1. The Role of Electrode Material in Applied Electrochemistry
Christos Comninellis
Swiss Federal Institute of Technology
ISP-GGEC-SB-EPFL
1015- Lausanne, Switzerland

2. OUTLINE OF THE PRESENTATION
1. Classification of electrochemical (anodic) reactions in aqueous media
Outer-sphere electron transfer reactions (facile)
Mn Mn e (M : transition metal complex)
Inner-sphere electron transfer reactions (demanding)
RH (RH)ads R* H e (RH : organic compound)
Electrochemical oxygen transfer (EOT) reactions (demanding)
RH H2O RO H e (RH : organic compound)
2. Cases studied:
Case I : Mediated oxidation of organics (in-cell or ex-cell)
Case II : Direct methanol fuel cell (DMFC)
Case III: Oxygen evolution in acid media
Case IV: Direct organic oxidation

3. Outer-sphere electron transfer anodic reactions
Ru(NH3) Ru(NH3) e-
Specific chemistry and interactions between the electrode and the reactant (product) is not important. The electrode is acting as a source or sink of electrons
The reactant and product are not necessarily adsorbed on the electrode surface.
In principle the kinetics of an outer-sphere reaction is not very sensible to the chemistry of electrode material (provided that the electrode is a good electronic conductor).
Electrocatalysis is not a predominant factor for outer-sphere reactions.
The standard electrochemical rate constant depends on the reorganization energy (Marcus theory) and the tunneling distance. In fact the standard electrochemical rate constant decrease exponentially with the distance from the electrode.

4. Anodic reactions in non-complexing aqueous acid media
Typical « pseudo » outer-sphere electron transfer reactions in applied electrochemistry
Anodic reactions in non-complexing aqueous acid media
Mn(II) Mn(III) e Eo = 1.5 V
Ce(III) Ce(IV) e Eo = 1.7 V
Co(II) Co(III) e Eo = 1.9 V
Ag(I) Ag(II) e Eo = 2.0 V
These reactions are usually fast and take place close to the thermodynamic potential (low overvoltage)

5. Case I :Application of outer-sphere electron transfer reactions
in applied electrochemistry
Pre-requirement conditions
Slow kinetics for oxygen evolution (main side reaction)
2H2O O H e Eo = 1.23 V
- High anodic stability in acid media
1M HClO4
25oC
Thermodynamics

6. Case I :Application of outer-sphere electron transfer reactions
in applied electrochemistry
Indirect in-cell oxidation using catalytic amounts of the outer sphere mediator (Application to the destruction of organic pollutants using Ag2+/Ag+ in HNO3)
Indirect ex-cell oxidation using stoichiometric amounts of Mn3+/Mn2+ in H2SO4

7. 2. Inner-sphere electron transfer anodic reactions (dehydrogenation)
Dissociative adsorption of the organic compound RH
RH (RH)ads (R*)ads (H*)ads
Discharge of adsorbed hydrogen
(H*)ads H e-
Specific chemistry and interactions between the anode ant the reactant (product) is important.
The reactant and product are adsorbed on the electrode surface.
Electrocatalysis is a predominant factor for inner-sphere reactions
Inner-sphere reactions are generally fast reactions at electrocatalytic electrodes (Pt)
Generally there are problems of electrode poisoning

8. Langmuir-Hinshelwood (L-H)
3. Electrochemical oxygen transfer (EOT) reactions in acid media
RH H2O RO H e-
A two step reaction:
I) Water activation
(H2O)ads (OH*)ads H e-
II) Reaction at the anode surface according to two posible mechanisms :
RH (OH*)ads RO H e (E-R)
(RH)ads (OH*)ads RO H e (L-H)
Eley-Rideal (E-R)
Langmuir-Hinshelwood (L-H)

9. 3. Electrochemical oxygen transfer (EOT) reactions in acid media
There are two possible mechanisms for water activation:
a) Dissociative adsorption of water followed by hydrogen discharge (E <Ethermodynamic)
(H2O)ads (OH*)ads (H*)ads
(H*)ads H e-
This is the case of electrocatalytic electrodes like Pt or Ru. The discharge can
take place at low potentials ( V/SHE) and OH* are strongly adsorbed.
b) Electrochemical water discharge (E > Ethermodynamic)
(H2O)ads (OH*)ads H e-
This is the case of non electrocatalytic electrodes like IrO2,SnO2,PbO2or BDD. The
discharge can take place at potentials above the thermodynamic potential (1.23 V/SHE)
and OH* are generally weekly adsorbed.

10. Case II : Methanol oxidation for DMFC application
Methanol oxidation at Pt nanoparticles
Dehydrogenation :Inner-sphere electron transfer (fast)
Pt3(CH3OH)ads Pt(CO)ads + 2Pt + 4H e-
Oxidation : Electrochemical oxygen transfer reaction (rds)
Pt(CO)ads H2O Pt CO H e-

11. Case II : Methanol oxidation for DMFC application
1 M HClO4
1 M HClO4 + Methanol (0.1-1M)
Thermodynamics
(0.046 V/RHE)

12. Electrochemical oxygen transfer from H2O to CO at Pt
Case II : Methanol oxidation for DMFC application
Electrochemical oxygen transfer from H2O to CO at Pt
Pt H2O Pt(OH*)ads Pt(H*)ads (1)
Pt(H*)ads Pt H e (fast) (2)
Pt(CO)ads Pt(OH*)ads Pt CO H e (3)
Pt does not activate water (react. 1) below 0.4 V/RHE
CO is strongly adsorbed on Pt blocking the active sites
Reaction (3) follows the Lagmuir- Hinshelwood mechanism
Thermo.

13. Case II : Methanol oxidation for DMFC application
Recherche of Pt -Metal alloys in order to decrease the activation energy of the electrochemical oxygen transfer reaction:
Pt(CO)ads H2O Pt CO H e-
Two main approaches:
Electronic effect (modification of the Pt work function)
Weakening of the Pt-CO bond due to the shift of the d electron from the alloy metal M to Pt.
Pt/Ni (1:1)
XPS Pt4f spectra for Pt alloy nanoparticles
Pt/Ni (3:1)
Pt/Ru/Ni (5:4:1) Pt
Pt/Ru (1:1)

14. Case II : Methanol oxidation for DMFC application
ii) Cooperative effect (bifunctiomal mechanism)
The second alloy metal activates water at low potentials and hence
promoting CO electro-oxidation. Pt M

15. Case II : Methanol oxidation for DMFC application
Electronegativity ( c ) of elements according to Pauling
Dc = cPt - cNi
Dc = 0 in case of Pt/Ru alloy small XPS shift no electronic effect
Dc = 0.4 in case of Pt/Ni alloy high XPS shift shift of d electrons from Ni to Pt
weakening of the Pt-CO bond
Remarks :
For the other noble metals Dc = no electronic effect
For the other d and sp metals Dc > electronic effect
Corrosion problems with non-noble metals in acid medium

16. Case II : Methanol oxidation for DMFC application
Candidate metals in the bifunctional mechanism
M-O (kJ/mol) A
Activity
Water activation (react. 1) is favorable at metals with high M-O dissociation energy
M + H2O MOH + H e (1)
CO oxydation (react.2) is favorable at metals with low M-O dissociation energy
MOH + CO M CO2 + H (2)
A : dissociation energy of H2O
H2O OH* H*

17. Case II : Methanol oxidation for DMFC application
Water activation on M in the M-Pt composite catalyst
1. Water adsorption on active sites M
M H2O M(H2O)ads
2. Water dissociation followed by oxidation of
the adsorbed hydrogen in a fast reaction.
M(H2O)ads M M(OH*)ads M(H*)ads
M(H*)ads M H e-
3. Interaction of (OH*)ads with the catalyst forming
an active oxide.
M(OH*)ads MO H e-
M(H2O)ads
M : non active
(M is not participating in the reaction)
M(OH*)ads
M: active
(M participates in the reaction) MO

18. CO oxidation on a non active M metal in the Pt/M catalyst
Case II : Methanol oxidation for DMFC application
CO oxidation on a non active M metal in the Pt/M catalyst
CO is adsorbed on M (the case of Ru)
Spillover of CO from Pt to Ru and reaction with OH* following the Lagmuir-Hinshelwood mechanism.
M(CO)ads M(OH*)ads M CO H e- M OH CO
CO is not adsorbed on M (the case of Sn)
In this case CO reacts with OH* at the Pt-M boundary.
Pt(CO)ads M(OH*)ads M Pt CO H e- M Pt OH CO

19. CO oxidation on an active metal in the Pt/MO catalyst
Case II : Methanol oxidation for DMFC application
CO oxidation on an active metal in the Pt/MO catalyst
The oxidic species MO are not covered by CO.
In case CO reacts with electrogenerated MO at the Pt- MO boundary.
M H2O MO H e-
Pt(CO)ads + MO M + Pt CO2 O O CO CO MO MO Pt Pt
Exemples of redox catalysis: WOx, MoOx,VOx,RuOx

20. Case II : Methanol oxidation for DMFC application
Methanol oxidation at Pt- M alloys (synergetic effect)
MeOH electrooxidation
% at. Pt
j / A g-1 Pt

21. Problems with Pt/M and Pt/MO composite catalysts
Case II : Methanol oxidation for DMFC application
Problems with Pt/M and Pt/MO composite catalysts
Limited solubility of M with Pt in case of Pt-M alloy formation (Os in Pt)
Difficulties in the preparation of a uniform M-Pt alloy.
The presence of M in the Pt/M composite catalyst can decrease dramatically
the catalytic activity of methanol dehydrogenation (modification of electronic
or/and structural properties of Pt)
Preparation of ternary (Pt-Ru-Sn) or quaternary (Pt-Ru-Ir-Os) alloys is complex
and can increase dramatically the production cost.
Corrosion problems in case of non-noble element alloy (M = Ni,Sn,Mo…)

22. Case III: oxygen evolution in acid media
1. Water adsorption on active sites M
M H2O M(H2O)ads
2. Water discharge
M(H2O)ads M(OH*)ads + H+ + e-
i) Non active electrode
2 M(OH*)ads M H2O2
H2O O H e-
ii) Active electrode
Interaction of (OH*)ads with the electrode forming
an active oxide.
M(OH*)ads MO H e-
2MO M O2
M(H2O)ads
Non active electrode
(is not participating in the reaction)
M(OH*)ads
Active electrode
(participates in the reaction) MO

23. Case III: oxygen evolution in acid media
On IrO2 anode : (Active)
On BDD anode : (Non-active)
Also the oxygen evolution reaction was studied at BDD and BDD/IrO2 electrodes. The cyclic voltammogram in perchloric acid shows the potential shift of about 1 V for the oxygen evolution reaction before and after IrO2 deposition for a IrO2 loading G = 6.4. This change can be explain by a changing in the mechanism of the oxygen evolution. In black you can see the mechanism path in case of a non active electrode like BDD. The first step is the discharge of water and the formation of hydroxyl radicals physisorbed on the surface. Afterwards, the hydroxyl groups react together to form oxygen. In case of an active electrode like IrO2, hydroxyl radicals physisorbed on the surface are introduced in the lattice and the metal oxide reaches a higher oxidation state passing to IV to VI. The active site returns to the initial oxidation state and oxygen is liberated. The fact that the OER on a BD/IrO2 electrode starts at 1.45 V, that corresponds to the redox potential of the couple Ir(VI)/Ir(IV) confirms this assumption. The catalytic activity of IrO2 towards the OER lead to a mechanism change also with a very low amount of oxide iridium on a BDD surface. - + e H ) OH (
IrO O 2 3 1
BDD + H O
BDD ( OH ) + H + + e - 2 1
BDD ( OH )
BDD O + H + + e - 2 2

24. Case III: oxygen evolution in acid media
Typical acive and non active electrodes in acid media
Active Electrodes:
RuO2 based electrodes : RuO2-TiO2
IrO2 based electrodes : IrO2-Ta2O5
Non-active Electrodes:
SnO2 based electrodes : SnO2-Sb2O5
TiO2 based electrodes : TiO2-NbOx
Diamond based electrodes : boron doped diamond (BDD)

25. Case IV:Oxalic acid oxidation
BDD (non-active) DE
IrO2(active)
1000
2000
3000
4000
5000 4 8 12
specific charge [Ah L -1 ]
IrO2
BDD
oxalic acid conc. [mol L-1]
The presence of oxalic acid in the solution also influences the overpotential for the electrolyte decomposition.
The fact that the response of a BDD electrode is more sensitive than that of a BDD/IrO2 to the organic concentration indicates a different mechanism for the reaction of organic oxidation.

26. Oxidation of organics on non-active (BDD) and active (IrO2) electrodes
The proposed model is valid for organic oxidation under oxygen evolution conditions.
The mechanism involves, as already sow in a preceding transparency, hydroxyl radicals previously physisorbed on the electrode surface.
In case of a BDD electrode, hydroxyl radicals act as intermediates in the oxidation of organic compounds.

27. Preparation of the DSA electrodes for Cl2 production
Electrode DSA-Cl2

28. Preparation of BDD electrodes by HF CVD
Growth rate : mm/h
Thickness : 1 mm
H H*
Filament
(2500oC)
p-Si substrate
(830oC)
(100 tor)
1% CH4 in H2
+ 3 ppm trimethylboron

29. Morphological characterization of BDD electrodes
SEM image of a BDD
HF-CVD technique (CSEM, Switzerland)
Silicon substrate:
p-type single crystal (Siltronix)
resistivity 1-3 mW cm
Now let ’s move on the experimental details about electrode preparation.
In case of a BDD, electrodes are prepared at the Swiss Center for Electronics and Microtecnology by the Hot Filament Chemical Vapor Deposition technique.
The substrate was a p-type single crystal of silicon with a resistivity of 1-3mW cm.
The diamond film obtained has a thickness of 1 mm. The SEM image shows the random textured, polycrystalline film of BDD
The raman spectrum shows three peaks corresponding (1) to the silicon substrate, (2) to the sp3 carbon (diamond) and (3) to the sp2 carbon (the non diamond). The non-diamond carbon (sp2) is less than 1% of the diamond carbon.
The boron level is around 8000 ppm in order to obtain a low resistivity of the film: around 15 mW cm.
Raman spectrum of a BDD: (1) p-Si substrate, (2) sp3 carbon and (3) sp2 carbon
BDD film:
thickness 1mm (± 10%)
non-diamond carbon < 1% of diamond carbon
ppm boron (resistivity 15 mW cm (± 10%))

30. Large scale (50x100 cm) production of BDD electrodes
CONDIAS GmbH D-Braunschweig, GERMANY
CSEM CH-Neuchâtel, SWITZERLAND

31. encapsulated nanoparticle
Preparation of Pt nanoparticles using dendrimeric polymers
Structure of the
dendrimaric polymers
PAMAM G4-NH2
Principe of nanoparticules using dendrimeric polymers pH ~ 5, molar ratio : Pt2+ / G4-NH2 = 30.
dendrimer
in solution
complex ion
metallic ion
(Pt2+)
BH4-
reducing agent
encapsulated nanoparticle

32. Preparation of Pt nanoparticles using dendrimeric polymers
Pt particle size distribution .
Freq. / %
d / nm

33. + Preparation of Pt and Pt alloy nanoparticles using the
microemulsion technique 3) 1) A +
NaBH4 2)
A = pure precursors or mixtures of precursors : H2PtCl6, H2RuCl6,SnCl4
3 % water; % n-heptane ; surfactant (BRIJ-30)

34. Preparation of Pt-Ru alloys by the microemulsion technique

35. Preparation of Pt nanoparticles using the microemulsion technique
Pt particle size distribution
XPS
CPS
Freq. / %
d / nm eV

36. Deposition of Pt and Pt alloys on BDD
Pt nanoparticles
BDD

37. 2-steps synthesis of Au nanoparticles
1) Sputtering of a thin Au film on diamond
2) Thermal decomposition in air at ~ 600 °C
20 s sputtering
40 s sputtering
50 s sputtering
Metallic nanoparticles on BDD. Application to electrocatalysis.

38. Electrochemical preparation of Pt nanoparticles on BDD
Pt loading : 50 mg /cm2, Average particle size : 200 nm