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Article

Copper-Decorated Ti3C2Tx MXene Electrocatalyst for Hydrogen Evolution Reaction

by
Buxiang Wang
,
Qing Shu
*,
Haodong Chen
,
Xuyao Xing
,
Qiong Wu
and
Li Zhang
Faculty of Materials Metallurgy and Chemistry, Jiangxi University of Science and Technology, Ganzhou 341000, China
*
Author to whom correspondence should be addressed.
Metals 2022, 12(12), 2022; https://doi.org/10.3390/met12122022
Submission received: 29 October 2022 / Revised: 19 November 2022 / Accepted: 23 November 2022 / Published: 25 November 2022
(This article belongs to the Special Issue Deposition of Metals and Their Application in Catalytic Processes of Energy Interest)
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Abstract

:
It remains a formidable challenge to prepare an economical and stable electrocatalyst for hydrogen evolution reaction using non-precious metals. In this study, MXene (Ti3C2Tx) nanosheets were prepared by high-energy ultrasound treatment, and Cu nanoparticles were prepared by NaBH4 as a reducing agent. Then, the electrocatalyst Cu/Ti3C2Tx, suitable for hydrogen evolution reaction (HER), was prepared by supporting Cu with Ti3C2Tx. The structure, morphology, crystal phase and valence state of the obtained catalyst were determined by a variety of characterization analysis methods, and the influence of these properties on the catalytic performance is discussed here. The results of Brunner–Emmet–Teller (BET) showed that Ti3C2Tx can effectively inhibit Cu agglomeration. Results of Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) showed that Cu has metallic and oxidized states. X-ray Photoelectron Spectroscopy (XPS) further revealed the existence of multivalent states in Cu, which would contribute to the formation of electron transfer channels and the enhancement of electrocatalytic activity. In addition, the Cu/Ti3C2Tx catalyst has strong hydrophilicity, as measured by contact angle, which is conducive to HER. Ti3C2Tx has acceptable electrocatalytic hydrogen evolution performance: under alkaline conditions, when the current density is 10 mA cm−2, HER overpotential is as low as 128 mV and the Tafel slope is as low as 126 mV dec−1. Meanwhile, Ti3C2Tx showed adequate stability for HER (94.0% of the initial mass activity after 1000 CV cycles). This work offers insights into the development of high-performance non-precious metal-based catalysts to achieve the high performance of HER in alkaline electrolytes.

1. Introduction

Hydrogen energy, which is clean, efficient, safe and sustainable, has attracted extensive attention from researchers around the world in the face of global energy shortages and increasingly serious environmental pollution [1,2]. It has potential in the field of transportation and industrial production, so it is considered as one of the most promising energy types of the 21st century. At present, electrolysis of water is considered a mature and promising method for hydrogen production. However, the cathodic reaction (hydrogen evolution reaction, HER) and anodic reaction (oxygen evolution reaction, OER) have the problems of high energy consumption and instability in the process of hydrogen production from water electrolysis [3]. The theoretical thermodynamic voltage E0 is 1.23 V when the electrolytic water reaction is carried out at a standard atmospheric pressure and 25 °C [4,5]. However, due to the influence of electrochemical resistance, transmission resistance, external circuit resistance and other factors, the voltage E applied at both ends of the electrode must be greater than 1.23 V. Therefore, the current technical research mainly focuses on how to reduce the energy loss and improve the energy conversion efficiency in the process of hydrogen production from water electrolysis.
Efficient electrocatalysts play an important role in the process of hydrogen production from water electrolysis, including reducing the reaction barrier, reducing energy consumption, increasing reaction efficiency and rate, etc. Therefore, the research of efficient electrocatalysts has become the most important task in the exploration of electrolysis water hydrogen production technology. At present, the main electrocatalysts used in water electrolysis for hydrogen production are precious metals (Pt, Ru, Pd, Ir, etc.) and their alloys. Although such electrocatalysts have low overpotential, they also have the defects of high production cost and unscalable production, which cannot reach the standard of industrial scale of water electrolysis for hydrogen production [6,7]. Therefore, researchers have turned their attention to the relatively inexpensive non-precious metals (Ni, V, Mo, Fe, Cr, W, etc.) and their alloys [8]. At present, Cu-based catalysts have been found to be effective in electrocatalytic oxidation of glucose and reduction of H2O2 [9,10]. However, there is still a gap in the electrocatalytic performance of Cu-based catalysts when it is compared with nickel-based and cobalt-based catalysts. This limits the application of Cu-based catalysts in HER and OER [11]. At present, with the unremitting efforts of researchers, the catalytic performance of Cu-based catalysts in electrolysis of water for hydrogen production has been improved. Hyunsik et al. obtained two-dimensional copper oxide (CuO) nanosheets on stainless steel plates by chemical bath deposition and air annealing. They were found to have high catalytic activity and stability when used in OER [12].
Metal catalysts are usually prepared by gas-phase reduction, that is, by adding H2 or CO under heating conditions to reduce the corresponding metal oxides. The reduction process will release a lot of heat, but the copper nanoparticles are extremely sintered (≥300 °C) to render deactivation caused by the low Taman temperature of 407 °C, and overheating will lead to the condensation of the microcrystals of Cu, resulting in the reduction in catalytic activity [13,14]. Additionally, the severe clustering of nanoparticles has a negative impact on the catalytic performance [15]. In order to reduce the agglomeration of copper during preparation, Zhou et al. prepared Cu-based catalysts by using NaBH4 as a reducing agent [16]. In contrast to the gas-phase reduction method, because this method is carried out in a solution at room temperature, the solvent in the solution can both disperse the reactants and rapidly absorb the heat generated by the exothermic reaction, thus slowing down Cu agglomeration during the preparation. In addition, some studies have found that suitable support not only provides a good support structure, but also can prevent the sintering of active substances in the process of use. It improves the heat resistance and prevents deactivation of the catalyst. Spencer found that Cu supported on ZnO can enhance the spillover effect of hydrogen atoms, adsorb water, disperse the active site of Cu, reduce the sintering rate of Cu particles and keep the small grains of copper in a metastable state [17]. MXenes are a diverse group of low-dimensional transition metal carbide and nitride with the general formula of Mn+1XnTx (n = 1–3), where M is the transition metal; X is a carbide, nitride or carbonitride; and Tx is the surface termination group. The surface of MXenes (Ti3C2Tx) is rich in O, OH and F groups, which provide abundant available sites for Cu adsorption [18]. Cu2+ can be loaded onto the surface of Ti3C2Tx nanosheets by electrostatic interaction [19]. Furthermore, Cu strongly reacts with oxygen-containing surface functional groups on the surface of Ti3C2Tx nanosheets, which further promotes Cu adsorption and fixation on MXenes. In addition, Cu nanoparticles are inserted into the interspace between the MXene layers, effectively inhibiting the accumulation of MXenes and expanding the layer spacing, exposing more active sites, thereby improving its electrocatalytic performance [20,21].
In this study, MXene (Ti3C2Tx) nanosheets were prepared by high-energy ultrasound treatment, and Cu nanoparticles were prepared by NaBH4 as a reducing agent. Then, the electrocatalyst Cu/Ti3C2Tx, suitable for HER, was prepared by supporting Cu with Ti3C2Tx. The structure, morphology, crystalline phase and valence states of the samples were analyzed by several characterization methods. The activity and stability of Cu/Ti3C2Tx for HER were studied. The reasons for the improvement of electrocatalytic performance are discussed here, such as the influence of the presence of multiple electron valence states in copper on electron transport, Ti3C2Tx on Cu agglomeration, and strong hydrophilicity of Ti3C2Tx catalyst on water splitting.

2. Materials

All reagents were of analytic reagent grade and used as received without additional purification. Copper nitrate trihydrate (Cu(NO3)2·3H2O), titanium aluminum carbide 312(Ti3AlC2), lithium fluoride (LiF) and sodium borohydride (NaBH4) were purchased from Aladdin Corporation (Ontario, CA, USA).

2.1. Preparation of Cu/Ti3C2Tx Catalyst

Ti3C2Tx: First, we put 1 g lithium fluoride (LiF) into a Teflon beaker containing 20 mL hydrochloric acid (HCl 12 mol/L). Then, we put the beaker containing the mixture on the constant-temperature magnetic stirrer and set the temperature to 55 °C and the rate of magnetic stirring to 100 rpm. After LiF was completely dissolved in hydrochloric acid, 1 g of titanium aluminum carbide (Ti3AlC2) powder was slowly placed into a beaker containing the mixture. The mixture was stirred continuously for 24 h to remove the Al metal layer. Second, the obtained suspension was decanted into a centrifuge tube and centrifuged at 6400 rpm for 10 min to stratify the suspension. The upper layer was clear liquid and the lower layer was sediment. We poured the supernatant out of the centrifuge tube, added a certain amount of deionized water and shook well, and then continued to centrifuge at the rate of 6400 rpm for 10 min, so that the suspension was stratified again. After that, we poured out the liquid on the top of the centrifuge tube, refilled it with deionized water and shook well. This process was repeated until the pH of the mixture reached 6 to ensure that residual HF and other impurities were removed. Finally, the suspension was filtered by using a suction filter, and the collected sediment was dried in an oven at 60 °C for 24 h to obtain Ti3C2Tx (Tx stands for O, F, OH functional group). The illustration of the Cu/Ti3C2Tx synthetic is shown in Figure 1.
Cu/Ti3C2Tx: In order to synthesize Ti3C2Tx nanosheets, 0.3 g Ti3C2Tx was poured into a beaker containing 30 mL deionized water and sonicated in an ice water bath for 3 h. We placed the beaker filled with Ti3C2Tx on the thermostatic magnetic stirrer, set the temperature to 25 °C, added 0.6 g of Cu(NO3)2 to the Ti3C2Tx solution, stirred at 800 RPM for 30 min and let stand for another 3 h until the solution was clear and layered (the top solution was light blue). We added a certain amount of NaBH4 until the solution was clear and layered (the upper layer was colorless and transparent), stirred for 30 min at 800 rpm and let stand for 12 h. The upper layer of the supernatant in the beaker was poured out and the remaining pellet was dried in a freeze dryer for 48 h to obtain Cu/Ti3C2Tx.
To expand the comparison scope, the preparation of Cu and Ti3C2Tx was similar to the above. Cu(NO3)2 solution was directly added to NaBH4 to obtain Cu, and Ti3C2Tx solution after ultrasonic dispersion was directly dried to obtain Ti3C2Tx.

2.2. Material Characterization

The crystalline phases of Ti3AlC2, Ti3C2Tx and Cu/Ti3C2Tx were analyzed by using XRD (Empyrean, Panalytical, Almelo, The Netherlands). The microstructure of Ti3C2Tx and Cu/Ti3C2Tx catalysts was characterized by SEM (MLA650F, FEI, Hillsboro, OR, USA) and TEM (Tecnai G2-20, FEI, Hillsboro, OR, USA). In addition, the elemental phases of the Cu/Ti3C2Tx catalyst were analyzed by Scanning Electron Microscopy coupled with Energy-Dispersive X-ray Spectroscopy (SEM-EDX, MLA650F, FEI, Hillsboro, OR, USA). Surface chemical states of the Cu/Ti3C2Tx catalyst were determined by XPS (ESCALAB Xi+, Waltham, MA, USA). The surface wetting performance of Ti3C2Tx, Cu and Cu/Ti3C2Tx was tested on the contact angle meter (JCY type, ShangHai Fangrui Instrument Co., Ltd., Shanghai, China). The specific surface areas of Ti3C2Tx, Cu and Cu/Ti3C2Tx were determined by a N2 physical adsorption instrument (ASAP2020, Micromeritics Instrument Corporation, Austin, TX, USA).

2.3. Electrochemical Measurements

The electrochemical properties of Cu, Ti3C2Tx and Cu/Ti3C2Tx were tested by using the electrochemical workstation (CHI660E). A carbon rod acted as the counter electrode (CE), a clip electrode coated with the sample material acted as the working electrode (WE) and a calomel electrode acted as the auxiliary electrode (AE). The catalyst was placed in a mixture of isopropyl alcohol and water solvent, and ultrasonic action was performed for 40 min. Then, it was coated on the carbon paper of the clip electrode and dried at 25 °C. All tests were conducted in a 1 M KOH solution and the test ambient temperature was controlled at 25 °C. All potential values were converted to standard electrode potential relative to the reversible hydrogen electrode (RHE), calculated as follows:
VRHE = V(SCE) + 0.0591 × pH + 0.2415
HER activities were measured by linear sweep voltammetry (LSV). The overpotential (η) of the cathode can be obtained from the LSV graph. In this work, the scanning rate in the LSV test was 5 mV s−l. In addition, the slope of the Tafel plot can also be calculated from the LSV graph. To quantitatively determine the double-layer capacitance (Cdl) of the catalyst, the current density of the catalyst was tested by using cyclic voltammetry (CV) and setting different sweep rates. Since the value of the electrochemical specific surface area (ECSA) is proportional to the value of Cdl, the ECSA of catalyst can be obtained from the Cdl value. To test the stability of the catalyst, long-term cyclic voltammetry was used.

3. Results and Discussions

3.1. Characterization of Cu/Ti3C2Tx Catalyst

3.1.1. SEM

The SEM images clarify the morphology and microstructures of Cu/Ti3C2Tx. Ti3C2Tx exhibits a distinct layered structure [22]. After HF etches, various surface terminations are formed on the surface of Ti3C2Tx [23], which can provide abundant sites for Cu atoms anchored on the Ti3C2Tx. Cu2+ can be reacted with surface terminations of Ti3C2Tx, which transforms into Cu1+ through self-induced redox reactions [24]. Figure 2a is the SEM image of Cu/Ti3C2Tx. These particles loaded on the surface of Ti3C2Tx are Cu, which can be inserted into the gaps between the layers [25]. In addition, Figure 2b displays the elemental distribution maps of Cu/Ti3C2Tx, showing the distribution of elements in Cu/Ti3C2Tx by SEM-EDX, and indicating the presence of C, O, Ti and Cu distributed uniformly throughout the catalyst surface. In addition, the estimated atomic contents of C, O, Ti and Cu are about 26.98%, 33.72%, 9.70% and 29.59%, respectively. The atomic proportion of O is the highest in the Cu/Ti3C2Tx, indicating a potential presence of oxidized copper.

3.1.2. TEM

In order to obtain more detailed structural information of the catalyst, the synthesized Ti3C2Tx and Cu/Ti3C2Tx nanostructures were investigated by TEM. Figure 3a shows a typical sheet-like morphology of Ti3C2Tx. Figure 3b shows structural characteristics of Cu/Ti3C2Tx. Numerous particles scatter across Ti3C2Tx. According to the analysis of the SEM results, Cu2+ in solution is reduced to Cu0 by NaBH4, which aggregates into small particles and covers the surface of Ti3C2Tx [25]. Moreover, gaps and long cones between these particles can be observed, which are formed by a large number of reduced Cu0 and Cu1+ ions that grow in a certain direction [26]. As can be seen from Figure 3c, the interplanar spacing of 0.21 nm can be assigned to the (111) crystallographic plane of Cu phase [27,28]. In Figure 3d, the interplanar spacing of 0.25 nm corresponds to the (111) plane of Cu2+1O phase [29], indicating the presence of metallic and oxidized states in Cu/Ti3C2Tx.

3.1.3. Physical Adsorption/Desorption Measurements of N2

The specific surface areas of Ti3C2Tx, Cu and Cu/Ti3C2Tx were determined by the Brunner–Emmet–Teller (BET) method, and the test was performed on a N2 physical adsorption instrument. N2 adsorption isotherms of Ti3C2Tx, Cu and Cu/Ti3C2Tx are given in Figure 4. The specific surface area of Ti3C2Tx is 2.9 m2 g−1. Ti3C2Tx is easy to stack together, thus losing the characteristic of a high specific surface area [30]. The specific surface area of Cu is 6.4 m2 g−1, which can be attributed to the easy agglomeration of nanoscale Cu0 [31]. The specific surface area of the Cu/Ti3C2Tx is 9.5 m2 g−1, which indicates the successful preparation of a Cu/Ti3C2Tx catalyst with a high specific surface area. This can be attributed to the fact that Cu nanoparticles effectively inhibit the accumulation of Ti3C2Tx [20,21]. It is worth noting that the number of active sites on the surface of a catalyst is positively correlated with its catalytic performance. Catalysts with large specific surface areas can provide more active sites for HER, helping to reduce the overpotential of electrochemical reactions [32]. Put another way, the actual surface of the catalyst is larger than the macroscopic geometric area, which can make it have more active sites and shorten the ion migration distance, thus facilitating the release of H2 in alkaline solution.

3.1.4. XRD

The crystal structure of Ti3AlC2, Ti3C2Tx and Cu/Ti3C2Tx can be reflected by the XRD diffraction pattern. As shown in Figure 5a, Ti3AlC2 has pronounced characteristic diffraction peaks at 2θ of 9.5°, 19.1° and 39.0°, which are consistent with the data of Ti3AlC2 (JCPDS. NO.52-0875), corresponding to Ti3AlC2 (002), (004) and (104) crystal planes, respectively. After etching, diffraction peaks at 9.5°, 19.1° and 39.0° (2θ) are all significantly attenuated. Moreover, a new peak appears at 6.1°, indicating that Ti3AlC2 has successfully transitioned into Ti3C2Tx [33,34]. The XRD pattern of Ti3C2Tx is shown in Figure 5b, and 10 characteristic diffraction peaks appear at 2θ of 34.0°, 36.6°, 39.1°, 41.6°, 48.3°, 56.7°, 60.0°, 65.3°, 70.2° and 73.9°, corresponding to Ti3AlC2 (101), (103), (104), (105), (107), (109), (110), (1011), (1012) and (108) crystal planes, respectively. At the same time, three characteristic diffraction peaks appeared at 2θ of 35.9°, 41.7° and 60.4°, basically consistent with TiC (JCPDS. NO.32-1383), corresponding to TiC (111), (200) and (220) crystal planes, respectively, which indicates that the inevitable presence of TiC and Ti3AlC2 impurities in the product is illustrated after etching [35]. In Figure 5c, four peaks appear at 2θ of 43.2°, 50.4°, 74.1° and 89.9°, respectively, and these characteristic peaks are consistent with the standard of Cu (JCPDS. NO.04-0836), corresponding to the (111), (200), (220) and (311) crystal planes of Cu, respectively. In addition, Cu/Ti3C2Tx also appeared at 2θ of 29.5°, 36.4°, 42.2°, 52.4°, 61.3°, 69.5°, 73.5° and 77.3°, and these peaks are related to Cu2+1O (JCPDS.NO.05-0667), corresponding to Cu2+1O (110), (111), (200), (211), (220), (310), (311) and (222) crystal planes, respectively. Additionally, a peak appears at 60.7° related to Ti3C2Tx [34], which suggests that Cu/Ti3C2Tx has been successfully synthesized. In addition, the analysis result of XRD is consistent with the results of the TEM, which further confirms the presence of metallic (Cu) and oxidized (Cu2+1O) states in Cu/Ti3C2Tx.

3.1.5. XPS

The surface chemical composition of the Cu/Ti3C2Tx catalyst was identified by XPS, which can further investigate the oxidation state and composition of Cu/Ti3C2Tx. In Figure 6a, the XPS survey scan band of the Cu/Ti3C2Tx catalyst reveals the occurrence of Cu, C, O and Ti elements in the catalyst. Figure 6b shows the XPS Cu 2p spectrum of the Cu/Ti3C2Tx catalyst; peaks at 935.1 eV (Cu 2p3/2) and 954.7 eV (Cu 2 p1/2) can be attributed to Cu2+ [36]. Peaks at Cu 2p1/2(952.0) and Cu 2p3/2(932.2 eV) correspond to Cu+ [37]. In addition, peaks at 932.1 eV (Cu 2p3/2) and 952.5 eV (Cu 2p1/2) can be attributed to Cu0 [27]. This indicates the presence of Cu and Cu2+1O in Cu/Ti3C2Tx materials [38]. The C1s spectrum of the Cu/Ti3C2Tx catalyst is shown in Figure 6c, containing C-Ti (281.8 eV), C-C (284.8 eV), O-C (286.4 eV) and O-C=O (289.0 eV) bonds [39,40]. Figure 6d shows the O 1s spectrum of the Cu/Ti3C2Tx catalyst. The peak at 532.9 eV conforms to H-O-H bonds [41], which are adsorbed on the surface of Cu2+1O-doped Cu [42,43]. The peak at 531.7 eV corresponds to O-H, while the peak at 530.5 eV may correspond to Cu2+1O. This shows that the surface of the catalyst contains not only Cu oxides (Cu-O), but also hydroxides (Cu-OH), and Cu oxide/hydroxide catalysts can effectively accelerate the dissociation of water [44,45,46]. Moreover, CuO is a hydrophilic material [47,48], which also verifies that the Cu/Ti3C2Tx catalyst has a certain hydrophilicity.

3.1.6. Contact Angle Test

The contact angle is a quantitative measure of the wettability of the surface. To further test the hydrophilicity of Ti3C2Tx, Cu and Cu/Ti3C2Tx catalysts, the contact angle between the drop and the catalyst surface was measured. In Figure 7a, the contact angle of Ti3C2Tx is 142.3°. In Figure 7b, the contact angle of Cu is 33.1°. In Figure 7c, the contact angle of Cu/Ti3C2Tx is 12.7°. The contact angle of Cu/Ti3C2Tx is much smaller than that of Ti3C2Tx and Cu, which indicates that the Cu/Ti3C2Tx catalyst has stronger hydrophilicity. In light of the SEM and TEM analysis results, Cu nanoparticles are inserted into the interspace between the Ti3C2Tx layers, effectively inhibiting the accumulation of Ti3C2Tx, exposing more hydrophilic functional groups, thereby improving its hydrophilicity. In the hydrogen evolution reaction, the stronger the hydrophilicity of the catalyst, the shorter the time that the bubbles generated by the reaction stay on the catalyst surface, and the smaller the size of the bubbles leaving the surface of the catalyst, which is more conducive to the improvement of the efficiency of the electrolysis water reaction [49].

3.2. HER Activity in Alkaline Electrolytes

The electrocatalytic properties of the synthesized Cu/Ti3C2Tx electrocatalysts were investigated by the CV measurement at different sweep speeds. As shown in Figure 8a, the area of the closed curve increases with increasing scanning speed. To expand the comparison scope, the Cdl of the catalysts was defined by the CV of the synthesized Cu, Ti3C2Tx and Cu/Ti3C2Tx. As shown in Figure 8b, the Cdl of the Cu/Ti3C2Tx is 29.4 mF cm−2. The Cdl values of the Ti3C2Tx and Cu are 9.8 and 2.1 mF cm−2, respectively. The Cdl of Cu/Ti3C2Tx is 3 times higher than that of Ti3C2Tx. Furthermore, the ECSA of the catalysts can be obtained from the Cdl value by a formulated methodology [50]. As shown in Figure 8c, the ECSA of Cu/Ti3C2Tx is 735 cm−2. The ECSA values of Ti3C2Tx and Cu are 245 and 52.5 cm−2, respectively. The ECSA of Cu/Ti3C2Tx is 3 times higher than that of Ti3C2Tx. The large ECSA value can usually be attributed to the unique structure of Cu/Ti3C2Tx. Ti3C2Tx supports small Cu nanoparticles, which not only effectively prevents Cu agglomeration, but also effectively improves the specific surface area of Cu/Ti3C2Tx catalyst.
The HER performance of Ti3C2Tx, Cu and Cu/Ti3C2Tx was investigated in 1 M KOH solution. In Figure 9a, Ti3C2Tx exhibits the worst HER activity and Cu/Ti3C2Tx exhibits the best HER performance. The catalytic activity exhibits the following trend: Cu/Ti3C2Tx > Cu > Ti3C2Tx. The improvement of the catalytic performance of Cu/Ti3C2Tx can be attributed to the insertion of Cu nanoparticles into Ti3C2Tx, which improves the catalytic activity and enhances the conductivity of Ti3C2Tx-based catalysts [51]. Cu/Ti3C2Tx shows the lowest overpotential (η10 = 128 mV), while that of Cu is 360 mV and that of Ti3C2Tx is 541 mV. In addition, the overpotential (η10) of Cu/Ti3C2Tx is lower than that of most of the non-precious catalysts for HER listed in Table 1 [52,53,54,55,56,57,58,59].
The Tafel slope is an important parameter in evaluating the mechanistic pathway of the hydrogen evolution reaction. The lower the value of Tafel, the stronger the HER catalytic activity of the catalyst. In Figure 9b, the Tafel slope (Cu/Ti3C2Tx) is 126 mV dec−1, which is less than that of Ti3C2Tx (468 mV dec−1) and Cu (290 mV dec−1), indicating that the dynamics for HER of Cu/Ti3C2Tx are faster [60]. In addition, the value of the Tafel slope infers the rate controlling step of the reaction, and the Tafel slope of the Cu/Ti3C2Tx catalyst is 126 mV dec−1, corresponding to the Volmer–Heyrovsky mechanism, while the rate controlling step of this reaction is a Volmer step [61,62].
It is worth noting that the HER under alkaline conditions involves the dissociation of water molecules (Volmer reaction), and water molecules must first adsorb and dissociate on the surface of the catalyst to form adsorption states of Hads and OH¯ [63]. In light of the SEM, TEM, XRD and XPS analysis results, CuO and Cu(OH)2 contained in Cu/Ti3C2Tx promote the water dissociation and improve the overall efficiency of water electrolysis. Figure 9c depicts the LSV curve after 1000 CV cycles under alkaline conditions with a current density of 9.4 mA cm−2 at an overpotential of 128 mV. Compared with the current density of Cu/Ti3C2Tx before the test, the current density of Cu/Ti3C2Tx at an overpotential of 128 mV after the test only decreases by 0.6 mA cm−2. As shown in Figure 9d, the mass activity of Cu/Ti3C2Tx at an overpotential of 128 mV is 16.95 A/gCu. After the test, the mass activity of Cu/Ti3C2Tx decreases by only 1.02 A/gCu, indicating that Cu/Ti3C2Tx has better stability.

4. Conclusions

An electrocatalyst for hydrogen evolution, Cu/Ti3C2Tx, was synthesized by loading Cu onto Ti3C2Tx. Cu/Ti3C2Tx shows efficient electrocatalytic performance. Under alkaline conditions, the HER overpotential is as low as 128 mV at a current density of 10 mA cm−2. The Tafel slope of the Cu/Ti3C2Tx catalyst is as low as 126 mV dec−1. In addition, the mass activity of Cu/Ti3C2Tx at an overpotential of 128 mV decreases by only 1.02 A/gCu after the stability test, which indicates that the Cu/Ti3C2Tx has acceptable stability. The high catalytic performance of Cu/Ti3C2Tx can be attributed to the following aspects: CuO and Cu(OH)2 accelerate water dissociation, and the presence of CuO makes the Cu/Ti3C2Tx catalyst more hydrophilic, which accelerates the release of bubbles and effectively improves the efficiency of the electrolyzed water reaction. This work provides insights into the development of high-performance non-precious metal-based catalysts to achieve the high performance of HER in alkaline electrolytes.

Author Contributions

Conceptualization, B.W. and Q.S.; methodology, Q.S.; software, Q.W. and L.Z.; validation, B.W.; formal analysis, B.W.; investigation, B.W.; resources, H.C. and X.X.; data curation, B.W.; writing—original draft, B.W.; writing—review and editing, Q.S. and B.W.; visualization, B.W.; supervision, Q.S.; project administration, B.W.; funding acquisition, Q.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by [National Natural Science Foundation of China] grant number [21766009].

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 ethical.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Metals 12 02022 g001 550
Figure 1. The illustration of the synthetic for Cu/Ti3C2Tx.
Figure 1. The illustration of the synthetic for Cu/Ti3C2Tx.
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Figure 2. SEM image of (a) Cu/Ti3C2Tx and (b) EDS mapping of Cu/Ti3C2Tx.
Figure 2. SEM image of (a) Cu/Ti3C2Tx and (b) EDS mapping of Cu/Ti3C2Tx.
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Figure 3. TEM image of (a) Ti3C2Tx, (b–d) Cu/Ti3C2Tx.
Figure 3. TEM image of (a) Ti3C2Tx, (b–d) Cu/Ti3C2Tx.
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Figure 4. N2 adsorption/desorption isotherms for Ti3C2Tx, Cu and Cu/Ti3C2Tx.
Figure 4. N2 adsorption/desorption isotherms for Ti3C2Tx, Cu and Cu/Ti3C2Tx.
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Figure 5. XRD pattern of (a) Ti3AlC2 and Ti3C2Tx; XRD pattern of (b) Ti3C2Tx and (c) Cu/Ti3C2Tx.
Figure 5. XRD pattern of (a) Ti3AlC2 and Ti3C2Tx; XRD pattern of (b) Ti3C2Tx and (c) Cu/Ti3C2Tx.
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Figure 6. XPS spectra of Cu/Ti3C2Tx: (a) full spectrum of Cu/Ti3C2Tx, (b) Cu 2p spectra of Cu/Ti3C2Tx, (c) C 1s spectra of Cu/Ti3C2Tx, (d) O 1s spectra of Cu/Ti3C2Tx.
Figure 6. XPS spectra of Cu/Ti3C2Tx: (a) full spectrum of Cu/Ti3C2Tx, (b) Cu 2p spectra of Cu/Ti3C2Tx, (c) C 1s spectra of Cu/Ti3C2Tx, (d) O 1s spectra of Cu/Ti3C2Tx.
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Figure 7. The contact angle test of Ti3C2Tx, Cu and Cu/Ti3C2Tx; (a) Ti3C2Tx, (b) Cu and (c) Cu/Ti3C2Tx.
Figure 7. The contact angle test of Ti3C2Tx, Cu and Cu/Ti3C2Tx; (a) Ti3C2Tx, (b) Cu and (c) Cu/Ti3C2Tx.
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Figure 8. HER performance of Ti3C2Tx, Cu and Cu/Ti3C2Tx series catalysts: (a) CV curves of Cu/Ti3C2Tx at the scan rate of 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 mV s−1, (b) comparisons of Cdl, (c) comparisons of ECSA.
Figure 8. HER performance of Ti3C2Tx, Cu and Cu/Ti3C2Tx series catalysts: (a) CV curves of Cu/Ti3C2Tx at the scan rate of 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 mV s−1, (b) comparisons of Cdl, (c) comparisons of ECSA.
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Figure 9. HER performance of Ti3C2Tx, Cu and Cu/Ti3C2Tx series catalysts: (a) LSV curves at the scan rate of 5mV s−1, (b) the corresponding Tafel plots, (c) stability test chart of Cu/Ti3C2Tx, (d) comparisons of mass activity of Cu/Ti3C2Tx.
Figure 9. HER performance of Ti3C2Tx, Cu and Cu/Ti3C2Tx series catalysts: (a) LSV curves at the scan rate of 5mV s−1, (b) the corresponding Tafel plots, (c) stability test chart of Cu/Ti3C2Tx, (d) comparisons of mass activity of Cu/Ti3C2Tx.
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Table
Table 1. The overpotential of non-precious metal-based HER catalysts.
Table 1. The overpotential of non-precious metal-based HER catalysts.
CatalystOverpotential (η10)References
Cu/Ti3C2Tx128 mVThis work
Mo-Ti2Cu3133 mV[52]
Cu2(OH)PO4/Co3(PO4)2·8H2O138 mV[53]
Cu2O/g-C3N4148.7 mV[54]
PEDOT@Mn-Salen COFEDA150 mV[55]
Cu3P155 mV[56]
fs-Cu/MoS2181 mV[57]
Cu(OH)2@FCN MOF/CF290 mV[58]
Cu2S330 mV[59]
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Wang, B.; Shu, Q.; Chen, H.; Xing, X.; Wu, Q.; Zhang, L. Copper-Decorated Ti3C2Tx MXene Electrocatalyst for Hydrogen Evolution Reaction. Metals 2022, 12, 2022. https://doi.org/10.3390/met12122022

AMA Style

Wang B, Shu Q, Chen H, Xing X, Wu Q, Zhang L. Copper-Decorated Ti3C2Tx MXene Electrocatalyst for Hydrogen Evolution Reaction. Metals. 2022; 12(12):2022. https://doi.org/10.3390/met12122022

Chicago/Turabian Style

Wang, Buxiang, Qing Shu, Haodong Chen, Xuyao Xing, Qiong Wu, and Li Zhang. 2022. "Copper-Decorated Ti3C2Tx MXene Electrocatalyst for Hydrogen Evolution Reaction" Metals 12, no. 12: 2022. https://doi.org/10.3390/met12122022

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