Unsaturated Single Atoms on Monolayer Transition Metal Dichalcogenides for Ultrafast Hydrogen Evolution
Yuting LuoShuqing ZhangHaiyang PanShujie XiaoZenglong GuoLei TangUsman KhanBaofu DingMeng LiZhengyang CaiYüe ZhaoWei LvQingliang FengXiaolong ZouJunhao LinHui‐Ming ChengBilu Liu
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Large scale implementation of electrochemical water splitting for hydrogen evolution requires cheap and efficient catalysts to replace expensive platinum. Molybdenum disulfide is one of the most promising alternative catalysts but its intrinsic activity is still inferior to platinum. There is therefore a need to explore new active site origins in molybdenum disulfide with ultrafast reaction kinetics and to understand their mechanisms. Here, we report a universal cold hydrogen plasma reduction method for synthesizing different single atoms sitting on two-dimensional monolayers. In case of molybdenum disulfide, we design and identify a new type of active site, i.e., unsaturated Mo single atoms on cogenetic monolayer molybdenum disulfide. The catalyst shows exceptional intrinsic activity with a Tafel slope of 35.1 mV dec-1 and a turnover frequency of ~10^3 s-1 at 100 mV, based on single flake microcell measurements. Theoretical studies indicate that coordinately unsaturated Mo single atoms sitting on molybdenum disulfide increase the bond strength between adsorbed hydrogen atoms and the substrates through hybridization, leading to fast hydrogen adsorption/desorption kinetics and superior hydrogen evolution activity. This work shines fresh light on preparing highly-efficient electrocatalysts for water splitting and other electrochemical processes, as well as provides a general method to synthesize single atoms on two-dimensional monolayers.Keywords:
Molybdenum disulfide
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Electrochemical corrosion measurements allow calculation of the instantaneous zinc corrosion rate via polarization resistances by using tafel factors. However, the determination of the actual tafel factor is problematic since it depends on the state of the formed zinc layers and the corrosion reactions taking place. Therefore, valid tafel factors are either determined in additional experiments via dynamic polarization or estimated by calculation. In most cases a constant value for tafel factors is assumed for simplification, without regard to the real conditions of the corroding system. Since naturally formed zinc layers are unstable using conventional test electrolyte solutions determination of tafel factors is hindered additionally and inaccurate interpretations can result. For some time now, the use of gel-type electrolytes in corrosion research has enabled minimally invasive investigation of zinc surface layers and thus offers new approaches to a scientifically proven determination of tafel factors. The paper presents a new method for the determination and evaluation of tafel factors using gel-type electrolytes and electrochemical frequency modulation technique (EFM). This relatively new electrochemical method offers the possibility to determine both polarization resistances and tafel factors within one measurement and in short measuring intervals. Starting from a comprehensive parameter study it is shown that a direct relationship between the two values exists that can be described mathematically. This contribution presents the determined tafel factors for the system gel-type electrolyte/zinc and discusses their applicability and their limits.
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Despite numerous experimental and theoretical studies devoted to the oxygen evolution reaction, the mechanism of the OER on transition metal oxides remains controversial. This is in part owed to the ambiguity of electrochemical parameters of the mechanism such as the Tafel slope and reaction orders. We took the most commonly assumed adsorbate mechanism and calculated the Tafel slopes and reaction orders with respect to pH based on microkinetic analysis. We demonstrate that number of possible Tafel slopes strongly depends on a number of preceding steps and surface coverage. Furthermore, the Tafel slope becomes pH dependent when the coverage of intermediates changes with pH. These insights complicate the identification of a rate-limiting step by a single Tafel slope at a single pH. Yet, simulations of reaction orders complementary to Tafel slopes can solve some ambiguities to distinguish between possible rate-limiting steps. The most insightful information can be obtained from the low overpotential region of the Tafel plot. The simulations in this work provide clear guidelines to experimentalists for the identification of the limiting steps in the adsorbate mechanism using the observed values of the Tafel slope and reaction order in pH-dependent studies.
Overpotential
Oxygen evolution
Reaction rate
Limiting
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The usually employed expression for the Tafel slope is modified to take into account the potential which is effective in charge transfer within the double layer. The exchange currents and cathodic Tafel slopes are obtained on noble metal electrodes having different number of holes in the d‐band. Using the conventional and modified Tafel slopes, possible paths and rate‐determining steps are suggested for the cathodic reduction of oxygen.
Exchange current density
Noble metal
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2005 is the 100th anniversary of the two original publications of the Tafel equation [1,2]. The international corrosion community is currently celebrating [3] the use of the corresponding Tafel slope (?), which is one of the most frequently used parameters in electrochemical corrosion. Even now, with the use of modern research and testing techniques, both electrochemists and corrosion engineers are frequently encountering this ‘Tafel constant’ in the technical literature and in instrumentation manuals (you can’t use an LPR meter without assuming beta values). Unfortunately, Tafel slopes are commonly misused and measured completely out of context. So, what is a Tafel slope? Perhaps most importantly, when is an apparent ‘Tafel’ slope a true Tafel slope?
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Tafel analysis is a widely accepted technique for corrosion studies in electrochemistry. A general literature search for one of the electronegative metals, zinc, revealed serious deviations in corrosion results. In order to understand the reasons behind these deviations, zinc metal was investigated at macro and micro levels during and after the Tafel corrosion analysis. In-situ macro surface investigation during the OCP period and Tafel analysis were performed, and it was found that the zinc surface undergoes proceeding corrosion attack following the immersion in 3.5 wt.% NaCl solution. In-situ macro surface analysis exhibited that the pre-oxidation of the surface proceeds as nonuniform at local regions. SEM-EDS and XRD analysis proved that the particular crystal planes of the zinc form ZnO with increasing immersion time. A linear sweep voltammetry (LSV) technique was applied to detect the oxygen removal and starting hydrogen evolution potentials. Three identical Tafel experiments were performed on samples without any treatment, and another three consecutive Tafel experiments were performed on the samples which applied pre-reduction potential. Obtained results revealed that in-situ pre-applied reduction potential just before the Tafel analysis cleaned the surface and allowed uniform oxide formation, resulting in the lowest standard deviation of the calculated Tafel elements.
Linear sweep voltammetry
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Distilled water
Ohmic contact
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Abstract Cost‐effective and high‐efficiency electrocatalysts for water splitting are important for the production of renewable energy conversion and storage systems. Herein, a 3 D NiO nanobelt electrode was synthesized by direct oxidation of nickel foam using the hydrothermal method. Electrochemical studies reveal that the 3 D NiO nanobelt electrode exhibits excellent electrocatalytic activities for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with low onset overpotential, small Tafel slope, and very eminent electrochemical stability in the standard aqueous solution of 1.0 m KOH. More importantly, the OER activity of 3 D NiO nanobelt can be enhanced by the wrapping of N‐doped carbon (NiO@C), which can achieve an overpotential of 500 mV at 100 mA cm −2 , a low Tafel slope of 92 mV dec −1 , and distinguished electrochemical stability. This strategy provides a general route to prepare 3 D nanostructured catalyst for water splitting by directly using transition‐metal foam as metal source and substrate.
Overpotential
Oxygen evolution
Non-blocking I/O
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Chromate conversion coating
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