A Comprehensive Review on Controlling Surface Composition of Pt‐Based Bimetallic Electrocatalysts

With increasing energy demands worldwide, significant efforts have been made to develop superior electrocatalysts for efficient energy conversion systems. Among all the electrocatalysts exploited, Pt‐based bimetallic nanomaterials stand out by virtue of their high catalytic activity and relatively low cost due to the introduction of a nonprecious metal component. It should be noted that electrocatalytic reactions only take place on the surface of catalysts, so investigations of the surface composition of Pt‐based bimetallic nanomaterials are necessary for practical electrocatalysts. In this review, recent studies on controlling the surface composition of Pt‐based bimetallic catalysts for the oxygen reduction reaction, formic acid electrooxidation, and ethanol electrooxidation are summarized. The controlling strategies, including the chemical method and the electrochemical method, are discussed. The impacts of surface composition compositions on the electrocatalytic performance are also discussed. Finally, the challenges and future directions for controlling the surface composition of Pt‐based bimetallic nanomaterials are addressed.     

Insights into Structural Evolution of Lithium Peroxides with Reduced Charge Overpotential in Li−O2 System

Reducing charge–discharge overpotential is of great significance to enhance the efficiency and cyclability of Li?O₂ batteries. Here a dramatically reduced charge overpotential (0.4 V) via a rational design of cathode architecture, which features highly uniform Pt and Pt₃Co nanocrystals embedding within nitrogen‐doped cobalt@graphene heterostructures is successfully achieved. Because of the improvement in the catalytic efficiency for optimizing the electrical and dynamic properties, the cathode design enables promising electrochemical performance. More importantly, the results reveal different overpotentials prompted by different structural evolution of bulk Li₂O₂, which largely depends on the Pt modification approach (surface‐coating and bulk‐doping). This dependence is found to be attributed to the influence of Pt nanocomponents and their dispersion on the formation and decomposition mechanism of Li₂O₂. Density functional theory calculations provide mechanistic insights into the promotional effect of Pt and Pt₃Co on the reduction of the charge overpotential. Finally, an inner relationship between overpotential and electrochemical kinetics is proposed.     

Metal–Organic Framework Templated Pd@PdO–Co3O4 Nanocubes as an Efficient Bifunctional Oxygen Electrocatalyst

The development of high‐efficiency bifunctional electrocatalyst for oxygen reduction and evolution reactions (ORR/OER) is critical for rechargeable metal–air batteries, a typical electrochemical energy storage and conversion technology. This work reports a general approach for the synthesis of Pd@PdO–Co₃O₄ nanocubes using the zeolite‐type metal–organic framework (MOF) as a template. The as‐synthesized materials exhibit a high electrocatalytic activity toward OER and ORR, which is comparable to those of commercial RuO₂ and Pt/C electrocatalysts, while its cycle performance and stability are much higher than those of commercial RuO₂ and Pt/C electrocatalysts. Various physicochemical characterizations and density functional theory calculations indicate that the favorable electrochemical performance of the Pd@PdO–Co₃O₄ nanocubes is mainly attributed to the synergistic effect between PdO and the robust hollow structure composed of interconnected crystalline Co₃O₄ nanocubes. This work establishes an efficient approach for the controlled design and synthesis of MOF‐templated hybrid nanomaterials, and provides a great potential for developing high‐performance electrocatalysts in energy storage and conversion.     

An Open‐Structured Matrix as Oxygen Cathode with High Catalytic Activity and Large Li2O2 Accommodations for Lithium–Oxygen Batteries

The nonaqueous lithium–oxygen (Li–O₂) battery is considered as one of the most promising candidates for next‐generation energy storage systems because of its very high theoretical energy density. However, its development is severely hindered by large overpotential and limited capacity, far less than theory, caused by sluggish oxygen redox kinetics, pore clogging by solid Li₂O₂ deposition, inferior Li₂O₂/cathode contact interface, and difficult oxygen transport. Herein, an open‐structured Co₉S₈ matrix with sisal morphology is reported for the first time as an oxygen cathode for Li–O₂ batteries, in which the catalyzing for oxygen redox, good Li₂O₂/cathode contact interface, favorable oxygen evolution, and a promising Li₂O₂ storage matrix are successfully achieved simultaneously, leading to a significant improvement in the electrochemical performance of Li–O₂ batteries. The intrinsic oxygen‐affinity revealed by density functional theory calculations and superior bifunctional catalytic properties of Co₉S₈ electrode are found to play an important role in the remarkable enhancement in specific capacity and round‐trip efficiency for Li–O₂ batteries. As expected, the Co₉S₈ electrode can deliver a high discharge capacity of ≈6875 mA h g^?1 at 50 mA g^?1 and exhibit a low overpotential of 0.57 V under a cutoff capacity of 1000 mA h g^?1, outperforming most of the current metal‐oxide‐based cathodes.     

Bimetallic Cobalt‐Based Phosphide Zeolitic Imidazolate Framework: CoPx Phase‐Dependent Electrical Conductivity and Hydrogen Atom Adsorption Energy for Efficient Overall Water Splitting

Cobalt‐based bimetallic phosphide encapsulated in carbonized zeolitic imadazolate frameworks has been successfully synthesized and showed excellent activities toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Density functional theory calculation and electrochemical measurements reveal that the electrical conductivity and electrochemical activity are closely associated with the Co₂P/CoP mixed phase behaviors upon Cu metal doping. This relationship is found to be the decisive factor for enhanced electrocatalytic performance. Moreover, the precise control of Cu content in Co‐host lattice effectively alters the Gibbs free energy for H* adsorption, which is favorable for facilitating reaction kinetics. Impressively, an optimized performance has been achieved with mild Cu doping in Cu_0.3Co_2.7P/nitrogen‐doped carbon (NC) which exhibits an ultralow overpotential of 0.19 V at 10 mA cm^–2 and satisfying stability for OER. Cu_0.3Co_2.7P/NC also shows excellent HER activity, affording a current density of 10 mA cm^–2 at a low overpotential of 0.22 V. In addition, a homemade electrolyzer with Cu_0.3Co_2.7P/NC paired electrodes shows 60% larger current density than Pt/RuO₂ couple at 1.74 V, along with negligible catalytic deactivation after 50 h operation. The manipulation of electronic structure by controlled incorporation of second metal sheds light on understanding and synthesizing bimetallic transition metal phosphides for electrolysis‐based energy conversion.     

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.     

Tailoring the Mechanical and Electrochemical Properties of an Artificial Interphase for High‐Performance Metallic Lithium Anode

Lithium metal is regarded as the “Holy Grail” of anode materials due to its low electrochemical potential and high theoretical capacity. Unfortunately, its unstable solid electrolyte interphase (SEI) leads to low Coulombic efficiency (CE) and serious safety issues. Herein, a hybrid nanoscale polymeric protective film with tunable composition and improved stiffness is developed by incorporating aluminum crosslinkers into the polymer chains. The Li plating/stripping process is regulated through the protective coating and the dendrite growth is effectively suppressed. Promisingly, the protected Li can deliver stable performance for more than 350 h with a cycling capacity of 2 mAh cm^?2 without a notable increase in overpotential. Moreover, a stable charge/discharge cycling in Li–O₂ batteries with the protected Li can be maintained for more than 600 h. This work provides guidance on the rational design of electrode interfaces and opens up new opportunities for the fabrication of next‐generation energy storage systems.     

Hierarchical Porous Carbonized Co3O4 Inverse Opals via Combined Block Copolymer and Colloid Templating as Bifunctional Electrocatalysts in Li–O2 Battery

Hierarchically organized porous carbonized‐Co₃O₄ inverse opal nanostructures (C‐Co₃O₄ IO) are synthesized via complementary colloid and block copolymer self‐assembly, where the triblock copolymer Pluronic P123 acts as the template and the carbon source. These highly ordered porous inverse opal nanostructures with high surface area display synergistic properties of high energy density and promising bifunctional electrocatalytic activity toward both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). It is found that the as‐made C‐Co₃O₄ IO/Ketjen Black (KB) composite exhibits remarkably enhanced electrochemical performance, such as increased specific capacity (increase from 3591 to 6959 mA h g^?1), lower charge overpotential (by 284.4 mV), lower discharge overpotential (by 19.0 mV), and enhanced cyclability (about nine times higher than KB in charge cyclability) in Li–O₂ battery. An overall agreement is found with both C‐Co₃O₄ IO/KB and Co₃O₄ IO/KB in ORR and OER half‐cell tests using a rotating disk electrode. This enhanced catalytic performance is attributed to the porous structure with highly dispersed carbon moiety intact with the host Co₃O₄ catalyst.     

Enhanced Cyclability of Lithium–Oxygen Batteries with Electrodes Protected by Surface Films Induced via In Situ Electrochemical Process

Although the rechargeable lithium–oxygen (Li–O₂) batteries have extremely high theoretical specific energy, the practical application of these batteries is still limited by the instability of their carbon‐based air‐electrode, Li metal anode, and electrodes, toward reduced oxygen species. Here a simple one‐step in situ electrochemical precharging strategy is demonstrated to generate thin protective films on both carbon nanotubes (CNTs), air‐electrodes and Li metal anodes simultaneously under an inert atmosphere. Li–O₂ cells after such pretreatment demonstrate significantly extended cycle life of 110 and 180 cycles under the capacity‐limited protocol of 1000 mA h g^?1 and 500 mA h g^?1, respectively, which is far more than those without pretreatment. The thin‐films formed from decomposition of electrolyte during in situ electrochemical precharging processes in an inert environment, can protect both CNTs air‐electrode and Li metal anode prior to conventional Li–O₂ discharge/charge cycling, where reactive reduced oxygen species are formed. This work provides a new approach for protection of carbon‐based air‐electrodes and Li metal anodes in practical Li–O₂ batteries, and may also be applied to other battery systems.     

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.     

Carbon Nanodot Surface Modifications Initiate Highly Efficient,Stable Catalysts for Both Oxygen Evolution and Reduction Reactions

Efficient, stable, and low‐cost electrocatalysts for the oxygen evolution and reduction reactions (OER and ORR) are essential components of energy conversion. Although much progress has been achieved in the development of platinum‐based electrocatalysts for ORR and iridium‐based electrocatalysts for OER, they are still not yet viable for large‐scale commercialization because of the high cost and scanty supply of the noble metals. Here, it is demonstrated that carbon nanodots surface‐modified with either phosphorus or amidogen can respectively achieve electrocatalytic activity approaching that of the benchmark Pt/C and IrO₂ /C catalysts for ORR and OER. Furthermore, phosphorus (amidogen)‐modified carbon nanodots with attached Au nanoparticles exhibit superior ORR (OER) activity better than commercial Pt/C (IrO₂/C) catalysts as well as excellent electrochemical stability under visible light.     

Lithiophilic CuO Nanoflowers on Ti‐Mesh Inducing Lithium Lateral Plating Enabling Stable Lithium‐Metal Anodes with Ultrahigh Rates and Ultralong Cycle Life

Lithium (Li) metal anodes are promising candidates for high‐energy‐density batteries. However, uncontrollable dendritic plating behavior and infinite volume expansion are hindering their practical applications. Herein, a novel CuO@Ti‐mesh (CTM) is prepared by microwave‐assisted reactions, followed by pressing on Li wafers, leading to Li/CuO@Ti‐mesh (LCTM) composite anodes. The lithiophilic CuO nanoflowers on Ti‐mesh provides evenly distributed nucleation sites, inducing uniform Li‐ion lateral plating, which can effectively inhibit the growth of Li dendrites and volume expansion during cycling. The as‐prepared LCTM composite anode exhibits high Coulombic efficiency (CE) of 94.2% at 10 mA cm^‐2 over 90 cycles. Meanwhile, the LCTM anode shows a low overpotential of 50 mV at 10 mA cm^‐2 over 16 000 cycles and a low overpotential of 90 and 250 mV even at ultrahigh current densities of 20 and 40 mA cm^‐2. When paired with Li₄Ti₅O₁₂ (LTO), it enhances the capacity retention of LTO/Li wafer full cells by about two times from 36.6% to 73.0% and 42.0% to 80.0% at 5C and 10C with long‐term cycling. It is hoped that this LCTM anode with ultrahigh rates and ultralong cycle life may put Li‐metal anode forward to practical applications, such as in Li–S, Li‐air batteries, etc.     

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.     

Exploiting Synergistic Effect by Integrating Ruthenium–Copper Nanoparticles Highly Co‐Dispersed on Graphene as Efficient Air Cathodes for Li–CO2 Batteries

Li–CO₂ batteries are attractive electrical energy storage devices; however, they still suffer from unsatisfactory electrochemical performance, and the kinetics of CO₂ reduction and evolution reactions must be improved significantly. Herein, a composite of ruthenium–copper nanoparticles highly co‐dispersed on graphene (Ru–Cu–G) as efficient air cathodes for Li–CO₂ batteries is designed. The Li–CO₂ batteries with Ru–Cu–G cathodes exhibit ultra‐low overpotential and can be operated for 100 cycles with a fixed capacity of 1000 mAh g^?1 at 200 and 400 mA g^?1. The synergistic effect between Ru and Cu not only regulates the growth of discharge products, but also promotes CO₂ reduction and evolution reactions by changing the electron cloud density of the surface between Ru and Cu. This work may provide new directions and strategies for developing highly efficient air cathodes for Li–CO₂ batteries, or even practical Li–air batteries.     

An Advanced Separator for Li–O2 Batteries: Maximizing the Effect of Redox Mediators

Although Li–O₂ batteries are promising next‐generation energy storage systems with superior theoretical capacities, they have a serious limitation regarding the large overpotential upon charging that results from the low conductivity of the discharge product. Thus, various redox mediators (RMs) have been widely studied to reduce the overpotential in the charging process, which should promote the oxidation of Li₂O₂. However, RMs degrade the Li metal anode through a parasitic reaction between the RM and the Li metal, and a solution for this phenomenon is necessary. In this study, an effective method is proposed to prevent the migration of the RM toward the anode side of the lithium using a separator that is modified with a negatively charged polymer. When DMPZ (5,10‐dihydro‐5,10‐dimethylphenazine) is used as an RM, it is found that the modified separator suppresses the migration of DMPZ toward the counter electrode of the Li metal anode. This is investigated by a visual redox couple diffusion test, a morphological investigation, and an X‐ray diffraction study. This advanced separator effectively maximizes the catalytic activity of the redox mediator. Li–O₂ batteries using both a highly concentrated DMPZ and the modified separator exhibit improved performance and maintained 90% round‐trip efficiency up to the 20th cycle.     

Exploiting the Condensation Reactions of Acetophenone to Engineer Carbon‐Encapsulated Nb2O5 Nanocrystals for High‐Performance Li and Na Energy Storage Systems

Efficient synthetic methods to produce high‐performance electrode‐active materials are crucial for developing energy storage devices for large‐scale applications, such as hybrid supercapacitors (HSCs). Here, an effective approach to obtain controllable carbon‐encapsulated T‐Nb₂O₅ nanocrystals (NCs) is presented, based on the solvothermal treatment of NbCl₅ in acetophenone. Two separate condensation reactions of acetophenone generate an intimate and homogeneous mixture of Nb₂O₅ particles and 1,3,5‐triphenylbenzene (TPB), which acts as a unique carbon precursor. The electrochemical performance of the resulting composites as anode electrode materials can be tuned by varying the Nb₂O₅/TPB ratio. Remarkable performances are achieved for Li‐ion and Na‐ion energy storage systems at high charge–discharge rates (specific capacities of ≈90 mAh g^?1 at 100 C rate for lithium and ≈125 mAh g^?1 at 20 C for sodium). High energy and power densities are also achieved with Li‐ and Na‐ion HSC devices constructed by using the Nb₂O₅/C composites as anode and activated carbon (YPF‐50) as cathode, demonstrating the excellent electrochemical properties of the materials synthesized with this approach.     

Hierarchical NiCo2S4@NiO Core–Shell Heterostructures as Catalytic Cathode for Long‐Life Li‐O2 Batteries

The critical challenges of Li‐O₂ batteries lie in sluggish oxygen redox kinetics and undesirable parasitic reactions during the oxygen reduction reaction and oxygen evolution reaction processes, inducing large overpotential and inferior cycle stability. Herein, an elaborately designed 3D hierarchical heterostructure comprising NiCo₂S₄@NiO core–shell arrays on conductive carbon paper is first reported as a freestanding cathode for Li‐O₂ batteries. The unique hierarchical array structures can build up multidimensional channels for oxygen diffusion and electrolyte impregnation. A built‐in interfacial potential between NiCo₂S₄ and NiO can drastically enhance interfacial charge transfer kinetics. According to density functional theory calculations, intrinsic LiO₂‐affinity characteristics of NiCo₂S₄ and NiO play an importantly synergistic role in promoting the formation of large peasecod‐like Li₂O₂, conducive to construct a low‐impedance Li₂O₂/cathode contact interface. As expected, Li‐O₂ cells based on NiCo₂S₄@NiO electrode exhibit an improved overpotential of 0.88 V, a high discharge capacity of 10 050 mAh g^?1 at 200 mA g^?1, an excellent rate capability of 6150 mAh g^?1 at 1.0 A g^?1, and a long‐term cycle stability under a restricted capacity of 1000 mAh g^?1 at 200 mA g^?1. Notably, the reported strategy about heterostructure accouplement may pave a new avenue for the effective electrocatalyst design for Li‐O₂ batteries.     

Cu,Co‐Embedded N‐Enriched Mesoporous Carbon for Efficient Oxygen Reduction and Hydrogen Evolution Reactions

Rational synthesis of hybrid, earth‐abundant materials with efficient electrocatalytic functionalities are critical for sustainable energy applications. Copper is theoretically proposed to exhibit high reduction capability close to Pt, but its high diffusion behavior at elevated fabrication temperatures limits its homogeneous incorporation with carbon. Here, a Cu, Co‐embedded nitrogen‐enriched mesoporous carbon framework (CuCo@NC) is developed using, a facile Cu‐confined thermal conversion strategy of zeolitic imidazolate frameworks (ZIF‐67) pre‐grown on Cu(OH)₂ nanowires. Cu ions formed below 450 °C are homogeneously confined within the pores of ZIF‐67 to avoid self‐aggregation, while the existence of Cu? N bonds further increases the nitrogen content in carbon frameworks derived from ZIF‐67 at higher pyrolysis temperatures. This CuCo@NC electrocatalyst provides abundant active sites, high nitrogen doping, strong synergetic coupling, and improved mass transfer, thus significantly boosting electrocatalytic performances in oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). A high half‐wave potential (0.884 V vs reversible hydrogen potential, RHE) and a large diffusion‐limited current density are achieved for ORR, comparable to or exceeding the best reported earth‐abundant ORR electrocatalysts. In addition, a low overpotential (145 mV vs RHE) at 10 mA cm^?2 is demonstrated for HER, further suggesting its great potential as an efficient electrocatalyst for sustainable energy applications.     

A Comparative Study of Redox Mediators for Improved Performance of Li–Oxygen Batteries

Redox meditators (RMs) are soluble catalysts located in an electrolyte that can improve the energy efficiency (reduced overpotential) and cyclability of Li–oxygen (Li–O₂) batteries. In this work, 20 RMs within a Li–O₂ system with dimethyl sulfoxide and tetraethylene glycol dimethyl ether electrolytes are studied and their electrochemical features such as redox potential, the separation of cathodic and anodic peaks, and their current intensities are measured using cyclic voltammetry (CV) experiments. Six RMs are selected as “primary” choices based on their electrochemical performance, and stability tests are then performed to examine their electrochemical responses after consecutive cycles. Moreover, galvanostatic cycling tests are performed within a Li–O₂ battery system assembled with selected six RMs for real case consistency investigations. It is found that results from CV to galvanostatic cycling tests are consistent for halides and organometallic RMs, where the former exhibit much higher stability. However, the organic RMs show high reversibility in CV but low in battery cycling results. Density functional theory calculations are carried out to gain more understanding of the stability and redox potentials of the RMs. This study provides comparative information to select the most reliable RMs for Li–O₂ batteries along with new fundamental understanding of their electrochemical activity and stability.