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Article Cite This: ACS Omega 2018, 3, 1710−1717

Cobalt-Doped Ba2In2O5 Brownmillerites: An Efficient Electrocatalyst for Oxygen Reduction in Alkaline Medium Chamundi P. Jijil,†,‡ Indrajit M. Patil,§ Bhalchandra Kakade,*,§ and R. Nandini Devi*,†,‡ †

Catalysis and Inorganic Chemistry Division, CSIR-National Chemical Laboratory, Pune 411008, India Academy of Scientific and Innovative Research, Ghaziabad 201002, India § SRM Research Institute, SRM Institute of Science and Technology, Kattankulathur, Chennai 603203, India ‡

S Supporting Information *

ABSTRACT: A series of compounds with cobalt doping in the indium site of Ba2In2O5 brownmillerites exhibited excellent oxygen reduction activity under alkaline conditions. Doping (25%) retains the brownmillerite structure with disorder in the O3 site in the two-dimensional alternate layer along the ab plane. Further substitution of cobalt in the indium site leads to the loss of a brownmillerite structure, and the compound attains a perovskite structure. Cobalt-doped samples exhibited far better oxygen reduction reaction (ORR) activity when compared to the parent Ba2In2O5 brownmillerite. Among the series of compounds, BaIn0.25Co0.75O3−δ with the highest Co doping and oxygen vacancies randomly distributed in the lattice exhibited the best ORR activity. BaIn0.25Co0.75O3−δ showed a 40 mV positive shift in the onset potential with better limiting current density and a nearly four-electron-transfer reduction pathway when compared to the parent Ba2In2O5 brownmillerite.

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INTRODUCTION Owing to the increased energy demand and high levels of pollution from the conventional fuels, an alternative energy source with minimal environmental impact is the need of the hour. Fuel cell technology is considered as one of the promising options with environmentally benign byproducts.1−3 An oxygen reduction reaction (ORR) in the cathode of low-temperature fuel cells has gained considerable interest in the past few decades because of its importance in the efficiency of the fuel cell.4 The ORR is, in general, a sluggish electrochemical reaction and requires precious metals as electrocatalysts to enhance activity and durability.5−7 The state-of-the-art electrocatalyst for the ORR in fuel cells is based on platinum (Pt). However, a widespread use of Pt is limited because of its high cost and scarcity.8−10 The ORR under alkaline conditions is more facile than under acidic conditions; moreover, a wide range of non-noble metal-based materials have been shown as potential candidates toward electrocatalysis under such conditions.11,12 Metal oxides involving perovskites, brownmillerites, spinels, pyrochlores, and so forth; heteroatom-doped carbon nanomorphologies; nitrogen-coordinated metals on carbon matrixes; and so forth are studied as ORR catalysts.13−20 Because of the low cost, high stability under strong alkaline conditions, high corrosion resistance, and facile synthesis procedure, perovskite-type oxides are investigated extensively in the past few years as electrocatalysts for the ORR.21 Transition-metal atoms in these oxides are believed to be the active center for the ORR. Perovskite-type oxides are also reported to exhibit enhanced catalytic activity toward the ORR © 2018 American Chemical Society

under cathodic polarization because of the formation of oxygen vacancy. Brownmillerites, A2B2O5, a class of compounds structurally similar to perovskites and having an inherent oxygen vacancy in their structure can be envisaged as potential electrocatalysts for the ORR. These compounds have oxygen vacancies ordered in the two-dimensional layer in the lattice.22 Doping “B” site with transition metals exhibiting redox properties can be envisaged to improve the ORR activity of the compounds. Moreover, doping will also lead to the random distribution of oxygen vacancy along the lattice, which will again add to the efficiency of the catalyst. In the present study, cobalt doping in the indium site of Ba2In2O5 brownmillerite and its effect on the structure as well as ORR activity under alkaline medium is investigated. The parent Ba2In2O5 is a thoroughly studied mixed ionic and electronic-conducting material showing excellent ionic conductivity at higher temperature where the orthorhombic structure of brownmillerite transforms to cubic with the oxygen vacancies disordered in the lattice.23 Cobalt (Co) is chosen as a dopant in this study mainly because of its ability not only to enhance conductivity and redox property but also to induce oxygen vacancy disorder in the brownmillerite lattice.24,25 Also, the incorporation of such a redox center in the lattice will make the catalyst tolerant to local changes in the oxygen concentration owing to the structural stability of brownmillerite and may result in an effective ORR active Received: October 27, 2017 Accepted: January 24, 2018 Published: February 9, 2018 1710

DOI: 10.1021/acsomega.7b01655 ACS Omega 2018, 3, 1710−1717

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Figure 1. (a) PXRD pattern of Ba2In2−xCoxO5−δ (x = 0.5, 1, and 1.5) and (b) enlarged portion of PXRD patterns of Co-doped Ba2In2O5 brownmillerite. The shoulder peaks are due to Kα2 radiations.

electrocatalyst. It was in fact observed that Co doping in Ba2In2O5 improved the ORR activity of the samples.

Table 1. Structural Parameters Obtained from the Rietveld Refinement of Ba2In2−xCoxO5−δ (x = 0.5, 1, and 1.5)

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Ba2In1.5Co0.5O5−δa

RESULTS AND DISCUSSION Ba2In2−xCoxO5−δ series of compounds was synthesized by a solid-state reaction, and the phase purity of the compounds was determined by powder X-ray diffraction (PXRD). Figure 1a represents the PXRD patterns of all the synthesized compounds. The PXRD pattern of Ba2In1.5Co0.5O5−δ reveals that the compound crystallizes in a tetragonal system, whereas further doping in the “B” site of Ba2In2O5 with cobalt leads to the formation of a cubic perovskite structure. Further, a closer observation into the PXRD patterns of the Co-doped Ba2In2O5 system reveals that with increase in the Co concentration, there is a gradual shift in the peaks toward higher 2θ as shown in Figure 1b, which indicates a decrease in the lattice parameters with higher Co doping. This can be attributed to the lower crystal radii of cobalt than that of indium. The detailed structural parameters of the synthesized materials were determined by Rietveld refinement of the PXRD patterns using GSAS−EXPGUI software. The structural parameters obtained after refinement of cobalt-doped Ba2In2O5 compounds are tabulated in Table 1. The Rietveld refinement plot of the compounds is shown in Figure S1 (Supporting Information). Rietveld refinement of the PXRD pattern of Ba2 In1.5Co0.5O5−δ was carried out using the structural parameters of hydrated Ba2In2O5 in P4/mmm space group (JCPDS file no. 01-089-9079).26 The In/Co ratio in the “B” site was manually assigned as 75 and 25% of the occupancy, respectively, and the refinement with these parameters proceeded smoothly. However, refinement of the patterns of systems with higher concentrations of Co with the same model was not fruitful. Hence, a model of cubic perovskite, BaZrO3 (JCPDS file no. 01-074-1299), was selected because the PXRD pattern of this model matches with that of Ba2In2−xCoxO5−δ (x = 1 and 1.5) with a small shift in 2θ values.27 Rietveld refinement with these parameters proceeded smoothly, and both BaIn0.5Co0.5O3−δ and BaIn0.25Co0.75O3−δ fitted to the cubic Pm3̅m space group. The polyhedral representations of the compounds are shown in Figure 2. In Ba2In1.5Co0.5O5−δ, the brownmillerite structure of hydrated Ba2In2O5 is retained. There is a disorder in the O3 site in the two-dimensional alternate layer along the ab plane evident from the two equivalent positions of the O3 site. Further substitution of cobalt in the indium site of Ba2In2O5,

Χ2 wRp (%) Rp (%) space group A (Å) B (Å) C (Å) O1 x y z occup O2 x y z occup O3 x y z occup

BaIn0.5Co0.5O3−δb

BaIn0.25Co0.75O3−δc

4.37 6.79 4.75 P4/mmm

4.19 5.92 4.33 Pm3̅m

3.75 4.92 3.18 Pm3̅m

4.22532(2) 4.22532(2) 8.46218(15) 0.5 0.5 0.2661 (12) 0.8687(18) 0 0.5 0.5 0.7715 (24) 0.3272 (32) 0 0 0.4032(42)

4.16337(2) 4.16337(2) 4.16337(2) 0.5 0 0 0.8176(36)

4.12006(1) 4.12006(1) 4.12006(1) 0.5 0 0 0.8187(22)

a

Ba2In1.5Co0.5O5−δ: Ba(0,0,0.277), In1(0.5,0.5,0.5), In2(0.5,0.5,0), Co1(0.5,0.5,0.5), Co2(0.5,0.5,0). bBaIn0.5Co0.5O3−δ: Ba(0.5,0.5,0.5), In(0,0,0), Co(0,0,0). cBaIn0.25Co0.75O3−δ: Ba(0.5,0.5,0.5), In(0,0,0), Co(0,0,0).

viz., BaIn0.5Co0.5O3−δ and BaIn0.25Co0.75O3−δ, leads to the loss of the brownmillerite structure, and the compound attains a perovskite structure with the B-site cation (In/Co) surrounded by six oxygen atoms in an octahedral geometry as shown in Figure 2b,c. The oxygen vacancies in these compounds are randomly distributed throughout the lattice. The lattice parameters of Co-doped Ba2In2O5 with variation in the Co concentration are depicted in Figure 3. For comparison with other cubic systems, the pseudocubic lattice parameter of tetragonal Ba2In1.5Co0.5O5−δ was calculated (see the Supporting Information for details). It is evident from the plot that cell lengths and cell volumes decrease with increase in Co concentration. This can be attributed to the smaller crystal radius of Co (0.72 and 0.68 Å for tetrahedral and octahedral coordination, respectively) when compared to that of In (0.76 and 0.94 Å for tetrahedral and octahedral coordination, respectively).28 1711


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Figure 2. Polyhedral representations of the compounds based on the Rietveld refinement parameters for (a) Ba2In1.5Co0.5O5−δ, (b) BaIn0.5Co0.5O3−δ, and (c) BaIn0.25Co0.75O3−δ.

X-ray photoelectron spectroscopy (XPS) measurements were carried out to study the oxidation state of metals. Figure 4 shows the XPS spectra obtained over the range of 772.5−800 eV. In this region, the Co 2p3/2 and Ba 3d5/2 peaks overlap around 780 eV and Co 2p1/2 and Ba 3d3/2 peaks overlap around 796 eV; however, individual peaks could be extracted through deconvolution.29 The Ba2+ peak lies at slightly higher binding energy when compared to Co ions. The spin−orbit splitting of Co 2p3/2 and Co 2p1/2 in all the compounds was found to be around 15.3 eV, indicating Co in the +3 oxidation state. Also, the absence of satellite features around 786 eV discards the chances of Co2+ on the surface.30 Further, the Brunauer−Emmett−Teller (BET) surface area of all the samples was measured using N2 adsorption study. It was observed that the surface area of the sample was in between

Figure 3. Cell length and volume of Ba2In(2−x)CoxO5−δ as a function of substitution of x.

Figure 4. XPS spectra of Ba 3d and Co 2p in (a) Ba2In1.5Co0.5O5−δ, (b) BaIn0.5Co0.5O3−δ, and (c) BaIn0.25Co0.75O3−δ. Data are represented in black circles, fitted spectra in red, deconvoluted peaks of Co and Ba in brown and pink, respectively, and the baseline in blue. 1712


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Figure 5. (a) Cyclic voltammograms of Co-doped Ba2In2O5 compounds under N2 (dotted)- and O2 (solid)-saturated solutions at a sweep rate of 20 mV s−1; (b) comparative linear sweep voltammograms (LSVs) for Ba2In2O5 and Co-doped Ba2In2O5 obtained at the rotating speed of 1600 rpm at a sweep rate of 5 mV s−1; (c) K−L plot of the ORR on Co-doped Ba2In2O5 at a constant potential of −0.49 V in O2-saturated solution; and (d) dependence of the electron-transfer number of Co-doped Ba2In2O5 on varying potentials; the experiments were done in 0.1 M KOH solution using Hg/HgO and Pt wire as the reference and counter electrodes, respectively.

0.1 and 0.6 m2 g−1 as expected from the high-temperature solidstate synthesis method employed for the preparation of the samples. The BET surface area of the samples is tabulated in Table S1 (Supporting Information). It can be observed that the surface area gradually increases with increase in the Co concentration, where various factors such as transition in the structure from tetragonal to cubic as well as intermediate grinding during the synthesis can attribute to this. To get an insight into the conductivity of the samples, electrochemical impedance spectroscopy (EIS) measurements using the pellets made from these oxide materials were performed at 300 °C under ambient atmosphere. The analysis of the ac impedance scans obtained from such materials can be done on the basis of the Bauerle model31 as mentioned by Jankovic et al. in similar compounds.32 Table S2 (in the Supporting Information) shows the total resistance of the Codoped Ba2In2O5 samples. It can be observed that the resistance decreases with increase in the Co concentration, indicating better conductivity of the samples with a higher concentration of Co. The enhancement in the conductivity is due to the enhanced amount of Co in the lattice, which also leads to the transition of the structure from tetragonal to cubic, which in fact facilitates the conductivity through the lattice. The improved conductivity in the samples will favor the transfer of charges from the catalyst to oxygen during the ORR.33 The detailed electrochemical studies on the series of compounds for the ORR activity were carried out in alkaline medium using cyclic voltammetry (CV) and rotating disk electrode (RDE) methods. All the samples were mixed with Vulcan XC-72 carbon to improve the electrical conductivity of the compounds. The contribution from carbon toward ORRs in

the catalyst composite is negligible, as evident from our previous studies in the brownmillerite−carbon composite.15 At first, the cyclic voltammogram of all the samples was recorded in N2-saturated 0.1 M KOH at a sweep rate of 20 mV s−1 and then under O2 saturation at the same sweep rate. Figure 5a represents the voltammograms of all the samples under both N2- and O2-saturated solutions. No characteristic peaks are observed under N2-saturated solution, whereas a clear reduction peak can be seen in all the samples under O2saturated solution. An increase in the reduction current after purging O2 in the solution indicates the ORR activity of the samples. A detailed investigation on the ORR activity of the samples was performed by the RDE method. Figure 5b represents the LSVs of Co-doped Ba2In2O5. It is evident from the figure that there is an improvement in the ORR activity with increase in the Co concentration. The onset potential of Co-doped samples is more positive than that of the parent Ba2In2O5, which was observed to be around −0.12 V versus Hg/HgO.16 Ba2In1.5Co0.5O5−δ exhibited an onset potential of −0.11 V which is 10 mV more positive than that of the parent Ba2In2O5. However, BaIn0.5Co0.5O3−δ and BaIn0.25Co0.75O3−δ exhibited onset potentials of −0.09 and −0.08 V, respectively, which are 30 and 40 mV higher than that of the parent Ba2In2O5. Also, it is evident from the figure that with increase in the Co concentration, the limiting current density increases, which can be attributed to the ability of cobalt to enhance the conductivity of the catalyst. The improved ORR activity of BaIn0.5Co0.5O3−δ and BaIn0.25Co0.75O3−δ when compared to that of the parent brownmillerite and with increase in the cobalt concentration can be attributed to the presence of catalytically active Co 1713


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Figure 6. (a) RRDE polarization curves for disk and ring currents and (b) corresponding percentage of the peroxide yield and the number of electron transfer derived from the RRDE voltammogram for BaIn0.25Co0.75O3−δ in the presence of O2-saturated 0.1 M KOH at a sweep rate of 10 mV s−1.

atoms in the “B” site. It is well-known that the transition metals in such oxides are responsible for ORR activity. Interestingly, Co doping in Ba2In2O5 gives a better voltammogram with a distinct mass transport region; these types of behaviors are not visible in many other brownmillerites and other oxide systems.16,34−36 This observation can be correlated to the studies carried out by Suntivich et al. where the authors have observed that an eg filling of 1 in the Co3+ ion in LaCoO3 enhances the ORR activity. Similar configuration can also be expected in our case because Co is mainly in the Co3+ state. Too little eg filling leads to strong B−O2 bonding, whereas too high eg filling leads to very weak B−O2 bonding; both the situations are not desirable for good ORR activity. However, moderate eg filling will lead to an ideal state where the bond strength between the B-site cation and O2 will be neither too strong nor too weak, leading to maximum activity as evident from Sabatier’s principle.13 LSVs of all the samples at different electrode rotation rates in O2-saturated 0.1 M KOH solution are represented in Figure S2 (Supporting Information). It is clear from the figure that the limiting current density increases with increase in the rotation rate of the electrode; this can be attributed to the enhanced rate of mass transport at a higher electrode rotation rate.37 Additional information such as number of electrons (n) involved in the ORR was determined by the Koutecky−Levich (K−L) equation38

concentration. In fact, at this potential, the number of electrons involved for the ORR on Ba2In1.5Co0.5O5−δ was found to be ∼3, whereas those for BaIn0.5Co0.5O3−δ was ∼3.5 and for BaIn0.25Co0.75O3−δ was ∼3.9. The number of electrons (4 or 2) involved in the ORR mechanism decides whether the reaction proceeds through a four-electron direct hydroxide pathway or two-electron peroxide intermediate pathway. Direct reduction of oxygen to hydroxide via the four-electron pathway is more efficient and desirable in the ORR.39 Hence, BaIn0.25Co0.75O3−δ with an electron-transfer number ∼4 and with more positive onset potential and the highest limiting current density exhibits better ORR activity among the series of compounds. The K−L plot of all the samples at different potentials is shown in Figure S3 (Supporting Information). The number of electrons involved in the ORR mechanism for all the samples at different potentials was calculated from the K−L equation. Figure 5d demonstrates the dependence of the applied potential on the electron-transfer number in these samples. It is clear from the figure that the number of electrons involved in the ORR for BaIn0.25Co0.75O3−δ is in between 3.7 and 4, indicating that the ORR in this system mainly proceeds via the four-electron mechanism involving direct hydroxyl formation. However, on the other hand, it is observed that the number of electrons involved in the ORR on Ba2In1.5Co0.5O5−δ is between 3 and 3.3. A value around 3 can be attributed to the simultaneous reaction involving both two-electron peroxide intermediate pathway and four-electron direct pathway.40 It is worthy to note that the ORR on BaIn0.5Co0.5O3−δ (n = ∼3.5) also proceeds through both two- and four-electron pathways, but four-electron direct reduction is more predominant. In addition to this, rotating ring-disk electrode (RRDE) experiments were performed on BaIn0.25Co0.75O3−δ to get a better insight into the mechanistic aspect of the ORR. Figure 6a represents the disk and ring current obtained in the RRDE measurements. A negligible ring current compared to the disk current gives a rough idea about the formation of inferior intermediate peroxide (HO2−) during the ORR process. Further, the number of electrons (n) involved in the reaction pathway and the percentage of peroxide generated were calculated from the RRDE data using the below equations:

1 1 1 = + j nFkCO2 0.62nFCO2DO2 2/3ν−1/6ω1/2

where “j” is the disk electrode current density, “k” is the reaction rate constant, “n” is the number of electrons exchanged per O2 molecule, “F” is the Faraday constant (96 500 C mol−1), “CO2” is the bulk oxygen concentration (1.2 × 10−6 mol L−1), “DO2” is the diffusion coefficient of molecular oxygen in 0.1 mol L−1 KOH solution (1.9 × 10−5 cm2 s−1), “ν” is the kinematic viscosity of the electrolyte (0.01 cm2 s−1), and “ω” is the electrode rotation speed in radians per second (=2πf = 2πrpm/ 60). A plot of the inverse of current (i−1) as a function of the inverse of the square root of the rotation rate (ω−1/2), which is known as the K−L plot, is a useful method to analyze the ORR kinetic parameters of an electrocatalyst. A comparison of the K−L plot of the various samples at −0.49 V is shown in Figure 5c. It is evident from the figure that the slope of the K−L plot changes along the series. This suggests that the number of electrons involved in the ORR in the series changes with Co

n=4×

ID ID + Ir /N

% HO2− = 200 × 1714

Ir / N ID + Ir /N DOI: 10.1021/acsomega.7b01655 ACS Omega 2018, 3, 1710−1717

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ACS Omega where ID is the disk current, Ir is the ring current, and N = 0.38 is the current collection efficiency of the Pt ring; the collection efficiency (N) was calculated by using a simple reversible couple of a ferrocyanide/ferricyanide system41 as reported in our previous studies.42 As evident from Figure 6b, RRDE results clearly show less than 12% of the peroxide yield that gives a direct value of the number of electrons involved in the ORR which ranges between 3.75 and 3.99 over the entire range of a potential window. The stability of the BaIn0.25Co0.75O3−δ catalyst was studied by a chronoamperometric stability (current vs time) test in alkaline media. As shown in Figure S4a in the Supporting Information, the BaIn0.25Co0.75O3−δ catalyst exhibits better current response (only 10% loss) after 600 s; however, under similar conditions, a commercial 40 wt % Pt/C catalyst reveals more than 50% degradation. Further, the inset of Figure S4a demonstrates an almost 30 mV negative shift in its half-wave potential after 2000 durable cycles, indicating poor stability under alkaline conditions. On the other hand, a methanol crossover test has been carried out in 1 M methanol in 0.1 M KOH solution for 600 s. Importantly, in methanol oxidation reaction (MOR) there is no considerable change in relative current with respect to time even after the addition of methanol in 0.1 M KOH solution for the BaIn0.25Co0.75O3−δ catalyst, whereas the Pt/C catalyst exhibits major changes in relative current values as evident from Figure S4b in the Supporting Information. Thus, the BaIn0.25Co0.75O3−δ catalyst shows better selectivity toward ORR kinetics under alkaline media. From these observations, it is clear that the incorporation of cobalt in the indium site of Ba2In2O5 improves the ORR activity of the sample. The ORR activity of Ba2In1.5Co0.5O5−δ is considerably enhanced from that of the parent Ba2In2O5, although both the systems have oxygen vacancies ordered in the alternate layers. Hence, the increase in the activity is solely attributed to the incorporation of active Co sites in the brownmillerite lattice. Further incorporation of Co, viz., BaIn0.5Co0.5O3−δ and BaIn0.25Co0.75O3−δ, results in the loss of a brownmillerite structure and formation of a cubic perovskite structure where the oxygen vacancies are randomly distributed in the lattice. This composition with the highest Co concentration exhibits the best ORR activity among the series. The enhancement in the ORR activity is due to the enhanced concentration of cobalt, but the role of random distribution of vacancies which will help in the O2 adsorption cannot be ruled out. The role of oxygen vacancy in the ORR is already reported in the literature and correlated to the ease of O2 adsorption and oxide ion conduction.16,43 The enhanced surface area and improved conductivity will also contribute to the better ORR activity of samples with a higher concentration of Co. Further, the ORR activity of these samples can be enhanced by heteroatom doping in the anionic site as observed in our previous studies on anion doping in a Ca2Fe2O5 system.44 The ORR activity of BaIn0.25Co0.75O3−δ is comparable with that of other perovskite-type oxides reported in the literature. The ORR performance of some of the perovskite catalysts is given in Table S3 (Supporting Information). On comparing with the state-of-the-art 40 wt % Pt/C catalysts, the onset potential of BaIn0.25Co0.75O3−δ is found to be 136 mV more negative (Figure S5, Supporting Information). However, high cost, scarcity, and huge instability45 of the Pt-based catalyst for the ORR in alkaline medium make them a poor choice in an alkaline fuel cell. Hence, cost-effectiveness, improved stability, and scope of fine-tuning these materials by different doping

strategies make them potential candidates for future fuel cell technology.

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CONCLUSIONS In summary, Ba2In2O5 brownmillerites with Co doped progressively in the In site were synthesized and tested for ORR activity in alkaline medium. Ba2In1.5Co0.5O5−δ, with the least concentration of Co, retained the brownmillerite structure with disorder in the O3 site in the two-dimensional alternate layer along the ab plane. However, higher concentrations of Co in the site of Ba 2 In 2 O 5 , viz., BaIn 0.5 Co 0.5 O 3−δ and BaIn0.25Co0.75O3−δ, lead to the loss of the brownmillerite structure, and the compound attains the perovskite structure with oxygen vacancy distributed randomly in the lattice. Both the cell length and cell volume gradually decreased with increase in Co concentration owing to the smaller crystal radius of Co when compared to that of In. XPS studies revealed that Co exists in the +3 oxidation state in the sample. Interestingly, Co-doped samples exhibited a far better ORR activity when compared to the parent Ba2In2O5 brownmillerite. They also exhibited a well-defined LSV which was not observed in the parent Ba2In2O5 system. The enhanced activity of Co-doped samples is attributed to the presence of catalytically active Co in the lattice. Among the doped systems with increase in the Co concentration, the ORR activity increases, which can be correlated to both the Co concentration and the random distribution of oxygen vacancy in the higher Co-doped systems. Also, the role of the improved surface area and conductivity with increase in the Co concentration can be attributed to the enhancement in the ORR activity in higher Co-doped samples. Moreover, the Co concentrations are found to have an impact on the reaction pathway. The parent brownmillerite is known to exhibit a two-electron reaction pathway, which proceeds through a peroxide intermediate. However, for Ba2In1.5Co0.5O5−δ, the number of electrons involved in the ORR was found to be ∼3, indicating that both two-electron peroxide intermediate pathway and four-electron direct pathway occur simultaneously. However, with increasing Co concentration, the four-electron direct pathway predominates with an electron-transfer number of ∼3.5 and ∼3.85 for BaIn0.5Co0.5O3−δ and BaIn0.25Co0.75O3−δ, respectively. This was further confirmed by the RRDE measurements, which reveal that the number of electrons involved in the reaction during the ORR for the entire potential window was between 3.75 and 3.99 with a maximum peroxide yield of 12%. Also, the samples were much more stable than the commercial Pt/C. The composition BaIn0.25Co0.75O3−δ with the highest Co concentration exhibited the best ORR activity among the series.

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EXPERIMENTAL SECTION

All the samples with composition Ba2In2−xCoxO5−δ (x = 0.5, 1, and 1.5) were synthesized by the solid-state method. Highpurity BaCO3 (99.98%, Sigma-Aldrich), In2O3 (99.99%, SigmaAldrich), and CoCO3 (Co 43−47%, Sigma-Aldrich) were used as the starting materials. Stoichiometric amounts of the starting materials were weighed and ball-milled at 300 rpm for 3 h in a Fritsch Pulverisette 6 Planetary Mill for homogeneous mixing. The samples were then calcined at 900 °C for 12 h. Subsequently, they were ground in a mortar and again calcined at 1300 °C for 18 h at a slow heating rate with intermediate grinding. The phase purities of all the samples were determined by PXRD in a PANalytical X’Pert Pro dual goniometer 1715


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ACS Omega diffractometer with Ni-filtered Cu Kα at 40 kV and 30 mA and an X’celerator solid-state detector with a step size of 0.008 and time per step 45.72 s. The diffraction pattern was obtained at room temperature in Bragg−Brentano geometry. Lattice parameters of the samples were calculated by the Rietveld refinement method in the PXRD pattern using GSAS− EXPGUI program.46 The oxidation state of cobalt in the samples was studied using XPS measurements carried out on a VG Micro Tech ESCA 3000 instrument at a pressure of >1 × 10−9 Torr with a pass energy of 50 eV, an electron takeoff angle of 60°, and an overall resolution of ∼0.1 eV. All binding energies were referenced to the C 1s peak (284.8 eV) arising from adventitious carbon. The surface areas of the samples were determined by N2 adsorption at the temperature of liquid nitrogen using the Autosorb iQ Quantachrome system. The samples were degassed at 300 °C under vacuum for 6 h prior to the analysis to remove the adsorbed moisture on the catalyst surface. The specific surface area was calculated using the BET model at a relative pressure of P/P0 = 0.05−0.3. The EIS measurements were performed using a CHI604E electrochemical analyzer (CH Instruments, Inc., USA). For making pellets for the measurement, 1 g of the synthesized oxide material was weighed and pressed in 13 mm pellet die. The ac impedance spectroscopy was performed over a frequency range of 100 mHz to 1 MHz with an amplitude of 50 mV. The electrochemical properties of the catalyst were measured by CV and LSV techniques using a CHI760E electrochemical workstation (CH Instruments, Inc., USA) in a conventional three-electrode test cell with Hg/HgO and Pt wire as the reference and counter electrodes, respectively, at room temperature. The catalyst was prepared by ball milling (300 rpm for 90 min) with a mixture of the sample and Vulcan XC72 carbon in the ratio of 4:1. A slurry of the catalyst was prepared by ultrasonically mixing 5.0 mg of the sample carbon composite in 960 μL of isopropanol−water (3:1) solution and 40 μL of 5 wt % Nafion solution for 30 min to get a homogeneous suspension. For preparing the working electrode for CV and RDE measurements, a glassy carbon (GC) electrode was first polished using 0.05 μm of alumina, and then the electrode was cleaned using Milli-Q water. Catalytic slurry (4 μL) was loaded onto the surface of the GC electrode of 3 mm diameter (0.0707 cm2 geometrical area). The slurry was allowed to dry slowly at room temperature to obtain a thin homogeneous catalyst film.47 The final catalyst loading on the electrode surface (geometrical area = 0.0707 cm2) was 283 μg cm−2. An aqueous solution of 0.1 M KOH (Aldrich, ≥85%) was used as the electrolyte for normal CV and RDE studies. Kinetics of ORRs of the catalyst were studied by using RDE in 0.1 M KOH using a three-electrode cell assembly at a scan rate of 5 mV s−1 at 400, 900, 1200, 1600, 2000, and 2500 rpm at room temperature. The yield of peroxide produced during the ORR and the number of electrons transferred per oxygen molecule were calculated by RRDE measurements at 1600 rpm in O2-saturated 0.1 M KOH solution.

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chronoamperometric stability, durability, and MOR stability test plot; comparative LSV with commercial Pt/C; BET surface area and EIS data of Co-doped Ba2In2O5 samples; comparative activity table of some perovskite-based catalysts; and calculation of the pseudocubic cell parameter of Ba2In1.5Co0.5O5−δ (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.K.). *E-mail: [email protected] (R.N.D.). ORCID

R. Nandini Devi: 0000-0002-6219-8089 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.P.J. thanks CSIR, India, for financial support. B.K. acknowledges the Department of Science and Technology Science and Engineering Research Board (DST-SERB; no. SB/FT/CS-120/ 2012) for instrumental facility. Authors thank Dama Srikanth for helping in surface area analysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01655. Rietveld refinement plot; LSVs under different electrode rotation rates; K−L plot under various potentials; 1716


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