Electrochemical Oxygen Reduction and Oxygen Evolution from DMSO Based Electrolytes

Abstract

In the present work the oxygen reduction reaction in organic electrolytes was investigated. Quantitative DEMS-studies (differential electrochemical mass spectroscopy) revealed that oxygen is reduced electrochemically to superoxide in TBAClO₄ (tetrabutyl ammonium perchlorate) containing, DMSO- (dimethyl sulfoxide) based electrolytes. This was derived from the fact that 1 electron is transferred in order to reduce one molecule of oxygen. 2 e^-/O₂ are transferred in the presence of Li⁺, hence, resulting in the formation of Li₂O₂. This is also true for NMP- (N-methyl-2-pyrolidone) based electrolytes.<br /> At smooth gold electrodes and at low overpotentials the z-value (number of electrons per reduced oxygen) is close to 1 e^-/O₂ even in lithium containing electrolytes. Under these conditions electrochemical oxygen reduction results in the formation of superoxide, which undergoes a chemical disproportionation to peroxide in the aftermath. This indirect pathway of peroxide formation shifts to a direct pathway of peroxide formation at higher overpotentials: Oxygen is reduced electrochemically to peroxide when the potential becomes more negative than 1.2 V (vs. Ag/Ag⁺) which is indicated by a z-value close to 2 e^-/O₂. This mechanism was also ascertained by RRDE (Rotating Ring Disc Electrode) measurements. These measurements show that during oxygen reduction at the disk electrode species are formed that are oxidised at the ring electrode (i.e. detection electrode). It is generally assumed that the species that reacts at the ring electrode under these conditions is superoxide. As the overpotential increases and the potential range of the indirect pathway of peroxide formation is entered, less current is observed at the ring electrode. <br /> It is a reasonable assumption that it is superoxide reacts at the ring electrode. However, it cannot be taken for granted. Therefore, a novel cell for RRDE like applications in combination with DEMS was designed. It allows proper identification of the species reacting at the detection electrode by means of mass spectroscopy. Employing the so-called 6-electrode cell showed that it is, indeed, superoxide that is detected at the ring electrode.<br /> Further support for the proposed mechanism of oxygen reduction at gold stem from eQCMB (electrochemical Quartz Crystal Microbalance) measurements: Only at potentials lower than 1.2 V the formation of a deposit takes place. This corresponds well to the formation of soluble superoxide at low overpotentials, whereas solid Li₂O₂ forms at high overpotentials. <br /> No such transition takes place when porous gold electrodes are employed. At these electrodes the z-value is always above 1.5 e^-/O₂. This difference has been ascribed to the structure of the porous gold electrode: Gold at open circuit catalyses a reaction that results in the oxidation of superoxide. Due to this reaction that takes place at electrical isolated gold particles in the porous gold electrode it is not possible to distinguish between the indirect and the direct pathway of peroxide formation. <br /> There are also other parameters that influence the pathway by which electrochemical oxygen reduction takes place: The electrode material, the presence of water and the cation of the electrolyte. At rhodium oxygen reduction proceeds via the direct pathway of peroxide formation irrespective of the applied overpotential. At ruthenium and glassy carbon a z-value of approximately 1.5 e^-/O₂ indicates that both the direct and the indirect pathway of peroxide formation take place in parallel. Much alike to rhodium, also platinum appears to favour the direct pathway of peroxide formation even at low overpotentials. However, the deposition of Li₂O₂ inhibits the ability of platinum to reduce oxygen to peroxide. Therefore, in the course of oxygen reduction the mechanism shifts from the direct to the indirect pathway of peroxide formation. At BDD- (Boron Doped Diamond) electrodes no oxygen reduction takes place. The impact of the electrode material on the oxygen reduction reaction reveals an electrocatalytic effect on both, the oxygen reduction to superoxide and to peroxide. This indicates that both oxygen and superoxide need to undergo specific interaction with the electrode material prior to any charge transfer. <br /> At gold electrodes the potential at which the transition from the indirect to the direct pathway takes place depends on the charge density of the cation in the electrolyte. As the charge density increases the transition takes place at lower overpotentials. No effect of the cation was found on the onset potential of oxygen reduction. Hence, the first charge transfer takes place without the involvement of the cation, whereas the second charge transfer is likely to take place after the cation has coordinated to superoxide. <br /> With increasing water contents the potential at which the transition from the indirect to the direct pathway takes place shifts to higher overpotentials. However, this was only observed at gold, the only electrode material at which a sharp transition from the indirect to the direct pathway of peroxide formation takes place. The origin of the water-effect is still elusive. <br /> During oxygen reduction, reduced oxygen species are deposited on the electrode. These are oxidised in the following anodic sweep. Oxygen evolution comes along with a z-value of 2 e^-/O₂ indicative for the oxidation of Li₂O₂ formed during oxygen reduction. There is no dependence of the onset potential of oxygen evolution on the used electrode material. However, the kinetics of this reaction is more sluggish at gold then at other electrode materials, which indicates also an electrocatalytic effect on the oxygen evolution reaction. <br /> At potentials more positive then 0.3 V evolution CO₂-evolution takes place which indicates electrolyte decomposition. The amounts of evolved CO₂ linearly depend on the amounts of reduced oxygen during the previous cathodic sweep. This indicates that electrolyte decomposition takes place during oxygen reduction and that CO₂ evolution is due to the oxidation of decomposition products. The deposition of decomposition products during oxygen reduction shows up in higher then expected m.p.e-values (mass per electron) observed via eQCMB measurements. The parallel deposition of Li₂O₂ and decomposition products leads to an inhomogeneous film on the electrode. This film-structure is responsible for the missing mass changes during oxygen evolution: When Li₂O₂ is oxidised the remaining decomposition products form cavities, which are filed up with electrolyte. The additional mass of the electrolyte offsets at least partially the mass losses due to Li₂O₂ oxidation.