Understanding water-oxidation catalysis at the atomic scale using in situ and ex situ transmission electron microscopy

Abstract

Direct observation of oxygen evolution reaction (OER) on catalyst surface in liquid in real time may significantly advance the current understanding of OER catalysis mechanism, which is fundamental for a variety of green-energy technologies with promising industrial potential such as artificial photosynthesis and hydrogen economy. Current water-splitting technology relies on precious metals to catalyze the sluggish and energy-demanding OER process as it is the bottleneck of overall water splitting. Large-scale water splitting requires non-precious catalysts, among which 3d transition metal oxides (TMOs) are highly attractive for their abundance and low cost. However, the progress of optimizing the catalytic activity of 3d TMOs is currently hindered by limited understanding of the atomic structures at 3d TMO surfaces and how they influence OER. Conventional mechanistic studies do not have sufficient spatial resolution to directly probe catalyst surfaces and unambiguously pinpoint the active reaction sites. Cutting-edge high-resolution microscopy techniques are thus needed to clarify the nature of the active reaction sites and the optimal surface structure, which will further lead the way to better catalysts suitable for industrial water-splitting applications. This project employs different manganese oxides (MnOx) nanocatalysts and, for the first time, realizes real-time nanoscale observation of chemical OER on Mn2O3 nanocatalyst surface, using an in situ liquid holder in a transmission electron microscope (TEM). With the controlled experimental condition, the oxygen evolution process can be directly captured by monitoring the development of oxygen nano-bubbles around the Mn2O3 nanocatalysts. The high spatial and temporal resolution of this in situ approach further enables us to unravel the real-time formation of a surface layer from initial nucleation to progressive extension on the Mn2O3 surface. The surface layers exhibit thickness oscillation, which represents a partially reversible surface restructuring relevant to OER catalysis and reflects the competition between the formation and filling of oxygen deficiency on the catalyst surface. Ex situ atomic-resolution TEM and electron energy-loss spectroscopy on the residual surface layer after OER reveal its amorphous nature with reduced Mn valence and oxygen coordination. Such newly formed amorphous layer which is presumably the active catalytic species develops on other MnOx catalyst surfaces too after chemical or electrochemical OER, including Mn2O3 microparticles and Mn3O4 and Mn5O8 nanoparticles. Besides shedding light on the dynamic OER catalysis as well as the atomic-scale structure characteristics on the MnOx nanocatalyst surface, this project also successfully demonstrates a powerful strategy combining in situ liquid TEM and atomic-resolution ex situ TEM for investigating various chemical reaction mechanisms in liquid. Exclusive information of catalyst surfaces achieved through this strategy, including local atomic structure, composition, and cation oxidation states, will offer not only a unique opportunity to understand OER catalysis at the atomic level, but also a timely guidance for designing and synthesizing more efficient catalysts for large-scale water splitting.