Electrochemical solid-liquid interfaces (SLI) are extremely complex and dynamic. SLI plays a significant role in dictating both charge transfer dynamics and selectivity of reaction pathways at any given electrochemical interface. Probing the structure and arrangement of interfacial ion and solvent molecules in-situ with nanoscale resolution is crucial to understand the SLI and develop efficient electrocatalysts.

Electrochemical atomic force microscopy (EC-AFM) technique offers in-situ/operando correlative measurements that can probe SLI with nanometer spatial resolution, providing access to nanoscale heterogeneities. In this contribution, I will highlight the use of EC-AFM as an interfacial force sensor to map the interfacial adhesion forces in addition to the local topography of the electrode under study. As the electrode surface charge is tuned by external applied bias, the local SLI structure is modulated, which leads to quantifiable changes in the adhesion forces mapped by the EC-AFM. Adhesion forces extracted from the AFM force curves (retract) provides direct insights on the interfacial energy associated with the molecular arrangement of the SLI.

I will discuss the presence of adhesion force inhomogeneities on the SLI when a (111) textured polycrystalline gold electrode is immersed in 10mM of sodium sulphate solution. We observe the presence of potential-induced hysteresis of adhesion forces as the potentials are cycled from low to high values and back. Additionally, increasing the ionic concentration of the electrolyte, reduces the adhesion hysteresis. Under applied anodic potentials the sulphate anions in the electrolyte adsorb on gold electrode, which leads to a substantial increase in the interfacial adhesion forces. A force minimum is observed before the onset of sulphate adsorption, which we attribute to the point of zero charge as the surface charge switches from negative to positive.

We also identify a negative correlation of electrode grain curvature and its mean adhesion force, highlighting the role of solid electrode asperities in influencing the interfacial energy of the SLI. The grain curvature also dictates the potential-dependent adhesion response of any individual grain at applied potentials; the flatter grain the higher is their adhesion response. Furthermore, we observe adhesion force inhomogeneities at the single grain level, characterized by two distinct surface states that exhibit similar potential-dependent responses.

Our work demonstrates the use of EC-AFM as a multimodal technique that can probe SLI arrangement in-operando with nanoscale resolution. This work opens the door to future work probing of the solid-liquid interface in the presence of different types of (non-)specifically adsorbing anions and cations, as well as studying the dynamics of EDL formation via measuring adhesion forces at applied potential pulses.