**2. Results and Discussion**

#### *2.1. Physcial Characterization*

All perovskite catalysts—PrBaCo2(1-x)Fe2xCo6-<sup>δ</sup> (x = 0, 0.2, and 0.5; denoted as PBC, PBCF82, and PBCF55, respectively)—are prepared via flame spray synthesis from which nano-scaled particles are obtained (Figure 1). Metal precursors are dissolved in combustible solution and injected into the flame at high temperature (possible up to ≈3000 ◦C) and the resulting precipitates are collected. Previously, we have established the benefits of this particular synthesis of perovskites for the electrocatalytic performance [2,27]. The physical and structural traits of such nanoparticles are observed using

transmission electron microscopy (TEM) (Figure 1). Figure 1a–c show the TEM images of the prepared nanoparticles of PBC, PBCF82, and PBCF55, respectively. All of the prepared nanoparticles are in sizes that range from 5–30 nm. Each inset of Figure 1a–c shows high-resolution (HR) TEM images of PBC, PBCF82, and PBCF55, respectively, and each reveals clear fringes, indicating the formation of crystalline structures.

**Figure 1.** Transmission electron microscopy (TEM) images of (**a**) PBC; (**b**) PBCF82; and (**c**) PBCF55. Each inset shows high-resolution TEM images revealing fringes.

Figure 2 shows the comparison of X-ray diffraction (XRD) patterns of the prepared layered double perovskite catalysts. The comparison of the overall XRD patterns (Figure 2a) of PBC, PBCF82, and PBCF55 reveal peaks that are well indexed to those characteristics of PrBaCo2O6-<sup>δ</sup> (P4/mmm) (JCPDS 00-053-0131) with a minor amount of side oxide phases. The broad XRD peaks render convolution of nearby peaks, and confirms the presence of nanoparticles as observed in TEM images. Apart from being in the same crystalline structure, the XRD peaks of PBCF82 and PBCF55 appear at a lower 2-theta value (Figure 2b) than PBC, which highlights that the perovskite structure exhibits larger lattice parameters upon the incorporation of Fe. The peaks of PBCF55—the one with the most Fe composition—appears at the lowest 2-theta value, suggesting that it exhibits the largest lattice parameters among them. Considering the nature of ordered perovskites, and which cations are oriented in layers, the partial substitution of Co with Fe may lead to B-site cation octahedral tilting, which is difficult to separate from cation ordering [8]. Overall, the comparison of XRD patterns confirms that the layered double perovskite structure is withheld upon Fe incorporation while lattices may expand.

**Figure 2.** (**a**) comparison of X-ray diffraction (XRD) patterns of PBC (black), PBCF82 (red), and PBCF55 (blue). The XRD patterns are well indexed to those of PrBaCo2O6-<sup>δ</sup> from literature (JCPDS 00-053-0131); (**b**) magnified view of the main XRD peak.

#### *2.2. Electrochemical Study*

Systematic electrochemical characterizations are conducted in order to assess the functional role of Fe in activity and stability of the layered double perovskite catalysts during the OER process. Figure 3a shows constructed Tafel plots from measured steady-state currents from series of chronoamperometry studies of the prepared catalysts. In the presence of Fe, PBCF82 and PBCF55 revealed different Tafel slopes as compared to the non-doped PBC. PBCF82 and PBCF55 both showed similar Tafel slopes (~50 mV dec<sup>−</sup>1) that are lower than that of the non-doped PBC (72 mV dec−1) (summarized in Table 1; CVs and steady-state current recorded during the series of chronoamperometry measurement is shown in Figure S1). Our recent study investigated the functional role of Fe in single randomly ordered Co-based perovskites, Ba0.5Sr0.5CoO3-<sup>δ</sup> and La0.2Sr0.8CoO3-δ, where remarkable enhancements in their OER activities were reported when doped with ~5 wt.% of Fe [24]. However, in this previous study, comparable Tafel slopes were observed for both non-doped and Fe-doped perovskites [24]. In contrast, here, the observed lower Tafel slope indicates that the catalyst would follow a different OER mechanism upon Fe incorporation, which may attribute to certain degree of changes of intrinsic physicochemical properties. PBCF82 and PBCF55 reveal higher current densities at 1.55 VRHE (17.1 and 19.7 A g−1, respectively) than compared to the non-doped PBC (13.8 A g−1). Likewise, lower Tafel slopes have been observed for Co based catalysts with increasing Fe composition in other studies [29,31,40]. All of the prepared layer perovskites revealed similar Brunauer–Emmett–Teller (BET) surface areas (Table S1), which shows that their differences in electrochemical activities are independent of their differences in surface area.

**Figure 3.** Electrochemical study comparing (**a**) Tafel plot of oxygen evolution reaction activities and (**b**) change in current densities with respect to the initial current density at every 25 cycles over 500 cycles between 1.0 and 1.6 VRHE of PBC (black), PBCF82 (red), and PBCF55 (blue).

As previous studies emphasize the importance of understanding the stabilities of electrode materials [2,41–43], the potential stability test was conducted during which steady-state currents are recorded as potential is stepped from 1.0 VRHE to 1.6 VRHE holding for 10 seconds at each potential for 500 cycles (Figure 3b). Here, the Fe-doping also showed a beneficial effect in functional stability where PBCF55 revealed the least amount of current density loss over the period of cycles (lost 32% of its initial current density), while PBC lost about 74%. PBCF82, which comprises less than half of Fe that is in PBCF55, demonstrated capability to retain about half of its initial current density at the end of the cycle (see Table 1). In this comparison, the most outstanding activity and stability are demonstrated by PBCF55 containing one-to-one ratio between Co:Fe in the B-site.

apparent mass specific exchange current density (j0), Tafel slope (b), activity expressed as mass specific current density at 1.54 V, and stability expressed as percent of initial mass-specific current density after the 500 cycles of stepping between 1.0 VRHE and 1.6 VRHE.

**Table 1.** Summary of electrochemical study results on oxygen evolution reaction activity and stability:


The functional stability of perovskite catalyst is described based on the ability to sustain its initial electrocatalytic activity. In relation to the structural integrity, the catalyst should demonstrate capabilities to serve as a suitable substrate for the prospect of surface oxy(hydroxide) layer formation [24]. In this regard, Pourbaix diagrams are constructed based on density-functional theory (DFT) calculations to assess the thermodynamically stable phases of PBC and PBCF in aqueous solution (Figure S2a,b, respectively). The Pourbaix diagrams of PBC and PBCF suggest the dissolution of Aand B-site cations. In fact, the cation dissolutions of perovskites are pointed out to be inevitable as governed by thermodynamics under the necessary OER conditions [41]. Nevertheless, it is essential to highlight that the degradation mechanism is also driven by kinetics. Hence, each perovskite catalyst

would reach the end of its service life through different pathways based on the defect chemistry under the OER conditions [2,12,43]. In the light of above findings, the incorporation of Fe seems to impede the rate of degradation, where the one with the highest Fe composition (i.e., PBCF55) showed the most enhanced potential stability. Nonetheless, it should be noted that a higher composition of Fe (i.e., Fe-rich; x > 0.5) may compromise the role of Co as the active center and intrinsically undergo a different OER mechanism than Co-rich PBCF (i.e., x < 0.5) which would dim the assessment of the functional role of Fe-doping [31,40].

#### *2.3. Operando X-ray Absorption Spectroscopy Study*

The results of electrochemical studies revealed increasing OER activity and stability with increasing Fe composition in the domain of 0 ≤ *x* ≤ 0.5. As mentioned, PrBaCo2(1-x)Fe2xCo6-<sup>δ</sup> with different Co to Fe composition ratios would follow different reaction pathways as they undergo different degradation processes under the OER condition. Therefore, operando X-ray absorption spectroscopy study is conducted in order to monitor the changes in local electronic and geometric structures of the prepared layered double perovskites during the OER process, through which the functional effects of Fe were highlighted.

In Figure 4a, Co K-edge energy positions of normalized X-ray absorption near edge structure (XANES) spectra of as-prepared double perovskites reveal that PBC and PBCFs have Co oxidation state between +2 and +3, for which the Co K-edge position of PBCF55 is positioned at the lowest energy level. Through comparison, PrBaCo2(1-x)Fe2xCo6-<sup>δ</sup> show reduced Co oxidation states with increasing Fe composition (i.e., *x*) in the following descending order: PBC > PBCF82 > PBCF55, each with ~0.3 eV difference in their Co K-edge energy position. With this information, the concentration of oxygen vacancy can be relatively estimated as listed in ascending order: PBC < PBCF82 < PBCF55 (refer to Figure S3). Note that all edge energy positions were determined at the half of the edge step. Figure 4b shows the comparison of the Fourier-transformed (FT) k3-weighted Co K-edge extended X-ray absorption fine structure (EXAFS) spectra of as-prepared PBC, PBCF82, and PBCF55. The peaks of FT-EXAFS spectra signify the presence of neighboring atomic shells at specific radial distances from the absorbing atom (i.e., Co). In Figure 4b, the first two major peaks at ~1.9 Å and ~2.8 Å are ascribable to the backscattering contributions from the nearest Co–O and Co–Co/Fe ligands, respectively. Here, it is noteworthy that the second peak is ascribable to Co–Co/Fe coordination shell of the edge-sharing polyhedra typically found in highly oxygen deficient perovskite oxides prepared by flame spray synthesis (see Figure S2) [2,44,45]. Inconveniently, this Co–Co/Fe ligand distance of edge-sharing polyhedra of PrBaCo2(1-x)Fe2xCo6-<sup>δ</sup> is in close vicinity to that of Co/Fe-oxy(hydroxide) (refer to Supplementary Information S5 for detailed explanation). The next appearing peaks located at ~3.5 Å corresponds to Pr/Ba neighbors. All of the FT-EXAFS profiles of PBC, PBCF82, and PBCF55 show similar peak locations to one another, suggesting that a similar local structure is maintained upon Fe incorporation. In Figure 4b, the first peak amplitude is observed to be decreased in the presence of Fe; listed in the descending order of amplitude: PBC > PBCF82 > PBCF55. Given that the first peak is ascribable to Co–O coordination shell, Co of the layered double perovskite seems to be bound to less oxygen atoms at that radial distance in the presence of Fe. This is further verified by the best fit of the Co–O peak of FT-EXAFS spectra of as-prepared catalysts, which shows the decrease in Co–O coordination number with a higher amount of Fe-doping (refer to Figure S10). Together with the comparison of XANES spectra, these findings lead to assert that more oxygen vacant sites are created with a higher amount of Fe-doping, and therefore reduces Co oxidation state in PrBaCo2(1-x)Fe2xCo6-δ. Moreover, the decrease of scattering intensities at farther radial distances observed in the FT-EXAFS spectra of both PBCF82 and PBCF55 is rationalized by octahedral distortions induced by doping of Fe (+3) into the B-site replacing Co (~+2.7) cations, which then weakens the backscattering from the neighboring atoms [35,46–53].

Furthermore, Figure 4c,d display comparisons of normalized XANES and FT-EXAFS spectra, respectively, of PBCF82 and PBCF55 recorded at the Fe K-edge. In Figure 4c, the edge energy positions

of Fe K-edge XANES spectra of PBCF82 and PBCF55 indicates that their Fe oxidation states are similar (between +3 and +4). More precisely, the Fe K-edge of PBCF55 is positioned at about ~0.1 eV lower energy than PBCF82, but this insignificant difference would make the comparison trivial. Figure 4d shows Fe K-edge FT-EXAFS spectra of both PBCF82 and PBCF55 with similar scattering patterns as those of Co K-edge FT-EXAFS spectra. This confirms that Fe is indeed well integrated into the B-site of perovskite structure.

**Figure 4.** Comparison of as-prepared PBC, PBCF82, and PBCF55 Co K-edge (**a**) X-ray absorption near edge spectra (XANES) spectra; (**b**) Fourtier-transformed extended X-ray absorption fine structure (FT-EXAFS) spectra; and Fe K-edge; (**c**) XANES spectra; and (**d**) FT-EXAFS spectra.

When a neutral oxygen vacancy is created during synthesis, the left behind electrons would be distributed to either to Co and/or Fe, which would lower their oxidation states. However, the electrons are not evenly distributed among them but rather more accepted by Fe [40,48]. Based on above evidence, the addition of Fe seems to aid Co to be stable in a lower oxidation state by providing charge stabilization through balancing oxygen non-stoichiometry, and therefore promotes formation of oxygen vacancies. In the presence of higher oxygen vacancy concentration, the lattice oxygen within the perovskite structure is more inclined to participate in the water oxidation reaction (i.e., LOER) and develops the OER active oxy(hydroxide) layer at the surface [3,27].

Figure 5a shows a shift of the Co K-edge XANES spectra to higher energy positions of PBC, PBCF82, and PBCF55 during the operando flow cell test, among which PBCF55 reveals the greatest extent of energy shift of ~0.7 eV at the highest potential, while PBC and PBCF82 display ~0.3 eV of edge shifts. Our previous studies showed that the positive Co K-edge shift during the OER process is attributed to the construction of the OER active oxy(hydroxide) layer, since the Co oxidation state of Co-oxy(hydroxide) layer is higher than those inherent in the perovskite oxide [2,27]. This also agrees with other DFT based studies [46,47,51]. Therefore, revisiting Figure 3a, the high OER activity of

PBCF55 can be explained by the large extent of increase in the Co oxidation state reading from the shift of the Co K-edge position. At this point, we emphasize that the rate of dissolution (i.e., degradation) of a perovskite oxide catalyst during the OER process is controlled by its kinetics; thereby, each catalyst would reach its end-of-service life at a different rate. In this context, Figure 5b shows Co K-edge shifts at each increasing applied potential with respect to the energy positions recorded at 1.2 VRHE during the anodic polarization in flow cell test. In Figure 5b, Fe-doped PBCF82 and PBCF55 reveal rapid positive Co K-edge shifts when polarized into the oxygen evolution regime (> 1.4 VRHE). The edge shift translates proportionally to the increase of Co oxidation state. This sudden increase of Co oxidation state when polarized above the OER onset signifies formation of the OER active oxy(hydroxide) layer, which is potential-induced. Meanwhile, in the absence of Fe, PBC shows a consistent increase even at potentials below the oxygen evolution regime, indicating that the oxidation of its Co species is triggered by chemical dissolution [2].

Particularly, even though the extent of Co K-edge shift of PBCF82 is smaller than that of PBCF55, PBCF82 demonstrated a similar OER activity as PBCF55. Nevertheless, PBCF82 showed a rapid yet subtle increase of the Co oxidation state when polarized above the OER onset as similar to PBCF55 (Figure 5b). This may suggest the development of Co oxy(hydroxide) layer occurs but still less than in the case of PBCF55. Based on these observations, PBCF82 and PBCF55 both show different Co oxidation behaviors than compared to PBC, suggesting that the catalyst would undergo a modified degradation pathway during the OER process upon Fe incorporation. Referring to the Tafel plot (Figure 3a), the difference in their Tafel slopes, where lower Tafel slopes are observed upon Fe-doping, may suggest that PBCF's would undergo different mechanism during OER than the non-doped PBC. Also referring to the stability test (Figure 3b), the inhibited degradation mechanism upon Fe incorporation is further supported by the improved current stabilities of PBCF82 and PBCF55. Although thermodynamics anticipate the dissolution of PrBaCo2(1-x)Fe2xCo6-δ, it is intriguing to observe an enhanced ability to retain the initial current density upon Fe-doping, where PBCF55 demonstrated the highest retention of current density over the course of 500 cycles. The above findings point out that Fe plays an important role in improving the stability owing to the retardation of degradation mechanism, where the potential-induced increase of Co oxidation state is more attributable to the development of Co-oxy(hydroxide) species. However, this does not mean that deterioration of the perovskite structure is completely avoided as the decreasing trend of current densities are observed during the stability test. In brief, while the rate of chemical dissolution is lagged upon the addition of Fe, the layered double perovskite can be sustained as a substrate for the development of OER active oxy(hydroxide) species.

Despite the clear indications as to which the construction of Co-oxy(hydroxide) is displayed by positive edge shifts in their Co K-edge XANES spectra, this development—along with other concurrent local structural changes—are not clearly manifested in the comparison of FT-EXAFS spectra collected at potentials below and above the OER onset (1.2 and 1.54 VRHE, respectively) (Figure S6a–c). Here, it is important to recapitulate the coinciding Co–Co radial distances between the edge-sharing polyhedra of PrBaCo2(1-x)Fe2xCo6-<sup>δ</sup> and that of the Co-oxy(hydroxide) layer, both of which are at the proximity of ~2.8 Å from their primary Co atoms. In this respect, the FT-EXAFS profiles with these concurring signals would confound the precise interpretation of local structural changes. Considering these challenges, only the first peak is fitted in order to verify the changes in the Co–O coordination during the OER process (summarized in Table S3 and Figure S10). The best fits show increased Co–O coordination numbers for all catalysts at the highest anodic potential (1.54 VRHE) and therefore further consolidates the increase of Co oxidation states during the OER process.

While the increase of Co oxidation states is evident, Fe K-edge XANES spectra collected during the operando measurement (Figure 5c) reveal insignificant edge shifts (~0.1 eV) throughout the course of anodic polarization. The Fe in both PBCF82 and PBCF55 are in a higher oxidation state (between +3 and +4) than Co (< +2.7), which allows Co to be in a more reduced state and thereby being more flexible in accommodating charge transfers [48]; this justifies the insignificant shift in the Fe K-edge. In this context, the formation of the active oxy(hydroxide) layer would therefore be accommodated more by Co polyhedra, and thereby less changes would be manifested in their Fe K-edge FT-EXAFS profiles (Figure S6).

**Figure 5.** (**a**) comparison of Co K-edge XANES spectra of PBC, PBCF82, and PBCF55 recorded at 1.2 and 1.54 VRHE; and (**b**) Co K-edge energy shift measured with respect to the edge position at 1.2 VRHE at each potential during operando flow cell tests; (**c**) comparison of Fe K-edge XANES spectra of PBCF82 and PBCF55 recorded at 1.2 and 1.54 VRHE.

In light of our findings, the Fe incorporation into the layered double perovskite catalyst leads to stabilizing Co in a lower oxidation state by providing a better charge distribution and promoting the formation of oxygen vacancies. Consequently, the degradation mechanism is inhibited such that the oxidation of Co is more induced by the increase of potential while its chemical dissolution is decelerated yet still unavoidable. The potential-induced Co oxidation behavior upon Fe-doping is indicative of the development of the OER active Co-oxy(hydroxide) surface layer. Despite these enhancements, no significant electronic and structural changes were detected with respect to Fe. Overall, the addition of Fe seems to enable the layered double perovskite catalyst to improve its structural integrity (similar as previously observed for NixFe1-xO2) [37] as a more suitable substrate for the construction of oxy(hydroxide) layer leading to enhanced OER activity and stability.
