High-performance energy storage is indispensable to address the intermittency of sustainable energy. Pseudocapacitors are a family of electrochemical energy storage devices that store electricity via redox reactions of pseudocapacitive materials supported on electrically conductive substrates (e.g., carbon). Commercially viable pseudocapacitor electrodes must contain active materials higher than 5 mg/cm2 [J. Electrochem. Soc. 164, A1487-A1488 (2017)]. On conventional carbon substrates, however, the high mass loading of electrically insulating pseudocapacitive materials often leads to low capacitances, due largely to the increased thickness of pseudocapacitive films and clogged pores, both of which severely retard ion accessibility and charge transport.

Our innovative solution to the high-mass-loading pseudocapacitor challenge comes from the disparate design and fabrication of carbon supports from macromolecular level, i.e., using a block copolymer, polyacrylonitrile-block-poly(methyl methacrylate) (PAN-b-PMMA). Our group's expertise in polymer chemistry ensures readily availability of high-quality electro-spun PAN-b-PMMA fibers. For energy storage, we hypothesize that the porous carbon fibers (PCFs) derived from PAN-b-PMMA are desirable pseudocapacitor electrode supports. This hypothesis is on the basis of the structure of PCFs: the uniform, interconnected mesopores and continuous carbon matrices resulted from the microphase-separation between PAN and PMMA, followed by the decomposition of PMMA and carbonization of PAN. Specifically, the abundant, interconnected mesopores contribute a large surface area (~600 m²/g) that keeps the thicknesses of the deposited pseudocapacitive films thin. The continuous carbon fibers facilitate electron transport in the support. The interconnected pores offer ions low-tortuosity diffusion channels. The relatively large mesopore sizes (>5 nm) ensure efficient ion transport when the thickness of pseudocapacitive layer is ~2 nm. All these characteristics are vital to the simultaneous achievement of high mass loading and ultrafast charge transport.

Figure. (a) Schematic illustration of the synthesis of porous carbon fibers (PCF) and loading of MnO2. (b) SEM image of porous carbon fibers after depositing MnO₂ for 2h (PCF@MnO₂-2h). (Inset) A photograph of a piece of PCF@MnO₂-2h next to a U.S. penny. (c) The pore size distributions of PCF, PCF@MnO₂-1h and PCF@MnO₂-2h. (d) The ion-diffusion resistivities of PCF@MnO₂ and other electrodes. (e) Mass loadings, gravimetric capacitances and areal capacitances of PCF@MnO₂ and other reported electrodes. The solid and open dots are capacitances based on the mass loadings of the entire electrodes and MnO₂, respectively.

In this work, we chose a low-cost, high-performance, and environment-benign pseudocapacitive material, MnO2, to be coupled with PCFs. We grew MnO2 nanosheets inside the mesopores of PCFs via a self-limiting redox reaction between carbon and KMnO4. A two-hour deposition resulted in a high mass loading approaching 7 mg/cm2. N2-physisorption revealed that the uniform mesopore size reduced from 11.7 nm to 9.3 nm after the deposition, suggesting the thickness of the MnO2 layer was less than 2 nm thick inside the mesopores. The partially filled mesopores contributed to the small ion-diffusion resistance below 2 Ω s0.5, which was significantly smaller than MnO2 electrodes prepared via other means. Owing to the high mass loading and fast charge transport, our MnO2-PCF composite electrode exhibited gravimetric and areal capacitances far exceeding those of conventional MnO2-based pseudocapacitive electrodes at comparable mass loadings.

Our work unequivocally signifies the promise of block copolymer-derived porous carbon materials in electrochemical energy storage. We speculate that PCFs will revolutionize the design of electrochemical devices beyond pseudocapacitors, including secondary batteries, desalination cells and heterogeneous catalysts.

The paper is available free of charge at https://www.nature.com/articles/s41467-019-08644-w.

Tianyu Liu

Postdoctoral Associate, Virginia Tech

Guoliang Liu

Assistant Professor, Virginia Tech