This paper, written by researchers from Sichuan University and others, discusses High-stable a-phase NiCo double hydroxide microspheres via microwave synthesis for supercapacitor electrode materials. The paper is published in an important journal < Chemical Engineering Journal >. IF：6.735.

In recent years, the research work of microwave chemical instrument used in the synthesis of materials has become a hot direction of scientific research, which has been paid great attention to by many scholars!

Fast and low-cost fabrication of high performance electrode materials is of great importance for the application of supercapacitors. Herein, in our research, three-dimensional (3D) flower-like NiCo double hydroxide (NiCo DH) microsphere was successfully synthesized via a rapid, inexpensive and energysaving microwave route without using any template or surfactant under atmospheric pressure. The asobtained NiCo DH microsphere endowed with a-phase structure with CNO_ ions intercalation in the interlayers (7.3 Å) was composed of ultrathin nanosheets with thickness less than 10 nm. Electrochemical test revealed the NiCo DH electrode showed a high specific capacitance of 1120 F g_1 at 1 A g_1 and remained 996 F g_1 at 10 A g_1 (88.9% retention). Moreover, after 2000 cycles, the capacitance reached 122.5% of its initial value at 10 A g_1 and still retained 93.8% at 30 A g_1 after another 1000 cycles, showing superb stability compared with reported a-phase hydroxides. The admirable stability could be attributed to the synergistic effect between Ni and Co elements, the ion exchange phenomenon between CNO_ and OH_ ions in the interlayer of NiCo DH during cycling test, and the coherent 3D superstructure. Besides, the asymmetric supercapacitor, with NiCo DH as positive electrode and activated carbon from coal as negative electrode, delivered a superior energy density of 42.5 Wh kg_1. Consequently, the pleasant synthesis procedure and excellent integrated performance of NiCo DH enable it to be a promising electrode material for the energy storage devices.

In summary, we have successfully fabricated the 3D flower-like NiCo DH microsphere superstructure via a simple, rapid and lowprice microwave assisted route. The microsphere composed of ultrathin nanosheets followed the oriented attachment growth process under microwave heating. The as-synthesized a-phase NiCo DH exhibited a high specific capacitance (1120 F g_1 at 1 A g_1), excellent rate capability (88.9% retention at 10 A g_1), and remarkable stability (122.5% of its initial value at 10 A g_1 after 2000 cycles and 93.8% retention at 30 A g_1 after another 1000 cycles). The synergistic effect between Ni and Co dramatically enhanced the stability of NiCo DH compared with unitary Ni (OH)2 or Co(OH)2. The coherent superstructure of NiCo DH and the ion-exchange phenomenon contributed to the increase of specific capacitance in KOH. Additionally, the as-fabricated ASC delivered a high energy density and good stability.

Typically, for the synthesis of NiCo DH sample, NiCl2_6H2O (1.5 mmol), CoCl2_6H2O (1.5 mmol), and urea (60 mmol) as precipitant were dissolved by deionized water (60 mL) in a 100 mL threenecked flask and stirred for 20 min to form transparent precursor solution. The resulting solution was placed in the microwave oven (XH-MC-1, Xianghu Co., Beijing) and heated under the microwave irradiation for various time (8–120 min) at 100 _C with the microwave power of 300W (it took about 2 min to reach 100 _C from room temperature). After reaction, the mixture aged for 1 h and the resulting precipitation was collected and washed by deionized water thoroughly. The as-obtained product was dried in a vacuum at 60 _C for 24 h to get the NiCo DH powder.