Hierarchical porous Nb2O5 cathodes for efficient electro-deoxidation in molten salts toward niobium powder preparation
The molten salts electro-deoxidation process, also known as the FFC-Cambridge process, offers substantial potential for the environmentally friendly synthesis of niobium powder in a relatively short process. However, this process suffers from uncontrolled intermediate particle coarsening and the resultant sluggish oxygen-ion diffusion kinetics. To address this, we engineered a novel hierarchically porous Nb 2 O 5 (HPS-Nb 2 O 5 ) cathode with interconnected macropores (∼10 2 μm), mesopores (∼10 1 μm), and micropores (submicron). Electrochemical analyses coupled with potentiostatic electrolysis reveal a four-step reduction pathway of Nb 2 O 5 in molten Na 3 AlF 6 -K 3 AlF 6 -AlF 3 salts: Nb 2 O 5 →Na 2 Nb 8 O 21 /NbO 2 →NbO→Nb, where solid-state oxygen diffusion remains rate-limiting. The novel cathode pore architecture optimizes the solid-liquid interfaces between the cathode and the electrolyte, facilitating uniform insertion of Na⁺/F⁻ ions into cathode particles to generate intermediate particles. Consequently, rhombohedral Na 2 Nb 8 O 21 intermediate with uniform size and morphology was formed, thereby effectively preventing the excessive coarsening of intermediate particles and associated oxygen diffusion path elongation that occurs in the conventional electro-deoxidation processes. Remarkably, complete reduction of 1 g Nb 2 O 5 to Nb powder was achieved in only 1.5 h at 2.5 V. This represents an 8-13-fold efficiency enhancement over the reported processes. The resultant niobium powder exhibits uniform submicron particles (200–500 nm) with near-spherical morphology. This pore-structure-regulated strategy overcomes fundamental bottlenecks in the FFC process and offers broad applicability for the efficient preparation of diverse refractory metals and alloys.
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