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Hanna Lyle, Suryansh Singh, Michael Paolino, Ilya Vinogradov,  and Tanja Cuk*

Physical Chemistry Chemical Physics, 2021, 33, 24984 Invited Perspective, DOI:  (Featured Inside Front Cover)

The conversion of diffusive forms of energy (electrical and light) into short, compact chemical bonds by catalytic reactions regularly involves moving a carrier (electron or hole) from an environment that favors delocalization to one that favors localization.  While delocalization lowers the energy of the carrier through its kinetic energy, localization creates a polarization around the carrier that traps it in a potential energy minimum.  The trapped carrier and its local distortion—termed a polaron in solids—can play a role as a highly reactive intermediate within energy-storing catalytic reactions but is rarely discussed as such.  Here, we present this perspective of the polaron as a catalytic intermediate through recent in-situ and time-resolved spectroscopic investigations of photo-triggered electrochemical reactions at material surfaces.  The focus is on hole-trapping at metal-oxygen bonds, denoted M-OH^*, in the context of the oxygen evolution reaction (OER) from water.  The potential energy surface for the hole-polaron defines the structural distortions from the periodic lattice and the resulting “active” site of catalysis.  This perspective will highlight how current and future time-resolved, multi-modal probes can use spectroscopic signatures of M-OH^* polarons to obtain kinetic and structural information on the individual reaction steps of OER.  A particular motivation is to provide the background needed for eventually relating this information to relevant catalytic descriptors by free energies.  Finally, the formation of the O-O chemical bond from the consumption of M-OH^*, required to release O₂ and store energy in H₂, will be discussed as the next target for experimental investigations.