Coralloid Co2P2O7 Nanocrystals Encapsulated by Thin Carbon Shells for Enhanced Electrochemical Water Oxidation

Hierarchical Porous Carbon Supported Co2P2O7 Nanoparticles for Oxygen Evolution and Oxygen Reduction in a Rechargeable Zn-Air Battery

The oxygen reduction/evolution reaction (ORR/OER) represents a pivotal process in metal-air batteries; however, it is constrained by the limitations of slow kinetics. Nevertheless, the creation of long-lasting and bifunctional catalysts represents a significant challenge. This study presents a series of hierarchical porous carbon-supported cobalt pyrophosphate (Co2P2O7-N/C-T) catalysts, prepared through the pyrolysis of porphyrin-based NTU-70 nanosheets with red phosphorus at varying temperatures. The Co2P2O7-N/C-800 not only demonstrates remarkable OER performance with an overpotential of only 290 mV at a current density of 10 mA cm-2 in 1 M KOH, but also exhibits an excellent ΔE of 0.74 V in 0.1 M KOH, which is lower than that of Pt/C + RuO2 (0.76 V). The utilization of Co2P2O7-N/C-800 as an air cathode in a rechargeable Zn-air battery (ZAB) results in a stable discharge voltage plateau of 1.405 V and a high gravimetric energy density of 801.2 mA h gZn-1. This work presents a promising strategy for the design of efficient bifunctional catalysts and demonstrates the critical importance of the interplay between the active center and the supported hierarchical porous carbon.

Porous Flower-Like Nanoarchitectures Derived from Nickel Phosphide Nanocrystals Anchored on Amorphous Vanadium Phosphate Nanosheet Nanohybrids for Superior Overall Water Splitting

Transition metal phosphides (TMPs) and phosphates (TM-Pis) nanostructures are promising functional materials for energy storage and conversion. Nonetheless, controllable synthesis of crystalline/amorphous heterogeneous TMPs/TM-Pis nanohybrids or related nanoarchitectures remains challenging, and their electrocatalytic applications toward overall water splitting (OWS) are not fully explored. Herein, the Ni2P nanocrystals anchored on amorphous V-Pi nanosheet based porous flower-like nanohybrid architectures that are self-supported on carbon cloth (CC) substrate (Ni2P/V-Pi/CC) are fabricated by conformal oxidation and phosphorization of pre-synthesized NiV-LDH/CC. Due to the unique microstructures and strong synergistic effects of crystalline Ni2P and amorphous V-Pi components, the obtained Ni2P/V-Pi/CC owns abundant active sites, suitable surface/interface electronic structure and optimized adsorption-desorption of reaction intermediates, resulting in outstanding electrocatalytic performances toward hydrogen and oxygen evolution reactions in alkaline media. Correspondingly, the assembled Ni2P/V-Pi/CC||Ni2P/V-Pi/CC electrolyzer only needs an ultralow cell voltage (1.44 V) to deliver 10 mA cm-2 water-splitting currents, exceeding its counterparts, recently reported bifunctional catalysts-based devices, and Pt/C/CC||IrO2/CC pairs. Moreover, the Ni2P/V-Pi/CC||Ni2P/V-Pi/CC manifests remarkable stability. Also, such device shows a certain prospect for OWS in acidic media. This work may spur the development of TMPs/TMPis-based nanohybrid architectures by combining structure and phase engineering, and push their applications in OWS or other clean energy options.

Optimum iron-pyrophosphate electronic coupling to improve electrochemical water splitting and charge storage

Tuning the electronic properties of transition metals using pyrophosphate (P2O7) ligand moieties can be a promising approach to improving the electrochemical performance of water electrolyzers and supercapacitors, although such a material's configuration is rarely exposed. Herein, we grow NiP2O7, CoP2O7, and FeP2O7 nanoparticles on conductive Ni-foam using a hydrothermal procedure. The results indicated that, among all the prepared samples, FeP2O7 exhibited outstanding oxygen evolution reaction and hydrogen evolution reaction with the least overpotential of 220 and 241 mV to draw a current density of 10 mA/cm2. Theoretical studies indicate that the optimal electronic coupling of the Fe site with pyrophosphate enhances the overall electronic properties of FeP2O7, thereby enhancing its electrochemical performance in water splitting. Further investigation of these materials found that NiP2O7 had the highest specific capacitance and remarkable cycle stability due to its high crystallinity as compared to FeP2O7, having a higher percentage composition of Ni on the Ni-foam, which allows more Ni to convert into its oxidation states and come back to its original oxidation state during supercapacitor testing. This work shows how to use pyrophosphate moieties to fabricate non-noble metal-based electrode materials to achieve good performance in electrocatalytic splitting water and supercapacitors.

Direct OC-CHO coupling towards highly C2+ products selective electroreduction over stable Cu0/Cu2+ interface

Electroreduction of CO2 to valuable multicarbon (C2+) products is a highly attractive way to utilize and divert emitted CO2. However, a major fraction of C2+ selectivity is confined to less than 90% by the difficulty of coupling C-C bonds efficiently. Herein, we identify the stable Cu0/Cu2+ interfaces derived from copper phosphate-based (CuPO) electrocatalysts, which can facilitate C2+ production with a low-energy pathway of OC-CHO coupling verified by in situ spectra studies and theoretical calculations. The CuPO precatalyst shows a high Faradaic efficiency (FE) of 69.7% towards C2H4 in an H-cell, and exhibits a significant FEC2+ of 90.9% under industrially relevant current density (j = -350 mA cm-2) in a flow cell configuration. The stable Cu0/Cu2+ interface breaks new ground for the structural design of electrocatalysts and the construction of synergistic active sites to improve the activity and selectivity of valuable C2+ products.

Surface Adsorption of Amorphous Phosphate on RuNi-Doped Molybdate for the Hydrogen Evolution Reaction

Developing highly active and cost-effective electrocatalysts is critical for enhancing the intrinsic performance of electrocatalytic water splitting. Oxoanion-based compounds, such as phosphates and molybdates, have emerged as promising electrocatalysts owing to their advantageous properties of nontoxicity, low price, and strong water adsorption ability. However, their relatively inferior activity has impeded extensive investigation into electrochemical applications. Herein, an amorphous phosphate-adsorbed and RuNi-doped molybdate (RuNiMo-P) composite is synthesized on nickel foam (NF) support by using a simple two-step method. Significantly, an acidic solution of phosphomolybdic acid (PMo12), containing a low concentration of Ru, can etch the NF, contributing to the in situ growth of the RuNi-doped molybdate precursor. Subsequent phosphating ensures the surface formation of the amorphous phosphate layer due to abundant oxygen in the precursor. The strong structural interaction between RuNi-doped molybdate and amorphous phosphate in RuNiMo-P prompts an enhanced hydrogen evolution reaction (HER) performance, delivering an overpotential of 38 mV at a current density of -10 mA cm-2, a Tafel slope of 53 mV dec-1, and good stability in an alkaline medium. Characterizations after HER reveal that RuNi doping, partial dissolution of phosphate and molybdate species, and newly formed NiOOH nanosheets can expose active sites, facilitate charge transfer, and modify electronic structures, thereby improving the HER performance effectively.

Amorphous and defective Co-P-O@NC ball-in-ball hollow structure for highly efficient electrocatalytic overall water splitting

Electrochemical water splitting using hollow and defect-rich catalysts has emerged as a promising strategy for efficient hydrogen production. However, the rational design and controllable synthesis of such catalysts with intricate morphology and composition present significant challenges. Herein, we propose a template-engaged approach to fabricate a novel ball-in-ball hollow structure of Co-P-O@N-doped carbon with abundant oxygen vacancies. The synthesis process involves the preparation of uniform cobalt-glycerate (Co-gly) polymer microspheres as precursors, followed by surface coating with ZIF-67 layer, adjustable chemical etching by phytic acid, and controllable pyrolysis at high temperature. The resulting ball-in-ball structure offers a large number of accessible active sites and high redox reaction centers, facilitating efficient charge transport, mass transfer, and gas evolution, which are beneficial for the acceleration of electrocatalytic reaction. Additionally, density functional theory (DFT) calculations indicate that the incorporation of oxygen and the presence of Co-P dangling bonds in CoP significantly enhance the adsorption of oxygenated species, leading to improved intrinsic electroactivity at the single-site level. As a sequence, the titled catalyst exhibits remarkable electrocatalytic activity and stability for water splitting in alkaline media. Notably, it only requires a low overpotential of 283 mV to achieve a current density of 10 mA cm^-2 for the oxygen evolution reaction. This work may provide some new insights into the design of complex hollow structures of phosphides with abundant defects for energy conversion.

Regulating the coordination capacity of ATMP using melamine: facile synthesis of cobalt phosphides as bifunctional electrocatalysts for the ORR and HER

Due to the complexity of the synthetic process of cobalt phosphides (CoP), ongoing efforts concentrate on simplifying the preparation process of CoP. In this work, amino tris(methylene phosphonic acid) (ATMP, L1) and melamine (MA, L2) are assembled into two-dimensional (2D) organic nanostructures by hydrogen bonding and ionic interactions via a supramolecular assembly, which greatly weakens the coordination ability of L1 with Co²⁺. As the introduced L2 is rich in carbon and nitrogen, it allows the cobalt-organophosphate complex to be placed under a strongly reducing atmosphere during the high-temperature calcination process to achieve an in situ phosphating purpose. The resulting catalyst (N-CoP/NC) exhibits the sought-after enhanced oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) performance. For the ORR in 0.1 M KOH, an enhanced onset potential (0.908 V vs. RHE) and diffusion limiting current (6.280 mA cm^-2) can be obtained, which is comparable to those of 20% Pt/C (0.911 V vs. RHE, 5.380 mA cm^-2). For the HER in 0.5 M H₂SO₄, an overpotential of 150 mV is required to drive a current of 10 mA cm^-2. Moreover, Density Functional Theory (DFT) calculations reveal the catalytic pathway of N-CoP/NC.

Supramolecular Polymer Intertwined Free-Standing Bifunctional Membrane Catalysts for All-Temperature Flexible Zn-Air Batteries

Rational construction of flexible free-standing electrocatalysts featuring long-lasting durability, high efficiency, and wide temperature tolerance under harsh practical operations are fundamentally significant for commercial zinc-air batteries. Here, 3D flexible free-standing bifunctional membrane electrocatalysts composed of covalently cross-linked supramolecular polymer networks with nitrogen-deficient carbon nitride nanotubes are fabricated (referred to as PEMAC@NDCN) by a facile self-templated approach. PEMAC@NDCN demonstrates the lowest reversible oxygen bifunctional activity of 0.61 V with exceptional long-lasting durability, which outperforms those of commercial Pt/C and RuO₂. Theoretical calculations and control experiments reveal the boosted electron transfer, electrolyte mass/ion transports, and abundant active surface site preferences. Moreover, the constructed alkaline Zn-air battery with PEMAC@NDCN air-cathode reveals superb power density, capacity, and discharge-charge cycling stability (over 2160 cycles) compared to the reference Pt/C + RuO₂. Solid-state Zn-air batteries enable a high power density of 211 mW cm^-2, energy density of 1056 Wh kg^-1, stable charge-discharge cycling of 2580 cycles for 50 mA cm^-2, and wide temperature tolerance from - 40 to 70 °C with retention of 86% capacity compared to room-temperature counterparts, illustrating prospects over harsh operations.

Supramolecular Gel-Derived Highly Efficient Bifunctional Catalysts for Omnidirectionally Stretchable Zn-Air Batteries with Extreme Environmental Adaptability

Most existing stretchable batteries can generally only be stretched uniaxially and suffer from poor mechanical and electrochemical robustness to withstand extreme mechanical and environmental challenges. A highly efficient bifunctional electrocatalyst is herein developed via the unique self-templated conversion of a guanosine-based supramolecular hydrogel and presents a fully integrated design strategy to successfully fabricate an omnidirectionally stretchable and extremely environment-adaptable Zn-air battery (ZAB) through the synergistic engineering of active materials and device architecture. The electrocatalyst demonstrates a very low reversible overpotential of only 0.68 V for oxygen reduction/evolution reactions (ORR/OER). This ZAB exhibits superior omnidirectional stretchability with a full-cell areal strain of >1000% and excellent durability, withstanding more than 10 000 stretching cycles. Promisingly, without any additional pre-treatment, the ZAB exhibits outstanding ultra-low temperature tolerance (down to -60 °C) and superior waterproofness, withstanding continuous water rinsing (>5 h) and immersion (>3 h). The present work offers a promising strategy for the design of omnidirectionally stretchable and high-performance energy storage devices for future on-skin wearable applications.

Defect-rich cobalt pyrophosphate hybrids decorated Cd0.5Zn0.5S for efficient photocatalytic hydrogen evolution: Defect and interface engineering

Photocatalysts with highly efficient charge separation are of critical significance for improving photocatalytic hydrogen production performance. Herein, a cost-effective and high-performance composite photocatalyst, cobalt-phosphonate-derived defect-rich cobalt pyrophosphate hybrids (CoPPi-M) modified Cd_0.5Zn_0.5S is rationally devised via defect and interface engineering, in which the co-catalyst CoPPi-M delivers a strong interaction with host photocatalyst Cd_0.5Zn_0.5S, rendering Cd_0.5Zn_0.5S/CoPPi-M with a remarkably improved efficiency of charge separation and migration. Besides, Cd_0.5Zn_0.5S/CoPPi-M exhibits a hydrophilic surface with ample access to electrons and a strong reduction ability of electrons. Benefiting from these advantages, the integration of defect-rich cobalt pyrophosphate and Cd_0.5Zn_0.5S enables Cd_0.5Zn_0.5S/CoPPi-M-5% with high photocatalytic H₂ production rate of 6.87 mmol g^-1h^-1, which is 2.46 times higher than that of pristine Cd_0.5Zn_0.5S, and the notable apparent quantum efficiency (AQE) is 20.7% at 420 nm. This work provides a promising route for promoting the photocatalytic performance of non-precious hybrid photocatalyst via defect and interface engineering, and advances energy-generation and environment-restoration devices.

Phytic Acid-Based FeCo Bimetallic Metal-Organic Gels for Electrocatalytic Oxygen Evolution Reaction

Electrocatalysts have been developed to improve the efficiency of gas release for oxygen evolution reaction (OER), and finding a simple and efficient method for efficient electrocatalysts has inspired research enthusiasm. Herein, we report bimetallic metal-organic gels derived from phytic acid (PA) and mixed transition metal ions to explore their performance in electrocatalytic oxygen evolution reaction. PA is a natural phosphorus-rich organic compound, which can be obtained from plant seeds and grains. PA reacts with bimetallic ions (Fe³⁺ and Co²⁺ ) in a facile one-pot synthesis under mild conditions to form PA-FeCo bimetallic gels, and the corresponding aerogels are further partially reduced with NaBH₄ to improve the electrocatalytic activity. Mixed valence states of Fe(II)/Fe(III) and Co(III)/Co(II) are present in the materials. Excellent OER performance in terms of overpotential (257 mV at 20 mA cm^-2 ) and Tafel slope (36 mV dec^-1 ) is achieved in an alkaline electrolyte. This reduction method is superior to the pyrolysis method by well maintaining the gel morphology structure. This strategy is conducive to the further improvement of the performance of metal-organic electrocatalysts, and provides guidance for the subsequent application of metal-organic gel electrocatalysts.

The Efficient K Ion Storage of M2 P2 O7 /C (M=Fe, Co, Ni) Anode Derived from Organic-Inorganic Phosphate Precursors

Metal phosphates have been widely explored in lithium ion batteries and sodium ion batteries owing to high theoretical capacities, mild toxicity and low cost. However, their potassium ion battery applications are less reported due to the limited conductivity and the slow diffusion kinetics. Considering these drawbacks, novel structured M₂ P₂ O₇ /C (M=Fe, Co, Ni) nanoflake composites are prepared through an organic-phosphors precursor-assisted solvothermal method and a subsequent high temperature annealing process. The designed Co₂ P₂ O₇ /C composite exhibits the highest rate capacity with 502 mAh g^-1 at 0.1 A g^-1 and good cyclability for 900 cycles at 1 A g^-1 and 2 A g^-1 when compared with Ni and Fe based composites. The superior electrochemical performance can be attributed to their unique nanoparticle-assembled nanoflake structure, which can afford enough active sites for K⁺ intercalation. In addition, the robust pyrophosphate crystal structure and the in situ formed carbon composition also have positive effects on enhancing the long-term cycling performance and the electrode's conductivity. Finally, this organic-phosphors precursor induced simple approach can be applied for easy fabrication of other pyrophosphate/carbon hybrids as advanced electrodes.

Structural design of cobalt phosphate on nickel foam for electrocatalytic oxygen evolution

The elaborate design and synthesis of low-cost, efficient and stable electrocatalysts for the oxygen evolution reaction (OER), which may alleviate the current energy shortage and environment pollution, is still a great challenge. Herein, metal phosphonate precursors with controllable morphologies were synthesizedin situon the surface of nickel foam with different solvents, and could be easily converted into carbon- and nitrogen-doped cobalt phosphate through a calcination method. The OER catalytic performance of the final products was studied in detail. The results showed that the nanowire shaped samples of CoPiNF-800 synthesized with deionized water under hydrothermal conditions had the strongest electrochemical performance. They exhibited extraordinary catalytic activity with a very low overpotential of 222 mV at 100 mA cm^-2, the smallest impedance and excellent electrochemical stability. These results not only demonstrate the possibility of preparing low-cost OER catalysts based on transition metal phosphate, but also aid our understanding of the controllable synthesis process of different morphologies.

In situ fabrication of a rose-shaped Co2P2O7/C nanohybrid via a coordination polymer template for supercapacitor application

Using a coordination polymer as the template, the porous rose-shaped Co₂P₂O₇/C-X were fabricated by in situ hybrid Co₂P₂O₇ nanoparticles and nanocarbon for supercapacitor applications.

Nitrogen-doped carbon integrated nickel–cobalt metal phosphide marigold flowers as a high capacity electrode for hybrid supercapacitors

The fabrication of advanced MOF-derived multicomponent NiCoP/NC marigold flowers electrode for high-performance hybrid supercapacitors.

Interfacial engineering of Mo2C-Mo3C2 heteronanowires for high performance hydrogen evolution reactions

Non-precious metal-based electrocatalysts with high activity and stability for efficient hydrogen evolution reactions are of critical importance for low-cost and large-scale water splitting. In this work, Mo₂C-Mo₃C₂ heteronanowires with significantly enhanced catalytic performance are constructed from an MoAn precursor via an accurate phase transition process. The structure disordering and surface carbon shell of Mo₂C-Mo₃C₂ heteronanowires can be precisely regulated, resulting in an enlarged surface area and a defect-rich catalytic surface. Density functional theory calculations are used to identify the effect of the defective sites and carbon shell on the free energy for hydrogen adsorption in hydrogen evolution. Meanwhile, the synergistic effect between different phases and the introduced lattice defects of Mo₂C-Mo₃C₂ are considered to enhance the HER catalytic performance. The designed catalyst exhibits optimal electrocatalytic activity in both acidic and alkaline media: low overpotentials of 134 and 116 mV at 10 mA cm^-2, a small Tafel slope of 64 mV dec^-1, and a long-term stability for 5000 cycles. This work will provide new insights into the design of high-efficiency HER catalysts via interfacial engineering at the nanoscale for commercial water splitting.

Self-Assembled Nickel Pyrophosphate-Decorated Amorphous Bimetal Hydroxides 2D-on-2D Nanostructure for High-Energy Solid-State Asymmetric Supercapacitor

To obtain a supercapacitor with a remarkable specific capacitance and rate performance, a cogent design and synthesis of the electrode material containing abundant active sites is necessary. In present work, a scalable strategy is developed for preparing 2D-on-2D nanostructures for high-energy solid-state asymmetric supercapacitors (ASCs). The self-assembled vertically aligned microsheet-structured 2D nickel pyrophosphate (Ni₂ P₂ O₇ ) is decorated with amorphous bimetallic nickel cobalt hydroxide (NiCo-OH) to form a 2D-on-2D nanostructure arrays electrode. The resulting Ni₂ P₂ O₇ /NiCo-OH 2D-on-2D array electrode exhibits peak specific capacity of 281 mA hg^-1 (4.3 F cm^-2 ), excellent rate capacity, and cycling stability over 10 000 charge-discharge cycles in the positive potential range. The excellent electrochemical features can be attributed to the high electrical conductivity and 2D layered structure of Ni₂ P₂ O₇ along with the Faradic capacitance of the amorphous NiCo-OH nanosheets. The constructed Ni₂ P₂ O₇ /NiCo-OH//activated carbon based solid-state ASC cell operates in a high voltage window of 1.8 V with an energy density of 78 Wh kg^-1 (1.065 mWh cm^-3 ) and extraordinary cyclic stability over 10 000 charge-discharge cycles with excellent energy efficiency (75%-80%) over all current densities. The excellent electrochemical performance of the prepared electrode and solid-state ASC device offers a favorable and scalable pathway for developing advanced electrodes.

Cobalt Phosphides Nanocrystals Encapsulated by P-Doped Carbon and Married with P-Doped Graphene for Overall Water Splitting

As one class of important functional materials, transition metal phosphides (TMPs) nanostructures show promising applications in catalysis and energy storage fields. Although great progress has been achieved, phase-controlled synthesis of cobalt phosphides nanocrystals or related nanohybrids remains a challenge, and their use in overall water splitting (OWS) is not systematically studied. Herein, three kinds of cobalt phosphides nanocrystals encapsulated by P-doped carbon (PC) and married with P-doped graphene (PG) nanohybrids, including CoP@PC/PG, CoP-Co₂ P@PC/PG, and Co₂ P@PC/PG, are obtained through controllable thermal conversion of presynthesized supramolecular gels that contain cobalt salt, phytic acid, and graphene oxides at proper temperature under Ar/H₂ atmosphere. Among them, the mixed-phase CoP-Co₂ P@PC/PG nanohybrids manifest high electrocatalytic activity toward both hydrogen and oxygen evolution in alkaline media. Remarkably, using them as bifunctional catalysts, the fabricated CoP-Co₂ P@PC/PG||CoP-Co₂ P@PC/PG electrolyzer only needs a cell voltage of 1.567 V for driving OWS to reach the current density at 10 mA cm^-2 , superior to their pure-phase counterparts and recently reported bifunctional catalysts based devices. Also, such a CoP-Co₂ P@PC/PG||CoP-Co₂ P@PC/PG device exhibits outstanding stability for OWS. This work may shed some light on optimizing TMPs nanostructures based on phase engineering, and promote their applications in OWS or other renewable energy options.

A perovskite oxide with a tunable pore-size derived from a general salt-template strategy as a highly efficient electrocatalyst for the oxygen evolution reaction

A porous perovskite oxide is fabricated by an inorganic salt-template strategy, which exhibits remarkable performance for the oxygen evolution reaction.

Ultrasmall Co2P2O7 nanocrystals anchored on nitrogen-doped graphene as efficient electrocatalysts for the oxygen reduction reaction

A novel composite electrocatalyst with ultrasmall Co₂O₂P₇ nanocrystals supported on nitrogen-doped graphene has been developed and exhibits remarkable ORR performance.

Hierarchical Porous Prism Arrays Composed of Hybrid Ni-NiO-Carbon as Highly Efficient Electrocatalysts for Overall Water Splitting

Searching for an economical and efficient water splitting electrocatalyst is still a huge challenge for hydrogen production. This work reports one-step synthesis of hierarchical porous prism arrays (HPPAs) composed of Ni-NiO nanoparticles embedding uniformly in graphite carbon (Ni-NiO/C HPPAs), which is derived from metal-organic framework (CPO-27-Ni) prism arrays grown on nickel foam (NF). Remarkable features of the prism arrays, synergistic effect of Ni-NiO/C, porous graphite carbon, high conductive NF, and good contact between catalyst and current collector result in excellent electrocatalytic performance of Ni-NiO/C HPPAs@NF. Ni-NiO/C HPPAs@NF shows a small overpotential of ∼49.48 mV at the current density of 10 mA cm^-2, low Tafel slope of 74 mV dec^-1 and robust stability for hydrogen evolution reaction (HER) in alkaline media. Especially, the overpotential for HER of Ni-NiO/C HPPAs@NF is only ∼132 mV at the current density of 185 mA cm^-2, almost the same as the value from the Pt/C. Furthermore, for oxygen evolution reaction in basic media, Ni-NiO/C HPPAs@NF shows better catalytic activity, lower Tafel slope and higher durability than precious IrO₂. The above finding offers an effective strategy to design the bifunctional electrocatalysts for overall water splitting.

Engineering Morphologies of Cobalt Pyrophosphates Nanostructures toward Greatly Enhanced Electrocatalytic Performance of Oxygen Evolution Reaction

Herein, a surfactant- and additive-free strategy is developed for morphology-controllable synthesis of cobalt pyrophosphate (CoPPi) nanostructures by tuning the concentration and ratio of the precursor solutions of Na4 P2 O7 and Co(CH3 COO)2 . A series of CoPPi nanostructures including nanowires, nanobelts, nanoleaves, and nanorhombuses are prepared and exhibit very promising electrocatalytic properties toward the oxygen evolution reaction (OER). Acting as both reactant and pseudo-surfactant, the existence of excess Na4 P2 O7 is essential to synthesize CoPPi nanostructures for unique morphologies. Among all CoPPi nanostructures, the CoPPi nanowires catalyst renders the best catalytic performance for OER in alkaline media, achieving a low Tafel slope of 54.1 mV dec-1 , a small overpotential of 359 mV at 10 mA cm-2 , and superior stability. The electrocatalytic activities of CoPPi nanowires outperform the most reported non-noble metal based catalysts, even better than the benchmark Ir/C (20%) catalyst. The reported synthesis of CoPPi gives guidance for morphology control of transition metal pyrophosphate based nanostructures for a high-performance inexpensive material to replace the noble metal-based OER catalysts.

Encapsulating Co2 P@C Core-Shell Nanoparticles in a Porous Carbon Sandwich as Dual-Doped Electrocatalyst for Hydrogen Evolution

A highly efficient and pH-universal hydrogen evolution reaction (HER) electrocatalyst with a sandwich-architecture constructed using zero-dimensional N- and P-dual-doped core-shell Co₂ P@C nanoparticles embedded into a 3 D porous carbon sandwich (Co₂ P@N,P-C/CG) was synthesized through a facile two-step hydrothermal carbonization and pyrolysis method. The interfacial electron transfer rate and the number of active sites increased owing to the synergistic effect between the N,P-dual-doped Co₂ P@C core-shell and sandwich-nanostructured substrates. The presence of a high surface area and large pore sizes improved the mass-transfer dynamics. This nanohybrid showed remarkable electrocatalytic activity toward the HER in a wide pH range with good stability. The computational study and experiments revealed that the carbon atoms close to the N and P dopants on the shell of Co₂ P@N,P-C were effective active sites for HER catalysis and that both Co₂ P and the N,P dopants gave rise to an optimized binding free energy of H on the active sites.

Self-Supported Hierarchical FeCoNi-LTH/NiCo2O4/CC Electrodes with Enhanced Bifunctional Performance for Efficient Overall Water Splitting

The development of advanced earth-abundant electrocatalysts for hydrogen production is highly desirable. In this paper, we report the design and synthesis of a novel and highly efficient electrode of NiCo₂O₄ nanoneedles decorated with FeCoNi layered ternary hydroxides supported on carbon cloth (FeCoNi-LTH/NiCo₂O₄/CC) by a facile and efficient two-step approach. It exhibits superior bifunctional catalytic activities for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline media, due to the special structure and strong synergies. The FeCoNi-LTH/NiCo₂O₄/CC obtains an onset overpotential of 240 mV and an overpotential of 302 mV at the current density of 50 mA cm^-2 for OER, which is superior to RuO₂. It also efficiently catalyzes HER with onset overpotential of 96 mV and overpotential of 151 mV to achieve a current density of 20 mA cm^-2. Serving as both cathode and anode in a two-electrode water splitting system, FeCoNi-LTH/NiCo₂O₄/CC only requires an overpotential of 1.65 V at current density of 50 mA cm^-2. The cell exhibits outstanding stability as well, indicating that FeCoNi-LTH/NiCo₂O₄/CC is a befitting material to be utilized as effective bifunctional catalysts for overall water splitting.

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

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

Interlayer Expansion of Layered Cobalt Hydroxide Nanobelts to Highly Improve Oxygen Evolution Electrocatalysis

The water oxidation reaction is known to be energy-inefficient and generally considered as a major bottleneck for water splitting. Exploring electrocatalysts with high-efficiency and at low cost is vital to widespread utilization of this technology, but is still a big challenge. Here we report an effective strategy based on an expanding interlayer of layered structures to realize a great enhancement of the catalytic performance of the oxygen evolution reaction from water splitting. Well-defined nanobelts of layer-structured cobalt benzoate hydroxide (Co(OH)(C₆H₅COO)·H₂O) are successfully prepared in terms of a simple hydrothermal process. Intercalation with benzoate ions induces the interlayer expansion of the cobalt hydroxide, which is useful for the accommodation of more electrolyte ions and favorable for their diffusion and transport. The as-prepared Co(OH)(C₆H₅COO)·H₂O nanobelts need significantly smaller overpotential (∼0.36 V) to reach 10 mA·cm^-2 of current density compared with their Co(OH)₂ (∼0.44 V) and Co₃O₄ (∼0.387 V) counterparts, and even favorably compare with most of the layered hydroxide-based electrocatalysts. Moreover, the Co(OH)(C₆H₅COO)·H₂O nanobelts retain a much higher stability than the RuO₂ reference in alkaline solution. This approach would be utilized to design and develop high-performance layered hydroxide-based electrocatalysts.

Hydrothermal synthesis of manganese phosphate/graphene foam composite for electrochemical supercapacitor applications

Manganese phosphate (Mn₃(PO₄)₂ hexagonal micro-rods and (Mn₃(PO₄)₂ with different graphene foam (GF) mass loading up to 150mg were prepared by facile hydrothermal method. The characterization of the as-prepared samples proved the successful synthesis of Mn₃(PO₄)₂ hexagonal micro-rods and Mn₃(PO₄)₂/GF composites. It was observed that the specific capacitance of Mn₃(PO₄)₂/GF composites with different GF mass loading increases with mass loading up to 100mg, and then decreases with increasing mass loading up to 150mg. The specific capacitance of Mn₃(PO₄)₂/100mg GF electrode was calculated to be 270Fg^-1 as compared to 41Fg^-1 of the pristine sample at a current density of 0.5Ag^-1 in a three-electrode cell configuration using 6M KOH. Furthermore, the electrochemical performance of the Mn₃(PO₄)₂/100mg GF electrode was evaluated in a two-electrode asymmetric cell device where Mn₃(PO₄)₂/100mg GF electrode was used as a positive electrode and activated carbon (AC) from coconut shell as a negative electrode. AC//Mn₃(PO₄)₂/100mg GF asymmetric cell device was tested within the potential window of 0.0-1.4V, and showed excellent cycling stability with 96% capacitance retention over 10,000 galvanostatic charge-discharge cycles at a current density of 2Ag^-1.