An apparatus for the synthesis and treatment of electrocatalyst particles in batch or continuous fashion is provided. In one embodiment, the apparatus is comprised of a three-electrode cell which includes a cell body electrode, a reference electrode and a floating counter electrode. The floating counter electrode may be disposed inside a counter electrode compartment which is adapted to float in a slurry containing a plurality of nanoparticles and an electrolyte.
 This invention was made with Government support under contract numbers DE-AC02-98CH10886 and DE-SC0012704 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.