According to a paper recently published on Chemical Communications, Prof. MENG Guowen's group from the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences (CAS) has made an efficient electrocatalytic material to reduce nitrate in water to nitrogen gas, providing a promising way to degrade nitrogen concentration in water.

"This material is a success in improving the selectivity of electrocatalytic reduction of nitrate for N₂ in water," said associate Prof. TANG Haibin, who was in charge of this study.

Nitrate, a dangerous water pollutant, is inclined to lead to water eutrophication, water bloom and other ecological and environmental problems. Among the nitrate removal methods and technologies, the electrocatalytic reduction method stands out as it can selectively convert nitrate (NO₃^-) into ammonium (NH₄⁺) or nitrogen gas (N₂), which can effectively reduce the concentration of total nitrogen in water, showing great application prospect for restoration of eutrophicated water body. However, the materials reported previously, although with preferential selectivity and efficiency towards ammonia/ammonium, are not suitable for application in practical water environments such as lakes and rivers.

In this study, a (111) preferentially oriented copper oxide (Cu₂O) film was prepared on the surface of a porous copper foam framework (Cu₂O@CF) by electrochemical deposition. Due to the conductive porous copper foam frame, the composite structure can be easily used as a cathode for electrocatalytic reduction of nitrate. The results showed that the catalytic reduction of nitrate to N₂ was significantly improved.  

"In alkaline solution, the removal rate of nitrate is 93% and the selectivity of N₂ is 99%; and in neutral solution, the removal rate of nitrate was 94.3% and the selectivity of N₂ is 49.2%", said Prof. TANG.

In order to verify the possible interference of other cations and anions in actual water, electrocatalytic tests were carried out using lake water of a local reservoir.  

"The removal rate reached as high as 91.1% for nitrate and the selectivity for N₂ rose up to 64.2%," said TANG, "both were higher than what we had expected."

The preparation of the cathode material in this study is simple, and it can realize high selectivity for nitrogen gas, indicating the application potential of Cu₂O in electrocatalytic reduction of nitrate to nitrogen gas.

This work provides valuable reference for further designs of stable and efficient electrocatalytic materials to remove nitrogen from water, which is helpful for the treatment and protection of water ecological environment.  

The research was financially supported by the National Natural Science Foundation of China, the Natural Science Foundation of Anhui Province, and the Key Research Program of Frontier Sciences of CAS.

(a-b) SEM images and (c) XRD pattern of the electrodeposited Cu₂O thin films on frame of Cu foam; (d) HRTEM image of the as-prepared Cu₂O thin films. (Image by TANG Haibin)

(a) Time-dependent concentration change of NO₃^--N, NO₂^--N and NH₄⁺-N; (b) N₂-selectivity and nitrate removal efficiency under the reaction conditions: 40 mg/L NO₃^-, 0.05 M K₂SO₄, pH=11, applied potential of -1.4 V (vs. Ag/AgCl). (Image by TANG Haibin)

(a) Time-dependent concentration evolution of NO₃^--N, NO₂^--N and NH₄⁺-N; (b) the N₂ selectivity and nitrate removal efficiency in spiked actual water with the reaction conditions: 40 mg/L NO₃^-, 0.05 M K₂SO₄, pH = 7, applied potential of -1.4 V (vs. Ag/AgCl). (Image by TANG Haibin)