Tip-Welded Ternary FeCo2S4 Nanotube Arrays on Carbon Cloth as Binder-Free Electrocatalysts for Highly Efficient Oxygen Evolution
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In the quest for many sustainable energy conversion technologies such as water splitting and fuel cells, developing inexpensive yet highly efficient and robust electrocatalysts for the oxygen evolution reaction (OER) is urgent but still an enormous challenge. Herein, tip-welded bimetallic iron–cobalt sulfide nanotube arrays with tunable morphology and composition were fabricated via a template-free method and directly grown on carbon cloth (FeCo2S4 NTA/CC) as a flexible binder-free catalytic electrode. Based on the well-defined hollow nanotube structure, more abundant active sites are exposed, which accelerates the charge transfer process. In addition, the composition of the catalysts also plays an important role in the electrochemical behavior. Benefited from the unique structure and synergistic effect of bimetallic sulfides, the obtained FeCo2S4 NTA/CC exhibits an outstanding electrocatalytic activity toward the OER with an extremely low overpotential of 317 mV to drive a current density of 100 mA cm–2, a small Tafel slope of 36 mV dec–1, and excellent durability during the alkaline water electrolysis in 1.0 M KOH.Keywords:
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Abstract The energy‐efficiency loss with high overpotential during hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), as well as economic inefficiencies including high‐cost materials and complicated processes, is considered the major challenge to the implementation of electrochemical water splitting applications. The authors present a new platform for electrocatalysts that functions in an unprecedented way to turn a catalyst into substrate. The NiFe alloy catalyzed substrate (NiFe‐CS) described herein is substantially active and stable electrocatalyst for both HER and OER, with low overpotential of 33 and 191 mV at 10 mA cm −2 for HER and OER, respectively. This structure enables not only the maximization of electrochemically active sites, but also the formation of hydroxyl species on the surface as the active phase. These outstanding results provide a new pathway for the development of electrocatalysts used in energy conversion technology.
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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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