Reutilizing Methane Reforming Spent Catalysts as Efficient Overall Water-Splitting Electrocatalysts

It is extremely prudent and highly challenging to design a greener bifunctional electrocatalyst that shows effective electrocatalytic activity and high stability toward electrochemical water splitting. As several hundred tons of catalysts are annually deactivated by deposition of carbon, herein, we came up with a strategy to reutilize spent methane reforming catalysts that were deactivated by the formation of graphitic carbon (GC) and carbon nanofibers (CNF). An electrocatalyst was successfully synthesized by in situ deposition of noble metal-free MoS₂ over spent catalysts via a hydrothermal method that showed exceptional performance regarding the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). At 25 mA cm^-2, phenomenal OER overpotentials (η₂₅) of 128 and 154 mV and modest HER overpotentials of 186 and 207 mV were achieved for MoS₂@CNF and MoS₂@GC, respectively. Moreover, OER Tafel slopes of 41 and 71 mV dec^-1 and HER Tafel slopes of 99 and 107 mV dec^-1 were obtained for MoS₂@CNF and MoS₂@GC, respectively. Furthermore, the synthesized catalysts exhibited good long-term durability for about 18 h at 100 μA cm^-2 with unnoticeable changes in the linear sweep voltammetry (LSV) curve of the HER after 1000 cycles. The carbon on the spent catalyst increased the conductivity, while MoS₂ enhanced the electrocatalytic activity; hence, the synergistic effect of both materials resulted in enhanced electrocatalysts for overall water splitting. This work of synthesizing enhanced nanostructured electrocatalysts with minimal usage of inexpensive MoS₂ gives a rationale for engineering potent greener electrocatalysts.