1 / 38

The Role of Electrode Material in Applied Electrochemistry

The Role of Electrode Material in Applied Electrochemistry. Christos Comninellis Swiss Federal Institute of Technology ISP-GGEC-SB-EPFL 1015- Lausanne, Switzerland. OUTLINE OF THE PRESENTATION. 1. Classification of electrochemical (anodic) reactions in aqueous media.

bono
Download Presentation

The Role of Electrode Material in Applied Electrochemistry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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+1 + 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 + 3H+ + 3e- (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)62+ Ru(NH3)63+ + 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. 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 O2 + 4H+ + 4e- 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. 3. Electrochemical oxygen transfer (EOT)reactions in acid media RH + H2O RO + 3H+ + 3e- 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 + 2H+ + 2e- (E-R) (RH)ads + (OH*)ads RO + 2H+ + 2e- (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 (0.3-0.5 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 DMFCapplication Methanol oxidation at Pt nanoparticles • Dehydrogenation :Inner-sphere electron transfer (fast) • Pt3(CH3OH)ads Pt(CO)ads + 2Pt + 4H+ + 4e- • Oxidation : Electrochemical oxygen transfer reaction (rds) • Pt(CO)ads + H2O Pt + CO2 + 2H+ + 2e-

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

  12. 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 2Pt + CO2 + 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 + CO2 + 2H+ + 2e- • Two main approaches: • Electronic effect (modification of the Pt work function) • Weakening of the Pt-CO bonddue to theshift 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 = 0 no electronic effect • For the other d and sp metals Dc > 0 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. OH CO OH CO M M M M OH OH CO CO M M Pt Pt 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 2M + CO2 + H+ + e- • 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 + CO2 + H+ + e-

  19. 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 MOat the Pt- MOboundary. M + H2O MO + 2H+ + 2e- Pt(CO)ads + MO M + Pt + CO2 O O CO CO MO MO Pt Pt Exemples of redox catalysis: WOx, MoOx,VOx,RuOx

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

  21. 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 2M + H2O2 H2O2 O2 + 2H+ + 2e- 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. + - + ® + + IrO H O IrO ( OH ) H e 2 2 2 1 + - ® + + ® + IrO ( OH ) IrO H e IrO IrO O 2 3 3 2 2 2 Case III: oxygen evolution in acid media On IrO2 anode : (Active) On BDD anode : (Non-active) + - + ® + + 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 DE 5000 DE IrO2 4000 3000 2000 BDD 1000 0 0 4 8 12 -1 specific charge [Ah L ] BDD (non-active) IrO2(active) oxalic acid conc. [mol L-1]

  26. Oxidation of organics on non-active (BDD) and active (IrO2) electrodes

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

  28. Preparation of BDD electrodes by HF CVD Growth rate : 0.24 mm/h Thickness : 1 mm H2 2H* 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 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 500-8000 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. BH4- reducing agent metallic ion (Pt2+) encapsulated nanoparticle dendrimer in solution complex ion 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.

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

  33. 1) NaBH4 3) 2) + A Preparation of Pt and Pt alloy nanoparticles using the microemulsion technique A = pure precursors or mixtures of precursors : H2PtCl6, H2RuCl6,SnCl4 3 % water; 80.4 % n-heptane ; surfactant (BRIJ-30)

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

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

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

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

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

More Related