Anodic H2O2 generation in carbonate-based electrolytes - Mechanistic insight from scanning electrochemical microscopy.

For the anodic H2O2 generation, it has been shown that the electrolyte composition can steer the reaction pathway toward increased H2O2 generation. Previous efforts made on composition optimization found that the impact of the molar fraction of carbonate species varies for different anodes, and therefore, controversies remain concerning the reaction pathways as well as the species involved in H2O2 formation. Considering that water oxidation results in the liberation of protons within the anode microenvironment, the corresponding acidification would cause an equilibrium shift between carbonate species, which in turn may modulate the reaction pathway. We determined the changes in the fraction of carbonate species in the vicinity of an anode by performing local pH measurements using a Au nanoelectrode positioned in close proximity to an operating anode by shear-force scanning electrochemical microscopy (SECM). It could be confirmed that the main anionic species at the interface is HCO3-, at potentials where H2O2 is preferentially formed, regardless of the pH value in the bulk. The simultaneous use of a Au-Pt double barrel microelectrode in generator-collector SECM measurements demon-strates that the local HCO3- concentration is collectively determined by the oxidation current, buffer capacity, and bulk pH of the electrolyte.

Unraveling the Link between Catalytic Activity and Agglomeration State with Scanning Electrochemical Microscopy and Atomic Force Microscopy.

In this article, we set up a methodology to investigate the relationship between the catalytic activity and the agglomeration state of platinum group metal-free ORR catalysts. To this end, we have developed a statistical approach based on scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM). Two catalysts are investigated at very low loadings in order to access their intrinsic activity. Differences in terms of dispersion, stability of the inks, and adherence on the substrate are observed, highlighting the importance of measuring the exact amount and agglomeration state of the materials under study. The agglomeration state of the deposits measured by AFM explains the differences in activity measured by SECM. The performances of the catalysts are compared, and the contributions of the intrinsic activity and the agglomeration state are identified. This work paves the way toward various applications ranging from the benchmarking of new catalysts to the optimization of an ink formulation, for ORR and beyond.