碳负载富氧空位的NiCo2O4用于电催化还原硝酸盐

水体中硝酸盐是一种广泛存在的污染物，因此，开发用于水中硝酸盐还原的高效电催化剂受到广泛关注.采用溶胶-凝胶法耦合硼氢化钠还原法制备了碳负载富氧空位的NiCo₂O_4-x/C电催化剂，并研究其去除硝酸盐的性能.结果发现，在该体系中，耦合导电碳载体可改善半导体型电催化剂导电性，构建氧空位可促使原子H^*生成，最终实现硝酸盐高效还原.NiCo₂O_4-x/C阴极在电流密度为20 mA·cm^-2、pH=7的条件下，可于3 h内去除水中94.8%的NO₃^--N，相较于NiCo₂O₄及NiCo₂O₄/C阴极，NO₃^--N去除率分别提高了36.6%和12.2%.此外，NiCo₂O_4-x/C阴极在连续5次还原硝酸盐过程中性能并未有明显变化.结合掩蔽实验和电子顺磁共振波谱分析证实，NiCo₂O_4-x/C阴极除了可通过直接电子还原硝酸盐外，还可以通过生成原子H^*间接还原硝酸盐.通过对实际废水处理进一步验证其转化硝酸盐的性能，研究表明，NiCo₂O_4-x/C阴极可有效去除焦化废水生物出水中硝酸盐.

Carbon-supported NiCo2O4 with oxygen-rich vacancies for electrocatalytic reduction of nitrate

Nitrate is a ubiquitous pollutant in water. Developing efficient electrocatalysts for nitrate removal has attracted widespread attention. In this study, we used the sol-gel coupled with sodium borohydride reduction method to prepare carbon-supported NiCo₂O₄ with oxygen-rich vacancies (NiCo₂O_4-x/C) electrocatalyst, and investigated its nitrate removal performance. In this system, the involvement of conductive carbon supports can improve the conductivity of semiconductor electrocatalysts, and the construction of oxygen vacancies can promote the generation of atomic H^*, which ultimately render the obtained-catalyst attractive in electrochemical reduction of nitrate. The resulting NiCo₂O_4-x/C cathode removed 94.8% of NO₃^--N in synthetic wastewater within 3 hours under the conditions of current density of 20 mA·cm^-2 and pH=7. The removal rate increased by 36.6% and 12.2% in comparison to the NiCo₂O₄ cathode and the NiCo₂O₄/C cathode, respectively. In addition, the performance of the NiCo₂O_4-x/C cathode towards nitrate reduction did not significantly change upon 5-cycle consecutive operation. The combination of the scavenger experiments and the electron paramagnetic resonance (EPR) measurements confirmed that the atomic H^*-mediated indirect pathway also plays a significant role in nitrate reduction, in addition to the direct reduction pathway with respect to the NiCo₂O_4-x/C cathode. Testing with the real industrial wastewater further verified its potential application for nitrate conversion, as evident from the fact that the NiCo₂O_4-x/C cathode can effectively remove nitrate in the bio-treated coking wastewater (BTCW).