Significance of Engineering the Octahedral Units to Promote the Oxygen Evolution Reaction of Spinel Oxides

The clean energy carrier, hydrogen, if efficiently produced by water electrolysis using renewable energy input, would revolutionize the energy landscape. It is the sluggish oxygen evolution reaction (OER) at the anode of water electrolyzer that limits the overall efficiency. The large spinel oxide family is widely studied due to their low cost and promising OER activity. As the distribution of transition metal (TM) cations in octahedral and tetrahedral site is an important variable controlling the electronic structure of spinel oxides, the TM geometric effect on OER is discussed. The dominant role of octahedral sites is found experimentally and explained by computational studies. The redox‐active TM locating at octahedral site guarantees an effective interaction with the oxygen at OER conditions. In addition, the adjacent octahedral centers in spinel act cooperatively in promoting the fast OER kinetics. In remarkable contrast, the isolated tetrahedral TM centers in spinel prohibit the OER mediated by dual‐metal sites. Furthermore, various spinel oxides preferentially expose octahedral‐occupied cations on the surface, making the octahedral cations easily accessible to the reactants. The future perspectives and challenges in advancing fundamental understanding and developing robust spinel catalysts are discussed.     

Enlarged Co?O Covalency in Octahedral Sites Leading to Highly Efficient Spinel Oxides for Oxygen Evolution Reaction

Cobalt‐containing spinel oxides are promising electrocatalysts for the oxygen evolution reaction (OER) owing to their remarkable activity and durability. However, the activity still needs further improvement and related fundamentals remain untouched. The fact that spinel oxides tend to form cation deficiencies can differentiate their electrocatalysis from other oxide materials, for example, the most studied oxygen‐deficient perovskites. Here, a systematic study of spinel ZnFe_xCo_2?xO₄ oxides (x = 0–2.0) toward the OER is presented and a highly active catalyst superior to benchmark IrO₂ is developed. The distinctive OER activity is found to be dominated by the metal–oxygen covalency and an enlarged Co? O covalency by 10–30 at% Fe substitution is responsible for the activity enhancement. While the pH‐dependent OER activity of ZnFe_0.4Co_1.6O₄ (the optimal one) indicates decoupled proton–electron transfers during the OER, the involvement of lattice oxygen is not considered as a favorable route because of the downshifted O p‐band center relative to Fermi level governed by the spinel's cation deficient nature.     

Efficient Ternary Mn-Based Spinel Oxide with Multiple Active Sites for Oxygen Evolution Reaction Discovered via High-Throughput Screening Methods (Small 2/2023)

The discovery of more efficient and stable catalysts for oxygen evolution reaction (OER) is vital in improving the efficiency of renewable energy generation devices. Given the large numbers of possible binary and ternary metal oxide OER catalysts, high-throughput methods are necessary to accelerate the rate of discovery. Herein, Mn-based spinel oxide, Fe₁₀Co₄₀Mn₅₀O, is identified for the first time using high-throughput methods demonstrating remarkable catalytic activity (overpotential of 310 mV on fluorine-doped tin oxide (FTO) substrate and 237 mV on Ni foam at 10 mA cm⁻²). Using a combination of soft X-ray absorption spectroscopy and electrochemical measurements, the high catalytic activity is attributed to 1) the formation of multiple active sites in different geometric sites, tetrahedral and octahedral sites; and 2) the formation of active oxyhydroxide phase due to the strong interaction of Co²⁺ and Fe³⁺. Structural and surface characterizations after OER show preservation of Fe₁₀Co₄₀Mn₅₀O surface structure highlighting its durability against irreversible redox damage on the catalytic surface. This work demonstrates the use of a high-throughput approach for the rapid identification of a new catalyst, provides a deeper understanding of catalyst design, and addresses the urgent need for a better and stable catalyst to target greener fuel.     

Ambient Fast Synthesis and Active Sites Deciphering of Hierarchical Foam‐Like Trimetal–Organic Framework Nanostructures as a Platform for Highly Efficient Oxygen Evolution Electrocatalysis

Metal–organic frameworks (MOFs) have attracted tremendous interest due to their promising applications including electrocatalysis originating from their unique structural features. However, it remains a challenge to directly use MOFs for oxygen electrocatalysis because it is quite difficult to manipulate their dimension, composition, and morphology of the MOFs with abundant active sites. Here, a facile ambient temperature synthesis of unique NiCoFe‐based trimetallic MOF nanostructures with foam‐like architecture is reported, which exhibit extraordinary oxygen evolution reaction (OER) activity as directly used catalyst in alkaline condition. Specifically, the (Ni₂Co₁)_0.925Fe_0.075‐MOF‐NF delivers a minimum overpotential of 257 mV to reach the current density of 10 mA cm^?2 with a small Tafel slope of 41.3 mV dec^?1 and exhibits high durability after long‐term testing. More importantly, the deciphering of the possible origination of the high activity is performed through the characterization of the intermediates during the OER process, where the electrochemically transformed metal hydroxides and oxyhydroxides are confirmed as the active species.     

Phosphorus Incorporation into Co9S8 Nanocages for Highly Efficient Oxygen Evolution Catalysis

The improvement of activity of electrocatalysts lies in the increment of the density of active sites or the enhancement of intrinsic activity of each active site. A common strategy to realize dual active sites is the use of bimetal compound catalysts, where each metal atom contributes one active site. In this work, a new concept is presented to realize dual active sites with tunable electron densities in monometal compound catalysts. Dual Co²⁺ tetrahedral (Co²⁺(T_d)) and Co³⁺ octahedral (Co³⁺(O_h)) coordination active sites are developed and adjustable electron densities on the Co²⁺(T_d) and Co³⁺(O_h) are further achieved by phosphorus incorporation (P‐Co₉S₈). The experimental results and density functional theory calculations show that the nonmetal P doping can systematically modulate charge density of Co²⁺(T_d) and Co³⁺(O_h) in P‐Co₉S₈ and simultaneously improve the electrical conductivity of Co₉S₈, which substantially enhances oxygen evolution reaction performance of P‐Co₉S₈.     

Novel MOF‐Derived Co@N‐C Bifunctional Catalysts for Highly Efficient Zn–Air Batteries and Water Splitting

Metal–organic frameworks (MOFs) and MOF‐derived materials have recently attracted considerable interest as alternatives to noble‐metal electrocatalysts. Herein, the rational design and synthesis of a new class of Co@N‐C materials (C‐MOF‐C2‐T) from a pair of enantiotopic chiral 3D MOFs by pyrolysis at temperature T is reported. The newly developed C‐MOF‐C2‐900 with a unique 3D hierarchical rodlike structure, consisting of homogeneously distributed cobalt nanoparticles encapsulated by partially graphitized N‐doped carbon rings along the rod length, exhibits higher electrocatalytic activities for oxygen reduction and oxygen evolution reactions (ORR and OER) than that of commercial Pt/C and RuO₂, respectively. Primary Zn–air batteries based on C‐MOF‐900 for the oxygen reduction reaction (ORR) operated at a discharge potential of 1.30 V with a specific capacity of 741 mA h g_Zn^–1 under 10 mA cm^–2. Rechargeable Zn–air batteries based on C‐MOF‐C2‐900 as an ORR and OER bifunctional catalyst exhibit initial charge and discharge potentials at 1.81 and 1.28 V (2 mA cm^–2), along with an excellent cycling stability with no increase in polarization even after 120 h – outperform their counterparts based on noble‐metal‐based air electrodes. The resultant rechargeable Zn–air batteries are used to efficiently power electrochemical water‐splitting systems, demonstrating promising potential as integrated green energy systems for practical applications.     

Ultrafine Co3O4 Nanoparticles within Nitrogen‐Doped Carbon Matrix Derived from Metal–Organic Complex for Boosting Lithium Storage and Oxygen Evolution Reaction

Transition metal oxides have recently received great attention for application in advanced lithium‐ion batteries (LIBs) and oxygen evolution reaction (OER). Herein, the ethylenediaminetetraacetic cobalt complex as a precursor to synthesize ultrafine Co₃O₄ nanoparticles encapsulated into a nitrogen‐doped carbon matrix (NC) composites is presented. The as‐prepared Co₃O₄/NC‐350 obtained by pyrolysis at 350 °C demonstrates superior rate performance (372 mAh g^?1 at 5.0 A g^?1) and high cycling stability (92% capacity retention after 300 cycles at 1.0 A g^?1) as anode for LIBs. When evaluated as an electrocatalyst for OER, the Co₃O₄/NC‐350 achieves an overpotential of 298 mV at a current density of 10 mA cm^?2. The NC‐encapsualted porous hierarchical structure assures fast and continuous electron transportation, high activity sites, and strong structural integrity. This works offers novel complex precursors for synthesizing transition metal–based electrodes for boosting electrochemical energy conversion and storage.     

Highly Active Core–Shell Carbon/NiCo2O4 Double Microtubes for Efficient Oxygen Evolution Reaction: Ultralow Overpotential and Superior Cycling Stability

Developing highly efficient electrocatalysts with earth abundant elements for oxygen evolution reaction (OER) is a promising way to store light or electrical energy in the form of chemical energy. Here, a new type of electrocatalyst with core–shell carbon/NiCo₂O₄ double microtubes architecture is successfully synthesized through a hydrothermal method combined with the calcination process with wet tissues as the template and carbon resource. The outer NiCo₂O₄ nanosheet arrays contain abundant defects, which come from reduction of the carbon in wet tissues. This indicates that carbon is a very excellent defect inducer. The inner carbon microtubes can act as the robust structure skeleton and these core–shell double microtubes provide abundant diffusion channels for oxygen and electrolyte, both of which contribute to improving the stability by avoiding damage to the electrode from produced O₂ bubbles and the collapse of the outer NiCo₂O₄ microtubes. Electrochemical results show that the electrode, core–shell carbon/NiCo₂O₄ double microtubes loaded on carbon cloth, exhibits prominent electrocatalytic activity with an overpotential of only 168 mV at 10 mA cm^?2 and a Tafel slope as low as 57.6 mV dec^?1 in 1.0 mol L^?1 KOH. This new type of electrocatalyst possesses great potential in water electrolyzers and rechargeable metal–air batteries.     

Hollow‐Structured Metal Oxides as Oxygen‐Related Catalysts

Metal oxide hollow structures with large surface area, low density, and high loading capacity have received great attention for energy‐related applications. Acting as oxygen‐related catalysts, hollow‐structured transition metal oxides offer low overpotential, fast reaction rate, and excellent stability. Herein, recent progress in the oxygen‐related catalysis (e.g., oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and metal–air batteries) of hollow‐structured transition metal oxides is discussed. Through a comprehensive outline of hollow‐structured spinels, perovskites, rutiles, etc., a rational design strategy is provided for an enhanced oxygen‐related catalysis performance from the viewpoint of crystal structures. Urgent challenges and further research directions are presented for hollow‐structured transition metal oxides toward excellent oxygen‐related catalysis.     

Web‐Like Interconnected Carbon Networks from NaCl‐Assisted Pyrolysis of ZIF‐8 for Highly Efficient Oxygen Reduction Catalysis

The oxygen reduction reaction (ORR) is under intense research due to its significance in energy storage and conversion processes. Recent studies show that interconnected and hierarchically porous structures can further enhance ORR kinetics as well as catalyst durability, but their preparation can be quite time and/or chemical consuming. Here, a simple approach is reported to prepare such complex structures by pyrolyzing composites containing NaCl and ZIF‐8. The templating effect of molten NaCl connects ZIF‐8 particles into web‐like carbon networks. During ORR activity measurements, it achieves a 0.964 V onset potential and a 38 mV dec^?1 Tafel slope, which are comparable to those of the benchmark Pt/C (0.979 V and 40 mV dec^?1). Due to the metal‐free feature, this catalyst exhibits a 16 mV shift in half‐wave potential after a 10 000‐cycle durability test, which is only 60% of that of Pt/C. The catalyst is also tested in Zn–air batteries and the assemblies are able to work at above 1.2 V for 140 h, which triples the life held by those with Pt/C. This study demonstrates a facile strategy to prepare metal‐free ORR catalysts with interconnectivity and hierarchical porosity, and proves their great potentials in ORR catalysis and Zn–air batteries.     

Co9S8 Nanoparticles‐Embedded N/S‐Codoped Carbon Nanofibers Derived from Metal–Organic Framework‐Wrapped CdS Nanowires for Efficient Oxygen Evolution Reaction

Metal–organic frameworks (MOFs) with tunable compositions and morphologies are recognized as efficient self‐sacrificial templates to achieve function‐oriented nanostructured materials. Moreover, it is urgently needed to develop highly efficient noble metal‐free oxygen evolution reaction (OER) electrocatalysts to accelerate the development of overall water splitting green energy conversion systems. Herein, a facile and cost‐efficient strategy to synthesize Co₉S₈ nanoparticles‐embedded N/S‐codoped carbon nanofibers (Co₉S₈/NSCNFs) as highly active OER catalyst is developed. The hybrid precursor of core–shell ZIF‐wrapped CdS nanowires is first prepared and then leads to the formation of uniformly dispersed Co₉S₈/N, S‐codoped carbon nanocomposites through a one‐step calcination reaction. The optimal Co₉S₈/NSCNFs‐850 is demonstrated to possess excellent electrocatalytic performance for OER in 1.0 m KOH solution, affording a low overpotential of 302 mV to reach the current density of 10 mA cm^?2, a small Tafel slope of 54 mV dec^?1, and superior long‐term stability for 1000 cyclic voltammetry cycles. The favorable results raise a concept of exploring more MOF‐based nanohybrids as precursors to induce the synthesis of novel porous nanomaterials as non‐noble‐metal electrocatalysts for sustainable energy conversion.     

Enhancing Oxygen Evolution Reaction through Modulating Electronic Structure of Trimetallic Electrocatalysts Derived from Metal–Organic Frameworks

The construction of efficient, durable, and non‐noble metal electrocatalysts for oxygen evolution reaction (OER) is of great value but challenging. Herein, a facile method is developed to synthesize a series of trimetallic (W/Co/Fe) metal–organic frameworks (MOFs)‐derived carbon nanoflakes (CNF) with various Fe content, and an Fe‐dependent volcano‐type plot can be drawn out for WCoFe_x ‐CNF. The optimized WCoFe_0.3‐CNF (when the feed ratio of Fe/Co is 0.3) demonstrates superior electrocatalytic performance with a low overpotential of only 254 mV@10 mA cm^?2 and excellent durability of 100 h. Further researches show that appropriate amount of iron doping can regulate the electronic structure, resulting in a favorable synergistic environment. This method may stimulate the exploration of electrocatalysts by utilizing MOFs as precursors while realizing electronic modulation by multimetal doping.     

Edge Sites with Unsaturated Coordination on Core–Shell Mn3O4@MnxCo3−xO4 Nanostructures for Electrocatalytic Water Oxidation

Transition‐metal oxides are extensively investigated as efficient electrocatalysts for the oxygen evolution reaction (OER). However, large‐scale applications remain challenging due to their moderate catalytic activity. Optimized regulation of surface states can lead to improvement of catalytic properties. Here, the design of Mn@Co_x Mn_3?x O₄ nanoparticles with abundant edge sites via a simple seed‐mediated growth strategy is described. The unsaturated coordination generated on the edge sites of Co_x Mn_3?x O₄ shells makes a positive contribution to the surface‐structure tailoring. Density functional theory calculations indicate that the edge sites with unsaturated coordination exhibit intense affinity for OH^? in the alkaline electrolyte, which greatly enhances the electrochemical OER performance of the catalysts. The resulting Mn@Co_x Mn_3?x O₄ catalysts yield a current density of 10 mA cm^?2 at an overpotential of 246 mV and a relatively low Tafel slope of 46 mV dec^?1. The successful synthesis of these metal oxides nanoparticles with edge sites may pave a new path for rationally fabricating efficient OER catalysts.     

Silk‐Derived 2D Porous Carbon Nanosheets with Atomically‐Dispersed Fe‐Nx‐C Sites for Highly Efficient Oxygen Reaction Catalysts

Controlled synthesis of highly efficient, stable, and cost‐effective oxygen reaction electrocatalysts with atomically‐dispersed Me–N_x–C active sites through an effective strategy is highly desired for high‐performance energy devices. Herein, based on regenerated silk fibroin dissolved in ferric chloride and zinc chloride aqueous solution, 2D porous carbon nanosheets with atomically‐dispersed Fe–N_x–C active sites and very large specific surface area (≈2105 m² g^?1) are prepared through a simple thermal treatment process. Owing to the 2D porous structure with large surface area and atomic dispersion of Fe–N_x–C active sites, the as‐prepared silk‐derived carbon nanosheets show superior electrochemical activity toward the oxygen reduction reaction with a half‐wave potential (E_1/2) of 0.853 V, remarkable stability with only 11 mV loss in E_1/2 after 30 000 cycles, as well as good catalytic activity toward the oxygen evolution reaction. This work provides a practical and effective approach for the synthesis of high‐performance oxygen reaction catalysts towards advanced energy materials.     

Engineering Bimetal Synergistic Electrocatalysts Based on Metal–Organic Frameworks for Efficient Oxygen Evolution

Benefiting from metal–organic frameworks (MOFs) unique structural characteristics, their versatility in composition and structure has been well explored in electrochemical oxygen evolution reaction (OER) processes. Here, a ligand/ionic exchange phenomenon of MOFs is reported in alkaline solution due to their poor stability, and the active species and reaction mechanism of MOFs are revealed in the OER process. A series of mixed Ni‐MOFs and Fe‐MOFs are synthesized by straightforward sonication and then directly used as catalyst candidates for OER in alkaline electrolyte. It can be confirmed via ex situ transmission electron microscopic images and X‐ray diffraction patterns analysis, that the bimetallic hydroxide (NiFe‐LDH) is generated in 1.0 m KOH in situ and acts as protagonist for oxygen evolution. The optimized catalyst (FN‐2) exhibits a lower overpotential (275 mV at a current density of 10 mA cm^?2) and excellent long‐term stability (strong current density for 100 h without fading). The revelation of the real active species of MOF materials may contribute to better understanding of the reaction mechanism.     

Tuning Surface Electronic Configuration of NiFe LDHs Nanosheets by Introducing Cation Vacancies (Fe or Ni) as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction

Intrinsically inferior electrocatalytic activity of NiFe layered double hydroxides (LDHs) nanosheets is considered as a limiting factor to inhibit the electrocatalytic properties for oxygen evolution reaction (OER). Proper defect engineering to tune the surface electronic configuration of electrocatalysts may significantly improve the intrinsic activity. In this work, the selective formation of cation vacancies in NiFe LDHs nanosheets is successfully realized. The as‐synthesized NiFe LDHs‐V_Fe and NiFe LDHs‐V_Ni electrocatalysts show excellent activity for OER, mainly attributed to the introduction of rich iron or nickel vacancies in NiFe LDHs nanosheets, which efficiently tune the surface electronic structure increasing the adsorbing capacity of OER intermediates. Density functional theory (DFT) computational results also further indicate that the OER catalytic performance of NiFe LDHs can be pronouncedly improved by introducing Fe or Ni vacancies.