One-step synthesis of honeycomb-like Ni/Mn-PMo12 ultra-thin nanosheets for high-performance asymmetric supercapacitors

Abstract

Polyoxometalates (POMs) are being increasingly applied in the field of electrochemistry owing to their excellent cluster-type open framework and ultra-high proton mobility. Honeycomb-like ultra-thin nanosheets of NiMn phosphomolybdate (Ni/Mn-PMo12) are successfully designed and synthesized on nickel foam via a one-step hydrothermal method. The Ni/Mn-PMo12 nanosheets at different ratios have similar morphology, with a three-dimensional structure consisting of ultrathin nanosheets that provides more electroactive sites and a shorter pathway for electron transfer and electrolyte diffusion. The optimized electrochemical performance of Ni/Mn-PMo12 is achieved using a Ni:Mn ratio of 2:1. The Ni/Mn-PMo12 nanosheets (Ni:Mn = 2:1) exhibit a high specific capacitance of 1950.5 F g−1 (975.3C g−1) at 1 A g−1, excellent property retention of 75.5% at 20 A g−1, and good cycle stability with 96.9% retention after 2000 cycles. The asymmetric supercapacitor assembled with Ni/Mn-PMo12 as the positive electrode and active carbon (AC) as the negative electrode shows a high energy density of 28.3 Wh kg−1 at a power density of 375 W kg−1, excellent rate performance, and good cycle performance (90.3% at a high current density of 2 A g−1 after 5000 cycles), suggesting that Ni/Mn-PMo12 nanosheets are highly promising for practical application in energy storage devices.