Investigations of Oxygen Reduction Reactions for Electrochemical Energy Storage and Conversion

High energy density portable power solutions have been of utmost importance for the advancement of modern day necessities such as data and voice communication, vehicular transportation, distributed power generation and storage of energy produced by sustainable power sources. Progress made in fuel cell and lithium-ion battery technologies over the past decade have opened opportunities to power electric and hybrid electric vehicles for long distance transportation. Alkaline membrane fuel cells (AEMFCs) are the new alternatives to proton exchange membrane fuel cells (PEMFCs), which require generous amounts of noble metal-based catalysts on their electrodes. Facile electrode kinetics on non-precious group metal catalysts in alkaline environments is the key factor which has promoted AEMFCs over PEMFCs. While the research on AEMFCs is vastly expanding, high energy density batteries are praiseworthy considering the high cost of hydrogen fuel. The state-of-the-art Li-ion batteries cannot reach the desirable capacity density to power electric vehicles capable of >300 miles on a single charge whereas Li-O2 batteries with a theoretical capacity more than ten times larger than that of Li-ion have become very promising for this application. Chapter 1 of this thesis provides a discussion of the background behind the fuel cell and battery technologies beyond Li-ion along with the electrochemical and analytical techniques employed throughout this investigation.;The major deterrent to AEMFC technology is its performance decrease by means of carbonate exchange of the membrane when exposed to carbon dioxide. The second Chapter deals with a quantitative determination of the influence of carbonate ions in the alkaline membrane on interfacial electrode reactions and reactant transport through the membrane. A Pt microelectrode investigation conducted on a commercial anion exchange membrane (AEM) (Tokuyama, A201) showed rather close kinetics for oxygen reduction reaction (ORR) with and without carbonate exchange as well as with a perfluorinated proton exchange membrane analog such as NafionRTM. Resolution of the mass transport into constituent components (diffusion coefficient and solubility) showed that the oxygen diffusion coefficient in the AEM exchanged with carbonate ions (CO3 2-) is lowered while the solubility remained unaffected. These results show remarkable agreement with polarization corrected fuel cell data, thus enabling a method to better resolve interfacial performance of an AEM fuel cell.;We have also investigated the kinetics of hydrogen oxidation reaction (HOR) and methanol oxidation reaction (MOR) at the Tokuyama (A201/A901) anion exchange membrane /Pt microelectrode interfaces using solid state electrochemical cells. Diffusion of hydrogen molecules through the membrane was not influenced by the carbonate ions due to the smaller size of the gaseous molecule. However, hydrogen concentration in the anion exchange membrane is low in the presence of carbonate ions. Methanol diffusion is facilitated in the anion exchange polymer electrolyte due to its high water content. A change of the diffusion path length in carbonate polymer electrolytes caused methanol permeability to drop significantly. The kinetic parameters obtained for the AEM in the carbonate form suggests that both hydrogen and methanol oxidation reactions proceed through the carbonate pathway. Therefore, the kinetic parameters obtained are significantly lower than what were observed at the AEM in the hydroxide form.;In the third Chapter I demonstrate that a microelectrode can be used as a diagnostic tool to determine O2 transport properties and redox kinetics in dimethyl sulfoxide (DMSO)--based electrolytes for non-aqueous Li-air batteries, and to elucidate the influence of ion-conducting salts on the O2 reduction reaction mechanism. Oxygen reduction/evolution reactions on a carbon microelectrode have been studied in dimethyl sulfoxide-based electrolytes containing Li+ and tetrabutylammonium ((C 4H9)4N+) ions. Analysis of chronoamperometric current-time transients of the oxygen reduction reactions in the series of tetrabutylammmonium (TBA) salt-containing electrolytes of TBAPF6, TBAClO4, TBACF3SO3, or TBAN(CF3SO 2)2 in DMSO revealed that the anion of the salt exerts little influence on O2 transport. Whereas steady-state ORR currents (with sigmoidal-shaped current-potential curves) were observed in TBA-based electrolytes, peak-shaped current-voltage profiles were seen in the electrolytes containing their Li salt counterparts. The latter response results from the combined effects of the electrostatic repulsion of the superoxide (O2 -) intermediate as it is reduced further to peroxide (O2 2-) at low potentials and the formation of passivation films of the O2 reduction products at the electrode. Raman spectroscopic data confirmed the formation of non-conducting Li2O2 and Li2O on the electrode surface at different reduction potentials in Li salt solutions. Out of the four lithium salt-containing electrolytes studied, namely LiPF6, LiClO4, LiCF3SO 3, or LiN(CF3SO2)2 in DMSO, the LiCF 3SO3/DMSO solution revealed the most favorable ORR kinetics and the least passivation of the electrode by ORR products.;The influence of lithium salts on O2 reduction reactions (ORR) in 1, 2-dimethoxyethane (DME) and tetraethylene glycol dimethyl ether (TEGDME) has been investigated in Chapter 4. Microelectrode studies in a series of tetrabutylammonium salt (TBA salt)/DME-based electrolytes showed that O 2 solubility and diffusion coefficient are not significantly affected by the electrolyte anion. The ORR voltammograms on microelectrodes in these electrolytes exhibited steady-state limiting current behavior. In contrast, peak-shaped voltammograms were observed in Li+-conducting electrolytes suggesting a reduction of the effective electrode area by passivating ORR products as well as migration-diffusion control of the reactants at the microelectrode as observed in DMSO-based electrolytes. FT-IR spectra have revealed that Li + ions are solvated to form solvent separated ion pairs of the type Li+(DME)nPF6- and Li +(TEGDME)PF6- in LiPF6--based electrolytes. On the other hand, the contact ion pairs (DME)mLi +(CF3SO3-) and (TEGDME)Li +(CF3SO3-) appear to form in LiSO3CF3-containing electrolytes. In the LiSO3 CF3--based electrolytes, the initial ORR product, superoxide (O2-), is stabilized in solution by forming [(DME) m-1(O2-)]Li+(CF3SO 3-) and [(TEGDME)(O2-)]Li +(CF3SO3-) complexes. These soluble superoxide complexes are able to diffuse away from the electrode surface reaction sites to the bulk electrolyte in the electrode pores where they decompose to form Li2O2. This explains the higher capacity obtained in Li/O2 cells utilizing LiCF3SO3/TEGDME electrolytes.;In Chapter 5 the synthesis of iron(II) phathlaocyanine (FePC)--based catalysts is presented. FePC embedded in a carbon support was heat-treated at a series of temperatures (300°C, 600°C and 800°C) and characterized by means of several spectroscopic and electrochemical techniques. Catalytic oxygen reduction recorded in the low Donor Number acetonitrile (MeCN)--based electrolytes have shown that the oxygen reduction reaction (ORR) mechanism is modified at the catalyst surface. Redox electrochemistry of FePC recorded in argon saturated electrolytes has confirmed that the iron is in the Fe(I) state at the O2 reduction potential in these electrolytes which is capable of stabilizing the superoxide leading to an inner--Helmholtz plane electron transfer reaction. In high Donor Number DMSO--based electrolytes the ORR was not influenced by the catalyst and this has been attributed to the oxidation state of iron being Fe(II) at the superoxide forming potential. The superoxide formed in such conditions are stabilized by the DMSO solvated softer Lewis acid Li+ as the Li+(DMSO) n--O2- ion pair in solution. The ORR reaction in this electrolyte proceeds through an outer Helmholtz plane electron transfer process despite the presence of the FePC catalyst in the electrode. Catalyzed carbon electrodes treated at 300 and 600°C were successfully employed in the low Donor Number tetra ethylene glycol dimethyl ether (TEGDME)--based electrolyte-containing Li-O2 cells. These cells showed significant reduction in the charging overpotentials and a doubling of the cell cycle life compared to uncatalyzed cells.;Chapter 6 sums up the conclusions of the research presented in this thesis and presents future directions for investigations.