Irene Sinisgalli, Nicolaas J. M. van Leth, and Antoni Forner-Cuenca. “Fluorine-free gas diffusion media for polymer electrolyte fuel cells Part I: Silicone-based microporous layers.” ChemRxiv (). https://doi.org/10.26434/chemrxiv.15008053/v1

Silicone-based microporous layers
SEM and EDS acquisition Top-view and cross-sectional images of GDLs with and without MPLs were recorded using a JEOL JSM IT-100 scanning electron microscope at 10 kV accelerating voltage, 35 pA probe current, and 10 mm working distance. Elemental mapping was conducted by EDS at 10 kV accelerating voltage, with the EDS detector inside the SEM chamber
MPL thickness, mass, and porosity inputs MPL mass was measured by weighing the GDL before and after MPL application. MPL thickness was measured from cross-sectional SEM images at five different locations, and the source states “the corresponding average value and standard deviation was calculated.” MPL porosity (ε(MPL)) was estimated from the calculated MPL composition and known material densities of carbon black, ρ(CB) = 1.9 g cm⁻³, PDMS, ρ(PDMS) = 1.03 g cm⁻³, and KR-220L, ρ(KR-220L) = 1.4 g cm⁻³, according to the equation continued on the following page
Calculated MPL porosity Equation 1: ε(MPL) = 1 − V(solids)/V(MPL) = 1 − {m(MPL) · [ω(carbon)/ρ(carbon) + ω(PDMS)/ρ(PDMS)]}/[d(MPL) · A]. V(solids) represents the true volume of all solid components in the MPL; V(MPL), the bulk volume of the MPL; m(MPL), the measured MPL mass; d(MPL), the measured MPL thickness; and A, the coating area, 3 × 3 cm². The PDF prints the multiplication sign as the letter x and the area exponent as a baseline numeral. The following prose defines ω(CB), while Equation 1 instead prints ω(carbon); likewise, p. 7 gives ρ(CB), while Equation 1 prints ρ(carbon). The parameters ω(CB) and ω(PDMS) denote the mass fractions of carbon black and PDMS in the MPL, respectively. Uncertainty was calculated by propagating the standard deviation of the thickness measurements while m(MPL), A, and ρ were treated as fixed values. Equation 2: Δε(MPL) = (1 − ε) Δd(MPL)/d(MPL). The source states that Δε(MPL) is the standard deviation of the thickness measurements
Mercury intrusion porosimetry acquisition Pore-size distributions were determined using an AutoPore IV 9500 mercury intrusion porosimeter with electrode samples of approximately 100 mg and a 5 cm³ penetrometer. Pore diameters were calculated assuming cylindrical pores and a mercury–carbon contact angle of 130° for both advancing and receding measurements. MPL porosity ε(MPL) was calculated from mercury-intrusion data at maximum saturation, corresponding to the highest applied capillary pressure of 50 MPa, using total cumulative pore volume normalized to sample mass V(pore), in mm³(pore)/g(sample), and bulk MPL volume V(MPL), in mm³(MPL)/g(sample). Equation 3: ε(MPL) = V(pore)/V(MPL) = v(pore)/[v(pore) + ω(carbon)/ρ(carbon) + ω(PDMS)/ρ(PDMS)]. The preceding prose defines uppercase V(pore), while the final ratio in Equation 3 instead prints lowercase v(pore)
Contact-angle acquisition The pronounced superhydrophobicity of the prepared MPL prevented stable droplet adhesion, rendering sessile-drop contact angle measurements not always possible. Surface wettability was therefore assessed using sessile-drop contact angle or sliding angle measurements according to the sample. Measurements were performed at room temperature using a KRÜSS DSA30S drop shape analyzer with a standard xyz stage for sessile-drop experiments and a motorized tilting-stage module for roll-off-angle measurements. The source states, “The instrument was fitted with a Basler high-resolution camera (1920 x 1200 px) and high-speed (2300 fps) and the software ADVANCE AD3221 for automated image analysis.” It then states again that the measurements were carried out at room temperature. For sessile-drop measurements, water droplets were dispensed initially at a fixed distance above the MPL surface and gradually lowered until first visible contact. The needle was withdrawn and contact angle recorded after droplet stabilization. Reported values are averages of three measurements at different locations on each sample using 5 µL water droplets
Sliding-angle acquisition For sliding angle measurements, samples were mounted horizontally on the motorized tilting stage and carefully leveled before testing. A 10 µL water droplet was gently deposited onto the surface using an automated syringe, after which the stage was tilted at a constant angular velocity of 1° s⁻¹. The roll-off angle was defined as the inclination angle at which the droplet initiated sustained motion across the surface. For each sample, three droplets were measured at different surface locations to account for local heterogeneity. The reported roll-off angle is given as the average value, with the corresponding standard deviation reported as the uncertainty
Sylgard-content sample identities The pastes and MPLs are referred to as S5%, S10%, and S25% according to nominal Sylgard content in the dry layers, although the Sylgard content in the precursor pastes differs from these values. The percentage signs appear without preceding spaces in these sample names
Binder-content morphology Sylgard 184-based MPLs with the three targeted binder contents were deposited on a commercial hydrophobized Freudenberg E15H GDL; SEM compared these coatings with the commercial reference material (Figure 2). SEM analysis revealed similarly thin MPLs for S5% and S10%, with image-derived thicknesses of 13–14 µm, whereas S25% exhibited a substantially thicker layer of 41 µm. Gravimetric measurements showed comparable loadings of approximately 0.6 mg cm⁻² for S5% and S10%, and a moderately higher loading of 0.8 mg cm⁻² for S25% (Table 1). Together, the modest increase in loading and pronounced increase in thickness translate into a higher calculated porosity for S25%. The PDF prints both inverse-area exponents as baseline numerals
Paste rheology and deposition behavior S5% and S10% pastes exhibited overlapping viscosity profiles over the entire investigated shear-rate range and showed approximately 1.43 Pa·s at 94.3 s⁻¹, the measured point closest to the shear-rate range relevant to blade coating, above 10² s⁻¹. S25% displayed markedly higher viscosity, reaching 2.54 Pa·s at the same shear rate. The PDF prints viscosity units with an ordinary space rather than a centered dot
Cross-sectional silicon distribution Cross-sectional EDS mapping assessed Si, used as a marker for the Sylgard-based binder, across GDM thickness; extracted Si intensity profiles are reported in Figure S2. Although Si signal was highest in the MPL region, it was also detected in the carbon-fiber substrate, suggesting partial penetration of the Sylgard-based paste into the porous GDL during coating. Because EDS intensity profiles across porous cross-sections can be affected by local morphology and sectioning quality, the data should be interpreted qualitatively rather than as quantitative concentration profiles
Binder-content wettability S5% and S10% exhibited external contact angles of approximately 134°, confirming the effectiveness of PDMS as a hydrophobic binder. For S25%, the 5 µL droplet could not be readily detached from the dispensing needle, preventing reliable contact-angle measurement by the same procedure; droplet volume was increased to 10 µL to facilitate deposition. Given the larger droplet and possible influence of gravitational deformation on measured contact angle, S25% wetting was evaluated by roll-off angle. S25% exhibited 4 ± 1°, indicating very weak droplet adhesion and pronounced water repellency. Increasing Sylgard content from 5 to 10 % did not substantially alter MPL wetting behavior, whereas increasing it to 25 % led to superhydrophobic behavior. The PDF prints spaces before the degree signs in both prose angle values and prints the two prose percentages without preceding spaces; validator-compatible forms are used here
Sylgard-based MPLs properties Table 2 columns are loading (mg cm⁻²), thickness (µm), porosity (%), crack (%), and contact angle (°): S5%, 0.6, 13 ± 1, 74 ± 2, 6.2, 134 ± 1; S10%, 0.6, 14 ± 1, 76 ± 2, 5.7, 134 ± 3; S25%, 0.8, 31 ± 4, 83 ± 2, 7.5, 4 ± 1. Coating loading was determined gravimetrically, MPL thickness from SEM cross-sections, coating porosity according to Equation 1, and cracked area by SEM image analysis using ImageJ. S5% and S10% contact angles used the sessile-drop method with 5 µL droplets (N=3); S25% used the tilting table method with 10 µL droplets (N=3). The Table 2 S25% thickness of 31 ± 4 µm conflicts with the p. 13 prose value of 41 µm
KR25% sample identity The paste and MPL prepared using KR-220L resin are referred to as KR25%. The percentage sign appears without a preceding space in this sample name
Binder-chemistry morphology and rheology KR25% loading is approximately twice that of S25% despite identical coating conditions. At 94.3 s⁻¹, apparent viscosity was 5.26 Pa·s for KR25% and 2.54 Pa·s for S25%. The PDF prints viscosity units with an ordinary space rather than a centered dot. Consistent with its higher areal loading, KR25% also exhibits greater thickness and a larger cracked area
Binder-chemistry silicon distribution Average cross-sectional Si intensity profiles extracted from EDS maps are shown in Figure S4. Although both S25% and KR25% showed detectable Si in the substrate region, the signal decreased more steeply for KR25%, indicating stronger localization of the KR-220L-based formulation near the outer MPL region. The more gradual S25% profile suggests broader distribution of the Sylgard-based coating across GDM thickness
MIP and geometrical MPL porosity MIP measurements compared S25% and KR25% with bare E15H GDL and commercial H15C14 GDM references. Nominal MPL porosities estimated from coating loading and cross-sectional thickness were 84 ± 2 % for S25% and 79 ± 2 % for KR25%; MIP-derived values were lower at 68 % and 72 %, respectively. The PDF prints these four prose percentages without preceding spaces. The geometrical estimate uses apparent layer volume and assumes a uniform coating, whereas MIP probes only mercury-accessible pore volume within the measured pore-size range. The discrepancy was more pronounced for S25%
Reference-material macropores The uncoated E15H substrate exhibits a dominant macropore contribution centered at approximately 20 µm, which is strongly attenuated in H15C14. H15C14 also has lower cumulative intruded pore volume, suggesting that its commercial MPL partially covers the fibrous substrate or restricts mercury access to its larger pores
Coated-substrate pore architecture By contrast, E15H substrates coated with S25% and KR25% MPLs retain cumulative mercury-accessible pore volumes comparable to the bare substrate. Both display a pronounced peak at larger pore diameters of 40–50 µm, with higher intensity for KR25% than S25%. These larger accessible features are likely associated with crack related pathways and/or coating–substrate voids rather than intrinsic MPL porosity, consistent with the larger cracked area observed for KR25%. The PDF prints the range with a hyphen-minus and “crack related” without a hyphen
Submicrometric pore contribution The submicrometric region contributes only marginally to total intruded pore volume. Bare E15H already exhibits a small peak centered around 60 nm, arising from carbon particles. This contribution becomes more pronounced in MPL-coated materials. The dV/dlogd area was integrated from 30 to 300 nm, selected to encompass the approximately 60 nm peak. In agreement with cumulative pore-volume differences over the same interval, integrated pore volume increased from 99 mm³ g⁻¹ for bare E15H to 154 mm³ g⁻¹ for S25%, 195 mm³ g⁻¹ for H15C14, and 238 mm³ g⁻¹ for KR25%. This confirms that MPL coating enhances the submicrometric pore contribution, most strongly for KR25%
Binder-chemistry wettability KR25% exhibited a slightly higher roll-off angle than S25%, which the source describes as indicating lower water droplet adhesion. Both materials displayed superhydrophobic behavior
Sylgard-based and KR-220L-based MPLs properties Table 3 columns are loading (mg cm⁻²), thickness (µm), porosity (%) by Equation 1, porosity (%) by Equation 3, crack (%), and roll-off angle (°): S25%, 0.8, 31 ± 4, 84 ± 2, 68, 7.5, 4 ± 1; KR25%, 1.6, 40 ± 3, 79 ± 2, 72, 11.7, 11 ± 2. Coating loading was determined gravimetrically, MPL thickness from SEM cross-sections, coating porosity according to Equation 1 and 3, and cracked area by SEM image analysis using ImageJ. Porosity footnote a is “Measured according to Equation 1 (weight/thickness)”; footnote b is “Measured according to Equation 3 (MIP)”; roll-off-angle footnote c is “Tilting table method, 10 µL droplets (N=3).” Table 2 on p. 15 reports S25% calculated porosity as 83 ± 2 %, while p. 18 prose and Table 3 report the Equation-1/geometrical value as 84 ± 2 %. In the final Table 3 column, the PDF prints the S25% entry as 4±1 with footnote c attached, but the KR25% entry as 11 ± 2 with footnote c separated by a space; validator-compatible uncertainty spacing is used here
Electrografted-substrate sample identities MPLs coated on commercial E15H substrates continue to be called S25% and KR25%; those coated on electrografted (“EG”) GDLs are denoted EG+S25% and EG+KR25%, respectively
Electrografted-substrate coating morphology For both binders, MPL loading was slightly higher on electrografted H15 than E15H. Cross-sectional images consistently suggest reduced penetration into the electrografted GDL. Cross-sectional Si profiles show broadly similar decay trends for EG+S25% and EG+KR25% and their E15H analogues, with at most slightly stronger surface localization for EG+KR25%. These profiles require caution because the electrografted substrate may contribute to Si background, limiting quantitative comparison of paste penetration from Si intensity. No clear overall layer-thickness or porosity changes were observed relative to E15H coatings. The main morphological variation was larger cracked area
Electrografted-substrate pore architecture MIP compared electrografted H15 without an MPL, EG+S25%, and EG+KR25% with corresponding S25% and KR25% materials on commercial E15H (Figure 6). Unlike E15H, electrografted H15 has no submicrometric-region peak, further supporting the hypothesis that the E15H feature originates from commercial substrate treatment. In Figure S7 cross-sectional EDS, fluorine maps of E15H-based materials show localized dense agglomerates consistent with PTFE-rich domains; no comparable agglomerates occur for the electrografted substrate in elemental maps or cross-sectional SEM. Electrografted H15 has a broader macropore contribution at approximately 20–40 µm, as expected for an uncoated fibrous substrate. Its cumulative intruded pore volume is higher than E15H, increasing from 1290 to 1890 mm³ g⁻¹, consistent with a more open mercury-accessible pore structure. After MPL deposition, EG+S25% largely retains this higher accessible volume, reaching 1750 mm³ g⁻¹ and remaining close to the bare electrografted substrate
Electrografted-substrate pore architecture continuation EG+KR25% shows lower cumulative intruded pore volume of 1450 mm³ g⁻¹, comparable to KR25% on E15H. This suggests that KR25%’s higher loading and greater thickness more strongly restrict access to substrate macropores, making measured pore structure less dependent on the underlying GDL. Loading/thickness and MIP porosity estimates in Table S1 are in closer agreement for EG+S25% and EG+KR25% than for corresponding MPLs on E15H, particularly for S25%. This suggests that on electrografted substrates mercury-accessible pore volume better reflects apparent MPL pore structure, consistent with limited MPL intrusion in cross-sectional images
Electrografted-substrate wettability EG+S25% and EG+KR25% roll-off angles were within experimental variability of their counterparts on E15H (Table S1), indicating that the electrografted GDL does not measurably alter MPL-surface wettability; both samples retain superhydrophobic behavior. On the GDL side, a stable droplet could not be deposited on E15H, while electrografted GDL contact angles were approximately 136–138° (Table S1). The PDF prints this range with a hyphen-minus. The electrografted substrate is therefore hydrophobic but has lower liquid-water repellency than E15H
Paste rheology acquisition Silicone-based MPL paste viscosity was measured as a function of shear rate using stress-controlled rheometer Anton Paar MCR 501 with a 50 mm serrated parallel-plate geometry (PP 50). A logarithmic shear-rate sweep was applied from 200 to 0.02 s⁻¹. Measurements were conducted at 15 °C to minimize solvent evaporation. The viscosity curves compared paste flow behavior
Electrografting cyclic-voltammetry behavior Cyclic voltammetry used H15 GDL in acetonitrile containing 0.1 M TBAPF₆ and 5 mM PDMS–NH₂; potential was cycled 20 times at 50 mV s⁻¹. An additional oxidation feature appeared around 0.5 V in the first cycle and progressively shifted to higher potentials in subsequent cycles. The significant exchanged-charge decrease over successive cycles indicates progressive surface passivation, consistent with successful electrografting
Bare E15H top-view SEM Carbonaceous particles are visible on and between the fibers. Images were acquired at 10.0 kV accelerating voltage, 10 mm working distance, and 35 pA probe current
Electrografted-GDL MPL morphology and wetting Table S1, “Sylgard-based and KR-220L-based MPLs coated onto electrografted GDLs properties,” has columns Loading (mg cm⁻²), Thickness (µm), two identically titled Porosity (%) columns marked with footnotes a and b, Crack (%), and Contact Angle (°). EG + S25%: 1.1, 28 ± 5, 78 ± 4, 73, 11.8, 2 ± 1 by footnote c, and 136 ± 4 by footnote d. EG + KR25%: 1.8, 39 ± 2, 67 ± 2, 72, 13.4, 10 ± 3 by footnote c, and 138 ± 1 by footnote d. Loading was determined gravimetrically, thickness from SEM cross-sections, porosity according to Equation 1 and 3, and cracked area by SEM image analysis using ImageJ. Footnote a: “Measured according to Equation 1 (weight/thickness)”; footnote b: “Measured according to Equation 3 (MIP)”; footnote c: “Tilting table method, 10 µL droplets (N=3)”; footnote d: “Sessile-drop method, 5 µL droplets (N=3), GDL side.” The single Contact Angle column therefore contains a tilting-table value followed by a separate GDL-side sessile-drop value for each sample. The PDF prints 2±1 compactly but spaces the other uncertainties; validator-compatible spacing is used here