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Precise control of elemental configurations within multi- metallic nanoparticles (NPs) could enable access to functional nanoma- terials with significant performance benefits. This can be achieved down to the atomic level by the disorder-to-order transformation of individual NPs. Here, by systematically controlling the ordering degree, we show that the atomic ordering transformation, applied to AuCu NPs, activates them to perform as selective electrocatalysts for CO[sub 2] reduction. In contrast to the disordered alloy NP, which is catalytically active for hydrogen evolution, ordered AuCu NPs selectively converted CO[sub 2] to CO at faradaic efficiency reaching 80%. CO formation could be achieved with a reduction in overpotential of ~200 mV, and catalytic turnover was enhanced by 3.2-fold. In comparison to those obtained with a pure gold catalyst, mass activities could be improved as well. Atomic-level structural investigations revealed three atomic gold layers over the intermetallic core to be sufficient for enhanced catalytic behavior, which is further supported by DFT analysis. [ABSTRACT FROM AUTHOR]

Modern energy challenges currently trigger an intense interest in catalysis of redox reactions -- electrochemical and photochemical -- particularly those involving small molecules such as water, hydrogen, oxygen, proton, carbon dioxide. A continuously increasing number of molecular catalysts of these reactions, mostly transition metal complexes, have been proposed, rendering necessary procedures for their rational benchmarking and fueling the quest for leading principles that could inspire the design of improved catalysts. The search of "volcano plots" correlating catalysis kinetics to the stability of the key intermediate is a popular approach to the question in catalysis by surface-active sites, with as foremost example the electrochemical reduction of aqueous proton on metal surfaces. We discussed here for the first time, on theoretical and experimental grounds, the pertinence of such an approach in the field of molecular catalysis. This is the occasion to insist on the virtue of careful mechanism assignments. Particular emphasis is put on the interest of expressing the catalysts' intrinsic kinetic properties by means of catalytic Tafel plots, which relate kinetics and overpotential. We also underscore that the principle and strategies put forward for the catalytic activation of the above-mentioned small molecules are general as illustrated by catalytic applications out of this particular field. [ABSTRACT FROM AUTHOR]

Postsynthetic ion exchange of [Co2(μ-Cl)2(btta)] (MAF-X27-Cl, H2bbta =1H,5H-benzo(1,2-d:4,5-d')bistriazole) possessing open metal sites on its pore surface yields a material [Co2(μ-OH)2(bbta)] (MAF-X27-OH) functionalized by both open metal sites and hydroxide ligands, giving drastically improved electrocatalytic activities for the oxygen evolution reaction (an overpotential of 292 mV at 10.0 mA cm-2 in 1.0 M KOH solution). Isotope tracing experiments further confirm that the hydroxide ligands are involved in the OER process to provide a low-energy intraframework coupling pathway. [ABSTRACT FROM AUTHOR]

Electrolytic water oxidation using earth-abundant elements is a key challenge in the quest to develop cheap, large surface area arrays for solar-to-hydrogen conversion. There have been numerous studies in this area in recent years, but there remains an imperative to demonstrate that the current densities reported are indeed due to the species under consideration and not due to the presence of adventitious (yet possibly highly active) contaminants at low levels. Herein, we show that adventitious nickel at concentrations as low as 17 nM can act as a water oxidation catalyst in mildly basic aqueous solutions, achieving stable (tens of hours) current densities of 1 mA cm-2 at overpotentials as low as 540 mV at pH 9.2 and 400 mV at pH 13. This nickel was not added to the electrolysis baths deliberately, but it was found to be present in the electrolytes as an impurity by ICP-MS. The presence of nickel on anodes from extended-time bulk electrolysis experiments was confirmed by XPS. In showing that such low levels of nickel can perform water oxidation at overpotentials comparable to many recently reported water oxidation catalysts, this work serves to raise the burden of proof required of new materials in this field: contamination by adventitious metal ions at trace loadings must be excluded as a possible cause of any observed water oxidation activity. [ABSTRACT FROM AUTHOR]

Molecular catalysis of carbon dioxide reduction using earth-abundant metal complexes as catalysts is a key challenge related to the production of useful products--the "solar fuels"--in which solar energy would be stored. A direct approach using sunlight energy as well as an indirect approach where sunlight is first converted into electricity could be used. A CoII complex and a FeIII complex, both bearing the same pentadentate N5 ligand (2,13-dimethyl-3,6,9,12,18-pentaazabicyclo- [12.3.1]octadeca-1(18),2,12,14,16-pentaene), were synthesized, and their catalytic activity toward CO2 reduction was investigated. Carbon monoxide was formed with the cobalt complex, while formic acid was obtained with the iron-based catalyst, thus showing that the catalysis product can be switched by changing the metal center. Selective CO2 reduction occurs under electrochemical conditions as well as photochemical conditions when using a photosensitizer under visible light excitation (λ > 460 nm, solvent acetonitrile) with the Co catalyst. In the case of the Fe catalyst, selective HCOOH production occurs at low overpotential. Sustained catalytic activity over long periods of time and high turnover numbers were observed in both cases. A catalytic mechanism is suggested on the basis of experimental results and preliminary quantum chemistry calculations. [ABSTRACT FROM AUTHOR]

Previous studies of Co catalysts for H2 evolution have shown opposite effects between the redox potentials of Co centers and their catalytic properties such as the overpotential and turnover frequency: Co catalysts with more positive reduction potentials from structural modification display insignificant changes in the overpotential for H2 evolution and require stronger acid for catalysis, and Co catalysts with lower overpotentials show decreased turnover frequency for H2 evolution. In order to explore the electronic effects of a ligand scaffold on the catalytic properties for H2 evolution by a Co complex with a pentadentate ligand, N,N-bis(2-pyridinylmethyl)-2,2'-bipyridine-6-methanamine (DPA-Bpy), we replaced the pyridyls in DPA-Bpy with more basic isoquinoline groups. In contrast to data from previously reported studies, in the current study, a Co complex with a more positive reduction potential, resulting from the replacement of pyridyls with isoquinoline groups, leads to a lower overpotential and higher turnover frequency for both electro- and photocatalytic H2 production in neutral aqueous solution. [ABSTRACT FROM AUTHOR]

CO2 conversion is an essential technology to develop a sustainable carbon economy for the present and the future. Many studies have focused extensively on the electrochemical conversion of CO2 into various useful chemicals. However, there is not yet a solution of sufficiently high enough efficiency and stability to demonstrate practical applicability. In this work, we use first-principles-based high-throughput screening to propose silver-based catalysts for efficient electrochemical reduction of CO2 to CO while decreasing the overpotential by 0.4-0.5 V. We discovered the covalency-aided electrochemical reaction (CAER) mechanism in which p-block dopants have a major effect on the modulating reaction energetics by imposing partial covalency into the metal catalysts, thereby enhancing their catalytic activity well beyond modulations arising from d-block dopants. In particular, sulfur or arsenic doping can effectively minimize the overpotential with good structural and electrochemical stability. We expect this work to provide useful insights to guide the development of a feasible strategy to overcome the limitations of current technology for electrochemical CO2 conversion. [ABSTRACT FROM AUTHOR]

High surface area tin oxide nanocrystals prepared by a facile hydrothermal method are evaluated as electrocatalysts toward CO2 reduction to formate. At these novel nanostructured tin catalysts, CO2 reduction occurs selectively to formate at overpotentials as low as ~340 mV. In aqueous NaHCO3 solutions, maximum Faradaic efficiencies for formate production of >93% have been reached with high stability and current densities of >10 mA/cm2 on graphene supports. The notable reactivity toward CO2 reduction achieved here may arise from a compromise between the strength of the interaction between CO2•– and the nanoscale tin surface and subsequent kinetic activation toward protonation and further reduction. [ABSTRACT FROM AUTHOR]

Solar-driven electrochemical transformations of small molecules, such as water splitting and CO2 reduction, pertinent to modern energy challenges, require the assistance of catalysts preferably deposited on conducting or semiconducting surfaces. Understanding mechanisms and identifying the factors that control the functioning of such systems are required for rational catalyst optimization and improved performance. A methodology is proposed, in the framework of rotating disk electrode voltammetry, to analyze the current responses expected in the case of a semigeneral reaction scheme involving a proton-coupled catalytic reaction associated with proton-coupled electron hopping through the film as rate controlling factors in the case where there is no limitation by substrate diffusion. The predictions concern the current density vs overpotential (Tafel) plots and their dependence on buffer concentration (including absence of buffer), film thickness and rotation rate. The Tafel plots may have a variety of slopes (e.g., F/RT ln 10, F/2RT ln 10, 0) that may even coexist within the overpotential range of a single plot. We show that an optimal film thickness exists beyond which the activity of the film plateaus. Application to water oxidation by films of a cobalt-based oxidic catalyst provides a successful test of the applicability of the proposed methodology, which also provides further insight into the mechanism by which these cobalt-based films catalyze the oxidation of water. The exact nature of the kinetic and thermodynamic characteristics that have been derived from the analysis is discussed as well as their use in catalyst benchmarking. [ABSTRACT FROM AUTHOR]

Practical implementation of one-dimensional semiconductors into devices capable of exploiting their novel properties is often hindered by low product yields, poor material quality, high production cost, or overall lack of synthetic control. Here, we show that a molten-salt flux scheme can be used to synthesize large quantities of high-quality, single-crystalline TiO2 nanowires with controllable dimensions. Furthermore, in situ dopant incorporation of various transition metals allows for the tuning of optical, electrical, and catalytic properties. With this combination of control, robustness, and scalability, the molten-salt flux scheme can provide high-quality TiO2 nanowires to satisfy a broad range of application needs from photovoltaics to photocatalysis. [ABSTRACT FROM AUTHOR]

The wide-scale implementation of solar and other renewable sources of electricity requires improved means for energy storage. An intriguing strategy in this regard is the reduction of CO2 to CO, which generates an energy-rich commodity chemical that can be coupled to liquid fuel production. In this work, we report an inexpensive bismuth-carbon monoxide evolving catalyst (Bi-CMEC) that can be formed upon cathodic polarization of an inert glassy carbon electrode in acidic solutions containing Bi3+ ions. This catalyst can be used in conjunction with ionic liquids to effect the electrocatalytic conversion of CO2 to CO with appreciable current density at overpotentials below 0.2 V. Bi-CMEC is selective for production of CO, operating with a Faradaic efficiency of approximately 95%. When taken together, these correspond to a high-energy efficiency for CO production, on par with that which has historically only been observed using expensive silver and gold cathodes. [ABSTRACT FROM AUTHOR]

Li2-O2 battery is regarded as one of the most promising energy storage systems for future appLi2cations. However, its energy efficiency is greatly undermined by the large overpotentials of the discharge (formation of Li22O2) and charge (oxidation of Li22O2) reactions. The parasitic reactions of electrolyte and carbon electrode induced by the high charging potential cause the decay of capacity and Li2mit the battery Li2fe. Here, a K-O2 battery is report that uses K+ ions to capture O2- to form the thermodynamically stable KO2 product. This allows for the battery to operate through the one-electron redox proceβ of O2/O2-. Our studies confirm the formation and removal of KO2 in the battery cycle test. Furthermore, without the use of catalysts, the battery shows a low discharge/charge potential gap of less than 50 mV at a modest current density, which is the lowest one that has ever been reported in metal-oxygen batteries. [ABSTRACT FROM AUTHOR]

A series of copper complexes based on the tris(2- pyridylmethyl)amine (TPA) ligand are examined for their oxygen reduction reaction (ORR) activity. Increasing the potential of the Cu, 1/11 couple from 0.23 V vs RHE for [Cu(TPA)(L)]2+ to 0.52 V for [Cu(T£PA)(L)]2+ (tris(2-pyridylethyi)amine) at pH 7 or adding a hydrogen-bonding secondary coordination sphere does not increase the onset potential from 0.69 V vs RHE for the ORR. The underlying mechanism for the ORR is determined to be first-order in 02 and secondorder in Cu. The rate-determining step is found to not be Cu11 to Cu1 reduction, as seen in other systems. The rate-determining step is also not the protonation of an intermediate, but may be the reduction of a hydroperoxo intermediate. Pyrolysis of the Cu complex of TPA affords an inactive material; activity is recovered through addition of intact TPA to the electrode surface. [ABSTRACT FROM AUTHOR]

Carbon dioxide reduction is an essential component of many prospective technologies for the renewable synthesis of carbon-containing fuels. Known catalysts for this reaction generally suffer from low energetic efficiency, poor product selectivity, and rapid deactivation. We show that the reduction of thick Au oxide films results in the formation of Au nanoparticles ("oxide-derived Au") that exhibit highly selective CO2 reduction to CO in water at overpotentials as low as 140 mV and retain their activity for at least 8 h. Under identical conditions, polycrystalline Au electrodes and several other nanostructured Au electrodes prepared via alternative methods require at least 200 mV of additional overpotential to attain comparable CO2 reduction activity and rapidly lose their activity. Electrokinetic studies indicate that the improved catalysis is linked to dramatically increased stabilization of the CO2•- intermediate on the surfaces of the oxide-derived Au electrodes. [ABSTRACT FROM AUTHOR]

We report a simple method of preparation of carbon paste ultramicroelectrodes (UMEs) for use as probe tips in scanning electrochemical microscopy (SECM). Carbon paste UMEs were prepared by packing the carbon paste into a chemically etched tip of a Pt-UME or a pulled glass capillary. Carbon-based UMEs are attractive in micrometer to nanometer gap experiments and in electrodeposition of single metal nanoparticles for electrocatalytic studies because of their high overpotential in proton and oxygen reduction. We have demonstrated the preparation of conically shaped carbon paste UMEs, appropriate for SECM measurements and micrometer to nanometer gap experiments. [ABSTRACT FROM AUTHOR]

The search for efficient catalysts to face modern energy challenges requires evaluation and comparison through reliable methods. Catalytic current efficiencies may be the combination of many factors besides the intrinsic chemical properties of the catalyst. Defining turnover number and turnover frequency (TOF) as reflecting these intrinsic chemical properties, it is shown that catalysts are not characterized by their TOF and their overpotential (η) as separate parameters but rather that the parameters are linked together by a definite relationship. The log TOF-η relationship can often be linearized, giving rise to a Tafel law, which allows the characterization of the catalyst by the value of the TOF at zero overpotential (TOF0). Foot-of-the-wave analysis of the cyclic voltammetric catalytic responses allows the determination of the TOF, log TOF-η relationship, and TOF0, regardless of the side-phenomena that interfere at high current densities, preventing the expected catalytic current plateau from being reached. Strategies for optimized preparative-scale electrolyses may then be devised on these bases. The validity of this methodology is established on theoretical grounds and checked experimentally with examples taken from the catalytic reduction of CO2 by iron(0) porphyrins. [ABSTRACT FROM AUTHOR]

Sodium periodate was characterized as a primary chemical oxidant for the catalytic evolution of oxygen at neutral pH using a variety of water-oxidation catalysts. The visible spectra of solutions formed from Cp*Ir(bpy)S04 during oxygen-evolution catalysis were measured. NMR spectroscopy suggests that the catalyst remains molecular after several turnovers with sodium periodate. Two of our [Cp*Ir(bis-NHC)][PF6]2 complexes, along with other literature catalysts, such as the manganese terpyridyl dimer, Hill's cobalt polyoxometallate, and Meyer's blue dimer, were also tested for activity. Sodium periodate was found to function only for water-oxidation catalysts with low overpotentials. This specificity is attributed to the relatively low oxidizing capability of sodium periodate solutions relative to solutions of other common primary oxidants. Studying oxygen-evolution catalysis by using sodium periodate as a primary oxidant may, therefore, provide preliminary evidence that a given catalyst has a low overpotential. [ABSTRACT FROM AUTHOR]

Modified Cu electrodes were prepared by annealing Cu foil in air and electrochemically reducing the resulting Cu2O layers. The CO2 reduction activities of these electrodes exhibited a strong dependence on the initial thickness of the Cu2O layer. Thin Cu2O layers formed by annealing at 130 °C resulted in electrodes whose activities were indistinguishable from those of polycrystalline Cu. In contrast, Cu2O layers formed at 500 °C that were ≥∼3 μm thick resulted in electrodes that exhibited large roughness factors and required 0.5 V less overpotential than polycrystalline Cu to reduce CO2 at a higher rate than H2O. The combination of these features resulted in CO2 reduction geometric current densities >1 mA/cm² at overpotentials <0.4 V, a higher level of activity than all previously reported metal electrodes evaluated under comparable conditions. Moreover, the activity of the modified electrodes was stable over the course of several hours, whereas a polycrystalline Cu electrode exhibited deactivation within 1 h under identical conditions. The electrodes described here may be particularly useful for elucidating the structural properties of Cu that determine the distribution between CO2 and H2O reduction and provide a promising lead for the development of practical catalysts for electrolytic fuel synthesis. [ABSTRACT FROM AUTHOR]

A new type of bioelectrochemical system for producing electrical power, called a microbial reverse-electrodialysis cell (MRC), was developed to increase voltages and power densities compared to those generated individually by microbial fuel cells (MFCs) or reverse electrodialysis (RED) systems. In RED systems, electrode overpotentials create significant energy losses due to thermodynamically unfavorable electrode reactions, and therefore a large number of stacked cells must be used to have significant energy recovery. This results in high capital costs for the large number of membranes, and increases energy losses from pumping water through a large number of cells. In an MRC, high overpotentials are avoided through oxidation of organic matter by exoelectrogenic bacteria on the anode and oxygen reduction on the cathode. An MRC containing only five pairs of RED cells, fed solutions typical of seawater (600 mM NaCl) and river water (12 mM NaCl) at 0.85 mL/min, produced up to 3.6 W/m² (cathode surface area) and 1.2-1.3 V with acetate as a substrate. Pumping accounted for <2% of the produced power. A higher flow rate (1.55 mL/min) increased power densities up to 4.3 W/m². COD removal was 98% with a Coulombic efficiency of 64%. Power production by the individual components was substantially lower with 0.7 W/m² without salinity driven energy, and <0.015 W/m² with reduced exoelectrogenic activity due to substrate depletion. These results show that the combination of an MFC and a RED stack synergistically increases performance relative to the individual systems, producing a new type of system that can be used to more efficiently capture salinity driven energy from seawater and river water. [ABSTRACT FROM AUTHOR]

The hydrogen evolution reaction (HER) at Pt-cathodes of microbial electrolysis cells (MEC) has been associated with overpotentials of several hundred millivolts. The high overpotentials challenge the sustainability of an MEC. This paper shows that the HER overpotential at MEC relevant pH values is reduced if buffer is present At 15 A/m2 and 50 mM buffer, the lowest overpotential for phosphate was -0.05 Vat pH 6.2, for ammonia was -0.05 V at pH 9.0, for carbonate was -0.09 V at pH 9.3, for Tris(hydroxymathyl)aminomathane was -0.07 V at pH 7.8, and for N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid was -0.08 V at pH 7.2. It was shown that the effect of buffer on the overpotantial is strongly pH dependent Furthermore, experimental data and a mass transport equation showed that by increasing the buffer concentration or linear flow speed (i.e., pump speed), or decreasing the currant density (i) the overpotential reduces and (iii) the minimum overpotential is reached at a pH that approaches the buffer dissociation constant (ρKa). Thus, to reduce the HER overpotential of an MEC, buffer (i.e., ρKa, buffer concentration, linear flow speed, and currant density must be well balanced with the expected operational pH. [ABSTRACT FROM AUTHOR]

Describes a chemistry classroom demonstration showing the dependence of hydrogen overpotential on the physical state of the metal employed. Immersion of platinum wire in hydrochloric acid; Atomic absorption analysis of the solution; Introduction of process of platinization and the concept of hydrogen overpotential.

Presents a laboratory experiment to demonstrate overvoltage in the hydrogen evolution reaction. Theoretical considerations; Design of electrodes; Analysis of current-potential curves; Tafel plots for different sulfuric aqueous solutions with platinum and mercury electrodes.

Evaluates the polarity of overpotential in electrochemistry. Definition of overpotential; Chemical reactions involving the polarity of the electrode and overpotential; Electrode mechanics.