High-Performance
Ionic Diode Membrane for Salinity
Gradient Power Generation
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Abstract
Salinity
difference between seawater and river water is a sustainable
energy resource that catches eyes of the public and the investors
in the background of energy crisis. To capture this energy, interdisciplinary
efforts from chemistry, materials science, environmental science,
and nanotechnology have been made to create efficient and economically
viable energy conversion methods and materials. Beyond conventional
membrane-based processes, technological breakthroughs in harvesting
salinity gradient power from natural waters are expected to emerge
from the novel fluidic transport phenomena on the nanoscale. A major
challenge toward real-world applications is to extrapolate existing
single-channel devices to macroscopic materials. Here, we report a
membrane-scale nanofluidic device with asymmetric structure, chemical
composition, and surface charge polarity, termed ionic diode membrane
(IDM), for harvesting electric power from salinity gradient. The IDM
comprises heterojunctions between mesoporous carbon (pore size ∼7
nm, negatively charged) and macroporous alumina (pore size ∼80
nm, positively charged). The meso-/macroporous membrane rectifies
the ionic current with distinctly high ratio of ca. 450 and keeps
on rectifying in high-concentration electrolytes, even in saturated
solution. The selective and rectified ion transport furthermore sheds
light on salinity-gradient power generation. By mixing artificial
seawater and river water through the IDM, substantially high power
density of up to 3.46 W/m<sup>2</sup> is discovered, which largely
outperforms some commercial ion-exchange membranes. A theoretical
model based on coupled Poisson and Nernst–Planck equations
is established to quantitatively explain the experimental observations
and get insights into the underlying mechanism. The macroscopic and
asymmetric nanofluidic structure anticipates wide potentials for sustainable
power generation, water purification, and desalination