Liquid Transport Regulation in Pore-Size-Gradient Carbonized Fiber Membranes for Enhanced Direct Methanol Fuel Cell Performance

Pore-size-gradient carbonized fiber membranes were prepared by sequential electrospinning and carbonization, and Ag nanoparticles were incorporated to reduce the electrical resistivity. Residual-pressure decay after pump shutdown was used to compare transient liquid water responses, complementing SEM characterization, four-probe measurements, and passive direct methanol fuel cell (DMFC) tests. For 80 μm membranes, the positive-gradient configuration, with small pores facing the inlet and large pores facing the outlet, generally promoted pressure release in bilayer and trilayer designs; this advantage weakened or reversed in four- and five-layer designs. The positive-gradient bilayer with a nominal G = 0.063 exhibited a residual pressure of 28.50 kPa and an average decay rate of 0.3583 kPa·min−1 over 60 min. The lowest resistivity among the tested Ag loadings was approximately 0.018 Ω·m at 4 wt.%. A separately selected 150 μm trilayer Ag-modified membrane achieved a DMFC peak power density of 1.10 ± 0.0366 mW·cm−2 at 3 mol·L−1 methanol, 43% and 12% higher than the nongradient and unmodified gradient controls, respectively. These comparisons support a combined modification effect under the tested conditions, while pore-scale mechanisms and the interactions between modifications remain unverified.