c

Jisoon Ihm

Recent progress in designing

hydrogen-storage nanostructures

October 22, 2009

Dept. of Physics and AstronomySeoul National University

HIGH EFFICIENCY

& RELIABILITY

ZERO/NEAR ZERO

EMISSIONS

6.5 wt%

1.전기자동차의 약진(미국의주력 차종)

하이브리드카 이후 :

매일경제 2009.8.13 A3면

전기모터 소재 Nd의 중국 편중 문제

수소 에너지 도입의 필요성

○ 전지구적인 환경문제

자동차 증가: 배기가스에 의한 환경 오염

이산화탄소 배출 증가로 인한 지구온난화

○ 에너지자원 위기

화석연료 수요의 급격한 증가(중국, 인도, …)

에너지자원 고갈 (국제 분쟁)

수소 에너지의 특성

○ 자원 의존적이지 않은 기술주도형 에너지

○ 자원의 지속성, 재생 가능(물에서 물로순환)

○ 높은 에너지 변환 효율

(원자력으로 물분해+수소연료전지)

○ 환경오염이 없음(CO2 배출 전무)

Biofuel의 문제점: CO2를 배출함

수소저장: 나노기술의 도전적과제

H2 production

H2 storage

Fuel cell

수소 저장 방법들

수소연료전지 자동차에 쓰임350기압 탱크(실용화)700기압 탱크(개발중)

Low energy density안전문제가 있음

가볍고, 부피가 작고,안전해야 함

고압 가스 탱크

고압가스 탱크

수소 저장: 액화 수소

온도 20K저장 특성이 좋음

일부 수소자동차에 쓰임(BMW, Linde AG)저온시스템 필요

액화 수소 보존 문제액화 비용이 비쌈

증발에 의한 수소 손실을 줄여야함

수소 저장: 메탈 하이드라이드 등

고온 화학적 흡착

수소 저장: 나노저장체

저온, 물리적 흡착

H. Lee, W. I. Choi, and J. Ihm, Phys. Rev. Lett. 97, 056104 (2006)

새로운 수소저장 나노물질: 금속이 부착된 폴리머

수소와의 접촉영역을 극대화하기 위해

가지모양으로 퍼져나감

참고: 두바이의 해안 도시 설계(interface의 증대)

SNU CNMP Group

Collaborators

• Moon-Hyun Cha, Manh Cuong Nguyen

• Prof. Seong Keun Oh

• Dr. Dong-Ok Kim

• Prof. Jihwa Lee

• Prof. Myunghyun Paik

SNU CNMP Group

Density Functional Theory (DFT)1

GGA (LDA+U)

Ultrasoft pseudopotential, PAW

PWscf, VASP (OpenMx)

2

3

4

Computational Methods

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Combinatorial metal decoration to polymers

Combinatorial

Decoration

Representatives for Polymers

Selected decorating atoms

Sc (Scandium)

Ti (Titanium)

V (Vanadium)

Cis-PA

Trans-PA

Polyaniline

Polypyrrole

H. Lee, W. I. Choi, and J. Ihm, Phys. Rev. Lett. 97, 056104 (2006)

H. Lee et al., Phys. Rev. B 76, 195110 (2007)

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Binding energy per H2 &zero-point vibration energy correction

Zero-point vibration energy is ~25% of the binding energy of H2

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Hydrogen adsorption-desorption:Adsorption number f as a function of P and T

(average) Adsorption number (f) as P and T

P1, T1P0, T0

Host

material

Usable hydrogen

00

/n

kTn

n

n

kTn

n

nn

egengf

Equilibrium thermodynamics with two phases

InZkTf

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f-P-T diagram : “Usable” hydrogen

Ti-cis-polyacetylene Sc-fullerene

00

/n

kTn

n

n

kTn

n

nn

egengf

Ti-polypyrrole

0.3 eV is most ideal for practical hydrogen storage

Adsorption condition : 30 atm and 25 oC (chemical potential = -0.22 eV)

Desorption condition : 3 atm and 100 oC (chemical potential = -0.38 eV)

EXAMPLES

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Research in Progress-I

Suppression of Ti-clustering or Polymer Crosslinking

Ex. Polybutadiene (C4H6)n

Each at infinite E = 0eV

Ex. Insertion of TiH2

instead of Ti

E = 3.93eV (No crosslinking)

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Research in Progress-II

High gravimetric capacity using functional groups

13 wt%

Metal-decorated ethane-1,2-diol (C2H6O2)

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Various geometries of functional groups

Ti clustering is suppressed as well.

Functional groups :-OH (optimal),-SH,-NH2,-CC-, -CO,-NCO

H. Lee et al., Solid State Commun. 146, 431 (2008)

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Splitting of Ti d orbitals into eg and t2g

(Octahedral ligand field)

Hybridization with H2 σ and σ* orbitals

Binding mechanism (Kubas interaction)

M. C. Nguyen et al., Solid State Commun. 147, 419 (2008)

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Ti dxy

Ti dxy H2 (σ*)

Ti dxy

H2 (σ*)

Exchange Field > Ligand Field Exchange Field < Ligand Field

1H2(0.25eV), 2H2 (0.16eV)3H2(0.45eV), 4H2 (0.43eV)

Ti dz2

Only up-spin hybridized Both up and down spin hybridized

Ti dz2

Binding mechanism : spin dependence

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(a) propane1,3-diol, (b) with one Ca, (c) with Ca and 7H2

PDOS of (b) above. Inset is the eigenstate at the peak position (arrow).

Mechanism in Ca decoration case

Nguyen et al. PRB 79,233408 (2009)

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PDOS of the inset atomic structure Eigenstates at peak 1 and 2

Mechanism in Ca decoration case (Cont’d)

There are 2 bonding channels:

• Hybridization of dx2-y2 of Ca and 4 orbitals of H2 (~ 0.06 eV)

• Dipole interaction:

• Hydrogen molecules are polarized by 0.667 Debye

• Ca, H (lone) and O are ionized by 0.8355, -0.4184, -0.5546 |e|

• Dipole interaction energy: 0.068 eV (~ 50% binding energy)

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New Material : Metal Organic Framework

Storage capacity: insufficient with MOF alone propose a metal-decorated MOF (Ca, Ti, …)

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2Ca -1.237 eV

1H2 -0.182 eV

4H2 -0.170 eV

5H2 -0.148 eV

Ca on MOF5

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Boron replaced MOF (optimum with Ca)

Ca -2.735 eV

4H2 -0.201 eV

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-0.392eV/H2

TiH3 on MOF

A strong candidate material!

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Conclusions

Computational design for optimal H2 storage medium

-High gravimetric capacity in low-dimensional materials and MOFs.

-Zero-point vibration energy : 20 - 25 % of the binding energy of H2.

-Ideal binding energy of H2 : 0.3 eV to maximize usable capacity.

-Understanding of H2-metal interaction : Kubas interaction or not?

-Strategy to suppress Ti clustering or to find other decorating metals.