An organic–inorganic hybrid polyoxoniobate decorated by a Co(III)-amine complex for electrocatalytic urea splitting

Abstract

The overall energy efficiency of electrochemical systems is significantly reduced by the conventional anodic oxygen evolution reaction (OER). It is feasible to improve energy efficiency by replacing the OER with the urea oxidation reaction (UOR), which has a lower thermodynamic potential. An organic–inorganic hybrid polyoxoniobate decorated by a Co(III)-amine complex, Na₄(H₂O)₁₅[Co(en)₃]₂{[Co(en)(Nb₆O₁₉)]₂}·34H₂O (Co₂Nb₆, en = ethylenediamine) with distinct physicochemical characteristics and well-defined single-crystal structure is reported. The structure of Co₂Nb₆ contains Lindqvist {[Co(en)(Nb₆O₁₉)]₂}¹⁰⁻ dimer and free [Co(en)₃]³⁺ complexes. Co₂Nb₆ exhibits remarkable catalytic activity for the UOR after being firmly attached to the surface of acetylene black by polyethyleneimine (PEI). To the best of our knowledge, this is the first instance of performing the electrocatalytic UOR based on Lindqvist polyoxoniobate clusters, which will pave the path for innovative concepts in the development of POM-based electrocatalysts.

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Introduction

In recent years, global attention has been drawn to the growing challenges of energy and environment.¹ One promising solution to these challenges is hydrogen, which offers high eco-friendliness, energy density, calorific value, and intermittent availability among renewable resources.² Electrocatalytic oxidation is a critical process for addressing environmental issues.³ The urea oxidation reaction (UOR) serves as a potential solution to purify urea-rich wastewater while generating hydrogen.⁴ It's worth noting that the cell potential required for urea electrolysis is theoretically much lower than that required for water electrolysis.⁵ In addition, the UOR is a key chemical process in artificial kidneys and direct urea fuel cells.⁶ However, the UOR has slow kinetics due to its complex intermediate transfers, 6e⁻ transfer process, and multiple gas-desorption steps.⁷ So far, precious metal-based catalysts, such as Pt, IrO₂, and RuO₂, have been proven to exhibit high catalytic activity for the UOR, similar to those used in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER).^8–10 Nevertheless, their high cost, scarcity, and poor stability limit their large-scale applications.¹¹ Therefore, there is a need to design and develop alternative catalysts with enhanced activity and lower cost for efficient hydrogen production by the UOR.

Recently, a variety of highly efficient catalysts for the UOR have been developed, including non-noble metal oxides, sulfides, phosphides, hydroxides, and nitrides.^12–16 However, the interfacial information of these nanocatalysts is often complex, posing challenges to establishing a solid structure–property-performance relationship of the catalysts.¹⁷ Furthermore, the aggregation of active sites in the catalysts can lead to reduced catalytic activity.¹⁸ As a result, creating a structurally stable precursor at the molecular level using the grafting method to efficiently disperse catalytically active sites has become a significant challenge for electrocatalysis.¹⁹ Based on this design concept, it is believed that crystalline catalysts with clear structures could provide an opportunity where this concept is put into practice and extend the possibilities of efficient hydrogen production.

Polyoxoniobates (PONbs), a subset of polyoxometalates, are a promising class of electrocatalytic nanomaterials due to their precise architecture and highly charged negative surface.^20,21 Specifically, the Lindqvist-type [Nb₆O₁₉]⁸⁻ cluster not only accelerates surface charge transport as a co-electron group but also maintains structural stability under strongly alkaline conditions with a pH value of 14, making it a potential electrocatalytic carrier under alkaline conditions.²² In addition, it can serve as an excellent O donor to bond transition metal (TM) ions.²³ However, TM ions tend to form suspended hydroxide that precipitates in alkaline media.^24,25 To address this issue, N-donor organic ligands are chosen to chelate with TM cations to produce TM-organic complexes, which can subsequently recombine with [Nb₆O₁₉]⁸⁻ clusters as a structural linker and modulator to establish charge homeostasis.²⁶ Furthermore, the [Nb₆O₁₉]⁸⁻ cluster plays a crucial role in dispersing TM-organic complexes, which can ultimately form stable organic–inorganic hybrid TM-PONbs.²⁷

It is well known that the catalytic activity of clusters is highly susceptible to their low stability in water or under reaction conditions.²⁸ Due to their strong adhesion to the electrode surface, the bubbles produced during the gas reaction tend to accelerate the detachment of active chemicals from the electrode surface, making it difficult to achieve high catalytic activity.^29–31 In order to improve the catalytic activity of clusters, grafting metal complexes onto polyoxometalates (POMs) at the molecular level has been proposed as an effective alternative. This approach improves the mechanical stability of associated metal complexes and the electron transfer rate, as the covalent bonds attached to POMs enhance the stability of POMs.^32–34 For instance, Gong used the cationic polymer polyethyleneimine (PEI) to immobilize Co₄POM on commercial TiO₂ nanoparticles, which demonstrated that highly reactive cobalt oxide is the main activity center of the OER.²⁵ Despite being promising electrocatalysts, polyoxometalates containing cobalt complexes have not yet been reported to electrocatalytically oxidize urea.³⁵

In this study, a polyoxoniobate decorated by a Co(III)-amine complex, Na₄(H₂O)₁₅[Co(en)₃]₂{[Co(en)(Nb₆O₁₉)]₂}·34H₂O (Co₂Nb₆, en = ethylenediamine), has been constructed from {[Co(en)(Nb₆O₁₉)]₂}¹⁰⁻ dimers and [Co(en)₃]³⁺ complexes. The strong adsorption ability of PEI was employed to anchor Co₂Nb₆ onto acetylene black (AB) as a UOR catalyst (Co₂Nb₆&AB). PEI was used to effectively reduce the adhesion of air bubbles on the catalyst surface, while acetylene black acted as a conductive substrate to enhance the catalytic activity of POMs by compensating for their weak conductivity. Due to the PONb clusters effectively partitioning the Co³⁺ active sites, Co₂Nb₆&AB shows excellent UOR catalytic activity and stability in alkaline electrolytes. Furthermore, our research highlights the advantages of creating crystal structures with well-defined electrocatalytic active sites.

Results and discussion

Crystal structure of Co₂Nb₆

The compound Co₂Nb₆ displays a one-dimensional organic–inorganic hybrid chain structure. X-ray single-crystal diffraction shows that compound Co₂Nb₆ crystallizes in the triclinic P1̄ space group and contains {[Co(en)(Nb₆O₁₉)]₂}¹⁰⁻ dimers and [Co(en)₃]³⁺ units. The crystal data and structure refinement are listed in Table S1.† Valence bond calculations show that Nb and Co atoms possess +5 and +3 valences, respectively (Table S2†). Two centrosymmetric [Nb₆O₁₉]⁸⁻ clusters are linked by two centrosymmetric [Co(en)]³⁺ units to form a cyclic {[Co(en)(Nb₆O₁₉)]₂}¹⁰⁻ dimer (Fig. 1a). The [Co(en)]³⁺ complexes are grafted onto the [Nb₆O₁₉]⁸⁻ clusters because Co³⁺ adopts a six-coordinated octahedral environment with four oxygen atoms from two [Nb₆O₁₉]⁸⁻ clusters and two nitrogen atoms from one en ligand (Fig. 1a). This is helpful for increasing the catalytic activity. Two different kinds of {[Co(en)(Nb₆O₁₉)]₂}¹⁰⁻ secondary building units exist in Co₂Nb₆. One of these is further linked by Na⁺ to create a one-dimensional zigzag chain structure running along the b axis (Fig. 1c), while the other is discrete and surrounded by two “V”-shaped {Na₃(H₂O)₁₃} clusters and eight [Co(en)₃]³⁺ cations. The isolated {[Co(en)(Nb₆O₁₉)]₂}¹⁰⁻ is located in the center of the regular parallelepiped formed by eight [Co(en)₃]³⁺ cations. In the packing structure (Fig. 1b), the 1D zigzag chains are arranged in parallel along the [100] direction in the –AAAA– sequence and the isolated {[Co(en)(Nb₆O₁₉)]₂}¹⁰⁻ and [Co(en)₃]³⁺ are distributed between the chains (Fig. 1d).

Fig. 1 (a) The structure of the {[Co(en)(Nb₆O₁₉)]₂}¹⁰⁻ dimer; (b) view of the regular parallelepiped formed by eight [Co(en)₃]³⁺ cations that encircle the {[Co(en)(Nb₆O₁₉)]₂}¹⁰⁻ dimer; (c) view of the 1D zigzag chain structure constructed from the {[Co(en)(Nb₆O₁₉)]₂}¹⁰⁻ dimer and Na⁺ cations in Co₂Nb₆; and (d) view of the packing structure of Co₂Nb₆. Atomic color code: Co, pink; O, red; Nb, blue; N, purple; C, grey; Na, yellow; polyhedral color code: NbO₆, blue.

The electronic interactions of surface elements in Co₂Nb₆ were investigated using X-ray photoelectron spectroscopy (XPS). The XPS survey spectra reveal the presence of Co, O, Nb, N, and C elements in Co₂Nb₆, as demonstrated in Fig. 2a. The Co 2p spectrum consists of two spin–orbit doublets and two satellite peaks, with binding energy values of 777.9 and 793.2 eV observed for Co 2p_3/2 and Co 2p_1/2 (Fig. 2b), respectively. These results suggest the presence of the cobalt ion in its highest oxidation state, i.e., Co³⁺.^36,37 The presence of Co³⁺ can generate more reactive intermediates (CoOOH) as effective active sites in the catalytic process, resulting in better OH ion capture and faster reaction kinetics.^37,38 The peaks of Nb 3d_5/2 and Nb 3d_3/2 are assigned to the Nb 3d spectrum of Co₂Nb₆ at 203.2 and 205.9 eV, respectively (Fig. 2c).³⁹ Additionally, as depicted in Fig. 2d, the fitted O 1s high-resolution spectra, and fitted peaks correspond to the Nb–O bonding (526.9 eV) and Co–O bonding (528.8 eV), respectively.^40,41 Other characterization results of Co₂Nb₆ are shown in the ESI, Fig. S1.†

Fig. 2 XPS spectra of Co₂Nb₆. (a) survey spectrum; (b) XPS spectrum of Co 2p; (c) XPS spectrum of Nb 3d; and (d) XPS spectrum of O 1s.

The incorporation of the “surface wetting state” concept in electrochemical reactions can be a valuable tool to improve the design of electrode surfaces for different electrochemical reactions.^42,43 Field emission scanning electron microscopy (FESEM) revealed the morphology of pristine samples. The Co₂Nb₆ shown in Fig. 3a exhibited a disordered nanostrip morphology with an average thickness of 4 μm. However, the inhomogeneous particulate matter and band morphology depicted in Fig. 3b were attributed to the coexistence of Co₂Nb₆ with AB (Co₂Nb₆/AB), indicating that they were not fully cross-linked. When the PEI was added to Co₂Nb₆ and AB (Fig. 3c), irregularly banded material under a smooth surface was revealed, which matched the morphology of Co₂Nb₆. This indicated that in the presence of PEI, Co₂Nb₆ and AB were effectively adherent to one another at this time (Co₂Nb₆&AB), assuring the electrochemical stability of the composite.⁴⁴ Furthermore, TEM images of Co₂Nb₆&AB showed distinct carbon layers encapsulating the crystalline nanoparticles, further demonstrating the effective bridging of Co₂Nb₆ on ABvia PEI (Fig. S2†). The elemental mapping diagram proved that the Co metal was uniformly dispersed in Co₂Nb₆&AB (Fig. S3†). The energy dispersive X-ray (EDX) spectrum (Fig. S4†) demonstrated that C, N, O, Co and Nb elements coexisted in Co₂Nb₆&AB and their specific weight distribution ratio was, respectively, about 13.34:20.56:43.33:5.48:15.29. Further analysis of the air contact angle was carried out. As shown in Fig. 3d–f, the air contact angles increased in order (136.5° < 139.5° < 148.5°) from Co₂Nb₆, Co₂Nb₆/AB to Co₂Nb₆&AB. In particular, Co₂Nb₆&AB was about to form a superhydrophobic surface (air contact angle ∼150°), which indicated that the bubbles can be quickly detached at the electrode surface during the gas generation reaction on its surface, allowing full contact between the catalyst surface and the electrolyte and also avoiding the loss of catalyst active material by stripping due to bubble adhesion.^29,45

Fig. 3 (a–c) SEM images of Co₂Nb₆, Co₂Nb₆/AB and Co₂Nb₆&AB, respectively; and (d–f) air contact angles of Co₂Nb₆, Co₂Nb₆/AB and Co₂Nb₆&AB, respectively.

The N₂ adsorption–desorption isotherms of Co₂Nb₆ and Co₂Nb₆&AB samples both exhibited the typical type IV curve, indicating the presence of inhomogeneous pores (Fig. 4a and c).⁴⁶ Although Co₂Nb₆ showed a very low specific surface area (1.762 m² g⁻¹), the pore size distribution revealed a microporous structure of less than 1 nm (Fig. 4b), which accorded well with the structural analysis of single crystal resolution. The bridging of acetylene black on Co₂Nb₆ could increase the surface area of Co₂Nb₆&AB 8-fold (Fig. 4d) with mesopores and macroporous structures of 20–70 nm. Co₂Nb₆ can successfully adhere to the surface of AB, creating pore channels for excellent electrolyte transport.³⁵ The relatively high surface area and rich pore structure of Co₂Nb₆&AB can facilitate the exposure of more active sites.⁴⁷

Fig. 4 N₂ adsorption–desorption isotherms of (a) Co₂Nb₆ and (c) Co₂Nb₆&AB; and pore size distribution curves for (b) Co₂Nb₆ and (d) Co₂Nb₆&AB.

Electrochemical tests

We evaluated the catalytic performance of samples for the urea oxidation reaction (UOR) using a conventional three-electrode system in alkaline media. The UOR and OER activities of Co₂Nb₆&AB catalysts were evaluated by linear sweep voltammetry (LSV) in an aqueous solution of 1 M KOH with 0.33 M urea. As shown in Fig. 5a, Co₂Nb₆&AB required a potential of 1.378 V to catalyze the UOR at 10 mA cm⁻², which was a considerable decrease of 270 mV compared to the OER (1.648 V). These results suggested that urea-assisted hydrogen production was more energy-efficient than the OER.⁴⁸ Additionally, the UOR performances of bare carbon cloth (CC), Nb₆&AB, CoO&AB, RuO₂ and Co₂Nb₆&AB were investigated under the same conditions, and the corresponding LSV curves are shown in Fig. 5b. The electrocatalytic activity of bare CC, Nb₆&AB and CoO&AB was found to be very low, whereas the Co₂Nb₆&AB electrode demonstrated significant UOR catalytic activity that was 113 mV lower than that of the RuO₂ electrode. To provide a more intuitive comparison of the catalysts’ capabilities, Fig. 5c depicts the potentials required by Co₂Nb₆&AB, Nb₆&AB, CoO&AB, and RuO₂ at various current densities. Moreover, the Co₂Nb₆&AB electrode exhibited a notably low Tafel slope of 58.7 mV dec⁻¹, which was lower than those of bare CC (237.7 mV dec⁻¹), Nb₆&AB (185.1 mV dec⁻¹), CoO&AB (148.6 mV dec⁻¹) and RuO₂ (127.4 mV dec⁻¹) electrodes (Fig. 5d). This finding suggested that the Co₂Nb₆&AB electrode exhibited faster UOR kinetics in practical applications. The turnover frequency (TOF) values of CoO&AB and Co₂Nb₆&AB are shown in Fig. S5† based on the number of metal Co sites. The TOF values of Co₂Nb₆&AB at different overpotentials (200 mV, 300 mV, 400 mV) were higher than those of CoO&AB, further demonstrating the higher atomic efficiency of metallic Co in Co₂Nb₆&AB. Remarkably, in comparison with previously reported Co-based catalysts, Co₂Nb₆&AB displayed excellent UOR performance (Table S3†). Meanwhile, Fig. 5e depicts the Nyquist diagram and corresponding equivalent circuit (inset), which were used to calculate the charge transfer resistance (R_ct) of the electrolyte/catalyst interface based on the fitted semicircle diameter. When compared to other catalysts, Co₂Nb₆&AB exhibited lower interfacial charge transfer resistance (the specific fitting parameters are shown in Table S4†). This result demonstrated that Co₂Nb₆&AB had faster charge transfer rate and better UOR kinetics.⁴⁹

Fig. 5 Electrochemical urea oxidation performance. (a) UOR and OER performances of the Co₂Nb₆&AB electrode in 1.0 M KOH solution with and without 0.33 M urea at a scan rate of 5 mV·s⁻¹; (b) UOR polarization curves for bare CC, Nb₆&AB, CoO&AB, RuO₂, and Co₂Nb₆&AB; (c) comparison of potentials at 10 mA cm⁻² and 20 mA cm⁻² for the corresponding electrodes; (d) Tafel plots; (e) Nyquist plots of the catalysts in 1 M KOH with 0.33 M urea (inset: Nyquist curve fitting equivalent circuit); and (f) chronoamperometric curves for Co₂Nb₆&AB at 10 mA cm⁻² constant current density in 1 M KOH with 0.33 M urea (inset: before and after a 3000 cycle CV scan, the LSV curves of Co₂Nb₆&AB).

In general, the larger the electrochemically active surface area (ECSA), the more active sites are thought to be exposed, and the higher the catalytic reaction activity. The ECSA of a catalyst is evaluated by the double-layer capacitance (C_dl). We have calculated the C_dl values based on the CV plots in non-Faraday regions (Fig. S6†), and the C_dl value of Co₂Nb₆&AB was 4.75 mF cm⁻², which was higher than those of bare CC (0.90 mF cm⁻²), Nb₆&AB (1.15 mF cm⁻²), CoO&AB (1.69 mF cm⁻²) and RuO₂ (3.47 mF cm⁻²), suggesting that the best intrinsic catalytic activity occurs on Co₂Nb₆&AB. These findings demonstrated that the synergistic interaction of AB and Co₂Nb₆ resulted in the UOR activity of Co₂Nb₆&AB. Apart from catalytic activity, long-term durability is an essential parameter to evaluate the potential of electrocatalysts for practical applications. The stability of the catalyst was evaluated by a chronopotentiometry test with a constant current density of 10 mA cm⁻². As shown in Fig. 5f, after 10 h of testing, the UOR performance of Co₂Nb₆&AB remained nearly stable (2 mV shift). The cyclic voltammetry measurement also displayed robust stability with negligible loss after 3000 cycles, as determined by the CV test. To verify the effective adhesion of Co₂Nb₆ to the AB surface using PEI, stability test studies on Co₂Nb₆/AB catalysts without PEI addition were performed under the same conditions. As a comparison, the LSV curve test (Fig. S7†) and the chronopotentiometry test (Fig. S8†) were performed on the Co₂Nb₆/AB sample. It was evident that activity decreased significantly at the beginning of the test, which may have been caused by active ingredient shedding.⁵⁰ Finally, XRD analysis of sample powders before and after testing revealed that the crystal structure of the Co₂Nb₆&AB catalysts remained essentially intact (Fig. S9†), emphasizing the remarkable stability of the composites under alkaline oxidation conditions.

To acquire a better understanding of the urea oxidation reaction (UOR) mechanism, cyclic voltammetry (CV) was carried out. As shown in Fig. 6a, the redox peaks observed in the CV curves confirmed the presence of active sites (CoOOH) during the UOR process, which was caused by the faradaic process of Co³⁺/Co²⁺ interconversion in the urea–alkaline electrolyte.⁵¹ This was likely due to the fact that the Co³⁺ ion in Co₂Nb₆&AB was unstable in an alkaline environment and was easily reduced to Co(OH)₂, which was then oxidized to CoOOH in a certain voltage state. CoOOH exhibited higher electrical conductivity than crystalline materials, which was thought to be the source of electrocatalytic activity. However, the characterization of the catalytic activity of CoOOH is very challenging because CoOOH is highly soluble and the amount of catalyst is currently unknown. Furthermore, the peak areas increased steadily as the pH value increased from 12 to 14 (Fig. 6b), indicating a favorable correlation between the oxidation behavior and OH⁻ concentration.⁵² This implied that the Co-active sites first absorbed the OH⁻ ion, producing the active CoOOH, and then interacted with the urea molecules. Furthermore, the Bode plot produced by contrasting frequency and phase angle revealed information about the electrochemical reaction processes. The nonhomogeneous charge distribution in the low frequency (10^–1 Hz–10¹ Hz) region of the Bode plot was widely attributed to the generation of intermediate species on the electrode surface. According to Fig. 6c, a characteristic peak with a center frequency of approximately 10⁰ Hz formed at 1.5 V in a 1 M KOH electrolyte. The peak shifted to higher frequencies and lower phase degrees as applied potentials increased, which could be attributed to the development of the oxygen evolution reaction on the surface of the Co₂Nb₆&AB catalyst. This peak shifted downward roughly 300 mV lower (1.20 V) with the addition of urea due to urea oxidation at the interface, indicating that the UOR was more preferable to the OER (Fig. 6d). According to the CV data, this peak was attributed to the adsorption of OH⁻ by the reactive Co atom. Another peak appeared at around 1.35 V in the low-frequency range (10⁰ to 10¹ Hz), which was associated with nonhomogeneous charge distributions caused by the UOR, and this was consistent with the lower onset potential given by the LSV curve above. It was worth noting that when the voltage increased, the peak frequency shifted significantly toward the higher frequency region, while the phase angle decreased greatly relative to that in the absence of urea, which may be due to the simultaneous electrolysis of water and urea.⁵³ These results implied that the Co₂Nb₆&AB catalyst improved urea dissociation kinetics and the electrooxidation of intermediates, demonstrating the catalyst's potential for promoting the UOR.

Fig. 6 (a) CV curves of Co₂Nb₆&AB in 1.0 M KOH with and without 0.33 M urea at a scan rate of 5 mV s⁻¹; (b) CV curves of the Co₂Nb₆&AB electrocatalyst in urea–alkaline electrolytes at various pH values; Bode plots of Co₂Nb₆&AB in 1 M KOH without (c) and with (d) 0.33 M urea at different potentials.

Conclusions

In this study, a facile and efficient approach was employed to synthesize a Co₂Nb₆&AB catalyst. In particular, the Co₂Nb₆ cluster is strategically attached to the surface of acetylene black via PEI. The Co₂Nb₆&AB catalyst exhibits outstanding performance in the urea oxidation reaction (UOR) in an alkaline electrolyte, which is found to be about 270 mV lower than that of the oxygen evolution reaction (OER) at a current density of 10 mA cm⁻². The catalyst also displays faster charge transfer rates and better UOR kinetics. Moreover, the long-term durability and stability of the catalyst in alkaline solutions are also satisfactory. These superior catalytic properties are attributed to several key factors: (i) improved conductivity of the catalyst due to the incorporation of acetylene black, which facilitates electron transport; (ii) enhanced dispersion of active sites on the surface of acetylene black via the use of PEI, which effectively suppresses the aggregation of Co atoms; and (iii) the exposure of abundant active sites, which accelerates charge transfer and bubble desorption.

Author contributions

Yan-Qiong Sun and Shou-Tian Zheng designed and led the project, revised the manuscript and acquired funding. Da-Huan Li performed material characterization and wrote the original draft. Nian Shi performed the crystal synthesis. Yong-Jiang Wang was responsible for the crystal structure determinations, and Ping-Wei Cai was responsible for electrochemical metrology analysis.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge financial support from the National Natural Science Foundations of China (No. 21971040, 21971039 and 21773029).

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Footnote

† Electronic supplementary information (ESI) available: Materials and methods, tables, supplementary plots including CV curves, chronopotentiometry curve, PXRD, TG and FT-IR. CCDC 2267557. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi01066j

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