Cobalt-Doped Nickel Phosphite for High Performance of Electrochemical Energy Storage

Critical Bimetallic Phosphide Layer Enables Fast Electron Transfer and Extra Energy Supply for Flexible Quasi-Solid-State Zinc Batteries

Nickel-based cathodes in aqueous nickel-zinc batteries typically suffer from sluggish reaction kinetics and limited energy density. In situ introduction of metal phosphides and rational construction of heterostructures can effectively promote electron/ion transport. However, the complex evolution of phosphidation and intractable phosphidizing degree greatly affect the composition of active phase, active sites, charge transfer rate, and ion adsorption strength of cathodes. Herein, the critical bimetallic phosphide layer (CBPL) is constructed on the NiCo-layered double hydroxide (NiCo-LDH) skeleton by a controllable anion-exchange strategy, yielding a novel nanohybrid cathode (NiCo-P1.0, 1.0 representing the mass ratio of Na2H2PO2 to NiCo-LDH). The high-conductivity CBPL with the inner NiCo-LDH forms extensive heterostructures, effectively regulating the electronic structure via charge transfer, thereby improving electrical conductivity. Remarkably, the CBPL exhibits unexpected electrochemical activity and synergizes with NiCo-LDH for electrode reactions, ultimately delivering extra energy. Benefiting from the bifunctional CBPL, NiCo-P1.0 delivers an optimal capacity of 286.64 mAh g-1 at 1C (1C = 289 mAh g-1) and superb rate performance (a capacity retention of 72.22% at 40C). The assembled NiCo-P1.0//Zn battery achieves ultrahigh energy/power density (503.62 Wh kg-1/18.62 kW kg-1, based on the mass loading of active material on the cathode), and the flexible quasi-solid-state pouch cell validates its practicality. This work demonstrates the superiority of bifunctional CBPL for surface modification, providing an effective and scalable compositing strategy in achieving high-performance cathodes for aqueous batteries.

Designing transition metal-based porous architectures for supercapacitor electrodes: a review

Over the past decade, transition metal (TM)-based electrodes have shown intriguing physicochemical properties and widespread applications, especially in the field of supercapacitor energy storage owing to their diverse configurations, composition, porosity, and redox reactions. As one of the most intriguing research interests, the design of porous architectures in TM-based electrode materials has been demonstrated to facilitate ion/electron transport, modulate their electronic structure, diminish strain relaxation, and realize synergistic effects of multi-metals. Herein, the recent advances in porous TM-based electrodes are summarized, focusing on their typical synthesis strategies, including template-mediated assembly, thermal decomposition strategy, chemical deposition strategy, and host-guest hybridization strategy. Simultaneously, the corresponding conversion mechanism of each synthesis strategy are reviewed, and the merits and demerits of each strategy in building porous architectures are also discussed. Subsequently, TM-based electrode materials are categorized into TM oxides, TM hydroxides, TM sulfides, TM phosphides, TM carbides, and other TM species with a detailed review of their crystalline phase, electronic structure, and microstructure evolution to tune their electrochemical energy storage capacity. Finally, the challenges and prospects of porous TM-based electrode materials are presented to guide the future development in this field.

Ultra-Low Pt Doping and Pt-Ni Pair Sites in Amorphous/Crystalline Interfacial Electrocatalyst Enable Efficient Alkaline Hydrogen Evolution

Noble metal doping can achieve an increase in mass activity (MA) without sacrificing catalysis efficiency and stability, so that alkaline hydrogen evolution reaction (HER) performance of the catalyst can be optimized to the maximum degree. However, its excessively large ionic radius makes it difficult to achieve either interstitial doping or substitutional doping under mild conditions. Herein, a hierarchical nanostructured electrocatalyst with enriched amorphous/crystalline interfaces for high-efficiency alkaline HER is reported, which is composed of amorphous/crystalline (Co, Ni)₁₁ (HPO₃ )₈ (OH)₆ homogeneous hierarchical structure with an ultra-low doped Pt (Pt-a/c-NiHPi). Benefiting from the structural flexibility of the amorphous component, extremely low Pt (0.21 wt.%, totally 3.31 µg Pt on 1 cm^-2 NF) are stably doped on it via a simple two-phase hydrothermal method. The DFT calculations show that due to the strongly electron transfer between the crystalline/amorphous components at the interfaces, electrons finally concentrate toward Pt and Ni in the amorphous components, thus the electrocatalyst has near-optimal energy barriers and adsorption energy for H₂ O^* and H^* . With the above benefits, the obtained catalyst exhibits an exceptionally high MA (39.1 mA µg^-1 _Pt ) at 70 mV, which is almost the highest level among the reported Pt-based electrocatalysts for alkaline HER.

Direct Electron Transfer Coordinated by Oxygen Vacancies Boosts Selective Nitrate Reduction to N2 on a Co-CuOx Electroactive Filter

Atomic hydrogen (H*) is used as an important mediator for electrochemical nitrate reduction; however, the Faradaic efficiency (FE) and selective reduction to N₂ are likely compromised due to the side reactions (e.g., ammonia generation and hydrogen evolution reactions). This work reports a Co-CuO_x electrochemical filter with CoO_x nanoclusters rooted on vertically aligned CuO_x nanowalls for selective nitrate reduction to N₂, utilizing the direct electron transfer between oxygen vacancies and nitrate to suppress the contribution by H*. At a cathodic potential of -1.1 V (vs Ag/AgCl), the Co-CuO_x filter showed 95.2% nitrate removal and 96.0% N₂ selectivity at an influent nitrate concentration of 20 N-mg L^-1. Meanwhile, the energy consumption and FE were 0.60 kW h m^-3 and 53.5%, respectively, at a permeate flux of 60 L m^-2 h^-1. The presence of abundant oxygen vacancies on Co-CuO_x was due to the change in the electron density of the Cu atom and a decrease of the coordination numbers of Cu-O via cobalt doping. Theoretical calculations and electrochemical tests showed that the oxygen vacancies coordinated nitrate adsorption and subsequent reduction reactions, thus suppressing the contribution of H* to nitrate reduction and leading to a thermodynamically favorable process to N₂ via direct electron transfer.

Atomic scaled modulation strategies and crystal phase transition of flower-like CoAl layered double hydroxide for supercapacitor

In the present work, cobalt aluminum layered double hydroxide and its derivatives were synthesized via hydrothermal method, alkali-etching method and phosphorization route to obtain different crystal phases. The structure, morphology...

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.

Metal-organic frameworks-derived hollow dodecahedral carbon combined with FeNx moieties and ruthenium nanoparticles as cathode electrocatalyst for lithium oxygen batteries

Owing to their high energy density, lithium-oxygen batteries (LOBs) have been drawn great attention as one of the promising electrochemical energy sources. However, the sluggish kinetics of oxygen reduction/evolution reaction (ORR/OER) hamper the widespread application of LOBs. Herein, an elaborate designed catalysts which are constructed by FeN_x moieties dispersed on the network-like hollow dodecahedral carbon and then decorated with Ru nanoparticles (FeN_x-HDC@Ru). Since the homogeneously dispersed FeN_x moieties could promote ORR performance, and the Ru nanoparticles could facilitate OER capability, the FeN_x-HDC@Ru nanocomposites used as cathode catalysts can significantly improve LOBs performance. A lower discharge and charge overpotentials of 0.15 V and 0.78 V can be detected in the first cycle, respectively, and an excellent cycle performance of 90 cycles at 200 mA g^-1 and 89 cycles at 500 mA g^-1 can be demonstrated. Herein, the charge transfer kinetics has been enhanced with the internal network-like hollow structure and a low impedance Li₂O₂/catalysts contact interface could be earned by the constructed Ru nanoparticles, these factors would lead to an efficient acceleration to the formation and decomposition of Li₂O₂ during discharge and charge process.

Recent Progress in Two-Dimensional Layered Double Hydroxides and Their Derivatives for Supercapacitors

High-performance supercapacitors have attracted great attention due to their high power, fast charging/discharging, long lifetime, and high safety. However, the generally low energy density and overall device performance of supercapacitors limit their applications. In recent years, the design of rational electrode materials has proven to be an effective pathway to improve the capacitive performances of supercapacitors. Layered double hydroxides (LDHs), have shown great potential in new-generation supercapacitors, due to their unique two-dimensional layered structures with a high surface area and tunable composition of the host layers and intercalation species. Herein, recent progress in LDH-based, LDH-derived, and composite-type electrode materials targeted for applications in supercapacitors, by tuning the chemical/metal composition, growth morphology, architectures, and device integration, is reviewed. The complicated relationships between the composition, morphology, structure, and capacitive performance are presented. A brief projection is given for the challenges and perspectives of LDHs for energy research.

Laser-Ablation-Produced Cobalt Nickel Phosphate with High-Valence Nickel Ions as an Active Catalyst for the Oxygen Evolution Reaction

Cost-effective, highly efficient and stable non-noble metal-based catalysts for the oxygen evolution reaction (OER) are very crucial for energy storage and conversion. Here, an amorphous cobalt nickel phosphate (CoNiPO₄ ), containing a considerable amount of high-valence Ni³⁺ species as an efficient electrocatalyst for OER in alkaline solution, is reported. The catalyst was converted from Co-doped Ni₂ P through pulsed laser ablation in liquid (PLAL) and exhibits a large specific surface area of 162.5 m²  g^-1 and a low overpotential of 238 mV at 10 mA cm^-2 with a Tafel slope of 46 mV dec^-1 , which is much lower than those of commercial RuO₂ and IrO₂ . This work demonstrates that PLAL is a powerful technology for generating amorphous CoNiPO₄ with high-valence Ni³⁺ , thus paving a new way towards highly effective OER catalysts.

Asymmetric supercapacitors with excellent rate performance by integrating Co(OH)F nanorods and layered Ti3C2Tx paper

Here we describe an aqueous asymmetric supercapacitor assembled using Co(OH)F nanorods on Ni foam (Co(OH)F@NF) as the positive electrode and layered Ti₃C₂T_x paper on Ni foam (Ti₃C₂T_x@NF) as the negative electrode. The Co(OH)F@NF was fabricated by a facile hydrothermal process. The nanorods were constructed as three-dimensional networks with stable structures and good durability in electrochemical reactions. As expected, the Co(OH)F@NF exhibited excellent capacitance (1265 mF cm⁻² at 1 mA cm⁻²) and wonderful rate performance (90% from 1 to 10 mA cm⁻²). The Co(OH)F@NF//Ti₃C₂T_x@NF asymmetric supercapacitor devices showed high capacitance of 376 mF cm⁻² and excellent rate capacity of 91% retention. When the two Co(OH)F@NF//Ti₃C₂T_x@NF devices were combined in series, a red led bulb could be lit for 12 minutes, further corroborating the possibility of the devices being used for practical applications.

Here we describe an aqueous asymmetric supercapacitor assembled using Co(OH)F nanorods on Ni foam (Co(OH)F@NF) as the positive electrode and layered Ti₃C₂T_x paper on Ni foam (Ti₃C₂T_x@NF) as the negative electrode.

Facile synthesis of Ni11(HPO3)8(OH)6/rGO nanorods with enhanced electrochemical performance for aluminum-ion batteries

The electrochemical behaviors of the ultrashort nickel phosphite nanorods supported on reduced graphene oxide (Ni₁₁(HPO₃)₈(OH)₆/rGO nanorods), as a candidate for cathodic applications in aluminum-ion batteries, are firstly investigated. Ni₁₁(HPO₃)₈(OH)₆/rGO nanorods are synthesized by a facile solvothermal process. Ni₁₁(HPO₃)₈(OH)₆ and Ni₁₁(HPO₃)₈(OH)₆/rGO cathodes both possess very high initial discharge capacities of 132.4 and 182.0 mA h g^-1 at a current density of 200 mA g^-1, respectively. In addition, the long-term cycling stability of the Ni₁₁(HPO₃)₈(OH)₆/rGO cathode is further evaluated, exhibiting a discharge capacity of 49.2 mA h g^-1 even over 1500 cycles. More importantly, the redox reaction mechanism of the Ni₁₁(HPO₃)₈(OH)₆ cathode for aluminum-ion batteries revealed that Ni₁₁(HPO₃)₈(OH)₆ is partially substituted with Al³⁺ to form Al_mNi_n(HPO₃)₈(OH)₆ and metallic Ni in the nanorod-like Ni₁₁(HPO₃)₈(OH)₆ cathodes during the discharge process. These findings are of great significance for the further development of novel materials for aluminum-ion batteries.

Ni-Doped Cobalt Phosphite, Co11(HPO3)8(OH)6, with Different Morphologies Grown on Ni Foam Hydro(solvo)thermally for High-Performance Supercapacitor

Ni-doped Co₁₁(HPO₃)₈(OH)₆ with different morphologies was directly grown on Ni foam hydro(solvo)thermally under different synthetic conditions. The optimum condition is solvothermal reaction for 6 h in an ethanol/water (EW) mixed solution, the molar ratio of NaH₂PO₂/Co(NO₃)₂ being 0.5:0.1, and the obtained S0.5-6 h-EW shows three-dimensional (3D) porous nanowire bundles. Whereas in the water-only solution, microrods are obtained, suggesting that the nanowires in bundles are aggregated together via the lateral (400) direction. Long reaction time and low molar ratio of reactants are all beneficial for the lateral growth of the nanowires, and the possible formation mechanism is proposed. All the obtained Ni-doped Co₁₁(HPO₃)₈(OH)₆/Ni foam samples are directly used as supercapacitor electrodes, and S0.5-6 h-EW shows the best electrochemical performance with a specific capacity of 159 mAh g^-1 at 0.5 A g^-1, which is close to the theoretical value of 212 mAh g^-1 for Co₁₁(HPO₃)₈(OH)₆, and it is the largest reported value so far. The excellent capacitive behavior of S0.5-6 h-EW is ascribed to the 3D porous nanowire bundles directly grown on a Ni foam collector without an additive and a binder, as well as to the doping of Ni into the cobalt phosphite. The S0.5-6 h-EW//activated carbon asymmetrical supercapacitor shows a maximum energy density of 58.7 Wh kg^-1 at a power density of 532 W kg^-1 and good cycling stability with the capacity retention of 90.5% after 10 000 charging-discharging cycles at 5.5 A g^-1.