This work reports for the first time an in situ synthesis of a NiAl-layered double hydroxide (NiAl-LDH) with a tunable interlayer spacing using a confined impinging jet microreactor (CIJ). It is found that CIJ allows simultaneous guest (water molecules and carbonate) intercalation and in situ growth of NiAl-LDH by imposing a proper micromixing scale in a space-confined mixing chamber. The final interlayer spacing of NiAl-LDH can be easily regulated from 0.9 to 3.6 nm by tuning the Reynolds number flow range from 4.8 X 10(3) to 6.7 X 10(2). The supercapacitor has been chosen as a model reaction to investigate the electrochemical activity of NiAl-LDHs. Results demonstrate that a higher interlayer space increases the electrochemical activity and enhances supercapacitor performances due to improvement in space accessibility of NiAl-LDHs during the faradaic redox reaction. NiAl-LDH with an interlayer space up to 3.6 nm presents fairly good performance as a supercapacitor electrode material in terms of specific capacitance (1285.2 F.g(-1) at 1 Ag-1) and stability (capacitance retention rates over 80% after 5000 cycles). This work develops a rapid and continuous flow methodology for a one-pot, in situ formation of NiAl-LDH with the controlled interlayer spacing via microreactor technology.