Presentation on theme: "The Role of Electrode Material in Applied Electrochemistry"— Presentation transcript:
1The Role of Electrode Material in Applied Electrochemistry Christos ComninellisSwiss Federal Institute of TechnologyISP-GGEC-SB-EPFL1015- Lausanne, Switzerland
2OUTLINE OF THE PRESENTATION 1. Classification of electrochemical (anodic) reactions in aqueous mediaOuter-sphere electron transfer reactions (facile)Mn Mn 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 H e (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 mediaCase IV: Direct organic oxidation
3Outer-sphere electron transfer anodic reactions Ru(NH3) Ru(NH3) 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 electronsThe 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.
4Anodic reactions in non-complexing aqueous acid media Typical « pseudo » outer-sphere electron transfer reactions in applied electrochemistryAnodic reactions in non-complexing aqueous acid mediaMn(II) Mn(III) e Eo = 1.5 VCe(III) Ce(IV) e Eo = 1.7 VCo(II) Co(III) e Eo = 1.9 VAg(I) Ag(II) e Eo = 2.0 VThese reactions are usually fast and take place close to the thermodynamic potential (low overvoltage)
5Case I :Application of outer-sphere electron transfer reactions in applied electrochemistryPre-requirement conditionsSlow kinetics for oxygen evolution (main side reaction)2H2O O H e Eo = 1.23 V- High anodic stability in acid media1M HClO425oCThermodynamics
6Case I :Application of outer-sphere electron transfer reactions in applied electrochemistryIndirect 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
72. Inner-sphere electron transfer anodic reactions (dehydrogenation) Dissociative adsorption of the organic compound RHRH (RH)ads (R*)ads (H*)adsDischarge 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 reactionsInner-sphere reactions are generally fast reactions at electrocatalytic electrodes (Pt)Generally there are problems of electrode poisoning
8Langmuir-Hinshelwood (L-H) 3. Electrochemical oxygen transfer (EOT) reactions in acid mediaRH H2O RO H e-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 H e (E-R)(RH)ads (OH*)ads RO H e (L-H)Eley-Rideal (E-R)Langmuir-Hinshelwood (L-H)
93. 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 cantake place at low potentials ( 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. Thedischarge can take place at potentials above the thermodynamic potential (1.23 V/SHE)and OH* are generally weekly adsorbed.
10Case II : Methanol oxidation for DMFC application Methanol oxidation at Pt nanoparticlesDehydrogenation :Inner-sphere electron transfer (fast)Pt3(CH3OH)ads Pt(CO)ads + 2Pt + 4H e-Oxidation : Electrochemical oxygen transfer reaction (rds)Pt(CO)ads H2O Pt CO H e-
11Case II : Methanol oxidation for DMFC application 1 M HClO41 M HClO4 + Methanol (0.1-1M)Thermodynamics(0.046 V/RHE)
12Electrochemical oxygen transfer from H2O to CO at Pt Case II : Methanol oxidation for DMFC applicationElectrochemical oxygen transfer from H2O to CO at PtPt H2O Pt(OH*)ads Pt(H*)ads (1)Pt(H*)ads Pt H e (fast) (2)Pt(CO)ads Pt(OH*)ads Pt CO H e (3)Pt does not activate water (react. 1) below 0.4 V/RHECO is strongly adsorbed on Pt blocking the active sitesReaction (3) follows the Lagmuir- Hinshelwood mechanismThermo.
13Case 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 CO H e-Two main approaches: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.Pt/Ni (1:1)XPS Pt4f spectra for Pt alloy nanoparticlesPt/Ni (3:1)Pt/Ru/Ni (5:4:1)PtPt/Ru (1:1)
14Case II : Methanol oxidation for DMFC application ii) Cooperative effect (bifunctiomal mechanism)The second alloy metal activates water at low potentials and hencepromoting CO electro-oxidation.PtM
15Case II : Methanol oxidation for DMFC application Electronegativity ( c ) of elements according to PaulingDc = cPt - cNiDc = 0 in case of Pt/Ru alloy small XPS shift no electronic effectDc = 0.4 in case of Pt/Ni alloy high XPS shift shift of d electrons from Ni to Ptweakening of the Pt-CO bondRemarks :For the other noble metals Dc = no electronic effectFor the other d and sp metals Dc > electronic effectCorrosion problems with non-noble metals in acid medium
16Case II : Methanol oxidation for DMFC application Candidate metals in the bifunctional mechanismM-O (kJ/mol)AActivityWater activation (react. 1) is favorable at metals with high M-O dissociation energyM + H2O MOH + H e (1)CO oxydation (react.2) is favorable at metals with low M-O dissociation energyMOH + CO M CO2 + H (2)A : dissociation energy of H2OH2O OH* H*
17Case II : Methanol oxidation for DMFC application Water activation on M in the M-Pt composite catalyst1. Water adsorption on active sites MM H2O M(H2O)ads2. Water dissociation followed by oxidation ofthe adsorbed hydrogen in a fast reaction.M(H2O)ads M M(OH*)ads M(H*)adsM(H*)ads M H e-3. Interaction of (OH*)ads with the catalyst formingan active oxide.M(OH*)ads MO H e-M(H2O)adsM : non active(M is not participating in the reaction)M(OH*)adsM: active(M participates in the reaction)MO
18CO oxidation on a non active M metal in the Pt/M catalyst Case II : Methanol oxidation for DMFC applicationCO oxidation on a non active M metal in the Pt/M catalystCO 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 M CO H e-MOHCOCO 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 H e-MPtOHCO
19CO oxidation on an active metal in the Pt/MO catalyst Case II : Methanol oxidation for DMFC applicationCO oxidation on an active metal in the Pt/MO catalystThe oxidic species MO are not covered by CO.In case CO reacts with electrogenerated MO at the Pt- MO boundary.M H2O MO H e-Pt(CO)ads + MO M + Pt CO2OOCOCOMOMOPtPtExemples of redox catalysis: WOx, MoOx,VOx,RuOx
20Case II : Methanol oxidation for DMFC application Methanol oxidation at Pt- M alloys (synergetic effect)MeOH electrooxidation% at. Ptj / A g-1 Pt
21Problems with Pt/M and Pt/MO composite catalysts Case II : Methanol oxidation for DMFC applicationProblems with Pt/M and Pt/MO composite catalystsLimited 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 dramaticallythe catalytic activity of methanol dehydrogenation (modification of electronicor/and structural properties of Pt)Preparation of ternary (Pt-Ru-Sn) or quaternary (Pt-Ru-Ir-Os) alloys is complexand can increase dramatically the production cost.Corrosion problems in case of non-noble element alloy (M = Ni,Sn,Mo…)
22Case III: oxygen evolution in acid media 1. Water adsorption on active sites MM H2O M(H2O)ads2. Water dischargeM(H2O)ads M(OH*)ads + H+ + e-i) Non active electrode2 M(OH*)ads M H2O2H2O O H e-ii) Active electrodeInteraction of (OH*)ads with the electrode formingan active oxide.M(OH*)ads MO H e-2MO M O2M(H2O)adsNon active electrode(is not participating in the reaction)M(OH*)adsActive electrode(participates in the reaction)MO
23Case III: oxygen evolution in acid media On IrO2 anode : (Active)On BDD anode : (Non-active)Also the oxygen evolution reaction was studied at BDD and BDD/IrO2 electrodes. The cyclic voltammogram in perchloric acid shows the potential shift of about 1 V for the oxygen evolution reaction before and after IrO2 deposition for a IrO2 loading G = 6.4. This change can be explain by a changing in the mechanism of the oxygen evolution. In black you can see the mechanism path in case of a non active electrode like BDD. The first step is the discharge of water and the formation of hydroxyl radicals physisorbed on the surface. Afterwards, the hydroxyl groups react together to form oxygen. In case of an active electrode like IrO2, hydroxyl radicals physisorbed on the surface are introduced in the lattice and the metal oxide reaches a higher oxidation state passing to IV to VI. The active site returns to the initial oxidation state and oxygen is liberated. The fact that the OER on a BD/IrO2 electrode starts at 1.45 V, that corresponds to the redox potential of the couple Ir(VI)/Ir(IV) confirms this assumption. The catalytic activity of IrO2 towards the OER lead to a mechanism change also with a very low amount of oxide iridium on a BDD surface.-+eH)OH(IrOO231BDD+HOBDD(OH)+H++e-21BDD(OH)BDDO+H++e-22
24Case III: oxygen evolution in acid media Typical acive and non active electrodes in acid mediaActive Electrodes:RuO2 based electrodes : RuO2-TiO2IrO2 based electrodes : IrO2-Ta2O5Non-active Electrodes:SnO2 based electrodes : SnO2-Sb2O5TiO2 based electrodes : TiO2-NbOxDiamond based electrodes : boron doped diamond (BDD)
25Case IV:Oxalic acid oxidation BDD (non-active)DEIrO2(active)100020003000400050004812specific charge [Ah L-1]IrO2BDDoxalic acid conc. [mol L-1]The presence of oxalic acid in the solution also influences the overpotential for the electrolyte decomposition.The fact that the response of a BDD electrode is more sensitive than that of a BDD/IrO2 to the organic concentration indicates a different mechanism for the reaction of organic oxidation.
26Oxidation of organics on non-active (BDD) and active (IrO2) electrodes The proposed model is valid for organic oxidation under oxygen evolution conditions.The mechanism involves, as already sow in a preceding transparency, hydroxyl radicals previously physisorbed on the electrode surface.In case of a BDD electrode, hydroxyl radicals act as intermediates in the oxidation of organic compounds.
27Preparation of the DSA electrodes for Cl2 production Electrode DSA-Cl2
28Preparation of BDD electrodes by HF CVD Growth rate : mm/hThickness : 1 mmH H*Filament(2500oC)p-Si substrate(830oC)(100 tor)1% CH4 in H2+ 3 ppm trimethylboron
29Morphological characterization of BDD electrodes SEM image of a BDDHF-CVD technique (CSEM, Switzerland)Silicon substrate:p-type single crystal (Siltronix)resistivity 1-3 mW cmNow let ’s move on the experimental details about electrode preparation.In case of a BDD, electrodes are prepared at the Swiss Center for Electronics and Microtecnology by the Hot Filament Chemical Vapor Deposition technique.The substrate was a p-type single crystal of silicon with a resistivity of 1-3mW cm.The diamond film obtained has a thickness of 1 mm. The SEM image shows the random textured, polycrystalline film of BDDThe raman spectrum shows three peaks corresponding (1) to the silicon substrate, (2) to the sp3 carbon (diamond) and (3) to the sp2 carbon (the non diamond). The non-diamond carbon (sp2) is less than 1% of the diamond carbon.The boron level is around 8000 ppm in order to obtain a low resistivity of the film: around 15 mW cm.Raman spectrum of a BDD: (1) p-Si substrate, (2) sp3 carbon and (3) sp2 carbonBDD film:thickness 1mm (± 10%)non-diamond carbon < 1% of diamond carbonppm boron (resistivity 15 mW cm (± 10%))
30Large scale (50x100 cm) production of BDD electrodes CONDIAS GmbH D-Braunschweig, GERMANYCSEM CH-Neuchâtel, SWITZERLAND
31encapsulated nanoparticle Preparation of Pt nanoparticles using dendrimeric polymersStructure of thedendrimaric polymersPAMAM G4-NH2Principe of nanoparticules using dendrimeric polymers pH ~ 5, molar ratio : Pt2+ / G4-NH2 = 30.dendrimerin solutioncomplex ionmetallic ion(Pt2+)BH4-reducing agentencapsulated nanoparticle
32Preparation of Pt nanoparticles using dendrimeric polymers Pt particle size distribution.Freq. / %d / nm
33+ Preparation of Pt and Pt alloy nanoparticles using the microemulsion technique3)1)A+NaBH42)A = pure precursors or mixtures of precursors : H2PtCl6, H2RuCl6,SnCl43 % water; % n-heptane ; surfactant (BRIJ-30)
34Preparation of Pt-Ru alloys by the microemulsion technique
35Preparation of Pt nanoparticles using the microemulsion technique Pt particle size distributionXPSCPSFreq. / %d / nmeV
36Deposition of Pt and Pt alloys on BDD Pt nanoparticlesBDD
372-steps synthesis of Au nanoparticles 1) Sputtering of a thin Au film on diamond2) Thermal decomposition in air at ~ 600 °C20 s sputtering40 s sputtering50 s sputteringMetallic nanoparticles on BDD. Application to electrocatalysis.
38Electrochemical preparation of Pt nanoparticles on BDD Pt loading : 50 mg /cm2, Average particle size : 200 nm