Progress in Bi₂WO₆-Based Materials for Electrochemical Sensing and Supercapacitor Applications

Abstract

Recently, the design and fabrication of novel electrode materials for electrochemical and electronic devices have received the widespread attention of the scientific community. In particular, electrochemical sensors and supercapacitors (SCs) involve the use of catalysts, which can enhance the electrochemical reactions at the surface of the electrode. Bismuth tungstate (Bi₂WO₆) is a cost-effective and efficient electrode material with decent optoelectronic properties and stability. The properties of Bi₂WO₆ can be improved by incorporating carbon-based materials, and the resulting composite may be a promising electrode material for electrochemical sensing and SCs. As per the available reports, Bi₂WO₆ has been combined with various nanostructured and conductive materials for electrochemical sensing and SC applications. This review discusses synthetic methods for the preparation of Bi₂WO₆. Progress in the construction of hybrid composites for electrochemical sensing and SC applications is reviewed. The Conclusion section discusses the role of electrode materials and their limitations with future perspectives for electrochemical sensing and SCs. It is believed that the present review may be useful for researchers working on Bi₂WO₆-based materials for electrochemical sensing and SC applications.

1. Introduction

Previously, nanostructured materials have received enormous interest for a variety of optoelectronic applications, such as electrochemical sensors [1], photocatalytic [2], optical [3], energy storage [4] and photovoltaics [5]. Nanostructured materials possess decent conductive properties, electrocatalytic properties, stability, and efficient specific surface area, which makes them desirable materials for such optoelectronic applications [6,7,8]. Semiconducting metal oxides are stable electrode materials for various electrochemical applications such as energy storage [9,10] and sensors/biosensors [11,12]. In particular, bismuth tungstate (Bi₂WO₆) is a semiconducting metal oxide and exhibits excellent physicochemical properties, such as photoluminescence [13,14], photocatalysis [15,16], pyroelectric [17], nonlinear dielectric induction [18], piezoelectric [19], and ferroelectric [20]. In previous years, various methods were utilized for the formation of Bi₂WO₆, including the solvothermal method [21,22], co-precipitation method [23], solid phase preparation method [24], ion exchange method [25], sol–gel method [26], ultrasonic spray pyrolysis method [27], hydrothermal method [28], ultrasonic chemical method [29], electro-spinning method [30], and microwave method [31]. It is well known that each method has its own limitations, disadvantages, and advantages. Bi₂WO₆ formed via different methods has different surface morphologies and particle sizes [32]. Thus, it is worth mentioning that the properties of Bi₂WO₆ can be tuned by employing a suitable synthetic method. In previous years, Bi₂WO₆ and its composites were explored in catalysis [33], energy conversion [34], gas sensors [35], dye degradation [36], hydrogen production [37], oxygen evolution reactions [38], CO oxidation [39], adsorption [40], and biomedical [41] applications. It is believed that Bi₂WO₆-based materials may be efficient electrode materials for the development of electrochemical sensors and energy storage devices.

At present, energy consumption has been significantly enhanced due to growth in the global population and the modernization of society [42]. Fossil fuels are extensively utilized to supply energy but have some limitations, such as limited abundance and the contribution of fossil fuels to environmental pollution [43]. Therefore, renewable energy-based technologies should be developed by exploring innovative approaches and strategies. In this regard, various energy technologies were developed, with supercapacitors (SCs) bridging the gap between batteries and traditional capacitors [44]. SCs have the potential to store and release energy within a short time, making them a desirable candidate for energy storage applications [45]. Numerous materials, such as transition metal oxides, polymers and semiconducting materials, are used for electrochemical applications [1,4]. It was observed that the utilization of new electrode materials for the fabrication of electrochemical devices is of particular importance. In this regard, Bi₂WO₆ emerged as a promising electrode material for sensing and SC applications. It is understood that Bi₂WO₆ offers several advantages, including environmental friendliness, decent stability, redox properties, and electrocatalytic properties. The incorporation of Bi₂WO₆ with other conductive materials also exhibits improved synergistic interactions and conductive properties, with more electrochemically active sites for redox reactions. Thus, it can be concluded that Bi₂WO₆ is a promising electrode material for electrochemical applications. Bi₂WO₆-based materials with semiconducting metal oxides, MXenes, LDH, rGO, etc., were utilized for electrochemical sensors and SC applications.

In this review, we report the progress in the design and construction of Bi₂WO₆-based materials with MXenes, semiconducting metal oxides, LDH, carbon nanotubes, rGO, etc., for sensing and SC applications. The examined literature on Bi₂WO₆-based materials for electrochemical sensing and SCs is shown in Chart 1a and Chart 1b, respectively. The reported literature on the synthesis of Bi₂WO₆ is summarized in Chart 1c. It can be observed that numerous reports are available on the synthesis of Bi₂WO₆, but limited reports are available on the fabrication of Bi₂WO₆-based electrochemical sensors and SCs.

2. Synthesis Methods

In the last few years, various methods, including electrochemical, hydrothermal, solvothermal, microwave, and sol–gel, were utilized for the preparation of Bi₂WO₆ and its composites. In this section, we briefly discuss the synthesis of Bi₂WO₆-based materials using different methods.

2.1. Hydrothermal Method

The hydrothermal method needs an autoclave/sealed container to improve the reaction between tungsten and bismuth precursors using water as a solvent. In a previous report, flower-like Bi₂WO₆ was synthesized through the hydrothermal method [46]. The bismuth nitrate and sodium tungstate precursors were dissolved in Milli-Q water and transferred to the autoclave system. The autoclave reactor was tightly sealed and kept at 160 °C for 12 h in a furnace, which yielded flower-like Bi₂WO₆. In contrast, the authors also observed that hydrothermal treatment at 200 °C for 24 h yielded Bi₂WO₆ NPs. It is clear that the morphology of Bi₂WO₆ can be tuned by optimizing the temperature, pH and reaction time. In another study [47], nickel (Ni)-doped Bi₂WO₆ was prepared through the hydrothermal method, possessing a hydrangea-like surface structure. The hydrothermal treatment at 160 °C for 4 h yielded Ni-doped Bi₂WO₆. The effects of pH were also studied by Wang et al. [48] to obtain Bi₂WO₆ NPs. However, this study used a relatively higher temperature of 200 °C for 20 h. It is noted that the hydrothermal method may be considered ecofriendly if the reaction is carried out at less than 100 °C with reduced time. But the abovementioned study involved 200 °C for 20 h, which is the major limitation for practical applications and scalability. Li et al. [49] reported the synthesis of Bi₂WO₆ using the hydrothermal method, which involves a relatively low temperature of 150 °C (time = 6 h). Gao et al. [50] utilized the hydrothermal method for the preparation of copper (Cu)-doped Bi₂WO₆. The authors also optimized the pH of the reaction solution, and hydrothermally grown Cu-doped Bi₂WO₆ exhibited a flower-like surface morphology under optimized conditions. Metal precursors are dissolved in water, and the pressure in the hydrothermal reactor facilitates the dissolution of precursors in solvents and promotes the nucleation process of the preparation of Bi₂WO₆. The hydrothermal method has various advantages, such as tuning the reaction time, pH, concentration of precursors, and temperature, and the high pressure in the autoclave has a significant role in optimizing the surface morphology of Bi₂WO₆.

2.2. Solvothermal Method

The solvothermal method is a similar approach for the preparation of nanostructured materials, except for the use of different solvents. In hydrothermal methods, water is utilized as a solvent, whereas the solvothermal method involves the use of solvents such as ethylene glycol. In a published research article, the solvothermal method was explored for the fabrication of a titanium dioxide (TiO₂) nanotube (NT)/Bi₂WO₆ composite [51]. Boron (B)-doped Bi₂WO₆ was obtained through the solvothermal method by utilizing ethylene glycol (EG) as a solvent [52]. The temperature for the solvothermal treatment was fixed at 433 K for 12 h. In another study [53], it was found that the solvothermal treatment of bismuth nitrate pentahydrate and sodium tungstate dehydrate in the presence of an EG solvent at 160 °C for 24 h yielded a hollow-shaped Bi₂WO₆ structure. Wu et al. [54] also explored novel strategies such as the microwave-assisted solvothermal approach towards the fabrication of Bi₂WO₆ nanosheets. The working principle for the solvothermal method is similar to the hydrothermal method. Thus, it is clear that the solvothermal method also offers several advantages, such as the optimization of pH, temperature, precursor concentration, and reaction time, to tune the surface morphology of the nanostructured materials.

2.3. Electrochemical Method

Electrodeposition, which is also called the electrochemical method, has been explored for the preparation of nanostructured materials and polymers. Zhu et al. [55] utilized the electrochemical method for the preparation of a Bi₂WO₆/rGO composite using a three-electrode assembly. The authors used a −1.2 V potential for the electrodeposition process. Indium tin oxide (ITO) was used as a working electrode substrate, whereas platinum foil was adopted as a counter electrode, with silver/silver chloride (Ag/AgCl) as a reference electrode. However, the calcination process involved a high sintering temperature of 420 °C for 1 h, which is a drawback for this study. The electrochemical method has various advantages, including low temperature, precise control over composition, formation of uniform films, cost-effectiveness, direct deposition on substrate, in situ oxidation and selective deposition. Orimolade et al. [56] proposed the electrodeposition method followed by a calcination process for the preparation of Bi₂WO₆. Therefore, future research may focus on the fabrication of Bi₂WO₆ films on conductive substrates through the electrochemical method by avoiding the calcination process.

2.4. Sol–Gel Method

Previously, Zhang et al. [57] adopted the sol–gel method for the synthesis of Bi₂WO₆. In brief, bismuth nitrate and citric acid were dissolved in distilled water, stirring the reaction solution at 80 °C, and ethylene diaminetetraacetic acid (EDTA) ammonia solution was added, followed by the addition of ammonium tungstate. The obtained colorless transparent aqueous solution was evaporated at 80 °C and formed dark-colored polymeric precursors/citric gels. The calcination at 350 °C for 4 h yielded a Bi₂WO₆ powder sample. A further study indicated that the sol–gel method can be used for the preparation of Bi₂WO₆/graphene thin films, which exhibited excellent catalytic properties for nitric oxide oxidation [58]. Mesoporous Bi₂WO₆ sheets were also prepared by Han et al. [59] by adopting the sol–gel method. The synthesized material showed a high surface area of 36.7 m² g⁻¹, with a pore volume of 0.07 cm³ g⁻¹. Therefore, Bi₂WO₆ displayed higher catalytic activity for photocatalytic applications. It is also understood that the sol–gel method also has numerous advantages, including homogeneous mixing, controlled composition, large-scale production, scalability, cost-effectiveness and coating capability.

2.5. Microwave Method

In 2023, the microwave method was adopted for the preparation of Bi₂WO₆ [60]. The microwave method involves the use of microwave irradiation to heat the precursor solution. A flower-like Bi₂WO₆ microstructure was also prepared using the rapid microwave-assisted method [61]. The authors also prepared a Bi₂O₃/Bi₂WO₆ composite using the microwave method. A Bi₂WO₆/Bi₂S₃ heterojunction was also developed by Liu et al. [62] for the reduction of chromium ions using the photocatalytic method. They suggested that the microwave method can also be used for the preparation of hybrid composite materials. It is also worth mentioning that the microwave method has several advantages, such as a shorter reaction time, fast heating process, uniform heating, and reproducibility. However, the large-scale production of Bi₂WO₆ via the microwave method is limited.

3. Progress in Electrochemical Sensors

Electrochemical sensors are the most promising next-generation devices, offering several advantages towards the monitoring of environmental pollutants and biomolecules. Electrochemical sensors can be utilized for environmental monitoring and clinical/healthcare systems. The selection of a suitable electrode modifier is of great significance for the development of highly sensitive and selective electrochemical sensors. In this context, a cost-effective and promising electrode material (Bi₂WO₆) was explored for electrochemical sensing applications. Peng et al. [63] explored the potential application of hydrothermally grown Bi₂WO₆ nanoplates (NPts) for the simultaneous detection of diethylstilbestrol (DES) and bisphenol A (BPA). The formation of the Bi₂WO₆ phase was authenticated using the X-ray diffraction (XRD) method, whereas its morphological features were evaluated via scanning electron microscopy (SEM). It is well understood that DES and BPA are environmental pollutants; therefore, the detection of DES and BPA is necessary. Electrochemical impedance spectroscopy (EIS)-based investigations revealed that a Bi₂WO₆ NPts-modified carbon paste electrode (CPE) exhibits a low charge transfer resistance (R_ct) value compared to the bare CPE. It suggests that Bi₂WO₆ NPts-modified CPE possesses higher electrical conductivity and electrocatalytic properties, which may be ascribed to the presence of Bi₂WO₆ NPts on the CPE surface. The cyclic voltammetry (CV) results shows that diffusion plays a crucial role for the electrochemical determination of DES and BPA. Differential pulse voltametric (DPV) analysis further suggested that redox peak current responses for DES and BPA linearly increase with increasing concentration. Thus, these authors were able to obtain limits of detection (L_OD) of 15 nM and 20 nM for the sensing of DES and BPA, respectively. The spike approach-based studies indicate that DES and BPA can be recovered from milk samples. Liu et al. [64] adopted the hydrothermal synthetic technique for the fabrication of three-dimensional (3D) Bi₂WO₆. The SEM results shows that the prepared Bi₂WO₆ is composed of flower-like microspheres (MSs), which were further used to immobilize horseradish peroxidase (HRP) for the construction of a hydrogen peroxide (H₂O₂) sensor. H₂O₂ is widely used in various industries, but its toxic effects on human health motivated the researcher to develop electrochemical sensors. The immobilization process is described in Figure 1a. The flower-like MSs structure of the prepared Bi₂WO₆ was confirmed through SEM analysis, as shown in Figure 1b. The immobilization of HRP with Bi₂WO₆ MSs was also confirmed, as seen in the SEM image (Figure 1c). The nafion/HRP/3D-Bi₂WO₆ was coated on a glassy carbon electrode (GCE) to determine the H₂O₂ using the amperometry (AMP) technique. The AMP response of the nafion/HRP/3D-Bi₂WO₆/GCE with the spike of different H₂O₂ concentrations is presented in Figure 1d. It was observed that the current response linearly increased with a spike in H₂O₂ (Figure 1e). The stability for 30 days was also checked for long-term applications, and further studies revealed good reproducibility for the detection of H₂O₂. The H₂O₂ recovery in real samples was also satisfactory, revealing the potential of the proposed nafion/HRP/3D-Bi₂WO₆/GCE for practical applications.

In other previous work [65], the voltametric determination of Sudan I was also reported using Bi₂WO₆ nanosheet (NS)-modified GCE. The formation of Bi₂WO₆ NS was checked using Raman spectroscopy, SEM, XRD, energy-dispersive X-ray spectroscopy (EDX) and Fourier transform infrared (FTIR) spectroscopy. The electrochemical oxidation of Sudan I involves a two-proton/two-electron transfer mechanism and diffusion-controlled process. The L_OD of 2 nM, sensitivity of 3.0563 µA µM⁻¹ and linear range (L_R) of 0.02 µM to 114.6 µM were achieved for Sudan I sensing. The authors also recovered Sudan I from food samples, including chili powder and apple juice. The dual-mode electrochemical/photoelectrochemical detection platform for hydrogen sulfide (H₂S) was explored by Xu et al. [66]. In this context, a titanium dioxide (TiO₂)-modified Bi₂WO₆/silver (Ag) heterojunction was developed for the reduction of H₂O₂. The proposed electrode exhibited an L_OD of 60 nM and L_R of 0.5 µM to 300 µM, with reasonably good stability and reproducibility. This electrode was also efficient and selective for the determination of H₂O₂ in the presence of many interfering substances, including selectivity, Na₂SO₄, Na₂S₂O₃, NaHSO₄, Na₂SO₃, ascorbic acid (AA), l-Cys, dopamine (DA), and 30 μM H₂S. The presence of synergistic interactions may be responsible for the improved selectivity and electrochemical performance of the proposed H₂O₂ sensor. Hu et al. [67] developed a sulfamethoxazole (SMX) electrochemical sensor by exploring Br-terminated 2D Bi₂WO₆ as a sensing layer. The authors proposed that the prepared Br-terminated 2D Bi₂WO₆ can be explored in electrochemical sensing and photochemical water treatments. Vitamins are organic compounds and are essential for the operation of the human body. Vitamin deficiency may cause diseases in the human body. Riboflavin (RF), which is also called vitamin B2 (RF-7, 8-dimethyl 10 ribityl-isoalloxazine), plays a vital role in the development of flavin mononucleotide (FMN) and flavin coenzyme, comprising flavin adenine dinucleotides (FADs), which are important for tissue aeration. It is well known that RF can facilitate the transformation of fats, proteins, and carbohydrates to ATP. RF deficiency may cause biological problems such as burning and scorching of the eyes. Therefore, monitoring the level of RF is important. In this regard, Bi₂WO₆ was prepared, and its morphological characteristics were optimized by using sodium hydroxide (NaOH), polyvinylpyrrolidone (PVP) and a mixture of ternary solvents. The hollow spheres of Bi₂WO₆ were obtained by employing PVP + NaOH as a solvent [68]. The Bi₂WO₆ hollow sphere-modified GCE exhibited improved electro-oxidation of RF using the DPV technique. The fabricated RF sensor displayed a reasonable L_OD of 3.65 nM, sensitivity of 1.14 µA µM⁻¹ cm⁻² and L_R of 0.03 µM to 457 µM for the reduction of RF, whereas the oxidation process showed an L_OD of 8.9 nM with a sensitivity of 0.95 µA µM⁻¹ cm⁻². The presence of hollow spheres acted as a tunnel for the travel of electrons and ions, which enhanced the electrochemical sensing of RF. In another report [69], Bi₂WO₆ was decorated on a paraffin-deposited graphite electrode (PGE) for the electrochemical determination of AA. AA is an efficient antioxidant and plays a vital role in the human body. Accurately monitoring the level of AA is of great significance. Bi₂WO₆/PGE shows acceptable electrochemical performance for the detection of AA using CV and DPV techniques, and observations revealed that the fabricated electrode has higher electrochemical activity for AA detection at a pH of 7.0. The current response was also linearly increased, and an L_OD of 77.85 mM was obtained, with a sensitivity of 0.26 mM/mA. The L_OD of the Bi₂WO₆/PGE-based AA sensor needs to be further improved by introducing novel strategies or combining Bi₂WO₆ with other conductive and high-surface-area materials. The misuse of antibiotics such as ampicillin (AMP) is dangerous for humans as their residues can enter the human body through the ecological and food chains and can exert negative effects on human health. Tang et al. [70] reported the fabrication of a novel AMP sensor by exploring the electrochemical properties of a Bi₂WO₆-based composite. The hydrothermally prepared Bi₂WO₆ was combined with carboxylated multi-walled carbon nanotubes (MWCNTs-COOH) by employing the ultrasonication method. Other steps involved the immobilization of metal β-lactamases (MBLs) + chitosan (CS) to develop the MBLs/CS/Bi₂WO₆/MWCNTs-COOH/GCE for the determination of AMP (Figure 2). The constructed MBLs/CS/Bi₂WO₆/MWCNTs-COOH/GCE showed improved stability, reproducibility, repeatability, selectivity and acceptable recovery of AMP in real samples. The L_OD of 0.17 nM and two L_R of 0.0007 to 0.05 µM and 0.1 to 10 µM were observed for AMP detection. The sensing mechanism for the electrochemical detection of AMP is explained in Figure 2.

Another study reported the benign wet-chemical co-precipitation method for the preparation of Bi₂WO₆ [71]. The prepared Bi₂WO₆ was characterized by XRD, which confirmed the formation of the desired product and matched with JCPDS number 73-1126. The authors observed that XRD analysis revealed the formation of orthorhombic Bi₂WO₆. The transmission electron microscopy (TEM) analysis indicated the presence of the polycrystalline nature of the prepared orthorhombic Bi₂WO₆. In further studies, it was observed that the Bi₂WO₆-modified electrode has relatively higher electrical conductivity compared to the bare electrode. The authors explored the potential of the Bi₂WO₆-modified electrode for the simultaneous determination of hydroquinone and resorcinol (HQ and RS) using the CV and DPV methods. The pH of the analyte solution was also optimized, and it was observed that the fabricated electrode is highly active at a pH of 7.0. In high basic or high acidic pH conditions, no significant peak was observed for the detection of HQ and RS. This sensor was also found to be highly stable, selective, and reproducible for the determination of HQ and RS. The real sample studies in tap water and ointment suggested its significance for the real-time monitoring of HQ and RS. Previously, a zinc vanadate-modified Bi₂WO₆ (Zn₃(VO₄)₂/Bi₂WO₆) composite was fabricated using the hydrothermal method, as shown in Figure 3a [72]. The prepared Zn₃(VO₄)₂/Bi₂WO₆ shows decent crystalline nature and was found to be in agreement with JCPDS number 24-1482 and 39-0256 for Zn₃(VO₄)₂ and Bi₂WO₆, respectively. It was also observed from XRD analysis that Zn₃(VO₄)₂ has a monoclinic phase, whereas Bi₂WO₆ possesses an orthorhombic crystal structure. The Zn₃(VO₄)₂/Bi₂WO_6-modified electrode demonstrated an L_OD of 1 pM and L_R of 1 nM to 5 µM for the determination of cotinine. The proposed cotinine sensor also showed good replicability and stability, as shown in Figure 3b and Figure 3c, respectively.

A single-step hydrothermal method was explored for the fabrication of a morphologically engineered biomimetic 3D biconcave-shaped Bi₂WO₆ [73]. The authors used aluminum (Al) foil as a substrate to develop the Bi₂WO₆-based nicotine electrochemical sensor. The Bi₂WO₆-modified Al foil-based nicotine sensor exhibited excellent selectivity, L_OD of 0.9 nM, sensitivity of 83.23 μA/(μM cm²), and acceptable stability/reproducibility. Another study reported the fabrication of a photoelectrochemical DA sensor using the benign approach [74]. In this connection, a cobalt tetraaminophthalocyanine covalently linked graphene oxide (CoTAPc-GO) composite was fabricated using simple strategies. Furthermore, CoTAPc-GO and Cu-doped Bi₂WO₆ microflowers (MFs) were combined by employing the ultrasonication method. The obtained material (CoTAPc-GO/Cu-Bi₂WO₆) was coated on an indium tin oxide (ITO) surface, and its sensing activity for DA was evaluated. The authors obtained an L_OD of 7.2 nM, with two L_R of 0.05 µM to 5 µM and 5 µM to 250 µM for the detection of DA. Xu et al. [75] reported a photoelectrochemical immunosensor for the determination of amyloid beta (Aβ). The flower-like amorphous structure of Bi₂WO₆ was obtained, which was further combined with Mn²⁺-doped cadmium selenide (CdSe). The obtained results exhibited a reasonable L_OD of 0.068 pg/mL with L_R of 0.2 pg/mL to 50 ng/mL. The flower-like Bi₂WO₆ was decorated on graphene nanoribbons (GNRs) by employing the microwave synthesis method [76]. The fabrication procedure for Bi₂WO₆@GNRs is depicted in Figure 4a. The Raman and X-ray photoelectron spectroscopy (XPS) studies confirmed the presence of Bi₂WO₆ on the GNR surface. Further studies revealed that a Bi₂WO₆@GNR-modified screen-printed electrode (SPE) has a good selective nature for the determination of HQ and CC. The impedance studies indicate the presence of enhanced electrical conductivity of the constructed Bi₂WO₆@GNRs/SPE. Therefore, Bi₂WO₆@GNRs/SPE shows an L_OD of 5.31 nM and 7.24 nM for CC and HQ detection, respectively. The proposed sensor was also effective for the detection of CC and HQ in a water sample and face cream sample, respectively. In addition, the proposed sensor exhibited decent selectivity for the determination of CC and HQ in the presence of various interfering substances (Ni⁺, Cu²⁺, Cr⁺, Na⁺, Zn²⁺, Cl⁻, Hg+, Cd⁺, DA, hydrazine (Hz), nitrobenzene (NBz), 4-nitrophenol (4-NP), 4-aminophenol (4-AP), 2-aminophenol (2-AP), glucose (glu), nitrite, resorcinol (RS), epinephrine, AA, vitamin B₆ and vitamin B₉). The enhanced performance of the Bi₂WO₆@GNRs/SPE was ascribed to the presence of synergistic interactions and the high surface area of the electrode material. Verma et al. [77] prepared a plate-like Bi₂WO₆-modified mesh-like graphitic carbon nitride (g-CN) composite, and rGO was loaded into this prepared hybrid material using the hydrothermal-assisted synthesis method. The authors tuned the percentage of g-CN, and the optimized sample BCN-200/rGO (Bi₂WO₆/g-CN-200 mg/rGO) exhibited enhanced catalytic properties. The formation of the BCN-200/rGO is shown in Figure 4b. The obtained Bi₂WO₆/g-CN/rGO composite was explored for the fabrication of an electrochemical sensor towards the detection of 4-NP. The CVs of the g-CN, Bi₂WO₆, BCN-200 and BCN-200/rGO-modified GCE are shown in Figure 4c.

It can be seen that BCN-200/rGO/GCE has a higher redox peak current response for the detection of 4-NP. The mechanism involved in the sensing of 4-NP is shown in Figure 4d. The BCN-200/rGO-modified GCE also showed excellent selectivity and stability for the sensing of 4-NP, as shown in Figure 4e and Figure 4f, respectively. The presence of synergy in the Bi₂WO₆, g-CN, and rGO enhanced the catalytic behavior of the BCN-200/rGO-modified GCE. Additionally, the fabricated 4-NP sensor delivered an L_OD of 14 nM, L_R of 0.2 μM to 100 μM and sensitivity of 12.86 μA.μM⁻¹.cm⁻². It is well known that 4-Nitroquinoline N-oxide (4-NQO) is an important tumorigenic organic compound, which has adverse effects on the human body. Thus, in a previous report, a Bi₂WO₆ nanosphere-decorated rGO composite was prepared using the sonication method [78]. The Bi₂WO₆/rGO composite was further deposited on the GCE surface and its electrochemical activities for the detection of 4-NQO using the AMP method. The proposed electrode shows an L_OD of 6.11 nM and excellent detection of 4-NQO in urine and human blood samples. The plasmonic Ag-induced metallic Ag Bi₂WO₆/TiO₂ composite was developed for oxygen/hydrogen evolution and DA electrochemical sensing applications [79]. The CV results show that the current response linearly increases with an increasing concentration of DA. Due to the multifunctional applications of the proposed material in a single report, the authors did not significantly focus on the sensing of DA. Thus, more in-depth studies are required to explore the potential of the above-proposed electrode material for DA sensing applications. In recent years, MXene-based materials have been widely used to enhance the physicochemical properties of seminconducting metal oxides and metal sulfides, etc. The incorporation of MXene materials enhanced the electrical conductivity of the resulting composite materials. In this context, niobium carbide (Nb₄C₃T_x) MXene was incorporated with Bi₂WO₆ (as first layer), and salmon ds-DNA was engineered as a second layer/biological recognition element on a conductive substrate [80]. Thus, these authors obtained a decent L_OD of 2.8 nM, L_R of 0.01 µM to 100 µM, and acceptable recovery of 98.7% to 103.6% for the determination of pemetrexed using Bi₂WO₆/Nb₄C₃Tx/ds-DNA/SPE. Another study [81] reported a novel sandwich-type SARS-CoV-2 (COVID-19) nucleocapsid protein immunosensor by employing simple strategies. The Bi₂WO₆/bismuth sulfide (Bi₂S₃) composite was utilized as an electrode platform, whereas g-CN/Au NPs/tungsten trioxide (WO₃) was adopted for signal amplification. The proposed sensor delivered an L_OD of fg/mL. Nitrite has a negative influence on human health and the environment due to its toxic properties. The determination of nitrite is necessary, and a previous report indicated that Co phthalocyanine (CoPc) NPs were covalently linked to Bi₂WO₆ and nickel tungstate (NiWO₄), which can used as sensing layers for the determination of nitrite [82]. The authors observed that CoPc covalently linked to Bi₂WO₆-modified GCE shows better performance compared to the CoPc covalently linked to NiWO₄-modified GCE. An interesting LOD of 0.063 µM to 1.7 µM was observed for the detection of nitrite. Kanamycin (KAN) is a well-known amino glycoside, which has serious negative impacts on human health. In this regard, it is useful to develop the KAN sensor by exploring Bi₂WO₆/Ti₃C₂ MXene QDs [83]. The electrostatically driven hydrothermal-assisted-formed Bi₂WO₆/Ti₃C₂ MXene QD composite was adopted as a sensing material. The proposed KAN sensor exhibited recoveries in the range of 94.2% to 101.3%, L_OD of 0.025 nM, and L_R of 0.01 nM to 50 nM. The dual 3D structural Bi₂WO₆/graphene hydrogel (GH)-modified molecularly imprinted polymer (MIP)-based electrode was developed for the detection of 4-NP in PM_2.5 [84]. The fabricated MIPs/Bi₂WO₆@GH/ITO electrode had an L_OD of 5.78 × 10⁻¹³ M and L_R of 5.0 × 10⁻¹² to 1.0 × 10⁻⁷ M for the determination of 4-NP. This enhanced electrochemical performance of the proposed electrode was attributed to the 3D flower-like morphology of Bi₂WO₆ and porous network of GH. It is proven that hexavalent chromium (Cr) has toxic effects, and exposure to Cr via ingestion or inhalation may cause allergies, affect internal organs, and contribute to cancer. Therefore, it is really of great significance to develop an electrochemical sensor for the monitoring of Cr. In this connection, the solvothermal method was adopted for the preparation of a β-Bi₂O₃/Bi₂WO₆/meso-tetraphenylporphyrin (H₂TPP) composite on a fluorine-doped tin oxide (FTO) substrate [85]. It was observed that the presence of H₂TPP enhances the active site density and provides an efficient surface area for effective adsorption by providing pyrrolic and pyridinic-N atoms to β-Bi₂O₃/Bi₂WO₆/H₂TPP. The β-Bi₂O₃/Bi₂WO₆/5 wt % H₂TPP exhibited excellent cyclic stability, sensitivity, reproducibility and an L_OD of 8 nM. The electrochemical sensing performances of the various reported electrochemical sensors are summarized in

Table 1. Performance of some previously reported electrochemical sensors.

Electrode Material	Analyte	LOD	Sensitivity	LR	Technique	References
Bi2WO6 nanoplates@SPE	BPA	20 nM	-	70 nM to 1300 nM	DPV	63
Bi2WO6 nanoplates@SPE	DES	15 nM	-	50 nM to 2100 nM	DPV	63
Nafion/HRP/3D-Bi2WO6/GCE	H2O2	180 nM	-	0.5 µM to 100 µM and 100 µM to 250 µM	AMP	64
Bi2WO6 NSs/GCE	Sudan I	5 nM	3.0563 µA µM−1	0.02 µM to 114.6 µM	DPV	65
TiO2/Bi2WO6/Ag/GCE	H2S	80 nM	-	0.5 μM to 300 μM	AMP (i-t)	66
Br-terminated 2D Bi2WO6 NSs	SMX	5.44 µM	-	-	DPV	67
Bi2WO6 (PVP + NaOH)/GCE	RF	12.151 nM	1.1415 µA µM−1 cm−2	0.03 µM to 457 µM	DPV	68
Bi2WO6	AA	77.85 mM	0.26 mM/mA	-	CV	69
MBLs/CS/Bi2WO6/MWCNTs-COOH/GCE	Ampicillin	0.17 nM	-	0.0007 µM to 0.05 µM and 0.1 µM to 10 µM	DPV	70
Bi2WO6	HQ	57 µM	-	200 µM to 5 mM	SWV	71
Bi2WO6	RS	4.3 µM	-	20 µM to 5 mM	SWV	71
Zn3(VO4)2/Bi2WO6	Cotinine	1 pM	-	1 nM to 5 µM	CV	72
Bi2WO6	Nicotine	13 nM	56.97 ± 2 μM–1	50 nM to 100 μM	I-V/Amp	61
CoTAPc-GO/Cu-Bi2WO6	DA	7.2 nM		0.05 µM to 5 µM and 5 µM to 250	PEC	74
ITO/Bi2WO6/CdS/Ab/BSA	Amyloid beta	0.068 pg mL−1	-	0.2 pg mL−1 to 50 ng mL−1	PEC	75
Bi2WO6@GNRs	HQ	7.51 nM	42.2 μA μM−1 cm−2	0.021 µM to 1550 µM	DPV	76
Bi2WO6@GNRs	CC	5.31 nM	54.14 μA μM−1 cm−2	0.021 µM to 1550 µM	DPV	76
BCN-200/rGO	4-NP	14 nM	12.86 μA μM−1 cm−2	0.2 μM to 100 μM	DPV	77
Bi2WO6/rGOS	4-NQO	0.006 µM	-	0.025 µM to 718 µM	Amp	78
Bi2WO6/Nb4C3Tx/ds-DNA/SPE	Pemetrexed	2.8 nM	-	0.01 µM to 100 µM	DPV	80
g-C3N4/Au/WO3	SARS-CoV-2 NP	3 fg mL−1	-	-	DPV	81
CoPc/Bi2WO6	Nitrite	0.063 µM	-	-	DPV/Amp	82
CoPc@Bi2WO6	Nitrite	0.12 µM	-	-	DPV/Amp	82
MIPs/Bi2WO6@GH/ITO	4-NP	5.78 × 10−13 M	-	5.0 × 10−12 M to 1.0 × 10−7 M	Amp	84

.

It can be summarized that the above-discussed electrochemical sensors demonstrated promising electrochemical performance for the determination of various environmental pollutants. It was observed that pristine Bi₂WO₆ exhibits moderate electrochemical performance in terms of sensitivity. In contrast, the electrochemical performance of pristine Bi₂WO₆ was significantly enhanced by combining it with conductive materials, such as rGO, MXenes (Nb₄C₃Tx, Ti₃C₂), MWCNTs, g-C₃N₄, NiO, or Ag NPs. It was observed that Bi₂WO₆/MWCNTs/MBLs/CS exhibits the lowest LOD of 0.17 nM for ampicillin detection due to the presence of synergistic interactions such as high surface area and more abundant active sites. On the other side, the Bi₂WO₆/g-C₃N₄/rGO composite-based electrode demonstrates enhanced sensitivity of 12.86 μA μM⁻¹ cm⁻² for 4-NP, which can be assigned to the high electron mobility and surface area of the electrode material. It can be understood that Bi₂WO₆-based hybrid composites with MXenes and carbonaceous materials (rGO or CNTs) are promising electrode materials compared to the other hybrid composites for electrochemical sensing applications.

4. Progress in SCs

The design and fabrication of novel electrode materials are of great importance for SCs. Bi₂WO₆-based composites were significantly explored for the construction of SCs. Herein, we compile the progress in the preparation of Bi₂WO₆-based materials for the development of next-generation SCs.

4.1. Bi₂WO₆-Based Materials for SCs

In a previous report, Bi₂WO₆ NPs were synthesized via the sonochemical approach to explore an ecofriendly and cost-effective SC device [86]. The electrochemical performance of the Bi₂WO₆ NPs was evaluated in different aqueous electrolytes (1 M NaOH, 1 M sodium sulfate (Na₂SO₄), and 1 M lithium hydroxide (LiOH)). It was revealed that the KOH electrolyte was effective compared to the other electrolytes. Thus, the authors also used 1 M and 6 M KOH solutions for further electrochemical studies with higher ionic mobility, smaller hydration sphere radius and lower equivalent series resistance. The galvanostatic charge–discharge (GCD) studies show a specific capacitance of 608 F/g, which can be obtained for Bi₂WO₆ in 1 M KOH at a current density of 0.5 mA/cm². The specific capacitance of 304 F/g at 3 mA/cm² was also observed in a 1 M KOH electrolyte system. This study suggested that Bi₂WO₆ NPs can be explored as negative electrode modifiers for SCs in a 1 M KOH electrolyte. In another study [87], Bi₂WO₆ was also synthesized using the hydrothermal method. The authors used different times (3, 6, 12 and 24 h) for the preparation of Bi₂WO₆. The synthesized Bi₂WO₆ samples at 3, 6, 12 and 24 h were named BWO1, BWO2, BWO3 and BWO4, respectively. The optimized studies show that Bi₂WO₆-based SCs exhibit a specific capacitance of 363 F/g in neutral electrolytes and 5000 cyclic stabilities. The 3D Bi₂WO₆ globules were vertically grown in nickel foam (NF) electrodes using benign conditions [88]. The 3D Bi₂WO₆-modified NF electrode was utilized as a negative electrode, and areal capacitance of 1.120 F/cm³ was observed at a current density of 1.5 mA/cm² with excellent cyclic stability of 1000. Shembade et al. [89] reported the hydrothermal preparation of mesoporous spherical nanoflower-like Bi₂WO₆. The Bi₂WO₆ was deposited on NF, and its electrochemical performance for SCs was examined using CV and GCD. It was found that Bi₂WO₆ NFs exhibit the presence of an orthorhombic phase in which atoms are present in a regular arrangement. The optimized electrode (Bi₂WO₆/NF-2 = BWO-2) showed a specific capacitance of 647 F/g in a 1 M KOH electrolyte, including stability of 5000 cycles. In another study, various nanostructures of Bi₂WO₆ were obtained by exploring hydrothermal treatment under various pH conditions [90]. pH 1-, 7- and 11-based synthesis exhibits the formation of Bi₂WO₆ with flower-like hierarchical structures (average diameter = 7 µm), irregular flake-like structures (average thickness = 90 nm) and uniform spherical structures (average size = 85 nm), respectively. The flower-like hierarchical, flake-like and spherical-shaped Bi₂WO₆ demonstrated specific capacitance of 255 F/g, 214 F/g, and 412 F/g, respectively. It can be noted that Bi₂WO₆ spheres exhibited the highest electrochemical performance for SCs. Karnan et al. [91] pioneered one-pot preparation of Bi₂WO₆ nanostructures for SCs. A benign ultrasonic-assisted synthesis method was adopted to fabricate bare Bi₂WO₆ (BWO-bare). The authors also heated Bi₂WO₆ at 600 °C, which can be labeled as BWO-600. Figure 5 illustrates the formation of BWO-600, and the obtained material was characterized by XRD and SEM to investigate its phase purity and morphological features. The SEM analysis revealed that BWO-600 consists of a nanostructure with a diameter of 50 to 310 nm. In contrast, BWO-bare does not show evident clusters. Under a 6 M KOH electrolyte system, the BWO-600-based electrode shows specific capacitance of 614 C/g in a three-electrode system, whereas the specific capacity of 376 C/g was observed in asymmetric SCs.

Other work adopted the solution combustion method for the preparation of Bi₂WO₆ NPs [92]. The authors used jackfruit extract as fuel in the synthesis of Bi₂WO₆ NPs. Nithya et al. [93] proposed the fabrication of the polyvinylpyrrolidone (PVP)-assisted synthesis of Bi₂WO₆ NPs through the sonochemical method. The concentration of the PVP was tuned in the range of 0.05 to 0.2 g. The optimized results for PVP-assisted Bi₂WO₆ demonstrated an acceptable specific capacitance value of 462 F/g, which is higher than pure Bi₂WO₆ (304 F/g) at a current density of 3 mA/cm². In another study, Nithya et al. [94] also proposed the preparation of the surfactant-assisted sonochemical synthesis of Bi₂WO₆ NPs, which exhibited specific capacitance of 708 F/g under the optimized conditions.

4.2. Bi₂WO₆-Based Compoiste Materials for SCs

The supercritical water method was also adopted for the fabrication of a Bi₂WO₆/rGO hierarchical composite [95]. The supercritical water approach requires much less time to complete the reaction for the preparation of the Bi₂WO₆/rGO composite. The proposed electrode material exhibited specific capacity of 1158 C/g at 3 A/g in a 6 M KOH electrolyte. A novel electrode material, i.e., a bismuth vanadate (BiVO₄)/Bi₂WO₆ microsphere composite, was also prepared using the microwave-assisted hydrothermal method [96]. It was stated that decoration of the BiVO₄ NPs onto the Bi₂WO₆ NSs may prevent the restacking of B_i2WO₆ NSs in a disorganized manner and facilitate the formation of a mesoporous BiVO₄/Bi₂WO₆ hierarchical microsphere composite. It was also found that the prepared BiVO₄/Bi₂WO₆ hierarchical microsphere composite has higher structural stability, larger surface area and enhanced electron/ion migration. Thus, due to the above promising properties, an interesting specific capacitance of 616.8 F/g was obtained at 1 A/g and a stability of 4000 cycles with 87% retention using BiVO₄/Bi₂WO₆ hierarchical microspheres as an electrode material. The synthesis of the Bi₂WO₆/ZnO/rGO composite is described in Figure 6 [97]. The Bi₂WO₆/ZnO/rGO composite was prepared via multiple steps. It was believed that the proposed ternary material may be a promising electrode modifier for SC applications. The ratio of the rGO was optimized to enhance the electrochemical activity of the Bi₂WO₆/ZnO/rGO composite for SC applications. The authors achieved a specific capacitance of 254.34 F/g at 2 A/g under optimized conditions. The presence of decent electrical conductivity, improved electrochemical activity and synergistic interactions enhanced the performance of the Bi₂WO₆/ZnO/rGO composite-based SCs.

Qadeer et al. [98] proposed that the electrochemical properties of the pristine Bi₂WO₆ can be further improved by combining it with titanium disulfide (TiS₂). Therefore, the Bi₂WO₆/TiS₂ composite was obtained via the hydrothermal method. The morphological characteristic properties of the Bi₂WO₆/TiS₂ composite were checked using SEM technique, which revealed that TiS₂ nanoplates are uniformly dispersed across the hierarchically opened surface of Bi₂WO₆. Thus, it was observed that the resulting Bi₂WO₆/TiS₂ composite exhibited improved porosity and specific surface area, which may facilitate the electron and ion transport. The GCD studies displayed specific capacitance of 453.3 F/g, 523.7 F/g and 1153.3 F/g for TiS₂, Bi₂WO₆, and Bi₂WO₆/TiS₂ composites, respectively. Another previous study proposed the incorporation of tin oxide (SnO₂) to the Bi₂WO₆ nanoflowers (NFs) using simple strategies, as shown in Figure 7 [99].

The SnO₂ QD-interspersed Bi₂WO₆ NFs were utilized as an electrode material, with a specific capacitance of 812 and 741 F/g at 7 A/g for Bi₂WO₆/SnO₂ QDs and Bi₂WO₆-based electrodes, respectively. The improved specific capacitance for the Bi₂WO₆/SnO₂ QD-based electrode may be ascribed to the quantum dot size of SnO₂ and the presence of synergistic interactions. This material offers great potential for SC applications. Nickel oxide (NiO) is a promising semiconducting metal oxide, with reasonably good electrochemical properties. The Bi₂WO₆/NiO composite was adopted as an electrode modifier for SCs [100]. The capacitive diffusion and Nyquist plots of Bi₂WO₆, NiO and Bi₂WO₆/NiO composite-based electrodes are shown in Figure 8a and Figure 8b, respectively. It can be seen that the Bi₂WO₆/NiO composite has decent electrical conductivity. The GCD curves of the Bi₂WO₆, NiO and Bi₂WO₆/NiO composite at different current densities are depicted in Figure 8c, Figure 8d and Figure 8e, respectively. It can be observed that the prepared materials have higher specific capacity at 2 A/g. The highest specific capacitance for the Bi₂WO₆/NiO composite-based electrode was attributed to the presence of synergism in the hybrid composite material. The capacitive retention/coulombic efficiency of the Bi₂WO₆/NiO composite is shown in Figure 8f. Thus, it is also suggested that the Bi₂WO₆/NiO composite has decent cyclic stability of 2000 for SCs.

Zheng et al. [101] proposed a benign one-pot hydrothermal method for the fabrication of a 3D porous Bi₂WO₆/rGO hydrogel composite. The electrochemical studies revealed that the 3D porous Bi₂WO₆/rGO hydrogel composite has higher capacitive properties and demonstrates specific capacitance of 268.7 F/g at an applied current density of 0.75 A/g. This material also displayed robust stability of 1000 cycles at 3 A/g, with capacitance retention of 81%. This improved performance may be due to the synergistic interactions, unique 3D porous rGO hydrogel-supported Bi₂WO₆ architecture, which provided good electrical conductivity and more channels for ion diffusion. Zhang et al. [102] also adopted a nano Bi₂WO₆-decorated rGO composite via the hydrothermal method. The proposed Bi₂WO₆/rGO composite shows improved electrical conductivity and increased charge transfer channels. Thus, it is expected that the presence of these properties may facilitate the electron transfer, and Bi (III) may oxidize to Bi (IV), which provide higher specific capacitance and stability. Therefore, the authors achieved a specific capacitance of 922 F/g at 3 A/g, which is higher than pristine Bi₂WO₆ (574 F/g at 2 A/g). The Al-based MOF (Materiaux De Lavoisier = MIL)-53 was prepared via the solvothermal method [103]. It is observed from these studies that MIL-53 (Al) has low conductivity, which restricts its potential applications in energy storage systems. Therefore, MIL-53 (Al) was incorporated with Bi₂WO_6, and its properties were evaluated via the CV, EIS and GCD methods. The MIL-53 (Al)/Bi₂WO₆-based electrode displayed a larger surface area and improved conductive nature. Therefore, an enhanced specific capacitance value of 220 F/g was observed at 0.5 A/g in the 3 M KOH electrolyte with excellent cyclic stability (5000 cycles; retention capacitance = 78%). Sheoran et al. [104] reported a novel electrode modifier comprising a carbon black (CB)-anchored bismuth tungstate-aniline complex composite ((Bi₂(WO₄)₃/Aniline/CB) (BTACB)). The authors reported a specific capacitance of 306 F/g at 1 A/g, which can be attributed to the synergism between aniline complex, CB and bismuth tungstate, which provide larger surface area and acceptable conductivity. A sandwich-like heterostructure of the Bi₂WO₆/g-CN composite was developed using benign approaches [105]. This fabricated Bi₂WO₆/g-CN composite was adopted as an electrode modifier for energy storage applications, and the obtained results showed specific capacitance of 158 mF/g at 1 mA/cm². It is well understood that 3D porous structures may promote the electron transport process and improve the electrochemical capacitive properties of the SCs. In this regard, the 3D aerogel/poriferous structure of Bi₂WO₆ sheets was combined with graphene NSs via benign hydrothermal treatment [106]. It was stated that the 3D multi-hole structure of the prepared hybrid aerogel may provide high surface area and promote electron transport and ion transmission. This may decrease the internal electrical resistance and enhance the capacitive properties of the fabricated electrode. The Bi₂WO₆/graphene aerogel composite demonstrated specific capacitance of 714 F/g at 4 A/g. In another previous report [107], MIL-53 (Al) and Bi₂WO₆ were obtained via the hydrothermal route, whereas the MIL-53 (Al)/Bi₂WO₆ composite was fabricated using the impregnation method. Due to the lower equivalent series resistance and synergistic interactions, the MIL-53 (Al)/Bi₂WO₆ composite demonstrated specific capacitance of 554 F/g at 0.5 A/g. Due to the high surface area and high porosity, copper MOF (Cu-MOF) has been utilized in the fabrication of hybrid materials for SCs. In this connection, a Cu-MOF/Bi₂WO₆ binary hybrid composite was developed through the solvothermal method [108]. The weight percentage was varied to optimize the electrochemical performance of the Cu-MOF/Bi₂WO₆ binary hybrid composite for SCs. The samples with different weight percentages were labeled as CuBW20, CuBW50 and CuBW80. 3 M KOH was explored as a suitable electrolyte, and CuBW80 demonstrated excellent specific capacitance of 1137 F/g, specific power of 4000 W/kg, specific energy of 11 Wh/kg at a current density of 0.5 A/g. This optimized electrode material was also suitable for stability of 1000 cycles. In another study [109], Bi₂O₃/Bi₂WO₆ (BO/BWO) and Au-decorated Bi₂O₃/Bi₂WO₆ (Au-BO/BWO) composites were developed through the hydrothermal method. The electrochemical performance of BO/BWO and Au-BO/BWO was evaluated via CV analysis, as shown in Figure 9a. It can be seen that A-BO/BWO displayed higher electrochemical activity compared to BO/BWO. Similarly, A-BO/BWO showed improved capacitance and suggested its potential for SCs (Figure 9b). The specific capacitance versus scan rate graph is shown in Figure 9c, whereas the specific capacitance versus current density graph is depicted in Figure 9d. The BO/BWO-based electrode demonstrated specific capacitance of 148.81 F/g, whereas the highest specific capacity of 495.05 F/g for the A-BO/BWO-based electrode was obtained at 1 A/g in a 1 M KOH electrolyte system. The improved specific capacity of the A-BO/BWO-based electrode may be attributed to the presence of Au NPs, which improved electrical conductivity and decent electrical contacts between the electrolyte and electrode, which may have improved the effective surface area. The multi-functional polyaniline (PANI)-wrapped Bi₂WO₆ 2D nanoplate/BiOCl (BW-BIC-PANI) composite was also prepared by Kavinkumar et al. [110].

The fabrication of the BW-BIC-PANI composite via the hydrothermal method followed by the chemical oxidative polymerization method is shown in Figure 9e. The authors proposed that a specific capacitance of 156 F/g can be obtained at 1 A/g by employing a BW-BIC-PANI₂₀₀-based electrode. The presence of synergistic interactions enhances the electrochemical capacitive properties of the BW-BIC-PANI₂₀₀ for SCs. The hydrothermal method was also utilized for the preparation of Ag-doped Bi₂WO₆ (ABW) [111]. Furthermore, ABW was incorporated with MXene to fabricate the ABW@MXene composite via the sonication method. The specific capacitances of 870 F/g and 1420 F/g for the ABW@MXene-based electrode were observed using CV and GCD, respectively. In contrast, MXene exhibited a specific capacitance value of 200 F/g and 413 F/g using CV and GCD, respectively. The improved performance for ABW@MXene was attributed to the presence of synergistic interactions between ABW and MXene. Liu et al. [112] prepared Bi₂S₃ NFs, which comprised 1D NRs. The Bi₂S₃ NFs were obtained through a topotactic transformation approach. The Bi₂WO₆ NFs were used as a precursor template for the preparation of Bi₂S₃ NFs. The Bi₂WO_6-derived Bi₂S₃ NFs exhibited decent specific capacitance and electrochemical stability for SCs. The electrochemical capacitive performance of the Bi₂WO_{6-based mat}erials is summarized in

Table 2. Electrochemical performance of previously reported SCs based on Bi₂WO₆ and its hybrid composites as electrode material.

Electrode Material	Specific Capacitance	Electrolyte	Current Density	Cyclic Stability	References
3D-Bi2WO6/NF	1.12 F/cm2	2 M KOH	1.5 mA/cm2	10,000	88
BWS/RGO-12	1058 C/g	6 M KOH	3A/g	10,000	95
3D BiVO4/Bi2WO6	616.8 F/g	1 M KOH	1 A/g	4000	96
Bi2WO6(BWO)/ZnO/rGO	254.34 F/g	2 M KOH	2 A/g	-	97
BW/T@40 %	1153.3 F/g	2 M KOH	1 A/g	-	98
Bi2WO6-SnO2 quantum dots	812 F/g	3 M KOH	7 A/g	3000	99
Bi2WO6-NiO	398.2 C/g	3 M KOH	10 mV/s	2000	100
Bi2WO6 (BWO-2)	647 F/g	1 M KOH	5 mA/cm2	5000	89
Flower-like hierarchical Bi2WO6	255 F/g	1 M KOH	0.5 mA/cm2	850	90
Flake-like Bi2WO6	214 F/g	1 M KOH	0.5 mA/cm2	850	90
Spherical structure Bi2WO6	412 F/g	1 M KOH	0.5 mA/cm2	850	90
3D Bi2WO6/rGO	268.7 F/g	6 M KOH	0.75 A/g	1000	101
Bi2WO6/graphene	922 F/g	6 M KOH	3 A/g	2000	102
MIL-53(Al)/Bi2WO6	220 F/g	3 M KOH	0.5 A/g	5000	103
Bi2(WO4)3/aniline/CB	306 F/g	1 M KOH	1 A/g	-	104
Bi2WO6-g-C3N4	158 mF/g	6 M KOH	1 mA/cm2	1000	105
Bi2WO6/graphene	714 F/g	6 M KOH	4 A/g	-	106
MIL-53(Al)/Bi2WO6	554 F/g	6 M KOH	0.5 A/g	-	107
BWO-600	614 C/g	6 M KOH	2 A/g	10,000	91
CuBW80	1137 F/g	3 M KOH	0.5 A/g	10,000	108
Bi2O3/Bi2WO6	495.05 F/g	1 M KOH	1 A/g	2000	109
BW-BIC-PANI200	156 A/g	3 M KOH	1 A/g	4000	110
ABW@MXene	1420 F/g	1 M KOH	1 A/g	-	111
PVP-assisted Bi2WO6	462 F/g	1 M KOH	3 mA/cm2	500	93
HMT assisted Bi2WO6 NPs	708 F/g	1 M KOH	3 mA/cm2	5000	94

.

5. Conclusions, Limitations and Perspectives

It is concluded that Bi₂WO₆ has been widely used for the fabrication of working electrodes for electrochemical sensing and SC applications. Bi₂WO₆ has low conductive properties, which can be enhanced by preparing Bi₂WO₆-based hybrid composites with conductive materials, such as carbon-based materials or MXenes. The Bi₂WO₆-based composites exhibited excellent sensing performance for the detection of various analytes, including DES, H₂O₂, H₂S, Sudan I, ampicillin, 4-NP, pemetrexed, etc. It was suggested in previous reports that DPV is the most widely used sensing technique for the determination of environmental pollutants. Bi₂WO₆ may exhibit a smaller hydration sphere radius, higher ionic mobility, and lower equivalent series resistance, which benefit researchers by producing a higher specific capacity for SCs. The presence of Bi³⁺ with 6s² lone pair electrons may improve the abundant active sites and electronic conductivity. Thus, Bi₂WO₆-based composites also exhibited excellent specific capacitance for SC applications. Despite several advantages, some challenges exist, which are mentioned below.

• Pristine Bi₂WO₆ suffers from low conductivity, which needs to be addressed.
• Although several efforts were made to enhance the conductivity of Bi₂WO₆ by combining it with conductive supports such as MXene or rGO, synthetic procedures are not cost-effective for practical applications.
• DPV is a more sensitive technique, but, in some cases, the presence of similar interfering substances affects the selectivity of electrochemical sensors.
• Real sample studies are conducted using the standard addition method, which exhibits accepted recovery. However, we believe that such methods are not reliable for real sample analysis. Thus, new methods should be developed.
• The depth mechanism for sensing applications should be studied.
• For SCs, stability is the major concern, which needs to be properly studied in depth.

We believe that future studies may involve the following key points for further research.

• Green and cost-effective methods need to be developed for scalability and practical applications.
• Electrochemical sensors can be combined with machine learning technology for the accurate analysis of environmental pollutants.
• Electrochemical sensors can be explored in smartphone-based devices for environmental monitoring.
• Flexible and wearable electrochemical sensors can be developed for environmental monitoring.
• MXene is a new material, and its potential is not fully explored. Thus, Bi₂WO₆ and MXene-based materials should be studied in depth, and a cost-effective and acid etching-free method should be developed for the transformation of MAX to MXene.