Three dimensional Ti3C2Tx and rGO hybrid supported Pt catalyst for the high performance hydrogen evolution reaction

Abstract The effective electrocatalysts for hydrogen evolution reaction (HER) in alkaline aqueous is critical point to reduce energy losses for water electrolysis. Here, a highly active and stable electrochemical catalyst of platinum (Pt) nanoparticles loaded on three-dimensional Ti3C2Tx-reduced graphite oxide (rGO) hybrid support (Pt/Ti3C2Tx-rGO3D) has been developed for hydrogen evolution reaction (HER). The large surface of 3 D support provides an adequate site for deposition of Pt nanoparticles, leading to superior HER activity with low overpotential about 41 mV and better stability than Pt/C. This study offers a hybrid support for the development of highly active electrocatalysts for hydrogen evolution reaction.


Introduction
Hydrogen is considered as an ideal energy carrier because of its advantages of large amounts, high efficiency, high energy density and low carbon footprint. [1,2] Among various strategies for hydrogen production, alkaline water electrolysis is an ideal candidate due to its unique advantages including good manufacturing safety, stable output and high product purity. However, the sluggish kinetics of hydrogen evolution reaction (HER) causes high overpotential and large energy consumption that limits water electrolysis application . [3][4][5][6][7][8] Pt is generally considered as the suitable electrocatalysts for HER because of its high exchange current density and low overpotential. [9][10][11][12] However, their scarcity and high cost seriously hinder their commercial potential. The development of active and stable Pt-based HER electrocatalysts for water electrolysis is a key step in the realization of a hydrogen economy.
The supported Pt nanoparticles (NPs) are typically used as HER catalyst. The geometry of the NPs is restricted to the majority of the Pt atoms to provide more surface atoms for the electrochemical reaction. However, the support materials structures are another useful way to improve the catalyst activity and stability. MoS 2 with a 2 D nano-sheet structure are potential carriers for single atoms. [13] The modification of Ni atoms enhanced the adsorption of H on the surface of MoS 2 , thus the efficiency of HER would be improved. Incorporation of nickel hydroxide (Ni (OH) 2 ) with Pt can synergistically catalyze HER in alkaline media due to Ni (OH) 2 provides active sites for cleaving H À OH bonds, and Pt facilitates the combination of generated hydrogen intermediates into H 2 molecules. [14] Graphene has the advantages of high specific surface area, light weight, and chemical inertness, has been extensively employed as a stable and excellent nanocatalysts support for synthesizing efficient heterogeneous catalysts. [15][16][17][18] 2 D transition metal carbides and/or nitrides (MXene) have attracted great attentions for electrochemical catalysts due to their kinetic-favorable layered nanostructure, high conductivity, and good surface reactivity. [19][20][21][22][23][24] However, MXene suffers from a tendency of sheets aggregation because of the hydrogen bonding and van der Waals attraction. [25] 3 D structure is an ideal way to inhibit the internal aggregation effectively, and 3 D skeleton has high specific surface area. [26] Moreover, the 3 D structure could benefit the ionic and electronic conductivity and improve the stability. Nevertheless, it is still a challenge to achieve the direct assembly of individual MXenes sheets into 3 D macrostructure with a stable interlinked network.
Here, we report a catalyst with Pt loaded on 3 D Ti 3 C 2 T x -rGO hybrid support via one-step hydrothermal method. The Pt/Ti 3 C 2 T x -rGO 3D catalysts exhibit good electrochemical properties that the overpotential at a current density of 10 mA cm À2 is only 41 mV, and the Tafel slope is 28 mV dec À1 . Moreover, the Pt/Ti 3 C 2 T x -rGO 3D catalysts outperformed the commercial Pt/C catalysts for stability. The overpotential of Pt/Ti 3 C 2 T x -rGO 3D catalyst is increased by only 30 mV after 30000 s stability test and 3 mV after 1000 cycles of voltammetry test. This strategy offers a novel support and a useful way to design and synthesis active and stable catalysts for HER in alkaline water electrolysis. Experimental Synthesis of Pt/Ti 3 C 2 T x -rGO 3D catalyst 80 mg Ti 3 C 2 T x -rGO 3D support was dispersed in 80 ml deionized water, and 1.34 ml 40 mg ml À1 H 2 PtCl 6 Á6H 2 O was added into the mixture. The 15 ml cold water with 300 mg NaBH 4 was dripped into the mixture slowly until no bubbles observed. The mixture was rinsed several times until pH$7, and the sediment was flited and vacuum dried at 60 C for 6 hours to achieve Pt/Ti 3 C 2 T x -rGO 3D catalyst as shown in Scheme S1. Pt/rGO 2D , Pt/rGO 3D , Pt/Ti 3 C 2 T x2D , Pt/ Ti 3 C 2 T x3D , Pt/Ti 3 C 2 T x -rGO 2D can be obtained by the same reduction method. Others experiments and characterization methods are shown in SI.

Results and discussions
The characteristic peak (002) of Ti 3 C 2 T x left shifted from 9.76 to 6.72 as shown in the XRD patterns ( Figure 1a). It indicated that the lamellar spacing increasing because of successful etching and intercalation. The characteristic peak of GO shifted from 9.39 to 21.47 because of layers distance decrease during hydrothermal and NaBH 4 reduction process. The broad diffraction peak presented reduced GO sheets with poor ordering along hydrothermal reaction. [27] The diffraction peaks of (111), (200) and (220) could be identified as face centered cubic (fcc) structured Pt particles. Meanwhile, disappearance of peak (002) of Ti 3 C 2 T x in Pt/ Ti 3 C 2 T x -rGO 3D represents the incorporation of rGO which led to an increase interplanar spacing of the MXene flakes when the hybrid support achieved. The Raman spectra in Figure 1b reveals two obvious peaks at 1360 cm À1 (D band) and 1580 cm À1 (G band). The I D /I G ratio (the intensity ratio between the D peak and the G peak) for Pt/Ti 3 C 2 T x -rGO 3D and Pt/C are 1.29 and 1.05, respectively. This higher I D /I G ratio indicates much more graphite domain or defects in the sample because of three-dimensional porous structure. Compare to activated carbon support used for Pt/C, the Ti 3 C 2 T x -rGO 3D supports possess better loading capacity, which facilitate the catalytic performance of Pt/ Ti 3 C 2 T x -rGO 3D . Figure 2a exhibits a continuous substrate formed by Ti 3 C 2 T x and rGO with uniformly dispersion of Pt nanoparticles. The uniform distribution of Ti, C, O, F and Pt elements has been evidenced by EDS (Figure 2b-e). Figure  S1 presents the freeze-dried Ti 3 C 2 T x -rGO 3D porous structure with the typical three-dimensional interconnected skeleton and continuous pores size, because Ti 3 C 2 T x connects to the rGO backbone through the interfacial interactions and forms strong p-p interaction. [28] The 3 D structure could effectively prevent the agglomeration of rGO and Ti 3 C 2 T x nanosheets during the electrolysis. Figure 3a exhibits the XPS survey spectrum of the elemental compositions of Pt/C and Pt/Ti 3 C 2 T x -rGO 3D . Pt 4f and C 1 s with binding energy of 74 eV and 285 eV are observed in Pt/C, respectively. The XPS survey of the Pt/Ti 3 C 2 T x -rGO 3D presents the element of Pt 4f, C 1 s, Ti 2p and O 1 s with binding energies of 74 eV, 285 eV, 456 eV and 531 eV, respectively. The binding energies of Pt/Ti 3 C 2 T x -rGO 3D at Pt 4f 7/2 and Pt 4f 5/2 were 71.43 eV and 74.68 eV as shown in Figure 3b, which were 0.25 eV and 0.5 eV lower than that of Pt/C, respectively. The lower binding energies may promote the electron transfer from Pt nanoparticles to Ti 3 C 2 T x -rGO 3D support and improve the HER catalytic activity. [29] The XPS survey are divided into two parts that ascribed to Pt metal and PtO, labeled as Pt0 and Pt 2þ , respectively. As shown in Figure 3c-d and Table S1, the binding energy of Pt/Ti 3 C 2 T x -rGO 3D have a lower value than Pt/C, which indicates a downshifted d-band center caused a weaker chemical adsorption energy with oxygen-containing species such as CO ads and OH ads . [30][31] Meanwhile, the less oxidation state (Pt 2þ ) of Pt/Ti 3 C 2 T x -rGO 3D are observed according to a higher utilization of Pt0, which benefits catalytic activity and utilization of Pt. In Figure S2a, the C 1 s XPS spectrum exhibits two obvious peaks at 284.8 eV and 285.8 eV, which are designated to sp2 carbon (C ¼ C) and C-Ti, respectively. As shown in Figure S2b, the two peaks at 530.6 and 539.9 eV are ascribed to O-Ti and surface hydroxyl groups, respectively. In Figure S2c, the two peaks with 458.8 eV and 464.7 eV binding energies are attributed to the lattice Ti-O bond in TiO 2 , and the peak at 459.5 eV binding energy is correspond to the Ti-C bond in Ti 3 C 2 .
The specific surface area of Ti 3 C 2 T x -rGO 3D was 100.05 m 2 g À1 , while that of Ti 3 C 2 T x -rGO 2D is only 80.295 m 2 g À1 as shown in Figure 4a. This result could be attributed to the interconnected porous network and mitigated restacking of Ti 3 C 2 T x nanosheets in the support. The pore sizes of Ti 3 C 2 T x -rGO 3D are between 3 and 5 nm as    shown in Figure 4b. Moreover, rGO embedded into the Ti 3 C 2 T x sheet layer could effectively alleviate the self-aggregation. The nano-size pores may benefit to the ion/electron/ gas transport.
A 41 mV overpotential at 10 mA cm À2 for HER of Pt/ Ti 3 C 2 T x -rGO 3D was achieved which is much lower than that of commercial Pt/C and Pt/Ti 3 C 2 T x -rGO 2D indicating higher kinetic efficiency of 3 D catalyst as shown in Figure  5 and Table S2. The 3 D porous skeleton support could provide controllable size of Pt particles and larger surface for particles deposition as shown in Figure S3. Therefore, the large number of accessible edges will serve as active catalytic sites to improve the HER activity. Furthermore, it enables the species rapidly diffuse in HER reaction. [1] The high catalytic performance of Pt/Ti 3 C 2 T x -rGO 3D may also be attributed to strong chemical and electronic coupling between the porous framework and Pt nanoparticles. [32 , 33] The Tafel slope of Pt/Ti 3 C 2 T x -rGO 3D is 28 mV dec À1 , which is the lowest value among all catalysts. This excellent performance could be attributed to the following advantages of Ti 3 C 2 T x -rGO 3D support: (a) the three-dimensional skeleton constructed by Ti 3 C 2 T x and rGO, could provide sufficient reaction sites; (b) the functional groups on Ti 3 C 2 T x and rGO 3D surfaces enhance the interaction between the Pt particles and support that improve the stability during the electrochemical reaction; (c) The mesoporous formed on the hybrid support promote the diffusion of elements.  As shown in Figure 5c, the internal resistance and charge transfer resistance of Pt/Ti 3 C 2 T x -rGO 3D are 7.8X and 34.0 X, respectively. The Pt/Ti 3 C 2 T x -rGO 3D catalysts could provide the high abundance of available catalytic active sites and lower the charge transfer resistance. After 30000 s stability test, Pt/Ti 3 C 2 T x -rGO 3D demonstrates significantly higher HER performance compared to the commercial Pt/C as Figure 5d shown, that 130 mV, 55 mV, 36, mV, 34 mV, and 30 mV potential drop for Pt/C, Pt/Ti 3 C 2 T x -rGO 2D, Pt/ rGO 3D, Pt/rGO 2D , Pt/Ti 3 C 2 T x -rGO 3D, correspondingly. As shown in Figure 6, there is only 3 mV potential drop for Pt/ Ti 3 C 2 T x -rGO 3D while 10 mV potential drop for Pt/C, indicating the better HER catalytic stability of Pt/Ti 3 C 2 T x -rGO 3D . The better stability may be attributed to the strong interaction between the deposited Pt nanoparticles and the supports. [34] Conclusion The 3 D porous Ti 3 C 2 T x -rGO 3D hybrid support is synthesized via a one-step hydrothermal step, and the catalyst was synthesized by an integration of Pt metal and 3 D frame support through a facile chemical method. Pt/Ti 3 C 2 T x -rGO 3D hybrid catalyst exhibits a HER activity with a low overpotential of 41 mV and Tafel slope of 28 mV dec À1 , because of the highly exposed edges and excellent electrical coupling to the hybrid support. What's more, Pt/Ti 3 C 2 T x -rGO 3D also exhibits notable stability. The high performance of HER activity of Pt/Ti 3 C 2 T x -rGO 3D could be attributed to the homogeneous dispersion of Pt nanoparticles and the strong interaction between Pt and hybrid support. Therefore, the approached Pt/Ti 3 C 2 T x -rGO 3D catalyst could provide an advanced performance for the HER reaction.