Achille De Battisti Carlos Alberto Martinez-Huitle Sergio Ferro Laboratory of Electrochemistry University of Ferrara, Italy Implementation of Advanced

Achille De Battisti Carlos Alberto Martinez-Huitle Sergio Ferro

Laboratory of Electrochemistry

University of Ferrara, Italy

Implementation of Advanced Implementation of Advanced Electrodes to the Wastewaters Electrodes to the Wastewaters

TreatmentTreatment

Palić, Serbia, 17 – 22 Sept., 2006

ESSEE 4

Urban legends against the electrochemical Urban legends against the electrochemical wayway

to wastewater remediationto 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 …

1. 1. Cl- + S S-Cl + e Volmer2. S-Cl + S-Cl 2 S + Cl2 Tafel

1. Cl- + S S-Cl + e 2'. S-Cl + Cl- S + Cl2 + e Heyrovsky

1. Cl- + S S-Cl + e 2".S-Cl S-Cl+ + e 3. S-Cl+ + Cl- S + Cl2

in all cases, S-Cl Clads

Krishtalik

the Chlorine evolution the Chlorine evolution reactionreaction

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

the Oxygen evolution the Oxygen evolution reactionreaction

DIAGNOSTIC PARAMETERS DIAGNOSTIC PARAMETERS Tafel slope (Tafel slope (bb ))

Reaction orders Reaction orders

Kinetic study of a Kinetic study of a reactionreaction

chemical chemical formationformation

of the oxide

electrochemical electrochemical formationformation

of the oxide

OO22 and Cl and Cl22 evolution reactions in electrochemical evolution reactions in electrochemical incinerationincineration

major links with the fundamentals of electrocatalysismajor links with the fundamentals of electrocatalysis

a solid background for developments from bench to commercial a solid background for developments from bench to commercial scalescale

Electrochemical incineration: how to follow it?Electrochemical incineration: how to follow it?

Traditional analytical approaches:Traditional analytical approaches: NMR, IR, UV-Vis., mass spectrometry, different NMR, IR, UV-Vis., mass spectrometry, different chromatographies…chromatographies…

The alternative:The alternative:

global analytical parameters, like COD and TOCglobal analytical parameters, like COD and TOC

efficiency parameters (ICE, EOD)efficiency parameters (ICE, EOD)

the biological (aerobic/anaerobic) treatmentthe biological (aerobic/anaerobic) treatment

biodegradable biodegradable effluenteffluent

toxic ortoxic ornon-biodegradablenon-biodegradable

effluenteffluent

3.1 3.1 DIRECT ELECTROCHEMICAL OXIDATIONDIRECT ELECTROCHEMICAL OXIDATION

Strongly oxidant hydroxyl radicals are formed at high oxygen overvoltage Strongly oxidant hydroxyl radicals are formed at high oxygen overvoltage anodes (PbOanodes (PbO2 2 , Sb(V) or F, Sb(V) or F-- doped-SnO doped-SnO2 2 , diamond electrodes…), diamond electrodes…)

MOx

MOx+1

H2O

H+ + e

1/2 O2

R

CO2 + z H+ + z e

1/2 O2 + H+ + e

H+ + e

MOx( OH)

R

RO

In case of metal-oxide electrodes, we In case of metal-oxide electrodes, we can distinguish two kind of electrode can distinguish two kind of electrode material: “material: “mineralizingmineralizing” and ” and ““convertingconverting” anodes, depending on ” anodes, depending on the available oxidation state of the the available oxidation state of the metal.metal.When the latter can increase its When the latter can increase its valence, radicals are stabilized by valence, radicals are stabilized by interaction with the electrode surface interaction with the electrode surface and their oxidation power is slower and their oxidation power is slower (effect: partial oxidation). On the (effect: partial oxidation). On the contrary, when the oxide lattice contrary, when the oxide lattice cannot be “expanded”, the hydroxyl cannot be “expanded”, the hydroxyl radicals exhibit larger reactivity radicals exhibit larger reactivity (effect: complete mineralization, i.e. (effect: complete mineralization, i.e. transformation into COtransformation into CO22).).

Ch. Comninellis, Electrochim. Acta, 39 (1994) 1857

3.2 IN3.2 INDIRECT ELECTROCHEMICAL OXIDATIONDIRECT ELECTROCHEMICAL OXIDATION

Due to a lower oxygen overvoltage (Due to a lower oxygen overvoltage (higher catalytic activity towards the higher catalytic activity towards the OEROER), other anodic materials generally exhibit low faradaic yields. It is ), other anodic materials generally exhibit low faradaic yields. It is the case of galvanic Pt and IrOthe case of galvanic Pt and IrO22-based DSA’s-based DSA’s®®. The performance of these . The performance of these

stable anodes can be improved by using stable anodes can be improved by using inorganic mediatorsinorganic mediators of the of the oxidationoxidation..

Active chlorineActive chlorine is of particular is of particular interest: oxidation of chlorides interest: oxidation of chlorides requires lower anode potentials, requires lower anode potentials, compared with those necessary compared with those necessary for OHfor OH formation. formation.The contemporaneous formation The contemporaneous formation of the two reactive species (Clof the two reactive species (Cl and OHand OH radicals) may produce radicals) may produce hypochlorous acid, which is a hypochlorous acid, which is a strong oxidant.strong oxidant.

Cl-, ClO3-, ClO-

ads

Cl-

CO2 + H2O + Cl-

Cl-

OH- + Cl2

MOx (HOCl)

e

e

OH-

R

MOx( OH)

MOx

A. De Battisti et al., J. Electrochem. Soc., 147 (2000) 592

A possible consequence of A possible consequence of chloride mediation: more chloride mediation: more electrode materials for electrode materials for electrochemical incinerationelectrochemical incineration


Redox Mediators:Redox Mediators:

OH + H+ + e H2O 2.74 V

O3 + 2H+ + 2e O2 + H2O2.07 VS2O8

= + 2e 2SO4= 2.05 V

Ag2+ + e Ag+ in HClO4 4N 1.987 V

Co3+ + e Co2+ in HNO3 3M 1.842 V

H2O2 + 2H+ + 2e 2H2O 1.776 V

Ce4+ + e Ce3+ in HClO4 1N 1.70 V

MnO4- + 4H+ + 3e MnO2 + 2H2O 1.679 V

HClO + H+ + e 1/2Cl2+ H2O1.63 VHBrO + H+ + e 1/2Br2 + H2O1.59 VMn3+ + e Mn2+ 1.51 VMnO4

- + 8H+ + 5e Mn2+ + 4H2O1.491 VCr2O7

= +14H+ + 6e 2Cr3+ + 7H2O 1.33 V

IO3- + 6H+ + 5e 1/2I2 + 3H2O

1.195 V


A. De Battisti et al., J. Electrochem. Soc., 147 (2000) 592

glucose 10 g/l in Na2SO4 1M and NaOH 0.01N

0 3 6 9 12 15 18 21 240

2000

4000

6000

8000

10000

12000

NaCl 3g/lNaCl 5g/l

NaCl 1g/lwithout NaCl

electrolysis time (h)

CO

D (

mg

/l o

f O

2)

3.2 IN3.2 INDIRECT OXIDATION DIRECT OXIDATION –– Role of the different Role of the different ParametersParameters

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

Glucose 10 g/l in 1M Na2SO4 + NaCl 5g/l; Ti/Pt at 1200 A/m2 and 25 °C



Glucose 10 g/l in 1M Na2SO4 + 0.1M NaOH + NaCl 3 g/l; Ti/Pt at 25 °C



Glucose 10 g/l in 1M Na2SO4 + 0.1M NaOH + NaCl 3 g/l; Ti/Pt at 1200 A/m2




3.2 IN3.2 INDIRECT OXIDATION DIRECT OXIDATION – – WHAT HAVE WE WHAT HAVE WE LEARNT?LEARNT?

relatively small amounts of chloride ions relatively small amounts of chloride ions may inhibitmay inhibit the OER, the OER, causingcausing an increase of the anode potential and therefore a higher reactivity an increase of the anode potential and therefore a higher reactivity ofof adsorbed hydroxyl and chloride/oxychloride radicalsadsorbed hydroxyl and chloride/oxychloride radicals

increasing the chloride concentration above a certain critical value increasing the chloride concentration above a certain critical value would causewould cause a potentiostatic buffering by the chlorine redox a potentiostatic buffering by the chlorine redox system,system, and consequently a decrease of the anode potentialand consequently a decrease of the anode potential

…may inhibit……would cause…

Further investigation is needed!

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 Electrochemical3.3 Electrochemical Oxidation Oxidation – – The electrode The electrode materialmaterial

Oxalic Acid – direct oxidationOxalic Acid – direct oxidationsubstrate concentration: 0.12M ; background electrolyte: 1N H2SO4

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-Ta2O5 active coatings

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

. 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: Ti/Ir0.67Ru0.33O2-2SnO2 electrode

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


Tafel plot for Oxalic acid electroxidation, in HClO4, at different electrode materials

[OA] = 750mM


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:

HH22O O ••OH + HOH + H++ + e + e- - (r.d.s.)(r.d.s.)

HH22CC22OO44 + + ••OHOH HCHC22OO44•• + H+ H22OO

HCHC22OO44•• COCO22 + + ••COOHCOOH

••COOH + COOH + ••OHOH COCO22 + H + H22OO

H2O•OH

•OH

•OH

•OH

HOOC-COOH

•OH

•OH

Ch. Comninellis et al., J. Appl. Electrochem., 30 (2000) 1345


0

0.5

1

1.5

2

2.5

E /

(V

vs.

SC

E)

Mildly ox BDDMildly ox BDD GCGC 11 Ti/IrO Ti/IrO22-2SnO-2SnO22 Pt Pt Ti/PtTi/Pt22 Ti/Ir Ti/Ir0.67 0.67 RuRu0.33 0.33 SnSn22OO66

33 Ti/IrO Ti/IrO22-Ta-Ta22OO55

11 22 33

JJ @ 1.0 mA/cm @ 1.0 mA/cm22

••OH radicalsOH radicals

mechanismmechanism

HH22CC22OO44 (H (H22CC22OO44))adsads ++ HH++ + e + e--

HH22O (OH)O (OH)adsads + H + H++ + e + e--

(H(H22CC22OO44))adsads + 2(OH) + 2(OH)adsads 2CO 2CO22 + +

2H2H22OO


Oxalic Acid – direct and mediated oxidation at Oxalic Acid – direct and mediated oxidation at bulkbulk Pt Ptsubstrate concentration: 0.12M ; NaX concentration: 5 g/l

background electrolyte: 0.25M NaOH + 0.5M Na2SO4A. D

e B

att

isti

et

al., Ele

ctro

chem

. S

olid

-Sta

te L

ett

., 8

(200

5)

D3

5


Tartaric Acid – direct and mediated oxidation at Tartaric Acid – direct and mediated oxidation at Ti/PtTi/Pt

substrate concentration: 0.10M ; NaX concentration: 5 g/l

background electrolyte: 0.5M H2SO4 or 0.25M NaOH + 0.5M Na2SO4


Effect of NaCl concentration Effect of NaCl concentration on the current/potential on the current/potential characteristics, attained at the Pt electrodecharacteristics, attained at the Pt electrode

supporting electrolyte: 0.25M NaOH + 0.5M Na2SO4

A. D

e B

att

isti

et

al., Ele

ctro

chem

. S

olid

-Sta

te L

ett

., 8

(200

5)

D3

5


Effect of NaBr concentration Effect of NaBr concentration on the current/potential on the current/potential characteristics, attained at the Pt electrodecharacteristics, attained at the Pt electrode


A. D

e B

att

isti

et

al., Ele

ctro

chem

. S

olid

-Sta

te L

ett

., 8

(200

5)

D3

5


Effect of NaF concentration Effect of NaF concentration on the current/potential on the current/potential characteristics, attained at the Pt electrodecharacteristics, attained at the Pt electrode


A. D

e B

att

isti

et

al., Ele

ctro

chem

. S

olid

-Sta

te L

ett

., 8

(200

5)

D3

5


A. D

e B

att

isti

et

al., Ele

ctro

chem

. S

olid

-Sta

te L

ett

., 8

(200

5)

D3

5

Halogenide-mediated (indirect) electrochemical Halogenide-mediated (indirect) electrochemical incineration (alkaline media):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: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

objectivesobjectives

sterilization of solutions sterilization of solutions for medical purposesfor medical purposes

low capacity municipal low capacity municipal plants plants

final treatmentfinal treatment

a real approach… for the potabilization of watera real approach… for the potabilization of water

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 5H2O8e 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 5H2O6e NH2OH + 7ОН

Cl 2H2O 5e ClO2 4 Н 2NO3 2H2O4e N2O42 + 4ОН

2SO42 2e S2O82 N2 + 5H + 4e N2H5

2H2CO32 2e C2O62 + 4H Fe2+ + 2e Fe

23 cm

29 cm CharacteristicsCharacteristics

produced water: 1 liter/minuteproduced water: 1 liter/minute

redox potential: -0.05 Vredox potential: -0.05 VSCESCE

(potable water: 0.3 ÷ 0.4 V(potable water: 0.3 ÷ 0.4 VSCE SCE ))

service life service life : 2.000.000 liters: 2.000.000 liters

(e.g. 20 l /day (e.g. 20 l /day 274 years!!) 274 years!!)



W

pH < 5 pH > 9

applications

Potable water

Wastewater

Swimming Pools, Spas, Hot tubs

Cooling Towers Disinfection

Agricultural Applications

Food Processing

DSA® (Dimensionally Stable Anodes)

support

film interlayer

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 featuresIdeal features

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-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

Limitations of DSAs used in the industry

Service life index:

- 1IrkAh g

3800 h = 158 d = 5 m

Service life index

Preparation (0 100% Mol IrO2 )

Characterization

• Microstructure(XRD) (SEM) (EDX) (AFM)

• Electrochemical Activity

• Service life

“Building Blocks”

“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)

35% Ir

1200X

Scanning Electron Microscopy Images

50% Ir

1200X

“cracked mud”

100% Ir

Increase of the Ir %

30000X

Complete absence of organisation

Element Weight%

Atomic%

O K 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

y = 0,9718x - 1,9033

R2 = 0,9947

0

20

40

60

80

100

0 20 40 60 80 100

Nominal composition (%Ir)

ED

X r

esu

lts

(%Ir

)

Correlation among EDX results and gravimetric data from precursor solutions

Energy Dispersive X-rays Analysis

The slope close to 1 indicates that no volatilization of Sn takes place during the

pyrolysis step

low thickness for the oxide film: 1m

(presence of Ti from the support)

35 % Ir

Atomic Force Microscopy Images

Ir 0% - Sn 100%

No formation of nanoaggregates

Formation of nanoaggregates

Ir 35% - Sn 65%

Atomic Force Microscopy Images

Formation of microaggregates

Ir 20% - Sn 80%

X-ray Diffraction Analysis

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

1 1 0

1 0 1

2 1 1

Ti

TiTi Ti

XRD: test of the Vegard law

% Ir a = b100 4.54760 4.60620 4.6881 4.71

Formation of a metastable solid solution

y = -0.0017x + 4.7149

R2 = 0.9934

4.52

4.56

4.6

4.64

4.68

4.72

4.76

0 20 40 60 80 100

% IrO2

cell

par

amet

er a

=b

(am

stro

ng

)

0 20 40 60 80 100

66

67

68

69

70

71

72

Cel

l Vol

ume

% IrO2

XRD - Particle size vs Composition

Particle Size:

35% 2.5 nm

Cell Volume

0 10 20 30 40 50 60 70 80 90 10020

30

40

50

60

70

80

<

D>

vol Å

% IrO2

Cyclovoltammetric Characterization

0.4 0.6 0.8 1.0 1.2 1.4

-40

-20

0

20

40

IrO2-SnO

2 35 % a/o in HClO

4 1 M @ 298 K

j (A

/m2 )

E (V vs NHE)

92 C/g Ir

( )x x n nIrO n H e IrO OH

Electrolyte: HClO4 1N

Potential range: 0.151.15 V

Scan rate: 100mV/sec


HClO4 1N - Potential window: 0.151.15 V - Scan rate: 100mV/sec

Effect of the catalyst composition on the anodic chargeEffect of the catalyst composition on the anodic charge


Supporting electrolyte: HClO4 1N - Potential window: 0.151.15 V

Effect of the catalyst loading on the anodic chargeEffect of the catalyst loading on the anodic charge

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

-5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.51.15

1.20

1.25

1.30

1.35

1.40

1.45

1.50

1.55

1.60

Polarization Curve 50% IrO2-SnO

2, HClO

41M @ 298 K

b = 43 mV/dec

Pot

entia

l (V

vs

SC

E)

log j (A/cm2)

Kinetic study: considerations

% Ir bexp

(mV/dec)

30 46

35 46

40 47

50 43

1eff

expOH

bb

bexp beff S-OH « 1

Optimal catalytic activity (high TurnOver number)

Kinetic study of the Oxygen Evolution Reaction

Hypothesis

• H2O (H2O)ads

• first step in equilibrium

• second step rate determining

• low overpotentials

• s-OH 0

2 1 2 exp 3 2 OHv K k F RT

K1 (reql constant)

Electrochemical Oxide formation

1

1

2

3

3

2

2 3

2

1)

2)

3)2 2

k

k

k

k

k

S H O S OH H e

S OH H O S O H O e

S O S O

22.303*

log 3

RT

j F

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

Documents

Achille De Battisti Carlos Alberto Martinez-Huitle Sergio Ferro Laboratory of Electrochemistry University of Ferrara, Italy Implementation of Advanced