We present an electrochemical impedance spectroscopy (EIS) technique that can detect and characterize single particles as they collide with an electrode in solution. This extension of single-particle electrochemistry offers more information than typical amperometric single-entity measurements, as EIS can isolate concurrent capacitive, resistive, and diffusional processes on the basis of their time scales. Using a simple model system, we show that time-resolved EIS can detect individual polystyrene particles that stochastically collide with an electrode. Discrete changes are observed in various equivalent circuit elements, corresponding to the physical properties of the single particles. The advantages of EIS are leveraged to separate kinetic and diffusional processes, enabling enhanced precision in measurements of the size of the particles. In a broader context, the frequency analysis and single-object resolution afforded by this technique can provide valuable insights into single pseudocapacitive microparticles, electrocatalysts, and other energy-relevant materials.

The interaction between catalyst and support is well known to influence the reactivity and stability of heterogeneous catalysts, and electrochemical hydrogen evolution catalysts based on amorphous or nanocluster MoS x have shown enhanced reactivity when supported on Au disk electrodes. However, it has been synthetically challenging to create strong interactions between the MoS x catalyst layer and the metallic support material while maintaining high surface area and solution dispersibility for the composite catalyst. In this work, we utilize colloidal ligand-exchange methods to adsorb a single layer of tetrathiometallate complex (MoS4 2–, WS4 2–) onto colloidal Au nanoparticles and characterize the influence of the Au support on the electronic and geometric properties of the surface MS x monolayer. Utilizing spectroscopic and computational methods, we show that the Au surface templates cross-linked oligomers of MoS x to generate highly active bridging disulfide moieties and tunes the hydrogen atom binding energies through strong covalent Au–S interactions. These Au@MoS4 nanoparticles are easily incorporated into high surface area electrodes and are able to achieve 100 mA/cm2 of hydrogen evolution current density at 171 mV of overpotential.