performance enhanced to 175 V and 10 µA. ZIF-62/Teflon charges a 1 µF capacitor in 150 s

and showed endurance for 21 h with good stability till a relative humidity of 60%. ZIF-62/

Teflon TENG was integrated with the human body for energy harvesting based on body

motions during exercises and the number of voltage peaks helps as a fitness tracker.

Hajraet al. investigated cyclodextrin (CD) MOF having Na as the metal centers [36].

This is a major development towards bio-compatible MOF TENG as the biocompatibility

of ZIF-based MOF is not consistent. They have studied the α, ß, and γ CD MOF as a

positive layer and Teflon as a negative layer in multiunit Z-shape vertical separation

mode. Kelvin probe force microscopy (KPFM) is used to characterize the surface poten­

tials of α, ß, and γ CD MOFs. They followed the trend α > γ > ß CD MOF. The difference in

surface potentials originated due to the structure of ligands as all CD MOFs showed

similar surface roughness at higher scan rates. The TENG performances of the devices

agree with KPFM analysis showing the high output voltage of 152 V for α CD MOF/

Teflon TENG with 1.3 µA current and 14.3 nC charge. The α CD MOF/Teflon TENG was

used to charge a capacitor using a bridge rectifier to convert the AC output of the TENG

to DC power. It was attached to the human body and shoes to harness the energy pro­

duced during daily activities like walking and jogging.

PENG works on the principle of piezoelectricity i.e., the voltage difference will be

generated across the materials under the application of force. The voltage across the

material will be generated due to the change in the center of mass of anions and cations in

the material. The reverse process is also possible where voltage difference across the

material will induce stress in the material named the inverse piezoelectric effect. The

applications of PENG are more versatile than TENG. They can be used in smart textiles,

ultrasonic transducers, and the transportation sector. The structure of the piezoelectric

device is very simple with piezoelectric material sandwiched between two electrodes.

Piezoelectric devices can be operated in different modes based on the nature of applied

pressure like compression, shear, and expansion. Ceramics that does not have a center of

symmetry such as quartz, ZnO, etc.; polymers such as PVDF, PTFE, etc.; and biomaterials

such as cellulose, onion skin, etc. show piezoelectric properties. Piezoceramics show

higher conversion efficiencies than piezopolymers, but they cannot handle high strains

due to their brittle nature. So, composites of both were developed to combine their

benefits. Multifunctional PENG sensors are more accurate than TENG-based sensors as

they suffer from static electricity, humidity, and temperature changes. MOFs are used as

nanofillers in piezopolymers owing to their crystalline nature, porosity, and surface

functionalities. MOFs have also been introduced in PENG to improve the properties of

PENG for different applications.

Moghadam et al. developed zirconium-based MOF PVDF composite PENG by the

electrospinning method for self-powered arterial pulse monitoring [37]. Incorporation of

MOF into PVDF increased the piezoelectric constant by 3.4 times with 3% MOF loading

and 4.4 times with 5% MOF loading owing to the increased order of crystallinity and

polar β phase PVDF content in the composite. PVDF MOF composite nanofibers were

sandwiched between copper-coated aluminum foils. The PVDF-MOF composite showed

a peak-to-peak voltage of 600 mV under an applied force of 5 N. They used it as a

wearable sensor for monitoring the pulse signals of a human, which is discussed in the

next section.

Ferroelectric materials are analogous to ferromagnetic materials that produce electric

field polarization in absence of an electric field. All ferroelectric materials show a pie­

zoelectric effect due to the absence of center symmetry. In this aspect, Roy et al. devel­

oped a self-polarized ferroelectric device for non-invasive health monitoring applications

Advancements in MOFs Based Nanogenerators

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