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Article

Rare-Earth/Manganese Oxide-Based Composites Materials for Electrochemical Oxygen Reduction Reaction

by
Stefan V. Panić
1,
Marijana R. Pantović Pavlović
1,2,
Miroslava M. Varničić
1,
Vojin Tadić
3,
Srećko Stopić
4,
Bernd Friedrich
4 and
Miroslav M. Pavlović
1,2,*
1
Department of Electrochemistry, Institute of Chemistry, Technology and Metallurgy, National Institute of the Republic of Serbia, University of Belgrade, Njegoševa 12, 11000 Belgrade, Serbia
2
Center of Excellence in Environmental Chemistry and Engineering-ICTM, University of Belgrade, Njegoševa 12, 11000 Belgrade, Serbia
3
Department for Experimental Testing of Precious Metals, Mining and Metallurgy Institute, Zeleni Bulevar 35, 19210 Bor, Serbia
4
Process Metallurgy and Metal Recycling, RWTH Aachen University, Intzestraβe 3, D52072 Aachen, Germany
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(6), 641; https://doi.org/10.3390/catal12060641
Submission received: 27 May 2022 / Revised: 9 June 2022 / Accepted: 10 June 2022 / Published: 12 June 2022
(This article belongs to the Special Issue Advanced Nanomaterials for Electrocatalysis: Synthesis, Characterization and Application)
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Abstract

:
The main objective of this research was a systematic development of advanced micro/nanostructured materials based on the most used metal-oxides for ORR and metal-oxides with an extremely low-loading of Pt for comparison. Hybrid composites compared were: MnO2, La2O3, mixed lanthanum manganese oxides (LMO), and mixed lanthanum manganese oxides with reduced platinum load (LMO-Pt). The influence of the reduced amount of noble metal, as well as single oxide activity toward ORR, was analyzed. The complete electrochemical performance of the hybrid materials has been performed by means of CV, LSV, and EIS. It was shown that all synthesized catalytic materials were ORR-active with noticeable reduction currents in O2 saturated 0.1 M KOH. The ORR behavior indicated that the La2O3 electrode has a different mechanism than the other tested electrode materials (MnO2, LMO, and LMO-Pt). The EIS results have revealed that the ORR reaction is of a mixed character, being electrochemically and diffusion controlled. Even more, diffusion is of mixed character due to transport of O2 molecules and the chemical reaction of oxygen reduction. O2 diffusion was shown to be the dominant process for MnO2, LMO, and LMO-Pt electrolytic materials, while chemical reaction is the dominant process for La2O3 electrolytic materials.

1. Introduction

Fuel cells, batteries, and supercapacitors are known as the most efficient and promising technologies for electrochemical energy conversion and storage. The main constraints for their production for practical use are primarily due to the high cost of materials, as follows: e.g., noble metals such as platinum, as well as rather poor operational stability in different climate environments. The long-lasting challenge is to provide an advanced low-cost catalyst with performance as efficient as noble metals or better. Therefore, the rational design of novel multicomponent materials with improved catalytic properties can lead to alternatives that can compete with existing technologies.
Alternative energy devices are one of the most efficient and clean energy-producing sources available [1]. Still, the sluggish kinetics of the electrochemical oxygen reduction reaction (ORR) is detected as the main limitation. Noble metals (e.g., Pt, Au) are used and considered the most efficient catalysts for their performance, but at the same time, due to their high cost and scarcity, commercialization of these energy devices is impeded [2,3,4]. In the last decades, the main idea has been to reduce the loading of noble metals in the electrode material but to keep the same activity that their bulk electrode provides for the desired reactions.
In particular, micro/nanomaterials with a suitable combination of the components were investigated as a promising strategy toward high-performance electrode materials. For example, transition metal oxides (TMO) such as MnO2, Co2O3, etc. have been studied, due to their rich oxidation states, high specific capacitances, low cost, and environmental friendliness [5,6]. Among TMOs, the focus is mostly based on MnO2, which is already indicated as a non-carbon material of potent catalytic activity improvements while being used as a noble metal carrier [7]. It has been reported that manganese oxide promotes oxygen disproportionation, resulting in an overall four-electron reduction of O2 on MnOx/Nafion modified electrodes [8]. Unfortunately, the electrochemical performance of single MnO2 electrodes is limited by poor electrical conductivity (10−5–10−6 S cm−1) [9,10]. An effective way to improve the utilization of MnO2 is to combine conductive materials (metals, oxides, carbon-based materials) to form composites as electrode materials with improved characteristics. For example, Xia et al. have reported the fabrication of composite materials with core-shell structures of Co2O3@Pt@MnO2 as high-performance electrode materials for supercapacitors [10]. Lahousse et al. have found an MnO2/TiO2-Pt electrocatalyst of high-measured catalytic activity in comparison to the noble metal catalysts [11]. Various combinations of mixed oxide materials have been investigated so far, e.g., 3D flower-like Co2O3@MnO2 [6], nanosheets of MnO2 [12], CuO/MnO2 [13], NiO/MnO2 [14], etc.
Additionally, among rare earth oxides, lanthanum oxide (La2O3) has been extensively analyzed for its excellent electronic, photocatalytic, and electrocatalytic properties [15,16]. It has been discovered that the existence of the interlayer defect structure of the oxides was helpful for the active oxygen adsorption and had a positive effect on catalyzing ORR [17]. On the other hand, despite its outstanding electronic structure, quite similar to MnO2, the single oxide has low electroconductivity. Therefore, lots of effort has been put into the development of a hybrid composite material that would combine all the benefits of the metals. In this regard, perovskite oxides with an ABO3 structure have attracted great attention as a replacement metal oxides. Various perovskites have been developed, such as LaMnO3, LaCoO3, LaNiO3, LaCrO3, and LaFeO3 [18,19,20]. In particular, Co and Mn-based perovskites have shown high and stable performance for ORR as well as for the oxygen evolution reaction (OER). This bifunctional property is due to its oxidation states and spin configuration [21]. Recently, LaMnO3 nanoparticles were found to achieve high energy and power densities, synthesized using natural lemon juice as a green surfactant [22]. However, due to various structures, a combination of metals, and the synthesis procedure reported, the systematic design and comparison of metal oxide-based electrode materials is necessary.
The main aim of this paper was a basic analysis and comparison of the lanthanum and manganese-based hybrid materials for ORR of reduced Pt loading. In addition, the utilization of cheap and stable carrier materials (e.g., TMOs) were used, which were supposed to contribute to the activity of a reduced number of noble metals.
The syntheses of hybrid electrocatalytic materials with detailed electrochemical characterization were conducted. The electrode materials based on La2O3/MnO2 (LMO) have been synthesized by one-pot synthesis, and the influence of the addition of Pt low loading has been investigated. Transition metal oxides were chosen as base support due to their double-role potential, one of which was to serve as stabilizing catalyst carriers while the other one had interactive participation in the enhancement of the catalytic properties of noble metals. Electrochemical characterizations have been carried out by means of cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) in 0.1 M KOH. ORRs were investigated in the 3-electrode set-up using a rotating disk (RDE) as a working electrode to provide controlled mass transport conditions.

2. Results and Discussion

Stable cyclic voltammograms (CVs) of La2O3, MnO2, LMO, and LMO–Pt thin-layer electrodes are shown in Figure 1.
There are no significant differences in the La2O3, LMO, and LMO-Pt CV responses. MnO2 exhibits slightly higher cathodic currents in the range from 1.0 to −0.2 V, which meets the value for LMO–Pt material at E = −0.2 V. For an LMO–Pt sample, the onset of the hydrogen evolution reaction is clearly visible around 0.0 V due to the presence of Pt. Although being quite weak, the broad redox pair peak around 0.6 V can be distinguished for La2O3, MnO2, and LMO-Pt. The only anodic shoulder-like peak that can be distinguished for LMO.
MnO2 cyclic voltammogram in the potential range of −0.2 V to 1.6 V in 0.10 M KOH electrolyte represents a typical voltammogram with a wide reduction peak related to the reduction reactions of MnO2 to Mn2O3, Mn3O4, and finally to Mn(OH)2. The anodic counterpart appears as the increase in currents at the potentials above 1.1 V. The broad peaks of other materials, La2O3, LMO, and LMO-Pt, belong to mutual redox reactions of rare earth metals and Mn oxides. These voltammograms show forms that are typical for the materials that could be associated with the pseudocapacitive response of these faradaic processes. The absence of ideal linearity and symmetry between the charge-discharge processes can be observed as wider discharge counterparts.
The prepared electrocatalytic materials were tested for their ORR activity by means of LSV in an O2-saturated 0.10 M KOH electrolyte. Figure 2 presents the ORR electrochemical performances of La2O3, MnO2, LMO, and LMO-Pt thin powder layers at the electrode rotating speed of 1000 rpm.
Noticeable reduction currents can be observed as a result of ORR. The ORR onset potential for MnO2, LMO, and LMO-Pt electrodes was approx. 0.7 V, which is comparable to previous work [23], while the onset potential for the La2O3 electrode was around 0.8 V. The MnO2, LMO, and LMO-Pt electrode materials show similar curve shapes and activity. For all of the materials, the curve appears stepped, with a transition around 0.4 V. This could also be an indication of different ORR mechanisms, with La2O3 electrode material being quite a different mechanism with respect to MnO2, LMO, and LMO-Pt electrode materials. It can be noticed that LMO electrode material has a more similar behavior to La2O3 electrode material in the potential range from 0.3 V to −0.2 V, while the former exhibits the highest activity below ca. 0.0 V. MnO2 and LMO-Pt have quite similar curve shapes in the mentioned potential range. The MnO2 electrode material has the smallest ORR activity amongst three named electrodes with a current plateau of around −1.9 mA cm−2, followed by LMO (−2.0 mA cm−2) and, finally, LMO with 5 mass% of Pt (−2.3 mA cm−2). The La2O3 electrode material has a different shape of polarization curve. This indicates that different ORR mechanisms could be taking place on the La2O3 thin layer; the ORR starts at more positive potentials and, consequently, the overpotential is lower at the potentials positive to ca. 0.65 V). This feature is similar to the ORR activity of commercial 20 mass% Pt@XC-72 catalysts [23,24]. However, the ORR current plateau is considerably lower (−1.3 mA cm−2) with respect to other samples.
Tafel analysis is generally utilized to compare the electrocatalytic activity of the electrodes and to discriminate the reaction mechanisms of the reactions with mass transfer limitations at low to moderate overpotentials.
Ohmic-drop-compensated pоtentiodуnаmic Tafel сurvеs for the ORR at La2O3, MnO2, LMO, and LMO-Pt materials in O2-saturated 0.10 M KOH are shown in Figure 3 (the overall current is examined since well-developed diffusion current density plateaus have not been registered to apply the typical Koutecky–Levich analysis).
It can be seen in Figure 3 that all the electrode materials exhibit Tafel slope at low overpotentials (up to 100 mV) between 53 and 60 mV dec−1 (lines). Tafel analysis leads to the following two important physical parameters: the Tafel slope and the exchange current density. Based on the theory of ORR mechanisms, the Tafel slope of 60-mV dec−1 determines the overall reaction rate when the surface adsorption of the species MOO and MOO− is most likely the rate-determining step [25,26]. One can observe that, in the measured overpotential range, La2O3 material has the highest currents. At lower overpotentials (0.8–0.9 V potential region), MnO2 has one of the highest currents among Mn-containing samples; at higher overpotentials (at potentials negative to ca. 0.75 V), the highest ORR currents are registered for Pt-free Mn oxide-based materials. In this high-overpotential range, LMO-Pt’s activity appears to approach the activities of MnO2 and LMO. Samples are of considerably lower ORR activity than bulk Pt (the Tafel slope of 52 mV dec−1 was registered (not shown)). The lower activity could be attributed to the fact that there is only 5 mass% of Pt in the LMO-Pt electrolytic material, which is quite low content to induce the ORR activities close to bulk Pt. Moreover, from Figure 2, it can be seen that La2O3 electrolytic material loses considerably its highest low-overpotential activity and reaches the lowest limiting currents.
The current densities per mol of a metal could be a useful indication of the specific activity of the components in the composites [23]. This parameter was calculated at the potential in the region of calculated slopes of Tafel dependencies, and the results are shown in Table 1.
It can be concluded that La improves the activity of Mn at lower overpotentials but not at higher ones.
To further analyze the ORR activities and their correlation to sample structures, potentiostatic electrochemical impedance spectroscopy (PEIS) measurements were performed at open circuit potential for all the samples in an N2-saturated 0.1 M KOH solution. Galvanostatic electrochemical impedance spectroscopy (GEIS) measurements were performed at OCP and the current from a low- to moderate-overpotential region of −0.765 mA cm−2 (with an amplitude of 0.510 mA cm−2) for all the samples in an O2-saturated 0.1 M KOH solution in order to evaluate the details of ORR activity and the electrochemical behavior of the tested materials.
The PEIS results, shown as the capacitance complex plane plots (CCPPs) of the synthesized materials, are given in Figure 4a. CCPPs show differences in capacitive features between the tested electrodes. The mutual characteristic is a well-resolved capacitive loop in the high-frequency domain followed by straight-line behavior in the low-frequency domain. The capacitive loop for the Pt-modified LMO electrode is of a higher diameter than LMO itself, and the highest among all of the samples, meaning the capacitance is considerably increased by Pt–modification. Samples involving single oxides–MnO2 and La2O3 exhibit similar EIS features of two indicatively overlapped capacitive loops of similar diameters. The region consisting of a loop-like response at high frequencies corresponds to pseudocapacitive redox transitions of a metal ion from the oxide. The straight-line behavior in the low-frequency region is attributed to the ionic exchange in the electrolyte/material interphase.
The impedance features from Figure 4a of the investigated electrodes have been simulated by the equivalent electrical circuit (EECs). The impedance characteristics of EECs are indicated by lines in the capacitance complex plane plots of Figure 4a. Samples’ PEIS responses are described by different circuits and are shown in Figure 4b–d. The framework for fitting was to quantify the EIS behavior through the simplest suitable EEC with the lowest possible fitting error. The EECs correspond to those applied in the literature [27,28] and are typical for porous structures. EECs are presented as resistors and constant phase elements (CPEs) through the branches. The Chi-squared calculated was below 0.004, while the relative error of parameter values did not exceed 34%. The equivalent electric circuits used to fit the data gave similar fitting quality for the electrode based on LMO and MnO2.
The results obtained for La2O3, MnO2, LMO, and LMO-Pt electrolytic material in N2-saturated 0.10 M KOH from fitting results using EECs are given in Table 2.
From the results shown in Figure 4 and Table 2, one can conclude that there are two time constants. CPE1 corresponds to the capacitive response of the outer surface of the electrode materials readily available to the electrolyte, and CPE2 corresponds to the capacitive response of the inner layer surfaces that are less accessible. The R1 resistance corresponds to the solution resistance. It can be seen that R3 is present in the EEC for MnO2 and LMO. It appears that R3 describes the feature related to MnO2 and its redox transitions to different oxidation states. This statement is supported by the value of R3 of LMO, in which the content of Mn is lower with respect to the pure MnO2 layer. The R3 value of LMO is almost three times higher than the R3 value of MnO2. If R2 is taken into consideration, the most inclined conclusion could be that it represents the redox resistance at the inner surfaces rather than the pore resistance, which should be comparatively negligible for Mn-containing samples (R2 is also higher for LMO in comparison to pure MnO2). On the other hand, R2 could be dominated by the pore resistance in La2O3 due to less pronounced pseudocapacitive behavior since R3 was not required in the EEC. Consequently, R2 is an order of magnitude higher for La2O3 in comparison to MnO2 and LMO. In the case of LMO-Pt, Pt causes these resistances to not affect the PEIS response.
The GEIS results for La2O3, MnO2, LMO, and LMO-Pt in O2-saturated 0.10 M KOH are shown in Figure 5.
With an amplitude of 0.510 mA cm−2, the GEIS measurements of La2O3 electrode material enter the limiting current region according to Figure 2. Two overlapped semicircles can be seen for all the materials. Taking the semicircle diameter as a measure of the charge transfer resistance, i.e., ORR activity, the activity series in the tested region is as follows: MnO2 > LMO-Pt > LMO > La2O3. The ratio of the two more or less overlapped semicircles for the materials differs too. For LMO-Pt and MnO2, the diameter of the high-frequency semicircle is a bit larger than that of the low-frequency one. For LMO, the two diameters are of similar values, whereas the diameter of the low-frequency semicircle is larger for La2O3. The ORR is of mixed limiting character, being electrochemically- and diffusion-controlled. High-frequency response corresponds to charge transfer, and low-frequency corresponds to mass transfer limitations. The authors are fond of believing, according to findings from PEIS measurements, that the limiting current for the tested materials is the consequence of the chemical character of the materials rather than mass transfer. Despite pronounced activity at low overpotentials, La2O3 has much larger mass transport limitations (the diameter of the low-frequency semicircle is considerably larger) in comparison to other samples. Apparently, this reflects the lowest position of the limiting current plateau of La2O3 from Figure 2. This most pronounced ORR limiting condition also affects the diameter of the high-frequency loop, which is not straightforwardly seen for Mn-containing samples. These features indicate that ORR at La2O3 is not subjected only to diffusion limitations, but to limitations related to some slow chemical reaction(s).
The fitting results gained by EEC (Figure 5b) for ORR at La2O3, MnO2, LMO, and LMO-Pt are given in Table 3.
Owing to finite diffusion of O2, the finite length Warburg short circuit terminus was introduced (Ws). This element is also known as a Generalized Finite Warburg element (GFW), which is an extension of the Finite-Length Warburg (FLW). To allow its full finite diffusion assignment, the Ws-P parameter was fixed to 0.5. The FLW represents the solution of the one-dimensional diffusion equation of diffusion species, which is completely analogous to wave transmission in a finite-length RC transmission line. Ws-T parameter equals L2/D, where L is the effective diffusion thickness, and D is the effective diffusion coefficient. The Ws-R parameter defines a finite resistivity to diffusion. At very low frequencies, Z’ approaches Ws-R while Z” tends to zero.
The Warburg element is dominant in the low-frequency region. The larger the Ws-T parameter, the greater the diffusion thickness of specific species. R2 is charge transfer resistance, which shows the activity of the electrode materials towards ORR. From the results given in Table 2, GEIS shows that at measured current density, MnO2, LMO, and LMO-Pt have almost the same activity towards ORR, while the activity of La2O3 is almost twice as low. The higher the resistance, the lower the activity. On the other hand, the W1-T time constant is in direct correlation with the thickness of the double layer. Table 2 shows that the effective diffusion thickness of La2O3 and MnO2 is double the value of LMO and LMO-Pt. Comparing the results from Table 2 and Table 3, it can be seen that the presence of oxygen doubles the capacitance of the double layer. The values are even higher if the material is more active towards ORR. This is reasonable since adsorbed oxygen-containing species occupy some space in a double layer.
Figure 5 shows that galvanostatic EIS gives a more reliable and accurate response in comparison to potentiostatic conditions (data not shown; there were difficulties in recording low-frequency responses). However, GEIS requires a large amplitude of currents, which can provoke a considerable impact on the nonlinear response. In order to check the influence of higher harmonics on the apparently linear response of investigated electrode materials, GEIS was recorded for La2O3, MnO2, LMO, and LMO-Pt in O2-saturated 0.10 M KOH at OCP with a current amplitude of 0.510 mA cm−2. This should induce the changes in potential output in a much wider range (and reasonably the widest) with respect to the GEIS measurements presented in Figure 5, and consequently a nonlinear muti-harmonic response. Since EIS theory is based on the linear response of the first harmonic, its value should be negligibly affected by the intensity of the third harmonic [29,30]. The modulus of the third harmonic of ORR on the investigated electrode material is analyzed and presented in Figure 6.
At higher frequencies (from 10 Hz to 100 kHz), the intensity of the third harmonic is weakly dependent on frequency, with LMO-Pt showing the most pronounced nonlinearity, followed by La2O3, MnO2, and LMO. At lower frequencies, the nonlinearity contributes gradually more due to diffusion impact. The MnO2 electrode material is of the lowest intensity of the third harmonic in the low-frequency domain, while the intensities of La2O3 and LMO increase to approach the intensity of LMO-Pt. Registered spectra of the third harmonic indicate that diffusion contributes much more to the nonlinearity than charge transfer does, whereas the increased number of components in the catalyst induces the nonlinearity of the charge transfer process. Combining the data for the activity from Table 3 (R2, activity series, as follows: LMO ≈ MnO2 > LMO-Pt >> La2O3) and those for CP1-T from Table 2 as a measure of active surface area (LMO-Pt > La2O3 > MnO2 > LMO), it can be concluded that Figure 6 clearly shows that the most ORR active samples, LMO and MnO2, are of the least expressed charge transfer nonlinearity. This also holds to a less pronounced extent for the region of diffusion control at low frequencies.
The influence of the third harmonic on the first is illustrated in Figure 7 for the LMO-Pt sample, which showed the highest intensities of the third harmonic. The third harmonic starts increasingly to influence the output at the frequencies below 1 Hz and reaches almost 20% of the complete output intensity at the lowest applied frequency. This indicates that the output intensity should be corrected for the contribution of the third harmonic to gain reliable GEIS data related to linear response. The impedance complex plane plots with the contribution of the third harmonic to the output excluded are shown in Figure 7. Loop-like dependences are obtained, which appear more or less elongated, depending on the sample, in the direction of increasing the imaginary impedance. This behavior could be associated with the inductive contribution, which is usually attributed to the adsorption phenomena in the ORR mechanism expressed in the low-frequency domain [31]. Figure 7 shows that more pronounced elongations are found for La-containing samples. This introduces La2O3 as the catalyst component that affects mostly the adsorption steps in the ORR mechanism and consequently facilitates the spill-over issues.

3. Materials and Methods

3.1. Synthesis

The synthesis of MnO2 was performed by mixing equimolar concentrations of commercial KMnO4 and MnCl2. The equimolar concentration was determinate via oxidation equation 2 KMnO4 (aq) + 2 MnCl2 (aq) → 4 MnO2 (s) + 2 KCl (aq) + Cl2 (g). The mixture was heated to 80 °C during 30 min with constant mixing.
LMO was prepared by mixing starting precursor solutions to give desired stoichiometric mole ratios La:Mn = 2:5. Commercial La2O3 and KMnO4 were used in such concentrations that they give mentioned La:Mn mole ratios. The chemicals were mixed with reflux and constant mixing at 100 °C during 3 h.
Preparation of LMO-Pt was conducted by mixing La2O3 and KMnO4 so that mole ratios would be La:Mn = 2:5 with the addition of 5 mass% H2PtCl4. The mixture was subsequently heated to 100 °C with reflux and constant mixing during 24 h.
All the chemicals used were commercial Sigma-Aldrich (Saint Louis, MO, USA) P.A.

3.2. Electrochemical Characterization

Electrochemical behavior was analyzed by cyclic voltammetry, linear sweep voltammetry, potentiostatic electrochemical impedance spectroscopy (PEIS), and galvanostatic electrochemical impedance spectroscopy (GEIS). Measurements were performed by three-electrode cell system in 0.1 M solution of KOH. Counter and reference electrodes were platinum wire and Hg|Hg2Cl2, respectively. Working electrode was prepared by putting 30 µL of desired solution on glassy carbon. The electrode ink solution was prepared by mixing the powder with ultrapure double distilled water at a concentration of 5 mg/mL. After 2 h of drying in the air atmosphere, the electrodes were covered with 20 µL of Nafion solution (in water, the ratio 1:100), and dried for 2 more hours. Before the CV and PEIS measurements, the electrolyte was purged with N2 for 30 min. Prior to LSV and GEIS measurements, the electrolyte was saturated with O2. For electrochemical measurements potentiostat/galvanostat (BioLogic SAS, SP-240, Grenoble, France) was used. CV measurements were conducted at scan rate of 50 mVs−1 between the potentials of −0.2 and 1.7 V vs. RHE at room temperature. PEIS measurements were conducted at open circuit potentials. LSV scan rate was 2 mVs−1 and electrode rotating rate was 1000 rpm. GEIS measurements were taken at current densities of −0.765 mA cm−2 with an amplitude of 0.510 mA cm−2. The same measuring parameters were used for all the samples. All the measurements in the experiments are presented vs. RHE.

4. Conclusions

Electrocatalytic materials based on metal oxides, La2O3, and MnO2 were synthesized and tested for their activity for oxygen reduction reaction (ORR).
The behavior in ORR indicated that La2O3 is of different ORR kinetics with respect to MnO2, La2O3/MnO2 (LMO), and LMO-Pt electrode materials. The investigation indicates that the surface adsorption of the oxygen species is most likely the rate-determining step for ORR. La2O3 has the highest activity towards ORR at low overpotentials, but it reaches the lowest limiting currents, while MnO2, LMO, and LMO-Pt electrolytic materials have similar activity for ORR.
The capacitance is considerably increased by Pt incorporation into the oxide mixture. MnO2 and La2O3-containing materials exhibit competitive contributions of redox transitions of Mn and pore resistance, depending on the composition. The capacitive response can be governed by Pt, which could appear much to populate the surface of oxide particles rather than the opposite.
Charge transfer resistance indicated the ORR activity series LMO ≈ MnO2 > LMO-Pt >> La2O3, with the oxide material showing the highest activity due to the interactive role of La oxide. The molar activity of Mn oxide is found to be doubled by interactivity. The introduction of a small amount of Pt was not found to considerably affect the ORR kinetics of oxides.

Author Contributions

S.V.P.: Investigation, data curation, methodology. M.R.P.P.: Conceptualization, validation, visualization, writing-original draft, writing-review. M.M.V.: Formal analysis, writing-original draft, writing–review. V.T.: Data curation. S.S.: Conceptualization. B.F.: Funding acquisition. M.M.P.: Supervision, visualization, writing-original draft, writing–review. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Grant No. 451-03-68/2022-14/200026). The authors would like to thank the Ministry of Education, Science and Technological Development of the Republic of Serbia and DAAD, Germany, for funding of the Project No.: 57334757.

Data Availability Statement

All available data are contained within the article.

Acknowledgments

The authors would like to thank Vladimir Panić and Jasmina Stevanović for help in results analysis.

Conflicts of Interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

Sample Availability

Samples of the La2O3, MnO2, LMO, and LMO–Pt powders are available from the authors.

References

  1. Wang, X.; Li, Z.; Qu, Y.; Yuan, T.; Wang, W.; Wu, Y.; Li, Y. Review of Metal Catalysts for Oxygen Reduction Reaction: From Nanoscale Engineering to Atomic Design. Chem 2019, 5, 1486–1511. [Google Scholar] [CrossRef]
  2. Li, Z.; Yang, Y.; Relefors, A.; Kong, X.; Siso, G.M.; Wickman, B.; Kiros, Y.; Soroka, I.L. Tuning morphology, composition and oxygen reduction reaction (ORR) catalytic performance of manganese oxide particles fabricated by γ-radiation induced synthesis. J. Colloid Interface Sci. 2021, 583, 71–79. [Google Scholar] [CrossRef]
  3. Kodama, K.; Nagai, T.; Kuwaki, A.; Jinnouchi, R.; Morimoto, Y. Challenges in applying highly active Pt-based nanostructured catalysts for oxygen reduction reactions to fuel cell vehicles. Nat. Nanotechnol. 2021, 16, 140–147. [Google Scholar] [CrossRef] [PubMed]
  4. Ren, X.; Lv, Q.; Liu, L.; Liu, B.; Wang, Y.; Liu, A.; Wu, G. Current progress of Pt and Pt-based electrocatalysts used for fuel cells. Sustain. Energy Fuels 2020, 4, 15–30. [Google Scholar] [CrossRef]
  5. Wang, Y.; Li, J.; Wei, Z. Transition-metal-oxide-based catalysts for the oxygen reduction reaction. J. Mater. Chem. A 2018, 6, 8194–8209. [Google Scholar] [CrossRef]
  6. Che, H.; Lv, Y.; Liu, A.; Mu, J.; Zhang, X.; Bai, Y. Facile synthesis of three dimensional flower-like Co3O4@MnO2 core-shell microspheres as high-performance electrode materials for supercapacitors. Ceram. Int. 2017, 43, 6054–6062. [Google Scholar] [CrossRef]
  7. Dessie, Y.; Tadesse, S.; Eswaramoorthy, R.; Abebe, B. Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review. J. Sci. Adv. Mater. Devices 2019, 4, 353–369. [Google Scholar] [CrossRef]
  8. Neburchilov, V.; Wang, H.; Martin, J.J.; Qu, W. A review on air cathodes for zinc–air fuel cells. J. Power Sources 2010, 195, 1271–1291. [Google Scholar] [CrossRef]
  9. Lv, J.; Zhang, Y.; Lv, Z.; Huang, X.; Wang, Z.; Zhu, X.; Wei, B. Strontium doped lanthanum manganite (LSM) effects on electrochemical performance of LSM/MnO2 composites for supercapacitor. J. Mater. Sci. Mater. Electron. 2017, 28, 17020–17025. [Google Scholar] [CrossRef]
  10. Xia, H.; Zhu, D.; Luo, Z.; Yu, Y.; Shi, X.; Yuan, G.; Xie, J. Hierarchically Structured Co3O4@Pt@MnO2 Nanowire Arrays for High-Performance Supercapacitors. Sci. Rep. 2013, 3, 2978. [Google Scholar] [CrossRef]
  11. Lahousse, C.; Bernier, A.; Grange, P.; Delmon, B.; Papaefthimiou, P.; Ioannides, T.; Verykios, X. Evaluation of gamma-MnO2 as a VOC removal catalyst: Comparison with a noble metal catalyst. J. Catal. 1998, 178, 214–225. [Google Scholar] [CrossRef]
  12. Zhang, T.; Ge, X.; Zhang, Z.; Tham, N.N.; Liu, Z.; Fisher, A.; Lee, J.Y. Improving the Electrochemical Oxygen Reduction Activity of Manganese Oxide Nanosheets with Sulfurization-Induced Nanopores. ChemCatChem 2018, 10, 422–429. [Google Scholar] [CrossRef]
  13. Huang, M.; Zhang, Y.; Li, F.; Wang, Z.; Alamusi; Hu, N.; Wen, Z.; Liu, Q. Merging of Kirkendall Growth and Ostwald Ripening: CuO@MnO2 Core-shell Architectures for Asymmetric Supercapacitors. Sci. Rep. 2014, 4, 4518. [Google Scholar] [CrossRef] [PubMed]
  14. Li, Y.; Peng, H.; Zhang, C.; Chu, M.; Xiao, P.; Zhang, Y. Branched ultra-fine nickel oxide/manganese dioxide core–shell nanosheet arrays for electrochemical capacitors. RSC Adv. 2015, 5, 77115–77121. [Google Scholar] [CrossRef]
  15. Zhang, X.; Xiao, Q.; Zhang, Y.; Jiang, X.; Yang, Z.; Xue, Y.; Yan, Y.-M.; Sun, K. La2O3 Doped Carbonaceous Microspheres: A Novel Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions with Ultrahigh Mass Activity. J. Phys. Chem. C 2014, 118, 20229–20237. [Google Scholar] [CrossRef]
  16. Badawy, A.; Ibrahim, S.M.I. The influence of La2O3-doping on structural, surface and catalytic properties of nano-sized cobalt–manganese mixed oxides. Int. J. Ind. Chem. 2016, 7, 287–296. [Google Scholar] [CrossRef] [Green Version]
  17. Wang, N.; Liu, J.; Gu, W.; Song, Y.; Wang, F. Toward Synergy of Carbon and La 2 O 3 in Their Hybrid as Efficient Catalyst for Oxygen Reduction Reaction. RSC Adv. 2016, 6, 77786–77795. [Google Scholar] [CrossRef]
  18. Sunarso, J.; Torriero, A.A.J.; Zhou, W.; Howlett, P.C.; Forsyth, M. Oxygen Reduction Reaction Activity of La-Based Perovskite Oxides in Alkaline Medium: A Thin-Film Rotating Ring-Disk Electrode Study. J. Phys. Chem. C 2012, 116, 5827–5834. [Google Scholar] [CrossRef]
  19. Zhu, H.; Zhang, P.; Dai, S. Recent Advances of Lanthanum-Based Perovskite Oxides for Catalysis. ACS Catal. 2015, 5, 6370–6385. [Google Scholar] [CrossRef]
  20. Chen, D.; Chen, C.; Baiyee, Z.M.; Shao, Z.; Ciucci, F. Nonstoichiometric Oxides as Low-Cost and Highly-Efficient Oxygen Reduction/Evolution Catalysts for Low-Temperature Electrochemical Devices. Chem. Rev. 2015, 115, 9869–9921. [Google Scholar] [CrossRef]
  21. Paulraj, A.R.; Kiros, Y. La0.1Ca0.9MnO3/Co3O4 for oxygen reduction and evolution reactions (ORER) in alkaline electrolyte. J. Solid State Electrochem. 2018, 22, 1697–1710. [Google Scholar] [CrossRef] [Green Version]
  22. Shafi, P.M.; Joseph, N.; Thirumurugan, A.; Bose, A. Enhanced Electrochemical Performances of Agglomeration-free LaMnO 3 Perovskite Nanoparticles and Achieving High Energy and Power Densities with Symmetric Supercapacitor Design. Chem. Eng. J. 2018, 338, 147–156. [Google Scholar] [CrossRef]
  23. Varničić, M.; Pavlović, M.M.; Eraković Pantović, S.; Mihailović, M.; Pantović Pavlović, M.R.; Stopić, S.; Friedrich, B. Spray-Pyrolytic Tunable Structures of Mn Oxides-Based Composites for Electrocatalytic Activity Improvement in Oxygen Reduction. Metals 2022, 12, 22. [Google Scholar] [CrossRef]
  24. Fink, M.; Eckhardt, J.; Khadke, P.; Gerdes, T.; Roth, C. Bifunctional α-MnO2 and Co3O4 Catalyst for Oxygen Electrocatalysis in Alkaline Solution. ChemElectroChem 2020, 7, 4822–4836. [Google Scholar] [CrossRef]
  25. Shinagawa, T.; Garcia-Esparza, A.T.; Takanabe, K. Insight on Tafel slopes from a microkinetic analysis of aqueous electrocatalysis for energy conversion. Sci. Rep. 2015, 5, 13801. [Google Scholar] [CrossRef] [Green Version]
  26. Marković, N.; Schmidt, T.; Stamenkovic, V.; Ross, P. Oxygen Reduction Reaction on Pt and Pt Bimetallic Surfaces: A Selective Review. Fuel Cells J. 2001, 1, 105–116. [Google Scholar] [CrossRef]
  27. Eraković, S.; Pavlović, M.M.; Stopić, S.; Stevanović, J.; Mitrić, M.; Friedrich, B.; Panić, V. Interactive promotion of supercapacitance of rare earth/CoO3-based spray pyrolytic perovskite microspheres hosting the hydrothermal ruthenium oxide. Electrochim. Acta 2019, 321, 134721. [Google Scholar] [CrossRef]
  28. Pavlović, M.M.; Pantović Pavlović, M.R.; Eraković Pantović, S.G.; Stevanović, J.S.; Stopić, S.R.; Friedrich, B.; Panić, V.V. The Roles of Constituting Oxides in Rare-Earth Cobaltite-Based Perovskites on their Pseudocapacitive Behavior. J. Electroanal. Chem. 2021, 897, 115556. [Google Scholar] [CrossRef]
  29. Weiner, D.D.; Spina, J.F. Sinusoidal Analysis and Modeling of Weakly Nonlinear Circuits: With Application to Nonlinear Interference Effects; Van Nostrand Reinhold Company: New York, NY, USA, 1980. [Google Scholar]
  30. Bensmann, B.; Petkovska, M.; Vidaković-Koch, T.; Hanke-Rauschenbach, R.; Sundmacher, K. Nonlinear Frequency Response of Electrochemical Methanol Oxidation Kinetics: A Theoretical Analysis. J. Electrochem. Soc. 2010, 157, B1279. [Google Scholar] [CrossRef]
  31. Singh, R.K.; Devivaraprasad, R.; Kar, T.; Chakraborty, A.K.; Neergat, M. Electrochemical Impedance Spectroscopy of Oxygen Reduction Reaction (ORR) in a Rotating Disk Electrode Configuration: Effect of Ionomer Content and Carbon-Support. J. Electrochem. Soc. 2015, 162, F489. [Google Scholar] [CrossRef]
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Figure 1. Stable cyclic voltammograms of La2O3, MnO2, LMO, and LMO-Pt thin-layer electrodes at scan rate of 50 mV s−1 in N2-saturated 0.10 M KOH at room temperature.
Figure 1. Stable cyclic voltammograms of La2O3, MnO2, LMO, and LMO-Pt thin-layer electrodes at scan rate of 50 mV s−1 in N2-saturated 0.10 M KOH at room temperature.
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Figure 2. ORR activities of La2O3, MnO2, LMO, and LMO-Pt electrode materials at the scan rate of 2 mV s−1 and electrode rotating speed of 1000 rpm; 0.1 M NaOH, 25 °C.
Figure 2. ORR activities of La2O3, MnO2, LMO, and LMO-Pt electrode materials at the scan rate of 2 mV s−1 and electrode rotating speed of 1000 rpm; 0.1 M NaOH, 25 °C.
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Catalysts 12 00641 g003 550
Figure 3. Ohmic-drop-compensated pоtentiodуnаmic Tafel сurvеs for the ORR at La2O3, MnO2, LMO, and LMO-Pt material in O2-saturated 0.10 M KOH, 25 °C.
Figure 3. Ohmic-drop-compensated pоtentiodуnаmic Tafel сurvеs for the ORR at La2O3, MnO2, LMO, and LMO-Pt material in O2-saturated 0.10 M KOH, 25 °C.
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Catalysts 12 00641 g004a 550Catalysts 12 00641 g004b 550
Figure 4. (a) Capacitance complex plane plots for synthesized materials at OCP in 0.10 M KOH and (b–d) equivalent circuits used to fit the experimental EIS data. Equivalent electric circuit data are presented by lines in the plots.
Figure 4. (a) Capacitance complex plane plots for synthesized materials at OCP in 0.10 M KOH and (b–d) equivalent circuits used to fit the experimental EIS data. Equivalent electric circuit data are presented by lines in the plots.
Catalysts 12 00641 g004aCatalysts 12 00641 g004b
Catalysts 12 00641 g005 550
Figure 5. (a) Nyquist diagrams of La2O3, MnO2, LMO, and LMO-Pt electrocatalytic materials in O2-saturated 0.10 M KOH at current of −0.15 mA with an amplitude of 0.10 mA and (b) equivalent circuits used to fit the experimental EIS data. Equivalent electric circuit data are presented by lines in the plots.
Figure 5. (a) Nyquist diagrams of La2O3, MnO2, LMO, and LMO-Pt electrocatalytic materials in O2-saturated 0.10 M KOH at current of −0.15 mA with an amplitude of 0.10 mA and (b) equivalent circuits used to fit the experimental EIS data. Equivalent electric circuit data are presented by lines in the plots.
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Figure 6. Third harmonics for ORR at La2O3, MnO2, LMO, and LMO-Pt in O2-saturated 0.10 M KOH gained by GEIS at OCP with current amplitude of 0.10 mA.
Figure 6. Third harmonics for ORR at La2O3, MnO2, LMO, and LMO-Pt in O2-saturated 0.10 M KOH gained by GEIS at OCP with current amplitude of 0.10 mA.
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Figure 7. (a) First and third harmonic for LMO-Pt electrode material and (b) Adjusted Nyquist diagrams of La2O3, MnO2, LMO, and LMO-Pt electrolytic material in O2-saturated 0.10 M KOH at currents that correspond to OCP.
Figure 7. (a) First and third harmonic for LMO-Pt electrode material and (b) Adjusted Nyquist diagrams of La2O3, MnO2, LMO, and LMO-Pt electrolytic material in O2-saturated 0.10 M KOH at currents that correspond to OCP.
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Table
Table 1. The current densities per mol of a metal for La2O3, MnO2, LMO, and LMO-Pt electrolytic material in O2-saturated 0.10 M KOH.
Table 1. The current densities per mol of a metal for La2O3, MnO2, LMO, and LMO-Pt electrolytic material in O2-saturated 0.10 M KOH.
jLa (A mol−1)jMn (A mol−1)jPt (A mol−1)
La2O30.78--
MnO2-4.14-
LMO13.010.3-
LMO-Pt9.717.65168.3
Table
Table 2. Electrochemical impedance spectroscopy data for La2O3, MnO2, LMO, and LMO-Pt electrocatalytic materials in N2-saturated 0.10 M KOH gained at OCP.
Table 2. Electrochemical impedance spectroscopy data for La2O3, MnO2, LMO, and LMO-Pt electrocatalytic materials in N2-saturated 0.10 M KOH gained at OCP.
CPE1-T (µF)CPE1-PCPE2-T (µF)CPE2-PR2 (Ω)R3 (Ω)
La2O356.70.8618.90.6354.89 × 104
MnO254.50.8521.00.6582.50 × 1031.31 × 105
LMO31.60.8823.40.5934.89 × 1033.74 × 105
LMO-Pt60.60.924.90.515
Table
Table 3. Galvanostatic electrochemical impedance spectroscopy data for La2O3, MnO2, LMO, and LMO-Pt in O2-saturated 0.10 M KOH obtained at Is = −0.765 mA cm−2 with an amplitude of 0.510 mA cm−2.
Table 3. Galvanostatic electrochemical impedance spectroscopy data for La2O3, MnO2, LMO, and LMO-Pt in O2-saturated 0.10 M KOH obtained at Is = −0.765 mA cm−2 with an amplitude of 0.510 mA cm−2.
CPE1-T (µF)CPE1-PW1-R (Ω)W1-T (s)R1 (Ω)R2 (Ω)
La2O3128.80.8418.90.5343.8518
MnO2167.50.8369.70.5345.7376
LMO143.70.83123.30.30146.7375
LMO-Pt149.20.8082.30.24143.1386
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Panić, S.V.; Pantović Pavlović, M.R.; Varničić, M.M.; Tadić, V.; Stopić, S.; Friedrich, B.; Pavlović, M.M. Rare-Earth/Manganese Oxide-Based Composites Materials for Electrochemical Oxygen Reduction Reaction. Catalysts 2022, 12, 641. https://doi.org/10.3390/catal12060641

AMA Style

Panić SV, Pantović Pavlović MR, Varničić MM, Tadić V, Stopić S, Friedrich B, Pavlović MM. Rare-Earth/Manganese Oxide-Based Composites Materials for Electrochemical Oxygen Reduction Reaction. Catalysts. 2022; 12(6):641. https://doi.org/10.3390/catal12060641

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Panić, Stefan V., Marijana R. Pantović Pavlović, Miroslava M. Varničić, Vojin Tadić, Srećko Stopić, Bernd Friedrich, and Miroslav M. Pavlović. 2022. "Rare-Earth/Manganese Oxide-Based Composites Materials for Electrochemical Oxygen Reduction Reaction" Catalysts 12, no. 6: 641. https://doi.org/10.3390/catal12060641

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