Highly Immobilized Bimetallic Fe/M‐N4 (M‐ Mg or Zn) Conductive Metal–Organic Frameworks on Nitrogen‐Doped Porous Carbon for Efficient Electrocatalytic Hydrogen Evolution and Oxygen Reduction Reactions

Herein, a simple method is proposed for developing bimetallic Fe/M‐N4/nitrogen‐doped porous carbon (NPC) (M‐Zn or Mg) conductive metal–organic framework (c‐MOF) composites because of their great potential in replacing conventional catalysts. The prepared composite MOF exhibits remarkable catalytic activity for both hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), surpassing the performance of the state‐of‐the‐art transition metal‐N4 cathode catalysts. These composites demonstrate excellent selectivity for a four‐electron transfer, facilitated by an associative reaction pathway that functions as the rate‐determining step. Therefore, they offer high half‐wave and onset potential values for ORR, i.e., 0.92 and 1.02 V for Fe/Mg‐N4‐NPC (hexaminobenzene (HAB)‐3@NPC) at a current density of 4.11 mA cm−2, and 0.89 and 0.99 V for Fe/Zn‐N4‐NPC (HAB‐2@NPC) at a current density of 3.8 mA cm−2, respectively. In addition, they provide low overpotentials of 21 and 64 mV at the current density of 10 mA cm−2 with Tafel slopes of 47.9 and 34.2 mV dec−1 for HER, respectively. Furthermore, when utilized as the cathode in bifunctional electrode assembly cells, they provide low cell voltages of 1.412 V at a current density of 20mA cm−2. In the membrane electrode assembly, the HAB‐3@NPC composite demonstrates an optimal power density of 0.861 Wcm−2, thus underscoring its potential in practical applications.


Introduction
[3] Water electrolysis is a promising technology for renewable energy sources, enabling efficient production, transportation, and storage of hydrogen energy.[6][7] However, in practical application, HER and OER display sluggish kinetics, ultimately resulting in low device efficiency.0] However, such metals are expensive and cannot be used to meet the requirements of industrial-scale hydrogen production.
Herein, a simple method is proposed for developing bimetallic Fe/M-N 4 / nitrogen-doped porous carbon (NPC) (M-Zn or Mg) conductive metal-organic framework (c-MOF) composites because of their great potential in replacing conventional catalysts.The prepared composite MOF exhibits remarkable catalytic activity for both hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), surpassing the performance of the state-of-the-art transition metal-N 4 cathode catalysts.These composites demonstrate excellent selectivity for a four-electron transfer, facilitated by an associative reaction pathway that functions as the rate-determining step.Therefore, they offer high half-wave and onset potential values for ORR, i.e., 0.92 and 1.02 V for Fe/Mg-N 4 -NPC (hexaminobenzene (HAB)-3@NPC) at a current density of 4.11 mA cm À2 , and 0.89 and 0.99 V for Fe/Zn-N 4 -NPC (HAB-2@NPC) at a current density of 3.8 mA cm À2 , respectively.In addition, they provide low overpotentials of 21 and 64 mV at the current density of 10 mA cm À2 with Tafel slopes of 47.9 and 34.2 mV dec À1 for HER, respectively.Furthermore, when utilized as the cathode in bifunctional electrode assembly cells, they provide low cell voltages of 1.412 V at a current density of 20mA cm À2 .In the membrane electrode assembly, the HAB-3@NPC composite demonstrates an optimal power density of 0.861 Wcm À2 , thus underscoring its potential in practical applications.
In addition, fuel cells are the most sought-after technology to convert chemical energy into electrical energy through an electrochemical reaction with high-energy conversion efficiency.Fuel cells comprise a cathode and an anode for achieving the ORR and hydrogen oxidation reaction, respectively.Moreover, these bifunctional oxygen reduction and hydrogen evolutionbased electrochemical reactions are always hindered by sluggish kinetics, which cause high overpotential and low round-trip efficiency. [11,12]To overcome this barrier, a bifunctional electrocatalyst was utilized and was active for both HER and ORR performance.However, most bifunctional catalysts could allow the HERs and ORRs to share similar active sites.Hence, the thermodynamically most favorable ORRs are predominant at the active site, which diminishes the HER-related catalytic activity.In general, it is believed that bifunctional electrocatalysts with decoupled active sites lead to HER and ORR at the cathode under reduction potential.[15][16] In recent years, significant theoretical and experimental research has been conducted on cations in MOF based on transition metals (TM), as decoupled active sites for bifunctional electrochemical performance.[23] In addition, the porous structures of these MOFs could result in high catalytic activity as they facilitate in the generation of charge transport pathways by using metal ions and linkers.[26][27][28] Recently, strategies, such as heteroatom doping, coordination number regulation, hybridization with carbonbased 2D MOF materials, have been used to modulate the d-orbitals (and d-band center position) of metal cations in TM-N 4 catalysts, thus allowing a significant enhancement of their bifunctional activity. [29,30][33][34] These guest moieties enhance charge conductivity by functioning as a bridge between the separate layers.The charge can travel out-of-plane of the separate layers of MOFs, thereby enhancing the charge conduction in the direction perpendicular to the stacked layers.Moreover, the inclusion of electron-rich N atoms in the N-containing carbon is advantageous for enhancing the catalyst activity.[37] The implantation of TM-N 4 , specifically into nitrogen-doped porous carbon (NPC)related structures, can significantly accelerate charge carrier transfer and shift the position and utilization of the d-band, thus improving the catalyst activity of the whole system.Therefore, we developed pyridinic/graphitic NPC materials to facilitate in the engineering of pristine Fe 3 (HAB) 2 as Fe-N 4 (HAB-1) catalysts.Additionally, we fabricated bimetallic catalysts, Fe/M-N 4 (M-Zn and Mg), where M represents elements from the d-d block (HAB-2) and s-d block (HAB-3).These catalysts have been designed and synthesized based on unique properties of the NPC support to enhance their catalytic performance in specific applications.The NPCs with three-dimensional porous carbon architectures achieved the efficient adsorption of Fe-N 4 , Fe/Zn-N 4 , and Fe/Mg-N 4 as HAB-1, HAB-2, and HAB-3 complexes, respectively, owing to their strong capillary forces.To fabricate the pyridinic/graphitic NPCs, an amine-functionalized metal-organic framework (nano-MOF-5) was first subjected to a series of processes involving solvothermal treatment and thermal pyrolysis processes.Subsequently, we immobilized the obtained NPCs onto the Fe/M-N 4 complexes.The N and C modulation in Fe-N 4 and Fe/M-N 4 complexes synergistically embedded them into the NPC matrix through strong interactions, resulting in Fe-N 4 /NPC and Fe/M-N 4 /NPC (M-Zn or Mg) coordination configuration.The resulting catalysts obtained from the aforementioned process are denoted as HAB-1@NPC, HAB-2@NPC, and HAB-3@NPC, and they exhibit higher conductivity and improved structural stability than their bimetallic counterparts HAB-2 and HAB-3.Furthermore, density functional theory (DFT) calculations and electron paramagnetic resonance (EPR) analysis were employed to confirm the metallic behavior and structural stability of Fe/M-N 4 and Fe/M-N 4 /NPC composites.These approaches allowed the establishment of a relationship between the density of states (DOSs) and the excellent electrical conductivity of these materials.In addition to their metallic behavior and structural stability, HAB-2@NPC and HAB-3@NPC catalysts exhibit remarkable electrocatalytic bifunctional activity for both the HER and ORR.These catalysts also demonstrate superior stability and robust methanol tolerance in alkaline environments.In particular, they outperform Fe-N 4 and Fe/M-N 4 (HAB-1, HAB-2, and HAB-3) catalysts, as well as the benchmark Pt/C catalyst.The results of HER/ORR for HAB-2@NPC and HAB-3@NPC showed an overpotential of 21 and 64 mV at 10 mA cm À2 , with Tafel slopes of 34.2 and 47.9 mV dec À1 (for HER).In addition, the E 1/2 half-wave potentials were 0.92 and 1.02 V and the onset potentials were calculated as 0.89 and 0.99 V (for ORR), respectively.Furthermore, excellent selectivity was exhibited for the 4e À pathways of ORR.The synergistic effects between Fe/M-N 4 and NPC composites resulted in a downshift of the d-band center, weakening the adsorption of intermediate species and improving the bifunctional activity of HER/ORR.Moreover, the carbon structures facilitated charge separation of the Fe-N 4 and Fe/M-N 4 systems, significantly reducing the electrochemical reaction barrier and resulting in the higher intrinsic activity of Fe/M-N 4 /NPC (HAB-2@NPC and HAB-3@NPC) catalysts, as confirmed by the Tafel slope/electrochemical impedance spectroscopy (EIS) analysis.We also conducted a mechanistic investigation of the bifunctional HER/ORR on the HAB-2@NPC and HAB-3@NPC catalysts, based on the cathodic transfer coefficient (αc) values derived through EIS.Moreover, we fabricated a membrane electrode assembly using HAB-3@NPC, which demonstrated a peak power density of 0.861 W cm À2 .The findings of this study suggest that HAB-2@NPC and HAB-3@NPC hold promise for electrochemical conversion and storage devices.

Structural and Physical Characterization of Electrocatalysts
The preparation process of the bimetallic Fe/M (M-Zn and Mg)-N 4 catalysts, i.e., HAB-2 and HAB-3, is illustrated in Figure 1a.Initially, a mixed solution of the metal precursors and organic linker was prepared in an aqueous ammonia solution and subjected to solvothermal conditions.The coordination of HAB (C 6 N 6 H 6 ) with Fe (II)/M (Mg, Zn-(II)) salts resulted in the formation of d-π bonds between Fe 2þ/ M (M-Mg/Zn) 2þ and the ÀNH group in the HAB ligand.This bridging bond between the Fe/M atoms and HAB nanosheets results in hexagonal lattice structures with a (P6/mmm) coordination symmetry, [38] as depicted in Figure 1a.The Fe 3 (HAB) 2 compound theoretically comprises six TM atoms and six HAB ligands, forming porous structures.The electronic state of the compound ranges from À0.5 to 0.5 eV, with the Fermi level representing the electronic state.The presence of porosity in the material facilitates electron transfer/release, which is beneficial for bifunctional HER/ORR applications.The incorporation of Fe and HAB molecules serves to address the charge-transfer issue and improve the properties of the pristine Fe-N 4 and bimetallic Fe/M-N 4 catalysts.To further enhance this catalyst, it is embedded within the NPC system.This incorporation into NPC not only addresses charge-transfer issues but also improves the bifunctional activity for both the HER and ORR reactions.The morphologies of as-prepared nano-MOF-5(Zn) and AF-nano-MOF-5(Zn), as well as HAB-1, HAB-2, and HAB-3 MOF catalysts are characterized by scanning electron microscopy (SEM) images, as shown in Figure 1b and Figure S1 and S2 (Supporting Information).The typical Figure 1.a) Schematic of the synthesis process of HAB-1 and HAB-1@NPC composites.b) FE-SEM images of HAB-3.c-e) HAADF-STEM images of HAB-3@NPC (bright and dark fields).f ) Element maps of HAB-2@NPC and g) SAED patterns of HAB-3@NPC.
field-emission SEM (FE-SEM) images of HAB-3 and HAB-2 bimetallic MOFs reveal uniform thin-sheet-like structures.The bimetallic modification (Mg and Zn) shows no changes to the intrinsic morphology of HAB-1 nanosheets.The corresponding energy-dispersive X-Ray element mapping is shown in Figure S2 and Table S1 (Supporting Information).The elemental mapping images indicate that Fe, Zn, C, and Mg signals are uniformly distributed over the entire architecture.Further, the morphologies of HAB-1@NPC, HAB-2@NPC, and HAB-3@NPC composites were investigated through high-resolution transmission electron microscopy (HR-TEM) and high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM), as shown in Figure 1c-g and Figure S3 (Supporting Information).The HR-TEM images of HAB-3@NPC indicate that nanosheets are dispersed into the carbon matrix and their d-spacing is 0.273 nm, as shown in Figure 1g.The selected-area electron diffraction (SAED) pattern exhibits low crystallinity for HAB-3@NPC.This result was confirmed through the X-Ray diffraction (XRD) patterns.In addition, HAADF-STEM images of HAB-3@NPC catalyst showed that nanosheets were strongly immobilized in the N-doped carbon matrix, which is beneficial for enhancing the conductivity and corrosion resistance of the materials.The bright-and dark-field HR-TEM analyses showed similar formation structures, as shown in Figure 1d,e.These Fe/M (M-Mg/Zn) sites, along with the C and N atoms, were observed and uniformly distributed on the carbon composites in HAB-1@NPC, HAB-2@NPC, and HAB-3@NPC MOF through elemental mapping.In addition, the positions of Fe and Mg, are identified in the corresponding electron energyloss spectroscopy chemical mapping (Figure S3, Supporting Information).
To investigate the crystal and chemical structural information of the compounds, several characterization techniques were employed, including XRD, X-Ray photoelectron spectroscopy (XPS), EPR, Fourier-transform infrared (FT-IR), and Raman spectroscopy.Figure 2a-c and Figure S4 (Supporting Information) present the XRD patterns of the pristine MOF (HAB-1), bimetallic MOFs (HAB-2 and HAB-3), and their composites (HAB-1@NPC, HAB-2@NPC, and HAB-3@NPC).In addition, the XRD pattern of Fe-N 4 (HAB-1) matches with a previously reported hexagonal 2D structure with AB parallel stacking of the 2D sheets. [39,40]The HAB-1 pattern exhibits distinct diffraction peaks (2θ), ranging from 7.89 to 30.8 and corresponding to the (100), (200), and (001) planes of the P6/mmm space group, Fe 3 (HAB) 2 .The diffraction peak at 2θ ≈7.89-15.9can be attributed to the (100) and (200) lattice planes, revealing a long-range order within the 2D plane.In addition, the broad peak at 24.8, corresponding to the (001) plane, suggests the stacking peak of These values indicate that HAB-3 has a smaller crystallite size, suggesting a large surface area-to-volume ratio, and thus the presence of more active sites, which could contribute to the improved electrocatalytic activity over those of HAB-1 and HAB-2.For the NPC, the narrow peak observed at 25.7 and 42.92 can be attributed to the (002) and (100) crystal planes of graphitic carbon with (JCPDS no.41-1487) (Figure 2a). [41]In the composites of HAB-1@NPC, HAB-2@NPC, and HAB-3@NPC, a narrow peak is observed at 24.03 corresponding to the (002) plane, indicating the low crystalline nature of the material.The absence of diffraction peaks in the HAB-1, HAB-2, and HAB-3 materials suggests the successful immobilization of bimetallic Fe/M-N 4 MOFs into NPC.This immobilization results in the improvement of the conducting properties of Fe/M-N 4 MOFs and the subsequent enhancement of their bifunctional activities.
The molecular structure and functional groups of HAB-1, HAB-2, and HAB-3 were identified through the FT-IR and Raman spectra, respectively, as shown in Figure S4 (Supporting Information).The full absorption-band peak in the FT-IR spectrum was observed at 500-3400 cm À1 .The peak at 3431 cm À1 corresponds to the N-H stretching vibration that might be attributed to Fe-N 4 MOF.In addition, the peak at 2933 and 1649 cm À1 was assigned to C-H and N-H bending vibration, whereas the peak at 1429 cm À1 was due to C-H stretching vibration of the aromatic rings. [22]Moreover, the peak at 1091 cm À1 was assigned to the symmetric and asymmetric stretching vibrations of the N-Fe-N group.The peak at 1207 cm À1 was related to the C-C vibration of the HAB linker.The characteristic peaks observed in HAB-2 and HAB-3 were similar to that of pristine HAB-1, suggesting the presence of metal nodes and various N-related functional groups in the bimetallic compositions. [13,42]However, compared to HAB-1, the intensity of the characteristic vibration peaks in the bimetallic HAB-2 and HAB-3 was reduced.These results confirm the presence of a functional group of pristine HAB-1 and bimetallic, HAB-2, and HAB-3.The characteristic peaks of HAB-1 are observed in the vibrational frequency range of 400-1800 cm À1 to highlight the lattice vibrational mode of the N group and its coordination with HAB and Fe metal atoms of the Fe 3 (HAB) 2 framework.The spectral range of 1537 cm À1 is associated with aromatic rings of the HAB group.The characteristic peak at 1446 cm À1 is assigned to the ring C-H doubling zone (in-plane deformation), C=C stretching quinoid rings, and benzoin units.In addition, the peak at 781 cm À1 is attributed to the N-H bending mode of the HAB linker. [43]No significant changes were observed in the HAB-2 and HAB-3 MOFs, suggesting that the incorporation of a second metal cation does not affect the HAB-1 structures.The Raman spectra were used to determine degree graphitization/defect of HAB-1@NPC, HAB-2@NPC, and HAB-3@NPC composite materials, as depicted in Figure 2b and Figure S5 (Supporting Information).Usually, the disorder level of the carbon-based catalyst was determined according to intensity ratio, I D /I G .The band at 1331.49cm À1 is assigned to the d-band, suggesting structural defects in carbon, and the band at 1591.68 cm À1 (G band) indicates the presence of a graphitic structure in carbon.As a result, the better graphitization is beneficial for enhancing the bifunctional activities of HER and ORR.
The elemental analyses based on XPS were used to detect the state of surface chemistries and electronic structures of HAB-1, HAB-2, and HAB-3 as well as HAB-1@NPC, HAB-2@NPC, and HAB-3@NPC.The XPS survey of composite materials confirmed the existence of Fe, Mg, Zn, N, and C elements (Figure S4, Supporting Information).For the deconvoluted Fe2p spectrum of HAB-1@NPC, Figure 2d shows two spin-orbital lines along with a satellite signal.In addition, we observed characteristic peaks of the Fe element around 710.7, 724.62, 713.36, and 726.63 eV and corresponding satellite peaks (719.48 and 729.44 eV), indicating the presence of Fe 2þ and Fe 3þ in Fe-N 4 complex. [44]The binding energy (B.E) at 1303 eV was related to that of Mg1s, confirming the presence of Mg in HAB-3@NPC (Figure 2e).In addition, The O element was observed from XPS spectra owing to a residual oxygen-containing group or absorbed during XPS experiments.Similarly, the Fe2p and Zn2p spectral peaks of the bimetallic HAB-2@NPC exhibit similar characteristic energies (Figure 2f,g).For example, the B.Es of Fe2p (lower) at 711.92 and 713.65 eV correspond to those at 719.42 eV, whereas the B.Es (higher) at 724.45 and 727.49eV correspond to the 729.11[47] The B.E of Zn2p peaks was deconvoluted to two major subpeaks at 1044.79 and 1021.56 eV, as related to Zn 2p1/2 and Zn 2p 3/2, respectively (Figure 2g).2i).Four types of N species were obtained in the deconvolution of the N1s spectrum at B.Es of 398.60, 400.49, 399.66, 401.58, and 404.04 eV, indexed to the B.Es of pyridinic-N, pyrrolic-N, metallic ÀN, graphitic N, and oxidized N, respectively. [48]The N-related bonding configuration is displayed in Figure 2h.The pyrrolic-N and pyridinic-N atom species are the main active species, which caused them to bond with two adjacent carbon atoms and function as anchoring sites for the Fe/M (Mg and Zn) metal nodes. [49]Then, the graphitic N-species (N-G) that substituted C atoms in the hexagonal ring of the carbon network, are considered as additional active sites that could help improve the electrochemical activity.In addition, high resolution of C1s spectra was used to analyze the defect degree of the three samples.Generally, 2D-based materials have two types of carbon atoms (i.e., sp 2 is a basal plane, and sp 3 is a defect carbon Figure 2. XRD patterns and b) Raman spectra of HAB-1@NPC, HAB-2@NPC, and HAB-3@NPC.c) EPR spectra of HAB-1, HAB-3, and HAB-3@NPC and d-i) high-resolution Fe2p, Mg1s, Zn2p, N1s, and C1s XPS for HAB-2@NPC and HAB-3@NPC.atom).Hence, the C-C (sp 2 )/C-C (sp 3 ) ratio can explore the defect degree of 2D materials (Figure S5, Supporting Information).The sp 2 /sp 3 values for HAB-1@NPC, HAB-2@NPC, and HAB-3@NPC were calculated as 2.2%, 2.17%, and 1.24%, respectively, revealing that lower defects provide higher activity; these results show similarities with the Raman results of the composites.Moreover, the carbon defects can facilitate reactant adsorption, promote charge transfer between the catalyst's surface and intermediates, and accelerate bifunctional kinetics.Furthermore, N can regulate metal atoms in catalysts, while graphitic N, pyridinic-N, and pyrrolic-N can offer more electrons and enhance the conductivity of the carbon matrix.Furthermore, XPS analysis was performed in the HAB-1, HAB-2, and HAB-3 samples, the details of which are given in Figure S5 (Supporting Information).The bimetallic Fe/M-N 4 contains multiple metal valences of Fe 2þ , Mg 2þ , and Zn 2þ , uniformly coordinated within HAB and coexisting with various functional groups of C-C, C-N, and N-C=O, respectively.The high valance state of Fe/M-N 4 with N and C groups confirms its excellent catalytic performance.To further understand the spin-state polarization and electronic properties of HAB-1, HAB-3, and HAB-3@NPC, their EPR spectra were developed (Figure 2c).The EPR spectrum exhibits that HAB-3@NPC has the highest peak intensity than those of HAB-3 and HAB-1, suggesting an increase in the number of unpaired electrons.Considering the introduction of NPC into the Fe/M-N 4 complex and its synergistic effects, HAB-3@NPC showed significant improvements with a charge carrier, HER/ORR intermediates, and enhanced intrinsic activity.

Study of the HER Activity of the Electrocatalyst
The electrochemical water-reduction performance of bimetallic MOF and their composites was evaluated in 1.0 M KOH through a three-electrode probe.As demonstrated in Figure 3a, all polarization curves of the samples were recorded at a scan rate of 5 mVs À1 .In addition, the mechanism of the catalytic activity of each component is described in detail.Interestingly, HAB-3@NPC-3 demonstrates superior activity than those of HAB-1, HAB-2, HAB-3, HAB-1@NPC, and HAB-2@NPC.The order of HER catalytic activity follows HAB-1 < HAB-2 < HAB-3 < HAB-1@NPC < HAB-2@NPC < HAB-3@NPC (Figure 3C).Specifically, the HAB-1 component requires a higher overpotential of 316 mV to achieve a geometric current density of 10 mA cm À2 .The bimetallic catalyst and their composites show high activity with η 10 reducing to 264, 189, 93, 64, and 21 mV for HAB-2, HAB-3, HAB-1@NPC, HAB-2@NPC, and HAB-3@NPC.The performance of HAB-3@NPC is noteworthy, surpassing many of the recently reported catalysts based on MOFs and non-noble metals for HER/ORR reactions.Table S4 (Supporting Information) summarizes the comparison report, including the performance of these catalysts.In addition, the Tafel slope was investigated by fitting the Tafel plot to the Tafel equation to analyze the catalytic activity and kinetics of HER (Figure 3b).The HAB-3@NPC affords a small Tafel slope of 34.2 mV dec À1 , which is much lesser than that of HAB-2@NPC (47.9 mV dec À1 ), HAB-1@NPC (68.2 mV dec À1 ), HAB-3(74.2mV dec À1 ), HAB-2 (98.7 mV dec À1 ), and HAB-1(165.2mV dec À1 ), revealing that HAB-3@NPC has a relatively higher HER kinetic process under alkaline conditions.Furthermore, the Tafel slope is used to describe the ratedetermining step (RDS) during the electrochemical HER process.In general, the alkaline HER pathways involve water dissociation of H 2 O to H* and OH À and subsequent production of H 2 .In other words, the Volmer step initiates the water-dissociation process, i.e., electron-coupling of H 2 O molecules to generate absorbed H intermediate on the electrode surface (H*).2] 2H 2H ads !H 2 ðTafel reactionÞ (2) where H ads reveals an H atom absorbed on the active sites of an electrocatalyst, associating with the theoretical value of the Tafel slope for the kinetic reaction, which was determined to be approximately 120, 40, and 30 mV dec À1 , for the Volmer, Heyrovsky, and Tafel reactions, respectively. [53]To be considered as outstanding alkaline HER electrocatalysts, the catalysts must be able to facilitate the dissociation of H 2 O into absorbed hydroxyl ions (OH À ) and hydrogen (H), while also providing moderate adsorption/ desorption abilities for intermediate hydrogen species (H*).In this study, pristine HAB-1 exhibited the Volmer reaction, in which two absorbed hydrogen species (H*) combined to form gaseous H 2 .This step is identified as the RDS, suggesting that the pristine HAB-1 catalyst possesses the capability to split water molecules H 2 O into H* species.Furthermore, Mg/Zn metal ions incorporated into Fe-HAB can result in an optimized H desorption to promote the Volmer-Heyrvosky steps for the HER and achieve high electrocatalytic activity.After that Fe/Mg-N 4 MOF immobilized into the NPC structures, the structures were able to accommodate two H atoms and provide H 2 recombination (Volmer Tafel reaction).The resulting HAB-3@NPC composites could achieve a balance between the adsorption/desorption of H 2 O as well as the recombination of H, resulting in a reduced HER overpotential.This enhanced HER kinetics could be attributed to the synergistic effects between Fe/Mg-N 4 and NPC.In addition, the exchange current density was calculated to probe the intrinsic electrochemical property, which can be obtained by extrapolating the corresponding Tafel plot.The HAB-3@NPC (9.8 mA cm À2 ) possesses higher current density than those of HAB-1 (1.22 mA cm À2 ), HAB-2 (2.51 mA cm À2 ), HAB-3 (3.44 mA cm À2 ), HAB-1@NPC (6.99 mA cm À2 ), and HAB-2@NPC (8.62 mA cm À2 ).Moreover, HAB-3@NPC provides an extraordinary intrinsic activity, indicating a higher electrochemical HER performance.
To further reveal the intrinsic activity on the exposed area of catalysts, the electrochemical surface area (ECSA) was evaluated through cyclic voltammetry (CV) in the non-Faradic region at different scan rates from 100 to 20 mVs À1 versus reversible hydrogen electrode (RHE) (Figure S6, Supporting Information).The CV curves did not display any redox peak formation, indicating an electrical double-layer capacitor.As shown in Figure 3f, the C dl evaluated by the current density ( j) plot against the potential scan rate and the attained slopes of these fitting line, conforms well with the determined ECSA value.The C dl values were calculated for HAB-3@NPC (23.7 mFcm À2 ), HAB-2@NPC (14.08 mFcm À2 ), HAB-1@NPC (13.47 mFcm À2 ), HAB-3 (6.7 mFcm À2 ), HAB-2 (5.8 mFcm À2 ), and HAB-1(3.4 mFcm À2 ).Furthermore, the ECSA analysis of the bimetallic c-MOF and composites showed a similar tendency with the results of C dl .In addition, the ECSA of HAB-3@NPC (592.5 cm 2 ) is seven times that of the ECSA of HAB-1 (85 cm 2 ), indicating a larger amount of exposed active sites in the HAB-3@NPC composites.Additionally, EIS was conducted to investigate the synergistic properties of the electrode as well as the interfacial chargetransfer process.Figure 3d presents the results, which correspond with that of an equivalent circuit used to model the electrocatalyst.An equivalent circuit comprising a constant phase element, double-layer resistance (R1), solution resistance (R s ), and charge-transfer resistance (R ct ) were used to prompt Nyquist plots.Figure 3d shows that the Nyquist plots of HAB-3@NPC (17.97 Ω) demonstrate a lower charge-transferring resistance (R ct ) than those of HAB-2@NPC Ω), HAB-1@NPC (62.32 Ω), HAB-3 (68.39 Ω), HAB-2 (72.95 Ω), and HAB -1(119.62Ω), suggesting fast electron transfer and improved intrinsic kinetic activity of the electrocatalyst.Moreover, the turnover frequency (TOF) was evaluated to determine the production capability of H per second per active site of the electrocatalyst by considering the linear sweep voltammetry (LSV) (Figure 3e) and CV curves (Figure S7 and Table S2, Supporting Information).The TOF value of HAB-3@NPC was calculated as 2.19 s À1 at the overpotential of 20 mV; this value is higher than those of the TOF valves of HAB-2@NPC (1.49 s À1 ), HAB-1@NPC (1.23 s À1 ), HAB-3 (0.80 s À1 ), HAB-2 (0.65 s À1 ), and HAB-1 (0.21 s À1 ).This confirms the presence of an active interface between Fe/Mg-N 4 and the N-carbon layer, which is uniformly distributed throughout the entire catalyst system.The mass activity was estimated according to the overpotential at 20 mV.Furthermore, the mass activity of HAB-1, HAB-2 HAB-3, HAB-1@NPC, HAB-2@NPC, and HAB-3@NPC were estimated as 19.9, 22.4, 26.5, 43.1,48.5, and 66.6Ag À1 , indicating the fastest HER conversion efficiency of HAB-3@NPC electrode (Figure 3e).
The long-term stability of the electrocatalysts is another important factor for practical applications.Therefore, we performed a stability test for HAB-3@NPC and Pt/C. Figure S8 (Supporting Information ) displays the chronoamperometry plot of HAB-3@NPC at the current density of 20 mA cm À2 for 50 h.After a long-term durability test, the surface chemical composition of the HAB-3@NPC material was analyzed through XPS (Figure S8, Supporting Information).As shown in Figure S9 (Supporting Information), HAB-3@NPC is well preserved in an alkaline condition and no obvious structural changes were observed after the stability test, confirmed through HR-TEM and elemental mapping.The Fe 2p and Mg1s XPS spectra of HAB-3@NPC after the stability test retained its earlier features, further confirming that the HAB-3@NPC catalyst is a good HER catalytic material.

Understanding the Mechanism and Catalytic Activity of HER on HAB-3@NPC and HAB-2@NPC
To gain a deeper understanding of the HER mechanism and activity difference between HAB-3@NPC and HAB-2@NPC, both EIS and DFT calculations were conducted.In HAB-3@NPC composites, the Fe/Mg metallic site was coordinated by four N and six C atoms and anchored on the carbon matrix.Similarly, in HAB-2@NPC composites, the Fe/Zn metallic site is coordinated by four N and six C atoms.Two model structures were confirmed through the XPS and DFT calculations.In general, the incorporation of a second metal atom has potential to further alter the electronic properties of a catalyst, resulting in an increase in the intrinsic activity.Herein, the s and d states of the second metal atom (Mg and Zn) were incorporated into Fe-N 4 and anchored onto N-carbon structures, thus exhibiting excellent the HER performance.The onset potential of HAB-3@NPC was determined as À0.01 V, whereas that for HAB-2@NPC was measured at À0.034 V. Based on the d-band theory, an upshift of the d-band center to the Fermi level translates into stronger binding between absorbates and catalysts, whereas a downshift of the d-band center from Fermi level decreases the binding strength of the reactants.This is because the DOS near Fermi level results in a larger concentration of charge carriers and higher electrical conductivity. [54]For example, the Zn sites of Fe/Zn-N-C are close to the Fermi level (ΔG HÃ = 0 eV), indicating that the d-band center moves downward to the Fe-HAB, resulting in a weak bonding of Fe/Zn-N-C with H* and the evolution of H 2 gas. [55]The DOS states near the Fermi level enhanced the conductivity of Fe-N 4 /C, as experimentally presented by the reduced charge-transfer resistance from EIS analysis.Similarly, the Mg sites of the Fe/M-N 4 /C composites charge of the d-band center move downward, and this is favorable for the enhanced adsorption of the intermediates and the intrinsic HER activity of HAB-3@NPC.In addition, the presence of the small atomic radius of Mg in Fe-N 4 /C allowed for stronger electronegativities that significantly improved the absorbed H*, generating into a larger evolution of H 2 than Fe/Zn-N 4 /C, which was confirmed through DFT calculations.Thus, these HAB-3@NPC and HAB-2@NPC composites can be considered as ideal electrocatalysts.The aforementioned HER suggests a Tafel mechanism for HAB-3@NPC and a Heyrvosky reaction for HAB-3 (Figure 3g).To further identify the reaction mechanism and RDS in the HER, EIS was performed for HAB-3@NPC and HAB-2@NPC at different applied potentials (0.023-0.63 V). Figure S10 (Supporting Information) shows the Nyquist plots that help determine the charge transport resistance (R ct ) (i.e., interface between catalyst surface and electrode), interfacial resistance (R int ) (i.e., interface between surface catalyst and electrolyte), and cathodic transfer coefficient, which are provided in Table S3 (Supporting Information).Figure S8 (Supporting Information) plots the logarithm of charge-transfer resistance (R ct ) against the overpotential.According to the Butler-Volmer theory, the slope of the log R ct versus overpotential (η) relationship can be used to calculate the cathodic transfer coefficient: [56] α c ¼ ðslopeÞ Ã ðÀRT=FÞ (4) The Tafel slope of HAB-3@NPC obtained from the impedance method revels that the Tafel mechanism (2H*!H 2 þ 2 Ã ) is involved in the HER.In addition, the obtained Tafel value suggests a strong bond between the absorbed H and the active site in HAB-3@NPC.The value is similar to the Tafel slope obtained from voltametric studies.Moreover, all results are in good agreement with EIS analysis.Hence, the 1/R ct versus the overpotential value demonstrates that HAB-3@NPC has higher conductivity, resulting in rapid ion/electron transport during HER.

ORR Activity of the Electrocatalysts
The ORR catalytic activity of these catalysts was studied using an apparatus with a three-electrode probe cast rotating disk electrode (RDE)/(RRDE) and 0.1 M KOH electrolyte under ambient temperature.The CV curves and LSV curves were measured to scrutinize the electrocatalytic performance of ORR.The applied potential range for ORR was 1.2-0.3V with respect to RHE, and the scan rate was 5 mV s À1 .As shown in Figure 4a, the CV curves of HAB-3@NPC, HAB-2@NPC, and HAB-3 were tested in N 2and O 2 -saturated alkaline electrolytes.In contrast, the pristine HAB, HAB-3@NPC, and HAB-2@NPC exhibited reduction peaks in the O 2 -saturated electrolyte suggesting their effectiveness in the ORR process.The HAB-3@NPC showed a potential of 0.90 V, which is significantly higher than those of other competent samples.This excellent ORR activity of HAB-3@NPC could be attributed to its metallic behavior.Furthermore, the N polycyclic aromatic chain in HAB and pyridinic/graphitic NPC offer electrons to oxygen, thus weakening the O-O bond, in turn resulting in a lower activation energy for ORR.Further, the ORR performance of the electrocatalyst was investigated according to the LSV polarization curves at 1600 rpm to evaluate the half-wave potential.As shown in Figure 4b,c, the LSV curves for the HAB-3@NPC show better catalytic performance in terms of half-wave (E 1/2 ) and onset potential, i.e., 0.92 and 1.02 V, respectively, and diffusion limited current densities ( J L ) of 12.25 mA cm À2 .This performance of HAB-3@NPC outperforms those of commercial Pt/C and the most recently reported MOF-based electrocatalyst, as summarized in Table S5 and S6 (Supporting Information).In contrast, the half-wave and onset potentials of the other samples are lower, i.e., HAB-2@NPC (0.89, 0.99 V), HAB-3 (0.83, 0.98 V), and HAB-2 (0.79, 0.86 V) besides, diffusion-limited current density ( J L )@ 0.90 V value of HAB-3@NPC (4.11 mA cm À2 ), HAB-2@NPC (3.9 mA cm À2 ), HAB-3 (3.68 mA cm À2 ), and HAB-2 (3.1 mA cm À2 ) at 1600 rpm, Figure 4c.The RDE tests were performed at various speeds from 200 to 2400 rpm for HAB-2, HAB-3, HAB-2@NPC, and HAB-3@NPC, as shown in Figure 4d,e.Among the resultants of ORR, HAB-3@NPC, and HAB-2@NPC exhibit the best catalytic performance, because coordination environment of Fe/Mg-N 4 / NPC and Fe/Zn-N 4 /NPC MOF composites improves the ORR activity with the following metrics.1) The bimetallic Fe/M (Mg and Zn) system displays a strong adsorption to oxygenated species, which could benefit the O 2 -reduction reaction.2) A highly N-doped carbon helps improve electron-absorbing ability that promotes oxygen adsorption, and 3) tunable porous structures and surface area are favorable for maintaining long-term stability of MOF.In addition, to verify the ORR kinetics and catalytic mechanism, the Tafel slope of these samples is calculated as shown in Figure 4g.The Tafel plot was obtained from the i k values versus potential, as displayed by the following KL equation: [57] 1 where i k and i DL denote the kinetic and limiting current densities, respectively.The HAB-3@NPC and HAB-2@NPC catalysts exhibit a smaller Tafel slope at 63 and 74 mV dec À1 than those of HAB-3 (107 mV dec À1 ) and HAB-2 (117 mV dec À1 ), indicating faster kinetics of the ORR reaction.Moreover, the Tafel slope helps understand the catalytic mechanism of HAB-3@NPC and reveals that fast initial electron transport is the rate-limiting step in the ORR process.Further, the HAB-3@NPC catalyst displays an optimal ORR performance in an alkaline medium, i.e., lower halfwave potential and Tafel slope among all the MOF-based catalysts, which is even better than those of commercial Pt/C catalyst.Furthermore, the current exchange density is a crucial factor in estimating the catalytic efficiency, which can be obtained from the intercept of the linear Tafel plot.The HAB-3@NPC exhibits 8.3 mA cm À2 higher I ec than those of HAB-2@NPC (7.62 mA cm À2 ), HAB-3 (5.32 mA cm À2 ), and HAB-2 (4.69 mA cm À2 ).
To understand the kinetic reaction of the ORR process on the HAB-3@NPC and other components, RDE tests were performed at various speeds from 1600 to 2400 rpm, as shown in Figure 4h and Figure S10 (Supporting Information).The Koutecky-Levich (K-L) plots of HAB-3@NPC were generated from the LSV curves at different potentials from 0.4 to 0.8 V.They exhibit good linearity and almost unchanged slope, following first-order kinetics.The slope of the K-L plots reveals that the electron transfer (n) of HAB-3@NPC, HAB-2@NPC, HAB-3, and HAB-2 is 4.4, 4.13, 3.88, and 3.53, respectively, which are close to the theoretical values of the four-electron reaction pathway.These results were further confirmed by the RRDE tests, as shown in Figure 4i.The H 2 O 2 yield of HAB-3@NPC, HAB-2@NPC, and HAB-3 was determined to be 4.5%, 15.4%, and 40% in the range of 0.2-0.90V, corresponding to an average electron transfer number of 3.99, 3.62, and 3.3, confirming the efficient four-electron ORR pathway and complete conversion of O 2 to H 2 O rather than H 2 O 2 .In addition to the kinetics and catalytic activity, stability is a crucial criterion to be established for the design of an advanced bifunctional catalyst in practical applications.The ORR stability of HAB-3@NPC and HAB-3 electrodes was determined through the chronoamperometry analysis, as shown in Figure 5a.The obtained results demonstrate that even after 80 h of operation in the ORR, the current density of the HAB-3@NPC electrode remains stable at 10 mA cm À2 .This performance indicates the structural stability of a catalyst in the alkaline condition.In addition, no obvious changes were observed in the morphology of HAB-3@NPC composites.Furthermore, the methanol tolerance test of HAB-3@NPC and Pt/C electrocatalysts was analyzed in an O 2 -saturated 0.1 M KOH solution at the constant potential of 0.90 V, as shown in Figure S11 (Supporting Information), after injecting 3 M (0.5 mL) of methanol into the electrolyte.In addition, the cathodic current of HAB-3@NPC showed only a slight variation than that of the Pt/C catalyst, demonstrating a notable tolerance to methanol.

Understanding the Mechanism and Catalytic Activity of ORR on HAB-3@NPC
In general, the ORR in alkaline electrolytes suggests a possible reaction pathway such as associative and dissociative.[60][61][62] Generally, the thermodynamic overpotential is a function of the B.Es of the intermediate species (*O, *OH, or *OOH). [62]Hence, the optimum B.E can be regulated to accelerate the sluggish kinetics and achieve high current density at low overpotential.Thus, the dentification of RDS in the ORR process on the electrode surface is of considerable research interest, as it optimizes the binding affinities to the reaction intermediates on the active sites of the catalytic surface.
To identify the RDS and reaction pathway of ORR for HAB-3@NPC and HAB-2@NPC, EIS was evaluated at various applied potentials.Further, to understand the electrocatalytic performance of HAB-3@NPC, the Tafel slope and transfer coefficients were measured.The cathodic charge transfer is calculated as follows: [63] where n r denotes the number of electrons transferred before RDS, n q is the number of electrons involved in the RDS, and β c is the symmetry factor (0.5).The value of n q is 0 or 1, depending on whether the chemical or electrochemical step becomes RDS in the ORR.Based on cathodic charge-transfer value, we conclude that if the RDS is not involved in the process, the Tafel slope of the overall reaction is 60 mV dec À1 and if a single electron is transferred, the Tafel slope will be 120 mV dec À1 .According to the accepted mechanism, ORR follows an associative reaction pathway, in which the stronger binding of OOH* with parallel orientation to the electrode surface tends to the O-O bond cleavage, which is referred to the RDS, i.e., OOH! * O* þ OH*, as confirmed by the Tafel slope and EIS analysis.Hence, the calculated Tafel slope values of 63 and 74 mV dec À1 further demonstrate an associative reaction mechanism in the ORR on the surface of HAB-3@NPC and HAB-2@NPC electrode, where the RDS is the chemical step of O-O bond cleavage.The α c values for the HAB-3@NPC and HAB-2@NPC based on the Tafel slope values were calculated as 0.91 and 0.89, indicating boosted charge-transfer capability to adsorption of reaction intermediates.In addition, the observed α c value is almost similar to the EIS results (Figure 5b-d), thus confirming the ORR reaction pathways.In addition, the calculated α c value based on the Tafel slope and EIS analysis easily provides information about the maximum applied energy (overpotential) that could drive the reaction forward on the surface of HAB-3@NPC and HAB-2@NPC electrodes of the RDS of the ORR by reducing the activation energy.The corresponding theoretical Tafel slope is 2.303RT/(n þ 0.5) F and is collected from a typical electrocatalyst.When the RDS is involved in the initial electron transfer (α c = 0.5) in the ORR, [64] the excepted Tafel slope is 119.1 mV dec À1 .Therefore, all our catalysts have the same RDS value, as verified using EIS.As shown in Figure 5 (logarithm of R ct varies vs applied potentials), the cathodic transfer coefficient was calculated using the Butler-Volmer formalism.The HAB-3@NPC and HAB-2@NPC catalysts exhibit 0.91 and 0.89, similar to those obtained through the Tafel slopes, thus conforming well with the results of the EIS analysis.Rotating-ring disk electrode experiments were conducted to provide further insight into the mechanism to understand the ORR pathway over the different catalysts.As the calculated electron transfer numbers per O 2 molecules (n = 3.9 and 3.6 at 0.4 to 0.9 V) of HAB-3@NPC and HAB-2@NPC, suggesting that ORR involves the direct four-electron pathway containing dual adsorption sites with O 2 molecules.The molecular O 2 was first absorbed on the surface HAB-3@NPC and HAB-2@NPC catalysts, and then the diffused O 2 molecules interact with the dual sites through two bonds forming a bridge model, which could weaken the stable O-O bond.In the second step, one electron The proposed ORR mechanism on the surface of HAB-3@NPC and HAB-2@NPC electrocatalysts is described in Figure 5g.These four-electron pathways depend on the downshifted d-band center of Fe/M-N 4 /C composites.Moreover, the results suggest that both the electrocatalysts improve O 2 absorption and possess weaker absorption energy for OOH* to follow the four-electron process for efficient fuel-cell application.Thus, the combination of bimetallic Fe/Zn (d/d block) and Fe/Mg (d/S block) elements plays a critical role in the ORR performance and NPC, which is beneficial for ORR catalytic activity.The strong electronic effects of Fe/M-(Mg and Zn)-N 4 /C composites could be maintained in the structures, resulting in excellent stability.

Membrane Electrode Assembly and Performance Evaluation of the Water-Splitting Test
Based on the excellent HER/ORR bifunctional electrocatalytic activity and durability demonstrated by HAB-3@NPC, we strongly encourage researchers to further investigate its performance in practical fuel-cell application by constructing an overall water-splitting cell (OWS) and membrane electrode assembly (MEA).The OWS cell is fabricated using Pt/C ||HAB-3@NPC and Pt/C ||HAB-3@NPC (Figure 5e).The Pt/C ||HAB-3@NPC couple exhibits the best catalytic performance.A cell voltage of 1.412 V was required to acquire a current density of 20 mA cm À2 , which is much lower than that of Pt/C ||HAB-3@NPC (1.679 V).In addition, the H 2 -O 2 system with a pressure of 1.0 bar (Figure 5f ), the HAB-3@NPC electrocatalyst exhibits a maximum power density (P max ) of 0.861 Wcm À2 .Moreover, our designed HAB-3@NPC electrocatalyst exhibits higher activity in MEA/OWS than other c-MOF-based catalysts.The results suggest that HAB-3@NPC is a promising candidate for industrial applications.In addition, it comprises highly accessible metallic Fe/Mg-N 4 /NC active sites and exhibits greatly enhanced bifunctional HER/ORR activity in an alkaline medium than that for the reported TM-N 4 and others.
To investigate the participation of orbitals in different samples, we calculated the partial density of state (PDOS) for all samples, as shown in Figure 6a,b.Interestingly, the samples demonstrated an obvious difference in PDOS in the presence of NPC.Before adding the NPC, no orbitals were observed around À15 eV and a big gap is seen between the orbitals.In contrast, when considering the presence of NPC, the gap around À15 eV was completely filled by carbon orbitals, suggesting that the contribution of active orbitals significantly increased after improving the structures with NPC. [38]Moreover, the introduction of a second metal greatly varies the spatial distributions of Fe orbitals and orbital energy levels, resulting in the rehybridization of Fe orbitals with Mg or Zn. [65]According to the PDOS diagrams, the distribution of electron density in bimetallic structures increases than that in the single atomic structures; this is beneficial for electron transfer in the electrochemical reaction.To achieve an insight into the chemical stability, the formation energy of the bimetallic samples was compared with that of others, as shown in Figure 6c.As such, the formation energy of HAB-3@NPC (À803.259eV) was demonstrated to be more negative than that of HAB-2@NPC (À796.255eV), confirming a more stable structure of HAB-3@NPC than that of HAB-2@NPC.The same trend was observed for the HAB structure without NPC.In addition, the values of the formation energies confirm that the addition of NPC to HAB generates more stable structures owing to a more negative formation energy than that of pure HAB samples. [66]These results conform with the experimental findings, thus explaining the higher performance of HAB-3@NPC than that of others (Figure S12 and S13, Supporting Information).Furthermore, the side view of HAB-3@NPC depicted in Figure S14 (Supporting Information) shows the carbonic structure of NPC and HAB, which tend to interact horizontally after coming into contact with each other.

Conclusion
In summary, we successfully developed 2D-based bimetallic conductive MOFs and their composites as bifunctional electrocatalysts for HER/ORR.The fabrication process involved the synthesis of 2D Fe-N 4 (HAB-1) and Fe/M-N 4 (HAB-2 and HAB-3) nanosheets by using ammonia-assisted solvothermal and exfoliation methods, followed by the immobilization of Fe/M-N4/C (HAB-1@NPC, HAB-2@NPC, and HAB-3@NPC) composites.The thin and uniform nanosheet structure, combined with the hierarchical porous nature of the NPC support and the presence of Fe/M (Mg and Zn)-N 4 active sites, contributed to the excellent ORR/HER activity, durability, and methanol tolerance of HAB-2@NPC and HAB-3@NPC in alkaline medium.These catalysts outperformed the commercial Pt/C catalyst.Furthermore, the HAB-2@NPC and HAB-3@NPC catalysts exhibited high maximum power density of 0.861 Wcm À2 in MEA fuel cells and 1.412 V at a current density of 20 mA cm À2 for overall water splitting.These exceptional properties highlight the synergistic effects between NPC and Fe/M-N 4 (M-Mg or Zn) complexes, which confer desirable ORR/HER catalytic performance.These findings open up opportunities for advancements in catalysis and energy-storage technologies in various cutting-edge areas.

Fe 2 (
HAB) 3 .With the incorporation of Mg and Zn into Fe-N 4 , the crystal structure of Fe-N 4 remains unchanged.The diffraction peaks of Fe/M (M-Zn and Mg)-N 4 (HAB-2 and HAB-3) MOFs exhibit the same intensities and positions as Fe-N 4 (HAB-1), with slight differences, indicating partial isomorphic substitution of Fe with M (Mg or Zn).The average crystalline size (d-spacing) was also determined using the Debye-Scherrer's equation.The d-spacing ranges from 0.67 to 0.69 nm for HAB-3 and HAB-2.
Regarding HAB-3@NPC, the high resolution of the C1s spectrum is assigned to C-C (Sp 2 ) at 284.48 eV, C-C (Sp 3 ) at 285.63 eV, C=N/C-O at 286.62 eV, and C-N/C-O at 287.77 eV as well as the π*-π* satellite at 291.457 eV (Figure
is transferred to O 2 to form OOH*, promoting the dissociation of OOH* to undergo a four-electron ORR, as indicated by the α c values based on both the EIS and Tafel slope analyses.This results in the formation of Fe/M (Mg, Zn)-OH ads and C-N-O ads .The second electron can then be transported to the C-N-O ads , followed by a further continuous electron transfer and hydrogenation of intermediate OOH*, which forms the H 2 O molecules.

Figure 5 .
Figure 5. a) Chronoamperometry curves of the HAB-3@NPC and HAB-3 at 10 mA cm À2 .EIS spectra of b,c) HAB-2@NPC and HAB-3@NPC at various applied potentials; d) charge transport resistance versus overpotential plot.e) Overall water-splitting steady-state polarization curves at 5 mV s À1 in 1 M KOH.f ) Membrane electrode assembly of HAB-3@NPC.g) The proposed ORR mechanism on the electrode surface of HAB-3@NPC electrocatalyst based on αc values.