Investigation of Fuel Cell Behavior with Different Catalyst Loadings at Varying Humidities and Temperatures

Investigation of fuel cell behavior with different catalyst loadings at varying humidities and temperatures Yurii V. Yakovlev * , Yevheniia V. Lobko , Miquel Gam´ on Rodríguez , Alina Madalina Darabut , Lucinda Blanco Redondo , David Kalabis , Iva Matolínov´ a Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holeˇ soviˇ ck´ ach 2, 18000 Prague 8, Czech Republic HIGHLIGHTS GRAPHICAL ABSTRACT •Systematic study of catalyst loading and ionomer/carbon ratio in PEMFCs. •Loading of 0.4 mg Pt cm −2 balances kinetics, proton resistance, and water management. •Peak performance shifts from 75 % RH (low loadings) to 50 % RH (high loadings). •Higher I/C ratio (0.8) enhances durability and water retention under dry operation. •Results provide design rules for reliable PEMFCs in low-humidity, portable applications. ABSTRACT Proton exchange membrane fuel cells (PEMFCs) suffer from severe performance losses under low-humidity conditions, which limits their deployment in portable and open-cathode systems without external humidifiers. In this study, we systematically investigate the combined effects of platinum (Pt) loading (0.2–1.0 mg Pt cm −2 ), ionomer-to-carbon (I/C) ratio (0.6 and 0.8), and operating conditions (0–100 % RH, 40–70 ◦C) using commercial catalyst materials. We find that performance does not peak at full humidification but instead at intermediate RH values (50–75 %), with the optimum shifting lower as Pt loading increases. An intermediate loading (0.4 mg Pt cm −2 ) maximizes performance by balancing ORR kinetics, proton resistance, and water management. Increasing I/C ratio to 0.8 impairs performance under high humidity but substantially improves both water retention and durability under dry conditions by slowing electrochemically active surface area loss. These results establish clear design principles linking catalyst layer composition to operating environment, providing guidelines for reliable PEMFC operation in practical low-humidity applications such as UAVs and portable devices. 1. Introduction Proton Exchange Membrane (PEM) fuel cells have emerged as a promising alternative to traditional power sources due to their high energy conversion efficiency, reduced emissions, and potential for clean energy generation [1]. However, one of the challenges associated with PEM fuel cells is their sensitivity to operating conditions, particularly at low humidity. In such an environment, especially at elevated temperatures, rapid dehydration of the ionomer phase and membrane dramatically reduces proton conductivity and, as a result, the overall performance [2,3]. In a dry environment, low proton conductivity in the catalyst layer renders the majority of catalyst particles inactive [4]. In * Corresponding author. E-mail address: [email protected] (Y.V. Yakovlev). Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour https://doi.org/10.1016/j.jpowsour.2025.238804 Received 28 August 2025; Received in revised form 27 October 2025; Accepted 7 November 2025 Journal of Power Sources 663 (2026) 238804 Available online 21 November 2025 0378-7753/© 2025 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). addition, membranes demonstrate high ohmic overpotential and low fuel cell performance at high current densities. Moreover, the durability of PEM fuel cells is significantly affected at low humidity levels [5] (see Table 1). External humidifiers are required to control the humidity levels and fuel performance [6]. However, the addition of humidifiers increases the cost and weight of the system and may be appropriate for stationary or high-power mobile systems. In contrast, medium-power solutions such as portable power generators or unmanned aerial vehicles often use open-cathode or air-breathing designs [7,8]. These designs face challenges in achieving proper water management and often exhibit poor performance due to dehydration. Improving the water retention properties of the membrane [9], gas diffusion [10–12], microporous [13,14], and catalyst layer [13,15–17], among others, can improve fuel cell performance at low humidity. Alternatively, adjusting the catalyst layer loading and thickness may also impact the performance. For example, increasing Pt loading improves reaction kinetics, as observed in a study on air-breathing microbial fuel cells, where increasing Pt loading from 0.2 to 2 mg cm −2 boosted fuel cell performance [18]. However, high loadings and thicker catalyst layers can impede mass transport, potentially affecting performance, as noted in Ref. [19]. Therefore, determining the optimal catalyst loading is necessary to achieve the best fuel cell performance [20]. While much of the recent research focus has been on achieving ultra-low Pt loadings (≤0.1 mg cm −2 ) [21–23], the durability and stable performance required for commercial and heavy-duty applications still rely on more moderate loadings ≥0.2 mg Pt cm −2 [24], due to their higher reliability [25,26]. The fundamental interactions between water management, ionomer content, and catalyst loading in these practically relevant systems are not fully understood. Our study addresses this specific knowledge gap. Thus, the objective of this study is to systematically investigate and map the interactions between these key parameters. Unlike recent studies that emphasize ultra-low Pt loadings (<0.1 mg cm −2 ), this work focuses on the practically relevant range of 0.2–1.0 mg Pt cm −2 , where commercial systems still operate for durability reasons. By varying the cathode catalyst loading, the ionomer-to-carbon (I/C) ratio, the operating humidity and temperature, we aim to elucidate the complex tradeoffs between reaction kinetics, mass transport, and water management. The goal is to establish clear design principles that connect catalyst layer composition to optimal performance and durability, providing a crucial guide for the development of reliable fuel cell systems. 2. Experimental 2.1. Catalyst ink preparation For catalyst ink preparation, 250 mg of catalyst powder (Pt 40 % wt@Vulcan) was prewetted with 2 ml of acetone in the glass vial to prevent a catalytic burning of isopropanol vapors. Then, 1.8 ml of the ionomer dispersion (D-521, 5 % PFSA dispersion) was poured in, followed by dilution in the mixture of acetone (8 ml) and isopropanol (10 ml). The prepared mixture was homogenised using a horn-type ultrasonicator (HD-3100, Bandeling SonoPulse) for 20 min with a 5 s on/off working cycle. To prevent the dispersion overheating the vial was cooled with an ice bath. The catalyst ink composition was optimized for the preparation of catalyst layers with an I/C ratio of 0.6. Formulation of the catalyst ink with I/C =0.8 differed in the increased amount of ionomer dispersion, 2.4 ml, with the rest of the routine same. 2.2. CCM preparation Catalyst-coated membranes were prepared by using ultrasonic spraying (ExactaCoat, Sono-Tek) of the catalyst ink onto the cation exchange membrane (Fumapem FS-715-RFS, 15 μ m of thickness. The ultrasonic nozzle has a diameter of 0.635 ±0.005 mm, a working frequency of 48 kHz, and a sonication power of 3 W. The spraying distance from the membrane surface was set to 42 mm. Catalyst ink was supplied using a syringe pump with a rate set at 0.50 ml min −1 . MEA active area was filed with a serpentine pattern, having 5 mm spacing, and the speed of the nozzle forward motion was set at 5 mm s −1 . Fast solvent evaporation from the catalyst layer was guaranteed by securing the membrane on the preheated plate to 65 ◦C. The spraying procedure was optimized to increase the catalyst loading by 0.05 mg Pt cm −2 every cycle. Catalyst loading for the cathode catalyst layer (CCL) had values of 0.2, 0.4, 0.8, and 1.0 mg Pt cm −2 . Anode catalyst layer loading was the same for all samples and equal to 0.3 mg Pt cm −2 . The desired loading was achieved by controlling the number of spraying cycles. A constant geometric area of 4 cm 2 was ensured by using a cover mask with a precut 2 ×2 cm 2 window. Prepared catalyst-coated membranes (CCMs) were stored in the box, and no additional pretreatment was used before measurements. 2.3. Fuel cell measurements The prepared CCMs were sandwiched between two pieces of gas diffusion layer (GDL) with a micro-porous layer (H24C5, Freudenberg) with an area of 4 cm 2 each. This membrane electrode assembly (MEA) was sealed in the graphite cell with a single serpentine flow field. The testing equipment allows constant temperature and humidity operation. Cell temperature was set at 40, 50, 60, and 70 ◦C; the relative humidity of gases was controlled by setting the proper dew point of the bubbler humidifier and balanced at 100, 75, 50, and 25 %. Experiments in a totally dry environment were conducted using a humidifier bypass valve. Conditioning of the fuel cell at each set of temperature and humidity is performed at a constant gas flow of 50 sccm for 6 h. Measurements of fuel cell polarization curves were done after the break-in process described in Ref. [27]. Polarization curves were measured in the galvanostatic mode with 5 mA cm −2 steps every 10 s with simultaneous voltage recording, using a potentiostat (PT2005; Kol ibrik.net). The fuel cell was fed by hydrogen and filtered air with gas-to-current stoichiometric ratios of 1.2 and 5, respectively. Polarization curves were repeated at least 5 times, and average values of peak power density and current density at a voltage of 0.6 V were used in the work. Cyclic voltammetry (CV) was measured using H 2 /N 2 feed with a flow rate of 40 sccm. The anode electrode, owing to its small overpotential, was used as the counter and reference electrode, due to its low overpotential, while the cathode was configured as a working electrode. Table 1 Summary of proporties of studied in the work catalyst layers with different composition. Pt loading (mg cm −2 ) I/C ratio Relative performance Performance stability in humidity range Notes 0.2 0.6 High Worst Thin catalyst layer; strongly affected by dehydration at low RH 0.4 0.6 Highest Good Best performance under moderate humidity (75-50 %RH) 0.4 0.8 Medium Best Improved water retention and stable operation under dry or open-cathode conditions 0.8 0.6 High Good Marginal power gain; increased mass-transport limitations 1.0 0.6 Medium Good Not cost-effective; no additional benefit Y.V. Yakovlev et al. Journal of Power Sources 663 (2026) 238804 2 Voltammograms were recorded at dV/dt rates of 100, 50, and 20 mV s −1 in the potential range from 0.08 to 1 V; measurements were repeated 15 times at each speed. For curves recorded at 50 mV s −1 , a total charge of the hydrogen desorption region was defined for curves recorded at 50 mV s −1 , where the upper limit of integration was 0.4 V, and linear extrapolation of the double layer region to the lower potentials was used as a baseline. Double-layer capacitance was retrieved from the slope of the current at 0.45 V vs. scan rate. Data used to calculate the total charge and double layer current calculation were sampled from the 3rd, 6th, 9th, and 12th scans and then averaged. Potentiostatic impedance spectroscopy (PEIS) was measured in the frequency range of 100 kHz–100 mHz with an excitation voltage amplitude of 5 mV and polarization at 0.45 V. Measurements were performed using H 2 /N 2 gas feed with flow rates of 50/100 sccm and 100 % of relative humidity for the anode and cathode sides, respectively. Both CV and PEIS were measured using the potentiostat SP-150 (BioLogic). 3. Results and discussion Cyclic voltammetry can be used for monitoring the electrochemically active surface of platinum catalyst and total electrode/electrolyte interface area, measured by integration of current of H UPD part and double layer capacitance (C dl ), respectively. During CV measurements no additional water is formed making it more straightforward to achieve equilibrium conditions. Results of such measurements for catalyst layers with different catalyst loadings and the same I/C ratio of 0.6 are presented in Fig. 1. As can be seen from the figure, both the H UPD charge (Fig. 1a) and the C dl (Fig. 1b) increase linearly with catalyst loading. This result indicates that the amount of catalyst active sites, as well as the electrode/electrolyte interface area, increases linearly with catalyst loading in the range of 0.2–1.0 mg Pt cm −2 with slopes (at 100 %RH) of 84.7 mC mg Pt −1 . This, however, is not the case for catalyst particles with low Pt concentration (Pt 20 wt%@Vulcan), where linearity can be broken at concentrations above 0.17 mg Pt cm −2 due to a thicker catalyst layer [28]. Linear dependency for H UPD charge and C dl is observed in the relative humidity range from 100 % to 25 %. However, the values of H UPD charge (unlike C dl ) decrease with humidity for all catalyst loadings in the mentioned range of humidities. Compared to H UPD values at 100 RH%, values at 25 RH% decreased by 30–50 % depending on the catalyst loading. The decrease of H UPD charge at low humidity correlates with recent findings [29], and could be a result of the ionomer shrinkage [30] as well as of less developed water channels, which, according to the recent MD study [31], play a significant role in proton transport. It is worth noting that different types of catalyst supports may influence the coverage of the ionomer, as seen in the cases of Ketjen Black and Vulcan [32,33]. Smaller Vulcan nanoparticles require less ionomer content to form an ionomer percolation network, which affects the optimal I/C ratio and humidity-related behaviour. . Values of C dl have, however, lesser dependency on humidity, which was also observed in Ref. [32]. Although reduced hydration of the catalyst layer could affect double-layer capacitance in a similar way as H UPD charge, the higher sensitivity of the latter may indicate more complicated effects of humidity on hydrogen adsorption. The effect of relative humidity can be further discussed in terms of the dependence of ionomer water content and electrochemical active surface area (ECSA) at different platinum loadings. An equilibrium water content, which is calculated from the relative humidity values (see eq. S(2)), in the ionomer has a dramatic effect on the ECSA values, as was previously observed in Refs. [32,34]. This effect is more pronounced in catalyst layers with lower ionomer content (5–30 wt%) and almost vanishes at high content (50 wt%) [32]. The empirical relationship between water content and ECSA can be written in the following form [34]: ECSA(λ) = ECSA0(1-e-aλ)(1) where ECSA 0 is the ECSA value at 100 %RH, a – constant, λ – equilibrium water content. Fitting of the experimental data by Eq. (1) (Fig. 2a) shows similar behavior for all studied catalyst loadings. The exponent factor, a, is in the range from 0.32 to 0.44, which is similar to the value of 0.32 (at 30 wt%) in Ref. [34]. However, contrary to the study [34], where the parameter a had a positive correlation with Pt loading, in a wider range of loadings, we found its value fluctuating. It is worth noting that the correlation may be hidden by measurement error, and a more delicate experiment is required. The temperature dependency of ECSA has been previously observed in a number of publications [35–37]. The decrease in the apparent value of ECSA can be explained by an increased rate of hydrogen desorption; thus, the maximum value of the hydrogen monolayer coverage at room temperature decreases from 0.77 to lower values [38]. Overall, an Arrhenius-type equation can be employed to describe such changes [34]: ECSA(T) = ECSA0,ref ⎛ ⎜ ⎝e EA R(1 T-1 Tref )⎞ ⎟ ⎠(2) where ECSA 0, ref is the ECSA value at T ref – reference temperature (298K for fitting purposes), E A is an activation energy term, R =8.31 J mol −1 K −1 is the gas constant. As can be seen in Fig. 2b, the experimental data can be fitted well using Eq. (2). at different humidities. The analysis of the data only Fig. 1. H UPD charge (a) and double layer capacitance (b) at different humidities calculated from cyclic voltammograms as a function of catalyst loadings measured at 70 o C. Y.V. Yakovlev et al. Journal of Power Sources 663 (2026) 238804 3 includes measurements at 100 and 75 %RH due to experimental limitations in setting low levels of both humidity and temperature. The fitting parameter, E A , which combines information on H-Pt adsorption energy and H-H repulsion, gets values of 2.77 and 3.67 kJ mol −1 for I/C ratios of 0.8 and 0.6, respectively. These values are lower than those calculated in Ref. [34], which may be related to the different types of supports used in studies. It should be noted that supported catalysts show deviations from thermodynamically predicted temperature dependencies [35] (see also Fig. S1, eq. S(3)). However, the overall effect of variation of humidity and temperature on the catalyst activity and, hence, fuel cell performance is different. While lowering the humidity level could imply a real decrease in the number of catalyst active sites (as indicated by the ECSA drop) and ultimately fuel cell performance, temperature variations, on the other hand, could alter apparent values of ECSA only, rendering the temperature dependency of this parameter inadequate for performance estimation. Increasing catalyst loading and, therefore, CL thickness prolongs proton pathways to the membrane. To assess the influence of loading on the proton conductivity in CL impedance spectroscopy was employed. As can be seen from the Nyquist plot (Fig. 3), all studied MEAs have the same high-frequency resistance (HFR), which corresponds to the resistance of the membrane. The same HFR for different catalyst loading, which also includes the contact resistance between the membrane and CL, indicates the absence of CL delamination at higher loading as well as the similarity of testing conditions. The impedance characteristics of a fuel cell measured with N 2 supply at the cathode typically have linear regions, and the real part of resistance at the transition between two regions can be used for the calculation of CL proton resistance [39]. The calculated proton resistance is shown in the inset of Fig. 3 are in accordance with literature [25] and appears to be a linear function of catalyst loading. The linear dependency of proton resistance in the catalyst layer on loading may indicate the fractal dimension of the proton-conducting network close to 1. This allows us to conclude that the proton-conducting channels in the catalyst layer are rather straight. It is worth noting that such a result is not universal and is dependent on the nature of the catalyst layer. In the work [40], non-linear resistance dependence with catalyst loading of IrO 2 -based electrodes. On the other hand, the Pt/C catalyst in fuel cell electrodes exhibits a dependence that is close to linear [25,41]. The overall performance depends on the efficiency of the proton, electron, and mass transport as well as on several active sites. Despite the plethora of parameters with complicated interdependencies defining final performance, it can be easily characterised by measuring polarization curves. Typically, the maximum power density is used for quick performance assessment, while performance characterization at 0.6 V is more practical. These results we present in Fig. 4, which maps the performance of fuel cells with different catalyst loading operating at different humidities (detailed polarization and power density curves presented in Fig. S2). As can be seen from the figure, the performance of fuel cells, irrespective of the catalyst loading, strongly depends on the operating humidity (peak power density and current density data with confidence intervals presented in the Fig. S5). Thus, all the studied systems demonstrated a decline in performance when working at lower humidities. Such a result is expected due to the fact of progressive ionomer dehydration in the drier environment. The proton conductivity of the ionomer decreases with the degree of hydration, leading to an increase in ohmic losses. However, the performance of the fuel cell at 100 %RH does not reach peak values (shown by “stars” in Fig. 4). Instead, peak power density can be reached at relative humidities between 75 and 50 % for all catalyst loadings. Moreover, with an increase in catalyst loading, the peak performance gravitates to lower relative humidity values. Assuming that the thickness of the catalyst layer is proportional to the catalyst loading, such behavior may be related to water management effects. In this regard, at reduced humidity, when saturation by water vapors is low, the formation of condensed water droplets is less favorable. This, in turn, improves gas diffusion in the tortuous CL pores, while the condensed water droplets may reduce or block gas transport. At lower humidities (25 % and 0 %RH), however, a drop of proton conductivity upon ionomer dehydration overrides a benign effect of water droplets evaporation on the fuel cell performance. As a result, MEAs with all catalyst loadings demonstrate a reduced performance. An increase in catalyst loading leads to several counteracting phenomena. On the one hand, a linear increase in the number of catalyst sites with loading (as shown in Fig. 1a) facilitates ORR reaction kinetics and so the total performance. On the other hand, proton transfer resistance also increases with loading, which could deteriorate the performance. Moreover, efficient mass transport at higher loadings is Fig. 2. ECSA dependencies on ionomer equilibrium water content (a) and temperature (b). Fig. 3. Impedance spectroscopy of MEA with the cathode catalyst loading in the range from 0.2 to 1.0 mgPt cm −2 at 70 ◦C and 100 %RH. Y.V. Yakovlev et al. Journal of Power Sources 663 (2026) 238804 4 hindered, leading to performance limitations [42,43]. Therefore, the best performance can be reached in MEA with an optimal concentration of catalyst. Examination of Fig. 4 reveals that MEA with a loading of 0.4 mgPt cm −2 has the best performance, indicating that trade-offs between many parameters are established. This corresponds to the optimal value of 0.5 mgPt cm −2 [44] demonstrated in the literature. Best performance of fuel cell at catalyst loading of 0.4 mgPt cm −2 was also demonstrated in Ref. [25]. Close catalyst loading 0.35 mgPt cm −2 was also optimal in Ref. [47]; best performance at 0.4 mgPt cm −2 was demonstrated for [45, 46]. To show a more detailed picture of environmental effects on fuel cell performance, the peak power density was measured at different working temperatures and relative humidities. As can be seen from Fig. 5, fuel cell performance is sensitive to both temperature and humidity (detailed polarization and power density curves presented in Fig. S3 and S4; performance data with confidence intervals presented in Fig. S6). Looking at the data measured under fully hydrated conditions (100 %RH), it can be concluded that increasing temperature has a positive effect on fuel cell performance as the peak power density increases from 0.79 to 0.92 W cm −2 as the temperature increases from 40 to 70 ◦C. This result is not surprising as the increase in temperature facilitates better proton mobility and higher catalyst activity. Moreover, the improved gas permeability of thin ionomer films covering catalyst particles and better gas diffusion in nanosized pores also contribute to better performance [42,43]. However, when the relative humidity decreases, water retention in the catalyst layer becomes more difficult; thus, proton transport drops dramatically. Therefore, peak performance at lower relative humidities tends to shift to lower temperatures where retention of water is easier and conductivity of protons can be maintained at sufficient levels. This behavior, however, changes dramatically when the catalyst layer has higher ionomer content. Increasing the I/C ratio to 0.8 results in worse fuel cell performance as compared to that of an I/C ratio of 0.6. Such performance deterioration can be explained by mass transport problems in the catalyst layer as ionomer forms thicker films over catalyst particles and blocks pores [42]. However, MEAs with an I/C ratio of 0.8 show lesser variation of performance with humidity at a given temperature. Some slight improvement at intermediate humidities (25–50 %RH) may also be related to the removal of water excess and CL pores de-blockage [47]. A remark can be added that, despite the temperature range of this study being limited to <70 ◦C results demonstrated for lower humidities can be particularly useful for applications with a higher temperature range (>80 ◦C), e.g. for stationary and automotive applications, where fuel cell MEA will work at reduced hydration. It can be concluded that samples with higher ionomer content better retain water and support a reasonable level of proton conductivity, influencing overall fuel cell performance. Water retention can be extremely important when a fuel cell operates at low humidity. This helps not only to improve fuel cell performance but also to prevent its deterioration in the long run. It is important to note that an improvement of water retention is, however, counterintuitive, provided hydrophobicity increases at the ionomer loadings of around 30 wt% [48]. Operation of the fuel cell at low humidity may result in the degradation of the ionomer part. As was shown previously, operation at low humidity provokes the formation of hydrogen peroxide, which provokes ionomer decomposition [49]. Therefore, it is practically important to study ways of mitigating the performance degradation for fuel cells working in low-humidity environments. To prevent dehydration in the catalyst layer addition of ionomer may be a sound strategy due to the hydrophilic treatment of the catalyst layer. Fuel cells with catalyst layers with I/C ratios of 0.6 and 0.8 were tested at constant load in the potentiostatic mode. Starting operation with fully hydrated MEA, the fuel cell was fed with totally dry gasses for 20 h. Therefore, the fuel cell was operating most of the time in the selfhumidification regime. Although the hydration of ionomer will depend on the produced current [50,51], we have chosen voltage control mode to approach realistic conditions. Such measurements were repeated three times, and the current density was recorded as a function of time and presented in Fig. 6. As can be seen in the figure, at the beginning of every 20-h cycle, fuel cell performance is higher. This higher performance is observed for both I/C ratios and is related to higher hydration of the ionomer and corresponding lesser proton resistance. As a fuel cell operates with dry gases, its performance gradually decreases. Interestingly, systems with lower ionomer content (I/C =0.6) show a continuous decrease of current density, whereas ones with an I/C ratio of 0.8 after 5 h of operation reach a stable operation. Moreover, apparent degradation can be observed for systems with I/C =0.6 as every new cycle started and finished at smaller currents than the previous. This contrasts with a system with I/C =0.8, where performance during three cycles shows small variation. To understand the reason for the performance variation after every cycle, we measured the ECSA of fully Fig. 4. Peak power density (a) and current density at 0.6 V (b) as a function of catalyst loading and relative humidity at a temperature of 70 ◦C. The points of the maximum performance at the given catalyst loading are indicated by stars. Fig. 5. Peak power densities of fuel cells with cathode catalyst loading of 0.4 mgPt cm −2 and I/C ratios of 0.6 and 0.8 as a function of temperature and humidity.David Kalabis, Writing – review & editing, Visualization, Investigation. Y.V. Yakovlev et al. Journal of Power Sources 663 (2026) 238804 5 hydrated samples, which corresponds to the number of active sites. Fig. 7 shows the changes in ECSA as the test in a dry environment progresses. The initial ECSA of the system. As can be seen from the figure, in both cases, ECSA values decrease over time. We assume that the decrease in the apparent ECSA values is not attributed to the catalyst degradation [42], but rather to the destruction of ionomer channels due to dry operation. SEM images (Figure S7 and Figure S8) of the catalyst layer did not reveal apparent catalyst layer degradation, however further investigation by sensitive to particle size methods is needed (e.g. SAXS, TEM). In other words, more catalyst particles failed to fulfil the conditions of the triple phase boundary [52,53]. It is remarkable that ionomer loading influences the rate of the ECSA degradation process. Catalyst layer with higher ionomer content (I/C =0.8) demonstrates (at least in the time frame of the experiment) more than twice the lower rate of decrease of ECSA (Fig. 7). There could be several reasons explaining such observation. Firstly, the catalyst layer with higher ionomer content is assumed to have better water retention. Therefore, the local hydration of ionomer can be higher, and its degradation is less. Secondly, a higher thickness of the ionomer layer adds more robustness to the catalyst layer even in the case of ionomer degradation. It turns out that an increase of ionomer content in the catalyst layer is a sound strategy for optimisation for dry operation of the fuel cell catalyst layer. The ionomer degradation hypothesis can be supported by EIS measurements performed before and after the dry test protocol. Fig. S9 reveals stability in HFR values, suggesting insignificant membrane degradation, which can be attributed to the higher local humidity in the operating MEA. However, proton conduction resistance in the catalyst layer (R ion ) shows more pronounced changes. For a system with an I/C ratio of 0.6, R ion grows from 0.141 Ohm cm 2 at the beginning of the test (“pristine”) to 0.201 Ohm cm 2 at the end of the test (“aged”). On the other hand, MEA with I/C of 0.8 shows lesser variation in R ion – from 0.117 to 0.135 Ohm cm 2 . It is worth noting that the resistance of the catalyst layer with a higher ionomer content is lower, due to a more developed proton-conductive percolation network (0.117 vs 0.141 Ohm cm 2 ). Such a network can bring redundancy in connecting catalyst which is advantageous for stable operation, when operating conditions can facilitate ionomer degradation. The intricate effect of different catalyst compositions on performance of fuel cells are summiraized and commented in Table 1 4. Conclusions This study demonstrates that PEMFC performance and durability can be optimized through a rational balance of Pt loading, ionomer content, and operating conditions. Key findings are: (i) active sites and proton transport resistance both scale linearly with Pt loading, resulting in an optimal 0.4 mgPt cm −2 that maximizes performance; (ii) fuel cell performance does not peak at full humidification, but rather at intermediate humidities (50–75 % RH), with the optimum shifting lower as catalyst layer thickness increases; (iii) increasing the I/C ratio from 0.6 to 0.8 improves performance retention under dry conditions and reduces electrochemically active surface area loss, highlighting the role of ionomer in water management and durability. Together, these insights establish practical design rules for PEMFC cathode catalyst layers, supporting the development of reliable systems for low-humidity applications such as portable and air-breathing fuel cells. CRediT authorship contribution statement Yurii V. Yakovlev: Writing – original draft, Visualization, Methodology, Formal analysis, Data curation, Conceptualization. Yevheniia V. Lobko: Writing – original draft, Validation, Investigation, Formal analysis. Miquel Gam´ on Rodríguez: Validation, Investigation, Data curation. Alina Madalina Darabut: Visualization, Methodology, Conceptualization. Lucinda Blanco Redondo: Visualization, Validation, Formal analysis. David Kalabis: Writing – review & editing, Visualization, Investigation. Iva Matolínov´ a: Writing – original draft, Validation, Resources, Project administration, Funding acquisition. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The work was financially supported the Czech Science Foundation, project No. 22-03643S, structural fund project PaC-NG No. CZ.02.1.01/ 0.0/0.0/16_025/0007414 and “The Energy Conversion and Storage”, funded as project No. CZ.02.01.01/00/22_008/0004617 by Programme Johannes Amos Comenius, call Excellent Research. Fig. 6. Fuel cell performance was measured at 70 ◦C, 0 %RH, constant voltage of 0.6 V for samples with I/C ratios of 0.6 and 0.8. Fig. 7. ECSA values measured for systems with I/C ratios of 0.6 and 0.8 during dry test protocol at 70 ◦C and 100 %RH. Y.V. Yakovlev et al. Journal of Power Sources 663 (2026) 238804 6 Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2025.238804. Data availability Dataset of "Investigation of Fuel Cell Behavior with Different Catalyst Loadings at Varying Humidities and Temperatures” (Original data) (Zenodo) References [1] M.A. Abdelkareem, K. Elsaid, T. 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