Enhanced Photoinduced Electrocatalytic Oxidation of Methanol Using Pt Nanoparticle-Decorated TiO₂–Polyaniline Ternary Nanofibers

Abstract

Herein, perylene-3,4,9,10-tetracarboxylic acid-doped polyaniline (PTP) nanofibers with/without photoreactive anatase TiO₂ (TiO₂–PTP and PTP, respectively) have been successively synthesized and subsequently decorated by Pt nanoparticles (Pt NPs) to prepare Pt–PTP and Pt–TiO₂–PTP composites. High-resolution transmission electron microscopy confirms the presence of ∼3 nm spherical-shaped Pt NPs on both the composites along with TiO₂ on Pt–TiO₂–PTP. Pt loading on the composites is deliberately kept similar to compare the methanol electro-oxidation in the two composites. The Pt nanocomposites along with the precursor polyanilines are characterized by optical characterization, X-ray diffraction study, X-ray fluorescence spectroscopy, and Raman spectroscopy. The ternary composite-modified (Pt–TiO₂–PTP) electrode demonstrates high electrocatalytic performance for methanol oxidation reaction in acid medium than Pt–PTP and Pt–TiO₂. The higher electrochemical surface area (1.7 times), high forward/backward current ratio, and the higher CO tolerance ability for Pt–TiO₂–PTP make it a superior catalyst for methanol oxidation reaction in the electrochemical process than Pt–PTP. Moreover, the catalytic activity of Pt–TiO₂–PTP is further enhanced significantly with light irradiation. The cooperative effects of photo- and electrocatalysis on methanol oxidation reaction in Pt–TiO₂–PTP enhance the methanol oxidation catalytic activity approximately 1.3 times higher in light illumination than in dark. Therefore, the present work will be proficient to get a light-assisted sustainable approach for developing the methanol oxidation reaction activity of Pt NP-containing catalysts in direct methanol fuel cells.

1. Introduction

The growing demands for sustainable energy lead to the continuous investigation of alternative energy sources. In this regard, direct methanol fuel cells (DMFCs) have engrossed noteworthy consideration as promising renewable energy sources for portable electronics, electric vehicles, and other transportation media owing to their superior utilization efficiency, low pollution rate, cost-effectiveness, ease of synthesis, low functioning temperature, and so on.¹⁻⁷ However, the utmost negative downside for a broad commercial application of DMFCs is the fabrication of high-performance and long-lasting anodic compounds for the methanol oxidation reaction (MOR).⁸ Till date, efficient noble metals like Pt nanoparticles (Pt NPs) are still being used as the most studied anodic materials as electrocatalysts for DMFCs owing to their outstanding electrocatalytic performance.^9,10 However, a pure Pt electrocatalyst easily suffers from some intrinsic drawbacks like high cost and less durability, as it possesses less resistance to carbon monoxide (CO) poisoning, which must be sorted out for the commercialization of DMFCs.^11,12 The catalyst poisoning is initiated by the robust interaction of intermediary-generated CO species with Pt NP active sites and leads to a rapid deactivation of the Pt catalyst to diminish its technological viability.^13,14 The oxidation of the adsorbed CO by the generated −OH species on the Pt surface at a higher potential efficiently slows down the MOR process significantly.¹⁵ Hence, the development of high CO tolerance with better stability and superb catalytic performance in electrocatalysts is the key issue herein. To evaluate the solution with regard to CO-based catalytic poisoning, coupling of Pt NPs with other metal oxides like TiO₂, SnO₂, ZnO, Co₃O₄, and carbon materials has been investigated to improve the electrocatalytic performance and durability of Pt NPs successfully in electrocatalytic methanol oxidation.¹⁶⁻²² The combined effect of Pt NPs and these materials is possibly the main reason for betterment. Metal oxides generally adsorb −OH and consequently convert the catalyst-adsorbed CO to CO₂ to restore the active sites of Pt NPs for continuous electrocatalytic activity and effectively mitigate CO poisoning of Pt NPs to improve the catalytic performance for methanol oxidation.

Among the photoexcited semiconducting metal oxides, TiO₂ is the most promising as a support material for electrocatalytic methanol oxidation owing to its enhanced photocatalytic activity, durability, cost-effectiveness, and self-cleaning property.²³⁻²⁷ As TiO₂ has a powerful oxidation characteristic under UV light excitation, it has attracted huge attention of the researchers since Kamat et al. first employed it as a support material for MOR under UV light as a photocatalyst.²⁸⁻³¹ TiO₂ can improve the MOR performance of the noble metal electrocatalyst by two ways; first, it assists the methanol oxidation by photoexcitation as it has a 3.2 eV band gap, and second, by its self-cleaning ability, it decreases the CO poisoning of the catalyst as stated earlier. The UV light excitation on TiO₂ excites the valence band (VB) electrons to the conduction band and consequently creates negative electron–positive hole pairs (e^––h⁺).³² The photogenerated e^––h⁺ pairs can effectively produce the hydroxyl radical (^•OH) which can also oxidize the adsorbed methanol to CO₂.

TiO₂, in spite of having several advantages of being employed as a support material for noble metal electrocatalysts for methanol oxidation, reduces the electronic conductivity of the electrode materials.³³ Thus, a lot of research is going on recently to investigate the proper conducting support materials which can work as electron transport media from anodes to metal electrocatalysts as well as a binder for TiO₂ and noble metal composites. Generally, conducting carbon materials, like reduced graphene oxides, carbon black, carbon nanotubes, and so forth, and conducting polymers are used for this purpose.^27,30,34,35 Among the conducting polymers, polyaniline (PANI) is the most interesting as it contains exceptionally high electrical conductivity in doped state, high stability, ecofriendliness, ease of synthesis, and cost-effectiveness.^36,37 Again, the amine (−NH−) moieties of the PANI main chain provide the nucleation centers for the Pt NPs to firmly anchor on top of the PANI surface and successively improve the dispersion of Pt NPs on the PANI matrix.^6,34 PANI extends the N-containing adsorption sites for Pt NPs in a composite, averts the Pt NP agglomeration, and efficiently improve the homogeneous dispersion of Pt NPs in a polymer matrix by trimming down the size.³⁸ Also, by interacting with water in electrolyte, it can assist the conversion of CO to CO₂ during MOR to increase the CO tolerance of the modified electrode material.^39,40 On the other hand, PANI exhibits great potential as a photosensitizer because of its low band gap and π–π* transition.⁴¹ Hence, doping of PANI with TiO₂ can reduce the band gap of TiO₂ and shift the optical response to be excited at the visible light region. Therefore, there is enormous scope to use controlled nanostructured PANI as a conducting support for electrocatalysts to further upsurge the photo-assisted electrocatalytic performance of Pt–TiO₂-based electrode materials.

Herein, in this report, we present an easy procedure for the decoration of Pt NPs by embedding them in high-aspect-ratio perylene-3,4,9,10-tetracarboxylic acid-doped PANI (PTP) nanofibers and PTP nanofibers preloaded with TiO₂ (Scheme 1). Earlier, Rana et al. reported high-aspect-ratio PTP nanofibers with a very high electrical conductivity of 10^–3 S cm^–1.⁴² The nanofiber structure of PTP can serve a very high surface area for the decoration of TiO₂ and Pt NPs herein. We modified the synthesis technique to prepare TiO₂–PTP nanofibers by in situ polymerization of PTP in the presence of nanostructured anatase TiO₂. The synthesized Pt–PTP and Pt–TiO₂–PTP composites have been characterized by X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy studies. The prepared surfactant-free Pt NPs on PTP nanofibers (Pt–PTP and Pt–TiO₂–PTP) have been employed as light-assisted electrocatalysts for MOR in acidic medium. The electrocatalytic properties of PTP-containing catalysts were also compared with a without PTP catalyst Pt–TiO₂ to evaluate the role of PTP. Interestingly, the electrocatalytic performance of methanol oxidation has been distinctly improved in Pt–TiO₂–PTP compared to Pt–PTP and Pt–TiO₂. With the aid of UV light irradiation on Pt–TiO₂–PTP, the methanol oxidation activity was significantly enhanced further. Therefore, the present article provides a highly proficient UV light-assisted worthwhile approach for the improvement of the MOR catalytic performance of Pt-based electrocatalysts in DMFCs.

Scheme 1. Schematic Representation of the Formation of (a) PTP and (b) Pt–TiO₂–PTP.

2. Results and Discussion

2.1. Morphological Studies

The synthesis of the PANI nanofiber PTP from aniline is shown in Scheme 1a. The TiO₂–PTP composite is prepared by the same procedure used for PTP, taking 20% (w/w) TiO₂ at the time of polymerization of aniline. The fabrication of Pt NPs on PTP of the TiO₂–PTP polymer fiber composite is illustrated in Scheme 1b.

The FESEM image study in Figure 1a,b reveals the formation of the TiO₂ nanosphere of the diameter range of ∼250–500 nm by the solution combustion route. Again, the FESEM study of PANIs in Figure 1c–f demonstrates one-dimensional (1D) PTP nanofibers with the diameter range of ∼50–100 nm (Figure 1c). However, the average diameter is slightly increased to >100 nm in TiO₂–PTP (Figure 1d), possibly because of the inclusion of bigger TiO₂ nanospheres which completely mingled with the PANI nanostructures to provide 1D nanofibers. After treatment with H₂PtCl₆ on PTP and TiO₂–PTP nanofibers, the basic febrile morphologies are retained along with the decoration of Pt NPs on the fibers.

Figure 1.

FESEM images of (a,b) TiO₂, (c) PTP, (d) TiO₂–PTP, (e) Pt–PTP, and (f) Pt–TiO₂–PTP.

The decoration of Pt NPs on PANIs is clearly envisioned by the TEM image studies in Figure 2. The TEM images of Pt–PTP and Pt–TiO₂–PTP composites disclose that the Pt NPs are firmly wedged to the PANI surface as fine NPs. As we discussed earlier, the N centers of PTP can interact with Pt NPs to uniformly disperse Pt NPs over the PTP and TiO₂–PTP polymer fibers. The diameter of the Pt NPs is ∼3 nm, and it remains the same in both the composites (insets of Figure 2a,f). The high-resolution fringe lattice image (inset image of Figures 2b and S1 and S2 in the Supporting Information) demonstrates that the layer distance for the (111) plane of Pt NPs is calculated to be ∼0.223 nm in the Pt–PTP composite, where Pt–TiO₂–PTP divulges the lattice fringes of ∼0.37 nm corresponding to the (101) plane of anatase TiO₂ along with the characteristic lattice fringes of Pt ∼0.223 nm (inset image of Figure 2g). The presence of Pt NPs on PTP fiber is confirmed from the red color (Figure 2d) by the mapping of the image of the selected area in Figure 2c. Similarly, TiO₂ and Pt NPs are confirmed in the Pt–TiO₂–PTP composite by mapping the image of the selected area in Figure 2h–i. The simultaneous appearance of red and green dots confirms the presence of Pt NPs and TiO₂, respectively. The elemental mapping of all elements in both the composites is shown in Figures S1 and S2 in the Supporting Information. The indexed selected area electron diffraction (SAED) patterns in Figures S1 and S2 confirm the formation of Pt NPs in Pt–PTP as the diffraction ring is prominent because of the (111) plane of Pt. The diffraction ring of the (111) plane of Pt NPs is merged with the (004) plane of TiO₂ in the SAED pattern of Pt–TiO₂–PTP along with the presence of spots for the (101) plane of TiO₂. The energy-dispersive X-ray (EDX) study of the Pt–PTP composite divulges the presence of C, N, O, and Pt (Figure 2e), whereas Pt–TiO₂–PTP (Figure 2j) shows extra Ti along with the aforementioned four elements. The EDX study also evaluates ∼11.00 wt % loading of Pt in the Pt–PTP composite, whereas the loading density of Ti and Pt in Pt–TiO₂–PTP is found to be 6.38 and 8.50 wt %, respectively.

Figure 2.

TEM, mapping images, and EDX patterns of (a–e) Pt–PTP and (f–j) Pt–TiO₂–PTP. HRTEM images of individual Pt NPs of Pt–PTP (inset of (a)) and the corresponding lattice fringe patterns of Pt NPs (inset of (b)). (c) Dark-field image of Pt–PTP with the selected area for mapping ; (d) corresponding mapping image of Pt ; and (e) EDX pattern of the Pt–PTP composite (e). HRTEM image of a single Pt NP of Pt–TiO₂–PTP (inset of (f)) and the corresponding lattice fringe patterns of Pt NPs and TiO₂ (inset of (g)). (h) Dark-field image of Pt–TiO₂–PTP with the selected area for mapping; (i) corresponding mapping image for Pt and Ti (j) EDX pattern of the Pt–TiO₂–PTP composite.

2.2. Structural and Spectroscopic Studies

The spectroscopic studies of the materials were executed by UV–vis study, fluorescence spectroscopy, and Raman spectroscopy. In the UV–vis absorption spectra of perylene-3,4,9,10-tetracarboxylic acid (PTCA) (Figure 3a), the absorption maxima is shown at 465 nm along with the other peaks at 433 and 510 nm, which are the characteristic peaks for the perylene core.⁴² For PANIs (PTP and TiO₂–PTP), along with the characteristic absorption peaks of PTCA, the spectra exhibit a weak band at 335 nm attributed to the π–π* transition of the π electrons in the benzenoid rings of PANI and a broad band at around 640 nm representing the excitation of electrons from the highest occupied molecular orbital of benzenoid rings to the lowest unoccupied molecular orbital of quinoid rings.⁴³ Again, after the formation of Pt NPs on PANIs in Pt–PTP and Pt–TiO₂–PTP, the 640 nm band of PANI is shifted to a low absorbance broad band at a higher wavelength of 725 nm because of the π–polaron transition of the quinoid rings on the PANI chains.⁴⁴ The decoration of Pt NPs on PANI creates an interaction between Pt and the nitrogen centers of the PANI chains, which is responsible for the π–polaron transition.

Figure 3.

(a) UV–vis spectra, (b) fluorescence spectra, (c) powdered XRD pattern, and (d) XRF spectra of the compounds. (c) Corresponding lattice planes of TiO₂ and Pt NPs assigned in the powder XRD plot.

The fluorescence spectra of PTCA and all the composites are shown in Figure 3b. The perylene-containing PTCA is only responsible for the emission in all the PANI materials here. Therefore, we fixed the same amount of PTCA in all composites. All the composites reveal the emission peaks at 491, 530, and 575 nm, which are the characteristic peaks for the perylene core. However, the emission intensities of PTCA in the composites have enhanced as compared to pristine PTCA, possibly because of the reduced aggregation and isolation of certain perylene cores in PANI chains.⁴²

The XRD patterns of PTCA, PTP, and TiO₂–PTP (Figure 3c) in the range of 5–85° show the presence of PTCA in PANIs along with the incorporation of TiO₂ in TiO₂–PTP. As evident from Figure 3c, TiO₂ in its pristine form, TiO₂–PTP, and Pt–TiO₂–PTP is found to crystallize in a pure-phase anatase structure (I4₁/amd, JCPDS no. 89-4921). All the characteristic peaks of TiO₂ are assigned to their corresponding planes in Figure 3c. Again, the presence of Pt NPs in Pt–PTP and Pt–TiO₂–PTP is confirmed by the appearance of four strong peaks for crystalline Pt NPs in both the samples. Strong crystalline peaks cantered at 2θ = 40°, 46.2°, 67.7°, and 81.5° correspond to the (111), (200), (220), and (311) crystal planes of the face-centered cubic (fcc) Pt NPs.^6,45,46 This result suggests that Pt species from H₂PtCl₆ are reduced to metallic NPs and embedded over PANI-supported composites. The intensity of the diffraction peaks for Pt NPs is quite similar in both the composites as the Pt loading weight % is somewhat similar in both the composites. The fringe distance or the distance between two similar crystal planes of Pt NPs has been calculated from Bragg’s equation. The fringe distances calculated from Bragg’s equation for the (111) crystal planes of Pt NPs and the (101) crystal plane of TiO₂ are ∼0.225 and ∼0.36 nm, respectively, which are remarkably similar to the TEM results (inset image of Figures 2b,g and S1).^6,47

XRF was performed to investigate the presence of Pt NPs and TiO₂ in the composites. The XRF spectra shown in Figure 3d confirm the presence of Ti in TiO₂–PTP and Pt–TiO₂–PTP as the bands for Ti Kα and Ti Kβ are fairly present in both the composites. On the other hand, the existence of Pt in the Pt–PTP and Pt–TiO₂–PTP composites is corroborated as Pt Lα and Pt Lβ bands are present at 9.2 and 11 keV, respectively.⁴⁸

Raman spectroscopy was also used to investigate the synthesized catalysts, and the corresponding plots are shown in Figure S3. It is evidenced from the literature that the pristine TiO₂ shows the most intense band at 145 cm^–1 (E_g) and weak bands at 393 (B_1g), 505 (A_1g), and 626 cm^–1 (E_g), owing to the Raman-active modes of the pure anatase phase.^32,49 The Raman spectra of TiO₂–PTP reveal that the pristine E_g, B1_g, and A1_g bands are red-shifted to 121, 370, 489, and 610 cm^–1. Again, the E_g, B_1g, and A_1g bands are slightly blue-shifted to 124, 375, and 515 cm^–1, respectively, after the Pt NP decoration on TiO₂-doped PANI because of the electronic interference from the plasmonic Pt NPs with the excitation laser wavelength as a result of the strong metal-support electronic interaction.³² The second E_g band is slightly red-shifted to 602 cm^–1, which might be due to the generation of defects in TiO₂ by the Pt NPs which hamper the E_g-based longitudinal optical phonons.

2.3. Electrocatalytic Oxidation of Methanol

The electrocatalytic methanol oxidation activities of the prepared Pt–PTP and Pt–TiO₂–PTP composites have been studied by the conventional three-electrode system. The cyclic voltammetry (CV) curves of the two prepared Pt nanocomposites recorded in 0.5 M H₂SO₄ solution at a scan speed of 50 mV S^–1 are shown in Figure 4a. Both the voltammograms clearly provide the evidence about the hydrogen adsorption/desorption on the surface of Pt NPs at ∼−0.2 to 0.1 V.^6,46,50 Alongside this, Pt–PTP shows Pt oxide formation and oxide reduction on the Pt NP surface at ∼0.48 V in forward scan.^6,51,52 The TiO₂-doped composite Pt–TiO₂–PTP reveals the same peak for Pt oxide at ∼0.63 V in forward scan. The potential Pt oxide reduction is slightly higher in Pt–TiO₂–PTP, most likely due to the presence of TiO₂ in the composite.

Figure 4.

(a) Cyclic voltammograms of Pt–PTP and Pt–TiO₂–PTP composites in 0.5 M H₂SO₄ with a scan speed of 50 mV S^–1 at room temperature. (b) CV curves of MOR on Pt–PTP- and Pt–TiO₂–PTP-modified GCE in the mixture of 0.5 M methanol and 0.5 M H₂SO₄ with a scan speed of 50 mV S^–1 at room temperature.

The electrochemical surface area (ECSA), which is a very significant parameter for the electrochemical performance of an electrocatalyst, can be calculated by integrating the charge (Q_H) passing the electrode during the hydrogen adsorption/desorption process.^6,50 The charge required to oxidize a hydrogen monolayer is 0.21 mC cm^–2. Generally, ECSA is calculated using the following equation^6,46,51,53

where “Q_H” is the total charge relating to H⁺ adsorption on an integrated peak area of hydrogen adsorption/desorption; “m_Pt” is the active mass of Pt-containing catalyst (g m^–2) on the glassy carbon electrode (GCE), and it is 0.157 g m^–2 for Pt–PTP and 0.121 g m^–2 for Pt–TiO₂–PTP. The calculated ECSA values for these two composites are ∼24.38 and 41.24 m² g^–1 (Table 1), respectively. The ECSA value for Pt–TiO₂–PTP is 1.7 times higher than that of Pt–PTP, which implies that the catalytic activity of Pt–TiO₂–PTP is superior to that of Pt–TiO₂ in the electrochemical process. We can also compare the ECSA values of the commercially available Pt/C catalyst with similar Pt loading (10%) in the reported literature under similar CV conditions.⁶ Mondal et al. reported the ECSA of commercial Pt/C in 10% Pt-loading catalyst to be ∼14.62 m² g^–1, which is much lower than that of our prepared catalysts Pt–PTP and Pt–TiO₂–PTP, which have the ECSA values of ∼24.38 and 41.24 m² g^–1, respectively.⁶ Hence, we can conclude that our prepared catalysts are more superior in catalytic activity than the 10% Pt-loaded commercial Pt/C catalyst.

Table 1. Summary of Preparation of Different Composites and Their Electrocatalytic Activity.

composites	PTP or TiO2–PTP (mg)	H2PtCl6 (mg)	Pt wt % loading	TiO2 wt %	Pt NP shape and size	ECSA (m2 g–1)	If/Ib dark	If change in light
Pt–PTP	10 mg PTP	10	11	0	spherical, ∼3 nm	24.38	2.58	no change
Pt–TiO2–PTP	10 mg TiO2–PTP	10	8.5	20	spherical, ∼3 nm	41.24	2.80	1.3 times
Pt–TiO2		10	10	80	spherical	32.21	1.96	2 times
Pt/Ca			10		spherical ∼2.3–3.0 nm	14.62

^aData were taken from ref (6) as the experiment was done under similar conditions.

MOR has been performed by two Pt NP-containing nanocomposite-modified GCE in 0.5 M H₂SO₄ electrolyte in the presence of 0.5 M methanol at 25 °C in the potential range of −0.2 to 1.0 V (vs Ag/AgCl). The cyclic voltammograms during MOR shown in Figure 4b are considerably different from the voltammetry behaviors shown in Figure 4a. MOR exhibits two well-defined oxidation peaks at forward and reverse scans. The first one is at ∼0.74 V in the forward scan and the second peak is at ∼0.60 V in the backward scan. In a typical methanol electro-oxidation reaction, the peak current density at the forward scan (I_f) denotes the dehydrogenation of adsorbed methanol to produce Pt-adsorbed carbonaceous species like CO. This Pt-adsorbed CO can work as a catalyst poison, whereas the peak current density at the backward scan (I_b) is mainly related with the oxidation of adsorbed carbonaceous species like CO.^6,54 The MOR can be represented by the following reactions:

In forward scan

In reverse scan

Importantly, the values of I_f, I_b, and the ratio of I_f/I_b are the important parameters which correspond to the catalytic efficiency and catalyst poisoning. Higher the value of peak current intensity and the ratio, higher will be the catalytic efficiency. It is clear from Figure 4b that the methanol oxidation current density of Pt–TiO₂–PTP (0.84 mA cm^–2) is higher than that of Pt–PTP (0.31 mA cm^–2). Moreover, the Pt–TiO₂–PTP catalyst exhibits a slightly higher I_f/I_b ratio compared with the Pt–PTP electrocatalyst (2.8 vs 2.58). This observation implies that the Pt–TiO₂–PTP catalyst is slightly efficient toward MOR than Pt–PTP under similar conditions owing to the presence of TiO₂ in the nanostructures, which can improve the catalytic activity, as discussed in Introduction. The I_f/I_b ratio in the catalysts remains reasonably similar (2.72 and 2.56, respectively, in the two catalysts) even after the 10th scan (Figure 5a,b), which implies that the catalyst poisoning is not an issue here and that the catalysts are very stable during MOR. Furthermore, to investigate the stability of the Pt–TiO₂–PTP catalyst, we performed the study up to 100 cycles, and the I_f/I_b ratio remained reasonably similar (2.8 in the 1st scan and 2.71 after the 100th scan) even after 100 scans (Figure S4 in the Supporting Information). Hence, we can conclude that the Pt–TiO₂–PTP catalyst exhibits good electrocatalytic stability for methanol electro-oxidation. The I_f/I_b ratio in our synthesized catalysts is significantly higher than that of the previously reported Pt– PANI-based catalysts for alcohol oxidation.^6,55−57 As the ratio is higher, the CO-tolerance ability is also higher in our catalysts compared to the other reported Pt– PANI composites. Perhaps, the presence of a large perylene core in the polymer nanostructures and the use of less Pt wt % increase the vacant space required for the CO liberation as well as make our catalysts superior with regard to CO poisoning than the previously reported Pt– PANI -based catalysts for methanol oxidation.

Figure 5.

CV curve of the first (black line) and 10th (gray line) cycles of MOR for (a) Pt–PTP- and (b) Pt–TiO₂–PTP-modified GCE in the mixture of 0.5 M methanol and 0.5 M H₂SO₄ with a scan speed of 50 mV S^–1 at room temperature. (c) CO-stripping linear sweep voltammetry (LSV) study of both the composites in 0.5 M H₂SO₄ with a scan speed of 50 mV S^–1 at room temperature.

To study the effect of scan rate during MOR, CV was performed during MOR at various scan rates, and the obtained voltammograms are depicted in Figure S5a,b. The value of I_f grows up by increasing the scan rate. The anodic peak current density (I_f) versus the square root of the scan rate in Figure S5c,d in the Supporting Information provides a linear relation in both the cases. From this linear plot, it can be concluded that the methanol oxidation on both the modified electrodes is a diffusion-controlled process.^58,59

To compare the CO tolerance of the Pt–PTP and Pt–TiO₂–PTP catalysts further, CO-stripping has been measured (Figure 5c). In the CO-stripping experiment, the peak at ∼0.85 V has been observed after feeding the catalyst-modified working electrode with CO gas externally for 15 min bubbling. Again, a similar peak is vanished upon purging of argon for 30 min. The obtained peak currents and the onsets in LSV measurements are more or less similar in both the composites, which indicate that both the composites are similarly effective for CO oxidation and CO tolerance.

2.4. Effect of PTP in the Catalyst

To evaluate the effect of the PANI-containing polymer PTP on the catalyst, we have prepared Pt–TiO₂ with 10 wt % Pt NP loading to compare the electrochemical properties with the PTP-containing catalysts. We have characterized the Pt–TiO₂ sample by powder XRD, XRF, FESEM, EDX analysis, and electrochemical characterizations, and the corresponding results are given in Figure S6 in the Supporting Information. The XRD study reveals the characteristic peaks for anatase TiO₂ along with the distinguishing peaks for Pt NP at 40°, 46.2°, 67.7°, and 81.5° corresponding to the (111), (200), (220), and (311) crystal planes of the fcc Pt NPs. The EDX spectra evaluate ∼10 wt %

Pt loading in the Pt–TiO₂ sample. The electrochemical analysis demonstrates that the ECSA value is 32.21 m² g^–1, which is quite lower than that of Pt–TiO₂–PTP. Again, the I_f/I_b ratio during MOR in acidic medium is 1.96, which is again quite lower than that of the PTP-containing polymer nanocomposite catalysts (2.80 for Pt–TiO₂–PTP and 2.58 for Pt–PTP). Hence, we can conclude that PTP has a definite role in homogeneously distributing Pt NPs for enhancing the ECSA value in the Pt–TiO₂–PTP catalyst as we stated earlier in Introduction. Due to the using the high surface area containing PTP fibers as a conducting support in ternary nanocomposite catalyst, Pt–TiO₂–PTP showing significantly higher ECSA and I_f/I_b ratio than previously reported Pt–TiO₂ containing catalyst with reduced graphene oxide as conduction support where the ECSA is only 0.81 cm² mg^–1 and I_f/I_b ratio in dark is 1.93.²⁷ Again, a higher I_f/I_b ratio during MOR in the Pt–TiO₂–PTP and Pt–PTP catalysts demonstrates a higher CO tolerance in PTP-containing materials, as PTP can interact with water molecules and assist in the oxidation of CO to CO₂ during MOR at room temperature to increase the CO tolerance of the PTP-containing modified electrode material.

2.5. Photo-Assisted Electrochemical Study

Alongside the electrocatalytic oxidation of methanol, a fascinating phenomenon was observed when the electrocatalytic oxidation was performed under UV light irradiance. As shown in Figure 6a,b and summarized in Table 1, the catalytic performance toward methanol oxidation of the Pt–TiO₂–PTP-modified GCE electrodes is considerably improved with the assistance of light illumination as the current density in forward scan (I_f) is amplified to 1.15 mA cm^–2 (Figure 6b). However, the light illumination has no effect on the catalytic activity of Pt–PTP as the I_f value of MOR in the Pt–PTP-modified electrode is not improved at all after the light illumination. Hence, compared to Pt–PTP, the catalytic methanol oxidation performance is improved by 1.3 times with the assistance of UV light. Again, under light illumination in the Pt–TiO₂ catalyst, I_f is enhanced by ∼2 times (Figure S6g), which is slightly higher than the 1.3 times increment of I_f in Pt–TiO₂–PTP under the same conditions. This higher degree of I_f enhancement in Pt–TiO₂ may be attributed to the higher wt % of photosensitive TiO₂ in Pt–TiO₂.

Figure 6.

Cyclic voltammograms during MOR for (a) Pt–PTP and (b) Pt–TiO₂–PTP-modified GCE in the mixture of 0.5 M methanol and 0.5 M H₂SO₄ with a scan speed of 50 mV S^–1 with and without light irradiation. (c) UV light effect on chronoamperometric (CA) studies of modified GCEs during MOR under similar conditions stated earlier.

To further investigate the light irradiation effects on the two catalysts, the CA measurement was also performed in the solution of 0.5 M H₂SO₄ containing 0.5 M CH₃OH at a constant potential of 0.74 V. Figure 6c shows the CA curve from 0 to 4500 s, exposing the electrodes to the light irradiation in each 500 s interval. Initially, the CA curves exhibit lowering of current because of the formation of intermediate species such as Pt catalyst–(CH₃OH)_ad, Pt catalyst–(CO)_ad, Pt catalyst–(OH)_ad, and so forth, which deactivate the Pt surface as well as the oxidation current. After a while, current deterioration effectively slows down because of the adsorption/desorption equilibrium of the intermediate species on the Pt surface. However, when the light irradiation was supplied on the Pt–TiO₂–PTP-modified electrode, the current density was amplified sharply and again decreased immediately when the light was turned off. This phenomenon proves that the light treatment has a positive effect on the MOR process in the Pt–TiO₂–PTP-modified electrode. Again, for Pt–PTP, the increment of the current density after light illumination is negligible.

The improvement of the photoelectrochemical MOR activity of Pt–TiO₂–PTP under light irradiation is accredited to the strong metal–support interaction (SMSI) between the Pt NPs and TiO₂ and the light-induced enhancement of charge transport properties within the Pt–TiO₂–PTP catalyst. The mechanistic model of photoinduced electrochemical methanol oxidation is shown in Figure 7. As described in Introduction, anatase TiO₂ is a kind of n-type semiconductor with a band gap of 3.2 eV. In TiO₂, the movement of the Fermi level would assist the charge separation because of the SMSI interaction of the Pt NPs. The positive applied bias and light irradiation would generate an electron (e^–) to photoexcite from its VB and leave a hole (h⁺) in the VB. The generated (e^–)–h⁺ pairs participate in the surface redox reaction to increase the photocurrent under light irradiation.³⁰ Again, the photogenerated h⁺ can migrate to the catalyst surface and transform the OH^–/H₂O species to ^•OH radicals which are strong oxidant species to oxidize the species (e.g., CH₃OH) adsorbed on the surface of the catalyst.^25−27,60

Figure 7.

Mechanisms of enhanced methanol oxidation activity for Pt–TiO₂–PTP-modified electrode under photo-assisted electrochemical condition.

The formation of ^•OH radicals in our catalytic system can be proved spectroscopically by the conversion of coumarin to 7-hydroxycoumarin (umbelliferone) under UV irradiation. In the presence of ^•OH radicals, coumarin is converted to 7-hydroxycoumarin, which shows fluorescence at a maximum wavelength of 452 nm. Figure S7 reveals that the emission pattern of coumarin is entirely changed to produce a broad emission maximum at 452 nm after UV light irradiation in a mixture of coumarin and Pt–TiO₂–PTP catalyst. This study confirms the formation of 7-hydroxycoumarin (umbelliferone) via the generation of ^•OH radicals upon irradiation of UV light in the presence of TiO₂-containing catalytic system.

In addition, the reactive ^•OH radicals could also oxidize the catalyst-adsorbed CO to suppress the catalyst poisoning. Here, the conducting PANI fibers play a key role in transporting the e^– from the TiO₂ conduction band to the electrode. Again, methanol may also easily react with the photogenerated h⁺ to form methoxy radicals, followed by the electron injection into the conduction band of TiO₂, resulting in the improvement of MOR activity of the Pt–TiO₂–PTP catalyst. The increment of the MOR catalytic activity is lacking in Pt–PTP because of the absence of photoactive TiO₂ in the Pt–PTP composite. The negligible enhancement of current density in CA measurement in Pt–PTP under light illumination is probably due to the enhancement of photocurrent, as perylene-containing PTCA is present in the composite.

3. Conclusions

In conclusion, we have successfully synthesized TiO₂-containing PTP and subsequently decorated the nanofibers by Pt NPs. The ternary composite-modified (Pt–TiO₂–PTP) electrode displays high electrocatalytic activity for methanol oxidation in acidic medium than that without its TiO₂ counterpart (Pt–PTP), having similar Pt-loading weight percentage. The calculated ECSA value for Pt–TiO₂–PTP is 1.7 times higher than that for Pt–PTP, which indicates the superior catalytic activity of Pt–TiO₂–PTP in the electrochemical process. The ECSA values of our synthesized catalysts are also higher than that of the commercially available Pt/C catalyst at the same Pt-loading condition. The high I_f/I_b ratio of the two synthesized catalysts indicates that the CO tolerance ability is also higher in the Pt–PTP-based catalyst compared to the other reported Pt– PANI composites. The catalytic activity of the photoreactive TiO₂-containing catalyst is further improved significantly with the aid of UV light irradiation. The synergistic effects of photo- and electrocatalysis on MOR in Pt–TiO₂–PTP enhance the MOR catalytic activity approximately 1.3 times higher in light illumination than in dark. Hence, this work provides a significant opportunity for developing a UV light-assisted simple, nimble, and viable strategy for improving the MOR activity of Pt-based catalysts in DMFCs.

4. Experimental Section

4.1. Materials

Aniline, chloroplatinic acid hexahydrate (H₂PtCl₆, 6H₂O, ≥37.50% Pt basis), perylene-3,4,9,10-tetracarboxylic dianhydride, ammonium persulfate [(NH₄)₂S₂O₈] (APS), and titanium(IV) isopropoxide [Ti{OCH(CH₃)₂}₄] were bought from Sigma-Aldrich and were used as supplied, except aniline which was distilled before use. Methanol (HPLC Grade) was supplied by S D Fine-Chem Limited. Deionized water (18 MU cm, Millipore Milli-Q water) was used to prepare solutions where necessary.

4.2. Synthesis of TiO₂

TiO₂ was synthesized from the starting material TiO(NO₃)₂ by an instantaneous single step, the solution combustion route. TiO(NO₃)₂ was initially prepared by hydrolyzing Ti{OCH(CH₃)₂}₄, followed by dissolving it in a least quantity of concentrated nitric acid. Colorimetric method was employed for the evaluation of the Ti ion concentration in TiO(NO₃)₂ solution. The fuel glycine was mixed with TiO(NO₃)₂ in a 1:1.11 ratio to stoichiometrically balance the oxidizing/reducing valencies of the oxidizer and the fuel according to the propellant chemistry. In a 300 mL borosilicate dish, 1 g of TiO(NO₃)₂ and 0.442 g of glycine were mixed together, and the dish was kept in a preheated muffle furnace at 450 °C to yield the voluminous and fluffy TiO₂.

4.3. Synthesis of PTCA

PTCA was synthesized according to the reported procedure.⁴² Perylene-3,4,9,10-tetracarboxylic dianhydride (0.20 g, 0.5 mmol) was solubilized in 10 mL of 5% KOH solution in water by nonstop stirring at 70 °C. After cooling the solution to room temperature, 0.1 M HCl was added drop by drop with continuous stirring till the pH reached 5–6. The product was collected by filtrating the precipitation and consequently drying in vacuum overnight to provide 0.18 g of PTCA as a red powder (yield of 91%).

¹H NMR (500 MHz, DMSO-d₆, room temperature): 8.60–8.64 (d, 4H), 8.04–8.06 (d, 4H).

MASS: (MALDI-TOF): found 430.21 [(M + H)⁺].

FTIR: γC=O stretch (1692 cm^–1), O–H bends (1504 cm^–1), C–O stretch (1298 cm^–1), and O–H stretch (2990 cm^–1); C=C (perylene core) 1591 cm^–1; C–H (perylene core) 803 and 729 cm^–1.

4.4. Preparation of PTP

PTP was synthesized according to the reported procedure.⁴² PTCA (48 mg, 0.11 mmol) was taken in 20 mL of water to solubilize with continuous stirring for 2 h at 25 °C. Aniline (102 mg, 100 mL, 1.1 mmol) was taken into the PTCA solution and stirred for another 1 h at the same temperature. The mixture was ice-cooled, and aqueous APS solution (50 mg in 10 mL) was added dropwise to the mixture at 5 °C, and after addition it was kept at 5 °C temperature for another 24 h without stirring. The precipitate was filtered and washed alternatively with water and methanol to eliminate the oligomers and excess APS. Finally, it was dried under vacuum at room temperature for 24 h to obtain PTP (102 mg) as a greenish black powder.

4.5. Preparation of TiO₂-Containing PTP

TiO₂–PTP was prepared by the abovementioned procedure used for the synthesis of PTP. To the PTCA (48 mg, 0.11 mmol) solution in 20 mL water, aniline (102 mg, 100 mL, 1.1 mmol) and the prepared TiO₂ (20 mg) was mixed together and stirred for 1 h. The mixture was polymerized by APS by the abovementioned procedure to provide 118 mg of TiO₂–PTP as a greenish black powder.

4.6. Preparation of the Pt–PTP and Pt–TiO₂–PTP Catalysts

Pt–PTP and Pt–TiO₂–PTP catalysts were synthesized from their corresponding PANI nanofiber precursors PTP and TiO₂–PTP. In a typical process, 10 mg of PTP or TiO₂–PTP polymer was dispersed in 10 mL of water. A 5 mL of H₂PtCl₆ solution (2 mg mL^–1) was added dropwise with continuous stirring. NaBH₄ (5 mg) was used as the reducing agent and stirred for 6 h at 25 °C. The synthesized Pt–PTP and Pt–TiO₂–PTP composites were precipitated by centrifugation at 5000 rpm and washed with water for five times. Drying the materials in vacuum provided 12.5 mg and 13.1 mg Pt–PTP and Pt–TiO₂–PTP, respectively, as black powders.

4.7. General Characterization

The UV–vis spectra of the compounds were studied using a JASCO/V-650 (190–900 nm) UV–vis spectrophotometer, taking the dimethylformamide solution of the compounds. Fluorescence studies were done in a JASCO/FP-6300 (190–900 nm) fluorescence spectrometer. Powder XRD was studied with Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). A typical scan was performed at a scan rate of 1° min^–1 with a step size of 0.02°. High-resolution TEM, EDX analysis, and bright-field imaging and mapping of Pt–PTP and Pt–TiO₂–PTP were performed on a UHR-FEG-TEM (JEOL, JEM 2100) instrument at 200 kV. Water dispersions of the samples were casted on a 200-mesh Cu-grid for TEM. FESEM imaging and EDX analysis were performed by FEI, Apreo S with a 20 kV operating voltage by taking a small amount of methanol-dispersed sample drop casted on a silicon wafer. The loading of Pt in the synthesized catalysts was monitored by energy-dispersive XRF (Epsilon 1; PANalytical). The Raman spectra were recorded by a UniRAM 3300 Raman microscope with a laser wavelength of 532 nm.

4.8. Electrochemical Characterization

All electrochemical experiments like CV and CA were executed by an Autolab potentiostat PGSTAT128N using a three-electrode system. The electrodes were Pt–PTP and Pt–TiO₂–PTP catalyst-modified GCE-based working electrode, a Pt wire counter electrode, and an Ag/AgCl reference electrode. The GCE, 3 mm in diameter, was polished gently with 1, 0.3, and 0.05 mm alumina powder, and a mirror-finish GCE was obtained after washing. A 5 μL solution of methanol-dispersed Pt–PTP and Pt–TiO₂–PTP composites (2 mg mL^–1) were drop-casted over on active surface of GCE and dried to make a film for electrochemical study. The methanol electro-oxidation was recorded in a mixture of 0.5 M H₂SO₄ and 0.5 M methanol.

4.9. Photo-Assisted Electrochemical Characterization

The photo-assisted methanol electro-oxidation was studied under UV light irradiation using a medium-pressure mercury vapor lamp (365 nm) of 125 W. The average energy of the illuminated light was 3.5 eV with a photon flux of 5.86 × 10^–6 mol photons/s. Water was circulated around the lamp to keep the reaction at room temperature and to cease the IR emission from the lamp. In the photo-assisted condition, the electrochemical measurements were done by the abovementioned process. The formation of the ^•OH radicals was confirmed by the coumarin study. In a typical procedure, 5 mg of the Pt–TiO₂–PTP catalyst was added to a 3.5 mL of 0.03 mM coumarin solution in a quartz cuvette and UV light-irradiated for 120 min. After UV light irradiation, 500 mg of KCl was added to the suspension and kept in dark for 12 h to get a clear solution, and the emission spectra were studied. The emission spectra of the formed 7-hydroxycoumarin (umbelliferone) were measured at an excitation wavelength of 332 nm.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02610.

• HRTEM, SEAD, and elemental mapping from TEM; Raman spectra of TiO₂–PTP and Pt–TiO₂–PTP; scan rate-dependent CV; and general and electrochemical characterization of Pt–TiO₂ (PDF)

The authors declare no competing financial interest.