Development of Thin Film Membrane Assemblies with Novel Nanostructured Electrocatalyst for Next Generation Fuel Cells

Transcription

1 Development of Thin Film Membrane Assemblies with Novel Nanostructured Electrocatalyst for Next Generation Fuel Cells Dr. Bala Haran and Dr. Branko N. Popov Department of Chemical Engineering, University of South Carolina, Columbia, SC Tel: (803) Fax: (803) E mail: popov@engr.sc.edu Web site:

2 The term nano corresponds to So one nanometer corresponds to one billionth of a meter. meter m m centimeter cm m millimeter mm m micrometer µm m nanometer nm m Nanotechnology as the name suggests is the study of materials of nanodimensions In general study of materials varying in size from 1 to 100 nm

3 What are nanostructured materials?? a broad class of materials, with microstructures modulated in zero to three dimensions on length scales less than 100 nm materials with atoms arranged in nanosized clusters, which become the constituent grains or building blocks of the material

4 The Scale of Things -- Nanometers and More Things Natural Dust mite 200 µm Human hair ~ µm wide Red blood cells with white cell ~ 2-5 µm ~10 nm diameter Ant ~ 5 mm Fly ash ~ µm ATP synthase The Microworld The Nanoworld 10-2 m 10-3 m 10-4 m 10-5 m 10-6 m 10-7 m 10-8 m 1 cm 10 mm 1,000,000 nanometers = 1 millimeter (mm) 0.1 mm 100 µm 0.01 mm 10 µm 1,000 nanometers = 1 micrometer (µm) Visible spectrum 0.1 µm 100 nm 0.01 µm 10 nm MicroElectroMechanical Devices µm wide Nanotube electrode Things Manmade Head of a pin 1-2 mm Red blood cells Pollen grain Nanotube transistor 21st Century Challenge O O S O O O O O O O O O O O S O S O S Combine nanoscale building blocks to make functional devices, e.g., a photosynthetic reaction center with integral semiconductor storage O S P O O O O S O O S O S 10-9 m 1 nanometer (nm) DNA ~2-1/2 nm diameter Atoms of silicon spacing ~tenths of nm m 0.1 nm Quantum corral of 48 iron atoms on copper surface positioned one at a time with an STM tip Carbon nanotube ~2 nm diameter

5 Classification of Nanostructured Materials Based on the structure Nanostructured materials vary from zero dimensional atom clusters to three dimensional equiaxed grain structure. Each class has at least one dimension in the nanometer range

6 Atom clusters and filaments are defined as zero modulation dimensionality and can have any aspect ratio from 1 to Multilayered materials with layer thickness in the nanometer range are classified as onedimensionally modulated Layers in the nanometer thickness range consisting of ultrafine grains are twodimensionally modulated The last class is that consisting of three dimensionally modulated microstructure s or nanophase materials R.W. Siegel, Nanophase Materials, Encyclopedia of Applied Physics, vol. 11, VCH Publishers 1994, p 173

7 Synthesis of Nanostructured Materials with Superior Corrosion and Electrocatalytic Properties Synthesis of Nanostructured Materials by Electrochemical Processes Underpotential Deposition (UPD) of monolayers of Zn, Ni, Bi onto hard alloys Novel autocatalytic reduction process for deposition of amorphous alloys (Ni-P, Ni-Co-P) Galvanostatic pulse treatments for deposition of ternary and quarternary composites based on Zn, Ni, Cd, P Superior corrosion and catalytic properties Superior electrocatalytic properties (long cycle life, low self discharge, high rate capabilities) Superior mechanical properties (low rates of hydrogen permeation and corrosion)

8 Underpotential Deposition of Nanostructured Monatomic Layers of Zn, Pb and Bi UPD occurs with a formation of monatomic layers at potentials more noble than the reversible Nernst potential. UPD has been optimized for Zn, Pb and Bi by using the work functions of these metals and the work function of the substrate. The Underpotential shift (E) when the monatomic layers are formed is determined by the difference in work functions in electron volts of both metals. UPD formed monatomic layers of Pb, Zn and Bi on steel surface inhibit corrosion due to lowering of the binding energy of the hydrogen adatoms on Zn, Pb and Bi adsorbates.

9 Autocatalytic Reduction Process for Deposition of Nanostructured Composites One step process No external current is used for deposition. High temperature and large concentration of reducing agent (hypophosphite) during encapsulation leads to hydrogen evolution. Evolved hydrogen penetrates the hydride particles in the bath and results in lowering the particle size. EPMA of cobalt encapsulated LaNi 4.27 Sn 0.24 alloy Nanosized amorphous layers of Co-P, Ni-P are deposited by controlling the substrate particle size, the concentration of Co ++ or Ni ++ in the electrolyte and by controlling the deposition rate (ph, temperature and presence of leveling agents). 1 µm

10 DC and Pulse Deposition of Nanostructured Multilayers The particle nucleation rate and the grain size is controlled by the peak cathodic potential, the pulse period and the relaxation period and the duty cycle. Thin films and nanostructured deposits have been deposited by optimizing the duty cycle and the concentration of leveling agents. The film grain size is proportional to the crystal growth rate and inversely proportional to the nucleation rate. Pulse deposition increases the nucleation rate, decreases the crystal growth rate. Nanostructured Zn-Ni-Cd 1 µm 1 µm Multiple Layers of Zn-Ni

11 Development of Thin Film Membrane Assemblies with Novel Nanostructured Electrocatalyst for Next Generation Fuel Cells Develop superior fuel cell electrodes with better utilization of electrocatalyst Construct membrane electrodes with nanoparticles of carbon and catalyst particles Catalysts - Pt/Ru/Fe/Ni Synthesize mixed alloy catalysts with Pt- Fe-C and Pt-Ni-C Nanostructures will lead to Better utilization of noble metal and hence low electrode cost Decrease cathode polarization

12 Objectives improve the kinetics of oxygen reduction by modifying the electronic and short range atomic order around Pt by developing Pt based binary and ternary (Pt-Fe, Pt-Cu, Pt-Fe-Mn etc) nanocluster assemblies, improve understanding of catalyst structural and electronic properties on kinetics of electrochemical reactions, optimize the structure of the catalyze layer and its interface with the polymer membrane by selective localization of nanostructured catalyst through electrodeposition, and develop a theoretical model, which will explain the processes occurring at the electrolyte/nanostructured electrode interfaces and will help to optimize the performance and activity of the membrane electrode.

13 Specific Tasks to Accomplish Goals Task 1: Chemical Reduction of Pt Binary and Ternary Catalysts on Carbon, Task 2: Synthesis of Pt Binary and Ternary Alloys Through Pulse and Pulse Reversal Electrodeposition, Task 3: Material Characterization of Nanostructured Catalysts, Task 4: Electrochemical Characterization of Thin Film Membrane Assemblies and, Task 5: Theoretical Modeling of Membrane Electrode Assembly.

14 Task 1: Chemical Reduction of Pt Binary and Ternary Catalysts on Carbon Colloidal Method for Nanomaterial Synthesis Previous Accomplishments Developed new technology based on colloidal method, for synthesizing RuO 2 /carbon nano-composite material. Increased specific capacitance and utilization of RuO 2 by decreasing particle size and dispersing evenly over carbon. Improved the power rate at high current discharge.

15 Electrode Preparation using the Colloidal Method Preparation of the colloidal solution using RuCl 3 xh 2 O (39.99 wt% Ru) and NaHCO 3 Adsorption of the colloidal particles using carbon black Filtration using a 0.45 µm filtering membrane Annealing in air Mixing with 5wt% PTFE Grounding to a pellet type electrode Cold pressing with two tantalum grids

16 TEM image of RuO 2 nh 2 O/carbon composite electrode (40 wt% Ru) 25 nm

17 SEM images of RuO 2.nH 2 O/carbon composite electrode 3 µm 3 µm (60 wt% Ru ) (80 wt% Ru)

18 SEM image of RuO 2 deposited on carbon particle by sol-gel method (10.6 wt% Ru) Y. Sato et al. Electrochem. Solid State Lett. 3 (2000) 113

19 Comparison of Preparation Techniques for Ruo 2 /Carbon Composite Electrode Specific capacitance of RuO 2 Particle Size Ru loading limit Structure Annealing temperature Colloidal method 863 F/g 3 nm 40 wt% Amorphous 100 o C Sol-gel method 720 F/g 38 nm 10 wt% Amorphous 150 o C Heat decomposition 330 F/g 2 nm 50wt% Crystalline 320 o C

20 Task 2: Synthesis of Pt Binary and Ternary Alloys Through Pulse and Pulse Reversal Electrodeposition Objective Reducing cost of electrode by decreasing Pt loading Presenting Pt on the surface contacting with polymer electrolyte. Decreasing particle size and increasing activity of Pt

21 Process of Pulse Electrodeposition The rate of nuclei formation, v: k = 2 v k exp 1 η Overpotential of DC deposition η DC = η ln 0 i i DC Overpotential of PC deposition 0 i η = η ln PC 0 i i = i a η PC DC = η DC a 0 t + η ln 0 t t + η ln 0 t off on off on i p : peak current density i a : average current density

22 Previous Accomplishments: Effect of Pulse and DC Electrodeposition at Current Density of 50 ma/cm 2 Pulse 20 nm DC 200 nm

23 Previous Accomplishments: Effect of Particle Size of Pt on the Performance of PEMFC A A B B

24 Task 3: Material Characterization of Nanostructured Catalysts The crystalline structure, particle sizes and unit cell parameters will be determined for all nanostructured materials synthesized in this study. The pore structure of the final materials will be characterized via BET surface area and pore volume, pore size distribution, and mercury porosimetry measurements. The morphology of the various microstructures in them will be determined using scanning, (SEM) and transmission electron microscopy (TEM). Qualititave estimate of the catalyst thickness will be determined using Electron Probe MicroAnalysis (EPMA). Using X-ray diffraction data an attempt will be made to estimate the nature and number of phases present in the final deposit.

25 Task 4: Electrochemical Characterization of Thin Film Membrane Assemblies Fuel Cell Test Station for Electrochemical Studies

26 Task 4: Electrochemical Characterization of Thin Film Membrane Assemblies Tafel and linear polarization will be done to determine the catalytic activity of the electrodes. Electrochemical impedance spectroscopy will be used to determine the structural changes and deterioration behavior of the electrode materials. The rate capability and polarization characteristics of the nanostructured catalysts will be studied as a function of crystalline structure, particle size and inter-atomic distance. The kinetics of the processes occurring at the electrode-electrolyte interface will be determined by using slow scan voltammetry. The power capability of different thin film MEAs will be determined as a function of particle size. This study will provide information on the reactivity of the anode and cathode surface and structural changes of the electrode and will provide means for optimization of the chemical composition of the cathode.

27 Task 5: Theoretical Modeling of Membrane Electrode Assembly We plan to develop a theoretical model, which will explain the processes occurring at the electrolyte/nanostructured hybrid electrode interfaces and will help to optimize the performance and activity of the membrane electrode. Using packing theory, the model will account for incorporating nanosized Pt alloy clusters and large carbon particles over Nafion membrane. Effects of active carbon content in the electrode, different types of active carbon with various internal and external surface areas, discharging current density, and electrolyte salt concentration on the system s performance, will be investigated.

28 Research in Other Areas

29 Development of Novel Supercapacitors Based on Hybrid Metal-C Nanoparticles Capacitors deliver frequent pulses of energy in several electronic circuits Electrochemical capacitors Carbon based - Double layer Metal oxide (Ni, Ru, Co, Mn) - Faradaic reactions Need new devices which bridge gap between double layer and metal oxide capacitors

30 Development of Novel Supercapacitors Based on Hybrid Metal-C Nanoparticles Incorporate nano-particles of Metal Oxide on high surface area carbons Advantages More energy than electrolytic capacitors Extremely high power density Lower resistance than metal oxide capacitors Applications Communication - cellular phones Power conversion - converters, power supplies Pulse power - actuators, air bag detonation

31 OPTIMIZATION OF THE CATHODE LONG-TERM STABILITY IN MOLTEN CARBONATE FUEL CELLS Supported by DOE-FETC OBJECTIVE: To reduce the corrosion of MCFC cathodes and current collectors at high temperature in the melt APPROACH Develop novel fuel cell cathodes with better utilization of electrocatalyst and lower corrosion Construct electrodes with nickel and Co nanoparticles Novel electrodes and nanostructures will inhibit electrode dissolution and hence enhance operation life of the fuel cell

32 DEVELOPMENT OF SUPERIOR CARBON ANODES FOR Li-ION BATTERIES Supported by Sandia National Laboratories OBJECTIVE: To reduce the irreversible capacity loss and capacity fade seen in carbon anodes APPROACH Modify the surface of carbon by incorporating nano-particles of Pd, Ni and Co. Optimize the metal loading on carbon Electrochemically characterize the hybrid metal-c particles Teflon Mould Working Electrode Swagelok three electrode cells Separator Li Foil Counter Electrode Li Foil Reference Electrode Electrolyte (1M LiPF6 in PC:EC:DMC (1:1:3) Current Collector

33 Experimental Ni-composite graphite development through surface modification by dispersing nanosized Ni-composite particles on the graphite Pd-P alloy deposited from an electroless bath PdCl 2, NH 4 OH, NH 4 Cl, NaH 2 PO 2 ; ph=9.0, T= 90 o C Various amounts of Pd were deposited on SFG75 graphite 5%, 8%, 10% and 25% Pd by weight Electrochemical characterization Swagelok three-electrode cell Electrolyte: PC-EC-DMC (ratio of 1:1:3) Physical characterization - SEM, BET

34 SEM Images for Bare and Ni- composite Coated KS10 (a) bare KS10 (b) 10 wt% Ni-composite KS10

35 Surface Morphologies of Pd Dispersed Graphite (a) Bare Graphite (b) 5 wt% Pd - Graphite

36 Initial Charge-Discharge Profiles for Bare and Ni-composite Coated KS10 4 Potential (V vs.li/li + ) A B bare KS10 3 wt% Ni-composite KS10 5 wt% Ni-composite KS Capacity (mah/g)

37 Self-discharge Performances for Bare and Ni-composite Coated KS10 Discharge capacity retained (%) Storage time (days) bare KS10 10 wt% Ni-composite