Boost the Utilization of Dense FeN4 Sites for High‐Performance Proton Exchange Membrane Fuel Cells

Iron‐nitrogen‐carbon (Fe‐N‐C) catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) have seriously been hindered by their poor ORR performance of Fe‐N‐C due to the low active site density (SD) and site utilization. Herein, we reported a melamine‐assisted vapor deposition approach to overcome these hindrances. The melamine not only compensates for the loss of nitrogen caused by high‐temperature pyrolysis but also effectively etches the carbon substrate, increasing the external surface area and mesoporous porosity of the carbon substrate. These can provide more useful area for subsequent vapor deposition on active sites. The prepared 0.20Mela‐FeNC catalyst shows a fourfold higher SD value and site utilization than the FeNC without the treatment of melamine. As a result, 0.20Mela‐FeNC catalyst exhibits a high ORR activity with a half‐wave potential (E1/2) of 0.861 V and 12‐fold higher ORR mass activity than the FeNC in acidic media. As the cathode in a H2‐O2 PEMFCs, 0.20Mela‐FeNC catalyst demonstrates a high peak power density of 1.30 W cm−2, outstripping most of the reported Fe‐N‐C catalysts. The developed melamine‐assisted vapor deposition approach for boosting the SD and utilization of Fe‐N‐C catalysts offers a new insight into high‐performance ORR electrocatalysts.


Introduction
Proton exchange membrane fuel cells (PEMFCs) have been considered as the promising energy conversion technology for a variety of applications, particularly for the electrification of the transportation vehicles and devices. [1]The commercial application of PEMFCs relies heavily on efficient and low-cost catalysts to drive the cathodic oxygen reduction reaction (ORR). [2]So far, platinum (Pt)based catalysts are still the benchmark electrocatalysts for ORR. [3]ron-nitrogen-carbon (Fe-N-C) catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) have seriously been hindered by their poor ORR performance of Fe-N-C due to the low active site density (SD) and site utilization.Herein, we reported a melamineassisted vapor deposition approach to overcome these hindrances.The melamine not only compensates for the loss of nitrogen caused by hightemperature pyrolysis but also effectively etches the carbon substrate, increasing the external surface area and mesoporous porosity of the carbon substrate.These can provide more useful area for subsequent vapor deposition on active sites.The prepared 0.20Mela-FeNC catalyst shows a fourfold higher SD value and site utilization than the FeNC without the treatment of melamine.As a result, 0.20Mela-FeNC catalyst exhibits a high ORR activity with a half-wave potential (E 1/2 ) of 0.861 V and 12-fold higher ORR mass activity than the FeNC in acidic media.As the cathode in a H 2 -O 2 PEMFCs, 0.20Mela-FeNC catalyst demonstrates a high peak power density of 1.30 W cm À2 , outstripping most of the reported Fe-N-C catalysts.The developed melamine-assisted vapor deposition approach for boosting the SD and utilization of Fe-N-C catalysts offers a new insight into high-performance ORR electrocatalysts.
vapor deposition method has been found to increase the SD and site utilization of Fe-N-C catalysts by twofold compared with the traditional synthesis method. [38,39]However, the CVD method is uncontrollable.It cannot increase the SD by increasing the metal content.Therefore, we consider that how to further increase the accessible SD of Fe-N-C catalyst by CVD.
Herein, we report a melamine-assisted vapor deposition approach to increase the external surface area and mesoporous porosity of the carbon substrate and subsequent vapor-deposit more O 2 -accessible active sites.Melamine was selected as a self-sacrificing template and nitrogen source.Co-pyrolysis with ZIF8 can not only increase the external surface area and mesoporous porosity of NC carrier but also can effectively supplement the nitrogen loss caused by high-temperature pyrolysis.This provides more useful areas for subsequent vapor deposition.The prepared 0.20Mela-FeNC catalyst showed an SD of 8.4 9 10 19 sites g À1 and site utilization of 43.37%, which was four times higher SD value and site utilization than that of the FeNC without the treatment of melamine.Benefiting from the high density and high accessibility of Fe-N 4 sites, 0.20Mela-FeNC catalyst exhibits a high E 1/2 of 0.861 V (vs reversible hydrogen electrode (RHE)) in acidic media, surpassing the majority of reported PGM-free electrocatalysts.Furthermore, 0.20Mela-FeNC catalyst demonstrates a large peak power density of 1.30 W cm À2 in a H 2 -O 2 PEMFCs, outstripping most of the reported Fe-N-C catalysts.

The Melamine-Assisted Vapor Deposition Approach for the Synthesis of 0.2Mela-FeNC
As shown in Figure 1a, the 0.20Mela-FeNC catalyst was synthesized using a melamine-assisted vapor deposition approach.Firstly, a ZIF-8 nanoparticle with a uniform size of ~200 nm was prepared.The ZIF-8 was then mixed with a certain amount of melamine and was evaporated to obtain the mixture (Figure S1, Supporting Information, marked as ZIF8@xMela).The addition of melamine did not change the morphology and crystal structure of ZIF-8, and ZIF-8 and melamine were uniformly mixed (Figures S2-S4, Supporting Information).Secondly, the dry powders (ZIF-8 or ZIF-8@xMela) were pyrolyzed for 2 h at 900 °C under an Ar atmosphere to acquire the corresponding NC support.Finally, 30 mg of anhydrous ferrous chloride (FeCl 2 ) was placed at the upper tuyere position, and 40 mg NC carrier was located at the lower tuyere to vapordeposit dense Fe-N x sites (Figures S5 and S6, Supporting Information).Transmission electron microscopy (TEM) results showed that the NC directly derived from ZIF-8 has maintained the rhombic dodecahedron with a relatively dense carbon structure (Figure 1b).But the rhombic dodecahedron of ZIF-8 was completely damaged and many holes appeared on the carbon carrier after melamine treatment (Figure 1c,d).The high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images and corresponding element mapping of 0.20Mela-FeNC have revealed the homogeneous distribution of N and Fe on the carbon support (Figure 1e).The high-resolution HAADF-STEM image (Figure 1f) demonstrated the individual iron atoms (marked by red circles) in 0.20Mela-FeNC.The electron energy loss spectroscopy (EELS) of 0.20Mela-FeNC (Figure 1g) further indicated that the isolated bright spot was atomic Fe species associated with nitrogen, suggesting the Fe-N x sites.

The Effect of Melamine for NC Support
To verify the effect of melamine, we treated NC carriers with different mass ratios of melamine (xMela-NC, x = 0.15, 0.20, 0.25) using the same pyrolysis condition.The Brunauer-Emmett-Teller (BET) results of precursors showed that the surface area of the precursors decreased rapidly with the increase of melamine content, especially the micropore area (Figure S7, Table S1, Supporting Information), indicating that melamine was mainly filled in the micropores of the precursors.After the same pyrolysis process, these NC supports exhibited distinct shapes.The NC without melamine treatment maintained the shape of a Energy Environ.Mater.2024, 7, e12611 rhombic dodecahedron well, whereas the morphology of xMela-NC was severely damaged, and the etching was more serious with an increase in melamine (Figure S8, Supporting Information).Next, nitrogen-sorption analysis was applied to access the porous properties of the NC and xMela-NC (Figure S9, Supporting Information).This analysis showed that xMela-NC had an obvious adsorption hysteresis loop, and it gradually became larger with the increase of melamine, indicating a significant change in the porosity of NC carriers.It was not difficult to see that the mesoporous and macroporous adsorption volume of xMela-NC increased significantly after melamine treatment (Figure 2a).Besides, the external surface area percentage of xMela-NC increased from 9.2% to 53.2% with the increase of melamine (Figure 2b).However, BET surface areas of xMela-NC decreased from 876.2 m 2 g À1 to 703.1 m 2 g À1 .This was because the melamine filled in the micropores could decompose to produce massive gas byproducts (such as N 2 , CO 2 , CO, and so on) above 350 °C, [41,42] causing the collapse of the micropores, transforming them into the mesopores and macropores.Raman spectra were further measured to evaluate the defect degree of NC carriers.The I D /I G values of NC and xMela-NC increased with the increase of melamine (Figure 2c, Figure S10, Supporting Information).This is due to the increase in the proportion of mesopores in the NC carriers, resulting in increased edge carbon sites. [43,44]Therefore, we believed that melamine-assisted pyrolysis method could significantly improve the external surface area and edge carbon number of NC carriers.To gain deep insight into the surface chemical composition of NC and xMela-NC, X-ray photoelectron spectroscopy (XPS) analyses were conducted.The total N content in xMela-NC has increased with the increase of melamine, and among the nitrogen species, the pyridinic-N content also showed an increasing trend (Figure 2d).As we all know, the serious N loss occurred when precursors were treated above 900 °C. [21,45]The nitrogen content of melamine was up to 66.7%, thus it supplemented the serious nitrogen loss caused by high-temperature pyrolysis.According to the XPS results, melamine mainly promoted the formation of more pyridinic-N.The pyridinic-N structure would create more active sites for the ORR due to the Lewis basic sites created by pyridinic-N. [46,47]In brief, melamineassisted pyrolysis approach not only effectively etched the carbon substrate, increased the external surface area and multilevel pores porosity of the NC substrate but also made up for the nitrogen loss caused by high-temperature pyrolysis (Figure 2e).A larger external surface area and richer nitrogen-containing carbon defects provided more effective sites for subsequent vapor deposition.

ORR and PEMFCs Performances of FeNC and xMela-FeNC Catalysts
To verify the feasibility of the melamine-assisted vapor deposition approach, FeNC (made by NC without melamine treatment) and xMela-FeNC (made by xMela-NC) catalysts were prepared under the same vapor deposition conditions.The ORR performance of all Fe-N-C samples was investigated by RDE and RRDE in an O 2 -saturated 0.1 M H 2 SO 4 solution.For comparison, a commercial Pt/C catalyst (20% Pt, JM) was measured in O 2 -saturated 0.1 M HClO 4 solution.The E 1/2 of 0.20Mela-NC was shifted positively by 52 mV compared with that of NC (Figure S11, Supporting Information), indicating more electrochemically available active pyridinic-N sites in 0.20Mela-NC, which was consistent with XPS results.As shown in Figure S12, Supporting Information, 0.20Mela-FeNC exhibited the best ORR performance among the xMela-FeNC.This could be attributed to the fact that the optimized hierarchical pore structure was more conducive to the improvement of the accessible SD and mass transport.However, the transition etching would lead to a serious collapse of the structure, which reduced the SD.Therefore, we chose the optimized 0.20Mela- FeNC to study the superiority of melamine-assisted vapor deposition method.As revealed in Figure 3a, 0.20Mela-FeNC catalyst demonstrated a high ORR activity with an onset potential (E onset ) of ~1.00 V.The E 1/2 of 0.20Mela-FeNC was up to 0.861 V (Figure 3b), which was much higher than that of the FeNC (0.800 V) and reported PGMfree electrocatalysts (Table S8, Supporting Information).Meanwhile, the kinetic current density (j k ) of 0.20Mela-FeNC reached 8.56 mA cm À2 at 0.85 V, which was 12.2 times higher than 0.70 mA cm À2 for FeNC.The corresponding Tafel slope of 0.20Mela-FeNC was 77 mV dec À1 , which was lower than 82 mV dec À1 for FeNC (Figure 3c), indicating that 0.20Mela-FeNC catalyst exhibited accelerated ORR kinetics.To quantify the ORR pathway, a rotating ring-disk electrode (RRDE) technique was used to calculate electron transfer number (n) and H 2 O 2 yield during the ORR process.The H 2 O 2 yield and n of 0.20Mela-FeNC were below 2% and 3.97 at the potential range from 0.2 to 0.8 V, respectively (Figure 3d, Figure S13, Supporting Information), which was much better than that of FeNC (<10% and 3.80), indicating the superior selectivity of oxygen reduction toward H 2 O. Therefore, we believed that the catalyst prepared by the melamine-assisted vapor deposition approach greatly enhanced the ORR kinetics and four-electron selectivity due to its hierarchical pore structure and richer edge active sites.Unfortunately, 0.20Mela-FeNC, like other high-performance Fe-N-C catalysts, also faced poor electrocatalytic durability in acid media.The E 1/2 of 0.20Mela-FeNC decreased by 33 mV after 10 000 cycles between 0.6 and 1.0 V in a 0.1 M H 2 SO 4 solution (Figure S14, Supporting Information).

Electronic Structure Analysis of FeNC and 0.20Mela-FeNC
To understand the origin of the excellent ORR activity of 0.20 Mela-FeNC, various characterization techniques were used for an in-depth analysis.The morphology and porosity of NC carriers remained almost unchanged before and after vapor deposition of Fe-N x sites (Figures S15 and S16, Table S1, Supporting Information).The FeNC exhibited a type I isotherm with an adsorption capacity of 63% in the low-pressure region of P/P 0 < 0.1, indicating the typical microporous material with a low amount of mesopores.By contrast, the adsorption capacity in the low-pressure region of P/P 0 < 0.1 was 46%, and there was an obvious hysteresis loop, indicating that 0.20Mela-FeNC maintained the hierarchical pore structure (Figure 4a).Moreover, the 0.20Mela-FeNC exhibited a fourfold higher external surface area than that of FeNC (327.3 vs 71.7 m 2 g À1 ).The significant increase in the external surface area was found to root from the change of surface morphology, that is, the existence of mesopores in the surface of 0.20Mela-FeNC (Figure 4b).Furthermore, 0.20Mela-NC was also demonstrated to have more active pyridinic-N distributed on the surface.Therefore, 0.20Mela-NC could theoretically deposit more abundant accessible Fe-N x sites through vapor deposition method (Figure 4c).The higher Fe weight content of 0.20Mela-FeNC, as proved by ICP-MS, also indicated the deposition of more abundant Fe-N x sites (Table S4, Supporting Information).The overall Fe content in FeNC was only 1.34 wt.%, which was 0.45 wt.% lower than that of 0.20Mela-FeNC.The chemical state of elements across the catalyst surface was studied by X-ray photoelectron spectrometry (XPS) (Figure S17 and Table S2, Supporting Information).The C 1s peak of 0.20Mela-FeNC could be mainly deconvoluted into three peaks (C sp 2 , C sp 3 , and C-N/O, respectively).As shown in Figure S17a,d, Supporting Information, the area ratio of the C sp 3 and C sp 2 was increased in 0.20Mela-FeNC, indicating that the C 1s in 0.20Mela-FeNC showed some C vacancies relative to FeNC. [48]The result was consistent with the Raman and TEM characterizations.Moreover, similar features of N species were found out for the catalyst (Figure S17b,e, Supporting Information), including pyridinic-N (398.3 eV), Fe-N (399.0 eV), Pyrrolic-N (399.8 eV), graphitic-N (401.7 eV), and oxidized-N (403.5 eV). [27]The Fe-N contents were increased from 16.66% to 20.55% in the FeNC and 0.20Mela-FeNC (Table S3, Supporting Information), which suggested that a larger external surface area was favorable for vapor deposition of more accessible Fe-N x sites.X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses were conducted to identify the Fe-N coordination moieties in the 0.20Mela-FeNC.As shown in Figure 4d, the near-edge absorption threshold of Fe K-edge in the 0.20Mela-FeNC dropped between the FePc and the Fe 2 O 3 , and preferred to FePc, indicating that atomically dispersed Fe species in 0.20Mela-FeNC carried a positive charge and the oxidation state of Fe was near +2.The FT-EXAFS spectra of 0.20Mela-FeNC have shown that no obvious peak was found out at the position of metallic Fe-Fe (2.20 A), but presented one primary peak at about 1.50 A in R space (Figure 4e), which was close to the iron phthalocyanine (FePc) reference.Moreover, wavelet transform (WT) was also used to investigate the FeK-edge EXAFS oscillations of 0.20Mela-FeNC and only one Energy Environ.Mater.2024, 7, e12611 maximum intensity was observed at about 3.8 A À1 , [49] which was close to that of Fe-N, but distinct from the feature of Fe foil (7.5 A À1 ).It further indicated that the Fe atoms existed as mononuclear centers without the Fe-derived crystalline structure (Figure S18, Supporting Information).For giving further insights into the chemical configuration of Fe, K-edge EXAFS fitting analysis in R and k space was carried out to investigate the structural parameters and evaluate the fitting quality (Figure 4f, Figure S19, Supporting Information).The best-fitting result of 0.20Mela-FeNC was in good consistency with the experimental data, demonstrating an average Fe-N bond length of 1.99 A and an average coordination number of 3.8 for the first shell (Fe-N) (see more details in Table S5, Supporting Information).The M€ ossbauer spectra were fitted with two doublets, D1 (HS O 2 -Fe III N 4 ) and D2 (LS or MS Fe II N 4 ) (Figure S20, Table S6, Supporting Information). [15]The fitting result was consistent with the EXAFS analysis.These atomic structure analyses indicated that the formation of Fe-N 4 sites in 0.20Mela-FeNC had occurred.
To further understand the effect of melamine-assisted vapor deposition approach on Fe-N 4 utilization and SD, we conducted quantitative characterization of the catalysts.Firstly, electrochemically active surface area (ECSA) was an important factor to provide more information on the active sites of the catalysts.The double-layer capacitances (C dl ) based on cyclic voltammetry (CV) measurements at different scan rates were used to measure the ECSA of different catalysts (Figure S21, Supporting Information).As shown in Figure 5a, the C dl of the xMela-FeNC had approximately a twofold improvement relative to the FeNC, indicating that the larger external surface area provided more electrochemically active reaction region.We further quantified SD by the in situ electrochemical method of nitrite absorption followed by reductive stripping.A large, well-defined nitrite reduction peak was observed for the 0.20Mela-FeNC in its poisoned CV between 0.29 and À0.30 V (RHE, reversible hydrogen electrode), whereas only a tiny reductive peak was observed in the corresponding CV of FeNC (Figure S22, Supporting Information).The total amount of charge associated with the NO stripping peak of 0.20Mela-FeNC was 67.25 C g À1 , which was over fourfold more than the FeNC of 15.83 C g À1 .The corresponding SD values for the FeNC and 0.20Mela-FeNC were 32.81 lmol g À1 and 139.45 lmol g À1 , respectively (Figure 5b).The 0.20Mela-FeNC showed an over fourfold SD higher and 12-fold ORR mass activity improvement with a 36% increase in Fe content compared with FeNC.In comparison to two high-performance Fe-N-C catalysts fabricated by CVD (Table S9, Supporting Information), the SD value of 0.20 Mela-FeNC had seen an increment of 40% and 43%, respectively.The turnover frequency (TOF) of 0.20Mela-FeNC was calculated to be approximately 2.37 s À1 at 0.82 V, twice higher than that of FeNC (Figure 5c), indicating that the active sites of 0.20Mela-FeNC possessed higher intrinsic activity than that of FeNC.[52] Moreover, 0.20Mela-FeNC showed a 3.6-fold Fe utilization improvement (43.37%) than that of FeNC (11.93%), indicating that the increased external surface area made the Fe-N 4 sites deposit near the external surface of the catalyst.These Fe-N 4 sites were easily involved in the ORR process, thus increasing the density and utilization of accessible Fe-N 4 sites.

PEMFCs Measurements of FeNC and 0.20Mela-FeNC
Under the DOE testing protocols in PEMFCs, a practical assessment of 0.20Mela-FeNC was conducted (details in Supporting Information PEM fuel cell testing). [53]Figure 6a revealed that the peak power density (P max ) for 0.20Mela-FeNC was 1.30 W cm À2 at 0.40 V under 2.5 bar H 2 -O 2 , which is notably higher than FeNC's P max of 0.35 W cm À2 .The remarkable difference in performance between the two catalysts was due to the hierarchical pore structure and increased external surface area of the melamine-assisted catalyst, leading to a significant fourfold increase in the SD.The hierarchical pore structure of the 0.20Mela-FeNC increased the abundance of edge sites, thus resulting in twice the TOF of the FeNC.In addition to the intrinsic activity of the catalyst, the formation of a stable mass-transport channel for gas-liquid two-phase flow was another essential element in improving the activity of the Fe-N-C catalyst.The mass-transport performance could be evaluated by the concentration overpotential (g C ).
The g C was determined by separating the cell voltage given by Tafel's equation from the iR-free voltage (Figure S25, Supporting Information). [54]Since the anodic Pt layer was very thin, if the anodic mass-transport loss was ignored, the lower g C meant the faster cathode mass transport.Figure 6b shows the variation in the g C of the 0.20Mela-FeNC and FeNC cathode over the whole discharge process.Obviously, the g C of the 0.20Mela-FeNC was much lower than that of FeNC.Furthermore, the high-frequency resistance (HFR) of 0.20Mela-FeNC was reduced by 71% compared with FeNC (Figure S26, Supporting Information).These enhancements may be attributed to the hierarchical pore structure formed by melamine-assisted treatment.To validate the use of 0.20Mela-FeNC for automobile applications, it was tested under 2 bar H 2 -air and exhibited a P max of 0.54 W cm À2 , as shown in Figure 6c, as well as a current density of 88 mA cm À2 at 0.80 V (Figure S27, Supporting Information).Figure 6d compares the MEA performance of 0.20Mela-FeNC with other high-performance Fe-based catalysts (Table S10, Supporting Information).It was not difficult to see that the MEA performance of 0.20Mela-FeNC was at the leading level.However, 0.20Mela-FeNC showed an 87% of current density loss in the 21 h constant voltage test, and the peak power density was reduced by half (Figure S28, Supporting Information).More work should be done to improve the poor durability of highperforming Fe-based catalysts.

Conclusion
In summary, a mela-assisted vapor deposition approach was demonstrated to enhance the ORR activity of the Fe-based catalysts prepared  by chemical vapor deposition.The mechanism of improvement was that the increase of external surface area and the construction of the hierarchical pore structure not only increased the fourfold SD and site utilization improvement of the 0.20Mela-FeNC but also strengthened the mass-transport performance of PEMFCs.As a result, 0.20Mela-FeNC catalyst exhibits a E 1/2 of 0.861 V and 12-fold higher ORR mass activity than FeNC in acidic media, which is comparable to Pt/C catalyst and outperforms those for noble metal-free electrocatalysts.As the cathode in a H 2 -O 2 PEMFCs, 0.20Mela-FeNC catalyst demonstrates a large peak power density of 1.30 W cm À2 , outstripping most of the reported Fe-N-C catalysts.Our work offered new insights for boosting the site density and site utilization toward high-performance ORR electrocatalysts.

Experimental Section
Detailed information related to the synthesis of active electrodes, physicochemical characterization, and electrochemical evaluation of bifunctional electrodes towards UOR and supercapacitor application is provided in Supporting Information.

Figure 1 .
Figure 1.Preparation and structural characterization of 0.20Mela-FeNC catalyst.a) Schematic illustration for synthetic procedure of 0.20Mela-FeNC.TEM images of b) FeNC and c, d) 0.20Mela-FeNC, e) the EDS mapping images (green, iron; red, carbon; yellow, nitrogen); f) high-magnification AC-HAADF-STEM image (Fe atoms marked by the yellow circles), g) intensity profiles of 0.20Mela-FeNC obtained in the dotted orange circle region in f).

Figure 2 .
Figure 2. Structural characterization of xMela-NC.a) Pore sizes distribution of NC and xMela-NC, b) the corresponding surface area, c) Raman spectra of NC, 0.15Mela-NC, 0.20Mela-NC and 0.25Mela-NC.d) XPS N 1s spectra.e) The fraction of various types of nitrogen, and f) pyridinic-N and total N content.

Figure 4 .
Figure 4. Structural analysis of FeNC and 0.20Mela-FeNC.a) Nitrogen adsorption-desorption isotherms; b) the corresponding pore sizes distribution; c) diagram of vapor deposition of FeNC and 0.20Mela-FeNC; d) Fe k-edge XANES spectra; e) Fe K-edge FT-EXAFS spectra; f) Fe K-edge EXAFS fitting analysis in R space of 0.20Mela-FeNC.

Figure 5 .
Figure 5.The electrochemical analysis of active site density.a) Evaluation of C dl values by plotting the j at 0.35 V versus scan rate; b) SD, Fe content by ICP-MS and mass activity at 0.85 V; c) TOF at 0.82 V and Fe utilization of FeNC and 0.20Mela-FeNC.

Figure 6 .
Figure 6.PEMFCs performance measurements.a) Polarization and power density curves of the catalysts.Test conditions: cathode loading 3.5 mg cm À2 for Fe-N-C and 0.2 mg Pt cm À2 for Pt/C, anode loading 0.4 mg Pt cm À2 , GORE-SELECTâ membrane (15 lm), 1.21 cm 2 electrode, 80 °C, 100% relative humidity (RH) and abs.2.5 bar H 2 -O 2 at flow rates of 0.3/0.4L min À1 ; b) The concentration overpotential (g C ); c) Polarization and power density curves of 0.20Mela-FeNC at loading of 3.5 mg cm À2 under abs. 2 bar H 2 -air and JM Pt/C at loading of 0.1 mg Pt cm À2 under abs.1.5 bar H 2air. H 2 /air flow rates is 0.3 and 1.5 L min À1 , respectively; d) MEA performance comparison between 0.20Mela-FeNC and the literature reported high-performance Fe-based catalysts.