Folded nanocrystalline-stacked WO3 with photo-enhanced functionality for high-performance ammonium-ion storage

The integration of light harvesting with electrochemical energy storage has been increasingly recognized as a promising pathway to address the global energy crisis. By directly coupling photon absorption with ion storage processes, photo-assisted energy storage systems offer unique advantages over conventional batteries and capacitors. This intrinsic synergy not only enhances device performance but also opens new opportunities for solar-rechargeable energy storage technologies. Among various aqueous energy-storage systems, ammonium-ion-based devices have recently emerged as promising candidates owing to the unique hydrogen-bond-driven insertion mechanism of NH4+, which enables fast ion transport and high reversibility in mild aqueous electrolytes. Integrating such NH4+ storage chemistry with photoactive electrodes could further expand the functionality of aqueous systems toward light-assisted charge storage. Married this context with ammonium-ion hybrid supercapacitors (AIHSs), this concept represents an attractive and novel aqueous energy storage system, thanks to their low cost, intrinsic safety, environmental benignity, and unique hydrogen-bond-driven insertion mechanisms. To achieve these distinctive features, AIHSs particularly need to be coupled with photoactive electrodes featuring both high-performance and light-enhanced charge storage.
This dissertation thus focuses on the rational design of WO3-based electrode owning photo-enhanced functionality for efficient ammonium-ion storage. Through the development of folded nanocrystalline–amorphous WO3 structures, WO3/TiO2 heterojunctions, and WO3/BDD composites, this work systematically establishes structure–property–performance relationship and demonstrates new design principles to construct high-performance, solar-responsive aqueous energy storage devices. Three experimental parts are detailed as follows.
I) Folded nanocrystalline–amorphous WO3 electrodes: The folded nanocrystalline-stacked amorphous WO3 (a-WO3) electrodes were synthesized via a rapid electrochemical deposition method. They own hierarchical frameworks where ~5 nm crystallites are loosely assembled within an amorphous matrix. Their comprehensive characterization was conducted using microscopy, spectroscopy, electrochemical techniques, and density functional theory simulations. More importantly, the relationship between structural architecture, defect chemistry, and NH4+ storage performance was investigated in detail. The a-WO3 electrodes demonstrated superior electrochemical activity with a capacitance of 2783 mF cm−2 and excellent rate performance compared to crystalline counterparts. The amorphous structure is confirmed to provide abundant grain boundaries, oxygen vacancies, and mixed-valence tungsten states, facilitating rapid NH4+ transport through hydrogen bonding mechanisms. As a demonstration, the a-WO3 electrode was employed to construct a full ammonium-ion hybrid supercapacitor with a polyaniline cathode. This supercapacitor delivers remarkable energy and power densities of 620 mWh cm−2 and 23,980 mW cm−2, respectively, further with 81.5% capacity retention after 3000 cycles. These results highlight the effectiveness of controlled amorphization as a strategy to overcome the intrinsic limitations of crystalline WO3 and achieve both high capacity and durability. All these are shown in Chapter 3.
II) WO3/TiO2 heterojunctions for light-assisted energy storage: The WO3/TiO2 heterojunction composites were designed with an aim to harness photo-enhanced energy storage capabilities through synergistic light and electrochemical properties. The band alignment between WO3 and TiO2 was utilized to engineer Type II heterojunctions, which leverage the superior photocatalytic activity of TiO2 and charge separation capability, eventually enhancing electrochemical performance of WO3. Through controlled electrochemical deposition parameters, island-like distributed WO3 nanocrystals have been coated on TiO2, ensuring simultaneous exposure of both materials to the electrolyte and meanwhile achieving true synergistic effects rather than simple serial coupling. This island-like growth mode maximizes the heterojunction interface area, promoting efficient charge separation and transport. The TiO2 component provides superior photocatalytic activity and extends the photoactive spectral range, while WO3 offers high-capacity NH4+ storage capability. Comprehensive characterization of these composite electrodes reveals their enhanced light absorption, improved charge carrier dynamics, and accelerated ion transport kinetics under illumination. Under light irradiation, the typical composite material showed 40% capacity enhancement compared to that under dark conditions, with a significantly reduced charge transfer resistance by a factor of 80-85%, a photocurrent density of 0.47 mA cm−2, and substantially improved carrier density and interfacial kinetics. All these validate the effectiveness of heterojunction design in achieving photo-assisted energy storage. These details are summarized in Chapter 4.
III) WO3/BDD p-n junction for light-assisted energy storage: This session addresses the persistent conductivity limitations of WO3 electrodes by incorporating boron-doped diamond (BDD), a carbon material known for its wide potential window, high conductivity, and chemical robustness. The strategic combination creates a p–n junction configuration that exploits the ultra-wide bandgap of diamond (5.5 eV) and the superior electronic properties of p-type BDD interfaced with n-type WO3. The intrinsic electric field at the junction enhances charge separation efficiency, while the exceptional conductivity of BDD complements the NH4+ storage capability of WO3. Compared with pristine WO3, the composite delivered a ~1.7-fold higher photocurrent density and the areal capacitance under light was enhanced by nearly 20%. In addition, the electrode maintained over 90% of its capacity after extended cycling, outperforming pristine WO3. Therefore, the WO3/BDD p–n junction delivers significantly enhanced photocurrent response, improved NH4+ storage capacity under illumination, and extended cycling lifetime compared to pristine WO3. The built-in electric field facilitates directional charge transport and suppresses recombination losses, resulting in superior photoelectrochemical performance. These results are demonstrated in Chapter 5.
In summary, this thesis provides several successful approaches to design and optimize WO3-based electrode materials for sustainable ammonium-ion energy storage applications in the way of both advanced structural engineering and innovative photo-enhancement strategies. By developing folded nanocrystalline-stacked architectures, engineering Type II and p-n heterojunctions, and integrating photocatalytic functionalities, the great potential of WO3 in achieving high-performance, environmentally friendly energy storage devices have been proved in this thesis. They also feature dual solar energy harvesting and electrochemical storage capabilities. Such systematic investigation of photo-assisted mechanisms clarifies that heterojunction formation enables efficient charge separation, reduces recombination losses, and significantly enhances electrochemical performance under illumination. This work establishes fundamental design principles for photo-responsive electrode materials. Beyond ammonium-ion storage, the strategies presented here open new avenues for multifunctional aqueous devices capable of simultaneously harvesting solar energy and storing ions.