Anti-Swelling Polyelectrolyte Hydrogel with Submillimeter Lateral Confinement for Osmotic Energy Conversion

Abstract

Harvesting the immense and renewable osmotic energy with reverse electrodialysis (RED) technology shows great promise in dealing with the ever-growing energy crisis. One key challenge is to improve the output power density with improved trade-off between membrane permeability and selectivity. Herein, polyelectrolyte hydrogels (channel width, 2.2 nm) with inherent high ion conductivity have been demonstrated to enable excellent selective ion transfer when confined in cylindrical anodized aluminum pore with lateral size even up to the submillimeter scale (radius, 0.1 mm). The membrane permeability of the anti-swelling hydrogel can also be further increased with cellulose nanofibers. With real seawater and river water, the output power density of a three-chamber cell on behalf of repeat unit of RED system can reach up to 8.99 W m−2 (per unit total membrane area), much better than state-of-the-art membranes. This work provides a new strategy for the preparation of polyelectrolyte hydrogel-based ion-selective membranes, owning broad application prospects in the fields of osmotic energy collection, electrodialysis, flow battery and so on.

1. Introduction

The development of sustainable energy sources, such as solar, wind, geothermal and other clean energy, is urgent to combat the energy crisis [1–3]. Among various renewable energy sources, the earth-abundant salt difference energy is exactly promising [4, 5]. In theory, the total amount of salinity gradient energy from the confluence of rivers and oceans can reach about 1.4–2.6 TW, which is close to the global electricity demand of about 2.8 TW in 2020 [6]. The salinity gradient energy [7] can be directly converted into electricity energy with membrane-based [8] reverse electrodialysis (RED) technology [9–11]. The maximum energy conversion efficiency can be high as 100% when anion and cation were driven by salinity gradient to separately pass through anion- and cation-selective membrane [12, 13]. During the energy conversion process, the conversion efficiency is highly affected by the ion selectivity of the membrane [14–16]. On the other hand, the converted electricity will be dissipated by the internal resistance of the membrane which lows the output power density [17–19]. Therefore, it is particularly important to optimize the membrane permeability and ion selectivity to achieve high energy conversion efficiency and output power density [13, 19]. Usually, the ion selectivity will decrease when the membrane permeability increases. Simultaneously achieving excellent ion selectivity and membrane permeability is still one of the main challenges to prepare high-performance ion-selective membrane [10, 20, 21].

Pore size at the level of Debye length and high surface charge density are two essential parameters to achieve excellent ion transport selectivity [22]. Ionic polymer is one of the earliest candidates to prepare ion-selective membrane owing to its low cost, charged 3D network structures and excellent ion transport capacity [23]. However, the ionic polymer is easy to swelling because of the strong water-absorbing capacity and weak polymer skeleton structure. In traditional heterogeneous ion-selective membranes, ionic polymer and resin is mixed together to inhibit the swelling of ionic polymer and ensure the special pore size for ion-selective transport [24]. The resin will increase the ionic resistance of the composite in plain sight. So low-cost heterogeneous ion-selective membranes mainly hold its own in the primary step of electrodialysis process [25]. Because of the low cost and easy preparation, efforts toward practical ionic polymer-based ion-selective membranes are never stopped [2, 26, 27]. For example, there have been reports of introducing sulfonated functional groups [28]