Carbon‐Extraction‐Triggered Phase Engineering of Rhodium Nanomaterials for Efficient Electrocatalytic Nitrate Reduction Reaction

Phase engineering plays a crucial role in tuning the physicochemical properties of noble metal nanomaterials. However, synthesis of high-purity unconventional-phase noble metal nanomaterials remains highly challenging via current wet-chemical methods. Herein, we develop a unique synthetic methodology to prepare freestanding unconventional hexagonal close-packed (2H) Rh nanoplates (NPLs) via a rationally designed two-step strategy. By extracting C from pre-synthesized rhodium carbide of different sizes and morphology, phase-controlled synthesis of Rh nanomaterials can be achieved. Impressively, the obtained parallelogram 2H Rh NPLs have high phase purity, well-defined 2H (0001)_h and (10 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mover><mml:mn>1</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:annotation>${\mathrm{\bar{1}}}$</mml:annotation></mml:semantics> </mml:math> 0)_h facets, and good thermostability (stable up to 300 °C). In the proof-of-concept electrocatalytic nitrate reduction reaction (NO₃RR), the 2H Rh NPLs achieve higher ammonia (NH₃) Faradaic efficiency (91.9%) and NH₃ yield rate (156.97 mg h^-1 mg_cat ^-1) with lower overpotentials compared to the conventional face-centered cubic (3C) Rh nanocubes with (100)_f facets. Density functional theory calculations reveal that the unconventional (0001)_h surface has energetically favored NO₃RR pathway and stronger H^* absorption ability compared to the (100)_f surface, which may lead to the higher activity and selectivity of NH₃ production on 2H Rh NPLs. This work opens new avenues to the rational synthesis of unconventional-phase metal nanomaterials and provides important guidelines to design high-performance electrocatalysts.