*3.5. Density-Functional Theory—Pourbaix Diagrams*

Density-functional theory (DFT) calculations are used to help understand the experimental data. In the first set of calculations, the stability of the perovskites in an aqueous environment was investigated by means of Pourbaix diagrams. This method has been previously used to investigate the stability of Ru-based perovskites in water as a descriptor for the stability in water to identify novel light harvesting materials [60]. Pourbaix diagrams show the phase diagram of solid and dissolved species as a function of pH and applied potential (VSHE). DFT provides the total energies for the solid bulk for the perovskites and the other solid competing phases as described in the Materials Project database [61]. Experiments provide the dissolution energies for the dissolved species [62,63]. This method is implemented in the Atomic Simulation Environment (ASE) package [64], and more details about the methodology can be found in the literature [65,66]. All the bulk structures have been fully relaxed using the Quantum ESPRESSO code [67], PBEsol as exchange-correlation functional [68] and the pseudopotentials from the Standard Solid State Pseudopotential library (SSSP accuracy) [69]. We used Hubbard U+ correction of 4 eV and applied to Co, Fe, and Pr elements.

## **4. Conclusions**

In here, a systematic study is conducted to assess the role of Fe in the highly OER active layered double perovskite catalyst, PrBaCo2(1-x)Fe2xCo6-δ, by comparing different compositions of Fe: PBC (*x* = 0), PBCF82 (*x* = 0.2), and PBCF55 (*x* = 0.5). These layered double perovskite catalysts are prepared via flame spray synthesis, where Fe is indeed incorporated into the B-site as verified by XRD and FT-EXAFS profiles. In comparison to PBC, Fe-doped PBCF82 and PBCF55 revealed enhanced OER activities and improved current stabilities as to better retain the initial current density. In the basis of our findings, such enhanced electrocatalytic performance is attributed to the addition of Fe, which provides charge stability as to compensate for the oxygen non-stoichiometry and allows Co to be in a lower oxidation state. This leads to alteration in the oxidation behavior of the layered double perovskite catalyst upon anodic polarization so that its Co oxidation is predominantly provoked by the increase of applied potential. In addition, the potential-induced Co oxidation upon Fe-doping is attributed to the formation of OER active oxy(hydroxide) layer at the surface. This also implies that the perovskite structure is prolonged under the OER conditions so that it serves as a substrate longer for the construction of oxy(hydroxide) layer at the surface. It should be noted that, even in the presence of Fe, the catalyst is not exempted from the inevitable cation dissolution under the OER conditions as governed by its thermodynamic nature. All of these effects lead to highlighting the constructive role of Fe in the layered double perovskite as an OER catalyst, which is evident from lower Tafel slopes and enhanced current density stabilities of PBCF82 and PBCF55. Particularly, PBCF55—with the highest Fe composition—demonstrated the greatest enhancement in both activity and stability as OER catalyst.

In summary, these results lead to concluding that a layered double perovskite catalyst is intrinsically modified upon Fe-doping so that it becomes more resilient to the cation dissolution, and thereby better supports the development of an oxy(hydroxide) surface layer. Such changes contribute to enhancements in the activity towards oxygen evolution reaction. As conventionally seen from many transition metal oxide catalysts, such electrocatalytic performance enhancements are attributed to the synergy between Fe and Co. In this regard, it should be emphasized that the incorporation of Fe would modify the degradation mechanism of the host transition metal. Thus, it should be generalized that the electrocatalytic performance of a doped-metal oxide would rely on the interaction between the host transition metal and the dopant metal. These findings present a systematic study method to engineer and design an ideal perovskite oxide as OER catalyst.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/9/3/263/s1, Figure S1: The initial cyclic voltammetry scanned at 10 mV sec−<sup>1</sup> from 1.0 to 1.7 VRHE, and the series of chronoamperometry measurements recording steady-state current at each step of potential: (a–b) PBC; (c–d) PBCF82; and (e–f) PBCF55; Figure S2: Density-functional theory (DFT) calculated Pourbaix diagrams of (a) PrBaCo2O6-<sup>δ</sup> and (b) PrBaCo2(1-x)Fe2xO6-<sup>δ</sup> (i.e., PBCF82 and PBCF55). Red lines mark the ranges of the working potential (on standard hydrogen electrode (SHE) potential scale) for the oxygen evolution reaction (OER) in at pH 13 used during the operando XAS study; Figure S3: Comparison of XANES spectra of PBC prepared via sol-gel (SG) method (black) and PBC (red), PBCF82 (blue), and PBCF55 (green) prepared via flame spray synthesis; Figure S4: Illustration of Co–Co distances in (a) Co octahedra, surrounded by six oxygen atoms in an octahedral structure sharing a single oxygen corner in a typical stoichiometric perovskite (ABO3/A'A"B'B"O6); and (b) B-site cation surrounded by less oxygen than stoichiometry (ABO3-δ/A'A"B'B"O6-δ); therefore, the network of polyhedra would stabilize by pivoting to share two oxygen atoms; Figure S5: Comparison of Fourier transformed (FT) k3-weighted EXAFS profiles at Co K-edge of γ-Co-O(OH) [3] with as-prepared layered double perovskite catalysts: PBC (black), PBCF82 (red), and PBCF55 (blue); Figure S6: Comparison of FT Co K-edge EXAFS spectra collected at 1.20 and 1.54 VRHE during the operando XAS measurements of (a) PBC, (b) PBCF82, and (c) PBCF55. Simultaneously collected Fe K-edge EXAFS spectra of (d) PBCF82 and (e) PBCF55; Figure S7: Fourier transformed k3-weighted Co K-edge EXAFS spectra of PBC (a) as-prepared, (b) at 1.2 VRHE anodic, and (c) 1.54 VRHE anodic. Black line is the FT-EXAFS spectrum, red line is the fitted spectrum, and blue is the window of the fitting; Figure S8: Fourier transformed k3-weighted Co K-edge EXAFS spectra of PBCF82 (a) as-prepared, (b) at 1.2 VRHE anodic, and (c) 1.54 VRHE anodic. Black line is the FT-EXAFS spectrum, red line is the fitted spectrum, and blue is the window of the fitting; Figure S9: Fourier transformed k3-weighted Co K-edge EXAFS spectra of PBCF55 (a) as-prepared, (b) at 1.2 VRHE anodic, and (c) 1.54 VRHE anodic. Black line is the FT-EXAFS spectrum, red line is the fitted spectrum, and blue is the window of the fitting; Figure S10: Comparison of coordination number (NCo–O) of the first peak of FT-EXAFS spectra of PBC (black), PBCF82 (red), and PBCF55 (blue). Filled markers and empty markers represent NCo–O of the as-prepared catalysts and at 1.54 VRHE during the anodic polarization, respectively; Table S1: Summary of Brunauer–Emmet–Teller (BET) surface areas of the prepared layered double perovskite catalysts; Table S2: Lattice parameters of PBC, PBCF82, and PBCF55 calculated from Rietveld refinement of their X-ray diffractions. With the estimated lattice parameter, the Co–Co distance of edge-sharing polyhedron is calculated; Table S3: Summary of best fit parameters of the FT k3-weighted Co K-edge EXAFS spectra of as-prepared and at 1.20 and 1.54 VRHE during the anodic polarization of (a) PBC, (b) PBCF82, and (c) PBC55.

**Author Contributions:** Conception and conceptualization, B.-J.K., E.F., and T.J.S.; synthesis, M.B. and T.G.; characterizations, electrochemical measurements, and investigation, B.-J.K.; DFT computations, I.E.,C.; B.-J.K., E.F., I.E.,C., M.B., T.G., M.N. and T.J.S. discussed the results and commented on the manuscript.

**Funding:** This research was funded by Swiss National Science Foundation, Innosuisse and the Swiss Competence Center for Energy Research (SCCER) Heat and Paul Scherrer Institute.

**Acknowledgments:** The authors gratefully acknowledge the Swiss National Science Foundation through its Ambizione Program and the National Centre of Competence in Research (NCCR) Marvel, the Swiss Competence Center for Energy Research (SCCER) Heat and Electricity Storage through Innosuisse, Switzerland, and the Paul Scherrer Institute for financial contributions to this work, respectively. The authors thank the Swiss Light Source for providing beamtime at the SuperXAS beamline.

**Conflicts of Interest:** The authors declare no conflict of interest.
