We present a combination of comprehensive experimental and theoretical evidence to unravel the mechanism of two-electron oxygen reduction reaction (ORR) on a catalyst composed of mildly reduced graphene oxide supported on P50 carbon paper (mrGO/P50). This catalyst is unique in that it shows >99% selectivity toward H2O2, the highest mass activity to date, and essentially zero overpotential in base. Furthermore, the mrGO catalytically active site is unambiguously identified and presents a unique opportunity to investigate mechanisms of carbon-based catalysis in atomistic detail. A wide range of experiments at varying pH are reported: ORR onset potential, Tafel slopes, H/D kinetic isotope effects, and O2 reaction order. With DFT reaction energies and known thermodynamic parameters, we calculate the potential and pH-dependent free energies of all possible intermediates in this ORR and propose simple kinetic models that give semiquantitative agreement with all experiments. Our results show that mrGO is semiconducting and cannot support the conventional mechanism of coherently coupled proton–electron transfers. The conducting P50 provides electrons for initiating the ORR via outer sphere electron transfer to O2(aq), while the semiconducting mrGO provides the active catalytic sites for adsorption of O2 –(aq) or HO2(aq), depending upon electrolyte pH. Due to this unique synergistic effect, we describe the mrGO/P50 as a co-catalyst. This concept implies departure from the traditional picture of predicting catalytic activity trends based on a single descriptor, and the co-catalyst design strategy may generally enable other semiconductors to function as electrocatalysts as well.

Electrochemical impedance spectroscopy, pressure change measurements, and scanning electron microscopy were used to investigate the nonaqueous Na−O 2 cell potential decrease and rise (sudden deaths) on discharge and charge, respectively. To fit the impedance spectra from operating cells, an equivalent circuit model was used that takes into account the porous nature of the positive electrode and is able to distinguish between the electrolyte resistance in the pores and the charge-transfer resistance of the pore walls. The results obtained indicate that sudden death on discharge is caused by, depending on the current density, either accumulation of large NaO 2 crystals that eventually block the electrode surface and/or a thin film of NaO 2 forming on the cathode surface at the end of discharge, which limits charge-transfer. The commonly observed sudden rise in potential toward the end of charge may be caused by a concentration depletion of NaO 2 dissolved in the electrolyte near the cathode surface and/or an accumulation of degradation products on the cathode surface.

Ionic liquids (ILs) have been proposed as promising solvents for Li-air battery electrolytes.Here, several ILs have been investigated using differential electrochemical mass spectrometry (DEMS) to investigate the electrochemical stability in a Li-O 2 system, by means of quantitative determination of the rechargeability (OER/ORR), and thereby the Coulombic efficiency of discharge and charge. None of the IL based electrolytes are found to behave as needed for a functional Li-O 2 battery but perform better than commonly used organic solvents. Also the extent of rechargeability/reversibility has been found to be strongly dependent on the choice of IL cation and anion as well as various impurities.

The maximum discharge capacity in non-aqueous Li-O 2 batteries has been limited to a fraction of its theoretical value, largely due to a conformal deposition of Li 2 O 2 on the cathode surface. However, it has recently been established that additives that increase the shielding of either O 2 − or Li + will activate the formation of toroidal shaped Li 2 O 2 , thereby dramatically increasing cell capacity. Here we apply porous electrode theory to electrochemical impedance measured at the Li-O 2 cathode to investigate changes in the surface-and ionic resistance within the pores under conditions where either the surface-mechanism or the solution-mechanism is favored. Our experimental observations show that (i) an additional charge transfer process is observed in the impedance spectrum where the solution-based mechanism is favored; (ii) that the changes in the ionic resistance in the cathode during discharge (related to Li 2 O 2 build up) is much greater in cells where the solution-based mechanism is activated and can qualitatively determine the extent of discharge product deposited within the pores of the cathode versus the deposition extent at the electrode/electrolyte interface; and (iii) that the observed "sudden-death" during discharge is a consequence of the increasing charge transfer resistance regardless of whether Li 2 O 2 forms predominantly through either the surface-or solution-based mechanism. The Li-O 2 battery has, since Jiang and Abraham's seminal 1996 report, 1 received significant attention due to its high theoretical specific energy and energy density of 3500 Wh/kg and 3400 Wh/L, respectively.2 These values are based on the overall cell reaction in non-aqueous electrolytes that can be described as 2Li + O 2 Li 2 O 2 , with the forward reaction corresponding to discharge and the reverse direction to charge. To understand how to more appropriately engineer a practical Li-O 2 cathode and electrolyte, the mechanism of Li 2 O 2 formation has been the focus of significant research in the past 10 years. Laorie et al. 3,4 initially showed that varying Lewis basicity of the electrolyte substantially influenced the electrochemical kinetics and reversibility of the Li/O 2 reaction. Deposition morphology of Li 2 O 2 was also found to be influenced by electrolyte properties 5 and current density 6,7 with large (∼500-1000 nm) toroids or thin (∼5 nm) conformal films of Li 2 O 2 forming under various conditions. These observations have been explained by two independent reaction mechanisms that appear to be influenced primarily by electrolyte Lewis acidity or basicity, 5,8,9 which can be tuned through solvent, 5 anion, 9 and additive 8 selection. These two main reaction pathways for Li 2 O 2 deposition are referred to here as either the surface-or solution-based mechanism. The surface mechanism (Figure 1a) occurs in electrolytes with low Lewis basicity and acidity, where LiO 2 is insoluble, disallowing diffusion away from its initial site of formation and resulting in conformal Li 2 O 2 film formation during discha...