Needles Beat Flowers: How One Deposition Trick Doubles Supercapacitor Power

Zinc hydroxide is one of the humblest compounds in the battery lab—a white, chalky precipitate that chemists usually wash away rather than celebrate. But a new comparative study from researchers at Savitribai Phule Pune University in India and Chonnam National University in South Korea, published in Frontiers in Batteries and Electrochemistry, shows that this unassuming material can become a serious contender for supercapacitor electrodes, and that the entire performance story hinges on a single decision made before the film ever dries: which deposition method you choose.

The team, led by A. A. Mancharkar, J. H. Kim, and Habib M. Pathan, grew thin films of Zn(OH)2 on stainless steel substrates using two low-cost, low-temperature techniques: chemical bath deposition (CBD) and successive ionic layer adsorption and reaction (SILAR). Both methods draw from the same precursor chemistry—zinc acetate and aqueous ammonia—yet they produce films with dramatically different architectures, crystallinities, and, ultimately, electrochemical behavior. When tested as supercapacitor electrodes in a 2 M KOH electrolyte, the CBD-grown needles delivered a specific capacitance of 329 F/g at a current density of 1 A/g, while the SILAR-grown microflowers managed 179 F/g under identical conditions. The needles also retained 78 percent of their original capacitance after 3,000 charge–discharge cycles at a scan rate of 100 mV/s, compared with 74 percent for the microflowers.

To understand why one morphology outperforms the other, it helps to look at how each method builds the film at the molecular level. In the CBD process, zinc acetate and ammonium hydroxide are combined in a single aqueous bath held at 50 °C for two hours. Ammonia acts as a complexing agent, binding Zn2+ ions into soluble zinc–ammonia complexes such as [Zn(NH3)4]2+. This controlled complexation suppresses rapid homogeneous precipitation and instead releases Zn2+ ions gradually as the equilibrium shifts, maintaining a low supersaturation level that favors heterogeneous nucleation directly on the stainless steel surface. Because the entire bath stays warm throughout, nucleation and crystal growth proceed continuously, allowing the nuclei to develop into well-defined needle-like Zn(OH)2 nanostructures with improved crystallinity, high porosity, and strong adhesion to the substrate.

The SILAR method, by contrast, is a stepwise affair. The substrate is dipped sequentially into three beakers: first a 0.1 M zinc acetate solution for 5 seconds to adsorb Zn2+ ions, then double-distilled water for 5 seconds to rinse away loosely bound species, and finally a 2 M ammonia solution held at 50 °C for 10 seconds, where the adsorbed zinc ions react with hydroxide ions to form Zn(OH)2. This sequence constitutes a single cycle, and 50 cycles were carried out to achieve uniform film growth. Crucially, only the alkaline solution is heated; the adsorption and rinsing steps occur at room temperature. Each cycle initiates nucleation afresh rather than sustaining it, resulting in discontinuous crystal growth with comparatively slower kinetics. The outcome is a microflower-like architecture with lower crystallinity and a smaller specific surface area.

X-ray diffraction confirmed that both films crystallized in the orthorhombic phase of Zn(OH)2, matching standard reference data (JCPDS No. 00-048-1066), with prominent reflections from the (100), (002), (101), (102), (110), and (103) crystallographic planes. But the CBD-derived film showed stronger diffraction intensities, indicating higher crystallinity—a direct consequence of its continuous growth process. Applying the Scherrer equation to the diffraction peaks, the team estimated average crystallite sizes of 34 nm for the CBD film and 27 nm for the SILAR film. The SILAR route also produced a wider optical bandgap: 3.34 eV versus 3.19 eV for the CBD film, as determined from Tauc plots of UV–visible absorption data. FTIR spectroscopy verified the hydroxide nature of both films through characteristic Zn–OH bending modes near 1,033, 896, and 856 cm−1, alongside O–H stretching vibrations in the 3,806–3,793 cm−1 region.

Scanning electron microscopy revealed the morphological divide in vivid detail. The CBD film consists of densely packed, vertically aligned needle-like structures uniformly distributed across the substrate, with an average width of approximately 0.5 μm. This architecture offers a larger surface area and a greater number of electrochemically active sites, facilitating efficient charge storage. The SILAR film, in contrast, displays a microflower-like texture composed of uniformly dispersed flower-shaped assemblies with lateral dimensions ranging from 300 to 600 nm, forming a porous network with abundant accessible surface sites. Energy-dispersive X-ray spectroscopy confirmed zinc and oxygen as the major constituents in both samples, with atomic ratios consistent with Zn(OH)2 formation.

Brunauer–Emmett–Teller (BET) analysis quantified the textural differences. The N2 adsorption–desorption isotherms of both samples displayed typical type-IV behavior with H3 hysteresis loops, indicating mesoporous structures. The CBD film achieved a specific surface area of 17.63 m²/g, roughly double the 8.59 m²/g of the SILAR film. Pore-size distribution analysis using the Barrett–Joyner–Halenda model yielded average pore diameters of 16.34 nm for the CBD film and 17.89 nm for the SILAR film. The enhanced surface area of the CBD sample is likely associated with its highly crystalline nature and distinctive needle-like architecture, which expose a greater number of surface sites to the electrolyte.

Electrochemical testing in a three-electrode configuration—using the Zn(OH)2 films as the working electrode, a platinum wire counter electrode, and a Hg/HgO reference electrode—confirmed the pseudocapacitive charge-storage mechanism. Cyclic voltammetry showed well-defined redox peaks, indicating that charge storage is governed predominantly by reversible Faradaic reactions at the electrode–electrolyte interface. During charging, Zn(OH)2 is oxidized to ZnOOH with the release of an electron; during discharge, the reverse reduction regenerates Zn(OH)2. From CV data at a scan rate of 5 mV/s, the specific capacitances were calculated to be 402 F/g for the CBD film and 238 F/g for the SILAR film. The team also fabricated five independent electrodes under identical conditions to verify reproducibility: the CBD electrodes ranged from 392 to 410 F/g, while the SILAR electrodes fell between 230 and 242 F/g, demonstrating excellent electrode-to-electrode consistency for both methods.

Kinetic analysis using a power-law model revealed b-values of 0.66 for the CBD electrode and 0.73 for the SILAR electrode, both between 0.5 and 1.0, indicating a combination of diffusion-limited ion transport and surface-controlled capacitive effects. At a scan rate of 5 mV/s, the diffusion-controlled contribution accounted for nearly 78 percent of the total current in the CBD electrode and approximately 71 percent in the SILAR electrode. As the scan rate increased to 100 mV/s, the surface-controlled capacitive processes became more pronounced, with the SILAR electrode exhibiting a larger capacitive contribution of 64 percent compared to 56 percent for the CBD electrode. Electrochemical impedance spectroscopy showed comparable equivalent series resistance values of 0.72 and 0.78 Ω/cm² for the CBD and SILAR electrodes, respectively, but the charge-transfer resistance was 25.36 Ω/cm² for the CBD film versus 31.98 Ω/cm² for the SILAR film, and the Warburg impedance was 0.0015 Ω/cm² versus 0.0082 Ω/cm², indicating faster ion diffusion in the needle-like film.

The superior performance of the CBD-grown electrode traces back to its distinct growth mechanism. The gradual release of Zn2+ ions from zinc–ammonia complexes promotes controlled nucleation and crystal growth, resulting in a highly porous needle-like structure with improved crystallinity and larger specific surface area. These characteristics enhance electrolyte penetration, increase the number of electroactive sites, and facilitate rapid ion diffusion. The SILAR process, with its successive adsorption and reaction cycles, produces microflower-like structures with comparatively lower crystallinity and surface area, leading to reduced electrolyte accessibility and electrochemical activity. Both electrodes demonstrated satisfactory cycling durability, retaining 78 and 74 percent of their original capacitance after 3,000 cycles, indicating suitability for prolonged energy-storage operation. The findings underscore that precise control over synthesis methodology is essential for improving the electrochemical performance of Zn(OH)2-based electrodes and advancing their application in high-performance supercapacitor systems.

Subject of Research: Comparative electrochemical study of CBD and SILAR grown Zn(OH)2 thin films for supercapacitor electrodes

Article Title: A comparative study of electrochemical properties of CBD and SILAR grown Zn(OH)2 thin films

Article References: Mancharkar, A. A., Bodke, M. R., Malavekar, D. B., Bachankar, S. S., Choi, H., Kim, J. H., & Pathan, H. M. (2026). A comparative study of electrochemical properties of CBD and SILAR grown Zn(OH)2 thin films. Frontiers in Batteries and Electrochemistry, 5, Article 1905059. https://doi.org/10.3389/fbael.2026.1905059

Image Credits: AI Generated

DOI: 10.3389/fbael.2026.1905059

Keywords: zinc hydroxide, supercapacitor, chemical bath deposition, SILAR, thin films, pseudocapacitor, specific capacitance, electrochemistry, nanostructures, energy storage, crystallite size, bandgap