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. 2024 Feb 21;16(8):10295–10306. doi: 10.1021/acsami.4c01794

Synthesis of a Mesoporous SnO2 Catalyst Support and the Effect of Its Pore Size on the Performance of Polymer Electrolyte Fuel Cells

Masanori Inaba †,*, Ryuichi Murase †, Tomohiro Takeshita †, Kazuhisa Yano †, Satoru Kosaka †, Naoko Takahashi †, Noritake Isomura †, Keiichiro Oh-ishi †, Wataru Yoshimune †, Kimihiro Tsuchiya ‡, Takeshi Nobukawa ‡, Kensaku Kodama †
PMCID: PMC10910439  PMID: 38379515

Abstract

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The aim of this study was to clarify the effectiveness and challenges of applying mesoporous tin oxide (SnO2)-based supports for Pt catalysts in the cathodes of polymer electrolyte fuel cells (PEFCs) to simultaneously achieve high performance and high durability. Recently, the focus of PEFC application in automobiles has shifted to heavy-duty vehicles (HDVs), which require high durability, high energy-conversion efficiency, and high power density. It has been reported that employing mesoporous carbon supports improves the initial performance by mitigating catalyst poisoning caused by sulfonic acid groups of the ionomer as well as by reducing the oxygen transport resistance through the Pt/ionomer interface. However, carbon materials in the cathode can degrade oxidatively during long-term operation, and more stable materials are desired. In this study, we synthesized connected mesoporous Sb-doped tin oxides (CMSbTOs) with controlled mesopore sizes in the range of 4–11 nm and tested their performance and durability as cathode catalyst supports. The CMSbTO supports exhibited higher fuel cell performance at a pore size of 7.3 nm than the solid-core SnO2-based, solid-core carbon, and mesoporous carbon supports under dry conditions, which can be attributed to the mitigation of the formation of the Pt/ionomer interface and the better proton conductivity within the mesopores even at the low-humidity conditions. In addition, the CMSbTO supports exhibited high durability under oxidative conditions. These results demonstrate the promising applicability of mesoporous tin oxide supports in PEFCs for HDVs. The remaining challenges, including the requirements for improving performance under wet conditions and stability under reductive conditions, are also discussed.

Keywords: polymer electrolyte fuel cells, catalyst support, mesoporous tin oxide, Pt/ionomer interface, oxygen reduction reaction, mass transport

1. Introduction

To achieve a carbon-neutral society, the anticipated increase in the utilization of hydrogen, generated from renewable energy sources, as an energy carrier is significant. Polymer electrolyte fuel cells (PEFCs), which convert the chemical energy of hydrogen into electricity, are crucial power sources for the electrification of various transportation systems, and they have been commercialized for passenger light-duty vehicle (LDV) applications.1 Recently, the focus for future PEFC applications in automobiles has shifted from LDVs to heavy-duty vehicles (HDVs), such as buses and trucks, owing to hydrogen’s high gravimetric energy density.2 The total operating time during the lifetime of HDVs notably surpasses that of LDVs; therefore, the durability of fuel cell components, including catalyst materials, is becoming increasingly important. High fuel cell performance is also crucial for minimizing heat generation due to limited space available for radiators, while the usage of Pt as a catalyst cannot be unconditionally increased owing to the limited amount of Pt resources. Thus, achieving a high level of compatibility between the performance and durability of the catalyst material, which often entails a trade-off,3 is essential.

Employing mesoporous carbon as a cathode catalyst support is a promising approach for enhancing the performance of PEFCs.4−6 In the cathode catalyst layer of a PEFC with a mesoporous carbon support, the Pt-based catalyst nanoparticles deposited inside the mesopores are not directly covered by the ionomer. In addition, protons can access the Pt nanoparticles inside the mesopores even without ionomers if the distance between the Pt nanoparticles and ionomer is not too large.7 This configuration reduces catalyst poisoning by the sulfonic acid groups of the ionomer8,9 and results in higher catalytic activity.10 Moreover, the high oxygen transport resistance through the Pt/ionomer interfaces11 can be alleviated with mesoporous carbon supports. Yarlagadda et al. demonstrated that excellent oxygen reduction reaction (ORR) activities and gas transport properties can be achieved without compromise by employing mesoporous carbon supports with a large volume of appropriate pore sizes (4–7 nm).4 Because of these benefits, mesoporous carbon supports have been commercially applied in LDVs.12

However, during long-term operation, the carbon support at the cathode, where carbon oxidation reaction is thermodynamically feasible, can corrode.13 Although operational condition adjustments can mitigate this degradation, enhancing the intrinsic durability of the material is desired to simplify the control system and extend the cell lifetime. As alternative support materials with high stability under oxidative and acidic conditions, conductive oxides, such as nonstoichiometric titanium dioxide (TiO2),14,15 Nb-doped TiO2,16−20 and M-doped tin oxides (SnO2, M = Nb, Sb, Ta, etc.),21−36 have been studied. Among these materials, M-doped SnO2 is particularly noteworthy for its adjustable electrical conductivity via dopant element and concentration variations.34

SnO2-based supports with various structures for PEFC cathodes have been reported. Kakinuma et al. synthesized Nb-doped SnO2 supports with a fused-aggregate network structure, enhancing cell performance through effective gas diffusion and electronic conducting pathways.22−25 Ozouf et al. evaluated the ORR activity and durability under oxidative conditions of Pt catalysts supported on Sb-doped SnO2 aerogels with specific surface areas of 80–90 m2 g–1 in a liquid electrolyte and found comparable activity and superior durability for the Pt/Sb-SnO2 aerogel catalyst to those of a Pt catalyst supported on high-surface-area carbon.26,27 He et al. deposited highly dispersed Pt nanoparticles on a Sb-doped SnO2 aerogel via the so-called Pt-anchoring method and obtained enhanced ORR activity owing to the strong metal support interaction (SMSI) effect.28 Similarly, Jiménez-Morales et al. reported improved ORR activity and stability of Pt catalysts supported on electrospun Ta-doped SnO2 nanofibers, attributed to the SMSI effect.29

Several SnO2 supports with mesoporous structures have also been reported.30,31,33 Zhang et al. conducted rotating disk electrode (RDE) and membrane electrode assembly (MEA) tests on mesoporous SnO2 supports with a pore size of 13.4 nm, showing comparable activity and better durability than Pt catalysts supported on high-surface-area carbon.30 Jalalpoor et al. prepared mesoporous Sb-doped SnO2 supports with a pore size range of 3–4 nm using a mesoporous silica template, investigating the impact of doped Sb on the ORR activity in a liquid electrolyte.31 Nonetheless, in these studies, the role of mesopores in SnO2 supports in cell performance has not been clarified. Furthermore, the effect of the pore size on SnO2-based supports has not yet been investigated.

In this study, we synthesized mesoporous SnO2-based supports with well-controlled pore sizes ranging from 4 to 11 nm using a carbon template. We systematically investigated the impact of the pore size on cell performance to validate the effectiveness of mesoporous structures in SnO2-based supports.

2. Results and Discussion

2.1. Synthesis and Characterization of the Connected Mesoporous M-Doped Tin Oxide (CMMTO) Support

The CMMTO support particles were synthesized using a mesoporous carbon template (Figure 1a), a method developed by Tatsuda et al.37 for synthesizing undoped mesoporous tin oxide. To verify the successful application of this template method for doped tin oxides, we meticulously analyzed the products at each step of the synthetic process. Small-angle X-ray scattering (SAXS) patterns of the carbon template and Sb-SnO2/carbon composites with different reaction times are shown in Figure 1b. The profile of the carbon template had a shoulder at q ∼1 nm–1, corresponding to a pore diameter of 2.8 nm. In addition, the profile exhibited a well-defined primary peak (q*) at q ∼2 nm–1, indicating an ordered mesopore structure of the carbon template with a hexagonally packed cylindrical morphology.38 With progressing reactions, the intensity of the shoulder at q ∼1 nm–1 increased, attributable to increased electron density in the pores from SnO2 phase formation. The SAXS profile changed up to 5 h, and no significant change was observed between 5 and 6 h, suggesting the reaction’s completion at 5 h. The small peak at q ∼2 nm–1 initially disappeared after 1 h, reemerging at 3 h, indicating initial random SnO2 phase filling in the pores, followed by hexagonally packed SnO2 phase formation.

Figure 1.

Figure 1

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(a) Synthesis scheme of connected mesoporous M-doped SnO2 (CMMTO). (b) SAXS patterns of the mesoporous carbon template (0 h) and the Sb-SnO2/carbon composites with a reaction time of 1–6 h. The inset shows a structural model of the SnO2/carbon composite. (c, d) SEM images of the CMSbTO calcined at 300 °C. (e) TEM image of the CMSbTO calcined at 300 °C.

After the calcination of the Sb-SnO2/carbon composite in air, the carbon template was completely eliminated. The scanning electron microscopy (SEM) images in Figure 1c,d show the resultant mesoporous Sb-doped tin oxide (CMSbTO) particles, revealing an aggregated structure of approximately 100 nm Sb-SnO2 particles. The high-resolution transmission electron microscopy (TEM) image (Figure 1e) demonstrates lattice fringes throughout the CMSbTO phase, indicating high crystallinity even at a low calcination temperature of 300 °C. Furthermore, the SAXS patterns after the calcination, as presented in Figure S5, do not exhibit a peak at q ∼2 nm–1, indicating the absence of an ordered structure in the CMSbTO after the template removal.

X-Ray diffraction (XRD) patterns of the Sb-SnO2/carbon composite and CMSbTO particles are shown in Figure 2a. Broad peaks at 2θ of 27°, 34°, and 53° in the XRD pattern of the Sb-SnO2/carbon composite correspond to the diffractions by the 110, 101, and 211 planes of the SnO2 crystal, respectively. These peaks became larger and sharper as the calcination temperature increased. CMSbTO particle crystallite sizes, estimated using the Sherrer equation, are listed in Table 1. All of the diffraction peaks observed for the CMSbTO particles were attributed solely to SnO2 (rutile-type structure), with no evidence of other oxides such as Sb2O3 and Sb2O5.

Figure 2.

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(a) XRD patterns of the Sb-SnO2/carbon composite and the CMSbTOs with a calcination temperature of 300–600 °C. (b) Pore size distributions of the CMSbTOs obtained using the BJH method.

Table 1. Summary of the Physical Properties of the CMSbTOs Calcined at Various Temperatures.

calcination temperature [°C] crystallite size (XRD) [nm] pore diameter (SAXS) [nm] pore diameter (BJH) [nm] surface area (BET) [m2 g–1] pore volume [cc g–1] conductivity [S cm–1]
300 2.5 4.0 3.5 213 0.22 1.8 × 10–3
400 2.8 5.1 4.4 179 0.23 7.2 × 10–3
500 3.8 7.9 6.6 107 0.20 2.2 × 10–2
600 4.8 9.3 10.9 93 0.28 7.1 × 10–3
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The mesopore size in CMSbTO particles was controlled by adjusting the calcination temperature. Nitrogen adsorption measurements were performed on the CMSbTOs, and their pore size distributions (Figure 2b) were obtained by analyzing the nitrogen adsorption isotherms (Figure S1a) using the Barrett–Joyner–Halenda (BJH) method. As summarized in Table 1, the mean pore diameter of the CMSbTOs was increased from 3.5 to 10.9 nm with increasing calcination temperature from 300 to 600 °C. The size of the mesopores was also analyzed by SAXS (Figure S5), and the obtained mean pore diameters for the CMSbTOs were in good agreement with those obtained from the nitrogen adsorption measurements (Table 1). It is important to note here that the pore size range of the CMSbTOs covered the range where the fuel cell performance is enhanced for carbon supports (4–7 nm).4 The surface areas determined using the Brunauer–Emmett–Teller (BET) method and the total pore volumes are summarized in Table 1. The specific surface area was 213 m2 g–1 at the calcination temperature of 300 °C, and it decreased as the calcination temperature increased. Nonetheless, the specific surface area of CMSbTO was much higher than that of the solid-core Sb-SnO2 support prepared in this study (solid-core Sb-doped tin oxide (SSbTO), 17 m2 g–1, Table S1) even when CMSbTO was calcined at 600 °C (93 m2 g–1).

High electrical conductivity is an indispensable feature for catalyst support material candidates. The apparent electrical conductivities of the CMSbTO particles were in the range of 1.8 × 10–3–2.2 × 10–2 S cm–1, as shown in Table 1, aligning with the literature value for Sb-doped SnO2 supports.31,39 Because the Sb atom doped in SnO2 acts as a donor by substituting the Sn atom in the crystal lattice, a correlation is expected between the concentration of the substituted Sb atoms and the conductivity of CMSbTO. To investigate this, we conducted X-ray absorption fine structure (XAFS) measurements on CMSbTOs using transmission mode. Additionally, we performed waveform separation of the X-ray absorption near edge structure (XANES) spectra based on first-principles simulation results, where the spectrum of Sb substituted at the Sn position was consistent with that of Sb2O5, whereas the spectrum of Sb located between the crystal lattice of SnO2 was consistent with that of Sb2O3.40 The ratio of Sb substituted at the Sn position was determined from the results of waveform separation, and the concentration of the substituted Sb was calculated using the total Sb concentration of CMSbTO determined by inductively coupled plasma optical emission spectrometry (ICP-OES) analysis (Table S3). As shown in Figure 3, a volcano-shaped trend was observed between the concentration of substituted Sb and the conductivity of CMSbTO. The conductivity reached its maximum at approximately 6 at. % of substituted Sb. The observed decrease in conductivity at higher substituted Sb concentrations can be ascribed to the decrease in carrier mobility.41

Figure 3.

Figure 3

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Electrical conductivity of CMSbTO as a function of the atomic concentration of substituted Sb determined from the XAFS and ICP analyses.

Nb, Ta, or W-doped CMMTOs have also been synthesized. All these variants exhibited a rutile SnO2 structure (Figure S2), and no significant structural differences were noted among them under identical calcination conditions (Table S2). However, their electrical conductivities were 3 orders of magnitude lower than that of CMSbTO (Table S2). Electrochemical impedance spectroscopy measurements of the powder compacts26 indicated that the difference in the electrical conductivity was not only due to the difference in the bulk conductivity but also due to the remarkably high resistance at the particle interfaces (Figure S3). The bulk conductivity is expected to be improved by optimizing the carrier density, as conducted for CMSbTO (Figure 3). Furthermore, a previous study has demonstrated that increasing the Pt loading on the SnO2-based support can effectively reduce their interfacial resistance,24 a method that might be similarly beneficial for CMMTOs.

2.2. Preparation of the Pt/CMSbTO Electrocatalyst

In the present study, we focused on evaluating the performance of the Sb-doped mesoporous SnO2 materials, CMSbTO, as a cathode catalyst support in PEFCs. Owing to its superior conductivity among the CMMTO supports, CMSbTO was selected to investigate the efficacy of the mesoporous structure in SnO2-based supports. We utilized CMSbTO supports with mean pore diameters of 4.4, 5.7, 7.3, and 11 nm and employed the colloidal method for Pt deposition,42 with the as-synthesized Pt nanoparticles averaging around 2 nm in diameter.

Figure 4a presents an SEM image (reflected electron image) of the Pt/CMSbTO catalyst with a mean pore diameter of 4.4 nm, showcasing highly dispersed Pt nanoparticles (visible as white dots) on the outer surface of the CMSbTO support.

Figure 4.

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(a) SEM image (reflected electron image) of the Pt/CMSbTO catalyst (20 wt %). (b) Schematic of a thin section of the Pt/CMSbTO catalyst fabricated for the STEM-EDS analysis. (c) STEM image of the thin section of the Pt/CMSbTO catalyst fabricated using FIB. (d–g) EDS mappings for (d) Sn, (e) O, (f) Sb, and (g) Pt of the catalyst’s thin section.

To investigate whether Pt nanoparticles were also deposited inside the mesopores of CMSbTO, a thin section (∼50 nm thick) of the Pt/CMSbTO catalyst was fabricated using the focused ion beam (FIB) method (Figure 4b) and analyzed via scanning transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy (STEM-EDS). A STEM image of a thin section of the Pt/CMSbTO catalyst is shown in Figure 4c, while the corresponding EDS mappings for Sn, O, Sb, and Pt within the same field of view are shown in Figure 4d–g. These mappings show that Pt was distributed throughout the cross-section of the Pt/CMSbTO catalyst. Further, EDS line and spot analyses of the thin section were conducted to quantitatively assess the Pt distribution. In the line analysis, constant intensities of Sn and Pt were observed throughout the catalyst particle’s cross-section (Figure S7a,b), suggesting a uniform thickness of the thin section and even distribution of Pt. Additionally, the spot analyses revealed approximately equal ratios of Sn and Pt at various positions within the thin section (Figure S7c,d), confirming that Pt nanoparticles were deposited not only on the outer surface of the CMSbTO support but also within its mesopores. Moreover, STEM-EDS analysis demonstrated a uniform distribution of Sb within the CMSbTO particles (Figure S7b).

2.3. Electrochemical Measurements for Evaluating Initial Performances

Initially, the electrochemical properties of the Pt/CMSbTO catalysts were characterized using the thin-film-rotating disk electrode (TF-RDE) method in a 0.1 M HClO4 solution at 25 °C. The cyclic voltammograms (CVs) of the Pt/CMSbTOs are comparatively presented alongside those of Pt/SSbTO and Pt/Vulcan in Figure 5a. The CVs for Pt/CMSbTOs and Pt/SSbTO displayed typical “butterfly-shapes” consisting of hydrogen adsorption and desorption peaks (below 0.35 VRHE) and Pt redox peaks (above 0.55 VRHE), similar to the CV of Pt/Vulcan. Additionally, the CVs of Pt/CMSbTOs and Pt/SSbTO exhibited characteristic redox peaks approximately 0.6–0.7 VRHE, attributed to the modification of the Pt surface by Sn adatoms.43,44

Figure 5.

Figure 5

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Results of the TF-RDE measurements. (a) Cyclic voltammograms of the Pt/CMSbTOs, Pt/SSbTO, and Pt/Vulcan catalysts (measured in Ar purged 0.1 M HClO4 solution, a scan rate of 0.05 V s–1, and a Pt loading of 10 μgPt cm–2geo). (b) Comparison of the ORR mass activity (MA) at 0.9 V obtained from the linear sweep voltammograms (measured in O2 purged 0.1 M HClO4 solution, a scan rate of 0.02 V s–1, a rotation rate of 1600 rpm, background and IR compensated, and I/S = 0.13).

The ORR mass activities (MA) at 0.9 VRHE of Pt/CMSbTOs, Pt/SSbTO, and Pt/Vulcan, determined by TF-RDE measurements, are shown in Figure 5b. The MA values for Pt/CMSbTO catalysts showed no dependency on the pore size in the liquid electrolyte and were approximately 70% of that for the Pt/Vulcan catalyst. Since MA is a product of the electrochemical surface area (ECSA) and ORR specific activity (SA; ORR activity per unit surface area of Pt), it can be decomposed into these two parameters. This breakdown indicated that the ECSA and SA for the Pt/CMSbTO catalysts were approximately 80 and 90%, respectively, of those for the Pt/Vulcan catalyst (Figure S8). This suggests that the comparatively lower MA values of the Pt/CMSbTO catalysts were primarily due to their lower ECSA. In fact, the ECSA values for the Pt/CMSbTO catalysts were lower than expected from the size of the Pt nanoparticles before deposition. Given that the Pt particle size for Pt/CMSbTO catalysts (2 nm) was smaller than that for Pt/Vulcan (∼3 nm), a higher ECSA was anticipated for the former. Two potential reasons for this observation include (i) agglomeration of Pt nanoparticles and (ii) encapsulation of Pt nanoparticles by the SnO2 phase, associated with the SMSI effect.45,46 The Pt/SSbTO catalyst exhibited the lowest MA value, which was attributed to its low ECSA stemming from the agglomeration of Pt nanoparticles due to the low surface area of the SSbTO support.

Subsequently, single-cell tests using the MEAs with Pt/CMSbTO catalysts in the cathode were carried out. The I–V curves for Pt/CMSbTO under both dry (30% relative humidity (RH)) and highly humidified (80% RH) conditions are shown in Figure 6a,b, respectively. For comparison, these figures also display the I–V curves for Pt/SSbTO (solid-core SnO2 support), Pt/Vulcan (solid-core carbon support), and Pt/CNovel (mesoporous carbon support). To avoid complexity, only the I–V curves for Pt/CMSbTO with a pore size of 5.7 nm are shown in the figures, while the I–V curves for Pt/CMSbTO catalysts with other pore sizes are provided in Figure S9. Under dry conditions, Pt/CMSbTO demonstrated superior I–V performance in both the low- and high-current-density regions compared with the other catalysts.

Figure 6.

Figure 6

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Results of the MEA tests. (a) I–V curves for the Pt/CMSbTO (5.7 nm), Pt/SSbTO, Pt/Vulcan, and Pt/CNovel catalysts measured at 30% RH, 82 °C (IR not corrected). (b) I–V curves measured at 80% RH, 60 °C (IR not corrected). (c) Comparison of the ORR mass activity (MA) at 0.84 V (IR corrected). (d) Comparison of the oxygen transport resistance Rother.

The I–V performance in the low-current-density region is mainly governed by the ORR activity of the catalyst. The ORR mass activities (MA) determined from the current density at 0.84 V (IR-corrected voltage) are shown in Figure 6c. The MA for Pt/CMSbTO (5.7 nm), whose I–V curve is shown in Figure 6, was 509 A g–1Pt under dry conditions, which was 4.7 and 1.8 times than those of Pt/SSbTO (109 A g–1Pt) and Pt/Vulcan (290 A g–1Pt), respectively. In addition, unlike the results from the RDE measurements, the MA of the Pt/CMSbTOs showed a pore size dependence. The MA for the Pt/CMSbTO under dry conditions is maximized at the mean pore size of 7.3 nm (754 A g–1Pt: 2.6 times that for the Pt/Vulcan).

These findings can be explained by the mitigation of catalyst poisoning by sulfonic acid groups of the ionomer, which have been demonstrated to deteriorate the ORR activity on the Pt surface by over 80% in the worst case.8,47 As discussed above, a substantial portion of the Pt nanoparticles was deposited inside the mesopores, and these Pt nanoparticles were sheltered from the ionomer coating. However, when the pore size is too large, the amount of ionomer penetrating the mesopores is expected to increase and the mitigation effect against ionomer-induced catalyst poisoning by the mesopores is weakened. Conversely, when the pore size is too small, the mesopores can be flooded by water produced through the ORR. Thus, Pt/CMSbTO has an optimum mean pore size of 7.3 nm, reflecting its mitigating effect against catalyst poisoning. The ORR activity of Pt/CMSbTO was independent of the pore size in the RDE test because of the low ionomer content in the catalyst layer employed in the RDE test (i.e., ionomer-to-support weight ratio (I/S) = 0.13 in the RDE test vs I/S = 0.26 in the MEA test). Consequently, the Pt/CMSbTO catalysts with appropriate pore sizes (5.7 and 7.3 nm) exhibited significantly higher ORR mass activity than the Pt/Vulcan catalyst in the MEA test under dry conditions despite their lower intrinsic mass activities shown in the RDE test.

The mitigation of ionomer adsorption on the Pt catalysts also contributed to the better I–V performance of Pt/CMSbTO in the high-current-density region under dry conditions. Figure 6d shows the pressure-independent component, Rother, of the total oxygen transport resistance, Rtotal, which was determined from diffusion-limited current measurements using diluted oxygen.48Rother arises from Knudsen diffusion through the pores in the micro porous layer (MPL) and the catalyst layer, and from diffusion through the ionomer.49 In particular, both molecular dynamics simulations and experiments with a microelectrode have shown that a dense layer of ionomer is formed at the interface between the ionomer and Pt surface11 and that the oxygen transport resistance through the Pt/ionomer interface is much higher than that of the bulk ionomer.50Rother for Pt/CMSbTO (5.7 nm) under dry conditions is 18.7 s m–1, which is much lower than those for Pt/CSSbTO (68.3 s m–1) and Pt/Vulcan (59.7 s m–1). The low Rother value for Pt/CMSbTO can be attributed to the mitigation of the formation of the Pt/ionomer interface, owing to the mesoporous structure of the CMSbTO support. However, no clear difference in the catalyst layer resistance, RH+, was observed among Pt/CMSbTO, Pt/SSbTO, and Pt/Vulcan (Figure S10c). Therefore, the primary factor contributing to the superior I–V performance of Pt/CMSbTO in the high-current-density region under dry conditions is the lower oxygen transport resistance.

In contrast, the Pt/CNovel catalyst, which employs a mesoporous carbon support, exhibited the lowest I–V performance under dry conditions (Figure 6a). This significant difference in performance compared to that of the CMSbTO support can be attributed to the inherently hydrophobic nature of the carbon support surface. The significantly reduced Pt utilization rate (Figure S10a) suggest that the hydrophobic nature of the carbon support hinders the efficient supply of protons to the Pt particles deposited inside the mesopores.51 In the case of CMSbTO support, the hydrophilic surface of the SnO2 pore walls may retain a partial water film that facilitates proton conductivity even under low-RH conditions.

Conversely, under highly humidified conditions (80% RH), Pt/CNovel exhibited the best I–V performance, as shown in Figure 6b. Its MA surpassed that of Pt/Vulcan (Figure 6c), and it exhibited a lower value of Rother than Pt/Vulcan (Figure 6d). This can be attributed to the effect of the mesoporous structure of the support, which effectively mitigates the formation of the Pt/ionomer interface, as previously discussed. By contrast, the I–V performance of Pt/CMSbTO (5.7 nm) under highly humidified conditions was found to be comparable to those of Pt/SSbTO and Pt/Vulcan (Figure 6b). The MA values for Pt/CMSbTOs were lower than those for Pt/Vulcan, although a pore size dependence was still observed (Figure 6c). Moreover, Rother values for Pt/CMSbTOs were almost equivalent to those for Pt/SSbTO and Pt/Vulcan. These observations under highly humidified conditions can be explained by the flooding inside the mesopores of the CMSbTO support caused by the water generated through the ORR. Given the hydrophilic nature of the SnO2 surface, it is plausible that flooding in the mesopores is likely to occur with the SnO2-based support. Additionally, the mitigation of ionomer-induced catalyst poisoning may be less significant under highly humidified conditions, as the sulfonate anions’ adsorptivity on the Pt surface decreases along with the hydration of the ionomer,52 i.e., the poisonings were not as severe under wet conditions as under dry conditions. Thus, the expected mitigation of the catalyst poisoning was canceled out by the flooding.

In summary of the MEA performances, promising properties of Pt/CMSbTO were demonstrated; however, the following points should be addressed in future studies. First, flooding should be avoided under wet conditions. This issue can be solved by reducing the hydrophilicity of the support material or by optimizing the amount of ionomer in the catalyst layer. Second, the utilization of Pt catalysts should be increased. The ECSA values under the highly humidified condition for Pt/CMSbTOs determined from the CO stripping voltammetry were approximately 60% of that for the Pt/Vulcan (Figure S10a), which was smaller than the values expected from the results of the RDE measurements (ca. 80%). Furthermore, under dry conditions, the ECSA values for Pt/CMSbTOs were 50% of those for the Pt/Vulcan. These results indicated that there is room for improvement of the Pt utilization rate of Pt/CMSbTOs. It may be difficult for protons to reach the Pt nanoparticles located deep inside the mesopores of the CMSbTO support, and therefore, the distance between the Pt nanoparticles and ionomer should be controlled for these catalysts. Third, a method for evaluating the ionomer and anion coverage on the Pt catalyst, which are key properties determining cell performance, should be developed for SnO2-based supports. In the present study, the CO displacement53 technique was applied to the Pt/CMSbTOs and Pt/SSbTO catalysts. However, as shown in Figure S10d, no clear correlation was observed between the ORR activity and anion coverage, in contrast to previous studies using carbon-supported Pt catalysts.10,48 The cause of this behavior is yet to be clarified and may be related to the nature of the Pt catalysts supported on SnO2-based materials, such as the properties for CO oxidation and adsorption; therefore, it is necessary to find appropriate conditions for the CO displacement for these catalysts.

2.4. Accelerated Stability Tests of MEAs

The durability of Pt/CMSbTO, Pt/Vulcan, and Pt/CNovel catalysts under high-potential conditions was tested by the accelerated stability test (AST) of potential cycles between 1.0 and 1.5 VRHE under inert gas conditions (Figure 7a, start–stop AST). Figure 7b,c shows the normalized ECSA (normalized by its maximum value) during the start–stop AST and the I–V curves before and after 2000 potential cycles, respectively. For the catalysts employing carbon supports (Pt/Vulcan and Pt/CNovel), there was a significant reduction in ECSA up to 49%, alongside notable deterioration in the I–V performances following the 2000 potential cycles. These performance losses can be attributed to the degradation of the carbon supports via a carbon oxidation reaction, leading to the detachment and coalescence of the Pt nanoparticles. The decrease in ECSA was less pronounced for Pt/CNovel compared to Pt/Vulcan, probably due to the heat treatment (2100 °C) applied for the CNovel support before the Pt deposition to graphitize the carbon.

Figure 7.

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(a) Potential profile for the start–stop AST. (b) Normalized ECSA (normalized by its maximum value) as a function of the number of potential cycles for the start–stop AST. (c) I–V curves (IR not corrected) before and after the start–stop AST (2000 cycles) measured at 80% RH, 60 °C. (d) Potential profile for the load cycle AST. (e) Normalized ECSA as a function of the number of potential cycles for the load cycle AST. (f) I–V curves (IR not corrected) before and after the load cycle AST (10,000 cycles) measured at 80% RH, 60 °C.

In contrast, the ECSA for Pt/CMSbTO did not decrease over 2000 potential cycles; instead, it exhibited a slight increase. A potential explanation for the observed slight increase in ECSA could be an enhancement in the Pt utilization rate due to a change in the ionomer distribution or wettability of the SnO2 support surface induced by the potential cycling. Moreover, the I–V performance of Pt/CMSbTO essentially did not deteriorate through the start–stop AST. These results affirm the exceptional durability of the CMSbTO support against high potentials, aligning with the stability demonstrated by other SnO2-based supports in the literature.21,22

In addition to the durability tests at high potentials, accelerated stress tests simulating load cycles (load cycle ASTs) were conducted for the three catalysts by applying square-wave potential cycles between 0.6 and 1.0 VRHE, as illustrated in Figure 7d. As shown in Figure 7e, ECSA retention for catalysts employing mesoporous supports (Pt/CMSbTO and Pt/CNovel) was superior to that for Pt/Vulcan throughout the load cycle AST. In a previous study, Padgett et al. reported that ECSA retention after a durability test (30,000 potential cycles between 0.6 and 0.95 V) enhanced with increasing porosity of the carbon support.54 They attributed this improvement to the protective effect of porous carbon supports against Pt particle coalescence. Our results suggest that the Pt catalysts deposited inside the mesopores of the CMSbTO support are protected by a similar effect. However, the degradation in the I–V performance following the load cycle AST was more pronounced for Pt/CMSbTO than for Pt/CNovel, as shown in Figure 7f. This deterioration could be attributed to catalyst poisoning, either by Sb ions31 leached from the CMSbTO support, or by a thin oxide layer formed on the Pt nanoparticles as a consequence of the SMSI effect.55 The first of these concerns might be mitigated by changing the dopant to more stable elements.

2.5. Durability Evaluations against Low Potentials

Finally, the stability of the CMSbTO supports at low potentials was investigated. The cathode potential in fuel cells can decrease to approximately 0 V vs reversible hydrogen electrode (RHE) due to hydrogen crossover, depending on the operating and startup/shutdown modes. It has been reported that Sb-doped SnO2 has stability issue under low potentials.31,56 For example, Jalalpoor et al. observed dissolutions of Sb below 0.3 VRHE in their in situ ICP experiment.31 Dissolved Sb ions in the catalyst layer of PEFC can cause catalyst poisoning and decrease the proton conductivity of the ionomer via ion exchange. The use of other doping elements may be effective in avoiding this problem. Nonetheless, there is another concern regarding the inherent stability of SnO2 itself at low potentials and low pHs13.

To assess the stability of SnO2 at low potentials, a dissolution test was performed on undoped CMTO and Pt/CMTO powders in 1 M HClO4 at 60 °C saturated with H2. Figure 8 shows the cumulative amounts of dissolved Sn from CMTO and Pt/CMTO, determined by inductively coupled plasma mass spectroscopy (ICP-MS), as a function of the duration of the dissolution test. After 3 h, the amount of dissolved Sn from Pt/CMTO was 1.6 mg g–1SnO2, which is 30 times of that from CMTO (0.05 mg g–1SnO2). This result indicates that the dissolution of SnO2 is mediated by supported Pt nanoparticles. The dissolution rate of Sn from Pt/CMTO decreased after 6 h, and the cumulative amount of dissolved Sn from Pt/CMTO after 12 h was 3.0 mg g–1SnO2. This is equivalent to an exchange ratio with the sulfonic acid groups in the Nafion membrane of 0.4% (see the Supporting Information for details of the estimation). Although this exchange ratio is not large, Sn cations can migrate to the cathode and significantly deteriorate the cell performance;57 therefore, it is necessary to develop countermeasures against Sn dissolutions under low-potential conditions. Kakinuma et al. reported that the dissolution of Sn from solid-core SnO2 supports synthesized by the flame combustion method in a 0.1 M H2SO4 aqueous solution at 80 °C with H2 bubbling can be significantly reduced by changing the dopant from Sb to Nb or Ta.22,58 Clarifying the mechanisms of improved stability can provide the strategy to improve the stability of the mesoporous SnO2 under low-potential conditions.

Figure 8.

Figure 8

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Cumulative amount of dissolved Sn from the CMTO and Pt/CMTO as a function of duration of the dissolution test duration in 1 M HClO4 at 60 °C with H2 bubbling.

3. Conclusions

Connected mesoporous Sb-doped SnO2 (CMSbTO) particles, with controlled mesopore sizes in the range of 4–11 nm, were synthesized using a mesoporous carbon template. Using CMSbTOs as the cathode catalyst support, the impact of the mesopores of the SnO2 support on the performance of PEFC was systematically investigated.

Under dry conditions, Pt/CMSbTO exhibited superior I–V performance compared with Pt/SSbTO (solid-core Sb-doped SnO2 support), Pt/Vulcan (solid-core carbon support), and Pt/CNovel (mesoporous carbon support). The ORR mass activity in the MEA for Pt/CMSbTO showed a pore size dependence, and it was maximized at the mean pore size of 7.3 nm. The highest activity for Pt/CMSbTO was 2.6 times of that for Pt/Vulcan, which suggests the inhibition of catalyst poisoning by the sulfonic acid group of the ionomer. In addition, the oxygen transport resistance Rother for Pt/CMSbTO was less than half that observed for Pt/SSbTO and Pt/Vulcan, and this result indicates that the formation of the Pt/ionomer interface was mitigated. However, under highly humidified conditions, the I–V performance of Pt/CMSbTO was comparable to that of Pt/SSbTO and Pt/Vulcan, while Pt/CNovel exhibited the best performance. The ORR mass activity of Pt/CMSbTO was even lower than that of Pt/Vulcan, probably due to floodings inside the mesopores.

Pt/CMSbTO demonstrated a significantly higher durability against high potentials than Pt/Vulcan and Pt/CNovel, which is attributed to the inherent oxidative resistance of the SnO2-based supports. Additionally, the ECSA retention for Pt/CMSbTO throughout the load cycle AST was superior to that for Pt/Vulcan, possibly due to the protective effect of mesopores against Pt particle coalescence. However, the challenge of mitigating the catalyst poisoning by Sb ions leached from the CMSbTO support still remains. In addition, ex situ dissolution tests under low-pH (pH = 0) and low-potential (∼0 VRHE) conditions revealed an increased dissolution of Sn in the presence of Pt deposited on the SnO2 support.

The results of this study show that employing a mesoporous SnO2-based support as a cathode catalyst support for PEFCs can mitigate the formation of Pt/ionomer interfaces and achieve both high initial performance and durability against high potentials. However, challenges remain regarding cell performances under highly humidified conditions and stability at low potentials. Consequently, mesoporous SnO2-based supports are a promising option for the component in HDVs, even though further material development or optimization of the operating conditions is needed.

4. Experimental Section

4.1. Chemicals, Materials, and Gases

The following chemicals from FUJIFILM Wako Pure Chemical Corporation were used in the synthesis of M-doped SnO2 supports and the deposition of Pt catalysts: tin(II) chloride (SnCl2, 99.9%), antimony(III) chloride (SbCl3, 99.9%), niobium(V) chloride (NbCl5, 95.0+%), tantalum(V) chloride (TaCl5, 90.0+%), tungsten(VI) chloride (WCl6), hydrochloric acid (HCl, 35%), ethylene glycol (EG, 99.5%), sodium hydroxide (NaOH, 97.0%), hexachloroplatinic(IV) acid hexahydrate (H2PtCl6·6H2O, 98.5%), and 60% nitric acid (HNO3). Deionized (DI) water (resistivity >18.2 MΩ·cm at 25 °C, total organic carbon (TOC) <5 ppb) from the Milli-Q system (Millipore) was used for the catalyst synthesis, acid dilutions, catalyst ink formulations, and electrochemical cell cleaning. 2-Propanol (IPA, 99.7+%), ethanol (EtOH, 99.5+%), propylene glycol (PG, 99.0+%), and Nafion dispersion (DE2020 CS, 20 wt %) were purchased from FUJIFILM Wako Pure Chemical Corporation and used for catalyst ink formulation. Perchloric acid (60%) (HClO4; Ultrapur, Kanto Chemical Co. Inc.) was used for the electrolyte preparation. The following gases from the Sogo Kariya Sanso Corporation were used for electrochemical measurements: Ar (99.999%), O2 (99.999%), N2 (99.99%), Air, CO (99.95%), and H2 (99.99999%).

4.2. Synthesis of the CMMTO Support

Connected mesoporous M-doped (M = Sb, Nb, Ta, and W) tin oxides (CMMTOs) were synthesized by modifying the synthesis method for mesoporous tin oxides reported by Tatsuda et al.37 The details of the synthesis method for the CMMTOs in the present study are described in the Supporting Information, and as an example, the method for the Sb-doped one (CMSbTO) is briefly described below. Initially, 0.12 g of SbCl3 was dissolved in 4 mL of aqueous HCl solution (35%). Subsequently, 36 mL of DI water, 5.0 g of SnCl2, and 0.1 g of mesoporous carbon template were added to the solution. After stirring the dispersion at 25 °C for 2 h, 200 mL of DI water was added, and the dispersion was further stirred for 4 h. Filtration and redispersion in DI water were then repeated twice. The residue was dried at 80 °C in a vacuum oven for 2 h to obtain a Sb-SnO2/carbon composite. The carbon template was removed by calcinating the composite at 300 °C in air for 24 h to obtain a bluish CMSbTO powder. The CMSbTO powder was further calcined at 400–600 °C in air for 3 h, as required. For comparison, solid-core Sb-doped tin oxide (SSbTO) was also synthesized as described in the Supporting Information.

4.3. Preparation of Pt/CMSbTO Electrocatalysts

The Sb-doped SnO2 supports, which exhibited high electric conductivities, were used for electrochemical measurements. Pt nanoparticles were deposited on the Sb-doped SnO2 supports via the colloidal method,59,60 which consisted of two main steps: preparation of a colloidal suspension of Pt nanoparticles via an alkaline EG route and subsequent loading of the nanoparticles onto the SnO2 support. The colloidal suspension of Pt nanoparticles (ca. 2 nm in diameter) was prepared by mixing 4 mL of solution of 0.4 M NaOH in EG with 4 mL of solution of 40 mM H2PtCl6·6H2O in EG in a microwave reaction vessel and subsequently heating the mixture for 3 min at 160 °C with a microwave reactor (Monowave 400, Anton Paar) while stirring at 600 rpm. Pt nanoparticles were loaded onto CMSbTO as follows: 91.5 mg of CMSbTO powder was added to a colloidal suspension of Pt nanoparticles, and the suspension was stirred at room temperature overnight. Then, 0.25 mL of 1 M HNO3 was added to the suspension and stirred for 1 h. This process was repeated four times. The suspension was then filtered, washed with DI water, and filtered again. The residue was dried at 80 °C in a vacuum oven for 2 h to obtain Pt/CMSbTO electrocatalyst powder with a Pt loading of 20 wt %. A Pt/SSbTO electrocatalyst was also prepared using the colloidal method, in which the Pt loading was controlled to be 12.5 wt %.

4.4. Characterization of the CMMTO Support and Pt/CMSbTO Electrocatalysts

The crystalline structures of the CMMTOs were analyzed by measuring the XRD patterns (Ultima IV, Rigaku, Cu Kα radiation, 1.6 kW). The nitrogen adsorption isotherms (Autosorb, Anton Paar) of the CMMTOs were measured, and the surface areas and pore size distributions were obtained using the BET method and BJH method, respectively. SAXS and XAFS measurements were conducted to examine the mesoscopic structure and electronic state of CMSbTOs, respectively (see the Supporting Information for details). The morphologies of the CMMTOs and Pt/CMSbTOs were observed using SEM (S-5500, Hitachi High-Tech, acceleration voltage of 2 kV) and TEM (JEM-2100F, JEOL). TEM-EDS analysis was performed on thin sections of Pt/CMSbTO particles fabricated by using the FIB method. The concentrations of the doped elements in the CMMTOs and the Pt loadings of the Pt/CMSbTOs were determined by using ICP-OES (PS3520UVDD II, Hitachi High-Tech). The electrical conductivities of the CMMTOs were measured using a compression cell61 and potentiostat (SP-300, Biologic) at a compression pressure of 2.4 MPa.

4.5. Electrochemical Measurements

The intrinsic activities of 20 wt % Pt/CMSbTOs, 12.5 wt % Pt/SSbTO, and a commercial 30 wt % Pt/Vulcan (TKK, TEC10 V30E) were evaluated in a 0.1 M HClO4 aqueous electrolyte solution at 25 °C via the TF-RDE technique.62 To eliminate the influence of mass transport occurring in the catalyst thin film on the glassy carbon disk electrodes (5 mm in diameter) on the measured catalytic activity, uniform thin films were fabricated using a previously reported spraying method.63 A potentiostat (Metrohm, Autolab PGSTAT 30) and a glass cell with a three-electrode configuration were employed for electrochemical measurements. A Pt mesh and RHE were used as the counter and reference electrodes, respectively. After solution resistance compensation and background correction, the ORR kinetic current density at 0.9 V was determined using the Koutecký-Levich equation.

The MEA tests were conducted by using a single cell with an electrode area of 1 cm2. The catalyst ink was prepared by mixing the catalyst powder with a Nafion dispersion and small amount of PG (50 wt % EtOH aqueous solution). The ionomer-to-support weight ratio (I/S) for the 20 wt % Pt/CMSbTO and 12.5 wt % Pt/SSbTO catalysts was 0.26 (equivalent to the ionomer-to-carbon ratio (I/C) of 1.0 when it is converted to the volume ratio), while the I/C ratio for the commercial 30 wt % Pt/Vulcan catalyst and 30 wt % Pt/CNovel catalyst6 (preparation detailed in the Supporting Information) was 1.0. A catalyst layer was fabricated by spreading the catalyst ink onto a poly(tetrafluoroethylene) (PTFE) sheet using a blade coater. The obtained catalyst layer (Pt loading of ca. 0.1 mgPt cm–2) was used as the cathode, and a Pt/C catalyst layer (Pt loading of 0.05 mgPt cm–2, I/C = 1.0) was used as the anode; both layers were decaled to a Nafion membrane (Dupont, NR211) by hot pressing to fabricate the MEA. Following a brake-in, I–V performance measurements, cyclic voltammetry, and electrochemical impedance spectroscopy64 analyses were performed at a cell temperature of 60 °C with 80% relative humidity (RH, wet condition) at both electrodes, and at 82 °C with 30% RH (dry condition). CO stripping voltammetry65 and diffusion-limited current measurements using diluted oxygen48 were also conducted to measure the ECSA of Pt and oxygen transport resistance, respectively. Accelerated stress tests simulating load cycles (load cycle AST, square-wave potential cycles between 0.6 V for 3 s and 1.0 V for 3 s) and start–stop cycles (start–stop AST, 2000 potential cycles between 1.0 and 1.5 V) were carried out at 60 °C with 80% RH under inert gas conditions. The comprehensive protocols for each of these measurements are described in the Supporting Information.

4.6. Ex Situ Dissolution Test of CMTO and Pt/CMTO

To evaluate the stability of the SnO2-based support materials under low potentials, undoped CMTO and Pt/CMTO were dispersed in H2-saturated 1 M HClO4 solution at 60 °C and the amount of dissolved Sn ions was measured via ICP-MS analysis (Agilent 8900, Agilent Technologies).

Acknowledgments

The SAXS experiments were conducted at the BL8S3 of Aichi Synchrotron Radiation Center, Aichi Science & Technology Foundation, Aichi, Japan (Proposal No. 202103088). The XAFS and HAXPES measurements were performed at the BL16B2 and BL16XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (Proposal Nos. 2022B5070, 2022B5371, 2023A5070, and 2023A5371). The standard spectra of Sb were utilized from SPring-8 BENTEN database. We acknowledge Dr. Yuji Kamitaka for his help with the preparation of the Pt/CNovel catalyst.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.4c01794.

  • Synthesis procedures, nitrogen adsorption–desorption isotherms, XRD patterns, SAXS profiles, XAFS spectra, STEM-EDS line and spot analyses, linear sweep voltammograms, Tafel plots, cyclic voltammograms, and I–V curves (PDF)

Author Contributions

M.I.: conceptualization, data curation, formal analysis, investigation, methodology, validation, visualization, and writing—original draft. R.M.: data curation, formal analysis, investigation, and writing—original draft. T.T.: conceptualization, investigation, and methodology. K.Y.: conceptualization and methodology. S.K.: data curation, formal analysis, and investigation. N.T.: data curation, formal analysis, and investigation. N.I.: formal analysis and investigation. K.O.: data curation, formal analysis, and investigation. W.Y.: data curation, formal analysis, investigation, methodology, and writing—original draft. K.T.: project administration and validation. T.N. project administration and validation. K.K.: conceptualization, project administration, supervision, validation, and writing—original draft. All authors: writing—review and editing.

The authors declare no competing financial interest.

Supplementary Material

am4c01794_si_001.pdf (2.1MB, pdf)

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