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Research Letter

Supercapacitor electrodes with high active mass loading R. Poon and I. Zhitomirsky, Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, L8S4L7, Canada Address all correspondence to I. Zhitomirsky at E-mail: [email protected] (Received 25 April 2018; accepted 6 August 2018)

Abstract We report the fabrication and testing of MnO2–carbon nanotube (NT) electrodes for supercapacitors (SCap) with high active mass (AM). Cetylpyridinium chloride surfactant was used as a capping agent for synthesis and a phase transfer agent for the liquid–liquid extraction. Water immiscible solvent, n-butanol, was used as a receiving and reducing medium for the synthesis of MnO2 from cetylpyridinium permanganate. Improved co-dispersion and nanoscale mixing of MnO2 and NT enabled the fabrication of advanced electrodes with mass loading of 42–61 mg/cm2, ratio of AM to current collector mass of 0.63–0.91, which showed the highest capacitance of 8.95 F/cm2.

Introduction MnO2 has gained significant interest as a supercapacitor (SCap) material due to its high gravimetric capacitance (Cg, F/g), large and stable voltage window in Na2SO4 aqueous electrolyte, low toxicity, and low cost.[1–6] The charging process is described by the below equation: MnO2 + R+ + e− ↔ MnO2 R

(1)

Small size of MnO2 particles and porous electrode microstructure enable good ion access (R+ = H+, Na+, and K+) to the particle surface.[7] Carbon nanotube (NT) additives allow for improved electronic conductivity of MnO2–NT composites.[8–10] Despite the impressive progress[4,8,11–14] achieved in the design of MnO2-based composites, there is a need for the manufacturing of advanced SCap electrodes with high active mass (AM) for practical applications. It is known that Cg decreases significantly with increasing AM.[15] Moreover,[15] limited electrolyte access to AM can result in reduction of areal capacitance (CS, F/cm2) with increasing AM. In this scenario, the bulk material exhibits low capacitance whereas surface region is highly capacitive, and the overall capacitance is reduced since total capacitance is governed by the capacitor with lower capacitance. A high CS must be achieved at low electrode resistance. Another important characteristic of the SCap electrodes is the ratio (δm) of AM to current collector mass. It should be noted that δm values are often about 0.5–2% for films with AM of 0.5–1.0 mg/cm2 on metal foils. Substantially higher δm values are required for advanced lightweight SCap. The objective of this study was the development of MnO2– NT electrodes with high AM and enhanced performance. Our conceptually novel manufacturing method is based on the synthesis of surfactant–MnO4 complex compound for the MnO2

fabrication. The surfactant inhibited MnO2 particle growth, enabled liquid–liquid extraction of MnO2, and formation of composites with improved morphology. This method eliminated the problems related to the MnO2 particle agglomeration during filtration and drying and their re-dispersion in the electrode proce