Oxygen evolution behavior of La_1−xSr_xFeO_3−δ electrodes in LiCl–KCl melt

Abstract

Electrochemical reduction processes of oxides in molten salt have been proposed as the carbon-free technologies in order to achieve carbon neutrality. The anodic behavior of La_1−xSr_xFeO_3−δ as an O₂ evolution anode in LiCl–KCl at 723 K was investigated. The results suggested that at 723 K, the electrical conductivity of La_1−xSr_xFeO_3−δ tended to increase with the Sr doping. The anodic reactions of the La_1−xSr_xFeO_3−δ electrodes were characterized by electrochemical measurements in LiCl–KCl + Li₂O at 723 K. Based on the cyclic voltammograms of the La_0.7Sr_0.3FeO_3−δ electrode, O₂ evolution has proceeded between 2.7 and 3.6 V. The potential of the La_0.7Sr_0.3FeO_3−δ electrode during galvanostatic electrolysis has conducted at 39 mA cm⁻² for 15 h has remained stable at 2.8 V, indicating that the stable evolution of O₂ gas was monitored. The corrosion rate was estimated to have the low value of 8.6 × 10⁻⁴ g cm⁻² h⁻¹. Electrode surface data obtained after electrolysis indicated that the La_0.7Sr_0.3FeO_3−δ electrode exhibited excellent chemical and physical stability in LiCl–KCl at 723 K. This indicates that the La_0.7Sr_0.3FeO_3−δ electrode is promising candidate material as inert anodes for oxide decomposition. As an application of the La_0.7Sr_0.3FeO_3−δ electrode, the electrolytic reduction of CO₂ was also successfully achieved.

Graphical Abstract

1 Introduction

The electrochemical reduction of metal oxides in molten salt has been proposed as an approach to the production of metals [1,2,3,4,5]. The proposed process of molten salt electrolysis requires the use of an inert anode, as described in the following reactions:

$${\text{In molten salt}}:{\text{ MO}}_{x} \to {\text{ M}}^{{{2}x + }} + x{\text{O}}^{{{2} - }}$$

(1)

$${\text{Cathode reaction}}:{\text{ M}}^{{{2}x + }} + { 2}x{\text{e}}^{ - } \to {\text{ M}}$$

(2)

or

$${\text{MO}}_{x} + { 2}x{\text{e}}^{ - } \to {\text{ M }} + x{\text{O}}^{{{2} - }}$$

(3)

$${\text{Inert anode reaction}}:{\text{ 2O}}^{{{2} - }} \to {\text{ O}}_{{2}} + {\text{ 4e}}^{ - }$$

(4)

$${\text{Over}}\,{\text{all}}\,{\text{cell}}\,{\text{reaction}}:{\text{MO}}_{x} \to {\text{M}} + {x \mathord{\left/ {\vphantom {x {2{\text{O}}_{2} }}} \right. \kern-0pt} {2{\text{O}}_{2} }}$$

(5)

As described above, oxides (MO_x) dissolve into molten salt as cations (M^2x+) and oxide ions (O²⁻). Subsequently, by conducting electrolysis, M^2x+ is electrochemically reduced to obtain the relevant element in metallic form at the cathode. This element can be alternatively obtained through the direct reduction of MO_x. O²⁻ is electrochemically oxidized such that O₂ gas evolves at the inert anode. However, certain anodic materials that can stably trigger the evolution of O₂ gas stably in molten salt are required because molten salt is highly corrosive.

Carbon anodes have been used in most experiments on the electrochemical reduction of oxides in molten salt. Notably, these anodes react with the evolved O₂ to mainly form CO₂. This reaction has several drawbacks. The carbon anode is gradually consumed and will require to replacement. Importantly, the extent of consumption of the carbon anode significantly exceeds the amount of CO₂ produced, as the said gas originating from the anode tends to erode the electrode. This reduces the current efficiency of the process and increases its energy consumption [6,7,8]. In this context, the development of nonconsumable O₂-evolving anodes for metal extraction or carbon from oxides (including CO₂) in molten salt is indispensable.

A few materials can act as nonconsumable O₂-evolving anodes in molten salts. The characteristics required of such inert anodes include physical stability at the operating temperature, electrical conductivity, resistance to attack by the molten salt, resistance to attack by chlorine and oxygen gases, resistance to attack by chloride and oxide ions, resistance to thermal shock, and their robustness. Three general types of materials have been investigated as anodic materials for the described process: metals, ceramics, and the cermet electrode. Metal electrodes exhibit high electrical conductivity and good thermal shock resistance [9,10,11,12]. Although metals react with oxygen at high temperatures, the protective oxide layer formed inhibit additional oxidation reactions [9, 10]. Cetain ceramics are known to be stable in molten chlorides. However, the ceramic materials are poor electrical conductors, although a few exhibits semiconducting properties that allow conduction at high temperatures [7, 8, 13, 14]. Studies focusing on ion-conductive ceramics have also been reported [15]. Cermet is a composite material of metal and ceramics, combining the characteristics of both components. The metallic component of cermet has been observed to react with oxygen at high temperatures [16].

In the present study, we focused on valency-controlled metal oxide semiconductors as anodic materials because of their high melting point, excellent resistance to oxidation, excellent thermal stability, and low electrical resistivity at high temperatures. Specifically, the p-type La_1−xSr_xFeO_3−δ semiconductor, which is produced under an O₂ partial pressure of 0.2 atm at a high temperature, was selected for nonconsumable O₂-evolving anodes in molten salt. The p-type La_1−xSr_xFeO_3−δ semiconductor exhibits low electrical resistivity at high temperatures [17]. If parameter x in La_1−xSr_xFeO_3−δ−δ is 0, LaFeO₃ is obtained, which is an insulator at 298 K. The substitution of the trivalent La³⁺ ions by a divalent rare-earth metal, such as Sr²⁺ ions, forces the Fe ions present in the structure to switch from the stable trivalent to the tetravalent states. The coexistence of the trivalent and tetravalent Fe ions causes to an increase in electronic conductivity compared with LaFeO₃. La_1−xSr_xFeO_3−δ also exhibits high ionic conductivity due to the formation of additional oxygen vacancies at high temperatures [18, 19].

In this study, we synthesized La_1−xSr_xFeO_3−δ electrodes of various compositions (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) and then compared their physical properties. Their electrode reactions were characterized by electrochemical measurements, such as cyclic voltammetry. Electrolysis experiments were conducted under different conditions, and the behavior of the anode was evaluated in molten LiCl–KCl at 723 K.

As an application of the La_0.7Sr_0.3FeO_3−δ electrode, CO₂ electrolysis was performed in a molten LiCl–KCl + 3.0 mol% Li₂O at 723 K. CO₂ decomposition technology will be an effective for combating global warming.

2 Experimental

2.1 Synthesis of La_1−xSr_xFeO_3−δ

La_1−xSr_xFeO_3−δ powder was synthesized using the solid phase method. La₂O₃ (99.9%, Kojundo Chemical Laboratory Co., Ltd.), SrO (98.0%, Kojundo Chemical Laboratory Co., Ltd.), and Fe₂O₃ (99.9%, Kojundo Chemical Laboratory Co., Ltd.) were selected as the oxide precursors. Aliquots of the mentioned metal oxide precursors were weighed to obtain mixtures where parameter x assumed the following values: 0.1, 0.2, 0.3, 0.4, and 0.5. The oxides powders were mixed in a mortar and ground using a bead mill (Easy Nano RMB; Aimex Co., Ltd.) with ZrO₂ balls (φ2 mm) at 2000 rpm in ethanol. The milled powders were examined using a particle size analyzer (SALD-2300; Shimadzu Co., Ltd.) to keep the average particle sizes less than 1 µm. Afterward, the milled powders were then pressed using a 50 mm × 10 mm metal mold at 70 MPa and a cold isostatic press (CIP; Dr.CIP, Kobe Steel Co., Ltd.) at 245 MPa. The formed pellets were sintered in the range of 1473–1723 K for 8 h under the O₂ partial pressure of 0.2 atm. The surfaces of the obtained La_1−xSr_xFeO_3−δ pellets were mechanically polished, first with SiC paper, then with diamond particles (below a size of 6 µm) prior to characterization. The image of a pellet is shown in Fig. 1. A 50 mm × 10 mm × 10 mm metal mold was used. However, the size of the sintered body was subject to shrinking because of the CIP and sintering process. The size of the sintered pellets was measured after the sintering process to be in the range of 46–48 mm × 8–9 mm × 8–9 mm.

Fig. 1

Configuration of the electrolysis experimental equipment to evaluate the anodic characteristics of the La_1−xSr_xFeO_3−δ electrodes

The density of the La_1−xSr_xFeO_3−δ pellets was determined using the Archimedes technique. The polished surfaces of the pellets were investigated by scanning electron microscopy (SEM; JSM-7001FD, JEOL Ltd.) and energy-dispersive X-ray spectroscopy (EDS; JED-2300, JEOL Ltd.). The crystal structures of the pellets were characterized by X-ray diffraction (XRD; RINT-2001 MultiFlex, Rigaku Co, Ltd.). Sample identification was achieved using the relevant diffraction pattern, and the average lattice constant was calculated. The thermoelectric properties of the La_1−xSr_xFeO_3−δ pellets were measured from room temperature to 1073 K using a four-terminal conductivity analyzer (RZ2001i, Ozawa Science Co, Ltd.).

2.2 Electrochemical measurements

Figure 1 shows a schematic view of the electrolytic apparatus used. A quartz holder comprising high-purity silica was used. The working electrode comprised a La_1−xSr_xFeO_3−δ electrode mirror-polished using diamond particles (below a size of 6 µm). Pt wire was used to connect the La_1−xSr_xFeO_3−δ electrode. The counter electrode was Ni wire in a spiral shape (99.35%, φ1.0 mm, Sumiden Fine Conductors Co., Ltd.). The reference electrode was an Ag wire immersed in a eutectic LiCl–KCl melt containing 1 mol% AgCl in a thin-bottomed alumina tube. The alumina tube was sufficiently thin enough to use the membrane of the reference because the alumina intrinsically included a small quantity of various metal ions, which afforded ion conductivity at the experimental temperatures. The potential of the reference electrode was calibrated in reference to that of the Li^I | Li electrode, which was prepared by electrodepositing Li metal on a nickel electrode.

Lithium chloride (LiCl, 99.0%, Fujifilm Wako Pure Chemical Co.) and potassium chloride (KCl, 99.5%, Fujifilm Wako Pure Chemical Co.) were mixed in a eutectic composition (58.5 mol% LiCl, 41.5 mol% KCl). The obtained mixture was dried under vacuum conditions for more than 24 h at 473 K to remove water from it. The eutectic mixture was prepared to a total weight of 200 g (LiCl, 89.0 g; KCl, 111.0 g). Thereafter, it was melted in a high-purity alumina crucible. Lithium oxide (Li₂O, 99.5%, Fujifilm Wako Pure Chemical Co.) was used as the source of the oxide ion and was directly added to the melt. The experimental processes were conducted in a glove box under dried argon atmosphere. Thus, the effect of moisture was practically negligible.

CO₂ electrolysis was performed in LiCl–KCl + 3.0 mol% Li₂O at 723 K. The cathode deposits after CO₂ electrolysis were analyzed by Raman spectroscopy analysis (NRS-5500, Jasco Co.) using a YAG laser (532 nm).

3 Results and discussion

3.1 Synthesis of La_1−xSr_xFeO_3−δ

La_1−xSr_xFeO_3−δ powder was synthesized using the solid phase method. La₂O_3, SrO, and Fe₂O₃ powders were selected as oxide precursors. LaFeO₃ comprises stable trivalent La and trivalent Fe, and it exhibits a perovskite crystal structure. La_1−xSr_xFeO_3−δ was synthesized by replacing the trivalent La ion with the divalent Sr ions, forcing the Fe ion to switch from the stable trivalent to the tetravalent states [18, 19].

$$\left( {1 - x} \right){\text{ La}}_{{2}} {\text{O}}_{{3}} + \, 2x{\text{SrO}} + \alpha {\text{ - Fe}}_{{2}} {\text{O}}_{{3}} \to \, 2{\text{ La}}_{1 - x} {\text{Sr}}_{x} {\text{FeO}}_{3 - \delta }$$

(6)

The oxide precursors of La_1−xSr_xFeO_3−δ were weighed such that values of x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5 were obtained for parameter x, and the obtained mixtures were ground using a bead mill and pressed using a CIP at 245 MPa. Subsequently, the formed pellets were sintered within the range of 1473–1623 K for 8 h under the O₂ partial pressure of 0.2 atm. Figure 2 shows the XRD spectra of the La_1−xSr_xFeO_3−δ samples obtained after the mentioned sintering process. The XRD patterns of the La_1−xSr_xFeO_3−δ samples (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) were indicative of single-phase (La, Sr) FeO_3−δ ceramics. The average lattice constants of the samples were calculated from the relevant XRD spectra calibrated using a reference sample (pure Si). The axis average lattice constants for the a-, b-, and c-axes of the La_1−xSr_xFeO_3−δ samples decreased with an increasing in parameter x. The values of the coefficient (R²) of each axis were calculated using a regression line to be 0.8632, 0.9595, and 0.8830 for the a-, b-, and c-axes, respectively (Fig. S1, supplementary information). The coefficient (R²) exceeded 0.85 for the three axes. The average lattice constants of the synthesized La_1−xSr_xFeO_3−δ semiconductors were confirmed to be correlated with the parameter x of La_1−xSr_xFeO_3−δ.

Fig. 2

X-ray diffraction patterns of La_1−xSr_xFeO_3−δ samples after sintering at 1498–1723 K for 8 h in atmospheric air (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5 at 1723, 1623,1623, 1623, 1523, and 1498 K, respectively)

The coexistence of Fe³⁺ and Fe⁴⁺ ion was achieved by substituting La³⁺ for Sr²⁺ [20]. As the ionic radius of Fe⁴⁺ is smaller than that of Fe³⁺, the average lattice constant decreased with an increase in parameter x (Fig. S1). Therefore, the formation of a substituted La_1−xSr_xFeO_3−δ solid solution was suggested.

3.2 Densification of La_1−xSr_xFeO_3−δ

The ceramic electrodes used for molten salt electrolysis need to be characterized by high density. In this study, the La_1−xSr_xFeO_3−δ samples with different x values (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) were sintered at various temperatures in the range of 1473–1723 K, to determine the most suitable sintering temperature for the densification of the samples. Notably, the densities (g cm⁻³) were measured using the Archimedes technique.

Table 1 shows the relationship between the sintering temperature and density of La_1−xSr_xFeO_3−δ. The densities of the Sr-doped samples with x = 0.1, 0.2, and 0.3 drastically increased at 1623 K. Contrarily, the densities of the Sr-doped samples with x = 0.4 and 0.5 reached a maximum at 1523 and 1498 K. As confirmed from the binary phase diagram of LaFeO₃–SrFeO_3−δ [21], the melting point tended to decrease as the Sr concentration (x) increased. Thus, the densification temperature decreased as the x value increased.

Table 1 Relationship between the sintering temperature and density of La_1−xSr_xFeO_3−δ

3.3 Electrical properties of La_1−xSr_xFeO_3−δ

Figure 3 shows data reflecting the electrical properties of the La_1−xSr_xFeO_3−δ samples characterized by the highest density for each composition ratio at various temperatures. The conductivities of each sample were measured using the four-terminal conductivity analyzer from room temperature to 1073 K. When the Sr composition ratio was changed from x = 0 to x = 0.5, at 723 K, the electrical conductivity of the material increased from 5.7 × 10⁻¹ to 1.6 × 10⁴ S/m. The substitution of trivalent La³⁺ ion by a divalent metal, such as Sr²⁺ ion, forced the Fe ion to switch from the stable trivalent to the tetravalent states. The coexistence of the trivalent and tetravalent Fe ions caused to an increase in electronic conductivity compared with the LaFeO₃ of the parent material. Furthermore, La_1−xSr_xFeO_3−δ (x > 0.1) also exhibits higher ionic conductivity than LaFeO₃ owing to the formation of additional oxygen vacancies at elevated temperatures [19, 22].

Fig. 3

Relationship between x and the electrical conductivity of the La_1−xSr_xFeO_3−δ electrodes

3.4 Electrochemical behavior of oxide ions on La_1−xSr_xFeO_3−δ electrodes

Cyclic voltammetry performed in a LiCl–KCl melt at 723 K to investigate the electrochemical behavior of oxide ions on the La_0.7Sr_0.3FeO_3−δ electrode. Figure 4 shows the cyclic voltammograms obtained using the La_0.7Sr_0.3FeO_3−δ electrode before and after the addition of Li₂O to the LiCl–KCl melt to final concentrations of 0.5 and 3.0 mol%. Notably, the solubility of Li₂O in the LiCl–KCl melt at 723 K has been reported to be approximately 1.0 mol% [23]. The optimal amount of Li₂O was selected to be 3.0 mol% to maintain the saturation of Li₂O in the molten salt during electrolysis.

Fig. 4

Cyclic voltammograms of the La_0.7Sr_0.3FeO_3−δ electrode in LiCl–KCl melts at 723 K. Scan rate: 100 mV s⁻¹

Before the addition of Li₂O to the LiCl–KCl melt, the oxidation current density began to increase at a value for the potential of 3.7 V. Contrarily, after the addition of Li₂O, the oxidation current density began to increase at a potential of 2.7 V. In previous studies [24, 25] conducted on the same electrolyte comprising molten LiCl–KCl and Li₂O, which included a boron-doped diamond electrode acting as the anode, O₂ and Cl₂ evolution was observed to proceed at 2.5 and 3.6 V, respectively. Therefore, in the present system with a La_0.7Sr_0.3FeO_3−δ electrode as the anode, the increase in oxidation current density at 3.7 V observed before the addition of Li₂O to the LiCl–KCl melt was attributed to the reaction leading to chlorine gas evolution or chlorine compounds. Contrarily, the increase in oxidation current density at 2.7 V observed after the addition of Li₂O was attributed to the reaction leading to O₂ gas evolution. In the LiCl–KCl melt containing Li₂O, O₂ gas evolution was assumed to occur between 2.7 and 3.6 V at 723 K.

The electrochemical behavior of oxide ions on the La_1−xSr_xFeO_3−δ electrode (x = 0.3, 0.4, and 0.5) was also investigated by comparing the cyclic voltammograms obtained at each x value, to determine the composition of the La_1−xSr_xFeO_3−δ electrode that affords the highest reactivity leading to O₂ evolution. Figure 5 shows the cyclic voltammograms of La_1−xSr_xFeO_3−δ electrodes (x = 0.3, 0.4, and 0.5) in the LiCl–KCl melt containing 3.0 mol% Li₂O at 723 K. The electrode surface area was defined as the geometric surface area. The different background currents shown in Fig. 5 were attributed to the differences in the real surface area. As shown in Fig. 3, these three electrodes (x = 0.3, 0.4, and 0.5) exhibited the highest electrical conductivity. The La_0.5Sr_0.5FeO_3−δ electrode exhibited the highest oxidation current density between 2.8 and 3.1 V, but the La_0.7Sr_0.3FeO_3−δ electrode exhibited the highest oxidation current density between 3.2 and 3.6 V (Fig. 5). The current density (Fig. 5) of La_0.5Sr_0.5FeO_3−δ electrode practically remained constant from 2.9 to 3.4 V. In a study by Kado et al. [25], the oxidation current between 2.5 and 3.6 V in LiCl–KCl at 723 K was due to O₂ generation. Therefore, the shape of the cyclic voltammograms of each electrode suggested that the O₂ evolution reaction on La_0.7Sr_0.3FeO_3−δ and La_0.6Sr_0.4FeO_3−δ were governed by the charge-transfer step, whereas that on La_0.5Sr_0.5FeO_3−δ was governed by the diffusion step. The differences in the shape of the cyclic voltammograms were due to differences in the O₂ evolution processes on the electrode/the molten salt interface. The different amounts of Sr added to La_1−xSr_xFeO_3−δ affected the catalytic activity of the O₂ evolution reaction.

Fig. 5

Cyclic voltammograms of La_1−xSr_xFeO_3−δ electrodes recorded at 723 K in LiCl–KCl melts containing Li₂O 3.0 mol%. Scan rate: 100 mV s⁻¹

We decided to use the La_0.7Sr_0.3FeO_3−δ electrode in electrolysis tests. This electrode exhibited the lowest electrical conductivity during the cyclic voltammetry experiments. This confirmed that an increase in electrical conductivity did not necessarily enhance the O₂ evolution reaction.

3.5 Electrolysis tests performed using the La_0.7Sr_0.3FeO_3−δ electrode

Based on the cyclic voltammetry results, the O₂ evolution process was concluded to have occurred between 2.7 and 3.6 V in the LiCl–KCl melt containing 3.0 mol% Li₂O at 723 K. Figure 6 shows the time transition of current densities measured during potentiostatic electrolysis experiments conducted at 2.9 V, 3.3 V, and 3.5 V. The anodic currents measured in association with the O₂ gas evolution process practically remained steady around 10 mA cm⁻² at 2.9 V and around 70 mA cm⁻² at 3.3 V and 3.5 V. This implied that the electrochemical reaction taking place at the La_0.7Sr_0.3FeO_3−δ electrode did not change during electrolysis. The current density fluctuation shown in Fig. 6 was caused by altering the electrode surface area because of the adsorption and desorption of O₂ gas bubbles evolved at the electrode/molten salt interface during the experiment.

Fig. 6

Time transition of current densities on the La_0.7Sr_0.3FeO_3−δ electrode at 2.9, 3.3, and 3.5 V for 1 h

The electrolysis conditions were selected based on Fig. 4. The O₂ evolution process was observed at the potentials ranging from 2.7 to 3.6 V in the LiCl–KCl melt containing 3.0 mol% Li₂O at 723 K. The current density corresponding to the potential range of the gas evolution was selected from 10 to 100 mA cm⁻² depending on the experiment. Figure 7 shows data on the time transition of the potential during galvanostatic electrolysis experiments conducted at current densities of 49, 46, 39 mA cm⁻² for 2, 5, 15 h, respectively. The potential at which the O₂ gas evolution process occurred fluctuated between 3.1 and 3.3 V during the experiments conducted at 49 and 46 mA cm⁻² for 2 and 5 h, respectively. For the galvanostatic electrolysis at 39 mA cm⁻², the potential remained stable at 2.8 V during the electrolysis (Fig. 7). Oppositely, as the currents density exceeded 39 mA cm⁻², the potential began to fluctuate. Therefore, an applied current density of 39 mA cm⁻² was chosen for the experiment.

Fig. 7

Variation of potentials on the La_0.7Sr_0.3FeO_3−δ electrode at 50 mA cm⁻² for 2, 5, and 15 h

The mechanism of the fluctuating potential shown in Fig. 7 was unclear. A possible explanation was that the effect of potential dependence on the behavior of the nucleation, growth, and detachment of O₂ gas on the electrode might have caused the difference in the current time transition.

In addition, the gas generated on the La_0.7Sr_0.3FeO_3−δ electrode during the electrolysis conducted at a current density of 39 mA cm⁻² for 15 h was analyzed by gas chromatography (GC-2014, Shimadzu Co, Ltd.). Figure 8 shows the gas chromatograms of gas samples collected before and after galvanostatic electrolysis. It was confirmed in advance that the peak attributed to O₂ gas had a retention time of approximately 3 min based on the results of injecting O₂ standard gas. Figure 8 shows that the intensity of the peak due to O₂ in the gas sample collected after electrolysis was increased. Based on the gas chromatograms, the current efficiency of the O₂ evolution reaction (Eq. 4) was estimated at 30.4%. The following reactions are also one possible explanation for the low current efficiency of O₂ gas. Oxide ion (O²⁻) was oxidized to form intermediate species (for example, O₂²⁻, O₂⁻, and O⁻):

$${\text{2O}}^{{{2} - }} \to {\text{ O}}_{{2}}^{{{2} - }} + {\text{ 2e}}^{ - }$$

(7)

$${\text{2O}}^{{{2} - }} \to {\text{ O}}_{{2}}^{ - } + {\text{ 3e}}^{ - }$$

(8)

$${\text{O}}^{{{2} - }} \to {\text{ O}}^{ - } + {\text{ e}}^{ - }$$

(9)

Fig. 8

Gas chromatogram of gas samples collected before and after galvanostatic electrolysis conducted at a current density of 39 mA cm⁻² for 15 h

These intermediates formed by the anodic reactions suppressed the current efficiency and the subsequent reactions should occur.

$${\text{O}}_{{2}}^{{{2} - }} \to {\text{ O}}_{{2}}^{ - } + {\text{ e}}^{ - }$$

(10)

$${\text{O}}^{{{2} - }} + {\text{ O}}^{ - } \to {\text{ O}}_{{2}}^{{{2} - }} + {\text{ e}}^{ - }$$

(11)

In addition, certain intermediates decomposed to form O and O²⁻ [26], and the atomic oxygen easily reacted with the metal wire in the cell.

$${\text{O}}_{{2}}^{ - } \to {\text{ O }} + {\text{ O}}^{{{2} - }}$$

(12)

The jump in potential led by the small increase in current density (Fig. 7) might also have been caused by changes in side reactions associated with O₂ ions, such as those in Eqs. (7)–(12). The relatively low electric conductivity of the La_0.7Sr_0.3FeO_3−δ (Fig. 3) may be another reason for the suppressed the current efficiency.

The sample subjected to electrolysis at 39 mA cm⁻² current density for 15 h had the longest time and highest total electrolysis charge in this study. The morphological surface changes exhibited by the La_0.7Sr_0.3FeO_3−δ electrode because of the electrolysis experiment were investigated by SEM, EDS, and XRD. Figure 9 shows the SEM images of the electrode surfaces and the results of the EDS elemental analyses conducted on the electrodes before and after electrolysis. The SEM images revealed tiny holes (several tens of nanometers) in the surface morphology after electrolysis. It was suggested that the tiny holes were produced by eroding the anode during O₂ evolution. The EDS results confirmed that there were practically no change in the elemental composition. The detected carbon attributed to carbon tape which was used to bond the samples to the observation table during SEM.

Fig. 9

Scanning electron microscopy images and energy-dispersive X-ray spectroscopy results of the surface of the La_0.7Sr_0.3FeO_3−δ electrode before and after electrolysis at 39 mA cm⁻² for 15 h

Figure 10 shows the XRD patterns of the La_0.7Sr_0.3FeO_3−δ electrode recorded before and after galvanostatic electrolysis experiments. A single-phase diffraction pattern attributed to (La,Sr)FeO_3−δ was observed in all the samples of before and after galvanostatic electrolysis experiments. The lack of change in the diffraction pattern before and after galvanostatic electrolysis indicated that the crystal structure of the (La,Sr)FeO_3−δ electrode had not changed. The La_0.7Sr_0.3FeO_3−δ electrode showed high corrosion resistance. As a mechanism for the high corrosion resistance, it is suggested that the La_0.7Sr_0.3FeO_3−δ electrode itself functions as an oxide protective layer. The La_0.7Sr_0.3FeO_3−δ electrode exhibited sufficient chemical and physical stability for use in molten salt electrolysis experiments.

Fig. 10

X-ray diffraction patterns of the La_0.7Sr_0.3FeO_3−δ electrode recorded before and after the galvanostatic electrolysis

The corrosion rate γ is another important property that defines the suitability of nonconsumable O₂-evolving anodes. In this study, the value of parameter γ (g cm⁻² h⁻¹) was estimated using Eq. (13) [8]:

$$\gamma = {{\Delta W} \mathord{\left/ {\vphantom {{\Delta W} {t \cdot S}}} \right. \kern-0pt} {t \cdot S}}$$

(13)

where ΔW is the difference between the weight (g) of the anode before and after electrolysis, t is the electrolysis time (h), and S is the surface area of the part of the electrode that is immersed in the electrolyte. The corrosion evaluation of the anode during galvanostatic electrolysis was conducted in a LiCl–KCl melt containing 3.0 mol% Li₂O at 723 K. The conditions are summarized in Table 2.

Table 2 Corrosion rate and weight change of the La_0.7Sr_0.3FeO_3−δ electrode in association with the performance of galvanostatic electrolysis, conducting electric quantity of La_0.7Sr_0.3FeO_3−δ electrode

As shown in Table 2, the corrosion rate of the La_0.7Sr_0.3FeO_3−δ electrode was calculated to have a value between 0 and 8.6 × 10⁻⁴ g cm⁻² h⁻¹. Considering that the weight loss of La_0.7Sr_0.3FeO_3−δ was not confirmed after immersion for 10 h without electrolysis in LiCl–KCl at 723 K (Table S1), the calculated values in Table 2 were the corrosion rate due to electrolytic corrosion. The weight loss of this anode was caused by eroding the anode during O₂ evolution because tiny holes (several tens of nanometers) in the surface morphology was observed in the SEM images for the sample after electrolysis at 39 mA cm⁻² for 15 h (Fig. 9). Fray et al. [8] deemed an anode inert as long as the value of the corrosion rate was below 1.5 × 10⁻³ g cm⁻² h⁻¹. Therefore, La_0.7Sr_0.3FeO_3−δ is an extremely promising candidate in the manufacture of nonconsumable O₂-evolving anodes for metal oxides or CO₂ decomposition.

3.6 CO₂ decomposition using the La_0.7Sr_0.3FeO_3−δ electrode

Cyclic voltammetry experiments were performed in a LiCl–KCl melt + 3.0 mol% Li₂O after CO₂ bubbling for 5 h at 723 K. Figure 11 shows the cyclic voltammograms recorded before and after the CO₂ bubbling for 5 h. The oxidation current density after CO₂ bubbling was smaller than before CO₂ bubbling. The reaction shown in Eq. (14) proceeded by CO₂ bubbling in an electrolytic bath.

$${\text{CO}}_{{2}} + {\text{ O}}^{{{2} - }} \to {\text{ CO}}_{{3}}^{{{2} - }}$$

(14)

Fig. 11

Cyclic voltammograms of electrochemical reaction before and after CO₂ bubbling for 5 h at 723 K. Scan rate: 10 mV s⁻¹

The decrease in oxide ions in the electrolytic bath led to a decrease in current density due to the O₂ evolution reaction (Eq. 4).

Potentiostatic electrolysis was performed at 3.23 V, the peak of oxidation current density.

Figure 12 shows the time transition of current densities measured during potentiostatic electrolysis experiments at 3.23 V. The anodic currents measured in association with the O₂ gas evolution process practically remained steady at 1 mA cm⁻².

Fig. 12

Time transition of current densities on the La_0.7Sr_0.3FeO_3−δ electrode at 3.23 V for 15 h

Black deposition observed on the Ni cathode after the 15 h electrolysis was analyzed by Raman spectroscopy. Figure 13 shows the Raman spectra of the electrodeposition. Two peaks attributed to carbon were confirmed at 1350 cm⁻¹ and 1584 cm⁻¹. The following cathodic reaction proceeded.

$${\text{CO}}_{{3}}^{{{2} - }} + {\text{ 4e}}^{ - } \to {\text{ C }} + {\text{ 3O}}^{{{2} - }}$$

(15)

Fig. 13

Image of the Ni cathode before and after electrolysis and Raman spectra of electrodeposition

CO₂ electrolysis was successfully achieved. However, the current density of the electrolysis was low in the present experimental system; therefore, the electrolysis bath requires improvement.

4 Conclusion

Dense La_1−xSr_xFeO_3−δ ceramic materials of different compositions (x = 0.1, 0.2, 0.3, 0.4, and 0.5) were obtained through a sintering process between 1498 and 1673 K for 8 h under an O₂ the partial pressure of 0.2 atm. The electrical conductivity of the La_1−xSr_xFeO_3−δ ceramics increased from 5.7 × 10⁻¹ to 1.6 × 10⁴ S/m as x increased from 0 to 0.5.

The suitability of the La_1−xSr_xFeO_3−δ electrodes for use as O₂-evolving anodes was investigated through a series of electrochemical measurements in LiCl–KCl melt containing Li₂O at 723 K. The cyclic voltammetry experiments were conducted on the La_1−xSr_xFeO_3−δ electrodes with x = 0.3, 0.4, and 0.5. The La_0.5Sr_0.5FeO_3−δ electrode exhibited the highest oxidation current density between 2.8 and 3.1 V, whereas the La_0.7Sr_0.3FeO_3−δ electrode exhibited the highest oxidation current density between 3.2 and 3.5 V. The current density of the La_0.5Sr_0.5FeO_3−δ electrode shown in Fig. 5 practically remained constant from 2.9 to 3.4 V, indicating that the O₂ evolution was governed by a diffusion control. As the applied potential range was equal to from 0.4 to 1.0 V expressed by overpotential, the applied potentials were far from the equilibrium potential of O₂ generation [25].

The La_0.7Sr_0.3FeO_3−δ electrode was subjected to electrolysis tests and exhibited the lowest electrical conductivity among the electrodes used in the cyclic voltammetry experiments. This confirmed that increases in electrical conductivity were not necessarily associated with enhancements of the reaction leading to O₂ gas evolution.

During galvanostatic electrolysis, the potential of the La_0.7Sr_0.3FeO_3−δ electrode remained stable between 2.8 and 3.3 V, regardless of the reaction conditions. Based on the gas chromatography results, the stable evolution of O₂ gas was confirmed to have occurred, and the current efficiency of the electrolysis process conducted at 39 mA cm⁻² current density for 15 h was estimated at 30.4%. The corrosion rate of La_0.7Sr_0.3FeO_3−δ was estimated at a very low value of 8.6 × 10⁻⁴ g cm⁻² h⁻¹. The XRD, SEM, and EDS results of the La_0.7Sr_0.3FeO_3−δ electrode before and after the electrolytic process showed that the electrode exhibited excellent chemical and physical stability in the LiCl–KCl melts containing Li₂O at 723 K. This study shows that La_0.7Sr_0.3FeO_3−δ is a promising candidate material for the manufacture of nonconsumable O₂-evolving anodes.

As an application of the La_0.7Sr_0.3FeO_3−δ electrode, CO₂ electrolysis was successfully achieved. However, the current density of the CO₂ electrolysis was low in the present experimental system; therefore, the electrolysis bath and electrode require improvement.