Nickel-Iron Layered Double Hydroxides/Nickel Sulfide Heterostructured Electrocatalysts on Surface-Modified Ti Foam for the Oxygen Evolution Reaction

Abstract

Electrochemical approaches for generating hydrogen from water splitting can be more promising if the challenges in the anodic oxygen evolution reaction (OER) can be harnessed. The interface heterostructure materials offer strong electronic coupling and appropriate charge transport at the interface regions, promoting accessible active sites to prompt kinetics and optimize the adsorption-desorption of active species. Herein, we have designed an efficient multi-interface-engineered Ni₃Fe₁ LDH/Ni₃S₂/TW heterostructure on in situ generated titanate web layers from the titanium foam. The synergistic effects of the multi-interface heterostructure caused the exposure of rich interfacial electronic coupling, fast reaction kinetics, and enhanced accessible site activity and site populations. The as-prepared electrocatalyst demonstrates outstanding OER activity, demanding a low overpotential of 220 mV at a high current density of 100 mA cm^-2. Similarly, the designed Ni₃Fe₁ LDH/Ni₃S₂/TW electrocatalyst exhibits a low Tafel slope of 43.2 mV dec^-1 and excellent stability for 100 h of operation, suggesting rapid kinetics and good structural stability. Also, the electrocatalyst shows a low overpotential of 260 mV at 100 mA cm^-2 for HER activity. Moreover, the integrated electrocatalyst exhibits an incredible OER activity in simulated seawater with an overpotential of 370 mV at 100 mA cm^-2 and stability for 100 h of operation, indicating good OER selectivity. This work might benefit further fabricating effective and stable self-sustained electrocatalysts for water splitting in large-scale applications.

Keywords: heterostructure; multi-interface; stability; synergistic effect; water splitting.

Figure 1

Schematic illustration for (a) titanate web layer formation mechanisms during hydrothermal processing via Ti foam surface modification. (b) Fabrication route of the Ni₃Fe₁ LDH/Ni₃S₂/TW heterostructure electrocatalyst.

Figure 2

SEM morphology for (a) titanate web layers generated from surface-modified TF, (b) Ni₃S₂/TW, and (c) Ni₃Fe₁ LDH/Ni₃S₂/TW hybrid interface heterostructures. (d) TEM image and (e) HR-TEM image for Ni₃Fe₁ LDH/Ni₃S₂/TW electrocatalysts. (f) HAADF-STEM image with associated elemental mapping for Ni₃Fe₁ LDH/Ni₃S₂/TW. (g) XRD patterns for the corresponding electrocatalyst. (h) Raman spectra of the as-synthesized electrocatalysts.

Figure 3

Local structure characterization of the electrocatalysts. The XANES spectra for the (a) Ni K-edge and (b) Fe K-edge of as-prepared electrocatalysts. The EXAFS spectra for the (c) Ni and (d) Fe R-space for the respective electrocatalysts.

Figure 4

Surface characterizations of the as-prepared electrocatalysts. XPS spectra of (a) Ni 2p and (b) Fe 2p for Ni₃Fe₁ LDH/TW and Ni₃Fe₁ LDH/Ni₃S₂/TW. (c) XPS spectra of S 2p for Ni₃S₂/TW and Ni₃Fe₁ LDH/Ni₃S₂/TW. (d) XPS spectra of O 1s for Ni₃Fe₁ LDH/Ni₃S₂/TW. (e) Scheme of the electronic interactions at the heterostructure interfaces.

Figure 5

(a) OER LSV polarization curve of electrocatalysts in 1 M KOH at a scan rate of 1 mV/s. (b) The respective Tafel plots of the electrocatalysts. (c) Electrochemical capacitive currents vs scan rate of the samples. (d) Nyquist plots for EIS with its related electrocatalysts. (e) Chronopotentiometric stability curves for Ni₃Fe₁ LDH/Ni₃S₂/TW, Ni₃Fe₁ LDH/TW, and Ni₃S₂/TW at a constant current density of 100 mA cm^–2. (f) Comparison of the overpotentials at 100 mA cm^–2 for Ni₃Fe₁ LDH/Ni₃S₂/TW with those of reported electrocatalysts.

Figure 6

XPS characterizations for comparison of the pre- and post-OER electrocatalysts. XPS spectra of (a) Ni 2p, (b) Fe 2p, (c) O 1s, and (d) S 2p of the as-prepared Ni₃Fe₁ LDH/Ni₃S₂/TW electrocatalyst. The in situ Raman spectra for (e) Ni₃Fe₁ LDH/Ni₃S₂/TW and (f) Ni₃S₂/TW measured in 1 M KOH solution. (g) Coupled multiple interfacial engineering and proposed mechanistic effects on the heterostructured electrocatalysts.

Figure 7

Electrochemical HER overall water splitting performance measurements. (a) Polarization curve of different samples for HER in a 1.0 M KOH solution. (b) Nyquist plots of as-synthesized electrocatalysts. (c) Diagram of the overall water splitting cell system with 1.55 V. (d) Polarization curve comparison for the Ni₃Fe₁ LDH/Ni₃S₂/TW||Ni₃Fe₁ LDH/Ni₃S₂/TW with other catalysts using H-type cell two-electrode systems for overall water splitting in 1 M KOH solution. (e) Stability test for the Ni₃Fe₁ LDH/Ni₃S₂/TW electrocatalyst for the overall water splitting system.