A high-density nickel–cobalt alloy embedded in nitrogen-doped carbon nanosheets for the hydrogen evolution reaction
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The development of novel non-noble electrocatalysts is critical for an efficient electrochemical hydrogen evolution reaction (HER). In this study, high-density nickel-cobalt alloy nanoparticles embedded in the bent nitrogen-doped carbon nanosheets are prepared as a high-performance catalyst. The optimized Ni7Co3/NC-500 catalyst displays quite a low overpotential of 90 mV at a current density of 10 mA cm-2, and a small Tafel slope of 64 mV dec-1 in alkaline medium, and even performs better than commercial 20% Pt/C at a high current density (η150 = 233 mV for Ni7Co3/NC-500 and η150 = 267 mV for 20% Pt/C). Specifically, the high-density nickel-cobalt alloy (with an average size of 6.2 nm and a distance of <3.0 nm) embedded in the bent carbon nanosheets provides plentiful active sites. Furthermore, in situ visualization of the produced hydrogen bubbles shows that the small size of hydrogen bubbles (d = 0.2 mm for Ni7Co3/NC-500 vs. d = 0.8 mm for 20% Pt/C) resulting from the small water contact angle and the bent nanosheet structure would inhibit the aggregation of H2 bubbles on the surface to facilitate efficient mass diffusion. Density functional theory calculations reveal that the formation of the nickel-cobalt alloy can effectively lower water dissociation energy barriers and optimize hydrogen adsorption Gibbs free energy, manifesting a high HER activity.Keywords:
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Molybdenum disulfide
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Two-dimensional (2D) nanocomposites are fabricated on reduced graphene oxide (Co–P–B/rGO) via a one-step method at room temperature. By tuning the molar ratio of phosphorus and boron sources in reactants, outstanding bi-functional catalysts for the oxygen evolution reaction (OER) can be obtained with an extremely low onset overpotential of 50 mV and a small Tafel slope of 68 mV dec−1, as well as for the hydrogen evolution reaction (HER) with onset overpotential of 168 mV and a Tafel slope of 82 mV dec−1 in neutral solution (pH = 7.0). The improved performance could be due to the high specific area, efficient electron transfer and the synergistic effects of P and B. We also tested these catalysts in seawater for both OER and HER, which showed favorable activities. This approach to form 2D structures on a reduced graphene oxide may inspire the development of more extraordinary electrochemical catalysts and devices.
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Author(s): Limsakoune, Vipawee | Advisor(s): Viney, Christopher | Abstract: MoS2 is a good candidate for Hydrogen Evolution Reaction (HER) catalyst because it has low overpotential at 10 mA/cm2 and low Tafel slope close to platinum, is inexpensive, and is abundant. High surface area and mass loading of the Friction Induced Structural Transformation of crumpled MoS2 (FIST c-MoS2) could lower the overpotential at 10 mA/cm2 and Tafel slope compared to c-MoS2 and chemically exfoliated MoS2 (ce-MoS2). In this work, MoS2 catalysts are synthesized at room temperature to preserve active basal plane for hydrogen adsorption. The overpotential at 10 mA/cm2 of the catalyst after cyclic voltammetry is -196 mV vs. Reversible Hydrogen Electrode (RHE); the Tafel slope is 91.9 mV/dec, and the exchange current density is 75.9 µA/cm2 in 0.5 M H2SO4 electrolyte. After cyclic voltammetry from +0.2 V to -0.3 V vs. RHE for 1,000 cycles, FIST c-MoS2 on ce-MoS2, c-MoS2 on ce-MoS2, and ce-MoS2, all on carbon cloth, show similar overpotential at 10 mA/cm2, and similar Tafel slope. MoS2 catalysts from this work exhibit among the lowest overpotentials at 10 mA/cm2 of MoS2 based catalysts. Capacitance data derived from EIS plots reveal that cyclic voltammetry leads to an increased surface area of all the MoS2 catalysts. The increased surface area promotes low overpotential at 10 mA/cm2, and high exchange current density.
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A Tafel slope of 40 mV dec-1 and a very small overpotential were measured for our NiSMoS2G nanocatalyst, prepared using a scalable approach, and consisting of NiS nanoparticles covered by a stabilizing coating of MoS2 nanosheets, on unsophisticated and easy to obtain physical exfoliated graphite. A careful study proves that it is possible to improve the Tafel slope and the overpotential through the optimization of amounts of the different components. The conductive nanocarbon network, the highly active to charge accumulation of NiS nanoparticles and the coupling with MoS2 nanosheets, exposing a large number of edges, result in a very high hydrogen production rate of 1.63 ml cm-2 h-1 at -0.12 V, measured by means of an on-line mass spectrometry analysis.
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Herein, a novel composite of small amounts of Ag nanoparticles (NPs) decorated urchin-like cobalt carbonate hydroxide hydrate (CCHH) was developed for highly-efficient alkaline oxygen evolution reaction (OER). Not only can Ag colloids, as template agents, modify the morphologies of urchin-like CCHH microspheres to expose more active sites available, but also the supported Ag NPs formed by Ag colloids can transfer the electron to CCHH surfaces, accelerating the transformation of surface CoII to CoIII/CoIV (proton-coupled electron transfer (PCET) process). The urchin-like Ag/CCHH (0.013 mmol) precatalyst (before cyclic voltammetry (CV) activation) exhibits a better OER performance (a low overpotential of 273 mV at 10 mA cm-2 and small Tafel slope of 65 mV dec-1) as compared with commercial RuO2. Furthermore, the dynamic surface self-reconstruction (surface CO32- and OH - exchange) can further enhance the activities of Ag/CCHH precatalysts. Consequently, the optimal Ag/CCHH (0.013 mmol) catalyst presents a superior activity (a lower overpotential of 267 mV at 10 mA cm-2 and markedly reduced Tafel slope to 56 mV dec-1) along with an excellent stability after CV cycles. The study provides a feasible strategy to fully realize the low overpotential of CCHH-based OER electrocatalysts.
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