Catalyst Synthesis and Characterization

As one kind of low-cost biomass waste, the membrane of gizzard (MG) from killed chicken consists of keratin derived from polycondensation of multi-amino acids (e.g., alanine, proline, glycine, methionine, etc.) with abundant C, N, and O atoms, which can be utilized as a good precursor for synthesizing heteroatom-doped carbon-based ASCs. Recently, Mn was widely chosen for constructing ORR active sites due to its combination of multifarious properties and advantages, including unique electronic structure, inhibition of Fenton reaction, cost-effectiveness, environmental compatibility, good stability, etc., which make it a promising alternative to other transition metals in fuel cell and metal-air battery applications.^{3e, 4a, 12} Therefore, in this work, we choose MG as the biomass precursor and Mn as the active single-atom sites to construct the ASCs. As shown in Figure 2a, N groups provided abundant adsorption sites to anchor Mn²⁺ ions during the impregnation process in MnCl₂ solution. During the carbonization treatment at 700–900 °C, thermal decomposition of organic compounds and formation of fundamental carbon skeleton simultaneously took place. Mn²⁺ was decomposed to Mn-based nanoparticles and single atoms anchored by N coordination. As demonstrated by electron microscope analysis (i.e., scanning electron microscope (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM)) and energy-dispersive X-ray spectroscopy (EDX) mapping tests shown in Figures S1 and 2, after carbonization treatment at 800 °C, the sample shows a plate-like structure, and Mn-based nanoparticles and clusters can be observed on the N-doped carbon host (Mn_NPs-NC). Meanwhile, MnCl₂ pyrolysis created homogeneous erosion on graphitic layer to generate abundant micropores within the plate-like structures, which can provide large specific surface area and fast ion transportation for electrochemical reactions.¹³ Then, Mn-based nanoparticles were removed by acid pickling and only single-atom sites were preserved. Interestingly, the coordination structure of Mn single atoms varied with the pyrolysis temperature. MnN₅, MnN₄ and MnN₃ moieties were obtained at 700, 800 and 900 °C, respectively. Moreover, the amount of carbon defect sites was also greatly affected by the pyrolysis temperature, among which the sample carbonized under 800 °C possesses the most carbon defective sites (i.e., MnN₄/C_d ASC) compared to those of the 700 and 900 °C samples (i.e., MnN₅/C_d and MnN₃/C_d ASC, respectively). Figures 2b, 2c, and Figures S3a, 3b, 4a, 4b show the SEM and TEM images of MnN₄/C_d, MnN₅/C_d and MnN₃/C_d samples. All the samples possess disordered plate-like structure with diameter ranging from hundreds of nanometers to several micrometers. The HRTEM images and corresponding selected area electron diffraction (SAED) patterns (Figures 2d, S3c and S4c) of the samples show the disordered texture structures with some obscure ribbons, indicating a certain degree of graphitization,¹⁴ as also testified by the powder X-ray diffraction (XRD) results. The XRD patterns of all MnN_x/C_d (x=3, 4, 5) samples shown in Figure S5 exhibit a broad peak at 25.2° and a hump-like peak at 42.6°, which can be assigned to the (002) and (100) diffraction planes of graphitic frameworks (JCPDS no. 41–1487), respectively. No bright nanoparticles or clusters are present in the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and elemental mapping images (Figures 2e, 2 f, Figures S6 and S7), which can also be ascertained by their corresponding SAED patterns, indicating that Mn, N and C species are uniformly distributed within the carbon skeletons. For comparison, the nitrogen-doped carbon (NC) sample prepared without MnCl₂ addition was also characterised, which showed similar morphology and graphitization structures with the MnN_x/C_d samples (Figures S8 and S9). In addition, the aberration-corrected HAADF-STEM (AC-HAADF-STEM) images of MnN₄/C_d (Figure 2g), MnN₅/C_d (Figure S10) and MnN₃/C_d (Figure S11) samples show a number of individual bright spots as marked with green circles, which elucidate the presence of isolated Mn atoms anchored on the N-doped carbon nanosheets.¹⁵ The mass loadings of Mn in Mn_NPs-NC, MnN₅/C_d, MnN₄/C_d and MnN₃/C_d was determined to be 5.9 wt %, 1.7 wt %, 1.3 wt %, 1.0 wt %, respectively, from the results of inductively coupled plasma mass spectrometer (ICP-MS). The surface area and porosity properties of the obtained samples were investigated by N₂-adsorption/desorption measurements (Figure S12). All of the samples show similar microporous structures with pore distribution below 3 nm, as well as the close Brunauer–Emmett–Teller (BET) specific surface areas of 662.0, 621.9 and 491.3 m² g⁻¹ for MnN₅/C_d, MnN₄/C_d and MnN₃/C_d, respectively, which are much higher than those of NC (19.1 m² g⁻¹) and Mn_NPs-NC (54.9 m² g⁻¹) due to the MnCl₂ activation and subsequent acid pickling treatments (Figure S13).

To identify the local electronic and geometric structures of Mn species, X-ray absorption fine structure (XAFS) measurements were systematically performed as shown in Figure 3. The X-ray absorption near-edge spectroscopy (XANES) (Figure 3a) features of the MnN_x/C_d samples are obviously different from Mn foil and Mn₂O₃, indicating their varied atomic structures. Nevertheless, the nearly coincident near-edge absorption. energies (E₀) of MnN_x/C_d samples and MnO reveals that the oxidation states of Mn atoms are close to Mn²⁺. Figure 3b presents the R-space data of k³-weighted Mn K-edge Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) spectra of MnN_x/C_d and the reference samples. All three MnN_x/C_d samples exhibit only one obvious FT peak located at about 1.6 Å attributed to the Mn−N coordination scattering. No peaks of Mn−Mn or Mn−O−Mn coordinations are observed compared to Mn foil, MnO and Mn₂O₃, demonstrating the isolated dispersion of Mn atoms on the NC substrate. As the Mn K-edge wavelet transform (WT)-EXAFS of the samples illustrated in Figure 3c and Figure S14, the WT contour plots of MnN_x/C_d samples display only one intensity maximum of Mn−N coordination at ~5 Å⁻¹, without other scattering paths comparing with the Mn foil WT plots. The above results further demonstrate the formation of Mn single-atom sites. To extract the precise structural parameters, quantitative FT-EXAFS fitting was performed and the results were exhibited in Figure 3d-f and Table S1. The first shell of the central Mn atom displayed Mn−N coordination numbers of 5.4, 4.3 and 2.8 with mean bond lengths of 2.17, 2.18 and 2.15 Å for MnN₅/C_d, MnN₄/C_d and MnN₃/C_d, respectively. In addition, as shown in the N 1s X-ray photoelectron spectroscopy (XPS) results in Figure 3g, the spectra consist of three fitting peaks at 398.7, 400.3 and 401.9 eV arising from the pyridinic-N, pyrrolic-N, and graphitic-N, respectively, and pyrrolic-N is the primary N species for all the three samples.¹⁶ Therefore, we can infer that the Mn atoms mostly coordinated with pyrrolic-N atoms to form single-atom MnN₅, MnN₄ and MnN₃ moieties on the NC matrix for MnN₅/C_d, MnN₄/C_d and MnN₃/C_d, respectively. For MnN₅ site, the Mn single atom could be coordinated by four surface pyrrolic-N atoms and one axial N ligand.

The carbon defects and graphitization characteristics of the MnN_x/C_d ASCs were further investigated by the C 1s XPS and Raman spectra analysis. As shown in Figure 3h, the C 1s spectra can be deconvoluted into three peaks centered around the binding energies of 284.9, 285.9 and 288.0 eV, corresponding to sp²-hybridized C, sp³-hybridized C and O-C=O bonds, respectively.¹⁷ Notably, MnN₄/C_d sample shows the highest content of sp³-hybridized C species (33.3 %) compared to MnN₅/C_d (23.8 %) and MnN₃/C_d (18.5 %), indicating the most sp³-hybridized defect sites in MnN₄/C_d. As shown in the Raman spectra of Figure 3i, all the samples display two intensive peaks centered at the wavenumber of 1360 cm⁻¹ (D-band) and 1600 cm⁻¹ (G-band). The D-band signifies the disordered structures induced by the carbon lattice defects, while the G-band refers to the stretching vibration of sp²-hybridized C−C bond in graphitic structure.¹⁸ The I_D/I_G values of NC, MnN₅/C_d, MnN₄/C_d and MnN₃/C_d are 0.72, 0.88, 1.06 and 0.86, respectively, indicating an enhanced degree of disorder for MnN₄/C_d with the most defective sites. Based on the above discussion, we know that the varying pyrolysis temperature can not only tune the coordination structure of Mn single-atom sites with MnN₅, MnN₄ or MnN₃ moieties, but also regulate the carbon defect amount and structure, which endows the products great potentials as superior ASCs for energy conversion and storage applications.

Electrochemical and ZAB Performance

The ORR properties of MnN_x/C_d ASCs were investigated with a typical three-electrode system in O₂-saturated 0.1 M KOH electrolyte. The polarization curves and Tafel plots are shown in Figure 4a, and Figure S15–S17, and the corresponding mass activities and half-wave potentials (E_1/2) are summarized in Figure 4b. These results demonstrate that the optimal ORR performance is achieved by MnN₄/C_d ASC with MnN₄ coordination and most carbon defective sites, which delivers the minimal Tafel slope of 42 mV dec⁻¹, the highest mass activity of 120.4 mA mg_metal⁻¹ and the most positive E_1/2 of 0.900 V compared with the MnN₅/C_d, MnN₃/C_d, Mn_NPs-NC, NC and Pt/C counterparts. Moreover, the ORR performance is better than many recently reported non-noble metal based single-atom catalysts (Table S2). The ORR pathway of MnN₄/C_d was evaluated by the rotating disk electrode (RDE) measurements. As shown in Figure S18a, the corresponding Koutecky–Levich (K–L) plot is linear, and the value of electron-transfer number (n) is calculated to be about 4.0 over the potential range from 0.3 to 0.8 V, suggesting the first-order reaction kinetics toward the concentration of dissolved oxygen and the four-electron ORR pathway. The methanol tolerances of MnN₄/C_d and commercial Pt/C were also tested by injecting methanol solution to the electrolyte during the chronoamperometric measurements. As shown in Figure S18b, after methanol injection, MnN₄/C_d still maintains a stable current density, while the Pt/C exhibits a sharp jump due to the methanol oxidation, revealing the excellent tolerance to methanol crossover of MnN₄/C_d sample. Moreover, in the accelerated durability test (Figure 4c), no obvious half-wave potential decay can be observed even after 15000 continuous cycles, demonstrating the superior long-term durability of MnN₄/C_d ASC. To reveal the N coordination number effect on the intrinsic activity of Mn single-atom sites, we studied the ORR mechanism on MnN_x (x=3, 4, 5) sites by employing density-functional-theory (DFT) calculations. According to the results in Figure S19, MnN₃ shows a higher energy barrier for the hydrogenation of OO (1.59 eV) than MnN₄ (1.21 eV), indicating the poor activity of MnN₃. Besides, MnN₅ shows a positive free energy of *OO (0.26 eV), indicating that the adsorption of OO is endothermic on MnN₅. Thus, the MnN₄ shows the best catalytic activity in ORR among all MnN_x systems, which agrees well with the experimental results.

To verify the ion enrichment effect of carbon defects through the electric double layer capacitance mechanism, we further tested the supercapacitive performance of the MnN_x/C_d and NC samples by cycling voltammetry (CV) and galvanostatic charge–discharge (GCD) in the three-electrode and two-electrode systems (Figure 4d and Figures S20–S27). The GCD curves of MG-derived samples at the current density of 1 A g⁻¹ are shown in Figure 4d. In comparison with other samples, MnN₄/C_d displays larger specific capacitance as ascertained by its longer discharge time. In addition, MnN₄/C_d exhibits linear symmetrical charge/discharge profiles with negligible voltage drops in the potential range of −1.1–0.1 V, which indicates a small internal resistance and good reversible capacitive behavior. As shown in Figure 4e, the specific capacitances of the samples at different current densities were calculated from their corresponding GCD curves. Notably, MnN₄/C_d presents the maximal specific capacitance (300.2 F g⁻¹ at 1 A g⁻¹) as compared to other samples. Even at an ultrahigh current density of 200 A g⁻¹, the MnN₄/C_d still retains a specific capacitance of 143.5 F g⁻¹, which is almost as twice-fold as that of MnN₅/C_d (e.g., 75.1 F g⁻¹) and more than triple-fold as those of MnN₃/C_d (e.g., 44.3 F g⁻¹). In contrast, NC displays a low specific capacitance of 65.1 F g⁻¹ at 1 A g⁻¹ and only 19.2 % of capacitance retention at 50 A g⁻¹ (compared with 1 A g⁻¹). All the supercapacitive results thoroughly demonstrate the superior electric double layer capacitance of MnN₄/C_d with the most sp³-hybridized carbon defect sites compared with those of MnN₅/C_d and MnN₃/C_d with less defective carbon sites. For further comparison, the carbonization temperature was increased to 1000 °C to prepare the Mn-NC-1000 sample, which shows the similar morphology, graphitization degree and diffraction peaks as those of MnN_x/C_d ASCs, and the Mn atoms are uniformly dispersed on the NC support (Figure. S28 and S29). However, the amount of sp³-hybridized C (16.1 %) species decreases significantly due to the excessive carbonization temperature (Figure S30a). Moreover, the pyrrolic-N species greatly decreases and the Graphitic-N become the primary N species (Figure S30b). As a result, Mn-NC-1000 delivers a E_1/2 of 0.8 V for ORR (Figure S31) and a low specific capacitance of 126.4 F g⁻¹ at 1 A g⁻¹ for supercapacitor (Figure S32). The poor electrochemical performances of Mn-NC-1000 as compared to MnN_x/C_d ASCs could be caused by its modified N coordination structure and less sp³-hybridized carbon defect sites. To demonstrate the ion enrichment effect of MnN₄/C_d sample on the ORR activity, we compared its ORR performance with Pt/C in KOH electrolytes with different concentrations from 0.05 to 1.0 mol L⁻¹. As shown in Figures 4f and S33, along with the KOH concentration decreases from 1.0 mol L⁻¹ to 0.05 mol L⁻¹, the E_1/2 value of Pt/C decreases from 0.892 V to 0.802 V with 90 mV fall-off due to the lowered ionic conductivity of electrolyte. Meanwhile, the MnN₄/C_d sample shows only 42 mV decrement of E_1/2 value from 0.912 V to 0.870 V. This phenomenon reveals the positive influence of the ion enrichment effect from carbon defects of MnN₄/C_d on the ORR performance. Moreover, the LSV curves show a decreased limiting current density along with the increasing KOH concentration, which can be due to the reduced dissolved oxygen concentration in high ion-concentration electrolyte.

To further verify the influence of ion enrichment on ZAB performances, concentration effect (i.e., 6 M, 4 M and 2 M) of KOH electrolyte on the performance of MnN₄/C_d-based ZABs was scrutinized. It is known that the 6 M KOH electrolyte of ZABs delivered maximum electrolyte conductivity, while electrolytes possessing reduced concentration (e.g., 4 M and 2 M) exhibited decreased ionic conductivity.¹⁹ As illustrated in Figure S34a, the initial open circuit voltages of the MnN₄/C_d-based liquid ZABs surpass 1.4 V across varying concentrations of KOH electrolyte. The open circuit voltage of ZAB with a 6 M KOH electrolyte remains virtually unchanged even after 120 hours. Remarkably, the ZAB open circuit voltage decreases only 0.06 and 0.12 V, respectively, in the cases of 4 and 2 M KOH concentrations. In sharp contrast, the open circuit voltages of the liquid ZABs utilizing the commercial Pt/C standard sample experience a significant decline as the electrolyte concentration decreases. The power density achieved by the MnN₄/C_d-based liquid ZAB with a 6 M KOH electrolyte is measured at 255 mW cm⁻² (Figure S34b). Additionally, strong power densities are observed in liquid ZABs with reduced electrolyte concentrations (200 mW cm⁻² for 4 M KOH and 128 mW cm⁻² for 2 M KOH). On the other hand, the power density of the liquid ZAB composed of the commercial Pt/C standard sample is 140 mW cm⁻² at 6 M KOH electrolyte, with a sharp decrease after reducing the electrolyte concentration (54 mW cm⁻² at 4 M KOH and 37 mW cm⁻² at 2 M KOH). The instantaneous discharge performances in the latter case are significantly lower than that exhibited by MnN₄/C_d-based liquid ZABs. These results collectively demonstrate the superior ZABs performance offered by MnN₄/C_d catalyst even when used in low-alkalinity electrolyte.

More importantly, the corresponding flexible solid-state ZABs were assembled to further assess the practical potential. A polyacrylic acid (PAA)-based hydrogel solid electrolyte that is resistant to low temperature was developed and soaked with varying concentrations of KOH. By using carbon cloth coated with MnN₄/C_d catalyst as the cathode and zinc foil as the anode, we were able to create flexible solid-state ZABs. As shown in Figure S35a, even after 40 hours, the open circuit voltage of MnN₄/C_d-based flexible ZABs utilizing solid electrolytes with different KOH concentrations remain stable above 1.35 V. For flexible ZABs composed of Pt/C standard sample, there is a significant decrease in open circuit voltage when electrolyte concentration decreases (i.e., 1.41, 1.3 and 0.98 V for 6, 4 and 2 M, respectively). The MnN₄/C_d-based flexible ZAB utilizing a concentration of 6 M has an excellent discharge power density of up to 155 mW cm⁻², which is higher than standard samples (115 mW cm⁻²) (Figure S35b). The electrochemical impedances of MnN₄/C_d and Pt/C-based flexible ZABs were measured to be 0.94 and 1.15 Ω (Figure S36), and the corresponding ionic conductivities were calculated to be 283.7 and 231.9 mS cm⁻¹, respectively. Even after reducing electrolyte concentration levels, this type of flexible battery still maintained high power densities (i.e., 88 and 48 mW cm⁻² for 4 and 2 M, respectively). In contrast, power densities for flexible ZABs with standard sample significantly declined once their respective concentrations were lowered (e.g., 41 mW cm⁻² for 4 M) or could no longer discharge altogether (at just 2 M).

Low temperature tolerance is an important characteristic of ZABs. To this end, at temperature as low as −40 °C, the MnN₄/C_d-based flexible ZABs still demonstrated good discharge capacity while maintaining open circuit voltages above 1.35 V and retaining some degree of power density (76 mW cm⁻² for the 6 M electrolyte) (Figures 4g and h), which is better than most flexible ZABs currently available, as shown in Table S3. To sum up, MnN₄/C_d ASC delivered comprehensive advantages for ZABs (Figure 4i),²⁰ regarding the superior electrocatalytic activity (i.e., mass activities and E_1/2 of ORR) and stability, and performance of liquid and flexible ZABs toward wide range of working conditions (e.g., low temperature and low alkalinity)

Mechanistic Study for the Electrochemical Performance by DFT

By employing DFT calculations, we demonstrated the ORR mechanism on MnN₄ with the assistance of K⁺ ion. Based on the electrical double layer effect observed in carbon-based electrodes, cations accumulate near the interface at negative voltages, and we find that sp³-hybridized carbon facilitates this process. Figure 5a shows the atomic configuration of MnN₄ on sp²- or sp³-hybridized C, while Figure 5b presents the adsorption energy of K⁺ ions on the graphene surface at sp²-C and sp³-C sites, as shown in Figure 5a. For sp²-C, the adsorption energies at different sites are all positive, indicating very weak adsorption of K⁺ ions at open-circuit potential. In contrast, the adsorption of K⁺ at sp³-C sites is more energetically favorable compared to sp²-C, which can be attributed to the quantum capacitance effect. Furthermore, as the distance between K⁺ and Mn decreases, the adsorption energy of K⁺ on sp³-C becomes more negative, suggesting that the MnN₄ active site also attracts K⁺ ions, as shown in Figure 5c.

To determine whether sp³-C or the accumulated K⁺ ions promote ORR catalytic activity, we show the projected density of states (PDOS) for sp²-C and sp³-C with and without K⁺ cations in Figure 5d, and Figure 5e displays the PDOS for the Mn ion. For sp²-C, a V-shaped dip at the Fermi level is observed, while two peaks appear at the Fermi level for sp³-C, which are the local defect states. Figure S37 shows that sp³-C exhibits a very high C_Q, whereas sp²-C contributes little to C_Q near zero, consistent with the experimental results (Figure 4d and e). After the adsorption of the K⁺ ion, the sp³-C becomes n-doped, which is further confirmed by the PDOS on Mn in Figure 5e. Compared with MnN₄ located on sp²-C, the PDOS on Mn in MnN₄/sp³-C remains unchanged, indicating that the sp³-hybridized C does not significantly affect the electronic properties of Mn. When the K⁺ ion is located at the sp³-C site, as shown in Figure 5e, the peaks above the Fermi level shift to lower energies, suggesting that the MnN₄/K⁺-sp³-C system exhibits higher catalytic activity. Therefore, the sp³-C accumulates K⁺ ions, and the presence of these K⁺ ions near the active site that plays an important role in promoting ORR, in agreement with the experimental findings.

Based on the adsorption energy in Figure 5b, K⁺ ions prefer to locate near MnN₄, suggesting that K⁺ ions can form bonds with intermediates during the ORR. Figure 5f shows the free energy landscape for the ORR process on MnN₄/sp²-C, MnN₄/sp³-C with the K⁺ ion at site1 and site3, respectively. The rate-limiting step in the ORR is the hydrogenation of the oxygen molecule on MnN₄, with a barrier of 1.21 V. When the K⁺ ion is adsorbed on the sp³-C at site1, which is away from the MnN₄ active site, the overpotential increases to 1.25 V, similar to that in MnN₄/sp²-C. However, when the K⁺ ion is adsorbed at sp³-C on site3, which is closer to the MnN₄ active site, the overpotential decreases significantly to 1.07 V. This decrease is attributed to the weakening of the O−O bond by the adsorbed K⁺ ion. The O−O bond lengths of the *OOH intermediate on pristine MnN₄, MnN₄ with K adsorbed at site1, and MnN₄ with K adsorbed at site3 are 1.48 Å, 1.47 Å, and 1.51 Å, respectively. Therefore, the adsorbed K⁺ cation participates in the catalytic process during ORR, leading to an enhancement in catalytic activity.