Wide Potential Window Supercapacitors Using Open‐Shell Donor–Acceptor Conjugated Polymers with Stable N‐Doped States

Abstract

IntroductionSupercapacitors are electrochemical energy storage devices that store charge through fast, reversible redox reactions, enable load-leveling, regenerative energy harvesting, and high power applications. [1][2][3][4][5][6] The energy density of a supercapacitor (E = CV 2 /2) is linearly dependent on the specific capacitance C and proportional to the square of the operational voltage V. The main strategies to increase energy depend upon the mechanism of charge storage, whether capacitive or Faradaic. Capacitive electrodes store charge on the surface; hence, research is focused on strategies to increase the surface area, typically based on high surface area carbons. [7][8][9] Faradaic materials undergo redox reactions and offer the ability to tune the operating voltage. There has been considerable success in tuning the cathode voltage and delivering high capacitance systems operating at the upper limit of prototypical nonaqueous electrolytes. [1,7] However, the device voltage and energy remain limited by a lack of complementary high power electrodes for the anode. [10,11] In the absence of new high voltage electrolytes, there is no room to further extend the cathode potential, because over-potential may lead to hazardous runaway reactions between the highly oxidized cathodes and flammable nonaqueous electrolytes. In contrast, it is generally safe for nonaqueous electrolytes to operate down to −2 V relative to the Ag/AgCl electrode, and the development of anode materials is critical to increase the energy densities of supercapacitors.Consequently, there is an urgent need for strategies aimed at extending the operational voltage toward negative potentials to improve the energy density and cell voltage of supercapacitors. As an alternative to metal oxides, redox-active macromolecules offer mechanical flexibility, low-cost, and scalability relevant for small and large scale applications. [12][13][14][15][16] In radical polymers, [17][18][19][20] the redox sites are at pendant radical groups distributed along a polymer backbone. Due to the insulating nature of the backbone and low conductivity, devices based on these materials require blending with additional conductive materials for electron transport, ultimately lowering the electrode energy Supercapacitors have emerged as an important energy storage technology offering rapid power delivery, fast charging, and long cycle lifetimes. While extending the operational voltage is improving the overall energy and power densities, progress remains hindered by a lack of stable n-type redox-active materials. Here, a new Faradaic electrode material comprised of a narrow bandgap donor−acceptor conjugated polymer is demonstrated, which exhibits an open-shell ground state, intrinsic electrical conductivity, and enhanced charge delocalization in the reduced state. These attributes afford very stable anodes with a coulombic efficiency of 99.6% and that retain 90% capacitance after 2000 charge-discharge cycles, exceeding other n-dopable organic materials. Redox cycling proce...

Open‐Shell Donor–Acceptor Conjugated Polymers with High Electrical Conductivity

“…[ 26,27 ] Substitution of the TQ acceptor with methyl ( P1 ), phenyl ( P2 ), and thiophene ( P3 ) moieties affords the capability to fine‐tune structural and electronic features. The polymers were synthesized using a modified microwave‐assisted Stille cross‐coupling copolymerization between (4,4‐dihexadecyl‐4 H ‐cyclopenta[2,1‐ b :3,4‐ b ′]dithiophene‐2,6‐diyl)bis(trimethylsta‐nnane) [ 32 ] and 4,9‐dibromo‐6,7‐dimethyl‐[1,2,5]thiadiazolo[3,4‐ g ]quinoxaline [ 33 ] for P1 , 4,9‐dibromo‐6,7‐diphenyl‐[1,2,5]thiadiazolo[3,4‐ g ]quinoxaline [ 34 ] for P2 , and 4,9‐dibromo‐6,7‐di(thiophen‐2‐yl)‐[1,2,5]thiadiazolo[3,4‐ g ]quinoxaline [ 35 ] for P3 . We used Pd(PPh 3 ) 4 as the catalyst (3.5 mol%) in xylenes and a reaction time of 30 min for P1 , and 80 min for P2 and P3 , followed by in situ chain‐end functionalization with thiophenes.…”

Highly Stable Indacenodithieno[3,2-b]thiophene-Based Donor–Acceptor Copolymers for Hybrid Electrochromic and Energy Storage Applications

“…1.2 g cm –3 of a conjugated polymer (thiophene- and EDOT-based, alkyl side chains containing polymers typically ranging between 1.0 and 1.4 g cm –3 ), 70 , 80 the C v of PIDTT–TBT, PIDTT–EBE, PIDTT–TBzT, and PIDTT–EBzE measured at the high scan rate of 100 mV s –1 corresponds to a C s of approximately 150, 110, 160, and 190 F g –1 , respectively, which are the highest values reported for all-polymer electrochromic supercapacitors (for comparison, see Table S4 and Figure S24, Supporting Information ). 38 , 46 , 48 , 63 It is then significant that the C s of PIDTT–EBzE reaches 260 F g –1 at the low scan rate of 5 mV s –1 and compares to that of the state-of-the-art polymer composite electrodes used in supercapacitors—as a single electro-optical material. 21 , 41 , 50 …”

Donor-Acceptor Conjugated Macrocycles with Polyradical Character and Global Aromaticity

“…The cross-conjugated donors with a highly planar molecular backbone have a high degree of electronic coherence through extended π-conjugation, thereby leading to a small bandgap ( London et al., 2017 , 2019 ; Wang et al., 2019 ; Foster et al., 2014 ). For example, the cyclopentadithiophene (CPDT) donor, when conjugated with thiadiazoloquinoxaline (TQ) acceptor unit, provides a vanishingly small singlet-triplet energy gap upon increasing oligomer length, resulting into triplet to be the ground state at a longer chain length ( London et al., 2019 ).…”

“…The π-conjugated open-shell organic semiconductors (OSCs) with one or more unpaired electrons in the degenerate or near-degenerate non-bonding molecular orbitals (NBMOs) exhibit unique electronic, optical, and magnetic properties ( Lewis, 2015 ; Stuyver et al., 2019 ; Rudebusch et al., 2016 ; Dressler et al., 2018 ; Gopalakrishna et al., 2018 ; London et al., 2019 ; Abe, 2013 ; Ni et al., 2020a ; Rickhaus et al., 2020 ; Sabuj and Rai, 2020 ). The open-shell OSCs with di- or polyradical character are of particular interest as their unique electronic structure makes them suitable for optoelectronic applications, such as nonlinear optics, photovoltaics, charge-storage, and spintronics devices ( Rajca, 1994 ; Abe, 2013 ; Nakano, 2017 ; Mas-Torrent et al., 2009 ; Wang et al., 2019 ). Despite their significance for many technologies, the synthetic routes are not well established for molecules with a higher radical (above diradical) characters.…”