Electrodeposition of Co4S3 on NiCo LDH nanosheet arrays for advanced hydrogen evolution
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Nanosheet
Overpotential
Alkaline water electrolysis
Electrolysis of water
Our present work demonstrated electrochemical oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) activities of ultrathin nickel cobalt hydroxide (NixCoy-OH, Ni/Co = x/y) nanosheet films. A Ni1Co4-OH nanosheet film showed the lowest overpotential of all Ni/Co ratios. When the layer number of Ni1Co4-OH nanosheet film was changed, the larger layer number gave the lower overpotential. However, the 1–4 layers of nanosheet showed larger decreases of the overpotentials per layer (−61 mV/layer) than the 4–12 layers (−9 mV/layer). Our present work demonstrated electrochemical oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) activities of ultrathin nickel cobalt hydroxide (NixCoy-OH, Ni/Co = x/y) nanosheet films. The larger layer number of Ni1Co4-OH nanosheet film gave a lower overpotential, however, 1–4 layers of nanosheet showed larger decreases of the overpotentials per layer (−61 mV/layer) than the 4–12 layers (−9 mV/layer).
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Oxygen evolution
Cobalt hydroxide
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Developing new electrocatalysts is essentially important for efficient water splitting to produce hydrogen. Two-dimensional (2D) materials provide great potential for high-performance electrocatalysts because of their high specific surface area, abundant active edges, and tunable electronic structure. Here, we report few-layer NiPS3 nanosheet-graphene composites for high-performance electrocatalysts for oxygen evolution reaction (OER). The pure NiPS3 nanosheets show an overpotential of 343 mV for a current density of 10 mA cm-2, which is comparable to that for IrO2 and RuO2 catalysts. More importantly, the NiPS3 nanosheet-graphene composites show significantly improved OER activity due to the synergistic effect. The optimized composite shows a very low overpotential of 294 mV for a current density of 10 mA cm-2, 351 mV for a current density of 100 mA cm-2, a small Tafel slope of 42.6 mV dec-1, and excellent stability. These overall performances are far better than those of the reported 2D materials and even better than those of many traditional materials even at a much lower mass loading of NiPS3.
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Oxygen evolution
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A wafer-scale 1.4 nm ultrathin Ni(OH)2 nanosheet was synthesized by ionic layer epitaxy. This free-standing Ni(OH)2 nanosheet was directly used to catalyze the oxygen evolution reaction (OER). At a current density of 10 mA cm-2, the overpotential reached 295 mV (vs. RHE) in Fe-rich 1 M NaOH. This 1.4 nm Ni(OH)2 nanosheet showed a very high turnover frequency of 5.47 s-1 and a mass activity of more than 2 × 104 A g-1 at an overpotential of 300 mV. Such a high electrocatalytic mass activity of the Ni(OH)2 nanosheet was more than 2 orders of magnitude higher than those of typical OER catalysts. The capability of producing wafer-scale nanometer-thick nanosheets offers a promising strategy to improve the mass efficiency of electrochemical catalysts, which is particularly valuable for preserving rare and precious catalyst materials.
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A Co-MOF nanosheet array on Ni foam (Co-MOF/NF) acts as a superior electrocatalyst for the oxygen evolution reaction, needing an overpotential of only 311 mV to drive a geometrical catalytic current density of 50 mA cm−2 in 1.0 M KOH.
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Bubble evolution induced overpotential for hydrogen production by water electrolysis is an important issue and the generated gas bubbles will cover the internal/external surface of the electrode. Herein, a flow-through electrode loaded with Co-based nanosheets is presented. Under the condition of flow-through mode, the catalysts can be adequately exposed because the generated bubbles can be timely removed from the catalyst surface. The overpotential can be reduced by about 100 mV for hydrogen evolution reaction (HER) and 57 mV for oxygen evolution reaction (OER) under the condition of 300 mA cm-2 and electrolyte flux of 339 m3 m-2 h-1. A novel electrolyzer assembled with flow-through electrode for hydrogen production by alkaline water electrolysis is conducted. The cell voltage can be decreased by 100 mV at 400 mA cm-2 at 6 M KOH, 338 K, and 1 atm, with the energy of 5 kWh Nm-3 required.
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Electrolysis of water
Alkaline water electrolysis
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Considerations about thermodynamic and kinetic requirements for water splitting at n-type semiconductors are presented. A main point in water photooxidation concerns the catalytic role that the semiconductor must play in order to minimize the overpotential for oxygen evolution. On the basis of our previous results about water splitting at n-TiO/sub 2/ electrodes, and of the literature data on the electrocatalytic evolution of oxygen at RuO/sub 2/, the best metallic catalyst known up to date for this reaction, the minimum overpotential for water photooxidation is estimated to be of the order of 0.6 eV, which fixes the minimum semiconductor bandgap at about 1.8 eV. Implications of the model in photoreactions competing with water splitting are discussed.
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Oxygen evolution
Photoelectrochemical cell
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High-performance water splitting electrocatalysts are urgently needed in the face of the environmental degradation and energy crisis. The first principles method was used in this study to systematically examine the electronic characteristics of transition metal (Sc, Ti, V, Cr, Mn, Fe, and Ru) doped WSi2N4(TM@WSi2N4) and its potential as oxygen evolution reaction (OER) catalysts. Our study shows that the doping of TM atoms significantly improves the catalytic performance of TM@WSi2N4, especially Fe@WSi2N4shows a low overpotential (ηOER= 470 mV). Interestingly, we found that integrated-crystal orbital Hamilton population and d-band center can be used as descriptors to explain the high catalytic activity of Fe@WSi2N4. Subsequently, Fe@WSi2N4exhibits the best hydrogen evolution reaction (HER) activity with a universal overpotential of 47 mV on N1sites. According to our research, Fe@WSi2N4offers a promising substitute for precious metals as a catalyst for overall water splitting with low OER and HER overpotentials.
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Converting solar energy into sustainable hydrogen fuel by photoelectrochemical (PEC) water splitting is a promising technology to solve increasingly serious global energy supply and environmental issues. However, the PEC performance based on TiO2 nanomaterials is hindered by the limited sunlight-harvesting ability and its high recombination rate of photogenerated charge carriers. In this work, layered SnS2 absorbers and CoOx nanoparticles decorated two-dimensional (2D) TiO2 nanosheet array photoelectrode have been rationally designed and successfully synthesized, which remarkably enhanced the PEC performance for water splitting. As the result, photoconversion efficiency of TiO2/SnS2/CoOx and TiO2/SnS2 hybrid photoanodes increases by 3.6 and 2.0 times under simulated sunlight illumination, compared with the bare TiO2 nanosheet arrays photoanode. Furthermore, the TiO2/SnS2/CoOx photoanode also presented higher PEC stability owing to CoOx catalyst served as efficient water oxidation catalyst as well as an effective protectant for preventing absorber photocorrosion.
Nanosheet
Nanochemistry
Nanomaterials
Visible spectrum
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Overpotential
Oxygen evolution
Electrolysis of water
Alkaline water electrolysis
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