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

Abstract

Heterometal-doped nickel-oxy-hydroxides or high-entropy multimetallic oxides show notable electrocatalytic activity. Herein, a readily available Anderson-type polyoxometalate (POM) anion, heptamolybdate ([Mo 7 O 24 ] 6− ), is taken as an inorganic ligand to stabilize the nickel(II)-doped iron-oxy-hydroxide nanocore. [Mo 7 O 24 ] 6−ligated Ni x Fe 1−x O(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 x Fe 1−x O(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 x O y @FeO(OH) and the POM-free bare FeO(OH) and Ni x Fe 1−x O(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 watersplitting 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 x Fe 1−x O(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.

Vacancy-Rich SnO2 Quantum Dot Stabilized by Polyoxomolybdate as Electrocatalyst for Selective NH3 Production

“…The binding energy values and the spin–orbit coupling constant (Δ3d) of 8.4 eV confirmed the Sn IV valence state, consistent for SnO 2 . The core-level Mo 3d envelop was broad but could be deconvoluted into two binding energy maxima at 235.22 and 232.2 eV corroborating the Mo 3d 3/2 and Mo 3d 5/2 spin–orbit components (Figure g), which supports the Mo VI of the Mo x cluster present in the material. , The core-level O 1s XP spectrum depicted that the envelope with a dominant maximum at 530.8 eV corresponds to the O 2– of SnO 2 core and POMs (Figure S11b). However, a small, albeit broad hump observed at 531.3 eV can be interpreted as the dominant O-vacancy sites of the SnO 2 core , that was further proved by X-ray absorption, electron paramagnetic resonance, and Raman spectroscopic studies.…”

“…Under a similar electrochemical condition, free [Mo 7 O 24 ] 6– exhibits the irreversible Mo VI/V redox response at −0.74 V (vs Ag/AgCl) (Figure S15c). , Approximately 140 mV anodic shift of the Mo VI/V redox peak for Mo x @SnO 2 , compared to the free POM, inferred coordination of POM to the SnO 2 core through [MoO] terminals, which makes the Mo centers more electropositive. The ESI-MS study with the material, after treating with TBABr followed by dispersing in CH 3 CN, confirmed identified ion peaks at m / z = 1378.56, 1498.59, 1877.89, 2178.67, and 2400.50 amu, which are well attributed to the molecular ions [(TBA)­(H) 5 (K)­(Mo 7 O 24 )] + , [(TBA)­(Mo 7 O 24 )] + , [(TBA) 5 (H)­(K) 3 (Na) 2 (Mo 7 O 24 )­(K)­2] + , [(TBA) 4 (H)­(K)­(Na)­(Mo 7 O 24 )] + , and [(TBA) 3 (H) 4 (Mo 7 O 24 )] + , respectively (Figure S16), confirming the presence of [Mo 7 O 24 ] 6– Mo x @SnO 2 .…”

“…In this context, the covalent linkage of the stabilizing ligand to the nanoparticle’s surface without affecting the reactivity would be desirable . Anderson-type polyoxomolybdate (NH 4 ) 6 [Mo 7 O 24 ] (POM) has recently been shown as an effective inorganic ligand to stabilize γ-FeO­(OH) and Ni x Fe 1– x O­(OH) quantum dot-sized nanoparticles, highly dispersed in water , The molybdenum POM clusters were covalently linked to the surface of the oxy-hydroxide nanocore to preserve its quantum confinement, and POMs did not block the activity of the nanocore during electrocatalytic oxygen evolution reaction (OER). , …”

Evolution of Mn–Bi2O3 from the Mn-doped Bi3O4Br electro(pre)catalyst during the oxygen evolution reaction

“…A higher Mn percentage indicates that during the synthesis all the Bi(NO 3 ) 3 precursor was present in the reaction in excess and Mn II was doped in the Bi 3 O 4 Br bulk lattice. 49…”

Enhancing Trifunctional Electrocatalytic Activities via Lanthanide Modulation in Fe–Co-Based Double Perovskite Oxides

“…A fantastic display of enhanced chemical as well as physical property (viz., activity) is observed for double perovskites where B-sites are tuned with alternate B and B′ cations owing to the coupling effect by intervening oxygen bridging in every B- and B′-sites. In addition, for metal-based catalysts, metal oxo-hydroxides (formed in situ from metal hydroxides or oxides) − are regarded as the active species, the formation of which gets impeded due to the formation of several M–M bonds . Thus, for the promotional effect in the OER activity of double perovskites, a strategy to accelerate the formation of active species (suppressing the M–M bonds) and optimize the e g electronic state, at the same time, is highly anticipated.…”