Surfactant-Assisted Catalyst Ink Dispersion for Enhanced Cell Performance of Proton Exchange Membrane Fuel Cells

Abstract

This study examines the effects of several commercial surfactants on the dispersion of catalyst inks for proton exchange membrane fuel cells (PEMFCs). Catalyst inks containing Pt/C were spray-coated and assembled into membrane electrode assemblies (MEAs) by hot pressing. The structural and electrochemical properties of the resulting catalyst layers were characterized through particle size analysis, zeta potential measurements, contact angle determinations, and single-cell performance tests. Among the formulations evaluated, the ink with non-ionic surfactant Triton X-100 (TX) delivered the best performance, achieving a current density of 1134 mA/cm² at 0.3 V—substantially higher than that of the surfactant-free control. These findings provide practical guidance for selecting appropriate surfactants to optimize catalyst-ink preparation and enhance PEMFC performance.

1. Introduction

The continued reliance on fossil fuels and the associated increase in greenhouse gas emissions have led to severe climate change and environmental degradation [1,2,3]. In response, the international community adopted the Paris Agreement, committing to limit the global average temperature rise to below 1.5–2 °C. In alignment with these global initiatives, South Korea has announced its goal of achieving carbon neutrality by 2050 and is actively advancing research and development in alternative and renewable energy technologies to meet this target [1,3]. Among various clean energy solutions, fuel cells have gained significant attention due to their high efficiency and near-zero greenhouse gas emissions. PEMFCs are emerging as promising power sources for both transportation and stationary energy applications, offering substantial reductions in carbon dioxide emissions [4,5,6].

In PEMFCs, hydrogen gas is oxidized at the anode, generating electrons that travel through an external circuit, while protons migrate through the electrolyte membrane to the cathode. There, the proton reacts with oxygen and the returning electrons to produce water as the sole byproduct [4,5,7]. A key component of PEMFCs is the MEA, which critically influences both performance and durability. Extensive research has focused on optimizing MEA design and functionality [7,8,9,10], including efforts to enhance catalyst activity, reduce mass transport resistance, and improve catalyst ink formulation through adjustments in catalyst loading and physicochemical properties [11,12,13].

For example, Yang et al. [14] investigated different methods of ionomer dispersion during catalyst ink preparation. Their approach involved separately dispersing ionomers and then re-dispersing other components using additional equipment. The study showed that improved ionomer mobility in the solvent led to better pore structure, reduced resistance, and a higher electrochemically active surface area, ultimately boosting fuel cell performance [13,15,16,17]. Regarding Nafion ionomer content, several studies [18,19,20] have reported that insufficient ionomer levels fail to establish a continuous network among catalyst particles, thereby reducing both electronic and ionic conductivity. On the other hand, excessive ionomer content can obstruct gas diffusion and diminish Pt catalyst activity, thus limiting mass transport. Optimal ionomer content, especially under low Pt loading conditions, has been reported to range between 20 and 35 wt.% [16,21,22]. Zhekun Chen et al. [23] evaluated the effects of surfactants and carbon materials on the structural and compositional uniformity of the microporous layer (MPL). They discovered that ink with a high degree of network growth produces an MPL with a low density of cracks. Conversely, ink with a high polydispersity index results in an MPL characterized by poor crack homogeneity. Additionally, both polar surfactants and the non-polar polymer polytetrafluoroethylene (PTFE) are insoluble, leading to a heterogeneous distribution of PTFE. On the other hand, Jülide Hazal Özdemir et al. [24] investigated the synthesis and characteristics of platinum (Pt) nanoparticles on immobilized carbon supports. They noted an approximately 12% reduction in the Pt content of the catalyst synthesized using Tween 80 as a surfactant. However, there was an enhancement of 85% in the electrochemically active surface area, which significantly improved the performance of the single-cell proton exchange membrane fuel cell (PEMFC).

In addition, the type and composition of dispersion solvents have been shown to significantly affect catalyst ink properties, including viscosity and dispersion uniformity [25,26,27]. These solvent characteristics influence catalyst particle aggregation within the catalyst layer (CL) and affect the distribution of ionomers in the ink. Therefore, optimizing the catalyst ink formulation is essential for improving MEA performance and overall energy efficiency [28,29]. Lee et al. [30] further studied the impact of surfactants on catalyst inks. Their findings revealed that surfactants facilitated uniform dispersion, leading to smoother and more homogeneous electrode surfaces. In contrast, inks without surfactants exhibited binder aggregation, as indicated by an increased fluorine and carbon content on the electrode surface. These structural differences had a notable impact on both the performance and durability of the electrodes, favoring those prepared with surfactants.

Despite extensive research on catalyst ink manufacturing [11,13,28,29], limited studies have examined the structural, physical, and performance-related effects of surfactant incorporation, even though surfactant-enhanced dispersion and performance improvements have been reported [31,32,33,34]. Therefore, further investigation into the use of surfactants with varying ionic properties during MEA fabrication is warranted. Non-uniform dispersion of catalyst particles often leads to agglomeration, which can prevent uniform ionomer coverage and reduce electrochemical activity, thereby degrading fuel cell performance. While some progress has been made in optimizing ink formulations, comprehensive investigations into the influence of surfactant type and concentration on dispersion uniformity and catalyst layer structure remain insufficient.

This study aims to improve MEA performance by enhancing catalyst ink dispersion through the incorporation of various surfactants during electrode fabrication. The physicochemical characteristics of the resulting electrodes were analyzed using scanning electron microscopy (SEM), particle size distribution, and contact angle measurements. The inclusion of surfactants effectively suppressed catalyst particle agglomeration and improved the utilization of Pt/C catalysts. Single-cell PEMFC performance tests further confirmed that surfactant-assisted dispersion led to enhanced catalyst distribution and significantly improved overall cell performance.

2. Results and Discussion

The addition of surfactants has been reported to inhibit particle aggregation in solution, as demonstrated in previous studies [31,34]. The dispersion quality of catalyst ink is influenced by the physicochemical properties of the surfactants, particularly their ionic charge, which can affect the ionomer–catalyst interaction and subsequently enhance the electrochemical activity of platinum catalysts after ionomer coating. Figure 1 presents a schematic illustration of the catalyst ink preparation process incorporating various surfactants during electrode fabrication. Compared to inks without a surfactant, those containing surfactants exhibit markedly reduced particle aggregation, resulting in more uniform dispersion within the ink.

Catalyst inks prepared with and without surfactants were evaluated for sedimentation behavior over time using a filter paper method. The control sample, designated as SD, contained no surfactant and was tested alongside formulations containing various surfactants for comparison. Due to the dark color of the catalyst ink, a direct visual assessment of sedimentation was challenging. To overcome this, the upper layer of the ink—containing relatively fewer particles after sedimentation—was carefully dropped (50 μL) onto white filter paper to assess color brightness.

A lighter color on the filter paper indicated greater sedimentation of catalyst particles and thus poorer dispersion stability, whereas a darker color suggested that more particles remained suspended, reflecting a better dispersion quality [35,36]. The results, presented in Figure 2, show the evolution of dispersion stability over time. Observations were initially recorded at intervals of 30 min, 1 h, and 3 h, followed by extended intervals at 12 and 24 h to evaluate long-term dispersion behavior. After 4 days, minimal changes in color intensity were observed across all samples, indicating that the dispersion stability of the catalyst inks had reached equilibrium and became comparable.

Particle size analysis based on volume distribution (

Table 1. Zeta potential and particle size data for samples according to the presence of ionomers.

Sample	Zeta Potential (mV)		Particle Size (µm)		Change (%)
	Without Ionomer	With Ionomer	Without Ionomer	With Ionomer
SD	−25.61	−20.12	0.712	0.359	49.58
DA	−21.65	−25.45	0.605	0.267	59.00
TX	−25.43	−21.69	0.555	0.248	55.31
BB	−24.51	−16.98	0.668	0.327	51.04
CC	−27.04	−30.52	0.672	0.558	16.96

and Figure 3) reveals that adding the ionomer reduces the mean particle diameter in every catalyst-ink sample. This decrease stems from Nafion’s amphiphilic nature: its hydrophobic polytetrafluoroethylene (PTFE) backbone (C₂F₄)_n and hydrophilic sulfonic acid (-SO₃H) side chains enable it to act like a surfactant [7,17,37,38]. Upon ionization, the sulfonic acid groups become negatively charged, generating electrostatic repulsion that suppresses aggregation and improves dispersion [14]. The greater size reduction produced by the ionomer than by the surfactants mainly reflects dosage differences: Nafion was introduced at 25 wt.% of the catalyst mass, whereas each surfactant accounted for only 1 wt.% of the total solids [31,32].

Comparing the surfactant-free SD ink with the surfactant-containing inks (Figure S3) shows that most surfactants further decreased particle size [31,33]. In particular, the DA and TX formulation displayed ~30% smaller particles than the SD control. By contrast, the CC formulation produces larger particles, and the BB formulation showed no meaningful change. These outcomes imply that, for CC and BB, the hydrophilic head groups, especially under cationic or zwitterionic conditions, diminish net electrostatic repulsion by accumulating positive charge, thus promoting re-aggregation rather than dispersion [39]. Particle size distribution curves (Figure S4) confirm a pronounced narrowing upon surfactant addition: both intensity and cumulative distributions shift toward smaller diameters. Among the surfactants tested, TX yields the narrowest distribution, whereas CC exhibits the broadest. This refinement underscores the effectiveness of appropriate surfactants in tailoring ink microstructures.

Zeta potential data (Table 1) provides complementary insights. In aggregated ionomer domains, sulfonic acid groups are largely buried; when the ionomer is well-dispersed, more sulfonic acid groups are exposed, dissociate, and adsorb onto the catalyst surface, driving the zeta potential to more negative values—an indicator of improved ionomer coverage and dispersion [14,40].

Figure 3a shows the electrostatic environment of the surfactant-free SD ink. After ionomer addition, the zeta potential shifts to more negative values, implying that Nafion’s sulfonate side chains orient toward the catalyst surface and increase the surface charge density. In the DA formulation (Figure 3b), the anionic surfactant’s negatively charged head groups repel the ionomer’s sulfonate moieties, further enhancing the net surface negativity and lowering the zeta potential [41]. For the TX ink (Figure 3c), the non-ionic surfactant introduces no additional charge, yielding a zeta potential comparable to that of the SD control; the slight decrease suggests that the ethoxylate head groups orient away from the hydrophobic Pt surface. In the BB sample (Figure 3d), the zwitterionic surfactant acquires cationic character under the mildly acidic measurement conditions, partially neutralizing the ionomer-derived negative charges and producing a less-negative zeta potential [31]. By contrast, in the CC formulation (Figure 3e), the cationic surfactant’s quaternary-ammonium head groups promote additional ionomer adsorption yet force the sulfonate groups to point outward, giving the most negative zeta potential of all samples [42].

Figure 4 displays SEM micrographs of electrodes fabricated from catalyst inks both with and without surfactants. In these images, the outer region of the catalyst layer (CL) appears brighter, whereas the CL interior is darker. To avoid any diverse effects on Pt activity, surfactant loading was limited to 1 wt.% of the combined mass of Pt/C and Nafion. At 30,000× magnification, no discernible structural differences emerge among the samples, indicating that the overall electrode morphology is essentially independent of surfactant presence or type in the ink formulation.

Figure 5 displays images of water droplets placed on the surfaces of electrodes prepared from different formulates, with noticeable variations in contact angles between the left and right sides of each droplet. The average of these two angles was used to assess and compare the surface characteristics of the electrodes. All samples exhibited contact angles around or above 130°, indicating that the electrode surfaces were predominantly hydrophobic [43,44]. This hydrophobic nature facilitates effective water removal during fuel cell operation, thereby reducing the risk of flooding [45]. In the case of the SD sample without surfactant (Figure 5a), a contact angle slightly below 130° was observed. In contrast, samples containing surfactants showed contact angles around 135°, suggesting that the surfactants—composed of both hydrophilic and hydrophobic groups, with a dominant hydrophobic component—enhanced the hydrophobicity of the electrode surfaces [46]. As shown in Figure 5b–e, the larger contact angles indicate improved surface hydrophobicity [43,44].

Electrodes prepared from catalyst inks—with and without surfactants—were assembled into MEAs and evaluated in a single-cell PEMFC. Figure 6a shows the polarization (I–V) curves and corresponding maximum current densities, whereas Figure 6b compares the mean current densities at 0.5 V and 0.3 V. The non-ionic Triton X-100 (TX) formulation delivered the best performance, reaching 1134 mA/cm² at 0.3 V; the cationic CC formulation followed closely at 1100 mA/cm². The remaining samples were ranked DA > SD > BB, although their differences were modest. At 0.5 V (Figure 6b), TX achieved 740 mA/cm²—about 3.4% higher than DA (716 mA/cm²). The advantage of TX persisted at 0.3 V, at which TX and CC reached 1134 and 1088 mA cm⁻², respectively. The power density curves Figure 6c corroborate these trends: the surfactant-free SD MEA attained a peak power density of 370 mW cm⁻² at 0.49 V, while TX outperformed all other inks with 390 mW cm⁻². By contrast, CC, DA, and BB yielded lower maxima of 357, 370, and 360 mW cm⁻², respectively. Kinetic analyses (Figure 6d) further highlight the efficacy of TX. The TX-based MEA exhibited the smallest Tafel slope (53.8 mV dec⁻¹), indicating the lowest kinetic overpotential. The slopes increased to 54.6 mV dec⁻¹ for SD, 55.7 mV dec⁻¹ for CC, and 60.0 mV dec⁻¹ for DA.

The open-circuit voltage (OCV, Figure S5) decreased when surfactants were introduced, most likely because surfactant adsorption partially blocked Pt active sites. The SD MEA retained the highest OCV (0.888 V). Among the surfactant-modified inks, TX displayed a moderate OCV of 0.868 V, DA remained comparable to SD, and CC showed the lowest value (0.844 V).

Overall, TX provides the most uniform catalyst dispersion and the best balance of kinetic and mass-transport performance, whereas CC incurs a larger kinetic penalty that offsets its otherwise favorable dispersion.

By comparing the physicochemical characteristics of catalyst inks and electrodes with their corresponding performance test results, we examined how variations in the MEA fabrication process influence the structure and functionality of catalyst layers. Figure 7a illustrates the relationship between particle size and current density at 0.3 V. In general, smaller average particle diameters (≈0.2–0.4 μm) correspond to a higher current density at 0.3 V. The CC electrode is an outlier: despite its larger mean size (>0.5 μm), it still delivers robust performance, implying that factors other than particle size—such as porosity or ionomer distribution—also govern cell efficiency [47]. Inks formulated without surfactants (e.g., SD) disperse reasonably well, yet the ionomer coats the Pt/C particles unevenly, as observed by So et al. [48]. By contrast, the TX, DA, and BB inks form smaller aggregates with more uniform ionomer coverage, which strengthens interfacial contact and boosts electrochemical activity. Although the CC ink yields larger agglomerates, the resulting macropores may enhance catalyst accessibility and gas transport, compensating for its coarser morphology.

Figure 7b presents the correlation between the electrode surface contact angle and current density at 0.3 V. Contact angles span 128–138°, and higher angles consistently correlate with better performance at 0.30 V [49]. Yu et al. [42] showed that fine pore networks (10–100 μm) favor high-current operation, whereas a modest fraction of macropores (100–1000 μm) improves mass transport under heavy loads [50]. Das et al. [51] further demonstrated that a more hydrophobic GDL—indicated by a larger contact angle—expedites water removal, lowers saturation, and eases oxygen transport, all of which raise overall PEMFC output. [52].

In the previous MEA test, the non-ionic surfactant Triton X-100 delivered the best performance. To refine its optimum loading, additional MEAs were fabricated with Triton X-100 at 0.5, 1.0, 2.0, and 3.0 wt.%. Figure 8a shows the polarization curves, while Figure 8b compares the mean current densities at 0.50 V and 0.30 V. The 0.5–2.0 wt.% samples displayed virtually identical behavior, each exceeding 1120 mA/cm² at 0.30 V. By contrast, the 3.0 wt.% sample reached only 1032 mA/cm², an ≈8.5% drop. The same trend appears in Figure 8b: the 3.0 wt.% electrode produced the lowest average current density, whereas performance improved progressively from 0.5 to 2.0 wt.%.

These findings indicate that moderate Triton X-100 loadings enhance catalyst ink dispersion and MEA output relative to the surfactant-free control (SD), but excessive surfactant is detrimental. Concentrations above about 3 wt.% likely disturb uniform catalyst agglomeration and raise internal mass-transfer resistance, as reported by So et al. [47]. The resulting denser catalyst layer impedes reactant transport and lowers overall cell performance.

3. Materials and Methods

Four commercial surfactants—each representing a distinct ionic class—were investigated: anionic 4-dodecylbenzenesulfonic acid (DA; Sigma-Aldrich, St. Louis, MI, USA); nonionic Triton X-100 (TX; Sigma-Aldrich, St. Louis, MI, USA); zwitterionic EMPIGEN^® BB detergent (BB; Sigma-Aldrich, St. Louis, MI, USA); and cationic cetyltrimethylammonium chloride solution (CC; Sigma-Aldrich, St. Louis, MI, USA). Catalyst inks were prepared according to a unified protocol and are identified hereafter by the abbreviation of the surfactant they contain (DA, TX, BB, or CC).

The electrocatalyst was 40 wt.% Pt supported on Vulcan XC-72 carbon (Premetek Co., Cherry Hill, NJ, USA). A 15 µm thick reinforced proton-exchange membrane (FCMT, Anyang-si, Republic of Korea) and a 5 wt.% Nafion ionomer solution (Chemours, Wilmington, DE, USA) was used as received. Isopropyl alcohol (IPA, Samchun Chemicals, Gangnam-gu, Seoul, Republic of Korea) and all other reagents were employed without further purification. Key properties of the surfactants are summarized in

Table 2. Types and characteristics of surfactants used in ink preparation.

Molecular Name	Molecular Formula	Types	Molecular Structure	Abbrev.
4-Dodecylbenzene sulfonic acid	C18H30O3S	Anionic HLB11.7		DA
Triton X-100	C14H22O(C2H4O)n (n = 9–10)	Nonionic HLB13.4		TX
EMPIGEN® BB detergent	C16H33NO2	Zwitterionic HLB11.4		BB
Cetyltrimethyl ammonium chloride	C19H42NCl	Cationic HLB15.8		CC

.

Catalyst inks were prepared by first dissolving each surfactant at 1 wt.% of the combined mass of Pt/C and Nafion. A surfactant stock solution (distilled water: IPA = 6: 4 v/v) was added to a vial containing ~0.05 g of Pt/C (40 wt.% Pt/Vulcan XC-72, Premetek, Cherry Hill, NJ, USA), followed by 10 min of ultrasonication. Nafion ionomer (5 wt.% solution, Chemours, Wilmington, DE, USA) was then introduced at 25 wt.% of the Pt/C mass, and the suspension was magnetically stirred for ≥1 h.

Uniform catalyst layers were formed by spray coating the ink onto gas-diffusion layers (AvCarb 1120, Fuel Cell Store, Bryan, TX, USA) and the membrane, using a hot plate/spray gun setup that enables manual control of spray width and speed, thus minimizing catalyst loss [53]. The target Pt loading was 0.30 mg cm⁻² over an active area of 25 cm². After pre-treating the proton-exchange membrane (15 µm reinforced membrane, FCMT, Republic of Korea; procedure in the Supporting Information), the anode GDL was hot-pressed to the membrane at 120 °C and 150 kgf cm⁻² for 1 min; the cathode GDL was added during final cell assembly (Figure S2).

Particle-size distributions were measured with a Mastersizer 3000 (Malvern Panalytical, Malvern, Worcestershire, UK), and zeta potentials were obtained by electrophoretic light scattering on an ELSZ neo (Otsuka Electronics, Osaka, Japan). Electrode surface morphology was examined by field-emission SEM (Hitachi SU8220, Minato-ku, Tokyo, Japan). Static contact angles were recorded with a Phoenix-Pico goniometer (SEO, Mumbai, India; 0–180° range, ±1° accuracy) by depositing pico-to-nanoliter droplets onto the electrode surface.

Single-cell hardware (Figure S3) comprised two end plates, current collectors, graphite flow-field plates, a 20 µm silicone gasket, and the MEA. Assembly followed Shrivastava et al. [2], using a compression pressure of 70–75 kgf cm⁻² (matched to the 180 µm GDL and 20 µm gasket) to ensure low contact resistance and gas-tight sealing.

Electrochemical testing was performed on a Smart2-PEM system (Wonatech, Seocho-gu, Seoul, Republic of Korea) controlled by WFTS (SMART, version 2.0) software. Hydrogen and air were supplied at stoichiometric ratios of 1.5 and 3.0, respectively. Gas and cell temperatures were set to 90 °C (H₂), 70 °C (air), and 80 °C (cell). Current–voltage and power density curves were recorded to compare the performance of the fabricated MEAs.

4. Conclusions

This study systematically evaluated how different surfactant types and loading influence catalyst ink dispersion and, in turn, PEMFCs’ performance. Adding surfactants together with the Nafion ionomer markedly decreased catalyst particle size and suppressed agglomeration, as confirmed by particle-size analysis and SEM imaging. The resulting inks produced catalyst layers (CLs) with denser, better-interconnected microstructures and more uniform ionomer coverage. Zeta-potential and contact-angle data showed that each surfactant’s physicochemical properties modulated interfacial interactions and surface wettability, thereby shaping CL morphology. Improved dispersion translated into higher catalyst utilization and enhanced cell performance. Among the four surfactants tested, non-ionic Triton X-100 (TX) delivered the best results, reaching 1134 mA/cm² at 0.30 V—surpassing anionic DA, zwitterionic BB, and cationic CC. These results corroborate earlier reports that optimized dispersion is critical for maximizing electrochemical efficiency, especially at low Pt loadings. Overall, the findings provide practical guidance on surfactant selection and underscore the importance of tailoring interfacial properties to further boost PEMFC performance in future applications.