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

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31 July 2023

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01 August 2023

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Abstract
This study presents the synthesis of graphitic carbon nitride (g−C3N4) and its nanostructures with cobalt ferrite oxide (CoFe2O4) and silver nanocubes (Ag) using the combined pyrolysis of melamine and polyol method. The resulted nanostructures were tested as electrocatalysts for hydrogen and oxygen evolution reactions in alkaline media. It was found that the Ag/CoFe2O4/g−C3N4 shows the highest current density and gives the lowest overpotential of −259 mV for HER to reach a current density of 10 mA cm−2 in 1 M KOH. Overpotentials to reach the current density of 10 mA·cm−2 for OER are 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 a bifunctional catalyst due to the notable performance towards HER and OER and for total water splitting in practical applications is a promising alternative to noble metal-based electrocatalysts.
Keywords: 
Subject: Chemistry and Materials Science  -   Applied Chemistry

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]. The challenge is to design and development novel, non-noble and low-cost bifunctional electrode materials with high efficiency for both HER and OER. Transition-metal boride/phosphide-based materials are attractive catalysts for H2 release due to their advantages of earth-abundant elements, considerable catalytic activity, high stability, and low cost [7]. Metal nanoparticles as catalysts have also attracted much attention over the last decades owing to their unique properties. However, metal nanoparticles tend to aggregate into clumps and ultimately into their bulk counterparts due to their high surface energy, thus leading to decreased catalytic activity and suffering in 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 in 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, graphitic carbon nitride (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 high bulk modulus, good thermal conductivity, 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 its high nitrogen (N) content. The production of this material does not require high costs and is easy to produce. A simple approach to obtain g–C3N4 is polymerization of cyanamide, dicyandiamide or melamine [8,9,10,11,12]. Depending on reaction conditions, different materials with different degrees of condensation, properties and reactivities can be obtained [8]. In this study, the g–C3N4 was synthesized using the 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 allows to obtain different morphologies of g–C3N4, ranging from nanosheets to rolled nanosheets, nanotubes, and nanoflakes with nanoparticles, depending on the thermal polymerization temperatures of 500, 520, and 540 oC, respectively [13].
However, optimal use of g–C3N4 for electrochemical applications requires the 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 using microwave-assisted processes, hydrothermal and solvothermal syntheses routes, sol-gel processes, chemical reduction, etc. The application of earth-abundant transition/noble metals-free (TMs, where M=Co, Ni, Fe, Mn, Mo) and TMs-based alloys or non-metallic (TMXs, where X=N, O, S, C, P, etc.) compounds as active electrocatalysts for HER/OER has been reported [14,15,16,17,18]. 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 [19,20,21,22]. Among them, Co-based electrocatalysts, including cobalt oxides [23], hydroxides [24], nitrides [25,26], sulfides [27], and selenides [28], phosphides [29], cobalt ferrite oxide [6,30,31], play a rather significant role in water splitting and are widely used in HER/OER [32,33]. 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 hence low charge transfer efficiency [32,33]. 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 a challenge.
In this study, we reported the synthesis of cobalt ferrite oxide (CoFe2O4)@g–C3N4 and silver nanocubes (Ag–Nc)@CoFe2O4/g–C3N4 nanostructures using the polyol method and their employment as electrocatalysts for HER and OER in alkaline media.

2. Materials and Methods

2.1. Materials and Synthesis

Melamine (99%), Fe(II) acetylacetonaye (C15H24FeO6, 99%, labeled as No. 1), Co(II) acetylacetonate (C10H14CoO4, 99%, labeled as No. 2), AgNO3 (99%), methanol (CH3OH, 99%), tetraethylene glycol (TEG, HO(CH2CH2O)3CH2CH2OH, 99%), 1,5-pentanediol, potasium hydroxide (KOH, 98.8%) were used for the synthesis.

2.1.1. Synthesis of g–C3N4

At first, the g–C3N4 was prepared using thermal annealing of melamine at a temperature of 520 oC 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.
The XRD pattern of as-prepared g–C3N4 exhibited a typical pattern, with two pronounced peaks centered approximately at 13.2o and 27.3o (Figure 2), which may be assigned to the (100) and (002) planes of the trigonal N bond of tri-s-triazine and the layered packing of conjugated aromatic units in g–C3N4, respectively [34,35,36].

2.1.2. Synthesis of CoFe2O4/g–C3N4 nanoparticles using the polyol method

0.112 mmol of reagent No. 1 (C15H24FeO6) and 0.056 mmol of reagent No. 2 (C10H14CoO4) were dissolved in 8 ml of TEG under ultrasonication. Then, 0.041 mmol of synthesized g–C3N4 was added to the reaction mixture. Scheme of reaction mixture is shown in Figure 3.
The resultant mixture was kept in a microwave reactor „Monowave 300“ (Anton Paar). Synthesis was carried out according to the following protocol: the temperature was increased up to 180 oC in 2 min followed by the temperature increase up to 270 oC in 3 min. Then, the synthesis was carried out at 270 oC for 58 min. The reaction mixture during synthesis was stirring with a magnetic stirrer. The obtained product was washed 4 times with methanol, separating the particles 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 [37]. 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 up to 175 oC and maintained for 10 min. Then the two precursor solutions were injected into the hot reaction flak at different rates: 0.5 mL of AgNO3 solution every minute and 0.25 mL of the PVP solution every 30 s. AgNO3 is 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 silver colloidal solution and kept for at least 1 day with occasional stirring in UG.

2.3. Electrochemical Measurements

The performance of synthesized samples was evaluated using a potentiostat/galvanostat PGSTAT100 (Metrohm Autolab B. V., Utrecht, The Netherlands). Standard three-electrode cell was used, where the working electrode was a glassy carbon (GC) electrode modified with the synthesized samples. A geometric surface area of GC electrode was 0.196 cm2. An Ag/AgCl (3 M KCl) and GC road were employed as the reference and counter electrodes, respectively. Linear sweep voltammograms (LSVs) were recorded in a 1 M KOH solution at a scan rate of 2 mV s−1. All reported potential values were referred to as “RHE” – reversible hydrogen electrode according to the following Eqn. 1:
ERHE = Emeasured + 0.059⋅pH + EAg/AgCl (3 M KCl)
where EAg/AgCl (3 M KCl) = 0.210 V.
Current densities for HER and OER presented in this paper were normalized to the geometric area of catalysts.

3. Results

The electrocatalytic activity of 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 3a, whereas data of electrochemical performance of the tested catalysts are given in Table 2.
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Figure 2. (a) HER polarization curves of g–C3N4, CoFe2O4/g–C3N4, and Ag/CoFe2O4/g–C3N4 catalysts in 1 M KOH solution at a potential scan rate of 2 mV s−1; (b) The corresponding Tafel slopes for each catalyst.
Figure 2. (a) HER polarization curves of g–C3N4, CoFe2O4/g–C3N4, and Ag/CoFe2O4/g–C3N4 catalysts in 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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As seen, the lowest onset potential (Eonset) of −0.161 V for the HER exhibits the Ag/CoFe2O4/g–C3N4 sample as compared with CoFe2O4/g–C3N4 and pure g–C3N4 (Table 2). Additionally, the latter catalyst shows the significantly higher current density and the lower overpotential of −259.0 mV for the HER to reach a current density of 10 mA cm−2 (η10) (Figure 3a) as 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 determined from the following equation (Eqn. 2) [38]:
η = b ⋅ log j/j0
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 proceed by either the Volmer–Heyrovsky or Volmer–Tafel mechanisms and in alkaline media it involves three main steps as shown in equations (3) to (5) [30]:
* + H2O + e– → *Hads + OH– (Volmer step)
*Hads + e– + H2O → H2 + OH– + * (Heyrovsky step)
2*Hads → H2 + * (Tafel step)
Hads denotes the H2 adsorbed to the metal sites, where * denotes 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 3b shows the Tafel slopes of g–C3N4, CoFe2O4/g–C3N4, and Ag/CoFe2O4/g–C3N4 samples pointing to the rate-determining step and the likely mechanism associated with electrocatalytic hydrogen generation. The Ag/CoFe2O4/g–C3N4 sample 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 predicts the favorable HER kinetics following the Volmer-Heyrovsky mechanism on the 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 Ag/CoFe2O4/g–C3N4 indicate that the addition of silver nanocubes to CoFe2O4/g–C3N4 increases the activity for HER.

3.3. Investigation of Electrocatalysts Activity for OER

The performance of catalysts for OER was further evaluated. Figure 4a,b presents the OER polarization curves and the corresponding Tafel slopes recorded on the g–C3N4, CoFe2O4/g–C3N4, and Ag/CoFe2O4/g–C3N4 at a slow scan rate of 2 mV s−1 in 1 M KOH solution. The summarized data are also given in Table 3.
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Figure 4. (a) OER polarization curves of g–C3N4, CoFe2O4/g–C3N4, and Ag/CoFe2O4/g–C3N4 catalysts in 1 M KOH solution at a potential scan rate of 2 mV s−1; (b) The corresponding Tafel slopes for each catalyst.
Figure 4. (a) OER polarization curves of g–C3N4, CoFe2O4/g–C3N4, and Ag/CoFe2O4/g–C3N4 catalysts in 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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Notably, pure g–C3N4 shows poor OER activity with a low current density, even at high overpo-tential. On the contrary, CoFe2O4/g–C3N4 and Ag/CoFe2O4/g–C3N4 gave much higher current densities and lower overpotentials compared to g–C3N4, meaning the significant improvement for OER catalytic activity. Eonset values were found in a gradual increasing order, as follows: Ag/CoFe2O4/g–C3N4 (1.4855 V) < CoFe2O4/g–C3N4 (1.5056 V) < g–C3N4 (1.6404 V) with overpotential values of 255.5, 275.6, and 410.4 mV, respectively (Table 3). Overpotentials to reach the current density of 10 mA·cm−2 were found as 370.2 and and 382.7 mV for Ag/CoFe2O4/g–C3N4 and CoFe2O4/g–C3N4, respectively (Table 3). The Tafel slope of Ag/CoFe2O4/g–C3N4 (48.1 mV dec−1) is lower than those of CoFe2O4 and g–C3N4 (Figure 3b, Table 3), indicating the better catalytic activity for the OER. A 4e− mechanism is widely accepted for the OER .process. The steps of the reaction in an alka-line media can be represented by Eqns. 6–9 [30,39,40]:
* + 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, during OER, the adsorbed intermediates are OH*, O*, and OOH*. The first step of the OER process denoted by equation (6) is the electrosorption of OH– onto the active sites of the catalyst’s surface. Higher oxidation state metal species are more susceptible to adsorb OH–, accelerate the multielec-tron transportation process and, hence, can therefore enhance the OER process [39,40]. The catalytic activity of the CoFe2O4/g–C3N4 and Ag/CoFe2O4/g–C3N4 materials is also compared with previously reported works and is presented in Table 4. These overpotentials are comparable to those previously reported for state-of-the-art non-precious metal catalysts for water splitting in an alkaline medium.
The above results demonstrate that CoFe2O4/g–C3N4 and Ag/CoFe2O4/g–C3N4 materials could act as a bifunctional catalyst due to the notable performance towards HER and OER and for total water splitting in practical applications is a promising alternative to noble metal-based electrocatalysts.

4. Conclusions

In this study, we reported the synthesis of cobalt ferrite oxide (CoFe2O4)@g–C3N4 and silver nanocubes@CoFe2O4/g–C3N4 nanostructures using the polyol method and their employment as electrocatalysts for HER and OER in alkaline media. It was found that the Ag/CoFe2O4/g–C3N4 shows the highest current density and gives the lowest overpotential of −259 mV for HER to reach a current density of 10 mA cm−2 in 1 M KOH. Overpotentials to reach the current density of 10 mA·cm−2 for OER are 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 a bifunctional catalyst due to the notable performance towards HER and OER and for total water splitting in practical applications is a promising alternative to noble metal-based electrocatalysts.

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.; 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 has receive funding from 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.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Structure of melamine.
Figure 1. Structure of melamine.
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Figure 2. XRD patterns for g–C3N4.
Figure 2. XRD patterns for g–C3N4.
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Figure 3. Scheme of synthesis of CoFe2O4/g–C3N4.
Figure 3. Scheme of synthesis of CoFe2O4/g–C3N4.
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Table 2. Electrochemical parameters of the investigated materials toward HER in alkaline media.
Table 2. Electrochemical parameters of the investigated materials toward HER in alkaline media.
Sample Eonset, V at j = −0.1 mA cm−2 η10*, mV Tafel slope, mV dec−1
g–C3N4 −0.40 − 182.3
CoFe2O4/g–C3N4 −0.280 −424.6 76.1
Ag/CoFe2O4/g–C3N4 −0.161 −259.0 62.9
* Overpotential at 10 mA cm−2.
Table 3. Electrochemical parameters of the investigated catalysts toward OER in alkaline media.
Table 3. Electrochemical parameters of the investigated catalysts toward OER in alkaline media.
Catalysts Eonset, V at j = 0.1 mA cm−2 ηonset, mV E, V at j = 10 mA cm−2 η10*, mV Tafel slope, mV dec−1
g–C3N4 1.6404 410.4 – – 139.9
CoFe2O4/g–C3N4 1.5056 275.6 1.6127 382.7 52.3
Ag/CoFe2O4/g–C3N4 1.4855 255.5 1.6000 370.2 48.1
Table 4. Electrochemical parameters of different Co-based gCN catalysts for HER and OER in alkaline media.
Table 4. Electrochemical parameters of different Co-based gCN catalysts for HER and OER in alkaline media.
Catalyst Electrolyte HER OER Ref.
η10*, mV Tafel slope, mV dec−1 η10*, mV Tafel slope, mV dec−1
CoFe2O4/g–C3N4 1 M KOH 424.6 76.1 382.7 52.3 This study
Ag/CoFe2O4/g–C3N4 1 M KOH 259.0 62.9 370.2 48.1 This study
CoFe2O4/gCN/NGQDs 1 M KOH 287 96 445 69 [30]
Co2FeO4@rGO (CFG-10) 1 M KOH 320 48 240at 20mA cm−2 51 [6]
Co2FeO4@PdO 1 M KOH 269 49 259at 20mA cm−2 59 [41]
CoNi2S4/gCN 1 M KOH 160 90.76 310at 30mA cm−2 49.86 [42]
Co-SCN/RGO 1 M KOH 150 94 250 96 [43]
Co3O4/g–C3N4 1 M KOH 313 169 315 67 [44]
Co3O4MoO3/g-C3N4 1 M KOH 125 94 206 60 [44]
* Overpotential at 10 mA cm−2.
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