Abbasi, R. et al. A roadmap to low-cost hydrogen with hydroxide exchange membrane electrolyzers. Adv. Mater. 31, 1805876 (2019).

Article  Google Scholar 

Cullen, D. A. et al. New roads and challenges for fuel cells in heavy-duty transportation. Nat. Energy 6, 462–474 (2021).

Article  CAS  Google Scholar 

Weber, A. Z., Balasubramanian, S. & Das, P. K. in Advances in Chemical Engineering (ed. Sundmacher, K.) Vol. 41, 65–144 (Academic Press, 2012).

Wang, X. X., Swihart, M. T. & Wu, G. Achievements, challenges and perspectives on cathode catalysts in proton exchange membrane fuel cells for transportation. Nat. Catal. 2, 578–589 (2019).

Article  CAS  Google Scholar 

Thompson, S. T. et al. ElectroCat: DOE’s approach to PGM-free catalyst and electrode R&D. Solid State Ion. 319, 68–76 (2018).

Article  CAS  Google Scholar 

Kramm, U. I. et al. On an easy way to prepare metal–nitrogen doped carbon with exclusive presence of MeN4-type sites active for the ORR. J. Am. Chem. Soc. 138, 635–640 (2016).

Article  CAS  PubMed  Google Scholar 

Jaouen, F. & Dodelet, J.-P. O2 reduction mechanism on non-noble metal catalysts for PEM fuel cells. Part I: experimental rates of O2 electroreduction, H2O2 electroreduction, and H2O2 disproportionation. J. Phys. Chem. C 113, 15422–15432 (2009).

Article  CAS  Google Scholar 

Leonard, N. D. et al. Deconvolution of utilization, site density, and turnover frequency of Fe–nitrogen–carbon oxygen reduction reaction catalysts prepared with secondary N-precursors. ACS Catal. 8, 1640–1647 (2018).

Article  CAS  Google Scholar 

He, Y., Liu, S., Priest, C., Shi, Q. & Wu, G. Atomically dispersed metal-nitrogen-carbon catalysts for fuel cells: advances in catalyst design, electrode performance, and durability improvement. Chem. Soc. Rev. 49, 3484–3524 (2020).

Article  CAS  PubMed  Google Scholar 

He, Y. & Wu, G. PGM-free oxygen-reduction catalyst development for proton-exchange membrane fuel cells: challenges, solutions, and promises. Acc. Mater. Res. 3, 224–236 (2022).

Article  CAS  Google Scholar 

Gewirth, A. A., Varnell, J. A. & DiAscro, A. M. Nonprecious metal catalysts for oxygen reduction in heterogeneous aqueous systems. Chem. Rev. 118, 2313–2339 (2018).

Article  CAS  PubMed  Google Scholar 

Zhang, H. et al. High-performance fuel cell cathodes exclusively containing atomically dispersed iron active sites. Energy Environ. Sci. 12, 2548–2558 (2019).

Article  CAS  Google Scholar 

Wang, X. X. et al. Nitrogen-coordinated single cobalt atom catalysts for oxygen reduction in proton exchange membrane fuel cells. Adv. Mater. 30, 1706758 (2018).

Article  Google Scholar 

Li, J. et al. Atomically dispersed manganese catalysts for oxygen reduction in proton-exchange membrane fuel cells. Nat. Catal. 1, 935–945 (2018).

Article  CAS  Google Scholar 

Zhang, H. et al. Single atomic iron catalysts for oxygen reduction in acidic media: particle size control and thermal activation. J. Am. Chem. Soc. 139, 14143–14149 (2017).

Article  CAS  PubMed  Google Scholar 

Wu, G. et al. Carbon nanocomposite catalysts for oxygen reduction and evolution reactions: from nitrogen doping to transition-metal addition. Nano Energy 29, 83–110 (2016).

Article  CAS  Google Scholar 

Chen, M., He, Y., Spendelow, J. S. & Wu, G. Atomically dispersed metal catalysts for oxygen reduction. ACS Energy Lett. 4, 1619–1633 (2019).

Article  CAS  Google Scholar 

Chen, G. et al. Highly accessible and dense surface single metal FeN4 active sites for promoting the oxygen reduction reaction. Energy Environ. Sci. 15, 2619–2628 (2022).

Article  CAS  Google Scholar 

Uddin, A. et al. High power density platinum group metal-free cathodes for polymer electrolyte fuel cells. ACS Appl. Mater. Interfaces 12, 2216–2224 (2020).

Article  CAS  PubMed  Google Scholar 

Zhang, S., Qin, Y., Ding, S. & Su, Y. A DFT study on the activity origin of Fe−N−C sites for oxygen reduction reaction. ChemPhysChem 23, e202200165 (2022).

Article  CAS  PubMed  Google Scholar 

Kattel, S. & Wang, G. A density functional theory study of oxygen reduction reaction on Me–N4 (Me = Fe, Co, or Ni) clusters between graphitic pores. J. Mater. Chem. A 1, 10790–10797 (2013).

Article  CAS  Google Scholar 

Liu, K., Wu, G. & Wang, G. Role of local carbon structure surrounding FeN4 sites in boosting the catalytic activity for oxygen reduction. J. Phys. Chem. C 121, 11319–11324 (2017).

Article  CAS  Google Scholar 

Zhao, X., Levell, Z. H., Yu, S. & Liu, Y. Atomistic understanding of two-dimensional electrocatalysts from first principles. Chem. Rev. 122, 10675–10709 (2022).

Article  CAS  PubMed  Google Scholar 

Holby, E. F., Wang, G. & Zelenay, P. Acid stability and demetalation of PGM-free ORR electrocatalyst structures from density functional theory: a model for ‘single-atom catalyst’ dissolution. ACS Catal. 10, 14527–14539 (2020).

Article  CAS  Google Scholar 

Li, J. et al. Identification of durable and non-durable FeNx sites in Fe–N–C materials for proton exchange membrane fuel cells. Nat. Catal. 4, 10–19 (2021).

Article  Google Scholar 

Liu, S. et al. Atomically dispersed iron sites with a nitrogen–carbon coating as highly active and durable oxygen reduction catalysts for fuel cells. Nat. Energy 7, 652–663 (2022).

Article  CAS  Google Scholar 

Liu, S., Shi, Q. & Wu, G. Solving the activity–stability trade-off riddle. Nat. Catal. 4, 6–7 (2021).

Article  Google Scholar 

Zhu, Y. et al. Engineering local coordination environments of atomically dispersed and heteroatom-coordinated single metal site electrocatalysts for clean energy-conversion. Adv. Energy Mater. 10, 1902844 (2020).

Article  CAS  Google Scholar 

Wang, Y. et al. Advanced electrocatalysts with single-metal-atom active sites. Chem. Rev. 120, 12217–12314 (2020).

Article  CAS  PubMed  Google Scholar 

Li, B. et al. Unraveling the mechanism of ligands regulating electronic structure of MN4 sites with optimized ORR catalytic performance. Appl. Surf. Sci. 595, 153526 (2022).

Article  CAS  Google Scholar 

Zhang, X. et al. Towards understanding ORR activity and electron-transfer pathway of M-Nx/C electro-catalyst in acidic media. J. Catal. 356, 229–236 (2017).

Article  CAS  Google Scholar 

Martinez, U., Komini Babu, S., Holby, E. F. & Zelenay, P. Durability challenges and perspective in the development of PGM-free electrocatalysts for the oxygen reduction reaction. Curr. Opin. Electrochem. 9, 224–232 (2018).

Article  CAS  Google Scholar 

Wu, G., More, K. L., Johnston, C. M. & Zelenay, P. High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt. Science 332, 443–447 (2011).

Article  CAS  PubMed  Google Scholar 

Tylus, U. et al. Elucidating oxygen reduction active sites in pyrolyzed metal–nitrogen coordinated non-precious-metal electrocatalyst systems. J. Phys. Chem. C 118, 8999–9008 (2014).

Article  CAS  Google Scholar 

Artyushkova, K., Serov, A., Rojas-Carbonell, S. & Atanassov, P. Chemistry of multitudinous active sites for oxygen reduction reaction in transition metal–nitrogen–carbon electrocatalysts. J. Phys. Chem. C 119, 25917–25928 (2015).

Article  CAS  Google Scholar 

Workman, M. J., Serov, A., Tsui, L.-K., Atanassov, P. & Artyushkova, K. Fe–N–C catalyst graphitic layer structure and fuel cell performance. ACS Energy Lett. 2, 1489–1493 (2017).

Article  CAS  Google Scholar 

Matter, P. H., Zhang, L. & Ozkan, U. S. The role of nanostructure in nitrogen-containing carbon catalysts for the oxygen reduction reaction. J. Catal. 239, 83–96 (2006).

Article