Metal Hydride Test Station (top) and alloy oxidation in bare metal hydrides (bottom) Bare LaNi Sn Cobalt coated LaNi 4.

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1 Surface Modification to Improve Hydrogen Entry Efficiency and Storage Capabilities of Metal Hydride Alloys Branko N. Popov, Durairajan Anand, Bala S. Haran and Ralph E. White Center For Electrochemical Engineering, University of South Carolina Objectives: To develop anode materials for NiMH cells with high capacity and constant cycle life Approach: (1) Recognize the importance of particle pulverization and corrosion processes in developing high cycle life alloys. () Modify the hydriding and dehydriding kinetics at the surface by electroless coatings. (3) Use EIS to understand the alloy oxidation during cycling. Accomplishments: (1) Developed a insitu technique to calculate particle size and evaluate pulverization in MH alloys during cycling. () Established a methodology for determination of thermodynamic, transport and kinetic parameters of hydride alloys (3) Developed superior electrodes with high capacity, long cycle life and good energy density by various electroless coatings Impact & Transition: (1) Would lead to the development of high stability, low Co content alloys () Modifying electrode kinetics would lead to the development of Magnesium based A B hydrides R ct (Ω) Bare Ni 4.7 Sn mah 1 mah 3 mah 3 mah 00 mah 340 mah 181 mah 340 mah Cobalt coated Ni 4.7 Sn Number of cycles Metal Hydride Test Station (top) and alloy oxidation in bare metal hydrides (bottom)

2 NiMH Battery At Ni Electrode NiOOH + H discharge + charge At MH Electrode discharge MH + OH M + HO + e charge Cell Reaction discharge NiOOH + MH Ni(OH) + M charge Overcharge Reactions 4OH O + e Ni(OH) OH O + H 1 H O + e H + O + OH 4e (0.401 V) (0.88 V) Loss in cycle life due to metal hydride electrode

3 Motivation To study various coatings in improving the performance of metal hydride alloys To develop an optimized coating for high alloy capacity and long cycle life through electrochemical and material characterization

4 Problem Definition Capacity Fade Capacity decreases during cycling. Causes for capacity decay Particle pulverization due to volume expansion/contraction during hydrogen intercalation. Alloy oxidation to inactive material and poor contact of the oxidized alloy Capacity (mah/g), Co, Al and Y (wt %) µ m >50 µ m N um ber of cycles Alloy Segregation Ni Co Y Al Number of cycles

5 Alloy Pulverization 300 µm 60 µm SEM of Hydride Alloy After 30 and 650 Cycles in Toshiba Cells

6 Corrosion Mechanism in Ni 5X Sn X Alloy ( OH) + Ni5xMx 3/ Ni + 5xMx + 3HO 3 H Ni 5xMx + 3OH 3 5x x 3e ( OH) + Ni M Ni + 3H O + 3e 3OH + 3/H O EPMA analysis showing lanthanum oxidation during cycling A. Durairajan, Bala S. Haran, B. N. Popov, and R. E. White, Pulverization and Corrosion of Bare and Cobalt Encapsulated Metal Hydride Electrodes, Journal of Power Sources, January 000.

7 Segregation of Hydride Alloys diffuses to the surface of the alloy and is oxidized

8 Alloy Segregation 0, Co, Al and Y (wt %) Ni Co Y Al Ni (wt %) Number of cycles

9 Capacity Decay for Ni 4.7 Sn 0.4 Alloy Capacity (mah/g) Number of cycles

10 Conventional Approach To Solve Capacity Fade PhilipsHughes Aircraft Co. and BNL tried with partial substitutions in the alloy to stabilize the electrode thermodynamics Limitations Does not eliminate the problem of alloy corrosion during cycling. Does not prevent the surface oxidation and subsequent segregation of active material. Substitutions decrease the capacity of the alloy.

11 Understand the alloy pulverization and optimize the particle size to reduce pulverization. Recognize the importance of corrosion processes in developing high cycle life alloys. Prevent the oxidation of the alloy during cycling. Eliminate any shedding of pulverized active material. Modify the kinetics of the hydriding/dehydriding reaction at the surface Our Approach

12 Objectives Develop novel techniques for electrochemical characterization of metal hydride alloys. Develop a technique for synthesizing AB 5 type hydrides loaded with nanosized catalytic particles of Cu, Pd, Ni, Co. Study the electrocatalytic ability and cycling performance of these novel materials. Optimize the loading of catalyst on the surface of hydrides

13 Plan of Study Alloys Studied Ni 4.7 Sn 0.4 Ni 4.5 Al 0.75 Ni 4.5 Al Ce 0.35 Ni 3.55 Co 0.75 Mn 0.4 Al 0.3 Encapsulation with Nanosized Amorphous Cu, Pd, Ni, Co Particles Material Characterization SEM, TEM Analysis BET Analysis EPMA, EDAX, XRD Electrochemical Characterization Cycling studies AC Impedance Cyclic Voltammetry, Linear Polarization

14 Impedance Model for Determination of Diffusion Coefficient and Particle Size discharge x, ads + xoh M + xho xe charge MH + Assuming Fickian diffusion and solving for the concentration of hydrogen in the particle c s = o c p a MH jwmhr V 1 FD ( ε) pr D Faradaic Impedance is given by sinh pr cosh D dη Z = dj pr D sinh pr D

15 Impedance at the interface Z ( ω) = η j s + ω coth ( 1 i) σ [( 1 + i) ψ] ( 1 i) ψ σ = ( j/ c ) s wmh ( j/ η ) a V( 1 ε) F D s MH Im = σ ω ( T + T ) ( ω) = Re i Im Z + Re ( ) 1 T + T 1 = σ ω ( T T ) ( ) 1 T + T 1 ψ = ωr D

16 Increasing the diffusion coefficient reduces the time for diffusion Hence the transition appears at high frequencies Imaginary Z (Ω) µm 5.0x10 10 cm /s Increasing ω.5x10 10 cm /s 1.5x10 10 cm /s 8.0x10 11 cm /s Real Z (Ω) 5.0x10 11 cm /s Semiinfinite diffusion Infinite diffusion Transition Imaginary Z (Ω) µm 0 µm µm 5 µm Real Z (Ω) Particle size in contrast to diffusion coefficient leads to larger diffusion times Hence transition shifts to lower frequencies

17 Particle Size Determination using EIS Imaginary Z Real Z Impedance response of cobalt coated alloy characterized by semiinfinite, transition and finite diffusion regimes.6 Exponential fit of Nyquist transition regime Imaginary Z Im = * e (0.4609*Re) Particle Size = 3 µm Real Z

18 Determination of hydrogen diffusion coefficient Governing Equation: Analytical Solution: c c s c c o o ( rc) ( rc) t = 1 + = D r ( ) a 1 π r n = 1 n nπ r a e Dn π a t Constant surface concentration: log 6 FD i = log ± o da Constant flux at the surface: D = a 1 Qo 15 i ( c c ) s π D 303. a t i a ; Q l = Qo τi= ± 15 D τ G. Zheng, B. N. Popov, and R. E. White, Electrochemical Determination of the Diffusion Coefficient of Hydrogen through a Ni 4.5 Al 0.75 Electrode in Alkaline Aqueous Solution, J. Electrochem. Soc., 14, (1995).

19 Hydrogen Diffusion Coefficient Determination ma/g 60 ma/g Potential (mv) D a 1 = Q 15 i Time (s) o = τ 4 X10 5 s 1

20 Particle Size Determination using EIS Imaginary Z Impedance response of cobalt coated alloy characterized by semiinfinite, transition and finite diffusion regimes Real Z.6.4. Exponential fit of Nyquist transition regime Imaginary Z Im = * e (0.4609*Re) Particle Size = 3 µm Real Z

21 Measurement of Electrochemical Kinetic Parameters Tafel: η = 3. RT 3. RT log i o log i c ; η = αnf αnf Microcurrent current density: i C ( 1 β ) F OH ( p) η RT H O ( p) = i o e C C OH ( b) H O ( b) C e β F η RT RT F ln i i o i C OH ( i) i = i o C OH ( b) e tanh K ( 1 β ) F η ( ) ( ) K C RT H O i C H O ( b) e β F η RT i K F i RT io LM e = + A FD C s H O H O e ( 1 ) β β F η η i RT C O ( b) G. Zheng, B. N. Popov and R. E. White, "Application of Porous Electrode Theory on Metal Hydride Electrodes in Alkaline Solution, "J. Electrochem. Soc., 143, 435 (1996).

22 Determination of Exchange Current Density 0.89 ( 1 β ) F η i RT i = io e e i F o i = η RT R p = RT Fi o β F RT η i Potential (V) % SOC 88 % SOC 63 % SOC 7 % SOC 14 % SOC Current (ma/g) B. N. Popov, G. Zheng, and R. E. White, Determination of Transport and Electrochemical Kinetic Parameters of MH Electrodes, Journal of Applied Electrochemistry, 6, , (1996).

23 Electrode Thermodynamics ch arg e M + xh O + xe MH + xoh Electrode potential: Φ M eq discharg e o RT a H = Φ ln F a M = Free Metal Sites x, ads Thermodynamic model for excess energy: G = RT ( 1 c ) γ + c [ ln ln γ ] E s M s H A ln γ H = RT ln γ A M = RT c ( 1 c ) Electrode potential considering nonidealities in in γ: γ: Φ eq o = Φ RT F ln c s ( 1 c ) s s s A + s F [ c 1] B. S. Haran, B. N. Popov and R. E. White, "Theoretical Analysis of Metal Hydride Electrodes: Studies on Equilibrium Potential and Exchange Current Density," J. Electrochem. Soc., 145, No. 5, (1998).

24 Change in Equilibrium Potential with SOC Experimental Model 0.90 Φ eq (V) State of Charge (%)

25 Electrode Kinetics Modified ButlerVolmer expression for hydride electrode: j = j o ( ) ( c β Φ Φ c ) s s 1 1 β e RT F c s β c ( c ) s e β F ( 1 β ) RT F Φ o A o A Φ c s F ( 1) ( ) ( β ) Concentration dependent exchange current density: ( 1 β ) ( 1 ) q β jo = jo, ref c s p cs e j o, j = o, ref 05. ( ) e p+ β q p [ ( ) ( ) ] 1 1 β + β A RT p c s q cs 50% SOC 05. A RT s [ p+ β ( q p) ]

26 Change in Exchange Current Density With State of Charge j o (ma/g) Experimental Model fit State of Charge (%)

27 Microencapsulation Surface treatment of the alloy by electroless plating of a porous thin film of copper, nickel, palladium or cobalt Why Microencapsulation? Microencapsulation of the alloy particles will: act as a barrier film for protecting the alloy surface from oxygen facilitate the charge transfer reaction on the alloy surface thereby improving the electrocatalytic activity increase the electrical and thermal conductivity of the alloy prevent shedding of active material from the electrode and hence reduce the effect of pulverization

28 Copper Microencapsulation Developed a deposition technique for coating AB 5 hydrides with Cu. Determined the electrochemical parameters for Cu coated Ni 4.7 Sn 0.4 alloys Symmetry Factor = 0.53 Theoretical Capacity = 75 mah/g D/a = 3.1 X 10 5 s 1 Exchange current density = 4 ma/g (15% above bare alloy) Cu coating acts as a microcurrent collector and facilitates the charge transfer on the surface of the particle. G. Zheng, B. N. Popov, and R. E. White, Determination of Transport and Electrochemical Kinetic Parameters of Bare and Copper Coated Ni 4.7 Sn 0.4 Electrodes in Alkaline Solution, J. Electrochem. Soc., 14, (1996).

29 Palladium Microencapsulation Synthesized Pd loaded Ni 4.5 Al 0.75 with different amounts of Pd on surface Studied the various resistances for Pd coated Ni 4.5 Al 0.75 electrode using electrochemical impedance spectroscopy Determined the electrochemical parameters for Pd coated Ni 4.5 Sn 0.75 alloys Hydrogen reaction order = 0. Theoretical Capacity = 95 mah/g Symmetry factor = 0.53 Exchange current density = 70 ma/g Pd coating increases the capacity of the metal hydride alloy. (95 mah/g compared to 1 mah/g) G. Zheng, B. N. Popov, and R. E. White, Electrochemical Investigations of Bare and PdCoated Ni 4.5 Al 0.75 Electrodes in Alkaline Solution, J. Appl. Electrochemistry, 14, June (1998).

30 + Nickel Microencapsulation Overall Reaction Ni + H PO + HO Ni + H + HPO3 + H Catalytic Decomposition of Hypophosphite Ion Catalytic surface H PO + HO H + HPO3 + Ni Ni Deposition + + e Ni + H + e H Deposition of Phosphorus + H PO + H + e P + HO + + e

31 After 3 minutes Ni Ni Ni P Pd Sn P P P Pd Pd Pd Pd Pd Pd Sn Pd Sn Pd Pd Sn Sn Pd Pd Sn Sn Sn Sn Sn Sn Ni Ni After 6 minutes Ni Ni Ni P P P P Pd Pd Pd Pd Pd Pd Sn Sn Sn Sn Pd Sn Sn Sn Pd Sn Pd Pd Sn Sn Sn Sn Ni Ni After 13 minutes Ni Ni Ni P P P P Pd Pd Pd Pd Pd Pd Sn Pd Sn Pd Sn Pd Pd Sn Pd Sn Sn Sn Sn Sn Sn Ni Ni 4 6 8

32 TEM and SEM analysis after complete Ni plating 16 µm

33 Novel Electroless Deposition Process Onestep process Does not involve expensive SnCl and PdCl Electroless coating involving no external current Activation and deposition are done in a single bath

34 BET Surface Areas Before and After Plating in m /g Particle Size Before After 538 µm µm µm µm > 50 µm

35 Co Microencapsulation Experimental Pellet electrode was prepared from cobalt plated Ni 4.7 Sn 0.4 alloy Cycling and characterization studies were done in a 3 electrode setup with Ni counter electrode and Hg/HgO reference electrode Electrode was activated with 10 chargedischarge cycles B. S. Haran, B.N. Popov and R. E. White, Studies of Electroless Cobalt Coatings for Microencapsulation of Hydrogen Storage Alloys, J. Electrochem. Soc., 145, No. 9, (1998).

36 Chargedischarge Studies Objective: to determine the dependence of exchange current density, equilibrium potential on the SOC charge the electrode under a constant current mode until the hydrogen content reaches its saturated value perform polarization studies after the electrode has stabilized discharge the electrode at a constant current for a specific period of time to reach the desired SOC repeat the studies uptil the electrode is discharged to a potential of 0.6 V

37 ChargeDischarge Characteristics of Bare and Cobalt Coated Ni 4.7 Sn Bare Discharge Potential (V) Bare charge Capacity (mah/g) Co plated Discharge Co plated charge 4

38 Discharge Characteristics of Cobalt Coated Ni 4.7 Sn 0.4 at different Charging Times Potential (V vs. Hg/HgO) hours 3 hours 5 hours 7.5 hours 10 hours Time (hours)

39 Effect of Nanosized CoP on Discharge Capacity Hydrogen Absorption/Desorption discharge x, ads + xoh M + xho xe charge MH + Cobalt Faradaic Reaction Oxdn Co + OH Co(OH) + Rdn E = 930 mv vs. Hg/HgO [1] e Cobalt Encapsulated Alloys Involved Reactions E = 800 mv vs. Hg/HgO [] Cobalt Faradaic Oxidation Hydrogen Absorption/Desorption Discharge Capacity Utilization

40 Discharge Characteristics of Pure Cobalt and Cobalt Coated Ni 4.7 Sn 0.4 Electrodes Potential (V vs. Hg/HgO) mah Pure Cobalt 1 Cobalt mixed Ni 4.7 Sn Capacity (mah/g)

41 Cobalt Utilization Studies Charge time (hours) Total discharge capacity (mah/g) Discharge capacity due to hydride (mah/g) Discharge capacity due to cobalt (mah/g)

42 CV s before Activation Current (ma/g) Co coated Co mixed Potential (V vs Hg/HgO)

43 CV s after Activation 00 Current (ma/g) cobalt coated cobalt mixed Potential (V vs Hg/HgO)

44 Cycle Life Studies 400 Capacity (mah/g) Co Mixed Bare Co encapsulated, Thickness = µm Co encapsulated, Thickness = µm Number of cycles

45 Cobalt Utilization Studies Coating thickness (µm) Capacity due to cobalt (mah/g) Cobalt utilization (%) Cycle life Capacity due to cobalt Cobalt utilization = x100% Theoretical Co capacity (900 mah/g) A. Duraiarajan, B. S. Haran, B. N. Popov and R. E. White, Cycle Life and Utilization Studies on Microencapsulated AB 5 Type Metal Hydride, Journal of Power Sources, January 000.

46 Change in Exchange Current Density of Ni 4.7 Sn 0.4 Alloy Before and After Coating Exchange current density (ma/g) Nickel plated NiCo plated Bare alloy CoNi plated State of charge (%)

47 Change in Equilibrium Potential of Ni and Co Coated Alloy Potential (mv) Nickel plated Bare alloy Cobalt plated NiCo plated SOC (%)

48 Cycling of Co coated Ni 4.7 Sn 0.4 alloy in 6 M KOH Capacity (mah/g) CoP plated CoNiP plated Bare alloy Number of cycles

49 Electron Probe Microscopic Analysis on Cobalt Encapsulated Alloy EPMA for Nickel (left) and nthanum (right). Nickel and nthanum are deficient on the surface. 80 µm EPMA for Cobalt (left) and Oxygen (right). Cobalt is uniformly coated over the surface and is present as Co(OH).

50 Pulverization in Bare and Cobalt Coated Ni 4.7 Sn 0.4 electrodes with cycling Number of cycles Particle Size, µm µm Bare Ni 4.7 Sn 0.4 Co coated 37 µm µm 34 mm 33 µm Number of cycles

51 Conclusions Potentiostatic and galvanostatic technique has been developed for determination of the diffusion coefficient of hydrogen through MH electrodes. Transport and electrochemical kinetic parameters of bare and Cu/Pd coated alloys were determined using porous electrode theory. Cu and Pd coatings increase the electrocatalytic activity for hydrogen entry into the alloy. Developed a theoretical model to study the performance of the hydride electrode. Studied the change in electrode parameters as a function of SOC.

52 Conclusions Developed a novel onestep plating process for microencapsulation of hydride alloys Deposited Co coatings on the surface of Ni 4.7 Sn 0.4 alloy using the novel deposition technique Studied the unique behavior of these coatings in alkaline solutions Developed superior electrodes with constant capacity and longer cycle life using CoP nanostructures A tenfold increase in cycle life and a 5% increase in capacity have been achieved using these coatings

53 Other Accomplishments Effect of temperature on performance of MH electrodes has been determined. The electrode capacity and utilization were determined for different electrode weights and binder contents. Did electrochemical and material characterization on NiCoP and CoNiP coated Ni 4.7 Sn 0.4 alloys Determined the hydrogen diffusion coefficient in hydride alloys using impedance spectroscopy

54 Future Directions Synthesize surface modified AB 5 and AB alloys Synthesis of nanocrystalline alloy electrodes through ball milling. Propose to synthesize A B alloys Mg doped with Ni, Co, Ti, Zr alloys. Form a Nirich surface layer and increase surface area, improve reactivity and corrosion resistance

55 Acknowledgments Financial support by the Exploratory Technology Research (ETR) Program, which is supported by the Office of Transportation Technologies (OTT) of the US Department of Energy (DOE), Subcontract No is acknowledged gratefully.