Overpotential

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

N-Doped Porous Molybdenum Carbide Nanobelts as Efficient Catalysts for Hydrogen Evolution Reaction

Abstract: Electrochemical water splitting has been highly valued as a clean and sustainable method to produce hydrogen with high purity and large quantity. Platinum (Pt) possesses excellent performance for hydrogen evolution reaction (HER), however, the expensiveness and rareness still restricts its broad application. Here, an environmentally-friendly and template-free method is developed to synthesize N-doped molybdenum carbide in three different forms: nanobelts, nanorods and nanoparticles. During the synthesis, no additional acid or alkali aqueous solution is used and the morphology is controlled by adjusting water content and treatment time. The corresponding physicohemical and electrochemical results display that the nanobelts with porous nanostructure exhibit excellent activity and good stability for HER both in acidic and alkaline electrolytes. In the latter case (1.0 M KOH aqueous solution), nanobelts show a small onset potential value (–52 mV), yield a current density of 10 mA cm−2 at a relatively low overpotential (110 mV) and exhibit a low Tafel slope (49.7 mV dec−1). From the obtained results is deduced that this is a facile way to prepare cost-effective molybdenum carbide with high efficiency for hydrogen evolution.

Mesoporous MoO 3- x Material as an Efficient Electrocatalyst for Hydrogen Evolution Reactions

Abstract: A unique approach for the synthesis of nonstoichiometric, mesoporous molybdenum oxide (MoO3–x) with nanosized crystalline walls by using a soft template (PEO-b-PS) synthesis method is introduced. The as-synthesized mesoporous MoO3–x is very active and stable (durability > 12 h) for the electrochemical hydrogen evolution reaction (HER) under both acidic and alkaline conditions. The intrinsic MoO3 serves as an HER electrocatalyst without the assistance of carbon materials, noble metals, or MoS2 materials. The results from transmission electron microscopy and N2 sorption techniques show that the as-synthesized mesoporous MoO3–x has large accessible pores (20–40 nm), which are able to facilitate mass transport and charge transfer during HER. In terms of X-ray diffraction, X-ray photoelectron spectroscopy, temperature-programmed oxidation, and diffusive reflectance UV–vis spectroscopy, the mesoporous MoO3–x exhibits mixed oxidation states (Mo5+, Mo6+) and an oxygen-deficient structure. The as-synthesized MoO3–x only requires a low overpotential (≈0.14 V) to achieve a 10 mA cm−2 current density in 0.1 m KOH and the Tafel slope is as low as 56 mV dec−1. Density functional theory calculations demonstrate a change of electronic structure and the possible reaction pathway of HER. Oxygen vacancies and mesoporosity serve as key factors for excellent performance.

Electrochemical Behavior of Highly Conductive Boron‐Doped Diamond Electrodes for Oxygen Reduction in Alkaline Solution

Abstract: Highly conductive boron-doped polycrystalline diamond thin films (ρ ≃ 10 -3 Ω cm) reduction prepared via microwave plasma chemical vapor deposition (CVD). The electrochemical behavior for oxygen reduction was examined in 0.1 M KOH using linear sweep voltammetry. Oxygen reduction was found to be highly inhibited, the cathodic voltammetric peak being observed at ∼ -1.2 V vs. Ag/AgCl, compared with the standard potential for the two-electron reduction of oxygen (O 2 + H 2 O + 2e - = HO 2 - + OH - , E°' = -0.234 V vs. Ag/AgCl at pH 13). This demonstrates that, even in the presence of dissolved oxygen, diamond retains a relatively wide potential window, which could be advantageous in certain types of analytical applications. Possible interpretations for the high overpotential for oxygen reduction include a lack of adsorption sites for oxygen and/or reduced intermediates, a low density of states or a potential drop within a thin were nm) surface layer, all of which have also been proposed for highly ordered pyrolytic graphite. The experimental data were fitted using digital simulation, which showed that the reduction peak appearing at ca. -1.2 V also consistent with an nantly due to the reduction of oxygen to peroxide. Rotating disk electrode measurements were also consistent with an overall two-electron process. Experiments involving the addition of superoxide dismutase also supported this conclusion. The oxygen reduction reaction is proposed to occur on the sp 3 carbon component of the surface, with a very small contribution from sp 2 carbon impurities at smaller overpotentials.

Fe3O4‐Decorated Co9S8 Nanoparticles In Situ Grown on Reduced Graphene Oxide: A New and Efficient Electrocatalyst for Oxygen Evolution Reaction

Abstract: Cobalt sulfide materials have attracted enormous interest as low-cost alternatives to noble-metal catalysts capable of catalyzing both oxygen reduction and oxygen evolution reactions. Although recent advances have been achieved in the development of various cobalt sulfide composites to expedite their oxygen reduction reaction properties, to improve their poor oxygen evolution reaction (OER) activity is still challenging, which significantly limits their utilization. Here, the synthesis of Fe3O4-decorated Co9S8 nanoparticles in situ grown on a reduced graphene oxide surface (Fe3O4@Co9S8/rGO) and the use of it as a remarkably active and stable OER catalyst are first reported. Loading of Fe3O4 on cobalt sulfide induces the formation of pure phase Co9S8 and highly improves the catalytic activity for OER. The composite exhibits superior OER performance with a small overpotential of 0.34 V at the current density of 10 mA cm−2 and high stability. It is believed that the electron transfer trend from Fe species to Co9S8 promotes the breaking of the Co–O bond in the stable configuration (Co–O–O superoxo group), attributing to the excellent catalytic activity. This development offers a new and effective cobalt sulfide-based oxygen evolution electrocatalysts to replace the expensive commercial catalysts such as RuO2 or IrO2.

Bimetallic Cobalt-Based Phosphide Zeolitic Imidazolate Framework: CoPx Phase-Dependent Electrical Conductivity and Hydrogen Atom Adsorption Energy for Efficient Overall Water Splitting

Abstract: Cobalt-based bimetallic phosphide encapsulated in carbonized zeolitic imadazolate frameworks has been successfully synthesized and showed excellent activities toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Density functional theory calculation and electrochemical measurements reveal that the electrical conductivity and electrochemical activity are closely associated with the Co2P/CoP mixed phase behaviors upon Cu metal doping. This relationship is found to be the decisive factor for enhanced electrocatalytic performance. Moreover, the precise control of Cu content in Co-host lattice effectively alters the Gibbs free energy for H* adsorption, which is favorable for facilitating reaction kinetics. Impressively, an optimized performance has been achieved with mild Cu doping in Cu0.3Co2.7P/nitrogen-doped carbon (NC) which exhibits an ultralow overpotential of 0.19 V at 10 mA cm–2 and satisfying stability for OER. Cu0.3Co2.7P/NC also shows excellent HER activity, affording a current density of 10 mA cm–2 at a low overpotential of 0.22 V. In addition, a homemade electrolyzer with Cu0.3Co2.7P/NC paired electrodes shows 60% larger current density than Pt/RuO2 couple at 1.74 V, along with negligible catalytic deactivation after 50 h operation. The manipulation of electronic structure by controlled incorporation of second metal sheds light on understanding and synthesizing bimetallic transition metal phosphides for electrolysis-based energy conversion.