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Communication

Manganese-Cobalt Spinel Nanoparticles Anchored on Carbon Nanotubes as Bi-Functional Catalysts for Oxygen Reduction and Oxygen Evolution Reactions

by
Yixiao Zhang
1,2,
Xinyu Xie
1,
Zhichuang Zheng
1,
Xian He
1,
Peng Du
1,2,*,
Ru Zhang
2,
Limin Guo
1,* and
Kai Huang
1,*
1
State Key Laboratory of Information Photonics and Optical Communications, School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, China
2
Beijing Key Laboratory of Space-Ground Interconnection and Convergence, Beijing University of Posts and Telecommunications (BUPT), Xitucheng Road No. 10, Beijing 100876, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2023, 13(23), 12702; https://doi.org/10.3390/app132312702
Submission received: 30 October 2023 / Revised: 12 November 2023 / Accepted: 23 November 2023 / Published: 27 November 2023
(This article belongs to the Special Issue Novel Study on Photochemistry and Electrochemistry of Composite Nanomaterials)
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Abstract

:
The pivotal role of oxygen electrocatalysis in the realm of energy conversion and storage is unmistakably significant. In an endeavor to diminish the reliance on precious metals, the development of innovative catalysts exhibiting exceptional bifunctional durability and heightened activity for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) has garnered considerable scholarly interest. Employing a straightforward two-step methodology, we have successfully synthesized uniformly distributed MnCo2O4 and CoMn2O4 nanoparticles of diminutive size, meticulously anchored onto carbon nanotubes (CNTs). Owing to the synergistic covalent interplay between the spinel oxide nanoparticles and CNTs, these nanocomposites demonstrate ORR activity on par with, and notably superior OER activity compared to, commercially available Pt/C catalysts. The onset potential of MnCo2O4-CNTs stands at 1.03 V vs. RHE, maintaining 95.76% of its initial current density following a 10,000-s chronoamperometry test. Furthermore, MnCo2O4-CNTs outperform CoMn2O4-CNTs in OER catalysis. The outstanding performance of MnCo2O4-CNTs is attributed to the higher content of Co3+ ions, which are active for the oxygen electrocatalysis.

1. Introduction

The swift exhaustion of traditional fossil fuels coupled with an escalating demand for energy has heightened the urgency to innovate sustainable energy conversion and storage technologies that are not only efficient and cost-effective but also environmentally benign [1,2,3]. Among various electrochemical energy systems, such as fuel cells and rechargeable metal–air batteries, the roles of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are critical as key performance indicators. Yet, the broader commercial adoption and efficacy of these energy devices are considerably hampered by the inherently slow kinetics and substantial overpotential required for these reactions [4,5]. To date, materials based on noble metals have been acknowledged as the most effective catalysts in the realm of commercial energy conversion. And it has been verified that oxides, alloys, and carbon materials of Ir, Ru, and Pt species have advantageous performance in OER and ORR catalysis [6,7,8,9]. In pursuit of minimizing the substantial costs associated with the extensive use of noble metals and to attain an impressive catalytic efficiency for both ORR and OER, considerable research has been directed towards the discovery and development of innovative, bifunctional catalysts based on non-precious metals [10,11,12,13].
Extensive research underscores the electrocatalytic prowess of transition metal oxides composed of constituent elements such as Ni, Cu, Fe, Co, and Mn, along with their nanostructured forms (including Co3O4, Mn3O4, NiCo2O4, ZnCo2O4, CoMn2O4, CoFe2O4, NiFe2O4), which have demonstrated remarkable performance in electrocatalysis [14,15,16,17,18,19,20,21,22]. Among various catalysts, the mixed valence transition metal oxides have exhibited great importance in applications of energy technologies because of their low cost, facile synthesis, considerable activity, and stability in aqueous alkaline solutions [23,24,25,26]. Notably, catalysts based on Co and Mn have garnered attention for their high bifunctional efficacy in ORR and OER. This largely ascribes to the high oxidation potential of Co and the enhanced electronic transport characteristics of Mn [27,28,29]. Nonetheless, the inherently low electronic conductivity of these mixed valence transition metal oxides remains a critical barrier, impeding further enhancements in catalytic performance. The recent innovation in carbon-support materials such as graphene, carbon fibers, and nanotubes, offers promising approaches to overcome this limitation [30,31]. Consequently, the integration of oxides with nanocarbons significantly amplifies the electrocatalytic activity and stability, a result of the exceptional electronic conductivity, remarkable flexibility, and expansive surface area of carbon-based materials. Furthermore, the fine dispersion of oxide particles and the robust interfacial coupling within these composites are crucial factors driving superior catalytic efficiency [32,33,34,35].
Due to the structural superiority, a series of cobalt-manganese mixed oxides have been explored, such as MnxCo3-xO4, MnCoOx, and CoMn2O4 systems [36,37]. However, the majority of previous work leaned towards the applications for anode materials, supercapacitor, and ORR electrocatalysts. Hence, developing the kind of nanocomposite catalysts with bi-functional electrocatalysis still remains a great challenge [38,39].
In this work, we have adeptly synthesized uniform MnCo2O4 and CoMn2O4 nanoparticles of reduced size, composed primarily of non-precious metals, seamlessly anchored onto carbon nanotubes (CNTs) using a straightforward, surfactant-free, two-step process with superior performance compared to Pt/C at a low price. We evaluated the catalytic efficiency of MnCo2O4-CNTs and CoMn2O4-CNTs towards ORR and OER. When contrasted with pure manganese cobalt oxide nanoparticles, the activity of the composites integrated with CNTs is markedly enhanced. Notably, the MnCo2O4-CNTs nanocomposites outshine all other prepared samples in ORR performance, displaying activity comparable to, and durability surpassing, that of the conventional Pt/C catalyst. Additionally, MnCo2O4-CNTs demonstrate a more pronounced positive onset potential, superior current density, and enhanced durability compared to the mere physical mixtures of MnCo2O4 and CNTs (MnCo2O4 + CNTs). This improvement is largely ascribed to the strengthened chemical bonding between MnCo2O4 and CNTs [40]. Furthermore, MnCo2O4-CNTs also exhibit significant catalytic performance in OER, positioning them as a formidable contender for bifunctional catalysts in the domain of energy conversion and storage.

2. Materials and Methods

2.1. Materials

Hydroxylated CNTs (CNTs-OH, hydroxyl content: 3.0 wt%, purity > 95 wt%, XFNANO), Manganese acetate (Mn(Ac)2, 98%, Sinopharm Chemical Reagent, Shanghai, China), Cobaltous acetate (Co(Ac)2, 98%, Sinopharm Chemical Reagent), ammonium hydroxide (NH4OH, 25 wt%, Merck Chemicals, Merck, Darmstadt, Germany) and ethanol (C2H6O, 99.7%, Aladdin, Shanghai, China) were employed in their pristine forms without necessitating further purification.

2.2. Synthesis of MnCo2O4-CNTs and CoMn2O4-CNTs

Typically, 20 mg of CNTs-OH was firstly dispersed in 65 mL of ethanol to achieve a homogeneous suspension solution with 30 min of sonication process. Also, 1 mmol of Mn(Ac)2 and 2 mmol of Co(Ac)2 were added into the suspension solution. After 10 min of continuous sonication, the mixture should be heated at 80 °C under the gentle magnetic stirring. In an hour, 1 mL of H2O and 1 mL of NH4OH (~25%) were added dropwise, and the mixture was steadfastly maintained at 80 °C for an additional 20 h. Thereafter, the reaction concoction was transferred into a 40 mL Teflon-lined stainless steel autoclave, where it underwent a hydrothermal reaction at 150 °C for 3 h. The product was isolated via centrifugation and subsequently purified with multiple washes using deionized water and ethanol. The MnCo2O4-CNTs composites were ultimately obtained after a drying period of 12 h at 40 °C under vacuum conditions. Employing an identical methodology but with an altered molar ratio of Mn(Ac)2 and Co(Ac)2, CoMn2O4-CNTs composites were similarly synthesized. For comparative purposes, MnCo2O4 and CoMn2O4 nanoparticles were also produced using the same protocol, albeit excluding the addition of CNTs-OH in the initial step.

2.3. Characterization

X-ray diffraction (XRD) analyses of the powdered samples were meticulously performed using a Panalytical X’pert diffractometer. Measurements spanned an angular range from 10° to 90°, employing a scanning velocity of 2° per minute, with Cu Kα radiation serving as the source. The microstructure and morphological attributes of the catalysts were extensively examined via a transmission electron microscope (TEM, JEM-ARM200F, JEOL Ltd., Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) assessments were conducted with an ESCALAB 250Xi spectrometer (Thermo Scientific, Waltham, MA, USA), aiming to precisely determine the binding energies associated with Mn 2p, Co 2p, O 1s, and C 1s.

2.4. Electrochemical Measurements

Electrocatalytic efficacy was rigorously evaluated utilizing an Autolab PGSTAT204 (Metrohm AG, Herisau, Switzerland) electrochemical workstation, structured in a classical three-electrode cell arrangement. This setup included a working electrode comprising a glassy carbon rotating disk electrode (RDE) with a diameter of 5 mm, a counter electrode fashioned from Pt foil, and a Hg/HgO reference electrode (referenced at 0.92 V vs. RHE). For preparatory steps, 5 mg of the catalyst was uniformly dispersed in a solvent blend comprising 300 μL of isopropanol, 700 μL of ultrapure water, and 30 μL of a 5.0 wt% Nafion solution. This dispersion was subjected to a 30 min sonication to ensure a well-homogenized ink. Specifically for ORR processes, the catalyst ink was similarly prepared, though 1 mg of Vulcan XC-72R carbon powder replaced 20 wt% of the catalyst. Subsequently, 10 μL of this catalyst ink was carefully applied onto the glassy carbon electrode, establishing a catalyst loading density of approximately 0.25 mg cm−2. The working electrode thus prepared was utilized in the evaluations. Electrolyte solutions, saturated, respectively, with O2 and N2, were used to examine ORR and OER activities, with both solutions containing aqueous 0.1 M KOH. For typical measurement protocols, cyclic voltammetry (CV) tests were conducted at a scan rate of 5 mV s−1, whereas linear sweep voltammetry (LSV) tests proceeded at a scan rate of 10 mV s−1. The stability of the electrocatalysts under chronoamperometric conditions was ascertained at a rotating speed of 1600 rpm and a potential of 0.7 V over a duration of 10,000 s.

3. Results

In this study, the uniform manganese-cobalt spinel nanoparticles and their composites with carbon nanotubes have been prepared through a straightforward two-step methodology. The structural formation and crystallization characteristics of the samples were initially verified through X-ray diffraction (XRD) analysis. Illustrations in Figure 1a demonstrate that the specific diffraction peaks’ positions and intensities correspond precisely with those of the cubic MnCo2O4 spinel structure (JCPDS no. 23-1237), devoid of any extraneous diffraction peaks indicating impurities. It is indicated that cubic spinel MnCo2O4 has been successfully synthesized via the two-step process with precursor Mn/Co molar ratio of 1:2 (Mn/Co). Meanwhile, the XRD patterns of the products prepared with a precursor Mn/Co molar ratio of 2:1 (Mn/Co) are presented in Figure 1b. The characteristic diffraction peaks can be indexed as the tetragonal CoMn2O4 phase (JCPDS no. 77-0471). This suggests the formation of CoMn2O4 characterized by a slightly altered spinel framework. Notably, the XRD patterns illustrate no marked distinction between the two manganese-cobalt spinel nanoparticles and MnCo2O4 (CoMn2O4)-CNTs composites, due to the limited amount of CNTs and presence of MnCo2O4 and CoMn2O4 in the composites [41,42].
Subsequent examinations of the microstructures and morphological characteristics of the synthesized samples were conducted using transmission electron microscopy (TEM). Figure 2a–c show the image of pure MnCo2O4, and uniform nanoparticles with an average diameter of approximately 7 nm can be observed. The high-resolution TEM (HRTEM) image, depicted in Figure 2c, distinctly displays lattice fringes, with the measured d-spacing of 0.25 and 0.48 nm aligning perfectly with the (311) and (111) planes of the cubic spinel MnCo2O4 structure, respectively [43]. Furthermore, in the MnCo2O4-CNT nanocomposites illustrated in Figure 2d–f, the MnCo2O4 nanoparticles, denoted as MCO, exhibit a reduced average size of about 4 nm and are evenly distributed on the carbon nanotubes’ surfaces. The TEM imagery of unadulterated CoMn2O4, presented in Figure S1a–c, shows CoMn2O4 nanoparticles mirroring the morphology of MnCo2O4 but with a larger average size of 9 nm. Additionally, the observed lattice fringes of the CoMn2O4 nanoparticles, exhibiting an interfringe spacing of 0.49 nm, correspond to the (101) plane of the tetragonal spinel CoMn2O4 phase, which is shown in Figure S1c [44]. Figure S1d–f depict the CoMn2O4-CNT nanocomposites, where carbon nanotubes are uniformly encapsulated by CoMn2O4 nanoparticles averaging 5 nm in size. Notably, MnCo2O4 and CoMn2O4 nanoparticles were not detected outside the CNTs following ultrasonication in ethanol for TEM analysis, suggesting a robust interaction between the CNTs and the manganese-cobalt spinel nanoparticles [45]. Additionally, it is noteworthy that the integration of CNTs curtails the size expansion of manganese-cobalt nanoparticles, attributable to the abundant hydroxyl functionalities on the CNTs-OH surfaces serving as prolific nucleation sites for the spinel nanoparticles [46].
To delve deeper into the composition and valence states of the surface elements, X-ray photoelectron spectroscopy (XPS) analyses were conducted. The outcomes are presented in Figure 3. Figure 3a displays the survey spectra for both MnCo2O4-CNTs and CoMn2O4-CNTs, definitively confirming the presence of Mn, Co, O, and C elements. Furthermore, the Mn/Co atomic ratios in each sample are conspicuously inverse, aligning with the ratios used in the initial precursors. Through Gaussian fitting, the primary Mn 2p spin-orbit peaks, Mn 2p1/2 at approximately 653.1 eV and Mn 2p3/2 at about 641.5 eV, were deconstructed into four distinct peak components. The binding energies at approximately 641.2 and 652.6 eV, as seen in Figure 3b, are indicative of Mn2+, while the peaks at around 643.0 and 653.9 eV can be ascribed to Mn3+ [47]. Likewise, the Co 2p spectrum, illustrated in Figure 3c, was optimally fitted assuming eight variants, including two pairs of spin-orbit doublets representative of Co2+ and Co3+ states, along with their respective four shakeup satellites (denoted Sat.) [47,48]. Thus, it is reasonable to conclude that the mixed-valence metal cations (Mn2+, Mn3+, Co2+ and Co3+) co-exist in both MnCo2O4-CNTs and CoMn2O4-CNTs nanocomposites at octahedral and tetrahedral stacking interstices [46]. It is significant to note that these fluctuating valence states of the metal cations are likely to augment the electrochemical performance of the materials [49]. As depicted in Figure 3d, the C 1s spectra exhibit a predominant peak at 284.3 eV, attributable to the C=C bonds within the CNTs, and two subsidiary peaks corresponding to oxidized carbon forms, such as C-O and C=O, underscoring the robust interactions between the manganese-cobalt nanoparticles and the CNTs [50,51].
The electrocatalytic efficacies of the synthesized catalysts for the oxygen reduction reaction (ORR) were initially evaluated using cyclic voltammetry (CV) in a 0.1 M KOH solution with a glassy carbon electrode. As depicted in Figure 4a, all O2-saturated curves displayed pronounced reduction current peaks compared to those recorded in N2-saturated electrolytes, thereby indicating the ORR catalytic capabilities of the manganese-cobalt spinel nanoparticles and their composites. Additionally, the results of the rotating disk electrode (RDE) linear sweep voltammetry (LSV) assays conducted in O2-saturated 0.1 M KOH solution at a rotation speed of 1600 rpm are showcased in Figure 4b. Notably, the MnCo2O4-CNTs and CoMn2O4-CNTs composite catalysts demonstrated superior catalytic performance relative to their individual constituents. This enhanced efficiency is credited to the CNTs’ hybridization of chemical bonds, which bolsters the electronic conductivity of the manganese-cobalt oxides [52]. Remarkably, the MnCo2O4-CNTs nanocomposites exhibited the most proficient catalytic activity for ORR, outstripping other catalysts in terms of onset potential and current density. The onset and half-wave potentials of the MnCo2O4-CNTs were measured at 1.03 and 0.73 V vs. RHE, respectively, rivaling the performance of commercial Pt/C catalysts, and also exhibited a substantial current density. The Co3+ at octahedral sites is known to be the catalytic sites for the ORR. [18] In the case of MnCo2O4-CNTs, Co3+ and Mn2+ are the dominant component in octahedral and tetrahedral interstices, thus favoring MnCo2O4-CNTs to exhibit a superior ORR catalytic activity [53].
Furthermore, the ORR performance of the MnCo2O4-CNTs composite catalyst significantly surpasses that of mere physical mixtures of MnCo2O4 and CNTs, as illustrated in Figure S2. This clearly indicates that robust hybrid coupling between the nanoparticles and carbon nanotubes plays a crucial role in enhancing ORR catalysis. [54,55]. To delve deeper into the pronounced ORR activity of the MnCo2O4-CNTs nanocomposite, linear sweep voltammetry (LSV) curves at various rotation rates ranging from 400 to 2000 rpm are depicted in Figure 4c. In these curves, the broad current plateaus indicative of the diffusion-limiting current density are readily apparent, with the limiting current density escalating and the rotation rate increasing, attributed to the reduced diffusion distance at higher rates. Additionally, the Koutecky–Levich (K-L) analysis, portrayed in Figure 4d at varying potentials from 0.20 to 0.55 V vs. RHE across rotation speeds from 400 to 2000 rpm, is performed to elucidate the underlying reaction mechanism of the MnCo2O4-CNTs catalyst. The impeccable linearity and nearly uniform slope across these plots corroborate the first-order reaction kinetics. Moreover, the calculated average number of electrons transferred (n) is approximately 3.95, indicating that the MnCo2O4-CNTs catalyst predominantly follows a quasi-four-electron (quasi-4e−) pathway for ORR, underscoring its efficient electron transfer mechanism.
The long-term durability of a catalyst is a critical factor for its practical application. Chronoamperometric tests for MnCo2O4-CNTs composites, MnCo2O4 + CNTs physical mixtures, and commercial Pt/C were performed at 0.7 V vs. RHE in an O2-saturated 0.1 M KOH environment. As illustrated in Figure S4, the current density of the MnCo2O4-CNTs composites exhibited a mere 4.24% decline following 10,000 s of continuous operation, in stark contrast to the 13.79% and 12.89% reductions observed in the MnCo2O4 + CNTs mixtures and Pt/C, respectively. Notably, the endurance of the nanocomposites surpasses that of the physical blends of MnCo2O4 and CNTs. This considerable enhancement in stability is largely ascribed to the robust chemical hybrid coupling between MnCo2O4 and CNTs, as substantiated via X-ray photoelectron spectroscopy (XPS) analysis. Consequently, the MnCo2O4-CNTs nanocomposites not only demonstrate competitive ORR activity but also superior durability compared to commercial Pt/C catalysts, positioning them as a viable alternative to platinum-based catalysts.
To explore the broader applications of manganese-cobalt nanoparticles–CNTs composites in electrochemical catalysis, oxygen evolution reaction (OER) experiments were carried out in a 0.1 M KOH solution. Displayed in Figure S5, the RDE polarization curves for MnCo2O4-CNTs, CoMn2O4-CNTs, and commercial Pt/C catalysts reveal significant findings. The OER onset potential for both MnCo2O4-CNTs and CoMn2O4-CNTs composites is approximately 1.53 V vs. RHE, showcasing a marked improvement over the Pt/C catalyst. Despite being slightly less efficient than elite precious metal oxides like RuO2 and IrO2, which currently represent the pinnacle of OER catalysts, these manganese-cobalt nanoparticles–CNTs nanocomposites emerge as promising candidates in active OER electrocatalysis. Notably, the overpotential required for MnCo2O4-CNTs to achieve OER is about 450 mV at 10 mA cm−2, which is more favorable than the CoMn2O4-CNTs composite (approximately 560 mV). This superior catalytic activity in OER exhibited by MnCo2O4-CNTs can be primarily credited to the presence of Co3+ species within the octahedral sites of the cubic MnCo2O4 spinel structure, playing a pivotal role in facilitating water oxidation [18].

4. Conclusions

In summary, manganese-cobalt spinel nanoparticles–carbon nanotubes composites of cubic MnCo2O4-CNTs and tetragonal CoMn2O4-CNTs have been effectively synthesized via a two-step method by involving the meticulous calibration of precursor ratios. Compared with a simple physical mixture of MnCo2O4 and CNTs (MnCo2O4 + CNTs), MnCo2O4-CNTs showed more positive starting potential, superior current density, and greater durability. These spinel oxides, characterized by their mixed valence properties, showcase a uniform dispersion of nanoparticles across the carbon nanotubes, anchored securely through robust hybrid coupling. Remarkably, these nanocomposites have demonstrated ORR activities on par with, and OER activities surpassing, those observed in the commercially available Pt/C catalysts. This work provides an attractive approach for designing efficient ORR/OER electrocatalysts and will open exciting avenues for exploring new strategies for the design of high-performance electrocatalysts for renewable energy applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app132312702/s1, Figure S1: TEM images of (a–c) CoMn2O4 nanoparticles, and (d–f) CoMn2O4-CNTs nanocomposites; Figure S2: ORR polarization curves of MCO, MCO-CNTs, and MCO + CNTs samples; Figure S3: The current density at the electrode potential of 0.8 V vs. RHE of MnCo2O4, MnCo2O4-CNTs, and CoMn2O4 + CNTs with the rotation of 1600 rpm; Figure S4: Chronoamperometric curves of MnCo2O4-CNTs, MnCo2O4 + CNTs, and Pt/C with the rotation of 1600 rpm; Figure S5: OER polarization curves of MnCo2O4-CNTs, CoMn2O4-CNTs, and commercial Pt/C catalyst at the rotation of 1600 rpm.

Author Contributions

The conceptualization and design of the experiments were the collective effort of K.H., Y.Z. and P.D. The experimental work was carried out by Y.Z., X.X. and P.D. X.H. and Z.Z. were responsible for data analysis. Reagents, materials, and analytical tools were provided by R.Z., K.H. and L.G. The manuscript was principally authored by Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research received funding from the National Natural Science Foundations of China (Grant Nos. 51902027, 61874014, 51976143, 61874013, 61672108, 61976025, 61674019, and 61974011), National Basic Research of China (Grant No. 2015CB932500), Fundamental Research Funds for the Central Universities (Grant No. 2019RC20, 2023ZCJH03), Teaching Reform Projects at BUPT (2022CXCY-B03), Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications, P.R. China) and BUPT Excellent Ph.D. Students Foundation (CX2022240).

Data Availability Statement

The data from this research are available upon request from the corresponding author. The data are not publicly available because they are part of the ongoing research of the authors.

Conflicts of Interest

The authors report no conflict of interest regarding this publication.

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Figure 1. XRD patterns of (a) MnCo2O4 and MnCo2O4-CNTs and (b) CoMn2O4 and CoMn2O4-CNTs.
Figure 1. XRD patterns of (a) MnCo2O4 and MnCo2O4-CNTs and (b) CoMn2O4 and CoMn2O4-CNTs.
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Figure 2. TEM images of (a–c) MnCo2O4 nanoparticles and (d–f) MnCo2O4-CNTs nanocomposites.
Figure 2. TEM images of (a–c) MnCo2O4 nanoparticles and (d–f) MnCo2O4-CNTs nanocomposites.
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Figure 3. XPS spectra of MnCo2O4-CNTs and MnCo2O4-CNTs. (a) Survey spectrum, (b) Mn 2p, (c) Co 2p, and (d) C 1s.
Figure 3. XPS spectra of MnCo2O4-CNTs and MnCo2O4-CNTs. (a) Survey spectrum, (b) Mn 2p, (c) Co 2p, and (d) C 1s.
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Figure 4. (a) CV curves of as-prepared samples (solid line for O2-saturated, dashed line for N2-saturated); (b) ORR polarization curves of MnCo2O4, MnCo2O4-CNTs, CoMn2O4, CoMn2O4-CNTs, and commercial Pt/C catalyst with the rotation of 1600 rpm; (c) LSV curves of MnCo2O4-CNTs nanocomposites in O2-saturated 0.1 M KOH electrolyte from 400 to 2000 rpm; (d) linear fitting of the Koutecky–Levich model at different potentials of MnCo2O4-CNTs nanocomposites.
Figure 4. (a) CV curves of as-prepared samples (solid line for O2-saturated, dashed line for N2-saturated); (b) ORR polarization curves of MnCo2O4, MnCo2O4-CNTs, CoMn2O4, CoMn2O4-CNTs, and commercial Pt/C catalyst with the rotation of 1600 rpm; (c) LSV curves of MnCo2O4-CNTs nanocomposites in O2-saturated 0.1 M KOH electrolyte from 400 to 2000 rpm; (d) linear fitting of the Koutecky–Levich model at different potentials of MnCo2O4-CNTs nanocomposites.
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MDPI and ACS Style

Zhang, Y.; Xie, X.; Zheng, Z.; He, X.; Du, P.; Zhang, R.; Guo, L.; Huang, K. Manganese-Cobalt Spinel Nanoparticles Anchored on Carbon Nanotubes as Bi-Functional Catalysts for Oxygen Reduction and Oxygen Evolution Reactions. Appl. Sci. 2023, 13, 12702. https://doi.org/10.3390/app132312702

AMA Style

Zhang Y, Xie X, Zheng Z, He X, Du P, Zhang R, Guo L, Huang K. Manganese-Cobalt Spinel Nanoparticles Anchored on Carbon Nanotubes as Bi-Functional Catalysts for Oxygen Reduction and Oxygen Evolution Reactions. Applied Sciences. 2023; 13(23):12702. https://doi.org/10.3390/app132312702

Chicago/Turabian Style

Zhang, Yixiao, Xinyu Xie, Zhichuang Zheng, Xian He, Peng Du, Ru Zhang, Limin Guo, and Kai Huang. 2023. "Manganese-Cobalt Spinel Nanoparticles Anchored on Carbon Nanotubes as Bi-Functional Catalysts for Oxygen Reduction and Oxygen Evolution Reactions" Applied Sciences 13, no. 23: 12702. https://doi.org/10.3390/app132312702

APA Style

Zhang, Y., Xie, X., Zheng, Z., He, X., Du, P., Zhang, R., Guo, L., & Huang, K. (2023). Manganese-Cobalt Spinel Nanoparticles Anchored on Carbon Nanotubes as Bi-Functional Catalysts for Oxygen Reduction and Oxygen Evolution Reactions. Applied Sciences, 13(23), 12702. https://doi.org/10.3390/app132312702

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