Solar-steam generation is recognized as a promising pathway to mitigate the global issue of clean water shortage. Preparation of high-performance photothermal materials and efficient design of advanced evaporators are two key factors which need to be enhanced to facilitate the practical application of solar steam generation for clean water production. In this work, polydopamine coated nickel–cobalt bimetal (Ni1Co3@PDA) nanosheets were synthesized and employed as photothermal materials for solar steam generation. Ni1Co3@PDA nanosheets were coated on the surface of a commercial sponge using sodium alginate as a binding agent. The obtained photothermal sponges exhibited excellent light absorption (>99%) which benefited the light-to-heat conversion and solar evaporation. A kerosene lamp-like evaporator, which spatially separated the bulk water and the evaporation surface, was adopted to evaluate the performance of the Ni1Co3@PDA sponge for solar steam generation. It was found that the photothermal sponges could be stacked to form 3D evaporators of adjustable heights to achieve superior evaporation rates while maintaining the same footprint. The stacked photothermal sponges with a height of 6.0 cm showed the highest water evaporation rate of 2.42 kg m−2 h−1 under 1.0 sun with corresponding energy efficiency (109%) beyond the theoretical limit. This was due to the eliminated heat conduction loss, reduced radiation and convection loss, as well as net energy gain from the environment. The salinity of the collected clean water is only 2.26 ppm. The overall system is cost effective and highly efficient, thus shows great potential for future real-world applications.