Abstract
AbstractKnown 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.
Highlights
Known for decades, Liebig’s carbon nitrides have evolved into a burgeoning far precluded the unambiguous realizaclass 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
Photo-excited electrons diffuse towards the surface, where they react with cations from the electrolyte, driving their intercalation into the host electrode
This intriguing property has been cast in the concept of “dark photocatalysis”, which we have introduced for the hydrogen evolution (HE) reaction and which has since been used for other photocatalytic and photosynthetic reactions, in photoredox catalysis (Figure 3a).[8b,24] The benefit of this concept lies in the fact that the production of solar fuels can be temporally decoupled from the availability of fluctuating solar energy, effectively buffering the intermittency of solar irradiation and with it, the generation of solar fuels
Summary
Tailored light-matter interactions are the basis for most energyconverting processes occurring in nature, enabling life as it has evolved on our planet. The process termed “photointercalation” (PI) by Tributsch captures the essentials of optoionic photoelectrochemical systems.[16] We will exemplify this process based on the photo-assisted intercalation of cationic species (H+, Li+, Cu2+, etc.) into p-type transition metal dichalcogenides such as MSe2 (M = Zr, Hf),[17] or of Cu+ into Cu6−xPS5I.[18] Here, photo-excited electrons diffuse towards the surface, where they react with cations from the electrolyte, driving their intercalation into the host electrode This electron localization process is driven by the formation of an intercalation band within the material’s band gap, which effectively lowers the energy of the excited charge carriers. We will show that the first steps in this direction have already been made and highlight potential development areas
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