Publications:

 

 

Published papers

[1]

Electrochemical Studies of ferrocene in a lithium ion conducting organic carbonate electrolyte.

Cormac O. Laoirea, Edward Plichtab, Mary Hendricksonb, Sanjeev Mukerjeea, K.M. Abrahama

a) Northeastern University, Department of Chemistry and Chemical Biology, Boston, MA 02115, USA
b) US Army CERDEC, Army Power Division, Ft. Monmouth, NJ 07703, USA

Electrochimica Acta 54 (2009) 6560–6564

 

a b s t r a c t
We carried out a detailed study of the kinetics of oxidation of ferrocene (Fc) to ferrocenium ion (Fc+) in the non-aqueous lithium ion conducting electrolyte composed of a solution of 1M LiPF6 in 1:1 EC:EMC solvent mixture. This study using cyclic (CV) and rotating disk electrode (RDE) voltammetry showed that the Fc0/Fc+ redox couple is reversible in this highly concentrated electrolyte. The ferrocene and ferrocenium ion diffusion coefficients (D) were calculated from these results. In addition, the electron transfer rate constant (k0) and the exchange current density for the oxidation of ferrocene were determined. A comparison of the kinetic data obtained from the two electrochemical techniques appears to show that
the data from the RDE experiments are more reliable because they are collected under strict mass transport control. A Tafel slope of c.a. 79 mV/decade and a transfer coefficient ˛ of 0.3 obtained fromanalysis of the RDE data for ferrocene oxidation suggest that the structure of the activated complex is closer to that of the oxidized specie due to strong interactions with the carbonate solvents. The experiments reported here are relevant to the study of redox reagents for the chemical overcharge protection of Li-ion batteries.

 

[2]

Electrocatalysis of Oxygen Reduction on Carbon Supported PtCo Catalysts Prepared by Water-in-Oil Micro-emulsion

Qinggang He, and Sanjeev Mukerjee*

Department of Chemistry and Chemical Biology, Northeasetrn University, 360 Huntington Avenue, Boston, MA 02115, USA

 

Electrochimica Acta 55(2010) 1709-1719


Abstract
Synthesis of carbon supported PtCo/C using micro-emulsion method including simultaneous procedure and sequential procedure in both acid and alkaline media was reported. UV-vis, electron microscopy were used to characterize the formation, surface morphology and distribution of PtCo nanoparticles. Crystallite structure of catalysts was analyzed from XRD patterns. Catalytic properties of PtCo/C catalysts synthesized were compared with commercial Pt/C using RDE based on both mass activity (M.A.) and specific activity (S.A.). PtCo/C catalysts prepared in both acidic and basic conditions showed better performance than commercial Pt/C catalyst. High temperature heat treatment was found useful only to PtCo/C by sequential procedure. The peroxide yield was explored also using RRDE technique. The H2O2 yield results were correlated with S.A. and R values (ratio of charge transferred about Co and Pt on the surface of catalyst) obtained from CVs in 1M KOH solution. A sacrificial Co oxidized effect on impediment of adsorption of OH may cause higher catalytic properties and higher H2O2 yield to Pt base alloy catalysts.

 

[3]

Enhanced activity and interfacial durability study of ultra low Pt based electrocatalysts prepared by ion beam assisted deposition (IBAD) method

N. Ramaswamya, T.M. Arrudaa, W. Wena, N. Hakima, M. Sahaa, A. Gulláb, S. Mukerjeea

a) Department of Chemistry and Chemical Biology, Northeastern University, 360 Huntington Avenue, Boston, MA 02115, USA
b) De Nora R&D Division, 625 East Street, Fairport Harbor, OH 44077, USA

 

Electrochimica Acta 54 (2009) 6756–6766

 

a b s t r a c t
Ultra low loading noble metal (0.04–0.12mgPt/cm2) based electrodes were obtained by direct metallization of non-catalyzed gas diffusion layers via dual ion beam assisted deposition (IBAD) method. Fuel cell performance results reported earlier indicate significant improvements in terms of mass specific power density of 0.297 gPt/kW with 250Ã… thick IBAD deposit (0.04mgPt/cm2 for a total MEA loading of 0.08mgPt/cm2) at 0.65V in contrast to the state of the art power density of 1.18 gPt/kW using 1mgPt(MEA)/cm2 at 0.65 V. In this article we report the peroxide radical initiated attack of the membrane electrode assembly utilizing IBAD electrodes in comparison to commercially available E-TEK (now BASF Fuel Cell GmbH) electrodes and find the pathway of membrane degradation as well. A novel segmented fuel cell is used for this purpose to relate membrane degradation to peroxide generation at the electrode electrolyte interface by means of systematic pre and post analyses of the membrane are presented. Also, we present the results of in situ X-ray absorption spectroscopy (XAS) experiments to elucidate the structure/property relationships of these electrodes that lead to superior performance in terms of gravimetric power density obtained during fuel cell operation.

 

[4]

Fundamental Investigation of Oxygen Reduction Reaction on Rhodium Sulfide-Based Chalcogenides
Joseph M. Ziegelbauer[†,‡], Daniel Gatewood§ Andrea F. Gulla ´,|, Maxime J.-F. Guinel
Frank Ernst, David E. Ramaker § and Sanjeev Mukerjee*,‡


Department of Chemistry and Chemical Biology, Northeastern UniVersity, Boston, Massachusetts 02115,
Department of Chemistry, The George Washington UniVersity, Washington, D.C. 20375, De Nora Research and
DeVelopment DiVision, 625 East Street, Fairport Harbor, Ohio 44077, and Department of Materials Science
and Engineering, Case Western ReserVe UniVersity, CleVeland, Ohio 44106

 

* To whom correspondence should be addressed. Phone: (617) 373-2382. Fax: (617) 373-8949. E-mail: s.mukerjee@neu.edu.
† Current Address: General Motors R&D Center, MC: 480-102-000, 30500 Mound Road, Warren, MI 48090.
‡ Northeastern University.
§ The George Washington University.
| De Nora Research and Development Division.
⊥ Case Western Reserve University.

 

J. Phys. Chem. C 2009, 113, 6955–6968


abstract
Synchrotron-based X-ray absorption spectroscopy (XAS), including the surface-specific ΔXANES technique,
is used to investigate the active reaction site for water activation and the oxygen reduction reaction (ORR)
on the novel, mixed-phase chalcogenide electrocatalyst RhxSy/C (De Nora). The specific adsorption of water,
OH, and O as a function of overpotential is reported. This study builds on a prior communication based
solely on interpreting the XAS spectra of RhxSy with respect to the metallic Rh3S4 phase. Here, a more extensive
overview of the electrocatalysis is provided on RhxSy/C, the thermally grown Rh2S3/C and Rh3S4/C preferential
phases and a standard 30 wt % Rh/C electrocatalyst, including results obtained by X-ray diffraction (XRD),
XAS, high-resolution transmission electron imaging, microanalysis, and electrochemical investigations. Heating
of the RhxSy catalysts to prepare the two preferential phases causes Rh segregation and the formation of Rh
metal particles, and immersion in TFMSA causes S dissolution and the formation of a Rh skin on the RhxSy
samples. It is shown that some Rh-Rh interactions are needed to carry out the ORR. This is present on the
Rh6 moieties in both the Rh3S4 and RhxSy catalysts, but a partial Rh skin (present from acid dissolution) is
also contributing to the ORR observed on RhxSy. This to our knowledge is the first time a reaction site in a
multiphase inorganic framework structure has been investigated in terms of electrocatalytic pathway for oxygen
reduction.

 

Carbon-supported PdM (M=Au and Sn) nanocatalysts for the electrooxidation of ethanol in high pH media
Qinggang He [a],Wei Chen[b], Sanjeev Mukerjeea[a*], Shaowei Chenb[b**], Frantiˇsek Laufek[c]
(a) Department of Chemistry and Chemical Biology, Northeastern University, 360 Huntington Avenue, Boston, MA 02115, United States
(b) Department of Chemistry and Biochemistry, University of California, 1156 High Street, Santa Cruz, CA 95064, United States
(c) Czech Geological Survey, Czech Republic

 

Journal of Power Sources 187 (2009) 298–304

a b s t r a c t
Carbon-supported Pd4Au- and Pd2.5Sn-alloyed nanoparticles were prepared by a chemical reduction method, and characterized by a wide array of experimental techniques including mass spectrometry, transmission electron microscopy, and X-ray diffraction spectroscopy. Ethanol electrooxidation on the as-synthesized catalysts and commercial Pt/C was then investigated and compared in alkaline media by cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy studies at room temperature. Voltammetric and chronoamperometric measurements showed higher current density and longer term stability in ethanol oxidation with the palladium alloy nanocatalysts than with the commercial one. Electrochemical impedance spectroscopy and Tafel plots were employed to examine the charge-transfer kinetics of ethanol electrooxidation. The results suggest that whereas the reaction kinetics might be somewhat more sluggish on the Pd-based alloy catalysts than on commercial Pt/C, the former appeared to have a higher tolerance to surface poisoning. Overall, the Pd-based alloy catalysts represent promising candidates for the electrocatalytic oxidation of ethanol, and Pd4Au/C displays the best catalytic activity among the series for the ethanol oxidation in alkaline media.

 

[5]

Carbon-Supported, Selenium-Modified Ruthenium- Molybdenum Catalysts for Oxygen Reduction in Acidic
Media Maxime J.-F. Guinel[b], Arman Bonakdarpour [a], Biao Wang[a, c], Panakkattu K. Babu [a], Frank Ernst[b], Nagappan Ramaswamy [d], Sanjeev Mukerjee[d], and Andrzej Wieckowski*[a]

 

[a] Dr. A. Bonakdarpour, Dr. B. Wang, Dr. P. K. Babu, Prof. A. Wieckowski Department of Chemistry
University of Illinois at Urbana–Champaign
600 South Mathews Avenue, Urbana, IL 61801 (USA)
Fax: (+1) 217-244-8068
E-mail: andrzej@scs.uiuc.edu
[b] Dr. M. J.-F. Guinel, Prof. F. Ernst
Department of Materials Science and Engineering
Case Western Reserve University
10900 Euclid Avenue, Cleveland, OH 44106-7204 (USA)
[c] Dr. B. Wang
College of Material Science and Engineering
Donghua University
1882 West Yan-an Road, Shanghai, 200051 (PR China)
[d] Dr. N. Ramaswamy, Prof. S. Mukerjee
Department of Chemistry and Chemical Biology
Northeastern University
360 Huntington Avenue, Boston, MA 02115 (USA)

 

ChemSusChem 2009, 2, 658 – 664

 

The stability and oxygen reduction activity of two carbon-supported catalyst materials are reported. The catalysts, Se/Ru and Se/ACHTUNGTRENUNG(Ru–Mo), were prepared by using a chemical reduction method. The catalyst nanoparticles were evenly dispersed onto globular amorphous carbon supports, and their average size was ca. 2.4 nm. Thermal treatment at 5008C for 2 h in an inert argon atmosphere resulted in coarsening of the nanoparticles, and also in some decrease of their activity. A gradual reduction of activity was also observed for Se/Ru during potential-cycle experiments. However, the incorporation of small amounts of Mo into the Se/Ru catalysts considerably improved the stability of the catalyst against dissolution. The Mo-containing samples showed excellent oxygen reduction activities even after cycling
the potential 1000 times between 0.7 and 0.9 V. Furthermore, they showed excellent fuel-cell behavior. The performance of
the Se/Ru catalysts is greatly improved by the addition of small amounts of elemental Mo. Possible mechanisms responsible for the improvement of the activity are discussed.

 

[6]

Influence of phosphate anion adsorption on the kinetics of oxygen reduction on low index Pt(hkl) single crystals
Qinggang Hea, Nagappan Ramaswamya, Sanjeev Mukerjeea* , Xiaofang Yangb, Frank Liub, Bruce Koelb, Wei Chenc, Shaowei Chenc* , Badri Shyamd, and David Ramakerd

a) Department of Chemistry & Chemical Biology, Northeastern University, 360 Huntington Ave, Boston, MA 02115 USA

b) Department of Chemistry and Center for Advanced Materials and Nanotechnology, Lehigh University, Bethlehem, PA 18015 USA

c) Department of Chemistry and Biochemistry, University of California, 1156 High Street, Santa Cruz,

CA 95064 USA

d) Department of Chemistry, George Washington University, 725 21st street N.W, Washington D.C. 20052 USA

J. Electrochim. Acta, (Under Review).

 

Abstract
The detrimental effects of phosphate anion adsorption on the oxygen reduction reactions (ORR) on low index Pt single crystal electrodes were studied in 0.1 M perchloric acid by using a hanging meniscus rotating disk electrode in the presence of varied concentrations of H3PO4. The kinetic current for ORR decreased dramatically on Pt(100), Pt(110), Pt(111), and PtSn(111) even with the addition of a small amount (1 mM) of H3PO4 into the perchloric acid solution, most probably due to the adsorption of phosphate anions onto the Pt active sites .

 

[7]

Promoting effect of CeO2 in the electrocatalytic activity of rhodium for ethanol electro-oxidation

Q. Hea, S. Mukerjeea, B. Shyamb, D. Ramakerb, S. Parres-Esclapezc, M.J. illan-Gomezc, A. Bueno-Lopezc*

(a) Department of Chemistry & Chemical Biology, Northeastern University, 360 Huntington Ave, Boston, 02115 MA USA
(b) Department of Chemistry, George Washington University, 725 21st Street N.W, Washington, DC 20052, USA
(c) Department of Inorganic Chemistry, University of Alicante, Ap. 99 E-03080, Alicante, Spain

 

Journal of Power Sources 193 (2009) 408–415


a b s t r a c t
The promoting effect of ceria in the electrocatalytic activity of rhodium for ethanol electro-oxidation in alkali media has been studied. Rh/C, CeO2/C and RhCeO2/C catalysts were synthesized and characterized by TEM, XRD, XPS, TG-MS, H2-TPR and XAS. The electrocatalytic activitywas studied by Cyclic Voltammetry (CV) and chronoam perometry. The onset potential of oxidation on RhCeO2/Cwas shifted negatively as compared to that on Rh/C, despite ceria itself does not show any electrocatalytic activity. The promoting effect of ceria has been attributed to the improved rhodium dispersion, and differences in the oxidation state of rhodium between Rh/C and RhCeO2/C were not found. The carbon support reduces rhodium species to Rh0, and also partially reduces ceria, during the samples preparation, and the surface of the carbon support is oxidised.

[8]

Influence of Nanoqueous Sovents on the Electrochemistry of Oxygen in the Rechargeab;eLithium -- Air Battery Cormac O. Laoire, Sanjeev Mukerjee, and K. M. Abraham


Department of Chemistry and Chemical Biology, Northeastern UniVersity, 360 Huntington AVenue, Boston, Massachusetts 02115


Edward J. Plichta and Mary A. Hendrickson
U.S. Army CERDEC, Army Power DiVision, Ft. Monmouth, New Jersey 07703

 

J. Phys. Chem. C 2010, 114, 9178-9186

 


ReceiVed: August 21, 2009; ReVised Manuscript ReceiVed: September 28, 2009 Unlocking the true energy capabilities of the lithium metal negative electrode in a lithium battery has until now been limited by the low capacity intercalation and conversion reactions at the positive electrodes. Abraham et al. (Abraham, K. M.; Jiang, Z. J. Electrochem. Soc.  1996, 143, 1-5) overcame this limitation by removing these electrodes and allowing lithium to react directly with oxygen in the atmosphere, forming the Li-air battery. The Li/O2 battery redox couple has a theoretical specific energy of 5200 W h/kg and represents the ultimate, environmentally friendly electrochemical power source. In this work, we report for the first time the intimate role of electrolyte, in particular the role of ion conducting salts, in determining the reversibility and kinetics of oxygen reduction in nonaqueous electrolytes designed for such applications. Such fundamental understanding of this high
energy density battery is crucial to harnessing its full energy potential. The kinetics and mechanisms of O2 reduction in solutions of hexafluorophosphate of the general formula A+PF6 -, where A ) tetrabutylammonium (TBA), K, Na, and Li, in acetonitrile are reported on glassy carbon electrodes using cyclic voltammetry (CV) and rotating disk electrode (RDE) techniques. The results show that the cations in the electrolyte strongly influence the reduction mechanism of O2. Larger cations represented by TBA salts displayed reversible O2/O2 - redox couple, in contrast to those containing the smaller Li (and other alkali metal) cations, where an irreversible one-electron reduction of O2 to LiO2, and other alkali metal superoxides, is shown to occur as the first process. It was also found the LiO2 formed initially decomposes to Li2O2. Electrochemical data support the view that alkali metal oxides formed via electrochemical and chemical reactions passivate the electrode surface, making the processes irreversible. The O2 reduction mechanisms in the presence of the different cations have been supplemented by kinetic parameters determined from detailed analyses of the CV and RDE data. The Lewis acid characteristics of the cation appear to be crucial in determining the reversibility of the system. The results of this study are expected to contribute to the rapid development of the Li-air battery.

[9]

Effect of RuOxHy Island Size on PtRu Particle Aging in Methanol Badri Shyam a, Thomas Arruda b, Sanjeev Mukerjee b and David E. Ramakera

(a) Department of Chemistry, The George Washington University, Washington D.C. 20052, USA
(b) Department of Chemistry and Chemical Biology, Northeastern University, Boston, MA 02115, USA

 

J.Phys.Chem.C 2009, 113, 19713-19721

 

Abstract
The aging properties of two different commercial PtRu black catalysts, Johnson-Matthey HiSpec6000 and Tanaka TEC90110, were observed in 1M trifluoromethanesulfonic acid (TFMSA) with 0.3 M methanol as a function of both time and potential cycling. Cu underpotential deposition, cyclic voltammetry and x-ray absorption spectroscopy, using both the EXAFS and Δμ-XANES analysis techniques, on samples after potential cycling between 0.02 and 0.8 V, revealed the PtRu aging mechanism, and offer an explanation why the two catalysts age differently. The Tanaka catalyst had relatively large RuOxHy islands on the surface and underwent little dissolution and agglomeration after 40 cycles. This had little detrimental effect on the CO oxidation properties as seen in the CO stripping data, and therefore this catalyst can be expected to be relatively more tolerant to poisoning in DMFCs. The Johnson-Matthey catalyst, on the other hand, was found to have much smaller Ru islands initially, underwent more Ru dissolution/agglomeration and showed definite signs of increase in Ru island size and partial oxidation to RuOxHy. The CO stripping data for this catalyst show a significant increase in onset potential with aging reflecting a change from the predominant bifunctional to ligand-effect mechanism
for the CO oxidation. Key words: PtRu aging, fuel cells, methanol, X-ray aborption spectroscopy

[10]

Contrast in Metal-Ligand Effects on PtnM Electrocatalysts with M equal Ru vs. Mo and Sn as Exhibited by in situ XANES and EXAFS Measurements in Methanol Frances J. Scott1, Sanjeev Mukerjee2, and David E. Ramaker1

1 Department of Chemistry, The George Washington University, Washington, DC 20052
2 Department of Chemistry, Northeastern University, Boston, MA 02115 s.mukerjee@neu.edu, ramaker@gwu.edu

 

J.Phys.Chem.C 2010, 114, 442-453

 


Abstract
In Situ X-Ray Absorption Spectroscopy (XAS) measurements, at the Pt L3 edge (XANES and EXAFS), were carried out on carbon-supported PtnMo and PtSn electrocatalysts in an electrochemical cell in 1 M HClO4 with 0.3 M methanol. The CO, OH, and H relative adsorbate coverages on Pt are determined as a function of the applied potential via the ΔXANES technique and compared with comparable data previously reported for PtnRu. The more reactive Sn and Mo atoms on the Pt surface form the oxide over the potentials of interest, while Ru has variable oxide content depending on Ru island size and potential. The strength of the electronic ligand effect appears to increase in the order Ru < MoOn < SnOn < RuOn, where the Pt-CO bond strength is found to decrease and the Pt-OH bond strength increase with ligand effect. In the Sn and Mo bimetallics, the ligand effect is found to be sufficiently strong to allow CO replacement by H2 at low potentials. These widely different ligand effects may provide a straightforward explanation for the previously observed anode behavior in fuel cells; PtnMo better in reformate but PtnRu better in methanol.

Keywords: X-Ray Absorption Spectroscopy, electrocatalysis, CO poisoning, XANES

 

[11]

Fundamental Aspects of Spontaneous Cathodic Deposition of Ru onto Pt/C Electrocatalystsand Membranes under Direct Methanol Fuel Cel Operating Conditions: An In Situ X-Ray Absorption Spectroscopy and Electron Spin Resonance Study Thomas M. Arruda1, Badri Shyam 2 ,Jamie S. Lawton 1, Nagappan Ramaswamy 1, David E. Budil 1, David E. Ramaker 2 and Sanjeev Mukerjee 1


1 Department of Chemistry & Chemical Biology, Northeastern University, 360 Huntington Ave, Boston, MA 02115
2 Department of Chemistry, George Washington University 725 21st street N.W, Washington D.C. 20052

 

J.Phys.Chem.C 2010, 114, 1028-1040
Abstract

In situ X-ray Absorption Spectroscopy (XAS), using both the EXAFS and -XANES analysis procedures, is utilized to examine Ru deposition onto Pt/C cathodes at millimolar concentration of Ru in a 1M HClO4 electrolyte. Also Electron Spin Resonance (ESR) spectroscopy is utilized to examine the effects of Ru3+ ion exchanged into Nafion membranes. The -XANES analysis of the XAS data allows a determination of the coverage of Ru with time (minutes to hours) and to determine the binding site of the deposited Ru species (atop/bridged at low coverage and 3-fold at higher coverage) apparently onto the corners and edges initially and at higher coverage onto the faces of the cubooctahdral clusters when exposed to Run+ at OCP. The deposition appears to be inhibited at potentials where adsorbates (such as H and OH) usually adsorb, and Coulomb enhanced at OCP when substantial O exists on the surface. The ESR analysis of Ru3+ in the Nafion membrane indicates significant detrimental changes to the membrane in the presence of Ru ions; namely a decrease in the water uptake and an increase in the microviscosity of the fluid regions. Together these data indicate the critical nature of keeping the fuel cell under potential control and avoiding an uncontrolled shut down.

[12]

Local structure of Ru1-xNixO2-y dioxide and its implications to electrocatalytic behavior – an XPS and XAS study Z. Bastl, J. Franc, K. Macounova, M. Makarova, V. Petrykin, S. Mukerjee2, N. Ramaswamy2. Spirovova and P. Krtil


1 J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i, Dolejskova 3, 18223 Prague, Czech Republic
2 Depratment of Chemistry and Chemical Biology, Northeastern University, 360 Huntington Ave., Boston, MA, USA

 

J.Phys.Chem.C 2009,113, 21657-21666

 


Abstract:
Chemical composition, crystal structure as well as short range atomic arrangement Ru1-xNixO2-y oxides with x ranging between 0 and 0.3 were studied using energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The prepared materials form single phase nanocrystals. Materials with nominal Ni content below 0.1 are stable under conditions of the electrocatalytic oxygen evolution in acid media. Materials with higher Ni content are subject to selective Ni dissolution. Regardless of the chemical composition the surface of Ru1-xNixO2-y oxides is Ni enriched with respect to overall chemical composition. The XPS does not confirm variability of the oxidation state of Ru in the studied materials. The Ni is present in both divalent and trivalent states with fraction of trivalent decreasing with increasing Ni content. The refinement of local structure using EXAFS data based on Ru and Ni K edge absorption spectra show that Ru preserves local arrangement characteristic for ruthenium dioxide within 6 Å from the absorbing Ru atom. The incorporated Ni shows a tendency to form clusters within a rutile structure which eventually become confined to shear planes. These shear planes manifest themselves on the surface as line or plane defects, which are the most likely structural features active in the electrocatalytic processes.

 

 

[13]

Elucidating the Mechanism of Oxygen Reduction for Lithium-Air Battery Applications


Cormac O. Laoire, Sanjeev Mukerjee, and K. M. Abraham*
Department of Chemistry and Chemical Biology, Northeastern UniVersity, 360 Huntington AVenue,
Boston, Massachusetts 02115
Edward J. Plichta and Mary A. Hendrickson
U.S. Army CERDEC, Army Power DiVision, Ft. Monmouth, New Jersey 07703

 

J. Phys. Chem. C 2009, 113, 20127–20134

 

Abstract


Unlocking the true energy capabilities of the lithium metal negative electrode in a lithium battery has until now been limited by the low capacity intercalation and conversion reactions at the positive electrodes. Abraham et al. (Abraham, K. M.; Jiang, Z. J. Electrochem. Soc. 1996, 143, 1-5) overcame this limitation by removing these electrodes and allowing lithium to react directly with oxygen in the atmosphere, forming the Li-air battery. The Li/O2 battery redox couple has a theoretical specific energy of 5200 W h/kg and represents the ultimate, environmentally friendly electrochemical power source. In this work, we report for the first time the intimate role of electrolyte, in particular the role of ion conducting salts, in determining the reversibility and kinetics of oxygen reduction in nonaqueous electrolytes designed for such applications. Such fundamental understanding of this high energy density battery is crucial to harnessing its full energy potential. The kinetics and mechanisms of O2 reduction in solutions of hexafluorophosphate of the general formula A+PF6
-, where A ) tetrabutylammonium (TBA), K, Na, and Li, in acetonitrile are reported on glassy carbon electrodes using cyclic voltammetry (CV) and rotating disk electrode (RDE) techniques. The results show that the cations in the electrolyte strongly influence the reduction mechanism of O2. Larger cations represented by TBA salts displayed reversible O2/O2
- redox couple, in contrast to those containing the smaller Li (and other alkali metal) cations, where an irreversible one-electron reduction of O2 to LiO2, and other alkali metal superoxides, is shown to occur as the first process. It was also found the LiO2 formed initially decomposes to Li2O2. Electrochemical data support the view that alkali metal oxides formed via electrochemical and chemical reactions passivate the electrode surface, making the processes irreversible. The O2 reduction mechanisms in the presence of the different cations have been supplemented by kinetic parameters
determined from detailed analyses of the CV and RDE data. The Lewis acid characteristics of the cation appear
to be crucial in determining the reversibility of the system. The results of this study are expected to contribute to
the rapid development of the Li-air battery.