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Article

Sodium Borohydride Treatment to Prepare Manganese Oxides with Oxygen Vacancy Defects for Efficient Oxygen Reduction

by
Shuo Sun
,
Haoran Yu
,
Lanlan Li
,
Xiaofei Yu
,
Xinghua Zhang
,
Zunming Lu
and
Xiaojing Yang
*
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China
*
Author to whom correspondence should be addressed.
Metals 2022, 12(7), 1059; https://doi.org/10.3390/met12071059
Submission received: 20 May 2022 / Revised: 14 June 2022 / Accepted: 20 June 2022 / Published: 21 June 2022
(This article belongs to the Special Issue Advancements on Functional Catalytic Materials with Noble-Metal-Like Characters)
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Abstract

:
Manganese oxides are often used as catalysts for oxygen reduction reactions due to their low price and high stability, and they have been extensively studied. However, the poor electrical conductivity and low intrinsic activity of manganese oxides restrict its application in oxygen reduction. In this paper, the manganese oxide octahedral molecular sieve is used as the research object, and the oxygen reduction performance of the material is adjusted by the surface reduction etching treatment of sodium borohydride. After being treated with 8 mmol/L sodium borohydride, the oxygen vacancy content of the manganese oxide octahedral molecular sieve was 26%. The manganese oxide octahedral molecular sieve showed the best performance, and its half-wave potential was 0.821 V. Tests show that the material has excellent electrical conductivity and high oxygen reduction kinetics. The generation of appropriate oxygen vacancies on the surface directly improves the chemical properties of the material surface, regulates the ratio of Mn3+/Mn4+ on the surface of the nanorod, and increases the oxygen reduction adsorption sites on the surface of the material. On the other hand, the electrical conductivity of the material is adjusted to increase the electron transfer rate during the oxygen reduction process, thereby enhancing the oxygen reduction activity.

1. Introduction

Today, when traditional energy is facing a crisis, the development and application of fuel cells is imminent, and a major problem that restricts the application of fuel cells is cost [1,2,3]. Cathode oxygen reduction reaction in fuel cells is restricted by multi-stage reactions, and the performance and stability need to be improved [4,5]. At the same time, platinum-based noble metal catalysts also have high-cost problems as oxygen reduction catalysts [6,7,8]. To solve these series of problems, non-precious metal catalysts have become the research object preferred by everyone [9,10,11,12]. Manganese oxides have attracted extensive attention due to their variable valence states, multiple crystal forms, abundant reserves, and low toxicity [13,14,15,16]. The valence state and crystal form of manganese oxides are variable, and the electronic state of manganese oxides greatly affects its electrochemical performance [17]. Among many manganese oxides, MnO2 has the most abundant crystal form and has the most research value [18,19]. Studies have shown that δ- < β- < amorphous- < α-MnO2-SF, α-MnO2 has the best activity among many crystal forms [20]. Excellent activity and variable structure make it valuable for research.
The factors limiting the catalytic activity of manganese oxides are mainly oxygen adsorption capacity and electrical conductivity. The oxygen reduction process involves interaction with oxygen and oxygen-containing intermediates. To promote the rapid progress of the oxygen reduction reaction, the manganese site needs to have a suitable adsorption force with oxygen to ensure the rapid adsorption and desorption of oxygen during the reaction process. The transfer of electrons is involved in the oxygen reduction process. As a semiconductor, manganese oxide itself has poor conductivity, which inhibits its catalytic activity. Studies have found that the conductivity of the catalyst can be enhanced by doping and compounding with other substances, thereby enhancing the catalytic activity. However, among many regulation methods, the method of oxygen vacancy regulation is the most simple and operable. Studies have shown that by constructing a new oxygen vacancy defect structure [21,22,23], on the one hand, the electronic distribution of the original material can be broken and the electronic structure of the material can be improved, thereby regulating the catalytic performance. On the other hand, the existence of an oxygen vacancy defect structure can regulate the intrinsic conductivity of the material, promote electron transfer, and then improve the catalytic activity. The existence of surface oxygen vacancy defect structures can effectively improve the chemical environment of the material surface, thereby regulating the adsorption of the active sites on the catalyst surface and oxygen intermediates [24]. Therefore, it has great potential to construct oxygen vacancy defect structures to improve the catalytic performance of materials [25]. The beneficial effect of oxygen vacancies on the catalytic activity of manganese oxides was confirmed by DFT [26]. Both Cao and Wei et al. enhanced the photocatalytic efficiency by regulating the bulk and surface oxygen vacancy content in titania [27,28]. Numerous studies have shown that oxygen vacancies play a positive role in the electrocatalytic performance of manganese oxides [29,30]. However, the generation of oxygen vacancies and the catalytic mechanism still need further research.
The research in this work is aimed at preparing high activity and high stability oxygen-containing vacancy materials, adjusting the oxygen vacancy content of manganese oxide octahedral molecular sieve materials, and regulating the electrocatalytic performance. The oxygen reduction mechanism of the NaBH4-OMS-2 catalyst, the relationship between oxygen vacancy content and catalytic performance were further explored. In the experiment, sodium borohydride was selected as the etchant, aiming to change the surface structure of nanorods and controllably generate oxygen vacancies through the reduction and etching effect of sodium borohydride on the surface of nanorods. The content of oxygen vacancies was regulated by controlling the concentration of sodium borohydride. This controllable, simple and operable method was used to obtain oxygen vacancies with different contents, so as to further explore the relationship between oxygen vacancy content and electrocatalytic performance.

2. Experimental Section

Synthesis of OMS-2 Nanorods: The preparation of OMS-2 is carried out by a typical hydrothermal method. First, 0.10 g of manganese sulfate and 0.24 g of potassium permanganate need to be dissolved, mixed and stirred for 30 min, and then react in an oven at 140 °C for 24 h. After the hydrothermal reaction was completed, the powder sample obtained by the reaction was taken out and washed. Finally, the OMS-2 sample was obtained by drying at 80 °C overnight.
Synthesis of NaBH4-OMS-2 Nanorods: Take 50 mg of the prepared OMS-2 sample and treat it with 20 mL of sodium borohydride solutions of different concentrations (4 mmol/L, 8 mmol/L, 12 mmol/L, 15 mmol/L), and place them on a stirring table. The reaction was carried out for 30 min, followed immediately by centrifugation. After separating the catalyst samples, further washing and centrifugation operations are required. After washing 5–6 times, the samples are placed in an oven for drying treatment, and the temperature is set to 60 °C to obtain the NaBH4-OMS-2 catalyst. According to the concentration of sodium borohydride from small to large, the samples were labeled as 1NaBH4-OMS-2, 2NaBH4-OMS-2, 3NaBH4-OMS-2, 4NaBH4-OMS-2 in turn.
Structural characterization: The instrument used for XRD was Bruker D8 Discover, Cu Kα (λ = 1.5406 Å) radiation, and the manufacturer of the instrument was Bruker, Germany. The catalysts were characterized by SEM using a scanning electron microscope of Quanta 450FEG of FEI Company. The material was characterized by a Talos F200S transmission electron microscope from FEI. The chemical state of the material was characterized by ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS).
Electrochemical measurements: The electrochemical performance test used a CHI760E electrochemical workstation and a ALS RRDE-3A rotating disk electrode. The electrochemical workstation three-electrode system was used for testing. The saturated calomel electrode is the reference electrode, the platinum wire is the counter electrode, and the glassy carbon electrode is the working electrode. The catalyst sample and XC-72 were added in a ratio of 3:2, 655 μL isopropanol, 325 μL deionized water and 20 μL Nation solution were added, and the catalyst ink was obtained by sonicating for 30 min. 3 μL of ink was dropped onto the working electrode for further electrocatalytic testing.
Cyclic voltammetry (CV) and the linear sweep voltammetry (LSV) tests were performed at potential ranges of −1 V and 0.2 V (vs. SCE). The upper and lower limits of the electrochemical impedance spectroscopy (EIS) test frequency were set to 0.1 Hz and 100 kHz, respectively, and the voltage was −0.23 V (vs. SCE). The Mott-Schottky test needs to set the open-circuit voltage plus or minus 1.0 V range for testing, set the potential increment to 0.01 V s−1, and the frequency to 1000 Hz. The voltage set for the i-t test was −0.5 V (vs. SCE).
The formula for calculating the Koutechy–Levich curve is as follows [31]:
i−1 = ik−1 + id−1
id = 0.62nFAC0DO22/3ν−1/6ω1/2
where i, ik, and id are the actual, kinetic and diffusion currents, respectively. F, C0, DO2, v, ω are Faraday’s constant, O2 bulk concentration, diffusion coefficient of O2 in electrolyte, electrolyte dynamic viscosity, and rotational angular velocity, respectively.

3. Results and Discussion

3.1. Structural Analysis

Figure 1 shows the XRD patterns of OMS-2 and OMS-2 treated with NaBH4 concentrations of 4 mmol/L, 8 mmol/L, 12 mmol/L and 15 mmol/L. The sodium borohydride solution here is alkaline, because H− in BH4− combines with H+ to generate H2 in an acidic medium. The reaction of H+ and H− is affected by pH. The alkaline medium contains a large amount of OH−, while the H in NaBH4 is −1, which can only be used as a reducing agent. It can be seen that OMS-2 has a 2 × 2 tunnel structure. It can correspond to the standard card of α-MnO2 (JCPDS NO.44-0141, the space group is I4/m), indicating that OMS-2 can be prepared by this simple hydrothermal method [32]. Analysis of the spectrum can be obtained, and the xNaBH4-OMS-2 obtained by different concentrations of the sodium borohydride treatment all maintain the structure of manganese oxide octahedral molecular sieve. Through the comparison of diffraction peaks 12.78°, 18.10°, 28.84°, 37.52°, 49.86°, etc., all of them match the crystals of (110), (200), (310), (211), (411) crystal plane of α-MnO2. The diffraction peak intensities of 1NaBH4-OMS-2, 2NaBH4-OMS-2 and 3NaBH4-OMS-2 decreased slowly after the sodium borohydride treatment, while the diffraction peak intensity of 4NaBH4-OMS-2 decreased significantly. This is because the sodium borohydride concentration is too high, resulting in serious damage to the structure of the surface of the material, so the crystallinity of the material is greatly reduced.
The microscopic morphologies of OMS-2 and NaBH4-OMS-2 can be characterized by scanning electron microscopy. Figure 2a,b shows the SEM images of OMS-2, showing nanorod-like morphology. OMS-2 has a diameter range of 30–90 nm and a relatively uniform length of about 500 nm. At the same time, the surface of the material is uniform and the crystallinity is high. Figure 2d is the SEM image of the 2NaBH4-OMS-2 sample. The sample treated with sodium borohydride maintains the nanorod shape as a whole, and the diameter and length are not much different from those before treatment, and there is no complete breakage. But a layered structure appeared on the surface of the nanorods, and the tips were thinned, as indicated by the yellow circles. To further illustrate the grain size and particle size of the material, XRD grain size analysis and TEM grain size distribution analysis were carried out. The XRD data were processed by Jade software and Scherrer formula, and the grain size of the obtained material was 17.0 nm. The TEM particle size distribution is shown in Figure S1. The particle size distribution map reflects the diameter distribution of the material between 30–90 nm, with less small size distribution. But the surface of the nanorods is no longer flat. The 2NaBH4-OMS-2 sample was further magnified and observed, as shown in Figure 2e. The treatment of sodium borohydride made the surface of the nanorods no longer smooth, a layered structure appeared, and the tips were thinned. At the same time, concave structures appear on the surface, which are all due to the reduction treatment of sodium borohydride. The composition of OMS-2 did not change before and after the sodium borohydride treatment. Figure S2 shows the EDS spectrum of 2NaBH4-OMS-2, and no other impurity elements appear in the sample after treatment. This is due to the short treatment time of sodium borohydride and the sufficient centrifugal washing after treatment.
Figure 2c,f and Figure S3 are the TEM and HRTEM spectra of OMS-2 and 2NaBH4-OMS-2, respectively. Compared with OMS-2, the surface of 2NaBH4-OMS-2 is uneven. It can be clearly seen that the surface of nanorods is partially thinned due to the existence of defects, especially the tip is more obvious, and the overall thickness is uneven. For nanorods, the tip energy is higher and more fragile, so the thinning effect is more pronounced. The thinning also starts from a certain point in the middle and expands to the front and back of the nanorod based on this point, so the layered structure on the nanorod appears. When the expansion is not complete, it will leave a fine block structure on the surface of the nanorod. Further observing the structural information of the nanorods, the measured lattice spacing is about 0.304 nm, which corresponds to the (310) crystal plane of the manganese oxide octahedral molecular sieve.
The morphologies of the OMS-2 samples treated with different concentrations of sodium borohydride are shown in Figure 3. The surface of the nanorods after the reduction treatment is no longer smooth, and there is a block structure and different degrees of thinning. With the increase in the sodium borohydride concentration, the surface reduction degree of nanorods intensified, and the number of bulk structures increased and became more finely divided. Specifically, the nanorods in the 1NaBH4-OMS-2, 2NaBH4-OMS-2 and 3NaBH4-OMS-2 samples are only affected by the surface, and the length of the nanorods has little effect. But for the 4NaBH4-OMS-2 sample, because the sodium borohydride concentration is too high, the fragmentation degree is more, and the nanorod length is reduced. This corresponds to the XRD results, which together indicate that the poor crystallinity of the material is due to the high degree of surface reduction in the nanorods. A certain concentration of sodium borohydride can cause oxygen vacancy defects on the surface of the nanorods, and the nanorod structure itself is basically unaffected. However, when the concentration of sodium borohydride is too high, the surface structure of the nanorods will be significantly broken, and the nanorods will be thinned and broken.
Due to the reduction effect of sodium borohydride, low-valence manganese is generated on the surface of the material, and oxygen vacancies are generated according to the principle of electrostatic equilibrium. Oxygen vacancies act as cations, balancing the surface charge of the material. Tests show that the sodium borohydride treatment is a reliable method to improve the surface structure of the materials and generate oxygen vacancies (Figure S4). Figure 4a is the XPS spectrum of O 1s; the peaks at the binding energies of 531.6 eV and 529.9 eV are attributed to oxygen vacancies (O2) and lattice oxygen (O1), respectively. With the increase in the sodium borohydride concentration, the oxygen vacancy content increased significantly. It can be seen from Figure 4b that the distance between the two peaks of Mn 2p1/2 and Mn 2p3/2 is 11.7 eV, indicating that the manganese in the manganese oxide before and after the treatment exists in both trivalent and tetravalent forms. However, to accurately determine the specific valence state of manganese in the material, it is also necessary to combine the XPS peak analysis. The 2p3/2 and 2p1/2 spin-orbit peaks of Mn3+ and the 2p3/2 and 2p1/2 spin-orbit peaks of Mn4+ were further identified, corresponding to 641.2 eV, 653.2 eV, 642.4 eV and 654.1 eV, respectively. With the increase in the treatment concentration, the content of Mn3+ increases gradually, and the content of Mn3+/Mn4+ also increases accordingly. To more clearly characterize the change of oxygen vacancies, the relative content of oxygen vacancies was calculated (Figure 4c). When the concentration of sodium borohydride is too high, the destruction of the surface structure of the sample results in no further increase in the content of oxygen vacancies.
It is preliminarily speculated from the charge balance inside the material that the change of oxygen vacancy content is related to the change of Mn3+ content, and the Mn3+ content will change regularly. Therefore, the corresponding data are displayed, and the relative content changes of Mn3+ and Mn4+ are shown in Figure 4d. The relative content of Mn3+ gradually increased from 36% to 56%, and the growth rate also showed a trend of first fast and then slow, while the corresponding relative content of Mn4+ also gradually decreased from 64% to 44%. The changing laws of the relative content of oxygen vacancies and Mn3+ prove each other, which together indicate that the concentration of sodium borohydride is an important factor affecting the occurrence of oxygen vacancies in the samples. During the oxygen reduction process, oxygen vacancies and changes in the relative content of Mn3+ play a crucial role, thus having an important impact on the oxygen reduction performance.

3.2. Electrocatalytic Performance

To further explore the effect of different oxygen vacancy defects on the performance of the catalyst, the electrochemical performance of the catalyst was evaluated. From the cyclic voltammetry curves (Figure S5), it can be seen that several samples all have oxygen reduction peaks, indicating that they all have oxygen reduction activities, but there are differences in the positions of the oxygen reduction peaks. It is believed that the corresponding potential of the oxygen reduction peak in the CV diagram has an inseparable relationship with the oxygen reduction performance. The more positive the oxygen reduction peak potential is, the better the oxygen reduction performance. It was observed that the linear sweep voltammetry (LSV) exhibited a standard S-shaped curve (Figure 5a). Figure 5b is a partially enlarged view of the LSV curve. It can be judged from the onset potential region that 2NaBH4-OMS-2 has the most positive onset potential (0.920 V). The onset potential is the basis for judging the performance of the catalyst. It is generally believed that the more positive the onset potential, the better the catalytic performance. It is preliminarily judged that the oxygen reduction performance of the material shows an increasing trend from OMS-2 to 2NaBH4-OMS-2, and a decreasing trend from 2NaBH4-OMS-2 to 4NaBH4-OMS-2. In addition, the 4NaBH4-OMS-2 sample exhibits a lower potential than OMS-2, and its performance is somewhat reduced.
The half-wave potential (E1/2) of the 2NaBH4-OMS-2 sample is significantly improved. The E1/2 of 2NaBH4-OMS-2 is 0.821 V, while the E1/2 of OMS-2 is 0.750 V, which is a 71 mV improvement in performance, which is attributed to the modulation of oxygen vacancies. In samples treated with different concentrations of sodium borohydride, both 1NaBH4-OMS-2 and 3NaBH4-OMS-2 showed an increase in half-wave potential. The half-wave potential of the 4NaBH4-OMS-2 sample is 9 mV lower than that of OMS-2, indicating that the excessive reduction in sodium borohydride is not conducive to the improvement of the oxygen reduction performance of the material. Figure 5c is the Tafel curve of the catalyst. The Tafel slope of 4NaBH4-OMS-2 is larger than that of OMS-2, while the other samples are opposite. The Tafel slope can reflect the catalytic kinetics. The results show that the excessive oxygen vacancy content is not conducive to the catalytic process, so it has poor catalytic kinetics, while the moderate oxygen vacancy content is of great significance to improve the catalytic performance.
The apparent electron transfer number of the catalyst is also a major factor affecting the electrochemical performance of the catalyst. To further characterize the electron transfer number of the oxygen reduction process, RRDE tests were carried out. Figure 5d shows the electron transfer number (n) and hydrogen peroxide yield (H2O2%) of the catalyst. n and H2O2% are not fixed but are in a dynamic process with the change of voltage, so the average n and H2O2% at 0.4–0.9 V potential are calculated. As shown in Figure S6, the average n of 2NaBH4-OMS-2 is 3.91, which is much higher than that of OMS-2 (n = 3.77), and it has a lower H2O2% value, showing excellent electrochemical performance. In addition, n can also be calculated from the K-L curve during the catalyst oxygen reduction process (Figure S7). The electron transfer numbers of OMS-2 and 2NaBH4-OMS-2 were obtained to be 3.76 and 3.92, respectively, which further indicated that the treated samples had excellent 4e− transfer. The close to 4e− transfer process reflects the excellent oxygen reduction performance of the material.
The quality and morphology of the material itself will affect the number of active sites exposed by the material, which in turn affects the activity of the material. To eliminate the problem of performance differences caused by the quality and area of the material, the specific activity of the material is further calculated. Among them, the means of evaluating the active area in the calculation of the area ratio activity are not the same, and each calculation method has disadvantages. Therefore, two methods, ECSA and BET, are used for comparative calculation [33,34]. Figure S8 is the CV test chart of different scan speeds, and the Cdl and ECSA of different materials are obtained by calculation, as shown in Table S1. The ECSA has the same trend as the performance change, reflecting the excellent catalytic performance of the catalyst. The area specific activity was further calculated by BET, and it can be seen from Figure 5e that both the mass specific activity and the area specific activity obey the same performance trend as LSV (Table S2). It is worth noting that the 2NaBH4-OMS-2 sample exhibits excellent mass specific activity and area specific activity, which fully demonstrates the great feasibility of preparing high-performance catalysts by adjusting the content of oxygen vacancies [35].
After the 20,000 s i-t test, the retention rate of the catalyst current density is as high as 89% (Figure 5f). The material structure did not change before and after the test, further illustrating the excellent stability of the 2NaBH4-OMS-2 catalyst (Figure S9). In the chronoamperometry test, an 1 M methanol solution was injected for 800 s (Figure S10). After that, the current density curve showed slight fluctuations and then recovered quickly without being affected by methanol poisoning, indicating that the 2NaBH4-OMS-2 catalyst has excellent methanol resistance. The excellent stability and methanol resistance of the 2NaBH4-OMS-2 sample is attributed to the unique stability of the nanorod-like structure. The tunnel structure of OMS-2 binds the internal ions, so that the ions can exist stably inside the material. At the same time, the interconnected structure of octahedron makes the material more stable, so NaBH4-OMS-2 shows better stability.

3.3. Catalytic Mechanism Research

OMS-2 is a non-stoichiometric oxide with a nanocages-like structure. The MnO6 octahedral units shared by the double-strand edges on the boundary are ordered and packed, and the hollow interior is a 2 × 2 (0.46 nm × 0.46 nm) one-dimensional tunnel structure, which can serve as a reservoir for ions. This unique inorganic system can trap other ions, leading to new physical and chemical properties. Potassium ions are located inside the channels of OMS-2, similar to the sodium ions or calcium ions in the channels built by SiO4 tetrahedra in the zeolite molecular sieve structure. In addition to compensating for the charge imbalance caused by the mixing of Mn4+ and Mn3+ in the tunnel, potassium ions also play a guiding role in maintaining the pore structure and size during the formation of octahedral molecular sieves.
The 2NaBH4-OMS-2 catalyst sample has excellent oxygen reduction performance, indicating that the sodium borohydride treatment plays an important role. The schematic diagram of the formation of oxygen vacancies is shown in Figure 6. For transition metal oxides, the electrocatalytic performance largely depends on the oxygen vacancy content. The generation of oxygen vacancies modulates the electronic structure and surface chemistry of the catalyst, so the effect of oxygen vacancies on performance will be explored from these two aspects.
The existence of oxygen vacancies modulates the electronic structure of the catalyst material, and it also affects the electrical conductivity of the material. Figure 7a shows the electrochemical impedance spectroscopy (EIS) of the catalyst. As the content of oxygen vacancies increases, the radius of the low-frequency region first decreases and then increases. An appropriate amount of oxygen vacancies can adjust the resistance of the material itself to a certain extent, which can promote the transport of electrons in the oxygen reduction process and improve the oxygen reduction activity. To further explore the change in conductivity, the material was tested with Mott-Schottky (M-S) curves (Figure 7b). It can be judged that the material is an n-type semiconductor by the positive slope of the M-S curve. The lower slope of the curve reflects the higher carrier concentration of the material, and 2NaBH4-OMS-2 also has a higher carrier concentration. The appropriate surface oxygen vacancy content adjusts the electronic structure of the surface, makes the electron transfer in the oxygen reduction process more rapid, reduces the transport resistance, and promotes the efficient and rapid oxygen reduction.
The defect treatment of the catalyst mainly occurs on the surface of the material, and the formed oxygen vacancy defect structure is dominated by surface oxygen vacancies. Surface oxygen vacancies will have an impact on the chemical properties of the catalyst surface. According to research, the oxygen vacancies on the catalyst surface change the element valence state and the number of active sites on the material surface. It can even exist as an active site for the oxygen reduction reaction and become a site for oxygen adsorption. Oxygen vacancies on the surface of the catalyst affect the manganese active sites on the surface of the nanorods, which play a role in adjusting the electronic structure of manganese elements, and then adjust the adsorption capacity of manganese active sites and oxygen reaction intermediates. At the same time, oxygen vacancies and manganese ion sites act synergistically as active sites to promote the activity of the oxygen reduction reaction.
Figure 7c shows the relationship between the oxygen vacancies and the oxygen reduction performance. As the content of generated oxygen vacancies increases, the performance first increases and then decreases, indicating that the catalytic performance is closely related to the content of oxygen vacancies. The formation of oxygen vacancies is generally accompanied by changes in the valence of other elements. During the formation of the oxygen vacancies, in order to maintain the charge balance on the surface of the material, the valence of Mn will change accordingly, and the electronic structure will be adjusted at the same time.
Figure 7d shows the relationship between oxygen vacancies and Mn3+ content. As the oxygen vacancy content increases, the Mn3+ content also increases gradually, and the two show a linear relationship, which is attributed to electrostatic equilibrium. Oxygen vacancies can be regarded as “cations”. To balance the generated “cations”, the tetravalent manganese element will gain electrons and become trivalent, so the corresponding Mn3+ content gradually increases with the increase in oxygen vacancy content. When the oxygen vacancy content is 26%, the value of Mn3+/Mn4+ is closer to 1. At this time, the Mn3+/Mn4+ redox couple value of the catalyst is the largest, and the catalyst has the most excellent catalytic performance. Some studies have shown that Mn4+ can promote the decomposition of peroxides and promote the oxygen reduction process through 2e− +2e− pathway, while the existence of pure Mn4+ is unfavorable for the oxygen reduction process. The coexistence of two valence states of Mn3+ and Mn4+ greatly promotes the oxygen reduction activity.
The high catalytic activity of Mn3+ is attributed to the fact that the trivalent manganese ion has electrons in the anti-bonding orbital, that is, the eg orbital is filled with electrons, which makes the active center of manganese and the reaction intermediate have proper adsorption during the catalytic process, which is beneficial to the oxygen reduction reaction [36]. At the same time, the filling of antibonding orbital electrons can lead to the Jahn-Teller (J-T) distortion in MnO6, which promotes the oxygen reduction activity. The oxygen reduction process mainly depends on the adsorption and desorption of oxygen species, so it is very important to adjust the electronic structure of the material. Figure 7e,f shows the oxygen reduction mechanism of manganese and the structure of OMS-2. The oxygen reduction process experienced four processes: O2 was adsorbed on the manganese site in the form of OO2−, protonated to OOH−, OOH− was decomposed to leave O2−, and re-protonated to OH−. The first step is considered to be the rate-controlling step that limits the oxygen reduction process, while Mn3+ with eg electrons transfers the electrons axially to oxygen, which promotes the kinetic exchange between O2 and OH−. This would improve covalentity, facilitate the exchange of manganese sites with oxygen intermediates, and promote oxygen reduction activity.
We further performed DFT simulation calculations to illustrate the role of oxygen vacancies more clearly. Figure 8 shows the optimized geometric structure model and energy band structure diagram of OMS-2 and OVs-OMS-2 corresponding to the oxygen vacancy content of 2NaBH4-OMS-2. It can be observed from the structure diagram that with the increase in oxygen vacancy content, the valence state of manganese in the material also changes accordingly. It is worth noting that when the content of oxygen vacancies is too high, the material cannot obtain a stable structure, which is consistent with the experimental results. When there are too many oxygen vacancies, the structural distortion of the material is obvious, so it cannot exist stably. From the energy band structure diagram, it can be determined that the oxygen vacancy content has a significant effect on the electrical conductivity of the material. The calculated band gap to OMS-2 is 0.45 eV. The band gap decreases significantly with increasing oxygen vacancy content. It is shown that the appropriate content of oxygen vacancies can effectively increase the electrical conductivity of the material, thereby affecting the electron transfer process in the oxygen reduction process and promoting the oxygen reduction process.

4. Conclusions

This study demonstrates a method to prepare OMS-2 with appropriate content of oxygen vacancies by NaBH4 treatment. Notably, this electrocatalyst exhibits excellent electrocatalytic performance at an oxygen vacancy content of 26%. The half-wave potential is as high as 0.821 V (vs. RHE) and has good stability. Oxygen vacancies play two roles in the oxygen reduction process. On the one hand, it modulates the electronic structure of the catalyst, modulates the ratio of Mn3+/Mn4+, improves the electron transfer rate during the oxygen reduction process, and optimizes the 4e− pathway of the oxygen reduction reaction, which promotes an efficient and rapid oxygen reduction process; on the other hand, it optimizes the chemical properties of the catalyst surface and improves the conductivity of the material. The generation of surface oxygen vacancies affects the element valence and the number of active sites on the catalyst surface, adjusts the adsorption capacity of manganese sites and oxygen reaction intermediates, and cooperates with manganese sites to increase the number of oxygen adsorption sites in the oxygen reduction process to promote the oxygen reduction reaction. This simple strategy of preparing catalysts with high oxygen reduction activity by manipulating oxygen vacancies has high application prospects.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/met12071059/s1, Figure S1: TEM spectrum and particle size distribution of 2NaBH4-OMS-2; Figure S2: EDS spectra of 2NaBH4-OMS-2; Figure S3: (a) TEM images of OMS-2 and (b) 2NaBH4-OMS-2; Figure S4: XPS images of (a) O 1s and (b) Mn 2p of OMS-2 and 2NaBH4-OMS-2; Figure S5: Cyclic voltammetry curves of OMS-2 and OMS-2 treated with different concentrations of NaBH4; Figure S6: Histogram of average electron transfer number and hydrogen peroxide yield at 0.4 V–0.9 V potential; Figure S7: LSV curves of (a,b) OMS-2 and (c,d) 2NaBH4-OMS-2 at different rotational speeds and linear plots at different potentials; Figure S8: CV plots at different scan rates and Cdl slope plots of electric double layer capacitance of (a,d) OMS-2, (b,e) 1NaBH4-OMS-2, (c,f) 2NaBH4-OMS-2, (g,i) 3NaBH4-OMS-2, (h,j) 4NaBH4-OMS-2; Figure S9: XRD patterns before and after testing; Figure S10: i-t with methanol added of 2NaBH4-OMS-2; Table S1: Cdl and ECSA of OMS-2 and OMS-2 treated with different concentrations of NaBH4; Table S2: Summary table of performance test of OMS-2 and OMS-2 treated with different concentrations of NaBH4.

Author Contributions

Conceptualization, S.S. and H.Y.; methodology, L.L.; software, X.Y. (Xiaofei Yu); validation, S.S., X.Z. and Z.L.; formal analysis, S.S.; investigation, X.Y. (Xiaofei Yu); resources, X.Y. (Xiaojing Yang); data curation, S.S.; writing—original draft preparation, S.S.; writing—review and editing, X.Y. (Xiaojing Yang). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 51871088, 22179032, 51771068, 51771067), and the Natural Science Foundation of Hebei Province (No. B2021202011, E2021202022).

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD patterns of OMS-2 and OMS-2 treated with different concentrations of NaBH4.
Figure 1. XRD patterns of OMS-2 and OMS-2 treated with different concentrations of NaBH4.
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Figure 2. SEM images of (a,b) OMS-2 and (d,e) 2NaBH4-OMS-2, high resolution TEM images of (c) OMS-2 and (f) 2NaBH4-OMS-2.
Figure 2. SEM images of (a,b) OMS-2 and (d,e) 2NaBH4-OMS-2, high resolution TEM images of (c) OMS-2 and (f) 2NaBH4-OMS-2.
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Figure 3. SEM images of (a) 1NaBH4-OMS-2, (b) 2NaBH4-OMS-2, (c) 3NaBH4-OMS-2, (d) 4NaBH4-OMS-2.
Figure 3. SEM images of (a) 1NaBH4-OMS-2, (b) 2NaBH4-OMS-2, (c) 3NaBH4-OMS-2, (d) 4NaBH4-OMS-2.
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Figure 4. XPS images of (a) O 1s and (b) Mn 2p; (c) relative contents of oxygen vacancies and lattice oxygen; (d) relative contents of Mn3+ and Mn4+ of OMS-2 treated with different concentrations of NaBH4.
Figure 4. XPS images of (a) O 1s and (b) Mn 2p; (c) relative contents of oxygen vacancies and lattice oxygen; (d) relative contents of Mn3+ and Mn4+ of OMS-2 treated with different concentrations of NaBH4.
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Figure 5. (a,b) Linear sweep voltammetry curves (1600 rpm and 0.005 V s−1), (c) Tafel plot, (d) Electron transfer number and hydrogen peroxide yield, (e) mass specific activity and area specific activity at 0.77 V potential of OMS-2 and OMS-2 treated with different concentrations of NaBH4, (f) i-t of 2NaBH4-OMS-2 at 0.5 V potential.
Figure 5. (a,b) Linear sweep voltammetry curves (1600 rpm and 0.005 V s−1), (c) Tafel plot, (d) Electron transfer number and hydrogen peroxide yield, (e) mass specific activity and area specific activity at 0.77 V potential of OMS-2 and OMS-2 treated with different concentrations of NaBH4, (f) i-t of 2NaBH4-OMS-2 at 0.5 V potential.
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Figure 6. Schematic diagram of the formation of oxygen vacancies by NaBH4 treatment.
Figure 6. Schematic diagram of the formation of oxygen vacancies by NaBH4 treatment.
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Figure 7. (a) Electrochemical impedance spectra; (b) Mott-Schottky curves of OMS-2 and OMS-2 treated with different concentrations of NaBH4, (c) Relationship between oxygen vacancy and electrocatalytic performance; (d) relationship between oxygen vacancy and Mn3+ content and Mn3+/Mn4+ content, (e) Schematic diagram of oxygen reduction mechanism of manganese (Arabic numerals represent steps, Roman numerals represent manganese valence states); (f) schematic diagram of oxygen reduction structure of OMS-2.
Figure 7. (a) Electrochemical impedance spectra; (b) Mott-Schottky curves of OMS-2 and OMS-2 treated with different concentrations of NaBH4, (c) Relationship between oxygen vacancy and electrocatalytic performance; (d) relationship between oxygen vacancy and Mn3+ content and Mn3+/Mn4+ content, (e) Schematic diagram of oxygen reduction mechanism of manganese (Arabic numerals represent steps, Roman numerals represent manganese valence states); (f) schematic diagram of oxygen reduction structure of OMS-2.
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Figure 8. Optimized structure diagram (left) and band structure diagram (right) of (a) OMS-2, (b) OVs-OMS-2.
Figure 8. Optimized structure diagram (left) and band structure diagram (right) of (a) OMS-2, (b) OVs-OMS-2.
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Sun, S.; Yu, H.; Li, L.; Yu, X.; Zhang, X.; Lu, Z.; Yang, X. Sodium Borohydride Treatment to Prepare Manganese Oxides with Oxygen Vacancy Defects for Efficient Oxygen Reduction. Metals 2022, 12, 1059. https://doi.org/10.3390/met12071059

AMA Style

Sun S, Yu H, Li L, Yu X, Zhang X, Lu Z, Yang X. Sodium Borohydride Treatment to Prepare Manganese Oxides with Oxygen Vacancy Defects for Efficient Oxygen Reduction. Metals. 2022; 12(7):1059. https://doi.org/10.3390/met12071059

Chicago/Turabian Style

Sun, Shuo, Haoran Yu, Lanlan Li, Xiaofei Yu, Xinghua Zhang, Zunming Lu, and Xiaojing Yang. 2022. "Sodium Borohydride Treatment to Prepare Manganese Oxides with Oxygen Vacancy Defects for Efficient Oxygen Reduction" Metals 12, no. 7: 1059. https://doi.org/10.3390/met12071059

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