From Metal Nitrides to High-Entropy Nitrides: Advances in Spectrally Selective Absorber Coatings

Abstract

Spectrally selective absorber coatings (SSACs) are critical for efficient photothermal conversion in solar-thermal systems. Traditional metal nitrides have long served as cornerstone materials for these coatings due to their tunable optical properties and high thermal stability, which yield excellent solar absorptance and low infrared emittance. However, as solar-thermal applications push toward more extreme operating conditions, conventional nitrides are reaching inherent limits, particularly in achieving broadband absorption and resisting high-temperature oxidation. In recent years, high-entropy nitrides (HENs) have emerged to address these challenges by incorporating multiple metal cations into a single-phase nitride. This multi-element design affords unprecedented control over optical properties and significantly enhances thermal stability and corrosion resistance. As a result, HEN coatings exhibit broadband solar absorption and exceptional high-temperature durability beyond the reach of traditional binary nitrides. This review charts the progression from classical nitrides to advanced HENs in SSACs. We discuss fundamental photothermal conversion mechanisms, compare fabrication techniques, and highlight performance breakthroughs that illustrate how entropy-engineered nitrides overcome the bottlenecks of conventional systems. Finally, we outline remaining challenges and future directions, offering a roadmap for the design of next-generation absorber coatings to meet the stringent demands of advanced solar-thermal technologies.