Transcript 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
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
…
the Chlorine evolution reaction
1. 1. Cl- + S
2. S-Cl + S-Cl
 S-Cl + e
 2 S + Cl2
Volmer
Tafel
1. Cl- + S
2'. S-Cl + Cl-
 S-Cl + e
 S + Cl2 + e
Heyrovsky
1. Cl- + S
2". S-Cl
3. S-Cl+ + Cl-
 S-Cl + e
 S-Cl+ + e
 S + Cl2
in all cases, S-Cl  Clads
Krishtalik
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
DIAGNOSTIC PARAMETERS
 Tafel slope (b )
 Reaction orders
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)
the biological (aerobic/anaerobic) treatment
biodegradable
effluent
toxic or
non-biodegradable
effluent
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).
MOx
H2O
RO
+
1/2 O2
H +e
+
C O2 + z H + z e
R
+
1/2 O2 + H + e
R
MOx( OH)
Ch. Comninellis, Electrochim. Acta, 39 (1994) 1857
MOx+1
+
H +e
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
OH
MOx
-
CO2 + H 2O + Cl
-
-
OH + Cl 2
e
R
-
-
Cl , ClO 3 , ClO
-
Cl
MOx (HOCl) ads
MOx( OH)
-
Cl
e
A. De Battisti et al., J. Electrochem. Soc., 147 (2000) 592
3.2 INDIRECT ELECTROCHEMICAL OXIDATION
Redox Mediators:
+ H+ + e
O3 + 2H+ + 2e
S2O8= + 2e
Ag2+ + e
Co3+ + e
H2O2 + 2H+ + 2e
Ce4+ + e
MnO4- + 4H+ + 3e
HClO + H+ + e
HBrO + H+ + e
Mn3+ + e
MnO4- + 8H+ + 5e
Cr2O7= +14H+ + 6e
IO3- + 6H+ + 5e
OH
 H2 O
 O2 + H2O
 2SO4=
 Ag+
in HClO4 4N
 Co2+ in HNO3 3M
 2H2O
 Ce3+ in HClO4 1N
 MnO2 + 2H2O
 1/2Cl2+ H2O
 1/2Br2 + H2O
 Mn2+
 Mn2+ + 4H2O
 2Cr3+ + 7H2O
 1/2I2 + 3H2O
2.74 V
2.07 V
2.05 V
1.987 V
1.842 V
1.776 V
1.70 V
1.679 V
1.63 V
1.59 V
1.51 V
1.491 V
1.33 V
1.195 V
3.2 INDIRECT ELECTROCHEMICAL OXIDATION
glucose 10 g/l in Na 2SO4 1M and NaOH 0.01N
12000
COD (mg/l of O2)
10000
8000
6000
4000
without NaCl
NaCl 1g/l
NaCl 3g/l
NaCl 5g/l
2000
0
0
3
6
9
12
15
18
21
24
electrolysis time (h)
A. De Battisti et al., J. Electrochem. Soc., 147 (2000) 592
3.2 INDIRECT OXIDATION – Role of the different Parameters
Glucose 10 g/l in 1M Na2SO4 + 0.1M NaOH; Ti/Pt at 1200 A/m2 and 25 °C
S. Ferro et al., Electrochim. Acta, 46 (2000) 305
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
3.2 INDIRECT OXIDATION – Role of the different Parameters
Glucose 10 g/l in 1M Na2SO4 + 0.1M NaOH + NaCl 3 g/l; Ti/Pt at 25 °C
S. Ferro et al., Electrochim. Acta, 46 (2000) 305
3.2 INDIRECT OXIDATION – Role of the different Parameters
Glucose 10 g/l in 1M Na2SO4 + 0.1M NaOH + NaCl 3 g/l; Ti/Pt at 1200 A/m2
S. Ferro et al., Electrochim. Acta, 46 (2000) 305
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
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
3.3 Electrochemical Oxidation – The electrode material
Oxalic Acid – direct oxidation
substrate concentration: 0.12M ; background electrolyte: 1N H2SO4
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.
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
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.
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
3.3 Electrochemical Oxidation – The electrode material
Tafel plot for Oxalic acid electroxidation, in HClO4, at different electrode materials
[OA] = 750mM
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
H2C2O4 + •OH
HC2O4•
•COOH + •OH
•OH
+ H+ + e-
(r.d.s.)
HC2O4• + H2O
CO2 + •COOH
CO2 + H2O
•OH
•OH
•OH
•OH
H2O
HOOC-COOH
•OH
•OH
Ch. Comninellis et al., J. Appl. Electrochem., 30 (2000) 1345
3.3 Electrochemical Oxidation – The electrode material
•OH radicals
mechanism
H2O
(OH)ads + H+ + e-
(H2C2O4)ads + 2(OH)ads
2.5
E / (V vs. SCE)
(H2C2O4)ads + H+ + e-
H2C2O4
2CO2 + 2H2O
2
1.5
1
1
0.5
2
3
0
Mildly ox BDD
GC
1 Ti/IrO2-2SnO2
2 Ti/Ir0.67 Ru0.33 Sn2O6
3 Ti/IrO2-Ta2O5
Pt
Ti/Pt
J @ 1.0 mA/cm2
A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35
3.3 Electrochemical Oxidation – The electrode material
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
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
A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35
3.3 Electrochemical Oxidation – The electrode material
Effect of NaCl concentration on the current/potential
characteristics, attained at the Pt electrode
supporting electrolyte: 0.25M NaOH + 0.5M Na2SO4
A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35
3.3 Electrochemical Oxidation – The electrode material
Effect of NaBr concentration on the current/potential
characteristics, attained at the Pt electrode
supporting electrolyte: 0.25M NaOH + 0.5M Na2SO4
A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35
3.3 Electrochemical Oxidation – The electrode material
Effect of NaF concentration on the current/potential
characteristics, attained at the Pt electrode
supporting electrolyte: 0.25M NaOH + 0.5M Na2SO4
A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35
3.3 Electrochemical Oxidation – The electrode material
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
a real approach… for the potabilization of water
objectives
sterilization of solutions
for medical purposes
low capacity municipal
plants
final treatment
Generation of fresh water anolyte and catholyte
Some chemical reactions that may take place in the electrochemical
treatment of potable water
Anode reactions
Cathode reactions

2H2O  4e  4H  O2
2H2O  2e  H2  2OH
2H2O  2e  2H  H2O2
О2  е  О2
O2 + Н2О  2e  O3  2 Н
О2  Н2О  2е  НО2  ОН
OH  e  HO
НО2  Н2О  е  HO  2ОН
3H2O  6e  O3  6H
О2  2 Н  2е  Н2О2
O2 + 2OH  3e  O3  H2O
ecathode + Н2О  еaq

Н2О  е  HO  Н

Н  еaq  H
Н2О2  е  НО2  Н
Н2О  еaq  H  ОН
3OH  2e  НО2  Н2О
CO32  6Н  4е  HCHO + Н2О
H2O  2e  2H  O
CO32  8Н  6е  CH3OH + 2Н2О
H2O  e  H  OH
2CO32  4Н  2е  C2O42 + 2Н2О
3OH  2e  HO2  H2O
2CO2  2Н  2е  Н2C2О4
2Cl  2e  Cl2
CO2  2Н  2е  НCОOH
Cl  H2O  2e  HClO  H
CO32 2 Н2О  2е  HCO2 + 3ОН
Cl 2 Н2О  4e  HClO2  3Н
2SO42 5H2O8e S2O32  10OH
HCl  2H2O  5e  ClO2  5H
2SO42 4Н  2е  S2O62  2Н2О
Cl 4OH  4e  ClO3 2 Н2О
SO42 4H2O  2e  SO32  2OH
Cl 4OH  5e  ClO2 2 Н2О
SO42 4 Н  2e  H2SO3  Н2О
Cl  2OH  2e  ClO  Н2О
NO3  5H2O6e  NH2OH + 7ОН
Cl  2H2O  5e  ClO2 4 Н
2NO3  2H2O4e  N2O42 + 4ОН
2SO42  2e  S2O82
N2 + 5H + 4e  N2H5
2H2CO32  2e  C2O62 + 4H
Fe2+ + 2e  Fe
a real approach… for the potabilization of water
23 cm
Characteristics
29 cm
produced water: 1 liter/minute
redox potential: -0.05 VSCE
(potable water: 0.3 ÷ 0.4 VSCE )
service life : 2.000.000 liters
(e.g. 20 l /day  274 years!!)
a real approach… for the potabilization of water
pH < 5
W
pH > 9
Potable water
Food Processing
Agricultural Applications
Cooling Towers Disinfection
applications
Swimming Pools, Spas,
Hot tubs
Wastewater
DSA® (Dimensionally Stable Anodes)
interlayer
film
support
Film
the Oxide
mixture
Support
Interlayer
Electrocatalytic Oxides (IrO2, RuO2, PtOx)
Valve-metal Oxides (SnO2, Ta2O5, TiO2)
A conductive metal, thermally stable (Ti, Ta)
A thin layer of a metal or oxide, having a high affinity
toward the catalytic film
 High surface area
 High electrical conductivity
Ideal features
 High electrocatalytic activity/selectivity
 Chemical and mechanical stability
 Low cost
 Health safety
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)
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-surfacearea (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
Limitations of DSAs used in the industry
Service life
index:
kAh gIr-1
Service life index
3800 h = 158 d = 5 m
“Building Blocks”

Preparation (0  100% Mol IrO2 )

Characterization
•
Microstructure(XRD) (SEM) (EDX) (AFM)
•
Electrochemical Activity
•
Service life
“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)
Scanning Electron Microscopy Images
1200X
“cracked mud”
Increase of
the Ir %
1200X
35% Ir
30000X
50% Ir
100% Ir
Complete absence
of organisation
Energy Dispersive X-rays Analysis
35 % Ir
Element
Weight
%
Atomic%
OK
26.29
68.70
Cl K
0.60
0.71
Ti K
15.14
13.22
Sn L
35.39
12.46
Ir M
22.57
4.91
Totals
100.00
 low thickness for the oxide film:  1m
(presence of Ti from the support)
EDX results (%Ir)
Correlation among EDX results and gravimetric data from precursor solutions
The slope close to 1
indicates that no volatilization
of Sn takes place during the
pyrolysis step
Nominal composition (%Ir)
Atomic Force Microscopy Images
Ir 0% - Sn 100%
No formation of
nanoaggregates
Ir 35% - Sn 65%
Formation of
nanoaggregates
Atomic Force Microscopy Images
Ir 20% - Sn 80%
Formation of
microaggregates
X-ray Diffraction Analysis
101
Ti
Ti
110
Ti
Ti
211
(i)
Gradual transition from the IrO2 rutile structure to
the rutile system of SnO2
(ii)
Progressive shift of the 2 values, varying the
percentage of Iridium
XRD: test of the Vegard law
% Ir
100
60
20
1
Formation of a metastable solid solution
a =b
4.547
4.606
4.688
4.71
XRD - Particle size vs Composition
80
60
Particle Size:
50
35% 2.5 nm
40
30
72
20
0
10
20
30
40
50
60
70
80
90
100
71
% IrO2
70
Cell Volume
Cell Volume
<D>vol Å
70
69
68
67
66
0
20
40
60
% IrO
80
100
Cyclovoltammetric Characterization
IrOx  n(H  e)  IrOxn OH n

Electrolyte: HClO4 1N
IrO2-SnO2 35 % a/o in HClO4 1 M @ 298 KPotential range: 0.151.15 V
Scan rate: 100mV/sec
40
2
j (A/m )
20
0
92 C/g Ir
-20
-40
0.4
0.6
0.8
1.0
E (V vs NHE)
1.2
1.4
Cyclovoltammetric Characterization
HClO4 1N
-
Potential window: 0.151.15 V
-
Scan rate: 100mV/sec
Effect of the catalyst composition on the anodic charge
Cyclovoltammetric Characterization
Supporting electrolyte: HClO4 1N
-
Potential window: 0.151.15 V
Effect of the catalyst loading on the anodic charge
Role of the anodic material on the rate of the
electrochemical process
Polarization Curve 50% IrO2-SnO2, HClO41M @ 298 K
1.60
1.55
Potential (V vs SCE)
1.50
1.45
1.40
1.35
1.30
1.25
b = 43 mV/dec
1.20
1.15
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
2
log j (A/cm )
-2.0
-1.5
-1.0
-0.5
Kinetic study: considerations
% Ir
bexp
(mV/dec)
30
46
35
46
40
47
50
43
bexp  beff
bexp 
beff
1  OH
S-OH « 1
Optimal catalytic activity (high TurnOver number)
Kinetic study of the Oxygen Evolution Reaction
Electrochemical Oxide formation
1) S  H 2O
k1
k1

2 RT
 2.303*
 log j
3F
S  OH  H   e
2) S  OH  H 2O  S  O  H 3O   e
k2
3)2S  O
k3
k3
2S  O2
Hypothesis
• H2O  (H2O)ads
v2  K1k2OH exp 3 F 2RT  • second step rate determining
• first step in equilibrium
• low overpotentials
• s-OH  0
K1 (reql constant)
Dependence of current density for o.e. on anodic
charge density: all electrode compositions
Dependence of current density for o.e. on anodic
charge density: same composition, different thickness
The Group!:
Martina Donatoni
Sergio Ferro
Fabio Galli
Carlos Alberto Martinez-Huitle
Davide Perelli
Lourdes Vazquez-Gomez