Literature Review: Pd and the ORR

1
Literature Review Pd and the ORR

• Kinetics of Oxygen Reduction on an Epitaxial Film
  of Palladium on Pt(111), J. Phys. Chem. B, 2000
  (Climent, Markovich, Ross)
• Investigation of Hydrogen Adsorption and
  Absorption in Palladium thin films, J.
  Electrochem. Soc. 2004 (Gabrielli, Grand, Lasia,
  Perrot)
• Simcha Singer
• EEL
• 2/9/05

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Literature CV of Pd in N2

• What is occuring???
• Hydrogen Desorption Hads?He-
• Oxide Formation (PdO is formed)
  H2O?Oads2H2e-
• (O2-?O or Pd?Pd2)
• Oxide reduction (PdO reduced) Oads2H2e- ?H2O
• (O ? O2-)
• 4. Hydrogen Adsorption and Absorption He- ?
  Hads

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Paper 1 Epitaxial Pd Film on Pt

• Best Catalysts for ORR are Pt group metals
  RhltPdltPt
• Kinetics also affected by adsorbed spectator
  anions in electrolyte
• Not many studies on Pd single crystal electrodes
  b/c Pd absorbs hydrogen and disorders single
  crystal surface
• Deposited epitaxial layer of Pd on Pt(111)
  surface instead of bulk single crystal Pd (111)
  which absorbs H2

4
Technique/Film Structure

• Mounted Pt(111) surface in RDE config. and
  deposited 1 ML of Pd by cycling electrode in 10-5
  M PdO in 0.05 M H2SO4 at 20 mV/s
• For Pd films gt 10 ML H2 is absorbed
• For Pd films lt 5 ML, H2 isnt absorbed, possibly
  b/c lattice constants of Pt and Pd differ by 0.8
  and strain of 1st layer prevents outer layers
  from expanding and letting in H2

5
CV in H2SO4 and HClO4

• Pt (dashed line) 1st peaks are H ads, 2nd are
  anion ads (HSO4- and OH-)
• For Pd ML In H2SO4 the peaks merge
• In HClO4 anion peak shifts negative (100 mV)
  relative to Pt in same electrolyte and double
  peak in Hads region

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CV in H2SO4 and HClO4

• Interpretation Anion adsorption peaks merging
  (H2SO4)/shifting(HClO4) indicates stronger
  interactions of anions with Pd ML than Pt, due to
  lower potential of zero charge for Pd
• PZC is when sm ss0 (excess charge on electrode
  and in solution)
• They claim multiple Hads peaks due to defect
  sites when forming layer

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ORR

• ORR can either proceed as O24H4e- ?2H2O
• or O22H2e- ?H2O2
• In H2SO4 on Pt ORR proceeds as 4 e- process above
  0.2 V vs RHE
• Pd film ORR inhibited below 0.65 V due to
  adsorbed HSO4-
• Adsorbed HSO4- inhibits initial O2 ads as well as
  formation of Pd pairs which allow breaking O2
  bonds, proceeds as 2e- process

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ORR cont.

• In HClO4 ORR is not as greatly inhibited as in
  H2SO4 b/c OHads occurs at 0.65 V
• Below 0.65 V, on positive sweep, theres no anion
  adsortion, so performance is same as Pt(111) ?
  Full 4 electron process

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K-L Plots

• As expected, lower intercepts for HClO4
  corresponding to higher kinetic currents
• These plots may serve as good comparison for our
  research

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Tafel Plots

• Lower Tafel slopes for HClO4
• Constant slope ? same rate determining step
• Higher Tafel slope for H2SO4
• Changes from 120 to 400 mV/decade
• They propose its due to rds changing from charge
  transfer to adsorption of oxygen (b/c its
  inhibited by anion adsorption)

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Conclusions

• Above 0.8 V vs RHE, kinetic currents are 2 times
  higher in HClO4 than in H2SO4
• Due to lower pzc of bisulfate anion, which is due
  to similarity in its structure with Pt and Pd
• Adsoption inhibition of ORR is higher on Pd film
  than on Pt single crystal alone
• My question is what about oxide
  formation/reduction? They didnt mention it

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Paper 2 Hydrogen Adsorption and Absorption on
Pd Thin Films

• Used 0.1 M H2SO4 electrolyte for RDE
• Electrodeposited Pd thin film on Au electrodes,
  thicknesses 10 ML-10 um
• Used bulk Pd foils for cpmparison, cleaned
  surface of oxides by applying a negative
  potential vs RHE
• Roughness factor of thin films varied from 1 to
  2.7 with thickness of layer

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SEM image of Pd Film
14
CV of Bulk and thin film Pd

• Hydrogen desorption extends to more positive
  potentials on bulk Pd due to diffusion from bulk
• On bulk Pd, adsorption and absorption peaks are
  indistinguishable
• Peaks at .155 and .265 correspond to adsorption
  while more negative peaks are absorption

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More CV data

• Increasing Anodic limit causes PdO reduction peak
  to shift to more negative values

16
Changing Layer Thickness

• Plot flipped to positive side
• As thickness increases, absorption current
  increases
• Absorption current swallows adsorption current

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10 ML Pd Film

• H adsorption starts at 0.4 V (peak at 0.27 V)
• H absorption starts at 0.132 V (peak at 0.04 V)
• H evolution starts at 0.025 V
• Integrating charge indicates more than a single
  adsorbed H layer

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Conclusions

• On bulk Pd (gt4000 ML, gt1.6 um), H adsorption and
  absorption begin at same potential (CV of wire)
• On very thin films, 10 ML, adsorption,
  absorption and evolution peaks are separated
  (fig. 6)
• On intermediate thicknesses, absorption and
  evolution are not separated on CV (fig. 4)