Nitrogen Doping in Oxygen-Deficient Ca - ACS Publications

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N Doping in Oxygen Deficient Ca2Fe2O5: A Strategy for Efficient Oxygen Reduction Oxide Catalysts Chamundi P Jijil, Moorthi Lokanathan, Sundaresan Chithiravel, Chandrani Nayak, Dibyendu Bhattacharyya, Shambhu Nath Jha, Peram Delli Babu, Bhalchandra Kakade, and R. Nandini Devi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11718 • Publication Date (Web): 24 Nov 2016 Downloaded from http://pubs.acs.org on November 24, 2016

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ACS Applied Materials & Interfaces

N Doping in Oxygen Deficient Ca2Fe2O5: A Strategy for Efficient Oxygen Reduction Oxide Catalysts Chamundi P Jijila§, Moorthi Lokanathanb, Sundaresan Chithiravelc§, Chandrani Nayakd, Dibyendu Bhattacharyyad, Shambhu Nath Jhad, P.D. Babue, Bhalchandra Kakade*b and R. Nandini Devi*a§ a

Catalysis and Inorganic Chemistry Division, CSIR-National Chemical Laboratory, Pune, India.

b

SRM Research Institute, SRM University, Chennai, 603203, India

c

Polymer and Advanced Materials Laboratory, CSIR-National Chemical Laboratory, Pune,

India. d

Atomic & Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai – 400 085,

India. e

UGC-DAE Consortium for Scientific Research, R-5 Shed, Bhabha Atomic Research Centre,

Mumbai - 400 085, India. §

Academy of Scientific and Innovative Research, New Delhi, India.Information.

ACS Paragon Plus Environment

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KEYWORDS: fuel cell, oxygen reduction reaction, brownmillerite, N-doping, Neutron diffraction, Rietveld refinement, EXAFS

ABSTRACT: Oxygen reduction reaction is increasingly being studied in oxide systems due to advantages ranging from cost effectiveness to desirable kinetics. Oxygen deficient oxides like brownmillerites are known to enhance ORR activity by providing oxygen adsorption sites. In parallel, N and Fe doping in C materials and consequent presence of catalytically active complex species like C-Fe-N is also suggested to be good strategies for designing ORR active catalysts. A combination of these features in a N doped Fe containing brownmillerite can be envisaged to present synergistic effects to improve the activity. This is conceptualized in this report through enhanced activity of N doped Ca2Fe2O5 brownmillerite when compared to its oxide parents. N doping is proved using neutron diffraction, UV-vis spectroscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Electrical conductivity is also found to be enhanced by N doping which influences the activity. Electrochemical characterizations using cyclic voltammetry, rotating disc electrode and rotating ring disk electrode indicates an improved oxygen reduction activity in N-doped brownmillerite with a 10 mV positive shift in the onset potential. RRDE measurements show that both the compound exhibit 4-electron reduction pathways with lower H2O2 production in N-doped system also N-doped sample exhibited better stability. The observations will enable better designing of ORR catalysts which are stable and cost effective.

INTRODUCTION One of the major drawbacks of electrochemical energy conversion devices such as fuel cells and metal air batteries is the slow kinetics of oxygen reduction reaction (ORR) at the cathode.1-4


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The state-of-the-art catalysts involve Pt and Pt based alloys which show high electrocatalytic activity towards ORR.5-7 However, the prohibitive cost and scarcity of such noble metals is deterrent for a widespread commercialization of these technologies. Among the various fuel cells, alkaline fuel cell (AFC) has the highest electrical efficiency and offers an alternative for reducing the cost by replacing Pt with noble metal free catalysts.8,9 Carbon nanotubes-supported metal particles,10,11 graphitic carbon nitride/carbon composite,12 N/B/P/S doped carbon nanotubes (CNTs) and graphenes13-15 and metal oxides including perovskites and brownmillerites16-18 are among the potential alternatives proposed as ORR catalysts. In metal oxides, especially in stoichiometric perovskites, the catalytic activity is attributed mostly to the B-site transition metal cation.19 The effect of d electrons of the transition metal ions in perovskites towards the electrocatalytic activity in oxygen evolution reaction was initially reported by Otagawa et al.20 The electrocatalytic properties were related to the number of delectrons and the occupancy of the antibonding orbitals of M-OH. Recently, Suntivich et al has observed M shaped relation between the number of d electrons and ORR activity.16 Further, perovskite type oxides are reported to exhibit enhanced catalytic activity towards ORR under cathodic polarization due to the formation of oxygen vacancy.21 Very recently, we demonstrated an interesting correlation of B-site symmetry and oxygen vacancy concentrations to ORR activity in brownmillerite class of compounds. The ORR activity in Ce doped Ba2In2O5 was investigated under alkaline condition and it was observed that more symmetric B-site environment along with oxygen vacancy enhances the catalytic activity.22 Among non-oxide systems, N doped C and related materials have shown immense potential as cost effective alternatives of Pt.23-25 These systems exhibit excellent ORR activity owing to their unique electronic properties resulting from the conjugation between the nitrogen lone pair electrons and


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graphene π system.26 A detailed understanding of the active sites and the role of nitrogen atom is yet to be achieved; however, the generally accepted mechanism is that a net positive charge is created on the carbon atoms adjacent to nitrogen in the matrix, which readily attracts electrons from the anode and also facilitates the O2 adsorption in the ORR process.23 Further development has shown that N and transition metal (Fe & Co) co-doped carbon morphologies exhibit better ORR activity under both acidic and alkaline condition.27-29 Among the transition metals, Fe containing systems are widely studied. Very recently Zitolo et al has demonstrated that the active sites in Fe-N-C systems are the porphyrinic FeN4C12 moieties. Further it was observed that the ORR activity can be enhanced on pyrolysis under ammonia because of the incorporation of highly basic N-group in Fe-N-C structure.30 In general it is observed that Fe-N-C site is active for ORR and enhances the activity of the catalyst.31,32 A very recent study reveals that CoO films synthesized under N2 atmosphere exhibit higher ORR activity than their N-free counterparts. Even though N doping is not unambiguously proved, the enhancement in the activity has been attributed to the Co-Nx active sites which may be present in the catalyst.33 Hence, it can be anticipated that catalyst systems incorporating the advantageous features of oxides with substantial oxygen vacancies and transition metal-N interaction in a single stable structure will have desirable ORR activity. Herein, we report the incorporation of Fe-N sites in stable and oxygen deficient brownmillerite along with their ORR studies in alkaline conditions. Iron based brownmillerite, Ca2Fe2O5 was selected and nitrogen doping was achieved by purging ammonia gas through the pristine sample at high temperatures. The N doped brownmillerite exhibited enhanced ORR activity compared to the corresponding oxide. To the best of our knowledge, nitrogen doped perovskites or brownmillerites are not yet reported for


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ORR activity and this observation offers a better path for development of noble metal free, highly active oxide electrocatalysts. EXPERIMENTAL SECTION Ca2Fe2O5 (CFO) was synthesized by solid state method. High purity CaCO3 (≥99%, SigmaAldrich) and Fe2O3 (99.98%, Sigma-Aldrich) were used as the starting materials. Stoichiometric amounts of CaCO3 and Fe2O3 were mixed and calcined at 500 ˚C for 2 h followed by calcination at 950 ˚C for 16 h. For nitrogen doping, the samples were heated under ammonia gas flow at 800 ˚C for 6 h. Ammonia gas was synthesized in-situ by drop wise addition of NaOH (1 M) to NH4Cl (4 M) solution and the gas was carried by a flow of N2 (20 mL/min) into the tubular furnace. The gas was passed through CaO just before entering the furnace in order to trap the H2O present in the mixture. Hereafter nitrogen doped Ca2Fe2O5 is mentioned as CFO-N. The phase purity of the synthesized sample was determined by powder X-ray diffraction (PXRD) in PANalytical X’Pert Pro dual goniometer diffractometer with Ni filtered Cu-Kα at 40 kV and 30 mA and 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. Neutron diffraction experiments were carried out at room temperature with the help of Focusing Crystal Diffractometer at Dhruva reactor using a wavelength of 1.48 Å. The scattered neutrons in this diffractometer were detected using an array of four 3He linear position sensitive detectors covering a range of 6˚−120˚. Lattice parameters from the neutron diffraction data were calculated by Rietveld refinement method by using GSAS-EXPGUI programme.34 UV-vis spectra of the samples in reflectance mode were recorded using Cary 5000 UV-vis-NIR spectrophotometer in a solid sample holder. Oxidation state of the elements and the presence of nitrogen in the doped sample were studied using X-ray Photoelectron Spectroscopic (XPS) measurements carried out


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on a VG Micro Tech ESCA 3000 instrument at a pressure of > 1 x 10-9 Torr with a pass energy of 50 eV, electron take off angle 60˚ and the overall resolution was ~ 0.1 eV. The experiments were carried out using Al Kα x-ray source (1486.6 eV). XPS profile of Fe 2p was fitted using XPSPEAK41 software. All binding energies were referenced to the C 1s peak (284.8 eV) arising from adventitious carbon. X ray absorption spectroscopy (XAS) measurements of the samples at Fe K-edge were carried out in transmission mode at the scanning extended X-ray absorption fine structure (EXAFS) Beamline (BL-9) at the Indus-2 Synchrotron Source (2.5 GeV, 100 mA) at the Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India.35,36 The analysis of the EXAFS data has been carried out using the standard procedure37,38 using the IFEFFIT software package39. The current-voltage (I-V) measurements were carried out in Keithley Semiconductor Characterization System 4200-SCS instrument. For making the substrate for I-V curve measurement, 10 mg of the sample was taken in 1 ml isopropyl alcohol and sonicated for 10 min. 200 µL of the solution was drop casted on an inter digitated Au micro electrodes and it was dried at 60 ˚C for 30 min. The current in a potential range of -100 to +100 V was measured under ambient condition in a 2 probe setup. The electrochemical properties of the catalysts were measured by cyclic voltammetry (CV) and rotating disk electrode (RDE) assembly using a CHI604E electrochemical analyser (CH Instruments, Inc., USA); and rotating ring disk electrode (RRDE) assembly using CHI760E bipotentiostat (CH Instruments, Inc., USA) in a conventional three-electrode test cell with Hg/HgO and platinum wire as the reference and counter electrodes, respectively under room temperature. All the potentials were converted to reversible hydrogen electrode (RHE) by VRHE = VHg/HgO + 0.098 + (0.059 x pH of electrolyte solution).40 The catalyst was prepared by ballmilling (300 rpm for 90 min) a mixture of sample and Vulcan XC-72 carbon in the ratio of 4:1.


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Slurry of catalyst was prepared by ultrasonically mixing 5 mg of the sample-carbon composite in 960 µL of isopropanol - water (3:1) solution and 40 µL of 5 wt % Nafion solution for 45 min in order to get a homogeneous suspension. For preparing the working electrode for CV, RDE and RRDE measurements, glassy carbon (GC) electrode was first polished using 0.05 µm alumina and then the electrode was cleaned using Milli-Q water. 4 µL of the catalytic slurry was loaded onto the surface of the GC electrode using a micropipette. The slurry was allowed to dry overnight under ethanol atmosphere at room temperature to obtain a thin homogeneous catalyst film. The final catalyst loading on the electrode surface was 283 µgcm-2 which was calculated based on the geometrical area of the electrode. An aqueous solution of 0.1 M KOH (Aldrich, ≥ 85%) was used as the electrolyte for normal CV and RDE studies. The number of electrons involved in the reaction and the yield of peroxide was calculated by RRDE measurement at 1600 rpm inO2 saturated 0.1 M KOH solution. The disk electrode was scanned at a rate of 5 mV s-1 and the ring potential was kept at 1.4 V vs. RHE. RESULTS AND DISCUSSION Pure Ca2Fe2O5 (CFO) was synthesized by solid state method and its phase purity was confirmed using PXRD. Nitrogen doping in CFO was achieved by passing ammonia gas through the synthesized sample at 800 ˚C for 6 h. The PXRD patterns of CFO is shown in Figure 1 and it matches with already reported pattern of Ca2Fe2O5 (JCPDS file No. 01-071-2264).41 The compound crystallizes in orthorhombic structure. PXRD patterns of nitrogen doped sample (Figure 1) matches with its parent brownmillerite indicating that the crystal structure is retained after nitrogen doping. Neutron diffraction data of CFO-N was collected to get a better insight into the position of oxide and nitride ions in the compound. Rietveld refinements on the neutron diffraction data were


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Figure 1. PXRD patterns of Ca2Fe2O5 (CFO) and its N doped counterpart (CFO-N). Blue line represents the expected peak position of Ca2Fe2O5. performed with already reported crystallographic data of CFO. Three different models were tried, in the first model all the anionic sites were occupied by oxygen, in the second all the anionic sites were exclusively assigned to nitrogen and in the third both oxygen (90% of the occupancy) and nitrogen (10% of the occupancy) were assigned for each anionic site in the beginning.42 The occupancy of each site was individually refined during the refinement process with a constraint that the total occupancy of each anionic site does not exceed 1. Of the three models used for the refinement, the third one with both oxygen and nitrogen in the anionic site produced the best fit with a χ2 of 7.454 when compared to oxygen only (χ2 of 10.01) and nitrogen only (χ2 of 14.8) models. This arrives at a substitution of 7 atom% of O with N. Figure S1 (Supporting Information) represents the Rietveld refinement plot of CFO-N. The detailed results obtained from the Rietveld refinement is given in Table 1. The polyhedral representation of CFO-N is shown in Figure S2 (Supporting Information). CFO-N, like its parent CFO, crystallizes in orthorhombic brownmillerite structure with alternate layers of FeO6 and FeO4 polyhedra. The


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Table 1. Rietveld refinement parameters of neutron diffraction data of CFO-N Fractional Coordinate Atom

Occ. X

Y

Uiso

Z

Ca

0.4789(20)

0.1092(5)

0.0288(16)

1

0.0154(12)

Fe

0

0

0

1

0.0143(14)

Fe

-0.0561(12)

0.25

-0.0679(12)

1

0.0154(15)

O1

0.2702(20)

-0.0167(4)

0.2257(16)

0.994(35)

0.0179(13)

O2

0.0268(13)

0.1422(4)

0.0721(11)

0.915(34)

0.0259(14)

O3

0.6032(22)

0.25

-0.1286(20)

0.822(48)

0.0120(16)

N1

0.2702(20)

-0.0167(4)

0.2257(16)

0.006(35)

0.0179(13)

N2

0.0268(13)

0.1422(4)

0.0721(11)

0.085(35)

0.0259(14)

N3

0.6032(22)

0.25

-0.1286(20)

0.178(48)

0.0120(16)

a = 5.4224(3), b = 14.7896(6), c = 5.5941(3); Space group = Pnma; χ2 =5.496, wRp = 4.8, Rp = 3.7

Figure 2. Ball and stick model of N-doped Ca2Fe2O5 obtained from the Rietveld refinement data. “V” represents possible positions of inherent vacancies available in the tetrahedral layer of the brownmillerite in the ac plane. oxygen vacancies in these compounds are orderly distributed in the two dimensional FeO4 tetrahedral layer in the ac plane. From the occupancy parameters, it is clear that O3, the apex


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oxygen of FeO4 tetrahedra is more labile for N substitution, when compared to octahedral oxygens. This interesting observation also points to the fact that N is in close proximity to the oxygen vacancies which in fact occupy the uncoordinated sites of the FeO4 tetrahedra. Hence a catalytic complex VO-Fe-N can be envisaged in this novel material (Figure 2).

Figure 3. Tauc plot of Ca2Fe2O5 (CFO) and its N doped counter-part (CFO-N). It was further observed that the color of the samples changed after nitrogen doping (Supporting Information Figure S3). This type of change in color is reported in nitrogen doped oxides as due to modulations in band gaps.43 Band gap of these materials were calculated from Tauc plot (Figure 3) which evidences a slight reduction in the band gap of CFO-N when compared to its parent brownmillerite. CFO-N has a band gap of 2.147 eV, which is 0.077 eV lesser than the parent CFO. It is known that in N doped oxides, N-2p orbitals present impurity acceptor states above the valence bands consisting of O-2p orbitals consequently reducing the band gap.44 Hence the reduction in band gap reveals N doping in CFO. XPS measurements were carried out to further confirm the presence of nitrogen in the doped samples. Figure S4 (Supporting Information) represents the XPS survey spectra of CFO and CFO-N. Very broad peaks spanning from 395 to 402 eV are observed indicating nitrogen in multiple bonding sites with varying chemical environments (Supporting Information Figure


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S5).45 Also a small contribution from the molecularly adsorbed N2 (~400 eV and ~402 eV) cannot be avoided since reactions were done under N2 atmosphere.46 The signal to noise ratio of the samples were very poor which may be because of low loading of nitrogen in the lattice. This is expected since we employed NH3 diluted with N2 for the synthesis.

Figure 4. Fe 2p XP spectra of Ca2Fe2O5 (CFO) and its N doped counterpart (CFO-N). Deconvolution of Fe 2p3/2 was performed considering Gupta and Sen model. Data in black circles, fitted spectra in red, deconvoluted peaks in blue, cyan, pink and brown. However, better insights can be obtained from Fe 2p XP spectra. Figure 4 displays the XP spectra of CFO and CFO-N. The Fe 2p3/2 peak for CFO and CFO-N was observed at 711.6 and 711.3 eV respectively. The spin orbit splitting of Fe 2p component for CFO and CFO-N was found to be 13.3 and 13.2 eV respectively. Also the satellite peak appeared around 8 eV above the binding energy of Fe 2p3/2 indicating the presence of Fe in 3+ oxidation state.47 The XPS data of early transition metals with high spin unpaired electrons show multiplet structure for the 2p spectra due to spin-orbital and electrostatic interactions.48,49 Based on the Gupta-Sen model and studies conducted by Grosvenor et al., the high spin Fe3+ species can be fitted to four main


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peaks.50-52 We have followed the same procedure and the Fe 2p specta obtained could be successfully fitted into four multiples, indicating the presence of Fe in 3+ oxidation state in the compounds. A closer look into the spectra reveals that there is a slight shift of around 0.3 eV towards lower binding energy of the Fe 2p peak in nitrogen doped samples. This can be attributed to the fact that when N replaces O in the FeOx geometry, owing to the lower electronegativity of N than O, there is a red shift of the Fe 2p peaks.53

Figure 5. Normalized Fe K-edge XANES spectra from CFO and CFO-N. Inset represents the enlarged portion of pre-edge region. Further XAS studies were performed to study the incorporation of nitrogen in brownmillerite lattice. Figure 5 represents the X-ray absorption near edge structure (XANES) spectra measured at Fe K-edge for CFO and CFO-N. The spectra obtained for Ca2Fe2O5 was similar to that reported in the literature.54 The absorption edges of Fe in the CFO and CFO-N samples lie close to that of the Fe2O3 standard around 7113.5 eV (Supporting Information Figure S6). Hence it can be concluded that the oxidation state of Fe in CFO and CFO-N are same as that of Fe2O3 standard i.e. in +3 oxidation state.55 The Fe K-edge XANES spectra of the CFO and CFO-N samples have similar features. Therefore, it can be inferred that no appreciable change occurs in


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the CFO lattice due to N doping. But a closer look into the pre-edge region of XANES (Figure 5 inset) shows a small shift in the peaks. Since the pre-edge is quite sensitive to the coordination number and orbital geometry, these change may be because of the perturbation in the covalent mixing of 3d orbital with ligand valance orbital.56 Figure S7 (Supporting Information) shows the experimental EXAFS (µ(E) vs. E) spectra of CFO and CFO-N samples measured at Fe K-edge. The detailed step involved in the fitting of the experimental data is given in Supporting Information.

Figure 6. Fourier transformed EXAFS spectra of CFO and CFO-N at the Fe K-edge (scatter points) and theoretical fit (solid line). The experimental χ(r) vs. r plots of CFO and CFO-N samples at Fe K-edge have been fitted with theoretically simulated χ(r) vs. r plots generated assuming the brownmillerite structure of CFO. The structural parameters for the CFO are obtained from the reported crystal data. There are two nonequivalent Fe sites present in a unit cell represented as Fe1 and Fe2 in the present text. Following the above structure, the experimental data have been fitted assuming three Fe-O shells at 1.84(4×) Å, 1.96(4×) Å, 2.12(2×) Å and two Fe-Ca shells at 3.06(6×) Å and 3.31(4×) Å.


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Table 2. Local Structural Parameters for CFO and CFO-N calculated by EXAFS measurements at Fe K-Edge. Path

Theoretical

R Factor

CFO

CFO-N

0.0014

0.0023

r (Å)

N

r (Å)

N

σ2

r (Å)

N

σ2

Fe1-O1

1.84

4

1.88(1)

4

0.001(1)

1.85(1)

4

0.001(1)

Fe2-O1

1.96

4

2.05(1)

4

0.006(2)

2.02(1)

4

0.003(1)

Fe2-O2

2.12

2

2.31(2)

2

0.005(2)

2.30(1)

2

0.003(2)

Due to the limitation in number of fitting parameters set by Nyquist criterion, Fe-N paths are not explicitly considered during fitting of N doped sample, instead the changes in the Fe-O bond distances are studied carefully to predict the effect of doping. Figure 6 shows the experimental χ(r) vs. r plots of the CFO and CFO-N samples at Fe-K edge along with the best fit theoretical plots and the fitting results have been tabulated in Table 2. From the EXAFS results it can be seen that the Fe1-O1( tetragonal Fe site) and Fe2-O1 (Octahedral Fe site) bond lengths are slightly lower in CFO-N sample than that in CFO sample which might be a result of nitrogen doping. EXAFS and DFT studies have been carried out on N doped TiO2 systems by Ceotto et al57 and Sahoo et al58 to predict the location of N dopant. It has been found that the interstitial doping of N in TiO2 lattice increases both the equatorial and axial Ti-O bond distances while, substitutional doping does not change the bond distances significantly. In our case we have not observed any expansion of equatorial and axial bonds or any extra peak that may correspond to the Fe-N bond. Therefore, the chances of N occupying interstitial positions may be excluded and the EXAFS results indicate that N substitutes O in the lattice.


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Thus a string of results comprising the neutron diffraction data, the change in the color of the N-doped samples along with the band gap reduction observed from the Tauc plot, XPS and XAS data enable us to conclude that the nitrogen doping in the brownmillerite was successful and brings forth the existence of active Fe-Nx sites in the samples. Further the current-voltage measurements on the parent and N-doped samples were performed to understand the effect of doping on the electrical properties of the samples (Supporting Information Figure S8). The conductivity of CFO-N was found to be 3.07 x 10-8 S cm-1, which is 100 times higher than that of the parent brownmillerite. The electrocatalytic activity of CFO and CFO-N for ORR was thoroughly studied in alkaline medium using CV, RDE and RRDE methods. To improve the conductivity, the samples were mixed with Vulcan XC-72. Our previous study has revealed that the effect of Vulcan XC-72 carbon towards the ORR activity in brownmillerite-carbon composite was negligible.18 The cyclic voltammogram of both the samples were recorded in N2 and O2 saturated 0.1 M KOH solution at a sweep rate of 50 mV s-1. Figure 7(a&b) represents the cyclic voltammogram of CFO and CFO-N; both the samples exhibited a nearly rectangular CV curve when the electrolyte was saturated with N2 as evident from the voltammogram. Interestingly, a distinct reduction peak ~ 0.65 V in O2 saturated electrolyte was observed for both the samples. An increase in the reduction current after O2 saturation indicates the ORR activity of both the samples. A reduction hump ~0.4 V could perhaps be due to redox reactions like Fe+3/+2 from the electrocatalyst. Also it is worth to note that a capacitive behaviour is observed in CV profile, which may be due to anion-based intercalation pseudocapacitance as a result of presence of anion vacancy in the lattice.59


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Figure 7. Cyclic voltammograms of (a) CFO & (b) CFO-N in O2 and N2 saturated solution at a sweep rate of 50 mVs-1; (c) RRDE voltammograms recorded on the disk and ring electrodes at an electrode rotating speed of 1600 rpm in O2 saturated solution at a sweep rate of 5 mVs-1; (d) Electron transfer number and the percentage of peroxide generated at different potentials calculated from RRDE data. All the experiments were performed in 0.1 M KOH solution. A more detailed investigation on the ORR activity of the samples was performed by RDE and RRDE method. Figure S9 (Supporting Information) represents the LSV’s of parent and N-doped CFO samples recorded at various electrode rotation rates in O2 saturated 0.1 M KOH solution in an RDE electrode. It is evident from the figure that the current density increases with increase in the rate of rotation. This can be attributed to enhanced rate of mass transport at higher electrode rotation. Figure 7c represents the voltammograms obtained on the disk and ring electrodes in O2


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saturated electrolyte at 1600 rpm in RRDE setup. The disk current reflects the ORR activity of the sample. A positive shift of 10 mV in the onset potential of CFO-N was observed when compared to the parent CFO having onset potential at 0.70V Vs RHE also the cathodic current for CFO-N was higher than the parent CFO throughout the potential window indicating better electrocatalytic activity of N-doped CFO. The increased activity of N-doped samples is likely due to the presence of Fe-Nx active sites and improved conductivity in these samples which favors the transfer of charges from the catalyst to oxygen.60 Further it can be observed that a current plateau characteristic for diffusion controlled region is not well defined in both the samples. This can be attributed to the fact that ORR is under a mixed kinetic-diffusion control over the whole potential range. Similar observations are previously reported on some of the nonPt systems including Fe containing oxide systems.27,61-63 The lack of well defined mass transport region in Fe containing perovskite system can be correlated to the study conducted by Suntivich et al.16 The authors have correlated the intrinsic ORR activity of perovskites (ABO3) of first row transition elements (B) as a function of the eg-filling of B site cations. They observed a volcano shape for the ORR activity as a function of the eg-filling of B site cations; the most active ORR ions (Mn, Ni & Co) are placed near the peak of the volcano. Ease of adsorption and desorption of O2 on B, through B-O2 bond strengths, is connected to eg - filling. Fe containing perovskites are placed at the base of the volcano plot (with eg > 1.5) and hence has poorer adsorptiondesorption capability. We propose this as the reason for poor diffusion-controlled behavior of most of the Fe containing oxides manifesting as lack of well defined voltammograms. From Figure 7c it is evident that the ring current, which is indicative of the yield of peroxide species formed during the reaction, is slightly more for the parent CFO brownmillerite.


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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 equation = 4 ×

(1) +

% = 200 ×

+

(2)

Where ID is disk current, Ir is ring current and N = 0.38 is current collection efficiency of Pt ring; the collection efficiency (N) was calculated by using simple reversible couple of ferrocyanide/ferricyanide system64 as reported in our previous studies (A brief description is given in Supporting Information).65 As evident from Figure 7d, it can be observed that for CFO, the number of electrons involved in the reaction is around 3.9 and that for CFO-N is around 3.95. This suggests that in both the compounds, ORR proceeds mainly through a four electron pathway. Further it can be observed that the peroxide yield is lesser for the N-doped sample when compared to the parent CFO brownmillerite which has a maximum of 8 % peroxide relative to the total product. Finally the stability of both the samples was evaluated by potentially cycling the catalyst at a scan rate of 50 mVs-1 in 0.1 M KOH for 5000 cycles. Figure S10 (Supporting Information) represents the LSV before and after the stability test. It is evident from the figure that in both cases there is a decrease in the cathodic current after 5000 cycles. The decrease in the activity can be attributed to the carbon corrosion and probably the weakening of carbon-oxide interface during potential cycling.66-68 But it can be observed that in the case of CFO, a drop of 30 mV in the onset potential is observed whereas in the case of CFO-N the onset potential is same indicating better stability for CFO-N.


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To understand the effect of structural parameters, if any, on the electrochemical activity, similar tests were conducted on N doped Fe2O3 synthesized by the same procedure. Fe2O3 is a simple binary oxide with each Fe atoms bonded to six oxygen atom and forming an octahedral geometry without any oxygen vacancies. Figure S11 (Supporting Information) represents the LSV of CFO-N and Fe2O3-N. Detailed studies on the reaction kinetics showed that the ORR mechanism proceeds initially by a 2 electron pathway and later by a 4 electron pathway. This variation in behavior of simple Fe2O3 compared to brownmillerites indicates a contribution from the structural parameters like oxygen vacancies on the surface which may act as oxygen absorption sites facilitating catalysis. On comparing with the state-of-the art 40 wt% Pt/C catalysts, the onset potential of CFO-N is 185 mV more negative (Figure S12, Supporting Information). However very recent study conducted by Anicet et al. has shown huge instability of the Pt based catalyst for ORR in alkaline medium.69 Cost effectiveness, high stability and scope of further bringing down the difference in the over potential of brownmillerite and Pt/C by doping in cationic site of oxides make these types of N-doped brownmillerites a potential candidate for future fuel cell technology. Nevertheless the main scope of this study was to design a strategy to improve the ORR activity of brownmillerite based oxide materials by nitrogen doping. It is known from literature that on doping, N ions substitute oxide ions or exist in interstitial sites. From the neutron dif-fraction experiments along with XPS and XAS in the present system, the presence of N in the lattice is confirmed. The presence of catalytically active Fe-Nx site along with the presence of O2 adsorption sites in the form of inherent oxygen vacancies influences the ORR activity. Further modification in the brownmillerite by B site doping with ORR active transition metals will lead to the disorder in the oxygen vacancy and will also bring positive shift


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in the overpotential. This type of shift in the overpotential on B-site doping is observed in our previous studies on Ce doped Ba2In2O5 system.22 Hence the current work can be envisaged as a potential strategy for improving the ORR activity of brownmillerite and other perovskite based oxide systems for future fuel cell technology. CONCLUSION In summary, ORR activity of a brownmillerite Ca2Fe2O5 is compared with its N doped counterpart. Oxides were synthesized by solid state method and further N-doping was achieved by passing ammonia through them. N doping could be discerned by visual comparison consequent to color changes in the sample. Rietveld refinement of neutron diffraction data with different models shows the presence of nitrogen in the structure as evident from the lowest χ2 obtained for the model containing both O and N in anionic site. O3, coordinated to FeO4 tetrahedra is more labile for N substitution. It is known that oxygen vacancies in CFO are ordered along the ac plane in the unsaturated coordination sites of the tetrahedra. Hence, creation of catalytic complexes of structure VO-Fe-N is unambiguously proved. XPS shows broad peaks for N1s but a closer look into the Fe 2p XP spectra revealed a slight reduction in the binding energy of Fe in the doped compounds, attributed to the lower electronegativity of N in the proximity of Fe. XAS studies further confirm the presence of N in the lattice. An enhancement in the electrical conductivity of N-doped samples was also observed. N-doped CFO-N exhibited enhanced ORR activity than the parent CFO. CFO-N exhibited 10 mV positive shift in the onset potential and higher cathodic current than the parent CFO. The enhancement in the activity is assigned to the presence of nitrogen in the proximity of Fe species forming a Fe-Nx active center and also due to the higher electrical conductivity of CFO-N which favors the transfer of charges


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from catalyst to oxygen. Here, we emphasize a new strategy to potentially enhance the electrocatalytic effect of brownmillerite oxides towards ORR.

ASSOCIATED CONTENT Supporting Information. Rietveld refinement plot, Polyhedral representation, photographic image, XPS survey spectra, N 1s XP spectra, Fe K-edge XANES spectra, Fe K-edge EXAFS spectra, current-potential profile, LSV’s under different rotation and stability results of CFO and its N-doped counterparts. Comparative LSV’s of CFO-N & Fe2O3 and CFO-N & 40 wt% Pt/C. The following files are available free of charge.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT C.P.J. thanks CSIR, India, for financial support. B.K. acknowledges Department of Science and Technology Science and Engineering Research Board (DST-SERB; No. SB/ FT/CS-120/2012) for instrumental facility.


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Nitrogen Doping in Oxygen-Deficient Ca - ACS Publications

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