An Efficient and Stable MXene-Immobilized, Cobalt-Based Catalyst for Hydrogen Evolution Reaction

Abstract

Hydrogen (H₂) is considered to be the best carbon-free energy carrier that can replace fossil fuels because of its high energy density and the advantages of not producing greenhouse gases and air pollutants. As a green and sustainable method for hydrogen production, the electrochemical hydrogen evolution reaction (HER) has received widespread attention. Currently, it is a great challenge to prepare economically stable electrocatalysts for the HER using non-precious metals. In this study, a Co/Co₃O₄/Ti₃C₂T_x catalyst was synthesized by supporting Co/Co₃O₄ with Ti₃C₂T_x. The results show that Co/Co₃O₄/Ti₃C₂T_x has excellent HER activity and durability in 1 mol L⁻¹ KOH, and the overpotential and Tafel slope at 10 mA·cm⁻² were 87 mV and 61.90 mV dec⁻¹, respectively. The excellent HER activity and stability of Co/Co₃O₄/Ti₃C₂T_x can be explained as follows: Ti₃C₂T_x provides a stable skeleton and a large number of attachment sites for Co/Co₃O₄, thus exposing more active sites; the unique two-dimensional structure of Ti₃C₂T_x provides an efficient conductive network for rapid electron transfer between the electrolyte and the catalyst during electrocatalysis; Co₃O₄ makes the Co/Co₃O₄/Ti₃C₂T_x catalyst more hydrophilic, which can accelerate the release rate of bubbles; Co/Co₃O₄ can accelerate the adsorption and deionization of H₂O to synthesize H₂. This study provides a new approach for the design and preparation of low-cost and high-performance HER catalysts.

1. Introduction

In order to deal with the energy crisis and environmental protection issues, countries around the world have been increasing their attention to new clean energy. Among many kinds of new clean energy sources available on the globe, hydrogen (H₂) is considered to be the best carbon-free energy carrier that can replace fossil fuels. In the current hydrogen production technology, the electrochemical decomposition of water to produce hydrogen is considered to be one of the best and the most efficient and greenest hydrogen production methods [1,2,3]. An electrochemical water decomposition reaction is composed of the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), in which the HER is the key step. A catalyst that can reduce the reaction barrier and energy consumption, increase the reaction efficiency and improve the reaction rate plays a very important role in HER process. Therefore, the study of efficient electrocatalysts has become the most important part of HER technology exploration.

Numerous studies have found that precious metals such as platinum (Pt), ruthenium (Ru), palladium (Pd), gold (Au) and iridium (Ir) have more suitable ΔG_H* values than other metallic elements. This may be due to the fact that precious metals have a surface defect, which makes the active substance easily adsorbed on their surface and facilitates the formation of active intermediates [4]. Pt-based material is currently recognized as the best HER electrocatalyst, and its ΔG_H* value is almost zero. However, the limited reserves and high price of Pt limit its large-scale application, and therefore, it is not suitable for commercial hydrogen production [5]. Therefore, it is urgent to develop high-efficiency and low-cost HER electrocatalysts.

At present, a large number of studies have found that transition metals (such as Fe, Co, Ni, etc.) and transition metal oxides show high electrocatalytic activity for HER in alkaline solutions and are attractive candidates for HER electrocatalyst. Compared with the small global reserves and high market prices of precious metals, transition metals have abundant global reserves and cost advantages. The ΔG_H* value of Co is moderate, and Co-based materials such as cobalt phosphide, cobalt selenide, cobalt sulfide and cobalt layered double hydroxides (Co-LDHs) have diversified characteristics in terms of their composition, crystal structure and reversible morphology. Results show that Co-based materials have great potential for the HER. However, despite the advantages of Co-based materials in terms of HER catalytic activity, there are still some shortcomings in their industrial applications. For example, cobalt oxide can dissolve in acidic solutions, resulting in the loss of active sites; agglomeration often occurs during the preparation of cobalt selenide; low conductivity can reduce the HER activity of cobalt sulfide, cobalt phosphide and Co-LDHs [6]. In order to realize the practical application of cobalt-based materials in the HER, some researchers have tried various strategies, such as constructing nanostructures, carbon material loading, morphological control and doping. For example, Wang et al. synthesized Co@CoO nanowires and found that they are excellent catalysts for hydrogen production at low overpotential [7]. He et al. improved the HER efficiency of Co-Ni phosphide (Co-Ni-P) in an alkaline solution by anchoring hollow Co-Ni-P nanospheres on reduced graphene oxide nanosheets (RGO) [8]. Gao et al. synthesized a nuclear/shell structure Co₃O₄@CoS₂ nanoarray with excellent HER activity in acidic solutions [9]. Zhang et al. synthesized a highly efficient HER electrocatalyst, Ni₂P/CoP@C-NSG, suitable for acidic solutions by loading highly dispersed Ni₂P and CoP onto carbon nanospheres and growing Ni₂P/CoP on a N and S co-doped graphene substrate [10]. In general, transition metal oxides are good for the dissociation of water but bad for the recombination of adsorbed hydrogen atoms (H_ad) to form H₂. However, transition metals can effectively adsorb hydrogen atoms and promote the conversion of H_ad to H₂. Therefore, researchers have set out to construct transition metal/metal oxide catalysts suitable for the HER. For example, Wang et al. reported a cobalt-based electrocatalyst, Co/CoO/CoMoO₃, with a highly dispersed metal/metal oxide active component and a tight series structure that together promoted the efficient conduct of the HER under a high current density in a medium/alkaline electrolyte. Through experiments and DFT calculations, it was found that the strong OH adsorption at CoO site promotes the dissociation of water, and the appropriate H adsorption at Co sites promotes the precipitation of hydrogen, thus achieving the high current density of the HER in alkaline and neutral media [11].

In recent years, a large number of studies have found that the introduction of suitable substrates to disperse and anchor Co-based catalytic species is a feasible way to improve its utilization efficiency, because it can expose as many active points on the catalyst surface as possible [12,13,14,15]. Therefore, the HER performance of Co-based materials can be improved by selecting a suitable carrier [16]. MXene (Ti₃C₂T_x) is the general term for a series of novel two-dimensional (2D) transition metal carbide and carbon nitride materials with graphene-like structures. Compared to traditional three-dimensional (3D) catalysts, Ti₃C₂T_x has a larger surface area and can provide a large number of attachment points for active substances. At the same time, the good electrical conductivity of Ti₃C₂T_x can accelerate the transfer of electrons from the electrode to the catalyst surface. Therefore, it may be a suitable carrier for loading cobalt-based active components. However, due to the influence of van der Waals forces, Ti₃C₂T_x nanosheets are easy to accumulate or stack, which easily leads to the catalytic active site being covered up. It is worth noting that the layer spacing of MXene obtained via HF etching of the MAX phase can be expanded. The large layer spacing facilitates the entry of active substances into the interior of Ti₃C₂T_x, and the loaded active components can play a role in supporting the structure to prevent the stacking of Ti₃C₂T_x. Therefore, this study attempted to use Ti₃C₂T_x loaded with Co-based active components. Through the above attempts, the HER activity of the Co-based active component can be improved by dispersing and anchoring Ti₃C₂T_x. On the other hand, the stacking of Ti₃C₂T_x can be prevented by Co-based active components. Therefore, it is expected that the electrocatalyst with high HER activity can be prepared through the synergistic action of the carrier and cobalt-based catalyst.

Metal-supported electrocatalysts are generally prepared via the high-temperature H₂ reduction of metal. However, due to the hightemperature, the H₂ reduction method has the characteristics of a strong heating effect, and it may accelerate the growth of particles on the catalyst surface and the agglomeration of metal active centers, resulting in the reduction of metal dispersion and specific surface area of the catalyst, which has a negative impact on the catalytic performance. In order to reduce the agglomeration of metal active centers during the reduction process, using NaBH₄ as a reducing agent is an effective strategy for preparing metal-supported electrocatalysts. Compared with the H₂ reduction method, the NaBH₄ reduction process is carried out in a solution at room temperature so that the solvent can disperse the reactants and quickly absorb the heat generated by the exothermic reaction, thus slowing down the agglomeration of metal active centers in the reduction process.

However, the synthesis of catalysts suitable for the HER by supporting cobalt-based active components with Ti₃C₂T_x is rarely reported. Therefore, in this study, Ti₃C₂T_x was prepared using ultrasonic-assisted HF etching of Ti₃AlC₂ and then used as the carrier to support the Co-based active component (Co/Co₃O₄). Subsequently, the Co/Co₃O₄/Ti₃C₂T_x catalyst was prepared through the liquid-phase reduction method using NaBH₄ as the reducing agent. The structure, morphology, crystal phase and elemental valence of the obtained catalyst were analyzed using various characterization methods. The activity and stability of Co/Co₃O₄/Ti₃C₂T_x during electrocatalysis of the HER were studied in electrolytes of 1 mol L⁻¹ KOH and 0.5 mol L⁻¹ H₂SO₄.

2. Materials and Methods

2.1. Reagents

The reagents used are as follows: Co(NO₃)₂·6H₂O (analytical grade, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), NaBH₄ (analytical grade, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), Ti₃AlC₂ (analytical grade, Zhejiang Yamei Nano Technology Co., Ltd., Jiaxing, China) and HF (analytical grade, Shanghai Aladdin Technology Co., Ltd., Shanghai, China).

2.2. Synthesis of Co/Co₃O₄/Ti₃C₂T_x Catalyst

Ti₃C₂T_x preparation was performed as follows: First, 20 mL of 48% aqueous HF solution was transferred to a Teflon beaker with a volume of 200 mL. Subsequently, 1.00 g of Ti₃AlC₂ powder was slowly added to the above-mentioned beaker within 30 min and electromagnetically stirred for 24 h at 150 rpm and 55 °C. The resulting solution was centrifuged at 6400 rpm for 10 min to yield the sediment and recycle the supernatant (including the HF solution and etched Al layers), respectively. The obtained sediment was cleaned with deionized water several times until the pH of the dispersion was close to 6. Finally, the resulting powder (Ti₃C₂T_x) was dried in a vacuum oven at 60 °C for 24 h. The dried powder was obtained and identified as Ti₃C₂T_x (T_x stands for O, F and OH functional groups).

The Co/Co₃O₄/Ti₃C₂T_x catalyst preparation occurred as follows: First, 0.30 g Ti₃C₂T_x was added to a beaker containing 25 mL of deionized water and treated with an ultrasonic device in an ice water bath for 2 to 3 h. Subsequently, the Ti₃C₂T_x solution was obtained after the above-mentioned solution was clearly stratified; second, 0.8 g of Co(NO₃)₂·6H₂O was added to the Ti₃C₂T_x solution and electromagnetically stirred for 30 min at 800 rpm and 25 °C. After standing for 3 h, the solution became clear and stratified to obtain a Co/Co₃O₄/Ti₃C₂T_x solution; third, some NaBH₄ was added until the solution was clear and layered (the upper layer was colorless and transparent), magnetically stirred at 800 rpm for 30 min and left for 12 h. The white liquid in the beaker was poured away, and the remaining sediment was dried in the freeze dryer for 48 h to obtain Co/Co₃O₄/Ti₃C₂T_x. The preparation process of Co/Co₃O₄/Ti₃C₂T_x is shown in Figure 1.

2.3. Catalyst Characterization

The functional groups of Ti₃C₂T_x were identified using Fourier infrared spectroscopy (FT-IR) (Nicolet 5700, Madison, WI, USA). The crystalline phases of the obtained catalyst were analyzed using X-ray diffraction (XRD) (PANalytical, Empyean, Almelo, The Netherlands) with Cu Kα radiation (λ = 0.15418 nm) at 40 kV and 40 mA over a 2θ range from 5° and 85°, and the XRD results of the samples were compared with existing standard cards to determine their phase structure. The surface morphologies and chemical compositions of the catalysts were investigated using a scanning electron microscope (SEM) (MLA650F, FEI Company, Hillsboro, OR, USA) equipped with an energy-dispersive X-ray spectroscope (EDS, Quantax 400, Bruker, Ettlingen, Germany) at an accelerating voltage of 20 kV. The size, shape and distribution of the obtained catalyst were analyzed using a transmission electron microscope (TEM) (TecnaiG2-20, FEI Company, Hillsboro, OR, USA). The chemical valence states of the elements in the catalyst were measured using X-ray photoelectron spectroscopy (XPS) (Escalab Xi+, Thermo Fisher Scientific Inc., Oxford, UK) under the vacuum condition of 8 × 10⁻¹⁰ Pa. A monochromatic Al Kα X-ray source (hv = 1486.6 eV) was used with a working voltage of 12 kV. The specific surface area and pore size distribution of the catalyst were measured using a N₂ physical adsorption instrument (ASAP 2020, Micromeritics Instrument Ltd., Norcross, GA, USA) at 77 K. The contact angle of the catalyst was measured using a contact angle measuring instrument (JY-Pha, Chengde Yote Instrument Manufacturing Co. Ltd., Chengde, China) to determine the surface wettability.

2.4. Electrocatalytic Hydrogen Evolution Test of Catalyst

For the preparation of test electrode, first, 1 mg of the catalyst was weighed and ground to powder; second, it was dispersed in a solution composed of 0.25 mL of isopropyl alcohol, 0.75 mL of deionized water and 20 μL of 5 wt% Nafion solution; third, the suspension was dispersed via ultrasonic treatment under the condition of an ice-water bath until 1 mL of uniformly dispersed suspension was formed. Finally, the solution containing the catalyst was coated on the surface of the carbon paper with an area of 1 cm², and the working electrode supported by the catalyst was obtained after completely drying.

The HER performance of the catalyst was tested on a CHI660E electrochemical workstation at 25 °C. A standard three-electrode system was adopted, with a spectral pure graphite rod as the auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode. The potentials are all relative to the reversible hydrogen electrode (RHE). The conversion formula is shown in Equation (1):

$${\mathrm{V}}_{(\mathrm{R}\mathrm{H}\mathrm{E})}={\mathrm{V}}_{(\mathrm{S}\mathrm{C}\mathrm{E})}+0.0591\times \mathrm{p}\mathrm{H}+0.2415$$

(1)

where V_(RHE) is the potential value (V) of the RHE, V_(SCE) is the potential value (V) of the SCE, and pH is the pH value of the solution.

In this study, the non-Faraday, dual-layer capacitance currents corresponding to the cyclic voltammetry curve (CV) of the catalyst under different scanning rates (10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 mV s⁻¹) were tested. After linear fitting, the cyclic voltammetry curves were developed. The capacitance of the electrochemical double-layer capacitance (C_dl) was calculated using Equation (2):

$$\mathrm{I}=\mathrm{V}\times {\mathrm{C}}_{\mathrm{d}\mathrm{l}}$$

(2)

where V is the scanning rate (mV s⁻¹), and I is the current density at different scanning rates (mA cm⁻²).

In order to evaluate the true electrocatalytic activity of the catalyst, the electrochemically active surface area (ECSA, cm⁻²) involved in the electrochemical catalytic reaction should be calculated. The ECSA value can be calculated from the C_dl value using the Randles–Sevcik equation, as shown in Equation (3):

$$\mathrm{E}\mathrm{C}\mathrm{S}\mathrm{A}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{d}\mathrm{l}}}{{\mathrm{C}}_{\mathrm{s}}}}$$

(3)

where the C_s value is the specific capacitance (mF) of the corresponding surface smooth sample under the same conditions. The C_s value is usually in the range of 20–60 mF [17], and the C_s value was set as 40 mF in this study.

In general, the electrocatalytic activities of different catalysts can be compared by their overpotential (η) values at a current density of 10 mA cm⁻². η values can be calculated using the linear sweep voltammetry (LSV) curve derived from the linear sweep voltammetry (LSV) method, which is usually applied to the electrode with a continuously linear voltage (−0.6 V vs. RHE), and the resulting current is monitored. Since this type of current depends on the rate of oxidation or reduction occurring at the electrode, the size of the current in the LSV curve can reflect the degree of HER. In this study, the scanning rate was set at 5 mV s^−l for LSV testing. Then, a line at y = −10 was made, and the abscissa of the intersection point of the LSV curve was the η value after taking the absolute value.

The Tafel slope (b) is the rate at which the current increases with respect to η, which is mainly determined by the transfer coefficient. The smaller the b value is, the faster the current density increases and the smaller the η change, indicating that the rate-determining step is at the end of the multi-electron transfer reaction, which usually marks an electrocatalyst with excellent performance. The current density and potential data of the LSV diagram are converted into a graph of log(current density i) and η values; that is, the Tafel diagram is obtained. Then, the linear fitting was carried out through Origin 8 software, and the b value was calculated using Equation (4):

$$\eta =\mathrm{b}\log \mathrm{I}+\mathrm{a}$$

(4)

where η is the overpotential (mV), I is the current density (mA cm⁻²), a is the constant, and b is the Tafel slope.

The larger the surface area of the catalyst, the more active sites are exposed to the electrocatalyst surface, and the mass activity (MA) value is more indicative of the true activity of the catalyst. In general, a smaller-sized catalyst has higher MA value because a smaller-sized particle means that the ratio of surface atoms to total atoms is larger, exposing more electrocatalytic active sites. In order to more intuitively compare the activities of different catalysts, the MA values can be calculated according to Equation (5):

$$\mathrm{M}\mathrm{A}=\mathrm{I}/\mathrm{M}$$

(5)

where MA (mA mg⁻¹) is the mass activity of the catalyst, I is the current density (mA cm⁻²), and M is the catalyst load (mg cm⁻²).

The stability of the catalyst was tested using a chronocurrent method (i − t), which is usually performed at a constant potential and monitors changes in the electrode current. In addition, the stability of the electrocatalyst was tested using the CV method with more than 1000 cycles, which was achieved by comparing the LSV data before and after the stability test. All CV, LSV, i − t and other tests were performed in electrolytes of 1 mol L⁻¹ KOH or 0.5 mol L⁻¹ H₂SO₄, and the obtained data were not corrected by IR compensation.

3. Results

3.1. Characterization of Catalyst

The FT-IR result of the Ti₃C₂T_x is shown in Figure 2. There are three characteristic peaks of Ti₃C₂T_x in Figure 2, corresponding to C=O stretching vibrations, O-H bending vibrations and O-H stretching vibrations at 1633, 1393 and 3446 cm⁻¹, respectively.

The surface morphologies of Ti₃C₂T_x and Co/Co₃O₄/Ti₃C₂T_x were characterized using an SEM, and the results are shown in Figure 3. It can be observed from Figure 3a that the Ti₃C₂T_x obtained via HF etching Ti₃AlC₂ has an obvious “organ-like” morphology, which is conducive to the active substance to penetrate the internal structure of Ti₃AlC₂. The morphology is formed due to the destruction of Ti-Al bonds between the etched layers, which is consistent with other researchers’ reports [18]. It can be clearly observed from Figure 3b that the surface of the obtained Co/Co₃O₄/Ti₃C₂T_x has particle clusters with different sizes and shapes. The formation of these particles can be attributed as follows: during the reduction reaction, a large amount of Co²⁺ is reduced by NaBH₄ to zero-valent Co, which accumulates and deposits on the surface of Ti₃C₂T_x [19].

In order to understand the types and distribution of elements in the catalyst, the Co/Co₃O₄/Ti₃C₂T_x catalyst shown in Figure 4a was analyzed using element surface scanning, and the results are shown in Figure 4b–f.

Many bright and evenly distributed areas can be seen in Figure 4b, indicating that the elements Co and O are evenly distributed on the surface of Ti₃C₂T_x, which helps expose more active sites on the catalyst surface. Then, six square regions were randomly selected on the surface of Co/Co₃O₄/Ti₃C₂T_x, and the element information of each square region was detected. The results are shown in Figure 4g. It can be seen from Figure 4g that the atomic proportion of the same element at each detection point is close, which further indicated that the distribution of the elements Co and O is relatively uniform. The average atomic contents of C, O, Ti and Co are 22.16%, 58.10%, 4.95% and 14.80%, respectively. The high O content in Co/Co₃O₄/Ti₃C₂T_x indicates the presence of cobalt oxide.

In order to further explore the structural information of catalysts, the morphologies of Ti₃C₂T_x and Co/Co₃O₄/Ti₃C₂T_x were observed using a TEM, and the results are shown in Figure 5. It can be seen from Figure 5a that Ti₃C₂T_x presents a unique two-dimensional sheet structure with many active edge sites, which can provide a large number of attachment sites for the loaded active substances. As shown in Figure 5b, a large number of clusters are attached to the surface of the layered Co/Co₃O₄/Ti₃C₂T_x. This may be because NaBH₄ reduced a large amount of Co²⁺ to Co, and then, Co particles gathered and deposited on the surface of Ti₃C₂T_x [20], which was consistent with the results of the SEM analysis.

The crystal structures and compositions of Ti₃AlC₂, Ti₃C₂T_x and Co/Co₃O₄/Ti₃C₂T_x were obtained using XRD. It can be seen from the XRD patterns of Ti₃AlC₂ and Ti₃C₂T_x shown in Figure 6a that after HF etching of Ti₃AlC₂, obvious characteristic diffraction peaks appeared at 2θ of 9.5°, 19.3° and 39.0°. By comparing the standard card of Ti₃AlC₂ (JCPDS NO.52-0875), it can be inferred that the above characteristic diffraction peaks correspond to the (002), (004) and (104) crystal faces, respectively. The characteristic diffraction peak (104) of Al in the MAX structure almost completely disappeared, indicating that the Al layer had been completely etched. In addition, Ti₃AlC₂ shows a new wide characteristic diffraction peak at 2θ of 6.1°, indicating that the (002) characteristic diffraction peak corresponding to MXene moved from 9.5° to a lower angle of 6.1°. According to the Bragg equation, the layer spacing of MXene obtained by etching the MAX phase was enlarged. The larger layer spacing facilitates the active substances to enter the interior of Ti₃C₂T_x, and the active components can support the structure and prevent the stacking of Ti₃C₂T_x [20,21]. The XRD pattern of Co/Co₃O₄/Ti₃C₂T_x is shown in Figure 6b. Four characteristic diffraction peaks appeared at 2θ of 41.6°, 44.6°, 47.4° and 75.5°, respectively. By comparing the standard card of Co (JCPDS NO.05-0727), it can be inferred that the above characteristic diffraction peaks correspond to the (100), (002), (101) and (110) crystal faces, respectively. At the same time, six characteristic diffraction peaks appeared at 2θ of 34.1°, 39.6°, 57.3°, 68.4, 71.9 and 85.4°, respectively. By comparing the standard card of CoO (JCPDS NO. 42-1300), it can be inferred that the above characteristic diffraction peaks correspond to the (111), (200), (220), (311), (222) and (400) crystal faces. In addition, the XRD pattern of Co/Co₃O₄/Ti₃C₂T_x shows that there are seven characteristic diffraction peaks at 2θ of 19.0°, 31.3°, 36.7°, 38.5°, 44.8°, 59.3° and 65.3°. Compared with the standard card of Co₃O₄ (JCPDS NO.43-1003), it can be inferred that the above characteristic peaks correspond to the crystal faces of (111), (220), (311), (222), (400), (511) and (440), respectively. This also proves that the Co element in Co/Co₃O₄/Ti₃C₂T_x exists in metallic and oxidized states (Co, CoO and Co₃O₄). In addition, Co/Co₃O₄/Ti₃C₂T_x showed eleven characteristic diffraction peaks corresponding to MXenes at 2θ of 9.5°, 19.0°, 33.9°, 36.6°, 39.1°, 41.6°, 48.3°, 56.7°, 60.0°, 65.3°, 70.2° and 73.9°, respectively. In summary, the Co/Co₃O₄/Ti₃C₂T_x catalyst was successfully prepared.

The elemental composition of the Co/Co₃O₄/Ti₃C₂T_x catalyst was analyzed using XPS. Figure 7a shows the full XPS spectrum of Co/Co₃O₄/Ti₃C₂T_x, and binding energy peaks corresponding to Co, O, Ti, C and other elements can be clearly seen in the full spectrum. Furthermore, the fine spectrum of C1s was obtained via narrow-area scanning (Figure 7b), which can be divided into three peaks with binding energies of 284.8 eV, 286.4 eV and 289.0 eV, corresponding to C-C, O-C and O-C=O, respectively [22,23]. Figure 7c shows the fine spectrum of Co2p, which can be divided into three peaks with binding energies of 780.1 eV, 781.1 eV and 782.7 eV, respectively, attributed to Co⁰, Co³⁺ and Co²⁺ [24], indicating that Co exists in the form of metallic Co and in the oxidation state as Co₃O₄, which is consistent with the XRD analysis results. It is worth noting that most of the Co2p peaks are Co²⁺. Figure 7d shows the fine spectrum of O1s, which can be divided into three characteristic peaks with binding energies of 530.7 eV, 531.2 eV and 532.2 eV, respectively, attributed to Co-O, −OH and O in the adsorbed H₂O [25], which means that O in the catalyst exists in two forms: lattice oxygen and surface hydroxyl oxygen. The surface of the catalyst contains both cobalt oxide (Co-O) and cobalt hydroxide (Co-OH), which can effectively accelerate the dissociation of water [26,27]. In addition, the appearance of the binding energy peak corresponding to oxygen in the adsorbed H₂O also indicates that the catalyst has a certain hydrophilicity.

The hydrophilicity of the catalyst has a great influence on the HER. Hence, the hydrophilicity of the catalyst was detected by measuring the contact angles of Ti₃C₂T_x, Co and Co/Co₃O₄/Ti₃C₂T_x, and the results are shown in Figure 8a–c. As can be seen from Figure 8a–c that the contact angles of Ti₃C₂T_x, Co and Co/Co₃O₄/Ti₃C₂T_x are 141.9°, 122.1° and 11.7°, respectively. The contact angle of Co/Co₃O₄/Ti₃C₂T_x is much smaller than those of Ti₃C₂T_x and Co, which indicates that the hydrophilicity of Co/Co₃O₄/Ti₃C₂T_x is much stronger than those of Ti₃C₂T_x and Co. The possible reasons are as follows: a portion of Co on the surface of Co/Co₃O₄/Ti₃C₂T_x is oxidized to Co₃O₄, which endows the catalyst to have a higher degree of hydrophilicity. When the HER occurs, the stronger the hydrophobicity of the catalyst surface, the shorter the retention time of the generated bubbles on the catalyst surface and the smaller the bubble size when leaving the surface, which means that the reaction speed of the HER is faster [28].

The specific surface area and aperture of Ti₃C₂T_x, Co and Co/Co₃O₄/Ti₃C₂T_x were measured using the specific surface area tester. Figure 9a shows the N₂ adsorption–desorption curve, in which the N₂ adsorption–desorption isothermal curve of Co/Co₃O₄/Ti₃C₂T_x is type IV, indicating that there was a H3 hysteresis loop at a relative pressure p/p⁰ of 0.45–0.9 caused by the capillary condensation of adsorbate. Therefore, the Co/Co₃O₄/Ti₃C₂T_x had a mesoporous structure, and the pore structure was irregular. The pore diameters and specific surface areas of the catalysts are listed in

Table 1. Specific surface areas (BET), pore volumes (single-point method) and average pore diameters of Ti₃C₂T_x, Co and Co/Co₃O₄/Ti₃C₂T_x.

Catalyst	Specific Surface Area (m2/g)	Pore Volume (cm3/g)	Average Pore Diameter (nm)
Ti3C2Tx	2.94	0.0076	10.71
Co	64.02	0.30	186.79
Co/Co3O4/Ti3C2Tx	17.64	0.044	10.07

. The specific surface area, pore volume and pore size of Ti₃C₂T_x were 2.94 m² g⁻¹, 0.0076 cm³ g⁻¹ and 10.71 nm. For comparison, the specific surface area of Co was 64.02 m² g⁻¹, the pore volume was 0.30 cm³ g⁻¹, and the pore size was 186.79 nm. When Ti₃C₂T_x was loaded with Co/Co₃O₄, the specific surface area of Co/Co₃O₄/Ti₃C₂T_x became 17.64 m² g⁻¹, and the pore volume was 0.04 cm³ g⁻¹; both of them were increased. The pore size of Co/Co₃O₄/Ti₃C₂T_x was 10.07 nm, which was similar to the pore size of Ti₃C₂T_x. It is worth noting that compared with Ti₃C₂T_x, the specific surface area, pore volume and pore diameter of Co/Co₃O₄/Ti₃C₂T_x are greatly increased, which may be due to Co/Co₃O₄ infiltrating the interior of Ti₃C₂T_x and effectively inhibiting the accumulation of Ti₃C₂T_x flakes. This helps widen the layer spacing of the catalyst, thereby increasing the contact area between the catalyst and electrolyte, providing more active sites for the HER. In addition, the pore size distributions of Ti₃C₂T_x, Co and Co/Co₃O₄/Ti₃C₂T_x are shown in Figure 9b. Compared with Ti₃C₂T_x and Co/Co₃O₄/Ti₃C₂T_x, the pore size distribution of Co is wider, and the mesoporous distributions of Co and Co/Co₃O₄/Ti₃C₂T_x were mainly in the range of 35–40 nm and 5–20 nm, respectively.

3.2. Electrocatalytic HER Performance Test in Acidic Electrolyte

The HER electrocatalytic activities of Ti₃C₂T_x, Co and Co/Co₃O₄/Ti₃C₂T_x were tested in a 0.5 mol L⁻¹ H₂SO₄ solution, and the LSV results are shown in Figure 10a. It can be seen from Figure 9a that the electrocatalytic activities of Ti₃C₂T_x and Co are very low. However, when Ti₃C₂T_x and Co formed Co/Co₃O₄/Ti₃C₂T_x, the electrocatalytic performance was significantly improved. Furthermore, the η values corresponding to Ti₃C₂T_x, Co and Co/Co₃O₄/Ti₃C₂T_x at a current density of 10 mA cm⁻² were obtained, and they were 599 mV, 517 mV and 86 mV. However, after only four LSV tests, the η value of Co/Co₃O₄/Ti₃C₂T_x was changed from an initial 86 mV to 225 mV (see Figure 10b). The results show that Co/Co₃O₄/Ti₃C₂T_x has poor durability in a 0.5 mol L⁻¹ H₂SO₄ solution and is not suitable for long-term use in an acidic medium. The reasons for the sharp decline in the catalytic activity of Co/Co₃O₄/Ti₃C₂T_x in an acidic electrolyte may be as follows: Co and Co₃O₄ were easily dissolved in strong acids. The stability test shows that the Co/Co₃O₄/Ti₃C₂T_x catalyst is not suitable for long-term use in an acidic medium.

3.3. Electrocatalytic HER Performance Test in Alkaline Electrolyte

The HER performances of Ti₃C₂T_x, Co and Co/Co₃O₄/Ti₃C₂T_x were tested in a 1 mol L⁻¹ KOH solution, and the obtained LSV results are shown in Figure 11a. The η values corresponding to the above catalysts at a current density of 10 mA cm⁻² were obtained: Co/Co₃O₄/Ti₃C₂T_x has a smaller η value (87 mV) than Co (443 mV) and Ti₃C₂T_x (541 mV). Thus, the trend of catalytic activity was Co/Co₃O₄/Ti₃C₂T_x > Co > Ti₃C₂T_x. In 1 mol L⁻¹ KOH, both Co and Ti₃C₂T_x require large η values to perform the HER, which indicates that both Co and Ti₃C₂T_x have poor HER activity. The CV test results of Ti₃C₂T_x, Co and Co/Co₃O₄/Ti₃C₂T_x under different scanning rates (10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 mV s⁻¹) are shown in Figure 11b–d. Based on the CV test results of each catalyst, their C_dl values were obtained according to Equation (2). The C_dl values of Ti₃C₂T_x, Co and Co/Co₃O₄/Ti₃C₂T_x were 9.86 mF cm⁻², 7.59 mF cm⁻² and 28.34 mF cm⁻², respectively (see Figure 11e). According to Equation (3), the C_dl value can be converted to the ECSA value, and the ECSA values of Ti₃C₂T_x, Co and Co/Co₃O₄/Ti₃C₂T_x were 246.50 cm² 189.75 cm² and 708.5 cm², respectively. It can be seen that Co/Co₃O₄/Ti₃C₂T_x has the largest electrochemically active area. The Tafel slope values of Ti₃C₂T_x, Co and Co/Co₃O₄/Ti₃C₂T_x in a 1 mol L⁻¹ KOH electrolyte were calculated according to Equation (4). The Tafel slope value of Co/Co₃O₄/Ti₃C₂T_x was 61.90 mV dec⁻¹, which is better than the 263.09 mV dec⁻¹ of Co and 324.46 mV dec⁻¹ of Ti₃C₂T_x (seen Figure 11f). The smaller Tafel slope value indicates that the Co/Co₃O₄/Ti₃C₂T_x catalyst has a fast kinetic process in a 1 mol L⁻¹ KOH solution. Meanwhile, the Tafel slope values indicate that the Co/Co₃O₄/Ti₃C₂T_x catalyst follows the Volmer–Heyrovsky mechanism. Co/Co₃O₄ can accelerate the adsorption and deionization of H₂O to synthesize H₂. When water molecules are adsorbed on the Co/Co₃O₄ interface, the Co₃O₄ phase helps to split the HO-H bond and produce adsorbed hydroxide ions (OH⁻) and hydrogen atoms (H_ads), while the Co phase can integrate H_ads to produce H₂ [29].

The long-term durability of the Co/Co₃O₄/Ti₃C₂T_x catalyst was tested in a 1 mol L⁻¹ KOH electrolyte, and the LSV results are shown in Figure 11g. Compared with the initial curve, the LSV curve did not change significantly after 1000 CV cycles, and the η value was only 8 mV different, indicating that Co/Co₃O₄/Ti₃C₂T_x had good cyclic stability in a 1 mol L⁻¹ KOH electrolyte. In addition, in order to better reflect the activity changes of Co/Co₃O₄/Ti₃C₂T_x before and after 1000 CV tests, the MA value of Co/Co₃O₄/Ti₃C₂T_x before and after 1000 CV cycles was calculated from Equation (5). Before the stability test, the MA value of Co/Co₃O₄/Ti₃C₂T_x was 67.57 mA (mgCo)⁻¹. After the stability test, the MA value of Co/Co₃O₄/Ti₃C₂T_x was 60.14 mA (mgCo)⁻¹. The MA value of Co/Ti₃C₂T_x did not change much after 1000 CV cycles, and the activity of Co/Co₃O₄/Ti₃C₂T_x could still be maintained at 89% after 1000 CV cycles. The results show that Co/Co₃O₄/Ti₃C₂T_x has good catalytic stability in a 1 mol L⁻¹ KOH solution.

The electrocatalytic performance for the HER of the synthesized Co/Co₃O₄/Ti₃C₂T_x was compared with that of MXene-based catalysts reported in the literature, and the results are shown in

Table 2. Comparison of electrocatalytic performances for HERs of representative MXene-based catalysts.

Type of Electrocatalyst	Electrolyte	Electrochemical Performance	Refs.
COF/Ti3C2Tx	0.5 M H2SO4	A very low onset potential of 19 mV and a small Tafel slope of 50 mV dec−1	[30]
MIL/Ti3C2Tx	0.5 M H2SO4	An operating overpotential of 107 mV at 10 mA/cm2	[31]
Mo2CTx	0.5 M H2SO4	A low overpotential of 283 mV at 10 mA cm−2 and an average TOF of −0.02 H2 s−1 at 200 mV	[32]
MoS2/Ti3C2/C	0.5 M H2SO4	A low overpotential of 135 mV at 10 mA cm−2 and a Tafel slope of 45 mV dec−1	[33]
Co-MoS2/Mo2CTx	1 M KOH	A low overpotential of 112 mV at 10 mA cm−2 and a Tafel slope of 82 mV dec−1	[34]
Co/Co3O4/Ti3C2Tx	1 M KOH	A low overpotential of 87 mV at 10 mA cm−2 and a Tafel slope of 105.17 mV dec−1	This work

. It could be seen from the table that the PtCo/Ti₃C₂T_x catalyst prepared in this study had better performance in terms of its overpotential and Tafel slope, making it a promising electrocatalyst for the HER.

4. Conclusions

In this study, the Co/Co₃O₄/Ti₃C₂T_x catalyst was synthesized by supporting Co/Co₃O₄ with Ti₃C₂T_x. In a 1 mol L⁻¹ KOH solution, the Co/Co₃O₄/Ti₃C₂T_x catalyst showed excellent HER performance, with a low overpotential of 87 mV at 10 mA cm⁻² and a Tafel slope of 105.17 mV dec⁻¹. The smaller Tafel slope value indicates that the Co/Co₃O₄/Ti₃C₂T_x catalyst has a fast kinetic process and follows the Volmer–Heyrovsky mechanism in a 1 mol L⁻¹ KOH solution. The results also show that Co/Co₃O₄/Ti₃C₂T_x has good catalytic stability in a 1 mol L⁻¹ KOH solution, and the activity of Co/Co₃O₄/Ti₃C₂T_x could still be maintained at 89% after 1000 CV cycles. It can be expected that these results provide a possible new way to design and prepare a low-cost, high-performance HER catalyst.