Fabrication of efficient nanostructured Co3O4-Graphene bifunctional catalysts: Oxygen evolution,hydrogen evolution,and H2O2 sensing

Nanostructured Co₃O₄-graphene hybrid catalysts are fabricated by a one-step vacuum kinetic spray technique from microparticles of Co₃O₄ and graphite powders. The Co₃O₄-graphene hybrid catalysts with various Co₃O₄ contents are studied concerning the oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) in 1.0 M KOH, as well as, H₂O₂ sensing in 0.1 M NaOH. We find that increasing graphene content in the hybrid catalysts results in an overall improvement of the OER electrocatalytic activity due to the enhancement in the charge transfer kinetics. The hybrid catalyst with 25 wt% Co₃O₄ reveals the optimum electrocatalytic activity toward the OER with the lowest overpotential (η) of 283 mV@ 10 mA cm⁻² and superior reaction kinetics with a low Tafel slope of 25 mV dec⁻¹. Besides, the OER stability at 50 mA cm⁻² for 50 h in 1.0 M KOH was verified. The hybrid catalyst with 50 wt% Co₃O₄ revealed the highest activity toward the HER with η of 108 mV@ 10 mA cm⁻², Tafel slope of 90 mV.dec⁻¹, and stability at 50 mA cm⁻² for nearly 30 h. Moreover, it reveals ultrahigh H₂O₂ amperometric detection with superior sensitivity of 18,110 μA mM⁻¹ cm⁻², linear detection range from 20 μM to 1 mM, and a limit of detection of 0.14 μM.     

The Heterojunction of Ni and Co9S8 was Synthesized and Anchored on Carbon Nanotubes to Improve the Performance of Water Electrolysis

Developing durable, low-cost water electrolysis catalysts plays a critical role in solving clean energy problems. Herein, we synthesized Ni/Co₉S₈@CNT micro flower comprising Ni/Co₉S₈ mesoporous nanosheets and carbon nanotubes (CNT). HRTEM images indicate that the Ni/Co₉S₈@CNT micro flower contains abundant Ni/Co₉S₈ heterojunctions. The band structure and coupling effect of the Ni/Co₉S₈ heterostructure optimizes the transfer of electrons, thus the synergistic hybrid effectively improves conductivity and electrocatalytic performance. Therefore, Ni/Co₉S₈@CNT shows good evolution reaction (OER) and hydrogen evolution reaction (HER) performance in an alkaline environment. Specifically, at a current density of 10 mA cm^?2, the overpotential of OER is 289 mV, and the overpotential of HER is 228 mV. The Tafel slope is 69 mV dec^?1. In addition, Ni/Co₉S₈@CNT in alkaline electrolyte only loses 10% of its activity after working for 10 h at a current density of 10 mA cm^?2. These results indicate that composite materials composed of transition metal sulfides and highly conductive carbon materials are a good choice for low-cost electrocatalysts.

Graphic abstract

Synthesize the heterojunction of metal Ni and Co9S8 and anchor it on the CNT to improve the performance of OER and HER.

     

MOFs derived (Ni0.75Co0.25)Se2 nanoparticles embedded in N-doped nanocarbon for hybrid supercapacitors

Bimetallic selenides have aroused great interest as the electroactive materials for energy storage because of their high conductivity, robust electrochemical activity, and the synergistic effect. Herein, (Ni_0.75Co_0.25)Se₂ nanoparticles embedded in N-doped nanocarbon ((Ni_0.75Co_0.25)Se₂@NC) hybrids were derived from nickel and cobalt bimetal-organic frameworks (NiCo-MOFs), which were synthesized by ethylene glycol solvothermal method. Due to the synergistic contributions and unique architecture, (Ni_0.75Co_0.25)Se₂@NC hybrids electrode presents a considerable specific capacity of 536.6 C g^?1 at a discharge current density of 1 A g^?1. In addition, an as-assembled (Ni_0.75Co_0.25)Se₂@NC//activated carbon (AC) hybrid supercapacitor (HSC) ((Ni_0.75Co_0.25)Se₂@NC//AC HSC) shows large specific capacitance (73.6 F g^?1 at 0.5 A g^?1), outstanding energy density (26.2 Wh kg^?1 at 400 W kg^?1) with superior cyclic performance (88.7% of capacity retention after 5000 cycles). Furthermore, a (Ni_0.75Co_0.25)Se₂@NC//AC device could drive a mini-fan running for 67 s. Thus, (Ni_0.75Co_0.25)Se₂@NC is an outstanding active material for electrochemical energy storage.     

Sodium-nickel pyrophosphate as a novel oxygen evolution electrocatalyst in alkaline medium

The development of robust and low-cost oxygen evolution reaction (OER) electrocatalysts is a challenging issue in electrochemical water-splitting technology. Tailoring the electrocatalysts through nano- and composition engineering is an effective strategy to enhance the intrinsic and extrinsic electrocatalytic activities. In this study, for the first time, sodium-nickel pyrophosphate (Na₂NiP₂O₇) is proposed as a novel OER electrocatalyst in alkaline environment. First, the electrocatalytic performance of micron-sized Na₂NiP₂O₇ (Na₂NiP₂O₇-micron) is evaluated. In addition, nanoscale Na₂NiP₂O₇ (Na₂NiP₂O₇-nano) and Fe-substituted Na₂NiP₂O₇-nano (Na₂Ni_1−xFe_xP₂O₇-nano) are synthesized to improve the electrocatalytic performance of Na₂NiP₂O₇. Although Na₂NiP₂O₇-micron exhibits low electrocatalytic OER activity in alkaline medium, its catalytic activity can be significantly improved through reducing the particle size and substituting Fe in the Ni sites. The synthesized Na₂Ni_0.75Fe_0.25P₂O₇-nano exhibits optimal catalytic activity with an overpotential of 300 mV at a current density of 10 mA/cm² and long-term durability with continuous O₂ generation over 100 hours in alkaline medium.     

Metal oxide/carbon nanosheet arrays derivative of stacked metal organic frameworks for triggering oxygen evolution reaction

Numerous clean energy systems rely on the oxygen evolution process (OER), which takes place during water splitting reaction. For this purpose, transition-metal oxides have garnered considerable attentions as a prominent OER electrocatalysts. In present study, we fabricate the nanosheet arrays of metal oxide/carbon (MOx/C; M = Fe, Ag, and Mn) fabricated via hydrothermal route. As templates, this approach employs the covered 2-dimensional (2D) metal-organic frameworks (2D-MOFs), and these MOx/C arrays made from 2D MOFs exhibit significant electrocatalytic activity and durability. Among all, Ag₂O/C showed the overpotentials of 270 mV at a current density (j) of 10 mA cm^?2, while the tafel slope is 45 mV dec^?1, that is lower than other metal oxide-based catalysts like MnO/C, and Fe₂O₃/C. It also shows 48 h high stability due to the conductive nature, larger surface area and the presence of carbon cage for easy transfer of electrons. The conceptual framework and synthetic strategy employed in this study can be applied to create more multi-metal oxide anchoring Ag₂O carbon matrix-based electrocatalysts that are extremely efficient, affordable, and perform significantly better in OER and other future applications.     

In situ coupling of lignin-derived carbon-encapsulated CoFe-CoxN heterojunction for oxygen evolution reaction

Exploring highly active and stable electrocatalysts for oxygen evolution reaction (OER) is crucial for developing water splitting and rechargeable metal-air batteries. In this study, a hybrid electrocatalyst of CoFe alloy and Co_xN heterojunction encapsulated and anchored in N-doped carbon support (CoFe-Co_xN@NC) was in situ coupled via the pyrolysis of a novel coordination polymer derived from lignin biomacromolecule. CoFe-Co_xN@NC exhibited outstanding OER activity with a low overpotential of 270 mV at 10 mA cm⁻² and stability with an increment of 20 mV, comparable to commercial Ir/C. DFT calculations showed that Co_xN and N-doped graphitic encapsulation can reduce the binding strength between *O and CoFe alloy, limit metal leaching and agglomeration, and improve electron transfer efficiency, considerably enhancing the OER activity and stability. In situ coupling approach for preparing alloy and nitride heterojunctions on N-doped lignin-derived carbon offers a promising and universal catalyst design for developing renewable energy conversion technologies.     

Low-temperature synthesized amorphous quasi-high-entropy carbonate electrocatalyst with superior surface self-optimization for efficient water oxidation

Anionic High-entropy materials have seldom been reported as a new library of water oxidation electrocatalysts owing to great difficulty in uniformly distributing multiple elements with different physicochemical properties and harsh synthesis conditions. Herein, a series of amorphous quasi-high-entropy carbonates for the first time is prepared via a facile low-temperature hydrothermal route. The optimized CoCrFeMnMoCO₃ with hydrothermal treatment of 6 h can serve as a promising oxygen evolution reaction (OER) electrocatalyst for water splitting on account of amorphous structure rendering more exposed active sites, superior synergistic effect realizing surface component self-optimization, high-valence ferritic species (Fe^(3+δ)+) providing high catalytic activity and high-entropy stabilization guaranteeing long-term OER performance, thus exhibiting the low overpotentials of 302 and 355 mV at the current densities of 10 and 100 mA cm⁻², respectively, the small Tafel slope of 36.7 mV dec⁻¹, and excellent durability longer than 38 h, dramatically exceeding its corresponding crystalline counterpart and benchmark RuO₂ catalyst, as well as yielding the current density of 10 mA cm⁻² with impressive low voltage of 1.56 V while used as bifunctional electrocatalyst for overall water splitting. This study lights a broad avenue to design other anionic high-entropy materials as promising OER catalysts.     

A medium-entropy perovskite oxide La0.7Sr0.3Co0.25Fe0.25Ni0.25Mn0.25O3-δ as intermediate temperature solid oxide fuel cells cathode material

In this study, we report a novel medium-entropy perovskite oxide of La_0.7Sr_0.3Co_0.25Fe_0.25Ni_0.25Mn_0.25O_3-δ (LSCFNM73) with high constitutive entropy (S_config) as the cathode material of intermediate temperature solid oxide fuel cells (IT-SOFCs). The intrinsic properties of phase structure, electrical conductivity, thermal expansion and oxygen adsorption capacity of La_1-xSr_xCo_0.25Fe_0.25Ni_0.25Mn_0.25O_3-δ (LSCFNM, x = 0, 0.1, 0.2, 0.3) oxides are evaluated in detail. The LSCFNM73 oxide exhibits the maximum electrical conductivity of 464 S cm⁻¹ at 800 °C and a relatively lower thermal expansion coefficient (TEC) of 15.34 × 10⁻⁶ K⁻¹, which is selected as the propriate cathode composition. The B-site of LSCFNM73 contains four elements which can increase the configuration entropy. Additionally, NiO-Yttria stabilized zirconia (YSZ) supported fuel cell is fabricated by tape casting, hot pressing-lamination, co-sintering and screen printing technologies. The fuel cell demonstrates a maximum power density of 1088 mW cm² at 800 °C, and excellent stability at 750 °C under 0.75V in 120 h and 10 times thermal cycling between 750 °C and 400 °C. Therefore, the medium-entropy LSCFNM73 oxide can be applied in IT-SOFCs as a competitive cathode material.     

Salt-Templated Nanoarchitectonics of CoSe2-NC Nanosheets as an Efficient Bifunctional Oxygen Electrocatalyst for Water Splitting

Recently, the extensive research of efficient bifunctional electrocatalysts (oxygen evolution reaction (OER) and hydrogen evolution reaction (HER)) on water splitting has drawn increasing attention. Herein, a salt-template strategy is prepared to synthesize nitrogen-doped carbon nanosheets encapsulated with dispersed CoSe₂ nanoparticles (CoSe₂-NC NSs), while the thickness of CoSe₂-NC NSs is only about 3.6 nm. Profiting from the ultrathin morphology, large surface area, and promising electrical conductivity, the CoSe₂-NC NSs exhibited excellent electrocatalytic of 10 mA·cm⁻² current density at small overpotentials of 247 mV for OER and 75 mV for HER. Not only does the nitrogen-doped carbon matrix effectively avoid self-aggregation of CoSe₂ nanoparticles, but it also prevents the corrosion of CoSe₂ from electrolytes and shows favorable durability after long-term stability tests. Furthermore, an overall water-splitting system delivers a current density of 10 mA·cm⁻² at a voltage of 1.54 V with resultants being both the cathode and anode catalyst in alkaline solutions. This work provides a new way to synthesize efficient and nonprecious bifunctional electrocatalysts for water splitting.     

Cobalt oxide nanocomposites and their electrocatalytic behavior for oxygen evolution reaction

In this work, we have described the simple preparation method of cobalt oxide nanocomposites where cobalt oxide nanoparticles were grown on the surface of carbon nanotube, graphene oxide and graphene (Co₃O₄@CNT, Co₃O₄@GO, Co₃O₄@G). The as-grown Co₃O₄@CNT, Co₃O₄@GO, Co₃O₄@G were investigated for H₂O oxidation. The nanoparticles displayed high activity toward oxygen evolution. Further, the stability of the catalysts were tested in alkaline solution, which exhibited good stability. Among all nanoparticles, Co₃O₄@G exhibited higher current density at lower overpotential and also exhibited lower Tafel slope (157.1 mV dec⁻¹) as compared to Co₃O₄@CNT and Co₃O₄@GO. The Co₃O₄@G delivered a current density of 10 mAcm⁻² at 0.8 V (overpotential 535 V versus Ag/AgCl) in 0.1 M KOH solution, which is superior than many electrocatalysts reported for oxygen evolution so far. The good electrocatalytic performance might be due to the structural features of Co₃O₄@G, which cause enhancement of oxygen evolution activity.     

Hierarchical CoP3/NiMoO4 heterostructures on Ni foam as an efficient bifunctional electrocatalyst for overall water splitting

Controllable synthesis strategies of the cost-effective and high-active non-noble metal bifunctional electrocatalysts for overall water splitting are imperatively required. Herein, the hierarchical heterostructure CoP₃/NiMoO₄ nanosheets on Ni foam (CoP₃/NiMoO₄–NF) are synthesized by hydrothermal, annealing and phosphorization treatment. The synergistic effect between CoP₃ and NiMoO₄ remarkably promotes the HER intrinsic activity. Moreover, the Ni foam promotes the vertical growth of well-aligned nanosheet arrays, which expose more active sites for HER and OER. The CoP₃/NiMoO₄–NF-2 (Co/Mo = 1/1) electrocatalyst reveals a low overpotential of 92 mV for HER and 347 mV for OER at 10 mA cm⁻² in 1.0 M KOH. Especially, the CoP₃/NiMoO₄–NF-2 exhibits exceptional performance for overall water splitting which presents a low cell voltage of 1.57 V at 10 mA cm⁻², and outstanding durability which could maintain over 12 h. The design strategy and controllable synthesis of the hierarchical heterostructure bifunctional electrocatalyst will be beneficial for efficient overall water splitting.     

Dual role of Fe boost lattice oxygen oxidation of Mo-based materials from kinetics and thermodynamics

High-valent Mo-based oxides are easily dissolved in alkaline electrolyte resulting in complete surface reconstruction of catalyst. Therefore, there are few researches on the oxygen evolution reaction (OER) process of this material, especially the reaction mechanism. Herein, Fe-Mo₂C@CN was synthesized by introducing 3d metal Fe into the Mo-based catalyst, which inhibited the complete dissolution of Mo. The overpotential is only 226 mV at a current density of 10 mA cm⁻². Experimental and density functional theory (DFT) results demonstrate that excellent electrocatalytic performance derives from the dual role of Fe and the thermodynamically favorable single-site lattice oxygen oxidation mechanism (LOM). Electronic-rich pure Fe inhibits the molybdenum dissolution while enhancing the reaction kinetics. And the doped Fe decreases the d‐band center, weakens the M-O (metal-oxygen) bond, and promotes the involvement of lattice oxygen in the OER process. This work provides theoretical basis for the engagement of Mo-based catalysts in water splitting.     

Physicochemical and electrocatalytic properties of LaNiO3 prepared by a low-temperature route for anode application in alkaline water electrolysis

LaNi_1–x Fe_xO₃ (x = 0, 0.25, 0.5) has been synthesized by the hydroxide solid solution precursor method for electrochemical characterization as oxygen anode in strongly alkaline medium. Studies were made at the oxide film, which was obtained by the oxide-slurry painting technique. The cyclic voltammetric study showed the formation of a diffusion-controlled quasireversible redox couple, Ni(ii)/Ni(iii), (E ⁰ - 430 ± 10 mV) at the oxide surface in 1 m KOH. The reaction was observed to follow approximately first-order kinetics in OH^– concentration. Values of the Tafel slope ranged between 59 and 86 mV decade^–1 with all the oxide film electrodes. The electrocatalytic activity was found to be greatest with the Ni/LaNi_0.75Fe_0.25O₃ electrode. A comparison was made between the electrocatalytic activities of LaNiO₃ prepared by the hydroxide solid solution precursor and by the hydroxide coprecipitation technique.     

Redox Potentials of High-Valent Iron-, Cobalt-, and Nickel-Oxido Complexes: Evidence for Exchange Enhanced Reactivity

The ferrocene titration method has been employed to determine the one-electron reduction potentials of a series of oxido-iron(IV), oxido-cobalt(IV) and oxido/hydroxido-nickel(III) complexes based on the same tetradentate TMG₃tren ligand (TMG₃tren=tris[2-(N-tetramethylguanidyl)ethyl]amine). The S=2 ground state of the [(TMG₃tren)Fe^IV=O]²⁺ complex allows an exchange enhanced reactivity, which enables it to perform efficient oxygen atom transfer (OAT) and hydrogen atom abstraction (HAA) reactions with a low one-electron reduction potential of 270 mV vs. SCE. In the absence of exchange enhanced reactivity, the OAT and HAA abilities of the S=3/2 [(TMG₃tren)Co^IV−O(Sc(OTf)₃)]²⁺, S=1/2 [(TMG₃tren)Ni^III−O(H)]²⁺ and the previously reported S=1 [(TMC)(CH₃CN)Fe^IV=O]²⁺ and [(N4Py)Fe^IV=O]²⁺ complexes can be directly correlated to their reduction potentials. Notably, [(N4Py)Fe^IV=O]²⁺ and [(TMG₃tren)Fe^IV=O]²⁺ exhibit similar OAT and HAA reactivities although the reduction potential of [(N4Py)Fe^IV=O]²⁺ is 0.24 V more positive than that of [(TMG₃tren)Fe^IV=O]²⁺. The present study therefore provides experimental evidence for exchange enhanced reactivity and rationalizes nature's choice for employing S=2 oxido-iron(IV) cores to achieve difficult oxidation reactions at biologically viable potentials.     

Surface Modulation of 3D Porous CoNiP Nanoarrays In Situ Grown on Nickel Foams for Robust Overall Water Splitting

The careful design of nanostructures and multi-compositions of non-noble metal-based electrocatalysts for highly efficient electrocatalytic hydrogen and oxygen evolution reaction (HER and OER) is of great significance to realize sustainable hydrogen release. Herein, bifunctional electrocatalysts of the three-dimensional (3D) cobalt-nickel phosphide nanoarray in situ grown on nickel foams (CoNiP NA/NF) were synthesized through a facile hydrothermal method followed by phosphorization. Due to the unique self-template nanoarray structure and tunable multicomponent system, the CoNiP NA/NF samples present exceptional activity and durability for HER and OER. The optimized sample of CoNiP NA/NF-2 afforded a current density of 10 mA cm⁻² at a low overpotential of 162 mV for HER and 499 mV for OER, corresponding with low Tafel slopes of 114.3 and 79.5 mV dec⁻¹, respectively. Density functional theory (DFT) calculations demonstrate that modulation active sites with appropriate electronic properties facilitate the interaction between the catalyst surface and intermediates, especially for the adsorption of absorbed H* and *OOH intermediates, resulting in an optimized energy barrier for HER and OER. The 3D nanoarray structure, with a large specific surface area and abundant ion channels, can enrich the electroactive sites and enhance mass transmission. This work provides novel strategies and insights for the design of robust non-precious metal catalysts.     

双功能yolk-shell钴@钴氮碳掺杂氧电极催化剂

以ZIF-67为模板,通过表面原位聚合多巴胺,与金属Co²⁺发生强烈螯合,释放出有机配体,得到中空的金属-有机结构材料（Co-PDA）。通过900℃高温处理得到类似蛋黄（yolk-shell）结构的金属氮掺杂碳材料（Co@Co-N/C）。这种特殊结构的材料具有优异的氧还原（ORR）和析氧反应（OER）电催化活性,在0.1 mol/L KOH电解液中,其ORR的半波电位为0.81 V,Tafel斜率为60 mV/dec;在电流密度为10 mA/cm²时,其OER过电位为390 mV,Tafel斜率为71 mV/dec,总的氧电极催化活性为0.82 V,是一种优良的双功能氧电极催化剂。     

Selective transfer of cations between two electrolytes using the intercalation properties of Chevrel phases

A device based on an electrochemical transfer junction (constituted by M_xMo₆S₈ or M_xMo₆Se₈) placed between two tanks allows the transfer of cations by application of current density controlled between electrodes placed in tanks. The transfer protocol was tested on different mixed electrolytes containing cations directly engaged in the batteries industry (M²⁺ = Co²⁺, Ni²⁺, Cd²⁺, Zn²⁺, Mn²⁺, Cu²⁺). Good performances of the process are provided until 1.6 mA cm⁻². The electrolysis through an electrochemical transfer junction made of Chevrel phases represents a suitable method for the selective extraction of cations with appreciable selectivity rates with an appropriate choice of the host lattice (sulfide or selenide). Remarkable separations between Co/Ni, Zn/Mn with Mo₆S₈ and Cd/Zn, Cd/Ni, Cd/Co and Zn/Ni, with Mo₆Se₈ were observed.     

Oxygen evolution on electrochemically generated cobalt spinel coating

The oxygen evolution reaction (OER) has been investigated in 40 wt % KOH at 80°C on a thick cobalt oxide coating obtained by potential cycling of a cobalt electrode for 0, 2, 4, 10 and 17h in the same electrolyte with and without dissolved strontium. The kinetic parameters of the OER were determined after preanodization for 1 h at 1 A cm^–2. Improved electrocatalytic activity for the OER, better mechanical strength of the coating and lower variation of the oxygen overpotential with time were noticed up to 70 h of polarization as the coating built up in the presence of dissolved strontium in the electrolyte. The beneficial effect of dissolved strontium on the electrocatalytic activity is ascribed to the accumulation of Co₃O₄, which results in a lower Tafel slope for the OER.     

MnxCu1−xCo2O4 used as bifunctional electrocatalyst in alkaline medium

Composite film electrodes containing mechanically mixed Mn_xCu_1−xCo₂O₄ (0 ≤ x ≤ 1) particles, carbon black Vulcan XC72R and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) were formed on the glassy carbon disk surface of a rotating ring-disk electrode (RRDE) and studied for the oxygen reduction and evolution reactions (ORR and OER, respectively) in 1 M KOH solution. The electrocatalytic activities for both reactions were observed to depend strongly on the Mn content in CuCo₂O₄. An opposite trend was observed for the apparent and intrinsic electrocatalytic activities for the ORR; the simultaneous presence of Cu and Mn was found to be detrimental to the intrinsic charge density, but beneficial to the geometric charge density with a maximum for Mn_0.6Cu_0.4Co₂O₄. The latter was characterized by the highest total number of electrons exchanged per O₂ molecule, n, close to 4, greater k₁ (4e⁻ process)/k₂ (2e⁻ process) ratios, and by a unique and low Tafel slope (−41 mV dec⁻¹). The results obtained for the OER showed that the intrinsic electrocatalytic activity is determined by the number of active sites (Co⁴⁺) electrochemically formed at the oxide surface prior to the OER, from Co³⁺ cations. The partial substitution of Cu by Mn in CuCo₂O₄ was found to decrease the OER activity.     

Effect of the partial replacement of Fe by Ni and/or Mn on the electrocatalytic activity for oxygen evolution of the CoFe2O4 spinel oxide electrode

The study of the electrochemical behaviour of mixed spinel oxide electrodes, obtained by the partial replacement of Fe by Ni and/or Mn in the cobalt ferrite CoFe₂O₄ is presented. The electrodes were prepared by brush painting of iron substrates with a suspension of the respective oxide, prepared by solid-state reaction. The influence of the substituent on the electrodes electrocatalytic activity towards the OER is analysed in terms of the kinetic parameters obtained by steady state measurements and the cationic distribution proposed for the oxides. The data show that the introduction of Ni brings about the presence of Co³⁺ tetrahedrally coordinated in addition to the Co³⁺/Co²⁺ couple in octahedral sites, giving rise to a better electrocatalyst for the OER. In contrast the presence of Mn produces electrodes with lower catalytic activity.