Project Portugal 2030

Summary

The main objective of the ReNOxCell project is to demonstrate a reversible solid oxide cell for efficient electrochemical elimination of toxic NOx in exhaust gases. The main innovation and originality of the concept is the design of a solid electrolyte cell with a symmetrical arrangement of electrodes/electrocatalysts with high selectivity to NOx conversion in repeated NO oxidation – storage – NO2 reduction cycles. The proposed concept (Fig.4) represents a flow-through planar cell comprising a porous ceramic solid electrolyte plate with symmetrical Ba-based electrocatalysts with mixed ionic-electronic conductivity, high selectivity for NOx conversion and NO2 storage ability. The electrodes will operate in alternating anodic/cathodic polarization cycles. Anodic polarization will promote the oxidation of NO to NO2 followed by the formation of nitrates and their storage in the form of nitrate. It is assumed that ceramic electrocatalyst material with a high content of Ba in the structure may form a thin (nanometer-size) surface layer or grains of nitrates. Then, polarity will be reversed, and cathodic polarization will promote the reduction of nitrates into nitrogen and recovery of the electrocatalyst surface. Other possible reactions may include the oxidation of soot particles and carbon-based pollutants into CO2 and competing processes of oxygen and water reduction. Fig.5 illustrates the effects of electrochemical polarization on the gas emissions at 700C predicted based on thermodynamic calculations. In addition to the common benefits of electrochemical solid oxide systems compared to other technologies, the proposed concept offers additional advantages: - Noble metal-free and therefore cost-effective system; - Anodic pre-oxidation of NO to NO2 is expected to increase selectivity in the presence of oxygen; - Long-term stability by avoiding the chemical and thermomechanical incompatibility issues between components; - Scalability: possibility to combine single cells in series into stacks to enhance the NOx conversion efficiency. The implementation of the project goal implies achieving the following objectives: i) Development of electrocatalysts with high affinity for NOx and redox reversibility, based on perovskite or perovskite-related structures, which are versatile to combine highly basic components (Ba, K) and oxides of transition metals (Ni, Co, Mn, Fe). Their redox ability with the co-existence of different valence states and related changes in oxygen stoichiometry will allow to design of electrocatalysts with suitable properties (mixed conductivity, oxygen storage ability) and reversible electrocatalytic activity. Basic A-site cations (Ba2+,K+) will play key roles by interacting with acidic NO2. ii) Fabrication of electrochemical cells based on solid oxide electrolyte membranes with tailored porous or cellular microstructure enabling continuous gas flow through the cell. The electrolyte plate must combine suitable oxygen-ionic transport properties with good thermochemical compatibility with the electrocatalyst and sufficient thermomechanical stability. iii) Demonstration of lab-scale prototypes of single cells and short stacks of cells (up to 5). The cells will be based on porous electrolyte plates with 15-20 mm diameter and thickness of around 1 mm. The cells are to operate at selected optimal temperature in the range of 300-550C. The target NOx conversion efficiency and selectivity are =90%.