Potential Dependent Ion Arrangement Near the Electrode/Electrolyte Interface

The electric double layer (EDL) is the ion arrangement evolving at electrolyte interfaces. Despite its importance in many applications such as supercapacitors, lithium-ion batteries (LIB), water purification, and stabilization of nanoparticles in solution, the structural motifs of the EDL are still under debate as direct structural evidence is rare. Additionally, in LIBs the EDL governs the interfacial speciation and thus the solid electrolyte interphase (SEI) formation which is still poorly understood. Both, the SEI and EDL are crucial in the mass/charge transfer across the interface and hence determine the performance of LIBs. Conclusively, an atomic level elucidation of the electrolyte/electrode interface and its rearrangement dynamics due to potential changes is of utmost importance for understanding the formation of the SEI in LIBs and transport properties across the interface, stabilization mechanisms of nanoparticles in solution and water desalination using electric potentials.<br/> <br/> We present a combined <i>in-situ</i> X-ray reflectivity (XR) and electrical impedance spectroscopy (EIS) study on the single-crystalline boron-doped diamond – electrolyte interface. The metal-like electric conductivity and wide electrochemical window of the single crystalline boron-doped diamond allow us to apply large potentials across the interface. Using XR we obtain density profiles normal to our interface by modeling the potential dependent EDL to reproduce our experimental data.<br/>Our results on CsCl(aq) show a clear trend of the XR profile on the applied potential of -1.2V – 1.2V vs Ag/AgCl. For the highest concentration of 1 molar CsCl(aq), we observe broad Kiessig fringes. Our analysis shows, that for increasing potentials the width of the EDL increases, while the total electron density decreases suggesting an exchange of Cesium-ions with Chlorine-ions at the interface. At the same time, we see a shift in the time constant of the capacitive surface processes. Our efforts allow us to challenge the current theoretical models and simulations and will help to understand the mechanisms involved at the electrode – electrolyte interfaces. Ultimately, better understanding the EDL will pave the way for improved interface engineering resulting in improved device performances and longevity.