Young Investigator Group Nanoscale Solid-Liquid Interfaces

Research Topics

We are interested in fundamental processes of solid-liquid interfaces which are specifically enhanced for nanomaterials. This involves understanding the nature of surface chemistry and of interfacial (or confined) water and other liquids as well as their respective role on charge transfer processes across the interface. We are targeting a better understanding of interfacial processes for applications in electrochemical energy storage and photo/electrochemical energy conversion as detailed below:

Surface chemistry

Due to their high surface-to-volume ratio, nanomaterials properties are often governed by their surface chemistry. We are particularly interested in understanding the following questions related to surface chemistries for the full range of nanomaterials that we are investigating:

• its control using chemical and physical surface treatments,

• its characterization under realistic (liquid/gaseous) conditions using spectroscopy techniques (FTIR, Raman, XAS),

• its local distribution determined by nanoscale chemical imaging (STXM),

• its role on liquid restructuring at the interface,

• its role on photo/electrochemical charge transfer.

   

Interfacial and confined water

The aqueous environment close to nanomaterials may strongly differ from bulk water due to local rearrangement of water molecules. This may affect the solvation of nanoparticles but can also enhance the chemical reactivity of water molecules or other adsorbed molecules at the interface. This latter point is particularly relevant in the context of photo/electrochemical energy conversion.

In layered nanomaterials such as MXenes, the strong confinement of the electrolyte between the layers also dramatically affects its hydrogen bonding, which impacts cation/proton solvation and diffusion. We aim at understanding the role of electrolyte confinement on pseudocapacitive electrochemical energy storage processes in such nanomaterials.

We therefore intend to characterize the water hydrogen bonding environment at the interface with nanomaterials. For this purpose, we mostly use XAS at the O K-edge and FTIR spectroscopies which are particularly sensitive to water hydrogen bonding structure. This work will be extended to other liquids such as organic electrolytes and ionic liquids in the near future.

Pseudocapacitive electrochemical energy storage

Pseudocapacitive materials, such as MXenes, store energy using reversible surface electrochemical reactions. They may provide solutions to achieve simultaneously high power and energy densities as they combine some benefits from capacitors (e.g. fast ion diffusion) and batteries (e.g. faradaic charge transfer). Nevertheless, pseudocapacitive energy storage mechanism remains generally poorly understood. MXenes are particularly interesting pseudocapacitive materials because their core and surface chemistry can be finely tuned as well as their interlayer spacing.

We are applying in situ/operando spectroscopy characterization of MXene during electrochemical cycling to investigate:

• Redox and faradaic reactions on MXene in different electrolytes,

• Proton/cation solvation in MXene interlayer space,

• Dynamics of proton/cation diffusion in MXene films,

• Local electrochemical processes over single MXene flakes.

To this aim, we use the high sensitivity of soft X-rays spectroscopies (XPS, XAS) to the surface functional groups and transition metal chemical environment. Local spectroscopy at the single MXene flake level is possible using X-ray microspectroscopy (X-PEEM and STXM). Raman spectroscopy brings further information on the carbide core and surface groups while FTIR spectroscopy is particularly sensitive to intercalated species.

Photo/electrochemical energy conversion

Solid-liquid interface has a predominant role in electrochemical reactions relevant for photo/electrochemical energy conversion, such as hydrogen evolution reaction (HER), oxygen evolution/reduction reaction (OER/ORR) and carbon dioxide/nitrogen reduction reactions (CO₂RR/NRR). Several carbon-based nanomaterials have been proposed as low-cost and metal-free catalysts for these reactions. Collaborating with material scientists and electrochemists experts in PEC energy conversion, we apply in situ/operando spectroscopy to:

• identify possible active sites and reaction intermediates,

• probe the role of aqueous microenvironment on the reaction path,

• unravel electronic processes induced by application of an external potential or light illumination.