Scanning Electrochemical Cell Microscopy Investigation of Single ZIF-Derived Nanocomposite Particles as Electrocatalysts for Oxygen Evolution in Alkaline Media

Scanning Electrochemical Cell Microscopy (SECCM) in Aprotic Solvents: Practical Considerations and Applications

Many applications in modern electrochemistry, notably electrosynthesis and energy storage/conversion take advantage of the "tunable" physicochemical properties (e.g., proton availability and/or electrochemical stability) of nonaqueous (e.g., aprotic) electrolyte media. This work develops general guidelines pertaining to the use of scanning electrochemical cell microscopy (SECCM) in aprotic solvent electrolyte media to address contemporary structure-electrochemical activity problems. Using the simple outer-sphere Fc^0/+ process (Fc = ferrocene) as a model system, high boiling point (low vapor pressure) solvents give rise to highly robust and reproducible electrochemistry, whereas volatile (low boiling point) solvents need to be mixed with suitable low melting point supporting electrolytes (e.g., ionic liquids) or high boiling point solvents to avoid complications associated with salt precipitation/crystallization on the scanning (minutes to hours) time scale. When applied to perform microfabrication-specifically the electrosynthesis of the conductive polymer, polypyrrole-the optimized SECCM set up produces highly reproducible arrays of synthesized (electrodeposited) material on a commensurate scale to the employed pipet probe. Applying SECCM to map electrocatalytic activity-specifically the electro-oxidation of iodide at polycrystalline platinum-reveals unique (i.e., structure-dependent) patterns of surface activity, with grains of specific crystallographic orientation, grain boundaries and areas of high local surface misorientation identified as potential electrocatalytic "hot spots". The work herein further cements SECCM as a premier technique for structure-function-activity studies in (electro)materials science and will open up exciting new possibilities through the use of aprotic solvents for rational analysis/design in electrosynthesis, microfabrication, electrochemical energy storage/conversion, and beyond.

Accessing the Electrochemical Activity of Single Nanoparticles by Eliminating the Heterogeneous Electrical Contacts

While single nanoparticle electrochemistry holds great promise for establishing the structure-activity relationship (SAR) of electroactive nanomaterials, as it removes the heterogeneity among individuals, successful SAR studies remain rare. When one nanoparticle is seen to exhibit better performance than the others, it is often simply attributed to better activity of the particular individual. By taking the ion insertion reaction of Prussian blue nanoparticles as an example, here we show that the electrical contact between nanoparticles and electrode, a previously overlooked factor, was greatly distinct from one nanoparticle to another and significantly contributed to the apparent heterogeneity in the reactivity and cyclability. An individual nanoparticle with intrinsically perfect structure (size, facet, crystallinity, and so on) could be completely inactive, simply due to poor electrical contacts, which blurred the SAR and likely caused failures. We further proposed a sputter-coating method to enhance the electrical contacts by depositing an ultrathin platinum layer onto the sample. Such an approach was routinely adopted in scanning electron microscopy to improve the electron mobility between nanoparticles and substrate. Elimination of heterogeneous contacts ensured that the electrochemical activity of single nanoparticles can be accessed and further correlated with their structural features, thus paving the way for single nanoparticle electrochemistry to deliver on its promises in SAR.

Patterning of BiVO4 Surfaces and Monitoring of Localized Catalytic Activity Using Scanning Photoelectrochemical Microscopy

There is a lot of interest in understanding localized catalytic activities at the micro and nanoscale and designing robust catalysts for photoelectrochemical oxidation of water to address the pressing energy and environmental challenges. Here, we demonstrate that scanning photoelectrochemical microscopy (SPECM) can be effectively employed as a novel technique (i) to modify a photocatalyst surface with an electrocatalyst layer in a matrix fashion and (ii) to monitor its localized activity toward the photoelectrochemical (PEC) water oxidation reaction. The three-dimensional SPECM image clearly shows that the loading of the FeOOH electrocatalyst on the BiVO₄ semiconductor surface strongly affects its local PEC reaction activity. The optimal photoelectrodeposition time of FeOOH on the BiVO₄ photocatalyst was found to be ∼20 min when FeOOH was employed as the electrocatalyst. The electrocatalyst optimization process was conducted on a single photoanode electrode surface, making the optimization process efficient and reliable. The morphology of the formed photocatalyst/electrocatalyst hybrid, inclusive of its localized activity toward the water oxidation reaction, was simultaneously probed. A photoanode surface comprising CuWO₄/BiVO₄/FeOOH was further prepared in this study and investigated. It was found that the localized photoactivity truly reflects the activity of the local area, differs from region to region, and is contingent on the morphology of the surface. Moreover, the Pt UME is determined as an efficient probe to analyze the photoactivity of the PEC water splitting reaction. This work highlights the novel SPECM technique for enhancement and examination of the catalytic activity of the nanostructured materials.

Array Microcell Method (AMCM) for Serial Electroanalysis

We describe a method for electrochemical measurement and synthesis based on the combination of a mobile micropipette and a microelectrode array, which we term the array microcell method (AMCM). AMCM has the ability to address single electrodes within a microelectrode array (MEA) and provides a simple, low-cost format to enable versatile electrochemical measurements. In AMCM, a droplet at the tip of a movable micropipette (inner diameter of 50 μm) functions as an electrochemical cell, in which the electrode area is defined by a microelectrode of the array. We also report carbon MEAs that are well suited for AMCM and are fabricated from pyrolyzed photoresist films (PPFs). PPF-MEAs with nominal electrode diameters of 5.5 μm are characterized by AMCM, standard macroscale electrochemical methods, and finite element modeling. The versatility of AMCM is demonstrated by measurement of single Pt microparticles and by electrodeposition of shapecontrolled Pt nanoparticles.

Probing Single-Particle Electrocatalytic Activity at Facet-Controlled Gold Nanocrystals

Electrocatalytic reduction reactions (i.e., the hydrogen evolution reaction (HER) and oxygen reduction reaction) at individual, faceted Au nanocubes (NCs) and nano-octahedra (ODs) expressing predominantly {100} and {111} crystal planes on the surface, respectively, were studied by nanoscale voltammetric mapping. Cyclic voltammograms were collected at individual nanoparticles (NPs) with scanning electrochemical cell microscopy (SECCM) and correlated with particle morphology imaged by electron microscopy. Nanoscale measurements from a statistically informative set of individual NPs revealed that Au NCs have superior HER electrocatalytic activity compared to that of Au ODs, in good agreement with macroscale cyclic voltammetry measurements. Au NCs exhibited more particle-to-particle variation in catalytic activity compared to that with Au ODs. The approach of single-particle SECCM imaging coupled with macroscale CV on well-defined NPs provides a powerful toolset for the design and activity assessment of nanoscale electrocatalysts.

A redox interaction-engaged strategy for multicomponent nanomaterials

The review article focuses on the redox interaction-engaged strategy that offers a powerful way to construct multicomponent nanomaterials with precisely-controlled size, shape, composition and hybridization of nanostructures.

High-Throughput Correlative Electrochemistry-Microscopy at a Transmission Electron Microscopy Grid Electrode

As part of the revolution in electrochemical nanoscience, there is growing interest in using electrochemistry to create nanostructured materials and to assess properties at the nanoscale. Herein, we present a platform that combines scanning electrochemical cell microscopy with ex situ scanning transmission electron microscopy to allow the ready creation of an array of nanostructures coupled with atomic-scale analysis. As an illustrative example, we explore the electrodeposition of Pt at carbon-coated transmission electron microscopy (TEM) grid supports, where in a single high-throughput experiment it is shown that Pt nanoparticle (PtNP) density increases and size polydispersity decreases with increasing overpotential (i.e., driving force). Furthermore, the coexistence of a range of nanostructures, from single atoms to aggregates of crystalline PtNPs, during the early stages of electrochemical nucleation and growth supports a nonclassical aggregative growth mechanism. Beyond this exemplary system, the presented correlative electrochemistry-microscopy approach is generally applicable to solve ubiquitous structure-function problems in electrochemical science and beyond, positioning it as a powerful platform for the rational design of functional nanomaterials.

Scanning Electrochemical Cell Microscopy Investigation of Single ZIF-Derived Nanocomposite Particles as Electrocatalysts for Oxygen Evolution in Alkaline Media

"Single entity" measurements are central for an improved understanding of the function of nanoparticle-based electrocatalysts without interference arising from mass transfer limitations and local changes of educt concentration or the pH value. We report a scanning electrochemical cell microscopy (SECCM) investigation of zeolitic imidazolate framework (ZIF-67)-derived Co-N-doped C composite particles with respect to the oxygen evolution reaction (OER). Surmounting the surface wetting issues as well as the potential drift through the use of a non-interfering Os complex as free-diffusing internal redox potential standard, SECCM could be successfully applied in alkaline media. SECCM mapping reveals activity differences relative to the number of particles in the wetted area of the droplet landing zone. The turnover frequency (TOF) is 0.25 to 1.5 s^-1 at potentials between 1.7 and 1.8 V vs. RHE, respectively, based on the number of Co atoms in each particle. Consistent values at locations with varying number of particles demonstrates OER performance devoid of macroscopic film effects.