Solution synthesis and activation of spinel CuAl2O4 film for solar water-splitting

•Facile solution synthesis of spinel CuAl2O4 (CAO) film with controlled porosity for the first time.•Development of an effective solution activation method to incorporate Li+ into the CAO.•Simultaneous improvement of light absorption, charge transport and transfer properties via the Li+ activation.•Achieving nine times higher photocurrent density than that of pristine CAO under simulated sunlight illumination.•Demonstration of a record photocurrent density of −5.1 mA/cm2 by coupling CuO underlayer.