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 cata...[ Read more ]

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.