Structural Design of Metal Nanomaterials for Highly Efficient Electrochemical Ammonia Synthesis

Abstract

Ammonia (NH₃), a crucial compound of the nitrogen cycle, is not only an indispensable chemical feedstock for nitrogen fertilizer production but also a promising carbon-free energy carrier. Electrocatalytic nitrate/nitrite reduction reaction (NO₃RR/NO₂RR) powered by renewable electricity has emerged as a promising strategy for NH₃ synthesis owing to their environment-friendly feature and energy-saving merit. Metal/alloy nanomaterials have attracted extensive research interests in NO₃RR/NO₂RR. However, they still suffer from poor Faradaic efficiency and limited NH₃ yield rate due to the sluggish reaction process, which involves a complex proton-coupled electron transfer. In this thesis, we focus on the structural design of metal/alloy nanomaterials by constructing two-dimensional nanostructures, modulating the atomic coordination environment and tuning the crystal phase for highly efficient NH₃ synthesis via NO₃RR/NO₂RR.

The first part describes the synthesis of ultrathin nanosheet-assembled RuFe nanoflowers (NFs) with low-coordinated Ru sites to enhance NO₃RR performances in neutral electrolyte. Significantly, RuFe NFs show outstanding NH₃ Faradaic efficiency of 92.9% and yield rate of 38.68 mg h^-1 mg_cat^-1 (64.47 mg h^-1 mg_Ru^-1) at -0.30 and -0.65 V (vs. reversible hydrogen electrode (RHE)), respectively. Meanwhile, RuFe NFs also demonstrate excellent electrocatalytic stability during consecutive electrolysis for 20 cycles. Experimental studies and theoretical calculations reveal that RuFe NFs with low-coordinated Ru sites are highly electroactive with an increased d-band center to guarantee efficient electron transfer, leading to low energy barriers of NO₃RR. In addition, the rechargeable zinc-nitrate batteries with large-specific capacity using RuFe NFs as the cathode were successfully demonstrated, indicating their great potential in next-generation electrochemical energy supply systems. This part provides a promising and feasible strategy to enhance the electrochemical NO₃RR performance via atomic coordination environment engineering of catalysts for highly efficient NH₃ synthesis.

The second part demonstrates the controlled synthesis of RuMo alloy nanoflowers (NFs) with unconventional face-centered cubic (fcc) phase and hexagonal close-packed/fcc heterophase for highly efficient NO₃RR. Remarkably, the fcc RuMo NFs demonstrate high Faradaic efficiency of 95.2% and a large yield rate of 32.7 mg h^-1 mg_cat^-1 toward NH₃ production at 0 and -0.1 V (vs. RHE), respectively. In addition, the outstanding catalytic durability of fcc RuMo NFs for NO3RR is confirmed by the 20 consecutive electrolysis cycles and the long term chronoamperometry test. In situ characterizations and theoretical calculations have unraveled that fcc RuMo NFs possess the highest d-band center with superior electroactivity, which originates from the strong Ru-Mo interactions and the high intrinsic activity of the unconventional fcc phase. The optimal electronic structures of fcc RuMo NFs supply strong adsorption of key intermediates with suppression of the competitive hydrogen evolution, which further determines the remarkable NO₃RR performance. The successful demonstration of high-performance zinc-nitrate batteries with fcc RuMo NFs suggests their substantial application potential in electrochemical energy supply systems. This part not only provides a feasible method to synthesize unconventional phase/heterophase RuMo alloy nanomaterials but also offers an effective strategy in promoting the NO₃RR performance toward NH₃ synthesis.

In the last part, the general one-pot synthesis of IrNi, IrRhNi and IrFeNi alloy nanobranches with unconventional hexagonal close-packed (hcp) phase is reported. Notably, the as-synthesized hcp IrNi nanobranches demonstrate excellent catalytic performance towards NO₂RR, with superior NH₃ Faradaic efficiency and yield rate of 98.2% and 34.6 mg h^-1 mg_cat^-1 (75.5 mg h^-1 mg_Ir^-1) at 0 and -0.1 V (vs. RHE), respectively. Electron paramagnetic resonance (EPR) and in-situ differential electrochemical mass spectrometry (DEMS) results indicate that hcp IrNi NBs can supply abundant active hydrogen for the hydrogenation step and significantly reduce the overpotential for NH₃ production. Theoretical calculations reveal that the Ir-Ni interactions within hcp IrNi alloy improve electron transfer to benefit both nitrite activation and active hydrogen generation, leading to a stronger reaction trend of NO₂RR by greatly reducing energy barriers of rate-determining step. This part not only demonstrates a general method to the controlled synthesis of unconventional phase metal alloy nanomaterials, but also offers a feasible strategy for the rational design of advanced NO₂RR electrocatalysts towards NH₃ synthesis.

Date of Award	7 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhanxi FAN (Supervisor)