Uniform copper-cobalt phosphides embedded in N-doped carbon frameworks as efficient bifunctional oxygen electrocatalysts for rechargeable Zn-air batteries

First-Row Transition Metals for Catalyzing Oxygen Redox

Rechargeable zinc-air batteries are widely recognized as a highly promising technology for energy conversion and storage, offering a cost-effective and viable alternative to commercial lithium-ion batteries due to their unique advantages. However, the practical application and commercialization of zinc-air batteries are hindered by the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Recently, extensive research has focused on the potential of first-row transition metals (Mn, Fe, Co, Ni, and Cu) as promising alternatives to noble metals in bifunctional ORR/OER electrocatalysts, leveraging their high-efficiency electrocatalytic activity and excellent durability. This review provides a comprehensive summary of the recent advancements in the mechanisms of ORR/OER, the performance of bifunctional electrocatalysts, and the preparation strategies employed for electrocatalysts based on first-row transition metals in alkaline media for zinc-air batteries. The paper concludes by proposing several challenges and highlighting emerging research trends for the future development of bifunctional electrocatalysts based on first-row transition metals.

A General "In Situ Etch-Adsorption-Phosphatization" Strategy for the Fabrication of Metal Phosphides/Hollow Carbon Composite for High Performance Liquid/Flexible Zn-Air Batteries

Benefiting from the admirable energy density (1086 Wh kg-1 ), overwhelming security, and low environmental impact, rechargeable zinc-air batteries (ZABs) are deemed to be attractive candidates for lithium-ion batteries. The exploration of novel oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) bifunctional catalysts is the key to promoting the development of zinc-air batteries. Transitional metal phosphides (TMPs) especially Fe-based TMPs are deemed to be a rational type of catalyst, however, their catalytic performance still needs to be further improved. Considering Fe (heme) and Cu (copper terminal oxidases) are nature's options for ORR catalysis in many forms of life from bacteria to humans. Herein, a general "in situ etch-adsorption-phosphatization" strategy is designed for the fabrication of hollow FeP/Fe2 P/Cu3 P-N, P codoped carbon (FeP/Cu3 P-NPC) catalyst as the cathode of liquid and flexible ZABs. The liquid ZABs manifest a high peak power density of 158.5 mW cm-2 and outstanding long-term cycling performance (≈1100 cycles at 2 mA cm-2 ). Similarly, the flexible ZABs deliver superior cycling stability of 81 h at 2 mA cm-2 without bending and 26 h with different bending angles.

A dual-confinement strategy to construct cobalt-based phosphide nanoclusters within carbon nanofibers for bifunctional water splitting electrocatalysts

It is a challenging task to explore highly active and stable noble-metal-free bifunctional electrocatalysts for water splitting, both in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, a new dual-confinement strategy for the fabrication of cobalt-base phosphide in the carbon nanofibers (CNFs) was proposed via electrospinning, followed by the corresponding pyrolysis. The ultrafine phosphides derived from the pore confinement of ZIF and space confinement of the polymer revealed abundant active sites and P defects. More importantly, by introducing a second metal element Ni or Cu, the electronic structure and synergistic effect were further enhanced, and the obtained bimetallic CoNiP_x-CNF electrocatalyst exhibited the remarkable performance for HER and OER, featuring the low η₁₀ values of 154 and 269 mV in 1.0 M KOH electrolyte, respectively. CoNiP_x-CNFs as a catalyst for both anode and cathode showed a current density of 10 mA cm^-2 at a voltage of 1.56 V, exceeding better stability, which is superior to most non-noble metal electrocatalysts reported in a previous research. The dual-confinement strategy is believed to provide an effective and simple approach for the synthesis of high-performance and cost-efficient bifunctional electrocatalysts for overall water splitting.

Fe incorporation-induced electronic modification of Co-tannic acid complex nanoflowers for high-performance water oxidation

The exploration of high-efficiency, cost-effective and earth-abundant non-noble metal electrocatalysts toward oxygen evolution reaction (OER) is of vital importance for the advancement of renewable energy conversion technologies. Herein, we report...

Functionality and design of Co-MOFs: unique opportunities in electrocatalysts for oxygen reduction reaction

The first report highlighting miraculous and intelligent electrocatalysts that can be tailored to form useful structures and morphologies with active sites for the oxygen reduction reaction.

In situ encapsulation engineering boosts the electrochemical performance of highly graphitized N-doped porous carbon-based copper-cobalt selenides for bifunctional oxygen electrocatalysis

Transition-metal selenides are gaining prominence as promising electrode materials for energy storage applications owing to their low electronegativity and environment-friendliness compared with metal sulfides/oxides. Herein, a CuCoSe@NC nanocomposite with copper-cobalt selenides embedded in highly graphitized N-doped porous carbon was synthesized by an in situ encapsulation strategy with metal-organic framework crystals (CuCo-BDC) as templates followed by selenization, and used as a bifunctional electrocatalyst for Zn-air batteries in lye. The result shows that the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic activity of the optimal CuCoSe@NC-2 was enhanced, and the assembled Zn-air batteries exhibited a remarkable electrochemical performance with the use of the CuCoSe@NC-2 electrode, including a high power density (137.1 mW cm^-2) and excellent charge-discharge cycling stability, which were better than those of the Pt/C + RuO₂ electrocatalyst. Such improvement is attributed not only to the higher porosity and larger specific surface area (342 m² g^-1) of the carbon matrix, which increased the contact area with oxygen-containing species, but also the encapsulation effect of the highly graphitized N-doped carbon layer and the high content of pyridine-N species also further improved the conductivity of selenide composites. This work has introduced N-doped bimetallic selenides as an ideal candidate for bifunctional electrocatalysts.

ZIF-67-based catalysts for oxygen evolution reaction

As a new type of crystalline porous material, the imidazole zeolite framework (ZIF) has attracted widespread attention due to its ultra-high surface area, large pore volume, and unique advantage of easy functionalization. Developing different methods to control the shape and composition of ZIF is very important for its practical application as catalyst. In recent years, nano-ZIF has been considered an electrode material with excellent oxygen evolution reaction (OER) performance, which provides a new way to research electrolyzed water. This review focuses on the morphological engineering of the original ZIF-67 and its derivatives (core-shell, hollow, and array structures) through doping (cation doping, anion doping, and co-doping), derivative composition engineering (metal oxide, phosphide, sulfide, selenide, and telluride), and the corresponding single-atom catalysis. Besides, combined with DFT calculations, it emphasizes the in-depth understanding of actual active sites and provides insights into the internal mechanism of enhancing the OER and proposes the challenges and prospects of ZIF-67 based electrocatalysts. We summarize the application of ZIF-67 and its derivatives in the OER for the first time, which has significantly guided research in this field.

Hydrophobic POM Electrocatalyst Achieves Low Voltage "Charge" in Zn-Air Battery Coupled with Bisphenol A Degradation

Zn-air batteriesare a perspective power source for grid-storage. But, after they are discharged at1.1 to 1.2 V, large overpotential is required for their charging (usually 2.5 V). This is due to a sluggish oxygen evolution reaction (OER). Incorporating organic pollutants into the cathode electrolyte is a feasible strategy for lowering the required charging potential. In the discharge process, the related oxygen reduction reaction, hydrophobic electrocatalysts are more popular than hydrophilic ones. Here, a hydrophobic bifunctional polyoxometalate electrocatalyst is synthesized by precise structural design. It shows excellent activities in both bisphenol A degradation and oxygen reduction reactions. In bisphenol A containing electrolyte, to achieve 100 mA ⋅ cm^-2 , its potential is only 1.32 V, which is 0.34 V lower than oxygen evolution reaction. In the oxygen reduction reaction, this electrocatalyst follows the four-electron mechanism. In both bisphenol A degradation and oxygen reduction reactions, it shows excellent stability. With this electrocatalyst as cathode material and bisphenol A containing KOH as electrolyte, a Zn-air battery was assembled. When "charged" at 85 mA ⋅ cm^-2 , it only requires 1.98 V. Peak power density of this Zn-air battery reaches 120.5 mW ⋅ cm^-2 . More importantly, in the "charge" process, bisphenol A is degraded, which achieves energy saving and pollutant removal simultaneously in one Zn-air battery.

Binder-Free Air Electrodes for Rechargeable Zinc-Air Batteries: Recent Progress and Future Perspectives

Designing an efficient air electrode is of great significance for the performance of rechargeable zinc (Zn)-air batteries. However, the most widely used approach to fabricate an air electrode involves polymeric binders, which may increase the interface resistance and block electrocatalytic active sites, thus deteriorating the performance of the battery. Therefore, binder-free air electrodes have attracted more and more research interests in recent years. This article provides a comprehensive overview of the latest advancements in designing and fabricating binder-free air electrodes for electrically rechargeable Zn-air batteries. Beginning with the fundamentals of Zn-air batteries and recently reported bifunctional active catalysts, self-supported air electrodes for liquid-state and flexible solid-state Zn-air batteries are then discussed in detail. Finally, the conclusion and the challenges faced for binder-free air electrodes in Zn-air batteries are also highlighted.

The cobalt carbide/bimetallic CoFe phosphide dispersed on carbon nanospheres as advanced bifunctional electrocatalysts for the ORR, OER, and rechargeable Zn-air batteries

It is very important, but also challenging to produce high-activity, high durability and affordable non-noble-metal-bifunctional-electrocatalysts for sustainable energy application. Here, one-pot synthesized iron covalent porphyrin polymers (FePor-CPP), with carefully placed Fe, N atoms, a regular porous structure, Co₃[Co(CN)₆]₂ and NaH₂PO₂ precursors were carbonized into N,P-doped carbon nanospheres with the active species of both bimetallic CoFe phosphides and CoC_x nanoparticles (denoted as CoC_x/(Co_0.55Fe_1.945)₂P@C). By employing the CoC_x/(Co_0.55Fe_1.945)₂P@C as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrode catalysts, superior catalytic activity is achieved with E_1/2 of 0.84 V for ORR, and overpotential of 0.39 V at 10 mA cm^-2 for OER in an alkaline medium, respectively. Furthermore, CoC_x/(Co_0.55Fe_1.945)₂P@C as air electrode for rechargeable Zn-air battery shows power density as high as 131 mW cm^-2 and charge-discharge cycle stability, and this suggests the potential application of CoC_x/(Co_0.55Fe_1.945)₂P@C in energy transformation systems. The high electrocatalytic performances are revealed to originate from the change of electronic structure of bimetallic (Co_0.55Fe_1.945)₂P via introducing P into the Co_0.55Fe_1.945 alloy, resulting in a decreased energy gap of CoC_x/(Co_0.55Fe_1.945)₂P@C relative to that of CoC_x/Co_0.55Fe_1.945@C. This work proposes a versatile strategy to develop multifunctional non-precious catalysts for this kind of energy-related electrocatalytic reactions.

MOF-derived Co/Cu-embedded N-doped carbon for trifunctional ORR/OER/HER catalysis in alkaline media

In this report, we demonstrate a bimetallic Co/Cu-embedded N-doped carbon structure for trifunctional catalysis of oxygen reduction, oxygen evolution and hydrogen evolution reactions in alkaline media. A hybrid catalyst synthesized through a metal-organic framework-based process (M-NC-CoCu) enables an active trifunctional catalysis due to its multi-faceted favorable characteristics. It is believed that a range of catalytically active sites are formed through the approach including well-dispersed tiny CuCo2O4 phases, a high concentration of pyridinic and graphitic N, and Cu-Ox, Cu-Nx and Co-Nx moieties. In addition, a high-surface-area morphology with a high concentration of sp2 bonding, which is beneficial for facilitated electron conduction, further contributes to the performance as an electrocatalyst.

Recent advances of noble-metal-free bifunctional oxygen reduction and evolution electrocatalysts

Bifunctional oxygen reduction and evolution constitute the core processes for sustainable energy storage. The advances on noble-metal-free bifunctional oxygen electrocatalysts are reviewed.

Cu/S-Occupation Bifunctional Oxygen Catalysts for Advanced Rechargeable Zinc-Air Batteries

The design and synthesis of low-cost and highly efficient bifunctional catalysts is an inevitable path for rechargeable zinc-air batteries (rZABs). In this work, double-carbon co-supported Co-based oxide with the Cu and S substitutions are synthesized by a one-step hydrothermal method and formed a unique honeycomb structure. As expected, the (Cu, Co)₃OS₃@CNT-C₃N₄ exhibits high oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity with low overpotential (0.86 V), high power density (215 mW cm^-2), and long-term discharge stability (115 h). The (Cu, Co)₃OS₃@CNT-C₃N₄-based rZAB also shows a stronger charge-discharge durability with a very low voltage gap of merely 0.5 V than that of Pt/C+RuO₂. The high catalytic performances are attributed to these following reasons: (i) the porous morphology and hierarchical structure with plentiful "catalytic buffer", which accelerates the mass transfer; (ii) a high-speed electronic transmission network established by C₃N₄ and carbon nanotube (CNT), enhancing the conductivity; (iii) the strong synergistic effect between (Cu, Co)₃OS₃@CNT and C₃N₄, which improves the kinetics of ORR/OER; and (iv) the controllable occupation of Cu ions and S ions, which effectively regulates the CoO₆ surface and increases the active site density. This work not only offers a promising ORR/OER electrode for rZAB but also provides a new pathway to understand the improvement mechanism for catalysts by the bi-ion substitutions.

Fe-MOF-Derived Efficient ORR/OER Bifunctional Electrocatalyst for Rechargeable Zinc-Air Batteries

The construction of an efficient oxygen reduction reaction and oxygen evolution reaction (ORR/OER) bifunctional electrocatalyst is of great significance but still remains a giant challenge for high-performance metal-air batteries. In this study, uniform FeS/Fe₃C nanoparticles embedded in a porous N,S-dual doped carbon honeycomb-like composite (abbr. FeS/Fe₃C@NS-C-900) have been conveniently fabricated by pyrolysis of a single-crystal Fe-MOF, which has a low potential gap ΔE of ca. 0.72 V, a competitive power density of 90.9 mW/cm², a specific capacity as high as 750 mAh/g_Zn, and excellent cycling stabilities over 865 h (1730 cycles) at 2 mA/cm² when applied as a cathode material for rechargeable zinc-air batteries. In addition, the two series-linked Zn-air batteries successfully powered a 2.4 V LED light as a real power source. The efficient ORR/OER bifunctional electrocatalytic activity and long-term durability of the obtained composite might be attributed to the characteristic honeycomb-like porous structure with sufficient accessible active sites, the synergistic effect of FeS and Fe₃C, and the N,S codoped porous carbon, which provides a promising application potential for portable electronic Zn-air battery related devices.

KB-templated in situ synthesis of highly dispersed bimetallic NiFe phosphides as efficient oxygen evolution catalysts

KB-templated in situ synthesized highly dispersed bimetallic NiFe phosphides function as efficient oxygen evolution catalysts.