Axial Sulfur-Coordination Engineering Boosting Fe‒N‒C Catalysts for High-Performance Proton Exchange Membrane Fuel Cells

Abstract

Fe-N-C catalysts have long suffered from kinetically sluggish oxygen reduction reaction (ORR) due to excessive adsorption strength toward oxygen intermediates and low site utilization. Heteroatom doping effectively accelerates ORR reaction kinetics through electronic structure modulation of metal sites for optimal intermediate adsorption, while chemical vapor deposition (CVD) enhances the turnover frequency (TOF) of active sites. Herein, we developed an FeSNC catalyst featuring abundant FeS₁N₄ sites via a dual-precursor CVD strategy. Experimental and theoretical analyses revealed that S incorporation disrupts the symmetric coordination of active sites, which optimizes OH* adsorption energies from 0.212 eV to 1.194 eV. Moreover, the TOF increased from 1.98 e^-1·site^-1·s^-1 to 6.32 e^-1·site^-1·s^-1, significantly enhancing the intrinsic activity of the catalyst. More notably, the hydrophilic character of S-containing species substantially improved hydrophilicity in the S-doped catalyst, thereby promoting mass transport of oxygen and proton delivery. As a result, the FeSNC catalyst exhibited an extremely high half-wave potential of 0.863 V in 0.1 mol·L^-1 HClO₄ and achieved a peak power density of 1.2 W·cm^-2 in H₂-O₂ PEMFCs. This work highlights the critical role of coordination engineering.

Keywords

Oxygen reduction reaction / Fe-N-C / Heteroatom doping / Electronic regulation / Mass transport

4. Conclusions

In summary, we developed a S-doped Fe-N-C (FeSNC) cathode catalyst for PEMFCs via a dual-source chemical vapor deposition strategy. The deliberate integration of S species serves to simultaneously engineer the electronic configuration of Fe active centers and optimize the interfacial wetting properties of the carbonaceous matrix. DFT calculations reveal that axial S coordination reduces the charge density at Fe sites, weakening OH* adsorption and accelerating its desorption. Moreover, S-containing moieties optimize water management in the cathode catalyst layer, facilitating triple-phase boundary formation and improving oxygen transport. As a result, the FeSNC demonstrated exceptional intrinsic ORR activity, achieving 55 mA·cm^-2 at 0.90 V_iR-free (H₂-O₂) and a peak power density of 1.2 W·cm^-2 in a MEA. This work highlights the dual functionality of heteroatom doping, precisely tuning electronic properties and interfacial wetting behavior, providing a novel design strategy for advancing PEMFC performance. Future studies will explore the scalability of this synthesis approach and its applicability to other heteroatom-coordinated M-N-C systems for sustainable energy technologies.

Acknowledgements

The work was supported by the National Key Research and Development program of China (2022YFB4004100), the National Natural Science Foundation of China (U22A20396), the University of Science and Technology of China, Academic Leading Talent Cultivation Program (KY2060000272) and the China Postdoctoral Science Foundation (2024M753087,GZC20241623). Thank the Shanghai Synchrotron Radiation Facility for conducting the X-ray absorption spectroscopic experiments at BL14W1 station.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

Data will be made available on request.

Author contributions

Lin Lin: writing, experiment; Xiu-Xuan Hou: DFT; Zhe-Chen Fan, Yi-Xuan Yin, Wei-Yi Zhao: Data curation, Investigation; Kai Wei: Formal analysis, Software; Yu-Die Zhou, Lina Hou: experiment; Yin Wang: Supervision; Hao Wan: Conceptualization, Supervision, Writing - review & editing; Jun-Jie Ge: Funding acquisition, Project administration, Supervision, Writing - review & editing.

References

[1] Jiao K, Xuan J, Du Q, Bao Z M, Xie B, Wang B W, Zhao Y, Fan L H, Wang H Z, Hou Z J, Huo S, Brandon N P, Yin Y, Guiver M D. Designing the next generation of proton-exchange membrane fuel cells[J]. Nature, 2021. 595(7867): 361-369. http://dx.doi.org/10.1038/s41586-021-03482-7.

[2] Scofield M E, Liu H, Wong S S. A concise guide to sustainable PEMFCs: recent advances in improving both oxygen reduction catalysts and proton exchange membranes[J]. Chem. Soc. Rev., 2015, 44(16): 5836-5860. http://dx.doi.org/10.1039/C5CS00302D.

[3] Sun K, Dong J C, Sun H, Wang X D, Fang J J, Zhuang Z B, Tian S B, Sun X M. Co(CN)₃ catalysts with well-defined coordination structure for the oxygen reduction reaction[J]. Nat. Catal., 2023, 6(12): 1164-1173. http://dx.doi.org/10.1038/s41929-023-01047-7.

[4] Shao M H, Chang Q W, Dodelet J P, Chenitz R. Recent advances in electrocatalysts for oxygen reduction reaction[J]. Chem. Rev., 2016, 116(6): 3594-3657. http://dx.doi.org/10.1021/acs.chemrev.5b00462.

[5] Zhao Y S, Wan J W, Yao H Y, Zhang L J, Lin K F, Wang L, Yang N L, Liu D B, Song L, Zhu J, Gu L, Liu L, Zhao H J, Li Y L, Wang D. Few-layer graphdiyne doped with sp-hybridized nitrogen atoms at acetylenic sites for oxygen reduction electrocatalysis[J]. Nat. Chem., 2018, 10(9): 924-931. http://dx.doi.org/10.1038/s41557-018-0100-1.

[6] Snitkoff-Sol R Z, Rimon O, Bond A M, Elbaz L. Direct measurement of the oxygen reduction reaction kinetics on iron phthalocyanine using advanced transient voltammetry[J]. Nat. Catal., 2024, 7: 139-147. http://dx.doi.org/10.1038/s41929-023-01086-0.

[7] Sa Y J, Seo D J, Woo J, Lim J T, Cheon J Y, Yang S Y, Lee J M, Kang D, Shin T J, Shin H S, Jeong H Y, Kim C S, Kim M G, Kim T Y, Joo S H. A general approach to preferential formation of active Fe-N_x sites in Fe-N/C electrocatalysts for efficient oxygen reduction reaction[J]. J. Am. Chem. Soc., 2016, 138(45): 15046-15056. http://dx.doi.org/10.1021/jacs.6b09470.

[8] Zeng Y Z, Li C Z, Li B Y, Liang J S, Zachman M J, Cullen D A, Hermann R P, Alp E E, Lavina B, Karakalos S, Lucero M, Zhang B, Wang M, Feng Z X, Wang G F, Xie J, Myers D J, Dodelet J P, Wu G. Tuning the thermal activation atmosphere breaks the activity-stability trade-off of Fe-N-C oxygen reduction fuel cell catalysts[J]. Nat. Catal., 2023, 6(12): 1215-1227. http://dx.doi.org/10.1038/s41929-023-01062-8.

[9] Gewirth A A, Varnell J A, DiAscro A M. Nonprecious metal catalysts for oxygen reduction in heterogeneous aqueous systems[J]. Chem. Rev., 2018, 118(5): 2313-2339. http://dx.doi.org/10.1021/acs.chemrev.7b00335.

[10] Huang Z, Li F H, Liu Y D, Chen S G, Wei Z D, Tang Q. The role of nitrogen sources and hydrogen adsorption on the dynamic stability of Fe-N-C catalysts in oxygen reduction reaction[J]. Chem. Sci., 2024, 15(3): 1132-1142. http://dx.doi.org/https://doi.org/10.1039/d3sc05378d.

[11] Xue D P, Yuan P F, Jiang S, Wei Y F, Zhou Y, Dong C L, Yan W F, Mu S, Zhang J N. Altering the spin state of Fe-N-C through ligand field modulation of single-atom sites boosts the oxygen reduction reaction[J]. Nano Energy, 2023, 105: 108020. http://dx.doi.org/https://doi.org/10.1016/j.nanoen.2022.108020.

[12] Sui R, Liu B, Chen C, Tan X, He C, Xin D Y, Chen B, Xu Z Y, Li J Z, Chen W X, Zhuang Z B, Wang Z B, Chen C. Constructing asymmetric Fe-Nb diatomic sites to enhance ORR activity and durability[J]. J. Am. Chem. Soc., 2024, 146(38): 26442-26453. http://dx.doi.org/10.1021/jacs.4c09642.

[13] Xie H, Xie X H, Hu G X, Prabhakaran V, Saha S, Gonzalez-Lopez L, Phakatkar A H, Hong M, Wu M L, Shahbazian-Yassar R, Ramani V, Al-Sheikhly M I, Jiang D E, Shao Y Y, Hu L B. Ta-TiO_x nanoparticles as radical scavengers to improve the durability of Fe-N-C oxygen reduction catalysts[J]. Nat. Energy, 2022, 7(3): 281-289. http://dx.doi.org/10.1038/s41560-022-00988-w.

[14] Li J Z, Zhang H G, Samarakoon W, Shan W T, Cullen D A, Karakalos S, Chen M L, Gu D M, More K L, Wang G, Feng Z, Wang Z, Wu G. Thermally driven structure and performance evolution of atomically dispersed FeN₄ sites for oxygen reduction[J]. Angew. Chem. Int. Ed., 2019, 58(52): 18971-18980. http://dx.doi.org/https://doi.org/10.1002/anie.201909312.

[15] Liu H, Jiang L Z, Khan J, Wang X X, Xiao J M, Zhang H D, Xie H J, Li L, Wang S Y, Han L. Decorating single-atomic Mn sites with FeMn clusters to boost oxygen reduction reaction[J]. Angew. Chem. Int. Ed., 2023, 62(3): e202214988. http://dx.doi.org/https://doi.org/10.1002/anie.202214988.

[16] Yang N, Li L, Li J, Ding W, Wei Z D. Modulating the oxygen reduction activity of heteroatom-doped carbon catalysts via the triple effect: charge, spin density and ligand effect[J]. Chem. Sci., 2018, 9(26): 5795-5804. http://dx.doi.org/10.1039/C8SC01801D.

[17] Wu S Q, Wei W, Li A Y. Theoretical studies on the bonding and separation performance of lanthanides and actinides with phenanthroline derived N,O/S-coordinated asymmetric hydrophilic ligands[J]. Comput. Theor. Chem., 2025, 1251: 115321. http://dx.doi.org/https://doi.org/10.1016/j.comptc.2025.115321.

[18] Fan H Y, Liu Y R, Li J X, Lv Z H, Wang C L, Liu R, Dong F, Tian C G, FengX, Yang W X, Wang B. Optimizing the mass transport and atomic fe intrinsic activity to achieve high-performing fuel cells[J]. J. Am. Chem. Soc., 2025, 147(22): 18953-18962. http://dx.doi.org/10.1021/jacs.5c03499.

[19] Ni W P, Gao Y, Zhang Y, Younus H A, Guo X, Ma C, Zhang Y, Duan J F, Zhang J, Zhang S G. O-doping boosts the electrochemical oxygen reduction activity of a single Fe site in hydrophilic carbon with deep mesopores[J]. ACS Appl. Mater. Interfaces, 2019, 11(49): 45825-45831. http://dx.doi.org/10.1021/acsami.9b18510.

[20] Mehmood A, Gong M, Jaouen F, Roy A, Zitolo A, Khan A, Sougrati M T, Primbs M, Bonastre A M, Fongalland D, Drazic G, Strasser P, Kucernak A. High loading of single atomic iron sites in Fe-NC oxygen reduction catalysts for proton exchange membrane fuel cells[J]. Nat. Catal., 2022, 5: 311-323. http://dx.doi.org/10.1038/s41929-022-00772-9.

[21] Kresse G, Furthmüller J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set[J]. Comput. Mater. Sci., 1996, 6(1): 15-50. http://dx.doi.org/https://doi.org/10.1016/0927-0256(96)00008-0.

[22] Kresse G, Hafner J. Ab initio molecular dynamics for liquid metals[J]. Phys. Rev. B, 1993, 47: 558-561. http://dx.doi.org/10.1103/PhysRevB.47.558.

[23] Blöchl P E. Projector augmented-wave method[J]. Phys. Rev. B, 1994, 50: 17953-17979. http://dx.doi.org/10.1103/PhysRevB.50.17953.

[24] Perdew J P, Burke K, Ernzerhof M. Generalized gradient approximation made simple[J]. Phys. Rev. Lett., 1996, 77: 3865-3868. http://dx.doi.org/10.1103/PhysRevLett.77.3865.

[25] Dudarev S L, Botton G A, Savrasov S Y, Humphreys C J, Sutton A P. Electron-energy-loss spectra and the structural stability of nickel oxide: An LSDA+U study[J]. Phys. Rev. B, 1998, 57: 1505-1509. http://dx.doi.org/10.1103/PhysRevB.57.1505.

[26] Jiao Y C, Han D D, Liu L, Ji L, Guo G N, Hu J H, Yang D, Dong A G. Highly ordered mesoporous few-layer graphene frameworks enabled by Fe₃O₄ nanocrystal superlattices[J]. Angew. Chem. Int. Ed., 2015, 54(19): 5727-5731. http://dx.doi.org/https://doi.org/10.1002/anie.201501398.

[27] Shen H J, Gracia-Espino E, Ma J Y, Tang H D, Mamat X, Wagberg T, Hu G Z, Guo S J. Atomically FeN₂ moieties dispersed on mesoporous carbon: A new atomic catalyst for efficient oxygen reduction catalysis[J]. Nano Energy, 2017, 35: 9-16. http://dx.doi.org/https://doi.org/10.1016/j.nanoen.2017.03.027.

[28] Wan-Me Z X, Zhang H T, Zhao Y, Wen H Z, Wan X Y, Xie Y L. Synthesis of S-doped mesoporous carbon and its use in advanced supercapacitors and sodium-ion batteries[J]. Carbon Trends, 2025, 21: 100558. http://dx.doi.org/https://doi.org/10.1016/j.cartre.2025.100558.

[29] Shen H J, Gracia-Espino E, Ma J, Zang K T, Luo J, Wang L, Gao S S, Mamat X, Hu G, Wagberg T, Guo S J. Synergistic effects between atomically dispersed Fe-N-C and C-S-C for the oxygen reduction reaction in acidic media[J]. Angew. Chem. Int. Ed., 2017, 56(44): 13800-13804. http://dx.doi.org/https://doi.org/10.1002/anie.201706602.

[30] Liu S W, Li C Z, Zachman M J, Zeng Y Z, Yu H R, Li B Y, Wang M Y, Braaten J, Liu J W, Meyer H M, Lucero M, Kropf A J, Alp E E, Gong Q, Shi Q, Feng Z X, Xu H, Wang G, Myers D J, Xie J, Cullen D A, Litster S, Wu G. Atomically dispersed iron sites with a nitrogen-carbon coating as highly active and durable oxygen reduction catalysts for fuel cells[J]. Nat. Energy, 2022, 7: 652-663. http://dx.doi.org/10.1038/s41560-022-01062-1.

[31] Jia Q Y, Ramaswamy N, Tylus U, Strickland K, Li J, Serov A, Artyushkova K, Atanassov P, Anibal J, Gumeci C, Barton S C, Sougrati M T, Jaouen F, Halevi B, Mukerjee S. Spectroscopic insights into the nature of active sites in iron-nitrogen-carbon electrocatalysts for oxygen reduction in acid[J]. Nano Energy, 2016, 29: 65-82. http://dx.doi.org/https://doi.org/10.1016/j.nanoen.2016.03.025.

[32] Jin Z Y, Jiao D X, Dong Y L, Liu L, Fan J, Gong M, Ma X C, Wang Y, Zhang W, Zhang L, Yu G Z, Voiry D, Zheng W T, Cui X Q. Boosting electrocatalytic carbon dioxide reduction via self-relaxation of asymmetric coordination in fe-based single atom catalyst[J]. Angew. Chem. Int. Ed., 2024, 136(6): e202318246. http://dx.doi.org/https://doi.org/10.1002/anie.202318246.

[33] Zhao Y L, Chen H C, Ma X, Li J Y, Yuan Q, Zhang P, Wang M M, Li J, Li M, Wang S F, Guo H, Hu R, Tu K H, Zhu W, Li X N, Yang X, Pan Y. Vacancy defects inductive effect of asymmetrically coordinated single-atom Fe-N₃S₁ active sites for robust electrocatalytic oxygen reduction with high turnover frequency and mass activity[J]. Adv. Mater., 2024, 36(11): 2308243. http://dx.doi.org/https://doi.org/10.1002/adma.202308243.

[34] Zhao L L, Cao Q, Wang A L, Duan J, Zhou W J, Sang Y H, Liu H. Iron oxide embedded titania nanowires - An active and stable electrocatalyst for oxygen evolution in acidic media[J]. Nano Energy, 2018, 45: 118-126. http://dx.doi.org/https://doi.org/10.1016/j.nanoen.2017.12.029.

[35] Liang Y Y, Li Y G, Wang H L, Zhou J, Wang J, Regier T, Dai H J. Co₃O₄ nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction[J]. Nat. Mater., 2011, 10(10): 780-786. http://dx.doi.org/10.1038/nmat3087.

[36] Wang W Y, Ren H, Guo J H, Wang S, Wang J, Wang Y Y, Sun W, Tang L, Wang X Q. Synergistically enhanced atomically dispersed Fe-N_x sites on carbon nanotubes by heteroatom S to boost electrochemical oxygen reduction for microbial fuel cell[J]. J. Power Sources, 2025, 629: 235960. http://dx.doi.org/https://doi.org/10.1016/j.jpowsour.2024.235960.

[37] Han H, Wang X M, Zhang X. A hierarchically ordered porous Fe, N, S tri-doped carbon electrocatalyst with densely accessible Fe-N_x active sites and uniform sulfur-doping for efficient oxygen reduction reaction[J]. J. Colloid Interface Sci., 2022, 615: 617-626. http://dx.doi.org/https://doi.org/10.1016/j.jcis.2022.02.030.

[38] Wang X, Chen W X, Zhang L, Yao T, Liu W, Lin Y, Ju H, Dong J C, Zheng L R, Yan W, Zheng X S, Li Z, Wang X Q, Yang J, He D S, Wang Y, Deng Z X, Wu Y, Li Y D. Uncoordinated amine groups of metal-organic frameworks to anchor single Ru sites as chemoselective catalysts toward the hydrogenation of quinoline[J]. J. Am. Chem. Soc., 2017, 139(28): 9419-9422. http://dx.doi.org/10.1021/jacs.7b01686.

[39] Malko D, Kucernak A, Lopes T. In situ electrochemical quantification of active sites in Fe-N/C non-precious metal catalysts[J]. Nat. Commun., 2016, 7: 13285. http://dx.doi.org/10.1038/ncomms13285.

[40] Liu Y R, Li J X, Lv Z H, Fan H, Dong F L, Wang C L, Chen X C, Liu R, Tian C A, Feng X, Yang W X, Wang B. Efficient proton-exchange membrane fuel cell performance of atomic Fe sites via p-d hybridization with Al dopants[J]. J. Am. Chem. Soc., 2024, 146(18): 12636-12644. http://dx.doi.org/10.1021/jacs.4c01598.

[41] Nonoyama N, Okazaki S, Weber A Z, Ikogi Y, Yoshida T. Analysis of oxygen-transport diffusion resistance in proton-exchange-membrane fuel cells[J]. J. Electrochem. Sci., 2011, 158(4): B416-B423. http://dx.doi.org/10.1149/1.3546038.

[42] Yin S H, Chen L, Yang J, Cheng X Y, Zeng H B, Hong Y H, Huang H, Kuai X X, Lin Y G, Huang R, Jiang Y X, Sun S G. A Fe-NC electrocatalyst boosted by trace bromide ions with high performance in proton exchange membrane fuel cells[J]. Nat. Commun., 2024, 15: 7489. http://dx.doi.org/10.1038/s41467-024-51858-w.

[43] Yang B L, Han Q, Han L K, Leng Y, O'Carroll T, Yang X X, Wu G, Xiang Z H. Porous covalent organic polymer coordinated single Co site nanofibers for efficient oxygen-reduction cathodes in polymer electrolyte fuel cells[J]. Adv. Mater., 2023, 35(1): 2208661. http://dx.doi.org/https://doi.org/10.1002/adma.202208661.