Non-isothermal melting of ice in the gas-diffusion layer of a proton-exchange-membrane fuel cell
Non-isothermal ice melting in the fibrous gas-diffusion layer (GDL) of a proton-exchange-membrane fuel cell (PEMFC) is investigated using differential scanning calorimetry (DSC). Non-isothermal ice-melting rates and ice-melting times are obtained from heat-flow measurements in water-saturated Toray GDLs at heating rates of 1, 2.5, 5, 10, and 25 K/min. In all cases, ice-melting times decrease nonlinearly with increasing heating rate. Nevertheless, melting temperatures remain at 272.9 ± 0.5 and 272.7 ± 0.4 K for bulk ice and ice within the GDL, respectively, reiterating that melting is thermodynamic-based at a rate limited by heat transfer. The slight GDL ice melting-point depression is consistent with the Gibbs–Thomson equation for equilibrium melting using an average pore diameter of 30 μm. Ice-melting endotherms are predicted from overall DSC energy balances coupled with a moving-boundary Stefan problem, where an ice-melting front within a GDL propagates with volume-averaged properties through an effective medium. Agreement between DSC experiment and theory is excellent. The proposed model accurately predicts ice-melting endotherms for Toray GDLs with two ice saturations and for bulk ice. Further, a pseudo-steady-state analysis obtains an analytical expression for ice-melting time, which is controlled by the time for heat addition to the propagating solid/liquid interface. Significantly, the new expression elucidates parameters controlling ice melting and allows for better design of both GDL materials and heating strategies to enhance the success of PEMFC cold-start.