Activating lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution

Abstract

Understanding how materials that catalyse the oxygen evolution reaction (OER) function is essential for the development of efficient energy-storage technologies. The traditional understanding of the OER mechanism on metal oxides involves four concerted proton-electron transfer steps on metal-ion centres at their surface and product oxygen molecules derived from water. Here, using in situ O isotope labelling mass spectrometry, we provide direct experimental evidence that the O generated during the OER on some highly active oxides can come from lattice oxygen. The oxides capable of lattice-oxygen oxidation also exhibit pH-dependent OER activity on the reversible hydrogen electrode scale, indicating non-concerted proton-electron transfers in the OER mechanism. Based on our experimental data and density functional theory calculations, we discuss mechanisms that are fundamentally different from the conventional scheme and show that increasing the covalency of metal-oxygen bonds is critical to trigger lattice-oxygen oxidation and enable non-concerted proton-electron transfers during OER.

Factors Controlling the Redox Activity of Oxygen in Perovskites: From Theory to Application for Catalytic Reactions

“…Although computational works [5,7] have proposed a mechanism mediated by the formation of surface oxygen vacancies, limited experimental evidence has been provided so far due to the lack of proper techniques to capture the transient nature of surface intermediates. Recently, combining oxygen isotopic labelling and in situ mass spectrometry, the demonstration was made that using negative charge transfer materials such as SrCoO 3 leads to the participation of the lattice oxygen in the OER reaction ( Figure 6) [6]. Similar lattice oxygen involvement was also proposed for the same materials by Stevenson et al [5].…”

“…As illustrated in Figure 3 and discussed previously elsewhere [6,23,28,63], accessing the redox of oxygen in perovskites materials will be possible at low energy corresponding to a potential at which H 2 O is usually oxidized to form gaseous oxygen. Therefore, the reactions of interest for which oxygen can be used as an active site are limited.…”

“…Hence, using surface oxygen as active site eventually induces the formation of surface oxygen vacancies, which can be compensated for by the formation of cationic defects and the dissolution of surface cations if the rate of oxygen insertion from the aqueous electrolyte is not fast enough. Moreover, the formation of surface oxygen vacancies can also be counterbalanced by the migration of lattice oxygen to the surface, as visualized by high resolution transmission electron microscopy (HRTEM) and also spotted by online electrochemical mass spectroscopy (OLEMS) (Figure 6a-e) [6,23]. Finally, other perovskites containing Ir 5+ were recently proposed as highly active OER catalysts in acidic conditions without further mention being made of their surface stability [84].…”

“…The second mechanism invoked the formation of an O-O bond by the reaction of OH − from the electrolyte with deprotonated oxygen atoms. Interestingly, when performing in situ mass spectrometry on 18 O-labeled perovskites, two type of gaseous oxygen were found: 34 O 2 coming from the reactivity of one oxygen from the electrolyte with lattice oxygen and 36 O 2 arising from the combination of two lattice oxygens [6]. Therefore, it is critical to further understand the electronic states respectively involved in these two types of O-O bond formation, denoted respectively acid-base and direct coupling reactions by the homogeneous catalysis community [81,82].…”

“…Due to the unique flexibility of this family in accepting a large variety of cations in A-or M-sites, the electronic conductivity of perovskites can be tuned from insulating to metallic, with numerous magnetic properties being reported [3]. In addition to the traditional physical properties of perovskites, the importance of ligand hole formation, studied in depth during the supraconducting era following the discovery of high-T c materials in 1986 [4], has recently being revisited since it can offer a unique opportunity to modify the catalytically active redox center on the surface of perovskites for the low temperature oxygen evolution reaction (OER), one half reaction of water splitting [5][6][7]. However, triggering and controlling the redox activity of oxygen on the surface of perovskite materials has been shown to be challenging, therefore requiring a deeper understanding of this phenomenon in order to develop new redox active compounds.…”