Vertical Diffusion of Ions within Single Particles during Electrochemical Charging

Scanning Electrochemical Microscopy Meets Optical Microscopy: Probing the Local Paths of Charge Transfer Operando in Booster-Microparticles for Flow Batteries

Understanding the oxidation/reduction dynamics of secondary microparticles formed from agglomerated nanoscale primary particles is crucial for advancing electrochemical energy storage technologies. In this study, the behavior of individual copper hexacyanoferrate (CuHCF) microparticles is explored at both global and local scales combining scanning electrochemical microscopy (SECM), for electrochemical interrogation of a single, but global-scale microparticle, and optical microscopy monitoring to obtain a higher resolution dynamic image of the local electrochemistry within the same particle. Chronoamperometric experiments unveil a multistep oxidation/reduction process with varying dynamics. On the one hand, the global SECM analysis enables quantifying the charge transfer as well as its dynamics at the single microparticle level during the oxidation/reduction cycles by a redox mediator in solution. These conditions allow mimicking the charge storage processes in these particles when they are used as solid boosters in redox flow batteries. On the other hand, optical imaging with sub-particle resolution allows the mapping of local conversion rates and state-of-charge within individual CuHCF particles. These maps reveal that regions of different material loadings exhibit varying charge storage capacities and conversion rates. The findings highlight the significance of porous nanostructures and provide valuable insights for designing more efficient energy storage materials.

Imaging Single Prussian Blue Nanoparticles with Extraordinary Low-Spin Iron Capacity

In attempts to obtain high-capacity Prussian blue nanomaterials, current efforts are predominantly focused on the particle-ensemble-level understanding of their structure-activity relationships. Complementarily, it would be insightful to screen out extraordinary individuals from the nanoparticle population. Using a simple and efficient technique of bright-field microscopy, this work enables, for the first time, quantitative characterization of the overall two-redox-center electrochemistry of single Prussian blue nanoparticles many at a time. Quantitative optical voltammograms with little interference from solvent breakdown and non-Faradaic electrode charging/discharging are extracted for each single nanoparticle, revealing clear heterogeneity among them. On this basis, the microscopic method allows a detailed comparative analysis between the two redox-active sites. It is found that while the synthesized nanoparticles show a similar specific capacity of the high-spin (HS-Fe) sites with STD/mean = 30%, most individual nanoparticles exhibit monodispersedly small capacities of the low-spin iron (LS-Fe) sites, only about 1 7 ± 1 of the HS-Fe capacity. Most importantly, it is discovered that there is always a small fraction (∼8%) of the single nanoparticles showing an impressively tripled LS-Fe capacity. Facilitated by optical imaging, the discovery of this easily overlooked extraordinary subpopulation confers alternative opportunities for targeted efforts for material chemists to improve synthesis and material design based on these unusual individuals, which in turn implies the general significance of nanoparticle screening.

Optical Approach for Mapping the Intercalation Capacity of Porous Electrodes

The intercalation capacity of a porous electrode in real batteries is not uniform spatially due to the inevitable structural and compositional inhomogeneity and site-dependent ion and electron transport features. Reliable methods to quantify the capacity distribution are highly desirable but absent so far in battery research. In this paper, a novel optical technique, oblique incident reflection difference (OIRD), was employed to monitor in situ the electrochemical ion (de)intercalation behavior of Prussian blue analogue (PBA) porous films. The OIRD signal responded synchronously to the ion (de)intercalation, and the change in the OIRD signal (ΔI) was positively correlated with the local electrochemical capacity, thereby enabling mapping of the spatially resolved ion storage capacity of the films. Optical analysis further showed that the OIRD response originated from the ion (de)intercalation-induced dielectric constant change of PBA films. This work therefore offers an intriguing in situ and spatially resolved tool for the study of rechargeable batteries.

Iontronic microscopy of a tungsten microelectrode: "seeing" ionic currents under an optical microscope

Optical methods for monitoring electrochemical reactions at an interface are advantageous because of their table-top setup and ease of integration into reactors. Here we apply EDL-modulation microscopy to one of the main components of amperometric measurement devices: a microelectrode. We present experimental measurements of the EDL-modulation contrast from the tip of a tungsten microelectrode at various electrochemical potentials inside a ferrocene-dimethanol Fe(MeOH)2 solution. Using the combination of the dark-field scattering microscope and the lock-in detection technique, we measure the phase and amplitude of local ion-concentration oscillations in response to an AC potential as the electrode potential is scanned through the redox-activity window of the dissolved species. We present the amplitude and phase map of this response, as such this method can be used to study the spatial and temporal variations of the ion-flux due to an electrochemical reaction close to metallic and semiconducting objects of general geometry. We discuss the advantages and possible extensions of using this microscopy method for wide-field imaging of ionic currents.

Plasmonic Scattering Imaging of Surface-Bonded Nanoparticles at the Solution-Solid Interface

Imaging nanoscale objects at interfaces is essential for revealing surface-tuned mechanisms in chemistry, physics, and life science. Plasmonic-based imaging, a label-free and surface-sensitive technique, has been widely used for studying the chemical and biological behavior of nanoscale objects at interfaces. However, direct imaging of surface-bonded nanoscale objects remains challenging due to uneven image backgrounds. Here, we present a new surface-bonded nanoscale object detection microscopy that eliminates strong background interference by reconstructing accurate scattering patterns at different positions. Our method effectively functions at low signal-to-background ratios, allowing for optical scattering detection of surface-bonded polystyrene nanoparticles and severe acute respiratory syndrome coronavirus 2 pseudovirus. It is also compatible with other imaging configurations, such as bright-field imaging. This technique complements existing methods for dynamic scattering imaging and broadens the applications of plasmonic imaging techniques for high-throughput sensing of surface-bonded nanoscale objects, enhancing our understanding of the properties, composition, and morphology of nanoparticles and surfaces at the nanoscale.

Single Particle Hopping as an Indicator for Evaluating Electrocatalysts

The design and screening of electrocatalysts for gas evolution reactions suffer from little understanding of multiphase processes at the electrode-electrolyte interface. Due to the complexity of the multiphase interface, it is still a great challenge to capture gas evolution dynamics under operando conditions to precisely portray the intrinsic catalytic performance of the interface. Here, we establish a single particle imaging method to real-time monitor a potential-dependent vertical motion or hopping of electrocatalysts induced by electrogenerated gas nanobubbles. The hopping feature of a single particle is closely correlated with intrinsic activities of electrocatalysts and thus is developed as an indicator to evaluate gas evolution performance of various electrocatalysts. This optical indicator diminishes interference from heterogeneous morphologies, non-Faradaic processes, and parasitic side reactions that are unavoidable in conventional electrochemical measurements, therefore enabling precise evaluation and high-throughput screening of catalysts for gas evolution systems.

Emerging Optical Microscopy Techniques for Electrochemistry

An optical microscope is probably the most intuitive, simple, and commonly used instrument to observe objects and discuss behaviors through images. Although the idea of imaging electrochemical processes operando by optical microscopy was initiated 40 years ago, it was not until significant progress was made in the last two decades in advanced optical microscopy or plasmonics that it could become a mainstream electroanalytical strategy. This review illustrates the potential of different optical microscopies to visualize and quantify local electrochemical processes with unprecedented temporal and spatial resolution (below the diffraction limit), up to the single object level with subnanoparticle or single-molecule sensitivity. Developed through optically and electrochemically active model systems, optical microscopy is now shifting to materials and configurations focused on real-world electrochemical applications.

Determining the depth of surface charging layer of single Prussian blue nanoparticles with pseudocapacitive behaviors

Understanding the hybrid charge-storage mechanisms of pseudocapacitive nanomaterials holds promising keys to further improve the performance of energy storage devices. Based on the dependence of the light scattering intensity of single Prussian blue nanoparticles (PBNPs) on their oxidation state during sinusoidal potential modulation at varying frequencies, we present an electro-optical microscopic imaging approach to optically acquire the Faradaic electrochemical impedance spectroscopy (oEIS) of single PBNPs. Here we reveal typical pseudocapacitive behavior with hybrid charge-storage mechanisms depending on the modulation frequency. In the low-frequency range, the optical amplitude is inversely proportional to the square root of the frequency (∆I ∝ f^−0.5; diffusion-limited process), while in the high-frequency range, it is inversely proportional to the frequency (∆I ∝ f⁻¹; surface charging process). Because the geometry of single cuboid-shaped PBNPs can be precisely determined by scanning electron microscopy and atomic force microscopy, oEIS of single PBNPs allows the determination of the depth of the surface charging layer, revealing it to be ~2 unit cells regardless of the nanoparticle size.

The surface charging layer in nanomaterials, which determines their pseudocapacitive behavior, is challenging to characterize. Here the authors perform Faradic electrochemical impedance spectroscopy measurements of single cuboid Prussian blue nanoparticles, displaying a hybrid charge storage mechanism, and determine the depth of the surface charging layer.

A microwell array-based approach for studying single nanoparticle catalysis with high turnover frequency

Measuring the catalytical activities of single catalysts in the case of high turnover frequency (TOF, realistic conditions) is highly desirable to accurately evaluate the functional heterogeneities among individuals and to understand the catalytic mechanism. Herein, we report a microwell array-based method to in operando measure the photocatalytic kinetics of single CdS nanoparticles (NPs) with high TOF. This was realized by sealing individual CdS NPs into separated micrometer-sized polydimethylsiloxane wells, thus eliminating the diffusion of products among individuals in the case of high concentration of reactants. This method allowed us to monitor the activities of single catalysts with an average TOF up to 2.1 × 10⁵ s^-1. Interestingly, two types of catalytical behaviors were revealed during single CdS photocatalysis: a rapid decline in activity for most CdS NPs and an initial increase in activity followed by a decrease for a minor population of individuals. The developed method will facilitate the investigation of catalytic activities of single particles under realistic conditions and hold great potential in the fields of photo/electro-catalysts, enzymes, functional bacteria, and so on.