Abstract
The ubiquity of nanomaterials in electrochemical energy conversion technologies has driven intense interest in unravelling the material's structure- function relationships. In response, single- entity electrochemistry has been developed as a technique to study the properties of individual nanoparticles, one at a time. This approach engenders a bottom-up understanding, linking the activities of the single particles to the emergent properties of the ensemble.
The "nano-impact" technique, a subset of single-entity electrochemistry, uses low-noise instrumentation to measure current transients arising from individual particles which stochastically collide with an ultramicroelectrode. These transients have been successfully used to measure the size of both insulating and redox-active nanoparticles, as well as the activity of single electrocatalysts. However, it remains challenging to study certain classes of materials on a single-entity basis. One such class is pseudocapacitors – materials which mix electronic and ionic conductivity, critical components of ion batteries and other next-generation energy storage technologies. While careful experimental design and data analysis have allowed the detection and qualitative characterization of single ion-intercalating particles, their nature as mixed conductors makes quantitative information difficult to obtain amperometrically.
This hurdle is due to the measured current response being dictated by one or more possible factors: electron transfer, ion diffusion within the particle or across the particle/electrolyte interface, or ion transfer in the bulk. To overcome this, electrochemical impedance spectroscopy (EIS) is commonly used to characterize bulk pseudocapacitors. In EIS, a small-magnitude sinusoidal (AC) voltage is applied to the working electrode and the current response is recorded. Different timescales can be probed by varying the AC frequency – the high frequency response is dominated by fast processes, such as capacitance, while the low frequency response contains information on electron transfers and mass transport. In this way, EIS gives a full picture of the electroactive material and can unravel the coexisting electronic and ionic conductivity of mixed conductors.
In this presentation, I will discuss our development of an EIS-based technique to detect and characterize single particles. We implemented fast-Fourier transform based EIS to rapidly acquire impedance spectra spanning several decades of frequencies. The spectra are fit to an equivalent circuit model and monitored as a function of time. Discrete changes in various equivalent circuit parameters are observed, corresponding to single particle-electrode impact events. Using a model system of polystyrene microbeads, we demonstrate that discrete increases in the charge-transfer resistance can be used to accurately measure the contact area between the individual bead and the ultramicroelectrode. I will also discuss our recent progress towards detecting single ion-intercalating nanoparticles and characterizing their charge storage ability on a single particle basis. These advances expand both the scope of single-entity electrochemistry and the depth of information gleaned from a single measurement of a single particle.