In Situ Electrochemistry of Rechargeable Battery Materials: Status Report and Perspectives

The development of rechargeable batteries with high performance is considered to be a feasible way to satisfy the increasing needs of electric vehicles and portable devices. It is of vital importance to design electrodes with high electrochemical performance and to understand the nature of the electrode/electrolyte interfaces during battery operation, which allows a direct observation of the complicated chemical and physical processes within the electrodes and electrolyte, and thus provides real‐time information for further design and optimization of the battery performance. Here, the recent progress in in situ techniques employed for the investigations of material structural evolutions is described, including characterization using neutrons, X‐ray diffraction, and nuclear magnetic resonance. In situ techniques utilized for in‐depth uncovering the electrode/electrolyte phase/interface change mechanisms are then highlighted, including transmission electron microscopy, atomic force microscopy, X‐ray spectroscopy, and Raman spectroscopy. The real‐time monitoring of lithium dendrite growth and in situ detection of gas evolution during charge/discharge processes are also discussed. Finally, the major challenges and opportunities of in situ characterization techniques are outlined toward new developments of rechargeable batteries, including innovation in the design of compatible in situ cells, applications of dynamic analysis, and in situ electrochemistry under multi‐stimuli. A clear and in‐depth understanding of in situ technique applications and the mechanisms of structural evolutions, surface/interface changes, and gas generations within rechargeable batteries is given here.     

Review of Recent Development of In Situ/Operando Characterization Techniques for Lithium Battery Research

The increasing demands of energy storage require the significant improvement of current Li‐ion battery electrode materials and the development of advanced electrode materials. Thus, it is necessary to gain an in‐depth understanding of the reaction processes, degradation mechanism, and thermal decomposition mechanisms under realistic operation conditions. This understanding can be obtained by in situ/operando characterization techniques, which provide information on the structure evolution, redox mechanism, solid‐electrolyte interphase (SEI) formation, side reactions, and Li‐ion transport properties under operating conditions. Here, the recent developments in the in situ/operando techniques employed for the investigation of the structural stability, dynamic properties, chemical environment changes, and morphological evolution are described and summarized. The experimental approaches reviewed here include X‐ray, electron, neutron, optical, and scanning probes. The experimental methods and operating principles, especially the in situ cell designs, are described in detail. Representative studies of the in situ/operando techniques are summarized, and finally the major current challenges and future opportunities are discussed. Several important battery challenges are likely to benefit from these in situ/operando techniques, including the inhomogeneous reactions of high‐energy‐density cathodes, the development of safe and reversible Li metal plating, and the development of stable SEI.     

Antioxidant technology for durability enhancement in polymer electrolyte membranes for fuel cell applications

While polymer electrolyte membrane fuel cells (PEMFCs) have surged in popularity due to their low environmental impact and high efficiency, their susceptibility to degradation by in-situ generated peroxide and oxygen radical species has prevented their widespread adoption. To alleviate chemical attack on components of PEMFCs, particularly on polymer electrolyte membranes (PEMs), antioxidant approaches have been the subject of enormous interest as a key solution because they can directly scavenge and remove detrimental peroxide and oxygen radical species. However, a consequence is that long-term PEMFC device operation can cause undesirable adverse degradation of antioxidant additives provoked by the distinctive chemical/electrochemical environment of low pH, electric potential, water flux, and ion exchange/concentration gradient. Moreover, changes in the physical state such as migration, agglomeration, and dissolution of antioxidants by mechanical or chemical pressures are serious problems that gradually deteriorate antioxidant activity and capacity. This review presents current opportunities for and limitations to antioxidant therapy for durability enhancement in PEMs for electrochemical device applications. We also provide a summary of advanced synthetic design strategies and in-depth analyses of antioxidants regarding optimizing activity-stability factors. This review will bring new insight into the design to realization of ideal antioxidant nanostructures for PEMs and open up new opportunities for enhancing proliferation of durable PEMFCs.     

The Absence and Importance of Operando Techniques for Metal‐Free Catalysts

Operando characterization techniques have played a crucial role in modern technological developments. In contrast to the experimental uncertainties introduced by ex situ techniques, the simultaneous measurement of desired sample characteristics and near‐realistic electrochemical testing provides a representative picture of the underlying physics. From Li‐ion batteries to metal‐based electrocatalysts, the insights offered by real‐time characterization data have enabled more efficient research programs. As an emerging class of catalyst, much of the mechanistic understanding of metal‐free electrocatalysts continues to be elusive in comparison to their metal‐based counterparts. However, there is a clear absence of operando characterization performed on metal‐free catalysts. Through the proper execution of operando techniques, it can be expected that metal‐free catalysts can achieve exceptional technological progress. Here, the motivation of using operando characterization techniques for metal‐free carbon‐based catalyst system is considered, followed by a discussion of the possibilities, difficulties and benefits of their applications.     

Efficient Sodium Storage in Rolled‐Up Amorphous Si Nanomembranes

Alloying‐type materials are promising anodes for high‐performance sodium‐ion batteries (SIBs) because of their high capacities and low Na‐ion insertion potentials. However, the typical candidates, such as P, Sn, Sb, and Pb, suffer from severe volume changes (≈293–487%) during the electrochemical reactions, leading to inferior cycling performances. Here, a high‐rate and ultrastable alloying‐type anode based on the rolled‐up amorphous Si nanomembranes is demonstrated. The rolled‐up amorphous Si nanomembranes show a very small volume change during the sodiation/desodiation processes and deliver an excellent rate capability and ultralong cycle life up to 2000 cycles with 85% capacity retention. The structural evolution and pseudocapacitance contribution are investigated by using the ex situ characterization techniques combined with kinetics analysis. Furthermore, the mechanism of efficient sodium‐ion storage in amorphous Si is kinetically analyzed through an illustrative atomic structure with dangling bonds, offering a new perspective on understanding the sodium storage behavior. These results suggest that nanostructured amorphous Si is a promising anode material for high‐performance SIBs.     

Fuel Cells: Structure‐Morphology‐Property Relationships of Non‐Perfluorinated Proton‐Conducting Membranes (Adv. Mater. 42/2010)

A fundamental understanding of structure‐morphology‐property relationships of proton exchange membranes (PEMs) is crucial in order to improve the cost, performance, and durability of PEM fuel cells (PEMFCs). In this context, there has been an explosion over the past five years in the volume of research carried out in the area of non‐perfluorinated, proton‐conducting polymer membranes, with a particular emphasis on exploiting phase behavior associated with block and graft copolymers. This progress report highlights a selection of interesting studies in the area that have appeared since 2005, which illustrate the effects of factors such as acid and water contents and morphology upon proton conduction. It concludes with an outlook on future directions.     

Structure‐Morphology‐Property Relationships of Non‐Perfluorinated Proton‐Conducting Membranes

A fundamental understanding of structure‐morphology‐property relationships of proton exchange membranes (PEMs) is crucial in order to improve the cost, performance, and durability of PEM fuel cells (PEMFCs). In this context, there has been an explosion over the past five years in the volume of research carried out in the area of non‐perfluorinated, proton‐conducting polymer membranes, with a particular emphasis on exploiting phase behavior associated with block and graft copolymers. This progress report highlights a selection of interesting studies in the area that have appeared since 2005, which illustrate the effects of factors such as acid and water contents and morphology upon proton conduction. It concludes with an outlook on future directions.     

Electrochemically-driven large amplitude pH cycling for acid-base driven DNA denaturation and renaturation

In this paper, we present an electrochemically driven large amplitude pH alteration method based on a serial electrolytic cell involving a hydrogen permeable bifacial working electrode such as Pd thin foil. The method allows solution pH to be changed periodically up to ±4~5 units without additional alteration of concentration and/or composition of the system. Application to the acid-base driven cyclic denaturation and renaturation of 290 bp DNA fragments is successfully demonstrated with in situ real-time UV spectroscopic characterization. Electrophoretic analysis confirms that the denaturation and renaturation processes are reversible without degradation of the DNA. The serial electrolytic cell based electrochemical pH alteration method presented in this work would promote investigations of a wide variety of potential-dependent processes and techniques.     

A Universal Strategy for Constructing Seamless Graphdiyne on Metal Oxides to Stabilize the Electrochemical Structure and Interface

The structural and interfacial stabilities of metal oxides (MOs) are key issues while facing the volumetric variation and intensive interfacial polarization in electrochemical applications, including lithium‐ion batteries (LIBs), supercapacitors, and catalysts. The growth of a seamless all‐carbon interfacial layer on MOs with complex dimensions is not only a scientific problem, but also a practical challenge in these fields. Here, the growth of graphdiyne under ultramild condition is successfully implemented in situ for coating MOs of complex dimensions. The seamless all‐carbon interface and conductive network are formed at the same time. This method cleverly avoids the structural degradation of MOs at a high temperature in the presence of traditional carbon materials. Under the protection of the high‐quality graphdiyne layer, the samples as LIB anodes deliver high performances in terms of Coulomb efficiency, capacity, long‐term retention, and structural and interfacial stabilities. Both experimental achievements and theoretical calculations demonstrate that the graphdiyne is a particular protection layer for MOs and plays a crucial role for preventing the structural and interfacial degradation of the electrode. Furthermore, the universality of this method will promote the potential applications of many promising MOs in other electrochemical fields.     

Suppressing Cation Migration and Reducing Particle Cracks in a Layered Fe‐Based Cathode for Advanced Sodium‐Ion Batteries

Sodium‐ion batteries have huge potential in large‐scale energy storage applications. Layered Fe‐based oxides are one of the desirable cathode materials due to abundance in the earth crust and high activity in electrochemical processes. However, Fe‐ion migration to Na layers is one of the major hurdles leading to irreversible structural degradation. Herein, it is revealed that distinct Fe‐ion migration in cycling NaFeO₂ (NFO) should be mainly responsible for the strong local lattice strain and resulting particle cracks, all of which results in the deterioration of electrochemical performance. More importantly, a strategy of Ru doping could effectively suppress the Fe‐ion migration and then reduce the local lattice strain and the particle cracks, finally to greatly enhance the sodium storage performance. Atomic‐scale characterization shows that NFO electrode after cycling presents the intense lattice strain locally, accompanied by the remarkable particle cracks. Whereas, Ru‐doped NFO electrode maintains the well‐ordered layered structure by inhibiting the Fe–O distortion, so as to eliminate the resulting side effect. As a result, Ru‐doped NFO could greatly improve the comprehensive electrochemical performance by delivering a reversible capacity of 120 mA h g^?1, about 80% capacity retention after 100 cycles. The findings provide new insights for designing high‐performance electrodes for sodium‐ion batteries.     

Ni2P@Carbon Core–Shell Nanoparticle‐Arched 3D Interconnected Graphene Aerogel Architectures as Anodes for High‐Performance Sodium‐Ion Batteries

To alleviate large volume change and improve poor electrochemical reaction kinetics of metal phosphide anode for sodium‐ion batteries, for the first time, an unique Ni₂P@carbon/graphene aerogel (GA) 3D interconnected porous architecture is synthesized through a solvothermal reaction and in situ phosphorization process, where core–shell Ni₂P@C nanoparticles are homogenously embedded in GA nanosheets. The synergistic effect between components endows Ni₂P@C/GA electrode with high structural stability and electrochemical activity, leading to excellent electrochemical performance, retaining a specific capacity of 124.5 mA h g^?1 at a current density of 1 A g^?1 over 2000 cycles. The robust 3D GA matrix with abundant open pores and large surface area can provide unblocked channels for electrolyte storage and Na⁺ transfer and make fully close contact between the electrode and electrolyte. The carbon layers and 3D GA together build a 3D conductive matrix, which not only tolerates the volume expansion as well as prevents the aggregation and pulverization of Ni₂P nanoparticles during Na⁺ insertion/extraction processes, but also provides a 3D conductive highway for rapid charge transfer processes. The present strategy for phosphides via in situ phosphization route and coupling phosphides with 3D GA can be extended to other novel electrodes for high‐performance energy storage devices.     

Self‐Supported Mesostructured Pt‐Based Bimetallic Nanospheres Containing an Intermetallic Phase as Ultrastable Oxygen Reduction Electrocatalysts

Developing highly active and stable cathode catalysts is of pivotal importance for proton exchange membrane fuel cells (PEMFCs). While carbon‐supported nanostructured Pt‐based catalysts have so far been the most active cathode catalysts, their durability and single‐cell performance are yet to be improved. Herein, self‐supported mesostructured Pt‐based bimetallic (Meso‐PtM; M = Ni, Fe, Co, Cu) nanospheres containing an intermetallic phase are reported, which can combine the beneficial effects of transition metals (M), an intermetallic phase, a 3D interconnected framework, and a mesoporous structure. Meso‐PtM nanospheres show enhanced oxygen reduction reaction (ORR) activity, compared to Pt black and Pt/C catalysts. Notably, Meso‐PtNi containing an intermetallic phase exhibits ultrahigh stability, showing enhanced ORR activity even after 50 000 potential cycles, whereas Pt black and Pt/C undergo dramatic degradation. Importantly, Meso‐PtNi with an intermetallic phase also demonstrated superior activity and durability when used in a PEMFC single‐cell, with record‐high initial mass and specific activities.     

Application of scanning electrochemical microscope in the study of corrosion of metals

Scanning electrochemical microscope (SECM) has become a very useful and powerful technique for probing a variety of electrochemical reactions in corrosion process due to its high spatial resolution and electrochemical sensitivity to characterize the topography and redox activities of the metal/electrolyte solution interface. Its capability for the direct identification of chemical species in localized corrosion processes with high spatial resolution would be more advantageous compared to other local probe techniques with only morphological characterization. In this review, the applications of the SECM in the study of early stages of localized corrosion, electroactive defect sites in passive films, local initiation of pits, degradation of coating properties on steels, and some combined methods through SECM integrated with other techniques have been summarized and commented. Finally, the optimization for SECM’s experiment design and operation as well as foreseeable application range has been proposed.     

In Situ TEM Electromechanical Testing of Nanowires and Nanotubes

The emergence of one‐dimensional nanostructures as fundamental constituents of advanced materials and next‐generation electronic and electromechanical devices has increased the need for their atomic‐scale characterization. Given its spatial and temporal resolution, coupled with analytical capabilities, transmission electron microscopy (TEM) has been the technique of choice in performing atomic structure and defect characterization. A number of approaches have been recently developed to combine these capabilities with in‐situ mechanical deformation and electrical characterization in the emerging field of in‐situ TEM electromechanical testing. This has enabled researchers to establish unambiguous synthesis‐structure‐property relations for one‐dimensional nanostructures. In this article, the development and latest advances of several in‐situ TEM techniques to carry out mechanical and electromechanical testing of nanowires and nanotubes are reviewed. Through discussion of specific examples, it is shown how the merging of several microsystems and TEM has led to significant insights into the behavior of nanowires and nanotubes, underscoring the significant role in‐situ techniques play in the development of novel nanoscale systems and materials.     

Local Degradation of PEDOT:PSS on Silicon Nanostructures Using Scanning Electrochemical Microscopy

Conducting polymers show attractive characteristics as electrode materials for micro-electrochemical energy storage (MEES). However, there is a lack of characterization techniques to study conjugated/conducting polymer-based nanostructured electrodes. Here, scanning electrochemical microscopy (SECM) is introduced as a new technique for in situ characterization and acceleration of degradation processes of conducting polymers. Electrodes of PEDOT:PSS on flat silicon, silicon nanowires (SiNWs) and silicon nanotrees (SiNTrs) are analyzed by SECM in feedback mode with approach curves and chronoamperometry. The innovative degradation method using SECM reduces the time required to locally degrade polymer samples to a few thousand seconds, which is significantly shorter than the time usually required for such studies. The degradation rate is modeled using Comsol Multiphysics. The model provides an understanding of the phenomena that occur during degradation of the polymer electrode and describes them using a mathematical constant A₀ and a time constant τ.     

Electrolytes and Electrolyte/Electrode Interfaces in Sodium‐Ion Batteries: From Scientific Research to Practical Application

Sodium‐ion batteries (SIBs) have drawn considerable interest as power‐storage devices owing to the wide abundance of their constituents and low cost. To realize a high performance–price ratio, the cathode and anode materials must be optimized. As essential components of SIBs, electrolytes should have wide electrochemical windows, high thermal stability, and exceptional ionic conductivity. Therefore, improved electrolytes, based on various materials and compositions, are developed to meet the practical demands of SIBs, including organic electrolytes, ionic liquids, aqueous, solid electrolytes, and hybrid electrolytes. Although mature organic electrolytes are currently used in production, aqueous and solid electrolytes show advantages for future applications, as discussed here in detail. Current efforts in modifying electrolytes to optimize their interfacial compatibility with electrodes, leading to longer battery lifetimes and greater safety, are described. The advanced characterization techniques used to investigate the properties of electrolytes and interfaces are introduced, and the reaction processes and degradation mechanisms of SIBs are revealed. Furthermore, the practical prospects of SIBs promoted by high‐quality electrolytes appropriately matched with electrodes are predicted and directions for developing next‐generation SIBs are suggested.     

Nitrogen‐Coordinated Single Cobalt Atom Catalysts for Oxygen Reduction in Proton Exchange Membrane Fuel Cells

Due to the Fenton reaction, the presence of Fe and peroxide in electrodes generates free radicals causing serious degradation of the organic ionomer and the membrane. Pt‐free and Fe‐free cathode catalysts therefore are urgently needed for durable and inexpensive proton exchange membrane fuel cells (PEMFCs). Herein, a high‐performance nitrogen‐coordinated single Co atom catalyst is derived from Co‐doped metal‐organic frameworks (MOFs) through a one‐step thermal activation. Aberration‐corrected electron microscopy combined with X‐ray absorption spectroscopy virtually verifies the CoN₄ coordination at an atomic level in the catalysts. Through investigating effects of Co doping contents and thermal activation temperature, an atomically Co site dispersed catalyst with optimal chemical and structural properties has achieved respectable activity and stability for the oxygen reduction reaction (ORR) in challenging acidic media (e.g., half‐wave potential of 0.80 V vs reversible hydrogen electrode (RHE). The performance is comparable to Fe‐based catalysts and 60 mV lower than Pt/C ‐60 μg Pt cm^?2). Fuel cell tests confirm that catalyst activity and stability can translate to high‐performance cathodes in PEMFCs. The remarkably enhanced ORR performance is attributed to the presence of well‐dispersed CoN₄ active sites embedded in 3D porous MOF‐derived carbon particles, omitting any inactive Co aggregates.     

Manganese‐Based Na‐Rich Materials Boost Anionic Redox in High‐Performance Layered Cathodes for Sodium‐Ion Batteries

To improve the energy and power density of Na‐ion batteries, an increasing number of researchers have focused their attention on activation of the anionic redox process. Although several materials have been proposed, few studies have focused on the Na‐rich materials compared with Li‐rich materials. A key aspect is sufficient utilization of anionic species. Herein, a comprehensive study of Mn‐based Na_1.2Mn_0.4Ir_0.4O₂ (NMI) O3‐type Na‐rich materials is presented, which involves both cationic and anionic contributions during the redox process. The single‐cation redox step relies on the Mn³⁺/Mn⁴⁺, whereas Ir atoms build a strong covalent bond with O and effectively suppress the O₂ release. In situ Raman, ex situ X‐ray photoelectron spectroscopy, and soft‐X‐ray absorption spectroscopy are employed to unequivocally confirm the reversibility of O₂^2? species formation and suggest a high degree of anionic reaction in this NMI Na‐rich material. In operando X‐ray diffraction study discloses the asymmetric structure evolution between the initial and subsequent cycles, which also explains the effect of the charge compensation mechanism on the electrochemical performance. The research provides a novel insight on Na‐rich materials and a new perspective in materials design towards future applications.     

Iron‐Free Cathode Catalysts for Proton‐Exchange‐Membrane Fuel Cells: Cobalt Catalysts and the Peroxide Mitigation Approach

High‐performance and inexpensive platinum‐group‐metal (PGM)‐free catalysts for the oxygen reduction reaction (ORR) in challenging acidic media are crucial for proton‐exchange‐membrane fuel cells (PEMFCs). Catalysts based on Fe and N codoped carbon (Fe–N–C) have demonstrated promising activity and stability. However, a serious concern is the Fenton reactions between Fe²⁺ and H₂O₂ generating active free radicals, which likely cause degradation of the catalysts, organic ionomers within electrodes, and polymer membranes used in PEMFCs. Alternatively, Co–N–C catalysts with mitigated Fenton reactions have been explored as a promising replacement for Fe and PGM catalysts. Therefore, herein, the focus is on Co–N–C catalysts for the ORR relevant to PEMFC applications. Catalyst synthesis, structure/morphology, activity and stability improvement, and reaction mechanisms are discussed in detail. Combining experimental and theoretical understanding, the aim is to elucidate the structure–property correlations and provide guidance for rational design of advanced Co catalysts with a special emphasis on atomically dispersed single‐metal‐site catalysts. In the meantime, to reduce H₂O₂ generation during the ORR on the Co catalysts, potential strategies are outlined to minimize the detrimental effect on fuel cell durability.     

In Situ Coupling Strategy for the Preparation of FeCo Alloys and Co4N Hybrid for Highly Efficient Oxygen Evolution

An in situ coupling approach is developed to create a new highly efficient and durable cobalt‐based electrocatalyst for the oxygen evolution reaction (OER). Using a novel cyclotetramerization, a task‐specific bimetallic phthalocyanine‐based nanoporous organic framework is successfully built as a precursor for the carbonization synthesis of a nonprecious OER electrocatalyst. The resultant material exhibits an excellent OER activity with a low overpotential of 280 mV at a current density of 10 mA cm^?2 and high durability in an alkaline medium. This impressive result ranks among the best from known Co‐based OER catalysts under the same conditions. The simultaneous installation of multiple diverse cobalt‐based active sites, including FeCo alloys and Co₄N nanoparticles, plays a critical role in achieving this promising OER performance. This innovative approach not only enables high‐performance OER activity to be achieved but simultaneously provides a means to control the surface features, thereby tuning the catalytic property of the material.