**1. Introduction**

Today, modern society is evolving to become more energy dependent. As the awareness of environmental impact from current energy systems is elevating, more efforts recently have been devoted towards mainstreaming renewable energy sources. However, the implementation of renewable energy technologies is challenging, as it requires an efficient energy storage system to mediate the intermittent generation and consumption of energy. The electrochemical splitting of water (i.e., water electrolysis) offers an effective method to produce large amounts of hydrogen (H2), which can be stored and used as an energy vector [1]. Therefore, an efficient oxygen evolution reaction (OER) is essential since it is the key reaction in water electrolysis. During the past decade, the members of the perovskite oxide family (ABO3) have been gaining vast attention for their promising activities as OER electrocatalysts under alkaline conditions, and thereby relieving the need of expensive precious metals such as iridium [2–7]. Generally, perovskite oxides are composed of rare-earth (e.g., lanthanides) or earth alkaline metals (e.g., Ba) in the A-site and 3D transition metals in the B-site (e.g., Ni, Co and/or

Fe). The perovskites intrinsic properties can be tailored through partial cation substitution [2,8,9]. This substitution can transpire in both A- and B-sites of the perovskite (i.e., A'1-*n*A"*n*B'1-*m*B"*m*O3; 0 ≤ *n*, *m* ≤ 1) either in ordered or random arrangement [10]. Likewise, cation ordering plays an important role in engineering the intrinsic properties of a perovskite such as electronic structure, ionic conductivity, and magnetic properties, all of which may change its electrocatalytic behavior [3,10,11]. The recently proposed OER mechanism emphasizes that the formation of oxy(hydroxide) layer at metal oxide surface is essential along the path of lattice oxygen evolution reaction (LOER) [12,13]. In this context, developing perovskite oxides as OER catalyst is advantageous, owing to its ability to exhibit high oxygen vacancy concentration upon cation substitution and ordering it so as to activate the LOER [14,15]. In contrast to the conventional OER mechanism [16,17], in the case where the lattice oxygen is directly involved (i.e., LOER), the high surface OH− coverage from the alkaline media is no longer necessary as loosely bonded lattice oxygen atoms act as the reaction intermediates itself; as a result, the overpotential is lowered [12,18]. In this regard, the use of layered double perovskite oxides (A'2(1-*n*)A"2*n*B'2(1-*m*)B"2*m*O6; 0 ≤ *n*, *m* ≤ 1) is beneficial as they tend to localize the oxygen vacancies into layers through A-site ordering and promote high oxygen mobility [14]. Among the layered double perovskite family, PrBaCo2O6-<sup>δ</sup> (PBC) has been appraised for its high OER activity [19–24]. Nevertheless, past studies point out the instability of PBC under OER conditions [2,13,23,25–27], raising queries regarding its degradation mechanism. In our recent study [2], we highlighted that the degradation is a kinetic process and every catalyst varies in how it reaches the end of its service life depending on the inherent properties.

In a different perspective, a single randomly ordered perovskite oxide, Ba0.5Sr0.5Co0.8Fe0.2O3-<sup>δ</sup> (BSCF), has been identified as another highly active OER catalyst [5,22,27,28]. More recently, we have reported that highly oxygen deficient BSCF prepared via flame spray synthesis would lead to the participation of lattice oxygen atoms (i.e., LOER) coupled with the OER process. Based on operando X-ray absorption spectroscopy (XAS) results [2,24,27] and density-functional theory (DFT) based calculations [2,24], BSCF is capable of facilitating the formation of a self-constructed oxy(hydroxide) surface layer during OER, owing to its thermodynamic nature of meta-stability under the OER condition. Intriguingly, in the means to understand each individual chemical component of BSCF, recent findings highlight the vital role of Fe in its B-site so as to pertain to the thermodynamic meta-stability, and provide charge stability [24]. Likewise, many studies have reported constructive effect of incorporating Fe into 3D transition metal oxide catalysts for OER [29–38].

Therefore, in this study, we incorporate Fe into the B-site of PBC in different ratios to yield PrBaCo2(1-*x*)Fe2*x*Co6-<sup>δ</sup> (*x* = 0.2 and 0.5; denoted as PBCF82 and PBCF55, respectively) for the purpose of tailoring the electrocatalytic performance with respect to OER activity and stability. Nanoparticles of all the materials under study are attained via flame spray synthesis [39]. Operando XAS is used to gain insights into changes in local electronic and geometric structures of the layered double perovskites upon Fe-doping. Combined with the thermodynamic nature inferred from DFT calculations, the roles of Fe in the layered double perovskite as OER catalyst are highlighted. Based on our findings, we underline the synergetic effect that Fe conveys and elucidate the enhanced OER performance of layered double perovskite catalyst upon Fe incorporation.
