Improving the charge storage capacity and lifetime and charging/discharging efficiency of battery systems is essential for large-scale applications such as long-term grid storage and long-range automobiles. While there have been substantial improvements over the past decades, further fundamental research would help provide insights into improving the cost effectiveness of such systems. For example, it is critical to understand the redox activities of cathode and anode electrode materials and stability and the formation mechanism and roles of the solid−electrolyte interface (SEI) that forms at the electrode surface upon an external potential bias. The SEI plays a critical role in preventing electrolyte decay while still allowing charges to flow through the system while serving as a charge transfer barrier. While surface analytical techniques such as X-ray photoelectron (XPS), X-ray diffraction (XRD), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and atomic force microscopy (AFM) provide invaluable information on anode chemical composition, crystalline structure, and morphology, they are often performed ex situ, which can induce changes to the SEI layer after it is removed from the electrolyte. While there have been efforts to combine these techniques using pseudo-in situ approaches via vacuum-compatible devices and inert atmosphere chambers connected to glove boxes, there is still a need for true in situ techniques to obtain results with improved accuracy and precision. Scanning electrochemical microscopy (SECM) is an in situ scanning probe technique that can be combined with optical spectroscopy techniques such as Raman and photoluminescence spectroscopy methods to gain insights into the electronic changes of a material as a function of applied bias. This Review will highlight the potential of SECM and recent reports on combining spectroscopic measurements with SECM to gain insights into the SEI layer formation and redox activities of other battery electrode materials. These insights provide invaluable information for improving the performance of charge storage devices.

Our recent study suggests that cobalt modification can greatly improve the photoelectrochemical (PEC) water oxidation activities of n-BiVO4 (ACS Appl. Energy Mater., 1(5), 2283 (2018)). This enhancement is attributed to (1) the formation of a top layer of Co3O4 at BiVO4 surface as a cocatalyst and (2) the enrichment in electronic conductivity of BiVO4 upon Co doping. Here, surface interrogation mode of scanning electrochemical microscopy (SI-SECM) technique equipped with an ultramicroelectrode (UME) tip was employed to in-situ interrogate and quantify the surface adsorbed reactive oxygen species at Co-BiVO4 generated during PEC water oxidation. Experimental conditions of PEC water oxidation including applied potential and illumination time were found to play an important role in determining the surface coverage of the oxygen species. Their spontaneous decay under the open circuit condition was also characterized by increasing the time delay before interrogating the Co-BiVO4 surface.