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Article

CuNb2O6 Particles Obtained via Solid-State Reaction and Application as Electrocatalyst for Oxygen Evolution Reaction

by
Kívia F. G. de Araújo
1,
Cleber S. Lourenço
2,
Vitor M. S. F. Souza
1,
Matheus D. da Silva
1,
Gabriel D. S. Vasconcelos
1,
Maria J. S. Lima
1,3,
Jakeline R. D. Santos
1,
Kelly C. Gomes
4,
Francisco J. A. Loureiro
5,6,
Marco A. Morales
2,* and
Uílame U. Gomes
1,2
1
Department of Materials Science and Engineering, Federal University of Rio Grande do Norte, Natal 59078-970, Brazil
2
Department of Theoretical and Experimental Physics, Federal University of Rio Grande do Norte, Natal 59078-970, Brazil
3
School of Science and Technology, Federal University of Rio Grande do Norte, Natal 59078-970, Brazil
4
Department of Renewable Energy Engineering, Federal University of Paraíba, João Pessoa 58051-900, Brazil
5
TEMA—Centre for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro, 3810-193 Aveiro, Portugal
6
LASI—Intelligent Systems Associate Laboratory, 4800-058 Guimarães, Portugal
*
Author to whom correspondence should be addressed.
Ceramics 2025, 8(2), 55; https://doi.org/10.3390/ceramics8020055
Submission received: 31 March 2025 / Revised: 7 May 2025 / Accepted: 9 May 2025 / Published: 13 May 2025
(This article belongs to the Special Issue Advances in Electronic Ceramics, 2nd Edition)
Browse Figures
Versions Notes

Abstract

Copper niobate (CuNb2O6) is an important compound due to its low cost and polymorphism, presenting monoclinic and orthorhombic phases, which leads to unique physical–chemical properties. The electrochemical performance of efficient electrocatalysts for the oxygen evolution reaction (OER) is of importance in order to produce hydrogen gas from water. In this context, this work reports the synthesis of CuNb2O6 particles by high-energy milling for 5 and 10 h, and subsequent thermal treatment at 900 °C for 3 h. The samples were characterized by XRD, XRF, FESEM, RAMAN, UV–Vis, and FT-IR techniques, and were applied as electrocatalysts for the OER. The samples had both monoclinic and orthorhombic crystalline phases. The band gaps were in the range of 1.92 to 2.06 eV. In the application for the OER, the particles obtained by 5 and 10 h of milling exhibited overpotentials of 476 and 347 mV vs. RHE at 10 mA cm−2, respectively. In chronopotentiometry experiments for 15 h, the samples exhibited excellent chemical stability. The electrochemical performance of the sample milled for 10 h showed superior performance (347 mV vs. RHE) when compared with electrocatalysts of the same type, demonstrating that the methodology used to synthesize the samples is promising for energy applications.

Graphical Abstract

1. Introduction

Recently, reports of environmental problems on a global scale have become paramount in the development of new technologies. This problem has increased the interest in the research of new materials for clean energy sources. Among the green energy sources, water electrolysis to produce hydrogen stands out [1,2,3].
Water splitting belongs to the field of electrocatalysis, which consists of the following two semi-reactions: the hydrogen evolution reaction (HER), which is the cathodic half-reaction (4H2O + 4e− ↔ 4OH− + 2H2) [4], and the oxygen evolution reaction (OER), which is the anodic half-reaction (4OH−(aq) ↔ 2H2O(l) + O2(g) + 4e−) [5,6,7,8]. The OER takes place in multiple steps that slows down the kinetic reaction and requires a high overpotential for the transfer of the four electrons in the reaction [9]. Therefore, the use of electrocatalysts to reduce the overpotential and to speed up the reaction is of great importance [10]. In this regard, one of the main challenges is to develop low-cost electrocatalysts that have high electrochemical performance and are abundant in nature [11]. Commercial electrocatalysts based on noble metals, such as ruthenium and iridium oxides, are very expensive and very scarce, which limits their application on a large scale [12].
A wide class of materials that has been highlighted for OER applications are oxide compounds based on niobium. Niobates have a high potential for catalysis [13,14,15], and are formed by the combination of Nb5+ with another metal cation (M) and oxygen anions, and such an oxide is of the type M2+Nb2O6. This oxide is made of lamellas of positive and negative layers [16]. In the case where the element M is a transition metal, like Cu, Zn, Ni, Mn, Co, or Fe, the niobate is classified as columbite with a perovskite-type crystalline structure [17,18]. These niobates have interesting physicochemical properties, including ferroelectricity, piezoelectricity, and luminescence [19].
The niobate CuNb2O6 is a low-cost semiconductor that is environmentally friendly and which has two main crystalline phases with different properties. CuNb2O6 can be found forming either monoclinic or orthorhombic structures, and the key parameter in the formation of these structures is the thermal treatment [20,21,22]. In both polymorphs, the Cu and Nb form octahedra arrangements of CuO6 and NbO6, which are interconnected through planar and axial oxygens [23,24]. In addition, CuNb2O6 has a high opto-electric coefficient, strong chemical and thermal stability, a strong antiphotocorrosive nature, greater transparency in the visible wavelength region [25,26], and nonlinear optical properties [20].
CuNb2O6 has several applications, for instance, in the photo-reduction of CO2 [16], as a photocatalyst for solar-driven H2 evolution over CuNb2O6 [27], as sensors [21], as photocathodes for photoelectrochemical water splitting and the OER [13,15], in fuel cells [16], as supercapacitors [28], in dielectric devices [20], in hydrogen evolution reaction processes [13,14], among others.
Recent work on the synthesis of niobates has shown that solid-state reaction is a useful method to produce single-phase CuNb2O6. Kamimura et al. [27] reported the synthesis of the monoclinic phase at a temperature of 650 °C, and the orthorhombic phase was produced at 900 °C; both thermal treatments were applied for 24 h. In another work, Priyadarshani et al. synthesized the orthorhombic CuNb2O6 phase when the powders were milled for 1 h and a thermal treatment was applied at 900 °C for 12 h [20].
High-energy milling (HEM) is a process that promotes an efficient mixing and homogenization of precursor powders for the production of nanocomposite materials [29,30]. In addition to being efficient, it is a simple process with the potential for large-scale production [30,31]. The nanostructures obtained from HEM are the result of deformation, fragmentation, and cold-welding phenomena, to which the particles are subjected in a cyclical manner during the milling process [30].
In this work, CuNb2O6 particles were obtained by HEM using 5 and 10 h of milling time. This work aims to produce electrocatalyst CuNb2O6 samples for water splitting with a low overpotential. The samples were characterized by XRD, XRF, FESEM, RAMAN, UV–Vis, FT-IR, and electrochemistry procedures for the oxygen evolution reaction (OER).

2. Materials and Methods

2.1. Structural, Vibrational, and Morphological Characterization

To determine the chemical composition of the samples in terms of the analyte percentages, an X-ray fluorescence spectrometer—the Shimadzu EDX-720 (Kyoto, Japan)—was used. To determine the structure and phase composition, X-ray diffraction (XRD) measurements were performed by using a Rigaku Miniflex II diffractometer (Tokyo, Japan), a CuKα radiation source with λaverage = 1.5481 Å, a scanning range of 20–70° (2θ) at a speed of 3°/min, and a 0.02° step size. The diffractograms were refined using the Rietveld method available in the MAUD software (version 2.999). The crystal structure was visualized by VESTA software (Visualization for Electronic and Structural Analysis, version 3.5.8). The morphology of the samples was studied by analyzing the scanning electron microscopy (SEM) images obtained in a field emission electron microscope—the FESEM, Carl Zeiss model Auriga 40 (Jena, Germany)—equipped with an energy-dispersive spectroscopy (EDS) facility. The vibrational study of the samples was performed with a Fourier transform infrared (FT-IR) spectrometer, the Bruker FT-IR VERTEX 70 spectrometer (Karlsruhe, Germany). Raman spectra were recorded using a HORIBA labRam HR Evolution (Kyoto, Japan) with the laser at a maximum power of 90 mW, a laser wavelength of 532 nm, and a spectral window between 100 and 1000 cm−1. To obtain information about the electronic transitions within the particles, a UV–Vis diffuse reflectance spectrometer from SHIMADZU model UV-2450 (Kyoto, Japan) was used, with wavelengths between 190 and 900 nm and a resolution of 0.1 nm in reflectance mode.

2.2. Electrochemical Characterization

With regard to the electrochemical measurements, linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and chronopotentiometry were carried out using a PGSTAT204 potentiostat/galvanostat with a FRA32M from Metrohm (Utrecht, The Netherlands) and a conventional three-electrode cell, consisting of the Ag/AgCl reference electrode, a platinum wire as the counter electrode, and the CuNb2O6 sample deposited on Ni foam, used as the working electrode, in a 1 M KOH electrolyte solution. Firstly, the Ni foam was cleaned in a solution of hydrochloric acid and dried. The catalytic ink was prepared by mixing 500 μL of isopropyl alcohol, 5 mg of the CuNb2O6 sample, and 50 μL of Nafion™ solution. The Nafion solution contained 5 wt.% of perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid-tetrafluoroethylene, 45 wt.% water, and 50 wt.% propanol. Then, the catalytic ink was added dropwise onto the Ni foam substrates and dried at room temperature for 24 h. The potential was converted from Ag/AgCl (concentration of KCl = 3 mol/L) to the reversible hydrogen electrode (RHE) according to the following Nerst Equation (1) with the electrolyte at pH = 13.6:
ERHE = EAg/AgCl + 0.059 pH + 0.1976 V
The overpotential values (η) were calculated using Equation (2), as follows:
η = ERHE − 1.23 V
Linear sweep voltammetry (LSV) was performed at 5 mV s−1. The cyclic voltammograms were obtained at different scan rates in a potential range (5 mV·s−1 to 200 mV·s−1). Chronopotentiometry measurements were performed at a fixed current density of 10 mA·cm−2 for a time interval of 15 h. Electrochemical impedance spectroscopy (EIS) was carried out using a frequency range (0.1 Hz–10 kHz) under DC potentials (0.5, 0.55, 0.6, 0.65, and 0.7 V vs. RHE). The specific capacitance can be converted into the electrochemical active surface area (ECSA) using the specific capacitance value (CS = 0.040 mF cm−2) with the following equation:
ECSA = CDL/CS

2.3. Synthesis Method

The CuNb2O6 samples were prepared via the solid-state reaction of commercial copper oxide (CuO) (Vetec, São Paulo, Brazil, II ICO PA, 99.0%, with an average particle diameter of 9.20 μm) and niobium pentoxide (Nb2O5) (Sigma Aldrich, São Paulo, Brazil,94.55%, with an average particle diameter of 1.88 μm), previously milled for 5 and 10 h, and the powders had a molar ratio of 1:1. Figure 1a shows the SEM image of the Nb2O5 sample, where it is possible to observe a plate-like morphology, whereas the copper oxide sample exhibits a morphology of agglomerates of ellipsoidal and rice-shaped very small particles (Figure 1b).

2.4. Synthesis via Solid-State Reaction

The powders were processed in a planetary mill, the FRITSCH model PULVERISETTE 7 (Rhineland-Palatinate, Germany), at a rotation speed of 400 rpm. The ratio of the powder mass to balls was 1⁄5. After pouring the powders and balls, the vessel was filled with ethyl alcohol. Wet milling was performed for 5 and 10 h. Afterwards, the dried powders were thermally treated in an oven at 900 °C for 3 h with a heating rate of 5 °C/min. The sintered samples obtained with powders milled for 5 and 10 h were labeled as CuNb2O6-5 h and CuNb2O6-10 h, respectively. The chemical reaction between the milled powders to produce CuNb2O6 is shown in Equation (4), as follows:
CuO + Nb2O5 → CuNb2O6

3. Results

3.1. XRF Results

Elemental analysis was realized by XRF. Table 1 provides the detected oxides with their respective atomic percentages. The composition of the samples was close to the nominal expected molar ratio of CuO:Nb2O5, at 1:1 [20,28]. The absence of contamination validates the synthesis method [32].

3.2. Structural and Morphological Study

The XRD results are shown in Figure 2. The results indicate that the synthesis processes using the two milling times and subsequent sintering were efficient in the formation of the CuNb2O6 phase, and no evidence of a second phase was observed. The majority of the observed peaks correspond to the monoclinic structure of the CuNb2O6 phase (ICSD 81046, space group P 1 21/c 1) [33]. However, the orthorhombic CuNb2O6 phase (ICSD 79455, space group P b c n) [34] was also present, and its peaks appeared at 25.08° and 30.35°.
Figure 2b shows a zoomed-in view of the main peaks of the copper niobate. The presence of the main peak of the orthorhombic phase was observed at 30.35°. However, for the orthorhombic phase, it was noted that the CuNb2O6-10 h sample had a broader diffraction peak and reduced intensity compared to one corresponding to the CuNb2O6-5 h sample. Figure 2c shows a pictorial view of the CuNb2O6 crystalline structure; it is formed by layers of NbO6 and CuO6 octahedra [28,35].
Table 2 presents the results of the Rietveld refinements of the diffractograms for both samples. The quality of fit, the Rwp, Rexp, and χ2 for both samples, indicated excellent results, meaning that the used crystal information file (CIF) charts agreed well with the experimental diffractograms [33,34]. Thus, the experimental data confirmed the presence of CuNb2O6 with a predominantly monoclinic structure of 87 wt.%, accompanied by a smaller fraction of an orthorhombic structure of 13 wt.%. The monoclinic structure had angles of α = γ = 90° and β > 90°, whereas the orthorhombic structure had α = β = γ = 90°. The presence of both crystalline phases in the samples was in agreement with earlier reports that indicated the synthesis at 900 °C with a 3 h isotherm favored the obtainment of CuNb2O6 with a mixed monoclinic and orthorhombic structure [20].
Figure 3 shows SEM images of the Nb2O5 and CuO milled for 5 and 10 h. After 5 h of milling, it is observed that the HEM process led to the fragmentation of the powders, resulting in smaller particles compared to the pristine powder (see Figure 1). In addition, the formation of sheets occurs due to plastic deformation and the formation of agglomerates, which is a consequence of the cold welding between the phases. With the increase in the milling time to 10 h, the intensification of the fragmentation and cold-welding phenomena is noted, leading to smaller and thinner sheets of Nb2O5-CuO composites.
The morphology and characteristics of the CuNb2O6 particles can be observed in Figure 4. In Figure 4a,b, it is evident that the longer milling time of the precursors led to smaller particles. Furthermore, for the CuNb2O6-10 h sample, some aggregated particles were observed; the organization of these particles formed porous sheets. These porous CuNb2O6 sheets appeared to be very thin. In Figure 4c,d, the sizes of some particles are shown, which shows a greater refinement of the particles for the CuNb2O6-10 h sample. It is also noted that the sizes of these particles were similar to that of 10 h milled precursors.
In Figure 4c, for the CuNb2O6-5 h sample, it is possible to observe the presence of two distinct morphologies, highlighted in yellow. The polyhedral-like particles may correspond to the orthorhombic phase, whereas the spherical particles may correspond to the monoclinic phase (Figure 4c). An important characteristic of the CuNb2O6-10 h sample was the formation of aggregates with the shape of thin sheets, having a thickness of approximately 0.2 μm (Figure 4f), whereas the aggregates of the CuNb2O6-5 h sample had a greater thickness of 1.6 μm (Figure 4e).

4. Discussion

4.1. Structural, Vibrational, and Morphological

The synthesis processes resulted in the production of copper niobate with a mixed crystalline structure formed by the monoclinic and orthorhombic phases, with a predominance of peaks related to the monoclinic phase (Figure 2a). The diffractogram of the CuNb2O6-10 h sample had a lower intensity for the peak at 30.35°, corresponding to the orthorhombic phase (Figure 2b). This is because the CuNb2O6-10 h sample had smaller crystallites due to the intense collisions between the milling bodies and powder during the HEM process, which generated plastic deformation, cold welding, fracture, and the rewelding of the particles [36]. Therefore, a longer HEM time contributed to a better organization of the crystalline structure during the production of CuNb2O6.
The results of the Rietveld refinements, observed in Table 2, show that the increase in the milling time promoted an increase in the values of the lattice parameters and the unit cell volume of the monoclinic and orthorhombic structures by 0.81% and 1.24%, respectively, in addition to reducing the crystallite sizes. This is because the HEM process generated tensions in the lattice, accompanied by significant microstructural modifications in the material, such as an increase in the number of defects and the redistribution of residual stresses. These combined factors contributed to the increase in the lattice parameter [37].
Figure 3 shows the reduction in the particle size and the formation of sheets as a result of the impacts among the ball, powder, and container at high speed, therefore promoting the welding and fragmentation of the powders [38,39]. As reported in the literature, HEM is a powerful tool to promote the greater homogenization between the phases and reduction in the particle size. In addition, HEM plays a fundamental role in solid-state synthesis, as it reduces the particle size and activates reactions that favor phase formation, thus facilitating the synthesis process [40]. Furthermore, optimizing solid-state reaction conditions contributes to reducing environmental impacts [41].
In Figure 4, the greater refinement for the samples prepared using a longer milling time is evident, and it is observed that the size of these particles was similar to that of the milled precursors (Figure 3a,b). This indicates that the homogenization promoted by HEM allowed for the solid-state reaction to occur predominantly in each particle of the milled powders. As reported in the literature, the morphology of the CuNb2O6 particles depends on the polymorph [27]. The polyhedral prismatic and spherical morphologies identified in Figure 4c were also observed in the samples prepared by other methods. For example, the CuNb2O6 samples prepared by the self-combustion method presented particles with an orthorhombic structure and with polyhedral morphologies [23]. Furthermore, Tang et al. obtained CuNb2O6 particles with a monoclinic crystal structure in samples produced by coprecipitation, and their morphologies were called corn-like aggregates [1]. In Figure 4e,f, it was observed that the thickness of the aggregates reduced from 1.6 μm to 0.2 μm when the milling time was extended to 10 h. Thus, a longer HEM time of the precursors favors the obtainment of CuNb2O6 with a greater surface area.
Figure 5a presents the FTIR spectrum for both CuNb2O6 samples. The identification of chemical bonds and vibrational modes of metal–oxygen bonds are shown in the figure. The peaks observed at 880, 850, 812, and 680 cm−1 were attributed to the NbO6 octahedra (edge-shared), while the peak at 610 cm−1 corresponded to bonds of the NbO6 octahedra (corner-shared) for the samples obtained from precursors milled for 5 h and 10 h [2,20,21,27]. Additionally, for wavenumbers smaller than 532 cm−1, there were several peaks associated with the bonds of the CuO6 octahedra (edge- and corner-shared) [20,21,42]. In fact, according to the XRD results, both of the samples have similar crystalline structures and chemical compositions. Besides that, equivalent chemical bonds and functional groups were expected [43,44,45].
The Tauc plots obtained from the UV–Vis reflectance spectra for both CuNb2O6 samples are shown in Figure 5b. The band gap was estimated from the Tauc plot (F(R)hv)n, with n = 0.5, since the CuNb2O6 was classified as an indirect energy band gap material, a p-type semiconductor [13,22,27,46,47]. The optical band gap values were 2.06 and 1.92 eV for the niobate obtained from the 5 and 10 h milled powders, respectively. This is consistent with the reported range for the monoclinic CuNb2O6 [2,13,17,46]; this result is mostly due to the main crystalline phase in both samples.
Figure 5c shows the vibrational study conducted through Raman spectroscopy. Both spectra have similar features, mainly because both samples have similar crystalline phases and compositions. It is important to mention that, for the CuNb2O6-10 h sample, no signal from the Nb2O5 phase was found. For instance, the Nb2O5 phase should exhibit two broad bands, centered at 660 and 261 cm−1, and two narrow peaks at 992 and 116 cm−1; as noticed in the Raman spectra, none of the samples had these peaks. For the CuO phase, one cannot rule out the presence of this phase in the Raman spectra; since the main peaks of this phase should appear at 296, 346, and 631 cm−1, these signals may be superposed with the peaks of the CuNb2O6 phase. It is clear that the XRD analysis discarded the possibility of the presence of Nb2O5 and CuO phases; therefore, one can conclude that, within the sensitivity of the XRD and Raman spectroscopy methods, the samples are single-phase. In fact, the band between 150 and 360 cm−1 (yellow region) contains the peaks due to the Cu-O and Nb–O stretching and vibrational modes due O–Nb–O arrangements [22,42,48]. Moreover, the peaks in the range from 360 to 780 cm−1 (white region) belong to the vibrational modes of Nb–O. For the CuNb2O6-5 h sample, the high intensity peaks at 502 and 901 cm−1 are due to the Nb-O vibrational mode and symmetric O–Nb–O stretching vibration, respectively. These peaks are also present in the CuNb2O6-10 h sample, but with a smaller intensity. There is a lack of detailed studies on the Raman spectra of the CoNb2O6 system; therefore, the reason for these intensity variations is unclear and may deserve additional theoretical work.

4.2. Electrocatalytic Performances

The electrocatalytic activities of the CuNb2O6 electrocatalysts were investigated using various electrochemical characterization techniques, such as linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and chronopotentiometry for the oxygen evolution reaction (OER).
According to the anodic polarization results (Figure 6a), the electrocatalysts made from CuNb2O6-5 h, CuNb2O6-10 h, and the blank Ni foam showed overpotential values of 476, 347, and 515 mV vs. RHE, respectively, at a current density J = 10 mA cm−2. The CuNb2O6-10 h electrocatalyst showed better catalytic performance for oxygen evolution. This may be related to the morphology of this sample; the sample with a longer milling time had smaller particles and, therefore, a larger surface area for reaction in the catalytic process. In fact, several studies [49,50,51,52] have shown that catalysts with smaller particles have a larger electrochemical surface area, favoring the OER phenomena [53]. The overpotential of the CuNb2O6-10 h sample was of 347 mV at 10 mA cm−2, and this value was smaller than the one displayed by commercial iridium oxide of 400 mV, at 10 mA cm−2 [54]. The performance of the CuNb2O6 electrocatalyst is satisfactory and comparable to the performance of various OER samples reported in the literature (Table 3).
Table 3 displays a comparative approach of the present results with earlier works on similar systems; all of these results were obtained in alkaline solutions containing KOH. It is important to mention that, to the best of our knowledge, there was a single work in which CuNb2O6 was used as an electrocatalyst for OER application. The majority of the earlier works were related to either pure CoNb2O6 or composites containing CoNb2O6. As pointed out in Table 3, in the present work, the CuNb2O6-10 h sample had the smaller overpotential of 347 mV at 10 mA cm−2. The Cu-Nb-O system, studied by Alves et al. [15], consisted in composite CuO/CuNb2O6 nanofibers obtained by solution blow spinning, and the overpotential for their sample was 380 mV [15]. Due to lack of works on CuNb2O6 samples, the comparison will be performed by considering other niobates. For instance, the pure CoNb2O6 sample provided an overpotential of 330 mV [55], whereas the CoNb2O6 phase doped with Ni or Ag provided lower overpotentials of 220 and 250 mV, respectively [55]. Therefore, the CuNb2O6-10 h sample had an electrocatalytic performance that was better than the one obtained for the equivalent phase sample.
Another parameter studied is the reaction kinetics of the electrocatalysts, using the Tafel equation (ղ = a + b log j), where ղ is the overpotential, a is the intercept relative to the exchange current density j, b is the Tafel slope, and j is the current density. In addition, the Tafel analysis reports that the adsorption of intermediate species is the rate-determining step for all electrocatalysts, which is in accordance with the Krasil’shchikov reaction model for the OER in alkaline media [57].
Figure 6b shows the Tafel slopes for the electrocatalysts CuNb2O6-5 h and CuNb2O6-10 h, and the Ni foam, where it is possible to observe values of 160, 70, and 168 mV dec−1, respectively. The CuNb2O6-10 h electrocatalyst showed the best reaction kinetics; since it had the lowest Tafel slope of 70 mV dec−1, it suggested a greater efficiency for oxygen evolution. The Tafel slope of 160 mV dec−1 for the electrocatalyst CuNb2O6-5 h indicated slower kinetics due to a limitation in the charge and mass transfer processes.
The electrical capacitance double-layer (Cdl) values were obtained in order to better understand the kinetics of the CuNb2O6 samples. The Cdl values were estimated using cyclic voltammetry curves (CV), using the linear relationship between the anodic current density (Ja) and scan rate (ʋ), as described by Ja = ʋ × Cdl [58]. The CuNb2O6-5 h and CuNb2O6-10 h samples had Cdl values of 2.55 and 2.60 mF cm−2, respectively (Figure 6c). Therefore, the higher Cdl value for the CuNb2O6-10 h sample corroborated a higher electrochemical active surface area (ECSA), as shown in Figure 6d, of 65 cm2. This result is directly related to a greater amount of active sites present on the electrode surface [15] for the CuNb2O6-10 h sample, which is mostly due to the presence of pores. The ECSA was estimated using the equation ECSA = Cdl/CS, where CS is the specific capacitance, with a value of CS = 0.040 mF cm−2 [59], since the electrocatalysts are copper-based in alkaline solution.
The stability of the electrocatalysts is of great importance, and this was evaluated by chronopotentiometry (CP) for 15 h, with a current density of 10 mA cm−2. Figure 6e shows the excellent mechanical and chemical stability for both electrocatalysts. The small increase in the overpotential at the beginning of the measurement was to maintain the required current throughout the test. This effect is attributed to the blocking of active sites by the accumulation of bubbles on the electrode surface [15,60,61]. In general, both electrocatalysts showed satisfactory stability during the 15 h of testing.
To better understand the electrochemical behavior of the CuNb2O6-5 h and the CuNb2O6-10 h samples, Figure 6f presents the representative Nyquist plots obtained by electrochemical impedance spectroscopy (EIS) under a DC potential of 0.55 V vs. RHE. Both Nyquist plots exhibit an off-set resistance, which describes the solution resistance, followed by a depressed semicircle that corresponds to the total polarization resistance (Rp) of the electrocatalyst due to the OER faradaic current, as well as the capacitive contribution arising from the formation of a double-layer phenomenon at the electrocatalyst/electrolyte interface (CPEdl, where CPE is a constant phase element) [62]. The impedance data were fitted using a typical Voight equivalent circuit model, i.e., Rs−(Rp||CPEdl) (inset in Figure 6f) [63].
The total polarization resistance (Rp) is depicted in Figure 6g as a function of the applied potential. The plot shows a decreasing tendency of Rp with an increasing DC bias for both samples, as expected due to the increased reaction processes. At the same time, the Rp of the CuNb2O6-10 h electrocatalyst was found to be lower under all of the applied potentials, which is in agreement with the increased OER kinetics noted for this sample. However, the estimated double-layer capacitance (Cdl, Figure 6g), which is associated with the solvated ions arising from the potential difference between the electrolyte solution and the electrocatalyst, decreases with an increasing DC bias. This is due to the well-known phenomenon of a transient gas bubble adhesion, which causes a temporary reduction in the electroactive surface area during the OER [62,64,65]. Meanwhile, the Cdl determined by EIS was found to be lower for the CuNb2O6-10 h electrocatalyst, despite the lower magnitude of its Rp term (Figure 6g). Likewise, this is suggested to be linked to the transient gas bubble formation. In this regard, although the CuNb2O6-10 h electrocatalyst possesses a smaller particle size (Figure 4) (i.e., an increased surface area), this area becomes momentarily decreased during the OER due to a vigorous gaseous oxygen formation, which is plausible, since more active sites are available for the reaction. Remarkably, this behavior does not seem to negatively affect the final performance of the CuNb2O6-10 h electrocatalyst, which still shows a lower Rp and, therefore, increased OER kinetics.

5. Conclusions

This study demonstrates the successful synthesis of high-purity CuNb2O6 particles by high-energy milling (HEM) and sintering at 900 °C for 3 h. The study on the morphology of the samples showed that finer particles with porous structures were formed when the milling time of the precursors was increased to 10 h, leading to a higher superficial area. With regard to the OER experiments and earlier published works, the CuNb2O6-10 h electrocatalyst showed the best catalytic activity observed for this system. In addition, the electrocatalysts showed superior long-term stability at J = 10 mA cm−2 for 15 h. The optical band gap values were 2.06 eV (CuNb2O6-5 h) and 1.92 eV (CuNb2O6-10 h). Thus, it can be concluded that solid-state reaction at a high temperature for milled CuO and Nb2O5 powders is an excellent method for producing single-phase copper niobates for the OER process.

Author Contributions

Conceptualization, K.F.G.d.A., C.S.L. and M.J.S.L.; methodology, K.F.G.d.A. and V.M.S.F.S.; formal analysis, K.F.G.d.A., C.S.L., M.D.d.S., G.D.S.V., J.R.D.S., F.J.A.L. and K.C.G.; investigation, K.F.G.d.A., C.S.L., G.D.S.V., M.D.d.S., J.R.D.S., F.J.A.L. and M.A.M.; resources, U.U.G., K.C.G., and M.A.M.; data curation, K.F.G.d.A., C.S.L., M.D.d.S., J.R.D.S. and F.J.A.L.; writing—original draft preparation, K.F.G.d.A., C.S.L., J.R.D.S., F.J.A.L. and M.A.M.; writing—review and editing, K.F.G.d.A., M.A.M., K.C.G., U.U.G. and M.J.S.L.; visualization, K.F.G.d.A., C.S.L., M.J.S.L. and M.A.M.; supervision, U.U.G. and M.A.M.; project administration, U.U.G. and M.A.M.; funding acquisition, U.U.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

Francisco J. A. Loureiro acknowledges the following grants/projects: 2020.02797.CEECIND/CP1589/CT0030, 2022.02498.PTDC, UIDB/00481/2020, and UIDP/00481/2020 from Fundação para a Ciência e a Tecnologia (FCT), and CENTRO-01-0145-FEDER-022083 from Centro Portugal Regional Operational Programme (Centro2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF). Kelly C. Gomes acknowledges the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the grant 310639/2022-0. Jakeline R. D. Santos acknowledges the CNPq for the grant 152799/2024-9. Marco A. Morales acknowledges the CNPq for the grants 421854/2021-9 and 317194/2021-6.

Conflicts of Interest

The authors declare no conflicts of interest.

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Ceramics 08 00055 g001
Figure 1. SEM images of the precursor powders: (a) Nb2O5; (b) CuO.
Figure 1. SEM images of the precursor powders: (a) Nb2O5; (b) CuO.
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Figure 2. (a) Diffractograms of the CuNb2O6 samples; (b) zoom-in showing the main peaks of the orthorhombic and monoclinic structures; (c) crystal structure of the monoclinic CuNb2O6, plotted using VESTA software.
Figure 2. (a) Diffractograms of the CuNb2O6 samples; (b) zoom-in showing the main peaks of the orthorhombic and monoclinic structures; (c) crystal structure of the monoclinic CuNb2O6, plotted using VESTA software.
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Figure 3. SEM images of the milled powders before annealing for (a) 5 h and (b) 10 h.
Figure 3. SEM images of the milled powders before annealing for (a) 5 h and (b) 10 h.
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Figure 4. SEM images of the CuNb2O6 particles: (a,c,e) CuNb2O6-5 h; (b,d,f) CuNb2O6-10 h.
Figure 4. SEM images of the CuNb2O6 particles: (a,c,e) CuNb2O6-5 h; (b,d,f) CuNb2O6-10 h.
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Ceramics 08 00055 g005
Figure 5. (a) FT-IR; (b) Tauc plots for the CuNb2O6 samples; (c) Raman spectra for the CuNb2O6 samples.
Figure 5. (a) FT-IR; (b) Tauc plots for the CuNb2O6 samples; (c) Raman spectra for the CuNb2O6 samples.
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Ceramics 08 00055 g006aCeramics 08 00055 g006b
Figure 6. OER performance testing of CuNb2O6-5 h, CuNb2O6-10 h, and Ni foam. (a) LSV curve, the grey line is a reference for the current density of 10 mA cm−1, (b) Tafel Slope, (c) double-layer capacitance (Cdl), (d) electrochemical surface area (ECSA), (e) chronopotentiometry at J = 10 mA cm−2 for 15 h, (f) representative Nyquist plots obtained by electrochemical impedance spectroscopy (EIS) under a DC potential of 0.65 V vs. RHE (the inset represents the equivalent circuit model used to fit the data), and (g) total polarization resistance (Rp) and double-layer capacitance (Cdl) as a function of the applied potential determined by electrochemical impedance spectroscopy (EIS).
Figure 6. OER performance testing of CuNb2O6-5 h, CuNb2O6-10 h, and Ni foam. (a) LSV curve, the grey line is a reference for the current density of 10 mA cm−1, (b) Tafel Slope, (c) double-layer capacitance (Cdl), (d) electrochemical surface area (ECSA), (e) chronopotentiometry at J = 10 mA cm−2 for 15 h, (f) representative Nyquist plots obtained by electrochemical impedance spectroscopy (EIS) under a DC potential of 0.65 V vs. RHE (the inset represents the equivalent circuit model used to fit the data), and (g) total polarization resistance (Rp) and double-layer capacitance (Cdl) as a function of the applied potential determined by electrochemical impedance spectroscopy (EIS).
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Table 1. Elemental analysis by XRF.
Table 1. Elemental analysis by XRF.
SampleCuO (mol.%)Nb2O5 (mol.%)
CuNb2O6-5 h47.452.6
CuNb2O6-10 h48.151.9
Table 2. Rietveld refinement results; angles are β values.
Table 2. Rietveld refinement results; angles are β values.
Crystal Structure Parameters Quality of Fit
SamplePhaseβ
(°)		Lattice Parameters (Å)Phase (wt.%)Cell Volume (Å3)Cryst. Size (nm)RwpRexpχ2
abc
CuNb2O6-5 hMono91.75.00514.1725.76187.4408.559.92.720.843.19
Ortho90.014.0875.6045.11612.6403.954.1
CuNb2O6-10 hMono91.75.01914.2115.77687.1411.858.32.880.833.47
Ortho90.014.1545.6255.13612.9408.942.6
Table 3. Comparison of the OER performance of niobate electrocatalysts containing copper and cobalt. Data referring to an overpotential to generate J = 10 mA cm−2 (ƞ10).
Table 3. Comparison of the OER performance of niobate electrocatalysts containing copper and cobalt. Data referring to an overpotential to generate J = 10 mA cm−2 (ƞ10).
ElectrocatalystElectrolyte (1.0 M)ղ10 (mV)Tafel (mV dec−1)Reference
CuNb2O6-10 hKOH34770This work
CuNb2O6-5 hKOH476160This work
CuO/CuNb2O6KOH380102[15]
CoNb2O6@NiKOH 22050[55]
CoNb2O6@AgKOH 25054[55]
CoNb2O6 KOH330100[55]
CoNb2O6@Ag0.6Ni0.4KOH400 *40[55]
CuO-NKOH38576[55]
CoNb2O6-600 °CKOH50382[56]
CoNb2O6-1000 °CKOH559122[56]
* J = 100 mA cm−2.
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de Araújo, K.F.G.; Lourenço, C.S.; Souza, V.M.S.F.; da Silva, M.D.; Vasconcelos, G.D.S.; Lima, M.J.S.; Santos, J.R.D.; Gomes, K.C.; Loureiro, F.J.A.; Morales, M.A.; et al. CuNb2O6 Particles Obtained via Solid-State Reaction and Application as Electrocatalyst for Oxygen Evolution Reaction. Ceramics 2025, 8, 55. https://doi.org/10.3390/ceramics8020055

AMA Style

de Araújo KFG, Lourenço CS, Souza VMSF, da Silva MD, Vasconcelos GDS, Lima MJS, Santos JRD, Gomes KC, Loureiro FJA, Morales MA, et al. CuNb2O6 Particles Obtained via Solid-State Reaction and Application as Electrocatalyst for Oxygen Evolution Reaction. Ceramics. 2025; 8(2):55. https://doi.org/10.3390/ceramics8020055

Chicago/Turabian Style

de Araújo, Kívia F. G., Cleber S. Lourenço, Vitor M. S. F. Souza, Matheus D. da Silva, Gabriel D. S. Vasconcelos, Maria J. S. Lima, Jakeline R. D. Santos, Kelly C. Gomes, Francisco J. A. Loureiro, Marco A. Morales, and et al. 2025. "CuNb2O6 Particles Obtained via Solid-State Reaction and Application as Electrocatalyst for Oxygen Evolution Reaction" Ceramics 8, no. 2: 55. https://doi.org/10.3390/ceramics8020055

APA Style

de Araújo, K. F. G., Lourenço, C. S., Souza, V. M. S. F., da Silva, M. D., Vasconcelos, G. D. S., Lima, M. J. S., Santos, J. R. D., Gomes, K. C., Loureiro, F. J. A., Morales, M. A., & Gomes, U. U. (2025). CuNb2O6 Particles Obtained via Solid-State Reaction and Application as Electrocatalyst for Oxygen Evolution Reaction. Ceramics, 8(2), 55. https://doi.org/10.3390/ceramics8020055

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