Background:
Nitrate (NO3) electroreduction to ammonia (NH3) provides an alternative to the energy-intensive Haber-Bosch process due to its high efficiency, mild operating conditions and low chemical inputs. NO3-containing wastewater produced by a variety of chemical industries poses a significant threat to the environment due to its toxicity and carcinogenicity. Therefore, the conversion of NO3 to NH3 is of great significance for obtaining useful products and mitigating environmental problems. Considering the high cost and limited availability, researchers have been looking for non-precious electrocatalysts with high NH4+-N selectivity, with a focus on iron-based electrocatalysts due to their low cost and non-toxicity.
To keep iron-based electrodes active and inhibit their spontaneous corrosion, researchers explored a variety of strategies, including encapsulating iron in carbon, alloying iron with transition metals, and doping other elements. However, a well-designed corrosion strategy may favor the generation of new active phases on the iron surface, thereby improving hydrogen evolution reaction (HER), oxygen evolution reaction, and hydrodechlorination performance. Well-designed iron auto-corrosion plays a positive role in the electrocatalytic nitrate reduction reaction (NO3 RR), but there is a lack of research on the identification of the active phase and reaction mechanism of the reconstructed interface.
Brief introduction of the results
Ammonia production from nitrate electroreduction is particularly important to reduce environmental pollution and obtain high value-added products. While non-toxic, inexpensive iron-based materials promise to be a promising catalyst for electrochemical nitric acid reduction, complex designs are required to ensure their continued high activity and inhibit spontaneous corrosion.
Recently,The research group of Prof. Xu Zhao, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, reported an economical self-corrosion method that uses Ni2+ ions in wastewater to control the formation of Nife layered dihydroxide active phase on iron surface, thereby obtaining a high nitrate conversion rate (972%) and ammonia selectivity (903%)。The combination of nitrate reduction and acid adsorption enables the conversion of NO3 to (NH4)2SO4 for applications such as fertilizer.
This unique "waste-to-treasure" perspective not only challenges the conventional wisdom that corrosion reduces the active phase, but also significantly improves catalytic efficiency while utilizing the valuable resources of wastewater, providing a practical way to convert nitrate into a useful ammonia product. This work was published in Nature Water, a top international journal, with the title of "intentional corrosion-induced reconstruction of defective nife layered double hydroxide boosts electrocatalytic nitrate reduction to ammonia". Congratulations!
**Reading guide
Figure 1Comparison of NO3 RR properties and schematic diagram of NiFe-LDH-OV growth on iron surface in wastewater with or without Ni2+ after corrosion
Figure 2Conversion performance of NO3 to NH4+ on different cathodes
In this paper, we report that the Fe surface can be reconstructed when treating No3 in electroplating wastewater containing Ni2+. Defective nickel-iron layered double hydroxide (NiFe-LDH-OV) nanosheets, as the main active phase, grow spontaneously on an iron substrate instead of the usual rust, resulting in enhanced conversion of NO3 to NH4+ without causing passivation of the Fe electrode. This study demonstrates the feasibility and scalability of self-corrosion reconstruction of other heavy metal ions (such as CO2+, ZN2+, and Mn2+) on Fe surfaces. By coupling NO3 RR with acid absorption, the authors achieved a complete conversion of NO3- to (NH4)2SO4 products for applications such as fertilizers. This work demonstrates a "waste-to-treasure" strategy to efficiently remediate NO3 by utilizing cheap iron cathodes and heavy metal ions in actual wastewater to facilitate the formation of efficient nitrate reduction catalysts.
Figure 3Test characterization
Figure 4Mechanism analysis of NO3 RR enhancement
The NO3 RR process consists of two main pathways: direct electron reduction and atomic H* reduction. Apparently, an increase in NO3 concentration attenuates the intensity of HAS* on FENI500 FF (Figure 4A), suggesting that active hydrogen (HAS*) plays a crucial role in promoting the reduction of NO3. Reducing the initial potential increases the HADS* yield at FENI500 FF. The electron spin resonance results further confirmed that the nine-line signal of H* on the surface of FeNi500 FF was more enhanced than that of Fe (Fig. 4B).
Subsequently, an increase in tert-butanol concentration adversely affected NO3-RR, suggesting that H* plays an integral role in NO3 RR. Although Ni exhibited the slowest reaction kinetics and lacked a significant cyclic voltammetry peak associated with HADS*, the incorporation of Ni2+ promoted the reconstitution of NiFe-LDH-OV nanosheets on the Fe surface, thereby promoting the production of HAs*. Therefore, the increase in NH4+-N yield in FENI500 FF may be attributed to the formation of the NIII-O NIII-OH species.
XPS analysis of the electrodes is shown in: 6 and 532There are three peaks at 3 EV, corresponding to lattice oxygen or hydroxyl (-OH) bonds, defect oxygen, defect oxygen, and adsorbed oxygen (OADS) (Figure 4C). After NO3 RR treatment, the OV density of the FEN500 FF cathode is 5237%, which is 500 of the original FENI1 FF27-fold, indicating that the regeneration of OVS after electrochemical reduction was conducive to electron transfer and increased the activity of NO3 RR. XPS spectra of Fe2P indicate the presence of FeIII and FEII in FeNi500 FF, and a positive binding energy shift is observed, indicating a charge redistribution between Fe and Ni. 856.4 and 874Two peaks of 3 EV confirmed that the nickel in the FENTI500 FF was predominantly Ni2+. After NO3 RR treatment, the spectral intensity of 2P was consistent with the original spectrum, confirming the stability of FENI500 FF. These results show that Ni2+ can effectively regulate the electronic structure of Fe.
Figure 5In situ Raman analysis
Figure 6Computational fluid dynamics** analysis and ammonia product collectionSummary and outlook
In summary, the authors propose a "waste-to-treasure" approach for the construction of high-performance iron-based electrodes for NO3-RR in wastewater. Due to the presence of heavy metal ions, self-corrosion occurs on the surface of Fe, forming layered dihydroxide nanosheets, which avoids traditional corrosion passivation. Experimental and simulation results show that -FeoOh and defective -Ni(OH)2 formed by electrochemically driven phase separation enhance the adsorption of NO3- and the generation of H* and the transfer of electrons, and promote the conversion of NO3- to NH4+. The combination of NO3-RR effluent and acid adsorption was used to successfully produce (NH4)2SO4 fertilizer products. Overall, this unique perspective is expected to expand the design of electrocatalysts and provide new insights into wastewater treatment applications.
Bibliographic information
intentional corrosion-induced reconstruction of defective nife layered double hydroxide boosts electrocatalytic nitrate reduction to ammonia. (nat. water 2023, doi: 10.1038/s44221-023-00169-3)