Oxygen-Coordinated Single Mn Sites for Efficient Electrocatalytic Nitrate Reduction to Ammonia.

Electrocatalytic nitrate reduction reaction has attracted increasing attention due to its goal of low carbon emission and environmental protection. Here, we report an efficient NitRR catalyst composed of single Mn sites with atomically dispersed oxygen (O) coordination on bacterial cellulose-converted graphitic carbon (Mn-O-C). Evidence of the atomically dispersed Mn-(O-C2)4 moieties embedding in the exposed basal plane of carbon surface is confirmed by X-ray absorption spectroscopy. As a result, the as-synthesized Mn-O-C catalyst exhibits superior NitRR activity with an NH3 yield rate (RNH3) of 1476.9 ± 62.6 µg h-1 cm-2 at - 0.7 V (vs. reversible hydrogen electrode, RHE) and a faradaic efficiency (FE) of 89.0 ± 3.8% at - 0.5 V (vs. RHE) under ambient conditions. Further, when evaluated with a practical flow cell, Mn-O-C shows a high RNH3 of 3706.7 ± 552.0 µg h-1 cm-2 at a current density of 100 mA cm-2, 2.5 times of that in the H cell. The in situ FT-IR and Raman spectroscopic studies combined with theoretical calculations indicate that the Mn-(O-C2)4 sites not only effectively inhibit the competitive hydrogen evolution reaction, but also greatly promote the adsorption and activation of nitrate (NO3-), thus boosting both the FE and selectivity of NH3 over Mn-(O-C2)4 sites.

Electrochemical ammonia synthesis by reduction of nitrate on Au doped Cu nanowires.

Electrochemical nitrate reduction reaction (NO3 -RR) to synthesize valuable ammonia (NH3) is considered as a green and appealing alternative to enable an artificial nitrogen cycle. However, as there are other NO3 -RR pathways present, selectively guiding the reaction pathway towards NH3 is currently challenged by the lack of efficient catalyst. Here, we demonstrate a novel electrocatalyst for NO3 -RR consisting of Au doped Cu nanowires on a copper foam (CF) electrode (Au-Cu NWs/CF), which delivers a remarkable NH3 yield rate of 5336.0 ± 159.2 µg h-1 cm-2 and an exceptional faradaic efficiency (FE) of 84.1 ± 1.0% at -1.05 V (vs. RHE). The 15N isotopic labelling experiments confirm that the yielded NH3 is indeed from the Au-Cu NWs/CF catalyzed NO3 -RR process. The XPS analysis and in situ infrared spectroscopy (IR) spectroscopy characterization results indicated that the electron transfer between the Cu and Au interface and oxygen vacancy synergistically decreased the reduction reaction barrier and inhibited the generation of hydrogen in the competitive reaction, resulting in a high conversion, selectivity and FE for NO3 -RR. This work not only develops a powerful strategy for the rational design of robust and efficient catalysts by defect engineering, but also provides new insights for selective nitrate electroreduction to NH3.