Activity of N-coordinated multi-metal-atom active site structures for Pt-free oxygen reduction reaction catalysis: Role of *OH ligands

We report calculated oxygen reduction reaction energy pathways on multi-metal-atom structures that have previously been shown to be thermodynamically favorable. We predict that such sites have the ability to spontaneously cleave the O2 bond and then will proceed to over-bind reaction intermediates. In particular, the *OH bound state has lower energy than the final 2 H2O state at positive potentials. Contrary to traditional surface catalysts, this *OH binding does not poison the multi-metal-atom site but acts as a modifying ligand that will spontaneously form in aqueous environments leading to new active sites that have higher catalytic activities. These *OH bound structures have the highest calculated activity to date.

kinetic barriers between intermediate states (as well as solvation and entropic effects) but is useful for comparing relative activities and predicting limiting steps in the ORR reaction pathway. Details of the DFT calculations can be found in the methods section.
The calculated reaction pathway for the multi-metal-atom Fe 2 N 5 structure is shown in Figure 1. Due to the spontaneous dissociation of O 2 , a dissociative pathway is assumed. It is readily apparent that the reaction pathway at 0 V potential is significantly lower than the ideal pathway, plotted in black, representing overbinding at all ORR intermediate steps. Due to the *OH bound state (reaction coordinate 5) being a lower energy than the ORR final state (2 H 2 O molecules, reaction coordinate 6), the value of V TH is 20.15 V, which indicates a very inactive ORR catalyst structure. We predict that the *OH bound state would spontaneously form in water bearing aqueous electrolytes and be persistent over a wide range of applied potentials.
In many heterogeneous catalyst systems, overbinding of *OH would be indicative of catalyst poisoning. The persistent *OH would serve to block active surface sites. The three-dimensional nature of the Fe 2 N 5 site, however, means that even with a persistent *OH ligand, the active site is not sterically hindered from reacting further with an O 2 molecule. As shown in Figure 2, the *OH is found to bind at the graphene edge host and the bridging N tilts perpendicular to    Treating the *OH bound Fe 2 N 5 site as the thermodynamically stable site in water-bearing electrolytes, we have calculated the reaction pathway energetics as shown in Figure 3. The effect of the *OH ligand and the resulting increased coordination of the Fe atoms drastically increases the calculated activity, leading to a V TH value of 0.72 V, an increase of 0.87 V versus the *OH free site. The potential determining step in this reaction pathway remains the final protonation step of the *OH intermediate. Additionally, the *OH ligand alters the O 2 binding such that spontaneous dissociation does not occur and so an associative ORR pathway becomes the more likely pathway.
Due to the multi-metal-atom nature of the predicted stable site, it is interesting to consider the effect of mixed-metal active site structures in similar structures. Such mixed-metal optimization adds an additional parameter that can be used to tune activity in multi-metalatom sites. It has previously been shown that the combination of Fe and Co can alter catalyst activity in synthesized systems 2 . Exchanging one Fe for a Co atom in the previously considered high activity Fe 2 N 5 (*OH) site leads to an FeCoN 5 (*OH) site structure. This structure and its calculated reaction pathway is shown in Figure 4. This structure has a calculated V TH value of 0.80 V which is the highest calculated value for any PFC structure to date. The potential determining step in this pathway is the protonation of the *OO intermediate. While direct comparison to experimental data is hindered by mass-transport contributions, this predicted highest activity structure presents an attractive PFC synthesis target.
In summary, stability calculations suggest that spontaneous *OH ligand formation on multi-metal-atom N-coordinated PFC active site structures are thermodynamically likely to be formed during synthesis. Calculated reaction pathways suggest that such sites are significantly more ORR active than single-metal-atom or metal-free sites previously considered in the literature. While it is not possible to definitively say that the presented multi-metal-atom sites are responsible for the high activity found in current state-of-the art PFCs due to a lack of direct observation, these structures are attractive targets for rational design synthesis approaches. Further studies are required in order to include the roles of kinetic barriers between intermediate states, metal atom spin states, solvation of bound intermediates, and zero-point and entropic contributions specific to multi-metal-atom edge sites.

Methods
Spin-polarized periodic density functional theory (DFT) as implemented in the Vienna ab initio Simulation Package (VASP) [24][25][26][27] was used to calculate the relaxed atomic and electronic structure of the candidate active sites as well as intermediate structures for the potential energy surfaces. The GGA method as implemented by Perdew, Burke, and Ernzerhof (PBE-GGA) 28,29 was used to describe the exchangecorrelation functional component of the Hamiltonian. An energy cut off of 400 eV was used for the plane-wave basis set. A convergence criteria for the electronic selfconsistent loop of 10 25 eV was utilized and structures were relaxed until all forces were below 0.02 eV/Å . 8 3 5 carbon pair zig-zag graphene nanoribbons were used as the C-host material for the active sites and vacuum of at least 20 Å was used between periodic ribbons to limit self-interaction. Gamma-point centered 5 3 1 3 1 K-point meshes were used to sample the Brillouin zone with the 1 values corresponding to the two vacuum directions. van der Waals interactions are included in all calculations through the use of the DFT-D2 method of Grimme 30 . A variety of models exist in the literature to include zero-point energy, entropy, and solvation effects. Of particular interest is the work of Lyalin, et al. 31 , as it includes the largely balancing effects of all three effects on a B-N surface. Unfortunately, a different reaction pathway than that considered in this work is considered. Szakacs, et al. 15 , have interpreted these values for the associative pathway considered here and found the major contribution for the intermediate states to be destabilization of the *O 1 2 H 1 1 2 e 2 1 H 2 O state (Reaction Coordinate 4) by 0.40 eV. Addition of this term does not change the potential determining intermediate states that lead to V TH for any of the three considered reaction pathways (Reaction Coordinates 5 R 6 and 2 R 3). Instead of adopting this potentially system and exchange-correlation functional dependent methodology, we assume full cancelation of these three effects and report pathways based solely on calculated binding energies from internal energies. This cancelation assumption has previously been shown a fair approximation for ORR intermediates on Pt surfaces 7,32 . Further justification of cancelation of ZPE, entropy, and solvation effects is the focus of future work.