High-Quality CoFeP Nanocrystal/N, P Dual-Doped Carbon Composite as a Novel Bifunctional Electrocatalyst for Rechargeable Zn-Air Battery

Cerium-Based Electrocatalysts for Oxygen Evolution/Reduction Reactions: Progress and Perspectives

Ce-based materials have been widely used in photocatalysis and other fields because of their rich redox pairs and oxygen vacancies, despite research on the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) remaining scare. However, most pristine cerium-based materials, such as CeO₂, are non-conductive materials. Therefore, how to obtain highly conductive and stable OER/ORR electrocatalysts is currently a hot research topic. To overcome these limitations, researchers have proposed a variety of strategies to promote the development of Ce-based electrocatalysts in recent years. This progress report focuses on reviewing new strategies concerning three categories of Ce-based electrocatalysts: metal-organic framework (MOF) derivatives, structure tuning, and polymetallic doping. It also puts forward the main existing problems and future prospects. The content of cerium in the crust is about 0.0046%, which is the highest among the rare earth elements. As a low-cost rare earth material, Ce-based materials have a bright future in the field of electrocatalysis due to replacing precious metal and some transition metals.

Flexible Solid-State Metal-Air Batteries: The Booming of Portable Energy Supplies

The rapid development of portable and wearable electronics has given rise to new challenges and provoked research in flexible, lightweight, and affordable energy storage devices. Flexible solid-state metal-air batteries (FSSMABs) are considered promising candidates, owing to their large energy density, mechanical flexibility, and durability. However, the practical applications of FSSMABs require further improvement to meet the demands of long-term stability, high power density, and large operating voltage. This Review presents a detailed discussion of innovative electrocatalysts for the air cathode, followed by a sequential overview of high-performance solid-state electrolytes and metal anodes, and a summary of the current challenges and future perspectives of FSSMABs to promote practical application and large-scale commercialization in the near future.

Designing High-Quality Electrocatalysts Based on CoO:MnO2@C Supported on Carbon Cloth Fibers as Bifunctional Air Cathodes for Application in Rechargeable Zn-Air Battery

To achieve the requirements of rechargeable Zn-air batteries (ZABs), designing efficient, bifunctional, stable, and cost-effective electrocatalysts is vital for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which still are struggling with unsolved challenges. The present research provides a concept based on the nanoscale composites which were engineered by using MnO₂@C, CoO@C, and CoO:MnO₂@C bifunctional electrocatalysts for fabrication of uniform carbon cloth (CC)-based electrodes. The CoO:MnO₂@C electrocatalyst represented more efficient electrochemical properties through ORR and OER processes with superior positive half-wave potential (E_1/2 = 0.78 V) and better limiting current density (i = 1.10 mA cm^-2) in comparison with MnO₂@C (E_1/2 = 0.71 V, i = 0.92 mA cm^-2) and CoO@C (E_1/2 = 0.69 V, i = 0.86 mA cm^-2) electrocatalysts. For the rechargeable ZABs fabricated by using CoO:MnO₂@C-CC as an O₂-breathing cathode, the specific capacity (SC), peak power density (P), open-circuit voltage (E_OCV), and gap of charge/discharge voltage resulted in values of 520 mAh g_Zn^-1, 210.0 mW cm^-2, and 1.45 and 0.45 V, respectively, that afforded greater electrochemical characters than what was obtained for ZABs based on MnO₂@C-CC (410 mAh g_Zn^-1, 195.0 mW cm^-2, 1.38 and 0.44 V) and CoO@C-CC (440 mAh g_Zn^-1, 165.0 mW cm^-2, 1.15 and 0.54 V). At the same time, lower E_i=10 (= 1.45 V) implied a more efficient OER in alkaline electrolyte solution for CoO:MnO₂@C than MnO₂@C (E_i=10 = 1.50 V) and CoO@C (E_i=10 = 1.39 V). Based on cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and X-ray photoelectron spectroscopy (XPS) results, it could be stated that the CoO:MnO₂@C catalytic surface could experience 30 and 32% lower charge transfer resistance (R_ct = 13.9 Ω) than MnO₂@C (R_ct = 20.1 Ω) and CoO@C (R_ct = 29.7 Ω), respectively, which empowers an enhancement in ORR/OER performance. Prominently, the design concept of proposed electrocatalysts could suggest clear horizon for the synthesis and development paradigms of bifunctional catalysts for energy storage materials and devices.

Cobalt-Doped Iron Phosphate Crystal on Stainless Steel Mesh for Corrosion-Resistant Oxygen Evolution Catalyst

We report an oxygen evolution reaction (OER) catalyst prepared by the incorporation of cobalt-doped iron phosphate on stainless steel mesh (SSM) through a one-step hydrothermal method. Compared to the catalytic property of bare SSM, our OER catalyst (0.84-CoFePi) showed a 42% improvement in current density at the potential of 1.9 V vs. RHE, and the onset potential was decreased by 26.5 mV. Furthermore, the loss in current density of bulk electrolysis after 12 h in 1 M KOH (pH 14) solution and 0.0441 wt% H2SO4 (pH ≈ 3) containing 0.1 M NaCl solution was negligible (3.1% and 3.2%, respectively). Moreover, our cobalt-doped iron phosphate on SSM exhibits the dramatic improvement in corrosion resistance to a basic, mild acidic solution and chloride ions compared to bare SSM.

A Superior and Stable Electrocatalytic Oxygen Evolution Reaction by One-Dimensional FeCoP Colloidal Nanostructures

Transition metal phosphides (TMPs) are expected to be excellent electrocatalysts for oxygen evolution reaction (OER) because of their high stability, highly conducting metalloid nature, highly abundant constituting elements, and the ability to act as a precatalyst due to in situ surface-formed oxy-hydroxide species. Herein, a "one-pot" colloidal approach has been used to develop a rod-shaped one-dimensional non-noble metal FeCoP electrocatalyst, which exhibits an excellent OER activity with an exceptionally high current density of 950 mA cm^-2, a turnover frequency value of 7.43 s^-1, and a low Tafel slope value of 54 mV dec^-1. The FeCoP electrocatalyst affords OER ultralow overpotentials of 230 and 260 mV at current densities of 50 and 100 mA cm^-2, respectively, in 1.0 M KOH, and demonstrates a superior catalytic stability of 10,000 cycles and durability up to 60 h at 50 mA cm^-2. An insight into the superior and stable electrocatalytic OER performance by the FeCoP nanorods is obtained by extensive X-ray photoelectron spectroscopy, X-ray diffraction, Raman and infrared spectroscopy, and cyclic voltammetry analyses for a mechanistic study. This reveals that a high number of electrocatalytically active sites enhance the oxygen evolution and kinetics by offering metal ion sites for utilitarian in situ surface formation and adsorption of *O, *OH, and *OOH reactive species for OER catalysis.

Significant Change of Metal Cations in Geometric Sites by Magnetic-Field Annealing FeCo2 O4 for Enhanced Oxygen Catalytic Activity

The application of magnetic fields in the oxygen reduction/evolution reaction (ORR/OER) testing for electrocatalysts has attracted increasing interest, but it is difficult to characterize on-site surface reconstruction. Here, a strategy is developed for annealing-treated FeCo₂ O₄ nanofibers at a magnetic field of 2500 Oe, named FeCo₂ O₄ -M, showing a right-shifted half-wave potential of 20 mV for the ORR and a left-shifted overpotential of 60 mV at 10 mV cm^-2 for the OER as compared with its counterpart. Magnetic characterizations indicate that FeCo₂ O₄ -M shows the spin-state transition of cations from a low-spin state to an intermediate-spin state compared with FeCo₂ O₄ . Mössbauer spectra show that the Fe³⁺ ion in the octahedral site (0.76) of FeCo₂ O₄ -M is more than that of FeCo₂ O₄ (0.71), indicating the effective stimulus of metal cations in geometric sites by magnetic-field annealing. Furthermore, theoretical calculations demonstrate that the d-band centers (ε_d ) of Co 3d and Fe 3d in the tetrahedral and octahedral sites of the FeCo₂ O₄ -M nanofibers shift close to the Fermi level, revealing the enhanced mechanism of the ORR/OER activity.

Frontiers and Structural Engineering for Building Flexible Zinc-Air Batteries

With the development of flexible devices, the demand for wearable power sources has increased and gradually become imperative. Zinc-air batteries (ZABs) have attracted lots of research interest due to their high theoretical energy density and excellent safety properties, which can meet the wearable energy supply requirements. Here, the flexibility of energy storage devices is discussed first, followed by the chemistries and development of flexible ZABs. The design of flexible electrodes, the properties of solid-state electrolytes (SSEs), and the construction of deformable structures are discussed in depth. The researchers working on flexible energy storage devices will benefit from the work.

Porous Indium Tin Oxide-Supported NiFe LDH as a Highly Active Electrocatalyst in the Oxygen Evolution Reaction and Flexible Zinc-Air Batteries

The oxygen evolution reaction (OER) is crucial for hydrogen production from water splitting and rechargeable metal-air batteries. However, the four-electron mechanism results in slow reaction kinetics, which needed to be accelerated by efficient catalysts. Herein, a hybrid catalyst of novel nickel-iron layered double hydroxide (NiFe LDH) on porous indium tin oxide (ITO) is presented to lower the overpotential of the OER. The as-prepared NiFe LDH@ITO catalyst showed superior catalytic activity toward the OER with an overpotential of only 240 mV at a current density of 10 mA/cm². The catalyst also offered high stability with almost no activity decay after more than 200 h of chronopotentiometry test. Furthermore, the applications of NiFe LDH@ITO in (flexible) rechargeable zinc-air batteries exhibited a better performance than commercial RuO₂ and can remain stable in cycling tests. It is supposed that the superior catalytic behavior originates from the ITO conductive framework, which prevents the agglomeration and facilitates the electron transfer during the OER process.

Anchoring Fe-N-C Sites on Hierarchically Porous Carbon Sphere and CNT Interpenetrated Nanostructures as Efficient Cathodes for Zinc-Air Batteries

Engineering efficient zinc-air batteries have attracted tremendous attention because of their essential role in the field of renewable energy systems. However, the sluggish reaction kinetics of the oxygen reduction reaction (ORR) at the air cathode impair the battery performance significantly. Recently, metal-N-C-based porous carbon nanoarchitectures have emerged as promising ORR electrocatalysts in zinc-air batteries. Herein, taking advantage of metal-organic complexation and mesoporous silica templates, we successfully anchor Fe-N-C sites on hierarchically porous carbon sphere and carbon nanotube interpenetrated nanostructures (Fe-N-C/HPCS@CNT) to serve as efficient cathodes for zinc-air batteries. Benefiting from its synergistic effects between the highly active Fe-N-C sites, ultrahigh surface areas, and unique hierarchically porous nanostructures, Fe-N-C/HPCS@CNT exhibits preferable ORR performance (E_1/2 = 0.873 V) compared to commercial Pt/C (E_1/2 = 0.841 V). Most importantly, when used as a cathode catalyst for homemade zinc-air batteries, Fe-N-C/HPCS@CNT exhibits gratifying peak power density (164.0 mW cm^-2), large specific capacity (762.0 mAh g^-1), superior long-term stability, extraordinary rate capability, and excellent charge/discharge performance. We believe that this report will not only offer new insights into the design of Fe-N-C-based catalysts but also promote the practical utilization of Fe-N-C-based cathodes for a wide range of energy applications.

Advanced Oxygen Electrocatalyst for Air-Breathing Electrode in Zn-Air Batteries

The electrochemical reduction of oxygen to water and the evolution of oxygen from water are two important electrode reactions extensively studied for the development of electrochemical energy conversion and storage technologies based on oxygen electrocatalysis. The development of an inexpensive, highly active, and durable nonprecious-metal-based oxygen electrocatalyst is indispensable for emerging energy technologies, including anion exchange membrane fuel cells, metal-air batteries (MABs), water electrolyzers, etc. The activity of an oxygen electrocatalyst largely decides the overall energy storage performance of these devices. Although the catalytic activities of Pt and Ru/Ir-based catalysts toward an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER) are known, the high cost and lack of durability limit their extensive use for practical applications. This review article highlights the oxygen electrocatalytic activity of the emerging non-Pt and non-Ru/Ir oxygen electrocatalysts including transition-metal-based random alloys, intermetallics, metal-coordinated nitrogen-doped carbon (M-N-C), and transition metal phosphides, nitrides, etc., for the development of an air-breathing electrode for aqueous primary and secondary zinc-air batteries (ZABs). Rational surface and chemical engineering of these electrocatalysts is required to achieve the desired oxygen electrocatalytic activity. The surface engineering increases the number of active sites, whereas the chemical engineering enhances the intrinsic activity of the catalyst. The encapsulation or integration of the active catalyst with undoped or heteroatom-doped carbon nanostructures affords an enhanced durability to the active catalyst. In many cases, the synergistic effect between the heteroatom-doped carbon matrix and the active catalyst plays an important role in controlling the catalytic activity. The ORR activity of these catalysts is evaluated in terms of onset potential, number of electrons transferred, limiting current density, and durability. The bifunctional oxygen electrocatalytic activity and ZAB performance, on the other hand, are measured in terms of potential gap between the ORR and OER, ΔE = E_j10^OER - E_1/2^ORR, specific capacity, peak power density, open circuit voltage, voltaic efficiency, and charge-discharge cycling stability. The nonprecious metal electrocatalyst-based ZABs are very promising and they deliver high power density, specific capacity, and round-trip efficiency. The active site for oxygen electrocatalysis and challenges associated with carbon support is briefly addressed. Despite the considerable progress made with the emerging electrocatalysts in recent years, several issues are yet to be addressed to achieve the commercial potential of rechargeable ZAB for practical applications.