Flexible iontronics based on 2D nanofluidic material

Iontronics focuses on the interactions between electrons and ions, playing essential roles in most processes across physics, chemistry and life science. Osmotic power source as an example of iontronics, could transform ion gradient into electrical energy, however, it generates low power, sensitive to humidity and can’t operate under freezing point. Herein, based on 2D nanofluidic graphene oxide material, we demonstrate an ultrathin (∼10 µm) osmotic power source with voltage of 1.5 V, volumetric specific energy density of 6 mWh cm−3 and power density of 28 mW cm−3, achieving the highest values so far. Coupled with triboelectric nanogenerator, it could form a self-charged conformable triboiontronic device. Furthermore, the 3D aerogel scales up areal power density up to 1.3 mW cm−2 purely from ion gradient based on nanoconfined enhancement from graphene oxide that can operate under −40 °C and overcome humidity limitations, enabling to power the future implantable electronics in human-machine interface.


Determination of Voc for the planar osmotic power source (Au/AgNO3/GO/rGO/Au)
The planar osmotic power source is an open system, and water or oxygen from the environment might participate in the redox reactions. Theoretically, the Nernst equation is used to calculate the voltage introduced by redox reactions. However, since our device is operating as a solid-state power source, the potential could only be estimated with the guide of the Nernst equation as below.
Ag + + e -⇌ Ag at the Au charge collector interface.

Determination of diffusion potential Ediff from ion gradient
As the negatively charged GO is cation-selective, it can transport cations (K + ) preferentially from the 3 high concentration rGO side to the low concentration GO side, generating the diffusion potential (Ediff).
Ediff originates from the ion selectivity of the GO, which can result in differences in the diffusive fluxes of anions and cations. Ediff can be expressed as where t+ and t-are the transference numbers for cation and anion, respectively. R, T, and F are the universal gas constant, absolute temperature, and Faraday constant, respectively. ahigh and alow are the activities of K + in the high concentration and low concentration sides, respectively. In the above equation, (t+ -t-) is called ion selectivity. It was reported that graphene nanopores 1 were found to preferentially transporting K + over its counter anions such as Clwith selectivity ratios over 100 and hydrated K + diffuses orders magnitude more quickly than most hydrated ions within the 2D nanofluidic channels.
Based on such prior art 1 , we calculated the ion selectivity of GO to K + is 0.99. The charge selectivity of GO to K + is very high (close to 0.99), and the Ediff for K + in the osmotic power sources (both the planar one and the 3D GO aerogel one) follow the same equation. The osmotic power source is operated as a solid state power source in contrast to those in traditional electrolyte. The measured Ediff is quite high, also indicating a very high selectivity of K + cations.

Calculation of the energy density of the planar osmotic power source
The energy density of the osmotic cell can be calculated by where I, U, t are the discharge current, the electric potential and discharge time, respectively. Maximum power of 3.39 μW can be calculated in Fig. 1e, and similarly the maximum volumetric specific power density of 28 mW cm -3 was obtained.

Calculation of areal power density of the 3D osmotic power source
The energy density of the osmotic cell can be calculated by where I, U, t are the discharge current, the electric potential and discharge time, respectively.
The areal energy density (Es) can be expressed as where S represent the surface area of the GO device.
The 3D osmotic source was made from the RTIL ionogel, which enables it made in a more compact space with area of 0.32 cm × 0.20 cm. The maximum power of 84 µW can be calculated from Fig. 4b