Metal‐Organic Framework‐Derived Non‐Precious Metal Nanocatalysts for Oxygen Reduction Reaction

By virtue of diverse structures and tunable properties, metal‐organic frameworks (MOFs) have presented extensive applications including gas capture, energy storage, and catalysis. Recently, synthesis of MOFs and their derived nanomaterials provide an opportunity to obtain competent oxygen reduction reaction (ORR) electrocatalysts due to their large surface area, controllable composition and pore structure. This review starts with the introduction of MOFs and current challenges of ORR, followed by the discussion of MOF‐based non‐precious metal nanocatalysts (metal‐free and metal/metal oxide‐based carbonaceous materials) and their application in ORR electrocatalysis. Current issues in MOF‐derived ORR catalysts and some corresponding strategies in terms of composition and morphology to enhance their electrocatalytic performance are highlighted. In the last section, a perspective for future development of MOFs and their derivatives as catalysts for ORR is discussed.     

MnMoO4 Electrocatalysts for Superior Long‐Life and High‐Rate Lithium‐Oxygen Batteries

Lithium‐oxygen batteries represent a significant scientific challenge for high‐rate and long‐term cycling using oxygen electrodes that contain efficient electrocatalysts. The mixed transition metal oxide catalysts provide the most efficient catalytic activity for partial heterogeneous surface cations with oxygen vacancies as the active phase. They include multiple oxidation states and oxygen vacancies. Here, using a combination of transmission electron microscopy, differential electrochemical mass spectrometry, X‐ray photoelectron spectroscopy, and electrochemical properties to probe the surface of the MnMoO₄ nanowires, it is shown that the intrinsic MnMoO₄ oxygen vacancies on the oxygen electrode are an effective strategy to achieve a high reversibility and high efficiency for lithium‐oxygen (Li‐O₂) batteries. The modified MnMoO₄ nanowires exhibit a highly stable capacity at a fixed capacity of 5000 mA h g_sp^?1 (calculated weight of Super P carbon black) during 50 cycles, a high‐rate capability at a current rate of 3000 mA g_sp^?1 during 70 cycles, and a long‐term reversible capacity during 188 cycles at a fixed capacity of 1000 mA h g_sp^?1. It is demonstrated that this strategy for creating mixed transition metal oxides (e.g., MnMoO₄) may pave the way for the new structural design of electrocatalysts for Li‐O₂ batteries.     

From Lithium‐Oxygen to Lithium‐Air Batteries: Challenges and Opportunities

Lithium ‐ air batteries have become a focus of research on future battery technologies. Technical issues associated with lithium‐air batteries, however, are rather complex. Apart from the sluggish oxygen reaction kinetics which demand efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts, issues are also inherited from the nature of an open battery system and the use of reactive metal lithium as anode. Lithium‐air batteries, which exchange oxygen directly with ambient air, face more challenges due to the additional oxidative agents of moisture, carbon dioxide, etc. which degrade the metal lithium anode, deteriorating the performance of the batteries. In order to improve the cycling performance one must hold a full picture of lithium‐oxygen electrochemistry in the presence of carbon dioxide and/or moisture and fully understand the fundamentals of chemistry reactions therein. Recent advances in the exploration of the effect of moisture and CO₂ contaminants on Li‐O₂ batteries are reviewed, and the mechanistic understanding of discharge/charge process in O₂ at controlled level of moisture and/or CO₂ are illustrated. Prospects for development opportunities of Li‐air batteries, insight into future research directions, and guidelines for the further development of rechargeable Li‐air batteries are also given.     

Egg‐Derived Mesoporous Carbon Microspheres as Bifunctional Oxygen Evolution and Oxygen Reduction Electrocatalysts

A bifunctional evolution reaction (OER) and oxygen reduction reaction (ORR) electrocatalysts are developed, based on codoped mesoporous carbon microspheres from ecofriendly biomass of eggs without the introduction of extrinsic dopants, via a facile and high‐throughput spray‐drying process. The obtained egg‐derived mesoporous carbon microspheres (egg‐CMS) present large specific surface area and high pore volume, as well as abundant dopant types including nitrogen, phosphorous, and iron that are originated from the innate protein and small organic molecule contents. When fabricated as OER or ORR catalysts, these egg‐CMS exhibit low onset potentials, high current densities, small Tafel slopes, and excellent stabilities. As a proof‐of‐concept, a rechargeable Zn‐air battery is demonstrated using the high‐active egg‐CMS as a bifunctional OER and ORR catalyst, suggesting the capability of utilizing full biomass materials for efficient energy storage and utilization.     

Optimization of Carbon‐ and Binder‐Free Au Nanoparticle‐Coated Ni Nanowire Electrodes for Lithium‐Oxygen Batteries

A Au nanoparticle‐coated Ni nanowire substrate without binder or carbon is used as the electrode (denoted as the Au/Ni electrode) for Li‐oxygen (Li‐O₂) batteries. A minimal amount of Au nanoparticles with sizes of <30 nm on a Ni nanowire substrate are coated using a simple electrodeposition method to the extent that maximum capacity can be utilized. This optimized, one body, Au/Ni electrode shows high capacities of 921 mAh g^?1_Au, 591 mAh g^?1_Au, and 359 mAh g^?1_Au, which are obtained at currents of 300 mAg^?1_Au, 500 mAg^?1_Au, and 1000 mAg^?1_Au respectively. More importantly, the Au/Ni electrode exhibits excellent cycle stability over 200 cycles.     

Eutectic‐Derived Mesoporous Ni‐Fe‐O Nanowire Network Catalyzing Oxygen Evolution and Overall Water Splitting

The development of efficient and abundant water oxidation catalysts is essential for the large‐scale storage of renewable energy in the form of hydrogen fuel via electrolytic water splitting, but still remains challenging. Based upon eutectic reaction and dealloying inheritance effect, herein, novel Ni‐Fe‐O‐based composite with a unique mesoporous nanowire network structure is designed and synthesized. The composite exhibits exceptionally low overpotential (10 mA cm^?2 at an overpotential of 244 mV), low Tafel slope (39 mV dec^?1), and superior long‐term stability (remains 10 mA cm^?2 for over 60 h without degradation) toward oxygen evolution reaction (OER) in 1 m KOH. Moreover, an alkaline water electrolyzer is constructed with the Ni‐Fe‐O composite as catalyst for both anode and cathode. This electrolyzer displays superior electrolysis performance (affording 10 mA cm^?2 at 1.64 V) and long‐term durability. The remarkable features of the catalyst lie in its unique mesoporous nanowire network architecture and the synergistic effect of the metal core and the active metal oxide, giving rise to the strikingly enhanced active surface area, accelerated electron/ion transport, and further promoted reaction kinetics of OER.     

Intrinsic Effects of Ruddlesden‐Popper‐Based Bifunctional Catalysts for High‐Temperature Oxygen Reduction and Evolution

Unveiling the intrinsic effects of Ruddlesden‐Popper (RP) series A_n+1B_nO_3n+1 (A = La, B = Ni, Co, Mn, Cu, n = 1, 2 and 3) catalysts is essential in order to optimize the activity of oxygen reduction reaction (ORR) and evolution reaction (OER). Here, it is demonstrated that the oxygen vacancy is not the key point for RP to realize high ORR and OER activity at high temperature. Instead, interstitial O^2? with high concentration and fast migration, and lattice oxygen with high activity are favorable for the high‐temperature catalytic activity. Aliovalent cation doping is an effective strategy to modify the catalytic activity. For the RP catalysts, low‐valence ion doping does not introduce oxygen vacancies, which suppresses the activity of lattice oxygen and decreases the interstitial O^2? concentration; whereas high‐valence ion doping enhances the interstitial O^2– concentration and the lattice oxygen activity. The evaluations of six RP series (La₂NiO₄, La₂CoO₄, La₃Co₂O₇, La₄Ni₃O₁₀, La₂MnO₄, and La₂CuO₄ based) and twenty samples as oxygen electrodes for solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs) demonstrate that this finding is applicable to all the selected RP series.     

High‐Performance Platinum‐Perovskite Composite Bifunctional Oxygen Electrocatalyst for Rechargeable Zn–Air Battery

Constructing highly active electrocatalysts with superior stability at low cost is a must, and vital for the large‐scale application of rechargeable Zn–air batteries. Herein, a series of bifunctional composites with excellent electrochemical activity and durability based on platinum with the perovskite Sr(Co_0.8Fe_0.2)_0.95P_0.05O_3?δ (SCFP) are synthesized via a facile but effective strategy. The optimal sample Pt‐SCFP/C‐12 exhibits outstanding bifunctional activity for the oxygen reduction reaction and oxygen evolution reaction with a potential difference of 0.73 V. Remarkably, the Zn–air battery based on this catalyst shows an initial discharge and charge potential of 1.25 and 2.02 V at 5 mA cm^?2, accompanied by an excellent cycling stability. X‐ray photoelectron spectroscopy, X‐ray absorption near‐edge structure, and extended X‐ray absorption fine structure experiments demonstrate that the superior performance is due to the strong electronic interaction between Pt and SCFP that arises as a result of the rapid electron transfer via the Pt? O? Co bonds as well as the higher concentration of surface oxygen vacancies. Meanwhile, the spillover effect between Pt and SCFP also can increase more active sites via lowering energy barrier and change the rate‐determining step on the catalysts surface. Undoubtedly, this work provides an efficient approach for developing low‐cost and highly active catalysts for wider application of electrochemical energy devices.     

Gadolinium‐Induced Valence Structure Engineering for Enhanced Oxygen Electrocatalysis

Rare earth doped materials with unique electronic ground state configurations are considered emerging alternatives to conventional Pt/C for the oxygen reduction reaction (ORR). Herein, gadolinium (Gd)‐induced valence structure engineering is, for the first, time investigated for enhanced oxygen electrocatalysis. The Gd₂O₃–Co heterostructure loaded on N‐doped graphene (Gd₂O₃–Co/NG) is constructed as the target catalyst via a facile sol–gel assisted strategy. This synthetic strategy allows Gd₂O₃–Co nanoparticles to distribute uniformly on an N‐graphene surface and form intimate Gd₂O₃/Co interface sites. Upon the introduction of Gd₂O₃, the ORR activity of Gd₂O₃–Co/NG is significantly increased compared with Co/NG, where the half‐wave potential (E_1/2) of Gd₂O₃–Co/NG is 100 mV more positive than that of Co/NG and even close to commercial Pt/C. The density functional theory calculation and spectroscopic analysis demonstrate that, owing to intrinsic charge redistribution at the engineered interface of Gd₂O₃/Co, the coupled Gd₂O₃–Co can break the OOH*–OH* scaling relation and result in a good balance of OOH* and OH* binding on Gd₂O₃–Co surface. For practical application, a rechargeable Zn–air battery employing Gd₂O₃–Co/NG as an air–cathode achieves a large power density and excellent charge–discharge cycle stability.     

Three‐Dimensional Smart Catalyst Electrode for Oxygen Evolution Reaction

A multifunctional catalyst electrode mimicking external stimuli–responsive property has been prepared by the in situ growth of nitrogen (N)‐doped NiFe double layered hydroxide (N–NiFe LDH) nanolayers on a 3D nickel foam substrate framework. The electrode demonstrates superior performance toward catalyzing oxygen evolution reaction (OER), affording a low overpotential of 0.23 V at the current density of 10 mA cm^?2, high Faradaic efficiency of ≈98%, and stable operation for >60 h. Meanwhile, the electrode can dynamically change its color from gray silver to dark black with the OER happening, and the coloration/bleaching processes persist for at least 5000 cycles, rendering it a useful tool to monitor the catalytic process. Mechanism study reveals that the excellent structural properties of electrode such as 3D conductive framework, ultra thickness of N–NiFe LDH nanolayer (≈0.8 nm), and high N‐doping content (≈17.8%) make significant contribution to achieving enhanced catalytic performance, while N–NiFe LDH nanolayer on electrode is the main contributor to the stimuli‐responsive property with the reversible extraction/insertion of electrons from/into N–NiFe LDH leading to the coloration/bleaching processes. Potential application of this electrode has been further demonstrated by integrating it into a Zn–air battery device to identify the charging process during electrochemical cycling.     

Sulfur/Oxygen Codoped Porous Hard Carbon Microspheres for High‐Performance Potassium‐Ion Batteries

Potassium‐ion batteries (KIBs) are very promising alternatives to lithium‐ion batteries (LIBs) for large‐scale energy storage. However, traditional carbon anode materials usually show poor performance in KIBs due to the large size of K ions. Herein, a carbonization‐etching strategy is reported for making a class of sulfur (S) and oxygen (O) codoped porous hard carbon microspheres (PCMs) material as a novel anode for KIBs through pyrolysis of the polymer microspheres (PMs) composed of a liquid crystal/epoxy monomer/thiol hardener system. The as‐made PCMs possess a porous architecture with a large Brunauer–Emmett–Teller surface area (983.2 m² g^?1), an enlarged interlayer distance (0.393 nm), structural defects induced by the S/O codoping and also amorphous carbon nature. These new features are important for boosting potassium ion storage, allowing the PCMs to deliver a high potassiation capacity of 226.6 mA h g^?1 at 50 mA g^?1 over 100 cycles and be displaying high stability by showing a potassiation capacity of 108.4 mA h g^?1 over 2000 cycles at 1000 mA g^?1. The density functional theory calculations demonstrate that S/O codoping not only favors the adsorption of K to the PCMs electrode but also reduces its structural deformation during the potassiation/depotassiation. The present work highlights the important role of hierarchical porosity and S/O codoping in potassium storage.     

Recent Progress on Stability Enhancement for Cathode in Rechargeable Non‐Aqueous Lithium‐Oxygen Battery

The pressing demand on the electronic vehicles with long driving range on a single charge has necessitated the development of next‐generation high‐energy‐density batteries. Non‐aqueous Li‐O₂ batteries have received rapidly growing attention due to their higher theoretical energy densities compared to those of state‐of‐the‐art Li‐ion batteries.To make them practical for commercial applications, many critical issues must be overcome, including low round‐trip efficiency and poor cycling stability, which are intimately connected to the problems resulting from cathode degradation during cycling. Encouragingly, during the past years, much effort has been devoted to enhancing the stability of the cathode using a variety of strategies and these have effectively surmounted the challenges derived from cathode deteriorations,thus endowing Li‐O₂ batteries with significantly improved electrochemical performances. Here, a brief overview of the general development of Li‐O₂ battery is presented. Then, critical issues relevant to the cathode instability are discussed and remarkable achievements in enhancing the cathode stability are highlighted. Finally, perspectives towards the development of next generation highly stable cathode are also discussed.     

Oxygen Reduction Reactions on Single‐ or Few‐Atom Discrete Active Sites for Heterogeneous Catalysis

The oxygen reduction reaction (ORR) is of great importance in energy‐converting processes such as fuel cells and in metal–air batteries and is vital to facilitate the transition toward a nonfossil dependent society. The ORR has been associated with expensive noble metal catalysts that facilitate the O₂ adsorption, dissociation, and subsequent electron transfer. Single‐ or few‐atom motifs based on earth‐abundant transition metals, such as Fe, Co, and Mo, combined with nonmetallic elements, such as P, S, and N, embedded in a carbon‐based matrix represent one of the most promising alternatives. Often these are referred to as single atom catalysts; however, the coordination number of the metal atom as well as the type and nearest neighbor configuration has a strong influence on the function of the active sites, and a more adequate term to describe them is metal‐coordinated motifs. Despite intense research, their function and catalytic mechanism still puzzle researchers. They are not molecular systems with discrete energy states; neither can they fully be described by theories that are adapted for heterogeneous bulk catalysts. Here, recent results on single‐ and few‐atom electrocatalyst motifs are reviewed with an emphasis on reports discussing the function and the mechanism of the active sites.     

Highly Efficient Oxygen Evolution by a Thermocatalytic Process Cascaded Electrocatalysis Over Sulfur‐Treated Fe‐Based Metal–Organic‐Frameworks

The oxygen evolution reaction (OER) is a bottleneck process for water splitting and finding highly efficient, durable, low‐cost, and earth‐abundant electrocatalysts is still a major challenge. Here a sulfur‐treated Fe‐based metal–organic‐framework is reported as a promising electrocatalyst for the OER, which shows a low overpotential of 218 mV at the current density of 10 mA cm^?2 and exhibits a very low Tafel slope of 36.2 mV dec^?1 at room temperature. It can work on high current densities of 500 and 1000 mA cm^?2 at low overpotentials of 298 and 330 mV, respectively, by keeping 97% of its initial activity after 100 h. Notably, it can achieve 1000 mA cm^?2 at 296 mV with a good stability at 50 °C, fully fitting the requirements for large‐scale industrial water electrolysis. The high catalytic performance can be attributed to the thermocatalytic processes of H⁺ capture by –SO₃ groups from *OH or *OOH species, which cascades to the electrocatalytic pathway and then significantly reduces the OER overpotentials.     

Molecular Design of Single‐Atom Catalysts for Oxygen Reduction Reaction

Fuel cells are highly attractive for direct chemical‐to‐electrical energy conversion and represent the ultimate mobile power supply solution. However, presently, fuel cells are limited by the sluggish kinetics of the cathodic oxygen reduction reaction (ORR), which requires the use of Pt as a catalyst, thus significantly increasing the overall cost of the cells. Recently, nonprecious metal single‐atom catalysts (SACs) with high ORR activity under both acidic and alkaline conditions have been recognized as promising cost‐effective alternatives to replace Pt in fuel cells. Considerable efforts have been devoted to further improving the ORR activity of SACs, including tailoring the coordination structure of the metal centers, enriching the concentration of the metal centers, and engineering the electronic structure and porosity of the substrate. Herein, a brief introduction to fuel cells and fundamentals of the ORR parameters of SACs and the origin of their high activity is provided, followed by a detailed review of the recently developed strategies used to optimize the ORR activity of SACs in both rotating disk electrode and membrane electrode assembly tests. Remarks and perspectives on the remaining challenges and future directions of SACs for the development of commercial fuel cells are also presented.