Bridging the Gap between Single Nanoparticle Imaging and Global Electrochemical Response by Correlative Microscopy Assisted By Machine Vision

Abstract

The nanostructuration of an electrochemical interface dictates its micro‐ and macroscopic behavior. It is generally highly complex and often evolves under operating conditions. Electrochemistry at these nanostructurations can be imaged both operando and/or ex situ at the single nanoobject or nanoparticle (NP) level by diverse optical, electron, and local probe microscopy techniques. However, they only probe a tiny random fraction of interfaces that are by essence highly heterogeneous. Given the above background, correlative multimicroscopy strategy coupled to electrochemistry in a droplet cell provides a unique solution to gain mechanistic insights in electrocatalysis. To do so, a general machine‐vision methodology is depicted enabling the automated local identification of various physical and chemical descriptors of NPs (size, composition, activity) obtained from multiple complementary operando and ex situ microscopy imaging of the electrode. These multifarious microscopically probed descriptors for each and all individual NPs are used to reconstruct the global electrochemical response. Herein the methodology unveils the competing processes involved in the electrocatalysis of hydrogen evolution reaction at nickel based NPs, showing that Ni metal activity is comparable to that of platinum.

The new era of high throughput nanoelectrochemistry

“…425 As also highlighted in SECCM multiscale measurements, sometimes, the optically-inferred information does not match the macroscopic electrochemical behaviors. For example, the cyclic voltammogram for the electrodeposition of a single Ni NP can be inferred from the optical monitoring presented in Figure 15B, 357 but the sum of all the individual voltammograms shows a mismatch in charge or electrochemical current overall. An interpretation is that the reduction of Ni 2+ and formation of Ni is accompanied by the electrocatalysis of water reduction at the Ni NP.…”

“…Several strategies have also been proposed, to convert the optical signature variation quantitatively into an optically-inferred electrochemical curve, such as an opticalvoltammogram. 23,324,[354][355][356][357] Light absorbance, or the extinction wavelength (for plasmonic NPs), are usually proportional to a local concentration or amount of electrochemically transformed material, i.e., an electrochemical charge. 358 The electrochemical current then often tracks the time derivative of local optical transients.…”

“…Large number of objects often need to be analyzed and automatized image analysis routines can be used. 357 These routines, often employing machine learning strategies, are now able to compare/recognize automatically images obtained from different microscopies, in order to find the same features, and classify them according to their activity, shape or structure.…”

“…354,[391][392][393][394] By differentiating the NP volume fluctuation, one can further calculate an 'optical current' related to the growth of single NPs. 354,392 Optical microscopy has been employed for imaging the electrodeposition of more complex materials such as Ni, 357,392 Co, 393 Li, 395,396 Mg, 397 and Zn 339,340 as metal and/or as metal oxides and hydroxides (Ni, 392 Mn 398 or Zn 398 ). These studies have implications in electrocatalysis and battery processes, as well as for studying the electrocrystallization of metal oxides for energy conversion and storage.…”

“…When individual object or domain activity is imaged optically, the sum of the optically-inferred activity of all the individual domains can be compared to the ensemble electrochemical response. This optical-electrochemical correlative strategy was achieved in a number of situations, most recently for the electrodeposition of Ag, 354,390,424 Ni NPs, 357 the electrogeneration of H2 nanobubbles, 361,408 the charging/discharging of MnO2 electrode for Zn/MnO2 batteries, 357 graphene layer oxidation and reduction. 425 As also highlighted in SECCM multiscale measurements, sometimes, the optically-inferred information does not match the macroscopic electrochemical behaviors.…”