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The Role of Electrode Material in Applied Electrochemistry Christos Comninellis Swiss Federal Institute of Technology ISP-GGEC-SB-EPFL 1015- Lausanne,

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Presentation on theme: "The Role of Electrode Material in Applied Electrochemistry Christos Comninellis Swiss Federal Institute of Technology ISP-GGEC-SB-EPFL 1015- Lausanne,"— 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 1. Classification of electrochemical (anodic) reactions in aqueous media Outer-sphere electron transfer reactions (facile) M n M n+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 + H 2 O RO + 3H + + 3e - (RH : organic compound) OUTLINE OF THE PRESENTATION 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 1.Outer-sphere electron transfer anodic reactions Ru(NH 3 ) 6 2+ Ru(NH 3 ) 6 3+ + 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 - E o = 1.5 V Ce(III) Ce(IV) + e - E o = 1.7 V Co(II) Co(III) + e - E o = 1.9 V Ag(I) Ag(II) + e - E o = 2.0 V These reactions are usually fast and take place close to the thermodynamic potential (low overvoltage)

5 Pre-requirement conditions -Slow kinetics for oxygen evolution (main side reaction) 2H 2 O O 2 + 4H + + 4e - E o = 1.23 V - High anodic stability in acid media 1M HClO 4 25 o C Thermodynamics Case I :Application of outer-sphere electron transfer reactions in applied electrochemistry

6 Case I :Application of outer-sphere electron transfer reactions in applied electrochemistry a)Indirect in-cell oxidation using catalytic amounts of the outer sphere mediator (Application to the destruction of organic pollutants using Ag 2+ /Ag + in HNO 3 ) b)Indirect ex-cell oxidation using stoichiometric amounts of Mn 3+ /Mn 2+ in H 2 SO4

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 + H 2 O RO + 3H + + 3e - A two step reaction: I) Water activation (H 2 O) 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 E thermodynamic ) (H 2 O) ads (OH * ) ads + H + + e - This is the case of non electrocatalytic electrodes like IrO 2,SnO 2,PbO 2 or BDD. The discharge can take place at potentials above the thermodynamic potential (1.23 V/SHE) and OH* are generally weekly adsorbed.

10 Methanol oxidation at Pt nanoparticles Dehydrogenation :Inner-sphere electron transfer (fast) Pt 3 (CH 3 OH) ads Pt(CO) ads + 2Pt + 4H + + 4e - Oxidation : Electrochemical oxygen transfer reaction (rds) Pt(CO) ads + H 2 O Pt + CO 2 + 2H + + 2e - Case II : Methanol oxidation for DMFC application

11 1 M HClO 4 1 M HClO 4 + Methanol (0.1-1M) Thermodynamics (0.046 V/RHE)

12 Electrochemical oxygen transfer from H 2 O to CO at Pt Pt + H 2 O Pt(OH*) ads + Pt(H*) ads (1) Pt(H*) ads Pt + H + + e - (fast) (2) Pt(CO) ads + Pt(OH*) ads 2Pt + CO 2 + 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. Case II : Methanol oxidation for DMFC application

13 Recherche of Pt -Metal alloys in order to decrease the activation energy of the electrochemical oxygen transfer reaction: Pt(CO) ads + H 2 O Pt + CO 2 + 2H + + 2e - Two main approaches: i)Electronic effect (modification of the Pt work function) Weakening of the Pt-CO bond due to the shift of the d electron from the alloy metal M to Pt. XPS Pt4f spectra for Pt alloy nanoparticles Pt/Ni (1:1) Pt/Ni (3:1) Pt/Ru/Ni (5:4:1) Pt/Ru (1:1)Pt Case II : Methanol oxidation for DMFC application

14 ii) Cooperative effect (bifunctiomal mechanism) The second alloy metal activates water at low potentials and hence promoting CO electro-oxidation. Pt M Case II : Methanol oxidation for DMFC application

15 Electronegativity (  ) of elements according to Pauling  in case of Pt/Ru alloy small XPS shift no electronic effect  in case of Pt/Ni alloy high XPS shift shift of d electrons from Ni to Pt weakening of the Pt-CO bond  Pt -  Ni Remarks : For the other noble metals  no electronic effect For the other d and sp metals  electronic effect Corrosion problems with non-noble metals in acid medium Case II : Methanol oxidation for DMFC application

16 Candidate metals in the bifunctional mechanism A A : dissociation energy of H 2 O H 2 O OH* + H* M-O (kJ/mol) Activity Water activation (react. 1) is favorable at metals with high M-O dissociation energy M + H 2 O MOH + H + + e - (1) CO oxydation (react.2) is favorable at metals with low M-O dissociation energy MOH + CO M + CO 2 + H + (2) Case II : Methanol oxidation for DMFC application

17 Water activation on M in the M-Pt composite catalyst 1. Water adsorption on active sites M M + H 2 O M(H 2 O) ads 2. Water dissociation followed by oxidation of the adsorbed hydrogen in a fast reaction. M(H 2 O) 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(H 2 O) ads M(OH*) ads MO M : non active (M is not participating in the reaction) M: active (M participates in the reaction) Case II : Methanol oxidation for DMFC application

18 CO oxidation on a non active M metal in the Pt/M catalyst a)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 + CO 2 + H + + e - b)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 + CO 2 + H + + e - MMMM OH CO MMPt OH CO Case II : Methanol oxidation for DMFC application

19 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 MO at the Pt- MO boundary. Pt(CO) ads + MO M + Pt + CO 2 M + H 2 O MO + 2H + + 2e - MO Pt OOCO Exemples of redox catalysis: WO x, MoO x,VO x,RuO x Case II : Methanol oxidation for DMFC application

20 Methanol oxidation at Pt- M alloys (synergetic effect) MeOH electrooxidation % at. Pt j / A g -1 Pt

21 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…) Case II : Methanol oxidation for DMFC application

22 Case III: oxygen evolution in acid media 1. Water adsorption on active sites M M + H 2 O M(H 2 O) ads 2. Water discharge M(H 2 O) ads M(OH*)a ds + H + + e - i) Non active electrode 2 M(OH*) ads 2M + H 2 O 2 H 2 O 2 O 2 + 2H + + 2e - ii) Active electrode Interaction of (OH*) ads with the electrode forming an active oxide. M(OH*) ads MO + H + + e - 2MO M + O 2 M(H 2 O) ads M(OH*) ads MO Non active electrode (is not participating in the reaction) Active electrode (participates in the reaction)

23 Case III: oxygen evolution in acid media   eH)OH(IrOOH 222   eH )OH(IrO 32 223 O 2 1    eH)OH(BDDOH 2 1 )OH(BDD  2 O 2    eH On IrO 2 anode : (Active) On BDD anode : (Non-active)

24 Typical acive and non active electrodes in acid media Active Electrodes: RuO 2 based electrodes : RuO 2 -TiO 2 IrO 2 based electrodes : IrO 2 -Ta 2 O 5 Non-active Electrodes: SnO 2 based electrodes : SnO 2 -Sb 2 O 5 TiO 2 based electrodes : TiO 2 -NbO x Diamond based electrodes : boron doped diamond (BDD) Case III: oxygen evolution in acid media

25 Case IV:Oxalic acid oxidation EE EE IrO 2 (active) BDD (non-active) 0 1000 2000 3000 4000 5000 04812 specific charge [Ah L ] IrO 2 BDD oxalic acid conc. [mol L -1 ]

26 Oxidation of organics on non-active (BDD) and active (IrO 2 ) electrodes

27 Preparation of the DSA electrodes for Cl 2 production Electrode DSA-Cl 2

28 Preparation of BDD electrodes by HF CVD 1% CH4 in H2 + 3 ppm trimethylboron (100 tor) Filament (2500 o C) p-Si substrate (830 o C) Growth rate : 0.24  m/h Thickness : 1  m H 2 2H*

29 Morphological characterization of BDD electrodes Silicon substrate: HF-CVD technique (CSEM, Switzerland) p-type single crystal (Siltronix) resistivity 1-3 m  cm thickness 1  m ( ± 10%) non-diamond carbon < 1% of diamond carbon 500-8000 ppm boron (resistivity 15 m  cm ( ± 10%)) BDD film: SEM image of a BDD Raman spectrum of a BDD: (1) p-Si substrate, (2) sp 3 carbon and (3) sp 2 carbon

30 Large scale (50x100 cm) production of BDD electrodes CONDIAS GmbH D-Braunschweig, GERMANY CSEM CH-Neuchâtel, SWITZERLAND

31 Structure of the dendrimaric polymers PAMAM G4-NH 2 Principe of nanoparticules using dendrimeric polymers pH ~ 5, molar ratio : Pt 2+ / G4-NH2 = 30. complex ion metallic ion (Pt 2+ ) BH 4 - reducing agent encapsulated nanoparticle dendrimer in solution Preparation of Pt nanoparticles using dendrimeric polymers

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

33 3 % water; 80.4 % n-heptane ; surfactant (BRIJ-30) Preparation of Pt and Pt alloy nanoparticles using the microemulsion technique 1) A + NaBH 4 2) 3) A = pure precursors or mixtures of precursors : H 2 PtCl 6, H 2 RuCl 6,SnCl 4

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 eV XPS CPS

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

37 Metallic nanoparticles on BDD. Application to electrocatalysis. 2-steps synthesis of Au nanoparticles 1) Sputtering of a thin Au film on diamond 2) Thermal decomposition in air at ~ 600 °C 20 s sputtering50 s sputtering40 s sputtering

38 Pt loading : 50  g /cm 2, Average particle size : 2  nm Electrochemical preparation of Pt nanoparticles on BDD


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