A Flexible Vertical-Section Wood/MXene Electrode with Excellent Performance Fabricated by Building a Highly Accessible Bonding Interface

Cross-section wood (CW) is generally used as a host for free-standing electrodes, as the abundant opened pores can provide large space for loading guest materials with high electrical conductivity and electrochemical activity. However, there is still a challenge for CW to be used in flexible supercapacitors (SCs) because of its low mechanical strength. Herein, as an alternative to CW, vertical-section wood (VW) with excellent mechanical strength and good flexibility is developed and used as a free-standing and flexible electrode by using Ti3C2Tx (MXene) with ultrahigh conductivity and good electrochemical activity as a guest material. In particular, the highly accessible bonding interface for Ti3C2Tx is first built by delignification on VW to generate abundant pores for continuously absorbing Ti3C2Tx and to expose cellulose with abundant oxygen-containing groups for stable combination with Ti3C2Tx. Then, cyclic pressing is used to form negative pressure to pump the Ti3C2Tx suspension into VW, combining with a preheating process to trigger layer-by-layer self-assembly of Ti3C2Tx nanosheets onto a wood cell wall by evaporating water in the suspension. As a result, the free-standing electrode has a large Ti3C2Tx loading mass proportion of 33 wt %, a high conductivity of 3.14 S cm-1, and good flexibility with much higher mechanical strength of 15.1 MPa than 0.4 MPa of CW. The symmetric SC delivers a good specific capacitance of 805 mF cm-2 at 0.5 mA cm-2, a remarkably high rate capability of 84% to 10 mA cm-2, and an energy density of 13.85 μW h cm-2 at 87.5 μW cm-2. Additionally, this SC shows a long lifespan of 90.5% after 10,000th charge and discharge cycles even at a constant bending angle of 90°, suggesting promising potential in flexible devices.