Environmental TEM Investigation of Electrochemical Stability of Perovskite and Ruddlesden–Popper Type Manganite Oxygen Evolution Catalysts

Abstract

The sluggish kinetics of the oxygen evolution reaction (OER) is a grand challenge for energy storage technologies. Several perovskites and other oxides of earth‐abundant elements are found to exhibit improved catalytic OER activity. However, less attention is paid to the electrochemical stability, an important factor for large‐scale application. The ongoing search for stable catalysts calls for characterizing active catalyst surfaces and identifying mechanisms of deactivation, activation, or repair. In situ techniques are indispensable for these tasks. This study uses environmental transmission electron microscopy on the highly correlated perovskite Pr1–xCaxMnO3 and the Ruddlesden–Popper Pr0.5Ca1.5MnO4 as model electrodes to elucidate the underlying mechanisms of the stability trends identified on rotating ring disk electrodes. An electron beam at fluxes well below those that would cause radiation damage is used to induce positive local electrode potentials due to secondary electron emission, driving electrochemical reactions in H2O vapor. The stability of the model systems increases with increasing ionic character of the MnO bond, while more covalent bonds are prone to corrosion, which is triggered by formation of point defects in the oxygen sublattice.

In Situ Preparation of Pr1-xCaxMnO3 and La1-xSrxMnO3 Catalysts Surface for High-Resolution Environmental Transmission Electron Microscopy

“…At the higher pressure of water vapor (p H2O = 25•10 −8 bar, Cold Trap OFF) the amorphous layer was transformed into randomly oriented PrO 2 particles as shown in Figure 9b (see also Figure S8). Similar structural transformations of initially amorphous areas were reported earlier for the ETEM experiments performed in water vapor at higher pressures of 5•10 −6 and 5•10 −5 bar [18], and this is consistent with the drop of OER activity in electrochemical experiments over time due to Mn leaching into the electrolyte [28]. After the cold trap was filled with liquid nitrogen and as soon as the partial pressure of water vapor dropped about an order of magnitude (p H2O = 25•10 −9 bar), the epitaxial recrystallization was observed (Figure 9c…”

“…The LSMO interlayer was grown in order to adjust the lattice misfit between STO and PCMO. The growth parameters and detailed structural characterization of studied films is published elsewhere [17,18,28].…”

“…In its turn, dedicated gaseous holders [13] allow performing studies at pressures up to 4-5 bars [14], but field of view, spatial resolution, quality of images and use of analytical tools are limited because of silicon nitride membranes especially at low accelerating voltages [8]. Due to these recent technical improvements, ETEM has become a valuable instrument to reveal structural transformations of nanoparticles and catalyst surfaces in reactive environments [15][16][17][18].…”

Undesired Bulk Oxidation of LiMn2O4 Increases Overpotential of Electrocatalytic Water Oxidation in Lithium Hydroxide Electrolytes

“…Based on the previous studies of the e g occupancy, Mn 3+ (e g occupancy of 1) is desirable in an electrocatalyst for the OER. However, a single parameter does not fully describe the catalytic properties of a material as complex as an oxide . Mixed manganese valences between Mn 3+ and Mn 4+ result in the highest activity, i. e. lowest overpotential, in many reports .…”

Self‐Assembled Ruddlesden–Popper/Perovskite Hybrid with Lattice‐Oxygen Activation as a Superior Oxygen Evolution Electrocatalyst

“…[ 18–22 ] Lately, their roles as OER electrocatalysts have received increasing attention since they possesses higher intrinsic activity than their simple perovskite counterparts, but their instability during OER remains a serious challenge. [ 23–27 ]…”