but still limited to noble metal, such as iridium (Ir) and ruthenium (Ru). [3] The high cost of noble metal and scarcity in resources have hindered their wide application as the catalytic electrode for H 2 production. Therefore it is urgent to obtain high performance OER catalysts based on the earth-abundant elements other than the noble metal-based catalysts (IrO 2 and RuO 2 ). [4] The layered double hydroxides (LDHs), [5] also known as hydrotalcite-like clays, is a typical non-noble-metal-based material that composed of layers of divalent and trivalent metal cations coordinated to hydroxide anions with guest anions (typically CO 3 2− ) intercalated between the layers. [6] The highly tunable structures, such as layer numbers, metal species, interlayer spacing, and more importantly, the specific cation arrangements in the hydroxide layers, make LDHs-based materials as promising OER catalysts to replace the state-of-the-art noble metal based catalysts. [7] The NiFe LDHs is nowadays known as one of the most active OER catalysts with a low overpotential and high electrolysis current. [8] However, it is still suffering from poor electronic conductivity that hindering its further improvement and practical application. [9] Recently, tremendous efforts have been devoted to enhance the electrocatalytic activity, such as, exfoliation to fabricate a single layer structure, [10] integrating the LDHs nanosheets to form array on a conductive substrate, [11] loading noble metal on the surface, [12] modification the interlayer anion to change the electronic structure, [13] sulfurization, [14] defect engineering to increase the active site, [15] topotactic reduction to metallic nanosheets, [16] and doping. [17] It is already known that Fe in NiFe LDHs is the key aspect for the high OER catalytic performance, [18] so we can modify the chemical environment of Fe by incorporating other valence variable transition metals (such as Co [17a] and Mn [17b] ) into NiFe LDHs laminate to enhance the catalyst activity. Vanadium is an earth-abundant element and its activity for electrochemical water oxidation has been explored as active sites in NiV LDHs. [19] However, as a typical transition metal with a series of valence states, the electron interaction and synergetic effect between V and other metals in the laminate of LDHs have never been touched. On the other hand, the integrating of nanosheets materials can enhance the intrinsic activity of OER by providing electron transfer pathways from electron-conductive Binary NiFe layer double hydroxide (LDH) serves as a benchmark non-noble metal electrocatalyst for the oxygen evolution reaction, however, it still needs a relatively high overpotential to achieve the threshold current density. Herein the catalyst's electronic structure is tuned by doping vanadium ions into the NiFe LDHs laminate forming ternary NiFeV LDHs to reduce the onset potential, achieving unprecedentedly efficient electrocatalysis for water oxidation. Only 1.42 V (vs reversible hydrogen electrode (RHE), ≈195 mV overpoten...
Single atom catalyst, which contains isolated metal atoms singly dispersed on supports, has great potential for achieving high activity and selectivity in hetero-catalysis and electrocatalysis. However, the activity and stability of single atoms and their interaction with support still remains a mystery. Here we show a stable single atomic ruthenium catalyst anchoring on the surface of cobalt iron layered double hydroxides, which possesses a strong electronic coupling between ruthenium and layered double hydroxides. With 0.45 wt.% ruthenium loading, the catalyst exhibits outstanding activity with overpotential 198 mV at the current density of 10 mA cm −2 and a small Tafel slope of 39 mV dec −1 for oxygen evolution reaction. By using operando X-ray absorption spectroscopy, it is disclosed that the isolated single atom ruthenium was kept under the oxidation states of 4+ even at high overpotential due to synergetic electron coupling, which endow exceptional electrocatalytic activity and stability simultaneously.
sunlight for sustainable energy conversion and storage. [1] For renewable and efficient hydrogen production, electrochemical water splitting employing renewable electrical energy is a promising route due to its inherent advantages, including readily available reactant, stable output, and feasibility of large-scale production. [2] However, the large overpotential (η) of both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) greatly limited their practical applications. Moreover, due to the thermodynamic limitation, the HER reaction usually prefers to be conducted under strong acidic solution while the OER is conducted under basic solution. [3] Extra energy needs to be supplied in order to maintain the pH differences between those two reactions. It is thus imperative to develop highly active electrocatalysts that can utilize under a wide pH range. At present, the state-ofthe-art electrocatalysts for HER are precious metal Pt-based materials and for OER are costly Ir-or Ru-based oxides. [4] But the high cost, scarcity and unsatisfactory durability of above mentioned catalysts further limit the practical utilization of the water splitting technology. [5] Water splitting requires development of cost-effective multifunctional materials that can catalyze both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) efficiently. Currently, the OER relies on the noble-metal catalysts; since with other catalysts, its operation environment is greatly limited in alkaline conditions. Herein, an advanced water oxidation catalyst based on metallic Co 9 S 8 decorated with single-atomic Mo (0.99 wt%) is synthesized (Mo-Co 9 S 8 @C). It exhibits pronounced water oxidization activity in acid, alkali, and neutral media by showing positive onset potentials of 200, 90, and 290 mV, respectively, which manifests the best Co 9 S 8 -based singleatom Mo catalyst till now. Moreover, it also demonstrates excellent HER performance over a wide pH range. Consequently, the catalyst even outperforms noble metal Pt/IrO 2 -based catalysts for overall water splitting (only requiring 1.68 V in acid, and 1.56 V in alkaline). Impressively, it works under a current density of 10 mA cm −2 with no obvious decay during a 24 h (0.5 m H 2 SO 4 ) and 72 h (1.0 m KOH) durability experiment. Density functional theory (DFT) simulations reveal that the synergistic effects of atomically dispersed Mo with Co-containing substrates can efficiently alter the binding energies of adsorbed intermediate species and decrease the overpotentials of the water splitting.
Introducing oxygen vacancies to metal oxide materials would improve their catalytic activity but usually needs reductive reagents (e.g., H2) and high temperatures (e.g., >600 °C), which is unsafe, complex, and time consuming. Herein, a fast (30 s) and facile (operated at ambient conditions) flame‐engraved method is used to introduce abundant oxygen vacancies and well‐defined hexagonal cavities with (110) edges to nickel–iron layered double hydroxides (NiFe‐LDH). Abundant oxygen vacancies, lower coordination numbers, and electron‐rich structures of Ni and Fe sites emerge in the flame‐engraved NiFe‐LDH array electrode, leading to its onset potential as low as 1.40 V (vs reversible hydrogen electrode) for oxygen evolution reaction. This highlights the importance and convenience of flame‐engraving method in preparing metal hydroxides with abundant oxygen vacancies, which can be used as efficient electrochemical catalysts.
Scheme 6. The [3+2] cycloaddition of cyclic nitrone with methyl propiolate.Figure 3. Rationalization of the high selectivity for the formation of 7 b. Angewandte Chemie 8949
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