Designing highly active bifunctional electrocatalysts from Earth-abundant elements has great prospects for substituting precious-metal based catalysts in energy conversion processes, such as water splitting. Here, we report a bifunctional catalyst comprising transition metal hydroxides (TMOHs) and transition metal sulphides (TMSs) grown on a nickel foam (NF) surface, denoted as NiFeOH/CoS_x/NF, that delivers high electrocatalytic activity for both the oxygen evolution reaction (OER: ultra-low overpotential of 211 mV at a current density of 50 mA cm⁻²) and the hydrogen evolution reaction (HER: overpotential of 146 mV at a current density of 10 mA cm⁻²) in alkaline media, representing one of the best bifunctional catalytic performances yet reported for a non-noble metal based system. From our experimental observations, the significant improvement of the catalytic activity emanates from the synergistic effects of NiFeOH and CoS_x, due to the optimization of their electronic configurations, thereby creating novel characteristics. Employing this catalyst system as both the anode and the cathode for overall water splitting in a water electrolyzer delivers 10 mA cm⁻² at a low cell potential of 1.563 V with excellent long-term electrocatalytic functionalities over 10 h of continuous operation. These findings represent the design principle for developing multi-component bifunctional electrocatalysts for overall water splitting.