The 4 th SUNBEAM Workshop Structural and Electronic properties of platinum nanoparticles studied by in situ x-ray x diffraction and in situ x-ray x absorption spectroscopy Hideto Imai Fundamental and Environmental Research Laboratories NEC Corporation
Micro Fuel Cells Portable batteries in the Ubiquitous Networking Society Potential successor to Li - ion batteries Higher energy capacity Longer usage period Smaller size Instantly rechargeable Fuel (Methanol or hydrogen) Anode catalysts Proton conducting membrane CO CH 3 OH + H O CO + 6H + + 6e - H + Cathode catalysts O + + 4H + + 4e - H O e - Fuel cell-powered laptop computer (A prototype) Air (O ) H O
Technological hurdles Energy Loss at the cathode electrode Cathode Reaction Oxygen Reduction Reaction Cathode 1. O + 4H + + 4e - = H O E 0 = 1.3 V Catalytic activity of the current catalyst (Pt) is not enough Potential (V) 0.8 0.6 0.4 Cell Voltage (PEFCs) Voltage loss at cathode Cell Voltage (DMFCs) Elucidation of ORR mechanism Surface morphology Anode Intermediate species Electronic state of platinum 0. H H H+ + e - CH OH + H 3 O CO + 6H+ + 6e - Current density (A/cm ) Current vs. Voltage (Polarization) curve for PEFCs and DMFCs
Stability of platinum in aqueous systems Fuel cell operating region Hydrogen adsorption Double layer Oxide formation.0 Potential (V) 1.5 - PtO 3 O +4H + +4e - =H O PtO Pt PtO or Pt(OH) H + +e - =H Current (A) - Pt-H Pt Pt-OH? H + + e - H Pt-O? H O O + 4H + + 4e - - 0 4 6 8 ph 10 1 14 -x10 - Potential (V vs. RHE) 1.5 The potential - ph equilibrium diagram for the system platinum - water Cyclic voltammogram for carbon supported platinum nanoparticles in M H SO 4
In situ XRD and XAFS measurements Platinum nanoparticles (supported on a carbon) ca. 3 nm, 50 wt% - Pt loading Potential control Home made three electrode electrochemical cell M H SO 4 electrolyte Potentiostatic mode In situ XRD BL16XU Photon energy 30 kev ( λ = 41nm ) Transmission mode, Imaging plate In situ XAFS BL16B L 3 and L absorption edges Transmission mode
In situ x-ray diffraction 10x10 6 10x10 6 Intensity (a.u.) 8 6 4 (111) (00) (0) V vs. RHE 0.4 V 0.7 V 0.9 V V 1. V 1.4 V Pt (bulk) PtO (bulk) (311) Intensity (a.u.) 8 6 4 V vs. RHE 0.4 V 0.7 V 0.9 V V 1. V 1.4 V Pt (bulk) PtO (bulk) (111) V, V (00) PtO PtO () PtO 0 8 10 1 14 16 θ (degree) 18 0 0 10 θ (degree) 1 The potential dependence of x-ray diffraction patterns for carbon supported platinum nanoparticles
In situ x-ray absorption spectroscopy L 3 absorption edge L absorption edge Absorption (a.u.) 1.5 V vs. RHE V Absorption (a.u.) 1.4 1. 0.8 0.6 0.4 V vs. RHE V 0. 11.5 11.6 11.7 Photon Energy (kev) 13.0 13.5 13.30 13.35 Photon Energy (kev) 13.40 The potential dependence of XAS spectra at the L 3 absorption edge The potential dependence of XAS spectra at the L absorption edge
Fourier transform of Pt L 3 EXAFS Fourier Fourier Transform Transform 30 0. 0 0.1 10 V vs. RHE V Pt-O Pt-O Pt-Pt Pt-Pt V vs. RHE V Fourier Transform 0. 0.1 Pt-O Pt-Pt V vs. RHE V 0 10 1 R (Å) 3 3 4 54 0 1 R (Å) 3 4 5 The k 3 -weighted Fourier transform of Pt L 3 EXAFS The k-weighted Fourier transform of Pt L 3 EXAFS
Structural models for the platinum nanoparticle surface ~ 0.7 V Clean Pt surface (Double layer region) Surface reconstruction cf. Pt-O distance.58å (bulk PtO ) on-top site Pt-OH ~.0Å hollow site Pt-O ~.06Å 1.1 ~ 1.4 V Amorphous like PtO x (several layers) 0.8 ~ V Irreversible OH (or atomic O) adsorption bridge site Pt-OH ~.6Å
XANES spectra 1. Absorption (a.u.) 1.5 V vs. RHE V Absorption (a.u.) 0.8 0.6 0.4 V vs. RHE V 0. 11.55 11.56 11.57 11.58 Photon Energy (kev) 11.59 13.5 13.6 13.7 13.8 13.9 Photon Energy (kev) 13.30 The potential dependence of XANES spectra at the L 3 absorption edge The potential dependence of XANES spectra at the L absorption edge
L 3 and L absorptions in platinum E Conduction band The fractional change in the number of unoccupied d states from that of bulk Pt E F 5d 5/ Spin-orbit coupling 1.6 ev 5d 3/ f d = h T h TPt = ( A 3 + 1.11 Α ) (A 3 + 1.11Α ) Pt h T = ( 1 + f d ) h TPt L 3 p 3/ L p 1/ A i : edge area for the i th edge A i = A isample -A ipt h T : total number of unoccupied d states h T = h Tsample h TPt h TPt = 0.30 (tight-binding + spin-orbit coupling)
Electronic states Unoccupied states in platinum 5d5 orbitals Pt 5d unoccupied states 0.45 0.40 0.35 0.30 bulk platinum Potential (vs. RHE) 1.5 8x10-3 6 4 0 - -4 Current (A) Q (C).5.0 1.5 - - 1.5V 6V Surface oxidation Pt-H PtO x Hydrogen adsorption V Pt-OH Potential (V vs. RHE) The potential dependence of unoccupied states in platinum 5d orbitals The total charge due to the hydrogen adsorption and the surface oxide formation estimated from the cyclic voltammogram
Summary In situ XRD and XAFS studies clearly showed the followings : In the oxygen reduction reaction potential region. Surface reconstruction, adsorption of oxygen species, and formation of platinum oxides Irreversible OH adsorption (0.8 ~ V) Pt-OH at on-top site Pt-O O at 3 fold or 4 fold hollow sites Surface oxide formation (1.1 ~ 1.4 V) amorphous like PtO x (several layers) Charge transfer from platinum The number of unoccupied states in Pt 5d5 orbitals linearly depends on the potential and the coverage of oxygen species The formation of surface oxygen species may significantly reduce the catalytic activity of platinum on oxygen reduction reaction