Ammonium ion intercalation and oxygen-rich vacancies in birnessite-type MnO2 for supercapacitors and oxygen evolution applications

Defect engineering is an effective strategy to improve the electrochemical and electrocatalytic properties of transition metal oxide-based electrode materials. In this work, NH4+ ion intercalated MnO2 nanoflowers (C-A-MnO2) were prepared from hydrothermally synthesized manganese dioxide (MnO2) using a simple temperature-controlled method, in which oxygen vacancies on/inside the C-A-MnO2 nanoflowers were generated by the reduction of Mn4+ species by NH4+ ions. The electrical conductivity and electrochemical properties were improved by modulating the content of oxygen vacancies. In addition, the intercalation of NH4+ ions further enlarged the layer spacing of MnO2, obtaining a larger specific surface area, exposing more active sites, and shortening the electron/ion transport paths. The specific capacitance of the 1 M-A-MnO2 samples exhibited a significant enhancement (from 182.2 F g−1 to 252.2 F g−1 at a current density of 0.5 A g−1). An ASC device (1 M-A-MnO2//La-MoO3/GQD) with an operating voltage of 1.9 V was assembled. The energy density of the device was 66.66 W h kg−1 at a power density of 475 W kg−1. 1 M-A-MnO2 also exhibited excellent cycling stability with 94.3% capacitance retention after 8000 cycles. Meanwhile, it exhibited a low overpotential (361 mV at 10 mA cm−2) and Tafel slope (61 mV dec−1) as the oxygen evolution reaction (OER) electrocatalyst. Thus, this work provides valuable insights for rational intercalation of NH4+ ions for controlled synthesis of supercapacitor electrode materials and OER catalysts.