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

Ov

erv

iew

Global warming – mean surface temperature 1850-2006

• Overview of hydrogen energy

Energy Consumption

Passenger vehicles are major consumption of fossil fuel

Energy consumption is outpacing production

Ov

erv

iew

Energy Consumption

Ov

erv

iew

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 States, 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 typeSources of greenhouse gases

Ov

erv

iew

Global warming Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade)

Northern Hemisphere ice trends

Relationship between [CO2] and temperature

Ov

erv

iew

Greenhouse Effect

Ov

erv

iew

Renewable energy Renewable energy is energy generated from natural resources—such as sunlight, wind, rain, tides and geothermal heat—which are renewable (naturally replenished). In 2006, about 18% of global final energy consumption came from renewables

wind turbines Monocrystalline solar cell

Ov

erv

iew

Hydrogen Energy

If the energy used to split 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

Ov

erv

iew

H2 Hydrogen is the only chemical energy carrier that has the potential to used without generating pollutants to the atmosphere.

Environmentally friendly.

Hydrogen fueled heat engines can be optimized by the manufacturer to operate at much higher thermal efficiencies than heat engines powered with traditional hydrocarbon fuels.

Efficient combustion.

Clean , Renewable and Sustainable .

“ The choice for the future .”

Why hydrogen ?

Ov

erv

iew

Ov

erv

iew

Building Hydrogen Economy

Ov

erv

iew

Ov

erv

iew

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 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.e. normal photosynthesis, to the production of hydrogen.

Fig. An algae bioreactor for hydrogen production.

Ov

erv

iew

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 photovoltaic 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

Ov

erv

iew

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

Ov

erv

iew

H2 storage Adsorbed hydrogen on materials with a large specific surface area (T<100 K) : carbon materials or zeolite

Adsorbed on 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 matrixFig. Carbon nanotube

Ov

erv

iew

H2 utilization (Fuel cell)

A fuel cell is an electrochemical 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

Ov

erv

iew

H2 on-board vehicle application A hydrogen vehicle is a vehicle that uses 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

Ov

erv

iew

• Introduction of hydrogen storage

Hyd

rog

en S

torag

e

Hydrogen storage is a key enabling technology for the advancement of hydrogen and fuel cell power technologies in transportation applications.The major bottleneck for commercializing fuel-cell vehicles is on-board hydrogen storage.

The goal is to pack H2 as close as possible.

Hydrogen Storage implies the reduction of an enormous volume of hydrogen gas. Compression of H2 gas.

What is Hydrogen Storage ?

Hyd

rog

en S

torag

e

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 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

Hyd

rog

en S

torag

e

Reversible on-board

The on board storage media require hydrogen in 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 TankFuel Cell Stacks

Air Pump

Power Control Unit

Hydrogen Filler Mouth

Hyd

rog

en S

torag

e

Current analysis activities is to optimize the trade-off among…

Weight, volume, cost, as well as life-cycle cost, energy efficiency, and environmental impact analyses.

Hydrogen Tank

Hydrogen Filler Mouth

The technical challenge is…

Storing sufficient hydrogen while meeting all consumer requirements without compromising passenger or cargo space.

Reversible on-board

Hyd

rog

en S

torag

e

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 situation 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 gasoline 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 today’s gasoline vehicles (greater than 300 mile driving range) and complete market penetration.

US DOE H2 storage system targets

Hyd

rog

en S

torag

e

6 wt% 9 wt%

Hyd

rog

en S

torag

e

Current approaches include:1.High pressure H2 cylinders2.Cryogenic and liquid hydrogen

3.High surface area sorbents4.Metal hydrides5.Complex metal hydrides

Hydrogen Storage

Methods

Conventional Storage

Advanced Solid Materials Storage

Increasing H2 density by Pressure and Temp. control.

Using little additional material to reach high H2 density.

Hyd

rog

en S

torag

e

1. HP H2 CylindersIntroduction…

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 compressed 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.

Hyd

rog

en S

torag

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 hydrogen 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…

Hyd

rog

en S

torag

e

2. Liquid H2 StorageIntroduction…

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 the 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.

Hyd

rog

en S

torag

e

2. Liquid H2 StorageIntroduction…

The energy required for liquefy hydrogen, over 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 storage capacities of these tanks, ‘Cryo-compresed’ tanks i.e. compressed cryogenic hydrogen or a combination of liquid hydrogen and high pressure hydrogen are developed.

Hyd

rog

en S

torag

e

3. High Surface Area SorbentsIntroduction…

Carbon nanotubes (CNTs), and several other high surface area sorbents (e.g. carbon nanofibers, 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:Temperature and pressure, micropore density, specific surface area

Hyd

rog

en S

torag

e

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 SWCNTs.

Hyd

rog

en S

torag

e

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 K.

Factor 3Specific Surface Area

Hyd

rog

en S

torag

e

Where is Hydrogen

Interplanar spacing

Inner surface

External surface

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 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.

H2 Molecules

Hydrogen Storage Active Materials

Hyd

rog

en S

torag

e

Other Possible Sorbents

Hyd

rog

en S

torag

e

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 CNTs.

A combination of physisorption and chemisorption.

HHHH

HH

To improve the hydrogen storage amount in CNTs

3. High Surface Area Sorbents

Example…

Criteria for efficient spillover effectsmetal catalyst

R.T. Yang et al. Energy Environ. Sci. 1 (2008) 268.

Intro

du

tion

Nano-sized metal particles (Ni, Pt, Pd, V etc.) with high surface area 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.

Hyd

rog

en S

torag

e

TEM observations Pd(hfa)2 : CNT = 1 : 1

Pd ~27.2 wt%

The CNTs were densely covered with slightly elliptic Pd deposited from 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.

Hyd

rog

en S

torag

e

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 sample.

Due to the spillover effect, the hydrogen storage amount is significantly increased by four times.

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%

En

han

cem

ent

via

spill

ove

r

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 hydrogen uptake amount is shown as a dotted line.

Hyd

rog

en S

torag

e

Hyd

rog

en S

torag

e

It’s 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 temperature (> 300 °C) to let go of it.

4. Metal hydridesIntroduction…

Hyd

rog

en S

torag

e

Again of the reversible hydrides simple magnesium does best. Magnesium is the world's 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 of dollars per kilo.

4. Metal hydridesIntroduction…

The temperature at which the metal hydrides release the hydrogen at standard pressure.

There's about a 30% penalty to heat the magnesium (30% of the fuel cell keeps the metal hot).

Hyd

rog

en S

torag

e

Alloys

Solidsolutions

Intermetalliccompounds

others

AB5 AB2 AB A2B

OtherAB3,A2B7

A2B17,etc.

Stable Metastable

Multiphase Quasiccrystalline Amorphous Nanocrystalline

4. Metal hydridesIntroduction…

Brief Category

The most important families of hydride-forming IMC. Element A has a high affinity to hydrogen and element B has a low affinity to hydrogen.

Hyd

rog

en S

torag

e

How to form Metal Hydrides

Hydrogen reacts at elevated temperatures with many transition metals and their alloys to form hydrides. The electropositive elements 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.

Hyd

rog

en S

torag

e

How to form Metal Hydrides

The lattice structure is that of a typical metal with hydrogen atoms on the 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 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.

Hyd

rog

en S

torag

e

About Metal Hydrides

Because of the phase transition, metal hydrides 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. Both hydrides belong to the complex hydrides family.

Hyd

rog

en S

torag

e

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 properties 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 light metal hydrides : AMH4 (A= alkali or alkali earth metal, M= third group elements) Unlike classic interstitial metal hydrides, the alanates desorb and absorb hydrogen through chemical decomposition and recombination reactions.

Alanates

Borohydrides

Table selected complex hydrides

Hydride H2 ( wt % ) Source

LiAlH4 10.5 Commercially available

NaAlH4 7.5 Commercially available

KAlH4 5.8 As described in J. Alloys Compd., 353 (2003) 310

Mg(AlH4)2 9.3 As described in Inorg. Chem., 9 (1970) 325

Ca(AlH4)2 7.7 As described in Inorg. Nucl. Chem., 1 (1955) 317

LiBH4 18.5 Commercially available

NaBH4 10.6 Commercially available

Mg(BH4)2 14.9 As described in Inorg. Chem., 11 (1972) 929

Ca(BH4)2 11.4 Synthetic procedure to be developed

Al(BH4)3 16.9 As described in J. Am. Chem. Soc., 75 (1953) 209

Complex Hydrides

Hyd

rog

en S

torag

e

Hyd

rog

en S

torag

e

Complex Hydrides

The method for improving hydrogen storage capacity

Destabilization of LiBH4 with MgH2!Although the storage density is promising, one of the major issues with many metal hydrides, due to the reaction enthalpies involves (e.g. ~40 kJ/mol H2), is thermal management during refueling. Approximately 0.5-1 MW of heat must be rejected during recharging on-board vehicular systems.

Reversibility and durability of these materials also needs to be demonstrated. Issues with handling, pyrophoricity, and exposure to air, humidity and contaminants also need to be addressed.

Material design of metal borohydride M(BH4)n or alanate M(AlH4)n.

Charge transfer from Mn+ to [BH4]- is a key feature for the stability of M(BH4)n, which can be estimated by value of Pauling electronegativity χP. The charge transfer becomes smaller with increasing value of χP, which makes ionic bond weaker.

χp of cation Mn+ ↑, ionic bond weaker, thermal desorption temperature↓

Fig. The desorption temperature Td as a function of the Pauling electronegativity χp.

Hyd

rog

en S

torag

e

Complex Hydrides

H2 storage in Mg2Ni alloy

Ball-milling vial

Mg powder

Ni powderL

Milling steel balls

Diameter: 5/16 inch, 2.10g

Ball to powder ratio, BPR= 5:1 , 10:1

＋

Ball-milling powder for 5, 10, 15, 20, 25hr in SPEX 8000

Hyd

rog

en S

torag

e

4. Metal hydridesExamples…

Experimental method - preparation of Mg2Ni

In 1atm N2 glove box

2 0 4 0 6 0 8 01 0 3 0 5 0 7 0

D iffra ctio n A n g le(2)

Rel

ativ

e In

ten

sity

5 h r ,B P R = 1 0

1 0 h r ,B P R = 1 0

M gN i

1 5 h r ,B P R = 1 0

2 0 h r ,B P R = 1 0

1 5 h r ,B P R = 5

M g 2N i

Fig. X-ray diffraction patterns of as-milled powders with different ball-milling conditions.

Hyd

rog

en S

torag

e

4. Metal hydridesExamples…

Mg2Ni can be prepared by ball-milling Mg and Ni powder over 10 hr. Prolonging milling time and enlarging ball-to-powder ratio are able to increase the crystallinity of Mg2Ni powders, reducing the particle size as well as grain size.

Powders Milling time (hr) BPR Hydrogenation density

Mg2Ni, Ni 10 10 1.57 wt.%Mg2Ni 15 10 2.76 wt.%Mg2Ni 20 10 2.89 wt.%

Table hydrogen capacities measured at 300 psi H2 and 573 K with different ball-milling conditions.

0 2 0 4 0 6 0 8 0 1 0 0

T im e (m in )

0

1

2

3

4

H2

Ab

sorp

tion

(w

t.%

)

10 h r15 h r

T h eoretica l H 2 a b sorp tio n d en sity o f M g 2N i

20 h r

Fig Hydrogen absorption rate among 10, 15, 20 hr ball-milled powders

Prolonging milling time from 10 to 20 hr increases hydrogen capacity over 1 wt% to around 2.9 wt%. Hydrogen absorption rate was improved obviously by prolonging milling time, especially for as-milled powder for 20 hr performing the best absorption rate.

Hyd

rog

en S

torag

e

NaAlH4 is a promising candidate for hydrogen storage, up to 5.6 wt% gravimetric capacity, easy accessibility. However, the high reactivity, sluggish kinetic and high desorption temperature makes it imperfect for application use.

The desorption properties can be described in 3 step:

NaAlH4→1/3Na3AlH6 + H2 + 2/3Al (210~240°C)

Dissociation enthalpy +37 kg/mol (3.7wt %)

1/3Na3AlH6 → NaH + 1/3Al + 1/2H2 (>250°C)

Dissociation enthalpy +47 kg/mol (1.85wt %)

NaH→ Na + 1/2H2 (up to 400 °C) (1.85wt%)

4. Complex hydridesExamples…

Hyd

rog

en S

torag

e

4. Complex hydridesExamples…

Enhance NaAlH4 thermodynamic properties by admixing with MWCNTs

Hyd

rog

en S

torag

e

Ball-milling vialNaAlH4 powder

Aldrich, 90%

Milling steel ballsDiameter: 5/16 inch, 2.10g 3/16 inch, 0.44g

Ball to powder ratio, BPR= 10:1

In 1atm N2-filled glove box

Volume = 75ml

High energy milling in SPEX 8000 Mixer/Mill for 1700r.p.m

1. As-received NaAlH4 mix with 10,20,30 wt% CNT and bill mill for 30min

4. Complex hydridesExamples…

Hyd

rog

en S

torag

e

Fig. 2 TGA results of as-received NaAlH4 and

10, 20 wt% MWCNTs-admixed NaAlH4

from room temperature to 350 °C.

Fig. 1 SEM micrographs of (a) as-received NaAlH4

and (b) 20 wt% MWCNTs-admixed NaAlH4.

(a)

(b)

Desorption temperature decrease from 220oC to 160oC

Fig. In-situ synchrotron XRD results of as-received NaAlH4 from room temperature to 330 °C. (λ=1.033209 Å)

In-situ synchrotron XRD experiment of NaAlH4

Hyd

rog

en S

torag

e

NaAlH4 Na3AlH6

AlNaH

Hyd

rog

en S

torag

e

Examples…NaAlH4 4. Complex

hydrides

Reversibility!!

1st desorption 2nd desorption (after reH2)

3.9wt% 1.9wt%

NaAlH4 admix MWCNTs under cyclic test can exhibited reversibility between Na3AlH6 and NaH phase (2nd desorption reaction ) at constant pressure 1000 psi/180oC for 5h

Su

mm

ary

The materials science challenge of hydrogen storage is to understand the interaction of hydrogen with other elements better, especially metals.

Hydrogen production, storage, conversion has reached a technological level, although plenty of improvements and new discoveries are still possible.

EN

D

Thank you for your kind attention.

Department of Materials Science and Engineering

National Cheng Kung UniversityCorrosion Prevention Laboratory