A novel mixed metal oxide of CuNiO_x has been successfully obtained via electrosynthesis and thermal conversion of a Hofmann-type MOF Cu(pz)₂[Ni(CN)₄] (pz = pyrazine). CuNiO_x has demonstrated a significant enhancement in light absorption within a visible light range, leading to an impressive increase in photocurrent density and photocatalytic activity during light irradiation compared to CuO. The exceptional photoelectrochemical properties of the photocathodes of CuNiO_x have been studied using multiple photoelectrochemical instrumental techniques, which proved the electrodes were stable over the whole water photolysis. The coupling of two metal centers has explained their outstanding performance.