Electrochemical tests of various electrocatalysts. (IMAGE)
Caption
Several single/binary/ternary MOx electrocatalysts (M = Fe, Co, Ni, Cu, FeCo, FeNi, FeCu, CoCu, FeCoNi and FeNiCu) as control samples were also prepared using the same Joule-heating synthesis method and electrochemical tested.
The CuOx electrocatalyst exhibits the most positive onset potential, suggesting the highest catalytic activity for NO3−RR. All Cu-containing oxide electrocatalysts (CuOx, CoCuOx, FeCuOx, FeNiCuOx and FeCoNiCuOx) have lower overpotentials than the samples without Cu (NiOx, CoOx, FeOx, and FeCoNiOx), which further confirms that the Cu element significantly contributes to improving the catalytic activity of NO3−RR. However, the RNH3/NO2− (0.13) of the CuOx electrocatalyst is the lowest of all samples. Furthermore, the CuOx electrocatalyst shows the highest Tafel slope (266 mV dec−1) in 0.1 M KOH electrolyte containing 0.1 M NaNO2, signifying a slow kinetics for the NO2− reduction and electron transfer rate. Therefore, Cu effectively facilitates the formation of NO2−, but the hydrogenation of NO2− to NH3 requires the synergy among Fe, Ni, and Co elements.
The CoOx electrocatalyst demonstrates the highest RNH3/NO2− (13.6), which confirms its superior selectivity for NH3. In the NO2−RR, the CoOx electrocatalyst has the lowest Tafel slope (106 mV dec−1), revealing that the Co has faster electron transfer efficiency, which is favorable for the reduction of NO2− to NH3. However, the high overpotential and the low rNH3 of the CoOx electrocatalyst suggest that the Co has a slow rate of reducing NO3− to NO2−, hindering the efficient reduction of NO3−. The RNH3/NO2−, rNH3 and FENH3 of the (FeCoNiCu)Ox electrocatalyst significantly exceed those of the (FeNiCu)Ox electrocatalyst, which further proves that the Co is the active sites for the reduction of NO2− to NH3 in the (FeCoNiCu)Ox electrocatalyst.
After adding Fe and Ni, the (FeCoNiCu)Ox and (FeCoNi)Ox electrocatalysts show significantly improved the NO3−RR performances than that of (CoCu)Ox. Compared to CuOx and CoOx electrocatalysts, the FeOx and NiOx electrocatalysts exhibit the lower Tafel slopes (118 mV dec−1 and 186 mV dec−1) in 0.1 M KOH electrolyte, which indicates that Fe and Ni are more favorable for water dissociation and proton generation.
Therefore, the synergistic effects of Fe, Co, Ni and Cu in the (FeCoNiCu)Ox electrocatalyst not only suppress the competition of HER but also significantly improve the selectivity and yield rate of NH3. Finally, the (FeCoNiCu)Ox electrocatalyst achieves the best FENH3 and rNH3.
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