α-Co(OH)2 with interlayer anions for photocatalytic/photoelectrocatalytic water splitting

•Two-step preparation to obtain Co(OH)2 with in situ intercalation of sulphate ions.•Sulphate ion intercalation modulates the band structure, light absorption, and electrical conduction of Co(OH)2 to enhance photocatalytic/photo-electrocatalytic water splitting.•The α-Co(OH)2 photoanode shows a photocurrent density of 4.36 mA·cm−2 at 1.23 VRHE.•Density functional theory calculations demonstrate the formation of Co active sites and the mechanism of water splitting.