Aqueous Ammonium‐Ion Supercapacitors with Unprecedented Energy Density and Stability Enabled by Oxygen Vacancy‐Enriched MoO₃@C

Abstract

The use of non-metal charge carriers such as ammonium (NH 4 + ) in electrochemical energy storage devices offers advantages in terms of weight, element abundance, and compatibility with aqueous electrolytes. However, the development of suitable electrodes for such carriers lags behind other technologies. Herein, we present a high-performance anode material for ammonium-ion supercapacitors based on a MoO 3 /carbon (MoO 3 @C) composite. The NH 4 + storage performance of such composites and their practical application prospects are evaluated both in a three-electrode configuration and as symmetric supercapacitors. The optimized material reaches an unprecedented specific capacitance of 473 F·g −1 (158 mAh·g −1 ; 1592 mF·cm −2 ) at a current density of 1 A·g −1 , and 92.7% capacitance retention after 5000 cycles in a three-electrode set-up. This outstanding performance is related to the presence of oxygen vacancies that enhance the composites' ionic/ electronic transportation and electrochemical reaction site, while at the same time facilitating the formation of hydrogen bonds between NH 4 + and the host material. Using the optimized composite, symmetric supercapacitors based on an (NH 4 ) 2 SO 4 gel electrolyte are fabricated and demonstrated to provide unmatched energy densities above 78 Wh·kg −1 at a power density of 929 W·kg −1 . Besides, such devices are characterized by extraordinary capacitance retention of 97.6% after 10,000 cycles.

Critical Advances of Aqueous Rechargeable Ammonium Ion Batteries

“…As a result, the oxygen‐deficient MoO 3 showed a specific capacitance of 208.8 F g −1 even at 20 A g −1 and a capacity retention rate of 92.7% after 5000 cycles. [ 56 ]…”

“…The electrochemical window of the electrolyte needs to be as wide and stable as possible. (NH 4 ) 2 SO 4 , [19,23,30,34,38,50,56,64,67] NH 4 Cl, [18,33] CH 3 COONH 4 , [37,43,54,59] NH 4 NO 3 , [13,35] etc. are used as electrolytes for aqueous ammonium ion (NH 4 þ ) batteries.…”

MoS2@Polyaniline for Aqueous Ammonium‐Ion Supercapacitors

“…Two issues are worth considering when designing an NH 4 + ‐insertion material: 1) It should form non‐ionic chemical bonds (H‐bonds) with NH 4 + to ensure stable ion insertion; and 2) The interlayer spacing should be large enough for smooth NH 4 + travel. In our previous work, [ 14 ] we demonstrated the role of oxygen vacancies in enhancing the conductivity of a MoO 3 ‐based composite and the formation of hydrogen bonds between NH 4 + and the host material. However, the limitations of metal oxides, including a mild rate capability, intrinsically poor electrical conductivity, and sluggish redox reaction kinetics, constrain the energy‐storage performance of the produced devices.…”

Long‐Cycle‐Life Sodium‐Ion Battery Fabrication via a Unique Chemical Bonding Interface Mechanism

“…[11][12][13] Many methods for introducing vacancies in metal-oxide electrode materials exist, including hightemperature vapor-phase reduction, solvothermal methods, and etching, which have been successfully applied to chalcogenide oxides, TiO 2, and MoO 3 , and have significantly improved the electrode ion-diffusion coefficient and electrochemical reaction kinetics. [13,14] Heteroelement doping strategies can fundamentally modulate the electronic structure and change the polarity of a material, providing new properties to the material and enhancing its electrochemical performance. Currently, various nonmetallic heteroelements, including N, O, S, B, and P, can be doped into carbon materials.…”