Heteroatom doping is widely used in the design of electrocatalysts as it can tune the electronic structure and create more active sites. However, it may simultaneously alter the wetting properties of the catalyst microenvironment, which plays a critical role in gas-involving reactions. Here, we report an interplay between the active sites and the microenvironment in the electrosynthesis of H2O2 via two-electron oxygen reduction on doped carbon. For both oxygen-doped and fluorine-doped carbon, rotating ring-disk electrode (RRDE) measurements indicated a monotonic increase of the intrinsic activity for H2O2 production with the doping level. In contrast, the H2O2 production rate in a gas-diffusion-electrode (GDE) flow cell reached the highest value on a moderately doped carbon catalyst but declined on catalysts with further increased doping. In both cases, the doping created more active sites in carbon but also changed its wetting characteristics. Only a microenvironment with moderate hydrophilicity or hydrophobicity could enable an optimal balance between gaseous O2 and liquid electrolyte in the GDE for high-rate electrosynthesis of H2O2.

Many electrochemical reactions for the development of renewable energy technologies are gas-evolving reactions, where the electrocatalytic performance is susceptible to the wetting properties of the catalyst microenvironment. Here, using N2H4 electro-oxidation to N2 on carbon-supported Pt nanocatalysts as a model reaction, we controlled the microenvironment using oxygen-doped and fluorine-doped carbon supports to make it more hydrophilic and more hydrophobic, respectively, and elucidated the effect on the reaction kinetics. The electrode with oxygen-doped carbon showed a 123% higher activity than that with pristine carbon, benefiting from the increased wetting and exposure of Pt catalytic sites to the electrolyte. Counterintuitively, the electrode with fluorine-doped carbon also exhibited a 46% higher activity than that with pristine carbon, despite its lower wetting of Pt. We found that the hydrophobic microenvironment accelerated the surface diffusion, coalescence, and detachment of the generated N2 gas bubbles, which would otherwise block the Pt active sites from catalyzing the reaction.

The catalytic reduction and subsequent reoxidation of methylene blue (MB) is a well-known reaction, commonly referred to as a "clock reaction". Specifically, the reduction process is accompanied by a color change from intense blue to clear and can be readily monitored by UV−vis spectroscopy via the decrease in absorbance at λ max = 664 nm, providing facile access to valuable reaction kinetics. Unsurprisingly, the reduction of methylene blue has become a widely used probe for heterogeneous catalyst reactivity. Yet, despite its broad utility, the mechanism of reduction is not well-understood. Herein, we report an experimental study on the mechanism of MB reduction using Pt/C nanoparticles. Through a multiparametric study incorporating in situ probing of reaction kinetics, pH, dissolved oxygen, and variable catalyst and reductant loading, the multiple factors impacting the reaction are disentangled. We demonstrate that the reaction steps that limit the reduction of MB are the H 2 evolution and reoxidation sequences. We limit these competitive reactions by optimizing reductant concentration and catalyst loading, N 2 purging, and restriction of O 2 redissolution. This achieves a highly competitive activity parameter of 25,740 min −1 g Pt −1 L.