Green hydrogen (H₂) is an import energy carrier due to the zero-carbon emission in the energy cycle. Nevertheless, green H₂ production based on electrolyzer and photovoltaics (EZ/PV) remains limited due to the highly pH-dependant and energy exhausting overall water splitting. Herein, we report a series of Ru-nanocluster-modified mesoporous nanospheres (Ru_x@mONC) as pH-universal electrocatalysts towards both hydrogen evolution reaction (HER) and hydrazine oxidation reaction (HzOR). The optimal catalyst Ru₂₀@mONC realizes remarkable catalytic activity and stability towards both HER and HzOR regardless of electrolytes. As a result, the electrode pair of Ru₂₀@mONC//Ru₂₀@mONC requires low cell-potentials of 39/429, 405/926, and 164/1,141 mV to achieve the current density of 10/100 mA·cm⁻², as well as the long-term stability for HzOR assisted electrochemical water splitting in alkaline, acidic, and neutral media, respectively. Those performances are more promising compared to the state-of-the-art electrocatalysts so far reported. A proof-of-concept test demonstrates an efficient production of green H₂ powered by a single-junction silicon solar cell, which may inspire the use of a cost-effective EZ/PV system. Furthermore, a combined spectroscopic and theoretical study verifies the formation of abundant Ru/NC heterointerfaces in Ru₂₀@mONC, which not only contributes to the balancing of H* adsorption/desorption in HER but also facilitates the *N₂H₂ dehydrogenation in HzOR.