In situ electrochemical synthesis of polypyrrole Cu2O MoO3 nanocomposite on graphene oxide nanosheets for hydrogen generation and supercapacitors

Abstract

In this work, a simple, single-step electrochemical method is used to decorate a ternary Polypyrrole-Cu₂O-MoO₃ (PPy-Cu₂O-MoO₃) nanocomposite on Graphene Oxide (GO) nanosheets formed on a Graphite Foil Electrode (GFE). The characterization confirmed successful synthesis of ternary nanocomposite, and electrochemical tests showed the electrode performed well as both a supercapacitor and a hydrogen evolution reaction (HER). The investigation of the hydrogen production reaction by the PPy-Cu₂O-MoO₃/GO/GFE shows that this electrode has a smaller Tafel slope and overpotential. Additionally, the PPy-Cu₂O-MoO₃/GO/GFE has a specific capacitance of 1010.30 mF cm⁻² at 1 mA cm⁻² in a 0.5 M H₂SO₄ electrolyte solution. The evaluation of the fabrication of a symmetric solid-state supercapacitor device shows that the constructed device has an excellent capacitance of 596.5 mF cm⁻² at 1 mA cm⁻² and a cyclic stability of 82.4% after 6000 GCD cycles. For hydrogen evolution reaction, the electrode demonstrated an overpotential of 361 mV at 10 mA cm⁻² and a Tafel slope of 142 mV dec⁻¹, indicating favorable electrocatalytic activity. These results highlight the potential of the synthesized nanocomposite as a multifunctional electrode material for both energy storage and clean energy production.

Introduction

Industrial development and increasing energy demand have always caused global warming and environmental pollution, which has become a global crisis¹. In addition, energy shortage problems are another of the most important challenges caused by the decline in fossil fuel resources². Using alternative sources of fossil fuels (clean energy sources) can solve the problems caused by environmental pollution and global warming^3,4. Hydrogen, as a clean energy & fuel source that has the advantages of low emissions, high efficiency, and production from renewable resources, is a promising candidate to replace fossil fuels^{5, 6–7}. Among the common methods of hydrogen production, electrochemical water splitting methods are recommended due to their low environmental impact and economic efficiency^8,9.

Also, the decline in fossil fuels and their adverse effects on the environment have led to increased attention being paid to energy storage devices that have high energy& power density. Supercapacitors are one of the energy storage devices that have received the most attention due to their features such as high charge/discharge speed, high energy and power density, excellent cyclic stability^{10, 11–12} and their usability in various fields and industries such as electronics, transportation, medicine, military, etc^13,14. This property of supercapacitors is highly dependent on the used electrode materials. Today, many supercapacitors are fabricated based on conductive polymers^15,16, metal oxides^17,18, carbon compounds¹⁹ etc., each of which has advantages and disadvantages. In general, supercapacitor materials are divided into two groups: electrical double layer capacitors (EDLCs) and pseudocapacitors (PC)^20,21. EDLCs have high cyclic stability and conductivity but have low energy storage capacity. EDLC materials include carbon compounds such as GO, graphite, CNT, MWCNT, etc^22,23. In contrast to EDLCs, PC materials consisting of conductive polymers and metal oxides have high capacitance and lower cyclic stability^{24, 25–26}. The combination of EDLCs and PC materials allows for the creation of a supercapacitor with excellent characteristics of high specific capacitance and good stability. So far, many conductive polymers have been used to fabricate supercapacitors, one of which is polypyrrole (PPy), which can be synthesized electrochemically and chemically²⁷. PPy has been composited with various metal oxides such as WO₃, MnO₂, NiO, etc., and has been investigated as a supercapacitor material²⁸.

Copper and its oxides, especially CuO and Cu₂O, play an important role in supercapacitor electrodes and hydrogen production technology^29,30. Its excellent electrochemical properties and abundance (easy availability) make it an attractive material for energy conversion and storage. CuO and Cu₂O have attracted attention in the fabrication of supercapacitor electrodes due to their high theoretical capacity, environmental friendliness, and cost-effectiveness³¹. Also, one of the most important features of this metal is its ability to be composited with other metal oxides and conductive polymers to create synergistic properties and increase capacitance. This metal is used as a catalyst or photocathode for water splitting due to its suitable band gap and catalytic activity in hydrogen production reactions. However, one of the biggest drawbacks of this metal is the low conductivity of its oxides, which can hinder electron transfer at electrodes. Another drawback of this metal is its low stability during redox reactions^32,33.

Molybdenum (Mo) metal and its oxide are another metal used in the construction of supercapacitor electrodes and hydrogen production^34,35. This metal has good energy storage capacity and mechanical stability, making it more popular for faricating supercapacitor electrodes²⁰. This metal has good energy storage capacity and mechanical stability, making it more popular for use in the fabrication of supercapacitor electrodes. Mo is more abundant than precious metals such as platinum and palladium, making it cost-effective to use in hydrogen production reactions, which Its most important property for use in hydrogen evolution reactions (HER) is its corrosion resistance³⁶. Molybdenum metal oxide, like copper metal, has low electrical conductivity³⁷so synthesizing the oxides of these two metals on substrates and materials with EDLC properties can increase their conductivity and make them suitable for use in the manufacture of supercapacitor electrodes and hydrogen generation reactions.

In this work, for the first time, the ternary PPy-Cu₂O-MoO₃ nanocomposite is deposited in a single step on GO nanosheets. For this purpose, GO nanosheets were first formed on GFE under the electrochemical anodizing process, in the next step the ternary nanocomposite was deposited from a solution containing all three types of materials through the electrochemical method and cyclic voltammetry (CV) technique. The fabricated electrode was used as a new electrode both in the role of energy storage as a supercapacitor and as a hydrogen generation electrode. The results of electrochemical studies showed that the fabricated electrode has the potential to be used in both fields. The development of this single-step nanocomposite electrodeposition method can be a starting point for the fabrication of other multi-layer nanocomposites and their use in different fields.

Experimental

Materials & reagents

All materials were used analytical grade without further purification. Pyrole (C₄H₄NH, ≥ 99%), ammonium heptamolybdate tetrahydrate ((NH₄)₆Mo₇O₂₄.4H₂O), Copper(II) sulfate (CuSO₄.9H₂O), Sulfuric acid (H₂SO₄ 97%), Potassium nitrate (KNO₃), and potassium sulfate (K₂SO₄) were purchased from Merck. A graphite foil with 98% purity and a thickness of 0.5 mm was purchased from Alpha Aesar Company. Double distilled water (DDW) was used for all experiments and reactions.

Physicochemical characterization techniques

The structural and synthetic properties of various electrodes were analyzed using a range of techniques. Surface morphology studies and determination of the elemental composition of the constructed electrodes were performed using ZEISS Sigma VP and MIRA3 TESCAN field emission scanning electron microscope (FE-SEM) and energy-dispersive X-ray spectroscopy (EDX). Brunauer-Emmett-Teller (BET) and pore size distribution Barrett Joyner Halenda (BJH) analysis were carried out using BEL SORP MINi II. To identify the molecular structure and characterize chemical bonds of the synthesized nanocomposite and fabricated electrodes, the Attenuated total reflection infrared spectrum (ATR- IR, Thermo Nicolet-USA) and the Raman spectra (Thermo Nicolet Almega Dispersive) of samples were recorded. The X-ray diffraction (XRD-XPert, PRO MPD, Cu Kα radiation, 2θ = 10–90^◦) technique was used to identify the phase and crystalline state of the synthesized nanocomposite compound onto the GO/GFE electrode. The electrochemical behavior of the constructed electrodes and assembled device were evaluated using an Autolab PGSTAT30.

Fabrication of PPy-Cu₂O-MoO₃ /GO/GFE

The PPy-Cu₂O-MoO₃ /GO/GFE electrode was constructed in two stages as follows: In the first stage, the GFE was anodized according to our previous works^38,39. Accordingly, the GFE was initially trimmed to the required dimensions of 1 × 4 cm², and then the electrode surfaces were cleaned of any contamination using ethanol and DDW in an ultrasonic bath, In the subsequent step, anodizing was performed in a K₂SO₄ solution (0.1 M) and two-electrode configuration using a DC power device. In this configuration, GFE was connected to the anode pole and Pt sheet to the cathode pole of the device, and the anodizing process was carried out for 2 min at a constant potential (10 V) and current (0.5 A). The resulting electrode was named GO/GFE was washed with DDW and dried in an oven at 70 °C for 4 h for use in the next step. In the second stage, the GO/GFE was used as a substrate with EDLCs properties to simultaneously electrodeposit of ternary PPy-Cu₂O-MoO₃ nanocomposite as a PC material. For this purpose, first 100 µl of pyrrole monomer were added to 25 ml of 0.5 M H₂SO₄ and sonicated, then 0.15, 0.15, and 0.2 gr of each of the KNO₃, CuSO₄.9H₂O, and (NH₄)₆Mo₇O₂₄.4H₂O were added to the Pyrrole- H₂SO₄ solution, respectively, and sonicated for 5 min. The prepared solution was used as a deposition solution for the synthesis of PPy-Cu₂O-MoO₃ nanocomposite on the GO/GFE electrode. The deposition process was carried out electrochemically under a three-electrode system using the CV technique (potential window − 0.9–1.3 V, scan rate 0.05 V s^− 1, and 10 cycles) in which GO/GFE, Pt sheet, and Ag/AgCl electrodes were selected as working, auxiliary, and reference electrodes, respectively. The PPy /GO/GFE electrode was prepared in the same manner as the PPy-Cu₂O-MoO₃ /GO/GFE but without the presence of CuSO₄.9H₂O, and (NH₄)₆Mo₇O₂₄.4H₂O. The fabricated electrodes were dried at 80 °C for 24 h.

Construction of solid-state symmetrical supercapacitor device

The supercapacitor device was assembled by connecting two PPy-Cu₂O-MoO₃/GO/GFE electrodes with a PVA/H₂SO₄ gel polymer electrolyte. For this purpose, first, PVA/H₂SO₄ gel polymer electrolyte was prepared by mixing 1 g of PVA, 10 ml of DDW, and 2 ml of H₂SO₄ (98%) on a magnetic stirrer at 70–80 °C. Then, after cooling the prepared gel, a few drops of it were placed between two PPy-Cu₂O-MoO₃ /GO/GFE electrodes with a surface area of ​​1 cm² and pressed tightly together. Finally, the fabricated supercapacitor device was dried for 24 h at room temperature to evaluate its electrochemical behavior and practical operation by electrochemical techniques.

Complying with relevant institutional, national, and international guidelines and legislation

The authors declare that all relevant institutional, national, and international guidelines and legislation were respected.

Results and discussion

Morphological & physicochemical characterization

FE-SEM analysis was used to study the surface morphology of the fabricated electrodes and synthesized PPy-Cu₂O-MoO₃ nanocomposite. As previously proven, electrochemical anodizing of GFE creates GO nanosheets on it. Figure 1a and b show the SEM images of the GFE and the GO/GFE, respectively. As shown in the SEM image of the GO/GFE electrode, GO is formed in the nanosheet form after the anodization process on the GFE. Figure 1c displays the SEM image of the PPy/GO/GFE surface, revealing the successful polymerization of the PPy polymer onto the GO nanosheets, evidenced by the formation of distinct pellet-like structures. Figure 1d and e depict SEM images of the PPy-Cu₂O-MoO₃/GO/GFE electrode surface captured at varying magnifications. Based on these figures, the electrosynthesis of ternary nanocomposite of PPy-Cu₂O-MoO₃ on GO nanosheets has been carried out successfully. Thus, the deposition of multiple metal oxides and PPy in pellet form has been successfully simultaneously synthesized on GO nanosheets for the first time. The deposited metal oxides are completely clear and shown in these figures. EDX analysis was performed to ensure the deposition of the ternary PPy-Cu₂O-MoO₃ nanocomposite on the GO/GFE electrode. Figure 1f shows the EDX spectrum of the PPy-Cu₂O-MoO₃/GO/GFE electrode surface. As can be seen from this figure, the presence of Cu, Mo, C, N, and O elements with atomic percentages of 0.99, 9.55, 48.71, 10.31, and 30.44%, respectively, confirms that this nanocomposite was successfully synthesized on the GO/GFE electrode. Also, Fig. 1g shows the distribution arrangement of each element (EDX mapping element) on the GO/GFE electrode surface.

Fig. 1 [Images not available. See PDF.]

FE-SEM images of (a) GFE, (b) GO/GFE, (c) PPy/GO/GFE, (d, e) PPy-Cu₂O-MoO₃/GO/GFE, electrodes, (f) EDX spectrum, (g), and EDX elemental mapping images of PPy-Cu₂O-MoO₃/GO/GFE, electrode.

Increasing the surface area of ​​the constructed electrodes used in supercapacitor and hydrogen generation systems is one of the most important parameters that can greatly affect their performance. BET analysis was performed to investigate the pore size, diameter, and surface area of GFE and ​​PPy-Cu₂O-MoO₃/GO/GFE electrodes. Figure 2a and b show the Nitrogen adsorption/desorption isotherm diagrams for GFE and ​​PPy-Cu₂O-MoO₃/GO/GFE electrodes, respectively. Also, Fig. 2c shows the BJH pore size distribution diagram for both electrodes. Table 1 shows the data extracted from both electrodes’ BET and BJH plots. As is clear from the obtained data, the average pore size of the GFE electrode surface after the anodizing process and deposition of the ternary nanocomposite has increased significantly from 18.59 to 34.4 nm. This increase in pore size can be attributed to the GO nanosheets and the ternary ​​PPy-Cu₂O-MoO₃ nanocomposite. Also, the pore volume of BJH has increased after the modification process of the GFE electrode surface, such that the pore volume of the GFE electrode surface has increased from 0.06 to 0.08 cm³ g^− 1, which is due to the multiple PPy-Cu₂O-MoO₃/GO nanocomposites on the GO/GFE electrode. Increasing the size and volume of pores by modifying the surface of the GFE electrode leads to increased performance of the electrode for H₂ generation and energy storage. This increase in H₂ generation efficiency and capacitance of the PPy-Cu₂O-MoO₃/GO/GFE electrode is due to the facilitation and smoothing of the diffusion process at the electrode-electrolyte interface⁴⁰. According to the obtained data in Table 1, by modifying the surface of the GFE electrode, its BET surface area decreased from 14.41 to 10.014 m² g^− 1, which is due to the coating of the GFE electrode surface with the accumulation of ternary PPy-Cu₂O-MoO₃ nanocomposite⁴¹. Finally, according to the obtained results from this analysis, it can be said that the fabricated electrode with mesoporous characteristics and high surface area can have appropriate performance for both H₂ generation and energy storage.

Figure 2d shows the X-ray diffraction pattern of the PPy-Cu₂O-MoO₃/GO/GFE electrode. According to this XRD pattern, the peaks appearing with very high intensity in the regions of 26.68 and 54.72° are related to the crystalline state of graphite^38,42,43. The peak appearing at the 2θ = 13.52° is related to (001) reflection of GO formed by the anodizing process onto GFE⁴⁴. The peak related to the PPy compound appeared in the form of a peak at 2θ = 20–40° due to its amorphous state, which overlapped with the GFE peak and increased the intensity of the peak at the region of 26° and followed the previous literature⁴⁵. Also, according to the XRD diffraction pattern for the PPy-Cu₂O-MoO₃/GO/GFE electrode, the peaks appearing at the 2θ = 76 and 87° are related to the (211) and (220) states of Mo in the nanocomposite structure⁴⁶. The peaks related to Cu₂O also appeared with very low intensity in the regions of 45, 63 and 73.2° are related to the (200), (311), and (222) states of Cu₂O according to previous study⁴⁷. However, the results obtained from XRD analysis, like previous analyses, confirmed the successful synthesis of the ternary PPy-Cu₂O-MoO₃ nanocomposite onto GO/GFE electrode.

Fig. 2 [Images not available. See PDF.]

BET N₂ adsorption-desorption isotherm of (a) GFE, (b) PPy-Cu₂O-MoO₃/GO/GFE, (c) BJH pore size distribution both electrodes, and (d) XRD diffraction pattern of PPy-Cu₂O-MoO₃/GO/GFE electrode.

Further verification of the synthesis of ternary PPy-Cu₂O-MoO₃ nanocomposite on the GO/GFE electrode was carried out using ATR-IR and Raman spectroscopy. Figure 3a and b show the ATR-IR spectra of the GFE and GO/GFE electrodes. After the anodizing process and the formation of GO sheets on the GO/GFE electrode, the peaks appearing at 3426, 2836, and 1702 cm^− 1 are related to the stretching vibrations of the hydroxyl group (-OH), the symmetric vibrations of the CH₂, and the stretching vibrations of the -C = O groups, respectively⁴⁸.

Also, the peaks appearing in the regions 1434, 1218, and 1070 cm^− 1 can be attributed to the stretching vibrations of the -O-H band, the stretching vibrations of the C-O epoxy group, and the stretching vibrations of the C-O alkoxy group, respectively^49,50 which all of these peaks that have appeared are related to graphene oxides formed on the GFE. According to Fig. 3c, the peak at 1444 cm^− 1 demonstrate the stretching vibrations of the C = N bond of the PPy polymer ring. Also, the appropriate absorption bands at 1114 (C–N stretch bending), 993 (= C–H out of plan vibration), 850 (C–C out of plan vibrations), and 696 cm^− 1 (C–H out of plan vibrations) are confirm the formation of PPy. All of these peaks with low shift and intensity have appeared in the ATR-IR spectra of the PPy-Cu₂O-MoO₃/GO/GFE electrode (Fig. 3d), which show the successful synthesis of ternary nanocomposite onto GFE^51,52.

Figure 3e shows the Raman spectra of GFE and PPy-Cu₂O-MoO₃/GO/GFE electrodes. As is clear in the Raman spectrum of the GFE electrode, there is a G band in the region of 1582 cm^− 1, which is due to the first-order scattering of the E_2g phonon of sp² C atoms^48,53. Also, the appearance of a peak in the region of 2725 cm^− 1 for the GFE electrode originates from a two-phonon resonance, which has disappeared in the Raman spectrum of the PPy-Cu₂O-MoO₃/GO/GFE electrode due to the covering of the GFE electrode surface with the PPy-Cu₂O-MoO₃ ternary nanocomposite and the formation of GO nanosheets⁵⁴.

In the Raman spectrum of the PPy-Cu₂O-MoO₃/GO/GFE electrode, the main and important peak (Ag) appearing in 1593 cm-1 is related to the symmetric C = C stretching of the PPy structure⁵⁵. For the PPy structure, the peak of (Ag), located at 1378 cm^− 1, is related to the inter-ring (C–C) stretching⁵⁶. Additionally, the peaks observed at 1254 and 1088 cm^− 1 correspond to the ring deformation mode (δ ring) Ag for PPy^55,56. The band at 930 cm^− 1 for Ag type is related to ring deformation associated with dication (dipolaron) for PPy structure^55,57. Also in the Raman spectra of the PPy-Cu₂O-MoO₃/GO/GFE electrode, the other observed peaks at 298 and 140 cm^− 1 are assigned to B_2g‒(O = Mo) and Cu₂O nanostructure^{58, 59–60}.

Fig. 3 [Images not available. See PDF.]

ATR-IR spectra of (a) GFE, (b) GO/GFE, (c) PPy/GO/GFE, (d) PPy-Cu₂O-MoO₃/GO/GFE, electrodes, (e) Raman spectra of GFE and PPy-Cu₂O-MoO₃/GO/GFE electrodes.

Evaluation of electrochemical behavior of PPy-Cu₂O-MoO₃/GO/GFE electrode

The electrochemical behavior of the PPy-Cu₂O-MoO₃/GO/GFE as a supercapacitor electrode was investigated in both three and two electrode systems (assembled symmetric device). Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and Electrochemical impedance spectroscopy (EIS) techniques were performed for both configurations. In the three-electrode electrochemical system studies, H₂SO₄ 0.5 M was selected as the electrolyte solution. The fabricated electrode, platinum sheet (Pt), and Ag/AgCl were chosen as the working, auxiliary, and reference electrodes. All tests were performed at ambient temperature and using Autolab PGSTAT30.

CV studies

CV is an important initial technique for investigating the capacitive nature of the synthesized electrodes. In this work, this technique was also used as the first electrochemical test. Figure 4a shows the CV curves of GFE, GO/GFE, PPy/GO/GFE, and PPy-Cu₂O-MoO₃/GO/GFE electrodes at 30 mV s^− 1 in the potential range of -0.3–1.2 V. As evident from this plot, the surface area beneath the CV curve of the PPy-Cu₂O-MoO₃/GO/GFE electrode is significantly greater than that of the other fabricated electrodes, which is evidence of the greater capacitance of the PPy-Cu₂O-MoO₃/GO/GFE electrode. For a detailed investigation, the capacitance of each electrode was calculated using Eq. 1. In this equation, C_sp, ΔV, v, i, and s are the specific capacitance (F cm^− 2), potential window (V), scan rate (V s^− 1), current (A), and electrode surface area (cm², respectively.

1

The Csp of the fabricated electrodes was calculated using Eq. 1 and the results are presented in Table 1. As evident from the results presented in this table, the PPy-Cu₂O-MoO₃/GO/GFE electrode has a much better capacitance ( 260.86 mF cm^− 2) at each stage of modification of the GFE electrode, which is due to (1) the formation of GO nanosheets on the surface of the GFE electrode, (2) The synthesized PPy on the GO/GFE surface, and (3) the synergistic effect of Cu₂O-MoO₃ metal oxide with PPy. Figure 4b illustrates the CV plot of the PPy-Cu₂O-MoO₃/GO/GFE electrode at 20 mV s^− 1 that the highlight redox peaks corresponding to the electroactive species on its surface. As is clear from this curve, two pairs of oxidation peaks (peaks 1 and 2) and two pairs of reduction peaks (peaks 3 and 4) are observed. The presence of these redox peaks can be attributed to the pseudocapacitive behavior of the PPy-Cu₂O-MoO₃ ternary nanocomposite. Peaks 1 and 4 are associated with the redox activity of the Cu₂O nanostructures. In contrast, peaks 2 and 3 correspond to the redox activity of the MoO₃-PPy, which overlap and are seen as an oxidation and reduction peak. Figure 4c and d display the CV plots of the PPy-Cu₂O-MoO₃/GO/GFE at different scan rates up and down, respectively. As evident from these plots, there is no significant difference in the shift of the redox peaks with increasing scan rate, which proves the reversible behavior and excellent capacitive nature of the PPy-Cu₂O-MoO₃/GO/GFE electrode. Figure 4e shows the calculated specific capacitance for the fabricated electrode at different scan rates. As can be seen from this plot, Csp decreases with increasing scan rate, indicating that the diffusion process controls the electron transfer kinetics at the electrode-electrolyte interface^61,62. This is also because there is enough time for diffusion-controlled processes at low scan rates, which leads to an increase in Csp, while at high scan rates, the opposite is true⁶³.

Fig. 4 [Images not available. See PDF.]

CV plots of (a) different constructed electrodes, (b) PPy-Cu₂O-MoO₃/GO/GFE electrode at 20 mV s^− 1 along with the related redox peaks, PPy-Cu₂O-MoO₃/GO/GFE electrode at (c) low and (d) high scan rates, and (e) calculated specific capacitance of the PPy-Cu₂O-MoO₃/GO/GFE electrode at different scan rates.

Table 1. Calculated C_sp of different fabricated electrodes from CV plot.

Electrodes	Csp (mF/cm2)
GFE	54.633
GO/GFE	130.48
PPy/GO/GFE	206.637
PPy-Cu2O-MoO3/GO/GFE	260.86

The EIS technique was investigated to complement and further confirm the capacitive behavior of the PPy-Cu₂O-MoO₃/GO/GFE electrode. Figure 5a shows the Nyquist plot of GFE, PPy/GO/GFE, and PPy-Cu₂O-MoO₃/GO/GFE electrodes in 0.5 M H₂SO₄ in the frequency range of 100 kHz to 0.1 Hz. As has been proven previously, three important parameters in the Nyquist plot are being evaluated to study the capacitive nature of the electrode and materials. One of these parameters is Rs or electrode-electrolyte resistance (the starting point of the Nyquist plot), the second is Rct, the charge transfer resistance (the diameter of the semicircle of the Nyquist plot), and the other is the Warburg line, which appears at low frequencies and diagonally with a slope of 45° to the real axis and indicates the diffusion behavior of ions or charge carriers in the electrolyte^14,39,64. According to Fig. 5a, the PPy-Cu₂O-MoO₃/GO/GFE electrode has a smaller Rs ​​and semi-circular diameter (Rct) than the other electrodes, which in a way indicates that the charge transfer resistance at the electrode-electrolyte interface is reduced due to the presence of the PPy-Cu₂O-MoO₃ ternary nanocomposite and it exhibits good capacitive behavior. Figure 5b presents the fitted Nyquist plot of the PPy-Cu₂O-MoO₃/GO/GFE electrode, along with its corresponding equivalent circuit. According to the figure, The Nyquist plot for this electrode is represented by an equivalent circuit that includes parameters such as Rs, Rct, W, and CPE. These parameters correspond to the solution resistance (Rs), the charge transfer resistance at the electrode-electrolyte interface (Rct), the Warburg impedance (W), and the constant phase element (CPE), each reflecting distinct electrochemical processes within the system (the CPE parameter has appeared due to the presence of graphite and GO compounds as a supercapacitor substrate, which causes the charge transfer or diffusion process to be inhomogeneous)^22,64,65 which is well fitted to the corresponding curve. The results of each of these parameters are reported in Table 2. Finally, the EIS technique, similar to CV, and GCD methods, further confirmed the outstanding capacitive performance of the PPy-Cu₂O-MoO₃/GO/GFE electrode.

Fig. 5 [Images not available. See PDF.]

(a) Nyquist plot of GFE, PPy/GO/GFE, and PPy-Cu₂O-MoO₃/GO/GFE electrodes in 0.5 M H₂SO₄and (b) fitted Nyquist plot of PPy-Cu₂O-MoO₃/GO/GFE electrode with the equivalent circuit related in 0.5 M H₂SO₄ electrolyte solution.

Table 2. The fitted equivalent circuit data of the Nyquist plot of PPy-Cu₂O-MoO₃/GO/GFE electrode.

Element	Fitted data
Rs (Ω)	2.017
Rct (Ω)	0.6
CPE (Ω−1 s)	0.1632
n	0.65
WP (Ω)	0.02

GCD was the next technique used to assess the capacitive behavior of the fabricated electrodes. Figure 5a shows the GCD curves of various fabricated electrodes in 0.5 M H₂SO₄ that were charge-discharged to 1 V. In this technique, a longer discharge time of the electrode is indicative of improved capacitive performance, as it suggests a more efficient charge storage and release process. This extended discharge time correlates directly with an enhanced Csp, reflecting the electrode’s superior ability to store and deliver charge during cycling. Thus, according to Fig. 6a, the PPy-Cu₂O-MoO₃/GO/GFE electrode has a much better capacitance than other fabricated electrodes. The reason for this good capacitance of the PPy-Cu₂O-MoO₃/GO/GFE electrode can be attributed to (1) GO nanosheets formed on the surface of the GFE, (2) the synergistic effect of the PPy-Cu₂O-MoO₃ ternary nanocomposite, and (3) the combination of EDLC materials (GF and GO) with pseudocapacitive materials (PPy-Cu₂O-MoO₃). The Csp of the electrodes and capacitive materials in the GCD technique is determined using Eq. 2, where I denotes the current density (A), Δt represents the discharge time (s), ΔV indicates the discharge potential window (V), and S refers to the surface area of the electrode (cm²)^66,67.

2

Csp for the fabricated electrodes were calculated using Eq. 2 and the results are reported in Fig. 6b. According to this figure, the PPy-Cu₂O-MoO₃/GO/GFE electrode has a much better capacitance (742.26 mF cm^− 2) than the other electrodes, which indicates its excellent capacitive nature. To further investigate the performance of the PPy-Cu₂O-MoO₃/GO/GFE electrode, its charge-discharge behavior was recorded at different current densities. Figure 6c and d show this electrode’s GCD curve and the calculated Csp at different current densities, respectively. As evident from the results, the PPy-Cu₂O-MoO₃/GO/GFE electrode exhibits a remarkable Csp of 1010.3 mF cm⁻² at 1 mA, underscoring its promising potential for application in the supercapacitor industry.

Fig. 6 [Images not available. See PDF.]

(a) GCD plots (b) specific capacitance of different fabricated electrodes at 4 mA cm^− 2, (c) GCD plots (d) specific capacitance of PPy-Cu₂O-MoO₃/GO/GFE electrode at different current densities (b) in 0.5 M H₂SO₄ solution.

Evaluation of electrochemical behavior of fabricated supercapacitor device

The practical performance of the PPy-Cu₂O-MoO₃/GO/GFE electrode was assessed through the construction of a symmetric solid-state supercapacitor device. The electrochemical characteristics of the assembled supercapacitor were analyzed using CV, GCD, and EIS techniques to evaluate its capacitive behavior. Figure 7a and b display the CV plots of the assembled device at different low and high scan rates, respectively. As evident from these curves, distinct redox peaks are observed at both scan rates, which signifies the superior capacitive performance of the ternary PPy-Cu₂O-MoO₃ nanocomposite deposited onto the GO/GFE electrode. A key observation from the CV data is that, even at higher scan rates, the Csp remains largely unchanged (Fig. 7c), suggesting stable capacitance and highlighting the excellent pseudocapacitive behavior and high-rate stability of the constructed device^14,68. This process proves that the structure of the ternary copper nanocomposite is such that (1) access to the active energy storage sites is not limited, (2) the ion transport resistance in the material is minimal, and (3) charge transfer occurs rapidly at the electrode surface. The advantages of this type of synthesized electrode behavior include a longer lifetime (high cyclic life), stable performance in high-power applications, and reduced energy dissipation^69,70. Accordingly, the Csp of the assembled device at different scan rates was calculated according to Eq. 2 and reported in Fig. 7c. According to the results, the fabricated device has an excellent capacitance of 103.81 mF cm^− 2 at 10 mV s^− 1. Figure 7d and e present the GCD profiles of the fabricated supercapacitor device at low (1–5 mA cm^− 2) and high (6–10 mA cm^− 2) current densities, respectively. The triangular and nearly symmetrical shape of these plots demonstrates the outstanding pseudocapacitive behavior of the supercapacitor device constructed with the PPy-Cu₂O-MoO₃/GO/GFE electrode. This characteristic shape is indicative of efficient charge storage and fast ion transport, further confirming the device’s superior electrochemical performance. Figure 7f shows the calculated Csp from the GCD curves for different current densities. Based on the obtained results, the fabricated supercapacitor device demonstrates an outstanding capacitance of 596.5 mF cm^− 2 at 1 mA cm^− 2, highlighting its superior charge storage capability. Also, the stable performance of the fabricated supercapacitor device for high-power applications was evaluated by drawing the Ragon plot. Figure 7g presents the Ragone plot of the fabricated device across various current densities. As illustrated in the plot, the supercapacitor exhibits a maximum power density of 7500 mW cm^− 2 and an energy density of 174 mWh cm^− 2, demonstrating its capability to function efficiently under high-power conditions. Furthermore, the long-term electrochemical stability of the device was evaluated through 6000 consecutive GCD cycles. Figure 7h shows the cyclic stability curve along with the initial and final 20 GCD cycles. According to the obtained results, the fabricated supercapacitor device showed a very good cyclic stability of 82.4% after 6000 charge-discharge cycles. Figure 7i depicts the Nyquist plot of the fabricated supercapacitor device both before and after cyclic stability testing. As observed in the plot, the device exhibits a relatively high Rs, which can be attributed to the use of a PVA/H₂SO₄ gel polymer electrolyte, compared to the H₂SO₄ electrolyte utilized in a three-electrode system, the increased Rs is primarily due to the charge transfer resistance associated with the gel polymer electrolyte. According to this curve, after 6000 GCD cycles, the resistance of the device has increased by a very small amount, which can be attributed to the very small degradation of the electrolyte or nanocomposite on the surface of the fabricated supercapacitor electrodes.

The electrochemical performance analysis of the fabricated supercapacitor device utilizing the PPy-Cu₂O-MoO₃/GO/GFE electrode indicates its superior efficiency compared to previously reported studies employing PPy conductive polymer and its nanocomposites. A detailed comparison of key performance parameters is provided in Table 3^{71, 72, 73, 74–75}.

Fig. 7 [Images not available. See PDF.]

Cyclic voltammograms at (a) low and (b) high scan rates, (c) specific capacitance at different scan rate, (d, e) Galvanostatic charge/discharge plot at different current densities, (f) specific capacitance at different current densities, (g) Ragon plot, (h) cyclic stability, and (i) Nyquist plots of PPy-Cu₂O-MoO₃/GO/GFE supercapacitor device before and after cyclic stability.

Table 3. Comparison performance of fabricated supercapacitor device by PPy-Cu₂O-MoO₃/GO/GFE electrode with other electrodes.

Electrodes	Electrolyte	Applied current/Scan rate	Capacitance	Stability cycles	Stability (%)	Ref
GO-PPy	0.5 M NaCl	1 mA/cm2	80 F/g	1500	53	71
PPy-WO3	3 M NaOH	0.76 mA/cm2	253 mF/cm2	4000	90.5	72
PPy/CuO/bacterial cellulose	2 M NaCl	0.8 mA/cm2	601 F/g	300	64	73
PPy/MnO2/carbon fiber	PVA/H3PO4 gel	0.1 A/cm3	69.3 F/cm3	1000	86.7	74
PPy/GO/ZnO	PVA/KOH gel	1 A/g	123.8 F/g	1000	90	75
PPy-Cu2O MoO3/GO/GFE	0.5 M H2SO4	1 mA/cm2	1010.30 mF/cm2	–	–	This work
PPy-Cu2O MoO3/GO/GFE	PVA/ H2SO4 gel	1 mA/cm2	596.5 mF/cm2	6000	82.4	This work

Electrochemical water splitting tests

The electrochemical behavior of the fabricated electrodes was investigated to evaluate their performance for the hydrogen evolution reaction (HER) in 0.5 M H₂SO₄ solution in a three-electrode system. In this system, GFE, GO/GFE, PPy/GO/GFE, and PPy-Cu₂O-MoO₃/GO/GFE electrodes were selected as working electrodes (the surface area of ​​the working electrodes was 1 cm², Pt and Ag/AgCl electrodes were selected as counter and reference electrodes. The voltages recorded against the Ag/AgCl electrode were converted to the RHE scale using Eq. 3^76,77 and their Tafel slope was calculated.

3

The Tafel slope was calculated from the LSV curves and through Eq. 4, where b is the Tafel slope and a is a constant⁷⁷.

4

Figure 8a shows the LSV curves of different electrodes. Figure 8b shows the Tafel slope calculated from Eq. 4 for different electrodes, which the Tafel plots were derived from the LSV curves within the region of kinetic control (prior to mass transport limitations). According to the obtained results, the Tafel slope for the PPy-Cu₂O-MoO₃/GO/GFE electrode is 142 mV dec^− 1, which is much lower than that of the GFE (268 mV dec^− 1), GO/GFE (214 mV dec^− 1), and PPy/GO/GFE (153 mV dec^− 1) electrodes. The low Tafel slope for the PPy-Cu₂O-MoO₃/GO/GFE electrode compared to other electrodes indicates the excellent kinetic efficiency of this electrode in the electrochemical reaction of HER. The good performance of the PPy-Cu₂O-MoO₃/GO/GFE electrode for the HER reaction compared to other electrodes can be attributed to the synergistic effect of the conductive PPy polymer with the Cu₂O-MoO₃ metal oxide, which in the presence of each other creates an active surface with greater accessibility and ultimately leads to increased water molecule adsorption⁷⁸. It is widely accepted that the overpotential at a current density of 10 mA cm⁻² (η₁₀) serves as a key benchmark for assessing the performance of HER catalysts⁷⁹. Accordingly, Fig. 8c and d show the zoomed image of the LSV curve and the calculated overpotential for different electrodes at -10 mA cm^− 2. As is also clear from these curves, the PPy-Cu₂O-MoO₃/GO/GFE electrode has a much lower overpotential than other fabricated electrodes, which once again confirms the better performance of this electrode in the HER reaction. Finally, the results of studying the electrochemical behavior of the PPy-Cu₂O-MoO₃/GO/GFE electrode showed that this fabricated electrode, in addition to its suitable performance as a supercapacitor electrode, can also function very well as an electrode for the HER reaction.

Fig. 8 [Images not available. See PDF.]

(a) LSV curves, (b) Tafel slope, (c) zoomed image of the LSV curve, and (d) calculated overpotential at 10 mA cm^− 2 for different fabricated electrodes.

Conclusions

In summary, the PPy-Cu₂O-MoO₃/GO/GFE electrode was successfully fabricated by anodizing the GFE, followed by the simultaneous electrochemical deposition of the ternary PPy-Cu₂O-MoO₃ nanocomposite using the CV technique on the GO/GFE electrode. This is the first report of using this technique for the simultaneous deposition of ternary PPy-Cu₂O-MoO₃ nanocomposites, offering advantages such as binder-free co-deposition, straightforward deposition, and uniform synthesis of nanocomposites on GO nanosheets. Morphological and chemical structure analyses of the fabricated electrode, conducted using various techniques (FE-SEM, BET, EDX, XRD, ATR-IR, and Raman), confirmed the successful synthesis of the new nanocomposite via a facile and rapid method. These results highlight the potential of this technique for synthesizing other multi-component nanocomposites. The investigation of the capacitive behavior of the PPy-Cu₂O-MoO₃/GO/GFE electrode demonstrated an excellent capacitance of 1010.30 mF cm⁻² in 0.5 M H₂SO₄ at 1 mA cm⁻². Furthermore, to evaluate its practical energy storage capability, a symmetrical supercapacitor device was fabricated. The performance results showed that this device exhibited a remarkable capacitance of 596.5 mF cm⁻² at 1 mA cm⁻² and maintained 82.4% of its cycling stability after 6000 GCD cycles, indicating the promising application of the fabricated electrode in the supercapacitor industry. Additionally, the application of the fabricated electrode for the water-splitting process revealed an overpotential of 361 mV and a Tafel slope of 142 mV dec⁻¹, indicating good performance in this field. Overall, the studies on the capacitive behavior and hydrogen production capabilities of the PPy-Cu₂O-MoO₃/GO/GFE electrode demonstrate its excellent performance and potential for applications in both energy storage and water-splitting technologies.

Acknowledgements

The authors wish to express thanks to the office of the vice chancellor of research of Urmia University – Iran for the financial support.

Author contributions

RD: methodology, investigation, data curation, writing original draft, writing—review and editing.MB: Supervise, write, review, and edit.KF: Supervise, write, review, and edit.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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