In-Situ Grown, Passivator-Modulated Anodization Derived Synergistically Well-Mixed Ni–Fe Oxides from Ni Foam as High-Performance Oxygen Evolution Reaction Electrocatalyst

Abstract

A simple one-step passivator-modulated nickel foam (NF) anodization process was developed to fabricate synergistically well-mixed NiO/α-Fe 2 O 3 @NF, which was demonstrated a high efficiency electrocatalyst for oxygen evolution reaction (OER), exhibiting ultralow overpotentials and excellent stability at high current densities. The NiO/α-Fe 2 O 3 @NF composite electrode, with NiO and α-Fe 2 O 3 well-mixed at nanoscale for much enhanced synergistic effects, exhibited extraordinary electrochemical activities with ultralow overpotentials of 244 and 334 mV at current densities of 50 and 500 mA/ cm 2 , respectively, for the OER in 1 M KOH. The NiO/α-Fe 2 O 3 @NF composite electrode was further tested as both the cathode and anode for overall water splitting, and achieved low working cell voltages of 1.76 and 1.99 V at the current densities of 50 and 500 mA/ cm 2 , respectively, outperforming the pairing of Pt/C@NF and IrO 2 @NF, 1.79 and 2.08 V, correspondingly. The stability of the NiO/α-Fe 2 O 3 @NF electrode was also outstanding, experiencing only a minor chronoamperometric decay of 5.6% after a continuous operation at 500 mA/cm 2 for 24 h. The success of the present catalyst design was attributed to the strong positive synergistic effects between the well mixed NiO and α-Fe 2 O 3 at nanoscale and between the nonconductive mixed oxide catalyst and the conductive nickel foam support.

Single Step Grown NiFe Sponges as Efficient Water Splitting Electrocatalysts in Alkaline Medium

“…The XRD patterns (Figure A) of all the samples exhibit well resolved and intense diffraction peaks confirming the crystalline nature of the synthesized sponges. The X‐ray diffraction pattern of Ni sponge consists of peaks characteristic of pure Ni as well as NiO in accordance with JCPDS 04–850 (44.4, 51.8 and 76.5° corresponding to (111), (200) and (220) planes of metallic Ni) and JCPDS 47–1049 (37.2, 43.3 and 62.9° corresponding to (111), (200), and (220) planes of cubic NiO) respectively, indicating that this sample consists of a pure Ni core and an oxide NiO as the shell . The XRD patterns of NiFe bimetallic sponge with different Fe concentrations have peaks corresponding to metallic Ni and NiO, along with characteristic peaks representing Fe 2 O 3 (Ref: JCPDS 39–1346, 30.2, 35.7, 57.5 and 79.7°, corresponding to (220), (311), (511) and (444) planes of cubic Fe 2 O 3 ) .…”

“…Though, the electrochemical water splitting performances of NiFe sponge are dependent on these oxide layers, the two competing effects associated with these oxide layers limit the electrocatalytic OER performances. Firstly, the thicker layer of oxides (NiO and Fe 2 O 3 ) at higher Fe content will increase the charge transfer resistance preventing the formation of active reaction intermediaries from the oxidation of oxides [e. g. the oxyhydroxides (NiOOH and FeOOH) is less feasible . Secondly, the availability of more oxides for oxidation leads to more active reaction intermediaries at the surface.…”

“…Secondly, the availability of more oxides for oxidation leads to more active reaction intermediaries at the surface. Thus, these two competing actions originate as a result of Fe doping leading to the strongest synergistic effect between NiO and Fe 2 O 3 towards electrochemical water oxidation activities …”

“…Very recently, Chuah et al . reported a simple one‐step nickel‐foam anodization approach to develop a synergistically well‐mixed Ni−Fe oxide on Ni‐foam as high performance water splitting electrocatalysts …”

“…The large porosity of as‐synthesized sponge catalysts, as evident from TEM, can help the mass transfer and also the transfer of gaseous products released during the reaction. A high resolution TEM (HRTEM) image (Figure d) shows the lattice fringes with an inter‐planar distance of 0.18 nm and 0.21 nm, which can be attributed to the (024) and (200) planes of cubic Fe 2 O 3 and NiO, respectively . The selected area electron diffraction (SAED) pattern (inset of Figure d) also indicates the apparent polycrystalline nature with well‐defined patterns.…”