Hydrogen Peroxide Production DOI: 10.1002/anie.200801004 H 2 O 2 Generation by Decamethylferrocene at a Liquid j Liquid Interface** Bin Su, Raheleh Partovi Nia, Fei Li, Mohamad Hojeij, Michel Prudent, ClØmence Corminboeuf, Zdenek Samec, and Hubert H. Girault* The reduction of molecular oxygen (O 2 ) is a technologically important research topic, particularly in the context of a hydrogen economy. The O 2 reduction reaction (ORR) can proceedbyadirectfour-electronreductiontoproducewater or a two-electron reduction to give hydrogen peroxide, with the former pathway being highly desirable for fuel cell applications.Thedevelopmentoffuelcellsforthecombined production of electricity and hydrogen peroxide (H 2 O 2 ) has been proposed recently. [1] H 2 O 2 is an industrially important productthatisusedonascaleofaboutthreemillionmetric tons per year worldwide. Its production is currently based almost exclusively on the anthraquinone hydrogenation and oxidation process. [2] Many alternative routes have also been developed, one of which is the electrochemical cathodic reduction of O 2 [2–4] in the presence of molecular electro- catalysts such as metalloporphyrins [5] and anthraquinones. [6] Herein we present an electrochemical method for pro- ducingH 2 O 2 atasoftmolecularinterfaceratherthanatasolid electrode. This approach relies on controlling the interfacial Galvanipotentialdifferencebetweentwoimmisciblephases, inotherwordsthepolarizationattheliquid j liquidinterface, toallowthereductionofO 2 toH 2 O 2 .WechosetheORRby ferrocene derivatives, a reaction that has been known for many years, to illustrate this principle. [7,8] Specifically, we studiedthereductionofO 2 bydecamethylferrocene(DMFc) in 1,2-dichloroethane (DCE) [9] in contact with an aqueous solutionofsulfuricacid.Asdescribedbelow,thepolarization of the interface can be controlled by the distribution of different salts. The main advantage of the present biphasic system is that the ORR can be stopped at the formation of H 2 O 2 ,whichcanbedirectlyextractedintotheaqueousphase during the reaction. H 2 O 2 is a strong oxidant that readily oxidizes ferrocene derivatives, which usually leads to the absence of H 2 O 2 in the final products of the homogeneous oxidationofferrocenederivativesbyO 2 . [8,10] Whenaninterfaceisformedbetweenanaqueouselectro- lyte containing hydrophilic ions and an organic solution containinglipophilicions,thisinterfacebecomespolarizable. This leads to a polarization potential window, the width of which is defined by the transfer reactions of the electrolyte ions across the interface. The potential dependence for ion transferacrosstheinterfacefollowsaNernstequation: D W O  ¼ D W O  O i þ RT z i F ln  a O i a W i  ð1Þ where D W O  O i represents the standard transfer potential, in other words the Gibbs energy of transfer expressed in a voltage scale. For example, using Li 2 SO 4 and bis(triphenyl- phosphoranylidene)ammonium tetrakis(pentafluorophenyl)- borate (BTPPATPFB) as the hydrophilic and lipophilic electrolytes, respectively, in water and DCE results in a potentialwindowfrom 0.5to0.4V,asshownbythedotted lineinFigure1.Thiswindowisdeterminedbythetransferof Li + and SO 4 2 ions from water to DCE at positive and negative potentials respectively, since BTPPA + and TPFB  ions are too lipophilic to transfer first. If an ion having a medium lipophilicity, for example decamethylferrocenium (DMFc + )inDCE,ispresentitwilltransferwithintheabove potentialwindow,andthistransfercanbemonitoredbycyclic voltammetry,asshownbythefulllineinFigure1. Figure 1. Polarizationofawater j DCEinterfacebyvariouscommon ions.Thedottedandfulllinesshowthepotentialwindowandtransfer ofDMFc + duetopolarizationbyexternalvoltages,respectively. [*] Dr.B.Su,R.P.Nia,Dr.F.Li,M.Hojeij,M.Prudent, Prof.H.H.Girault Laboratoired’ElectrochimiePhysiqueetAnalytique EcolePolytechniqueFØdØraledeLausanne(EPFL) Station6,1015Lausanne(Switzerland) Fax:(+ 41)21-693-3667 E-mail:hubert.girault@epfl.ch Homepage:http://lepa.epfl.ch Prof.C.Corminboeuf LaboratoryforComputationalMolecularDesign EcolePolytechniqueFØdØraledeLausanne(EPFL) BCH,1015Lausanne(Switzerland) Prof.Z.Samec DepartmentofElectrochemistry J.HeyrovskyInstituteofPhysicalChemistry AcademyofSciencesoftheCzechRepublic Dolejskova3,18223Prague8(CzechRepublic) [**] ThisworkwassupportedbytheEPFLandEuropeanCOSTAction (D36/007/06).WethankProfessorsX.HuandJ.Waserforfruitful discussions.ThetechnicalassistanceofV.Devaudisalso acknowledged. SupportinginformationforthisarticleisavailableontheWWW underhttp://www.angewandte.orgorfromtheauthor. Angewandte Chemie 4675 Angew. Chem. Int. Ed. 2008, 47, 4675–4678 # 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim