The electrochemical nitrate reduction reaction (NO3RR) is a promising approach for the conversion of nitrate contaminants into ammonia (NH3), a desirable chemical nowadays produced with the energy-intensive Haber Bosch process. Cu remains one of the most effective catalysts for NO3RR, however, its NH3 selectivity and production rate are still limited. The fundamental reactivity/structure/composition relationships need further elucidation to move the field forward. Herein, we study the impact of size, shape and composition of Cu catalysts in NO3RR using a library of well-defined and shaped tunable Cu and Cu oxide nanocrystals. These catalysts exhibit different performance, which needs to be explained considering their status during operation. This proposal aims at using operando X-Ray absorption spectroscopy (XAS) to follow dynamic changes in coordination environment and oxidation state of Cu under NO3RR