Computer simulation of active layer of fuel cell electrode with polymer electrolyte: Complete combined carbon support grains, calculation of overall characteristics

Abstract

Full computer simulation of the active layer of a fuel cell cathode with polymer electrolyte and complete combined carbon support grains is carried out. The active layer structure included two types of equal-size cubic grains (combined support grains and voids) together forming a cubic lattice. Also, the structure of combined grains was modeled; a carbon cluster was formed in them, with the oxygen reduction process occurring on its surface; the rest of the grain volume was filled by polymer electrolyte. The completeness of the grains consisted in the fact that they were characterized by 3D electron conductivity, ability to take part in the transport of protons in the active layer and the carbon cluster in the grains had the maximum possible surface area. Calculation of overall currents of oxygen cathodes with full combined carbon support grains, Nafion, and platinum yielded the following result. At t = 80°C, pressure p* = 101 kPa, cathode potential E0 = 0.8 V, and optimum active layer thickness Δ* = 20 μm, maximum overall current Imax = 0.38 A/cm2, maximum power density Wmax = 0.31 W/cm2. At potential E0 = 0.7 V, Δ* = 9.8 μm, Imax = 1.13 A/cm2, Wmax = 0.79 W/cm2. At potential E0 = 0.6 V, Δ* = 3.8 μm, Imax = 2.95 A/cm2, Wmax = 1.76 W/cm2. At potential E0 = 0.5 V, Δ* = 1.4 μm, Imax = 7.71 A/cm2, Wmax = 3.86 W/cm2. The overall current values are higher than those observed experimentally at the given cathode potentials. The discrepancy is explained by the fact that calculations of active cathode layers with a practically regular structure were carried out. All combined support grains in them are full and identical, while in fact the active layer structure is not characterized by the properties of fullness and equivalence. The second circumstance is that experimental active layers rarely have a strictly optimum thickness. Meanwhile deviation from this optimum results in losses in current. Transition to cathodes with combined grains has additional advantages. (1) In such grains, all platinum participates in current generation, the catalyst utilization degree reaches 100%. (2) Oxygen can enter the active layer not through small Knudsen pores, but through large (with the diameter of hundreds and more nm) gas pores, in which usual molecular gas diffusion occurs, so that diffusion limitations in the active layer become less significant. 3. In the active layer, the danger of gas pore flooding by evolving water decreases. Now, water vapor is much more easily removed from large gas pores directing then into the gas-diffusion layer pores.