Benchmarking three-dimensional metal foam electrodes for the electrochemical reduction of nitrate

Abstract

The electrochemical reduction of nitrate (ERN) is a promising and sustainable strategy for addressing the critical issue of nitrate pollution in water sources. The rational design of electrocatalysts has spotlighted metal-based three-dimensional (3D) electrodes such as Cu and Ni foams. Metallic Cu foam showcases promising kinetics for ERN conversion, while Ni foam serves as a robust support material for self-standing catalyst evaluation. This review underscores the nuances and challenges in ERN research when exploiting 3D electrodes, emphasizing the overlooked roles of pore per inch (PPI) and electrochemically active surface area (ECSA), as well as the lack of standardization and inconsistent reporting practices, which hinders direct performance comparison among different ERN studies. The PPI value controls reactant and products mass transfer at the interface and impacts on the magnitude and extension of the solution alkalinization at the electrode surface, while ECSA is essential for accurately comparing the specific electrocatalytic activity of metal foam electrodes for ERN. Particular attention is devoted to modified self-standing 3D electrode materials, where the catalytic performance on either Cu or Ni foams is altered by adding other metals and/or metal oxides (Cufoam/M) and (Nifoam/M), respectively. A detailed analysis of selected modified Cu and Ni foam electrocatalysts available in the literature is provided to demonstrate the lack of standardization reporting ERN performance at present. Thus, we propose the adoption of more rigorous characterization and reporting practices to advance the development of efficient and scalable ERN systems for sustainable water treatment. This includes PPI value, ECSA determination and additional engineering figures of merit encompassing both selectivity and conversion such as NH3 generation efficiency (%) and ERN energy efficiency (%). Parameters such as nitrate conversion, faradaic efficiency and selectivity, often used as benchmarks, are considered not suitable enough for comparing different ERN studies because they depend on the initial nitrate concentration, the total charge circulated and the type of electrolysis (galvanostatic or potentiostatic).