Inhibition of polymeric densification at platinum/ionomer interface via enlarging tetrafluoroethylene spacing in perfluorinated sulfonic-acid ionomer

Inhibition of polymeric densification at platinum/ionomer interface via enlarging tetrafluoroethylene spacing in perfluorinated sulfonic-acid ionomer | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Inhibition of polymeric densification at platinum/ionomer interface via enlarging tetrafluoroethylene spacing in perfluorinated sulfonic-acid ionomer Chi-Young Jung, Wonyoung Choi, Hyunguk Choi, Youngje Park, Seowon Choi, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5891522/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Polymer electrolyte fuel cells hold great potential for powering heavy-duty vehicles (HDVs) run by clean hydrogen, but a major challenge lies in the ionomer poisoning of scarce platinum (Pt) catalysts, which hinders the Pt utilization and cell efficiency. Here, we report a simple yet effective approach to mitigate polymeric densification at the Pt/ionomer interface, via enlarging tetrafluoroethylene (TFE) spacing between neighboring side chains. Ionomers with weaker confinement to Pt, arising from strengthened hydrophobic interactions, suppress the specific adsorption and lead to less-densified ionomer morphology. Despite having a lower ion-exchange capacity, they exhibited high accessibilities (over 80%) and a significant reduction of 22–8% in sulfonate coverage, hence resulting in two-fold improvements in activity and local transport towards the oxygen reduction reaction. This strategy offers a key solution to unlock the full potential of Pt, offering seamless integration into current manufacturing processes, thus accelerating the sustainability and scalability of fuel cell technology. Physical sciences/Energy science and technology/Fuel cells Physical sciences/Chemistry/Electrochemistry/Fuel cells Polymer Electrolyte Fuel Cell Perfluorinated Sulfonic Acid Ionomer Platinum Catalyst Tetrafluoroethylene Spacing Platinum/Ionomer Interface Polymeric Densification Ionomer Accessibility Sulfonate Coverage Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Polymer electrolyte fuel cells (PEFCs) fed by clean hydrogen (H 2 ) coupled with atmospheric oxygen (O 2 ) are one of the most attractive alternatives to conventional internal combustion engines, driving a shift towards complete electrification in the global transportation sector [ 1 ]. PEFCs offer decisive advantages over lithium-ion batteries, such as faster fueling ( 500 miles), which are particularly beneficial for heavy-duty vehicle (HDV) applications [ 2 ]. However, the spread of this technology is hindered by the high cost and supply scarcity of platinum (Pt) or platinum group metal (PGM). Although state-of-the-art fuel cell electric vehicles (FCEVs) have succeeded in reducing PGM usage from 36 to 19.2 g, a substantial reduction of PGM materials by over four times is still necessary to achieve sustainability [ 3 , 4 ]. This has led to extensive research and development of oxygen (O 2 ) reduction reaction (ORR) catalysts with excellent activity per Pt mass; however, all developed materials have failed to realize their full potential, as demonstrated in the rotating disk electrode (RDE) in a half-cell setup when implemented in a real-scale membrane electrode assembly (MEA) at the single-cell level [ 5 ]. Furthermore, the local transport resistance of O 2 through the perfluoro sulfonic acid (PFSA) ionomer film covering Pt has become unexpectedly larger after the use of highly active materials with reduced Pt deployment [ 6 ], which poses the performance discrepancy between RDE and MEA as one foremost technical hurdle remaining for broader commercialization. One primary origin of this notorious phenomenon, also referred to as the RDE vs. MEA challenge [ 7 ], is believed to be the highly densified polymeric matrixes at Pt/ionomer interface in the ORR electrode. To the best of our knowledge, recent evidence states that the highly concentrated formulation of polymeric ionomer may lead to stronger adsorption of the sulfonate terminal group (-SO 3 H) onto Pt, specifically occurring at onset potentials around 0.4 to 0.5 V versus reference hydrogen electrode (RHE), thus intensifying ionomer poisoning [ 8 , 9 ] as well as interfacial O 2 transport resistance [ 10 – 12 ]. To address this issue, several efforts have been made to eliminate the direct contact between Pt and ionomer [ 13 – 17 ]. To obviate the direct contact, one has proposed the mesoporous carbons as accessible and porous supporting materials [ 13 ] while others have devised the Pt-protective layer by forming a porous polymeric film or porous graphitic carbon layer covering Pt [ 14 , 15 ], as well as by depositing ionic liquids exhibiting considerable proton conductivity onto Pt [ 16 , 17 ]. This improves ORR activity at the MEA scale by securing larger numbers of active sites from the specific adsorption of sulfonate groups. However, this strategy inevitably suffers from significantly lower ionomer accessibility, particularly at lower relative humidity (RH), which drastically reduces the electrochemically available surface area (ECSA) by greater extent without the assistance of water. To enhance ionomer accessibility, studies have reported the promotion of ionomer homogeneity deposited onto the conventional Pt catalysts supported by carbon black (Pt/C) by controlling Coulombic interaction through the introduction of electro-negative functionalities onto the surfaces of carbon supports near Pt [ 18 – 21 ]. One effective outcome demonstrated that nitrogen dopants and their neighboring carbons are beneficial in loosening the ionomer structure at the Pt/ionomer interface, by attracting the sulfonate groups in ionomer against Pt [ 18 ]. Recently, Pt supported by Vulcan (Pt/V) has been reported to be more favorable in exploiting the Pt/ionomer interface than supported by Ketjen black (Pt/KB), due to less micro-porous structures with better accessibility [ 20 ]. However, there remains a technical challenge regarding that adsorption strength of sulfonate group onto carbon support can be notably weakened particularly at the O 2 -containing defect sites, which results in its recombination onto Pt surfaces [ 22 , 23 ]. Consequently, an insufficient number of nitrogen atoms spatially adjacent to the Pt catalyst may also disturb the effectiveness of this supportive interaction. In this article, we present a novel approach to mitigate undesired polymeric densification at the Pt/ionomer interface, by strengthening the hydrophobic interaction between the ionomer and Pt/V within the electrode slurry, which may significantly improve the performance of the ORR electrode. Our primary strategy involved altering the ionomer backbone chemistry by increasing the tetrafluoroethylene (TFE) spacing between neighboring side chains, as illustrated in Scheme 1 . The proposed method promotes molecular attraction between the TFE backbone and Vulcan carbon in a water-rich environment, which in turn, improves the adsorption affinity of the ionomer onto carbon over Pt during the slurry preparation, deposition, and annealing processes. Loosened polymeric morphology at the Pt/ionomer interface can be attributed to the as-fabricated electrode microstructure, thereby providing two beneficial features: (i) alleviating the potential-dependent specific adsorption of sulfonate groups onto Pt, and (ii) decreasing the local transport resistance of O 2 at the Pt/ionomer interface. Based on these improvements, it was confirmed that the electrode ionomers with enlarged TFE spacing exhibited not only an excellent coverage of the ionomer but also an incredibly lower coverage of specifically adsorbed sulfonate groups onto Pt, as evaluated by dry proton accessibility (DPA) and CO displacement measurements, respectively. Consequently, these features allow us to attain the boosted reaction kinetics and mass transport properties in the ORR electrode, thereby pushing forward the overall MEA performance to the next step. Here reported findings demonstrate that enlarging the TFE spacing in ionomer is a key route to address the challenges derived from ionomer densification, thereby unlocking the full catalytic potential of Pt and door to the sustainable commercialization of FCEVs. Mitigation of polymeric densification at Pt/ionomer interface via enlarging TFE spacing PEFC electrodes are typically manufactured using slurry-processing techniques, which involve both deposition and curing processes. The dispersion behavior in the slurry significantly governs the Pt/ionomer interface of the as-fabricated electrode [ 24 ]. Typically, the electrode slurry comprises a blend of the PFSA ionomer and Pt/V or Pt/KB catalyst in a water-alcohol mixture used as dispersing solvent, leading to complex interactions among the slurry components. As illustrated in Fig. 1 a, the ionomer’s amphiphilic nature, stemming from the hydrophobic TFE backbone and hydrophilic sulfonate-terminated perfluoro ether side chains, gives rise to three key interactions among the dispersing particles: (i) hydrophobic interactions between the TFE backbone and carbon support (Fig. 1 b), which results in ionomers being adsorbed onto the outer surface of the carbon support [ 25 ], (ii) electrostatic attractions between the sulfonate side chains and Pt particles, locating the sulfonate group towards the Pt (Fig. 1 c) [ 26 ], and (iii) ionic interactions between the negatively charged side chains of neighboring ionomers, which are influenced by the concentration of the ionomer (Fig. 1 d) [ 27 , 28 ]. Among these complex interactions, ionomers can either be adsorbed onto the surfaces of Pt/C particles (i.e., adsorbed ionomers) or remain unattached to Pt/C and dispersed in the slurry (i.e., free ionomers) [ 29 ]. To reduce the specific adsorption of sulfonate groups on the Pt surface, mitigation of the polymeric chains onto Pt surface can be a key solution which is origin form strengthen the hydrophobic interactions between the TFE backbone and carbon support. This can be comparable to or even stronger than that between the sulfonic acid side chain and the Pt catalyst [ 22 , 30 ]. Pt surface is also covered with a native oxide layer in the liquid slurry, which may repel the sulfonic acid side chain under such non-polarized conditions [ 31 ]. Moreover, the carbon support, showcasing a larger exterior surface area than Pt ( Supplementary Fig. 1 ), suggests that carbon/ionomer interactions predominantly dictate solution-level interactions compared to Pt/ionomer interactions. Thus, we hypothesize that enlarging the TFE spacing can be a key solution to reduce specific adsorption of sulfonate group onto Pt surface via strengthening the hydrophobic attraction between TFE backbone and carbon support. To confirm our strategy, we prepared three PFSA ionomers with identical side chain compositions but different TFE spacings, where the molecular distances between the neighboring side chains were modified as illustrated in Fig. 1 e. Short side chain (SSC) ionomers are selected over long side chain (LSC) ionomers, as they are well known to successively suppress the specific adsorption sulfonate group. Here, the SSC ionomers were modified to exhibit short (S-TFE spacing), intermediate (I-TFE spacing), and long TFE-spacing (L-TFE spacing), which can be described by the number of repeating units (m) in the backbone (Fig. 1 f ) . The m values of the S-TFE, I-TFE and L-TFE spacing ionomers were 4.4, 5.5, and 7.0, respectively (Fig. 1 g). The gel permeation chromatography (GPC) results are provided in Fig. 1 h, ensuring that all the samples exhibited similar degrees of polymerization with the number of the repeating units (n) ranging from 4.9 to 5.1. The increase in TFE spacing may lead to an increase in the molecular fraction of the TFE backbone over sulfonic acid side chain, thus resulting in an enlarged domain of the hydrophobic region (Fig. 1 i). Therefore, by adjusting the TFE spacing, the ionomer’s adsorption preference can be selectively controlled either onto Pt or carbon support, which is primarily attributed to the promotion of hydrophobic attraction in practical water-rich slurry environments. Dependence of TFE Spacings on Ionomer’s Adsorption Affinity The TFE spacings may serve as a decisive factor in determining the adsorption preference of the PFSA ionomer, either onto the Pt or carbon support, thereby governing the ionomer morphology at the Pt/ionomer interface. Particularly in a water-rich environment, ionomer with larger TFE spacing may develop stronger adsorption onto the carbon support than Pt, due to the increased hydrophobic interactions [ 32 ]. Figure 2 presents the rheological measurements of the electrode slurries employing the S-TFE, I-TFE, and L-TFE spacing ionomers to explore how the varying adsorption behaviors affect the slurry rheology. As shown in Fig. 2 a, the viscosity values were taken from the steady-shear characteristics ( Supplementary Fig. 2) at a low shear rate of 0.01 sec − 1 , with varying ionomer contents, to confirm the dispersion behaviors in the slurry. U-shaped profiles were developed as a function of ionomer content. In the initial part of the low ionomer contents (0-0.25 mmol SO3− g C −1 ), the slurry viscosity dramatically decreased by two orders of magnitude after a small amount of ionomer was applied to the Pt/C dispersion without ionomer. The ionomer adsorbed onto the Pt/C prevents the severe aggregation by promoting electrostatic repulsion [ 33 ]. It is also noteworthy that the slurry employing L-TFE spacing ionomer exhibited a notably reduced viscosity, from 2.78 × 10 5 cP down to 7.77 × 10 4 at 0.25 mmol SO3− g C –1 , when compared with that employing S-TFE ionomer. At the intermediate ionomer contents (0.25–0.75 mmol SO3− g C –1 ), plateaus in slurry viscosity were observed with an increase in ionomer content for all three samples, indicating that the additional amounts contributed to the formation of free ionomers. Subsequently, at the high ionomer contents (0.75–1.25 mmol SO3− g C –1 ), a rapid increase in viscosity was recorded as a result of the excessively higher concentration in the slurry, thereby enhancing the ionic strength between the sulfonate groups and dispersing solvent molecules [ 30 , 33 ]. The reduced viscosities for all ionomer contents indicate that the larger TFE spacing ionomers were successfully adsorbed onto Pt/V with an increased surface coverage, due to the enhanced interaction between the TFE backbone and carbon support, as presented in Fig. 1 b. The competitive formation of adsorbed and non-adsorbed ionomers can also be elucidated using three-interval thixotropy test (3ITT), as shown in Fig. 2 b. We sheared the three slurries at a low shear rate of 0.1 s − 1 for 60 s (rest phase), then at a high shear rate of 100 s − 1 for 10 s (shear phase), and lastly at a low rate of 0.1 s − 1 for 60 s (recovery phase), during which the time-dependent viscosity variations were monitored in the recovery phase to analyze the thixotropic behaviors. Interestingly, the slurry employing L-TFE spacing ionomer exhibited the largest viscosity upshoot of approximately 10.9 × 10 3 cP, within 1 s after the shear was removed. This rapid viscosity recovery can be attributed to the elastic rebound of ionomers that are primarily adsorbed onto Pt/C with higher surface coverage ratios [ 34 ]. Later, at a high ionomer content of 1.25 mmol SO3 g C −1 , the recovery time was significantly reduced from over 30 s to less than 5 s, due to less entanglement of the polymeric chain resulting from deficient free ionomers ( Supplementary Fig. 3 ). This was further supported by a dynamic light scattering (DLS) analysis, where the slurries of interest were diluted to 0.1 wt% to ensure transparency (Fig. 2 c). It is clearly demonstrated that the larger TFE spacing ionomers are beneficial in suppressing the formation of larger agglomerates in diameter of > 1 µm, that is pronouncedly originated from the reduced concentration of free ionomers dispersed in the solvent. The amplitude sweep measurements were conducted to show how the adsorption affinity of the ionomer affects the internal structure of the electrode slurry (Fig. 2 d–f). Three different ionomer concentrations of 0.125, 0.75, and 1.25 mmol SO3− g C −1 were tested for the TFE spacing of each ionomer. Amplitude sweep measurements provide information on slurry particle interaction by analyzing the storage (G') and loss modulus (G''). In the low strain region, we observe a linear viscoelastic (LVE) region, denoted by G' outstripping G'', signifying elastic particle interactions. As the strain increased, the internal structure of the slurry weakened, leading to a diminution of both G' and G'', culminating in strain-induced softening. Consequently, the slurry transitions towards liquid-like behavior as G'' overtakes G'. The LVE region, which indicates the internal strength of the slurry, expanded with increasing ionomer content and decreasing ionomer TFE spacing. When examining the transition to a solution-like state with a consistent ionomer content, it was observed that this transformation occurred at higher rates of deformation as the TFE spacing of the ionomer decreased. This observation can be attributed to the presence of free ionomers in the slurry, which induce attractive interactions between the particles, resulting in a gel-like behavior of the slurry [ 35 ]. The tube inversion test provided a visual insight into the variations in the internal structure of the slurry due to the different TFE spacings and ionomer contents (Fig. 2 d–f). Therefore, the result suggests that the concentration-controlled slurry with enlarged TFE spacing not only promotes the orientation of the adsorbed ionomer towards carbon rather than Pt but also suppresses the excessive presence of free ionomer remaining in the solvent. Local microstructure and spatial distribution of ionomer in the electrode Typically, the PFSA ionomers are either adsorbed onto Pt/C or remain non-adsorbed in the dispersing solvent, depending on the ionomer’s chemical structure and the solvent composition [ 22 , 36 ]. This behavior ultimately determines the formation of adsorbed and deposited ionomers in the as-fabricated electrode [ 37 ]. Hence, the microstructural properties, including electrode surface morphology and ionomer distribution, are significantly influenced by the adsorption behavior of the ionomer in electrode slurry. For visual verification, scanning electron microscopy (SEM) was performed to observe the electrode microstructures using ionomers with varying TFE spacing. The molality of the ionomer was fixed at an optimum value of 0.75 mmol SO3− g C −1 , as discussed in Supplementary Figs. 4 and 5 . As shown in Fig. 3 a, large white particles, with diameters ranging from 1 to 2 µm are observed, presumed to be bulky ionomer aggregates when the S-TFE ionomer was applied. In contrast, the size and number of aggregate particles notably decreased on the electrode surfaces after the use of the I-TFE and L-TFE spacing ionomers (Fig. 3 b and 3 c). Furthermore, the formation of surface cracks was significantly reduced as the TFE spacing increased, as shown in Supplementary Fig. 6 , which may result from the well-dispersed electrode slurry with fewer agglomerated structures [ 39 ]. Comparison of the secondary electron (SE) and backscattered electron (BSE) images at an identical location can be a more precise way of analyzing the spatial ionomer distribution. Unlike SE mode, BSE signals originate from deeper regions of the electrode, making them advantageous for distinguishing the bulky ionomer aggregates deposited onto the nano-sized adsorbed ionomer layer on Pt/C [ 40 ]. In electrodes employing the S-TFE ionomer, a large number of white dots corresponding to Pt nanoparticles are missing in the SE image (Fig. 3 d). However, they are clearly visible in the corresponding region (marked with blue lines) in the BSE image (Fig. 3 e). This discrepancy is primarily due to the deposition of larger ionomer aggregates, with sizes of several tens of nanometers, on the Pt surface [ 40 ], which may have interrupted the Pt signals in the SE mode. In contrast, after using larger TFE-spacing ionomers, the Pt nanoparticles in the SE image correspond well with those in the BSE image, after the use of larger TFE spacing ionomers (Fig. 3 f–i). This suggests that, with an increase of TFE spacing, the considerably thin electrode ionomers are uniformly distributed onto the electrode, which highlights the potential existence of a well-established carbon support/ionomer interface. The formation of intact interfaces between carbon support and ionomer was further investigated by scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDS), as shown in Fig. 4 a–c. By examining the elemental map of fluorine (F, green) signals overlaid on either the STEM image or the Pt map, the local correspondence between carbon support and ionomer can be quantified, as presented Fig. 4 d–f. A strong overlap between F and Pt was frequently observed in the S-TFE electrode, with the highest local correspondence ratio of 80% (Fig. 4 a, d ) . Additionally, multiple regions with higher ionomer concentration were detected when a shorter TFE spacing was applied, as excessive amounts of free ionomers in the slurry turned into bulky ionomer aggregates deposited onto the electrode. The ionomer aggregates dominantly increase their size and number density at higher ionomer contents, as displayed in Supplementary Fig. 7 . This result is primarily attributed by the highly densified TFE backbones in ionomer near Pt, due to the nano-confinement effect [ 41 ]. Consequently, the specific adsorption of sulfonate groups onto the Pt catalyst can be exacerbated under typical fuel cell operation in the rated cell. As the TFE spacing becomes larger, the overlap of F and Pt signals significantly weakened, and simultaneously, the F signals displayed a stronger locational correspondence within the carbon support (Fig. 4 b, c ) . Figure 4 f shows that the local correspondence ratio was dramatically reduced to 51% in the L-TFE electrode, which is well consistent with the F and Pt mapping results. Based on these findings, it was demonstrated that the larger TFE-spacing ionomers coordinate more strongly onto the surfaces of the carbon support, which is beneficial for separating sulfonate groups from the Pt surfaces. This creates a weakly confined Pt/ionomer interface with a less densified polymeric structure. Electrochemical characterization Finally, electrochemical characterizations were conducted in a single cell with the cathode Pt loadings of 0.2 ± 0.01 mg Pt cm − 2 , to meet the US DOE 2030 target for PGM loadings required for HDV applications [ 42 ]. Figure 5 a–c presents the H₂-air polarization curves with high-frequency resistances (HFRs) for the S-TFE, I-TFE, and L-TFE MEAs with varying ionomer contents. The maximum current densities for S-TFE, I-TFE, and L-TFE MEAs at ionomer contents ranging from 0.25 to 1.25 mmol SO3− g C −1 were recorded as 1.48, 1.60, and 1.84 A cm − 2 at a cell voltage of 0.6 V, respectively, when the ionomer content was fixed at 0.75 mmol SO3− g C −1 . Interestingly, the L-TFE MEA with the lowest ionomer content, exhibited a notably low HFR of 44 mΩ cm 2 , significantly lower than that of the S-TFE MEA (65 mΩ cm 2 ), which is attributed to better connectivity of ionomer pathways in the electrode. Combined with both SEM and transmission electron microscopy (TEM) analyses (Figs. 3 and 4 ), it can be concluded that a larger TFE spacing is effective for achieving electrode ionomers with higher coverage and uniform distribution onto Pt/C. Moreover, the cell performance of L-TFE MEA was 24% higher than that of S-TFE MEA, along with a 15% increase in peak power density as shown in Fig. 5 d. We also performed electrochemical impedance spectroscopy (EIS) obtained at an output current density of 2.0 A cm –2 to analyze the origin of overall cell resistance as displayed in Figs. 5 e. Nyquist plots were fitted using equivalent circuit model ( Supplementary Figs. 8a ) to obtain HFR, charge transport resistance (CTR), and mass transport resistance (MTR), which are listed in the bar graphs (Figs. 5 f ) . The L-TFE MEA showed a considerable reduction in CTR by 44.9% when compared with S-TFE MEA, where the increased ionomer coverage onto Pt/C is responsible for an enhanced ORR process [ 43 ]. Furthermore, the MTR of L-TFE MEA was dramatically reduced by over 60% (from 284 to 111 mΩ cm 2 ), which is mainly due to the improved local O₂ transport over bulk transport, considering that the electrode porosity and pore size distribution were found to be quite similar ( Supplementary Table 2 ). The main cause of this phenomenon is presumed to be the notably loosened polymeric-chain morphology at the Pt/ionomer interface, which enables better nanophase segregation behavior in the weak confinement regime [ 26 ]. To understand the effect of TFE spacing on the ionomer distribution and coverage, the ECSA was calculated via CO stripping measurements under RH of 20% and 100%, in obtain the DPA (ECSA RH20 /ECSA RH100 ). This approach allows the measurement of the proximity of the ionomer sulfonate group, which directly contacts the Pt particles [ 18 ]. Since proton accessibility reaches approximately 100% at very high ionomer content, comparing proton accessibility at low ionomer contents is more suitable for studying the effects of TFE spacing on the ionomer coverage. As shown in Fig. 6 a, the dry proton accessibility for S-TFE, I-TFE, and L-TFE MEAs at 0.5 mmol SO3− g C −1 was estimated to be 63%, 71%, and 81%, respectively. The high proton accessibility for L-TFE MEA indicates that a large amount of proton-reaching areas were formed on the Pt surface, considering that the same sulfonate molarity was applied to the cathode. In contrast, the shorter TFE MEAs showed lower ionomer coverage on the Pt surface due to the presence of bulky ionomer agglomerates, in line with the aforementioned electrode microstructures. Furthermore, at 0.75 mmol SO3− g C −1 , the proton resistance of L-TFE MEA was 83%, which is higher than that of S-TFE MEA (76%). These results imply that increasing the TFE spacing facilitates the adsorption of ionomers along the carbon support rather than on Pt, mitigating the presence of ionomers on Pt and promoting a more homogeneous ionomer distribution onto Pt/C. The catalytic activity for performance-optimized MEAs at 0.75 mmol SO3− g C −1 was measured from the H₂-O₂ polarization curve. In Fig. 6 b, the mass activities of the S-TFE, I-TFE, and L-TFE MEAs were 66, 88, and 121 Ag Pt −1 , respectively, indicating that L-TFE MEA was twice larger than that of the S-TFE MEA. The result of Tafel slope for L-TFE MEA was 70.1 mV dec − 1 , corresponding to the theoretical value based on the transfer coefficient unity at 80°C ( Supplementary Fig. 9) . However, the similar ECSA values under RH 100% for S-TFE, I-TFE, and L-TFE MEAs were estimated to be 59.4, 60.1, and 62.5 m² g Pt ⁻¹, respectively. This indicates that a proton reachable Pt surface was formed across all MEAs. To quantify the sulfonate group adsorption on Pt active site, the CO-displacement was investigated, as shown in Fig. 6 c. In the test, the adsorbed sulfonate group was displayed by linear CO adsorption, and the hydroxyl groups covering the Pt surface were negligible at 0.4 V RHE [ 44 ]. The L-TFE MEA showed a remarkable reduction in sulfonate group coverage of 8.2% than that of S-TFE MEA (22.3%). Figure 6 c also shows the H₂-N₂ curve measured at RH 50% to obtain the proton resistance, which can be estimated from 1/3 of the total resistance. The equivalent circuit was given in Supplementary Fig. 8b . The proton resistance of the L-TFE MEA (15.3 mΩ cm²) was 3.5-folds lower than that of the S-TFE MEA (52 mΩ cm²), despite the less effective chemical structure of L-TFE ionomer [ 11 ]. These results demonstrate that the enlarging TFE spacing allows to mitigate the polymeric densification at Pt surface while simultaneously maintaining the ORR active site, thus releasing the intrinsic ORR activity at the MEA level. To measure the local O₂ transport resistance ( \(\:{\text{R}}^{\text{l}\text{o}\text{c}\text{a}\text{l}})\) in the ionomer film onto the Pt surface, limiting current measurements were conducted, as displayed in Fig. 6 e. The \(\:{\text{R}}^{\text{local}}\:\) was obtained by distinguishing the pressure-dependent MTR (R PD ) and the pressure-independent MTR (R PI ) based on the limiting current measurements. This clearly shows that the R PD remained constant, as similar porosity results were observed for the electrodes ( Supplementary Fig. 4b and Supplementary Table 2 ). However, the R PI for S-TFE, I-TFE, and L-TFE MEAs were 0.272, 0.157, and 0.109 s cm − 1 , respectively. The R PI for L-TFE MEA was 2.5 folds lower than that for the S-TFE MEA. This dramatic reduction in the R PI of the L-TFE MEA can be attributed to the loosening of ionomer from the Pt surface [ 45 ]. Thus, the result strongly supports that enlarging TFE spacing is beneficial for loosening the polymeric densification at Pt, promoting O₂ permeability to most Pt particles in the MEA. Conclusion Here, we demonstrate a dedensification of the polymeric chain at the Pt/ionomer interface which enables the realization of intrinsic ORR performance in the MEA via strengthening the interaction between TFE backbone in ionomer and carbon support in Pt/C. Enlarging the TFE spacing has been proven beneficial for tuning the binding affinity of the ionomer towards carbon over Pt by hydrophobic attraction under water-rich slurry. These benefits enable the ionomer to remain adsorbed on the carbon support while simultaneously reducing polymer densification on the Pt surface. Rheological analysis of the electrode slurry showed that enlarging TFE spacing significantly reduced the viscosity and led to pronounced viscoelastic behavior, resulting from the ionomer forming excellent coverage along the carbon and enhancing electrostatic repulsion on the surface of the dispersing particles. Therefore, mitigation of polymeric densification at Pt/ionomer interface was successfully achieved in the as-fabricated electrode microstructures. As a result, sulfonate group coverage on the Pt surface was significantly reduced to only 8% in the L-TFE MEA compared to 22% in the S-TFE MEA in the CO displacement measurements. Moreover, the L-TFE MEA exhibited a 2-fold increase in ORR activity and a 2.5-fold reduction in local O 2 transport resistance when compared with the S-TFE MEA, thereby presenting remarkable improvement of the cell performance. This approach offers a key solution to unlock the catalytic potential at the MEA level by loosening the polymeric densification at the Pt/ionomer interface. This technology can be readily applied to the typical mass production of FCEVs, facilitating fuel cell market penetration and addressing the decarbonization challenges of the transportation sector. Experimental Procedures Materials SSC ionomers with different TFE spacing (D72-25BS, D83-24B, and D98-25BS, Solvay), 1-propanol (nPA) (HPLC grade, Sigma Aldrich), ultrapure deionized (DI) water (Milli-Q, resistivity of 18.4 MΩ), and 40 wt% Pt loaded Pt/C catalyst (TEC10V40E, Tanaka) were used to prepare the electrode slurry. All ionomers were used as electrode binders and electrolytes (25 wt% of ionomers over total solids). Vulcan XC72 was chosen as the catalyst support over high surface area carbon (HSC, e.g. Ketjen Black EC300J or EC600). Vulcan-type support offers advantages at high proton accessibility. A 130 µm thick polytetrafluoroethylene (PTFE)-coated glassy fiber sheet (AF055, Alphaflon) was used in the blade coating process. The Nafion membrane (NR-211, Chemours) was used as the polymer electrolyte membrane. 300 µm thick polytetrafluoroethylene film (PTFE, Alphaflon) and 50 µm thick polyimide (Kapton, Alphaflon) film were used as decal substrates in the MEA preparation. Lastly, the purity of the gases is given as O 2 (> 99.999% purity), N 2 (> 99.999% purity), H 2 (> 99.999% purity), CO (1%, N 2 balance), and O 2 mixture (1.5%, 3%, 7%, and 10%, N 2 balance) were used for single-cell test. Electrode slurry preparation : To ensure sufficient wettability between the ionomer and the solution, the ionomer dispersion was dispersed in a mixture of DI water and nPA for 24 hours (a final DI water-to-nPA weight ratio of 1:1). Then, the electrode slurry was prepared by blending the Pt/C catalyst and ionomer dispersion by using a high-energy ball mill (Pulverisette 7, Fritsch) at 100 rpm for 24 h; the ionomer concentration was set within a range of sulfonic acid molality from 0 to 1.25 mmol SO3− g C −1 and these were quantified as sulfonic acid molarity relative to the carbon support weight (mmol SO3− g C −1 ); Sulfonic acid molarity corresponding to ionomer-to-carbon support ratio (I/C) was presented in Supplementary Table 3 . Moreover, all samples were prepared with a consistent carbon weight to solvent volume ratio (C g S ml −1 ), of 0.1 C g S ml −1 . MEA fabrication and cell assembly The slurry was coated onto a PTFE-coated glass fiber sheet using the blade coating method applying shear rate of 100 s − 1 . The coated decals were annealed at 80°C for 2 h in a nitrogen atmosphere. The Pt loadings of the electrodes were fixed at 0.2 ± 0.01 mg Pt cm −2 for both the cathode and anode, which were confirmed using a portable X-ray fluorescence (XRF) analyzer (Niton XL5, Thermo Scientific). After that, the MEA for the single-cell test was prepared by utilizing the decal transfer method. The 25 µm-thick Nafion 211 membrane was assembled between the cathode and anode coated decal. Subsequently, the assembly was hot-pressed at 110°C and 65 bar for 10 min, followed by rapid cooling 5 min. For the single-cell test, an air-cooled single-cell with an active area of 25 cm 2 (K-cell, CNL) and a graphite gas flow field with 9 channels ( Supplementary Fig. S10a–b ) were utilized. A gas diffusion layer (GDL) (Sigracet 22 BB, SGL), with a total thickness of 215 µm and a 5 wt% PTFE resin content in the microporous layer (MPL) was employed. In the single-cell assembly, the compression rate of the GDL was set to approximately 25% using a PTFE stopper [ 46 ], and the clamping pressure was set to 7 Nm. A 25 cm² MEA was utilized for electrochemical evaluation, while a 1 cm² MEA was used for limiting current measurements ( Supplementary Fig. S10c–d ). Ionomer characterization To confirm the degrees of polymerization of each ionomer, number-average molecular weight ( M n) was measured by using gel permeation chromatography (GPC) (HPC-8420, Tosoh) and Toyo soda kogyo (TSK) gel α-M column. N-methyl-2-pyrrolidinone (NMP) containing 50 mmol L − 1 was used as an eluent at a flow rate of 0.5 mL min − 1 at 50°C. Rheological measurements The rheological measurements were conducted using a stress-controlled rheometer (Hybrid Rheometer, DHR3 TA Instruments) equipped with a 40 mm diameter stainless steel parallel-plate geometry set at a gap distance of 500 µm and a temperature of 25°C. To prevent solvent evaporation, a solvent saturation trap was used during all measurements. Prior to the rheological measurements, the slurry was preconditioned to eliminate any sample loading history by shearing shear at 500 s − 1 for 1 min followed by resting for 1 min [ 47 ]. The steady-shear measurements were conducted by imposing a stress sweep in logarithmic steps ranging from 0.001 to 1,000 s − 1 . To evaluate the degree of particle interaction and agglomeration, the viscosity values were summarized, and the representative viscosity value was obtained in the low shear rate range of 0.01 s − 1 , where the slurry exhibits Brownian motion. Amplitude sweep measurements were performed by increasing stress from 0.0001 to 200 Pa with a fixed predetermined frequency of 0.5 Hz. The LVE region was obtained in the amplitude sweep measurements. The 3ITT was performed in three stages. First, the slurry was sheared at 0.1 s − 1 for 60 s to form a stationary state. The shear rate was then increased to 100 s − 1 for 10 s to confirm the cluster deformation behavior. Finally, the slurry entered a reconstruction stage by shearring at 10 − 1 for 60 s. MEA characterizations High-resolution field emission SEM (Verios 5 UC, Thermo Fisher Scientific) was utilized in both SE and BSE modes to study the microstructure of the agglomerated particles on the electrode surface and to measure the thickness of the electrode cross-section. The acceleration voltage was set to 10 kV. A 3D laser-scanning confocal microscope (VK-X1000, Keyence) was used to determine the number and size of the crack in the electrode surface. The crack calculation method was described in Supplementary Fig. 11 . TEM analysis (Tecnai F20 G2, FEI), combined with EDS, provides insights into the distribution and morphology of the ionomers on the electrode by identifying elements C, Pt, S, and F. For TEM specimen preparation, the 100 nm-thick cross-sections of the electrodes ( Supplementary Fig. 12b) , were prepared through cryo-focused ion beam (FIB) (Helios NanoLab 600, FEI) milling using a Ga + ion source at a 52° angle, operating at an energy of 30 kV and a current of 93 pA ( Supplementary Fig. 12a) . Prior to the FIB milling, a thin tungsten film was deposited onto the surface of the sample at a controlled temperature under liquid nitrogen to minimize damage to the electrode samples [ 48 ]. Mercury intrusion porosimeter (ASAP 2020, Micromeritics) was used to assess the porosity and pore size distribution of the electrodes by applying pressures ranging from 0.1 to 60,000 psi with a 1,000 mmHg transducer. The electrode porosity was calculated by subtracting the membrane thickness from the overall MEA thickness. Electrochemical measurements Electrochemical measurements were conducted using a 100 W PEFC testing station (G20, Greenlight Innovation). Active area of the tested MEAs were adjusted to 25 cm². Prior to electrochemical evaluation, the cells were activated at 80°C with a potential range of 0.4 to 0.8 V at a step rate of 0.1 V. The polarization curve was conducted at 80°C and 100% RH, with a back pressure of 150 kPa, while maintaining the reaction gas flow of H 2 /air (SR 1.5 / 2.0). After calibrating the H 2 crossover current, the mass activity was calculated at 0.9 V from the iR-corrected H 2 -O 2 polarization curve [ 49 ]. To measure ionomer accessibility, the DPA was calculated using the ECSA obtained from CO stripping at 100% and 20% RH (DPA = ESA 20 /ESA 100 ). To quantify the sulfonic acid groups adsorbed on the catalyst surface, CO displacement measurement was conducted. The displacement coverage (θ dis ) was found using θ dis = 2 × charge recorded during the adsorption of CO (q dis, CO )/ electrical charge (q strip, CO , generated by the oxidation of CO in the monolayer is 420 µC cm − 2 ), two electrons are required to oxidize CO to CO 2 [ 50 ]. To obtain local O 2 transport resistance occurring at Pt/ionomer interface, the limiting current measurements were conducted with varying O 2 mole fractions (1.5%, 3%, 5%, 7%, and 10%, N 2 balance) and cell pressures (150 kPa, 200 kPa, and 300 kPa). References Shaw, W. J. et al. A US perspective on closing the carbon cycle to defossilize difficult-to-electrify segments of our economy. Nat. Rev. Chem. 8 , 376-400 (2024). Cullen, D. A. et al. New roads and challenges for fuel cell in heavy-duty transportation. Nat. Energy 6 , 462–474 (2021). Yoshida, T. & Kojima, K. Toyota MIRAI fuel cell vehicle and progress toward a future hydrogen society. Electrochem. Soc. Interface 24 , 45–49 (2015). 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Electrochem. 29 , 100744 (2021). Berlinger, S. A. et al. Impact of platinum primary particle loading on fuel cell performance: insights from catalyst/ionomer ink interactions. ACS Appl. Mater. Interf. 14 , 36731-36740 (2022). Mewis, J. & Wagner, N. J. Thixotropy. Adv. Colloid Interface Sci. 147–148 , 214–227 (2009). K Sano. Et al. A mechanically adaptive hydrogel with a reconfigurable network consisting entirely of inorganic nanosheets and water. Nature Commun . 11 , 6026 (2020) Kushner D. I., Kusoglu A., Podraza N. J. & Hickner M.A. Substrate‐Dependent Molecular and Nanostructural Orientation of Nafion Thin Films. Adv. Funct. Mater . 29 , 1902699 (2019) Harada. M. et al. Distinguishing Adsorbed and Deposited Ionomers in the Catalyst Layer of Polymer Electrolyte Fuel Cells Using Contrast-Variation Small-Angle Neutron Scattering. ACS Omega 6 , 15257–15263 (2021) Ahn, C. Y. et al. Enhancement of service life of polymer electrolyte fuel cells through application of nanodispersed ionomer. 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Characterizing electrolyte and platinum interface in PEM fuel cells using CO displacement. J. Electrochem. Soc. 164 , F60-F64 (2017). Scheme 1 Scheme 1 is available in the Supplementary Files section. Additional Declarations There is NO Competing Interest. Supplementary Files Scheme1.png Scheme 1. Illustration of the MEA fabrication procedures employing two different PFSA ionomers with shorter and larger TFE spacing | Adsorption-driven morphology and distribution of ionomer on the Pt/C catalyst that can be modulated by backbone chemistry during the electrode coating and post-deposition annealing process. SupplementaryInhibitionofpolymericdensificationatplatinumionomerinterfaceW.Y.Choietal.docx SUPPLEMENTARY INFORMATION Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5891522","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":418540521,"identity":"b3d5c0c3-4688-4ab3-a79d-879b710a9aa2","order_by":0,"name":"Chi-Young Jung","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAzElEQVRIiWNgGAWjYHCCBCC2MWCD8gyI1ZJGmhYQOAxXSVgL/4yEZ9IFv84b87H3PmCuqGAwNm8goEXiRkKa9My+22ZsPMcNGM+cYTCTOUBAi4EEUAtvz20bNok0BsbGNgYbCUIOg2o5Z8Mm/wyo5R+xWnh+HDBjk2ADamlgMCOoReLMg2Rr3oZkYzaeNIaDDcckjAlq4W/PSbzN88fOcH77McaHDTU2hjMIaWEQyElgYGyDsA8AbSWoAWjNcaDCP0QoHAWjYBSMgpELAOdrNZUJdSwBAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0003-3393-2333","institution":"Korea Institute of Energy Research","correspondingAuthor":true,"prefix":"","firstName":"Chi-Young","middleName":"","lastName":"Jung","suffix":""},{"id":418540522,"identity":"0d0f3876-5226-43f8-a46e-e7415d8aa882","order_by":1,"name":"Wonyoung Choi","email":"","orcid":"https://orcid.org/0000-0002-6775-1824","institution":"Korea Institute of Energy Research","correspondingAuthor":false,"prefix":"","firstName":"Wonyoung","middleName":"","lastName":"Choi","suffix":""},{"id":418540523,"identity":"51af755d-4f7f-4939-b0f5-c5496d539817","order_by":2,"name":"Hyunguk Choi","email":"","orcid":"","institution":"Korea Institute of Energy Research","correspondingAuthor":false,"prefix":"","firstName":"Hyunguk","middleName":"","lastName":"Choi","suffix":""},{"id":418540524,"identity":"e3bf1a81-8a13-40ff-807a-90cc8c0a4d7c","order_by":3,"name":"Youngje Park","email":"","orcid":"","institution":"Korea Institute of Energy Research","correspondingAuthor":false,"prefix":"","firstName":"Youngje","middleName":"","lastName":"Park","suffix":""},{"id":418540525,"identity":"f7fa260b-9b6b-4973-ab9f-b3e7f418080f","order_by":4,"name":"Seowon Choi","email":"","orcid":"","institution":"Korea Institute of Energy Research","correspondingAuthor":false,"prefix":"","firstName":"Seowon","middleName":"","lastName":"Choi","suffix":""},{"id":418540526,"identity":"b7560e33-f5c5-45c7-b7eb-dd1a6ddeeb9a","order_by":5,"name":"Hyeon E Cho","email":"","orcid":"","institution":"Korea Institute of Energy Research","correspondingAuthor":false,"prefix":"","firstName":"Hyeon","middleName":"E","lastName":"Cho","suffix":""},{"id":418540527,"identity":"38315c42-8e07-4410-8274-5052451d3c8a","order_by":6,"name":"Namjin Lee","email":"","orcid":"","institution":"Korea Institute of Energy Research","correspondingAuthor":false,"prefix":"","firstName":"Namjin","middleName":"","lastName":"Lee","suffix":""},{"id":418540528,"identity":"4f512821-ef8f-42e6-9ee1-dde81922bd62","order_by":7,"name":"Younggi Yoon","email":"","orcid":"","institution":"Korea Institute of Energy Research","correspondingAuthor":false,"prefix":"","firstName":"Younggi","middleName":"","lastName":"Yoon","suffix":""},{"id":418540529,"identity":"1cfa7403-c5f7-4950-8fef-5b022c37bdc5","order_by":8,"name":"Sungchul Yi","email":"","orcid":"https://orcid.org/0000-0003-1132-509X","institution":"Hanyang University","correspondingAuthor":false,"prefix":"","firstName":"Sungchul","middleName":"","lastName":"Yi","suffix":""},{"id":418540530,"identity":"b4c09a0d-8a21-49cc-b649-9b4b011bc247","order_by":9,"name":"Min Jae Ko","email":"","orcid":"https://orcid.org/0000-0002-4842-3235","institution":"Hanyang University","correspondingAuthor":false,"prefix":"","firstName":"Min","middleName":"Jae","lastName":"Ko","suffix":""}],"badges":[],"createdAt":"2025-01-24 01:17:11","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5891522/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5891522/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":77215227,"identity":"13566c6b-f778-43a0-8f90-a5ed0b83bc17","added_by":"auto","created_at":"2025-02-26 09:28:03","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":549106,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePhysical concept of PFSA ionomer with varying TFE spacing | a.\u003c/strong\u003e Chemical structure of PFSA ionomer; black, green, red, yellow, and white represent C, F, O, S, and H, respectively. Interactions between slurry component; \u003cstrong\u003eb-d.\u003c/strong\u003e Hydrophobic interaction (carbon support-TFE backbone) (b), electrostatic interaction (Pt-sulfonate group) (c), and ionic interaction (sulfonate group- sulfonate group, H-bond) (d). \u003cstrong\u003ee. \u003c/strong\u003eIllustrations of the ionomer with respect to TFE spacing \u003cstrong\u003ef.\u003c/strong\u003e Chemical structure of the ionomer, representing the repeating TFE unit as ‘m’ and the degree of polymerization of ‘n’. \u003cstrong\u003eg. \u003c/strong\u003em values for S-TFE, I-TFE, and L-TFE spacing ionomers. \u003cstrong\u003eh. \u003c/strong\u003ePeak molecular weight distribution obtained by GPC. \u003cstrong\u003ei.\u003c/strong\u003e Weight fraction of TFE backbone and side chains with respect to ionomer TFE spacing.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-5891522/v1/351933b842ce3e193bfd6070.png"},{"id":77215559,"identity":"e171f532-c149-46d0-a38a-d90f25c2a064","added_by":"auto","created_at":"2025-02-26 09:36:03","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":520594,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eRheological measurement of electrode slurry |\u003c/strong\u003e \u003cstrong\u003ea. \u003c/strong\u003eViscosity profile under low a shear rate of 0.01 s\u003csup\u003e-1\u003c/sup\u003e for S-TFE spacing, I-TFE spacing, and L-TFE spacing ionomer-based electrode slurries with varying ionomer concentrations. \u003cstrong\u003eb-c.\u003c/strong\u003e Rest stage of 3ITT (b) and hydrodynamic diameter of agglomerates (c) in electrode slurry at 0.75 mmol\u003csub\u003eSO3-\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e-1\u003c/sup\u003e. \u003cstrong\u003ed-f.\u003c/strong\u003e Amplitude sweep measurement of S-TFE spacing (black) (d), I-TFE spacing (red) (e), and L-TFE spacing (blue) (f) ionomers derived slurry at concentrations of 0.125 (▲), 0.75 (●), and 1.25 (◆) mmol\u003csub\u003eSO3-\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e-1\u003c/sup\u003e, highlighting the gel-to-solution transition point (circle) and a snapshot of a tube inversion test taken 5 s after inverting a 2 mL slurry.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-5891522/v1/da3e5382154be33147d9e435.png"},{"id":77215561,"identity":"ce61664a-18ed-4ab2-98fd-eb147c42b542","added_by":"auto","created_at":"2025-02-26 09:36:03","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1383136,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMicrostructure of as-fabricated electrodes│a-c. \u003c/strong\u003eSEM images of electrode with varying TFE spacing at ionomer contents of\u003cstrong\u003e \u003c/strong\u003e0.75 mmol\u003csub\u003eSO3-\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e-1\u003c/sup\u003e. SE images of S-TFE spacing (a), I-TFE spacing (b), and L-TFE spacing (c) ionomers at 10k magnification. d-i. SE and BSE images captured at identical locations for S-TFE spacing (d, e), I-TFE spacing (f, g), and L-TFE spacing (h, i) ionomers at 200k magnification.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-5891522/v1/45df85fe38a007b1aedcac8d.png"},{"id":77215229,"identity":"188be9f8-f44a-48de-a08c-67a5daf33eb7","added_by":"auto","created_at":"2025-02-26 09:28:03","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":874742,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSpatial distribution of PFSA ionomers and their correspondence onto Pt and carbon support in the ORR electrode│a-c. \u003c/strong\u003eSTEM bright-field images and elemental maps of Pt (red), F (green), and S (yellow) obtained from the cathode side of cross-sectioned MEA with ionomer content at 0.75 mmol\u003csub\u003eSO3-\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e-1\u003c/sup\u003e: S-TFE spacing (a), I-TFE spacing (b), and L-TFE spacing (c)\u003cstrong\u003e \u003c/strong\u003eionomers. \u003cstrong\u003ed-f.\u003c/strong\u003e Local correspondence ratios of the ionomer signal overlaid Pt or carbon support calculated by using RGB values extracted from the STEM images with elemental maps: S-TFE spacing (d), I-TFE spacing (e), and L-TFE spacing (f) ionomers.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-5891522/v1/1414bbb9137c73c70d3b65b7.png"},{"id":77215234,"identity":"b842b371-5aa3-416e-8df5-6f3b76068e71","added_by":"auto","created_at":"2025-02-26 09:28:03","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":538149,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectrochemical analysis of the single cells | a-c. \u003c/strong\u003eH₂–air polarization curves of 25cm\u003csup\u003e2\u003c/sup\u003e MEAs with S-TFE spacing (black) (a), I-TFE spacing (red) (b), and L-TFE spacing (blue) (c) ionomers MEAs with varying ionomer contents. \u003cstrong\u003ed.\u003c/strong\u003e Peak power density and performance (at 0.6 V) of optimized MEAs at 0.75 mmol\u003csub\u003eSO3-\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e–1\u003c/sup\u003e. \u003cstrong\u003ee-f.\u003c/strong\u003e EIS curves with fitted data (e) and bar graph (f) of CTR (green) and MTR (red) measured at a current density of 2.0 Acm\u003csup\u003e-2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-5891522/v1/ee2e6599e172553add623c71.png"},{"id":77215568,"identity":"29a6e1e7-d3a3-4914-9e2e-911cbe4cd78a","added_by":"auto","created_at":"2025-02-26 09:36:03","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":279896,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEffect of TFE spacing on the electrochemical properties | a. \u003c/strong\u003eECSA value calculated from CO stripping curve under 100% and 20% RH conditions, along with DPA value (= ECSA\u003csub\u003eRH20\u003c/sub\u003e/ECSA\u003csub\u003eRH100\u003c/sub\u003e). \u003cstrong\u003eb.\u003c/strong\u003e ECSA, mass activity (MA), and specific activity (SA) obtained at varying TFE spacing MEAs with 0.75 mmol\u003csub\u003eSO3-\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e-1\u003c/sup\u003e. \u003cstrong\u003ec.\u003c/strong\u003e CO-displacement current density measured at 0.4 V\u003csub\u003eRHE\u003c/sub\u003e. \u003cstrong\u003ed.\u003c/strong\u003e H\u003csub\u003e2\u003c/sub\u003e-N\u003csub\u003e2\u003c/sub\u003e EIS with fitting curve under 50% RH for both An-Ca. \u003cstrong\u003ee.\u003c/strong\u003e local O₂ transport resistance by limiting current measurement with the operating condition of the flow rate of 1,000 sccm H\u003csub\u003e2\u003c/sub\u003e and 2,000 sccm O₂/N₂ (1.5%, 3%, 7%, and 10% O\u003csub\u003e2\u003c/sub\u003e in N\u003csub\u003e2\u003c/sub\u003e), p\u003csub\u003ea\u003c/sub\u003e = p\u003csub\u003ec\u003c/sub\u003e = 150, 200, and 300 kPa\u003csub\u003eabs\u003c/sub\u003e at 100% RH\u003csub\u003eAn/Ca\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-5891522/v1/c3b76d3c4f928592683fda85.png"},{"id":79344061,"identity":"2a468e1c-3e88-4b32-bf10-5bfddc63c142","added_by":"auto","created_at":"2025-03-27 09:18:59","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5321868,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5891522/v1/0e205244-7e1c-442b-9fb1-479a708946b6.pdf"},{"id":77215252,"identity":"bed6fd19-c808-4da1-b870-d4e5364742ba","added_by":"auto","created_at":"2025-02-26 09:28:04","extension":"png","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":474037,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eScheme 1. Illustration of the MEA fabrication procedures employing two different PFSA ionomers with shorter and larger TFE spacing | \u003c/strong\u003eAdsorption-driven morphology and distribution of ionomer on the Pt/C catalyst that can be modulated by backbone chemistry during the electrode coating and post-deposition annealing process.\u003c/p\u003e","description":"","filename":"Scheme1.png","url":"https://assets-eu.researchsquare.com/files/rs-5891522/v1/eef78668b9ec6d09d0250ee5.png"},{"id":77215255,"identity":"103f794f-f8de-4497-80cc-c549ac9f884d","added_by":"auto","created_at":"2025-02-26 09:28:04","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":9984077,"visible":true,"origin":"","legend":"SUPPLEMENTARY INFORMATION","description":"","filename":"SupplementaryInhibitionofpolymericdensificationatplatinumionomerinterfaceW.Y.Choietal.docx","url":"https://assets-eu.researchsquare.com/files/rs-5891522/v1/453188ae56b77e8f720e9737.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Inhibition of polymeric densification at platinum/ionomer interface via enlarging tetrafluoroethylene spacing in perfluorinated sulfonic-acid ionomer","fulltext":[{"header":"Introduction","content":"\u003cp\u003ePolymer electrolyte fuel cells (PEFCs) fed by clean hydrogen (H\u003csub\u003e2\u003c/sub\u003e) coupled with atmospheric oxygen (O\u003csub\u003e2\u003c/sub\u003e) are one of the most attractive alternatives to conventional internal combustion engines, driving a shift towards complete electrification in the global transportation sector [\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e]. PEFCs offer decisive advantages over lithium-ion batteries, such as faster fueling (\u0026lt;\u0026thinsp;5 min) and extended driving ranges (\u0026gt;\u0026thinsp;500 miles), which are particularly beneficial for heavy-duty vehicle (HDV) applications [\u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e]. However, the spread of this technology is hindered by the high cost and supply scarcity of platinum (Pt) or platinum group metal (PGM). Although state-of-the-art fuel cell electric vehicles (FCEVs) have succeeded in reducing PGM usage from 36 to 19.2 g, a substantial reduction of PGM materials by over four times is still necessary to achieve sustainability [\u003cspan class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e]. This has led to extensive research and development of oxygen (O\u003csub\u003e2\u003c/sub\u003e) reduction reaction (ORR) catalysts with excellent activity per Pt mass; however, all developed materials have failed to realize their full potential, as demonstrated in the rotating disk electrode (RDE) in a half-cell setup when implemented in a real-scale membrane electrode assembly (MEA) at the single-cell level [\u003cspan class=\"CitationRef\"\u003e5\u003c/span\u003e]. Furthermore, the local transport resistance of O\u003csub\u003e2\u003c/sub\u003e through the perfluoro sulfonic acid (PFSA) ionomer film covering Pt has become unexpectedly larger after the use of highly active materials with reduced Pt deployment [\u003cspan class=\"CitationRef\"\u003e6\u003c/span\u003e], which poses the performance discrepancy between RDE and MEA as one foremost technical hurdle remaining for broader commercialization.\u003c/p\u003e\n\u003cp\u003eOne primary origin of this notorious phenomenon, also referred to as the RDE vs. MEA challenge [\u003cspan class=\"CitationRef\"\u003e7\u003c/span\u003e], is believed to be the highly densified polymeric matrixes at Pt/ionomer interface in the ORR electrode. To the best of our knowledge, recent evidence states that the highly concentrated formulation of polymeric ionomer may lead to stronger adsorption of the sulfonate terminal group (-SO\u003csub\u003e3\u003c/sub\u003eH) onto Pt, specifically occurring at onset potentials around 0.4 to 0.5 V versus reference hydrogen electrode (RHE), thus intensifying ionomer poisoning [\u003cspan class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e] as well as interfacial O\u003csub\u003e2\u003c/sub\u003e transport resistance [\u003cspan class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e]. To address this issue, several efforts have been made to eliminate the direct contact between Pt and ionomer [\u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e]. To obviate the direct contact, one has proposed the mesoporous carbons as accessible and porous supporting materials [\u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e] while others have devised the Pt-protective layer by forming a porous polymeric film or porous graphitic carbon layer covering Pt [\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e], as well as by depositing ionic liquids exhibiting considerable proton conductivity onto Pt [\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e]. This improves ORR activity at the MEA scale by securing larger numbers of active sites from the specific adsorption of sulfonate groups. However, this strategy inevitably suffers from significantly lower ionomer accessibility, particularly at lower relative humidity (RH), which drastically reduces the electrochemically available surface area (ECSA) by greater extent without the assistance of water.\u003c/p\u003e\n\u003cp\u003eTo enhance ionomer accessibility, studies have reported the promotion of ionomer homogeneity deposited onto the conventional Pt catalysts supported by carbon black (Pt/C) by controlling Coulombic interaction through the introduction of electro-negative functionalities onto the surfaces of carbon supports near Pt [\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e]. One effective outcome demonstrated that nitrogen dopants and their neighboring carbons are beneficial in loosening the ionomer structure at the Pt/ionomer interface, by attracting the sulfonate groups in ionomer against Pt [\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]. Recently, Pt supported by Vulcan (Pt/V) has been reported to be more favorable in exploiting the Pt/ionomer interface than supported by Ketjen black (Pt/KB), due to less micro-porous structures with better accessibility [\u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e]. However, there remains a technical challenge regarding that adsorption strength of sulfonate group onto carbon support can be notably weakened particularly at the O\u003csub\u003e2\u003c/sub\u003e-containing defect sites, which results in its recombination onto Pt surfaces [\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e]. Consequently, an insufficient number of nitrogen atoms spatially adjacent to the Pt catalyst may also disturb the effectiveness of this supportive interaction.\u003c/p\u003e\n\u003cp\u003eIn this article, we present a novel approach to mitigate undesired polymeric densification at the Pt/ionomer interface, by strengthening the hydrophobic interaction between the ionomer and Pt/V within the electrode slurry, which may significantly improve the performance of the ORR electrode. Our primary strategy involved altering the ionomer backbone chemistry by increasing the tetrafluoroethylene (TFE) spacing between neighboring side chains, as illustrated in Scheme \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. The proposed method promotes molecular attraction between the TFE backbone and Vulcan carbon in a water-rich environment, which in turn, improves the adsorption affinity of the ionomer onto carbon over Pt during the slurry preparation, deposition, and annealing processes. Loosened polymeric morphology at the Pt/ionomer interface can be attributed to the as-fabricated electrode microstructure, thereby providing two beneficial features: (i) alleviating the potential-dependent specific adsorption of sulfonate groups onto Pt, and (ii) decreasing the local transport resistance of O\u003csub\u003e2\u003c/sub\u003e at the Pt/ionomer interface. Based on these improvements, it was confirmed that the electrode ionomers with enlarged TFE spacing exhibited not only an excellent coverage of the ionomer but also an incredibly lower coverage of specifically adsorbed sulfonate groups onto Pt, as evaluated by dry proton accessibility (DPA) and CO displacement measurements, respectively. Consequently, these features allow us to attain the boosted reaction kinetics and mass transport properties in the ORR electrode, thereby pushing forward the overall MEA performance to the next step. Here reported findings demonstrate that enlarging the TFE spacing in ionomer is a key route to address the challenges derived from ionomer densification, thereby unlocking the full catalytic potential of Pt and door to the sustainable commercialization of FCEVs.\u003c/p\u003e\n\u003ch3\u003eMitigation of polymeric densification at Pt/ionomer interface via enlarging TFE spacing\u003c/h3\u003e\n\u003cp\u003ePEFC electrodes are typically manufactured using slurry-processing techniques, which involve both deposition and curing processes. The dispersion behavior in the slurry significantly governs the Pt/ionomer interface of the as-fabricated electrode [\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e]. Typically, the electrode slurry comprises a blend of the PFSA ionomer and Pt/V or Pt/KB catalyst in a water-alcohol mixture used as dispersing solvent, leading to complex interactions among the slurry components. As illustrated in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea, the ionomer\u0026rsquo;s amphiphilic nature, stemming from the hydrophobic TFE backbone and hydrophilic sulfonate-terminated perfluoro ether side chains, gives rise to three key interactions among the dispersing particles: (i) hydrophobic interactions between the TFE backbone and carbon support (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb), which results in ionomers being adsorbed onto the outer surface of the carbon support [\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e], (ii) electrostatic attractions between the sulfonate side chains and Pt particles, locating the sulfonate group towards the Pt (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ec) [\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e], and (iii) ionic interactions between the negatively charged side chains of neighboring ionomers, which are influenced by the concentration of the ionomer (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed) [\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003eAmong these complex interactions, ionomers can either be adsorbed onto the surfaces of Pt/C particles (i.e., adsorbed ionomers) or remain unattached to Pt/C and dispersed in the slurry (i.e., free ionomers) [\u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e]. To reduce the specific adsorption of sulfonate groups on the Pt surface, mitigation of the polymeric chains onto Pt surface can be a key solution which is origin form strengthen the hydrophobic interactions between the TFE backbone and carbon support. This can be comparable to or even stronger than that between the sulfonic acid side chain and the Pt catalyst [\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e30\u003c/span\u003e]. Pt surface is also covered with a native oxide layer in the liquid slurry, which may repel the sulfonic acid side chain under such non-polarized conditions [\u003cspan class=\"CitationRef\"\u003e31\u003c/span\u003e]. Moreover, the carbon support, showcasing a larger exterior surface area than Pt (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;1\u003c/strong\u003e), suggests that carbon/ionomer interactions predominantly dictate solution-level interactions compared to Pt/ionomer interactions. Thus, we hypothesize that enlarging the TFE spacing can be a key solution to reduce specific adsorption of sulfonate group onto Pt surface via strengthening the hydrophobic attraction between TFE backbone and carbon support.\u003c/p\u003e\n\u003cp\u003eTo confirm our strategy, we prepared three PFSA ionomers with identical side chain compositions but different TFE spacings, where the molecular distances between the neighboring side chains were modified as illustrated in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ee. Short side chain (SSC) ionomers are selected over long side chain (LSC) ionomers, as they are well known to successively suppress the specific adsorption sulfonate group. Here, the SSC ionomers were modified to exhibit short (S-TFE spacing), intermediate (I-TFE spacing), and long TFE-spacing (L-TFE spacing), which can be described by the number of repeating units (m) in the backbone (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ef\u003cstrong\u003e)\u003c/strong\u003e. The m values of the S-TFE, I-TFE and L-TFE spacing ionomers were 4.4, 5.5, and 7.0, respectively (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eg). The gel permeation chromatography (GPC) results are provided in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eh, ensuring that all the samples exhibited similar degrees of polymerization with the number of the repeating units (n) ranging from 4.9 to 5.1. The increase in TFE spacing may lead to an increase in the molecular fraction of the TFE backbone over sulfonic acid side chain, thus resulting in an enlarged domain of the hydrophobic region (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ei). Therefore, by adjusting the TFE spacing, the ionomer\u0026rsquo;s adsorption preference can be selectively controlled either onto Pt or carbon support, which is primarily attributed to the promotion of hydrophobic attraction in practical water-rich slurry environments.\u003c/p\u003e\n\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003eDependence of TFE Spacings on Ionomer\u0026rsquo;s Adsorption Affinity\u003c/h2\u003e\n \u003cp\u003eThe TFE spacings may serve as a decisive factor in determining the adsorption preference of the PFSA ionomer, either onto the Pt or carbon support, thereby governing the ionomer morphology at the Pt/ionomer interface. Particularly in a water-rich environment, ionomer with larger TFE spacing may develop stronger adsorption onto the carbon support than Pt, due to the increased hydrophobic interactions [\u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e]. Figure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e presents the rheological measurements of the electrode slurries employing the S-TFE, I-TFE, and L-TFE spacing ionomers to explore how the varying adsorption behaviors affect the slurry rheology. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea, the viscosity values were taken from the steady-shear characteristics (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;2)\u003c/strong\u003e at a low shear rate of 0.01 sec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, with varying ionomer contents, to confirm the dispersion behaviors in the slurry. U-shaped profiles were developed as a function of ionomer content. In the initial part of the low ionomer contents (0-0.25 mmol\u003csub\u003eSO3\u0026minus;\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e), the slurry viscosity dramatically decreased by two orders of magnitude after a small amount of ionomer was applied to the Pt/C dispersion without ionomer. The ionomer adsorbed onto the Pt/C prevents the severe aggregation by promoting electrostatic repulsion [\u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e]. It is also noteworthy that the slurry employing L-TFE spacing ionomer exhibited a notably reduced viscosity, from 2.78 \u0026times; 10\u003csup\u003e5\u003c/sup\u003e cP down to 7.77 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e at 0.25 mmol\u003csub\u003eSO3\u0026minus;\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e\u0026ndash;1\u003c/sup\u003e, when compared with that employing S-TFE ionomer. At the intermediate ionomer contents (0.25\u0026ndash;0.75 mmol\u003csub\u003eSO3\u0026minus;\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e\u0026ndash;1\u003c/sup\u003e), plateaus in slurry viscosity were observed with an increase in ionomer content for all three samples, indicating that the additional amounts contributed to the formation of free ionomers. Subsequently, at the high ionomer contents (0.75\u0026ndash;1.25 mmol\u003csub\u003eSO3\u0026minus;\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e\u0026ndash;1\u003c/sup\u003e), a rapid increase in viscosity was recorded as a result of the excessively higher concentration in the slurry, thereby enhancing the ionic strength between the sulfonate groups and dispersing solvent molecules [\u003cspan class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e]. The reduced viscosities for all ionomer contents indicate that the larger TFE spacing ionomers were successfully adsorbed onto Pt/V with an increased surface coverage, due to the enhanced interaction between the TFE backbone and carbon support, as presented in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb.\u003c/p\u003e\n \u003cp\u003eThe competitive formation of adsorbed and non-adsorbed ionomers can also be elucidated using three-interval thixotropy test (3ITT), as shown in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb. We sheared the three slurries at a low shear rate of 0.1 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 60 s (rest phase), then at a high shear rate of 100 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 10 s (shear phase), and lastly at a low rate of 0.1 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 60 s (recovery phase), during which the time-dependent viscosity variations were monitored in the recovery phase to analyze the thixotropic behaviors. Interestingly, the slurry employing L-TFE spacing ionomer exhibited the largest viscosity upshoot of approximately 10.9 \u0026times; 10\u003csup\u003e3\u003c/sup\u003e cP, within 1 s after the shear was removed. This rapid viscosity recovery can be attributed to the elastic rebound of ionomers that are primarily adsorbed onto Pt/C with higher surface coverage ratios [\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e]. Later, at a high ionomer content of 1.25 mmol\u003csub\u003eSO3\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e, the recovery time was significantly reduced from over 30 s to less than 5 s, due to less entanglement of the polymeric chain resulting from deficient free ionomers (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;3\u003c/strong\u003e). This was further supported by a dynamic light scattering (DLS) analysis, where the slurries of interest were diluted to 0.1 wt% to ensure transparency (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec). It is clearly demonstrated that the larger TFE spacing ionomers are beneficial in suppressing the formation of larger agglomerates in diameter of \u0026gt;\u0026thinsp;1 \u0026micro;m, that is pronouncedly originated from the reduced concentration of free ionomers dispersed in the solvent.\u003c/p\u003e\n \u003cp\u003eThe amplitude sweep measurements were conducted to show how the adsorption affinity of the ionomer affects the internal structure of the electrode slurry (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed\u0026ndash;f). Three different ionomer concentrations of 0.125, 0.75, and 1.25 mmol\u003csub\u003eSO3\u0026minus;\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e were tested for the TFE spacing of each ionomer. Amplitude sweep measurements provide information on slurry particle interaction by analyzing the storage (G\u0026apos;) and loss modulus (G\u0026apos;\u0026apos;). In the low strain region, we observe a linear viscoelastic (LVE) region, denoted by G\u0026apos; outstripping G\u0026apos;\u0026apos;, signifying elastic particle interactions. As the strain increased, the internal structure of the slurry weakened, leading to a diminution of both G\u0026apos; and G\u0026apos;\u0026apos;, culminating in strain-induced softening. Consequently, the slurry transitions towards liquid-like behavior as G\u0026apos;\u0026apos; overtakes G\u0026apos;. The LVE region, which indicates the internal strength of the slurry, expanded with increasing ionomer content and decreasing ionomer TFE spacing. When examining the transition to a solution-like state with a consistent ionomer content, it was observed that this transformation occurred at higher rates of deformation as the TFE spacing of the ionomer decreased. This observation can be attributed to the presence of free ionomers in the slurry, which induce attractive interactions between the particles, resulting in a gel-like behavior of the slurry [\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e]. The tube inversion test provided a visual insight into the variations in the internal structure of the slurry due to the different TFE spacings and ionomer contents (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed\u0026ndash;f). Therefore, the result suggests that the concentration-controlled slurry with enlarged TFE spacing not only promotes the orientation of the adsorbed ionomer towards carbon rather than Pt but also suppresses the excessive presence of free ionomer remaining in the solvent.\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003eLocal microstructure and spatial distribution of ionomer in the electrode\u003c/h3\u003e\n\u003cp\u003eTypically, the PFSA ionomers are either adsorbed onto Pt/C or remain non-adsorbed in the dispersing solvent, depending on the ionomer\u0026rsquo;s chemical structure and the solvent composition [\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e]. This behavior ultimately determines the formation of adsorbed and deposited ionomers in the as-fabricated electrode [\u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e]. Hence, the microstructural properties, including electrode surface morphology and ionomer distribution, are significantly influenced by the adsorption behavior of the ionomer in electrode slurry.\u003c/p\u003e\n\u003cp\u003eFor visual verification, scanning electron microscopy (SEM) was performed to observe the electrode microstructures using ionomers with varying TFE spacing. The molality of the ionomer was fixed at an optimum value of 0.75 mmol\u003csub\u003eSO3\u0026minus;\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e, as discussed in \u003cstrong\u003eSupplementary Figs.\u0026nbsp;4 and 5\u003c/strong\u003e. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea, large white particles, with diameters ranging from 1 to 2 \u0026micro;m are observed, presumed to be bulky ionomer aggregates when the S-TFE ionomer was applied. In contrast, the size and number of aggregate particles notably decreased on the electrode surfaces after the use of the I-TFE and L-TFE spacing ionomers (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb and \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec). Furthermore, the formation of surface cracks was significantly reduced as the TFE spacing increased, as shown in \u003cstrong\u003eSupplementary Fig.\u0026nbsp;6\u003c/strong\u003e, which may result from the well-dispersed electrode slurry with fewer agglomerated structures [\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003eComparison of the secondary electron (SE) and backscattered electron (BSE) images at an identical location can be a more precise way of analyzing the spatial ionomer distribution. Unlike SE mode, BSE signals originate from deeper regions of the electrode, making them advantageous for distinguishing the bulky ionomer aggregates deposited onto the nano-sized adsorbed ionomer layer on Pt/C [\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e]. In electrodes employing the S-TFE ionomer, a large number of white dots corresponding to Pt nanoparticles are missing in the SE image (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed). However, they are clearly visible in the corresponding region (marked with blue lines) in the BSE image (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ee). This discrepancy is primarily due to the deposition of larger ionomer aggregates, with sizes of several tens of nanometers, on the Pt surface [\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e], which may have interrupted the Pt signals in the SE mode. In contrast, after using larger TFE-spacing ionomers, the Pt nanoparticles in the SE image correspond well with those in the BSE image, after the use of larger TFE spacing ionomers (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ef\u0026ndash;i). This suggests that, with an increase of TFE spacing, the considerably thin electrode ionomers are uniformly distributed onto the electrode, which highlights the potential existence of a well-established carbon support/ionomer interface.\u003c/p\u003e\n\u003cp\u003eThe formation of intact interfaces between carbon support and ionomer was further investigated by scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDS), as shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea\u0026ndash;c. By examining the elemental map of fluorine (F, green) signals overlaid on either the STEM image or the Pt map, the local correspondence between carbon support and ionomer can be quantified, as presented Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed\u0026ndash;f. A strong overlap between F and Pt was frequently observed in the S-TFE electrode, with the highest local correspondence ratio of 80% (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea, d\u003cstrong\u003e)\u003c/strong\u003e. Additionally, multiple regions with higher ionomer concentration were detected when a shorter TFE spacing was applied, as excessive amounts of free ionomers in the slurry turned into bulky ionomer aggregates deposited onto the electrode. The ionomer aggregates dominantly increase their size and number density at higher ionomer contents, as displayed in \u003cstrong\u003eSupplementary Fig.\u0026nbsp;7\u003c/strong\u003e. This result is primarily attributed by the highly densified TFE backbones in ionomer near Pt, due to the nano-confinement effect [\u003cspan class=\"CitationRef\"\u003e41\u003c/span\u003e]. Consequently, the specific adsorption of sulfonate groups onto the Pt catalyst can be exacerbated under typical fuel cell operation in the rated cell. As the TFE spacing becomes larger, the overlap of F and Pt signals significantly weakened, and simultaneously, the F signals displayed a stronger locational correspondence within the carbon support (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb, c\u003cstrong\u003e)\u003c/strong\u003e. Figure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ef shows that the local correspondence ratio was dramatically reduced to 51% in the L-TFE electrode, which is well consistent with the F and Pt mapping results. Based on these findings, it was demonstrated that the larger TFE-spacing ionomers coordinate more strongly onto the surfaces of the carbon support, which is beneficial for separating sulfonate groups from the Pt surfaces. This creates a weakly confined Pt/ionomer interface with a less densified polymeric structure.\u003c/p\u003e\n\u003ch3\u003eElectrochemical characterization\u003c/h3\u003e\n\u003cp\u003eFinally, electrochemical characterizations were conducted in a single cell with the cathode Pt loadings of 0.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 mg\u003csub\u003ePt\u003c/sub\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, to meet the US DOE 2030 target for PGM loadings required for HDV applications [\u003cspan class=\"CitationRef\"\u003e42\u003c/span\u003e]. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea\u0026ndash;c presents the H₂-air polarization curves with high-frequency resistances (HFRs) for the S-TFE, I-TFE, and L-TFE MEAs with varying ionomer contents. The maximum current densities for S-TFE, I-TFE, and L-TFE MEAs at ionomer contents ranging from 0.25 to 1.25 mmol\u003csub\u003eSO3\u0026minus;\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e were recorded as 1.48, 1.60, and 1.84 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at a cell voltage of 0.6 V, respectively, when the ionomer content was fixed at 0.75 mmol\u003csub\u003eSO3\u0026minus;\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e. Interestingly, the L-TFE MEA with the lowest ionomer content, exhibited a notably low HFR of 44 mΩ cm\u003csup\u003e2\u003c/sup\u003e, significantly lower than that of the S-TFE MEA (65 mΩ cm\u003csup\u003e2\u003c/sup\u003e), which is attributed to better connectivity of ionomer pathways in the electrode. Combined with both SEM and transmission electron microscopy (TEM) analyses (Figs.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e), it can be concluded that a larger TFE spacing is effective for achieving electrode ionomers with higher coverage and uniform distribution onto Pt/C. Moreover, the cell performance of L-TFE MEA was 24% higher than that of S-TFE MEA, along with a 15% increase in peak power density as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ed.\u003c/p\u003e\n\u003cp\u003eWe also performed electrochemical impedance spectroscopy (EIS) obtained at an output current density of 2.0 A cm\u003csup\u003e\u0026ndash;2\u003c/sup\u003e to analyze the origin of overall cell resistance as displayed in Figs. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ee. Nyquist plots were fitted using equivalent circuit model (\u003cstrong\u003eSupplementary Figs.\u0026nbsp;8a\u003c/strong\u003e) to obtain HFR, charge transport resistance (CTR), and mass transport resistance (MTR), which are listed in the bar graphs (Figs. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ef\u003cstrong\u003e)\u003c/strong\u003e. The L-TFE MEA showed a considerable reduction in CTR by 44.9% when compared with S-TFE MEA, where the increased ionomer coverage onto Pt/C is responsible for an enhanced ORR process [\u003cspan class=\"CitationRef\"\u003e43\u003c/span\u003e]. Furthermore, the MTR of L-TFE MEA was dramatically reduced by over 60% (from 284 to 111 mΩ cm\u003csup\u003e2\u003c/sup\u003e), which is mainly due to the improved local O₂ transport over bulk transport, considering that the electrode porosity and pore size distribution were found to be quite similar (\u003cstrong\u003eSupplementary Table\u0026nbsp;2\u003c/strong\u003e). The main cause of this phenomenon is presumed to be the notably loosened polymeric-chain morphology at the Pt/ionomer interface, which enables better nanophase segregation behavior in the weak confinement regime [\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003eTo understand the effect of TFE spacing on the ionomer distribution and coverage, the ECSA was calculated via CO stripping measurements under RH of 20% and 100%, in obtain the DPA (ECSA\u003csub\u003eRH20\u003c/sub\u003e/ECSA\u003csub\u003eRH100\u003c/sub\u003e). This approach allows the measurement of the proximity of the ionomer sulfonate group, which directly contacts the Pt particles [\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]. Since proton accessibility reaches approximately 100% at very high ionomer content, comparing proton accessibility at low ionomer contents is more suitable for studying the effects of TFE spacing on the ionomer coverage. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003ea, the dry proton accessibility for S-TFE, I-TFE, and L-TFE MEAs at 0.5 mmol\u003csub\u003eSO3\u0026minus;\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e was estimated to be 63%, 71%, and 81%, respectively. The high proton accessibility for L-TFE MEA indicates that a large amount of proton-reaching areas were formed on the Pt surface, considering that the same sulfonate molarity was applied to the cathode. In contrast, the shorter TFE MEAs showed lower ionomer coverage on the Pt surface due to the presence of bulky ionomer agglomerates, in line with the aforementioned electrode microstructures. Furthermore, at 0.75 mmol\u003csub\u003eSO3\u0026minus;\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e\u003csub\u003e,\u003c/sub\u003e the proton resistance of L-TFE MEA was 83%, which is higher than that of S-TFE MEA (76%). These results imply that increasing the TFE spacing facilitates the adsorption of ionomers along the carbon support rather than on Pt, mitigating the presence of ionomers on Pt and promoting a more homogeneous ionomer distribution onto Pt/C.\u003c/p\u003e\n\u003cp\u003eThe catalytic activity for performance-optimized MEAs at 0.75 mmol\u003csub\u003eSO3\u0026minus;\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e was measured from the H₂-O₂ polarization curve. In Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eb, the mass activities of the S-TFE, I-TFE, and L-TFE MEAs were 66, 88, and 121 Ag\u003csub\u003ePt\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e, respectively, indicating that L-TFE MEA was twice larger than that of the S-TFE MEA. The result of Tafel slope for L-TFE MEA was 70.1 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, corresponding to the theoretical value based on the transfer coefficient unity at 80\u0026deg;C (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;9)\u003c/strong\u003e. However, the similar ECSA values under RH 100% for S-TFE, I-TFE, and L-TFE MEAs were estimated to be 59.4, 60.1, and 62.5 m\u0026sup2; g\u003csub\u003ePt\u003c/sub\u003e⁻\u0026sup1;, respectively. This indicates that a proton reachable Pt surface was formed across all MEAs. To quantify the sulfonate group adsorption on Pt active site, the CO-displacement was investigated, as shown in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003ec. In the test, the adsorbed sulfonate group was displayed by linear CO adsorption, and the hydroxyl groups covering the Pt surface were negligible at 0.4 V\u003csub\u003eRHE\u003c/sub\u003e [\u003cspan class=\"CitationRef\"\u003e44\u003c/span\u003e]. The L-TFE MEA showed a remarkable reduction in sulfonate group coverage of 8.2% than that of S-TFE MEA (22.3%). Figure \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003ec also shows the H₂-N₂ curve measured at RH 50% to obtain the proton resistance, which can be estimated from 1/3 of the total resistance. The equivalent circuit was given in \u003cstrong\u003eSupplementary Fig.\u0026nbsp;8b\u003c/strong\u003e. The proton resistance of the L-TFE MEA (15.3 mΩ cm\u0026sup2;) was 3.5-folds lower than that of the S-TFE MEA (52 mΩ cm\u0026sup2;), despite the less effective chemical structure of L-TFE ionomer [\u003cspan class=\"CitationRef\"\u003e11\u003c/span\u003e]. These results demonstrate that the enlarging TFE spacing allows to mitigate the polymeric densification at Pt surface while simultaneously maintaining the ORR active site, thus releasing the intrinsic ORR activity at the MEA level.\u003c/p\u003e\n\u003cp\u003eTo measure the local O₂ transport resistance (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{R}}^{\\text{l}\\text{o}\\text{c}\\text{a}\\text{l}})\\)\u003c/span\u003e\u003c/span\u003e in the ionomer film onto the Pt surface, limiting current measurements were conducted, as displayed in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003ee. The \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{R}}^{\\text{local}}\\:\\)\u003c/span\u003e\u003c/span\u003ewas obtained by distinguishing the pressure-dependent MTR (R\u003csup\u003ePD\u003c/sup\u003e) and the pressure-independent MTR (R\u003csup\u003ePI\u003c/sup\u003e) based on the limiting current measurements. This clearly shows that the R\u003csup\u003ePD\u003c/sup\u003e remained constant, as similar porosity results were observed for the electrodes (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;4b\u003c/strong\u003e and \u003cstrong\u003eSupplementary Table\u0026nbsp;2\u003c/strong\u003e). However, the R\u003csup\u003ePI\u003c/sup\u003e for S-TFE, I-TFE, and L-TFE MEAs were 0.272, 0.157, and 0.109 s cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. The R\u003csup\u003ePI\u003c/sup\u003e for L-TFE MEA was 2.5 folds lower than that for the S-TFE MEA. This dramatic reduction in the R\u003csup\u003ePI\u003c/sup\u003e of the L-TFE MEA can be attributed to the loosening of ionomer from the Pt surface [\u003cspan class=\"CitationRef\"\u003e45\u003c/span\u003e]. Thus, the result strongly supports that enlarging TFE spacing is beneficial for loosening the polymeric densification at Pt, promoting O₂ permeability to most Pt particles in the MEA.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eHere, we demonstrate a dedensification of the polymeric chain at the Pt/ionomer interface which enables the realization of intrinsic ORR performance in the MEA via strengthening the interaction between TFE backbone in ionomer and carbon support in Pt/C. Enlarging the TFE spacing has been proven beneficial for tuning the binding affinity of the ionomer towards carbon over Pt by hydrophobic attraction under water-rich slurry. These benefits enable the ionomer to remain adsorbed on the carbon support while simultaneously reducing polymer densification on the Pt surface. Rheological analysis of the electrode slurry showed that enlarging TFE spacing significantly reduced the viscosity and led to pronounced viscoelastic behavior, resulting from the ionomer forming excellent coverage along the carbon and enhancing electrostatic repulsion on the surface of the dispersing particles. Therefore, mitigation of polymeric densification at Pt/ionomer interface was successfully achieved in the as-fabricated electrode microstructures. As a result, sulfonate group coverage on the Pt surface was significantly reduced to only 8% in the L-TFE MEA compared to 22% in the S-TFE MEA in the CO displacement measurements. Moreover, the L-TFE MEA exhibited a 2-fold increase in ORR activity and a 2.5-fold reduction in local O\u003csub\u003e2\u003c/sub\u003e transport resistance when compared with the S-TFE MEA, thereby presenting remarkable improvement of the cell performance. This approach offers a key solution to unlock the catalytic potential at the MEA level by loosening the polymeric densification at the Pt/ionomer interface. This technology can be readily applied to the typical mass production of FCEVs, facilitating fuel cell market penetration and addressing the decarbonization challenges of the transportation sector.\u003c/p\u003e"},{"header":"Experimental Procedures","content":"\u003cp\u003e\u003cstrong\u003eMaterials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSSC ionomers with different TFE spacing (D72-25BS, D83-24B, and D98-25BS, Solvay), 1-propanol (nPA) (HPLC grade, Sigma Aldrich), ultrapure deionized (DI) water (Milli-Q, resistivity of 18.4 MΩ), and 40 wt% Pt loaded Pt/C catalyst (TEC10V40E, Tanaka) were used to prepare the electrode slurry. All ionomers were used as electrode binders and electrolytes (25 wt% of ionomers over total solids). Vulcan XC72 was chosen as the catalyst support over high surface area carbon (HSC, e.g. Ketjen Black EC300J or EC600). Vulcan-type support offers advantages at high proton accessibility. A 130 \u0026micro;m thick polytetrafluoroethylene (PTFE)-coated glassy fiber sheet (AF055, Alphaflon) was used in the blade coating process. The Nafion membrane (NR-211, Chemours) was used as the polymer electrolyte membrane. 300 \u0026micro;m thick polytetrafluoroethylene film (PTFE, Alphaflon) and 50 \u0026micro;m thick polyimide (Kapton, Alphaflon) film were used as decal substrates in the MEA preparation. Lastly, the purity of the gases is given as O\u003csub\u003e2\u003c/sub\u003e (\u0026gt;\u0026thinsp;99.999% purity), N\u003csub\u003e2\u003c/sub\u003e (\u0026gt;\u0026thinsp;99.999% purity), H\u003csub\u003e2\u003c/sub\u003e (\u0026gt;\u0026thinsp;99.999% purity), CO (1%, N\u003csub\u003e2\u003c/sub\u003e balance), and O\u003csub\u003e2\u003c/sub\u003e mixture (1.5%, 3%, 7%, and 10%, N\u003csub\u003e2\u003c/sub\u003e balance) were used for single-cell test.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrode slurry preparation\u003c/strong\u003e: To ensure sufficient wettability between the ionomer and the solution, the ionomer dispersion was dispersed in a mixture of DI water and nPA for 24 hours (a final DI water-to-nPA weight ratio of 1:1). Then, the electrode slurry was prepared by blending the Pt/C catalyst and ionomer dispersion by using a high-energy ball mill (Pulverisette 7, Fritsch) at 100 rpm for 24 h; the ionomer concentration was set within a range of sulfonic acid molality from 0 to 1.25 mmol\u003csub\u003eSO3\u0026minus;\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e and these were quantified as sulfonic acid molarity relative to the carbon support weight (mmol\u003csub\u003eSO3\u0026minus;\u003c/sub\u003eg\u003csub\u003eC\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e); Sulfonic acid molarity corresponding to ionomer-to-carbon support ratio (I/C) was presented in \u003cstrong\u003eSupplementary Table\u0026nbsp;3\u003c/strong\u003e. Moreover, all samples were prepared with a consistent carbon weight to solvent volume ratio (C\u003csub\u003eg\u003c/sub\u003eS\u003csub\u003eml\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e), of 0.1 C\u003csub\u003eg\u003c/sub\u003eS\u003csub\u003eml\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMEA fabrication and cell assembly\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe slurry was coated onto a PTFE-coated glass fiber sheet using the blade coating method applying shear rate of 100 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The coated decals were annealed at 80\u0026deg;C for 2 h in a nitrogen atmosphere. The Pt loadings of the electrodes were fixed at 0.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 mg\u003csub\u003ePt\u003c/sub\u003ecm\u003csup\u003e\u0026minus;2\u003c/sup\u003e for both the cathode and anode, which were confirmed using a portable X-ray fluorescence (XRF) analyzer (Niton XL5, Thermo Scientific). After that, the MEA for the single-cell test was prepared by utilizing the decal transfer method. The 25 \u0026micro;m-thick Nafion 211 membrane was assembled between the cathode and anode coated decal. Subsequently, the assembly was hot-pressed at 110\u0026deg;C and 65 bar for 10 min, followed by rapid cooling 5 min. For the single-cell test, an air-cooled single-cell with an active area of 25 cm\u003csup\u003e2\u003c/sup\u003e (K-cell, CNL) and a graphite gas flow field with 9 channels (\u003cstrong\u003eSupplementary Fig. S10a\u0026ndash;b\u003c/strong\u003e) were utilized. A gas diffusion layer (GDL) (Sigracet 22 BB, SGL), with a total thickness of 215 \u0026micro;m and a 5 wt% PTFE resin content in the microporous layer (MPL) was employed. In the single-cell assembly, the compression rate of the GDL was set to approximately 25% using a PTFE stopper [\u003cspan class=\"CitationRef\"\u003e46\u003c/span\u003e], and the clamping pressure was set to 7 Nm. A 25 cm\u0026sup2; MEA was utilized for electrochemical evaluation, while a 1 cm\u0026sup2; MEA was used for limiting current measurements (\u003cstrong\u003eSupplementary Fig. S10c\u0026ndash;d\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eIonomer characterization\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo confirm the degrees of polymerization of each ionomer, number-average molecular weight (\u003cem\u003eM\u003c/em\u003en) was measured by using gel permeation chromatography (GPC) (HPC-8420, Tosoh) and Toyo soda kogyo (TSK) gel \u0026alpha;-M column. N-methyl-2-pyrrolidinone (NMP) containing 50 mmol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was used as an eluent at a flow rate of 0.5 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 50\u0026deg;C.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eRheological measurements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe rheological measurements were conducted using a stress-controlled rheometer (Hybrid Rheometer, DHR3 TA Instruments) equipped with a 40 mm diameter stainless steel parallel-plate geometry set at a gap distance of 500 \u0026micro;m and a temperature of 25\u0026deg;C. To prevent solvent evaporation, a solvent saturation trap was used during all measurements. Prior to the rheological measurements, the slurry was preconditioned to eliminate any sample loading history by shearing shear at 500 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 1 min followed by resting for 1 min [\u003cspan class=\"CitationRef\"\u003e47\u003c/span\u003e]. The steady-shear measurements were conducted by imposing a stress sweep in logarithmic steps ranging from 0.001 to 1,000 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. To evaluate the degree of particle interaction and agglomeration, the viscosity values were summarized, and the representative viscosity value was obtained in the low shear rate range of 0.01 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, where the slurry exhibits Brownian motion. Amplitude sweep measurements were performed by increasing stress from 0.0001 to 200 Pa with a fixed predetermined frequency of 0.5 Hz. The LVE region was obtained in the amplitude sweep measurements. The 3ITT was performed in three stages. First, the slurry was sheared at 0.1 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 60 s to form a stationary state. The shear rate was then increased to 100 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 10 s to confirm the cluster deformation behavior. Finally, the slurry entered a reconstruction stage by shearring at 10\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 60 s.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMEA characterizations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eHigh-resolution field emission SEM (Verios 5 UC, Thermo Fisher Scientific) was utilized in both SE and BSE modes to study the microstructure of the agglomerated particles on the electrode surface and to measure the thickness of the electrode cross-section. The acceleration voltage was set to 10 kV. A 3D laser-scanning confocal microscope (VK-X1000, Keyence) was used to determine the number and size of the crack in the electrode surface. The crack calculation method was described in \u003cstrong\u003eSupplementary Fig.\u0026nbsp;11\u003c/strong\u003e. TEM analysis (Tecnai F20 G2, FEI), combined with EDS, provides insights into the distribution and morphology of the ionomers on the electrode by identifying elements C, Pt, S, and F. For TEM specimen preparation, the 100 nm-thick cross-sections of the electrodes (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;12b)\u003c/strong\u003e, were prepared through cryo-focused ion beam (FIB) (Helios NanoLab 600, FEI) milling using a Ga\u003csup\u003e+\u003c/sup\u003e ion source at a 52\u0026deg; angle, operating at an energy of 30 kV and a current of 93 pA (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;12a)\u003c/strong\u003e. Prior to the FIB milling, a thin tungsten film was deposited onto the surface of the sample at a controlled temperature under liquid nitrogen to minimize damage to the electrode samples [\u003cspan class=\"CitationRef\"\u003e48\u003c/span\u003e]. Mercury intrusion porosimeter (ASAP 2020, Micromeritics) was used to assess the porosity and pore size distribution of the electrodes by applying pressures ranging from 0.1 to 60,000 psi with a 1,000 mmHg transducer. The electrode porosity was calculated by subtracting the membrane thickness from the overall MEA thickness.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrochemical measurements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eElectrochemical measurements were conducted using a 100 W PEFC testing station (G20, Greenlight Innovation). Active area of the tested MEAs were adjusted to 25 cm\u0026sup2;. Prior to electrochemical evaluation, the cells were activated at 80\u0026deg;C with a potential range of 0.4 to 0.8 V at a step rate of 0.1 V. The polarization curve was conducted at 80\u0026deg;C and 100% RH, with a back pressure of 150 kPa, while maintaining the reaction gas flow of H\u003csub\u003e2\u003c/sub\u003e/air (SR 1.5 / 2.0). After calibrating the H\u003csub\u003e2\u003c/sub\u003e crossover current, the mass activity was calculated at 0.9 V from the iR-corrected H\u003csub\u003e2\u003c/sub\u003e-O\u003csub\u003e2\u003c/sub\u003e polarization curve [\u003cspan class=\"CitationRef\"\u003e49\u003c/span\u003e]. To measure ionomer accessibility, the DPA was calculated using the ECSA obtained from CO stripping at 100% and 20% RH (DPA\u0026thinsp;=\u0026thinsp;ESA\u003csub\u003e20\u003c/sub\u003e/ESA\u003csub\u003e100\u003c/sub\u003e). To quantify the sulfonic acid groups adsorbed on the catalyst surface, CO displacement measurement was conducted. The displacement coverage (\u0026theta;\u003csub\u003edis\u003c/sub\u003e) was found using \u0026theta;\u003csub\u003edis\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;2 \u0026times; charge recorded during the adsorption of CO (q\u003csub\u003edis, CO\u003c/sub\u003e)/ electrical charge (q\u003csub\u003estrip, CO\u003c/sub\u003e, generated by the oxidation of CO in the monolayer is 420 \u0026micro;C cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e), two electrons are required to oxidize CO to CO\u003csub\u003e2\u003c/sub\u003e [\u003cspan class=\"CitationRef\"\u003e50\u003c/span\u003e]. 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Soc.\u003c/em\u003e \u003cstrong\u003e164\u003c/strong\u003e, F60-F64 (2017).\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Scheme 1","content":"\u003cp\u003eScheme 1 is available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Polymer Electrolyte Fuel Cell, Perfluorinated Sulfonic Acid Ionomer, Platinum Catalyst, Tetrafluoroethylene Spacing, Platinum/Ionomer Interface, Polymeric Densification, Ionomer Accessibility, Sulfonate Coverage","lastPublishedDoi":"10.21203/rs.3.rs-5891522/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5891522/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003ePolymer electrolyte fuel cells hold great potential for powering heavy-duty vehicles (HDVs) run by clean hydrogen, but a major challenge lies in the ionomer poisoning of scarce platinum (Pt) catalysts, which hinders the Pt utilization and cell efficiency. Here, we report a simple yet effective approach to mitigate polymeric densification at the Pt/ionomer interface, via enlarging tetrafluoroethylene (TFE) spacing between neighboring side chains. Ionomers with weaker confinement to Pt, arising from strengthened hydrophobic interactions, suppress the specific adsorption and lead to less-densified ionomer morphology. Despite having a lower ion-exchange capacity, they exhibited high accessibilities (over 80%) and a significant reduction of 22\u0026ndash;8% in sulfonate coverage, hence resulting in two-fold improvements in activity and local transport towards the oxygen reduction reaction. This strategy offers a key solution to unlock the full potential of Pt, offering seamless integration into current manufacturing processes, thus accelerating the sustainability and scalability of fuel cell technology.\u003c/p\u003e","manuscriptTitle":"Inhibition of polymeric densification at platinum/ionomer interface via enlarging tetrafluoroethylene spacing in perfluorinated sulfonic-acid ionomer","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-26 09:27:58","doi":"10.21203/rs.3.rs-5891522/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"19b7cbbd-1f7a-4bef-84a0-ad4c5e9058c1","owner":[],"postedDate":"February 26th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":44625197,"name":"Physical sciences/Energy science and technology/Fuel cells"},{"id":44625198,"name":"Physical sciences/Chemistry/Electrochemistry/Fuel cells"}],"tags":[],"updatedAt":"2025-03-27T09:10:50+00:00","versionOfRecord":[],"versionCreatedAt":"2025-02-26 09:27:58","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5891522","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5891522","identity":"rs-5891522","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}