Halide-Doping Effect of Strontium Cobalt Oxide Electrocatalyst and the Induced Activity for Oxygen Evolution in an Alkaline Solution

Perovskites of strontium cobalt oxyhalides having the chemical formulae Sr2CoO4-xHx (H = F, Cl, and Br; x = 0 and 1) were prepared using a solid-phase synthesis approach and comparatively evaluated as electrocatalysts for oxygen evolution in an alkaline solution. The perovskite electrocatalyst crystal phase, surface morphology, and composition were examined by X-ray diffraction, a scanning electron microscope, and energy-dispersive X-ray (EDX) mapping. The electrochemical investigations of the oxyhalides catalysts showed that the doping of F, Cl, or Br into the Sr2CoO4 parent oxide enhances the electrocatalytic activity for the oxygen evolution reaction (OER) with the onset potential as well as the potential required to achieve a current density of 10 mA/cm2 shifting to lower potential values in the order of Sr2CoO4 (1.64, 1.73) > Sr2CoO3Br (1.61, 1.65) > Sr2CoO3Cl (1.53, 1.60) > Sr2CoO3F (1.50, 1.56) V vs. HRE which indicates that Sr2CoO3F is the most active electrode among the studied catalysts under static and steady-state conditions. Moreover, Sr2CoO3F demonstrates long-term stability and remarkably less charge transfer resistance (Rct = 36.8 ohm) than the other oxyhalide counterparts during the OER. The doping of the perovskites with halide ions particularly the fluoride-ion enhances the surface oxygen vacancy density due to electron withdrawal away from the Co-atom which improves the ionic and electronic conductivity as well as the electrochemical activity of the oxygen evolution in alkaline solution.


Introduction
One of the most scientific challenges in energy conversion technologies is the discovery of cost-effective, highly active electrodes to implement the key electrochemical oxygen evolution (OER) and oxygen reduction reactions (ORR) in direct-solar and electricity-driven water splitting, fuel cells, and metal-air batteries [1][2][3][4]. However, the electrochemical oxygen evolution reaction (OER) in alkaline solution occurs through multistep protoncoupled electron transfers (4OH − → 2H 2 O + 4e − + O 2 in alkaline media) and it is kinetically slow [5,6], therefore, the development of highly active, abundant, and low-cost catalysts for water oxidation is one of the main tasks for large scale applications [7][8][9].
Among the non-precious metal-based catalysts replacements, perovskite-based materials (ABO 3 ) have shown good activity as bifunctional electrocatalysts for oxygen evolution (OER) and oxygen reduction (ORR) owing to good cation ordering, which boosts the mobility of oxygen ions for enhanced catalytic performance [10][11][12]. Furthermore, their composition, crystal structure, and morphology can be designed to further enhance their electronic and catalytic activities [13,14]. There has been much effort to identify appropriate A-and B-site elements as well as the anions for perovskite structures (ABO 3 ) that could replace and offer higher catalytic performance for ORR and OER [15][16][17][18][19][20][21]. Within this context, the structural investigations of perovskites have shown that the weakening of the metal-oxygen bond by fully or partial substitution of the B-site enhances the ionic and electronic conductivity as well as the activity of OER and ORR [22][23][24][25]. Consequently, the oxygen vacancy density, as well as the valance and concentration of the B-cation, are changed within the structure improving the electronic properties and enhancing the catalytic activity [26,27]. Within this context, the doping of cobalt [28] or nickel [29] within the iron-molybdenum-based perovskite structure improves the electrochemical activity for the ORR in solid oxide fuel cell applications. The second strategy to achieve the desired chemical and physical properties of perovskite oxide is by partially exchanging the O-sites in the ABO 3 perovskite structure with anions such as Cl - [30,31], N 3- [32], F - [33,34], or S 2- [35]. The incorporation of such anions into the perovskite structure may decrease the coulombic force between oxygen and the B-site atom which promotes the lattice oxygen activity and migration [34,36]. Furthermore, due to the structural versatility of the perovskite family, layered perovskites and double perovskites have attracted more attention as competitive candidates as bifunctional catalysts in ORR and OER catalysis [37][38][39][40]. Takeguchi [42]. Previously, our group reported the effect of chloride-doping of a layered iron oxy-chloride perovskite (Sr 2 FeO 2 Cl) that exhibited good oxygen electrocatalytic activity as well as a bifunctional OER/ORR electrocatalyst. Interestingly, the lower the oxygen and higher chloride content in this type of oxy-chloride perovskite materials significantly enhanced the OER/ORR activity [38]. Within this context, Miyahara et al. reported a highly active OER electrocatalyst with layered cobalt perovskite oxychlorides of Sr 2 CoO 3 Cl and Sr 3 Co 2 O 5 Cl 2 . They attributed the high OER/ORR electrocatalytic activity to the incorporation of a chloride anion into oxygen sites that upshifted the O p-band center of the Fermi level [39]. Nevertheless, the research on perovskites anion-doping and the influence on the structural oxygen deficiency and activity for oxygen evolution reaction is limited, therefore, this work reports a comparative and systematic investigation of the effect of halide ions doping (F -, Cl -, and Br -) on strontium cobalt layered perovskite (Sr 2 CoO 4 ) electrocatalyst and the induced performance of the electrochemical activity towards OER in an alkaline solution compared to the parent oxide. Figure 1 shows the XRD patterns of the crystal structures of Sr 2 CoO 4 , Sr 2 CoO 3 F, Sr 2 CoO 3 Cl, and Sr 2 CoO 2 Br catalysts. All the prepared oxyhalides showed identical peaks of the desired compounds, confirming that the resultant oxyhalides had a layered perovskite structure, except for a minor 5.6% impurity assigned to Sr 6 Co 5 O 15 and other unidentified impurities [42][43][44][45]. The XRD results reveal the main crystal structure phase has a tetragonal structure that is equivalent to the Sr 2 CoO 4 RP-type structure with a space group of I4/mmm with the Co atom in octahedral coordination with six oxygen/halide anions (JCPDS ICDD card NO. 01-070-3734). The oxygen/halide anions are disordered at the apical sites and the Sr cation occupies an interstitial space of the eight corner-sharing of the octahedral Co atom [24,25,46]. Figure 2 presents the SEM images of the surface morphology of the prepared Sr 2 CoO 4 , Sr 2 CoO 3 F, Sr 2 CoO 3 Cl, and Sr 2 CoO 3 Br catalysts showing that the catalysts have similar micrometer-sized irregular shaped particles, with no significant difference in the surface morphology observed for the oxyhalides from the Sr 2 CoO 4 parent catalyst. As an example, the SEM-EDX mapping images of the Sr 2 CoO 3 F catalyst are shown in Figure 3 which reveals the uniform distribution of the constituent elements of Sr, Co, O, and F throughout the sample. Intensity, a. u.  Figure 2 presents the SEM images of the surface morphology of the prepared Sr2CoO4, Sr2CoO3F, Sr2CoO3Cl, and Sr2CoO3Br catalysts showing that the catalysts have similar micrometer-sized irregular shaped particles, with no significant difference in the surface morphology observed for the oxyhalides from the Sr2CoO4 parent catalyst. As an example, the SEM-EDX mapping images of the Sr2CoO3F catalyst are shown in Figure 3 which reveals the uniform distribution of the constituent elements of Sr, Co, O, and F throughout the sample.     The elemental composition of the overall oxyhalides was determined by SEM-EDX mapping analysis and Table 1 summarises the elemental composition analysis of the strontium cobalt oxyhalide catalysts in addition to the parent oxide of Sr2CoO4. The results  The elemental composition of the overall oxyhalides was determined by SEM-EDX mapping analysis and Table 1 summarises the elemental composition analysis of the strontium cobalt oxyhalide catalysts in addition to the parent oxide of Sr 2 CoO 4 . The results revealing good agreement between the EDX elemental analysis and the molecular formula of Sr 2 CoO 4 , Sr 2 CoO 3 F, Sr 2 CoO 3 Cl, and Sr 2 CoO 3 Br, except for an impurity within 2.0 wt.% attributed to the unidentified strontium cobalt oxide phase as confirmed by X-ray analysis.

Structure and Morphology of the Sr 2 CoO 3-x H x Catalysts
Furthermore, Figure S1 shows the wide XPS spectra of the oxyhalide catalysts that reveal the surface chemical functionality and the oxygen atom bonding upon incorporating F − , Cl − or Br − anions. The XPS wide spectra clearly show the peaks for the C 1s that commonly originates from the carbon surface contamination and the main constituent elements of Sr 3d, Co 2p, and O 1s. Figure 4 shows the deconvoluted core spectra analysis of the O 1s peak for the Sr 2 CoO 3 F, Sr 2 CoO 3 Cl, Sr 2 CoO 3 Br, and parent Sr 2 CoO 4 while Table 2 reports the deconvoluted O 1s peaks binding energy and oxygen bonding ratio values of the studied oxyhalide catalysts. The O 1s fitted core spectra in Figure 4 reveal the presence of a peak at lower binding energy (peak 1) that can be assigned to the O 2− anion of the lattice oxygen and the peak in the middle is related to the surface O 1− ions (peak 2), while the peak at higher binding energy (peak 3) is due to the chemically adsorbed oxygen species (O chem ) at the surface [47,48]. However, the O 1s spectrum of the parent Sr 2 CoO 4 catalyst (Figure 4d) can only be fitted by two peaks that are assigned for O 2− anion (peak 1) and O 1− /O chem (peak 2), possibly because of a small amount of chemically adsorbed oxygen species. From the data in Table 2 and upon the incorporation of halide ions, the overall peaks' binding energy is shifted to lower values in the order F − > Cl − > Br − which indicates a weaker Co-O bonding due to withdrawn the electron density away from the Co-atom, consequently enhances the lattice oxygen (O 2− ) reactivity and reduces the activation energy required for O 2− activation [47][48][49]

Electrocatalytic Activity of Cobalt Oxyhalide Catalysts in OER
The electrochemical behavior and activity of the produced strontium cobalt oxyhalide (Sr 2 CoO 3-x H x ) catalysts were evaluated for the OER in 1.0 M NaOH by cyclic voltammetry. The catalyst particles were uniformly deposited as thin films on the carbon paper electrodes via electrophoretic deposition at predefined catalyst loading. Figure 5a shows cyclic voltammograms (CV) at 20 mV s −1 of the Sr 2 CoO 3-x H x (x = 1) compared to carbon paper in the narrow potential window in 1.0 M NaOH. The blank carbon paper electrode (grey line) shows the standard capacitance CV, while the oxyhalide voltammograms exhibit the well-known shape of the Faradic cobalt redox Co(II)/Co(III) system. Although the oxyhalide catalysts have almost similar cobalt wt.% (Table 1 above), the CV indicates that the Sr 2 CoO 3 F electrode exhibits a significantly higher electrochemical Co(II)/Co(III) redox current than Sr 2 CoO 4 , Sr 2 CoO 3 Cl, and Sr 2 CoO 3 Br, suggesting that the Sr 2 CoO 3 F electrode has a higher electroactive surface of Co(II)/Co(III) sites. Figure 5b compares the linear sweep voltammetry (LSV) at 20 mV s −1 for the strontium cobalt oxyhalide (Sr 2 CoO 3-x H x ) compared to the parent Sr 2 CoO 4 in 1.0 M NaOH electrolyte at a catalyst loading of 0.8 mg/cm 2 . In the case of the oxyhalides electrodes, significant catalytic activity towards the OER was observed, as evidenced by less positive onset potential and the higher current density compared to the pristine Sr 2 CoO 4 (black line) catalyst. Table 3 reports the OER catalyst characteristic parameters of onset potential (the potential at 1.0 mA/cm 2 ), the potential at a current density of 10 mA cm −2 (η 10% ) that corresponds to 10% solar to the energy efficiency of the cobalt oxyhalides. The incorporation of halide anions of F − , Cl − , or Br − into the Sr 2 CoO 4 substrate enhanced the OER, with the onset potential shifting to less positive in the order of F − (1.50) < Cl − (1.52) < Br − (1.61) < Sr 2 CoO 4 (1.64) V vs. HRE. Moreover, the (η 10 ) values similarly decrease as Sr 2 CoO 4 (1.73) > Sr 2 CoO 3 Br (1.67) > Sr 2 CoO 3 Cl (1.60) > Sr 2 CoO 3 F (1.56) V, indicating that Sr 2 CoO 3 F is the most active electrode among the studied catalysts, presumably due to its higher electroactive surface area and/or conductivity as shown by the impedance analysis below. Moreover, the electrochemical behavior of the oxyhalides catalysts was investigated under steady-state conditions using rotating disk electrode (RDE) of glassy carbon, and the results of LSVs at a rotation speed of 1600 rpm and 10 mV s −1 are shown in Figure S2. The RDE results and as shown in Table 3 revealed no significant change in the values of the OE onset potential and (η 10% ) under steady-state condition confirming the electrode kinetic is the predominant during oxygen evolution reaction. Moreover, a comparable activity trend towards the oxygen evolution was achieved confirming the Sr 2 CoO 3 F catalyst is the most active under either static or steady-state conditions.
To gain more insight on the enhanced OER activity of the oxyhalide catalysts, Tafel plots were used to examine the catalysis kinetics for the OER and are plotted in Figure 5c as extracted from the LSV shown in Figure S2 (Supporting Information) that performed at a scan rate of 10 mV s −1 and rotation rate of 1600 rpm. The Tafel plots describe the relationship between the overpotential and the logarithm of the current (I), which can provide important information about water oxidation including the electronic and geometric enhancement in the activity of the electrocatalyst [49]. As shown in Table 3     Electrochemical impedance spectra (EIS) analysis was also employed to evaluate the charge carrier transfer resistance during the OER for the oxyhalide catalysts of Sr 2 CoO 3 F, Sr 2 CoO 3 Cl, and Sr 2 CoO 3 Br compared to pristine Sr 2 CoO 4 catalyst. Figure 6 presents typical Nyquist plots of the impedance data obtained for the strontium cobalt oxyhalide (Sr 2 CoO 4-x H x ) compared to the pristine Sr 2 CoO 4 catalyst at 1.55 V versus HRE and within the frequency range of 0.01 to 1.0 MHz with a 20-mV s −1 amplitude in 1.0 M NaOH. The Nyquist diagram of the oxyhalide electrodes can be fitted to the standard Rs/Q 1 /R ct equivalent circuit illustrated in the inset of Figure 6, where the R s is the solution resistance, R ct , the charge transfer resistance, and Q 1 is the double-layer capacitance at the catalyst/electrolyte interface. The impedance parameters obtained from the fitted equivalent circuit of the oxyhalide catalyst are reported in Table 3. The most significant characteristic observed in Figure 6 and Table 4 is the radii of the arc that related to the charge transfer resistance (R ct ) on the EIS Nyquist plots of Sr 2 CoO 3 F (36.8 Ω) is significantly smaller compared to Sr 2 CoO 3 Cl (52.9 Ω), Sr 2 CoO 3 Br (128.5 Ω) and pristine Sr 2 CoO 4 (296.1 Ω) , catalysts, suggesting that the Sr 2 CoO 3 F catalyst possesses a remarkably lower charge transfer resistance (R ct ) at the catalyst/electrolyte interface during the OER. Electrochemical impedance spectra (EIS) analysis was also employed to evalu charge carrier transfer resistance during the OER for the oxyhalide catalysts of Sr2 Sr2CoO3Cl, and Sr2CoO3Br compared to pristine Sr2CoO4 catalyst. Figure 6 presents Nyquist plots of the impedance data obtained for the strontium cobalt oxyhalide (S xHx) compared to the pristine Sr2CoO4 catalyst at 1.55 V versus HRE and within t quency range of 0.01 to 1.0 MHz with a 20-mV s −1 amplitude in 1.0 M NaOH. The N diagram of the oxyhalide electrodes can be fitted to the standard Rs/Q1/Rct equival cuit illustrated in the inset of Figure 6, where the Rs is the solution resistance, charge transfer resistance, and Q1 is the double-layer capacitance at the catalyst/elec interface. The impedance parameters obtained from the fitted equivalent circuit of yhalide catalyst are reported in Table 3. The most significant characteristic obser Figure 6 and Table 4 is the radii of the arc that related to the charge transfer resistan on the EIS Nyquist plots of Sr2CoO3F (36.8 Ω) is significantly smaller compa Sr2CoO3Cl (52.9 Ω), Sr2CoO3Br (128.5 Ω) and pristine Sr2CoO4 (296.1 Ω), catalysts, su ing that the Sr2CoO3F catalyst possesses a remarkably lower charge transfer resistan at the catalyst/electrolyte interface during the OER.  Furthermore, the values of the double-layer capacitance (Q 1 ) indicate that the Sr 2 CoO 3 F catalyst shows higher capacitance than the other oxyhalide electrodes, which agrees with the results obtained from linear sweep voltammetry above. The ISE analysis implies that the Sr 2 CoO 3 F catalyst accelerates the charge transfer kinetics and acts as a highly effective OER catalyst. The enhanced electrochemical oxygen evolution performance of the oxyhalides over parent Sr 2 CoO 4 catalyst can be attributed to the doping of Cl − , Br − and in particular the F − ions enhanced the O 1− and O chem sites concentration that improve the ionic and electronic conductivity as well as the electrochemical activity.
The long-term stability of the strontium cobalt oxyhalide (Sr 2 CoO 3-x H x ) catalysts during OER in alkaline solution was investigated by chronoamperometry and chronopotentiometry measurements. Figure 7   The long-term stability of the strontium cobalt oxyhalide (Sr2CoO3-xHx) catalysts during OER in alkaline solution was investigated by chronoamperometry and chronopotentiometry measurements. Figure 7 presents the chronoamperometric curves of the Sr2CoO4, Sr2CoO3F, Sr2CoO3Cl, and Sr2CoO3Br catalysts in 1.0 M NaOH (pH = 14) at a constant potential of 1.65 V vs. HRE for 12 h, showing that the Sr2CoO3F catalyst exhibits outstanding stability with no obvious current density decrease and maintained about 95% of the oxygen evolution activity after 12 h of electrolysis. In contrast, Sr2CoO3Cl, Sr2CoO3Br, and Sr2CoO4 suffer reduced activity and the current density decreased from 44.8 to 35 mA cm −2 , 32.2 to 24.6 mA cm −2 , and 18.3 to 7.5 mA cm −2 , respectively, after only 4 h or less during electrolysis. Presumably, during the longterm stability test in alkaline solution and under applied potential, the halide-doped strontium cobalt oxide catalysts are partially converted to oxyhydroxides due to the intercalation of OH − groups with the fluoride-doped catalyst having the most stable O 2− /OH − /F − coordination and bonding with Co atom due to the stronger electron affinity of fluoride ion [48,49]. However, more work is required to understand the oxyhalide catalysts' struc- In contrast, Sr 2 CoO 3 Cl, Sr 2 CoO 3 Br, and Sr 2 CoO 4 suffer reduced activity and the current density decreased from 44.8 to 35 mA cm −2 , 32.2 to 24.6 mA cm −2 , and 18.3 to 7.5 mA cm −2 , respectively, after only 4 h or less during electrolysis. Presumably, during the long-term stability test in alkaline solution and under applied potential, the halidedoped strontium cobalt oxide catalysts are partially converted to oxyhydroxides due to the intercalation of OH − groups with the fluoride-doped catalyst having the most stable O 2− /OH − /F − coordination and bonding with Co atom due to the stronger electron affinity of fluoride ion [48,49]. However, more work is required to understand the oxyhalide catalysts' structural changes and the corresponding intrinsic activity during prolonged electrolysis in alkaline solution. In conclusion, the electrochemical measurements suggest that the Sr 2 CoO 3 F is the most active and stable catalyst for OER applications among the studied strontium cobalt oxyhalide catalysts.

Electrochemical Performance of the Sr 2 CoO 3 F Catalyst
The Sr 2 CoO 3 F catalyst activity during OER in alkaline solution was optimized by studying the catalyst loading effect, NaOH concentration, and long-term stability at a predefined constant potential or current density. Figure 8a shows the linear sweep voltammograms at 20 mV s −1 of the Sr 2 CoO 3 F catalyst at different catalyst loadings. Moreover, Figure 8b illustrates the effect of the catalyst loading on the potential at 10 mA/cm 2 (black line) and the OER current density achieved at 1.8 V vs. HRE (red line). From Figure 8b, the Sr 2 CoO 3 F catalyst loading around 0.8 mg/cm 2 is optimal to achieve the best OER activity, that is, the smallest potential at 10 mA/cm 2 and the highest current density at 1.8 V vs. HRE. In addition, Figure 8c illustrates the LSV at 20 mV s −1 of the Sr 2 CoO 3 F catalyst (at 0.8 mg loading) in different concentrations of NaOH electrolyte, showing that the OER onset potential is significantly shifted to a less positive potential from 0.66 to 0.52 V vs. SCE by increasing the NaOH from 0.1 to 5.0 M, respectively. In addition, the OER current density substantially increased at a more positive potential indicating the enhancement of OER in concentrated alkaline solution. The oxygen evolution at a less positive potential in concentrated alkaline solution could be attributed to the Nernst effect and the current density enhancement due to an increase of OH − ion concentration at the catalyst/electrolyte interface. EIS analysis was also employed to evaluate the charge carrier transfer resistance of the most active oxyhalides of Sr 2 CoO 3 F at the different applied potentials. Figure 8d presents typical Nyquist plots of the impedance data obtained for the Sr 2 CoO 3 F at Sr 2 CoO 3 F at different applied potential and frequency ranges of 0.01 to 1.0 MHz with a 20 mV amplitude, showing that the radii of the arc in the EIS Nyquist plots of Sr 2 CoO 3 F decrease with the increasing applied potential from 1.55 to 1.62 , suggesting that the charge transfer resistance of OER at the Sr 2 CoO 3 F catalyst decreases with increasing applied potential indicating higher OER performance. Furthermore, Table 5 reports the impedance parameter values of solution resistance (R s ), double-layer capacitance (Q 1 ), and charge transfer resistance (R 1 ) derived from the fitting of the equivalent circuit (shown in Figure 8d inset) for the impedance spectra at a different applied potential of the Sr 2 CoO 3 F catalyst, indicating that charge transfer resistance (R 1 ) significantly decreased from 36.8 to 4.34 ohm by increasing the applied potential from 1.55 to 1.62 V, confirming the enhancement of the charge transfer and the acceleration of the OER kinetics at the Sr 2 CoO 3 F catalyst. Finally, Figure 9 shows the chronoamperometry and chronopotentiometry at a predefined applied potential and current density during the prolonged OER at the Sr 2 CoO 3 F catalyst in 1.0 M NaOH. The Sr 2 CoO 3 F catalyst exhibits long-term stability with an almost stable current and potential during water oxidation in an alkaline solution. The catalyst recorded a potential of 1.58 V vs. HRE to achieve a current density of 10 mA/cm 2 that is required for 10% efficiency of solar to hydrogen production which outperforms the benchmarked catalysts prepared by other approaches.
For the evaluation of the Sr 2 CoO 3 F against the benchmark of perovskites and iridium oxide catalysts, Table S1 reports our strontium cobalt oxyhalides catalysts activities of OER onset overpotential and the overpotential required to achieve 10 mA/cm 2 current density (η 10 mA/cm2 ) in comparison to the benchmarking catalysts that reported in the literature. The Sr 2 CoO 3 F catalyst exhibits superior OER onset overpotential (270 mV) lower than that of well-known precious metal IrO 2 catalyst (320 mV) [S11] in an alkaline solution. Moreover, the Sr 2 CoO 3 F shows better OER activity than that of perovskite cobalt-based catalysts such as Sr 3  potential is significantly shifted to a less positive potential from 0.66 to 0.52 V vs. SCE by increasing the NaOH from 0.1 to 5.0 M, respectively. In addition, the OER current density substantially increased at a more positive potential indicating the enhancement of OER in concentrated alkaline solution. The oxygen evolution at a less positive potential in concentrated alkaline solution could be attributed to the Nernst effect and the current density enhancement due to an increase of OH − ion concentration at the catalyst/electrolyte interface. EIS analysis was also employed to evaluate the charge carrier transfer resistance of the most active oxyhalides of Sr2CoO3F at the different applied potentials. Figure 8d presents typical Nyquist plots of the impedance data obtained for the Sr2CoO3F at Sr2CoO3F at different applied potential and frequency ranges of 0.01 to 1.0 MHz with a 20 mV amplitude, showing that the radii of the arc in the EIS Nyquist plots of Sr2CoO3F decrease with the increasing applied potential from 1.55 to 1.62, suggesting that the charge transfer resistance of OER at the Sr2CoO3F catalyst decreases with increasing applied potential indicating higher OER performance. Furthermore, Table 5 reports the impedance parameter values of solution resistance (Rs), double-layer capacitance (Q1), and charge transfer resistance (R1) derived from the fitting of the equivalent circuit (shown in Figure 8d inset) for the impedance spectra at a different applied potential of the Sr2CoO3F catalyst, indicating that charge transfer resistance (R1) significantly decreased from 36.8 to 4.34 ohm by increasing the applied potential from 1.55 to 1.62 V, confirming the enhancement of the charge transfer and the acceleration of the OER kinetics at the Sr2CoO3F catalyst.   For the evaluation of the Sr2CoO3F against the benchmark of perovskites and iridium oxide catalysts, Table S1 reports our strontium cobalt oxyhalides catalysts activities of OER onset overpotential and the overpotential required to achieve 10 mA/cm 2 current density (η10 mA/cm2) in comparison to the benchmarking catalysts that reported in the literature. The Sr2CoO3F catalyst exhibits superior OER onset overpotential (270 mV) lower than that of well-known precious metal IrO2 catalyst (320 mV) [S11] in an alkaline solution. Moreover, the Sr2CoO3F shows better OER activity than that of perovskite cobalt-based catalysts such as Sr3Co2O5Cl2 [S3] hydrated Sr3Co2O5(OH)2 [S1] and defected SrCo0.8Fe0.5-xO3-δ [S5]. Finally, the catalysts the Sr2CoO3F catalyst reveal OER activity that is comparable to the catalysts of nickel-cobalt-fluoride [S6] and nickel-iron-oxyfluoride [S10].

Catalyst Preparation Characterizations
A set of the perovskite oxyhalides of strontium cobalt oxyhalides (Sr2CoO3-xHx, where H = F, Cl, or Br, and x = 0 or 1; Sr2CoO4, Sr2CoO3F, Sr2CoO3Cl, and Sr2CoO2Br) were prepared by solid-phase synthesis following the method reported in the literature [45,46]. A stoichiometric molar ratio of SrCO3 (99,9%,Aldrich, Dorset, UK), Co3O4 (99.9%, Kojundo Chemical Laboratory, Mölndal, Sweden), and the potassium precursor of chloride, bromide, or fluoride (Aldrich, 99.95%) were ground and heated at 850 °C under a nitrogen atmosphere in a platinum crucible for 24 days. The products were cooled to room temperature, reground, and heated to 550 °C for a further 12 h. The product was cooled to room temperature, reground, and stored for further characterization.
The crystallography and phase data of the fabricated catalysts were analyzed by powder X-ray diffraction (XRD, Rigaku Miniflex 600, Rigaku Corporation, Tokyo, Japan) using Cu Kα radiation. The surface morphology and elemental composition of the fabricated catalysts were examined by a scanning electron microscope (SEM; S-4800, Hitachi, Oberkochen, Germany) with a fitted energy dispersive X-ray spectrometer detector (EX350, Horiba).

Catalyst Preparation Characterizations
A set of the perovskite oxyhalides of strontium cobalt oxyhalides (Sr 2 CoO 3-x H x , where H = F, Cl, or Br, and x = 0 or 1; Sr 2 CoO 4 , Sr 2 CoO 3 F, Sr 2 CoO 3 Cl, and Sr 2 CoO 2 Br) were prepared by solid-phase synthesis following the method reported in the literature [45,46]. A stoichiometric molar ratio of SrCO 3 (99,9%,Aldrich, Dorset, UK), Co 3 O 4 (99.9%, Kojundo Chemical Laboratory, Mölndal, Sweden), and the potassium precursor of chloride, bromide, or fluoride (Aldrich, 99.95%) were ground and heated at 850 • C under a nitrogen atmosphere in a platinum crucible for 24 days. The products were cooled to room temperature, reground, and heated to 550 • C for a further 12 h. The product was cooled to room temperature, reground, and stored for further characterization.
The crystallography and phase data of the fabricated catalysts were analyzed by powder X-ray diffraction (XRD, Rigaku Miniflex 600, Rigaku Corporation, Tokyo, Japan) using Cu K α radiation. The surface morphology and elemental composition of the fabricated catalysts were examined by a scanning electron microscope (SEM; S-4800, Hitachi, Oberkochen, Germany) with a fitted energy dispersive X-ray spectrometer detector (EX350, Horiba).

Electrochemical Measurements
All electrochemical characterizations were performed using a potentiostat/galvanostat (Autolab PGSTAT302) electrochemical system in a three-electrode glass cell using 1.0 M NaOH (>99%, Loba Chemie PVT Ltd., Mumbai, India) electrolyte. Carbon paper (SIGRACET ® , GDL-24BC, SGL CARBON SE, Bonn, Germany) or glassy carbon rotating disk electrode (GC-RDE, 3 mm diameter, Metrohm, Herisau, Switzerland) was employed as a working electrode, and the saturated calomel electrode (SCE) and Pt-mesh (0.5 × 1.0 cm 2 ) were used as the reference and counter electrodes, respectively. The oxyhalide catalyst was deposited by electrophoretic deposition (EPD) on the CP substrate. Briefly, the catalyst powder (15 mg) was dispersed in 15 mL acetone containing 40 mg iodine using an ultrasonic probe for 10 min. The electrophoretic deposition cell was constructed using a 50 mL glass beaker with two-parallel CP (1.0 × 1.0 cm 2 ) electrodes immersed in the acetone-catalyst suspension 1.0 cm apart. Then, +10 V bias was applied across the two electrodes for a predefined deposition time using a potentiostat (model VSP, BioLogic, Göttingen, Germany) and the electrocatalyst particles were coated on the negative electrode (cathode). After deposition, the working electrode was washed with distilled water and dried in an oven at 80 • C K for 2.0 h under an open atmosphere. The average weight of all mesoporous catalysts deposited on CP was approximately 0.8 mg.
Chronoamperometry and chronopotentiometry measurements were conducted to evaluate the durability of the fabricated electrode materials by fixing the current density or potentials. Electrochemical Impedance Spectroscopy (EIS) analysis was performed in the frequency range of 10 −2 to 200 kHz with a 20-mV amplitude at a bias of 1.5 V vs. RHE in 1.0 M NaOH. The electrolyte solutions were deaerated with purged nitrogen gas for 15 min before the electrochemical measurements and the electrochemical experiments were repeated independently at least three times for each catalyst.

Conclusions
In summary, the effect of halide anions (F − , Cl − , and Br − ) doping on the structure and the induced electrochemical activity of the Sr 2 CoO 4 perovskite catalyst has been systematically and comparatively examined using a range of characterization techniques. The physicochemical characterization results revealed that the halide anions doping, in particular the fluoride ion, enhanced the density of the surface oxygen vacancy defects, presumably due to weakening of the Co-O bonding and the withdraw of the electron density away from the Co-atom by halide ions. The comparative electrochemical activity performance of the oxyhalide-doped (F − , Cl − , and Br − ) catalysts revealed that the oxygen evolution reaction (OER) in alkaline was significantly improved, and the fluoridedoped exhibits the most superior activity among the investigated catalysts under static and steady-state conditions. The OE onset potential as well as the potential at current density of 10 mA/cm 2 were shifted to a lower values in the order of Sr 2 CoO 4 (1.64, 1.73) > Sr 2 CoO 3 Br (1.61, 1.65) > Sr 2 CoO 3 Cl (1.53, 1.60) > Sr 2 CoO 3 F (1.50, 1.56) V vs. HRE. Moreover, Sr 2 CoO 3 F demonstrates remarkably less charge transfer resistance (R ct = 36.8 ohm) and long-term stability than the other oxyhalide counterparts during the OER. The doping of the perovskites with halide ions, particularly the fluoride-ion, enriches the oxygen vacancy defects due to electrons pulling away from the B-site of Co-atom that enhance the ionic and electronic conductivity as well as the electrochemical activity of the oxygen evolution reaction in alkaline solution. Generally, the strategy of halide anions doping at O-sites is an effective route to improve the electrochemical performance of perovskite-based oxides for oxygen evolution reaction.