Maneuvering the Physical Properties and Spin States to Enhance the

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Maneuvering the Physical Properties and Spin States to Enhance the Activity of La-Sr-Co-Fe-O Perovskite Oxide Nanoparticles in Electrochemical Water Oxidation Rahul Majee, Sudip Chakraborty, Hemant G. Salunke, and Sayan Bhattacharyya ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00531 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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ACS Applied Energy Materials

Maneuvering the Physical Properties and Spin States to Enhance the Activity of La-Sr-Co-Fe-O Perovskite Oxide Nanoparticles in Electrochemical Water Oxidation Rahul Majee,1 Sudip Chakraborty,2 Hemant G. Salunke,3 and Sayan Bhattacharyya1,* 1

Department of Chemical Sciences, and Centre for Advanced Functional Materials, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur - 741246, India 2 Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Box516, Uppsala, SE-75120, Sweden 3 Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India * Email for correspondence: [email protected] Abstract Perovskite oxides have attracted considerable attention as durable electrocatalysts for metal-air batteries and fuel cells due to their precedence in oxygen electrocatalysis in spite of the complexities involved with their crystal structure, spin states and physical properties. Here we report optimization of the activity of a model perovskite system La1-xSrxCo1-yFeyO3-δ (LSCF; x = 0.301, y = 0.298 and δ = 0.05-0.11) towards electrochemical water oxidation (OER) by altering the calcination temperature of the non-aqueous sol-gel synthesized nanoparticles (NPs). Our results show that an improved OER activity is the result of a synergism between its morphology, surface area, electrical conductivity, and spin state of the active transition metal site. With an eg orbital occupancy of 1.26, the inter-connected ~90 nm LSCF NPs prepared at 975oC (LSCF-975) outperforms the other distinguishable LSCF morphologies, requiring 440 mV overpotential to achieve 10 mA/cm2, a performance comparable

to

the

best-performing perovskite

oxide electrocatalysts.

While

the

interconnected NP morphology increases the propensity of electronic conduction across crystalline grain boundaries, the morphology-tuned high spin Co3+ ions increases the probability of binding reaction intermediates at the available surface sites. Density functional theory (DFT) based work function modeling further demonstrate that LSCF-975 is the most

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favorable OER catalyst among others in terms of a moderate work function and Fermi energy level facilitating the adsorption and desorption of reaction intermediates. Keywords: Perovskite oxide; Nanoparticles; Water Oxidation; Morphology; Spin state Introduction The surge for renewable energy relies on the deployment of energy conserved within chemical bonds and water splitting is one of the most desired options.1 Among the two halfreactions, oxygen evolution, being a sluggish multi-electron uphill process, remains the bottleneck for water splitting, artificial photosynthesis and metal-air batteries.2 The performance of devices comprising OER is constrained due to the electrocatalysts requiring high overpotential.3 A majority of reported OER catalysts suffer from below par activity or stability for example, the expensive RuO2 and IrO2 have stability issues over longer durations,4 and the non-precious heteroatom doped carbon catalysts have poor operational durability due to corrosion in the electrolyte.5,6 In comparison, the Fe, Co and Ni based transition metal oxides and hydroxides have demonstrated promising activity and stability in basic medium.7,8 Among the robust mixed oxide systems, perovskite oxides have emerged as alternative state-of-the-art catalysts.9-12 The perovskite oxides accommodated with oxygen vacancies are extensively used in solid oxide fuel cells or in metal-air batteries due to having an impeccable bifunctional activity towards oxygen electrocatalysis.13,14 Among the composition modulated systems, doped barium and lanthanum cobalt ferrite [(Ba/La)1xSrxCo1-yFeyO3],

doped lanthanum nickelate (LaCo1-xNixO3-δ) and double perovskites

demonstrate significantly high activity.15-17 The complex crystal structure and spin states of bulk and nanoscale perovskite oxides have a profound influence on their physical properties such as electrical conductivity and oxygen mobility, which however can be tuned by altering their synthesis conditions and chemical composition. In the perovskite oxides with formula ABO3 where A is the trivalent


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lanthanide and B is the trivalent transition metal site, the transition metal B-site is mainly responsible for their electrocatalytic activity.18 A realistic doping approach can tune the ratio of different high/low spin states of the transition metal centers suitable for effective adsorption and desorption of reaction intermediates for enhanced OER activity.19 In fact, the eg orbital filling close to 1.2 has been shown to be ideal for electrochemical OER.20 In this respect the transition metal spin states of nanoparticulate systems are more susceptible to the NP-size than their bulk counterparts, which alter the activity of catalytically active surface sites.19 Because of its cubic crystal structure, perovskite oxide catalysts allow facile charge transfer through the metal centers. Although the BO63- unit mainly controls their electrical conductivity, A3+ site can also influence the electrical properties when doped partially with a bivalent ion of comparable size that upsets the cubic lattice and transforms it to orthorhombic or rhombohedral crystal structures.10,12 Although catalytic activity depends on the conducting environment surrounding the surface binding sites,21 charge transportation within the catalyst core intricately relies on the electronic conduction pathways across interconnected NPs through their grain boundaries. The electrically conducting material backbone allows feasible transport of electrons approaching from the electrode to the active surface site where the reactant species would bind. High temperature annealing can generate well-defined lattice planes which in turn improve the conductivity, provided there is no change in crystal structure.22 Improving the crystallinity however compromises the surface area to volume ratio which shrinks the pores of porous matrices, thus limiting the facile binding of adsorbates on the catalyst surface. The transport of charge and ion carriers can still be enhanced by employing three-dimensional conducting supports such as Ni foam or carbon fiber paper (CFP),23 carbon nanostructures with the catalyst ink,21 or by modulating the morphology of the catalyst nanostructures.24 The best electrocatalytic activity of a catalyst system can be achieved when all the parameters



namely its electrical conductivity, surface area, electrochemically active surface area (ECSA) and spin state of the active metal sites are optimized. From materials chemistry perspective, the synthesis methodology and calcination temperature play vital roles in controlling the activity through morphology tuning and by altering the electronic environment at the atomic scale.19,22 Herein we have presented an analogy between these factors by designing perovskite oxide NP catalysts at different calcination temperatures. La1-xSrxCo1-yFeyO3-δ (LSCF; x = 0.301, y = 0.298 and δ = 0.05-0.11) is chosen as the model OER catalyst and its three distinguishable morphologies demonstrate different OER activities owing to the interplay of their physical parameters. The LSCF NPs synthesized at 975oC serves as the optimal parameterized electrocatalyst requiring 440 mV overpotential to achieve 10 mA/cm2, and this performance is comparable to the best performing perovskite oxide electrocatalysts. Results and Discussion Structural Characterization A non-aqueous sol-gel route, using ethylene glycol (EG) solution of metal acetates in stoichiometric ratio, was adapted to prepare ~8 nm colloidal NPs of amorphous LSCF (Figure 1). The colloidal NPs were calcined at 750, 975 and 1200oC to give the crystalline phase of LSCF-750, LSCF-975 and LSCF-1200, respectively. Inductively coupled plasma – mass spectrometry (ICP-MS) analyses elucidate a similar elemental composition of the LSCF NPs synthesized at different calcination temperatures whereby the average atomic ratio of La : Sr : Co : Fe is 69.9 : 30.1 : 70.2 : 29.8. The quantification of oxygen non-stoichiometry in La0.699Sr0.301Co0.702Fe0.298O3-δ reveals δ = 0.05, 0.08 and 0.11 for LSCF-750, LSCF-975 and LSCF-1200, respectively (Discussion S1 and Table S1, Supporting Information). The X-ray diffraction (XRD) patterns show that the LSCF catalysts crystallize in rhombohedral crystal structure with space group, ܴ3തܿ (Figure 2a), a common occurrence with doped perovskite oxides when La3+ is the A-site cation in ABO3.25 The crystallite size increases gradually with 4 ACS Paragon Plus Environment

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synthesis temperature as 17, 20 and 24 nm for LSCF-750, LSCF-975 and LSCF-1200, respectively. Although there are no apparent changes in the crystal structure with calcination temperature, Rietveld refinement of the XRD patterns of phase-pure LSCF samples provide further insight into their lattice modulations (Figure S1). The bond lengths, bond angles between metal centers and oxygen, and unit cell volume were deduced from the refinement results obtained with minimum possible values of residual weight percentage (Rwp) and goodness of fit (χ2) values (Table 1). Of special mention is the unit cell parameter c and Co– O–Co (or Fe–O–Fe) bond angle of LSCF-975, which are 13.40335 Å and 174.7o, respectively, comparably higher than those of LSCF-750 and LSCF-1200. The contraction of the unit cell volume in spite of elongation along the z-axis and the bond angle closer to 180o minimizes the local strain in LSCF-975,26 feasible for better conduction of the electrons. The unit cell schematics shown in Figure 2b are obtained from the CIF files of Rietveld analyses. Figure 2c and Figure S2 show the transmission electron microscope (TEM) image of the representative ~90 nm diameter LSCF-975 anisotropic NPs. Before calcination, although the colloidal NPs are dispersed within a gel-matrix formed by EG, which minimize their aggregation, heat treatment almost eliminates the surface functional groups and renders crystallinity to the NPs,25 in addition to increasing the inter-grain connectivity. High resolution TEM image in Figure 2c-i show the 0.5-1.5 nm thick crystalline grain boundaries generated at a slow growth rate during calcination. From the plot profile of a selected region in Figure 2c-ii, the interplanar spacing is found to be 0.375 nm corresponding to the (100) reflection, whereas the 60o angle between the crystallographic planes is characteristic of a typical rhombohedral crystal system. The selected area electron diffraction (SAED) pattern (Figure 2d) demonstrates high crystallinity of the NPs. Field emission scanning electron microscope (FESEM) images in Figure 2e show the 3-dimensional morphology of the differently calcined LSCF NPs. While LSCF-750 has



relatively dispersed NPs with average diameter of ~28 nm, the LSCF-975 NPs are aggregated indicating a transition phase during sintering from a nanometric domain to the bulk structure, shown vividly in the TEM images and elemental mapping of LSCF-975 and LSCF-750 NPs in Figure S2 and Figure S3, respectively. Indeed LSCF-1200 represents a sintered sample having interconnected particle diameter ranging from 500 nm to 2 µm as confirmed from TEM imaging (Figure S4). Since inter-particle connectivity is undoubtedly a determining factor in tuning the feasibility of conduction of electrons and their overall catalytic performance,24 the interconnectivity was tested further by bath-sonicating the samples in ethanol for 2 h at room temperature to break open the grain boundaries and separate the individual NPs. Atomic force microscope (AFM) images (Figure 2f) of these dispersed NPs on Si wafers show that the NP sizes are significantly reduced by this method, namely LSCF750 has discrete particles of diameter 25 nm whereas the predominantly interconnected NPs of LSCF-975 are ~50 nm in diameter, calculated from the peak width of height profiles (insets of Figure 2f). TEM imaging shows that the grain boundaries of LSCF-750 or LSCF950 samples suffer from the lack of a crystalline inter-particle junction, unlike LSCF-975 (Figure S3 and Figure S5). An extension of the periodic arrangement of atoms from the NP core to the grain boundary helps in lowering the barrier for inter-particle electron hopping or quantum mechanical tunneling.27 OER Performance To evaluate the electrochemical OER activity in 1 M KOH solution, the LSCF catalyst inks were deposited on CFP (Figure 1), whereby the catalyst morphology remains intact post deposition (Figure S6). The linear sweep voltammetry (LSV) polarization curves in Figure 3a demonstrate the superior OER activity of LSCF-975 NPs as compared to LSCF750 and LSCF-1200 samples. LSCF-975 requires an overpotential of 440 mV to achieve 10 mA/cm2 which is comparable or better than several reported perovskite electrocatalysts


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(Table S2).13,19,28-40 In comparison, LSCF-750 and LSCF-1200 require 470 and 540 mV, respectively. The Tafel equation (η = b log (j) + a, where η is the overpotential, j is the current density and b the tafel slope) is employed to derive the Tafel plots from LSV polarization curves (Figure 3b), where Tafel slope in the faradaic region indicates the ease of OER kinetics on the catalyst surface. LSCF-975 NPs demonstrate the least Tafel slope of 109 mV dec-1 as compared to 110 and 121.3 mV dec-1 for LSCF-750 and LSCF-1200, respectively. The better OER activity of LSCF-750 than LSCF-1200 cannot be explained by their similar Tafel slopes since the Tafel slopes are potential dependent especially for multistep electrochemical reactions,7 and due to potential dependence of the catalytic mechanism, there might be more than one Tafel slopes in different overpotential intervals. The activity trend is further investigated with different substrate scope. When Cu mesh is used as the substrate, a similar electrocatalysis trend of LSCF samples is observed (Figure S7a), and the better activity at Cu mesh than CFP is due to its metallic conductivity and more uniform dispersion of the catalyst ink. The glassy carbon electrode however shows a lower performance than CFP (Figure S7b). The Nyquist plots from electrochemical impedance spectroscopy (EIS) analysis also show that LSCF-975 has the least charge transfer resistance of 89.4 Ω (Figure 3c) which thereby attests to its favorable intrinsic activity. ECSA is calculated from the double layer capacitance derived from cyclic voltammetry (CV) plots at different scan rates (Figure S8), provides an estimate of the surface sites exposed to the electrolyte and their accessibility for electron transfer processes. ECSA calculated for LSCF-750, LSCF-975 and LSCF-1200 are 1.88, 2.15 and 0.06 cm2, respectively which again relates to the superior performance of LSCF-975 NPs. The mass activity of the catalysts (Figure S9) also shows a similar trend to that of the current density in Figure 3a. Chronoamperometric stability of the best performing LSCF-975 NP catalyst measured at a constant applied potential of 1.7 V vs RHE is shown in



Figure 3d. While all the LSCF catalysts show excellent stability even after 22 h (Figure S10), a slight increase in overpotential after chronoamperometric analysis is probably due to the leaching of catalyst NPs from the electrode under operational conditions. Preciously so, the LSV plot taken after 70 h stability test for LSCF-975 shows an increase in overpotential by 9 mV at 10 mA/cm2 with the catalyst morphology however remaining intact (Figure 3d insets). On the other hand LSCF-750 shows an increase in overpotential by 38 mV, whereas LSCF1200 shows increase of 21 mV after 22 h. Two of the major factors that lead to poor stability of electrocatalysts are the loss of active sites and leaching of catalyst NPs from the substrate. Firstly, during chronoamperometric analysis, the comparably less active LSCF-1200 is likely to adsorb more hydroxyl groups on the surface which in turn decreases the total number of available catalytic sites temporally. Secondly, there are lesser chances for the bulk particles in LSCF-1200 to effectively anchor to CFP as like LSCF-975 NPs and hence the chance of leaching in the former is more. Quantification of the evolved O2 gas during 1-3 h bulk electrolysis was performed by eudiometry whereby the Faradaic efficiency (φ) is measured by comparing the theoretical quantity of gas, calculated from the amount of charge passed at different time intervals, with the experimentally measured values (Figure 3e). Near unity φ values are obtained as 97, 99 and 98 % for LSCF-750, LSCF-975 and LSCF-1200, respectively denoting the absence of any allied parasitic reactions and maximum consumption of charge during OER. Elucidation of the LSCF Electrocatalysts The relative OER performance of LSCF electrocatalysts is likely the result of a culmination of various physical attributes. Firstly, any electrochemical reaction depends on the intrinsic conductivity of the catalyst material. From the inverse slopes of current vs. potential plots at room temperature (Figure 4a), the electrical resistance is measured to be 47.2, 28.1 and 27.1 Ω for LSCF-750, LSCF-975 and LSCF-1200, respectively. Given that a


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similar material composition and crystal structure will not have a profound impact on the variation of electrical resistance, the increasingly interconnected particle network with increasing calcination temperature is likely to improve the conduction of electrons by minimizing the defects and grain boundaries (Figure 1). The oxygen vacancies should also influence the electronic conduction, however a close to similar non-stoichiometry of δ = 0.05-0.11 is unlikely to alter the catalytic performance. Secondly, although ECSA has a direct implication on the OER activities, the Brunauer-Emmett-Teller (BET) surface area also plays a vital role in guiding the availability of surface binding sites and mass transport. In the presence of mesopores of diameter ~3.6 nm (as observed for LSCF-750 in Figure 4b inset),41,42 the surface area varies as 8.8, 2 and 0.05 m2/g for LSCF-750, LSCF-975 and LSCF-1200, respectively (Figure 4b) which however does not follow the ECSA trend. The surface area / volume sacrificial growth of the NPs during calcination blocks the available pores rendering electrolyte diffusion to the active sites difficult.43 The intrinsic activity is reflected by normalizing the activity with respect to the physical surface area and ECSA whereby LSCF-975 shows a better performance in comparison to LSCF-750 (Figure S11). BET surface area normalized specific activity shows overpotential of 510 and 460 mV to achieve 1 mV/cm2 for LSCF-750 and LSCF-975, respectively. ECSA normalized specific activity on the other hand shows an increase in overpotential to 432 mV for LSCF-975 but a decrease to 460 mV for LSCF-750. While the BET surface area normalized activity considers equal abundance of active sites, ECSA normalization refers to the intrinsic activity treating all active centers having equal electronic abundance. The above comparison implies the descriptive role of electrical conductivity on the electrochemical activity of the catalysts wherein apart from its surface area midway between the other two morphologies, LSCF-975 has an added advantage of its electrical conductivity close to the bulk LSCF-1200.



Apart from the morphological influence on catalytic behavior, the OER performance also depends on the specificity of eg electron density on transition metal d-orbital since the optimum energy for adsorption and desorption of HO-/HO./O2 or other reactant intermediates is governed by the electronic environment of transition metal sites. Molecular orbital insight into the perovskite oxide materials shows that an eg orbital filling close to 1.2 has superior performance in electrochemical OER.20 While one of the ways to optimize the transition metal eg electron density is to choose the right metal ion e.g. Mn3+ with eg1 configuration in high spin (HS) state or transition metal-site doping to provide a desirable altered electron density,44 the other facile method is to tune the NP size providing alternative surface stressed oxidation states to obtain an advantageous high/low spin ratio at the transition metal centers.19 The transition metal spin states are calculated from the plots of temperature dependent magnetization (Figure 4c,d). The calculated effective magnetic moment (µeff) of both LSCF and LaCoO3 (LCO) decreases linearly with increasing calcination temperature. In case of LCO, µeff arises due to the HS fraction of Co3+, from where eg orbital occupancy is calculated. In La0.699Sr0.301Co0.702Fe0.298O3, since Fe3+ ions preferably get oxidized to Fe4+ due to Sr2+ doping at La3+ site, all three ions Co3+, Fe3+ and Fe4+ contribute to its µeff value (Figure 4).15,16,45 Taking oxygen non-stoichiometry, the contribution of Fe3+ and Fe4+ ions and overall charge neutrality in to account, the fraction of Co3+ is calculated from the µeff values. The total eg occupancy is calculated by multiplying the atomic fraction of respective ions with their number of eg electrons (Table S3 and S4). The eg occupancy of transition metal centers in LSCF decreases with increase in calcination temperature as 1.66, 1.26 and 0.67 for LSCF750, LSCF-975 and LSCF-1200, respectively. LCO shows a similar trend whereby the eg occupancy varies as 0.93, 0.92 and 0.71 for LCO-750, LCO-975 and LCO-1200, respectively. The probable reason for relative increase in µeff and so do the eg occupancy for LSCF in comparison to LCO is due to the presence of Fe centers, within the FeO6 octahedral


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site, having HS electronic configuration. With an increase in crystallinity with higher calcination temperature, the fraction of HS Co3+ electronic configuration decreases, which also reduces the eg occupancy.19 LSCF-975 with the best OER activity has an eg occupancy close to 1.2 which matches with previous reports where trivalency and a proximal eg1 configuration are found to be important parameters in improving the electrochemical performance.20,46 The relatively better OER activity of LSCF-975 is therefore attributed to its optimized electrical conductivity, surface area and favorable spin state of the transition metal ion centers. With the highest surface area while LSCF-750 enjoys a larger number of surface active binding sites, its disadvantageous electrical conductivity due to having amorphous grain boundaries increases the electrochemical impedance. On the other hand in the better conducting LSCF-1200, charge transfer suffers due to reduced surface active sites and the electrochemical impedance increases among all three samples. LSCF-975 has the advantage of an interconnected NP morphology, which although affects the surface area but the propensity of electronic conduction across crystalline grain boundaries increases the electrical conductivity, close to that of LSCF-1200. This allows better charge transfer through the crystallites and increases the probability of binding of reaction intermediates at the available surface sites, reflected from the Nyquist plot (Figure 3c) and highest ECSA, respectively. In addition the flatter ∠Co-O-Co of 174.72o, closer to 180o, facilitates the charge transfer more than LSCF-750 and LSCF-1200. The OER activity of LSCF-975 is also intrinsically related to its rational design with a NP size of about 90 nm and an optimized electronic structure of the transition metal ions. Along with morphological advantages of LSCF-975 NPs, the sizetuned HS state of Co3+ ions provides a decently stable perovskite oxide catalyst with lower overpotential, smaller Tafel slope and lower charge transfer resistance. Work Function Modeling based on DFT



In order to find the correspondence between experimental outcome of OER activity with its theoretical counterpart and probing the reason for different activities in three different LSCF samples, systematic first principles electronic structure calculations based on DFT framework was performed. Our prediction of OER activity is based on the work function (φ) hypothesis, as proposed in previous reports,47-49 where a correlation has been derived between φ of a system corresponding to electron transfer between the surface and the intermediate adsorbate of the reaction pathway of water splitting. Earlier it has been demonstrated that electron transfer is strongly dependent on φ of the corresponding catalytic surface, whereby the activation energy of hydrogen evolution reaction on a surface is inversely proportional to the φ value,47 which can be perceived correctly as the value corresponding to the energy required for the electron moving away from the surface. Keeping a similar assumption in mind,47-49 we have determined the work function of La0.699Sr0.301Co0.702Fe0.298O3-δ systems with δ = 0.05, 0.08 and 0.11, that correspond to LSCF-750, LSCF-975 and LSCF-1200, respectively. The surface along [001] direction is constructed for all the three systems and after obtaining their minimum energy configuration, the corresponding φ is determined, which is essentially the difference between energy of an electron being at rest in vacuum near the surface and Fermi energy level (EF) of the system. The left panel of Figure 5a represents the variation of φ where LSCF-975 is the most balanced one in terms of its potential and the corresponding EF, and therefore the value of φ derived from this difference, also lies between LSCF-750 and LSCF-1200. The potential values for LSCF-750, LSCF-975 and LSCF-1200 are 5.032, 5.048 and 5.266 eV, whereas the corresponding EF values are 1.292, 1.273 and 1.222 eV, respectively. This gives rise to the corresponding φ as 3.739, 3.775 and 4.044 eV. With the increase in calcination temperature, since EF decreases gradually, the electron deficiency also increases and therefore the adsorption as well as binding energy of OH- on the surface will gradually increase from LSCF-750 to LSCF-1200. At the same time, the 12 ACS Paragon Plus Environment

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prerequisite condition for desorption of O2 from the catalyst surface is supplying more electrons from metal d-orbital to antibonding orbital of oxygen which is more favorable for the catalyst having lower ϕ value.19,20 Thus by following Sabatier principle of optimum binding between catalyst and reactants, LSCF-975 having optimized EF and ϕ, is balanced in terms of the potential levels and emerges as the best possible OER active system. This theoretical observation supports the experimental outcome of the OER activity for the 3 systems, where LSCF-975 is validated as the best choice as OER catalyst. The right panel of Figure 5b is depicting the bird’s eye and side view of the LSCF structure. In addition structural transformations are likely to occur during OER which can be determined by applying free energy calculations,50 along with the theoretical determination of eg electrons and the rate determining step over various intermediates, which will be considered for future studies. Conclusions In conclusion, we have demonstrated an analogy between different factors such as morphology, surface area, electrical conductivity, and spin state of the active transition metal site to enhance the OER activity of a perovskite oxide nanocatalyst. The colloidal NPs of La0.699Sr0.301Co0.702Fe0.298O3-δ (δ = 0.05-0.11) prepared by an advantageous non-aqueous solgel route were calcined at three different temperatures, among which LSCF-975 could accommodate crystalline grain boundaries in between the interconnected NPs. The LSCF-975 NPs outperforms LSCF-750 and bulk LSCF-1200, requiring 440 mV overpotential at 10 mA/cm2. With an electrical conductivity close to LSCF-1200 and a midway surface area less than LSCF-750, LSCF-975 NPs could gain a larger ECSA, smaller Tafel slope, lower charge transfer resistance and eg orbital occupancy close to 1.2 that could enhance the probability of binding reaction intermediates. Combining all these factors along with a decent stability, LSCF-975 NPs emerge to be a suitable and robust OER catalyst for metal-air batteries and 13 ACS Paragon Plus Environment


allied applications. DFT calculations corroborate the experimental results whereby LSCF-975 NPs have moderate EF and ϕ values facilitating easier binding of OH- and desorption of O2 from the catalyst surface. Our strategy can be extended to other perovskite oxide electrocatalysts to gain the highest activity for a particular elemental composition. Experimental Section Materials All chemicals used in this work were used without further purification. To synthesize LSCF NPs, lanthanum (III) acetate tetrahydrate (Sigma Aldrich, 99%), strontium (II) acetate hydrate (Alfa aesar, 99%), cobalt (III) acetate hydrate (Merck, 99%), iron (III) acetate hydrate (Merck, 99%) and EG (Merck, 99%) were used. Toray CFP (Alfa Aesar), Cu-mesh (Alfa Aesar, 0.11mm), nafion (Sigma Aldrich, 5 wt%) and potassium hydroxide (KOH; Merck, ≥ 85 %) were used for electrochemical measurements. In all experiments, double distilled water was used. Methods Synthesis of LSCF NPs LSCF NPs were synthesized using our previously reported non-aqueous sol gel method.25 In brief, stoichiometric amount of metal acetate salts of the corresponding elements of LSCF were dissolved in EG by stirring at 80°C for 1 h. The resulting clear solution was further stirred for 10 h at 120°C followed by evaporating EG at 120°C to obtain a dry powder. This powder was calcined at three different temperatures viz. 750°C for 6 h, 975°C for 6 h and at 1200°C for 4 h in air to obtain LSCF-750, LSCF-975 and LSCF-1200, respectively. Calcination was also performed at 950°C for 6 h in air to obtain LSCF-950 for comparison. Pristine LaCoO3 NPs were prepared by the above protocol in absence of Sr- and Fe-acetates, followed by a similar calcination protocol. Physicochemical characterizations


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Powder XRD measurements were carried out using Rigaku (mini flex II) powder Xray diffractometer with Cu Kα radiation (λ = 1.54059 Å) with scan speed 2θ = 1o per min. Rietveld analysis of the XRD patterns was performed with the General Structure Analysis System (GSAS) software (Los Alamos National Laboratory Report, 2004). The 3dimensional views of crystal structures were obtained using VESTA 3 software. Oxygen nonstoichiometry was calculated using thermogravimetric method.51 FESEM images were recorded with Carl Zeiss SUPRA 55VP FESEM instrument. The ICP-MS measurements were carried out in a Perkin-Elmer Optima 2100 DV instrument. AFM imaging was performed using NT-MDT Nova AFM, whereby the ethanolic dispersion of the samples were drop-casted on Si wafer. TEM images were obtained with a model UHR-FEG-TEM, JEOL, JEM 2100 F instrument using 200 kV electron source, the DST-FIST facility at IISER Kolkata. Surface area was measured by N2 adsorption desorption analysis using a Micromeritics Gemini VII surface area analyzer. Before measuring the surface area the samples were kept at 170oC for 6 h for degassing. The electrical resistance of the LSCF samples was measured using a two-electrode setup using 0.2 mm thick pressed dry pellets connected to the potentiostat with silver paste. The voltage range for measuring the resistance was maintained at 1-2 V. Temperature dependant magnetization measurements were performed using the Cryogenics - Physical Property Measurements System (PPMS) with VSM probe. Electrochemical measurements All electrochemical measurements were performed in a customized three-neck glass cell in a conventional three-electrode configuration controlled by a 2-channel electrochemical workstation supplied by BioLogic Scientific Instruments, VSP300. CFP with ~0.5 cm2 geometric area was used as the working electrode, Pt rod as a conventionally used counter electrode for OER measurements and Ag/AgCl in 1 M KCl as reference electrode. The



electrolyte used for OER measurements was 1 M KOH. All applied potentials were referenced to a reversible hydrogen electrode (RHE) using the equation ERHE = EAg/AgCl + 0.059pH. The applied potential was reported after iR correction to eliminate the charge compensation due to solvent resistance using the following equation: EiR-corrected = Eapplied – i.Ru where, i is current on the electrode and Ru is the uncompensated resistance. For each measurement iR-correction was done prior to plotting the dataset. The catalyst ink was made by dispersing 5 mg of the ground powder in 1 ml 1:1 ethanol : water mixture, followed by adding 10 µl 5% nafion as binder. The mixture was mixed well in ultrasonic bath for 30 min to make an uniform dispersion. 5 µl of catalyst ink was uniformly drop casted on each side of the CFP in such a way that it covers a geometric area ~0.25 cm2. The catalyst loading on the electrode was maintained at ~0.2 mg/cm2. The catalyst ink loaded on CFP was dried at room temperature prior to use for electrochemical measurements. The measurements were performed with only the catalyst deposited area dipped inside the electrolyte solution. Prior to measuring the LSVs, pre-electrolysis was performed by normal CV scans of 20 cycles to ensure the removal of surface impurities. Before performing measurements the uncompensated solution resistance (Ru) of all electrodes was calculated using potentiostatic impedance measurement at zero open circuit potential and Ru was manually corrected to the respective applied potential during data plotting as shown above. Tafel plots were obtained by plotting overpotential with respect to the logarithmic value of obtained polarization current (log i). Electrochemical AC current impedance was measured at bias potential 1.6 V against RHE and in a frequency range from 10 mHz to 1 MHz. ECSA was calculated by measuring the capacitance of electrochemically active surface by CV in non-faradaic current zone (1.2-1.25 V). The current at same potential with different scan rates results in a straight line and the slope denotes capacitance of the active surface (Cs). The ratio of Cs to the


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specific capacitance for the standard oxide surface (C0) is the ECSA of our interest.7 Chronoamperometric tests were performed with catalyst loaded on CFP at an applied potential 1.7 V for 70 h for best catalyst and for other catalysts at 1.7 V for 22 h. Faradaic efficiencies of the LSCF samples were calculated using eudiometric gas collection techniques with homemade setup, where the evolved O2 from electrode was collected and the volume of the gas measured using an inverted sealed burette. Computational Methodology In order to support the experimental outcome of OER activity of the LSCF systems (750, 975 and 1200), we have systematically performed Density Functional Theory (DFT) based electronic structure calculations to determine the work function of three synthesized La0.699Sr0.301Co0.702Fe0.298O3-δ systems with δ = 0.05 (LSCF-750), 0.08 (LSCF-975) and 0.11 (LSCF-1200). The unit cell representing the crystal structure of the LSCF system consists of total 120 atoms, distributed as 16 Lanthanum (La), 8 Strontium (Sr), 16 Cobalt (Co), 8 Iron (Fe), and 72 Oxygen (O). After finding the minimum energy configuration of the bulk counterpart of the three systems through geometry optimization, we have created 15 Å vacuum in z-direction in order to make their (001) surface structures. Our modeling to support the experimental results is based upon the assumption of work function calculations which we determine from the surface structure of LSCF-750, LSCF-975 and LSCF-1200. The geometry optimization throughout the electronic structure calculations were performed using Vienna Ab-initio Simulation Package (VASP).52,53 We have used generalized gradient approximation (GGA) type Perdew-Burke-Ernzerhof (PBE) exchange correlation functional in this work.54 The ionic relaxation have been performed until the Hellman-Feynman forces are getting smaller than 0.001 eV/Ao for each of the individual systems. We have used 5×5×5 Monkhorst Pack k-points for the geometry optimization of bulk structure, whereas 5×5×1 for the surface structures. The spin-polarized calculations have been performed throughout this 17 ACS Paragon Plus Environment


work along with the consideration of van der Waals dispersion correction of DFT-D3 type as proposed by Grimme, to take into account the dispersive forces between the layers of LSCF systems. Supporting Information Oxygen non-stoichiometry determination; Rietveld refined XRD patterns; TEM images of LSCF-750 and LSCF-950 NPs; FESEM images of catalyst ink on Cu-mesh; Comparative table with reported perovskite oxide catalysts; Comparison with glassy carbon electrode; ECSA determination; Mass activity; Calculation of eg orbital occupancy. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements RM thanks Council of Scientific & Industrial Research (CSIR) for his fellowship. The financial support from DST-SERB under sanction no. EMR/2016/001703 is duly acknowledged. PRACE computing facility is acknowledged for performing the computation.


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Table 1: Rietveld refinement data.

LSCF-750

Rwp Co-O χ2 Volume 3 (Å) (Å) 5.49530 5.49530 13.35029 349.013 3.739 4.27 1.946

Co-O-Co Angle 170.75

LSCF-975

5.42999 5.42999 13.40335 342.248 3.244 4.34 1.927

174.72

LSCF-1200

5.47024 5.47024 13.30239 344.703 2.470 5.29 1.932

174.69

Sample

a

B

c



Figure 1: Schematics showing preparation of LSCF electrocatalysts for water splitting, the ease of electron transport through the interconnected NP morphology and spin states of the transition metal ion centers. The TEM image shows the 8 nm colloidal NPs of LSCF-975 deposited on a C-coated Cu-grid. Inset shows the size histogram.


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Figure 2: (a) XRD patterns and (b) 3-D crystallographic images with corresponding Co-OCo bond angles obtained from the CIF files of Rietveld analyses. (c) TEM image of representative LSCF-975 NPs. The high resolution images of the selected areas show (i) the grain boundaries by dotted lines and (ii) the lattice fringes corresponding to a rhombohedral system. The plot profile from a selected area in (ii) identifies the interplanar distance. (d) SAED pattern. (e) FESEM images of LSCF-750, LSCF-975 and LSCF-1200. Insets show the size histograms. (f) The corresponding AFM images with the height profiles (insets) showing interconnectivity of the particles.



Figure 3: (a) LSV polarization curves for OER. (b) Tafel plots for the OER process. (c) Nyquist plots obtained by EIS at 1.6 V vs RHE for OER. Inset shows the equivalent circuit. (d) Chronoamperometric stability test of LSCF-975 NPs on CFP in 1 (M) KOH at a constant overpotential of potential of 1.7 V vs RHE. Insets shows the LSV curves before and after the durability test, and FESEM image of LSCF-975 NPs after the OER durability test. (e) Faradaic efficiency of LSCF-750, LSCF-975 and LSCF-1200 showing the theoretically calculated and experimentally measured O2 gas with time, at 1.7 V vs RHE.


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Figure 4: (a) Current-voltage plots of the LSCF catalysts to determine their electrical resistance from inverse slope of the linear fits. (b) N2 adsorption and desorption isotherms. Inset shows the pore size distribution of LSCF-750 and LSCF-975 NPs. Temperature dependence of inverse magnetic susceptibility where the straight lines show data fitting to the Currie-Weiss law for (c) LSCF and (d) LCO samples.



Figure 5: Left panel shows the variation of work function (φ) values in LSCF-750, LSCF975 and LSCF-1200. Right panel shows the bird’s eye view (top) and side (bottom) perspective of LSCF systems.


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Table of Contents (TOC)



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Maneuvering the Physical Properties and Spin States to Enhance the

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