Synergistic Carbon Support Engineering in Composite Catalyst Layer for High‐Performance PEM Fuel Cells

Abstract

ABSTRACT

This study introduces an innovative composite cathode catalyst layer (CCL) design for proton exchange membrane fuel cells (PEMFCs), combining Pt‐supported by Vulcan carbon (Pt/V) and Ketjenblack carbon (Pt/KB) to overcome mass transport limitations and ionomer‐induced catalyst poisoning. The composite architecture strategically positions Pt/V layer with lower ionomer‐to‐carbon ratio (I/C = 0.6) near the proton exchange membrane to maximize surface Pt accessibility and oxygen transport efficiency, whereas Pt/KB layer (I/C = 0.9) adjacent to the gas diffusion layer leverages its porous structure to shield Pt from sulfonate group poisoning and enhance proton conduction under low‐humidity conditions. This synergistic carbon support engineering achieves a balance between reactant accessibility and catalyst utilization, as demonstrated by improved power density, reduced transport resistance, and higher Pt utilization under dry conditions. These findings establish a new paradigm for low‐Pt CCL design through rational carbon support hybridization and ionomer gradient engineering, offering a scalable solution for high‐performance PEMFCs in energy‐critical applications.

Introduction

The depletion of fossil fuel reserves and urgent climate challenges have accelerated global demand for renewable energy technologies such as wind, solar, and geothermal systems [1]. Proton exchange membrane fuel cells (PEMFCs), recognized as the forefront of clean energy conversion technologies, exhibit remarkable efficiency and minimal emissions [2], holding the potential to revolutionize the fields of transportation and stationary power applications [3–6]. However, the journey towards widespread commercialization of PEMFCs is constrained by a multitude of challenges, including high costs, performance degradation, and durability [7–10]. Particularly, the catalyst layer (CL), being the core component within PEMFCs, plays a crucial role in determining the cell's overall efficiency and stability, with its performance having a direct and profound impact [11, 12].

As the core component of PEMFCs, the membrane electrode assembly (MEA) performs three critical functions: (1) sustaining electrochemical reactions, (2) managing reactant/product flows [12], and (3) facilitating charge transport through both electrons and ions [13]. To achieve these functions simultaneously, it is essential to precisely control the material properties across multiple length scales, making electrode design an important area of research [14]. Therefore, MEA's structures and material choices directly affect efficiency, durability, and the overall performance of the fuel cell, making it a key focus area in the development of advanced fuel cell technologies [15, 16]. Although considerable attention in catalyst research has focused on enhancing the oxygen reduction reaction (ORR) at the cathode due to its sluggish kinetics and the requirement for precise metals like platinum (Pt) [17], addressing losses associated with electrode transport limitations represents another effective strategy for advancing the development of PEMFCs [18].

Within MEA, the CL, composed of electrocatalysts bound and interconnected by ionomers, serves as the place for electrocatalytic reactions to take place. At the cathodic catalyst layer (CCL), where ORR occurs, oxygen is reduced with transported protons to form water. This process, occurring within the porous CCL, constitutes a fully coupled reaction–diffusion phenomenon, necessitating enhanced pathways for the transport of oxygen, protons, water, and electrons to the three-phase boundary [19, 20]. Although the development of CCL has demonstrated satisfactory performance, several critical challenges remain. Specifically, the performance of the CCL is often hindered by mass transport limitations, particularly at higher current densities, which leads to a drop in cell voltage and overall efficiency [21]. Within the CCL, carbon particles, functioning as electrocatalysts support, are randomly distributed and interconnected, thereby establishing channels for electronic transfer [22]. Recent studies have shown that the specific surface area, pore structure, conductivity, and interfacial characteristics of carbon supports play a crucial role in the mass transport performance of the CCL [23, 24]. High specific surface area carbon supports can provide more catalytic sites, thereby increasing the contact opportunities for the reactants [25, 26]. However, compared to the popular solid carbons like Vulcan, within porous carbons such as Ketjenblack, the interior Pt nanoparticles have restricted access to protons and oxygen [25, 27, 28].

In addition to the effect of carbon supports on the mass transport performance of the CCL, ionomer is another key component that facilitates proton conduction and plays a crucial role in the overall electrochemical processes [29]. Ionomers, typically perfluorosulfonic acid (PFSA) polymers, such as Nafion, are known for their excellent proton conductivity and chemical stability [30]. However, there are inherent limitations that affect their performance in practical applications [31]. One significant challenge is the dependence of proton conductivity on water content [32]; ionomers, which are capable of creating a hydrophilic environment, play a crucial role in retaining water, a key factor for proton conduction. However, if the water retention is not efficiently managed, it may lead to an excess of water, causing flooding that can obstruct gas pathways and severely restrict mass transport [33]. On the other hand, a deficiency of ionomers can result in dry regions within the CL, which in turn reduces proton conductivity and ultimately diminishes the overall performance [34–37]. Additionally, ionomers, despite their importance in proton conductivity, can inadvertently hinder the performance of electrocatalysts through processes such as ionomer poisoning, which is a critical issue that needs to be addressed [38]. Therefore, it is significant to fine-tune the ionomer-to-carbon ratio (I/C) of the constituent materials and their interactions within the dispersion medium to optimize the complex interfaces between the catalyst–support, catalyst–ionomer, and gas–liquid interactions. These interfaces, which are often overlooked but highly significant, play a crucial role in determining the overall mass transport within the porous electrode, thereby directly influencing the overall efficiency and performance of the system [39–41].

Recent advancements in mass transport development have shown promise in addressing the intricate composition and ordering CCL structures, primarily through carbon support modification, ionomer distribution, and the implementation of nanoconfinement effects. For instance, Kongkanand et al. demonstrated that the careful selection of appropriate pore sizes can effectively reduce the transport resistance of gases within the CL, thereby increasing the power density of PEMFCs [42]. In a related study, Zhang et al. indicated that the surface characteristics of the carbon support have a profound impact on both ionomer distribution and oxygen transport resistance within the CL [43]. To address this, they employed magnesium oxide as a pore-forming agent, which led to the creation of additional oxygen transport pathways within the CL [44]. Orfanidi et al. developed strategies to decrease local oxygen resistance by optimizing ionomer distribution through the functionalization of carbon support with nitrogen-containing surface groups [45, 46]. Neyerlin et al. [47] and Doo et al. [48] found that appropriate catalyst ink properties can lead to better ionomer agglomeration in solvents, resulting in optimal CL structures that facilitate efficient oxygen transport. Recent studies have demonstrated the potential of improving nanoconfinement within the CL by incorporating ionic liquids, which are known for their considerable oxygen solubility and good ionic conductivity, thereby aiding the overall mass transport within the CL [49]. Furthermore, optimizing the microstructure of the ML can improve the distribution of reactants and enhance the electrochemical interface, thereby mitigating mass transport issues [50]. Recent studies have demonstrated that ordered CLs can improve the utilization of precious metal catalysts, such as Pt, by increasing the effective surface area accessible to reactants. This architecture not only enhances the electrochemical reaction rates but also contributes to greater durability by minimizing the degradation of catalyst particles through reduced agglomeration and leaching effects [51–53]. By controlling the porosity and permeability of the CL, the design of ordered CLs can be tailored to optimize the ionic and electronic conductivity, thereby enhancing the overall electrochemical performance of the fuel cell [54]. It is possible to balance the transport of ions and gases, ensuring that the electrochemical reactions occur efficiently under varying operational conditions [54].

The performance of the MEA is intimately linked to the mass transport within the CL, especially at high current densities (HCDs), where the electrochemical reaction necessitates a significantly increased supply of oxygen and protons. In such demanding conditions, any deficiency in local mass transport can lead to reactant starvation and product accumulation, phenomena that can markedly diminish the cell performance. In light of these challenges, this study presents a systematic approach to constructing a composite CCL designed to enhance the overall performance of PEMFCs. Through the employment of a duo of commercially available electrocatalysts, each characterized by its unique carbon porosities, and carefully incorporating them with the ionomer using a variety of I/C ratios, we aim to engineer a CCL that not only sustains elevated electrocatalytic activity but also concurrently demonstrates enhanced mass transport capabilities. This innovative approach is expected to culminate in a marked augmentation of the overall performance of PEMFCs. This work holds significant promise for contributing to the advancement of PEMFC technology, facilitating its integration into practical applications, and promoting the transition towards sustainable energy solutions.

Experimental Methods

Fabrication of Catalyst-Coated Membranes

Traditional Electrodes Preparation

Commercial Pt-supported by Vulcan carbon (Pt/V) (TEC10EA50E, TKK, Pt loading 47 wt%) and Pt-supported by Ketjenblack carbon (Pt/KB) (TEC10E50E, TKK, 46 wt%) were individually dispersed in a solvent mixture composed of deionized water (DI) and n-propanol (nPA) at a mass ratio of 7:3, resulting in a solution with a Pt concentration of 0.84 mg_Pt/mL. Subsequently, Nafion solutions (DuPont D2020) were individually added into the above two catalyst inks to achieve I/C mass ratios of 0.6 and 0.9 for Pt/V and Pt/KB, respectively. The two catalyst inks were first dispersed and subjected to ultrasonication using an ultrasonic cell crusher for an initial period of 10 s. Following this, the prepared inks were transferred to an ice water bath and further ultrasonicated for 20 min. To prepare electrodes containing Pt/V or Pt/KB, each catalyst ink was subsequently sprayed onto Nafion 211 membranes using a Hangzhou Pansolen ultrasonic precision sprayer, aiming for a target catalyst loading of 0.10 mg_Pt/cm².

Composite Electrodes Preparation

The configuration and ultrasonic ink steps are identical to those described above. The Pt/V catalyst ink was initially sprayed onto the Nafion 211 membrane, followed by the application of the Pt/KB catalyst ink onto the Pt/V coated Nafion 211 membrane. This procedure yielded a composite CL membrane electrode with a total Pt loading of 0.10 mg_Pt/cm², and equal Pt loading for both the Pt/V and Pt/KB layers.

The anodic CLs were prepared using Pt/V dispersed in a DI/nPA mixture (with a DI/nPA mass ratio of 7:3), featuring an I/C ratio of 0.6 and a Pt loading of 0.05 mg_Pt/cm².

Characterization of CCL

The cross-sectional morphology of the catalyst-coated membrane (CCM) was examined using a scanning electron microscope (SEM, Tescan Mira4). The porosity gradient within the composite CCL was visualized using the Xradia 515 Versa 3D X-ray microscope. Brunauer–Emmett–Teller (BET) analyses were used to measure the specific surface area of the catalyst powders.

In Situ Electrochemical Diagnostics

The MEAs were prepared using Freudenberg H23C8 gas diffusion layers (GDLs) at 18% compression. Electrochemical cell tests were performed on membrane electrodes with an active area of 5 cm² and a cathodic loading of 0.10 mg_Pt/cm² using a serpentine flow field to quantify the cell performance.

Break-In Procedures

First, the cell was heated to 80°C and held at an open-circuit voltage (OCV) in H₂/air, followed by a series 5/10/5 voltage cycles between 0.6 and 0.9 V.

Voltage Recovery

The voltage recovery (VR) step exposed the cells to H₂/air (950/500 sccm) at 0.1 V for 4 h at 40°C and 150% relative humidity (RH). This process was proved to be a valuable step in removing sulfate, improving electrochemical performance over the entire potential range after significant degradation of MEA [55].

H₂/O₂ Polarization Curves

The test protocol consisted of measuring the I–V curves from 0.75 V to OCV at 80°C and under 100, 100 kPa O₂ partial pressure (150 kPa total pressure), with a flow rate of H₂/O₂ = 0.5/2.0 L/min, and RH set to 100% and 25%, respectively. Data collection was conducted for 4 min at each data point, with the final values being averaged over the last minute.

H₂/Air Performance

The test protocol consisted of measuring the I–V curve (80°C, 150 kPa) using a constant current mode with a flow rate of H₂/O₂ = 0.5/2.0 L/min. The RH was set to 100% and 25%, respectively. The anode and cathode flow rates were maintained at 0.5 and 2 L/min, respectively. Data collection was conducted for 4 min at each data point, with the final values being averaged over the last minute. Polarization curve measurements were performed in the anode direction, starting from low potential to OCV.

CO Stripping Voltammetry

Pt's electrochemical surface area (ECSA) was determined by integrating the CO stripping charge (q_strip) obtained by cyclic voltammetry (CV) after CO displacement experiments at 0.09 V. The utilization of the Pt catalyst at different RH conditions was measured by CO stripping at 0.09 V at 80°C. Once the MEA reached the specified temperature and RH values, the cathode was purged with nitrogen (N₂) gas for 20 min, followed by multiple CV scans between 0.08 and 1.0 V to clean the electrode. Subsequently, the 10% CO (with equilibrium N₂) was introduced into the cathode for 60 s while holding the potential at 0.09 V. Finally, the cathode was purged with N₂ for an additional 10 min to remove residue CO. CV scans were performed between 0.08 and 1.2 V to conduct the CO stripping. The ECSA was calculated by integrating the CO stripping peak, assuming a charge density of 420 μC/cm²_Pt. The CO stripping measurements were repeated across a range of RH values, from 25% to 100%. Subsequently, the Pt utilization was calculated as the ratio of the charge by CO stripping at each respective RH value to the charge obtained at 100% RH.

CO Displacement Chronoamperometry

Initially, a CV scan from 0.08 to 1.0 V at a scan rate of 100 mV/s was performed to clean the cathode under N₂, followed by a potential holding at 0.30 V for at least 2 min to establish a steady-state current. Afterward, CO gas (10% CO in N₂) was fed to the cathode at 10 sccm. The obtained displacement charge (q_dis) from the CO displacement curve was normalized by stripping charge (q_strip) from the CO stripping curve to calculate the sulfonate coverage. Since CO oxidation is a 2e⁻ process and CO substitution involves one e⁻, there is a stoichiometric number of 2. The coverage (θ_dis) is determined by the following equation: 1${\theta }_{\mathrm{dis}}=\frac{2q_{\mathrm{dis}}}{q_{\mathrm{strip}}}$

H⁺ Transport Resistance Test

Electrochemical impedance spectroscopy (EIS) analysis was performed using a Donghua DH7002A workstation to quantify the H⁺ transport resistance within the CCL ($R_{{\mathrm{H}}^{+}}$). The potentials of the N₂ working electrode (cathode) and the H₂ reference electrode (anode) were 0.2 V. The frequency range spanned from 100,000 to 0.1 Hz, with voltage perturbations of 3 mV (amplitude). Anode H₂ and cathode N₂ were supplied at a constant flow rate of 1000 sccm at a cell temperature of 80°C and under a back pressure of 50 kPa.

Ideally, in the H₂/N₂ Nyquist plot, the 45° linear region below the high frequency corresponds to the migration of H⁺ through the ionomer phase in the CL. The lower frequency line, which appears below this region, exhibits a 90° phase angle attributed to the total capacitance. For simplicity, the resistance to proton transport through the CL ($R_{{\mathrm{H}}^{+}}$) can be determined from the length of the Warburg-like region ($R_{{\mathrm{H}}^{+}}$/3) projected onto the real axis (Z′). In practical tests, the curves deviate from the ideal 45° and 90° lines, which can be attributed to the inhomogeneous distribution of the H⁺ resistance within the CL. $R_{{\mathrm{H}}^{+}}$/3 is obtained by extrapolating an asymptote from the low-frequency line on Z′ and measuring the intersection point, from which $R_{{\mathrm{H}}^{+}}$ can be derived.

O₂ Limiting Current Experiments

Limiting current measurements were performed at 80°C and 75%, RH, with oxygen mole fractions of 0.02, 0.03, and 0.05, using a differential cell setup. Limiting currents were obtained at total cell pressures of 150, 200, 250, and 300 kPa, while maintaining constant voltages of 0.30, 0.24, 0.18, 0.12, and 0.06 V for 3 min each. Because of the effect of hydrogen precipitation on current density below 0.1 V, the maximum current density value obtained above 0.12 V is reported as the limiting current (i_lim).

The total oxygen transfer resistance, R_Total, is determined by the following equation: 2$R_{\mathrm{Total}}=\frac{4F{c}_{{\mathrm{O}}_2}}{i_{\lim }}=R_{\mathrm{CH}}+R_{\mathrm{DM}}+R_{\mathrm{MPL}}$where $F$ is Faraday's constant and $c_{{\mathrm{O}}_2}$ is the oxygen concentration. R_Total consists of both the pressure-dependent (slope, R_P) and the pressure-independent which is also known as non-Fickian oxygen transport (intercept, R_NP) components. R_P is mainly derived from the intermolecular diffusion of oxygen through the gas channels of the bipolar plate and GDL, whereas R_NP originates from Knudsen diffusion or diffusion through liquid water or ionomers. R_CH, R_DM, and R_MPL represent the oxygen transport resistances in channels of flow field plate, diffusion media, and micro-porous layer, respectively.

Fuel Cell Single-Cell Simulation

Model Construction

Using the fuel cell module of COMSOL Multiphysics software, a model of the sandwich structure containing the GDL, gas diffusion electrode (GDE), proton exchange membrane (PEM), and flow channel is built, with the size and shape referenced to the actual single-cell flow channel design. The total number of meshes is about 450,000, with key areas refined.

Parameter Configuration

According to the actual material properties, the material parameters such as electrical conductivity, thermal conductivity, porosity, diffusion coefficient, etc. for each part are set. Boundary conditions such as flow rate, pressure and mole fraction of oxygen are set at the inlet of the flow channel, pressure conditions are set at the outlet, and other boundaries are set according to the operating standard conditions. The initial mole fraction of oxygen is close to the inlet value, and other variables are set as reasonable estimated values.

Simulation

The steady-state solver is used to optimize the iteration accuracy and convergence conditions and then carry out the calculation, and monitor and deal with the numerical problems in real time. The results of physical field distributions such as oxygen mole fraction and current density are extracted after convergence.

Analysis and Validation of Results

The distribution patterns of oxygen mole fraction across different regions are examined, and the relationships with the flow channel structure, current density, and other factors are analyzed. By comparing the simulation results with the actual test data, if significant deviation is observed, the model is iteratively refined and troubleshooted until the results closely align, thereby verifying the model's validity.

Results and Discussion

Construction of Composite CCL

Carbon supports, which are integral to the efficacy of ORR electrocatalysts, function as conductive scaffolds that not only facilitate electron transfer but also disperse the electrocatalytic materials, thereby maximizing the surface area and enhancing the accessibility of reactants to the active sites. Among the common carbon supports utilized in PEMFC applications, Vulcan carbon and Ketjenblack carbon stand out as the most prevalent, each possessing a unique set of properties that significantly influence their performance as catalyst supports in the context of ORR catalysts, thereby having substantial implications for the overall efficiency of PEMFC [56]. Vulcan carbon, frequently designated as Vulcan XC-72, is a commercially accessible form of carbon black that has garnered a reputation for its exemplary electrical conductivity, expansive surface area, and ideal pore structure, all of which are pivotal for its role as a catalyst support [57, 58]. Owing to its capacity to accommodate a substantial quantity of Pt nanoparticles on its outer surface, the CCL engineered with Vulcan-supported electrocatalysts demonstrates an enhanced capability for local oxygen and proton transport, which translates to an elevated power density under HCD, thereby solidifying Vulcan's status as a favored catalyst support in PEMFCs [4]. However, the utilization of Vulcan carbon has its drawbacks, particularly its heightened susceptibility to ionomer poisoning, a consequence of the increased likelihood of sulfonate ions from the ionomer being specifically adsorbed onto Pt nanoparticles, which in turn diminishes the electrocatalytic activity [59]. In contrast, Ketjenblack carbon, classified as a porous carbon, is distinguished by its high surface area and distinctive hierarchical structure, which endows it with an elevated level of porosity and remarkable electrical conductivity [4, 58, 60, 61]. These properties are capable of augmenting the dispersion of Pt catalysts and circumventing Pt poisoning by the ionomer, achieved by sequestering Pt nanoparticles within the pores [59]. Nevertheless, the porous structure of Ketjenblack carbon, while beneficial in activation overpotential of electrodes, also imposes limitations on the accessibility of interior Pt nanoparticles to exterior reactants and ionomers, thereby markedly curtailing local mass transport and adversely affecting the HCD performance. By weighing the merits and demerits of these two carbon support types, the proposed strategy of this study is to harness the synergistic benefits of both Vulcan and Ketjenblack carbons, thereby constructing a composite CCL that amalgamates their respective advantages.

Therefore, as shown in Figure 1A, by employing two types of commercially available catalysts, Pt/V and Pt/KB, our strategy is predicated on the distinct impacts of these two carbon support structures on both electrocatalytic activity and mass transport phenomena within the CCL. Notably, Vulcan is particularly adept at enhancing the local mass transport of oxygen but is prone to causing Pt poisoning; in contrast, Ketjenblack, while effective in safeguarding Pt from ionomer-induced poisoning, demonstrates a less robust mass transport of reactants. Furthermore, the design of the CCL meticulously considers the concentration gradients of oxygen and protons, ensuring a heightened proton concentration adjacent to the PEM and an elevated oxygen concentration in proximity to the GDL. Consequently, our design of the composite CCL strategically positions the Pt/V layer closer to the PEM, utilizing a lower I/C of 0.6. The abundance of exterior Pt nanoparticles aids in diminishing the local transport of oxygen and protons, while the reduced ionomer content attenuates its deleterious poisoning effect on the exterior Pt nanoparticles. In contrast, the Pt/KB layer is strategically positioned closer to the GDL, where the elevated oxygen concentration can partially ameliorate the issue of oxygen transport within the porous carbon. Moreover, the increased ionomer content, with a higher I/C of 0.9, facilitates proton conduction particularly in regions distant from the PEM, and the porous structure of Ketjenblack is capable of significantly suppressing internal Pt poisoning. As shown in Figure 1B, the composite Pt/V + Pt/KB CCL has a total thickness of approximately 6 µm. The distinct porous structures (Figure 1C) are observed near the center of the CCL, which corresponds to the results of the BET test (Table S1). This results from the disparate carbon supports confirm the successful construction of this composite layer.

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MEA Performance

Reference CCLs with identical Pt loading were meticulously crafted using individual Pt/KB and Pt/V compositions for comparison. Figure 2A,B provides a comprehensive summary of the geometric performance characteristics of Pt/KB, Pt/V, and the composite Pt/V + Pt/KB CCLs during H₂/O₂ polarization experiments. These experiments were conducted under both 100% and 25% RH conditions, and the data presented reflect the maximum performance achieved post the application of VR cycles (H₂/air, 0.1 V, 4 h, 40°C, 150% RH). Consistent with the findings of prior research [62], as depicted in Figure 2C, the Pt/KB CCL demonstrates superior kinetic performance, with mass activity (MA) and specific activity (SA) that are more than twice as high as those measured for the Pt/V CCL. The rationale behind this disparity lies in the unique structure of porous Ketjenblack supports, which allows for Pt to be distributed not only on their surface but also within their intricate pore network. This distribution shields the Pt within the carbon's pores from the detrimental effects of sulfonate anion functional groups present in Nafion, thereby enhancing its performance [59]. It is noteworthy that, as illustrated in Figure 2C, the composite CCL of Pt/V + Pt/KB demonstrates kinetic performance comparable to that of the Pt/KB CCL, in terms of both MA and SA. This equivalence can be attributed to the inclusion of a porous carbon substrate within the composite CCL, which mitigates the poisoning effects on Pt.

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Due to the limitations of H₂/O₂ polarization curves in providing insights into proton transport and, particularly, gas transport-related losses, a comprehensive analysis was conducted using H₂/air polarization curves under both wet and dry conditions, as depicted in Figure 2D,E. The composite CCL consisting of Pt/V + Pt/KB demonstrates a superior power density at HCD, which is indicative of an enhanced mass transport of reactants. Under saturated conditions of 100% RH, the composite CCL of Pt/V + Pt/KB manifests a peak power density of 1.03 W cm⁻², surpassing the performance of Pt/KB CCL and Pt/V CCL, which exhibit power densities of 0.78 and 0.89 W cm⁻², respectively. This enhancement in HCD performance becomes particularly pronounced under dry conditions, where proton transport is significantly constrained. Its merits emphasize that, under 25% RH, the system experienced an acute scarcity of water, resulting in suppressed proton transport and diminished overall MEA performance; notwithstanding, the MEA performance of the composite Pt/V + Pt/KB CCL was markedly superior to that of the other two CCLs. As encapsulated in Figure 2F, at a current density of 1.5 A cm⁻², the composite Pt/V + Pt/KB CCL exhibits a considerably higher cell potential (0.31 V) and the corresponding power density (0.46 W cm⁻²), whereas Pt/KB CCL demonstrates 0.16 V and 0.23 W cm⁻², and Pt/V CCL demonstrates 0.22 V and 0.34 W/cm⁻². The enhanced HCD performance can be attributed to the gradient in carbon porosity and the variation in Nafion content within the composite CCL, which collectively contribute to the development of a more conductive structure, thereby significantly augmenting the overall performance of the MEA and enhancing its efficiency and effectiveness under various operating conditions.

Improved Proton Conduction

In this study, the porous carbon carrier Ketjenblack EC3000J is utilized as the primary carbon carrier, with Pt nanoparticles dispersed both internally and externally. Pt nanoparticles can be accessed through tiny microporous openings on the carbon surface, measuring approximately 1–2 nm, whereas ionomer chains, typically larger than 10 nm, are generally inaccessible due to their larger size [23]. Consequently, the proton accessibility of Pt nanoparticles within the carbon carrier pores is significantly influenced by the presence of condensate in the micropores and mesopores. The utilization of Pt nanoparticles within the microporous and mesoporous regions can be quantified through electrochemical CO adsorption/dissolution measurements, which require water as a reactant (Pt-CO_ads + H₂O → CO₂ + 2H⁺ + 2e⁻); this water can be provided either by condensate molecules in the CL or by bound or unbound water within the ionomers. CO adsorption/dissolution measurements were conducted as a function of RH to elucidate the accessibility of Pt on the inner and outer surfaces of the carbon primary particles. Pt utilization refers to the effective ECSA of Pt, quantified by CO adsorption/dissolution measurements. At each RH condition, Pt utilization is calculated by dividing the ECSA value for CO dissolution at that specific RH by the value at 100% RH. This approach accounts for the accessibility of Pt sites, influenced by both the ionomer distribution and transport properties, such as proton conduction and water retention (Figure S1). Because the electrochemical oxidation of CO requires water and proton accessibility either through ionomer or water condensed on the catalyst, as the RH decreases, Pt nanoparticles that are not in proximity to ionomer will no longer participate in the reaction. The structure of Ketjenblack carbon carrier is predominantly microporous rather than mesoporous, and it tends to lack water at low RH, resulting in poor proton conduction to the catalysts not covered by ionomers, as illustrated in Figure 3A. Consistent with previous findings [23], at 25% RH, the Pt utilization in the Pt/KB CCL is limited to only 30%. In contrast, within the Pt/V CCL, where the majority of Pt nanoparticles are strategically positioned on the carbon surface and readily accessible to ionomers, the Pt utilization at dry conditions is observed to be higher than that of Pt/KB CCL. However, in the specific context of our study, the Pt accessibility at 25% RH is approximately 73%, a figure that falls somewhat short of the values reported in prior studies [23]. This discrepancy may be contributed to the suboptimal distribution of ionomers within the CL in our MEA fabrication system. In the composite Pt/V + Pt/KB CCL, the Pt utilization rate can achieve 56% at 25% RH, a significant improvement compared to the Pt/KB CCL and more closely aligning with the performance observed in the Pt/V CCL. This improved Pt accessibility with the addition of Pt/V layer, compared with the single Pt/KB CCL, is mainly attributed to the additional proton transport channels provided by the Vulcan-based structure. Indeed, proton transfer on the catalyst surface primarily occurs through two mechanisms: the Grotthuss mechanism, which utilizes sulfonic acid groups as carriers, and the carrier mechanism, which relies on water for proton transport [63]. The findings indicate that the improved proton transfer across the composite Pt/V + Pt/KB CCL is likely influenced by both mechanisms in tandem.

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The improved proton transport of the composite CCL was further validated through EIS, a widely employed technique to assess the effective $R_{{\mathrm{H}}^{+}}$ within the CCL. The corresponding equivalent circuit diagram is shown in Figure S2. Figure 3B presents the Nyquist plots for the three MEAs under two distinct humidity levels: 100% RH and 50% RH. The impedance response of the Pt/V CCL demonstrates a slower increase at low frequencies compared to CCLs of Pt/KB and Pt/V + Pt/KB, indicating a higher $R_{{\mathrm{H}}^{+}}$ for the Pt/V layer. As shown in Figure 3C, at 100% RH, the $R_{{\mathrm{H}}^{+}}$ values for CCLs of Pt/KB, Pt/V, and Pt/V + Pt/KB were 64, 80, and 50 mΩ cm², respectively, confirming that incorporating higher amount of Nafion (I/C = 0.9) into the Pt/KB CCL significantly decreased the proton conduction resistance compared to the Pt/V layer with I/C ratio of 0.6. Furthermore, the design of the composite Pt/V + Pt/KB CCL resulted in a resistance comparable to that of the Pt/KB CCL, while being significantly lower than that of the Pt/V CCL, indicating a more efficient and smoother proton conduction pathway within the composite CCL.

The proton transport within the CCLs, particularly under dry conditions, is significantly influenced by both the local concentration of protons and the structure of ionomer networks. To investigate the effects of local proton concentration and ionomer loading on proton conduction, we initially fabricated two MEAs, each featuring a CCL with a thickness of 3 µm. One CCL consisted of Pt/V with an I/C of 0.6, whereas the other consisted of Pt/KB with an I/C of 0.9. Owing to the reduced thickness of the CCL and the higher local proton concentration, the proton transport impedances for both configurations are fairly similar, as illustrated in Figure S3. This suggests that near the PEM attachment, the higher local proton concentration facilitates efficient proton conduction within the CCL adjacent to the PEM, despite the lower Nafion content (I/C = 0.6) in the Pt/V CCL. On the other hand, the proton conduction at the end further away from the PEM was evaluated by constructing two types of Pt/V CCLs; as shown in Figure S4, the I/C ratio in the CL closer to the PEM was 0.6, whereas the I/C ratios in the CL closer to GDL were varied to 0.6 and 0.9, respectively. As shown in Figure S5, in the area close to the GDL end, under conditions of reduced local proton concentration, the Pt/V CL with a higher ionomer loading (I/C = 0.9) exhibits lower proton transport impedance, suggesting that an increased ionomer loading enhances the completeness of the ionomer network, thereby facilitating proton conduction. Consequently, as illustrated in Figure 3D, when the CCL is thicker and both the proton concentration gradient and the ionomer network affect proton conduction, the Pt/KB CCL, owing to its higher I/C ratio, exhibits superior proton conduction compared to the Pt/V CCL. Regarding the composite Pt/V + Pt/KB CCL, since proton conduction near the PEM is minimally influenced by the I/C ratio and the partial layer further from the PEM utilizes a higher I/C ratio (I/C = 0.9), the composite CCL demonstrates effective proton conduction by leveraging variations in ionomer loading.

Improved Oxygen Transport

Within the cathode of PEMFCs, oxygen molecules, driven by a convective pattern, traverse the flow channel and subsequently permeate through the GDL, ultimately diffusing towards the CCL as a result of the concentration gradient that exists [64]. Eventually, in the final stage, oxygen molecules must dissolve into the ionomer film that coats the surface of the catalyst particles, a process that can only be achieved after they have successfully surmounted a series of complex oxygen transfer barriers that hinder their progress [65]. Consequently, the substantial oxygen transport resistance at the ionomer–Pt interface has long been theorized to be the primary culprit behind the significant performance loss observed at HCD for PEMFC with low Pt loading [31]. Furthermore, the transport of oxygen is even more severely restricted in catalysts that are supported by Ketjenblack, primarily because a significant proportion of Pt nanoparticles are situated deep within the inner pores of the support materials, making it exceedingly difficult for oxygen to access the active sites [59]. Within the CCL, not only do ionomers play a crucial role in binding together carbon particles and their agglomerates, forming a variety of porous structures that differ in size, but also the surface of the Pt nanoparticles is typically enveloped by the thin PFSA film, with a thickness usually ranging from 7 to 10 nm [66]. As a result, oxygen must first diffuse through the intricate network of pores within the CCL before it can permeate through the PFSA film to reach the catalytic surface. Lopez-Haro et al. [67] found that the I/C ratio exhibits a positive proportionality with the extent of Nafion layer coverage on carbon supports, a finding that has led to the general acceptance that adjusting the I/C ratio can serve as an effective tool for controlling oxygen transport within these systems [13].

To better understand the relationship between oxygen transport and ionomer/catalyst interactions, a combination of in situ CO displacement chronoamperometry and limiting current measurement was performed. Figure 4A shows the conversion currents of CO displacement corrected using the steady-state background current. It becomes evident that, despite the lower ionomer amount (I/C = 0.6), the CCL of Pt/V exhibits a larger peak area compared to that of the CCL of Pt/KB, implying that a significantly higher number of Pt sites are occupied by a sulfonate group, primarily due to the greater density of Pt sites on the outer surface of the carbon. The extent of sulfonate group coverage was quantitatively determined by dividing the charge in CO displacement by the charge in CO stripping (Figure S6). According to Figure 4B and Table S2, the sulfonate group coverage for the CCL of Pt/V stands at a substantial 20%, which is significantly higher than the 9% observed for the CCL of Pt/KB. For the composite CCL, the overall sulfonate group coverage amounts to 12%, and when compared to the CCL of Pt/V, the mitigated specific adsorption of sulfonate anion can be attributed to the protection effect conferred by the porous structure of Ketjenblack support.

[IMAGE OMITTED. SEE PDF]

To investigate the oxygen transport issue within the CCL thoroughly, the R_Total was assessed for all samples using the limiting current technique. Figure 4C shows the R_Total of the cathode electrode consisting of both the R_P and the R_NP. R_P primarily results from the intermolecular diffusion of oxygen through the gas channels of the bipolar plate and the GDL, whereas R_NP originates from Knudsen diffusion or diffusion through liquid water or ionomers [68]. Pt/KB exhibits a higher R_Total than Pt/V within the studied backpressure range, indicating that the increased presence of ionomers impedes oxygen transport. Additionally, Pt nanoparticles located within the pores are unlikely to participate in the oxygen transport process, resulting in a greater oxygen transport resistance. As shown in Figure 4D, the R_NP of the Pt/KB CCL is 0.48 s cm⁻¹, which is significantly higher than that of the Pt/V CCL (0.35 s cm⁻¹). This is consistent with a previous study suggesting that the ionomer is an important factor contributing to the increase in R_NP at low Pt loading [31]. A similar R_NP to that of the Pt/V CCL was observed for the composite Pt/V + Pt/KB CCL. Given that R_NP acts as the highest transport-related voltage loss mechanism [31], the differences in R_NP likely arise from variations in the local ionomer/catalyst interface. In our case, the composite CCL exhibits a similar R_NP to the Pt/V CCL due to the presence of solid carbon support with higher Pt accessibility [42] and a thinner ionomer film adjacent to Pt sites [31]. Taken together through Figure 4D, the composite CCL has significant advantages in terms of mitigating Pt poisoning, proton transport, and localized oxygen transport.

Another factor that the design principle of the composite CCL leverages is the oxygen concentration gradient across the entire CCL. To gain a deeper understanding of oxygen diffusion across the CCL, we conducted a fuel cell single-cell simulation of oxygen diffusion at the cathode (Figure S7). This simulation is a comprehensive single-phase diffusion model that considers both the transport process and the concentration distribution of oxygen in the fluid flow within porous media. As shown in Figure 4E, near the gas feeding inlet, apparently, all CCLs exhibit a pronounced gradient distribution of oxygen concentration within the CL, extending from the GDL towards the PEM. However, the membrane electrode structure featuring a Pt/KB CCL displays a less uniform distribution of oxygen concentration throughout the reaction system. The steeper gradient of oxygen concentration in the Pt/KB CCL can be resulted from the higher ionomer loading, which tends to obstruct oxygen diffusion to the catalyst and leads to a relatively lower oxygen concentration at the PEM end. The lower R_NP of the Pt/V CCL (Figure 4D) also suggests superior interfacial resistance, either at the gas/ionomer interface or at the ionomer/Pt interface, compared to those of the Pt/KB CCL [69]. During the construction of the composite Pt/V + Pt/KB CCL, the inclusion of a Pt/V layer with lower ionomer content within the Pt/V portion leads to enhanced oxygen concentration throughout the CL. The trend of the oxygen concentration gradient near the gas outlet is observed to be similar to that near the gas inlet (Figure S8). Therefore, it can be concluded that, compared to the Pt/KB CCL, the composite Pt/V + Pt/KB CCL performs to improve the utilization of oxygen and consequently contributes to the improved cell performance.

Conclusion

In this work, we leveraged the structural advantages of solid carbon support (Vulcan) and porous carbon support (Ketjenblack), in conjunction with the interactions between ionomers and the transport of proton and oxygen, to construct a composite CCL of Pt/V + Pt/KB. The Pt/V layer, situated closer to the PEM, features a substantial amount of Pt nanoparticles on the carbon support surface, thereby enhancing proton accessibility, especially under low RH conditions. Additionally, the lower amount of ionomer minimizes the transport resistance of oxygen. In contrast, the Pt/KB layer, positioned closer to the GDL, features a unique porous structure that protects Pt from poisoning by sulfonic acid groups, while the higher ionomer content facilitates proton transfer away from the PEM end. The enhanced performance of the composite CCL originates from synergistic effects: the surface Pt on Vulcan improves reactant accessibility near the membrane, while porous structure of Ketjenblack protects catalytic sites from poisoning. This architecture establishes continuous proton pathways while maintaining effective gas diffusion channels. This design provides a novel approach for optimizing the CL design based on the geometry of carbon supports, thereby enhancing electrode mass transfer and kinetic performance.

Acknowledgments

The authors would like to thank ZEISS in China for the x-ray microscope analysis. The work was financially supported by National Natural Science Foundation of China (22202124 and UA22A20429), Shanxi Scholarship Council of China (2023-008 and 2023-009), Shanxi Outstanding Project Selection and Support Program for Overseas Scientific and Technological Activities (20230002), Science and Technology Innovation Teams of Shanxi Province (202304051001023), the Key Research and Development Program of Shanxi Province (No. 202302060301009), Qingdao New Energy Shandong Laboratory Open Project (QNESL OP), and Shandong Provincial Natural Science Foundation (Nos. ZR2024QB175 and ZR2023LFG005).

Conflicts of Interest

The authors declare no conflicts of interest.

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