More from Less but Precise: Industry-relevant Pseudocapacitance by Atomically-precise Mass-loading MnO2 within Multifunctional MXene Aerogel

Abstract

Ever-increasing mass loading of transition metal oxides (TMOs) yields high pseudocapacitance in laboratory electrochemical capacitors. However, their performance based on whole electrode mass is still far from industry standards. Highly-promising solution based on loading TMOs into 3D porous electrodes causes a yet unresolved challenge to find ways to achieve ultimate energy storage by atomically precisely loading less active material. Inspired by single-atom catalysis, we propose a new “more from less but precise” concept of homogeneously dispersing a common MnO2 TMO via atomic sites to maximize atom redox reaction efficiency for industry-relevant pseudocapacitance. The concept is materialized by multifunctional MXene aerogel with super-hydrophilicity and surface functional groups, which provides 3D atomic nucleation sites to homogeneously load in-situ-formed, covalently-bonded MnO2 nanosheets. The gravimetric capacitance of MnO2 is largely enhanced, yielding superior pseudocapacitance of >400 F/g at > 5 mg/cm2 that is typically achieved at 10 times lower loadings. Outstanding electrode areal capacitance is achieved using 2–3 times less MnO2 mass, demonstrating industry-relevant pseudocapacitance almost twice higher than in state-of-the-art devices. MnO2/MXene//MXene asymmetric supercapacitor shows practically-high energy (~50.1 Wh/kg) and power (~10.0 kW/kg) densities, among the best MnO2 pseudocapacitors. Capacitive-mechanism-controlled redox reactions, rarely achievable in diffusion-controlled porous pseudocapacitive electrodes, are revealed.