INTRODUCTION
Background, properties, and applications of lead dioxide coatings
Lead dioxide (PbO2) has received continuous attention because of its outstanding electrochemical stability, high electrical conductivity, and strong oxygen-evolution activity. These properties make it an essential component in lead–acid batteries, where a stable PbO2 layer improves corrosion resistance and extends the service life of the electrode [1,2,98]. Beyond batteries, PbO2-based anodes are applied in electrocatalytic oxidation processes such as wastewater treatment and CO2 electroreduction, which are crucial for sustainable energy and environmental technologies [3–5,99]. The performance of PbO2 coatings depends strongly on synthesis parameters such as current density, bath composition, temperature, and the presence of organic or inorganic additives [6,100]. Therefore, optimizing electrodeposition conditions and incorporating functional additives remain central to enhancing PbO2 performance.
Fig. 1 illustrates the anodic electrodeposition of lead dioxide, showing the sequential formation of intermediate species (Pb(OH)⁺, Pb(OH)22+) and their conversion to the final PbO2 phase on a Sn–Sb-coated Ti substrate. Variations in current density or electrolyte chemistry can significantly influence the nucleation and growth rate of PbO2 crystals, thereby affecting the coating morphology, compactness, and electrochemical behavior [4,7].
In the past decade, surfactants have become vital modifiers for tailoring PbO2 film properties. They regulate surface tension, control the double-layer structure, and influence the diffusion of ionic species during growth. Anionic surfactants such as sodium dodecyl sulfate (SDS) and SDBS, cationic surfactants like CTAB, and nonionic species including PEG or Triton X-100 have been widely employed to refine crystal size, increase coating density, and reduce defect formation [8–10]. Recent works demonstrate that surfactant molecular structure—particularly head-group charge and chain length—governs nucleation kinetics and phase composition of β-PbO2 layers. By modifying adsorption behavior at the electrode–electrolyte interface, surfactants enhance charge-transfer efficiency and electrocatalytic activity toward oxygen-evolution reactions (OER).
Growing environmental concerns have encouraged the development of green surfactants derived from biodegradable or bio-based precursors such as rhamnolipids, saponins, and amino-acid surfactants. These eco-friendly agents can reduce surface energy without releasing toxic residues, offering sustainable alternatives for PbO2 electrode fabrication [11,12]. In parallel, doped PbO2 anodes—modified with metals (Ni, Co, Ce) or conductive polymers (PANI, PPY)—have demonstrated superior conductivity, mechanical integrity, and catalytic stability. When combined with appropriate surfactants, such composite electrodes display synergistic effects: improved adhesion, enhanced grain refinement, and extended service life under high-potential operation [13–17]. However, to tailor these strategies effectively, it is essential to understand the fundamental electrochemical reactions and their thermodynamic–kinetic behavior that govern PbO2 growth. In other words, the chemical or “green” surfactant that modifies the bath composition ultimately exerts its influence by altering the rates and energetics of the underlying anodic reactions.
Based on prior studies, the anodic formation of lead dioxide (PbO2) in nitrate-based electrolytes involves a sequence of oxidation steps and the generation of hydroxyl radicals. These reactions proceed through both thermodynamically driven and kinetically controlled pathways that determine the nucleation rate, film density, and crystalline phase of the resulting PbO2 coating. The overall electrodeposition process is governed by parameters such as current density, temperature, Pb2+ concentration, pH, and deposition duration [18,19]. The principal anodic reactions are as follows:
1. Water oxidation (formation of surface hydroxyl radicals):
This reaction initiates the process by generating adsorbed hydroxyl species (OHads) on the anode surface. Thermodynamically, it occurs once the applied potential exceeds the standard potential for water oxidation (~1.23 V vs SHE). Kinetically, the surface energy and catalytic activity of the Ti/Sn–Sb substrate influence the reaction rate.
2. Formation of lead hydroxide intermediate:
The electrostatic interaction between Pb2+ and OHads promotes localized nucleation. The diffusion coefficient of Pb2+ and the structure of the electrical double layer — often modified by surfactants — determine how uniformly this step occurs across the electrode.
3. Electrochemical conversion to dihydroxylated complex:
This oxidation step is rate-determining and controlled by electron-transfer kinetics. The charge-transfer coefficient (α) and overpotential strongly influence the growth rate of PbO2 nuclei.
4. Formation of crystalline lead dioxide:
This final reaction is thermodynamically favored and marks the transformation of amorphous intermediates into the stable β-PbO2 phase. The release of protons lowers local pH, influencing the crystal habit and surface roughness. The overall deposition follows a progressive nucleation and three-dimensional growth mechanism, where PbO2 initially forms a uniform monolayer and subsequently thickens through layer-by-layer crystallization [7,20]. The interplay between thermodynamic driving forces (electrode potential, Gibbs free energy of formation) and kinetic constraints (mass transfer, adsorption, electron transfer) ultimately dictates coating morphology, compactness, and phase composition. Understanding these reaction dynamics provides the scientific foundation for designing next-generation surfactant-assisted and doped PbO2 anodes. By adjusting surfactant concentration, molecular structure, or dopant type, researchers can tune interfacial kinetics, lower nucleation barriers, and achieve compact, stable, and eco-friendly PbO2 coatings suitable for advanced energy and environmental systems.
Therefore, a comprehensive understanding of how surfactant chemistry interacts with the thermodynamic and kinetic stages of PbO2 electrodeposition is essential for designing coatings with tailored properties. This review systematically summarizes recent progress in the application of surfactants during PbO2 synthesis, emphasizing how their molecular characteristics—such as charge type, chain length, and critical micelle concentration—govern nucleation, morphology, conductivity, and electrocatalytic performance. By analyzing more than thirty recent studies, this work integrates scattered findings into a unified framework that links interfacial chemistry with macroscopic electrode behavior. Furthermore, particular attention is given to environmentally benign or “green” surfactants and to synergistic effects achieved when surfactants are combined with dopants such as Ag, F, or conductive polymers like PANI and graphene derivatives. The review also highlights existing research gaps, including the limited mechanistic modeling of surfactant–ion interactions and the need for in-situ characterization of growth kinetics. These insights aim to guide future efforts toward the rational design of sustainable, high-performance PbO2-based anodes for energy conversion and wastewater treatment applications.
LEAD DIOXIDE ELECTROCHEMICAL DEPOSITION PROCESS
Lead Dioxide Bath Composition
The composition of the electrolyte critically determines the morphology and quality of electrodeposited PbO2 coatings. A standard electrolyte typically contains lead nitrate (Pb(NO3)2) dissolved in nitric acid (HNO3). The concentration of Pb(NO3)2 usually ranges from 0.1 to 0.5 M, while the HNO3 concentration is maintained between 0.5 and 2.0 M, with 1.0 M being common [21–23].
Higher concentrations of lead nitrate generally increase the deposition rate and improve crystal orientation. Wang et al. reported that increasing Pb(NO3)2 from 0.1 to 0.5 mol·L–1 enhanced film conductivity due to improved crystallinity and fewer crystal defects [24].
The pH of the electrolyte is another key parameter influencing phase formation. An acidic medium stabilizes Pb2+ ions and favors the formation of β-PbO2 over intermediate species such as PbO or Pb2O3 [25]. Wang et al. found that maintaining the pH between 1.0 and 2.0 yields dense and adherent coatings, while higher pH promotes undesirable lead hydroxide phases [26]. Thus, optimizing both the electrolyte composition and surfactant addition is essential for achieving coatings with superior electrocatalytic and mechanical properties.
Fig. 2 shows representative SEM micrographs of PbO2 coatings deposited under different electrolyte conditions. Variations in Pb2+ concentration, acid content, and pH lead to distinct surface morphologies, reflecting changes in nucleation density and crystal growth kinetics. In particular, high acid concentration produces smoother and denser surfaces, while low-acid baths result in coarse, porous morphologies due to limited ion mobility and increased overpotential. These structural differences directly influence coating conductivity, oxygen evolution activity, and long-term stability.
Apart from electrolyte composition, the deposition parameters—particularly current density and temperature— play decisive roles in defining the structure and performance of PbO2 coatings.
Current density directly influences the nucleation rate and grain growth of PbO2. In typical electrodeposition, the current density ranges from 5 to 50 mA·cm–2 [27]. Increasing current density accelerates the formation of Pb4+ species, leading to faster crystal growth and denser coatings. Liu et al. [28] reported that raising the current density from 10 to 30 mA·cm–2 produced thicker and more compact films with improved electrocatalytic activity. However, excessively high current densities may induce internal stress and promote oxygen evolution, resulting in rough or porous deposits.
Temperature also affects the kinetics of PbO2 formation by altering ion mobility and reaction rate. Moderate heating generally enhances deposition efficiency and coating adhesion. Nevertheless, if the temperature exceeds the optimal range, the morphology deteriorates due to uncontrolled crystal coarsening and enhanced side reactions. Wang et al. [29] found that the most favorable deposition temperature lies between 45°C and 65°C, where the coatings exhibit high conductivity and strong adhesion.
Fig. 3 summarizes the combined effects of current density and temperature on current efficiency during PbO2 electrodeposition. At 10 mA·cm–2, current efficiency decreases sharply as temperature increases, indicating intensified oxygen evolution at low current densities. Conversely, at 50 mA·cm–2, efficiency remains relatively stable with temperature, suggesting that higher current densities mitigate parasitic reactions. These trends confirm that an optimized balance between temperature and current density is essential to achieve compact, highly efficient PbO2 coatings suitable for electrocatalytic and industrial applications.
The duration of electrodeposition strongly affects the final thickness, uniformity, and adhesion of PbO2 coatings. Longer deposition times generally yield thicker layers; however, excessive growth may introduce internal stress, microcracks, and poor mechanical integrity [22,30]. Therefore, an optimized deposition duration is essential to balance coating thickness with structural stability.
Overall, the interplay between electrolyte composition, current density, temperature, and deposition time governs the nucleation kinetics and crystallinity of PbO2 films. A thorough understanding of these parameters enables the fabrication of dense, adherent, and conductive coatings suitable for energy storage, electrocatalysis, and wastewater treatment applications. Nevertheless, beyond these electrochemical parameters, the role of additives—especially surfactants—remains a critical but often underestimated factor. Surfactants can dramatically alter surface energy, promote uniform nucleation, and refine crystal morphology, thereby improving the mechanical and electrochemical performance of PbO2 coatings. The following section discusses in detail how different classes of surfactants influence film growth mechanisms and functional properties.
OVERVIEW OF VARIOUS SURFACTANTS
Main roles of surfactants
Surfactants play a crucial role in the electrochemical deposition of PbO2 coatings by modifying interfacial tension and influencing the nucleation and crystal-growth mechanisms. By adsorbing onto the electrode surface or complexing with Pb2+ ions, surfactants alter the local electric field, diffusion rate, and energy barrier for nucleation. These effects lead to finer grain structures, smoother surfaces, and denser coatings with enhanced stability and electrical conductivity.
Different classes of surfactants—cationic, anionic, and nonionic—interact with the growing PbO2 layer in distinct ways depending on their molecular structure. The head-group charge determines the electrostatic interaction with the anode surface and ionic species in the electrolyte. For example, cationic surfactants such as cetyltrimethylammonium bromide (CTAB) can adsorb onto negatively charged active sites, suppressing uncontrolled growth and promoting compact β-PbO2 formation. Conversely, anionic surfactants like sodium dodecyl benzene sulfonate (SDBS) preferentially interact with Pb2+ ions, regulating nucleation density and enhancing coating adhesion [31,32].
The hydrophobic chain length of a surfactant governs the degree of surface coverage and micelle stability. Longer alkyl chains typically increase adsorption strength and improve the compactness of the film, but excessively long chains may hinder ion transfer and reduce deposition rate. Furthermore, the critical micelle concentration (CMC) defines the balance between free surfactant molecules and micelles in solution [33,34]. Operating near the CMC ensures optimal surface activity—providing sufficient adsorption to stabilize crystal growth while avoiding excessive aggregation that can cause morphological irregularities.
Fluorinated surfactants and hybrid organic–inorganic additives have recently been shown to enhance both corrosion resistance and mechanical toughness of PbO2 coatings while preserving high conductivity [35]. Thus, surfactant selection must consider molecular structure, concentration, and bath chemistry to achieve the desired combination of electrochemical and mechanical properties.
Fig. 4 conceptually summarizes the multifaceted roles of surfactants in PbO2 electrodeposition. Each function— ranging from crystal-growth regulation to corrosion resistance and conductivity retention—stems from molecular-level interactions discussed above. The diagram highlights how precise control of surfactant type and concentration allows researchers to tailor coating morphology, minimize defects, and extend service life. This integrative view reinforces that optimizing surfactant chemistry is not merely an additive adjustment but a central design strategy for advanced PbO2-based electrodes.
A representative example of the influence of surfactants on PbO2 morphology was reported by Duan et al. [36]. In their study, the addition of sodium dodecyl benzene sulfonate (SDBS), an anionic surfactant, into the Pb(NO3)2 bath significantly refined the grain structure of electrodeposited PbO2. Scanning electron microscopy (SEM) revealed a pronounced reduction in crystal size and a transition from coarse polygonal grains to compact, uniform microstructures. This morphological refinement was attributed to the strong adsorption of SDBS molecules onto active growth sites on the anode surface, which effectively inhibited the aggregation of PbO2 nuclei. The sulfonate head groups of SDBS interact electrostatically with Pb2+ ions, while the hydrophobic tails form an ordered interfacial layer that limits excessive grain coalescence. As a result, the nucleation rate increases while lateral grain growth is suppressed, producing denser coatings with improved adhesion and electrocatalytic activity.
Fig. 5 presents the SEM micrographs from Duan’s research, illustrating the progressive reduction in grain size with increasing SDBS concentration. These images clearly demonstrate how controlled surfactant chemistry can tailor PbO2 morphology, which is essential for enhancing anode performance in electrochemical degradation and wastewater treatment systems.
Surfactants used in PbO2 electrodeposition are generally classified into four categories—anionic, cationic, nonionic, and zwitterionic—each exhibiting distinct electrochemical behaviors that influence nucleation, crystal growth, and surface morphology. Anionic surfactants, such as sodium dodecyl sulfate (SDS) and sodium lauryl sulfate (SLS), contain negatively charged sulfate groups that interact with Pb2+ ions and the positively polarized anode surface. This electrostatic interaction improves ion dispersion and stabilizes the electrolyte, thereby promoting uniform nucleation and compact PbO2 crystal growth [37,38]. The presence of SDS in the bath reduces surface tension and inhibits the formation of large grains, leading to dense, fine-grained coatings with enhanced adhesion.
Cationic surfactants, exemplified by cetyltrimethylammonium bromide (STAB), carry positively charged head groups that adsorb strongly onto negatively polarized sites of the electrode. This adsorption alters the local surface potential and modifies double-layer structure, facilitating smoother and more cohesive film formation [39]. The long hydrophobic tail of STAB also assists in orienting PbO2 crystallites, resulting in better control over grain orientation and film uniformity.
Nonionic surfactants, including polyethylene glycol (PEG) and Triton X-100, contain polar but uncharged functional groups. They primarily affect the physical properties of the deposition bath by increasing viscosity and stabilizing colloidal suspensions without introducing additional charge. PEG molecules adsorb onto growing PbO2 surfaces via hydrogen bonding, which moderates the growth rate and suppresses roughness, yielding coatings with enhanced smoothness and structural integrity [40,41].
Zwitterionic surfactants, possessing both positive and negative functional groups, exhibit amphoteric behavior. Their dual charge enables dynamic adsorption depending on local pH and potential, leading to more balanced charge distribution during deposition. As a result, they enhance coating homogeneity and reduce internal stress, improving both mechanical and electrochemical properties [42].
In summary, each surfactant class affects PbO2 electrodeposition through a unique combination of electrostatic and molecular interactions. Selecting the appropriate surfactant—considering head-group charge, hydrophobic chain length, and concentration—allows precise control over coating texture, density, and conductivity. Table 1 provides representative examples of these surfactants along with their key structural characteristics and applications.
Functioning principles of surfactants
Surfactants fundamentally control the nucleation, growth, and morphology of PbO2 crystals during electrochemical deposition [60]. By modifying the interfacial tension between the electrode and the electrolyte, these additives determine the distribution of active nucleation sites and the subsequent rate of crystal growth. Anionic surfactants such as SDS effectively reduce surface tension, thereby increasing nucleation density and producing fine-grained, compact coatings with enhanced uniformity and stability [61].
Beyond interfacial effects, surfactants can also influence the electrochemical environment by stabilizing reactive intermediates and facilitating ion transport within the deposition bath. For example, fluorinated surfactants have been shown to accelerate Pb2+ oxidation kinetics while simultaneously improving corrosion resistance and mechanical strength [62]. Similarly, cationic surfactants exhibit concentration-dependent behavior: at lower concentrations, they favor hemispherical crystal structures, whereas higher concentrations promote the formation of elongated or cylindrical grains [63].
A representative study by Luk’Yanenko et al. (2020) [61] investigated the effect of sodium laureth sulfate (SLES), an anionic surfactant, on the phase structure and morphology of PbO2 coatings. X-ray diffraction (XRD) analysis (Fig. 6) revealed that SLES significantly alters the crystal structure of electrodeposited PbO2. The characteristic peaks of β-PbO2 (110) and α-PbO2 (111), which are distinct in undoped coatings, almost disappeared with increasing SLES content. Concurrently, a new peak corresponding to the β-PbO2 (022) plane emerged, indicating a phase transformation and formation of PbO2–SLES composites. Increasing the SLES concentration led to reduced intensity of all major peaks, suggesting smaller crystal domains and a partial transition toward amorphous structure.
These structural changes are consistent with the surface morphology observed by scanning electron microscopy (SEM). As shown in Fig. 7, the pure PbO2 sample exhibits coarse, angular grains, whereas the incorporation of 3.2 wt% SLES produces a denser microstructure with sub-micron particles. Further increasing the surfactant content to 10.2 wt% SLES results in a uniform, nano-sized grain morphology. The correlation between the XRD and SEM results clearly demonstrates that surfactant molecules strongly affect nucleation kinetics and crystal growth pathways. Such control over structure and texture is essential for enhancing the electrocatalytic activity and durability of PbO2-based anodes used in energy storage and wastewater treatment systems.
EFFECT OF THE SURFACTANTS ON THE CHARACTERISTICS OF LEAD DIOXIDE ANODES
Electrical Conductivity
The electrical conductivity of PbO2 coatings is strongly affected by the type and concentration of surfactants present in the electrolyte during electrodeposition. Surfactants influence not only the crystal morphology but also the density of grain boundaries and the quality of the interfacial contact between the coating and the substrate.
Among different surfactant types, anionic surfactants, particularly sodium dodecyl sulfate (SDS), have been widely reported to enhance the electrical conductivity of PbO2 and related metal-oxide electrodes. Zhang et al. (2014) [64] demonstrated that the incorporation of SDS into the deposition bath significantly improved the coating’s conductivity by refining the crystal structure and reducing structural defects. Electrochemical impedance spectroscopy (EIS) results, as shown in Fig. 8, revealed that electrodes prepared with SDS exhibit smaller semicircles in the high-frequency region of the Nyquist plot, indicating a clear reduction in charge-transfer resistance. This improvement can be attributed to increased crystallinity, enhanced intergrain connectivity, and a denser coating structure.
However, as Zhang and co-workers pointed out, this beneficial effect is concentration-dependent. The optimum SDS concentration (~0.2 M) yielded the lowest charge-transfer resistance and highest capacitance, while excessive SDS addition led to molecular aggregation and increased porosity, ultimately reducing overall conductivity and coating stability. These findings highlight the necessity of maintaining the surfactant concentration below its critical micelle concentration (CMC) to ensure efficient charge transport and uniform crystal growth.
In parallel, recent researches [65,66] reported that cationic surfactants such as cetyltrimethylammonium bromide (CTAB) also improve electrical performance, though through a different mechanism. CTAB modifies the surface charge of the substrate, thereby facilitating more efficient ion transfer and accelerating electrochemical reactions during deposition. The corresponding data, illustrated in Fig. 9, demonstrate that increasing CTAB concentration enhances the specific conductivity of the electrolyte. Moreover, conductivity further increases with temperature due to improved ion mobility and reduced electrostatic interactions between the surfactant molecules and the substrate. This dual influence of surfactant concentration and temperature underscores the kinetic control that cationic additives exert on charge transport within the electrolyte. In contrast, nonionic surfactants such as polyethylene glycol (PEG) provide structural stability by preventing particle agglomeration, but their lack of ionic charge results in slightly lower conductivity compared to ionic surfactants. Therefore, anionic surfactants like SDS are generally more effective for enhancing electrical conductivity, while cationic surfactants such as CTAB excel in improving charge mobility and deposition uniformity. Selecting an appropriate surfactant type and concentration thus plays a key role in optimizing the electrical and electrocatalytic performance of PbO2 coatings.
In recent studies, surfactant-assisted synthesis has proven to be a critical strategy for controlling the electrical conductivity and electrochemical behavior of metal chalcogenides. Varun Sarathi et al. (2024) [97] systematically examined the effect of three surfactants—cetyltrimethylammonium bromide (CTAB), ethylenediaminetetraacetic acid (EDTA), and urea—on the structural and electrochemical properties of FeSe2 electrodes designed for supercapacitor applications. Their findings revealed a clear structure–property relationship linking the molecular nature of the surfactant to the charge transport efficiency of the resulting material.
Effect of CTAB (Cationic Surfactant)
The CTAB-modified FeSe2 exhibited the most favorable electrochemical performance among the investigated samples. The positively charged quaternary ammonium head group of CTAB interacts electrostatically with negatively charged selenium sites during nucleation, effectively directing the growth of FeSe2 nanostructures into uniform, amorphous nanospherical aggregates. This self-assembly mechanism results in a higher density of interconnected conductive pathways and shorter electron transport distances. As Varun Sarathi and colleagues (2024) [97] reported, the CTAB-assisted FeSe2 showed a dramatically reduced chargetransfer resistance (≈1.5 Ω) compared to the pristine sample (≈3 Ω). The corresponding Nyquist plot (Fig. X) clearly shows the smallest semicircle diameter for the CTAB-based electrode, reflecting faster ion diffusion and lower interfacial impedance. Consequently, the CTAB sample achieved the highest specific capacitance of ~401 F g–1 at 10 mV s–1, confirming its superior conductivity and charge-storage ability.
Effect of EDTA (Chelating Agent)
The introduction of EDTA into the synthesis medium altered the crystallization behavior of FeSe2 through its strong chelating interaction with Fe2+ ions. This coordination limited the rate of Fe–Se bond formation and promoted the growth of larger, cube-like crystallites (~32 nm). The resulting structure exhibited enhanced crystallinity but lower electron mobility due to fewer grain boundaries acting as conductive bridges. As a result, the EDTA-assisted sample presented moderate conductivity and a specific capacitance of ~155 F g–1, representing a balance between ordered structure and charge transport efficiency. The higher crystallinity improved mechanical stability, yet the reduced density of defect-related conduction channels led to higher charge-transfer resistance than the CTAB-modified electrode.
Effect of Urea (Nonionic Surfactant)
Unlike CTAB and EDTA, urea, a neutral molecule, influenced FeSe2 formation primarily through hydrogen bonding and surface energy modulation rather than ionic interaction. However, this weak adsorption at the FeSe2/electrolyte interface caused particle agglomeration and incomplete charge percolation throughout the material. The urea-assisted sample exhibited the highest resistance (≈6 Ω) and the lowest specific capacitance (~58 F g–1). The large semicircular region in the corresponding Nyquist plot confirmed substantial impedance, which was attributed to poor ion diffusion and limited electronic connectivity. These results suggest that neutral surfactants lacking electrostatic charge cannot effectively facilitate charge transfer or promote compact, conductive microstructures.
From a molecular perspective, the distinct performance trends observed among CTAB, EDTA, and urea can be attributed to their head-group chemistry and interaction modes with FeSe2 precursors.
· CTAB, with its positively charged quaternary ammonium head and long hydrophobic alkyl chain, provides both strong electrostatic adsorption and steric control over nucleation. This dual functionality fosters a well-connected conductive network, improving both electronic and ionic transport.
· EDTA, being a multidentate anionic ligand, forms stable Fe–EDTA complexes that slow nucleation and lead to large-grain structures with limited conduction pathways—beneficial for crystallinity but detrimental to charge mobility.
· Urea, with neutral polar groups, lacks the ability to generate interfacial charge separation or guide oriented growth, resulting in randomly aggregated particles and reduced conductivity.
This comparison underscores a direct correlation between molecular polarity, charge distribution, and electrochemical performance. Cationic surfactants such as CTAB, due to their strong electrostatic coupling and chain-mediated self-organization, outperform neutral and chelating agents in producing highly conductive FeSe2 electrodes. These findings reinforce the general principle that surface-active molecules with ionic head groups and controlled chain length can optimize charge transfer, minimize resistance, and enhance capacitive behavior in both chalcogenide and oxide-based electrode systems.
The comparative electrochemical results summarized above are further validated by the impedance spectra presented in Fig. 10. The Nyquist plots clearly illustrate how the choice of surfactant governs the interfacial charge-transfer processes within the FeSe2 electrodes. The smallest semicircle observed for the CTAB-assisted electrode reflects the lowest charge-transfer resistance, consistent with its highly interconnected nanostructure and enhanced electronic coupling between Fe and Se sites. In contrast, the larger semicircles corresponding to EDTA- and urea-assisted samples indicate sluggish ion diffusion and limited electron mobility due to their less favorable surface coordination environments. The linear Warburg region at low frequencies in the CTAB curve further confirms improved ionic diffusion, supporting the hypothesis that cationic surfactants promote more efficient electron–ion transport. Together, these impedance characteristics quantitatively substantiate the structure–property relationships discussed above and demonstrate how molecular-level surfactant chemistry directly translates into macroscopic electrical performance.
Electrocatalytic Activity
The incorporation of surfactants during the electrochemical deposition of PbO2 significantly influences its electrocatalytic activity, particularly in the oxygen evolution reaction (OER). By modifying surface energy and crystal growth mechanisms, surfactants generate more porous morphologies and expose a larger number of catalytically active sites. This enhancement of electrochemically active surface area (ECSA) directly reduces oxygen evolution overpotential and improves current density during anodic operation.
In the study by Ming Zhou et al. (2016) [67], the addition of anionic surfactant sodium dodecyl sulfate (SDS) into the deposition bath substantially improved the oxygen evolution behavior of PbO2 electrodes. The linear sweep voltammetry (LSV) curves in Fig. 11(a) show a clear leftward shift in the OER onset potential with increasing SDS content, indicating lower overpotential and faster oxygen evolution kinetics. Correspondingly, Fig. 11(b) illustrates that the minimum overpotential occurs near the optimal SDS concentration of 0.2 M, beyond which molecular aggregation begins to hinder charge transport. This behavior demonstrates that an appropriate surfactant concentration enhances the accessibility of active sites while maintaining compactness in the coating microstructure.
Further confirmation of this trend is provided by the long-term stability test presented by Ming Zhou and colleagues [67], shown in Fig. 12. The potential–time profiles reveal that electrodes containing the optimized SDS concentration maintain a stable potential over prolonged operation, whereas surfactant-free samples experience noticeable potential drift due to progressive surface passivation. The observed improvement in electrochemical durability is attributed to the surfactant’s ability to suppress crack formation and stabilize Pb–O intermediates during repeated oxygen evolution cycles.
Complementary observations were reported by Luk’Yanenko et al. (2020) [65], who employed sodium laureth sulfate (SLES) as an anionic surfactant during PbO2 electrodeposition. As depicted in Fig. 13, the steady-state polarization curves show that increasing the SLES content from 3.2 wt.% to 10.2 wt.% results in a progressive decrease in OER overpotential and a concurrent rise in current density. The enhancement arises from the transformation of coarse, crystalline deposits into fine-grained, nanoparticulate PbO2–SLES composites. This morphology increases the density of reactive oxygen species and facilitates rapid charge transfer across the electrode–electrolyte interface.
Moreover, Shmychkova et al. (2021) [68] demonstrated that fluorinated surfactants further enhance the electrocatalytic characteristics of PbO2 electrodes. The presence of –CF2 and –CF3 groups promotes the formation of hydrophobic, corrosion-resistant surfaces while simultaneously increasing the local concentration of reactive oxygen intermediates. This dual effect strengthens ion transport and stabilizes oxygen-containing species, yielding superior OER efficiency and electrode longevity.
Collectively, these findings reveal a consistent mechanistic principle: the molecular architecture of surfactants—particularly head-group charge, hydrophobic tail length, and concentration—dictates both surface morphology and active-site distribution. Surfactants with polar or fluorinated groups not only modulate PbO2 crystal nucleation but also sustain the redox-active oxygen species crucial for long-term catalytic performance.
Also, Zhou et al. (2024) [39] reported a remarkable improvement in the electrocatalytic and structural performance of PbO2 electrodes through the incorporation of the cationic surfactant stearyl trimethyl ammonium bromide (STAB). Their findings revealed that the long hydrophobic alkyl chain of STAB promoted the growth of compact, fine-grained β-PbO2 films with fewer surface defects. This modification enhanced charge transport within the oxide layer and reduced oxygen evolution overpotential by approximately 70 mV. Likewise, Zou et al. (2022) investigated the effect of cetyltrimethylammonium bromide (CTAB) and found that its cationic head group directed preferential crystal orientation along the (301) and (211) planes. This alignment improved electron mobility and provided strong interfacial bonding between the oxide and substrate, leading to superior electrochemical durability. Both studies concluded that the electrostatic interaction between positively charged surfactant heads and negatively charged oxygen sites on the electrode surface governs the compactness and conductivity of the PbO2 layer, which directly enhances its catalytic efficiency toward oxygen evolution reactions (OER).
In a complementary manner, Saoudi et al. (2020) [69] and Luk’Yanenko et al. (2020) [65] compared anionic and fluorinated surfactants, highlighting that the negatively charged groups in agents such as sodium dodecyl sulfate (SDS) or sodium tripolyphosphate (STPP) favor uniform crystal nucleation and smoother surfaces, while fluorinated surfactants improve chemical stability and suppress parasitic oxygen evolution. These structural refinements reduce active-site passivation and extend electrode lifespan under long-term electrolysis. More recently, Neeraj et al. (2024) [70] confirmed that surfactant-assisted PbO2 coatings exhibit enhanced degradation kinetics for organic pollutants, attributed to their higher electroactive surface area and accelerated hydroxyl radical generation. Collectively, these findings reveal that surfactant molecular design—particularly head-group charge and chain polarity—plays a decisive role in balancing conductivity, surface reactivity, and catalytic lifetime. Cationic surfactants ensure superior charge transfer and film cohesion, while anionic and fluorinated types strengthen chemical stability and resistance to oxygen evolution side reactions. Together, these insights underline that rational surfactant selection provides an effective strategy for tuning the electrocatalytic performance of PbO2-based anodes, marking an essential pathway for future development of high-efficiency electrochemical materials.
Density and compactness of the coating
The density and compactness of lead dioxide coatings are strongly influenced by the type and concentration of surfactant additives used during the electrodeposition process. Surfactants modify the nucleation behavior and control the subsequent growth of PbO2 crystals, resulting in coatings with distinct morphologies and densities. Anionic surfactants such as sodium dodecyl sulfate (SDS) promote uniform nucleation through electrostatic repulsion between negatively charged micellar heads and lead ions, thereby forming compact, fine-grained β-PbO2 structures [71]. This effect leads to a reduction in void formation and enhances coating integrity. In contrast, cationic surfactants such as cetyltrimethylammonium bromide (CTAB) tend to produce larger, columnar grains due to localized aggregation of surfactant molecules, yielding coatings with slightly lower density and less uniform texture [72]. Meanwhile, nonionic surfactants like polyethylene glycol (PEG) act as mild structure-directing agents; by reducing interfacial energy and facilitating smoother film growth, PEG can improve both adhesion and density within an optimal concentration range.
The findings of Hossain et al. (2020) [73] provide direct experimental evidence of these mechanisms. Their SEM analysis (Fig. 14) compared PbO2 coatings electrodeposited with and without SDS as an additive. The SDS-modified electrode displayed a compact microstructure with smaller grain size, reduced porosity, and superior layer uniformity. This microstructural refinement significantly improved charge-transfer efficiency and discharge density, underscoring the role of surfactants in producing mechanically robust and electrochemically efficient anodes. In contrast, the surfactant-free PbO2 films exhibited irregular and loosely packed grains, leading to reduced mechanical stability and inferior electrochemical performance. Therefore, it can be concluded that the molecular characteristics of surfactants—specifically head-group charge and hydrophobic chain length—govern crystal nucleation kinetics, surface compaction, and ultimately the density-dependent electrocatalytic behavior of PbO2 coatings.
Building upon the improvements in coating density and grain compactness discussed above, the electrochemical performance of PbO2 electrodes is also strongly influenced by the presence of surfactants during cyclic operation. In the study by Hossain et al. (2020) [73], the galvanostatic charge–discharge profiles (Fig. 15) revealed a gradual increase in current density with successive cycles. This progressive enhancement indicates improved electrical connectivity and mechanical integrity of the PbO2 films synthesized with sodium dodecyl sulfate (SDS). The presence of SDS facilitated the formation of finer and more uniform grains, which maintained structural stability during repeated charge–discharge processes and minimized internal stress or layer detachment.
This behavior suggests that surfactant-assisted PbO2 coatings not only achieve higher initial conductivity but also exhibit superior long-term durability under electrochemical cycling. The compact morphology generated by SDS acts as an efficient pathway for charge transport and prevents localized degradation, thus sustaining electrode performance over extended operation. In summary, surfactants such as SDS play a dual role—enhancing both the morphological stability and electrochemical resilience of PbO2 coatings—ultimately leading to denser, more uniform, and high-performance anodes suitable for energy storage and catalytic applications.
Recent studies have further confirmed the critical role of surfactant-type additives in determining the compactness and uniformity of PbO2 coatings. Tang et al. (2022) [74] reported that the incorporation of dicationic ionic liquids during the electrodeposition process acts analogously to molecular surfactants by improving ion dispersion and promoting ordered crystal growth. Their modified PbO2 films exhibited denser microstructures and enhanced adhesion compared to conventional anodes, attributed to the dual electrostatic and hydrophobic interactions between ionic liquid molecules and lead cations. Similarly, Duan et al. (2023) [75] developed a Y-mediated 3D graphene–PbO2 composite anode and demonstrated that surfactant-like surface modifiers could regulate crystal nucleation on hierarchical substrates, leading to smaller grain size and superior film compactness. These results align well with the earlier findings of Hossain et al. (2020) [73] and Zhou et al. (2024) [47], where anionic and cationic surfactants such as SDS and CTAB were shown to influence β-PbO2 orientation and density by altering interfacial energy and charge distribution during nucleation.
Comparatively, Tang’s use of dicationic ionic liquids emphasizes molecular charge balance and electrostatic alignment to achieve compact PbO2 growth, whereas Duan’s approach leverages structural templating and rare-earth mediation to enhance surface coverage and eliminate voids. When analyzed alongside conventional surfactants like SDS or PEG, these newer strategies reveal a convergent mechanism: surface-active molecules with tailored polarity and chain length minimize lattice defects, accelerate nucleation kinetics, and suppress crack formation, thereby yielding denser and more conductive PbO2 coatings. Collectively, these advances highlight that beyond conventional organic surfactants, next-generation ionic or hybrid surfactant systems can effectively tune the morphology–conductivity relationship of PbO2 anodes, offering promising pathways for achieving long-term electrocatalytic stability.
In summary, recent findings by Hossain, Tang, and Duan clearly demonstrate that surfactants play a decisive role in controlling the nucleation, growth, and compactness of PbO2 coatings. Whether traditional types like SDS and CTAB or newer ionic and hybrid systems, these additives enhance coating density, grain uniformity, and electrochemical stability by tuning interfacial energy and crystal orientation.
The diagram below conceptually summarizes these effects, showing how variations in surfactant charge, chain length, and polarity influence the morphology and electrocatalytic performance of PbO2 anodes. This framework highlights the importance of molecular design in achieving dense, conductive, and durable PbO2 coatings for advanced electrochemical applications.
Characteristics of Common Surfactants
This section discusses the main surfactants commonly used in the electrochemical deposition of lead dioxide (PbO2) coatings and analyzes their structural characteristics and functional impact on coating formation.
1. Triton X-100, a nonionic surfactant with the general formula C₁₄H22O and an amphiphilic structure comprising a hydrophobic octylphenol group and a hydrophilic polyethylene glycol (PEG) chain, has attracted considerable interest due to its ability to balance surface tension and enhance interfacial stability. Its nonionic nature minimizes interference with charged species in the electrolyte, allowing for controlled adsorption on the electrode surface.
From a mechanistic viewpoint, Triton X-100 facilitates the formation of uniform PbO2 and similar oxide films by lowering surface tension and stabilizing intermediate colloids during electrochemical growth. Studies have shown that its inclusion during electrodeposition, such as in MnO2 and PbO2 systems, improves film homogeneity, refines crystal size, and increases the electrochemically active surface area [76,77]. As illustrated in Fig. 17, the specific capacitance of the deposited oxide increases with Triton X-100 concentration up to the critical micelle concentration (CMC), beyond which micelle formation reduces the number of active molecules at the electrode–solution interface. This transition highlights the importance of optimizing surfactant concentration to achieve maximum coating uniformity and compactness.
In essence, Triton X-100 acts as a structure-directing agent: its long alkyl chain promotes lateral packing and grain densification, while its PEG segment regulates ion diffusion near the surface. Consequently, this surfactant enables the deposition of PbO2 layers with enhanced compactness, improved surface morphology, and higher electrocatalytic efficiency — a trend consistent with previous findings for SDS and CTAB systems, though achieved through a distinct nonionic interaction mechanism.
2. Polyethylene glycol (PEG) is a nonionic polymeric surfactant with a variable molecular weight typically ranging from 200 to 20,000 g/mol. Its linear structure, composed of repeating ethylene oxide units, provides both hydrophilic and flexible characteristics, allowing PEG to act as an effective surface modifier during the electrodeposition of PbO2 coatings [78].
From a mechanistic perspective, PEG serves primarily as a leveling and structure-directing agent. During deposition, PEG molecules adsorb onto active growth sites, regulating the diffusion of Pb2+ ions and moderating the local current density. This controlled adsorption minimizes rapid, uncontrolled crystal growth and helps suppress the formation of pinholes or rough areas on the coating. As a result, PbO2 films grown in the presence of PEG exhibit smoother surfaces, improved adhesion to the substrate, and greater thickness uniformity compared to PEG-free films [79].
Furthermore, PEG’s polymeric chains enhance the mobility of surface species and promote gradual nucleation, leading to compact grain boundaries and a reduced number of structural defects. This results in improved electrical contact and long-term stability, properties particularly valuable for electrodes in oxygen evolution reactions (OER), fuel cells, and wastewater treatment applications. The effect of PEG, though less electrostatically driven than that of ionic surfactants like SDS or CTAB, relies on steric stabilization and hydrogen-bonding interactions, which collectively improve the homogeneity and compactness of PbO2 coatings.
In summary, PEG acts as a mild yet effective surfactant that balances surface tension, refines microstructure, and enhances film uniformity. Its flexible chain structure allows precise control over coating morphology, complementing the effects of ionic surfactants and contributing to the overall densification and performance improvement of PbO2 anodes.
3. Sodium dodecyl sulfate (SDS) is an anionic surfactant with the chemical formula C12H25NaO4S, commonly used in electrochemical systems due to its strong surface activity and ability to modify interface properties. Structurally, SDS consists of a long hydrophobic alkyl chain (C12H 25 − O 3 −
One of SDS’s key advantages lies in its strong ability to lower surface tension in aqueous media, which improves wetting and mass transport at the electrode surface [80]. During the electrodeposition of PbO2, SDS molecules adsorb on high-energy crystal facets, suppressing uncontrolled growth and leading to the formation of fine-grained, compact, and well-adhered coatings [81,82]. This controlled nucleation not only enhances coating uniformity but also increases the electrochemically active surface area, thereby improving reaction kinetics for both oxygen and chlorine evolution reactions.
The study by Li et al. (2017) [43] demonstrated the direct relationship between SDS concentration and the electrocatalytic activity of PbO2 electrodes in the oxidation of Acid Red G (ARG). As shown in Fig. 18, the PbO2–SDS electrode containing 5.0 vol% surfactant achieved the highest performance, with 83.5% decolorization and 53.9% COD removal within 60 minutes of electrolysis. These improvements were attributed to the electrode’s higher oxygen evolution potential (OEP), lower charge-transfer resistance (Rf), and enhanced film adhesion. Beyond this concentration, micelle aggregation limited surfactant efficiency, indicating the existence of an optimal range near the critical micelle concentration (CMC).
Overall, SDS serves as an efficient anionic modifier that balances surface charge and interfacial energy, enabling the formation of dense, stable PbO2 coatings. Compared to nonionic surfactants such as PEG and Triton X-100, SDS offers superior electrochemical activity and stability due to its stronger electrostatic interaction with Pb2+ ions, which promotes uniform nucleation and enhances catalytic lifetime during prolonged electrolysis.
Building upon the previous discussion of SDS as an anionic surfactant capable of modulating nucleation and improving PbO2 electrode morphology (Li et al., 2017) — where optimal SDS concentration enhanced decolorization and COD removal of organic dyes — recent studies have further substantiated its pivotal role in electrochemical systems. In a recent investigation by Hadavand et al. (2025) [37], the incorporation of SDS during the electrodeposition of β-PbO2 on Ti/TiO2 substrates markedly improved both the structural compactness and electrocatalytic efficiency of the resulting anode toward the degradation of Cephalexin. The presence of SDS promoted the formation of dense, pyramidal crystal grains and a highly uniform surface layer (Fig. 19), leading to reduced charge-transfer resistance (Rₚ = 37.9 Ω) and enhanced electron transport across the electrode–electrolyte interface. Moreover, the modified electrode exhibited an increased oxygen evolution potential (OEP ≈ 1.7 V vs. Ag/AgCl), effectively suppressing the parasitic oxygen evolution reaction and favoring hydroxyl radical (•OH) generation as the dominant oxidative pathway. Consequently, the Ti/TiO2–βPbO2–SDS anode achieved 97.6% Cephalexin degradation and 70.3% COD removal, with negligible Pb2+ leaching (< 0.01 mg L–1), demonstrating outstanding stability and environmental compatibility.
These findings reinforce the mechanistic insights discussed earlier — namely that SDS acts not merely as a morphological modifier but as a functional interfacial regulator, optimizing both the electronic structure and surface reactivity of PbO2-based electrodes. In contrast to nonionic surfactants such as PEG or Triton X-100, the strong electrostatic interaction between the negatively charged sulfate head of SDS and Pb2+ ions promote uniform crystal growth and prolonged catalytic lifetime, highlighting SDS as one of the most effective surface-active agents for advanced electrochemical oxidation systems.
4. Sodium dodecylbenzene sulfonate (SDBS) is an anionic surfactant characterized by its aromatic benzene ring and long hydrophobic alkyl chain. The sulfonate head group (–S O 3 −
During the electrodeposition of β-PbO2, SDBS molecules adsorb preferentially on the crystal facets with higher surface energy, thereby regulating the rate of nucleation and suppressing irregular grain growth. As a result, the deposited coatings exhibit smaller and more uniform grains with improved compactness and adhesion [31,36]. This structural refinement significantly enhances charge transfer and reduces internal resistance, contributing to better electrochemical performance, particularly for oxygen evolution and organic oxidation reactions.
The influence of SDBS concentration on PbO2 electrode behavior was investigated by Wei et al. (2021) [31] through cyclic voltammetry (CV) in 0.5 M H2SO4. As illustrated in Fig. 20, the voltammetric charge quantity (q*) increased notably with the addition of SDBS, reaching a maximum at 10 mg L–1. This concentration yielded the highest peak current density during oxidation and reduction cycles, signifying a substantial increase in electroactive surface area and improved charge storage capability. The results indicate that moderate SDBS levels facilitate optimal micelle formation and efficient ion transport at the electrode interface, while excessive concentrations (> 20 mg L–1) may cause micellar aggregation, limiting accessibility of active sites.
In summary, the incorporation of SDBS leads to a pronounced improvement in both the morphology and electrocatalytic properties of β-PbO2 coatings by promoting dense, fine-grained microstructures and enhanced electron mobility. When compared with other anionic surfactants such as SDS, SDBS exhibits stronger aromatic adsorption and higher surface charge density, resulting in superior electrochemical activity and long-term stability for wastewater oxidation applications.
5. Cetyltrimethylammonium bromide (CTAB) is a cationic surfactant containing a positively charged quaternary ammonium headgroup and a long hydrophobic alkyl chain (C16H33–). This amphiphilic structure allows CTAB molecules to adsorb selectively onto negatively charged surfaces, such as lead dioxide or conductive substrates, during electrodeposition. The electrostatic attraction between CTAB⁺ and negatively charged oxygen-containing sites facilitates the formation of a compact, uniform coating with enhanced adhesion.
From a structural perspective, the balance between CTAB’s hydrophilic quaternary ammonium group and hydrophobic alkyl tail promotes ordered micelle formation near the electrode interface. These micelles regulate ionic mobility and crystal nucleation, leading to smaller β-PbO2 grains and a denser film [83]. This structural refinement enhances electron transport, lowers internal resistance, and provides a larger electrochemically active surface area [84].
The influence of CTAB on the electrocatalytic behavior of PbO2 coatings was systematically evaluated by Fang Mu et al. (Duan et al., 2012) [85], who investigated hydroxyl radical (•OH) generation during the electrochemical oxidation of organic pollutants. Their fluorescence spectroscopy results demonstrated that PbO2–CNT–CTAB electrodes exhibited significantly higher fluorescence intensity at 425 nm compared to unmodified PbO2, reflecting a higher concentration of •OH radicals formed on the electrode surface (Fig. 21a). This enhancement was attributed to CTAB’s ability to produce smoother and more compact coatings, thus providing a greater density of catalytically active sites.
As shown in Fig. 21b, the rate constant (k) for hydroxyl radical formation was markedly higher for CTAB-modified electrodes than for pure PbO2, confirming the improved oxidative capability. This improvement stems from CTAB’s molecular structure—its positively charged head enhances electrostatic interactions with reactive intermediates, while its hydrophobic tail promotes ordered crystal alignment, yielding superior electron transfer kinetics.
Overall, CTAB acts as a structural and electrochemical enhancer during PbO2 deposition, combining micelle-mediated growth control with strong surface adsorption to increase film density and electrocatalytic efficiency. Compared to anionic surfactants like SDS or SDBS, CTAB demonstrates distinct advantages in forming compact coatings and improving radical-mediated oxidation efficiency, particularly in wastewater treatment applications.
6. Although gelatin is not conventionally categorized as a synthetic surfactant, it functions analogously to nonionic or zwitterionic surfactants due to its amphoteric molecular structure. Gelatin, a biopolymer derived from collagen, contains both hydrophilic amino acid residues and hydrophobic peptide segments, allowing it to adsorb effectively at the solid–liquid interface during electrochemical deposition. This dual functionality enables gelatin molecules to reduce interfacial tension, control nucleation, and stabilize the growth of lead dioxide crystals in a manner comparable to synthetic surfactants [86].
At the molecular level, gelatin’s amphiphilic segments orient themselves such that hydrophobic groups adhere to the substrate while hydrophilic chains extend into the electrolyte. This configuration facilitates a more homogeneous distribution of Pb2+ ions near the electrode surface, reducing the likelihood of uneven deposition and defect formation. Consequently, gelatin acts as a structure-directing additive that enhances both the smoothness and compactness of β-PbO2 coatings.
Studies by Wang et al. [86] and Zhang et al. [87] reported that incorporating gelatin in the deposition bath significantly improved the mechanical integrity and coating density of PbO2 films. The presence of gelatin led to the formation of smaller, tightly packed crystallites with reduced intergranular voids, which translated into improved adhesion and durability. Moreover, gelatin-modified coatings exhibited enhanced electrochemical stability and lower charge-transfer resistance, reflecting their superior interfacial conductivity.
In a comparative perspective, unlike conventional surfactants such as SDS or CTAB that rely primarily on electrostatic interactions, gelatin influences deposition through hydrogen bonding and steric stabilization. Its zwitterionic nature allows reversible interaction with charged species in the electrolyte, which provides a buffering effect that promotes balanced nucleation rates. This property contributes to uniform film growth, resulting in a denser and mechanically resilient PbO2 layer.
Overall, gelatin serves as an environmentally friendly, biodegradable additive capable of improving both the morphology and electrochemical performance of lead dioxide coatings. Its natural polymeric backbone ensures stable adsorption without generating harmful by-products, making it an appealing “green” alternative to synthetic surfactants in PbO2 electrode fabrication.
Building on the mechanistic understanding discussed above, Table 2 consolidates key findings from prior studies comparing different classes of surfactants employed in PbO2 electrodeposition. The comparative analysis highlights that nonionic surfactants such as Triton X-100 improve coating uniformity primarily through surface-tension reduction, whereas polymeric additives like PEG significantly enhance compactness and reduce defect density due to steric stabilization effects. In contrast, anionic surfactants such as SDS exert strong electrostatic control over nucleation, leading to the formation of fine-grained, highly active β- PbO2 surfaces with superior electrocatalytic performance.
The qualitative assessments presented in Table 2—such as decreased porosity, enhanced coating density, reduced crystal size, and improved electrocatalytic performance—were derived from a systematic evaluation of experimental data reported in previous studies. Specifically, the evaluation of porosity reduction and coating densification was based on a comparative analysis of scanning electron microscopy (SEM) images, where the distribution and frequency of pores and microdefects were quantified. Similarly, the assessment of crystal size reduction was guided by X-ray diffraction (XRD) analyses and the Debye–Scherrer equation, allowing for the estimation of average crystallite sizes in PbO2 films prepared with and without surfactant additives.
To transform quantitative data from these diverse sources into unified qualitative descriptors (“low,” “moderate,” “high,” “remarkable,” “very high”), the research team established a comparative scale. This scale reflects the relative percentage improvement in each property of surfactant-modified electrodes compared to the reference PbO2 anode synthesized without surfactants (the “witness sample”). Each qualitative category corresponds to a defined range of percentage enhancement or reduction observed in the experimental literature, as summarized below in Table 3. This methodological framework ensures consistent interpretation across datasets and provides a reliable basis for cross-comparison among different surfactant systems.
Following the comparative tables, the bar chart presented below visualizes the relative influence of each surfactant on the structural and electrochemical properties of PbO2 coatings. The horizontal axis represents the evaluated surfactants, while the vertical axis corresponds to the percentage improvement observed for each property relative to the unmodified PbO2 reference electrode. This graphical analysis serves to consolidate the findings from Tables 2 and 3, providing a clear comparative framework for understanding the distinct functional roles of the different surfactant classes.
As illustrated, CTAB exhibits the most pronounced enhancement in electrocatalytic activity. Its cationic nature facilitates the adsorption of negatively charged intermediates at the electrode surface, promoting uniform crystal alignment and faster charge-transfer kinetics. The quaternary ammonium head of CTAB strongly interacts with the lead dioxide lattice, improving electron mobility and reducing overpotential during oxygen evolution. This explains the sharp increase in catalytic efficiency observed for CTAB-modified anodes in recent electrochemical evaluations.
Conversely, anionic surfactants such as SDS and SDBS are particularly effective in reducing porosity and increasing coating compactness. Their negatively charged sulfate and sulfonate groups coordinate with Pb2+ ions in the deposition bath, enhancing nucleation control and producing densely packed β-PbO2 grains. Between the two, SDBS demonstrates slightly superior compactness, attributed to its aromatic ring structure, which stabilizes crystal nuclei through π–cation interactions and promotes the formation of uniform, tightly packed layers. However, these same electronic characteristics limit its surface catalytic activity compared to CTAB, as excessive negative charge density may inhibit the optimal adsorption of reactive intermediates.
In contrast, gelatin, due to its polymeric and zwitterionic nature, provides a dual benefit—improving both coating density and mechanical integrity. The presence of amine, hydroxyl, and carboxyl functional groups enables multiple coordination sites with lead ions, allowing gelatin chains to fill microvoids and bridge grain boundaries. This molecular reinforcement reduces defect density, enhances film cohesion, and improves long-term stability of the coating. The results align with Zhang et al. [87], who reported significant suppression of surface microcracks and enhanced adhesion in gelatin-modified PbO2 electrodes.
Overall, the comparative analysis underscores that the molecular structure and charge polarity of each surfactant dictate its specific influence on coating formation and performance. Cationic CTAB primarily enhances electron transfer and catalytic efficiency; anionic SDBS and SDS reduce porosity through ionic coordination and growth regulation; and biopolymeric gelatin strengthens compactness by structural stabilization. Together, these findings provide an integrated mechanistic understanding of how tailored surfactant chemistry can be leveraged to optimize PbO2 electrode performance in electrochemical applications.
CHALLENGES AND FUTURE DIRECTIONS
The incorporation of surfactants in the electrochemical deposition of lead dioxide (PbO2) coatings presents both opportunities and challenges. While these additives can significantly enhance coating performance, their complex chemical interactions and practical limitations still hinder full industrial adoption. Understanding the mechanisms governing surfactant–substrate interactions, optimizing surfactant concentration, and ensuring process reproducibility remain critical research needs. Nevertheless, translating these advances from laboratory studies to industrial practice offers substantial potential benefits. From a practical standpoint, surfactant-engineered PbO2 electrodes hold strong promise for large-scale applications. In electrochemical wastewater treatment systems, optimized surfactant-assisted coatings can significantly enhance oxygen evolution efficiency and electrode lifespan, thereby reducing operational costs and maintenance frequency [91–93]. Similarly, in metal electrowinning and electrorefining processes, denser and more conductive PbO2 layers improve current efficiency and energy utilization [94,95]. Beyond these established fields, integrating green surfactants and doped PbO2 composites may enable scalable fabrication of robust electrodes for advanced electrolyzers, flow batteries, and hybrid energy-conversion devices [96]. Bridging laboratory-scale research with pilot-plant validation will therefore be a key step toward the commercialization of high-performance, sustainable PbO2-based anodes.
Major challenges
· Complex interfacial interactions: The molecular interactions between surfactants, lead ions, and the growing oxide layer remain highly intricate and system-dependent. Variations in the surfactant’s headgroup charge, chain length, and hydrophobic–hydrophilic balance strongly affect the nucleation and crystal growth of PbO2. For example, Yanenko et al. [88] demonstrated that anionic surfactants can alter the β/α phase ratio of PbO2, modifying its conductivity and stability. However, the exact mechanisms governing these interfacial phenomena—such as competitive adsorption and micellar templating—are not yet fully elucidated, making predictive coating design difficult.
· Concentration sensitivity and process optimization: Surfactant concentration critically affects coating quality. At sub-optimal levels, surfactants enhance wettability and uniformity; however, excessive concentrations can increase porosity or inhibit adhesion due to micelle oversaturation and uncontrolled hydrogen evolution. Establishing universal concentration–performance relationships remain a key experimental challenge, particularly when scaling from laboratory to pilot-scale electrochemical cells.
· Environmental and sustainability concerns: Most high-performance surfactants used in PbO2 electrode fabrication are petroleum-derived, raising environmental and biodegradability concerns. Decomposition products may introduce secondary pollutants into aquatic systems. Consequently, the development of ecofriendly or bio-based surfactants—for example, those derived from amino acids, polysaccharides, or plant oils—represents a critical future direction toward sustainable electrode fabrication.
· Scalability and economic feasibility: Reproducible large-scale deposition requires strict control over temperature, pH, and current density, parameters that are easily destabilized by surfactant-induced changes in electrolyte viscosity and conductivity. Moreover, several advanced surfactants, despite excellent performance in lab studies, are cost-prohibitive for industrial use. Future work should prioritize identifying low-cost, multifunctional surfactants or synergistic blends that retain performance while remaining economically viable.
· Limited mechanistic understanding: Despite progress in recent years, a comprehensive molecular-level understanding of how specific surfactant structures influence nucleation kinetics and electronic properties is still lacking. Advanced in-situ diagnostic tools—such as electrochemical quartz crystal microbalance (EQCM), atomic force microscopy (AFM), and operando Raman spectroscopy—should be leveraged to bridge this gap. Such mechanistic insights will be vital for rational design of next-generation surfactant systems.
· Industrial Applications: Industrial-scale implementation of surfactant-modified PbO2 anodes remains limited despite their promising laboratory performance. In large electrochemical reactors used for waste-water oxidation, metal electrowinning, and chloride-based electrolyzers, optimized surfactant-assisted PbO2 coatings can significantly reduce energy consumption by lowering OER overpotential and improving masstransfer efficiency. Additionally, surfactant-engineered microstructures offer better resistance to particle shedding and mechanical degradation during long-term operation, which is critical for industrial systems that often operate at high current densities (50–200 mA cm–2). Tailoring surfactant chemistry therefore provides a practical and scalable pathway for producing robust PbO2 anodes with longer service lifetimes, lower maintenance costs, and higher operational safety in full-scale environmental and metallurgical plants.
Future research directions
Future research in surfactant-assisted electrodeposition of PbO2 coatings should address both material innovation and practical deployment to bridge the gap between laboratory results and industrial application.
· Development of green surfactants: There is a critical need to develop biodegradable, non-toxic surfactants that maintain high performance while reducing environmental footprint. For example, while many current studies focus on petrochemical-derived surfactants, work by A. Velichenko et al. (2020) [89] demonstrated the use of fluorinated surfactants for PbO2, albeit with sustainability concerns remaining.
· Surfactant combinations and synergistic formulations: exploring mixed-surfactant systems represents a highly promising strategy for overcoming the intrinsic limitations of individual surfactants. While single surfactants often improve only one or two coating parameters, synergistic formulations can compensate for these weaknesses by integrating complementary functionalities. For example, combining a nonionic surfactant that enhances wetting and surface uniformity with an ionic surfactant that strongly regulates nucleation density can simultaneously promote smoother film formation, better adsorption behavior, and more controlled crystal growth. Such mixed systems may also improve electrical conductivity by facilitating more homogeneous grain orientation and reducing defect density. Therefore, the rational design of multi-component surfactant mixtures should be considered a key direction for future research, particularly for developing PbO2 coatings with enhanced electrical properties, long-term stability, and optimized morphology suitable for industrial applications.
· Advanced characterization methods and mechanism elucidation: To design next-generation coatings, mechanistic insight is needed. Techniques such as AFM, XPS, operando electrochemical impedance, and insitu SEM should be applied to map how surfactant molecules influence nucleation rates and crystal growth pathways. For instance, O. Saoudi et al. (2020) [69] used CV and impedance measurements to study SDS and CTAB effects on PbO2/PbSO4 electrodes.
· Application-driven research and scalability: Research must move beyond small-scale trials toward real industrial systems — for example wastewater treatment reactors, electrowinning cells, or lead-acid battery anodes. In such contexts, surfactants must perform under high current density, long-term operation, and harsh chemical environments. For example, review work by Q. Zhou et al. (2022) [90] notes that PbO2 electrodes are used for organic pollutant degradation at industrial scale.
· Integrated additive systems and tailor-made formulations: Future studies should investigate how surfactants can be integrated with dopants (e.g., Bi, Ce) or nanostructured supports (e.g., CNT, graphene, MXene) to co-enhance electrical conductivity, catalytic activity, and mechanical stability. Research showing positive effects for PbO2 composite coatings suggests this is a promising path.
· Environmental impact and lifecycle assessment: As new surfactants are introduced, lifecycle assessments must ensure that modifications do not introduce new hazards. Environmentally benign formulations that deliver performance and safety will strengthen adoption in industry.
In conclusion, a conceptual framework summarizing these research directions is shown in Fig. 23, illustrating the interplay between surfactant chemistry, electrode structure, application environment, and scaling. This roadmap aims to encourage targeted innovation in surfactant design and bring high-performance PbO2 electrodes into broader industrial deployment.
CONCLUSION
Surfactants play a crucial role in tailoring the microstructure, electrochemical performance, and durability of anodically deposited lead dioxide (PbO2) coatings. This review highlights how different surfactant types— such as PEG, SDS, SDBS, CTAB, Triton X-100, and gelatin—modulate nucleation, crystal growth, and film compactness through distinct interfacial chemistries. By enabling control over morphology and surface properties, surfactant-assisted electrodeposition offers a versatile platform for optimizing PbO2 electrodes in energy storage, electrocatalysis, and wastewater treatment.
Despite notable progress, the mechanistic understanding of surfactant–ion interactions remain incomplete, and most current studies rely on empirical observations. Future research should integrate kinetic modeling with in-situ and operando diagnostics (e.g., EQCM, Raman, or electrochemical AFM) to unravel dynamic adsorption and nucleation processes. Moreover, the use of biodegradable, renewable biosurfactants will be essential to advance environmentally sustainable electrode fabrication. Combining surfactant-assisted growth with dopants or hybrid materials such as PbO2–graphene or PbO2–PANI composites may further enhance conductivity, stability, and industrial viability.
In summary, rational design of surfactant-assisted PbO2 electrodeposition represents a promising pathway toward high-efficiency, long-life, and eco-friendly electrochemical systems. Bridging mechanistic insight with scalable processing will be key to realizing next-generation PbO2-based anodes for industrial wastewater treatment, metal electrowinning, and advanced energy conversion technologies.
PDF Links
PubReader
Full text via DOI
Download Citation
