Synthesis of Co 3 O 4 /Co 3 V 2 O 8 /Ni nanocomposites. The Ni foam substrate covered by Co 3 O 4 nanowires was then used as the “sub- strate”to guide the growth of Co 3 V 2 O 8 nanowires in the hy- drothermal reaction. Next step was to dissolve 1.6 g Na 3 VO 4 ·12H 2 O and 1.55 g CoCl 2 ·6H 2 O in 160 mL deionized water, then took the homogeneous solution into a 200 mL Teflon-lined stainless steel autoclave with insertion of the obtained Co 3 O 4 /Ni samples at 150 °C for 6 h growth. The formation mechanism of Co 3 V 2 O 8 was: 2Na 3 VO 4 +3CoCl 2 → Co 3 V 2 O 8 +6NaCl. After that, the sam- ples were dried at 200 °C for 12 h under vacuum to obtain the Co 3 O 4 /Co 3 V 2 O 8 /Ni nanocomposites.