Operando characterization of interfacial charge transfer processes.

Interface science has become a key aspect for fundamental research questions and for the understanding, design and optimization of urgently needed energy and information technologies. As the interface properties change during operation, e.g. under applied electrochemical stimulus, and because multiple bulk and interface processes coexist and compete, detailed operando characterization is needed. In this perspective, I present an overview of the state-of-the art and challenges in selected X-ray spectroscopic techniques, concluding that among others, interface-sensitivity remains a major concern in the available techniques. I propose and discuss a new method to extract interface-information from nominally bulk sensitive techniques, and critically evaluate the selection of X-ray energies for the recently developed meniscus X-ray photoelectron spectroscopy, a promising operando tool to characterize the solid-liquid interface. I expect that these advancements along with further developments in time and spatial resolution will expand our ability to probe the interface electronic and molecular structure with sub-nm depth and complete our understanding of charge transfer processes during operation.