Enhancing the Stability of a Pt‐Free ORR Catalyst via Reaction Intermediates

Finding a platinum‐free cathode catalyst that effectively models the oxygen reduction reaction (ORR) of a proton‐exchange membrane (PEM) fuel cell cathode better than the current commercial Pt/C catalyst has been a major shortcoming in fuel cell technology. Overall, a promising platinum‐free cathode catalyst must offer great ORR activity, ORR selectivity, and acid stability. Due to their enticing ORR activity and selectivity to the preferred four‐electron ORR pathway, the possible dissolution reactions and oxygen‐intermediate reactions of iron phthalocyanine monolayer supported on a pristine graphene (GFePc) and boron‐doped graphene substrate (BGFePc) have been studied to determine the stability as a function of potential and pH through spin‐polarized density functional theory (DFT) calculations at both infinitesimally low (10−9 m) and 1 m Fe2+/Fe3+ ionic concentrations. BGFePc offers higher stability in both concentrations than GFePc. In both cases, the oxygen‐intermediates are more stable than the bare catalytic surface due to the metal d‐band center shifting further away from the Fermi level in the valence band state (higher energy of antibonding). Moreover, at an Fe2+ ionic concentration, both catalysts would be stable in the potential and pH regions at the operating conditions of rotating disk electrode (RDE) experiments and PEM fuel cells.


Computational Methods
The present study carried out spin-polarized density functional theory (DFT) calculations using the Vienna ab-initio simulation package (VASP) and projected-augmented wave (PAW) Finding a platinum-free cathode catalyst that effectively models the oxygen reduction reaction (ORR) of a proton-exchange membrane (PEM) fuel cell cathode better than the current commercial Pt/C catalyst has been a major shortcoming in fuel cell technology. Overall, a promising platinum-free cathode catalyst must offer great ORR activity, ORR selectivity, and acid stability. Due to their enticing ORR activity and selectivity to the preferred four-electron ORR pathway, the possible dissolution reactions and oxygenintermediate reactions of iron phthalocyanine monolayer supported on a pristine graphene (GFePc) and boron-doped graphene substrate (BGFePc) have been studied to determine the stability as a function of potential and pH through spin-polarized density functional theory (DFT) calculations at both infinitesimally low (10 −9 m) and 1 m Fe 2+ /Fe 3+ ionic concentrations. BGFePc offers higher stability in both concentrations than GFePc. In both cases, the oxygen-intermediates are more stable than the bare catalytic surface due to the metal d-band center shifting further away from the Fermi level in the valence band state (higher energy of antibonding). Moreover, at an Fe 2+ ionic concentration, both catalysts would be stable in the potential and pH regions at the operating conditions of rotating disk electrode (RDE) experiments and PEM fuel cells.

Introduction
The oxygen reduction reaction (ORR) of a proton exchange membrane (PEM) fuel cell cathode, which currently uses platinum-based catalysts, offers sluggish kinetics and is a limiting factor in the advancement of fuel cell technology. Catalysts free of precious metals have been explored in recent years. A suitable alternative to platinum-based or platinum-group metal (PGM) catalysts would require surpassing the integrity, stability, and oxygen reduction performance that the current catalysts offer. pseudopotentials with a plane-wave cut-off of 400 eV. The Perdew-Burke-Erzenhof (PBE) [12,13] form of the generalized gradient approximation (GGA) [14,15] was used to describe the interactions between valence electrons and frozen cores. A semiempirical scheme proposed by Grimme (DFT-D2) was used to account for long-range dispersion corrections, such as van der Waal interactions. [16] It is to be noted that methods that correct the intra-band Coulomb interaction, such as DFT+U, were not presently considered in our calculations. Grimme's DFT-D2 method without DFT+U has been implemented previously and was able to successfully describe the electronic properties of graphene systems. [17] To facilitate convergence, the Gaussian smearing method was employed a width of 0.05 eV around the fermi level. Electronic energies were converged to 10 −6 eV. Ionic relaxations were performed until the residual forces on the atom were less than 0.02 eV Å −1 .
The single layer, slab model graphene sheet with an optimized C-C bond length (1.42 Å) was used for both the pristine (GFePc) and boron-doped iron phthalocyanine functionalized graphene (BGFePc). The GFePc surface has a cell periodic in both the x and y directions with dimensions of 17.07 × 14.78 Å. Due to the 2D nature of the monolayer graphene-based catalyst, the z direction is 25 Å to ensure negligible interactions between periodic images. To sample the Brillouin zone, a 3 × 3 × 1 Monkhorst-Pack k-point mesh was used.
Following the methodology outlined by Holby et al., [8] the dissolution of metal species of C-hosted M-N y structures follow a general reaction. This general reaction has been specified to GFePc system and is shown in reaction Equation (1 Since the energy of gas phase O 2 is poorly described through DFT, researchers have been moving towards using molecular H 2 O reference states for the removal of intermediate states. [6] To remedy this, the free energy of O 2 was directly obtained from the reaction O 2 + 2H 2 = 2H 2 O; the free-energy change for this equation is 4.92 eV. Additionally, the free energies of hydrogen molecules were calculated using a combination of both computational and experimental results. The free energy of H 2 O was calculated in the gas phase with a pressure of 0.035 bar that is in equilibrium with liquid water at 300 K. [18] The free energies of surface-bound species were calculated using the TAMkin [19] program by implementing finite temperature thermodynamic model calculations. In these calculations, the nitrogen atoms surrounding the metal center cavity, along with the metal, added hydrogens, or intermediate species were unfrozen. The remaining atoms were frozen at their relaxed state to limit computational cost. However, this small contribution due to remaining frequency calculations would cancel out during the reaction free energy evaluation, which were calculated as energy differences between reactants and products. When the metal dissolves into solution, the phthalocyanine cavity can support up to two hydrogens in the metals place. Because of this, n+a must be equal or less than two for all dissolution reactions. In most cases, the iron metal ion will dissolve to the Fe 2+ state. The free energy of a single dissolved Fe ion can be described by Equation (2) as utilized by previous works [7,8] Where + Feaq c x is the concentration of ionic Fe x+ , c o is the standard ionic Fe x+ concentration used when G exp was reported. It is noted that G exp can sometimes be reported as a dissolution potential. In the case of Fe 2+ , −0.96 eV/Fe was used [20] at 1 m standard ionic concentration was used. In order to increase the accuracy of this value, phonon calculations of bulk α-Fe were done to obtain DFT,Febulk E . For the oxygen reduction reaction, the intermediates necessary to complete the direct four-electron pathway are: *OOH, *OH, *O 2 , and *O. The bare catalyst surface, GFePc, and the intermediates present in ORR were chosen to encompass the stability of GFePc as an ORR catalyst. The dissolution reactions considered in our model are shown in Table S1 in the Supporting Information.
To define the lines of equilibrium from these electrochemical reactions, the equations outlined by previous work from Persson et al. [7] and Holby et al., [8] the pH equilibrium potential for x≠n is determined by Equation (3) ( ) Minding that n = 0 will yield a horizontal line on the diagram; it offers no pH dependence and represents a purely electric potential-mediated dissolution reaction. For x = n, the respective equation results in an equilibrium pH, which appears as a vertical line on the diagram, can be modeled by Equation In Equations (3) and (4), G products and G reactants are respectively defined as the total energy of products and reactants for a single metal atom dissolution reaction. k B is the Boltzmann constant, T is temperature (unit dependent on k B ), and ln (10) is from the log base difference between the Nernst equation and the pH scale. All potentials listed in this paper are versus the computational hydrogen electrode (vs CHE). [21] The ligation/intermediate reactions between GFePc and the intermediates *OH, *OOH, *O 2 , and *O were considered to obtain the equilibrium lines in areas of the overall stability diagram that had overlapping stable intermediates. These are shown in Table 1. Note that some of the stability lines of the reaction may not be present in the final diagram but play a part in knowing which intermediate is the most stable at that particular potential and pH.
The calculations were performed for both the pristine (GFePc) and boron-doped (BGFePc) graphene systems. The equations for both systems remain the same as boron does not directly take part in the reaction as it is located in the substrate.

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However, as noted in a previous work, it is expected that boron does influence the electronic properties of GFePc and can be indirectly beneficial to the reaction. [11] The previous works on GFePc have shown that the catalyst system follows the direct four-electron pathway due to the high dissociation energy of the bond cleavage step. [10] Hence, the two-electron reaction pathway which leads to the production of H 2 O 2 and how it effects the stability of GFePc has not been considered in this study. A wide range of ionic concentrations (10 −9 to 1 m) of Fe x+ were considered as the free ionic concentration of Fe x+ can drastically change the stability range of the catalyst and its intermediates. It is expected that as the free ionic concentration of Fe x+ increases, the metalated material or intermediate become more stable as less metal wants to dissolve into solution. [8]

Results and Discussion
Phthalocyanine is an extremely stable molecule on its own by also being supported on the graphene substrate, it is suspected that the stability of GFePc and BGFePc encompasses a large range. Phthalocyanine is known to only host up to two hydrogens in its center, H 2 Pc. Since less dissolution reactions are possible than with traditional FeN 4 catalysts because of its limitation of hydrogens, its stability range is hypothesized to be increased.
To calculate the stability and dissolution of GFePc, the free energies of the pristine catalyst, the demetalated catalyst, and the addition of subsequent hydrogens on the phthalocyanine are required. In Figure 1, the structures of FePc, Pc, HPc, and H 2 Pc are shown without the presence of the graphene substrate to easily visualize the molecule by itself. Figure 2, shows the structures with the presence of the graphene substrate. Figure 2 shows that the cavity (GPc) and systems with the added hydrogen atoms offer no visible distortion of the graphene substrate or phthalocyanine which are held together by van der Waal dispersion forces. This is also true for the boron doped case, which is shown in Figure 3. The π-π stacking interactions that immobilize FePc molecules on graphene is said to improve the catalyst's stability. [9] This interaction was expected to be considered with Grimme's DFT-D2 semiempirical scheme [16] in which other long-range dispersion forces were considered.

Hydrogen Binding Energies
The hydrogen binding energies (E BE ) of both the pristine and boron-doped systems were calculated using Equation (5), along with the reference state mentioned previously and the TAMkin values generated previously at 300 K.
The binding energy per hydrogen can be calculated by dividing Equation (4) by n. Table 2 shows the calculated binding energies per hydrogen present at 300 K. Equation (5) does not take into account potential, pH, or electron transfer so it is only applicable for T = 300 K, pH = 0, and U = 0 V and serves as a relative comparison between these structures. As shown in Table 2, the binding energy per hydrogen is slightly increased in the boron-doped system for both H-bound structures. However, with there being less than a 0.1 eV difference between the binding energies of H 2 GPc and H 2 BGPc,there serves to be no major difference in the hydrogen binding energy of these two systems. However, this means that any difference in stability between these systems arises from their difference in GFePc/BGFePc or intermediate (*OH, *OOH, *O 2 , and *O) structures. These were discussed in our previous publication. [22]

Stability of GFePc and BGFePc
The stability of just the bare catalyst structures of both GFePc and BGFePc at concentrations between 10 −9 and 1 m ionic Fe 2+ concentration is shown in Figures 6 and 7. For both cases, as the stability of GFePc/BGFePc increases with ionic Fe 2+ concentration. As more Fe 2+ ions are in solution, the more favorable the GFePc/BGFePc states become. At high potentials and extremely low pH values, the GFePc/BGFePc catalysts dissolute into Fe 2+ and phthalocyanine cavity H-bound states.
The equilibrium pH of GFePc lies between 3.51 and −0.99 for 10 −9 and 1 m, respectively. The equilibrium pH of BGFePc lies between 2.16 and −2.33 for 10 −9 and 1 m, respectively. Noting that a pH less than −2 is extremely unlikely during normal operating conditions of RDE experiments and PEM fuel cell operations. BGFePc was also found to be slightly more stable than the GFePc catalyst when comparing their dissolution reactions. This may be the result of the boron-doped graphene substrate gaining charge while the pristine graphene substrate losing charge when in the BGFePc/GFePc configuration as displayed by our previous Bader charge analysis calculations. [22] The boron-doping seemingly increases the stability of iron phthalocyanine functionalized graphene at lower pH values and slightly higher potentials.

Stability of *OH, *OOH, *O 2 , and *O Intermediates
The stability of ORR intermediates should be incorporated into an overall stability diagram. This along with the ligation reactions between GFePc/BGFePc and its intermediates allow for a more complete stability diagram that mirrors a traditional Pourbaix diagram. Relative stability plots for *OH, *OOH, *O 2 , and *O are shown in Figure 8 for GFePc and Figure 9 for

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BGFePc at Fe 2+ ionic concentrations of 10 −9 m. In Figures S1 and S2 (Supporting Information), the same plots are shown for an Fe 2+ ionic concentration of 1 m. The ORR intermediates of the catalyst are more stable than the bare surface of GFePc/BGFePc when Figure 6 is compared with Figure 8 and Figure 7 is compared with Figure 9. This results clearly consistent with the previous observations for the FeN 4 C 138 catalyst. [8] It can be argued that the presence of these ORR intermediates stabilizes Fe against dissolution, and it can suggest that the catalyst active site has a greater chance of dissolution during turnover cycles. [8] This could be due to the shifting of the metal d-band center (shown in Table S3 in the Supporting Information) further away from the Fermi level in the valence band state which causes the higher the energy level of antibonding state of the ORR intermediates, make them more stable than the bare catalyst surface.

Ligation Reactions Between Intermediates
There is a lot of overlapping stability regions between intermediates. To remedy this, the ligation reactions listed in Table 1 are used to find which intermediate is most favorable at any given potential and pH. The stability plots of these intermediate pairs are shown in Figures S3-S6 (Supporting Information) for both GFePc and BGFePc systems.

Overall Stability Diagrams
Overall stability diagrams of both GFePc and BGFePc can be created by combining the stability regions of the bare catalyst surface, its ORR intermediates, and the separate ligation reactions between them. This can be seen in Figure 10 for an Fe 2+ ionic concentration of 10 −9 m to encompass the smallest stable regions of all intermediates (since the 1 m Fe 2+ ionic concentration has a larger stability range). It is important to note that Figure 10 does not display all locations at which that intermediate is stable, but rather the location that it is the most stable intermediate. The extra lines present GFePc's overall stability diagram is due to the slightly decreased stability of some its intermediates that leads to the visualization of ligation reactions that are not visible in BGFePc for the range of pH and potentials chosen.

Fe 3+ Consideration
As pointed out by Holby et al., for potentials above ≈0.77 V at standard conditions the Fe 3+ is the stable ionic phase. [8] Iron (II) phthalocyanine can only directly dissolute into Fe 2+ . However, there could be extra reactions occurring that can form Fe 3+ . Because of this, the formation of Fe 3+ is considered in this section. The intermediates are stable at higher potentials than the bare surface and will also have dissolution reactions using Fe 3+ considered during calculation. It is noted that G exp for Fe 3+ , −0.11 eV/Fe [23] at 1 m standard ionic concentration was used. The all intermediates were investigated as they all have stability ranging above ≈0.77 V. Any dissolution reaction that occurred over 0.77 V was changed to incorporate Fe 3+ . The intermediate with the highest stability, O*, saw a drop in equilibrium potential in both GFePc and BGFePc. For O-GFePc, at 10 −9 m Fe 3+ ionic concentration, the equilibrium potential is 1.65 V (vs the 2.05 V for the 10 −9 m Fe 2+ ionic concentration previously calculated). This means that O-GFePc loses ≈0.40 V of stability and the stability diagram above that potential will dissolute into Fe 3+ rather than Fe 2+ . For O-BGFePc, the stability equilibrium potential drops from 2.23 V at 10 −9 m Fe 2+ ionic concentration to 1.77 V at 10 −9 m Fe 3+ ionic concentration (≈0.46 V difference). Note that these are high potentials and it is unlikely to reach that at normal operating conditions during RDE experiments or PEM fuel cell operations.
The relative stability diagrams of GFePc/BGFePc and their intermediates at Fe 3+ ionic concentrations of 10 −9 and 1 m are shown in Figures S7-S12 in the Supporting Information. Note that the equilibrium pH stays the same as it is located below the ≈0.77 V threshold. The updated overall stability diagram which

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includes the Fe 3+ dissolution consideration is shown in Figure 11 for both GFePc and BGFePc. The O* intermediate stability seems to be the main difference when comparing Figures 10  and 11. However, as mentioned previously, these high potentials are unlikely to be reached during normal operating conditions of RDE experiments or PEM fuel cell operations.

Comparison to Experimental Data
We have not come across any experimental data studying the stability of specifically GFePc and BGFePc as a function of pH and potential. However, Jiang et al. recently studied iron phthalocyanine supported by reduced graphene in alkaline media in which they noted that when held at 0.85 V for 10 000 s in a 0.1 m KOH, 84% of relative current was preserved after a current-time chronoamperometric response time study. This was higher than their Pt/C control catalyst which retained 71.6% of its relative current. [9] Our DFT-calculated individual stability diagrams show that bare GFePc is unstable below pH values of 3.51 and bare BGFePc is unstable below pH values of 2.16 in 10 −9 m Fe 2+ ionic concentration. This means that if there are no ORR intermediates present, both GFePc and BGFePc would dissolute into Fe 2+ until an Fe 2+ ionic concentration of 1 m is achieved, where the stability region increases to pH values of −0.99 and −2.33 for GFePc and BGFePc, respectively. Meaning that in most cases, some of the catalyst will always dissolute into the used aqueous solution. However, when the ORR intermediates are present on the catalyst surface, the catalyst is stable at these low pH values (pH< −2), where the bare surface is not. Given that these intermediates likely evolve spontaneously and can play a major role in the electrocatalysis of Fe-N-C ORR catalysts. [8,[24][25][26] Our present study on both GFePc and BGFePc systems suggest that the ORR intermediates play a major role in the overall stability of these catalysts against metal dissolution.

Conclusion
The current study computationally determines stability as a function of potential and pH for iron phthalocyanine functionalized graphene (GFePc), its boron-doped derivative (BGFePc), and their ORR intermediates (*O, *O 2 , *OH, and *OOH). Spin-polarized DFT calculations were used in the present study to determine the stability ranges of GFePc/BGFePc for future use in applications such as RDE experiments and PEM fuel cell operations.
A promising ORR catalyst requires great ORR activity, ORR selectivity, and acid stability. GFePc and BGFePc have been previously studied for their enticing ORR activity and selectivity to the favorable 4-electron ORR pathway. Being able to computationally describe the stability by utilizing dissolution reactions at differing metal charge states and ORR intermediate reactions enables us to fully define promising ORR catalysts computationally.

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As shown in the present work, BGFePc and its ORR intermediates were slightly more stable in lower pH values and higher potentials than GFePc and in both cases, the bare catalyst is less stable than the intermediates. However, the stability of both GFePc and BGFePc ranges to high potentials that are unlikely to be reached during normal operating conditions of RDE experiments or PEM fuel cell operations. Though having low acid stability (pH < 2) at an Fe 2+ /Fe 3+ ionic concentration of 10 −9 m, the stability increases to encompass the lower pH (pH > −2) values when the Fe 2+ /Fe 3+ ionic concentration increases to 1 m. Additionally, the presence of the ORR intermediates offers some protection against Fe dissolution as they broaden the stability range of GFePc. This could be due to the shifting of metal d-band center of ORR intermediates further away from the Fermi level in the valence band state causes the higher the energy of antibonding state. Without these ORR intermediates, the bare catalyst would be more prone to its lower stability range and be subject to irreversible Fe dissolution. To understand how other substrate dopants and ligand exchanges effect the stability of iron phthalocyanine functionalized graphene is beyond the scope of our current study and should be explored in the future. Direct experimental Pourbaix stability diagrams should be generated for GFePc and BGFePc to compare our DFT-computed stability plots.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.