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Properties of Ca1−xHoxMnO3 perovskite-type electrodes
Electrochimica Acta 54 (2009) 5902–5908
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Properties of Ca1−x Hox MnO3 perovskite-type electrodes B.M. Ferreira a , M.E. Melo Jorge b,∗ , M.E. Lopes c , M.R. Nunes b , M.I. da Silva Pereira b a
Dep. de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal Dep. de Química e Bioquímica, Centro de Ciências Moleculares e Materiais, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal c Dep. de Química, Centro de Química de Évora, Universidade de Évora, Évora, Portugal b
a r t i c l e
i n f o
Article history: Received 8 April 2009 Received in revised form 29 April 2009 Accepted 13 May 2009 Available online 21 May 2009 Keywords: Nanocrystalline manganites Holmium doping Electrical properties Electrochemical characterization Manganite electrodes
a b s t r a c t In this work, Ca1−x Hox MnO3 (x = 0, 0.1 and 0.2) perovskite oxide pelleted electrodes were prepared from the respective powders obtained by the citrate route method at 1173 K. The electrodes exhibit particle size that decreases with the holmium content in the oxide. All the samples reveal semiconductor behaviour and the presence of holmium induces a marked decrease in the electrical resistivity. The results can be well attributed to the changes in the Mn4+ /Mn3+ ratio. Electrodes were characterized by cyclic voltammetry and chronopotentiometry. Cyclic voltammetric studies indicate a similar behaviour of the electrodes, irrespective of their composition. Two pairs of peaks were identified and associated, one to the Mn4+ /Mn3+ redox couple and the other to the Mn7+ /Mn4+ and Mn6+ /Mn4+ redox couples. The voltammetric data provide evidence that the electrodes roughness factor increases with the introduction of Ho-ions in the oxide structure, what is consistent with the crystallite size obtained by X-ray diffraction (XRD) and the morphology observed by scanning electron microscopy (SEM). The Hosubstituted electrodes present higher current density when compared with CaMnO3 electrodes what can be attributed both to higher electrical conductivity and smaller particle size. The chronopotentiometric studies have shown that the discharge occurs by different mechanisms for the oxide electrodes with and without Ho. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Many perovskite-type oxides ABO3 with B = Fe, Co and Mn, exhibit high electronic conductivity. This is a result of the hopping mechanism due to the B metal ions mixed valence, the oxide ions high mobility in the crystal and the easy changes in the oxygen contents in their lattices, leading to electrode materials with potential technological applications. A great variety of oxides, with perovskite-type structure, has been exploited in the past [1–8], namely as electrode materials (La1−x Srx FeO3 ) [1], electrolyte materials for solid oxide fuel cells (La1−x Srx MnO3 and LaMO3 , with M as a transition metal) [2,3], cathodes in alkaline batteries (lanthaniumsubstituted CaMnO3 ) [4] and as potential catalysts alternative to noble metals ((La,Sr)(Co,M)O3 with M = Cu, Fe, Ni, Cr and Mn) [5,6]. These distinct applications are an outcome of the good electrical conductivity, electrocatalytic activity and thermal and chemical stabilities. Until now many detailed studies have been published involving different hole-doped manganites Ln1−x Ax MnO3 bulk samples and films (Ln = trivalent rare-earth cation; A = divalent alkaline earth cation). In contrast there are only a few reports on electron-doped
∗ Corresponding author. E-mail address: [email protected] (M.E. Melo Jorge). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.05.054
materials, corresponding to Mn4+ -rich composition. This work is inserted in this context; more precisely it regards the effect of the partial replacement of Ca2+ ion by a trivalent ion on the properties of CaMnO3 . This substitution induces a reduction of Mn4+ ions to Mn3+ ions due to charge compensation and consequently modifies the oxide properties. Due to their ionic radius and valence stability the lanthanides are the trivalent ions adequate to substitute Ca2+ in the A-site of the perovskite structure. Studies concerning the Ca1−x Cex MnO3−ı oxide system properties, prepared by the ceramic method [9,10], indicate these materials as possible candidates for cathode materials in alkaline batteries. Recently we have prepared Ca1−x Cex MnO3 (x = 0, 0.1, 0.2) pellet electrodes, from the powders synthesized by the citrate route [11]. The study has shown that the presence of the Ce4+ ion enhances the oxide electrical conductivity and the surface electrochemistry is governed by the Mn4+ /Mn3+ redox couple. In addition, the data indicate that the prepared Ca1−x Cex MnO3 electrodes could be promising cathode materials for alkaline batteries [12]. Lately we have reported that substituted perovskite-type oxides Ca1−x Hox MnO3 (x = 0.1, 0.2), prepared by the citrate route [13,14], are chemically stable and compared with the parent CaMnO3 , show a decrease in grain size and higher electrical conductivities from room temperature up to 873 K. These properties make these oxides potentially interesting as electrode materials.
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Fig. 1. X-ray diffraction patterns for Ca1−x Hox MnO3 (x = 0, 0.1, 0.2).
Based on the idea of using them as cathode materials in alkaline batteries and considering that little information on the aqueous electrochemistry of these compounds exists up to now, the properties of the Ca1−x Hox MnO3 (x = 0, 0.1 and 0.2) electrodes in alkaline solution have been investigated. The citrate route method has been chosen to synthesize the oxide powders, based on the encouraging results of our previous studies [12–14]. Cyclic voltammetry (CV) and chronopotentiometry were employed to study the electrochemical behaviour of the Ca1−x Hox MnO3 (x = 0, 0.1 and 0.2) oxide electrodes. 2. Experimental Bulk ceramic samples Ca1−x Hox MnO3 (0 ≤ x ≤ 0.2) were prepared from appropriate amounts of CaCO3 (Aldrich > 99.995%), Ho2 O3 (Aldrich > 99.9%) and Mn(NO3 )2 ·4H2 O (Riedel-de Haën, p.a.) by the citrate route method as described in previous papers [13,14]. However a different heat treatment was performed. After the decomposition at 873 K, for 6 h, the resulting amorphous powder was grounded and heated in air at 1173 K for 18 h in alumina crucibles (Alsint 99.7). The oxide powder (≈200 mg) was uniformly distributed onto an inserted Pt mesh and pressed, into 1 mm thick, ≈2 cm2 platelet pellets under 40 bar pressure and sintered at 1173 K,
Fig. 2. SEM micrographs (15,000×) of Ca1−x Hox MnO3 system: (a) x = 0, (b) x = 0.1 and (c) x = 0.2.
in air, for 6 h. The electrical contact was made by welding the Pt mesh to a copper wire. The samples were then mounted in a glass tube with Araldite epoxy resin, so that the electrolyte could only make contact with the oxide. Three specimens were prepared for each composition, two for the electrochemical experiments and one for morphological characterization.
Table 1 Lattice parameters, crystallite and particle size for Ca1−x Hox MnO3 . x
a (Å)
b (Å)
c (Å)
V (Å3 )
Crystallite size XRD (D) (nm)
Particle size SEM (nm)
0 0.1 0.2
5.2785 (4) 5.2923 (5) 5.3314 (5)
7.4613 (5) 7.4623 (5) 7.4721 (5)
5.2644 (4) 5.2781 (4) 5.2840 (4)
207.33 (4) 208.44 (5) 210.50 (5)
79 45 40
250 100 80
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Fig. 3. Arrhenius plots of log (T) vs. 1/T for the samples Ca1−x Hox MnO3 (x = 0, 0.1, 0.2). Table 2 Room temperature electrical conductivity ( 300 K ), activation energies (Ea ) and valence state of Mnx+ ions for Ca1−x Hox MnO3 . x 0 0.1 0.2
300 K (−1 cm−1 ) 0.21 15.32 6.21
Temperature range used for obtaining Ea (K)
Ea (meV)
Mnx+
298–600 623–873 298–873 298–873
73 358 88 95
3.98 3.9 3.8
Phase identification and structural analysis of the polycrystalline powder and the electrodes were carried out by XRD at room temperature using a Philips PW 1730 X-ray powder diffractometer, operated at 40 kV and 30 mA, employing a monochromatized Cu K␣ radiation in Bragg–Brentano geometry. The data were collected at 0.02◦ step in 2 over the range 20◦ ≤ 2 ≤ 90◦ with a counting time of 0.8 s per step and the lattice parameters were determined. The average crystallite size (D) was evaluated from the XRD data by applying the Scherrer’s formula D = k/ˇ cos , where k is a constant depending on the grain shape (k = 0.89, for circular grain),
the wavelength of Cu K␣ radiation, the Bragg diffraction angle 2 − ˇ2 where of the most intense peak and ˇ is defined as ˇ2 = ˇm s ˇm and ˇs are the experimental full width at half maxima (FWHM) and the FWHM of a standard silicon sample, respectively [15]. The samples surface morphology and grain size were investigated by scanning electron microscopy using a JEOL (JSM-35C). The oxygen stoichiometry in each sample was determined by iodometric titration. Potassium iodide and hydrochloric acid solutions were added to dissolve the sample in a flask; the solution was titrated with a standard sodium thiosulfate. All samples can be considered stoichiometric within the experimental error and consequently, the effect from oxygen vacancies should be negligible. The variation of the electrical resistivity with temperature was measured using a four-probe technique in the temperature range of ∼300–900 K. The oxides electrochemical behaviour was studied by means of open circuit potential measurements and cyclic voltammetry, in 1 mol dm−3 KOH solutions. Solutions were prepared from AnalaR reagents with Millipore Milli-Q water and degassed with nitrogen, 99.999% purity gas supplied by Air Liquide. The electrochemical experiments were performed in a two-compartment three-electrode glass cell at room temperature. The counter electrode was a graphite rod and, as reference, an Hg/HgO (0.099 V vs. SHE) was used. Voltammetric studies were carried out using a low noise operational amplifier potentiostat incorporated with positive feedback IR compensation, programmed by a Bank VSG 83 waveform generator and a Kipp & Zonen Pró-1 recorder and the chronopotentiometric studies with a Voltalab 32 Radiometer apparatus connected to an IMT 102 interface, controlled by a personal computer through the VoltaMaster 2 software. 3. Results and discussion 3.1. X-ray powder diffraction XRD results revealed the formation of perovskite-type single phases (Fig. 1) for all the samples and the diffractograms are characteristic of the orthorhombic symmetry, space group Pnma. The increase in the values of the cell parameters (Table 1) is caused by an expansion of the octahedral site (B-site) that overcomes the contraction on the A-site due to an increase in Mn3+ ion content, as discussed in the literature [13,14] and according with the ionic radius values [16]. Our data are in excellent agreement with previously reported results for the same system synthesized by different methods [13,14,17–22]. The average crystallite size (D) was evaluated from the broadening of the XRD line width by applying the Scherrer’s formula. From the estimated average crystallite size, as shown in Table 1, one finds that this parameter decreases with increasing holmium content. Similar tendency has been found previously for samples with identical composition [13,14]. 3.2. Scanning electron microscopy
Fig. 4. Cyclic voltammograms for the Ca0.9 Ho0.1 MnO3 electrodes in 1 mol dm−3 KOH at a sweep rate of 10 mV s−1 , for different positive and negative potential limits.
The SEM images shown in Fig. 2 clearly indicate that the introduction of Ho-ion in the oxide samples led to smaller grains, although the shape remains similar. Since the compounds were sintered under the same conditions, we can conclude that the difference in the grain size results mainly from the influence of the holmium content on the grain growth. The crystallite size obtained by XRD is small compared to the grain size observed by SEM (see Table 1). This difference is probably due to the crystallite aggregation. As it has been referred previously, a grain may consist of several crystallite domains which appear due to twinning and other structure defects in the grain [23]. The SEM images show a similar trend on the grain size variation with the Ho-content as reported by us in a previous work [13,14].
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conductivity and the activation energy for conduction. The linear dependence of log (T) vs. 1/T is characteristic of the polaron hopping transport mechanism, for which the conductivity can be represented by the function = (A/T) exp (−Ea /kT) [24]. The electrical conductivity increases with the increase in temperature, for all compounds, indicating its semiconducting nature. Moreover, the calcium partial substitution by holmium causes a significant increase in the electrical conductivity. Our results confirm those already published and discussed [14]. 3.4. Open circuit potential measurements The open circuit potential of the Ca1−x Hox MnO3 (x = 0, 0.1 and 0.2) oxide electrodes was measured in KOH 1 mol dm−3 . An average value of 0.097 ± 0.025 V vs. Hg/HgO has been obtained. No meaningful changes were observed by the presence of Ho-ions in the oxide. The value approaches the thermodynamic value calculated for the Mn4+ /Mn3+ solid-state redox couple, 0.089 V vs. Hg/HgO [25]. This result indicates that the Mn4+ /Mn3+ redox couple is the determinant on the surface equilibrium reaction, what is in accordance with previous studies on CaMnO3 -based electrodes [9,12]. 3.5. Cyclic voltammetric studies
Fig. 5. Cyclic voltammograms for the Ca1−x Hox MnO3 (x = 0, 0.1, 0.2) electrodes in 1 mol dm−3 KOH at a sweep rate of 10 mV s−1 .
3.3. Electrical conductivity studies Fig. 3 presents Arrhenius plots of log (T) vs. 1/T, for all the samples, and Table 2 summarizes the room temperature electrical
Cyclic voltammograms were recorded for the different oxides, in 1 mol dm−3 KOH at a sweep rate of 10 mV s−1 , between the open circuit potential and different positive and negative limits. Fig. 4 shows a representative family of voltammograms for the Ca0.9 Ho0.1 MnO3 . Taking into account this result, it is possible to associate the anodic peaks A1 and A2 to the cathodic peaks C1 and C2, respectively. The safe voltage window was also established and cyclic voltammograms were run between +0.5 V and −0.3 V for the different electrodes. The obtained curves are displayed in Fig. 5. Comparing the different curves, an increase in the current density is observed for the samples containing Ho. This result could be due to an increase of the oxide surface area by the presence of Ho. Table 3 presents the peak potentials and the calculated formal potential (Ef ) of the redox couples associated to the A1/C1 and A2/C2, estimated from the anodic and cathodic peak potentials, based on the following equation, Ef = (Epa + Epc )/2. The results show that the presence of Ho induces a shift on the anodic (A2) and cathodic (C2) peaks, towards more negative and positive potential values, respectively. This behaviour can be associated with the changes on the samples electrical properties. In what concerns peaks A1 and C1, this behaviour is not observed. An additional cathodic peak C appears in the cyclic voltammograms of the Hocontaining electrodes associated with the increase in current at the positive potential limit. Based on the potential values, the pair of peaks A2/C2 has been assigned to the Mn4+ /Mn3+ solid-state surface redox couple that compares well with the calculated equilibrium potential, 0.089 V vs. Hg/HgO. Peaks A1/C1 were associated with soluble Mn species with higher valence states Mn4+ /Mn6+ and/or Mn4+ /Mn7+ , which slightly depart from the calculated thermodynamic potentials at pH 14 that are 0.504 and 0.491 V vs. Hg/HgO, respectively [25]. This is in accordance with the purple colour appearing in the test solution, near
Table 3 Peak and formal potentials for the redox couples associated to the voltammetric peaks with the electrodes of Ca1−x Hox MnO3 . x
0 0.1 0.2
E (V) vs. Hg/HgO A1
C1
Ef
A2
C2
Ef
0.381 ± 0.038 0.337 ± 0.023 0.340 ± 0.017
0.291 ± 0.009 0.257 ± 0.025 0.273 ± 0.043
0.336 ± 0.038 0.297 ± 0.025 0.307 ± 0.043
0.110 ± 0.053 0.060 ± 0.029 0.047 ± 0.013
−0.070 ± 0.019 −0.053 ± 0.054 0.060 ± 0.031
0.020 ± 0.053 0.004 ± 0.054 −0.007 ± 0.031
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the electrode surface before the oxygen evolution, indicating that Mn is lost from the oxide. The effect of the potential scan rate (), on the cyclic voltammograms, was investigated and Fig. 6a presents the respective curves for the Ca0.9 Ho0.2 MnO3 electrode recorded for sweep rates of 5, 10, 15, 20 and 25 mV s−1 . As it can be seen the peak current, for both anodic and cathodic waves, increases with the increasing of potential scan rate. A linear dependence of jp vs. 1/2 is observed for the anodic peak A2, for all the studied electrodes, which is characteristic of a diffusion type process (Fig. 6b). The peak potential separation (Ep ), between the anodic and cathodic peaks, increases linearly with sweep rate for all the compositions. The extrapolation of Ep to zero sweep rate gives the values 52, 1 and 17 mV for the oxides with x = 0, 0.1 and 0.2, respectively. It is interesting to note that the lower value is obtained for the Ca0.9 Ho0.1 MnO3 electrode. Ep is usually taken as a measure of the ohmic drop, including two components: the uncompensated ohmic drop between the electrode surface and the Luggin capillary tip and the ohmic drop across the oxide [26]. Since all the experiments were carried out in the same solution and with the same geometric cell arrangement, the component due to the uncompensated resistance of the electrolyte can be left behind and it can be concluded that the observed differences are due to the ohmic drop across the oxide. These results are in perfect agreement with the electrical conductivity data, at room temperature as Fig. 6 shows, and provide a striking demonstration of the importance of the electrical conductivity in determining the electrochemical response of the perovskite electrodes, as Fig. 6c shows. Fig. 7a presents cyclic voltammograms obtained between 0 and 0.2 V vs. Hg/HgO. The shape of the voltammograms indicates a typical capacitive behaviour for the three materials and become more rectangular in shape for the sample containing Ho. Also the current response when the sweep direction changed becomes faster what is in accordance with the enhanced oxide electrical conductivity and marginal solution resistance. A linear variation between the current density, measured at E = 0.1 V vs. Hg/HgO and the sweep rate is observed for all the samples as Fig. 7b also displays. The capacitance was determined according to the equation C = dj/d(dV/dt), where j is the current density and dV/dt the potential sweep rate. Assuming a value of 60 F cm−2 for the capacitance of the oxide/aqueous solution interface, the electrodes roughness factors have been also calculated [27]. The calculated specific capacitance and roughness factors are presented in Table 4. A simple analysis shows that the capacitance increases with increasing the amount of Ho-ions in the samples, however a more precise analysis indicates that for x = 0.1 an increase of 140% occurs in relation to the CaMnO3 oxide while when the Ho-ions amount changes from 0.1 to 0.2 the increase in capacitance is only 14%. Therefore it can be conclude that the important factor is the presence of Ho-ions in the samples and not so much the amount. This is consistent with the crystallite size obtained by XRD and the morphology observed by SEM for the Ho-containing electrodes. Therefore the capacitance increase for the samples containing Ho-ions can be associated with the enhancement of the oxides surface area. Rf values are higher than those published by us for the Ca1−x Cex MnO3 (x = 0, 0.1, 0.2) electrodes. The differences are attributed to the lower synthesis temperature and shorter heat treatment. 3.6. Chronopotentiometric studies The potential–time curves for the Ca1−x Hox MnO3 (x = 0, 0.1 and 0.2) cathodes, at 1 mA cm−2 in the potential range of 0 to −1.6 V vs. Hg/HgO, are presented in Fig. 8. A qualitative analysis of the curves shows three distinct regions, for all the electrodes. Initially (Fig. 8 inset) a decrease on the potential occurs followed by a well-
Fig. 6. (a) Family of cyclic voltammograms at sweep rates 5, 10, 15, 20 and 25 mV s−1 for the Ca0.9 Ho0.1 MnO3 electrode in 1 mol dm−3 KOH. (b) Variation of the peak current (A2) with the sweep rate. (c) Variation of the peak separation with the room temperature electrical conductivity.
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Fig. 7. (a) Cyclic voltammograms obtained in the double-layer region and (b) doublelayer charging current as a function of sweep rate for the Ca1−x Hox MnO3 (x = 0, 0.1, 0.2) electrodes in 1 mol dm−3 KOH at a sweep rates of 1, 2, 4, 6, 8 and 10 mV s−1 .
defined plateau, between −0.300 and −0.400 V vs. Hg/HgO. The initial potential decrease has been assigned to the reduction of the Mn4+ ion to Mn3+ ion and the plateau to the reduction of the Mn3+ ion to Mn2+ ion [28]. As it can be seen the CaMnO3 sample shows the plateau potential for the longest period what is in accordance with the higher Mn3+ /Mn2+ ratio at the electrode surface. After the plateau, the discharge for the CaMnO3 occurs in a single reduction stage, together with hydrogen evolution. For the Ho-ions containing oxides an inflection is observed between −0.400 and −0.800 V vs. Hg/HgO, indicating the occurrence of two successive processes. The behaviour found for the Ho-containing oxides could be related with the Mn3+ ions originally present in the lattice site for these specific samples. Actually, after the plateau there are two kinds of Mn3+ ions that are not equivalent, the ones originally in the lattice sites and the others that come from the reduction of the Mn4+ ions, giving rise to an energetic differentiation on the Mn3+ cations. The differences observed on the discharge curves, may reflect the changes in the perovskite ionic composition induced by the Hoions, which allow the existence of Mn3+ ions in the structure. It is expected that Mn3+ ions, originally in the lattice, are more difficult to reduce that the ones resulting from the Mn4+ ions reduction. As a consequence its reduction should occur at higher negative potential, giving rise to the potential inflection. The differences between the samples with x = 0.1 and 0.2, reflect the electrical conductivity changes. The plateau observed for potential values lower than −1.400 V vs. Hg/HgO, has been attributed to the deposition of metallic Mn, confirmed by the electrodes metallic luster after the discharge. The curve obtained for the CaMnO3 is similar to those reported previously by Esaka et al. for different CaMnO3 -based systems, namely Ca1−x Cex MnO3−ı [9,10] and Ca1−x Lax MnO3−ı [4,29,30], where a single plateau appears in the same potential region,
Fig. 8. Discharge curves for the Ca1−x Hox MnO3 (x = 0, 0.1 and 0.2) electrodes in 1 mol dm−3 KOH. Applied current −1.5 mA cm−2 : (a) global curves; (b) expanded view of first half-hour of discharge time. Table 4 Capacitance and roughness factor for the Ca1−x Hox MnO3 oxide electrodes in 1 mol dm−3 KOH solutions. x
C (F cm−2 )
Rf
0 0.1 0.2
0.2235 ± 0.0079 0.5382 ± 0.0142 0.6261 ± 0.0173
3725 ± 125 8970 ± 237 1043 ± 289
although the plateau assignment made by the authors is ambiguous. Assuming the end potential of the discharge to be −0.400 V vs. Hg/HgO, the calculated discharge capacity values are 334, 191 and 200 C g−1 for the oxide electrodes with x = 0, 0.1 and 0.2, respectively. These values indicate that the replacement of small amounts of Ca2+ by Ho3+ in the CaMnO3 slightly decreases its capacity. Similar variations have been reported for other CaMnO3 -based oxides. In the case of Ca1−x Cex MnO3−ı the discharge varies from 860 C g−1 (x = 0) to 720 C g−1 (x = 0.1) [9] while for the Ca1−x Lax MnO3−ı system (x = 0.05, 0.10 and 0.15) the larger discharge capacity, 700 C g−1 has been obtained for the oxide with x = 0.10 in 15% KOH solution at 25 ◦ C [4,29]. Assuming the discharge end potential at −1000 mV, the calculated discharge capacity values are 334, 273 and 229 C g−1 , for the oxide electrodes with x = 0, 0.1 and 0.2, respectively. 4. Conclusions The physical and electrochemical behaviour of a novel Ho-based CaMnO3 electrode has been studied using various techniques. The
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partial substitution of Ca2+ by an ion with a higher valence state (Ho3+ ) in the CaMnO3 phase was achieved and induced an increase in the cell parameters (more visible in the a–c plane) due to the reduction of some Mn4+ to Mn3+ . The presence of Ho significantly increases the oxides electrical conductivity and decreases the grain size, for the same preparation conditions, giving rise to higher surface area electrodes and consequently to higher current intensity and capacitance values. The open circuit potential indicates that the Mn4+ /Mn3+ redox couple is determinant on the surface equilibrium reaction. The chronopotentiometric studies have shown that the discharge occurs by different mechanisms for the oxide electrodes with and without Ho indicating that the oxide ionic composition determines the electrodes behaviour. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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Properties of Ca1−x Hox MnO3 perovskite-type electrodes B.M. Ferreira a , M.E. Melo Jorge b,∗ , M.E. Lopes c , M.R. Nunes b , M.I. da Silva Pereira b a
Dep. de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal Dep. de Química e Bioquímica, Centro de Ciências Moleculares e Materiais, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal c Dep. de Química, Centro de Química de Évora, Universidade de Évora, Évora, Portugal b
a r t i c l e
i n f o
Article history: Received 8 April 2009 Received in revised form 29 April 2009 Accepted 13 May 2009 Available online 21 May 2009 Keywords: Nanocrystalline manganites Holmium doping Electrical properties Electrochemical characterization Manganite electrodes
a b s t r a c t In this work, Ca1−x Hox MnO3 (x = 0, 0.1 and 0.2) perovskite oxide pelleted electrodes were prepared from the respective powders obtained by the citrate route method at 1173 K. The electrodes exhibit particle size that decreases with the holmium content in the oxide. All the samples reveal semiconductor behaviour and the presence of holmium induces a marked decrease in the electrical resistivity. The results can be well attributed to the changes in the Mn4+ /Mn3+ ratio. Electrodes were characterized by cyclic voltammetry and chronopotentiometry. Cyclic voltammetric studies indicate a similar behaviour of the electrodes, irrespective of their composition. Two pairs of peaks were identified and associated, one to the Mn4+ /Mn3+ redox couple and the other to the Mn7+ /Mn4+ and Mn6+ /Mn4+ redox couples. The voltammetric data provide evidence that the electrodes roughness factor increases with the introduction of Ho-ions in the oxide structure, what is consistent with the crystallite size obtained by X-ray diffraction (XRD) and the morphology observed by scanning electron microscopy (SEM). The Hosubstituted electrodes present higher current density when compared with CaMnO3 electrodes what can be attributed both to higher electrical conductivity and smaller particle size. The chronopotentiometric studies have shown that the discharge occurs by different mechanisms for the oxide electrodes with and without Ho. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Many perovskite-type oxides ABO3 with B = Fe, Co and Mn, exhibit high electronic conductivity. This is a result of the hopping mechanism due to the B metal ions mixed valence, the oxide ions high mobility in the crystal and the easy changes in the oxygen contents in their lattices, leading to electrode materials with potential technological applications. A great variety of oxides, with perovskite-type structure, has been exploited in the past [1–8], namely as electrode materials (La1−x Srx FeO3 ) [1], electrolyte materials for solid oxide fuel cells (La1−x Srx MnO3 and LaMO3 , with M as a transition metal) [2,3], cathodes in alkaline batteries (lanthaniumsubstituted CaMnO3 ) [4] and as potential catalysts alternative to noble metals ((La,Sr)(Co,M)O3 with M = Cu, Fe, Ni, Cr and Mn) [5,6]. These distinct applications are an outcome of the good electrical conductivity, electrocatalytic activity and thermal and chemical stabilities. Until now many detailed studies have been published involving different hole-doped manganites Ln1−x Ax MnO3 bulk samples and films (Ln = trivalent rare-earth cation; A = divalent alkaline earth cation). In contrast there are only a few reports on electron-doped
∗ Corresponding author. E-mail address: [email protected] (M.E. Melo Jorge). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.05.054
materials, corresponding to Mn4+ -rich composition. This work is inserted in this context; more precisely it regards the effect of the partial replacement of Ca2+ ion by a trivalent ion on the properties of CaMnO3 . This substitution induces a reduction of Mn4+ ions to Mn3+ ions due to charge compensation and consequently modifies the oxide properties. Due to their ionic radius and valence stability the lanthanides are the trivalent ions adequate to substitute Ca2+ in the A-site of the perovskite structure. Studies concerning the Ca1−x Cex MnO3−ı oxide system properties, prepared by the ceramic method [9,10], indicate these materials as possible candidates for cathode materials in alkaline batteries. Recently we have prepared Ca1−x Cex MnO3 (x = 0, 0.1, 0.2) pellet electrodes, from the powders synthesized by the citrate route [11]. The study has shown that the presence of the Ce4+ ion enhances the oxide electrical conductivity and the surface electrochemistry is governed by the Mn4+ /Mn3+ redox couple. In addition, the data indicate that the prepared Ca1−x Cex MnO3 electrodes could be promising cathode materials for alkaline batteries [12]. Lately we have reported that substituted perovskite-type oxides Ca1−x Hox MnO3 (x = 0.1, 0.2), prepared by the citrate route [13,14], are chemically stable and compared with the parent CaMnO3 , show a decrease in grain size and higher electrical conductivities from room temperature up to 873 K. These properties make these oxides potentially interesting as electrode materials.
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Fig. 1. X-ray diffraction patterns for Ca1−x Hox MnO3 (x = 0, 0.1, 0.2).
Based on the idea of using them as cathode materials in alkaline batteries and considering that little information on the aqueous electrochemistry of these compounds exists up to now, the properties of the Ca1−x Hox MnO3 (x = 0, 0.1 and 0.2) electrodes in alkaline solution have been investigated. The citrate route method has been chosen to synthesize the oxide powders, based on the encouraging results of our previous studies [12–14]. Cyclic voltammetry (CV) and chronopotentiometry were employed to study the electrochemical behaviour of the Ca1−x Hox MnO3 (x = 0, 0.1 and 0.2) oxide electrodes. 2. Experimental Bulk ceramic samples Ca1−x Hox MnO3 (0 ≤ x ≤ 0.2) were prepared from appropriate amounts of CaCO3 (Aldrich > 99.995%), Ho2 O3 (Aldrich > 99.9%) and Mn(NO3 )2 ·4H2 O (Riedel-de Haën, p.a.) by the citrate route method as described in previous papers [13,14]. However a different heat treatment was performed. After the decomposition at 873 K, for 6 h, the resulting amorphous powder was grounded and heated in air at 1173 K for 18 h in alumina crucibles (Alsint 99.7). The oxide powder (≈200 mg) was uniformly distributed onto an inserted Pt mesh and pressed, into 1 mm thick, ≈2 cm2 platelet pellets under 40 bar pressure and sintered at 1173 K,
Fig. 2. SEM micrographs (15,000×) of Ca1−x Hox MnO3 system: (a) x = 0, (b) x = 0.1 and (c) x = 0.2.
in air, for 6 h. The electrical contact was made by welding the Pt mesh to a copper wire. The samples were then mounted in a glass tube with Araldite epoxy resin, so that the electrolyte could only make contact with the oxide. Three specimens were prepared for each composition, two for the electrochemical experiments and one for morphological characterization.
Table 1 Lattice parameters, crystallite and particle size for Ca1−x Hox MnO3 . x
a (Å)
b (Å)
c (Å)
V (Å3 )
Crystallite size XRD (D) (nm)
Particle size SEM (nm)
0 0.1 0.2
5.2785 (4) 5.2923 (5) 5.3314 (5)
7.4613 (5) 7.4623 (5) 7.4721 (5)
5.2644 (4) 5.2781 (4) 5.2840 (4)
207.33 (4) 208.44 (5) 210.50 (5)
79 45 40
250 100 80
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Fig. 3. Arrhenius plots of log (T) vs. 1/T for the samples Ca1−x Hox MnO3 (x = 0, 0.1, 0.2). Table 2 Room temperature electrical conductivity ( 300 K ), activation energies (Ea ) and valence state of Mnx+ ions for Ca1−x Hox MnO3 . x 0 0.1 0.2
300 K (−1 cm−1 ) 0.21 15.32 6.21
Temperature range used for obtaining Ea (K)
Ea (meV)
Mnx+
298–600 623–873 298–873 298–873
73 358 88 95
3.98 3.9 3.8
Phase identification and structural analysis of the polycrystalline powder and the electrodes were carried out by XRD at room temperature using a Philips PW 1730 X-ray powder diffractometer, operated at 40 kV and 30 mA, employing a monochromatized Cu K␣ radiation in Bragg–Brentano geometry. The data were collected at 0.02◦ step in 2 over the range 20◦ ≤ 2 ≤ 90◦ with a counting time of 0.8 s per step and the lattice parameters were determined. The average crystallite size (D) was evaluated from the XRD data by applying the Scherrer’s formula D = k/ˇ cos , where k is a constant depending on the grain shape (k = 0.89, for circular grain),
the wavelength of Cu K␣ radiation, the Bragg diffraction angle 2 − ˇ2 where of the most intense peak and ˇ is defined as ˇ2 = ˇm s ˇm and ˇs are the experimental full width at half maxima (FWHM) and the FWHM of a standard silicon sample, respectively [15]. The samples surface morphology and grain size were investigated by scanning electron microscopy using a JEOL (JSM-35C). The oxygen stoichiometry in each sample was determined by iodometric titration. Potassium iodide and hydrochloric acid solutions were added to dissolve the sample in a flask; the solution was titrated with a standard sodium thiosulfate. All samples can be considered stoichiometric within the experimental error and consequently, the effect from oxygen vacancies should be negligible. The variation of the electrical resistivity with temperature was measured using a four-probe technique in the temperature range of ∼300–900 K. The oxides electrochemical behaviour was studied by means of open circuit potential measurements and cyclic voltammetry, in 1 mol dm−3 KOH solutions. Solutions were prepared from AnalaR reagents with Millipore Milli-Q water and degassed with nitrogen, 99.999% purity gas supplied by Air Liquide. The electrochemical experiments were performed in a two-compartment three-electrode glass cell at room temperature. The counter electrode was a graphite rod and, as reference, an Hg/HgO (0.099 V vs. SHE) was used. Voltammetric studies were carried out using a low noise operational amplifier potentiostat incorporated with positive feedback IR compensation, programmed by a Bank VSG 83 waveform generator and a Kipp & Zonen Pró-1 recorder and the chronopotentiometric studies with a Voltalab 32 Radiometer apparatus connected to an IMT 102 interface, controlled by a personal computer through the VoltaMaster 2 software. 3. Results and discussion 3.1. X-ray powder diffraction XRD results revealed the formation of perovskite-type single phases (Fig. 1) for all the samples and the diffractograms are characteristic of the orthorhombic symmetry, space group Pnma. The increase in the values of the cell parameters (Table 1) is caused by an expansion of the octahedral site (B-site) that overcomes the contraction on the A-site due to an increase in Mn3+ ion content, as discussed in the literature [13,14] and according with the ionic radius values [16]. Our data are in excellent agreement with previously reported results for the same system synthesized by different methods [13,14,17–22]. The average crystallite size (D) was evaluated from the broadening of the XRD line width by applying the Scherrer’s formula. From the estimated average crystallite size, as shown in Table 1, one finds that this parameter decreases with increasing holmium content. Similar tendency has been found previously for samples with identical composition [13,14]. 3.2. Scanning electron microscopy
Fig. 4. Cyclic voltammograms for the Ca0.9 Ho0.1 MnO3 electrodes in 1 mol dm−3 KOH at a sweep rate of 10 mV s−1 , for different positive and negative potential limits.
The SEM images shown in Fig. 2 clearly indicate that the introduction of Ho-ion in the oxide samples led to smaller grains, although the shape remains similar. Since the compounds were sintered under the same conditions, we can conclude that the difference in the grain size results mainly from the influence of the holmium content on the grain growth. The crystallite size obtained by XRD is small compared to the grain size observed by SEM (see Table 1). This difference is probably due to the crystallite aggregation. As it has been referred previously, a grain may consist of several crystallite domains which appear due to twinning and other structure defects in the grain [23]. The SEM images show a similar trend on the grain size variation with the Ho-content as reported by us in a previous work [13,14].
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conductivity and the activation energy for conduction. The linear dependence of log (T) vs. 1/T is characteristic of the polaron hopping transport mechanism, for which the conductivity can be represented by the function = (A/T) exp (−Ea /kT) [24]. The electrical conductivity increases with the increase in temperature, for all compounds, indicating its semiconducting nature. Moreover, the calcium partial substitution by holmium causes a significant increase in the electrical conductivity. Our results confirm those already published and discussed [14]. 3.4. Open circuit potential measurements The open circuit potential of the Ca1−x Hox MnO3 (x = 0, 0.1 and 0.2) oxide electrodes was measured in KOH 1 mol dm−3 . An average value of 0.097 ± 0.025 V vs. Hg/HgO has been obtained. No meaningful changes were observed by the presence of Ho-ions in the oxide. The value approaches the thermodynamic value calculated for the Mn4+ /Mn3+ solid-state redox couple, 0.089 V vs. Hg/HgO [25]. This result indicates that the Mn4+ /Mn3+ redox couple is the determinant on the surface equilibrium reaction, what is in accordance with previous studies on CaMnO3 -based electrodes [9,12]. 3.5. Cyclic voltammetric studies
Fig. 5. Cyclic voltammograms for the Ca1−x Hox MnO3 (x = 0, 0.1, 0.2) electrodes in 1 mol dm−3 KOH at a sweep rate of 10 mV s−1 .
3.3. Electrical conductivity studies Fig. 3 presents Arrhenius plots of log (T) vs. 1/T, for all the samples, and Table 2 summarizes the room temperature electrical
Cyclic voltammograms were recorded for the different oxides, in 1 mol dm−3 KOH at a sweep rate of 10 mV s−1 , between the open circuit potential and different positive and negative limits. Fig. 4 shows a representative family of voltammograms for the Ca0.9 Ho0.1 MnO3 . Taking into account this result, it is possible to associate the anodic peaks A1 and A2 to the cathodic peaks C1 and C2, respectively. The safe voltage window was also established and cyclic voltammograms were run between +0.5 V and −0.3 V for the different electrodes. The obtained curves are displayed in Fig. 5. Comparing the different curves, an increase in the current density is observed for the samples containing Ho. This result could be due to an increase of the oxide surface area by the presence of Ho. Table 3 presents the peak potentials and the calculated formal potential (Ef ) of the redox couples associated to the A1/C1 and A2/C2, estimated from the anodic and cathodic peak potentials, based on the following equation, Ef = (Epa + Epc )/2. The results show that the presence of Ho induces a shift on the anodic (A2) and cathodic (C2) peaks, towards more negative and positive potential values, respectively. This behaviour can be associated with the changes on the samples electrical properties. In what concerns peaks A1 and C1, this behaviour is not observed. An additional cathodic peak C appears in the cyclic voltammograms of the Hocontaining electrodes associated with the increase in current at the positive potential limit. Based on the potential values, the pair of peaks A2/C2 has been assigned to the Mn4+ /Mn3+ solid-state surface redox couple that compares well with the calculated equilibrium potential, 0.089 V vs. Hg/HgO. Peaks A1/C1 were associated with soluble Mn species with higher valence states Mn4+ /Mn6+ and/or Mn4+ /Mn7+ , which slightly depart from the calculated thermodynamic potentials at pH 14 that are 0.504 and 0.491 V vs. Hg/HgO, respectively [25]. This is in accordance with the purple colour appearing in the test solution, near
Table 3 Peak and formal potentials for the redox couples associated to the voltammetric peaks with the electrodes of Ca1−x Hox MnO3 . x
0 0.1 0.2
E (V) vs. Hg/HgO A1
C1
Ef
A2
C2
Ef
0.381 ± 0.038 0.337 ± 0.023 0.340 ± 0.017
0.291 ± 0.009 0.257 ± 0.025 0.273 ± 0.043
0.336 ± 0.038 0.297 ± 0.025 0.307 ± 0.043
0.110 ± 0.053 0.060 ± 0.029 0.047 ± 0.013
−0.070 ± 0.019 −0.053 ± 0.054 0.060 ± 0.031
0.020 ± 0.053 0.004 ± 0.054 −0.007 ± 0.031
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the electrode surface before the oxygen evolution, indicating that Mn is lost from the oxide. The effect of the potential scan rate (), on the cyclic voltammograms, was investigated and Fig. 6a presents the respective curves for the Ca0.9 Ho0.2 MnO3 electrode recorded for sweep rates of 5, 10, 15, 20 and 25 mV s−1 . As it can be seen the peak current, for both anodic and cathodic waves, increases with the increasing of potential scan rate. A linear dependence of jp vs. 1/2 is observed for the anodic peak A2, for all the studied electrodes, which is characteristic of a diffusion type process (Fig. 6b). The peak potential separation (Ep ), between the anodic and cathodic peaks, increases linearly with sweep rate for all the compositions. The extrapolation of Ep to zero sweep rate gives the values 52, 1 and 17 mV for the oxides with x = 0, 0.1 and 0.2, respectively. It is interesting to note that the lower value is obtained for the Ca0.9 Ho0.1 MnO3 electrode. Ep is usually taken as a measure of the ohmic drop, including two components: the uncompensated ohmic drop between the electrode surface and the Luggin capillary tip and the ohmic drop across the oxide [26]. Since all the experiments were carried out in the same solution and with the same geometric cell arrangement, the component due to the uncompensated resistance of the electrolyte can be left behind and it can be concluded that the observed differences are due to the ohmic drop across the oxide. These results are in perfect agreement with the electrical conductivity data, at room temperature as Fig. 6 shows, and provide a striking demonstration of the importance of the electrical conductivity in determining the electrochemical response of the perovskite electrodes, as Fig. 6c shows. Fig. 7a presents cyclic voltammograms obtained between 0 and 0.2 V vs. Hg/HgO. The shape of the voltammograms indicates a typical capacitive behaviour for the three materials and become more rectangular in shape for the sample containing Ho. Also the current response when the sweep direction changed becomes faster what is in accordance with the enhanced oxide electrical conductivity and marginal solution resistance. A linear variation between the current density, measured at E = 0.1 V vs. Hg/HgO and the sweep rate is observed for all the samples as Fig. 7b also displays. The capacitance was determined according to the equation C = dj/d(dV/dt), where j is the current density and dV/dt the potential sweep rate. Assuming a value of 60 F cm−2 for the capacitance of the oxide/aqueous solution interface, the electrodes roughness factors have been also calculated [27]. The calculated specific capacitance and roughness factors are presented in Table 4. A simple analysis shows that the capacitance increases with increasing the amount of Ho-ions in the samples, however a more precise analysis indicates that for x = 0.1 an increase of 140% occurs in relation to the CaMnO3 oxide while when the Ho-ions amount changes from 0.1 to 0.2 the increase in capacitance is only 14%. Therefore it can be conclude that the important factor is the presence of Ho-ions in the samples and not so much the amount. This is consistent with the crystallite size obtained by XRD and the morphology observed by SEM for the Ho-containing electrodes. Therefore the capacitance increase for the samples containing Ho-ions can be associated with the enhancement of the oxides surface area. Rf values are higher than those published by us for the Ca1−x Cex MnO3 (x = 0, 0.1, 0.2) electrodes. The differences are attributed to the lower synthesis temperature and shorter heat treatment. 3.6. Chronopotentiometric studies The potential–time curves for the Ca1−x Hox MnO3 (x = 0, 0.1 and 0.2) cathodes, at 1 mA cm−2 in the potential range of 0 to −1.6 V vs. Hg/HgO, are presented in Fig. 8. A qualitative analysis of the curves shows three distinct regions, for all the electrodes. Initially (Fig. 8 inset) a decrease on the potential occurs followed by a well-
Fig. 6. (a) Family of cyclic voltammograms at sweep rates 5, 10, 15, 20 and 25 mV s−1 for the Ca0.9 Ho0.1 MnO3 electrode in 1 mol dm−3 KOH. (b) Variation of the peak current (A2) with the sweep rate. (c) Variation of the peak separation with the room temperature electrical conductivity.
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Fig. 7. (a) Cyclic voltammograms obtained in the double-layer region and (b) doublelayer charging current as a function of sweep rate for the Ca1−x Hox MnO3 (x = 0, 0.1, 0.2) electrodes in 1 mol dm−3 KOH at a sweep rates of 1, 2, 4, 6, 8 and 10 mV s−1 .
defined plateau, between −0.300 and −0.400 V vs. Hg/HgO. The initial potential decrease has been assigned to the reduction of the Mn4+ ion to Mn3+ ion and the plateau to the reduction of the Mn3+ ion to Mn2+ ion [28]. As it can be seen the CaMnO3 sample shows the plateau potential for the longest period what is in accordance with the higher Mn3+ /Mn2+ ratio at the electrode surface. After the plateau, the discharge for the CaMnO3 occurs in a single reduction stage, together with hydrogen evolution. For the Ho-ions containing oxides an inflection is observed between −0.400 and −0.800 V vs. Hg/HgO, indicating the occurrence of two successive processes. The behaviour found for the Ho-containing oxides could be related with the Mn3+ ions originally present in the lattice site for these specific samples. Actually, after the plateau there are two kinds of Mn3+ ions that are not equivalent, the ones originally in the lattice sites and the others that come from the reduction of the Mn4+ ions, giving rise to an energetic differentiation on the Mn3+ cations. The differences observed on the discharge curves, may reflect the changes in the perovskite ionic composition induced by the Hoions, which allow the existence of Mn3+ ions in the structure. It is expected that Mn3+ ions, originally in the lattice, are more difficult to reduce that the ones resulting from the Mn4+ ions reduction. As a consequence its reduction should occur at higher negative potential, giving rise to the potential inflection. The differences between the samples with x = 0.1 and 0.2, reflect the electrical conductivity changes. The plateau observed for potential values lower than −1.400 V vs. Hg/HgO, has been attributed to the deposition of metallic Mn, confirmed by the electrodes metallic luster after the discharge. The curve obtained for the CaMnO3 is similar to those reported previously by Esaka et al. for different CaMnO3 -based systems, namely Ca1−x Cex MnO3−ı [9,10] and Ca1−x Lax MnO3−ı [4,29,30], where a single plateau appears in the same potential region,
Fig. 8. Discharge curves for the Ca1−x Hox MnO3 (x = 0, 0.1 and 0.2) electrodes in 1 mol dm−3 KOH. Applied current −1.5 mA cm−2 : (a) global curves; (b) expanded view of first half-hour of discharge time. Table 4 Capacitance and roughness factor for the Ca1−x Hox MnO3 oxide electrodes in 1 mol dm−3 KOH solutions. x
C (F cm−2 )
Rf
0 0.1 0.2
0.2235 ± 0.0079 0.5382 ± 0.0142 0.6261 ± 0.0173
3725 ± 125 8970 ± 237 1043 ± 289
although the plateau assignment made by the authors is ambiguous. Assuming the end potential of the discharge to be −0.400 V vs. Hg/HgO, the calculated discharge capacity values are 334, 191 and 200 C g−1 for the oxide electrodes with x = 0, 0.1 and 0.2, respectively. These values indicate that the replacement of small amounts of Ca2+ by Ho3+ in the CaMnO3 slightly decreases its capacity. Similar variations have been reported for other CaMnO3 -based oxides. In the case of Ca1−x Cex MnO3−ı the discharge varies from 860 C g−1 (x = 0) to 720 C g−1 (x = 0.1) [9] while for the Ca1−x Lax MnO3−ı system (x = 0.05, 0.10 and 0.15) the larger discharge capacity, 700 C g−1 has been obtained for the oxide with x = 0.10 in 15% KOH solution at 25 ◦ C [4,29]. Assuming the discharge end potential at −1000 mV, the calculated discharge capacity values are 334, 273 and 229 C g−1 , for the oxide electrodes with x = 0, 0.1 and 0.2, respectively. 4. Conclusions The physical and electrochemical behaviour of a novel Ho-based CaMnO3 electrode has been studied using various techniques. The
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partial substitution of Ca2+ by an ion with a higher valence state (Ho3+ ) in the CaMnO3 phase was achieved and induced an increase in the cell parameters (more visible in the a–c plane) due to the reduction of some Mn4+ to Mn3+ . The presence of Ho significantly increases the oxides electrical conductivity and decreases the grain size, for the same preparation conditions, giving rise to higher surface area electrodes and consequently to higher current intensity and capacitance values. The open circuit potential indicates that the Mn4+ /Mn3+ redox couple is determinant on the surface equilibrium reaction. The chronopotentiometric studies have shown that the discharge occurs by different mechanisms for the oxide electrodes with and without Ho indicating that the oxide ionic composition determines the electrodes behaviour. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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