Anomalous pH‐Dependent Nanofluidic Salinity Gradient Power

Abstract

Previous studies on nanofluidic salinity gradient power (NSGP), where energy associated with the salinity gradient can be harvested with ion-selective nanopores, all suggest that nanofluidic devices having higher surface charge density should have higher performance, including osmotic power and conversion efficiency. In this manuscript, this viewpoint is challenged and anomalous counterintuitive pH-dependent NSGP behaviors are reported. For example, with equal pH deviation from its isoelectric point (IEP), the nanopore at pH < IEP is shown to have smaller surface charge density but remarkably higher NSGP performance than that at pH > IEP. Moreover, for sufficiently low pH, the NSGP performance decreases with lowering pH (increasing nanopore charge density). As a result, a maximum osmotic power density as high as 5.85 kW m can be generated along with a conversion efficiency of 26.3% achieved for a single alumina nanopore at pH 3.5 under a 1000-fold concentration ratio. Using the rigorous model with considering the surface equilibrium reactions on the pore wall, it is proved that these counterintuitive surface-charge-dependent NSGP behaviors result from the pH-dependent ion concentration polarization effect, which yields the degradation in effective concentration ratio across the nanopore. These findings provide significant insight for the design of next-generation, high-performance NSGP devices.

A review: applications of ion transport in micro‐nanofluidic systems based on ion concentration polarization

“…The device demonstrated rapid consumption of fluorescent dye samples and can be used as an effective microfluidic concentrator for fluorescent dyes to achieve a 60‐fold concentration enrichment in 200 s. Yeh et al . revealed an abnormal counterintuitive pH‐dependent nanofluid salinity gradient power (NSGP) behaviour . Their results showed that at a concentration of 1000, the conversion efficiency of a single alumina nanopore reacheed 26.3% at pH 3.5, which can produce a maximum osmotic power density of up to 5.85 kW m −2 .…”

Improved Rectification and Osmotic Power in Polyelectrolyte-Filled Mesopores

“…In this case, the osmotic current is estimated as . On the other hand, the osmotic voltage can be obtained by letting [ 44 ]. Figure 4 a illustrates the simulated current–voltage and the power density–voltage curves of the considered mesopore system in 1000 mM/1 mM salinity gradient where the power density is calculated as .…”

“…This can be attributed to the significant ICP effect arising from the high space charge density of a PE layer [ 48 ]. Hence, we calculate the maximum osmotic power density ( ) as the highest value of the P–V curve in the present manuscript, instead of using the simple formula that the previous studies adopted [ 44 , 63 , 64 ], …”

“…Here, we theoretically investigate the mesoscale ionic diode and osmotic power generator by considering a cone-shaped mesopore whose narrow opening is filled with a positively charged polyelectrolyte (PE) layer. Compared with all the earlier theoretical works focusing primarily on the nanofluidic powers [ 44 , 45 , 46 , 47 ], this is the first report studying the osmotic power generator at the mesoscale. The effect of the most key parameter to the system, the space charge density of a PE layer ( ), on the rectification ability and osmotic power conversion is systematically discussed.…”

“…In the absence of a voltage bias and/or a concentration gradient through a mesopore, the resulting ionic current can be evaluated by where S denotes the either end of the reservoirs. Then the transference number of anions ( ) under a concentration gradient is calculated as [ 44 ] where and are the osmotic current contributed from cations and anions, respectively. In general, the more significant deviation of from 0.5 to 1, the more significant the anion selectivity.…”