Nanostructured Ti 0.7 Mo 0.3 O 2 Support Enhances Electron Transfer to Pt : High-Performance Catalyst for Oxygen Reduction Reaction

Transcription

1 Nanostructured Ti 0.7 Mo 0.3 O 2 Support Enhances Electron Transfer to Pt : High-Performance Catalyst for Oxygen Reduction Reaction Seonbaek Ha Professor : Carlo U. Segre Department of Chemical and Biological Engineering Illinois Institute of Technology 1

2 Outline Introduction and background of proton exchange membrane fuel cell Challenges toward reality in fuel cell The understanding of the paper - Experimental design : the electrochemical improvement of using Ti 0.7 Mo 0.3 O 2 - Application of X ray absorption spectroscopy : XANE, EXAF 2

3 Background of PEMFCs (Proton Exchange Membrane Fuel Cells) Polymer Electrolyte Membrane Fuel Cell Anode Cathode Polymer electrolyte Catalyst and catalyst support Fuel Anode : H 2 (fuel) 2H + + 2e - E o = 0 V Cathode : ½ O 2 + 2H + + 2e - 2H 2 O E o = 1.229V Overall : ½ O 2 + H 2 2H 2 O 3 Source : US DOE

4 Challenges of Catalyst and Catalyst Support in PEMFCs 20 wt % Pt/C Catalyst : platinum Support : Carbon black Challenges : Catalytic activity - oxygen reduction reaction (ORR) Stability (durability) - carbon corrosion - loss of Pt during operation Carbon corrosion reaction C + 2H 2 O CO 2 + 4H + + 4e - (E = V vs. RHE) 4

5 Approach to Find New Catalyst Support in PEMFCs High Surface Area (BET measurement) (~ m 2 /g carbon ) High Porosity ( nm pore size carbon ) Electronic conductivity ( > 1 S/cm carbon ) Stable under electrochemical conditions potential cycling at V cyclic voltammetry (CV), linear sweep voltage (LSV) Stable in acidic media (ph = 1, 2) 0.1 M HClO 4, 0.5M H 2 SO 4 Conducting metal oxides are a promising candidate due to higher stability

6 The Research Purpose and Experiment Preparation of Pt/Ti 0.7 Mo 0.3 O 2 in PEMFCs Van Thi Thanh Ho et al., Nanostructured Ti 0.7 Mo 0.3 O 2 Support Enhances Electron Transfer to Pt : High-Performance Catalyst for Oxygen Reduction Reaction, J. Am. Chem. Soc., 133 (2011) Target : high activity of Pt/Ti 0.7 Mo 0.3 O 2 catalyst as compared Pt/C : higher stability of Pt/Ti 0.7 Mo 0.3 O 2 catalyst Electrochemical Measurment NHE (reference electrode), 0.5M H 2 SO 4, 0.1M HClO 4, 7 μl of catalyst ink with 0.5 wt% Nafion and 6.2 mg of Pt/mL Cyclic Volammetry ( V, 25 mv/sec) ORR measurement (0 1.1V, 1 mv/sec) at 1600 rpm Sample preparation 12 mm MoCl 5 and 28 mm TiCl 4 in Teflon-lined autocave (at 200 C, 10 C/min, 2hr) Ti 0.7 Mo 0.3 O 2 and hexachloroplatinic acid in ethylene glycol + NaOH (ph 11) sonication for 30min and then heated (160 C) in micro wave oven Analysis Instruments : XRD, TEM, XANES, EXAFS 6

7 Metal Doped TiO 2 Ti 0.7 Mo 0.3 O 2 Ti 0.7 W 0.3 O 2 TiO 2 phase Annealing temperature Anatase TiO 2 Rutile TiO C 750 C BET Surface area 230 m 2 /g (232 m 2 /g, carbon) unknown Electrical conductivity S/cm (Ti 0.7 Mo 0.3 O 2 ) S/cm (undoped TiO 2 ) 7 Van Thi Thanh Ho et al., J. Am. Chem. Soc., 133 (2011) unknown Deli Wang et al., J. Am. Chem. Soc. 9 (2010)

8 Introduction to X-Ray Absorption Spectroscopy ln (I 0 /I) = µ(e) x µ : absorption coefficient X : sample thickness Energy of absorbed radiation at edge Borh Atomic Model Binding energy of electrons in the K, L, M,.. shells of the absorbing elements 8 Jens Als-Nielsen et al., Elements of Modern X-ray Physics

9 Normalized Absorption Schematics of X-Ray Absorption Spectroscopy XANES : ± 10 ev of edge (x-ray absorption near edge structure) K, L, M edge E 0 : binding energy NEAXFS : within 10 ev 50 ev of edge (near edge x-ray absorption fine structure) EXAFS : 50 ev 1000 ev above edge (extended x-ray absorption fine structure) Coordination number Oxidation state Geometry 9 Local electronic and atomic structure of sample

10 XANE Result of Support Material Mo : 4d5 5S1 MoO 3 :1s 4d (at pre-edge) Average valence state of Mo in Ti 0.7 Mo 0.3 O 2 = Ebbinghaus, S.et al., J. Solid State Chem.156 (2001)194

11 XANE Results of Catalyzed Support Decrease in d-band vacancy of Pt/Ti 0.7 Mo 0.3 O 2 facile e - donation from Ti 0.7 Mo 0.3 O 2 to Pt 11

12 EXAFS Results of Catalyzed Support FCC Pt bulk 12

13 ORR Activity of Catalyzed Support ECSA (electrochemical surface area) Q Pt [C/cm 2 ]= Q total - Q dl Q pt Q dl Pt/Ti 0.7 Mo 0.3 O 2 the highest performance 13

14 Stability Evaluation of Catalyzed Support ~ 8% degradation ~ 50.6% degradation ~ 25.8% degradation

15 Conclusion Higher ORR activity and higher stability of Pt/Ti 0.7 Mo 0.3 O 2 X ray spectroscopy has predictable value in guessing electrochemical performance of catalyst/support for the application of PEMFC Strong metal/support interaction between Pt and Ti 0.7 Mo 0.3 O 2