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1、氫能源Hydrogen energy,材料系 蔡文達 教授,October 20th,2011,工學院次能源專長,二十一世紀前五十年 人類將面臨之十大問題,ENERGY WATER FOOD ENVIRONMENT POVERTY TERRORISM&WAR DISEASE EDUCATION DEMOCRACY POPULATION,Greenhouse Effect,Overview,Global warming mean surface temperature 1850-2006,Overview of hydrogen energy,Energy Consumption,Passeng
2、er vehicles are major consumption of fossil fuel,Energy consumption is outpacing production,Overview,Energy Consumption,Overview,Pollution of Fossil Fuel,Fossil fuel burning has produced approximately three-quarters of the increase in CO2 from human activity over the past 20 years.In the United Stat
3、es,more than 90%of greenhouse gas emissions come from the combustion of fossil fuels.Combustion of fossil fuels also produces other air pollutants,such as nitrogen oxides,sulfur dioxide,volatile organic compounds and heavy metals.,Global fossil carbon emission by fuel type,Sources of greenhouse gase
4、s,Overview,Global warming,Since 1979,land temperatures have increased about twice as fast as ocean temperatures(0.25C per decade against 0.13C per decade),Northern Hemisphere ice trends,Relationship between CO2 and temperature,Overview,Greenhouse Effect,Overview,Renewable energy,Renewable energy is
5、energy generated from natural resourcessuch as sunlight,wind,rain,tides and geothermal heatwhich are renewable(naturally replenished).In 2006,about 18%of global final energy consumption came from renewables,wind turbines,Monocrystalline solar cell,Overview,Hydrogen Energy,If the energy used to split
6、 the water were obtained from renewable or Nuclear power sources,and not from burning carbon-based fossil fuels,a hydrogen economy would greatly reduce the emission of carbon dioxide and therefore play a major role in tackling global warming.,2H2O O2+4H+4e-,2H+2e-H2,Overview,Clean,Renewable and Sust
7、ainable.“The choice for the future.”,Why hydrogen?,Overview,Overview,Building Hydrogen Economy,Overview,Overview,H2 production,Hydrogen is commonly produced by extraction from hydrocarbon fossil fuels via a chemical path.Hydrogen may also be extracted from water via biological production in an algae
8、 bioreactor,or using electricity(by electrolysis),chemicals(by chemical reduction)or heat(by thermolysis)Biological production:Biohydrogen can be produced in an algae bioreactor.In the late 1990s it was discovered that if the algae is deprived of sulfur it will switch from the production of oxygen,i
9、.e.normal photosynthesis,to the production of hydrogen.,Fig.An algae bioreactor for hydrogen production.,Overview,H2 production,Electrolysis:Hydrogen can also be produced through a direct chemical path using electrolysis.With a renewable electrical energy supply,such as hydropower,wind turbines,or p
10、hotovoltaic cells,electrolysis of water allows hydrogen to be made from water without pollution.Chemical production:By using sodium hydroxide as a catalyst,aluminum and its alloys can react with water to generate hydrogen gas.Al+3 H2O+NaOH NaAl(OH)4+1.5 H2,Solar Energy,Fig.Photoelectrochemical cell,
11、Overview,H2 storage,High pressure gas cylinders(up to 800bar)Liquid hydrogen in cryogenic tanks(at 21 K),Fig.Liquid hydrogen tank for a hydrogen car,Fig.gas cylinders,Overview,H2 storage,Adsorbed hydrogen on materials with a large specific surface area(T100 K):carbon materials or zeolite Adsorbed on
12、 interstitial sites in a host metal(at ambient pressure and temperature):metal hydride Chemically bond in covalent and ionic compounds(at ambient pressure,high activity):complex metal hydride,Fig.Hydrogen in metal matrix,Fig.Carbon nanotube,Overview,H2 utilization(Fuel cell),A fuel cell is an electr
13、ochemical conversion device.It produces electricity from fuel(on the anode side)and an oxidant(on the cathode side),which react in the presence of an electrolyte.,Fig.Direct-methanol fuel cell,Fig.Scheme of fuel cell,Overview,H2 on-board vehicle application,A hydrogen vehicle is a vehicle that uses
14、hydrogen as its on-board fuel for motive power.The term may refer to a personal transportation vehicle,such as an automobile,or any other vehicle that uses hydrogen in a similar fashion,such as an aircraft.,Fig.Hydrogen station,Overview,Introduction of hydrogen storage,Hydrogen Storage,What is Hydro
15、gen Storage?,Hydrogen Storage,Reversible on-board vs.Regenerable off-board,System that bind H2 with low binding energy(less than 20-25 kJ/mol H2)can undergo relatively easy charging and discharging of hydrogen under moderate conditions that are applicable.While in stronger bonds(typically in excess
16、of 60-100 kJ/mol H2),once the hydrogen is released,recharging with H2 under operating conditions convenient at a refueling station is problematic.,On-board,Off-board,Vehicular hydrogen storage approaches:,Definitions,Hydrogen Storage,Reversible on-board,The on board storage media require hydrogen in
17、 liquid or gaseous form under different pressures,depending on specifications of the on-board technology.,“Reversible”on-board?because these methods may be recharged with hydrogen on-board the vehicle,similar to refueling with gasoline today.,Hydrogen Tank,Fuel Cell Stacks,Air Pump,Power Control Uni
18、t,Hydrogen Filler Mouth,Hydrogen Storage,Current analysis activities is to optimize the trade-off amongWeight,volume,cost,as well as life-cycle cost,energy efficiency,and environmental impact analyses.,Hydrogen Tank,Hydrogen Filler Mouth,The technical challenge isStoring sufficient hydrogen while me
19、eting all consumer requirements without compromising passenger or cargo space.,Reversible on-board,Hydrogen Storage,To achieve comparable driving range may require larger amount of H2.,On a weight basis,hydrogen has nearly three times the energy content of gasoline.However,on a volume basis the situ
20、ation is reversed and hydrogen has only about a quarter of the energy content of gasoline.,Why Challenge?,For the successful commercialization and market acceptance of hydrogen powered vehicles,the performance targets developed are based on achieving similar performance and cost levels as current ga
21、soline fuel storage systems for light-duty vehicles.,Gasoline or Hydrogen.,Gasoline or Hydrogen,The 2015 targets represent what is required based on achieving similar performance to todays gasoline vehicles(greater than 300 mile driving range)and complete market penetration.,US DOE H2 storage system
22、 targets,Hydrogen Storage,6 wt%,9 wt%,Hydrogen Storage,Current approaches include:,High pressure H2 cylindersCryogenic and liquid hydrogenHigh surface area sorbentsMetal hydridesComplex metal hydrides,Hydrogen Storage Methods,Conventional Storage,Advanced Solid Materials Storage,Increasing H2 densit
23、y by Pressure and Temp.control.,Using little additional material to reach high H2 density.,Hydrogen Storage,1.HP H2 Cylinders,Introduction,70 MPa H2 storage cylinders?,The most common storage system is high pressure gas cylinders.Carbon fiber-reinforced composite tanks for 350 bar and 700 bar compre
24、ssed hydrogen are under development and already in use in prototype hydrogen-powered vehicles.The cost of high-pressure compressed gas tanks is essentially dictated by the cost and the amount of the carbon fiber that must be used for structural reinforcement for the composite vessel.,Hydrogen Storag
25、e,1.HP H2 Cylinders,Volumetric density of compressed H2 as a function of gas pressure.,The safety of pressurized cylinders is a concern.Industry has set itself a target of a 110 kg,70 MPa cylinder with a gravimetric storage density of 6 wt%and a volumetric density of 30 kg/m3.The relatively low hydr
26、ogen density together with the very high gas pressures in the system are important drawbacks of this technically simple method.,The volumetric density increases with pressure and reaches a maximum above 1000 bar,depending on the tensile strength of the material.,Introduction,Hydrogen Storage,2.Liqui
27、d H2 Storage,Introduction,Primitive phase diagram for hydrogen.,Liquid H2 only exists between the solid line and the line from the triple point at 21.2 K and the critical point at 32 K.,Liquid hydrogen(LH2)tanks can,in principle,store more hydrogen in a given volume than compressed gas tanks,since t
28、he volumetric capacity of liquid hydrogen is 0.070 kg/L(compared to 0.039 kg/L at 700 bar).Key issue with LH2 tanks are hydrogen boil-off,the energy required for hydrogen liquefaction,as well as tank cost.,Hydrogen Storage,2.Liquid H2 Storage,Introduction,The energy required for liquefy hydrogen,ove
29、r 30%of the lower heating value of hydrogen,remains a key issue and impacts fuel cost as well as fuel cycle energy efficiency.The large amount of energy necessary for liquefaction and the continuous boil-off of hydrogen limit the use of liquid hydrogen storage system.,LH2 tank system,To increase the
30、 storage capacities of these tanks,Cryo-compresed tanks pressed cryogenic hydrogen or a combination of liquid hydrogen and high pressure hydrogen are developed.,Hydrogen Storage,3.High Surface Area Sorbents,Introduction,Carbon nanotubes(CNTs),and several other high surface area sorbents(e.g.carbon n
31、anofibers,graphite materials,metal-organic frameworks,aerogels,etc.)are being studied for hydrogen storage.The process for hydrogen adsorption in high surface area sorbents is physisorption,which is based on weak Van der Waals forces between adsorbate and adsorbent.Some factors investigated:Temperat
32、ure and pressure,micropore density,specific surface area,Hydrogen Storage,3.High Surface Area Sorbents,Factor 1Temp.and Pressure,Hydrogen adsorption isotherms at room temperature and at 77 K fitted with a Henry type and a Langmuir type equation,respectively(a)for activated carbon,(b)for purified SWC
33、NTs.,Hydrogen Storage,3.High Surface Area Sorbents,Factor 2Micropore Density,Correlation between the hydrogen storage capacity at 77 K and the pore volume for pores with diameter 1.3 nm.,Relation between hydrogen storage capacity of the different carbon samples and their specific surface area at 298
34、 K.,Factor 3Specific Surface Area,Hydrogen Storage,Where is Hydrogen,The long path for hydrogen diffusion into interior of CNTs is a challenge.Generally,the H2 storage capacity under moderate conditions was at or below 1 wt%.Physisorption alone is not sufficient to reach the high capacity at ambient
35、 temperature.The big advantages of physisorption for hydrogen storage are the low operating pressure,the relatively low cost of the material involved,and the simple design of the system.The rather small gravimetric and volumetric hydrogen density on carbon are significant drawbacks.,Hydrogen Storage
36、,Other Possible Sorbents,Hydrogen Storage,In recent literatures,hydrogen spillover is a promising approach to enhance the hydrogen storage amount of carbon nanostructures.,Hydrogen molecules dissociate to atomic hydrogen on a metal catalyst and subsequently migrate from the metal to the surface of C
37、NTs.,A combination of physisorption and chemisorption.,To improve the hydrogen storage amount in CNTs,3.High Surface Area Sorbents,Example,Criteria for efficient spillover effects,R.T.Yang et al.Energy Environ.Sci.1(2008)268.,Introdution,Nano-sized metal particles(Ni,Pt,Pd,V etc.)with high surface a
38、rea and good hydrogen-dissociation catalytic activity.,High dispersion and uniform distribution of the catalysts on hydrogen adsorbents.,Intinate contact between catalysts and the hydrogen adsorbents.,Hydrogen Storage,Pd 27.2 wt%,The CNTs were densely covered with slightly elliptic Pd deposited from
39、 scCO2 fluid.,10 nm,Even at the heavy loading condition,highly dispersion of the nano-particles can be still obtained;no formation of Pd film was observed.,Hydrogen Storage,Since the 4%-Pd sample has more H2 dissociation sites,its hydrogen capacity(1.31 wt%)is higher than that(1.18 wt%)of the 2%-Pd
40、sample.,At 25oC and 6.9 MPa c.4%Pd/CNTs:1.31 wt%b.2%Pd/CNTs:1.18 wt%a.Pristine CNTs:0.33 wt%,Hydrogen storage capacityof the Pd-decorated CNTs,If all the 4 wt%Pd in CNTs was converted to PdH0.75,this hydride will contribute a hydrogen capacity of 0.03 wt%to the overall material.The additivity of the
41、 hydrogen uptake amount is shown as a dotted line.,Hydrogen Storage,Hydrogen Storage,Its a chemical compound or form of a bond between hydrogen with a metal.Metals hydrize at certain temperatures and pressures.Magnesium Hydride,MgH2,stores the largest density of hydrogen but requires high temperatur
42、e(300 C)to let go of it.,4.Metal hydrides,Introduction,Hydrogen Storage,Again of the reversible hydrides simple magnesium does best.Magnesium is the worlds third most abundant metal.Iron titanium comes next for price.Pretty much everything else is an exotic designer alloy as of now:tens of thousands
43、 of dollars per kilo.,4.Metal hydrides,Introduction,The temperature at which the metal hydrides release the hydrogen at standard pressure.Theres about a 30%penalty to heat the magnesium(30%of the fuel cell keeps the metal hot).,Hydrogen Storage,4.Metal hydrides,Introduction,Brief Category,The most i
44、mportant families of hydride-forming IMC.Element A has a high affinity to hydrogen and element B has a low affinity to hydrogen.,Hydrogen Storage,How to form Metal Hydrides,Hydrogen reacts at elevated temperatures with many transition metals and their alloys to form hydrides.The electropositive elem
45、ents are the most reactive,i.e.Sc,Yt,lanthanides,actinides,and members of the Ti and Va groups.The binary hydrides of the transition metals are predominantly metallic in character.,Hydrogen Storage,How to form Metal Hydrides,The lattice structure is that of a typical metal with hydrogen atoms on the
46、 interstitial sites;and for this reason they are also called interstitial hydrides.The type is limited to the composition This type of structure is limited to the compositions of MH,MH2,and MH3.,The ternary system ABxHn,element A is usually a rare earth or an alkaline earth metal and tends to form a
47、 stable hydride.Element B is often a transition metal and forms only unstable hydrides.Some well defined ratios of B:A,where x=0.5,1,2,5,have been found to form hydrides with a hydrogen to metal ratio of up to two.,Hydrogen Storage,About Metal Hydrides,Because of the phase transition,metal hydrides
48、can absorb large amounts of hydrogen at a constant pressure.One of the most interesting features of metallic hydrides is the extremely high volumetric density of hydrogen atoms present in the host lattice.The highest volumetric hydrogen density reported is about 150 kg/m3 in Mg2FeH6 and Al(BH4)3.Bot
49、h hydrides belong to the complex hydrides family.,Hydrogen Storage,About Metal Hydrides,Metal hydrides are very effective at storing large amounts of hydrogen in a safe and compact way,but the gravimetric hydrogen density is shown to less than about 3 wt%.It remains a challenge to explore the proper
50、ties of lightweight metal hydrides.Complex hydrides?Group 1,2,and 3 light metals,e.g.Li,B,and Al,give rise to a large variety of metal-hydrogen complexes.They are especially interesting because of their light weight and the number of hydrogen atoms per metal atom,which is two in many cases.,Complex
