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 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

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.

Anodic reactions in non-complexing aqueous acid media 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)

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

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

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

Langmuir-Hinshelwood (L-H) 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)

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.

Case II : Methanol oxidation for DMFC application 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-

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

Electrochemical oxygen transfer from H2O to CO at Pt 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.

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 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 nanoparticles Pt/Ni (3:1) Pt/Ru/Ni (5:4:1) Pt Pt/Ru (1:1)

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

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

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*

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

CO oxidation on a non active M metal in the Pt/M catalyst 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- M OH CO 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- M Pt OH CO

CO oxidation on an active metal in the Pt/MO catalyst 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 MO at the Pt- MO boundary. 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

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

Problems with Pt/M and Pt/MO composite catalysts 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…)

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

Case 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. - + ® e H ) OH ( IrO O 2 3 1 BDD + H O ® BDD ( OH ) + H + + e - 2 1 BDD ( OH ) ® BDD + + O + H + + e - 2 2

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)

Case IV:Oxalic acid oxidation BDD (non-active) DE IrO2(active) 1000 2000 3000 4000 5000 4 8 12 specific charge [Ah L -1 ] IrO2 BDD oxalic 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.

Oxidation 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.

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

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

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 Now 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 BDD The 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 carbon BDD film: thickness 1mm (± 10%) non-diamond carbon < 1% of diamond carbon 500-8000 ppm boron (resistivity 15 mW cm (± 10%))

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

encapsulated nanoparticle 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. dendrimer in solution complex ion metallic ion (Pt2+) BH4- reducing agent encapsulated nanoparticle

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

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

Preparation of Pt-Ru alloys by the microemulsion technique

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

Deposition of Pt and Pt alloys on BDD Pt nanoparticles BDD

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 sputtering 40 s sputtering 50 s sputtering Metallic nanoparticles on BDD. Application to electrocatalysis.

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