Effect of β-MnO₂ on Controlled Polymorphism of VO₂(x) (x = A, B, M Polymorphs) Microstructures Anchored on Two-Dimensional Reduced Graphene Oxide Nanosheets for Overall Water Splitting

Herein, nonprecious bifunctional composite materials made of vanadium dioxide (VO 2 ) anchored on two-dimensional reduced graphene oxide (2D-rGO) nanosheets (VO 2 /2D-rGO) via a one-step in situ hydrothermal method were developed. In particular, different polymorphs of VO 2 microstructures, that is, VO 2 (A), VO 2 (B), VO 2 (M), and so on, obtained by the influence of β-MnO 2 have been paid much attention because of their wide range of potential applications, especially, in electrochemical water splitting. Accordingly, the electrochemical activity of the VO 2 /2D-rGO composite for the oxygen evolution reaction and hydrogen evolution reaction (OER and HER) was dramatically amended by tuning the electronic structure of VO 2 (A) using a cationic additive such as β-MnO 2 . Moreover, the obtained VO 2 (A)/2D-rGO (MVAG) composite was used as a precursor for attaining different polymorph-based composite materials by altering the additive KMnO 4 . The crystallographic phase transition of the VO 2 ( x ) microstructure was systematically studied and confirmed by powder X-ray diffractometer patterns. By varying the additive concentration, three different composite materials were prepared: β-MnO 2 -VO 2 (M)/2D-rGO (MVMG), β-MnO 2 -VO 2 (B)/2D-rGO (MVBG), and β-MnO 2 -V 2 O 5 /2D-rGO (MVOG). Benefiting from the prismatic type of the VO 2 (B) microstructure, the MVBG composite material shows enhanced OER (273 mV) and HER (178 mV) activities to achieve a current density of 10 mA cm –2 . Remarkably, the two-electrode system has been constructed using one of the polymorphs such as MVBG||MVBG to study the cell voltage of 1.62 V to reach a current density of 10 mA cm –2 and displayed a Tafel slope of 108 mV dec –1 . In addition, the MVBG||MVBG system showed outstanding long-term stability for 35 h. This work offers simple and convenient protocols for achieving cost-effective bifunctional composite materials for overall water splitting.