  Besides the dominant NaCl, natural seawater/river water contains trace
multivalent ions, which can provide effective screening to surface charges.
Here, in both negatively and positively charged nanopores, influences from
divalent ions as counterions and coions have been investigated on the
performance of osmotic energy conversion (OEC) under natural salt gradients. As
counterions, trace Ca2+ ions can suppress the electric power and conversion
efficiency significantly. The reduced OEC performance is due to the bivalence
and low diffusion coefficient of Ca2 ions, instead of the uphill transport of
divalent ions discovered in the previous work. Effectively screened charged
surfaces by Ca2+ ions induce enhanced diffusion of Cl ions which simultaneously
decreases the net ion penetration and ionic selectivity of the nanopore. While
as coions, Ca2+ ions have weak effects on the OEC performance. The promotion
from charged exterior surfaces on OEC processes for ultra-short nanopores is
also studied, which effective region is ~200 nm in width beyond pore boundaries
independent of the presence of Ca2+ ions. Our results shed light on the
physical details of the nanofluidic OEC process under natural seawater/river
water conditions, which can provide a useful guide for high-performance osmotic
energy harvesting.
