Nanoscale electrochemical kinetics & dynamics: the challenges and opportunities of single-entity measurements

Abstract

The development of nanoscale electrochemistry since the mid-1980s has been predominately coupled with steady-state voltammetric (i-E) methods. This research has been driven by the desire to understand the mechanisms of very fast electrochemical reactions, by electroanalytical measurements in small volumes and unusual media, including in vivo measurements, and by research on correlating electrocatalytic activity, e.g., O2 reduction reaction, with nanoparticle size and structure. Exploration of the behavior of nanoelectrochemical structures (nanoelectrodes, nanoparticles, nanogap cells, etc.) of a characteristic dimension λ using steady-state i-E methods generally relies on the well-known relationship, λ2 ∼ Dt, which relates diffusional lengths to time, t, through the coefficient, D. Decreasing λ, by performing measurements at a nanometric length scales, results in a decrease in the effective timescale of the measurement, and provides a direct means to probe the kinetics of steps associated with very rapid electrochemical reactions. For instance, steady-state voltammetry using a nanogap twin-electrode cell of characteristic width, λ ∼ 10 nm, allows investigations of events occurring at timescales on the order of ∼100 ns. Among many other advantages, decreasing λ also increases spatial resolution in electrochemical imaging, e.g., in scanning electrochemical microscopy, and allows probing of the electric double layer. This Introductory Lecture traces the evolution and driving forces behind the "λ2 ∼ Dt" steady-state approach to nanoscale electrochemistry, beginning in the late 1950s with the introduction of the rotating ring-disk electrode and twin-electrode thin-layer cells, and evolving to current-day investigations using nanoelectrodes, scanning nanocells for imaging, nanopores, and nanoparticles. The recent focus on so-called "single-entity" electrochemistry, in which individual and very short redox events are probed, is a significant departure from the steady-state approach, but provides new opportunities to probe reaction dynamics. The stochastic nature of very fast single-entity events challenges current electrochemical methods and modern electronics, as illustrated using recent experiments from the authors' laboratory.

Oxygen Evolution Electrocatalysis of a Single MOF‐Derived Composite Nanoparticle on the Tip of a Nanoelectrode

“…[4] They also allow monitoring kinetics of very fast electrochemical events in small volumes and unusual electrolytes. [5] Theenhanced mass transport rate makes it possible to circumvent ensemble and film effects, which are usually encountered when electrode reactions are analyzed macroscopically.…”

Bioelectrical understanding and engineering of cell biology

“…The use and combination of such techniques have already resulted in the development of specific and powerful bio-electrochemical tools. Multi-electrode arrays allow the measurement of electrochemical events occurring at time scales of the order of microseconds [55] and at the tissue level [56], while scanning ion conductance (SICM) and scanning electrochemical microscopy (SECM) allow mapping of ionic conductivity and redox reactions, respectively, at nano-scale to single-cell levels [57]. Electrochemical approaches can also be used to modify the chemical composition of the cell microenvironment in a selective and controlled manner by producing or delivering reactant species that will trigger specific processes [58,59] or by exposing cells to electrical fields and pulses [60][61][62].…”

Theoretical modeling of electrochemical nucleation and growth of a single metal nanocluster on a nanoelectrode

“…The time dependence of the radius of a hemispherical nanocluster, which formed at time t 0 and grows under constant overpotential (growth is controlled by diffusion), may be easily obtained from eqn (9) and 13:…”

“…This allows us to study nanoobjects from one atom to several nanolayers. [5][6][7][8][9][10] Different approaches are used now to model the electrochemical nucleation and growth processes. New data may be obtained both by the computational methods (methods of quantum chemistry, molecular dynamics, Monte Carlo, etc.)…”