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Article

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

by
Mohamed A. Ghanem
1,*,
Mabrook S. Amer
1,2,
Abdullah M. Al-Mayouf
1,2,
Prabhakarn Arunachalam
1 and
Mark T. Weller
3
1
Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia
2
K.A.CAR Energy Research and Innovation Center at Riyadh, King Saud University, Riyadh 11451, Saudi Arabia
3
Chemistry Department, Cardiff University, Cardiff CF10 3AT, UK
*
Author to whom correspondence should be addressed.
Catalysts 2021, 11(11), 1408; https://doi.org/10.3390/catal11111408
Submission received: 17 October 2021 / Revised: 15 November 2021 / Accepted: 16 November 2021 / Published: 20 November 2021
(This article belongs to the Special Issue Advancements in Electrochemical Energy Catalysts)
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Abstract

:
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.

1. 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 proton-coupled electron transfers (4OH− → 2H2O + 4e− + O2 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 (ABO3) 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 (ABO3) 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 ABO3 perovskite structure with anions such as Cl– [30,31], N3– [32], F– [33,34], or S2– [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 et al. reported a reversible oxygen electrocatalyst consisting of layered perovskite of LaSr3Fe3O10 [41] and Grimaud et al. fabricated double perovskite composed of cobalt-type (PrBaCo2O5+d) that exhibited even higher OER performance than the BSCF type [42]. Previously, our group reported the effect of chloride-doping of a layered iron oxy-chloride perovskite (Sr2FeO2Cl) 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 Sr2CoO3Cl and Sr3Co2O5Cl2. 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 (Sr2CoO4) electrocatalyst and the induced performance of the electrochemical activity towards OER in an alkaline solution compared to the parent oxide.

2. Results and Discussion

2.1. Structure and Morphology of the Sr2CoO3-xHx Catalysts

Figure 1 shows the XRD patterns of the crystal structures of Sr2CoO4, Sr2CoO3F, Sr2CoO3Cl, and Sr2CoO2Br 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 Sr6Co5O15 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 Sr2CoO4 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 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.
Table 1 SEM-EDX elemental composition analysis of the strontium cobalt oxyhalide catalysts in comparison to the parent oxide of Sr2CoO4.
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 revealing good agreement between the EDX elemental analysis and the molecular formula of Sr2CoO4, Sr2CoO3F, Sr2CoO3Cl, and Sr2CoO3Br, except for an impurity within 2.0 wt.% attributed to the unidentified strontium cobalt oxide phase as confirmed by X-ray analysis.
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 Sr2CoO3F, Sr2CoO3Cl, Sr2CoO3Br, and parent Sr2CoO4 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 O2− anion of the lattice oxygen and the peak in the middle is related to the surface O1− ions (peak 2), while the peak at higher binding energy (peak 3) is due to the chemically adsorbed oxygen species (Ochem) at the surface [47,48]. However, the O 1s spectrum of the parent Sr2CoO4 catalyst (Figure 4d) can only be fitted by two peaks that are assigned for O2− anion (peak 1) and O1−/Ochem (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 (O2−) reactivity and reduces the activation energy required for O2− activation [47,48,49]. Moreover the O 1s fitted peaks’ area ratio of [O1− /O2−] as well as the [(O1− + Ochem)/O2−] / O2−] were much higher in case of Sr2CoO3F (3.1, 13.20) than for Sr2CoO3Cl (2.9, 3.83), Sr2CoO3Br (1.28, 1.91) and the parent Sr2CoO4 (0.63, 0.63) catalysts, respectively. Therefore, the doping of halide ions particularly fluoride ion enhanced the O1− and Ochem site concentration that directly related to the density of the surface oxygen vacancy defects [48,49].

2.2. Electrocatalytic Activity of Cobalt Oxyhalide Catalysts in OER

The electrochemical behavior and activity of the produced strontium cobalt oxyhalide (Sr2CoO3-xHx) 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 Sr2CoO3-xHx (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 Sr2CoO3F electrode exhibits a significantly higher electrochemical Co(II)/Co(III) redox current than Sr2CoO4, Sr2CoO3Cl, and Sr2CoO3Br, suggesting that the Sr2CoO3F 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 (Sr2CoO3-xHx) compared to the parent Sr2CoO4 in 1.0 M NaOH electrolyte at a catalyst loading of 0.8 mg/cm2. 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 Sr2CoO4 (black line) catalyst. Table 3 reports the OER catalyst characteristic parameters of onset potential (the potential at 1.0 mA/cm2), the potential at a current density of 10 mA cm−2 (η10%) 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 Sr2CoO4 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) < Sr2CoO4 (1.64) V vs. HRE. Moreover, the (η10) values similarly decrease as Sr2CoO4 (1.73) > Sr2CoO3Br (1.67) > Sr2CoO3Cl (1.60) > Sr2CoO3F (1.56) V, indicating that Sr2CoO3F 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 (η10%) 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 Sr2CoO3F 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, the Tafel slope values decrease in the order of Sr2CoO4 (110 mV/dec) > Sr2CoO3Br (103 mV/dec) > Sr2CoO3Cl (98 mV/dec) > Sr2CoO3F (88 mV/dec), indicating that the OER kinetics are more favourable at the Sr2CoO3F electrode than Sr2CoO3Cl, Sr2CoO3Br and the pristine Sr2CoO4 catalyst.
Electrochemical impedance spectra (EIS) analysis was also employed to evaluate the charge carrier transfer resistance during the OER for the oxyhalide catalysts of Sr2CoO3F, Sr2CoO3Cl, and Sr2CoO3Br compared to pristine Sr2CoO4 catalyst. Figure 6 presents typical Nyquist plots of the impedance data obtained for the strontium cobalt oxyhalide (Sr2CoO4-xHx) compared to the pristine Sr2CoO4 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/Q1/Rct equivalent circuit illustrated in the inset of Figure 6, where the Rs is the solution resistance, Rct, the charge transfer resistance, and Q1 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 (Rct) on the EIS Nyquist plots of Sr2CoO3F (36.8 Ω) is significantly smaller compared to Sr2CoO3Cl (52.9 Ω), Sr2CoO3Br (128.5 Ω) and pristine Sr2CoO4 (296.1 Ω), catalysts, suggesting that the Sr2CoO3F catalyst possesses a remarkably lower charge transfer resistance (Rct) at the catalyst/electrolyte interface during the OER.
Furthermore, the values of the double-layer capacitance (Q1) indicate that the Sr2CoO3F 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 Sr2CoO3F 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 Sr2CoO4 catalyst can be attributed to the doping of Cl−, Br− and in particular the F− ions enhanced the O1− and Ochem sites concentration that improve the ionic and electronic conductivity as well as the electrochemical activity.
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 long-term 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 O2−/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 Sr2CoO3F is the most active and stable catalyst for OER applications among the studied strontium cobalt oxyhalide catalysts.

2.3. Electrochemical Performance of the Sr2CoO3F Catalyst

The Sr2CoO3F 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 Sr2CoO3F catalyst at different catalyst loadings. Moreover, Figure 8b illustrates the effect of the catalyst loading on the potential at 10 mA/cm2 (black line) and the OER current density achieved at 1.8 V vs. HRE (red line). From Figure 8b, the Sr2CoO3F catalyst loading around 0.8 mg/cm2 is optimal to achieve the best OER activity, that is, the smallest potential at 10 mA/cm2 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 Sr2CoO3F 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 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.
Finally, Figure 9 shows the chronoamperometry and chronopotentiometry at a predefined applied potential and current density during the prolonged OER at the Sr2CoO3F catalyst in 1.0 M NaOH. The Sr2CoO3F 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/cm2 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 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/cm2 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].

3. Materials and Methods

3.1. 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).

3.2. 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 cm2) 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 cm2) 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.

4. Conclusions

In summary, the effect of halide anions (F−, Cl−, and Br−) doping on the structure and the induced electrochemical activity of the Sr2CoO4 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 fluoride-doped 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/cm2 were shifted to a lower 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. Moreover, Sr2CoO3F demonstrates remarkably less charge transfer resistance (Rct = 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.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal11111408/s1, Figure S1: Comparative XPS survey spectra of Sr2CoO4, Sr2CoO3Br, Sr2CoO3Cl, and Sr2CoO3F catalysts, Fig. S2 linear sweep voltammetry (LSV) of Sr2CoO4, Sr2CoO3Br, Sr2CoO3Cl, and Sr2CoO3F electrodes at rotation speed of 1600 rpm, in N2 deaerated 1.0 M NaOH at a scan rate of 10 mV s−1 and catalyst loading of 0.8 mg/cm2, Table S1: Electrochemical activity comparison of strontium cobalt oxyhalides catalysts activities of OER onset overpotential and the overpotential required to achieve 10 mA/cm2 current density (η10 mA/cm2) with the benchmarking catalysts that reported in the literature.

Author Contributions

M.A.G. proposed the conceptualization plan, wrote the results and discussion, and submit the final manuscript; M.S.A. performed the experimental structure and electrochemical characterization and wrote the original draft; A.M.A.-M. supervise the whole project and revise the manuscript; P.A. carried out the data analysis and wrote the manuscript; M.T.W. prepared the catalysts, analyze and wrote the crystal structure. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Plan for Science, Technology, and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award Number, Award Number 13-ENE-1227-02.

Acknowledgments

This project was funded by the National Plan for Science, Technology, and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award Number, Award Number 13-ENE-1227-02.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 11 01408 g001 550
Figure 1. XRD patterns of strontium cobalt oxyhalide catalysts.
Figure 1. XRD patterns of strontium cobalt oxyhalide catalysts.
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Catalysts 11 01408 g002 550
Figure 2. SEM images of (a), (b) Sr2CoO3F, (c) Sr2CoO3Cl, and (d) Sr2CoO3Br catalysts.
Figure 2. SEM images of (a), (b) Sr2CoO3F, (c) Sr2CoO3Cl, and (d) Sr2CoO3Br catalysts.
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Catalysts 11 01408 g003 550
Figure 3. SEM-EDX composition map analysis of Sr2CoO3F catalyst.
Figure 3. SEM-EDX composition map analysis of Sr2CoO3F catalyst.
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Catalysts 11 01408 g004 550
Figure 4. The core-level of O 1s spectra of (a) Sr2CoO3F, (b) Sr2CoO3Cl, (c) Sr2CoO3Br and (d) parent Sr2CoO4 catalyst.
Figure 4. The core-level of O 1s spectra of (a) Sr2CoO3F, (b) Sr2CoO3Cl, (c) Sr2CoO3Br and (d) parent Sr2CoO4 catalyst.
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Catalysts 11 01408 g005 550
Figure 5. (a) cyclic voltammetry in the narrow potential window at 20 mV s−1 for the strontium cobalt oxy-halide (Sr2CoO3-xHx, H = F, Cl or Br and x = 1) catalysts in 1.0 M NaOH electrolyte and catalyst loading of 0.8 mg/cm2, (b) linear sweep voltammetry within OER region at 20 mV s−1 for the strontium cobalt oxyhalides catalysts in 1.0 M NaOH electrolyte and catalyst loading = 0.8 mg cm−2, and (c) Tafel slopes of the oxyhalides catalysts as extracted from LSV shown in Figure S2 (supporting Information) and performed at a scan rate of 10 mV s−1 and rotation rate of 1600 rpm.
Figure 5. (a) cyclic voltammetry in the narrow potential window at 20 mV s−1 for the strontium cobalt oxy-halide (Sr2CoO3-xHx, H = F, Cl or Br and x = 1) catalysts in 1.0 M NaOH electrolyte and catalyst loading of 0.8 mg/cm2, (b) linear sweep voltammetry within OER region at 20 mV s−1 for the strontium cobalt oxyhalides catalysts in 1.0 M NaOH electrolyte and catalyst loading = 0.8 mg cm−2, and (c) Tafel slopes of the oxyhalides catalysts as extracted from LSV shown in Figure S2 (supporting Information) and performed at a scan rate of 10 mV s−1 and rotation rate of 1600 rpm.
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Catalysts 11 01408 g006 550
Figure 6. EIS Nyquist diagram and equivalent circuit of the oxyhalides and parent Sr2CoO4 electrodes (loading of 0.8 mg/cm2) in 1.0 M NaOH solution at 1.55 V versus HRE and within the frequency range of 0.01 to 1.0 MHz with a 20 mV amplitude. The solid lines are the fitted curves and the symbols are representing the experimental data.
Figure 6. EIS Nyquist diagram and equivalent circuit of the oxyhalides and parent Sr2CoO4 electrodes (loading of 0.8 mg/cm2) in 1.0 M NaOH solution at 1.55 V versus HRE and within the frequency range of 0.01 to 1.0 MHz with a 20 mV amplitude. The solid lines are the fitted curves and the symbols are representing the experimental data.
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Catalysts 11 01408 g007 550
Figure 7. Chronoamperometric curves of the Sr2CoO4, Sr2CoO3F, Sr2CoO3Cl, and Sr2CoO3Br catalysts (0.8 mg/cm2) in 1.0 M NaOH (pH = 14) at constant potential of 1.65 V vs. HRE.
Figure 7. Chronoamperometric curves of the Sr2CoO4, Sr2CoO3F, Sr2CoO3Cl, and Sr2CoO3Br catalysts (0.8 mg/cm2) in 1.0 M NaOH (pH = 14) at constant potential of 1.65 V vs. HRE.
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Catalysts 11 01408 g008a 550Catalysts 11 01408 g008b 550
Figure 8. (a) LSV at 20 mVs−1 for the Sr2CoO3F catalyst in 1.0 M NaOH electrolyte with different catalyst loading, (b) plot for the relation of the catalyst loading against the potential at 10 mA/cm2 (black line) and the OER current density achieved at 1.8 V vs. HRE (red line), (c) the LSV at 20 mV s−1 of the Sr2CoO3F catalyst (loading of 0.8 mg/cm2) in different concentration of NaOH electrolyte, and (d) EIS Nyquist diagram and equivalent circuit for the Sr2CoO3F catalyst at the different applied potential within the frequency range of 0.01 to 1.0 MHz and a 20 mV amplitude and in 1.0 M NaOH solution.
Figure 8. (a) LSV at 20 mVs−1 for the Sr2CoO3F catalyst in 1.0 M NaOH electrolyte with different catalyst loading, (b) plot for the relation of the catalyst loading against the potential at 10 mA/cm2 (black line) and the OER current density achieved at 1.8 V vs. HRE (red line), (c) the LSV at 20 mV s−1 of the Sr2CoO3F catalyst (loading of 0.8 mg/cm2) in different concentration of NaOH electrolyte, and (d) EIS Nyquist diagram and equivalent circuit for the Sr2CoO3F catalyst at the different applied potential within the frequency range of 0.01 to 1.0 MHz and a 20 mV amplitude and in 1.0 M NaOH solution.
Catalysts 11 01408 g008aCatalysts 11 01408 g008b
Catalysts 11 01408 g009 550
Figure 9. (a) chronoamperometry at different applied potential and (b) chronopotentiometry at a different applied current density of the Sr2CoO3F catalyst (loading 0.8 mg/cm2) in 1.0 M NaOH electrolyte during prolonged water oxidation.
Figure 9. (a) chronoamperometry at different applied potential and (b) chronopotentiometry at a different applied current density of the Sr2CoO3F catalyst (loading 0.8 mg/cm2) in 1.0 M NaOH electrolyte during prolonged water oxidation.
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Table
Table 1. The EDX elemental composition analysis of the strontium cobalt oxyhalide catalysts and the parent oxide of Sr2CoO4.
Table 1. The EDX elemental composition analysis of the strontium cobalt oxyhalide catalysts and the parent oxide of Sr2CoO4.
Catalyst Sr wt. % ± 2.0%Co wt. % ± 2.0%O wt. % ± 2.0%H wt. % ± 2.0%
Sr2CoO459.06 19.64 21.30--
Sr2CoO3F58.18 19.56 15.936.33
Sr2CoO3Cl55.15 18.55 15.1311.17
Sr2CoO3Br49.63 16.70 13.6020.07
Table
Table 2. Analysis values of the deconvoluted O 1s peaks of the oxyhalide catalysts.
Table 2. Analysis values of the deconvoluted O 1s peaks of the oxyhalide catalysts.
Catalyst Peak BE (O2−)/eVPeak 1 Area %Peak 2 (O1−)/eVPeak Area %Peak 3 (Ochem)/eVPeak 3 Area %[O1−/ O2−][O1− + Ochem/O2−]
Sr2CoO4532.9061.4534.238.6----0.630.63
Sr2CoO3Br530.3434.3533.4543.9535.0621.81.281.91
Sr2CoO3Cl530.7420.7533.1460.1534.7219.22.903.83
Sr2CoO3F 529.966.7531.1420.8532.0172.53.1013.20
Table
Table 3. The oxygen evolution onset potential and overpotential at a current density of 10 mA/cm2 (η10%) and Tafel slope of the strontium cobalt oxyhalides catalysts.
Table 3. The oxygen evolution onset potential and overpotential at a current density of 10 mA/cm2 (η10%) and Tafel slope of the strontium cobalt oxyhalides catalysts.
CatalystOnset Potential ± 0.005 vs. RHE/ VOnset Potential ± 0.005 vs. RHE/ V (RDE = 1600 rpm)Potential at 10 mA/cm2Potential at 10 mA/cm2 (RDE = 1600 rpm)Tafel slope, mV/dec (at 1.0 mV s−1 Using GCE)
Sr2CoO3F1.501.451.561.5688
Sr2CoO3Cl1.531.491.601.5798
Sr2CoO3Br1.611.511.651.60103
Sr2CoO41.641.551.731.65110
Table
Table 4. Impedance parameter values derived from the fitting of the equivalent circuit for the impedance spectra of the oxyhalides catalysts recorded in 1.0 M NaOH solution at 1.55 V vs. HRE and within the frequency range of 0.01 to 1.0 MHz with a 20 mV amplitude.
Table 4. Impedance parameter values derived from the fitting of the equivalent circuit for the impedance spectra of the oxyhalides catalysts recorded in 1.0 M NaOH solution at 1.55 V vs. HRE and within the frequency range of 0.01 to 1.0 MHz with a 20 mV amplitude.
CatalystRs (ohm)Q1 (mF)R1 (ohm)
Sr2CoO44.1821.680296.1
Sr2CoO3Br4.3721.890128.5
Sr2CoO3Cl4.1722.31052.9
Sr2CoO3F4.2338.30036.8
Table
Table 5. Impedance parameter values derived from the fitting of the equivalent circuit for the impedance spectra of Sr2CoO3F at different applied potential recorded in 1.0 M NaOH solution.
Table 5. Impedance parameter values derived from the fitting of the equivalent circuit for the impedance spectra of Sr2CoO3F at different applied potential recorded in 1.0 M NaOH solution.
Potential (V) vs. HRERs (ohm)Q1 (mF)R1 (ohm)
1.554.2338.30036.8
1.574.2238.60015.9
1.584.1341.75010.40
1.604.1941.7606.30
1.62 4.1641.5104.34
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Ghanem, M.A.; Amer, M.S.; Al-Mayouf, A.M.; Arunachalam, P.; Weller, M.T. Halide-Doping Effect of Strontium Cobalt Oxide Electrocatalyst and the Induced Activity for Oxygen Evolution in an Alkaline Solution. Catalysts 2021, 11, 1408. https://doi.org/10.3390/catal11111408

AMA Style

Ghanem MA, Amer MS, Al-Mayouf AM, Arunachalam P, Weller MT. Halide-Doping Effect of Strontium Cobalt Oxide Electrocatalyst and the Induced Activity for Oxygen Evolution in an Alkaline Solution. Catalysts. 2021; 11(11):1408. https://doi.org/10.3390/catal11111408

Chicago/Turabian Style

Ghanem, Mohamed A., Mabrook S. Amer, Abdullah M. Al-Mayouf, Prabhakarn Arunachalam, and Mark T. Weller. 2021. "Halide-Doping Effect of Strontium Cobalt Oxide Electrocatalyst and the Induced Activity for Oxygen Evolution in an Alkaline Solution" Catalysts 11, no. 11: 1408. https://doi.org/10.3390/catal11111408

APA Style

Ghanem, M. A., Amer, M. S., Al-Mayouf, A. M., Arunachalam, P., & Weller, M. T. (2021). Halide-Doping Effect of Strontium Cobalt Oxide Electrocatalyst and the Induced Activity for Oxygen Evolution in an Alkaline Solution. Catalysts, 11(11), 1408. https://doi.org/10.3390/catal11111408

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