When excess water (40 µL) was added, voltage generates immediately but decreases to zero once water migrated to the dry end (Figure ). The reason is that the solid-water interfaces are asymmetric at the initial state (i.e., CBNP-water vs Cu/CBNP-water) but become the same (i.e., Cu/CBNP-water) when water migrates to the bottom of the dry-end Cu strip. This demonstrates the vital role of asymmetric solid-water interfaces in electricity generation. It also implies that the CBNP layer at the wet end is not necessary for voltage generation since it forms an undesirable internal conductive path between the dry CBNP and wet Cu strip, which would reduce the output voltage of the TWIEG. Therefore, we further designed an asymmetric WIEG (AWIEG) with the MCE membrane half coated by CBNPs (denoted as CBNP h /MCE membrane), as shown in Figure . The obtained CBNP h /MCE exhibits desirable flexibility (Figure ). The output voltage of the AWIEG was recorded with a pair of Cu d -Cu w (Figure ). The effect of the areal loading of CBNP on the output performance of the AWIEG was first investigated. As shown in Figure and Figure , the average output voltage of the AWIEG increases obviously when the CBNP loading increases from 0.02 to 0.08 mg cm -2 due to the decreased resistance of the CBNP layer. Thereafter the loading of CBNPs is chosen to be 0.08 mg cm -2 in the following experiments. The SEM image of the corresponding CBNP h /MCE is shown in Figure , which exhibits a uniform and dense distribution of CBNPs on the surface of MCE membrane. The thickness of the CBNP layer is about 650 nm (Figure ). We also investigated the relationship between the area of the CBNP layer and the output performance of the AWIEG. As shown in Figure , when the area of CBNP layer increases, the output voltage shows no significant difference while the output current increases accordingly. In addition, the output voltages of the TWIEG and AWIEG with the same areal loading of CBNP (0.02 or 0.08 mg cm -2 ) were compared. As shown in Figure , the AWIEG generates a higher voltage than the TWIEG, which demonstrates the advantage of the asymmetric configuration that avoids the internal short-circuit issue of the TWIEG. The underlying mechanism of the electricity generation process for AWIEG is further investigated. In previous works, water evaporation was considered as the main cause of electricity generation for CBNP-based WIEGs. To elucidate the role of water evaporation, we designed a specific setup as illustrated in Figure . The AWIEG half-wetted by water was hung in a waterfilled beaker sealed by a polyethylene membrane, where humidity can become saturated (100%) to significantly suppress the water evaporation. As shown in Figure , the generated voltage of the AWIEG decreases fast in ambient air but remains very stable in the sealed beaker with almost no water evaporation. After taking the AWIEG out of the beaker, the voltage decreases gradually to zero due to the evaporation of water. The observations indicate that the electricity generation in the AWIEG designed in this research does not rely on water evaporation. Therefore, the water evaporation rate and humidity would not have a significant effect on the output performance of the AWIEG if water is continuously supplied. These observations demonstrate that the generated voltage is mainly attributed to the electrochemical potential difference between the two asymmetric solid-water interfaces. The results also imply that the AWIEG can work under extremely humid conditions when evaporation is greatly suppressed.