Boosting the electrochemical performance of MnO2 composite carbon electrodes via ion insertion confined by carbonized wood pores

Abstract

Manganese dioxide (MnO₂) exhibits high theoretical specific capacity and has low cost. However, its actual capacitance is much lower than the theoretical value; moreover, its practical application is limited by its intrinsically poor electrical conductivity and unsatisfactory cycling stability. In this work, we synthesized a series of wood-carbon-based composite electrode active materials (δ-MnO₂-PPy-WC-m-v-t) by electrodeposition of δ-MnO₂ on PPy-coated delignified wood (PPy-DW) by varying the concentrations of Mn(CH₃COO)₂ aqueous solution (m) under a set deposition voltage (v) for different durations (t = 1, 2 or 3 h), followed by carbonization. δ-MnO₂-PPy-WC-1.5-3-2, abbreviated as δ-MnO₂-PPy-WC, showed the highest areal specific capacitance at varied areal current densities among these δ-MnO₂-PPy-WC-m-v-t materials. The electrochemical performance of δ-MnO₂-PPy-WC was boosted by an ion insertion strategy, i.e., in situ cyclic voltammetry sweeps in a series of electrolytes with varying ion sizes (LiCl, NaCl, and KCl) to insert ions of different sizes into the layers of δ-MnO₂. Among the three kinds of cation (Li⁺, Na⁺, and K⁺) inserted δ-MnO₂-PPy-WC, the Li⁺ inserted δ-MnO₂-PPy-WC electrode ((Li⁺_0.47Mn³⁺_0.47Mn⁴⁺_0.53)O²⁻₂-PPy-WC) shows the best electrochemical performance of the highest areal capacitance (12.6 F cm⁻²), the best conductivity (R_s = 1.35 Ω), and the highest cycle stability (95.2% retention after 10 000 cycles) along with good rate capability (54.0%) due to the optimal match between the Li⁺ hydrated diameter (6.8 Å) and the the wood pores confined interlayer distance of δ-MnO₂ (6.9 Å). The confinement effect of wood pores on ion insertion into δ-MnO₂ layers has been confirmed by the significant disparity in electrochemical performance between cation-inserted δ-MnO₂@CC and δ-MnO₂-PPy-WC. Furthermore, a symmetric supercapacitor, assembled using (Li⁺_0.47Mn³⁺_0.47Mn⁴⁺_0.53)O²⁻₂-PPy-WC as the anode and cathode active materials, exhibited a high areal capacitance of 3.40 F cm⁻² at 1.0 mA cm⁻², high energy density of 1.88 mWh cm⁻² at 1.00 mW cm⁻² and excellent capacitance retention of 96.4% after 10 000 long-term cycles. This work sheds light on a feasible strategy for developing supercapacitors with high areal energy density and long cycling life for energy storage.