| Popis: |
With the advancements in fuel cell technology, PEM fuel cells have seen tremendous improvements in terms of durability, enhanced catalytic performance and reliability. The introduction of the microporous layer (MPL) has been proven to be highly beneficial in for water management especially at high current densities. Over the years, MPLs fabricated with various carbon blacks were compared and key relationships were reported based mainly on porosity. However, it is also known that carbon blacks have different properties such as structure and particle size which results in porosity differences. This work aims to develop a clearer understanding of the impact of surface porosity and pore size in the carbons on the overall MPL performance by attempting to delineate these properties from other properties intrinsic to different carbon blacks. To achieve this, commercial acetylene black (AB) was activated under the presence of CO₂ for different durations to increase porosity. In addition, colloid-imprinted carbons (CICs) with different pore sizes were synthesized to obtain carbons with tunable pore sizes. Results revealed that AB activated for longer durations resulted in higher surface area and pore volume. Subsequently, fuel cell testing showed that the ABs with increasing porosity had the poorer mass transport polarization performance at 80 °C and 100%RH (hot and the humid conditions). This is attributed to the higher water vapor absorbance shown by higher porosity ABs. CIC synthesis with various colloidal silica (CS) sizes exhibited distinct pore patterns reflecting the diameter of the CS particles. By varying the CS to mesophase pitch (carbon precursor) ratio, CICs with variable pore volume and surface areas were obtained with an increasing ratio generally resulting in increased porosity. Non-heat treated CICs were more hydrophilic than the heat treated CICs due to the higher percentage of surface oxygen functional groups and showed increased water vapor sorption. CIC12 (pore diameter of ~15 nm) had the highest water sorption with 93% mass increase. Accordingly, fuel cell polarization performances illustrated that CIC12 had the poorest performance at hot and humid conditions due to enhanced water retention capability. This performance significantly improved when CIC12 was heat treated to increase its hydrophobicity. |
