Metal Phosphides and Phosphates-based Electrodes for Electrochemical Supercapacitors

Evaluation of the effect of precursor ratios on the electrochemical performances of binder-free NiMn-phosphate electrodes for supercapattery

Binary metal phosphate electrodes have been widely studied for energy storage applications due to the synergistic effects of two different transition elements that able to provide better conductivity and stability. Herein, the battery-type binder-free nickel-manganese phosphate (NiMn-phosphate) electrodes were fabricated with different Ni:Mn precursor ratios via microwave-assisted hydrothermal technique for 5 min at 90 °C. Overall, NiMn3P electrode (Ni:Mn = 1:3) showed an outstanding electrochemical performance, displaying the highest specific (areal) capacity at 3 A/g of 1262.4 C/g (0.44 C/cm2), and the smallest charge transfer resistance of 108.8 Ω. The enhanced performance of NiMn3P electrode can be ascribed to the fully grown amorphous nature and small-sized flake and flower structures of NiMn3P electrode material on the nickel foam (NF) surface. This configuration offered a higher number of active sites and a larger exposed area, facilitating efficient electrochemical reactions with the electrolyte. Consequently, the NiMn3P//AC electrode combination was chosen to further investigate its performance in supercapattery. The NiMn3P//AC supercapattery exhibited remarkable energy density of 105.4 Wh/kg and excellent cyclic stability with 84.7% retention after 3000 cycles. These findings underscored the superior electrochemical performance of the battery-type binder-free NiMn3P electrode, and highlight its potential for enhancing the overall performance of supercapattery.

Harnessing morphological alteration from microflowers to nanoparticles and cations synergy (Co:Ni) in binder-free cobalt nickel vanadate thin film cathodes synthesized via SILAR method for hybrid supercapacitor devices

Electrode materials must be rationally designed with morphologies and electroactive sites manipulated through cations' synergy in bimetal compounds in order to maximize the performance of energy storage devices. Therefore, the present study emphasizes binder-free scalable preparation of cobalt nickel vanadate (CNV) thin films by a facile successive ionic layer adsorption and reaction (SILAR) approach with specific cations (Co:Ni) alternation. Increasing the Ni cation content in the CNV notably transforms its microflower structure comprising nanoflakes (252 nm) into nanoparticles (74 nm). An optimized S-CNV5 thin film cathode with Co:Ni molar ratio of ∼ 0.4:0.6 and a high specific surface area of 340 m2 g-1, provided the excellent specific capacitance (Csp) and capacity (Csc) of 1382 F g-1 and 691 C g-1, respectively at 1 A g-1 current density. A hybrid aqueous supercapacitor (HASc) device with positive and negative electrodes comprising optimized CNV and reduced graphene oxide (rGO), respectively, in a 1 M KOH electrolyte delivered a Csp of 133 F g-1 and a specific energy (SE) of 53 Wh kg-1 at a specific power (SP) of 2261 kW kg-1. Additionally, a fabricated hybrid solid-state supercapacitor (HSSc) device with the same electrodes applying PVA-KOH gel electrolyte displayed a Csp of 119 F g-1, and SE of 46 Wh kg-1 at SP of 1184 W kg-1. This boosted electrochemical activity is due to the synergetic effects of Ni and Co species in the CNV thin film electrodes, emphasizing the potential of CNV electrodes as cathodes in hybrid energy storage devices.

Reactive P and S co-doped porous hollow nanotube arrays for high performance chloride ion storage

Developing stable, high-performance chloride-ion storage electrodes is essential for energy storage and water purification application. Herein, a P, S co-doped porous hollow nanotube array, with a free ion diffusion pathway and highly active adsorption sites, on carbon felt electrodes (CoNiPS@CF) is reported. Due to the porous hollow nanotube structure and synergistic effect of P, S co-doped, the CoNiPS@CF based capacitive deionization (CDI) system exhibits high desalination capacity (76.1 mgCl- g-1), fast desalination rate (6.33 mgCl- g-1 min-1) and good cycling stability (capacity retention rate of > 90%), which compares favorably to the state-of-the-art electrodes. The porous hollow nanotube structure enables fast ion diffusion kinetics due to the swift ion transport inside the electrode and the presence of a large number of reactive sites. The introduction of S element also reduces the passivation layer on the surface of CoNiP and lowers the adsorption energy for Cl- capture, thereby improving the electrode conductivity and surface electrochemical activity, and further accelerating the adsorption kinetics. Our results offer a powerful strategy to improve the reactivity and stability of transition metal phosphides for chloride capture, and to improve the efficiency of electrochemical dechlorination technologies.

A Membrane-Free Decoupled Water Electrolyzer Operating at Simulated Fluctuating Renewables with Tri-Functional NiCo-P Electrode

Water electrolysis has been considered a promising technology for the conversion of renewables to hydrogen. However, preventing mixing of products (H2 and O2 ) and exploring cost-efficient electrolysis components remains challenging for conventional water electrolyzers. Herein, we designed a membrane-free decoupled water electrolysis system by using graphite felt supported nickel-cobalt phosphate (GF@Nix Coy -P) material as a tri-functional (redox mediator, hydrogen evolution reaction (HER), oxygen evolution reaction (OER)) electrode. The versatile GF@Ni1 Co1 -P electrode obtained by a one-step electrodeposition not only exhibits high specific capacity (176 mAh g-1 at 0.5 A g-1 ) and long cycle life (80 % capacity retention after 3000 cycles) as a redox mediator, but also has relatively outstanding catalytic activities for HER and OER. The excellent properties of the GF@Nix Coy -P electrode endow this decoupled system with more flexibility for H2 production by fluctuating renewable energies. This work provides guidance for multifunctional applications of transition metal compounds between energy storage and electrocatalysis.

Cold plasma synthesis of phosphorus-doped CoFe2O4 with oxygen vacancies for enhanced OER activity

Spinel-type metal oxides are promising electrocatalysts for the oxygen evolution reaction (OER) due to their unique electronic structure and low cost. Herein, we induced oxygen vacancies and doped phosphorus into CoFe2O4 using cold plasma. The abundant oxygen vacancies enhanced hydrophilicity and modified the electronic structure of CoFe2O4, while the phosphorus doping formed numerous new active centers. The doped P and formed FeP promoted the charge transfer and improved the conductivity of the catalyst. The phosphorus-doped CoFe2O4 exhibited exceptional OER activity with an overpotential of 180 mV at 10 mA cm-2 and a Tafel slope of 65.8 mV dec-1 in an alkaline electrolyte. DFT calculations confirmed that phosphorus doping can improve the charge distribution near the Fermi level and optimize the d-band center position.

Hierarchical Cube-in-Cube Cobalt-Molybdenum Phosphide Hollow Nanoboxes Derived from the MOF Template Strategy for High-Performance Supercapacitors

The design of hierarchical hollow nanostructures with complex shell architectures is an attractive and effective way to obtain a desirable electrode material for energy storage application. Herein, we report an effective metal-organic framework (MOF) template-engaged method to synthesize novel double-shelled hollow nanoboxes, in terms of chemical composition and structure complexity, for supercapacitor application. Starting from cobalt-based zeolitic imidazolate framework (ZIF-67(Co)) nanoboxes as the removal template, we developed a rational preparation approach to synthesize cobalt-molybdenum-phosphide (CoMoP) double-shelled hollow nanoboxes (donated as CoMoP-DSHNBs) through (i) ion-exchange reaction, (ii) template etching, and (iii) phosphorization treatment, respectively. Notably, despite the previously reported works, the phosphorization was simply done using the facile solvothermal method, without employing annealing and high-temperature conditions, which can be considered as one of the merits of the current work. CoMoP-DSHNBs showed excellent electrochemical properties owing to their unique morphology, high surface area, and optimal elemental composition. In a three-electrode system, the target material showed a superior specific capacity of 1204 F g^-1 at 1 A g^-1 with a remarkable cycle stability of 87% after 20000 cycles. The hybrid device formed of activated carbon (AC) as the negative electrode and CoMoP-DSHNBs as the positive electrode exhibited a high specific energy density of 49.99 W h kg^-1 and a maximum power density of 7539.41 W kg^-1 with a great cycling stability of 84.5% after 20,000 cycles.

Lithium Manganese Sulfates as a New Class of Supercapattery Materials at Elevated Temperatures

To make supercapattery devices feasible, there is an urgent need to find electrode materials that exhibit a hybrid mechanism of energy storage. Herein, we provide a first report on the capability of lithium manganese sulfates to be used as supercapattery materials at elevated temperatures. Two compositions are studied: monoclinic Li₂Mn(SO₄)₂ and orthorhombic Li₂Mn₂(SO₄)₃, which are prepared by a freeze-drying method followed by heat treatment at 500 °C. The electrochemical performance of sulfate electrodes is evaluated in lithium-ion cells using two types of electrolytes: conventional carbonate-based electrolytes and ionic liquid IL ones. The electrochemical measurements are carried out in the temperature range of 20-60 °C. The stability of sulfate electrodes after cycling is monitored by in-situ Raman spectroscopy and ex-situ XRD and TEM analysis. It is found that sulfate salts store Li⁺ by a hybrid mechanism that depends on the kind of electrolyte used and the recording temperature. Li₂Mn(SO₄)₂ outperforms Li₂Mn₂(SO₄)₃ and displays excellent electrochemical properties at elevated temperatures: at 60 °C, the energy density reaches 280 Wh/kg at a power density of 11,000 W/kg. During cell cycling, there is a transformation of the Li-rich salt, Li₂Mn(SO₄)₂, into a defective Li-poor one, Li₂Mn₂(SO₄)₃, which appears to be responsible for the improved storage properties. The data reveals that Li₂Mn(SO₄)₂ is a prospective candidate for supercapacitor electrode materials at elevated temperatures.

Activation-Assisted High-Concentration Phosphorus-Doping to Enhance the Electrochemical Performance of Cobalt Carbonate Hydroxide Hydrate

P-doping into metal oxides has been demonstrated as a valid avenue to ameliorate electrochemical performance because it can tune the electronic structures and increase the active sites for an electrochemical reaction. However, it usually results in a low P-doping concentration via the commonly used gas phosphorization method. In this work, an activation-assisted P-doping strategy was explored to significantly raise the P-doping concentration in cobalt carbonate hydroxide hydrate (CCHH). The activation treatment increased active sites for electrochemical reaction and endowed the sample with a high P content in the subsequent gas phosphorization process, thereby greatly enhancing the conductivity of the sample. Therefore, the final CCHH-A-P electrode exhibited a high capacitance of 6.62 F cm^-2 at 5 mA cm^-2 and good cyclic stability. In addition, the CCHH-A-P//CC ASC with CCHH-A-P as the positive electrode and carbon cloth as the negative electrode provided a high energy density of 0.25 mWh cm^-2 at 4 mW cm^-2 as well as excellent cycling performance with capacitance retention of 91.2% after 20,000 cycles. Our work shows an effective strategy to acquire Co-based materials with high-concentration P-doping that holds great potential in boosting the electrochemical performance of electrode materials via P-doping technology.

Heterogeneous Ni-Co phosphide/phosphate with a specific hollow sea-urchin-like structure for high-performance hybrid supercapacitor and alkaline zinc-metal battery applications

Constructing well-defined nanostructures consisting of the multiple components with distinctive features are a promising but challenging strategy to develop advanced electroactive materials for energy storage applications. Herein, heterogeneous Ni-Co phosphide/phosphate with a specific hollow sea-urchin-like structure has been synthesized as advanced electroactive materials for both hybrid supercapacitor (HSC) and alkaline zinc-metal battery (AZB) applications. The heterogeneous Ni-Co phosphide/phosphate combines the merits of improved electrolyte interfacial property from the specific hollow sea-urchin-like structure, high electron-conductivity of phosphide, and better ion adsorption and solid diffusion property of phosphate. As a result, the Ni-Co phosphide/phosphate achieves a high capacity to 180.7 mA h g^-1 at 1 A g^-1, excellent rate capability of 51% capacity retention when the specific current increases by 50 times, and stable cycling stability of 85% capacity retention when cycled for 1000 cycles. Ex situ test was conducted to investigate the formation mechanism for the hollow and sea-urchin-like structure, which can be ascribed to the anion exchange reaction between pre-formed hydroxide and CO₃^2- ions. When used to assemble HSCs with reduced graphene oxide (RGO), the HSCs exhibit a high specific energy of 49.4 W h kg^-1, an ultrahigh specific power to 11.7 kW kg^-1, and an eminent cycling stability over 10,000 cycles. Meanwhile, Ni₂Co-P/PO_x-based AZB also achieves both high-energy and high-power performance with the specific energy of 308.0 W h kg^-1 at 828.4 W kg^-1 and 117.4 W h kg^-1 at 30.8 kW kg^-1. These results above suggest that heterogeneous Ni-Co phosphide/phosphate has great potential to be used as a candidate for both HSC and AZB applications.

Matching CP@NCOH/NF Cathode and GH/FNP/NF Anode for High-Performance Asymmetric Supercapacitor

It is extremely crucial to design and match high-quality cathode and anode for achieving high-performance asymmetric supercapacitors (ASCs). Herein, Co₃ (PO₄ )₂ @NiCo-LDH/Ni foam (CP@NCOH/NF) cathode with hierarchical morphology and graphene hydrogel/Fe-Ni phosphide/Ni foam (GH/FNP/NF) anode with the robust and porous structure are elaborately designed and prepared, respectively. Owing to their unique and profitable structures, both CP@NCOH/NF and GH/FNP/NF electrodes yield the superior capacity (10760 and 2236 mC cm^-2 at 2 mA cm^-2 , respectively), good rate capability (63% retention at 200 mA cm^-2 and 52% retention at 50 mA cm^-2 , respectively), and excellent cycling stability (72% and 74% retention after 10 000 cycles, respectively). Benefiting from their matchable electrochemical performances, the configured solid-state CP@NCOH/NF//GH/FNP/NF ASC outputs both competitive energy density (80.2 Wh kg^-1 /4.1 mWh cm^-3 ) and power density (14563 W kg^-1 /750 mW cm^-3 ), companied by remarkable cyclability (71% retention after 10 000 cycles), manifesting its great promise for large-scale integrated energy-storage system.

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.

The electrosorption behavior of shuttle-like FeP: performance and mechanism

Owing to its high electrochemical ability, the FeP is envisioned to be the potential electrode for capacitive deionization (CDI) with enhanced performance. However, it suffers from poor cycling stability due to the active redox reaction. In this work, a facile approach has been designed to prepare the mesoporous shuttle-like FeP using MIL-88 as the template. The porous shuttle-like structure not only alleviates the volume expansion of FeP during the desalination/salination process but also promotes ion diffusion dynamics by providing convenient ion diffusion channels. As a result, the FeP electrode has demonstrated a high desalting capacity of 79.09 mg g⁻¹ at 1.2 V. Further, it proves the superior capacitance retention, which maintained 84% of the initial capacity after the cycling. Based on post-characterization, a possible electrosorption mechanism of FeP has been proposed.

In this work, mesoporous shuttle-like FeP for electrosorption is prepared. As an electrode, it achieves a high salt adsorption capacity of 79.09 mg g⁻¹ and superior capacitance retention. The conversion of Fe^II to Fe^III is responsible for the removal of salty ions.

Status review of nickel phosphides for hybrid supercapacitors

Transition metal phosphides are a new class of materials that have attracted enormous attention as a potential electrode for supercapacitors (SCs) compared to metal oxides/hydroxides and metal sulfides due to their strong redox-active behaviour, good electrical conductivity, layered structure, low cost, and high chemical and thermal stability. Recently, several efforts have been made to develop nickel phosphides (Ni_xP_y) (NPs) for high-performance SCs. The electrochemical properties of NPs can be easily tuned by several innovative approaches, such as heteroatom doping, defect engineering, and developing a hollow architecture. The prospects of NPs as a positive electrode in hybrid SCs are summarized to understand the material's practical relevance. Finally, the challenges and perspectives are provided for the development of high-performance NPs for SCs. The thorough elucidation of the structure-property-performance relationship offers a guide for developing NP-based next-generation energy-storage devices.

A Review on the Application of Cobalt-Based Nanomaterials in Supercapacitors

Among many electrode materials, cobalt-based nanomaterials are widely used in supercapacitors because of their high natural abundance, good electrical conductivity, and high specific capacitance. However, there are still some difficulties to overcome, including poor structural stability and low power density. This paper summarizes the research progress of cobalt-based nanomaterials (cobalt oxide, cobalt hydroxide, cobalt-containing ternary metal oxides, etc.) as electrode materials for supercapacitors in recent years and discusses the preparation methods and properties of the materials. Notably, the focus of this paper is on the strategies to improve the electrochemical properties of these materials. We show that the performance of cobalt-based nanomaterials can be improved by designing their morphologies and, among the many morphologies, the mesoporous structure plays a major role. This is because mesoporous structures can mitigate volume changes and improve the performance of pseudo capacitance. This review is dedicated to the study of several cobalt-based nanomaterials in supercapacitors, and we hope that future scholars will make new breakthroughs in morphology design.

Boosting the Photoelectrochemical Water Oxidation Performance of TiO2 Nanotubes by Surface Modification Using Silver Phosphate

Photoelectrocatalytic approaches are fascinating options for long-lasting energy storage through the transformation of solar energy into electrical energy or hydrogen fuel. Herein, we report a facile method of fabricating a composite electrode of well-aligned TiO2 nanotubes (TNTs) decorated with photodeposited silver phosphate (Ag3PO4) nanoparticles. Assessment of the optical, physiochemical and photoelectrochemical features demonstrated that the fabricated TNTs/Ag3PO4 films showed a substantially boosted photocurrent response of 0.74 mA/cm2, almost a 3-fold enrichment in comparison with the pure TNTs. Specifically, the applied bias photon-to-current efficiency of the fabricated TNTs/Ag3PO4 composite electrode was 2.4-fold superior to that of the pure TNTs electrode. In these TNTs/Ag3PO4 photoanodes, the introduction of Ag3PO4 over TNTs enhanced light absorption and improved charge transfer and surface conductivity. The developed process can be generally applied to designing and developing efficient contact interfaces between photoanodes and numerous cocatalysts.

Ultra-fast green microwave assisted synthesis of NaFePO4-C nanocomposites for sodium ion batteries and supercapacitors

Sodium ion batteries are favored in stationary and large scale power storage due to their low cost and nontoxicity. As the lithium is replaced with sodium due to the cost motive, a cheap processing method is needed to maintain the cell price as low as possible. We report an ultra-fast synthesis method that utilizes the high microwave absorbance of silicon carbide content in rice straw ash. Amorphous/maricite mixtures of sodium iron phosphates-carbon composites (NaFePO₄-C) are synthesized, crystallized, and carbon coated using one-step microwave heating. The sodium ion electroactive composites are prepared using different microwave heating durations ranging from 30 to 100 s. High purity inert gases are not needed during synthesis, processing, and even at cell assembly. The materials are characterized by elemental analysis techniques, X-ray diffraction (XRD), scanning/transmission electron microscope (SEM/TEM), and Raman spectroscopy. The electrochemical performance of the synthesized nanocomposites is examined as sodium ion battery cathode and as symmetric supercapacitors. The optimum synthesis time is 60 s for the application as sodium ion batteries and as a supercapacitor. The maximum specific capacity is 108.4 mA h g^-1 at 0.2 C in the case of using it as a battery cathode. While the capacitance is 86 F g^-1 at 0.5 A g^-1 as a supercapacitor. The capacity retention is 92.85% after 40 cycles at 0.2 C as sodium ion battery electrode. For supercapacitor, the capacity retention is 81.7% after 1000 cycles.

Lamellar structured Ni3P2O8: first-ever use to design 1.8 V operated flexible all-solid-state symmetric supercapacitor

Increasing demand for microelectronic devices necessitates the development of highly flexible energy storage technologies with a wide operating voltage. Thus, flexible electrodes and their devices with the requisite mechanical and electrochemical characteristics have prime importance. In this regard, the present article demonstrates the feasibility of designing a flexible all-solid-state supercapacitor using a chemically grown Ni₃P₂O₈ lamellar microstructured electrode embedded with carboxy methyl cellulose-Na₂SO₄ (CMC-Na₂SO₄) gel electrolyte. The formed symmetric device impressively exhibited a maximum working voltage window of 1.8 V with a high specific energy of 44.7 W h kg^-1 and specific power of 3.3 kW kg^-1 along with prolonged cycle life. Also, the device's high deformation tolerance (95%) when bent at 170° with a flashing light-emitting diode (LED) working demonstration showcases its viability for advanced flexible energy storage applications.

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.

In Situ Generation of NiCoP Nanoparticles on a Bimetal-Organic Framework for High-Performance Supercapacitors

The rational exploration of hybrid materials with well-defined compositions and structures/morphologies is essential for achieving high-performance electrodes for supercapacitors. Here, in situ dispersion and anchoring of NiCoP nanoparticles (NPs) on a bimetal-organic framework (Co₁Ni₂-MOF) by a controllable partial phosphorization approach are reported. The phosphating temperature and time significantly affect the specific capacitance of NiCoP/Co₁Ni₂-MOF-X-Y (where X and Y represent the phosphating temperature and time, respectively). Co₁Ni₂-MOF provides anchoring sites for confining NiCoP NPs, effectively improving the stability of NiCoP NPs. Highly dispersed NiCoP NPs facilitate OH^- adsorption, boosting the redox reaction kinetics. NiCoP/Co₁Ni₂-MOF-350-2 with optimized phosphating conditions exhibits a high specific capacitance of 525 F g^-1 at 0.5 A g^-1, which is superior to that of the precursor of Co₁Ni₂-MOF. Moreover, a hybrid supercapacitor constructed with NiCoP/Co₁Ni₂-MOF-350-2 and activated carbon shows a high specific capacitance and outstanding long-term stability.

NiCoP-CeO2 composites for efficient electrochemical oxygen evolution

In this study, a novel NiCoP-CeO2 composite was constructed on a Ni foam by a simple hydrothermal method and thermal phosphating strategy. In the OER test, NiCoP-CeO2 exhibited a low overpotential of 217 mV at 10 mA cm-2, 45 mV dec-1 of Tafel slopes. With the help of theoretical calculations and experimental characterization, the reason for performance improvement was analyzed in depth. The results show that CeO2 leads to a confinement effect, maintaining the nanosheet morphology of NiCo-LDHs, which contributes to sustaining the catalyst in favourable contact with H2O and minimizing the OER potential. Furthermore, by loading CeO2 onto NiCoP, the hydrophilicity of the catalyst is significantly enhanced. Our work provides an ingenious synthesis strategy for the preparation of efficient and inexpensive electrocatalytic materials.

First principles study of optoelectronic and photocatalytic performance of novel transition metal dipnictide XP2 (X = Ti, Zr, Hf) monolayers

Low cost and highly efficient two dimensional materials as photocatalysts are gaining much attention to utilize solar energy for water splitting and produce hydrogen fuel as an alternative to deal with the energy crisis and reduce environmental hazards. First principles calculations are performed to investigate the electronic, optical and photocatalytic properties of novel two dimensional transition metal dipnictide XP2 (X = Ti, Zr, Hf) monolayers. The studied single layer XP2 is found to be dynamically and thermally stable. TiP2, ZrP2 and HfP2 systems exhibit semiconducting nature with moderate indirect band gap values of 1.72 eV, 1.43 eV and 2.02 eV, respectively. The solar light absorption is found to be in energy range of 1.65-3.3 eV. All three XP2 systems (at pH = 7) and the HfP2 monolayer (at pH = 0) that straddle the redox potentials, are promising candidates for the water splitting reaction. These findings enrich the two dimensional family and provide a platform to design novel devices for emerging optoelectronic and photovoltaic applications.

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.

Super-Hydrophilic Leaflike Sn4P3 on the Porous Seamless Graphene-Carbon Nanotube Heterostructure as an Efficient Electrocatalyst for Solar-Driven Overall Water Splitting

Water splitting using renewable energy resources is an economic and green approach that is immensely enviable for the production of high-purity hydrogen fuel to resolve the currently alarming energy and environmental crisis. One of the effective routes to produce green fuel with the help of an integrated solar system is to develop a cost-effective, robust, and bifunctional electrocatalyst by complete water splitting. Herein, we report a superhydrophilic layered leaflike Sn₄P₃ on a graphene-carbon nanotube matrix which shows outstanding electrochemical performance in terms of low overpotential (hydrogen evolution reaction (HER), 62 mV@10 mA/cm², and oxygen evolution reaction (OER), 169 mV@20 mA/cm²). The outstanding stability of HER at least for 15 days at a high applied current density of 400 mA/cm² with a minimum loss of potential (1%) in acid medium infers its potential compatibility toward the industrial sector. Theoretical calculations indicate that the decoration of Sn₄P₃ on carbon nanotubes modulates the electronic structure by creating a higher density of state near Fermi energy. The catalyst also reveals an admirable overall water splitting performance by generating a low cell voltage of 1.482 V@10 mA/cm² with a stability of at least 65 h without obvious degradation of potential in 1 M KOH. It exhibited unassisted solar energy-driven water splitting when coupled with a silicon solar cell by extracting a high stable photocurrent density of 8.89 mA/cm² at least for 90 h with 100% retention that demonstrates a high solar-to-hydrogen conversion efficiency of ∼10.82%. The catalyst unveils a footprint for pure renewable fuel production toward carbon-free future green energy innovation.

In-situ generated NiCo2O4/CoP polyhedron with rich oxygen vacancies interpenetrating by P-doped carbon nanotubes for high performance supercapacitors

Supercapacitor with high storage capacity and small volumes are the development trends of miniaturization and portable energy storage systems. Herein, we design a novel self-supporting P-doped carbon nanotube (P-CNT) intercalating NiCo₂O₄/CoP core-shell polyhedron film. P-CNT is an ideal substrate with high electrical conductivity and interconnected porous architecture, which can enable the electrons transport to an external circuit from the electroactive component. NiCo₂O₄/CoP core-shell fluffy polyhedrons are derived from metal-organic frameworks with rich oxygen vacancies and abundant characteristics of pseudocapacitance, as well as better wettability. The self-supporting composite film readily achieves an ultra-high gravimetric and volumetric capacitance of 1918.4 F g^-1 and 1074.3 F cm^-3 at 1 A g^-1. Accordingly, as-assembled hybrid supercapacitors using two binder-free electrodes, i.e., a self-supporting composite film as the positive electrode and P-doped CNT integrating graphene film as the negative electrode, harvest a remarkable gravimetric/volumetric energy density of 68.6 W h kg^-1 (41.8 W h L^-1) at 800 W kg^-1 (488 W L^-1). Our work suggests that the rational-designed NiCo₂O₄/CoP@P-CNTs electrode is a competitive candidate for designing next-generation supercapacitors with high volumetric energy density.

Droplet Flow Assisted Electrocatalytic Oxidation of Selected Alcohols under Ambient Condition

This study reports using a droplet flow assisted mechanism to enhance the electrocatalytic oxidation of benzyl alcohol, 2-phenoxyethanol, and hydroxymethylfurfural at room temperature. Cobalt phosphide (CoP) was employed as an active electrocatalyst to promote the oxidation of each of the individual substrates. Surface analysis of the CoP electrocatalyst using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), as well as electrochemical characterization, revealed that it had excellent catalytic activity for each of the substrates studied. The combined droplet flow with the continuous flow electrochemical oxidation approach significantly enhanced the conversion and selectivity of the transformation reactions. The results of this investigation show that at an electrolysis potential of 1.3 V and ambient conditions, both the selectivity and yield of aldehyde from substrate conversion can reach 97.0%.

Co2P decorated Co3O4 nanocomposites supported on carbon cloth with enhanced electrochemical performance for asymmetric supercapacitors

An effective strategy is demonstrated to promote electrochemical performance by the combination of Co3O4 with Co2P to form a composite electrode.

Hierarchical flower-like architecture of nickel phosphide anchored with nitrogen-doped carbon quantum dots and cobalt oxide for advanced hybrid supercapacitors

The exploitation of hybrid supercapacitors with excellent electrochemical properties is of great significance for energy storage systems. Herein, a three-dimensional hierarchical flower-like architecture of nickel phosphide (Ni₂P) decorated with nitrogen-doped carbon quantum dots (N-CQDs) and cobalt oxide (Co₃O₄) is constructed by an effective two-step hydrothermal strategy followed by in situ phosphorization process. Introducing N-CQDs with superior electrochemical characteristics can not only induce the formation of N-CQDs deposited nickel hydroxide (Ni(OH)₂) flower-like architecture but also significantly enhance the electrochemical features of Ni(OH)₂ nanosheets. After combination with Co₃O₄ nanoparticles and phosphorization treatment, an advanced cathode of Ni₂P/Co₃O₄/N-CQDs with enriched surface phosphate ions is obtained, which possesses an ultra-high capacity of 1044 C g^-1 (2088 F g^-1) at 1 A g^-1 with a splendid rate capacity of 876 C g^-1 (1752 F g^-1) at 20 A g^-1. Moreover, a device assembled by Ni₂P/Co₃O₄/N-CQDs hierarchical flower-like architecture and p-phenylenediamine functionalized reduced graphene oxide (PPD/rGO) nanosheets depicts a commendable energy density of 53.5 Wh kg^-1 at 772.9 W kg^-1. This work provides a novel hierarchical multi-component electrode material with decent electrochemical capacities for hybrid supercapacitors, which has a broad prospect in energy storage devices.

Towards advanced aqueous zinc battery by exploiting synergistic effects between crystalline phosphide and amorphous phosphate

High-performance aqueous zinc batteries are expected to be realized, rooting from component synergistic effects of the hierarchical composite electrode materials. Herein, hierarchical crystalline Ni-Co phosphide coated with amorphous phosphate nanoarrays (C-NiCoP@A-NiCoPO₄) self-supporting on the Ni foam are constructed as cathode material of an aqueous zinc battery. In this unique core-shell structure, the hexagonal phosphide with high conductivity offers ultra-fast electronic transmission and amorphous phosphate with high stability, and open-framework provides more favorable ion diffusivity and a stable protective barrier. The synergistic effects of this intriguing core-shell structure endow the electrode material with outstanding reaction kinetics and structural stability, which is theoretically confirmed by density functional theory (DFT) calculations. As a result, the C-NiCoP@A-NiCoPO₄ electrode exhibits a higher specific capacity of 350.6 mA h g^-1 and excellent cyclic stability with 92.6% retention after 10 000 cycles. Moreover, the C-NiCoP@A-NiCoPO₄ is coupled with Zn anode to assemble an aqueous pouch battery that delivers ultra-high energy density (626.33 W h kg^-1 at 1.72 kW kg^-1) with extraordinary rate performance (452.05 W h kg^-1 at 33.56 kW kg^-1). Moreover, the corresponding quasi-solid flexible battery with polyacrylamide hydrogel electrolyte exhibits favorable durability under frequent mechanical strains, which indicates the great promise of crystalline/amorphous hierarchical electrodes in the field of energy storage.

Conversion of Amorphous MOF Microspheres into a Nickel Phosphate Battery-Type Electrode Using the "Anticollapse" Two-Step Strategy

Metal-organic frameworks (MOFs) have attracted great attention as templates for preparation of functional porous materials owing to their adjustable structures, rich porosity, and controllable components. However, collapsed templates during the conversion process hinder their application and synthesis of derivatives. In this study, we demonstrate a novel two-step etching strategy during which amorphous MOF microspheres are initially transformed into nickel hydroxide and then subsequently transformed into microspherical nickel phosphates. Through this strategy, the prepared nickel phosphates maintain the microspherical morphology of MOFs but with no MOF residuals, exhibiting ultrahigh specific surface area, uniform pore size, and good structural robustness. Examined as a supercapacitor electrode, they show an outstanding specific capacity of 820 C g^-1 at 0.5 A g^-1 and remarkable cycling stability of 88% capacity retention after 10 000 cycles. Moreover, an asymmetric supercapacitor constructed utilizing reduced graphene cross-linked with p-phenylenediamine oxide (PPD-rGO) as the cathode displays a preeminent energy density of 64.56 Wh kg^-1 at a power density of 507 W kg^-1. This strategy has important significance in guiding the preparation of high-performance MOF-derived electrodes.

Fabrication of Vertical-Standing Co-MOF Nanoarrays with 2D Parallelogram-like Morphology for Aqueous Asymmetric Electrochemical Capacitors

Metal organic frameworks (MOFs) have been considered as one of the most promising electrode materials for electrochemical capacitors due to their large specific surface area and abundant pore structure. Herein, we report a Co-MOF electrode with a vertical-standing 2D parallelogram-like nanoarray structure on a Ni foam substrate via a one-step solvothermal method. The as-prepared Co-MOF on a Ni foam electrode delivered a high area-specific capacitance of 582.0 mC cm⁻² at a current density of 2 mA cm⁻² and a good performance rate of 350.0 mC cm⁻² at 50 mA cm⁻². Moreover, an asymmetric electrochemical capacitor (AEC) device (Co-MOF on Ni foam//AC) was assembled by using the as-prepared Co-MOF on a Ni foam as the cathode and a active carbon-coated Ni foam as the anode to achieve a maximum energy density of 0.082 mW cm⁻² at a power density of 0.8 mW cm⁻², which still maintained 0.065 mW cm⁻² at a high power density of 11.94 mW cm⁻². Meanwhile, our assembled device exhibited an excellent cycling stability with a capacitance retention of nearly 100% after 1000 cycles. Therefore, this work provides a simple method to prepare MOF-based material for the application of energy storage and conversion.

Exploring the electrochemical performance of copper-doped cobalt-manganese phosphates for potential supercapattery applications

The significant electrochemical performance in terms of both specific energy and power delivered via hybrid energy storage devices (supercapattery) has raised their versatile worth but electrodes with flashing electrochemical conduct are still craved for better performance. In this work, binary and ternary metal phosphates based on copper, cobalt, and manganese were synthesized by a sonochemical method. Then, the compositions of copper and cobalt were optimized in ternary metal phosphates. The structural studies and morphological aspects of synthesized materials were scrutinized by X-ray diffraction and scanning electron microscopy. Furthermore, the electrochemical characterizations were performed in three- and two-cell configurations. The sample with equal compositions of copper and cobalt (50/50) demonstrates the highest specific capacity of 340 C g⁻¹ at a current density of 0.5 A g⁻¹ among all. This optimized composition was utilized as a positive electrode material in a supercapattery device that reveals a high specific capacity of 247 C g⁻¹. The real device exhibits an excellent energy density of 55 W h kg⁻¹ while delivering a power density of 800 W kg⁻¹. Furthermore, the device was able to provide an outstanding specific power of 6400 W kg⁻¹ while still exhibiting a specific energy of 19 W h kg⁻¹. The stability potential of the device was tested for 2500 continuous charge and discharge cycles at 8 A g⁻¹. Excellent capacitive retention of 90% was obtained, which expresses outstanding cyclic stability of the real device. A theoretical study was performed to investigate the capacitance and diffusion-controlled contribution in the device performance using Dunn's model. The maximum diffusion-controlled contribution of 85% was found at 3 mV s⁻¹ scan rate. The study demonstrates the utilization of ternary metal phosphates as self-supported electrode materials for potential energy storage applications.

The optimized copper-doped cobalt–manganese phosphate was utilized as a positive electrode in an asymmetric architecture (supercapattery device), which yields enhanced specific energy and power.

Ultrathin and Highly Crumpled/Porous CoP Nanosheet Arrays Anchored on Graphene Boosts the Capacitance and Their Synergistic Effect toward High-Performance Battery-Type Hybrid Supercapacitors

Constructing novel electrode materials with supernal specific capacitance and cycle stability is important for the practical applications of supercapacitors. Herein, ultrathin and highly crumpled CoP/reduced graphene oxide (rGO) nanosheet arrays are grown on nickel foam (NF) through a hydrothermal-phosphidation route. Benefitting from the synergistic effects of CoP with large specific capacity and rGO with high conductivity and ultrathin nanosheet arrays structure, CoP/rGO shows extraordinary electrochemical performance. The CoP/rGO electrode possesses a superior specific capacity of 1438.0 C g^-1 (3595.0 F g^-1) at 1 A g^-1, which is 3.43, 2.05, and 2.26 times larger than those of Co(OH)₂/rGO, Co₃O₄/rGO, and bare CoP. In particular, the CoP/rGO nanosheet arrays show the highest specific capacities among the monometallic phosphide-based nanostructures reported so far. The CoP/rGO retains 1198.9 C g^-1 (2997.2 F g^-1) at 10 A g^-1, revealing the outstanding rate capability of 83%. Theoretical calculations reveal that rGO can adequately reduce the absorption energy of OH^- on CoP, which makes CoP/rGO have strong adsorption capacity of OH^-, resulting in boosting electrochemical performance. A hybrid supercapacitor of CoP/rGO/NF//AC was designed, which presents a superior energy density of 43.2 Wh kg^-1 at a power density of 1010.5 W kg^-1. After 10 000 cycles, the CoP/rGO/NF//AC supercapacitor reveals excellent cycling durability with a capacitance retention of 89%. This work provides a new insight into the design of high-performance electrode materials by combining high capacitive metal phosphides with conductive carbon, which is of great significance for energy storage systems.

Phosphate-modified Co-Ni phosphide heterostructure formed by interfacial and electronic tuning for boosted faradaic properties

Rational structural and compositional modulation endows electrode materials with unique physicochemical characteristics due to their adjustable electronic properties. Herein, a phosphate-modified hierarchical nanoarray consisting of a heterojunction with a well-aligned cobalt phosphide nanowire core and nickel phosphide nanosheet shell on flexible carbon cloth (denoted as CoP@Ni₂P-CC) is engineered. The phosphate-modulated heterogeneous phosphide with a tuned electronic structure, additional heterojunction interfaces, and high degree of covalency in the chemical bonds accelerates the reaction kinetics and enhances the energy storage performance. Due to these reasons, the as-obtained phosphide-based heterostructured CoP@Ni₂P-CC electrode delivers a capacity of 475.9 C g^-1 at 0.5 A g^-1 with a satisfying rate capability, which is greatly superior to that of its transition metal counterparts (sulfide, selenide, and oxide). After being assembled into a flexible hybrid supercapacitor (FHSC), a wide operating voltage (1.8 V), high energy/power densities (49.8 W h kg^-1/8.6 kW kg^-1), and long-term stability (85.1% capacity retention after 10 000 cycles) were achieved. This work may provide a general method from multiple strategies for designing reliable pseudocapacitive materials for flexible electronics.

Sulfur and phosphorus co-doped nickel-cobalt layered double hydroxides for enhancing electrochemical reactivity and supercapacitor performance

Layered double hydroxides (LDHs) have drawn significant interest as emerging active materials for advanced energy storage devices; however, their low electric and ionic conductivity limit their applications. In this study, we report sulfur (S) and phosphorus (P) co-doped NiCo LDH nanoarrays prepared via a facile phosphor-sulfurization process to impart diverse co-doping effects. Combining the benefits of their unique hierarchical structure and reduced charge transfer resistance, the S and P co-doped NiCo LDH (NiCo LDH-SP) nanoarrays realize faster and more efficient redox reactions and achieve enhanced surface reactivity, thereby resulting in a performance superior to that of pristine NiCo LDH. Therefore, a NiCo LDH-SP shows an ultra-high specific capacitance of 3844.8 F g-1 at a current density of 3 A g-1 and maintains a specific capacitance of 2538.8 F g-1 at a high current density of 20 A g-1. Additionally, an asymmetric supercapacitor, assembled with the NiCo LDH-SP as the cathode and activated carbon (AC) as the anode (NiCo LDH-SP//AC), shows a high energy density of 74.5 W h kg-1 at a power density of 0.8 kW kg-1 and outstanding cycling stability, thereby retaining ∼81.3% of its initial specific capacitance after 5000 cycles. This study presents a facile and promising strategy for developing LDH-based electrode materials with excellent electrochemical performance for advanced energy storage applications.

Super-Hydrophilic Hierarchical Ni-Foam-Graphene-Carbon Nanotubes-Ni2P-CuP2 Nano-Architecture as Efficient Electrocatalyst for Overall Water Splitting

Water splitting via an electrochemical process to generate hydrogen is an economic and green approach to resolve the looming energy and environmental crisis. The rational design of multicomponent materials with seamless interfaces having robust stability, facile scalability, and low-cost electrocatalysts is a grand challenge to produce hydrogen by water electrolysis. Herein, we report a superhydrophilic homogeneous bimetallic phosphide of Ni₂P-CuP₂ on Ni-foam-graphene-carbon nanotubes (CNTs) heterostructure using facile electrochemical metallization followed by phosphorization without any intervention of metal-oxides/hydroxides. This bimetallic phosphide shows ultralow overpotentials of 12 (HER, hydrogen evolution reaction) and 140 mV (OER, oxygen evolution reaction) at current densities of 10 and 20 mA/cm² in acidic and alkaline mediums, respectively. The excellent stability lasts for at least for 10 days at a high current density of 500 mA/cm² without much deviation, inferring the practical utilization of the catalyst toward green fuel production. Undoubtedly, the catalyst is capable enough for overall water splitting at a very low cell voltage of 1.45 V @10 mA/cm² with an impressive stability of at least 40 h, showing a minimum loss of potential. Theoretical study has been performed to understand the reaction kinetics and d-band shifting among metal atoms in the heterostructure (Ni₂P-CuP₂) that favor the HER and OER activities, respectively. In addition, the catalyst demonstrates an alternate transformation of solar energy to green H₂ production using a standard silicon solar cell. This work unveils a smart design and synthesizes a highly stable electrocatalyst against an attractive paradigm of commercial water electrolysis for renewable electrochemical energy conversion.