Quasi-in situ Observation of MnO2 Nanorods by Electrochemical Transmission Electron Microscopy for Oxygen Reduction Reaction Process

Understanding the electrode materials’ surface is of fundamental importance for catalytic studies as most electrochemical reactions take place there. Although several operando techniques have been used to monitor the electrocatalytic process, real-time imaging techniques for observing the surface change on electrode materials are still a challenge and limited to a few stable catalytic systems. Herein, the quasi-in situ electrochemical transmission electron microscopy (TEM) was carried out to track the morphological and local structure evolution during the oxygen reduction reaction (ORR) on manganese dioxide (MnO₂) for the first time. The α-MnO₂ nanorods exhibit comparable ORR electrocatalytic activity (half-wave potential, E_1/2: 0.83 vs. 0.85 V vs. RHE; diffusion-limiting current density, J_d: −5.46 vs. −5.52 mA cm⁻²) and better methanol tolerance than Pt/C. An electrochemical TEM chip assembled with a three-electrode system was used to perform the electrochemical experiments similar to typical testing procedures. The ex situ and quasi-in situ TEM images consistently showed that MnO₂ nanorods had undergone surface roughening, and lattice expansion with 0.97% and 1.97% in the a and c-axis, respectively as ORR proceeded. The quasi-in situ electrochemical TEM fills the gap between ex situ characterization and operando spectroscopies and deepens the mechanistic understanding of electrocatalytic processes.