On the Lattice Oxygen Evolution Mechanism: Avoiding Pitfalls

Abstract

The oxygen evolution reaction (OER) is often designated as the enigma in water electrolysis because the development of active and stable OER catalysts is a challenging and formidable task. While ab initio theory in the density functional theory approximation initially focused on the mechanistic description via the OH, O, and OOH adsorbates, in recent years the lattice oxygen evolution reaction (LOER) mechanism attracted increasing attention, given that the LOER is seen as the main reason for catalyst instability under anodic potential conditions. The present concept article critically analyzes the LOER and indicates pitfalls in the interpretation of this mechanistic pathway. A method to assess the energetics of the LOER in relation to conventional OER mechanisms by the compilation of free‐energy diagrams is introduced, which may contribute to enhance our understanding of the competing LOER and OER on the atomic scale. Further works are urgently needed to comprehend the interrelationship for the evolution of gaseous oxygen from the electrolyte or the crystal lattice.

Method to Determine the Bifunctional Index for the Oxygen Electrocatalysis from Theory

“…This is corroborated by the fact that the *OOH intermediate has been identified spectroscopically [88,89] . Yet, we would like to note that also other mechanistic pathways have been reported in the literature, including bifunctional and binuclear mechanisms [90] or lattice oxygen evolution [91–93] . These mechanistic steps have not been considered in our approach, which can be related to the fact that the above‐specified mechanisms are thermodynamically not restrained by scaling relations.…”

“…[88,89] Yet, we would like to note that also other mechanistic pathways have been reported in the literature, including bifunctional and binuclear mechanisms [90] or lattice oxygen evolution. [91][92][93] These mechanistic steps have not been considered in our approach, which can be related to the fact that the above-specified mechanisms are thermodynamically not restrained by scaling relations. Therefore, from a pure thermodynamic standpoint, these pathways are energetically favored over the mononuclear description, and thus, our approach may be seen as an upper bound analysis to capture the BI by a dedicated evaluation scheme of adsorption free energies referring to the mononuclear description.…”

On the Role of Interfacial Water Dynamics for Electrochemical Stability of RuO2 and IrO2

“…This is because the primary emphasis of research community has been on understanding OER activity, while atomic-scale investigations of OER-driven corrosion have started to emerge only recently. [8][9][10][11][12][13][14][15][16] It is believed that dissolution of metal-oxide electrocatalysts is triggered by OER based on the observations of the coincident onsets of OER and dissolution; however, mechanistic details remain under intense discussion. Among various factors of lattice instability, the so-called lattice oxygen mechanism (LOM) is considered [3,12,14,16,17] and was directly correlated with dissolution of Ir in iridium oxides.…”

“…[8][9][10][11][12][13][14][15][16] It is believed that dissolution of metal-oxide electrocatalysts is triggered by OER based on the observations of the coincident onsets of OER and dissolution; however, mechanistic details remain under intense discussion. Among various factors of lattice instability, the so-called lattice oxygen mechanism (LOM) is considered [3,12,14,16,17] and was directly correlated with dissolution of Ir in iridium oxides. [10,11] In a recent study, a quantitative assessment of the lattice oxygen involvement and the degree of Ir dissolution was performed for rutile IrO 2 .…”

“…[18] Overall, lattice oxygen involvement in OER was observed in a wide range of metal-oxide electrocatalysts, but its actual role in materials stability is not well understood. [16] A related question concerning LOM and its role in catalyst degradation is what happens with the newly formed defect site after O 2 is released from the oxide lattice. It was discussed that the evolved lattice oxygen species can be replenished either through diffusion of oxygen atoms from the bulk to the surface or via dissociation of water molecules producing oxygenated species that refill the vacancy sites (likely a much more facile process).…”