Flexible Asymmetric Supercapacitors Based on Nitrogen‐Doped Graphene Hydrogels with Embedded Nickel Hydroxide Nanoplates

Abstract

To push the energy density limit of supercapacitors (SCs), new electrode materials with hierarchical nano-micron pore architectures are strongly desired. Graphene hydrogels that consist of 3 D porous frameworks have received particular attention but their capacitance is limited by electrical double layer capacitance. In this work, we report the rational design and fabrication of a composite hydrogel of N-doped graphene (NG) that contains embedded Ni(OH) nanoplates that is cut conveniently into films to serve as positive electrodes for flexible asymmetric solid-state SCs with NG hydrogel films as negative electrodes. The use of high-power ultrasound leads to hierarchically porous micron-scale sheets that consist of a highly interconnected 3 D NG network in which Ni(OH) nanoplates are well dispersed, which avoids the stacking of NG, Ni(OH) , and their composites. The optimal SC device benefits from the compositional features and 3 D electrode architecture and has a high specific areal capacitance of 255 mF cm at 1.0 mA cm and a very stable, high output cell voltage of 1.45 V, which leads to an energy density of 80 μW h cm even at a high power of 944 μW cm , considerably higher than that reported for similar devices. The devices exhibit a high rate capability and only 8 % capacitance loss over 10 000 charging cycles as well as excellent flexibility with no clear performance degradation under strong bending.

Graphene‐Based Planar Microsupercapacitors: Recent Advances and Future Challenges

“…The capacitance of graphene‐based supercapacitors can be improved via heteroatom doping, e.g., nitrogen, boron, phosphor and sulfur . However, the preparation of homogeneously doped graphene films for MSCs over a large scale is challenging.…”

Comment on “Flexible Asymmetric Supercapacitors Based on Nitrogen‐Doped Graphene Hydrogels with Embedded Nickel Hydroxide Nanoplates”

“…[1],h owever,g oes beyondt his misleading practice. This is always misleading because ac urvaturer adius of zero is strictly impossible.…”

“…[1],t he larger total voltage drop is instead enteredi nto E = C (DV) 2 /2, which considerably increases E through its explicit quadratic dependence on DV. It is therefore incorrect to calculate the energy density (E)f or ad ifferent region.I nt he case of Ref.…”

“…[1] claims that the capacitor can be operated at ah igh operating output voltage of almost1 .5 V. As tated operating voltagei mplies that the device can be charged to this voltage to reach the stated energy density.Thus, the reported C is aparameter that involves only the smaller voltage drop of 1.1 V, and it is not an obtainable capacitance if operating in the stated range. [1] claims that the capacitor can be operated at ah igh operating output voltage of almost1 .5 V. As tated operating voltagei mplies that the device can be charged to this voltage to reach the stated energy density.Thus, the reported C is aparameter that involves only the smaller voltage drop of 1.1 V, and it is not an obtainable capacitance if operating in the stated range.…”

Two‐Step Deposition/Reduction Synthesis of Porous Lamellar β‐Ni(OH)2/Reduced Graphene Oxide Composites with Large Capacitance for Supercapacitors

“…Xie et al. fabricated a hydrogel of N‐doped graphene that contained embedded Ni(OH) 2 nanoplates . The asymmetric capacitor made from the composite hydrogels achieved a high area specific capacitance of 254.5 mF cm −2 at 1.0 mA cm −2 , and cycling stability with 92 % capacitance retention over 10000 charging cycles.…”