Computational Studies of the Stability of Carbon-based Fuel Cell Materials

With the increase in global pollution and electricity use, a clean energy sources and energy conversion technologies are needed. An attractive alternative is using a fuel cell to convert chemical energy into electrical energy. Still, a durability is a critical concern for fuel cell development. In this thesis, two degradation concerns related to the materials in the fuel cell are investigated by density functional theory (DFT).

Firstly, the stability of a single metal atom catalyst (M/N/C) is investigated. The M/N/C catalyst is a promising catalyst at the cathode of the fuel cell. However, under an acid condition of the fuel cell, it has poor stability. The thermodynamic model of the dissolution reaction for the M/N/C catalyst with various local carbon structures and metal atoms is investigated. The results reveal a significant role of both local carbon structure and the metal atom in the stability. The computational screening considering both activity and stability suggests the MN₄ site on the bulk graphene, and graphene edge with M= Fe, Co, and Ru as a stable catalyze in the acid condition. The adsorption of the anion in the electrolyte is then further included in the activity and stability calculations for the MN₄ site on bulk graphene and graphene edge. Under the ORR-related condition, the electrolyte anions compete with water on the single metal site, in some cases either poisoning or modifying the catalyst activity and thermodynamic stability. The catalytic activity and stability descriptor suggest promising electrolyte and metal atom combinations that result in an active and durable catalyst.

Finally, the alkaline stability of the polybenzimidazole molecule is investigated. The polybenzimidazole molecule is a promising material for anion exchange membranes in alkaline fuel cells, but the degradation at a higher KOH concentration limits its practical use. It is found that the polybenzimidazole molecule undergoes deprotonation, forming the ionized molecule in an alkaline solution. In highly alkaline conditions, the fully deprotonated molecule is dominant and unlikely to degrade. In contrast, the non- and partially deprotonated molecules can undergo degradation. The effective energy barrier depends on the KOH concentration and becomes lower as the concentration increases. Also, it is found that the barrier along the reaction pathway and the availability of the vulnerable species in the solution both can affect the degradation rate, suggesting a significant role of pK_a value in the degradation.