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

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 …

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

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

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)

biodegradable effluent the biological (aerobic/anaerobic) treatment toxic or non-biodegradable effluent 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). Ch. Comninellis, Electrochim. Acta, 39 (1994) 1857

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

Redox Mediators: 3.2 INDIRECT ELECTROCHEMICAL OXIDATION OH + H+ + e  H2O 2.74 V O3 + 2H+ + 2e  O2 + H2O 2.07 V S2O8= + 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+ H2O 1.63 V HBrO + H+ + e  1/2Br2 + H2O 1.59 V Mn3+ + e  Mn2+ 1.51 V MnO4- + 8H+ + 5e  Mn2+ + 4H2O 1.491 V Cr2O7= +14H+ + 6e  2Cr3+ + 7H2O 1.33 V IO3- + 6H+ + 5e  1/2I2 + 3H2O 1.195 V

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

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 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 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

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 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

Oxalic Acid – direct oxidation 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

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

(H2C2O4)ads + 2(OH)ads 2CO2 + 2H2O 3.3 Electrochemical Oxidation – The electrode material •OH radicals mechanism H2C2O4 (H2C2O4)ads + H+ + e- H2O (OH)ads + H+ + e- (H2C2O4)ads + 2(OH)ads 2CO2 + 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 J @ 1.0 mA/cm2

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

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

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

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

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

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

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

O 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

a real approach… for the potabilization of water 23 cm 29 cm Characteristics 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 pH > 9 W

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

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

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:

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

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)

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

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

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

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

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

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

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

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

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

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)

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)

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

Dependence of current density for o. e 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