Overpotential

About: Overpotential is a research topic. Over the lifetime, 16474 publications have been published within this topic receiving 616632 citations.

Activity Descriptors for CO2 Electroreduction to Methane on Transition-Metal Catalysts

Abstract: The electrochemical reduction of CO2 into hydrocarbons and alcohols would allow renewable energy sources to be converted into fuels and chemicals. However, no electrode catalysts have been developed that can perform this transformation with a low overpotential at reasonable current densities. In this work, we compare trends in binding energies for the intermediates in CO2 electrochemical reduction and present an activity “volcano” based on this analysis. This analysis describes the experimentally observed variations in transition-metal catalysts, including why copper is the best-known metal electrocatalyst. The protonation of adsorbed CO is singled out as the most important step dictating the overpotential. New strategies are presented for the discovery of catalysts that can operate with a reduced overpotential.

Ultrathin Spinel-Structured Nanosheets Rich in Oxygen Deficiencies for Enhanced Electrocatalytic Water Oxidation

Abstract: Electrochemical water splitting is a clean technology for H2 fuels, but greatly hindered by the slow kinetics of the oxygen evolution reaction (OER). Herein, a series of spinel-structured nanosheets with oxygen deficiencies and ultrathin thicknesses were designed to increase the reactivity and the number of active sites of the catalysts, which were then taken as an excellent platform for promoting the water oxidation process. Theoretical investigations showed that the oxygen vacancies confined in the ultrathin nanosheet could lower the adsorption energy of H2O, leading to increased OER efficiency. As expected, the NiCo2O4 ultrathin nanosheets rich in oxygen vacancies exhibited a large current density of 285 mA cm−2 at 0.8 V and a small overpotential of 0.32 V, both of which are superior to the corresponding values of bulk samples or samples with few oxygen deficiencies and even higher than those of most reported non-precious-metal catalysts. This work should provide a new pathway for the design of advanced OER catalysts.

Origin of Outstanding Stability in the Lithium Solid Electrolyte Materials: Insights from Thermodynamic Analyses Based on First-Principles Calculations

Abstract: First-principles calculations were performed to investigate the electrochemical stability of lithium solid electrolyte materials in all-solid-state Li-ion batteries. The common solid electrolytes were found to have a limited electrochemical window. Our results suggest that the outstanding stability of the solid electrolyte materials is not thermodynamically intrinsic but is originated from kinetic stabilizations. The sluggish kinetics of the decomposition reactions cause a high overpotential leading to a nominally wide electrochemical window observed in many experiments. The decomposition products, similar to the solid-electrolyte-interphases, mitigate the extreme chemical potential from the electrodes and protect the solid electrolyte from further decompositions. With the aid of the first-principles calculations, we revealed the passivation mechanism of these decomposition interphases and quantified the extensions of the electrochemical window from the interphases. We also found that the artificial coating layers applied at the solid electrolyte and electrode interfaces have a similar effect of passivating the solid electrolyte. Our newly gained understanding provided general principles for developing solid electrolyte materials with enhanced stability and for engineering interfaces in all-solid-state Li-ion batteries.

Hierarchical NiCo2S4 Nanowire Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions

Abstract: A recent approach for solar-to-hydrogen generation has been water electrolysis using efficient, stable, and inexpensive bifunctional electrocatalysts within strong electrolytes. Herein, the direct growth of 1D NiCo2S4 nanowire (NW) arrays on a 3D Ni foam (NF) is described. This NiCo2S4 NW/NF array functions as an efficient bifunctional electrocatalyst for overall water splitting with excellent activity and stability. The 3D-Ni foam facilitates the directional growth, exposing more active sites of the catalyst for electrochemical reactions at the electrode–electrolyte interface. The binder-free, self-made NiCo2S4 NW/NF electrode delivers a hydrogen production current density of 10 mA cm–2 at an overpotential of 260 mV for the oxygen evolution reaction and at 210 mV (versus a reversible hydrogen electrode) for the hydrogen evolution reaction in 1 m KOH. This highly active and stable bifunctional electrocatalyst enables the preparation of an alkaline water electrolyzer that could deliver 10 mA cm–2 under a cell voltage of 1.63 V. Because the nonprecious-metal NiCo2S4 NW/NF foam-based electrodes afford the vigorous and continuous evolution of both H2 and O2 at 1.68 V, generated using a solar panel, they appear to be promising water splitting devices for large-scale solar-to-hydrogen generation.

Hydrogen-evolution catalysts based on non-noble metal nickel-molybdenum nitride nanosheets.

Abstract: Hydrogen production through splitting of water has attracted great scientific interest because of its relevance to renewable energy storage and its potential for providing energy without the emission of carbon dioxide. Electrocatalytic systems for H2 generation typically incorporate noble metals such as Pt in the catalysts because of their low overpotential and fast kinetics for driving the hydrogen evolution reaction (HER). However, the high costs and limited world-wide supply of these noble metals make their application in viable commercial processes unattractive. Several non-noble metal materials, such as transition-metal chalcogenides, carbides, and complexes as well as metal alloys have been widely investigated recently, and characterized as catalysts and supports for application in the evolution of hydrogen. Nitrides of early transition-metals have been shown to have excellent catalytic activities in a variety of reactions. One of the primary interests in the applications of nitrides in these reactions was to use them in conjunction with low-cost alternative metals to replace group VIII noble metals. For example, the function of molybdenum nitride as a catalyst for hydrocarbon hydrogenolysis resembles that of platinum. The catalytic and electronic properties of transition-metal nitrides are governed by their bulk and surface structure and stoichiometry. While there is some information concerning the effect of the bulk composition on the catalytic properties of this material, there is currently little known about the effects of the surface nanostructure. Nickel and nickel–molybdenum are known electrocatalysts for hydrogen production in alkaline electrolytes, and in the bulk form they exhibited exchange current densities between 10 6 and 10 4 Acm , compared to 10 3 Acm 2 for Pt. Jaksic et al. postulated a hypo-hyper-d-electronic interactive effect between Ni and Mo that yields the synergism for the HER. Owing to their poor corrosion stability, few studies in acidic media have been reported.With the objective of exploiting the decrease in the overpotential by carrying out the HER in acidic media, we have developed a low-cost, stable, and active molybdenum-nitride-based electrocatalyst for the HER. Guided by the “volcano plot” in which the activity for the evolution of hydrogen as a function of the M H bond strength exhibits an ascending branch followed by a descending branch, peaking at Pt, we designed a material on the molecular scale combining nickel, which binds H weakly, with molybdenum, which binds H strongly. Here we report the first synthesis of NiMo nitride nanosheets on a carbon support (NiMoNx/C), and demonstrate the high HER electrocatalytic activity of the resulting NiMoNx/C catalyst with low overpotential and small Tafel slope. The NiMoNx/C catalyst was synthesized by reduction of a carbon-supported ammonium molybdate [(NH4)6Mo7O24·4H2O] and nickel nitrate (Ni(NO3)2·4H2O) mixture in a tubular oven in H2 at 400 8C, and subsequent reaction with NH3 at 700 8C. During this process, the (NH4)6Mo7O24 and Ni(NO3)2 precursors were reduced to NiMo metal particles by H2, and then they were mildly transformed to NiMoNx nanosheets by reaction with ammonia. The atomic ratio of Ni/Mo was 1/4.7 determined by energy dispersive X-ray spectroscopy (EDX) on the NiMoNx/ C sample. The transmission electron microscopy (TEM) images, as shown in Figure 1a, display NiMo particles that are mainly spherical. The high-resolution TEM image, as shown in the inset of Figure 1a, corroborated the presence of an amorphous 3 to 5 nm Ni/Mo oxide layer (see Figure S4 in the Supporting Information for resolved image), whereas NiMoNx is characterized by thin, flat, and flaky stacks composed of nanosheets with high radial-axial ratios (Figure 1b and Figure S5 in the Supporting Information for a magnified image). Figure 1c shows that some of the nanosheets lay flat on the graphite carbon (as indicated by the black arrows), and some have folded edges that show different layers of NiMoNx sheets (white arrows). The thickness of the sheets ranged from 4 to 15 nm. The average stacking number of sheets measured from Figure 1b is about [*] Dr. W.-F. Chen, Dr. K. Sasaki, Dr. J. T. Muckerman, Dr. R. R. Adzic Chemistry Department, Brookhaven National Laboratory Upton, NY 11973 (USA) E-mail: ksasaki@bnl.gov