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Although rapid advances have been made in micro/nano-scale devices, there is still a lack of clean and sustainable power source for them. Here we propose a high voltage nanofluidic energy generator inspired by electrical eel using ion-concentration gradients, which converts Gibbs free energy into electricity without any pollutants. The high voltage can be induced by alternatively multi-stacking cation and anion-exchange nanochannel network membranes (CE-NCNMs and AE-NCNMs) in a confined microscale space. These membranes were constructed by in situ self-assembled nanoparticles with hydroxyl and amine groups, respectively. The multiple stacks of CE-NCNMs & AE-NCNMs were successfully realized by precisely guiding the nanodrops with the suspended positively or negatively charged nanoparticles into the desired positions in the multilayered microchannel platform. The performance of the proposed nanofluidic energy generator was quantitatively investigated by changing nanoparticle species, intermembrane distance (IMD), and environmental temperature. Interestingly, we found that our optimized IMD (~80 µm) is very similar to the inter-cell membrane distance of electrocytes in natural electric eels and the diffusion potential of a single full cell at this IMD (~138 mV) is also similar to the net potential across a single electrocyte (~150 mV). This optimized IMD is verified through not only electrical measurement but also by fluorescent tracing and numerical analysis using multiphysics simulation. The high voltage of up to 1 V achieved by stacking 20 full cells is, to the authors’ knowledge, the highest value yet obtained by microfluidic systems harnessing ion-concentration gradients. [ABSTRACT FROM AUTHOR]

Renewable energy processes based on salinity gradient differences possess spreading interests as an alternative to energy production methods based on fossil fuels. The reverse electrodialysis (RED) process is used for energy production by mixing seawater and river water. In the proposed solution, the spent battery effluent is applied as a high-concentrated solution for the first time, generating a potential difference in the RED stack. The high-selective cation exchange membrane for cobalt transportation in the RED system was a demanding barrier controlling the separation behaviour. The initial research confirmed the energy production ability from mixing high concentrated (HC) solution of Co2+, Li+ and Ni2+ nitrates and low concentrated (LC) solution containing chloric acid. The maximum extracted energy was estimated at 0.44 W/m2 with an energy efficiency of 45.5%. The presented method is a promising candidate for dealing with battery waste management and converting it into a valuable product such as cobalt salts with the potential for power harvesting. • Connection of power harvesting and cobalt salt recovery. • Battery spent effluent as a promising candidate for power production. • Application of high-selective cation exchange membrane for cobalt cations separation. [ABSTRACT FROM AUTHOR]

Nanofluid reverse electrodialysis (RED) is considered the most promising technology for harvesting osmotic energy. As materials used for RED, natural nanofluid materials are renewable but suffer from low power densities due to weak surface charge densities or irregular ion transport channels, while advanced materials from sophisticated manufacturing are too expensive. Here, a pair of novel natural nanofluid membranes with channel-like nanopores for highly improved selective ion transport were developed by assembling negatively and positively charged bacterial cellulose nanofibers via a space-confined flattened extrusion process. The regular internal channel-like nanopores with strongly increased surface charges assured less electrical imperfectness and therefore high ion selectivity, leading to RED systems with a high output power density of 0.72 W m−2 (5.58 W m−2 of negatively charged membranes), far superior to other existing natural nanofluidic RED systems. This strategy of efficiently enhancing the selective ion transport will open up a new route for the development of nanofluidic RED devices, and allows a wide range of their applications in other energy fields. [Display omitted] • The cylindrical channel-like nanopore structures are more electrically "perfect". • Modification of natural materials to achieve very high Zeta potential. • The power density is much higher than reported pure natural nanofluidic RED devices. [ABSTRACT FROM AUTHOR]

Abstract: Reverse electrodialysis (RED) is a technology to generate power from mixing waters with different salinity. The net power density (i.e. power per membrane area) is determined by 1) the membrane potential, 2) the ohmic resistance, 3) the resistance due to changing bulk concentrations, 4) the boundary layer resistance and 5) the power required to pump the feed water. Previous power density estimations often neglected the latter three terms. This paper provides a set of analytical equations to estimate the net power density obtainable from RED stacks with spacers and RED stacks with profiled membranes. With the current technology, the obtained maximum net power density is calculated at 2.7W/m2. Higher power densities could be obtained by changing the cell design, in particular the membrane resistance and the cell length. Changing these parameters one and two orders of magnitude respectively, the calculated net power density is close to 20W/m2. [Copyright &y& Elsevier]

Blue energy conversion, where the chemical energy stored in salinity gradients can be converted into electricity with ion-selective nanochannel membranes, has considered to be one of the most promising renewable energies. Conventional understanding on this energy suggests that as to largely reduce the resistance, ultrashort channel membranes are required to gain high-energy output. To understand the channel-length-dependent blue energy conversion in detail, we engineered a series of highly ordered and uniform ~23.0 nm in diameter alumina nanochannel membranes with various lengths. Most anomalously, our experiments however show that for sufficiently short nanochannels, the shorter the channel length, regardless of surface charge nature, the smaller the generated power, violating the past understanding. The anomalous channel-length-dependent blue energy conversion is well supported by our rigorous model. The modeling reveals that ultrashort nanochannels will induce the significant ion concentration polarization effect, which appreciably undermines effective salinity ratio and ion selectivity in the nanochannel. If this effect dominates, the nanofluidic osmotic power turns into a decrease with decreasing channel length. Both the experimental and theoretical results reported consistently highlight the importance of osmotic ion transport especially in ultrashort nanochannels, and this finding shed light on the design of high-efficiency blue energy harvesters. [Display omitted] • Alumina nanochannel membranes with highly ordered channel arrays of various lengths were engineered for blue energy. • Experiments show that the osmotic power has a local-maximum dependence on channel length, violating the past understanding. • The significant ion concentration polarization in ultrashort nanochannels results in the anomalous decrease in osmotic power. [ABSTRACT FROM AUTHOR]