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A single metal site incorporated in N-doped carbon (M/N/C) is a promising electrocatalyst. Here, we perform a computation investigation of the effect of electrolyte anion adsorption on the activity and stability of single-atom catalysts (MN4) with M as transition metal and p-block metal. The MN4 site on two different graphene structures (bulk graphene and graphene edge) is studied under electrochemical conditions for the oxygen reduction reaction (ORR) and the CO2 reduction reaction (CO2RR). Because of the two-dimensional nature of the catalyst, reaction intermediates and electrolyte ions can interact with both sides of the single-atom catalyst. As a result, the electrolyte anions compete with water and adsorbate on the single metal site, in some cases either poisoning or modifying the catalyst activity and thermodynamic stability. We find most electrolyte anions adsorbs on the single metal site under ORR conditions but not at the lower potentials for the CO2RR. Still, the adsorption of water and gas molecules can occur under CO2RR conditions. For example, under ORR conditions, the thermodynamic driving force of the *SO4-FeN4 site in the 0.1 M H2SO4 solution is about 0.47–0.56 eV lower than the *O-FeN4 site in water, depending on the local carbon structure. Additionally, the stabilization by electrolyte anions depends on the nature of the metal atom. Our study demonstrates the important role of electrolytes and the coordination environment for the activity and stability of the M/N/C catalyst.

Polybenzimidazole doped with aqueous KOH has emerged as an attractive electrolyte system for high-rate alkaline water electrolysis since it combines high ion conductivity with low H2 permeability. The lifetime is, however, limited to a few weeks under operating conditions due to degradation modes leading to chain scission. In this work, the underlying degradation mechanisms are explored by monitoring the chemical changes of a series of small-molecule arylene-linked bis-benzimidazoles treated under extreme caustic conditions for nearly 6 months at 80 °C. Degradation products and degradation pathways are identified experimentally and supported by density functional theory calculations. Based on the experimental and theoretical data, it is suggested that the degradation mainly proceeds via the remaining fraction of neutral benzimidazole and that stability can be improved by increasing the degree of deprotonation along the molecule.

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 deproton

Metal-nitrogen carbon (M-N-C) catalysts, atomically dispersed and nitrogen-coordinated MNx sites embedded in carbon planes, have exhibited encouraging oxygen reduction reaction activity in an acidic environment. However, one challenge for these materials is their insufficient long-term stability in the acid environment. Herein, we systematically investigate both catalytic activity toward ORR and stability under acid conditions using density functional theory (DFT). Various local atomic structures around the MNx site and different metal atoms (M = Cr, Mn, Fe, Co, Ni, and Ru) are considered in this study to understand the relation between atomic structures, stability, and catalytic activity. The stability of the M-N-C catalyst is considered from the propensity of the metal atom center to dissolve from the carbon host structure. The calculations reveal that the considered MNx sites are thermodynamically unstable in acid ORR conditions. However, based on the calculated thermodynamic driving force toward the metal dissolution, the MN4 sites with Fe, Co, Ni, and Ru metal atoms embedded on the graphene plane and at the graphene edge are more stable in the acid ORR condition than the other considered MNx structures. Combining the stability and catalytic activity descriptor, we propose some acid-stable and active MNx structures toward ORR. This computational study provides helpful guidance for the rational modification of the carbon matrix hosting MNx moieties and the appropriate selection of a metal atom for optimizing the activity and stability toward the ORR reaction.

The development of efficient electrocatalyst for the oxygen reduction reaction (ORR) is essential for the commercialization of fuel cell technologies. Iron carbide encapsulated in N-doped graphene (NG/Fe 3 C) has been recently recognized as a promising ORR catalyst. In this study, the stability and catalytic activity of N-doped graphene supported on metal-iron carbide (NG/M_Fe 3 C) toward ORR are investigated using density functional theory (DFT) calculations. The NG/M_Fe 3 C heterostructure is modeled by substituting Fe atoms in the Fe 3 C substrate near the NG/Fe 3 C interface by metal atoms M (M = Cr - Mn, Co - Zn, Nb - Mo, Ta - W). The calculations show that the introduction of the metal atoms M alters the work function of the overlayer N-doped graphene, which is found to correlate with the binding strength of the ORR intermediates. We find that introducing Ni or Co atoms at the interface improves the ORR activity of the NG/Fe 3 C as well as stabilize the heterostructure. The ORR activity increases as the concentration of Ni or Co atoms near the interface increases, and the stable heterostructure is available in a wide range of substituted concentrations. These results suggest approaches to improve the ORR activity of NG/Fe 3 C catalysts.