Optoelectronics Meets Optoionics: Light Storing Carbon Nitrides and Beyond

Abstract

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.

Sol−Gel Processing of Water‐Soluble Carbon Nitride Enables High‐Performance Photoanodes**

“…Finally, in contrast to conventional metal oxide‐based photoelectrodes at which high product selectivities are typically difficult to achieve, the excellent selectivities (∼100 %) of photoelectrocatalytic transformations demonstrated for this novel class of PCN photoanodes derived from water‐soluble PCN constitute a particularly remarkable feature with a great promise for future practical applications [7,38] . The robust binder‐free films derived from sol−gel processing of water‐soluble PCN thus establish a new paradigm for high‐performance ‘soft‐matter’ photoelectrocatalytic systems and pave the way for further applications in which high‐quality PCN films are required, such as electrochemical sensors [39] or (photo)batteries [36b] …”

Light/Electricity Energy Conversion and Storage for a Hierarchical Porous In2S3@CNT/SS Cathode towards a Flexible Li‐CO2 Battery

“…Energy shortage and environmental pollution arising from the large use of fossil energy directly affect sustainable economic and social development . Thus, for the sake of reducing the CO 2 emissions and the dependence on fossil fuels, the development of new energy storage systems and renewable energy has served as a critical technological driving force . Recently, rechargeable Li‐CO 2 batteries have gained intense attention because of the conversation ability of CO 2 to sustainable electricity, especially in CO 2 ‐rich environments, such as Mars exploration, in which the atmosphere contains 96 % CO 2 .…”

Conductivity mechanism in ionic 2D carbon nitrides: from hydrated ion motion to enhanced photocatalysis

“…[70] Diffusion lengths of 0.10 (for K-and Li-PHI) to 0.35 Å (for Na-PHI) were calculated, which will be significantly higher under aqueous conditions, but presumably with a preserved relative trend (Figure 3c). This however means that within the measured timescales, only short range motion is enabled, which is likely sufficient to reach and stabilize an electron residing nearby (polaronic effect), [65] and hence is most efficient for Na-PHI with the highest ion conductivity. A diffusion distance corresponding to the layer separation (3.2 Å) would require for example roughly 22 ns for Na-PHI, which is too long to be detected by PL, since the radiative decay signal being used as measure for the charge recombination process is shorter lived.…”

“…This generated charge gradient was described as the driving force for the alkali ions in solution to follow the electrons. [64][65][66] In photocatalysis experiments with suspended particles of similar size (500-1000 nm) we hypothesize that the mobile cations in the pores of PHI act to stabilize the trapped electrons through Coulombic interactions, while the pore water assists in dielectric screening and acts as a vehicle to facilitate movement of the cations through the pores additionally. As outlined above, the ionic conductivity is enhanced in humid environment and is therefore assumed to be the highest in an aqueous environment, approximating 100 % RH, as applicable during photocatalysis in aqueous suspension.…”