English

• To address the challenges of increasing energy crisis, the exploration of efficient and sustainable energy technologies has become a top priority. Water electrolysis that is powered by surplus green electricity has emerged as a promising avenue for the large-scale generation of hydrogen fuel [1-6]. However, the anodic half-reaction normally has the sluggish kinetics, leading to the limited energy conversion efficiency of the conventional water electrolysis [7-9]. Recently, more efforts have been made to improve the efficiency of water electrolysis by coupling much thermodynamically favorable anodic reactions with cathodic hydrogen evolution reaction (HER) [10-13]. Among them, the electrocatalytic glycerol oxidation reaction (GOR) has attracted increasing attention due to the simultaneous advantages for the co-production of high value-added chemicals from by-product glycerol of biodiesel industry and green hydrogen from water [14-17]. However, the development of low-cost and highly efficient electrodes is of great significance for the large-scale application of this catalytic system.

  Although noble metal-based materials have high activity towards both HER and GOR, the dilemma is their high prices and low reserves [18,19]. In this regard, the earth-abundant 3d transition metal-based materials, such as nitrides, phosphides and alloys, have been emerging as ideal alternatives benefiting from their economical, high conductivity, tunable electronic structure and compositions [20-22]. For instance, a carbon shell-encapsulated Mn-CoN/NF electrode is prepared as bifunctional electrocatalyst, achieving a small overpotential of 31 mV at 10 mA/cm² for HER and ultralow potential of 1.37 Ⅴ vs. RHE at 400 mA/cm² for GOR [23]. Moreover, hierarchically structured CuNiP/CuO_x-VP, Ni₃N/Co₃N heterostructure, Ni-M-N nanosheets, N-doped carbon coated Ni-Mo-N/NF electrodes are also rationally synthesized as efficient electrocatalysts for the co-production of formate and H₂ [24-27]. Therefore, utilizing various regulation strategies to optimize the structure and composition of non-noble metal-based materials plays a critical role in improving the catalytic performance of glycerol upgrading with water electrolysis.

  Although molybdenum nitride (Mo₂N) is one of important materials for HER due to its robust stability and good adsorption energy of H* intermediate, its anodic GOR activity is unsatisfactory [28,29]. Therefore, it is highly desired to improve the catalytic performance of Mo₂N toward HER and GOR by reasonable composition and structural design. Herein, we develop a hierarchical heterostructural electrode by in-situ growing metallic Co mediated molybdenum nitride nanorods on nickel foam (Co@Mo₂N/NF), which exhibits promising catalytic activity and stability for both HER and GOR. Remarkably, the built HER||GOR electrolyzer by using Co@Mo₂N/NF as both the anode and the cathode realizes high Faradaic efficiency of formate and H₂ and good stability for glycerol-assisted water electrolysis. The optimized electronic structure and Gibbs free energy of reaction mediates on Co@Mo₂N heterostructure are confirmed by DFT calculation, while the formation of active catalytic materials and key reaction intermediates are revealed by systematic in-situ spectroscopy analysis.

  The synthetic process of Co@Mo₂N/NF hierarchical electrode is illustrated in Fig. S1 (Supporting information). Firstly, the CoMoO_x•0.9H₂O/NF precursor is obtained by hydrothermal reaction, and it is converted to Co@Mo₂N/NF by annealing at 500 ℃ in ammonia atmosphere. The crystal information for those materials is confirmed by X-ray diffraction (XRD) analysis. In Fig. S2 (Supporting information), the diffraction peaks of those prepared samples can match well with the standard materials of CoMoO_x•0.9H₂O (PDF#14–0086), Co (PDF#15–0806) and Mo₂ N (PDF#25–1368). For the Co@Mo₂N/NF sample, the XRD pattern exhibits the mixed composition of Co (PDF#15–0806), Co (PDF#05–0727) and Mo₂N (PDF#25–1368). Subsequently, transmission electron microscopy (TEM) image shows the morphology of nanorods with rough surface for Co@Mo₂N, which is different from the TEM images of Co and Mo₂N (Fig. 1a, Figs. S3 and S4 in Supporting information). The high-resolution TEM (HR-TEM) images of Co@Mo₂N exhibit the lattice fringes with a spacing of 0.203 nm and 0.200 nm, which can be assigned to the (111) plane of Co and (004) plane of Mo₂N, respectively (Figs. 1b and c). Besides, the energy dispersive X-ray spectroscopy (EDS) analysis suggests the uniform existence of Co, Mo, N elements in the whole Co@Mo₂N nanorod (Fig. 1d).

  Figure 1

  

  Figure 1.  (a) The TEM and (b, c) HRTEM images of Co@Mo₂N. (d) Element mapping images of Co@Mo₂N sample. The high-resolution XPS spectra of (e) Mo 3d, (f) Co 2p and (g) N 1s for the Co, Mo₂N and Co@Mo₂N samples.

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  X-ray photoelectron spectroscopy (XPS) analysis is further used to determine the chemical composition and valence states of the prepared samples [30-32]. The XPS survey of those three samples is shown in Fig. S5 (Supporting information) and confirm their different compositions. In Fig. 1e, the Mo 3d for the Co@Mo₂N/NF can be divided into three pairs of peaks. The peaks at 229.7, 232.2 and 235.2 eV can be assigned to Mo 3d of Mo³⁺, Mo⁴⁺and Mo⁶⁺ [33]. The high-resolution spectra of Co 2p for the Co@Mo₂N/NF can be deconvoluted into four peaks (Fig. 1f). The peaks located at 781.0 and 797.1 eV belong to Co 2P_3/2 and Co 2p_1/2 of Co²⁺, while the other two peaks with binding energies of 786.0 and 803.5 eV are satellite peaks [34]. As shown in the N 1s XPS spectrum, the two peaks at 395.8 and 397.6 eV can be attributed to Mo 3p and metal Mo-N bonds (Fig. 1g) [35]. Based on above result, the Co/NF, Mo₂N/NF and Co@Mo₂N/NF samples are successfully prepared.

  Linear sweep voltammetry (LSV) curves are used to study the catalytic performance of those synthesized electrodes in 1.0 mol/L KOH with and without 0.1 mol/L glycerol. As shown in Fig. 2a, comparing to the traditional OER process, the Co@Mo₂N/NF has a reduced potential of 320 mV at 100 mA/cm², indicating its intrinsic high catalytic activity for GOR. Subsequently, the catalytic product is further analyzed by ¹H nuclear magnetic resonance (¹H NMR). In Figs. S6 and S7 (Supporting information), the peaks of formate (8.4 ppm) can be observed from the ¹H NMR spectra after GOR electrolysis at different potentials. There are no C₃ or C₂ by-products, or their contents are below the detection line, indicating that the reaction intermetidates can be rapidly converted to formate product. Moreover, the Co@Mo₂N/NF realizes a maximum FE_formate of 95.03% at 1.35 Ⅴ vs. RHE (Fig. 2b) and high formate yield of 2.16 mmol h^-1 cm^-2 at 1.5 Ⅴ vs. RHE (Fig. 2c and Table S3 in Supporting information). In addition, the CoMoO_x•0.9H₂O/NF, Co/NF, Mo₂N/NF, and Co@Mo₂N/NF electrodes are also used for comparison. In Fig. S8 (Supporting information), the Co@Mo₂N/NF shows best catalytic performance for GOR among all prepared samples. The Co@Mo₂N/NF shows a small potential of 1.28 Ⅴ vs. RHE at a current density of 100 mA/cm², which is slightly lower than CoMoO_x•0.9H₂O/NF (1.40 Ⅴ vs. RHE), Co/NF (1.38 Ⅴ vs. RHE) and Mo₂N/NF (1.44 Ⅴ vs. RHE), indicating its improved electrochemical activity (Fig. 2d).

  Figure 2

  

  Figure 2.  (a) The LSV curves of Co@Mo₂N/NF in 1 mol/L KOH with and without 0.1 mol/L glycerol. (b) Faradaic efficiency and (c) yield of formate at different potentials. (d) Potential comparison of CoMoO_x•0.9H₂O/NF, Co/NF, Mo₂N/NF, and Co@Mo₂N/NF towards GOR at different current density. (e) Comparison of the Faradaic efficiency and yield of formate on Co/NF, Mo₂N/NF, and Co@Mo₂N/NF samples at 1.35 Ⅴ vs. RHE. (f) The Faradaic efficiency of formate during the consecutive cycling tests at 1.35 Ⅴ vs. RHE. (g) HER polarization curves and (h) Tafel slopes of CoMoO_x•0.9H₂O/NF, Co/NF, Mo₂N/NF, and Co@Mo₂N/NF. (i) The stability test of Co@Mo₂N/NF at −100 mA/cm².

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  Compared with CoMoO_x•0.9H₂O/NF (104 mV/dec), Co/NF (97 mV/dec) and Mo₂N/NF (131 mV/dec), the Co@Mo₂N/NF has a lower Tafel slope of 60 mV/dec, suggesting its enhanced reaction kinetics of GOR (Fig. S9 in Supporting information). This result is consistent with the result from the electrochemical impedance spectroscopy (EIS), where the Co@Mo₂N/NF has the smallest interface charger resistance (Fig. S10 and Table S1 in Supporting information) [36-40]. Furthermore, the comparison of FE_formate and yield of formate are performed on Co/NF, Mo₂N/NF, and Co@Mo₂N/NF electrodes. In Fig. 2e, the Co@Mo₂N/NF delivers the higher FE_formate and yield of formate than those of Co/NF and Mo₂N/NF at 1.35 Ⅴ vs. RHE, suggesting the promotion effect of Co@Mo₂N/NF heterostructure on catalytic performance. Notably, the Co@Mo₂N/NF also has superior catalytic stability for GOR, which can be demonstrated by consecutive cycles and chronoamperometry tests (Fig. 2f and Fig. S11 in Supporting information). Meanwhile, the influence of the concentration of glycerol on the GOR activity is also studied and the optimal concentration of glycerol is confirmed to be 0.1 mol/L (Fig. S12 in Supporting information).

  The HER performance of those samples is further evaluated. In Fig. 2g, the Co@Mo₂N/NF catalyst only requires an overpotential of 49 mV to achieve the current densities of 50 mA/cm², which is better than CoMoO_x•0.9H₂O/NF (288 mV), Co/NF (226 mV) and Mo₂N/NF (208 mV). Meanwhile, the Tafel slope of Co@Mo₂N/NF is 75 mV/dec (Fig. 2h), which is lower than CoMoO_x•0.9H₂O/NF (144 mV/dec), Co/NF (123 mV/dec) and Mo₂N/NF (106 mV/dec). Moreover, the lower charge transfer resistance of Co@Mo₂N/NF than that of the other catalysts in Nyquist plots has also been proven (Fig. S13 and Table S1 in Supporting information). Those results suggest that the Co@Mo₂N/NF has enhanced catalytic performance for HER benefiting from the formation of heterostructure (Fig. S14 and Table S2 in Supporting information). The electrochemical active surface area (ECSA) of those catalysts is obtained from the CV curves (Figs. S15 and S16 in Supporting information), and the ECSA normalized performance of Co@Mo₂N/NF is also higher than the other three electrodes, indicating its intrinsic high catalytic activity [41-44]. In addition, the robust stability of Co@Mo₂N/NF is also confirmed by 1000 cycling cycles and chronoamperometry tests (Fig. 2i and Fig. S17 in Supporting information). For the optimal synthesis conditions, the HER and GOR catalytic performance of a series of Co@Mo₂N/NF are performed in Fig. S18 (Supporting information), which indicates the annealing temperature of 500 ℃ is the optimal.

  In situ Raman spectroscopy is carried out during GOR process at various potentials (1.25–1.55 Ⅴ vs. RHE). Figs. 3a-d show the Raman characteristic peaks extracted from Co@Mo₂N/NF, Co/NF and Mo₂N/NF. Two distinct Raman peaks emerge at 454 and 555 cm⁻¹, and gradually increase with the rise of the applied potentials. These Raman signals belong to the E_g mode and A_1g modes of the Co-O vibrations in the formed CoOOH species [45]. Notably, the peak intensity of those two peaks is lower for Co@Mo₂N/NF than Co/NF, suggesting the rapid consumption of active CoOOH species in the glycerol oxidation process. No signals of CoO₂ species can be detected in the measured in-situ Raman spectra. Therefore, the electro-oxidation formation of CoOOH species should be acted as active materials and that further be reduced by glycerol to construct the catalytic cycle process (Fig. 3e). In addition, the generated formate product can be confirmed from the appearance of characteristic peak around 1353 cm^-1 [46]. The Co@Mo₂N/NF shows an earlier starting potential of 1.25 Ⅴ vs. RHE to produce formate than the other two catalysts, indicating its improved reaction kinetic towards the conversion of glycerol oxidation to formate [47]. The catalytic reaction pathway of GOR is further determined by in-situ FT-IR spectroscopy. The FT-IR spectra of Co@Mo₂N/NF within the potential range of 1.2–1.55 Ⅴ vs. RHE and the extracted characteristic peaks are shown in Fig. 3f. The peak at 1660 cm^-1 gradually intensifies with the increase of the potential, which is attributed to the formation of glyceraldehyde [48]. Meanwhile, the formation of -COOH and COO* intermediates can be observed at 1581 and 1535 cm^‒1 [49]. After 1.3 Ⅴ vs. RHE, the absorption bands of formate at 1381 and 1353 cm^‒1 become larger, suggesting that the terminal hydroxyl group of glycerol is first oxidized to aldehyde-based intermediates, and then further oxidized to formate product [50,51].

  Figure 3

  

  Figure 3.  (a) Potential-dependent in-situ Raman spectra and the extracted Raman characteristic peaks of (b) Co@Mo₂N/NF, (c) Co/NF and (d) Mo₂N/NF for GOR. (e) Schematic illustrations of the catalytic cycle process on the Co@Mo₂N/NF. (f) The in-situ FT-IR characteristic peaks of the Co@Mo₂N/NF samples for GOR. (g) The differential charge density redistribution in Co@Mo₂N. The calculated adsorption energy and structure of (h) H₂O molecule and (i) H* intermediates on Co and Co@Mo₂N.

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  The relationship between structure and activity is investigated by DFT calculations on Co (111) and Co (111) and Mo₂N (004) heterostructure. First of all, the charge density difference analysis confirms the strong interaction between Co and Mo₂N, with effective charge redistribution at interface (Fig. 3g). The strong charge redistribution is beneficial to improving the adsorption and activation of active species for better HER activity [52]. In addition, the initial water dissociation and adsorption of H* intermediate are critical for alkaline HER. For the Co and Co@Mo₂N, the calculated adsorption energy of H₂O molecule is shown in Fig. 3h. The Co@Mo₂N heterostructure has a negative value than the simple Co material, suggesting the significantly improved dissociation kinetics of water ionization [53]. On the other hand, the conversion kinetic of H* to final H₂ is also promoted on Co@Mo₂N heterostructure, which can be confirmed by the lower Gibbs free energy of H* adsorption in Fig. 3i [54]. Therefore, the formation of Co@Mo₂N heterostructure contributes to improving the water dissociation as well as the faster transformation of H* to H₂.

  Given the promising performance of Co@Mo₂N/NF towards HER and GOR, a membrane-free flow cell is constructed by using Co@Mo₂N/NF as the cathode and anode (Fig. 4a). The LSV curves (without IR correction) of the Co@Mo₂N/NF-based electrolyzer are recorded in 1.0 mol/L KOH electrolyte with and without adding 0.1 mol/L glycerol (Fig. 4b). The HER||GOR electrolyzer has a low voltage of 1.61 Ⅴ to reach a high current density of 50 mA/cm², which is much lower than that of HER||OER (1.87 Ⅴ), suggesting the significant improvement of energy saving by the substitution of GOR [55,56]. Moreover, the FE_formate reaches the highest value of 94.35% at a low voltage of 1.5 Ⅴ for HER||GOR electrolyzer, while the yield of formate increases with the elevating voltages and delivers the highest value of 0.87 mmol h^-1 cm^-2 at 1.65 Ⅴ (Figs. 4c and d). Meanwhile, the Faradaic efficiency of cathodic H₂ gas is also evaluated and demonstrates the high potential for sustained and stable hydrogen production with stable FE_H2 above 96% during a long-term electrolysis process (Fig. 4e). This constructed HER||GOR electrolyzer can operate stably at 50 mA/cm² for more than 120 h, suggesting the promising application potential towards the efficient glycerol upgrading with water electrolysis (Fig. 4f).

  Figure 4

  

  Figure 4.  (a) Schematic diagram of the membrane-free flow cell. (b) LSV curves of the Co@Mo₂N/NF-based electrolyzer in 1.0 mol/L KOH electrolyte with and without 0.1 mol/L glycerol. (c, d) Faradaic efficiency and yield of formate on Co@Mo₂N/NF under different potentials and (e) cathodic Faradaic efficiency of H₂ with continuous electrolysis. (f) The stability test of the Co@Mo₂N/NF-based electrolyzer at 50 mA/cm².

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  In summary, we have highlighted a Co@Mo₂N/NF heterostructural electrode towards GOR and HER by a two-step approach of hydrothermal and nitriding reaction. Benefiting from the optimized electronic structure, exposed catalytic active sites and reaction kinetics, the Co@Mo₂N/NF electrode displays higher intrinsic catalytic activity than the other control samples. Impressively, this kind of Co@Mo₂N/NF can be further used for a two-electrode electrolyzer and realizes the synchronous glycerol electro-oxidation with water electrolysis. The HER||GOR electrolyzer exhibits high Faradaic efficiency of formate (94.35% at 1.5 Ⅴ) and H₂ (over 96%), a high yield of formate (0.87 mmol h^-1 cm² at 1.65 Ⅴ), along with good catalytic stability within 120 h at 50 mA/cm². In situ Raman and FT-IR spectra confirm the formation of active CoOOH species as well as the key aldehyde-based intermediates, while DFT calculations reveal the effective charge redistribution at interface as well as the adsorption free energy of water and H* intermediate on Co@Mo₂N heterostructure, thus contributing to the superior catalytic performance for both GOR and HER.

  Declaration of competing interest

  All authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Yiming Guo: Investigation, Formal analysis, Data curation. Zhouhong Yu: Validation, Software, Resources. Bin He: Writing – original draft, Visualization, Funding acquisition. Pengzuo Chen: Supervision, Funding acquisition, Conceptualization.

  Acknowledgments

  This work is financially supported by the National Natural Science Foundation of China (No. 22205205), the Natural Science Foundation of Zhejiang Province (No. LQ24E040002), the Science Foundation of Zhejiang Sci-Tech University (ZSTU) (Nos. 21062337-Y, LW-YP2024076).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111010.

  
  
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  Figure 1  (a) The TEM and (b, c) HRTEM images of Co@Mo₂N. (d) Element mapping images of Co@Mo₂N sample. The high-resolution XPS spectra of (e) Mo 3d, (f) Co 2p and (g) N 1s for the Co, Mo₂N and Co@Mo₂N samples.

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  Figure 2  (a) The LSV curves of Co@Mo₂N/NF in 1 mol/L KOH with and without 0.1 mol/L glycerol. (b) Faradaic efficiency and (c) yield of formate at different potentials. (d) Potential comparison of CoMoO_x•0.9H₂O/NF, Co/NF, Mo₂N/NF, and Co@Mo₂N/NF towards GOR at different current density. (e) Comparison of the Faradaic efficiency and yield of formate on Co/NF, Mo₂N/NF, and Co@Mo₂N/NF samples at 1.35 Ⅴ vs. RHE. (f) The Faradaic efficiency of formate during the consecutive cycling tests at 1.35 Ⅴ vs. RHE. (g) HER polarization curves and (h) Tafel slopes of CoMoO_x•0.9H₂O/NF, Co/NF, Mo₂N/NF, and Co@Mo₂N/NF. (i) The stability test of Co@Mo₂N/NF at −100 mA/cm².

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  Figure 3  (a) Potential-dependent in-situ Raman spectra and the extracted Raman characteristic peaks of (b) Co@Mo₂N/NF, (c) Co/NF and (d) Mo₂N/NF for GOR. (e) Schematic illustrations of the catalytic cycle process on the Co@Mo₂N/NF. (f) The in-situ FT-IR characteristic peaks of the Co@Mo₂N/NF samples for GOR. (g) The differential charge density redistribution in Co@Mo₂N. The calculated adsorption energy and structure of (h) H₂O molecule and (i) H* intermediates on Co and Co@Mo₂N.

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  Figure 4  (a) Schematic diagram of the membrane-free flow cell. (b) LSV curves of the Co@Mo₂N/NF-based electrolyzer in 1.0 mol/L KOH electrolyte with and without 0.1 mol/L glycerol. (c, d) Faradaic efficiency and yield of formate on Co@Mo₂N/NF under different potentials and (e) cathodic Faradaic efficiency of H₂ with continuous electrolysis. (f) The stability test of the Co@Mo₂N/NF-based electrolyzer at 50 mA/cm².

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