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High-performance organic pseudocapacitors via molecular contortion

Abstract

Pseudocapacitors harness unique charge-storage mechanisms to enable high-capacity, rapidly cycling devices. Here we describe an organic system composed of perylene diimide and hexaazatrinaphthylene exhibiting a specific capacitance of 689 F g−1 at a rate of 0.5 A g−1, stability over 50,000 cycles, and unprecedented performance at rates as high as 75 A g−1. We incorporate the material into two-electrode devices for a practical demonstration of its potential in next-generation energy-storage systems. We identify the source of this exceptionally high rate charge storage as surface-mediated pseudocapacitance, through a combination of spectroscopic, computational and electrochemical measurements. By underscoring the importance of molecular contortion and complementary electronic attributes in the selection of molecular components, these results provide a general strategy for the creation of organic high-performance energy-storage materials.

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Fig. 1: Synthesis, structure and performance of the electroactive polymer PHATN.
Fig. 2: Three-electrode CV and charging mechanism of PHATN.
Fig. 3: Three-electrode electrochemical characterization of pseudocapacitive behaviour and kinetics of PHATN.
Fig. 4: Characterization of two-electrode button cells assembled from PHATN(−) and AC(+) electrodes.

Data availability

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

This work was supported by the US National Science Foundation Division of Materials Research under Award number DMR-2002634 and the Office of Naval Research (ONR) under Award no. N00014-16-1-2921. The electrochemical measurement apparatus was purchased with the help of the US Air Force Office of Scientific Research (AFOSR) Grant No. FA9550-18-1-0020. C.N. thanks S. Buckler and D. Buckler for their generous support. J.C.R. and S.R.P. are supported by the US Department of Defense through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program. V.A.P. is supported by the National Science Foundation Graduate Research Fellowship Program (NSF GRFP #2019279091). D.A.R. thanks the Columbia Nano Initiative for postdoctoral fellowship support. L.E.M. thanks Columbia University for lab startup funding. Y.Y. acknowledges support from the Air Force Office of Scientific Research (FA9550-20-1-0233).

Author information

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Authors

Contributions

J.C.R., Y.Y., X.R., C.N. and S.R.P. designed the experiments. S.R.P. and J.G. synthesized and spectroscopically characterized the materials. J.C.R. and V.A.P. fabricated the devices and performed the electrochemical measurements. R.M. and L.E.M. performed solid-state NMR measurements. D.A.R. and S.R.P. performed the gas adsorption analysis. H.Z. provided the activated carbon electrodes. S.R.P. and M.L.S. performed the DFT modelling. All authors discussed the data and contributed to writing the manuscript.

Corresponding authors

Correspondence to Xavier Roy or Colin Nuckolls or Samuel R. Peurifoy.

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Competing interests

The authors declare no competing interests.

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Peer review information Nature Materials thanks Olivier Fontaine and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Solid state 13C NMR of PHATN.

a, Cyclized PHATN (2) and the thermolyzed product (PHATN). Note the retention of the characteristic aromatic material peaks between 𝛿 200-100 ppm and the disappearance of the alkyl peaks between 𝛿 75-0 ppm, indicating the near-quantitative removal of the alkyl chains during thermolysis. b, Solid-state 13C NMR of the PHATN material incorporated into an electrode (see Methods for details), both as-fabricated (orange trace) and soaked in electrolyte (purple trace), showing a shift assigned to ion association to a carbonyl within the material. Asterisks denote spinning sidebands at magic angle spinning frequency of 18 kHz.

Extended Data Fig. 2 DFT model of extended PHATN.

(a) Top-view and (b) side-view of the DFT energy-minimized structure of Extended PHATN composed of multiple units of PDI and HATN. The accessible space provided by molecular contortion is clearly visible.

Extended Data Fig. 3 Specific capacitance as a function of current density for PHATN and a suite of benchmark materials.

Green symbols are carbon-based materials;35,49,50,51 red symbols are conducting polymer-based materials;5,27,31,32,33,34,52,53,54,55,56,57,58,59,60,61,62,63,64 purple symbols are hybrid organic/inorganic materials;65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82 and blue symbols are inorganic materials83,84,85,86,87,88,89,90,91,92. PHATN outperforms nearly all other pure organic materials at lower rates, and at higher rates achieves performance unprecedented in any material class besides inorganic compounds. PHATN values and all reference values are taken from three-electrode measurements.

Extended Data Fig. 4 Gas adsorption measurements of PHATN and PA-PDI.

Adsorption (filled symbols) and desorption (open symbols) isotherms of CO2 for PHATN (dark green) and PA-PDI (blue) collected at −78 °C. Analysis of the PHATN isotherm (dark green) shows a Brunauer-Emmett-Teller surface area of 131 m2/g, calculated using the pressure range 50–227 torr, and a Langmuir surface area of 671 m2/g, calculated using the pressure range of 227–647 torr. However, as PHATN can likely undergo further structural distortions under operating conditions, these values are intended to be an estimate of the surface area and are included here for reporting purposes only. Analysis of the PA-PDI isotherm (blue) shows a Brunauer-Emmett-Teller surface area of 12 m2/g, calculated using the pressure range 50–227 torr. This is indicative of extremely low porosity, consistent with our hypothesis that contortion is crucial to the characteristic porosity shown in PHATN.

Extended Data Fig. 5 EIS measurements of PHATN performed at a range of hold potentials.

a, High frequency region of the Nyquist plots displaying potential dependence of the diameter of the semicircular segment. The change in diameter of the semicircle with potential is indicative of a change in charge transfer resistance, as expected from a pseudocapacitive process93. The dotted lines are guides for the eye. b, Low frequency region of the Nyquist plots, which shows steeper Warburg regions at more negative potentials. c, Frequency dependence of the specific capacitance, which shows a low-frequency plateau forming near 800 F/g – near the maximum measured capacitance (689 F/g) and approaching the theoretical capacitance of the material (996 F/g). The potentials are in V vs Hg/HgO.

Extended Data Fig. 6 Self-discharge of PHATN in a three-electrode configuration.

(a), (b), and (c) show the effect of holding the electrode at the max charge voltage before allowing to relax, with (a) in linear time, (b) in log time, and (c) in root time. (d), (e), and (f) show the effect of the max charge voltage on the self-discharge behavior, with (d) in linear time, (e) in log time, and (f) in root time.

Extended Data Fig. 7 Electrochemical characterization of PA-PDI, an uncontorted control material.

a, CV of PA-PDI shows similar reversible redox peaks to PHATN, though sharper and less broad. b, GCD of PA-PDI shows relatively high IR drop and less ideal triangular capacitor shape. All measurements were performed in 6 M KOH aqueous electrolyte.

Extended Data Fig. 8 Direct electrochemical comparison of PA-PDI with PHATN.

a, CV at 50 mV/s and b, GCD at 1 A/g. c, Nyquist plot of PA-PDI, when compared to PHATN, displays a less steep Warburg slope in the low-frequency region, indicating less capacitive character. Both measurements are performed at −0.7 V vs Hg/HgO. Frequency range is from 100 kHz to 20 mHz.

Extended Data Fig. 9 Rate and cycling performance of PA-PDI.

a, Specific capacitance values vs. rate for PHATN and PA-PDI. The latter has consistently lower performance, especially at high rates, attributable to the absence of contortion and resulting internal space which enable ion movement. b, Capacity retention and coulombic efficiency vs. number of cycles for PA-PDI. The material maintains the same high stability over 10,000 cycles as PHATN, indicating that the polymeric material is well-formed and not affected by repeated charging and discharging.

Extended Data Fig. 10 Galvanostatic intermittent titration technique (GITT) for PHATN and PA-PDI.

GITT discharge curves as function of time for (a) PHATN and (b) PA-PDI. The measurements were performed at 2 A/g with 20 s current pulse and 1 min open circuit relaxation. c, Schematic interpretation of the GITT data to estimate the diffusion constant (see below equation). d, Diffusion coefficient (D) of the charge carrying species measured from GITT for PHATN and PA-PDI. Comparing the diffusion coefficients across the potential range, we observe that the diffusion coefficient of PHATN (~1 × 10−7 cm2/s) is nearly one order of magnitude larger than that of PA-PDI (~3 × 10−8 cm2/s), indicating that the charge carrying species (K+) diffuses through the internal space created by contortion much more quickly, leading to the superior performance of PHATN. These values of ionic diffusion coefficients are comparable to reported values measured with aqueous electrolyte in porous polymeric materials94,95.

Supplementary information

Supplementary Information

Supplementary Figs. 1–30, Tables 1–8, Schemes 1–6, XYZ coordinates of DFT energy-minimized structures and discussion.

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Russell, J.C., Posey, V.A., Gray, J. et al. High-performance organic pseudocapacitors via molecular contortion. Nat. Mater. (2021). https://doi.org/10.1038/s41563-021-00954-z

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