Proton exchange membrane fuel cells (PEMFCs) are environmentally benign power generation devices that can effectively convert chemical energy to electricity, and therefore have potential application in electric vehicles and stationary power stations ^{[1, 2]}. However, various technical and economic challenges hinder the widespread use of PEMFCs ^{[3, 4]}; for example, because of the sluggish kinetics of the oxygen reduction reaction (ORR), a highly active cathode catalyst is needed ^{[5, 6]}. At present,Pt and its alloys are still the most effective catalysts for the ORR ^[7], but the limited supply and high cost of Pt are the main obstacles to the commercialization of PEMFCs ^{[8, 9]}. Non-noble-metal catalysts with high activity in the ORR have therefore attracted much attention.

Various materials have been explored as non-noble-metal catalysts for the ORR, including heteroatom (N,B,S)-doped carbons ^[10-14], transition-metal- and nitrogen-codoped carbons ^[15-17], transition-metal chalcogenides ^{[18, 19]}, and transition-metal oxides ^[20]. Among these materials, metal- and nitrogen-codoped carbons, i.e.,M-N-C materials (M is usually Fe or Co), have shown great promise ^[21-24], with ORR activity approaching those of Pt-based catalysts. However, these materials are generally poorly stable in acidic media.

It is generally accepted that metal-nitrogen coordination structures (e.g.,M-N₄) are the main catalytic sites in Fe-N-C materials for the ORR ^{[25, 26]}. The poor stability of this class of materials is caused by exchange of the metal ions in the M-N₄ structures with protons and/or oxidative attack by the peroxide intermediates formed in the ORR ^[27]. Metalloporphyrins and metallophthalocyanines are the two main types of metal macrocyclic compounds containing M-N₄ coordination structures. And they are excellent molecular catalysts for various reactions, especially ORRs in alkaline media ^[28-32]. But they have poor activity and stability in acidic solutions ^[33-40].

We used density functional theory (DFT) calculations to investigate possible ion-exchange reactions between metal ions in various metalloporphyrins and metallophthalocyanines and protons in strongly acidic solutions (pH = 1) to determine whether the replacement of metal ions by protons degrades the performances of M-N-C materials and metal macrocyclic compounds in acidic media. The results show that ORR-active Fe and Co macrocyclic molecular catalysts are stable against ion-exchange with protons; this suggests that the performance degradation of these molecular catalysts and the corresponding M-N-C catalysts is not caused by replacement of metal ions by protons.

2 Computational details

Spin-polarized DFT calculations implemented with the DMol³ module ^[41] in the BIOVIA Accelrys Materials Studio program were used. The exchange-correlation functionals were based on the Perdew-Burke-Ernzerhof functional within the generalized gradient approximation. Core treatment was adopted for all electron relativistic methods to obtain metal relativistic effects. Double numerical plus polarization function basis sets were used in the calculations. A smearing of 0.005 Ha (1 Ha = 27.21 eV) was applied to the orbital occupation. Geometric optimizations were performed using the Broyden-Fletcher-Goldfarb-Shanno algorithm and the geometric convergence tolerance for the energy change, maximum force, and maximum displacement were 1 × 10^-5 Ha, 0.002 Ha/Å (1 Å = 0.1 nm), and 0.005 Å, respectively. Self-consistent-field procedures were performed with a convergence criterion of 1 × 10^-6 Ha on the total energy to achieve accurate electronic convergence. The total energy (E) of all electrons involved in the system was obtained from geometric optimization of the molecular structure, and the zero-point energy (E_ZPE) and entropy (S) corrections (at 0.1 MPa, 298.15 K) were calculated using frequency analysis.

3 Results and discussion

3.1 Thermodynamic scheme for investigating stability of metal macrocyclic molecules against ion exchange with protons

Ion exchange between metal ions and protons was investigated in a thermodynamic scheme by calculating the free energy of the following reaction:

$MP+2{{H}^{+}}={{H}_{2}}P+{{M}^{2+}}$

(1)

where M = Cr,Mn,Fe,Co,Ni,Cu, or Zn, and P = phthalocyanine (Pc) or porphyrin (P). The free energy of reaction (1) can be obtained from the chemical potentials (μ) of the species involved:

$\Delta G={{\mu }_{{{H}_{2}}P}}+{{\mu }_{{{M}^{2+}}}}-2{{\mu }_{{{H}^{+}}}}$

(2)

The chemical potentials of the metal-coordinated and protonated macrocyclic molecules, i.e.,μ_MPand μ_H2P, can be estimated from the DFT-calculated total energy (E), zero-point energy (E_ZPE), and entropy (S) of the corresponding molecules, i.e.,μ_H2Pc= E_H2Pc-total+ E_H2Pc-ZPVE- TS_H2Pc. The chemical potential of M²⁺ in solution,μ_M2+, can be estimated from

${{\mu }_{{{M}^{2+}}}}={{\mu }_{M}}+2eU_{{{M}^{2+}}/M}^{0}+RT\ln {{c}_{{{M}^{2+}}}}$

(3)

where c_M2+is the concentration of M²⁺ in the medium,U⁰_M2+/Mis the standard equilibrium electrode potential of M²⁺ + 2e^- = M (these can be found in Ref. ^[42]), and μ_Mis the chemical potential of metal atom M and can be estimated from the DFT-calculated cohesive energy (E_M), zero-point energy (E_M-ZPE), and vibrational entropy (S_M) of metal M, i.e.,μ_M= E_M + E_M-ZPE - TS_M.

The chemical potential of a proton in the solution ^[43] can be calculated as

${{\mu }_{{{H}^{+}}}}=1/{{2}_{{{\mu }_{{{H}_{2}}}}}}+RT\ln {{c}_{{{H}^{+}}}}$

(4)

where c_H+is the proton concentration, which is 0.1 mol/L for pH = 1, and μ_H2is the chemical potential of H₂ gas at 0.1 MPa. Here, 1/2 μ_H2is used as a substitute for the standard chemical potential of H⁺ in solution (μ_H+); they have the same value because the zero reference potential is set at the potential of the standard hydrogen electrode. The chemical potential of H₂ can be estimated from the DFT-calculated total energy (E), zero-point energy (E_ZPVE), and entropy (S), i.e.,μ_H2= E_H2-total+ E_H2-ZPVE-TS_H2. In the above equations,T is the experimental temperature, i.e., 298.15 K.

Eqs. (2)-(4) can be used to calculate the equilibrium concentration of M²⁺ in the solution by setting ΔG = 0, at which exchange equilibrium is reached between MP molecules and protons. The amount of MP molecules that undergo degradation through demetalation is then Vc_M2+M_M , where V is the volume of the solution used in the measurement, which is assumed to be 50 mL here, and M_MP is the molar mass of the corresponding MP. The percentage of degraded MPs can be estimated as

$x \%=100V{{c}_{{{M}^{2+}}}}{{\overline{M}}_{Mp}}/{{m}_{t}}$

(5)

where m_t is the total mass of MP loaded on the electrode, which is usually from a few tens to hundreds of micrograms per square centimeter.

3.2 Effects of metal ions on stability of metal macrocyclic compounds

Table 1 gives the equilibrium concentrations of M²⁺ and the demetalation percentages for various MPs and MPcs on an electrode of diameter 5 mm, estimated according to the procedure described in section 3.1 (assuming that the loading of MPs on electrode is 150 μg/cm²). The data show that MPs with Cr,Mn, and Zn metal centers are less stable than other MPs under strongly acidic conditions; for example, the equilibrium concentrations of Cr²⁺,Mn²⁺, and Zn²⁺ are greater than 0.07 mol/L when the corresponding MPcs are in contact with a solution of pH = 1. This corresponds to a demetalation amount of about 2.0x 10⁶ mg, and is several orders of magnitude higher than the total mass of MPcs normally loaded on the electrode for electrocatalysis. The MPs with Fe²⁺,Co²⁺,Ni²⁺, and Cu²⁺ metal centers are fairly stable under strongly acidic conditions, with ion equilibrium concentrations less than 3.2 x 10^-15 mol/L, which are negligibly low and below the possible detection limit. Fe- and Co-based metal macrocyclic molecules are usually used as ORR electrocatalysts, therefore we can conclude that the exchange of metal ions with protons is not the main cause of performance degradation for these molecular catalysts. Similar conclusions can be drawn for carbon materials doped with Fe (Co) and nitrogen, which are believed to contain similar metal-nitrogen coordination structures to those in macrocyclic molecules as ORR catalytic centers.

表 1 (Table 1)

Table 1 Equilibrium concentrations of exchanged metal ions, mass of demetalated MPs (m), and demetalation percentages on 5-mm electrode in solution of pH = 1 (assuming mt = 150 mg/cm2). MPs/MPcs cM2+/(mol/L) m/mg x/% CrPc ~7 × 10-2 ~1 × 106 ~7 × 104 MnPc ~5 × 10-1 ~1 × 107 ~5 × 105 FePc ~1 × 10-23 ~4 × 10-16 ~1 × 10-17 CoPc ~4 × 10-30 ~1 × 10-22 ~4 × 10-24 NiPc ~4 × 10-29 ~1 × 10-21 ~4 × 10-23 CuPc ~4 × 10-22 ~1 × 10-14 ~4 × 10-16 ZnPc ~2 × 10 ~5 × 108 ~1 × 107 CrP ~2× 10-4 ~3 × 103 ~1 × 102 MnP ~1 × 108 ~3 × 1015 ~9 × 1013 FeP ~3 × 10-15 ~6 × 10-8 ~2 × 10-9 CoP ~3 × 10-22 ~6 × 10-15 ~2 × 10-16 NiP ~1 × 10-21 ~2 × 10-14 ~6 × 10-16 CuP ~4 × 10-18 ~8 × 10-11 ~3 × 10-12 ZnP ~2 × 103 ~4 × 1010 ~1 × 109

Table 1 Equilibrium concentrations of exchanged metal ions, mass of demetalated MPs (m), and demetalation percentages on 5-mm electrode in solution of pH = 1 (assuming mt = 150 mg/cm2).

A possible reason for the poor stability of MPs with central Cr,Mn, and Zn atoms is that the 3d orbitals of these three metals are half filled (3d⁵) or fully filled (3d¹⁰), which reduces the ability of the 3d orbitals to coordinate with nitrogen and other atoms. For a given metal,MPc is more stable than MP in strongly acidic solutions. This is because π-conjugation is greater for phthalocyanines than for porphyrins, which results in shorter and stronger M-N bond. The MP stability order is CoPc < NiPc < FePc < CuPc and CoP < NiP < CuP < FeP, which is approximately consistent with the Lewis acidity with the bivalent cations: Co²⁺ < Ni²⁺ < Fe²⁺ < Cu^{2+ [44]}.

3.3 Effects of substituents on stability of metal macrocyclic compounds

The electronic structures of metal macrocyclic molecules can be altered by introducing substituents. We therefore investigated the effects of various substituents on the stability of Fe and Co phthalocyanines in acidic media. Fe and Co phthalocyanines with halogen (F-,Cl-), nitro (NO₂-), methyl (Me-), tertiary butyl (tBu-), and amino (NH₂-) substituents were considered (Fig. 1), respectively. The structures are denoted by 16F-MPc, 16Cl-MPc, 4NO₂-MPc, 4F-MPc, 4Cl-MPc, 4NH₂-MPc, 4Me-MPc, and 4tBu-MPc (M = Fe or Co), based on the type and number of substituents. Our DFT geometric optimization showed that 16Cl-FePc has a distorted structure, different to those of the other substituted FePcs.

Fig. 1

Fig. 1. Molecular structures of substituted metal phthalocyanines: M = Fe or Co; R = F or Cl; R1 = NH2,Me,tBu,NO2, or Cl,F.

Table 2 gives the equilibrium concentrations of M²⁺ and the demetalation percentages for various substituted metallophthalocyanines on electrodes of diameter 5 mm, estimated using the procedure described in section 3.1 (assuming that the MP loading on the electrode is 150 μg/cm²). A comparison of the results shown in Tables 1 and 2 shows that the demetalation percentages of FePc and CoPc with electron-donating substituents (Me-,NH₂-,tBu-) are lower than those of the parent compounds, which indicates that the stability of FePc and CoPc can be enhanced by introducing electron-donating substituents into the macrocycles.

表 2 (Table 2)

Table 2 Equilibrium concentrations of exchanged metal ions, mass of demetalated MPs (m), and demetalation percentages on 5-mm electrode in solution of pH = 1 (assuming mt = 150 mg/cm2). MPs cM2+/(mol/L) m/mg x/% 4Me-FePc ~1 × 10-26 ~4 × 10-19 ~1 ×10-20 4NH2-FePc ~5 × 10-26 ~2 × 10-18 ~5 × 10-20 4tBu-FePc ~5 × 10-28 ~2 × 10-20 ~6 × 10-22 4Cl-FePc ~7 × 10-25 ~2 × 10-17 ~8 × 10-19 4F-FePc ~5 × 10-25 ~2 × 10-17 ~6 × 10-19 4NO2-FePc ~7 × 10-21 ~3 × 10-13 ~9 × 10-15 16Cl-FePc ~5 × 10-25 ~3 × 10-17 ~9 × 10-19 16F-FePc ~4 × 10-14 ~2 × 10-6 ~6 × 10-8 4Me-CoPc ~5 × 10-33 ~2 × 10-25 ~6 × 10-27 4NH2-CoPc ~3 × 10-33 ~9 × 10-26 ~3 × 10-27 4tBu-CoPc ~3 × 10-33 ~1 × 10-25 ~4 × 10-27 4Cl-CoPc ~4 × 10-30 ~2 × 10-22 ~5 × 10-24 4F-CoPc ~3 × 10-30 ~1 × 10-22 ~3 × 10-24 4NO2-CoPc ~1 × 10-32 ~4 × 10-25 ~1 × 10-26 16Cl-CoPc ~9 × 10-30 ~5 × 10-22 ~2 × 10-23 16F-CoPc ~1 × 10-21 ~6 × 10-14 ~2 × 10-15

Table 2 Equilibrium concentrations of exchanged metal ions, mass of demetalated MPs (m), and demetalation percentages on 5-mm electrode in solution of pH = 1 (assuming mt = 150 mg/cm2).

Electron-withdrawing substituents, i.e., 4Cl-, 4F-, and 16Cl-, enhance the stability of FePc to some extent; however, strongly electron-withdrawing groups such as 4NO₂- and 16F- decrease the stability of FePc. Electron-withdrawing substituents all decrease the stability of CoPc, and the destabilization increases with increasing electron-withdrawing ability. However, regardless of whether the substituents were electron-withdrawing or electron-donating, the equilibrium concentrations of Fe²⁺ and Co²⁺ were lower than 4.4 × 10^-14 mol/L when the corresponding MPcs were in contact with a solution of pH = 1. This corresponds to negligible demetalation of the corresponding MPcs for electrodes under normal electrocatalytic conditions.

3.4 Further discussion

As already stated in the introduction, demetalation via exchange of metal ions with protons during the ORR is a possible cause of degradation of metal-N₄ compounds in acidic media. Our calculations show that ORR-active metal-N₄ macrocyclic compounds have considerable resistance to exchange with protons. Although our results cannot confirm whether such molecular catalysts are stable in ORRs in acidic media, they show that the replacement of metal ions by protons is not the reason for the performance degradation of ORR-active Fe and Co macrocyclic molecular catalysts and carbon materials doped with Fe (Co) and nitrogen, which are believed to contain similar metal-nitrogen coordination structures to those of macrocyclic molecules as ORR catalytic centers.

The oxygenated species produced during the ORR, e.g., peroxide intermediates, could also cause degradation of metal-N₄ macrocyclic compounds. The ORR mechanism needs to be investigated to fully understand the stability of macrocyclic molecular catalysts for ORRs in acidic media; and this is a complicated and challenging task. It is difficult for us to cover this in the present paper, but our calculation results suggest that the degradation of these molecular catalysts may be caused by attack by peroxide intermediates formed during the ORR.

Hydrogen peroxide can destroy materials by forming hydroxyl radicals in the presence of metal ions through the Fenton reaction ^{[45, 46]}:

$F{{e}^{2+}}+{{H}_{2}}{{O}_{2}}+{{H}^{+}}\to F{{e}^{3+}}+\centerdot OH+{{H}_{2}}O$

(6)

The negligible demetalation of ORR-active Me-N₄ molecules shown by the present calculations suggests that peroxide attack on these Me-N₄ compounds would not be strong, as is generally believed, unless the medium is contaminated by metal ions.

4 Conclusions

A theoretical scheme was established to investigate exchange between metal ions of metallophthalocyanines and metalloporphyrins with protons in acidic media. The equilibrium concentrations of metal ions in solution were obtained, using DFT calculations, for various transition-metal phthalocyanines and porphyrins in contact with a strongly acidic solution, i.e., pH = 1. The results show that Fe,Co,Ni, and Cu phthalocyanines and porphyrins are stable against ion exchange with protons in acidic media, but Cr,Mn, and Zn ions in phthalocyanines and porphyrins are easily replaced by protons. In addition, it was found that electron-donating substituents enhance the stability of metal phthalocyanines. These results suggest that the exchange of metal ions with protons is not the main cause of performance degradation of Fe/Co-based macrocyclic molecules and Fe/Co-N-C materials, which are being widely investigated as non-noble-metal electrocatalysts for the ORR to replace Pt in fuel cells.

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