Sustainable High-Performance Energy Storage: The Solar Battery

This innovative solar battery is based on cyanamide-functionalized polyheptazine-imide (NCN-PHI), a two- or three-dimensional carbon nitride material (Inlet Fig. 1) that uniquely integrates light absorption and energy storage in a single component. Unlike conventional systems, where energy conversion and storage occur separately, NCN-PHI directly captures sunlight, stores the resulting photoinduced electrons, and releases them as electrical energy on demand.

The material functions as a photoanode in an aqueous electrolyte, where light excitation generates electron-hole pairs (Fig. 1). While holes are extracted to a cathode via a redox shuttle, electrons are trapped within the NCN-PHI structure, stabilized by alkali ions such as Na⁺, K⁺, or Li⁺. This mechanism prevents immediate recombination, allowing charge retention for extended periods.

With a high overpotential for hydrogen evolution, NCN-PHI avoids unwanted water reduction, expanding the operational voltage window and improving energy density. The device achieves high durability (Fig. 2) and can be charged using sunlight or conventional electrical methods, demonstrating versatility in energy storage applications.

The shift toward renewable energy faces a key challenge: efficient and sustainable energy storage. Solar energy, while abundant, requires reliable storage solutions to compensate for its intermittent availability. Conventional methods rely on separate photovoltaic systems and batteries, leading to energy losses, high costs, and material constraints. Lithium-ion batteries, though widely used, depend on scarce resources and have limited cycle life.

Aqueous battery systems offer a safer alternative but struggle with low energy densities due to the electrochemical limitations of water. Additionally, most current storage technologies require immediate discharge, reducing flexibility. An ideal solution would integrate solar energy capture with direct electrical storage in a single material. Such an approach could improve efficiency, scalability, and sustainability while reducing dependence on fossil-based charging sources.

• Integrated Functionality: Simultaneous light harvesting and energy storage within a single material.
• High Performance: High energy density, long charge retention, and operational stability (Fig. 2).
• Scalability: Suitable for large-scale production using cost-effective, abundant materials.
• Flexibility: Compatible with multiple ions (e.g., Na⁺, K⁺, Li⁺, and ammonia) in aqueous electrolytes.
• Environmentally Friendly: Operates sustainably based on earth-abundant, cost-effective materials.

• Storage systems for renewable energy grids to balance supply and demand.
• Solar-powered mobile devices and remote systems.
• Photovoltaic power generation with integrated storage capabilities.
• Advanced energy solutions for off-grid applications.
• Specialized detectors requiring energy storage and release mechanisms.

PCT (WO2020143912A1; 09.01.2019), active in EP, JP, US

Podjaski, F., Kröger, J., & Lotsch, B. V. (2018). Toward an aqueous solar battery: direct electrochemical storage of solar energy in carbon nitrides. Advanced Materials, 30(9), 1705477.