3.1 Surface design of Pt-based catalysts

The morphology, crystal facet, surface electronic structure, and particle size of Pt‑based catalysts directly determine the adsorption strength, spatial distribution, and conformation of ionomers on the catalyst surface, which further dominate the formation, uniformity, and mass‑transport properties of the ionomer coating. Therefore, understanding the structural characteristics of catalysts with different crystal morphologies and their synthesis routes is not only a prerequisite for regulating catalyst–ionomer interfacial interactions but also a core basis for realizing precise ionomer coating engineering. This section focuses on the structural design and controllable synthesis of Pt‑based catalysts, aiming to clarify the relationship between catalyst surface characteristics and ionomer adsorption behavior, so as to provide material design principles for the subsequent regulation of coating structure and performance optimization.

The morphology of nano catalysts significantly influences their adsorption behavior toward ionomers, primarily through factors such as crystallographic facet type, defect density, and surface curvature [57]. Isotropic nanospheres, with their complex facet composition and high density of defect sites, often induce strong and heterogeneous adsorption of ionomers [58]. In contrast, anisotropic nanorods exhibit preferential facet exposure: their side surfaces are typically composed of strongly adsorbing facets, while the ends consist of weakly adsorbing facets, resulting in distinct direction-dependent adsorption behavior [59]. One-dimensional nanowires, characterized by continuous atomic ordering and low defect density, facilitate more uniform ionomer adsorption and promote the formation of efficient mass transport channels [60, 61]. Among shape-controlled polyhedral nanocrystals, cubes are predominantly enclosed by strongly adsorbing facets, whereas octahedra and icosahedra are dominated by weakly adsorbing facets [62]. Truncated polyhedra allow precise tuning of facet ratios to optimize adsorption strength. Therefore, rational catalyst design—favoring the exposure of weakly adsorbing facets and reducing highly active defect sites—represents a key strategy for enhancing resistance to ionomer poisoning.

Significant progress has been made in the design and synthesis of Pt-based alloy catalysts with various morphologies, such as nanocubes, nanowires, and polyhedra, aimed at optimizing their electrocatalytic activity and stability [63]. However, compared to the systematic investigation into the relationship between morphology and catalytic performance, studies on the adsorption behavior between these alloy catalyst surfaces and ionomers, along with the underlying mechanisms, remain insufficient. Most research focuses on the enhancement of ORR performance through alloy composition, while overlooking issues such as interfacial poisoning and mass transport limitations caused by ionomer molecules under realistic catalytic conditions. In particular, there is still a lack of systematic understanding and experimental validation—from the atomic level to macroscopic performance—regarding how the surface characteristics of differently shaped alloys influence the strength, distribution, and structure–property relationships of ionomer adsorption. Therefore, further research into the correlation between the morphology, electronic structure, and anti-adsorption capability of alloy catalysts toward ionomers is of crucial scientific and practical importance for the design of high-performance, poisoning-resistant fuel cell electrode materials.

Previous studies have extensively explored the structural design of Pt-based catalysts. Based on first-principles calculations, researchers have systematically investigated the segregation energy (E_seg) of solute atoms migrating from the bulk to the surface in various heavy-metal/transition-metal alloy systems. A correlation has been established between E_seg and the difference in hydrogen binding energy (Δ|BE_H| = |BE_H^sol| – |BE_H^host|) between pure solute metals and pure host metals, thereby revealing the manipulation mechanisms underlying the structural stability of near-surface alloys (NSAs) in hydrogen environments. As shown in Figure 3A, when Δ|BE_H| > 0, the solute metal exhibits stronger hydrogen adsorption than the host metal; otherwise, it is weaker [64]. This parameter reflects the modification of the surface electronic structure by solute elements and is of key importance for understanding the catalytic performance of alloys. Furthermore, by analyzing the coupling between the transition state energy (ETS) and the hydrogen binding energy during H₂ dissociation on pure noble metal and NSA surfaces, studies have linked kinetic energy barriers to thermodynamic adsorption strength, establishing a descriptor framework for catalytic activity (Figure 3B). This relationship provides an important basis for the rational design of high-performance catalysts: optimal catalytic performance typically occurs near the peak of the volcano curve, where moderate hydrogen binding energy and low reaction barriers are achieved. Research has also shown that the ORR activity of Pt₃M alloys is closely related to the d-band center position of their surfaces, a trend observed for both Pt-skin and Pt-skeleton surface structures (Figure 3C) [65]. Through combined experimental and theoretical approaches, a curve has been constructed demonstrating how the Pt–Pt distance systematically influences the specific activity of the catalysts (Figure 3D) [23]. Building on this in-depth understanding of the structure–performance relationships in Pt-based catalysts, numerous researchers have employed controlled synthesis methods to realize these desired structural features.

Figure 3 Design of Pt-based alloy structure catalyst. (A) The effect of hydrogen-induced segregation on the stability of near-surface alloys (NSAs). Alloy systems are represented as solute/matrix element pairs. The x-axis E_seg denotes the energy required for solute atoms to migrate from the matrix interior to the surface, while the y-axis Δ|BE_H| represents the absolute difference in hydrogen binding energy between the solute (|BE_H^sol|) and matrix (|BE_H^host|) close-packed surfaces (θ_H = 1/4 monolayer). The shaded regions indicate predicted ranges of hydrogen-induced segregation, with * marking surface overlayer structures and others representing subsurface alloys (see schematic in Panel A). The color scheme corresponds to different matrix metal categories and remains consistent in subsequent figures. (B) Correlation analysis between transition-state energy (E^TS) and hydrogen binding energy (BE_H) for H₂ dissociation (noble metals and near-surface alloy systems). Relationships between the catalytic properties and electronic structure of Pt₃M alloys. (C) Pt-skin and (D) Relationship between the Pt–Pt nearest neighbor distance and specific activity

In recent years, a variety of synthesis strategies have been successfully developed for the controlled morphology and composition of platinum-based and non-precious metal catalysts. Huang et al. [66] employed a surfactant-free and external magnetic field-free hydrothermal method to precisely control the composition of Pt alloy nanoparticles by adjusting the molar ratio of platinum and palladium precursors, successfully obtaining tetrahedral crystals enriched with Pt (111) facets (Figure 4A). Zhang et al. [67] synthesized well-defined platinum nanocubes via thermal decomposition in an organic phase, combined with high-temperature annealing to effectively remove surfactants. By varying precursor addition methods (co-dissolution vs. stepwise dripping), heating rate, and reaction time, extending this approach to the preparation of Pt–Co, Pt–Fe, and Pt–Ni alloy cubes (Figure 4B). Kong et al. [68] developed a solvothermal morphology-control strategy using I⁻ as a shape-directing agent. By tuning the metal precursor ratio, they successfully synthesized composition-tunable octahedral PtCu-based binary and ternary alloy nanocrystals (Figure 4C).

Figure 4 The relationship between the morphology and particle size of active crystal stacking and its activity. (A) The (111) facets of the Pt alloy Nanotetrahedras (NTDs). Reproduced with permission from Ref. [66]. Copyright 2019, ACS. (B) High-magnification transmission electron microscopy (TEM) images of the nanocubes. Reproduced with permission from Ref. [67]. Copyright 2015, ACS. (C) TEM images of the as-prepared octahedral Pt nanocrystals. Reproduced with permission from Ref. [68]. Copyright 2020, ACS. (D) High-resolution TEM image of a truncated octahedron showing the (111) lattice. Reproduced with permission from Ref. [69]. Copyright 2021, ACS. (E) Bright-field STEM images for a single nanocrystal. Reproduced with permission from Ref. [70]. Copyright 2016, ACS. (F) TEM image of networked Pt_5.7Pb NWs. Reproduced with permission from Ref. [71]. Copyright 2016, ACS. (G) TEM images of the Pt nanostructures. Reproduced with permission from Ref. [72]. Copyright 2008, ACS. (H) TEM image of the Fe/N–C catalyst. Reproduced with permission from Ref. [73]. Copyright 2022, ACS. (I) Correlation between specific activity (SA) and fraction of surface terrace atom%, and (J) mass activity (MA) at 0.9 V vs. RHE in 0.1 M HClO₄

Wu et al. [69] proposed a mild surface treatment process using n-butylamine at room temperature to simultaneously achieve ligand exchange and carbon support loading, avoiding structural damage from high-temperature treatment. This method directly converted morphology-specific Pt₃Ni nanocrystals into practical catalysts ready for electrocatalytic testing (Figure 4D). Wang et al. [70] reported a template-based synthesis of hollow platinum nanoframes: slow injection of a platinum precursor enabled atomically smooth epitaxial growth to form a uniform Pt shell, followed by oxidative etching to selectively remove the Pd template, ultimately yielding high-surface-area platinum nanoframes that retained an icosahedral morphology (Figure 4E). Feng et al. [71] utilized a solvothermal system with Polyvinylpyrrolidone (PVP) and citric acid as modulating agents. By precisely controlling the reaction temperature, they successfully prepared composition-tunable networked Pt alloy nanowires (Figure 4F). Feng et al. [72] employed a seed-mediated growth method to fabricate Pt nanorods (Figure 4G). Wang et al. [73] found that optimizing the acid leaching process effectively reduces iron impurities in FeNC catalysts, thereby mitigating degradation induced by Fenton reactions. This treatment resulted in an approximate doubling of fuel cell activity and a significant enhancement in stability, although improvements were not evident in half-cell tests. (Figure 4H). Shinozaki et al. [74] systematically investigated the effect of platinum nanocrystal size on electrocatalytic specific activity and mass activity, providing a theoretical basis for particle size optimization (Figure 4I, J).

Jiang et al. [32] employed first-principles calculations to investigate the changes in reaction kinetics of Pt facets before and after adsorption of the ionomer side chain −SO₃⁻. The study reveals that the transition of the sulfonic acid group from an adsorbed to a non-adsorbed state triggers abrupt changes in both the system energy and the side-chain conformation, leading to a classification of the Pt/ionomer interface into contact and non-contact structures (Figure 5A). When −SO₃⁻ is in contact with the Pt surface, it causes a downward shift in the d-band center of Pt, weakens the interfacial electric field, and disrupts the hydrogen-bond network, thereby exerting a blocking effect on the ORR, and a loss of mass activity (Figure 5B). Huang et al. [75] used density functional theory to calculate the effect of transition-metal doping on the adsorption energy of Pt(111). Their results indicate that doping with transition metals lowers the adsorption energy of oxygen-adsorption sites on the surface, thereby enhancing the desorption rate of reaction intermediates (Figure 5C). These findings demonstrate that polar oxygen-containing groups in the ionomer backbone can spontaneously adsorb onto Pt facets, and that modifying the interfacial surface energy of Pt can regulate such adsorption behavior. Yoshimune et al. [76] applied small-angle scattering to examine the influence of Pt density on the carbon support surface on the ionomer adsorption layer. They observed that increasing Pt density thickens the ionomer adsorption layer but reduces its density, indicating that specific adsorption between the ionomer and Pt increases the layer thickness while altering the orientation of the side chains, which in turn affects the layer density. Furthermore, by modifying the surface with nitrogen-based groups, they found that the dense ionomer adsorption layer thickened from 1 to 3 nm and exhibited increased density compared to the unmodified interface [77]. Collectively, these studies highlight that the specific adsorption of the ionomer side-chain −SO₃⁻ group is a key factor governing the characteristics of the adsorption layer.

Figure 5 The influence of sulfonic acid group adsorption from ionomer side chains on Pt surfaces on reaction kinetics. (A) The ΔE during the separation process of the S atom (Nafion) from the Pt atom. (B) The half-wave potential in the Linear Sweep Voltammetry (LSV) curves before and after the S atom moves away from the Pt particles. (C) Oxygen adsorption site binding energy on the Pt₃Ni (111) facet and the Mo‑doped Pt₃Ni (111) facet

In addition to transition-metal-doped alloy catalysts, high-entropy catalysts offer distinct local coordination environments and electronic structures, enabling modulation of the adsorption configurations of ionomer functional groups, thereby synergistically enhancing selective ion transport and proton conduction at electrochemical interfaces. Chen et al. [78] designed a high-entropy catalyst via density functional theory (DFT) calculations that circumvents the limitations imposed by the Sabatier principle. By adjusting the Pt concentration within the alloy, the adsorption energy on the alloy surface was effectively tuned (Figure 6A, B). Batchelor et al. [79] applied DFT to compute the adsorption energies of oxygen-containing intermediates on high-entropy alloys with varied compositions. Their results demonstrated that altering the types of transition metals in high-entropy alloys modifies the surface adsorption energy, indicating the potential to regulate ionomer adsorption behavior (Figure 6C, D).

Figure 6 Changes in adsorption energy of high entropy alloys and carbon shell catalysts. (A) Schematic diagram of high entropy alloy designs. (B) Adsorption energies (ΔE_H*) for high-entropy alloy catalysts with varying Pt concentrations. (C) Design schematic diagram of high entropy alloy with heavy metal atoms. Reproduced with permission from Ref. [80]. Copyright 2021, ACS. (D) Adsorption energy of different atoms for oxygen-containing functional groups (* OH). (E) TEM images of catalysts with different carbon shell thicknesses. (F) Charge density modifications induced by −NH₂-functionalized carbon shells. Reproduced with permission from Ref. [81]. Copyright 2024, ACS

Carbon-shell catalysts, with their tunable shell structures and electronic properties, enable stable encapsulation of active sites and precise control of the reaction microenvironment. While improving catalytic stability and selectivity, carbon shells effectively suppress sintering, leaching, and poisoning of the metal components. The presence of carbon shells also inhibits specific adsorption between the ionomer side chain −SO₃⁻ and the crystal facets. Kim et al. [80] synthesized Pt-based catalysts coated with carbon shells of varying thicknesses using organic ligands, suggesting that carbon shells can be utilized to mitigate −SO₃⁻–facet specific adsorption (Figure 6E). Wan et al. [81] performed DFT calculations to examine the charge density on carbon-shell-coated catalyst surfaces. Their findings reveal a significant enhancement in surface charge density for catalysts with −NH₂-functionalized carbon shells. The chemisorption between −NH₂ and −SO₃⁻ further provides an effective strategy for tailoring ionomer adsorption behavior (Figure 6F).

In summary, the performance of the catalyst–ionomer interface in proton exchange membrane fuel cells hinges on regulating the adsorption behavior of sulfonate groups on the catalyst surface. By modifying catalyst morphology, size, alloy composition, and even introducing high-entropy alloys or carbon-shell structures, the surface adsorption energy can be systematically tuned, thereby shifting sulfonate groups from excessively strong chemisorption toward a dynamically moderate adsorption state that facilitates proton conduction. Combined with density functional theory calculations, the influence of catalyst parameters on adsorption energy can be quantitatively elucidated, providing theoretical guidance and a design basis for the rational engineering of highly efficient and stable catalyst–ionomer composite interfaces and for optimizing mass transport within the triple-phase boundary region.

3.2 Design and optimization of carbon carrier structure

The physicochemical properties of carbon supports play a critical role in modulating the CBIL [82]. The physicochemical properties of carbon supports—including morphology [83], pore structure [84], specific surface area [85], graphitization degree [86], and surface functional groups—critically modulate the adsorption behavior of the ionomer and the interfacial mass transport in the catalyst layer. [87–89] High specific surface area provides more adsorption sites, while pore architecture affects ionomer accessibility and reactant transport. Support morphology influences the uniformity of ionomer coverage, and graphitization level governs electrical conductivity and interfacial stability [90]. Surface functional groups further tune adsorption configuration and binding strength through hydrogen bonding or acid‑base interactions [91]. Systematic studies using various carbon materials and advanced techniques have quantified adsorption thermodynamics and kinetics, revealing how hydrophobic, hydrogen‑bonding, and electrostatic forces collectively regulate the interfacial assembly and stability of ionomer layers [92–94]. Although molecular‑level insights into ionomer adsorption on carbon surfaces have been advanced, translating these findings to the complex multiphase environment of practical fuel cell electrode inks remains a key challenge.

In fuel cell electrode systems, spherical carbon particles (30–50 nm in diameter) are widely used due to their excellent electrical and thermal conductivity. Studies have shown that spherical carbon supports with different structures—including carbon black, highly graphitized carbon, mesoporous carbon, and solid carbon spheres—exhibit significant differences in pore size distribution, specific surface area, and degree of graphitization. These structural parameters directly influence the loading behavior of Pt nanoparticles and the adsorption characteristics of the ionomer. The work by Park et al. [90] systematically revealed this structure–performance relationship: although the porous structure of conventional carbon black provides a large specific surface area conducive to the dispersion of Pt particles within the pores, the tortuous pore channels lead to non-uniform ionomer adsorption, thereby hindering oxygen transport. On highly graphitized carbon black surfaces, Pt particles tend to aggregate at graphitic stripes or defect sites, resulting in uneven ionomer coverage and discontinuous ion transport pathways. AB800, a mesoporous carbon precursor, possesses a well-ordered pore structure that enables uniform Pt loading; however, due to size exclusion effects, ionomers are unable to penetrate deep into the pores, leaving some Pt active sites without effective contact with the ionomer. In contrast, solid carbon spheres allow uniform distribution of both Pt nanoparticles and the ionomer. Electrochemical tests further confirmed that these distinct interfacial structures significantly affect the ORR kinetics and mass transport behavior (Figure 7A). Beyond spherical carbons, carbon nanotubes and their modified derivatives also exhibit unique interfacial properties (Figure 7B, C) [71, 95]. Recent studies indicate that well-designed mesoporous carbon structures can synergistically optimize ORR kinetics and mass transport, offering a promising strategy to overcome the inherent limitations of solid carbon spheres (excessive ionomer coverage) and porous carbons (tortuous mass transfer paths) (Figure 7D–F) [96].

Figure 7 The influence of carbon carrier morphology on the ionomer layer. (A) Schematic diagram of proton and oxygen transport pathways during the reaction process of Pt/C catalysts with different morphologies of carbon supports after adsorbing ionomers. (B) SEM image of carbon nanotubes. Reproduced with permission from Ref. [95]. Copyright 2023, ACS. (C) Schematic diagram of modifying carbon nanotubes with N groups and loading Pt particles. (D) Conventional vs. mesoporous carbon particle architectures. (E) Local-O₂-transport resistances and ORR mass activities of Pt catalysts on several carbon supports as a function of dry proton accessibility as determined by CO stripping measurement. (F) O₂ and proton accessibility as a function of the volume of the carbon mesopores in the range of 4−7 nm for porous carbons. Open symbols represent catalysts with accessible porous carbons. Reproduced with permission from Ref. [96]. Copyright 2018, ACS

In the study of carbon surface functionalization, the introduction of groups such as −SO₃⁻ and −NH₂⁺ can significantly alter the chemical properties of the carbon surface, thereby modulating the adsorption behavior of the ionomer (Figure 8A, B) [97]. Specifically, −NH₂⁺ groups can form specific adsorption through electrostatic attraction with the −SO₃⁻ side chains of the ionomer, enhancing interfacial binding [98]. In contrast, −SO₃⁻ groups suppress adsorption due to electrostatic repulsion with the ionomer side chains. Beyond hydrophilic modification, Forouzandeh et al. [99] achieved hydrophobic modification by grafting pentafluorophenyl groups (−PhF₅) onto the carbon surface, strengthening the hydrophobic interactions of carbon particles in liquid environments (Figure 8C, D). Furthermore, the introduction of aniline or benzenesulfonic groups via diazonium reactions allows fine-tuning of the interaction mode between the carbon surface and the ionomer (Figure 8E) [100]. More notably, the strategy of directly anchoring and grafting ionomer monomers onto the carbon surface enables the carbon support to serve dual functions of proton conduction and binding. This approach not only alters the conventional interaction mechanism between the ionomer and carbon but also restructures the mass transport and reaction kinetics across the entire three-phase interface (Figure 8F) [101].

Figure 8 Functionalized carbon supports for Pt catalysis. (A) Functionalization and Pt-loading design of carbon supports. (B) Schematics of high specific surface area carbon (HSC) and Monarch carbon carriers. (C, D) Branch-like ‒PhF₅ functional groups on carbon surfaces. (E) Charged branch-like functional groups on carbon surfaces. (F) Polymer-modified branch structures on carbon support surfaces

In recent years, carbon-based materials derived from metal–organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs) have attracted widespread attention as novel catalyst supports [102]. By precisely controlling pyrolysis conditions—such as temperature, atmosphere, and heating rate—MOF/ZIF precursors can be transformed into porous carbon materials with high specific surface area, tunable pore size distribution, and abundant active sites [103]. Among them, metal-free carbon supports exhibit excellent electrical conductivity and surface activity by preserving the porous structure of the precursor and incorporating nitrogen doping [104]. In contrast, metal-doped carbon supports utilize metal nodes from the precursor or externally introduced metal species to form atomically dispersed metal–nitrogen–carbon (M–N–C) active sites during pyrolysis. These materials not only inherit the structural advantages of the MOF/ZIF precursors but also achieve synergistic control over the pore structure, surface chemical properties, and distribution of active sites through ligand carbonization and metal reduction/aggregation during pyrolysis [105]. The unique surface chemistry (e.g., M–Nₓ sites and N-doping), hierarchical pore architecture (micro–meso–macro pore synergy), and modulated electronic structure (e.g., degree of graphitization and d-band center) of MOF/ZIF-derived carbon supports influence the interaction mechanisms with the ionomer—such as hydrogen bonding, electrostatic interactions, coordination bonds, and π–π stacking—at the molecular scale, thereby governing the adsorption thermodynamics and mass transport kinetics at the three-phase interface [106, 107].

3.3 Design and configuration adjustment of polymer molecular chains

In PEMFC electrodes, the ionomer serves as a key polymeric material that combines ionic conductivity with structural regulation [108, 109]. It not only forms a continuous proton-conducting network within the CL through its sulfonate groups (−SO₃⁻), enabling efficient proton transport, but also modulates the microenvironment of the reaction interface via interactions between its side chains and the catalyst surface (e.g., Pt) [110]. The nanoscale coating layer formed by the ionomer on the carbon-supported catalyst directly governs the construction of the triple-phase boundary: an excessively thick or nonuniform coating impedes oxygen diffusion, leading to mass transport losses at high current densities, while insufficient coating reduces proton conduction efficiency and increases interfacial resistance [111–113]. Moreover, the chemical structure, side-chain length, equivalent weight (EW), and adsorption configuration of the ionomer on the catalyst surface collectively determine the proton conductivity, gas permeability, water management, and long-term chemo-mechanical stability of the CL [108, 114]. Therefore, the rational design and interfacial engineering of ionomers represent a core research direction for overcoming the performance and durability limitations of high-performance fuel cell electrodes [115, 116].

Ionomers are typically sulfonated perfluorinated polymers (e.g., Nafion^®) or partially fluorinated/non-fluorinated polymers. Based on the number of −C₃F₆O− units (x) and −CF₂− units (y) in the side chains, ionomers can be classified into types such as Nafion, Aciplex, Flemion, 3M, and short side chain ionomer (SSC) (Figure 9A) [108]. The EW of an ionomer, defined as the polymer mass per mole of ion-exchange groups, is jointly determined by the number of −CF₂− repeat units between two side chains (m) and the molecular structure of the side chains (Figure 9B) [108]. The EW value directly influences the ionic conductivity, water uptake, and mechanical stability of the material, serving as a key parameter for regulating ion transport efficiency and membrane-electrode durability in proton exchange membrane fuel cells. Furthermore, beyond typical ionomers with polytetrafluoroethylene backbones, ionomers possessing special backbone structures also exist. Their non-linear monomer architectures can reduce material density while also affecting proton conductivity (Figure 9C–E) [117].

Figure 9 Chemical structural parameters of ionomers. (A) Monomer configuration, degree of polymerization, and side-chain structural parameters. (B) Relationship between the number of tetrafluoroethylene repeat units and EW. (C–E) Fundamental physicochemical parameters of ionomers with different backbone/side-chain structures

The application process of ionomers in electrode CLs involves their dissolution and dispersion in solvents, adsorption onto catalyst agglomerate surfaces, and dehydration/de-alcoholization during drying. Therefore, studying the dispersion state of ionomers in solvents is crucial. Wei et al. [118] investigate the relationship between the chain distribution of ionomers in solvents and their characteristic small-angle scattering exponent. They found that when ionomers exhibit poor solubility and structural collapse, the exponent is –3; when the chains follow a Gaussian distribution, the exponent is –2; and when the ionomers are fully swollen with well-extended chain structures, the exponent is –1.7 (Figure 10A). Gupit et al. [115] employed rotational rheometry to examine the relationship between ionomer solution mass fraction and zero-shear viscosity. Based on linear fitting, the ionomer dispersions were categorized into dilute (< 2.5 wt%), semi-dilute (2.5 wt%–10 wt%), concentrated (10 wt%–30 wt%), and gel (> 30 wt%) regimes (Figure 10B), clarifying the dispersion characteristics of ionomers at different stages of electrode fabrication, such as slurry preparation and drying. Welch et al. [116] also used small-angle scattering to study the morphological structure of ionomers in different solvents. In ethylene glycol and glycerol, the ionomers form rod-like structures (Figure 10C); in N-Methyl-2-pyrrlidone (NMP), they adopt extended disordered coil structures (Figure 10D); and in water/isopropanol solvents, they exhibit a disk-like morphology with entangled central chain segments and extended peripheral segments (Figure 10E).

Figure 10 Morphological characteristics of ionomer dispersions. (A) Small‑angle scattering exponents corresponding to different dispersion states. (B) Effect of ionomer concentration on the dispersion state. Reproduced with permission from Ref. [115]. Copyright 2020, ACS. (C) One‑dimensional small‑angle scattering data and corresponding morphology in glycerol solvent. (D) One‑dimensional small‑angle scattering data and corresponding morphology in ethylene glycol solvent. (E) One‑dimensional small‑angle scattering data and corresponding morphology in a 1:1 (m/m) water/isopropanol mixed solvent. Reproduced with permission from Ref. [116]. Copyright 2012, ACS

Ngo et al. [26] investigated the influence of alcohol content on the dispersion behavior of ionomers using cryo-transmission electron microscopy and dynamic light scattering. The results reveal that in pure alcohol solvent (100 wt%), the solubility parameters of the ionomer closely match those of the solvent, leading to the formation of relatively small particles. When the alcohol content is reduced to 70 wt%, ionomer aggregates emerge, which is attributed to hydrogen‑bonding interactions between the −SO₃⁻ groups on the ionomer side chains and water molecules. As the alcohol content decreases further to 60 wt%, the ionomers form dispersed rod‑like aggregates. At an alcohol content of 50 wt%, these aggregates begin to interconnect, resulting in secondary agglomeration (Figure 11A). Dynamic light scattering measurements show that increasing water content shifts the radius‑of‑gyration distribution of the ionomer from a unimodal to a bimodal profile, indicating the coexistence of two distinct size populations: smaller and larger particles. This suggests that the rise in water content induces conformational collapse of the ionomer, reducing the primary particle size, while simultaneously promoting secondary aggregation through hydrogen‑bonding interactions, thereby yielding larger secondary agglomerates. Yang et al. [119] also investigated the influence of water content in the solvent on the dispersion state of ionomers. As the water content increases, the zeta potential of ionomer particles decreases, with a pronounced decline observed when the water mass fraction exceeds 0.7. As the water mass fraction rises from 0.1 to 0.5, the excluded volume fraction of the ionomers gradually increases; however, when the water mass fraction reaches 0.7, the excluded volume fraction drops abruptly (Figure 11B, C). The increase in excluded volume indicates a more extended conformation of the ionomers, while the sharp decrease suggests a collapse of the ionomer conformation (Figure 11D). Therefore, clarifying the molecular weight, molecular configuration, functional group characteristics, and solvation conformation of the ionomer is crucial for regulating the structure of the adsorption layer.

Figure 11 Effect of water‑alcohol mixed solvents with varying compositions on the dispersion state of ionomers. (A) Schematic diagram of agglomerate morphology of ionomers in isopropyl alcohol (IPA)/Water mixed solvents with different mass fractions. (B) Reta potential and (C) excluded volume fraction of ionomer aggregates in water‑alcohol solvents as functions of water content. (D) Schematic illustration of the influence of water content in solvents on the fractal morphology of ionomers. Reproduced with permission from Ref. [119]. Copyright 2025, ACS

Collectively, the surface characteristics of Pt-based catalysts, the physicochemical properties of carbon supports, and the molecular structure and solvation behavior of ionomers form three interrelated core factors that jointly determine the interaction strength and adsorption configuration at the catalyst–ionomer interface. Clarifying the independent and synergistic effects of these three components lays a solid foundation for understanding the formation mechanism of ionomer adsorption layers in practical electrode inks. On this basis, we can further investigate the dynamic evolution of ionomer adsorption and deposition processes, identify key parameters that dominate coating morphology and uniformity, and then propose targeted regulation strategies, thus achieving a smooth logical transition from material property analysis to interfacial process control and finally to performance-oriented coating structure optimization.

4.1 Study on the adsorption state of ionomer in liquid-phase environments

The formation of the ionomer overlayer in electrodes primarily comprises two stages: multilayer adsorption of the ionomer on the surface of catalyst agglomerates, followed by its deposition [120]. The multilayer adsorption behavior is jointly regulated by the intrisic structure of the ionomer, the physicochemical state of the agglomerate surface, and the properties of the solvent (Figure 12A) [77, 114]. In contrast, the deposition process is closely associated with factors such as component concentration, particle diffusion–repulsion–aggregation, capillary action, and particle network formation (Figure 12B). Among these, the multilayer adsorption of the ionomer in the liquid phase serves as the key controlling step that determines the final ionomer coating structure at the triple‑phase boundary of the electrode [121–123].

Figure 12 Multilayer adsorption and deposition of ionomers on catalyst surfaces across key manufacturing steps.​ (A) Slurry preparation. (B) Electrode fabrication

Gao et al. [124] performed DFT calculations to determine the adsorption energies of different segments of ionomers on Pt surfaces (Figure 13). The results indicate that the ether linkages (−O−) in the middle section of long-side-chain (LSC) ionomers can adsorb onto Pt crystal surfaces, whereas the shorter side chains of SSC ionomers restrict the contact between ether groups and the Pt surface. Specific adsorption of the sulfonate group from the ionomer side chain onto Pt primarily occurs via oxygen atoms in −SO₃⁻. When two oxygen atoms face the Pt surface, the adsorption energy is ΔE = −2.6013 eV; when only one oxygen atom is oriented toward Pt, the adsorption energy is ΔE = −2.2066 eV. The adsorption energy of the LSC ionomer side chain on Pt is ΔE = −2.8558 eV, while that of the SSC ionomer is ΔE = −2.8871 eV. However, for the entire ionomer monomer, the adsorption energy on Pt is ΔE = −5.8321 eV for the LSC type and ΔE = −5.5823 eV for the SSC type. Although the more concentrated polar groups (−O−, −SO₃⁻) on the short side chain enhance the adsorption energy of the side chain itself, the overall rigidity of the SSC ionomer reduces the adsorption energy of the full monomer. In contrast, the flexible LSC ionomer exhibits stronger overall adsorption on the Pt surface.

Figure 13 DFT adsorption energy calculation of Pt surface for different segments of long side chain and short side chain ionomers. The adsorption energies of CF₃SO₃, CF₃OCF₂CF(CF₃)O(CF₂) ₂SO₃, CF₃O(CF₂)₂SO₃, LSC, and SSC on the Pt surface

Berlinger et al. [92] The molecular mechanisms and dynamic regulation of ionomer-catalyst interactions in porous electrodes were systematically elucidated through multiscale characterization techniques. The thermodynamic behavior of ionomer adsorption on model Pt/C surfaces under different solvent environments was quantitatively analyzed using quartz crystal microbalance (QCM) and isothermal titration calorimetry (ITC). QCM measurements revealed that an ionomer with an EW of 620 g mol⁻¹ exhibited higher adsorbed mass on Pt surfaces compared to an ionomer with an EW of 1,000 g mol⁻¹, and the adsorbed mass increased with higher water content in alcohol–water solvent mixtures (Figure 14A).

Figure 14 QCM-D testing and ITC testing for the adsorption process of ionomers. (A) Frequency shifts (Δf) and corresponding mass changes (Δm) measured at the third harmonic during the adsorption of ionomers (equivalent weights, EW = 620 and 1,000 g mol⁻¹ per SO₃⁻ group) from water/n-propanol mixtures with varying ratios onto model surfaces (alkane thiols and platinum). (B) Absolute values of the entropy (|TΔS|) and enthalpy (|ΔH|) contributions to the adsorption free energy of ionomers on pristine catalyst materials (Pt and carbon). (C) The distribution density of ionomer side chains in alcohol water solvents. (D, E) The quantity and distance of −SO₃⁻ at the interface of ionomer aggregates in solvents with different dielectric constants. Reproduced with permission from Ref. [126], Copyright 2019, ACS

ITC results indicated that ionomer adsorption on both Pt and C surfaces is driven by combined enthalpic and entropic contributions, with entropy-driven behavior predominating on carbon surfaces. On Pt surfaces, the adsorption mechanism was influenced by ionomer EW: for EW values of 620 and 1,000 g mol⁻¹, adsorption was primarily entropy-driven, whereas for an EW of 825 g mol⁻¹, the enthalpic contribution became significantly more pronounced (Figure 14B). Novy et al. [125] investigated the influence of alcohol-to-water ratio on the surface side-chain density of ionomer aggregates. They observed that the side-chain density increased with increasing alcohol content in the solvent (Figure 14C). Tarokh et al. [126] further demonstrated through simulations that an increase in solvent dielectric constant leads to a reduction in the number of side‑chain −SO₃⁻ groups at the ionomer aggregate interface, while also increasing the distance between adjacent side‑chain −SO₃⁻ groups (Figure 14D, E). In summary, the distribution density of ionomer side‑chain −SO₃⁻ not only influences its adsorption mechanism but also results in different morphologies and spatial distributions under solvent‑driven conditions.

Yoshimune et al. [77] investigated the adsorption mechanism of the ionomer on nitrogen (N)-group-modified carbon surfaces and its influence on the adsorption layer structure. The ionomer backbone adsorbs onto the carbon surface via hydrophobic interactions, undergoing conformational collapse to form a compact adsorption layer approximately 1 nm thick and an extended layer of about 30 nm. After N‑group modification, chemical adsorption occurs between the −SO₃⁻ groups on the ionomer side chains and the N‑groups, resulting in a significant increase in both the thickness and density of the compact layer (Figure 15A, B). Furthermore, they employed small‑angle X‑ray scattering (SAXS) to examine the influence of Pt density on the carbon surface on the structure of the ionomer adsorption layer. The results reveal that increased Pt density enhances the interaction between −SO₃⁻ and Pt, leading to a decrease in the density of the ionomer adsorption layer and an increase in its thickness (Figure 15C, D) [76]. Kodama et al. [127] systematically studied the adsorption behavior of perfluorosulfonic acid-based compounds (including small-molecule analogs and ionomers) on platinum surfaces and their impact on the ORR using electrochemical voltammetry and surface-enhanced infrared absorption spectroscopy (SEIRAS). The results reveal that compounds containing ether bonds (−O−) exhibit significantly stronger adsorption on both Pt (111) single crystals and polycrystalline Pt films compared to those without ether groups, indicating a specific interaction between the ether group and the Pt surface (Figure 15E–G). Cleve et al. [128] examined the influence of water content in the solvent on ionomer aggregation and adsorption. Their findings show that in water-rich environments, ionomers form smaller aggregates, strengthening the interaction between sulfonate groups and Pt. In alcohol-rich solvents, however, ionomers assemble into larger aggregates, weakening the ionomer/catalyst surface interaction (Figure 15H). Furthermore, LSC ionomers, with higher flexibility, allow easier orientation of ether groups toward the Pt surface, whereas short-side-chain ionomers, being more rigid, hinder effective contact (Figure 15I) [129].

Figure 15 Morphology and structure of polymer adsorption layer. (A) Schematic illustration of ionomer adsorption conformations on carbon surfaces and nitrogen‑group‑modified carbon surfaces. (B) Schematic illustration of physical adsorption of the ionomer backbone and chemical adsorption of side chains. (C) Effect of Pt loading in Pt/C catalysts on the density and thickness of the ionomer overlayer on the catalyst surface. (D) Schematic of ionomer adsorption on carbon surfaces with different Pt density distribution. (E–G) The influence of side chain ether bonds and stiffness of ionomers on their adsorption on catalyst surfaces. Reproduced with permission from Ref. [127]. Copyright 2017, ACS. (H) Influence of water content in the solvent on the conformation of the ionomer adsorption layer. Reproduced with permission from Ref. [128]. Copyright 2019, ACS. (I) Effect of solvent properties on ionomer conformation and the structure of its adsorption layer

The type of carbon support also influences the adsorption amount of ionomers. Ma et al. [130] investigated the ionomer adsorption behavior on the surfaces of different carbon particles (Figure 16A). The ionomer rapidly reached the first plateau on XC‑72 carbon, indicating that graphitization promotes ionomer adsorption. Among Vulcan XC‑72, Shawinigan Black, Ketjenblack, and Hispec 1000, Shawinigan Black exhibited the highest saturated adsorption capacity for the ionomer, followed by Hispec 1000, Vulcan XC‑72, and then Ketjenblack, which showed the lowest adsorption.

Figure 16 Ionomer adsorption amounts on different types of support surfaces. (A) Vulcan XC‑72, Shawinigan Black; Ketjenblack, Hispec 1000 (Pt Partical, 9 nm, 100 wt% Metal), Hispec 6000 (Pt/Ru, 2.77 nm, 100 wt% Metal), Hispec 9000 (Pt/C, 5.47 nm, 57.84 wt% Metal), Hispec 9100 (Pt/C, 2.48 nm, 56.76 wt% Metal), Hispec 10000 (Pt/Ru/C, 2.58 nm, 56.76 wt% Metal), Hispec 10100 (Pt/Ru/C, 2.39 nm, 58.61 wt% Metal), and TEC10 (Pt/C, 3.60 nm, 66.30 wt% Metal). (B) Ionomer adsorption amounts before and after Pt loading

Compared to Hispec 1000, the alloying of Pt with Ru in Hispec 6000 lowers the adsorption energy of Pt facets, thereby reducing the ionomer adsorption amount. In contrast to Hispec 1000, the deposition of Pt particles on carbon particles in Hispec 9000 enhances ionomer adsorption. Relative to Hispec 9000, the reduction of Pt particle size from 5.47 to 2.48 nm in Hispec 9100 leads to a notable decrease in saturated ionomer adsorption. Compared to Hispec 6000, supporting Pt/Ru alloys on carbon carriers in Hispec 10000 results in a significant increase in saturated ionomer adsorption. In contrast to Hispec 10000, the change in support type for Hispec 10100 causes a marked decline in adsorption capacity. Relative to Hispec 9100, the larger specific surface area of TEC10 substantially promotes ionomer adsorption. In summary, alloying reduces ionomer adsorption, whereas decreasing Pt particle size and enhancing carbon‑support properties promote ionomer adsorption. Wu et al. [129] investigated the adsorption of ionomers on CB/PBI and CB/PBI/Pt surfaces before and after Pt loading. After Pt deposition, the first adsorption plateau of the ionomer increased from 0.1 to 0.25, whereas the second plateau exhibited a smaller change in magnitude but a narrower plateau range (Figure 16B). This indicates that Pt loading modifies the adsorption behavior of the ionomer on the Pt‑containing surface.

In summary, ionomers undergo both physical and chemical adsorption at interfaces in liquid‑phase environments. The backbone adsorbs physically via entropy‑driven hydrophobic interactions, whereas the side chains—containing polar groups such as −O− and −SO₃⁻ —participate in both physical and chemical adsorption on carbon‑supported metal crystallites, with chemical adsorption dominating the final adsorbed configuration. Building on this, previous studies have modulated ionomer adsorption behavior by altering the surface chemical composition of catalysts to control the strength of chemical adsorption.

4.2 Tailoring the electrodes’ ionomer overlayer structure through adsorption behavior regulation

Extensive studies have been conducted to optimize the ionomer coating structure at the triple‑phase boundary of electrodes, with a focus on the morphology and surface chemistry of carbon supports. Due to the size effect of ionomer particles, which range from tens to hundreds of nanometers upon solvation, their adsorption and penetration into support pores are strongly influenced by pore dimensions.

Takeshita et al. [131] prepared carbon supports with pore sizes ranging from 3.8 to 9.8 nm using a templating method and investigated ionomer penetration within the pores (Figure 17A). Their results show that when the pore size is smaller than 3.8 nm, ionomers cannot penetrate the pores and instead form a coating around the support particles. As the pore size increases, ionomers gradually infiltrate the pores, leading to higher ionomer coverage on the Pt surface (Figure 17B) and a gradual decrease in the mass activity of the catalyst (Figure 17C). Ott et al. [132] modified the surface chemistry of porous carbon supports with nitrogen (N) groups, which enhanced the chemical adsorption between the −SO₃⁻ side chains of the ionomer and the support surface (Figure 17D). This promoted uniform adsorption of the ionomer on the catalyst surface (Figure 17E) and prevented ionomer penetration into the mesopores, thereby effectively reducing the oxygen transport resistance within the electrode (Figure 17F). Sun et al. [133] examined the influence of O‑, S‑, and N‑group‑modified carbon supports on the ionomer coating structure, finding that N‑groups facilitate the formation of a uniform thin ionomer overlayer on the catalyst surface, optimizing the transport pathways for O₂ and water (Figure 17G). Electrochemical tests further demonstrated that N‑group modification significantly improves the mass activity and electrochemical surface area of the catalyst and enhances the performance of the membrane electrode assembly under 20% relative humidity (Figure 17H).

Figure 17 Carbon modified regulation of ionomer coating structure. (A) Energy dispersive X-ray analysis (EDS) of ionomer distribution on carbon particles with pore sizes of 3.8 and 9.8 nm. (B) Effect of ionomer coverage on mass activity for Pt/C catalysts supported on carbons with different pore sizes. (C) Relationship between support pore size and mass activity of Pt/C catalysts. (D) Schematic of ionomer adsorption on N‑group‑modified Pt/C surfaces. (E) Formation of a continuous and uniform ionomer overlayer on catalyst surfaces within the CL. (F) Influence of support modification on the pressure‑independent term of oxygen transport resistance in Membrane Electrode Assemblies (MEAs). (G) Schematic illustrating the effect of ionomer adsorption on N‑group‑modified porous catalyst surfaces on oxygen and water transport. (H) Electrochemical Surface Area (ECSA) of Rotating Disk Electrode (RDE) and MEA before and after N modification N

The ionomer overlayer not only significantly influences the transport of O₂, water, and protons, but its side‑chain –SO₃⁻ adsorption on Pt particles also occupies active reaction sites, thereby reducing the catalytic activity of Pt [134]. Previous studies have examined the impact of −SO₃⁻ adsorption on the steps of the ORR and improved the catalytic performance of membrane electrode assemblies by suppressing −SO₃⁻ adsorption on Pt. Jiang et al. [32] investigated the diffusion, adsorption, and desorption processes of oxygen and its intermediates on Pt facets (Figure 18A) and found that −SO₃⁻ adsorption markedly increases the energy barriers of each elementary step (Figure 18B) and lowers the exchange current density of the ORR (Figure 18C). Li et al. [135] innovatively introduced an ionic liquid at the ionomer‑Pt interface (Figure 18D), enabling the ionic conductor [MTBD][beti] to bond with −SO₃⁻, thereby effectively reducing −SO₃⁻ coverage on Pt facets (Figure 18E). This ionic conductor also provides proton conductivity, compensating for the proton transport loss due to ionomer bonding and significantly enhancing the mass activity of the electrode (Figure 18F). Chen et al. [136] explored the use of cyclohexanol to tailor the ionomer coating structure. This approach effectively hinders −SO₃⁻ adsorption on Pt, substantially lowering −SO₃⁻ coverage on the Pt surface (Figure 18G). Moreover, the cyclic structure of cyclohexanol reduces the density of the ionomer overlayer, further improving the catalytic activity of the membrane electrode assembly (Figure 18H).

Figure 18 Toxic mechanism and inhibition methods of ionomers. (A) Reaction steps of the ORR on Pt surfaces. (B) Energy barriers for each step of the ORR. (C) Exchange current densities for the ionomer side‑chain –SO₃⁻ in contact and non‑contact configurations with Pt. (D) Schematic of an ionic liquid (IL, [MTBD][beti]) inhibiting ionomer adsorption on Pt. (E) Ionomer coverage determined from CO displacement charge and CO stripping charge as a function of applied potential for Pt (111) (black squares), Pt (111)+Nafion (red circles), and Pt (111)+Nafion+IL (green triangles). (F) ORR polarization curves in O₂‑saturated 0.1 M HClO₄ for Pt (111) (black), Pt (111)+Nafion (red), and Pt (111)+Nafion+IL (green). Reproduced with permission from Ref. [135]. Copyright 2020, ACS. (G) Schematic of cyclohexanol inhibiting −SO₃⁻–Pt contact. (H) The impact of sulfonate‑group coverage (percentage) coverage on ECSA

4.3 Innovation in materials and structures of three-phase interface ionomer layers

In addition to tailoring the ionomer adsorption process to design the coating structure at the triple‑phase boundary, previous studies have also focused on innovating ionomer structures and optimizing the overlayer to enhance the structural and transport properties of the interface. Jinnouchi et al. [117] synthesized a high‑oxygen‑permeability ionomer using cyclic monomers and employed molecular dynamics simulations to compare the adsorption layers of Nafion and this ionomer. The ring‑shaped structure introduces steric hindrance that reduces the density of the ionomer overlayer, thereby promoting oxygen transport (Figure 19A). Computational results show that compared to Nafion, the cyclic ionomer exhibits lower −SO₃⁻ coverage on Pt surfaces, indicating reduced poisoning of Pt facets (Figure 19B). Moreover, the oxygen diffusion coefficient within this ionomer layer is higher and its diffusion resistance lower at humidities above 20%RH (Figure 19C). Yang et al. [137] developed an α‑aminoketone‑covalent organic framework ionomer interwoven with Nafion (Figure 19D). This approach utilizes synergistic hydrogen‑bonding to retain water molecules, enhancing hydration and proton transport while reducing oxygen transport resistance (Figure 19E). The Am‑COF‑3‑SO₃H matrix linked via α‑aminoketone bonds stabilizes water through cooperative hydrogen bonds, enabling efficient Grotthuss‑type proton hopping. This accelerates oxygen‑proton transport in fuel cells, mitigates Pt poisoning, and significantly improves power density at medium‑to‑high temperatures (Figure 19F).

Figure 19 Innovation in polymer structure. (A) Schematic diagram of the density of the coating layer formed by Nafion and HOPI on Pt surface. (B) Surface coverage of sulfonate anions in ionomers determined (RH ≈ 60%, λ = 6). (C) Oxygen diffusion coefficients in Nafion and HOPI. (D) Schematic of proton and oxygen transport within a breathable open‑framework ionomer in PEMFCs. (E) Oxygen transport resistance for Nafion and the breathable open‑framework ionomer. (F) Power densities of MEAs with Nafion and the breathable open‑framework ionomer at different temperatures

In addition to innovating the intrinsic physicochemical properties of ionomers, previous studies have also employed technical strategies to engineer porous ionomer overlayers, maintaining proton transport while effectively reducing oxygen transport resistance. Doo et al. [138] fabricated a nanoscale mask on Pt particles via nanoengineering to prevent ionomer adsorption, thereby preventing complete coverage of Pt by the ionomer. After chemically removing the mask, a partially covered ionomer structure was obtained (Figure 20A). This structure significantly reduced oxygen transport resistance at low‑to‑medium temperatures and effectively enhanced the power density of the fuel cell (Figure 20B). Cheng et al. [139] utilized the mutual solubility between polyethylene glycol and Nafion, along with its self‑generated water leaching effect, to prepare an ionomer coating with abundant pore structures (Figure 20C). This architecture allows oxygen to reach the Pt surface directly through pores rather than relying on bulk diffusion through the ionomer. Although the transport resistance of this glycol‑doped Nafion layer increases rapidly with thickness, and its bulk transport resistance is notably high, its unique porous structure reduces the local oxygen transport resistance in the electrode by 4–5 times (Figure 20D). Zhang et al. [140] introduced a COF‑type cyclic organic compound at the ionomer‑Pt interface, which avoided direct coverage of Pt particles by the ionomer (Figure 20E). Electrochemical tests showed that this structure significantly improves the mass activity of the electrode and greatly lowers oxygen transport resistance (Figure 20F).

Figure 20 Innovation in the structure of polymer coating layer. (A) Schematic of the ionomer forming full‑coverage and partial‑coverage structures on Pt surfaces. (B) Oxygen transport resistance of full‑coverage and partial‑coverage MEAs at different temperatures. (C) Oxygen transport schematics for triple‑phase boundary structures with non‑porous and porous ionomer overlayers. (D) Local transport resistance. (E) Schematic diagram of COF optimized ionomer coating structure in Pt/C@COF‑Nafion. (F) Local transport resistance

In summary, it is evident that numerous factors influence the adsorption behavior of ionomers (Figure 21). For the adsorbate—the ionomer itself—its molecular structure, degree of polymerization, EW, backbone/side-chain architecture, and solvation conformation collectively determine its adsorption characteristics. For the adsorbent—the metal-crystal-supported catalyst—the morphology, composition, particle size, crystallographic facets, electronic and geometric structure of the metal crystals, as well as the morphology, pore structure, degree of graphitization, surface area, and functional groups of the support, all contribute to defining the adsorption sites. These diverse factors do not act independently but interact synergistically to modulate interfacial adsorption thermodynamics, including adsorption energy, binding strength, and adsorption equilibrium, which further dominate the final coverage, uniformity, and nanostructure of the ionomer coating. From these coupled effects, the most influential parameters include the crystal facet and surface electronic structure of Pt catalysts, the pore structure and surface functional groups of carbon supports, and the equivalent weight and side-chain configuration of ionomers. These key parameters provide direct guidance for the rational design of catalyst surfaces, support microstructures, and ionomer molecular architectures toward optimized adsorption behavior and coating structure. Studying the adsorption behavior of ionomers on catalyst surfaces in liquid environments thus requires the integration of multiple variables, which presents considerable complexity. Focusing on interfacial adsorption thermodynamics offers an effective approach to disentangle these factors through combined physicochemical experiments and computational simulations. This strategy enables the identification of key parameters governing ionomer adsorption and represents the most efficient pathway toward achieving controlled ionomer adsorption on catalyst surfaces. Such an approach structurally optimizes the EDL processes in fuel cell electrodes, thereby facilitating the coordinated transport of protons, electrons, and gases—ultimately leading to breakthroughs in fuel cell performance.

Figure 21 Schematic diagram of directional adsorption control of ionomers on two-component adsorption substrates. The green, purple, and red entries represent the physicochemical properties of ionomers, metal crystals, and ionomers, respectively, which are the key factors affecting the adsorption thermodynamics at the interface of ionomers