Regulating the spin density of Co^III using boron-doped carbon dots for enhanced electrocatalytic nitrate reduction

Abstract

Controlling the electron spin state on the catalyst surface can regulate the reaction activity, rate, and selectivity of surface reactions. However, there are still many challenges in exploring new strategies for electron spin regulation and mechanisms of the electron spin effect. Herein, we reported a method of using BCDs loading to change the electron spin density of magnetic Co₃O₄. Octahedral Co³⁺(O_h)–O with a t_2g⁶e_g⁰ configuration transformed into Co²⁺(O_h)–O with a t_2g⁵e_g² configuration after BCDs loading showed excellent activity in the electrocatalytic nitrate reduction reaction, achieving a maximum NH₄⁺ Faradaic efficiency of 94.6 ± 0.9% and high stability. Experimental results and theoretical calculations have found that high activity is attributed to the spin density regulation changing the rate-determining step and reducing the energy it needs to overcome. This work expands the spin density regulation methods and provides a new perspective for understanding the mechanisms of carbon dot materials in electrocatalytic reactions.

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1. Introduction

In electrocatalytic reactions, changes in the electron spin can affect the chemical properties and reactivity of surface atoms or molecules on the catalyst, thereby affecting the rate and selectivity of the reaction.^1,2 For example, on the metal surface, the change of electron spin will affect the electron cloud density and binding energy of surface atoms, thus affecting the interaction and reactivity between atoms and reactants.³ Secondly, the electron spin effect has an important impact on the reaction mechanism and reaction kinetics of catalysts. For example, in some catalytic reactions, electron spin can affect the generation and conversion of reaction intermediates, thereby affecting the rate and selectivity of the reaction.⁴ At the same time, electron spin can also affect the stability and activation energy of the transition state of the reaction, thus affecting the dynamic characteristics and thermodynamic stability of the reaction.⁵ Overall, there is a close relationship between electron spin and surface catalysis. By controlling the electron spin state on the catalyst surface, the reaction activity, rate, and selectivity of surface reactions can be regulated, providing new ideas and methods for preparing efficient and highly selective electrocatalysts.^6,7

Researchers have proposed some electron spin regulation strategies, which can achieve high efficiency and selectivity of the catalyst by adjusting the state of electron spin in the catalyst.^8–10 One strategy is to regulate electron spin through external electromagnetic fields.¹¹ Researchers used external magnetic fields to regulate the state of electron spin in the catalyst, thereby controlling the catalytic efficiency and selectivity of the catalyst.¹² Another strategy is to achieve the regulation of electron spin through metal doping or alloying.¹³ In this strategy, researchers doped other metal or semiconductor materials into the catalyst, thereby changing the state of electron spin in the catalyst and achieving the regulation of catalytic efficiency and selectivity.¹⁴ These electron spin control strategies can effectively influence the properties of catalysts and improve the performance in applications.¹⁵

Herein, we reported the discovery of a new method for altering the electron spin density using non-metallic composites. Spin density modulation of Co-based spinel oxide (Co₃O₄) can be achieved by loading B-doped carbon dots (BCDs), that is octahedral Co³⁺(O_h)–O with a t_2g⁶e_g⁰ electron configuration will be transformed into Co²⁺(O_h)–O with a high spin t_2g⁵e_g² configuration. The spin density-regulated BCDs/Co₃O₄/CC catalyst exhibited excellent activity in the electrocatalytic nitrate reduction reaction (NO₃RR) for ammonia synthesis,^16,17 achieving a maximum NH₄⁺ Faradaic efficiency (FE) of 94.6 ± 0.9% at −1.3 V vs. SCE and high stability. This work demonstrates the feasibility of adjusting the spin density using BCDs, displaying the ability of carbon dots to change the catalytic performance by affecting metal spin states and providing new strategies for expanding the application and universality of carbon dot materials.

2. Results and discussion

2.1. Morphology and structural characterization

Before preparing the precursor catalyst, BCDs were synthesized from C₆H₉BClNO₂ using a hydrothermal method.¹⁸ The detailed synthesis steps are described in the ESI.† To verify the successful preparation of BCDs, we performed transmission electron microscopy (TEM) characterization. The TEM morphological image in Fig. S1† shows that BCDs have a typical shape of quantum dots with an average diameter of around 4.1 nm. X-ray diffraction (XRD) measurements in Fig. S2† displays an obvious broad diffraction peak centered at 22.8°, which corresponds to the (100) crystal plane of graphitized carbon in BCDs.¹⁹ Various related functional groups (C–C, C=C, C–N, B–O, and O–H) on the surface of BCDs were detected using Fourier transform infrared spectroscopy (FT-IR, Fig. S3†).²⁰ Additionally, X-ray photoelectron spectroscopy (XPS) was used to further understand the chemical structure and elemental composition of BCDs. As shown in Fig. S4,† the survey spectrum displayed that the BCDs were mainly composed of C, N, O and B. In the high resolution C 1s, N 1s, O 1s and B 1s spectra, the assignment of different functional groups corresponds well with the FT-IR results.²¹ Based on the above results, we can confirm the successful preparation of BCDs.

The heterostructured BCDs/Co₃O₄/CC electrocatalyst was prepared by a two-step process as illustrated in Fig. 1a. First, a precursor catalyst BCDs/Co(OH)_x/CC supported on carbon cloth was synthesized through a hydrothermal method. The scanning electron microscopy (SEM) image in Fig. 1b shows the morphology of nanowires grown on carbon cloth fibers. Then, the as-prepared precursor catalyst was annealed at 450 °C for 6 h under an Ar atmosphere to obtain the BCDs/Co₃O₄/CC electrocatalyst. The SEM image of BCDs/Co₃O₄/CC in Fig. 1c still displays the morphology of nanowires without significant surface changes, which is similar to the changes in the control sample of Co₃O₄/CC (Fig. S5†). In order to further analyze the surface morphology and structure of BCDs/Co₃O₄/CC, we conducted TEM characterization. As shown in Fig. 1d, the primary structure of the nanowire morphology is nanoparticles with a diameter of 50–100 nm, which is connected one by one. Such a morphology is conducive to exposing more active sites than smooth nanowires. The high-resolution TEM (HRTEM) image shows that the sample has two distinct regions (Fig. 1e). One is the inner region with a highly crystalline lattice. The lattice spacing is 0.24 nm, corresponding to the (311) crystal plane of Co₃O₄.^22,23 The other region is a carbonized surface layer, which is formed by the BCDs at high annealing temperatures. In addition, we analyzed the composition and distribution of elements in the sample, and the results shows that Co, O, C, and B were evenly distributed on the catalyst (Fig. 1f).

Fig. 1 Synthesis and characterization of BCDs/Co₃O₄/CC. (a) Schematic illustration of the synthesis of BCDs/Co₃O₄/CC. (b) SEM image of the precursor catalyst. (c–e) SEM, TEM and HRTEM images of BCDs/Co₃O₄/CC. (f) Element mapping images of BCDs/Co₃O₄/CC.

As a comparison, we also prepared Co₃O₄/CC using the same method without adding BCDs as a reactant. The XRD patterns of BCDs/Co₃O₄/CC, Co₃O₄/CC and CC are shown in Fig. S6a.† The diffraction peaks centered at 31.3, 36.9, 59.5 and 65.3° correspond to the Co₃O₄ phase (JCPDS No. 73-1701),²⁴ while the diffraction peak of graphitized carbon in the BCDs is covered by the broad diffraction peak of carbon cloth in the range of 20–30°. Raman tests were also used to analyze the structure of the three samples (Fig. S6b†). CC showed characteristic D and G peak signals at 1355 and 1601 cm⁻¹, respectively. After loading Co₃O₄ on CC, additional signals caused by Co₃O₄ appear in the region of 400–700 cm⁻¹.²⁵ Furthermore, XPS was used to analyze the element composition and surface chemical states of the catalyst. The survey spectrum of BCDs/Co₃O₄/CC shows the presence of Co, C, O, and B in the catalyst (Fig. S7†). The high-resolution Co 2p XPS spectra of BCDs/Co₃O₄/CC and Co₃O₄/CC in Fig. 2a show two main peaks (denoted by grey) at 781.6 and 797.1 eV, which could be ascribed to the Co 2p_3/2 and Co 2p_1/2 of Co²⁺ species.²⁶ The other two peaks denoted by red can be attributed to Co³⁺ species. In addition, the shake-up process of Co²⁺ species in high spin states leads to the appearance of two adjacent satellite peaks centered at 787.4 and 802.9 eV.¹⁴ The proportion of Co³⁺ and Co²⁺ species in Co₃O₄/CC was calculated to be 62.1% and 37.9% (Fig. 2b), respectively, while this proportion changed to 34.4% and 65.6% in BCDs/Co₃O₄/CC (Fig. 2c). This result indicates an increased proportion of Co²⁺ species as a function of BCDs loading. In addition, the O 1s XPS spectra display peaks at 533.1, 530.9 and 529.3 eV due to C=O species, surface –OH groups and the lattice oxygen of Co–O in Co₃O₄, respectively (Fig. 2d).²⁷ Notably, the characteristic peak of B–O centered at 531.8 eV in BCDs/Co₃O₄/CC can be observed, further indicating the successful loading of BCDs. Moreover, the B 1s XPS spectrum of BCDs/Co₃O₄/CC also confirm the above results (Fig. 2e).²⁸

Fig. 2 Surface property characterization. (a) XPS spectra of the Co 2p region of the Co₃O₄/CC and BCDs/Co₃O₄/CC. (b and c) Calculated ratio of Co³⁺ and Co²⁺ in Co₃O₄/CC and BCDs/Co₃O₄/CC, respectively. (d) O 1s region of Co₃O₄/CC and BCDs/Co₃O₄/CC. (e) B 1s region of BCDs/Co₃O₄/CC. (f) Crystal structure of Co₃O₄. (g) Scheme of the valence electron configurations and spin states of octahedral Co influenced by the BCDs.

On the basis of the above analysis, the valence electron configurations of Co with and without BCDs in Co₃O₄ are identified. Typically, as illustrated in Fig. 2f, Co₃O₄ has two types of Co–O species, that is, octahedral Co³⁺(O_h)–O coordinated by six oxygen atoms (denoted by grey) and tetrahedral Co²⁺(T_d)–O coordinated by four oxygen atoms (denoted by purple).²⁹ The theoretical proportion of Co²⁺ in Co₃O₄ inferred from the above crystal structure is 33.3%, which is close to the Co²⁺ proportion of 37.9% in Co₃O₄/CC. However, this proportion underwent a significant change to 65.6% after the loading of BCDs, indicating that the BCDs can act as an electron bridge to accelerate electron transfer from BCDs to Co³⁺(O_h), forming the Co²⁺(O_h) species. The valence electron configuration of Co³⁺(O_h) in Co₃O₄/CC is t_2g⁶e_g⁰, while Co²⁺(O_h) in BCDs/Co₃O₄/CC possesses a t_2g⁵e_g² configuration with three unpaired electrons, as illustrated in Fig. 2g.³⁰ Specifically, the π-symmetry t_2g of Co³⁺(O_h) is fully occupied, which would result in electron repulsion between Co³⁺ and the reactant.⁷ As for the unpaired electron in Co²⁺(O_h), π-donation via O–Co–O would be strengthened by BCDs loading. In summary, the BCDs can alter the electronic structure of Co₃O₄ and regulate the spin state of Co³⁺(O_h) to active Co²⁺(O_h) species.

2.2. Electrocatalytic performances

The spin density-regulated BCDs/Co₃O₄/CC electrocatalyst was applied for nitrate reduction reaction measurements. We used a typical H-type three electrode reactor for performance testing, where 50 mL of 0.5 M K₂SO₄ and 0.5 M K₂SO₄ + 200 ppm NO₃⁻–N electrolyte were added to the anode and cathode chambers, respectively.^31–33 As shown in Fig. 3a, there is no significant difference between the linear sweep voltammetry (LSV) curves of CC before and after the addition of nitrate, which indicates that the influence and contribution of the substrate material to the reaction performance are basically zero. As for the BCDs/Co₃O₄/CC and Co₃O₄/CC electrocatalysts, the current density of the LSV curves significantly increased after the adding of nitrate. Meanwhile, BCDs/Co₃O₄/CC shows a higher current density compared to Co₃O₄/CC. Additionally, the catalytic performance of BCDs/CC was also tested (Fig. S8†), and the low current density indicated that the activity of single-component BCDs was weak in electrocatalytic nitrate-to-ammonia conversion. The above results revealed that nitrate participates in the reaction and the spin density of Co₃O₄ regulated by BCDs effectively improves the performance of the reaction.³⁴ Then, we tested nitrate reduction at fixed potentials, and measured the concentration of each ion in the electrolyte after the reaction using ion chromatography (IC, Fig. S9–S11†).^35,36 As displayed in Fig. 3b, with the increase of applied potentials, the FE of NH₄⁺ exhibits a volcanic trend of first increasing and then decreasing, and can reach a maximum FE of 94.6 ± 0.9% at −1.3 V vs. SCE. Furthermore, the NH₄⁺ generation rate gradually increases with the lowering of potentials, reaching 193.3 μmol h⁻¹ cm⁻² at −1.4 V vs. SCE (Fig. 3c).

Fig. 3 Electrocatalytic nitrate reduction performances. (a) LSV curves of BCDs/Co₃O₄/CC, Co₃O₄/CC, and CC. (b and c) NH₄⁺ FEs and NH₄⁺ yield rates of BCDs/Co₃O₄/CC at different applied potentials. (d and e) Stability test (repeated for four times) and the corresponding NH₄⁺ FEs of BCDs/Co₃O₄/CC at a potential of −1.3 V vs. SCE. (f) ¹H NMR spectra of electrocatalytic nitrate-to-ammonia conversion on BCDs/Co₃O₄/CC at −1.3 V vs. SCE by using ¹⁴NO₃⁻ and ¹⁵NO₃⁻ as the nitrogen source.

In order to determine whether the spin density-regulated BCDs/Co₃O₄/CC electrocatalyst can work effectively for a long time, we conducted stability tests at −1.3 V vs. SCE.³⁷ As shown in Fig. 3d and e, under four consecutive cyclic tests, the FE of NH₄⁺ and the current density remained basically unchanged, indicating that the BCDs/Co₃O₄/CC electrocatalyst can maintain good stability during the electrocatalytic process. In addition, to confirm that the ammonia generated during the reaction process comes from the reduction of nitrate, we used isotope labeling experiments to detect the source of nitrogen in the ammonia.^38–40 Under the same reaction conditions, ¹⁵NO₃⁻ was used to replace the original ¹⁴NO₃⁻ as the reactant, and the ammonia was generated by a continuous reaction at −1.3 V vs. SCE. When ¹⁴NO₃⁻ was used, the ¹H nuclear magnetic resonance (NMR, 600 MHz) spectrum showed a typical triple peaks for ¹⁴NH₄⁺. When ¹⁵NO₃⁻ was used, the ¹H NMR spectrum displayed a double peaks for ¹⁵NH₄⁺ (Fig. 3f).⁴¹ Therefore, we determined that the NH₄⁺ obtained by the reaction was reduced from the added NO₃⁻, excluding the interference of the reaction system or other nitrogen-containing compounds.

2.3. Density functional theory (DFT) calculations

DFT calculations were utilized to further understand the activity of BCDs/Co₃O₄/CC for the electrocatalytic NO₃RR.⁴² First, we calculated the direction and amount of charge transfer on the composite catalyst using Bader charge analysis. As shown in Fig. 4a, the B atoms will transfer 1.76 and 1.82 electrons to the adjacent metal Co on the BCDs/Co₃O₄/CC catalyst. Therefore, BCDs can serve as a bridge for electron transfer from B atoms to the octahedral Co³⁺(O_h), forming active high-spin Co²⁺(O_h) with a t_2g⁵e_g² configuration.⁴³ Furthermore, thecompetitive hydrogen evolution reaction is the main reason for the decrease in the FE of ammonia in the electrocatalytic NO₃RR.⁴⁴ Therefore, we also calculated the adsorption energy for the interaction between nitrate and H⁺ on the catalyst surface. As displayed in Fig. 4b, the adsorption energies for nitrate on BCDs/Co₃O₄/CC and Co₃O₄/CC are 3.00 and 2.47 eV, respectively, indicating that BCDs/Co₃O₄/CC has a stronger adsorption effect on nitrate. This is beneficial for the adsorption and occupation of nitrate on the catalyst surface and improves the catalytic reaction performance. Meanwhile, the adsorption energies for H⁺ on BCDs/Co₃O₄/CC and Co₃O₄/CC were 0.32 and 0.19 eV, respectively, much lower than the adsorption energies of nitrate. This indicates that the hydrogen evolution reaction can be effectively suppressed on the surfaces of these two catalysts, which is consistent with the high FE of the test results.

Fig. 4 Mechanism investigation. (a) Charge density difference and Bader charge of BCDs/Co₃O₄. The yellow and blue electronic clouds indicate charge accumulation and depletion. (b) The adsorption energy for NO₃⁻ and H⁺ on Co₃O₄ and BCDs/Co₃O₄. Insets show the charge-density difference for NO₃⁻ adsorption. (c) Free-energy diagrams of nitrate reduction to ammonia on the surface of Co₃O₄ and BCDs/Co₃O₄. Insets show the optimized configurations for adsorbates on the surface of BCDs/Co₃O₄.

Based on our experimental testing and computational findings, we have deduced a possible reaction pathway for nitrate reduction to generate ammonia (Fig. 4c).⁴⁵ The most favorable pathway is as follows:

For the Co₃O₄/CC catalyst without spin regulation, the rate-determining step of the reaction is the conversion of NO* to NOH*, while for the BCDs/Co₃O₄/CC catalyst with spin regulation, the rate-determining step of the reaction is the hydrogenation of NO₃*. Therefore, for BCDs/Co₃O₄/CC, the rate-determining step has changed and the energy to be overcome in this step has also decreased, resulting in a significantly improved electrocatalytic performance.

3. Conclusion

In summary, we report a method of using BCDs loading to alter the electron spin density of magnetic metal oxides. Octahedral Co³⁺(O_h)–O with a t_2g⁶e_g⁰ electronic configuration transforms into octahedral Co²⁺(O_h)–O with a t_2g⁵e_g² configuration after BCDs loading. The BCDs/Co₃O₄/CC catalyst with spin density regulation showed excellent activity in the electrocatalytic NO₃RR for ammonia synthesis, achieving a maximum NH₄⁺ FE of 94.6 ± 0.9% and high stability at −1.3 V vs. SCE. The high activity is attributed to the spin density regulation changing the rate-determining step and reducing the energy it needs to overcome. This work expands the methods of spin density regulation and provides a new perspective on the mechanism of carbon dot materials in catalytic reactions.

Author contributions

Conceptualization: J.C. and S.L.; methodology: J.Y. and A.C.; investigation: J.H., X.L., H.S., and Y.W.; writing – original draft: J.H. and X.L.; writing – review and editing: J.C., S.Q.Z., and S.L.; funding acquisition: J.C., S.L., and S.Q.Z.; supervision: S.L.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22205213), the China Postdoctoral Science Foundation (2020M682332, 2021T140614), the Natural Science Foundation of Henan Province (222300420287), and the Henan Postdoctoral Foundation (202003015).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi00865g

‡ These authors contributed equally to this work.

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