Stabilization of Nickel-Doped Iron-oxy-hydroxide Core in Water by Heptamolybdate Ions to Improve the Electrochemical Oxygen Evolution Reaction

Heterometal-doped nickel-oxy-hydroxides or high-entropy multimetallic oxides show notable electrocatalytic activity. Herein, a readily available Anderson-type polyoxometalate (POM) anion, heptamolybdate ([Mo₇O₂₄]^6-), is taken as an inorganic ligand to stabilize the nickel(II)-doped iron-oxy-hydroxide nanocore. [Mo₇O₂₄]^6--ligated Ni_xFe_1-xO(OH) nanomaterials with different ratios of Ni(II) and Fe(III) in the core (1-3) are prepared via a hydrothermal route. ICP-MS and the subsequent PXRD study of the materials have found out that approximately 1.5-2% nickel is incorporated into the γ-FeO(OH) core without altering its two-dimensional-layered lattice structure. The presence of numerous POMs covalently linked on the surface of 4-5 nm highly crystalline Ni_xFe_1-xO(OH) core is proven by multiple spectroscopic and microscopic techniques. Negative zeta potential of 1-3 infers the ionic surface of the materials due to the presence of negatively charged POMs which makes them highly dispersed and stable in water. Using 1-3 as electrocatalysts, oxygen evolution reaction (OER) is studied under alkaline condition. For catalytic OER, 1-3 on the nickel foam (NF) electrode require almost 20 mV less overpotential compared to the undoped core material Mo_xO_y@FeO(OH) and the POM-free bare FeO(OH) and Ni_xFe_1-xO(OH). The better OER activity can be correlated to better electrokinetics, realized from the Tafel slope and charge-transfer resistance (R_ct). The fabricated electrode 1@NF not only shows a long-term stability under the OER condition but also can be fabricated to a water-splitting electrolyzer using a graphite rod as the cathode to produce green hydrogen with Faradaic efficiency of ca. 72%. In this study, Anderson-type POM is used as a potential ligand to derive the quantum-dot-sized Ni_xFe_1-xO(OH) core as a reactive electrocatalyst for OER. In a broad context, this strategy, i.e., the use of POM as a pure inorganic ligand to stabilize a reactive metal oxide nanocore, can further be adapted to design a variety of multimetallic or mixed-valence metal oxide materials.