Molybdenum titanium carbide (Mo2TiC2Tx) MXene coated carbon electrodes for vanadium redox flow batteries

Abstract

Carbon-based electrodes are the most commonly used electrode materials for vanadium redox flow batteries (VRFBs). Due to the use of aqueous electrolytes in VRFBs, the first challenge is the hydrophobicity properties of carbon-based electrodes, and the second challenge is that the desired redox reaction on the positive side, VO²⁺/VO₂⁺, competes with the oxygen evolution reaction. Therefore, a proper surface treatment is needed. In the present work, three different brands of carbon papers (Sigracet 28AA, Toray 060, and Freudenberg H23) were treated with heat treatment and an MXene coating. For the latter, a two-dimensional (2D) molybdenum titanium carbide (Mo₂TiC₂T_x) was chosen and the drop-casting method was used for coating on carbon papers. Scanning electron microscopy (SEM) confirmed MXene distribution and X-ray photoelectron spectroscopy (XPS) showed the presence of Mo and Ti on the electrode surface. Cyclic voltammetry tests revealed that the vanadium reaction rate, 7.76 × 10⁻⁴ cm s⁻¹, and diffusion coefficient, 5.51 × 10⁻⁵ cm² s⁻¹, using Mo₂TiC₂T_x MXene-coated carbon papers are comparable with when heat-treated carbon paper, 1.41 × 10⁻³ cm s⁻¹ and 1.32 × 10⁻⁴ cm² s⁻¹, is used. VRFB tests were conducted over 150 cycles. Although a higher resistance of 1.2 Ω cm² was observed for VRFB using Mo₂TiC₂T_x MXene-coated carbon papers versus heat-treated, 0.8 Ω cm², the energy efficiency of 71% was reasonably comparable to 79% for the system using heat-treated electrodes. More importantly, the same discharge capacity retention of 75% was achieved for both systems. The chemical stability of the Mo₂TiC₂T_x MXene coating was confirmed by XPS post-analysis of electrodes where similar peaks for the freshly coated electrodes were observed. This work further broadens the potential applications of MXene coating as a treatment for carbon electrodes.