Asymmetric tapered multistage solar still with optimized mass transfer equilibrium for ultrahigh water production

Solar membrane distillation offers a highly promising and sustainable solution to the global freshwater crisis. However, its widespread practical application is currently hampered by a key challenge: Pursuing high water production. This bottleneck stems from a mismatch between the evaporation and condensation capacities in existing systems, where vapor may not be condensed in time due to insufficient condensation capacity, or the available condensation capacity may be underutilized when evaporation is inadequate. Here we show an asymmetric tapered multistage solar still that enables ultrahigh water production by introducing a design principle based on optimizing the mass transfer equilibrium between evaporation and condensation. By systematically optimizing the ratio of condensation-to-evaporation areas through a tunable mass transfer gap, the system achieves a state of ultrahigh-production equilibrium, in which evaporation and condensation processes are maximally coupled. Based on this principle, an optimized eight-stage passive solar still device is built to get a total water production of 4.32 L·m^-2·h^-1 and total ${\eta }_c$ of 81% under 1 kW·m^-2 illumination (with 3.1 wt% natural seawater), which ranks among the highest values reported in existing literature. It exhibits stable performance under varying light conditions and salt resistance, producing 34.2 L·d^-1 in outdoor tests.