"In Situ Spectroscopy and Nanoscale Imaging of Electrochemical Energy Conversion and Storage Systems" 
Associate Professor Justin Sambur - Department of Chemistry, Colorado State University
Host: Bo Zhang

Prof. Sambur's research team is developing in situ microscopy and spectroscopy techniques that advance critical knowledge in basic energy science. They tackle fundamental scientific problems in two vital research areas, solar energy conversion and electrochemical energy storage. The first part of Prof. Sambur's talk will focus on solar energy conversion. The fundamental problem that limits the solar energy conversion efficiency of conventional semiconductors such as Si is that all absorbed photon energy above the band gap is lost as heat. The critical question that the team's research addresses is: Can we avoid energy losses in semiconductors? Hot-carrier systems that avoid such losses have tremendous potential in photovoltaics and solar fuels production, with theoretical efficiencies of 66% (well above the detailed-balance limit of 33%). Ultrathin 2D semiconductors such as monolayer (ML) MoS2 and WSe2 have unique physical and photophysical properties that could make hot-carrier energy conversion possible. The specific knowledge gap in the field is how the energy levels of 2D semiconductors move with applied potential and/or illumination, making the driving force for charge transfer (DG0´) unclear. Since DG0´ governs the hot-carrier extraction rate (kET), understanding how and why DG0´ changes under solar fuel generation conditions is critical to controlling kET relative to the cooling rate. Prof. Sambur's research team has employed photocurrent spectroscopy, steady-state absorption spectroscopy, and in situ femtosecond transient absorption spectroscopy as a function of applied potential to characterize underlying steps in a ML MoS2 photoelectrochemical cell. The rich data set informs them on the timescales for hot-carrier generation/cooling and exciton formation/recombination, as well as the magnitudes of changes in exciton energy levels, exciton binding energies, and the electronic band gap. These findings open the possibility of tuning the hot-carrier extraction rate relative to the cooling rate to ultimately utilize hot-carriers for solar energy conversion applications. The second part of Prof. Sambur's talk will focus on elucidating charge storage mechanisms in nanoscale materials, which underlies the performance of electrochemical technologies such as batteries and smart windows. He will discuss the team's high-throughput electro-optical imaging method that measures the battery-like and capacitive-like (i.e., pseudocapacitive) charge storage contributions in single metal oxide nanoparticles. Prof. Sambur will present the team's single particle-level measurements that show (1) individual particles exhibit different charge storage mechanisms at the same applied potential and (2) particle size-dependent pseudocapacitive charge storage properties.