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 (VO₂) anchored on two-dimensional reduced graphene oxide (2D-rGO) nanosheets (VO₂/2D-rGO) via a one-step in situ hydrothermal method were developed. In particular, different polymorphs of VO₂ microstructures, that is, VO₂(A), VO₂(B), VO₂(M), and so on, obtained by the influence of β-MnO₂ have been paid much attention because of their wide range of potential applications, especially, in electrochemical water splitting. Accordingly, the electrochemical activity of the VO₂/2D-rGO composite for the oxygen evolution reaction and hydrogen evolution reaction (OER and HER) was dramatically amended by tuning the electronic structure of VO₂(A) using a cationic additive such as β-MnO₂. Moreover, the obtained VO₂(A)/2D-rGO (MVAG) composite was used as a precursor for attaining different polymorph-based composite materials by altering the additive KMnO₄. The crystallographic phase transition of the VO₂(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-MnO₂-VO₂(M)/2D-rGO (MVMG), β-MnO₂-VO₂(B)/2D-rGO (MVBG), and β-MnO₂-V₂O₅/2D-rGO (MVOG). Benefiting from the prismatic type of the VO₂(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.