Abstract
Electrocatalytic reactions such as oxygen evolution (OER) and oxygen reduction reactions (ORR) are one of the most complex heterogeneous charge transfer processes because of the involvement of multiple proton-coupled-electron transfer steps over a narrow potential range and the formation/breaking of oxygen-oxygen bonds. Obtaining a clear mechanistic picture of these reactions on some highly active strongly-correlated oxides such as MnOx, NiOx, and IrOx has been challenging due to the inherent limitations of the common spectroscopic tools used for probing the reactive intermediates and active sites. This perspective article briefly summarizes some of the key challenges encountered in such probes and describes some of unique advantages of confocal near-infrared photoluminescence (NIR-PL) technique for probing surface and bulk metal cation states under in-situ and ex-situ electrochemical polarization studies. Use of this technique opens up a new avenue for studying changes in the electronic structure of metal oxides occurring as a result of perturbation of defect equilibria, which is crucial in a broad range of heterogeneous systems such as catalysis, photocatalysis, mineral redox chemistry, and batteries.
Highlights
Electrocatalytic oxygen evolution and oxygen reduction reactions underpin the efficient operation of many reversible electrochemical energy conversion and storage devices such as solar cells, regenerative fuel cells (Adler, 2004; Gasteiger et al, 2005), and rechargeable metal-air batteries (Cao et al, 2012) that have theoretical energy densities (10–12 kWh/kg) on par with the energy density of gasoline (∼13 kWh/kg) (Yang et al, 2011)
Mechanism of OER/oxygen reduction reactions (ORR)-Nature of Reaction Intermediates. It has been well-established that OER and ORR on metal oxides proceed via formation of a series of reaction intermediates that involves the binding of the electrolyte ions and H2O molecules to the metal and oxygen (O−2 ) lattice sites that serve as the catalytic active center for the reaction
When the intermediates involve the sequential formation of ∗OH, ∗O, ∗OOH, and ∗OO (Figure 1A) at the metal cation site (∗, active site), which undergoes an increase in the oxidation state, the mechanism is commonly known as the adsorbate evolution mechanism (AEM)
Summary
Electrocatalytic oxygen evolution and oxygen reduction reactions underpin the efficient operation of many reversible electrochemical energy conversion and storage devices such as solar cells, regenerative fuel cells (Adler, 2004; Gasteiger et al, 2005), and rechargeable metal-air batteries (Cao et al, 2012) that have theoretical energy densities (10–12 kWh/kg) on par with the energy density of gasoline (∼13 kWh/kg) (Yang et al, 2011).
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