Microwave-assisted hydrothermal annealing of binary Ni–Co oxy-hydroxides for asymmetric supercapacitors

• MAHA is promising in tuning the crystal phase/size of Ni–Co oxy-hydroxides at low temperatures in short time.
• The crystalline index quantifies the degree of phase transformation from Ni–Co hydroxides to NiCo2O4 crystals.
• The larger size of NiCo2O4 crystals is, the longer activation time is needed but higher reversibility is obtained.
• Importance of potential window for evaluating CS of battery-like materials is demonstrated.

Binary nickel–cobalt oxy-hydroxides (denoted as NCOH) with tunable crystal phases are prepared from an atomic mixture of Ni(OH)2 + Co(OH)2 in the 1:2 ratio (denoted as (Ni/2Co)(OH)2) through a microwave-assisted hydrothermal annealing (MAHA) method. The (Ni/2Co)(OH)2 powders are synthesized through a sol–gel process from an aqueous solution containing NiCl2·6H2O and CoCl2·6H2O. The MAHA process is demonstrated to be a novel and promising method for transforming/tuning the crystal phase and size of NCOH at relatively low annealing temperatures in short time. The MAHA temperature and time are varied to transform the crystalline phases from a hydroxide mixture to NiCo2O4 nanocrystals, which can be quantified by a crystalline index (denoted as CI) value obtained from X-ray diffraction (XRD) patterns. The higher content and larger size of NiCo2O4 nanocrystals are, the longer electrochemical activation time is needed. The rate of phase transformation can be enhanced by adding microwave adsorbents, graphene oxide (GO), to obtain a NiCo2O4/RGO composite with reduced crystal size (RGO: reduced GO). The variation in the crystalline index and the effect of the potential window on the specific capacitance of NCOH powders are systematically compared and discussed in this work.

We proposed the crystalline index obtained from X-ray diffraction (XRD) patterns to quantify the degree of phase transformation from Ni–Co hydroxides to NiCo2O4 nanocrystals. The specific capacitance of Ni–Co oxy-hydroxides reaches a maximum value when the CI value is around 0.30 which corresponds to NCOH150 (MAHA temperature and time respectively equal to 150 °C and 15 min). In addition, the total specific capacitance of a NiCo2O4/RGO composite is about 410 F g−1, corresponding to the specific capacitance of NiCo2O4 equal to 730 F g−1, revealing a high utilization of electroactive materials when tiny NiCo2O4 nanocrystals are evenly dispersed on the RGO nanosheets.Figure optionsDownload as PowerPoint slide