Nitrogen doped CuCo2O4 nanoparticles anchored on beaded-like carbon nanofibers as an efficient bifunctional oxygen catalyst toward zinc-air battery

Excellent Bifunctional Oxygen Evolution and Reduction Electrocatalysts (5A1/5)Co2O4 and Their Tunability

Hastening the progress of rechargeable metal-air batteries and hydrogen fuel cells necessitates the advancement of economically feasible, earth-abundant, inexpensive, and efficient electrocatalysts facilitating both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, a recently reported family of nano (5A1/5)Co2O4 (A = combinations of transition metals, Mg, Mn, Fe, Ni, Cu, and Zn) compositionally complex oxides (CCOs) [Wang et al., Chemistry of Materials, 2023,35 (17), 7283-7291.] are studied as bifunctional OER and ORR electrocatalysts. Among the different low-temperature soft-templating samples, those subjected to 600 °C postannealing heat treatment exhibit superior performance in alkaline media. One specific composition (Mn0.2Fe0.2Ni0.2Cu0.2Zn0.2)Co2O4 exhibited an exceptional overpotential (260 mV at 10 mA cm-2) for the OER, a favorable Tafel slope of 68 mV dec-1, excellent onset potential (0.9 V) for the ORR, and lower than 6% H2O2 yields over a potential range of 0.2 to 0.8 V vs the reversible hydrogen electrode. Furthermore, this catalyst displayed stability over a 22 h chronoamperometry measurement, as confirmed by X-ray photoelectron spectroscopy analysis. Considering the outstanding performance, the low cost and scalability of the synthesis method, and the demonstrated tunability through chemical substitutions and processing variables, CCO ACo2O4 spinel oxides are highly promising candidates for future sustainable electrocatalytic applications.

Facile Synthesis of Co Nanoparticles Embedded in N-Doped Carbon Nanotubes/Graphitic Nanosheets as Bifunctional Electrocatalysts for Electrocatalytic Water Splitting

Developing robust and cost-effective electrocatalysts to boost hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs) is crucially important to electrocatalytic water splitting. Herein, bifunctional electrocatalysts, by coupling Co nanoparticles and N-doped carbon nanotubes/graphitic nanosheets (Co@NCNTs/NG), were successfully synthesized via facile high-temperature pyrolysis and evaluated for water splitting. The morphology and particle size of products were influenced by the precursor type of the cobalt source (cobalt oxide or cobalt nitrate). The pyrolysis product prepared using cobalt oxide as a cobalt source (Co@NCNTs/NG-1) exhibited the smaller particle size and higher specific surface area than that of the pyrolysis products prepared using cobalt nitrate as a cobalt source (Co@NCNTs/NG-2). Notably, Co@NCNTs/NG-1 displayed much lower potential -0.222 V vs. RHE for HER and 1.547 V vs. RHE for OER at the benchmark current density of 10 mA cm-2 than that of Co@NCNTs/NG-2, which indicates the higher bifunctional catalytic activities of Co@NCNTs/NG-1. The water-splitting device using Co@NCNTs/NG-1 as both an anode and cathode demonstrated a potential of 1.92 V to attain 10 mA cm-2 with outstanding stability for 100 h. This work provides a facile pyrolysis strategy to explore highly efficient and stable bifunctional electrocatalysts for water splitting.

Electrospun Carbon Nanofibers and Their Applications in Several Areas

Carbon nanofibers (CNFs) have a broad spectrum of applications, including sensor manufacturing, electrochemical catalysis, and energy storage. Among different manufacturing methods, electrospinning, due to its simplicity and efficiency, has emerged as one of the most powerful commercial large-scale production techniques. Numerous researchers have been attracted to improving the performance of CNFs and exploring new potential applications. This paper first discusses the working theory of manufacturing electrospun CNFs. Next, the current efforts in upgrading the properties of CNFs, such as pore architecture, anisotropy, electrochemistry, and hydrophilicity, are discussed. The corresponding applications due to the superior performances of CNFs are subsequently elaborated. Finally, the future development of CNFs is discussed.

Synergetic Nanostructure Engineering and Electronic Modulation of a 3D Hollow Heterostructured NiCo2O4@NiFe-LDH Self-Supporting Electrode for Rechargeable Zn-Air Batteries

Developing electrocatalysts that integrate the merits of the hollow structure and heterojunction is an attractive but still challenging strategy for addressing the sluggish kinetics of oxygen evolution reaction (OER) in many renewable energy technologies. Herein, a 3D hierarchically flexible self-supporting electrode with a hollow heterostructure is intentionally constructed by assembling thin NiFe layered double hydroxide (LDH) nanosheets on the surface of metal-organic framework-derived hollow NiCo₂O₄ nanoflake arrays (NiCo₂O₄@NiFe-LDH) for rechargeable Zn-air batteries (ZABs). Theoretical calculations demonstrate that the interfacial electron transfer from NiFe-LDH to NiCo₂O₄ induces the electronic modulation, improves the conductivity, and lowers the reaction energy barriers during OER, ensuring high catalytic activity. Meanwhile, the 3D hierarchically hollow nanoarray architecture can afford plentiful catalytic active sites and short mass-/charge-transfer pathways. As a result, the obtained catalyst exhibits remarkable OER electrocatalytic performance, showing low overpotentials (only 231 mV at 10 mA cm^-2, 300 mV at 50 mA cm^-2) and robust stability. When assembling liquid and flexible solid-state ZABs with NiCo₂O₄@NiFe-LDH as the OER catalyst, the ZABs achieve excellent power density, high specific capacity, superior cycle durability, and good bending flexibility, exceeding the RuO₂ + Pt/C benchmarks and other previously reported self-supporting catalysts. This work not only constructs an advanced hollow heterostructured catalyst for sustainable energy systems and wearable electronic devices but also provides insights into the role of interfacial electron modulation in catalytic performance enhancement.

Introducing non-bridging ligand in metal-organic framework-based electrocatalyst enabling reinforced oxygen evolution in seawater

Using seawater as the replacement of freshwater for electrolysis, with the integration of renewable energy, is deemed as an attractive manner to harvest green hydrogen. However, the complexity of seawater puts forward stricter requirement to the electrocatalyst to alleviate the chlorine electrochemistry and corrosion. Herein, a nanosheet array of NiFe-MOF@Ni₂P/Ni(OH)₂ is devised by partially substituting terephthalic acid (H₂BDC) ligand by ferrocenecarboxylic acid (FcCA). Tailoring the active site into an under-coordinated fashion affords NiFe-MOF@Ni₂P/Ni(OH)₂ excellent performance towards oxygen evolution reaction (OER), only requiring the overpotentials of 302 mV and 394 mV in alkaline seawater to drive the current densities of 100 and 1000 mA cm^-2, respectively. Moreover, the as-obtained electrocatalyst showed robust durability for operating more than 120 h at 500 mA cm^-2 under harsh condition (6 M KOH + 1.5 M NaCl, 60 ℃). Density functional theory (DFT) calculations confirmed that tuning the coordination environment of Ni in NiFe-MOF by incorporating the non-bridging FcCA ligands could boost the formation of more active catalytic sites, which can simultaneously enhance the electronic conductivity and accelerate OER kinetics. This work provides beneficial enlightenment of combining MOF-based electrocatalyst with direct electrolysis of seawater.

Electronic tuning of Ni-Fe-Co oxide/hydroxide as highly active electrocatalyst for rechargeable Zn-air batteries

As a bifunctional oxygen electrocatalyst (oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)), spinel copper cobaltite (CuCo₂O₄) is attracting significant research interest owing to the tailored Co, Cu electronic structure and ease of adjusting the electrochemically active area. However, its poor OER performance (>300 mV at 10 mA cm^-2) limits its practical application for rechargeable zinc-air batteries. Therefore, we construct a CuCo₂O₄/NiFe LDH oxide/hydroxide interface to tune the properties of Ni, Fe and Co for enhancing OER activity and decreasing the charging overpotential of rechargeable zinc-air batteries. The obtained electrocatalysts show a low overpotential of 251 mV (10 mA cm^-2), which is 91 mV lower than the overpotential (342 mV) of CuCo₂O₄. By in situ Raman, XPS and electrochemical analyses, we ascribe the enhanced OER activity to the increasing Ni/Fe oxidation state triggered by the charge transfer of Ni/Fe and Co, which prompts CuCo₂O₄/NiFe LDH to rapidly form an active surface layer. Benefiting from enhanced OER performance, zinc-air batteries with a CuCo₂O₄/NiFe LDH electrode display a high round-trip efficiency with a low voltage gap of ∼0.78 V (10 mA cm^-2) due to the obvious decrease in the charging overpotential. These results suggest the importance of tuning the charge transfer on interfaces for designing high-efficiency electrocatalysts.

Construction of 3D Hierarchical Co3O4@CoFe-LDH Heterostructures with Effective Interfacial Charge Redistribution for Rechargeable Liquid/Solid Zn-Air Batteries

Constructing three-dimensional (3D) hierarchical heterostructures is an appealing but challenging strategy to improve the performance of catalysts for electrical energy devices. Here, an efficient and robust flexible self-supporting catalyst, interface coupling of ultrathin CoFe-LDH nanosheets and Co₃O₄ nanowire arrays on the carbon cloth (CC/Co₃O₄@CoFe-LDH), was proposed for boosting oxygen evolution reaction (OER) in rechargeable liquid/solid Zn-air batteries (ZABs). The strong interfacial interaction between the CoFe-LDH and Co₃O₄ heterostructures stimulated the charge redistribution in their coupling regions, which improved the electron conductivity and optimized the adsorption free energy of OER intermediates, ultimately boosting the intrinsic OER performance. Besides, the 3D hierarchical nanoarray structure facilitated the exposure of catalytically active centers and rapid electron/mass transfer during the OER process. As such, the CC/Co₃O₄@CoFe-LDH catalyst achieved excellent OER catalytic activity in alkaline medium, with a small overpotential of 237 mV at 10 mA cm^-2, a low Tafel slope of 35.43 mV dec^-1, and long-term durability of up to 48 h, significantly outperforming the commercial RuO₂ catalyst. More impressively, the liquid and flexible solid-state ZABs assembled by the CC/Co₃O₄@CoFe-LDH hybrid catalyst as the OER catalyst presented a stable open circuit voltage, large power density, superb cycling life, and satisfactory flexibility, indicating great potential applications in energy technology. This work provides a good guidance for the development of advanced electrocatalysts with heterostructures and an in-depth understanding of electronic modulation at the heterogeneous interface.

Electrospun Hollow Carbon Nanofibers Decorated with CuCo2O4 Nanowires for Oxygen Evolution Reaction

In recent years, spinel-type structural cobalt salts (NiCo2O4, CuCo2O4, etc.) have been widely used electrocatalysis because of their superior properties such as large crustal reserves, low cost, environmental friendliness, high electrochemical activity, abundant oxidation valence, and stable chemical properties. In this paper, hollow carbon nanofibers loaded CuCo2O4 nanowires (CuCo2O4@CNFs) were prepared by electrospinning technique and solvothermal method. The CuCo2O4@CNFs exhibit enhances electrocatalytic activity for oxygen evolution reaction (OER), requiring an overpotential of 273 mV in a 1.0 M KOH solution to achieve a current density of 10 mA cm−2. In addition, the overpotential remained almost constant after 3000 cycles of voltammetry measurements. The enhanced electrocatalytic activity may be attributed to the unique one-dimensional hollow nanostructure of CNFs and high dispersion of CuCo2O4 nanowires, which enhanced the charge transfer and improved the diffusion of the electrolyte ions at the surface.

Efficient iron-cobalt oxide bifunctional electrode catalysts in rechargeable high current density zinc-air batteries

Iron-cobalt (FeCo) oxides dispersed on reduced graphene oxide (rGO) were synthesized from nitrate precursors at loading levels from 10 wt% to 60 wt%. These catalysts were tested in lab-scale zinc-air batteries (ZABs) at a high current density of 100 mA cm^-2 of the cathode area for the first time, cycling between 60 min of discharging and 60 min of charging. The optimum loading level for the best ZAB cycling performance was found to be 40 wt%, at which CoFe₂O₄ and CoO nanocrystals were detected. A discharge capacity of at least 90% was maintained for about 60 cycles with FeCo 40 wt%, demonstrating superior stability over amorphous FeCo oxides with FeCo 10 wt% despite similar performance at electrochemical tests. At a high current density of 100 mA cm^-2, OER catalytic activity was found to be the limiting factor in ZAB's cyclability. The discrepancies between the ORR/OER catalytic activities by electrochemical and battery cycling test results highlight the role and importance of rGO in improving electrical conductivity and activation of metal oxide electrocatalysts under high current density conditions. The difference of battery cycling test results from traditional electrochemical test results suggests that electrochemical tests conducted at low current densities may be inadequate in predicting practical battery cycling performance.

Nickel-Cobalt Hydrogen Phosphate on Nickel Nitride Supported on Nickel Foam for Alkaline Seawater Electrolysis

Developing high-performance non-noble bifunctional catalysts is pivotal for large-scale seawater electrolysis but remains a challenge. Here we report a sandwichlike NiCo(HPO₄)₂@Ni₃N/NF (denoted by NiCoHPi@Ni₃N/NF) catalyst. Vertical Ni₃N nanosheet arrays are first grown and supported on nickel foam, and then a bimetallic NiCoHPi coating is decorated on Ni₃N nanosheets by one-step electrodeposition. The hierarchical sandwich like structure offers a large surface area and plenty of catalytic active sites, and the coupling of interconnected Ni₃N and NiCoHPi accelerates the electron transfer. Moreover, the surficial hydrogen phosphate ions contribute to a proper OH^- absorption capacity due to the Lewis acid-base reaction. As a result, the NiCoHPi@Ni₃N/NF catalyst exhibits good OER and HER activity, requiring overpotentials of 365 mV (for OER) and 174 mV (for HER) to deliver 100 mA cm^-2 in the alkaline simulated seawater electrolyte. When assembled the NiCoHPi@Ni₃N/NF catalyst as both the anode and cathode, it only needs 1.86 V to reach 100 mA cm^-2 in alkaline simulated seawater electrolyte. This work may inspire the design and exploration of self-supported hierarchical composite electrocatalysts for hydrogen production from the electrolysis of seawater.