The Effect of Ni Doping on the Performance and Electronic Structure of

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The Effect of Ni Doping on the Performance and Electronic Structure of LSCF Cathodes Used for IT-SOFCs Alessandro Longo, Leonarda Francesca Liotta, Dipanjan Banerjee, Valeria La Parola, Fabrizio Puleo, Chiara Cavallari, Christoph J. Sahle, Marco Moretti Sala, and Antonino Martorana J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07626 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 24, 2017

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The Journal of Physical Chemistry

The Effect of Ni Doping on the Performance and Electronic Structure of LSCF Cathodes Used for IT-SOFCs Alessandro Longo[a,b]*, Leonarda F. Liotta[b]*, Dipanjan Banerjee[c], Valeria La Parola[b], Fabrizio [b]

[d]

[d]

[d]

Puleo , Chiara Cavallari , Christoph J. Sahle , Marco Moretti Sala [a]

and Antonino Martorana

[e]

Netherlands Organization for Scientific Research (NWO), Dutch-Belgian Beamline, ESRF - The

European Synchrotron, CS40220, 38043, 71 Avenue des Martyrs, 38000 Grenoble, France. [b]

Istituto per lo Studio dei Materiali Nanostrutturati (ISMN)-CNR, UOS Palermo, Via Ugo La Malfa,

153, 90146 Palermo, Italy. [c]

Dutch-Belgian Beamline (DUBBLE), ESRF – The European Synchrotron, CS40220, 38043

Grenoble Cedex 9, France. [d]

European Synchrotron Radiation Facility, The European Synchrotron, 71 Avenue des Martyrs,

38000 Grenoble, France. [e]

Dipartimento di Fisica e Chimica, Università di Palermo, Viale delle Scienze Ed. 17, 90128 Palermo,

Italy.

Abstract: We investigated the effect of nickel doping on the electronic structure and performance of nanostructured La0.6Sr0.4Co0.2Fe0.8-0.03Ni0.03O3-δ prepared by the one pot sol-gel method. The commercial undoped La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF0.8) was used as reference. Moreover, for comparison, Ni (3 mol%) was deposited by wetness impregnation over the La0.6Sr0.4Co0.2Fe0.8O3-δ. We show by in-situ X-ray absorption spectroscopy (XAS) at 900°C under air flow that nickel enters the B perovskite site of the material and favors the stabilization of the cobalt oxidation state, as evidenced by the delay in the decrease of the average Co valence with respect to undoped samples. Our results are further supported by in-situ X-ray Raman spectroscopy (XRS) that allowed us to monitor the temperature evolution of the O K-edge. XRS evidences that nickel-doped LSCF shows unmodified O2p-TM3d density of states, which proves that the Co oxidation state is preserved. Electrochemical impedance spectroscopy (EIS) measurements were carried out over half-cell systems consisting

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of LSCF-based materials deposited onto a Ce0.8Gd0.2O2-δ electrolyte. The improvement

of

the

electrochemical

performances

of

the

Ni-doped

La0.6Sr0.4Co0.2Fe0.8-0.03Ni0.03O3-δ sample with respect to a reference Ni-impregnated LSCF is attributed to the stabilization of the TM-O6 structural units, which were recently proposed as the functional units for oxygen reduction.

Introduction

The research on solid oxide fuel cells (SOFCs) is presently focused on the design of new materials to overcome the high-temperature regimes. The achievement of operative temperatures in the range of 500-700 °C, usually designated as intermediate-temperature (IT) regime, involves significant efforts to improve the mixed electronic-ionic conductivity (MIEC) at the electrodes. These efforts concern in particular the cathodic materials that constitute the bottleneck towards the attainment of efficient IT-SOFC devices.1 Lanthanum strontium cobalt ferrites (LSCF), at different compositions, are still the most studied cathodic materials for IT-SOFCs.1,2 The ionic conductivity of LSCF depends very much on the stoichiometric composition. The oxide ion conductivity is believed to essentially depend on the Sr concentration, whose insertion in the A perovskite site of the parent lanthanum ferrite determines the creation of oxygen vacancies for charge compensation.3 On the other hand, the oxidation state of the Bsite transition metals, which affects the overall defect equilibrium of LSCF, is closely related to the oxygen partial pressure and affects the electronic charge carrier concentration.3 A detailed analysis of the B-site oxidation state, also in operative conditions,4 was carried out in some recent papers, showing that Co is more


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susceptible to changes of valence than Fe as a function of reductive environment.5 It is well recognized that the electronic conduction in LSCF is p-type and that, when oxygen vacancies are created as a consequence of decreased O2 partial pressure, electrons are injected into the Co 3d/O2p hybridized state, thus affecting the hole conductivity.6 The crucial role played by the TM3d-O2p band in the redox behavior of LSCF is highlighted by Mueller and coworkers. The authors demonstrated, by in situ X-ray Absorption Near Edge Spectroscopy (XANES) measurements at the O K-edge in controlled oxygen pressure close to real operating mode, that the surface transition-metal (TM)-O6 octahedron is the redox-active building block of LSCF and other perovskite oxides.7 Among the B-site dopant, the lower oxygen activation temperature and enhanced desorption rate mainly motivated the use of Ni as B-site additional dopant for lanthanum strontium cobaltite (LSC).8-10 However, the effect of the Ni in the electronic structure of Ni doped LSC was controversially reported in literature. Hjalmarsson et al. suggested that nickel doping does not influence the electronic configuration of cobalt dramatically.8 This stands in contrast to Rao et al.9 who claimed that the low spin to high spin transition is suppressed by nickel doping. However, the amount of Ni used as co-dopant is very different in the two cited studies and the difference could explain the contrasting results reported.8,9 Motivated by these observations, the present study aims to highlight the effect of Ni on the local environment and electronic structure of La0.6Sr0.4Co0.2Fe0.8-0.03Ni0.03O3−δ perovskite. Notably, to the best of our knowledge, the use of Ni as co-dopant in LSCF system is not reported in literature. We demonstrate by in-situ XANES performed under air flow that Ni3+ hinders the formation of oxygen vacancies near Co, thus stabilizing the Co higher oxidation state.



Finally, in-situ XRS measurements at the O K-edge prove that Ni helps to retain the hybridization between the O2p and TM3d orbital. Ultimately, this effect hampers the modification of the TM-O6 octahedron, which plays a fundamental role by catalyzing the process of oxygen incorporation and evolution. We observe that the effect is not only limited to the surface of the electrocatalyst but also, involves a deep modification of the electronic structure in the bulk. Therefore, we conclude that insertion of Ni into the perovskite structure provides higher stability of the Co environment in a wider temperature range than for un-promoted or Ni-impregnated LSCF and, according to our over potential measurements, we propose nickel-doped LSCF as an improved MIEC material for fuel cell applications.

EXPERIMENTAL Preparation of LSCF-Ni powders All the chemical reagents supplied by Sigma-Aldrich were analytical reagents grade. The La0.6Sr0.4Co0.2Fe0.77Ni0.03O3-δ (LSCF08-Ni) perovskite was synthesized by the one-pot sol-gel citrate method as previously reported.11,12 The metal nitrate precursors weighted in order to obtain the desired nominal composition were dissolved in a minimum quantity of deionized water. Citric acid (molar ratio of citric acid/metals = 1.5) was added to this solution and finally ammonia (28 - 30%) was dripped until pH 9 – 10. The resulting solution was dehydrated in an oil bath at 90 °C to form a sol, followed by a further heating at 120 °C to yield a gel. The so obtained gel was calcined (heating rate 5 °C/min) at 800 °C for 4 h. A portion of the calcined powder was further treated (heating rate 5 °C/min) from room temperature (RT) up to 1100 °C for 2 h in order to stabilize the perovskite structure at the same temperature used for the button cell preparation (see Electrochemical Impedance Spectroscopy


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paragraph). Finally, a portion of the sample calcined at 1100 °C was aged in furnace at 1300 °C for 4 h in order to simulate aging conditions. The amount of Ni (3 mol%) in the perovskites corresponds to ∼0.8 wt%. A Ni-free catalyst with composition La0.6Sr0.4Co0.2Fe0.8O3-δ (labeled as LSCF08) commercially provided was used as reference. Moreover, in order to investigate the effect of the Ni preparation method, Ni (3 mol%) was deposited by wetness impregnation technique over a portion of the above mentioned La0.6Sr0.4Co0.2Fe0.8O3-δ perovskite. Typically, the wetness impregnation technique involves that the active metal precursor is dissolved in few mL of water and added to the support. The volume of the aqueous solution must correspond to the pore volume of the support, so that capillary action draws the solution into the pores. After drying overnight at 100 °C, the catalyst labeled as LSCF08-Ni imp was calcined at 800 °C for 4 h and then up to 1100 °C for 2h or at 1300 °C for 4 h. Table 1 summarizes the synthesized samples and their nominal composition. Elemental analysis of the so prepared perovskite oxides was performed by inductively coupled plasma optical emission spectroscopy (ICP-OES), using an ICP Perkin-Elmer Optima 3000 DV spectrometer. The powders were dissolved in concentrated HNO3 at 60 °C and then evaporated almost until dryness. Diluted water solutions of the extracted metal nitrates were analyzed quantitatively by comparison with standard solutions. The real composition corresponded to the nominal content within ± 5%. The specific surface area of the above synthesized LSCF and LSCF-Ni powders calcined at 1100 °C ranged between 13.0-15.0 m2/g as determined by the BET method using a Sorptomatic 1900 Carlo Erba Instrument. The corresponding values decreased to 3.0-5.0 m2/g after calcination at 1300 °C.



All the characterizations described below have been recorded on the samples after calcination at 1100 °C or at 1300 °C, as specified in the text. Table 1. Sample description. sample

Composition

LSCF08

La0.6Sr0.4Co0.2Fe0.8O3-δ

LSCF08 imp

Ni/La0.6Sr0.4Co0.2Fe0.8O3-δ

LSCF08-Ni

La0.6Sr0.4Co0.2Fe0.77Ni0.03O3-δ

X-ray Diffraction (XRD) XRD measurements on the starting powders were carried out with a Bruker D5000 vertical goniometer equipped with Cu anode (Kα radiation λ=1.5418 Å) and a graphite monochromator. A proportional counter and a 0.03° step size in 2θ were used. The integration time was 40 seconds per step and the scan range was from 20 to 90° in 2θ. Rietveld refinement of the XRD data was performed using the GSAS program.13,14 X-ray Absorption Fine Structure (XAFS) X-ray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectra of LSCF08-Ni pellets (diluted with Boron Nitride) were collected at BM26A, the Dutch-Belgian Beam Line (DUBBLE) at the European Synchrotron Radiation Facility (ESRF) at the Fe, Co and Ni K-edge (7112, 7709 and 8333 eV respectively).15 The energy of the X-ray beam was tuned by a double-crystal monochromator operating in fixed-exit mode using a Si(111) crystal pair. Static measurements of the samples were performed in a closed-cycle N2-cryostat (Oxford Instruments) at 77 °C to minimize the damping effect induced by thermal vibrations. The EXAFS spectra of the samples were collected in fluorescence mode using a 9-


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element Ge detector (Ortec Inc.); reference spectra of the metals and of the respective oxides (Sigma Aldrich) were measured in transmission mode using Ar/Hefilled ionization chambers at ambient temperature and pressure. The “in situ” XAS measurements were registered at a heating rate of 5 °C/min from RT to 800 or 950 °C under air flow, using the microtomo cell available from the sample environment laboratory of ESRF.16 In situ X-ray Raman Scattering All X-ray Raman scattering (XRS) spectroscopy data were gathered at the beamline ID20 of the ESRF. The pink beam from four U26 undulators was monochromatized using a cryogenically cooled Si(111) monochromator and focused to a spot size of approximately 10 µm x 20 µm (V x H) at the sample position using a mirror system in Kirkpatrick-Baez geometry. The large solid angle spectrometer at ID20 was used to collect XRS data with 36 spherically bent Si(660) analyzer crystals. The data were treated with the XRStools program package as described elsewhere.17 The samples were ground to fine powder, pressed into pellets (13 mm in diameter), and placed onto an in-situ high-temperature furnace (microtomo) at an incident beam angle of 10 degrees. All measurements were collected at room temperature, 400 °C, 800 °C and back to room temperature. Acquisition scans lasted around 6 hours per temperature. Samples were heated with a heating ramp of 10 °C/min, kept at the target temperature during the measurement and finally cooled down to room temperature. In short, for each temperature we collected several scans of the oxygen K-edge by scanning the incident beam energy to create energy losses in the vicinity of the oxygen K-edge. All scans were checked for consistency and signals from different analyzer crystals were averaged over. The overall energy resolution was 1.0 eV and the mean momentum transfer was 6.2 pm 0.4 Å-1.



X-ray Photoemission Spectroscopy (XPS) The XPS analyses were performed with a VGMicrotech ESCA 3000 Multilab, equipped with a dual Mg/Al anode. The spectra were collected using a nonmonochromatized Al Kα source (1486.6 eV). The analyzer was operated in constant analyzer energy (CAE) mode. For the individual peak energy regions, step energy of 20 eV was used. Survey spectra were measured at 50 eV energy step. The sample powders were mounted on a double-sided adhesive tape. The pressure in the analysis chamber was in the range of 10-8 Torr during data collection. All peak energies were calibrated to the C 1s binding energy of adventitious carbon at 285.1 eV. The invariance of the peak shapes and widths at the beginning and at the end of the analyses ensured absence of differential charging. Peak fitting procedures were performed with the software provided by VG, based on non-linear least squares fitting program using a weighted sum of Lorentzian and Gaussian component curves after background subtraction according to Shirley and Sherwood.18 Atomic concentrations were calculated from peak intensity using the sensitivity factors provided with the software. The binding energy values are reported with a precision of ± 0.15 eV and the atomic percentages with a precision of ± 10%. Thermogravimetric analysis (TGA) Thermogravimetric analyses (TGA) were performed with a TGA/DSC1 STAR system Mettler Toledo in order to evaluate the oxygen vacancies content of the samples in terms of desorbed α and β oxygen species. To this aim, the samples (~10 mg) were pretreated in N2 (30ml/min) at 750 °C for 60 min to clean the surface and remove any chemisorbed species, then were cooled down to room temperature in O2 (21 vol%)/N2 (30 ml/min), followed by purging with N2 (30 ml/min) for 15 min aimed to


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eliminate all the physisorbed oxygen species. Finally, the weight loss, corresponding to the desorption of oxygen species, was recorded under N2 (30 ml/min) by heating from room temperature up to 1000 °C at a rate of 5 °C /min. TGA analyses were also carried out in oxygen flow. In this case, the samples (~10 mg) were pretreated in N2 (30ml/min) at 250 °C for 30 min in order to remove any chemisorbed H2O or CO2. After cooling down to room temperature, the weight loss was recorded by heating under O2 (21 vol%)/N2 (30ml/min) from room temperature up to 800°C at a rate of 5 °C /min. Fabrication of symmetric cells and AC impedance measurements Electrolyte substrates were prepared from Ce0.8Gd0.2O2-δ (GDC) that was synthesized from Ce(NO3)36H2O and Gd(NO3)36H2O precursors in presence of 0.5 M hexamethylenetetramine solution, then dried at 100 °C overnight and further calcined at 700 °C 4h. The so obtained GDC was pressed into disks at 10 ton/cm2 for 10 minutes, then, sintered at 1500ºC for 5 h. The resulting electrolyte disks had a diameter of 1.1 cm and a thickness of 0.1 cm. Finally, LSCF or Ni-doped LSCF cathode materials were mixed with few drops of PEG-PPG-PEG, Pluronic® P-123 (average Mn ~5,800, Sigma Aldrich) and ethanol in order to obtain a suspension that was deposited on each side of GDC electrolytes, via slurry coating technique. After sintering at 1100ºC for 2 h, the thickness of the two electrodes deposited, acting as working electrode and counter electrode, respectively, was 30 µm with a geometric area of 0.95 cm2. The so prepared symmetric cells, LSCF(Ni-doped LSCF)/GDC/LSCF(Ni-doped LSCF), were placed inside a ProboStat test station, and the cathode electrochemical performances were evaluated at open circuit voltage (OCV) by using a potentiostat coupled to a frequency response analyzer (Autolab PGSTAT302N) in a frequency



range between 0.01 Hz and 100 kHz and applying a perturbation of 10 mV. Before the measurement, the symmetric cells were heated from room temperature to 400°C with a rate of 5°C /min and keep in 400°C for 1 h. The impedance spectra were recorded in the range 400-800 °C by using Pt meshes placed, as current collectors, on the surface of the two electrodes. The experimental data were analyzed by using the software NOVA 1.10 Metrohm Autolab.

Results and discussion XRD and EXAFS: structural considerations XRD data and Rietveld refinements are shown in Figure 1. The obtained structural parameters are reported in Table S3, in the Supporting Information file (SI). LSCF samples can be refined in the rhombohedral R3തch or in the cubic Pm3m space group. In this respect, the presence of the (113) peak at 38.5° (see Figure S5a in SI), diagnostic of the rhombohedral arrangement,11 supports the structural refinement in the R3തch space group. It is worth noticing that irrespective of the thermal treatment, the XRD patterns of all the specimens were refined by using the rhombohedral R3തch space group indicating the presence of one single crystallographic phase. Further details are reported in the supporting information file. By comparing the structural parameters of the Ni-free LSCF08 and LSCF08-Ni perovskites calcined at 1100 °C reported in Table S3, important differences in the lattice parameters a and c can be observed. The reported contraction of the lattice parameters could be explained with the insertion low spin Ni3+ which has a shorter ionic radius (0.56 Å) with respect to the high spin Co3+ and Fe3+ cations (0.61 Å and 0.645 Å, for Co3+ and Fe3+ respectively) in the structure.19 Such differences are retained after calcination at 1300 °C.


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Figure 1. XRD patterns and Rietveld refinement (black and red lines, respectively) for the samples calcined at 1300 °C. The inset of panel B shows the high-angle portion of the patterns.

EXAFS In order to unravel the local environment of Ni, EXAFS measurements were collected at the Fe, Co and Ni K edges. The EXAFS data analysis for all the samples at the three different edges was carried out with the GNXAS package. The details are reported in the SI file.20 For sake of clarity only the samples LSCF08-Ni, LSCF08 Ni imp together with nickel oxide used as reference compound measured at the Ni K edge are discussed in this section. Interestingly, the analysis of EXAFS data evidenced the presence of a small amount of NiO, which has not been observed in the XRD pattern (see Figure 1). Even though the Ni-O distance at ~ 1.89 Å cannot be separated from that belonging to the perovskite, the twelve-fold Ni-Ni distance at ~



2.98 Å, is unequivocally detectable in the EXAFS signal and produces an enhanced intensity in the Fourier Transform (FT) peak at ~ 3.0 Å. It is worth noticing that the partitioning of Ni between the perovskite and the NiO phases (see Table S4 in SI), seems to be a function of the Fe/Co ratio and, for a given composition, of the calcination temperature.21 According to the GNXAS method, two-body (γ2) and three-body (γ3) configurations are needed to fit the data.20 Therefore, to model the perovskite structure, the six fold Ni-O distance at ∼1.9 Å, the eightfold Ni-coordination at 3.34 Å, and one six fold three-body configuration, arising from the Ni-O-M alignment (θ4=170°) with a Ni-M (M=Co or Fe) long bond at ∼3.8 Å, were taken into account. This model was also used to fit the data collected at the Fe and Co K-edge (Figure S6 in SI). In order to carry out a quantitative assessment of the two Ni environments, the respective coordination numbers and the first-shell Debye-Waller factor were fixed in the fitting procedure and parameterized as a function of the total Ni amount. The fitting results are shown in Figure 2. Notably, the presence of the FT peak at 3.8 Å, (highlighted with an arrow in Figure 2), corresponds to the distance of Ni-M (M=Co, Fe) of two corner sharing octahedra enhanced by a strong multiple scattering contribution, allows us to confirm the Ni-insertion in the B-site. Indeed, the Ni-O-La distance would be observed at higher R value if Ni was hosted in the A-site. The maximum amount of Ni inserted in the B-site corresponds to 80 wt % (±10%) of the total Ni content. Notably, after the thermal treatment at 1300 °C, a negligible small variation of the oxide component has been observed, as highlighted by a reduction of the Ni-Ni peak and indicating a good stability of the sample LSCF08-Ni. More details of the EXAFS analysis are reported in Table S4 of the SI file and are shown in Figure S6 (see SI file).


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Figure 2. Fourier transforms, uncorrected for the phase shift, of the observed (black) and calculated (red) EXAFS spectra at the Ni K-edge for all the samples calcined at 1100 °C (left panel) and 1300 °C (right panel), respectively. The arrow indicates the three body configuration of the perovskite arrangement whose amplitude is enhanced by the multiple scattering contributions.

Electronic characterization: X-ray Absorption Near Edge Spectroscopy (XANES) and XRS measurements XANES Static XANES spectra at the Ni K-edge for the samples calcined at 1100 and at 1300 °C respectively are reported in Figure 3. All the experimental details are reported in the supporting information file (SI). The Ni K-edge of LSCF08-Ni shows a shift of 2-3 eV with respect to NiO.22,23 The edge shift is consistent with the insertion of Ni3+ in the perovskite structure. Indeed, similar edge shift differences have been reported by Wolley et al. for LaNiO3 and by Bevilacqua et al. for LaFe0.4Ni0.6O3.23,24 It



is worth noticing the presence of a well-defined pre-edge peak at 8334 eV in all the LSCF spectra reported in Figure 3. It appears more pronounced and centered at 8332 eV in pure NiO. This pre-edge feature has been widely discussed in literature.24-26 Remarkably, after thermal treatment at 1300 °C in LSCF08-Ni, it is unchanged, suggesting a good thermal stability of Nickel into the perovskite lattice.

Figure 3. XANES spectra at Ni K-edge of the LSCF08-Ni, imp and the NiO. Black: as prepared samples calcined at 1100 °C; red: samples treated at 1300 °C; blue: NiO reference. The dashed circle indicates the observed edge shift. The arrow indicates the change in the spectrum.

To elucidate the Ni local environment and the implications of its insertion in the electronic structure, XANES simulations have been performed by using the FDMNES program.27 The results are reported in Figure 4.


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Figure 4. XANES simulations of NiO and LSCF08-Ni. The perovskite arrangements used for the calculation are drawn in the inset: Ni atoms in gray, oxygen red, lanthanum green, cobalt/iron blue.

In the calculations, the possibilities that Ni is hosted either at the A or B site of the perovskite was considered. The Ni local environment of LSCF08-Ni was described using a cluster of 8 Å radius. The insertion in the A site (see Figure S8 in the SI) did not reproduce the XANES spectra and consequently was discarded. According to the calculation reported so far, the features present in the spectrum fit reasonably well with the R3തch atomic arrangement. It is worth noticing that an evident nickel oxide contribution to the XANES spectrum of LSCF08-Ni cannot be recognized confirming that the NiO content is small. The Ni K-edge spectra of LSCF08-Ni and LSCF08-Ni imp collected during in situ treatments from room temperature to 800 °C under heating in air (Figure S9, SI) did not evidence any noticeable modification of



the Ni local environment; this indicates that nickel retains its average oxidation state as Ni3+. On the other hand, the Fe K-edge XANES spectra (reported in Figure 5), collected in the same temperature range, show a decrease of the white line and a slight shift of the edge onset to lower energy in all samples when the temperature is raised. Similar X-ray absorption results have been reported by Harvey et al. for Barium Strontium Cobalt Ferrite (BSCF) samples for temperatures up to 1000 °C in air.28

Figure 5. XANES spectra registered at the Fe K-edge during in situ thermal treatment up to 800 °C under air flow. Panel A: LSCF08; panel B: LSCF08-Ni imp; panel C: LSCF08-Ni. Inset: zoom of the pre-edge region.

By looking at the pre-edge peak in the samples (inset in each of the panel in Figure 5), its position remains centered at about 7112 eV, suggesting that the initial oxidation state of iron in LSCF08 and LSCF08-Ni is mostly Fe3+, possibly with a minor amount of Fe4+, formed during the thermal treatment (see TGA curves


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discussed in SI). This stability of Fe has been reported for LSCF and Barium ferrite doped with strontium and cobalt (BSCF) perovskites with different Co/Fe ratio. Irrespective of the Ni concentration and conceivably due to the preferential oxygen vacancy formation between the Fe and Co atoms,29 an important reduction of symmetry around Fe was expected. However, no significant changes have been observed in the local environment of iron. The effect of Ni insertion into the LSCF structure is strikingly evidenced by the XANES spectra collected at the Co K-edge reported in Figure 6 and 7. The spectra have been recorded during the thermal treatment in air from room temperature (RT) up to 800 °C. By increasing the temperature, a shift of the edge to lower energies occurs in the LSCF08 and LSCF08-Ni imp samples. The shift is mainly due to the decrease of the Co average valence with the consequent oxygen vacancies creation.30-32 The LSCF08 and LSCF08-Ni imp show an edge shift of about ~5 eV similar to that exhibited by pure LaCoO3.30 A similar energy shift has been reported by Harvey et al. for BSCF catalysts and it is consistent with the reduction of Co(III) to Co(II).28 Other authors reported an energy shift of ~1 eV for LSCF systems with different composition (Co=0.8 , Fe=0.2), and attributed this effect to a decrease of the average Co valence toward Co(III) or lower average valence.33-37 It is well acknowledged that Co(IV) is formed in the lanthanum cobaltite (LCO) when La3+ is replaced with Sr2+, inducing an increase of the average Co oxidation state and a straightening of the octahedral cage.9,10,31,38 Indeed, the presence of Co(IV) has been reported by several authors also for LSCF and BSCF at different Co/Fe ratios. 34-37, 28 Due to the large edge shift shown in Figure 6 (∼ 4 eV), it can be concluded that Co(III) and Co(IV) present in the structure are mostly reduced to Co(II), while



evidences of the mere reduction of Co(IV) to Co(III) are not recognizable. Remarkably, only a negligible energy shift (< 0.5 eV), measured in the same temperature range of unpromoted catalysts (see Figure 6) is observed for the sample LSCF08-Ni. To induce energy shifts similar to those observed in unpromoted samples, the temperature had to be raised to 950 °C. Moreover, to achieve complete reduction, a treatment of thirty minutes was required. The structural kinetics is reported in Figure 7. These results demonstrate that doping of LSCF with nickel prevents the formation of oxygen vacancies and stabilizes the higher Co oxidation states. The overall effect is the stabilization of the whole network of TM-O6 octahedra in the perovskite. A quantitative assessment of this effect is reported in Figure 8, where the difference between the i-th Co K-edge energy E0i and the starting E00 value during the temperature ramp has been reported as a function of the reaction time and temperature.


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Figure 6. XANES spectra registered at the Co K-edge during in situ thermal treatment up to 800 °C under air flow. Panel A: LSCF08. Panel B: LSCF08-Ni imp; panel C: LSCF08-Ni. In the inset: zoom of the pre-edge zone. The circle highlights the absence of variation in the edge.



Figure 7. Thermal treatment of LSCF08-Ni at 950 °C. The red line is the end spectrum at 800 °C. The solid blue is the last spectrum recorded at 950 °C.

A strong interdependence between Sr and Ni in Ni/Sr doped lanthanum cobaltite has been reported in literature.10 In particular, it was suggested that, for a given Sr content, an optimum Ni amount determines the highest electronic conductivity and affects the oxygen self-diffusion coefficient. Since the iron ions in strontium doped lanthanum ferrite remain essentially in the 3d5 configuration,39 even at high Sr concentration, the proposed Ni/Sr interdependence in lanthanum cobaltites can be extended, after normalization for the Co content, to LSCF08-Ni. In this context, a key role is attributed to the Ni atoms which, together with the transformation of Co3+ to Co4+, contribute to the overall electro-neutrality by oxidation from Ni2+ to Ni3+. As Ni3+ mainly exists at the low spin state (ls) in perovskite-type oxides,9,10,40 the low spin Ni3+ t2g6eg1 octahedral state interacts more covalently than high spin (hs) Co3+ with


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the oxygen 2p orbital.10,40 By consequence, Ni3+ (ls) controls the formation of O2vacancies and stabilizes the oxygen lattice.10 The capability of Ni to stabilize oxygen vacancies was already reported for Nidoped strontium titanate. In this latter case, the effect was attributed to the electronic structure of Ni3+, which has one electron occupying one of the eg orbitals geometrically directed towards the vacancy and therefore, as observed by EPR spectroscopy, can form stable Ni3+-VO complexes.41,42 Ultimately, the stabilization of the oxygen vacancy and the better covalent interaction with the lattice oxygen strengthen the TM-O6 configurations.

Figure 8. Variation of the E0 Co K-edge energy as a function of time (left panel) and temperature (right) for unpromoted LSCF08 (black dots), LSCF08-Ni imp (red) and LSCF08-Ni (blue). The arrow indicates the starting temperature of the structural change of the LSCF08 and LSCF08-Ni imp samples.



“In situ” XRS: the O K-edge The X-ray absorption data support the idea that the inclusion of Ni in the B site of LSCF produces a substantial modification in its electronic structure compared to the undoped sample. This interpretation is also corroborated by XRS measurements at the O K-edge as reported in Figure 9A. O K-edge excitations correspond to electronic transitions from the O 1s core level to 2p states hybridized with the TM 3d states, thereby reflecting the extent of covalence between O 2p and TM 3d orbitals (TM=Ni, Fe, Co).43 Accordingly, the first double feature in the pre-edge region between 525 and 530 eV (peaks A and A' in Figure 9B) is attributed to bands of mixed O 2p and TM 3d character. The second peak (B) corresponds to bands deriving from Sr 4d/La 5d electronic states and the broad, intense peak at 544 eV (C) is attributed to bands of mixed O 2p and TM 4s and 4p character.


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Figure 9A. “in situ” XRS spectra collected at O K edge. The main features are labeled A, A', B, C (see text for details).

Figure 9B Zoom of the pre-edge region which shows the variation of the eg↑ and t2g↓ states respectively.

The A, A’ spectra, at RT and up to 400 °C, are similar for all the samples. On the other hand, there is a clear and systematic change as a function of the thermal treatment, in particular for the features A, A’ and C, which are connected with the hybridization between the O 2p and the TM 3d states. At 800 °C we observe an abrupt increase of intensity in the pre-edge of LSCF08 and LSCF08 Ni imp samples. In addition, the extra spectral weight of the pre-edge features A and A’ is preserved in the spectra collected at RT after the thermal treatment cycle (RT, 400 °C, 800 °C, RT), therefore suggesting that the electronic structure changes are not reversible. On the contrary, the pre-edge features in Ni-doped LSCF show only a modest variation



with temperature and the initial RT spectrum is partially recovered after thermal treatment. It is worth of note that slight variations are observed for the peaks B and C in the spectra measured at RT after a thermal cycle for the LSCF-Ni and LSCF08. On the other hand, the peak C for the sample LSCF08-Ni imp undergoes an important modification, indicating a reorganization of the TM-O states. The features A and A' are associated to the σ* (π*) molecular orbital in LSCF, which originates from the anti-bonding mixture of TM (Fe/Co) 3d eg↑ (t2g↓) and O 2p states. The variation in the vacancy concentration can modify the pre-edge features A and A'. An enhanced spectral weight of these is indicative of a depopulation of the corresponding electronic states near the Fermi level. The presence of Ni in the perovskite structure, retaining the electronic density on the eg↑ states during the treatment in air, inhibits these effects, which instead are predominant in the undoped LSCF08 and LSCF08 Ni impregnated samples. Notably, the degree of covalency, whose variation is connected with the variation of population of the t2g↓ states7 (Figure 10D) is almost preserved in the LSCF08-Ni sample, corroborating the conclusion that minor changes occur in the filled states with O 2p character.42 Thus, during the thermal treatment in the LSCF08-Ni sample, the lattice oxygen keeps its reduced state remaining bonded with the TM, so that the oxygen vacancies formation is considerably hampered. In conclusion, the difference of the spectra collected at 800 °C proves the role of Ni to maintain the electronic structure of LSCF in a wide temperature range. To further elucidate the influence of Ni on the electronic structure of LSCF08, XANES simulations at the O K-edge with the FDMNES program package, for both Ni-doped and undoped structures have been performed.27 According to the results reported so far, only B-site insertion of Ni has been considered (see Figure 10, a-d)


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The details of the calculations are reported in the SI file. Remarkably, in order to obtain a good agreement between the simulations and the oxygen XRS spectrum measured at RT the parameter (dilatorb), which modifies the valence orbitals dilating or contracting them. Then, this parameter allows considering the oxygen in its ionic state in the structure.

Figure 10. Right upper panel: simulations of the O K edge. a: simulation of Ni doped and undoped perovskite (supercell 2x2) dilatorb=0, no experimental convolution; b: effect of dilatorb equal to 0, 0.1, 0.2, 0.3 for respectively including experimental convolution. For comparison reason, the simulation A (red dot) is reported without convolution; c: simulations of Ni doped and undoped perovskite with the same dilatorb equal 0.3 versus the LSCF08-Ni measured at RT; d: simulation of undoped structure dilatorb 0.0 versus LSCF08 measured at 800 °C.



This parameter tunes the height of the pre-edge peaks and their position with respect to the main edge.27 To simulate the spectra of both Ni-doped and undoped LSCF08 measured at RT, the best value of dilatorb was 0.35. Despite the fact that the Ni-doped model shows lower pre-edge peak, the difference with the undoped structure is hardly significant. Indeed, due to the SCF approximation used in the calculation, the evaluation of the electronic density in the d states is not accurate enough. So, the result is not decisive to determinate unequivocally the role of the dopant in the electronic structure (see Figure 10A). A similar appraisal can be drawn for the simulations including oxygen vacancies (see Figure S10 in SI). However, it is worth noticing that, to simulate the LSCF08 spectrum at 800 °C, the dilatorb parameter was strikingly reduced (0.005). This indicates less ionic character of oxygen and a decreased covalency, which is ultimately connected with the depletion of the TM d orbital.42,43 The difference of the two results obtained for the measured spectra at RT and at 800 °C respectively, is consistent with the better covalent interaction between Ni and oxygen in the structure and provides an average information on the role of Ni in the electronic structure.40 In conclusion, Ni induces a better stability of the TM-O bonds in the whole structure during the thermal treatment, by trapping the existing oxygen vacancies. On the contrary, the undoped structure undergoes an important electronic modification which is highlighted at 800 °C.

Electrochemical performances The electrochemical activity of LSCF08, LSCF08-Ni imp and LSCF08-Ni oxides was evaluated by impedance measurements registered in the range of temperature


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400-800 °C. In Figure 11, experimental and fitting spectra recorded under air atmosphere, at 700 °C and open circuit are shown superimposed. A simple equivalent circuit was calculated by using the software NOVA 1.10 Metrohm Autolab for the interpretation of the impedance results obtained. This circuit gave a reasonable fitting (less than 10% error) and included a resistor Rs connected in series with a subcircuit, Rp//Q as shown in the inset of Figure 11. Rs was associated with the charge transfer at the cathode/electrolyte interface. The subcircuit, Rp//Q, was related to the bulk ions transport mechanism. Moreover, a Q element (constant phase element) was used instead of a pure capacitance to take into account the depressed shape of the impedance arcs. As shown in Figure 11, an enhancement of the cathodic electrocatalytic activity with a decrease of the area specific resistance (ASR) measured for LSCF was observed for the LSCF08 in

presence of Ni and the effect was a function of the method for Ni introduction: for LSCF08 the ASR was equal to 0.69 Ωcm2, while for LSCF08 Ni imp and LSCF08-Ni Rp decreased up to 0.53 and 0.37Ωcm2, respectively. It emerges clearly that Ni promotion greatly enhances the electrocatalytic activity of LSCF08 and such effect strongly depends on the preparation method, Ni doping by one pot sol-gel method having the strongest influence. Considering recent results,44 an ASR value of ∼ 0.5

Ωcm2 was reported for La0.6Sr0.4Co0.2Fe0.8O3-δ /GDC at 650 °C,45 other authors found for La0.58Sr0.4Co0.2Fe0.8O3-δ/GDC ASR value of 3.86 Ωcm2 at 800 °C that decreased

to 1.9 and to 1.22 Ω cm2 by impregnating the cathode material with Ag (2wt%) or Cu (2wt%), respectively.46 Taking into account the different experimental conditions, our results are in reasonable agreement with the literature.



Figure 11. Experimental EIS spectra registered in air at 700 °C and open circuit conditions for LSCF08, LSCF08-Ni imp and LSCF08-Ni cathode materials. In the inset the equivalent circuit used to fit the data is reported.

The activation energy values calculated in the range of temperature 400-800 °C were equal to 1.44, 1.41 and 1.36 eV for LSCF08, LSCF08-Ni imp and LSCF08-Ni respectively. In Figure 12 the Arrhenius plot is reported, showing that Ni promotion favors the electrocatalytic reduction of oxygen, especially for LSCF08-Ni that has the lowest value. The calculated values are close to those reported in literature and point out the improvement of oxygen transport properties played by Ni, once incorporated into the perovskite lattice.


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8

LSCF08-Ni LSCF08 Ni imp LSCF08

6 -1

ln σ T(S cm K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 2 0 -2 -4

Ea = 1.36 eV Ea = 1.41 eV Ea = 1.44 eV

-6 0,9

1,0

1,1

1,2

1,3

1,4

1,5

1000/T (1/K) Figure 12. Arrenhius plot calculated in the range of temperature 400-800 °C for LSCF08, LSCF08-Ni imp and LSCF08-Ni.

Conclusions The present study reports the effect of Ni insertion on the electronic structure of LSCF08 as evidenced by X-ray absorption spectroscopy. We have proved by “in situ” XANES experiments carried out under air flow in the range of RT- 950 °C that B-site Ni dopant stabilizes the oxygen vacancies in LSCF08 and delays their formation. By carrying out X-ray Raman scattering spectroscopy to study the O K edge, we conclude that Ni helps to retain the hybridization between the O2p and TM3d orbital, which hinders the modification of the TM-O6 octahedron. This appears to be a key feature in order to maintain the redox properties of LSCF over a wide temperature range, so improving the oxygen reduction reaction in the intermediate temperature range. In La0.6Sr0.4Co0.2Fe0.8-0.03Ni0.03O3-δ, the effect seems to involve the bulk structure and is not only confined to the oxide surface.



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Then, the better electrochemical performances of La0.6Sr0.4Co0.2Fe0.8-0.03Ni0.03O3-δ, are correlated to the Ni ability in stabilizing the lattice oxygen, the latter being a crucial red-ox partner to molecular oxygen in the global balance of oxygen vacancies present in the structure.

Support Information file The

complete

analysis

of

X-ray

Photoelectron

Spectroscopy

(XPS)

and

Thermogravimetric analysis (TGA) are discussed. Additional XRD, EXAFS figures and XANES simulation are reported. The full tables of XRD and EXAFS data analysis are showed.

AUTHOR INFORMATION Corresponding Authors: *A. Longo. E-mail address: [email protected].

ORCID: Longo Alessandro: 0000-0002-8819-2128 *L.F. Liotta. E-mail address: [email protected].

ORCID: Leonarda F. Liotta: 0000-0001-5442-2469

Acknowledgements NOW and PON-Project Teseo are acknowledged. The authors are grateful to Dr. Francesco Giordano (ISMN-CNR) for XRD measurements, to Mr. Giovanni Ruggieri (ISMN-CNR) for assistance in EIS measurements and to Mr Hugo Vitoux (ESRF) for the excellent help during the set-up of the in situ cell. Dirk Detollenaere and Florian Ledrappier (DUBBLE) are also acknowledged. We kindly thank the ESRF for providing synchrotron radiation and C. Henriquet for providing technical support.


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Keywords: IT-SOFC, XRS, XAFS, in situ

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The Effect of Ni Doping on the Performance and Electronic Structure of

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