A team led by Prof. YU Shuhong and Prof. HOU Zhonghuai from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences developed a theory-guided microchemical engineering (MCE) approach to manipulate the reaction kinetics and thus optimize the electrocatalytic performance of methanol oxidation reaction (MOR) in 3D ordered and crossed-linked channel (3DOC). The study was published in the Journal of the American Chemical Society.
In micro-nano scale chemical engineering, two primary factors generally affect electrocatalytic kinetics at the electrode-electrolyte interface, i.e., the reaction on the electrode surface and mass transfer from the electrolyte to the near-surface and within the diffusion layer.
The surface reaction can be optimized by designing catalysts to nanoscale and increasing porosity to increase the active sites, as well as by adjusting the electronic structure and binding energy to increase the intrinsic activity of active sites. For macro catalyst-involved electrocatalysis, the mass transfer from the bulk electrolyte to the catalyst surface is fast enough due to the negligible characteristic length of the diffusion layer compared to the catalyst size.
However, as the catalyst downsizes to the nanoscale, the mass transfer deviates greatly from the prediction by traditional theory owing to the comparable diffusion layer length. Therefore, a novel methodology of optimizing the kinetics of given catalysts remains urgent to maximize the electrocatalytic performances.
In this study, the researchers proposed an MCE approach involving catalyst process optimization.
They selected platinum nanotubes (Pt NTs) as the model catalyst, employed air-liquid interface assembly and in-situ electrochemical etching to construct an ideal 3D ordered and crossed-linked channel, and used MOR as the model reaction to test the electrocatalytic performance of 3DOC. The measurement results indicated that there is an optimal channel size of 3DOC for MOR.
Besides, based on the free energy density function of the electrode surface, the researchers established a comprehensive kinetic model coupling the surface reaction and mass transfer to accurately regulate the kinetics and optimize the MOR performance. The results showed that increasing the channel size of 3DOC promoted the mass transfer from the bulk electrolyte onto the catalyst surface, and weakened the vertical electron flow of the reaction in 3DOC.
This competition between the mass transfer and surface reaction led to the best MOR performance on 3DOC with a specific size. Under the optimized channel size, mass transfer and surface reaction in the channeled microreactor were both well regulated.
This structural optimization, different from the traditionally thermodynamic catalyst design, ensures a significant increase in heterogeneous electrocatalytic performance. Using proposed MCE coupling mass transfer and surface reaction, kinetic optimization in electrocatalysis can be realized. This MCE strategy will bring about a leap forward in structured catalyst design and kinetic modulation.
Journal Information: Qing-Xia Chen et al, Microchemical Engineering in a 3D Ordered Channel Enhances Electrocatalysis, Journal of the American Chemical Society (2021). DOI: 10.1021/jacs.1c04653