Effect of the Dopant and Carbonaceous Support of the Perovskite Type LaNi_0.9X_0.1O₃ (X = Fe, Mn or Pd) on the Performance of Zn–Air Battery

Abstract

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are two processes that occur during the operation of the cathodic electrode in Zn–Air batteries, which enable the integration of alternative energy sources into electrical energy distribution systems. Transition metal oxides, such as perovskites based on LaNiO₃, are promising electrocatalysts for the ORR and OER in alkaline medium due to their versatile structure, allowing for the substitution of certain atoms with dopants, which enhances the catalytic activity for both reactions. This work reports an electrochemical study of the catalytic activity toward ORR and OER of perovskite catalysts (LaNiO3 doped with transition metals (Fe, Mn, or Pd)) in the presence of carbon-based materials as supports (multiwalled carbon nanotubes (MWCNT), graphene oxide nanosheets (GO), and graphitic carbon (C)). The results revealed interesting catalytic properties in both reactions, particularly La(Ni_0.9Pd_0.1)O₃/MWCNT, which showed an ORR activation potential of 0.87 V vs. RHE, comparable to that of the commercial Pt/C catalyst (0.99 V vs. RHE), while the overpotential for OER was lower than that of the Pt/C catalyst (1.68 V vs. RHE for La(Ni_0.9Pd_0.1)O₃/MWCNT and 1.79 V vs. RHE for the commercial Pt/C).

1. Introduction

The energy distribution deficit is accentuated globally due to population increase and rising energy demand, coupled with limited resources for energy production. Unlimited sources of energy, such as solar, wind, and geothermal energy, are classified as alternative solutions. The main disadvantage that needs to be overcome for these sources is their lack of capacity to store surplus energy. One way to accomplish this is by coupling it with an energy storage system before integrating it into the electrical energy grid [1]. Among the known energy storage systems, metal–air batteries have attracted the attention of the research community because of the use of abundant oxygen from the environment as one of the reagents, making metal–air batteries a higher energy density option compared to other batteries [2]. In addition, metal–air batteries have promising applications as a source of energy for hybrid electric vehicles [3,4], and the absence of fossil fuels in this device can reduce the carbon footprint of the energy storage/conversion process. However, the main drawbacks of metal–air batteries are the stability of the electrodes, cost, and reversibility of the reactions [3,5].

Different research groups have focused on the study of catalysis and the mechanism of the reactions taking place at the metal–air battery cathode, (i) the oxygen reduction reaction during the discharge cycle and (ii) the oxygen evolution reaction during the charging cycle (ORR and OER, respectively) [6,7]. Both reactions show slow kinetics and require considerable overpotential, which causes a decrease in the efficiency of the device [8,9]. The ORR can occur via different routes, which can be due to the partial reduction of oxygen that involves the transfer of two electrons or by a total reduction that involves four electrons for each oxygen molecule [10]. On the one hand, it is preferable to conduct the ORR via the total reduction route since the two-electron route produces intermediates such as HO₂⁻, which compromises battery stability and the mixed potential at the electrode, which decreases the theoretical potential of the overall device [11,12]. On the other hand, OER requires electrocatalysts with a high surface area and low overpotential to obtain a certain current density that does not limit the recharging process; however, this problem can be mitigated when electrocatalysts with high active sites per geometric unit are used [13]. The ORR and OER are often conducted in alkaline media since carbon corrosion and metal redissolution are promoted in acidic media. Moreover, ORR and OER catalysts exhibit better performance in alkaline media owing to intermediate stabilization and electric double-layer effects [14,15,16], showing lower overpotentials and better kinetic rates. Alkaline media allow a wide variety of earth-abundant catalysts, such as perovskites and spinels.

Among metallic catalysts with high catalytic activity, platinum-based materials have been reported to have the best performance as ORR catalysts, but they exhibit a high overpotential toward OER [17,18]. However, iridium-based catalysts exhibit the best performance at the OER but show low catalytic activity toward ORR, limiting the bifunctional behavior of the air cathode electrode of the metal–air battery [19]. The ORR has slow reaction kinetics and a sluggish mechanism, which leads to high overpotentials and undesired by-products [20]. Although Pt/C and Ir/C are considered benchmark catalysts for ORR and OER, their high cost, scarcity, and inability to maintain bifunctional performance under alkaline conditions, as 3D Ostwald ripening and leaching occur at both noble catalysts, limit the development of metal–air battery technology [21,22].

Zn–air batteries offer a few advantages over other metal–air batteries, such as a low self-discharge rate, long energy storage duration, and economic and easy preparation. In addition to the availability of Zn, its non-toxic behavior, etc. However, the formation of dendritic structures from the redissolution of Zn, which causes perforation of the separator and a short circuit in the battery, is the main disadvantage of this electrical storage system [23].

Perovskite oxide materials are an alternative to electrocatalysts as cathodic electrodes in metal–air batteries. Perovskites have an ABO₃ crystalline structure, where the B site can be substituted with other atoms, introducing changes in the structure and catalytic activity [24]. The perovskite LaNiO₃ exhibits chemical and structural flexibility and has been reported for the catalysis of ORR and OER, and could be classified as a “bifunctional catalyst” [25]. LaNiO₃ can be synthesized using a wide variety of methods and thus has excellent physical and chemical properties that can be tuned [26,27,28]. Zhang et al. reported perovskite catalysts LaNiO₃ mixed with different proportions of Fe on site B (LaNi_1−xFe_xO₃) [29]. The authors mentioned that the substitution of this metal represses the formation of Ni^II on the surface of the perovskite, so that the Ni–O bond remains strong. In addition, at lower proportions of Fe in LaNi_1−xFe_xO_3, the catalyst showed higher activity toward ORR and OER, with the ORR taking place via four-electron transfer even though there were changes in structure and properties [29]. Compared to platinum group metals, it has been shown that perovskites are more active for the OER compared to the ORR [30]. Another limitation is that LaNiO₃ perovskites have a fairly low electrical conductivity, a low surface area at ambient temperatures, and suffer from ohmic losses under polarization conditions, which results in higher overpotentials in the ORR against Pt-based catalysts [31].

Poux et al. investigated electrodes based on perovskite oxides and pyrolytic carbon, aiming to observe the influence of the electrocatalyst and electrocatalysis of ORR [32]. The authors revealed that carbon is required to improve the electrical contact between the perovskite particles and the current collector to ensure a high surface area of the perovskite [32]. Sakthivel et al. studied the proportions of carbon added to the LaNiO₃ catalyst, which showed better overall activity when the mass proportion of catalyst to support was 60%: 40% [33].

The role of carbon in catalysis is to increase the efficiency of metal oxides, and various studies have utilized different structural carbon materials with the goal of introducing supports that are more stable and provide higher catalytic activity, particularly under OER conditions, where carbon can be electrochemically oxidized, compromising the performance of the device [32,33]. Carbon nanotubes (MWCNT) provide a higher catalytic activity to the composite metal oxide–carbon support with better conductive properties and a high surface area [34]. Graphene oxide nanosheets (GO) are another interesting material that can improve the electrochemical performance of perovskites, since GO can be highly dispersed on the surface of the support, decreasing nanoparticle agglomeration and increasing the number of active sites [35].

In recent years, there has been an increase in the design of composites with synergistic effects that can improve the catalytic activity of reactions in alkaline conditions, such as doping carbon with nitrogen and coupling with FeNi₃ [36], co-doping MWCNT with heteroatoms such as nitrogen and sulfur, and then coupling with Co₃Fe₇ nanoparticles [37], and bimetallic NiCo core-shell nanoparticles on N—doped MWCNT [38]. In all cases, Pt/C and Ir/C were used as the catalysts reported as models for comparison, as these commercial catalysts exhibited the best performance at the ORR and OER, respectively.

Herein, an electrochemical study of catalysts based on La(Ni_0.9M_0.1)O₃, where M = Fe, Mn, and Pd, with different carbonaceous supports (MWCNT, GO, and graphitic carbon) toward the ORR and OER in an alkaline medium was performed, as well as their stability and selectivity for potential application as air electrodes in metal–air batteries.

2. Materials and Methods

Perovskite fine powders were synthesized using the glycine-nitrate process (GNP), as previously reported [39]. Commercial graphitic carbon was purchased from Fuel Cell Store (Bryan, TX, USA), while the MWCNTs were synthesized by the spray-pyrolysis method in a tubular oven with minor modifications using a ferrocene/toluene solution as the catalyst and a quartz tube as the growth substrate [40]. Subsequently, the MWCNTs were functionalized with an HNO₃/H₂SO₄ solution in a 3:1 molar ratio, both reagents were purchased from Sigma Aldrich (St. Louis, MO, USA). GO was obtained using the modified Hummers method, which involves oxidizing graphite in a strongly acidic medium [41].

The catalytic inks were prepared by mixing the perovskite catalyst with the carbon support in mass proportions of 100:0 wt% and 50:50 wt% and dispersed in 550 μL of ethanol and 175 μL of Nafion 117^® solution, 20 μL of the prepared ink was deposited on the surface of a glassy carbon rotating disk electrode drop by drop (GCRDE, 5 mm of diameter (Pine Research, Durham, NC, USA)) that was previously polished with a 3 µm alumina solution and left to dry to obtain the modified electrode, obtaining a smooth catalyst ink film.

Electrochemical characterization of the reactions that take place at the cathode of the Zn–Air battery was performed in a conventional three-electrode cell connected to a Biologic VMP–300 potentiostat/galvanostat (Seyssinet–Parisett, FR) with the modified GCRDE as working electrode, a platinum spiral as counter electrode, and Hg/HgO/KOH_sat (Pine Research, Durham, NC, USA) as reference electrode with KOH 0.1 M as the electrolyte. The protocol for the catalytic activity test of both reactions consists of the activation of the electrode surface by performing 30 voltametric cycles under N₂ saturation of the KOH 1.0 M electrolyte. ORR catalytic activity was performed with polarization curves recorded from 1.23 V to 0.00 V, under O₂ saturation conditions in a hydrodynamic regime (from 0 rpm to 2000 rpm) at 5 mV s⁻¹. On the other hand, OER catalytic activity was recorded in polarization curves recorded from 1.0 V to 2.0 V, under N₂ saturation conditions in hydrostatic regime at 5 mV s⁻¹. For comparison, all potentials referred to in this work were converted to the Reversible Hydrogen Electrode (RHE).

Stability test at ORR and OER were recorded by chronoamperometric curves, by imposing E_1/2 at 1500 rpm and E_{J=5 mA cm⁻²} at hydrostatic condition, for ORR and OER, respectively.

The catalytic activity of the LaNiO₃-based electrocatalysts was compared with the commercial Pt/C 20% and Ir/C 20% (both acquired from Fuel Cell Store (Bryan, TX, USA)), since the high catalytic activity toward ORR and OER, respectively. The selection of those catalysts is based on the high catalytic activity toward ORR and OER, which stands as a high-performance baseline for the implied reactions in an independent approach, since few research groups use a mixed Pt/C–Ir/C air cathode for Zn–air battery [42].

The tests were studied in a secondary Zn–air battery composed of KOH 6 M+ Zn(O₂CCH₃)₂ 0.2 M electrolytic solution, a Zn foil anode, and the bifunctional air electrode with the catalyst investigated was the anode and the cathode, respectively. The battery test was performed in a home-made battery of acrylic pieces designed to hold the electrodes and electrolyte in place, using a vent to allow air to flow through the cathode. The fixture had a small reservoir where the electrolyte is deposited with a syringe, and nuts and bolts were used to hold all pieces together. The electrodes have a geometric active area with the electrolyte of 0.78 cm², and aluminum tape is used as the electrical conductor.

3. Results

3.1. Physicochemical Characterization

For the physicochemical characterization of the obtained oxides, particle size, morphology, and elemental analysis were studied using a scanning electron microscope, TESCAN, VEGA3 (Brno, Czech Republic), equipped with a Bruker XFlash detector 4010 for energy dispersive spectroscopy (EDS). Figure 1a–d shows the bright-field SEM images of the four perovskites with a view field of 1 μm and voltage operation of 20 kV. All showed particle sizes ranging between 110 and 130 nm, which is attributed to the atomic radius of the transition metal mixed in the B-site cation. Figure S1a–d show the dark-field images of the same perovskites. In the EDS analysis (Figure 2a–d), the intensity peaks for lanthanum and oxygen are the highest as they are the most abundant in the lattice, while the peak for nickel and the other transition metal (Fe, Mn or Pd) are less intense specifically the metal added to the perovskite matrix (Fe, Mn or Pd) that is mixed. However, the EDS spectra confirmed the presence of substituted metals in the structure of the perovskite.

Figure S2 shows the BET analysis performed on the synthesized perovskites and MWCNT support. The specific surface area was estimated using BET analysis (Quantachrome, Autosorb IQ, Anton Paar, Los Angeles, CA, USA). It is noticeable that almost the same surface area was obtained for all perovskites, and no relevant changes were observed for the doped La(Ni_0.9M_0.1)O₃. However, the specific surface area of MWCNT is around three times that of perovskites; hence, it is expected that the incorporation of MWCNT in the catalyst matrix increases the surface area. Table S1 shows the surface area values for the LaNiO₃ perovskites and MWCNT.

Figure 3 shows the structural phases of the patterns obtained by X-ray diffraction (XRD). All diffractograms show a rhombohedral structure with no impurity phases. The lattice parameters were a = 5.452 Å, b = 5.452 Å, and c = 13.157 Å for LaNiO₃, La(Ni_0.9Fe_0.1)O₃, and La(Ni_0.9Mn_0.1)O₃ (PDF no. 01–070–5757), while for La(Ni_0.9Pd_0.1)O₃, the parameters were a = 5.452 Å, b = 5.452 Å, and c = 6.513 Å (PDF no. 00–034–1181).

3.2. Effect of the Carbon Support on ORR and OER

In the electrochemical characterization of the ORR, Figure 4a presents the linear sweep voltammograms of the non-doped perovskites with different supports in O₂-saturated KOH solution at a rotating rate of 1500 rpm. As expected, the addition of a carbon structure increases the electrical conductivity between perovskite particles, as observed in the Nyquist plots shown in Figure S3, where the resistance attributed to the interface electrode/electrolyte decreases in LaNiO₃ when MWCNTs are incorporated. Another piece of evidence that the carbon addition enhances the electrical conductivity of LaNiO₃ is observed by the increasing limiting current density when compared to the pure perovskite in Figure 4a. Important parameters considered for the catalysis were the half-wave potential and the activation potential, as they provide insight into the energy required for the reaction to proceed. Since the ORR is a slow-kinetic reaction, agitation of the solution is required to transport oxygen to the electrode surface, where the rotating disk electrode theory comes into play by varying the rotation rate of the electrode while performing the LSVs to obtain a variation in the limiting current density. The Koutecky–Levich analysis was performed to calculate the slopes and compared with the calculated theoretical slopes for the ORR 2 and 4 electron routes, which are presented in Figure 4c. These values are listed in

Table 1. Limiting current density, half-wave potential, activation potential, KL slopes, Tafel slopes, and exchange current density of LaNiO₃ with different carbon supports.

Catalyst	Jlim (mA cm−2)	E1/2 (V vs. RHE)	Eact (V vs. RHE)	KL Slope (cm2 rad1/2 s−1/2 mA−1)	Tafel Slope (mV dec−1)	J0 (mA cm−2)
LaNiO3	−0.45	0.57	0.71	2.6576	207.1	2.2 × 10−3
LaNiO3/C	−3.40	0.59	0.78	1.4415	158.4	5.9 × 10−3
LaNiO3/MWCNT	−3.42	0.60	0.79	1.557	93.1	2.8 × 10−3
LaNiO3/GO	−2.04	0.66	0.81	2.3424	261.5	8.9 × 10−3
Pt/C 20%	−5.31	0.81	1.01	0.8255	100.8	3.12 × 10−4
Ir/C 20%	−4.94	0.64	0.91	0.8594	300.0	6.79 × 10−2

, and it can be observed that Vulcan and carbon nanotubes yield a more limiting density, while graphene oxide nanosheets have more positive potentials, indicating that the addition of carbon decreases the ohmic resistance of the catalyst layer. All the slopes are closer to a value corresponding to four electrons, indicating that these catalysts perform the ORR through this route. The theoretical KL slope values are 5.2 and 2.6 for two and four electrons transferred, respectively. It is important to note that the KL analysis is based on a one-step process under steady state in the laminar flow regime and mass-transfer limited conditions for flat and smooth surface electrodes [43].

Stability chronoamperometric tests were performed by imposing a half-wave potential constant for 30 min, and the results are presented in Figure 4b. Again, the current density increased noticeably when carbon was added, and the presence of carbon nanotubes showed a more negative current density while remaining stable during the test period. Finally, by studying the charge transfer, Tafel plots were obtained to calculate the charge transfer coefficient and exchange current density (Figure 4d). The Tafel slope is a critical parameter that describes the amount of overpotential required to achieve a given current density. A lower Tafel slope indicates that the catalyst requires a lower overpotential to develop the kinetic reaction. Assuming that no changes occur in the ORR mechanism for the LaNiO₃-based catalysts, the Tafel slope of LaNiO₃/MWCNT indicates that this catalyst possesses the highest catalytic activity compared to the others.

The OER polarization curves are presented in Figure 5a, and the addition of carbon nanotubes increased the activity of the catalyst. At a current density of 5 mA cm⁻², the overpotential for LaNiO₃/MWCNT was 1.73 V and the overpotential for LaNiO3/GO was 1.78 V vs. RHE, which showed better OER activity than Pt/C and was only surpassed by the OER activity of Ir/C. The chronoamperograms in Figure 5b show that nanotubes provide a more positive current flow and relative stability, even in comparison with Ir/C. Finally, the Tafel slopes (Figure 5c) show that the OER is normally a more energy-demanding process. The Tafel slope tends to be higher than that of the ORR in most cases, and the catalyst shows a variation from 200 to 350 mV dec⁻¹. The slopes decrease when a support is added, indicating that carbon has a strong influence on the OER mechanism.

Table 2. E_{J = 5 mA cm⁻²}, Tafel slope, and exchange current density of LaNiO₃ with different carbon supports.

Catalyst	EJ = 5 mA cm−2 (V vs. RHE)	Tafel Slope (mV dec−1)	J0 (mA cm−2)
LaNiO3	–	416.9	1.29 × 10−2
LaNiO3/C	–	243.6	9.11 × 10−2
LaNiO3/MWCNT	1.73	200.2	7.38 × 10−2
LaNiO3/GO	1.78	207.6	3.88 × 10−2
Pt/C 20%	1.79	458.3	5.34 × 10−2
Ir/C 20%	1.49	266.7	8.54 × 10−2

lists the catalytic values for LaNiO₃ with different carbon supports for the OER.

3.3. Effect of the Substituted Transition Metal La(Ni_0.9M_0.1)O₃ on ORR and OER

Since the LaNiO₃ perovskite possesses a flexible structure, where the B site can be doped with transition metals, it is important to determine which transition metals increase the catalytic activity toward ORR and OER. A wide variety of transition metals were investigated, and the results are listed in Table S2. The selection of a few catalysts for bifunctional air cathodes is a complex task, since there are many catalytic features and mechanistic parameters to consider. However, the potential terms (E_1/2, E_act, E_{J = 5 mA cm⁻²}) are associated with the activation energy of the involved reactions, while the Tafel slope is related to the kinetic rate reaction and the mechanism. Among the transition metals, La(Ni_0.9Fe_0.1)O₃, and La(Ni_0.9Mn_0.1)O₃ exhibited better kinetic parameters toward ORR (Figure S4). These catalysts exhibited more positive E_1/2 and E_act values and a lower Tafel slope during the ORR (Table S3).

In the LSV (Figure 6a), the Pd-doped perovskite showed the most positive half-wave and activation potentials, which are highly comparable to the Pt/C catalyst. It is probable that Pd is in a +4 oxidation state with a 4d⁶ low-spin configuration, and it has been known that the intrinsic activity is correlated with the occupied e_g orbital and the covalent bonding on the B-site cation [44]. No significant changes were observed in the Nyquist plots (Figure S3) of the different La(Ni_0.9M_0.1)O₃-based catalyst; hence, no changes in the mechanism or conductivity were observed. In the chronoamperometric graphs (Figure 6b), LaNiO₃/MWCNT showed the most negative current density. The commercial catalysts (Pt/C and Ir/C) showed a dramatic reduction in the cathodic current density associated with the decreasing activity of the catalysts, whereas the LaNiO₃-based catalysts exhibited a very stable current density. The KL slopes are once again closer to a value corresponding to four electrons, indicating a preferable route where the O₂ molecule is completely reduced to OH⁻ ions (Figure 6c). The Tafel slope confirmed that the Pd mixed perovskites with carbon nanotubes display lower Tafel slopes close to 100 mV dec⁻¹, such as Pt/C and relatively high J₀, indicating that they are indeed more intrinsically active (Figure 6d). Pd-doped perovskites are the most interesting catalysts in this study. Their activity is comparable to that of the Pt/C catalyst and exhibits the most attractive electrochemical parameters, warranting a larger focus on these perovskites for ORR. The kinetic parameters for the LaNi_0.9M_0.1O₃/MWCNT-based materials are listed in

Table 3. Limiting current density, half-wave potential, activation potential, KL slopes, Tafel slopes, and exchange current density of La(Ni_0.9M_0.1)O₃ type perovskite (where M = Fe, Mn, and Pd) with MWCNT.

Catalyst	Jlim (mA cm−2)	E1/2 (V vs. RHE)	Eact (V vs. RHE)	KL Slope (cm2 rad1/2 s−1/2 mA−1)	Tafel Slope (mV dec−1)	J0 (mA cm−2)
LaNiO3/MWCNT	−3.42	0.60	0.79	1.557	93.1	0.6351
La(Ni0.9Fe0.1)O3/MWCNT	−2.47	0.60	0.79	1.3511	190.8	0.3099
La(Ni0.9Mn0.1)O3/MWCNT	−3.52	0.60	0.79	1.5413	444.9	0.1329
La(Ni0.9Pd0.1)O3/MWCNT	−5.02	0.61	0.90	1.3911	102.7	0.5757
Pt/C 20%	−5.31	0.81	1.01	0.8255	100.8	3.12 × 10−4
Ir/C 20%	−4.94	0.64	0.91	0.8594	300.0	6.79 × 10−2

. As expected, Pt/C exhibited a higher ORR activation potential (1.01 V vs. RHE), which is closer to the theoretical activation potential (1.23 V vs. RHE), indicating a lower overpotential for the ORR. Among the perovskites, the best catalytic activity is shown by La(Ni_0.9Pd_0.1)O₃, which is predicted in terms of ORR volcano plots for metal catalysts, where Pt and Pd are the metals with the highest catalytic activity, transition metals that bind oxygen too strongly, and the activity is limited by proton electron transfer to O*or OH*. In contrast, for metals that bind oxygen too weakly, the activity is limited by proton electron transfer to O₂*or splitting of the O–O bond [45].

Similar to ORR, the catalytic activity toward OER of all the catalysts cited in Table S2 was investigated, where LaNi_0.9Mn_0.1O₃ exhibited the best performance (Figure S5) according to the lower E_{J = 5 mA cm⁻²} and Tafel slope at OER (Table S3).

Conversely, for the OER study, the conditions adversely affect the catalyst performance of standalone perovskites without carbon addition, as observed in other studies that have highlighted the importance of the synergy of the mixed ions in the B site that enhances the OER activity [46] as well as the addition of a stable carbon lattice [34,35,36,37,38,44,45,46,47]. The OER polarization curves are presented in Figure 7a, and the Fe-doped perovskite is the most active for the OER due to its stable configuration, which avoids the formation of NiO, which is beneficial, as stated by Suntivich et al. [30]. The overpotential for La(Ni_0.9Fe_0.1)O₃/MWCNT was 1.60 V vs. RHE, which is the lowest of all the catalysts tested in this study. The perovskite catalysts with MWCNT exhibited an exchange current density of a higher order of magnitude and lower Tafel slopes (200–260 mV dec⁻¹) in comparison with Pt/C (Figure 7c). Moreover, LaNi_0.9Fe_0.1O₃/MWCNT showed the highest current in the chronoamperometric curves, which suggests the capability of this catalyst to work in harsh conditions compared to Ir/C, which had the lowest activation potential but a rapid deterioration of the current density. Under these considerations, La(Ni_0.9Fe_0.1)O₃/MWCNT enhanced OER activity significantly.

Table 4. E_{J = 5 mA cm⁻²}, Tafel slope, and exchange current density of La(Ni_0.9M_0.1)O₃ type perovskite (where M = Fe, Mn, and Pd) with MWCNT.

Catalyst	EJ = 5 mA cm−2 (V vs. RHE)	Tafel Slope (mV dec−1)	J0 (mA cm−2)
LaNiO3/MWCNT	1.73	200.2	7.38 × 10−2
La(Ni0.9Fe0.1)O3/MWCNT	1.60	238.5	1.19 × 10−1
La(Ni0.9Mn0.1)O3/MWCNT	1.74	219.4	7.69 × 10−2
La(Ni0.9Pd0.1)O3/MWCNT	1.68	253.3	1.08 × 10−1
Pt/C 20%	1.79	458.3	5.34 × 10−2
Ir/C 20%	1.49	266.7	8.54 × 10−2

shows the catalytic features of the LaNi_0.9M_0.1O₃/MWCNT catalysts for the OER.

Table 5. Catalytic activity comparison of different catalysts for ORR and OER reported in the literature.

	ORR Parameter			OER Parameter
Catalyst	Jlim (mA cm−2)	E1/2 (V vs. RHE)	Eact (V vs. RHE)	EJ = 5 mA cm−2 (V vs. RHE)	Ref
LaNiO3/MWCNT	−3.42	0.60	0.79	1.56	This work
La(Ni0.9Pd0.1)O3/MWCNT	−5.02	0.61	0.90	1.68	This work
La(Ni0.9Fe0.1)O3/MWCNT	−2.47	0.60	0.79	1.60	This work
LaCoO3/C	−3.51	0.64	0.78	1.62	[27]
La(Ni0.9Fe0.1)O3/C	−4.01	0.31	0.41	1.31	[29]
LaCoO3/C	−2.32	0.79	0.89	unknown	[32]
LaNiO3/N–CNT	−5.59	0.74	0.88	1.59	[34]
La0.6Ca0.4MnO3/GO	−3.25	0.59	0.85	unknown	[35]
FeNi3/N–C	−5.86	0.84	0.92	1.49	[36]
Co–MnO2/C	−5.01	0.76	0.87	1.63	[42]
Co3O4@Co/N–CNT	−5.00	unknown	0.90	1.61	[48]

shows the catalytic features of the LaNi_0.9M_0.1O₃/MWCNT catalysts for the ORR and OER against the catalytic values of different non-noble catalysts reported in the literature, where it is observed that the catalysts synthesized in this work exhibited competitive values in both reactions.

3.4. Charge–Discharge at Zn–Air Battery

Figure 8a shows a schematic of the components and arrangement of the battery used in this study. The charge–discharge plot (Figure 8b) indicates that the perovskite materials possessed good reversibility at low current densities, displaying ΔV values closer to 800 mV. Interestingly, during discharge at 0.7 V, the LaN_i0.9Fe_0.1O₃/MWCNT catalyst displayed a density similar to that of noble metals. Power density peak during the discharge cycle is shown in Figure 8c, where LaNi_0.9Fe_0.1O₃/MWCNT exhibited a competitive value compared to Ir/C and Pt/C. The stability of the Zn–Air battery was tested at different percentages of the maximum current achieved during discharge, where the maximum current densities were 21 mA cm⁻², 31 mA cm⁻², 33 cm⁻², and 44 mA cm⁻² for La(Ni_0.9Pd_0.1)O₃, La(Ni_0.9Fe_0.1)O₃, Pt/C, and Ir/C, respectively (Figure 8d). The reference materials exhibited equivalent activity to that at the beginning of the test. Nevertheless, the LaNi_0.9Fe_0.1O₃/MWCNT catalyst showed activation of active sites at the end of the test due to the potential at 2% demand current was higher than that at the beginning when 1% was demanded. In this regard, it is possible that some surface reconstruction occurs during the stability test, which could enhance its durability in real-life long-term battery testing. Regarding the galvanic charge/discharge test (Figure 8e), the perovskites displayed an 2.3 V of initial charge potential and a discharge potential of 1 V. After 50 cycles (approximately 16 h), the electrode voltage gap was 1.60 V (the charge potential increased to 2.4 V and the discharge potential decreased to 0.8 V). Yoo et al. reported a galvanic charge/discharge test on a Zn–Air battery with a Mo@ZrS₂/rGO electrocatalyst as the air electrode with an electrode voltage gap of 1.86 V after 16 h under similar conditions (the charge potential was practically constant at 2.57 V, while the discharge potential decreased to 0.71 V) [49]. This group also performed galvanic charge/discharge tests on a Zn–Air battery with noble metals as electrocatalysts for the air electrode (Au–Ag/rGO and Pt@Au–Ag/rGO), where Pt@Au–Ag/rGO exhibited a higher power density peak and higher stability [50].

4. Conclusions

Zn–air batteriesare an electrochemical technology for storage/conversion energy that offers many advantages, such as low discharge rate, minimal security requirements, and easy assembly. One of the main topics in Zn–Air batteries is the comparison of the performance of non-noble catalysts for the integration of bifunctional air cathodes. LaNiO₃ is a flexible material that can be easily synthesized and is an affordable option as a bifunctional catalyst. Since the performance, stability, and mechanism are dependent on the carbon support and the introduction of the dopant in the B site, a systematic study was conducted in this work.

MWCNTs were found to be the ideal support material in this study and were used to study the ORR and OER capabilities of the LaNiO₃ catalyst doped with Fe, Pd, and Mn. LaN_i0.9Pd_0.1O₃/MWCNT was very active toward ORR, and LaN_i0.9Fe_0.1O₃/MWCNT was the best LaNiO₃-based catalyst for OER. These two catalysts, along with control catalysts (Pt/C 20% and Ir/C 20%), were tested in a Zn–Air battery configuration. Although Ir/C 20% showed polarization curves comparable to those of the LaN_i0.9Fe_0.1O₃/MWCNT and LaN_i0.9Pd_0.1O₃/MWCNT catalysts, long-term stability testing showed the superiority of the perovskite catalysts. This shows the effects of mixing the B-site cation with different transition metals, similar to a previous work that investigated even more transition metal mixed perovskites that are not included in this study. The perovskite-based Zn–Air battery, especially LaNi_0.9Fe_0.1O₃/MWCNT, started the cycle test with a voltage comparable to that of the noble metal-based batteries but showed reduced voltage after 30 cycles, indicating reactivation of the cathode structure, which could be key to unlocking long-term stability for rechargeable metal–air batteries.