Run Xiong,Bincheng Xu,Zhixuan Chen,Chen Cheng,Ying Wang
Electrocatalytic nitrate (NO 3 − ) reduction reaction (NitRR) provides us a green solution to synthesize ammonia (NH 3 ) and ease environmental problems. However, the development of robust electrodes with high NH 3 yield rates and Faradaic efficiencies (FEs) is still challenging. Herein, we report a self-supporting electrode incorporating vertically aligned Co-based metal-organic framework (MOF) nanosheets (Co-NDC/NF), which facilitates electron and mass transfer for efficient NitRR to NH 3 within a wide range of NO 3 − concentration (0.05−1.0 M). In 1.0 M KOH + 1.0 M KNO 3 electrolytes, such Co-NDC/NF reaches a notable NH 3 yield rate of 19,200 ug h −1 cm −2 , a high FE of 93.9% and excellent electrochemical stability, which is superior to most electrocatalysts. A flow-through reactor with large-area Co-NDC/NF also achieves the continuous production of 1096 mg L −1 NH 3 under an industrial current density. Furthermore, the rapid transformation of *NO 2 − to *NO and selective conversion of *NO to *NOH played critical roles in NitRR toward ultrafast NH 3 production. Our research not only offers a facile fabrication procedure for high-performance electrode, but also gives new insights into the enhanced electron transfer and mass transport within MOF-based electrode in electrochemical reactions.
A novel self-organized nanoporous V O -Co 3 O 4 /Co cathode was prepared via anodization and plasma treatment and obtained a significant nitrate reduction efficiency. In the anodization, an oxide layer with the nano-sized pore structure initially grew in-situ on the Co substrate and showed a better surface area. Subsequently, He-plasma increased surface oxygen vacancies (V O ) from 24 % to 57 %. Electrons in vacancies were charged into empty e g orbital of low-spin Co 3+ (O h , octahedral) and firstly generated high-spin Co 2+ (O h ) with the configuration of t 2g 6 e g 1 , accounting for 71.7 % of cobalt species. Accordingly, two original mechanisms (Vo-catalyzed and Co 2+ (O h )-catalyzed) were concluded in this study. Oxygen vacancies increased the charge intensity and served as absorption sites in nitrate reduction. Meanwhile, massive Co 2+ (O h ) provided electrons in the e g orbital with a higher energy state and mediated the faster electron transfer through a Co 2+ –Co 3+ –Co 2+ redox cycle, compared with Co 2+ (T d , tetrahedral). Ultimately, a faster reaction kinetic of 0.0220 min −1 was achieved by V O -Co 3 O 4 than other cathodes e.g., Co 3 O 4 (0.0150 min −1 ). Such V O -Co 3 O 4 /Co cathode-based denitrification strategy displayed great advantages in engineering application and completely removed 90 % of TN from actual wastewater.
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