Durability of Graphene-Supported Iron Phthalocyanine in Acidic and Basic Media: A Theoretical Study

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The oxygen reduction reaction (ORR) is a crucial chemical process, which contributes significantly to the energy efficiency of fuel cells and metal-air batteries. Although Pt and its alloys are known as the most efficient catalysts for ORR, its high costs and scarcity limits their applications. Alternatively, Fe–N–C catalysts have attracted the most interest owing to their low-cost and facile preparation methods. Metal macrocycles, such as iron phthalocyanines (FePc), are the precursors most widely used to prepare the ORR catalysts in fuel cells and metal-air batteries. However, limited progress has been made in improving their electrocatalytic activity and durability. In particular, FePc has demonstrated to suffer from severe degradation in the acidic fuel cell environments, commonly attributed to demetalation and/or degradation by ORR intermediates. The typical life cycle for FePc catalysts is less than 100 cycles in either acidic or basic media.In this work we use ab initio molecular dynamic (MD) simulations to investigate the degradation of graphene-supported FePc in acidic and basic media at operational conditions. The acidic media includes hydrochloric, acetic, sulfuric and phosphoric acids, whereas the basic media includes sodium hydroxide. The solution concentration in all cases is 1M. The MD simulations were performed by dispersion-corrected DFT calculations, using periodic boundary conditions, at 80°C through a simulation time of 2 ps. Our results show that demetalation is not the cause of the FePc degradation, instead we find that for acetic and hydrochloric acids, the acid anion binds to the Fe center, with binding energies of -1.1 and -6.5 eV, respectively. The formation of this ligand would hinder the O2 adsorption on the metal center, limiting the FePc catalytic activity. On the contrary, for phosphoric and sulfuric acids, as well as for basic media, the ion-Fe binding is energetically unlikely, enabling the O2-metal interaction. Therefore, our results suggest that the catalytic activity and degradation of FePc are connected to the acidic media involved.This work was supported by ANID/FONDECYT under Grant No. 1170480