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.

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Ammonium‐ion aqueous supercapacitors are raising notable attention owing to their cost, safety, and environmental advantages, but the development of optimized electrode materials for ammonium‐ion storage still lacks behind expectations. To overcome current challenges, here, a sulfide‐based composite electrode based on MoS2 and polyaniline (MoS2@PANI) is proposed as an ammonium‐ion host. The optimized composite possesses specific capacitances above 450 F g−1 at 1 A g−1, and 86.3% capacitance retention after 5000 cycles in a three‐electrode configuration. PANI not only contributes to the electrochemical performance but also plays a key role in defining the final MoS2 architecture. Symmetric supercapacitors assembled with such electrodes display energy densities above 60 Wh kg−1 at a power density of 725 W kg−1. Compared with Li+ and K+ ions, the surface capacitive contribution in NH4+‐based devices is lower at every scan rate, which points to an effective generation/breaking of H‐bonds as the mechanism controlling the rate of NH4+ insertion/de‐insertion. This result is supported by density functional theory calculations, which also show that sulfur vacancies effectively enhance the NH4+ adsorption energy and improve the electrical conductivity of the whole composite. Overall, this work demonstrates the great potential of composite engineering in optimizing the performance of ammonium‐ion insertion electrodes.