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

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 NiWO₄ lattice to improve the electrocatalytic efficiency. The co-existence of atomically dilute quantities of Pt⁰ and Pt^IV atoms in the NiWO₄ 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 NiWO₄ and a physical mixture of Pt⁰ (2 wt %) and NiWO₄ exhibit poor oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and ammonia oxidation reaction (AOR) activities, 3-4% Pt-enriched NiWO₄ depict improved electrocatalytic performances with at least 50 mV overpotential drop for both the OER and HER. The 3%Pt/NiWO₄ 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 H₂ production. The onset potential for the AOR is also cathodically shifted for 3%Pt/NiW and 4%Pt/NiW compared to the NiWO₄ 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 [NO₂]^- as the major product. The electrolysis of 1 M NH₃ using 4%Pt/NiW/NF as a working electrode produces predominantly [NO₂]^- (FE: 53%) and [NO₃]^- (FE: 30%). The improved electrocatalytic activity of 3-4% Pt-enriched NiWO₄ can be due to the low Tafel slope and charge transfer resistance (R_ct). Pt⁰ being electron-rich induces facile electronic conduction during electrocatalysis and enhances a better binding of the analytes such as H₂O, [OH]^-, and NH₃. 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 NiWO₄ material is presented here as a multimodal electrocatalyst that can efficiently electrolyze water or ammonia to produce hydrogen.