1. Implementation of Advanced Electrodes to the Wastewaters Treatment
ESSEE 4
Palić, Serbia, 17 – 22 Sept., 2006
 Implementation of Advanced Electrodes to the Wastewaters Treatment
Achille De Battisti
Carlos Alberto Martinez-Huitle
Sergio Ferro
Laboratory of Electrochemistry
University of Ferrara, Italy

2. to wastewater remediation
Urban legends against the electrochemical way
to wastewater remediation
it’s not useful in case of poorly-conducting electrolytes
it requires a huge consumption of electricity
electrodes are expensive
chemicals are easily available
reactors are complex and/or difficult to manage …

3. the Chlorine evolution reaction
1. Cl- + S  S-Cl + e Volmer
2. S-Cl + S-Cl  2 S + Cl2 Tafel
1. Cl- + S  S-Cl + e
2'. S-Cl + Cl-  S + Cl2 + e Heyrovsky
2". S-Cl  S-Cl+ + e
3. S-Cl+ + Cl-  S + Cl2
in all cases, S-Cl  Clads
Krishtalik

4. electrochemical formation
the Oxygen evolution reaction
1. S + H2O  S-OH + H+ + e
2. S-OH + S-OH  S-O + S + H2O
3. S-O + S-O  2 S + O2
1. S + H2O  S-OH + H+ + e
2'. S-OH  S-O + H+ + e
3. S-O + S-O  2 S + O2
chemical formation
of the oxide
electrochemical formation
of the oxide
Kinetic study of a reaction
Tafel slope (b )
Reaction orders
DIAGNOSTIC PARAMETERS

5. O2 and Cl2 evolution reactions in electrochemical incineration
major links with the fundamentals of electrocatalysis
a solid background for developments from bench to commercial scale
Electrochemical incineration: how to follow it?
Traditional analytical approaches:
NMR, IR, UV-Vis., mass spectrometry, different chromatographies…
The alternative:
global analytical parameters, like COD and TOC
efficiency parameters (ICE, EOD)

6. biodegradable effluent
the biological (aerobic/anaerobic) treatment
toxic or
non-biodegradable
effluent
biodegradable effluent

7. 3.1 DIRECT ELECTROCHEMICAL OXIDATION
Strongly oxidant hydroxyl radicals are formed at high oxygen overvoltage anodes (PbO2 , Sb(V) or F- doped-SnO2 , diamond electrodes…)
In case of metal-oxide electrodes, we can distinguish two kind of electrode material: “mineralizing” and “converting” anodes, depending on the available oxidation state of the metal.
When the latter can increase its valence, radicals are stabilized by interaction with the electrode surface and their oxidation power is slower (effect: partial oxidation). On the contrary, when the oxide lattice cannot be “expanded”, the hydroxyl radicals exhibit larger reactivity (effect: complete mineralization, i.e. transformation into CO2).
Ch. Comninellis, Electrochim. Acta, 39 (1994) 1857

8. 3.2 INDIRECT ELECTROCHEMICAL OXIDATION
Due to a lower oxygen overvoltage (higher catalytic activity towards the OER), other anodic materials generally exhibit low faradaic yields. It is the case of galvanic Pt and IrO2-based DSA’s®. The performance of these stable anodes can be improved by using inorganic mediators of the oxidation.
Active chlorine is of particular interest: oxidation of chlorides requires lower anode potentials, compared with those necessary for OH formation.
The contemporaneous formation of the two reactive species (Cl and OH radicals) may produce hypochlorous acid, which is a strong oxidant.
A possible consequence of chloride mediation: more electrode materials for electrochemical incineration
A. De Battisti et al., J. Electrochem. Soc., 147 (2000) 592

9. Redox Mediators: 3.2 INDIRECT ELECTROCHEMICAL OXIDATION
OH + H+ + e  H2O V
O3 + 2H+ + 2e  O2 + H2O V
S2O8= + 2e  2SO4= V
Ag2+ + e  Ag+ in HClO4 4N V
Co3+ + e  Co2+ in HNO3 3M V
H2O2 + 2H+ + 2e  2H2O V
Ce4+ + e  Ce3+ in HClO4 1N V
MnO4- + 4H+ + 3e  MnO2 + 2H2O V
HClO + H+ + e  1/2Cl2+ H2O V
HBrO + H+ + e  1/2Br2 + H2O V
Mn3+ + e  Mn V
MnO4- + 8H+ + 5e  Mn2+ + 4H2O V
Cr2O7= +14H+ + 6e  2Cr3+ + 7H2O V
IO3- + 6H+ + 5e  1/2I2 + 3H2O V

10. 3.2 INDIRECT ELECTROCHEMICAL OXIDATION
A. De Battisti et al., J. Electrochem. Soc., 147 (2000) 592

11. Glucose 10 g/l in 1M Na2SO4 + 0.1M NaOH; Ti/Pt at 1200 A/m2 and 25 °C
3.2 INDIRECT OXIDATION – Role of the different Parameters
Glucose 10 g/l in 1M Na2SO M NaOH; Ti/Pt at 1200 A/m2 and 25 °C
S. Ferro et al., Electrochim. Acta, 46 (2000) 305

12. Glucose 10 g/l in 1M Na2SO4 + NaCl 5g/l; Ti/Pt at 1200 A/m2 and 25 °C
3.2 INDIRECT OXIDATION – Role of the different Parameters
Glucose 10 g/l in 1M Na2SO4 + NaCl 5g/l; Ti/Pt at 1200 A/m2 and 25 °C
S. Ferro et al., Electrochim. Acta, 46 (2000) 305

13. Glucose 10 g/l in 1M Na2SO4 + 0.1M NaOH + NaCl 3 g/l; Ti/Pt at 25 °C
3.2 INDIRECT OXIDATION – Role of the different Parameters
Glucose 10 g/l in 1M Na2SO M NaOH + NaCl 3 g/l; Ti/Pt at 25 °C
S. Ferro et al., Electrochim. Acta, 46 (2000) 305

14. 3.2 INDIRECT OXIDATION – Role of the different Parameters
Glucose 10 g/l in 1M Na2SO M NaOH + NaCl 3 g/l; Ti/Pt at 1200 A/m2
S. Ferro et al., Electrochim. Acta, 46 (2000) 305

15. 3.2 INDIRECT OXIDATION – WHAT HAVE WE LEARNT?
relatively small amounts of chloride ions may inhibit the OER, causing
an increase of the anode potential and therefore a higher reactivity of
adsorbed hydroxyl and chloride/oxychloride radicals
increasing the chloride concentration above a certain critical value
would cause a potentiostatic buffering by the chlorine redox system,
and consequently a decrease of the anode potential
…may inhibit…
…would cause…
Further investigation is needed!
S. Ferro et al., Electrochim. Acta, 46 (2000) 305

16. The electrode material is becoming the main character
…It means one more “tough” variable
We have to simplify the experimental approach
Let’ study a simpler substrate:
Oxalic Acid

17. Oxalic Acid – direct oxidation
3.3 Electrochemical Oxidation – The electrode material
Oxalic Acid – direct oxidation
substrate concentration: 0.12M ; background electrolyte: 1N H2SO4

18. Anodic oxidation of Oxalic Acid (OA) at different electrode materials:
IrO2-Ta2O5 active coatings
Polarization curves for a Ti/IrO2-Ta2O5 electrode, at different OA concentrations.Inset: elaboration of data in terms of Tafel plot.

19. Anodic oxidation of Oxalic Acid (OA) at different electrode materials:
IrO2-2SnO2 active coatings
Polarization curves for a Ti/IrO2-2SnO2 electrode, at different OA concentrations

20. Anodic oxidation of Oxalic Acid (OA) at different electrode materials: Ti/Ir0.67Ru0.33O2-2SnO2 electrode
. Polarization curves for a Ti/Ir0.67Ru0.33O2-2SnO2 electrode,
at different OA concentrations. Inset: elaboration of data in terms of Tafel plot.

21. Anodic oxidation of Oxalic Acid (OA) at different electrode materials: IrO2-2SnO2 active coatings
Polarization curves for a Ti/IrO2-2SnO2 electrode, at different OA concentrations

22. 3.3 Electrochemical Oxidation – The electrode material
Tafel plot for Oxalic acid electroxidation, in HClO4, at different electrode materials
[OA] = 750mM

23. H2O 3.3 Electrochemical Oxidation – The electrode material
Considering the BDD anode material, Comninellis et al. [ref] have proposed a mechanism of OA oxidation that involves the participation of hydroxyl radicals generated at the electrode surface:
H2O •OH + H+ + e (r.d.s.)
H2C2O4 + •OH HC2O4• + H2O
HC2O4• CO2 + •COOH
•COOH + •OH CO2 + H2O
H2O OH
HOOC-COOH
Ch. Comninellis et al., J. Appl. Electrochem., 30 (2000) 1345

24. (H2C2O4)ads + 2(OH)ads 2CO2 + 2H2O
3.3 Electrochemical Oxidation – The electrode material
•OH radicals
mechanism
H2C2O (H2C2O4)ads + H+ + e-
H2O (OH)ads + H+ + e-
(H2C2O4)ads + 2(OH)ads CO2 + 2H2O 1 2 3
1 Ti/IrO2-2SnO2
Mildly ox BDD GC
Ti/Pt Pt
2 Ti/Ir0.67 Ru0.33 Sn2O6
3 Ti/IrO2-Ta2O5
1.0 mA/cm2

25. 3.3 Electrochemical Oxidation – The electrode material
A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35
Oxalic Acid – direct and mediated oxidation at bulk Pt
substrate concentration: 0.12M ; NaX concentration: 5 g/l
background electrolyte: 0.25M NaOH + 0.5M Na2SO4

26. Tartaric Acid – direct and mediated oxidation at Ti/Pt
3.3 Electrochemical Oxidation – The electrode material
Tartaric Acid – direct and mediated oxidation at Ti/Pt
substrate concentration: 0.10M ; NaX concentration: 5 g/l
background electrolyte: 0.5M H2SO4 or 0.25M NaOH + 0.5M Na2SO4

27. supporting electrolyte: 0.25M NaOH + 0.5M Na2SO4
3.3 Electrochemical Oxidation – The electrode material
A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35
Effect of NaCl concentration on the current/potential characteristics, attained at the Pt electrode
supporting electrolyte: 0.25M NaOH + 0.5M Na2SO4

28. supporting electrolyte: 0.25M NaOH + 0.5M Na2SO4
3.3 Electrochemical Oxidation – The electrode material
A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35
Effect of NaBr concentration on the current/potential characteristics, attained at the Pt electrode
supporting electrolyte: 0.25M NaOH + 0.5M Na2SO4

29. supporting electrolyte: 0.25M NaOH + 0.5M Na2SO4
3.3 Electrochemical Oxidation – The electrode material
A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35
Effect of NaF concentration on the current/potential characteristics, attained at the Pt electrode
supporting electrolyte: 0.25M NaOH + 0.5M Na2SO4

30. 3.3 Electrochemical Oxidation – The electrode material
A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35

31. Volume reaction of the substrate with electrogenerated
Halogenide-mediated (indirect) electrochemical incineration (alkaline media):
Volume reaction of the substrate with electrogenerated
strong oxidants (ClO2, HClO, ClO-, BrO3-);
Surface reaction of the adsorbed substrate with
electrosorbed species (e.g.: oxy-chloro radicals);
Inhibition of the oxygen evolution reaction.
“Direct” electrochemical incineration:
Concomitant with oxygen evolution reaction;
Good faradaic yields at high-oxygen overvoltage anodes;
Weakly adsorbed hydroxyl radicals are the main factor
leading to electrochemical incineration;
As an extreme case, hydroxyl radicals may act within a
reaction cage nearby the electrode surface.
A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35

32. O a real approach… for the potabilization of water objectives
sterilization of solutions for medical purposes
low capacity municipal plants
final treatment

33. Generation of fresh water anolyte and catholyte

34. a real approach… for the potabilization of water
23 cm
29 cm
Characteristics
produced water: 1 liter/minute
redox potential: VSCE
(potable water: 0.3 ÷ 0.4 VSCE )
service life : liters
(e.g. 20 l /day  274 years!!)

35. a real approach… for the potabilization of water
pH < 5
pH > 9 W

36. Agricultural Applications Cooling Towers Disinfection
Food Processing
Potable water
applications
Swimming Pools, Spas,
Hot tubs
Agricultural Applications
Wastewater
Cooling Towers Disinfection

37. DSA® (Dimensionally Stable Anodes)
interlayer
film
support
Support
the Oxide mixture
Electrocatalytic Oxides (IrO2, RuO2, PtOx)
Valve-metal Oxides (SnO2, Ta2O5, TiO2)
Film
A conductive metal, thermally stable (Ti, Ta)
Interlayer
A thin layer of a metal or oxide, having a high affinity toward the catalytic film
High surface area
High electrical conductivity
High electrocatalytic activity/selectivity
Chemical and mechanical stability
Low cost
Health safety
Ideal features

38. The accelerated service-life test for oer DSA
Need for quicker diagnostics, e.g.: 1-4 months
Typical test example :
Solution: 3 M H2SO4
Temperature: 60°C
Galvanostatic conditions, j = 10 – 50 kA m-2
Test end upon 1 V increase in cell potential
(polarization curves and CV’s recorded during the experiments)

39. The accelerated service-life test for oer DSA.
A possible way to analize the results
The passivation (deactivation) time can be misleading (the catalyst loading (film thickness) is not properly considered).
Charge consumption per unit electrode-surface-area (e.g. kAh m-2) is more meaningful than time;
Normalization to film thickness (catalyst loading) is mandatory.
At fixed coating composition the amount of noble-metal (e.g.: g Ir m-2) can be used as normalizing factor

40. Limitations of DSAs used in the industry
Service life index:

41. Service life index
3800 h = 158 d = 5 m

42. “Building Blocks” Preparation (0  100% Mol IrO2 ) Characterization
Microstructure(XRD) (SEM) (EDX) (AFM)
Electrochemical Activity
Service life

43. Preparation (0  100% Ir, constant-mass deposits)
“Building Blocks”
Preparation (0  100% Ir, constant-mass deposits)
Ti-support etching by conc. NaOH
(acidic treatments lead to shorter s.l.)
Interlayer deposition (thermal methods)
precursor deposition (Ir(IV) and Sn(IV) chloro-aceto
complexes, colloidal suspensions)
Thermal decomposition: 450 °C)

44. Scanning Electron Microscopy Images
35% Ir
1200X
“cracked mud”
Increase of the Ir %
50% Ir
1200X
30000X
100% Ir
Complete absence
of organisation

45. Energy Dispersive X-rays Analysis
35 % Ir
Element
Weight%
Atomic%
O K
26.29
68.70
Cl K
0.60
0.71
low thickness for the oxide film:  1m
(presence of Ti from the support)
Ti K
15.14
13.22
Sn L
35.39
12.46
Ir M
22.57
4.91
Totals
100.00
Correlation among EDX results and gravimetric data from precursor solutions
Nominal composition (%Ir)
EDX results (%Ir)
The slope close to 1 indicates that no volatilization of Sn takes place during the pyrolysis step

46. Atomic Force Microscopy Images No formation of nanoaggregates
Ir 0% - Sn 100%
No formation of nanoaggregates
Ir 35% - Sn 65%
Formation of nanoaggregates

47. Formation of microaggregates
Atomic Force Microscopy Images
Ir 20% - Sn 80%
Formation of microaggregates

48. X-ray Diffraction Analysis
1 1 0
1 0 1
2 1 1 Ti
Gradual transition from the IrO2 rutile structure to the rutile system of SnO2
(i )
(ii )
Progressive shift of the 2 values, varying the percentage of Iridium

49. XRD: test of the Vegard law
Formation of a metastable solid solution

50. XRD - Particle size vs Composition
35% 2.5 nm
Cell Volume

51. Cyclovoltammetric Characterization
Electrolyte: HClO4 1N
Potential range: 0.151.15 V
Scan rate: 100mV/sec
92 C/g Ir

52. Cyclovoltammetric Characterization
HClO4 1N Potential window: 0.151.15 V Scan rate: 100mV/sec
Effect of the catalyst composition on the anodic charge

53. Cyclovoltammetric Characterization
Supporting electrolyte: HClO4 1N Potential window: 0.151.15 V
Effect of the catalyst loading on the anodic charge

54. Role of the anodic material on the rate of the electrochemical process

55. Optimal catalytic activity (high TurnOver number)
Kinetic study: considerations
% Ir
bexp
(mV/dec) 30 46 35 40 47 50 43
S-OH « 1
bexp  beff
Optimal catalytic activity (high TurnOver number)

56. Electrochemical Oxide formation
Kinetic study of the Oxygen Evolution Reaction
Electrochemical Oxide formation
Hypothesis
H2O  (H2O)ads
first step in equilibrium
second step rate determining
low overpotentials
s-OH  0
K1 (reql constant)

57. Dependence of current density for o. e
Dependence of current density for o.e. on anodic charge density: all electrode compositions

58. Dependence of current density for o. e
Dependence of current density for o.e. on anodic charge density: same composition, different thickness

59. The Group!: Martina Donatoni Sergio Ferro Fabio Galli
Carlos Alberto Martinez-Huitle
Davide Perelli
Lourdes Vazquez-Gomez