Verónica
Celorrio
a,
Laura
Calvillo
b,
Ellie
Dann
a,
Gaetano
Granozzi
b,
Ainara
Aguadero
c,
Denis
Kramer
d,
Andrea E.
Russell
e and
David J.
Fermín
*a
aSchool of Chemistry, University of Bristol, Cantocks Close, BS8 1TS, Bristol, UK. E-mail: David.Fermin@bristol.ac.uk
bDipartimento di Scienze Chimiche, Università di Padova, Via Marzolo 1, 35131 Padova, Italy
cDepartment of Materials, Imperial College, SW7 2AZ, London, UK
dEngineering Sciences, University of Southampton, SO17 1BJ, Southampton, UK
eChemistry, University of Southampton, SO17 1BJ, Southampton, UK
First published on 13th July 2016
The mean activity of surface Mn sites at LaxCa1−xMnO3 nanostructures towards the oxygen reduction reaction (ORR) in alkaline solution is assessed as a function of the oxide composition. Highly active oxide nanoparticles were synthesised by an ionic liquid-based route, yielding phase-pure nanoparticles, across the entire range of compositions, with sizes between 20 and 35 nm. The bulk vs. surface composition and structure are investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES). These techniques allow quantification of not only changes in the mean oxidation state of Mn as a function of x, but also the extent of A-site surface segregation. Both trends manifest themselves in the electrochemical responses associated with surface Mn sites in 0.1 M KOH solution. The characteristic redox signatures of Mn sites are used to estimate their effective surface number density. This parameter allows comparing, for the first time, the mean electrocatalytic activity of surface Mn sites as a function of the LaxCa1−xMnO3 composition. The ensemble of experimental data provides a consistent picture in which increasing electron density at the Mn sites leads to an increase in the ORR activity. We also demonstrate that normalisation of electrochemical activity by mass or specific surface area may result in inaccurate structure–activity correlations.
LaMnO3+δ has been reported as one of the most active perovskite materials for the ORR.7,16,18 A growing number of publications on the electrocatalytic properties of Mn-based oxides have emerged in recent years,19–24 proposing composition–activity relationships which may appear somewhat contradictive. For instance, Stoerzinger et al. concluded that Mn3+ sites are the most active for ORR in La1−x(Ca, Sr)xMnO3 (011)-oriented films.24 In a subsequent work, it is proposed that the most active composition of La(1−x)SrxMnO3 contains 33% Sr, promoting a mixed Mn3+/4+ state.23 Du et al. also concluded that a mixed valency of Mn3+/4+ is the most active in CaMnO3−δ with 0 < δ < 0.5.25 The interesting review by Stoerzinger et al. shows a significant spread in the activity of Mn oxide catalysts, based on kinetic current normalized by mass at a given potential, spanning by several orders of magnitude.18
The complexity of these systems arises not only from bulk structural properties such as phase purity, but also from off-stoichiometric surface composition due to preferential elemental segregation.12,26 For instance, Lee et al. showed that the tendency of LaxA1−xMnO3 to segregate is dependent on the size between the host and the dopant following the trend Ba > Sr > Ca.12 Consequently, it is a real challenge to estimate the electroactive surface area of these materials not only in the case of nanoparticles supported on porous carbon layers, but also in continuous thin films.
This work assesses, for the first time, the mean activity of individual surface Mn-sites in a family of LaxCa1−xMnO3 nanoparticles towards the ORR in alkaline solutions. Oxide nanoparticles across the entire composition range were synthesised with a high degree of phase purity employing an ionic liquid-based approach.16 Quantitative analysis based on X-ray diffraction, X-ray photoemission spectroscopy and X-ray absorption near edge spectroscopy allowed uncovering of the bulk and surface structures of the oxide nanostructures, as well as the mean Mn oxidation state. Kinetic current values normalised by either the mass or specific surface area of the catalysts showed that these materials rank among the most active for the ORR, although the composition dependence is somewhat unclear employing these standard normalisation parameters. However, a clearer trend emerges when the kinetic responses are normalised by the mean Mn surface number density obtained from electrochemical analysis. We conclude that the activity of surface Mn sites increases as the mean oxidation state decreases promoted by increasing La content on the A-site.
The crystal structure of LaMnO3 was accurately refined in the cubic Pm3m space group without the need for any added structural distortion. Every feature in the XRD pattern was accurately refined justifying the assignment of the most symmetrical group. However, it is important to note that the relatively broad XRD features arising from the small particle sizes may introduce a degree of uncertainty in the identification of the LaMnO3 phase group, which has been previously assigned to the orthorhombic Pbnm phase at room temperature.28 On the other hand, CaMnO3 and the mixed compositions LaxCa1−xMnO3 were accurately refined in the orthorhombic Pnma space group. Table S1† summarizes the unit-cell parameters and discrepancy factors after the refinements. It could be seen that the unit cell volume shows a non-monotonic dependence on the La content due to the influence of a variety of parameters such as contrast in ionic radii, the oxidation state of Mn and the presence of oxygen vacancies. Table S2† shows the atomic composition of the materials as obtained by SEM-EDX, which closely reflects the composition of the ionic liquid precursor. Table S3† outlines the corresponding specific surface area (SSA) estimated for the various oxide nanostructures. In view of the relatively narrow size distribution and low porosity of the nanoparticles, the SSA was estimated employing a geometric approximation and the material density which was obtained from the XRD refinement analysis.
Core level photoemission spectra of La 3d, Ca 2p, O 1s and Mn 2p regions for the various LaxCa1−xMnO3 samples are displayed in Fig. 2. The La 3d lines (Fig. 2a) show typical double splitting due to the spin–orbit interaction and the shake-up excitation of an oxygen valence band electron to the empty La 4f level. The La 3d5/2 is located at 834.4 eV, corresponding to La3+ compounds,29,30 and does not change with the addition of Ca.
The Ca 2p photoemission line can be deconvoluted into two components (Fig. 2b and Table S4†). Ca2+ at the perovskite lattice exhibits a binding energy (BE) shift from 344.8 eV to 346.3 eV with increasing La content. This shift has already been observed in the literature but no clear explanation has been reported.29,31 In principle, this effect can be attributed to the changes in the nearest neighbors of Ca atoms: the variation of the amount of La is expected to change the electronic structure of the oxygens and consequently that of the Ca atoms. The second Ca 2p3/2 component identified in the range of 346.7–347.4 eV is attributed to the formation of CaCO3 and/or CaO at the oxide surface due to Ca segregation.20,32 The presence of a C 1s component at 289.6 eV (Fig. S4a†) and O 1s at 532.2 eV is consistent with the generation of CaCO3 and CaO at the oxide surface.
The O 1s photoemission peak is deconvoluted into five contributions described by symmetrical Voigt functions (see Fig. 2c and Table S5†). The lower BE component is assigned to oxygen in the perovskite lattice (metal–oxygen bonds) centred at 528.5 eV.33 In the case of the samples containing La, this component becomes very small due to the segregation of La and consequent formation of La2O3 at the surface. The presence of the La2O3 species is represented by the component at 529.3 eV. The third component at 530.9–531.1 eV is attributed to hydroxyl groups, whereas that at about 532.1 eV can be associated with CaO and/or CaCO3 formed at the surface due to Ca segregation, as well as with carbonyl groups. The component at higher BE is associated with adsorbed water.20,30,34
The photoemission lines in the Mn 2p region are displayed in Fig. 2d, with the Mn 2p3/2 peak located at 642 eV. This broad peak contains the contribution from Mn3+ (641.9 eV) and Mn4+ (642.2 eV) signals.29 The maximum BE value of this signal shifts towards lower energies as x increases (see also Fig. S4†), which is consistent with an increase in the Mn3+/Mn4+ ratio. Quantitative deconvolution of the two Mn oxidation states is rather challenging given the small differences in BE values. As discussed below, XANES analysis provides a far more accurate determination of the mean Mn oxidation state.
The surface composition of LaxCa1−xMnO3 obtained from the photoemission studies is summarised in Fig. 2e (see also Table S6†). Due to the polycrystalline nature of the samples, the surface composition can only be considered as an effective value rather than a specific facet of the nanoparticles. The first interesting observation is that the surface atomic ratio of La and Ca is different from the bulk ratio (dotted line). For x values between 0.4 and 0.6, the surface La content is higher than that in the bulk, demonstrating the preferential segregation of this cation, which forms La2O3 at the surface. It is also observed that the atomic ratio of A- to B-sites significantly deviates from the bulk values as the La content increases. A-site segregation has been consistently observed in perovskite materials prepared at high temperature.13,35,36 However, the A-site surface segregation trend seems to break down at high x-values in LaxCa1−xMnO3, with LaMnO3 showing a higher surface Mn content than La0.81Ca0.19MnO3. As demonstrated further below, this trend is also consistent with electrochemical responses sensitive to the surface density of Mn sites.
The mean oxidation state of Mn sites as a function of the perovskite composition is estimated from the XANES spectra displayed in Fig. 3a. The main Mn K-edge in the case of LaMnO3 is shifted by 4 eV to lower energies in comparison to CaMnO3. A similar difference is observed between MnO2 and Mn2O3 standards (Fig. S5†), which is consistent with Mn3+ and Mn4+ oxidation states.37,38 The inset in Fig. 3a shows that the intensity of the pre-edge feature decreases with increasing La content, further confirming a decrease in the mean Mn oxidation state with increasing La content.37,39
The Mn K-edge position can also be affected by local distortion of the MnO6 octahedra, leading to an apparent edge shift even in the absence of an oxidation state change.40–42 Consequently, the Mn valency was probed from the pre-edge energy position following the approach reported by Croft et al.43Fig. 3b shows a linear relationship between the pre-edge position (established from the first derivative of the spectra) and the Mn oxidation state of the standards MnO2, Mn2O3 and Mn3O4, in agreement with literature values.39,43 The mean Mn oxidation state of LaxCa1−xMnO3 was estimated from the trend shown in Fig. 3b, providing values consistent with the composition of the A-site. For instance, the pre-edge position of CaMnO3 was very close to that of the standard MnO2, confirming the predominance of a Mn4+ oxidation state. It can also be seen that increasing values of x lead to a decrease in the mean Mn oxidation state. It is interesting to note that Mn exhibits a 2.8 oxidation state in the case of LaMnO3, strongly indicating the presence of oxygen vacancies.
Fig. 4a shows characteristic cyclic voltammograms of the various LaxCa1−xMnO3 nanostructures supported on a mesoporous carbon layer (Vulcan) in argon-saturated 0.1 M KOH solution at 0.010 V s−1. The oxide loading was kept constant at 250 μg cm−2 in all experiments. The faradaic charges associated with the anodic and cathodic peaks are very similar, and the responses remain stable in this potential range. These results suggest that the stability of these materials is not significantly compromised within the timescale of these experiments. Investigations carried out over a wide potential range revealed that the stability of the oxide appears to be compromised at potentials above 1.7 V. LaMnO3 features two cathodic reduction peaks located at 0.90 and 0.50 V, which have been described in terms of the formation of a partially reduced state, prior to the reduction of Mn2+.11,16,44,45 The voltammogram of CaMnO3 is characterised by a broad reduction peak centred at 0.80 V, ascribed to the reduction of Mn4+ to Mn2+.44,45 A small feature at ∼0.5 V is also observed superimposed to the main broad voltammetric peak, which may suggest the presence of Mn3+-like sites at the surface of CaMnO3. A systematic increase in current with increasing Ca2+ content in the A-site is observed. This trend is also a reflection of the dependence of surface B-site depletion on the composition of the oxide. Although there are a number of interesting features in these voltammetric responses, which are still under investigation, we shall confine our analysis to the faradaic charges involved in the reduction of surface Mn sites. A recent study has also shown comparable voltammetric responses of various Mn oxides, including LaMnO3, although the assignment of redox states is not compatible with our observations.15
Fig. 4 Cyclic voltammograms of CaMnO3, La0.37Ca0.63MnO3, La0.60Ca0.40MnO3, La0.81Ca0.19MnO3 and LaMnO3 nanoparticles supported on a mesoporous carbon electrode in Ar-saturated 0.1 M KOH solution at 0.010 V s−1 (a). The oxide content in each electrode was 250 μg cm−2. As the Ca content increases, the intensity of the voltammetric features increases. The traces of La0.81Ca0.19MnO3 and LaMnO3 effectively overlap in this scale. Dependence of the effective Mn atomic surface density with La content (b). The number density of Mn sites was estimated from integrating the voltammetric responses. Details of these calculations are given in the ESI.† |
The cyclic voltammograms in Fig. 4a allow estimating the mean Mn surface number density (ΓMn) for the various oxide compositions as shown in Fig. 4b.16 The charge is obtained from integration of the voltammetric responses across the potential range in Fig. 4a, which reflect changes from the initial oxidation state of Mn sites to a Mn2+ state. It is important to note that the open circuit potential for all of the oxides is located at potentials slightly more positive than the onset of the reduction wave located around 0.9 V. Consequently, the effective redox state estimated from XANES can be considered as the initial state in the potential range investigated. Fig. S6† illustrates the background current correction used for integration of the voltammograms. Due to the large background current in the voltammetric responses, a self-consistent approach was implemented involving: (i) balancing the charges of oxidation and reduction features, and (ii) systematic variation of the background threshold. The error bars associated with the charge in Table S7† and Mn number density (Fig. 4b) include this systematic analysis of the background current. Table S7† summarises the values obtained from the integration of the voltammetric signals and the methodology used for estimating ΓMn, employing density values obtained from the XRD refinement and the mean particle size. It is rather remarkable that ΓMn (Fig. 4b) and the B/A-site ratio obtained from XPS (Fig. 2e) show a very similar composition dependence. This observation demonstrates that voltammetric analysis can provide useful information on the surface composition of these oxides. However, it should also be considered that the penetration depth of XPS (in the range of 6 nm) probes a larger portion of the oxide composition with respect to the electrochemical signal, which is sensitive to the oxide/electrolyte boundary.
Disk (iDISK, bottom panel) and ring (iRING, top panel) currents obtained at 1600 rpm and 0.010 V s−1 in an O2-saturated 0.1 M KOH solution are contrasted in Fig. 5 for all of the carbon-supported oxides. The ORR onset potential is significantly more positive than that of the Vulcan support (ca. 0.7 V),16 confirming that the reaction mainly takes place at the oxide particles at low overpotentials. Substantial peroxide detection at the ring electrode is not observed until 0.7 V, further indicating that ORR mainly occurs through a four-electron mechanism. The most positive onset potential is observed for LaMnO3, while CaMnO3 requires a higher overpotential to initiate the reaction. As illustrated in Fig. S7,† the shift in ORR onset potential appears to mirror the displacement in the first peak of Mn reduction. This observation is consistent with our previous studies linking the ORR kinetics with increasing electron density at the Mn site under operational conditions.16
The ring and disk current responses as a function of the rotation rate for CaMnO3 are displayed in Fig. S8.† In Fig. S9,† it can be seen that the effective number of transferred electrons (n) and the hydrogen peroxide yield (% HO2−) are above 3.5 and below 25% for all of the perovskite nanostructures. The Koutecky–Levich plots in the range of 0.53 to 0.64 V (Fig. S10a–e†) are characterised by slopes consistent with the four-electron reduction process. From these plots, the kinetic current as a function of the applied potential can be extracted as shown in Fig. S10f.†
Fig. 5b displays the kinetic current at 0.80 V normalised by the effective number of surface Mn sites (Table S7†). For the first time, we can demonstrate a clear trend in the activity of surface Mn sites with the mean oxidation state. To put these values in the context of conventional benchmarking, Fig. S11† shows the normalisation of the kinetic current by catalyst mass and specific surface area. At first, the data show that these materials are among the most active reported for the ORR.6 Interestingly, normalisation by these parameters leads to non-monotonic dependencies with La content. Consequently, we conclude that systematic analysis of catalyst performance as a function of the oxide composition requires explicit determination of the A-site surface segregation and oxygen vacancies (affecting the B-site oxidation state).
Finally, the physical principles responsible for the correlation between ORR catalytic performance and the oxidation state of the Mn site remain to be elucidated. From a simplistic phenomenological point of view, it could be postulated that higher electron density at the Mn-site leads to a higher capacity for O–O bond breaking. It should also be considered that the effective electron density of the surface Mn sites is affected not only by the valency of the A-site and oxygen vacancies, but crucially by the electrode potential. Although this description offers guiding principles for optimisation of Mn-based perovskites, the physical rationalisation of this observation requires complex theoretical modelling capable of including the effect of water.
We strongly believe that any attempt to establish structure–reactivity relationships should carefully consider the complex surface chemistry of these oxides. Normalisation by the mass of the catalysts can provide useful information in terms of overall performance. However, this approach does not consider phenomena such as surface segregation, which is strongly dependent on the nature of the A-site and the oxide phase formation temperature. In this particular family of compounds, La3+ promotes the optimum oxidation state for Mn although the overall activity of the catalysts is somewhat compromised by the strong tendency of La3+ to segregate at the oxide surface in comparison to Ca2+. Based on these observations, we can predict that promoting the highly electron-rich Mn site, crystallised at low temperatures, may lead to a substantial increase in the ORR activity.
Further details of the crystal structure investigation(s) may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen (Germany), on quoting the depository numbers CSD 431077, 431082, 431086, 431090 and 431091.
Footnote |
† Electronic supplementary information (ESI) available: TEM images, particle size distributions, H-Res TEM images, Mn 2p3/2 inset, XANES spectra for standard Mn compounds, calculation of the number of Mn sites, cyclic voltammetry in Ar-saturated solution, RRDE data for CaMnO3 at different rotation rates, calculation of the number of electrons and HO2− yield, Koutecky–Levich plots and different normalizations for ik. See DOI: 10.1039/c6cy01105e |
This journal is © The Royal Society of Chemistry 2016 |