Review on in situ Sum Frequency Generation Vibrational Spectroscopy Studies of Liquid-Solid Interfaces in Electrochemical Systems

The demand for safe, sustainable, and cheap energy production technologies and storage devices has grown at an overwhelming rate since the birth of the commercial lithium ion battery in the 1990s. As our research in electrochemical systems advances, we face fundamental questions arising from the electrified electrode–electrolyte interface that requires optical methods that can characterize the underlying structure and composition of intermediate species under reaction conditions. Sum frequency generation vibrational spectroscopy (SFGVS) emerges as a prominent optical characterization method to probe electrified interfaces owing to its surface-specific, nondetrimental, and sensitivity properties. In this article, we first introduce the basic theory of SFGVS and then present current SFGVS work associated with renewable energy studies on lithium ion batteries and fuel cells. We show in this article that SFGVS is an excellent technique for probing the electrode's surface/interface under reaction condition, when an external potential is applied, owing to its surface specificity with the molecular level sensitivity. Finally, we briefly contextualize the challenges in developing advanced SFG techniques and address future applications.