Advanced electrocatalysts for nitrate reduction reaction

Abstract

Electrochemical nitrate reduction (NO₃RR) has become an appealing approach for sustainable NH₃ synthesis owing to the “waste-to-wealth” conversion, mild operating conditions, and zero-carbon emission. However, the complicated eight-electron transfer process involving multiple N-containing species with various chemical states seriously drags the reaction rate. Furthermore, the competition from hydrogen evolution reaction (HER) makes it difficult to achieve a high Faradaic efficiency (FE). Although some noble metals have shown good activity for this reaction, the scarce resource and high cost limit their practical application. Electrocatalysts with high NO₃RR activity and FE, and low cost are urgently demanded. In this thesis, efforts are devoted to developing non-noble metal-based catalysts for selective NH₃ synthesis under ambient conditions. In the first work, surface modulation is adopted to suppress the competing HER by constructing CoO_x nanosheets enriched with adsorbed oxygen. The CoO_x nanosheets show excellent catalytic performance with the maximum ammonia yield of 82.4 mg h^-1 mg_cat^-1 and a FE of 93.4% at -0.3 Vin an alkaline electrolyte. Density functional theory (DFT) calculations reveal that the high FE is correlated with the adsorbed oxygen, which could suppress HER by strengthening hydrogen adsorption and offer a more exothermic reaction pathway. In the second work, Cu-doped Fe₃O₄ is successfully constructed as a non-noble metal-doped catalyst for NO₃RR. The maximum FE of ~100% and a high NH₃ yield of 179.6 mg h^-1 mg_cat^-1 at -0.6 V are found, which are superior to Fe₃O₄ and Cu-Fe₃O₄ with different Cu contents. In situ Raman spectroscopy verifies the formation of NH₃ on the Cu-Fe₃O₄. In the third work, single-atom catalysts (SACs) are developed to maximize the metal utilization and lower the cost of catalysts. The Co SAC with a high Co content achieves the highest NH₃ yield of 60.08 mg h^-1 mg_cat^-1 at -0.6 V and a maximum FE of 91.74% at -0.3 V. The NO₃RR performance of Co SAC is superior to that of Cu SAC and Ni SAC. At last, BiFeO₃ is also successfully synthesized and evaluated as a catalyst for NO₃RR. The catalyst attains a FE of 96.85% and an NH₃ yield of 90.45 mg h^-1 mg_cat^-1 at -0.6V. During the nitrate reduction reaction, the crystalline BiFeO₃ rapidly converts into an amorphous phase, which is stable in the long-term reaction. This study develops several non-noble metal-based catalysts with excellent NO₃RR performance by surface modulation, heteroatom doping, and constructing single atom catalysts, which pave the way for the rational design and mechanism study of future electrocatalysts.

Date of Award	2022
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Minhua SHAO (Supervisor) & Meng Gu (Supervisor)