Multiscale Electrochemistry of Lithium Manganese Oxide (LiMn2O4): From Single Particles to Ensembles and Degrees of Electrolyte Wetting.

Scanning electrochemical cell microscopy (SECCM) facilitates single particle measurements of battery materials using voltammetry at fast scan rates (1 V s-1), providing detailed insight into intrinsic particle kinetics, otherwise obscured by matrix effects. Here, we elucidate the electrochemistry of lithium manganese oxide (LiMn2O4) particles, using a series of SECCM probes of graded size to determine the evolution of electrochemical characteristics from the single particle to ensemble level. Nanometer scale control over the SECCM meniscus cell position and height further allows the study of variable particle/substrate electrolyte wetting, including comparison of fully wetted particles (where contact is also made with the underlying glassy carbon substrate electrode) vs partly wetted particles. We find ensembles of LiMn2O4 particles show voltammograms with much larger peak separations than those of single particles. In addition, if the SECCM meniscus is brought into contact with the substrate electrode, such that the particle-support contact changes from dry to wet, a further dramatic increase in peak separation is observed. Finite element method modeling of the system reveals the importance of finite electronic conductivity of the particles, contact resistance, surface kinetics, particle size, and contact area with the electrode surface in determining the voltammetric waveshape at fast scan rates, while the responses are relatively insensitive to Li+ diffusion coefficients over a range of typical values. The simulation results explain the variability in voltammetric responses seen at the single particle level and reveal some of the key factors responsible for the evolution of the response, from ensemble, contact, and wetting perspectives. The variables and considerations explored herein are applicable to any single entity (nanoscale) electrochemical study involving low conductivity materials and should serve as a useful guide for further investigations of this type. Overall, this study highlights the potential of multiscale measurements, where wetting, electronic contact, and ionic contact can be varied independently, to inform the design of practical composite electrodes.

Correlating the Local Electrocatalytic Activity of Amorphous Molybdenum Sulfide Thin Films with Microscopic Composition, Structure, and Porosity.

Thin-film electrodes, produced by coating a conductive support with a thin layer (nanometer to micrometer) of active material, retain the unique properties of nanomaterials (e.g., activity, surface area, conductivity, etc.) while being economically scalable, making them highly desirable as electrocatalysts. Despite the ever-increasing methods of thin-film deposition (e.g., wet chemical synthesis, electrodeposition, chemical vapor deposition, etc.), there is insufficient understanding on the nanoscale electrochemical activity of these materials in relation to structure/composition, particularly for those that lack long-range order (i.e., amorphous thin-film materials). In this work, scanning electrochemical cell microscopy (SECCM) is deployed in tandem with complementary, colocated compositional/structural analysis to understand the microscopic factors governing the electrochemical activity of amorphous molybdenum sulfide (a-MoSx) thin films, a promising class of hydrogen evolution reaction (HER) catalyst. The a-MoSx thin films, produced under ambient conditions by electrodeposition, possess spatially heterogeneous electrocatalytic activity on the tens-of-micrometer scale, which is not attributable to microscopic variations in elemental composition or chemical structure (i.e., Mo and/or S bonding environments), shown through colocated, local energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analysis. A new SECCM protocol is implemented to directly correlate electrochemical activity to the electrochemical surface area (ECSA) in a single measurement, revealing that the spatially heterogeneous HER response of a-MoSx is predominantly attributable to variations in the nanoscale porosity of the thin film, with surface roughness ruled out as a major contributing factor by complementary atomic force microscopy (AFM). As microscopic composition, structure, and porosity (ECSA) are all critical factors dictating the functional properties of nanostructured materials in electrocatalysis and beyond (e.g., battery materials, electrochemical sensors, etc.), this work further cements SECCM as a premier tool for structure-function studies in (electro)materials science.

Correlative Electrochemical Microscopy of Li-Ion (De)intercalation at a Series of Individual LiMn2 O4 Particles.

The redox activity (Li-ion intercalation/deintercalation) of a series of individual LiMn2 O4 particles of known geometry and (nano)structure, within an array, is determined using a correlative electrochemical microscopy strategy. Cyclic voltammetry (current-voltage curve, I-E) and galvanostatic charge/discharge (voltage-time curve, E-t) are applied at the single particle level, using scanning electrochemical cell microscopy (SECCM), together with co-location scanning electron microscopy that enables the corresponding particle size, morphology, crystallinity, and other factors to be visualized. This study identifies a wide spectrum of activity of nominally similar particles and highlights how subtle changes in particle form can greatly impact electrochemical properties. SECCM is well-suited for assessing single particles and constitutes a combinatorial method that will enable the rational design and optimization of battery electrode materials.

Metal support effects in electrocatalysis at hexagonal boron nitride.

A scanning electrochemical droplet cell technique has been employed to screen the intrinsic electrocatalytic hydrogen evolution reaction (HER) activity of hexagonal boron nitride (h-BN) nanosheets supported on different metal substrates (Cu and Au). Local (spatially-resolved) voltammetry and Tafel analysis reveal that electronic interaction with the underlying metal substrate plays a significant role in modulating the electrocatalytic activity of h-BN, with Au-supported h-BN exhibiting significantly enhanced HER charge-transfer kinetics (exchange current is ca. two orders of magnitude larger) compared to Cu-supported h-BN, making the former material the superior support in a catalytic sense.

In-situ synthesis of highly efficient visible light driven stannic oxide/graphitic carbon nitride heterostructured photocatalysts.

Novel and efficient visible-light-driven stannic oxide/graphitic carbon nitride heterostructured photocatalysts are prepared via a simple in-situ solvothermal method. Characterization results demonstrate that there exist strong interactions between SnO2 nanoparticles and g-C3N4 matrix, which indicates the formation of SnO2/g-C3N4 heterojunction. The as-synthesized SnO2/g-C3N4 composite exhibits improved efficiency for photodegradation of rhodamine B in aqueous solutions, with an apparent rate constant 6.5 times higher than that of commercial TiO2 (Degussa P25). The enhanced photocatalytic activity is attributed to synergistic effect between SnO2 and g-C3N4, resulting in effective interfacial charge transfer and prolonged charge-hole separation time. Moreover, SnO2/g-C3N4 composite photocatalysts possess excellent durability and stability after 6 recycling runs, and a possible photocatalytic mechanism is also proposed. This research highlights the promising applications of two dimensional g-C3N4 based composite photocatalysts in the field of waste water disposal and environmental remediation.

Sandwich-like nitrogen-doped porous carbon/graphene nanoflakes with high-rate capacitive performance.

Sandwich-like nitrogen-doped porous carbon/graphene nanoflakes (NPCFs) are prepared via a two-step approach, firstly by using in situ polymerization of pyrrole (Py) on the surface of graphene oxide (GO) and then by KOH activation under an Ar atmosphere. As the shape-directing agent and conductive matrix, graphene sheets play an important role in enhancing NPCFs' electrochemical performance. The NPCFs exhibit high specific surface area (2502 m(2) g(-1)), short ion diffusion path (ca. 30 nm), high conductivity (72 S m(-1)) and a considerable nitrogen level (6.3 wt%). These intriguing features render NPCFs a promising electrode material for electrochemical supercapacitors, which displays high specific capacitance (341 F g(-1)), excellent rate capability (over 71% retention ratio at 50 A g(-1)) and outstanding cycling stability (almost no capacitance loss after 2000 cycles) in a 30 wt% KOH aqueous electrolyte. Besides, the assembled symmetrical supercapacitor delivers a high gravimetric energy density of 11.3 Wh kg(-1) in an aqueous electrolyte and 66.4 Wh kg(-1) in an organic electrolyte.