Benchmarking pH-field coupled microkinetic modeling against oxygen reduction in large-scale Fe–azaphthalocyanine catalysts

Molecular metal–nitrogen–carbon (M–N–C) catalysts with well-defined structures and metal-coordination environments exhibit distinct structural properties and excellent electrocatalytic performance, notably in the oxygen reduction reaction (ORR) for fuel cells. Metal-doped azaphthalocyanine (AzPc) catalysts, a variant of molecular M–N–Cs, can be structured with unique long stretching functional groups, which make them have a geometry far from a two-dimensional geometry when loaded onto a carbon substrate, similar to a “dancer” on a stage, and this significantly affects their ORR efficiency at different pH levels. However, linking structural properties to performance is challenging, requiring comprehensive microkinetic modeling, substantial computational resources, and a combination of theoretical and experimental validation. Herein, we conducted pH-dependent microkinetic modeling based upon ab initio calculations and electric field-pH coupled simulations to analyze the pH-dependent ORR performance of carbon-supported Fe–AzPcs with varying surrounding functional groups. In particular, this study incorporates large molecular structures with complex long-chain “dancing patterns”, each featuring >650 atoms, to analyze their performance in the ORR. Comparison with experimental ORR data shows that pH-field coupled microkinetic modeling closely matches the observed ORR efficiency at various pH levels in Fe–AzPc catalysts. Our results also indicate that assessing charge transfer at the Fe-site, where the Fe atom typically loses around 1.3 electrons, could be a practical approach for screening appropriate surrounding functional groups for the ORR. This study provides a direct benchmarking analysis for the microkinetic model to identify effective M–N–C catalysts for the ORR under various pH conditions.


Computational Methods
Supplementary Table 1.Summary of the DFT-calculated energies, experimental Gibbs free formation energies, and entropic contributions at standard conditions: T=298 K and pressure=1 bar.

Common procedures
Matrix assisted laser deposition / ionization time of flight mass spectroscopy (MALDI-TOF-MS) measurement of catalytic molecules were performed using REFLEXIII, Bruker Daltonics with alpha-Cyano-4-hydroxycinnamic acid (CHCA) as a matrix.UV-Vis spectra of catalyst molecules dissolved in DMSO (WAKO/GR) have been measured by using UV-Vis-NIR spectrometer, V-670, Jasco, Tokyo, Japan.Each sample was dispersed in DMSO by ultrasonication for 10 min and the spectra of supernatants, whose wavelength ranging from 300 to 800 nm, were measured.
Synthesis of FeAzPc-8N-8Me: 19.8 g of iron(II) chloride tetrahydrate was added to 300 mL of 1pentanol with stirring.By heating, 100 mL of 1-pentanol was distilled off under normal pressure.The mixture was allowed to cool to 130 °C, and 60.1 g of 5,6-dimethyl-2,3-pyrazinedicarbonitrile was added.The temperature was set to 155 °C and the reaction was carried out for 15 hours.The reaction S5 mixture was cooled to 100 °C, 100 mL of 6N hydrochloric acid was added, and stirred for 2 hours at 100 °C.The mixture was cooled to 70 °C, and the precipitated solid was filtered off.After washing with methanol, the solid was washed with 2N hydrochloric acid, followed by methanol, and finally acetone.
The product was dried at 60 °C under reduced pressure to obtain the desired product.Yield: 49.2 g (75.2 %).Synthesis of 5-Bromo-2,3-pyridinedicarboxylic acid dimethyl ester: 30 g quinolinic acid (179.5 mmoL) was added to 250 mL methanol and stirred.10 mL of concentrated sulfuric acid was added dropwise, followed by the addition of 20 mL of methyl orthoformate and refluxed.After one day refluxing, 20 mL of methyl orthoformate was added, and the solution was refluxed overnight.The solution was cooled to 40 °C, and 16 mL of bromine was added dropwise, and after one day, another 8 mL of bromine was added, and the bath temperature was set at 58 °C.The reaction was carried out for another two days.After cooling, 300 mL of ethyl acetate and 150 mL of water were added, and the solution was neutralized with sodium hydrogen carbonate, and after separation, the organic phase was washed twice with saturated sodium hydrogen carbonate solution.The organic phase was dehydrated with anhydrous sodium sulfate, concentrated by evaporation, and the product was crystallized with cold isopropyl alcohol.The resulting crystals were filtered out, washed with cold isopropyl alcohol, and dried to obtain the product.Yield: 34.4 g (69.9 %) Synthesis of 5-(2-ethylhexylsulfanyl)-2,3-pyridinedicarboxylic acid dimethyl ester: 10.0 g 2ethylhexanethiol (68.4 mmoL), 18.7 g dimethyl 5-bromopyridine-2,3-dicarboxydimethyl (68.2 mmoL), and 14.7 g potassium carbonate were added to 80 mL DMF, the solution was heated at 80 ˚C under N2 atmosphere overnight.After reaction, the solvent was distilled off under reduced pressure.

Characterization
Water and ethyl acetate were added, and the organic phase was separated and washed twice with dilute hydrochloric acid, followed by washing with water, dehydration with anhydrous sodium sulfate, and concentrated under reduced pressure to obtain the product.Yield: 20.0 g (86.1%).

Synthesis of 5-(2-ethylhexylsulfonyl
)-2,3-pyridinedicarboxylic acid dimethyl ester: 120 mL acetic acid and 780 mg sodium tungstate, dihydrate were added to 15.5 g 5-(2-ethylhexylsulfonyl)-2,3pyridinedicarboxylic acid dimethyl ester (45.7 mmoL) and heated to 40 ˚C.A half of 11.4 mL of 30% hydrogen peroxide aqueous solution was added dropwise to the solution, and the mixture was stirred for 30 minutes.After the temperature was set to 75 °C, the remaining hydrogen peroxide aqueous solution was slowly added dropwise.The reaction was carried out at 75 °C for 2 hours and then, cooled to room temperature.Ethyl acetate and water were added, and organic phase was separated from the mixture.The organic phase was washed three times with sodium sulfite solution, followed by three washes with saturated sodium bicarbonate solution, and the organic phase was dehydrated with anhydrous sodium sulfate.The organic phase was concentrated, and the residue was purified by silica gel column chromatography to give the product as a colorless oil.Yield: 14.5 g (85.4 %).
Synthesis of 5-(2-ethylhexylsulfonyl)-2,3-pyridinedicarboxylic acid: 100 mL of methanol was added to 6.0 g (16.2 mmoL) of 5-(2-ethylhexylsulfonyl)-2,3-pyridinedicarboxylic acid and stirred at room temperature.3.0 g of lithium hydroxide monohydrate was added to this solution, and the reaction was carried out at room temperature for 1 hour.After confirming the end of the reaction by TLC, the solution was acidified with concentrated hydrochloric acid and the solvent was removed under reduced pressure.The residue was extracted with ethyl acetate, washed three times with water, and the organic phase was concentrated to give a colorless oil-like product, which solidified after standing overnight.
Synthesis of FeAzPc-4N-TS: 3.0 g of urea and 25.3 mg of ammonium molybdate, hexahydrate was added to 1.0 g of 5-(2-ethylhexylsulfonyl)-2,3-pyridinedicarboxylic acid, and the mixture was stirred at 160 °C for 30 minutes under nitrogen flow.400 mg of iron(III) chloride, hexahydrate, and 3.0 g of urea were added to this reaction mixture, and the reaction was carried out at 200 °C for 3 hours.After cooling to room temperature, water was added and the precipitated solid was filtered off, washed with water, washed with methanol, and dried to give the crude product.This was purified by silica gel column chromatography to obtain the desired product.Yield: 201 mg (21.6%).water and ethyl acetate were added for separation.The organic phase was washed with water three times and concentrated under reduced pressure until it dried, desired object was obtained.Yield: 4.2 g (91.6 %).

S10
Synthesis of FeAzPc-4N-TM: 6.3 g of urea and 53.1 mg of ammonium molybdate, tetrahydrate were added to 1.9 g of 5-(2-ethylhexylsulfanyl)-2,3-pyridinedicarboxylic acid and the mixture was stirred at 160 °C for 30 minutes under nitrogen flow.840 mg of iron(III) chloride, hexahydrate, and 6.3 g of urea were added to the reaction mixture and the reaction was carried out at 195 °C for 5 h.After cooling to 100 °C, 100 mL of water was added, and the mixture was stirred overnight.The precipitated solid was filtered out, washed with water, washed with methanol, and purified by silica gel column chromatography.Yield: 858 mg (48.9 %).4) FeAzPc-8N-OB (Iron,[1,2,3,4,10,11,12,13,19,20,21,22,28,29,30,4,10,13,19,22,28, Water was added and the precipitated solid was filtered off, the resulting solid was heated and dissolved in ethyl acetate, 300 mg of activated carbon was added and stirred for 10 minutes.The insoluble material was removed by celite filtration, the filtrate was concentrated, and the objective was obtained by crystallization from a mixture of ethyl acetate and isopropanol.Yield: 3.89 g (53.1 %).

Characterization
Synthesis of FeAzPc-8N-OB: 104 mg of iron(II) chloride, tetrahydrate was added 20 mL of 1-S12 pentanol and stirred.After 10 mL of 1-pentanol was distilled off at atmospheric pressure, the mixture was cooled to 130 °C.After 4 hours, the mixture was cooled to room temperature and the precipitated solid was filtered off.After 4 hours, the mixture was cooled to room temperature and the precipitated solid was filtered out and washed with methanol.The resulting solid was purified by silica gel column chromatography (chloroform/methanol elution) to afford the desired product.Yield: 81 mg (10.6%).
The dispersion was sonicated with a homogenizer for 5 min and then suction-filtered to collect the samples.The samples were washed three times each with methanol and chloroform and then dried in vacuo.