Nanoscale Materials: Exploring the Energy Frontier

Transcription

1 Nanoscale Materials: Exploring the Energy Frontier Prashant N. Kumta Department of Materials Science and Engineering, Biomedical Engineering Carnegie Mellon University, Pittsburgh, PA 15213

2 Need for Energy Storage Systems Hybrid Electric Vehicle Backup Power Source Telecommunication Electronic Devices Laser Military Use Particle Accelerator Nissan Diesel Sizuki Electric Power Systems

3 Potential Energy Storage Systems Li-ion ion Batteries * Super-capacitors * Direct Methanol Fuel Cells * Hydrogen Storage *Focus of the current presentation

4 Lithium-Ion Systems: Anodes and Cathodes Comparison of Battery Energy Density (gravimetric and volumetric) Considerable improvement in area of cathodes with identification of new layered compounds, LiFePO 4 and LiNi 0.5 Mn 0.5 O 2, LiMn 1/3 Ni 1/3 Co 1/3 O 2. Improvements in the anode area are still warranted.

5 Introduction and Background Li-alloy (Li x M) : Anodes for Li ion cells 3 - Intermetallic phases containing lithium Li x M xli + + xe - + M (M = Mg, Ca, Al, Si, Ge, Sn, Sb, Bi, Zn, etc) 1 - Higher theoretical capacity than graphite (LiC 6 : 372 mah/g, 832 Ah/L, Li 22 Sn 5 : 990 mah/g, Ah/L Li 22 Si 5 : 4000 mah/g, Ah/L) 1, 2 Major problems of Li-alloys for use as anodes - Poor reversibility caused by large volume changes (Sn: 593%, Si: 412% volume change) - Cracking or crumbling during cycling 1. M. Winter & J. O. Besenhard, Electrochimica Acta, 1999, 45, B. A. Boukamp & R. A. Huggins, J. Electrochemical Soc., 1981, 128, L.Y. Beaulieu, K.W. Eberman, R.L. Turner, L.J. Krause and J.R. Dahn, Elect. Sol. St. Lett. 2001, A137.

6 Amorphous active phase - No secondary phase observed at the interface Si/Transition metal non-oxide system is stable during HEMM. Nanoscale structure is the key to attaining the high capacity 2 nm 20nm 20nm Bright Field Dark Field

7 Electrochemical Response(Sn:C=1:1) (Tetraethyl tin+ps-resin powder heat-treated at 600 o C for 5h in UHP-Ar, current rate = 100µA/cm 2 ) Sp.Capacity (m Ah/g) Graphite 500 Cycle between 0.02 and 1.2V Cycle Number Vol.Capacity (Ah/l) dq/dv ( mah/g.v ) Cell Potential (V) - Theoretical Capacity = 636 mah/g - Higher capacity than graphite (~480mAh/g or 1450Ah/l) with stability (0.15%loss/cycle).

8 Nanostructured inactive matrix can endure large volumetric stresses of the active material due to possible superplastic deformation that can provide compressive stresses to accommodate large strains *. HEMM derived Si-C nanocomposites n -p ε = σ d D o exp( Q/RT) * M.J. Mayo, Nanostructured Materials, Vol. 9 (1997) 717

9 XTEM of sputter deposited Si film (250 nm as-deposited film) Si 100 nm 2 nm Cu Conventional XTEM High Resolution XTEM

10 Galvanostatic Cycling of 250 nm Si Thin Film (C/2.5 and 2C rates) Also note low irreversible loss ~14% and 0.1 % loss per cycle. Increase of capacity with cycle # for the 2C sample may be due to kinetic limitation of the amorphous Si sample in the initial cycles, which is mitigated gradually in later cycles by morphological changes in the electrode, decreasing the effective diffusion distance for the lithium ions from the electrolyte. Specific Capacity (mah/g) High Capacity and Excellent Rate Capability of 250 nm Silicon Thin Film Anode (Area = 1 cm 2 ) 4199 mah/g : Theoretical Capacity of Pure Silicon Anode Material C/2.5 rate (100uA/cm 2 ) 2C rate (500uA/cm 2 ) C/2.5 Charge C/2.5 Discharge 2C Charge 2C Discharge mah/g : Theoretical Capacity of Conventional Graphite Anode Material Used in Commercial Lithium-Ion Batteries Cycle Number 2 µm As-Deposited SEM : C/2.5 rate 30 cycles 2 µm 2 µm Si EDAX : C/2.5 rate 30 cycles

11 Multilayer Film Schematic Silicon (250 nm) Carbon (50nm) Cr (10 nm) Copper Foil (25 µm) Note: Drawing not to scale Note: E carbon ~ GPa

12 Cycling of Multilayer Film, C/2.5 rate 3500 Specific Capacity (mah/g-si) dq/dv (mah/gv) discharge charge Cell Potential (V) Cycle Number SEM image of a 500 C-4h annealed 250 nm Si/50nm C/10 nm Cr/Cu multilayer thin film cycled at 0.4 C after 75 cycles, 5000x. Interface dynamics control of the nanoscale films is the key to improved electrochemical performance

13 Supercapacitors: Potential Energy Storage Systems Power Density (W/kg) s 40s 6 min Supercapacitor 1 h 10 h Pb/Acid Zinc-Air NiMH 100 h Alkaline Li-ion Ni-Cd Fuel Cell Primary Li 1000 h Leclanche Electrochemical Capacitor Ultracapacitor Supercapacitor Primary Battery Leclanche Primary Li Alkaline Secondary Battery Pb/Acid Zinc-Air Ni-Cd NiMH Li-ion Energy Density (Wh/kg) Fuel Cell Ragone Plot

14 ψ 0 _ + + ψ d ( ζ ) Gouy-Chapman Diffuse Layer 1/k Stern Layer Outer Helmholtz Layer Inner Helmholtz Layer H 2 O (Solvent) ψ = ln ze + 1 γ exp( κx) Cation _ Anion Neutral Molecule Non-Faradaic : No electron transfer Ex) Dielectric Capacitor Electric Double Layer Capacitor σ = (8k B 2 k B Tc o T ε ε ) o r 1+ γ exp( κx) zeϕo γ = tanh 4kBT sinh ze 2k 1/ 2 ϕo K B : Boltzmann constant ϕ 0 : Potential at (x=0) z : Ion valency x : Distance from the surface T : Temperature ε o : Permittivity of free space ε r : Solvent dielectric constant C o : Electrolyte concentration B T

15 (g = 0) (g > 0) θ 1 θ θ B = f (V) θ A = f (V) C φ (= q dθ/dv) V 0.5 C φ θ 2 V 0.5 C φ Ox + ze Red Faradaic : electron transfer Ex) Battery, Pseudocapacitor θ : Coverage V : Voltage K : Reaction rate constant g : Adsorption isotherm constant F : Faraday constant C φ θ = 1 θ dθ = dv VF K exp exp( ± gθ ) RT F RT VF K exp RT VF 1+ K exp RT 2

16 Competitive Systems Material Gravimetric Capacitance (F/g) Electrolyte Potential Window (V) Scan rate (mv/s) Reference Carbon Multi-walled carbon nano-tubes Carbon nano-tubes Mesoporous Carbon ~ wt% H SO wt% H SO 2 4 1M LiPF mA 5 Niu et al. Ma et al. Zhou et al. Transition Metal Oxide RuO 2 RuO 2 xh 2 O RuO 2 on Carbon Tape Amorphous MnO 2 H 2 O NiO x Co(OH) Xerogel 2 Fe 3 O ~ M H SO M H SO M H SO 2 4 1M KCl 1M KOH 1M KOH 1M Na SO Raistrick et al. Zheng et al. Long et al. Lee et al. Liu et al. Lin et al. Wu et al. Polymer Poly3- (3,5-difluorophenylthiophene) Poly(dithienothiophene)-pDTT pdtt Polyaniline (PANI) Polyaniline (PANI) on Stainless Steel 23 ~ 30mAh/g ~1300 Poly(acrylonitrile) Poly(ethylene oxide) PEO Poly(methylmethacrylate) PMMA 1M LiClO 4 in PC Searson et al. Arbizzani et al. Clement et al. Prasad et al.

17 Transition Metal Nitrides: Novel Capacitor Materials Good Electrical Conductivity High rate electrochemical response of the capacitor depends on electrical conductivity of the electrode and the ionic conductivity of the electrolyte Chemical Stability Cost The electrolyte widely used are corrosive (H 2 SO 4 or KOH) High Mass Density (g/cm 3 ) To increase volumetric capacitance (F/cc) High Specific Surface Area (m 2 /g ) To increase the electrode-electrolyte interface area for charge storage

18 Material Structure Density (g/cm 3 ) Electronic Conductivity σ 10-2 Ω - 1 m -1 M.W. RuO 2 Tetragonal (P4 2 /mnm) Carbon - < 1 < TiN Cubic (Fm3m) VN Cubic (Fm3m) ZrN Cubic (Fm3m) NbN Cubic (Fm3m) Mo 2 N Cubic (Pm3m) TaN Cubic (Fm3m) Ta 3 N 5 Orthorhombic (Cmcm) < WN Cubic (Pn3m) Ref: 1) J. P. Zheng, P. J. Cygan, and T. R. Jow, J. Electrochem. Soc., 142(8), (1995) p ) M. Wixom, L. Owens, J. Parker, J. Lee, I. Song, and L. Thompson, Electrochem. Soc. Proc., 96 (25) (1999) p. 63 3) P. T. Shaffer, Handbook of High-Temperature Materials, Vol.1, pp Plenum Press, New York, (1964)

19 TEM Analysis: Morphology of the Nanocrystalline Nitirdes VN500 o C VN-400 o C

20 Electrochemical Response of the Nanocrystalline Nitrides Gravimetric Current (I/g) mV/s 50mV/s 25mV/s 10mV/s 2mV/s Potential V (vs. Hg/HgO) Gravimetric Capacitance (F/g) mg/cm mg/cm mg/cm mg/cm mg/cm Scan Rate (mv/s) Figure 2 The (a) cyclic voltammogram of nanocrystallites synthesized at 400 o C and (b) gravimetric capacitance (F/g) versus VN nanocrystallites loading scanned at various rates (2~100mV/s) in 1M KOH electrolyte.

21 25 x 10 3 O 1s x 10 3 O 1s V 2p3 16 V 2p3 Counts 15 V 2p1 Counts V 2p (a) 6 4 (b) Binding Energy (ev) Binding Energy (ev) XPS: Presence of V 2 O 5 before cycling Transmittance (%) (a) (b) (c) OH stretching VN 400 o C VN 400 o C in 1M KOH for 24h VN 400 o C cycled 10times at 2mV/s XPS: Formation of V 2 O 3 after 200 cycles 970 V=O 798 V-O The FTIR spectra of (a) VN powder, immersed in 1M KOH solution for 24h and cycled 200 times and XPS spectra of VN powder (b) before and (c) after electrochemically cycled for 200 times. Controlled oxidation is the key minimal reduction in electronic conductivity Wavenumber (cm -1 )

22 Major Benefits of Direct Methanol Fuel Cells High Efficiency of Energy Conversion Enhanced utilization of fuel Lower carbon dioxide emissions Direct Generation of Electrical Energy Losses in mechanical to electrical energy conversion is avoided Low temperature operation Low emissions of nitrogen oxides, carbon monoxide and particulates

23 Potential solution for portable applications Near Ambient temperature operation High energy density required Easy to handle fuel 2W 20W W 500W 1-5kW DIRECT METHANOL FUEL CELL SYSTEM METHANOL FUEL CELL

24 CO 2 Emissions and Efficiency for Traditional ICE and methanol or hydrogen fed Fuel Cells G. Cacciola et al. J. Power Sources 100 (2001) 67-79

25 Drawbacks Expensive materials Platinum catalysts, fluoropolymers Components that are expensive to manufacture Lack of fuel flexibility Requires clean fuel; e.g. sulfur free Electrochemical reactions are sensitive to poisons in the environment

26 Direct Methanol Fuel Cell Concept Anode (Pt-Ru) CH 3 OH + H 2 O CO 2 + 6H + + 6e - - LOAD + 6e - 6e - CO 2 <3% Methanol / water CO 2 3H 2 O + H + H + H 2 0, N 2, O 2 6H + H + H + H + H + FUEL 3% Methanol / water H 2 O + CH 3 OH H + H + H + H + H + H + 6H + + 3/2 O 2 OXIDANT Air (O 2 ) - Electrode (Anode) + Electrode (Cathode) Cathode (Pt) PROTON EXCHANGE MEMBRANE (PEM) 6H O 2 + 6e - 3H 2 O

27 Synthesis of Catalysts Most synthetic approaches in the literature are based on the reduction of Halide precursors of Pt and Ru Limitations: Need for several washing steps to eliminate unwanted by-products Tedious process leading to loss of active material Poisoning of the catalyst Need for improved catalyst using non-halogen containing precursors Objective: Develop novel sol-gel based methods using non-halogen precursors for generating Pt-Ru catalysts High surface area Pt to Ru ratio close to unity High electrochemical activity (low polarization of the electrode)

28 Morphology Characterization Transmission Electron Microscopy (TEM) TEM micrographs of the powder possessing the highest surface area ( m 2 /g) with the best electrochemical activity (optimized sample) Diffraction pattern indicates single phase Pt-Ru composition HRTEM images show 2-4 nm sized crystallites

29 Electrochemical Characterization Anode polarization data Results of electrochemical tests conducted on the powders derived using a large excess of tetramethylammonium hydroxide [TMAH : (Pt-acac+Ru-acac) = 1.75:1 and Pt:Ru = 1:1] Potential vs H 2 /H SSA = 97.9 m 2 /g E (25 o C, measured) E' (25 o C, corrected) E (60 o C, measured) E' (60 o C, corrected) y = x R= y = x R= Potential vs H 2 /H SSA = m 2 /g E (25 o C, measured) E' (25 o C, corrected) E (60 o C, measured) E' (60 o C, corrected) y = x R= y = x R= Ln(mA/g) Ln(mA/g) (a) Two step heat treatment of 400/3h/1%O 2 (3 g) (0.5g)300/3h/1%O 2 IV-1-1 (b) Two step heat treatment of 300/2h/1%O 2 (3 g) (0/5g)300/4h/1%O 2 IV-1-2

30 Comparison of catalytic activity Sample Specific surface area (m 2 /g) Intercept on y axis in polarization curve Slope in polarization curve Cell potential (V) at ln(ma/g)=8 (60 C) 25 C 60 C 25 C 60 C JPL Giner Johnson- Matthey TM AH:acac=1.25:1 Pt:Ru = 1: TM AH:acac=1.25:1 Pt:Ru = 1: TM AH:acac=1.25:1 Pt:Ru = 1: TMAH:acac=1.75:1 Pt:Ru = 1:1

31 Sample identification Surface area(m 2 /g) Phase/phases present Lattice parameter (nm) Carbon content (%) JM catalyst 70 Pt(Ru) JM catalyst- (600 o C/6h in Ar) - Pt(Ru) ) +Ru Sample A (500 0 C/6h in Ar) 1 Pt(Ru) ) + C Sample B (Sample A: C/2h in Ar- 1% O2) 158 Pt(Ru) ) + C Sample C (Sample B: C/2h in Ar- 1% O 2 ) 120 Pt(Ru)

32 Prospects for Future Nanomaterials show potential for energy storage Nanoscale phenomena in batteries, fuel cells and supercapacitors still need to be studied Considerable potential for integration, theoretical and experimental studies