Atypical performance of CoO-accelerated interface tweaking in hierarchical cobalt phosphide/oxide@P-doped rGO heterostructures for hybrid supercapacitors

Designing two-dimensional (2D) heterostructures based on suitable energy materials is a promising strategy to achieve high-performance supercapacitors with hybridized transition metal and carbonaceous-based electrodes. The influence of each component and its content on the capacitor performance necessitates deeper insights. In this study, a 2D/2D heterostructure made of hierarchical pseudocapacitive cobalt phosphide/oxide and P-doped reduced graphene oxide (PrGO) nanosheets (CoP/CoO@PrGO) was fabricated using porous zeolitic-imidazolate framework precursor. The decoration of 2D leaf-like CoP/CoO hybrid onto PrGO could create a unique interface with a large number of active sites, CoO-driven creation of pseudocapacitive surface PO_x species, and high P content (∼3 at.%) in PrGO, thus promoting the Faradaic reaction, electrical conductivity, and overall charge storage. This framework yields a high specific capacitance of 405 F g^-1 at 5 A g^-1 and excellent cycling stability (over 100 % after 10,000 cycles), superior to those of pristine CoP@PrGO (300 F g^-1 at 5 A g^-1). Furthermore, the fabricated asymmetric supercapacitor delivers reasonable energy density of 4.2 Wh kg^-1 at a power density of 785 W kg^-1 and cycling stability of ∼100 % after 10,000 cycles. Therefore, CoP/CoO@PrGO with its unique interfacial properties can promote the development of heterostructure electrode for high-performance supercapacitors.

Multimetallic transition metal phosphide nanostructures for supercapacitors and electrochemical water splitting

Transition metal phosphides (TMPs) have recently emerged as an important class of functional materials and been demonstrated to be outstanding supercapacitor electrode materials and catalysts for electrochemical water splitting. While extensive investigations have been devoted to monometallic TMPs, multimetallic TMPs have lately proved to show enhanced electrochemical performance compared to their monometallic counterparts, thanks to the synergistic effect between different transition metal species. This topical review summarizes recent advance in the synthesis of new multimetallic TMP nanostructures, with particular focus on their applications in supercapacitors and electrochemical water splitting. Both experimental reports and theoretical understanding of the synergy between transition metal species are comprehensively reviewed, and perspectives of future research on TMP-based materials for these specific applications are outlined.

The Preparation and Electrochemical Pseudocapacitive Performance of Mutual Nickel Phosphide Heterostructures

Transition metal phosphide composite materials have become an excellent choice for use in supercapacitor electrodes due to their excellent conductivity and good catalytic activity. In our study, a series of nickel phosphide heterostructure composites was prepared using a temperature-programmed phosphating method, and their electrochemical performance was tested in 2 mol L−1 KOH electrolyte. Because the interface effect can increase the catalytic active sites and improve the ion transmission, the prepared Ni2P/Ni3P/Ni (Ni/P = 7:3) had a specific capacity of 321 mAh g−1 under 1 A g−1 and the prepared Ni2P/Ni5P4 (Ni/P = 5:4) had a specific capacity of 218 mAh g−1 under 1 A g−1. After the current density was increased from 0.5 A g−1 to 5 A g−1, 76% of the specific capacity was maintained. After 7000 cycles, the capacity retention rate was above 82%. Due to the phase recombination effect, the electrochemical performance of Ni2P/Ni3P/Ni and Ni2P/Ni5P4 was much better than that of single-phase N2P. After assembling the prepared composite and activated carbon into a supercapacitor, the Ni2P/Ni3P/Ni//AC had an energy density of 22 W h kg−1 and a power density of 800 W kg−1 and the Ni2P/Ni5P4//AC had an energy density of 27 W h kg−1 and a power density of 800 W kg−1.

Synergistically enhanced performance of transition-metal doped Ni2P for supercapacitance and overall water splitting

Cost-effective and readily available catalysts applicable for electrochemical conversion technologies are highly desired. Herein, we report the synthesis of dithiophosphonate complexes of the type [Ni{S₂P(OH)(4-CH₃OC₆H₄)}₂] (1), [Co{S₂P(OC₄H₉)(4-CH₃OC₆H₄)}₃] (2) and [Fe{S₂P(OH)(4-CH₃OC₆H₄)}₃] (3) and employed them to prepare Ni₂P, Co-Ni₂P and Fe-Ni₂P nanoparticles. Ni₂P was formed by a facile hot injection method by decomposing complex 1 in tri-octylphosphine oxide/tri-n-octylphosphine at 300 °C. The prepared Ni₂P was doped with Co and Fe employing complexes 2 and 3, respectively, under similar experimental conditions. Doping Ni₂P with Co and Fe demonstrated synergistic improvement of Ni₂P performance as an electrocatalyst in supercapcitance, hydrogen evololution and oxygen evolution reactions in alkaline medium. Cobalt doping improved the Ni₂P charge storage capacity with a supercapacitance of 864 F g^-1 at 1 A g^-1 current density. Fe doped Ni₂P recorded the lowest overpotential of 259 mV to achieve a current density of 10 mA cm^-2 and a Tafel slope of 80 mV dec^-1 for OER, better than the undoped Ni₂P and the benchmark IrO₂. Likewise, Fe-doped Ni₂P electrode required the lowest overpotential of 68 mV with a Tafel slope of 110 mV dec^-1 to attain the same current density for HER. All catalysts showed excellent stability in supercapacitance and overall water splitting reactions, indicating their practical use in energy conversion technologies.

3D hierarchical rose-like Ni2P@rGO assembled from interconnected nanoflakes as anode for lithium ion batteries

In recent years, anode materials of transition metal phosphates (TMPs) for lithium ion batteries have drawn a vast amount of attention, due to their high theoretical capacity and comparatively low intercalation potentials vs. Li/Li⁺.

Rationally designed Ni2P/Ni/C as a positive electrode for high-performance hybrid supercapacitors

Ni₂P/Ni/C is fabricated a simple simultaneous carbonization and phosphidation process. It displays exceptional rate performance with excellent cycling ability, mainly resulting from accelerated charge transfer ability and stable porous structure.

High-Performance Flexible Solid-State Asymmetric Supercapacitors Based on Bimetallic Transition Metal Phosphide Nanocrystals

Transition metal phosphides (TMPs) have recently emerged as an important type of electrode material for use in supercapacitors thanks to their intrinsically outstanding specific capacity and high electrical conductivity. Herein, we report the synthesis of bimetallic Co_xNi_1-xP ultrafine nanocrystals supported on carbon nanofibers (Co_xNi_1-xP/CNF) and explore their use as positive electrode materials of asymmetric supercapacitors. We find that the Co:Ni ratio has a significant impact on the specific capacitance/capacity of Co_xNi_1-xP/CNF, and Co_xNi_1-xP/CNF with an optimal Co:Ni ratio exhibits an extraordinary specific capacitance/capacity of 3514 F g^-1/1405.6 C g^-1 at a charge/discharge current density of 5 A g^-1, which is the highest value for TMP-based electrode materials reported by far. Our density functional theory calculations demonstrate that the significant capacitance/capacity enhancement in Co_xNi_1-xP/CNF, compared to the monometallic NiP/CNF and CoP/CNF, originates from the enriched density of states near the Fermi level. We further fabricate a flexible solid-state asymmetric supercapacitor using Co_xNi_1-xP/CNF as positive electrode material, activated carbon as negative electrode material, and a polymer gel as the electrolyte. The supercapacitor shows a specific capacitance/capacity of 118.7 F g^-1/166.2 C g^-1 at 20 mV s^-1, delivers an energy density of 32.2 Wh kg^-1 at 3.5 kW kg^-1, and demonstrates good capacity retention after 10000 charge/discharge cycles, holding substantial promise for applications in flexible electronic devices.

Electrochemical fabrication of nickel phosphide/reduced graphene oxide/nickel oxide composite on nickel foam as a high performance electrode for supercapacitors

Three-dimensional (3D) nickel phosphide/reduced graphene oxide (rGO)/nickel oxide composite on nickel foam (Ni₂P/rGO/NiO/NF) is fabricated as a supercapacitor (SC) electrode material via the two-step electrochemical deposition of graphene oxide (GO) and nickel phosphide on the nickel foam. Typically, rGO/NiO/NF is fabricated at first by the electrochemical treatment of nickel foam at 10 V in 0.1 M sulfuric acid with GO for 10 min. The result reveals that NiO nanosheets are vertically grown on the surface of nickel foam and rGO is deposited on the surface of NiO/NF, leading to the enhancement of capacity. Secondly, nickel phosphide is electrochemically deposited on the surface of rGO/NiO/NF in the sodium hypophosphite-based aqueous solution at 10 mA cm^-2 to yield the Ni₂P/rGO/NiO/NF. The deposition of Ni₂P leads to a much higher capacity. The optimal areal and mass specific capacities are obtained as 3.59 C cm^-2 and 742 C g^-1 at the electrochemical deposition time of 30 and 10 min, respectively. The high capacity reveals that the proposed two-step electrochemical fabrication process is facile and effective. In addition, the Ni₂P/rGO/NiO/NF electrode-based all-solid-state asymmetric SC was fabricated and could successfully turn on a light-emitting diode light. This revealed its feasibility in practical application and confirmed that the resulting 3D Ni₂P/rGO/NiO/NF has a great potential as the SC electrode material.

MOF-derived NiO/Ni architecture encapsulated into N-doped carbon nanotubes for advanced asymmetric supercapacitors

In the second step, the Ni-MOF was transformed into Ni-MOF-600 with low NiO content via calcination under nitrogen protection at 600 °C

Hierarchical NiCo2S4@NiCoP core-shell nanocolumn arrays on nickel foam as a binder-free supercapacitor electrode with enhanced electrochemical performance

A novel hierarchical core-shell nanocolumn array, with NiCo₂S₄ hollow nanowire (NiCo₂S₄ H-NW) as the core and NiCoP nanosheet (NiCoP NS) as the shell, has been directly synthesized on nickel foam (NF) as self-supported, binder-free electrode for high-performance supercapacitors. The morphological characterizations reveal that the diameter of NiCo₂S₄ H-NW core is ∼100 nm and the diameter of single NiCo₂S₄@NiCoP core-shell nanocolumn is ∼250 nm. Through a series of electrochemical tests and the analysis of charge storage kinetics, hierarchical NiCo₂S₄@NiCoP/NF electrode presents high areal specific capacitance of 5.98 F/cm² at 1 mA/cm², outstanding rate capability (70.29% capacitance retention with the current density increased from 1 to 50 mA/cm²) and superior cycling stability (92.94% of original capacity is retained after 5000 cycles at 10 mA/cm²). The prominent performance of NiCo₂S₄@NiCoP/NF electrode could be resulted from their unique hierarchical core-shell nanocolumn structure, which could offer abundant active sites near the interface for fast electrochemical reaction, and validly avoid the collapse of internal structure for the stability of whole structure in the repeated electrochemical measurement. The novel NiCo₂S₄@NiCoP/NF electrode offers a new method for future electrochemical energy storage devices with high-stability.

Water Splitting-Biosynthetic Hybrid System for CO2 Conversion using Nickel Nanoparticles Embedded in N-Doped Carbon Nanotubes

CO₂ reduction has drawn increasing attention owing to the concern of global warming. Water splitting-biosynthetic hybrid systems are novel and efficient approaches for CO₂ conversion. Intimate coupling of electrocatalysts and biosynthesis requires the catalysts possess both high catalytic performance and excellent biocompatibility, which is a bottleneck of developing such catalysts. Here, a complex of Ni nanoparticles embedded in N-doped carbon nanotubes (Ni@N-C) is synthesized as a hydrogen evolution reaction electrocatalyst and is coupled with a hydrogen oxidizing autotroph, Cupriavidus necator H16, to convert CO₂ to poly-β-hydroxybutyrate. In Ni@N-C, the Ni nanoparticles are encapsulated in N-C nanotubes, which prevents bacteria from direct contact with Ni and inhibits Ni²⁺ leaching. As a result, Ni@N-C exhibits excellent biocompatibility and stability. This work demonstrates that electrocatalysts and biosynthesis can be intimately coupled through rational catalyst design.

Graphene-supported platinum/nickel phosphide electrocatalyst with improved activity and stability for methanol oxidation

In this paper, we report a novel catalyst using Ni₂P as a cocatalyst of Pt supported on graphene for methanol oxidation.

NiCo2O4/NiCoP nanoflake-nanowire arrays: a homogeneous hetero-structure for high performance asymmetric hybrid supercapacitors

Transition metal phosphides (TMPs) represent an important class of compounds with metalloid characteristics and good electrical conductivity, which are of great benefit to enhance electrochemical performances.

Recent progress in layered double hydroxide based materials for electrochemical capacitors: design, synthesis and performance

As representative two-dimensional (2D) materials, layered double hydroxides (LDHs) have received increasing attention in electrochemical energy storage and conversion because of the facile tunability between their composition and morphology. The high dispersion of active species in layered arrays, the simple exfoliation into monolayer nanosheets and chemical modification offer the LDHs an opportunity as active electrode materials in electrochemical capacitors (ECs). LDHs are favourable in providing large specific surface areas, good transport features as well as attractive physicochemical properties. In this review, our purpose is to provide a detailed summary of recent developments in the synthesis and electrochemical performance of the LDHs. Their composites with carbon (carbon quantum dots, carbon black, carbon nanotubes/nanofibers, graphene/graphene oxides), metals (nickel, platinum, silver), metal oxides (TiO₂, Co₃O₄, CuO, MnO₂, Fe₃O₄), metal sulfides/phosphides (CoS, NiCo₂S₄, NiP), MOFs (MOF derivatives) and polymers (PEDOT:PSS, PPy (polypyrrole), P(NIPAM-co-SPMA) and PET) are also discussed in this review. The relationship between structures and electrochemical properties as well as the associated charge-storage mechanisms is discussed. Moreover, challenges and prospects of the LDHs for high-performance ECs are presented. This review sheds light on the sustainable development of ECs with LDH based electrode materials.

Metal Phosphides and Phosphates-based Electrodes for Electrochemical Supercapacitors

Phosphorus compounds, such as metal phosphides and phosphates have shown excellent performances and great potential in electrochemical energy storage, which are demonstrated by research works published in recent years. Some of these metal phosphides and phosphates and their hybrids compare favorably with transition metal oxides/hydroxides, which have been studied extensively as a class of electrode materials for supercapacitor applications, where they have limitations in terms of electrical and ion conductivity and device stability. To be specific, metal phosphides have both metalloid characteristics and good electric conductivity. For metal phosphates, the open-framework structures with large channels and cavities endow them with good ion conductivity and charge storage capacity. In this review, we present the recent progress on metal phosphides and phosphates, by focusing on their advantages/disadvantages and potential applications as a new class of electrode materials in supercapacitors. The synthesis methods to prepare these metal phosphides/phosphates are looked into, together with the scientific insights involved, as they strongly affect the electrochemical energy storage performance. Particular attentions are paid to those hybrid-type materials, where strong synergistic effects exist. In the summary, the future perspectives and challenges for the metal phosphides, phosphates and hybrid-types are proposed and discussed.

Three-Dimensional Cobalt Phosphide Nanowire Arrays as Negative Electrode Material for Flexible Solid-State Asymmetric Supercapacitors

Despite the great progress that has been accomplished in supercapacitors, the imbalance of the development of positive and negative electrode materials still remains a critical issue to achieve high energy density; therefore, exploring high-performance negative electrode materials is highly desirable. In this article, three-dimensional cobalt phosphide (CoP) nanowire arrays were synthesized on a carbon cloth and were utilized as a binder-free supercapacitor negative electrode. The as-synthesized CoP nanowire arrays presented a high capacitance of 571.3 mF/cm² at a current density of 1 mA/cm². By using CoP nanowire arrays as the negative electrode and MnO₂ nanowire arrays as the positive electrode, a flexible solid-state asymmetric supercapacitor has been fabricated and has exhibited excellent electrochemical performance, such as a high energy density of 0.69 mWh/cm³ and a high power density of 114.2 mW/cm³. In addition, the solid-state asymmetric supercapacitor shows high cycle stability with 82% capacitance retention after 5000 charge/discharge cycles. This work demonstrates that CoP is a promising negative electrode material for high-performance supercapacitor applications.

Synthesis of graphene-transition metal oxide hybrid nanoparticles and their application in various fields

Single layer graphite, known as graphene, is an important material because of its unique two-dimensional structure, high conductivity, excellent electron mobility and high surface area. To explore the more prospective properties of graphene, graphene hybrids have been synthesised, where graphene has been integrated with other important nanoparticles (NPs). These graphene-NP hybrid structures are particularly interesting because after hybridisation they not only display the individual properties of graphene and the NPs, but also they exhibit further synergistic properties. Reduced graphene oxide (rGO), a graphene-like material, can be easily prepared by reduction of graphene oxide (GO) and therefore offers the possibility to fabricate a large variety of graphene-transition metal oxide (TMO) NP hybrids. These hybrid materials are promising alternatives to reduce the drawbacks of using only TMO NPs in various applications, such as anode materials in lithium ion batteries (LIBs), sensors, photocatalysts, removal of organic pollutants, etc. Recent studies have shown that a single graphene sheet (GS) has extraordinary electronic transport properties. One possible route to connecting those properties for application in electronics would be to prepare graphene-wrapped TMO NPs. In this critical review, we discuss the development of graphene-TMO hybrids with the detailed account of their synthesis. In addition, attention is given to the wide range of applications. This review covers the details of graphene-TMO hybrid materials and ends with a summary where an outlook on future perspectives to improve the properties of the hybrid materials in view of applications are outlined.

Preparation and characterization of activated carbon derived from the Borassus flabellifer flower as an electrode material for supercapacitor applications

Activated carbon is prepared by a two stage process with H₃PO₄ activating agent using the precursor material Borassus flabellifer flower as an electrode material for supercapacitors.

Direct aqueous solution synthesis of an ultra-fine amorphous nickel–boron alloy with superior pseudocapacitive performance for advanced asymmetric supercapacitors

This study reports a facile aqueous solution synthesis of an ultrafine amorphous nickel–boron alloy and its applications as a novel positive electrode material for asymmetric supercapacitors.

A Ni-P@NiCo LDH core–shell nanorod-decorated nickel foam with enhanced areal specific capacitance for high-performance supercapacitors

In this study, nickel phosphide (Ni-P) was combined with NiCo LDH via facile phosphorization of Ni foam and subsequent electrodeposition, forming core–shell nanorod arrays on Ni foam. The obtained Ni-P@NiCo LDH delivered excellent capacitive performance.

Three-Dimensional Hierarchical NixCo1-xO/NiyCo2-yP@C Hybrids on Nickel Foam for Excellent Supercapacitors

Active materials and special structures of the electrode have decisive influence on the electrochemical properties of supercapacitors. Herein, three-dimensional (3D) hierarchical Ni_xCo_1-xO/Ni_yCo_2-yP@C (denoted as NiCoOP@C) hybrids have been successfully prepared by a phosphorization treatment of hierarchical Ni_xCo_1-xO@C grown on nickel foam. The resulting NiCoOP@C hybrids exhibit an outstanding specific capacitance and cycle performance because they couple the merits of the superior cycling stability of Ni_xCo_1-xO, the high specific capacitance of Ni_yCo_2-yP, the mechanical stability of carbon layer, and the 3D hierarchical structure. The specific capacitance of 2638 F g^-1 can be obtained at the current density of 1 A g^-1, and even at the current density of 20 A g^-1, the NiCoOP@C electrode still possesses a specific capacitance of 1144 F g^-1. After 3000 cycles at 10 A g^-1, 84% of the initial specific capacitance is still remained. In addition, an asymmetric ultracapacitor (ASC) is assembled through using NiCoOP@C hybrids as anode and activated carbon as cathode. The as-prepared ASC obtains a maximum energy density of 39.4 Wh kg^-1 at a power density of 394 W kg^-1 and still holds 21 Wh kg^-1 at 7500 W kg^-1.

Shape-Controlled Synthesis of Co2P Nanostructures and Their Application in Supercapacitors

Co2P nanostructures with rod-like and flower-like morphologies have been synthesized by controlling the decomposition process of Co(acac)3 in oleylamine system with triphenylphosphine as phosphorus source. Investigations indicate that the final morphologies of the products are determined by their peculiar phosphating processes. Electrochemical measurements manifest that the Co2P nanostructures exhibit excellent morphology-dependent supercapacitor properties. Compared with that of 284 F g(-1) at a current density of 1 A g(-1) for Co2P nanorods, the capacitance for Co2P nanoflowers reaches 416 F g(-1) at the same current density. Furthermore, an optimized asymmetric supercapacitor by using Co2P nanoflowers as anode and graphene as cathode is fabricated. It can deliver a high energy density of 8.8 Wh kg(-1) (at a high power density of 6 kW kg(-1)) and good cycling stability with over 97% specific capacitance remained after 6000 cycles, which makes the Co2P nanostructures potential applications in energy storage/conversion systems. This study paves the way to explore a new class of cobalt phosphide-based materials for supercapacitor applications.

Hierarchical polypyrrole/Ni3S2@MoS2 core–shell nanostructures on a nickel foam for high-performance supercapacitors

Hierarchical polypyrrole/Ni₃S₂@MoS₂ core–shell nanostructures have been successfully designed and constructed on a Ni foam substrate through a facile two-step solution synthesis protocol and exhibit high performance as a supercapacitor.

High performance electrochemical capacitor materials focusing on nickel based materials

Of the two major capacitances contributing to electrochemical storage devices, pseudo-capacitance, which results from the reversible faradaic reactions, can be much higher than the electric double layer capacitance.

Controllable synthesis of hierarchical NiCo2S4@Ni3S2 core–shell nanotube arrays with excellent electrochemical performance for aqueous asymmetric supercapacitors

Unique NiCo₂S₄@Ni₃S₂ core–shell nanotube arrays with a hierarchical structure, as promising positive electrodes for supercapacitors, were designed and synthesized on the surface of Ni foam via a hydrothermal reaction and electrodeposition method.

Facile one-pot synthesis of a NiMoO4/reduced graphene oxide composite as a pseudocapacitor with superior performance

A hybrid NiMoO₄/rGO composite was successfully synthesized by a facile one-pot hydrothermal method.