Advanced Asymmetric Supercapacitors Based on Ni(OH)2/Graphene and Porous Graphene Electrodes with High Energy Density

Hierarchical flowerlike nickel hydroxide decorated on graphene sheets has been prepared by a facile and cost‐effective microwave‐assisted method. In order to achieve high energy and power densities, a high‐voltage asymmetric supercapacitor is successfully fabricated using Ni(OH)₂/graphene and porous graphene as the positive and negative electrodes, respectively. Because of their unique structure, both of these materials exhibit excellent electrochemical performances. The optimized asymmetric supercapacitor could be cycled reversibly in the high‐voltage region of 0–1.6 V and displays intriguing performances with a maximum specific capacitance of 218.4 F g^?1 and high energy density of 77.8 Wh kg^?1. Furthermore, the Ni(OH)₂/graphene//porous graphene supercapacitor device exhibits an excellent long cycle life along with 94.3% specific capacitance retained after 3000 cycles. These fascinating performances can be attributed to the high capacitance and the positive synergistic effects of the two electrodes. The impressive results presented here may pave the way for promising applications in high energy density storage systems.     

Bifunctional Porous NiFe/NiCo2O4/Ni Foam Electrodes with Triple Hierarchy and Double Synergies for Efficient Whole Cell Water Splitting

A 3D hierarchical porous catalyst architecture based on earth abundant metals Ni, Fe, and Co has been fabricated through a facile hydrothermal and electrodeposition method for efficient oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The electrode is comprised of three levels of porous structures including the bottom supermacroporous Ni foam (≈500 μm) substrate, the intermediate layer of vertically aligned macroporous NiCo₂O₄ nanoflakes (≈500 nm), and the topmost NiFe(oxy)hydroxide mesoporous nanosheets (≈5 nm). This hierarchical architecture is binder‐free and beneficial for exposing catalytic active sites, enhancing mass transport and accelerating dissipation of gases generated during water electrolysis. Serving as an anode catalyst, the designed hierarchical electrode displays excellent OER catalytic activity with an overpotential of 340 mV to achieve a high current density of 1200 mA cm^?2. Serving as a cathode catalyst, the catalyst exhibits excellent performance toward HER with a moderate overpotential of 105 mV to deliver a current density of 10 mA cm^?2. Serving as both anode and cathode catalysts in a two‐electrode water electrolysis system, the designed electrode only requires a potential of 1.67 V to deliver a current density of 10 mA cm^?2 and exhibits excellent durability in prolonged bulk alkaline water electrolysis.     

p‐Si/SnO2/Fe2O3 Core/Shell/Shell Nanowire Photocathodes for Neutral pH Water Splitting

Silicon is one of the promising materials for solar water splitting and hydrogen production; however, it suffers from two key factors, including the large external potential required to drive water splitting reactions at its surface and its instability in the electrolyte. In this study, a successful fabrication of novel p‐Si/n‐SnO₂/n‐Fe₂O₃ core/shell/shell nanowire (css‐NW) arrays, consisting of vertical Si NW cores coated with a thin SnO₂ layer and a dense Fe₂O₃ nanocrystals (NCs) shell, and their application for significantly enhanced solar water reduction in a neutral medium is reported. The p‐Si/n‐SnO₂/n‐Fe₂O₃ css‐NW structure is characterized in detail using scanning, transmission, and scanning transmission electron microscopes. The p‐Si/n‐SnO₂/n‐Fe₂O₃ css‐NWs show considerably improved photocathodic performances, including higher photocurrent and lower photocathodic turn‐on potential, compared to the bare p‐Si NWs or p‐Si/n‐SnO₂ core/shell NWs (cs‐NWs), due to increased optical absorption, enhanced charge separation, and improved gas evolution. As a result, photoactivity at 0 V versus reversible hydrogen electrode and a low onset potential in the neutral solution are achieved. Moreover, p‐Si/n‐SnO₂/n‐Fe₂O₃ css‐NWs exhibit long‐term photoelectrochemical stability due to the Fe₂O₃ NCs shell well protection. These results reveal promising css‐NW photoelectrodes from cost‐effective materials by facile fabrication with simultaneously improved photocathodic performance and stability.     

Oriented Multiwalled Organic–Co(OH)2 Nanotubes for Energy Storage

In energy storage materials, large surface areas and oriented structures are key architecture design features for improving performance through enhanced electrolyte access and efficient electron conduction pathways. Layered hydroxides provide a tunable materials platform with opportunities for achieving such nanostructures via bottom‐up syntheses. These nanostructures, however, can degrade in the presence of the alkaline electrolytes required for their redox‐based energy storage. A layered Co(OH)₂–organic hybrid material that forms a hierarchical structure consisting of micrometer‐long, 30 nm diameter tubes with concentric curved layers of Co(OH)₂ and 1‐pyrenebutyric acid is reported. The nanotubular structure offers high surface area as well as macroscopic orientation perpendicular to the substrate for efficient electron transfer. Using a comparison with flat films of the same composition, it is demonstrated that the superior performance of the nanotubular films is the result of a large accessible surface area for redox activity. It is found that the organic molecules used to template nanotubular growth also impart stability to the hybrid when present in the alkaline environments necessary for redox function.     

Efficient and Stable Bifunctional Electrocatalysts Ni/NixMy (M = P,S) for Overall Water Splitting

Development of easy‐to‐make, highly active, and stable bifunctional electrocatalysts for water splitting is important for future renewable energy systems. Three‐dimension (3D) porous Ni/Ni₈P₃ and Ni/Ni₉S₈ electrodes are prepared by sequential treatment of commercial Ni‐foam with acid activation, followed by phosphorization or sulfurization. The resultant materials can act as self‐supported bifunctional electrocatalytic electrodes for direct water splitting with excellent activity toward oxygen evolution reaction and hydrogen evolution reaction in alkaline media. Stable performance can be maintained for at least 24 h, illustrating their versatile and practical nature for clean energy generation. Furthermore, an advanced water electrolyzer through exploiting Ni/Ni₈P₃ as both anode and cathode is fabricated, which requires a cell voltage of 1.61 V to deliver a 10 mA cm^?2 water splitting current density in 1.0 m KOH solution. This performance is significantly better than that of the noble metal benchmark—integrated Ni/IrO₂ and Ni/Pt–C electrodes. Therefore, these bifunctional electrodes have significant potential for realistic large‐scale production of hydrogen as a replacement clean fuel to polluting and limited fossil‐fuels.     

Rock Salt Ni/Co Oxides with Unusual Nanoscale‐Stabilized Composition as Water Splitting Electrocatalysts

The influence of nanoscale on the formation of metastable phases is an important aspect of nanostructuring that can lead to the discovery of unusual material compositions. Here, the synthesis, structural characterization, and electrochemical performance of Ni/Co mixed oxide nanocrystals in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is reported and the influence of nanoscaling on their composition and solubility range is investigated. Using a solvothermal synthesis in tert ‐butanol ultrasmall crystalline and highly dispersible Ni _x Co_{1? x} O nanoparticles with rock salt type structure are obtained. The mixed oxides feature non‐equilibrium phases with unusual miscibility in the whole composition range, which is attributed to a stabilizing effect of the nanoscale combined with kinetic control of particle formation. Substitutional incorporation of Co and Ni atoms into the rock salt lattice has a remarkable effect on the formal potentials of NiO oxidation that shift continuously to lower values with increasing Co content. This can be related to a monotonic reduction of the work function of (001) and (111)‐oriented surfaces with an increase in Co content, as obtained from density functional theory (DFT+U) calculations. Furthermore, the electrocatalytic performance of the Ni _x Co_{1? x} O nanoparticles in water splitting changes significantly. OER activity continuously increases with increasing Ni contents, while HER activity shows an opposite trend, increasing for higher Co contents. The high electrocatalytic activity and tunable performance of the nonequilibrium Ni _x Co_{1? x} O nanoparticles in HER and OER demonstrate great potential in the design of electrocatalysts for overall water splitting.     

Novel Core–Shell FeOF/Ni(OH)2 Hierarchical Nanostructure for All‐Solid‐State Flexible Supercapacitors with Enhanced Performance

Well‐controlled core–shell hierarchical nanostructures based on oxyfluoride and hydroxide are for the first time rationally designed and synthesized via a simple solvothermal and chemical precipitation route, in which FeOF nanorod acts as core and porous Ni(OH)₂ nanosheets as shell. When evaluated as electrodes for supercapacitors, a high specific capacitance of 1452 F g^?1 can be obtained at a current density of 1 A g^?1. Even as the current density increases to 10 A g^?1, the core–shell hybrid still reserves a noticeable capacitance of 1060 F g^?1, showing an excellent rate capacity. Furthermore, all‐solid‐state flexible asymmetric supercapacitor based on the FeOF/Ni(OH)₂ hybrid as a positive electrode and activated carbon as a negative electrode shows high power density, high energy density, and long cycling lifespan. The excellent electrochemical performance of the FeOF/Ni(OH)₂ core–shell hybrid is ascribed to the unique microstructure and synergistic effects. FeOF nanorod from FeF₃ by partial substitution of fluorine with oxygen behaves as a low intrinsic resistance, thus facilitating charge transfer processes. While the hierarchical Ni(OH)₂ nanosheets with large surface area provide enough active sites for redox chemical reactions, leading to greatly enhanced electrochemical activity. The well‐controllable oxyfluoride/hydroxide hybrid is inspiring, opening up a new way to design new electrodes for next‐generation all‐solid‐state supercapacitors.     

Heterogeneous Bimetallic Phosphide Ni2P-Fe2P as an Efficient Bifunctional Catalyst for Water/Seawater Splitting

Developing high-performance and cost-effective bifunctional electrocatalysts for large-scale water electrolysis is desirable but remains a significant challenge. Most existing nano- and micro-structured electrocatalysts require complex synthetic procedures, making scale-up highly challenging. Here, a heterogeneous Ni₂P-Fe₂P microsheet is synthesized by directly soaking Ni foam in hydrochloric acid and an iron nitrate solution, followed by phosphidation. Benefiting from high intrinsic activity, abundant active sites, and a superior transfer coefficient, this self-supported Ni₂P-Fe₂P electrocatalyst shows superb catalytic activity toward overall water splitting, requiring low voltages of 1.682 and 1.865 V to attain current densities of 100 and 500 mA cm⁻² in 1 m KOH, respectively. Such catalytic performance is superior to the benchmark IrO₂ || Pt/C pair and also places this electrocatalyst among the best bifunctional catalysts reported thus far. Furthermore, its enhanced corrosion resistance and hydrophilic surface make it suitable for seawater splitting. It is able to achieve current densities of 100 and 500 mA cm⁻² in 1 m KOH seawater at voltages of 1.811 and 2.004 V, respectively, which, together with its robust durability, demonstrates its great potential for realistic seawater electrolysis. This work presents a general and economic approach toward the fabrication of heterogeneous metallic phosphide catalysts for water/seawater electrocatalysis.     

Macroporous Array Induced Multiscale Modulation at the Surface/Interface of Co(OH)2/NiMo Self-Supporting Electrode for Effective Overall Water Splitting

Outstanding electrocatalysts for high-efficiency water splitting demand not only the high intrinsic activity determined by the electronic structure but also a favorable mass transfer (electrolyte diffusion and bubble desorption) and strong structural stability. Here, a 3D core–shell electrocatalyst consisting of Co(OH)₂ cavity array-encapsulated NiMo alloy on the flexible carbon cloth substrate (Co(OH)₂/NiMo CA@CC) is proposed. Density functional theory reveals that coupling NiMo with Co(OH)₂ can better optimize the water adsorption/dissociation and hydrogen adsorption energies in hydrogen evolution reaction, and also accelerate the kinetics of oxygen evolution reaction. Based on this, the open porous structure of the outer Co(OH)₂ cavity array further promotes the diffusion of the electrolyte into the heterogeneous interface between NiMo and Co(OH)₂, significantly shortening the electron transfer pathways and exposing multiple active sites. In addition, the macroporous array structure accelerates the bubble evolution and desorption process, thus ensuring a rapid mass transfer. When served as bifunctional electrocatalysts toward alkaline overall water splitting, Co(OH)₂/NiMo CA@CC delivers a current density of 10 mA cm⁻² at a low cell voltage of 1.52 V. Results support the multiscale optimization of the surface/interface engineering induced by the macroporous array.     

A Novel Photocathode Material for Sunlight‐Driven Overall Water Splitting: Solid Solution of ZnSe and Cu(In,Ga)Se2

Thin films of a solid solution of ZnSe and CuIn_0.7Ga_0.3Se₂ ((ZnSe) _x (CIGS) _1–x ) are prepared by co‐evaporation. Structural characterization reveals that the ZnSe and CIGS form a solid solution with no phase separation. (ZnSe)_0.85(CIGS)_0.15‐based photocathodes modified with Pt, Mo, Ti, and CdS exhibit a photocurrent of 7.1 mA cm^?2 at 0 V_RHE, and a relatively high onset potential of 0.89 V_RHE under simulated sunlight. A two‐electrode cell containing a (ZnSe)_0.85(CIGS)_0.15 photocathode and a BiVO₄‐based photoanode has an initial solar‐to‐hydrogen conversion efficiency of 0.91%, which is one of the highest values reported for a photoanode–photocathode combination. Thus, (ZnSe)_0.85(CIGS)_0.15 is a promising photocathode material for efficient photoelectrochemical water splitting.     

A High Energy Density Asymmetric Supercapacitor from Nano‐architectured Ni(OH)2/Carbon Nanotube Electrodes

The demand for advanced energy storage devices such as supercapacitors and lithium‐ion batteries has been increasing to meet the application requirements of hybrid vehicles and renewable energy systems. A major limitation of state‐of‐art supercapacitors lies in their relatively low energy density compared with lithium batteries although they have superior power density and cycle life. Here, we report an additive‐free, nano‐architectured nickel hydroxide/carbon nanotube (Ni(OH)₂/CNT) electrode for high energy density supercapacitors prepared by a facile two‐step fabrication method. This Ni(OH)₂/CNT electrode consists of a thick layer of conformable Ni(OH)₂ nano‐flakes on CNT bundles directly grown on Ni foams (NFs) with a very high areal mass loading of 4.85 mg cm^?2 for Ni(OH)₂. Our Ni(OH)₂/CNT/NF electrode demonstrates the highest specific capacitance of 3300 F g^?1 and highest areal capacitance of 16 F cm^?2, to the best of our knowledge. An asymmetric supercapacitor using the Ni(OH)₂/CNT/NF electrode as the anode assembled with an activated carbon (AC) cathode can achieve a high cell voltage of 1.8 V and an energy density up to 50.6 Wh/kg, over 10 times higher than that of traditional electrochemical double‐layer capacitors (EDLCs).     

Mechanistic Insights on Ternary Ni2−xCoxP for Hydrogen Evolution and Their Hybrids with Graphene as Highly Efficient and Robust Catalysts for Overall Water Splitting

Searching the high‐efficient, stable, and earth‐abundant electrocatalysts to replace the precious noble metals holds the promise for practical utilizations of hydrogen and oxygen evolution reactions (HER and OER). Here, a series of highly active and robust Co‐doped nickel phosphides (Ni_2?xCo_xP) catalysts and their hybrids with reduced graphene oxide (rGO) are developed as bifunctional catalysts for both HER and OER. The Co‐doping in Ni₂P and their hybridization with rGO effectively regulate the catalytic activity of the surface active sites, accelerate the charge transfer, and boost their superior catalytic activity. Density functional theory calculations show that the Co‐doped catalysts deliver the moderate trapping of atomic hydrogen and facile desorption of the generated H₂ due to the H‐poisoned surface active sites of Ni_2?xCo_xP under the real catalytic process. Electrochemical measurements reveal the high HER efficiency and durability of the NiCoP/rGO hybrids in electrolytes with pH 0–14. Coupled with the remarkable and robust OER activity of the NiCoP/rGO hybrids, the practical utilization of the NiCoP/rGO‖NiCoP/rGO for overall water splitting yields a catalytic current density of 10 mA cm^?2 at 1.59 V over 75 h without an obvious degradation and Faradic efficiency of ≈100% in a two‐electrode configuration and 1.0 m KOH.     

General Formation of Monodisperse IrM (M = Ni,Co, Fe) Bimetallic Nanoclusters as Bifunctional Electrocatalysts for Acidic Overall Water Splitting

The development of bifunctional electrocatalysts for overall water splitting in acidic media is vital for polymer electrolyte membrane (PEM) electrolyzers, but still full of obstacles. Here, highly efficient acidic overall water splitting is realized by utilizing ultrasmall, monodispersed Iridium (Ir)‐based nanoclusters (NCs) as the candidate, via a surfactant‐free, wet‐chemical, and large‐scalable strategy. Benefiting from the high specific surface area, clean surface, and strong binding between NCs and supports, the IrM NCs exhibit attractive activities and durability for both oxygen evolution reaction and hydrogen evolution reaction in acidic electrolytes, with IrNi NCs showing the best performance. More significantly, in the overall water splitting, IrNi NCs reach 10 mA cm^?2 at a cell voltage of only 1.58 V in 0.5 m H₂SO₄ electrolyte, holding promises for potential implementation of PEM water electrolysis. This work opens a new avenue toward designing bifunctional “acidic stable” catalysts for efficient overall water splitting.