Effect of Temperature and Pressure on the Kinetics of the Oxygen Reduction Reaction

Fundamental understanding of the oxygen reduction reaction in aqueous medium at temperatures above 100 °C is lacking due to the practical limitations related to the harsh experimental conditions. In this work, the challenge to suppress water from boiling was overcome by conducting the electrochemical investigation under pressurized conditions. A striking improvement in the kinetics of the electrocatalytic reduction of O₂ by about 150 fold relative to room temperature and pressure was recorded under an O₂ pressure of 3.4 MPa at 200 °C in basic aqueous environment. To deconvolute the combined effect of temperature and pressure, the underlying variables that dictate the observed O₂ reduction kinetics of Pt and carbon electrodes were examined individually. O₂ availability at the electrode–solution interface was controlled by the interplay between the diffusion coefficient and concentration of O₂. Accurate knowledge of the temperature and pressure dependence of O₂ availability at the electrode surface, the Tafel slope, the transfer coefficient, and the electrochemical active surface area was required to correctly account for the enhanced O₂ reduction kinetics

High Activity Oxygen Evolution Reaction Catalysts from Additive-Controlled Electrodeposited Ni and NiFe Films

Electrodeposition of Ni or NiFe films exhibiting fractal-like behavior from plating baths containing an inhibitor, such as 3,5-diamino-1,2,4-triazole (DAT), is found to yield oxygen evolution reaction (OER) catalysts for alkaline solutions exhibiting high current densities (100 mA/cm²), high mass activity (∼1200 A/g of catalyst), high stability (>72 h), and low overpotentials (∼300 mV). By changing electrodeposition time, the activity of the catalyst can be tuned, with longer times yielding higher activities. The electrodeposition method works with any conductive substrate yielding unprecedented performance and providing an easy route to high activity catalysts

Origins of Less Noble Behavior by Au during CO Adsorption

The behavior of the CO interaction with gold in an electrochemical environment is presented in this work by means of the SERS technique. The results show spectroscopic evidence that the adsorbed CO promotes the formation of oxidic species even at potentials where it is not thermodynamically favorable (lower than 0.6 V vs RHE), explaining the low-overpotential CO electrooxidation reaction onset (@ ca. 0.2 V). At high potentials (<1.3 V), the CO displays an anomalous behavior, persisting adsorbed on the surface at the high coverage oxide film, which allows us to use the CO molecule as a probe and get information about the electrode surface on the course of the reaction as well as suggests gold oxide to be an active catalyst in small organic alcohol oxidation

Elucidating Zn and Mg Electrodeposition Mechanisms in Nonaqueous Electrolytes for Next-Generation Metal Batteries

Cyclic voltammetry and linear sweep voltammetry with an ultramicroelectrode (UME) were employed to study Zn and Mg electrodeposition and the corresponding mechanistic pathways. CVs obtained at a Pt UME for Zn electroreduction from a trifluoromethylsulfonyl imide (TFSI^–) and chloride-containing electrolyte in acetonitrile exhibit current densities that are scan rate independent, as expected for a simple electron transfer at a UME. However, CVs obtained from three different Mg-containing electrolytes in THF exhibit an inverse dependence between scan rate and current density. COMSOL-based simulation suggests that Zn electrodeposition proceeds via a simple one-step, two-electron transfer (E) mechanism. Alternatively, the Mg results are best described by invoking a chemical step prior to electron transfer: a chemical–electrochemical (CE) mechanism. The chemical step exhibits an activation energy of 51 kJ/mol. This chemical step is likely the disproportionation of the chloro-bridged dimer [Mg₂(μ–Cl)₃·6THF]⁺ present in active electrodeposition solutions. Our work shows that Mg deposition kinetics can be improved by way of increased temperature

Mo-V‑O Based Electrocatalysts for Low Temperature Alcohol Oxidation

There is a growing interest in alcohol oxidation electrochemistry due to its role in renewable energy technologies. The goal of this work was to develop active non- precious metal electrocatalysts based on the Mo-V-(M)-O (M is Nb, Te) lattice. Selective gaseous alkane oxidation had been previously observed on these catalysts at elevated temperatures above 300 °C. In this study, the activity of the catalysts at lower temperatures, 25–60 °C, was investigated. Hydrothermal conditions were used to synthesize the Mo-V-(M)-O mixed oxides. Physical characterization of the catalysts were obtained by powder X-ray diffraction (XRD), scanning electron micrography (SEM) equipped with energy dispersive X-ray (EDX), transmission electron micrography (TEM), and X-ray photoelectron spectroscopy (XPS). The catalytic activity for the oxidation of cyclohexanol was studied electrochemically. Chronoamperometric studies were used to evaluate the long-term performance of the catalysts. The onset of alcohol oxidative current was observed between 0.2 and 0.6 V versus Ag/AgCl. Gas chromatography–mass spectrometry analysis was used to determine the nature of the oxidative products. The mild oxidation products, cyclohexanone and cyclohexene, were observed after oxidation at 60 °C. The catalytic activity increased in the order Mo-V-O < Mo-V-Te-O < Mo-V-Te-Nb-O. Mo-V-(Te,Nb)-O based electrocatalysts efficiently catalyzed the oxidation of alcohols at low temperatures

Effect of pH and Azide on the Oxygen Reduction Reaction with a Pyrolyzed Fe Phthalocyanine Catalyst

The active site of pyrolyzed Fe/N/C electrocatalysts for the oxygen reduction reaction (ORR) has been a source of debate since the initial discovery that these materials demonstrated activity toward the ORR. Here we utilize carbon-supported iron­(II) phthalocyanine (FePc) that has been pyrolyzed at 800 °C in the absence and presence of azide in acidic, neutral, and basic environments in order to probe the ORR activity and mechanism of pyrolyzed Fe/N/C materials. The presence of azide served to enhance the ORR activity of this material in neutral electrolyte while having no effect in acid or base. Tafel slope differences in addition to the azide enhancement suggest an iron-centered active site for the ORR in pyrolyzed FePc and potentially other Fe/N/C electrocatalysts

ZnAl_<i>x</i>Co_2–<i>x</i>O₄ Spinels as Cathode Materials for Non-Aqueous Zn Batteries with an Open Circuit Voltage of ≤2 V

Rechargeable Zn batteries are promising energy storage alternatives for Li-ion batteries in part because of the high specific and volumetric capacities of Zn anodes, as well as their low cost, improved prospects for safety, and the fact that they are environmentally friendly. Development efforts, however, have focused mostly on aqueous electrolyte systems, which are intrinsically limited by the narrow electrochemical potential window of water. As a consequence, the use of alternative non-aqueous electrolytes has attracted a growing level of interest with the hope that they may provide higher operational voltages, which potentially could provide viable pathways to high-energy and high-power density Zn batteries. With regard to the latter, the considerable progress made in developing useful non-aqueous electrolyte chemistries for Zn anodes has not been matched by correlated progress regarding the development of useful cathode materials. In this work, a new series of spinels, ZnAl_<i>x</i>Co_2–<i>x</i>O₄, are reported and their utility as cathode materials for non-aqueous Zn-ion batteries is demonstrated. Full cells constructed using this new spinel as a cathode paired with a metal anode showed capacities over 100 cycles of 114 mAh/g and an onset potential of 1.95 V, which is the highest OCV yet reported for a non-aqueous Zn-ion battery system. The data show that the Zn²⁺ ions reversibly intercalate into the spinel structure during the charge–discharge processes, a compositional transformation directly correlated with a reversible conversion between Co⁴⁺ and Co³⁺ oxidation states within the lattice. The data illustrate that the Al³⁺-doped spinel structure is a robust candidate material for use in non-aqueous Zn batteries, suggesting guidelines for the design of more efficient multivalent cathode materials

In Situ Raman Spectroscopy of Sulfur Speciation in Lithium–Sulfur Batteries

In situ Raman spectroscopy and cyclic voltammetry were used to investigate the mechanism of sulfur reduction in lithium–sulfur battery slurry cathodes with 1 M lithium bis­(trifluoromethane sulfonyl)­imide (LiTFSI) and tetraethylene glycol dimethyl ether (TEGDME)/1,3-dioxolane (DIOX) (1/1, v/v). Raman spectroscopy shows that long-chain polysulfides (S₈^2–) were formed via S₈ ring opening in the first reduction process at ∼2.4 V vs Li/Li⁺ and short-chain polysulfides such as S₄^2–, S₄^–, S₃^•–, and S₂O₄^2– were observed with continued discharge at ∼2.3 V vs Li/Li⁺ in the second reduction process. Elemental sulfur can be reformed in the end of the charge process. Rate constants obtained for the appearance and disappearance polysulfide species shows that short-chain polysulfides are directly formed from S₈ decomposition. The rate constants for S₈ reappearance and polysulfide disappearance on charge were likewise similar. The formation of polysulfide mixtures at partial discharge was found to be quite stable. The CS₂ additive was found to inhibit the sulfur reduction mechanism allowing the formation of long-chain polysulfides during discharge only and stabilizing the S₈^2– product

Exploring Salt and Solvent Effects in Chloride-Based Electrolytes for Magnesium Electrodeposition and Dissolution

We describe in this work Mg electrodeposition and dissolution from a wide range of inorganic ethereal electrolytes consisting of MgCl₂ and a second chloride salt. Systematic variations of the cosalt reveal two broad classes of electrolytes, namely, the group 13 electrolytes, which require electrolytic cycling to improve their performance, and electrolytes based on heavy p-block chlorides, which exhibit Mg intermetallic formation. Results from electrospray ionization mass spectrometry demonstrate that Mg deposition and stripping only occur in electrolytes containing Mg multimers. We also explore the role of solvent in determining the electrochemical performance of chloride-based electrolytes. Our analysis establishes thermodynamic parameters that dictate the ability of a solvent to support Mg electrochemistry in the MgCl₂–AlCl₃ system. In their totality, these results illustrate important electrolyte design guidelines for future Mg-ion batteries

<i>In Situ</i> Electrochemical Stress Measurements Examining the Oxygen Evolution Reaction in Basic Electrolytes

<i>In situ</i> electrochemical stress measurements are used to interrogate changes in oxide structure before and during the oxygen evolution reaction (OER) from Ir, Ni, Co, Au, and Pt electrodes in alkaline electrolyte. Stress evolution during potential cycling reports on changes in oxidation state and oxide forms. Hysteresis observed in the potential-dependent stress from Ir, Au, and Pt electrodes is associated with chemical irreversibility in electrode composition and roughness. Alternatively, Ni and Co exhibit reversible conversion between hydroxide and oxyhydroxide forms during cycling. From the experimentally determined stress, charge passed during electrode oxidation, and Young’s modulus, the change in strain exhibited by Ni and Co electrodes during hydroxide-oxyhydroxide conversion is calculated to be 7.0% and 8.4%, respectively. We also show that the magnitude of change in stress is proportional to the amount of material that is further oxidized