Fig. 6: Energy harvesting driven by moisture and steam based on ion concentration gradients.

a Schematic illustration of a single moisture-electric generator (MEG) unit featuring AlgCa/Na network embedded within a PVA gel. The water gradient is defined as the difference in moisture content between upper and lower sides of the MEG sample. bThe open-circuit voltage (V_oc) (red) of an MEG device over time measured under open environment with fluctuating RH with synchronously recorded ambient RH(black). a, b Reproduced with permission from ref. ¹⁴. Copyright 2024, Springer Nature. c Structure of a single ionic hydrogel moisture-electric generator (IHMEG) device features asymmetric moisture penetration layers. d Continuous DC open-circuit voltage (V_oc) output (red) of an IHMEG device over time under ambient environmental conditions, with ambient relative humidity (blue) and temperature (black) recorded synchronously. c, d Reproduced with permission from ref. ²²⁶. Copyright 2022, Wiley. e Structure of a nanocellulose-based hydrogel moisture-electric nanogenerator (N-HMEG) with asymmetric moisture penetration layers. Reproduced with permission from ref. ²²⁷. Copyright 2024, Elsevier. f Schematic illustration depicts moisture-enabled electric generation in the bilayer of polyelectrolyte films (BPFs), which enables spontaneous water adsorption and ion dislocation from moist air and the schematic of the HMEG device. Reproduced with permission from ref. ²⁴⁵. Copyright 2021, Springer Nature. g Schematic diagram of a single Ion-Exchange Membrane Moisture-Electric Generator (IEM-MEG) (left) and working principle of IEM-MEG (right). Reproduced with permission from ref. ⁸⁷. Copyright 2025, Wiley. h Schematic illustration of the moisture-activated electricity generator. i Current density measurement of MEGs under different flow conditions. Synchronized flow (left), water flow only (center) and desynchronized flow (right). h, i Reproduced with permission from ref. ⁸⁵. Copyright 2025, Wiley.