Ambient energy harvesting has great potential to contribute to sustainable development and address growing environmental challenges. Converting waste energy from energy-intensive processes and systems (e.g. combustion engines and furnaces) is crucial to reducing their environmental impact and achieving net-zero emissions. Compact energy harvesters will also be key to powering the exponentially growing smart devices ecosystem that is part of the Internet of Things, thus enabling futuristic applications that can improve our quality of life (e.g. smart homes, smart cities, smart manufacturing, and smart healthcare). To achieve these goals, innovative materials are needed to efficiently convert ambient energy into electricity through various physical mechanisms, such as the photovoltaic effect, thermoelectricity, piezoelectricity, triboelectricity, and radiofrequency wireless power transfer. By bringing together the perspectives of experts in various types of energy harvesting materials, this Roadmap provides extensive insights into recent advances and present challenges in the field. Additionally, the Roadmap analyses the key performance metrics of these technologies in relation to their ultimate energy conversion limits. Building on these insights, the Roadmap outlines promising directions for future research to fully harness the potential of energy harvesting materials for green energy anytime, anywhere.

The ratio of the Zeeman splitting to the cyclotron energy ($M=\mathrm{\Delta }{E}_Z/\hslash {\omega }_c$) for holelike carriers in bismuth has been quantified with great precision by many experiments performed during the past five decades. It exceeds 2 when the magnetic field is along the trigonal axis and vanishes in the perpendicular configuration. Theoretically, however, $M$ is expected to be isotropic and equal to unity in a two-band Dirac model. We argue that a solution to this half-a-century-old puzzle can be found by extending the $\mathit{k}·\mathit{p}$ theory to multiple bands. Our model not only gives a quantitative account of the magnitude and anisotropy of $M$ for holelike carriers in bismuth, but also explains its contrasting evolution with antimony doping and pressure, both probed by new experiments reported here. The present results have important implications for the magnitude and anisotropy of $M$ in other systems with strong spin-orbit coupling.

Polycrystalline ${\text{Mo}}_9$ cluster chalcogenides ${\text{Ag}}_x{\text{Mo}}_9{\text{Se}}_{11}$ $\left(3.4\le x\le 3.9\right)$ have been prepared by powder metallurgy techniques, sintered by spark plasma sintering and characterized by x-ray diffraction. Their thermoelectric properties (electrical resistivity, thermopower, thermal conductivity) have been determined in the 300–800 K temperature range. The ${\text{Ag}}_x{\text{Mo}}_9{\text{Se}}_{11}$ compounds show p-type conduction characteristics. The outstanding low lattice thermal conductivities give rise to a rather high value of the dimensionless thermoelectric figure of merit $ZT$ of $\sim 0.65$ at 800 K for $x=3.8–3.9$ , making this family of materials particularly promising for thermoelectric power generation applications.

An innovative design of a thermoelectric micro-generator with integrated wavy-shaped Bi₂Te₃-based materials yields the highest output power achieved so far for an in-plane device.