Direct Probing of the Oxygen Evolution Reaction at Single NiFe₂O₄ Nanocrystal Superparticles with Tunable Structures

Abstract

Due to the precisely controllable size, shape, and composition, self-assembled nanocrystal superlattices exhibit unique collective properties and find wide applications in catalysis and energy conversion. Identifying their intrinsic electrocatalytic activity is challenging, as their averaged properties on ensembles can hardly be dissected from binders or additives. We here report the direct measurement of the oxygen evolution reaction at single superparticles self-assembled from ∼8 nm NiFe 2 O 4 and/or ∼4 nm Au nanocrystals using scanning electrochemical cell microscopy. Combined with coordinated scanning electron microscopy, it is found that the turnover frequency (TOF) estimated from single NiFe 2 O 4 superparticles at 1.92 V vs RHE ranges from 0.2 to 11 s −1 and is sensitive to size only when it is smaller than ∼800 nm in diameter. After the incorporation of Au nanocrystals, the TOF increases by ∼6-fold and levels off with further increasing Au content. Our study demonstrates the first direct single entity electrochemical study on individual nanocrystal superlattices with tunable structures and unravels the intrinsic structure−activity relationship that is not accessible by other methods.

Monodisperse Nanocrystal Superparticles through a Source–Sink Emulsion System

“…82,83 The source−sink emulsion approach leads to the formation of functional NC superparticles of various sizes, shapes, and morphologies in just a few minutes, paving the way to a more extended use of these complex artificial solids toward applications in photonics, 6,53 magnetotherapy, 21,22 energy storage, 84 and catalysis. 85 ■ MATERIALS AND METHODS NC Synthesis. Detailed synthetic procedures are provided in the Supporting Information.…”

Unraveling the Structural Sensitivity of CO2 Electroreduction at Facet-Defined Nanocrystals via Correlative Single-Entity and Macroelectrode Measurements

“…To realize the knowledge-guided design of next-generation CO 2 RR electrocatalysts with superior activity, selectivity, and durability, the measurement of intrinsic electrocatalytic parameters at the single-nanocrystal level and comparison with macroelectrode data to establish reliable structure–activity correlations are critical. With such correlations, the impact of extrinsic factors such as electrode morphology, local pH variation, and mass transfer hindrance can be further elucidated. − Scanning electrochemical cell microscopy (SECCM) is especially adept at achieving single-entity electrochemical measurements. − SECCM has been used to resolve the spatial heterogeneity of MoS 2 , and Fe 4.5 Ni 4.5 S 8 catalysts for HER and polycrystalline Au catalysts for CO 2 RR. , SECCM has also been used recently to extract kinetic information for oxygen reduction at Pt electrodes and probing oxygen evolution at superparticles and ZIF-derived composites …”

Heterogeneity between and within Single Hematite Nanorods as Electrocatalysts for Oxygen Evolution Reaction

“…Coupling nanoscale scanning electrochemical probe microscopy with colocalized structure characterization represents an exciting approach to address the structure–activity correlation in electrocatalysis. − It offers both the advantage of single entity measurement and the high throughput. − Herein, we use scanning electrochemical cell microscopy (SECCM), coupled with colocalized imaging, to reveal the distribution of the electrocatalytic activity at the single catalyst level with hematite nanorods as an OER catalyst. Higher-resolution SECCM mapping further reveals the spatial heterogeneity of OER activity within a single nanorod structure.…”