Improving Performance of a Passive Direct Methanol Fuel Cell by Hydrophobic Treatment for Cathode Current Collector

Abstract

This study employs hydrophobic modification of the current collector to optimize cathode water management and enhance the performance of passive DMFCs. The surface of the cathode current collector was hydrophobized by polytetrafluoroethylene (PTFE) coating and titanium dioxide/polydimethylsiloxane (PDMS) composite coating. The experimental results showed that the surface hydrophobic treatment significantly improved the cell performance at low methanol concentration and marginally improved the cell performance at high methanol concentration. Among them, the DMFC with bilayer TiO₂/PDMS hydrophobic-treated cathode current collector with a contact angle of 153.2° showed the best performance, which achieved superhydrophobicity and led to a peak power density that was 27.25% higher compared to the DMFC with an untreated current collector. With the gradient-based hydrophobic treatment for the cathode current collector, the best performance was achieved when double-layer TiO₂/PDMS was used on the MEA side and PTFE coating on the air side.

1. Introduction

Since the Industrial Revolution, the large-scale consumption of fossil fuels has led to a continuous rise in global greenhouse gas concentrations, contributing to climate change characterized by rising temperatures and increased frequency of extreme weather events and rising sea levels [1,2]. Therefore, the development of clean energy technologies, the construction of new power systems, and the promotion of carbon emission reduction technologies are not only strategic pivots for countries to fulfil their ‘carbon neutral’ commitments but also inevitable choices to safeguard the destiny of sustainable development of mankind [3]. With the transformation of the global energy structure to low-carbon, the large-scale application of renewable energy sources such as wind energy and solar energy has significantly reduced the intensity of carbon emissions in the power sector. However, the intermittency and uneven geographical distribution of renewable energy sources have created an urgent need for energy storage systems such as supercapacitors and flow batteries, as well as secondary energy conversion technologies [4]. Hydrogen, as an ideal clean energy carrier, is still a bottleneck for industrialization in terms of storage and transportation safety and infrastructure costs [5,6]. In this context, a direct methanol fuel cell (DMFC) provides economic feasibility for the commercialization of DMFC by virtue of its easy access to fuel (which can be produced through coal/natural gas reforming, biomass conversion to bio-methanol, etc.), benefiting from stable fuel supply, lower cost, and simplified methanol handling and easier storage and transportation [7,8,9].

Passive DMFC, as a miniaturized energy device that does not require external auxiliary equipment and relies on natural diffusion to achieve fuel supply, consists of a methanol fuel chamber (storing liquid methanol and supplying it by diffusion), a membrane electrode assembly (MEA containing an anode diffusion layer, an anode catalyst, a proton-exchange membrane, and a cathode diffusion layer of a cathode catalyst, where an electrochemical reaction occurs), a cathode end plate (an open-ended structure to achieve oxygen convection in the air), and a current collector (conductive and structural support), as shown in Figure 1, and shows broad application prospects in portable electronic equipment, drones, and field emergency power supply [10,11,12,13].

The schematic of the passive DMFC test apparatus is depicted in Figure 2. The methanol solution in the methanol storage tank on the anode side enters the electrode through natural diffusion from the anode current collector, during which it flows through the gas diffusion layer (GDL) of the anode, and then flows into the catalytic layer (CL) of the anode, where the methanol undergoes oxidation in the CL of the anode and is decomposed to generate CO₂, H⁺, and electrons, wherein the generated H⁺ passes through the proton-exchange membrane (PEM) to reach the CL of the cathode, and the generated electrons pass through the external circuit for the transmission of electrical energy. At the same time, O₂ starts from the cathode current collector, passes through the cathode GDL, reaches the CL, and undergoes a reduction reaction with H⁺ in the cathode CL, combining to form H₂O [14].

However, the cathode current collector is prone to water flooding during operation, which leads to the obstruction of oxygen transport, the intensification of methanol infiltration, and performance degradation and severely restricts the output stability and energy efficiency of the cell [14,15,16,17]. To address this challenge, many scholars have optimized cell water management by optimizing the cell structure and materials and modulating the surface hydrophilicity [18,19,20,21,22]. Xu et al. [23] improved the water management capability and reduced the internal resistance by adding a water management layer (WML) and a hydrophobic air filtration layer (AFL), as well as by using thinner membranes, which significantly improved the cell performance and fuel efficiency and showed stable performance in long-term operation. Deng et al. [24] used carbon nanotube (CNT) paper as a cathode gas diffusion layer (GDL). The unique structure of the CNT paper enhanced the reverse diffusion of water and reduced cathode waterlogging; it also improved oxygen transfer efficiency, improved water management, and reduced methanol infiltration. Xue et al. [25] designed a new type of cathode support layer using stainless steel fiber mats as a substrate and a polytetrafluoroethylene (PTFE) hydrophobic treatment on the outside, which reduced the cathode water accumulation phenomenon, improved water back diffusion to improve water management, and significantly improved the cell performance at high methanol concentration.

The current collector is a key component of passive DMFC, and its hydrophobic treatment is a key technical means to optimize water management and improve comprehensive performance. The core objective of current collector hydrophobic treatment is to reduce the retention of liquid water at the electrode interface through surface modification, so that the water generated during the cell reaction can be discharged from the current collector in a timely manner, and at the same time, the oxygen diffusion channel can be maintained unobstructed. In this paper, the surface of the cathode stainless steel current collector is hydrophobically treated with PTFE and titanium dioxide/polydimethylsiloxane (TiO₂/PDMS) by constructing a hydrophobic coating in order to improve the cell performance. The cathode current collector is subjected to gradient hydrophobic treatment to achieve the optimal effect by regulating the hydrophobicity of its different parts.

2. Experimental

The current collector material used in this paper is stainless steel 316L, and the current collector structure is a perforated current collector with 25 cylindrical through holes and an opening rate of 50.2%.

2.1. Fabrication of MEA

The MEA fabricated in this work utilized a gas diffusion electrode (GDE) configuration [26]. Carbon paper pretreatment was initiated by sequential immersion in acetone and ethanol (Aladdin Chemical Reagent Co., Ltd., Beijing, China) for 15 min, followed by drying at 80 °C, to establish a clean substrate for the GDL. Subsequently, a pre-prepared microporous layer (MPL) slurry was uniformly deposited onto the pretreated carbon paper via a spray-coating technique, with careful control of the spray gun flow rate to ensure homogeneous distribution. This MPL slurry was formulated by mixing carbon powder and 5 wt.% PTFE in an 85:15 mass ratio within a reagent bottle. An appropriate amount of isopropanol was added as a dispersant, followed by homogenization via bath sonication. Following coating, the MPL-coated carbon paper was dried at 80 °C for 2 h and subsequently sintered in a muffle furnace at 370 °C for 30 min. The catalyst layer (CL) slurry was prepared by blending the catalyst material (40 wt.% Pt/C for the cathode and 60 wt.% PtRu/C for the anode, both at a loading of 2 mg cm⁻²) with Nafion solution in a 7:3 ratio. Isopropanol was again employed as a dispersant, and the mixture was dispersed using bath sonication. This CL slurry was then spray-coated onto the prepared MPL-GDL substrate. The coated electrodes were dried at 80 °C for 2 h, resulting in the formation of distinct catalyst layers on the GDL surfaces. Finally, the completed anode and cathode GDEs were hot-pressed together with a pretreated proton exchange membrane to assemble the full MEA.

2.2. PTFE Hydrophobic Treatment

First, the stainless steel surface was pretreated. It was then uniformly polished with 400~800 mesh sandpaper to remove the oxide layer and increase the roughness. It was sequentially washed with acetone and anhydrous ethanol ultrasonically for 15 min to remove grease and impurities. It was then soaked with dilute hydrochloric acid for 5 min (to remove the oxide film), followed by rinsing with deionized water. The stainless steel sheets were dried in an oven at 80 °C for 30 min to avoid residual moisture on the surface.

Immediately following the configuration of PTFE dispersion, the PTFE emulsion was diluted with deionized water at a certain volume ratio and magnetically stirred for 30 min until uniformly dispersed, finally generating a 12 wt.% PTFE solution. A certain amount of isopropyl alcohol ultrasonic dispersion was added, and the solution was loaded into the spray gun (EVV-2, Schutze Co., Ltd., Jena, Germany). The air pressure was adjusted to 0.3 MPa. The stainless steel sheet was 20–30 cm far from the spray gun for the uniform formation of a thin layer. After each layer was sprayed, it was dried at room temperature for 10 min, and spraying was repeated 2–3 times to increase the uniformity of the coating. The impregnation method can also be used, wherein the pre-treated stainless steel sheet is immersed in 12 wt.% PTFE solution and left to stand for 1–2 min, during which time it is slowly lifted to avoid sagging. Pre-curing was carried out in an oven at 80 °C for 30 min to remove the solvent and initial film formation. The pre-cured samples were transferred to a muffle furnace and sintered at 370 °C for 30 min to melt the PTFE to form a continuous hydrophobic coating. The PTFE hydrophobic-treated stainless steel current collector can be obtained by removing it after natural cooling to room temperature (as shown in Figure 3) [27].

2.3. TiO₂/PDMS Hydrophobic Treatment

Prior to the hydrophobic treatment of the stainless steel current collector surface, the modified nano TiO₂ was carried out. Two grams of nano TiO₂ (hydrophilic at this point) and 0.03 g of sodium hexametaphosphate (Aladdin Chemical Reagent Co., Ltd., Beijing, China) were weighed using an electronic balance and dispersed in 200 mL of ultrapure water. The pH was adjusted to about 5.0 by continuous stirring via a magnetic stirrer, while 0.1 M hydrochloric acid solution was added dropwise. Subsequently, 0.1 g of sodium laurate was added as a surfactant, and the reaction was carried out in a magnetic stirrer in a constant temperature water bath for 2 h. The reaction product was dehydrated by a vacuum drying chamber for 8 h and then finely ground using a mortar and pestle to produce hydrophobically modified nano-TiO₂ powder for spare parts. Immediately following the surface pretreatment of the stainless steel current collector, the surface was polished using 600-mesh sandpaper and then washed with acetone and anhydrous ethanol for 10 min, cleaned and degreased, and dried and set aside [28].

To construct a monolayer hydrophobic structure, modified TiO₂ was added to 10 mL of toluene and ultrasonically stirred for 10 min, and then PDMS and curing agent were added to it, and it was stirred thoroughly until a mixed gel was formed. The mixed gel was coated on the surface of the pre-treated stainless steel current collector using the drop-coating or dipping method and then dried and cured to form a single-layer TiO₂/PDMS coating (as shown in Figure 4).

Firstly, PDMS was mixed with a curing agent at a mass ratio of 10:1 to form a primer film on the substrate surface by the spin-coating method, and the primer layer was formed after initial curing (80 °C, 2 h). Subsequently, the TiO₂/PDMS composite gel was coated on the surface of the substrate layer, the interfacial chemical bonding was achieved by secondary curing (80 °C, 3 h), and the final bilayer TiO₂/PDMS hydrophobic coating with micro-nano hierarchical structure was obtained (as shown in Figure 5).

2.4. Gradient Hydrophobic Treatment

Gradient hydrophobic structure means that the hydrophobicity of a material surface shows continuous or graded gradient changes along a certain direction or region, and by regulating the difference in wettability of different regions, it can achieve the functions of directional water transport, prevention of water pooling, or enhancement of interfacial stability [28]. The graded hydrophobic treatment of the cathode current collector enhances the capillary force-driven directional water transport by modulating the hydrophobicity of different parts of it to achieve optimal results [29]. In this section, by regulating the surface hydrophilicity of both sides of the cathode current collector, a more hydrophobic coating is used near the MEA side and a less hydrophobic coating is used near the air side, such that a gradient distribution of hydrophobicity is formed in the current collector orifice, which creates the capillary driven force so it better reduces the retention of liquid water at the electrode-current collector interface and maintains the smoothness of the oxygen diffusion channel.

2.5. Physical Characterization

In this paper, a field emission scanning electron microscope SEM (Hitachi Regulus 8100, Tokyo, Japan) was used to observe the surface micromorphology of stainless steel 316L after hydrophobic treatment with PTFE and treatment with TiO₂/PDMS. The contact angle of the surface coating of stainless steel 316L was measured using an optical contact angle meter (DCAT21, Dataphysics, Filderstadt, Stuttgart, Germany) to probe the hydrophobicity of the current collector surface after hydrophobic treatment.

2.6. Electrochemical Testing

In this paper, an electrochemical workstation (CHI604E, Chenhua Equipments, Shanghai, China) is used to perform EIS characterization of passive DMFCs, which can effectively resolve the impedance distribution characteristics of the various interfaces inside the cell and the differences in their electrode reaction kinetics. During the experiment, a three-electrode test system was established after the cell system was stabilized: the working electrode was connected to the cathode current collector lugs, the reference electrode was connected to the anode current collector lugs together with the counter electrode, and a DC bias voltage of 0.3 V was maintained. The tests were conducted in a wide frequency domain scanning mode with a scanning range of 20 kHz to 0.01 Hz, and the impedance response characteristics of the system were recorded synchronously under the condition of applying a 5 mV sinusoidal waveform perturbation signal. The environmental control parameters (including temperature, electrolyte state, etc.) of this experiment were strictly maintained at the same baseline conditions as those used in the previous polarization curve determination.

3. Results and Discussion

3.1. SEM Characterization

The surface morphology of the PTFE coatings formed by the two hydrophobic treatments and the TiO₂/PDMS coatings are shown in Figure 6. As shown in Figure 6a,b, PTFE coatings exhibit a fibrous 3D network structure on the coating surface, and the surface is relatively flat, with fiber diameters distributed in micro- and nano-scale. The hydrophobicity relies mainly on the low surface energy property of PTFE material, and the basic hydrophobicity is achieved by reducing the solid-liquid interfacial interaction. In contrast, the TiO₂/PDMS composite coating exhibits a significantly differentiated micro- and nano-multilevel structure: the modified TiO₂ nanoparticles are uniformly dispersed in the PDMS matrix without agglomeration, forming dense micro- and nano-scale clusters. High-magnification SEM images further reveal that the TiO₂ clusters have multiple grooves and pore structures on the surface, and their geometrical features significantly enhance the surface roughness. The micro-nano hierarchical structure in TiO₂/PDMS (Figure 6c,d) achieves superhydrophobicity via the Cassie–Baxter model [28]:

$$\mathit{cos}{\theta }^{\ast }=f_1\mathit{cos}{\theta }_1+f_2\cos {\theta }_2$$

where ${\theta }^{\ast }=153.2°$ (measured contact angle), ${\theta }_1=122.7°$ (intrinsic contact angle of PDMS), ${\theta }_2=180°$ (contact angle of water on air), and $f_2=0.89$ (area fraction of air trapped in surface grooves). This air-entrapping morphology reduces the solid-liquid contact area by 89% compared to PTFE’s fibrous network, directly explaining the minimized droplet adhesion observed in Figure 6.

3.2. Contact Angle Test

The contact angle of the hydrophobically treated current collector surface was tested using a measuring instrument, as shown in Figure 7. The contact angle of the stainless steel current collector surface treated with single-layer TiO₂/PDMS was 151.1°, which showed superhydrophobicity (Figure 7a), whereas with double-layer TiO₂/PDMS treatment, the contact angle was gradually increased to 153.2° (Figure 7b), which greatly improved the hydrophobicity. Since stainless steel 316L does not possess hydrophobicity per se without surface hydrophobicity treatment, after PTFE coating treatment the contact angle was changed to 136.1° (Figure 7c), which is in line with the hydrophobic material properties. After coating treatment using PDMS as a binder, the contact angle of stainless steel 316L was increased to 122.7° (Figure 7d), but the surface was not rough enough to achieve higher hydrophobicity, and compounding it with modified TiO₂ could further improve the hydrophobicity.

3.3. Comparison of Electrochemical Properties Under Different Hydrophobic Treatments

The polarization curves of passive DMFCs after four hydrophobic treatments in a 2 M methanol environment were first tested, as shown in Figure 8a. It can be seen that the performance of the current collector with surface hydrophobic treatment is significantly higher than that of the current collector without surface hydrophobic treatment, which is mainly reflected in the maximum current density. The maximum current density of the cathode current collector with PTFE hydrophobic treatment increased from 12.5 mA cm⁻² to 20 mA cm⁻², which is a 60% enhancement compared to the untreated current collector, and the maximum current density of the cathode current collector with both single- and double-layer TiO₂/PDMS hydrophobic treatments increased to 22.5 mA cm⁻², which is an 80% enhancement. In the 4 M-methanol environment (Figure 8b), the PTFE hydrophobic treatment increased the maximum current density from 22.5 mA cm⁻² to 30 mA cm⁻², an improvement of 33.3%, and both the single- and double-layer TiO₂/PDMS hydrophobic treatments increased the maximum current density to 40 mA cm⁻², an improvement of 77.8%. The improved cell performance can be attributed to the formation of a hydrophobic film of PTFE on the surface of the current collector, which reduces the retention of liquid water at the electrode interface at low methanol concentration while maintaining the oxygen diffusion channel, resulting in a more adequate reaction, reduced concentration polarization, and improved current density.

The polarization curves of passive DMFC with different hydrophobic treatments at 6 M and 8 M methanol concentrations were then tested, as shown in Figure 8c and Figure 8d, respectively. The maximum power density of PTFE coating at 6 M methanol concentration reached 4.24 mW cm⁻² with 19.13% performance improvement compared with the DMFC with untreated cathode current collector, which reached 4.39 mW cm⁻² with 23.31% performance improvement with single-layer treatment, and with double-layer treatment, it reached 4.53 mW cm⁻² with 27.53% performance improvement. At 8 M methanol concentration, the maximum current density of the cell almost did not change, and the maximum power density enhancement was 11.49%, 12.64%, and 16.48%, respectively. At high methanol concentrations, the anode reaction rate is fast, and the water generated by the cathode oxygen reduction reaction further increases, leading to a significant increase in the risk of cathode flooding. The main reason for the improvement in cell performance at this time is that the hydrophobic coating makes it difficult for liquid water to adhere, forming droplets that are quickly carried out by the airflow and preventing the water film from blocking the oxygen transport path. Reducing flooding ensures that the pores of the cathode GDL and catalytic layer are unobstructed, enhancing the oxygen transfer rate and reducing the concentration polarization at high current densities.

In order to further investigate the current collector with several surface hydrophobic treatments, the EIS of the cell was tested at a concentration of 6 M methanol, as shown in Figure 9. The EIS curve consists of two parts, the high frequency semicircle, which represents the charge transfer resistance, and the low frequency semicircle, which represents the mass transfer resistance. From this figure, it can be observed that there is no significant difference in the charge transfer resistance for several surface hydrophobic treatments, while the mass transfer resistance in the low frequency semicircle has a significant difference. The ohmic resistance of the current collector with surface hydrophobic treatment is 0.2 Ω, while the ohmic resistance of the untreated current collector is 0.17 Ω. The results show that the total resistance of the current collector with surface hydrophobic treatment decreases, which can be attributed to the fact that the surface of the current collector with surface hydrophobic treatment is much rougher compared with that of the untreated surface, which increases the surface contact resistance but at the same time makes the mass transfer resistance of the cell decrease significantly, suggesting that the hydrophobic treatment can be used not only to reduce the charge-transfer resistance but also to reduce the mass-transfer resistance. The mass transfer resistance of the cell with surface hydrophobic treatment on the cathode current collector was significantly reduced, indicating that the hydrophobic treatment allowed the cathode water to be discharged in a timely pattern, thus optimizing the oxygen transfer and reducing the concentration polarization, which is consistent with previous analyses.

To quantify the mechanism behind the performance enhancement, the EIS spectra in Figure 9 were fitted using an equivalent circuit model, with the detailed parameters summarized in

Table 1. Summary of equivalent circuit fitting parameters (under 6 M methanol operation).

Parameters	Untreated	PTFE-Treated	Single-Layer TiO2/PDMS	Double-Layer TiO2/PDMS	Unit
Rs	0.1325	0.17259	0.17784	0.17970	Ω
Rct	2.865	2.635	2.335	2.153	Ω
Rmt	3.843	3.386	2.725	2.074	Ω

. The fitting results reveal a dual-pathway mechanism: First, the charge transfer resistance (R_ct) progressively decreased from 2.865 Ω for the untreated sample to 2.153 Ω for the bilayer TiO₂/PDMS one (a 24.8% reduction), indicating a moderate improvement in the oxygen reduction reaction (ORR) kinetics, likely due to the increased accessibility of active sites as flooding was mitigated. More critically, the mass transport resistance (R_mt) exhibited the most pronounced and monotonic decline, dropping substantially from 3.843 Ω to 2.074 Ω (a total reduction of 46.0%). This parameter change provides the most direct evidence that the performance gain is predominantly attributable to optimized cathode water management: the superhydrophobic surface efficiently expels liquid water, preventing pore blockage and thereby facilitating unimpeded oxygen transport to the catalytic sites. This quantitative analysis is in excellent agreement with the performance observed in the polarization curves (Figure 8), particularly the mitigation of concentration polarization at high current densities.

Comparing these hydrophobic treatments, it is obvious that the current collector with bilayer TiO₂/PDMS hydrophobic treatment has the best electrochemical performance. This is mainly attributed to its superhydrophobicity and the largest contact angle compared to other coatings. The larger contact angle reduces the contact area and adhesion between the liquid and the surface, and the low adhesion property of the surface reduces the resistance of the droplets in the process of movement, so that even tiny droplets can be quickly detached from the surface by gravity or diffusion, which avoids cathodic flooding and optimizes the water management.

3.4. Electrochemical Properties Under Graded Hydrophobic Treatment

In order to investigate the effect of gradient hydrophobic treatment of the cathode current collector on the cell performance, three different gradient hydrophobicity treatments were applied to the current collector. The first gradient hydrophobicity treatment (TD-1) was a more hydrophobic double-layer TiO₂/PDMS coating on the side close to the MEA and a less hydrophobic PDMS coating on the side close to the air; the second gradient hydrophobicity treatment (TD-2) was a more hydrophobic double-layer TiO₂/PDMS coating near the MEA side and a less hydrophobic PTFE coating near the air side; the third gradient hydrophobicity treatment (TD-3) was the use of a more hydrophobic bilayer TiO₂/PDMS coating near the MEA side and a single-layer TiO₂/PDMS coating near the air side, which is also more hydrophobic.

The cell performance of the three gradient hydrophobic-treated current collectors at different methanol concentrations was tested, and the polarization curves are shown in Figure 10. The results show that the gradient hydrophobic treatment can significantly enhance the cell performance under high methanol concentration conditions. Compared to the cathode current collector without gradientation treatment, the gradient hydrophobic treatment did not enhance the maximum current density, and the enhancement of the maximum power density was extremely small in the low methanol concentration environment. While in the environment of high methanol concentration, the maximum power density has some enhancement. This is mainly due to the fact that the hydrophobic treatment near the MEA side makes it difficult for liquid water to wet the walls of the flow channel and promotes the rapid expulsion of water in the form of droplets by the air flow instead of forming a film of water blocking the orifices, which significantly reduces flooding and ensures the efficient transport of oxygen to the reaction site. Hydrophobic treatment near the air side reduces surface tension and prevents liquid water from collecting on the current collector surface to form larger droplets after flowing out of the orifice, while the formation of different hydrophobic properties in the orifice drags the water forward, enhancing the drainage rate, reducing water flooding, and improving the oxygen diffusion, which significantly improves the cathode reaction kinetics and the overall cell efficiency. Super-hydrophobicity on the MEA side is able to reduce the water retention, while moderate hydrophobicity on the air side is able to balance the drainage and gas transport to the reaction site. Hydrophobicity can balance the drainage and gas flow resistance. When comparing these three gradient hydrophobic treatments, TD-2 showed the best performance, while TD-3 showed the worst performance. Especially at 6 M concentration, the voltage of TD-2 was higher than that of the other two gradient hydrophobic treatments in both high and low current density regions. The quantitative performance comparisons of these treatments are summarized in

Table 2. Performance comparison of hydrophobic treatments at 6 M methanol.

Treatment	Max. Current Density (mA/cm2)	Peak Power Density (mW/cm2)	Power Improvement (%)
Untreated	12.5	3.55	0
PTFE coating	20.0	4.24	+19.4
Single-layer TiO2/PDMS	22.5	4.39	+23.7
Bilayer TiO2/PDMS	22.5	4.53	+27.6
Gradient TD-1	25.1	4.80	+35.2
Gradient TD-2	25.8	4.92	+38.6
Gradient TD-3	24.3	4.82	+35.8

, and the maximum power density of TD-2 reached 4.92 mW cm⁻², with a performance improvement of 7.36%, while that of TD-1 reached 4.80 mW cm⁻², with a performance improvement of 5.96%, and that of TD-3 reached 4.82 mW cm⁻², with a performance improvement of 6.40%. TD-2 is superior to TD-1 mainly due to the PTFE coating’s contact angle being larger than that of the PDMS coating, so the hydrophobicity effect is better. For TD-2 vs. TD-3, although the contact angle of the single-layer TiO₂/PDMS coating is the largest, it is more similar to that of the coatings close to the MEA side, and the gradient between the two is smaller, resulting in a 15.2% reduction in the capillary force formed in the pores and a lower drainage rate. Fundamentally, the directional water transport is driven by the Laplace pressure difference (Δp) across the gradient surface:

$$\Delta p ={\displaystyle \frac{2\gamma }{R}} \left(\cos {\theta }_{\mathrm{a}\mathrm{i}\mathrm{r}}-\cos {\theta }_{\mathrm{M}\mathrm{E}\mathrm{A}}\right)$$

where $\gamma =72.8$ mN/m (water surface tension at 25 °C), R = 1.6 mm (radius of current collector pores), ${\theta }_{\mathrm{a}\mathrm{i}\mathrm{r}}=136.1°$ (PTFE side), and ${\theta }_{\mathrm{M}\mathrm{E}\mathrm{A}}=153.2°$ (TiO₂/PDMS side). For the TD-2 configuration, the significant contact angle difference generates substantial Laplace pressure to propel water toward the air side (Figure 10), with ΔP values orders of magnitude higher than in uniform coatings.

4. Conclusions

To address cathode flooding issues in current collectors in operation resulting in oxygen transmission obstruction then performance degradation, which seriously limits the output stability of the fuel cell, and to provide more effective water management for passive DMFCs, some well-designed experiments were carried out in this paper. They included three kinds of hydrophobic treatments for a cathode current collector: a PTFE coating, a single-layer TiO₂/PDMS coating, and a double-layer TiO₂/PDMS coating on the surface of pre-treated current collector, respectively. The hydrophobic treatment forms a low surface energy coating on the surface of the stainless steel 316L current collector, which significantly increases the water contact angle and effectively inhibits the accumulation of liquid water. The SEM structural characterization clearly shows that the stainless steel surface generates micro-nano-structures as PTFE forms a fibrous 3D network structure, while TiO₂ is uniformly attached in PDMS to form a low surface energy structure. The contact angle tests of these coatings reveal that the surface contact angles of the stainless steel current collector treated with single-layer and double-layer TiO₂/PDMS are 151.1° and 153.2°, respectively, which shows superhydrophobicity, while that of the stainless steel current collector treated with PTFE coating is 136.1°, which is in line with the hydrophobic material properties.

The electrochemical performance of the three surface hydrophobic-treated cathode current collectors and the cathode current collector without any surface hydrophobic treatment were firstly tested in combination with the C2-type current collector, respectively, and the maximum current density of the hydrophobic-treated cathode current collector was significantly increased at 2 M and 4 M methanol concentrations, whereas the maximum current density did not increase with 6 M and 8 M methanol, but the voltage drop was slowed down, resulting in a significant increase in the maximum power density. The reason for this is analyzed as the coating contact angle increases, the larger contact angle reduces the contact area and adhesion between the liquid and the surface, the low adhesion property of the surface reduces the resistance of the droplets in the process of movement, and even tiny droplets can be quickly detached from the surface by gravity or diffusion, avoiding cathodic flooding and optimizing the water management. Comparing the three surface hydrophobic treatments, it is obvious that the improvement in cell performance is almost linearly related to the contact angle, especially for the superhydrophobic surface, which has a stronger repulsion of water and better drainage effect.

The effect of gradient hydrophobic treatment on the cell performance was also investigated for the cathode current collector electrode, with double-layer TiO₂/PDMS coatings on both sides near the MEA side and PDMS coatings, PTFE coatings, and single-layer TiO₂/PDMS coatings on the air-facing side, respectively. The electrochemical test results show that the PTFE coating on the air-facing side achieves the best performance, as the contact angle of the PTFE coating is larger than that of the PDMS coating, so the hydrophobic effect is better, while the contact angle of the single-layer TiO₂/PDMS coating is the largest, but it is quite similar to that of the coating on the side near the MEA, so the gradient between these two is smaller, and the drainage effect is reduced. Therefore, optimal air-side hydrophobic treatment requires a significant hydrophobicity gradient across the thickness of the current collector and a contact angle that is large enough, with the PTFE coating being optimal. In summary, three pivotal advancements are achieved: (1) a gradient hydrophobicity mechanism with bilayer TiO₂/PDMS (θ = 153.2°) on the MEA side and PTFE (θ = 136.1°) on the air side drives directional water transport via the Laplace pressure difference; (2) the micro-nano hierarchical structure in bilayer TiO₂/PDMS enables record superhydrophobicity, yielding 27.25% peak power density gain; and (3) cost-effective manufacturability using spray-coated stainless steel 316L ($5.62 total cost) cuts production time by >60% versus graphite. These advancements would resolve the water-oxygen trade-off problem for portable DMFCs with >6 M methanol solution.