The global demand for sustainable energy sources necessitates the development of efficient and cost-effective energy storage solutions. Solar energy, a clean and abundant resource, is a promising candidate for driving various chemical reactions and physical processes. However, the fabrication of high-performance energy storage devices often involves complex and expensive procedures. This research introduces a novel approach to solar energy storage using plasmon-activated water (PAW). Water, with its unique hydrogen bonding network, is a ubiquitous and environmentally benign solvent. However, the rapid equilibrium of hydrogen bond (HB) formation and breakage in bulk water limits its utility as an energy storage medium. This study explores the possibility of leveraging the transient nature of hot electrons generated in plasmon-activated Au nanoparticles to disrupt the equilibrium of HBs in water and create a metastable, energetic state. The goal is to demonstrate that this plasmon-activated water can act as a new energy storage resource with enhanced chemical reactivity and a longer lifetime than the picosecond scale of the initial hot electron transfer process. The novelty lies in utilizing the inherent properties of water, modified by plasmonic excitation, as a simple and inexpensive method for solar energy storage, surpassing the limitations of traditional approaches.

Previous research has extensively explored the use of Au nanoparticles (AuNPs) in various applications due to their localized surface plasmon resonance (LSPR) properties. These applications include surface-enhanced Raman scattering (SERS), photothermal ablation of tumors, and photochemical catalytic reactions. Studies have demonstrated that hot electron transfer (HET) from illuminated AuNPs can initiate chemical reactions. However, the transient nature of these processes (picosecond timescale) limited their practical utilization for energy storage. Other research has focused on developing stable energy storage materials, such as polymer phase-change materials embedded with Fe3O4-functionalized graphene nanosheets and solar-driven pseudocapacitors using ZnO@NiO nanorod arrays. However, these systems often involve complex fabrication processes. A prior study by the authors demonstrated that plasmon-activated water (PAW) could be created using green LEDs to excite AuNPs. This work builds upon that foundation by using natural sunlight to generate PAW and investigate its potential as a new solar energy storage medium.

AuNP-coated ceramic rods were prepared by immersing ceramic rods in a solution containing 50 ppm AuNPs (approximately 10 nm in diameter) for one day. The AuNP-coated rods were then rinsed and dried. PAW solutions were prepared by placing the AuNP-coated ceramic rods in vials containing 250 mL of 0.1 M KCl-containing deionized (DI) water, exposing them to direct sunlight for 3 hours. Two types of PAW solutions were prepared: in situ (electrolyte added before sunlight exposure) and ex situ (electrolyte added after sunlight exposure). The evaporation rates of the PAW solutions and DI water were measured. The specific heats of the solutions were determined by measuring the rate of temperature increase under constant heating. Nuclear magnetic resonance (NMR) relaxation times (T1) were measured to assess the hydrogen bonding structure. Electrochemical linear sweep voltammetry (LSV) was used to evaluate the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in both PAW solutions and DI water. Cyclic voltammetry (CV) was employed to investigate the electrochemical behavior of K3Fe(CN)6 in the PAW and DI water solutions. The energy conversion efficiency was estimated by considering the energy required to break hydrogen bonds and raise the temperature of the PAW solution.

The PAW solutions exhibited distinct properties compared to DI water. Evaporation rates were significantly higher in the PAW solutions (approximately 10% higher for both in situ and ex situ preparations, even after aging for two days). Specific heat measurements revealed reduced specific heat capacities for the PAW solutions (approximately 17% reduction for in situ PAW compared to DI water). NMR relaxation time (T1) measurements confirmed a decrease in the strength of hydrogen bonds within the PAW solutions. Electrochemical measurements demonstrated that the PAW solutions enhanced both OER and HER. The in situ PAW solution showed particularly significant improvements in OER, with current increases of approximately 220% compared to DI water. This enhanced activity persisted for up to two days. The energy conversion efficiency from solar energy to the PAW solution was estimated to be approximately 6.7%, which is comparable to other, more complex energy storage systems. Furthermore, the enhanced activity in the PAW solution was attributed to increased diffusion coefficients and electron transfer rate constants of electroactive species in PAW, as measured by CV studies with potassium ferricyanide. The effect of sunlight was further demonstrated with electrochemical experiments on Au substrates, showing an enhanced electrochemical reaction with increased illumination.

The findings demonstrate that the use of sunlight-illuminated AuNPs to generate PAW offers a simple and efficient method for solar energy storage. The enhanced evaporation rate, reduced specific heat, and altered NMR relaxation times collectively indicate a weakening of the hydrogen bonding network in PAW, resulting in a higher chemical potential. The enhanced OER and HER performance highlights the potential of PAW as a catalyst for water-splitting reactions. The relatively long lifetime of the enhanced PAW properties (approximately two days) makes it a practical energy storage solution. The energy conversion efficiency of approximately 6.7% is comparable to more complex systems, showcasing the promise of this simple and cost-effective approach. The observed changes in electrochemical behavior are consistent with increased diffusion coefficients and electron transfer rates. The observed effects are attributable to the interaction between hot electrons generated by plasmonic excitation in the AuNPs and the hydrogen bonding network of water molecules, creating a metastable state with increased energy and chemical reactivity. These findings contribute to the advancement of solar energy storage technologies and open up new avenues for research in this field.

This study successfully demonstrates the feasibility of using plasmon-activated water (PAW) as a novel solar energy storage medium. The simple and cost-effective method of generating PAW via sunlight-excited AuNPs offers a practical solution for storing solar energy. The enhanced chemical potential of PAW results in improved performance in water-splitting reactions (OER and HER). Future research should focus on strategies to further extend the lifetime of the PAW’s enhanced properties and exploring other potential applications of PAW in various water-related fields.

The energy conversion efficiency of 6.7% is a preliminary estimate and may be subject to variations depending on environmental factors such as sunlight intensity. The study primarily focused on the effects of PAW on OER and HER, and further investigation is required to explore its impact on other water-related chemical reactions. Long-term stability studies should be conducted to evaluate the longevity of the PAW's enhanced properties under various conditions. The study's findings are based on experiments using a specific type of AuNPs and ceramic rods; therefore, further research is needed to determine the generalizability of these findings to different materials and configurations.