Translating insights from experimental analyses with single-crystal electrodes to practically-applicable material development strategies for controlling the Pt/ionomer interface in polymer electrolyte fuel cells

Ionomers are used in polymer electrolyte fuel cells (PEFCs) catalyst layers to improve proton conduction. Recent analytical studies have clarified that the adsorption of the ionomer on the surface of a Pt catalyst deteriorates the catalytic activity for the oxygen reduction reaction and oxygen transport properties near the catalyst surface. These findings have led to the development of new materials, such as mesoporous carbon support and highly oxygen-permeable ionomer, which are now commercially used. In this review article, we summarize recent analytical studies of the Pt/ionomer interface focusing on half-cell experiments with single-crystal electrodes. We also present promising approaches for mitigating ionomer adsorption, as well as the remaining challenges in the application of these approaches to PEFCs.


Introduction
Electrification of transportations is imperative for realizing a sustainable economy and solving the issue of global warming. Polymer electrolyte fuel cells (PEFCs) are crucial devices as power sources for vehicles and have been applied in passenger light-duty vehicles (LDVs) [1]. The initial cost of LDVs with PEFCs is still higher than that of LDVs with internal combustion engines. Therefore, a reduction in both the cell size and platinum usage for the electrode catalysts is required. The performance and durability of the electrodes usually deteriorate with decreasing Pt usage [2]. Thus, improving the intrinsic activity and durability of the catalyst, as well as the mass transport properties of the electrodes, has been an important theme in PEFC research. These properties are, however, subject to trade-off relationships [2] that must be overcome through technical breakthroughs for the widespread use of PEFCs in LDV applications.
Recently, the focus of future PEFC applications has shifted to the class of heavy-duty vehicles (HDVs). In HDV applications, the total cost of ownership is more important than the initial cost [3], and the requirement for reducing Pt usage is alleviated. The acceptable Pt usage, however, depends on the capacity of the Pt supply, the progress of Pt recycling technologies, and the degree of the spread of PEFCs. In addition, the Pt supply capacity is influenced by the Pt reserve and the geographical and geopolitical factors [4]. Therefore, the Pt usage cannot be unconditionally increased. In addition, the total operating time in the lifetime of HDVs is much longer than that of LDVs (e.g. five times [3]); thus, the cell components must be highly durable. Accordingly, significant progress in overcoming the trade-off between performance and durability is still strongly desired in the development of PEFC components for HDVs.
The performance of PEFCs is strongly affected by the catalytic activity and mass transport properties of the cathode, where the oxygen reduction reaction (ORR) occurs ( figure 1(a)). There have been many reports on improving the activity of cathode catalysts. One example is the (111)-facet-preferred shape-controlled Pt-alloy nanocatalysts [5], which were inspired by a model experiment showing the extremely high activity of the Pt 3 Ni (111) surface [6]. These state-of-the-art catalysts suffer from low stability in potential cycles [7].
Doping Pt-Ni nanoparticles with a third element, such as Rh and In [8,9], and forming an intermetallic Pt alloy phase [10] improved the catalyst durability. However, the durability tests in these studies were conducted at room temperature, and further analyses are necessary to test their stability under practical conditions (>60 • C).
Another approach for improving the performance and durability of cathode catalysts is to control the surrounding conditions. In aqueous electrolytes, the ORR kinetics [11,12] and Pt dissolution rate [13] are significantly affected by electrolyte properties, such as ion type, concentration, and temperature. In the cathode of a PEFC, the ORR kinetics and the Pt dissolution rate are expected to be affected by the interfacial properties between the Pt catalyst and ionomer. Therefore, designing an interfacial structure should be effective for improving cell performance and durability.
To understand the phenomena at the interface, experimental studies using Pt single-crystal electrodes are a powerful tool because their well-defined surface structures exhibit clear signals depending on the type of adsorbed species and adsorption sites [14,15]. This method has been successfully applied in the analysis of Pt/ionomer interfaces and the material development for controlling the interfacial properties of PEFC cathodes. The details of these studies are reviewed in the following sections. Figure 1(a) illustrates the microstructure of the cathode catalyst layer, which is composed of catalysts, supports, ionomers and pores. Perfluorinated sulfonic acid polymers (e.g. Nafion) have been used as ionomers because of their high chemical stability and strong acidity. While the sulfate anion (SO 4 2− ) in aqueous sulfuric acid solution was well known to be strongly adsorbed on the surface of Pt [16], the adsorptivity of the sulfonate anions of the ionomers was not clarified before the 2010s. In addition, attention was not paid to the adsorptivity of the sulfonates of the ionomers. This is because one of the low-molecular-weight perfluoro-sulfonic-acid anions, trifluoro methane sulfonic acid anion (CF 3 SO 3 − ), was reported to be non-adsorbing on the surface of Pt [17]. In addition, the access to the Pt surface was expected to be restricted for the sulfonate bound to a polymer chain. Several groups [18][19][20] have pointed out the possibility that the sulfonates of ionomers are adsorptive on the surface of Pt. However, direct evidence, such as voltammetric peaks, was not obtained because the experiments were conducted using polycrystalline Pt electrodes with undefined surface structures.

Fundamental studies of ionomer adsorption on Pt surface
To address this issue, Subbaraman et al [14] coated the surfaces of Pt single-crystal electrodes with a Nafion thin film and examined their electrochemical properties in the configuration of the rotating disk electrode (RDE) ( figure 1(b)). They observed redox peaks in the double-layer region (0.4-0.6 V vs. reversible hydrogen electrode) in the cyclic voltammogram (CV) of the Nafion-coated Pt (111) surface (red curve in figure 1(c)) and assigned these peaks to the adsorption/desorption of anions through the so-called CO-displacement method [21]. Notably, the cleanliness of the experimental system was ensured by the CV profile (e.g. the preservation of the plateau peaks for hydrogen adsorption below 0.4 V after the Nafion coating). Therefore, the redox peaks were attributed to the sulfonate of the ionomer rather than to anionic impurities. Consequently, the sulfonate anions of Nafion were proven to be adsorbed on the Pt surface. In addition, Subbaraman et al [22] observed the suppression of ORR activity on the Pt surface by the Nafion coating.
Kodama et al [23][24][25][26] explored the experimental method and quantitatively analyzed the ionomer effect. They established a method for uniformly coating the entire Pt surface with an ionomer film (figure 2(a)). Uniformity of the ionomer film is crucial for quantifying the ionomer effect without the contribution of bare surface. In addition, they applied a Pt single-crystal electrode that exhibited clear butterfly peaks for the OH formation/reduction between 0.6 and 0.85 V (black dashed curve in figure 2(b)), and thus, is defect-free. The following information was obtained in the experiment wherein the defect-free Pt (111) surface was uniformly coated with Nafion film. The anion adsorption/desorption peaks of the CV were sharper and larger, and the suppression of the OH formation/reduction butterfly peaks by the Nafion-coating was more significant in the study by Kodama et al [24][25][26] (figure 2(b)) than in the study by Subbaraman et al [14,22] (figure 1(c)). These observations indicate that the uniformity of the Nafion thin film on the Pt surface might be insufficient in the study by Subbaraman et al [14,22]; thus, the Nafion effects might be underestimated. In the study with the uniform Nafion thin film by Kodama et al [24], the sulfonate coverage was estimated as 0.09 monolayers from the electric charge of the anion adsorption/desorption peaks in the CV (figure 2(b)) by assuming a one-electron-transfer process for sulfonate adsorption. This result indicates that the ratio of the number of Pt atoms masked by sulfonate group atoms to that of all Pt atoms at the electrode surface is smaller than 30%. For example, even if the sulfonate moiety was adsorbed onto three Pt atoms through three oxygen atoms, the ratio is 27%. However, the ORR activity measurement (figure 2(c)) revealed more  [2], with permission from Springer Nature). (b) Approach for analyzing the Pt/ionomer interface in PEFCs with the model system using an ionomer-coated Pt single-crystal electrode established by Subbaraman et al [14] (c) CVs for Pt (111) surface coated with Nafion thin film (red solid curve) and bare Pt (111) surface in perchloric acid (black dashed curve) and in sulfuric acid (green dashed curve). The Nafion-coated surface in perchloric acid exhibits redox peaks in the double layer region (0.4-0.6 V) like the bare surface in sulfuric acid, where sulfate anion is known to be adsorbed on the Pt surface (Adapted with permission from [14]. Copyright (2010) American Chemical Society). significant ORR suppressions: 84% at 0.82 V and 60% at 0.90 V (i.e. only less than 50% of active sites are available for ORR).
The origin of the gap between the sulfonate coverage and ORR suppression degree was investigated from a molecular-scale viewpoint [24]. Cyclic voltammetry was conducted on a bare Pt (111) electrode in electrolytes consisting of low-molecular-weight compounds of perfluoro (2-ethoxyethane) sulfonic acid and nonafluorobutane sulfonic acid, which are characterized by the presence and absence of an ether group, respectively (figure 3(a)). The results indicated that the adsorptivity was higher for the perfluoro-sulfonate with an ether group than for that without an ether group, as shown by the larger anion peaks and lower peak potentials for the former. The adsorption states were further analyzed using surface-enhanced infrared absorption spectroscopy (SEIRAS) [27] with a polycrystalline Pt film as the working electrode. In SEIRAS, there is a surface selection rule, in which only vibrational modes perpendicular to the surface are observable. Information on the molecular orientation was obtained from the surface selection rule and the observed absorption bands. As shown in figure 3(b), the asymmetric C-O-C band in the SEIRA spectrum at 0.7 V is weaker than the corresponding band in the bulk solution spectrum. According to the surface selection rule, the weak asymmetric C-O-C band indicates a parallel orientation of the perfluoro-sulfonic acid anion to the Pt surface. This orientation is presumably caused by the interaction between the ether group and Pt surface  [26]. Copyright © 2018 Toyota Central R&D Labs., Inc. All rights reserved) and (c) ORR polarization curves for the defect-free Pt (111) surface uniformly coated with a Nafion thin film and a bare Pt (111) surface (Adapted from [25]. CC BY 4.0. © 2022 The Authors. Electrochemical Science Advances published by Wiley-VCH GmbH and Adapted with permission from [26]. Copyright © 2018 Toyota Central R&D Labs., Inc. All rights reserved). (b), (c) Adapted from [25]. CC BY 4.0. © 2022 The Authors. Electrochemical Science Advances published by Wiley-VCH GmbH and Adapted with permission from [26]. Copyright © 2018 Toyota Central R&D Labs., Inc. All rights reserved.
(figure 3(c)). This adsorption mechanism can explain the origin of the significant ORR suppression by the ionomer coating on the Pt surface as well as the gap between the suppression degree and sulfonate coverage discussed above, that is, not only the sulfonate moiety but also the perfluoro-alkyl part of the side chain blocks the ORR sites.
In addition to Pt (111) surface, Subbaraman et al [22] analyzed the effects of Nafion coating on other low-index facets ((100) and (110)) and reported that ORR is also suppressed on these low-index facets. Kodama et al [25] studied the effect of (111) terrace width on Nafion adsorption using stepped Pt single-crystal electrodes with the (111)-terrace atomic width of n = 2-9 (n is the number of atoms). As shown in figure 4(a), the sulfonate adsorption on the stepped surfaces appears in the form of (a) anion adsorption/desorption peaks in the double-layer region (e.g. on Pt (554)) or (b) in the hydrogen adsorption/desorption region (e.g. on Pt (331)), or (c) the suppression of the hydrogen adsorption/desorption and oxide formation/reduction peaks without clear anion-induced peaks (e.g. Pt (221)). The third form is a typical sign of anion adsorption on Pt electrodes in acidic electrolytes [28]. Figure 4(b) shows the ORR activities for the bare and Nafion-coated stepped Pt single-crystal electrodes as a function of the step density. The results indicate that the activities with the Nafion thin film are much lower than those without it while both series exhibit a volcano-like trend. Figure 4(c) shows the ratio of the activity for the Nafion-coated surface to that for the corresponding bare surface as a function of the step density. The results indicate that the Nafion coating suppresses the ORR activity by more than 50%, regardless of the terrace width. The effects of a Nafion coating on Pt nanoparticles supported on carbon (Pt/C) were studied using the RDE method by Shinozaki et al [29]. Figure 5(a) shows the normalized specific activity for the Nafion-coated Pt/C catalyst with the supports of high-surface-area carbon (HSC), the solid-core carbon of Vulcan, or low-surface-area solid-core carbon (LSC) to the corresponding Nafion-free Pt/C catalyst as a function of the weight ratio of the ionomer to the carbon support (I/C). The results indicate that the activity is suppressed with increasing I/C ratio because of ionomer adsorption on the Pt catalysts. The degree of ORR suppression is less significant for the Pt catalyst supported on the HSC probably because some of the Pt nanoparticles located in the pores of the HSC are not in contact with the ionomer. The ORR activity for the Pt catalysts on the solid-core carbon supports (Vulcan and LSC), where a majority of the Pt nanoparticles are expected to be in contact with the ionomer, was significantly suppressed (40%-60%) by the Nafion film at high I/Cs. In summary, ORR activity suppression on the surface of Pt by Nafion coating is universal for all surface morphologies.
As discussed above, the significant effects of Nafion-coating on Pt catalysts have been clarified with RDE experiments, where the ionomer-coated electrode surface is immersed in an aqueous electrolyte; therefore, the ionomer is fully hydrated. In PEFCs, however, only solid electrolytes are used and the ionomer is not necessarily fully hydrated. For example, a humidifier is not used in the LDV system of the Toyota Mirai fuel cell vehicle [1]. Although ionomer and membrane drying can be alleviated by internal humidification system [1], the ionomer is not fully hydrated in some cases, depending on the operating mode. In addition, the ionomer tends to be dehydrated in HDV applications, where the fuel cell stack can be operated at high temperatures (>90 • C) [3]. Thus, clarifying the effects of the ionomer coating on the Pt surface in non-fully hydrated states is important. Ionomer adsorption without an aqueous electrolyte has been studied with model systems and membrane-electrode-assembly (MEA) configurations. Kodama et al [30] devised a solid-state cell for Pt single-crystal electrodes, where a Nafion-coated Pt single-crystal electrode was pressed onto a Nafion membrane instead of being immersed in an aqueous electrolyte so that its CV could be measured under non-fully humidified conditions ( figure 5(b)). The adsorption/desorption peaks of sulfonate anions on the Pt (111) surface shifted to lower potentials with the dehydration of the ionomer (figure 5(c)) and to higher potentials with the rehydration of the ionomer ( figure 5(d)). This observation indicates that the adsorptivity of sulfonate increases with ionomer dehydration. The same trend was observed in subsequent MEA tests [31,32]. Temperature is another important factor that affects ionomer adsorption. Nishikawa et al [33] demonstrated that ORR suppression by sulfate anions is alleviated with increasing temperature, and the same trend is expected to be applied to anionic moieties of ionomers. Thus, the combined effects of humidity and temperature on the anion adsorptivity of ionomers must be considered to understand the PEFC properties.

Ionomer effects in practical systems
The effects of ionomers on Pt nanoparticles in MEAs have been studied extensively. Ikeda et al [34] established a voltammetric method using a fluorocarbon fluid to measure the ionomer coverage on Pt/C catalysts. They found that the ionomer coverage is lower for a Pt catalyst with the porous carbon support of Ketjenblack than that with the solid-core carbon support of Vulcan. Iden et al [35] applied electrochemical impedance spectroscopy and reported that the ionomer coverage of Pt catalysts is lower on Ketjenblack than on less porous graphitized Ketjenblack. Garrick et al [36] applied the CO displacement method [21] to measure the sulfonate anion coverage on a cathode Pt catalyst and demonstrated that the anion coverage is lower for Pt catalyst with porous carbon support than for that with solid-core carbon support. However, these studies still need to clarify the relationship between the coverage and ORR activity of the cathode. Subsequently, Takeshita et al [37] applied CO stripping voltammetry combined with the use of a fluorocarbon fluid and examined the relationship between the ionomer (Nafion) coverage and ORR activity of Pt catalysts with various carbon supports in the MEA configuration. The results indicated that the activity increased with decreasing Nafion coverage (figure 6(a)). They also quantified the ratio of the ORR activity of the Pt catalyst that was completely covered by a Nafion film to that of a bare Pt catalyst by extrapolating the data with a straight line ( figure 6(a)). It was found that the degree of ORR suppression by the Nafion film was significant, as in the case of the RDE. Lower ionomer coverages and higher ORR activities were achieved with porous carbons, and their origins are discussed later in the present article.
The ionomer effect on oxygen transport is a crucial topic in the development of PEFCs with low Pt loadings; thus, it has been intensively studied mainly by industrial research groups. Initially, low Pt loadings were anticipated to only cause an increase in the activation overpotential, which appears as a uniform voltage drop over the entire current density region [40]. However, an unexpected concentration overpotential appeared in the high current density region in addition to the activation overpotential because of the increased oxygen transport resistance [41]. Greszler et al [42] found that the oxygen transport resistance in a cathode catalyst layer is nearly inversely proportional to the catalyst surface area of the cathode. This result indicates that the additional resistance stems from the ionomer that covers the Pt surface. Electrochemical analyses using subscale cells [43][44][45] and microelectrode half-cells [46,47] narrowed the location of the resistance to the Pt/ionomer or ionomer/gas interface. Through molecular dynamics (MD) simulations, Jinnouchi et al [38] revealed that oxygen transport is inhibited by a dense ionomer layer thinner than 0.5 nm formed near the Pt surface ( figure 6(b)). The presence of Pt/ionomer interfacial resistance requires increasing the specific surface area of the cathode catalyst in addition to increasing the mass activity to achieve high cell performance with low Pt loadings [48]. It is a big challenge to satisfy this requirement while improving the durability of the catalyst, which usually deteriorates with increasing the specific surface area [49].
Owing to the presence of a thin resistive layer near the Pt surface, not only the catalytic activity but also the oxygen transport can be affected by the adsorption of the sulfonate moieties. If the length scale of the resistive-layer thickness is much longer than that of the area covered by the adsorbate, the diffusion-limited flux is independent of the surface coverage of the adsorbate. This is because the concentration distribution is almost completely determined by the thick resistive layer. However, as discussed above, the thickness of the resistive layer is only 0.5 nm, which is possibly comparable to the length scale of the adsorbate area. In this case, the flux can be influenced by the geometrical pattern of the adsorbates. The effect of adsorbates on the diffusion-limited oxygen flux near the Pt surface was examined by Suzuki et al [39] using a local diffusion model. Figure 6(c) shows the effectiveness factor of the flux to the Pt surface, which is defined as the ratio of the flux with adsorbate to that without adsorbate, as a function of the ratio of the width of the masking area (w) to the resistive layer thickness (t), calculated for an adsorbate coverage of 0.5, using the local diffusion model. The result indicates that the effectiveness factor significantly decreases when the width of the area masked by an adsorbate is comparable to or longer than the thickness of the resistive layer. The width of the  [37], Copyright (2020), with permission from Elsevier). (b) Snapshot of the Pt/ionomer interface (top) and the ionomer density distribution and free energy profile of O2 molecules (bottom) obtained from a molecular dynamics simulation [38] (Adapted from [2], with permission from Springer Nature). (c) Effectiveness factor of the flux to the Pt surface (H), defined as the ratio of the flux with adsorbates to that without adsorbate, as a function of the ratio of the width of the masking area (w) to the resistive layer thickness (t) calculated for the adsorbate coverage of 0.5 by using the local diffusion model described in [39]. (d) Potential-dependency of the oxygen transport resistance near Pt surface (R other ) at the Pt loading of 0.238 mg · cm −2 experimentally obtained with a subscale cell. (Reproduced from [39]. © IOP Publishing Ltd.CC BY 3.0). masking area is presumably ca. 1 nm in length for the adsorption of the sulfonate anion of the ionomer [24] since the perfluoro-alkyl part of the side chain also masks the Pt surface, as discussed above ( figure 3(c)). This width is greater than the thickness of the resistive layer (<0.5 nm) [38]. Hence, oxygen flux can be significantly reduced by sulfonate adsorption. Suzuki et al [39], indeed, experimentally observed that oxygen transport resistance near the Pt surface (R other ), which is proportional to the inverse of the diffusion-limited flux, increased with increasing the potential from 0.3 V and was maximized in the potential range of 0.6-0.7 V (figure 6(d)), where the sulfonate coverage is also expected to be maximized [24]. The agreement between the model and experiment supports the presence of the resistive layer with a sub-nanometer thickness predicted by the MD simulation [38]. The observed increase in R other at a Pt loading of 0.238 mg cm −2 , ca. 12 s m −1 (figure 6(d)), is also practically significant regarding the cell performance. For example, Yamada et al [50] observed the cell voltage drop of ca. 30 mV in a middle current density region (ca. 1.5 A cm −2 ) when R other was increased by 12 s m −1 .

Strategies for mitigating ionomer adsorption
As discussed above, the adsorption of the ionomer on the Pt catalyst surface in a PEFC can significantly deteriorate both the catalytic activity and power density of the cell. Mitigating these negative effects is one of the key technologies for developing PEFCs. In this area, relevant new materials, highly oxygen-permeable ionomer (HOPI) and mesoporous carbon support have been commercially applied in the new MIRAI from Toyota [51]. In the following paragraphs, the effectiveness of these materials and future options are discussed.  [26]. Copyright © 2018 Toyota Central R&D Labs., Inc. All rights reserved and Adapted from [52]. CC BY 4.0.
A straightforward approach to mitigate ionomer adsorption on a Pt surface is to modify the molecular structure of the ionomer. The anion adsorptivity and ORR activity of various ionomers were examined using a Pt (111) electrode [23,24,26,52]. The results are summarized in figure 7. The sulfonimide ionomer of NBC4, which has two sulfonimide acid groups terminated by a perfluoro-alkyl chain in the side chain, exhibits broader and weaker peaks in the double-layer region of the CV (figure 7(b)) and higher ORR activity (figure 7(c)) than Nafion. Therefore, an ionomer with bulky anionic moieties with a delocalized charge distribution and the termination by a perfluoro-alkyl chain is effective for mitigating ionomer adsorption on the surface of Pt [23]. Interestingly, Aquivion exhibits lower anion adsorptivity and higher ORR activity than Nafion as shown in figures 7(b) and (c) although both ionomers have a terminal sulfonic acid group in the side chain. This result can be explained by the SEIRAS results ( figure 3). Nafion has two ether groups at the root and midpoint of its side chain, which facilitate sulfonate adsorption on the Pt surface via the interaction between the surface and ether at the midpoint without causing significant strain on the main chain ( figure 8(a)). In contrast, Aquivion has only one ether group at the root of its side chain. Hence, the sulfonate adsorption via the ether-mediated interaction requires a strain of the main chain in this ionomer ( figure 8(b)). Consequently, the adsorptivity of sulfonate was weaker for Aquivion than for Nafion [24]. These adsorption mechanisms suggest that the adsorption of sulfonate is further mitigated by applying ionomers whose main chains are rigid and less subject to strain. Incorporating cyclic ring structures is one such approach. By the incorporation, the main chain loses the flexibility to allow adsorption of the sulfonate group. The so-called HOPIs have such main-chain structures. An example of a HOPI is shown in figure 7(a). The results of the electrochemical measurements with Pt (111) indicate that the anion adsorptivity is weaker, and the ORR activity is higher for the HOPI than for Aquivion as shown in figures 7(b) and (c) [52]. Thus, ionomers with less flexible molecular structures are effective for improving the activity of Pt catalysts in PEFCs. The high activity of the HOPI was also realized in an MEA with a real Pt nanoparticle catalyst. The activity of Pt/C by HOPI was 1.4-2.0 times higher than that of Nafion, and the cell voltage in the low-medium current density region by HOPI was up to about 30 mV higher than that of Nafion [52]. The agreement between the trend in the experiment with the single-crystal electrode and that in the MEA experiment demonstrates the applicability of analytical works using the model surface for developing practical PEFCs. It should also be noted that the HOPI incorporating the cyclic ring structure mitigates the dense ionomer resistive layer, significantly reducing the interfacial oxygen permeation resistance [52]. Accordingly, the suppression of the flexibility of the main chain enhances oxygen transport as well as catalytic activity.
In addition to modifying the molecular structure of the ionomer, the application of mesoporous carbon supports can improve both catalytic activity and oxygen transport properties. Transmission electron microscopy observations indicated that a large portion of the Pt nanoparticles are located in the pores of the porous carbon particles [53]. Electrochemical measurements [37] and MD simulations [54] indicated that these Pt particles are not covered by ionomers because the ionomer molecules do not easily penetrate the pores. In addition, protons can access the Pt nanoparticles in the pore through liquid water without an ionomer [55]. Consequently, the specific activity of Pt catalyst with porous carbon supports is higher than that with solid-core carbon supports, as shown in figure 6(a), and the accessibility of oxygen to the Pt particles in porous carbons is highly enhanced [56]. Yarlagadda et al [56] reported an optimized pore size range of 4-7 nm to improve both the activity and mass transport properties. In addition to pore size, the location of Pt nanoparticles in the pores [57], uniformity of the ionomer films outside the carbon particles [57,58], and continuity of the pores [59] are important for improving the proton and oxygen accessibility to Pt catalysts on porous carbons. Remarkable negative effects have not been reported in the application of mesoporous carbons, and this option will be a key technology in the development of PEFCs, at least in the near future.
Ionomer-induced ORR suppression can also be mitigated by modifying the catalyst surface with foreign materials. Yamada et al [50,60] applied a dopamine coating on a Pt catalyst surface followed by heat treatment to form a thin carbon layer [61] and carefully examined the effects of the thin carbon layer on the catalyst properties. Figure 9(a) shows that the sulfonate coverage on the Pt/C catalyst decreased, and the mass activity increased with an increase in the amount of dopamine. In addition, the modification improved the catalyst durability, as shown by the higher retention ratio of the electrochemical Pt surface area (ECSA) throughout the potential cycle durability test for the heat-treated dopamine-modified catalyst than for the heat-treated and non-heat-treated unmodified catalysts ( figure 9(b)). However, the oxygen transport resistance near the catalyst surface was increased by the modification because the thin carbon layer was highly resistant to oxygen transport. Therefore, countermeasures are still necessary to apply this concept to PEFCs that require high power densities.
Reducing coverage of modifiers while maintaining the mitigation effect against ionomer adsorption can solve this issue. Anion adsorption can be mitigated by partially covering the Pt surface with cyanide [62], hydrophobic cation [63], or ionic liquid (IL) [25,64]. Figure 9(c) shows the CVs of the Nafion-coated Pt (443) surface pre-modified with the IL of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide before and after ORR activity measurements [25]. The anion adsorption/desorption peaks on (111) terrace sites at 0.5 V and hydrogen adsorption/desorption peaks on (110) step sites at 0.12 V disappeared by the IL pre-modification. These results indicate that the contact of the ionomer with the Pt surface was mitigated by the IL molecules selectively adsorbed on the step sites, as depicted in figure 9(d). This effect can also be beneficial for increasing the intrinsic catalytic activity and preventing the dissolution of low-coordinated Pt atoms at the edges and corners, which have lower dissolution potentials than the terrace Pt atoms [65,66]. However, figure 9(c) indicates that the IL molecules were desorbed, and the Pt surface re-contacted the ionomer film during the ORR activity measurements, as indicated by the re-appearance of the hydrogen adsorption/desorption peaks on the (110) steps and anion adsorption/desorption peaks on the (111) terraces. Therefore, countermeasures against IL leaching during the long-term operation of PEFCs are still necessary. In another unique approach, Pt nanoparticles were masked with alkanethiol molecules before ink preparation, and the alkanethiol molecules were electrochemically removed after MEA fabrication [67]. The resulting Pt catalyst exhibited a low population of ionomers on the Pt surfaces and a higher power density due to an improved oxygen transport property near the catalyst surfaces. Although the process cost can increase with the additional masking and removal steps, nanoscale engineering for optimizing the ionomer distribution near the catalyst surfaces can become a key strategy for improving cell performance.

Conclusion
The properties of the Pt/ionomer interface in the cathode catalyst layer of PEFCs are now recognized to significantly affect cell performance. In clarifying the ionomer effects on the ORR, experimental analyses with Pt single-crystal electrodes have played a crucial role, showing significant ORR activity suppressions by the ionomer coating on the Pt surface. In this review article, we summarized these analytical studies and demonstrated the linkage between the results of the model electrodes and real nanocatalysts in MEAs. The MEA results were reasonably explained by the results obtained using the model electrodes. In addition, the negative effects of the adsorption of anionic moieties in the ionomer on the oxygen transport properties in MEAs were discussed from a molecular-scale perspective. We also summarized the material development strategies for controlling the Pt/ionomer interface in practical PEFCs by reviewing recently reported studies. Modifying the molecular structure of the ionomer and applying mesoporous carbon supports have been demonstrated to be effective in improving activity and oxygen transport properties. These approaches have been commercially applied. Further technical breakthroughs are, however, necessary for wider applications of PEFCs. Promising future options such as surface modification of Pt catalysts by foreign materials have been reported. Although some technical issues, such as the negative effect on mass transport or the low stability of the modifier itself, need to be solved, these approaches will be key technologies in future PEFCs.

Data availability statement
The data that support the findings of this study are available upon reasonable request from the authors.