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Article

Synthesis of Silver Nanocubes@Cobalt Ferrite/Graphitic Carbon Nitride for Electrochemical Water Splitting

by
Ausrine Zabielaite
*,
Olegas Eicher-Lorka
,
Zenonas Kuodis
,
Ramunas Levinas
,
Dijana Simkunaite
,
Loreta Tamasauskaite-Tamasiunaite
* and
Eugenijus Norkus
Center for Physical Sciences and Technology (FTMC), LT-10257 Vilnius, Lithuania
*
Authors to whom correspondence should be addressed.
Crystals 2023, 13(9), 1342; https://doi.org/10.3390/cryst13091342
Submission received: 28 July 2023 / Revised: 27 August 2023 / Accepted: 30 August 2023 / Published: 2 September 2023
(This article belongs to the Special Issue New Materials for Electrochemical Energy Storage Systems and Catalysis)
Browse Figures
Versions Notes

Abstract

:
This study presents the synthesis of graphitic carbon nitride (g-C3N4) and its nanostructures with cobalt ferrite (CoFe2O4) and silver nanocubes (Ag) when using the combined pyrolysis of melamine and the polyol method. The resulted nanostructures were tested as electrocatalysts for hydrogen and oxygen evolution reactions in alkaline media. It was found that Ag@CoFe2O4/g-C3N4 showed the highest current density and gave the lowest overpotential of −259 mV for HER to reach a current density of 10 mA cm−2 in a 1 M KOH. The overpotentials for reaching the current density of 10 mA·cm−2 for OER were 370.2 mV and 382.7 mV for Ag@CoFe2O4/g-C3N4 and CoFe2O4/g-C3N4, respectively. The above results demonstrated that CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4 materials could act as bifunctional catalysts due to their notable performances and high stabilities toward hydrogen and oxygen evolution reactions (HER and OER). Total water splitting in practical applications is a promising alternative to noble-metal-based electrocatalysts.

1. Introduction

The development of green hydrogen production technologies by water electrolysis (water splitting) has become one of the major current priorities [1,2,3,4,5,6,7]. Large-scale green H2 ecosystems are highly desirable, but challenging to implement. The development of green H2 ecosystems is severely constrained by limitations such as the relatively low production and storage efficiencies of renewable technologies; purification; and high transportation and utilization costs (which include safety concerns regarding the handling of H2 [8]). Green hydrogen production via the renewable method of water electrolysis (2H2O → 2H2 + O2, E° = 1.23 V vs. RHE) can result in pure H2 and O2 without carbon emissions, and it is based on the OER and HER taking place at the anode and cathode, respectively [1,3,8]. The main challenge is to design and develop novel, non-noble, and low-cost bifunctional electrode materials with high efficiency for both HER and OER. It is also important to address the environmental impact of engineered nanomaterials and the difficulty of recovering them, as well as the possibility of recycling, regeneration, and reuse [9]. Transition-metal boride/phosphide-based materials are attractive catalysts for H2 release due to their advantages of being earth-abundant elements, as well as their considerable catalytic activity, high stability, and low cost [10]. Metal nanoparticles as catalysts have also attracted a great deal of attention over the last few decades owing to their unique properties. However, due to their high surface energy, metal nanoparticles tend to aggregate into clumps and eventually into their bulk counterparts, resulting in reduced catalytic activity and long-term stability. Dispersing or anchoring the metal nanoparticles onto certain supporting materials with a large surface area to form a supported catalyst can improve the stability of the catalyst by averting the aggregation of nanoparticles. Therefore, selecting suitable supports is crucial for obtaining stable and catalytically active catalysts. So far, many supporting materials (metal oxides, organic polymer, porous materials, carbon-based materials, etc.) have been broadly investigated to stabilize metal nanoparticles. Recently, g-C3N4 has been widely used as a base carrier for the deposition of nanoparticles of various metals (Ni, Co, Mn, Cu, Fe, etc.) and their oxides. G-C3N4 has excellent properties, such as a high bulk modulus, good thermal conductivity, a small mechanical friction coefficient, high elasticity, and chemical inertness. Moreover, g-C3N4 is also a very promising material that can replace the commonly used carbon for the production of catalysts due to its high nitrogen (N) content. Among the many carbon-based materials, g-C3N4 has great potential in various photocatalytic and electrocatalytic applications due to its tunable physicochemical and electrical properties, as well as its non-toxic and metal-free nature, as well as its visible light optical response, environmental friendliness, biocompatibility, etc. [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]. Currently, there is a strong scientific interest in the polymeric g-C3N4 as a photocatalyst given its outstanding characteristics, such as its 2D graphite planar structure and its tunable visible energy band gap of 2.7 eV (with a valence band maximum of 1.6 eV and a conduction band minimum of 1.1 eV vs. NHE, which allows for the absorption of visible light up to 460 nm in addition to the facile synthesis methods used [17,26,27,28]). This has led to its widespread use in the photocatalytic degradation of many environmental pollutants for the purposes of CO2 reduction, bacterial disinfection, and the degradation of harmful organic substances [11,12,14,16,26]. Currently, a great deal of focus is on the photocatalytic production of H2 via water splitting [26,27,29,30,31,32,33,34,35,36,37]. However, the practical application of g-C3N4 for photocatalytic hydrogen evolution is limited due to several drawbacks, including its high recombination rate of photogenerated electron–hole pairs, low surface area, low number of active sites, its limited absorption of light in the visible range of 460 only, its low movement of photogenerated charge carries, the slow kinetics of its surface reaction, its rapid backward and side reactions, the degree of dissolved oxygen, product separation, reproducibility, etc. [19,26,27,28,29,38].
Equally, due to the strong covalent bonds between the carbon and N atoms, g-C3N4 can effectively act as an electrocatalyst for water splitting, which is active for both OER and HER reactions [15,20,23]. However, the low conductivity and limited availability of redox sites limit the applicability of pristine g-C3N4 [19]. The production of g-C3N4 material is cheap and does not require high costs [14]. A simple approach to obtain g-C3N4 is the polymerization of cyanamide, dicyandiamide, or melamine [14,25,39,40,41,42,43]. Depending on the reaction conditions, different materials with different degrees of condensation, properties, and reactivities can be obtained [39]. In this study, g-C3N4 was synthesized with melamine as a precursor. Figure 1 presents the structure of melamine, which is a kind of three-triazine heterocyclic organic compound.
Heating the melamine at different temperatures—depending on the respective thermal polymerization temperatures of 500, 520, and 540 °C [44]—allows for the different morphologies of g-C3N4 to be obtained, which range from nanosheets to rolled nanosheets, nanotubes, and nanoflakes with nanoparticles. These morphologies have different properties that can significantly affect the functionality of g-C3N4 [44]. However, the optimal use of g-C3N4 for electrochemical applications requires an improvement of its poor conductivity, which can be increased in several ways: physically mixing g-C3N4 with conductive carbon materials; immobilizing g-C3N4 on carbon bases (carriers) or depositing metal nanoparticles via microwave-assisted processes; hydrothermal and solvothermal syntheses routes; sol–gel processes; chemical reduction, etc. The application of earth-abundant transition/noble metal-free (TMs, where M = Co, Ni, Fe, Mn, Mo) and TM-based alloys or non-metallic (TMXs, where X = N, O, S, C, P, etc.) compounds as active electrocatalysts for HER/OER has been reported [45,46,47,48,49]. TMXs have received much attention due to their distinctive structural features, abundant active sites, tunable electronic properties, compositions, and ease of employment for large-scale production. Co, Ni, and Fe are typically characterized as the most powerful materials for water splitting [50,51,52,53]. Among them, Co-based electrocatalysts, including cobalt oxides [54], hydroxides [55], nitrides [56,57], sulfides [58], selenides [59], phosphides [60], and cobalt ferrite oxide [6,61,62], play a rather significant role in water splitting and are widely used in HER/OER [63,64]. However, their catalytic performance and stability do not yet meet the requirements for use in practical applications. Many electrocatalysts based on cobalt suffer from poor electrical conductivity and low-charge transfer efficiency [63,64]. Efficient and stable Co-based electrocatalytic materials with sufficient intrinsic electronic structure and an unlimited number of active sites on the surface for optimized water splitting remain challenging.
Recently, CoFe2O4 as a type of mixed valence transition metal oxide with spinel structure has received considerable attention due to its chemical stability, high coercivity, large saturation magnetization, mechanical hardness, as well as—most importantly—being a promising catalyst in terms of low cost, high catalytic activity, and environmental friendliness with excellent electrical conductivities [65,66]. This makes it suitable for use in applications such as catalysis, photocatalysis [65,67], supercapacitors [68], fuel cells [69], batteries [70], and many other areas. Although CoO and CoFe2O4 improve the performance of bare g-C3N4 in a variety of processes, including water oxidation, the activity required for practical applications has yet to be achieved [71]. Coupling with noble metal Ag nanoparticles is appropriate for overcoming insufficient catalytic activity. Ag nanoparticles have been extensively studied among metallic nanoparticles due to their versatility in synthesis, ease of processing, fast kinetic reaction rate, high thermal and chemical stability, good conductivity, etc. [14,72]. Recently, an eightfold increase in the catalytic activity of Co2P by forming an Ag@CoxP core–shell nanostructure for OER has been reported [72]. It was assumed that synergistic interactions between the Ag core and shell determine the OER as the Ag core modulates the electronic structure of CoxP, thus leading to enhanced catalytic activity. The g-C3N4-decorated with BiVO4/Ag2CO3 was investigated for HER and OER, and showed admirable activity and good stability for OER with a very low overpotential of 136 mV in an alkaline medium [73]. The outstanding activity of the electrocatalyst was attributed to the decoration and electronic interaction of g-C3N4 with the BiVO4/Ag2CO3 nanocomposite, in which its surface area, active sites, and charge transfer were increased, and its resistance was decreased. Ag@CoFe2O4/g-C3N4 as a photocatalyst showed an improvement in unprecedented photocatalytic activity toward H2 production [74]. The photocatalytic activity of the p-CFO/n-CN heterojunction was enhanced after adding Ag nanoparticles on the surface of photocatalysts via the formation of a plasmonic heterojunctions between Ag and CFO (CoFe2O4). This resulted in photocatalytic improvements by providing active sites for the adsorption of water molecules for the light-driven catalytic reactions that produced both H2 and O2.
Based on these findings, we reported the synthesis of CoFe2O4/g-C3N4 and silver nanocube Ag@CoFe2O4/g-C3N4 nanostructures with the polyol method, as well as their employment as electrocatalysts for HER and OER in alkaline media.

2. Materials and Methods

2.1. Materials and Synthesis

Melamine (99%); Fe(II) acetylacetonate (C15H24FeO6, 99%); Co(II) acetylacetonate (C10H14CoO4, 99%), AgNO3 (99%); methanol (CH3OH, 99%); tetra ethylene glycol (TEG); HO(CH2CH2O)3CH2CH2OH, 99%); polyvinylpyrrolidone (PVP); 1,5-pentanediol, potassium hydroxide (KOH, 98.8%); and Cu(II) chloride (CuCl2, 99%) were purchased from Merck KGaA (Darmstadt, Germany) and used for the synthesis.

2.1.1. Synthesis of g-C3N4

Firstly, the g-C3N4 was prepared via the thermal annealing of melamine at a temperature of 520 °C for 4 h. The precursor was placed in a closed high-alumina crucible and heated to temperature with a rate of 5 °C/min. After the synthesis, it was ground into a fine powder.

2.1.2. Synthesis of CoFe2O4/g-C3N4 Nanoparticles Using the Polyol Method

A total of 0.112 mmol of C15H24FeO6 and 0.056 mmol of C10H14CoO4 were dissolved in 8 mL of TEG under ultrasonication. Then, 0.041 mmol of synthesized g-C3N4 was added to the reaction mixture. The scheme of the reaction mixture is shown in Figure 2.
The resultant mixture was kept in a “Monowave 300” microwave reactor (Anton Paar, Graz, Austria). Synthesis was carried out according to the following protocol: the temperature was increased to 180 °C within 2 min, followed by a temperature increase to 270 °C within 3 min. Then, the synthesis was carried out at 270 °C for 58 min. The reaction mixture during synthesis was stirred with a magnetic stirrer. The obtained product was washed four times with methanol, and the particles were separated with a neodymium magnet. The final colloid of nanoparticles was diluted with methanol to 1.5 mL.

2.1.3. Synthesis of Ag Nanocubes

Ag nanocubes were synthesized according to the procedure described in [75]. Briefly, 2.94 mmol of AgNO3 and 0.0064 µmol of CuCl2 were dissolved in 12.5 mL of 1,5-pentanediol. In a separate flask, 2.215 mmol of PVP was dissolved in 12.5 mL of 1,5-pentanediol. Using a temperature-controlled silicone oil bath, a reaction flask containing 20 mL of 1,5-pentanediol was heated to 175 °C and maintained for 10 min. Then, the two precursor solutions were injected into the hot reaction flask at different rates: 0.5 mL of the AgNO3 solution every minute, and 0.25 mL of the PVP solution every 30 s. AgNO3 was poured 7 times, PVP-14. The reaction was stopped by simply removing it from the heat source and waiting for it to cool down.
Additionally, methanol was added for dilution. Particles were deposited by centrifugation at 8000 rpm for 8 min. After deposition, the final product was washed with methanol 3 times by mixing the particles in an ultrasonic bath. The resulting nanoparticle colloid was diluted with methanol to 3 mL.

2.1.4. Ag@CoFe2O4/g-C3N4

To obtain Ag@CoFe2O4/g-C3N4, 250 μL of prepared CoFe2O4/g-C3N4 solution was mixed with 100 μL of a silver colloidal solution and then kept for at least one day with occasional stirring in an ultrasonic bath.

2.2. Characterization of Catalysts

The XRD patterns of the studied powders were measured using an X-ray diffractometer D2 PHASER (Bruker, Karlsruhe, Germany). The measurements were conducted in the 2θ range of 10–70°.
A transmission electron microscope Tecnai G2 F20 X-TWIN (FEI, Eindhoven, The Netherlands) was used to characterize catalyst shape and size.
The SEM images and distribution of elements in the catalysts were analyzed using a scanning electron microscope TM4000Plus with an AZetecOne detector (Hitachi, Tokyo, Japan).

2.3. Electrochemical Measurements

The performance of the synthesized samples was evaluated using a PGSTAT100 potentiostat/galvanostat (Metrohm Autolab B.V., Utrecht, The Netherlands). A standard three-electrode cell was used, where the working electrode was a glassy carbon (GC) electrode modified with the synthesized samples. The geometric surface area of the GC electrode was 0.196 cm2. A Ag/AgCl (3 M KCl) and GC road were employed as the reference and counter electrodes, respectively. Linear sweep voltammograms (LSVs) were recorded in an N2-saturated 1 M KOH solution at a scan rate of 2 mV s−1. All reported potential values were referred to as “ERHE”-reversible hydrogen electrode according to the following Equation (1):
ERHE = Emeasured + 0.059·pH + EAg/AgCl (3 M KCl)
where EAg/AgCl (3M KCl) = 0.210 V.
The current densities for HER and OER presented in this paper were normalized to the geometric area of the catalysts.
The stability tests of the CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4 catalysts were conducted for 1000 cycles at a constant scan rate of 100 mV s−1 at a rotation speed of 1600 rpm, after which the stable polarization curves were recorded at 2 mV s−1 for comparison with the initial curve.

3. Results and Discussion

The structure of the synthesized CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4 was examined by XRD. Figure 3 presents the XRD patterns of g-C3N4 (a), CoFe2O4/g-C3N4 (b), and Ag@CoFe2O4/g-C3N4 (c). The XRD pattern of the as-prepared g-C3N4 exhibited a typical pattern with a pronounced peak, which was centered approximately at 27.3° and had several small diffraction peaks at 13.2°, 18°, 22°, 44°, and 56.5° (Figure 3); this may be assigned to the (002), (100), (101), (111), (103), and (004) planes of the trigonal N bond of tri-s-triazine and the layered packing of the conjugated aromatic units in g-C3N4, respectively [44,76,77].
Strong diffraction peaks at the 2θ values of 27.6° were seen in both the XRD patterns of CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4 indicating that the g-C3N4 structure had not changed (Figure 3b,c). The XRD pattern of CoFe2O4/g-C3N4 consisted of well-indexed peaks at 18.3°, 30.3°, 35.5°, 43.2°, 57.1°, and 62.2°, which corresponded to the (h k l) planes 111, 220, 311, 400, 511, and 440, respectively. These matched well with a cubic spinel phase of CoFe2O4 according to COD 1,533,163 (Figure 3b). The same peaks were also observed in the XRD pattern of Ag@CoFe2O4/g-C3N4 (Figure 3c). However, two characteristic diffraction peaks of Ag at 2θ values of 38.2° and 44.4° were seen in the XRD pattern for Ag@CoFe2O4/g-C3N4 (Figure 3c). Those peaks corresponded to the 111 and 200 crystalline planes of Ag with a cubic symmetry (COD1509146).
The morphology of CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4 was observed by SEM. The SEM images of both catalysts showed irregular spherical agglomerated particles of different sizes (Figure 4a and Figure 5a). The EDS spectra for CoFe2O4/g-C3N4 (Figure 4b) and Ag@CoFe2O4/g-C3N4 (Figure 5b) confirmed the existence of metal species Co and Fe on CoFe2O4/g-C3N4, as well as0 Co, Fe, and Ag on Ag@CoFe2O4/g-C3N4. The elemental mapped images of CoFe2O4/g-C3N4 (Figure 4c–e) and Ag@CoFe2O4/g-C3N4 (Figure 5c–f) showed a homogeneous distribution of Co, Fe, and O, as well as Co, Fe, O, and Ag, respectively, thus indicating the great uniformity of the synthesized CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4.
It can be seen from Figure 6 that Ag nanocubes with a size of 30–40 nm were loaded on the CoFe2O4/g-C3N4.
The electrocatalytic activity of the prepared catalysts was investigated for HER and OER in an alkaline medium. The HER polarization curves recorded on the g-C3N4, CoFe2O4/g-C3N4, and Ag@CoFe2O4/g-C3N4 samples in alkaline media are shown in Figure 7a, whereas the data of the electrochemical performance of the tested catalysts are given in Table 1.
As shown in Table 1, the Ag/CoFe2O4/g-C3N4 sample exhibited the lowest onset potential (Eonset) of −0.161 V for the HER when compared with CoFe2O4/g-C3N4 and pure g-C3N4. Additionally, the latter catalyst showed a significantly higher current density and a lower overpotential of −259.0 mV for the HER to reach a current density of 10 mA cm−2 (η10) (Figure 5a) when compared to that of CoFe2O4/g-C3N4 (−424.6 mV).
The reaction kinetics and mechanism of the as-prepared catalysts can be evaluated on the basis of Tafel slopes, which were determined from the following equation (Equation (2)) [78]:
η = b·log j/j0
where η is the overpotential, b is the Tafel slope, j is the experimental current density, and j0 is the exchange current density. The plot of η versus log j represents the Tafel slope. It is widely accepted that HER proceeds by either the Volmer–Heyrovsky or Volmer–Tafel mechanisms, and, in alkaline media, it involves three main steps as shown in Equations (3)–(5) [61]:
* + H2O + e− → *Hads + OH− (Volmer step)
*Hads + e− + H2O → H2 + OH− + * (Heyrovsky step)
2*Hads → H2 + * (Tafel step)
where Hads denotes the H2 adsorbed to the metal sites and * indicates the metal sites. The theoretical Tafel slopes in the aforementioned reaction steps are 120 mV dec−1, 40 mV dec−1, and 30 mV dec−1, respectively. Figure 5b shows the Tafel slopes of the g-C3N4, CoFe2O4/g-C3N4, and Ag@CoFe2O4/g-C3N4 samples, thus pointing to the rate-determining step and the likely mechanism associated with electrocatalytic hydrogen generation. The Ag@CoFe2O4/g-C3N4 catalyst was found to have the lowest Tafel slope of 62.9 mV dec−1 compared to CoFe2O4/g-C3N4 (79.1 mV dec−1) and g-C3N4 (182.3 mV dec−1). This predicted the favorable HER kinetics that apply following the Volmer–Heyrovsky mechanism on CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4.
Among the investigated catalysts, the lower Eonset of −0.161 V, a small overpotential of −259 mV at 10 mA cm−2, and a low Tafel slope of 62.9 mV dec−1 in Ag@CoFe2O4/g-C3N4 indicated that the addition of Ag nanocubes to CoFe2O4/g-C3N4 increased the activity for HER.

Investigation of Electrocatalyst Activity for OER

The performance of catalysts for OER was further evaluated. Figure 8a,b presents the OER polarization curves and the corresponding Tafel slopes recorded in the g-C3N4, CoFe2O4/g-C3N4, and Ag@CoFe2O4/g-C3N4 at a slow scan rate of 2 mV s−1 in a 1 M KOH solution. The summarized data are also given in Table 2.
Notably, pure g-C3N4 shows poor OER activity with a low current density, even at a high overpotential. On the contrary, CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4 gave much higher current densities and lower overpotentials compared to g-C3N4, meaning a significant improvement for OER catalytic activity. Eonset values were found in a gradually increasing order as follows: Ag@CoFe2O4/g-C3N4 (1.4855 V) < CoFe2O4/g-C3N4 (1.5056 V) < g-C3N4 (1.6404 V). Furthermore, these had overpotential values of 255.5, 275.6, and 410.4 mV, respectively (Table 3). The overpotentials required to reach the current density of 10 mA·cm−2 were found as 370.2 and 382.7 mV for Ag@CoFe2O4/g-C3N4 and CoFe2O4/g-C3N4, respectively (Table 2). The Tafel slope of Ag@CoFe2O4/g-C3N4 (48.1 mV dec−1) was lower than those of CoFe2O4 and g-C3N4 (Figure 8b, Table 2), indicating a better catalytic activity for the OER. A 4e− mechanism was widely accepted for the OER process. The steps of the reaction in an alkaline media can be represented by Equations (6)–(9) [58,61,79]:
* + OH− → e− + OH*
OH− + OH* → e− + O* + H2O
OH− + O* → e− + OOH*
OH− + OOH* → H2O +*+ e− + O2
where * denotes the electrocatalyst’s adsorption site. Similarly, the adsorbed intermediates were OH*, O*, and OOH* during OER. The first step of the OER process, denoted by Equation (6), was the electrosorption of OH− onto the active sites of the catalyst’s surface. Higher-oxidation-state metal species are more susceptible to adsorbing OH−, thus they accelerate the multielectron transportation process; therefore, they can enhance the OER process [58,79].
Additionally, the stability of the CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4 catalysts was examined for 1000 cycles at a constant scan rate of 100 mV s−1, and at a rotation rate of 1600 rpm in N2-saturated 1 M KOH for HER and OER. Figure 9 presents the initial and after 1000 cycles LSVs that were recorded at 2 mV s−1 at 1600 rpm for both processes on the investigated materials. CoFe2O4/g-C3N4 exhibited a negligible shift of approximately 20 mV after 1000 potential cycles (Figure 9a), indicating good stability in alkaline media. In contrast, Ag@CoFe2O4/g-C3N4 showed a higher HER activity after 1000 cycles by the potential shift of approximately 17 mV to more positive potential values (Figure 9c). In the case of OER, the LSV curves for both catalysts exhibited almost no differences from the initial test before the CV cycles (Figure 9b,d) that occurred at a constant current density of 10 mA cm−2, as well as at a minimal change of approximately 9 and 20 mV at 120 mA cm−2 for Ag@CoFe2O4/g-C3N4 and CoFe2O4/g-C3N4, respectively (which indicated high stability for OER). The obtained data show the potential of CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4 catalysts for practical application.
The above results demonstrated that CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4 materials could act as bifunctional catalysts due to their notable performances and high stabilities toward HER and OER.
The catalytic activity of the CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4 materials was also compared with previously reported works and is presented in Table 3. These overpotentials are comparable to those previously reported for state-of-the-art non-precious metal catalysts for water splitting in an alkaline medium. It was seen that Ag@CoFe2O4/g-C3N4 exhibited lower HER overpotential values at 10 mA cm−2 and a lower Tafel slope when compared to CoFe2O4/gCN/NGQDs and Co3O4/g-C3N4, whereas the CoNi2S4/gCN, Co-SCN/RGO, and Co3O4MoO3/g-C3N4 catalysts exhibited lower overpotential values but higher Tafel slopes. In the case of OER, Ag@CoFe2O4/g-C3N4 achieved a lower overpotential value at 10 mA cm−2 and a lower Tafel slope of 48.1 mV dec−1 compared to CoFe2O4/gCN/NGQDs, but it had lower overpotential values at 10 mA cm−2. Meanwhile, higher Tafel slope values were exhibited by Co2FeO4@rGO (CFG-10), Co2FeO4@PdO, CoNi2S4/gCN, Co-SCN/RGO, Co3O4/g-C3N4, and Co3O4MoO3/g-C3N4 when compared with those for Ag@CoFe2O4/g-C3N4.

4. Conclusions

In this study, we reported the synthesis of CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4 nanostructures, which was achieved with the polyol method, as well as their employment as electrocatalysts for HER and OER in alkaline media. It was found that Ag@CoFe2O4/g-C3N4 shows the highest current density and gives the lowest overpotential of −259 mV for HER in terms of reaching a current density of 10 mA cm−2 in a 1 M KOH. The overpotentials for reaching the current density of 10 mA·cm−2 for OER were 370.2 mV and 382.7 mV for Ag@CoFe2O4/g-C3N4 and CoFe2O4/g-C3N4, respectively. The above results demonstrate that CoFe2O4/g-C3N4 and Ag@CoFe2O4/g-C3N4 materials could act as bifunctional catalysts due to their notable performances and high stabilities toward HER and OER.

Author Contributions

Conceptualization, A.Z., E.N. and L.T.-T.; methodology, R.L. and O.E.-L.; validation, R.L. and O.E.-L., data curation, D.S. and A.Z.; investigation, Z.K.; formal analysis, Z.K.; writing—original draft preparation, A.Z., D.S., R.L. and L.T.-T.; writing—review and editing, E.N., D.S. and A.Z.; supervision, A.Z. and E.N.; project administration, E.N. All authors have read and agreed to the published version of the manuscript.

Funding

This project received funding from the European Social Fund (project No. 09.3.3-LMT-K-712-23-0188) under a grant agreement with the Research Council of Lithuania (LMTLT).

Data Availability Statement

Not applicable.

Acknowledgments

The authors are grateful to Audrius Drabavičius from the Department of Characterization of Materials Structure, Center for Physical Sciences and Technology for their materials characterization via TEM.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

The following abbreviations were used in this manuscript:
CoFe2O4Cobalt ferrite
EDSEnergy-dispersive X-ray analysis
EonsetOnset potential
ηonsetOnset overpotential
g-C3N4Graphitic carbon nitride
HERHydrogen evolution reaction
jCurrent density (mA cm−2)
NHENormal hydrogen electrode
LSVsLinear sweep voltammograms
PVPPolyvinylpyrrolidone
OEROxygen evolution reaction
RHEReversible hydrogen electrode
SEMScanning electron microscopy
TEGTetra ethylene glycol
TEMTransmission electron microscopy
XRDX-ray diffraction

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Crystals 13 01342 g001
Figure 1. Structure of melamine.
Figure 1. Structure of melamine.
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Crystals 13 01342 g002
Figure 2. Scheme of the synthesis of CoFe2O4/g-C3N4.
Figure 2. Scheme of the synthesis of CoFe2O4/g-C3N4.
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Crystals 13 01342 g003
Figure 3. XRD patterns for the (a) g-C3N4, (b) CoFe2O4/g-C3N4, and (c) Ag@CoFe2O4/g-C3N4 catalysts.
Figure 3. XRD patterns for the (a) g-C3N4, (b) CoFe2O4/g-C3N4, and (c) Ag@CoFe2O4/g-C3N4 catalysts.
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Crystals 13 01342 g004
Figure 4. (a) SEM image and (b) corresponding EDS pattern of CoFe2O4/g-C3N4. (c–e) The elemental mapped images.
Figure 4. (a) SEM image and (b) corresponding EDS pattern of CoFe2O4/g-C3N4. (c–e) The elemental mapped images.
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Crystals 13 01342 g005
Figure 5. (a) SEM image and (b) corresponding EDS pattern of Ag@CoFe2O4/g-C3N4. (c–f) The elemental mapped images.
Figure 5. (a) SEM image and (b) corresponding EDS pattern of Ag@CoFe2O4/g-C3N4. (c–f) The elemental mapped images.
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Figure 6. TEM image of the Ag@CoFe2O4/g-C3N4 catalyst.
Figure 6. TEM image of the Ag@CoFe2O4/g-C3N4 catalyst.
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Figure 7. (a) HER polarization curves of the g-C3N4, CoFe2O4/g-C3N4, and Ag@CoFe2O4/g-C3N4 catalysts in a 1 M KOH solution at a potential scan rate of 2 mV s−1. (b) The corresponding Tafel slopes for each catalyst.
Figure 7. (a) HER polarization curves of the g-C3N4, CoFe2O4/g-C3N4, and Ag@CoFe2O4/g-C3N4 catalysts in a 1 M KOH solution at a potential scan rate of 2 mV s−1. (b) The corresponding Tafel slopes for each catalyst.
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Crystals 13 01342 g008
Figure 8. (a) OER polarization curves of the g-C3N4, CoFe2O4/g-C3N4, and Ag@CoFe2O4/g-C3N4 catalysts in a 1 M KOH solution at a potential scan rate of 2 mV s−1. (b) The corresponding Tafel slopes for each catalyst.
Figure 8. (a) OER polarization curves of the g-C3N4, CoFe2O4/g-C3N4, and Ag@CoFe2O4/g-C3N4 catalysts in a 1 M KOH solution at a potential scan rate of 2 mV s−1. (b) The corresponding Tafel slopes for each catalyst.
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Crystals 13 01342 g009aCrystals 13 01342 g009b
Figure 9. LSV curves before and after 1000 cycles for HER and OER for CoFe2O4/g-C3N4 (a,b) and Ag@CoFe2O4/g-C3N4, which were (c,d) recorded at 2 mV s−1 at 1600 rpm.
Figure 9. LSV curves before and after 1000 cycles for HER and OER for CoFe2O4/g-C3N4 (a,b) and Ag@CoFe2O4/g-C3N4, which were (c,d) recorded at 2 mV s−1 at 1600 rpm.
Crystals 13 01342 g009aCrystals 13 01342 g009b
Table 1. Electrochemical parameters of the investigated materials toward HER in alkaline media.
Table 1. Electrochemical parameters of the investigated materials toward HER in alkaline media.
SampleEonset, V at j = −0.1 mA cm−2η10 *, mVTafel Slope, mV dec−1
g-C3N4−0.40−182.3
CoFe2O4/g-C3N4−0.280−424.676.1
Ag@CoFe2O4/g-C3N4−0.161−259.062.9
* Overpotential at 10 mA cm−2.
Table 2. Electrochemical parameters of the investigated catalysts toward OER in alkaline media.
Table 2. Electrochemical parameters of the investigated catalysts toward OER in alkaline media.
CatalystsEonset, V at j = 0.1 mA cm−2ηonset, mVE, V at j = 10 mA cm−2η10 *, mVTafel Slope, mV dec−1
g-C3N41.6404410.4−−139.9
CoFe2O4/g-C3N41.5056275.61.6127382.752.3
Ag@CoFe2O4/g-C3N41.4855255.51.6000370.248.1
* Overpotential at 10 mA cm−2.
Table 3. Electrochemical parameters of the different Co-based gC3N4 catalysts for HER and OER in alkaline media.
Table 3. Electrochemical parameters of the different Co-based gC3N4 catalysts for HER and OER in alkaline media.
CatalystElectrolyteHEROERRef.
η10 *, mVTafel Slope, mV dec−1η10 *, mVTafel Slope, mV dec−1
CoFe2O4/g-C3N41 M KOH424.676.1382.752.3This study
Ag@CoFe2O4/g-C3N41 M KOH259.062.9370.248.1This study
CoFe2O4/gCN/NGQDs1 M KOH2879644569[61]
Co2FeO4@rGO (CFG-10)1 M KOH32048240
at 20mA cm−2		51[6]
Co2FeO4@PdO1 M KOH26949259
at 20mA cm−2		59[80]
CoNi2S4/gCN1 M KOH16090.76310
at 30mA cm−2		49.86[81]
Co-SCN/RGO1 M KOH1509425096[82]
Co3O4/g-C3N41 M KOH31316931567[83]
Co3O4MoO3/g-C3N41 M KOH1259420660[83]
* Overpotential at 10 mA cm−2.
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Zabielaite, A.; Eicher-Lorka, O.; Kuodis, Z.; Levinas, R.; Simkunaite, D.; Tamasauskaite-Tamasiunaite, L.; Norkus, E. Synthesis of Silver Nanocubes@Cobalt Ferrite/Graphitic Carbon Nitride for Electrochemical Water Splitting. Crystals 2023, 13, 1342. https://doi.org/10.3390/cryst13091342

AMA Style

Zabielaite A, Eicher-Lorka O, Kuodis Z, Levinas R, Simkunaite D, Tamasauskaite-Tamasiunaite L, Norkus E. Synthesis of Silver Nanocubes@Cobalt Ferrite/Graphitic Carbon Nitride for Electrochemical Water Splitting. Crystals. 2023; 13(9):1342. https://doi.org/10.3390/cryst13091342

Chicago/Turabian Style

Zabielaite, Ausrine, Olegas Eicher-Lorka, Zenonas Kuodis, Ramunas Levinas, Dijana Simkunaite, Loreta Tamasauskaite-Tamasiunaite, and Eugenijus Norkus. 2023. "Synthesis of Silver Nanocubes@Cobalt Ferrite/Graphitic Carbon Nitride for Electrochemical Water Splitting" Crystals 13, no. 9: 1342. https://doi.org/10.3390/cryst13091342

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