Trimetallic Zeolitic imidazolite framework-derived Co nanoparticles@CoFe-nitrogen-doped porous carbon as bifunctional electrocatalysts for Zn-air battery

Coupling CoOx@NC with NiFe LDH Enhances Oxygen Electrocatalysis for Rechargeable High-Efficiency Zn-Air Batteries

Developing bifunctional catalysts with the high oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalytic activities is of great significance in improving the efficiency of rechargeable Zn-air batteries. Herein, this study adopts simple one-pot pyrolysis and subsequent hydrothermal strategies to prepare a high-performance bifunctional composite catalyst by coupling a layered hydroxide NiFe LDH with Co and N codoped carbon framework, composed of cobalt oxide and metallic Co nanoparticles distributed on interconnected porous carbon nanosheets and carbon nanotubes (CNTs). Benefiting from the exposure of more active sites, high conductivity, and beneficial charge transport advantages, the best catalyst (CoOx@NC)2─NiFe demonstrates excellent bifunctional electrocatalytic activities and stability for oxygen-related electrochemical reactions, exhibiting a low potential gap of 0.757 V. Additionally, the rechargeable Zn-air battery assembled with the (CoOx@NC)2─NiFe catalyst performs a peak power density of 136 mW cm-2, and a specific capacity of 803 mAh gZn -1 at 10 mA cm-2, and outstanding stability (over 200 hours). This work presents a simple and low-cost approach for synthesizing bifunctional catalysts to realize efficient and durable rechargeable zinc-air batteries.

Robust Spray Combustion Enabling Hierarchical Porous Carbon-Supported FeCoNi Alloy Catalyst for Zn-Air Batteries

Rechargeable Zn-air batteries (RZABs) are poised for industrial application, yet they require low-cost, high-performance catalysts that efficiently facilitate both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). The pivotal challenge lies in designing multimetal active sites and optimizing the carbon skeleton structure to modulate catalyst activity. In this study, we introduce a novel hierarchical porous carbon-supported FeCoNi bifunctional catalyst, synthesized via a spray combustion method. The carbon, derived from sucrose, was tailored into a hierarchical porous morphology through etching with NO3- ions and NaCl, thereby significantly increasing the surface area for the interaction of the O2 and electrolyte interaction. The in situ formation of FeCoNi alloy nanoparticles ensures their uniform dispersion and anchoring, facilitating electron transport. The strong interaction and charge transfer at the heterogeneous FeCoNi alloy interfaces, along with nitrogen doping, which enhances the OER/ORR activity, endow the FeCoNi/N-PC catalyst with exceptional bifunctional catalytic properties, characterized by an activity parameter of 0.73 V. Furthermore, the RZAB assembled with this catalyst demonstrates outstanding cycling stability and reversibility, with a minimal round-trip efficiency decay of 7.6% over 1380 cycles (460 h) at 10 mA cm-2.

Enhanced oxygen evolution and power density of Co/Zn@NC@MWCNTs for the application of zinc-air batteries

Rechargeable zinc-air batteries (ZABs) are viewed as a promising solution for electric vehicles due to their potential to provide a clean, cost-effective, and sustainable energy storage system for the next generation. Nevertheless, sluggish kinetics of the oxygen evolution reaction (OER), the oxygen reduction reaction (ORR) at the air electrode, and low power density are significant challenges that hinder the practical application of ZABs. The key to resolving the development of ZABs is developing an affordable, efficient, and stable catalyst with bifunctional catalytic. In this study, we present a series of bifunctional catalysts composed of Co/Zn nanoparticles uniformly embedded in nitrogen-doped carbon (NC) and multi-walled carbon nanotubes (MWCNTs) denoted as Co/Zn@NC@MWCNTs. The incorporation of MWCNTs using a facile and non-toxic method significantly decreased the overpotential of the OER from 570 to 430 mV at 10 mA cm-2 and the peak power density from 226 to 263 mW cm-2. Besides, the electrochemical surface area measurements and electrochemical impedance spectroscopy indicate that the three-dimensional (3D) network structure of MWCNTs facilitates mass transport for ORR and reduces electron transfer resistance during OER, leading to a small potential gap of 0.86 V between OER and ORR, high electron transfer number (3.92-3.98) of the ORR, and lowest Tafel slope (47.8 mV dec-1) of the OER in aqueous ZABs. In addition, in-situ Raman spectroscopy revealed a notable decrease in the ID/IG ratio for the optimally configured Co/Zn@NC@MWCNTs (75:25), indicating a reduction in defect density and improved structural ordering during the electrochemical process, which directly contributes to enhanced ORR activity. Hence, this study provides an excellent strategy for constructing a bifunctional catalyst material with a 3D MWCNTs conductive network for the development of advanced ZAB systems for sustainable energy applications.

Spray Pyrolysis Regulated FeCo Alloy Anchoring on Nitrogen-Doped Carbon Hollow Spheres Boost the Performance of Zinc-Air Batteries

Low-cost and high-efficiency non-precious metal-based oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) bifunctional catalysts are the key to promoting the commercial application of metal-air batteries. Herein, a highly efficient catalyst of Fe0.18Co0.82 alloy anchoring on the nitrogen-doped porous carbon hollow sphere (FexCo1-x/N-C) is intelligently designed by spray pyrolysis (SP). The zinc in the SP-derived metal oxides and metal-organic framework volatilize at high temperature to construct a hierarchical porous structure with abundant defects and fully exposes the FeCo nanoparticles which uniformly anchor on the carbon substrate. In this structure, the coexistence of Fe0.18Co0.82 alloy and binary metal active sites (Fe-Nx/Co-Nx) guarantees the Fe0.2Co0.8/N-C catalyst exhibiting an excellent half-wave potential (E1/2 ═ 0.84 V) superior to 20% Pt/C for ORR and a suppressed overpotential (280 mV) than RuO2 for OER. Assembled rechargeable Zn-air battery (RZAB) demonstrates a promising specific capacity of 807.02 mAh g-1, peak power density of 159.08 mW cm-2 and durability without electrolyte circulation (550 h). This work proposes the design concept of utilizing an oxide core to in situ consume the porous carbon shell for anchoring metal active sites and construct defects, which benefits from spray pyrolysis in achieving precise control of the alloy structure and mass preparation.

Design Principles and Mechanistic Understandings of Non-Noble-Metal Bifunctional Electrocatalysts for Zinc-Air Batteries

Zinc-air batteries (ZABs) are promising energy storage systems because of high theoretical energy density, safety, low cost, and abundance of zinc. However, the slow multi-step reaction of oxygen and heavy reliance on noble-metal catalysts hinder the practical applications of ZABs. Therefore, feasible and advanced non-noble-metal electrocatalysts for air cathodes need to be identified to promote the oxygen catalytic reaction. In this review, we initially introduced the advancement of ZABs in the past two decades and provided an overview of key developments in this field. Then, we discussed the working mechanism and the design of bifunctional electrocatalysts from the perspective of morphology design, crystal structure tuning, interface strategy, and atomic engineering. We also included theoretical studies, machine learning, and advanced characterization technologies to provide a comprehensive understanding of the structure-performance relationship of electrocatalysts and the reaction pathways of the oxygen redox reactions. Finally, we discussed the challenges and prospects related to designing advanced non-noble-metal bifunctional electrocatalysts for ZABs.

Electronic-State Modulation of Metallic Co-Assisted Co7 Fe3 Alloy Heterostructure for Highly Efficient and Stable Overall Water Splitting

Manipulating electronic structure of alloy-based electrocatalysts can eagerly regulate its catalytic efficiency and corrosion resistance for water splitting and fundamentally understand the catalytic mechanisms for oxygen/hydrogen evolution reactions (OER/HER). Herein, the metallic Co-assisted Co7 Fe3 alloy heterojunction (Co7 Fe3 /Co) embeds in a 3D honeycomb-like graphitic carbon is purposely constructed as a bifunctional catalyst for overall water splitting. As-marked Co7 Fe3 /Co-600 displays the excellent catalytic activities in alkaline media with low overpotentials of 200 mV for OER and 68 mV for HER at 10 mA cm-2 . Theoretical calculations reveal the electronic redistribution after coupling Co with Co7 Fe3 , which likely forms the electron-rich state over interfaces and the electron-delocalized state at Co7 Fe3 alloy. This process changes the d-band center position of Co7 Fe3 /Co and optimizes the affinity of catalyst surface to intermediates, thus promoting the intrinsic OER/HER activities. For overall water splitting, the electrolyzer only requires a cell voltage of 1.50 V to achieve 10 mA cm-2 and dramatically retains 99.1% of original activity after 100 h of continuous operation. This work proposes an insight into modulation of electronic state in alloy/metal heterojunctions and explores a new path to construct more competitive electrocatalysts for overall water splitting.

Facile synthesis of a MOF-derived Co-N-C nanostructure as a bi-functional oxygen electrocatalyst for rechargeable Zn-air batteries

A novel catalyst obtained from the pyrolysis of a Co/Fe/Zn zeolitic imidazolite framework was prepared as an oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalyst. The Co-N-C-900 catalyst displays promising ORR and OER activity with E_1/2 = 0.854 V and E_j=10 = 1.780 V. The rechargeable Zn-air battery equipped with a Co-N-C-900 cathode electrocatalyst illustrates a high peak power density of 275 mW cm^-2, which is much superior than that of commercial 20% Pt/C. Significantly, the designed Zn-air battery with the Co-N-C-900 catalyst presents good cycling stability for 180 h in the rechargeable Zn-air battery.

Conductivity-enhanced porous N/P co-doped metal-free carbon significantly enhances oxygen reduction kinetics for aqueous/flexible zinc-air batteries

Heteroatom-doped metal-free carbon catalysts for oxygen reduction reactions have gained significant attention because of their unusual activity and economic cost. Here, a novel N/P co-doped porous carbon catalyst (NPPC) with a high surface area for oxygen reduction reaction (ORR) is constructed by a facile high-temperature calcination method employing ZIF-8 as the precursor and red phosphorus as the phosphorus source. In particular, ZIF-8 is firstly calcined to obtain N-doped carbon (NC) followed by further calcination with red phosphorus to obtain NPPC. Ultraviolet photoelectron spectroscopy (UPS) analysis shows that the ultra-low amount of P doping could significantly decrease the work function from 4.32 to 3.86 eV. The resultant catalyst exhibits a promising electrocatalytic activity with a half-wave potential (E_1/2) of 0.87 V and a limiting current density (J_L) of 5.15 mA cm^-2. Besides, it also shows improved catalytic efficiency and excellent durability with a negligible decay of J_L after 2000 CV cycles. Moreover, aqueous and solid-state flexible zinc-air batteries (ZAB) using the catalyst show a promising application potential. This work provides new insight into developing P/N-doped metal-free carbon ORR catalysts.

Advances in Titanium Carbide (Ti3C2T x ) MXenes and Their Metal-Organic Framework (MOF)-Based Nanotextures for Solar Energy Applications: A Review

Introducing new materials with low cost and superior solar harvesting efficiency requires urgent attention to solve energy and environmental challenges. Titanium carbide (Ti3C2T x ) MXene, a 2D layered material, is a promising solution to solve the issues of existing materials due to their promising conductivity with low cost to function as a cocatalyst/support. On the other hand, metal-organic frameworks (MOFs) are emerging materials due to their high surface area and semiconducting characteristics. Therefore, coupling them would be promising to form composites with higher solar harvesting efficiency. Thus, the main objective of this work to disclose recent development in Ti3C2T x -based MOF nanocomposites for energy conversion applications to produce renewable fuels. MOFs can generate photoinduced electron/hole pairs, followed by transfer of electrons to MXenes through Schottky junctions for photoredox reactions. Currently, the principles, fundamentals, and mechanism of photocatalytic systems with construction of Schottky junctions are critically discussed. Then the basics of MOFs are discussed thoroughly in terms of their physical properties, morphologies, optical properties, and derivatives. The synthesis of Ti3C2T x MXenes and their composites with the formation of surface functionals is systematically illustrated. Next, critical discussions are conducted on design considerations and strategies to engineer the morphology of Ti3C2T x MXenes and MOFs. The interfacial/heterojunction modification strategies of Ti3C2T x MXenes and MOFs are then deeply discussed to understand the roles of both materials. Following that, the applications of MXene-mediated MOF nanotextures in view of CO2 reduction and water splitting for solar fuel production are critically analyzed. Finally, the challenges and a perspective toward the future research of MXene-based MOF composites are disclosed.

Metal-organic frameworks/ hydrotalcite/graphene oxide sandwich composites derived Fe-Ce@GSL hierarchical materials as highly efficient catalysts for rechargeable Zn-air batteries

The fabrication of efficient bi-functional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) applied in energy storage and conversion devices like Zn-air batteries to solve the growing energy and environmental crises has attracted great attentions. In this work, the Fe-Ce@GSL catalysts have developed by first constructing the MOF/LDH/GO templates with multi-stage mixed growth method followed by calcining the template at high temperature. Fe-Ni-LDH (hydrotalcite) plays the role of linking the metal organic frameworks (Fe-Ce-MOF) and graphene oxides (GO), avoiding the separation of MOFs derivatives and GO sheets during pyrolysis process. Rare-earth metal oxide (CeO₂) featuring with abundant oxygen vacancies dispersed on the surface of transition-metal oxide can efficiently improve the stability of catalysts. The optimal Fe₇-Ce₁@GSL-800 catalysts exhibit excellent ORR/OER performances with the potential gap between ORR (E_1/2 = 0.87 V) and OER (E_J=10 = 1.59 V) of 0.720 V. The aqueous Zn-air battery assembled with Fe₇-Ce₁@GSL-800 catalysts exhibits outstanding performances with high open circuit voltage (1.56 V), large specific capacity (801.1 mAh/g@10 mA.cm^-2), and good charge-discharge cycle performances (>500 h). The Fe₇-Ce₁@GSL-800 based solid-state Zn-air battery also delivers an excellent performance with high specific capacity (791.7 mAh/g@5 mA.cm^-2) and long cycle stability (>230 h).

Conversion of rice husk biomass into electrocatalyst for oxygen reduction reaction in Zn-air battery: Effect of self-doped Si on performance

An outstanding oxygen reduction reaction (ORR) electrocatalyst is firstly developed deriving from sustainable rice husk (RH) biomass. Benefiting from self-doped Si in RH, the higher proportion of pyridine N, graphite N and expecially Fe-N_x as well as thiophene S contents were produced in Si-Fe/S/N-RH₃ in comparison with those of Si-free Fe/S/N-RH_3. Consequently, the half-wave potential of 0.89 V and the onset potential of 0.96 V are achieved for Si-Fe/S/N-RH₃, outperforming the benchmark electrocatalyst Pt/C and other Fe-based electrocatalysts reported in alkaline media. Furthermore, it is found that the exisentence of self-doped Si can improve the graphitization degree of the catalyst, leading to the long-term stability (larger than 85% retention after 40000 s) and prominent methanol tolerance for Si-Fe/S/N-RH₃. In addition, Si-Fe/S/N-RH₃ shows a power density of 86.2 mW cm^-2 and excellent durability in Zn-air battery. The work highlights the potential to develop sustainable and cost-effective ORR electrocatalysts from waste biomass as the substitute for precious metal catalysts.

Constructing Porous Carbon Electrocatalysts from Cobalt Complex-Decorated Micelles of Mesoporous Silica for Oxygen Reduction/Evolution Reaction

The construction of a porous carbon structure with a high specific surface area is conducive to enhanced electrocatalytic activity due to the accessibility of active sites and improvement of the mass transfer. Herein, we explored the possibility of using micelles of mesoporous silica (MCM-48) as the carbon source to generate porous carbon under the confinement of MCM-48 channels. The complexes formed by Co²⁺ and 4,4'-bipyridine were in situ incorporated into the micelles to derive Co-related active sites (Co-N_x, Co, and Co₃O₄) for catalyzing the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). After pyrolysis in the N₂ atmosphere and subsequent removal of the MCM-48 skeleton, the target porous carbon electrocatalyst was obtained, which exhibited promising performance for both ORR and OER and has great potential as the cathode material for Zn-air battery application. This work not only confirms the effectiveness of using the micelles of MCM-48 as the carbon source for preparing the porous carbon materials, but also provides a new platform for design and synthesis of structurally controllable materials for energy-related electrocatalytic applications.

Zn, Co, and Fe Tridoped N-C Core-Shell Nanocages as the High-Efficiency Oxygen Reduction Reaction Electrocatalyst in Zinc-Air Batteries

Transition metal-nitrogen-carbon (TM-N-C) nanomaterials are promising platinum-based substitutes for the oxygen reduction reaction (ORR). However, large-scale commercial production of high-efficiency, durable TM-N-C catalysts remains a formidable challenge. In this work, a facile ″ZIF-on-ZIF″ strategy is first adopted to design ZIF-8@ZIF-67 core-shell polyhedral nanocages, and then, ferrocene (Fc) is added to form ZIF-8@ZIF-67@Fc double-layer encapsulating polyhedral nanocages. Finally, Zn, Co, and Fe tridoped N-C nanocages (ZnCoFe-N-C) as the high-efficiency ORR electrocatalyst are prepared through high-temperature annealing. Benefiting from the trimetal, nitrogen and carbon species bond to each other to form highly efficient active sites, and the material exhibits outstanding performance in 0.1 M KOH, onset potential and half-wave potential of up to 0.95 and 0.878 V (vs RHE), respectively, and long-term durability and methanol tolerance. Furthermore, when utilizing as a zinc-air battery (ZAB) air electrode, it exhibits wonderful indicators, reflected in an open circuit voltage of 1.525 V, power density of 350.2 mW cm^-2, and specific capacity of 794.7 mAh g_zn^-1, which outperforms the benchmark Pt/C catalyst. This work provides a facile and effective strategy to obtain a highly efficient and stable TM-N-C electrocatalyst for the ORR in ZABs.

Boron-nitrogen-doped carbon dots on multi-walled carbon nanotubes for efficient electrocatalysis of oxygen reduction reactions

Cost-effective production of metal-free catalysts for the oxygen-reduction reaction (ORR), to supersede Pt-based catalysts, is challenging. Here, a three-dimensional nanocatalyst was prepared by compounding multi-wall carbon nanotubes (MWCNTs) with easily modified and doped carbon dots (CDs) as sources of B and N. The catalyst has high conductivity and a large specific surface area similar to the MWCNTs, allowing exposure of many CDs with rich edge active sites and enhancing electron transfer. The catalyst exhibits excellent ORR performance, with 0.92 V of E_onset vs reversible hydrogen electrode (RHE). The E_1/2 value exhibits a reduction of 50 mV compared with that of Pt/C (0.85 V) with a limited current density of 5.95 mA cm^-2. The enhanced catalytic performance is attributed to the synergy of pyridine N and BC₃. This work describes a simple and economical strategy for metal-free catalysts, and promotes the development of such catalysts for metal-air batteries and fuel cells.