Science 14Feb2020

INSIGHTS | PERSPECTIVES

sciencemag.org SCIENCE

GRAPHIC: DACE GAO

Recently, hydrogels and ionogels have
been used as highly transparent and de-
formable electrodes in stretchable iono-
tronic devices ( 7 ), including artificial skin
and muscle, light emission ( 8 ), power gen-
eration ( 9 ), and human-machine interfaces.
The cross-linked polymer matrix in hydro-
gel endows the material with a solid phase
and freestanding nature, and the electrolyte
provides ionic conductivity for signal trans-
mission or energy coupling. Solid-state
ionic diodes can be read-
ily devised by bringing two
hydrogels, each doped with
oppositely charged poly-
electrolytes, into contact
to form an asymmetric in-
terface ( 10 ). However, the
durability of hydrogels is
limited by water evapo-
ration. Also, most of the
soft diodes have involved
Faradaic currents gener-
ated from electrochemical
processes and are often plagued by water
electrolysis and corrosion issues, which are
impediments to stable operation.
Kim et al. overcome the above-mentioned
challenges by rationally designing and
synthesizing a pair of ionoelastomers that
excludes the use of volatile solvent. The sta-
tionary polyelectrolyte networks are either
positively or negatively charged, and the as-
sociated counterions (ionic liquid moieties)
are free to move in response to concentra-

tion gradients or electric fields. The authors

exploited the interface between polyanionic

and polycationic ionoelastomers to build a

polyanion-polycation heterojunction with

an ionic double layer (IDL) to create a solid-

state ionic diode with inherent stretchabil-

ity, stability, and robust ionic rectification.

At the heterojunction, entropic diffusion

of mobile counterions forms a depletion

layer in the form of an IDL, similar to the

depletion region in a semiconductor p-n

junction (see the figure).

The remaining immobi-

lized charges on polyelec-

trolyte chains give rise to

a drift current opposite to

the diffusion current un-

til equilibrium is reached,

and induce a strong elec-

trostatic adhesion between

the two layers. The authors

confirmed the existence of

the IDL by measuring the

built-in potential in the de-

pletion layer and by monitoring the build-

up or collapse of the IDL when the junction

was subjected to either reverse or forward

biases. The non-Faradaic current conduc-

tion of the solvent-free charged couples,

which also have a wide electrochemical

window (±3 V), circumvents the shortcom-

ings of hydrogel- or solution-based ionic

diodes in which loss of liquid electrolyte is

unavoidable. The stretchability of the iono-

elastomer makes it compatible with stretch-

able and sustainable ionic logic circuitry in

future ionotronic devices.

In ionotronic devices, a high rectification

ratio of the soft diodes is desirable. For ex-

ample, a diode could serve as a selector de-

vice for isolation in series with the memory

element in stackable memory arrays. This

could be tailored from the contributions of

IDL and electrical double-layer (EDL) ca-

pacitance. The latter stems from the metal-

ionoelastomer interface. We expect future

generations of ionoelastomers to achieve

better conductivity by using weakly asso-

ciated counterions and facile segmental

motion of polymeric chains for ionic move-

ments. Developing ionic conductors that

are self-healing and thermally stable ( 11 )

would broaden the impact of ionotronics.

Prevention of water or moisture absorption

under ambient conditions will require the

development of hydrophobic polyanion and

polycation networks to avoid hygroscopic-

ity. Other possible hindrances, such as the

parasitic formation of ionic liquid by the

combination of two counterions near the

junction or possible salt formation under

certain circumstances that may cause varia-

tions in diode performance or junction

breakdown, deserve further attention.

The mechanoelectrical response of

the deformable ionic junction provides

a promising pathway to realize stimulus-

responsive and tunable rectification. Simi-

lar flexible ionic diodes for low-frequency

mechanical energy harvesting have been

demonstrated ( 12 ). This property is attrac-

tive for developing a digitally autonomous

programmable approach for controlling

the preferential conduction of ions and

even molecules. Analogous to applications

of field-effect transistors, the ionic modu-

lation of reconfigurable ionic circuits—and

eventually the digital manipulation of bio-

molecules—is plausible. j

REFERENCES AND NOTES

1. E. Gouaux, R. Mackinnon, Science 310 , 1461 (2005).


2. H. Chun, T. D. Chung, Annu. Rev. Anal. Chem. 8 , 441
   (2015).


3. H. J. Kim, B. Chen, Z. Suo, R. C. Hayward, Science 367 ,
   773 (2020).


4. B. Lovrecek, A. Despic, J. Bockris, J. Phys. Chem. 63 , 750
   (1959).


5. Q. Pu, J. Yun, H. Temkin, S. Liu, Nano Lett. 4 , 1099 (2004).


6. W. Guan, R. Fan, M. A. Reed, Nat. Commun. 2 , 506 (2011).


7. C. Yang, Z. Suo, Nat. Rev. Mater. 3 , 125 (2018).


8. J. Wang et al., Adv. Mater. 28 , 4490 (2016).


9. K. Parida et al., Adv. Mater. 29 , 1702181 (2017).


10. O. J. Cayre, S. T. Chang, O. D. Velev, J. Am. Chem. Soc. 129 ,
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11. J. Lee et al., Adv. Mater. e1906679 (2019).


12. Y. Hou et al., Adv. Energy Mater. 7 , 1601983 (2017).

ACKNOWLEDGMENTS

The authors are supported by the National Research

Foundation, Prime Minister’s Office, Singapore under awards

NRF-NRFI2016-05 and NRF-CRP13-2014-02.

10.1126/science.aba6270

Electrostatic adhesion

Reverse bias

Equilibrium state

Polycation

Polyanion

The ionic heterojunction is formed by balancing two concurrent
processes: the difusion of mobile ions driven by entropy, and the
drift of mobile ions due to the built-in potential i (yellow curve,
right) induced by the remaining fxed ions.

Interfacial attraction is

generated from the

remaining fxed ions.

Built-in potential

i

Depletion region builds up and no charge

fows (red curve below; creates a barrier).

Forward bias

Depletion region collapses and charge fows

(blue curve above).

Fixed cation

Fixed anion

Mobile cation

Mobile anion

Depleted

Neutral

Neutral

junction

Neutral

+

++

––

“...an ionic analogy

to p-n junctions is

expected to bring

about unconventional

circuits that simulate

the nervous system...”

736 14 FEBRUARY 2020 • VOL 367 ISSUE 6479

Formation of an ionic heterojunction
Analogous to the p-n junction in solid-state electronics, Kim et al. constructed an ionic heterojunction
at the coherent interface between a polyanionic and a polycationic ionoelastomer.

Published by AAAS