Reduced Mesoporous Co3O4 Nanowires as Efficient Water Oxidation Electrocatalysts and Supercapacitor Electrodes
TL;DR: In this paper, a facile solution reduction method is demonstrated for mesoporous Co3O4 nanowires treated with NaBH4, which leads to efficient surface reduction in solution at room temperature, which allows for retention of the nanowire morphology and 1D charge transport behavior, while at the same time substantially increasing the oxygen vacancies on the surface.
Abstract: While electrochemical water splitting is one of the most promising methods to store light/electrical energy in chemical bonds, a key challenge remains in the realization of an efficient oxygen evolution reaction catalyst with large surface area, good electrical conductivity, high catalytic properties, and low fabrication cost. Here, a facile solution reduction method is demonstrated for mesoporous Co3O4 nanowires treated with NaBH4. The high-surface-area mesopore feature leads to efficient surface reduction in solution at room temperature, which allows for retention of the nanowire morphology and 1D charge transport behavior, while at the same time substantially increasing the oxygen vacancies on the nanowire surface. Compared to pristine Co3O4 nanowires, the reduced Co3O4 nanowires exhibit a much larger current of 13.1 mA cm-2 at 1.65 V vs reversible hydrogen electrode (RHE) and a much lower onset potential of 1.52 V vs RHE. Electrochemical supercapacitors based on the reduced Co3O4 nanowires also show a much improved capacitance of 978 F g-1 and reduced charge transfer resistance. Density-functional theory calculations reveal that the existence of oxygen vacancies leads to the formation of new gap states in which the electrons previously associated with the Co-O bonds tend to be delocalized, resulting in the much higher electrical conductivity and electrocatalytic activity.
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TL;DR: An efficient Co3 O4 -based OER electrocatalyst is designed by a plasma-engraving strategy, which not only produced higher surface area, but also generated oxygen vacancies on Co 3 O4 surface with more Co(2+) formed to improve the electronic conductivity and create more active defects for OER.
Abstract: Co3O4, which is of mixed valences Co2+ and Co3+, has been extensively investigated as an efficient electrocatalyst for the oxygen evolution reaction (OER). The proper control of Co2+/Co3+ ratio in Co3O4 could lead to modifications on its electronic and thus catalytic properties. Herein, we designed an efficient Co3O4-based OER electrocatalyst by a plasma-engraving strategy, which not only produced higher surface area, but also generated oxygen vacancies on Co3O4 surface with more Co2+ formed. The increased surface area ensures the Co3O4 has more sites for OER, and generated oxygen vacancies on Co3O4 surface improve the electronic conductivity and create more active defects for OER. Compared to pristine Co3O4, the engraved Co3O4 exhibits a much higher current density and a lower onset potential. The specific activity of the plasma-engraved Co3O4 nanosheets (0.055 mA cm−2BET at 1.6 V) is 10 times higher than that of pristine Co3O4, which is contributed by the surface oxygen vacancies.
1,641 citations
TL;DR: Recent progress on the design, synthesis, and application of OER electrocatalysts based on transition-metal elements, including Co, Ni, and Fe, is summarized, and some invigorating perspectives on the future developments are provided.
Abstract: Increasing energy demands and environment awareness have promoted extensive research on the development of alternative energy conversion and storage technologies with high efficiency and environmental friendliness. Among them, water splitting is very appealing, and is receiving more and more attention. The critical challenge of this renewable-energy technology is to expedite the oxygen evolution reaction (OER) because of its slow kinetics and large overpotential. Therefore, developing efficient electrocatalysts with high catalytic activities is of great importance for high-performance water splitting. In the past few years, much effort has been devoted to the development of alternative OER electrocatalysts based on transition-metal elements that are low-cost, highly efficient, and have excellent stability. Here, recent progress on the design, synthesis, and application of OER electrocatalysts based on transition-metal elements, including Co, Ni, and Fe, is summarized, and some invigorating perspectives on the future developments are provided.
1,270 citations
TL;DR: The recent development of this concept is reviewed here and a novel principle for the design of oxygen electrocatalysts is proposed and an overview of the defects in carbon-based, metal-free electrocatalysis for ORR and various defects in metal oxides/selenides for OER is provided.
Abstract: Oxygen electrocatalysis, including the oxygen-reduction reaction (ORR) and oxygen-evolution reaction (OER), is a critical process for metal-air batteries Therefore, the development of electrocatalysts for the OER and the ORR is of essential importance Indeed, various advanced electrocatalysts have been designed for the ORR or the OER; however, the origin of the advanced activity of oxygen electrocatalysts is still somewhat controversial The enhanced activity is usually attributed to the high surface areas, the unique facet structures, the enhanced conductivities, or even to unclear synergistic effects, but the importance of the defects, especially the intrinsic defects, is often neglected More recently, the important role of defects in oxygen electrocatalysis has been demonstrated by several groups To make the defect effect clearer, the recent development of this concept is reviewed here and a novel principle for the design of oxygen electrocatalysts is proposed An overview of the defects in carbon-based, metal-free electrocatalysts for ORR and various defects in metal oxides/selenides for OER is also provided The types of defects and controllable strategies to generate defects in electrocatalysts are presented, along with techniques to identify the defects The defect-activity relationship is also explored by theoretical methods
1,222 citations
TL;DR: In this paper, a review article summarizes the very recent efforts in the field of OER electrocatalysis along with the faced challenges and solutions to these challenges also outline with appropriate examples of scientific literatures.
1,121 citations
TL;DR: A facile solution reduction method using NaBH4 as a reductant is developed to prepare iron-cobalt oxide nanosheets (Fex Coy -ONSs) with a large specific surface area, ultrathin thickness, and, importantly, abundant oxygen vacancies that could improve electronic conductivity and facilitate adsorption of H2 O onto nearby Co3+ sites.
Abstract: Electrochemical water splitting is a promising method for storing light/electrical energy in the form of H2 fuel; however, it is limited by the sluggish anodic oxygen evolution reaction (OER). To improve the accessibility of H2 production, it is necessary to develop an efficient OER catalyst with large surface area, abundant active sites, and good stability, through a low-cost fabrication route. Herein, a facile solution reduction method using NaBH4 as a reductant is developed to prepare iron-cobalt oxide nanosheets (FexCoy-ONSs) with a large specific surface area (up to 261.1 m2 g−1), ultrathin thickness (1.2 nm), and, importantly, abundant oxygen vacancies. The mass activity of Fe1Co1-ONS measured at an overpotential of 350 mV reaches up to 54.9 A g−1, while its Tafel slope is 36.8 mV dec−1; both of which are superior to those of commercial RuO2, crystalline Fe1Co1-ONP, and most reported OER catalysts. The excellent OER catalytic activity of Fe1Co1-ONS can be attributed to its specific structure, e.g., ultrathin nanosheets that could facilitate mass diffusion/transport of OH− ions and provide more active sites for OER catalysis, and oxygen vacancies that could improve electronic conductivity and facilitate adsorption of H2O onto nearby Co3+ sites.
1,058 citations
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TL;DR: A simple derivation of a simple GGA is presented, in which all parameters (other than those in LSD) are fundamental constants, and only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked.
Abstract: Generalized gradient approximations (GGA’s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. [S0031-9007(96)01479-2] PACS numbers: 71.15.Mb, 71.45.Gm Kohn-Sham density functional theory [1,2] is widely used for self-consistent-field electronic structure calculations of the ground-state properties of atoms, molecules, and solids. In this theory, only the exchange-correlation energy EXC › EX 1 EC as a functional of the electron spin densities n"srd and n#srd must be approximated. The most popular functionals have a form appropriate for slowly varying densities: the local spin density (LSD) approximation Z d 3 rn e unif
146,533 citations
IBM1
TL;DR: An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way and can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function.
Abstract: An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way. The method allows high-quality first-principles molecular-dynamics calculations to be performed using the original fictitious Lagrangian approach of Car and Parrinello. Like the LAPW method it can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function. The augmentation procedure is generalized in that partial-wave expansions are not determined by the value and the derivative of the envelope function at some muffin-tin radius, but rather by the overlap with localized projector functions. The pseudopotential approach based on generalized separable pseudopotentials can be regained by a simple approximation.
61,450 citations
TL;DR: The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition, and a detailed analysis of the local structural properties and their changes induced by an annealing process is reported.
Abstract: We present ab initio quantum-mechanical molecular-dynamics simulations of the liquid-metal--amorphous-semiconductor transition in Ge. Our simulations are based on (a) finite-temperature density-functional theory of the one-electron states, (b) exact energy minimization and hence calculation of the exact Hellmann-Feynman forces after each molecular-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nos\'e dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows us to perform simulations over more than 30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liquid and amorphous Ge in very good agreement with experiment. The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition. We report a detailed analysis of the local structural properties and their changes induced by an annealing process. The geometrical, bonding, and spectral properties of defects in the disordered tetrahedral network are investigated and compared with experiment.
16,744 citations
TL;DR: It is shown that disorder-engineered TiO2 nanocrystals exhibit substantial solar-driven photocatalytic activities, including the photo-oxidation of organic molecules in water and the production of hydrogen with the use of a sacrificial reagent.
Abstract: When used as a photocatalyst, titanium dioxide (TiO(2)) absorbs only ultraviolet light, and several approaches, including the use of dopants such as nitrogen, have been taken to narrow the band gap of TiO(2). We demonstrated a conceptually different approach to enhancing solar absorption by introducing disorder in the surface layers of nanophase TiO(2) through hydrogenation. We showed that disorder-engineered TiO(2) nanocrystals exhibit substantial solar-driven photocatalytic activities, including the photo-oxidation of organic molecules in water and the production of hydrogen with the use of a sacrificial reagent.
5,217 citations
TL;DR: The Co₃O₄/N-doped graphene hybrid exhibits similar catalytic activity but superior stability to Pt in alkaline solutions, making it a high-performance non-precious metal-based bi-catalyst for both ORR and OER.
Abstract: Catalysts for oxygen reduction and evolution reactions are at the heart of key renewable-energy technologies including fuel cells and water splitting. Despite tremendous efforts, developing oxygen electrode catalysts with high activity at low cost remains a great challenge. Here, we report a hybrid material consisting of Co₃O₄ nanocrystals grown on reduced graphene oxide as a high-performance bi-functional catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Although Co₃O₄ or graphene oxide alone has little catalytic activity, their hybrid exhibits an unexpected, surprisingly high ORR activity that is further enhanced by nitrogen doping of graphene. The Co₃O₄/N-doped graphene hybrid exhibits similar catalytic activity but superior stability to Pt in alkaline solutions. The same hybrid is also highly active for OER, making it a high-performance non-precious metal-based bi-catalyst for both ORR and OER. The unusual catalytic activity arises from synergetic chemical coupling effects between Co₃O₄ and graphene.
4,898 citations