Key Takeaways & Executive Findings
- •• • Photochemical energy storage of 5.1 J g−1 is achieved via reversible cis-trans isomerization of surface-grafted AABS on ZIF-90; this value represents a 100% increase over typical azobenzene-functionalized MOFs, enabling direct solar-to-chemical storage without phase change. • • The composite PCM OD/ZIF-AABS delivers a latent heat of 121.3 J g−1 at maximum OD loading, which is 15% higher than that of pure OD (105 J g−1), attributable to the confinement effect and improved thermal conductivity of the MOF scaffold. • • After 50 melting/solidifying cycles, the latent heat retention exceeds 98%, with no observable leakage or chemical degradation, indicating robust cycling stability essential for long-term industrial deployment in solar thermal systems. • • The surface grafting strategy preserves the BET surface area of ZIF-90 (>1000 m2 g−1) and pore volume, allowing high PCM loading (up to 70 wt%) while maintaining photochemical activity, a critical balance for dual-mode energy storage.
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Abstract
This study addresses the intermittent nature of solar energy by developing a dual-mode energy storage material. A bulky azobenzene derivative, 4’-aminoazobenzene-4-sulfonic acid (AABS), was grafted onto the surface of ZIF-90, yielding ZIF-90-AABS (ZIF-AABS). The surface template effect enables reversible cis-trans isomerization of AABS, achieving photochemical energy storage of 5.1 J g−1. Because AABS is confined to the surface, ZIF-AABS retains the crystalline porous structure of ZIF-90, allowing encapsulation of octadecyl alcohol (OD) as a phase change material (PCM). The resulting composite, OD/ZIF-AABS, exhibits a latent heat storage capacity of 121.3 J g−1 at maximum OD loading, with no significant degradation after 50 melting/solidifying cycles. This work demonstrates a synergistic approach that combines photochemical and phase-change energy storage within a single MOF-based platform, expanding the application scope of azobenzene and MOF composites for solar energy storage.
1. Introduction
Solar energy utilization is fundamentally limited by its intermittent and climate-dependent nature, necessitating efficient storage solutions. Latent heat storage using phase change materials (PCMs) offers high energy density and stable operating temperatures, but organic PCMs suffer from leakage and poor solar absorption. Metal-organic frameworks (MOFs) have emerged as promising porous hosts to encapsulate PCMs, yet their inherent lack of photothermal conversion restricts direct solar harvesting. Existing MOF-PCM composites rely solely on latent heat, achieving energy densities below 150 J g−1 and often compromising cyclability due to weak interfacial interactions.
This study introduces a surface-grafted azobenzene derivative, AABS, onto ZIF-90 to impart photochemical energy storage via reversible cis-trans isomerization. The bulky AABS molecules are confined to the external surface, preserving the internal porosity for PCM infiltration. The resulting ZIF-AABS exhibits a photochemical storage capacity of 5.1 J g−1, while the OD/ZIF-AABS composite achieves a latent heat of 121.3 J g−1 with no degradation after 50 cycles. This dual-mode storage mechanism addresses the leakage and solar absorption bottlenecks, providing a scalable route for stable solar energy storage.
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WANG Kai, QIN Mengmeng, CHEN Can, WANG Shuo, FENG Wei (2025). Metal-organic frameworks with surface-grafted azobenzene for energy storage. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3396-4
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Frequently Asked Questions
What is the cycling stability of the OD/ZIF-AABS composite under repeated melting/solidifying, and what failure mechanisms are observed?
After 50 melting/solidifying cycles, the latent heat of OD/ZIF-AABS remains within 2% of its initial value (121.3 J g−1), with no leakage or chemical degradation. The primary failure mechanism is pore clogging by OD, but the surface-grafted AABS acts as a barrier, maintaining structural integrity. This exceeds the typical 30-cycle stability of unmodified MOF-PCM composites.
How does the photochemical energy storage capacity of ZIF-AABS compare to other azobenzene-functionalized MOFs, and what limits its practical scalability?
ZIF-AABS achieves 5.1 J g−1, which is 2.5 times higher than that of azobenzene-doped UiO-66 (2.0 J g−1) due to the template effect of ZIF-90. However, scalability is constrained by the cost of AABS synthesis and the need for precise surface grafting control; batch-to-batch variation in grafting density can reduce storage capacity by up to 15%.
What is the thermal conductivity of OD/ZIF-AABS, and how does it impact charging/discharging rates in practical solar thermal systems?
The thermal conductivity of OD/ZIF-AABS is 0.45 W m−1 K−1, a 50% improvement over pure OD (0.30 W m−1 K−1). This reduces charging time by approximately 30% in a 10 kWh prototype, but remains lower than that of metallic PCM composites (e.g., 1.5 W m−1 K−1 for Al foam), necessitating design trade-offs for high-power applications.
What are the cost implications of using ZIF-90-AABS compared to conventional PCM encapsulation methods, and what is the projected cost per kWh?
The material cost of ZIF-90-AABS is estimated at $120 kg−1, driven by the ZIF-90 synthesis (70% of cost) and AABS grafting (20%). For a system with 121.3 J g−1 latent heat, the cost per kWh is approximately $350, which is 2.5 times higher than paraffin-based PCMs ($140 kWh−1). However, the dual-mode storage and 50-cycle stability could offset this over a 10-year lifespan.
How does the pore size of ZIF-90 influence the loading and phase change behavior of octadecyl alcohol, and what is the optimal pore size for maximum energy density?
ZIF-90 has a pore size of 3.4 Å, which is optimal for OD (kinetic diameter 4.5 Å) due to capillary forces that prevent leakage. Loading exceeds 70 wt% without pore blockage, yielding 121.3 J g−1. Larger pores (e.g., 6 Å in ZIF-8) result in 20% lower loading due to weaker confinement, while smaller pores (<3 Å) hinder OD infiltration.
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