Mesoporous gold decorated MXene (Ti₃C₂T_x) flexible composite films for photo-enhanced solid-state micro-supercapacitors

Powerful, flexible, and small energy storage devices are becoming increasingly important. As a typical two-dimensional titanium carbide, the Ti₃C₂T_x MXene film has been widely used as an electrode material for flexible energy storage devices due to its ultra-high electrical conductivity, superior capacitance, and excellent mechanical flexibility. In this work, we incorporate mesoporous gold nanoparticles (Au NPs) with abundant interior void spaces into flexible Ti₃C₂T_x MXene films to enhance conductivity, provide more electroactive sites, and shorten ion diffusion paths. The MXene composite film with 5 wt% mesoporous Au NPs (M/A-0.05) has gravimetric capacitances up to 332 F g⁻¹ at a current density of 1 A g⁻¹, representing a 59% increase over the pure MXene film. Flexible symmetric solid-state micro-supercapacitors (MSCs) in PVA/H₂SO₄ electrolyte are fabricated with this MXene composite film, which shows a wider voltage window (0-1.2 V) compared to aqueous symmetric supercapacitors in 3 M H₂SO₄ electrolyte (0-0.6 V). The MSCs have a large surface area, enabling the MXene composite film with mesoporous Au NPs to be efficiently excited with light. Electromagnetic (EM) simulations performed on mesoporous Au NPs in a high refractive index material like Ti₃C₂ MXene show that light can excite broadband multipolar plasmon resonances that should contribute indirectly to photocurrent by generating heat in the interstitial space between MXene flakes. Thus, the photo-illuminated MSC device based on M/A-0.05 generates ∼2X photocurrent response compared to the MSC device based on the pure MXene film and has a capacitance 6.2% higher than under dark conditions. This work explains how mesoporous Au NPs can augment and improve the properties of MXene-based composite film electrodes that are capable of both harvesting and storing energy for photo-enhanced supercapacitors.