Capturing Visible Light in Low-bandgap C4N Derived Responsive Bifunctional Air Electrodes for Solar Energy Conversion/Storage.

Abstract

Herein, we report facile synthesis and multiple inherent properties of low-bandgap mesoporous C 4 N particles and showcase its previously-unexplored use as universal responsive bifunctional oxygen catalysts for constructing visible-light-sensitive (VLS) rechargeable Zn-air battery (RZAB) and polymer-air battery (RPAB) towards solar energy conversion/storage. Compared to widely-studied g-C 3 N 4 , as-obtained C 4 N shows a smaller bandgap of 1.99 eV with larger photocurrent response and can function simultaneously as visible-light-harvesting antenna and bifunctional oxygen reduction/evolution (ORR/OER) catalyst, enabling effective photo-coupling to tune oxygen catalysis. In addition to good energy storage performance, the C 4 N-enabled VLS-RZAB displays a low charge voltage of 1.35 V under visible light - below the theoretical RZAB voltage of 1.65 V, corresponding to a high energy efficiency of 97.78 %. Pairing C 4 N cathode with polymer anode also endows an entirely-new all-polymer VLS-RPAB with light-boosted charge performance, implying the generality of our strategy. Combined theoretical and experimental studies unveil that the ORR and OER active sites in C 4 N are separate carbon sites near pyrazine-nitrogen atoms and photo-generated energetic holes can activate OER for improved reaction kinetics. This study not only highlights a rarely-explored carbon nitride polymer and its new property and application but also opens new opportunities to develop organic semiconductors for photo-coupled rechargeable batteries towards solar energy utilization.