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 1 ¯ ${\mathrm{\bar{1}}}$ 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 mgcat -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.