Impacts of transmembrane pH gradient on nanofluidic salinity gradient energy conversion

Solution pH can impact the nanochannel surface charge density, thus, to regulate the transmembrane ion transportation. Here, the performance of nanofluidic salinity gradient energy conversion is investigated, where the solution pH at the high/low concentration side varies separately, rendering asymmetric pH configurations. Results reveal that the solution pH at the low concentration side (pHL) exhibits a significant impact on the salinity gradient energy conversion. When pHL < Isoelectric Point (IEP), the energy conversion indicators are most stable under various solution pH at the high concentration side (pHH). The surface charge density of the nanochannel at the low concentration end determines the transmembrane ion transportation characteristics, dominating the energy conversion performance. The energy conversion performance via nanochannels of different lengths under asymmetric pH configurations is also studied. There exists optimal nanochannel length corresponding to the maximum electric power, which differs under various pH configurations. For longer nanochannels, the ion concentration polarization effect is weakened, allowing for a more effective energy extraction performance via pH-adjusted salinity gradients. At the length of 2000 nm, the electric power and energy conversion efficiency under the co-direction configuration is 78% and 97.9%, respectively, larger than those under the reverse direction configuration.