Investigation of bifunctionality of FePc-functionalized graphene for enhanced ORR/OER activity

Abstract

Iron phthalocyanine monolayer supported on a graphene substrate (GFePc) is a promising Pt-free oxygen reduction reaction catalyst. The electronic properties of GFePc can be tuned via the ligand exchange of FePc, substate doping, and defect incorporation into the graphene substrate. Boron and nitrogen were chosen as substrate dopants due to their comparability to carbon in size. Two of the most common graphene defects, single and double (585) vacancies, were chosen to represent defects that can naturally occur on the graphene substrate. Tetra-substituted phthalocyanines (carboxy-, nitro-, and amino-) and hexadeca-substituted phthalocyanines (chloro‑, fluoro-, and amino-) were tested due to their availability in the current market. Utilizing ab initio spin polarized density functional theory (DFT) calculations, the dominating pathways of both the oxygen reduction (ORR) and oxygen evolution reactions (OER) have been simulated along with defining their rate-limiting step to quantify the ORR/OER overpotentials (indicator of catalyst reactivity/performance). The DFT results show that ∼1 at.% Boron doping in conjunction with a mono-vacancy graphene substrate offers the best ORR performance but a worse OER performance when compared to the pristine GFePc. Generally, an increase in ORR activity exhibited a decrease in OER activity. A qualitative volcano correlation between electronic descriptors and ORR overpotential for modified GFePc systems was observed. The present study allows for expanded understanding and definition of substituted iron phthalocyanine functionalized graphene and the effects of both substrate modification and ligand exchange on GFePc's performance as a potential bifunctional single-metal-atom based electro-catalyst.