Molecular dynamics simulation of the effect of trace moisture on the decomposition mechanism of a C4F7N/CO2 gas mixture under an electric field

As an alternative insulating medium to SF6, the C4F7N/CO2 gas mixture combines the relatively high dielectric strength with favorable environmental compatibility. However, under practical operating conditions, the combined effects of electric fields and trace moisture can induce its decomposition, thereby compromising the operational reliability of gas-insulated equipment. To elucidate how trace moisture affects the decomposition behavior of the C4F7N/CO2 gas mixture under an electric field, a trace moisture-containing C4F7N/CO2 system is constructed using the reactive force field molecular dynamics method. The evolution characteristics of the parent molecules, key free radicals, major products, reaction pathways, total particle number, and particle species number were systematically investigated under different electric field strengths and trace moisture contents. The results demonstrate that the applied electric field significantly promotes the decomposition of the C4F7N/CO2 system. It accelerates the consumption of C4F7N and CO2 while enhancing the formation of key species, including CF3, O, and CO. Among the identified decomposition pathways, C4F7N => CN + C3F7 is the dominant route. Under an electric field of 0.02 V/Å, increasing the trace moisture content further intensifies the decomposition of C4F7N/CO2 and increases the overall abundance of reactive particles such as F and O. In the presence of trace moisture, the initial bond-cleavage pathways of C4F7N are further promoted, including C4F7N → C3F4N + CF3. Meanwhile, F radicals are preferentially consumed through reactions with H2O and H2O-derived radicals rather than through direct participation in C4F7N decomposition. This shift promotes HF formation and indicates that trace moisture restructures the radical reaction network, thereby accelerating the conversion of fluorinated intermediates into stable acidic products. Accordingly, the decomposition regime is gradually shifted from parent-molecule-dominated self-decomposition to radical-mediated synergistic decomposition. In addition, trace moisture not only increases the total number of particles in the system but also significantly increases the number of particle species, thereby enhancing the complexity of the reaction network. Trace moisture-assisted decomposition in the C4F7N/CO2 system is further amplified under electric-field exposure, thereby increasing the potential risk of dielectric performance degradation. The role of trace moisture in the electric-field-induced decomposition of C4F7N/CO2 mixtures is clarified by these results, and mechanistic support is provided for moisture management and condition assessment in environmentally friendly gas-insulated equipment.