Low Pt Loading on Wolframite-Type NiWO₄ to Excel the Electrocatalytic Water Splitting and Ammonia Oxidation Reaction

Abstract

Hydrogen production via water-splitting or ammonia electrolysis using transition metal-based electrodes is one of the most cost-effective approaches. Herein, ca. 1–4% of Pt atoms are stuffed into a wolframite-type NiWO 4 lattice to improve the electrocatalytic efficiency. The co-existence of atomically dilute quantities of Pt 0 and Pt IV atoms in the NiWO 4 without altering the lattice structure is established via powder X-ray diffraction, inductively coupled plasma mass spectrometry (ICP-MS), core-level X-ray photoelectron spectroscopy, and other spectroscopic studies. While the undoped NiWO 4 and a physical mixture of Pt 0 (2 wt %) and NiWO 4 exhibit poor oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and ammonia oxidation reaction (AOR) activities, 3–4% Pt-enriched NiWO 4 depict improved electrocatalytic performances with at least 50 mV overpotential drop for both the OER and HER. The 3%Pt/NiWO 4 electrode showcases a long-term (for 110 h) chronoamperometric/chronopotentiometric OER and HER performance, delivering high current at a low working potential. The bifunctional behavior of the material leads to the development of a water-splitting electrolyzer, 3%Pt/NiW / NF(−)/(+)3%Pt/NiW/NF, achieving >90% Faradaic efficiency for H 2 production. The onset potential for the AOR is also cathodically shifted for 3%Pt/NiW and 4%Pt/NiW compared to the NiWO 4 itself. Electrokinetic study through a rotating ring-disk electrode (RRDE) experiment and the Koutecký–Levich study provides an observed rate constant ( k obs ) of 1.68 × 10 –3 cm s –1 of AOR with a 6e – count from the kinetic current region, highlighting [NO 2 ] − as the major product. The electrolysis of 1 M NH 3 using 4%Pt/NiW/NF as a working electrode produces predominantly [NO 2 ] − (FE: 53%) and [NO 3 ] − (FE: 30%). The improved electrocatalytic activity of 3–4% Pt-enriched NiWO 4 can be due to the low Tafel slope and charge transfer resistance ( R ct ). Pt 0 being electron-rich induces facile electronic conduction during electrocatalysis and enhances a better binding of the analytes such as H 2 O, [OH] −, and NH 3 . At the same time, the Pt IV centers present adjacent to the Ni II sites can polarize the electron density to stabilize Ni III species and enhance the possibility of OER and AOR. This study demonstrates the effect of hetero-metal doping to tune the electronic structure to improve the electrochemical activity. The low-Pt-doped NiWO 4 material is presented here as a multimodal electrocatalyst that can efficiently electrolyze water or ammonia to produce hydrogen.

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