In situ Structural Evolution of Transition Metal-Based Electrocatalysts Probed by Electrochemical Atomic Force Microscopy

Electrocatalysis presents a promising pathway for sustainably converting intermittent renewable energy into value-added chemicals and fuels, while simultaneously mitigating environmental pollutants such as carbon dioxide (CO2) and nitrate (NO3 ⁻ ).Despite its potential, the practical deployment of key electrocatalytic processes, including the CO2 electroreduction reaction (CO2RR), NO3 ⁻ reduction reaction (NO3RR), and oxygen evolution reaction (OER), remains limited by challenges in achieving high selectivity, activity, and long-term stability.These performance metrics are intrinsically linked to the catalyst's surface structure, which evolves under reaction conditions.While catalyst surface reconstruction driven by dynamic processes can facilitate the in situ formation of active sites, excessive or uncontrolled restructuring may compromise catalyst stability.Moreover, such structural transformations can be reversible, complicating post-reaction characterization due to a possible relaxation back to the resting state.To overcome these challenges, in situ and operando surface-sensitive techniques with high spatial resolution are essential for directly visualizing local structural and morphological changes at the solid-liquid interface and decoupling their impact on catalytic performance from bulk properties.However, the intense gas evolution associated with many electrocatalytic reactions imposes significant obstacles to in situ surface characterization.Beyond merely capturing these structural transformations, it is critical to elucidate the fundamental factors governing surface evolution and to establish their correlation with catalytic activity, selectivity, and stability.Such insights are vital for the rational design of efficient and robust electrocatalysts, as well as for developing strategies to actively control catalytic properties under operando conditions.This thesis investigates Co-and Cu-based model electrocatalysts in the form of thin films and single-crystals, which enable precise control over surface properties while minimizing the intrinsic complexity of the catalytic system.By combining these well-defined model systems with advanced in situ and operando characterization techniques, particularly electrochemical atomic force microscopy (EC-AFM), this work elucidates how initial electrode faceting, local chemical environment, and applied potential govern the in situ morphological and structural evolution of electrocatalysts under reaction conditions, spanning mesoscopic to atomic length scales.This includes having a key role on determining how long is needed till the active state is created or a "steady-state" is achieved.Building on these insights into catalyst surface structural evolution, the thesis further explores surface pre-functionalization with organic modifiers as a strategy to regulate in situ restructuring and other critical properties, ultimately enhancing catalytic performance.The findings presented in this thesis challenge conventional interpretations and theoretical models based on static surfaces assumptions and contribute to establishing reliable correlations between catalyst properties, in situ surface structures, and catalytic behavior.EC-AFM is demonstrated as a powerful and versatile technique for advancing mechanistic understanding of transition metal (oxide)-based heterogeneous electrocatalysts.Its broader applicability to the study of functional materials under operational electrocatalytic conditions is also highlighted.