RESUMO
The difference between solution pH and local pH near an electrode surface greatly determines the electrocatalytic performance. However, there is still a lack of a facile and universal method for the local pH detection of various electrode reactions, leaving the origin of local pH changes unclear. Herein, by using phosphate species in phosphate buffer solution (PBS) as the pH probe, we demonstrate a universal local pH detection strategy through in situ Raman spectroscopy for various electrode reactions. Oxygen evolution is chosen as the example to detect the potential-dependent local pH change. Then the strategy extends to nitrate reduction, nitrobenzene reduction, and benzylamine oxidation. By comparing the local pH changes in different reactions, we reveal that the local pH change is strongly dependent on the reaction current, the ability of the system to replenish the local H+/OH-, and the number of H+/OH- per electron transfer of the electrode reaction.
RESUMO
Heteroatom-doped carbon materials have been widely used in many electrocatalytic reduction reactions. Their structure-activity relationships are mainly explored based on the assumption that the doped carbon materials remain stable during electrocatalysis. However, the structural evolution of heteroatom-doped carbon materials is often ignored, and their active origins are still unclear. Herein, taking N-doped graphite flake (N-GP) as the research model, we present the hydrogenation of both N and C atoms and the consequent reconstruction of the carbon skeleton during the hydrogen evolution reaction (HER), accompanied by a remarkable promotion of the HER activity. The N dopants are gradually hydrogenated and almost completely dissolved in the form of ammonia. Theoretical simulations demonstrate that the hydrogenation of the N species leads to the reconstruction of the carbon skeleton from hexagonal to 5,7-topological rings (G5-7) with thermoneutral hydrogen adsorption and easy water dissociation. P-, S-, and Se-doped graphites also show similar removal of doped heteroatoms and the formation of G5-7 rings. Our work unveils the activity origin of heteroatom-doped carbon toward the HER and opens a door to rethinking the structure-performance relationships of carbon-based materials for other electrocatalytic reduction reactions.
RESUMO
Heteroatom-doped carbon materials are widely used as metal-free electrocatalysts and supporting substrates for many metal-based composites. However, almost all the current researches are based on the assumption of the self-stability of the heteroatom-doped carbon materials, neglecting their possible structural evolution during electrocatalysis, especially under harsh oxygen evolution reaction (OER) conditions. Besides, previous researches always focused on the dropcast carbon-based materials with only a few participated dopants, leading to unobservable structural evolution during the electrolysis. Here, heteroatom-doped graphite flakes (GP) with a large quantity of participated dopants are chosen as the research model to multiply the transformation during the electrolysis. Through the combination of theoretical calculations and experiments, we present the nearly complete dissolution of the heteroatoms in N-, P-, and Se-doped carbon materials in the form of the high-valence oxoanions during OER. The oxygen-abundant residues are proven to be responsible for the OER activity. Among the oxygen-containing functional groups in the residues, the ortho-quinone moieties, whose structures change with the doping elements, are finally identified as the active sites. Heteroatom-doped carbon materials as the supporting substrates for the metal-based composite experience a similar transformation, leading to unexpectedly different activity origins. Our work not only reveals the real active sites of the heteroatom-doped carbon materials for OER but also provides new insight into understanding the heteroatom-doped carbon materials as the supporting substrates for other anodic reactions.
