Electrostatics and Structure at Electrode:Electrolyte Interfaces from Nonlinear Optics

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With support from the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program in the Division of Chemistry, Professor Franz Geiger of Northwestern University is using advanced laser methods to study how charged species (ions) and water molecules are arranged in solution near a metal electrode. The textbook model describing this arrangement goes back to the 19th century but describes only the location of the ions and it does not account for their motion. The actual situation is more complicated because the water molecules are arranged differently near the metal surface than they are in the liquid, and the ions move in and out of this interfacial layer near the electrode, called the electrical double layer, or EDL. However, studying the EDL is challenging, as it is only a few nanometers thick, or about 100,000 times thinner than a sheet of paper. Professor Geiger and his students are using laser techniques to directly observe the structure of the water molecules and the ions near the metal surface. Their discoveries could provide fundamental insight that advances technologies ranging from clean energy generation and energy storage to environmental remediation. The project is also contributing to the development of the Nation's science, technology, engineering, and mathematics (STEM) workforce by creating research opportunities for students, and the team is integrating their research into the Evanston high school chemistry curriculum to increase participation in STEM through a science ambassadorship program. By measuring the amplitude and phase of heterodyne-detected second harmonic generation (HD-SHG) signals from electrode:electrolyte interfaces, Professor Geiger and his students can directly probe the structural and electrostatic properties of the Stern and diffuse layers . Using this technique, the team is studying the dynamic exchange of ions between these two components of the electrical double layer that occurs in response to external stimuli (applied potential, pH, electrolyte concentration) and measuring the total interfacial potential drop across the EDL. The HD-SHG methodology is also being implemented in a wide-field microscopy mode capable of imaging spatial variations in the EDL across an electrode surface during electrochemical reactions. By quantifying the Coulombic and non-Coulombic contributions to the interfacial potential drop, and by learning about the structure of interfacial water molecules and ions, they are aiming to improve century-old models for describing electrode processes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.