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Page 14 of 20 Dang et al. Chem Synth 2023;3:14 https://dx.doi.org/10.20517/cs.2022.33

By leveraging the piezoelectric property, selenium nanomaterials serve as excellent wearable sensors for
mechanical deformation and physiological diagnosis. Wu et al. developed a Se nanowire-based piezoelectric
nanogenerator (PENG) that consisted of PDMS layers, electrodes derived from Ag nanowires, and
piezoelectric Se nanowires, as presented in Figure 6D . Here, the PDMS layer serves two purposes: one is
[85]
to act as the insulating layer between the Se and Ag nanowires, and the other is to encapsulate the Se and Ag
nanowires to prevent oxidation and performance deterioration [Figure 6D (i)]. This device was flexible,
stretchable, and robust enough to withstand mechanical deformation without breaking. Therefore, PENGs
could be adhered to the skin of the human body to detect very faint deformations such as finger bending.
The piezoelectric output voltage increases with the bending angle, allowing the finger movement to be
recognized. Moreover, the nanogenerator could also be attached to a human wrist to detect the pulses of the
radial artery in real time. Three distinct peaks are observed in the piezoelectric current output curve for the
cardiac cycle. The P1, P2, and P3 represent the early systolic peak pressure, late systolic augmentation
shoulder, and diastolic pulse waveform, respectively. These peaks can be valuable indicators to quantify the
physiological information about the cardiovascular system of the wearer.

Flexible batteries
Recent studies have also demonstrated the huge potential of Se nanomaterials for high-performance
electrodes in energy storage devices. Typical lithium-sulfur (Li-S) batteries show high theoretical specific
capacities and energy densities. However, two major issues have hindered their practical application. First,
sulfur is not a preferable electrode material because of its low electronic and ionic conductivity. Second,
electrolyte-soluble polysulfide intermediate products are dissolvable in liquid electrolytes when a battery is
working, leading to the significant shuttle effect and capacity loss [95-98] .

Se is an alternative to sulfur and has been used as a potential electrode material for Li-Se batteries [99-101] .
However, bulky Se cathodes exhibit poor cycle performance and low Coulombic efficiency in comparison
with S cathodes . One strategy to overcome this issue is to incorporate Se into a 3D interconnected
[102]
[103]
mesoporous carbon nanofibers (CNFs) . This nanostructured Se/CNF enabled high-performance
electrodes [Figure 7A]. The resulting Li-Se batteries can deliver a reversible capacity of 516 mAh g after 900
-1
-1
cycles without any capacity loss at 0.5 A g .
Moreover, self-standing nanostructured graphene-Se@CNT film electrodes for Li-Se batteries have been
synthesized . The electrode displays high flexibility and bendability, as shown in Figure 7B. The flexible
[104]
film cathode delivers a high reversible discharge capacity of 400 mAh g that is slightly reduced to 315 mAh
-1
g , as well as a stable Coulombic efficiency above 96% even after 100 charging-discharging cycles.
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Nanostructured Se has also been integrated into Na-Se batteries for improved performance. For example,
impregnating Se into microporous multichannel carbon nanofibers (MCNF) forms a self-standing cathode
-1
film in a Na-Se battery [Figure 7C] that exhibits an extraordinary discharge capacity (596 mA h g at the
100th cycle at 0.1 A g ), excellent rate capability (379 mA h g at 2 A g ), and long-time capacity durability
-1
-1
-1
[105]
over 300 charging-discharging cycles . With more advances in performance, flexible batteries based on
nanostructured Se might constitute a powerful platform for powering flexible and electronic devices and
systems instead of using conventional heavy and rigid counterparts.
CONCLUSION AND OUTLOOK
Se nanomaterials are experiencing a flourishing research momentum. Advances in materials synthesis and
fabrication, control over morphologies and structure, fundamental understanding of structure-properties
relationship, and methodologies of devices integration offer a new class of Se nanomaterial-based flexible
and wearable electronics, with wide-ranging applications in smart sensing, health care and energy. While
the translation of nanoscale materials into macroscopic real devices is truly exciting, some challenges
relevant to fundamental and applied science still exist.