Ion transport control in nanofluidics is paramount for applications such as water purification, biosensing, energy storage, and conversion. Designing stable nanofluidic channels that selectively allow specific ions to pass is essential. The human neuron serves as a model, transmitting electrical signals via cation transport for high-speed communication relevant to neuromorphic computing. This research presents a novel concept of neuro-inspired energy harvesting using confined van der Waals crystals, aiming to maximize ion diffusion flux for electromotive force (EMF) generation. The key challenge lies in creating nanofluidic channels that are stable in liquid environments while exhibiting ion selectivity. Current methods, such as using restacked 2D materials like graphene oxide, MXene, and MoS2, have shown promise in ion-selective membranes, but improvements in stability and efficiency are needed. This study proposes a graphene oxide (GO) based system that addresses these challenges by mimicking the key features of biological ion channels: appropriate pore size, surface charge for selectivity, and structural stability in liquid phase. By achieving these, the researchers aim to demonstrate a significant advancement in passive-type, large-scale energy generation.

Numerous studies have explored selective ion transport techniques for various applications. These include osmotic energy conversion using 2D porous structures or 3D heterogeneous composites. Membranes using restacked 2D materials like graphene oxide, MXene, and MoS2 have shown potential for ion selectivity. Methods focusing on oxidative functionalization and composite membranes have aimed to tune surface charge density for increased selectivity. However, creating robust channels that maintain their function over prolonged periods remains a challenge. Existing robust channels, such as those using organic membranes or epoxy, demonstrate enhanced structural stability, but often compromise on other crucial aspects like selectivity or efficiency. This research aims to overcome these limitations by designing a system that combines all three critical features of a biological ion channel: appropriate pore size, surface charge, and structural stability.

The researchers fabricated a confined channel from a graphene oxide (GO) layer physically confined with epoxy. This channel connected two reservoirs with different NaCl concentrations (C1 and C2). The epoxy confinement prevents the typical expansion of GO layers in aqueous solutions, maintaining a consistent channel height comparable to the size of cations. X-ray diffraction (XRD) was used to characterize the interlayer spacing in both confined and unconfined GO. The ionic transport properties were investigated by measuring I-V characteristics with different cations (Na+, K+, Ca2+) at identical concentrations and under applied voltage. The effect of channel length on conductance was also studied. To establish a chemical potential gradient, different NaCl concentrations were used in the two reservoirs. I-V response was measured to determine short-circuit current (Isc) and open-circuit voltage (Voc), providing insights into ion selectivity and energy conversion efficiency. Elemental distribution (Na+ and Cl-) was analyzed using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) to assess ion selectivity during diffusion. Fick's first law was used to determine the diffusion coefficient of Na+ ions. The influence of pH on conductance and surface charge was studied by varying pH and measuring zeta potential. Molecular dynamics (MD) simulations were performed to investigate ion dehydration energy barriers at different carboxyl group protonation states. Radial distribution functions were used to analyze the hydration state of ions in the confined channel and in bulk water. The stability of the confined GO membrane was tested under different humidity conditions, including fully wet, fully dried, and partially wetted states. Finally, the practical application potential was tested by discharging multiple series-connected cells at a constant current, and by powering LEDs using a series of cells. Real seawater and river water were tested to assess applicability in real-world scenarios.

The epoxy-confined GO channel exhibited a free spacing of 4.5 Å, comparable to the size of biological ion channels. Ionic conductance increased with decreasing hydrated ion size (NaCl > KCl > CaCl2). Under a concentration gradient of 1 M-1 mM NaCl, the system achieved a short-circuit current (Isc) and open-circuit voltage (Voc) resulting from Na+ transport and the thermodynamic activity difference, showing a high ion selectivity of 95.8%. The energy conversion efficiency was calculated as 41.4%, and the power density was 5.26 W/m². The open-circuit voltage (Voc) remained stable for over 18 hours, showcasing the channel's robustness and the system's potential for long-term operation. SEM-EDS mapping confirmed high Na+ selectivity during diffusion. The diffusion coefficient of Na+ ions in the confined channel was 1.75–2.3 times higher than in bulk water, confirming the enhanced ion mobility in the confined environment. Conductance increased with increasing pH, and MD simulations showed decreased Na+ dehydration energy barriers at higher pH due to carboxyl group deprotonation. Radial distribution functions indicated partial dehydration of Na+ ions in the channel, contributing to enhanced mobility. The neuromorphic cell maintained stable Voc at 60% relative humidity and above, due to capillary condensation in the hydrophilic nanopores. Discharge tests with series-connected cells exhibited stable performance, and cells successfully powered LEDs, showcasing the system's practical applicability. The system demonstrated functionality when using real seawater and river water, further demonstrating real-world application potential.

This research successfully demonstrates a bio-inspired energy harvesting system using confined van der Waals crystals. The system mimics key features of biological ion channels to achieve high ion selectivity, energy conversion efficiency, and long-term stability. The high ion selectivity, exceeding 90% even at high concentrations, is a significant advancement. The achieved energy conversion efficiency and power density are among the highest reported for osmotic energy systems. The long-term stability, exceeding 150 hours of operation, addresses a critical limitation of previous nanofluidic energy conversion devices. The ability to operate under different humidity conditions, including near-dry conditions, broadens the range of potential applications. The demonstration of powering LEDs using series-connected cells showcases the system's viability for real-world applications. This system offers significant potential for large-scale, passive energy generation and can be potentially applied in various fields such as powering wearable electronics and remote sensors.

This study presents a novel neuro-inspired energy generation system using confined GO van der Waals crystals. The system demonstrates high ion selectivity, energy conversion efficiency, and long-term stability, surpassing existing nanofluidic energy conversion technologies. The successful powering of LEDs using series-connected cells highlights its practical potential. Future research could focus on optimizing channel design for even higher efficiency, exploring different 2D materials, and investigating scalability for large-scale energy generation applications.

While the study demonstrates impressive results, several limitations should be acknowledged. The current design uses NaCl as the electrolyte, and further investigation is needed to explore compatibility with other electrolytes. The long-term stability is evaluated up to 150 hours; longer-term tests are required for a more complete evaluation. The scalability of the device for large-scale power generation remains to be fully explored. The effect of varying environmental conditions beyond humidity should also be evaluated. Although the system is inspired by biological ion channels, there are structural differences that require further investigation.