High-performance ternary alkali nitrides for renewable energy applications

Rapid decline in fossil fuel energy necessitates the immediate need for renewable energy resources. Here, we report a previously unexplored class of nitrides AMN(2) (A=Na, K, Rb, Cs, M=V, Nb, Ta) keeping renewable energy applications in mind. Using a detailed structure and stability analysis using first principles simulation, we discovered twelve such compounds (few of which were already synthesized before), which are chemically, mechanically, and dynamically stable. These twelve compounds were then evaluated for their suitability for three renewable energy applications: (i) photovoltaics, (ii) water splitting, and (iii) thermoelectrics. Careful analysis of electronic structure reveals high optical transition strength resulting in a sharp rise in absorption. This in turn yields high short circuit current and hence excellent solar efficiency for a few compounds namely CsVN2 and RbVN2. Along with excellent absorption quality, some compounds show favorable band edge positions compared to water redox levels and hence are promising as photoelectrodes in photo(electro)chemical water splitting devices. Mixture of flat and dispersive bands in the band structure yields both high Seebeck and electrical conductivity, thus an excellent power factor for six compounds. Simulated lattice thermal conductivity shows moderate to ultralow values and thus the possibility of achieving a high thermoelectric figure of merit (ZT), even at lower temperatures. From the experimental perspective, we discuss the possible challenges that may arise while utilizing these compounds for the desired applications and suggest possible pathways to overcome them. We believe such theoretical prediction of promising materials is extremely useful for new materials discovery and anticipate rapid response from the experimental community.