Heterogeneous electrocatalytic reactions are believed to occur at a minority of coordination sites through a series of elementary reactions that are balanced by minor equilibria. These features mask changes in reaction sites, making it challenging to directly identify and analyze reaction sites or intermediates while studying reaction mechanisms. Systematic perturbations of a reaction system often yield systematic changes in material properties and behavior. Correlations between measurable changes in parameters describing the structure and behavior, therefore, serve as powerful tools for distinguishing active reaction sites. This review explores structure–property correlations that have advanced understanding of behavior and reaction mechanisms in heterogeneous electrocatalysis. It covers correlations that have advanced understanding of the contributions of the local reaction environment to reactivity, of structure and bonding within solid-state materials, of geometric or mechanical strain in bonding environments, and of the impact of structural defects. Such correlations can assist researchers in developing next generation catalysts by establishing catalyst design principles and gaining control over reaction mechanisms.

Hydrogels are smart-swelling 3D structures capable of incorporating/expelling water while maintaining their structures. When combined with electroactive materials, such as conducting polymers, the resulting composite may present tunable properties. Herein, the preparation and characterization of alginate-polypyrrole composite hydrogels is described using chemical polymerization to form polypyrrole inside and around alginate beads, employing two simple protocols. These materials were qualitatively tested as extraction phases, using the solid-phase extraction technique, for the pre-concentration of contaminants of emerging concern (atrazine, caffeine, and progesterone). Compared to alginate alone, the composite materials showed a modified extraction capacity, especially for the extraction of progesterone. It was shown that the alginate matrix also contributes to the extraction, not only acting as a support but also as an active extraction media, evidencing a good combination of materials.