PtNi alloy catalyst for ORR

1. Background

2. Background
Transpotation
depends primarily on
petroleum fuel base
with a total world
usage of refined
petroleum products
of 90.87 million
barrels a day

3. A Stable Hydroxide-Conducting Polymer
2015.07.05
히엔
3
J. Am. Chem. Soc. 2012, 134, 10753−10756
Midterm presentation

4. Pt cost
efficiency
Loss Pt
surface
Major problems of ORR
- Platinum (Pt) or Pt-based
catalysts often contribute
20-25% to the cost of
PEMFC systems.
- Efficiencies of present
PEMFC systems range
between 35-45%, although
the theoretical limits are
nearly 80% efficiency for
low temperature fuel cells
like PEMFC.
- The dissolution and/or loss
of Pt surface area in the
cathode.
Introduction
OH– bonds tightly to Pt surface
atoms, leaving less room for O2 to
adsorb onto Pt active sites.
 Blocking active sites results in
hinders the rate of cathodic reaction.

5. Approach
This research examined selected cathode materials with well-
characterized surfaces so that the mechanism of action can be
attributed to a specific property (at the atomic and molecular
level) of the surface.
In this way, we can determine:
(i) Whether the kinetics of the ORR are structure-sensitive
(ii) The composition of the topmost surface atomic layers (the
segregation profile)
(iii) How alloying [usually described in terms of the ligand effect
or/and ensemble effect] alters the chemical properties of the
surfaces.

6. Result & Discussion
Surface characterization of the
Pt3Ni single crystals
(LEEDS) low-energy electron
diffraction spectroscopy. (D to F)
(LEIS) low-energy ion scattering (B)
 Composition of the outemost
layer: pure 100% Pt (Pt skin
structures)
(UPS) synchrotron-based high-
resolution ultraviolet (C)  ≠
surface electronic structure
(CV) Cyclic voltammetry as
compared to the voltammetry of
the corresponding Pt single crystal
(gray curves). RHE reversible
hydrogen electrode.
Fig1. A combination of in situ and ex situ surface-sensitive probes and density
functional theory (DFT) calculations was used to study ORR on Pt3Ni(hkl) single-
crystal surfaces.
1. Which surface properties govern the variations in reactivity of PtNi catalysts?
2. How surface structures, surface segregation, and intermetallic bonding affect
the ORR kinetics?
Answer

7. Result & Discussion
LEEDS surface characterization of the Pt3Ni(111). The green dots in this LEEDS
pattern (left) for a single crystal of Pt3Ni(111) reveal a tightly packed
arrangement of surface atoms that repels platinum-grabbing hydroxide
ions and boosts catalytic performance.
 Selling point of Fig 1

8. Result & Discussion
8
(2A,A’) Surface x-ray scattering (SXS) was used to characterize
the position and near-surface composition of the alloy in situ
 1st layer 100% Pt; 2nd layer 52% Ni; 3th layer 87% Pt.
At 0.05V the potential was cycled  both the Pt3Ni(111)
surface structure and the segregation profile are completely
stable.
(2B) Surface coverage calculated from cyclic voltammograms
of Pt3Ni(111) and Pt(111). with the onset of adsorption
on Pt(111)-skin, which consists of the same surface density of
Pt atoms as Pt(111), a dramatic negative shift (≈0.15 V) in Hupd
formation and positive shift (≈0.1 V) in OHad formation
occurred relative to Pt(111).
on Pt3Ni(111), the fractional coverages by Hupd and Ohad
were dramatically reduced by 50% relative to Pt(111), which is
in agreement with the large downshift (0.34 eV) of the d-band
center position on the Pt-skin structure (1C)
(2C) Position of the d-band centers to the fractional coverages of adsorbed hydrogen (H+ + e–
= ӨHupd, where Hupd refers to the underpotentially deposited hydrogen) between 0.05 < E
< 0.4 V, where E is the applied potential,
Hydroxyl species (2H2O = OHad + H3O+ + e–, where OHad is the adsorbed hydroxyl layer) above
0.6 V

9. The DFT calculations show a positive
shift of ∆U° = 0.10 V when the
sublayer has 50% Ni atoms.
 The experiment and theory thus
reach an excellent and quantitative
agreement in this case and clearly
establish an electronic effect of
subsurface Ni on the Pt-OH chemical
bonding.
The kinetics of O2 reduction are
determined by the number of free Pt
sites available for the adsorption of O2
(1 – Өad) and by the ∆Gad of O2 and
reaction intermediates on metal
surfaces precovered by OHad.
the change in the reversible potential the rate of the ORR:

10. (E) ORR currents measured on Pt3Ni(111),
Pt(111), polycrystalline Pt surfaces.
The positive potential shift of 100 mV in
electrode half-potential (∆E1/2) between
ORR polarization curves measured on Pt
poly and Pt3Ni(111) surfaces.
 ӨOHad ↓ on the Pt-skin structure, the
key parameter that determines the unusually
catalytic activity of Pt3Ni(111) is the low
coverage by OHad [i.e., the (1 – Өad) equation
2].
(D) Green scale refers to hydrogen peroxide
production in designated potential region
Because of the lower coverage by Hupd, the
production of peroxide is substantially
attenuated on the Pt-skin surface.
on Pt3Ni(111) the fuel cell relevant
potentials (E > 0.8 V), the observed
catalytic activity for the ORR is the
highest that has ever been observed on
cathode catalysts, including the
Pt3Ni(100) and Pt3Ni(110) surfaces.
Result & Discussion

11. Fig 3. Influence of the surface morphology and electronic
surface properties on the kinetics of ORR.
Result & Discussion
Pt3Ni(100)-skin < Pt3Ni(110)-
skin <<< Pt3Ni(111)-skin
activities increasing in the order
Pt(100) << Pt(111) < Pt(110) (Fig.
3).
Different electronic structure
(|Dd[111]| = 0.34 eV, where
|Dd[hkl]| is the d-band center
shift), the ORR is being
enhanced by factor of 10 on
Pt(111)-skin relative
to that on Pt(111).

12.  The Pt3Ni(111) surface has an unusual electronic
structure (d-band center position) and
arrangement of surface atoms in the near-surface
region. This causes a weakening of the bonds
between the Pt surface atoms and the OH–
molecules. The weakening increases the number
of active sites available for O2 adsorption.
 The overall effect generates an increase in
specific activity for cathodic reaction:
10 times more active than the Pt(111) surface .
90 times more active than state-of-the-art Pt/C
catalysts currently used in fuel cells.
 The next step is to engineer nanoparticle catalysts
with electronic and morphological properties that
mimic the surfaces of pure single crystals of
Pt3Ni(111).  the amount of Pt will be reduced
without a loss in cell voltage, while also
maintaining the maximum power density.
Conclusion

14. S. Guo, S. Zhang, S. Sun, Angew. Chem. Int. Ed. 2013, 52, 8526-8544.

15. Z.-Y. Yang et al. J. Mater Chem. A. 2014, 2, 2623-2627.
Bean pod shaped Fe-C-N Graphene based Fe-C-N
H. R. Byon et al. Chem. Mater. 2011, 23, 3421-3428.
Fe N doped graphene
K. Parvez et al. ACS Nano 2012, 6, 9541-9550.

17. Update researchs
Kinetic activities of the main Pt-based electrocatalyst systems at 0.9 V vs.
Reversible Hydrogen Electrode (RHE): (a) Activities are measured by
rotating disc electrode (RDE) and (b) Activities are measured in
membrane electrode assemblies (MEAs) at 80 °C and 150 kPa saturated
O2. Reprinted with permission from Ref. [9]. Copyright 2012, Nature
Publishing Group.

18. TEM-HRTEM micrographs and their corresponding
particle size distribution (histograms were fitted using
the log-normal function) of the nanostructured AuPt (20
wt%) supported on (a) Vulcan XC 72R and (b)
Ketjenblack EC-600JD. (a) Reprinted and adapted with
permission from Ref. [103]. Copyright 2014, John Wiley &
Sons, Inc. (b) Reprinted and adapted with permission
from Ref. [102]. Copyright 2014, John Wiley & Sons, Inc.
Update research

19. Schematic representation of the
oxygen reduction reaction (ORR)
mechanism by direct pathway
(A: adsorption parallel to the
surface) and indirect pathway (B:
adsorption perpendicular to the
surface). Reprinted and adapted
with permission from Ref. [125].
Copyright 1997, Elsevier.
Update research

20. (a) Koutecky–Levich and (b) jk−1 plot for the determination of jL.
Inset in (b) shows the Tafel plots: data are extracted from ORR after
the durability test in 0.1 M HClO4. (c) Comparison of jk at 0.9 V vs.
RHE in 0.1 M HClO4 and 0.1 M NaOH.
Update research

21. A novel, hollow Fe–N–C hybrid
nanostructures that act as active and
durable electrocatalytic materials
J.H. Lee et al. / Inorganica Chimica Acta 422 (2014) 3–7
Update research

22. 2NH3 3H2 + N2 △H = 46kJ/mol
Ammonia
decomposition
Hien’s Research objective

23. 23