Designed NiMoC@C and NiFeMo2C@C core-shell nanoparticles for oxygen evolution in alkaline media

Abstract

Electrochemical water splitting is one of the most promising and clean ways to produce hydrogen as a fuel. Herein, we present a facile and versatile strategy for synthesizing non-precious transition binary and ternary metal-based catalysts encapsulated in a graphitic carbon shell. NiMoC@C and NiFeMo₂C@C were prepared via a simple sol-gel based method for application in the Oxygen Evolution Reaction (OER). The conductive carbon layer surrounding the metals was introduced to improve electron transport throughout the catalyst structure. This multifunctional structure showed synergistic effects, possess a larger number of active sites and enhanced electrochemical durability. Structural analysis indicated that the metallic phases were encapsulated in the graphitic shell. Experimental results demonstrated that the optimal core-shell material NiFeMo₂C@C exhibited the best catalytic performance for the OER in 0.5 M KOH, reaching a current density of 10 mA cm^-2 at low overpotential of 292 mV for the OER, superior to the benchmark IrO₂ nanoparticles. The good performances and stability of these OER electrocatalysts, alongside an easily scalable procedure makes these systems ideal for industrial purposes.

Keywords: OER; binary transition metal; core-shell structure; graphitic carbon; metal-metal carbide hybrid; nanoparticles; ternary transition metal; urea glass route.

SCHEME 1

The two semi-reactions in the electrolysis of water.

FIGURE 1

Schematic representation of the urea-glass route.

FIGURE 2

XRD patterns of as-prepared NiMo- (A) and NiFeMo- (B) based samples. (C) Raman spectra of the NiMoC@C and NiFeMo₂C@C. The reference patterns of MoC (ICDD: 04–013–9931), NiFe (ICDD: 04–008–8265), Mo₂C (ICDD: 04–003–6378), Ni (ICDD: 04–001–0091) are reported as vertical full lines, for comparison. The peak marked with * and • are attributed to carbon (ICDD: 04-15-2407), and MoC, respectively.

FIGURE 3

EM images of NiMoC@C (A–H) and NiFeMo₂C@C (I–P) samples. (A, I) SEM images (B, J) low resolution TEM images. (C–H) HRTEM images of core-shell NiMoC@C samples. (F–G) HRTEM images of selected area in Figure 3D. (F) from area highlighted in green in Figure 3D. (G) from area highlighted in blue in Figure 3D. (H) HRTEM image of highlighted area (in orange) from Figure 3E. (K–P) HRTEM images of core-shell NiFeMo₂C@C. (M) HRTEM image of selected area in Figure 3L, from area highlighted in red. (O) HRTEM image of selected area in Figure 3N: from area highlighted in pink. (P) HRTEM image of selected area in Figure 3N: from area highlighted in orange.

FIGURE 4

HRTEM image/bright field TEM images and the corresponding elemental mapping of C, Ni, Mo and O for NiMoC@C (A–G) and C, Ni, Fe, Mo and O for NiFeMo₂C@C (H–O).

FIGURE 5

(A) High-resolution XPS spectra of C in NiMoC@C and NiFeMo₂C@C, (B) Ni2p in NiMoC@C and NiFeMo₂C@C, (C)High-resolution XPS spectra of Mo3d, (D) High-resolution XPS spectra of Fe2p.

FIGURE 6

Electrochemical performance of as-prepared NiMoC@C, NiFeMo₂C@C catalysts and commercial IrO₂ NPs toward the OER in 0.5 M KOH solution. (A) CV curves of 10th and 1000th cycles of NiMoC@C, NiFeMo₂C@C at a scan rate of 5 mV/s. (B) iR-compensated LSV curves of NiMoC@C, NiFeMo₂C@C catalysts and commercial IrO₂ NPs at a scan rate of 5 mV/s. (C) Summary of overpotential at j = 10 mA cm^-2 and 20 mA cm^-2. (D) Tafel plots determined from the LSV curves. (E) EIS measurements at a constant potential of 1.544 V and 1.522 V vs. RHE for NiMoC@C, NiFeMo₂C@C respectively.

FIGURE 7

The chronopotentiometry tests at 10 mA cm-2 in 0.5 M KOH for 24 h (A). TEM images of NiFeMo2C@C after OER process (B, C).