Abstract
Printed flexible energy storage devices such as micro-supercapacitors require high electrochemical performance for practical applications. Here, we report a high volumetric energy density of up to 1.13 × 10−3 Wh cm−3 at a power density of 0.11 W cm−3 by inkjet printing of Fe-doped MnO2 nanosheets inks as active materials on polyimide substrates. The enhancement results from atomic-level substitutional doping of 3d metal ions (Co, Fe, Ni) in sub-nanometer thick 2D MnO2 nanosheets. Substitutional doping introduces new electronic states near the Fermi level, thereby enhancing the electronic conductivity and contributing to the formation of redox-active 3d surface states. Fe-doped MnO2 showed the best performance in terms of specific areal and volumetric capacitance. Our finding suggests that the rational doping at atomic scale shows great promise for achieving high energy and power density flexible energy storage devices.
Highlights
Two-dimensional (2D) materials have attracted great attention for supercapacitors (SCs) because of their unique physical and chemical properties induced by the dimensional reduction [1,2,3]
The doping concentrations were estimated by energy dispersive X-ray spectroscopy (EDS) and were found to be about 5.3%, 5.3% and 4.7% for Fe, Co and Ni-doped MnO2 nanosheets, respectively (Fig. S9, Supporting Informa tion), in accordance with the nominal element compositions targeted during synthesis
The higher specific capacitance of the Fe-doped MnO2 electrode compared with Co and Ni-doped MnO2 is confirmed by the galvanostatic charge/discharge (GCD) curves between 0 and 1 V at varying current densities (Fig. 2b–c, Fig. S11, Supporting Information)
Summary
Two-dimensional (2D) materials have attracted great attention for supercapacitors (SCs) because of their unique physical and chemical properties induced by the dimensional reduction [1,2,3]. The performance of MnO2 nanosheets in SCs is limited by poor electronic conductivity. One of the strategies to improve the electronic conductivity and electrochemical performance of MnO2 electrodes is to combine it with highly conductive materials such as graphene, carbon nanotubes or carbon fibers [14]. Versatile fabrication techniques have been utilized to directly deposit materials on different substrates to fabricate interdigitated electrode patterns for MSCs, such as laser scribing, electrochemical deposition, conversion reaction and inkjet printing [25]. The excellent performance could be accomplished by atomic-level substitu tional doping of 3d metal ions (Co, Fe, Ni) into 2D MnO2 nanosheets by a facile bottom-up method, which introduced new electronic states near the Fermi level, thereby enhancing the electronic conductivity within the nanosheets and contributing to the formation of redox-active 3d surface states. The influence of substitutional doping on band structure and the excellent performance of Fe doping in particular was explained using first principles calculations, which demonstrates that substitutional doping can largely improve the electrochemical performance of 2D oxide materials
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