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Vol. 8. Issue 1.
Pages 759-765 (January - March 2019)
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Vol. 8. Issue 1.
Pages 759-765 (January - March 2019)
Original Article
DOI: 10.1016/j.jmrt.2018.05.019
Open Access
Lanthanum titanate nanometric powder potentially for rechargeable Ni-batteries: Synthesis and electrochemical hydrogen storage
J. Henaoa,
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Corresponding author.
, Y. Pachecob, O. Sotelob, M. Casalesb, L. Martinez-Gómezb
a CONACyT-CIATEQ A.C., Parque Industrial Bernardo Quintana, Av. Manantiales 23-A, El Marqués, Querétaro C.P.76246, Mexico
b Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México (UNAM), Avenida Universidad s/n, 62210 Cuernavaca, Morelos, Mexico
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La2Ti2O7 nanosized powder was prepared and evaluated as novel negative electrode material for Ni/oxide rechargeable batteries. La2Ti2O7 nanosized powder was synthesized at 850°C within 5h by the well-known citric-acid sol–gel route. The synthetized powder had an irregular morphology with a particle size below 100nm and a tendency to form clusters. La2Ti2O7 had a perovskite-type layered structure and presented a maximum electrochemical hydrogen uptake capacity of 224mAh/g corresponding to 0.84wt.% of hydrogen. Electrochemical studies revealed that the La2Ti2O7 compound can be considered an alternative electrode material for Ni/oxide rechargeable batteries since it exhibits a good cycling stability after activation.

Ni batteries
Hydrogen absorption
Full Text

The demand for reliable and efficient battery technology has led to increased interest in the development of new type of materials. Particularly, efforts have been made to determine new materials for nickel battery technology that can provide hydrogen storage under different operational conditions. These materials include intermetallic alloys, amorphous alloys, perovskite-type oxides, and carbon materials [1–4]. Metallic alloys (intermetallic and amorphous) are usually used as negative electrode materials in the well-known nickel metal hydride (Ni/MH) battery technology [1]. Alternatively, metal oxides and perovskite-type oxides have become a prominent option to replace conventional metallic alloys in Ni/oxide rechargeable battery technology [2,5]. Oxide compounds present advantageous properties over metallic materials since they are able to operate in strong basic environments and at high temperatures without losing their electrochemical performance. In recent years, a large amount of development has focused on Ni/oxide batteries using perovskite-type oxides with the general formula ABO3[2]. Hydrogen uptake in perovskite-type materials is possible due to the insertion of hydrogen protons into the oxygen-related positions within the crystalline structure of perovskites. Some examples are the doped La0.8Sr0.2FeO3 and La0.6Sr0.4Co0.2Fe0.8O3 compositions with an electrochemical capacity of 50 and 51mAh/g at room temperature, respectively [5,6].

Previous studies focused on improving the electrochemical properties of ABO3 perovskite electrodes have reported that nanometric powders present better hydrogen absorption properties than micrometric ones since the electrolyte–electrode contact area is increased, leading to enhanced efficiency of the electrochemical reaction [7]. One example of this finding is the nanometric LaFeO3 perovskite-type powder that presented an electrochemical capacity of 130mAh/g at room temperature; this result was remarkably higher than the 80mAh/g achieved from similar micrometric powder [7,8].

Alternatively, lanthanide titanium oxides with the general formula RE2Ti2O7 (RE=rare earth) have received a large amount of attention in the past years because these oxides present interesting physical properties, including ferroelectric, optical, and photocatalytic properties [9,10]. Although the formula A2B2O7 is usually associated with the pyrochlore structure [11], lanthanide titanium oxides belong to the perovskite-layered structure family (for A=La to Nd) which possesses narrow depletion layers and high quantum yields and therefore provides strong photocatalytic activity and high thermal stability [12]. To the best of our knowledge, most of the perovskite-type lanthanide titanium oxides, especially the La2Ti2O7 oxide, have been involved in applications related to dielectric and ferroelectric properties (photocatalysis), but no papers have investigated its hydrogen storage property.

In the present study, the suitability of using La2Ti2O7 perovskite-type nanometric powder as a novel negative electrode material for Ni/oxide batteries has been prioritized. Herein, the citric acid sol–gel (CASG) route has been used to obtain La2Ti2O7 nanometric powder. The CASG route is advantageous over the conventional synthesis route since it is a low-temperature synthesis method that produces homogeneous powders. The synthetized powder was used to prepare cold-pressed electrodes that were then evaluated via electrochemical measurements.

2Experimental2.1Synthesis of La2Ti2O7 nanometric powder and preparation of electrodes

Tetrabutyl titanate (Ti(OBu)4) (Aldrich) and La2O3 (99.9% purity, Aldrich) were used as the precursors of Ti and La in the CASG synthesis route. The molar ratio of La/Ti was fixed at 1/1. La2O3 powder (0.005mol) was dispersed in a spot of water, and then 5mL of concentrated HNO3 was added to it in droplets to dissolve the oxide. Subsequently, citric acid (0.04mol) was dissolved in 5mL of H2O and then poured into the solution. The mixture was thoroughly stirred by the magnetic mixer. At this point, Ti(OBu)4 (0.01mol) was added with vigorous stirring and in a 60°C liquid bath. After 0.5h, a homogeneous transparent solution was formed. The solution was heated at 120°C to vaporize the water. Four hours later, the mixture solution changed from a colorless solution to a white gel. Then, the gel was taken out and calcined in a Thermo Scientific Thermolyne furnace (F30428C-80 XL) at 850°C for 5h in air. The preheating ramp used was 7°C/min for about 2h up to 850°C. After the calcination, the samples were cooled to room temperature within the furnace for 12h at a cooling rate of 1.1°C/min.

The La2Ti2O7 electrodes were prepared using the so-called “latex” method. The La2Ti2O7 nanometric powder was mixed with black carbon to ensure good conductivity and polytetrafluoroethylene (PTFE) to provide the compound with more elasticity. The weight ratio of the mixture was 90:5:5 for La2Ti2O7, carbon black, and PTFE, respectively. Ethyleneglycol was added to the mixture in order to form a viscoelastic paste. The paste was deposited onto a 1cm2 Ni-mesh and then this was dried for 2h to evaporate the solvent. The electrodes were finally obtained by cold-pressing the deposited mixture onto the Ni-mesh at a pressure of 40MPa.

2.2Physical and electrochemical measurements

Differential and thermogravimetric curves (DSC–TGA) were evaluated on a simultaneous thermal analysis instrument (DSC-TGA-SDT Q600, TA Instruments) from room temperature up to 900°C and at a heating rate of 10°C/min. X-ray diffraction (XRD) measurements were conducted using a specular reflection mode (Bruker D8 Advanced, Cu Ka radiation, k=0.154056nm) over the angular range of 20°<Θ<80° with a step size of 0.008° and a measuring time of 300s per step. The particle size distribution of the La2Ti2O7 powder was determined using a laser diffraction equipment (LS1057083 Laser Diffraction, Malvern Instruments). The morphology of the product was determined by scanning electron microscopy (SEM, JEOL 7600). The electrochemical performance of the prepared lanthanum titanate oxide was tested in a conventional three-electrode open-air-cell (Potentiostat-Galvanostat, Gamry 1000E) using La2Ti2O7 as the working electrode, Hg/HgO as the reference electrode, Ni(OH)2 as the counter-electrode, and KOH (6M) solution as the electrolyte. The charge–discharge properties of the negative electrode were investigated using the chronopotentiometry technique. The working electrodes were fully charged and discharged at constant current densities of 10, 30, 60, and 125mA/g. The charge–discharge properties of the negative electrode were also investigated using cyclic voltammetry experiments and a chronoamperometry technique. Cyclic voltammograms were recorded between −0.4 and −1.3V compared with the Hg/HgO reference electrode at a scan rate of 1mV/s. A chronoamperometry technique was applied to estimate the hydrogen diffusion coefficient at different states of charge, after activation. Linear polarization scans were also recorded at the rate of 0.1mV/s from −5 to +5mV with respect to the open circuit potential.

3Results and discussion3.1Characterization of the synthetized powder

TGA–DSC measurements were carried out on the precursor gel to reveal the synthesis temperature corresponding to the La2Ti2O7 oxide obtained by the CASG route. Fig. 1a shows the TGA–DSC curves of the La2Ti2O7 precursor gel. The DSC curve has four peaks. The first endothermic peak (91°C) with relatively large weight loss (60%) is caused by the decomposition of the aqueous products [13,14]. The second endothermic peak (230°C) is attributed to the evaporation and burning of the organic substance. A third exothermic peak (395°C) is also observed and is caused by the combustion of the remaining organic substance, while the fourth exothermic peak (457°C) is related to the crystal lattice energy released during the formation of the perovskite-type structure. The last exothermic peak is associated with the final formation of the La2Ti2O7 compound. The measured overall weight loss was 77.26% and indicates the complete removal of all of the citric acid and all of the organic substance. No further weight loss was found from 600 up to 1000°C, suggesting that the La2Ti2O7 compound can be successfully fabricated in this temperature range. Consequently, the temperature selected to prepare La2Ti2O7 was 850°C to ensure complete transformation. Fig. 1b displays the XRD pattern of the prepared powder at 850°C (2h), which is quite consistent with a single perovskite-type layered phase of La2Ti2O7. The XRD pattern is indexed based on a monoclinic lattice (a=7.8262Å, b=12.8914Å, c=5.49Å, and β=98.081°) (JCPDS 81-1066). Fig. 1c and d shows the SEM and LS results of La2Ti2O7 prepared by CASG, respectively. The obtained powder presents an irregular morphology typical of chemical methods; it also displays a nanometric particle size (<100nm) with a tendency for agglomeration. The LS results prove that La2Ti2O7 particles agglomerate into clusters of about 475nm. Overall, the present results show that it is feasible to produce La2Ti2O7 nanopowders using CASG. This powder is usually fabricated by a conventional solid-state method at a high temperature (1100°C) and long calcination time (50h) [15]; however, the CASG results are advantageous because the La2Ti2O7 powder can be successfully prepared at a relatively low temperature (850°C) and a shortened calcination time (5h). The solid-state reaction is a two-step process involving a mechanical mixture of the precursors followed by heat treatment. To induce the mobility of the atoms to form the perovskite-type compound, calcination times are often long, between 12 and 50h, and at temperatures ranging from 1100 to 1400°C [16,17]. Thus, the prolonged times and high temperatures involved in the solid-state method represent higher energy consumption in comparison to the CASG route, which involves shorter times and lower calcination temperatures.

Figure 1.

Characterization results: (a) DSC–TGA curve of the La2Ti2O7 precursor gel; (b) XRD pattern of the La2Ti2O7 powder; (c) SEM micrograph of the powder obtained; (d) LS analysis.

3.2Electrochemical performance

Cyclic voltammetry experiments were carried out to reveal the electrochemical reduction–oxidation reactions of the La2Ti2O7 electrode in 6M KOH aqueous solution. Fig. 2 shows the cyclic voltammogram of the La2Ti2O7 negative electrode. A remarkable oxidation peak appears at −0.75V with respect to the Hg/HgO reference electrode, and a strong reduction peak appears at −0.825V with respect to the Hg/HgO reference electrode. This pair of redox peaks suggests that a reversible electrochemical reaction is occurring on the La2Ti2O7 working electrode. Interestingly, these peaks exhibit similar potential values compared with metal hydride electrodes [18]. The desorption peak of hydrogen appears prior to the electrochemical oxidation peak of hydrogen, suggesting the possible existence of strong hydrogen chemisorption. Moreover, the reduction peak becomes more defined with increasing number of cycles, indicating that the La2Ti2O7 electrode, unlike most ABO3-pervoskite-type oxides reported up to date [2], must be activated before reaching its maximum capacity (see also Fig. 4).

Figure 2.

Cyclic voltammogram of the La2Ti2O7 electrode.


Fig. 3 displays the typical charge–discharge curves for the La2Ti2O7 electrode. Interestingly, the charge–discharge curves of the cell are comparable to those of conventional hydrogen storage alloys [19] and those of conventional ABO3-pervoskite oxides [2,20], presenting an electrochemical discharge capacity of about 224mAh/g. The charge curve shows a long and flat potential plateau, possibly due to the formation of stable chemical bonds between hydrogen atoms and oxygen in the oxide structure (see inset in Fig. 3). Interestingly, the discharge potential plateau of the La2Ti2O7 electrode starts at approximately −0.75V, which is significantly lower than the potential plateau values in conventional ABO3-perovskite electrodes (∼−1.2V) [2]. This difference between ABO3 and A2B2O7 perovskite structures could be caused by the presence of storing sites in the La2Ti2O7 structure with stronger repulsive forces from the local environment, resulting in an increase in site energy and, therefore, in a lower potential compared with ABO3 electrodes [21].

Figure 3.

Charge–discharge curve of the La2Ti2O7 electrode.


Fig. 4 shows the cycling performance of the La2Ti2O7 electrode at a constant current density of 30mAh/g at room temperature. This test is usually performed to reveal the stability of electrode materials as a function of cycle number. The initial electrochemical discharge capacity of the La2Ti2O7 electrodes was 178mAh/g, corresponding to ∼0.66wt.% hydrogen. During the fourth to seventh cycle, the discharge capacities increased up to a maximum value of 224mAh/g at the eighth cycle. This result reflects the electrochemical activation process of La2Ti2O7 that is necessary to achieve the intrinsic hydrogen storage capabilities of the electrode. From the eighth cycle to the 20th cycle, the reversible capacity was kept around 200mAh/g; however, after the 20th cycle, an important capacity decay occurs, corresponding to a 47% decay (53% capacity retention). Despite the significant drop in the discharge capacity after the 20th cycle, the electrochemical capacity stabilized at approximately 114mAh/g for more than 20 charge–discharge cycles up to the end of the test (50th cycle). This long, stable capacity values as a function of cycle number is common in perovskite-type materials [2].

Figure 4.

Cycling performance of the La2Ti2O7 electrode.


To compare the electrode performance at different discharge currents, the high rate dischargeability (HRD) was evaluated. HRD is a parameter that reflects the discharge capability at different discharge current densities and can be dominated by the charge transfer rate at the electrode surface and the diffusion rate of hydrogen atoms in the bulk of the electrode [22]. HRD can be calculated by the following equation [23]:

Herein, Cn is the discharge capacity at a discharge current density of nmA/g and Cmax is the discharge capacity at the current density of 125mA/g. The experimental HRD curve is shown in Fig. 5. According to these results, the performance of the La2Ti2O7 electrode depends on the discharge rate, experiencing a progressive reduction in its electrochemical capacity when the discharge current density increases. The La2Ti2O7 electrode presents an electrochemical capacity of 379mAh/g at a current density of 10mA/g; nevertheless, its discharge ability is reduced only to 60.23mAh/g at 125mA/g (HRD16%). This behavior was also seen in hydrogen storage alloys and in perovskites in previous studies [22,24] and has been ascribed to the increase of electrochemical reaction kinetics on the electrode surface when the discharge rate increases. In terms of discharge capacity, the La2Ti2O7 electrode presents values that are close to those capacity values in ABO3-perovskite oxides. For instance, LaFeO3, La0.6Sr0.4FeO3, and La0.6Sr0.4Co0.2Fe0.8O3 have capacity values between 50 and 80mAh/g at a current density of 125mAh/g and at room temperature [2,5].

Figure 5.

High rate dischargeability of the La2Ti2O7 electrode as a function of the discharge current density.


The hydrogen diffusion coefficient of the La2Ti2O7 oxide was also estimated in the present study by the potential-step discharge technique. This method consists of applying a potential step while monitoring the anodic current–time response. The semi-logarithmic curve obtained from this experiment can be divided into two-time regions [25]. In the first-time region, the current experiences a rapid decrease due to the fast consumption of hydrogen on the surface of the electrode. This region is strongly governed by the charge transfer kinetics and/or by charge transfer and diffusion (mixed kinetics). However, in the second-time region, the current gradually decreases in a linear manner with time and is controlled by the hydrogen diffusion. Fig. 6 shows the semi-logarithmic curve of the anodic current–time response for the La2Ti2O7 electrode at 50 and 100% SOC. As expected, the current–time response of the La2Ti2O7 electrode presents both regions (mixed and diffusion controlled), which are decaled to each other due to the effect of SOC. According to the model proposed by Zheng et al. [26], the hydrogen diffusion coefficient DH can be estimated from the diffusion-controlled region in the electrode bulk by the following equation:

Figure 6.

Chronoamperometric semi-logarithm curve of the La2Ti2O7 electrode.


where i is the anodic current density (A/g), DH is hydrogen diffusion coefficient (cm2/s), d is density of the compound (g/cm3), a is the average radius of the particles, C0 is the initial hydrogen concentration in the bulk of the electrode (mol/cm3), Cs is the surface hydrogen concentration of the electrode (mol/cm3), and t is the discharge time (s). According to Eq. (2), DH was estimated as 3.87×10−14 and 1.51×10−13 for the La2Ti2O7 electrode at 50 and 100% SOC, respectively, that is, DH decreases with the state of charge. These results can be ascribed to the fact that diffusion paths are associated with oxygen-related positions within the perovskite-oxide structure. When the electrode begins the charge process, many of these oxygen positions become occupied by hydrogen in a well-established O–H chemical bond. However, when the number of H+ ions inserted into the unit cell of perovskite oxides increases, the O–H bond reduces its strength and consequently the activation energy necessary to break the O–H bond, facilitating hydrogen ion transportation [27]. Interestingly, the La2Ti2O7 electrode presents similar DH values compared with other perovskite-type oxide electrodes and has lower DH values than intermetallic compounds (<DH10−10cm2/s), which is due to the nature of the ion insertion and transport in perovskites with respect to intermetallic alloys [2,5,27].

In order to reveal more information about the kinetics of the process, linear polarization curves were obtained. Fig. 7 shows the linear polarization curve of the La2Ti2O7 electrode. Fig. 7 shows that the polarizing current has a linear relationship with overpotential in the overpotential range from −5 to 5mV. The linear polarization technique involves the hydrogen reduction–oxidation reactions on the surface layer of the oxide electrode and, as a result, the charge transfer rate at the electrode surface can be evaluated by the exchange current density (I0) according to the following equation [28]:

Figure 7.

Linear polarization curve of the La2Ti2O7 electrode.


where R is the gas constant (R=8.314Jmol−1K−1), T is the absolute temperature (K), F is the Faraday constant (F=96485Cmol−1), and Rp is the polarization resistance (mΩ) (i.e., the reciprocal of the slope in Fig. 7). According to Eq. (3), the exchange current density of the La2Ti2O7 electrode is 27.8mA/g. This value has the same order of magnitude as the exchange current density values reported for perovskite-oxides in previous reports [2]. For instance, the nano-LaFeO3 perovskite-type electrode has a I0 value of 28mA/g and a hydrogen diffusion coefficient on the order of 10−14 at room temperature [7]. These results suggest that the kinetics and transport properties of the La2Ti2O7 electrode are both quite similar to those reported for ABO3-perovskite oxides.


Nanometric La2Ti2O7 powder was prepared by means of the citric acid sol–gel route, which is currently a suitable technique to reduce energy consumption with respect to the conventional solid-state synthesis route. The as-prepared nanometric La2Ti2O7 powder presented hydrogen-storage properties in alkaline solution at room temperature. La2Ti2O7 had promising hydrogen-absorption capabilities with a total maximum discharge capacity of 224mAh/g and only 47% capacity decay, corresponding to 114mAh/g, after 50 cycles at room temperature. These results are notably better than the hydrogen-storage properties of some ABO3-perovskite oxides at room temperature. Thus, La2Ti2O7 can be regarded as a potential candidate for negative electrode material that can be used with rechargeable Ni/oxide batteries. The present study represents a first approach for developing A2B2O7-perovskite-layered oxides as negative electrode material for Ni/oxide batteries. Therefore, further studies are encouraged to further analyze the hydrogen-absorption mechanisms as well as application differences with respect to conventional ABO3-perovskite oxides.

Author contribution statement

All the authors have made a significant contribution to this manuscript.

Conflicts of interest

The authors declare no conflict of interests regarding the publication of this manuscript.


This work was supported by the National Science and Technology Council and the Secretary of Energy of Mexico “CONACYT-SENER-Sustentabilidad Energética” (Project No. 232611). Eng. Y. Pacheco would like to thank the support of the National Science and Technology Council (CONACYT) (grant No. 445176).

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