Abstract

The optimum composition of cathode catalyst has been studied for rechargeable zinc air battery application. La_0.9Ca_0.1Co_1−xFe_xO₃   perovskite powders were prepared using the citrate method. The substitution ratio of Co²⁺ with Fe³⁺ cations was controlled in the range of 0–0.4. The optimum substitution ratio of Fe³⁺ cations was determined by electrochemical measurement of the air cathode composed of the catalyst, polytetrafluoroethylene (PTFE) binder, and Vulcan XC-72 carbon. The substitution by Fe enhanced the electrochemical performances of the catalysts. Considering oxygen reduction/evolution reactions and cyclability, we achieved optimum substitution level of in La_0.9Ca_0.1Co_1−xFe_xO₃.

1. Introduction

The oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in aqueous solutions occur at a high overpotential . This irreversibility of the reactions is the main problem for developing a cathode of zinc air battery. In addition, bifunctional catalysts for the cathode are an important prerequisite for developing zinc air secondary batteries. Many types of electrocatalysts have been investigated for the ORR and OER: perovskite, spinel, noble metal, and pyrochlore [1–6]. However, most of the catalysts cannot provide suitable performance for both the OER and ORR. A transition metal oxide can be used only for an ORR catalyst, while metal oxide and spinel compounds can be used for OER catalysts.

Compared with noble metal catalysts, metal oxides offer the advantages of avoiding gassing on the zinc electrode when acting as bifunctional electrodes for the ORR and OER [7, 8]. The noble metal catalysts, Pd, Ag, and Pt, exhibit good performance for the ORR; however, these catalysts undergo a sharp degradation in the OER caused by the dissolution of the catalysts [9].

Perovskites, which have the general formula ABO₃, exhibit excellent catalytic performance in the ORR and OER because of their high oxide ion mobility. Partial substitution of A or B by a cation A′ or B′ with a different valence leads to ionic defects or changes in the valence state of the catalyst, which affects the catalytic activity and conductivity of the catalyst [1]. The substitution at catalytically active “B” sites in the ABO₃ structure by some transition metal cation can enhance the catalytic activity by leading to valence changes, which causes non-stoichiometry-related microstructural defects in the lattice [10]. Therefore, changing the oxygen content leads to structural rearrangement; increasing the lattice vacancies leads to higher oxide ion mobility, the synergistic role of mixed valence states of cations at “B” sites, and the segregation of active metal components at the surface layer [10, 11].

Fe-doped perovskite-type complex oxides exhibit excellent electron-ion mixed conductivity; therefore, many research groups are interested in these materials for electrode applications [12–16]. In particular, because these materials exhibit electric conductivities over 102 Ω⁻¹cm⁻¹ and ionic conductivities of 10⁻²–1.0 Ω⁻¹cm⁻¹ over 800°C, Fe-doped perovskite-type complex oxides have attracted growing attention [17]. Shimizu et al. synthesized La_1−xCo_1−yO₃ (A′: Ca, B′: Mn, Fe, Ni, and Cu) using the citrate method and reported that the perovskite catalysts exhibited the best performance for the ORR and cyclability when the Co cation was substituted by a Fe cation [18].

In a previous investigation, La_0.9Ca_0.1Co_1−xFe_xO₃ () oxides were synthesized and applied to the cathode for zinc air secondary batteries. These catalysts exhibited excellent electrochemical properties at room temperature [19].

2. Experimental

La_0.9Ca_0.1Co_1−xFe_xO₃ () oxide was synthesized using the citrate method for the cathode catalyst of zinc air secondary batteries. As starting materials, La(NO₃)₂6H₂O, Ca(NO₃)₂4H₂O, Co(NO₃)₂6H₂O, and Fe(NO₃)₃9H₂O were used. Metal nitrates were mixed in a stoichiometric molar ratio of La_0.9Ca_0.1Co_1−xFe_xO₃ () and dissolved in deionized water. Then, citric acid was added as a chelating agent while stirring constantly. After mixing the starting materials, the solution containing a mixture of citric acid and constituent metal nitrates was heated at 90°C for several hours to evaporate the excess solvents and promote polymerization. When the gel precursor was attained, it was heated at 100–120°C until it burned up followed by calcination for 2 hours at 700°C (Figure 1).

Figure 1 

Manufacturing process of the cathode catalyst.

The air electrode was prepared as illustrated in Figure 2. First, a polytetrafluoroethylene (PTFE) dispersion (60%, Dupont 30-J) was mixed with deionized water and ultrasonically dispersed for 10 min. Vulcan XC-72 carbon black and prepared La_0.9Ca_0.1Co_1−xFe_xO₃ () perovskite catalyst powder were poured into the PTFE-water mixture. The mixture was stirred for 2 h and dried at 120°C. The dried electrode powder was mixed again with some drops of isopropyl alcohol and kneaded to make a cathode sheet using a roll press. This cathode sheet was attached to one side of a Ni mesh, and a PTFE sheet for the gas diffusion electrode was placed at the other side of the Ni mesh. Figure 2 shows the process for preparing the air electrode. The compositions of the air cathode were 42.5% La_0.9Ca_0.1Co_1−xFe_xO₃ () perovskite, 42.5% Vulcan XC-72 carbon black, and 15% PTFE dispersion.

Figure 2 

Manufacturing process of the cathode.

The prepared disk-type cathode was used as a working electrode and was placed into an electrochemical cell. On the opposite side, the platinum mesh as a counter electrode was placed with an Hg/HgO electrode as the reference electrode. An 8.5 M potassium hydroxide (KOH) solution was used as the electrolyte.

A potentiostat (VMP3, Princeton Applied Research) was used for electrochemical measurement of the half-cell. The scan rate for the linear sweep voltammograms (LSV) and cyclic voltammograms (CV) was 2 mV s⁻¹ and 5 mV s⁻¹, respectively.

The surface morphology of the cathode was examined using a scanning electron microscope (S-2700, Hitachi, Japan). The XRD patterns were obtained using an X-ray diffractometer (1830 X-ray diffractometer, Philips) using Ni-filtered Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of 20°–90° at a scan rate of 0.1° s⁻¹.

3. Results and Discussion

Based on the XRD analysis presented in Figure 3, we verified that the powder calcined at 700°C contained a single phase of La_0.9Ca_0.1CoO₃ perovskite structures in all of the Fe-doped levels. Figure 4 shows the representative morphology of the La_0.9Ca_0.1Co_1−xFe_xO₃ () catalysts. The powders exhibit an average particle size of 50 nm (primary particles).

Figure 3 

XRD patterns of La_0.9Ca_0.1Co_1−xFe_xO₃ () catalyst after calcination.

Figure 4 

SEM images of catalyst.

Figures 5 and 6 present the polarization curves for a series of Fe-doped perovskites, La_0.9Ca_0.1Co_1−xFe_xO₃ (). The substitutions of cobalt cations by iron cations could improve the catalytic activity; thus, reduced cathodic overpotentials were obtained with the Fe-doped La_0.9Ca_0.1CoO₃ cathode compared with the nondoped materials. Neburchilov et al. reported that the Fe-doped perovskite catalyst has high activity for the decomposition of the in the two-electron pathway in ORR that occurred in alkaline solution [6, 20]. Similar results were observed for the anodic polarization characteristics, as demonstrated in Figure 6. Considering the cathodic and anodic polarization characteristics, the optimum composition based on the Fe doping level was La_0.9Ca_0.1Co_0.9Fe_0.1O₃.

Figure 5 

Cathodic polarization curves of La_0.9Ca_0.1Co_1−xFe_xO₃ () cathodes.

Figure 6 

Anodic polarization curves of La_0.9Ca_0.1Co_1−xFe_xO₃ () cathodes.

As observed in Figures 5 and 6, the doping of La_0.9Ca_0.1CoO₃ with Fe could lead to improvements in the oxygen evolution reaction rather than the oxygen reduction reaction. According to the results obtained by Jörissen, well-crystallized materials improve the oxygen reduction reaction, whereas less crystalline materials are beneficial for the oxygen evolution reaction [9]. The particle size of the synthesized Fe-doped La_0.9Ca_0.1CoO₃ was approximately 30 nm, which was smaller than that of the non-Fe-doped La_0.9Ca_0.1CoO₃, and the XRD intensity of the Fe-doped La_0.9Ca_0.1CoO₃ was smaller. Therefore, it can be concluded that the Fe-doped La_0.9Ca_0.1CoO₃ alloys synthesized in this study were less crystalline. Therefore, these alloys were beneficial for the oxygen evolution reaction.

When Co in the La_0.9Ca_0.1CoO₃ is substituted with Fe, the lattice contains non-stoichiometry-related microstructural defects due to synergistic valence changes. Therefore, the catalytic activity may be improved. However, high contents of Fe lead to decreased ion mobility because the binding energy of Fe-O is stronger than that of Co-O [12]. Therefore, as illustrated in Figures 5-6, highly Fe-doped La_0.9Ca_0.1CoO₃ exhibits a larger cathodic or anodic overpotential.

Figures 7 and 8 present the anodic and cathodic polarization curves of La_0.9Ca_0.1Co_1−xFe_xO₃ () catalysts as a function of the Fe doping level. In these results, the potential differences between charging (oxygen evolution, = 50 mA cm⁻²) and discharging (oxygen reduction, = 50 mA cm⁻²) were 929 mV for , 712 mV for , 717 mV for , 725 mV for , and 735 mV for .

Figure 7 

Anodic and cathodic polarization curves of La_0.9Ca_0.1Co_1−xFe_xO₃ () cathodes.

Figure 8 

Anodic and cathodic polarization potentials at current density of 50 mA cm⁻² as a function of Fe doping level.

The fact that the substitutions of cobalt with iron could improve the catalytic activity was demonstrated by several observations. However, as observed in Figure 9, upon increasing the Fe content, the cyclability worsened, which was caused by degradation of the structural properties of the cathode during cycling. Oxygen reduction can take place through two pathways. One leads to water or OH⁻ through a four-electron reduction; the other one leads to peroxide through a two-electron reduction. It is known that the perovskite catalyst has four-electron pathway reaction; however at high current density region, it follows two-electron pathway as the following two steps:

(a)

(b)

(c)

(d)

(a)
(b)
(c)
(d)

Figure 9 

Cyclic voltammograms of La_0.9Ca_0.1Co_1−xFe_xO₃ cathode at a sweep rate of 5 mV s⁻¹: (a) , (b) , (c) , and (d) .

The peroxide formed during the two-electron reaction diffuses into the bulk solution. During diffusion, the peroxide oxidizes the Teflon bonding between the catalyst and carbon. Decomposition of Teflon bonding blocks the cathode pore and results in increment of internal resistance. This is the main reason of capacity degradation [6]. It is presumed that the two-electron path ratio is increased along with the doping level of Fe in La_0.9Ca_0.1Co_1−xFe_xO₃. Therefore the cathode structure becomes unstable.

4. Conclusions

To improve the catalytic activity, Co in La_0.9Ca_0.1CoO₃ was substituted with Fe. In the anodic and cathodic polarization measurements, the substitution by Fe enhanced the electrochemical performances of the catalysts due to synergistic valence changes and resultant non-stoichiometry-related microstructural defects introduced into the lattice. However highly Fe-doped La_0.9Ca_0.1Co_1−xFe_xO₃ lost their structural stability.

Overall, by simultaneously considering oxygen reduction/evolution reactions and cyclability, La_0.9Ca_0.1Co_0.9Fe_0.1O₃ could be applied as a good bifunctional catalyst for cathodes.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

This work was supported by the National Research Foundation of Korea grant funded by the Korean Government (MEST) (NRF-2012-M1A2A2-028739).