A proton exchange membrane fuel cell (PEMFC) can directly convert hydrogen energy into electricity and is considered as a promising power source for portable electronic devices and electric vehicles owing to its high efficiency, high energy density, and low emission [1, 2]. However, durability and high cost are the critical issues that have hindered its commercialization and widespread application.

The catalyst support plays a significant role in the activity and durability of the catalyst in a PEMFC. Therefore, the exploration of advanced catalyst supports has been a promising strategy for improving the performance of electrocatalysts [3-6]. Carbon black is a traditional support material for fuel-cell catalysts because of its high electrical and thermal conductivity, high surface area, and porosity [7, 8]. However, carbon black is prone to corrosion under the operating conditions of fuel cells, which results in rapid degradation of the cell performance and durability. Therefore, extensive efforts have been made in the last ten years to develop new carbon-based materials and other alternative support materials for fuel-cell catalysts [9-15]. Transition metal oxides such as TiO₂ [16, 17], SnO₂ [18], WO₃ [19], and CeO₂ [20, 21] have been employed as alternatives to carbon supports to enhance the stability and durability of electrocatalysts. Among them, TiO₂ is more attractive owing to its inherent stability in an electrochemical environment and strong interactions with other metals [22-25]. However, its low electrical conductivity and small surface area have limited its widespread application in fuel cells. It is very necessary to design new catalyst support materials that can further enhance the electrical conductivity and surface area of TiO₂ when it is used as a catalyst support for fuel cells. Meanwhile, graphene, a single layer of sp²-bonded carbon atoms, has remarkable electrical conductivity and a high surface area[26]. It has been a potential material for fuel cells and other energy conversion and storage devices [27-29]. The high tendency of pristine graphene sheets to agglomerate is inevitable owing to the intense van der Waals forces [30], which lead to the aggregation of the fuel-cell catalysts and reduced performance.

Fortunately, the intercalation of TiO₂ with carbon materials has proven to be a good way to improve the electrochemical performance because of the synergistic effect between the materials [31-38]. For examples, Zhu et al. [31] successfully fabricated the nitrogen-doped graphitized carbon/TiO₂ supports by heat-treatment of polypyrrole/TiO₂ composites and the addition of nitrate as a graphitization catalyst, and then loading Pt nanoparticles by using a microwave-assisted polyol method in an ethylene glycol solution. The Pt/nitrogen-doped graphitized carbon/TiO₂ catalyst had high activity for the methanol oxidation reaction compared with the commercial Johnson Matthey catalyst. El-Deen et al. [35] prepared an rGO/TiO₂ composite with different TiO₂ loadings by using the alkaline hydrothermal method. Although these catalysts exhibited higher electrocatalytic performance than Pt/C, the preparation of these catalysts through the above-mentioned methods consumed a lot of time, which is not beneficial for mass production. Recently, some new synthesis methods have been developed. Jiang et al. [33] synthesized Pt/TiO₂-C catalysts with different particle sizes and TiO₂content by a microwave-assisted polyol process. Zhao et al. [34] synthesized Pt/graphene-TiO₂ hybrid catalysts through a facile one-pot solvothermal method. Ye et al. [38] prepared a Pt/TiO₂/graphene composite under microwave irradiation. The catalysts showed higher catalytic activity and better tolerance to CO poisoning.

In this work, a more convenient, facile, and rapid method for preparing the catalyst Pt/graphene-TiO₂ is proposed. A graphene-TiO₂ composite support material and Pt catalyst were successfully prepared. X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were carried out to determine the crystalline structure and composition. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) techniques were also used to examine the electrochemical performance of the Pt/graphene-TiO₂ catalysts.

All chemicals including graphite powder (100 μm, Qingdao Henglide Graphite Co., Ltd.), KMnO₄, KNO₃, H₂SO₄ ((95–97) wt%), H₂O₂ 30 wt% (m/m), K₂S₂O₈, P₂O₅, tetrabutyl titanate (TBT), deionized (DI) water, acetic acid (HAc), 1-butyl-3- methylimidazolium tetrafluoroborate ([bmin]BF₄), and H₂PtCl₆·6H₂O were purchased from Sinopharm, China. The Nafion solution (5 wt%) was purchased from Dupont. All chemicals were of analytical grade and used as received.

The graphene oxide (GO) was synthesized using a modified Hummer's method [39]. Briefly, 5 g of graphite powder was pre-oxidized in a mixture of 4.2 g K₂S₂O₈, 4.2 g P₂O₅, and 60 mL H₂SO₄ at 80 ℃ for 4.5 h. The pre-oxidized sample was added into 120 mL H₂SO₄ at 0 ℃, and then 2.5 g KNO₃ and 16 g KMnO₄ were slowly added. After the mixture was stirred for 2 h at 35 ℃, 250 mL DI water was added, followed by the addition of 30 mL 30 wt% H₂O₂. The resultant mixture was washed with DI water, and then filtrated with HCl solution (1 mol/L) until the filtrate was almost neutral. Finally, the filter cake was dried at 60 ℃ in air.

Anatase titanium dioxide [40] was prepared using a microwave-assisted ionothermal method. Specifically, 1 mL [bmin]BF₄, 2.5 mL H₂O, and 40 mL HAc were mixed well and then 1 mL TBT was added. The solution appeared light white and turned colorless and transparent after ultrasonic treatment for 30 min. The solution was then transferred into a microwave reactor (Explorer48, CEM) with a volume capacity of 50 mL and heated at 180 ℃ for 25 min. When it was cooled to room temperature, the white precipitate was isolated by centrifugation, rinsed several times with DI water and ethanol, and dried in a vacuum oven at 100 ℃ for 6 h.

Pt/graphene-TiO₂ catalysts with different TiO₂ content were synthesized by using a microwave-assisted hydrothermal method. Typically, a calculated amount of GO and TiO₂ were dispersed into a mixture of ethylene glycol (EG) under ultrasonic treatment for 30 min, and kept stirring for 20 min. Then, the H₂PtCl₆-EG solution was added into the solution with agitation for 30 min. The pH of the solution was then adjusted to approximately 13 and the solution was transferred into a microwave reactor with a volume capacity of 50 mL and heated at 140 ℃ for 3 min. After cooling to room temperature, the pH of the solution was adjusted to 4. The mixture was washed repeatedly with ultrapure water. The obtained Pt/graphene-TiO₂ catalysts were dried at 80 ℃ in a vacuum oven for 5 h and subsequently treated in a reductive atmosphere at 150 ℃ for 2 h.

The TiO₂ content in the catalysts was relative to the total mass of the mixed support. For the sake of convenience, Pt/graphene-TiO₂ catalysts with 20 wt%, 40 wt%, and 60 wt% TiO₂ were denoted as Pt/graphene-TiO₂-20%, Pt/graphene- TiO₂-40%, and Pt/graphene-TiO₂-60%, respectively. The Pt metal loading of the hybrid catalysts was approximately 20 wt%. The metal loading of the samples was determined by burning off the carbon support and verifying the metal content by XRD. Samples of 50–100 mg were prepared to ensure good reproducibility and to minimize any errors.

The XRD patterns of the as-prepared catalysts were obtained with a D/max-RB diffractometer (Japan) using a Cu K_α X-ray source operating at 45 kV and 100 mA and a scanning rate of 4° /min with an angular resolution of 0.05° of the 2θ scan. A scanning electron microscope (JSM-6510, JEOL, Japan) coupled with an energy dispersive spectrometer (EDS) was used to examine the surface morphology and elemental composition of the samples. TEM images were acquired to characterize the morphology of the samples using a Tecnai-20 G2 transmission electron microscope (USA), which was operated at 200 kV. The samples were finely ground and ultrasonically dispersed in alcohol, and a drop of the resultant dispersion was deposited and dried on a standard copper grid coated with a carbon film.

Electrochemical experiments were carried out on a potentiostat (Solatron 1287, Associate Scribner) with a three- electrode-type cell. A glassy carbon (GC) disk with a geometrical area of 0.1256 cm² was used as a working electrode. A saturated calomel electrode (SCE) and platinum wire were used as reference and counter electrodes, respectively. The working electrodes in the electrochemical experiments were prepared as follows. The catalyst (8 mg) and 80 μL Nafion aqueous solution were dispersed in a solution of 1500 μL DI water and 420 μL ethanol and then the ink was ultrasonicated for 30 min. Ten microliters of this dispersion was pipetted onto the GC substrate of the disc and dried at room temperature. Prior to any electrochemical measurements, the working electrode was cycled in the potential range from 0 to 1.3 V with respect to SCE at a sweep rate of 50 mV/s to activate the sample. The rotating disk electrode (RDE) was rotated at 1600 r/min throughout the experiment at a sweep rate of 5 mV/s. The CV and LSV measurements were performed in solutions containing 0.5 mol/L aq. H₂SO₄ saturated with N₂ and O₂. All the experiments were conducted at room temperature and all the potentials shown in this paper were converted to the reversible hydrogen electrode (RHE) scale.

The XRD patterns of the GO, TiO₂ and Pt/graphene-TiO₂ catalysts are shown in Fig. 1. The pattern in Fig. 1(1) displays a sharp diffraction peak at 11.4°, which was attributed to GO (001) plane reflection and indicated that GO was successfully prepared through the chemical oxidation of the graphite powder. The XRD pattern of TiO₂ shown in Fig. 1(2) was perfectly indexed with anatase TiO₂ in JCPDS card (84-1285). All the peaks in the spectrogram were extremely sharp and exactly straight lines. These characteristics indicated that the TiO₂ was highly crystallinity and were very consistent with a single-crystalline structure [39]. As shown in Fig. 1(3), the diffraction peak at 25° was owing to the carbon (002) plane. Fig. 1(4) and (5) possess a broad diffraction peak at 25° corresponding to the (002) plane of graphene and the sharp diffraction peak at 11.4° disappears, indicating that the GO was reduced to graphene. From Fig. 1(3), (4), and (5), it is clear that the diffraction peaks at 40°, 46°, and 67.5° represent the Pt (111), (200), and (220) planes observed in the spectrogram, which suggested that the prepared Pt exhibited a face-centered cubic (fcc) crystal structure [34]. Moreover, the characteristic diffraction peak of anatase TiO₂ is clearly evident in Fig. 1(5), which demonstrated that TiO₂ was integrated with the graphene sheets.

The SEM images of graphene, TiO₂, graphene-TiO₂-20%, graphene-TiO₂-40% and graphene-TiO₂-60% and the corresponding particle size distributions of TiO₂ are shown in Fig. 2. As shown in Fig. 2(a), the GO was transparent and consisted of a lamellar nanosheet with wrinkles. Fig. 2(b) shows anatase TiO₂, which forms well-dispersed cubes with good uniformity in size distribution. As shown in Fig. 2(c), (d) and (e), the TiO₂ cubes were uniformly distributed on the graphene sheets.

The corresponding histograms of the TiO₂ particle size distribution are shown in Fig. 2, which quantitatively reveals the particle size distribution of the catalyst support. Clearly, most of the cubic TiO₂ particle size was 60 nm in size. Highly dispersed and narrowly distributed TiO₂ cubes were obtained. After mixing with graphene, there was no change in the particle size of TiO₂, which implied that the crystallinity, uniformity, and cubic shape of TiO₂ were well-preserved during the microwave treatment.

Fig. 3 shows the TEM and HRTEM images of the Pt/graphene-TiO₂ catalysts. From Fig. 3(a), it is clear that Pt and TiO₂ were uniformly deposited on the graphene sheets. More interestingly, Fig. 3(b) shows that most of the Pt nanoparticles seem to preferentially anchor between TiO₂ and graphene and form a unique triple junction structure, which indicated that there was a strong interaction between TiO₂ and the precious metal. Fig. 3(c) shows the high-resolution TEM image of the Pt and TiO₂ particles. Regular lattice fringes with a spacing of 0.35 nm and 0.22 nm are clearly evident, which was extremely consistent with the (101) plane of the anatase TiO₂ and (111) plane of Pt, respectively. The TEM images and particle size distributions of the Pt nanoparticles are shown in Fig. 3(d–f). Highly dispersed, homogeneous, and spherical metal clusters were anchored on the composite support. The average particle size of the Pt particles in Pt/graphene-TiO₂-20%, Pt/graphene-TiO₂-40%, and Pt/graphene-TiO₂-60% was 2.4, 2.3, and 2.4 nm, respectively. The average size of the Pt was almost the same, which could not exert any influences on their activity [34]. The EDS analysis of the Pt/graphene-TiO₂ catalyst further proved the coexistence of Pt and TiO₂, as shown in Fig. 4. The content of each element is listed in the inset table.

The CV technique is very useful for obtaining information about the stability in the reaction media and the participation of the active sites on the electrode surfaces [32]. Fig. 5 shows the CV curves of Pt/C, Pt/graphene-TiO₂-20%, Pt/graphene- TiO₂-40%, and Pt/graphene-TiO₂-60% electrocatalysts in the N₂-saturated 0.5 mol/L aq. H₂SO₄ at a scan rate of 50 mV/s in the potential region of 0 and 1.3 V (vs. RHE). It is clear that all the catalysts show peaks related to hydrogen adsorption/desorption between 0.05 and 0.35 V (vs. RHE). The electrochemical surface area (ECSA) was 72.21, 77.36, 22.23, and 12.34 m²/g for Pt/C, Pt/graphene-TiO₂-20%, Pt/graphene- TiO₂- 40% and Pt/graphene-TiO₂-60%, respectively. The Pt/graphene-TiO₂-20% showed the highest ECSA among these catalysts, which indicated that the addition of cubic TiO₂ can mitigate the aggregation of the Pt particles and more Pt sites were available for the oxygen reduction reaction compared with the Pt/C catalyst. The ECSA significantly decreased with increasing TiO₂ content as a result of high electronic resistance.

Fig. 6 presents the CV curves before and after the accelerated potential cycling test (APCT) in the N₂-saturated 0.5 mol/L aq. H₂SO₄ at a scan rate of 50 mV/s in the potential range from 0 to 1.3 V (vs. RHE). Fig. 6(a) shows that there is a sharp decline in the hydrogen adsorption/desorption area of the Pt/C catalyst after 400 cycles because of agglomeration of the Pt nanoparticles and corrosion of the carbon powder [39]. However, as shown in Fig. 6(b, c, d), the attenuation in ECSA of the Pt/graphene-TiO₂ catalysts was not distinct compared with that of the Pt/C catalyst. To check the stability of the four catalysts, the ECSAs of these catalysts were calculated after APCT. After 2000 cycles, the ECSA degraded by 95% and 55% for Pt/C and Pt/graphene-TiO₂-20%, respectively. The decay rate of the Pt/graphene-TiO₂-40% and Pt/graphene-TiO₂-60% catalysts was almost the same, but slightly lower than that of the Pt/graphene-TiO₂-20%. The stability of the catalyst was indeed improved with increasing TiO₂ content. This indicated that the Pt/graphene-TiO₂ catalysts exhibited higher stability than the Pt/C catalyst, which was attributed to the excellent durability of the TiO₂ in acidic media and the strong metal–support interactions between the Pt particles and cubic TiO₂, mitigating the agglomeration of Pt metal particles [41-43]. The graphene with a single layer of carbon atom array structure also exhibited higher chemical stability than carbon powder [26-29]. In addition, the uniform dispersion of cubic TiO₂ nanoparticles on graphene sheets could offer more durable passages for ion and electrolyte transport [40].

The oxygen reduction reaction (ORR) curves for the Pt/graphene-TiO₂ catalysts and Pt/C in the O₂-saturated 0.5 mol/L aq. H₂SO₄ are presented in Fig. 7. Fig. 7(a) shows that the initial voltage for ORR on the Pt/C, Pt/graphene-TiO₂-20%, Pt/graphene-TiO₂-40%, and Pt/graphene-TiO₂-60% catalyst was 0.95, 0.99, 0.99 and 0.99 V, respectively. The limiting current of the Pt/graphene-TiO₂-20% was much closer to that of the Pt/C catalyst, whereas the limiting currents of the other two catalysts were much lower than that of Pt/C because of the high electrical resistance resulting from the high loading of TiO₂. As presented in Fig. 7(b), the onset potentials for ORR on Pt/C, Pt/graphene-TiO₂-20%, Pt/graphene-TiO₂-40%, and Pt/graphene-TiO₂-60% catalysts were 0.90, 0.99, 0.99, and 0.99 V after 2000 cycles, respectively. The negative shift of the LSV curve of the Pt/graphene-TiO₂ catalysts was much lower than that of the Pt/C catalyst. Hence, the Pt/graphene-TiO₂ catalyst was more durable than Pt/C. In our work, Pt/graphene-TiO₂ with 20 wt% TiO₂ exhibited the highest activity and stability.

The Pt/graphene-TiO₂ hybrid catalyst was synthesized through an environmentally friendly and novel microwave-assisted solvothermal method. The microstructure, composition, and electrochemical performance of the prepared catalyst were examined by physical and electrochemical techniques. The integration of graphene sheets and cubic TiO₂ enhanced the activity and stability of the Pt nanoparticles, which was attributed to the strong metal-support interactions and the synergetic effect between TiO₂ and graphene.

Acknowledgements

The author would like to thank Dr. Mingkai Liu for providing technological support. We thank Zoran Dinev, PhD, from Liwen Bianji, Edanz Group China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.