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CNT yarn-based supercapacitors 245

1/2
square root of the scan rate (v ) [20b]. These electrode materials involving
typical diffusion-limited redox reactions can be more strictly classified as
batteries instead of SCs. In general, the specific capacitances of pseudo-
capacitors are much higher than those of carbon materials based on the
EDLC mechanism.

10.1.2 Active materials
10.1.2.1 Metal-organic frameworks
Metal-organic frameworks (MOFs), also known as porous coordination
polymers, are receiving increasing attention in the field of electrochemical
energy storage due to their diverse structures, highly porous frameworks,
and tunable chemical compositions that can be designed at the molecu-
lar level [21, 22]. MOFs can be obtained from relatively cheap precursors.
Inorganic salts (nitrates, sulfates, and chlorides) are typical metal-ion pre-
cursors. Organic linkers are usually multidentate organic ligands such as
carboxylates, azoles or nitriles. Solvothermal synthesis, microwave-assisted
synthesis, and surfactant-assisted synthesis are common methods to fabricate
MOFs [23, 24]. Ni-based MOF shows a pseudocapacitive behavior with
two redox peaks at 0.32 and 0.17 V when applied to a saturated calomel
electrode (SCE) [25]. Zn-doping can be introduced into MOFs to achieve
a specific capacitance as high as 1620 F/g at a scan rate of 0.25 A/g [26].

10.1.2.2 Covalent organic frameworks
Covalent organic frameworks (COFs) represent a fascinating new type of
covalent porous crystalline polymers enabling the integration of organic
building blocks into an ordered structure with atomic precision [27]. In
COFs, the organic building units are held together by strong covalent bonds,
such as BO, CN, BN, and BOSi [28, 29]. Similar to MOFs, the COFs
have high surface areas, controllable pore sizes, and highly flexible molecular
designs. To rapidly and efficiently synthesize COFs, microwave and solvo-
thermal reactions are employed to prepare large-scale COFs [30, 31]. A
pyridine containing COF (TaPa-Py COF) exhibits reversible electrochem-
ical processes in a 1 M H 2 SO 4 electrolyte [32]. A capacitance of 180.5 F/g
−1
for the TaPa-Py COF could be obtained at 20 mV s . At a current den-
sity of 0.5 A/g, it delivered a capacitance of 209 F/g with a combination
of Faradaic capacitance and double-layer capacitance originated from the
pyridine units and the ordered porous structure of the TaPa-Py COF. The
symmetric SC based on the TaPa-Py COF showed excellent cycling stabil-
ity after 6000 charge/discharge cycles with 92% capacitance retention.