Electrocatalytic effects of two-dimensional (2D) layered titanate nanosheets (Ti2-x/4□x/4O4-x; □ = vacancy, x = 0.67) modified electrode for V3+/V2+ redox reactions

Two-dimensional (2D) electrocatalysts have attracted considerable recent research interest owing to the exceptional electrical properties arising from their high anisotropy. An ideal application of 2D materials is in the catalysis of essential electrochemical reactions for energy storage. To this effect, we explored the electrocatalytic properties of exfoliated 2D layered titanate nanosheets (Ti_2-x/4□_x/4O₄^-x; □ = vacancy, x  = 0.67, herein abbreviated as LTNS) for V³⁺/V²⁺ redox reaction as a model negative electrode of vanadium redox flow battery (VRFB). The dip-withdraw-dry method was used to fabricate the LTNS@CF electrode by coating pristine carbon felt (P-CF) with a very small amount (∼ 0.0043 wt% loading amount on electrode) of exfoliated LTNS colloidal aqueous suspension equivalent to cover the surface of the carbon fibers. The morphology of LTNS@CF exhibited a relatively rough surface covered by the firmly attached anisotropic LTNS compared to the smooth surface of P-CF. The electrocatalytic properties of these electrodes for V³⁺/V²⁺ redox reactions were analyzed by electrochemical impendence spectroscopy (EIS), cyclic voltammetry (CV), and VRFB single-cell, with LTNS@CF showing significantly lower charge transfer resistance (R_ct) and improved kinetics for V³⁺/V²⁺ redox reaction compared to that of P-CF. This is attributed to the concurrence of desired electrocatalytic properties of the LTNS: the 2D anisotropic layered shape that facilitates charge mobility and firm attachment to the surface of carbon fibers, the negative charge of LTNS that provides electrostatic redox reaction sites for the positively charged vanadium ions, corrosion resistance of Ti-O bond in strong acidic (H₂SO₄) electrolyte, and its polar nature that enhances the wettability of the electrode.