Electronic configuration regulation of single-atomic Mn sites mediated by Mo/Mn clusters for an efficient hydrogen evolution reaction

Abstract

Tuning the electron distribution of metal single-atom active sites via bimetallic clusters is an effective way to enhance their hydrogen evolution reaction (HER) activity, but remains a great challenge. A biochar-based electrocatalyst (BC_MoMn800-2) with both MnN₄ active sites and Mo₂C/Mn₇C₃ clusters was synthesized using in situ enriched Mo/Mn biomass as a precursor to trigger the HER. Various characterization and density functional theory (DFT) calculation results indicated that the presence of Mo₂C/Mn₇C₃ clusters in BC_MoMn800-2 effectively induced the redistribution of charges at MnN₄ sites, reducing the energy of H* activation during the HER. In 0.5 M H₂SO₄, the overpotential was 27.4 mV at a current density of 10 mA cm⁻² and the Tafel slope was 31 mV dec⁻¹, and its electrocatalytic performance was close to that of Pt/C. The electrocatalyst also exhibited excellent electrocatalytic stability and durability. This work might provide a new strategy for solid waste recycling and constructing efficient HER electrocatalysts.

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Introduction

Excessive use of fossil fuels has caused climate change and energy crisis, which have aroused widespread concern.^1,2 To solve these problems, new renewable energy sources that are carbon-neutral are continuously being explored.³ Hydrogen (H₂), as one of the perfect substitutes for fossil fuels, has gradually become an indispensable part of the future sustainable and clean energy system due to its excellent properties in terms of energy density, sustainability and cleanliness.^4,5 Currently, the traditional hydrogen production processes are pyrolysis of fossil fuels and reforming of natural gas.^6,7 In contrast, the production of green H₂ by water electrolysis possesses various advantages and is of great significance for global carbon neutrality.^8,9 Moreover, compared with alkaline electrolytes, water exhibits higher current densities and energy conversion efficiencies in acidic electrolytes, which could enhance the efficiency of H₂ production in large-scale industries.^10,11 However, in such an environment of acidic corrosion or strong oxidation potential, most catalysts are prone to dissolve and fall off from the electrode, resulting in a significant decrease in their catalytic activity and stability, thus limiting their practical applications.¹² Therefore, the synthesis of high-efficiency catalysts that could achieve high current density and long-term stability is of great significance for the large-scale application of water electrolysis hydrogen production technology.

Traditional noble metal catalysts, such as Pt, exhibit excellent electrocatalytic performance for the hydrogen evolution reaction (HER), but inherent defects (high cost and limited reserves) severely hinder their availability in practical industrial applications.^13,14 Therefore, there is an urgent need to develop a non-precious metal-based HER electrocatalyst with high efficiency, acid resistance, and long-term stability. In particular, metal cluster/single-atom catalysts have received widespread attention due to the ability to use metal clusters to tune the electronic structure of single atoms.^15–17 Generally, the increase of the metal content could effectively improve the HER electrocatalytic performance of the catalyst, while the excess metal is prone to agglomeration during the pyrolysis process, thereby seriously reducing the HER activity of the catalyst.^18,19 Therefore, improving the intrinsic activity of a single active site is an effective way to enhance the electrocatalytic activity for the HER. It has been reported that carbides, such as molybdenum carbide²⁰ and manganese carbide,²¹ are superior HER electrocatalysts with excellent electrical conductivity, resistance to acid, and alkali environments, and exhibit excellent long-term HER stability. Specifically, carbide clusters could induce changes in the local electronic structure, and reduce the adsorption energy of H* and the energy barrier of H* activation by regulating the electron density of d orbitals and TDOS.^22,23 Besides, the introduction of metal single-atom sites could not only regulate the surrounding electronic structure but also serve as the adsorption sites of H*, which facilitate the adsorption/desorption of H* and reduce the energy barrier of the HER process.^24,25 Based on the above analysis, it was speculated that the coupling of metal clusters and single atoms might exhibit unexpected HER electrocatalytic properties. Most of the current studies mainly focus on the optimization of the active sites of single atoms and metal clusters, and there are a large number of reports on the synergistic effect between single atoms and metal clusters.^26–29 Non-noble metals including Mo and Mn have been proven to exhibit good performance in electrocatalytic hydrogen evolution. The pyrolysis of Mo and C in a reducing atmosphere could generate Mo_xC. Mo_xC has a d-band electronic structure similar to Pt, and its flexible controllability and high conductivity ensure its high activity and stability in a wide pH range, thus showing promise as an HER catalyst.^11,30 Mn single atoms are an efficient candidate in the research of manganese-based catalyst materials and could be used in many challenging catalytic reactions. The three-dimensional unoccupied orbitals in Mn single atoms could accommodate foreign electrons, thereby effectively reducing the bonding strength with H* intermediates during the HER process. In addition, the introduction of Mn single atoms in carbon-based catalysts could promote the formation of Mn–N–C and enhance HER catalytic activity and stability.¹⁵ Previous studies reported that the coupling of Mo_xC with metals or non-metals is an effective method to improve the kinetics and activity of the HER.³¹ There are relatively few studies on electrocatalysts with the synergistic effects of Mo and Mn doped together in biochar. How the coordination conditions or structures between Mo_xC and Mn single atoms affect the HER performance is still a question that needs to be explored. Therefore, it is necessary to further evaluate the HER performance of the catalyst formed by the two and explore its HER mechanism.

Herein, a novel biochar-based HER electrocatalyst (BC_MoMn800-2) with Mo₂C/Mn₇C₃ clusters and atomically dispersed MnN₄ was prepared from in situ enriched Mo/Mn biomass as a precursor. HAADF-STEM and X-ray absorption near-edge spectroscopy results demonstrate the formation of atomically dispersed MnN₄ single atoms and Mo₂C/Mn₇C₃ clusters. Density functional theory (DFT) calculations further elucidated the strong electronic interaction between Mo₂C/Mn₇C₃ clusters and single-atom Mn sites, as well as the electrocatalytic mechanism of the HER. According to the above structural features, BC_MoMn800-2 exhibited excellent electrocatalytic activity for the HER. In 0.5 M sulfuric acid, the overpotential was 27.4 mV at a current density of 10 mA cm⁻² and the Tafel slope was 31 mV dec⁻¹, thus the electrocatalytic performance was close to that of Pt/C. Moreover, excellent long-term stability and durability were exhibited. Overall, experiments and theoretical calculations confirmed that BC_MoMn800-2 with metal clusters and single atoms is a low-cost and promising electrocatalyst for the HER.

Results and discussion

Characterization of electrocatalysts

The evolution of the catalyst structure at different pyrolysis temperatures is shown in Fig. 1a. In this study, biomass precursors containing Mo and Mn were prepared by plant absorption and further pyrolyzed at 700, 800, and 900 °C to obtain BC_MoMn with a graphene-like structure and abundant defects. Notably, the preparation method of BC_MoMn precursors using plant-absorbed metal ions displayed the following advantages. (1) Unlike the conventional impregnation pyrolysis method, this preparation method was beneficial for the uniform distribution of metal elements in biochar. (2) Evenly distributed metal elements were beneficial to the regulation of the local microenvironment of the catalyst. (3) N absorbed during plant growth enhanced the formation of metal single atoms during pyrolysis.

Fig. 1 Catalyst synthesis and structural characterization. (a) Schematic illustration of catalyst structure evolution at different preparation temperatures. (b) XRD spectra of BC_MoMn700-2, BC_MoMn800-2, and BC_MoMn900-2. (c) FESEM, (d) TEM, (e) HRTEM, (f) dark-field HAADF-STEM, (g) corresponding EDS line-scan elemental of BC_MoMn800-2, and (h–l) EDS mappings of BC_MoMn800-2.

The crystalline phases of the synthesized catalysts were detected by X-ray diffraction (XRD). As illustrated in Fig. 1b and S1,† an obvious characteristic diffraction peak was observed at 26.4° in all samples, which matched with graphitic carbon (C, PDF#41-1487).²⁴ With the increase of pyrolysis temperature, the intensity of the diffraction peak gradually increased, indicating that high temperature could transform amorphous carbon into graphitic carbon under the action of Mn/Mo. More importantly, XRD results demonstrated the formation of Mo₂C and Mn₇C₃ in the synthesized samples. Characteristic diffraction peaks of Mo₂C (PDF#35-0787) at 2θ = 38.0, 39.4, 52.1, 61.5, 69.6, 74.6, and 75.5° are ascribed to the (002), (101), (102), (110), (103), (112), and (201) facets, respectively.³² Additionally, characteristic diffraction peaks of Mn₇C₃ (PDF#36-1269) at 2θ = 44.6, 45.3, 49.8, 50.3, and 52.8° are ascribed to the (211), (300), (301), (202), and (220) facets, respectively.³³ Notably, after increasing the temperature to 900 °C, the diffraction peak of Mn₇C₃ almost disappeared, which may be due to the fact that Mn₇C₃ clusters participated in the growth of the carbon layer during the high-temperature graphitization of biochar.³⁴ Combining XRD patterns analyzed from samples pyrolyzed at 600 °C (Fig. S1b†) and the literature,^35,36 it could be speculated that the Mo⁰/Mn⁰ formed by reduction at 600 °C acted as a catalyst to start promoting the formation of graphene-like structure. By increasing the temperature to 700–800 °C, the Mo⁰/Mn⁰ gradually transformed into Mo₂C/Mn₇C₃ and were gradually wrapped by graphene-like structure. By further raising the temperature to 900 °C, Mn₇C₃ clusters were gradually consumed and combined in the stable form of graphene-like structure and Mn₇C₃.

The microstructure of the synthesized samples was characterized by field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). FESEM results showed that the BC_MoMn800-2 sample possessed a smooth surface structure (Fig. 1c). Compared with the control biochar, the introduction of Mn/Mo did not induce obvious changes in the morphology of biochar. Additionally, TEM images further confirmed the smooth surface structure of BC_MoMn800-2 (Fig. 1d). These results indicated that the preparation method achieved a uniform distribution of Mn/Mo in biochar, thereby suppressing the agglomeration of metal nanoparticles during pyrolysis. The HRTEM image clearly exhibited the metal cluster size (only 0.5–1.5 nm) and lattice fringe properties on the biochar substrate (yellow circles and split lines highlighted, Fig. 1e). Notably, two kinds of lattice fringes could be observed with distinct interfaces (dashed yellow lines highlighted). According to the Fast Fourier Transform (FFT) of the cluster lattice fringes (Fig. 1e inset), lattice spacings of 0.23 and 0.18 nm were calculated, corresponding to the (003) crystal plane of Mo₂C and (004) crystal plane of Mn₇C₃, respectively. As reported, the preparation method of plant enrichment could achieve a uniform distribution of metal species in biochar rather than loading them on the surface.³⁷ Of note, obvious carbon stripes were observed at the edge of the lattice (Fig. 1e inset), suggesting that the Mo₂C/Mn₇C₃ clusters were wrapped by the carbon layer. The distance between close bright spots was 0.59 nm (Fig. 1e inset), which was significantly larger than the atomic diameter of Mo/Mn atoms (0.278/0.254 nm), suggesting that the Mo or Mn element was distributed as a single atom in BC_MoMn800-2.³⁸ The HAADF-STEM images clearly revealed metal single atoms (highlighted in red circles) and clusters (highlighted in yellow circles) on the biochar matrix (Fig. 1f), indicating the coexistence of single-atom dispersed M–N–C sites and metal clusters.³⁹ The positional relationship of Mo/Mn in metal clusters was further proved by EDS line-scan elemental of BC_MoMn800-2 (Fig. 1g), which was consistent with FFT analysis. Additionally, the energy-dispersive X-ray spectroscopy (EDS) images of BC_MoMn800-2 confirmed the uniform distribution of N, Mo, and Mn elements on the biochar. More details on electron microscopy characterization of other catalysts are provided in Fig. S2.†

The carbon structural defects of the synthesized catalysts were characterized by Raman spectroscopy. The peaks at 1350 (D band) and 1580 (G band) cm⁻¹ were ascribed to defective/disordered carbon and graphitic structures, respectively, while the ratio of D and G bands (I_D/I_G) could reflect the defect degree of carbon-based materials.⁴⁰ As reported, different metal crystalline phases could lead to differences in the structural defects of carbon layers.⁴¹ By calculation, the I_D/I_G values of BC_MoMn700-2, BC_MoMn800-2, and BC_MoMn900-2 were 1.00, 1.12, and 1.07 (Fig. 2a), respectively, indicating that more carbon defect structures were formed in BC_MoMn800-2, which were conducive to the adjustment of the surface electronic structure, thereby enhancing the interfacial electron exchange.⁴² Additionally, the loading amount of Mo/Mn also affected the structure of the carbon (Fig. S3 and Table S1†). The I_D/I_G values of BC_MoMn800-1 and BC_MoMn800-3 were 1.05 and 1.18, suggesting that carbon structural defects gradually increased as the content of Mo and Mn increased.

Fig. 2 Chemical composition characterization. The (a) Raman, (b) high-resolution Mo 2p, (c) Mn 2p, and (d) N 1s spectra of BC_MoMn700-2, BC_MoMn800-2, and BC_MoMn900-2. (e, g) Mo and (f, h) Mn K-edge XANES and Fourier transforms of the EXAFS spectra of BC_MoMn800-2. (i) Wavelet transforms for the k³-weighted EXAFS signals of Mo foil, Mn foil, and BC_MoMn800-2.

The surface elemental composition and chemical speciation of the synthesized catalysts were detected by X-ray photoelectron spectroscopy (XPS). XPS further confirmed the existence of C, O, N, Mo, and Mn elements on the catalyst's surface (Fig. S4a†). Specifically, all synthesized catalysts exhibited similar C 1s and O 1s XPS spectra (Fig. S4b and c†). As illustrated in Fig. 2b, the high-resolution N 1s spectra of BC_MoMn700-2 were fitted with four peaks at 403.1, 400.8, 400.0, and 398.4 eV, corresponding to oxidized-N, graphitic-N, pyrrolic-N, and pyridinic-N.^43,44 Notably, the high-resolution N 1s spectra of BC_MoMn800-2 and BC_MoMn900-2 were fitted with a Mn–N peak at 399.3 eV and 399.8 eV instead of the pyrrolic-N peak,⁴⁵ and the negative electron shift of 0.5 eV indicated that BC_MoMn800-2 possessed a higher N electron density than BC_MoMn900-2. Additionally, BC_MoMn800-2 exhibited a higher relative content of graphite-N (42.6%) and Mn–N (21.4%) than the others (in Table S2†), which was beneficial for the accelerated electron transfer during the HER, thereby increasing the current density.^46,47 The high-resolution Mo 3d spectra of BC_MoMn700-2 could be fitted with three groups of peaks at 228.0/231.0, 228.5/232.1, and 232.2/235.3 eV (Fig. 2c), corresponding to Mo₂C, MoO₂, and MoO₃,^31,48 where MoO₂ and MoO₃ may be caused by surface oxidation.^49,50 In particular, the Mo₂C peaks in BC_MoMn800-2 and BC_MoMn900-2 were positively shifted by 0.4 and 0.2 eV, indicating that Mo₂C on BC_MoMn800-2 exhibited a significantly electron-deficient feature than the other two. The high-resolution Mn 2p spectra of BC_MoMn700-2 could be fitted with two groups of peaks at 641.2/653.1 and 642.5/654.0 eV (Fig. 2d), which were ascribed to Mn²⁺ and Mn³⁺, respectively.⁴⁵ Similarly, the Mn 2p spectra of BC_MoMn800-2 and BC_MoMn900-2 exhibited a positive shift, especially BC_MoMn800-2 (0.3 eV), indicating the electron loss tendency on Mn sites.^51,52 According to the above results, it could be speculated that the Mn single atoms on the graphene-like structure could promote the electron transfer from the Mo₂C/Mn₇C₃ clusters to the graphene-like structure, thereby resulting in the strengthening of electronic metal–support interaction between the Mo₂C/Mn₇C₃ clusters and the carbon layer. This speculation was further confirmed by DFT calculations (Fig. S5–S7†).

The electronic structures and coordination environments of Mo and Mn species in BC_MoMn800-2 were detected by X-ray absorption fine structure spectroscopy (XAFS). As illustrated in Fig. 2e, the Mo k-edge X-ray absorption near-edge structure (XANES) spectrum showed that the absorption edge position of Mo in BC_MoMn800-2 was between Mo foil and MoO₂, indicating that the average valence state of Mo was between 0 and +4. Moreover, Fourier-transformed X-ray absorption fine structure (FT EXAFS) of Mo k-edge revealed obvious Mo–C and Mo–Mo coordination at 1.3 and 2.4 Å (Fig. 2g), further confirming the generation of Mo₂C in BC_MoMn800-2.⁵³ Additionally, the weak Mo–O (1.7 Å) coordination may be due to the surface oxidation of the sample in air. As illustrated in Fig. 2f, the Mn k-edge XANES spectrum of BC_MoMn800-2 was in the vicinity of Mn₂O₃, indicating that the valence state of Mn was around +3. In the analysis of Mn k-edge FT EXAFS (Fig. 2h), the peaks at 1.4 and 2.3 Å could be ascribed to Mn–C and Mn–Mn coordination, which were the key to prove the existence of Mn₇C₃.^33,54 More importantly, the Mn–N coordination at 1.7 Å was detected in FT EXAFS of BC_MoMn800-2.⁵⁵ As reported,¹⁶ due to the complexity of metal–nitrogen–carbon materials, the average coordination number between the metal and nitrogen could not directly prove the specific structure of Metal–N_x. The calculated average coordination number of Mn–N was about 2.0, which may be due to the reduction of the coordination number of N by the Mn atoms adjacent to the cluster heterojunction. Notably, Mo–Mn coordination was also detected at 2.1 Å in the FT EXAFS of both Mo and Mn k-edges, which could be due to the interfacial coupling of clusters formed by Mo₂C and Mn₇C₃. Combined with HAADF-STEM and XRD analysis, it was further confirmed that Mo₂C and Mn₇C₃ in BC_MoMn800-2 existed in the form of clusters instead of nanoparticles. The wavelet transform (WT) of the k³-weighted EXAFS spectrum could further reconfirm the above coordination structure (Fig. 2i). According to the above results, it was inferred that the possible structure of BC_MoMn800-2 was MnN₄ fixed on the graphene layer plane and Mo₂C/Mn₇C₃ clusters below the graphene layer. This structure contained 4 Mn–N bonds and 2 Mn atoms (Fig. 2i inset), so the average coordination number of Mn–N was about 2.0, which was consistent with the detection and calculation results. Mn–N also existed in BC_MoMn900-2, but not in BC_MoMn700-2 (Fig. S8†). It was speculated that Mn single atoms were generated when the pyrolysis temperature was greater than 700 °C.⁵⁶ Moreover, Mn single atoms could facilitate the electron transfer from the Mo₂C/Mn₇C₃ cluster to the graphene-like structure by DFT calculation, thereby enhancing the electronic metal–support interaction (Fig. S6†).

HER catalytic performances

To evaluate the HER electrocatalytic performance of the synthesized catalysts, electrochemical tests were performed in 0.5 M H₂SO₄ electrolyte. As illustrated in Fig. 3a, the linear sweep voltammetry (LSV) image revealed that BC_MoMn800-2 exhibited a higher current density than others, indicating its superior HER catalytic performance. Notably, at a current density of 10 mA cm⁻² (Fig. 3b), the overpotential of BC_MoMn800-2 was only 27.4 mV, which was close to that of Pt/C (26.4 mV), but much lower than that of BC_MoMn700-2 (309.4 mV), BC_MoMn800-1 (316.4 mV), BC_MoMn800-3 (335.4 mV), and BC_MoMn900-2 (201.4 mV). Additionally, even at current densities of 20, 50, and 100 mA cm⁻², BC_MoMn800-2 still exhibited smaller overpotentials than other catalysts. Subsequently, the HER kinetics of the catalysts were evaluated by calculating the Tafel slope (Fig. 3c). The Tafel slope of BC_MoMn800-2 was the smallest, only 31 mV dec⁻¹, which was much smaller than that of BC_MoMn700-2 (113 mV dec⁻¹), BC_MoMn800-1 (251 mV dec⁻¹), BC_MoMn800-3 (196 mV dec⁻¹), and BC_MoMn900-2 (80 mV dec⁻¹), and close to that of Pt/C (29 mV dec⁻¹). These results indicated that the HER process of the catalysts followed the Volmer–Heyrovsky mechanism, and the Tafel step was rate-limiting.⁵⁷ Electrochemical impedance spectroscopy (EIS) further revealed the interfacial electron transfer kinetics of the catalysts in the HER process (Fig. 3d), and the resistance of BC_MoMn800-2 was the smallest (only 27.38 Ω), indicating the fastest charge transfer at the interface with the electrolyte. Compared with some of the recently reported advanced Mo/Mn catalysts, BC_MoMn800-2 exhibited excellent overpotential and Tafel slope in the HER process compared with the recently reported advanced Mo/Mn-containing catalysts (Fig. 3e).

Fig. 3 Electrochemical HER performance in 0.5 M H₂SO₄ solution. (a) HER polarization curves, (b) comparison of overpotential changes, (c) the corresponding Tafel plots, and (d) electrochemical impedance spectroscopy of BC_MoMn700-2, BC_MoMn800-1, BC_MoMn800-2, BC_MoMn800-3, BC_MoMn900-2, and Pt/C. (e) Comparison of the Tafel slope and overpotential at 10 mA cm⁻² for BC_MoMn800-2 with other recently reported HER catalysts. (f) The differences in current density (Δj_1/2) at 0.118 V (vs. RHE) as a function of scan rate for HER catalysts. (g) The electrolysis stability test of BC_MoMn800-2 at different current densities in 0.5 M H₂SO₄.

To evaluate the abundance of active sites, the effective electrochemical surface area (ECSA) of the catalyst was detected by the electrochemical double-layer capacitance method (Fig. 3f and S9†). Among the synthesized catalysts, BC_MoMn800-2 possessed the largest double-layer capacitance of 34.6 mF cm⁻², indicating its larger ECSA and more abundant active sites than others. Overall, from the overpotential, Tafel slope, resistance, and ECSA, it could be obviously shown that BC_MoMn800-2 exhibited superior catalytic activity in the HER process. The HER performance of electrodes is not only related to ECSA, but also affected by electrode composition, active surface area, and hydrophilicity.⁵⁸ Among them, the hydrophilicity of the electrode surface is a key factor affecting the movement behavior of H₂ bubbles, thus determining the kinetic performance of the HER reaction. In principle, the formation of small uniform H₂ bubbles facilitates its desorption from the electrode without causing polarization of the electrode. The contact angles of BC_MoMn800-2 were 32.6, 27.1, 24.0, 19.8, and 14.3° at 0, 100, 200, 500, and 1000 ms, respectively, which exhibited excellent hydrophilicity (Fig. S10†). The optimal hydrophilicity of the BC_MoMn800-2 electrode might be due to its abundant mesopores, channels, and defect structures, which accelerated the separation of the generated H₂ bubbles and enabled the acidic electrolyte to reach the active catalytic sites.⁵⁹ From Fig. S11,† the electrode surface of BC_MoMn700-2 and BC_MoMn900-2 adsorbed a large number of large bubbles generated, resulting in the coverage of active sites on the catalyst surface, thereby reducing the mass transfer rate at the electrode/electrolyte interface. In contrast, the electrode surface of BC_MoMn800-2 generated uniform small bubbles and desorbed rapidly, exhibiting superior H₂ production efficiency. Therefore, electrodes with smaller contact angles were beneficial for the generation of small/uniform bubbles and their rapid desorption from the electrode surface, thereby maintaining the mass transfer rate at the electrode/electrolyte interface, the effectiveness and stability of electrode activity.⁶⁰

Cyclic voltammetry (CV) tests were performed to evaluate the electrochemical stability of BC_MoMn800-2 during the acidic HER process. After 1000 cycles, HER polarization curves changed slightly, indicating its superior electrochemical stability (Fig. S12†). Additionally, further characterization analysis of the tested materials (Fig. S13 and S14†), including SEM, TEM, XRD, etc., showed that the surface of BC_MoMn800-2 maintained good microstructure and crystal phase characteristics. These results indicated that BC_MoMn800-2 exhibited superior electrochemical stability during the HER process. Long-term electrolytic stability tests at different current densities were performed to further evaluate the application prospect in the actual process (Fig. 3g). During the continuous experiment of 60 h, even if different current densities were applied, the current density of the reaction system remained stable. Notably, at a current density of 10 mA cm⁻², the current density of the BC_MoMn800-2 system decreased by less than 10%, indicating its excellent electrochemical stability for acidic HER, showing broad application prospects.

To evaluate the HER electrocatalytic active sites of the synthesized catalysts, the turnover frequency (TOF) was calculated. As illustrated in Fig. S15,† at an overpotential of 250 mV, the BC_MoMn800-2 electrode exhibited a superior TOF value of 74.2 s⁻¹ compared to other electrodes, indicating that the BC_MoMn800-2 electrode possessed an excellent intrinsic HER electrocatalytic activity. Combined with characterization analysis, this may be due to the synergy between Mo₂C–Mn₇C₃ clusters and MnN₄. In detail, Mo₂C–Mn₇C₃ clusters on BC_MoMn800-2 provided electrons for H* activation, while MnN₄ on graphene was beneficial for electron aggregation and H* adsorption/desorption. It is known that the HER activity of catalysts is not only dependent on the type of active sites but also closely related to the number and size of active sites. Compared with BC_MoMn800-2, BC_MoMn800-1 exhibited a lower electrocatalytic activity for the HER, which may be attributed to the lower number of active sites due to lower metal loading.⁶¹ In contrast, BC_MoMn800-3 possessed a higher metal content (Table S1†), but its HER electrocatalytic activity was significantly lower than that of BC_MoMn800-2, which might be due to the agglomeration of Mo₂C–Mn₇C₃ clusters caused by the excessive metal loading, reducing the HER electrocatalytic activity of the catalyst. Moreover, from the SEM and HRTEM images (Fig. S2†), the particle size of BC_MoMn800-3 was significantly increased compared to that of BC_MoMn800-2, demonstrating that the increase of particle size suppressed the electrocatalytic activity of the HER, thus further proving the speculation.

Mechanism and theoretical calculation

To elucidate the mechanism of the superior HER activity of the synthesized catalysts from the atomic scale, DFT calculations were performed. Based on HAADF-STEM and XAFS characterization analysis, the atomic models of Mo₂C–Mn₇C₃/graphene, Mo₂C–Mn₇C₃/MnN₄-graphene, and Mo₂C/Mn₇C₃–MnN₄-graphene were constructed and optimized (Fig. 4a and S5†), corresponding to BC_MoMn700-2, BC_MoMn800-1, and BC_MoMn900-2, respectively. As reported,⁶² a positive value of the Gibbs free energy of H* adsorption indicated that the HER catalyst exhibited poor adsorption of H*, while a negative value might result in difficulty in the desorption of the generated H₂ molecules. In acidic systems, among Mo₂C–Mn₇C₃/graphene, Mo₂C–Mn₇C₃/MnN₄-graphene, and Mo₂C/Mn₇C₃–MnN₄-graphene, Mo₂C–Mn₇C₃/MnN₄-graphene exhibited the best Gibbs free energy of H* adsorption (ΔG_H* = −0.03 eV, Fig. 4b), which was almost close to the ideal value of 0, indicating its excellent H* adsorption/desorption kinetics.⁶³ Notably, from the charge density difference diagram, the Mo₂C–Mn₇C₃ heterostructure formed in Mo₂C–Mn₇C₃/MnN₄-graphene exhibited a large local lattice distortion, resulting in changes in the local electronic structure, thereby increasing the activity of electrons. The Bader charge analysis further demonstrated the superior electron donating ability of the Mo₂C–Mn₇C₃ heterojunction (−0.293 e) compared to Mo₂C (−0.197 e, Fig. S6†), and electrons were transferred from Mo₂C–Mn₇C₃ to graphene, which was consistent with XPS analysis. Moreover, the density of states (DOS) at the Fermi level of the metal d orbitals in the Mo₂C–Mn₇C₃ heterostructure was significantly weakened (Fig. S7†), thereby resulting in a weakened adsorption capacity. Besides, the MnN₄ on graphene enhanced the charge transfer from Mo₂C–Mn₇C₃ to the graphene layer.

Fig. 4 DFT calculations. (a) Optimized atomic structures of Mo₂C–Mn₇C₃/graphene, Mo₂C–Mn₇C₃/MnN₄-graphene, and Mo₂C/Mn₇C₃–MnN₄-graphene. (b) The Gibbs free energy diagrams of the HER in an acid electrolyte at the equilibrium potential (inset: DFT calculations of H⁺ adsorption on different substrates). Calculation of (c) Bader charge, adsorption energies, (d) COHP, and (e) PDOS for H* adsorption on Mo₂C–Mn₇C₃/graphene, Mo₂C–Mn₇C₃/MnN₄-graphene, and Mo₂C/Mn₇C₃–MnN₄-graphene. (f) Schematic diagram of the HER mechanism.

Moreover, the electron transfer of different models during the H* adsorption process was investigated. H atoms adsorbed on Mo₂C–Mn₇C₃/MnN₄-graphene exhibited a smaller charge transfer of −0.148 e than Mo₂C–Mn₇C₃/graphene and Mo₂C/Mn₇C₃–MnN₄-graphene, corresponding to the smaller adsorption energy (E_ads) of −0.227 eV (Fig. 4c). The charge transfer of H atoms exhibited a significant linear relationship with E_ads (R² = 0.98, Fig. S16†), indicating that the low charge transfer on hydrogen atoms resulted in the weaker adsorption of H*, thereby accelerating the desorption of H*.

To evaluate the contribution of the bonding and antibonding states of H uptake at the Mo₂C–Mn₇C₃/MnN₄-graphene interface, the projected crystal orbital Hamilton population (pCOHP) analysis was performed.⁶⁴ The integral COHP (ICOHP) of Mo₂C–Mn₇C₃/MnN₄-graphene was maximum (−0.54 vs. −8.08 of Mo₂C–Mn₇C₃/graphene, −1.48 of Mo₂C/Mn₇C₃–MnN₄-graphene) in all models (Fig. 4d), corresponding to a moderate bonding strength, which was more conducive to the dissociation of H*. Moreover, the overlapping peaks of the H partial density of states on Mo₂C–Mn₇C₃/MnN₄-graphene were more pronounced than Mo₂C–Mn₇C₃/graphene and Mo₂C/Mn₇C₃–MnN₄-graphene in the PDOS calculations (Fig. 4e), indicating that it was closer to the Fermi level. So, it was more favorable for the activation of H atoms. Overall, these theoretical results indicated the synergistic effect between Mo₂C–Mn₇C₃ clusters and MnN₄ to accelerate the adsorption/dissociation of H*, resulting in its excellent HER performance in acidic solution. Correspondingly, a schematic diagram of the HER mechanism is proposed, as shown in Fig. 4f.

Conclusion

In summary, Mo/Mn biochars (BC_MoMn) were prepared from in situ enriched Mo/Mn biomass as precursors at different pyrolysis temperatures, especially at 800 °C. The synthesized BC_MoMn800-2 exhibited excellent HER electrocatalytic activity. In an acidic electrolyte, the overpotential of BC_MoMn800-2 was 27.4 mV at a current density of 10 mA cm⁻², and the Tafel slope was 31 mV dec⁻¹, which was close to that of Pt/C. This was ascribed to the unique properties arising from the interaction between MnN₄ and Mo₂C/Mn₇C₃ clusters in BC_MoMn800-2. DFT calculations revealed that the heterojunction of Mo₂C/Mn₇C₃ clusters in BC_MoMn800-2 induced changes in the local electronic structure and charge redistribution of the clusters. It was an indirect electron transfer, resulting in a decrease in charge at the MnN₄ site and an increase in electron density around C–N. Moreover, the presence of Mo₂C/Mn₇C₃ clusters reduced the energy required for H* activation and indirectly facilitated the desorption of H*. Crucially, the optimized electronic structure of MnN₄ sites facilitated the adsorption/desorption of H*, thereby promoting the kinetic process of the HER. These results indicated that BC_MoMn800-2 exhibited superior electrocatalytic performance for the HER to that of BC_MoMn700-2 and BC_MoMn900-2, which might be mainly derived from the synergistic effect of MnN₄ and Mo₂C/Mn₇C₃ clusters rather than the pure increase of metal content. In this study, we successfully synthesized an efficient and stable HER electrocatalyst, clarified the synergy and catalytic mechanism between clusters and single atoms, and provided a new idea for solid waste recycling and the synthesis of electrocatalysts.

Experimental section

Materials

Anhydrous ethanol (AR, 99.7%) and Nafion117 solution (5 wt%) were purchased from Aladdin Holdings Group Co., Ltd. Hoagland nutrient solution was purchased from Fuzhou Feijing Biological Technology Co., Ltd. Sodium molybdenum oxide (Na₂MoO₄, 99%), manganese chloride (MnCl₂, 99%) and platinum on carbon (Pt/C, 20%wt) were obtained from Meryer (Shanghai) Chemical Technology Co., Ltd. Sulfuric acid (H₂SO₄, AR) was purchased from Beijing Chemical Co., Ltd. All chemicals were of analytical grade and used as received without further purification.

Synthesis of biochar electrocatalysts in situ doped with Mo₂C and single-atomic Mn

The source of electrocatalysts: Reed seedlings were bought with the same biological quality, the roots of reed seedlings were rinsed with running water, and then fixed with white stones into buckets. Subsequently, they were cultured with Hoagland nutrient solution, Na₂MoO₄ and MnCl₂ (300, 500, and 800 mg L⁻¹), and changed every week to ensure the concentration of Mo and Mn ions, which would last for three months. The reed roots were washed with deionized water several times after the completion of growth, and the biomass containing Mo and Mn ions was obtained by drying for 24 h. The reeds were crushed with a grinder for 5 min before use.

Synthesis of Mo₂C, Mn₃C₇, and single-atomic Mn nanostructures: The required electrocatalysts were obtained by the pyrolysis of biomass in a vacuum tube furnace at 700, 800, and 900 °C for 2 h, the pyrolysis rate was 3 °C min⁻¹. The obtained biochar was stirred in 1 M H₂SO₄ for 24 h and then pumped and filtered to obtain a black solid, which was washed with deionized water several times until it approached the pH of deionized water. After that, the extracted biochar was dried for 48 h at a temperature of 80 °C in an oven and marked as BC_Mo/Mn700-2, BC_Mo/Mn800-1, BC_Mo/Mn800-2, BC_Mo/Mn800-3, and BC_Mo/Mn900-2. The numbers 1, 2, and 3 represent the increase of metal content in catalysts. Among them, the contents of Mo and Mn in BC_MoMn800-2 were detected by inductively coupled plasma optical emission spectrometry (ICP-OES) to be 6.53 and 8.61 wt%, respectively (Table S1†).

Characterization

Scanning electron microscopy (SEM, ZEISS GeminiSEM 300, Germany) and transmission electron microscopy (TEM, FEI Talos F200X, America) were used to record the microscopic morphology of the catalysts. X-ray Diffraction (XRD, Bruker D8 Advance, Germany) was used to analyze the crystal structure of the catalysts. A contact angle/surface tension meter (Lauda Scientific LSA100, Germany) was used to detect the hydrophilicity and hydrophobicity of the catalysts. The surface elemental composition and valence state of the catalysts were determined by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, America). Raman spectroscopy (Raman, Horiba LabRAM HR Evolution, Japan) was used to analyze the degree of graphitization of catalysts. Metal elements including Mo and Mn were detected by inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent 5110, America). The X-ray absorption fine structure spectra (XAFS) at the Mo/Mn K-edge of electrocatalysts were recorded at beamlines BL14W1 and BL11B in the Shanghai Synchrotron Radiation Facility (SSRF). Electrochemical measurements are provided in Text S1 of the ESI.†

Theoretical calculations

Spin-polarized DFT calculations were performed using the Vienna ab initio simulation package (VASP). The generalized gradient approximation proposed by Perdew, Burke, and Ernzerhof (GGA-PBE) was selected for the exchange–correlation potential. The pseudo-potential was described by the projector-augmented-wave (PAW) method. The geometry optimization was performed until the Hellmann–Feynman force on each atom was smaller than 0.02 eV Å⁻¹. The energy criterion was set to 10⁻⁶ eV in the iterative solution of the Kohn–Sham equation.

Data availability

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

Author contributions

Chengyu Zhang: conceptualization, validation, methodology, investigation, writing-original draft. Xiangyang Wang, Renyuan Zhao: methodology, validation, investigation, writing-review & editing. Fabrice Ndayisenga: supervision, writing-review & editing. Zhisheng Yu: conceptualization, supervision, writing-review & editing, funding acquisition.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This investigation was funded by the Binzhou Institute of Technology, (GYY-DTFZ-2022-003), the Fundamental Research Funds for the Central Universities (E2E40503X2) and the Science and Technology Service Network Initiative Project of the Chinese Academy of Sciences (KFJ-STS-QYZX-112).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3sc06053e

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