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Page 6 of 47 Zhou et al. Soft Sci. 2026, 6, 10

promoting the directional migration of counterions along the direction of water flow. This forms a potential
gradient at both ends of the material and ultimately generates a measurable continuous current in the
external circuit .
[13]

The material and structure of the upper and lower electrodes in MEG devices significantly influence ion
dissociation within the power-generating layer. Ideal electrode materials must possess high electrical
conductivity, excellent chemical stability, and ease of integration to enhance electrochemical performance
and ensure long-term stability. Selecting electrode materials with high specific surface area increases
adsorption sites for water molecules while simplifying the manufacturing process. In recent years, strategies
for constructing asymmetric electrodes using different materials or structures have garnered significant
attention . By employing sandwich structures with asymmetric electrodes [such as carbon (C)/aluminum
[78]
(Al) or bimetallic electrodes] , ion concentration gradients can be significantly enhanced and ion migration
[80]
pathways effectively optimized . Increasing material thickness can raise voltage; however, beyond a certain
[81]
limit, it impedes ion transport, leading to a decrease in current . Internal ionic concentration gradients
[82]
within devices are primarily achieved through two strategies . First, intrinsic chemical gradients can be
[12]
established within the material to form spontaneous ion diffusion pathways via asymmetric functional group
modifications [83,84] . This method offers excellent stability, but precise design and controllable preparation
remain challenging. Second, directional water introduction into the material, achieved through asymmetric
wetting structures or unidirectional water transmission paths, can generate ion concentration gradients [34,85] .
This approach imposes minimal requirements on the material itself, offering high flexibility but relying on
external humidity differences for maintenance. Both strategies can be employed synergistically to enhance
the device’s output performance and environmental adaptability.

Ion migration
Driven by internal water or ionic concentration gradients, freely mobile ions within the power-generating
layer undergo directed migration, generating electrical output in the connected external circuit . The water
[66]
gradient is a core factor driving ion migration within the power generation layer, particularly during the
initial moisture absorption phase and in asymmetric electrode structures. During initial moisture absorption
[Figure 2C], water molecules interact with material functional groups, forming a significant water gradient
that serves as the primary driving force for directed ion migration. Taking graphene oxide film (GOF) as an
example, water molecules adsorb directionally and promote the dissociation of a large number of hydrogen
ions (H ) from GOF. Subsequently, under the synergistic action of water gradient formation and diffusion,
+
H undergoes directional movement, generating an electric current . The porous structure of graphene
+
[86]
oxide (GO) efficiently adsorbs water molecules to form a moisture gradient. Based on this principle, an
asymmetric structure constructed with a moisture-proof substrate and a porous upper electrode significantly
enhances this gradient . The upper electrode absorbs large amounts of water and dissociates ions, while the
[60]
moisture-proof substrate maintains relatively low humidity on the underside. Driven by this difference, ions
migrate directionally toward the substrate, generating a stable directional current.

Establishing an ion concentration gradient is another effective strategy for driving directional ion migration
[Figure 2D]. To validate this approach, Huang et al. employed directional laser irradiation technology . By
[87]
controlling the laser intensity to decrease with increasing GO depth, they achieved a gradient-based
redistribution of oxygen-containing functional groups (Ocfgs), ultimately constructing the desired ion
concentration gradient within the GO. In this asymmetric structure, Ocfgs react with water molecules,
generating free ions. Due to the gradient distribution of the functional groups, the concentration of free ions
also exhibits a gradient, effectively driving directional ion migration. Building upon this, the selection of
electrode materials can further increase the ionic concentration gradient. For instance, replacing the top inert
electrode with an Al electrode allows Al to react with water, releasing additional ions . This significantly
[81]