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Article

Alternative Supports for Electrocatalysis of the Oxygen Evolution Reaction in Alkaline Media

by
Gwénaëlle Kéranguéven
1,*,
Ivan Filimonenkov
2,
Thierry Dintzer
1 and
Matthieu Picher
3
1
Institut de Chimie et des Procédés Pour l’Energie, l’Environnement et la Santé (ICPEES), CNRS, Université de Strasbourg, UMR 7515, 25 Rue Becquerel, 67087 Strasbourg, France
2
Technological Institute for Superhard and Novel Carbon Materials of National Research Centre Kurchatov Institute, 7A Tsentralnaya Street, 108840 Moscow, Russia
3
IPCMS Département Surfaces et Interfaces (DSI), 23 Rue du Lœss, 67034 Strasbourg, France
*
Author to whom correspondence should be addressed.
Electrochem 2025, 6(3), 23; https://doi.org/10.3390/electrochem6030023
Submission received: 18 March 2025 / Revised: 11 June 2025 / Accepted: 17 June 2025 / Published: 25 June 2025
(This article belongs to the Topic Electrocatalytic Advances for Sustainable Energy)
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Abstract

The anodic stability of tungsten carbide (WC) and iron oxide with a spinel structure (Fe3O4) were compared against similar data for nanostructured, boron-doped diamond (BDD), and the benchmark Vulcan XC72 carbon, in view of their eventual application as alternative supports for the anion exchange membrane electrolyzer anode. To this end, metal oxide composites were prepared by the in situ autocombustion (ISAC) method, and the anodic behavior of materials (composites as well as supports alone) was investigated in 1 M NaOH electrolyte by the rotating ring–disc electrode method, which enables the separation oxygen reduction reaction and materials’ degradation currents. Among all supports, BDD has proven to be the most stable, while Vulcan XC72 is the least stable under the anodic polarization, with Fe3O4 and WC demonstrating intermediate behavior. The Co3O4-BDD, -Fe3O4, -WC, and -Vulcan composites prepared by the ISAC method were then tested as catalysts of the oxygen evolution reaction. The Co3O4-BDD and Co3O4-Fe3O4 composites appear to be competitive electrocatalysts for the OER in alkaline medium, showing activity comparable to the literature and higher support stability towards oxidation, either in cyclic voltammetry or chronoamperometry stability tests. On the contrary, WC- and Vulcan-based composites are prone to degradation.

Graphical Abstract

1. Introduction

One of the major challenges of sustainable development is the replacement of fossil fuels by renewable energy. The intermittency of renewable energies (solar, wind) poses an issue of energy storage. Hydrogen produced by water electrolysis is seen as a promising green fuel allowing the storage of renewable energies. Two reactions occur in parallel during water electrolysis: hydrogen production at the cathode and oxygen at the anode, the latter known as the oxygen evolution reaction (OER, Equation (1)). However, the oxygen evolution reaction is a sluggish kinetics reaction responsible for the low electrochemical performance of the electrolyzer. One of the challenges in deploying this technology is to improve the OER kinetics and develop innovative and efficient materials without noble metals, as reviewed in [1]. Transition metal oxides with a spinel structure are promising electrocatalysts for the OER in alkaline media, because they are (i) easy to synthesize, (ii) inexpensive, and (iii) their properties can be widely modified by changing their elemental composition.
4HO− → O2 + 4e− + 2H2O
Due to its high activity and stability, Co3O4 oxide with spinel structure is extensively studied for the OER [2,3,4]. However, because of its low electronic conductivity, it is often either mixed with or supported on carbon black, containing a high content of sp2 carbon and giving it high electronic conductivity [5,6,7]. The addition of carbon black allows the improvement of the apparent electrocatalytic activity of the oxide nanoparticles; specifically, it increases the electronic conductivity of the catalyst thanks to the improvement of the surface utilization of the oxide nanoparticles due to the increase in the number of accessible active sites [6,7]. However, carbon black is oxidized under the OER condition (see Carbon Oxidation Reaction (COR) (Equation (2))), which prevents their use in water electrolysis systems. To this end, we have recently investigated the stability of carbon materials in alkaline media [4].
C + 4HO− → CO2 + 4e− + 2H2O
In this work, we investigate nanostructured, boron-doped diamond (BDD), tungsten carbide (WC), and iron oxide with a spinel structure (Fe3O4) as alternative catalyst supports for preparing Co3O4-based OER composites.
BDD consists of sp3 carbon with a negligible contribution of sp2 carbon [8], unlike the Vulcan XC72 carbon from Cabot, containing a high content of sp2 carbon, used as a benchmark. There is considerable interest in using BDD as an electrode material (see the Table S1 in Supplementary Information). Electrodes based on BDD have been investigated in the literature [9]. It was concluded that BDD offers a wide electrochemical window, high anodic stability [9], chemical inertness [9], low capacitance, and a fast electrochemical response [10]. It is known to withstand corrosive and high temperature/pressure environments [10]. Even though the BDD is more expensive than the Vulcan XC72 carbon, we recently observed that the apparent lifetime for the BDD is more than an order of magnitude higher than the lifetime of Vulcan XC72 by applying Faraday’s law, as explained in [5].
Tungsten carbide (WC) is a combination of carbon and tungsten atoms with the high strength and rigidity of a covalent compound, the high melting point of an ionic crystal, and the electromagnetism of a transition metal. This ceramic has been studied in chemical and electrochemical catalysis since the discovery of its Pt-like character, as reported by Levy and Boudart [11]. Tungsten carbide’s low electrical resistivity of about 0.2 µΩ·m is comparable with that of some metals (e.g., vanadium). Several publications have reviewed WC as an electrocatalyst, but misunderstandings about its stability persist. For example, Zhu et al. show that WC can be used as a stable carrier in fuel cells, where it has shown superior stability and conductivity to commercial carbon products, such as Vulcan [12]. Supported metal catalysts on WC have shown stability and high activity towards HER [13,14] and other reactions, such as methanol oxidation or OER [15,16]. Weidman et al. demonstrated that the potential stability window of WC is much wider in acidic than alkaline electrolytes [17]. Finally, Göhl et al. recently warned the users of WC regarding the degradation of WC in acid in the potential interval relevant to fuel and electrolysis cells [18].
Fe3O4 is a rather inexpensive and highly conductive material. Although most transition metal oxides are semiconductors, Fe3O4 (magnetite) has low electrical resistivity 0.3 mΩ m [19], which is significantly lower than that of Fe2O3 (~kΩ m). This is ascribed to electron exchange between the Fe(II) and Fe(III) ions in Fe3O4. Recently, Fe3O4-core CoFe2O4-shell nanoparticles showed high stability and high OER activity in alkaline medium per Co unit mass, suggesting synergetic interaction between the core and the shell [20,21].
To evaluate the suitability of WC and Fe3O4 as alternative supports for the transition metal oxide-based electrocatalysts in OER condition, we studied their anodic behavior in the potential interval 1.53–2.03 V vs. RHE in NaOH 1 M and compared it to the similar measurements published for Vulcan XC72 and BDD [22]. Since at high anodic potentials the electrochemical degradation of materials (such as COR for carbons) occurs in parallel with OER, the rotating ring–disc electrode (RRDE) was used to separate these two contributions. Previously, we evaluated the limits of applicability of the RRDE during OER in alkaline media for determining the amount of oxygen [5,23]. It has been shown that the RRDE is a suitable approach for separating the ORR and the support oxidation reaction in the absence of vigorous oxygen bubble formation.
Furthermore, Co3O4-WC, -Fe3O4, -BDD, and -Vulcan XC72 composites were prepared by an in situ autocombustion method described in our previous publications [6,7]. Once synthesized, the materials were analyzed using X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), and low-temperature nitrogen adsorption (analyzed in the framework of Brunauer–Emmett–Teller approximation, BET method). These analyses are used to define the structure of the synthesized metal oxide, the composition of the composite (metal oxide support), and the active surface of materials. Then these materials were electrochemically studied using the RRDE to determine their OER activity and the oxygen faradaic efficiency. Stability tests were performed at 1.66 and 1.58 V vs. RHE to assess the tolerance of composites to degradation.

2. Materials and Methods

2.1. Materials

A 5 mg mL−1 suspension of BDD in deionized water was purchased from Adama Nanotechnology. The morphological and structural characteristics of mesoporous BDD are summarized in Table 1, with a BET specific surface area of 181 m2 g−1, a BJH pore volume of 0.4 cm3 g−1, and a BJH adsorption mean pore width (“mean pore size” in Table 1) of 8.5 nm. The morphological and structural characteristics of BDD have been extensively studied in the literature [22,24,25,26]. The specifications of BDD used in this work are summarized in Table S1 in the Supplementary Information.
WC powder was purchased from Merck. WC consists of nanoparticles with an irregular and spiky menhir-like shape [22,24,25,26]. It is a mesoporous material with a low specific surface (4 m2 g−1 by BET), a low BJH volume of pores (0.010 cm3 g−1) compared to BDD nanoparticles, and a mean pore size of 8.5 nm (Table 1).
Fe3O4 (magnetite) has a cubic inverse spinel group structure, which consists of a cubic close packed array of oxide ions where all the Fe(II) ions occupy half of the octahedral sites, and the Fe(III) are split evenly across the remaining octahedral and the tetrahedral sites. Fe3O4 was purchased from Merck. It is a mesoporous material with a lower porosity than BDD nanoparticles. Its specific surface area is 36 m2 g−1, its BJH volume of pores is 0.045 cm3 g−1, and its mean pore size is 4 nm (see Table 1).
Vulcan XC72 is a mesoporous carbon black purchased from Cabot Company, with a similar porosity than BDD. Its specific surface, determined by the BET method, is 192 m2 g−1. Its BJH volume of pores is 0.3 cm3 g−1 and the mean pore size is 11 nm (Table 1). Vulcan XC72 nanoparticles are spherical and have a regular shape [7].
The unsupported Co3O4 was synthesized by autocombustion (AC), as detailed in [22] and the following Equation (3):
3Co(NO3)2 + Ψ NH2CH2COOH → Co3O4 + 2Ψ CO2 + 5Ψ/2H2O + (6 + Ψ)/2N2 + (7 − 9/4Ψ)O2
To prepare the composites within the in situ autocombustion (ISAC) route, an appropriate amount of support material (either BDD or WC, Fe3O4, Vulcan XC72) was added depending on the desired composition. Only the WC composite was calcined at 300 °C for 30 min, as indicated by the addition of “calc” at the end of the composite name, to ensure the formation of a spinel phase. The nomenclature used to designate the composites is explained in [6,7,22] and summarized in Scheme 1. The composite designation begins with the weight % of Co3O4 in the final material (determined by TGA), then the active phase (Co3O4 in this work), the synthesis (AC or ISAC), and finally the support material (BDD or WC or Fe3O4 or Vulcan). The unsupported Co3O4 synthesized by the autocombustion is named Co3O4AC. High-purity water (18.2 MΩ cm, <1 ppb TOC, ELGA Purelab, UK) was used to prepare all aqueous solutions in this study.

2.2. Material Characterisation

The synthesized materials were characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM), N2-physisorption, and thermogravimetric analysis (TGA) to investigate the phase purity, the morphology, and the specific surface area (SBET) using the Brunauer, Emmett, and Teller theory and by the Barrett, Joyner, and Halenda method for the volume (VBJH) and the surface area (SBJH) of the pores and the weight percentage of Co3O4 in the composites, respectively. Details of the equipment used and the conditions for analysis are given in [22]. Moreover, a Gatan spectrometer and an ultrahigh vacuum spectrometer (equipped with a RESOLVE 120 MCD5 hemispherical electron analyzer, using the Al Kα line (1486.6 eV) of a dual anode X-ray source as incident radiation, and recording the survey and high-resolution spectra in constant pass energy mode at 100 and 20 eV respectively) were used to determine the presence of cobalt element in the supported and the unsupported spinel (and the presence of iron element in the 26Co3O4ISACFe3O4 composite). Spectral maps were produced using a 30 mm2 EDAX Octane 9 silicon detector mounted on a Zeiss GeminiSEM 500 scanning electron microscope (SEM). A working distance of 13 mm enabled both the microanalysis (EDX) and backscattered electron (BSD) detectors to be used for electron imaging. A working voltage (EHT) of 12 kV was used to make the K lines of Fe and Co visible.

2.3. Electrochemical Measurements

The electrocatalytic OER activity of the Co3O4-supported composites was evaluated using a thin-film approach. The preparation of electrode and the conditions of the electrochemical measurements are described in [22,27]. The following equation was used to determine the oxygen faradaic efficiencies (F.E.):
F.E.(%) = 100 × (4IR/nORR)/(N × ID)
where ID and IR are the disc and the ring currents, nORR = 2 for a gold ring [28], and N is the collection factor. N factor determination is estimated using the well-known reversible ferro-, ferricyanide redox couple, as explained in [22], where N = 0.22 at 1600 rpm.
The potential was iR-corrected after the measurements using the estimated cell resistance, as described in [22].
Aging tests were performed by either staying at a constant potential of 1.66 V vs. RHE for 3 h or at 1.58 V vs. RHE for 30 min at 10 mV s−1, depending on the composite studied.

2.4. The Methodology for Measuring the OER Activity and Selectivity

While numerous publications report on the activities and behavior of transition metal oxide catalysts in the OER, for the same catalyst composition, the reported activity values often show significant variation. Thus, the following questions can be asked: why the data are so different, and is the OER activity properly determined? The OER activity is conventionally determined in liquid electrolyte three-electrode electrochemical cell using a thin-film rotating disc (RDE) approach, which was proposed by Schmidt et al. for carbon-supported Pt [29] and later extended by Suntivich et al. [30] to transition metal oxides. It should be noted, however, that the assessment of electrocatalytic activity by non-planar thin-film RDE is by no means trivial, since it is based on several assumptions, among them are (i) homogeneity of the film (ii) and the catalyst effectiveness factor equal to unity (100%) (for more information the reader is referred to [31] and references therein). The latter is only valid if the catalyst is conductive, and either current or potential gradients are absent in the catalyst film. Since many of transition metal oxides are semiconductors, the conductivity of the oxide film may not be sufficient for ensuring 100% catalyst utilization. Even for conductive materials (e.g., carbon-supported oxide nanoparticles) for thick catalyst layers, potential and current gradients may arise (subject to the catalyst film thickness and hence loading on the RDE) in the film compromising activity evaluation.
A convenient approach to check whether the electrocatalytic activity is correctly determined is to study the influence of the catalyst loading on the measured current. For example, Figure 1A shows OER current measured at 1.55 V vs. RHE versus the oxide loading on the GC disc for the 29Co3O4ISACVulcan composite. It appears clearly that the disc current is proportional to the oxide loading up to 100 µg cm−2 and then shows high scatter either levelling off or decaying. The plot confirms that catalytic activity cannot be correctly determined at high catalyst loadings. In Figure S1 in the Supplementary Information, the OER current measured at 1.55 V vs. RHE versus the oxide loading on the GC disc for the 16Co3O4ISACBDD, 32Co3O4ISACWC, and 26Co3O4ISACFe3O4 composites in NaOH media are presented. The disc currents are at least proportional to the Co3O4 loading up to 40 µg cm−2. Considering this, in what follows, we used the oxide loadings below 40 µgoxide cm−2.
Another important characteristic related to the OER studies is the faradaic efficiency (F.E.), since the measured current does not necessarily correspond to the OER alone but may include electrochemical degradation currents of the support or the metal oxide itself. The faradaic efficiency of the OER (or O2%) can be studied, for example, by the RRDE approach. The RRDE method is widely used in electrochemistry for the detection of electroactive reaction intermediates, since the current measured at the ring gives an indication of the amount of species formed at the disc. However, the RRDE technique is applicable for the detection of oxygen produced at the disc only under specific conditions, notably in the absence of oxygen bubble formation. We estimated a critical disk current density, i.e., 450 µA cm−2, at which the oxygen bubble formation starts to affect the measured OER efficiency and explained that this critical current density value likely depends on the microstructure and porosity of the IrO2 supported on glassy carbon layer [23]. Thus, the ring of the RRDE can be used as an O2 sensor only when the rate of the O2 formation is not too high, which requires the application of low catalyst loadings and avoiding high electrode potentials. For example, Figure 1B represents the OER faradaic efficiency (F.E.) versus time for the 29Co3O4ISACVulcan sample measured at a constant potential of 1.58 V vs. RHE (with ERing = 0.3 V vs. RHE) at two selected oxide loadings (2 and 15 µgoxide cm−2). One can see that the faradaic efficiency is close to 100% for the lower loading of oxide but decays down to 70% at 15 µg cm−2 loading. Note that the noise in the transient is due to bubble formation, the latter escaping the ring and resulting in the underestimation of the F.E. Figure 1C is associated to Figure 1B and represents disc currents versus time. It confirms that the underestimation of the F.E. at 15 µg cm−2 oxide loading is due to the highest disc current, hence the high rate of O2 production. Then a critical disk current may be estimated, in this work, at around 200 µA cm−2 from the Figure 1C.
Thus, we conclude that to compare the anodic behavior of various materials, i.e., OER activity and F.E., it is essential, in addition to avoid high electrode potentials, to optimize the loading of material on the GC disk of an R(R)DE. Firstly, the loading must not be too high to ensure a correct determination of the electrocatalytic activity (shown in Figure 1A) and the formation of bubbles of oxygen during the OER, thus avoiding errors in the measurement of the F.E. (shown in Figure 1B) [22]. Secondly, the loading must be high enough to cover the surface of the disc (in our case the GC) surface and avoid the contribution of the GC to the total measured current, as discussed in more detail in Section 3.2.1.

3. Results and Discussion

3.1. Morphology and Structure of Synthesized Materials

The formation of a crystalline Co3O4 spinel phase (ICDD card n°042-1467) designated by a “#” symbol both for the unsupported oxide synthesized with the AC method and for ISAC composites was evidenced by XRD in Figure 2. Moreover, Fe3O4 oxide (COD 1010369) is designated by a “§” symbol, WC by a “$” symbol, BDD by a “£” symbol, and Vulcan XC72 by a “&” symbol. We can notice, in the XRD pattern of 16Co3O4ISACBDD and the 32Co3O4ISACWCcalc composites, respectively, a small amount of the CoB phase (ICDD card n°75-1066) at 2θ = 49.3° designated with the @ symbol and a small amount of the tungsten trioxide WO3 phase (COD 2311041) at 2θ = 23.6–24.4° designated with the “/” symbol (Figure 1). Hatel et al. showed that tungsten trioxide can be formed by oxidation and/or the thermal treatment of WC [32]. The size of the spinel oxide crystals is listed in Table 1 (applying the Scherrer equation with the shape factor K = 0.89).
In Table 1, one may notice similar crystal size of Co3O4 in the 16Co3O4ISACBDD, 26Co3O4ISACFe3O4, 32Co3O4ISACWCcalc, and Co3O4AC samples (around 22 nm) and a twice smaller one (around 9 nm) in the 29Co3O4ISACVulcan composite. Previously in [6,7], we investigated LaMnO3 perovskite oxide–carbon composites prepared by the ISAC method and concluded that the specific surface area, pore size, and size distribution of carbon supports affect nucleation and the growth of oxide nanoparticles, with Vulcan XC72 offering the best characteristics among studied carbon materials. In this work, it seems also that among the studied materials, the porosity of Vulcan XC72 is best suited for the nucleation and growth of Co3O4 nanoparticles resulting in the smallest XRD derived average particle size.
Figure 3 shows TEM images of composites and bare supports. Note that in Figure 3, different magnifications are deliberately used to better observe the support and the oxide components, whose particle size varies in the composites studied. Vulcan XC72 carbon consists of spherical nanoparticles of around 25 nm (Figure 3C). BDD nanoparticles look like irregular spiky menhir (see Figure 3I and images in [24,25]). Fe3O4 comprises spherical nanoparticles of 15–30 nm (Figure 3L). WC material comprises menhir-like shape structures of a few hundred nanometers (Figure 3E,F). In the unsupported Co3O4AC sample, 15–50 nm size spherical nanoparticles are strongly agglomerated (Figure 2C of [22]), while in the Vulcan-supported sample (Figure 3A,B), BDD-supported sample (Figure 3G,H), Fe3O4-supported sample (Figure 3J,K), and WC-supported sample (Figure 3D), spherical nanoparticles are observed on the support surface with a smaller size around 5–10 nm. We have to notice that for the 26Co3O4ISACFe3O4 composite it is difficult to discern Fe3O4 and Co3O4 particles, since both crystallize in a similar spinel structure. However, based on the images of bare Fe3O4 support, one may hypothesize that the size of Co3O4 is around 5–10 nm; whereas, the size of Fe3O4 is higher, i.e., 15–30 nm. EDS elemental mapping and the corresponding spectra help us identify the presence of iron, cobalt, and oxygen in the 26Co3O4ISACFe3O4 composite (see Figure S2 in the Supplementary Information). EDS mapping shows that oxygen is distributed homogeneously throughout the composite. The iron and cobalt elements appear separately but in the presence of oxygen. The atomic composition of the oxygen and metals is consistent with the formation of spinel oxides (i.e., Co3O4 and Fe3O4). EDS mapping overlay shows an enrichment of Co at the surface of the composite, indicating that the nanoparticles of Co3O4 spinel recover the nanoparticles of Fe3O4 spinel during the ISAC synthesis. This is consistent with previous studies in which metal oxide nanoparticles were dispersed within the pores and at the surface of the support [7,22]. It should be noted that traces of carbon are observed in the EDS spectra, which may be due to carbonaceous residues formed during the ISAC process, as explained in our previous papers [6,7]. From the XRD, TEM, and EDS elemental data (Figure 2 and Figure 3 and Figure S2 in Supplementary Information), we conclude that the ISAC route is well suited to support Co3O4 nano-sized particles on Fe3O4 and WC. This agrees with Co3O4 supported on BDD and Vulcan XC72 carbon [22]. The fact that for BDD-, Fe3O4-, and WC-based composites, the TEM-size of Co3O4 nanoparticles is substantially smaller than the DRX-size can be explained by the existence of a fraction of larger particles in these composites resulting in narrowing of the XRD peaks. The formation of the unsupported Co3O4 spinel and supported composites was confirmed by XPS and EELS in Figure S3 and discussed in the Supplementary Information.
Table 1 also provides values of the BET surface area, the surface and area of mesopores and micropores, and the mean pore size for the supports and the synthesized composites. Note that the specific surface area determined by the BET method corresponds to the total surface area of the metal oxide and the supports accessible to N2 adsorption and thus does not show direct correlation with the size of metal oxide crystallites. ISAC synthesis allows the development of high specific surface composites of several tens m2 g−1. However, we have to note that if the specific surface area of the support is low, the specific surface area of the corresponding composite is also low. For example, for the 26Co3O4ISACFe3O4 composite, which has a specific surface of 7 m2 g−1, the specific surface area of Fe3O4 material support is 36 m2 g−1. Indeed, in a previous paper [7], we suggested that the morphology and the porous structure of carbon materials influence the growth of the metal oxide nanoparticles in the composites prepared by in situ autocombustion. Further experiments should be carried out to improve the synthesis and increase the specific surface area of Co3O4 composite based on Fe3O4.

3.2. Electrochemical Characterisation

3.2.1. Anodic Behavior of Vulcan XC72, BDD, WC, and Fe3O4

To investigate the anodic stability of WC and Fe3O4 and thus evaluate their suitability as support for the Co3O4 OER catalysts, we used RRDE and applied successive potential steps of 1.53–2.03 V vs. RHE in increments of 50 or 100 mV to the disc for 15 min and then measured the resulting current transients (Figure 4). By measuring the current of the disk (Figure 4A) and the current of the ring (Figure 4B), the OER current faradaic efficiency (F.E. calculated using Equation (4) and plotted in Figure 4C) can be calculated. Then the OER currents can be separated from the electrochemical support degradation currents shown in Figure 4D. The results obtained were then compared with the recently published data [22] for Vulcan XC72 carbon and BDD supports plotted in Figure 4 for convenience. Note that for Vulcan XC72, which produced the highest current at the disc (Figure 4A), a lower loading (91 μg cm−2) was used compared to other materials (183.4 µg cm−2). Indeed, in our previous publication, we have shown that for BDD (which is much more stable against anodic degradation than conventional carbon black materials), the utilization of low loading results in an overestimation of degradation currents, since the GC support (the reader is referred to [22] for further details), unless fully screened by the thin layer, may also contribute to the measured currents (see dotted curves of GC in Figure 4). Therefore, the loading of BDD was optimized at 183.4 µg cm−2, and a similar loading was also used for Fe3O4 and WC materials.
Figure 4A shows that disc currents increase with potential for all the studied materials with the highest anodic currents detected for Vulcan XC72 and the lowest for the BDD and WC powders, which is consistent with the chemical and electrochemical inertness previously reported in the literature [9,10,17,20,21]. One should keep in mind however that high anodic currents do not necessarily reflect fast material’s degradation, since the OER in alkaline electrolytes may occur on the studied materials even without Co3O4. This is reflected by the ring currents (Figure 4B) and the faradaic efficiencies (Figure 4C) calculated from the disc and ring currents (see Equation (4)). The ring currents (Figure 4B) increase with potential for Vulcan XC72 in all the potential interval. For WC, Fe3O4, and BDD materials the ring currents increase from 1.53 to 1.83 V vs. RHE and decrease from 1.83 to 2.03 V vs. RHE. The highest ring currents are detected for Vulcan XC72 and the lowest one for the WC and BDD powders, similarly to the disc currents (Figure 4A). Then depending on the ratio of ring and disc currents, the faradaic efficiencies are calculated (Figure 4C) and increase from 1.58 V to 1.73 V vs. RHE for all materials, reaching ~85–95% for Vulcan XC72, 60% for WC, ~70% for BDD, and 80% for Fe3O4. At a potential greater than 1.73 V, the OER efficiencies decrease.
Since the disk currents (Figure 4A) is the sum of the OER and the support degradation currents, the latter can be calculated from the overall disk current as jDisk (1–FE). Figure 4D represents the support degradation currents normalized to the support mass (the support degradation currents normalized to the geometric electrode area are presented in Figure S4 in the Supplementary Information). One may notice (i) degradation currents are observed on all the supports used in this study and (ii) lower mass-normalized support degradation currents on BDD, WC, and Fe3O4 compared to Vulcan XC72 in Figure 4D. For example, at 1.58 V vs. RHE the mass-normalized support degradation currents equal 500 mA g−1 for Vulcan XC72 against 65 mA g−1 for WC, 35 mA g−1 for Fe3O4, and 5 mA g−1 for BDD. Note also that the support degradation currents may be overvalued, since it is not possible to exclude the contribution of the surface of GC to the measured RRDE transients (see dotted curves of GC in Figure 4 and Figure S4 in the Supplementary Information). One may notice that for Fe3O4, which is thermodynamically unstable in the whole interval of investigated potentials (see Pourbaix diagram of iron), degradation kinetics is slower at low potential and increases at higher potentials (Figure 4D and Figure S4 in the Supplementary Information). The observed slow degradation kinetics agrees with the electrochemical inertness of Fe3O4 reported in the literature [20,21] and may be attributed to the formation of a thin passivating layer of γ-Fe2O3 on the surface of Fe3O4 nanoparticles [33] and reconstruction of the Fe3O4 surface [34]. Note that the thickness of the γ-Fe2O3 layer may be particle size-dependent as reported by Baaziz et al. [35]. At higher potentials, Fe3O4 could be oxidized into soluble species containing iron in a higher oxidation state (e.g., FeO42−) (see Pourbaix diagram of iron).
Regarding the WC material, even if the disc currents remain low (Figure 4A), the low and stable F.E. of the OER (Figure 4C) results in substantial currents of degradation. At a potential less than 1.63 V vs. RHE, WC degrades faster than Fe3O4, and at potential less than 1.93 V vs. RHE (Figure 4D), WC degrades faster than BDD. This agrees with the paper of Weidman et al. stating that WC is prone to dissolution in alkaline pH at OER potential [36]. Moreover, Göhl et al. observed that WC showed significant corrosion within the potential window for the oxygen evolution reaction, especially at oxidative potentials higher than 0.80 V vs. RHE. After reaching this threshold potential, the surface oxidizes continuously, and only partial passivation is achieved, even in acid electrolyte [18]. BDD shows the lowest degradation currents at potentials less than 1.93 V vs. RHE, where it degrades slightly more than WC. Anyway, BDD, Fe3O4, and WC show lower degradation currents compared to Vulcan XC72, with BDD being the most stable among the material supports.

3.2.2. Electrochemical Properties of Composite Materials

Cyclic voltammetry (CV) in a supporting electrolyte under nitrogen atmosphere is a convenient tool for in situ exploring of the interfacial properties of electrocatalytic materials. In Figure 5, iR-corrected CVs and chronoamperograms of unsupported Co3O4AC and ISAC composites are presented. Figure 5B shows an enlargement of the current of the disc in Figure 5A. Note that the disc currents of Co3O4AC and the Vulcan XC72, BDD, Fe3O4, and WC supports are shown in Figure S5 in the Supplementary Information for comparison. As outlined in previous publications [37,38,39,40,41], the redox peak observed around 1.45 V vs. RHE in Figure 5B can be assigned to the Co3+/Co4+ redox couple. It is interesting to note that the Co2+/Co3+ redox peak around 1.15 V vs. RHE is clearly observed only for the 26Co3O4ISACFe3O4 composite. Neither the Co2+/Co3+ nor Co3+/Co4+ redox couple is observed for 32Co3O4ISACWC composite, and one can note that the shape of the CV significantly differs from those of the other composites. Indeed, the pseudocapacitive behavior is not observed above 1.2 V vs. RHE, and oxidative currents are detected on both the positive and the negative CV scans, which agrees with the oxidation of WC at anodic potential, as previously reported by Göhl et al. [18] and Weidman et al. [36].
The charge corresponding to the redox peaks of the Co3+/Co4+ couple increases in the following series: Co3O4AC < 29Co3O4ISACVulcan < 26Co3O4ISACFe3O4 < 16Co3O4ISACBDD. The charge of the cathodic peak was used (due to difficulties to separate the anodic OER from pseudocapacitive currents), estimated to 0.01, 24.4, 10.0, and 4.8 C g−1oxide for Co3O4AC, 16Co3O4ISACBDD, 26Co3O4ISACFe3O4, and 29Co3O4ISACVulcan, respectively, and summarized in Table 1. The increase in the charge of the Co3+/Co4+ redox peaks observed for the ISAC composites, compared to the unsupported Co3O4, suggests that the addition of support increases the accessible sites of Co3O4 nanoparticles on the surface of ISAC composites. Previously, we explained this trend by a superior dispersion, an inferior agglomeration area, and an improvement of the charge transport between the Co3O4 particles and the current collector [22].

3.2.3. Electrochemical Activity of Composite Materials

The RRDE method was used to investigate the electrochemical activity of Co3O4 composites in Figure 5, to separate the support degradation and the OER currents. In Figure 5C,D, one can observe the ring currents and the oxygen faradaic efficiency, respectively, of ISAC composites associated to the disc currents shown in Figure 5A,B. The current of OER observed at the disc (Figure 5A) is supported by the appearance of the current of ORR at the ring (Figure 5C). We observe that the currents of the ring decrease in the following series: 29Co3O4ISACVulcan > 26Co3O4ISACFe3O4 > 16Co3O4ISACBDD > 32Co3O4ISACWCcalc > Co3O4AC, suggesting decrease of the OER activity in this series. Meanwhile, for Co3O4AC, both the currents of the disc and ring are much lower, confirming its lower OER activity. The oxygen F.E. can be calculated with reasonable accuracy at potentials above ca. 1.46 and 1.52 V (Figure 5D) for the Co3O4 composites and Co3O4AC, respectively. The OER “onsets” (Figure 5D) increase similarly to the ring currents, i.e., 29Co3O4ISACVulcan > 26Co3O4ISACFe3O4 > 16Co3O4ISACBDD > 32Co3O4ISACWCcalc > Co3O4AC. The 32Co3O4ISACWCcalc composite shows disc currents higher than those of 26Co3O4ISACFe3O4 and 16Co3O4ISACBDD but much lower ring currents and hence a lower faradaic efficiency. This apparent discrepancy can be attributed to oxidation of WC in the OER potential interval, as previously observed [18,36]. This observation also highlights the fact that it may not be appropriate to access the OER activity from disc currents alone.
We have determined that the potentiostatic mode is the most appropriate method for estimating the faradaic efficiency of the OER using the RRDE technique. This is since the fact that the slow diffusion of oxygen in the pores of the catalytic layer can result in a delay in the detection of oxygen at the ring, which can compromise the reliability of the faradaic efficiency measurement [23]. Thus, RRDE transients at a constant disc potential of 1.58 V vs. RHE for the unsupported Co3O4AC and for Co3O4 composites are compared in Figure 6 (note that a suitable catalyst loading was used to decrease current and avoid oxygen bubble formation). As demonstrated in Figure 6A,B, the currents of the disc and the ring of the Co3O4AC sample are relatively low. This is in agreement with the low cobalt redox peak charge. However, the currents (disc and the ring) and the F.E. increase for composites significantly (see Figure 6C, which is coherent with Figure 5D). The faradaic efficiency for the 16Co3O4ISACBDD, 26Co3O4ISACFe3O4, and 29Co3O4ISACVulcan composites with values close to 90–98% is the highest (this difference could be considered within the accuracy of the measurements), suggesting that these composites are stable against oxidative degradation, at least for RRDE transients at the disk potential of 1.58 V vs. RHE and/or under short-term polarization conditions. High faradaic efficiency observed for 26Co3O4ISACFe3O4 and 29Co3O4ISACVulcan may seem surprising considering the Fe3O4 and Vulcan XC72 degradation currents discussed in Section 3.2.1. This related inconsistency may be explained by (i) the different conditions such as the polarization duration, i.e., only 10 min in Figure 6 (longer polarization duration, under industrially relevant electrolysis conditions, are required to better evaluate the electrocatalyst’s potential for electrolysis applications), (ii) the slow degradation kinetics observed for the Fe3O4 and Vulcan XC72 supports at 1.58 V vs. RHE (see Figure S4 in Supporting Information, for Fe3O4 support equal to about 10 µA cm−2 and for Vulcan XC72 support of 35 µA cm−2), and (iii) a “protecting effect” of Co3O4 nanoparticles deposited on the Fe3O4 and Vulcan supports. Regarding the latter, in several papers, authors reported an improved carbon durability due to the presence of oxide in the OER oxide–carbon composite [5,42,43]. For example, we studied a composite OER catalyst consisting of a perovskite oxide mixed with carbon (Sibunit-152, Vulcan XC-72R, and acetylene black) and discovered that the oxide component protected the carbon counterpart [5]. In [42], the authors show that the addition of Ni perovskite to the nitrogen-doped carbon nanotubes (N-CNTs) improves the galvanostatic stability for the OER by almost two orders of magnitude. The nanoscale geometries of the perovskites and the CNTs enhance the number of metal–support and charge transfer interactions and thus the activity. Finally, in [43], the authors explained that, in the literature, high oxygen evolution reaction (OER) active electrocatalysts often exhibit improved OER durability in the presence of carbon oxidation reaction (COR). The coincidence of activity and durability in OER electrocatalysts is theoretically understood as preferential depolarization in galvanostatic situations. For a system involving multiple independent and competitive reactions at constant-current conditions, the overpotential is most dominantly determined by the most facile reaction, meaning that the most facile reaction is responsible for a dominant portion of the overall current. Therefore, higher OER activity improves durability by mitigating the current responsible for COR. This coincidence was then proven experimentally by comparing two catalysts with the same chemical identity of different dimensions (5 and 100 nm). The smaller-dimension catalyst, having a greater number of active sites per fixed mass, was found to be more durable due to the decrease in carbon corrosion.
For 32Co3O4ISACWCcalc composite the disc and ring currents are much smaller, suggesting a lower OER activity. Moreover, we can note that in Figure 6C the faradaic efficiency value is low at short times (~20%, which is similar of the one of Figure 5D) and then increases with time and reaches a maximum of 70%, suggesting a transition phenomenon with slow kinetics possibly due to the WC degradation. Moreover, we have to underline that the faradaic efficiency of all materials may be underestimated due to the involvement of the underlying GC surface (cf. Section 3.2.1). This explains the low faradaic efficiency of the unsupported Co3O4 sample.
Table 2 shows the OER activities, normalized to the mass obtained at 1.58 V vs. RHE, of the synthesized materials of this work and of the materials selected from the literature for comparison. Table 2 includes also the corresponding Tafel slopes (for this work, the reader can refer to our previous publication [22] for the procedure used to determine the Tafel slopes). Note that the activity and the Tafel slopes in Table 2 are not consistent with each other. This may be due to the divergent experimental conditions (loading of catalyst, thickness of the catalytic layer, addition of conductive additive, electrolyte, etc.) and to the material morphology or structure (e.g., crystallography of the surface, defects, etc.) of the nanoparticles of Co3O4. For example, in this work, the activity per mass of the composites is similar to that of a core-shell Fe3O4@CoFe2O4 nanoparticles [20] and higher than the activity of mesoporous Co3O4 supported in acetylene black [2], the Co3O4 impregnated on the graphene [3] and Co3O4 coated on carbon cloth [4]. One may notice that contrary to this work, in many publications, the activity of OER is only determined using the disk current, and so potential support and oxide degradation (if any) are neglected of the activity of OER.
In Table 2, the Tafel slope values vary from ~48 to 160 mV dec−1. For ISAC composites, the smaller Tafel slope (~50 mV dec−1) corroborates ohmic limitations in the film and lack of mass transport, as explained in our previous work [22]. Additionally, the highest Tafel slopes, observed for 32Co3O4ISACWCcalc and Co3O4AC, result from the lack of electronic conductivity due to ohmic limitation in the film, as well as with mesoporous Co3O4 supported in acetylene black and Co3O4 impregnated on graphene [2,3]. We can note that the high value of Tafel slope observed for 32Co3O4ISACWCcalc (134 mV dec−1) is consistent with the degradation of WC.
Table 2 also includes the electrochemical active surface area (ECSA). ECSA values were estimated considering the cathodic peak of cobalt at 1.45 V vs. RHE of cyclic voltammograms in Figure 5B, the mean surface density of Co cations estimated from crystallographic data of Co3O4 spinel of 1.27 × 1015 cm−2 (see Table S2 in the Supplementary Information), and the electron charge of 1.6 × 10−19 C. The ECSA calculation is explained in [22].

3.2.4. Stability Tests of ISAC Composites

Considering the previous results, a three-hour stability test of 29Co3O4ISACVulcan, 26Co3O4ISACFe3O4, and 16Co3O4ISACBDD was performed under a high constant potential of 1.66 V vs. RHE. Figure 7 shows CVs measured on the disc (black curves) and after 3 h of polarization (red curves) of 29Co3O4ISACVulcan (A), 26Co3O4ISACFe3O4 (B), and 16Co3O4ISACBDD (C). In Figure S6 in the Supplementary Information, the corresponding ring currents are presented. First, we observe a good reproducibility of the disc and ring currents for Fe3O4 (Figure 7B and Figure S6B) and BDD-based composites (Figure 7C and Figure S6C). As mentioned above, the observed stability of the Fe3O4 composite may be tentatively assigned to a “protecting effect” (see Section 3.2.3) of Co3O4 or by the slow degradation kinetics of the Fe3O4 supports observed at potentials less than 1.68 V vs. RHE (see Figure S4 in the Supporting Information, for Fe3O4 support equal to about 60 µA cm−2). Finally, we observed a significant loss in both the disc and ring currents (Figure 7A and Figure S6A) for the Vulcan XC72-based composite after three hours of the polarization test at 1.66 V vs. RHE. It shows that (i) the degradation of the Vulcan XC72 carbon support at 1.66 vs. RHE, and (ii) there is no “protecting effect” of Co3O4 oxide in these experimental conditions (i.e., 3 h at 1.66 V vs. RHE).
As WC degradation had previously been observed in this study, a simple 30 min stability test was performed on the 32Co3O4ISACWCcalc composite at a lower potential of 1.58 V vs. RHE. Figure S7 in the Supplementary Information shows a significant loss in the disc and ring currents, confirming the degradation of the WC-based composite in OER conditions.
However, long-term durability studies under industrially relevant electrolysis conditions, such as higher temperatures and the use of polymer rather than liquid electrolytes, are required to better evaluate the electrocatalyst’s potential for electrolysis applications.

4. Conclusions

To conclude, tungsten carbide (WC) and iron oxide with spinel structure (Fe3O4) were investigated as possible substitutes for carbon support in transition metal oxide composites for the oxygen evolution reaction and compared with recent results obtained for BDD and Vulcan XC72. To do so, we prepared composites of WC and Fe3O4, with Co3O4 (active for the OER) by the in situ autocombustion (ISAC) route. We observed that (i) the nanoparticles of Co3O4, of ca. 5–10 nm, are well supported on WC and Fe3O4, and (ii) the deposition of Co3O4 nanoparticles on support significantly increases the number of accessible sites of spinel surface and the increase in the charge of the Co4+/Co3+ redox peak.
The stability of support materials (alone) and the corresponding Co3O4-based composites and their OER activity were studied with the rotating ring–disc electrode (RRDE), using the ring electrode as an oxygen sensor. We demonstrated the importance of using RRDE method to discriminate between the OER and the electrochemical degradation currents.
Investigation of the degradation of support at anodic potential confirms that the stability of Fe3O4 support against oxidation is better than the one of Vulcan XC72 carbon at potential below 1.68 V vs. RHE, but lower than the one of BDD. In contrast, WC support does not seem to show much advantage over Vulcan XC72 in terms of its anodic stability until the applied potential is increased up to ~1.9 V vs. RHE.
A significant improvement in the mass-normalized OER activity of the ISAC composites is demonstrated compared to the unsupported Co3O4. Then the Co3O4 alternative support composites show OER activity comparable or even superior to the literature data for Co3O4.
Moreover, three-hour stability tests at 1.66 V vs. RHE confirm the higher stability of BDD- and Fe3O4-based ISAC composites; whereas, the composites based on Vulcan XC72 carbon and WC (for this latter, a 30 min stability test at 1.58 V vs. RHE was performed) suffers from support degradation.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/electrochem6030023/s1, Table S1: Summary of specifications of BDD; Table S2: Summary of crystallographic data for the calculation of the mean surface of cation estimated from crystallographic data of Co3O4 spinel; Figure S1: Plot of current disc obtained at 1.55 V vs. RHE versus the oxide loading for three composites 16Co3O4ISACBDD, 26Co3O4ISACFe3O4 and 32Co3O4ISACWC. On the left in 1M NaOH electrolyte and on the right in 10−2 M NaOH. Error bars were estimated based on at least two successive measurements; Figure S2: (A) MEB image of 26Co3O4ISACFe3O4 composite, (B, C and D) EDS elemental mapping (K lines) of iron, cobalt, oxygen, € EDS elemental mapping (K lines) of an overlay of cobalt and iron, (F, G, H) EDS spectra of area 1, area 2 and the full area, respectively, where the atomic percentages (K lines) of the iron, cobalt, oxygen are shown in the insert; Figure S3: (A) Co-2p high resolution X-ray photoelectron spectra of the Co3O4AC spinel and ISAC composites. (B) Co L2,3 edge EEL spectra of the Co3O4AC spinel and ISAC composites. Co3O4AC (blue), 16Co3O4ISACBDD (black), 29Co3O4SACVulcan (light green), 32Co3O4ISACWC (cyan), 26Co3O4ISACFe3O4 (dark green) and Fe3O4 commercial (orange); Figure S4: Transients obtained by RRDE of the GC support and GC-supported materials in 1M of NaOH saturated with N2 at 1600 rpm and 25 °C. The support oxidation currents were calculated from Figure 4A,C transients and normalized to the geometric surface area. Vulcan XC72 (91 µg cm−2) is in red, BDD (183.4 µg cm−2) is in blue, WC (183.4 µg cm−2) is in cyan, Fe3O4 (183.4 µg cm−2) is in green and GC is dotted magenta curve; Figure S5: RRDE disc currents of Co3O4 (pink), Vulcan (red), BDD (dark blue), Fe3O4 (green) and WC (light blue). The loadings are 15 µg cm−2 for Co3O4AC, 91 µg cm−2 loading for Vulcan and BDD, 40 µg cm−2 loading for Fe3O4 and WC; Figure S6: RRDE voltammograms of ring currents (associated to the disc currents of the Figure 7) obtained at ERing = 0.3 V vs. RHE for 29Co3O4ISACVulcan (A), 26Co3O4ISACFe3O4 (B) and 16Co3O4ISACBDD (C) at 1600 rpm and 10 mV s−1 in N2 saturated NaOH 1 mol L−1. All ring currents were corrected for the background ring currents (oxygen traces reduction). The loading of oxide in the composites is 12 µgoxide cm−2geo. Black curves have been obtained before the three-hours stability test of ISAC composites studied using chronoamperometry at 1.66V vs. RHE and the red curves have been obtained after the test; Figure S7: RRDE voltammograms of disc and ring currents obtained at ERing = 0.3 V vs. RHE for 32Co3O4ISACWCcalc at 1600 rpm and 10 mV s−1 in N2 saturated NaOH 1 mol L−1. All ring currents were corrected for the background ring currents (oxygen traces reduction). The loading of Co3O4 in 32Co3O4ISACWC is 40 µgoxide cm−2geo. Black curves have been obtained before the 30-min stability test of 32Co3O4ISACWC studied using chronoamperometry at 1.58V vs. RHE and the red curves have been obtained after the test. References [44,45,46,47,48] are cited in Supplementary Materials.

Author Contributions

Conceptualization, G.K.; methodology, G.K. and I.F.; validation, G.K. and I.F.; formal analysis, G.K., T.D., M.P. and I.F.; investigation, G.K.; resources, G.K.; data curation, G.K.; writing—original draft preparation, G.K.; writing—review and editing, G.K., T.D., M.P. and I.F.; funding acquisition, G.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by HYPE-FRCR2020 project (G.K.) and DAEMONHYC H2 PEPR project supported by the “France 2030” government investment plan managed by the French National Research Agency under reference “ANR-22-PEHY-0010” (G.K. and T.D).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article or Supplementary Materials.

Acknowledgments

Tokuyama company is acknowledged for providing alkaline ionomer. The authors are indebted to Corinne Ulhaq-Bouillet for the discussion of TEM and EELS data, Christophe Mélard and Sécou Sall for assistance with TGA measurements, Fabrice Vigneron for assistance with BET measurements, and Alain Rach for his technical support.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
BDD Boron-doped diamond
VulcanVulcan XC72 carbon
OERoxygen evolution reaction
CORcarbon oxidation reaction
ORRoxygen evolution reaction
HERhydrogen evolution reaction
RRDErotating ring disc electrode
RDErotation disc electrode
XRDX-ray diffraction
TGAthermogravimetric
TEMtransmission electron microscopy
EELSelectron energy loss spectroscopy
XPSX-ray photoelectron spectroscopy
BETBrunauer–Emmett–Teller approximation
BJHBarrett–Joyner–Halenda
ACautocombustion
ISACin situ autocombustion
calccalcined
TOCtotal organic carbon
UHVultrahigh vacuum
SBETspecific surface area
GCglassy carbon
RHEreversible hydrogen electrode
F.E.faradaic efficiency
ICDDInternational Centre for Diffraction Data
CODcrystal open database
ECSAelectrochemically active surface area
CV(s)cyclic voltammogram(s)

References

  1. Kumar, R.S.; Austeria, P.M.; Neethinathan, C.S.S.; Ramakrishnan, S.; Sekar, K.; Kim, A.R.; Kim, D.H.; Yoo, P.J.; Yoo, D.J. Highly mixed high-energy d-orbital states enhance oxygen evolution reactions in spinel catalysts. Appl. Surf. Sci. 2023, 641, 158469. [Google Scholar] [CrossRef]
  2. Sun, T.; Liu, P.; Zhang, Y.; Chen, Z.; Zhang, C.; Guo, X.; Ma, C.; Gao, Y.; Zhang, S. Boosting the electrochemical water splitting on Co3O4 through surface decoration of epitaxial S-doped CoO layers. Chem. Eng. J. 2020, 390, 124591. [Google Scholar] [CrossRef]
  3. Chou, S.-C.; Tso, K.-C.; Hsieh, Y.-C.; Sun, B.-Y.; Lee, J.-F.; Wu, P.-W. Facile synthesis of Co3O4@CoO@Co gradient Core@Shell nanoparticles and their applications for oxygen evolution and reduction in alkaline electrolytes. Materials 2020, 13, 2703. [Google Scholar] [CrossRef] [PubMed]
  4. Zhang, L.; Zhang, T.; Dai, K.; Zhao, L.; Wei, Q.; Zhang, B.; Xiang, X. Ultrafine Co3O4 nanolayer-shelled CoWP nanowire array: A bifunctional electrocatalyst for overall water splitting. RSC Adv. 2020, 10, 29326. [Google Scholar] [CrossRef] [PubMed]
  5. Filimonenkov, I.S.; Bouillet, C.; Kéranguéven, G.; Simonov, P.A.; Tsirlina, G.A.; Savinova, E.R. Carbon materials as additives to the OER catalysts: RRDE study of carbon corrosion at high anodic potentials. Electrochim. Acta 2019, 321, 134657. [Google Scholar] [CrossRef]
  6. Kéranguéven, G.; Royer, S.; Savinova, E. Synthesis of efficient Vulcan–LaMnO3 perovskite nanocomposite for the oxygen reduction reaction. Electrochem. Commun. 2015, 50, 28–31. [Google Scholar] [CrossRef]
  7. Kéranguéven, G.; Bouillet, C.; Papaefthymiou, V.; Simonov, P.A.; Savinova, E.R. How key characteristics of carbon materials influence the ORR activity of LaMnO3- and Mn3O4-carbon composites prepared by in situ autocombustion method. Electrochim. Acta 2020, 353, 136557. [Google Scholar] [CrossRef]
  8. Panich, A.M.; I Shames, A.; Goren, S.; Froumin, N.; Shenderova, O. Magnetic resonance study of lightly boron-doped diamond. Mater. Res. Express 2019, 6, 75612. [Google Scholar] [CrossRef]
  9. Einaga, Y. Development of electrochemical applications of boron-doped diamond electrodes. Bull. Chem. Soc. Jpn. 2018, 91, 1752–1762. [Google Scholar] [CrossRef]
  10. McLaughlin, M.H.S.; Corcoran, E.; Pakpour-Tabrizi, A.C.; de Faria, D.C.; Jackman, R.B. Influence of temperature on the electrochemical window of boron doped diamond: A comparison of commercially available electrodes. Nat. Sci. Rep. 2020, 10, 15707. [Google Scholar] [CrossRef]
  11. Levy, R.B.; Boudart, M. Platinum-Like Behavior of Tungsten Carbide in Surface Catalysis. Science 1973, 181, 547. [Google Scholar] [CrossRef] [PubMed]
  12. Zhu, W.; Ignaszak, A.; Song, C.; Baker, R.; Hui, R.; Zhang, J.; Nan, F. Nanocrystalline tungsten carbide (WC) synthesis/characterization and its possible application as a PEM fuel cell catalyst support. Electrochim. Acta 2012, 61, 198–206. [Google Scholar] [CrossRef]
  13. Chang, Q.; Zhang, X.; Yang, Z. First-principles study on the Cu-Au alloy monolayer supported on WC for hydrogen evolution. Appl. Surf. Sci. 2021, 565, 150568. [Google Scholar] [CrossRef]
  14. Koverga, A.A.; Flórez, E.; Rodriguez, J.A. Pushing Cu uphill of the volcano curve: Impact of a WC support on the catalytic activity of copper toward the hydrogen evolution reaction. Int. J. Hydrogen Energy 2021, 49, 25092–25102. [Google Scholar] [CrossRef]
  15. Liu, Y.; You, H.; Kimmel, Y.C.; Esposito, D.V.; Chen, J.G.; Moffat, T.P. Self-terminating electrodeposition of Pt on WC electrocatalysts. Appl. Surf. Sci. 2020, 504, 144472. [Google Scholar] [CrossRef]
  16. Wang, Y.; Su, J.; Dong, L.; Zhao, P.; Zhang, Y.; Wang, W.; Jia, S.; Zang, J. A novel hybrid of Ni and WC on new-diamond supported Pt electrocatalyst for methanol oxidation and oxygen reduction reactions. ChemCatChem 2017, 9, 3982–3988. [Google Scholar] [CrossRef]
  17. Weidman, M.C.; Esposito, D.V.; Hsu, I.J.; Chen, J.G. Electrochemical Stability of Tungsten and Tungsten Monocarbide (WC) Over Wide pH and Potential Ranges. J. Electrochem. Soc. 2010, 157, F179–F188. [Google Scholar] [CrossRef]
  18. Göhl, D.; Mingers, A.M.; Geiger, S.; Schalenbach, M.; Cherevko, S.; Knossalla, J.; Jalalpoor, D.; Schüth, F.; Mayrhofer, K.J.; Ledendecker, M. Electrochemical stability of hexagonal tungsten carbide in the potential window of fuel cells and water electrolyzers investigated in a half-cell configuration. Electrochim. Acta 2018, 270, 70–76. [Google Scholar] [CrossRef]
  19. Itai, R.; Shibuya, M.; Matsumura, T.; Ishi, G. Electrical resistivity of Magnetite anodes. J. Electrochem. Soc. 1971, 118, 1709. [Google Scholar] [CrossRef]
  20. Royer, L.; Makarchuk, I.; Hettler, S.; Arenal, R.; Asset, T.; Rotonnelli, B.; Bonnefont, A.; Savinova, E.; Pichon, B.P. Core–shell Fe3O4@CoFe2O4 nanoparticles as high-performance anode catalysts for enhanced oxygen evolution reaction. Sustain. Energy Fuels 2023, 7, 3239–3243. [Google Scholar] [CrossRef]
  21. Royer, L.; Bonnefont, A.; Asset, T.; Rotonnelli, B.; Velasco-Vélez, J.-J.; Holdcroft, S.; Hettler, S.; Arenal, R.; Pichon, B.; Savinova, E. Cooperative Redox Transitions Drive Electrocatalysis of the Oxygen Evolution Reaction on Cobalt–Iron Core–Shell Nanoparticles. ACS Catal. 2023, 13, 280–286. [Google Scholar] [CrossRef]
  22. Kéranguéven, G.; Filimonenkov, I.S.; Savinova, E.R. Investigation of the stability of the boron-doped diamond support for Co3O4-based oxygen evolution reaction catalysts synthesized through in situ autocombustion method. J. Electroanal. Chem. 2022, 916, 116367. [Google Scholar] [CrossRef]
  23. Filimonenkov, I.S.; Istomin, S.Y.; Antipov, E.V.; Tsirlina, G.A.; Savinova, E.R. Rotating ring-disk electrode as a quantitative tool for the investigation of the oxygen evolution reaction. Electrochim. Acta 2018, 286, 304–312. [Google Scholar] [CrossRef]
  24. Costa, G.C.; Shenderova, O.; Mochalin, V.; Gogotsi, Y.; Navrotsky, A. Thermochemistry of nanodiamond terminated by oxygen containing functional groups. Carbon 2014, 80, 544–550. [Google Scholar] [CrossRef]
  25. Shenderova, O.A.; McGuire, G.E. Science and engineering of nanodiamond particle surfaces for biological applications. Biointerphases 2015, 10, 030802. [Google Scholar] [CrossRef]
  26. Kondo, T.; Kato, T.; Miyashita, K.; Aikawa, T.; Tojo, T.; Yuasa, M. Boron-Doped Diamond Powders for Aqueous Supercapacitors with High Energy and High Power Density. J. Electrochem. Soc. 2019, 166, A1425–A1431. [Google Scholar] [CrossRef]
  27. Poux, T.; Napolskiy, F.; Dintzer, T.; Kéranguéven, G.; Istomin, S.Y.; Tsirlina, G.; Antipov, E.; Savinova, E. Dual role of carbon in the catalytic layers of perovskite/carbon composites for the electrocatalytic oxygen reduction reaction. Catal. Today 2012, 189, 83–92. [Google Scholar] [CrossRef]
  28. Bard, A.J.; Faulkner, L.R. Electrochemical Methods—Fundamentals and Applications, 2nd ed.; Wiley: Phoenix, TX, USA, 2001; ISBN 978-0-471-04372-0. [Google Scholar]
  29. Schmidt, T.J.; Gasteiger, H.A.; Stäb, G.D.; Urban, P.M.; Kolb, D.M.; Behm, R.J. Characterization of High-Surface-Area Electrocatalysts Using a Rotating Disk Electrode Configuration. J. Electrochem. Soc. 1998, 145, 2354–2358. [Google Scholar] [CrossRef]
  30. Suntivich, J.; Gasteiger, H.A.; Yabuuchi, N.; Shao-Horn, Y. Electrocatalytic Measurement Methodology of Oxide Catalysts Using a Thin-Film Rotating Disk Electrode. J. Electrochem. Soc. 2010, 157, B1263. [Google Scholar] [CrossRef]
  31. Savinova, E.R.; Oshchepkov, A.G. Benchmarking in electrocatalysis. In Comprehensive Inorganic Chemistry III, 3rd ed.; Elsevier: Amsterdam, The Netherlands, 2023; pp. 492–550. [Google Scholar]
  32. Hatel, R.; Baitoul, M. Nanostructured Tungsten Trioxide (WO3): Synthesis, structural and morphological investigations. J. Phys. Conf. Ser. 2019, 1292, 012014. [Google Scholar] [CrossRef]
  33. Fleischer, K.; Mauit, O.; Shvets, I.V. Stability and capping of magnetite ultra-thin films. Appl. Phys. Lett. 2014, 104, 192401. [Google Scholar] [CrossRef]
  34. Grumelli, D.E.; Wiegmann, T.; Barja, S.; Reikowski, F.; Maroun, F.; Allongue, P.; Balajka, J.; Parkinson, G.S.; Diebold, U.; Kern, K.; et al. Electrochemical Stability of the Reconstructed Fe3O4(001) Surface. Angew. Chem. Int. Ed. 2020, 59, 21904–21908. [Google Scholar] [CrossRef] [PubMed]
  35. Baaziz, W.; Pichon, B.P.; Fleutot, S.; Liu, Y.; Lefevre, C.; Greneche, J.-M.; Toumi, M.; Mhiri, T.; Begin-Colin, S. Magnetic Iron Oxide Nanoparticles: Reproducible Tuning of the Size and Nanosized-Dependent Composition, Defects, and Spin Canting. J. Phys. Chem. C 2014, 118, 3795–3810. [Google Scholar] [CrossRef]
  36. Weidman, M.C.; Esposito, D.V.; Hsu, Y.-C.; Chen, J.G. Comparison of electrochemical stability of transition metal carbides (WC, W2C, Mo2C) over a wide pH range. J. Power Sources 2012, 202, 11–17. [Google Scholar] [CrossRef]
  37. Acedera, R.A.E.; Gupta, G.; Mamlouk, M.; Balela, M.D.L. Solution combustion synthesis of porous Co3O4 nanoparticles as oxygen evolution reaction (OER) electrocatalysts in alkaline medium. J. Alloys Compd. 2020, 836, 154919. [Google Scholar] [CrossRef]
  38. Reikowski, F.; Maroun, F.; Pacheco, I.; Wiegmann, T.; Allongue, P.; Stettner, J.; Magnussen, O.M. Operando Surface X-Ray Diffraction Studies of Structurally Defined Co3O4 and CoOOH Thin Films During Oxygen Evolution. ACS Catal. 2019, 9, 3811–3821. [Google Scholar] [CrossRef]
  39. Bergmann, A.; Martinez-Moreno, E.; Teschner, D.; Chernev, P.; Gliech, M.; de Araújo, J.F.; Reier, T.; Dau, H.; Strasser, P. Unified structural motifs of the catalytically active state of Co(oxyhydr)oxides during the electrochemical oxygen evolution reaction. Nat. Catal. 2018, 1, 711–719. [Google Scholar] [CrossRef]
  40. Koza, J.A.; He, Z.; Miller, A.S.; Switzer, J.A. Electrodeposition of Crystalline Co3O4—A Catalyst for the Oxygen Evolution Reaction. Chem. Mater. 2012, 24, 3567–3573. [Google Scholar] [CrossRef]
  41. Faisal, F.; Bertram, M.; Stumm, C.; Cherevko, S.; Geiger, S.; Kasian, O.; Lykhach, Y.; Lytken, O.; Mayrhofer, K.J.J.; Brummel, O.; et al. Atomically Defined Co3O4(111) Thin Films Prepared in Ultrahigh Vacuum: Stability Under Electrochemical Conditions. J. Phys. Chem. C 2018, 122, 7236–7248. [Google Scholar] [CrossRef]
  42. Alexander, C.T.; Abakumov, A.M.; Forslund, R.P.; Johnston, K.P.; Stevenson, K.J. Role of the Carbon Support on the Oxygen Reduction and Evolution Activities in LaNiO3 Composite Electrodes in Alkaline Solution. ACS Appl. Energy Mater. 2018, 1, 1549–1558. [Google Scholar] [CrossRef]
  43. Yang, J.; Park, S.; Choi, K.Y.; Park, H.-S.; Cho, Y.-G.; Ko, H.; Song, H.-K. Activity-Durability Coincidence of Oxygen Evolution Reaction in the Presence of Carbon Corrosion: Case Study of MnCo2O4 Spinel with Carbon Black. ACS Sustain. Chem. Eng. 2018, 6, 9566–9571. [Google Scholar] [CrossRef]
  44. Liu, B.; Zheng, Y.; Peng, H.-Q.; Ji, B.; Yang, Y.; Tang, Y.; Lee, C.-S.; Zhang, W. Nanostructured and Boron-Doped Diamond as an Electrocatalyst for Nitrogen Fixation. ACS Energy Lett. 2020, 5, 2590–2596. [Google Scholar] [CrossRef]
  45. Biesinger, M.C.; Payne, B.P.; Grosvenor, A.P.; Lau, L.W.M.; Gerson, A.R.; Smart, R.S.C. Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni. Appl. Surf. Sci. 2011, 257, 2717–2730. [Google Scholar] [CrossRef]
  46. Biesinger, M.C.; Payne, B.P.; Lau, L.W.M.; Gerson, A.; Smart, R.S.C. X-ray photoelectron spectroscopic chemical state quantification of mixed nickel metal, oxide and hydroxide systems. Surf. Interface Anal. 2009, 41, 324. [Google Scholar] [CrossRef]
  47. Yang, J.; Liu, H.; Martens, W.N.; Frost, R.L. Synthesis and Characterization of Cobalt Hydroxide, Cobalt Oxyhydroxide and Cobalt Oxide Nanodiscs. J. Phys. Chem. C 2010, 114, 111–119. [Google Scholar] [CrossRef]
  48. Wang, Z.L.; Yin, J.S.; Jiang, Y.D. EELS analysis of cation valence states and oxygen vacancies in magnetic oxides. Micron 2000, 31, 571–580. [Google Scholar] [CrossRef]
Electrochem 06 00023 sch001
Scheme 1. Nomenclature used to designate the composites synthesized in this study.
Scheme 1. Nomenclature used to designate the composites synthesized in this study.
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Figure 1. (A) Plot of disc current of 29Co3O4ISACVulcan obtained at 1.55 V vs. RHE versus the oxide loading. (B) Faradaic efficiency and (C) disc currents obtained after RRDE chronoamperometry (ERing = 0.3 V vs. HRE) of 29Co3O4ISACVulcan at 1.58V vs. RHE at 1600 rpm in N2-saturated 1 M NaOH (2 or 15 µgoxide cm2geo). Error bars were estimated based on at least two successive measurements.
Figure 1. (A) Plot of disc current of 29Co3O4ISACVulcan obtained at 1.55 V vs. RHE versus the oxide loading. (B) Faradaic efficiency and (C) disc currents obtained after RRDE chronoamperometry (ERing = 0.3 V vs. HRE) of 29Co3O4ISACVulcan at 1.58V vs. RHE at 1600 rpm in N2-saturated 1 M NaOH (2 or 15 µgoxide cm2geo). Error bars were estimated based on at least two successive measurements.
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Figure 2. XRD patterns recorded for the synthesized oxide-support composites based on (A) Fe3O4, (B) WC, (C) BDD, and (D) Vulcan, the corresponding support and the unsupported Co3O4AC. In khaki Fe3O4, black 26Co3O4ISACFe3O4, orange 32Co3O4ISACWC, red WC, cyan BDD, pink 16Co3O4ISACBDD, grey Vulcan XC72, blue 29Co3O4ISACVulcan, green Co3O4AC. Co3O4 spinel (ICDD card n°042-1467) oxide phase is designated by the “#” symbol, Fe3O4 oxide (COD 1010369) by the “§” symbol, CoB phase (ICDD card n°75-1066) is designated with the symbol @, WO3 phase (COD 2311041) is designated with the “/” symbol, the supports Vulcan XC72, WC, and BDD are designated by “&”, “$”, and “£” symbols, respectively.
Figure 2. XRD patterns recorded for the synthesized oxide-support composites based on (A) Fe3O4, (B) WC, (C) BDD, and (D) Vulcan, the corresponding support and the unsupported Co3O4AC. In khaki Fe3O4, black 26Co3O4ISACFe3O4, orange 32Co3O4ISACWC, red WC, cyan BDD, pink 16Co3O4ISACBDD, grey Vulcan XC72, blue 29Co3O4ISACVulcan, green Co3O4AC. Co3O4 spinel (ICDD card n°042-1467) oxide phase is designated by the “#” symbol, Fe3O4 oxide (COD 1010369) by the “§” symbol, CoB phase (ICDD card n°75-1066) is designated with the symbol @, WO3 phase (COD 2311041) is designated with the “/” symbol, the supports Vulcan XC72, WC, and BDD are designated by “&”, “$”, and “£” symbols, respectively.
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Figure 3. TEM images of 29Co3O4ISACVulcan (A,B), Vulcan XC72 (C), 32Co3O4ISACWCcalc (D,E), WC (F), 16Co3O4ISACBDD (G,H), BDD (I), 26Co3O4ISACFe3O4 (J,K), and Fe3O4 (L).
Figure 3. TEM images of 29Co3O4ISACVulcan (A,B), Vulcan XC72 (C), 32Co3O4ISACWCcalc (D,E), WC (F), 16Co3O4ISACBDD (G,H), BDD (I), 26Co3O4ISACFe3O4 (J,K), and Fe3O4 (L).
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Figure 4. Transients obtained by RRDE of the GC support and GC-supported materials in 1M of NaOH saturated with N2 at 1600 rpm and 25 °C: (A) The geometric area-normalized disk currents; (B) the geometric area of the disc-normalized ring currents at a ring potential 0.3 V vs. RHE; (C) the oxygen faradaic efficiency. A 5-min polarization, at 0.93 V vs. RHE, is applied before and after each anodic potential. The currents of the ring are corrected to the background currents to reduce the traces of oxygen and normalized to the ring collection efficiency and the number of electrons transferred during the ORR at the ring (see Equation (4)). The support oxidation currents were calculated from A and C transients and normalized to the weight of the support material (D). Vulcan XC72 (91 µg cm−2) is in red, BDD (183.4 µg cm−2) is in blue, WC (183.4 µg cm−2) is in cyan, Fe3O4 (183.4 µg cm−2) is in green, and GC is the dotted magenta curve.
Figure 4. Transients obtained by RRDE of the GC support and GC-supported materials in 1M of NaOH saturated with N2 at 1600 rpm and 25 °C: (A) The geometric area-normalized disk currents; (B) the geometric area of the disc-normalized ring currents at a ring potential 0.3 V vs. RHE; (C) the oxygen faradaic efficiency. A 5-min polarization, at 0.93 V vs. RHE, is applied before and after each anodic potential. The currents of the ring are corrected to the background currents to reduce the traces of oxygen and normalized to the ring collection efficiency and the number of electrons transferred during the ORR at the ring (see Equation (4)). The support oxidation currents were calculated from A and C transients and normalized to the weight of the support material (D). Vulcan XC72 (91 µg cm−2) is in red, BDD (183.4 µg cm−2) is in blue, WC (183.4 µg cm−2) is in cyan, Fe3O4 (183.4 µg cm−2) is in green, and GC is the dotted magenta curve.
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Figure 5. RRDE voltammograms of Co3O4 (pink), 29Co3O4ISACVulcan (red), 16Co3O4ISACBDD (dark blue), 26Co3O4ISACFe3O4 (green), and 32Co3O4ISACWCcalc (light blue) at 1600 rpm and at the scan rate of 10 mV s−1 in 1M of NaOH saturated with N2. (A) is the current of disc, with (B) the zoom of (A). (C) is the current of ring at the ring potential of 0.3 V vs. RHE with a zoom in insert. (D) is the oxygen faradaic efficiency with a zoom in insert. All the currents of the ring were corrected to the background currents of the ring to reduce the traces of oxygen). The loadings are 15 µgoxide cm−2 for Co3O4AC, 15 µgoxide cm−2 loading for 29Co3O4ISACVulcan (or 51.7 µgcatalyst cm−2), for 16Co3O4ISACBDD (or 93.8 µgcatalyst cm−2), for 26Co3O4ISACFe3O4 (or 57.7 µgcatalyst cm−2), and for 32Co3O4ISACWcalc (or 46.9 µgcatalyst cm−2).
Figure 5. RRDE voltammograms of Co3O4 (pink), 29Co3O4ISACVulcan (red), 16Co3O4ISACBDD (dark blue), 26Co3O4ISACFe3O4 (green), and 32Co3O4ISACWCcalc (light blue) at 1600 rpm and at the scan rate of 10 mV s−1 in 1M of NaOH saturated with N2. (A) is the current of disc, with (B) the zoom of (A). (C) is the current of ring at the ring potential of 0.3 V vs. RHE with a zoom in insert. (D) is the oxygen faradaic efficiency with a zoom in insert. All the currents of the ring were corrected to the background currents of the ring to reduce the traces of oxygen). The loadings are 15 µgoxide cm−2 for Co3O4AC, 15 µgoxide cm−2 loading for 29Co3O4ISACVulcan (or 51.7 µgcatalyst cm−2), for 16Co3O4ISACBDD (or 93.8 µgcatalyst cm−2), for 26Co3O4ISACFe3O4 (or 57.7 µgcatalyst cm−2), and for 32Co3O4ISACWcalc (or 46.9 µgcatalyst cm−2).
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Figure 6. RRDE transients of Co3O4 (pink), 29Co3O4ISACVulcan (red), 16Co3O4ISACBDD (dark blue), 26Co3O4ISACFe3O4 (green), 32Co3O4ISACWCcalc (light blue) at 1600 rpm in 1M of NaOH saturated with N2 at the disk potential of 1.58V vs. RHE: (A) currents of disc normalized to the geometric area; (B) currents of ring normalized to the geometric area of disc at ERing = 0.3 V vs. RHE. (C) oxygen faradaic efficiency (FE%). All currents of ring were corrected for the background currents of ring (reduction in oxygen traces). The loadings are 15 µgoxide cm−2 for Co3O4AC, 2 µgoxide cm−2 loading for 29Co3O4ISACVulcan (or 7 µgcatalyst cm−2), 15 µgoxide cm−2 for 16Co3O4ISACBDD (or 93.8 µgcatalyst cm−2), for 26Co3O4ISACFe3O4 (or 57.7 µgcatalyst cm−2), and for 32Co3O4ISACWCcalc (or 46.9 µgcatalyst cm−2).
Figure 6. RRDE transients of Co3O4 (pink), 29Co3O4ISACVulcan (red), 16Co3O4ISACBDD (dark blue), 26Co3O4ISACFe3O4 (green), 32Co3O4ISACWCcalc (light blue) at 1600 rpm in 1M of NaOH saturated with N2 at the disk potential of 1.58V vs. RHE: (A) currents of disc normalized to the geometric area; (B) currents of ring normalized to the geometric area of disc at ERing = 0.3 V vs. RHE. (C) oxygen faradaic efficiency (FE%). All currents of ring were corrected for the background currents of ring (reduction in oxygen traces). The loadings are 15 µgoxide cm−2 for Co3O4AC, 2 µgoxide cm−2 loading for 29Co3O4ISACVulcan (or 7 µgcatalyst cm−2), 15 µgoxide cm−2 for 16Co3O4ISACBDD (or 93.8 µgcatalyst cm−2), for 26Co3O4ISACFe3O4 (or 57.7 µgcatalyst cm−2), and for 32Co3O4ISACWCcalc (or 46.9 µgcatalyst cm−2).
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Figure 7. RRDE voltammogram disc currents of 29Co3O4ISACVulcan (A), 26Co3O4ISACFe3O4 (B), and 16Co3O4ISACBDD (C) at 1600 rpm and 10 mV s−1 in N2 saturated 1 mol L−1 NaOH. The loading of oxide in the composites is 12 µgoxide cm−2geo. Black curves have been obtained before the three-hours stability test of ISAC composites studied using chronoamperometry at 1.66V vs. RHE, and the red curves have been obtained after the test.
Figure 7. RRDE voltammogram disc currents of 29Co3O4ISACVulcan (A), 26Co3O4ISACFe3O4 (B), and 16Co3O4ISACBDD (C) at 1600 rpm and 10 mV s−1 in N2 saturated 1 mol L−1 NaOH. The loading of oxide in the composites is 12 µgoxide cm−2geo. Black curves have been obtained before the three-hours stability test of ISAC composites studied using chronoamperometry at 1.66V vs. RHE, and the red curves have been obtained after the test.
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Table 1. Morphological, structural, and electrochemical characteristics of Fe3O4 commercial, WC commercial, Vulcan XC72, BDD, Co3O4AC, and ISAC composites.
Table 1. Morphological, structural, and electrochemical characteristics of Fe3O4 commercial, WC commercial, Vulcan XC72, BDD, Co3O4AC, and ISAC composites.
Support or
Composite		SBET/m2 g−1SBJH/
m2 g−1		VBJH/cm3 g−1Smicroa/m2 g−1Vmicro a/cm3 g−1Mean Pore Size/nmDDRX b/nmQ c/
C g−1
WC commercial44.90.010--8.5--
Fe3O4 commercial3643.40.045--4.2--
Vulcan XC-721941170.31185911--
BDD1811840.4--8.5--
16Co3O4ISACBDD43.142.50.140.0003-2124.40
29Co3O4ISACVulcan8046.70.3334.80.014628.594.80
26Co3O4ISACFe3O4711.40.0410-142310.00
32Co3O4ISACWCcalc1920.60.05221.3-10.122-
Co3O4AC1.470.560.00671.180.0006-240.01
SBET: BET surface area. SBJH: BJH surface area, VBJH: BJH volume and mean mesopore size were measured by B.J.H method. B.J.H. and BET methods are applied to the N2 adsorption branch of isotherm. a Vmicro and Smicro: volume and surface of micropores determined from the t-plot of the N2 adsorption branch of the isotherm. b The Scherrer equation with the shape factor K = 0.89 was used to estimate the average size of the Co3O4 crystallite from the XRD data. c Determined from electrochemistry, see Section 3.2.2 for details.
Table 2. Summary of the Co3O4 oxygen evolution activities obtained in this work and compared with literature data.
Table 2. Summary of the Co3O4 oxygen evolution activities obtained in this work and compared with literature data.
MaterialDesignation
(Oxide: Support
wt Ratio)		Oxide Loading/µg cm−2geoMediaECSA */m2 g−1Tafel Slope/
mV dec−1		Mass-Normalized
OER Activity
at 1.58 V RHE/
A g−1oxide		Ref.
Co3O4ACUnsupported Co3O4 nanoparticles151 M
NaOH		~5 × 10−3 *1191.3this work and [22]
16Co3O4ISACBDDBDD-supported Co3O4 nanoparticles (16:84)151 M
NaOH		13 *6514.2 [12.6]this work and [22]
29Co3O4ISACVulcanVulcan XC72
carbon-supported Co3O4 nanoparticles (29:71)		151 M
NaOH		2.4 *5955.7 [54.6]this work and [22]
26Co3O4ISACFe3O4Fe3O4-supported Co3O4 nanoparticles (26:74)151 M
NaOH		4.9 *4824.8 [23.3]this work
32Co3O4ISACWCcalcWC-supported Co3O4 nanoparticles (32:68)151 M
NaOH		-13419.7 [13.4]this work
Fe3O4@CoFe2O4Core-shell Fe3O4@CoFe2O4
nanoparticles		1.430.1 M NaOH-63 ± 228.8[18]
16Co3O4ISACSibunit152Sibunit-supported Co3O4 nanoparticles (16:84)151 M
NaOH		24 *8028.1 [18.5][22]
Co3O4Acetylene Black-
supported
mesoporous Co3O4		3801 M
NaOH		 1613.5[2]
Co3O4 (a)Carbon fiber paper-supportedCo3O4
nanoparticles		3001 M
KOH		 61.811[37]
Co3O4 (b)Co3O4 impregnated on the graphene127.40.1 M
KOH		 14719.6[3]
Co3O4 (c)Co3O4 post coated on carbon cloth81001M
KOH		 900.4[4]
Co3O4(111) (d)Co3O4 film electrodeposited on Au(111)150.1 M NaOH 65-[38]
* This estimation value has been calculated based on the charge of the Co4+/Co3+ reduction. In the square bracket: activity corrected of the support degradation. Data are given at 25 °C except for (a) at RT, (c) unknown, and (d) at RT, and potentials are iR-corrected except for (a), (b), and (c), where potentials are not iR-corrected.
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Kéranguéven, G.; Filimonenkov, I.; Dintzer, T.; Picher, M. Alternative Supports for Electrocatalysis of the Oxygen Evolution Reaction in Alkaline Media. Electrochem 2025, 6, 23. https://doi.org/10.3390/electrochem6030023

AMA Style

Kéranguéven G, Filimonenkov I, Dintzer T, Picher M. Alternative Supports for Electrocatalysis of the Oxygen Evolution Reaction in Alkaline Media. Electrochem. 2025; 6(3):23. https://doi.org/10.3390/electrochem6030023

Chicago/Turabian Style

Kéranguéven, Gwénaëlle, Ivan Filimonenkov, Thierry Dintzer, and Matthieu Picher. 2025. "Alternative Supports for Electrocatalysis of the Oxygen Evolution Reaction in Alkaline Media" Electrochem 6, no. 3: 23. https://doi.org/10.3390/electrochem6030023

APA Style

Kéranguéven, G., Filimonenkov, I., Dintzer, T., & Picher, M. (2025). Alternative Supports for Electrocatalysis of the Oxygen Evolution Reaction in Alkaline Media. Electrochem, 6(3), 23. https://doi.org/10.3390/electrochem6030023

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