A new concept of electrocatalytic reactor for ultimate NOx decomposition by nanostructural and compositional control of a NiO-YSZ electrocatalytic electrode has been proposed. Here, we present the electrochemically induced formation of nanosize Ni grains surrounded by nanopores in the NiO/YSZ interface of an electrocatalytic electrode under the reactor operation. In such an electrochemically assembled nanostructure, the reversibly produced 10-50 nm Ni grains are an effective place for selective NOx decomposition. Oxygen ionic current passed through the network of YSZ particles surrounding the Ni grains removed the produced oxygen species and permitted the reaction to reoccur. In such reactors, the electrical power required for NO decomposition in the presence of 10% of oxygen is at least 60 times lower than for traditional cells. The reactors' efficiency should lead to them replacing catalytic systems for exhaust gas purification. (C) 2004 Elsevier B.V. All rights reserved.