Optoelectronics Meets Optoionics: Light Storing Carbon Nitrides and Beyond.

Known for decades, Liebig's carbon nitrides have evolved into a burgeoning class of macromolecular semiconductors over the past 10+ years, front and center of many efforts revolving around the discovery of resource‐efficient and high‐performance photocatalysts for solar fuel generation. The recent discovery of a new class of "ionic" 2D carbon nitrides—poly(heptazine imide) (PHI)—has given new momentum to this field, driven both by unconventional properties and the prospect of new applications at the intersection between solar energy conversion and electrochemical energy storage. In this essay, key concepts of the emerging field of optoionics are delineated and the "light storing" ability of PHI‐type carbon nitrides is rationalized by an intricate interplay between their optoelectronic and optoionic properties. Based on these insights, key characteristics and general principles for the de novo design of optoionic materials across the periodic table are derived, opening up new research avenues such as "dark photocatalysis", direct solar batteries, light‐driven autonomous systems, and photomemristive devices. [ABSTRACT FROM AUTHOR]