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

Abstract

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.

Figure 1

Schematic illustration for utilizing the spent reforming catalyst for electrochemical water splitting.

Figure 2

(a) SEM image of the spent methane reforming catalyst with graphitic carbon (GC), (b) FESEM of the spent catalyst after deposition of MoS₂, i.e., MoS₂@GC, (c) EDS elemental distribution graph for MoS₂@GC, and (d, e) EDS elemental mapping of MoS₂@GC.

Figure 3

(a) FESEM images of the spent methane reforming catalyst with carbon nanofibers (CNF), (b) FESEM image of MoS₂ nanoflakes deposited on CNF, (d) EDS elemental distribution graph for MoS₂@CNF, and (e, f) EDS elemental mapping of MoS₂@CNF.

Figure 4

(a) XRD spectrum of GC, CNF, MoS₂@GC, and MoS₂@CNF. (b) FT-IR spectrum of the spent methane reforming catalyst with MoS₂, GC, CNF, MoS₂@GC, and MoS₂@CNF.

Figure 5

Electrochemical HER measurements of different catalysts. (a) Linear sweep voltammetry (LSV) polarization curve for bare Ni foam, GC, CNF, MoS₂@GC, and MoS₂@CNF; (b) corresponding Tafel slopes of spent GC and CNF, MoS₂@GC, and MoS₂@CNF; (c) overpotential comparison required at 25 mA cm^–2; and (d) current density of spent GC and CNF, and synthesized catalysts, i.e., MoS₂@CNF and MoS₂@GC at 200 mV versus RHE for the OER.

Figure 6

Electrochemical OER measurements of different catalysts. (a) Linear sweep voltammetry polarization curve; (b) corresponding Tafel slopes of spent GC and CNF, MoS₂@CNF, and MoS₂@GC; (c) overpotential required at 25 mA cm^–2; and (d) current density of spent GC and CNF, and synthesized catalysts, i.e., MoS₂@CNF and MoS₂@GC at 200 mV versus RHE for the OER.

Figure 7

(a) Cyclic voltammetry (CV) curve of spent GC, CNF, and synthesized catalysts, i.e., MoS₂@GC and MoS₂@CNF. (b) Chronopotentiometry (CP) curve for MoS₂@CNF and MoS₂@GC for 18 h. (c) Nyquist plot for GC, CNF, MoS₂@CNF, and MoS₂@GC with the equivalent circuit diagram. (d) Linear sweep voltammetry results after the stability test.