To obtain a supercapacitor with a remarkable specific capacitance and rate performance, a cogent design and synthesis of the electrode material containing abundant active sites is necessary. In present work, a scalable strategy is developed for preparing 2D‐on‐2D nanostructures for high‐energy solid‐state asymmetric supercapacitors (ASCs). The self‐assembled vertically aligned microsheet‐structured 2D nickel pyrophosphate (Ni2P2O7) is decorated with amorphous bimetallic nickel cobalt hydroxide (NiCo‐OH) to form a 2D‐on‐2D nanostructure arrays electrode. The resulting Ni2P2O7/NiCo‐OH 2D‐on‐2D array electrode exhibits peak specific capacity of 281 mA hg−1 (4.3 F cm−2), excellent rate capacity, and cycling stability over 10 000 charge–discharge cycles in the positive potential range. The excellent electrochemical features can be attributed to the high electrical conductivity and 2D layered structure of Ni2P2O7 along with the Faradic capacitance of the amorphous NiCo‐OH nanosheets. The constructed Ni2P2O7/NiCo‐OH//activated carbon based solid‐state ASC cell operates in a high voltage window of 1.8 V with an energy density of 78 Wh kg−1 (1.065 mWh cm−3) and extraordinary cyclic stability over 10 000 charge–discharge cycles with excellent energy efficiency (75%–80%) over all current densities. The excellent electrochemical performance of the prepared electrode and solid‐state ASC device offers a favorable and scalable pathway for developing advanced electrodes.

traditional battery systems. [10,11] Practically, lower energy density bounds the applicability of supercapacitors over battery systems, leaving more room in developing high energy density supercapacitors without having to compromise the power competence. [12-14] A well-known approach offering such outstanding energy density is via fabrication of asymmetric supercapacitor device that focusses on the positive materials excellency exhibiting high specific capacitance and providing a broad potential window when combined with a double-layer type negative electrode material. [15-17] Co 3 O 4 , a pseudocapacitive metal oxide, belonging to the family of spinel is a promising positive active material in electrochemical energy storage devices owing to its high specific capacitance (3560 F g −1) theoretically with being earthabundant, cost-effective, and also environment friendly. [18-20] Pseudocapacitors follow faradaic redox electrochemical reactions on the material's surface through continuous intercalation/deintercalation of electrons or ions which makes the surface vulnerable to destruction resulting in lower efficiency of the electroactive material affecting the electrochemical cycles. [21-23] Therefore, much effort has been put forward to increase the efficiency and stability of the Co 3 O 4 structures through nanostructured, [18] heterostructured, [24,25] and core-shell type structures [26] to reduce the structural deterioration from Co 3 O 4 and increase the overall specific capacitances. For instance, a 3D hierarchical structure of CoWO 4 /Co 3 O 4 was developed that exhibited significantly high specific capacitance of 1728 F g −1 at a current density of 1 A g −1 retaining about 85.9% specific capacitance after 5000 cycles. [27] Paliwal and Meher design a heterostructure of Co 3 O 4 /NiCo 2 O 4 perforated nanosheets that delivers specific capacitance of 1767 F g −1 at a current density of 0.5 A g −1 maintaining 552 F g −1 capacitance at high current density 16 A g −1. Additionally, the asymmetric device composed of Co 3 O 4 /NiCo 2 O 4 ||N-rGO retains 93.8% areal capacitance after 10 000 operating cycles. [28] An excellent core-shell type CoO@Co 3 O 4 nanocrystals were grown solvothermally which delivered 3377 F g −1 specific capacitance at current density 2 A g −1 and the capacity retention was about 58.6% after 4000 charge-discharge cycles. [29] Lu et al. reported Co 3 O 4 /CoS core-shell nanosheets grown over Ni-foam by room temperature sulfurization process. The structure showed an improved specific capacitance as high as 1658 F g −1 at 1 A g −1 Designing of multicomponent transition metal oxide system through the employment of advanced atomic layer deposition (ALD) technique over nanostructures obtained from wet chemical process is a novel approach to construct rational supercapacitor electrodes. Following the strategy, core-shell type NiO/Co 3 O 4 nanocone array structures are architectured over Ni-foam (NF) substrate. The high-aspect-ratio Co 3 O 4 nanocones are hydrothermally grown over NF following the p...

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The large‐scale application of supercapacitors (SCs) for portable electronics is restricted by low energy density and cycling stability. To alleviate the limitations, a unique interface engineering strategy is suggested through atomic layer deposition (ALD) and nitrogen plasma. First, commercial carbon cloth (CC) is treated with nitrogen plasma and later inorganic NiCo2O4 (NCO)/NiO core–shell nanowire arrays are deposited on nitrogen plasma–treated CC (NCC) to fabricate the ultrahigh stable SC. An ultrathin layer of NiO deposited on the NCO nanowire arrays via conformal ALD plays a vital role in stabilizing the NCO nanowires for thousands of electrochemical cycles. The optimized NCC/NCO/NiO core–shell electrode exhibits a high specific capacitance of 2439 F g−1 with a remarkable cycling stability (94.2% over 20 000 cycles). Benefiting from these integrated merits, the foldable solid‐state SCs are fabricated with excellent NCC/NCO/NiO core–shell nanowire array electrodes. The fabricated SC device delivers a high energy density of 72.32 Wh kg−1 at a specific capacitance of 578 F g−1, with ultrasmall capacitance decline rate of 0.0003% per cycle over 10 000 charge–discharge cycles. Overall, this strategy offers a new avenue for developing a new‐generation high‐energy, ultrahigh stable supercapacitor for real‐life applications.

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