Environmental TEM Study of Electron Beam Induced Electrochemistry of Pr_0.64Ca_0.36MnO₃ Catalysts for Oxygen Evolution

Abstract

Environmental Transmission Electron Microscopy (ETEM) studies offer a great potential for gathering atomic scale information on the electronic state of electrodes in contact with reactants but also pose big challenges due to the impact of the high energy electron beam. In this article, we present an ETEM study of a Pr 0.64 Ca 0.36 MnO 3 (PCMO) thin film electro-catalyst for water splitting and oxygen evolution in contact with water vapor. We show by means of off-axis electron holography and electrostatic modelling that the electron beam gives rise to a positive electric sample potential due to secondary electron emission. The value of the electric potential depends on the primary electron flux, sample-conductivity and grounding, and gas properties. We present evidence that two observed electro-chemical reactions are driven by the beam induced electrostatic potential of the order of a volt. The first reaction is an anodic electrochemical oxidation reaction of oxygen depleted amorphous PCMO which results in a recrystallization of the perovskite structure. The second reaction is oxygen evolution which can be detected by the oxidation of a silane additive and formation of SiO 2-x at catalytically active surfaces. Recently published in-situ XANES observation of subsurface oxygen vacancy formation during oxygen evolution at a positive potential [ 32 ] is confirmed in this work. The quantification of beam induced potentials is an important step for future controlled electro-chemistry experiments in an ETEM. 1. Introduction: Electro-catalysts are of high importance in speeding up electro-chemical reactions via the control of reaction steps and activation barriers [ 1 ]. Typically, electro-catalysts are used in the form of electrodes that enable the transfer of electrical charges into absorbed reactants. An ideal catalyst is not involved in the catalysed electro-chemical reaction. However, high catalytic performance typically requires materials with a sufficiently flexible atomic and electronic surface structure in order to affect the reaction processes [ 2 , 3 ]. In their active state, catalysts often undergo significant changes in surface and defect structure. A recent example even shows that a water splitting electro-catalyst can be formed during its activity [ 4 ]. Whether the catalyst forms desired highly active states, undergoes undesired changes which reduce its activity or even shows irreversible corrosion can sensibly depend on the specific ambient conditions. In-situ atomic scale studies of electro-catalysts in working conditions can contribute substantially to provide insights into the underlying mechanism [ 5 , 6 ].

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

“…In previous ETEM and in situ XANES experiments in H 2 O vapor under positive bias, surface reduction of P-PCMO at intermediate and high Ca doping was observed despite nominally oxidizing conditions due to formation of V O acting as electron donors. [26,34,41] Phase decomposition of P-PCMO into AO and MnO rich phases above a critical V O concentration has also been observed for vacuum annealed thin films. [52] By contrast, the increased ionic character of the MnO bond in Pr-rich P91-PCMO suppresses significant OER involvement of lattice O by preventing the formation of O 1− species and V O .…”

“…[26,34,41] This was concluded from the experimentally observed Mn reduction at the surface under strongly oxidizing conditions, which can be explained by the formation of V O and surface protonation acting as electron donors to Mn. This scenario was proven by the observation of vacancy ordering, showing that redox activity of lattice oxygen can open up a corrosion channel.…”

“…However, ETEM observations cannot be directly compared to ex situ electrochemical stability analysis due to differences in composition and conductivity of the electrolyte, pH value, pressure, and beam-induced effects. [34,41,48] Thus, distinguishing between electrochemical and beam damage effects is an important challenge of ETEM studies. Inelastic scattering of beam electrons has two main effects: First, the emission of secondary electrons creates a positive space charge within the material, which can drive electrochemical surface reactions.…”

“…The investigated TEM specimens can therefore be estimated to have a ground resistance of roughly 500 MΩ. The surface potential under illumination by an electron beam of 4 nA is evaluated to a value of ≈0.7 ± 0.1 V by using equation (7) in Mildner et al [41] The relatively large error in this order of magnitude estimation stems from uncertainties in estimated electric resistance of the porous thin lamella areas as well as from the deviation of the studied samples from planar lamella geometry underlying equation (7). Nevertheless, the estimated potential is electrochemically relevant for either driving material dependent electrode redox processes or even the OER.…”

“…This scenario was proven by the observation of vacancy ordering, showing that redox activity of lattice oxygen can open up a corrosion channel. [41] V O formation during OER is facilitated by increasing Ca concentration and was interpreted by an increasing contribution of O 2p holes at the upper valence band edge based on band structure calculations. A comparative study of the perovskite (P) manganite Pr 1−x Ca x MnO 3 and the related Ruddlesden-Popper (RP) system Pr 1−x Ca 1+x MnO 4 at equal doping levels of x Ca = 0.5, 0.75, and 1 shows much higher stability of the RP structure, which was interpreted by a reduced O 2p character of the acceptor states, i.e., lower covalence factor, measured by X-ray spectroscopy at the O K-edge.…”