Pt/C-TiO 2 as Oxygen Reduction Electrocatalysts against Sulfur Poisoning

: Proton exchange membrane (PEM) fuel cells using Pt-based materials as electrocatalysts have achieved a decent performance, represented by the launched Toyota Mirai vehicle. The ideal PEM fuel cells consume stored pure hydrogen and air. However, SO 2 , as a primary air contaminant, may be fed along with air at the cathode, leading to Pt site deactivation. Therefore, it is important to improve the SO 2 tolerance of catalysts for the stability of the oxygen reduction reaction (ORR). In this work, we develop the Pt/C-TiO 2 catalyst against SO 2 poisoning during ORR. Impressively, the hybrid Pt/C-TiO 2 catalyst with 20 mass % TiO 2 shows the best ORR and anti-toxic performance: the kinetic current density of ORR is 20.5% higher and the degradation rate after poisoning is 50% lower than Pt/C. The interaction between Pt and TiO 2 as well as the abundant hydroxyl groups on the surface of TiO 2 are both revealed to account for the accelerated removal of poisonous SO 2 on Pt surfaces.


Introduction
The PEM fuel cell has been considered as one of the most promising automobile power supplies owing to its high energy efficiency and environmental benefits [1][2][3]. Delightfully, the first-generation Toyota Mirai has provided an excellent example, using PtCo as a cathode catalyst, achieving a max power with 114 kW and an endurance mileage of up to 502 km. However, the widespread implementation of PEM fuel cells is facing a greatly challenging poisoning/degradation problem caused by impurities such as SO 2 and NO x in the air, which is ideally supplied directly to the cathodes [4,5]. In particular, SO 2 can strongly adsorb onto the Pt surface and deactivate the active sites for the oxygen reduction reaction (ORR), leading to the degradation of ORR performance [6][7][8][9][10][11][12][13]. For example, it has been reported that the continuous feeding of 2.5 ppm SO 2 /air mixture for 45 h at the cathode significantly reduced the fuel cell performance by 53% and, importantly, the subsequent continuous clean air feeding for 20 h could not completely recover the performance [14]. This is because the widely accepted Pt/C catalysts are not efficient enough to remove the S x species once the coverage is higher than 14%, where up to 95% of mass activity can be lost [15]. In addition, an extremely harsh potential range of 1.2 V~1.5 V is required to completely oxidize SO x into water-soluble SO 2− 4 and/or HSO − 4 , which is much higher than the working potential of a PEM fuel cell (0.6 V~0.8 V) [6,16,17]. In this regard, the poisoning SO 2 species is almost irreversible under the normal operating conditions of a PEM fuel cell [18]. This fact triggers us to explore innovations in developing advanced anti-poisoning catalysts.
Numerous efforts have been made to prevent Pt-based catalysts from SO 2 poisoning [19]. For example, Baturina and coworkers [20,21] found that the adsorbed SO 2 is more easily oxidized and removed on Pt 3 Co/C than Pt/C, although the alloying Pt 3 Co/C catalyst is more sensitive to the poisoning (The same concentration of SO 2 has a more serious effect on the ORR performance of Pt 3 Co/C). They found that H 2 O is more likely to decompose into hydroxyl (-OH) on Pt 3 Co/C, which is beneficial for accelerating the electro-oxidation of SO 2 . In addition, Xu et al. [22] found that the SO 2 -poisoned 5 mass % WO x -Pt/C can cause a higher loss of SO 2 than commercial Pt/C after the first potential cycling up to 1.5 V, which might be due to the synergistic effect between WO x and Pt. The OH group on the hydrophilic WO x surface and the hydrogen spillover effect promote the electro-oxidation of sulfur-containing species. In their latter work, the Pt/CeO 2 /C catalyst was prepared by one-pot synthesis, enhancing the oxygen migration and thus improving SO 2 electro-oxidation and ORR activity [23,24]. In our previous work, we found that Ru efficiently modified the electronic structure of Pt in PtRu alloy catalysts, leading to a weaker interaction between Pt and SO 2 [25]. In this work, we will focus on the effect of metal oxides to the ORR performance and SO 2 tolerance of Pt.
To tone-up the positive effects of oxidative hydroxyl and synergy, we herein develop a hybrid Pt/C-TiO 2 catalyst against SO 2 poisoning during ORR. The selection of TiO 2 is based on its high hydrophilicity and outstanding chemical and electrochemical stability, as well as strong interaction with the Pt component [26][27][28][29][30][31][32][33]. Importantly, based on the above discussions, the abundant hydroxyl groups (-OH) on the hydrophilic TiO 2 surface may benefit poisonous sulfur removal. Accordingly, the electro-oxidation of SO 2 is significantly accelerated on such a Pt/C-TiO 2 catalyst to reduce the SO 2 poisoning. To our knowledge, the effect of TiO 2 on suppressing SO 2 poisoning of Pt-based ORR catalysts has not been reported. In this work, our hybrid Pt/C-TiO 2 catalysts have higher ORR activity and better sulfur tolerance compared to traditional Pt/C. Figure 1 shows the X-ray diffraction (XRD) patterns of the as-prepared Pt/C, Pt/C-20TiO 2 catalysts and TiO 2 nanoparticles. For the Pt/C sample, the broad X-ray diffraction peak in the range of 20 •~3 0 • corresponds to the amorphous structure of XC-72 carbon black, while the peaks at 39.8 • , 46.2 • , 67.5 • , and 81.3 • correspond to Pt(111), (200), (220), and (311) facets, respectively (JCPDS No. 04-0802). The commercial TiO 2 sample shows sharp peaks, which can be indexed to the anatase TiO 2 (JCPDS No. 21-1272). The XRD pattern of Pt/C-20TiO 2 has the X-ray diffraction peaks of both Pt and TiO 2 . The above facts demonstrate that the Pt/C and Pt/C-20TiO 2 were synthesized successfully.

Characterization of Catalysts
As the high-resolution transmission electron microscopy (HRTEM) image in Figure 2a shows, the Pt nanoparticles are uniformly dispersed on the carbon supports with an average size of about 2.74 nm ( Figure S1). A small Pt particle size and uniform dispersion potentially ensure good catalytic performance [34]. Figure 2b shows TEM images of the as-prepared Pt/C-20TiO 2 catalyst. The lattice fringes of 0.35 nm (red tags) and 0.22 nm (yellow tags) are assigned to the TiO 2 (101) plane and the Pt (111) plane, respectively. It can be seen that the TiO 2 and Pt particles are well contacted with each other.

Electrocatalytic Performance
The cyclic voltammogram (CV) and linear sweep voltammetry (LSV) curves of all pristine catalysts (before poisoning) are shown in Figure 3a,b, respectively. The electrochemical surface areas (ECSAs) of all the catalysts were calculated based on the H 2 adsorption/desorption region at 0~0.3 V in CV curves. All the relevant data recorded and calculated from electrochemical measurement are listed in Table 1. As the high-resolution transmission electron microscopy (HRTEM) image in Figure  2a shows, the Pt nanoparticles are uniformly dispersed on the carbon supports with an average size of about 2.74 nm ( Figure S1). A small Pt particle size and uniform dispersion potentially ensure good catalytic performance [34]. Figure 2b shows TEM images of the as-prepared Pt/C-20TiO2 catalyst. The lattice fringes of 0.35 nm (red tags) and 0.22 nm (yellow tags) are assigned to the TiO2 (101) plane and the Pt (111) plane, respectively. It can be seen that the TiO2 and Pt particles are well contacted with each other.   As the high-resolution transmission electron microscopy (HRTEM) image in Figure  2a shows, the Pt nanoparticles are uniformly dispersed on the carbon supports with an average size of about 2.74 nm ( Figure S1). A small Pt particle size and uniform dispersion potentially ensure good catalytic performance [34]. Figure 2b shows TEM images of the as-prepared Pt/C-20TiO2 catalyst. The lattice fringes of 0.35 nm (red tags) and 0.22 nm (yellow tags) are assigned to the TiO2 (101) plane and the Pt (111) plane, respectively. It can be seen that the TiO2 and Pt particles are well contacted with each other.  The results show that the addition of TiO 2 has little effect on the ECSA of the Pt/C catalyst. Compared with that of Pt/C, the current peak, representing the oxygen reduction of the Pt/C-TiO 2 catalysts, shifts positively when TiO 2 ≤ 30%; Pt/C-20TiO 2 has the most positive oxygen reduction peak, which likely indicates the best ORR activity, consistent with the LSV results in Figure 3b. The activity towards ORR is improved by increasing TiO 2 content and then started to decrease after reaching a maximum value, i.e., Pt/C-20TiO 2 . Too much TiO 2 (≥30 mass %) hinders the ORR activity, which is due to the low electron conductivity of TiO 2 . On the other hand, the half-wave potential (E 1/2 ) of Pt/C-20TiO 2 is 0.867 V, presenting an increase by 0.014 V compared with Pt/C (0.853 V), and the i k @ 0.9 V is 2.337 mA/cm 2 , 20.5% higher than Pt/C (1.939 mA/cm 2 ). Based on the Koutechy-Levich (K-L) equation (see Section 3.3), the ORR polarization curves of Pt/C-20TiO 2 were

Electrocatalytic Performance
The cyclic voltammogram (CV) and linear sweep voltammetry (LSV) curves of all pristine catalysts (before poisoning) are shown in Figure 3a,b, respectively. The electrochemical surface areas (ECSAs) of all the catalysts were calculated based on the H2 adsorption/desorption region at 0~0.3 V in CV curves. All the relevant data recorded and calculated from electrochemical measurement are listed in Table 1.     Loss of i k @ 0.9 V after Poisoning ECSA (m 2 g Pt −1 ) The first CV scans of each catalyst after poisoning in Ar-saturated 0.1 M HClO 4 solution are shown in Figure 4a, where the ECSA of all the catalysts decreases in a similar way because the strongly adsorbed SO 2 blocks the active site of Pt. In addition, clear oxidation peaks are observed in the potential range of 0.8 V~1.5 V, due to the electrochemical oxidation of SO 2 . Notably, there is a negative shift in the SO 2 oxidation peak of Pt/C-TiO 2 catalysts compared with Pt/C, indicating the higher activity of Pt/C-TiO 2 catalysts to oxidize SO 2 . Particularly, the Pt/C-20TiO 2 has the largest negative shift by about 50 mV in SO 2 oxidation, as shown in Figure 4b. The oxide reduction potential region of Figure 4b is magnified and shown in Figure 4c to compare the catalytic performance of Pt/C and Pt/C-20TiO 2 before and after poisoning. It is clear that the reduction peaks of Pt/C-20TiO 2 before or after poisoning all positively shift compared with the Pt/C catalyst, indicating that the Pt/C-20TiO 2 has a better electrocatalytic performance to reduce oxygen-bearing species than Pt/C at initial and after sulfur poisoning.
SO2. Particularly, the Pt/C-20TiO2 has the largest negative shift by about 50 mV in SO2 oxidation, as shown in Figure 4b. The oxide reduction potential region of Figure 4b is magnified and shown in Figure 4c to compare the catalytic performance of Pt/C and Pt/C-20TiO2 before and after poisoning. It is clear that the reduction peaks of Pt/C-20TiO2 before or after poisoning all positively shift compared with the Pt/C catalyst, indicating that the Pt/C-20TiO2 has a better electrocatalytic performance to reduce oxygen-bearing species than Pt/C at initial and after sulfur poisoning. The LSV curves of ORR after SO2 poisoning are shown in Figure 5a. After SO2 poisoning, the ORR activity of all catalysts is decreased, although the Pt/C-20TiO2 catalyst still has the best ORR activity. The @ 0.9V comparison between the fresh and poisoned catalysts is shown in Figure 5b. With the addition of TiO2, the decrease percentage of kinetic current density is significantly lowered. The Pt/C-10TiO2 and Pt/C-20TiO2 catalysts have the smallest activity decay by only 34.4% and 35.1%, respectively, in contrast to 67.8% for Pt/C. Table 1 shows that the E1/2 of Pt/C-20TiO2 went down by 53 mV negatively after poisoned, much lower than the 71 mV of Pt/C. All this indicates that the SO2 tolerance of Pt/C-20TiO2 is greatly improved than Pt/C.  The LSV curves of ORR after SO 2 poisoning are shown in Figure 5a. After SO 2 poisoning, the ORR activity of all catalysts is decreased, although the Pt/C-20TiO 2 catalyst still has the best ORR activity. The i k @ 0.9 V comparison between the fresh and poisoned catalysts is shown in Figure 5b. With the addition of TiO 2 , the decrease percentage of kinetic current density is significantly lowered. The Pt/C-10TiO 2 and Pt/C-20TiO 2 catalysts have the smallest activity decay by only 34.4% and 35.1%, respectively, in contrast to 67.8% for Pt/C. Table 1 shows that the E 1/2 of Pt/C-20TiO 2 went down by 53 mV negatively after poisoned, much lower than the 71 mV of Pt/C. All this indicates that the SO 2 tolerance of Pt/C-20TiO 2 is greatly improved than Pt/C. oxidation, as shown in Figure 4b. The oxide reduction potential region of Figure 4b is magnified and shown in Figure 4c to compare the catalytic performance of Pt/C and Pt/C-20TiO2 before and after poisoning. It is clear that the reduction peaks of Pt/C-20TiO2 before or after poisoning all positively shift compared with the Pt/C catalyst, indicating that the Pt/C-20TiO2 has a better electrocatalytic performance to reduce oxygen-bearing species than Pt/C at initial and after sulfur poisoning. The LSV curves of ORR after SO2 poisoning are shown in Figure 5a. After SO2 poisoning, the ORR activity of all catalysts is decreased, although the Pt/C-20TiO2 catalyst still has the best ORR activity. The @ 0.9V comparison between the fresh and poisoned catalysts is shown in Figure 5b. With the addition of TiO2, the decrease percentage of kinetic current density is significantly lowered. The Pt/C-10TiO2 and Pt/C-20TiO2 catalysts have the smallest activity decay by only 34.4% and 35.1%, respectively, in contrast to 67.8% for Pt/C. Table 1 shows that the E1/2 of Pt/C-20TiO2 went down by 53 mV negatively after poisoned, much lower than the 71 mV of Pt/C. All this indicates that the SO2 tolerance of Pt/C-20TiO2 is greatly improved than Pt/C.

Mechanism of SO 2 Tolerance
The X-ray photoelectron spectroscopy (XPS) spectra of the O 1s, Ti 2p and Pt 4f regions in TiO 2 , Pt/C, and Pt/C-20TiO 2 are shown in Figure 6. As shown in the O 1s region (Figure 6a), the peaks at 533.5 ± 0.1 eV, 532.2 ± 0.1 eV and 530.6 ± 0.1 eV are assigned to physically absorbed water (O1), chemisorbed OH species (O2), and lattice oxygen in TiO 2 and/or PtO (O3) [26,35]. In addition, it can be seen that the Ti 2p binding energy of Pt/C-20TiO 2 has a negative shift by 0.4 eV compared with Pt/C (Figure 6b). The Pt 4f regions for Pt/C and Pt/C-20TiO 2 can be fitted into three sets of doublets (Figure 6c and/or PtO (O3) [26,35]. In addition, it can be seen that the Ti 2p binding energy of Pt/C-20TiO2 has a negative shift by 0.4 eV compared with Pt/C (Figure 6b). The Pt 4f regions for Pt/C and Pt/C-20TiO2 can be fitted into three sets of doublets (Figure 6c). For the Pt/C, Pt 4f peaks at 71.4 eV and 74.7 eV are assigned to Pt (0), 72.8 eV and 76.1 eV to Pt (II) in PtO or Pt(OH)2-like species, and 74.0 eV and 77.3 eV to Pt(IV) in PtO2. Interestingly, the binding energy of the Pt/C-20TiO2 catalyst is negatively shifted by 0.3 eV compared to the Pt/C catalyst, indicating an electronic interaction between Pt and TiO2. The electron transfer from TiO2 to Pt increases the electron density of Pt, which was reported as a necessity for effective ORR [36]. In addition, the binding energy between Pt and OH-like oxygen-containing species is weakened, which accelerates the reaction of O, OH, and H + to generate H2O and improves the ORR activity [37,38]. Jaksic et al. [39] found that in aqueous media, OH on the surface of metal oxides such as WO3 and TiO2 can be transferred to the Pt surface to form Pt-OH, which is the so-called spillover effect. Reasonably, such an OH spillover effect of TiO2 will repel the OH groups on Pt, leading to decreased binding energy between OH and Pt and promoting ORR activity [40]. On the other hand, the electronic interaction between Pt and TiO2 is able to weaken the adsorption of poisoning species on Pt surfaces [25,41]. At the same time, the spillover effect of oxidative OH on the TiO2 surface plays a significant role in promoting the electro-oxidation (removal) of SO2 [22]. Figure 7 is provided to illustrate this mechanism. For the Pt/C-TiO2 catalyst: Pt provides electrocatalytic active sites for oxygen reduction; C supports Pt nanoparticles and conducts electrons; and TiO2 modifies the electronic structure of Pt to enhance the ORR activity and SO2 tolerance. The electron transfer from TiO 2 to Pt increases the electron density of Pt, which was reported as a necessity for effective ORR [36]. In addition, the binding energy between Pt and OH-like oxygen-containing species is weakened, which accelerates the reaction of O, OH, and H + to generate H 2 O and improves the ORR activity [37,38]. Jaksic et al. [39] found that in aqueous media, OH on the surface of metal oxides such as WO 3 and TiO 2 can be transferred to the Pt surface to form Pt-OH, which is the so-called spillover effect. Reasonably, such an OH spillover effect of TiO 2 will repel the OH groups on Pt, leading to decreased binding energy between OH and Pt and promoting ORR activity [40]. On the other hand, the electronic interaction between Pt and TiO 2 is able to weaken the adsorption of poisoning species on Pt surfaces [25,41]. At the same time, the spillover effect of oxidative OH on the TiO 2 surface plays a significant role in promoting the electro-oxidation (removal) of SO 2 [22]. Figure 7 is provided to illustrate this mechanism. For the Pt/C-TiO 2 catalyst: Pt provides electrocatalytic active sites for oxygen reduction; C supports Pt nanoparticles and conducts electrons; and TiO 2 modifies the electronic structure of Pt to enhance the ORR activity and SO 2 tolerance.     TEM and HRTEM images were taken on JEOL JEM-2100 with an acceleration voltage of 300 kV. The morphologies and crystal structure of the Pt/C, Pt/C-TiO 2 catalyst and TiO 2 nanoparticles were characterized by XRD (Netherlands, Panalytical X'PERT) using Cu Kα radiation. XPS measurements were performed by a Physical Electronics PHI model 5700 instrument with an electron energy analyzer, with half-spherical precision using Al Kα radiation (1486.6 eV). The binding energy of all samples was calibrated vs. the C 1s value of 284.6 eV.

Synthesis of Catalysts
The Pt/C catalyst (20 mass % Pt) was prepared by the microwave-assisted polyol reduction method [42]. Briefly, 40 mg Vulcan XC-72 carbon black was well-dispersed into 60 mL ethylene glycol (EG) and isopropanol (V/V = 4:1) under ultrasonic treatment for 2 h. Then H 2 PtCl 6 -EG solution was added to the slurry and stirred constantly for 2 h. Next, 1 mol/L NaOH-EG solution was dropped into the slurry under continuous stirring until the pH was stable at 12. Argon was injected into the slurry for 20 min to remove oxygen, which was then placed in a microwave oven (2450 MHz, 800 W) and heated for 2 min. After the solution was cooled down to room temperature, a certain amount of dilute nitric acid was added to pH = 3, followed by stirring for 12 h. After washing with plenty of boiling ultra-pure water (Milli-Q, 18 MΩ cm), the Pt/C catalyst was dried in a vacuum oven at 70 • C for 12 h.

Electrochemical Measurements
All electrochemical measurements were carried out in a three-electrode system with an electrolyte of 0.1 mol/L HClO 4 aqueous solution at room temperature, using the glassy carbon rotating-disk electrode (GC-RDE, 5 mm in diameter) as the working electrode, a saturated Ag-AgCl electrode as the reference electrode, and a piece of Pt foil (1 cm 2 ) as the counter electrode. 10 µL of catalyst ink was dropped onto the polished GC-RDE. Then the electrodes were air-dried at room temperature. The catalyst loading was about 10 µg/cm 2 .
The ECSA of the catalyst is calculated based on the charge number of H adsorption or desorption, as shown in Equation (1).
where Q H (C) represents the charge number of H adsorption or desorption and can be obtained by integrating the CV curve in Ar-saturated 0.1 mol L −1 HClO 4 solution between 0.05 and 0.4 V; and m Pt (g) is the Pt load in the catalyst. Assuming there is a one-to-one relationship between the Pt atom and H atom of the hydrogen monatomic layer on the Pt/C catalyst, the charge amount of the hydrogen monatomic layer on the Pt/C catalyst surface is 210 µC cm −2 .
Before recording ORR activity, ultra-pure argon gas was injected into the electrolyte for 20 min to remove dissolved oxygen. CV scans were then performed at the scan rate of 50 mV s −1 in a potential range of 0.05 V~1.5 V for 10 cycles at room temperature. The ORR activity of the catalyst was then measured by LSV from 0.05 V to 1 V at a scan rate of 10 mV s −1 at 1600 rpm in O 2 -saturated 0.1 mol/L HClO 4 solution. The kinetic current Catalysts 2022, 12, 571 8 of 10 density at 0.9 V and transferred electron number were calculated from the ORR polarization following the Koutechy-Levich (K-L) equation, Equation (2).
where i k and i d are the kinetic current density and limited diffusion current density, n is the number of electrons transferred, F is the Faraday constant (96,487 C mol −1 ), A is the geometric surface area of the RDE, c is the bulk concentration of O 2 (2.64 × 10 −4 mol cm −3 ), D is the diffusion coefficient of O 2 (1.93 × 10 −5 cm 2 S −1 ), ν is the kinematic viscosity of the solution (0.01 cm 2 S −1 ), and ω is the angular velocity. The half-wave potential (E 1/2 ) is read from the ORR curve of the catalyst, which is the potential corresponding to the half of the limited diffusion current density. The poisoning of catalysts was carried out by immerging the working electrode at the potential of 0.65 V for 1 min in a 0.5 mmol/L Na 2 SO 3 + 0.1 mol/L HClO 4 electrolyte. Then, the GC-RDE was rinsed with ultra-pure water and transferred to a fresh 0.1 mol/L HClO 4 electrolyte without SO 2 . The ORR activity of the poisoned electrode was evaluated in an O 2 -saturated 0.1 mol/L HClO 4 electrolyte using LSV at 1600 rpm. To investigate the oxidation behavior of adsorbed SO 2 , a CV scan was performed between 0.05 V and 1.5 V for 10 cycles at a sweep rate of 50 mV/s. All of the potentials were reported with respect to the reversible hydrogen electrode (RHE) in this work.

Conclusions
Hybrid Pt/C-TiO 2 catalysts were prepared by mixing Pt/C catalyst and TiO 2 nanoparticles. By adding TiO 2 , the ORR performances of fresh and SO 2 -poisoned catalysts were all improved compared to the Pt/C and the activity loss caused by SO 2 poisoning was remarkably reduced. Our detailed observations indicate that the improved SO 2 tolerance is due to the accelerated electro-oxidation of SO 2 in the presence of optimal TiO 2 . We suggest that the electronic interaction between Pt and TiO 2 and the OH spillover effect on TiO 2 might be crucial for not only enhancing the ORR activity, but also removing the poisoning SO 2 . These results indicate that the TiO 2 modified Pt/C catalyst is a desired ORR catalyst against sulfur poisoners in PEM fuel cells.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/catal12050571/s1, Figure S1: TEM images of Pt/C and particle size distributions; Figure S2: The ORR polarization curves of Pt/C-20TiO 2 measured at different electrode rotation speeds and the K-L plots from the polarization curve.