Regulating Electronic Descriptors for the Enhanced ORR Activity of FePc-Functionalized Graphene via Substrate Doping and/or Ligand Exchange: A Theoretical Study

This work analyzes the optimization of a potential Pt-free cathode fuel cell catalyst, an iron phthalocyanine monolayer supported on a graphene substrate (GFePc) by regulating the e(g) orbital filling states via substrate doping and/or ligand exchange. In the present work, the substrate doping (B-doping and N-doping) of graphene and/or ligand exchanges (F- and NH2-) of FePc are explored to tune the performance of GFePc using ab initio spin-polarized density functional theory (DFT) calculations. The catalytic steps for the dominating pathway for GFePc (associative mechanism) were approximated for the entire oxygen reduction reaction (ORR), followed by the subsequent water formation reaction (WFR). The rate-limiting step of the mechanism is the initial reduction of O-2 to the O-OH reaction. The DFT results show that in a water-solvated environment boron doping of similar to 1 atom % of graphene lowers the overpotential of the rate-limiting step to similar to 0.49 eV, which is on par with the Pt cathode, at normal fuel cell operating conditions. This work also reveals that the scaling relationship can be broken via substrate doping and/or via ligand exchange for superior ORR activity, and this will guide the development of single-metal atom-based next-generation Pt-free fuel cell cathode catalysts.

Automated summary

This work analyzes the optimization of a potential Pt-free cathode fuel cell catalyst, GFePc, by regulating the e(g) orbital filling states via substrate doping and/or ligand exchange. The results show that boron-doped graphene can lower the overpotential of the rate-limiting step, making its activity comparable to that of the Pt cathode. Additionally, substrate doping and ligand exchange can also enhance the ORR activity.