Self-templated and triethanolamine-induced hollow MnO2 nanoboxes with abundant active Mn3+ and oxygen vacancies for high-performance Na-ion pseudocapacitors

The electrochemical performance of MnO2 is limited by its low surface area, sluggish kinetics, and poor stability. Herein, hollow MnO2 nanoboxes (H-MnO2-TEA) are successfully prepared using Prussian blue analogs (KMn[FeCN6], Mn-PBA) via a facile triethanolamine-induced strategy to tackle the abovementioned hurdles. Mn-PBA not only acts as a self-sacrificial template, but also triggers the “self-redox reaction” to in-situ form MnO2, in which the dissociated [FeCN6]3- oxidates Mn2+ in the alkalic medium. Besides, triethanolamine is employed as an intermediate modulator to induce the formation of hollow MnO2 and adjust the coordination environment of H-MnO2-TEA. Experimental results and DFT calculations demonstrate that abundant active Mn3+ and oxygen vacancies in H-MnO2-TEA are beneficial to providing more active sites, improving electrical conductivity, and boosting Na+ diffusion kinetics. Based on the improved structural and electrical properties, H-MnO2-TEA delivers a high specific capacitance of 316.8 F g−1 at 0.5 A g−1, superior rate performance, and excellent capacitive retention of 96.1 % after 10,000 cycles. This work provides an efficient way to construct hollow metal oxides for high-performance energy storage devices along with their underlying mechanism.