Nanofluidic Energy Harvesting Research Articles

Optimizing Nanofluidic Energy Harvesting in Synthetic Clay-based Membranes by Annealing Treatment.

Nanofluidic energy harvesting from salinity gradients is studied in 2D nanomaterials-based membranes with promising performance as high ion selectivity and fast ion transport. In addition, moving forward to scalable, feasible systems requires environmentally friendly materials to make the application sustainable. Clay-based membranes are attractive for being environmentally friendly, non-hazardous, and easy to manipulate materials. However, achieving underwater stability for clay-based membranes remains challenging. In this work, the synthetic clay Laponite is used to prepare clay-based membranes with high stability and excellent performance for osmotic energy harvesting. The Laponite membranes (Lap-membranes) are stabilized by low-temperature annealing treatment to effectively reduce the interlayer space, achieving a continuous operation under salinity gradients. Furthermore, the Lap-membranes conserve integrity while soaking in water for more than one month. The output power density improves from ≈4.97W m-2 on the pristine membrane to ≈9.89W m-2 in the membrane treated 12 h at 300°C from a 30-fold concentration gradient. Especially, It is found that the presence of interlayer water to be favorable for ion transport. Different mechanisms are proposed in the Lap-membranes involved for efficient ion selectivity and the states found with varying annealing temperatures. This work demonstrates the potential application of Laponite based nanomaterials for nanofluidic energy harvesting.

Advanced Janus membrane (MXene/CoAl-LDH) for efficient asymmetric ion transport and nanofluidic energy harvesting

Two-dimensional (2D) material membranes with nanometer or sub-1 nanometer confined space present enormous potential for water permeation, ion sieving and transport. In addition, electrolyte flowing through the asymmetric channels exhibits ion asymmetric transport behavior which provides a basis to control the ion flow. Therefore, it is an opportunity to develop Janus membranes by 2D materials for alleviating the concentration polarization and enhancing the performance in nanofluidic energy harvesting. Whereas the previously reported 2D material-based Janus laminated membranes were adjusted interlayer charge distribution by modifying positively charged polymer, due to the intrinsically negative surface charge of these 2D nanosheets. Thus, design of 2D/2D Janus membrane from positive and negative changed 2D materials will have great opportunity for efficient asymmetric ion transport and nanofluidic energy harvesting. In this work, Janus MXene/LDH (MXLDH) membranes containing negatively charged MXene and positively charged CoAl-LDH were designed without additional modifying process for efficient ion asymmetric transport and neutralization energy harvesting. The ratio of MXene/LDH draws a significant effect on ion rectification factor, osmotic voltage, ion selectivity and energy conversion efficiency. The Janus membrane with 86 wt% LDH (MXLDH-86) revealed the highest rectification factor of ∼6.2 in 0.1 mM NaCl aqueous solution. And the osmotic power density of MXLDH-86 under 1 M HCl/ 1 M NaOH can reach ∼8.75 W/m2, ∼17.7 % higher than that of the pristine MXene membrane, indicating that the Janus membrane with asymmetric charges is a promising material for harvesting neutralization energy.

Nanoengineered nanochannels for thermally ionic nanofluidic energy harvesting

This work demonstrates thermal-to-electric energy conversion based on ionic nanofluidic transport in nanochannels inducted by a temperature gradient. Two types of highly periodic and high aspect ratio nanochannels have been fabricated in a silicon (Si) substrate and in an aluminium oxide (Al2O3) membrane. Silicon nanochannels with diameter of 100 nm and height of 300 μm have been produced by metal-assisted chemical etching process (MACE), while nanochannels with the dimensions of the 10 nm and 3 μm respectively, were fabricated in a Al2O3 membrane by the anodic aluminum oxidation (AAO) process. Moreover, a novel approach of thermally nanofluidic energy harvesting was proposed and conducted. The performance of the thermally ionic nanofluidic energy harvesting system in cases with and without nanochannels has been evaluated and compared. The advantage of the presence nanochannels on energy conversion has been confirmed. The electrolyte concentration dependence on the output power has been determined. Also, the effect of nanochannel materials, including alumina and silicon materials, on the power density has been verified. The ionic Seebeck coefficient was enhanced by 7.3 times when the nanochannels were present. The silicon substrate with nanochannels demonstrated the highest performance for energy harvesting. Its power density reaches approximately 1.47 mW/m2, which was 13.3 times larger than the case without nanochannels. This investigation may open new opportunities for the future thermoelectric generators based on ionic transport in nanochannels.

The synergistic effect of space and surface charge on nanoconfined ion transport and nanofluidic energy harvesting

Diverse nanoconfined ion transport behaviors have attracted increasing attention in constructing ion-channel-inspired nanoconfined systems for high nanofluidic energy conversion performance. Conventional nanofluidic systems cannot simultaneously obtain high ion-selectivity and high ion-flux in a large region due to the decreasing effect of the electrical double layer (EDL) formed on the charged surface. Here, space charge is introduced in the nanoconfined system to effectively solve this problem by employing the synergistic effect of surface and space charges. The charge effect contribution on controlled-ion-transport is further decoupled into surface charge, space charge and synergistic effects for the first time. The surface charge induces charge separation within the EDL in a smaller diameter channel while the space charge dominates the ion-selectivity by the ion-exchange mechanism in a larger diameter channel. The synergistic effect, contributed by EDL and ion-exchange effects, is extremely important in large (around 100 nm), short nanochannel (around 100 nm), and high concentration fold (around 1000-fold) conditions to achieve high ion-selectivity, ion-flux, and nanofluidic energy conversion performances, which is systematically analyzed by theoretical simulation results. These results reveal the mechanism for enhanced ion transport behaviors and provide a solid foundation for designing high-performance nanofluidic devices in fields of desalination, sensor, energy conversion and storage.

Nanofluidic energy harvesting through a biological 1D protein-embedded nanofilm membrane by interfacial polymerization

Abstract The natural ion transportation in living cells under a chemical potential difference provides diverse clues for scientific developments. In particular, selective ion flux through protein nanochannels can generate high electrical power such as seen in electric eels. However since typical purified biological proteins (e.g., Na/K nanochannels) are unstable and costly, this study is the first to use biological 1D protein channels (tobacco mosaic virus, TMV) isolated from plants. For cost-effective 1D TMV-embedded nanofilm membranes, an interfacial polymerization method was introduced with m-phenylenediamine and trimesoyl chloride. The use of m-phenylenediamine helps the self-assembly of TMVs and also assists the diffusion of TMV for vertical alignment. The TMV-embedded membrane shows the highest power density of 21.5 W/m2 under a 1000-fold concentration difference. This membrane thus holds remarkable potential for facile fabrication and alignment of biological 1D protein-embedded nanofilm membranes.

Effects of heat transfer and the membrane thermal conductivity on the thermally nanofluidic salinity gradient energy conversion

Originating from advances in nanofabrication, functional materials have been developed to construct bioinspired or mimic nanochannels for efficient nanofluidic energy conversion. In previous studies regarding nanofluidic energy conversion, the material thermal conductivity has been never taken into consideration, however, which shall be addressed with transmembrane temperature difference applied. Thermal conductivities of widely used bulk materials for fabricating nanopores vary from 0 to 120 W/(m·K). Heat transfer occurs both in the liquid solution and solid membrane, impacting trans-channel solution temperature distribution and ion transportation, and yielding counterintuitive phenomena. Under positive temperature differences, the electric power shifts from inhibition to promotion as the membrane thermal conductivity increases. Larger membrane thermal conductivity evens temperature distribution in the nanochannel, weakening ion aggregations or depletions, which result in degraded electric power improvement under a negative temperature difference and upgraded power enhancement under a positive one. At a temperature difference of 30 K, the electric power is enhanced by 56.22% for a thin PET membrane (length = 50 nm) under the negative temperature difference. If the membrane thermal conductivity can be well tuned to nearly thermal insulation, the electric power can be enhanced by 120.07%. To step further, we proposed a criterion for membrane selection based on thermal conductivities in the thermally nanofluidic energy conversion: For thick membranes, materials of large thermal conductivity with a positive temperature difference are preferred for obvious power and energy efficiency enhancement. For thin membranes, materials of small thermal conductivity with a negative temperature difference are appealing. These findings reveal the importance of a long-overlooked factor, membrane thermal conductivity, in nanofluidic energy harvesting and can serve as a guidance for selecting appropriate membrane materials and developing high-performance nanofluidic power devices.

Ionic thermal up-diffusion in nanofluidic salinity-gradient energy harvesting.

Advances in nanofabrication and materials science give a boost to the research in nanofluidic energy harvesting. Contrary to previous efforts on isothermal conditions, here a study on asymmetric temperature dependence in nanofluidic power generation is conducted. Results are somewhat counterintuitive. A negative temperature difference can significantly improve the membrane potential due to the impact of ionic thermal up-diffusion that promotes the selectivity and suppresses the ion-concentration polarization, especially at the low-concentration side, which results in dramatically enhanced electric power. A positive temperature difference lowers the membrane potential due to the impact of ionic thermal down-diffusion, although it promotes the diffusion current induced by decreased electrical resistance. Originating from the compromise of the temperature-impacted membrane potential and diffusion current, a positive temperature difference enhances the power at low transmembrane-concentration intensities and hinders the power for high transmembrane-concentration intensities. Based on the system's temperature response, we have proposed a simple and efficient way to fabricate tunable ionic voltage sources and enhance salinity-gradient energy conversion based on small nanoscale biochannels and mimetic nanochannels. These findings reveal the importance of a long-overlooked element—temperature—in nanofluidic energy harvesting and provide insights for the optimization and fabrication of high-performance nanofluidic power devices.

Enhancing the efficiency of energy harvesting from salt gradient with ion-selective nanochannel

The development of a nanofluidic energy harvesting system plays a fundamental role in harvesting osmotic power from Gibbs free energy within the salt concentration gradient, which is considered to be a clean and renewable energy source for the future. In this study, a silica-nanochannel based nanofluidic energy harvesting system was fabricated and the output power density reached 705 W m−2 under suitable KCl concentration bias which exceeded—by almost two orders of magnitude—the results obtained by previous work. The enhancement of energy harvesting was mainly ascribed to the appropriate length of nanochannel that provides a good balance between the desirable ion selectivity and the unfavorable large resistance from the nanochannel. This high-performance nanofluidic energy device could be used in a variety of applications, including to power tiny biomedical devices or for constructing future clean-energy recovery plants.

Blue energy: Current technologies for sustainable power generation from water salinity gradient

“Salinity energy” stored as the salinity difference between seawater and freshwater is a large-scale renewable resource that can be harvested and converted to electricity, but extracting it efficiently as a form of useful energy remains a challenge. With the development of membrane science and technology, membrane-based techniques for energy extraction from water salinity, such as pressure-retarded osmosis and reverse electro-dialysis, have seen tremendous development in recent years. Meanwhile, many other novel methods for harvesting exergy from water mixing processes, such as electrochemical capacitor and nano-fluidic energy harvesting systems, have been proposed. In this work, an overview and state-of-the-art of the current technologies for sustainable power generation from the water salinity gradient are presented. Characteristics of these technologies are analyzed and compared for this particular application. Based on these entropic energy extracting methods, the water salinity, as the “blue energy”, will be another source of renewable energy to satisfy the ever-growing energy demand of human society.

Towards understanding the nanofluidic reverse electrodialysis system: well matched charge selectivity and ionic composition

The widespread use of tiny electrical devices, from microelectromechanical systems (MEMS) to portable personal electronics, provides a new challenge in the miniaturization and integration of power supply systems. Towards this goal, we have recently demonstrated a bio-inspired nanofluidic energy harvesting system that converts salinity gradient energy from the ambient environment into sustainable electricity with single ion-selective nanopores (Adv. Funct. Mater. 2010, 20, 1339). The nanofluidic reverse electrodialysis system (NREDS) significantly improves the performance of conventional membrane-based reverse electrodialysis systems due to a higher ionic flux and a lower fluidic resistance. However, the fundamental working mechanism of the NREDS has been largely unexplored in the literature. In this work we have systematically investigated the performance of the NREDS in relation to the electrolyte type and the charge selectivity of the nanofluidic channel using both experimental and theoretical approaches. Experimental results show that the short-circuit current, the open-circuit voltage, and the resulting electric power of the NREDS are very sensitive to the ionic composition of the electrolyte solution. Through an in-depth theoretical analysis, two dominant factors that govern the charge separation and ion selectivity of the nanochannels were identified. The results prove that, with well-matched electrolyte types and nanopore charge selectivity, the harvested electric power and energy conversion efficiency can be improved by nearly two orders of magnitude.