A preliminary study of the pseudo-capacitance features of strontium doped lanthanum manganite

Abstract

The intrinsically poor electrical conductivity of transition metal oxides is a critical challenge to obtain high electrochemical performance when these materials are used as the electrode of supercapacitors. In this study, the pseudo-capacitance features of perovskite (La_0.75Sr_0.25)_0.95MnO_3−δ (LSM) having a high electric conductivity of 44.9 S cm⁻¹ at room temperature was preliminarily investigated. The easily prepared LSM electrode was characterized by cyclic voltammetry and galvanostatic charge–discharge method to evaluate its electrochemical performances. The results show that LSM has pseudo-capacitance features and the specific capacitance is 56 F g⁻¹ at a scanning rate of 2 mV s⁻¹. This is the first report on the pseudo-capacitance value of LSM. Furthermore, the electrodes show no obvious capacitance degradation after 1000 cycles.

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1. Introduction

Improvement of the utilization of energy is an effective approach to resolve the worldwide energy shortage and environmental pollution issues. There is an urgent need to develop high efficiency energy conversion and storage devices.^1,2 Supercapacitors have attracted much attention due to their high power density, long life expectancy, high efficiency, environmental friendliness and high safety.^3,4 So far, supercapacitors have been extensively applied in various fields, such as electric hybrid vehicles, portable devices, industrial uninterruptible power supplies and military field.^5–8 According to the principle of energy storage, supercapacitors can be divided into two categories, i.e. the electric double-layer capacitors (EDLCs) and the pseudo-capacitors.^1,9–11 EDLCs store charges via electrostatic interaction, which is a pure physical process. In this mode, energy storage merely relies on charge accumulation of static electricity at the interface of electrode/electrolyte. The pseudo-capacitors store charges through redox reactions of electrode material during charging and discharging processes.^9,12–16 The pseudo capacitance process takes place not only at the surface of electrodes but inside the electrodes. Compared with EDLCs, the pseudo-capacitors naturally possess a higher capacitance.^1,17 Carbon materials, conducting polymers and metal oxides are extensively investigated as electrode materials for supercapacitors.^18,19 Among electrode materials, conducting polymers and transition metal oxides are employed to improve the performance of supercapacitors due to their redox effect, such as polyaniline (PANI), polypyrrol (PPy), manganese dioxide, cobalt oxide, nickel oxide and their composite materials.^1,9 However, the intrinsically poor electrical conductivity of transition metal oxides is a critical challenge to obtain high electrochemical performance, for example the electrical conductivity of the commonly used MnO₂ is as low as 10⁻⁵ to 10⁻⁶ S cm⁻¹.⁵

Strontium doped lanthanum manganite (LSM) have long been considered as one of the most attractive cathode materials for solid oxide fuel cells (SOFCs). LSM belongs to the pervoskite-type material family with a high electrochemical catalytic activity for oxygen reduction and especially a higher electrical conductivity than that of the transition metal oxides. The electrical conductivity of (La_0.75Sr_0.25)_0.95MnO_3−δ (LSM) reaches to 44.9 S cm⁻¹ at room temperature, which is higher than that of MnO₂ by about six orders of magnitude. The conductivity of electrodes plays an important role in the improvement of the capacitance of supercapacitors. It is interesting to evaluate the LSM as electrode material for supercapacitors due to its high electrical conductivity and excellent electrochemical activity. However, there is a lack of report on the capacitance features of the LSM. Here, pure (La_0.75Sr_0.25)_0.95MnO_3−δ (LSM) without any pre-treatment was selected as the starting materials to fabricate electrodes. The specific capacitance and cycling stability of the LSM electrode were tested and discussed.

2. Experimental

Commercial (La_0.75Sr_0.25)_0.95MnO_3−δ powder (Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Science, China) was used as starting material in this study. The LSM electrode was fabricated by a cost-effective and simple dipping process. Firstly, LSM powders and binder were ultrasonically dispersed in absolute ethyl alcohol for 20 min to get a stable LSM suspension (25 mg mL⁻¹). Then, a piece of carbon paper (1 cm × 2 cm) was immersed into the LSM-suspension and held certain time to ensure the suspension soak through the carbon paper. The carbon paper was finally dried by hair dryer in order to solidify binder. The dipping and drying process was repeated three times to increase the LSM loading. In this work, the LSM active material loading in the carbon paper was about 0.303 mg cm⁻².

Powder X-ray diffraction patterns of (La_0.75Sr_0.25)_0.95MnO_3−δ were recorded by a D/max-rB (12 kW) X-ray diffractometer with Cu_kα radiation (λ = 1.5406 Å).⁸ The morphology of LSM powder and electrode was observed by a scanning electron microscopy (SEM, SU8010). The cyclic voltammetry (CV) and charge–discharge cycles of LSM electrode were characterized by a standard three-electrode configuration in 1.0 M Na₂SO₄ aqueous solution, where LSM electrode, Ag/AgCl and a piece of platinum foil (2 cm × 2 cm) were used as working electrode, reference electrode and counter electrode, respectively. CHI604C electrochemical workstation (Shanghai, Chenhua, China) was used to record the electrochemical data.

3. Results and discussion

Fig. 1 shows the typical XRD patterns of LSM powders. All the diffraction peaks can be indexed to the perovskite LSM (PDF#54-1195) without any secondary peaks. The LSM has a rhombohedral crystal structure with a space group of R3m̄c (167). Both the stronger diffraction peaks and the narrow half-peak breadth as shown in Fig. 1 indicate a high crystallinity of LSM powders.

Fig. 1 XRD patterns of (La_0.75Sr_0.25)_0.95MnO_3−δ powders.

Fig. 2(a) and (b) show the high- and low-resolution scanning electron microscopy (SEM) images of LSM powders, respectively. It can be seen from Fig. 2(a) that LSM powder is mainly composed of initial particles with a size of ∼0.5 μm. Fig. 2(b) clearly exhibits that the small initial LSM particles tend to aggregate as bigger particles with a particle size up to 10 μm. The aggregation of the initial powder will drastically reduce the specific surface area of LSM powders. The specific surface area of LSM powders used in this work is 21.13 m² g⁻¹.

Fig. 2 (a) High- and (b) low-resolution scanning electron microscopy images of LSM powders.

To evaluate the pore structure of LSM powders, nitrogen adsorption and desorption measurements were performed. Fig. 3 shows the N₂ adsorption–desorption isotherm of LSM powders. The isotherm feature of LSM demonstrates the presence of mesopore in this sample, classified as type IV as defined by the International Union of Pure and Applied Chemistry (IUPAC).^19,20 Moreover, a distinct hysteresis loop can be observed in the P/P₀ range of 0.7–0.99, which is a typical mesopore feature.²¹ The capillary condensation occurs at higher pressure, indicating the LSM sample has the mesoporous and macroporous simultaneously. According to the IUPAC classification, the observed loop can be ascribed as type H1 loops,¹³ which indicates that pores in the sample may be caused by particles stacking. The most probable aperture and Brunauer–Emmertt–Teller (BET) surface area of the LSM are around 19.006 nm and 21.13 m² g⁻¹, respectively. The pore structure of electrode material is contributed to the electrochemical active surface area. These results show that the formation of aggregates in LSM drastically decreases mesoporosity as well as the specific surface area.¹⁹ It should be pointed out that here we focus on the pseudo capacitance features of LSM, the fine starting LSM powders with more even particle size distribution and enhanced specific surface will be adopted in the future investigation.

Fig. 3 The adsorption–desorption isotherm of LSM powders.

Fig. 4(a) and (c) show the cyclic voltammetry (CV) curves of carbon paper–LSM electrode and pure carbon paper electrode at different scanning rate in 1.0 M Na₂SO₄ aqueous solution, respectively, in order to clarify the contribution of carbon paper to the total capacitance. The specific capacitance of the electrode was calculated from the CV data according to the following equation:^7,22

where C_s (F g⁻¹) is the specific capacitance, m (g) is the mass of the LSM in the electrodes, i (A) is the current, υ (mV s⁻¹) is the scanning rate, and ΔV (V) is the potential window. As can be seen from Fig. 4(a), all the CV curves exhibit almost ideal rectangular shapes in the range of −0.1 V to 0.9 V, implying a reversible capacitive behavior. The size of the rectangle increases with an increase in sweep rate for all LSM samples.²³ Mn is the sole variable valence element in the formula of LSM. The charge storage mechanism of LSM electrode may originate from the valence variation of Mn, similar to MnO₂. As the scanning rate increases, the peak currents increase but all the CV curves still remain a rectangular-like with symmetric shape, even at a high scanning rate of 50 mV s⁻¹.

Fig. 4 The cyclic voltammetry (CV) curves of (a) carbon paper–LSM electrode and (c) carbon paper at different scan rate; the relationship between the specific capacitance and the scan rate of (b) carbon paper–LSM electrode and (d) LSM without carbon paper contribution.

The specific capacitance of the carbon paper–LSM electrode and the pure carbon paper electrode were calculated by integrating the surrounded area of CVs according to the eqn (1). It should be noted that the data presented in Fig. 4(a) and (b) involve the contribution from the pure carbon paper. The detail experiments and results on the carbon paper contribution to the total capacitance were provided in the ESI.† As shown in the ESI (Fig. S1†), the yttria-stabilized zirconia (YSZ) without pseudo-capacitance feature was selected to fabricate the carbon paper–YSZ electrode to simulate the situation of carbon paper in the carbon paper–LSM electrode, by this way the capacitance comes from the carbon paper can be thoroughly clarified. As can be seen from Fig. S1,† the pure carbon paper electrode, the carbon paper–binder electrode and the carbon paper–YSZ electrode show similar CV curves, indicating that the influence of the coated electrode active material and binder on the wettability between electrolyte and carbon paper is negligible in this study, and thus the accurate capacitance of LSM can be obtained from the data shown in Fig. 4(a) and (c). The net capacitances originated from LSM dependence on the scanning rate were plotted in Fig. 4(d). It can be seen that, at a lowest scanning rate of 2 mV s⁻¹, the highest specific capacitance of carbon paper–LSM electrode is 108 F g⁻¹ while the net capacitance comes from LSM is 56 F g⁻¹. The current increase with the increasing of scanning rate, while the shape of the CV curve deviates from the rectangular shape of ideal capacitor and the specific capacitance decreases. The loading LSM particles, especially the portion far away from the electrode surface, do not play a role in the charge storage at high scanning rate.⁵ Therefore, it is a possible reason for this specific capacitance decline. The second possible reason comes from the reduced penetration distance of electrolyte ions inside the electrode at high scanning rate.²⁴ The diffusion of ions from electrolyte to the electrolyte/electrode interface cannot compensate the consumption of ions during the charge–discharge process, leading to the non-ideal rectangular shape of CV curves and the decline of specific capacitance.^7,25–29 As previous reports, the large specific surface and suitable porosity are extremely important to get high electrode capacity.³⁰ The LSM powders used in this work exhibits serious aggregation, which results in a reduced specific surface area. This will reduce utilization ratio of LSM. Furthermore, the low porosity of the LSM electrode also reduces the effective contact area when the LSM particle is in contact with an electrolyte. This will be unfavorable for ions adsorption and diffusion at the interface of electrode/electrolyte. Nanoparticles or nanorods show an extreme high specific surface area and surface-to-volume ratio. It is reasonable to believe that LSM nanoparticles applied in electrode will improve the specific capacitance significantly. In situ fabrication of LSM nanoparticles and nanorods inside the carbon paper current collector is underway and will be reported later.

Further understanding of the fast ion diffusion in LSM electrode was investigated via AC impedance at a frequency range of 100 mHz to 100 kHz (ref. 31) at open-circuit voltage. The Nyquist plot for the carbon paper–LSM electrode was shown in Fig. 5. The impedance of electrode is composed of a well-defined semicircle in the high frequency range and a straight sloping line in the low frequency regions. On the Nyquist plot, the intercept at the real part (Z′) called equivalent series resistance (ESR) is 3.07 Ω. The diameter of the semicircle corresponds to the interfacial charge transfer resistance (R_ct), which usually represents the resistance of the electrochemical reactions at the electrode surface.^32,33 The R_ct resistance value extracted from the curves is 3.58 Ω, which reflects that LSM has a good conductivity. The straight line represents the diffusion resistance of electrolyte ions transfer in the electrode pore and the proton diffusion in electrode materials at the low frequency.^32,34 The angle with the real axis is close to 45° (Warburg impendence), corresponding to the ion diffusion processes.^11,35 This is a typical characteristic of pseudo-capacitance feature.

Fig. 5 Nyquist plot of carbon paper–LSM electrode.

The galvanostatic charge–discharge curves of the carbon paper–LSM electrode at 5 mA current were plotted in Fig. 6(a). The charging curves are symmetric with their corresponding discharging counterparts, and exhibit a linear variation of potential with time during charging and discharging processes.²³ The cycling stability of the carbon paper–LSM electrode was tested at a constant current of 5 mA. As shown in Fig. 6(b), the specific capacitance shows a trend of increase gradually with the increasing of cycle numbers and finally reaches saturated state. The continually improvement of polarization state of LSM electrode at the initial stage results in a capacitance increase. In addition, the wettability between the LSM electrode and electrolyte gets better with the increasing of cycles, and thus more active materials can be utilized to produce capacitance. The initial stable specific capacitance of carbon paper–LSM electrode is about 52 F g⁻¹ in the cycling process, then decreases to ∼51 F g⁻¹, retaining 98% of their initial specific capacitance after 1000 cycles. The LSM electrodes possess high electrochemical stability in 1.0 M Na₂SO₄ electrolyte, which is essentially required for supercapacitors.²³ The inductive coupled high frequency plasma (ICP) results indicate that the solubility of LSM in the neutral electrolyte of Na₂SO₄ (1.0 M) is negligible (see the ESI, Table S1†), which benefits the long-term stability of LSM-based electrode.

Fig. 6 (a) Galvanostatic charge–discharge curves of the carbon paper–LSM electrode (b) cycling performance of carbon paper–LSM electrode at 5 mA current, cycled between −0.1 and 0.9 V.

4. Conclusions

In this work, the capacitance feature of the LSM was preliminary investigated. The results indicate that LSM exhibits a pseudo-capacitance effect owing to the valence change of the manganese transition metal (Mn³⁺/Mn⁴⁺). The LSM electrode shows a specific capacitance of 56 F g⁻¹ at scanning rate of 2 mV s⁻¹. This specific capacitance originates from the charge adsorption on the surface of LSM particles and the charge transfer process of Mn in LSM between Mn³⁺ and Mn⁴⁺. The results of this work reveal that the LSM material possesses a capacitance characteristic and the LSM electrode shows a good cycling stability at the constant current of 5 mA for 1000 cycles. A significant improvement of specific capacitance can be realized through modifying a LSM electrode with nanoparticles or nanorods structure.

Acknowledgements

The Natural Science Foundation of China (21103037, 51372057) and the Project (HIT. NSRIF.2009059) supported by Natural Scientific Research Innovation Foundation in Harbin Institute of Technology are gratefully acknowledged. All the authors are grateful for the assistance of Dr Shijie You during the ICP testing.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra13583k

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