Table 1

Comparison of different types of fuel cell and their fundamental information."

Fig 1

Scheme for the operation process of PEMFCs."

Table 2

Recent progress on PGM-free PEMFCs."

Fig 2

Scheme for the cathode catalyst layer. Reproduced with permission 48, Copyright 2003, Elsevier B.V."

Fig 3

The performance comparison of state-of-the-art PGM-free PEMFCs and DOE 2025 targets. *: The Ohmic resistance is corrected. Reproduced with permission 53. Copyright 2019, Springer Nature."

Fig 4

Scheme for the M-N-C based FEMFC designing targets for large-scale applications."

Fig 5

(a) Scheme for pore-edge Fe-N4 sites model; (b) PEMFC performance in H2-air condition The peak power density was 0.43 W·cm-2; (c, d) correlation between the mass activity and Fe (D1) or Fe (D3) contents without (c) or with (d) correction of surface area. (a, b) Reproduced with permission 28. Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (c, d) Reproduced with permission 62. Copyright 2014, American Chemical Society."

Fig 6

(a) Schematic model of the presumed active site between two carbon crystallites after pyrosis; (b) performance comparison of the PGM-free catalyst with PGM catalyst in H2-O2 PEMFC test. Reproduced with permission 64. Copyright 2009, American Association for the Advancement of Science."

Fig 7

(a) Synthetic process of TPI@Z8(SiO2)-650-C; (b) schematic illustration of active sites on TPB in TPI@Z8(SiO2)-650-C; (c) comparison of pore size distributions in TPI@Z8(SiO2)-650-C and TPI@Z8-650-C; (d) polarization curves of H2-O2 fuel cells with TPI@Z8(SiO2)-650-C in different loadings, showing the current density at 0.9 V and the volumetric activity at 0.8 V (the Ohmic has been corrected). Reproduced with permission 39. Copyright 2018, Springer Nature."

Fig 8

(a, b) SEM images and (c) pore size distribution and pore volume of PANI-Fe-C and (CM+PANI)-Fe-C; (d) polarization and power density curves of MEAs prepared by Pt/C and (CM + PANI)-Fe-C with different Nafion contents. Reproduced with permission 29. Copyright 2017, American Association for the Advancement of Science."

Fig 9

(a, b) SEM images of C-ZIF-8-950 (a) and C-FeZIF-1.44-950 (b); (c) LSV curves at 0.1 mol·L-1 HClO4; (d) polarization and power density curves of MEAs prepared with C-FeZIF-1.44-950 and Pt/C in H2-air PEMFC tests. Reproduced with permission 38. Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim."

Fig 10

(a) Schematic illustration of macropores for mass transfer and micropores for hosting active sites; (b) SEM images; (c) power density curves, measured in PEMFC test at 200 kPa H2-O2 condition. Reproduced with permission 40. Copyright 2015, National Academy of Sciences."

Fig 11

(a) SEM images of MEA with GDL and catalyst layers; (b) iR-free polarization curves measured in H2-air fuel cells for the MEAs with various amounts of ionomers; (c) plots of CCL ionic conductance (black line) and cell performance (red line) as a function of relative humidity; (d) power density curves of fuel cells with various amounts of ionomers in H2-air. (a, b) Reproduced with permission 78. Copyright 2017, IOP Publishing Ltd. (c) Reproduced with permission 80. Copyright 2017, Elsevier. (d) Reproduced with permission 81. Copyright 2018, American Association for the Advancement of Science."

Fig 12

(a) Nano-CT images of ionomer distribution in CCLs with various catalyst particle sizes; (b) schematic illustration of catalyst aggregates; (c) EIS plots for CCL with various catalyst particle sizes; (d, e) polarization curves of MEAs prepared with catalysts in various particle sizes; (f) polarization curve of MEA fabricated with the catalyst in 100 nm, measured in the H2-O2 fuel cell. Reproduced with permission 83. Copyright 2020, American Chemical Society."

Fig 13

(a) Morphologies of three kinds of CCLs; (b) current density curves with water saturation for CCM, GDE-CCM, GDE, GDE(E). Reproduced with permission 84. Copyright 2019, American Chemical Society."

Fig 14

(a, b, c) Schematic illustration of various catalysts; (d) polarization curves at H2-O2 condition. Solid line: initial curves; dashed line: after first 50 h test; (e) current density curves in the stability tests at H2-O2 condition. Reproduced with permission 92. Copyright 2016, American Chemical Society."

Fig 15

(a) Schematic illustration of the protonation process; (b) current density changes in stability test at H2-air condition; (c) comparison of N 1s XPS spectra of NMCC-800 and NMCC-1000; (d) schematic illustration of anion-binding effects. (a, b, c) Reproduced with permission 93. Copyright 2009, IOP Publishing Ltd. (d) Reproduced with permission 94. Copyright 2011, American Chemical Society."

Fig 16

(a) Schematic illustration of the carbon corrosion; (b) polarization curve in H2-air fuel cell with the catalysts on different carbon supports; (c) current density curves of various catalysts in 500 h stability test in H2-air fuel cell; (d) schematic illustration of the growth process of Fe-N-CNT-2; (e) polarization and power density curve, and (f) current density curves in the stability test for Fe-N-CNT-2 and Fe-ZIF in H2-O2 fuel cell test. (a) Reproduced with permission 96, Copyright 2014, Royal Society of Chemistry; (b, c) Reproduced with permission 102, Copyright 2013, Royal Society of Chemistry; (d, e, f) Reproduced with permission 103, Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim."

Fig 17

Chronoamperometry curve showing the degradation behaviors of PGM-free PEMFC. Reproduced with permission 106. Copyright 2016, Elsevier BV."

Fig 18

(a) Schematic comparison for hydrophilic and hydrophobic catalysts against water flooding and carbon corrosion; (b, c) stability comparison of Fe/N/C (black) and Fe/N/C-F (purple) in the H2-O2 fuel cell tests at 0.5 V (b) and 0.6 V (c). Reproduced with permission 98, Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim."