Interplay of Active Sites and Microenvironment in High-Rate Electrosynthesis of H₂O₂ on Doped Carbon

Abstract

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.

Thermoplasmonic In Situ Fabrication of Nanohybrid Electrocatalysts over Gas Diffusion Electrodes for Enhanced H2O2 Electrosynthesis

“…In detail, TiN/NF@Toray-irr is the most selective electrocatalyst for H 2 O 2 production, showing a constant selectivity around 80% between 0.6 and 0.3 V vs RHE. Similar performances were recently observed in RRDE configuration for highly active O-and F-doped carbons creating an engineered reaction microenvironment for H 2 O 2 electrosynthesis . At lower potentials, the selectivity decreased reaching a still considerable 40% at 0.05 V vs RHE.…”

Sensitivity of Gas-Evolving Electrocatalysis to the Catalyst Microenvironment

“…The atomic percentage of oxygen in the pristine carbon black is found to be 4.1%, which increases to 13.1% in the C-O-12 sample. While the pristine carbon black does not contain fluorine, the atom percentage of fluorine in the C-F-800 sample is around 3.1% …”

“…For example, for the electroreduction of CO 2 , the reaction rate is often limited by the mass transport of CO 2 at high overpotentials due to the slow diffusion of CO 2 dissolved in aqueous electrolyte . The CO 2 electrolysis can be improved by creating a hydrophobic microenvironment, which can trap CO 2 gas bubbles near the catalysts and accelerate CO 2 mass transport. , Therefore, for reactions with a gaseous reactant, a moderately hydrophobic microenvironment may be beneficial because it can establish and maintain a balance between liquid electrolyte and gaseous reactant in the catalyst layer, so that the interaction between the three phases (solid catalyst, liquid electrolyte, and gaseous reactant) can be further optimized for efficient electrocatalysis. , …”

“…On the other hand, electrolyzers are being developed for renewably powered electrosynthesis of chemicals and fuels, such as water splitting to generate hydrogen and CO 2 reduction to produce hydrocarbons and oxygenates. , The major challenge for developing such technologies still lies in the rational design of active, selective, and durable electrocatalysts . However, in addition to solid catalysts, the electrocatalytic processes typically involve liquid electrolyte (including molecules and ions dissolved in the electrolyte) and gaseous species, bringing an intrinsic complexity to the local environment around catalytic sites, which may have a significant impact on the distribution and mass transport of reaction species and thus affect the electrocatalytic performance. − …”

“…6,8 Therefore, for reactions with a gaseous reactant, a moderately hydrophobic microenvironment may be beneficial because it can establish and maintain a balance between liquid electrolyte and gaseous reactant in the catalyst layer, so that the interaction between the three phases (solid catalyst, liquid electrolyte, and gaseous reactant) can be further optimized for efficient electrocatalysis. 10,13 In gas-evolving reactions such as hydrogen evolution reaction (HER) and hydrazine (N 2 H 4 ) oxidation to N 2 , the formation and detachment of gas bubbles impact the exposure of catalytic sites to liquid electrolyte, 14−19 and thus affect the reaction kinetics and mass and energy transfer. 20,21 The dynamics of gas bubbles depend on the adhesion force between the bubbles and the electrode surface, which further depends on the wetting properties and morphology of the electrodes.…”