High-Efficiency Oxygen Reduction to Hydrogen Peroxide Catalyzed by Oxidized Mo₂TiC₂ MXene

Abstract

The two-electron oxygen reduction reaction (2e⁻ORR) pathway electrochemical synthesis to H₂O₂ has the advantages of low investment and environmental protection and is considered to be a promising green method. Herein, the oxidized Mo₂TiC₂ MXene (O-Mo₂TiC₂) was successfully synthesized by a facile hydrothermal method as an electrocatalyst in electrocatalytic H₂O₂ production. The O-Mo₂TiC₂ achieved the 90% of H₂O₂ selectivity and 0.72 V vs. RHE of the onset potential. Moreover, O-Mo₂TiC₂ showed high charge transfer ability and long-term stable working ability of 40 h. This significantly enhanced electrocatalytic H₂O₂ production capacity is assigned the oxidation treatment of Mo₂TiC₂ MXene to generate more oxygen-containing groups in O-Mo₂TiC₂. This work provides a promising catalyst candidate for the electrochemical synthesis of H₂O₂.

1. Introduction

Hydrogen peroxide (H₂O₂) is a green and crucial oxidant that has been widely used in the chemical industry, environmental remediation, and textile manufacturing [1,2,3]. Currently, the industrial production of H₂O₂ mainly relies on the energy-intensive anthraquinone technology, which is a multi-step process that not only requires complex and large-scale facilities but also generates large amounts of waste chemicals [4,5,6]. In addition, the high-concentration H₂O₂ may pose high cost and safety issues during storage and transportation [7,8,9,10]. In fact, in most applications, only diluted H₂O₂ (0.1–3% g∙L⁻¹) is required [11,12,13]. For these reasons, it is an emerging trend to develop an energy-efficient route that reduces the cost of H₂O₂ synthesis, storage, and transportation [14,15,16,17]. Recently, the electrochemical generation of H₂O₂ from the oxygen reduction reaction (ORR) via a 2e⁻ transfer has attracted the attention of the academic community [18,19,20,21]. Besides, the electrochemical generation of H₂O from the ORR via a 4e⁻ transfer is the crucial pathway in fuel cell applications and metal-air batteries [22,23,24,25,26]. Therefore, the development of highly selective and performance 2e⁻ ORR electrocatalysts is the prerequisite for producing H₂O₂ [27,28,29].

For this purpose, noble metal and alloy catalysts have been verified to have high ORR activity and H₂O₂ selectivity, such as Pt, Pd, and Au-based catalysts [6,30,31]. However, their high cost and scarcity hinder their large-scale applications [32,33]. So far, two-dimensional (2D) carbon-based materials have shown good performance in the electrochemical synthesis of H₂O₂ due to their abundant reserve, tunable electronic structures, and composition versatility [34,35,36,37]. Recently, researchers have been developing other potential electrocatalysts, such as MXenes, which are two-dimensional metal carbides or nitrides [38,39,40]. MXenes are considered promising catalysts for the generation of H₂O₂ via 2e⁻ transfer [41,42,43,44,45].

MXenes have recently attracted great attention in the field of electrocatalysis due to their tunable composition and excellent chemical properties [35,46,47]. For instance, Yury et al. used Mo₂TiC₂Tx to support Pt single atom, showing excellent hydrogen evolution reaction (HER) performance [48]. Additionally, Xiao Huang et al. prepared Ti₃C₂T_x, V₂CT_x, and Nb₂CT_x for H₂O₂ electrosynthesis and found that MXenes are inherent 2e⁻ORR catalysts with high H₂O₂ selectivity [42]. Not only that, MXenes can be used in other applications. For instance, Tang et al. found that the MXenes and their fluorinated/hydroxylated derivative materials were advantageous materials for Li-ion battery applications [49]. Additionally, Xu et al. also developed a strategy to prepare rGO/Ti₃C₂Tx electrodes using Ti₃C₂Tx as the active conductive binder between rGO nanosheets [50]. Apart from this, Li et al. reported MXene quantum dots and graphitic carbon nitride nanosheets for the preparation of heterostructured g-C₃N₄@Ti₃C₂ quantum dots, which showed improved photocatalytic ability [51]. It can be seen that MXenes have a wide range of applications; however, its research of 2e⁻ORR in the electrochemical synthesis of H₂O₂ should be further strengthened.

In the present work, we have developed a facile synthetic method for preparing oxidized Mo₂TiC₂ MXene (O-Mo₂TiC₂) catalysts. O-Mo₂TiC₂ materials provide high selectivity and are active in the electrochemical synthesis of hydrogen peroxide under alkaline conditions. Catalyst evaluation for different pH environments also reveals that pH has an effect on performance. Furthermore, the electrocatalytic H₂O₂ production performance of the catalyst is indeed affected by the variation of catalyst loading on the working electrode, which has been demonstrated. This study opens up new directions in search of more active and selective electrocatalysts for the efficient production of H₂O₂.

2. Results and Discussion

The preparation of the O-Mo₂TiC₂ is schematically demonstrated in Scheme 1. Firstly, the Mo₂TiAlC₂ was added into hydrofluoric acid to etch Al to obtain a layered Mo₂TiC₂ MXene, which was further exfoliated by adding tetrabutylammonium hydroxide. Finally, the O-Mo₂TiC₂ was obtained via hydrothermal treatment with high concentrations of potassium hydroxide.

To investigate the structural features of Mo₂TiAlC₂, Mo₂TiC₂ MXene, and O-Mo₂TiC₂, we performed X-ray diffraction (XRD) analysis on these three materials (Figure 1a,b). Compared with Mo₂TiAlC₂, the (002) diffraction peak of Mo₂TiC₂ MXene has shifted to a lower angle (2θ from 9° to 7°), indicating that the interlayer spacing increases after the addition of tetrabutylammonium hydroxide. In addition, there is a characteristic weak peak of the (004) crystal plane, which is consistent with the formation of the MXene phase. As shown in Figure 1b, the diffraction peaks of MoO₃ and TiO₂ are observed in O-Mo₂TiC₂, which is consistent with literature reports [52,53,54,55], indicating the successful synthesis of oxidized Mo₂TiC₂. According to Figure S1, compared with the Mo₂TiC₂ EDX spectrum, there is no Al peak in the Mo₂TiC₂ MXene, indicating that the aluminum element was completely removed during the etching process. Figure 1c,d depict the morphology of the O-Mo₂TiC₂ forming process. As shown in Figure 1c, Mo₂TiC₂ MXene has obvious layered and sheet-like structures. The morphology and structure of O-Mo₂TiC₂ synthesized by the hydrothermal method did not change in Figure 1d. Compared with the Mo₂TiC₂ MXene, the O-Mo₂TiC₂ still maintains the corresponding layered structure, indicating that the layered structure and crystallinity of Mo₂TiC₂ MXene are hardly influenced by the oxidation process. In order to further determine the morphology and structural characteristics of Mo₂TiC₂ MXene and O-Mo₂TiC₂, we also carried out transmission electron microscopy (TEM) analysis, and the analysis results are shown in Figure 1e,f. It can be seen from the TEM images that the O-Mo₂TiC₂ and Mo₂TiC₂ MXene maintain a nearly similar layered structure.

In order to verify the distribution of C, O, Ti, and Mo, we used energy dispersive X-ray spectrometer (EDX) elemental mapping to analyze the O-Mo₂TiC₂. In Figure 2a–g, we can clearly see that the distribution of C, O, Ti, and Mo in the O-Mo₂TiC₂ is very uniform, indicating that the O element has been successfully introduced into the O-Mo₂TiC₂. The elemental contents (atomic %) for EDX analysis is: C = 38.70%, O = 31.95%, Ti = 9.91%, Mo = 19.45%, and the mass ratio: C = 14.01 wt%, O = 15.41 wt%, Ti = 14.31 wt%, Mo = 56.27 wt%. The specific surface areas of Mo₂TiC₂ MXene and O-Mo₂TiC₂ were measured via N₂ adsorption–desorption in Figure 2h, and the Brunauer–Emmett–Teller (BET) specific surface area is 5.41 m² g⁻¹ and 12.80 m² g⁻¹, respectively, suggesting that the oxidation process is in favor of improving the specific surface area of Mo₂TiC₂ MXene. The Raman spectra of the O-Mo₂TiC₂ are recorded in Figure 2i. Raman modes can be observed at around 170, 245, 310, and 770 cm⁻¹ in all samples [56]. These modes closely match previous reports on the Raman spectra of Mo₂TiC₂, giving further evidence of the successful synthesis of Mo₂TiC₂. It has been reported that the peak around 170 cm⁻¹ results from the Eg vibration of both Mo and Ti atoms, and the peak at around 245 cm⁻¹ corresponds directly to the Eg vibration of the O atoms, which suggests the presence of Mo-O in this MXene. The peaks at 310 and 770 cm⁻¹ are all thought to mostly originate from the vibrations of C atoms in the MXene. The sharp bands at 385 and 442 cm⁻¹ correspond to Raman active modes of TiO₂ [57]. The presence of Mo-O bands can be confirmed since its characteristic main bands at 282,666 and 710 cm⁻¹ (A_g) cm⁻¹ are observed, which is in accordance with previous analysis [57,58,59].

We also conducted X-ray photoelectron spectroscopy (XPS) analysis of the Mo₂TiC₂ MXene and O-Mo₂TiC₂, from which the chemical composition and valence state of each element during the oxidation reaction could be determined, which clearly confirmed the surface functional groups of the samples before and after the reaction. The XPS survey spectrum (Figure 3a,b) shows the presence of O, Ti, C, and Mo as the main components of Mo₂TiC₂ MXene and O-Mo₂TiC₂.

Figure S2 and Figure 4a–d show the high-resolution XPS spectra of Mo₂TiC₂ MXene and O-Mo₂TiC₂, where the changes of C 1s, Mo 3d, Ti 2p, and O 1s before and after the oxidation process are clearly observed. The three peaks of C 1s are C-O (286.1 eV), C-C (384.5 eV), and Mo(Ti)-C (282.0 eV) in Figure S2a [56]. Mo 3d has peaks of Mo-C (234.3 eV and 231.1 eV) and Mo-Ox (237.4 eV and 234.7 eV) in Figure S2b [56]. The three peaks of Ti 2p are Ti-O (458.9 eV), Ti-C (463.4 eV and 457.4 eV) in Figure S2c [56]. The three peaks of O 1s are Mo₂TiC₂-OH (531.4 eV), Mo₂TiC₂-Ox (529.9 eV), and Mo(Ti)-Ox (529.0 eV) in Figure S2d [56]. For O-Mo₂TiC₂ in Figure 4a–d, C 1s is mainly a new peak COO (288.3 eV) and an increase in C-O (285.6 eV) content in Figure 4a [48]. Figure 4b,c show that the original oxygen-containing peak areas of the other two elements also increased to varying degrees, and new oxygen-containing peaks also increased. After the oxidation treatment, the new peaks of Mo-Ox (235.3 eV and 232.0 eV in Figure 4b) and Ti-Ox (463.7 eV in Figure 4c) were introduced [44]. We can conclude that the oxidation process of O-Mo₂TiC₂ introduces more oxygen-containing groups. These oxygen-containing groups, including Mo-Ox and Ti-Ox, are important for the electrocatalytic process. Therefore, both Mo and Ti are important for the electrocatalytic process.

The electrocatalytic H₂O₂ production activity of the as-prepared O-Mo₂TiC₂ was evaluated by cyclic voltammetry (CV) in alkaline and neutral electrolytes, respectively, in Figure S3. Figure S3a shows almost no characteristic curve in the N₂-saturated 0.1 M KOH solution, while a distinct reduction peak appears in the O₂-saturated 0.1 M KOH solution (Figure S3b). Moreover, similar CV test results can also be observed in neutral solutions (Figure S3c,d), indicating that the O-Mo₂TiC₂ has a remarkable electrocatalytic activity for oxygen reduction. Especially in 0.1 M KOH electrolyte, the reduction peak is very obvious, indicating that the ORR activity is significantly enhanced in an alkaline solution. Following the above results, we evaluated the electrocatalytic activity of the O-Mo₂TiC₂ using an RRDE. Because the ORR can be divided into 4e⁻pathway and 2e⁻pathway, the electrocatalytic production of H₂O₂ is a typical 2e⁻ORR pathway. To study the electrocatalytic H₂O₂ production capability of O-Mo₂TiC₂ under both alkaline and neutral conditions, we employed two electrolytes: 0.1 M KOH solution (pH~13) and 0.1 M Na₂SO₄ solution (pH~7). Figure 5a and Figure S4 show the electrochemical results of the catalysts in the two electrolytes (the rotation speed of the RRDE electrode at 1600 rpm), where the oxygen reduction current (solid line) was measured on the disk electrode, and the H₂O₂ oxidation current (dotted line) were measured on platinum ring electrodes. According to Figure 5a and Figure S4, whether in 0.1 M KOH solution or 0.1 M Na₂SO₄ solution, compared with Mo₂TiC₂ MXene, O-Mo₂TiC₂ has a stronger ability to electrochemically synthesize H₂O₂, that is, higher ring current and corrected onset potential (0.1 M KOH solution: 0.72 V vs. RHE; 0.1 M Na₂SO₄ solution: 0.33 V vs. RHE). Therefore, combined with the above analysis, the electrocatalytic activity of Mo₂TiC₂ for H₂O₂ production is much higher than that of Mo₂TiC₂ MXene. This is because the surface oxidation treatment makes Mo₂TiC₂ MXene generate more oxygen-containing functional groups, which greatly increases the active sites of the reaction and improves the electrocatalytic ability of the catalyst O-Mo₂TiC₂ to produce H₂O₂.

As shown in Figure 5b, the number of transferred electrons of O-Mo₂TiC₂ is closer to 2e⁻. Therefore, O-Mo₂TiC₂ is easier to carry out the 2e⁻ORR process so as to achieve the purpose of H₂O₂ production. The H₂O₂ selectivity of O-Mo₂TiC₂ is maintained above 83% (Figure 5c), and the H₂O₂ selectivity reached the peak of 90% at 0.7 V vs. RHE, which is much higher than the highest H₂O₂ selectivity (70%) of Mo₂TiC₂ MXene. Similar results were observed in neutral solution (0.1 M Na₂SO₄ solution). The H₂O₂ selectivity of O-Mo₂TiC₂ reached 83% (Figure S5), which is higher than the 59% selectivity of Mo₂TiC₂ MXene in a neutral solution. Moreover, the number of transferred electrons also confirms this result in Figure S6. We also calculated the Faradaic efficiency (FE) of H₂O₂ in a 0.1 M KOH solution; the FE of O-Mo₂TiC₂ is 81% at 0.7 V vs. RHE (Figure 5d), which is much higher than Mo₂TiC₂ MXene (56%). The FE of O-Mo₂TiC₂ (Figure S7) is 70% in 0.1 M Na₂SO₄ solution, which is also much higher than the 42% of Mo₂TiC₂ MXene. Therefore, the above results indicate that the O-Mo₂TiC₂ exhibits a higher electrocatalytic H₂O₂ production capacity than the Mo₂TiC₂ MXene in both neutral and alkaline solutions. The impedance and interfacial electron transfer ability of O-Mo₂TiC₂ and Mo₂TiC₂ MXene in alkaline solution and neutral solution were analyzed via electrochemical impedance spectroscopy (EIS). According to Figure 5e and Figure S8, O-Mo₂TiC₂ showed lower charge transfer resistance, indicating the better electrochemical performance and fast dynamics, and the EIS fitting results are shown in Table S1. The internal resistance (R₁) is consistent, while the transfer resistance (R₂) is smaller than that of Mo₂TiC₂ MXene. Therefore, it is indicated that the O-Mo₂TiC₂ achieves a fast faradaic process and excellent reaction kinetics due to the introduction of oxygen element, which is consistent with the above-mentioned results of H₂O₂ activity and selectivity analysis. The stability is an important basis for studying whether the catalyst can be commercialized. Combined with the above analysis and test results, the O-Mo₂TiC₂ exhibited a high electrocatalytic ability to produce H₂O₂ in 0.1 M KOH solution. Therefore, we mainly tested the stability of O-Mo₂TiC₂ in an alkaline solution. As shown in Figure 5f, the O-Mo₂TiC₂ can continue to work for 40 h under the voltage of 0.7 V vs. RHE, and no obvious overpotential change is observed. Therefore, the O-Mo₂TiC₂ show good stability and great potential for large-scale practical application, which makes O-Mo₂TiC₂ a promising candidate catalyst to further enhance the potential of electrocatalytic H₂O₂ production at the industrial level.

The effects of loading amount on the H₂O₂ selectivity were also investigated in alkaline and neutral solutions (Figure 6a and Figure S9). The LSV curves of ring current and disk current in alkaline solution are consistent with the neutral solution. The onset potential of the LSV curve is the highest when the O-Mo₂TiC₂ loading is 50 μg cm⁻² and the H₂O₂ oxidation current is the highest. In alkaline solution, the different loadings of O-Mo₂TiC₂ have obvious effects on the number of transferred electrons, H₂O₂ selectivity, and Faradaic efficiency in Figure 6b–d and Figure S10–S12. With the decrease in the O-Mo₂TiC₂ loading, the ability of 2e⁻ORR to produce H₂O₂ gradually increased, the number of transferred electrons gradually approached 2e⁻, and the H₂O₂ selectivity and Faradaic efficiency also gradually increased. In 0.1 M KOH solution, when the catalyst loading gradually decreased, the number of transferred electrons gradually approached 2e⁻ (from 3.2e⁻ to 2e⁻) in Figure 6b, the H₂O₂ selectivity was gradually increased from 41% to 90% (Figure 6c), and the FE increased gradually from 25% to 80% (Figure 6d). Similar to an alkaline solution, when the catalyst loading was gradually decreased in neutral solution, the number of transferred electrons gradually approached 2e⁻ (from 3.4 e⁻ to 2.3e⁻) in Figure S10, and the H₂O₂ selectivity gradually increased from 32% to 83% (Figure S11), and the FE increased gradually from 19% to 70% (Figure S12). Based on the above experimental results, the O-Mo₂TiC₂ loading was very low, such as 50 μg cm⁻², and the O-Mo₂TiC₂ showed lower 4e⁻ORR activity and higher H₂O₂ activity. Therefore, we believe that when the O-Mo₂TiC₂ layer is very thin, the generated H₂O₂ would quickly escape from the active site of the O-Mo₂TiC₂ layer and avoid being further reduced.

In order to verify the effect of thickness on catalytic performance, we tested the electrocatalytic performance of O-Mo₂TiC₂ with a larger thickness. Figure S13 shows an SEM image of O-Mo₂TiC₂ with a thicker thickness. The test results are shown in Figures S14–S17. Comparing the results of Figures S14–S17 with Figure 5a–d and Figure 6a–d, it can be seen that the thicker O-Mo₂TiC₂ has a lower ring current and smaller onset potential (onset potential: 0.64 V vs. RHE). The H₂O₂ selectivity of the thicker thickness O-Mo₂TiC₂ is 41% in Figure S15, which is lower than that of the thinner thickness O-Mo₂TiC₂. In addition, the results of the number of transferred electrons (3.2 e⁻) in Figure S16, and the Faradaic efficiency (25%) in Figure S17, were calculated. Combined with the above analysis, the electrocatalytic activity of the thinner O-Mo₂TiC₂ for H₂O₂ production is much higher than that of the thicker O-Mo₂TiC₂. The reason is that the thinner O-Mo₂TiC₂ provides more reaction sites, greatly improving the electrocatalytic activity of O-Mo₂TiC₂.

3. Materials and Methods

3.1. Reagents and Chemicals

Potassium hydroxide (KOH, AR) was purchased from Shanghai McLean (Shanghai Aladdin Biochemical Technology Co., Ltd, Shanghai, China), aqueous hydrofluoric acid (HF, 40 wt%) was purchased from Fuchen (Fuchen Chemical Reagent Co., Ltd, Tianjin, China), carbon-aluminum-titanium-molybdenum (Mo₂TiAlC₂, AR) was purchased from Yiyi Technology (Jilin Province Yiyi Technology Co., Ltd, Jilin, China), argon gas (High purity) from Beijing Millennium (Beijing Millennium Jingcheng Gas Co., Ltd, Beijing, China), and tetrabutylammonium hydroxide (C₁₆H₃₇NO, 40 wt%) was purchased from Energy Chemical (Sarn Chemical Technology Co., Ltd, Shanghai, China). The ultrapure water used in all experiments was purified with a Milli-Q system.

3.2. Synthesis of Mo₂TiC₂ MXene and O-Mo₂TiC₂

3.2.1. Synthesis of Mo₂TiC₂ MXene

Mo₂TiAlC₂ (1 g) powder was slowly added to 40 wt% HF (20 mL), and the solution was stirred in an oil bath at 55 °C for 72 h. After 72 h, the solution was cooled at room temperature and then washed and centrifuged with deionized water continuously. The multilayered Mo₂TiC₂ MXene powder was collected when the decanted supernatant pH was neutral. The as-synthesized dry multilayer Mo₂TiC₂ MXene powder was dispersed into 60 mL of deionized water, and then, 1 mL of 40 wt% tetrabutylammonium hydroxide (TBAOH) was added. After sonication for 1 h under the continuous flow of argon and centrifugation at 3000 rpm for half an hour, the supernatant in the test tube was collected to obtain a layered Mo₂TiC₂ MXene solution. The solution was frozen overnight in a refrigerator and dried with a freeze dryer to obtain a layered Mo₂TiC₂ MXene powder, which was collected and frozen for later use.

3.2.2. Synthesis of Oxidized Mo₂TiC₂-Based MXene Materials

In this experiment, the hydrothermal method was used to prepare the oxidized Mo₂TiC₂-based MXene material; that is, the Mo₂TiC₂ MXene powder was oxidized with a high concentration (10 M) KOH solution in a polytetrafluoroethylene kettle. Briefly, about 50 mg of the previously prepared Mo₂TiC₂ powder was dispersed in 40 mL of 10 M KOH solution, stirred by ultrasonic for 30 min, and then transferred to an oven for 12 h at 180 °C. After 12 h, the product solution was naturally cooled to room temperature and then washed and centrifuged with deionized water and absolute ethanol continuously. When the pH of the poured supernatant was neutral, the product was transferred to a vacuum drying oven at 60 °C for about 10 h to dry it completely. The prepared sample is simply referred to as O-Mo₂TiC₂ in the rest of the paper.

3.3. Characterization of Mo₂TiC₂ MXene and O-Mo₂TiC₂

Scanning electron microscopy (SEM) measurements were performed with a Hitachi SU8010 scanning electron microscope at 200 kV. Transmission electron microscopy (TEM) was measured with a Tecnai F20 at 200 kV. Wide-angle X-ray diffraction (XRD) was performed with a Burker D8-advance X-ray diffractometer (operating current: 40 mA, operating voltage: 40 KV) under Cu-Kα (λ = 0.15406 nm) radiation. X-ray photoelectron spectroscopy (XPS) was measured with Mg-KR radiation (BE) at 1253.6 eV. Nitrogen adsorption–desorption isotherms were measured with a micromertics ASAP 2460 analyzer (USA) at liquid nitrogen temperature (77 K), and the samples were measured after degassing in a vacuum at 80 °C for 6 h. The surface area was obtained using the Brunauer–Emmett–Teller (BET) method. Raman spectra were collected using a Raman spectrometer (HORIBA labRAM HR Evolution).

3.4. Electrochemical Performance Tests

The tests on electrocatalytic hydrogen peroxide (H₂O₂) production in this paper were all completed by a CHI760E electrochemical workstation and rotating disk electrode device. The working electrode was a rotating ring disk electrode (RRDE) assembly (AFE7R9GCPT, Pine Research Instrumentation Inc, Shanghai, China) composed of a glassy carbon rotating disk electrode (area: 0.196 cm²) and a platinum ring, with a theoretical collection efficiency of 35%. The counter electrode is a carbon rod, and the reference electrode is Hg/HgO electrode. Test in normal temperature and pressure environment, test two different electrolytes: 0.1 M KOH solution (alkaline, “pH = 13”) and 0.1 M Na₂SO₄ solution (neutral, “pH = 7”).

In order to ensure the accuracy of the experiment, we determined the collection efficiency of the RRDE electrode used by a specific experiment, that is, measuring the RRDE electrode in a nitrogen-saturated solution of 1 M KNO₃ and 10 mM K₃Fe(CN)₆ (Macklin, AR, >99.5%) Apparent collection efficiency (N) in as shown in Figure S18 the apparent collection efficiency N was 34.3% at 1600 rpm. Because the apparent collection efficiency of the RRDE electrode is only related to the electrode itself and has nothing to do with other conditions such as catalyst and electrolyte, the measured data (N = 34.3%) can be directly used in subsequent experiments.

Here is a full description of the experiment:

(1)

Catalyst preparation:

Add 600 μL ultrapure water, 400 μL absolute ethanol, and 10 μL Nafion solution (5 wt%) to a 2 mL centrifuge tube. Another 5 mg of catalyst was weighed and mixed with it. Sonicate for half an hour to form a uniform ink and be ready to use. Use a pipette to drop the ink onto a glass carbon disk (surface area 0.196 cm², the catalyst loading can be adjusted at any time according to the experimental needs, about 500 μg cm⁻²~50 μg cm⁻²), and dry at room temperature. On the glassy carbon disk electrode, the catalyst layer is uniform with no obvious pinholes or exposed edges.

(2)

RRDE measurement:

Before the electrochemical performance test, in order to eliminate the air in the electrolyte as much as possible, it is necessary to ventilate the electrolyte with N₂ for 30 min. Then, the cyclic voltammetry (CV) curve was tested at a scan rate of 50 mV∙s⁻¹, with at least 40 cycles until the CV curve remained stable. Then ventilate the electrolyte with O₂ for 30 min. After that, the cycle was repeated 20 times at a scan rate of 10 mV∙s⁻¹ until the CV curve remained stable. Finally, the linear sweep voltammogram (LSV) in the O₂-saturated electrolyte was measured by polarization curves and a rotating ring disk electrode (RRDE). The ORR polarization curve was saved by adjusting the rotating disk electrode device to keep the electrode rotation speed at 1600 rpm and the scan rate at 10 mV∙s⁻¹ for measurement.

To detect the generated H₂O₂ while avoiding other ORR currents, the Pt ring potential was kept at 1.4 V vs. RHE during LSV. All LSV curves were corrected with resistance compensation and potential scales given relative to a reversible hydrogen electrode (RHE).

From the disc current (ID) and ring current (IR) results, determine the H₂O₂ selectivity and transfer number of electrons (n) with the following formulas:

$${\mathrm{H}}_2{\mathrm{O}}_2\%=200\frac{{\mathrm{I}}_{\mathrm{R}}/\mathrm{N}}{{\mathrm{I}}_{\mathrm{D}}+{\mathrm{I}}_{\mathrm{R}}/\mathrm{N}}$$

(1)

$$\mathrm{n}=4\frac{{\mathrm{I}}_{\mathrm{D}}/\mathrm{N}}{{\mathrm{I}}_{\mathrm{D}}+{\mathrm{I}}_{\mathrm{R}}/\mathrm{N}}$$

(2)

The formula for calculating the Faradaic efficiency (FE) of H₂O₂:

$${\mathrm{Faradaic}\mathrm{efficiency}\mathrm{of}\mathrm{H}}_2{\mathrm{O}}_2\%=100\frac{{\mathrm{I}}_{\mathrm{R}}/\mathrm{N}}{\left|{\mathrm{I}}_{\mathrm{D}}\right|}$$

(3)

(3)

Electrochemical impedance spectroscopy (EIS) was acquired in the range of 106 Hz to 0.1 Hz, measured in an oxygen-saturated 0.1 M KOH aqueous solution at 0.65 V vs. RHE. All measurement potentials using the three-electrode setup are manually 100% compensated.

(4)

The stability of the catalyst in this experiment was tested by the potentiostatic method, and the catalyst was tested under fixed voltage conditions for 40 h (0.7 V vs. RHE).

3.5. Mechanism

The 2e⁻ORR pathway for H₂O₂ production proceeds through Equations (4) and (5):

$${\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\to \mathrm{OOH}*+\mathrm{OH}$$

(4)

$$\mathrm{OOH}*+{\mathrm{e}}^{-}\to {\mathrm{HO}}_2^{-}$$

(5)

First, hydrogenation of oxygen occurs on active sites via proton-electron transfer to form OOH* intermediate, and then, OOH* intermediate is reduced to HO₂⁻ with second electron transfer. Therefore, the OOH* intermediate plays a key role in the 2e⁻ORR for H₂O₂ formation, and the adsorption energy of OOH* is used as the descriptor to evaluate the catalytic activity of different active sites [60,61].

4. Conclusions

Mo₂TiC₂ MXene was synthesized using hydrofluoric acid as an etchant and then oxidized with a high concentration of KOH solution by hydrothermal method to obtain O-Mo₂TiC₂. The H₂O₂ selectivity of O-Mo₂TiC₂ in alkaline solution reached up to 90%, and the onset potential reached 0.72 V vs. RHE. Moreover, O-Mo₂TiC₂ exhibited high charge transfer ability and long-term stable working ability (40 h). The significantly enhanced electrocatalytic H₂O₂ production is mainly due to the oxidation treatment of Mo₂TiC₂ MXene to generate more oxygen-containing groups in O-Mo₂TiC₂, which are beneficial to the improvement of electrocatalytic H₂O₂ production performance via increasing the active sites. It was also found that the electrocatalytic H₂O₂ production performance of the catalysts was indeed affected by the variation of the catalyst loading on the working electrode. This work provides a promising catalyst for the electrochemical synthesis of H₂O₂.