2.1 Chemicals

Titanium (IV) chloride (TiCl₄; 99.5%) was obtained from Aladdin. Tungsten hexachloride (WCl₆; 99.9%) and hexachloroplatinic acid (H₂PtCl₆·xH₂O) were purchased from Sigma-Aldrich, USA. Absolute ethanol (C₂H₅OH; 99.9%) and ethylene glycol [(CH₂OH)₂; 99.5%] were procured from Merck, Belgium.

2.2 Preparation of Ti_0.8W_0.2O₂ nanoparticles

Ti_0.8W_0.2O₂ nanoparticles were synthesized through a one-pot solvothermal route, and WCl₆ and TiCl₄ were used as the starting precursors for W and Ti, respectively [19]. WCl₆ (0.16 g) was first dissolved in 50 mL of absolute ethanol, and 0.176 mL of TiCl₄ was then added to the as-obtained solution. The as-prepared solution was then transferred to a Teflon-lined autoclave and placed in an oven at 200 °C for 10 h. The obtained suspension was then naturally cooled to room temperature and washed with acetone and purified water. Finally, the drying step was performed in an oven at 80 °C to collect Ti_0.8W_0.2O₂ nanoparticles for further analysis.

2.3 Preparation of 18.5 wt % Pt/Ti_0.8W_0.2O₂ electrocatalyst

The 18.5 wt % Pt/Ti_0.8W_0.2O₂ electrocatalyst was synthesized through a simple and rapid microwave-assisted polyol route, and ethylene glycol was used as a reducing agent. Ti_0.8W_0.2O₂ nanoparticles (110 mg) were first added to 25 mL of ethylene glycol and then magnetically stirred until complete dispersion. The as-obtained suspension was then ultrasonicated for around 30 min, and H₂PtCl₆ (0.05 M) (2.56 mL) was then mixed with the as-prepared suspension and continuously stirred for 20 min. The pH of the as-obtained mixture was adjusted to 11 by NaOH solution. The reduction reaction was carried out in a microwave oven (800 W, 2450 MHz, Electrolux EMS2047X) at 160 °C and 240 W for 2 min. Finally, the as-obtained product was rinsed several times with acetone and distilled water and then dried at 80 °C overnight for further analysis. The 20 wt % Pt/C electrocatalyst was prepared under the same conditions for comparison.

2.4 Material properties

The structural phases of the as-obtained samples were examined by X-ray diffraction (XRD, D2 PHASER; Bruker, Germany) in the 2θ range of 20°–80°. The surface morphologies and particle sizes of the samples were revealed by transmission electron microscopy (TEM, JEM 1400; JEOL). The elemental compositions of the as-prepared samples were examined by X-ray fluorescence (XRF) analysis. N₂ adsorption/desorption isotherms were recorded on a NOVA 1000e instrument at 77 K. For electrical conductivity measurement, the pellets of diameter 10 mm and thickness 1 mm were made from dry samples using a hydraulic press under 300-MPa pressure and then dried at 105 °C overnight.

2.5 Electrochemical characterization

The electrochemical properties of the as-synthesized 18.5 wt % Pt/Ti_0.8W_0.2O₂ catalyst were measured using an EC-LAB electrochemistry device equipped with a silver/silver chloride (saturated KCl) reference electrode, a Pt counter electrode, and a glassy carbon working electrode (diameter of 5 mm). To prepare the catalyst ink, the catalyst powder was ultrasonicated for 30 min in a solution consisting of ethanol absolute and 0.5% Nafion. The surface of the glassy carbon disk was polished with 0.5-μm Al₂O₃ polishing powder (BAS) and then washed with ethanol and purified water. The catalyst electrode was activated for 100 cycles at a sweep rate of 50 mV/s. To examine the ECSA of the as-obtained catalyst, the electrochemical test was carried out in a nitrogen-saturated 0.5 M H₂SO₄ solution at a sweep rate of 50 mV/s. Furthermore, to investigate the electrocatalytic activity and stability toward EOR of the as-obtained electrocatalysts, the CV and the accelerated durability tests were performed in a nitrogen-saturated 10 v/v% C₂H₅OH/0.5 M H₂SO₄ aqueous solution at a sweep rate of 50 mV/s. In the present work, all electrochemical potentials were described against the normal hydrogen electrode (NHE) scale.

3.1 Characterization of Ti_0.8W_0.2O₂ nanoparticles

The structural phases of Ti_0.8W_0.2O₂ and undoped TiO₂ nanoparticles were revealed by XRD measurements with respect to the standard spectra of anatase-TiO₂ (JCPDS 084–1286) and WO₃ (JCPDS 020–1324) (Fig. 1). Ti_0.8W_0.2O₂ nanoparticles possessed the structure of anatase-TiO₂ with typical diffraction peaks of the (101), (004), (200), (211), and (204) facets at 25.3°, 37.8°, 48.0°, 55.1°, and 62.7°, respectively. The typical peaks for the (001), (020), and (200) facets of WO₃ at 23.08°, 23.70°, and 24.09° were not detected in the XRD profile of Ti_0.8W_0.2O₂ nanoparticles. Moreover, the phase segregation between W and anatase-TiO₂ was not observed in the XRD patterns of Ti_0.8W_0.2O₂ nanoparticles, which signifies that W was successfully incorporated into TiO₂ lattices. The crystallite sizes of Ti_0.8W_0.2O₂ and undoped TiO₂ nanoparticles estimated based on the (101) peak were found to be 7.36 nm and 14.35 nm, respectively (calculated by Scherrer equation [20–22]). The decrease in the crystallite size of Ti_0.8W_0.2O₂ can be ascribed to the incorporation of W into TiO₂ lattices [23,24].

Fig. 2 displays TEM images of the as-prepared nanoparticles. It is evident that spherical-shaped Ti_0.8W_0.2O₂ and undoped TiO₂ nanoparticles had an average diameter of about 9 nm and 15 nm, respectively. The decrease in the particle size of Ti_0.8W_0.2O₂ can be attributed to the incorporation of W into TiO₂ lattices which hinders the grain growth [25–27]. In addition, less Ti_0.8W_0.2O₂ nanoparticle agglomeration was observed in comparison with previous findings [28–30], which can be attributed to the one-pot solvothermal route using inorganic starting precursors without using any surfactant/stabilizer or further heat treatment process. Therefore, smaller particle size and less particle agglomeration imply that the as-obtained Ti_0.8W_0.2O₂ nanoparticles possessed a large surface area and played an important role in the uniform dispersion of the Pt-based catalyst. Moreover, the high-resolution transmission electron microscopy (HR-TEM) image in Fig. 2 (b) (inset) displays well-defined fringes of 3.4-Å size corresponding to the (101) crystal plane of anatase-TiO₂, which closely agrees with XRD patterns (Fig. 1).

Fig. 3 (a) depicts the results of XRF spectroscopy for Ti_0.8W_0.2O₂ nanoparticles, and the proportions of Ti and W were found to be 79.40 and 20.60, respectively; hence, it signifies that the elemental compositions of Ti_0.8W_0.2O₂ nanoparticles were effortlessly controlled by adjusting the proportions of starting precursors. Furthermore, a relatively uniform distribution of compositional elements in Ti_0.8W_0.2O₂ nanoparticles was observed by elemental mapping (Fig. 3 (b–d)).

The N₂ adsorption/desorption isotherms of Ti_0.8W_0.2O₂ and undoped TiO₂ nanoparticles exhibited hysteresis loops of type IV curves (Fig. 4). It indicates that the as-obtained nanoparticles possessed a porous structure. The surface areas of Ti_0.8W_0.2O₂ and undoped TiO₂ nanoparticles were calculated as 152.32 m²/g and 125.51 m²/g, respectively (Fig. 4 (a, c)); hence, the increase in the surface area of Ti_0.8W_0.2O₂ nanoparticles could be ascribed to its smaller particle size in comparison to undoped TiO₂ (Fig. 2 (a–d)). Furthermore, the pore sizes of Ti_0.8W_0.2O₂ and undoped TiO₂ were measured to be ∼2.46 nm and ∼2.68 nm, respectively, by the Barrett–Joyner–Halenda (BJH) analysis (Fig. 4 (b, d)). This suggests that the as-synthesized Ti_0.8W_0.2O₂ and undoped TiO₂ were mesoporous materials, thus facilitating the uniform distribution of Pt and the electron transfer during electrochemical reactions [7,31]. Therefore, the as-obtained Ti_0.8W_0.2O₂ nanoparticles possessing a larger surface area than noncarbon supports [32–38] (Fig. 5 (a)) were promising supports for the Pt nanocatalysts.

In a fuel cell, one of the most important requirements for catalyst support is electrical conductivity, which facilitates electron transmission during anodic oxidation. The as-prepared Ti_0.8W_0.2O₂ nanoparticles yielded an electrical conductivity of 1.90 × 10⁻² S/cm, which is about 10⁴-fold higher than that of undoped TiO₂ nanoparticles (6.23 × 10⁻⁶ S/cm²) (Fig. 5 (b)). This suggests the successful incorporation of W into anatase-TiO₂ and the formation of the “aliovalent ions” effect [39,40]. Furthermore, the as-synthesized Ti_0.8W_0.2O₂ exhibited much higher electrical conductivity than previously reported noncarbon supports [35,37,41], implying that the Ti_0.8W_0.2O₂ nanoparticles were promising supports for the Pt-based electrocatalyst.

3.2 Properties of 18.5 wt % Pt/Ti_0.8W_0.2O₂ electrocatalyst

Fig. 6 compares the XRD pattern of the as-obtained Ti_0.8W_0.2O₂-supported Pt electrocatalyst with those of anatase-TiO₂ (JCPDS 084–1286) and face-centered cubic (fcc) Pt (JCPDS 04–0802). The as-prepared Ti_0.8W_0.2O₂-supported Pt electrocatalysts exhibited the typical Pt peaks corresponding to (111), (200), and (220) crystal planes observed at 2θ positions of 39.76°, 46.24°, and 67.45°, respectively. It is noteworthy that the phase separation between W and anatase-TiO₂ was not detected, which implies Ti_0.8W_0.2O₂ nanoparticles had a stable structure.

The morphology and particle size of the anchored Pt nanoform over the Ti_0.8W_0.2O₂ support were examined by TEM. TEM images of the Pt/Ti_0.8W_0.2O₂ electrocatalyst at different scale bars are exhibited in Fig. 7 (a–c). Pt nanoforms anchored on Ti_0.8W_0.2O₂ nanoparticles exhibited a spherical-shaped morphology with an average diameter of several nanometers. Furthermore, relatively uniform distribution and less agglomeration of Pt nanoforms over Ti_0.8W_0.2O₂ nanoparticles were also observed, which can be attributed to the microwave-assisted polyol route that incorporates rapid reduction and fast formation of metal nuclei, thus restricting further particle growth [42,43]. Moreover, the H-TEM image of the 18.5 wt % Pt/Ti_0.8W_0.2O₂ electrocatalyst exhibited a well-defined fringe of 2.3-Å size corresponding to the (111) crystal plane of fcc platinum. The formation of Pt nanoforms along the (111) facet improved the catalytic activity of 18.5 wt % Pt/Ti_0.8W_0.2O₂ toward EOR because of thier low poisoning rate [44].

To investigate the distribution of Pt over Ti_0.8W_0.2O₂ nanoparticles, the scanning electron microscope (SEM)/energy-dispersive X-ray spectroscopy (EDX) mapping measurement was used. The amount of Pt loading over Ti_0.8W_0.2O₂ nanoparticle surfaces was measured as 18.45 wt %, which is approximately equal to the theoretical Pt loading (18.5 wt %) (Fig. 8 (b)). Moreover, elemental mapping in Fig. 8 (c–e) demonstrates the relatively uniform dispersion of Pt on Ti_0.8W_0.2O₂ nanoparticles. These results suggest that the adopted simple and rapid microwave-assisted polyol approach was a suitable process to anchor Pt on Ti_0.8W_0.2O₂ nanoparticles. Moreover, the dispersion of Ti and W before and after Pt deposition remained almost unchanged (Fig. 8 (c, d)), which implies that the as-synthesized Ti_0.8W_0.2O₂ nanoparticles possessed a stable structure in the reduction environment.

The electrochemical characterization of the Ti_0.8W_0.2O₂-supported Pt electrocatalyst and the conventional Pt/C electrocatalyst was performed by CV (Fig. 9). The CV curves of different electrocatalysts revealed the presence of multiple peaks in hydrogen adsorption/desorption regions, thus exposing different Pt crystallographic planes and the high crystallinity of the electrocatalyst [3,45]. The ECSA values of 18.5 wt % Pt/Ti_0.8W_0.2O₂ and 20 wt % Pt/C were measured as 42.72 m²/g_Pt and 51.79 m²/g_Pt, respectively; therefore, the decrease in ECSA value of the 18.5 wt % Pt/Ti_0.8W_0.2O₂ catalyst can be attributed to its low Pt loading. The CV curve of the Pt/Ti_0.8W_0.2O₂ electrocatalyst exhibited the positively shifted onset potential of oxidation and the significantly suppressed intensity relative to the Pt/C catalyst, thus implying the weaker binding between OH species and the 18.5 wt % Pt/Ti_0.8W_0.2O₂ catalyst which was evidenced as beneficial effect toward oxygen reduction reaction (ORR) [46,47].

The electrocatalytic activity of the Ti_0.8W_0.2O₂-supported Pt electrocatalyst and the conventional Pt/C electrocatalyst toward EOR was examined by CV measurement (Fig. 10). The ethanol electrooxidation peaks for 18.5 wt % Pt/Ti_0.8W_0.2O₂ and 20 wt % Pt/C were observed at 0.90 V and 1.28 V, respectively, versus NHE (Fig. 10 (a)). Therefore, the 18.5 wt % Pt/Ti_0.8W_0.2O₂ electrocatalyst exhibited the facile removal of adsorbed carbonaceous intermediates and manifested a better CO tolerance than the carbon-supported Pt electrocatalyst [48,49]. The 18.5 wt % Pt/Ti_0.8W_0.2O₂ electrocatalyst displayed a current density of 20.90 mA/cm², which was comparable with that of the conventional 20 wt % Pt/C electrocatalyst (25.78 mA/cm²). Moreover, the onset potential of the 18.5 wt % Pt/Ti_0.8W_0.2O₂ catalyst was found as to be 0.70 V versus NHE, which was about 1.23-fold lower than that of the 20 wt % Pt/C electrocatalyst (0.80 V vs NHE) (Fig. 10 (b)); hence, it indicates that the 18.5 wt % Pt/Ti_0.8W_0.2O₂ electrocatalyst exhibited better ethanol electrooxidation than the conventional 20 wt % Pt/C electrocatalyst. The moderate activity of 18.5 wt % Pt/Ti_0.8W_0.2O₂ toward EOR can be attributed to the strong interactions between Pt nanoparticles and the TiO₂-based support and manifested beneficial effects on catalytic activity/selectivity [6,50]. Furthermore, the I_f/I_b value (the ratio between positive-going scan [I_f] and negative-going sweep [I_b]) was measured to examine the poisoning resistances of the as-obtained electrocatalysts during the anodic EOR. The 18.5 wt % Pt/Ti_0.8W_0.2O₂ electrocatalyst resulted in an I_f/I_b value (1.49) about 2.12 times higher than the conventional 20 wt % Pt/C electrocatalyst (0.70).

The electrocatalytic stability of the 18.5 wt % Pt/Ti_0.8W_0.2O₂ electrocatalyst and the conventional 20 wt % Pt/C electrocatalyst was investigated based on 2000 cycling tests in an acidic ethanol medium (Fig. 11). The Ti_0.8W_0.2O₂-supported Pt electrocatalyst exhibited superior electrocatalytic stability toward EOR in comparison with the conventional carbon-supported Pt electrocatalyst. After 2000 cycling tests, the current density of the 18.5 wt % Pt/Ti_0.8W_0.2O₂ electrocatalyst dropped from 20.90 mA/cm² to 18.23 mA/cm² (a degradation of 12.77%). However, the current density of the conventional 20 wt % Pt/C electrocatalyst dropped from 25.478 mA/cm² to 19.29 mA/cm² (a decay of 25.17%). Therefore, the superior chemical stability of the Ti_0.8W_0.2O₂-supported Pt electrocatalyst can be ascribed to the initial structural and chemical durability of the TiO₂-based support in the acidic media [41]. Moreover, the incorporation of W into the anodic electrocatalyst caused several beneficial effects: (i) It provided necessary OH_ad species at less positive potential than Pt (bifunctional mechanism) and facilitated the oxidation of carbonaceous intermediate species on the electrocatalyst surface; (ii) it modified the electronic interplays between Pt and the CO adsorbate (electronic effect); and (iii) it increased the catalytic activity through the “hydrogen spillover” effect [13,51–53].