Codoping-Induced, Rhombus-Shaped Co3O4 Nanosheets as an Active Electrode Material for Oxygen Evolution

Development of vertically aligned trimetallic Mg-Ni-Co oxide grass-like nanostructure for high-performance energy storage applications

Direct growth of nanostructured trimetallic oxide on substrate is considered as one of the promising electrode fabrication for high-performance hybrid supercapacitors. Herein, binder-free one-dimensional grass-like nanostructure was constructed on nickel foam by using electrodeposition approach. The admirable enhancement in rate capability was observed by the substitution of Mg and Ni in cobalt oxide crystallite. The prepared nickel cobalt oxide (NCO) and cobalt oxide (CO) electrode exhibited a rate capability of 57% and 58% (2 to 10 A g^-1) respectively. Interestingly, the rate capability was increased to 87% by the substitution of Mg and Ni simultaneously. The novel vertically aligned trimetallic Mg-Ni-Co oxide (MNCO) grass-like nanostructure electrode exhibited a high specific capacity of 846 C g^-1 at 2 A g^-1_, retained 97.3% specific capacity and showed an outstanding coulombic efficiency of 99% after 10,000 charge-discharge cycles. Moreover, we assembled hybrid supercapacitor (HSC) device for practical applications by using MNCO and activated carbon (AC) as the positive and negative electrode materials, respectively. HSC device exhibited a high specific capacity of 144 C g^-1 at 0.5 A g^-1. The high energy density of 31.5 Wh kg^-1 and the power density of 7.99 kW kg^-1 were achieved. All these interesting and attractive results demonstrate the significance of the vertically aligned electrode material towards practical applications.

Metal-Organic Framework-Derived Co3ZnC/Co Embedded in Nitrogen-Doped Carbon Nanotube-Grafted Carbon Polyhedra as a High-Performance Electrocatalyst for Water Splitting

The development of efficient, low-cost, and stable electrocatalysts for overall water splitting is of great significance for energy conversion. Transition-metal carbides (TMCs) with high catalytic activity and low cost have attracted great interests. Nevertheless, utilizing an efficient catalyst for overall water splitting is still a challenging issue for TMCs. Herein, we report the synthesis of a high-performance electrocatalyst comprising Co₃ZnC and Co nanoparticles embedded in a nitrogen-doped carbon nanotube-grafted carbon polyhedral (Co₃ZnC/Co-NCCP) by the pyrolysis of bimetallic zeolitic imidazolate frameworks in a reductive atmosphere of Ar/H₂. The Co₃ZnC/Co-NCCP exhibits remarkable electrochemical activity in catalyzing both the oxygen evolution reaction and hydrogen evolution reaction, in terms of low overpotential and excellent stability. Furthermore, the Co₃ZnC/Co-NCCP catalyst leads to a highly performed overall water splitting in the 1 M KOH electrolyte, delivering a current density of 10 mA cm^-2 at a low applied external potential of 1.65 V and shows good stability without obvious deactivation after 10 h operation. The present strategy opens a new avenue to the design of efficient electrocatalysts in electrochemical applications.

In-situ structure reconstitution of NiCo2Px for enhanced electrochemical water oxidation

Gaining insight into the structure evolution of transition-metal phosphides during anodic oxidation is significant to understand their oxygen evolution reaction (OER) mechanism, and then design high-efficiency transition metal-based catalysts. Herein, NiCo₂P_x nanowires (NWs) vertically grown on Ni foam were adopted as the target to explore the in-situ morphology and chemical component reconstitution during the anodic oxidation. The major factors causing the transformation from NiCo₂P_x into the hierarchical NiCo₂P_x@CoNi(OOH)_x NWs are two competing reactions: the dissolution of NiCo₂P_x NWs and the oxidative re-deposition of dissolved Co²⁺ and Ni²⁺ ions, which is based primarily on the anodic bias applied on NiCo₂P_x NWs. The well balance of above competing reactions, and local pH on the surface of NiCo₂P_x NW modulated by the anodic oxidation can serve to control the anodic electrodeposition and rearrangement of metal ions on the surface of NiCo₂P_x NWs, and the immediate conversion into CoNi(OOH)_x. Consequently, the regular hexagonal CoNi(OOH)_x nanosheets grew around NiCo₂P_x NWs. Benefiting from the active catalytic sites on the surface and the sufficient conductivity, the resultant NiCo₂P_x@CoNi(OOH)_x arrays also display good OER activity, in terms of the fast kinetics process, the high energy conversion efficiency, especially the excellent durability. The strategy of in-situ structure reconstitution by electrochemical reaction described here offers a reliable and valid way to construct the highly active systems for various electrocatalytic applications.

Hybrid Reduced Graphene Oxide Nanosheet Supported Mn-Ni-Co Ternary Oxides for Aqueous Asymmetric Supercapacitors

Hybrid reduced graphene oxide (RGO) nanosheet supported Mn-Ni-Co ternary oxides (MNCO) are prepared through a facile coprecipitation reaction with a subsequent calcination process as electrodes for supercapacitors. Electrochemical measurements prove that RGO can significantly improve the supercapacitive behaviors, compared with the pure MNCO electrode. A high specific capacity of 646.1 C g^-1 at 1 A g^-1 can be achieved and about 89.6% of the capacity can be remained at 30 A g^-1 relative to that of the low-current capacity, indicating attractive rate capability of the RGO-MNCO electrode. Moreover, an asymmetric supercapacitor (ASC) device is fabricated with nitrogen-enriched RGO as the negative electrode and the synthesized RGO-MNCO as the positive electrode. Electrochemical performances investigated at different potential range reveal that the ASC device presents excellent capacitive behavior and reversibility. A maximum energy density of 35.6 Wh kg^-1 at power density of 699.9 W kg^-1 can be delivered. Furthermore, stable cycle capability with 100% Coulombic efficiency and 77.2% the capacitance retention is also achieved after 10000 cycles. The achieved outstanding electrochemical properties indicate that the obtained RGO-MNCO electrode materials are fairly ideal for progressive supercapacitors.

One-Step Electrodeposition of Nanocrystalline ZnxCo3-xO4 Films with High Activity and Stability for Electrocatalytic Oxygen Evolution

The development of highly active, environmentally friendly, and long-term stable oxygen evolving catalysts at low costs is critical for efficient and scalable H₂ production from water splitting. Here, we report a new and facile one-step electrodeposition of nanocrystalline spinel-type Zn_xCo_3-xO₄ films from an alkaline Zn²⁺-Co²⁺-tartrate solution. The electrodeposited Zn_xCo_3-xO₄ electrode could be directly used as the anode for the water electrolysis without any post treatment. The Zn_xCo_3-xO₄ film shows a low and stable overpotential of ∼0.33 V at 10 mA cm^-2 (and ∼0.35 V at 20 mA cm^-2) for over 10 h and a Tafel slope of ∼39 mV dec^-1 toward the oxygen evolution reaction (OER) in 1 M NaOH, comparable to the best performance of the nonprecious OER catalysts reported for alkaline media. The enhanced OER activity of Zn_xCo_3-xO₄ compared to Co₃O₄ could be attributed to the surface structural modification and higher density of the accessible active Co³⁺ sites induced by the incorporation of Zn²⁺. The electrodeposition method in this paper could also be used to synthesize other binary and ternary metal oxide based catalytic electrodes for reactions such as the OER and oxygen reduction reaction (ORR).

Facile Synthesis of Unique Hexagonal Nanoplates of Zn/Co Hydroxy Sulfate for Efficient Electrocatalytic Oxygen Evolution Reaction

Cost-effective, highly active water oxidation catalysts are increasingly being demanded in the field of energy conversion and storage. Herein, a simple modified hydrothermally (MHT) synthesized zinc and cobalt based hydroxyl double salt, that is, Zn_4-xCo_xSO₄(OH)₆·0.5H₂O (ZCS), has been exfoliated for the first time as an efficient electrocatalyst for oxygen evolution reaction (OER) in alkaline medium. Morphology investigation suggests the evolution of unique hexagonal nanoplates of ZCS material. As OER catalyst, it requires only 370 and 450 mV overpotential to achieve 10 and 100 mA cm^-2 current density, respectively. More importantly, performance at the overpotential over 400 mV and durability of the designed material have been found to be superior to those of commercial RuO₂ catalyst. In the designed ZCS material trace amounts of cobalt species lead to higher mass activity of 146 A g^-1, compared to that of the RuO₂ catalyst (83 A g^-1) at the same overpotential of 370 mV. The outstanding activity and stability of the cost-effective material emerges from the promotional effect of Zn ions, which are present as the principal constituent in the electrocatalyst, and they also protect the cobalt ions in the matrix during its long-term electrochemical test. It is important to note that an appropriate ratio of zinc and cobalt ions synergistically helps to create an economically viable and environmentally suitable electrocatalyst in comparison to other related transition metal based materials.

Three-dimensional ordered mesoporous Co3O4 enhanced by Pd for oxygen evolution reaction

Considerable efforts have been devoted recently to design and fabrication of high performance and low cost electrocatalysts for oxygen evolution reaction (OER). However, catalytic activity of current electrocatalysts is usually restricted by high onset potential and limited active sites. Herein, we fabricated three-dimensional (3D) highly ordered mesoporous Pd-Co₃O₄ composite materials as excellent electrocatalysts for OER in alkaline solution with high activity and stability. Three-dimensional highly ordered mesoporous Co₃O₄ material was firstly synthesized using mesoporous silica KIT-6 as hard template. Then, Pd-Co₃O₄ nanomaterials were prepared by a simple reduction method. The as-prepared 3D mesoporous Pd-Co₃O₄ catalysts have ordered mesoporous structure with a high surface area of 81.0 m² g^-1. Three-dimensional highly ordered mesoporous structure can facilitate diffusion and penetration of electrolyte and oxygen. Moreover, the catalysts can also keep catalyst particles in a well dispersed condition with more catalytic active sites. Electrochemical measurements reveal that the 3D mesoporous Pd-Co₃O₄ catalysts exhibit superior performance in alkaline solution with low onset potential (0.415 V vs. SCE) and excellent long-duration cycling stability.

Transition-Metal (Co, Ni, and Fe)-Based Electrocatalysts for the Water Oxidation Reaction

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.

A noble and single source precursor for the synthesis of metal-rich sulphides embedded in an N-doped carbon framework for highly active OER electrocatalysts

Metal-rich sulphide (Co₉S₈ and Ni₃S₂) embedded in N-doped carbon (NC) frameworks were synthesized from novel Tris(ethylenediamine) Metal (ii) Sulfate complex whereas counter sulphate (SO₄²⁻) ion is the source of S. Both the hybrids show superior OER activity compared to commercial RuO₂.

Comprehensive Study of an Earth-Abundant Bifunctional 3D Electrode for Efficient Water Electrolysis in Alkaline Medium

We report efficient electrolysis of both water-splitting half reactions in the same medium by a bifunctional 3D electrode comprising Co3O4 nanospheres nucleated on the surface of nitrogen-doped carbon nanotubes (NCNTs) that in turn are grown on conductive carbon paper (CP). The resulting electrode exhibits high stability and large electrochemical activity for both oxygen and hydrogen evolution reactions (OER and HER). We obtain a current density of 10 mA/cm(2) in 0.1 M KOH solution at overpotentials of only 0.47 and 0.38 V for OER and HER, respectively. Additionally, the experimental observations are understood and supported by analyzing the Co3O4:NCNT and NCNT:CP interfaces by ab initio calculations. Both the experimental and the theoretical studies indicate that firm and well-established interfaces along the electrode play a crucial role on the stability and electrochemical activity for both OER and HER.