Massively Enhanced Charge Selectivity, Ion Transport, and Osmotic Energy Conversion by Antiswelling Nanoconfined Hydrogels

Developing a nanofluidic membrane with simultaneously enhanced ion selectivity and permeability for high-performance osmotic energy conversion has largely been unexplored. Here, we tackle this issue by the confinement of highly space-charged hydrogels within an orderedly aligned nanochannel array membrane. The nanoconfinement effect endows the hydrogel-based membrane with excellent antiswelling property. Furthermore, experimental and simulation results demonstrate that such a nanoconfined hydrogel membrane exhibits massively enhanced cation selectivity and ion transport properties. Consequently, an amazingly high power density up to ∼52.1 W/m2 with an unprecedented energy conversion efficiency of 37.5% can be reached by mixing simulated salt-lake water (5 M NaCl) and river water (0.01 M NaCl). Both efficiency indexes surpass those of most of the state-of-the-art nanofluidic membranes. This work offers insights into the design of highly ion-selective membranes to achieve ultrafast ion transport and high-performance osmotic energy harvesting.


Fabrication of ANM
2] In a typical procedure, a high-purity aluminum foil (99.9995%; thickness: 0.127 mm, Strem Chemicals) was electropolished using a perchloric acid/ethanol solution (1:4 v/v).The first anodization step involved using 0.3 M oxalic acid (Showa) electrolyte at 20 ℃ under a voltage of 50 V for 30 min.This was followed by a chemical etching process using a mixture of 6 wt% phosphoric acid (Honeywell) and 1.8 wt% chromium (VI) oxide (Showa) at 60 ℃ for 1 h.The second anodization step was then performed for 1, 2, or 4 h to regulate the membrane thicknesses 3 under the same conditions as the first anodization used.The channel sizes can be further adjusted by a channel-widening process using 5 wt% phosphoric acid at 30 ℃ for 30 or 50 min. 4

Fabrication of PAMPS@ANM
The nanoconfined hydrogels (PAMPS@ANM) were constructed by infiltrating a mixed AMPS monomer solution into straight nanopores of an ANM, followed by a UV curing process.
The mixed AMPS monomer solution consisted of AMPS (2.07 g, 10 mmol), N,N′methylenebisacrylamide (MBAA, 7.7 mg, 0.05 mmol) as a cross-linker, 2-oxoglutaric acid (2-OA, 1.46 mg, 0.01 mmol) as photo-initiator, and 5 mL deionized (DI) water, where the used concentration of the AMPS monomer solution (C AMPS ) was calculated to be 29.2 wt%.To increase the wettability of the ANM surface with the mixed AMPS monomer solution, a pretreatment, aimed at enhancing the hydrophilicity and surface density of hydroxyl functional groups on the ANM channels 5 , was carried out by immersing the ANM in a 30 wt% hydrogen peroxide (H 2 O 2 , Honeywell) solution for 30 min.The H 2 O 2 -treated ANM was then immersed in the AMPS solution for 2 min, after which any excess monomer solution on the outer surfaces of the ANM was carefully removed using Kimwipe tissues through capillary suction.To fabricate the PAMPS@ANM, the AMPS solution within the nanochannels of ANM was cured by exposing it to a 365-nm UV irradiation for 30 min, achieved using a BK-Areacure-100100 LED module (Brightek) equipped with a planar UV light source at a power of 100 mW.

Material Characterizations
The morphologies of ANM and PAMPS@ANM were characterized using a JSM-7900F SEM (JEOL) at an accelerating voltage of 15 kV.For ion selectivity tests, the PAMPS@ANM was immersed in 5 M NaCl solution for 24 h and then rinsed with DI water.Afterward, the elemental analyses on the cross-section of the samples before and after NaCl immersion were conducted by the same SEM equipped with an EDX detector.Side-view images of water droplets on the ANM and PAMPS@ANM surfaces were captured using an OSA60-G contact angle instrument system (Ningbo NB Scientific Instruments) equipped with an optical surface analyzer.A 7 μL water droplet was used as the probing liquid for the measurements.The FTIR spectroscopy of PAMPS hydrogels was performed using a FT/IR-4X spectrometer (JASCO) employing the attenuated total reflection (ATR) method with a diamond crystal.The measurement was carried out in the spectral range of 650-4000 cm −1 with a resolution of 4 cm −1 .

Swelling Tests
The swelling tests were conducted by immersing PAMPS hydrogels or PAMPS@ANM in NaCl solutions with various concentrations (0, 0.01, 0.5, 1, and 5 M) at room temperature for different periods.The gel fraction (GF), defined by m dry /[m cured × C AMPS ], was measured to be 0.82, where m dry and m cured are the masses of dried hydrogel after swelling and as-cured hydrogel, respectively.The swelling degree (Q m ) in mass is defined as m gel /m dry and can be rewritten into 4.14×(m gel / m cured ), considering the values of GF and C AMPS .Specifically, for the PAMPS@ANM measurement, bare ANM (m ANM ), as-cured PAMPS@ANM (m cured+ANM ), and swollen PAMPS@ANM (m gel+ANM ) were recorded, sequentially.The Q m for PAMPS@ANM can be calculated based on the following equation: The equilibrium swelling degree is defined by the Q m at equilibrium state after immersion in NaCl solutions for 1 day.

Electrical Measurement
The ion transport property and osmotic energy harvesting performance of the PAMPS@ANM were evaluated using a Keithley 6487 picoammeter (Keithley Instruments) connected to a custom-made conductive cell with a pair of Ag/AgCl electrodes.The membranes were positioned between two half-units of the conductive cell.To assess the voltage-driven ion transport property, current-voltage (I-V) curves were recorded in KCl with various concentrations.For the evaluation of the osmotic energy conversion performance, the working electrode was placed in the higher-concentration unit with various NaCl concentration gradients.The practical osmotic energy output can be estimated by transferring the produced power to an external resistor with a tunable resistance (R t ) and using the equation, where I is the measured current. 1 The effects of varying membrane thicknesses (~21, ~36, and ~60 μm) and channel sizes (~40, ~75, and ~100 nm) were investigated using the PAMPS@AMMs under a 50-fold NaCl concentration gradient while maintaining a constant low concentration of 0.01 M NaCl on one side.Furthermore, the impacts of different salt species (LiCl, NaCl, and KCl), salinity gradients (50-fold, 100-fold, 500-fold), and pH levels (3, 6, and 9) on the system were explored employing the PAMPS@ANM with a fixed channel size of 75 nm and membrane thickness of 36 μm.[8]

Electrode Calibration
In reference to the equivalent circuit diagram depicted in Figure S5a, the open-circuit voltage (V oc ) measured during the application of a salinity gradient across a selective membrane system can be decomposed into two distinct components: the osmotic potential (V osm ), and the redox potential (V red ) resulting from the non-uniform potential distribution at the two electrodes in the presence of a salinity gradient, which satisfy the following equation: To determine the pure salinity gradient-driven V osm and osmotic current (I osm ), we initiated the electrode calibration using V red .During this process, we recorded current-voltage curves while applying sweeping voltages ranging from −0.2 V to 0.2 V in increments of 0.01 V, both in the presence and absence of PAMPS@ANM.When PAMPS@ANM was absent, the measured voltage was designated as V red .The recorded values of V red were 46.7 and 78.3 mV in NaCl under the gradients of 50-and 500-fold, respectively.

Evaluation of Cation Selectivity and Maximum Energy Conversion Efficiency
The cation selectivity of the PAMPS@ANM can be determined by calculation of the cation transference number (t + ), which is obtained using the following equation: where g represents the activity coefficients of salty solutions, and the subscripts H and L refer to the properties of salty solutions in high-and low-concentration reservoirs, respectively.In general, a t + value of 0.5 indicates a non-ion-selective membrane, while a t + value of 1.0 signifies an ideally cation-selective membrane.
Upon obtaining t + , we can determine the maximum osmotic energy conversion efficiency (η max ) of the PAMPS@ANM using the following equation:

Theoretical Model
To reduce computational cost, we simplified the PAMPS hydrogel plugs in ANM (surface charge: +0.08 C/m 2 ) 9 as dense polyelectrolyte layers with uniformly high space charge density of −1.7×10 8 C/m 3 .Figure S7 depicts the simulated PAMPS@ANM system under consideration, and the ANM system is similar to Figure S7 in the absence of polyelectrolyte hydrogel plugs.
Osmotic ion transport and energy conversion of the above systems can be described by the coupled Poisson-Nernst-Planck and Stokes-Brinkman equations, 10-12 In the above, gel r is the space charge density of the PAMPS hydrogel; V , F , and R are the electrical potential, Faraday constant, and gas constant, respectively; e , T , u, p , and m are the permittivity, temperature, velocity, pressure, and dynamic viscosity of fluid, respectively; i z , i C , J i , and i D are the valence, concentration, flux, and diffusivity of i th ionic species ( for cations and 2 i = for anions); F is the region function ( 1 F = represents the region inside the hydrogel and 0 F = represents the region outside the hydrogel); gel l is the softness degree of hydrogel.For the bare ANM system, we assumed 0 The ionic current through the simulation system can be calculated by using where S denotes either end of the two reservoirs.The total meshes used in the modeling are about 500,000.Table S3.Hydrated radii and diffusion coefficients of the four monovalent alkali cations considered in the present work.P max is the achieved maximum power density measured under a 500 mM/10 mM concentration gradient of using the corresponding monovalent chloride salt.Table S4.Comparisons of the practical power density produced from the present nanoconfined hydrogels (PAMPS@ANM) with that from the ion-selective membranes without [15][16][17][18][19][20][21][22] and with [23][24][25][26][27][28][29] the ionic diode effect under a 5 M/0.01 M NaCl gradient at room temperature with the same testing area of 0.03 mm

Figure S1 .
Figure S1.Synthesis pathway of the PAMPS hydrogel through UV curing.

Figure
Figure S5.(a) Scheme depicting the equivalent circuit diagram of the considered osmotic

Figure S6 .
Figure S6.Effect of the testing area on the generated power density of PAMPS@ANM.The

Figure S7 .
Figure S7.Schematic representation of the simulated PAMPS@ANM system (not to actual

Figure S13 .
Figure S13.Illustrated current and power densities of PAMPS@ANM under a 50-fold RbCl

Figure S16 .
Figure S16.Demonstration of practical applications by the developed nanoconfined

Figure S17 .
Figure S17.Durability tests of the osmotic energy harvesting performance of

Figure S18 .
Figure S18.SEM image of PAMPS@ANM captured after the durability test, showing

Table S1 .
Equilibrium swelling degrees of PAMPS and PAMPS@ANM in NaCl solutions with various concentrations.

Table S2 .
Osmotic energy conversion efficiencies of the ANM without and with confined PAMPS tested under a 50-fold NaCl gradient. 2.