperatureregime desired for automotive applications, however,dispersive/分散的/ forces cannot facilitate substantial/大量的实质的/ hydrogenuptake. The modular nature of metalorganic frameworksallows for the facile, ordered incorporation of new func

8、tionalitiesto enhance the hydrogen storage properties. Here,we review the current state of hydrogen storage in metalorganic frameworks, focusing on strategies for improvingthe storage capacity of these compounds. The design ofnew frameworks depends on a detailed chemical understandingof the interact

9、ion of hydrogen at sites within the structure.We therefore also discuss briefly some spectroscopic toolsthat are available to interrogate hydrogen binding in thesesystems.H2 adsorption in metalorganic frameworksExcess versus total uptakeMost articles dealing with hydrogen storage in metalorganicfram

10、eworks report the H2 uptake capacity at a pressure ofca. 1 bar, where excess and total adsorption values are nearlyidentical. However, since pressures of up to 100 bar aredeemed safe for automotive applications, measurements athigher pressures, where these two quantities can differconsiderably, have

11、 become common. Excess adsorption refersto the amount of H2 taken up beyond what would becontained, under identical conditions, within a free volumeequivalent to the total pore volume of the sample. Thus,this quantity approximates the amount of H2 adsorbedon the surfaces within the material. Since t

12、he efficiency ofpacking and compressing gas molecules within the confines ofthe pores of a microporous solid is less than that achieved in afree volume, the excess adsorption will reach a maximum atsome pressure （typically 2040 bar） and then decrease. Despitethe decrease, measurements at pressures a

13、bove the maximumin excess adsorption are of value for assessing the compressibilityof H2 within the material and evaluating the total1295one should be careful to employ an accurate intermolecular H2potential energy function18 and to ensure that the comparisondata are for an authentic sample.19 These

14、 studies indicate thepresence of just van der Waals-type interactions between H2and most frameworks, consistent with the approximatecorrelation of H2 uptake at 77 K with surface area and thevery low storage capacities observed at 298 K. Indeed, withjust two electrons, H2 forms extremely weak van der

15、 Waalsbonds, resulting in isosteric heats of adsorption that aretypically in the range 47 kJ mol_1.Partial charges, either positive or negative, on the metalorganic framework surface can provide a means of strengtheningthe binding of H2 through dipoleinduced dipoleinteractions.6,20 Only a few comput

16、ational studies have dealtwith frameworks exhibiting such heterogeneous surfacepotentials. These have focused mainly on the chief experimentalstrategy adopted, that of utilizing frameworks with exposedmetal cation sites on the surface. An added complicationin performing calculations on frameworks be

17、aring opentransition metal coordination sites stems from the fact thatthese metals sometimes have open-shell electron configurations,for which assignment of the spin state can be difficult.For instance, the relatively strong metalH2 interactions withinMn3（Mn4Cl）3（BTT）82 （H3BTT = benzene-1,3,5-tris（1

18、Htetrazole）,which exhibits an isosteric heat of adsorption of10.1 kJ mol_1 at zero coverage,21 have been attributedvariously to a spin-state change upon binding22 or to aclassical Coulombic attraction.23 Understanding metalH2interactions of this type is instrumental to the design ofimproved storage

19、materials, and the development of computationalapproaches that can reliably handle interactionswith open-shell metal ions would present an important stepforward.Clearly, increasing the H2 binding energy within metalorganic frameworks is the most important challenge forcreating hydrogen adsorbents th

20、at operate at 298 K. Recentwork has addressed this issue and predicted optimalparameters for hydrogen storage in microporous materials.First, Langmuir isotherms were employed to derive equationsthat allow the calculation of an optimal adsorption enthalpy,DHopt, for a given adsorption temperature.24

21、According to thismodel, which can be reduced to the empirical equation_DHopt/RT = 6.1, a microporous adsorbent operatingbetween 1.5 and 100 bar at 298 K would ideally have anadsorption enthalpy of 13.6 kJ mol_1 over the entire H2uptake curve. Similarly, the model allows one to calculatethe optimal o

22、perating temperature for an adsorbent with agiven enthalpy of adsorption. For instance, it predicts that atypical metalorganic framework with an average adsorptionenthalpy of 6 kJ mol_1 would function optimally at atemperature of 131 K.The aforementioned model has recently been adjustedthrough intro

23、duction of an entropyenthalpy correlationterm.25 Whereas DSads had previously been assumed to beconstant and equal to _8R, the new model argues thatLangmuir adsorption is in fact governed by a positivecorrelation between entropy and enthalpy. Taking thisempirical correlation into account suggests th

24、at a materialoperating between 1.5 and 30 bar at 298 K requires a DHopt of2225 kJ mol_1, which is significantly higher than thatobtained with the previous model. Thus, for pressures rangingup to 100 bar, one would like to create new metalorganicframeworks featuring surfaces with a DHopt of ca.20 kJ

25、mol_1, representing an enhancement by a factor of3 or 4 over simple physisorption.As expected, in a microporous material where physisorptionand weak van der Waals forces dominate the adsorptionpicture, the storage density is also greatly dependent on thesize of the pore. Calculations on idealized ho

26、mogeneousmaterials, such as graphitic carbons and carbon nanotubes,predict that microporous materials with 7 A -wide pores willexhibit maximal H2 uptake at room temperature. In effect,a 7 A -wide slit-shaped pore maximizes the van der Waalspotential by allowing exactly one layer of H2 molecules toad

27、sorb on opposing surfaces, with no space left in between.Notably, at 77 K a layer sandwiched in between these twoopposing surface monolayers becomes favorable, and the idealpore size for maximum volumetric H2 uptake at 100 bar ispredicted to be 10 A , regardless of whether a slit shape orcylindrical

28、 pore shape is considered.26Finally, an ideal hydrogen storage material would be stableto any potential impurities that might commonly be present inH2 gas （e.g., H2S, carbonsulfur compounds, CO, CO2, N2,H2O, and hydrocarbons）, and to accidental exposure to theatmosphere. Indeed, metalorganic frameworks exhibitingsome of the best performance characteristics, such asZn4O（BDC）3 and Mn3（Mn4Cl）3（BTT）82, are known todecompose in air,19,21,27,28 which would need to be accountedfor in the design of a storage system. However, by producingframeworks featuring strong m